Pharmacology & Therapeutics 121 (2009) 20–28

Contents lists available at ScienceDirect

Pharmacology & Therapeutics

j ourna l homepage: www.e lsev ie r.com/ locate /pharmthera

The thyrotropin-releasing hormone (TRH)–immune system homeostatic hypothesis

J. Kamath a,⁎, G.G. Yarbrough b, A.J. Prange Jr. c, A. Winokur a

a University of Connecticut Health Center, Department of Psychiatry, 263 Farmington Avenue, Farmington, CT 06030, United Statesb TRH Therapeutics LLC, 2366 NW Pettygrove Street, Portland, OR 97210, United Statesc University of North Carolina, Department of Psychiatry, 6503 Meadow View Road, Hillsborough, NC 27278, United States

⁎ Corresponding author. University of Connecticut HeFloor, Farmington CT 06030-6415, United States. Tel.: 86

E-mail address: kamath@psychiatry.uchc.edu (J. Kam

0163-7258/$ – see front matter © 2008 Elsevier Inc. Aldoi:10.1016/j.pharmthera.2008.09.004

a b s t r a c t

a r t i c l e i n f o

Keywords:

TRH

Decades of researchhave estafar beyond its role as a regula

Thyrotropin-releasing hormoneImmunecytokineInflammationHomeostasis

Abbreviations:CNS, central nervous systemCRH, corticotropin-releasing hormoneD2, type 2 deiodinaseDMV, dorsal motor nucleus of the vagusHPA, hypothalamic-pituitary–adrenocorticalHPT, hypothalamic-pituitary–thyroidIEL, intraepithelial lymphocytesIFN-γ, interferon gammaIL-1, interleukin-1IL-2, interleukin-2IL-6, interleukin-6LPS, lipopolysaccharideNTS, nucleus tractus solitariusPBMC, peripheral blood mononuclear cellsPRL, prolactinPVN, paraventricular nucleiSRBC, sheep red blood cellsTNF-α, tumor necrosis factor-alphaTRH, thyrotropin-releasing hormoneTRH-R, TRH receptorTSH, thyroid stimulating hormoneVLM, ventrolateral medulla

blished that thebiological functions of thyrotropin-releasinghormone (TRH) extendtor of the hypothalamic-pituitary–thyroid axis. Gary et al. [Gary, K.A., Sevarino, K.A.,

Yarbrough, G.G., Prange, A.J. Jr., Winokur, A. (2003). The thyrotropin-releasing hormone (TRH) hypothesis ofhomeostatic regulation: implications for TRH-based therapeutics. J Pharmacol Exp Ther 305(2):410–416.] andYarbrough et al. [Yarbrough, G.G., Kamath, J., Winokur, A., Prange, A.J. Jr. (2007). Thyrotropin-releasing hormone(TRH) in theneuroaxis: therapeutic effects reflect physiological functions andmolecular actions.MedHypotheses69(6):1249–1256.] provided a functional framework, predicated on its global homeostatic influences, toconceptualize the numerous interactions of TRH with the central nervous system (CNS) and endocrine system.Herein, we profer a similar analysis to interactions of TRH with the immune system.Autocrine/paracrine cellular signalingmotifs of TRHandTRH receptors are expressed in several tissues andorgansof the immune system. Consistentwith this functional distribution, invitro and invivo evidence suggests a criticalrole for TRHduring the developmental stages of the immune systemaswell as its numerous interactionswith thefully developed immune system. Considerable evidence supports a pivotal role for TRH in the pathophysiology ofthe inflammatory process with specific relevance to the “cytokine-induced sickness behavior” paradigm. Thesefindings, combined with a number of documented clinical actions of TRH strongly support a potential utility ofTRH-based therapeutics in select inflammatory disorders.Similar to its global role in behavioral and energy homeostasis a homeostatic role for TRH in its interactions withthe immune system is consonant with the large body of available data. Recent advances in the field ofimmunology provide a significant opportunity for investigation of the TRH-immune system homeostatichypothesis. Moreover, this hypothesis may provide a foundation for the development of TRH-based therapeuticsfor certain medical and psychiatric disorders involving immune dysfunction.

© 2008 Elsevier Inc. All rights reserved.

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212. The expression of thyrotropin-releasing hormone and thyrotropin-releasing hormone-receptor in the immune

system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213. In vitro evidence for interactions between thyrotropin-releasing hormone and the immune system. . . . . . 224. In vivo evidence for interactions between thyrotropin-releasing hormone and the immune system . . . . . . 22

4.1. Effects of thyrotropin-releasing hormone on the immune system. . . . . . . . . . . . . . . . . . . 224.2. Effects of cytokines on thyrotropin-releasing hormone systems . . . . . . . . . . . . . . . . . . . 234.3. Role for thyrotropin-releasing hormone in the regulation of immune responses. . . . . . . . . . . . 23

alth Center, University of Connecticut School of Medicine, Department of Psychiatry, 10 Talcott Notch Road, East Lobby, 3rd0 679 6727; fax: 860 679 6781.ath).

l rights reserved.

21J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

5. Connections between thyrotropin-releasing hormone and the immune system . . . . . . . . . . . . . . . 235.1. Anatomical framework for thyrotropin-releasing hormone–immune system interactions . . . . . . . 235.2. Experimental evidence for vagus-dependent thyrotropin-releasing hormone–immune system

interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.3. Experimental evidence for vagus-independent thyrotropin-releasing hormone–immune system

interactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 245.4. Role of thyrotropin-releasing hormone in the pathophysiology of the inflammatory process. . . . . . 25

6. Thyrotropin-releasing hormone-based therapeutics in inflammatory disorders . . . . . . . . . . . . . . . 256.1. Cytokine-induced sickness syndrome as a therapeutic target in inflammatory disorders . . . . . . . . 266.2. Challenges and advances in the development of thyrotropin-releasing hormone-based therapeutics . . 27

7. Future investigations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27

1. Introduction

Ader first used the term “psychoneuroimmunology” in 1980(Ader, 1980) to describe the increasing body of evidence aboutinteractions between the brain and the immune system. Since thenresearch has demonstrated significant involvement of the endocrinesystem in brain-immune interactions in both health and diseasestates. These multi-directional interactions can occur at many stagesof development, and they are a continual part of the drive tomaintain homeostasis. Defects or deficiencies in one or more of thesesystems can lead to a specific disorder or aggravate a variety of otherdisorders.

Interactions between the central nervous system (CNS), the en-docrine system and the immune system are mediated at multiplelevels. These mediators include secreted chemical messengers such ashormones, cytokines, neurotransmitters and neuropeptides actingdirectly or via the nervous system. Evidence indicates interactions atthe level of receptors (e.g., the presence of neuroendocrine peptidereceptors on immune cells), at the level of secretory function (e.g., thesynthesis and secretion of neuroendocrine peptides by immune cells),and at the level of signal transduction.

Interactions of specific endocrine systems (e.g., the hypothalamic-pituitary–adrenocortical [HPA] axis) with the CNS and the immunesystem have been extensively described (Sternberg, 1995; Eskandari &Sternberg, 2002). Hypophysectomized rats and mice exhibit decrea-sed antibody response, decreased lymphocyte proliferation, reducedspleen natural killer cell activity, and prolongation of graft survival(Keller et al., 1988; Nagy & Berczi, 1978). Despite the increasing evi-dence in individual reports, few reviews have attempted to formulateinteractions between the hypothalamic-pituitary–thyroid (HPT) axis,the CNS and the immune system (Pawlikowski et al., 1994; Kruger,1996). To date, no review has delineated the interactions of one of thecritical hormones of the HPT axis, thyrotropin-releasing hormone(TRH), with immune and other systems. In the present report, we firstreview and discuss some of the pivotal evidence for interactions be-tween TRH, the CNS and the immune system and then propose afunctional framework in which to conceptualize the accumulatingevidence. We propose a TRH-immune system homeostatic hypothesis.The TRH-immune system homeostatic hypothesis states that TRH-mediated mechanisms respond to many elements of the immunesystem and affect them in ways that tend to maintain or restorehomeostasis.

Finally, we describe potential implications of this framework forthe pathophysiology of certain disorders and provide a rationale forTRH-based therapeutics.

The tripeptide thyrotropin-releasing hormone (TRH) is known tocontrol the synthesis and secretion of pituitary thyrotropin (thyroidstimulating hormone, TSH) and prolactin (PRL) (comprehensive re-view in Nillni & Sevarino, 1999). TRH-secreting neurons are located inthe medial portions of the paraventricular nuclei (PVN) of the hypo-thalamus; their axons terminate in the medial portion of the external

layer of the median eminence (Guillemin, 1978). Originally discoveredin the hypothalamus, consistent with its classical role as a hypotha-lamic hypophysiotrophic factor, TRH is now known to be distributedextensively in extrahypothalamic brain structures (Winokur & Utiger,1974; Yarbrough,1979) and in other organs and tissues (Lechan,1993).Similarly, receptors for TRH are found throughout the central andperipheral nervous system as well as in other organs and tissues (Sunet al., 2003). The widespread distribution of TRH and its receptorssuggests other important functions for this tripeptide, including pos-sible critical interactions with other biological systems (Gary et al.,2003; Yarbrough et al., 2007). The TRH receptors (TRH-R) belong to theseven transmembrane-spanning, G protein-coupledmembrane recep-tor family (Sun et al., 2003). Two receptor isoforms, TRH receptor R1(TRH-R1) and TRH receptor R2 (TRH-R2) have been identified (Ger-shengorn&Osman,1996). In the brainstem, TRH-R1has been shown tobe present in the dorsal motor nucleus of the vagus (DMV) and thenucleus tractus solitarius (NTS),while TRH-R2has been localized to thereticular formation, dorsal tegmental nucleus and spinal trigeminalnucleus (Heuer et al., 2000). TRH signaling occurs mainly via thephosphatidylinositol–calcium–protein kinase C transduction pathway,with subsequent elevations in intracellular calcium, and modulationof K+ channel conductance (Gershengorn & Osman, 1996). Notably,increasing evidence (Mellado et al., 1999; Montagne et al., 1999; Matreet al., 2003) supports the distribution of TRH and TRH receptors in theimmune system and a number of studies provide data supportingpotential interactions of TRH with the immune system, even at thelevel of regulation of transcription.

2. The expression of thyrotropin-releasing hormoneand thyrotropin-releasing hormone-receptor in the immune system

In addition to its classical function as a hypothalamic hypophysio-trophic factor, the widespread distribution of TRH and its receptorssuggests that the tripeptide plays important roles in other systems.TRH has been identified throughout the CNS, including retina andspinal cord (Martino et al., 1980; Gary et al., 2003). TRH immunor-eactivity has been detected in several peripheral tissues. Polymerasechain reaction (PCR) amplification analyses detected the expressionof TRH in testes, adrenal glands, lymphoid organs, thymus, and spleen(Montagne et al., 1999). Immunohistochemistry analyses of rat ad-renal gland extracts showed that TRH identified in this tissue issynthesized in mast cells (Montagne et al., 1997). It is possible thatTRH identified in other peripheral tissues may, in fact, be synthesizedthere in the cells of the immune system. Similar to the expression ofTRH, the expression of TRH receptors has been detected in severalextrahypothalamic brain structures and peripheral tissues (Sun et al.,2003). TRH receptors have been detected in hematopoietic tissuesrelated to the immune system, including thymus, bone marrow andlymphoid tissue (Sun et al., 2003). Northern blot analyses have iden-tified TRH-R mRNA in immune cells (Raiden et al., 1995). Western blotanalyses of extracts of rat lymphoid organs showed expression of TRH-

22 J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

R in thymus, mesenteric lymph nodes and spleen extracts (Melladoet al., 1999; Montagne et al., 1999). Expression analyses of rat tissuesusing monoclonal anti-TRH specific receptor antibodies have detectedthe presence of TRH receptors in several peripheral tissues relatedto the immune system, including thymus and lymphoid tissues(Fukusumi et al., 1995; Wang et al., 1997; Mellado et al., 1999; Bilek,2000; Yamada et al., 2000). Analysis of human peripheral blood withthese monoclonal antibodies detected TRH receptor expression innon-activated and phytohemagglutinin-activated T and B lympho-cytes (Mallado et al.,1999). TRH receptor expression has been detectedin both peripheral blood mononuclear cells (PBMC)-derived and intonsil-derived B and Tcells (Mallado et al., 1999). Altogether, it appearsthat TRH and its receptors exist and function as autocrine/paracrinesystems in the immune system and other peripheral tissues andorgans, perhaps analogous to its extrahypothalamic neurotransmitter/neuromodulatory networks in the CNS.

TRH may also influence the immune network indirectly. Thetripeptide stimulates TSH (and thyroid hormones) and PRL, and bothTSH and PRL have robust immunomodulatory properties (Kelley et al.,2007). Many different cells of the immune network have been shown toproduce TSH. These include T cells, B cells, splenic dendritic cells, bonemarrow hematopoietic cells, intestinal epithelial cells and lymphocytes(Klein, 2006). TRH was reported to stimulate the release of TSH fromimmune cells, and this effect was completely blocked by triiodothyr-onine (T3) administration (Komorowski et al., 1993). The presence ofTSH receptors has been documented on multiple cells of the immunesystem, including lymphoid and myeloid cells, on select immune cellpopulations in the bone marrow, and on intestinal T cells (Klein, 2003).Similarly, expression of PRL has been shown in many different types ofimmune cells, andPRL can beproducedby T lymphocytes andother cellsof the immune system (Ben-Jonathan et al., 1996). Detailed discussionsof interactions of TSH, thyroid hormones, and PRL with the immunesystem have been provided by Klein (2006) and Yu-Lee (2002).

3. In vitro evidence for interactions betweenthyrotropin-releasing hormone and the immune system

Several studies have investigated the role of TRHwithin the immunesystem. Lesnikov et al. (1992) reported that a stereotactic lesion of theanterior hypothalamic area in mice produced rapid involution of thethymus and a reduction of lymphocytes in peripheral blood. This effectwas prevented by the post-operational administration of TRH ormelatonin and seemed to reflect direct activity of TRH on thymic targetsor binding sites on lymphocytes. Consistent with this evidence, TRH hasbeen reported to increase thymocyte cell proliferation in rats (Pawli-kowski et al., 1992;Winczyk & Pawlikowski, 2000) and has been shownto antagonize the involution of the thymus produced by prednisolone(Pierpaoli & Yi, 1990). Thymectomy in pubertal rats resulted in sig-nificant depression of both TRH and TSH concentrations (Serebrov et al.,1992). TRH receptors have been detected on rat splenocytes (Raidenet al., 1995), and TRH has been shown to stimulate splenocyte pro-liferation (Raiden et al., 1995). Conversely, Kunert-Radek et al. (1991)reported suppression of spontaneous splenocyte proliferation by TRH.Studies in athymic mice indicate that TRH and TSH significantly in-fluence the development of lymphoid cells associated with intestinalintraepithelial lymphocytes (IEL) (Wang & Klein, 1995). TRH receptorshave been detected on IEL, and these receptors have been reported to beinvolved in the synthesis and secretion of TSH from intestinal T cells(Wang et al., 1997). Mice with congenitally mutant TSH receptors havebeen found to have a selectively impaired intestinal T cell repertoire(Wang et al., 1997).

In vitro studies of effects of TRH on immune system function havesuggested both stimulatory and inhibitory effects (Pawlikowski et al.,1994). For example, at low concentrations, TRH enhanced the T cell-independent antigen-induced antibody response via the production ofTSH (Kruger et al., 1989). In contrast, Hart et al. (1990) demonstrated a

decrease in IgG production and inhibition of monocytes by TRH.Moreover, Grasso et al. (1998) reported a stimulatory effect of TRH onin vitro interferon gamma (IFN-γ) production by human PBMCs. Incontrast, human whole blood cells stimulated by mitogen, when in-cubated with TRH and imipramine, showed suppression of IFN-γ andinterleukin-10 (IL-10) production (Kubera et al., 2000). Matre et al.(2003) established a novel functional link between TRH and theimmune system by providing evidence that the human TRH-R1 re-ceptor is transcriptionally regulated by the hematopoietic transcrip-tion factor c-Myb.

In summary, TRH interacts with the immune system both during itsdevelopment and in its fully developed state. These interactions are bi-directional and occur at all levels of the HPT axis. The effects of TRH onthe immune systemcanbe either stimulatoryor inhibitoryandare state-dependent. It is important to note that these interactions have beeninvestigated, to date, in the normal or inflammatory state; they have notbeen evaluated in the immunosuppressed state. The emergence ofevidence suggesting interactions of these systems at the level oftranscriptional regulation/signal transduction provides an opportunityto identify novel links as well as potential therapeutic targets.

4. In vivo evidence for interactions betweenthyrotropin-releasing hormone and the immune system

Similar to the in vitro evidence, the in vivo evidence suggests bothstimulatory and inhibitory interactions between TRH and the immunesystem.

4.1. Effects of thyrotropin-releasing hormone on the immune system

Consistent with the in vitro evidence, intravenous TRH showedstimulatory effects on IFN-γ production in five normoprolactinemicwomen (Grasso et al., 1998). In healthy controls, intravenous TRH ledto an increase in interleukin-2 (IL-2) concentrations (Komorowskiet al., 1994; Trejbal et al., 2001), while in patients with hypothyroidism(with high baseline IL-2 concentrations), intravenous TRH caused adecrease in IL-2 concentrations (Trejbal et al., 2001).

Studies conducted in animal models as well as in humans havesuggested a role for TRH interactions in the pathophysiology of specificdisorders involving changes in the immune system. Preliminary resultshave suggested a therapeutic potential for TRH analogs in thetreatment of these disorders. TRH exerted a powerful protective effectinmice challenged with encephalomyelitis virus (Pierpaoli & Yi, 1990).It decreased the intensity of fungal invasion, decreased mortalityrate, and increased survival time in a mouse model of experimentalcandidosis (Błaszkowska et al., 2004). Shimanko et al. (1992) reportedprotective effects of TRH in the treatment of edematous and des-tructive forms of acute pancreatitis. Intravenous and intra-lymphaticvessel administration of TRH in 15 patients with acute pancreatitis ledtodecreased edema anddecreased amylasemia (Shimanko et al.,1992).In a series of experiments in rats, Yoneda et al. (2003, 2005a) reportedstimulation of hepatic and pancreatic blood flowwith microinjectionsof a TRH analog in the dorsal vagal complex. The stimulatory effect onhepatic blood flowwas completely blocked by left cervical and hepaticbranch vagotomy but not by right cervical vagotomy (Yoneda et al.,2003). Similarly stimulation of pancreatic blood flow was blocked bycervical vagotomyon the side ofmicroinjection but not on the oppositeside or by subdiaphragamtic vagotomy (Yoneda et al., 2005a). Theeffect was also blocked by pretreatment with atropine or N(G)-nitro-L-arginine-methyl-ester (L-NAME), suggesting involvement of vagal-cholinergic and nitric oxide-dependent pathways (Yoneda et al.,2005a). The same group reported protective effects of a centrallyadministered TRH analog on cerulin-induced acute pancreatitis in ratsand blocking of this protective effect by subdiaphragmatic vagotomyorby pretreatment with L-NAME (Yoneda et al., 2005b). Taché et al.(2006) reported similar effects of central TRHadministration on gastric

23J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

function. TRH injected into the DMNor cisternamagna increased vagalefferent discharge, activated cholinergic neurons in gastric submucosaland myenteric plexuses and induced a vagal-dependent, atropine-sensitive stimulation of gastric secretory and motor functions. TRHantibody or TRH-R1 oligodeoxynucleotide antisense pretreatment inthe DMN or cisterna magna completely abolished this effect (Tachéet al., 2006). A TRH analogue was shown to induce gastric hyperemiavia degranulation of mast cells (Kawakubo et al., 2005; Santos et al.,1996). Central TRH administration was shown to induce acute gastriclesions via vagal stimulation of ulcerogenic factors (acid, pepsin,motility, histamine) (Taché & Yoneda, 1993; Stephens et al., 1988). Inthe same set of experiments TRH was shown to have a cytoprotectiveeffect against ethanol-induced gastric lesions by vagal-cholinergicstimulation of protective factors (prostaglandin, increased blood flow)(Taché & Yoneda, 1993).

4.2. Effects of cytokines on thyrotropin-releasing hormone systems

The immune system,mainly involving cytokines, similarly seems toexert stimulatory or inhibitory effects on TRH systems. Prepro-TRHmRNA levels did not change in the acute phase of the lipopolysacchar-ide (LPS)-induced model of inflammation (Boelen et al., 2004). How-ever, a significant increase in the expression of type 2 deiodinase (D2)mRNAwas seen in the hypothalamus. The authors suggested that thisenhanced D2 activity is a precursor for decreased hypothalamic TRHvia increased local T3 generation due to negative feedback. This wasconfirmed in subsequent experiments conducted by Boelen et al.(2006) and by Pekary et al. (2007). Prepro-TRH mRNA concentrationssignificantly decreased 48 h after LPS administration in the PVN, andthis correlated with increased expression of the pro-inflammatorycytokine interleukin-1 beta (IL-1β) in the hypothalamus (Boelen et al.,2006; Pekary et al., 2007). Boelen et al. (2006) noted that the D2 pro-moter region contains multiple nuclear factor (NF-kB) binding sites,suggesting a novel interaction at the level of transcriptional regulationbetween the immune system and TRH in the hypothalamus.

Multiple pro-inflammatory cytokines have been tested for their effectson the HPT axis. Notably, these cytokines affect the HPT axis at multiplelevels, leading to decreases in TRH expression, plasma TSH and thyroidhormone concentrations. However, the most dramatic effect seems to beon hypothalamic TRH expression. Continuous intraperitoneal infusion ofinterleukin-1 (IL-1) led to a 73% decrease in hypothalamic pro-TRHmRNAconcentrations (van Haasteren et al., 1994). Similarly, a single intravenousinjection of tumor necrosis factor-alpha (TNF-α; cachectin) reducedhypothalamic TRH (Panget al.,1989), and increasing daily doses of TNF ledto further significant reduction inhypothalamic TRH concentrations (Panget al., 1989). IL-6 has been shown to inhibit TRH-stimulated PRL secretionand has also been shown to inhibit TRH-stimulated free cytosolic calciumincrease (Schettini et al., 1991).

In summary, the in vivo evidence, like the in vitro evidence,demonstrates bi-directional interactions between TRH and the immunesystem, occurring at all levels of the HPT axis. Evidence supports thenotion that these interactions (stimulatory vs. inhibitory) may be state-dependent, suggesting a homeostatic role for TRH in these interactions.However, just as with the in vitro evidence, effects of cytokines onTRH and on the HPT axis have been evaluated only in the normal orinflammatory state. A homeostatic role of TRH in these interactions canbe conclusively established only after evaluations in the immunosup-pressed state.

4.3. Role for thyrotropin-releasinghormone in the regulation of immune responses

In contrast to the LPS model of nonspecific inflammation (T-cellindependent response), where an immediate suppression of TRH isseen, the T-cell dependent antigens, i.e. sheep red blood cells (SRBC),elicited a rapid increase in hypothalamic TRH and pituitary TRH-R

mRNAs in the early phase (4–24 h post immunization). Notably, adecrease in levels similar to the LPS-induced, i.e. T-cell independent,response followed this initial rise (Perez et al., 1999). The initialincrease in TRH was accompanied by a rise in plasma PRL levels.

Intracerebroventricular injection of antisense oligonucleotide com-plementary to rat TRH mRNA resulted in a significant inhibition ofspecific antibody production and concomitant inability to produce thepeak in plasma PRL levels in this model. This suggests that the T cell-dependent immune response and clonal expansion of T cells forappropriate antibody generation is critically dependent on the earlyactivation of TRH (Perez et al., 1999). It is unclear which pathwaysmediate this early rise in hypothalamic TRH seen in the T-cell dependentresponse. It is possible that other neuromodulators (e.g., NPY, CART,glutamate, and vasopressin), which are activated during this immuneresponse, may play roles in this early rise in TRH (Wittman, 2008).Evidence suggests that this early increase in TRH is then overridden bythe direct suppressive effects of proinflammatory cytokines on TRH inthe PVN, as in the LPS model. In summary, evidence also supports acritical role of TRH in the T-cell dependent immune response. Furtherexploration in this arena may lead to other therapeutic applications.

5. Connections betweenthyrotropin-releasing hormone and the immune system

Thedelineationof twoparadigmspertaining to interactionsbetweenthe CNS, the immune system and the endocrine systemhas significantlyadvanced the field of psychoimmunology. One of these paradigmspertains to interactions between these systems in the LPS-inducedsickness response. The second paradigm involves interactions betweenthese systems to control gastric function and control of other visceralorgans.We have already reviewed some evidence for a potential role forTRH in both of these paradigms. To avoid repetition,wewill now reviewonly the most critical in vivo evidence. We first describe the dynamicinteractions between the CNS, the immune system and the endocrinesystem, and then elucidate a potential role for TRH in these interactionsbased on the evidence presented. Finally, we establish a foundation forTRH-based therapeutics in specific illnesses.

Gary et al. (2003) were the first to provide an anatomical andfunctional framework to conceptualize diverse TRH pathways andTRH-mediated physiological and behavioral effects. The four distinctyet functionally integrated systems described by Gary et al. (2003) andYarbrough et al. (2007) provided a framework for the authors topropose a pivotal role for TRH in the regulation of CNS homeostasis. Inthe current section, we delineate a role for TRH in a set of interactionsthat integrate the immune system with the CNS and the endocrinesystem. On the basis of the evidence provided, we propose that someof the physiological and behavioral events observed in disorders ofimmune function are mediated by effects exerted on TRH systems.

5.1. Anatomical framework forthyrotropin-releasing hormone–immune system interactions

Current evidence suggests that the hypothalamus and brainstemserve as the epicenters of immune system interactions with othersystems (Pavlov & Tracey, 2004). These two epicenters also serve tointegrate overall brain responses to immune-derived signals from theperiphery. This integration is achieved via multiple, mainly catechola-minergic, projections from these epicenters to the forebrain and otherbrain regions (Gaykema et al., 2007; Pavlov & Tracey, 2004). The criticalareas within these epicenters that are involved in these interactionsinclude: the PVN in hypothalamus and the NTS, the ventrolateralmedulla (VLM) and the dorsal motor nucleus of vagus (DMN) in thebrainstem. The PVN receives significant, mainly catecholaminergic,input from the lower brainstem centers (i.e., NTS, VLM and DMN)(Sawchenko et al., 2000). Select neuromodulators (e.g., corticotropin-releasing hormone [CRH], neuropeptide Y, histamine, and leptin) also

Fig. 1. Overview of documented TRH and immune system interactions.

Table 1Comparison of cytokine-induced sickness effects with clinical effects of TRH

Domains Cytokine-induced sicknessmodela

In-vivo TRH effectsb

Neurovegetative Fatigue, psychomotorretardation, sleep alterations,anorexia

Arousal, vigilance, reversal ofsedation, improved motorfunction

Cognitive Decreased attention andconcentration, memorydifficulties

Improved cognition includingimproved memory

Affective Depressed mood, anxiety,anhedonia

Antidepressant effects, improvedemotional lability

Somatic Pain, gastrointestinaldisturbances

Pain modulation, gastriccytoprotective effects

a From animal models and human use of cytokines (see Dantzer and Kelley, 2007 andRaison et al., 2006 for details).

b Based on the in vivo data in animal models and human use (see Gary et al., 2003 andYarbrough et al., 2007 for details).

24 J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

play important roles in the pathophysiology of LPS-induced sicknesssyndrome. For example, significant increases in CRH in the PVN areobserved after LPS administration.

TRH and TRH receptors have a strong presence in both the hypo-thalamic and brainstem centers. Neurons in the dorsal vagal complex,including the DMN, express TRH receptors and are innervated by TRHfibers originating from TRH synthesizing neurons localized exclusively inthe brainstem nuclei (Bayliss et al., 1994; Lynn et al., 1991). These brain-stem nuclei, which contain TRH synthesizing neurons, namely the raphepallidus, raphe obscurus, and parapyramidal regions, also harbor vagaland sympathetic preganglionic motor neurons involved in thermal, car-diovascular, gastrointestinal, and pancreatic regulation (Wittman, 2008;Taché et al., 2006).

5.2. Experimental evidence for vagus-dependentthyrotropin-releasing hormone–immune system interactions

As described earlier, TRH administration in specific brainstem areashas been shown to impact functions of several visceral organs via vagalafferent-efferent pathways. Morrow et al. (1995) showed that a TRHanalog (RX-77368) administered in the DVC induced gastric contractility.Microinjection of IL-1 beta (IL-1β) in the DVC (along with RX-77368)completely blocked this effect. Intracisternal injection of an IL-1 receptorantagonist abolished the inhibitory effect of IL-1β on the TRH analog-induced gastric contractility (Morrow et al., 1995). Hermann and Rogers(1995) showed a similar inhibitory effect of another pro-inflammatorycytokine, TNF-α, on TRH stimulated gastric motility. Compared to the IL-1β inhibitoryeffect (30–120minpostinjection), the TNF-α effectwas bothimmediate (within 30 s) and longer lasting. The TNF-α inhibitory effectwas dose-dependent and required an intact vagal pathway (Hermann &Rogers, 1995). Hermann et al. (1999) demonstrated a similar inhibitoryeffect by intravenous administration of LPS. This inhibitory effect of LPS onTRH-induced gastric motility was reversed when endogenous TNF-αproduction was selectively suppressed (Hermann et al., 1999). Addition-ally, intravenous injections of bethanechol, a peripheral cholinergic ago-

nist, were still able to elicit usual increases in gastric motility in the LPSmodel. These experiments confirmed that the inhibition of TRH-inducedgastric motility seen in the LPS model was due to the central effects ofendogenously produced proinflammatory cytokines, primarily TNF-α.

Behavioral and physiological responses to LPS are achieved bycommunication of cytokine signals via vagal afferents to brain stemcenters and further to the hypothalamus via catecholaminergic and otherbrainstem pathways (Fig. 1). The brain stem and hypothalamic centerscoordinate the responsesof variousneuromodulators includingTRHto thecytokine signals. These responsesare thencommunicated to theperipheryvia vagal efferents and probably other, so far, unknown pathways.

5.3. Experimental evidence for vagus-independentthyrotropin-releasing hormone–immune system interactions

Vagal afferents clearly play a major role in the communication ofimmune signals from the periphery to the CNS. However, vagotomy

Fig. 2. TRH and immune system interactions in health and disease; focus on LPS/cytokine-induced sickness response.

25J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

was shown to block the behavioral (i.e. decreased social exploration)effects of IL-1β injected intraperitonially but not intravenously,suggesting that there are alternative pathways by which the cytokinescan mediate effects on the CNS (Bluthé et al., 1996). Similarly, in-traperitoneal IL-1β or LPS-induced physiological and behavioralresponses were only slightly attenuated in subdiaphragmaticallyvagotomized rats (Wieczorek et al., 2005). Porter et al. (1998) showedthat neither the vagal nor the nonvagal (splanchnic) afferent nervesfrom the upper gut are necessary for the anorexia produced by intra-peritoneal IL-1β and LPS. Similar to the vagal afferents, the brainstemcatecholaminergic pathways play a role in the activation of hypotha-lamic CRH neurons during the LPS-induced immune response(Ericsson et al., 1994). However, Fekete et al. (2005) showed thatthese brainstem pathways are not required in the LPS-induced sup-pression of TRH in the PVN.

In a comprehensive review, Wittman (2008) suggests that thebrainstem–catecholamine neurons–PVN pathway is necessary for theincrease in CRH, but not for the TRH suppression in the LPS paradigm. Itis important to note that, despite their large molecular size, cytokinescan be transported into the brain, endogenously synthesized in thebrain, or transmit signals to the brain via mechanisms independent ofvagal afferents (Pavlov & Tracey, 2004). Wittman (2008) suggests thatthe suppression of TRH in the PVN occurs as the result of direct effectsof proinflammatory cytokines, which may involve negative feedbackdue to increased local T3 production, as discussed earlier (Boelen et al.,2006). From an evolutionary perspective, itmay be important to have adirect pathway for suppression of TRH (Fig. 1). The suppression of TRHmay be necessary for survival during an adaptive sickness response(Kelley et al., 2003).

5.4. Role of thyrotropin-releasing hormonein the pathophysiology of the inflammatory process

Both in vitro and in vivo evidence suggests that cytokine signal-ing to the brain may involve direct suppression of TRH by the pro-inflammatory cytokines in key brain centers such as the PVN and theDVC. The findings reviewed above suggest a major role for TRH in thecore pathophysiology of the LPS-induced immune response. Suppres-

sion of TRH may represent one of the critical events during aninflammatory process. It may be the event that drives the behavioralchanges (e.g. social withdrawal, fatigue) and physiological changesthat are typical of the cytokine-induced sickness response.

The numerous clinical actions of TRH and TRH analogs (see Garyet al., 2003 and Yarbrough et al., 2007 for comprehensive reviews)such as arousal induction, enhancing cognitive function, improvingmotor function, and increasing gastric motility seem to be oppositeto what is observed in the cytokine-induced sickness behaviorparadigm (Table 1). The suppression of TRH seen in the LPS/pro-inflammatory cytokine model is consistent with the behavioral andphysiological effects seen in the cytokine-induced sickness model(Fig. 2).

In summary, the accumulated evidence (Fig. 1 and Table 2)strongly suggests significant suppressive effects of pro-inflammatorycytokines on TRH systems throughout the HPT axis during aninflammatory process. The suppressive effects on TRH are observedboth in the hypothalamus (PVN) and in the brainstem (DVC) via orindependent of vagal pathways. The clinical actions of TRH (Table 1)combined with the strong suppressive effects of proinflammatorycytokines on TRH systems during an inflammatory process suggest apotential role for TRH-based therapeutics in certain inflammatorydisorders (Fig. 2).

6. Thyrotropin-releasinghormone-based therapeutics in inflammatory disorders

The recognition and delineation of “proinflammatory cytokine-induced sickness behavior” has advanced the field of psychoimmunol-ogy in many ways. Dantzer and Kelley (2007) suggest that the commonsymptoms of sickness driven by proinflammatory cytokines – fatigue,anorexia, sleepiness, withdrawal from social activities, gastric stasis,fever, aching joints – are part of a “relative homeostasis” as a survivalresponse to the trigger (e.g. infection). This acute sickness response is nolonger adaptive if it is out of proportion to the insult or is unnecessarilyprolonged (Elenkov et al., 2005). Increasing evidence suggests that suchout of proportion or prolonged sickness behavior occurs in manydisorders and inflicts serious physical and emotional consequences. One

Table 3Future investigations of TRH-immune system interactions (not an exhaustive list)

In vitro – Investigation of TRH role in the development of organs/tissueswith autocrine/paracrine TRH networks– Investigation of TRH role and source in the peripheral organs/tissues with autocrine/paracrine TRH networks– Investigation of differential expression of TRH-R1 and TRH-R2receptors in the immune system and in other peripheral organswith TRH networks– Effects of TRH on immune cells and on cytokine production inimmunosuppressed state

In vivo in animalmodels

– Investigation of development of TRH system/HPT axis in theinflammatory vs. immunosuppressed state– Investigation of TRH system in the immunosuppressed state ofthe fully developed immune system– Behavioral and other effects of TRH in the inflammatory vs.immunosuppressed state– Behavioral and other effects of receptor specific TRH analogs(TRH-R1 vs. TRH-R2) on the immune system in general and in theinflammatory vs. immunosuppressed state

26 J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

possible cause is the use of cytokines (interferons or interleukins) totreat certain types of cancers and hepatitis C (Raison et al., 2006). Othercauses include the autoimmune disorders (e.g. rheumatoid arthritis,psoriasis, multiple sclerosis, ankylosing spondilytis, and inflammatorybowel diseases) (Elenkov et al., 2005). Theprolonged sickness syndromealso has been observed after chemotherapy or radiation for cancer andaftermyocardial infarction (Kelley et al., 2003; Elenkov et al., 2005). Thefatigue and depression observed in a subpopulation of patients withthese disorders has been associated with a chronic inflammatoryprocess (Miller et al., 2008). A number of recent studies associatedthe idiopathic fatigue reported by cancer patients with increase inspecific pro-inflammatory cytokine levels (Bower, 2007). Even localradiation treatments in otherwise healthy cancer patients can causesignificant fatigue. This radiation-induced fatigue has also beenassociated with increased plasma pro-inflammatory cytokine concen-trations in patients with cancer (Jacobsen & Thors, 2003). Marquetteet al. (2003) showed that the hypothalamus and certain other areas of

Table 2Experimental evidence of bi-directional interactions between TRH and immune system

Stimulating effects Inhibitory effects

In vitro – TRH stimulated thymocyte cellproliferation and inhibitedprednisolone-induced thymusinvolutiona

– TRH suppressed spontaneoussplenocyte proliferationa

– TRH stimulated splenocyteproliferationa

– TRH caused decreased IgGproduction and inhibited monocytes

– Thymectomy caused depressedthyroid function and decreased TRH,TSH levels⁎

– Mitogen-stimulated human wholeblood cells when incubated withTRH and imipramine showedsuppression of IFN-γ and IL-10production

– TRH enhanced T cell dependent,antigen induced antibody response– TRH stimulated IFN-α productionby human peripheral bloodmononuclear cells

In vivo inanimalmodels

– Central TRH analogueadministration caused gastrichyperemia via degranulation of mastcells

– TRH showed protective effect inmice challenged withencephalomyelitis virus

– Central TRH analogueadministration increased gastric,pancreatic and hepatic blood flow

– Centrally administered TRHanalogue showed protective effectsin cerulin-induced pancreatitis

– Central TRH analogueadministration enhanced gastricmotor and secretory function ⇒showed ulcerogenic potential

– Central TRH analogueadministration ⇒ showedcytoprotective effects in ethanol-induced gastric ulcers

– T cell dependent antigen response:In the early phase (b24 h) showedincreased hypothalamic TRH levels(required for adequate antibodyproduction)

– Central administration (in DVC) ofIL-1β and TNF-α blocked TRH-induced gastric motility– Intravenous LPS administrationblocked TRH-induced gastricmotility– Intravenous LPS administrationcaused suppression of hypothalamicTRH– Intraperitoneal IL-1 andintravenous TNF-α causedsuppression of hypothalamic proTRH mRNA levels– Intravenous IL-6 inhibited TRH-stimulated prolactin secretion– T cell dependent antigen response:In the late phase (N24 h) showedsuppression of hypothalamic TRHlevels

In vivo inhumans

– Intravenous TRH showedstimulatory effect on IFN-γ and IL-2levels in healthy subjects

– Intravenous TRH caused decreasedIL-2 levels in patients withhypothyroidism with baseline highIL-2 levels– Intravenous and intralymphaticvessel administration of TRH showedtherapeutic effects in the treatmentof acute pancreatitis

aIn developmental stages.

– Investigation of TRH interactions with other criticalneuromodulators (i.e. CRF, vasopressin, prolactin) in differentialimmune states– Investigation of TRH-induced immunomodulatory effect in selectdisorders (for example acute pancreatitis, spinal chord injury, andAlzheimer's disease) and its contribution to the therapeutic effectsassociated with TRH and its analogs in these disorders– Investigation of therapeutic effects of TRH and receptor specificTRH analogs in select animal models of inflammatory disorders

In vivo inhumans

– Investigation of immune system in the hyper vs. hypothyroidstates– Investigation of TRH system/HPT axis in the immunosuppressedstate– Investigation of therapeutic effects of TRH and its analogs inselect inflammatory disorders

the brain of partial-body irradiated rats show high levels of proin-flammatory cytokines, specifically IL-1β, TNF-α, and IL-6. This increasein cytokine levels was prevented by vagotomy before irradiation,confirming the importance of the vagal pathway for cytokine signaling(Marquette et al., 2003).

6.1. Cytokine-induced sickness syndrome asa therapeutic target in inflammatory disorders

Corticosteroids, known to suppress the inflammatory process, arefrequently used in palliative treatment for their anti-fatigue, anti-cachexia and anti-anorexia effects (Shih & Jackson, 2007). The firstidentification of the “sickness syndrome” as a therapeutic target camefrom the behavioral data generated by the clinical use of TNFinhibitors (recombinant soluble form of TNF receptor). The introduc-tion of these new anti-TNF drugs (etanercept, infliximab, andadalimumab) has significantly advanced the field of rheumatology.These drugs treat not only the specific aspects of an illness but alsocause an overall improvement in patient functioning and quality oflife. In clinical studies with these agents, patients reported less fatigue,improved physical function and better emotional and mental function(Nash & Florin, 2005). The “anti-sickness” effects of these agents havebeen reported in an array of autoimmune diseases includingrheumatoid arthritis, ankylosing spondylitis, Wegeners granuloma-tosis, psoriasis, and inflammatory bowel diseases (Nash & Florin,2005). In a recent pilot study, Monk et al. (2006) confirmed anti-fatigue effects of etanercept in cancer patients undergoing che-motherapy. Unfortunately, these agents have been associated withsignificant side effects. For some of these agents, the Food and DrugAdministration has added a “black box” warning of risk of seriousinfections (Rychly & DiPiro, 2005). Thus, more specific therapeuticagents are needed to counteract the behavioral consequences ofautoimmune disorders. TRH-based therapeutics may provide onesuch approach based on the hypothesis that some of the behavioral

27J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

symptoms of inflammatory disorders are due to the suppressiveeffects of cytokines on TRH systems. This hypothesis needs furtherevaluation in in vitro and in vivo settings.

6.2. Challenges and advances in the developmentof thyrotropin-releasing hormone-based therapeutics

The development of TRH-based therapeutics has been hamperedby the short half-life of TRH and its limited access to the CNS afterperipheral administration. However, it is important to emphasize that,in contrast to the recent novel treatments for inflammatory disorders(i.e. TNF blockers) with potentially serious side effects, TRH has beenin clinical use since 1974. It has a favorable safety record both inclinical use and in research studies, including studies in patients withserious illnesses (Gary et al., 2003; Yarbrough et al., 2007). Given thelimitations of native TRH, it is important to utilize metabolically stableTRH analogs with better access to the CNS. Two TRH analogs, TA-0910(Ceredist) and CG-3703, possessing these properties, have shownclinical promise (Gary et al., 2003). Ceredist has been used in Japansince 2001 for the treatment of spinocerebellar degeneration. Clinicalinvestigations can be conducted to test these and related compoundsfor the treatment of behavioral and other aspects of certain inflam-matory disorders (i.e. autoimmune diseases, inflammatory boweldiseases, cancer-related fatigue or depression, and Alzheimer'sdisease). Additionally, the development of new TRH analogs, espe-cially receptor subtype selective compounds, represents an area ofsignificant research potential.

7. Future investigations

Future investigations should include evaluation of TRH-basedtherapeutics in inflammatory disorders, especially for the behavioralaspects of the sickness syndrome associatedwith these disorders. On aconceptual level, it is important to investigate whether the homeo-static role for TRH in the CNS put forth by Gary et al. (2003) extends tothe regulation of the immune system. Studies to evaluate thedifferential roles of the two types of TRH receptors in TRH-immunesystem interactions are also a matter of high priority. As mentionedearlier, TRH and TRH analogs have shown clinical promise in certaindisorders (Gary et al., 2003). Many of these disorders are associatedwith inflammatory processes, and immunomodulationmay play a rolein the therapeutic effects associated with TRH or its analogs. It isimportant to test this hypothesis in animal models of these disorders.Potential experiments to investigate these hypotheses are described inTable 3.

8. Conclusions

TRH has numerous interactions with the immune system in theCNS as well as in the periphery and during multiple stages of devel-opment. These interactions can be direct or indirect, i.e. via otherneuromodulators. A large body of in vitro and in vivo evidence sup-ports a homeostatic role for TRH in its interactions with the immunesystem, extending the hypothesis previously proposed by Gary et al.(2003) and Yarbrough et al. (2007) of a homeostatic regulatory role forTRH to include effects on immune system function.

The TRH-immune system homeostatic hypothesis states that TRH-mediated mechanisms respond to many elements of the immunesystem and affect them in ways that tend to maintain or restorehomeostasis. Some aspects of this hypothesis may be tested by the useof TRH or its congeners to treat patients with certain inflammatorydiseases or to affect animal models of those diseases. The critical roleof TRH in the cytokine-induced sickness paradigm presents excellentopportunities for further exploration of this hypothesis and providespromising targets for TRH-based therapeutics of immune system-related medical and psychiatric disorders.

References

Ader, R. (1980). Presidential address: psychosomatic and psychoimmunologic research.Psychosom Med 42, 307−322.

Bayliss, D. A., Viana, F., Kanter, R. K., Szymeczek-Seay, C. L., Berger, A. J., & Millhorn, D. E.(1994). Early postnatal development of thyrotropin-releasing hormone (TRH)expression, TRH receptor binding, and TRH responses in neurons of rat brainstem.J Neurosci 14(2), 821−833.

Ben-Jonathan, N., Mershon, J., Allen, D., & Steinmetz, R. (1996). Extrapituitary prolactin:distribution, regulation, functions and clinical aspects. Endocr Rev 17, 639−669.

Bilek, R. (2000). TRH-like peptides in prostate gland and other tissues. Physiol Res 49(Suppl 1), 519−526.

Błaszkowska, J., Pawlikowski, M., Komorowski, J., & Kurnatowski, P. (2004). Effect ofthyroliberin on the course of experimental candidosis in mice. Mycoses 47(3–4),115−120.

Bluthé, R. M., Michaud, B., Kelley, K. W., & Dantzer, R. (1996). Vagotomy blocksbehavioral effects of interleukin-1 injected via the intraperitoneal route but not viaother systemic routes. Neuroreport 7(15–17), 2823−2827.

Boelen, A., Kwakkel, J., Thijssen-Timmer, D. C., Alkemade, A., Fliers, E., &Wiersinga,W.M.(2004). Simultaneous changes in central and peripheral components of thehypothalamus–pituitary–thyroid axis in lipopolysaccharide-induced acute illnessin mice. J Endocrinol 182(2), 315−323.

Boelen, A., Kwakkel, J., Wiersinga, W. M., & Fliers, E. (2006). Chronic local inflammationin mice results in decreased TRH and type 3 deiodinase mRNA expression in thehypothalamic paraventricular nucleus independently of diminished food intake.J Endocrinol 191(3), 707−714.

Bower, J. E. (2007). Cancer-related fatigue: links with inflammation in cancer patientsand survivors. Brain Behav Immun 21(7), 863−871.

Dantzer, R., & Kelley, K. W. (2007). Twenty years of research on cytokine-inducedsickness behavior. Brain Behav Immun 21(2), 153−160.

Elenkov, I. J., Iezzoni, D. G., Daly, A., Harris, A. G., & Chrousos, G. P. (2005). Cytokinedysregulation, inflammation and well-being. Neuroimmunomodulation 12(5),255−269.

Ericsson, A., Kovács, K. J., & Sawchenko, P. E. (1994). A functional anatomical analysis ofcentral pathways subserving the effects of interleukin-1 on stress-relatedneuroendocrine neurons. J Neurosci 14(2), 897−913.

Eskandari, F., & Sternberg, E. M. (2002). Neural-immune interactions in health anddisease. Ann N Y Acad Sci 966, 20−27.

Fekete, C., Singru, P. S., Sarkar, S., Rand, W. M., & Lechan, R. M. (2005). Ascendingbrainstem pathways are not involved in lipopolysaccharide-induced suppression ofthyrotropin-releasing hormone gene expression in the hypothalamic paraventri-cular nucleus. Endocrinology 146(3), 1357−1363.

Fukusumi, S., Ogi, K., Onda, H., & Hinuma, S. (1995). Distribution of thyrotropin-releasing hormone receptor mRNA in rat peripheral tissues. Regul Pept 57(2),115−121.

Gary, K. A., Sevarino, K. A., Yarbrough, G. G., Prange, A. J., Jr., & Winokur, A. (2003). Thethyrotropin-releasing hormone (TRH) hypothesis of homeostatic regulation:implications for TRH-based therapeutics. J Pharmacol Exp Ther 305(2), 410−416.

Gaykema, R. P., Chen, C. C., & Goehler, L. E. (2007). Organization of immune-responsivemedullary projections to the bed nucleus of the stria terminalis, central amygdala,and paraventricular nucleus of the hypothalamus: evidence for parallel viscer-osensory pathways in the rat brain. Brain Res 1130(1), 130−145.

Gershengorn, M. C., & Osman, R. (1996). Molecular and cellular biology of thyrotropin-releasing hormone receptors. Physiol Rev 76(1), 175−191.

Grasso, G., Massai, L., De Leo, V., & Muscettola, M. (1998). The effect of LHRH and TRH onhuman interferon-gamma production in vivo and in vitro. Life Sci 62(22),2005−2014.

Guillemin, R. (1978). Peptides in the brain: the new endocrinology of the neuron.Science 202, 390−402.

Hart, R., Wagner, F., Steffens, W., Lersch, C., Dancygier, H., Duntas, L., et al. (1990). Effectof thyrotropin-releasing hormone on immune functions of peripheral bloodmononuclear cells. Regul Pept 27(3), 335−342.

Hermann, G., & Rogers, R. C. (1995). Tumor necrosis factor-alpha in the dorsal vagalcomplex suppresses gastric motility. Neuroimmunomodulation 2(2), 74−81.

Hermann, G. E., Tovar, C. A., & Rogers, R. C. (1999). Induction of endogenous tumornecrosis factor-alpha: suppression of centrally stimulated gastric motility. AmJ Physiol 276(1 Pt 2), R59−68.

Heuer, H., Schäfer, M. K., O'Donnell, D., Walker, P., & Bauer, K. (2000). Expression ofthyrotropin-releasing hormone receptor 2 (TRH-R2) in the central nervous systemof rats. J Comp Neurol 428(2), 319.

Jacobsen, P. B., & Thors, C. L. (2003). Fatigue in the radiation therapy patient: currentmanagement and investigations. Semin Radiat Oncol 13(3), 372−380.

Kawakubo, K., Akiba, Y., Adelson, D., Guth, P. H., Engel, E., Taché, Y., et al. (2005). Role ofgastric mast cells in the regulation of central TRH analog-induced hyperemia in rats.Peptides 26(9), 1580−1589.

Keller, S. E., Schleifer, S. J., Liotta, A. S., Bond, R. N., Farhoody, N., & Stein,M. (1988). Stress-induced alterations of immunity in hypophysectomized rats. Proc Natl Acad Sci U S A85(23), 9297−9301.

Kelley, K. W., Bluthé, R. M., Dantzer, R., Zhou, J. H., Shen, W. H., Johnson, R. W., et al.(2003). Cytokine-induced sickness behavior. Brain Behav Immun 17(Suppl 1),S112−118.

Kelley, K. W., Weigent, D. A., & Kooijman, R. (2007). Protein hormones and immunity.Brain Behav Immun 21(4), 384−392.

Klein, J. R. (2003). Physiological relevance of thyroid stimulating hormone andthyroid stimulating hormone receptor in tissues other than the thyroid.Autoimmunity36(6–7), 417−421.

28 J. Kamath et al. / Pharmacology & Therapeutics 121 (2009) 20–28

Klein, J. R. (2006). The immune system as a regulator of thyroid hormone activity. ExpBiol Med (Maywood) 231(3), 229−236.

Komorowski, J., Stepień, H., & Pawlikowski, M. (1993). The evidence of thyroliberin/triiodothyronine control of TSH secretory response from human peripheral bloodmonocytes cultured in vitro. Neuropeptides 25(1), 31−34.

Komorowski, J., Stepien, H., & Pawlikowski, M. (1994). Increased interleukin-2 levelsduring standard TRH test in man. Neuropeptides 27, 151−156.

Kruger, T. E. (1996). Immunomodulation of peripheral lymphocytes by hormones of thehypothalamus–pituitary–thyroid axis. Adv Neuroimmunol 6(4), 387−395.

Kubera, M., Kenis, G., Bosmans, E., Jaworska-Feil, L., Lasoń, W., Scharpe, S., et al. (2000).Suppressive effect of TRH and imipramine on human interferon-gamma andinterleukin-10 production in vitro. Pol J Pharmacol 52(6), 481−486.

Kruger, T. E., Smith, L. R., Harbour, D. V., & Blalock, J. E. (1989). Thyrotropin:anendogenous regulator of the in vitro immune response. J Immunol 142(3), 744−747.

Kunert-Radek, J., Pawlikowski, M., Stepien, H., & Janecka, A. (1991). Inhibitory effect ofthyrotropin releasing hormone on spontaneous proliferation of mouse spleenlymphocytes in vitro. Biochem Biophys Res Commun 181(2), 562−565.

Lechan, R. M. (1993). Update on thyrotropin-releasing hormone. Thyroid Today 16, 1−11.Lesnikov, V. A., Korneva, E. A., Dall'ara, A., & Pierpaoli, W. (1992). The involvement of

pineal gland and melatonin in immunity and aging: II. Thyrotropin-releasinghormone and melatonin forestall involution and promote reconstitution of thethymus in anterior hypothalamic area (AHA)-lesioned mice. Int J Neurosci 62(1–2),141−153.

Lynn, R. B., Feng, H. S., Han, J., & Brooks, F. P. (1991). Gastric effects of thyrotropin-releasing hormone microinjected into the dorsal vagal nucleus in cats. Life Sci 48(13),1247−1254.

Marquette, C., Linard, C., Galonnier, M., Van Uye, A., Mathieu, J., Gourmelon, P., et al.(2003). IL-1beta, TNFalpha and IL-6 induction in the rat brain after partial-bodyirradiation: role of vagal afferents. Int J Radiat Biol 79(10), 777−785.

Martino, E., Nardi, M., Vaudagna, G., Simonetti, S., Cilotti, A., Piunchera, A., et al. (1980).Thyrotropin-releasing hormone-like material in human retina. J Endocrinol Invest 3,267−271.

Matre, V., Høvring, P. I., Fjeldheim, A. K., Helgeland, L., Orvain, C., Andersson, K. B., et al.(2003). The human neuroendocrine thyrotropin-releasing hormone receptorpromoter is activated by the haematopoietic transcription factor c-Myb. BiochemJ 372(Pt 3), 851−859.

Mellado, M., Fernández-Agulló, T., Rodríguez-Frade, J. M., San Frutos, M. G., de la Peña, P.,Martínez-A, C., et al. (1999). Expression analysis of the thyrotropin-releasinghormone receptor (TRHR) in the immune system using agonist anti-TRHRmonoclonal antibodies. FEBS Lett 28(451(3)), 308−314.

Miller, A. H., Ancoli-Israel, S., Bower, J. E., Capuron, L., & Irwin, M. R. (2008).Neuroendocrine-immune mechanisms of behavioral comorbidities in patientswith cancer. J Clin Oncol 26(6), 971−982.

Monk, J. P., Phillips, G., Waite, R., Kuhn, J., Schaff, L. J., Otterson, G. A., et al. (2006).Assessment of tumor necrosis factor alpha blockade as an intervention to improvetolerability of dose-intensive chemotherapy in cancer patients. J Clin Oncol 24(12),1852−1859.

Montagne, J. J., Ladram, A., Grouselle, D., Nicolas, P., & Bulant, M. (1997). Thyrotropin-releasing hormone immunoreactivity in rat adrenal tissue is localized in mast cells.J Histochem Cytochem 45(12), 1623−1627.

Montagne, J. J., Ladram, A., Nicolas, P., & Bulant, M. (1999). Cloning of thyrotropin-releasing hormone precursor and receptor in rat thymus, adrenal gland, and testis.Endocrinology 140(3), 1054−1059.

Morrow, N. S., Quinonez, G., Weiner, H., Taché, Y., & Garrick, T. (1995). Interleukin-1 betain the dorsal vagal complex inhibits TRH analogue-induced stimulation of gastriccontractility. Am J Physiol 269(2 Pt 1), G196−202.

Nagy, E., & Berczi, I. (1978). Immunodeficiency in hypophysectomized rats. ActaEndocrinol 89(3), 530.

Nash, P. T., & Florin, T. H. (2005). Tumour necrosis factor inhibitors. Med J 183(4),205−208.

Nillni, E. A., & Sevarino, K. A. (1999). The biology of pro-thyrotropin-releasing hormone-derived peptides. Endocr Rev 20(5), 599−648.

Pang, X. P., Hershman, J. M., Mirell, C. J., & Pekary, A. E. (1989). Impairment ofhypothalamic-pituitary–thyroid function in rats treated with human recombinanttumor necrosis factor-alpha (cachectin). Endocrinology 125(1), 76−84.

Pavlov, V. A., & Tracey, K. J. (2004). Neural regulators of innate immune responses andinflammation. Cell Mol Life Sci 61(18), 2322−2331.

Pawlikowski, M., Stepien, H., & Komorowski, J. (1994). Hypothalamic-pituitary–thyroidaxis and the immune system. Neuroimmunomodulation 1(3), 149−152.

Pawlikowski, M., Zerek-Mełeń, G., & Winczyk, K. (1992). Thyroliberin (TRH) increasesthymus cell proliferation in rats. Neuropeptides 23(3), 199−202.

Pekary, A. E., Stevens, S. A., & Sattin, A. (2007). Lipopolysaccharide modulation ofthyrotropin-releasing hormone (TRH) and TRH-like peptide levels in rat brain andendocrine organs. J Mol Neurosci 31(3), 245−259.

Perez, C. C., Penaleva, R., Paez, P. M., Renner, U., Reul, J. M., Stalla, G. K., et al. (1999). Earlyactivation of thyrotropin-releasing-hormone and prolactin plays a critical roleduring a T cell-dependent immune response. Endocrinology 140(2), 690−697.

Pierpaoli, W., & Yi, C. (1990). The involvement of pineal gland and melatonin inimmunity and aging. I. Thymus-mediated, immunoreconstituting and antiviralactivity of thyrotropin-releasing hormone. J Neuroimmunol 27(2–3), 99−109.

Porter, M. H., Hrupka, B. J., Langhans, W., & Schwartz, G. J. (1998). Vagal and splanchnicafferents are not necessary for the anorexia produced by peripheral IL-1beta, LPS,and MDP. Am J Physiol 275(2 Pt 2), R384−389.

Raiden, S., Polack, E., Nahmod, V., Labeur, M., Holsboer, F., & Arzt, E. (1995). TRH receptoron immune cells: in vitro and in vivo stimulation of human lymphocyte and ratsplenocyte DNA synthesis by TRH. J Clin Immunol 15(5), 242−249.

Raison, C. L., Capuron, L., & Miller, A. H. (2006). Cytokines sing the blues: inflammationand the pathogenesis of depression. Trends Immunol 27(1), 24−31.

Rychly, D. J., & DiPiro, J. T. (2005). Infections associated with tumor necrosis factor-alphaantagonists. Pharmacotherapy 25(9), 1181−1192.

Santos, J., Saperas, E., Mourelle, M., Antolín, M., & Malagelada, J. R. (1996). Regulation ofintestinal mast cells and luminal protein release by cerebral thyrotropin-releasinghormone in rats. Gastroenterology 111(6), 1465−1473.

Sawchenko, P. E., Li, H. Y., & Ericsson, A. (2000). Circuits and mechanisms governinghypothalamic responses to stress: a tale of two paradigms. Prog Brain Res 122,61−78.

Schettini, G., Grimaldi, M., Landolfi, E., Meucci, O., Ventra, C., Florio, T., et al. (1991). Roleof interleukin-6 in the neuroendocrine system. Acta Neurol (Napoli) 13(4), 361−367.

Serebrov, V., Zobnina, M. N., & Tikhonova, N. M. (1992). Structural–functional state ofthe thyroid gland after thymectomy. Biull Eksp Biol Med 114(9), 329−332.

Shih, A., & Jackson, K. C., 2nd (2007). Role of corticosteroids in palliative care. J PainPalliat Care Pharmacother 21(4), 69−76.

Shimanko, I. I., Limarev, V. M., Ashmarin, I. P., Lelekova, T. V., & Sanzhieva, L. T. (1992).The use of thyrotropin-releasing hormone in clinical practice as a lymphaticstimulator in the treatment of acute pancreatitis. Khirurgiia (Mosk)(1), 64−66.

Stephens, R. L., Ishikawa, T., Weiner, H., Novin, D., & Tache, Y. (1988). TRH analogue, RX77368, injected into dorsal vagal complex stimulates gastric secretion in rats. AmJ Physiol 254(5 Pt 1), G639−643.

Sternberg, E. M. (1995). Neuroendocrine factors in susceptibility to inflammatory disease:focus on the hypothalamic-pituitary–adrenal axis. Horm Res 43(4), 159−161.

Sun, Y., Lu, X., & Gershengorn, M. C. (2003). Thyrotropin-releasing hormone receptors—similarities and differences. J Mol Endocrinol 30(2), 87−97.

Taché, Y., & Yoneda, M. (1993). Central action of TRH to induce vagally mediated gastriccytoprotection and ulcer formation in rats. J Clin Gastroenterol 17(Suppl 1), S58−63.

Taché, Y., Yang, H., Miampamba, M., Martinez, V., & Yuan, P. Q. (2006). Role of brainstemTRH/TRH-R1 receptors in the vagal gastric cholinergic response to various stimuliincluding sham-feeding. Auton Neurosci 125(1–2), 42−52.

Trejbal, D., Petrasova, D., & Wagnerova, H. (2001). Prolactin and interleukin 2concentrations before and after i.v. TRH application in primary hypothyroidismand in controls. Bratisl Lek Listy 102(9), 417−419.

van Haasteren, G. A., van der Meer, M. J., Hermus, A. R., Linkels, E., Klootwijk, W.,Kaptein, E., et al. (1994). Different effects of continuous infusion of interleukin-1and interleukin-6 on the hypothalamic-hypophysial-thyroid axis. Endocrinology135(4), 1336−1345.

Wang, J., & Klein, J. R. (1995). Hormonal regulation of extrathymic gut T celldevelopment: involvement of thyroid stimulating hormone. Cell Immunol 161(2),299−302.

Wang, J., Whetsell, M., & Klein, J. R. (1997). Local hormone networks and intestinal T cellhomeostasis. Science 275(5308), 1937−1939.

Winokur, A., & Utiger, R. D. (1974). Thyrotropin-releasing hormone: regional distribu-tion in rat brain. Science 185, 265−266.

Wittman, G. J. (2008). Regulation of hypophysiotropic corticotropin-releasing hormoneand thyrotropin-releasing hormone-synthesizing neurons by brainstem catecho-laminergic neurons. Neuroendocrinol 0(7), 952−960.

Wieczorek, M., Swiergiel, A. H., Pournajafi-Nazarloo, H., & Dunn, A. J. (2005).Physiological and behavioral responses to interleukin-1beta and LPS in vagoto-mized mice. Physiol Behav 85(4), 500−511.

Winczyk, K., & Pawlikowski, M. (2000). Time of day-dependent effects of thyroliberinand thyrotropin on thymocyte proliferation in rats. Neuroimmunomodulation 7(2),89−91.

Yamada, M., Shibusawa, N., Hashida, T., Ozawa, A., Monden, T., Satoh, T., et al. (2000).Expression of thyrotropin-releasing hormone (TRH) receptor subtype 1 in mousepancreatic islets and HIT-T15, an insulin-secreting clonal beta cell line. Life Sci 66(12),1119−1125.

Yarbrough, G. G. (1979). On the neuropharmacology of thyrotropin releasing hormone(TRH). Prog Neurobiol 12(3–4), 291−312.

Yarbrough, G. G., Kamath, J., Winokur, A., & Prange, A. J., Jr (2007). Thyrotropin-releasinghormone (TRH) in the neuroaxis: therapeutic effects reflect physiological functionsand molecular actions. Med Hypotheses 69(6), 1249−1256.

Yoneda, M., Hoshimoto, T., Nakamura, K., Tamori, K., Yokohama, S., Kono, T., et al. (2003).Thyrotropin-releasing hormone in the dorsal vagal complex stimulates hepaticblood flow in rats. Hepatology 38(6), 1500−1507.

Yoneda, M., Goto, M., Nakamura, K., Yokohama, S., Kono, T., Tanamo, M., et al. (2005).Thyrotropin-releasing hormone in the dorsal vagal complex stimulates pancreaticblood flow in rats. Regul Pept 131(1–3), 74−81.

Yoneda, M., Goto, M., Nakamura, K., Shimada, T., Hiraishi, H., Terano, A., et al. (2005).Protective effect of central thyrotropin-releasing hormone analog on cerulein-induced acute pancreatitis in rats. Regul Pept 125(1–3), 119−124.

Yu-Lee, L. Y. (2002). Prolactin modulation of immune and inflammatory responses.Recent Prog Horm Res 57, 435−455.