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Hypothalamic Circadian Organization in Birds. I.

Anatomy, Functional Morphology, and Terminology

of the Suprachiasmatic Region

Roland Brandstatter* and Ute Abraham

Max-Planck-Research Centre for Ornithology, Andechs, Germany

ABSTRACT

In mammals, the master clock controlling circadian rhythmicity is located in the

hypothalamic suprachiasmatic nuclei (SCN). Until now, no comparable structure has

been unambiguously described in the brain of any nonmammalian vertebrate. In birds,

early anatomical and lesioning studies described a SCN located in the anterior

hypothalamus, but whether birds possess a nucleus equivalent to the mammalianSCN remained controversial. By reviewing the existing literature it became evident that

confusion in delineation and nomenclature of hypothalamic cell groups may be one of

the major reasons that no coherent picture of the avian hypothalamus exists. In this

review, we attempt to clarify certain aspects of the organization of the avian

hypothalamus by summarizing anatomical and functional studies and comparing

them to immunocytochemical results from our laboratory. There is no single cell

group in the avian hypothalamus that combines the morphological and neurochemical

features of the mammalian SCN. Instead, certain aspects of anatomy and morphology

suggest that at least two anatomically distinct cell groups, the SCN and the lateral

hypothalamic nucleus (LHN), bear some of the characteristics of the mammalian SCN.

Key Words: Circadian system; House sparrow (Passer domesticus); Lateral

hypothalamic nucleus; Hypothalamus; Suprachiasmatic nucleus.

*Correspondence: Dr. Roland Brandstatter, Department of Biological Rhythms and Behaviour, Max-

Planck-Research Centre for Ornithology, Von-der-Tann-Strasse 7, D-82346 Andechs, Germany;

E-mail: [email protected].

CHRONOBIOLOGY INTERNATIONAL

Vol. 20, No. 4, pp. 637655, 2003

DOI: 10.1081=CBI-120023343 0742-0528 (Print); 1525-6073 (Online)

Copyright# 2003 by Marcel Dekker, Inc. www.dekker.com

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INTRODUCTION

The hypothalamus of the vertebrate brain is a complex structure responsible for

integrating innumerable endocrine, autonomic, and behavioral responses that are involved

in regulating metabolism, homeostasis, and reproduction. Exploratory, ingestive, thermo-

regulatory, aggressive, defensive, sexual, and parental behaviors are all involved, and their

expression is tied in a fundamental way to a circadian rhythm of activity and rest

(Bjorklund et al., 1987). In mammals, this rhythmicity is controlled by a master

clock acting as a circadian pacemaker that is located in the hypothalamic suprachiasmatic

nucleus (SCN), a paired cell group adjacent to the third ventricle, just above the optic

chiasm. It is characterized by specific molecular, morphological, and physiological

features (for recent reviews, see Ibata et al., 1999; van Esseveldt et al., 2000):

morphologically, the SCN can be divided into a small rostral and a large caudal part

(van den Pol, 1980). The latter is composed of a ventrolateral part, termed core, and a

dorsomedial part, termed shell. The core, which receives photic input from the retina

(Moore, 1973), rhythmically expresses neuropeptides including vasoactive intestinal

polypeptide (VIP) (Laemle et al., 1995; Shinohara et al., 1994) and gastrin-releasing

peptide (Isobe and Muramatsu, 1995). Vasopressin is rhythmically produced in the shell

(Earnest and Sladek, 1986; Isobe and Muramatsu, 1995; Tominaga et al., 1992; Yamase

et al., 1991). Light pulses induce expression of immediate early response genes in the core

of the SCN, depending on the phase of the circadian oscillator (Kornhauser et al., 1990;

Rusak et al., 1990). The role of the SCN as a circadian pacemaker in mammals has been

clearly demonstrated by lesions that abolish circadian rhythms at the whole-organism level

(Moore and Eichler, 1972; Rusak and Zucker, 1979) as well as by transplantation of SCN

tissue into the brains of SCN-lesioned mammals that restored rhythmicity (Lehman et al.,1987; Ralph et al., 1990).

Until now, no comparable structure has been unambiguously identified in the brain of

any nonmammalian vertebrate, and little is known about the exact localization and specific

properties of the hypothalamic circadian oscillator in nonmammalian vertebrates. In this

article, we shall emphasize insight into the organization of the anterior suprachiasmatic

hypothalamus in birds, based on information from a variety of bird species gained over the

last six decades by various research groups as well as recent investigations performed by

the authors focusing on anatomical and morphological properties of the suprachiasmatic

hypothalamus in songbirds, particularly that of the house sparrow (Passer domesticus).

The Circadian System of Birds

In variance to mammals, birds have the capacity to perceive information about the

photic environment by retinal, pineal, as well as by deep encephalic photoreceptors

(Cassone and Menaker, 1984; Foster and Soni, 1998; Kojima and Fukada, 1999; Menaker

and Tosini, 1995; Silver et al., 1988). Depending on the species, circadian pacemaking at

the whole-organism level is organized by autonomous and anatomically distinct oscillators

localized in the retina (Pierce et al., 1993; Underwood et al., 1990), the pineal gland

(Binkley et al., 1978; Brandstatter et al., 2000; Gaston and Menaker, 1968; Zimmerman

and Menaker, 1979), and the hypothalamus (Takahashi and Menaker, 1982). Several lines

of evidence suggest that these components interact with each other to produce a stable

638 Brandstatter and Abraham

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circadian rhythmicity of the animal (for recent reviews, see Brandstatter, 2002; Gwinner

and Brandstatter, 2001) [Fig. 1(A)].

Initial functional evidence for a hypothalamic circadian pacemaker in birds is mostly

coming from lesioning studies. Takahashi and Menaker (1982) demonstrated that house

sparrows bearing lesions of the anterior suprachiasmatic hypothalamus gradually lost their

locomotor activity rhythms in constant conditions. Further indirect evidence was found in

pigeons that had been both pinealectomized and blinded and still showed residual

rhythmicity for a while after transfer to constant conditions before they became arrhythmic

(Ebihara et al., 1987). Circadian activity rhythms of pinealectomized house sparrows did

Figure 1. Diagram of the major components of the avian circadian pacemaking system and the

anterior suprachiasmatic hypothalamus. Oscillatory components are indicated by clock symbols;

photoreceptive structures are indicated by open arrows. Signal pathways are demonstrated by hatched

arrows. ep encephalic photoreceptors. The diagram is not indicative for a certain species but

summarizes information that has been obtained in chicken, house sparrow, pigeon, and Japanese

quail. Circadian oscillators that may act as pacemakers are present in the pineal gland, the retina, and

the hypothalamus. Oscillators may interact with each other by hormonal signals (hatched arrow from

pineal to hypothalamus), neural signal pathways (hatched arrow from hypothalamus to pineal) or

both (cross-hatched arrow from retina to hypothalamus). For detailed information see references

(Gwinner and Brandstatter, 2001) and (Brandstatter, 2002). The insert (B) shows the anatomical

position of the hypothalamic cell groups shown in C. The two cell groups that have been assumed to

represent the functional equivalent to the mammalian hypothalamic oscillator are drawn in grey (right

hemisphere) and indicated by grey arrows. GLV, ventro-lateral geniculate nucleus; LHRN=vSCN,

lateral hypothalamic retinorecipient nucleus (also called visual SCN); LHy, lateral hypothalamic

area; OC, optic chiasm; PON, preoptic nucleus; SCN, suprachiasmatic nucleus; V, third ventricle.

Hypothalamic Circadian Organization in Birds. I 639

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not disappear immediately but damped out over a series of transient cycles following

release into constant conditions (Gaston and Menaker, 1968; Zimmerman and Menaker,

1979), suggesting that there remained at least one damped oscillator after removal of the

pineal gland. It is likely that the presence of a hypothalamic oscillator is a common feature

of the avian circadian pacemaking system, since disruptive effects of hypothalamic lesions

on the circadian rhythms of birds were found in all further species investigated until now,

including the Java sparrow and the Japanese quail (Ebihara and Kawamura, 1981; Simpson

and Follett, 1981). However, the exact localization and specific properties of the

hypothalamic circadian oscillator in birds remained obscure. Several reasons may have

contributed to the lack of detailed knowledge about the avian equivalent of the mammalian

hypothalamic circadian oscillator, including incomplete and inconsistent anatomical

descriptions of the avian hypothalamus, confusion in delineation as well as terminology

of cell groups, and the lack of a molecular approach defining key genes and=or

transcription factors of circadian oscillators.

The Hypothalamus of Birds

The avian hypothalamus forms the ventral portion of the diencephalon on either side

of the third ventricle. It has been divided into a large number of more or less distinct cell

groups and it is traversed by several majorfiber systems (Crosby and Showers, 1969). The

parcellation of the hypothalamus as well as the delineation of cell groups has been highly

variable in the different bird species investigated, and the terminology applied has differed

greatly. A survey of nomenclatures used for the avian hypothalamus has been given by

Kuenzel and van Tienhoven (1982). Following the Nomina Anatomica Avium (Breazileand Kuenzel, 1993), the hypothalamus of birds can be divided into three main portions

the preoptic region, the medial (tuberal) region, and the caudal (mammillary) region, with

19 nuclei and two areas. However, we still lack a clear and generally applicable

demarcation of cell groups as well as a uniform terminology that would facilitate

comparison among species. Due to the rather high species-specific diversity of brain

morphology, different nomenclatures used, section planes shown that are not always

comparable, and inconsistency in the description of different cell groups, a uniform picture

of the avian hypothalamus does not exist. This is exemplified by anatomical descriptions of

the anterior hypothalamus in frequently used atlases of the avian brain (Fig. 2).

Major problems for a comprehensive understanding of the avian hypothalamus are the

rather high species-specific diversity of brain morphology as well as the fact that

functionally distinct cell types do not always respect the boundaries of traditional cellgroups delineated in Nissl stainings. Thus, functional studies and anatomical descriptions

often are in variance to each other. This ambiguity is also true for those cell groups that

have been associated with circadian function, i.e., that possibly represent the equivalent to

the mammalian SCN. Additionally, the term SCN has become a synonym for a

hypothalamic circadian oscillator or pacemaker and, thus, has sometimes been used to

describe a structure that shows evidence for the presence of a rhythmic parameter rather

than being used in its original anatomical meaning. Whether an anatomical equivalent to

the mammalian SCN is present in all vertebrate groups and whether it is the site of a

hypothalamic circadian oscillator in all species remains to be determined. Hence, when

identifying the avian SCNas recently claimed by Yoshimura et al. (2001)it is not


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Figure 2. Diagrams of the anterior hypothalamus as shown in frequently used atlases of the avian

brain. (A), (B): Schematic drawings of selected coronal sections through the anterior suprachiasmatic

hypothalamus of the pigeon (A: level A 8.50; B: level A 7.75; redrawn from (Karten and Hodos,

1967). (C), (D) Schematic drawings of selected coronal sections through the anterior suprachiasmatic

hypothalamus of the chicken (C: level A 9.0; D: level A 8.5; redrawn from (Nieuwenhuys et al.,

1998), adapted from (van Tienhoven and Juhasz, 1962)). (E), (F): Schematic drawings of selected

coronal sections through the anterior suprachiasmatic hypothalamus of the canary (E: level A.2.8; F:

level 2.6; redrawn from (Stokes et al., 1974)). Scale bar1 cm. Variability in the description of

hypothalamic organization becomes evident from differences in delineation and nomenclature of cell

groups. In the pigeon atlas, a supraoptic nucleus is described at the lateral angle of the ventricle (A)

where the SCN is shown in the chicken atlas (C). Neither a supraoptic nor a SCN are shown in the

atlas of the canary (E), possibly due to the lack of a section plane available frontal to the supraoptic

decussation. The LHy is mentioned but not demarcated in the pigeon (B), delineated in the chicken

(D) but not present at all in the atlas of the canary (F). AM, nucleus anterior medialis hypothalami;

CO, chopt, chiasma opticum; LA, nucleus lateralis anterior thalami; LHy, nulhy, nucleus lateralis

hypothalami; POM, nucleus preopticus medialis; PPM, nucleus preopticus paraventricularis magno-

cellularis; PVM, nucleus periventricularis magnocellularis; QF, tractus quintofrontalis; SO, nucleus

supraopticus; TSM, TrSM, tractus septomesencephalicus; VLT, nucleus ventrolateralis thalami. flt,

fasciculus lateralis telencephali; nuhyma, nucleus medialis hypothalami anterioris; nuprmc, nucleus

preopticus magnocellularis; nupvhy, nucleus paraventricularis hypothalami; nusc, nucleus supra-

chiasmaticus; pra, nucleus preopticus anterior. DS, dso, decussatio Supraoptica; DSD, decussatio

supraoptica dorsalis; FDB, fasciculus diagonalis brocae; GLV, nucleus geniculatus lateralis, pars

ventralis; POA, nucleus preopticus anterioris; TeO, tectum opticum; TrO, tractus opticus.


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sufficient to identify the site of circadian oscillations but to characterize a defined cell

group showing anatomical and=or morphological features justifying the term suprachias-

matic nucleus. On the other hand, identifying the site of a hypothalamic oscillator that

may act as a pacemaker requires the characterization of oscillatory components, either

molecular or physiological, that may be attributed to circadian function, independent of

whether these oscillations are present in the SCN or any other structure. Thus, whether

nonmammalian vertebrates do have a SCN and whether this cell group then contains a

circadian oscillator can only be answered when anatomical and functional studies are

combined, preferably in the same species, and then compared with different species. It is

important to accept that birds and mammals had a long parallel evolution and that speci fic

properties of the mammalian brain are not necessarily present in birds or other vertebrates,

particularly when regarding the high plasticity of the vertebrate brain and the enormous

diversity of brain morphology within and among vertebrate groups.

Anatomical and Morphological Features of the Suprachiasmatic

Hypothalamus in Birds

The hypothalamus of birds has been investigated in a large number of studies, many of

them either focusing on general anatomy, the neurosecretory hypothalamo-hypophyseal

system, retino-hypothalamic projections, or the distribution of various neurotransmitters

and neuropeptides. Early anatomical studies described a paired cell group adjacent to the

base of the third ventricle directly above the optic chiasm, termed SCN because of its

anatomical similarity to the mammalian SCN (Crosby and Showers, 1969; van Tienhovenand Juhasz, 1962). Later experimental lesioning studies suggested that this cell group

might indeed represent the functionally equivalent structure to the mammalian SCN

(Ebihara and Kawamura, 1981; Simpson et al., 1981; Takahashi and Menaker, 1982).

However, a delineation of this cell group and a clear description of its rostro-caudal

extension as well as its cytoarchitectonic properties in different bird species are still not

available. This cell group has been described at various levels of the rostro-caudal

extension of the hypothalamus, sometimes even as far caudal as at the height of the

supraoptic decussation. This cell group has not only been termed SCN (Abraham et al.,

2002; Bons, 1980; Brandstatter et al., 2001; Ebihara and Kawamura, 1981; Hartwig, 1974;

Takahashi and Menaker, 1982) but also supraoptic nucleus (Ebihara et al., 1987), medial

hypothalamic nucleus (Norgren and Silver, 1989), medial hypothalamic retinorecipient

nucleus (Shimizu et al., 1994), and medial SCN (King and Follett, 1997).Another cell group located in the more lateral suprachiasmatic hypothalamus has also

been claimed to possibly represent the equivalent to the mammalian SCN and to be the site

of the avian hypothalamic circadian oscillator. This assumption was primarily based on the

presence of retinal projections (see below), the presence of melatonin binding sites

(Cassone and Brooks, 1991), and presumed rhythms of metabolic activity (Cassone,

1988; Lu and Cassone, 1993). This cell group has been called zone 1 to 3 (Meier, 1973),

supraoptic nucleus (Ebihara and Kawamura, 1981), SCN (Cooper et al., 1983), lateral

hypothalamus (Ehrlich and Mark, 1984), visual SCN (vSCN) (Cassone and Moore, 1987;

King and Follett, 1997) or lateral hypothalamic retinorecipient nucleus (LHRN) (Norgren

and Silver, 1989; Shimizu et al., 1994) [Figs. 1(B), (C)].


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The analysis of coronal sections of the house sparrow hypothalamus by utilizing

traditional Nissl-staining methods as compared to immunocytochemistry with an antibody

against a neuron-specific nuclear protein revealed that neuron-specific staining is a helpful

tool to identify and demarcate cellular aggregations (Fig. 3). A description of the gross

morphology of neuronal aggregations in the house sparrow hypothalamus is given in

Fig. 4. A clearly demarcated SCN can first be observed at the height of the anterior portion

of the preoptic nucleus as small aggregations of nerve cells at each lateral angle of the third

ventricle interconnected by a small subventricular queue of neurons. Thus, together with

the anterior preoptic nucleus and the anteroventral preoptic nucleus, the SCN is the most

rostral cellular aggregation that can be found in the house sparrow hypothalamus as well as

in other songbirds (Fig. 4). The morphological appearance of the SCN changes throughout

its longitudinal extension by increasing in diameter until about half of its total length and

decreasing in diameter towards its caudal end. The caudal SCN becomes less clearly

distinguishable from surrounding neurons until the nucleus is finally no longer demarcated

in coronal sections, whereas horizontal sections allow a very clear demarcation of this cell

group (Fig. 5). Lateral to the SCN, an aggregation of neurons is present at the dorsal

border of the optic chiasm partly extending into it. Along its rostro-caudal extension this

cell group increases in size and extends into the lateral hypothalamic area. A detailed

Figure 3. Coronal sections through the anterior suprachiasmatic hypothalamus of the house

sparrow. Comparable section planes Nissl stained with cresyl violet (A) or immunocytochemically

processed to label cells that are immunoreactive for a neuron-specific nuclear protein (B; for details

see Abraham et al., 2002) demonstrate that neuron-specific immunocytochemistry allows a better

recognition of cellular aggregations that can hardly be demarcated in Nissl stainings. Scale

bar200 mm.


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Figure 4. Gross morphology of neuronal aggregations in the suprachiasmatic hypothalamus of

songbirds. Coronal sections through the suprachiasmatic hypothalamus of the house sparrow from

rostral (A) to caudal (E). Left: Micrographs of coronal sections immunocytochemically processed to

label cells immunoreactive for a neuron-specific nuclear protein (for details see Abraham et al.,

2002). Right: Digitally averaged images to enhance stained areas as a tool to visualise areas of

aggregated neurons. AV, anteroventral preoptic nucleus; GLV, ventrolateral geniculate nucleus; LHN,

lateral hypothalamic nucleus; PONa, anterior preoptic nucleus; PONm, medial preoptic nucleus;

PPN, periventricular preoptic nucleus; SCN, suprachiasmatic nucleus; TEL, telencephalon. Asterisks

indicate neuronal aggregations that do not respect the boundaries of traditional cell groups. The

presence of a SCN in other songbird species is indicated by coronal sections through the anterior

hypothalamus of the stonechat (F), the blackbird (G), and the zebra finch (H). Scale bar200 mm.


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Figure 5. Demarcation of the SCN in horizontal sections through the suprachiasmatic hypotha-

lamus of the house sparrow. Horizontal sections from ventral (A) to dorsal (G) demonstrate the

presence of a clearly demarcated SCN. It is located in the most anterior part of the hypothalamus

extending to about the height of the rostral border of the supraoptic decussation. It is laterally flanked

(C, D) or overlapped (E, F) by magnocellular neurons (asterisks) that are present throughout the

anterior hypothalamus but do not respect the boundaries of the traditional cell groups. Scale

bar200 mm. The rostro-caudal axis within the sections is indicated in (A). DSO supraoptic

decussation; LHN lateral hypothalamic nucleus; r, rostral; c, caudal; OC, optic chiasm; PPN,

periventricular preoptic nucleus; TEL, telencephalon.


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morphological analysis revealed that this cell group may be subdivided into two parts, a

ventral and a dorsal one. The ventral portion of this cell group is characterized by a dense

aggregation of neurons along the dorsal border of the optic chiasm, whereas a more diffuse

aggregation of neurons extends dorsally into the lateral hypothalamic area. The ventral

portion, representing the previously described LHRN (Norgren and Silver, 1989) or vSCN

(Cassone and Moore, 1987), does not appear to be anatomically distinct from the dorsal

part. Because of its anatomical localization and its extension into the lateral hypothalamic

area, this cell group has recently been termed lateral hypothalamic nucleus (LHN)

(Abraham et al., 2002). From rostral to caudal, neuronal aggregations that do not respect

the anatomical borders of the identified cell groups are present in the preoptic, suprachias-

matic, and lateral hypothalamus, indicating that functionally distinct cell populations may

overlap in the hypothalamus. These neurons sometimes complicate a clear delineation of

cell groups or may even result in misinterpretation of the borders of cell groups. This

exemplifies that anatomical investigations allow only a limited view on how complex

neural systems such as the hypothalamus are organized. However, aggregation of cells or

even neurons is without any doubt an indicator of a certain functional relationship but does

not prove that cells are more functionally coupled than less densely aggregated or

surrounding cells and, thus, the investigation of functional features is a prerequisite for

a more comprehensive understanding of the avian hypothalamic organization.

Retinal Projections to the Avian Hypothalamus

Based on the fact that retinohypothalamic transmission of photic information to the

SCN is a crucial step for the entrainment of the hypothalamic pacemaker in mammals,tract-tracing techniques were used in a number of studies in birds in an attempt to

localize the avian hypothalamic oscillator. Dense arborizations of retinal fibers were

found in the SCN of the pigeon (Shimizu et al., 1994), whereas retinal input to the SCN

in the same species has not been found (Cooper et al., 1983; Meier, 1973). Such

inconsistent results were also obtained in house sparrow, quail, and starling, where either

retinal input was detected (Hartwig, 1974; Norgren and Silver, 1989), or no retinal fibers

could be found in the SCN (Cassone and Moore, 1987; King and Follett et al., 1997).

Consistently, a substantial number of retinal projections were found in the ventral portion

of the LHN in all species investigated, but no uniform nomenclature was used for this

terminal field of retinal projections (Cassone and Moore, 1987; Ebihara and Kawamura,

1981; Ebihara et al., 1987; Ehrlich and Mark, 1984; Hartwig, 1974; King and Follett,

1997; Meier, 1973; Norgren and Silver, 1989; Shimizu et al., 1994). However, retinalinput is not necessarily a unique feature of a hypothalamic oscillator, since, in contrast to

mammals, light input to the hypothalamus of birds is not only mediated by retinal

projections. There is convincing evidence that photoreceptors located within the brain are

able to synchronize circadian oscillations in birds. Thus, retinal input to the suprachias-

matic hypothalamus may, at least in most species, be of less functional significance for

circadian pacemaking in birds than in mammals. A more useful tool to elucidate which

cell groups in the avian hypothalamus respond to photic stimulation is the investigation

of immediate early response gene (IEG) expression. Only few such studies were

performed in birds until now. They demonstrated IEG expression following a light

stimulus or in response to visual stimulation in the ventral part of the LHN but not in the


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SCN of chicken, starling, and quail. No differences were found when IEGs were induced

during either day or night (King and Follett, 1997; Wallman et al., 1994). In the house

sparrow hypothalamus, c-fos immunoreactive cells could be detected in and close to the

most rostral portion of the SCN as well as in the preoptic nucleus, the lateral

hypothalamic nucleus, and in the ventro-lateral geniculate nucleus of birds following

exposure to a one-hour light pulse during the night (Fig. 6). These preliminary data

indicate that the light signal is rapidly spread over the anterior hypothalamus of the house

sparrow, including regions that receive prominent retinal input such as the LHN and the

ventro-lateral geniculate nucleus, but also regions that receive no or only little direct

retinal input such as the preoptic nucleus and the SCN.

Neurochemical Properties of the Hypothalamus in Birds

The hypothalamic distribution of neurotransmitters and neuropeptides has been

investigated in a variety of bird species. In the following paragraph, particular attention

will be drawn to vasotocin (VT), the avian equivalent to vasopressin, and VIP, because both

neuropeptides have been associated with circadian function in the mammalian SCN

(reviewed in Abrahamson and Moore, 2001; van Esseveldt et al., 2000). In various bird

species, VT-immunoreactive cells were found in the anterior and medial preoptic nucleus

as well as in the periventricular preoptic nucleus, in the paraventricular nucleus and in the

lateral hypothalamus [e.g., Japanese quail (Bons, 1980; Panzica, 1985), house sparrow

(Cassone and Moore, 1987), chicken (Panzica, 1985), canary (Kiss et al., 1987), zebra

finch (Voorhuis and De Kloet, 1992), ring dove (Norgren and Silver, 1990)]. The

distribution of vasotocinergic cells in the avian hypothalamus mostly resembles theneurosecretory hypothalamo-hypophyseal system of mammals (Bjorklund et al., 1987)

and appears to be highly similar to the different avian species studied. In a few studies,

vasotocinergic cells were described in the SCN (e.g., Bons, 1980; Panzica, 1985).

However, the population of vasotocinergic cells in the periventricular preoptic nucleus

found in most of the other studies is localized in very close vicinity to the SCN, and

whether vasotocinergic cells are indeed present in the SCN of birds remains to be

established. In most studies VT-immunoreactivity was not found in the SCN, whereas

the LHN contains a prominent population of vasotocinergic cells in all birds studied.

Vasoactive intestinal polypeptide has been demonstrated in the hypothalamus of a

number of bird species, varying considerably not only among species but also within the

same species. This might be partly due to the fact that some authors used colchicin-

treatment, which has been shown to increase VIP-immunoreactivity (Cloues et al., 1990;Norgren and Silver, 1990; Yamada and Mikami, 1982). In the chicken, Kuenzel and

Blahser (1994) found VIPergic fibers in the periventricular region of the preoptic

hypothalamus and VIPergic cells in the latero-caudal hypothalamus as well as in the

paraventricular and preoptic nucleus, whereas Esposito et al. (1993) demonstrated few

VIPergic perikarya in the SCN, but no VIP-immunoreactivity in the lateral hypothalamus.

Yamada and Mikami (1982), Aste et al. (1995) and Teruyama and Beck (2001) localized

VIPergic cells andfibers scattered throughout the periventricular preoptic region and in the

paraventricular nucleus of the Japanese quail. The latter two studies also mentioned

VIPergic neurons in the lateral hypothalamus. The VIPergic system in the hypothalamus of

columbiform birds is characterized by a varying number of cells and fibers in the


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Figure6.

Functionalfeaturesoftheanteriorsuprac

hiasmatichypothalamusofthehousesparrow.Coronalsectionsfrom

rostral(A)

tocaudal(C)demonstratingc-fosimmunoreactive

cellsintherostralSCN

aswellaslater

altoit(blackarrows),theLHN,the

preopticnu

cleus(blackarrowheads),andtheGLVfollowinglightexposureduringnight.TEL

,telencephalon.Immunoreactivityfor

VT(D),so

matostatin(E),andgrowth-hormone(F)

indicatesimilarfunctionalpropertiesoftheventral(LHNv)anddorsal(LHNd)

portionsofthelateralhypothalamicnucleus,suggestingacertaindemarcationofthiscellgr

oup.Scalebars

200mm.


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periventricular preoptic region, the paraventricular nucleus, and the lateral hypothalamus

[ring dove (Cloues et al., 1990; Norgren and Silver, 1990), collared dove (den Boer-Visser

and Dubbeldam, 2002), pigeon (Hof et al., 1991)]. Norgren and Silver (1990) found VIP-ir

cell populations directly above the SCN and dorsomedial from the ventral portion of the

LHN. Similar results were obtained in the house sparrow, where a population of small

VIPergic cells was found medial to the ventral portion of the LHN (Cassone and

Moore, 1987).

Taken together, neither the avian SCN nor the LHN nor both combine anatomical,

morphological, or functional features of the mammalian SCN. Thus, the findings of

existing literature as compared to our own investigations can be summarized as follows:

1. A SCN has been described in many but not all studies investigating the avian

hypothalamus. If present, it has been variably termed by different investigators. In the

house sparrow, a clearly demarcated SCN is present that can be well delineated according

to neuron-specific staining in coronal and horizontal sections (Figs. 35). This nucleus can

also be found in various other species, including zebrafinch, blackbird, and stonechat

(Fig. 4). The reason for the lack of a consistent description of the SCN may be that

traditional Nissl stainings do not allow a very good distinction of hypothalamic cell

groups. Additionally, the SCN might have been overlooked in some studies because, in

variance to mammals, it is located in the most anterior part of the hypothalamus and, thus,

possibly represents the anatomical equivalent to the rostral portion of the mammalian SCN

(van den Pol, 1980) or suprachiasmatic preoptic nucleus (Bjorklund et al., 1987) rather

than the caudal portion of the mammalian SCN.

2. Lateral to the SCN, a morphologically heterogenous cell group is present that consistsof a ventral portion, previously described as LHRN (Norgren and Silver, 1989) or vSCN

(Cassone and Moore, 1987), and a dorsal part extending into the lateral hypothalamic area.

Immediate early response gene expression and the distribution of VT, somatostatin, and

growth hormone (Fig. 6) as well as the expression of the putative clock gene per2

(Abraham et al., 2003) do not support a separation into distinct cell groups, but rather

indicate that the dorsal and the ventral part are indeed a functionally coupled cell group.

This cell group is anatomically distinct from the SCN and, until now, no neural

connections between the SCN and the LHN could be found in any avian species. Thus,

the term vSCN, based on the presence of retinal input, melatonin receptors, and

rhythmic metabolic activity (Cassone, 1988; Cassone and Moore, 1987; Cassone and

Brooks, 1991; Lu and Cassone, 1993; King and Follett, 1997), should be avoided in future

studies. Neither is this cell group a part of the SCN nor is retinohypothalamic input or thepresence of melatonin binding sites a valuable justification for the term SCN. The vSCN

is part of a much larger lateral hypothalamic cell group that extends into the lateral

hypothalamic area and that shows more similarities to the mammalian supraoptic nucleus

than to the SCN (Oksche et al., 1974).

3. The SCN and the LHN are flanked and=or overlapped by neurons that stain positively

for VT and some other neuropeptides and that appear to be part of the neurosecretory

hypothalamo-hypophyseal system. Whether there exists any functional relation between

this system and circadian oscillations in the SCN and the LHN (Abraham et al., 2002;

Abraham et al., 2003) remains to be established in future studies.


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4. Whether the avian SCN receives direct retinal input or not is still elusive, although it

has been investigated in a rather large number of bird species. In contrast to that, retinal

projections to the LHN were found in all species studied so far. However, neither tracing

retinal projections, nor the investigation of immediate early response gene expression

allowedfinal conclusions on whether the SCN or the LHN might represent the equivalent

to the mammalian SCN. In the house sparrow, photic stimulation during night induces

c-fos expression in the rostral part of the SCN, the ventral and dorsal portion of the LHN,

the preoptic nucleus, and the ventro-lateral geniculate nucleus, indicating that the light

signal may be rapidly distributed over the suprachiasmatic and preoptic hypothalamus of

birds independent of whether direct retinal input is present or not (Fig. 6).

5. The avian hypothalamus has been studied regarding the distribution of various neuro-

transmitters and neuropeptides in a large number of species, but the results were too

inconclusive to assign circadian oscillator function to any particular cell group. Neither VT

nor VIP were found in the SCN of birds. Instead, VTwas found in the preoptic nucleus, close

to the SCN in the periventricular preoptic nucleus, as well as in the LHN of most species

investigated; whereas VIP-immunoreactive cells were only found close to or within the LHN.

In conclusion, a combination of anatomical, morphological, and functional observa-

tions is essential for a better understanding and an unambiguous identification of

functional cell groups in the avian hypothalamus. Investigations at selected section planes

may be misleading when neglecting the full rostro-caudal extension of a cell group, since

certain functional compartmentalizations may be present and particular properties may

only occur in a certain portion of a cell group, as exemplified by retinal input to the ventral

portion of the LHN, immediate early response gene expression that is exclusively presentin the most rostral portion of the SCN following a light stimulus (Fig. 6), and a complex

spatio-temporal pattern of per gene expression in the SCN (Brandstatter et al., 2001;

Abraham et al., 2002; Abraham et al., 2003). Morphological demarcation of cell groups

based on Nissl staining or even more sophisticated techniques, such as neuron-specific

immunocytochemistry, allow insight that is limited to structural and cytoarchitectonic

properties. However, functionally distinct cell types do not always respect the boundaries

of traditional cell groups that have been delineated in Nissl stainings, as exempli fied by

neurons of the neurosecretory hypothalamo-hypophyseal system (Oksche et al., 1974).

Thus, an integrative approach utilizing different techniques is needed to obtain unambig-

uous insight into the organization of complex neural systems. In the avian suprachiasmatic

hypothalamus, neither light responsiveness nor rhythmic clock gene expression is confined

to a single cell group, as is the case in mammals, but can be found in the SCN as well as inthe LHN. Functional studies are needed now to detect possible interaction mechanisms

among these cell groups and their function in circadian pacemaking, and to clarify possible

integration mechanisms of the melatonin signal to better understand how the complex

circadian system of birds works.

ACKNOWLEDGMENTS

Thanks are due to T. Roenneberg for coordinating the circadian Schwerpunkt-

programm of the German Research Council and his never ending commitment to


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promote chronobiology. Financial support by the German Research Council (DFG grants

no. Br-1899=1-1 and Br-1899=1-2) is gratefully acknowledged.

REFERENCES

Abraham, U., Albrecht, U., Gwinner, E., Brandstatter, R. (2002). Spatial and temporal

variation of passer Per2 gene expression in two distinct cell groups of the

suprachiasmatic hypothalamus in the house sparrow (Passer domesticus). Eur. J.

Neurosci. 16:429436.

Abraham, U., Albrecht, U., Brandstatter, R. (2003). Hypothalamic circadian organization

in birds. II. Clock gene expression. Chronobiol. Int. 20:657669.

Abrahamson, E. E., Moore, R. Y. (2001). Suprachiasmatic nucleus in the mouse: retinal

innervation, intrinsic organization and efferent projections. Brain Res. 916:172191.

Aste, N., Viglietti-Panzica, C., Fasolo, A., Panzica, G. C. (1995). Mapping of neurochemi-

cal markers in quail central nervous system: VIP- and SP-like immunoreactivity.

J. Chem. Neuroanat. 8:87102.

Binkley, S. A., Riebman, J. B., Reilly, K. B. (1978). The pineal gland: a biological clock

in vitro. Science 202:11981201.

Bjorklund, A., Hokfelt, T., Swanson, L. W. (1987). Handbook of Chemical Neuroanatomy.

Vol. 5. Integrated Systems of the CNS, Part I, Hypothalamus, Hippocampus,

Amygdala, Retina. Amsterdam: Elsevier Science Publishers.

Bons, N. (1980). The topography of mesotocin and vasotocin systems in the brain of the

domestic mallard and Japanese quail: immunocytochemical identification. Cell Tiss.Res. 213:3751.

Brandstatter, R. (2002). The circadian pacemaking system of birds. In: Kumar, V., ed.

Biological Rhythms. New Delhi, India: Narosa Publishing House, pp. 144163.

Brandstatter, R., Kumar, V., Abraham, U., Gwinner, E. (2000). Photoperiodic information

acquired and stored in vivo is retained in vitro by a circadian oscillator, the avian

pineal gland. Proc. Natl. Acad. Sci. 97:1232412328.

Brandstatter, R., Abraham, U., Albrecht, U. (2001). Initial demonstration of rhythmic Per

gene expression in the hypothalamus of a non-mammalian vertebrate, the house

sparrow. Neuroreport 12:11671170.

Breazile, J. E., Kuenzel, W. J. (1993). Systema nervosum centrale. In: Baumel, J. J.,

King, A. S., Breazile, J. E., Evans, H. E., Vanden Berge, J. C., eds. Handbook of

Avian Anatomy: Nomina Anatomica Avium. Cambridge: Nuttal Ornithol Club 23,pp. 493554.

Cassone, V. M. (1988). Circadian variation of [14C] 2-deoxyglucose uptake within the

suprachiasmatic nucleus of the house sparrow, Passer domesticus. Brain Res.

459:178182.

Cassone, V. M., Menaker, M. (1984). Is the avian circadian system a neuroendocrine loop?

J. Exp. Zool. 232:539549.

Cassone, V. M., Moore, R. Y. (1987). Retinohypothalamic projection and suprachiasmatic

nucleus of the house sparrow, Passer domesticus. J. Comp. Neurol. 266:171182.

Cassone, V. M., Brooks, D. S. (1991). Sites of melatonin action in the brain of the house

sparrow, Passer domesticus. J. Exp. Zool. 260:302309.


p

y


16/19

Cloues, R., Ramos, C., Silver, R. (1990). Vasoactive intestinal polypeptide-like immuno-

reactivity during reproduction in doves: influence of experience and number of

offspring. Horm. Behav. 24:215231.

Cooper, M. L., Pickard, G. E., Silver, R. (1983). Retinohypothalamic pathway in the dove

demonstrated by anterograde HRP. Brain Res. Bull. 10:715718.

Crosby, E. C., Showers, M. J. L. (1969). Comparative anatomy of the preoptic and

hypothalamic areas. In: Haymaker, W., Anderson, E., Nauta, W. J. H., eds. The

Hypothalamus. Springfield, III: Ch. C. Thomas Publ., pp. 61135.

den Boer-Visser, A. M., Dubbeldam, J. L. (2002). The distribution of dopamine,

substance P, vasoactive intestinal polypeptide and neuropeptide Y immunoreactiv-

ity in the brain of the collared dove, Streptopelia decaocto. J. Chem. Neuroanat.

23(1):127.

Earnest, D. J., Sladek, C. D. (1986). Circadian rhythms of vasopressin release from

individual rat suprachiasmatic explants in vitro. Brain Res. 382:129133.

Ebihara, S., Kawamura, H. (1981). The role of the pineal organ and the suprachiasmatic

nucleus in the control of circadian locomotor rhythms in the Java sparrow, Padda

oryzivora. J. Comp. Physiol. A 141:207214.

Ebihara, S., Oshima, I., Yamada, H., Maki, G., Koji, S. (1987). Circadian organization in

the pigeon. In: Hiroshige, T., Honma, K., eds. Comparative Aspects of Circadian

Clocks. Sapporo: Hokkaido University Press, pp. 8494.

Ehrlich, D., Mark, R. (1984). An atlas of the primary visual projections in the brain of the

chick Gallus gallus. J. Comp. Neurol. 223:592610.

Esposito, V., De Girolamo, P., Gargiulo, G. (1993). Immunoreactivity to vasoactive

intestinal polypeptide (VIP) in the hypothalamus of the domestic fowl, Gallus

domesticus. Neuropeptides 25:8390.Foster, R. G., Soni, B. G. (1998). Extraretinal photoreceptors and their regulation of

temporal physiology. Rev. Reprod. 3:145150.

Gaston, S., Menaker, M. (1968). Pineal function: the biological clock in the sparrow?

Science 160:11251127.

Gwinner, E., Brandstatter, R. (2001). Complex bird clocks. Phil. Trans. R. Soc. Lond. B

356:18011810.

Hartwig, H. G. (1974). Electron microscopic evidence for a retinohypothalamic pro-

jection to the suprachiasmatic nucleus of Passer domesticus. Cell Tiss. Res.

153:8999.

Hof, P. R., Dietl, M. M., Charnay, Y., Martin, J.-L., Bouras, C., Palacios, J. M.,

Magistretti, P. J. (1991). Vasoactive intestinal peptide binding sites and fibers in

the brain of the pigeon Columba livia: an autoradiographic and immunohistochem-ical study. J. Comp. Neurol. 305:393411.

Ibata, Y., Okamura, H., Tanaka, M., Tamada, Y., Hayashi, S., Iijima, N., Matsuda, T.,

Munekawa, K., Takamatsu, T., Hisa, Y., Shigeyoshi, Y., Amaya, F. (1999). Func-

tional morphology of the suprachiasmatic nucleus. Front. Neuroendocrin.

20:241268.

Isobe, Y., Muramatsu, K. (1995). Daynight differences in the contents of vasoactive

intestinal peptide, gastrin-releasing peptide and Arg-vasopressin in the suprachias-

matic nucleus of rat pups during postnatal development. Neurosci. Lett.

188:4548.


p

y


17/19

Karten, H. J., Hodos, W. (1967). A Stereotaxic Atlas of the Brain of the Pigeon (Columbia

livia). Baltimore, Maryland: The John Hopkins Press.

King, V. M., Follett, B. K. (1997). C-fos expression in the putative avian suprachiasmatic

nucleus. J. Comp. Physiol. A 180:541551.

Kiss, J. Z., Voorhuis, T. A. M., van Eekelen, J. A. M., De Kloet, E. R., De Wied, D. (1987).

Organization of vasotocin-immunoreactive cells and fibers in the canary brain.

J. Comp. Neurol. 263:347364.

Kojima, D., Fukada, Y. (1999). Non-visual photoreception by a variety of vertebrate

opsins. Novart. Fdn. Symp. 224:265279.

Kornhauser, J. M., Nelson, D. E., Mayo, K. E., Takahashi, J. S. (1990). Photic and

circadian regulation of c-fos gene expression in the hamster suprachiasmatic

nucleus. Neuron 5:127134.

Kuenzel, W. J., van Tienhoven, A. (1982). Nomenclature and location of avian hypotha-

lamic nuclei and associated circumventricular organs. J. Comp. Neurol.

206:293313.

Kuenzel, W. J., Blahser, S. (1994). Vasoactive intestinal polypeptide (VIP)-containing

neurons: distribution throughout the brain of the chick (Gallus domesticus) with

focus upon the lateral septal organ. Cell Tissue Res. 275:91107.

Laemle, L. K., Ottenweller, J. E., Fugaro, C. (1995). Diurnal variation in vasoactive

intestinal polypeptide-like immunoreactivity in the suprachiasmatic nucleus of

congenitally anophtalmic mice. Brain Res. 688:203208.

Lehman, M. N., Silver, R., Gladstone, W. R., Kahn, R. M., Gibson, M., Bittman, E. L.

(1987). Circadian rhythmicity restored by neural transplant. Immunocytochemical

characterisation of the graft and its integration with the host brain. J. Neurosci.

7:16261638.Lu, J., Cassone, V. M. (1993). Pineal regulation of circadian rhythms of 2-deoxy[14C]-

glucose uptake and 2[125I]iodomelatonin binding in the visual system of the house

sparrow, Passer domesticus. J. Comp. Physiol. A 173:765774.

Meier, R. E. (1973). Autoradiographic evidence for a direct retino-hypothalamic projection

in the avian brain. Brain Res. 53:417421.

Menaker, M., Tosini, G. (1995) The evolution of vertebrate circadian systems. In:

Hiroshige, T., Honma, K., eds. Proceedings of the Sixth Sapporo Symposium on

Biological Rhythms. Sapporo: Hokkaido University Press, pp. 3952.

Moore, R. Y. (1973). Retinohypothalamic projections in mammals: a comparative study.

Brain Res. 49:403409.

Moore, R. Y., Eichler, V. B. (1972). Loss of circadian adrenal corticosterone rhythm

following suprachiasmatic nucleus lesions in the rat. Brain Res. 42:201206.Nieuwenhuys, R., Ten Donkelaar, H. J., Nicholson, C. (1998). The Central Nervous System

of Vertebrates. Vol. 3. Berlin Heidelberg: Springer-Verlag.

Norgren, R., Silver, R. (1989). Retinohypothalamic projections and the suprachiasmatic

nucleus in birds. Brain. Behav. Evol. 34:7383.

Norgren, R. B., Silver, R. (1990). Distribution of vasoactive intestinal peptide-like and

neurophysin-like immunoreactive neurons and acetylcholinesterase staining in the

ring dove hypothalamus with emphasis on the question of an avian suprachiasmatic

nucleus. Cell Tissue Res. 259:331339.


p

y


18/19

Oksche, A., Kirschstein, H., Hartwig, H. G., Oehmke, H. J. (1974). Secretory parvocel-

lular neurons in the rostral hypothalamus and in the tuberal complex of Passer

domesticus. Cell Tiss. Res. 149:363369.

Panzica, G. C. (1985). Vasotocin-immunoreactive elements and neuronal typology in the

suprachiasmatic nucleus of the chicken and Japanese quail. Cell Tiss. Res.

242:371376.

Pierce, M. E., Sheshberadaran, H., Zhang, Z., Fox, L. E., Applebury, M. L., Takahashi, J. S.

(1993). Circadian regulation of iodopsin gene expression in embryonic photorecep-

tors in retinal cell culture. Neuron 10:579584.

Ralph, M. R., Foster, R. G., Davis, F. C., Menaker, M. (1990). Transplanted suprachias-

matic nucleus determines circadian period. Science 247:975978.

Rusak, B., Zucker, I. (1979). Neural regulation of circadian rhythms. Physiol. Rev.

59:449526.

Rusak, B., Robertson, H. A., Wisden, W., Hunt, S. P. (1990). Light pulses that shift rhythms

induce gene expression in the suprachiasmatic nucleus. Science 248:12371240.

Shimizu, T., Cox, K., Karten, H. J., Britto, L. R. G. (1994). Cholera toxin mapping of

retinal projections in pigeons (Columba livia), with emphasis on retinohypothalamic

connections. Visual Neurosci. 11:441446.

Shinohara, K., Honma, S., Katsuno, Y., Abe, H., Honma, K. (1994). Circadian rhythms in

the release of vasoactive intestinal polypeptide and arginine-vasopressin organotypic

slice culture of rat suprachiasmatic nucleus. Neurosci. Lett. 170:183186.

Silver, R., Witkovsky, P., Horvath, P., Alones, V., Barnstable, C. J., Lehman, M. N. (1988).

Coexpression of opsin- and VIP-like-immunoreactivity in CSF-contacting neurons

of the avian brain. Cell Tissue Res. 253:189198.

Simpson, S. M., Follett, B. K. (1981). Pineal and hypothalamic pacemakers: their role inregulating circadian rhythmicity in Japanese quail. J. Comp. Physiol. A

144:381389.

Stokes, T. M., Leonard, C. M., Nottebohm, F. (1974). The telencephalon, diencephalon,

and mesencephalon of the Canary, Serinus canaria, in stereotaxic coordinates.

J. Comp. Neurol. 156:337374.

Takahashi, J., Menaker, M. (1982). Role of the suprachiasmatic nuclei in the circadian

system of the house sparrow, Passer domesticus. J. Neurosci. 2:815828.

Teruyama, R., Beck, M. M. (2001). Double immunocytochemistry of vasoactive intestinal

peptide and cGnRH-I in male quail: photoperiodic effects. Cell Tiss. Res.

303:403414.

Tominaga, K., Shinohara, K., Otori, Y., Fukuhara, C., Inouye, S. I. T. (1992). Circadian

rhythms of vasopressin content in the suprachiasmatic nucleus of the rat.Neuroreport 3:809812.

Underwood, H. Barrett, R. K., Siopes, T. (1990). The quails eye: a biological clock. J. Biol.

Rhythms 5:257265.

van den Pol, A. N. (1980). The hypothalamic suprachiasmatic nucleus of rat: intrinsic

anatomy. J. Comp. Neurol. 191:661702.

van Esseveldt, L. K. E., Lehman, M. N., Boer, G. J. (2000). The suprachiasmatic nucleus

and the circadian time-keeping system revisited. Brain Res. Rev. 33:3477.

van Tienhoven, A., Juhasz, L. P. (1962). The chicken telencephalon, diencephalon and

mesencephalon in stereotaxic coordinates. J. Comp. Neurol. 118:185198.


p

y


19/19

Voorhuis, T. A. M., De Kloet, E. R. (1992). Immunoreactive vasotocin in the zebra finch

brain (Taeniopygia guttata). Dev. Brain Res. 69:110.

Wallman, J., Saldanha, C. J., Silver, R. (1994). A putative suprachiasmatic nucleus of birds

responds to visual motion. J. Comp. Physiol. A 174:297304.

Yamada, S., Mikami, S. I. (1982). Immunohistochemical localization of vasoactive

intestinal polypeptide containing neurons in the hypothalamus of the Japanese

Quail Coturnix-coturnix. Cell Tissue Res. 226:1326.

Yamase, K., Takahashi, S., Nomura, K., Haruta, K., Kawashima, S. (1991). Circadian

changes in arginine vasopressin level in the suprachiasmatic nuclei in the rat.

Neurosci. Lett. 130:255258.

Yoshimura, T., Yasuo, S., Suzuki, Y., Makino, E., Yokota, Y., Ebihara, S. (2001).

Identification of the suprachiasmatic nucleus in birds. Am. J. Physiol. Regulatory

Integrative Comp. Physiol. 280:R1185R1189.

Zimmerman, N. H., Menaker, M. (1979). The pineal gland: a pacemaker within the

circadian system of the house sparrow. Proc. Natl. Acad. Sci. 76:9991003.


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