BRAIN RESEARCH

E L S E V I E R Brain Research 661 (1994) 83-90

Research report

The actions of 5-HT 1 agonists and antagonists on nociceptive processing in the rat spinal cord: results from behavioural

and electrophysiological studies

Zahid Ali d, Gang Wu b Alexei Kozlov c, Stephen Barasi a,, Department of Physiology, UniL'ersity of Wales, Cardiff CF1 ISS, UK

b Department of Clinical Studies, Huddinge Unit.'ersity Hospital, Karolinska Institute, S-141 86 Huddinge, Sweden c P.K. Anokhin Institute for Normal Physiology, Gertzen Street, Moscow 10300, Russia

J Department of Neuroscience, Johns Hopkins Unit,ersity, 600 North Wolfe Street, Baltimore, MD 21205, USA

Accepted 19 July 1994

Abstract

We have developed a technique which allows drugs to be microinjected intrathecally in anaesthetised rats whilst single unit recordings are made from dorsal horn neurones. Using this technique together with recordings of tail flick latency (TFL) elicited from lightly anaesthetised rats we have found that the specific 5-HT~a agonist 8-OH DPAT (15, 150, 300 nmol) increases nociceptive responses recorded from single dorsal horn neurones and decreases TFL. The non-specific 5-HTlb agonist TFMPP (300 nmol) and the general 5-HT 1 agonist 5-CT (0.3, 3.0, 30 nmol) both decreased nociceptive responses and has inconsistent effects on TFL. Intrathecally applied 5-HT (130, 260 nmol) generally reduced nociceptive neuronal responses and increased TFL. In a minority of experiments, however, 5-HT increased nociceptive responses and it is suggested that this effect is associated with activation of 5-HT~a receptors. Activity at 5-HTIb receptors has the effect of suppressing or reducing responsiveness. The increased responsiveness of dorsal horn neurones to noxious stimulation associated with activity at 5-HT~a receptors may be associated either with increases in receptive field size, promotion of spinal nocifensive reflexes or the facilitation of the rostral transmission to specific brainstem sites.

Keywords: Serotonin 5-HT x receptor; Nociception; Dorsal horn; Intrathecal

I. Introduct ion

Nociceptive processing is influenced by activity in a number of descending spinal pathways. Despite receiv- ing a great deal of attention the function of the de- scending 5-HT pathway remains poorly understood. Serotonergic fibres originate from the 5-HT-containing neurones in the brainstem [3,7,23], and dorsal horn immunoreactive variscosities are concentrated in the superficial [13,17,27] and deeper layers [27]. More re- cently information concerning different binding sites for 5-HT receptor subtypes have become available (for review see Palacios et al. [24]).

* Corresponding author. Fax: (44) 0222-874094. E-maih Barasi(a: Cardiff.ac.uk.

0006-8993/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0006-8993(94)00898-1

5-HT released from raphe-spinal neurones was ini- tially thought exclusively to suppress spinal nociceptive transmission. Thus the iontophoretic application of 5-HT has been reported to inhibit nociceptive re- sponses of dorsal horn neurones [2,12,31] an effect consistent with other studies showing that intrathecally applied 5-HT produces a dose-dependent analgesia [5,6,11,14,28,33].

There is, however, considerable diasagreement con- cerning the effect of different 5-HT ligands in a variety of different experimental models. The existence of distinct populations of 5-HT receptors in the dorsal horn has been demonstrated and suggests the potential for functionally different action at different sites.

Pharmacological and electrophysiological studies in- vestigating the action of 5-HT~a ligands have produced conflicting results. Intrathecal application of the 5 -HT~

g4 Z. ,4li ct aL / Brain Research 661 (1994) 8 3 - 9 0

agonist 8-OH-DPAT has been reported to increase [21,35] decrease [8] or have no effect [22] in a series of behavioural tests.

EI-Yassir [10] reported a non-selective inhibition of all responses to sensory stimulation following the ion- tophoretic application of 8-OH-DPAT. In contrast ap- plied iontophoretically the agonist had no effect on excitatory amino acid evoked activity in wide dynamic range neurones [34].

The 5-HTtb agonist Ru 24969 has been reported to have an antinociceptive action in various nocifensive reflexes [l,6,22]. However, reflexes have also been en- hanced [29] or unaffected [18,19]. The iontophoretic application of Ru 24969 has been reported to reduce reponses of dorsal horn neurones to both pinch [10] and to excitatory amino acid evoked responses [35].

In an attempt to clarify the action of 5-HT~ agonists in the spinal cord we have adopted a single route of application for the drugs in both behavioural and elec- trophysiological studies and have specifically attempted to compare results from the two paradigms, Many studies have reported the effects of drugs that have been microinjected via chronically implanted cannulae in different behavioural experiments. This study ap- pears to be the first in which drugs have been applied in this manner whilst recordings are being made from single units.

confirmed histologically a note was made of the depth at which each study was performed. This approach was adopted since preliminm3' studies revealed no obvious relationship between study depth and drug effect.

Noxious thermal stimuli were applied to tile plantar surface ot the hind paw using a Peltier thermode. The characteristics of the heat stimulus (rise time, duration and intensity) were computer controlled. Typically the rise time and on time for the st imulus were both set at 10 s and the plateau temperature was 51-52°C. Base skin temperature was maintained at 28°-30°C. Neuronal responses were displayed on a polygraph and also captured by computer. Following computer analysis it was possible to note the number of action potentials produced in response to each heat stimulus.

In those tail flick studies in which drug effects elevated TFLs beyond the cut-off value of 6 s the non-parametric Mann--Whitney U-test was applied. Where data was normally distributed pre and post drug TFLs were compared using the Student t-test (two tailed). In view of the wide variation in number of action potentials recorded per nociceptive response, the non-parametric Wilcoxon matched- pairs test was employed in the analysis of the electrophysiological studies.

After at least three stable control responses, 5 /zl of drug was microinjected. The following drugs were used: 5-hydroxytryptamine creatinine sulphate complex (5-HT), the 5-HTi-like agonist 5-carbox- yamidotryptamine (5-CT), the 5-HTIb agonist l-(3-trifluoromethyl) phenlylpiperazine (TFMPP), the 5-HTIa agonist 8-hydroxy-2-(di-n- propylamino)tetralin (8-OH-DPAT), and the 5-HTi-like antagonist cyanopindolol, all dissolved in 0.9% saline. Cyanopindolol and higher concentrations of 8 -OH-DPAT were assisted into solution by adding 5 tzl glacial acetic acid in 2 ml of saline. All drugs were dissolved at room temperature immediately before use.

2. Materials and methods

All experiments were performed on male Wistar rats (200-300 g). The animals were kept on a constant 12 h l ight-dark cycle and experiments always performed between similar hours each day. An intrathecal cannula was introduced via the atlanto occipital mem- brane until the tip was judged to be close to the lumbar enlargment. In earlier experiments checks were made to confirm the location of the cannula tip following the injection of blue dye post-mortem. The volume of each cannula was measured and adjusted to approximately 5 gl . Drugs were microinjected over a period of approximately 2 min in a volume of 5 /z l with a saline flush of similar volume.

Animals prepared for tail flick testing were initially anaesthet ised with Fluothane. Subequently the rats were transferred and main- tained on the steroidal anaesthetic Saffan administered intravenously via the femoral vein. Anaes thes ia was mainta ined at a level at which corneal reflexes were absent; however, after blackening the under- side of the tail, animals responded with a tail flick when a light beam was applied. An electronic counter recorded the tail flick latency (TFL). An hour after surgery had been completed, reproducible (within 5-10%) TFLs were recorded at intervals of 4 rain.

Other rats used for electrophysiological studies were anaes- thetised with halothane (Fluothane) for the entire procedure. Blood pressure was recorded via the carotid artery and body temperature was mainta ined using a feed back control heating blanket. A lumbar laminectomy was performed, care being taken to leave the dura intact. If there were signs of raised CSF pressure a drain was provided by making a small slit in the dura at the distal margin of the exposed cord. A tungsten microeleetrode were advanced into the cord through a very small (less than 1 mm) hole made in the dura. Recordings were made from spontaneously active neurones. Al- though the exact location of the microelectrode recording tip was not

3. Results

3.1. Effect of 5-HT

3.1.1. Tail flick test 5-HT was microinjected intrathecally using four dif-

ferent doses. At doses of 32 nmol (n = i 1) and 64 nmol (n = 9), no significant changes in TFL were recorded. At the higher doses of 130 nmol (n = 11) and 260 nmol (n = 9), a statistically significant increase in TFL was recorded over a period of approximately 12 min (see Fig. 1A). Recovery to control levels of TFL was ob- tained about 50 min after microinjection of 5-HT. Control microinjectons of saline (n = 5 ) w e r e without significant effect on TFL.

3.1.2. Dorsal horn neurones Responses were recorded from twenty multirecep-

tive neurones in the dorsal horn responding to noxious heating of the plantar surface of the ipsilaterai paw. Changes in firing rate were not generally observed until the skin temperature exceeded approximately 47°C. The majority of responses consisted of an in- crease in firing rate which outlasted the period of heating. A number of neurones exhibited reductions in firing rate following heating, these responses were gen- erally unstable and difficult to study.

z. Ali et al. /Brain Research 661 (1994) 83-90 85

Noc icep t ive responses were r e c o r d e d t h roughou t the dorsa l horn and no c lear r e l a t ionsh ip was found in any of the e lec t rophys io log ica l s tudies b e t w e e n drug effect and dep th at which record ings were made . T h e major - ity of r ecord ings were o b t a i n e d be tween 400-600 /xM and the range was 100-1100 /xM. In app rox ima te ly 30% of s tudies noc icept ive responses exh ib i ted desen- s i t isat ion to noxious s t imula t ion desp i t e ca re be ing taken to increase the in te r s t imulus interval . Such s tud- ies were d i scon t inued .

A

B

C

5-HT

200

150

100

50

0

200

150

100

50

0

0 1tO 20 3i0 40 50

2 ¢.o

21 I I I I I 0 40 60 80 100

TIME(min)

Fig. 1. A: effects of intrathecally applied 5-HT (260 nmol n = 9) on tail flick latencies (TFL). Each point represents mean TFL(s) +_standard error of the mean. Significance of drug effects was determined by comparing TFLs recorded during the control period (4-12 min) against TFLs recorded following application of 5-HT (16-24 min) using the Mann-Whitney U-test. Levels of significance are indicated by * P < 0.05 and ** P < 0.01. B: example of the re- duction in response of a dorsal horn neurone to noxious thermal stimulation following the intrathecal application of 5-HT (260 nmol). Each point represents the total number of action potentials recorded during a single response. The arrow represents the time of intrathe- cal drug application. C: enhanced responsiveness of a single dorsal horn neurone to noxious thermal stimuli following intrathecal appli- cation of 260 nmol 5-HT. Each point represents the total number of action potentials recorded in response to a single noxious thermal stimulation. The arrow represents the time of intrathecal drug appli- cation. Following application of 5-HT, responses are briefly de- pressed (35-50 rain) and subsequently enhanced (50-120 min).

In four expe r imen t s 5 -HT was mic ro in jec ted in- t r a theca l ly at a dose of 130 nmol. In t h ree expe r imen t s nocicept ive responses were s ignif icant ly reduced . W h e n t es ted with the s t a n d a r d dose (260 nmol), th i r t een s tudies exhib i ted a s ignif icant r educ t ion in nocicept ive responses (see Fig. 1B). The admin i s t r a t ion of 5 -HT was of ten a c c o m p a n i e d by a t e m p o r a r y fall of b lood pressure . This hypotens ive effect was r e c o r d e d dur ing bo th increases and dec reases in nocicept ive responses .

In t h ree o the r exper iments , 5 -HT e n h a n c e d re- sponses to noxious s t imula t ion (see Fig. 1C). Some indica t ions of the complex na tu re of the r e sponse is i nd ica t ed by small r educ t ions in each response be fore the more d o m i n a n t inhibi t ion. Note that the dura t ion of the effect on nocicept ive responses is much g rea t e r than on TFLs .

Cont ro l app l i ca t ions of sal ine (n = 6) had no consis- ten t effect on nocicept ive responses .

3.2. Effects o f 5 -CT

3.2.1. Tail f l ick test Twenty-n ine rats rece ived one of t h r ee doses (2.5

nmol, n = 13; 25 nmol, n = 8; 250 nmol, n = 8) of the gene ra l 5 -HT, r e c e p t o r agonist . Tai l flick la tencies were not a f fec ted by 5-CT at any dose and no signs of m o t o r activity were noted .

3.2.2. Dorsal horn neurones Fol lowing micro in jec t ion of 5-CT, nocicept ive re-

sponses were r e d u c e d in all twelve exper iments . Fig. 2A shows a con t inuous excerp t of po lyg raph records d e m o n s t r a t i n g the effect of 0.3 nmol on nocicept ive responses r e c o r d e d f rom a dorsa l horn neurone . Fol- lowing the i.t. mic ro in jec t ion of 5-CT, the respons ive- ness of the n e u r o n e to noxious hea t was cons ide rab ly r e d u c e d and s p o n t a n e o u s activity abol i shed . Full re- covery of r e sponses was o b t a i n e d app rox ima te ly 30 min af ter the micro in jec t ion . A subsequen t micro in jec t ion of the gene ra l 5 -HT 1 an tagonis t s cyanop indo lo l did not effect the nocicept ive responses but abo l i shed the ac- t ions of a subsequen t app l i ca t ion of 5-CT. Pool ing of the raw da ta o b t a i n e d f rom all twelve s tudies r evea led tha t nocicept ive responses were s ignif icant ly r educed ( P < 0.05) fol lowing the micro in jec t ion of 0.3 nmol 5-CT (see Fig. 2B).

3.3. Effects o f TFMPP

3.3.1. Tail f l ick test The effect of the 5-HTlb agonis t T F M P P app l i ed

in t ra theca l ly was inves t iga ted at two doses: 300 nmol (n = 7) and 600 nmol (n = 9). A l t h o u g h the re a p p e a r e d to be a modes t inc rease in TFL , s ta t is t ical analysis of the da ta i nd i ca t ed that the effect was not s ignif icant .

86 Z. Ali et al, /Bra in Research 661 (1994) 83-90

A 140r . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . - .... 60L- ~ . . . . . . . . . . . . . . - : - : ' : . ' . ' . ~ . ' . . . . . . . . . . . . . ~ - - - . " '

• • A • • • • • •

. . . . 5'0 .......... ' 5-C"T(O.3nmol)

1 ( ) 0

CyanoP'indolol(10nmol) 5[CT(O.3nmol)

(, ...... ,.k ............. L. I ................... ..... ...... L L, ' ' 1 ~ o 40

Time(rain)

Fig. 2. A: reduction in nociceptive responses recorded from a dorsal horn neurone following the initial intrathecal injection of 5-CT (0,3 nmol). Neuronal firing rate and blood pressure are shown. Arrow heads indicate application of the ramped noxious thermal stimuli to the hind paw, rising from 30 to 48°C (15 s rise time, 5 s on time). The bars indicate period of drug application. Responses were not markedly affected by 10 nmol Cyanopindolol. A second application of 0.3 nmol 5-CT was not accompanied by a reduction in responses. B: the pooled responses of all 7 neurones to intrathecally applied 5-CT. Significance of the 5-CT effects was determined by comparing the mean response of each neurone during the control period (0 -7 min) with mean responses of each neurone during maximal drug effects (28-42 min bracketed responses) using the Wilcoxon test ( P < 0.05). The asterisk beneath the bar indicates responses which were significantly reduced.

3.3.2. Dorsal horn studies At a dose of 300 nM, TFMPP reduced nociceptive

responses in seven out of eight experiments. Fig. 3A shows the effect of the agonist on all seven neurones. Each ribbon represents one study. Following the i.t. microinjection of the 5-HTlb agonist, nociceptive re- sponses were reduced for approximately 30 min. Full recovery was obtained in all studies. The averaged effect is illustrated in Fig. 3B in which the pooled data are displayed. It is clear that there has been a signifi- cant (P < 0.05) reduction in nociceptive response.

B

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~.. 2000

v

10oo

ol, ,, i ,*i , , , , 0 14 28 42 56 70

TIME(min)

Fig. 2 (continued).

3.4. Effect o f 8-OH-DPA T

3.4.1. Tail flick test Since preliminary results suggested that the 5-HT~

receptor agonist 8-OH-DPAT reduced TFL, the heat stimulus intensity was reduced to lengthen control TFLs. Fig. 4A illustrates the reduction in TFLs follow, ing the intrathecal administration of 150 nmol of the agonist. This reduction was statistically significant (P < 0.05) and lasted for approximately 8 min. The graph shows the averaged responses from seven rats.

3. 4. 2. Dorsal horn studies Low doses (15 nmol) of 8-OH-DPAT were without

effect on nociceptive responses recorded from dorsal horn neurones. However, responses were enhanced in six of eight experiments in which 150 nmol was admin- istered and in two further studies 300 nmol also in- creased nociceptive responses, The three-dimensional plot (Fig. 4B) shows results obtained from all six stud- ies and the meaned effect is illustrated in Fig. 4C.

Table 1 summarises the effect of 5-HT ligands on nociceptive responses recorded from dorsal horn neu- rones.

Z. Ali et al. /Brain Research 661 (1994) 83-90 87

4. Discussion A

This is the first report demonstrating that drugs can be applied intrathecally whilst nociceptive responses are recorded from single dorsal horn neurones. This route of application has the advantage of delivering drugs relatively locally but not limiting exposure to the immediate environment of a single neurone. It may more accurately mimic the effects of endogenous trans- mitter release from a descending spinal pathway. This study has investigated further the actions of 5-HT on the processing of nociceptive information in the rat lumbar spinal cord. In line with earlier reports suggest- ing that 5-HT released synaptically [10] or applied exogenously [34,28,14] have antinociceptive actions, we also found that applied intrathecally 5-HT increases TFL in a dose-related fashion.

Using the same dose and route of administration as in the behavioural studies, we noted that in the major- ity of studies 5-HT reduced nociceptive responses recorded from neurones located throughout the dorsal

>,.. 0 Z w

....i v

<

A

B

1 0 0 0

Z 0

100 w

co 10 w

0

300nmol TFMPP

TIME(min)

w o9 300nmol TFMPP Z o

3000

2000

1000

~: o . . . . . . . . . . 0 14 28 42 56

O I.- TIME(min)

Fig. 3. A: reduction in nociceptive responses recorded from dorsal horn neurones following intrathecal injection of TFMPP (300 nmol). Each ribbon represents one of the 7 neurones in which responses were reduced following i.t. 300 nmol TFMPP. Each point on the ribbon represents the total number of action potentials produced in response to a single noxious thermal stimulus. Following application of TFMPP, responses were reduced in all seven neurones illustrated. The responses of one neurone were not affected by the application of TFMPP (not shown)• B: each time point represents the pooled responses from all eight neurones tested with 300 nmol i.t. TFMPP. The statistical significance of drug effects was determined by com- paring responses recorded during the control period (0-14 min) with responses recorded during maximal drug effects (28-49 min brack- eted responses were significant, P < 0.05)) using the Wilcoxon test.

150nmot 8-OH DPAT

1

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B

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0 8 16 24 32 4'0

TIME(min)

uJ 150nmol 8-OH DPAT "m co

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200 ~

0 8 TIME(min)

C w co

2000

w 1000

co w

0

150nmol 8-OH DPAT

0 14 28 42 56 70

TlME(min)

Fig. 4. A: reduction in TFL following the intrathecal application of 8 -OH-DPAT (150 nmol ) (n = 7). Each point represents mean TFLs(s) _+standard error of the mean. The statistical significance of drug effects was determined by comparing TFLs recording during the control period (0-8 min) against TFLs recorded following applica- tion 8 -OH-DPAT (12-20 rain) using the Student t-test. Significance is indicated by * P < 0.05. B: effects of intrathecally applied 8-OH- D P A T (150 nmol) on responses of lumbar dorsal horn neurones to noxious thermal stimulation of the hind paw. Each ribbon represents responses of one of the six neurones with enhanced responsiveness following application of 8-OH-DPAT. Each point on the ribbon represents the total number of action potentials produced in re- sponse to a single stimulation. The magni tude and duration of effect varied between the different neurones investigated. Responses of one neurone were not affected by the agonist. C: pooled responses of all seven neurones included in the statistical analysis. Significance of 8 -OH-DPAT effects was determined by comparing the mean control response for each neurone (0-14 min) with mean responses for each neurone during maximal drug effects (21-35 min bracketed re- sponses) using the Wilcoxon test (P < 0.05).

horn. Interestingly response enhancement in a limited number of studies indicates that 5-HT can both inhibit and facilitate the transmission of noxious information

8~ Z. All el al. /Brain Research 661 (1994) 83-90

Table l Action of 5-HT ligands on nociceptive responses recorded from dorsal horn nenrones

Drug Dose n Effect on nociceptive response

(nmol) Increase Decrease No effect

5-HT 130 4 - 3 I 260 16 3 13 0

5-CT 0.3 7 - 7 - 3.0 3 - 3 -

30.0 2 - 2

TFMPP 300 8 - 7 I

8-OH-DPAT 15.0 2 - - 2 150 8 6 1 1 300 2 2 - -

in the dorsal horn. The possibility that descending tryptaminergic systems may have facilitatory effects on sensory processing has also been proposed by Zhuo and Gebhar t [36] who reported such effects following electrical stimulation of nucleus reticularis gigantocel- lularis (nrgc) and nrgc pars alpha.

In the present experiments the general 5-HT~ ago- nist 5-CT and the non-specific 5-HT~D agonist TFMPP both consistently reduced nociceptive neuronal re- sponses. The 5-CT effect was blocked following the microinjection of cyanopindolol. Although used as a 5-HT~-like antagonist, cyanpoindolol has clear /~- adrenoceptor antagonist propert ies [16]. However such receptors are not thought to play a significant part in spinal nociceptive processing. Thus the 5-CT-mediated block of nocicpetive responses was probably associated with an action at 5-HTlb receptors. Although TFMPP has affinity for both 5-HTta and 5-HT~b receptors [32], the present results showing that the agonist reduces noeiceptive responses supports the idea that it is the action at 5-HT~u receptors which is related to this effect. These results are consistent with earlier electro- physiological studies [34,10]. That activation of 5-HT1b receptors can lead to a reduction in responsiveness to noxious stimuli is further supported by our finding that 8 -OH-DPAT has the functionally opposite effect thus providing some explanation for the mixed effects of 5-HT. Nevertheless we cannot exclude the possibility that TFMPP was also having some action at 5-HTIc, 5-HT 2 and 5-HT 3 receptor sub types.

TFMPP produced no clear reduction in TFL which, based on our single-neurone studies, might be expected to accompany the administration of 5-HTlb agonists. Previous reports indicate that the 5-HT~b agonist R U 24%9 reduces nocifensive reflexes [35,29,6,8,1]. How- ever, Crisp at al [6] noted that TFMPP was without effect in the tail flick test whereas the reflex associated with the hot plate test was suppressed. It remains unclear however why TFMPP was inactive in the tail

flick test reported in this paper. It may be that the tail flick test is not sufficiently sensitive to reflect the more subtle changes in sensory processing associated with TFMPP.

Activation of 5-HTIb receptors in the spinal cord is associated with a decrease in 5-HT release [4]. In common with other regions of the CNS spinal 5-HT~h receptors can function as presynaptic autoreceptors [29,30]. However, since lesions of descending trypta- minergic pathways do not reduce the 5-HT)b receptor population [4], it seems that most receptors are postsy- naptically located. Alternatively, these receptors may also be located on the terminals of non-tryptaminergic terminals and modulate the release of other transmit- ters. There is evidence that 5-HTib ligands reduce the release of acetylcholine in the hippocampus [15]. How- ever, if such an action occurs in the spinal cord the resulting reduced release of acelycholine would be predicted to increase neuronal responsiveness to nox- ious stimulation rather than the accepted inhibitory effect. Therefore, the current view appears to be that 5-HTib ligands act primarily post synaptically to reduce the tranmission of nociceptive information.

The consistent reduction in TFL following the ad- ministration of 8 -OH-DPAT is consistent with some earlier studies [35,6,1]. However, other reports indicate a reduction [9,15] or no effect [19] in behavioural tests. Despite some evidence that 8-OH D P A T decreases TFL secondarily in increases in tail skin temperature (skin blood flow), unpublished results from his labora- tory indicate that TFL decreases can be dissociated from an increase in tail skin temperature. Further- more, in the electrophysiological studies in which the agonist induced a functionally consistent effect, skin temperature was maintained at present levels by the feedback circuits of the Peltier thermode. Under these circumstances 8 -OH-DPAT could not have influenced paw skin temperature under the thermode.

Activation of 5-HT~a receptors has been reported to either non-specifically reduce reponses to dorsal horn neurones to a range of sensory stimuli [10] or to be without effect on glutamate evoked responses recorded from wide dynamic range neurones [33]. The enhanced neuronal responses reported here are consistent with the reduction in nocifensive reflex latency and suggests that activation of 5-HTIa agonists enhances the trans- mission of nociceptive information in the dorsal horn. Activation of 5-HT~a receptors results in membrane hyperpolarisation. In dorsal horn neurones [20,30] and thalamus [25] intracellular recordings, voltage current relationships demonstrate inward rectification in the hyperpolarised direction. Moreover the activation propert ies of this current may be shifted by 5-HT so that the increased inward current may be so great as to raise the neurone to its firing threshold [31]. The pharmacology of this effect in the thalamus has been

Z. Ali et al. /Brain Research 661 (1994) 83-90 89

associated with a 5-HT 1 like effect [25]. This provides a possible mechanism for the 5-HT~-mediated increase in responsiveness to afferent input.

The predominant action of spinally released 5-HT is to suppress or reduce responses of dorsal horn neu- rones to noxious thermal stimulation. However, the enhancement of transmission associated with 5-HT~ activity may encourage the rostral projection to supra- spinal centres concerned with endogenous antinocicep- tive control. In this way an "open channel" system may operate which continually provides information con- cerning peripheral stimulation to form part of a feed- back loop. There may also be increases in dorsal horn receptive field size and a facilitation of spinal nocifen- sive reflexes observed in this study.

Acknowledgements

We are grateful to the Royal Society and Wellcome Trust for financial support and to Phil Blanning for expert technical assistance.

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