The role of P2X₇ in pain and inflammation

The P2X₇ purinoceptor is unique amongst the P2X receptor family in that its activation is able to stimulate the release of mature, biologically active interleukin-1β (IL-1β), as well as a variety of other proinflammatory cytokines. Coupled with the predominate localisation of this receptor to immunocytes of haemopoetic origin, this receptor is an obvious candidate to play a major and pivotal role in processes of pain and inflammation. Using genetically modified animals that lack the P2X₇ receptor, several investigators have shown that these mice do indeed demonstrate a blunted inflammatory response, and fail to develop pain following both inflammatory and neuropathic insult. These animals also show altered cytokine production in response to inflammatory stimulus, which is far broader than merely modulation of IL-1β release. In this short article, we review the role of the P2X₇ receptor in modulating the release of cytokines and other mediators, and discuss the findings made from P2X₇ receptor-deficient animals. As well as highlighting outstanding questions regarding this intriguing receptor, we also speculate as to the potential therapeutic benefit of P2X₇ receptor modulation.

Transgenic models of pain: a brief review

Early validation of novel targets for the treatment of pain remains a key challenge, despite the recent technological advances which aid more rapid generation of pharmacological tools and molecular techniques that allow in vivo manipulation of gene transcription. Use of genetically modified animals, particularly 'knockouts', has been critical in evaluating the function of targets for which pharmacological manipulation has proved difficult. This review discusses recent findings from such investigations for three of the larger target classes, and highlights the relative merits and drawbacks of this methodology compared with other methods for the functional investigation of the role of individual molecular targets in pain.

Disruption of the P2X7 purinoceptor gene abolishes chronic inflammatory and neuropathic pain

The P2X(7) purinoceptor is a ligand-gated cation channel, expressed predominantly by cells of immune origin, with a unique phenotype which includes release of biologically active inflammatory cytokine, interleukin (IL)-1beta following activation, and unique ion channel biophysics observed only in this receptor family. Here we demonstrate that in mice lacking this receptor, inflammatory (in an adjuvant-induced model) and neuropathic (in a partial nerve ligation model) hypersensitivity is completely absent to both mechanical and thermal stimuli, whilst normal nociceptive processing is preserved. The knockout animals were unimpaired in their ability to produce mRNA for pro-IL-1beta, and cytometric analysis of paw and systemic cytokines from knockout and wild-type animals following adjuvant insult suggests a selective effect of the gene deletion on release of IL-1beta and IL-10, with systemic reductions in adjuvant-induced increases in IL-6 and MCP-1. In addition, we show that this receptor is upregulated in human dorsal root ganglia and injured nerves obtained from chronic neuropathic pain patients. We hypothesise that the P2X(7) receptor, via regulation of mature IL-1beta production, plays a common upstream transductional role in the development of pain of neuropathic and inflammatory origin. Drugs which block this target may have the potential to deliver broad-spectrum analgesia.

The cyclooxygenase-2 inhibitor GW406381X [2-(4-ethoxyphenyl)-3-[4-(methylsulfonyl)phenyl]-pyrazolo[1,5-b]pyridazine] is effective in animal models of neuropathic pain and central sensitization

The pathogenic form of the cyclooxygenase (COX) enzyme, COX-2, is also constitutively present in the spinal cord and has been implicated in chronic pain states in rat and man. A number of COX-2 inhibitors, including celecoxib and rofecoxib, are already used in man for the treatment of inflammatory pain. Preclinically, the dual-acting COX-2 inhibitor, GW406381X [2-(4-ethoxyphenyl)-3-[4-(methylsulfonyl)phenyl]-pyrazolo[1,5-b]pyridazine, where X denotes the free base], is as effective as rofecoxib and celecoxib in the rat established Freund's Complete Adjuvant model with an ED(50) of 1.5 mg/kg p.o. compared with 1.0 mg/kg p.o. for rofecoxib and 6.6 mg/kg p.o. for celecoxib. However, in contrast to celecoxib (5 mg/kg p.o. b.i.d.) and rofecoxib (5 mg/kg p.o. b.i.d.), which were without significant effect, GW406381X (5 mg/kg p.o. b.i.d.) fully reversed mechanical allodynia in the chronic constriction injury model and reversed thermal hyperalgesia in the mouse partial ligation model, both models of neuropathic pain. GW406381X, was also effective in a rat model of capsaicin-induced central sensitization, when given intrathecally (ED(50) = 0.07 mug) and after chronic but not acute oral dosing. Celecoxib and rofecoxib had no effect in this model. Several hypotheses have been proposed to try to explain these differences in efficacy, including central nervous system penetration, enzyme kinetics, and potency. The novel finding of effectiveness of GW406381X in these models of neuropathic pain/central sensitization, in addition to activity in inflammatory pain models and together with its central efficacy, suggests dual activity of GW406381X compared with celecoxib and rofecoxib, which may translate into greater efficacy in a broader spectrum of pain states in the clinic.

The behavioural effect of middle cerebral artery occlusion on apolipoprotein-E deficient mice

The behavioural effects of middle cerebral artery occlusion (MCAO) in apolipoprotein-E deficient (Apo-E KO) mice were investigated using a modified SHIRPA protocol and compared with effects in wild type littermate controls. The MCA was permanently occluded by insertion of an intraluminal filament to its origin on the Circle of Willis and behavioural responses were observed 24 h later. MCAO treatment caused a range of changes in the wild type mice whereas, few differences were observed in the Apo-E KO mice in the behavioural observation. In the rotarod task, MCAO operated wild type mice showed a significant reduction in performance compared with sham-operated and non-operated animals. In contrast, both sham and MCAO operated Apo-E KO mice showed significant impairment compared with non-operated controls. A significant reduction in performance was also observed in sham-operated Apo-E KO compared with sham-operated wild type mice. In locomotor activity tests, no significant reduction in activity was observed between non-operated and sham-operated wild type controls, whereas a significant reduction was found between sham operated and MCAO operated mice. In the Apo-E KO mice, both sham and MCAO-operated animals showed a reduction in locomotor activity compared with non-operated mice. Furthermore, Apo-E KO MCAO mice showed a worsened deficit in locomotor activity, which was significantly correlated with exacerbated cortical lesion volume, unlike wild-type MCAO mice. This study shows that Apo-E KO animals demonstrate an impaired functional recovery post surgery which may be further compounded by post experimental stroke and also demonstrates the utility of the SHIRPA test system for investigating behavioural changes in functional outcome post stroke.

Effects of centrally administered orexin-B and orexin-A: a role for orexin-1 receptors in orexin-B-induced hyperactivity

RATIONALE

Orexin-A and orexin-B are hypothalamic neuropeptides derived from a 130-amino acid precursor, prepro-orexin, and are potent agonists at both the orexin-1 (OX1) and orexin-2 (OX2) receptors. Orexin-A has been ascribed a number of in vivo functions in the rat after intracerebroventricular (ICV) administration, including hyperphagia, neuroendocrine modulation and a role in the regulation of sleep-wake function. The in vivo role of orexin-B is not as clear.

OBJECTIVES

To investigate the behavioural, endocrine and neurochemical effects of orexin-B in in-vivo tests. In a number of experiments, these effects were compared with those of orexin-A.

METHODS

Experiments were carried out in male, Sprague-Dawley rats with a guide cannula directed towards the lateral ventricle. The effects of orexin-B (ICV) upon grooming behaviour were compared with those of orexin-A. The effects of orexin-B upon the motor activity response to both novel and familiar environments were assessed in an automated activity monitor. Orexin-B was tested upon startle reactivity and body temperature. Further, plasma hormones and [DOPAC+ HVA]/[DA] and [5-HIAA]/[5-HT] ratios in six brain areas were measured 40 min post-orexin-B or orexin-A.

RESULTS

The clearest behavioural response to orexin-B was increased motor activity in both novel and familiar environments. Orexin-B-induced hyperactivity was blocked by an OX1 receptor antagonist, SB-334867-A, implicating OX1 receptors in this behavioural response. In common with orexin-A, orexin-B reduced plasma prolactin and failed to influence startle reactivity. However, in contrast with orexin-A, orexin-B increased head grooming but failed to cause a robust whole body grooming response or increase plasma corticosterone levels. Further, orexin-B, but not orexin-A, increased plasma TSH and increased hypothalamic and striatal [5-HIAA]/[5-HT] ratios.

CONCLUSIONS

The present study has demonstrated a number of behavioural, neuroendocrine and neurochemical effects of orexin-B that distinguish it from orexin-A. Further, we have demonstrated a role for OX1 receptors in the actions of orexin-B upon motor activity.

Vanilloid receptor-1 is essential for inflammatory thermal hyperalgesia

The vanilloid receptor-1 (VR1) is a ligand-gated, non-selective cation channel expressed predominantly by sensory neurons. VR1 responds to noxious stimuli including capsaicin, the pungent component of chilli peppers, heat and extracellular acidification, and it is able to integrate simultaneous exposure to these stimuli. These findings and research linking capsaicin with nociceptive behaviours (that is, responses to painful stimuli in animals have led to VR1 being considered as important for pain sensation. Here we have disrupted the mouse VR1 gene using standard gene targeting techniques. Small diameter dorsal root ganglion neurons isolated from VR1-null mice lacked many of the capsaicin-, acid- and heat-gated responses that have been previously well characterized in small diameter dorsal root ganglion neurons from various species. Furthermore, although the VR1-null mice appeared normal in a wide range of behavioural tests, including responses to acute noxious thermal stimuli, their ability to develop carrageenan-induced thermal hyperalgesia was completely absent. We conclude that VR1 is required for inflammatory sensitization to noxious thermal stimuli but also that alternative mechanisms are sufficient for normal sensation of noxious heat.

Orexin A activates locus coeruleus cell firing and increases arousal in the rat

The localization of orexin neuropeptides in the lateral hypothalamus has focused interest on their role in ingestion. The orexigenic neurones in the lateral hypothalamus, however, project widely in the brain, and thus the physiological role of orexins is likely to be complex. Here we describe an investigation of the action of orexin A in modulating the arousal state of rats by using a combination of tissue localization and electrophysiological and behavioral techniques. We show that the brain region receiving the densest innervation from orexinergic nerves is the locus coeruleus, a key modulator of attentional state, where application of orexin A increases cell firing of intrinsic noradrenergic neurones. Orexin A increases arousal and locomotor activity and modulates neuroendocrine function. The data suggest that orexin A plays an important role in orchestrating the sleep-wake cycle.

The effects of dopamine D3/D2 receptor agonists on intracranial self stimulation in the rat

The effects of the dopamine D3/D2 receptor agonists quinpirole, quinelorane and (+/-)7-OH-DPAT [(+/-) 7-hydroxy-2(N,N-di-n-propylamino) tetralin] on intracranial self-stimulation (ICSS) were investigated. Rats implanted with bipolar electrodes into the lateral hypothalamus were trained to lever press on a continuous reinforcement schedule for positively reinforcing trains of electrical stimulation. Three measures of responding were calculated; the frequency at which responding was 50% of the maximum (M50), the asymptotic response rate and the total area under the curve (AUC) for each frequency sweep. Quinpirole (2.2-66.0 microg/kg SC) significantly increased M50 and reduced both asymptote and AUC. Quinelorane (0.25-79.0 microg/kg SC) had no significant effect on M50 values but significantly reduced both asymptote and AUC. (+/-)7-OH-DPAT (2.5-74.0 microg/kg) did not significantly affect any of the measures. The data show that low doses of quinpirole and quinelorane, but not (+/-)7-OH-DPAT, inhibit ICSS maintained by electrodes in the lateral hypothalamus. Either dopamine D2 or dopamine D3 receptor stimulation may play a role in mediating ICSS inhibition, but studies with more selective receptor agonists and antagonists are required to define the role of each receptor.

5-HT4 receptor antagonism does not affect motor and reward mechanisms in the rat

5-HT4 receptors are concentrated in areas of the brain which are rich in dopamine neuronal markers, which may suggest that they influence motor and reward processes. We tested this hypothesis by examining the effects of a 5-HT4 receptor antagonist, 8-amino-7-chloro-(N-butyl-4-piperidyl)methylbenzo-1,4-dioxan-5-car boxylate hydrochloride (SB-204070-A) on amphetamine- and nicotine-induced locomotor stimulation in intact rats. In rats with unilateral 6-hydroxydopamine-induced lesions of the ascending nigrostriatal dopaminergic projection, SB-204070-A was tested for its effects on amphetamine-induced rotation. SB-204070-A was also tested for its effects on rewarded behaviour maintained by intracranial self-stimulation. SB-204070-A did not alter behaviour under any of these conditions, suggesting a lack of involvement of the 5-HT4 receptor in motor and reward processes.

Sabcomeline (SB-202026), a functionally selective M1 receptor partial agonist, reverses delay-induced deficits in the T-maze

Sabcomeline, (SB-202026 [R-(Z)-alpha-(methoxyimino)-1 -azabicyclo [2.2.2] octane-3-acetonitrile]), a functionally selective muscarinic M1 receptor partial agonist, was tested in rats trained to perform a delayed, reinforced alternation task in a T maze, a test of short-term spatial memory. For comparison the cholinesterase inhibitor tacrine (THA-9-amino- 1,2,3,4-tetrahydroaminoacridine) and the non-selective muscarinic receptor agonist RS86 (2-ethyl-8-methyl-2,8 diazospiro [4.5]-decane-1,3-dione hydrobromide) were also tested and all three compounds were also compared using a conditioned taste aversion (CTA) task. Sabcomeline (0.001-1.0 mg/kg i.p.) significantly reversed the T-maze choice accuracy deficit induced by a 20-s delay at 0.03 and 0.1 mg/kg. RS86 (0.1-3.0 mg/kg i.p.) reversed the deficit at 1.0 mg/kg and THA (0.1-3.0 mg/kg i.p.) had no effect at any dose. All three compounds induced conditioned taste aversion with minimum effective doses (MED) of 0.3, 1.0 and 3.0 mg/kg, respectively. The results show that sabcomeline reverses delay induced deficits in T-maze choice accuracy in a rewarded alternation task at doses approximately 10 times lower than those required to induce conditioned taste aversion. RS86 was equipotent in both tests. These data support the findings of clinical studies which have shown that SB-202026 provides significant symptomatic improvement in patients with probable Alzheimer's disease at doses which do not induce cholinergic side effects.

Chronic mild stress-induced reductions in saccharin intake depend upon feeding status

The consumption of a weak saccharin solution was examined in rats subjected to chronic mild stress (CMS). Intake of saccharin was reduced in stressed animals compared to isolated and group control animals but saccharin preference was not affected. Removal of water deprivation from the CMS schedule did not alter the effects of CMS upon saccharin intake. However, when food deprivation was omitted entirely from the CMS schedule the reduction in saccharin intake was eliminated. Similarly, in animals habituated to the full CMS procedure, reduction in saccharin intake was abolished by omitting food deprivation or by delaying the intake test for 24 h. Both CMS and food deprivation reduced water intake but had no effect on food consumed during the fluid-intake test. The presence of food deprivation during CMS appears to be a key factor affecting saccharin intake. Our data suggest that saccharin intake is not an appropriate measure of stress and anhedonia.

SB 202026: a novel muscarinic partial agonist with functional selectivity for M1 receptors

The finding that ascending cholinergic systems are severely degenerated in Alzheimer's disease has driven the search for a cholinomimetic therapy. Adverse effects observed with cholinesterase inhibitors and high-efficacy muscarinic agonists led us to design compounds with an improved profile. SB 202026 (R-(Z)-(+)-alpha-(methoxyimino)-1-azabicyclo[2.2.2] octane-3-acetonitrile) displaced [3H]-oxotremorine-M from muscarinic receptors in the rat brain with high affinity (IC50 = 14 nM), a potency similar to that of oxotremorine-M itself (IC50 = 13 nM), but exhibited low affinity for cholinergic nicotinic receptors and other neuroreceptors. In studies using cloned human muscarinic receptors, SB 202026 possessed approximately equal affinity in displacing [3H]-quinuclidinyl benzilate from all muscarinic receptor subtypes. In functional models in vitro, SB 202026 caused maximal depolarization of the rat superior cervical ganglion at low concentrations (300 nM) (M1-mediated effect), while producing a lower maximal effect than the high-efficacy agonists oxotremorine-M and carbachol on M2-mediated release of ACh and M3-mediated smooth muscle contraction (guinea pig ileum), respectively. The functional selectivity and partial agonist profile seen in vitro were reflected in vivo through potent cognition-related activity (M1-induced increase in hippocampal EEG power) combined with low efficacy, compared with arecoline or oxotremorine, on induction of bradycardia (M2-mediated response), hypotension (via M3-mediated vasorelaxation) and tremor (thought to be mediated by M3 receptors). The foregoing profile of SB 202026 predicted that cognition-enhancing activity would be achieved at doses below those that initiate undesirable side effects, and this has subsequently been demonstrated in rodents, marmosets and humans.

Stimulation of 5-HT1B receptors causes hypothermia in the guinea pig

The selective, brain penetrant, 5-HT(1B/D) (formerly 5-HT(1D beta/alpha)) receptor agonist SKF-99101H (3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate) (30 mg/kg i.p.) causes a dose related fall in rectal temperature in guinea pigs which previous studies have shown to be blocked by the non-selective 5-HT(1B/D) receptor antagonist GR-127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1'biphenyl]-4-carboxamide oxalate). The present study shows that the hypothermic response to SKF-99101H is dose-dependently blocked by SB-224289G (1'-methyl-5-(2'-methyl-4'-[(5-methyl-1,2,4-oxadiazol-3-yl)bipheny l-4-yl]carbonyl)-2,3,6,7-tetrahydrospiro[furo[2,3-f]indole-3,4'-pi peridone] hemioxalate) (0.3-10.0 mg/kg p.o.) (ED50 3.62 mg/kg), which is the first compound to be described which is more than 60 fold selective for the 5-HT1B receptor over the 5-HT1D receptor. SB-216641A (N-[3-(2-dimethylamino) ethoxy-4-methoxy-phenyl] 2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-car boxamide hydrochloride) (0.6-20.0 mg/kg i.p.), which is somewhat less selective (30 fold) for the 5-HT1B receptor over the 5-HT1D receptor had a similar effect (ED50 4.43 mg/kg). The brain penetrant 5-HT1D selective receptor antagonist, BRL-15572 (4-(3-chlorophenyl)-alpha-(diphenylmethyl)-1-piperazineethanol+ ++ dihydrochloride) (0.3-100.0 mg/kg i.p.) was inactive. When administered alone neither BRL-15572 (0.1-10 mg/kg i.p.) nor SB-224289G (2.2-22 mg/kg p.o.) had an effect on body temperature. These data demonstrate that 5-HT1B (formerly 5-HT(1D beta)) and not 5-HT1D (formerly 5-HT(1D alpha)) receptors mediate the hypothermic response to SKF-99101H (30 mg/kg i.p.) in guinea pigs. The compounds described are useful pharmacological tools for distinguishing responses to 5-HT1B and 5-HT1D receptors.

A computer-controlled apparatus for measuring and analysing myoclonic jerk activity in guinea pigs

Guinea pigs respond to 5-hydroxytryptophan (5-HTP) with a complex behavioural syndrome which includes distinctive whole-body jerks (myoclonic jerks). These are species-specific, involve all major muscle groups, are highly rhythmic and can occur with relatively high frequency (1-2/s). Current methods of quantifying the response, which rely on observer ratings are unsatisfactory. We report the development of a fully automated, PC-based system for detecting and analysing myoclonic jerks. Floor-mounted accelerometers generate a movement signal and detection is performed by an algorithm based on a detailed analysis of the topography of myoclonic jerk responses. The system can monitor the output from 10 purpose-built test enclosures and it provides for pseudorandom treatment allocation, flexible control of experimental parameters and automated data output to a spreadsheet which generates appropriate graphs, statistical summaries and data analysis. This robust response provides a useful means of exploring the role of novel 5-HT receptor subtypes in guinea-pig behaviour.

The role of 5-HT1D and 5-HT1A receptors in mediating 5-hydroxytryptophan induced myoclonic jerks in guinea pigs

Systemic administration of 5-hydroxytryptophan (5-HTP) to guinea pigs causes species-specific, rhythmic, whole body jerks (myoclonic jerks), the frequency and amplitude of which were measured in an automated apparatus. The brain penetrant 5-HT1D receptor agonist 3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate (SKF 99101H) (3-30 mg/kg i.p.) and the selective 5-HT1A receptor agonist (+/-)8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) (0.3-3 mg/kg s.c.) dose dependently potentiated the frequency and intensity of myoclonic jerks caused by 5-HTP (100 mg/kg). Cotreatment of guinea pigs with 8-OH-DPAT (3 mg/kg s.c.) and SKF 99101H (30 mg/kg i.p.), which were inactive when given alone, gave a marked myoclonic jerk response. Conversely, the myoclonic jerk response to higher doses of 5-HTP (150 mg/kg i.p.) was dose dependently blocked by the 5-HT1D receptor antagonist GR 127935 (N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1 ,2,4-oxadiazol-3-yl)[1,1'-biphenyl]4-carboxamide oxalate) (ED50 0.32 mg/kg i.p.) and the selective 5-HT1A receptor antagonist WAY 100635 (N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride) (ED50 0.33 mg/kg i.p.). The response to 5-HTP (150 mg/kg i.p.) was also blocked by ritanserin (0.01-0.3 mg/kg i.p.). Our data therefore confirm previous reports concerning the effects of 5-HT2A/2C receptor blockade on 5-HTP induced myoclonic jerks and suggest that both 5-HT1D and 5-HT1A receptors play an important role in mediating this behavioural response.

The 5 -HT3 receptor antagonists, granisetron and ondansetron, do not affect cocaine-induced shifts in intra-cranial self-stimulation thresholds

The effects of the 5-HT( 3) receptor antagonists, granisetron and ondansetron, were investigated on behaviour maintained by intracranial self-stimulation (ICSS). Rats, implanted with bipolar electrodes in the lateral hypothalamus, were trained to lever press on a continuous reinforcement schedule for positively reinforcing trains of electrical stimulation. The frequency at which responding reached 50% of maximum (M50) and the maximum rate of responding (asymptote) were used to measure drug effects. Granisetron (0.01-0.1 mg/kg i.p ) and ondansetron (0.03-0.3 mg/kg i.p ) had no effect on either parameter. In contrast, cocaine (20 mg/kg i.p ) potentiated rewarded responding, reducing M50 values, but neither granisetron (0.01-3.0 mg/kg i.p ) nor ondansetron (0.03-0.3 mg/kg i.p ) blocked this effect. Neither did granisetron (0.1-10.0 mg/kg i.p ) alter the effect of lower doses of cocaine (10 mg/kg i.p.). These data suggest that 5 -HT( 3) receptors do not play a significant role in mediating responding maintained by ICSS in the rat through hypothalamic electrodes. Neither do they modulate cocaine-induced potentiation of the behaviour.

5-HT 1D receptors mediate SKF 99101H-induced hypothermia in the guinea pig

The selective, brain penetrant, 5-HT(1D) receptor agonist SKF 99101H (10-30 mg/kg i.p.) caused a dose-related fall in rectal temperature in guinea pigs which lasted longer than 2 h. Sumatriptan (1.0-100 mg/kg i.p.), a selective 5-HT(1D) agonist which does not penetrate the brain, did not produce hypothermia, suggesting that peripheral mechanisms are not critically involved in the response. The hypothermia induced by SKF 99101H (30 mg/kg i.p.) was dose-dependently blocked by the 5-HT(1D) receptor antagonists GR 127935 (0.01-1 mg/kg i.p.) and GR 125743 (0.01-3 mg/kg i.p.), confirming the role of 5-HT(1D) receptors. Mianserin (0.3-10.0 mg/kg i.p.) and granisetron (0.1-3.0 mg/kg i.p.) were inactive, suggesting that 5-HT(2A/2B/2C) or 5-HT( 3) receptors play no significant role in the generation of the hypothermic response. Nor was the hypothermia reversed by prazosin (0.03-1.0 mg/kg i.p.), idazoxan (0.03-1.0 mg/kg i.p.) or scopolamine (0.01-0.3 mg/kg i.p.), thereby excluding mediation by α(1)- and α(2)-adrenoceptors and muscarinic receptors. WAY 100635 (0.1-1.0 mg/kg) significantly potentiated the effect of SKF 99101H. The antagonists, when given alone, had no effect on body temperature, with the exception of prazosin (0.1 and 1.0 mg/kg). Three days of treatment with parachloroamphetamine (30 mg/kg i.p.) depleted forebrain 5-HT by ∼ 75% in frontal cortex, hypothalamus, hippocampus and striatum, but failed to alter the hypothermic response to SKF 99101H. The hypothermia is, therefore, unlikely to be mediated by 5-HT(1D) receptors located on 5-HT neurons. SKF 99101H-induced hypothermia in the guinea pig may serve as a useful model for investigation of centrally acting 5-HT( 1D) receptor antagonists.