Estrogen rapidly modulates 5-hydroxytrytophan-induced visceral hypersensitivity via GPR30 in rats.

BACKGROUND & AIMS: Sex hormones have been reported to modulate visceral hypersensitivity (VH). Estrogen regulates neurons not only by binding to estrogen receptors (ERalpha and ERbeta) to initiate transcription but also via the G-protein coupled receptor GPR30, which binds and rapidly mediates actions of estrogen. We examined the role of sex hormones in a VH model without colonic inflammation. METHODS: 5-Hydroxytryptophan (5HTP) was injected subcutaneously into awake female rats to induce VH; the 5HT3 antagonist (granisetron) or saline (control) were injected 30 minutes later. Immunohistochemistry was used to quantify calcitonin gene-related peptide-immunoreactive (CGRP-IR) neurons in the dorsal root ganglion (DRG). 5HTP-induced VH was evaluated in ovariectomized rats injected with 17beta-estradiol, progesterone, or both. ER alpha/beta agonist, GPR30 agonist, ER antagonist (ICI-182,780) or GPR30 antisense oligodeoxynucleotide were given to 5HTP-primed, estrogen-treated ovariectomized rats. RESULTS: Rats given 5HTP had increased VH that was inhibited by granisetron, accompanied by a decrease in CGRP-IR in the DRG. Ovariectomy eliminated 5HTP-induced VH, whereas estrogen and the combination of estrogen and progesterone, but not progesterone alone, restored the VH. The GPR30 agonist, but not the ERbeta agonist, rapidly restored VH. VH was preserved by coadministration of ICI-182,780 and estrogen but was absent after administration of the GPR30 antisense oligodeoxynucleotide. GPR30 colocalized with 5HT3 in DRG neurons; no significant inflammation occurred in colonic mucosa. CONCLUSIONS: In the absence of mucosal inflammation, estrogen can rapidly modulate 5HTP-induced VH. Loss of gonad hormones suppresses VH, whereas estrogen replacement restores it. Estrogen-mediated VH appears to act through GPR30.

5-Hydroxytryptophan: A precursor of serotonin influences regional blood-brain barrier breakdown, cerebral blood flow, brain edema formation, and neuropathology.

5-Hydroxytryptophan (5-HTP), a precursor of serotonin, is therapeutically used for several psychiatric disorders such as anxiety and depression in the clinic. However, severe side effects, including abnormal mental functions, behavioral disturbances and intolerance are associated with this treatment. 5-HTP-induced elevation of plasma and brain serotonin levels may affect blood-brain barrier (BBB) breakdown, edema formation and regional cerebral blood flow (CBF) disturbances. Breakdown of BBB to serum proteins leads to vasogenic brain edema formation and cellular injuries. However, 5-HTP-neurotoxicity is still not well known. In this investigations 5-HTP induced elevation of endogenous plasma and brain serotonin levels and its effect on BBB breakdown, edema formation neuronal injuries was examined in a rat model. Furthermore, potential role of oxidative stress and nitric oxide (NO) was evaluated. In addition, several neurochemical agents such as p-CPA (5-HT synthesis inhibitor) indomethacin (prostaglandin synthase inhibitor), diazepam (ant stress drug), cyproheptadine, ketanserin (5-HT2 receptor antagonists) and vinblastine (inhibitor of microtubule function) were examined on 5-HT neurotoxicity. Our observations suggest that 4h after 5-HTP administrations, the endogenous serotonin levels increased by fourfold (150mg/kg) in the plasma and brain associated with profound hyperthermia (+3.86±0.24°C, oxidative stress and NO upregulation. Breakdown of the BBB to Evans blue albumin (EBA) in 8 brain regions and to [131]Iodine in 14 brain regions was observed. The CBF exhibited marked reduction in all the brain regions examined. Brain edema and cellular injuries are present in the areas associated with BBB disruption. Drug treatments reduced the BBB breakdown, edema formation NO production and brain pathology. These observations are the first to point out that 5-HTP-neurotoxicity caused by BBB breakdown, edema formation and NO production is instrumental in causing adverse mental and behavioral abnormalities, not reported earlier.

Failure to monoaminergic and cholinergic receptor blockers to prevent prostaglandin E2-induced luteinizing hormone release.

Receptor blocking drugs were used to determine whether adrenergic, dopaminergic, serotoninergic, or cholinergic synapses are involved in mediating the LH release induced by intraventricularly injected PGE2. Prostaglandin E2 (5mug) was injected into the 3rd ventricle (3rd V) of ovariectomized rats, and plasma LH concentrations before and after treatment were determined by radioimmunoassay. Phentolamine, 20 or 30 mug, or pronethalol, 20 mug (alpha and beta adrenergic receptor blockers, respectively) injected into the 3rd V failed to alter the elevation of plasma LH evoked by PGE2 injected into the ventricle 10 min later. Likewise, LH release following PGE2 was not changed when a dopaminergic blocker, pimozide (0.63 mg/kg, SC), was injected 2 h prior to PGE2. Two antagonists of serotonin, methysergide maleate (3 mg/kg ip) or cinanserin HC1 (1 mg/kg iv) given 2 h or 45 min before PGE2, respectively, failed to alter the action of PGE2. Atropine (100 or 250 mug) injected into the 3rd V 10 min prior to PGE2 was also ineffective in blocking the increase in plasma LH following PGE2. The results of this study indicate that the effect of PGE2 on LH release is not mediated by adrenergic, dopaminergic, serotoninergic, or cholinergic receptors. They also suggest that PGE2 is not acting trans-synaptically but probably directly on the LHRH neuron to induce the discharge of LHRH into the hypophysial portal vessels which then evokes release of LH from the adenohypophysis.

Peripheral serotonin2 receptor blockade does not inhibit 5-hydroxytryptophan-induced aldosterone stimulation.

To assess the effects of serotonin receptor blockade on 5-hydroxytryptophan (5HTP)-induced aldosterone secretion, we studied six normal men using the serotonin antagonists ketanserin and methysergide. The subjects were studied on three separate occasions, and pretreatment with dexamethasone was given before each study. On two occasions, the pretreatment period also included administration of a serotonin antagonist, either ketanserin (120 mg/day) or methysergide (6 mg/day). On the day of study, the subjects were given a single oral 200-mg dose of 5HTP. Plasma levels of aldosterone increased significantly after 5HTP treatment compared to basal levels during each stage of the study. No significant difference in response in the three studies was found. We conclude that peripheral blockade of serotonin2 receptors does not abolish 5HTP-induced aldosterone stimulation, and that this stimulation is most likely mediated by central pathways.

Blockade of a 5-hydroxytryptophan-induced animal model of depression with a potent and selective 5-HT2 receptor antagonist (LY53857).

To test the hypothesis that a new potent and selective 5-HT2 receptor antagonist would be an excellent blocker of D,L-5-hydroxytryptophan (5-HTP)-induced response suppression in an animal model of depression, we administered LY53857 60 min prior to 5-HTP injections into rats working on an operant schedule for milk reinforcement. As predicted, LY53857 pretreatment significantly blocked 5-HTP depression (90%) in doses as low as 0.1 mg/kg ip. When the dose of LY58357 was further reduced to 0.025 mg/kg, blockade of 5-HTP-induced depression was still greater than 30%. In doses as high as 5.0 mg/kg, LY53857 alone had no effect on the baseline performance of rats working a VI 1 schedule. Pretreatment with desipramine (2.5 mg/kg), an antidepressant characterized as having major noradrenergic effects, did not significantly block the 5-HTP-induced depression. These data suggest that the 5-HTP-induced depression is mediated by serotonergic mechanisms involving 5-HT2 receptors, as LY53857 is a selective antagonist of these receptors. These data also support the suggestion, based on other published data from this laboratory, that some antidepressants are antagonizing 5-HT2 receptors in our animal model of depression and may also act in a similar manner in depressed patients. Thus, this new drug could be of interest as a possible antidepressant agent of the general type that was proposed earlier by Aprison and Hingtgen (1981).

Long-term suppression of central serotonergic activity by corticosteroids: a possible model of steroid-responsive myoclonic disorders.

Chronic administration of cortisol succinate (12.5 mg per kilogram per day) to guinea pigs suppressed jumping behavior induced by 1-5 hydroxytryptophan and abolished diurnal threshold variations of this behavior. Chronic corticosteroid administration did not alter threshold or diurnal variations of apomorphine-induced stereotypy. These observations suggest that the efficacy of corticosteroids in some human myoclonic movement disorders may be related to central serotonergic inhibition.

The head-twitch response to intraperitoneal injection of 5-hydroxytryptophan in the rat: antagonist effects of purported 5-hydroxytryptamine antagonists and of pirenperone, an LSD antagonist.

The putative 5-hydroxytryptamine (5-HT) antagonists 2-bromo-LSD, cinanserin, cyproheptadine, pizotifen, methysergide, metitepine, mianserin and metergoline were found to reduce the frequency of the head-twitch response induced by intraperitoneal injections of 320 mg/kg of 5-hydroxytryptophan (5-HTP) in the rat. The antagonist dose-effect curve of these agents was biphasic. It consisted of an initial, steep, phase and a subsequent, shallower, phase. Analysis of the data by means of quantitative and quantal methods yielded different rank orders of potency of antagonist drugs. Only pirenperone, a drug identified earlier as a pure antagonist, produced a simple, monophasic dose-effect curve in antagonizing the effects of 5-HTP. The antagonist effects of pirenperone, and the first phase of the curve of the putative 5-HT antagonists, may reflect antagonist activity at 5-HT2 receptors. The data are consistent with earlier behavioural evidence that the putative 5-HT antagonists act complexly as mixed agonist-antagonists; only pirenperone exerted behavioural effects that suggest it to be a pure antagonist.

Tricyclics with analgesic and antidepressant activity. 2. [[(Alkylamino)ethyl]thio]dibenzo[b,f]thiepins and 10,11-dihydro derivatives.

A series of [[(alkylamino)ethyl]thio]dibenz[b,f]thiepins (III) and their 10,11-dihydro derivatives (IV) was synthesized and subjected to broad analgesic/CNS screening. Preliminary results indicated a combination of analgesic/antidepressant profiles, similar to that observed for the [[(alkylamino)ethyl]thio]dibenz[b,f]oxepins (I) and their corresponding dihydro derivatives (II). The most active congener from the present series, 10b, shows an antinociceptive potency in the pentazocine range as assessed by phenyl-p-quinone-induced writhing (PQW) and tail flick in mice. It is also more than twice as active as imipramine in preventing tetrabenazine-induced ptosis (TBZ), a test widely recognized to be of predictive value for clinically efficacious antidepressants.

Indoleamine antagonists: relative potencies as inhibitors of tryptamine- and 5-hydroxytryptophan-evoked responses.

Four indoleamine antagonists were evaluated for relative potencies as inhibitors of tryptamine-induced forepaw clonus and 5-hydroxytryptophan-evoked head twitches. Methergoline was approximately three times more potent against the forepaw clonus than the head twitch response, whereas methysergide exhibited nearly equal activity in both tests. Cyproheptadine and cinanserin showed a profile opposite to methergoline and a greater degree of selectivity, being 25 to 40 times more potent as inhibitors of the 5-hydroxytryptophan- than of the tryptamine-induced response. These findings clearly demonstrate that the rank order of potency of indoleamine antagonists varies greatly depending upon the test procedure employed.

Increased defecation during stress or after 5-hydroxytryptophan: selective inhibition by the 5-HT(4) receptor antagonist, SB-207266.

5-HT(4) receptor antagonism prevents the ability of exogenous 5-HT or 5-HTP to sensitize the intestinal peristaltic reflex and increase the rate of defecation, generally without affecting non-stimulated intestinal function. In this study we confirmed the ability of the selective 5-HT(4) receptor antagonist SB-207266 1 - 1000 microg kg(-1) p.o., to prevent the increase in defecation evoked over a 60 min period by 5-HTP 10 mg kg(-1) s.c. in conscious mice, in the absence of an apparent constipating action. The role of endogenous 5-HT in the mechanisms of increased defecation and/or diarrhoea was then investigated in conscious, fed rats. This was evoked by 180 min exposure to restraint stress, which increased both the number and mean weight of formed, faecal pellets excreted over the entire time period. SB-207266 1 - 1000 microg kg(-1) p.o. (dosed 30 min before restraint) did not affect the increase in defecation evoked during the first 60 min of restraint stress, but significantly and dose-dependently reduced or prevented the increased defecation during the remaining 120 min of the experiment; this action occurred in the absence of an apparent constipating action of SB-207266. In fasted rats exposed to restraint stress, watery diarrhoea developed and although there was a tendency for SB-207266 1 - 1000 microg kg(-1) p.o. (dosed 30 min before restraint) to reduce the incidence of diarrhoea, this inhibition was not complete. We conclude that selective 5-HT(4) receptor antagonism prevents disruptions in defecation behaviours caused by exogenous or endogenous enteric 5-HT and that this activity is not accompanied by a concomitant suppression of activity (constipation-like) within the intestine itself.

Selectivity of (3)H-MADAM binding to 5-hydroxytryptamine transporters in vitro and in vivo in mice; correlation with behavioural effects.

1. Binding of the novel radioligand (3)H-2-(2-dimethylaminomethyl-phenylsulphanyl)-5-methyl-phenylamine ((3)H-MADAM) to the serotonin transporter (SERT) was used to characterise a range of selective serotonin re-uptake inhibitors (SSRIs) in vitro and in vivo. 2. (3)H-MADAM bound with high affinity in a saturable manner to both human SERT expressed in CHO cells (K(d)=0.20 nm (pK(d)=9.74+/-0.12), B(max)=35+/-4 fmol mg(-1) protein) and mouse cerebral cortex membranes (K(d)=0.21 nm (pK(d)=9.66+/-0.10), B(max)=50+/-24 fmol mg(-1) protein). 3. Binding of (3)H-MADAM was highly selective for SERT in vitro as demonstrated by the in vitro profile of MADAM tested at 75 different receptors, ion channels and transporters. This was further substantiated by the pharmacological profile of the binding. Hence, the binding of (3)H-MADAM was potently inhibited by SSRIs but not by selective inhibitors of noradrenaline transport and dopamine transport. Likewise, a 5-HT(2A/2C) receptor antagonist did not inhibit (3)H-MADAM binding. 4. (3)H-MADAM binding in vivo was inhibited only by compounds which also inhibited the binding of (3)H-MADAM in vitro (the SSRIs, mixed SERT/noradrenaline transport inhibitors and clomipramine), confirming the selectivity of (3)H-MADAM for SERT also in vivo. Moreover, compounds effective in inhibiting (3)H-MADAM binding were the only ones found to be active in the mouse 5-HTP potentiation test confirming the model as a behavioural correlate to in vivo 5-HT uptake. 5. Finally, it was found that a SERT occupancy of 85-95% was necessary to produce 50% of the maximum behavioural response (ED(50)).

Effect of pindolol on the L-5-HTP-induced increase in plasma prolactin and cortisol concentrations in man.

Previous studies with direct-acting serotonin (5-HT) agonists and antagonists have demonstrated that stimulation of 5-HT1A, 5-HT1C and 5-HT2 receptors may promote cortisol and prolactin (PRL) secretion in man. There is also evidence that 5-HT1C/2 receptor stimulation contributes to the cortisol and PRL responses following administration of the 5-HT precursor, L-5-hydroxytryptophan (L-5-HTP), in man. To clarify the possible contribution of 5-HT1A receptor stimulation to the ability of L-5-HTP to stimulate cortisol and PRL secretion in man, the effect of pindolol, a beta adrenoceptor antagonist that is also a 5-HT1A partial agonist, on the L-5-HTP-induced increases in cortisol and PRL secretion, was examined in 12 normal male volunteers. Pretreatment with pindolol, 30 mg orally, significantly inhibited the PRL but not the cortisol response to L-5-HTP, 200 mg PO. Pindolol alone decreased basal plasma PRL levels and increased basal plasma cortisol levels, possibly due to 5-HT1A antagonist and agonists effects, respectively. These data, coupled with observations from other studies, suggest that the L-5-HTP-induced increase in PRL but not cortisol secretion requires 5-HT1A receptor activation. PRL secretion due to 5-HT formed from exogenous L-5-HTP may require the availability of both intact 5-HT1A and 5-HT2/5-HT1C receptors, since blockade of either receptor type inhibited the PRL response to L-5-HTP. The implication of this synergistic effect for interpretation of neuroendocrine studies involving the serotonergic system in man is discussed.

Behavioral and 5-HT antagonist effects of ritanserin: a pure and selective antagonist of LSD discrimination in rat.

The newly synthesized compound and putative 5-HT2 antagonist ritanserin, but not the structurally related compound R 56413, resembles pirenperone in that it acts as a pure antagonist in an LSD-saline drug discrimination assay in the rat. Ritanserin exceeded pirenperone in terms of behavioral specificity; the lowest effective dose of ritanserin in antagonizing LSD was one order of magnitude higher than that of pirenperone, but the compound depressed rate of operant responding only at doses that were about 1000-fold higher than those at which pirenperone was effective. Ritanserin exerted effects in an open field test which were reminiscent of anxiolytic drug activity in the rat; its effects were greater than those of pirenperone, R 56413 and the benzodiazepines chlordiazepoxide and diazepam. The results of experiments on antagonism of 5-HT-induced hypothermia and of the 5-HTP-induced head-twitch response fail to support the possibility that the putative anxiolytic effects of ritanserin in the rat can be ascribed simply to a pharmacologically defined action at 5-HT receptors.

Evidence for an involvement of 5-HT1B receptors in the inhibition of male rat ejaculatory behavior produced by 5-HTP.

The administration of the 5-hydroxytryptamine (5-HT) precursor 5-hydroxytryptophan (5-HTP) (25 mg/kg i.p.), in combination with an inhibitor of peripheral 5-HTP decarboxylase, produced a dose-dependent increase in the ejaculation latency of male rats, and this effect was enhanced by additional treatment with the 5-HT1 receptor antagonist (-)-pindolol (2 mg/kg s.c.). The 5-HT2A/C receptor agonist (+/-) 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (0.125-0.5 mg/kg s.c.) did not by itself affect male ejaculatory behavior, but additional treatment with (-)-pindolol (2 mg/kg s.c.) produced a dose-dependent decrease in number of ejaculating animals. The increased ejaculation latency produced by 5-HTP was fully antagonized by treatment with the 5-HT1B receptor antagonist isamoltane (4 mg/kg s.c.), but not by ritanserin (2 mg/kg s.c.) treatment. The selective 5-HT1A receptor antagonist WAY-100635 (0.15 mg/kg s.c.) enhanced the inhibitory actions of 5-HTP on the male rat ejaculatory behavior, and this dose of WAY-100635 fully antagonized 8-OH-DPAT-induced facilitation (0.25 mg/kg s.c.) of the ejaculatory behavior. WAY-100635 (0.04-0.60 mg/kg s.c.) did not, by itself, significantly affect male rat sexual behavior. Taken together, the results suggest an inhibitory role for postsynaptic 5-HT1B receptors in the effects produced by 5-HTP on male rat ejaculatory behavior. Furthermore, 5-HTP-induced inhibition of male rat ejaculatory behavior is partially controlled by stimulation of inhibitory 5-HT1A autoreceptors, since the effects of 5-HTP were accentuated by treatment with (-)-pindolol, as well as by the more selective 5-HT1A receptor antagonist WAY-100635.

Increased gastric cholinergic activity evoked by 5-hydroxy-L-tryptophan in the rat.

In gastrointestinal tissues such as rat stomach, exogenous 5-hydroxytryptamine (5HT) has little or no ability to affect nerve activity. However, endogenous 5HT might act differently, and this was investigated by stimulating 5HT synthesis using 5-hydroxy-L-tryptophan (5HTP). In longitudinal strips of rat forestomach, 5HTP (50 and 500 microM) increased cholinergically mediated contractions evoked by electrical field stimulation, probably by facilitating acetylcholine release; contractions evoked by exogenous acetylcholine were unaffected by 5HTP. The ability of 5HTP to increase electrically evoked contractions was long-lasting, required the presence of pyridoxal (a monoamine decarboxylase cofactor) and was reduced by the decarboxylase inhibitor carbidopa, but not by 6-hydroxydopamine. In the presence of paroxetine and nialamide, the 5HTP-induced increase in cholinergically mediated contractions was short-lasting. In anaesthetised rats 5HTP caused stimulation of gastric motility, which was blocked or reduced by atropine. These findings suggest that in the rat 5HTP stimulates gastric cholinergic activity, by increasing the concentration of 5HT at sites which normally synthesise 5HT.

Antagonism by lisuride and 8-OH-DPAT of 5-HTP-induced prolongation of the performance of male rat sexual behavior.

Both lisuride and 8-OH-DPAT dose dependently antagonized the 5-HTP-induced inhibition of male rat sexual behavior. The increase in the number of intromissions and/or the ejaculation latency produced by 5-HTP, 25 mg/kg i.p. (-60 min) in combination with the peripheral 5-HTP decarboxylase inhibitor benserazide, 25 mg/kg i.p. (-90 min), were antagonized by lisuride, 0.05-0.1 mg/kg i.p. (-15 min) and by 8-OH-DPAT, 0.025-0.05 mg/kg i.p. (-15 min). Thus, in this model lisuride and 8-OH-DPAT behave as 5-HT antagonists.

1-5-Hydroxytryptophan-induced flat body posture in the rat: antagonism by ritanserin and potentiation after 5,7-dihydroxytryptamine.

1-5-Hydroxytryptophan (1-5-HTP)-induced flat body posture (FBP) was antagonized by ritanserin in doses that were lower than those needed to antagonize head-twitches (HTW) and forepaw treading (FPT). 5,7-Dihydroxytryptamine (5,7-DHT) potentiated 1-5-HTP-induced FBP but not HTW or FPT. Ritanserin interacted with 5-HT2 and 5-HT1c receptors. 1-5-HTP-induced FBP could be mediated by postsynaptic 5-HT1c receptors and could serve as a behavioral model of postsynaptic 5-HT1c receptor stimulation in the CNS.

Comparison of clenbuterol enantiomers using four psychopharmacological tests sensitive to beta-agonists.

The effects of the enantiomers of clenbuterol were compared in four psychopharmacological tests in which beta-adrenergic agonists are known to be active. In mice (+/-)-clenbuterol 0.06 mg/kg decreased motor activity and antagonized the hypothermia induced by 16 mg/kg of apomorphine; at 0.5 mg/kg it increased head-twitches and tremors induced by 1-5-hydroxytryptophan (4 mg/kg) and decreased the number of bites of a food pellet by mice which had been maintained on a water only diet for 24-27 h ('Tantale' test). The present results show that the (-) form is responsible for this activity and that the (+) form has no effect per se and does not antagonize the (-) enantiomer.

Failure to antagonize the 8-hydroxy-2-di-n-propylamino)tetralin-induced facilitation of male rat sexual behavior by the administration of 5-HT receptor antagonists.

The administration of the putative 5-hydroxytryptamine (5-HT) agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-hydroxytryptophan (5-HTP) produced a facilitation and an inhibition of male rat sexual behavior, respectively. The 5-HTP-induced inhibitory effects were at least partially antagonized by the administration of metergoline, methiothepine or pirenperone. However, none of these agents were able to counteract any facilitatory effects produced by 8-OH-DPAT. It is concluded that 8-OH-DPAT does not facilitate the expression of masculine sexual behavior in the rat by stimulation of 5-HT1 or 5-HT2 receptors.

Central and peripheral mechanisms contribute to the antiemetic actions of delta-9-tetrahydrocannabinol against 5-hydroxytryptophan-induced emesis.

Delta-9-tetrahydrocannabinol (delta-9-THC) prevents cisplatin-induced emesis via cannabinoid CB(1) receptors. Whether central and/or peripheral cannabinoid CB(1) receptors account for the antiemetic action(s) of delta-9-THC remains to be investigated. The 5-hydroxytryptamine (5-HT=serotonin) precursor, 5-hydroxytryptophan (5-HTP), is an indirect 5-HT agonist and simultaneously produces the head-twitch response (a centrally mediated serotonin 5-HT(2A) receptor-induced behavior) and emesis (a serotonin 5-HT(3) receptor-induced response, mediated by both peripheral and central mechanisms) in the least shrew (Cryptotis parva). The peripheral amino acid decarboxylase inhibitor, carbidopa, prevents the conversion of 5-HTP to 5-HT in the periphery and elevates 5-HTP levels in the central nervous system (CNS). When administered i.p. alone, a 50 mg/kg dose of 5-HTP failed to induce either behaviour while its 100 mg/kg dose produced robust frequencies of both head-twitch response and emesis. Pretreatment with carbidopa (0, 10, 20 and 40 mg/kg) potentiated the ability of both doses of 5-HTP to produce the head-twitch response in a dose-dependent but bell-shaped manner, with maximal potentiation occurring at 20 mg/kg carbidopa. Carbidopa dose-dependently reduced the frequency of 5-HTP (100 mg/kg)-induced emesis, whereas the 10 mg/kg dose potentiated, and the 20 and 40 mg/kg doses suppressed the frequency of vomits produced by the 50 mg/kg dose of 5-HTP. The peripheral and/or central antiemetic action(s) of delta-9-THC (0, 1, 2.5, 5, 10 and 20 mg/kg) against 5-HTP (100 mg/kg)-induced head-twitch response and emesis were investigated in different groups of carbidopa (0, 10 and 20 mg/kg) pretreated shrews. Irrespective of carbidopa treatment, delta-9-THC attenuated the frequency of 5-HTP-induced head-twitch response in a dose-dependent manner with similar ID(50) values. Although delta-9-THC also reduced the frequency of 5-HTP-induced emesis with similar ID(50s), at the 5 mg/kg delta-9-THC dose however, 5-HTP induced significantly less vomits in the 10 and 20 mg/kg carbidopa-treated groups relative to its 0 mg/kg control group. Moreover, increasing doses of carbidopa significantly shifted the inhibitory dose-response effect of delta-9-THC in protecting shrews from 5-HTP-induced emesis to the left. Relatively, a large dose of delta-9-THC (20 mg/kg) was required to significantly reduce the number of vomits produced by direct acting serotonergic 5-HT(3) receptor agonists, serotonin and 2-methylserotonin. Low doses of delta-9-THC (0.1-1 mg/kg) nearly completely prevented 2-methylserotonin-induced, centrally mediated, head-twitch and ear-scratch responses. The results indicate that delta-9-THC probably acts pre- and postsynaptically to attenuate emesis produced by indirect and direct acting 5-HT(3) receptor agonists via both central and peripheral mechanisms. In addition, delta-9-THC prevents 5-HTP-induced head-twitch and emesis via cannabinoid CB(1) receptors since the CB(1) receptor antagonist, SR 141716A [N-piperidino-5-(4-chlorophenyl)-1-(2,4-dichlorophenyl)-4-methylpyrazole-3-carboxamide], countered the inhibitory actions of an effective dose of delta-9-THC against both behaviours.