Biochemical and pharmacological characterization of CGS 12066B, a selective serotonin-1B agonist

Serotonin modulates ethanol-induced depression in cerebellar Purkinje neurons

In the present study, we found that local application of serotonin (5-HT) potentiated ethanol-induced depressions of the spontaneous activity of Purkinje neurons in urethane-anesthetized rats. 5-HT also potentiated depressions induced by gamma-aminobutyric acid; however, this modulatory response was quantitatively smaller than 5-HT-induced potentiation of ethanol depression. Previous reports suggested that the release of 5-HT can be regulated by presynaptic 5-HT autoreceptors. We found that local application of methiothepin, which may induce 5-HT overflow through the inhibition of presynaptic autoreceptors, facilitated ethanol-mediated responses. This methiothepin effect was greatly diminished in neonatally 5,7-dihydroxytryptamine-lesioned animals, suggesting a presynaptic mechanism was involved. We also found that the 5-HT1A antagonist UH301 did not attenuate 5-HT-facilitated ethanol reactions. On the other hand, local application of 5-HT1B agonist CGS12066B potentiated ethanol-induced depression. Taken together, our data suggest that 5-HT can modulate ethanol-mediated electrophysiological depression, possibly mediated through 5-HT1B receptors in the cerebellum.

Methylenedioxymethamphetamine-induced inhibition of neuronal firing in the nucleus accumbens is mediated by both serotonin and dopamine

Methylenedioxymethamphetamine (MDMA) is a mood-altering, legally restricted drug that has been reported to inhibit glutamate-evoked firing of cells in the nucleus accumbens. This study used extracellular recording combined with microiontophoresis to examine whether the inhibitory effect of MDMA on neuronal firing in the nucleus accumbens is mediated by serotonin and/or dopamine. Serotonin and serotonin agonists with relative selectivity for the receptor subtypes 5-HT1A, 5-HT1B, 5-HT2A/2C and 5-HT3 all significantly (P < 0.01) inhibited glutamate-evoked firing of cells in the nucleus accumbens compared to the effects of an acidic saline control solution (30-60 nA, 60 s ejection currents for all). The current (dose)-dependent inhibition produced by the serotonin agonists did not differ significantly from the inhibition produced by MDMA except for the 5-HT1A agonist 8-hydroxy-(2-di-n-propylamino) tetralin, which inhibited glutamate-evoked firing significantly more than MDMA or any of the other serotonin agonists. At the highest ejection current tested (60 nA, 60 s), glutamate-evoked firing was inhibited by MDMA in 94% of tested cells, by serotonin in 80% of tested cells and by the serotonin receptor subtype agonists in 95-100% of the tested cells. In addition to being mimicked by serotonin and serotonin agonists, MDMA-induced inhibition of glutamate-evoked firing in the nucleus accumbens was partially blocked by the serotonin antagonists ketanserin (100% of tested cells), methysergide (80% of tested cells), methiothepin (100% of tested cells) and WAY100135 (100% of tested cells). Furthermore, application of the serotonin uptake blocker fluoxetine, which prevents MDMA-induced serotonin release, also significantly attenuated MDMA-induced inhibition of glutamate-evoked firing in all of the cells that were tested. These observations suggest that MDMA-induced inhibition of nucleus accumbens cell firing is at least partially mediated by serotonin. Depletion of dopamine by pretreatment with the neurotoxin 6-hydroxydopamine and the synthesis inhibitor alpha-methyl-p-tyrosine blocked the inhibition of glutamate-evoked firing produced by MDMA applied with low ejection currents (30-40 nA, 60 s). However, this dopamine depletion had no effect on inhibition of glutamate-evoked firing produced by serotonin ejected with low or high currents (20-60 nA, 60 s). These results suggest that both dopamine release and an intermediate step of MDMA-induced serotonin release are necessary for the inhibitory effects of MDMA on neuronal excitability in the nucleus accumbens. The dopamine- and serotonin-mediated inhibitory effects of MDMA on glutamate-evoked firing of nucleus accumbens cells may play a role in the mood-altering properties of this increasingly popular drug.

Novel selective and partial agonists of 5-HT3 receptors. Part 1. Synthesis and biological evaluation of piperazinopyrrolothienopyrazines

A series of piperazinopyrrolo[1,2-a]thieno[3,2-e]- and -[2,3-e]pyrazine derivatives were prepared and evaluated in order to determine the necessary requirements for high affinity on the 5-HT3 receptors and high selectivity versus other 5-HT receptor subtypes. Various substitutions on the piperazine and the thiophene ring of the pyrrolothienopyrazine moieties were systematically explored as well as replacement of the piperazine by other cyclic amines. The best compounds are in the nanomolar range of affinity of 5-HT3 receptors with high to very high selectivity (up to 10,000 for 14b). These high-affinity compounds have in common a benzyl- or allylpiperazine substituent with no substitutions on the thiophene ring. Five of these compounds (1a, 4b, 13a,b, and 14b) have been evaluated on the Von Bezold-Jarisch reflex and were characterized as partial agonists. One of them, 13a, has shown in vivo at very low dose a potent anxiolytic-like activity in the light/dark test.

Characterization of 5-hydroxytryptamine receptors mediating contractions in basilar arteries from stroke-prone spontaneously hypertensive rats

1. The 5-hydroxytryptamine (5-HT) induced-contraction in ring preparations of basilar arteries from Wistar-Kyoto rats (WKY) and stroke-prone spontaneously hypertensive rats (SHRSP) was pharmacologically characterized in vitro. 2. Contractile responses to 5-HT (1 nM-100 nM) and their pD2 values in arteries from SHRSP at 6 months of age were significantly greater than those in age-matched WKY, although the maximum response did not differ between the two groups. 3. There were no significant differences in contractile responses to U-44619, endothelin-1, neuropeptide Y, and angiotensin II between WKY and SHRSP arteries. 4. Spiperone (1 nM-1 microM, a 5-HT2 receptor antagonist), produced biphasic displacement of the 5-HT curves in WKY and SHRSP arteries. The response to high concentrations of 5-HT was concentration-dependently antagonized by spiperone, while the response to low concentrations of 5-HT was resistant to blockade by spiperone, and the spiperone-resistant contractile responses induced by 5-HT were greater in SHRSP than in WKY. Ketanserin (1-100 nM, 5-HT2) also produced a biphasic shift of the 5-HT curves for both arteries. 5. Methiothepin (10 and 100 nM, 5-HT1 and 5-HT2) potently inhibited 5-HT-induced contractions in both groups. In addition, methiothepin (100 nM) produced a parallel shift to the right of the component of 5-HT-induced contractile responses that was resistant to blockade by spiperone in both groups. 6. The contractile effects of 5-HT in WKY and SHRSP arteries were not affected by MDL 72222 (1 microM, 5-HT3) and SDZ 205-557 (1 microM, 5-HT4). In addition, cocaine (10 microM), pargyline (50 microM), prazosin (10 microM), indomethacin (3 microM) and SQ 29,548 (1 microM) did not affect the contractile effects of 5-HT in either artery. 7. Contractile responses to 5-carboxamidotryptamine, CGS 12066B, pindolol and propranolol were greater in SHRSP arteries than in WKY arteries, whereas contractions in response to 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), alpha-methyl-5-HT and 2-methyl-5-HT did not differ between the two groups. Cisapride failed to contract basilar arteries in both groups. Furthermore, a correlation analysis showed a highly significant correlation between the pD2 values of 5-HT agonists in WKY and SHRSP arteries and their published binding affinities at the 5-HT1B subtype. 8. These findings suggest that 5-HT elicits vasoconstriction in rat basilar arteries by stimulation of a mixed receptor population of 5-HT2 and 5-HT1-like receptors (similar to the 5-HT1B receptor subtype), and that the contraction mediated by 5-HT1-like receptors is enhanced in the basilar artery from SHRSP.

GR 127935 blocks the locomotor and antidepressant-like effects of RU 24969 and the action of antidepressants in the mouse tail suspension test

The 5-HT1A/B agonist RU 24969 induces hyperactivity in rodents and also shows antidepressant-like effects in some animal models of depression. We have examined the effects of selective antagonists at 5-HT1A and 5-HT1B/D receptors (WAY 100135 and GR 127935, respectively) on both the hyperlocomotor and anti-immobility effects of RU 24969. While a high dose of WAY 100135 (10 mg/kg) had no effect in either paradigm, GR 127935 attenuated the behavioural effects of RU 24969 in both. WAY 100135 was also without effect on the antidepressant effect of paroxetine, while GR 127935 blocked the effects of paroxetine (1 mg/kg) and imipramine (10 mg/kg). Furthermore, while coadministration of paroxetine or imipramine enhanced the effects of RU 24969 in the mouse tail suspension test, imipramine had no effect on the locomotor activating effects of the 5-HT1B agonist, suggesting different neural substrates may underly the effects in the different tests. These studies indicate a role for the 5-HT1B/D receptor in the mediation of the effects of antidepressant treatment.

Sleep and EEG power spectrum effects of the 5-HT1A antagonist NAN-190 alone and in combination with citalopram

The sleep and waking and EEG power spectrum effects of the putative 5-HT1A antagonist NAN-190 (0.5 mg/kg, i.p.) were studied alone and in co-administration with the selective serotonin re-uptake inhibitor citalopram (5.0 mg/kg, i.p.) in the rat. Citalopram, as in a prior dose-response study, reduced REM sleep. In addition, a slight increase in NREM sleep was observed. Citalopram reduced NREM fronto-parietal (FP) EEG power density in the 5-20 Hz range. When administered alone, NAN-190 suppressed REM sleep in the first 2 h, and reduced SWS-2 in the first 4 after administration. NAN-190 also suppressed selectively NREM sleep slow-wave activity in both fronto-frontal (FF) and FP EEG power spectrum. When administered in combination with citalopram, an attenuation of the power density reduction in the 7-15 Hz range in the FF EEG of citalopram alone, was observed. However, the EEG power spectral density and REM sleep suppressive effects of NAN-190 were both augmented. The results are compatible with the notion that serotonin is involved in the modulation of the slow wave activity in the EEG during NREM sleep. The results are cordant with other data suggesting that postsynaptic 5-HT1A stimulation might increase slow wave activity in the NREM EEG, and that serotonergic stimulation of other receptor subtypes (possibly 5-HT2) may decrease slow wave activity in the NREM EEG.

Role of 5-HT1b receptor in the pressure-induced behavioral and neurochemical disorders in rats

When human divers and experimental animals are exposed to increasing environmental pressure, they develop the high-pressure neurologic syndrome (HPNS) that has been recently demonstrated to include an increase in striatal dopamine (DA) release. This increase has been correlated with enhanced locomotor and motor activity (LMA). In the present study, we investigated the effect of the 5-HT1b receptor antagonist (+/-)cyanopindolol, which has been shown to block at normal pressure the increase in striatal DA release induced by the administration of the 5-HT1b receptor agonist CGS 12066B. Our data clearly showed that the administration of (+/-)cyanopindolol partially blocked both the pressure-induced increase in striatal DA release and the development of LMA. These results suggest the contribution of the 5-HT neurotransmission in the DA-related neurochemical and behavioral disorders that occur in rats exposed to high pressure.

Species differences in 5-HT autoreceptors

Release of 5-HT in the CNS is under the control of autoreceptors. These autoreceptors fall into two categories: cell body autoreceptors and terminal autoreceptors. The former inhibit 5-HT release through inhibition of cell firing; the latter through direct inhibition of release at the terminal. Cell body (or somatodendritic) autoreceptors belong to the 5-HT1A receptor subtype in all species studied so far. In the rat and mouse, the terminal autoreceptor is known to be a 5-HT1B receptor, whereas in human, pig, rabbit, and guinea pig, the terminal autoreceptor is thought to belong to the 5-HT1D receptor subtype. Until recently, the absence of a potent and selective 5-HT1D receptor antagonist has hindered this classification. We now present data with the novel 5-HT1D receptor antagonist, GR 127935, which demonstrates that in guinea pig cerebral cortex the terminal autoreceptor is a 5-HT1D receptor. In vitro [3H]5-HT release studies demonstrate that 5-HT inhibition of [3H]5-HT release is attenuated by GR 127935. In vivo, using the technique of microdialysis, GR 127935 and the non-selective antagonist methiothepin, when administered down the dialysis probe, potentiate extracellular levels of 5-HT. Both the in vitro and in vivo effects of these compounds are consistent with terminal autoreceptor blockade. However, when GR 127935 and methiothepin were administered systemically, both compounds inhibit extracellular levels of 5-HT. The most plausible explanations for this effect, such as partial agonism or activation of somatodendritic 5-HT1A receptors, are discussed.

Interactions of descending serotonergic systems with other neurotransmitters in the modulation of nociception

The effects of 5-hydroxytryptamine (5-HT) and ligands selective for particular 5-HT receptor subtypes on the transmission of nociceptive information in the spinal cord are complex. In these studies, we have focused on their interactions with two endogenous mediators of pain suppression, noradrenaline (NA) and adenosine. Spinal antinociception by 5-HT is blocked by alpha-adrenoreceptor antagonists and depletion of endogenous NA by 6-hydroxydopamine, while it is potentiated by blockade of NA reuptake with desipramine. These observations provide evidence for a 5-HT receptor-mediated increase in the release of NA from the spinal cord. This action appears to be due to activation of a 5-HT1-like receptor as it is mimicked by some 5-HT1 receptor ligands (mCPP, TFMPP and 5-Me-O-DMT), but not by DOI (5-HT2) or 2-Me-5-HT (5-HT3). An additional component of 5-HT action is via release of adenosine. Antinociception by 5-HT is blocked by the adenosine receptor antagonist 8-phenyltheophylline, and 5-HT has been shown to release adenosine from the spinal cord in in vitro and in vivo paradigms. Methylxanthine-sensitive antinociception is seen with some 5-HT1 receptor ligands (CGS 12066B, mCPP), but not with others or with DOI or 2-Me-5-HT. Further characterization of the 5-HT receptor subtype involved in adenosine release will require the use of additional approaches.

Effects of selective activation of the 5-HT1B receptor with CP-94,253 on sleep and wakefulness in the rat

The effects of the 5-HT1B receptor agonist CP-94,253 were compared with those of the mixed beta-adrenoceptor and 5-HT1A/B receptor antagonist (+/-)pindolol in rats implanted for chronic sleep recordings. CP-94,253 (5.0-10.0 mg/kg) significantly increased waking and reduced slow wave sleep (SWS) and REM sleep (REMS). At 2.0-4.0 mg/kg (+/-)pindolol reduced REMS. Pretreatment with (+/-)pindolol (2.0-4.0 mg/kg) reversed the effect of CP-94,253 on waking and SWS, while REMS remained suppressed. It is suggested that the 5-HT1B receptor together with other 5-HT receptor subtypes may have a direct regulatory action on sleep and waking in the rat.

The role of various calcium and potassium channels in the regulation of somatodendritic serotonin release

We prepared slices from midbrain containing the raphe nuclei and from hippocampus of rats. The brain slices were loaded with [3H]serotonin and superfused in order to measure the release of radioactivity at rest and in response to electrical stimulation. No difference was observed in the resting and stimulated fractional release of tritium in the somatodendritic and axon terminal parts of serotonergic neurons. The selective 5-HT1A receptor agonist 8-OH-DPAT decreased the electrically induced tritium efflux from raphe nuclei slices preloaded with [3H]serotonin, and this inhibition was reversed by the 5-HT1A receptor antagonist (+)WAY-100135. The 5-HT1B receptor agonist CGS-12066B but not 8-OH-DPAT, inhibited the stimulation-evoked tritium efflux from hippocampal slices after labeling with [3H]serotonin. The electrical stimulation-evoked tritium efflux in raphe nuclei slices incubated with [3H]serotonin was completely external Ca(2+)-dependent, and omega-conotoxin GVIA and Cd2+, but not diltiazem, inhibited the tritium overflow. In raphe nuclei slices 4-aminopyridine enhanced the electrical stimulation-induced tritium release in a concentration-dependent manner. The inhibition of tritium efflux by 8-OH-DPAT was abolished with 4-aminopyridine. Glibenclamide or tolbutamide proved to be ineffective. These data indicate that (1) different 5-HT receptor subtypes (5-HT1A and 5-HT1B) regulate dendritic and axon terminal 5-HT release; (2) serotonin release from the dendrites may be regulated by the voltage-sensitive N-type Ca2+ channels; (3) the 5-HT1A receptor-mediated inhibition of serotonin release may be due to opening of voltage-sensitive K+ channels.

Differential effects of CGS 12066B and CP-94,253 on murine social and agonistic behaviour

Although it has been previously proposed that 5-HT1B agonism specifically attenuates rodent agonistic behaviour, more recent investigations have indicated that such influences may be ancillary to an anxiogenic effect. The present study examined the influences of two 5-HT1B agonists, CGS 12066B and CP-94,253, on murine agonistic behaviour. In a resident-intruder paradigm, CGS 12066B (0.5-5.0 mg/kg) decreased resident offensive aggression, social interest, and exploration while dose-dependently enhancing defensive behaviours across the dose range tested. CP-94,253 (2.5-10.0 mg/kg) also reduced elements of resident offensive behaviour whereas defensive behaviours were largely unchanged. Some elements of resident nonsocial and social behaviour were enhanced at 2.5 and 5.0 mg/kg but decreased at 10.0 mg/kg. The behavioural profile of CP-94,253, but not CGS 12066B, supports the proposal that 5-HT1B receptors inhibit agonistic behaviour without concomitant sedative or anxiogenic effects. Findings are discussed in relation to 5-HT1A/1B/2C receptors involved in agonistic behaviour and anxiety.

Effects of selective serotonergic ligands on posthypoxic audiogenic myoclonus

Male Sprague-Dawley rats underwent cardiac arrest and resuscitation, subsequently exhibiting posthypoxic myoclonus. The audiogenic posthypoxic myoclonus in these animals could be attenuated with the following drugs: 5-hydroxytryptophan (5-HTP, serotonin [5-HT] precursor), N-(3-trifluoro-methylphenyl)piperazine hydrochloride (TFMPP, 5-HT1B/1C/2 agonist), (+/-)-2,5-dimethoxy-4-iodoamphetamine hydrobromide (DOI, 5-HT2 agonist), and 1-(m-chlorophenyl)-biguanide hydrochloride (m-CPBG, 5-HT3 agonist). In contrast, the following drugs were ineffective: (+/-)-8-hydroxy-dipropylaminotetralin hydrobromide (8-OH-DPAT, 5-HT1A agonist), buspirone hydrochloride (5-HT1A agonist), 7-trifluoromethyl-4(4-methyl-l-piperazinyl)-pyrrolo[1,2- a]quinoxaline maleate (CGS 12066B, 5-HT1B agonist), ketanserin tartrate (5-HT2 antagonist), methysergide maleate (5-HT2 antagonist), fluoxetine (5-HT uptake blocker), and saline (vehicle). The data suggest that enhancement of serotonergic activity, particularly through 5-HT2 and 5-HT3 receptors, have therapeutic potential for the treatment of posthypoxic myoclonus.

Enhanced oral activity responses to intrastriatal SKF 38393 and m-CPP are attenuated by intrastriatal mianserin in neonatal 6-OHDA-lesioned rats

Enhanced oral activity is induced in neonatal 6-hydroxydopamine- (6-OHDA-) lesioned rats by systemic administration of the dopamine (DA) D1 receptor agonist SKF 38393 and serotonin (5-HT) 5-HT2A,2C agonist m-chlorophenylpiperazine (m-CPP). The DA D1 receptor antagonist SCH 23390 effectively attenuates the effect of SKF 38393 but not m-CPP. The 5-HT2 antagonist mianserin attenuates the effects of both m-CPP and SKF 38393, suggesting that DA agonist effects are mediated by 5-HT neurochemical systems. To test whether DA and 5-HT agonist effects and interactions might occur within the neostriatum, rats were implanted with permanent injection cannulae, with tips in the ventral striatum. One group of rats was lesioned at 3 days after birth with 6-OHDA HBr (100 micrograms salt form, in each lateral ventricle; desipramine HCl pretreatment, 20 mg/kg IP, base form, 1 h), while controls received the vehicle in place of 6-OHDA. Cannulae were implanted when rats weighed 200-250 g. During a 1-h observation session SKF 38393 (5 nmol per side) produced 74.3 +/- 19.2 oral movements in intact rats and 310.7 +/- 97.0 oral movements in 6-OHDA-lesioned rats. m-CPP (10 nmol per side) produced 72.6 +/- 15.1 and 274.5 +/- 65.0 oral movements in these respective groups. These responses were several-fold greater than the 25.3 +/- 7.3 and 41.8 +/- 9.5 oral movements in the same groups after saline (0.5 microliter per side) (P < 0.05). Mianserin (6 nmol per side) alone had no effect on oral activity but attenuated responses to both SKF 38393 and m-CPP in intact and 6-OHDA-lesioned rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Interactions between 5-hydroxytryptamine receptor subtypes: is a disturbed receptor balance contributing to the symptomatology of depression in humans?

The purpose of this review is to describe the consequences of antidepressant treatment on the behaviour of rodents after activation of serotonin (5-hydroxytryptamine, 5-HT) receptor subtypes. In a summary table, the involvement of 5-HT receptors in inducing behavioural changes are described. It is emphasized that these effects are not always only exclusively linked to serotonergic functions nor that they are only initiated by central 5-HT receptors. Hereafter, the complex mutual inhibitory effects of 5-HT receptor subtype-mediated processes are discussed by interpreting effects of antagonists and describing the different effects of low and high doses of mixed 5-HT1C/5-HT2 receptor agonists. Mutual influences are seen particularly with 5-HT1A, 5-HT1C and 5-HT2, but not with 5-HT1B, 5-HT1D or 5-HT3 receptor-mediated effects. It is shown that the behavioural consequences of 5-HT1A, 5-HT1C and 5-HT2 receptor stimulation may be changed by brain lesions or chronic treatment with drugs. Among these drugs are the antidepressants. Finally, 5-HT receptor function in depressed patients is discussed, and the hypothesis is proposed that an important function of antidepressants is to restore a disturbed balance between 5-HT1A, 5-HT1C and 5-HT2 receptors in depressed patients.

Spinal supersensitivity to 5-HT1, 5-HT2 and 5-HT3 receptor agonists following 5,7-dihydroxytryptamine

The present study examined functional supersensitivity to 5-hydroxytryptamine (5-HT) and 5-HT ligands selective for 5-HT1, 5-HT2 and 5-HT3 receptors in two tests for nociception following the spinal administration of 5,7-dihydroxytryptamine (5,7-DHT). Intrathecal pretreatment with 5,7-DHT 30-100 micrograms (following desipramine) produced a selective depletion of spinal cord 5-HT levels of > 80% and augmented the antinociceptive action of 5-HT in the tail flick and hot plate tests. The tail flick test was the more sensitive test for expression of this action. Supersensitivity was observed with the 5-HT1 receptor ligands CGS 12066B (7-trifluoromethyl-4-(4-methyl-1-piperazinyl-pyrrolo[1,2-a] quinoxalinedimaleate), RU 24969 (5-methoxy-3-(1,2,4,6-tetrahydro-4-pyridinyl)1H indole succinate), TFMPP (m-trifluoromethylphenyl-piperazine HCl), mCPP (1-(3-chlorophenyl)piperazine dihydrochloride) and 5-Me-ODMT (5-methoxy-N,N-dimethyltryptamine hydrogen oxalate) but not with the 5-HT2 receptor ligand DOI ((+/-)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane HCl) or the 5-HT3 receptor ligand 2-Me-5-HT (2-methyl-5-hydroxytryptamine maleate) in the tail flick test. In the hot plate test, supersensitivity was observed only with 5-Me-ODMT. Intrathecal pretreatment with fluoxetine, a 5-HT uptake inhibitor, potentiated the action of 5-HT but not any of the other 5-HT1 receptor ligands examined. These results indicate that supersensitivity occurs with 5-HT and 5-HT1 receptor ligands but not with 5-HT2 or 5-HT3 receptor ligands. Both the loss of uptake sites and receptor upregulation may contribute to enhanced activity of 5-HT, but for other ligands, only the latter mechanism appears to occur.

Release of [3H]5-hydroxytryptamine from the intermediate area of rat thoracic spinal cord is modulated by presynaptic autoreceptors

Serotonin (5-HT) nerve terminals innervate sympathetic preganglionic neurons of the intermediolateral cell column (IML); however, neither the depolarization-induced release of 5-HT nor the presence of presynaptic modulatory autoreceptors have been directly studied in this system. We used in vitro superfusion of the microdissected intermediate area (including the intermediolateral cell column, intercalated nucleus, and central autonomic nucleus) of the rat thoracic spinal cord to measure basal and stimulated release of preloaded [3H]5-HT. Elevated K+ evoked a concentration- and Ca(2+)-dependent release of [3H]5-HT. Exogenous 5-HT and the 5-HT1B agonist, CGS-12066B, both decreased the K(+)-stimulated release of [3H]5-HT. A 5-HT1B antagonist (methiothepin) blocked the 5-HT- and the CGS-12066B-induced inhibition of K(+)-evoked release of [3H]5-HT. A 5-HT1A antagonist (NAN-190) did not alter the inhibitory actions of exogenous 5-HT. Moreover, a 5-HT1A agonist (8-OH-DPAT), a 5-HT2A/2C agonist [(+/-)-DOI hydrochloride), and a 5-HT3 agonist (2-methyl-5-HT) did not alter the K(+)-evoked release of [3H]5-HT. These data demonstrate that 5-HT is released from the intermediate area of the rat thoracic spinal cord. The 5-HT receptor subtype involved in the inhibition of the evoked release of [3H]5-HT is of the 5-HT1B subtype. These findings may help clarify the complex role of 5-HT in spinal regulation of the sympathetic nervous system.

Effects of the selective 5-HT1B agonist, CGS 12066B, on sleep/waking stages and EEG power spectrum in rats

Sleep/waking stages and EEG power spectra were studied in rats for 8 h following intraperitoneal administration of CGS 12066B, a selective 5-HT1B agonist. Waking was increased and rapid eye movement (REM) sleep decreased in a dose-dependent manner. Total slow-wave sleep (TSWS) was reduced, but only in the first 2 h period. The latencies to REM sleep and stable sleep were increased dose-dependently. The drug also induced profound behavioural changes that may account for some of the sleep/waking changes. EEG power densities in waking and TSWS were reduced dose-dependently from 7 to 20 Hz after CGS 12066B, suggesting a tendency towards general deactivation. The increase in waking together with a general deactivation suggest complex effects of CGS 12066B on the sleep/waking axis.

Potential antidepressant properties of SR 57746A, a novel compound with selectivity and high affinity for 5-HT1A receptors

SR 57746A, 4-(3-trifluoromethylphenyl)-N-[2-(naphth-2-yl)ethyl]-1,2,3,6- tetrahydropyridine HCl, was studied for its specific 5-HT1A receptor agonist action and antidepressant-like effects in the rat. The compound showed a high affinity for 5-HT1A specific binding sites in the rat hippocampus (IC50 3 nM), moderate affinity (10(-7)-10(-6) M) for dopamine D2 receptor, 5-HT uptake, 5-HT2 and alpha 1-adrenoceptor binding sites and practically no effect on binding sites of monoamine, GABAA, benzodiazepine and histamine receptors. It inhibited forskolin-stimulated adenylate cyclase activity in rat hippocampal membranes at concentrations of 10(-6) and 10(-5) M. The effect of 10(-6) M SR 57746A on forskolin-stimulated adenylate cyclase activity was completely antagonized by 10(-6) M (-)-propranolol. Administered per os as a three-dose course to rats, SR 57746A significantly increased struggling in the forced swimming test at doses from 0.3 to 3 mg/kg. Single doses had no such effect. The effect of a three-dose course with 1 mg/kg SR 57746A on rats' struggling was antagonized by pretreatment with 5 mg/kg i.p. metergoline, a non-selective 5-HT receptor antagonist, and by 20 mg/kg i.p. (-)-propranolol, an antagonist at 5-HT1 receptors. Three oral doses of 100 mg/kg parachlorophenylalanine, an inhibitor of 5-HT synthesis, and 100 mg/kg i.p. (+/-)-sulpiride, an antagonist at dopamine D2 receptors, also antagonized the effect of SR 57746A in the forced swimming test. The results show that SR 57746A has selectivity and high affinity for 5-HT1A receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Evidence that RU 24969-induced locomotor activity in C57/B1/6 mice is specifically mediated by the 5-HT1B receptor

1. The behavioural effects of the 5-HT1B receptor agonists, RU 24969 and CGS 12066B, have been investigated in C57/B1/6 mice. 2. RU 24969 (1-30 mg kg-1) produced intense and prolonged hyperlocomotion and other behavioural changes. 3. CGS 12066B caused similar effects, but they were much less pronounced, inconsistent and transient irrespective of whether this drug was given i.p. (1-15 mg kg-1) or i.c.v. (0.2-40 micrograms). However, CGS 12066B (7.5 and 15 mg kg-1) caused a dose-related inhibition of RU 24969 (7.5 mg kg-1)-induced hyperlocomotion indicating that the former is a 5-HT1B partial agonist. 4. RU 24969 (7.5 mg kg-1 i.p.)-induced hyperlocomotion was inhibited by the (-)-, but not (+)-isomers of pindolol (4 mg kg-1) and propranolol (20 mg kg-1) but not by metoprolol (10 mg kg-1) or ICI 118,551 (5 mg kg-1), consistent with an involvement of 5-HT1A or 5-HT1B receptors. 5. The response was not altered by the selective 5-HT1A receptor antagonist, WAY 100135 (5 mg kg-1, s.c.), the 5-HT2A/5-HT2C receptor antagonist, ritanserin (0.1 mg kg-1), the selective 5-HT3 receptor antagonist, ondansetron (1 mg kg-1) or the non-selective 5-HT receptor antagonists methysergide (3 mg kg-1) and metergoline (3 mg kg-1). 6. Although spiroxatrine (0.1 mg kg-1) and ketanserin (1 mg kg-1) inhibited RU 24969-induced hyperlocomotion, these effects were probably due to antagonism of dopamine D2 receptors and alpha 1-adrenoceptors respectively. 7. Taken together, these results indicate that RU 24969-induced hyperlocomotion results specifically from activation of central 5-HTIB receptors.8. Lesioning of 5-HT neurones with 5,7-dihydroxytryptamine (75 microg, i.c.v.) or depletion with pchlorophenylalanine(200 mg kg-1, i.p. for 14 days) had no effect on RU 24969-induced hyperlocomotiondemonstrating that the 5-HTIB receptors involved are postsynaptic and that they do not show super sensitivity.9. The involvement of other monoamine neurotransmitter systems in RU 24969-induced hyperlocomotionwas also examined. The response was inhibited by the al-adrenoceptor antagonist, prazosin(1 mg kg-1), the dopamine DI receptor antagonist, SCH 23390 (0.05 mg kg-1) and the dopamine D2 receptor antagonist, BRL 34778 (0.03 mg kg-1), but not by the M2-adrenoceptor antagonist, idazoxan(1 mg kg-1). Lesioning noradrenergic neurones with N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine(100 mg kg-1) markedly attenuated this behaviour. These results show that the hyperlocomotion is expressed via noradrenergic and dopaminergic neurones acting on alpha 1-adrenoceptors, DI and D2 receptors.10. RU 24969 decreased brain concentrations of 5-hydroxyindoleacetic acid whilst simultaneously increasing 5-HT, consistent with the reduction of 5-HT neuronal activity by activation of 5-HTlA and 5-HTIB autoreceptors. RU 24969 increased brain 3-methoxy-4-hydroxyphenylglycol, but not noradrenaline, concentrations which supports the involvement of noradrenergic neurones in the expression of hyperlocomotion. RU 24969 did not alter dopamine, dihydroxyphenylacetic acid or homovanillic acid concentrations in the nucleus accumbens suggesting that the dopaminergic neurones terminating there are not directly involved.

Age-dependence of a 6-hydroxydopamine lesion on SKF 38393- and m-chlorophenylpiperazine-induced oral activity responses of rats

Neonatal 6-hydroxydopamine (6-OHDA) treatment is associated with destruction of dopamine (DA) fibers and subsequent sprouting of serotonin (5-HT) fibers in the striatum of rats. Enhanced oral activity responses to SKF 38393 and m-chlorophenylpiperazine (m-CPP), respective agonists for the DA D1 receptor complex and 5-HT2C receptor complex, ensue. To study the ontogenetic nature of this effect, rats were treated at birth, 3 days, 7 days, 10 days or 14 days with 6-OHDA-HBr (200 micrograms i.c.v.; salt form), following desipramine-HCl pretreatment (20 mg/kg i.p., 1 h; base form). Another group of rats was treated at 35 days and again at 42 days with 6-OHDA-HBr (300 micrograms i.c.v.), following desipramine-HCl (20 mg/kg i.p., 1 h) and pargyline-HCl (50 mg/kg i.p., 30 min). In rats treated from birth to 10 days, 6-OHDA reduced striatal DA content at 5 months by > or = 94%. Striatal 5-HT content was elevated by 28% to 51%, but only in rats treated with 6-OHDA at 7 days from birth or earlier. An enhanced oral activity response to SKF 38393-HCl (0.03 to 1.0 mg/kg i.p.) was absent in rats treated 7 days or later, and the change in SKF 38393 effect was correlated with a change in striatal DA content. An enhanced response to m-CPP.2HCl (0.3 to 6.0 mg/kg i.p.) was absent after treatment at 14 or 35 days, when striatal DA content was reduced only 44% to 63% and 5-HT content was not changed.(ABSTRACT TRUNCATED AT 250 WORDS)

Cardiovascular effects of intrathecal administration of agents active at 5-hydroxytryptamine1-receptors in the rat: modulation by substance P and a substance P antagonist

The intrathecal (i.th., T8-10) administration in conscious rats of the 5-hydroxytryptamine (5-HT)1A agonist 8-OH-DPAT (10 nmol), and to a lesser extent the 5-HT1B agonist CGS 12066B (6 nmol), resulted in a blood pressure reduction and a bradycardia. These responses were prevented by the i.th. pretreatment with substance P (SP) (6.5 nmol) and enhanced following pretreatment with the non-peptide SP antagonist CP-96,345 (10 nmol). The partial 5-HT1A agonist 8-MeO-CLEPAT (10 nmol) had by itself small cardiovascular effects. Following the pretreatment with SP, 8-MeO-CLEPAT caused a pressor response and a tachycardia whereas the opposite effects were observed following pretreatment with the SP antagonist. These results support the notion of a functional interaction between serotonergic and SP mechanisms at the level of the preganglionic sympathetic nerves in the spinal cord.

Withdrawal from continuous or intermittent cocaine: behavioral responsivity to 5-HT1 receptor agonists

Research on chronic cocaine administration indicates that both the dose and route of administration influences the effects of chronic cocaine. Rats were pretreated with 40 mg/kg/day cocaine for 14 days by either SC injections or osmotic minipumps. Rats were then withdrawn from the pretreatment regimen for 7 days and their behavior rated following injections of 5-hydroxytryptamine1A (5-HT1A) or 5-HT1B agonists. In Experiment 1, rats received 0- to 4.0-mg/kg IP injections of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a selective 5-HT1A receptor agonist. In Experiment 2, rats received 0- to 16.0-mg/kg IP injections of 7-trifluoromethyl-4(4-methyl-1-piperazinyl)- pyrrolo[1,2a]quinoxaline (CGS 12066B), a selective 5-HT1B receptor agonist. The results of Experiment 1 indicated that rats receiving cocaine via osmotic minipumps exhibited marked 5-HT1A receptor subsensitivity. In contrast, rats receiving daily cocaine injections sometimes demonstrated evidence of 5-HT1A supersensitivity and sometimes demonstrated evidence of 5-HT1A normosensitivity. The results of Experiment 2 indicated there were no consistent differences between the pretreatment groups in the behavioral response to CGS 12066B, although there were significant differences at single, isolated time points. Overall, the results indicate that, at least in the present behavioral paradigm, the effects of chronic cocaine administration are mediated by changes in 5-HT1A receptor sensitivity but not by changes in 5-HT1B receptor sensitivity.

Effects of 5-HT1A receptor agonists on patterns of rat motor activity in relation to effects on forebrain monoamine synthesis

The effects of the 5-HT1A receptor agonists 8-OH-DPAT (0.15-2.5 mumol kg-1 subcutaneously), flesinoxan (0.6-10.0 mumol kg-1 subcutaneously) and buspirone (1.9-30.0 mumol kg-1 subcutaneously) on spontaneous motor activity in male Sprague-Dawley rats was examined in a photocell-equipped open-field arena. Following motor activity observations, the cerebral aromatic L-amino acid decarboxylase inhibitor NSD-1015 (100 mg kg-1 intraperitoneally) was administered and 30 min. later the animals were decapitated for subsequent analysis of the accumulated forebrain DOPA and 5-HTP levels, as an estimate of the rate of monoamine synthesis. 8-OH-DPAT and flesinoxan produced a similar and characteristic pattern of changes of the spontaneous motor activity in normal animals i.e. a moderate decrease in locomotor activity, a marked suppression of rearing and an increase in the relative amount of forward locomotion and of activity in the periphery of the open-field arena. This behavioural profile was closely related to a decrease in forebrain 5-HTP accumulation, indicating 5-HT receptor stimulation. In agreement with these observations buspirone also produced an increase in peripheral activity and a suppression of rearing. In contrast to effects by 8-OH-DPAT and flesinoxan, however, buspirone produced a further reduction of locomotor activity and reduced the forward locomotion. This difference in behavioural profile between buspirone and the other two compounds is probably explained by its DA receptor blocking properties, as indicated by an increased DOPA accumulation in the neostriatum. At least partially, 8-OH-DPAT, flesinoxan and buspirone, all antagonized reserpine-induced (5 mg kg-1 subcutaneously--16 hr) suppression of locomotor activity. This stimulation of locomotor activity in reserpine-treated rats is in all probability related to 5-HT1A receptor stimulation since concomitant DA D2 receptor blockade, in the case of buspirone, did not markedly affect this behavioural response.

Serotonin and the mammalian circadian system: I. In vitro phase shifts by serotonergic agonists and antagonists

The primary mammalian circadian clock, located in the suprachiasmatic nuclei (SCN), receives a major input from the raphe nuclei. The role of this input is largely unknown, and is the focus of this research. The SCN clock survives in vitro, where it produces a 24-hr rhythm in spontaneous neuronal activity that is sustained for at least three cycles. The sensitivity of the SCN clock to drugs can therefore be tested in vitro by determining whether various compounds alter the phase of this rhythm. We have previously shown that the nonspecific serotonin (5-HT) agonist quipazine resets the SCN clock in vitro, inducing phase advances in the daytime and phase delays at night. These results suggest that the 5-HT-ergic input from the raphe nuclei can modulate the phase of the SCN circadian clock. In this study we began by using autoradiography to determine that the SCN contain abundant 5-HT1A and 5-HT1B receptors, very few 5-HT1C and 5-HT2 receptors, and no 5-HT3 receptors. Next we investigated the ability of 5-HT-ergic agonists and antagonists to reset the clock in vitro, in order to determine what type or types of 5-HT receptor(s) are functionally linked to the SCN clock. We began by providing further evidence of 5-HT-ergic effects in the SCN. We found that 5-HT mimicked the effects of quipazine, whereas the nonspecific 5-HT antagonist metergoline blocked these effects, in both the day and night. Next we found that the 5-HT1A agonist 8-OH-DPAT, and to a lesser extent the 5-HT1A-1B agonist RU 24969, mimicked the effects of quipazine during the subjective daytime, whereas the 5-HT1A antagonist NAN-190 blocked quipazine's effects. None of the other specific agonists or antagonists we tried induced similar effects. This suggests that quipazine acts on 5-HT1A receptors in the daytime to advance the SCN clock. None of the specific agents we tried were able either to mimic or to block the actions of 5-HT or quipazine at circadian time 15. Thus, we were unable to determine the type of 5-HT receptor involved in nighttime phase delays by quipazine or 5-HT. However, since the dose-response curves for quipazine during the day and night are virtually identical, we hypothesize that the nighttime 5-HT receptor is a 5-HT1-like receptor.

Possible antidepressant dihydroergosine preferentially binds to 5-HT1B receptor sites in the rat hippocampus

The binding affinity of a possible antidepressant drug, dihydroergosine, for various 5-HT1 receptor subtypes was studied in the hippocampal rat brain membranes. Dihydroergosine displaced the binding of [3H]5-HT to the whole population of hippocampal 5-HT1 receptors with high affinity (Ki = 4.8 nM). The displacement curve was shallow and the slope factor less than unity, suggesting the interaction of dihydroergosine with multiple binding sites. When 8-OH-DPAT (100 nM) + chlorpromazine (500 nM), CGS 12066 B (200 nM) + ritanserin (500 nM), and (+/-)pindolol (1 microM) were included to block 5-HT1A + 5-HT1C, 5-HT1B + 5-HT1C, and 5-HT1A + 5-HT1B receptor subtype respectively, the competition studies have shown that under these selective conditions dihydroergosine binds with the highest affinity for 5-HT1B (Ki = 0.48 nM), with 8.7 times lower affinity for 5-HT1A (Ki = 4.2 nM) and with a moderate affinity for 5-HT1C (Ki = 156 nM) receptor subtype. While our previous studies suggested that dihydroergosine stimulates 5-HT1A and inhibits 5-HT2 receptors, this study suggests that the high affinity of this drug for 5-HT1B receptors should not be neglected.

LSD has high efficacy relative to serotonin in enhancing the cationic current Ih: intracellular studies in rat facial motoneurons

The effects of LSD (d-lysergic acid diethylamide) on rat facial motoneurons were compared to those of 5-hydroxytryptamine (5-HT) in brain slices by means of current clamp and single-electrode voltage-clamp recordings. As previously reported, 5-HT, in part by decreasing a resting potassium conductance, produced a reversible depolarization (approximately 5 mV), an increase in input resistance, and an enhancement in electrical excitability. LSD also produced an increase in electrical excitability, although with a much slower onset and longer duration. However, in contrast to 5-HT, LSD produced only a slight depolarization (1-2 mV). Moreover, in the presence of LSD the depolarizing effect of 5-HT was markedly attenuated. The 5-HT2/5-HT1C agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) produced effects intermediate between LSD and 5-HT. The LSD-induced increase in electrical excitability was completely reversed by spiperone, a 5-HT2/5-HT1A antagonist, and by ritanserin, a 5-HT2/5-HT1C antagonist; the effects of 5-HT were also reduced by these 2 antagonists, but complete blockade did not occur at the concentrations and durations tested. Surprisingly, LSD was found to enhance the hyperpolarization-activated nonspecific cation current Ih to a greater extent than did 5-HT; this enhancement was blocked by both spiperone and ritanserin. These results indicate that, despite having low efficacy relative to 5-HT in decreasing resting potassium conductance, LSD has high efficacy in enhancing the Ih current in rat facial motoneurons; possible mechanisms for this difference are discussed.

Serotonin 1B receptors in the developing somatosensory and visual cortices are located on thalamocortical axons

Serotonin (5-HT)-immunoreactive axons are densely distributed in the primary visual and somatosensory cortices of rats, mice, and hamsters for the first 2 weeks of life, and a recent study from this laboratory has demonstrated that 5-HT1B receptors assume a pattern that exactly matches that of the serotoninergic axons. The differential distribution of these receptors is also transient. In the present study, we combined receptor binding autoradiography with neurochemical ablation of 5-HT axons or electrolytic lesions of the dorsal thalamus in an effort to determine the neural elements upon which the 5-HT1B receptors were located. Subcutaneous injections of the toxin 5,7-dihydroxytryptamine, made on the day of birth, totally eliminated the dense and patterned 5-HT innervation of the somatosensory and striate cortices of rats killed on postnatal day 8 but had no qualitative effect upon the distribution or density of 5-HT1B receptors in either of these cortical regions in animals killed at the same age. Conversely, electrolytic lesions of the dorsal thalamus made on postnatal day 6 resulted in a complete loss of the dense and patterned distribution of 5-HT1B receptors in rats killed on postnatal day 8. These results indicate that thalamocortical axons transiently express 5-HT1B receptors.

Noradrenergic mediation of spinal antinociception by 5-hydroxytryptamine: characterization of receptor subtypes

The present study examined the involvement of spinal noradrenergic mechanisms in spinal antinociception by the 5-hydroxy-tryptamine (5-HT) receptor-selective agonists CGS 12066B (5-HT1B; 7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a]quinoxaline), TFMPP (5-HT1C; M-trifluoromethylphenyl-piperazine) and DOI (5-HT2; 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane) using the rat hot plate test. Effects of alpha-adrenoreceptor antagonists (phentolamine, yohimbine), the adrenergic neurotoxin 6-hydroxydopamine, and the selective noradrenergic uptake blocker desipramine were determined. CGS 12066B, TFMPP and DOI produced dose-related antinociception. The antinociceptive effect of each agent was reduced by pretreatment with both phentolamine and yohimbine (15-60 micrograms). Pretreatment with 6-hydroxydopamine (100 micrograms, intrathecal) for 7-10 days, which reduced spinal cord levels of noradrenaline by 87%, inhibited the action of TFMPP (and 5-HT), but not CGS 12066B or DOI. Pretreatment with desipramine (25 mg/kg, systemic) potentiated the action of TFMPP but not CGS 12066B or DOI (or 2-methyl-5-HT). These results suggest that antinociception by TFMPP is dependent on release of endogenous noradrenaline from the spinal cord, while that produced by CGS 12066B and DOI is not. As TFMPP exhibits a close similarity to 5-HT in these experiments, the 5-HT receptor subtype being activated to induce noradrenaline release may either be a 5-HT1C or a 5-HT1S subtype. Other mechanisms account for the observed blockade of the action of CGS 12066B and DOI by alpha-adrenoreceptor antagonists.

Mechanistic investigation of the stimulus properties of 1-(3-trifluoromethylphenyl)piperazine

Using a standard two-lever operant procedure with rats trained to discriminate 1-(3-trifluoromethylphenyl)piperazine (TFMPP) (0.5 mg/kg) from saline, tests of stimulus antagonism and stimulus generalization were performed to better understand the stimulus properties of this agent. The agents examined for ability to antagonize the TFMPP stimulus were prazosin, quipazine, zacopride, buspirone, 8-hydroxy-2-(di-N-propylamino) tetralin (8-OH-DPAT), 1-(2-methoxyphenol)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190), haloperidol, and 1-(2-pyrimidinyl)piperazine (1-PP); only buspirone attenuated the response to TF-MPP. In separate experiments, the lowest nondisrupting dose of buspirone (1.2 mg/kg) caused a rightward shift of the TFMPP dose-response curve (TFMPP alone, ED50 = 0.19 mg/kg; TFMPP + buspirone, ED50 = 0.43 mg/kg). In addition, 3-(1,2,5,6-tetrahydropyrid-4-yl)pyrrolo[3,2-b]pyrid-5-one (CP 93, 129), 7-trifluoromethyl-4-(4-methyl-1-piperazinyl)pyrolo[1,2-a]quinox ali ne (CGS 12066B), 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), 3-chlorophenylbiguanide (mCPBG), NAN-190, nisoxetine, zacopride, 1-PP, (+)-N-allylnormetazocine ((+)-NANM), and N-methyl-1-(3,4-methylenedioxyphenyl)-2-aminopropane (MDMA) were analyzed in tests of stimulus generalization. The TFMPP stimulus generalized only to CGS 12066B (ED50 = 4.2 mg/kg) and (+)-NANM (ED50 = 8.8 mg/kg). Tests with DOI and MDMA resulted in partial generalization. Up to doses that disrupted behavior, all other agents had little effect on TFMPP-appropriate responding. The results of these and other published studies suggest roles for 5-hydroxytryptamine 1B (5-HT1B), 5-HT1C, and, possibly, sigma-receptors in the mediation of the TFMPP stimulus and indicate a lack of involvement of 5-HT1A, 5-HT2, dopaminergic, and adrenergic mechanisms in this behavior.

A review and reevaluation of the role of serotonin in the modulation of lordosis behavior in the female rat

The role of serotonin (5-HT) in the modulation of sexual receptivity (lordosis) in the female rat is reviewed and reevaluated. The effects on lordosis of drug treatments that decrease or increase the activity and availability of central 5-HT are first discussed, and this is followed by an evaluation of the effects of drugs that act directly at 5-HT receptors. In order to shed light on the physiological significance of effects of serotonergic drugs on lordosis, there is also a review of what is known of changes in levels of serotonergic activity and densities of 5-HT receptors in the female rat brain that take place through the estrous cycle and in response to administration of behaviorally effective doses of gonadal steroids. Serotonin has generally been thought to have a tonic, inhibitory effect on lordosis. However, it is concluded that 5-HT can either inhibit or facilitate lordosis depending on which subtypes of central 5-HT receptors become activated. Because of a lack of consistent or compelling evidence of effects of ovarian hormones on serotonergic activity or 5-HT receptors in critical areas of the brain, it is stated that there is at present no basis to conclude that the effects of pharmacological manipulations of serotonergic activity on lordosis reflect an important, physiological role of 5-HT in the modulation of lordosis behavior in the female rat.

Molecular structural basis of ligand selectivity for 5-HT2 versus 5-HT1C cortical receptors

A molecular structural criterion of ligand selectivity for the 5-HT2 versus 5-HT1C receptor was hypothesized on the basis of radioligand binding data. Despite the large number of compounds which have been tested at both receptors, analysis of published data led to the identification of only five agents which are greater than 10-fold selective for the 5-HT2 versus the 5-HT1C receptor. Comparison of the two-dimensional structures revealed that, although these five compounds represent three distinct structural classes, they share a common structural feature located in the region hypothesized to be involved in receptor binding: a carbonyl or carboxyl oxygen interposed spatially between an aromatic ring and nitrogen atom. This structural feature was used to predict the relative selectivity of compounds that had not previously been analyzed at both the 5-HT2 and 5-HT1C receptors. All six drugs tested which contain the identified reactive carbonyl or carboxyl group were found to be selective for the 5-HT2 versus the 5-HT1C receptor with selectivity ratios ranging from 26 to 380. By contrast, three agents which are structurally similar but do not contain the reactive carbonyl or carboxyl group displayed equally high affinity for both receptor binding sites. Since the physiological roles of the 5-HT2 and 5-HT1C receptor are markedly different, it would be of potential clinical and scientific value to utilize this molecular structural feature to further identify chemical compounds which would selectively interact with only one of the two receptors.

The influence of serotoninergic drugs on dopaminergic neurotransmission in rat substantia nigra, striatum and limbic forebrain in vivo

The effects of serotoninergic drugs on dopaminergic neurotransmission in the substantia nigra, the striatum and the limbic forebrain of rat have been investigated. The accumulation of 3-methoxytyramine (3-MT) following inhibition of monoamine oxidase with pargyline was used as an indirect measure of dopamine (DA) activity in vivo. The effects of the following serotoninergic drugs were tested: the 5-HT1A receptor agonist 8-OH-DPAT, the 5-HT1B receptor agonist trifluoromethyl-phenylpiperazine (TFMPP), CGS 12066 B and RU 24969, the 5-HT1A/1B antagonist (+/-)pindolol, the 5-HT2/1C receptor antagonist ritanserin, the 5-HT2/1C receptor agonist DL-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI), the 5-HT3 receptor antagonist BRL 43694, the unselective 5-HT receptor antagonist methiothepin, and carbidopa + L-5-hydroxytryptophan (L-5-HTP) to achieve a general, unselective stimulation of multiple 5-HT receptors. In the substantia nigra, carbidopa + 5-HTP treatment increased the 3-MT accumulation by 26% and decreased the DA concentration to 67% of controls, tentatively suggesting a 5-HTP-induced displacement of nigral DA. A minor, non dose-related reduction in nigral 3-MT was seen after the 5-HT1A receptor agonist 8-OH-DPAT. None of the other serotonin receptor acting drugs induced any pronounced effect on the nigral 3-MT accumulation. Taken together, the findings provide little support for the idea that one single 5-HT receptor subtype serves a modulatory function on DA activity in the substantia nigra. In the striatum and the limbic forebrain, trifluoromethyl-phenylpiperazine dose-dependently increased the 3-MT accumulation to maximally 200%-220% of controls.(ABSTRACT TRUNCATED AT 250 WORDS)

Serotonin is a directly-acting hyperalgesic agent in the rat

In this study, we have investigated serotonin hyperalgesia employing the mechanical paw withdrawal nociceptive threshold test in the rat. Intradermally injected serotonin was found to produce a dose-dependent hyperalgesia that was not attenuated by procedures which eliminate the known indirect mechanisms of hyperalgesia such as sympathectomy, polymorphonuclear leukocyte depletion or cyclooxygenase inhibition. In addition, the latency to onset of serotonin hyperalgesia is extremely short, with maximal hyperalgesia observed in less than 1 min, a similar temporal onset to direct-acting hyperalgesic agents such as prostaglandin E2. The results suggest, therefore, that the hyperalgesic effects of serotonin in our animal model are exerted by direct action on primary afferent neurons. Only the intradermal injection of selective serotonin (5-hydroxytryptamine; 5-HT) agonists for the 1A receptor subset (5-HT1A), (+/-)-2-dipropylamino-8-hydroxy-1,2,3,4-tetrahydronaphthaline hydrobromide and N,N-dipropyl-5-carboxamido-tryptamine maleate, produced dose-dependent hyperalgesia. No hyperalgesia was seen after 5-HT1B, CGS-12066B maleate and m-trifluoromethylphenyl-piperazine hydrochloride; 5-HT2+IC, alpha methyl 5HT and (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl; or 5-HT3, 2-methyl-5-hydroxytryptamine maleate and phenylbiguanide, agonists. Similarly, only the 5-HT1A antagonists, spiroxatrine and spiperone, attenuated the hyperalgesia induced by intradermally injected serotonin. 5-HT2+IC antagonists, mesulergine and ketanserin, and 5-HT3 antagonists, quipazine and 3-tropanyl-indole-3-carboxylate, did not significantly attenuate 5-HT hyperalgesia. We conclude that serotonin produces hyperalgesia by a direct action on the primary afferent neuron via the 5-HT1A subset of serotonin receptors.

Characterization of [3H]CP-96,501 as a selective radioligand for the serotonin 5-HT1B receptor: binding studies in rat brain membranes

3-(1,2,5,6-Tetrahydro-4-pyridyl)-5-n-propoxyindole (CP-96,501) was found to be more selective ligand at the serotonin 5-HT1B receptor than the commonly used 5-HT1B agonist, 3-(1,2,5,6-tetrahydro-4-pyridyl)-5-methoxyindole (RU 24969). In rat brain membranes, the tritiated derivative, [3H]CP-96,501, was found to bind with a high affinity (KD, 0.21 nM) to a single binding site (nH, 1.0). The receptor density of this site (Bmax, 72 fmol/mg of protein) matched that of the 5-HT1B receptor determined with [3H]5-HT. Competition curves of 16 serotonergic compounds in [3H]CP-96,501 binding also indicated a single binding site. The rank order of their binding affinities with this new radioligand showed a high degree of correlation with their affinities at the 5-HT1B receptor determined with [3H]5-HT or [125I]iodocyanopindolol. Serotonergic compounds displayed competitive inhibition of [3H]CP-96,501 binding. In the presence of 5'-guanylylimidodiphosphate [Gpp(NH)p], [3H]CP-96,501 to displace [125I]iodocyanopindolol binding was also decreased. These findings are consistent with the agonist nature of CP-96,501. The results of this study suggest that [3H]CP-96,501 is a useful agonist radioligand for the 5-HT1B receptor.

Centrally active 5-HT receptor agonists and antagonists

Eleven subtypes of central 5-HT receptor have so far been postulated, four of which have been cloned (5-HT1A, 5-HT1C, 5-HT1D and 5-HT2) and a fifth (the 5-HT3 receptor) purified. The present review discusses the agonists and antagonists which act at these subtypes with respect to their degree of selectivity and in vivo potency. Selective agonists exist for the 5-HT1A, 5-HT1B and 5-HT3 receptors and selective antagonists for the 5-HT2 and 5-HT3 receptors.

Serotonin-norepinephrine interactions: a voltammetric study on the effect of serotonin receptor stimulation followed in the N. raphe dorsalis and the Locus coeruleus of the rat

In vivo voltammetry with carbon fibre electrodes was used to study the effect of the serotoninergic (5-HT) neuronal system on the noradrenergic (NE) system in the Locus coeruleus of the rat. The voltammetric DOPAC signal in the Locus coeruleus, used as a measure of NE neuronal activity, was increased after systemic application of the 5-HT1B agonist CGS-12066B, the 5-HT2 antagonist ritanserin, and, to a lesser extent, by ipsapirone, a 5-HT1A agonist. The findings suggest that the NE neuronal system of the Locus coeruleus is stimulated by 5-HT1A and 5-HT1B receptor activation and inhibited by 5-HT2 receptors. Likewise the 5-HT releaser and uptake inhibitor fenfluramine increased the DOPAC level in the Locus coeruleus. In contrast to the 5-HT1 agonists, which reduced 5-hydroxyindoleacetic acid (5-HIAA) in the Nucleus raphe dorsalis, ritanserin increased the 5-HIAA signal in this nucleus. This finding could help to explain the action of ritanserin as sleep-modulating substance.

Effect of serotonergic agonists in the nucleus accumbens on d-amphetamine-stimulated locomotion

Serotonergic projections from the raphe nuclei are thought to modulate locomotor activity in the rat, and serotonin injection into the nucleus accumbens attenuates the hypermotility elicited by amphetamine. The purpose of the present study was to characterize the effects of various classes of serotonergic agonists administered into the nucleus accumbens on d-amphetamine-stimulated locomotor activity in order to determine which serotonin receptor subtypes are involved. Administration of the nonselective 5-HT agonist quipazine, the 5-HT-1 agonist mCPP, the 5-HT-1a agonist 8-OH-DPAT, the 5-HT-1b agonist CGS-12066B, and the 5HT-1c/2 agonist DOI did not inhibit d-amphetamine-stimulated locomotor activity. Pronounced lateral head weaving was noted after 8-OH-DPAT administration. The combination of the 5-HT-1a agonist 8-OH-DPAT and the 5-HT-1b agonist CGS-12066B, however, did inhibit d-amphetamine-stimulated locomotor activity. In contrast, the 5-HT-3 agonist 1-phenylbiguanide enhanced the locomotor effect of d-amphetamine. This effect was partially reversed by the 5-HT-3 antagonist MDL-7222. These studies suggest that serotonin has complex and multiple effects on the regulation of locomotor activity within the nucleus accumbens.

Effect of 5-HT1A receptor agonists in two models of anxiety after dorsal raphe injection

The purpose of the present study was two-fold. Firstly, to present a more comprehensive analysis of the disinhibitory effects of 5-HT1A receptor agonists after discrete dorsal raphe (DRN) injections (Higgins et al. 1988). Secondly, the effects of the 5-HT1B receptor agonist CGS12066B and the 5-HT1B/1C agonist mCPP were examined following injection into this nucleus. The increases in social interaction (SI) induced by intra-raphe injections of 8-OH DPAT (0.02-1 micrograms), buspirone (0.04-0.2 microgram), ipsapirone (0.2 microgram) and gepirone (0.2-1 micrograms) under a high light unfamiliar paradigm (HLU) were typically due to increased bout frequency, duration and a higher incidence of sniff, follow, allogroom behaviour. These increases were qualitatively similar to those seen in control animals tested under low light/familiar (LLF) conditions, thus supporting the belief that the drug-induced increases in SI reflected decreases in anxiety. Furthermore, at doses effective under the HLU condition, 8-OH DPAT, buspirone and gepirone failed to modify SI under conditions of minimal suppression (LLF paradigm). At doses which significantly increased punished responding in a water-lick conflict test 8-OH DPAT, ipsapirone and gepirone tended to also increase unpunished rates of drinking. However, in drug untreated rats, prior habituation to the test apparatus also increased unpunished drinking, suggesting some neophobia-induced suppression. At a comparatively high dose, the 5-HT1B agonist CGS12066B (2.5 micrograms), but not the putative 5-HT1B/1C agonist mCPP (0.5-12.5 micrograms), increased SI under the HLU condition.(ABSTRACT TRUNCATED AT 250 WORDS)

Differential effects of novel ligands for 5-HT receptor subtypes on nonopioid defensive analgesia in male mice

The effects of a number of 5-HT receptor ligands were examined on nonopioid defensive analgesia in male DBA/2 mice. MDL 73005EF (0.05-1.0 mg/kg), a selective 5-HT1A receptor agonist, potently and dose-dependently inhibited the analgesic consequences of social defeat. CGS 12066B (0.5-10.0 mg/kg) and MK-212 (0.3-10.0 mg/kg), selective agonists for 5-HT1B and 5-HT1C sites, respectively, failed to influence this particular form of adaptive pain inhibition. Two 5-HT2/1C receptor antagonists, ritanserin (0.05-10.0 mg/kg) and ICI 169.369 (0.3-10.0 mg/kg), were also devoid of specific effects upon defensive analgesia. Both ritanserin and ICI 169,369 were found to have intrinsic analgetic efficacy and to induce behavioural changes indicative of increased defensiveness. These data, together with previous findings, confirm the specific involvement of 5-HT1A receptor mechanisms in the analgesic consequences of social defeat in male mice. Results are discussed in relation to the role of anxiety in adaptive pain inhibition.

Isolation increases a behavioral response to the selective 5-HT 1B agonist CGS 120 66B

The effect of two serotonergic drugs, CGS 120 66B acting specifically and TFMPP acting preferentially onto 5-HT1B receptors, was compared in preisolated and in pregrouped mice. Two mice put under an inverted beaker attempt to escape. The number of escape attempts of mice preisolated for 7 days was half that of pregrouped mice. In preisolated mice, TFMPP and CGS 120 66B increased the number of escape attempts up to, respectively, 200% and 300% of that of preisolated control mice. In pregrouped mice, CGS 120 66B was nearly inactive and TFMPP exerts a smaller effect. These results suggest that isolation increases the apparent responsiveness to 5-HT1B stimulants.

A new 5-hydroxy-indole derivative with preferential affinity for 5-HT1B binding sites

The affinities of several 5-hydroxy-indole derivatives for serotonin-1 (5-HT1) binding site subtypes, labeled with 2 nM [3H]5-HT, were assessed by quantitative autoradiography on rat brain sections. The results obtained with known ligands, namely 5-hydroxytryptamine (5-HT), 5-methoxytryptamine (5-Me-OT), 5-methoxy-N,N- dimethyl-tryptamine (5-Me-ODMT), 5-hydroxy-N,N-dimethyl-tryptamine (bufotenine) and 8-hydroxy-2-[di-N-propylamino]tetralin (8-OH-DPAT) demonstrate the reliability and the advantages of this technique for pharmacological studies. Novel serotonin derivatives were synthesized by carboxymethylation of the hydroxyl group. One of those new ligands, serotonin-O-carboxy-methyl- glycyl-tyrosinamide (S-CM-GTNH2), inhibited 2 nM [3H]5-HT binding to the substantia nigra with an IC50 of 22.4 nM, a value which is 22 times lower than that found in the dentate gyrus and choroid plexus. This demonstrates the preferential affinity of S-CM-GTNH2 for 5-HT1B versus 5-HT1A and 5-HT1C binding sites. S-CM-GTNH2 contains a tyrosine residue, which may be useful for the synthesis of a radioactive iodinated molecule and for the preparation of 'long-lasting ligands' linked through peptide bonds with a protein. These derivatives could be of great interest for ultrastructural and behavioral studies relevant to 5-HT1B sites.

Is there a relationship between 5-HT1B receptors and the mechanisms of action of antidepressant drugs in the learned helplessness paradigm in rats?

The present study was undertaken to investigate, in the learned helplessness paradigm, (1) the effects of CGS 12066B, a selective 5-HT1B receptor agonist administered alone, and (2) the effects of CGS 12066B co-administered with tricyclics (clomipramine, imipramine) or 5-HT re-uptake blockers (citalopram, fluvoxamine). The results showed that (1) the selective 5-HT1B agonist failed to reverse the escape deficit, and (2) in the drug interaction studies, CGS 12066B did not modify the ability of tricyclics to reverse the escape deficit. The 5-HT1B agonist, however, did significantly reduce the ability of 5-HT re-uptake blockers to reverse the escape deficit. These results suggest that modulation of 5-HT1B sites might modify the enhancement of 5-HT neurotransmission by antidepressants, particularly 5-HT re-uptake blockers.

Serotonergic modulation of the rat pup ultrasonic isolation call: studies with 5HT1 and 5HT2 subtype-selective agonists and antagonists

A modulatory role for serotonin has been described for the development and expression of the ultrasonic call of infant rat pups during brief maternal separations. In previous studies, serotonin reuptake inhibitors selectively reduced the rate of calling following acute administration to 9-11-day-old pups and a serotonin neurotoxin (MDMA) systematically disrupted the development of ultrasonic vocalizations but not other measures of motor development. In the current studies, we extended our investigations to include drugs with purported receptor subtype selectivities. Consistent with previous reports, acute administration of 5HT1A agonists buspirone and 8-OH-DPAT [+/-)-8-hydroxy-2-(di-N-propylamino)tetralin) reduced the rate of calling at doses which did not affect motor activity or core body temperature. The rate reducing effects of buspirone persisted up to 1 but not 2 h after injection. Administration of purported 5HT1B receptor agonists, CGS12066B (7-trifluoromethyl-4(4-methyl-1-piperazinyl)-pyrrolo[1,2-a] quinoxaline) and TFMPP (1-[3-fluoromethyl)phenyl]-piperazine) increased the rate of calling depending on the specificity of the drug for the 5HT1B receptor. d,l-Propranolol, a 5HT1 receptor antagonist, blocked the effects of both 8-OH-DPAT and TFMPP. m-CPP (1-(3-chlorophenyl)piperazine) and DOI [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane), drugs with putative actions at 5HT1C and 5HT2 receptor sites both decreased calling but differed according to their effects on motor activity. Ritanserin, a 5HT2 and 5HT1C antagonist, produced a dose-related increase in call rate. A dose of ritanserin with no apparent intrinsic effects effectively antagonized DOI rate reducing effects but potentiated the rate reducing effects of m-CPP.(ABSTRACT TRUNCATED AT 250 WORDS)