5-HT7 receptor-mediated dilatation in the middle meningeal artery of anesthetized rats

Topical administration of 5-carboxamidotryptamine (5-CT; 0.01-1000 microM) to the exposed dura mater encephali of anesthetized rats produced decreases in blood pressure and dilatation in the middle meningeal artery. Pretreatment with the 5-HT(1B/1D) receptor antagonist, N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl) [1,1-biphenyl]-4-carboxamide hydrochloride monohydrate (GR-127935; 1 mg/kg, i.v.), unmasked meningeal dilator responses to lower concentrations of 5-CT, and attenuated those to higher concentrations; GR-127935 also inhibited 5-CT-induced hypotension. The 5-HT7 receptor antagonist, (R)-1-{(3-hydroxyphenyl)sulfonyl}-2-{2-(2-(4-methyl-1-piperidinyl) ethyl} pyrrolidine (SB-269970; 1 mg/kg, i.v.), strongly inhibited dilator and hypotensive responses to 5-CT; the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) further inhibited meningeal and hypotensive responses. Thus, 5-CT may produce dilatation in the middle meningeal artery via 5-HT7 receptors; complex effects appear to involve 5-HT(1B/1D) receptors.     

The effect of SB-269970, a 5-HT(7) receptor antagonist, on 5-HT release from serotonergic terminals and cell bodies

1. The presence of 5-HT(7) receptor mRNA and protein in 5-HT neurons suggests that this receptor may act as a 5-HT autoreceptor. In this study, the effect of the 5-HT(7) receptor antagonist, SB-269970 ((R)-1-[3-hydroxy phenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine), was investigated on 5-HT release in the guinea-pig and rat cortex and the rat dorsal raphe nucleus (DRN), using the techniques of in vitro [(3)H]-5-HT release or fast cyclic voltammetry, respectively. 2. Cortical slices were loaded with [(3)H]-5-HT and release was evoked by electrical stimulation. 5-CT inhibited the evoked release of [(3)H]-5-HT in a concentration-dependent manner. SB-269970 had no significant effect on [(3)H]-5-HT release while the 5-HT(1B) receptor antagonist, SB-224289 significantly potentiated [(3)H]-5-HT release. In addition, SB-269970 was unable to attenuate the 5-CT-induced inhibition of release while SB-224289 produced a rightward shift of the 5-CT response, generating estimated pK(B) values of 7.8 and 7.6 at the guinea-pig and rat terminal 5-HT autoreceptors respectively. 3. Rat DRN slices were electrically stimulated and the evoked 5-HT efflux detected by voltammetric analysis. 8-OH-DPAT inhibited evoked 5-HT efflux and was fully reversed by WAY 100635. SB-269970 had no effect on either 5-HT efflux per se or 8-OH-DPAT-induced inhibition of 5-HT efflux. In addition, 5-CT inhibited 5-HT efflux in a concentration-dependent manner. SB-269970 was unable to attenuate the 5-CT-induced inhibition of 5-HT efflux. 4. In conclusion, we were unable to provide evidence to suggest a 5-HT autoreceptor role for 5-HT(7) receptors. However, investigations with more selective 5-HT(7) receptor agonists are needed to confirm the data reported here.     

The hypothermic effect of 5-CT in mice is mediated through the 5-HT7 receptor

The 5-HT(7) receptor is a recent addition to the 5-HT receptor family and to date there is no clear idea as to its potential role in the CNS. The receptor has been mapped by in situ hybridization and 5-HT(7)-like immunoreactivity and has been detected in discrete areas of the brain including the hypothalamus (Oliver et al., 1999). This suggests the receptor may be involved in temperature regulation and have shown that a selective 5-HT(7) receptor antagonist reverses the hypothermic effect of 5-CT in guinea-pigs. The current study confirmed that the 5-HT(7) receptor antagonists, SB-269970 (1-30 mg/kg, i.p.) and SB-258719 (5-20 mg/kg, i.p.), but not the 5-HT(1A) receptor antagonist, WAY 100635(0.1-1 mg/kg, s.c.), or the 5-HT(1B/D) antagonist, GR127935 (1.25-5 mg/kg, i.p.), reversed the hypothermic effect of 5-CT in mice. In addition the effect of 5-CT on body temperature was examined on 5-HT(7) receptor null mutant mice. 5-CT (0.1-1 mg/kg, i.p.) significantly reduced rectal temperature in wildtype but not 5-HT(7) receptor knockout mice. This suggests that the hypothermic effects of 5-CT are mediated through the 5-HT(7) receptor. All procedures were carried out in accordance with the UK Animals (Scientific Procedures) Act (1986).     

SB-656104-A,a novel selective 5-HT7 receptor antagonist,modulates REM sleep in rats

1 (6-((R)-2-[2-[4-(4-Chloro-phenoxy)-piperidin-1-yl]-ethyl]-pyrrolidine-1-sulphonyl)-1H-indole hydrochloride) (SB-656104-A), a novel 5-hydroxytryptamine (5-HT(7)) receptor antagonist, potently inhibited [(3)H]-SB-269970 binding to the human cloned 5-HT(7(a)) (pK(i) 8.7+/-0.1) and 5-HT(7(b)) (pK(i) 8.5+/-0.2) receptor variants and the rat native receptor (pK(i) 8.8+/-0.2). The compound displayed at least 30-fold selectivity for the human 5-HT(7(a)) receptor versus other human cloned 5-HT receptors apart from the 5-HT(1D) receptor ( approximately 10-fold selective). 2 SB-656104-A antagonised competitively the 5-carboxamidotryptamine (5-CT)-induced accumulation of cyclic AMP in h5-HT(7(a))/HEK293 cells with a pA(2) of 8.5. 3 Following a constant rate iv infusion to steady state in rats, SB-656104 had a blood clearance (CL(b)) of 58+/-6 ml min(-1) kg(-1) and was CNS penetrant with a steady-state brain : blood ratio of 0.9 : 1. Following i.p. administration to rats (10 mg kg(-1)), the compound displayed a t(1/2) of 1.4 h with mean brain and blood concentrations (at 1 h after dosing) of 0.80 and 1.0 micro M, respectively. 4 SB-656104-A produced a significant reversal of the 5-CT-induced hypothermic effect in guinea pigs, a pharmacodynamic model of 5-HT(7) receptor interaction in vivo (ED(50) 2 mg kg(-1)). 5 SB-656104-A, administered to rats at the beginning of the sleep period (CT 0), significantly increased the latency to onset of rapid eye movement (REM) sleep at 30 mg kg(-1) i.p. (+93%) and reduced the total amount of REM sleep at 10 and 30 mg kg(-1) i.p. with no significant effect on the latency to, or amount of, non-REM sleep. SB-269970-A produced qualitatively similar effects in the same study. 6 In summary, SB-656104-A is a novel 5-HT(7) receptor antagonist which has been utilised in the present study to provide further evidence for a role for 5-HT(7) receptors in the modulation of REM sleep.     

SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), a novel 5-ht5A receptor-selective antagonist, enhances 5-HT neuronal function: Evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain

This study utilised the selective 5-ht(5A) receptor antagonist, SB-699551-A (3-cyclopentyl-N-[2-(dimethylamino)ethyl]-N-[(4'-{[(2-phenylethyl)amino]methyl}-4-biphenylyl)methyl]propanamide dihydrochloride), to investigate 5-ht5A receptor function in guinea pig brain. SB-699551-A competitively antagonised 5-HT-stimulated [35S]GTPgammaS binding to membranes from human embryonic kidney (HEK293) cells transiently expressing the guinea pig 5-ht5A receptor (pA2 8.1+/-0.1) and displayed 100-fold selectivity versus the serotonin transporter and those 5-HT receptor subtypes (5-HT(1A/B/D), 5-HT2A/C and 5-HT7) reported to modulate central 5-HT neurotransmission in the guinea pig. In guinea pig dorsal raphe slices, SB-699551-A (1 microM) did not alter neuronal firing per se but attenuated the 5-CT-induced depression in serotonergic neuronal firing in a subpopulation of cells insensitive to the 5-HT1A receptor-selective antagonist WAY-100635 (100 nM). In contrast, SB-699551-A (100 or 300 nM) failed to affect both electrically-evoked 5-HT release and 5-CT-induced inhibition of evoked release measured using fast cyclic voltammetry in vitro. SB-699551-A (0.3, 1 and 3 mg/kg s.c.) did not modulate extracellular levels of 5-HT in the guinea pig frontal cortex in vivo. However, when administered in combination with WAY-100635 (0.3 mg/kg s.c.), SB-699551-A (0.3, 1 or 3 mg/kg s.c.) produced a significant increase in extracellular 5-HT levels. These studies provide evidence for an autoreceptor role for the 5-ht5A receptor in guinea pig brain.     

Characterization of SB-269970-A, a selective 5-HT(7) receptor antagonist

The novel 5-HT(7) receptor antagonist, SB-269970-A, potently displaced [(3)H]-5-CT from human 5-HT(7(a)) (pK(i) 8.9+/-0.1) and 5-HT(7) receptors in guinea-pig cortex (pK(i) 8.3+/-0.2). 5-CT stimulated adenylyl cyclase activity in 5-HT(7(a))/HEK293 membranes (pEC(50) 7.5+/-0.1) and SB-269970-A (0.03 - 1 microM) inhibited the 5-CT concentration-response with no significant alteration in the maximal response. The pA(2) (8.5+/-0.2) for SB-269970-A agreed well with the pK(i) determined from [(3)H]-5-CT binding studies. 5-CT-stimulated adenylyl cyclase activity in guinea-pig hippocampal membranes (pEC(50) of 8.4+/-0.2) was inhibited by SB-269970-A (0.3 microM) with a pK(B) (8.3+/-0.1) in good agreement with its antagonist potency at the human cloned 5-HT(7(a)) receptor and its binding affinity at guinea-pig cortical membranes. 5-HT(7) receptor mRNA was highly expressed in human hypothalamus, amygdala, thalamus, hippocampus and testis. SB-269970-A was CNS penetrant (steady-state brain : blood ratio of ca. 0.83 : 1 in rats) but was rapidly cleared from the blood (CLb=ca. 140 ml min(-1) kg(-1)). Following a single dose (3 mg kg(-1)) SB-269970 was detectable in rat brain at 30 (87 nM) and 60 min (58 nM). In guinea-pigs, brain levels averaged 31 and 51 nM respectively at 30 and 60 min after dosing, although the compound was undetectable in one of the three animals tested. 5-CT (0.3 mg kg(-1) i.p.) induced hypothermia in guinea-pigs was blocked by SB-269970-A (ED(50) 2.96 mg kg(-1) i.p.) and the non-selective 5-HT(7) receptor antagonist metergoline (0.3 - 3 mg kg(-1) s.c.), suggesting a role for 5-HT(7) receptor stimulation in 5-CT induced hypothermia in guinea-pigs. SB-269970-A (30 mg kg(-1)) administered at the start of the sleep period, significantly reduced time spent in Paradoxical Sleep (PS) during the first 3 h of EEG recording in conscious rats.     

Influence of serotonin 5-HT(7) receptor blockade on the behavioral and neurochemical effects of imipramine in rats

The aim of the present study was to examine the effect of the selective 5-HT(7) receptor antagonist (2R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrrolidine (SB-269970), administered alone or in combination with imipramine, on the immobility time of rats in the forced swim test as well as on the extracellular levels of dopamine (DA), noradrenaline (NA), serotonin (5-HT) and their metabolites in the prefrontal cortex of freely moving rats. Both compounds were administered intraperitoneally (ip). Like imipramine (30 mg/kg, but not 20 mg/kg), SB-269970 (1.25 and 2.5 mg/kg, but not 0.625 mg/kg) significantly shortened the immobility time of rats without affecting their exploratory locomotor activity measured in the open field test. SB-269970 (0.625 and 1.25 mg/kg) raised the extracellular levels of DA, NA, 5-HT and their metabolites in rat prefrontal cortex. In that structure, imipramine (20 mg/kg) produced an increase in all the neurotransmitters measured, but failed to affect the levels of their metabolites. A combination of the inactive doses of SB-269970 (0.625 mg/kg) and imipramine (20 mg/kg) found in the forced swim test produced antidepressant-like effect, which did not stem from the increased exploratory locomotor activity. At the same time, that combination voked a vast increase in the output of NA - but not DA and 5-HT - compared to the effects of both those drugs given alone. These results open up a possibility that the stimulating effect of SB-269970 on DA, NA and 5-HT transmission in the prefrontal cortex plays some role in the antidepressant-like activity of this compound. Moreover, these findings suggest that the increase in cortical NA level seems to account for the anti-immobility action observed after joint administration of the selective 5-HT(7) receptor antagonist and imipramine in rats.     

Effect of combined administration of 5-HT1A or 5-HT1B/1D receptor antagonists and antidepressants in the forced swimming test

In the present study, we examined effects of the selective serotonin (5-hydroxytryptamine, 5-HT) reuptake inhibitor citalopram, the 5-HT/noradrenaline reuptake inhibitor imipramine, the selective noradrenaline reuptake inhibitor desipramine or the monoamine oxidase-A inhibitor moclobemide, administered in combination with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635) or the 5-HT(1B/1D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-[1,2,4]oxadiazol-3-yl)1,1'-biphenyl-4-carboxamide (GR 127935) and the 5-HT(1B) receptor antagonist N-[3-(2-dimethylamino) ethoxy-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-(1,1'-biphenyl)-4-carboxamide (SB 216641) in the forced swimming test in rats. When given alone, citalopram (20 and 30 mg/kg), imipramine (20 mg/kg), desipramine (20 mg/kg), moclobemide (20 mg/kg), WAY 100635 (0.1 and 1 mg/kg), GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) did not shorten the immobility time of rats. Co-administration of WAY 100635 (0.1 and 1 mg/kg) and citalopram (20 mg/kg), or imipramine (20 mg/kg), or moclobemide (20 mg/kg) did not affect the immobility time of rats, whereas WAY 100635 given jointly with desipramine (20 mg/kg) induced a weak anti-immobility effect. GR 127935 (10 and 20 mg/kg) or SB 216641 (2 mg/kg) co-administered with imipramine, desipramine or moclobemide, but not citalopram, produced a significant anti-immobility action in the forced swimming test in rats. These results indicate that the blockade of 5-HT(1B) rather than 5-HT(1A) receptors may facilitate the anti-immobility effect of imipramine, desipramine or moclobemide in the forced swimming test. No interaction was observed between 5-HT(1A) or 5-HT(1B/1D) receptor antagonists and citalopram.     

Differential effects of 5-HT2C receptor ligands on place conditioning and locomotor activity in rats

Effects of the 5-hydroxytryptamine (5-HT)(1A/1B/2C) receptor agonist N-[3-(trifluoromethyl)phenyl] piperazine (TFMPP, 0-3.0 mg/kg s.c.) and the 5-HT2C receptor agonist 8,9-dichloro-2,3,4,4a-tetrahydro-1H-pyrazino[1,2-a]quinoxalin-5(6H)-one (WAY 161503, 0-3.0 mg/kg s.c.) in place conditioning were measured in male Sprague-Dawley rats. Effects of TFMPP, alone and with the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl] ethyl]-N-2-pyridinyl-cyclohexanecarboxamine (WAY 100635), the 5-HT(1B) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl) phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-1,1'-biphenyl-4-carboxamide (GR 127935) or the 5-HT2C receptor antagonist 6-chloro-5-methyl-1-[[2-(2-methylpyrid-3-yloxy)pyrid-5-yl]carbamoyl]indoline (SB 242084) and of WAY 161503 alone and with SB 242084 on locomotor activity were also assessed. Neither TFMPP nor WAY 161503 induced place conditioning. WAY 161503 (1.0 and 3.0 mg/kg s.c.) decreased locomotor activity; SB 242084 (1.0 mg/kg i.p.) blocked this effect. Reduced locomotor activity following TFMPP was blocked by SB 242084 but not WAY 100635 (0.1 mg/kg s.c.) or GR 127935 (3.0 mg/kg s.c.). Behaviourally relevant levels of 5-HT2C receptor stimulation may not exert reinforcing effects, although other studies indicate that such manipulations alter reinforcing effects of drugs of abuse.     

The anxiolytic-like effect of 5-HT1B receptor ligands in rats: a possible mechanism of action

We have examined the effect of lesions of 5-hydroxytryptamine (5-HT) neurons, produced by p-chloroamphetamine (p-CA; 2 x 10 mg kg(-1)), and the influence of flumazenil (Ro 15-1788, 10 mg kg(-1)), a benzodiazepine receptor antagonist, on the anxiolytic-like activity of CP 94253 (5-propoxy-3-(1,2,3,6-tetrahydro-4-pyridinyl)-1H-pyrrolo[3,2-b]pyridine), a 5-HT1B receptor agonist, SB 216641 (N-[3-[3-(dimethylamino)ethoxy]-4-methoxyphenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-[1,1'-biphenyl]-4-carboxamide), a 5-HT1B receptor antagonist, and GR 127935 (N-[4-methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)-l, l'-biphenyl-4-carboxamide), a 5-HT1B/1D receptor antagonist, in the Vogel conflict drinking test in rats. Diazepam was used as a reference compound. CP 94253 (2.5 mg kg(-1)), SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)) significantly increased the number of shocks accepted during experimental sessions in the conflict drinking test in vehicle- and p-CA-pretreated rats. Flumazenil did not change the anxiolytic-like effect of CP 94253 (2.5 mg kg(-1)), but wholly blocked the anxiolytic-like effects of SB 216641 (2.5 mg kg(-1)), GR 127935 (10 mg kg(-1)) and diazepam (5 mg kg(-1)). p-CA and flumazenil alone were inactive in the conflict drinking test. The results suggested that the anxiolytic-like effect of the 5-HT1B receptor ligands CP 94253, SB 216641 and GR 127935 was possibly linked to the postsynaptic 5-HT1B receptors or/and 5-HT1B heteroreceptors. The results suggested also that benzodiazepine receptors were indirectly involved in the effects of SB 216641 and GR 127935 (but not of CP 94253), which might have been due to a possible interaction between the 5-HT and the GABA/benzodiazepine systems.     

5-HT7 receptor stimulation by 8-OH-DPAT counteracts the impairing effect of 5-HT(1A) receptor stimulation on contextual learning in mice

The principal 5-HT(1A) receptor agonist 8-Hydroxy-2-(dipropylamino)tetralin (8-OH-DPAT) impairs several different types of learning. Besides 5-HT(1A) receptors, 8-OH-DPAT stimulates 5-HT(7) receptors, but it is not known whether 5-HT(7) receptors contribute to the impairments. The 5-HT(7) receptor antagonist (2R)-1-[(3-Hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl] pyrrolidine (SB-269970) was combined with 8-OH-DPAT to dissociate 5-HT(1A) from 5-HT(7) receptor-mediated effects, in the passive avoidance task for emotional learning. SB-269970 intensified impairments caused by 8-OH-DPAT. SB-269970 alone had no effect on memory performance, but moderately decreased retention under suboptimal learning conditions. These findings indicate that 5-HT(7) receptor stimulation by 8-OH-DPAT counteracts 5-HT(1A) receptor-mediated impairments in hippocampal-dependent contextual learning.     

No involvement of 5-HT(7) or 5-HT(1D) receptors in the (R)-8-OH-DPAT-induced depression of the monosynaptic reflex in spinalized rats

(R)-8-Hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) depressed the monosynaptic reflex. This effect was not antagonized by 5-HT(1A) receptor antagonists. We examined whether 5-HT(1D) and 5-HT(7) receptors are involved in (R)-8-OH-DPAT-induced inhibition of the monosynaptic reflex in spinalized rats. Pretreatment with methiothepin and mesulergine, but not clozapine, inhibited (R)-8-OH-DPAT-induced monosynaptic reflex depression. Pretreatment with 2a-(4-phenyl-1,2,3,6-tetrahydropyridal)butyl)-2a,3,4,5-tetrahydrobenzo[c,d]indol-2(1H)-one (DR4004) and (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl)ethyl]pyrolidine (SB-269970), new selective 5-HT(7) receptors antagonists, and N-[methoxy-3-(4-methyl-l-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl-1,2,4-oxadiazol-3-yl)[1,1-biphenyl]-4-carboxamide (GR127935), a selective 5-HT(1D) receptor antagonist, had no effect on (R)-8-OH-DPAT-induced depression. These results suggested that 5-HT(7) and 5-HT(1D) receptors are not involved in (R)-8-OH-DPAT-induced monosynaptic reflex depression.     

8-OH-DPAT acts on both 5-HT1A and 5-HT7 receptors to induce hypothermia in rodents

Studies using selective drugs and knockout mice have demonstrated that the 5-HT(7) receptor plays an instrumental role in serotonin-induced hypothermia. There is also evidence supporting an involvement of the 5-HT(1A) receptor, although mainly from studies using 8-hydroxy-2(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT(1A/7) receptor agonist. Here we studied the effects of 8-OH-DPAT and selective antagonists for the 5-HT(1A) and 5-HT(7) receptors on body temperature in rats, wild-type (5-HT(7)(+/+)) mice and knockout (5-HT(7)(-/-)) mice. At lower doses (0.3-0.6 mg/kg, i.p.), 8-OH-DPAT decreased body temperature in 5-HT(7)(+/+) mice but not in 5-HT(7)(-/-) mice. At a higher dose (1 mg/kg, i.p.) 8-OH-DPAT induced hypothermia in both 5-HT(7)(-/-) and 5-HT(7)(+/+) mice. The 5-HT(1A) receptor antagonist (S)-N-tert-butyl-3-(4-(2-methoxyphenyl)piperazine-1-yl)-2-phenylpropanamide (WAY-100135) (10 mg/kg, i.p.) inhibited the effect of 8-OH-DPAT at all doses in rats and mice. In 5-HT(7)(+/+) mice the selective 5-HT(7) receptor antagonist (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970) (10 mg/kg, i.p.) fully inhibited the hypothermia induced by 0.3 mg/kg 8-OH-DPAT, but not that of higher doses. In rats, SB-269970 caused a 60% inhibition of the hypothermia induced by 0.3 mg/kg 8-OH-DPAT. Thus, both 5-HT(7) and 5-HT(1A) receptors are involved in a complex manner in thermoregulation, with the 5-HT(7) receptor being more important at lower, possibly more physiological, concentrations.     

5-HT7, but not 5-HT2B, receptors mediate hypotension in vagosympathectomized rats

This study evaluated the possible involvement of 5-HT(2B) receptors in long-lasting hypotension to 5-hydroxytryptamine (5-HT), which is predominantly mediated by 5-HT7 receptors, in anaesthetised vagosympathectomized rats. Intravenous injections of 5-HT and 5-carboxamidotryptamine (5-CT) elicited a dose-dependent hypotension that was dose-dependently antagonised by (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl) ethyl] pyrrolidine (SB-269970; a selective 5-HT7 receptor antagonist), but not by saline. Interestingly, alpha-methyl-5-(2-thienylmethoxy)-1H-indole-3-ethanamine (BW723C86; a 5-HT(2B) receptor agonist) produced vasopressor responses without affecting hypotension to 5-HT. These results suggest that hypotension to 5-HT and 5-CT is mainly mediated by 5-HT7 receptors, whilst the role of 5-HT(2B) receptors seems unlikely.     

5-HT7 receptors modulate synchronized network activity in rat hippocampus.

In the CA3 region of rat hippocampal slices gamma-amino-butyric acid (GABA)(A/B) receptor antagonists induce low frequency bursting activity that was either inhibited (in 21% of slices) or increased by the selective 5-HT receptor agonists 5-carboxy-tryptamine (0.1-1 microM) and 8-hydroxydipropylaminotetralin (8-OH-DPAT). The selective 5-HT1A receptor antagonist N-(2-(4-(2-methoxyphenyl)-1-piperazinyl)ethyl)-N-(2-pyridinyl)cyclohexane carboxamide (WAY 100635) reversed the depression of bursting activity whereas the 5-HT7 receptor antagonist, (R)-3-(2-(2-(4-methylpiperidin-1-yl)-ethyl)pyrrolidine-1-sulfonyl)phenol (SB-269970; 1-10 microM), but not the 5-HT1A, 4 or 6 receptor antagonists WAY100635 (10 microM), SB-204070 (10 microM) and SB-271046 (10 microM), reversed the increase in bursting activity. The apparent -log10 K(D) value (8.4) for the effect of SB-269970 was consistent with a selective action at 5-HT7 receptors. Accompanying the 5-CT-induced increase in bursting frequency there was a shortening of the burst event waveform and a reduction in the after-hyperpolarization following each bursting event both of which were inhibited by SB-269970. These effects appeared to result predominantly from a direct 5-HT(7) receptor-mediated inhibition of a Ca2+ activated K+ channel.     

5-HT(1B/D) receptor agonist, SKF99101H, induces locomotor hyperactivity in the guinea pig

Previous studies in guinea pigs have shown that while a serotonin 5-HT(1B/D) receptor agonist, GR46611, does not induce locomotor activation when given alone, it markedly enhances the locomotor response to selective 5-HT(1A) receptor agonists, 8-OH-DPAT and buspirone. In these studies, we found that another 5-HT(1B/D) agonist, 3-(2-dimethylaminoethyl)-4-chloro-5-propoxyindole hemifumarate (SKF99101H), significantly elevated locomotor activity in guinea pigs when given alone. We assessed the relative contribution of 5-HT1(1A) and 5-HT(1B/D) receptors in the mediation of this effect.Activity was measured by photobeam interrupts in opaque Perspex cylinders linked to a computer. SKF99101H (20 mg/kg s. c.) significantly increased the locomotor activity in guinea pigs. The locomotor stimulant effect of SKF99101H (20 mg/kg s.c) was reversed by the selective 5-HT(1B/D) receptor antagonist N-[4-methoxy-3-(4-methyl-1-piperazinyl)phenyl]-2'-methyl-4'-(5-methyl -1,2,4-oxadiazol-3-yl)[1,1biphenyl]4-carboxamide (GR127935; 0.06-0. 25 mg/kg s.c.). The 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY100635; 0.05-0.25 mg/kg s.c.), slightly but significantly attenuated the hyperactivity induced by SKF99101H. These findings suggest that 5-HT(1B/D) receptor agonists may require concomitant activation of 5-HT(1A) receptors to induce locomotor activity in guinea pigs. The 5-HT(2A) receptor antagonist 6[2-[4-[bis(4-fluorophenyl)methylene]-1-piperidinyl]-ethyl]-7-methyl- 5H-thiazol[3,2-a]pyrimidin-5-one (ritanserin) had no effect on SKF99101H-induced hyperactivity, suggesting that these receptors are not involved in the mediation of SKF99101H-induced hyperactivity. SKF99101H-induced hyperactivity was significantly attenuated by the D(1) dopamine receptor antagonist SCH 23390 (0.005-025 mg/kg), but not by the D(2) dopamine receptor antagonist raclopride (0.25-1.0 mg/kg), possibly suggesting the selective involvement of D(1) dopaminergic receptors in the mediation of the stimulant actions of the 5-HT(1B/D) agonist.     

GR46611 potentiates 5-HT1A receptor-mediated locomotor activity in the guinea pig.

5-HT(1B/D) receptor agonists such as GR46611 (3-[3-(2-Dimethylaminoethyl)-H-indol-5-yl]-N-(4-methoxybenzyl)acrylamide ) are known to lower body temperature in guinea pigs. Although stimulation of their functional analogs in rats, the 5-HT1B receptor induces hyperlocomotion, this effect has yet to be demonstrated with 5-HT(1B/D) receptor agonists in the guinea pig. Previous studies have shown that 5-HT1A agonists increase locomotor activity in guinea pigs. The current study set out to examine the effects of 5-HT(1B/D) receptor stimulation on locomotor activity in the guinea pig and to examine the interaction between 5-HT1A and 5-HT(1B/D) receptor stimulation on locomotor activity in that species. The full agonist at 5-HT1A receptors, 8-OH-DPAT (R(+)-8-Hydroxy-dipropylaminotetralin HBr) dose-dependently increased locomotor activity in guinea pigs (0.3-1.25 mg kg(-1) s.c.), as to a lesser extent, did the partial agonist, buspirone (8-[4-[4-(2-Pyramidinyl)-1-piperazinyl]butyl]-8-azaspiro[4.5 ]decane-7,9-dione HCl) (5.0-20.0 mg kg(-1) s.c.). The 5-HT(1B/D) receptor agonist GR46611 had no effect on locomotor activity in guinea pigs at doses up to 40 mg kg(-1) s.c. 8-OH-DPAT-induced behavioural activation was reversed by the selective 5-HT1A receptor antagonist WAY100635 (N-[-2-[4-(-methoxyphenyl)-1-piperazinyl]ethyl]-N-(pyrinidyl) cyclo hexanocarboxamide trihydro-chloride), with a minimum effective dose of 0.006 mg kg(-1), but not by the 5-HT(1B/D) receptor antagonist GR127935 (2'-methyl-4-(5-methyl-[1,2,4]oxadiazol-3-yl)-biphenyl-4-carboxyli c acid [4-methoxy-3-(4-methyl-piperazin-1-yl)phenyl]-amide) (0.25-1.0 mg kg(-1)). GR46611, at doses that were without effect given alone (0.5-2.5 mg kg(-1)), significantly enhanced the locomotor response to subthreshold doses of 8-OH-DPAT (0.5 mg kg(-1)) and buspirone (10 mg kg(-1)). The effect of GR46611 on 8-OH-DPAT-induced hyperactivity was reversed by pretreatment with GR127935 and with WAY 100635 indicating that activation of both receptors was required for the expression of locomotor hyperactivity. These findings suggest that activation of 5-HT(1B/D) receptors alone may not stimulate locomotor activity but it does potentiate the locomotion induced by 5-HT1A receptor stimulation in guinea pigs.     

Evidence that hypophagia induced by d-fenfluramine and d-norfenfluramine in the rat is mediated by 5-HT2C receptors

The present series of studies is the first to investigate the pharmacological mechanisms underlying d-fenfluramine- and d-norfenfluramine-induced hypophagia in the rat using highly selective serotonin 5-HT2 receptor antagonists. Administration of d-fenfluramine, and its major metabolite d-norfenfluramine, suppresses food intake in animals. Both compounds stimulate the release of serotonin and are potent inhibitors of the re-uptake of 5-HT into nerve terminals. In addition, d-norfenfluramine also acts as a direct 5-HT(2B/2C) receptor agonist. Pre-treatment with the selective 5-HT2C receptor antagonist, SB-242084 (0.3-3 mg/kg), dose-dependently inhibited both d-fenfluramine- (3 mg/kg) and d-norfenfluramine-induced (2 mg/kg) hypophagia. In contrast, the hypophagic effect of d-fenfluramine and d-norfenfluramine was unaffected by prior treatment with the highly selective 5-HT2B receptor antagonists, SB-215505 (0.3-3 mg/kg) and RS-127445 (1-3 mg/kg) or the 5-HT2A receptor antagonists MDL 100,907 (0.003-0.03 mg/kg) and ketanserin (0.2, 0.5 mg/kg). In addition, the 5-HT1A receptor antagonist WAY-100635 (0.3, 1 mg/kg) and the 5-HT1B receptor antagonists GR-127935 (1, 2 mg/kg) and SB-224289 (2-10 mg/kg) did not affect d-fenfluramine-induced hypophagia. These data provide unequivocal evidence for an important role of the 5-HT2C receptor in the mediation of d-fenfluramine and d-norfenfluramine-induced hypophagia in the rat and do not support the involvement of 5-HT1A/1B/2A/2B receptors.     

The effect of the 5-HT1A receptor agonist, 8-OH-DPAT, on motion-induced emesis in Suncus murinus

In the present study we evaluated the role of 5-HT(1A) receptors in mediating the inhibitory action of 8-OH-DPAT, a 5-HT(1A) receptor agonist, in motion sickness in Suncus murinus. 8-OH-DPAT (0.1 mg/kg, i. p) attenuated motion-induced emesis which was associated with an increase in the latency of the onset to the first emetic episode. Pre-treatment with methysergide (a 5-HT(1/2/7) receptor antagonist, 1.0 mg/kg, i. p.), WAY-100635 (a 5-HT(1A) receptor antagonist, 1.0 mg/kg, i. p.), SB269970A (a 5-HT(7) receptor antagonist, 1.0 and 5.0 mg/kg, i. p.), ondansetron (a 5-HT(3) receptor antagonist, 1.0 mg/kg, i. p) or GR13808 (a 5-HT(4) receptor antagonist, 0.5 mg/kg, i. p) failed to modify the inhibitory action of 8-OH-DPAT on motion sickness. Furthermore, the application of either methysergide, WAY-100635, SB269970A, ondansetron or GR13808 alone had no effect on motion sickness in its own right. These data indicate that neither 5-HT(1A) nor any 5-HT(2) receptor subtypes, 5-HT(3), 5-HT(4) and 5-HT(7) receptors are likely to be involved in the inhibition of motion-induced emesis mediated by 8-OH-DPAT.     

The effect of the acute administration of various selective 5-HT receptor antagonists on focal hippocampal seizures in freely-moving rats

In this study, we assessed the effects of the acute administration of various 5-HT receptor antagonists on hippocampal partial seizures generated by low-frequency electrical stimulation in male Wistar rats. The seizure threshold and severity were determined by measuring the pulse number threshold and primary and secondary afterdischarges, respectively, and the latency of secondary discharge was also determined. The administration of either the selective 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazineyl]ethyl]-N-(pyridinyl)-c yclohe xanecarboximimde 3 HCl (WAY 100635, 0.1-1 mg/kg i.p.), the selective 5-HT(3) receptor antagonist granisetron (0.3-3 mg/kg i.p.), the selective 5-HT(2A) receptor antagonist R-(+)-a-(2, 3-dimethoxyphenyl)-1-[2-(4-fluorophenyl) ethyl]-4-piperidine-methanol (MDL 100907, 0.3-3 mg/kg i.p.) or the 5-HT(2B,C) receptor antagonist antagonist N-(1-methyl-5-indolyl)-N'-(3-pyridyl) urea HCl (SKB 200646A, 5-50 mg/kg i.p.) did not alter the pulse number threshold compared to vehicle-treated animals. However, the acute administration of WAY 100635 (0.3 mg/kg) and M100907 (1 mg/kg) significantly increased, whereas granisetron (1 mg/kg) decreased, the primary afterdischarge duration compared to vehicle-treated animals. The latency of secondary after discharge was significantly decreased by WAY 100635 (1 mg/kg) and granisetron (3 mg/kg) compared to vehicle-treated animals. These results suggest that in this model, the antagonism of 5-HT(1A), 5-HT(2A), 5-HT(3) or 5-HT(2B,C) receptors do not lower or raise seizure threshold. However, the antagonism of 5-HT(1A) receptors may increase or augment seizure severity.