5-HT 1A/1B receptor-mediated effects of the selective serotonin reuptake inhibitor, citalopram, on sleep: studies in 5-HT 1A and 5-HT 1B knockout mice.

Selective serotonin reuptake inhibitors (SSRIs) are extensively used for the treatment of depression. Aside from their antidepressant properties, they provoke a deficit in paradoxical sleep (PS) that is most probably mediated by the transporter blockade-induced increase in serotonin concentration in the extracellular space. Such an effect can be accounted for by the action of serotonin at various types of serotonergic receptors involved in PS regulation, among which the 5-HT(1A) and 5-HT(1B) types are the best candidates. According to this hypothesis, we examined the effects of citalopram, the most selective SSRI available to date, on sleep in the mouse after inactivation of 5-HT(1A) or 5-HT(1B) receptors, either by homologous recombination of their encoding genes, or pharmacological blockade with selective antagonists. For this purpose, sleep parameters of knockout mice that do not express these receptors and their wild-type counterparts were monitored during 8 h after injection of citalopram alone or in association with 5-HT(1A) or 5-HT(1B) receptor antagonists. Citalopram induced mainly a dose-dependent inhibition of PS during 2-6 h after injection, which was observed in wild-type and 5-HT(1B)-/- mice, but not in 5-HT(1A)-/- mutants. This PS inhibition was fully antagonized by pretreatment with the 5-HT(1A) antagonist WAY 100635, but only partially with the 5-HT(1B) antagonist GR 127935. These data indicate that the action of the SSRI citalopram on sleep in the mouse is essentially mediated by 5-HT(1A) receptors. Such a mechanism of action provides further support to the clinical strategy of antidepressant augmentation by 5-HT(1A) antagonists, because the latter would also counteract the direct sleep-inhibitory side-effects of SSRIs.     

Pharmacological characterisation of the decrease in 5-HT synthesis in the mouse brain evoked by the selective serotonin re-uptake inhibitor citalopram

The selective serotonin re-uptake inhibitor (SSRI) citalopram decreases the synthesis of 5-hydroxytryptamine (5-HT) in the mouse brain in vivo. The underlying mechanism was studied by recording the accumulation of 5-hydroxytryptophan (5-HTP) in hypothalamus and hippocampus after inhibition of the aromatic amino acid decarboxylase activity with m-hydroxybenzylhydrazine (NSD 1015). Depletion of 5-HT with reserpine markedly reduced the citalopram-induced decrease of 5-HTP but not that evoked by the 5-HT1A receptor agonist 8-OH-DPAT, which indicates that the presence of endogenous 5-HT is necessary for full effect of citalopram. In contrast to the almost complete antagonism of the decrease in 5-HT synthesis induced by 8-OH-DPAT, the 5-HT1A receptor antagonist WAY-100,635 only slightly affected the citalopram-evoked decrease in 5-HT synthesis. Likewise, the 5-HT1B receptor antagonists NAS-181 and GR127935 only slightly antagonised the citalopram effect although they strongly inhibited the decrease in 5-HT synthesis induced by the 5-HT1B receptor agonist anpirtoline. Combined treatment with 5-HT1A and 5-HT1B receptor antagonists did not produce any additive antagonistic effect on the citalopram-induced decrease in 5-HT synthesis. The 5-HT2A/2C receptor antagonist ketanserin, the 5-HT3 receptor antagonist ondansetron and the 5-HT4 receptor antagonist RS-39604 had no effect on the citalopram-induced decrease in 5-HT synthesis. The same was found for several other non-selective 5-HT receptor antagonists, e.g. cyproheptadine, dihydroergotamine, methiothepin, methysergide, metergoline and mianserin. It is concluded that the citalopram-induced decrease in 5-HT synthesis differs in sensitivity from that mediated by 5-HT1A or 5-HT1B receptor agonists and citalopram also seems to require endogenous 5-HT for its full effect.     

Effects of chronic treatment with fluoxetine and citalopram on 5-HT uptake, 5-HT1B autoreceptors, 5-HT3 and 5-HT4 receptors in rats

The effect in rats of chronic treatment with two specific 5-HT reuptake inhibitors (SSRI) with antidepressant properties, citalopram (10 mg/kg, i.p. twice a day for 14 days, one day washout) and fluoxetine (15 mg/kg, p.o. twice a day for 21 days, 7 days washout), was evaluated on some mechanisms involved in central 5-HT neurotransmission. No adaptive modifications of brain 5-HT uptake (sites) were found by measuring functional [³H]5-HT uptake and [³H]citalopram binding in cortical and hippocampal synaptosomes, and by [³H]citalopram binding autoradiography in the raphe nuclei (5-HT cell bodies) and the ventral tegmental area (5-HT axonal pathway). Chronic treatments had no effect on presynaptic 5-HT_1B autoreceptors, functionally evaluated by measuring 5-HT_1B-mediated inhibition of depolarization-induced [³H]5-HT release from cortical and hippocampal synaptosomes. Chronic citalopram or fluoxetine did not significantly affect the binding of [³H]BRL-43694 to 5-HT₃ receptors in the rat brain cortex. Citalopram had no effect on [¹²⁵I]SB-207710 binding to 5-HT₄ receptors, measured by autoradiography in the substantia nigra. Negative results, such as those reported in the present study, could be due to a number of variables including the animal species, the treatment schedule or the brain areas considered, thus explaining the differences from some previous reports of significant effects of SSRI. However, our negative data are in agreement with many other published studies, suggesting that adaptive modifications of brain 5-HT transporters, terminal 5-HT_1B receptors, 5-HT₃ and 5-HT₄ receptors may not be a general effect induced by all SSRI. Received: 9 December 1996 / Accepted: 1 March 1997     

5-HT1A and 5-HT1B agonists play a differential role on the respiratory frequency in rats

The effect of the putative 5-HT1A agonists 8-OH-DPAT and ipsapirone and 5-HT1B agonists TFMPP and m-CPP on respiratory activity in rats has been examined. In chloral hydrate-anesthetized rats, respiratory counts were decreased in a dose-dependent manner by both TFMPP and m-CPP, with an ED50 of 0.30 mg/kg (1.1 mumol/kg) and 3.0 mg/kg (11.0 mumol/kg) respectively. In contrast, both 5-HT1A agonists tested, 8-OH-DPAT and ipsapirone, produced an increase in respiratory rate at all doses tested. Moreover, the TFMPP-induced decrease in respiratory rate was antagonized by 8-OH-DPAT. The 5-HT2 antagonist ketanserin had no effect on the TFMPP-induced decrease in respiratory activity. However, methylsergide (5-HT1/5-HT2 antagonist) and (-)-cyanopindolol (5-HT1B antagonist) antagonized the TFMPP-induced respiratory rate decrease. The results of these experiments, coupled with the predominant presence of 5-HT1B receptors in the lower brainstem, pons, and medulla, as established by autoradiography studies, suggest a possible involvement of the 5-HT1B receptor subtype in the control of respiratory mechanisms, especially those involved in respiratory rhythmicity.     

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.     

Effect of co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 and selective 5-HT1B/1D receptor antagonist GR 127,935 on anxiolytic effect of citalopram in conditioned fear stress in the rat

This study investigated the effect of co-administration of the selective 5-HT1A receptor antagonist WAY 100,635 and selective 5-HT(1B/1D) receptor antagonist GR 127,935 with a subactive dose of citalopram [selective serotonin (5-HT) reuptake inhibitor (SSRI)] on the expression of conditioned freezing, an index of fear. In the present study, acute administration of citalopram (s.c.) reduced freezing significantly at high doses (10, 30 and 100 mg/kg), while showing no significant effect at low doses (1 and 3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg) reduced freezing markedly and significantly, as compared with either drug alone. However, the addition of GR 127,935 (4 mg/kg) did not potentiate the effects of citalopram (3 mg/kg) on freezing and did not enhance the effect of WAY 100,635 (0.15 mg/kg) with citalopram (3 mg/kg). Co-administration of WAY 100,635 (0.15 mg/kg) or GR 127,935 (4 mg/kg) gave no effect on high-dose citalopram (30 mg/kg)-induced inhibition of freezing behavior. These results suggest that co-administration of WAY 100,635 (0.15 mg/kg) strengthens the anxiolytic effect of citalopram (3 mg/kg) by facilitating central 5-HT neurotransmission. Since GR 127,935 (4 mg/kg) failed to accelerate the inhibition of freezing induced by citalopram (3 mg/kg) with WAY 100,635 (0.15 mg/kg) or citalopram (3 mg/kg) alone, it is suggested that blocking 5-HT1A receptors is more effective in facilitating the anxiolytic effect of citalopram than blocking 5-HT1B/1D receptors.     

Enhancement of 5-HT1B and 5-HT1D receptor antagonist effects on extracellular 5-HT levels in the guinea-pig brain following concurrent 5-HT1A or 5-HT re-uptake site blockade.

The effects of selective serotonin re-uptake inhibitor (SSRI), paroxetine, and 5-HT1A, 5-HT1B and 5-HT1B/1D receptor antagonists on in vivo extracellular 5-HT levels in the guinea-pig frontal cortex and dorsal hippocampus were investigated using the technique of microdialysis. The aim of the study was to further investigate the autoreceptor roles of the 5-HT1A, 5-HT1B and 5-HT1D receptors in the median vs dorsal raphe nuclei. In the frontal cortex, 5-HT1A (WAY 100635, 1 mg/kg i.p.) or 5-HT1B (SB-224289, 4 mg/kg i.p.) receptor antagonists had no effect on extracellular levels of 5-HT, whilst the mixed 5-HT1B/1D receptor antagonist (GR 127935, 0.3 mg/kg i.p) produced a significant decrease in extracellular 5-HT levels. Paroxetine (10 microM) significantly increased extracellular 5-HT levels when perfused locally into the cortex. Administration of SB-224289, followed 120 min later by WAY 100635, had no effect on extracellular 5-HT levels. In contrast, sequential administration of either WAY 100635 and GR 127935, or SB-224289 and paroxetine significantly increased extracellular 5-HT levels. In the dorsal hippocampus, whilst 5-HT1A receptor antagonism elicited by administration of WAY 100635 had no effect, both 5-HT1B and mixed 5-HT1B/1D receptor blockade significantly increased extracellular 5-HT levels. Administration of SB-224289 followed 120 min later with WAY 100635, or WAY 100635 followed 30 min later with GR 127935, potentiated the effect of the three compounds alone, significantly increasing extracellular 5-HT levels. These data demonstrate that to simultaneously increase extracellular 5-HT in both frontal cortex and dorsal hippocampus of the guinea-pig brain concurrent 5-HTA1A, 5-HT1B and 5-HT1D receptor blockade is required. Whereas in the dorsal hippocampus, 5-HT1B receptor blockade is sufficient to elicit an increase in extracellular 5-HT levels.     

Hypothermia induced by the putative 5-HT1A agonists LY165163 and 8-OH-DPAT is not prevented by 5-HT depletion

The putative 5-HT1A agonist 1-[2-(4-aminophenyl)ethyl]-4-(3-trifluoromethylphenyl)piperazine (LY165163, PAPP) (1, 2, 4, 10 mg/kg s.c.) caused a significant and dose-dependent hypothermia in rats, 30 and 60 min after injection. The decreases of temperature were less marked than that caused by 8-OH-DPAT 1 mg/kg s.c.). Depletion of brain serotonin (5-HT) by 91% following pretreatment with p-chlorophenylalanine (pCPA) (150 mg/kg i.p. on three successive days) significantly enhanced the hypothermic effects of both 8-OH-DPAT (0.25 mg/kg s.c.) and LY165163 (4 mg/kg s.c.). LY165163-induced hypothermia was also somewhat enhanced following depletion of hypothalamic 5-HT by 76% after infusion of 5,7-dihydroxytryptamine (5,7-DHT) (150 micrograms) into the third ventricle. Results indicate that the hypothermia induced by the putative 5-HT1A agonists LY165163 and 8-OH-DPAT in the rat is not dependent on presynaptic 5-HT stores and is therefore probably mediated by postsynaptic 5-HT receptors.     

The 5-HT1A receptor antagonist (S)-UH-301 augments the increase in extracellular concentrations of 5-HT in the frontal cortex produced by both acute and chronic treatment with citalopram

In a recent study, utilizing single cell recording techniques, we have shown that administration of 5-HT_1A receptor antagonists, e.g. (S)-UH-301, to rats concomitantly treated, acute or chronically, with the selective serotonin reuptake inhibitor (SSRI) citalopram significantly increases the activity of 5-hydroxytryptamine (5-HT) containing neurons in the dorsal raphe nucleus (DRN). Here we report correlative experiments using microdialysis in freely moving animals to measure extracellular levels of 5-HT and its metabolite 5-hydroxyindole acetic acid (5-HIAA) in the frontal cortex, a major projection area for DRN-5-HT neurons. Acute administration of (S)-UH-301 (2.5 mg/kg s.c.) or citalopram (2.0 mg/kg s.c.) increased 5-HT concentrations with a maximum of about 70% and 185%, respectively, above baseline. However, when (S)UH-301 was administered 30 min before citalopram the maximal increase in 5-HT levels was approximately 400%. In rats chronically treated with citalopram (20 mg/kg/day i.p. for 14 days) basal 5-HT concentrations in the frontal cortex were significantly increased and 5-HIAA concentrations were decreased when measured 10–12 h, but not 18–20 h, after the last injection of citalopram, as compared to basal 5-HT and 5-HIAA concentrations in chronic saline-treated rats. When (S)-UH-301 (2.5 mg/kg s.c.) was administered 12 h, but not 20 h, after the last dose of citalopram it produced a significantly larger increase in extracellular concentrations of 5-HT than in control rats. However, in rats pretreated with a single, very high dose of citalopram, 20 mg/kg i.p., administration of (S)-UH-301 at 12 h after citalopram did not increase 5-HT levels.The augmentation by (S)-UH-301 of the increase in brain 5-HT output produced by acute administration of citalopram is probably due to antagonism of the citalopram induced feedback inhibition of 5-HT cells in the DRN, as previously suggested. However, the capacity of (S)-UH-301 to further increase the already elevated extracellular concentrations of 5-HT in brain in animals maintained on a chronic citalopram regimen, in which significant tolerance to the initial feedback inhibition of DRN-5-HT cells had developed, represents a novel finding. Generally, the reduced feedback inhibition of 5-HT neurons obtained with chronic citalopram treatment, and the associated elevation of brain 5-HT concentrations, may be related to functional desensitization of somatodendritic 5-HT_1A autoreceptors in the DRN. This phenomenon may also largely explain the larger increase in 5-HT output produced by (S)-UH-301 in chronic citalopram treated animals as compared to its effect in control animals. Yet, a contributory factor may be a slight, remaining feedback inhibition of the 5-HT cells caused by residual citalopram at 12, but not 20 h after its last administration.Previous clinical studies suggest that addition of a 5-HT_1A receptor antagonist to an SSRI in the treatment of depression may accelerate the onset of clinical effects. Moreover, in therapy-resistant cases maintained on SSRI treatment, addition of a 5-HT_1A receptor antagonist may improve clinical efficacy. Since the therapeutic effect of SSRIs in depression has been found to be critically linked to the availability of 5-HT in brain, our experimental results support, in principle, both of the above clinically based notions.     

Interaction between 5-HT(1A) and 5-HT(1B) receptors: effects of 8-OH-DPAT-induced hypothermia in 5-HT(1B) receptor knockout mice

To test for adaptive compensatory changes that may have occurred in the functional activity of somatodendritic 5-HT(1A) receptors during the development of constitutive "knockout" mice lacking the 5-HT(1B) receptor subtype (5-HT(1B) -/- KO), we assayed for decrease in body temperature induced by an acute subcutaneous injection of the 5-HT(1A) receptor agonist, 8-hydroxy 2(di-n-propyl(amino)tetralin (8-OH-DPAT), either alone or in the presence of a selective 5-HT(1A) receptor antagonist, N-[4-(2-methoxyphenyl)-1-piperazinyl]-N-(2-pyridinyl) cyclo-hexanecarboxamide (WAY 100635). We compared dose-response curves, time course study, calculated ED(50) values (potency), maximal response to 8-OH-DPAT (efficacy) as well as measurements of the dose-dependent blockade of this response by WAY 100635 between wild-type controls and mutant mice. We found a higher efficacy of 8-OH-DPAT-induced hypothermia in 5-HT(1B) -/- KO compared to wild-type mice suggesting that an adaptive thermoregulatory process involving the functional activity of somatodendritic 5-HT(1A) receptors is altered in mutant mice lacking 5-HT(1B) receptors.     

5-HT2C receptors are involved in the discriminative stimulus effects of citalopram in rats

 Rats were trained on a fixed ratio 10, food-reinforced schedule to recognize a discriminative stimulus (DS) elicited by the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram (2.5 mg/kg, IP). The preferential, high efficacy agonist at 5-HT_2C receptors, Ro60-0175, dose-dependently generalized to citalopram with an ED₅₀ of 0.3 mg/kg, IP. Further, the selective 5-HT_2C receptor antagonist, SB242,084, dose-dependently (ED₅₀=0.1 mg/kg, IP) blocked the citalopram DS. These data suggest that 5-HT_2C receptors are involved in the DS properties of the SSRI, citalopram, in rats. They do not, however, exclude a potential role of other 5-HT receptor types. Received: 15 October 1998 / Final version: 10 December 1998     

Effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline in the rat frontal cortex: augmentation via 5-HT1A receptor antagonism.

Venlafaxine is a novel serotonin/noradrenaline reuptake inhibitor (SNRI) which has been shown clinically to be an effective antidepressant (AD) with a faster onset of action than serotonin specific reuptake inhibitors (SSRI). Preclinically, venlafaxine has been shown to potently inhibit dorsal raphe neuronal (DRN) firing through a 5-HT1A mediated mechanism, in a similar manner to SSRIs. Here we demonstrate the acute neurochemical effects of venlafaxine on extracellular concentrations of 5-HT and noradrenaline (NA) from the rat frontal cortex using in vivo microdialysis. Administration of venlafaxine (3-50 mg/kg s.c.) resulted in a significant dose-dependent increase in extracellular NA, but produced no significant increase in 5-HT concentrations. Combination treatment with the selective 5-HT1A antagonist WAY100635 produced a dose-dependent augmentation of venlafaxine-induced (3-30 mg/kg s.c) extracellular 5-HT concentrations, but had no further effect on NA above that produced by venlafaxine alone. WAY100635, at doses as low as 0.03 mg/kg s.c., maintained this potentiation effect. The beta-adrenergic/5-HT1A receptor antagonist (+/-)pindolol and the selective 5-HT1B/D antagonist GR127935 produced no significant augmentation of venlafaxine-induced changes in either 5-HT or NA. Using the alpha1 and alpha2-adrenoceptor antagonists, prazosin and idazoxane, we also demonstrate the role of the alpha-adrenoceptors in the augmentation of venlafaxine-induced changes. The possible mechanisms underlying venlafaxines improved clinical AD action and the potential for further enhancement of this SNRIs clinical effects are discussed.     

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

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

Effect of reserpine on behavioural responses to agonists at 5-HT1A, 5-HT1B, 5-HT2A, and 5-HT2C receptor subtypes.

Rats were given a single dose of reserpine (5 mg/kg s.c.) and behavioural responses to agonists at 5-HT receptor subtypes compared with those of control animals 21 days later. The following effects of activating postsynaptic 5-HT1A receptors by the agonist 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) were significantly increased: tail-flick, reciprocal forepaw treading, flat body posture. The hyperphagic effect of activating presynaptic 5-HT1A receptors by 8-OH-DPAT tended to increase and hypothermia on activating postsynaptic 5-HT1A sites tended to decrease. The hyperlocomotor effect of activating 5-HT1A sites also tended to decrease possibly as a result of a dependence of this response on the known depletion of catecholamines by reserpine. Head shakes on activating 5-HT2A receptors by 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and two effects of activating 5-HT2C receptors by 1-(3-chlorophenyl) piperazine (mCPP) were significantly increased (hypophagia, anxiety) and a third effect, hypolocomotion tended to increase but hypophagia on activating postsynaptic 5-HT1B receptors by CP-94, 253 was significantly attenuated. The results are discussed with particular reference to altered 5-HT function in 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.     

Citalopram reduces social interaction in rats by activation of serotonin (5-HT)(2C) receptors

The selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram (2.5 mg/kg, s.c.), reduced social interaction (SI) in rats. This action was abolished by the 5-HT(2B/2C) receptor antagonist, SB206, 553 (0.63 mg/kg, s.c.), and the 5-HT(2C) receptor antagonist, SB242, 084 (0.04 mg/kg, i.p.), but not by the 5-HT(2A) antagonist, MDL100, 907 (0.04 mg/kg, s.c.), the 5-HT(1A) antagonist, WAY100,635 (0.16 mg/kg, s.c.), or the 5-HT(3) antagonist, ondansetron (0.16 mg/kg, s. c.). These data suggest that 5-HT(2C) receptors are involved in the "anxiogenic" actions of citalopram.     

Pharmacological evidence that 5-HT(1B/1D) receptors mediate hypotension in anesthetized rats

5-Carboxamidotryptamine (5-CT; 0.003-310 microg/kg, i.v.) produced dose-dependent hypotensive responses which were blocked in a complex manner by the 5-HT(7) receptor antagonist, (R)-1-[(3-hydroxyphenyl)sulfonyl]-2-[2-(4-methyl-1-piperidinyl) ethyl] pyrrolidine (SB-269970; 1 mg/kg, i.v.), in anesthetized vagosympathectomized rats. Interestingly, 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.), also inhibited 5-CT-induced hypotension but the effect was clearly noncompetitive. Finally, the combination of GR-127935+SB-269970 (1 mg/kg, i.v., each) produced a further decreased of 5-CT-induced responses as compared to the effect of individual treatments. These data suggest that, in addition to 5-HT(7) receptors, 5-HT(1B/1D) receptors may also mediate hypotension in rats.     

5-HT2A receptors in the orbitofrontal cortex facilitate reversal learning and contribute to the beneficial cognitive effects of chronic citalopram treatment in rats

Chronic stress is a risk factor for depression, and chronic stress can induce cognitive impairments associated with prefrontal cortical dysfunction, which are also major components of depression. We have previously shown that 5 wk chronic intermittent cold (CIC) stress induced a reversal-learning deficit in rats, associated with reduced serotonergic transmission in the orbitofrontal cortex (OFC) that was restored by chronic treatment with a selective serotonin reuptake inhibitor (SSRI). However, the mechanisms underlying the beneficial cognitive effects of chronic SSRI treatment are currently unknown. Thus, the purpose of this study was to investigate the potential modulatory influence specifically of 5-HT2A receptors (5-HT2ARs) in the OFC on reversal learning, and their potential contribution to the beneficial cognitive effects of chronic SSRI treatment. Bilateral microinjections of the selective 5-HT2AR antagonist, MDL 100,907 into OFC (0.02-2.0 nmol) had a dose-dependent detrimental effect on a reversal-learning task, suggesting a facilitatory influence of 5-HT2ARs in the OFC. In the next experiment, rats were exposed to 5 wk CIC stress, which compromised reversal learning, and treated chronically with the SSRI, citalopram (20 mg/kg.d) during the final 3 wk of chronic stress. Chronic citalopram treatment improved reversal learning in the CIC-stressed rats, and bilateral microinjection of MDL 100,907 (0.20 nmol, the optimal dose from the preceding experiment) into OFC once again had a detrimental effect on reversal learning, opposing the beneficial effect of citalopram. We conclude that 5-HT2ARs in the OFC facilitate reversal learning, and potentially contribute to the beneficial cognitive effects of chronic SSRI treatment.     

Involvement of somatodendritic 5-HT(1A) receptors in Delta(9)-tetrahydrocannabinol-induced hypothermia in the rat

Previously, it has been reported that modulating serotonergic neurones by use of selective serotonin reuptake inhibitors (SSRI) can alter the hypothermic response produced by Delta(9)-tetrahydrocannabinol (Delta(9)-THC). The aim of the present study was to investigate the effect that activation or antagonism of 5-hydroxytryptamine (5-HT(1A)) receptors has on Delta(9)-THC-induced hypothermia. Delta(9)-THC (0.5, 2 and 5 mg/kg iv) decreased body temperature in a dose-related manner. Whilst having no significant effect on body temperature when administered 40 min prior to vehicle injection, the 5-HT(1A) receptor antagonist N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridinyl) cyclohexanecarboxamide trihydrochloride (WAY 100635; 1 mg/kg sc) significantly potentiated the hypothermia produced by 2 and 5 mg/kg Delta(9)-THC. In order to investigate whether this effect was due to antagonism at somatodendritic autoreceptors in midbrain raphe nuclei, WAY 100635 or the 5-HT(1A) agonist 8-hydroxy-(di-n-propylamino) tetralin (8-OH-DPAT) was microinjected into either the median raphe nuclei (MRN) or dorsal raphe nuclei (DRN) 40 min prior to Delta(9)-THC injection. Following microinjection into the DRN, neither WAY 100635 (0.5 nmol/0.5 microl/10 s) nor 8-OH-DPAT (15.2 nmol/0.5 microl/10 s) had any significant effect on Delta(9)-THC-induced hypothermia. However, WAY 100635 when microinjected into the MRN significantly potentiated Delta(9)-THC-induced hypothermia, and 8-OH-DPAT microinjected into the MRN significantly inhibited Delta(9)-THC-induced hypothermia. It is suggested from these studies that the potentiation of Delta(9)-THC-induced hypothermia by WAY 100635 when administered peripherally is mainly due to antagonism at somatodendritic 5-HT(1A) autoreceptors in the MRN.     

Effects of chronic citalopram treatment on 5-HT1A and 5-HT2A receptors in group- and isolation-housed mice

Selective serotonin reuptake inhibitors (SSRI) are characterized by high clinical effectiveness and good tolerability. A 2-3 week delay in the onset of effects is caused by adaptive mechanisms, probably at the serotonergic (5-HT) receptor level. To analyze this in detail, we measured 5-HT(1A) and 5-HT(2A) receptor bindings in vitro after 3 weeks of citalopram treatment (20 mg/kg i.p. daily) in group-housed as well as isolation-housed mice, reflecting neurobiological aspects seen in psychiatric patients. Isolation housing increased somatodendritic (+52%) and postsynaptic (+30-95%) 5-HT(1A) as well as postsynaptic 5-HT(2A) receptor binding (+25-34%), which confirms previous findings. Chronic citalopram treatment did not induce alterations in raphe 5-HT(1A) autoreceptor binding, independent of housing conditions. Housing-dependent citalopram effects on postsynaptic 5-HT(1A) receptor binding were found with increases in group- (+11-42%) but decreases in isolation-housed (-11 to 35%) mice. Forebrain 5-HT(2A) receptor binding decreased between 11 and 38% after chronic citalopram administration, independent of housing conditions. Citalopram's long-term action comprises alterations at the postsynaptic 5-HT(1A) and 5-HT(2A) receptor binding levels. Housing conditions interact with citalopram effects, especially on 5-HT(1A) receptor binding, and should be more strongly considered in pharmacological studies. In general, SSRI-induced alterations were more pronounced and affected more brain regions in isolates, supporting the concept of a higher responsiveness in "stressed" animals. Isolation-induced receptor binding changes were partly normalized by chronic citalopram treatment, suggesting the isolation housing model for further analyses of SSRI effects, especially at the behavioral level.