Effects of serotonin (5-HT)2 receptor ligands on depression-like behavior during nicotine withdrawal

A pronounced withdrawal syndrome including depressed mood prevents cigarette smoking cessation. We tested if blockade or activation of serotonin (5-HT)₂ receptors affected the time of immobility (as an indirect measure of depression-like behavior) in naïve animals and in those withdrawn from chronic nicotine in the forced swim test (FST). The antidepressant imipramine was used as a control. In the FST, the selective 5-HT_2A receptor antagonist M100,907 (1-2 mg/kg, but not 0.5 mg/kg), the selective 5-HT_2C receptor antagonist SB 242,084 (0.3-1 mg/kg, but not 0.1 mg/kg), the 5-HT_2C receptor agonists Ro 60-0175 (10 mg/kg, but not 3 mg/kg) and WAY 163,909 (1.5-10 mg/kg, but not 0.75 mg/kg) as well as imipramine (30 mg/kg, but not 15 mg/kg) decreased the immobility time while the non-selective 5-HT₂ receptor agonist DOI (0.1-1 mg/kg) was inactive in naïve rats. We found an increase in immobility time in rats that were withdrawn from nicotine exposure after 5 days of chronic nicotine treatment. This effect increased from day 1 until day 10 following withdrawal of nicotine, with maximal withdrawal effects on day 3. M100,907 (1 mg/kg), SB 242,084 (0.3 mg/kg), Ro 60-0175 (3 mg/kg), WAY 163,909 (0.75-1.5 mg/kg) and imipramine (15-30 mg/kg) shortened the immobility time in rats that had been removed from nicotine exposure for 3 days. Locomotor activity studies indicated that the effects of SB 242,084 might have been non-specific, as we noticed enhanced basal locomotion in naïve rats. This data set demonstrates that 5-HT_2A receptor antagonist and 5-HT_2C receptor agonists exhibited effects similar to antidepressant drugs and abolished the depression-like effects in nicotine-withdrawn rats. These drugs should be considered as adjuncts to smoking cessation therapy, to ameliorate abstinence-induced depressive symptoms.     

A role for serotonin in the antidepressant activity of N-Nitro-L-arginine, in the rat forced swimming test

The present study determined regional serotonin (5-HT) synthesis and metabolism changes associated with the nitric oxide synthase (NOS) inhibitor N^G-nitro-L-arginine (L-NA) and the influence of 5-HT receptor blockade in the antidepressant-like actions of L-NA in the forced swimming test (FST). Regional effects of L-NA (5,10 and 20 mg/kg i.p.) on tryptophan hydroxylase (TPH) activity, the rate limiting enzyme for 5-HT synthesis, were determined by measuring accumulation of the transient intermediate 5-hydoxytryptophan (5-HTP) following in vivo administration of the amino acid decarboxylase inhibitor, NSD 1015 (100 mg/kg). L-NA (5-20 mg/kg) dose dependently increased 5-HTP accumulation, particularly in the amygdaloid cortex, following exposure to the FST. L-NA also provoked an increase in regional brain 5-HIAA concentrations and in the 5-HIAA:5-HT metabolism ratio. Co-treatment with NSD-1015 failed to consistently modify the antidepressant-like effects of L-NA in the FST. Sub-active doses of L-NA (1 mg/kg) and the 5-HT re-uptake inhibitor fluoxetine (2.5 mg/kg) acted synergistically to increase swimming in the test. Co-treatment with the non-selective 5-HT receptor antagonist metergoline (1, 2 and 4 mg/kg), attenuated the L-NA (20 mg/kg)-induced reduction in immobility and increase in swimming behaviours. Metergoline alone however provoked an increase in immobility and reduction in swimming behaviours in the test. A similar response was obtained following co-treatment with the preferential 5-HT_2A receptor antagonist ketanserin (5 mg/kg) and the 5-HT_2C receptor antagonist RO-430440 (5 mg/kg). Co-treatment with the 5-HT_1A receptor antagonist WAY 100635 (0.3 mg/kg) or the 5-HT_1B receptor antagonist GR 127935 (4 mg/kg) failed to influence the antidepressant-like activity of L-NA. Taken together these data provide further support for a role for 5-HT in the antidepressant-like properties of NOS inhibitors.     

Antidepressant-like activity of 5-HT1A agonists measured with the forced swim test

This study examined the abilities of 5-hydroxytryptamine (5-HT) agonists with varying selectivity for different subtypes of 5-HT receptors to produce antidepressant-like behavioral effects in the forced swim test in rats. The 5-HT_1A agonists 8-OH-DPAT (0.125–1.0 mg/kg, SC) and tandospirone (SM-3997) (5–20 mg/kg, SC) both produced dose-related decreases in immobility time following subchronic treatment in rats. These effects were similar to those of the tricyclic antidepressants imipramine (5–15 mg/kg) and desipramine (5–15 mg/kg). In addition, the 5-HT_1A agonists, buspirone (20 mg/kg), gepirone (20 mg/kg) and ipsapirone (10 and 20 mg/kg) demonstrated antidepressant-like effects. Other groups of rats treated subchronically with each of the 5-HT_1A agonists or antidepressants showed no increase in locomotor activity, so that general changes in activity could not account for the reduction of immobility time in the forced swim test. 5-HT agonists selective for other receptor subtypes, such as the 5-HT_1B/1C agonistm-CPP (5 mg/kg) and the 5-HT_2/1C agonist DOB (1 mg/kg), were not effective in this behavioral test. The benzodiazepine diazepam (5 mg/kg) also failed to reduce immobility time, suggesting that anxiolytic properties of 5-HT_1A agonists did not mediate this behavioral effect. A common metabolite of some of the 5-HT_1A agonists, 1-PP, was ineffective in reducing immobility time. The stimulantd-amphetamine (2 mg/kg) significantly reduced immobility time but also significantly increased locomotor activity. Pretreatment with the 5-HT synthesis inhibitor PCPA alone did not alter immobility time, and did not alter the antidepressant-like effects of 8-OH-DPAT or tandospirone, suggesting that the 5-HT_1A agonists are producing their antidepressant-like effects through postsynaptic 5-HT_1A receptors. These results suggest that 5-HT_1A agonists may have antidepressant efficacy and act as a novel class of antidepressant drug.     

Investigation of the SSRI augmentation properties of 5-HT(2) receptor antagonists using in vivo microdialysis

Recent evidence that 5-HT₂ receptors exert a negative influence on central 5-hydroxytryptamine (5-HT) neurones suggests that 5-HT₂ receptor antagonists may augment the effects of serotonin selective reuptake inhibitors (SSRIs). The present study investigated whether pre-treatment with 5-HT₂ receptor antagonists enhances the effect of SSRI administration on hippocampal extracellular 5-HT of freely moving rats. Administration of the SSRI citalopram at a low (2 mg kg⁻¹) and higher (4 mg kg⁻¹) dose, increased dialysate 5-HT by 5- and 8-fold, respectively. Pre-treatment with the 5-HT₂ receptor antagonist ketanserin (4 mg kg⁻¹) augmented the effect of 4 mg kg⁻¹ but not 2 mg kg⁻¹ citalopram. The effect of 4 mg kg⁻¹ citalopram was also augmented by pre-treatment with either the 5-HT_2C receptor antagonist SB 242084 (0.5 mg kg⁻¹) or the 5-HT_2A receptor antagonist MDL 100907 (0.5 mg kg⁻¹). As with citalopram, fluoxetine elevated dialysate 5-HT at both a low (5 mg kg⁻¹) and higher (20 mg kg⁻¹) dose. However, neither dose of fluoxetine was augmented by ketanserin (4 mg kg⁻¹). These results confirm recent findings that 5-HT₂ receptor antagonists augment the effect of citalopram on extracellular 5-HT, and indicate the involvement of 5-HT_2C and possibly 5-HT_2A receptors. The lack of augmentation of fluoxetine might reflect the intrinsic 5-HT₂ receptor antagonist properties of this drug.     

Differential influence of selective 5-HT5A vs 5-HT1A, 5-HT1B,or 5-HT2C receptor blockade upon light-induced phase shifts in circadian activity rhythms: Interaction studies with citalopram

Though serotonergic mechanisms modulate circadian rhythms, roles of individual serotonin (5-HT) receptors remain uncertain since data are lacking for antagonists. Herein, both the 5-HT_5A receptor antagonist, A843277 (10 mg/kg), and the 5-HT_1B antagonist, SB224289 (1 mg/kg), inhibited light-induced phase advances in hamster circadian wheel-running rhythms. Conversely, though 5-HT_1A and 5-HT₇ receptors are likewise implicated in circadian scheduling, their blockade by WAY100635 (0.5 mg/kg) and SB269970 (1 mg/kg), respectively, was ineffective. Since actions of 5-HT reuptake inhibitors are modified by antagonists, we evaluated their influence on suppression of phase advances by citalopram (10 mg/kg). Its action was potentiated by WAY100635 and the 5-HT_2C antagonist, SB242084 (1 mg/kg), but not by A842377, SB224289, SB269970, and antagonists at 5-HT_2A (MDL100907) and 5-HT₆ (SB399885) receptors. In conclusion, this is the first in vivo evidence for an influence of 5-HT_5A receptors upon circadian rhythms, but no single class of 5-HT receptor mediates their control by citalopram.     

Involvement of serotonin receptor subtypes in the antidepressant-like effect of trim in the rat forced swimming test

Depression is a common illness with severe morbidity and mortality. Nitric oxide synthase (NOS) inhibitors are shown to elicit antidepressant-like effect in various animals models. It is widely known that serotonin plays an important role in the antidepressant-like effect of drugs. The aim of this study is to investigate the involvement of 5-HT₁ and 5-HT₂ receptor subtypes in the antidepressant-like effect of TRIM, a nNOS inhibitor, in the rat forced swimming test (FST). TRIM displays an antidepressant-like activity in FST which is blocked by pretreatment with the NOS substrate l-arginine. Depletion of endogenous serotonin using para-chlorophenylalanine (pCPA; 3 × 150 mg/kg, i.p.) partially attenuated TRIM (50 mg/kg)-induced reductions in immobility time in FST. Pretreatment with methiothepin (0.1 mg/kg, i.p, a non-selective 5-HT receptor antagonist), cyproheptadine (3 mg/kg i.p, a 5-HT₂ receptor antagonist) or ketanserin (5 mg/kg i.p, a 5HT_2A/2C receptor antagonist) prevented the effect of TRIM (50 mg/kg) in the FST. WAY 100635 (0.1 mg/kg i.p, a selective 5-HT_1A receptor antagonist) and GR 127935 (3 mg/kg i.p, a selective 5-HT_1B/1D receptor antagonist) slightly reversed the immobility-reducing effect of TRIM in the FST, but this failed to reach a statistically significant level. The results of this study demonstrate that antidepressant-like effect of TRIM in the FST seems to be mediated, at least in part, by an interaction with 5-HT₂ receptors while non-significant effects were obtained with 5-HT₁ receptors.     

Indorenate produces antidepressant-like actions in the rat forced swimming test via 5-HT1A receptors

Abstract Rationale. Indorenate has been proposed to possess antihypertensive, anorectic, stimulus control and anxiolytic-like actions. This compound has affinity mainly for the serotonergic_1A/1B receptors, hence it could possess antidepressant-like activity. Objectives. The general purpose of this study was to explore the possible antidepressant-like effects of the serotonergic compound indorenate in the forced swimming test (FST). Methods. In a first approach, a comparison of the actions of several doses of indorenate (2.5, 5.0, 10 mg/kg) with those of other 5-HT_1A agonists, buspirone (5.0, 10.0 mg/kg) and 8-OH-DPAT (0.25, 0.50, 1.0 mg/kg), was performed in the FST. Secondly, in order to determine the serotonergic receptors that are participating in indorenate's action, different doses of serotonergic antagonists were administered. The compounds used were the 5-HT_1A/1B and β-adrenergic antagonist pindolol (2.5, 5.0 mg/kg), the 5-HT_1B receptor antagonist GR 55562 (0.75, 1.5, 3.0 mg/kg), the 5-HT_1A antagonist WAY 100635 (0.25, 0.5, 1.0 mg/kg) and the 5-HT₂ antagonist ketanserin (1.0, 2.0, 4.0 mg/kg). Results. Indorenate (10 mg/kg), 8-OH-DPAT (1.0 mg/kg) and buspirone (5.0 and 10.0 mg/kg) reduced immobility behaviour in the FST, considered as an antidepressant-like effect. Both doses of pindolol (2.5 and 5.0 mg/kg) and WAY 100635 (0.5 and 1.0 mg/kg) antagonised the antidepressant-like effect of indorenate. Neither 5-HT_1B (GR55562) nor 5-HT₂ (ketanserin) antagonists produced changes in the effect of indorenate in the FST. Conclusions. Indorenate produces antidepressant-like actions in the FST that are mediated by the stimulation of 5-HT_1A receptors. Electronic Publication     

Differential regulation of rat peripheral 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2B</Subscript> receptor systems: influence of drug treatment

Most studies of 5-HT₂ receptor regulation have been carried out on the central nervous system (CNS) (which expresses 5-HT_2A and 5-HT_2C receptors); very few in vitro studies have addressed the peripheral receptors 5-HT_2A and 5-HT_2B. The aim of this investigation was to compare the possible short- and long-term processes regulating these peripheral receptors in the rat.The in vitro contractile response elicited by serotonin (5-HT, 10 µM) in the rat gastric fundus (5-HT_2B receptor system) was rapid and followed by a partial fade to a steady state, in contrast with the rat thoracic aorta response (5-HT_2A receptor system), which was more stable, slower and sustained. To characterize drug-receptor interactions, cumulative concentration/response curves (CCRCs) for 5-HT were constructed ex vivo for rat tissues treated with drugs acting at these receptors. Rats were examined 4 or 24 h after a single, i.p. administration of (±)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [(±)DOI, 1 or 2.5 mg/kg], clozapine, cyproheptadine or rauwolscine (10 mg/kg), 48 h after a single i.p. administration of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine (10 mg/kg) or 24 h after the last of with 15 daily i.p. administrations of (±)DOI (1 or 2.5 mg/kg), clozapine, cyproheptadine or rauwolscine (10 mg/kg). In the aorta, E_max (the maximum response elicited by 5-HT) was unchanged 4 h after a single dose of any of the drugs tested. However, 24 h after a single dose, E_max was lower in animals treated with (±)DOI (2.5 mg/kg), clozapine or cyproheptadine than in controls, whilst 48 h after a single dose of (±)DOI (2.5 mg/kg), clozapine or cyproheptadine there was no difference in E_max between experimental and control animals. After chronic treatment with (±)DOI (2.5 mg/kg), clozapine and cyproheptadine, E_max was lower than in controls. In the gastric fundus, E_max 4 h after a single dose of each drug was lower than in controls, and the response recovered by 24 or 48 h. Following chronic treatment, E_max was significantly lower than in controls for each drug used.These findings suggest first, that regulation of peripheral 5-HT₂ receptors (5-HT_2A and 5-HT_2B) is a functionally significant phenomenon in vivo, and occurs after administration of both agonists and antagonists. Second, the kinetics of peripheral 5-HT₂ receptor regulation were similar in both in vivo and ex vivo experiments. The 5-HT_2B receptors in rat gastric fundus are more sensitive to drug-induced regulation than the 5-HT_2A rat aortic receptors. Finally, long-term regulation of both receptors stabilizes short-term desensitization for longer.     

Blockade of the antidepressant-like effects of 8-OH-DPAT,buspirone and desipramine in the rat forced swim test by 5HT1A receptor antagonists

This study examined whether the antidepressant-like effect of serotonin (5-HT)_1A receptor agonists in the forced swim test (FST) is mediated by 5-HT_1A receptors. The 5-HT_1A receptor agonists 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and buspirone decreased immobility in the FST. The effect of 8-OH-DPAT was blocked by the 5-HT_1A receptor antagonists NAN 190, BMY 7378 and pindolol. The effect of buspirone was blocked by NAN 190 and pindolol. The antagonists produced no effects on their own. The norepinephrine (NE) uptake inhibitor desipramine (DMI) also reduced immobility, and this was also blocked by NAN 190, BMY 7378 and pindolol. The ₁, ₁ and ₂ adrenergic antagonists prazosin, betaxolol and ICI 118,551 did not block either 8-OH-DPAT or DMI, and produced no effects on their own. These results provide evidence that the antidepressant-like effects of 5-HT_1A receptor agonists in the FST are mediated through 5-HT_1A receptors, probably located postsynaptically. The finding that the 5-HT_1A receptor antagonists blocked the effect of DMI suggests that the NE and 5-HT systems interact in the FST.     

The effects of A 5-HT1A receptor agonist and antagonist on the 5-hydroxytryptamine release in the central nucleus of the amygdala: a microdialysis study with flesinoxan and WAY 100635

The modulation of extracellular 5-hydroxytryptamine (5-HT) in the central nucleus of the amygdala (CeA) by 5-HT_1A receptors was studied by intracerebral microdialysis in awake and freely moving rats. Local administration of 1 μM tetrodotoxin (TTX), 60 mM K⁺ and perfusion with Ca²⁺-free Ringer containing EGTA confirmed that the major part of dialysate 5-HT levels from the CeA is of neuronal origin. Administration of 300 nM of RU 24969, a 5-HT_1B receptor agonist, through the probe into the CeA decreased dialysate 5-HT levels to 67.2% of the baseline value. Systemic administration of the 5-HT_1A receptor agonists 8-OH-DPAT and flesinoxan dose-dependently decreased 5-HT levels in the CeA. The effect of 0.3 mg/kg of flesinoxan could be completely antagonized by systemic administration of 0.05 mg/kg WAY 100635, a 5-HT_1A receptor antagonist. WAY 100635 alone had only minimal effects at this dose. These data show that a major part of the extracellular 5-HT in the CeA stems from 5-HT neurons and that the amount of 5-HT released into this brain region can be modulated by 5-HT_1A receptors. Received: 11 September 1996 / Accepted: 25 November 1996     

Vortioxetine dose-dependently reverses 5-HT depletion-induced deficits in spatial working and object recognition memory: A potential role for 5-HT1A receptor agonism and 5-HT3 receptor antagonism

We previously reported that the investigational multimodal antidepressant, vortioxetine, reversed 5-HT depletion-induced memory deficits while escitalopram and duloxetine did not. The present report studied the effects of vortioxetine and the potential impact of its 5-HT_1A receptor agonist and 5-HT₃ receptor antagonist properties on 5-HT depletion-induced memory deficits. Recognition and spatial working memory were assessed in the object recognition (OR) and Y-maze spontaneous alternation (SA) tests, respectively. 5-HT depletion was induced in female Long-Evans rats using 4-cholro-DL-phenylalanine methyl ester HCl (PCPA) and receptor occupancies were determined by ex vivo autoradiography. Rats were acutely dosed with vortioxetine, ondansetron (5-HT₃ receptor antagonist) or flesinoxan (5-HT_1A receptor agonist). The effects of chronic vortioxetine administration on 5-HT depletion-induced memory deficits were also assessed. 5-HT depletion reliably impaired memory performance in both the tests. Vortioxetine reversed PCPA-induced memory deficits dose-dependently with a minimal effective dose (MED) ≤0.1 mg/kg (∼80% 5-HT₃ receptor occupancy; OR) and ≤3.0 mg/kg (5-HT_1A, 5-HT_1B, 5-HT₃ receptor occupancy: ∼15%, 60%, 95%) in SA. Ondansetron exhibited a MED ≤3.0 μg/kg (∼25% 5-HT₃ receptor occupancy; OR), but was inactive in the SA test. Flesinoxan had a MED ≤1.0 mg/kg (∼25% 5-HT_1A receptor occupancy; SA); only 1.0 mg/kg ameliorated deficits in the NOR. Chronic p.o. vortioxetine administration significantly improved memory performance in OR and occupied 95%, 66%, and 9.5% of 5-HT₃, 5-HT_1B, and 5-HT_1A receptors, respectively. Vortioxetine′s effects on SA performance may involve 5-HT_1A receptor agonism, but not 5-HT₃ receptor antagonism, whereas the effects on OR performance may involve 5-HT₃ receptor antagonism and 5-HT_1A receptor agonism.     

Involvement of 5-HT1C-receptors in drug-induced penile erections in rats

Drug-induced penile erections (PE) were initially suggested to be 5-HT_1B receptor mediated. However, since the discovery of the 5-HT_1C receptor a number of compounds, considered to be 5-HT_1B selective, appear to bind more strongly to the 5-HT_1C receptor and this prompted a re-evaluation of the receptor subtype involved in PE induction. PE could be induced by the 5-HT agonists mCPP (0.22–2.2 mg/kg), TFMPP (0.46–1.0 mg/kg) and MK 212 (0.1–1.0 mg/kg). The 5-HT agonist DOI (0.022–0.22 mg/kg) did not induce PE in placebo-pretreated rats but in rats pretreated with various 5-HT₂ antagonists it did. These compounds have in common a strong affinity for the 5-HT_1C receptor. mCPP (0.46 mg/kg)-induced PE could be antagonized by the 5-HT antagonists metergoline, cyproheptadine, mesulergine, mianserin, ritanserin and ketanserin. Their ED₅₀s were 0.04, 0.4, 0.03, 0.06, 0.4 and 2 mg/kg, respectively. The potency of both the agonists to induce, and the antagonists to inhibit, PE was found to be dependent on their selectivity for the 5-HT_1C receptor versus the 5-HT₂ receptor. Spiperone (0.1–1.0 mg/kg) and GR 38032F (1–10 mg/kg) did not antagonise mCPP-induced PE. 8-OH-DPAT and 5MeODMT counteracted mCPP (0.46 mg/kg)-induced PE. Their ED₅₀s were 0.03 and 0.4 mg/kg, respectively. DOI counteracted mCPP induced PE only at doses above 1 mg/kg, whereas CGS 12066B (1.0–10 mg/kg) was inactive. The results suggest that PE are induced by activation of the 5-HT_1C receptor and are functionally inhibited by activation of 5-HT_1A or 5-HT₂ receptors.     

Pharmacological profile of 5-hydroxytryptamine-induced inhibition on the pressor effect elicited by sympathetic stimulation in long-term diabetic pithed rats

We analysed the type and/or subtype of 5-hydroxytryptamine (5-HT) receptors involved in the inhibitory mechanisms of 5-HT on the pressor responses induced by stimulation of sympathetic vasopressor outflow in long-term diabetic pithed rats. Diabetes was induced in male Wistar rats by a single subcutaneous injection of alloxan. Eight weeks later, rats were anaesthetized, pre-treated with atropine, and pithed. The effect of 5-HT on the pressor responses elicited by stimulation of the sympathetic outflow was analysed in eight-week alloxan-induced diabetic pithed rats. 5-HT (20 μg/kg/min) reduced the pressor action obtained by electrical stimulation of the sympathetic outflow. However, there was no effect on exogenous noradrenaline-induced pressor responses. 5-CT (5 μg/kg/min), 8-OH-DPAT (5 μg/kg/min), and α-methyl-5-HT (5 μg/kg/min), selective 5-HT₁, 5-HT_1A and 5-HT₂ receptor agonists, respectively, reproduced the 5-HT inhibitory action. Nevertheless, infusion of 5 μg/kg/min of 1-phenylbiguanide, CGS-12066B, L-694,247, BW273C86 or MK212 (5-HT₃, 5-HT_1B, 5-HT_1D, 5-HT_2B and 5-HT_2C receptor agonists, respectively) had no effect on the pressor responses elicited by stimulation of the sympathetic outflow. Methiothepin (100 μg/kg) and a cocktail of WAY-100,635 (100 μg/kg) and spiperone (125 μg/kg) blocked the 5-HT inhibitory effect on the pressor action obtained by sympathetic stimulation. Moreover, WAY-100, 635 abolished the 8-OH-DPAT inhibitory effect and spiperone blocked α-methyl-5-HT action. In conclusion, this study revealed that long-term experimental diabetes induces changes in the receptor type/subtype involved in the 5-HT inhibitory action on the sympathetic pressor responses produced by electrical stimulation. This is mainly mediated by pre-junctional 5-HT_1A and 5-HT_2A receptors.     

Differences in the effects of 5-HT1A receptor agonists on forced swimming behavior and brain 5-HT metabolism between low and high aggressive mice

Rationale Male wild house-mice genetically selected for long attack latency (LAL) and short attack latency (SAL) differ in structural and functional properties of postsynaptic serotonergic-1A (5-HT_1A) receptors. These mouse lines also show divergent behavioral responses in the forced swimming test (FST, i.e., higher immobility by LAL versus SAL mice).Objectives We investigated whether the line difference in 5-HT_1A receptors is associated with a difference in brain 5-HT metabolism, and whether acute administration of a 5-HT_1A receptor agonist could differentially affect the behavioral responses of LAL and SAL mice.Methods 5-HT and 5-hydroxyindoleacetic acid (5-HIAA) levels were measured in homogenates of several brain regions using high-performance liquid chromatography. The behavioral effect of the full 5-HT_1A receptor agonist, 8-OH-DPAT, and of the somatodendritic 5-HT_1A autoreceptor agonist, S-15535, was examined in the FST. The effect of 8-OH-DPAT on forced swimming-induced 5-HT metabolism in brain homogenates was determined.Results In most brain regions, 5-HT and 5-HIAA levels and 5-HT turnover were not significantly different between LAL and SAL mice. 8-OH-DPAT abolished the behavioral line difference in the FST by reducing immobility in LAL mice and reducing climbing in SAL mice. S-15535 induced a similar behavioral effect to 8-OH-DPAT in SAL mice, but did not alter the behavior of LAL mice. Compared with LAL, forced swimming elicited in SAL mice a higher brain 5-HT turnover, which was potently attenuated by 8-OH-DPAT.Conclusions It is unlikely that the difference in 5-HT_1A properties between LAL and SAL mice is an adaptive compensatory reaction to changes in 5-HT metabolism. Although unspecific motor effects, at least in SAL mice, cannot be ruled out, it is suggested that the behavioral effects of 8-OH-DPAT and S-15535 may be mediated by predominant activation of postsynaptic 5-HT_1A receptors in LAL mice and by presynaptic 5-HT_1A receptors in SAL mice.     

Differential autoreceptor control of extracellular 5-HT in guinea pig and rat: species and regional differences

Rationale Central 5-hydroxytryptamine (5-HT) release is regulated by inhibitory 5-HT autoreceptors, including 5-HT_1A and 5-HT_1B receptors.Objectives The purpose of this study was to use combinations of selective autoreceptor antagonists to elucidate the role of these receptors in controlling extracellular 5-HT in terminal areas.Methods Microdialysis was carried out in awake rats and guinea pigs to measure extracellular 5-HT in the frontal cortex and dentate gyrus. Using the selective 5-HT_1A receptor antagonist, WAY-100635, and the selective 5-HT_1B receptor antagonist, SB-224289, we have compared the roles of 5-HT_1A and 5-HT_1B autoreceptors in controlling extracellular 5-HT.Results SB-224289 (4 mg/kg i.p.) alone produced a significant 50% increase in extracellular 5-HT in the dentate gyrus of guinea pigs, but not in the frontal cortex of the same animals. Co-administration of WAY-100635 (0.3 mg/kg s.c.), did not change the SB-224289-induced increase in dentate gyrus 5-HT but did produce a significant augmentation (60% increase) of guinea pig frontal cortex 5-HT. In contrast, neither autoreceptor antagonist, alone or in combination, affected extracellular 5-HT in the frontal cortex or dentate gyrus of rats.Conclusions These data indicate that there is a species difference in the autoreceptor control of 5-HT release. Furthermore, in the guinea pig there is a divergence between dorsal and median raphe innervated brain regions. On the basis that antagonism of 5-HT_1A and 5-HT_1B receptors produced an immediate increase in extracellular 5-HT in multiple brain regions in the guinea pig, it is suggested that this might be a novel mechanism for achieving antidepressant efficacy.     

Implication of 5-HT<Subscript>2</Subscript> receptor subtypes in the mechanism of action of antidepressants in the four plates test

Rationale The selective serotonin reuptake inhibitors (SSRIs) and the serotonin and noradrenaline reuptake inhibitors (SNRIs) increase synaptic levels of serotonin, leading to an increased activation of a multitude of specific postsynaptic 5-HT receptors. However, it is not yet known which 5-HT receptor subtypes mediate the therapeutic effects of antidepressants.Methods The effects of the SSRI, paroxetine and the SNRI, venlafaxine were evaluated in the mouse four plates test (FPT).Results Paroxetine administered intraperitoneally (IP) (0.5, 2–8 mg/kg) potently augmented the number of punished passages accepted by mice in this paradigm. The effects of paroxetine (8 mg/kg) were not reversed by the selective 5-HT_2C receptor antagonist, RS 10-2221 (0.1 mg/kg and 1 mg/kg) or the selective 5-HT_2B/2C receptor antagonist SB 206553 (0.1 mg/kg and 1 mg/kg), at doses which lack an effect when administered alone. In contrast, the selective 5-HT_2A receptor antagonist, SR 46349B (0.1 mg/kg and 1 mg/kg) completely abolished the paroxetine-induced increase in punished passages. The acute administration of venlafaxine induced an anxiolytic-like effect in the FPT at the doses of 2–16 mg/kg. This effect was reversed by the 5-HT_2B/2C receptor antagonist as did SR 46349B, for both doses administered. Our results strongly suggest that activation of 5-HT_2A receptors is critically involved in the anxiolytic activity of paroxetine, whereas the 5-HT_2A and 5-HT_2B receptors are involved in the anti-punishment action of venlafaxine in the FPT. The co-administration of selective 5-HT_{2A, 2B, 2C} receptor agonists (DOI, 0.06 mg/kg and 0.25 mg/kg; BW 723C86, 0.5 mg/kg and 2 mg/kg and RO 60-0175, 0.06 mg/kg and 0.25 mg/kg), respectively, was subsequently investigated. The effects of sub-active doses of paroxetine (0.25 mg/kg and 1 mg/kg) were augmented by BW 723C86 and RO 60-0175 receptor agonist challenge. The anti-punishment effects of venlafaxine (0.25 mg/kg and 1 mg/kg) were potentialised only by DOI co-administration.Conclusion These results indicate that the co-administration of 5-HT₂ receptor agonists with paroxetine and venlafaxine may provide a powerful tool for enhancing the clinical efficacy of these antidepressants.     

Role of 5-HT<Subscript>2A</Subscript> and 5-HT<Subscript>2C/B</Subscript> receptors in the acute effects of 3,4-methylenedioxymethamphetamine (MDMA) on striatal single-unit activity and locomotion in freely moving rats

Rationale Like amphetamine, a locomotor-activating dose of 3,4-methylenedioxymethamphetamine (MDMA) predominantly excites striatal single-unit activity in freely moving rats. Although both D₁- and D₂-like dopamine (DA) receptors play important roles in this effect, MDMA, unlike amphetamine, strongly increases both DA and serotonin (5-HT) transmission. Objectives This study was conducted to investigate the 5-HT receptor mechanisms underlying the striatal effects of MDMA. Methods We recorded the activity of >200 single units in the striatum of awake, unrestrained rats in response to acute MDMA administration (5 mg/kg) combined with the selective blockade of either 5-HT_2A or 5-HT_2C/B receptors. Results Prior administration of SR-46349B (a 5-HT_2A antagonist 0.5 mg/kg) blocked nearly all MDMA-induced striatal excitations, which paralleled its significant attenuation of MDMA-induced locomotor activation. Conversely, prior administration of SB-206553 (a 5-HT_2C/B antagonist 2.0 mg/kg) had no effect on the amount of MDMA-induced locomotor activation or the distribution of single-unit responses to MDMA. However, a coefficient-of-variation analysis indicated significantly less variability in the magnitude of both MDMA-induced neuronal excitations and inhibitions in rats that were pretreated with SB-206553 compared to vehicle. Analysis of concurrent single-unit activity and behavior confirmed that MDMA-induced striatal activation was not merely due to behavioral feedback, indicating a primary action of MDMA. Conclusion These results support and extend our previous findings by showing that 5-HT_2A and 5-HT_2C/B receptors differentially regulate the expression of MDMA-induced behavioral and striatal neuronal responses, either directly or through the modulation of DA transmission.     

Melatonin affects the immobility time of rats in the forced swim test: The role of serotonin neurotransmission

The efficacy of melatonin or its derivatives in depressive patients has been recently considered for clinical application. However, the evidence for its effect on experimental models of depression is not consolidated. Here, the effects of melatonin on the model of forced swim test (FST) paradigm were studied in male rats of the Wistar strain after acute intraperitoneal (i.p.) administration of 0.1, 0.5 or 1 mg/kg of the hormone. Melatonin at doses of 0.5 and 1 mg/kg, but not of 0.1 mg/kg, decreased the immobility of rats in the FST paradigm suggesting a possible antidepressant-like activity. The dose of 0.5 mg/kg appeared to be as potent as clomipramine 50 mg/kg in reducing the immobility time of rats in the FST paradigm. The effect of melatonin on immobility time of rats in the FST paradigm was abolished by the simultaneous injection of the non-selective melatonin antagonist, luzindole (0.25 mg/kg, subcutaneously). Similarly, administration of small quantities of serotonin (5-HT, 5 ng/1 μl) or of the 5-HT_2A/5-HT_2C receptor agonist 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (2 ng/1 μl) injected into the amygdale totally suppressed the reduction of immobility time in the FST paradigm induced by melatonin 0.5 mg/kg. These results may suggest that effects of melatonin on the behavioral reaction of rats in the FST paradigm are due to an interaction of the hormone with central 5-HT neurotransmission.     

On the role of brain 5-HT7 receptor in the mechanism of hypothermia: comparison with hypothermia mediated via 5-HT1A and 5-HT3 receptor

Intracerebroventricular administration of selective agonist of serotonin 5-HT₇ receptor LP44 (4-[2-(methylthio)phenyl]-N-(1,2,3,4-tetrahydro-1-naphthalenyl)-1-pyperasinehexanamide hydrochloride; 10.3, 20.5 or 41.0 nmol) produced considerable hypothermic response in CBA/Lac mice. LP44-induced (20.5 nmol) hypothermia was significantly attenuated by the selective 5-HT₇ receptor antagonist SB 269970 (16.1 fmol, i.c.v.) pretreatment. At the same time, intraperitoneal administration of LP44 in a wide range of doses 1.0, 2.0 or 10.0 mg/kg (2.0, 4.0, 20.0 μmol/kg) did not cause considerable hypothermic response. These findings indicate the implication of central, rather than peripheral 5-HT₇ receptors in the regulation of hypothermia. The comparison of LP44-induced (20.5 nmol) hypothermic reaction in eight inbred mouse strains (DBA/2J, CBA/Lac, C57BL/6, BALB/c, ICR, AKR/J, C3H and Asn) was performed and a significant effect of genotype was found.In the same eight mouse strains, functional activity of 5-HT_1A and 5-HT₃ receptors was studied. The comparison of hypothermic responses produced by 5-HT₇ receptor agonist LP44 (20.5 nmol, i.c.v.) and 5-HT_1A receptor agonist 8-OH-DPAT 1.0 mg/kg, i.p. (3.0 μmol/kg), 5-HT₃ receptor agonist m-CPBG (40.0 nmol, i.c.v.) did not reveal considerable interstrain correlations between 5-HT₇ and 5-HT_1A or 5-HT₃ receptor-induced hypothermia. The selective 5-HT₇ receptor antagonist SB 269970 (16.1 fmol, i.c.v.) failed to attenuate the hypothermic effect of 8-OH-DPAT 1.0 mg/kg, i.p. (3.0 μmol/kg) and m-CPBG (40.0 nmol, i.c.v.) indicating that the brain 5-HT₇ receptor is not involved in the hypothermic effects of 8-OH-DPAT or m-CPBG. The obtained results suggest that the central 5-HT₇ receptor plays an essential role in the mediation of thermoregulation independent of 5-HT_1A and 5-HT₃ receptors.     

Serotonin Receptors in the Brain of Rats Treated Chronically with Imipramine or RU24969: Support for the 5-HT1B Receptor Being a 5-HT Autoreceptor

Abstract: Rats were treated by intraperitoneal injection for four weeks with either RU24969, a 5-HT_1B and 5-HT_1A agonist or imipramine, a 5-HT uptake inhibitor. Pre- and postsynaptic 5-HT receptors were measured to compare the effect of direct or indirect stimulation of the 5-HT autoreceptor (5-HT_1B receptor). The 5-HT transport protein (5-HT uptake site), labelled with [³H]paroxetine, was unaffected after treatment with either one of the drugs. The density of 5-HT2 receptors, labelled with [³H]ketanserin, we found increased after treatment with RU24969 (B_max=161 fmol/mg protein) and decreased after treatment with imipramine (B_max=109 fmol/mg protein) as compared with control rats (B_max=134 fmol/mg protein). The 5-HT_1B receptor was found decreased both by the imipramine treatment (B_max=106 fmol/mg protein) and the treatment with RU24969 (B_max=105 fmol/mg protein), compared with control rats (B_max=130 fmol/mg protein). The 5-HT_1A receptor was found to be decreased after treatment with RU24969 (control: B_max=62 fmol/mg protein; RU24969-treated: 49 fmol/mg protein), but unchanged after treatment with imipramine (B_max=58 fmol/mg protein). These results correspond to what could be expected, if the 5-HT_1B receptor is the 5-HT autoreceptor.