Involvement of 5-HT1B receptors in the anticonflict effect of m-CPP in rats

Summary The anticonflict activity of m-CPP, a non-selective agonist of 5-HT receptors, was studied in the drinking conflict test in rats. m-CPP administered in doses of 0.125–0. 5 mg/kg increased the number of punished licks, the maximum effect having been observed after a dose of 0.25 mg/kg. The anticonflict effect of m-CPP (0.25 mg/kg) was antagonized by the non-selective 5-HT antagonist metergoline (1–4 mg/kg) and by the -adrenoceptor blocker SDZ 21009 (2 and 4 mg/kg) with affinity for 5-HT_1A and 5-HT_1B receptors. On the other hand, the 5-HT_1A receptor antagonist NAN-190 (0.5 and 1 mg/kg), the 5-HT₂ receptor antagonist ritanserin (0.25 and 0.5 mg/kg), and the -blockers betaxolol (8 mg/kg) and ICI 118,551 (8 mg/kg) with no affinity for 5-HT receptors did not affect the effect of m-CPP. The effect of m-CPP was not modified, either, in animals with the 5-HT lesion produced by p-chloroamphetamine.These results suggest that the anticonflict effect of m-CPP described above results from stimulation of 5-HT_1B receptors — most probably these which are located postsynaptically.     

Ex vivo inhibitory effect of the 5-HT uptake blocker citalopram on 5-HT synthesis

Summary Serotonin (5-hydroxytryptamine, 5-HT) synthesis was determined in vivo by measuring the accumulation of 5-hydroxytryptophan (5-HTP) in rat frontal cortex after inhibition of aromatic amino acid decarboxylase by administration of m-hydroxybenzylhydrazine (NSD 1015) (100 mg/kg, i.p.). The selective 5-HT reuptake inhibitor, citalopram, the 5-HT_1a agonists, (±)8-hydroxy-2-(di-n-propylamino)-tetralin (8-OH-DPAT), ipsapirone, gepirone and the 5-HT_1a/b agonist, 7-trifluoromethyl-4(4-methyl-1-piperazinylpyrolo[1,2-a]-quinoxaline (CGS 12066B), the 5-HT_1a/b ligands and -adrenoceptor antagonists, (±)pindolol and (±)alprenolol, and the non-selective 5-HT ligands, m-chlorophenylpiperazine (mCPP) and metergoline, all inhibited the synthesis of 5-HT. The 5-HT_1a /5-HT₂ antagonist, spiperone, alone, had no effect on basal 5-HT synthesis, however it attenuated the effect of 8-OH-DPAT by 56% and CGS 12066B by 39% but only barely that of citalopram by 17%. The selective 5-HT_1a antagonist, WAY 100635, which did not modify by itself 5-HT synthesis, had no effect on citalopram-induced reduction of 5-HT synthesis. Neither the 5-HT₂ agonist, (±)1-(2,5-dimethoxy-4-indophenyl)-2-aminopropane (DOI) nor the 5-HT₂ antagonist, ritanserin, had any effect on the synthesis of 5-HT. In addition, ritanserin did not modify the inhibitory effect of citalopram. Methiothepin was the only compound to increase 5-HT synthesis. These results suggest that the effect of citalopram on the synthesis of 5-HT is not mediated by 5-HT_1a or 5-HT₂ receptors and that other receptors may be involved.     

Pharmacological effects of milnacipran, a new antidepressant, given repeatedly on the α1-adrenergic and serotonergic 5-HT2A systems

Summary. Milanacipran (MIL) is a representative of a new class of antidepressants (SNRIs) which inhibit selectively the reuptake of serotonin and noradrenaline but, in contrast to tricyclics, show no affinity for neurotransmitter receptors. The present study was aimed at determining whether repeated MIL treatment induced adaptive changes in the α₁-adrenergic and serotonergic 5-HT_2A systems, similar to those reported by us earlier for tricyclic antidepressants. The experiments were carried out on male mice and rats. MIL was administered at a dose of 10 or 30 mg/kg p.o. once or repeatedly (twice daily for 14 days). The obtained results showed that MIL administered repeatedly potentiated the clonidine-induced aggressiveness and the methoxamine-induced exploratory hyperactivity, the effects mediated by α₁-adrenoceptors. MIL did not change the number or affinity (B_max and K_D) of α₁-adrenergic receptors in the cerebral cortex for [³H]prazonsin, however, the ability of the α₁-adrenoceptor agonist phenylephrine to compete for these sites was significantly enhanced. MIL given repeatedly (but not acutely) inhibited both the head twitch reaction induced by L-5-HTP or (±)DOI, the effects mediated by serotonergic 5-HT_2A receptors. MIL also decreased the binding (B_max) or [³H]-ketanserin to 5-HT_2A receptors in the cerebral cortex. The above results indicate that repeated MIL administration increases the responsiveness of α₁-adrenergic system (behavioural and biochemical changes) and decreases the responsiveness of the serotonergic 5-HT_2A receptors (especially behavioural changes) as tricyclics do. It may be concluded that the lack of MIL affinity for neurotransmitter receptors is of no importance to the development of adaptive changes in the studied systems, observed after repeated treatment with antidepressants. Received August 12, 1999; accepted April 6, 2000     

Functional correlates for 5-HT1A receptors in maternally deprived rats displaying anxiety and depression-like behaviors

Maternal separation is known to induce long-term changes in neuroendocrine and emotional responsiveness to stress in a large variety of models. We examined an animal model of early deprivation in Sprague–Dawley rats consisting of separating litters from their mothers and littermates 3 h daily during postnatal days 2 to 15. In adulthood, maternally deprived rats in comparison with non-deprived controls exhibited an increase in anxiety and depression-related behaviors in the open-field and forced swim tests. Because serotonin (5-HT) 5-HT_1A receptors seem to play an important role in the pathophysiology of major depression and in the mechanism of action of antidepressants, we investigated if 5-HT_1A receptor function is altered in deprived rats. Although the hypothermic response to the 5-HT_1A receptor agonist 8-OH-DPAT was increased in adult deprived rats compared to non-deprived control group, no differences between groups were found in the effect of the systemic 8-OH-DPAT administration on serotoninergic cell firing in dorsal raphe nucleus and in the 5-HT release at the ventral hippocampus levels. These results suggest that 5-HT_1A receptors are not substantially affected in adult Sprague–Dawley rats that were subjected to a maternal deprivation 3 h daily during the neonatal period.     

Differential effects of 5-HT1A receptor deletion upon basal and fluoxetine-evoked 5-HT concentrations as revealed by in vivo microdialysis

An involvement of serotonin (5-HT) 1A receptors in the etiology of psychiatric disorders has been suggested. Hypo-responsiveness of the 5-HT_1A receptor is linked to anxiety and constitutive deletion of the 5-HT_1A receptor produces anxiety-like behaviors in the mouse. Evidence that 5-HT_1A receptor inactivation increases the therapeutic effects of antidepressants has also been presented. The present studies used in vivo microdialysis and homologous recombination techniques to examine the contribution of 5-HT_1A autoreceptors to these effects. Basal and fluoxetine-evoked extracellular concentrations of 5-HT were quantified in the striatum, a projection area of dorsal raphe neurons (DRN), of wild-type (WT) and 5-HT_1A receptor knock out (KO) mice. The density of 5-HT transporters was also determined. Basal 5-HT concentrations did not differ in WT and KO mice. Fluoxetine (10 mg/kg) increased 5-HT concentrations in both genotypes. This increase was, however, 2-fold greater in KO mice. In contrast, no differences in K⁺-evoked 5-HT concentrations were seen. Similarly, neither basal nor stimulation-evoked DA differed across genotype. Autoradiography revealed no differences between genotype in the density of 5-HT transporters or post-synaptic 5-HT_2A receptors, an index of 5-HT neuronal activity. These experiments demonstrate that, under basal and KCl stimulated conditions, adaptive mechanisms in the 5-HT system compensate for the lack of 5-HT_1A autoreceptor regulation of DRN. Furthermore, they suggest that the absence of release-regulating 5-HT_1A autoreceptors in the DRN can not account for the anxiety phenotype of KO mice. The enhanced response to fluoxetine in KO mice is consistent with pharmacological studies and suggests that adaptive mechanisms that occur in response to 5-HT_1A receptor deletion are insufficient to oppose increases in 5-HT concentrations produced by acute inhibition of the 5-HT transporter.     

Potent serotonin (5-HT)2A receptor antagonists completely prevent the development of hyperthermia in an animal model of the 5-HT syndrome

The serotonin (5-HT) syndrome is the most serious side effect of antidepressants, and it often necessitates pharmacotherapy. In the present study, the efficacy of several drugs was evaluated in an animal model of the 5-HT syndrome. When 2 mg/kg of clorgyline, a type-A monoamine oxidase inhibiting antidepressant, and 100 mg/kg of 5-hydroxy- -tryptophan, a precursor of 5-HT, were administered intraperitoneally to rats to induce the 5-HT syndrome, the rectal temperature of the rats increased to more than 40°C, and all of the animals died by 90 min after the drug administration. The noradrenaline (NA) levels in the anterior hypothalamus, measured by microdialysis, increased to 15.9 times the preadministration level. Pretreatment with propranolol (10 mg/kg), a 5-HT_1A receptor antagonist as well as a beta-blocker, and dantrolene (20 mg/kg), a peripheral muscle relaxant, did not prevent the death of the animals, even though these two drugs suppressed the increase in rectal temperature to some extent. Chlorpromazine and cyproheptadine prevented the lethality associated with the 5-HT syndrome only at high doses. By contrast, pretreatment with ritanserin (3 mg/kg) and pipamperone (20 mg/kg), both potent 5-HT_2A receptor antagonists, completely prevented the increase in rectal temperature and death of the animals, and the hypothalamic NA levels in these two groups increased less than that in the other groups. These results suggest that potent 5-HT_2A receptor antagonists are the most effective drugs for treatment of the 5-HT syndrome, and that NA hyperactivity occurs in the 5-HT syndrome.     

Evidence for 5-HT4 receptor involvement in the enhancement of acetylcholine release by p-chloroamphetamine in rat frontal cortex

The roles of endogenous serotonin (5-HT) and 5-HT receptor subtypes in regulation of acetylcholine (ACh) release in frontal cortex of conscious rats were examined using a microdialysis technique. Systemic administration (1 and 3 mg/kg, i.p.) of the 5-HT-releasing agent p-chloroamphetamine (PCA) elevated ACh output in a dose-dependent manner. Depletion of endogenous 5-HT by p-chlorophenylalanine significantly attenuated the facilitatory effect of PCA on ACh release. The PCA (3 mg/kg)-induced increase in ACh release was significantly inhibited by local application of the 5-HT₄ receptor antagonists RS23597 (50 μM) and GR113803 (1 μM), while the 5-HT_1A antagonist WAY-100135 (10 mg/kg, i.p.; 100 μM), 5-HT_1A/1B/β-adrenoceptor antagonists (−)-pindolol (8 mg/kg, i.p.) and (−)-propranolol (150 μM), 5-HT_2A/2C antagonist ritanserin (1 mg/kg, i.p.; 10 μM) and 5-HT₃ antagonist ondansetron (1 mg/kg, i.p.; 10 μM) failed to significantly modify the effect of PCA. These results suggest that PCA-induced enhancement of 5-HT transmission facilitates ACh release from rat frontal cortex at least in part through 5-HT₄ receptors.     

Pre- and post-synaptic effects of the 5-HT3 agonist 2-Methyl-5-HT on the 5-HT system in the rat brain

Microiontophoretic applications of 5-HT and of the 5-HT₃ agonist 2-methyl-5-HT produced a current-dependent suppression of firing activity of both hippocampal (CA₁ and CA₃) and cortical neurons in anesthetized rats. Concomitant microiontophoretic applications of the 5-HT₃ antagonists BRL 46470A and S-zacopride, as well as their intravenous injection, did not antagonize the inhibitory effect of 5-HT and 2-methyl-5-HT. In contrast, the 5-HT_1A antagonist BMY 7378, applied by microiontophoresis or administered intravenously, significantly reduced the inhibitory action of 5-HT and 2-methyl-5-HT. The firing activity of dorsal raphe 5-HT neurons was also reduced by 5-HT, 2-methyl-5-HT and the 5-HT_1A agonist 8-OH-DPAT applied by microiontophoresis. While BRL 46470A (0.1 and 1 mg/kg, i.v.) did not antagonize the inhibitory effect of the three 5-HT agonists on 5-HT neuronal firing activity, only that of 8-OH-DPAT was attenuated by the 5-HT_1A antagonist (+) WAY 100135. R-zacopride significantly reduced the duration of suppression of firing activity of CA₃ pyramidal neurons induced by the electrical stimulation of the ascending 5-HT pathway, and this reducing effect was prevented by the three 5-HT₃/5-HT₄ antagonists renzapride, S-zacopride and tropisetron, but not by BRL 46470A. Finally, in in vitro superfusion experiments, both BRL 46470A and S-zacopride antagonized the enhancing action of 2-methyld-HT on the electrically-evoked release of [³H]-5-HT in both rat frontal cortex and hippocampus slices. These findings suggest that, in vivo, the suppressant effect of 2-methyl-5-HT on the firing activity of dorsal hippocampus pyramidal, somatosensory cortical, and dorsal raphe 5-HT neurons is not mediated by 5-HT₃ receptors, but rather by 5-HT_1A receptors. The attenuating effect of R-zacopride on the effectiveness of the stimulation of the ascending 5-HT pathway is not mediated by 5-HT₃ receptors. In contrast, in vitro, the enhancing action of 2-methyl-5-HT on the electrically-evoked release of [³H]5-HT in both frontal cortex and hippocampus slices is mediated by 5-HT₃ receptors. © 1995 Wiley-Liss, Inc.     

Effects of antipsychotic drugs on dopamine and serotonin contents and metabolites, dopamine and serotonin transporters, and serotonin1A receptors

Summary. The effects of neuroleptics have been attributed to dopamine (DA) receptor blockade; however, other neurotransmitters, in particular serotonin (5-HT), have also been implicated. In this study, we examined the effects of clozapine and haloperidol on the distribution of DA and 5-HT transporters, on endogenous DA, 5-HT and their major metabolites, and on 5-HT_1A receptors. Adult male Sprague-Dawley rats were treated with either haloperidol (1 mg/kg/day, i.p.), clozapine (20 mg/kg/day, i.p.) or saline for 21 days, and following 3 days of withdrawal, the brains were removed. Tissue levels of DA and 5-HT and their metabolites were measured by high-performance liquid chromatography in 16 brain regions, while quantitative autoradiography with [¹²⁵I]RTI-121, [³H]citalopram and [³H]8-OH-DPAT was employed to label DA transporters, 5-HT transporters and 5-HT_1A receptors, respectively. After haloperidol, densities of 5-HT transporters were increased in the dorsal insular cortex and in the ventral half of caudal neostriatum, while 5-HT_1A receptors augmented in cingulate cortex but decreased in the entorhinal area. After clozapine, [³H]citalopram labelling was increased in ventral hippocampus, ventral caudal neostriatum and nucleus raphe dorsalis, but decreased in medio-dorsal and latero-dorsal neostriatum as well as in substantia nigra. Binding of [³H]8-OH-DPAT following clozapine was decreased in frontal, parietal, temporal and entorhinal cortices but increased in the CA3 division of Ammon's horn. The changes in 5-HT transporters in nucleus raphe dorsalis and substantia nigra, as well as the 5-HT_1A receptor down-regulations caused by clozapine but not by haloperidol, may explain effects obtained with clozapine and other atypical neuroleptics. There were no modifications in densities of DA transporters, nor of tissue DA levels, after the chronic neuroleptic treatments. The results are in line with previous suggestions that a certain degree of tolerance to neuroleptics develops, in spite of profound D₁ and D₂ receptor changes that persist during the entire chronic treatment with these psychotropic agents. Received September 2, 1997; accepted July 9, 1998     

The synergistic effect of fluoxetine on the locomotor hyperactivity induced by MK-801, a non-competitive NMDA receptor antagonist

Summary It was found previously that the MK-801 (an uncompetitive NMDA receptor antagonist)-induced locomotor hyperactivity in rats was potently increased by antidepressant drugs. The present paper analysed the locomotor hyperactivity induced by combined treatment with fluoxetine + MK-801 in male Wistar rats. The MK-801 hyperactivity was increased by citalopram (the latter effect was prevented by zacopride and ketanserin), sertraline, p-chloramphetamine, 8-OH-DPAT and TFMPP. The hyperlocomotion caused by fluoxetine + MK-801 was antagonized by tropisetron and zacopride and, to a lesser extent, by ketanserin, ritanserin and NAN-190, but not by WAY 100135, pindolol, metergoline or mianserin. Sulpiride and clozapine were able to inhibit the fluoxetine + MK-801 hyperlocomotion. The hyperlocomotion induced by D-amphetamine or apomorphine was not modified by fluoxetine or citalopram. Fluoxetine increased the release of dopamine (measured by a microdialysis method) in the striatum, induced by MK-801. The obtained results indicate that fluoxetine increases the MK-801-induced locomotor hyperactivity via activation of 5-HT₃ receptors and, to a lesser degree, 5-HT₂ ones.     

Hypothermia induced by m-trifluoromethylphenylpiperazine or m-chlorophenylpiperazine: an effect mediated by 5-HT1B receptors?

Summary In the present study we examined the effect of different drugs on the m-trifluoromethylphenylpiperazine (TFMPP)- and m-chlorophenylpiperazine (m-CPP)-induced hypothermia in mice. Both the hypothermias studied are blocked or reversed by pindolol, cyanopindolol and compound 21-009, but not by atenolol. Neither hypothermia is antagonized by 5-HT_1A antagonists (ipsapirone, spiperone), a 5-HT_1C antagonist (mesulergine), 5-HT₂ antagonists (cyproheptadine, mianserin, methysergide), 5-HT₃ antagonists (ICS 205930, metoclopramide). The examined hypothermias are not antagonized by other antihypothermic agents (pimozide, idazoxan, atropine). The 8-OH-DPAT-induced hypothermia is not affected by cyanopindolol or compound 21009. The obtained results indicate that the TFMPP- and m-CPP-induced hypothermias in mice are mediated by 5-HT_1B. These hypothermias may be a good screening test for evaluation of the 5-HT_1B-agonistic and 5-HT_1B-antagonistic activity.     

Dose-dependent effects of buspirone on behavior and cerebral glucose metabolism in rats

In this study we compared the effects of the anxiolytic buspirone on behavior and regional cerebral metabolic rates for glucose (rCMRglc) with those of the reference serotonin (5-HT)_1A agonist 8-hydroxy-2(di-N-propylamino)tetralin (DPAT). Behavioral effects were assessed by scoring the 5-HT syndrome. rCMRglc was measured in 56 brain regions by using the quantitative autoradiographic [¹⁴C]2-deoxyglucose technique, at 10 min after i.p. injection of DPAT (1 mg/kg) or buspirone (0.4, 4 and 40 mg/kg) in awake male Fischer-344 rats. Whereas DPAT produced an intense 5-HT syndrome, buspirone had no behavioral effect. A low dose (0.4 mg/kg) of buspirone reduced rCMRglc in 18 brain areas (32%), more markedly in limbic areas and raphe nuclei. These were the only rCMRglc effects buspirone had in common with the potent 5-HT_1A agonist DPAT and suggest that low dose buspirone activates preferentially 5-HT_1A receptors. Hence, this receptor subtype may mediate buspirone functional effects on the limbic system and, given the role of these brain areas in mood control, possibly buspirone therapeutic actions. High doses (4 and 40 mg/kg) of buspirone produced widespread rCMRglc decreases in 46 (82%) and 44 (79%) of the areas studied and increased rCMRglc in one brain area, the lateral habenula, that was not affected by DPAT or a low dose of buspirone. The topographic distribution and direction of rCMRglc changes by high doses of buspirone differ from those produced by the 5-HT_1A agonist DPAT. Instead these changes resemble the rCMRglc effects of dopaminergic D₂ antagonists like haloperidol and are consistent with some pharmacological and binding properties of buspirone. In summary, this study suggests that buspirone produces dual, dose-dependent rCMRglc effects: (i) at a low dose rCMRglc reductions in limbic areas and raphe nuclei, probably due to preferential activation of 5-HT_1A receptors, and (ii) at higher doses widespread rCMRglc reductions along with a rCMRglc increase in the lateral habenula resulting from dopamine D₂ receptor blockade.     

Molecular and cellular mechanisms of antidepressant drugs

The mechanism of action of the antidepressants was reviewed through the summer of 1995. Antagonism of monoamine transport is the primary cellular action associated with many antidepressant medications. However, an increased synaptic concentration of monoamines is not the actual mechanism of antidepressant effects as suggested by the time lapse between treatment initiation and the onset of clinical antidepressant response. Chronic administration of all antidepressants increases the efficiency of 5-HT transmission in the synapse, albeit by different mechanisms. Serotonin transporter antagonists enhance serotonergic neurotransmission by decreasing the functional activity of the 5-HT_1A and 5-HT_1B autoreceptors. Tricyclic antidepressants and electroconvulsive therapy enhance 5-HT neurotransmission by increasing the responsiveness of postsynaptic 5-HT receptors. Monoamine oxidase inhibitors and the 5-HT_1A agonists increase serotonergic function by desensitizing the somatodendritic autoreceptor; the terminal 5-HT autoreceptor is not affected by these agents. Evidence for both enhanced and diminished noradrenergic transmission following antidepressant treatment exists, although the evidence more strongly supports decreased noradrenergic transmission. Ongoing investigations into intracellular adaptations (e.g., steroid receptors, growth factors, etc.) During chronic antidepressant administration offer the promise of furthering our understanding of the mechanism of action of antidepressants. Depression and Anxiety 4:153–159, 1996/1997. © 1997 Wiley-Liss, Inc.     

Evidence for the involvement of 5-HT1A receptor in the acute antidepressant-like effect of creatine in mice

Creatine was previously shown to produce an antidepressant-like effect in the tail suspension test through a modulation of the dopaminergic system. In this study, the mechanisms underlying its antidepressant-like effect were further evaluated by investigating the involvement of the serotonergic system in its effect. The anti-immobility effect of creatine (1 mg/kg) was prevented by the pretreatment of mice with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, i.p., for 4 consecutive days, an inhibitor of serotonin (5-HT) synthesis). Creatine (0.01 mg/kg, sub-effective dose) in combination with sub-effective doses of WAY100635 (0.1 mg/kg, s.c., a 5-HT_1A receptor antagonist), 8-OH-DPAT (0.1 mg/kg, i.p., a 5-HT_1A receptor agonist) or selective serotonin reuptake inhibitors fluoxetine (5 mg/kg, p.o.), paroxetine (0.1 mg/kg, p.o.), citalopram (0.1 mg/kg, p.o.) and sertraline (3 mg/kg, p.o.) reduced the immobility time in the tail suspension test as compared with either drug alone. These results indicate that the antidepressant-like effect of creatine is likely mediated by an interaction with 5-HT_1A receptors. Of note, the present results also indicate that creatine improves the effectiveness of the selective serotonin reuptake inhibitors, a finding that may have therapeutic implications for the treatment of depressive disorders.     

Study of mechanisms of calcitonin analgesia in mice: Involvement of 5-HT3 receptors

The analgesic effect of calcitonin when serotonin (5-HT) concentration is increased and the involvement of some 5-HT receptors were studied using the writhing test in mice. 5-hydroxytryptophan (5-HTP) administration increased both 5-HT levels in the central nervous system (CNS) and calcitonin analgesia. The 5-HT_1A agonist (±)-8-hydroxy-2-dipropylaminotetralin hydrobromide (8-OH-DPAT) diminished calcitonin analgesia, this effect being antagonised by the 5-HT_1A antagonist (WAY 100, 135). As the stimulation of 5-HT_1A autoreceptors reduces the turnover of 5-HT, the effect of 8-OH-DPAT on calcitonin analgesia may be attributed to this decrease. The 5-HT_2A–2C agonist (±)-1-(4-iodo-2,5-dimethoxyphenyl)-2-aminopropane hydrochloride (DOI) diminished calcitonin analgesia. A sub-analgesic dose of the 5-HT_2A antagonist ketanserin failed to prevent this effect. The 5-HT₃ agonist (±)-2-methyl-5-hydroxytryptamine maleate (2-methyl-5-HT) potentiated calcitonin analgesia, whereas it was significantly reduced by the 5-HT₃ antagonist tropisetron. The effect of 2-methyl-5-HT on calcitonin analgesia was also reversed by tropisetron, This result suggests that the 5-HT₃ receptor may play an important role in the relationship between calcitonin and the serotonergic system. Tropisetron also reversed the analgesia induced by calcitonin plus 5-HTP corroborating importance of the 5-HT₃ receptors.     

Serotoninergic Mechanisms of Immunomodulation Under Different Psychoemotional States: I. A role of 5-HT1a Receptor Subtype

The present study demonstrates the involvement of serotonin (5-HT) receptors of the 5-HT_1A type in immunoinhibitory effect of 5-HTergic system of the brain. A selective agonist of 5-HT_1A receptors 8-OH-DPAT (1 mg/kg) induces the immunosuppression, whereas 5-HT_1A blockade with WAY-100635 (1 mg/kg) resulted in immunostimulation. It is also shown that immunomodulating effects of the drugs were dependent on psychoemotional status of animals acquired aggressive or submissive behavior under social conflict conditions. Activation of 5-HT_1A receptors produced a decrease of the immunity in aggressive mice, whereas 5-HT_1A receptor blockade caused immunostimulation in submissive animals.     

Serotonergic agents modulate antidepressant-like effect of the neurosteroid 3α-hydroxy-5α-pregnan-20-one in mice

The present study demonstrated the antidepressant-like effect of neurosteroid 3α-hydroxy-5α-pregnan-20-one (3α, 5α THP) in mouse forced swim test of depression and its modulation by different serotonergic agents. Pretreatment with the selective serotonin reuptake inhibitor, fluoxetine (5 mg/kg, i.p.), the 5-HT releaser, fenfluramine (10 mg/kg, i.p.), the 5-HT_1A receptor agonist, 8-OH-DPAT (0.1 mg/kg, s.c.), the 5-HT_1B/1C receptor agonist, TFMPP (4 mg/kg, s.c.) and the 5-HT_2A/1C receptor agonist, DOI (2 mg/kg, s.c.) potentiated the antidepressant-like effect of 3α, 5α THP. At these doses the serotonergic agents per se did not modify the duration of immobility. However, fluoxetine (20 mg/kg, i.p.), fenfluramine (20 mg/kg, i.p.) or imipramine (5 or 20 mg/kg, i.p.) not only reduced immobility but also enhanced the antidepressant-like effect of 3α, 5α THP. Such a potentiating effect of the 5-HT_1A or the 5-HT_2A/1C receptor agonist was not antagonized by the sub-effective dose (0.1 mg/kg, s.c.) of their respective antagonists p-MPPI or ketanserin. Pretreatment with p-CPA (300×3 mg/kg, i.p.), a depleter of 5-HT neuronal store failed to block the influence of fluoxetine and fenfluramine on antidepressant-like effect of 3α, 5α THP. The accelerated effect of 3α, 5α THP in presence of serotonergic agents was antagonized by the GABA_A receptor antagonist, bicuculline (1 mg/kg, i.p.) or the 3α-hydroxysteroid oxidoreductase enzyme inhibitor, indomethacin (5 mg/kg, i.p.). These findings for the first time demonstrate that serotonergic agents potentiate the antidepressant-like action of 3α, 5α THP, by enhancing the GABAergic tone as a likely consequence of increased brain content of this neurosteroid.     

In vivo serotonin-sensitive binding of [11C]CUMI-101: a serotonin 1A receptor agonist positron emission tomography radiotracer

Positron emission tomography studies of 5-hydroxytryptamine (5-HT)_1A receptors have hitherto been limited to antagonist radiotracers. Antagonists do not distinguish high/low-affinity conformations of G protein-coupled receptors and are less likely to be sensitive to intrasynaptic serotonin levels. We developed a novel 5-HT_1A agonist radiotracer [¹¹C]CUMI-101. This study evaluates the sensitivity of [¹¹C]CUMI-101 binding to increases in intrasynaptic serotonin induced by intravenous citalopram and fenfluramine. Two Papio anubis were scanned, using [¹¹C]CUMI-101 intravenous bolus of 4.5±1.5 mCi. Binding potential (BP_F=B_avail/K_D) was measured before (n=10) and 20 minutes after elevation of intrasynaptic serotonin by intravenous citalopram (2 mg/kg, n=3; 4 mg/kg, n=3) and fenfluramine (2.5 mg/kg, n=3) using a metabolite-corrected arterial input function. Occupancy was also estimated by the Lassen graphical approach. Both citalopram and fenfluramine effects were significant for BP_F (P=0.031, P=0.049, respectively). The Lassen approach estimated 15.0, 30.4, and 23.7% average occupancy after citalopram 2 mg/kg, 4 mg/kg, and fenfluramine 2.5 mg/kg, respectively. [¹¹C]CUMI-101 binding is sensitive to a large increase in intrasynaptic serotonin in response to robust pharmacological challenges. These modest changes in BP_F may make it unlikely that this ligand will detect changes in intrasynaptic 5-HT under physiologic conditions; future work will focus on evaluating its utility in measuring the responsiveness of the 5-HT system to pharmacological challenges.     

5-HT receptor subtypes involved in the stimulatory effect of 5-HT on the peristaltic reflex in vitro

The effect of serotonin (5-HT) and of more selective 5-HT agonists on the peristaltic reflex evoked in the isolated guinea-pig ileum was investigated. Using the Trendelenburg technique, peristaltic contractions were elicited by increasing intraluminal pressure, and rhythmic contractions of the longitudinal and circular muscle were measured after serosal administration of the drugs. 5-HT potently stimulated contractions of the longitudinal muscle. The effect of 5-HT was partly antagonized by the 5-HT₄ receptor antagonist SDZ 205–557. Of the potent 5-HT_1A receptor agonists, 8-OH-DPAT, 5-carboxamidotryptamine (5-CT) and dipropyl-5-CT (DP-5-CT), only 5-CT caused a substantial stimulation. Of the 5-HT_1C-/5-HT₂ receptor agonists DOI and 5-methoxytryptamine (5-MeOT), DOI was inactive, whereas 5-MeOT potently stimulated contractions. 5-HT_1D receptor agonists (5-CT₁ sumatriptan) had a stimulatory effect. The effect of sumatriptan was antagonized by the 5-HT_1D receptor antagonist metitepine but not by the 5-HT₄ receptor antagonist SDZ 205–557. The 5-HT₃ receptor agonist 2-methyl-5-HT and the antagonists ICS 205–930 and granisetron did not influence the peristaltic reflex. 6-OH-indalpine, a 5-HT_1P agonist, was inactive. This data suggest that 5-HT stimulates the peristaltic reflex in the isolated guinea-pig ileum by activation of 5-HT₄- and 5-HT_1D receptors; other 5-HT receptor subtypes appear not to play a significant role in the modulation of this reflex.     

Long-term administration of the dopamine D3/2 receptor agonist pramipexole increases dopamine and serotonin neurotransmission in the male rat forebrain

Background

Long-term administration of the dopamine (DA) D2-like (D3/2) receptor agonist pramipexole (PPX) has been previously found to desensitize D2 autoreceptors, thereby allowing a normalization of the firing of DA neurons and serotonin (5-HT)_1A autoreceptors, permitting an enhancement of the spontaneous firing of 5-HT neurons. We hypothesized that PPX would increase overall DA and 5-HT neurotransmission in the forebrain as a result of these changes at the presynaptic level.

Methods

Osmotic minipumps were implanted subcutaneously in male Sprague-Dawley rats, delivering PPX at a dose of 1 mg/kg/d for 14 days. The in vivo electrophysiologic microiontophoretic experiments were carried out in anesthetized rats.

Results

The sensitivity of postsynaptic D2 receptors in the prefrontal cortex (PFC) remained unaltered following PPX administration, as indicated by the unchanged responsiveness to the microiontophoretic application of DA. Their tonic activation was, however, significantly increased by 104% compared with the control level. The sensitivity of postsynaptic 5-HT_1A receptors was not altered, as indicated by the unchanged responsiveness to the microiontophoretic application of 5-HT. Similar to other antidepressant treatments, long-term PPX administration enhanced the tonic activation of 5-HT_1A receptors on CA3 pyramidal neurons by 142% compared with the control level.

Limitations

The assessment of DA and 5-HT neuronal tone was restricted to the PFC and the hippocampus, respectively.

Conclusion

Chronic PPX administration led to a net enhancement in DA and 5-HT neurotransmission, as indicated by the increased tonic activation of postsynaptic D2 and 5-HT_1A receptors in forebrain structures.