Agonist-induced desensitization of 5-HT2 receptors on cultured calf aortic smooth muscle cells

[³²P]Phosphatidic acid (PA)-formation was quantified in calf aortic smooth muscle cultures for measuring the activation of the signal transducing system coupled to the 5-hydroxytryptamine₂-(5-HT₂) receptor. [³²P]PA-formation was increased upon stimulation of smooth muscle cells with serotonin (5-HT) and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM), but not with the 5-HT₁ agonists N,N-dipropyl-8-hydroxy-2-aminotetralin and RU 24969. The potency of drugs to inhibit the 5-HT induced [³²P]PA-formation closely corresponded to their binding affinity for 5-HT₂ receptors. 24-Hour treatment of smooth muscle cultures with 5-HT or DOM resulted in a substantial decrease of 5-HT induced [³²P]PA-formation. In contrast to the anomalous 5-HT₂ receptor regulation in vivo, 5-HT₂ receptors on smooth muscle cells appeared to be desensitized by agonist treatment.     

Protein kinase C translocation in human blood platelets

Protein kinase C (PKC) activity and translocation in response to the phorbol ester, phorbol 12-myristate, 13-acetate (PMA), serotonin (5-HT) and thrombin was assessed in human platelets. Stimulation with PMA and 5-HT for 10 minutes or thrombin for 1 minute elicited platelet PKC translocation from cytosol to membrane. The catecholamines, norepinephrine or epinephrine at 10 microM concentrations did not induce redistribution of platelet PKC. Serotonin (0.5-100 microM) and the specific 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) (10-100 microM) but not the 5-HT1A or 5-HT1B agonists, (+/-) 8-hydroxy-dipropylamino-tetralin (8-OH-DPAT) or 5-methoxy-3-3-(1,2,3,6-tetrahydro-4-pyridin) 1H-indole succinate (RU 24969) induced dose-dependent PKC translocations. Serotonin-evoked PKC translocation was blocked by selective 5-HT2 receptor antagonists, ketanserin and spiroperidol. These results suggest that, in human platelets, PMA, thrombin and 5-HT can elicit PKC translocation from cytosol to membrane. Serotonin-induced PKC translocation in platelets is mediated via 5-HT2 receptors.     

Serotonin Inhibition of the NMDA Receptor/Nitric Oxide/Cyclic GMP Pathway in Rat Cerebellum: Involvement of 5-Hydroxytryptamine2C Receptors

Abstract: Previous studies have shown that 5-hydroxytryptamine (5-HT) can potently inhibit glutamatergic transmission in rat cerebellum through the activation of multiple 5-HT receptors. The aim of this study was to subclassify the 5-HT₂ receptor mediating inhibition of the cyclic GMP response elicited by N -methyl- d -aspartate in adult rat cerebellar slices. Seven receptor antagonists, endowed with relative selectivities for the 5-HT_2A, 5-HT_2B, and 5-HT_2C subtypes, differentially affected the inhibition by (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane of the cyclic GMP response, suggesting that the receptor involved belongs to the 5-HT_2C subtype.     

Effects of 5-hydroxytryptamine on cat spinal motoneurons

The effects of 5-hydroxytryptamine (5-HT) on spinal motoneurons were examined in pentobarbital-anaesthetized cats and in nonanaesthetized decerebrate cats by intracellular recording and extracellular iontophoresis of 5-HT. 5-HT first induced a depolarization and then a long-lasting hyperpolarization (up to 60 min) with unchanged input resistance. The slow hyperpolarization was prevented by the 5-HT antagonists ketanserin (5-HT2), methysergide, and spiperone (5-HT1,2) and mimicked by the agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (5-HT2). The post-spike after hyperpolarization was enhanced after application of 5-HT. A depolarization was induce by the 5-HT agonists (+/-)-8-hydroxy-(2)-(di-n-propylamino)tetralin (5-HT1A) and 1-(2-methoxyphenyl)piperazine (5-HT1). Possible mechanisms for the 5-HT-induced hyperpolarization and its intracellular medication are discussed. The present data suggest multiple effects of 5-HT on cat spinal motoneurons.     

The activation of 5-HT receptors in prefrontal cortex enhances dopaminergic activity

Atypical antipsychotics show preferential 5-HT 2A versus dopamine (DA) D2 receptor affinity. At clinical doses, they fully occupy cortical 5-HT2 receptors, which suggests a strong relationship with their therapeutic action. Half of the pyramidal neurones in the medial prefrontal cortex (mPFC) express 5-HT 2A receptors. Also, neurones excited through 5-HT 2A receptors project to the ventral tegmental area (VTA). We therefore hypothesized that prefrontal 5-HT 2A receptors can modulate DA transmission through excitatory mPFC-VTA inputs. In this study we used single unit recordings to examine the responses of DA neurones to local (in the mPFC) and systemic administration of the 5-HT 2A/2C agonist 1-[2,5-dimethoxy-4-iodophenyl-2-aminopropane] (DOI). Likewise, using microdialysis, we examined DA release in the mPFC and VTA (single/dual probe) in response to prefrontal and systemic drug administration. The local (in the mPFC) and systemic administration of DOI increased the firing rate and burst firing of DA neurones and DA release in the VTA and mPFC. The increase in VTA DA release was mimicked by the electrical stimulation of the mPFC. The effects of DOI were reversed by M100907 and ritanserin. These results indicate that the activity of VTA DA neurones is under the excitatory control of 5-HT 2A receptors in the mPFC. These observations may help in the understanding of the therapeutic action of atypical antipsychotics.     

Suppressive effect of nantenine,isolated from Nandina domestica Thunberg,on the 5-hydroxy-L-tryptophan plus clorgyline-induced head-twitch response in mice

We investigated the effects of nantenine (9,10-Methylenedioxy-1,2 dimethoxyaporphine), a major alkaloid isolated from the fruit of Nandina domestica Thunb (Berberidaceae), on the 5-HT2A receptor-mediated head-twitch response (HTR) in mice. Intraperitoneal (i.p.) injection of nantenine (13.3, 20 and 30 mg/kg) as well as the 5-HT2A receptor antagonist ketanserin (0.0625, 0.25 and 1 mg/kg) inhibited the 5-hydroxy-L-tryptophan (l-5-HTP; 75 mg/kg, i.p.) plus monoamine oxidase inhibitor, clorgyline (1 mg/kg, i.p.)-induced HTR in a dose-dependent manner. In contrast, neither l-5-HTP plus clorgyline nor 5-HT1A agonist, 8-hydroxy-2-(di-n-propylamino)tetraline (8-OH-DPAT; 5 microg/mouse, i.c.v.)-induced head weaving was affected by nantenine or ketanserin. Furthermore, neither nantenine (up to 30 mg/kg) nor ketanserin (up to 1 mg/kg) affect on the locomotor activity. In the receptor binding studies, nantenine showed affinity to the 5-HT2A receptors (Ki = 0.4 microM), while it had less affinity toward alpha1-adrenergic (Ki = 2.1 microM) and D2-dopaminergic (Ki = 1.7 microM) receptors of the mouse brain. These results suggest that nantenine inhibits l-5-HTP plus clorgyline-induced head- twitch response by blocking 5-HT2A receptors in the central nervous system.     

Discriminative stimulus properties of the serotonergic agent 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI)

Using a two-lever drug discrimination procedure, six rats were trained to discriminate 0.5 mg/kg of racemic 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) from saline. Once trained, the animals demonstrated a dose-related decrease in discriminative performance upon administration of lower doses of DOI (ED50 = 0.16 mg/kg). DOI-stimulus generalization occurred with the putative 5-HT2 agonist DOM (ED50 = 0.49 mg/kg), but not with the 5-HT1A agonist 8-OH DPAT, or the 5-HT1B agonist TFMPP. Furthermore, the DOI stimulus could be antagonized by pretreatment of the animals with the 5-HT2 antagonist ketanserin. The present results, coupled with the prior demonstration that DOI possesses a significant affinity and selectivity for 5-HT2 binding sites, suggest that the discriminative stimulus effects of DOI may be 5-HT2-mediated.     

Rapid desensitization and resensitization of 5-HT2 receptor mediated phosphatidyl inositol hydrolysis by serotonin agonists in quiescent calf aortic smooth muscle cells

Agonist regulation of 5-hydroxytryptamine2 (5-HT2) receptors was studied in calf aortic smooth muscle cultures incubated in a quiescent, defined synthetic medium that does not stimulate cell proliferation, but that provides cells with supplements that maintain cell viability. In these cells, 5-hydroxytryptamine (5-HT)-induced [3H]inositol phosphates accumulation showed the characteristics of a 5-HT2 receptor coupled transducing system according to the inhibition of the response by 5-HT2 antagonists at nanomolar concentrations. The 5-HT2 receptor coupled response became rapidly desensitized during continued incubation with 5-HT and 1-(2,5-dimethoxy-4-methylphenyl)-2- aminopropane (DOM); nearly full desensitization was obtained in two hours with 10 microM 5-HT and DOM pretreatment. The recovery of the response had a half-live of 5 hours after 2 hours pretreatment and of 9.5 to 12.5 hours after 24 to 96 hours agonist pretreatment. The DOM-induced desensitization of the 5-HT2 receptor coupled response was fully blocked by 0.1 microM cinanserin. Cinanserin alone did not induce desensitization or up-regulation of the 5-HT2 receptor coupled response at 0.1 microM. It may be that the down-regulation of central 5-HT2 receptors by antagonists in vivo is a heterologous process due to mediators which are triggered by 5-HT2 antagonistic action.     

Dual 5-HT1A agonists and 5-HT re-uptake inhibitors by combination of indole-butyl-amine and chromenonyl-piperazine structural elements in a single molecular entity

The dual serotonin (5-HT) re-uptake inhibitor and 5-HT(1A) receptor agonist vilazodone was found to increase central serotonin levels in rat brain. In the course of structural modifications of vilazodone 3-[4-[4-(2-oxo-2H-1-benzopyran-6-yl)-1-piperazinyl]-butyl]-1H-indole-5-carbonitrile 8i and its fluorine analogue 6-[4-[4-(5-fluor-3-indolyl)-butyl]-1-piperazinyl]-2H-1-benzopyran-2-one have been identified. These unsubstituted chromenones are equally potent at the 5-HT(1A) receptor and 5-HT transporter. The implementation of nitrogen functionalities in position 3 of the chromenones resulted in compounds acting as agonists at the 5-HT(1A) receptor and as 5-HT re-uptake inhibitors like vilazodone. Ex vivo 5-HT re-uptake inhibition and in vitro 5-HT agonism were determined in the PCA- and GTPgammaS-assay, respectively. The potential of these chromenones to increase central 5-HT levels was measured in microdialysis studies and especially the derivatives 3-[4-[4-(3-amino-2-oxo-2H-chromen-6-yl)-piperazin-1-yl]-butyl]-1H-indole-5-carbonitrile 8f, ethyl (6-[4-[4-(5-cyano-1H-indol-3-yl)-butyl]-piperazin-1-yl]-2-oxo-2H-chromen-3-yl)-carbamate 8h and N-(6-[4-[4-(5-cyano-1H-indol-3-yl)-butyl]-piperazin-1-yl]-2-oxo-2H-chromen-3-yl)-acetamide 8k give rise to rapid development of increased serotonin levels in rat brain cortex, lasting longer than 3h.     

Evidence for 5-HT2 involvement in the mechanism of action of hallucinogenic agents

The affinities (Ki values) of twenty two psycho-active agents, including LSD, 5-OMe DMT and a series of phenalkylamine derivatives, for cortical 5-HT1 and 5-HT2 binding sites were compared with two measures of behavioral activity. It was found that a significant correlation (r = 0.938) exists between the 5-HT2 binding affinities of these agents and their ED50 values as determined in tests of stimulus generalization using 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM) as the training drug. Furthermore, for fifteen of these agents where human data were available, a significant correlation (r = 0.924) also exists between 5-HT2 binding affinities and their human hallucinogenic potencies. The results of this study suggest that the mechanism of action of these agents involves 5-HT2-related events.     

Nitric oxide synthase inhibitors enhance 5-HT2 receptor-mediated behavior, the head-twitch response in mice.

The purpose of this study was to characterize behavioral interactions between nitric oxide synthase (NOS) inhibitors and serotonergic 5-HT2 receptors. In the present study, NOS inhibitors, N(G)-nitro-L-arginine, N(G)-nitro-L-arginine methylester, N(G)-monomethyl-L-arginine, 7-nitroindazole, trifluoperazine and NO scavenger, methylene blue markedly enhanced 5-hydroxytryptamine (5-HT)-induced selective serotonergic behavior, the head twitch response (HTR), in mice. However NO generators, sodium nitroprusside, 3-morpholinosydnonimine and S-nitroso-N-acetylpenicillamine as well as NO precursor, L-arginine markedly inhibited 5-HT induced HTR in mice. In the previous study, it was demonstrated that the N-methyl-D-aspartate (NMDA) receptor antagonists markedly enhanced 5-HT-induced selective serotonergic behavior, HTR, whereas NMDA itself inhibited 5-HT-induced HTR in mice. In the present study, it was demonstrated that the inhibition by a NMDA receptor agonist, NMDA of 5-HT-induced HTR was reversed by the treatment with NOS inhibitors, N(G)-nitro-L-arginine and N(G)-nitro-L-arginine methylester. The suppressive action by a NO generator, S-nitroso-N-acetylpenicillamine of 5-HT-induced HTR was also reversed by the treatment with NMDA receptor antagonists, MK-801 and dextromethorphan. These results have shown that the NO system is located down stream of NMDA receptors involved in modulation of 5-HT2-mediated HTR. Therefore, the enhanced effects of NOS inhibitors on 5-HT-induced HTR support experimental evidence for the NO/5-HT2 as well as NMDA/5-HT2 receptor interactions indicating that NO plays an important role in the glutamatergic modulation of the serotonergic function at the 5-HT2 receptor.     

High-affinity agonist binding correlates with efficacy (intrinsic activity) at the human serotonin 5-HT2A and 5-HT2C receptors: evidence favoring the ternary complex and two-state models of agonist action

Many modern models of receptor-G protein function assume that there is a direct relationship between high-affinity agonist binding and efficacy. The validity of this assumption has been recently questioned for the serotonin 5-HT2A receptor. We examined the intrinsic activities of various ligands in activating phosphoinositide hydrolysis and measured their respective binding affinities to the high- and low-affinity states of the 5-HT2C (VNV isoform) and 5-HT(2A) receptors. Ligand binding affinities for the high-affinity state of the receptors were determined using 1-(4-[125I]iodo-2,5-dimethoxyphenyl)2-aminopropane, whereas [3H]mesulergine and N-[3H]methylspiperone were used, in the presence of excess guanine nucleotide [guanosine 5'-O-(3-thiotriphosphate)], to define binding to the low-affinity state of the 5-HT2C and 5-HT2A receptors, respectively. Antagonists labeled the high- and low-affinity states of each receptor with comparable affinities. Previously identified inverse agonists of the 5-HT2C receptor behaved as silent antagonists in our systems even when the receptor was overexpressed at a relatively high density. In contrast, the ability of agonists to bind differentially to the high- and low-affinity states of the 5-HT2A and 5-HT2C receptors was highly correlated (r2 = 0.86 and 0.96, respectively) with their intrinsic activities. These data suggest that high-affinity agonist states can account for agonist efficacy at human 5-HT2A or 5-HT2C receptors without the need for considering additional transition or active states of the receptor-ligand complex. The procedure described herein may expedite drug discovery efforts by predicting intrinsic activities of ligands solely from ligand binding assays.     

Two amino acids within the alpha4 helix of Galphai1 mediate coupling with 5-hydroxytryptamine1B receptors.

We previously reported that residues 299-318 in Galphai1 participate in the selective interaction between Galphai1 and the 5-hydroxytryptamine1B (5-HT1B) receptor (Bae, H., Anderson, K., Flood, L. A., Skiba, N. P., Hamm, H. E., and Graber, S. G. (1997) J. Biol. Chem. 272, 32071-32077). The present study more precisely defines which residues within this domain are critical for 5-HT1B receptor-mediated G protein activation. A series of Galphai1/Galphat chimeras and point mutations were reconstituted with Gbetagamma and Sf9 cell membranes containing the 5-HT1B receptor. Functional coupling to 5-HT1B receptors was assessed by 1) [35S]GTPgammaS binding and 2) agonist affinity shift assays. Replacement of the alpha4 helix of Galphai1 (residues 299-308) with the corresponding sequence from Galphat produced a chimera (Chi22) that only weakly coupled to the 5-HT1B receptor. In contrast, substitution of residues within the alpha4-beta6 loop region of Galphai1 (residues 309-318) with the corresponding sequence in Galphat either permitted full 5-HT1B receptor coupling to the chimera (Chi24) or only minimally reduced coupling to the chimeric protein (Chi25). Two mutations within the alpha4 helix of Galphai1 (Q304K and E308L) reduced agonist-stimulated [35S]GTPgammaS binding, and the effects of these mutations were additive. The opposite substitutions within Chi22 (K300Q and L304E) restored 5-HT1B receptor coupling, and again the effects of the two mutations were additive. Mutations of other residues within the alpha4 helix of Galphai1 had minimal to no effect on 5-HT1B coupling behavior. These data provide evidence that alpha4 helix residues in Galphai participate in directing specific receptor interactions and suggest that Gln304 and Glu308 of Galphai1 act in concert to mediate the ability of the 5-HT1B receptor to couple specifically to inhibitory G proteins.     

Brain region-specific alterations of 5-HT2A and 5-HT2C receptors in serotonin transporter knockout mice

The aim of the present studies was to determine the effects of reduced or absent serotonin (5-HT) transporters (5-HTTs) on 5-HT2A and 5-HT2C receptors. The density of 5-HT2C receptors was significantly increased in the amygdala and choroid plexus of 5-HTT knockout mice. On the other hand, the density of 5-HT2A receptors was significantly increased in the hypothalamus and septum, but reduced in the striatum, of 5-HTT knockout mice. However, 5-HT2A mRNA was not changed in any brain region measured. 5-HT2C mRNA was significantly reduced in the choroid plexus and lateral habenula nucleus of these mice. The function of 5-HT2A receptors was evaluated by hormonal responses to (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI). Oxytocin, but not adrenocorticotrophic hormone or corticosterone, responses to DOI were significantly greater in 5-HTT knockout mice. In addition, Gq and G11 proteins were not significantly changed in any brain region measured. The present results suggest that the constitutive alteration in the function of 5-HTTs changes the density of 5-HT2A and 5-HT2C receptors in a brain region-specific manner. These changes may not be mediated by alterations in their gene expression or in the level of Gq/11 proteins. The alterations in these receptors may be related to the altered behaviors of 5-HTT knockout mice.     

Enhancement of hippocampal cholinergic neurotransmission through 5-HT1A receptor-mediated pathways by repeated lithium treatment in rats

Hippocampal cholinergic neuronal activity is reported to be regulated, at least partly, through serotonin1A (5-HT1A) receptors. Chronic lithium treatment has been shown to alter both behavioral and neurochemical responses mediated by postsynaptic 5-HT1A receptors. We investigated whether long-term lithium treatment affects central cholinergic neurotransmission through 5-HT1A receptor-mediated pathways. Changes in acetylcholine (ACh) release induced by 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), a 5-HT1A receptor agonist, in the rat hippocampus were measured using a microdialysis technique and a radioimmunoassay for ACh. Administration of lithium for 21 days resulted in a serum lithium concentration of 1.03 mM and caused little change in density or affinity of [3H]8-OH-DPAT binding sites in the hippocampus. The local application of 8-OH-DPAT into the hippocampus of lithium treated rats increased the ACh efflux in both the absence and the presence of physostigmine, a cholinesterase (ChE) inhibitor, in the perfusion fluid. The basal ACh efflux of lithium treated rats was not different from that of the control rats under normal conditions, but was significantly higher than that of the controls when ChE was inhibited. These results demonstrate that chronic lithium treatment increases spontaneous ACh release in the hippocampus under conditions of ChE inhibition, but not under normal conditions, and enhances cholinergic neurotransmission through 5-HT1A receptor-mediated pathways, and suggest that activation of 5-HT1A receptor function by lithium is related to the enhancement of hippocampal cholinergic neurotransmission.     

Discriminative stimulus properties of the serotonin agonist 1-(3-trifluoromethylphenyl)piperazine (TFMPP)

Using a standard two-lever drug discrimination procedure, twelve rats were trained to discriminate 1.0 mg/kg of the serotonin (5-HT) agonist TFMPP from saline. Once trained, the animals displayed a dose-related decrease in discriminative performance upon administration of lower doses of TFMPP. Tests of stimulus generalization were performed using the purported 5-HT agonist RU-24, 969 and 1-(2,5-dimethoxy-4-methylphenyl)-2-aminopropane (DOM). While TFMPP produced stimulus effects similar to those of RU-24,969, these effects seem to be dissimilar to those of DOM. The results of the present study suggest that the discriminative stimulus effects of TFMPP may involve a 5-HT1-related mechanism.     

Effects of 5-HT1A and 5-HT2A receptor stimulation on temporal differentiation performance in the fixed-interval peak procedure

We examined the effects of the 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) and 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI) on performance on the fixed-interval peak procedure, and the sensitivity of these effects to 5-HT1A and 5-HT2A receptor antagonists (N-[2-(4-[2-methoxyphenyl]-1-piperazinyl]ethyl]-N-2-pyridinylcyclohexanecarboxamide [WAY-100635] and ketanserin). Rats were trained to press a lever for food reinforcement in 50 min sessions consisting of 32 trials in which the lever was continuously available, separated by 10 s inter-trial intervals. In 16 trials, reinforcement was delivered following the first response after 30 s had elapsed since trial onset (fixed-interval 30 s). In 16 randomly interposed (peak/probe) trials, reinforcement was omitted, and the lever remained in the operant chamber for 120 s. Response rate in probe trials was plotted against time from trial onset. Time to peak response rate (t(peak)) and the Weber fraction were derived from modified Gaussian curves fitted to each rat's data. 8-OH-DPAT (0.05 mg kg(-1)) reduced t(peak) and increased the Weber fraction; the effect on t(peak) was antagonized by WAY-100635 (0.1 mg kg(-1)). DOI (0.25 mg kg(-1)) also reduced t(peak) and increased the Weber fraction; the reduction of t(peak) was antagonized by ketanserin (2 mg kg(-1)). Stimulation of 5-HT1A and 5-HT2A receptors alters temporal differentiation in qualitatively similar ways.     

Enhanced activation of Akt and extracellular-regulated kinase pathways by simultaneous occupancy of Gq-coupled 5-HT2A receptors and Gs-coupled 5-HT7A receptors in PC12 cells

The most commonly prescribed antidepressants, the serotonin (5-HT) selective reuptake inhibitors, increase 5-HT without targeting specific receptors. Yet, little is known about the interaction of multiple receptor subtypes expressed by individual neurons. Specifically, the effect of increases in cAMP induced by Gs-coupled 5-HT receptor subtypes on the signaling pathways modulated by other receptor subtypes has not been studied. We have, therefore, examined the activation of the extracellular-regulated kinase (ERK) and Akt pathways by Gs-coupled 5-HT7A receptors and Gq-coupled 5-HT2A receptors, which are co-expressed in discrete brain regions. Agonists for both receptors were found to activate ERK and Akt in transfected PC12 cells. 5-HT2A receptor-mediated activation of the two pathways was found to be Ca2+-dependent. In contrast, 5-HT7A receptor-mediated activation of Akt required increases in both [cAMP] and intracellular [Ca2+], while activation of ERK was inhibited by Ca2+. The activation of ERK and Akt stimulated by simultaneous treatment of cells with 5-HT2A and 5-HT7A receptor agonists was found to be at least additive. Cell-permeable cAMP analogs mimicked 5-HT7A receptor agonists in enhancing 5-HT2A receptor-mediated activation of ERK and Akt. A role was identified for the cAMP-guanine exchange factor, Epac, in this augmentation of ERK, but not Akt, activation. Our finding of enhanced activation of neuroprotective Akt and ERK pathways by simultaneous occupancy of 5-HT2A and 5-HT7A receptors may also be relevant to the interaction of other neuronally expressed Gq- and Gs-coupled receptors.     

The effects of serotonin on glucocorticoid receptor binding in rat raphe nuclei and hippocampal cells in culture

The raphe-hippocampal serotonin (5-HT) system is involved in the regulation of the hypothalamus-pituitary-adrenal axis. The purpose of this study was to determine and compare the roles of 5-HT in the regulation of glucocorticoid receptor (GR) binding in the raphe nuclei and in the hippocampus. The effects of 5-HT, 5-HT agonists, and the 5-HT reuptake inhibitor citalopram on GR binding sites were studied in primary cultures of the fetal raphe nuclei and the hippocampus. Exposure of hippocampal cells to 5-HT, (+/-)-2,5-dimethoxy-4-iodoamphetamine (DOI; a 5-HT2 agonist), or citalopram resulted in an increase in number of GR binding sites. The effect of DOI was blocked by ketanserin (a 5-HT2 antagonist). Specific and saturable GR binding was found in raphe cells. Exposure of raphe cells to 5-HT, (+/-)-8 hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; a 5-HT1A agonist), or citalopram induced a significant decrease in number of GR binding sites. The effect of 8-OH-DPAT was reversed by WAY 100135 [N-tert-butyl-3-[1-[1-(2-methoxy)phenyl]piperazinyl]-1-phenylpropiona mide; a 5-HT1A antagonist]. These results show that the regulation of GRs during fetal life is structure-dependent and involves different 5-HT receptor subtypes. Moreover, the regulation of hippocampal GRs by citalopram suggests an action of antidepressants independent of their effects on monoamines.     

Selective reduction by isolation rearing of 5-HT1A receptor-mediated dopamine release in vivo in the frontal cortex of mice

Serotonin (5-HT)1A receptors modulate in vivo release of brain monoaminergic neurotransmitters which may be involved in isolation-induced aggressive behavior. The present study examined the effect of isolation rearing on the 5-HT1A receptor-mediated modulation of dopamine (DA), 5-HT and noradrenaline (NA) release in the frontal cortex of mice. The selective 5-HT1A receptor agonist (S)-5-[-[(1,4-benzodioxan-2-ylmethyl)amino]propoxy]-1,3-benzodioxole HCl (MKC-242) increased the release of DA and NA and decreased the release of 5-HT in the frontal cortex of mice. The effect of MKC-242 on DA release was significantly less in isolation-reared mice than in group-reared mice, while effects of the drug on NA and 5-HT release did not differ between both groups. The effect of the other 5-HT1A receptor agonist 8-hydroxy-2-(di-n-propylamino)tetralin on cortical DA release was also less in isolation-reared mice than in group-reared mice, and that of the drug on cortical 5-HT release did not differ between both groups. In contrast to MKC-242-induced DA release, amphetamine-induced increase in cortical DA release in vivo was greater in isolation-reared mice. The present findings suggest that isolation rearing enhances the activity of cortical dopaminergic neurons and reduces selectively the 5-HT1A receptor-mediated release of DA in the cortex.