5-HT1A receptor-mediated inhibition of nucleus accumbens neurons activated by stimulation of parafascicular nucleus of thalamus

 Electrophysiological studies using chloral hydrate-anesthetized rats were performed to elucidate the role of serotonin_1A (5-HT_1A) receptors in the regulation of neuronal activity of nucleus accumbens (Acc) neurons receiving input from the parafascicular nucleus of the thalamus (Pf). Extracellular neuronal activities were recorded in Acc using a glass microelectrode attached along a seven-barreled micropipette, each barrel of which was filled with dopamine, 5-HT, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT: 5-HT_1A agonist) hydrobromide, 1-(2-methoxyphenyl)-4-[4-(2-phthalimido)butyl]-piperazine (NAN-190: 5-HT_1A antagonist) hydrobromide, glutamate and 2 M NaCl. These drugs were microiontophoretically applied to the immediate vicinity of the target neuron. Spikes elicited by Pf stimulation were inhibited by iontophoretically applied dopamine, 5-HT and 8-OH-DPAT in a dose-dependent manner. In these neurons, firing induced by iontophoretic application of glutamate was also suppressed by dopamine, 5-HT and 8-OH-DPAT. The 5-HT or 8-OH-DPAT-induced inhibitions of the glutamate-induced firing were antagonized by concomitant application of NAN-190. These findings suggest that the dopamine-sensitive Acc neurons receiving input from Pf are inhibited by 5-HT via 5-HT_1A receptors located on postsynaptic Acc neurons. Received: 6 November 1996/Final version: 9 July 1997     

(-)-Propranolol blocks the inhibition of serotonergic dorsal raphe cell firing by 5-HT1A selective agonists

The ability of the beta-adrenoceptor antagonist propranolol to block the effects of serotonin (5-HT) and 5-HT1A-selective agonists on the spontaneous firing of serotonergic dorsal raphe neurons was assessed. During microiontophoretic application, (-)- but not (+)-propranolol rapidly and reversibly blocked the suppressant effects of the 5-HT1A-selective agonists ipsapirone (TVX Q 7821) and 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT). However, (-)-propranolol was a relatively weak antagonist of 5-HT itself, suggesting that the endogenous neurotransmitter may have actions on dorsal raphe neurons in addition to those mediated by 5-HT1A receptors.     

Chronic MAO A and MAO B inhibition decreases the 5-HT1A receptor-mediated inhibition of forskolin-stimulated adenylate cyclase

The effect of chronic administration of various monoamine oxidase (MAO) inhibitors on the ability of 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT) to inhibit forskolin-stimulated adenylate cyclase activity was studied. Groups of 12 rats were given either saline, (E)-beta-fluoromethylene-m-tyrosine (MDL 72394 0.25 mg/kg p.o.), clorgyline (1 mg/kg p.o.), selegiline (1 mg/kg p.o.) or tranylcypromine (5 mg/kg p.o.) once a day for 21 days. Biochemical determinations were made 72 h after the final dose. MDL 72394 and tranylcypromine produced a nonselective inhibition of MAO but clorgyline and selegiline selectively inhibited MAO A and MAO B respectively. All treatments that inhibited MAO A also increased tissue levels of 5-HT. Chronic treatment with MDL 72394, clorgyline or tranylcypromine reduced the ability of 8-OH-DPAT to inhibit forskolin-stimulated adenylate cyclase activity. These data suggest that chronic nonselective and chronic MAO A inhibition causes a down-regulation of the 5-HT1A-mediated inhibition of forskolin-stimulated adenylate cyclase activity.     

Inhibition of 5-HT2 receptor-mediated head-twitch response by cocaine via indirect stimulation of adrenergic alpha 2 and serotonergic 5-HT1A receptors.

Cocaine inhibits the 5-HT2-mediated (+/-)-DOI-induced head-twitch response (HTR) in mice in a dose-dependent manner. In order to investigate the possible inhibitory mechanism(s) of cocaine on 5-HT2 receptor function, we studied the effects of the selective adrenergic alpha 2 receptor antagonist yohimbine and the beta-adrenergic/5-HT1 receptor antagonist alprenolol, and the 5-HT3 antagonist ICS 205-930 on the inhibitory action of cocaine on the (+/-)-DOI-induced HTR. Neither yohimbine (0.1 and 0.5 mg/kg) nor alprenolol (10 mg/kg) pretreatment had any significant effect on the (+/-)-DOI-induced HTR. However, both antagonists prevented the inhibitory effects of cocaine on the (+/-)-DOI-induced HTR. The 5-HT3 antagonist ICS 205-930 neither produced HTR nor decreased the (+/-)-DOI-induced HTR frequency. The present results suggest that cocaine inhibits 5-HT2 receptor function by increasing the synaptic concentration of norepinephrine and serotonin via inhibition of their uptake and thus indirectly stimulating the respective inhibitory adrenergic alpha 2 and serotonergic 5-HT1A receptors. Furthermore, cocaine's 5-HT3 antagonist properties appear not to play a role in the inhibition of head-twitch behavior.     

Effects of cold stress on some 5-HT1A, 5-HT1C and 5-HT2 receptor-mediated responses.

The purpose of the present study was to analyze the influence of stress (24-h cold exposure) on presynaptic 5-HT1A receptors, and on postsynaptic 5-HT1A, 5-HT1C and 5-HT2 receptors. Cold exposure for 24 h affected neither pargyline-induced decreases in 5-hydroxyindoleacetic acid (5-HIAA) levels in midbrain and rest of brain, nor plasma glucose and corticosterone levels. Treatment with the 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT; 0.5-1 mg/kg), 3-5 h after the end of cold exposure triggered less intense flat body posture and forepaw treading in cold-exposed rats than in controls. On the other hand, 15- and 30-min plasma glucose responses to 8-OH-DPAT (0.25-0.5 mg/kg, 3-5 h after cold) or to the alpha 2-adrenoceptor agonist, clonidine (0.025 mg/kg), were not affected by cold, while the 15-min, but not the 30 min, plasma corticosterone response to 8-OH-DPAT was slightly amplified in cold-exposed rats. Cold exposure affected neither the inhibitory effect of 8-OH-DPAT (0.25-0.5 mg/kg, 3-5 h after cold) on midbrain 5-HIAA levels, nor the hypothermic effect of 8-OH-DPAT (0.5-1 mg/kg, 3-5 h after cold). Lastly, the hypoactivity elicited by the 5-HT1C receptor agonist, m-chlorophenyl-piperazine (1.5-3 mg/kg, 3-5 h after cold), or head shakes elicited by the 5-HT2 receptor agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (1-2 mg/kg, 3-5 h after cold), were of similar intensities in control and in cold-exposed rats.     

Selective cross-tolerance to 5-HT1A and 5-HT2 receptor-mediated temperature and corticosterone responses

The repeated administration of 5-methoxy-N,N-dimethyltryptamine (5-MeODMT, 3 mg/kg, twice daily for 14 days) significantly diminished hypothermia and corticosterone secretion induced by an acute challenge with the 5-HT1A agonist 8-OH-DPAT (0.1 mg/kg) when compared to the responses in animals treated chronically with the solvent vehicle. In contrast, the chronic administration of 5-MeODMT did not alter the magnitude of hyperthermia or corticosterone secretion induced by the acute administration of MK-212 (1.0 mg/kg). The repeated administration of the 5-HT2 agonist DOI (1.0 mg/kg, daily for 7 days) significantly reduced the increase in corticosterone, but not body temperature, produced by MK-212. Chronic treatment with DOI did not alter the hypothermia or increase in corticosterone secretion elicited by 8-OH-DPAT. These data are consistent with other evidence that these physiological effects of 8-OH-DPAT and MK-212 are mediated by 5-HT1A and 5-HT2 receptors, respectively. Thus, data presented in these studies are suggestive that the chronic administration of 5-MeODMT diminishes the responsiveness of 5-HT1A receptor-mediated changes in body temperature and corticosterone secretion without altering the responses mediated by 5-HT2 receptors. In contrast, the chronic administration of DOI selectively diminishes the magnitude of 5-HT2 receptor-mediated changes in corticosterone secretion without affecting the responsiveness of those receptors involved in thermoregulatory responses. These selective changes in receptor responsiveness following the chronic administration of these 5-HT agonists further establishes the independence of 5-HT1A and 5-HT2 receptor-mediated pharmacological effects.     

Anxiolytic-like effect of a serotonergic ligand with high affinity for 5-HT1A, 5-HT2A and 5-HT3 receptors

S-(-)-2-[[4-(napht-1-yl)piperazin-1-yl]methyl]-1,4-dioxoperhydropyrrolo[1,2-alpha]-pyrazine (CSP-2503) is a serotonin (5-HT) receptor ligand with selectivity and high affinity for 5-HT1A, 5-HT2A and 5-HT3 receptors. CSP-2503 reduced rectal temperature and 5-HT neuronal hypothalamic activity in mice, decreased electrical activity of raphe nuclei cells in rats and blocked the enhancement of adenylate cyclase activity induced by forskolin in HeLa cells transfected with the human 5-HT1A receptor. This compound also blocked head-twitches induced by the 5-HT(2A/2C) receptor agonist 2,5-dimethoxy-4-iodoamphetamine (DOI). Contractions of guinea pig ileum induced by the 5-HT3 receptor agonist 2-methyl-5-HT were prevented by CSP-2503. Moreover, it reduced the bradycardia reflex induced by 2-methyl-5-HT in anaesthetized rats. In the light/dark box and social interaction tests, CSP-2503 presented anxiolytic activity, an action shared by 5-HT1 agonists and 5-HT3 antagonists. Taken together, these results suggest that CSP-2503 is a new 5-HT1 receptor agonist with 5-HT2A and 5-HT3)receptor antagonist activities that might be useful in a number of conditions associated with anxiety.     

Nicotine withdrawal leads to increased sensitivity of serotonergic neurons to the 5-HT1A agonist 8-OH-DPAT

In order to explore the neurophysiology of nicotine withdrawal, we examined the activity of serotonergic neurons in the dorsal raphe nucleus in rats undergoing withdrawal from chronic exposure to nicotine. Animals were exposed to nicotine (6 mg/kg per day base) via SC implanted osmotic minipumps. After 12 days the pumps were removed and the animals allowed to go through spontaneous withdrawal. Rats were anesthetized on various days of the procedure and the effect of the 5-hydroxytryptamine (5-HT)-1A agonist 8-OH-DPAT on the single-unit activity of serotonergic neurons in the dorsal raphe nucleus was examined. The sensitivity of serotonergic neurons to 8-OH-DPAT was not changed by the chronic administration of nicotine or saline; slightly increased on day 2 of withdrawal; significantly increased on days 3 and 4 of withdrawal; and no longer significantly increased by day 7 of withdrawal. These results indicate that serotonergic neurons in the dorsal raphe nucleus have an increased sensitivity to systemically administered 8-OH-DPAT in rats undergoing nicotine withdrawal and that the serotonergic system may play a role in the symptoms of nicotine withdrawal. Received: 13 September 1996/Final version: 29 April 1997     

Modulation of the 5-HT1C receptor-mediated behavior by 5-HT2, but not 5-HT1A, receptor activation.

The effects of 5-HT1A-receptor agonists 8-hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and gepirone, a 5-HT1A/5-HT2-receptor agonist 5-methoxy-N,N-dimethyltryptamine (5-MeODMT) and a 5-HT2-receptor agonist (+-)1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane ((+/)DOI) on the 5-HT1C-receptor-mediated exploratory hypoactivity in rats, induced by m-trifluoromethylphenylpiperazine (TFMPP) or m-chlorophenylpiperazine (m-CPP), were studied in the open field test. (+/-)DOI attenuated the effects of TFMPP and abolished those of m-CPP (not dose-dependently). 5-MeODMT showed a weak antagonistic action only at one, intermediate dose. The effects of TFMPP or m-CPP were not changed by 8-OH-DPAT or gepirone. At the same time, 8-OH-DPAT, gepirone, 5-MeODMT and (+/-)DOI themselves practically did not change the exploratory activity of rats. The obtained results permit an assumption that a functional interaction exists between 5-HT1C- and 5-HT2-receptors, but not between 5-HT1C- and 5-HT1A-ones.     

5-HT1A receptor-mediated activation of G-protein-gated inwardly rectifying K+ current in rat periaqueductal gray neurons

5-hydroxytryptamine (5-HT) has been reported to modulate analgesia produced by opioids or electrical stimulation of the periaqueductal gray (PAG). 5-HT increases K+ conductance and inhibits the firing activity of the PAG neurons. We examined the electrophysiological and pharmacological characteristics of the K+ current involved in 5-HT-induced hyperpolarization of dissociated rat PAG neurons. Among the neurons tested, 5-HT activated inward K+ currents in 30-40%, whilst the remaining 60-70% did not respond to 5-HT. 5-HT activated an inwardly rectifying K+ current (I5-HT) in a concentration- and voltage-dependent manner. I5-HT was mimicked by a 5-HT1A receptor selective agonist, 8-OH-DPAT, and was reversibly blocked by a 5-HT1A receptor antagonist, piperazine maleate, but not by a 5-HT2 receptor antagonist, ketanserin. I5-HT was sensitive to K+ channel blockers such as quinine and Ba2+, but insensitive to 4-aminopyridine, Cs+ and tetraethylammonium. I5-HT was inhibited by GDP(beta)s and was irreversibly activated by GTP(gamma)s. I5-HT was significantly suppressed by N-ethylmaleimide and pertussis toxin, but not by cholera toxin. Second messenger modulators such as staurosporin, forskolin, and phorbol-12-myristate-13-acetate did not alter I5-HT. The present study indicates that 5-HT-induced hyperpolarization of the PAG neurons results from activation of the pertussis toxin-sensitive G-protein-coupled inwardly rectifying K+ currents through 5-HT1A receptors.     

Food restriction and streptozotocin treatment decrease 5-HT1A and 5-HT2A receptor-mediated behavioral effects in rats

Food restriction and hypoinsulinemia can affect the synthesis, turnover, and receptor function of serotonin (5-HT) in brain. This study explored the effects of food restriction and streptozotocin treatment on behavioral effects related to 5-HT1A (+)-8-hydroxy-2-(dipropylamino)tetralin hydrobromide (8-OH-DPAT) and 5-HT2A [(+/-)-2,5-dimethoxy-4-iodoamphetamine hydrochloride (DOI)] receptor activation. Lower lip retraction and flat body posture (8-OH-DPAT) and head twitching (DOI) were measured in rats during free feeding, food restriction, after treatment with streptozotocin, and finally after insulin replacement. 8-OH-DPAT induced lower lip retraction and flat body posture whereas DOI induced head twitching. One week of food restriction (10 g/day) decreased 8-OH-DPAT-induced lower lip retraction, 8-OH-DPAT-induced flat body posture, and DOI-induced head twitching. Subsequently, 1 week of free access to food restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. Finally, 1 week after streptozotocin, 8-OH-DPAT-induced flat body posture and DOI-induced head twitching were markedly reduced whereas 8-OH-DPAT-induced lower lip retraction was unchanged. One week of insulin replacement restored sensitivity to 8-OH-DPAT and DOI-induced behavioral effects. These results show that modest food restriction or experimentally induced diabetes can profoundly affect sensitivity to drugs acting at 5-HT1A or 5-HT2A receptors; these results could be relevant to understanding the comorbidity of depression and diabetes.     

Opioid hyperalgesia and tolerance versus 5-HT1A receptor-mediated inverse tolerance

In addition to analgesia, opioids also produce paradoxical hyperalgesic effects following acute and chronic treatment. In this article, we review the occurrence of this hyperalgesia under several conditions, and discuss the potential mechanisms and clinical implications. We also review recent evidence that paradoxical analgesia and inverse tolerance induced by stimulation of 5-HT(1A) receptors, which is a mirror image of opioid-induced hyperalgesia and tolerance, might achieve clinically significant analgesia in chronic pain.     

Agonist action at 5-HT1C receptors facilitates 5-HT1A receptor-mediated spontaneous tail-flicks in the rat

In rats lightly restrained in plastic cylinders, subcutaneous administration of the selective, high efficacy 5-HT1A receptor agonist, 8-hydroxy-2-(di-n-propylamino)tetralin (8-OH-DPAT), induced spontaneous tail-flicks, that is, tail-flicks in the absence of extraneous stimulation. The putative 5-HT1B receptor agonist, CGS 12066B, the mixed 5-HT1B/1C receptor agonists, 1-((3-(trifluoromethyl)phenyl]piperazine (TFMPP) and 1-(3-chlorophenyl)piperazine (mCPP), the 5-HT1C/2 receptor agonist, [+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) and the 5-HT1B/1C/2 receptor agonist, quipazine, did not, in contrast, elicit tail-flicks when applied alone. However, TFMPP, mCPP, DOI and quipazine, but not CGS 12066B, each potentiated the action of 8-OH-DPAT. Further, in the presence of TFMPP, mCPP and DOI, the dose-response curve for the induction of tail-flicks by 8-OH-DPAT was both steeper and shifted to the left. Tail-flicks induced by another high efficacy 5-HT1A receptor agonist, lisuride, were also enhanced by TFMPP, mCPP and DOI. The 5-HT1A receptor partial agonists, buspirone and (+/-)-flesinoxan, evoked tail-flicks only in the presence of TFMPP, mCPP or DOI. The mixed 5-HT1C/2 receptor antagonists, ritanserin and ICI 169,369, did not modify the action of 8-OH-DPAT alone but abolished the potentiation of 8-OH-DPAT-induced tail-flicks by DOI and TFMPP. Further, the selective 5-HT1A receptor antagonist, BMY 7378, blocked tail-flicks induced by both 8-OH-DPAT alone and 8-OH-DPAT plus DOI or TFMPP. A common property of those drugs potentiating 8-OH-DPAT-induced tail-flicks is an agonist action at 5-HT1C receptors and the data indicate that it is this mechanism which underlies the facilitation of tail-flicks.     

5-HT1 and 5-HT2 binding profiles of the serotonergic agents alpha-methylserotonin and 2-methylserotonin

alpha-Methyl-5-hydroxytryptamine (alpha-Me-5-HT; 2) and 2-methyl-5-hydroxytryptamine (2-Me-5-HT; 3) are considered to be 5-HT2-selective and 5-HT3-selective agents, respectively. These agents were synthesized and examined at serotonin (5-HT) binding sites because there is relatively little documentation as to their selectivity and because they have not been previously examined at the newly discovered 5-HT1D and 5-HT1E sites. As previously reported, 2-Me-5-HT possesses a low affinity (Ki greater than 500 nM) for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT2 sites; this agent also displays a low affinity for 5-HT1D (Ki = 1220 nM) and 5-HT1E (Ki greater than 10,000 nM) sites. However, alpha-Me-5-HT displays little selectivity for 5-HT1A, 5-HT1B, 5-HT1C, and 5-HT1D sites (Ki = 42, 85, 150, and 150 nM, respectively) and a very low affinity for 5-HT1E (Ki greater than 10,000 nM) sites. Depending upon the radioligand used to label the sites, alpha-Me-5-HT displays either a low affinity (Ki = 880 nM with [3H]ketanserin) or a high affinity (Ki = 3 nM with [3H]DOB) for 5-HT2 sites. These results suggest that alpha-Me-5-HT is not as selective as previously considered and that caution should be used when employing this agent in pharmacological studies because it may act as mixed 5-HT1/5-HT2 agonist.     

Serotonin 5-HT1 receptors mediate inhibition of cyclic AMP production in neurons

In purified striatal and cortical neurons in primary culture, serotonin (5-HT) stimulated basal cyclic AMP production (EC50, 0.5 microM) 2.5- and 1.5-fold, respectively. The 5-HT1 selective agonists, RU 24969 and 8-hydroxy-2-(di-n-propylamino)tetralin (PAT), did not stimulate cyclic AMP production. However, 5-HT, RU 24969 and PAT inhibited VIP-stimulated cyclic AMP formation in a dose-dependent manner. The actions of selective agonists and antagonists at 5-HT receptors mediating attenuation of cyclic AMP production suggest that they may be of the 5-HT1 subtype.     

豚鼠肠系膜下神经节细胞的5—HT1P受体介导的慢去极化反应

目的：观察不同５－羟色胺受体亚型拮抗剂对５－ＨＴ去极化反应的作用,并观察－５ＨＴ１Ｐ受体激动剂对肠系膜下神经节细胞膜曜闰的作用。方法：离体豚鼠ＩＭＧ细胞内生长电记录。结果：赛庚啶和ＢＲＬ ２４９２４可逆地阻抑５－ＨＴ慢去极化反应;压力注射ＭＣＰＰ则使大部分５－ＨＴ敏感细胞出现慢去极化反应。     

The distribution of serotonergic 5-HT1A and non-5-HT1A recognition sites along the dorso-ventral axis of the rat hippocampus

The distribution of serotonergic 5-HT1 binding sites along the hippocampal dorso-ventral axis was compared with [3H]5-HT uptake considered as a marker of the quantity of serotonergic nerve terminals. [3H]5-HT binding to 5-HT1A and non-5-HT1A sites, of high and low affinity for spiperone, respectively, was investigated in three parts of the hippocampus: dorsal, medial and ventral. In each hippocampal part, 5-HT1A sites constituted approximately 70% of binding sites. [3H]5-HT uptake and [3H]5-HT binding to 5-HT1A and non-5-HT1A sites were significantly higher in the ventral than in the dorsal part of the hippocampus. The dorso-ventral hippocampal gradient in [3H]5-HT binding to both 5-HT1A and non-5-HT1A sites was due to an increase of the density (Bmax) of these sites along the longitudinal hippocampal axis, but not their affinity (Kd) for [3H]5-HT. The results are discussed in the context of the previously described mismatch between the localization of serotonergic nerve terminals and serotonergic receptors in the hippocampus.     

The 5-HT(1A) receptor antagonist robalzotan completely reverses citalopram-induced inhibition of serotonergic cell firing.

5-HT(1A) receptor antagonists have been suggested to increase the efficacy of selective serotonin (5-hydroxytryptamine; 5-HT) reuptake inhibitors in the treatment of depression by enhancing the increase in brain 5-HT induced by 5-HT reuptake blockade. Here, the novel 5-HT(1A) receptor antagonist robalzotan [(R)-3-N, N-dicyclobutylamino-8-fluoro-3, 4-dihydro-2H-1-benzopyran-5-carboxamide hydrogen (2R, 3R) tartrate monohydrate] (12.5, 25, 50, 100 microg/kg, i.v.) was found to completely reverse the acute inhibitory effect of citalopram (300 microg/kg i.v.) or paroxetine (100 microg/kg, i.v.) on the activity of 5-HT neurons in the dorsal raphe nucleus in rats. Robalzotan (5, 50 microg/kg, i.v.) by itself increased the firing rate of the majority of 5-HT cells studied. The present results suggest that robalzotan may indeed augment the increases in 5-HT output induced by selective 5-HT reuptake inhibitors by antagonizing the feedback inhibition of 5-HT cell firing produced by such drugs. Thus, robalzotan may be effective by enhancing the action of selective 5-HT reuptake inhibitors or as monotherapy in the treatment of depression.