Electrophysiological evidence for a functional interaction between 5-HT1A and 5-HT2A receptors in the rat medial prefrontal cortex: An lontophoretic study

In this study, we examined the interaction of 5-HT_1A and 5-HT_2A receptors in the rat medial prefrontal cortex (mPFc) using the techniques of extracellular single unit recording and microiontophoresis. The iontophoresis of the selective 5-HT_1A receptor agonist (±)-8-hydroxy-2-(di-n-propylamino) tetralin (8-OHDPAT) produced a current-dependent suppression (2.5-20 nA) of the basal firing rate of spontaneously active mPFc cells. The iontophoretic (5-10 nA) and systemic administration (0.1-0.5 mg/kg, i.v. ) of the 5-HT_2A/5-HT_2C receptor antagonist ritanserin and the selective 5 HT_2A receptor antagonist MDL 28727 significantly potentiated and prolonged 8-OHDPATs suppressant action. In addition, the systemic administration of another selective 5-HT_2A antagonist MDL 100907, but not its less active enantiomer MDL 100009, also potentiated and prolonged 8-OHDPATs action. The potentiating effect of the 5-HT_2A receptor antagonists on the action of 8-OHDPAT is specific in that neither the iontophoresis of ritanserin nor MDL 28727 altered the suppressant action produced by the iontophoresis of the 5-HT₃ receptor agonist 2-methylserotonin onto mPFc cells. Moreover, the suppressant action of 8-OHDPAT was not altered by the systemic administration of the selective 5-HT₃ receptor antagonist granisetron (0.1-0.5 mg/kg, i.v.). On the other hand, the iontophoresis of a low current (0.5 nA) of the 5-HT_2A,2C receptor agonist (±)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane (DOI) potentiated the excitation induced by the iontophoresis of 1-glutamate on quiescent mPFc cells. The iontophoresis of 8-OHD-PAT at a current that had no effect on the firing rate of 1-glutamate activated when administered alone significantly attenuated the excitatory action produced by the iontophoresis of DOI. Overall these results confirm and extend the hypothesis that there is an interaction between 5-HT_1A and 5-HT_2A receptors in the mPFc at the neuronal level. © 1994 Wiley-Liss, Inc.     

Comparison of the effects of various typical and atypical antipsychotic drugs on the suppressant action of 2-methylserotonin on medial prefrontal cortical cells in the rat.

In this study, we report the effects of various typical and atypical antipsychotic drugs (APDs) on the suppressant action of microiontophoretically applied 2-methylserotonin (2-Me-5HT, a 5-HT3 agonist) on medial prefrontal cortical (mPFc) cells. The microiontophoresis of 2-Me-5HT (10-80 nA) produced a current-dependent suppression of mPFc cells' firing, and this effect was blocked by various 5-HT3 antagonists. The microiontophoresis of the atypical APDs clozapine and a structurally related compound, RMI 81,582, mimicked the action of the 5-HT3 antagonists. In addition, the intravenous administration of clozapine and RMI 81,582 antagonized the suppressant action produced by the iontophoretic application of 2-Me-5HT on mPFc cells. However, the suppressant action of 2-Me-5HT was not blocked by the typical APDs haloperidol and chlorpromazine. The putative atypical APDs risperidone, setoperone, CL 77328, SCH 23390, CGS 10746B, 1-sulpiride, and thioridazine were ineffective in antagonizing 2-Me-5HT's action. Overall, our results suggest that the majority of putative atypical APDs do not interact with 5-HT3 binding sites in the brain. Whether the interaction of clozapine and RMI 81,582 with 5-HT3 sites is correlated with their therapeutic efficacy or lower potential to induce neurological side effects remains to be determined.     

5-HT3-like receptors in the rat medial prefrontal cortex: an electrophysiological study.

In this study, we have identified and characterized 5-HT₃-like receptors in the rat medial prefrontal cortex (mPFc), an area with a moderate density of 5-HT₃ binding sites, using the techniques of single unit recording and microiontophoresis. The microiontophoresis of the 5-HT₃ receptor agonist 3-methylserotonin (2-Me-5HT), similar to the action of 5-HT), produced a current-dependent (10–80 nA) suppression of the firing rate of both spontaneously active and glutamate (GLU)-activated (quiescent) mPFc cells. Phenylbiguanide (PBG), another 5-HT₃ receptor agonist, suppressed the firing rate of mPFc cells but was less effective compared to 2-Me-5HT. The continuous iontophoresis (10–20 min) of 1 M magnesium chloride markedly attenuated the suppressant effect produced by electrical stimulation of the ascending 5-HT pathway, but did not alter 2-Me-5HT's action, suggesting that the action of 2-Me-5HT is a direct one. The suppressant action of 2-Me-5HT on mPFc cells was blocked by a number of structurally diverse and selective 5-HT₃ antagonists, with a rank order of effectiveness as follows: JCS 205930=(±)-zacopride>granisetron=ondansetron= LY 278584 >MDL72222. Furthermore, the intravenous administration of(±)-zacopride antagonized the action of 2-Me-5HT and PBG on mPFc cells. In contrast to the effects of the 5-HT₃ receptors antagonists, other receptor antagonists such as metergoline (5-HT_1A,1B,1C,2), (±)-pindolol (5-HT_1A,1B,β), SCH 23390 (5-HT_1C,2, D1), 1-sulpiride (D2) or SR 95103 (GABA_A) failed to block 2-Me-5HT's action. These results combined suggest that 2-Me-5HT's suppressive action on mPFc cells is mediated directly by 5-HT₃-like receptors.     

The action of(±)-MDMA on medial prefrontal cortical neurons is mediated through the serotonergic system

The mechanism of action of systemitically administered(±)-MDMA (3, 4-methylenedioxymethamphetamine) on spontaneously active neurons in the medial prefrontal cortex (mPFc) of chloral hydrate anesthetized rats was examined using standard single unit extracellular recording techniques. Intravenously administered MDMA dose-dependently decreased the firing rates of the majority of mPFc neurons in control rats. In contrast, in rats that were pretreated withp-chlorophenylalanine (PCPA), which depletes the brain serotonin (5-hydroxytryptamine, 5-HT) content by inhibiting tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of 5-HT, MDMA was largely ineffective in inhibiting the firing of mPFc cells. In PCPA-treated animals, the administration of 5-hydroxytryptophan (5-HTP), which presumably restored the brain 5-HT content, but notl-DOPA, reinstated MDMA's inhibitory action in PCPA-treated rats. In rats that were pretreated withα-methyl-p-tyrosine (AMPT), which depletes the brain dopamine (DA) content by inhibiting tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of DA, MDMA inhibited the firing of all of the mPFc cells. MDMA's effect on mPFc neurons was reversed by 5-HT receptor antagonists such as granisetron and metergoline. These results strongly suggest that MDMA exerts its action on mPFc cells indirectly by releasing endogenous 5-HT.     

The action of (+/-)-MDMA on medial prefrontal cortical neurons is mediated through the serotonergic system.

The mechanism of action of systemically administered (+/-)-MDMA (3,4-methylenedioxymethamphetamine) on spontaneously active neurons in the medial prefrontal cortex (mPFc) of chloral hydrate anesthetized rats was examined using standard single unit extracellular recording techniques. Intravenously administered MDMA dose-dependently decreased the firing rates of the majority of mPFc neurons in control rats. In contrast, in rats that were pretreated with p-chlorophenylalanine (PCPA), which depletes the brain serotonin (5-hydroxytryptamine, 5-HT) content by inhibiting tryptophan hydroxylase, the rate-limiting enzyme in the synthesis of 5-HT, MDMA was largely ineffective in inhibiting the firing of mPFc cells. In PCPA-treated animals, the administration of 5-hydroxytryptophan (5-HTP), which presumably restored the brain 5-HT content, but not L-DOPA, reinstated MDMA's inhibitory action in PCPA-treated rats. In rats that were pretreated with alpha-methyl-p-tyrosine (AMPT), which depletes the brain dopamine (DA) content by inhibiting tyrosine hydroxylase, the rate-limiting enzyme in the synthesis of DA, MDMA inhibited the firing of all of the mPFc cells. MDMA's effect on mPFc neurons was reversed by 5-HT receptor antagonists such as granisetron and metergoline. These results strongly suggest that MDMA exerts its action on mPFc cells indirectly by releasing endogenous 5-HT.     

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.     

Local infusion of the serotonin antagonists ritanserin or ICS 205,930 increases in vivo dopamine release in the rat medial prefrontal cortex

Previous research has indicated that atypical antipsychotic drugs like clozapine preferentially increase dopamine (DA) release from the mesocortical, relative to the nigrostriatal, system. While these drugs generally have weak affinity for the D2 receptor subtype, they are potent antagonists of the 5-hydroxytryptamine₂ (5-HT2) receptor. Research into neurotransmitter interactions indicates that 5-HT modulates DA release, but the nature of this interaction may depend upon the specific 5-HT receptor subtype and the neuronal location of that subtype. The present research tested the hypothesis that 5-HT2 receptors localized near mesocortical DA nerve terminals regulate DA release. This was accomplished by infusing the specific 5-HT2 antagonist ritanserin directly into the medial prefrontal cortex through reverse dialysis in vivo in the rat. Cortical extracellular fluid was then extracted by microdialysis and DA was subsequently assayed by HPLC with electrochemical detection. These results were compared to the systemic administration of ritanserin (1.0–5.0 mg/kg i.p.) and the local application of ICS 205,930, an antagonist at the 5-HT3/4 receptor subtypes. Both 5-HT antagonists increased cortical DA levels when infused locally at concentrations of 100 μM (12 nmoles/60 min), and these results were concentration-dependent. Systemically administered ritanserin also dose-dependently increased cortical DA efflux. These results indicate that atypical antipsychotic drugs may increase mesocortical DA release by antagonizing 5-HT receptors located in the prefrontal cortex. Furthermore, 5-HT may normally inhibit cortical DA release by actions at the 5-HT2 receptor subtype. © 1996 Wiley-Liss, Inc.     

Effects of antipsychotic drugs on 5-HT2 receptors in the medial prefrontal cortex: microiontophoretic studies

Microiontophoretic application (10-80 nA) of the relatively selective serotonin 2 (5-HT2) agonist, 1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane [+/-)-DOI), suppressed medial prefrontal cortical (mPFc) cell's firing rate in a dose-dependent manner. This effect was antagonized by the antipsychotic drugs (APDs) clozapine (CLOZ) and spiperone (SPIP), with CLOZ displaying a greater potency in blocking DOI's action in the mPFc. In contrast, the typical APDs haloperidol (HAL) and L-sulpiride (SUL) failed to block DOI's effect. These results suggest that certain APDs interact with 5-HT2 receptors. Although 5-HT2 antagonism alone may not distinguish typical versus atypical APDs, it is possible that their 5-HT2/dopamine 2 (D2) binding ratio is important.     

Modulation of the firing activity of rat serotonin and noradrenaline neurons by (+/-)pindolol.

BACKGROUND: (+/-)Pindolol is a beta-adrenergic/5-HT1A receptor antagonist used in combination with certain antidepressant drugs to accelerate the onset of the antidepressive response. METHODS: The aim of the present study was to assess, using an in vivo electrophysiologic paradigm, the effect of (+/-)pindolol on the spontaneous firing activity of rat dorsal raphe serotonin (5-HT) and locus coeruleus noradrenaline (NA) neurons. RESULTS: (+/-)Pindolol did not modify the firing activity of dorsal raphe 5-HT neurons at low doses (10 and 200 micrograms/kg, i.v.), but it prevented the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD, 10 micrograms/kg, i.v.) but not that of the 5-HT1A receptor 8-hydroxy-N,N-dipropyl-aminotetralin (8-OHDPAT, 5 micrograms/kg, i.v.). At a higher dose (500 micrograms/kg, i.v.), (+/-)pindolol decreased 5-HT neuronal firing and this effect was reversed by the selective 5-HT1A receptor antagonist WAY 100635 (100 micrograms/kg, i.v.), suggesting that it could act as a partial 5-HT1A autoreceptor agonist. In the locus coeruleus, the high dose of (+/-)pindolol decreased the firing activity of NA neurons and this effect was reversed by the 5-HT2A receptor antagonist MDL 100907 (200 micrograms/kg, i.v.). Finally, both a lesion of NA neurons and the administration of MDL 100907 prevented the suppressant effect of (+/-)pindolol on the firing of 5-HT neurons. CONCLUSIONS: It is suggested that, at low doses, (+/-)pindolol acts as a somatodendritic 5-HT1A autoreceptor antagonist whereas at a higher dose, it decreases the tonic excitatory input from NA neurons to 5-HT neurons.     

Serotonin excitation of facial motoneurons: receptor subtype characterization

The receptor subtype(s) mediating the enhancement of facial motoneuron excitability by serotonin (5-HT) was evaluated by means of single-cell recording in vivo (in the anesthetized rat) and in vitro in brain slices. In vivo, microiontophoretic application of the broad-spectrum 5-HT1 agonist 5-carboxamidotryptamine (5-CT), the 5-HT2/5-HT1C agonist 1-[2,5-dimethoxy-4-methylphenyl]-2-aminopropane (DOM), but not the selective 5-HT1A agonist 8-OH-2[di-n-propylamino]tetralin (8-OH-DPAT), produced a 5-HT-like enhancement of facial motoneuron excitability. Intravenous administration of the 5-HT2/5-HT1C antagonists ritanserin and LY 53857 in vivo blocked the facilitatory effects of 5-HT and DOM, but not norepinephrine (NE). Similarly, in brain slices, bath application of ritanserin blocked the effects of 5-HT, DOM, and 5-CT, but not NE on facial motoneurons. Intracellular recordings showed that DOM induced a slow depolarization and an increase in evoked spikes, but these effects were of lesser magnitude and longer duration than those produced by 5-HT. Taken together, these results indicate a role for 5-HT2 and/or 5-HT1C but not 5-HT1A receptors in serotonergic enhancement of facial motoneuron excitability since 5-HT's effect was 1) at least partially mimicked by the selective 5-HT2/5-HT1C agonist DOM, 2) mimicked by the broad-spectrum 5-HT1 agonist 5-CT but not the selective 5-HT1A agonist 8-OH-DPAT, and 3) blocked by the 5-HT2/5-HT1C antagonists ritanserin and LY 53857.     

Vagal afferent-mediated inhibition of a nociceptive reflex by i.v. serotonin in the rat. II. Role of 5-HT receptor subtypes

In the rat, intravenous (i.v.) serotonin (5-HT) is a noxious stimulus which produces distinct vagal afferent-mediated pseudoaffective responses, a passive avoidance behavior, a vagal afferent-mediated inhibition of the nociceptive tail-flick (TF) reflex and a complex triad of cardiovascular responses. In the present study, we have used a variety of 5-HT receptor antagonists to characterize the receptor subtype(s) in the rat that mediate (1) inhibition of the TF reflex and (2) the cardiovascular responses produced by i.v. 5-HT. 5-HT produced a dose-dependent (3-72 micrograms/kg, i.v.) inhibition of the TF reflex (ED50 = 15.3 +/- 0.7 micrograms/kg). Following administration of the 5-HT2 receptor-selective antagonists ketanserin (50-250 micrograms/kg, i.v.) or xylamidine (10-100 micrograms/kg, i.v.), or the 5-HT3 receptor-selective antagonists ICS 205-930 (50-250 micrograms/kg, i.v.) or MDL 72222 (25-250 micrograms/kg, i.v.), there appeared to be a parallel shift of the 5-HT dose-response curve to the right. Following co-administration of xylamidine (50 micrograms/kg, i.v.) with ICS 205-930 (100 micrograms/kg, i.v.), the 5-HT-induced inhibition of the TF reflex was completely abolished at all doses of 5-HT tested (3-288 micrograms/kg, i.v.). In contrast, administration of the centrally acting 5-HT2 receptor-selective antagonist LY 53857 (10-100 micrograms/kg, i.v.) or the non-specific receptor antagonist methysergide (25-500 micrograms/kg, i.v.) resulted in a dose-dependent, but not parallel shift of the 5-HT dose-response curve to the right. The maximal doses of LY 53857 and methysergide tested (250 micrograms/kg and 500 micrograms/kg, respectively) completely abolished the effects of 5-HT (3-288 micrograms/kg, i.v.). Administration of the alpha 1-adrenoceptor antagonist prazosin (25-100 micrograms/kg, i.v.) failed to alter the 5-HT dose-response curve, indicating that the effects of ketanserin were due to blockage of 5-HT2 receptors rather than alpha 1 receptors. Administration of each of the antagonists also produced marked, but selective effects on components of the complex cardiovascular response to i.v. 5-HT. Each of the 5-HT3 receptor selective antagonists (ICS 205-930 or MDL 72222) produced a dose-dependent attenuation of the Bezold-Jarisch reflex-mediated hypotension and bradycardia, and each of the 5-HT2 receptor selective antagonists (xylamidine, ketanserin or LY 53857) produced a dose-dependent attenuation of the pressor response. The non-specific 5-HT receptor antagonist methysergide produced a dose-dependent attenuation of the 5-HT-induced pressor response.(ABSTRACT TRUNCATED AT 400 WORDS)     

Effect of drugs influencing 5-HT function on ethanol drinking and feeding behaviour in rats: Studies using a drinkometer system

In the present study, we have investigated how various 5-HT agonists (m-chlorophenylpiperazine (mCPP) (0.1-1 mg/kg), 8-hydroxy 2-(di-N-propylamino) tetralin (8-OH DPAT) (0.125-2 mg/kg) and 5-HT (0.5-2 mg/kg)), the 5-HT uptake blocker sertraline (1-10 mg/kg), and the 5-HT uptake blocker and releaser dexfenfluramine (0.5-2.5 mg/kg), affect ethanol intake in a continual access paradigm using Wistar rats. By means of a drinkometer system the effect of each drug on microdrinking parameters (e.g., drink latency, number, and duration of drinking bouts) was assessed. The effect of various 5-HT antagonists (metergoline, ritanserin, ondansetron, and xylamidine) against the dexfenfluramine-induced suppression was studied. Furthermore, threshold doses for the anorectic and the suppressant effects of mCPP, sertraline and dexfenfluramine on ethanol intake were identified. From these studies, it seemed that similar mechanisms may be responsible for the suppressant effects of the various 5-HT agonists studied (direct and indirect) on ethanol and food intake. The 5-HT3 receptor antagonist, ondansetron, also reduced ethanol (but not food) intake. However, the profile of this effect may suggest an alternative means by which 5-HT3 receptors regulate ethanol intake in the rat by comparison to the various 5-HT agonists studied.     

Short-term constant light potentiation of large-magnitude circadian phase shifts induced by 8-OH-DPAT: effects on serotonin receptors and gene expression in the hamster suprachiasmatic nucleus

Nonphotic phase-shifting of mammalian circadian rhythms is thought to be mediated in part by serotonin (5-HT) acting in the suprachiasmatic nucleus (SCN) circadian clock. Previously we showed that brief (1-3 days) exposure to constant light (LL) greatly potentiates nonphotic phase-shifting induced by the 5-HT agonist, (+/-)2-dipropyl-amino-8-hydroxyl-1,2,3,4-tetrahydronapthalene (8-OH-DPAT). Here we investigated potential mechanisms for this action of LL, including 5-HT receptor upregulation and SCN clock gene and neuropeptide gene expression. Autoradiographic analysis of ritanserin inhibition of [3H]8-OH-DPAT binding indicated that LL (approximately 2 days) did not affect 5-HT7 receptor binding in the SCN or dorsal raphe. Measurement of 5-HT1A autoreceptors in the median raphe and 5-HT1B receptors in the SCN also showed no effect of LL. In experiment 2, hamsters held under a 14-h light : 10-h dark photocycle (LD) or exposed to LL for approximately 2 days received an intraperitoneal injection of 8-OH-DPAT or vehicle at zeitgeber time (ZT) 6 or 0 and were killed after 2 h of dark exposure. 8-OH-DPAT suppressed SCN Per1 and Per2 mRNAs at both ZTs, as assessed by in situ hybridization. Per1 mRNA was also suppressed by LL alone. In addition, in situ hybridization of arginine vasopressin (AVP) mRNA and vasoactive intestinal polypeptide mRNA showed that LL significantly suppressed the former but not the latter. The LL-induced suppression of SCN Per1 mRNA and AVP mRNA may be involved in LL-induced potentiation of pacemaker resetting, especially as these data provide additional evidence that LL suppresses circadian pacemaker amplitude, thus rendering the clock more susceptible to phase-shifting stimuli.     

MDMA: further evidence that its action in the medial prefrontal cortex is mediated by the serotonergic system.

Systemically administered (+/-)-MDMA (3,4-methylenedioxymethamphetamine, 'Ecstasy') suppressed the firing rates of the majority of neurons in the medial prefrontal cortex (mPFc). The responses of mPFc cells to (+/-)-MDMA is mimicked by (+)-MDMA but not (-)-MDMA. Furthermore, pretreatment with fluoxetine (a specific 5-HT uptake blocker) but not GBR 12909 (a specific dopamine uptake blocker) prevented the suppressant action of MDMA. These data support the notion that the 5-HT system mediates (+/-)-MDMA's action.     

LSD has high efficacy relative to serotonin in enhancing the cationic current Ih: Intracellular studies in rat facial motoneurons

The effects of LSD (d-lysergic acid diethylamide) on rat facial motoneurons were compared to those of 5-hydroxytryptamine (5-HT) in brain slices by means of current clamp and single-electrode voltage-clamp recordings. As previously reported, 5-HT, in part by decreasing a resting potassium conductance, produced a reversible depolarization (～5 mV), an increase in input resistance, and an enhancement in electrical excitability. LSD also produced an increase in electrical excitability, although with a much slower onset and longer duration. However, in contrast to 5-HT, LSD produced only a slight depolarization (1-2 mV). Moreover, in the presence of LSD the depolarizing effect of 5-HT was markedly attenuated. The 5-HT₂/5-HT_1c agonist 1-(2,5-dimethoxy-4-io-dophenyl)-2-aminopropane (DOI) produced effects intermediate between LSD and 5-HT. The LSD-induced increase in electrical excitability was completely reversed by spiperone, a 5-HT₂/5-HT_1A antagonist, and by ritanserin, a 5-HT₂/5-HT_1c antagonist; the effects of 5-HT were also reduced by these 2 antagonists, but complete blockade did not occur at the concentrations and durations tested. Surprisingly, LSD was found to enhance the hyperpolarization-activated nonspecific cation current I_h to a greater extent than did 5-HT; this enhancement was blocked by both spiperone and ritanserin. These results indicate that, despite having low efficacy relative to 5-HT in decreasing resting potassium conductance, LSD has high efficacy in enhancing the I_h current in rat facial motoneurons; possible mechanisms for this difference are discussed. © 1993 Wiley-Liss, Inc.     

Inhibition of NMDA-receptor mediated response in the rat medial prefrontal cortical pyramidal cells by the 5-HT3 receptor agonist SR 57227A and 5-HT: Intracellular studies

The techniques of intracellular recording and single-electrode voltage-clamp were used to study the effect of serotonin (5-HT) and the selective 5-HT₃ receptor agonist SR 57227A on N-methyl-D-aspartic acid (NMDA)-evoked responses in pyramidal cells of the rat medial prefrontal cortex (mPFC) in in vitro brain slice preparations. Bath application of 5-HT or SR 57227A produced a concentration-dependent inhibition of NMDA-induced membrane depolarization, action potentials, and inward current. The depressant action of 5-HT and SR 57227A had a slow onset and showed no signs of receptor desensitization. This action was markedly attenuated or completely blocked by the selective 5-HT₃ receptor antagonists granisetron and BRL 46470A, but not other receptor antagonists. In addition to inhibiting NMDA-evoked responses, SR 57227A also depressed significantly pharmacologically isolated, NMDA receptor-mediated, monosynaptic excitatory postsynaptic currents (EPSCs) elicited by electrical stimulation of the forceps minor; this inhibitory action was blocked by BRL 46470A but not other 5-HT receptor antagonists. Perfusion of Ca²⁺-free or low Ca²⁺ plus Cd²⁺ artificial cerebrospinal fluid prevented electrical stimulation-induced EPSCs, but did not affect the inhibitory action of 5-HT and SR 57227A. In conclusion, we demonstrate for the first time that 5-HT and SR 57227A interact with 5-HT₃-like receptors to produce a direct inhibitory action on NMDA receptor-mediated response in pyramidal cells of the mPFC. Synapse 29:257–268, 1998. © 1998 Wiley-Liss, Inc.     

Differential effects of novel ligands for 5-HT receptor subtypes on nonopioid defensive analgesia in male mice

The effects of a number of 5-HT receptor ligands were examined on nonopioid defensive analgesia in male DBA/2 mice. MDL 73005EF (0.05-1.0 mg/kg), a selective 5-HT1A receptor agonist, potently and dose-dependently inhibited the analgesic consequences of social defeat. CGS 12066B (0.5-10.0 mg/kg) and MK-212 (0.3-10.0 mg/kg), selective agonists for 5-HT1B and 5-HT1C sites, respectively, failed to influence this particular form of adaptive pain inhibition. Two 5-HT2/1C receptor antagonists, ritanserin (0.05-10.0 mg/kg) and ICI 169.369 (0.3-10.0 mg/kg), were also devoid of specific effects upon defensive analgesia. Both ritanserin and ICI 169,369 were found to have intrinsic analgetic efficacy and to induce behavioural changes indicative of increased defensiveness. These data, together with previous findings, confirm the specific involvement of 5-HT1A receptor mechanisms in the analgesic consequences of social defeat in male mice. Results are discussed in relation to the role of anxiety in adaptive pain inhibition.     

Evidence for serotonin receptor subtypes involvement in agmatine antidepressant like-effect in the mouse forced swimming test

This study investigated the involvement of 5-HT(1) and 5-HT(2) receptors in the antidepressant-like effect of agmatine in the mouse forced swimming test (FST). Pretreatment with p-chlorophenylalanine methyl ester (PCPA; 100 mg/kg, intraperitoneally (i.p.), an inhibitor of serotonin synthesis, for 4 consecutive days), methysergide (5 mg/kg, i.p., a serotonin (5-HT) antagonist), pindolol (32 mg/kg, i.p., a 5-HT(1A/1B) receptor/beta-adrenoceptor antagonist), N-[2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl]-N-(2-pyridynyl)cyclohexanecarboxamide (WAY 100635; 0.3 mg/kg, subcutaneously (s.c.), a selective 5-HT(1A) receptor antagonist), 1-(2-methoxyphenyl)-4[-(2-phthalimido)butyl]piperazine) (NAN-190; 0.5 mg/kg, i.p., a 5-HT(1A) receptor antagonist), 1-(2-(1-pyrrolyl)-phenoxy)-3-isopropylamino-2-propanol (isamoltane; 2.5 mg/kg, i.p., a 5-HT(1B) receptor antagonist), cyproheptadine (3 mg/kg, i.p., a 5-HT(2) antagonist) or ketanserin (5 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist), but not with propranolol (2 mg/kg, i.p., a beta-adrenoceptor antagonist), prevented the effect of agmatine (10 mg/kg, i.p.) in the FST. A subeffective dose of agmatine (0.001 mg/kg, i.p.) produced a synergistic antidepressant-like effect with pindolol (32 mg/kg), NAN-190 (0.5 mg/kg, i.p.), WAY 100635 (0.03 mg/kg, s.c.), (+)-8-hydroxy-2-(di-n-propylamino)tetralin HBr (8-OH-DPAT; 0.01 mg/kg, i.p., a 5-HT(1A) receptor agonist), R(-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane HCl (DOI; 1 mg/kg, i.p., a preferential 5-HT(2A) receptor agonist), or fluoxetine (10 mg/kg, i.p., a selective serotonin reuptake inhibitor, SSRI) but not with isamoltane (2.5 mg/kg, i.p.), ritanserin (4 mg/kg, i.p., a 5-HT(2A/2C) receptor antagonist) or ketanserin (5 mg/kg, i.p.). Taken together, the results firstly demonstrate that agmatine antidepressant-like effects in the FST seem to be mediated, at least in part, by an interaction with 5-HT(1A/1B) and 5-HT(2) receptors.     

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.     

Effects of sustained (+/-)pindolol administration on serotonin neurotransmission in rats

OBJECTIVE: Given reports that (+/-)pindolol, a beta-adrenergic-5-HT1A/1B receptor antagonist, accelerates the onset of the therapeutic effect of certain antidepressant drugs in major depression, the aim of this study was to assess the effect of sustained (+/-)pindolol administration on the sensitivity of pre- and postsynaptic 5-HT1A receptors, terminal 5-HT1B autoreceptors and on overall 5-HT neurotransmission. DESIGN: Prospective animal study. ANIMALS: Sprague-Dawley rats. OUTCOME MEASURES: Modifications of the sensitivity of somatodendritic and postsynaptic 5-HT1A receptors using in vivo electrophysiological paradigms in animals treated with vehicle or (+/-)pindolol (20 mg/kg/day, subcutaneously) through osmotic minipumps for 2 weeks. RESULTS: (+/-)Pindolol attenuated the suppressant effect of the 5-HT autoreceptor agonist lysergic acid diethylamide (LSD) on the firing activity of 5-HT neurons, suggesting that (+/-)pindolol antagonized somatodendritic 5-HT1A autoreceptors in the dorsal raphe nucleus. However, following a 2-day washout period, the suppressant effect of LSD was still attenuated, indicating rather a desensitization of 5-HT1A autoreceptors had occurred. In the CA3 region of the dorsal hippocampus, (+/-)pindolol treatment did not modify the responsiveness of postsynaptic 5-HT1A receptors to microiontophoretic applications of 5-HT. Moreover, such a treatment modified neither the effectiveness of the electrical stimulation of 5-HT fibers nor the function of terminal 5-HT autoreceptors. Finally, the administration of the selective 5-HT1A receptor antagonist WAY 100635 (100 micrograms/kg, intravenously) did not increase the firing activity of dorsal hippocampus CA3 pyramidal neurons in rats treated with (+/-)pindolol, thus failing to reveal the enhanced tonic activation of postsynaptic 5-HT1A receptors associated with major classes of antidepressant treatments. CONCLUSION: Prolonged administration of (+/-)pindolol by itself is not sufficient to enhance overall 5-HT neurotransmission; pindolol should therefore not be endowed with intrinsic antidepressant activity. Although pindolol is capable of antagonizing the 5-HT1A autoreceptor upon the initiation of a 5-HT reuptake-blocker treatment, it also induces a desensitization of this 5-HT1A autoreceptor, which could explain why patients do not relapse upon its discontinuation when they continue taking a 5-HT reuptake blocker.