Presynaptic 5‐HT₃ receptor‐mediated modulation of synaptic GABA release in the mechanically dissociated rat amygdala neurons

Abstract

Nystatin‐perforated patch recordings were made from mechanically dissociated basolateral amygdala neurons with preserved intact native presynaptic nerve terminals to study the mechanism of 5‐HT3 receptor‐mediated serotonergic modulation of GABAergic inhibition. The specific 5‐HT3 agonist mCPBG (1 μM) rapidly facilitated the frequency of GABAergic miniature inhibitory postsynaptic currents (mIPSCs) and this facilitation desensitized within 1 min. Tropisetron (30 nM), a specific 5‐HT3 antagonist, blocked the mCPBG effect. mCPBG augmented mIPSC amplitude. However, no direct postsynaptic serotonergic currents were evoked by mCPBG. Neither GABA‐evoked current amplitude nor the kinetics of individual GABAergic mIPSCs were affected by mCPBG. Therefore, the augmentation is unlikely to be due to postsynaptic effects evoked by mCPBG. At higher concentrations mCPBG produced shorter‐duration facilitation of miniature events. While mCPBG increased the mIPSC frequency in calcium‐containing solution with Cd2+, this increase was absent in Ca2+‐free external solution. It appears that the Ca2+ influx through voltage‐dependent calcium channels was not as crucial as that through 5‐HT3 receptors for synaptic GABA release. When two pulses of mCPBG (each 1 μM, 1 min) were given, the response to the second pulse elicited full recovery when the interval between pulses was at least 9 min. Protein kinase A (PKA) activation by 8‐Br‐cAMP (300 μM) shortened and PKA inhibition by Rp‐cAMP (100 μM) prolonged the recovery time. PKA activity did not affect the time course of fast desensitization. Our results suggest that a 5‐HT3‐specific agonist acts on presynaptic nerve terminals facilitating synaptic GABA release without postsynaptic effects. The facilitation requires calcium influx through presynaptic 5‐HT3 receptors. PKA modulates the recovery process from desensitization of presynaptic 5‐HT3 receptor‐mediated regulation of synaptic GABA release.

Stress Impairs 5-HT2A Receptor-Mediated Serotonergic Facilitation of GABA Release in Juvenile Rat Basolateral Amygdala

“…We then investigated the extent to which the 5-HT 2A receptor contributed to serotonergic facilitation of GABA release in the BLA, given that the 5-HT 3 receptor has also been shown to mediate serotonergic facilitation of GABAergic synaptic transmission in the BLA (Koyama et al, 2000;Stein et al, 2000). We first examined the effect of 10 mM serotonin on sIPSCs in the presence of the 5-HT 2A receptor antagonist MDL 100,907 (150 nM) alone.…”

“…One major concern is the extent to which the 5-HT 2A receptor contributes to serotonergic facilitation of GABA release, given that activation of another serotonin receptor, the 5-HT 3 receptor, has also been shown to facilitate GABA release in dissociated BLA neurons (Koyama et al, 2000(Koyama et al, , 2002. To address this concern, we examined the effect of serotonin in the presence of the 5-HT 2A receptor antagonist alone.…”

“…Second, the 5-HT 3 receptor is present only in a subset of GABAergic terminals and the effect of 5-HT 3 receptor activation on GABA release is mainly transient in nature and quickly desensitized (Koyama et al, 2000(Koyama et al, , 2002. To see the effect mediated by the 5-HT 3 receptor, a fast perfusion system (in which the exchange rate of the perfusion is in the millisecond range) was required to apply the 5HT 3 agonist (Koyama et al, 2000(Koyama et al, , 2002. Therefore, when serotonin was being applied using our bath perfusion system in which the exchange rate of bath solution is in the range of 1-2 min, a significant amount of the 5-HT 3 receptors may have been desensitized before the agonist reached the effective concentration.…”

“…Thus, the effect of 5-HT 3 receptor activation may not be evident with the 5-HT 1 receptor unantagonized. Second, the 5-HT 3 receptor is present only in a subset of GABAergic terminals and the effect of 5-HT 3 receptor activation on GABA release is mainly transient in nature and quickly desensitized (Koyama et al, 2000(Koyama et al, , 2002. To see the effect mediated by the 5-HT 3 receptor, a fast perfusion system (in which the exchange rate of the perfusion is in the millisecond range) was required to apply the 5HT 3 agonist (Koyama et al, 2000(Koyama et al, , 2002.…”

Nicotine Enhancement of Dopamine Release by a Calcium-Dependent Increase in the Size of the Readily Releasable Pool of Synaptic Vesicles

“…As observed by recording synaptic events in postsynaptic neurons, the enhancement appears as an increase in the frequency of quantal, or miniature, events. Examples of such enhancement include nicotinic acetylcholine receptor (nAChR) modulation of glutamate release in hippocampus (Gray et al, 1996) and of GABA release in thalamus (Léna and Changeux, 1997), NMDA receptor-mediated glutamate, GABA, and 5-HT release, and 5-HT 3 receptor-mediated increases in GABA release in amygdala (Koyama et al, 2000). In addition, biochemical evidence indicates that presynaptic ionotropic receptor activation promotes the release of the monoamines dopamine (Rapier et al, 1988;Grady et al, 1992) and norepinephrine (Clarke and Reuben, 1996).…”