5-HT1A receptor-mediated inhibition and 5-HT2 as well as 5-HT3 receptor-mediated excitation in different subdivisions of the rat amygdala

The techniques of extracellular single cell recording and microiontophoresis were used to study the effects of serotonin (5-HT) and of 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptor agonists on the spontaneous activity of amygdaloid neurons in rats anesthetized with urethane. The background discharge rate was modified by 5-HT as well as by 5-HT agonists in about two-thirds of neurons tested in different nuclei of the amygdaloid complex. Whereas the 5-HT(2) and 5-HT(3) agonists significantly increased the neuronal discharge rate in nearly all subdivisions of the amygdala, the 5-HT(1A) agonist significantly inhibited the firing rate. Co-administration of bicuculline and 5-HT receptor agonists prevented the 8-OH-DPAT-induced increases in the firing rate in most cases tested, as well as the inhibitory effects of DOI or 2-methyl-5HT. Therefore, GABAergic interneurons seem to be involved in the mediation of serotonergic effects. The action of 5-HT agonists on the neuronal discharge rate was blocked by different receptor-specific antagonists. The results support the hypothesis that 5-HT exerts control throughout the amygdala by acting at least on 5-HT(1A), 5-HT(2A/2C) and 5-HT(3) receptors seemingly located both on projection and interneurons.