The amygdala regulates the pituitary-adrenocortical response and release of hypothalamic serotonin following electrical stimulation of the dorsal raphe nucleus in the rat

The amygdala is known to modulate the function of the hypothalamo-pituitary-adrenocortical (HPA) axis, but the mechanism of this effect is still not clear. In the present study we examined the specific role of the serotonin (5-HT) system in mediating the effect of the amygdala on the activity of the HPA axis. Bilateral lesions of the amygdala in rats reduced the adrenocorticotropin (ACTH) and corticosterone responses to electrical stimulation of the dorsal raphe nucleus, where the cell bodies of serotonergic neurons are located. Amygdala lesions had no effect on the ACTH and corticosterone responses to administration of a 5-HT _1A receptor agonist directly into the paraventricular nucleus (PVN) of the hypothalamus, indicating that there was no impairment in the activity of post-synaptic 5-HT _1A receptors in the hypothalamus. In vivo microdialysis showed that amygdala lesions markedly attenuated the effect of electrical stimulation of the dorsal raphe to increase extracellular secretion of 5-HT in the PVN. This is the first demonstration that the amygdala has a facilitatory effect on the function of dorsal raphe 5-HT neurons which project to the PVN, and suggests a mechanism by which the amygdala may modulate the function of the HPA axis.