Cholinergic influences on hypothalamic-pituitary-adrenocortical activity of stressed rats: an approach utilizing agonists and antagonists.

Cholinergic involvement in the regulation of the hypothalamic-pituitary-adrenocortical (HPA) system of male rats was evaluated using muscarinic (atropine and methacholine) and nicotine (mecamylamine and nicotine) agents, which were selected for their specificity on cholinergic receptors (ChR). They were administered either intracerebroventricularly (icv) to produce central effects, or ip to produce both central and peripheral effects, prior to subjecting the animals to either auditory or hypercapnic stress for 1 h. Plasma corticosterone was used as an index of HPA activity. The results suggest that central muscarinic ChR are involved in inhibiting HPA activity in both non-stressed and stressed animals, whereas central nicotinic ChR are excitatory during stress but inactive in the non-stressed state. Stimulation of peripheral nicotinic ChR appeared to potentiate the HPA response to hypercapnia, and to inhibit the central excitatory nicotinic ChR when the latter were activated in non-stressed and auditory stress rats. These data suggest that during auditory stress the HPA system is more dependent upon the cholinergic system for its activation than during non-stressed and hypercapnic states.