The role of brain norepinephrine in clonidine suppression of isolation-induced distress in the domestic chick.

Distress vocalizations (DV) induced by social isolation were measured in 1-day-old domestic chicks after intracerebroventricular injections of drugs believed to exert their effects through the noradrenergic system. The alpha-adrenoreceptor agonist clonidine reliably suppressed DV rates at doses low as 0.08 micrograms. When given alone, phentolamine, phenoxybenzamine, propranolol, sotalol, and yohimbine (adrenoreceptor antagonists) did not reliably alter DV rates at doses that were not toxic. The clonidine DV suppression was reliably reversed by yohimbine (1.75 micrograms), but by none of the other adrenoceptor antagonists or naloxone. 6-Hydroxydopamine at doses as high as 120 micrograms, which essentially eliminated forebrain norepinephrine, failed to suppress DV rates reliably when given alone and, when given in combination with clonidine (0.1 micrograms) or morphine (0.05 micrograms), failed to reverse the severe DV suppression imposed by these drugs. The results suggest that clonidine suppresses DV rates in or by some means other than prejunctional alpha 2-adrenoreceptor stimulation.