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 α-adreno-receptor agonist clonidine reliably suppressed DV rates at doses low as 0.08 μg. When given alone, phentolamine, phenoxybenzamine, propranolol, sotalol, and yohimbine (adrenoreceptor antagonists) did not reliably after DV rates at doses that were not toxic. The clonidine DV suppression was reliably reversed by yohimbine (1.75 μg), but by none of the other adrenoceptor antagonists or naloxone. 6-Hydroxydopamine at doses as high as 120 μg, which essentially eliminated forebrain norepinephrine, failed to suppress DV rates reliably when given alone and, when given in combination with clonidine (0.1 μg) or morphine (0.05 μg), failed to reverse the severe DV suppression imposed by these drugs. The results suggest that clonidine suppresses DV rates in chicks through interaction with postsynaptic adrenoreceptors or by some means other than prejunctional α2-adrenoreceptor stimulation.