Ontogeny of two cholinergically mediated central effects: stereotyped yawning and potentiation of head-shaking

The ontogenetic course of two cholinergically mediated central neuropharmacological effects, yawning and potentiation of head-shaking induced by D-amphetamine (5 mg/kg), was explored in developing rats. Physostigmine (0.1 mg/kg) and pilocarpine (4 mg/kg) evoke stereotyped yawning in neonatal rats, the effect declining in the middle of the second week of life. Both cholinomimetic drugs strongly potentiate amphetamine induced head-shaking between the 4th and 10th postnatal days. Pilocarpine per se is capable of inducing head-shaking, in the absence of amphetamine, in rats from 8 to 12 days. Infant rat yawning and head-shaking are blocked by scopolamine (5 mg/kg). Nicotine (0.1 mg/kg) potentiates head-shaking but inhibits yawning. Yawning is also depressed by D-amphetamine. The early maturation of these cholinergic effects is discussed in comparison to the later maturation of several forebrain cholinergic systems.