Emerging cholinergic mechanisms and ontogeny of response inhibition in the mouse

Mice, 7, 11, 15, 19, and 85-115 (adult) days of age, served as subjects in experiments assessing effects of anticholinergics on the development of behavioral inhibition. The centrally active anticholinergic scopolamine produced a dose-dependent elevation in locomotor activity in 19-day-old and adult mice. Acquisition and retention of a step-off passive avoidance response (PAR) was initially studied in nondrugged subjects. Mice as young as 7 days of age learned and retained the PAR for 1 hr. Twenty-four-hour savings, however, were not observed until 19 days of age. Simple PAR performance deficits following scopolamine injection were first seen at 15 days of age. Mice in those age groups exhibiting 24-hr retention (19-day-olds and adults) were used to assess carry-over effects of scopolamine on retest. Only in the case of juveniles did scopolamine, injected prior to training, disrupt 24-hr retest performance. Since methylscopolamine, a peripherally active anticholinergic, had no effect on activity and PAR performance, it is assumed that scopolamine's effects were of central origin. The results suggest that behavioral suppression comes under cholinergic control during the second and third postnatal weeks but that cholinergic mechanisms may not mediate response inhibition uniformly throughout development.