A biphasic effect of chlordiazepoxide on animal locomotor activity

Centrally acting drugs often act 'biphasically', especially on animal locomotor activities: the same drug stimulates and depresses, depending either on dose or on time since administration. We propose a third, hitherto neglected, type of biphasic effect, well illustrated by chlordiazepoxide, which depends, instead, on the length of time an animal is exposed to a test environment: in an unfamiliar Y-maze, rats' spontaneous locomotion (number of entries into the arms of the Y) was stimulated to begin with, but switched to depression in a few minutes. The stimulant phase appeared to be independent of dose, while the depressant phase showed log linear dose dependence.