Behavioral effects of yohimbine and chlordiazepoxide: dependence on the rat's previous familiarization with the test conditions.

The effects of yohimbine and chlordiazepoxide on locomotor activity in an open field were found to depend on the rat's previous habituation to the test environment: chlordiazepoxide induced a locomotor activation in nonhabituated, but not in habituated rats; yohimbine activated in habituated rats only. In a further study, carried out in the rat's homecage (habituated conditions), the effect of both drugs on competition for sucrose pellets was assessed: yohimbine, but not chlordiazepoxide, helped so-called poor-performing rats to temporarily overcome their typical abstention from competition. The presence or absence of drug effects, obviously depending on the rat's previous familiarization with the test environment, are discussed with view to Gray's concept of the behavioral inhibition system.