Changes in tonic immobility and the GABA-benzodiazepine system in response to handling in the chick

Changes in the GABA-benzodiazepine system were investigated following regular handling of male chicks. Compared with handling-naive chicks, those exposed to 10 days of gentle handling required a larger number of inductions and had a lower duration of tonic immobility. Corresponding biochemical changes occurred, with handling-habituated chicks having a significantly lower basal [14C]GABA release from archistriatal slices and a reduction in the Bmax of [3H]muscimol binding in the forebrain. Benzodiazepine binding in the archistriatum was investigated using in vitro quantitative receptor autoradiography. Binding was localised in the anterior, mediale, dorsalis, and ventralis intermedium nuclei of the archistriatum, and there was significantly more binding in the anterior and ventralis intermedium/mediale archistriatum nuclei than in the dorsalis intermedium archistriatum nuclei. Benzodiazepine binding was not altered after handling in any of the investigated nuclei of the archistriatum. The results suggest that whereas several days of gentle handling in chicks leads to a decrease in forebrain GABAA receptors and a decrease in GABA release from the archistriatum, there are no accompanying changes in benzodiazepine receptors. Regular handling exerts a specific effect on chicks: it reduces their fear or human beings but not that of novel places or objects. It is possible that the pattern of biochemical changes observed in the present study may be specifically associated with this particular behavioural modification rather than with a change in general fearfulness.