Haloperidol differentially potentiates tonic immobility, the dorsal immobility response, and catalepsy in the developing rat.

The effects of a single subcutaneous injection of haloperidol (1 or 10 mg/kg) or the vehicle solution on three kinds of behavioral inhibition were investigated in rats of 10, 15, and 20 days of age. The behaviors measured were tonic immobility, a response induced by placing an animal on its back supported by a V-shaped trough; the dorsal immobility response, which can be elicited in young rodents by grasping the skin of the dorsal surface of the neck and lifting the animal into the air; and catalepsy, a form of immobility in which animals remain in unusual postures but are capable of righting themselves, and which can be elicited by drugs which block dopamine receptors. Haloperidol (10 mg/kg) significantly potentiated the duration of tonic immobility only at 10 days of age; potentiated catalepsy at all age levels, but significantly less in the 15-day-old rats than in the 10- or 20-day-old rats; and potentiated the duration of the dorsal immobility response in the 10- and 20-day-old animals but not in the 15-day-olds. These results are discussed with respect to the development of neurotransmitter systems in the brain.