Lesions of ventral tegmental dopamine neurons delay the development of tolerance to morphine catalepsy

Selective lesions of the dopamine (DA) neurons of the ventral tegmental area (VTA) were found to substantially delay the development of tolerance to morphine-induced catalepsy, in comparison with sham-operated controls receiving morphine. Lesioned subjects receiving vehicle injections showed no catalepsy. The data suggest that tolerance to morphine catalepsy requires intact VTA DA neurons. Furthermore, since the acute cataleptic response was intact in lesioned animals, the data suggest that the mechanisms involved in the cataleptic response to morphine are dissociable from those which bring about tolerance to that response.