Effects of lesions of the ventromedial hypothalamus on naloxone-induced morphine withdrawal in rats.

A previous report indicated that lesions of the hypothalamic ventromedial nucleus suppressed autonomic signs of withdrawal from morphine. The present experiment was designed to determine whether such lesions suppressed a second class, skeletal responses of withdrawal, as well. Naloxone-precipitated withdrawal was compared in previously-addicted VMN-lesioned and sham animals, and drug-naive VMN-lesioned and sham animals. The lesion failed to suppress any signs of withdrawal in the addicted lesioned animals as compared with the shams. An attempt was made to explain the discrepancy between these and earlier published results on the basis of differing experimental designs. Implications of these results for possible dissociation of tolerance and dependence effects or possible plasticity of the brain in regard to drug effects were discussed. Further experiments were proposed in which time of lesion is manipulated in relation to the induction of dependence.