Hypophysectomy produces analgesia and paraventricular lesions have no effect on formalin-induced pain

Chemical destruction of the pituitary gland has been shown to alleviate severe cancer pain in a substantial proportion of patients. The underlying mechanisms, however, remain controversial. The present study investigated the effects of hypophysectomy in the formalin test, which provides an animal model of tonic pain, and attempted to determine a possible neural mechanism to explain the effectiveness of the procedure. Hypophysectomized rats displayed significantly less pain behavior in the formalin test than control rats that underwent a sham hypophysectomy, implicating the pituitary gland in the modulation of tonic pain. Since the paraventricular nucleus of the hypothalamus (PVN) provides a major source of input to the pituitary gland, the effects of electrolytic lesions of the PVN on tonic pain were examined. The results failed to show a significant effect of PVN lesions on pain responses in the formalin test. The results suggest that the pituitary gland modulates tonic pain in the formalin test and that the test provides an animal model to study possible mechanisms which underlie the relief of severe cancer pain by hypophysectomy. However, since PVN lesions did not affect the response of rats in the formalin test, it is possible that the PVN is only one of multiple interacting neural and endocrine structures that influence the functions of the pituitary.