HYPOPHYSECTOMY PRODUCES ANALGESIA AND PARAVENTRICULAR LESIONS HAVE NOEFFECT ON FORMALIN-INDUCED PAIN

Chemical destruction of the pituitary gland has been shown to alleviat e severe cancer pain in a substantial proportion of patients, The unde rlying mechanisms, however, remain controversial, The present study in vestigated the effects of hypophysectomy in the formalin test, which p rovides an animal model of tonic pain, and attempted to determine a po ssible 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 hypophysecto my, implicating the pituitary gland in the modulation of tonic pain. S ince the paraventricular nucleus of the hypothalamus (PVN) provides a major source of input to the pituitary gland, the effects of electroly tic 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 m odel to study possible mechanisms which underlie the relief of severe cancer pain by hypophysectomy. However, since PVN lesions did not affe ct the response of rats in the formalin test, it is possible that the PVN is only one of multiple interacting neural and endocrine structure s that influence the functions of the pituitary. (C) 1995 Academic Pre ss, Inc.