Functional and Morphological Changes in the Hypothalamus-Pituitary Posterior Lobe System after Hypophysectomy in the Dog

Abstract

Acute diabetes insipidus-like symptoms have been reported as a complication after hypophysectomy in dogs. These symptoms are believed to be the consequence of deficiency of arginine vasopressin (AVP) secretion. The symptoms spontaneously resolve within 2 weeks, but the mechanism is unclear. In the present study, AVP secretion related to increases in Na⁺ concentration and serum osmotic pressure was measured, and immunohistochemical analysis in the paraventricular and supraoptic nuclei was perfomed after hypophysectomy in normal dog. In the hypertonic saline test, the plasma AVP concentration slightly increased in hypophysectomized dogs, although the increase was markedly smaller than that in normal dogs. An immunohistochemical study of the hypothalamus nucleus revealed that, AVP-positive cells tended to decrease after hypophysectomy. It suggests that excision of the posterior lobe by surgery injured the axon of magnocellular neuron in the hypothalamus. A decrease in the function and the number of AVP-producing and -secreting magnocellular neurons after hypophysectomy, suggests that the clinical improvement of postoperative diabetes insipidus-like symptoms may not be related to the recovery of AVP secretion.