Olfactory bulbectomy increases vasopressin, but not corticotropin-releasing hormone, content in the external layer of the median eminence of male rats

Removal of the olfactory bulbs results in numerous physiological and behavi oral changes in rats. The most frequent and characteristic change is an abn ormally high level of corticosterone in the blood, possibly due to changes in the activity of the hypothalamic neurons which synthesize corticotrophin -releasing hormone (CRH). Some of these neurons also synthesize vasopressin (AVP). They are located in the parvocellular part of the paraventricular n ucleus of the hypothalamus, which projects into the external layer of the m edian eminence. We investigated whether there was such a change in activity by studying the synthesis and storage activity of CRH neurons in bulbectom ized rats. CRH and AVP axon terminals in frozen sections of the external la yer of the median eminence were labeled by immunofluorescence techniques an d the degree of labeling was analyzed semi quantitatively. There was no dif ference in the area or intensity of CRH-labeling in control and bulbectomiz ed rats. However, a significantly larger area was stained for AVP in the bu lbectomized than in control rats. We also used in situ hybridization, with single- and double-labeling, to study the effects of bulbectomy on expressi on of the genes encoding CRH and AVP. No significant difference was found i n the levels of mRNA for CRH and the number of CRH+/AVP+ cell bodies was si milar in the parvocellular part of the paraventricular nucleus in bulbectom ized and normal rats. Our results suggest that the hypothalamopituitary-adr enal (HPA) axis changes observed after olfactory bulbectomy may be due to p lastic changes in hypothalamic CRH neurons, resulting in greater storage of increased AVP in CRH neurosecretory nerve terminals in the external layer of the median eminence. (C) 1999 Elsevier Science Ireland Ltd. All rights r eserved.