SEX DIFFERENCE IN GLUCOCORTICOID REGULATION OF VASOPRESSIN MESSENGER-RNA IN THE PARAVENTRICULAR HYPOTHALAMIC NUCLEUS

1. Arginine vasopressin (AVP) is synthesized in specific brain regions including the magnocellular and parvocellular divisions of the parave ntricular nucleus (PVN), Whereas magnocellular AVP responds to osmotic stimuli and functions mainly-although not exclusively-as an antidiure tic hormone, that produced in the parvocellular region controls the hy pothalamus-pituitary-adrenal (HPA) axis, in conjunction with CRF. 2. I n view of the reported sex differences in control of the HPA axis, we studied if these also pertain to AVP mRNA in the PVN of ovariectomized -estrogenized female rats and male rats determined by in situ hybridiz ation, AVP mRNA was measured in intact rats, adrenalectomized (ADX) ra ts and ADX receiving dexamethasone (DEX) of both sexes, 3. Computerize d autoradiography showed that in both sexes, AVP mRNA levels in the pa rvocellular division of the PVN increased after adrenalectomy and decr eased following DEX, However, the reduction by DEX was more pronounced in female rats. No changes were found for the magnocellular region. G rain counting analysis of the medial-medial (MMP) and medial-lateral ( MLP) subdivisions of the parvocellular region showed that the average number of grains per cell area in the MMP region of adrenally intact f emale rats was higher than that in males. However, in females there wa s no clear cut effect of adrenalectomy on AVP mRNA levels, although th e reduction after DEX treatment was again greater than that in male ra ts. Frequency histograms constructed by plotting the number of cells v s the number of grains per area substantiated the enhanced glucocortic oid negative control of AVP mRNA in the MMP and MLP of female rats. 4. The results indicated a sexual dimorphism in the glucocorticoid-depen dent plasticity of AVP mRNA levels in the PVN. Because AVP mRNA expres sion differs between sexes under basal levels, after adrenalectomy, an d after DEX treatment, these plastic changes may differentially condit ion the response to stress. Taking into consideration that stress and AVP may play a role in neurogenic hypertension, the possibility of sex ual dimorphisms in AVP control may be important to assess the role of sex hormones in stress and steroid-derived hypertension.