Increased colocalization of corticotropin-releasing factor and arginine vasopressin in paraventricular neurones of the hypothalamus in lactating rats: Evidence from immunotargeted lesions and immunohistochemistry

In lactating female rats, tonically elevated glucocorticoid secretion is ac companied by blunted stress responsiveness, reduced expression of hypothala mic corticotropin-releasing factor (CRF) mRNA and modest increases in argin ine vasopressin (AVP) expression in the paraventricular nucleus (PVN). To d etermine the relative contribution of CRF and AVP to parvocellular function , we performed selective CRF (CRF-Tx) or AVP (AVP-Tx) lesions in the PVN ne urones of ovariectomized virgin or lactating females (day 2 of lactation) b y using ricin A associated with monoclonal antibodies directed towards CRF or AVP. We also performed double immunohistochemical labelling of CRF and A VP in the PVN of control rats injected with immunoglobulin (Ig)Gs associate d with the ricin A (IgG-Tx). Brains were collected 12 days after the lesion and processed for in situ hybridization of CRF and AVP mRNA or for double fluorescence CRF and AVP immunohistochemistry. We found that lactating fema les exhibit a high degree of CRF and AVP colocalization in parvocellular PV N neurones, hypothalamic processes and median eminence terminals compared t o virgins. While CRF mRNA is significantly reduced in lactating rats, AVP m RNA and protein levels are greatly enhanced in parvocellular PVN neurones d uring lactation. Hypothalamic CRF or AVP ricin-A lesions significantly redu ced both CRF and AVP expression (15-35% decrease) as well as peptide immuno reactivity in PVN neurones in both groups of females. The specificity of th e lesions varied between virgins and lactators since in virgin females, AVP -Tx did not affect CRF mRNA expression whereas in lactating females, this s ame lesion significantly reduced CRF mRNA expression, suggesting that parvo cellular PVN neurones are more sensitive to the effects of the lesions duri ng lactation. In both virgins and lactators, lesion with CRF-Tx tended to i ncrease AVP mRNA expression; however, in virgins, parvocellular PVN neurone s were possibly compensating for the loss of CRF synthesis by increasing AV P expression and immunoreactivity. We conclude that lactation is associated with a high degree of CRF and AVP colocalization in parvoPVN neurones and that the increased AVP production in these neurones increases their sensiti vity to immunotargeted lesions. The opposite regulation of CRF and AVP gene expression during lactation might provide a useful model to study differen tial sensitivity to glucocorticoid feedback or hypothalamic activation of t ranscription factors.