Regulation of Basal Corticotropin-Releasing Hormone and Arginine Vasopressin Messenger Ribonucleic Acid Expression in the Paraventricular Nucleus: Effects of Selective Hypothalamic Deafferentations*

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Abstract
Three exists considerable evidence to suggest that CRH and arginine vasopressin (AVP)-secreting parvocellular neurosecretory neurons of the hypothalamic paraventricular nucleus (PVN) are central integrators of negative feedback effects evoked by circulating glucocorticoid hormones. Most evidence suggests that these neurons may be receptive to circulating glucocorticoid levels, either via glucocorticoid receptors indigenous to these cells and/or via extrahypothalamic glucocorticoid-receptor neurons interacting with the PVN secretory cell. In an effort to address this issue, we performed anterior (ANT), posterior (POST) and total (TOT) deafferentations of the PVN region in male Sprague-Dawley rats using microknives fashioned from narrow-gauge spinal needles. Effective knife cuts were verified immunohistochemically, and deemed acceptable only if they avoided damage to the PVN proper and fibers of CRH and AVP-containing neurons coursing through the hypothalmus en route to the median eminence, while effectively eliminating neuronal input into the PVN region. Subsequent to surgery, levels of mRNA encoding for CRH and AVP in the parvocellular and magnocellular PVN were assayed via semiquantitative in situ hybridization histochemistry. Results indicate that TOT deafferentations resulted in significant increases in CRH mRNA expression in the PVN, and a slight but noticeable induction of AVP mRNA in the medial parvocellular but not posterior magnocellular divisions of the PVN. ANT lesions also produced an up-regulation of CRH and AVP mRNA relative to operated control rats. POST lesions did not produce clear induction in either CRH or AVP mRNA. The data indicate that in the absence of neuronal input coming from anterior structures, CRH mRNA expression is up-regulated, suggesting that local effects of glucocorticoids on teh PVN neuron are ineffective in maintaining normal CRH mRNA expression. These results support a role for neuronal feedback in regulation of the CRH neuron. The limited up-regulation (compared with adrenalectomized rats) AVP mRNA in the TOT group suggests that while neuronal input may have some control of AVP mRNA expression, local glucocorticoid feedback is clearly able to restrict AVP message to levels considerably less than those seen in steroid-deficient animals. Analysis of knife-cut effects on plasma corticosterone and ACTH levels reveals that POST and TOT, but not ANT, deafferentations prohibit the secretory activity of the hypothalamo-pituitary-adrenocortical (HPA) axis seen pursuant to the anesthesia/thoracotomy in lesion and operated control groups. These data implicate a role for posterior structures in secretion of CRH and/or AVP in response to adequate stimulation.