Hyponatremia-Induced Inhibition of Magnocellular Neurons Causes Stressor-Selective Impairment of Stimulated Adrenocorticotropin Secretion in Rats*

Abstract

Chronically hyponatremic rats were subjected to various stressors in order to evaluate the possible contribution of magnocellular neurons to the regulation of ACTH secretion, since such rats have markedly inhibited secretion and synthesis of magnocellular arginine vasopressin (AVP) and oxytocin (OT). Stress caused by a novel environment or by insulin-induced hypoglycemia resulted in moderate increases in plasma ACTH, which were of similar magnitude in both hyponatremic and normonatremic rats, and these stressors caused no increase in plasma AVP and OT levels in either group of rats. However, when exposed to ether, hyponatremic rats exhibited a significantly blunted ACTH response compared to normonatremic controls (331 +/- 49 vs. 740 +/- 124 pg/ml; P less than 0.01, respectively), and plasma AVP levels were markedly increased in the normonatremic, but not in the hyponatremic, rats. Intravenous infusion of 2 M NaCl also caused an ACTH release in hyponatremic rats that was significantly smaller than that in their normonatremic counterparts (228 +/- 52 vs. 479 +/- 85 pg/ml; P less than 0.05, respectively), and in this case both plasma AVP and OT levels were markedly increased in the normonatremic, but not in the hyponatremic, rats. However, hyponatremic rats exhibited greatly increased plasma ACTH levels 2 and 96 h after adrenalectomy (ADX), which were statistically equivalent to the increases in ACTH levels in normonatremic rats after ADX. Seven days after ADX parvocellular neurons of the paraventricular nucleus showed strongly increased CRF-41 and AVP-neurophysin, but not OT-neurophysin, immunoreactivities in both normonatremic and hyponatremic rats. These results show that parvocellular CRF-41/AVP-producing neurons in the paraventricular nucleus are not inhibited by chronic hyponatremia, in contrast to magnocellular neurons, and suggest that ACTH secretion induced by ether or hypertonic saline, but not by novel environment or insulin-induced hypoglycemia, is partially mediated by magnocellular AVP and/or OT.