MEDIATION OF OSMOREGULATORY INFLUENCES ON NEUROENDOCRINE CORTICOTROPIN-RELEASING FACTOR EXPRESSION BY THE VENTRAL LAMINA TERMINALIS

Chronic exposure to a hyperosmolar challenge invokes coordinate, differential, and ostensibly adaptive alterations in the expression of mRNA encoding corticotropin-releasing factor (CRF) in the endocrine hypothalamus. Rats maintained on 2% (wt/vol) saline for 7 days displayed the expected reduction in CRF mRNA levels in the parvocellular neurosecretory compartment of the paraventricular nucleus, as well as a concomitant increase in CRF transcripts in oxytocin-containing magnocellular neurosecretory neurons. Also detected in salt-loaded animals was a prominent induction of the immediate-early gene product Fos in magnocellular neurosecretory cell groups and in several brain regions that are known to provide major projections to the endocrine hypothalamus. These included a triad of cell groups making up the lamina terminalis of the third ventricle, and, to a lesser extent, catecholaminergic cell groups in the caudal brain stem. Discrete transections of descending projections from structures associated with the lamina terminalis, as well as excitotoxin lesions centered in one lamina terminalis-associated structure, the organum vasculosum, abolished the effects of salt loading in both the magno- and parvocellular neurosecretory systems. Knife cuts in the lamina terminalis complex that spared only projections from the organum vasculosum region or cuts that disrupted ascending catecholaminergic projections failed to modify either effect of salt loading. The results suggest the existence of a simple circuit through which osmotic influences on gene expression in the magnocellular and parvocellular neurosecretory systems are effected.