C-FOS EXPRESSION IN ARCUATE NUCLEUS FOLLOWING INTRACEREBROVENTRICULARHYPERTONIC SALINE INJECTIONS

Experiments were done in conscious rats to investigate the effect of i .c.v. infusions of hypertonic NaCl solutions on the induction of the p rotein Fos in the arcuate nucleus (Are). Neurons containing Fos-like i mmunoreactivity were observed throughout the rostrocaudal extent of Ar e after i.c.v. infusions of hypertonic saline solutions (337-744 mM). However, most of the labelled neurons were confined to the middle thir d of the nucleus, in the region of the dorsomedial and ventromedial su bnuclei. Few, if any Fos-labelled neurons were observed in Are of anim als that received i.c.v. infusions of isotonic (142 mM) or mild hypert onic (173 mM) saline solutions or a hyperosmotic (660 mOsm/kg) saline solution of mannitol. No Fos-labelled neurons were found in the subfor nical organ, although a few were observed scattered throughout the org anum vasculosum laminae terminalis (OVLT) in all the animals studied. The density nor the distribution pattern of Fos-labelled neurons in OV LT was altered in animals receiving i.c.v. infusions of hypertonic sal ine or hyperosmotic solutions. These data demonstrate that Are neurons are activated during a hypertonic saline challenge and suggest that A re may function as a sodium-sensitive structure that is involved in bo dy-fluid and circulatory homeostasis.