CENTRAL C-FOS EXPRESSION IN NEONATAL AND ADULT RATS AFTER SUBCUTANEOUS INJECTION OF HYPERTONIC SALINE

Centrally-mediated responses to plasma hyperosmolality include compens atory drinking and pituitary secretion of vasopressin and oxytocin in both adult and neonatal rats. However, the anorexia that is produced b y plasma hyperosmolality in adult rats is not evident in neonates, per haps due to functional immaturity of osmoresponsive hindbrain circuits . To examine this possibility, the present study compared treatment-in duced brain expression of the immediate-early gene product c-Fos as a marker of neural activation in adult and two-day-old rats after subcut aneous injection of 2 M NaCl (0.1 ml/10 g body weight). This treatment produced marked hypernatremia in adult and two-day-old rats without a ltering plasma volume. Several brain regions (including components of the lamina terminalis, the paraventricular and supraoptic nuclei of th e hypothalamus, and the area postrema) were activated to express c-Fos similarly in adult and two-day-old rats after 2 M NaCl injection, con sistent with previous reports implicating a subset of these regions in osmotically-stimulated drinking and neurohypophyseal secretion. In co ntrast, other areas of the brain that were activated to express c-Fos in adult rats after 2 M NaCl injection were not activated in neonates; these areas included the central nucleus of the amygdala, the parabra chial nucleus and catecholamine cell groups within the caudal medulla. This study demonstrates that certain brain regions that are osmorespo nsive in adult rats (as defined by induced c-Fos expression) are not o smoresponsive in two-day-old rats. When considered in the context of k nown differences between the osmoregulatory capacities of adult and ne onatal rats, our results are consistent with the idea that osmorespons ive forebrain centres are primarily involved in osmotically-stimulated compensatory drinking and neurohypophyseal secretion, whereas osmores ponsive regions of the hindbrain are important for concomitant inhibit ion of feeding and gastric emptying. (C) 1997 IBRO. Published by Elsev ier Science Ltd.