FOS EXPRESSION FOLLOWING ISOTONIC VOLUME EXPANSION OF THE UNANESTHETIZED MALE-RAT

Cardiopulmonary afferents, baroreceptor afferents, or atrial natriuret ic peptide binding to circumventricular organs may mediate the central response to volume expansion, a condition common to pregnancy, exerci se training, and congestive heart failure. This study used Fos immunoc ytochemistry to examine brain regions activated by volume expansion. M ale Sprague-Dawley rats were infused with isotonic saline equal to 10% of their body weight in 10 min followed by a maintenance infusion of 0.5 ml/min for 110 min. Control animals received 2-h infusions at 0.01 ml/min. Five minutes after the start of volume expansion, central ven ous pressure of expanded animals was significantly greater than contro l animals. The volume-expanded group exhibited significantly greater F os activation (P < 0.05) in the area postrema, nucleus of the solitary tract, caudal ventrolateral medulla, paraventricular nucleus, supraop tic nucleus, and perinuclear zone of the supraoptic nucleus. Double la beling indicates that oxytocinergic neurons in the supraoptic nucleus are activated. Neurons in brain regions known to inhibit both sympathe tic activity and vasopressin release show increased Fos expression fol lowing isotonic volume expansion.