PARVOCELLULAR NEURONS OF THE PARAVENTRICULAR NUCLEUS ARE INVOLVED IN THE REDUCTION IN RENAL NERVE DISCHARGE DURING ISOTONIC VOLUME EXPANSION

The parvocellular neurons of the paraventricular nucleus (PVN) of the hypothalamus are known to be involved in the control of central autono mic outflow. While the PVN is also known to be involved in the control of fluid-balance, most of the studies examining this nucleus have emp hasized the magnocellular neurons, which are involved in the humoral c ontrol of fluid-balance and related hemodynamics. The present study to ok advantage of the differential sensitivity of these two cell types t o kainic acid as a means of investigating the role of the parvocellula r neurons in the reflex reduction of renal sympathetic nerve discharge (RSND) during acute isotonic volume expansion. Kainic acid (18 pmol), which destroys parvocellular but not magnocellular neurons in the PVN , was microinjected (20 nl) bilaterally at sites in and adjacent to th e PVN 3-4 days prior to acute isotonic volume expansion. In anesthetiz ed rats RSND decreased by 59% at the completion of acute isotonic volu me expansion (10% of body weight) in the vehicle-injected control grou p; on the other hand, it decreased by 33% in the kainic acid-treated g roup. The effect of destruction of the parvocellular neurons on the ba roreceptor reflex was also examined. Neither the renal nerve component (Delta%RSND/Delta AP), nor the heart rate component (Delta HR/Delta A P), of the baroreceptor reflex were different in the kainic acid-treat ed group (3.1 +/- 0.4, and 1.1 +/- 0.1, respectively) than in the vehi cle-injected control group (2.9 +/- 0.7, and 0.8 +/- 0.1, respectively ). We conclude that the parvocellular neurons of the PVN are an import ant synaptic relay site in the reflex are that is activated during iso tonic volume expansion, but not in the baroreceptor reflex.