Effect of muscimol and L-NAME in the PVN on the RSNA response to volume expansion in conscious rabbits

In the present study, we have investigated whether the hypothalamic paraventricular nucleus (PVN) contributed to the reflex reduction in renal sympathetic nerve activity (RSNA) normally elicited by volume expansion in the conscious rabbit. RSNA was monitored after volume expansion (Dextran 70, 2 ml/min for 30 min) in animals microinjected into, and outside, the PVN with muscimol (10 nmol), to acutely inhibit neuronal function. Because nitric oxide within the PVN inhibits RSNA, we also examined the effect of NG-nitro-L-arginine methyl ester (L-NAME; 20 nmol) to block nitric oxide synthase. Compared with vehicle, the reduction in RSNA elicited by volume expansion was abolished by injection of muscimol into the PVN. The effect was specific to the PVN because microinjections of muscimol outside the PVN had no effect on the response. L-NAME microinjected into or outside the PVN had no effect on the RSNA response. The findings suggest that the PVN is essential in the central pathways mediating the renal sympathetic nerve response elicited by elevations in plasma volume but that nitric oxide does not play a major role.