Glutamatergic input in the PVN is important in renal nerve response to elevations in osmolality

Elevations in plasma osmolality elicit reflex humoral and neural responses. The hypothalamic paraventricular nucleus (PVN) is important in humoral responses. We have investigated whether the PVN contributed to the renal nerve reduction that is normally elicited by increased plasma osmolality in the conscious rabbit. Renal sympathetic nerve activity (RSNA) was monitored after an intravenous infusion of hypertonic saline (1.7 M NaCl, 2 ml/min for 7 min). The responses were examined in animals microinjected with muscimol (10 nmol) into, and outside, the PVN to acutely inhibit neuronal function or with kynurenate (25 nmol) to block glutamate receptors. Compared with vehicle, the maximum reduction in RSNA elicited by hypertonic saline was significantly less with muscimol or kynurenate pretreatment into the PVN. A similar study with kynurenate was also performed in sinoaortically denervated rabbits, and similar effects were observed. The effect was specific to the PVN because microinjections of the drugs outside the PVN had no effect on the response. The findings suggest that excitatory inputs into the PVN may be important in the neural responses elicited by elevations in plasma osmolality.