Effects of plasma hypernatremia on nucleus tractus solitarius neurons

Experiments were done in chloralose-anesthetized, paralyzed, and artificially ventilated rats to investigate the effect of plasma hypernatremia and baroreceptor activation on the excitability of nucleus tractus solitarius (NTS) neurons. Extracellular, single unit recordings were made from 67 histologically verified neurons in the region of NTS. The firing frequency of 29 (43%) NTS neurons was increased by the intracarotid infusion of hypertonic saline (0.5 M). The acute activation of baroreceptors after the intravenous infusion of phenylephrine resulted in the excitation of seven (31.8%) or the inhibition of five (22.7%) of the units responsive to plasma hypernatremia. Thirteen of the hypertonic saline-responsive neurons were recorded in animals in which the organum vasculosum lamina terminalis and subfornical organ were lesioned and/or the left vagus nerve was cut. These data suggest that within NTS there exists a pool of neurons that alter their discharge rate in response to changes in plasma sodium levels and that these neurons may also function in cardiovascular regulation.