Activity in rat nucleus tractus solitarius after recovery from sodium deprivation

Sodium depletion has powerful effects on ingestive behavior. Depleted rats consume NaCl avidly at first, but decrease their intake to normal levels as they restore their sodium balance. However, vestiges of the depletion experience are expressed as a more rapidly induced and robust sodium consumption when the rat is challenged with a second depletion. Thus, the salience of sodium to the rat is modified in a lasting manner by severe deprivation. Sodium depletion also causes changes in the responses of taste cells in the nucleus tractus solitarius (NTS). In the present study, we examined whether gustatory-evoked responses in rat NTS continue to reflect the condition induced by sodium deprivation after sodium balance is restored. Single-unit recordings were made in response to 13 taste stimuli in two groups of rats: an experimental group that underwent 10-16 days of sodium deprivation followed by a 2-week recovery period, and a control group that never experienced deprivation. Experimental animals were tested for daily intake of 0.5 M NaCl before and after deprivation; they demonstrated a clear salt appetite only on the first day of the recovery period. Electrophysiological recordings revealed no significant differences between the two groups in response to any single stimulus. Neurons from each group of rats were categorized into three subtypes: sugar-sensitive, salt-sensitive, and nonsugar cells. A comparison of responses in these three subtypes offered no significant differences across groups. Thus, as rats restore depleted sodium levels following deprivation, the responsiveness of cells in the NTS also returns to a predeprivation state.