Lick rate analysis of sodium taste-state combinations

The combined effects of taste and physiological state on lick rate for NaCl were systematically evaluated in two experiments. In experiment 1 the lick rates of six groups of rats (Sprague-Dawley) were recorded during the first 3 min of a NaCl drinking bout, where each group received a different concentration. Half of the rats were depleted by injections of the diuretic-natriuretic furosemide (10 mg ip), whereas the other half were not. The inverted U-shaped concentration-response functions for sodium-replete rats peaked approximately at isotonicity. The replete lick rate function was uniformly elevated for rats examined in the sodium-depleted state. In experiment 2 each rat received access to all six solutions within a 40-min test session. Rats were tested twice, once in positive and once in negative sodium balance. The solutions were repeatedly presented in counterbalanced order in a series of 10-s exposures. The replete and deplete functions of the within-group study closely resembled those of the between-group study. When the deplete rats' function was divided into three segments coinciding with the beginning, middle, and end of the 40-min session, the functions were shifted down vertically but retained their shape. In general, sodium-state level dramatically affected the absolute lick rates but did not appear to shift the location of the concentration-response function along the horizontal (i.e., concentration) axis. The form of the function appears to have been determined by oral factors alone.