Effects of sodium depletion on competition and summation between rewarding effects of salt and lateral hypothalamic stimulation in the rat.

The effect of sodium depletion on the value of lateral hypothalamic (LH) stimulation and on competition and summation between saline reward and LH reward was assessed. Sodium depletion did not alter curves relating the number of LH rewards earned per trial to the number of stimulation pulses per train, suggesting that negative sodium balance does not alter the reward value of the stimulation. In contrast, sodium depletion increased the ability of saline (0.9% NaCl) to compete with LH stimulation in a forced-choice preference test. Summation between the effects of the 2 rewards was also seen under sodium depletion. These findings imply that the rewarding effects of saline and LH stimulation are evaluated within a common system of measurement and that negative sodium balance modulates the value of the saline reward upstream from the point where the rewarding effects of saline and LH stimulation are combined.