Abstract

Peripheral signals of energy balance modulate central reward processing and reward-seeking. Prior work has shown that negative energy balance potentiates the reward effectiveness of electrical stimulation evoked from the lateral hypothalamus (LH) when the electrode is located in the perifornical region of the LH but not neighbouring LH sites. Recent work has revealed however that the measurement methods by which these findings were obtained are relatively ambiguous and insensitive. Here we employed a new three-dimensional (3D) method, called the reward-mountain paradigm, which offers additional information on reward processing by measuring operant behaviour as a function of both the strength and cost of the reward. Ten male Long-Evans rats were trained to hold down a lever for electrical stimulation of the LH. Subjects performed reward-mountain sessions during four phases designed to manipulate long-term energy balance: a baseline phase during which subjects were fed ad libitum; a chronic food restriction phase that lasted until subjects reached 75% of their baseline body weight; a stable restriction phase during which subjects were maintained at the target weight; a recovery phase during which subjects were returned to an ad lib diet. During stable restriction, short-term energy balance was varied by feeding rats either before or after test sessions. Both chronic food restriction and meal time yielded mixed effects, supporting the conclusion drawn by prior studies that there exist functionally heterogeneous reward substrates in the LH which are differentially modulated by signals of energy availability.