Increasing evidence suggests that the subthalamic nucleus (STN) is involved in the modulation of motivation. Recent data obtained in the monkey have shown that STN neurons are sensitive to reward (Darbaky et al., 2005 Neuroreport16: 1241-4). In rats, STN lesions increase motivation for food, but decrease motivation for cocaine (Baunez et al., 2005 Nat. Neurosci. 8(4): 484-9). To better understand the role of the STN in the coding of reward, we recorded STN neuronal activity in rats during cocaine self-administration (SA) or during operant responses for sucrose solutions of various concentrations (10%, 4% versus 32%). Male rats were trained to press a lever to self-administer cocaine (i.v., 0.25 mg/infusion) contingent upon a FR1 schedule of reinforcement where each lever press was accompanied by compound conditioned stimuli (CS) (light and tone) and the onset of the pump in daily 2 hour sessions. Then, they were transferred to a modified FR-5 schedule of reinforcement and upon a FR1 schedule with delays between the lever press, the CS and onset of the pump. The same schedules were also used with a 10% sucrose solution as the reward.
In another set of experiments, rats were trained to press a lever down and wait until the onset of a light presented either to the left (predicting a 4% sucrose solution) or to the right (predicting a 32% sucrose solution) of the lever. 0.5sec after the light, a tone was presented to trigger the lever release and then the appropriate pump was activated to deliver the predicted reward in the magazine.
To date, our data indicate that STN neurons show a reactivity to the motor action (lever), to the CS and to the reward when the rats work for sucrose solution, but exhibit mainly a motor related activity when working for cocaine. These results show a differential coding within the STN with regards to the type of reward involved and support the dissociation between natural and cocaine reward observed after STN lesions.