Mesencephalic substrate of reward: lesion effects

Psychophysical studies suggest that reward-relevant neurons in the posterior mesencephalon (PM) form a caudal extension of the axonal pathway that mediates the rewarding effectiveness of electrical stimulation of the medial forebrain bundle. The present study sought to further characterize the reward-relevant functional link between these two regions by assessing changes to the rewarding effectiveness of caudal medial forebrain bundle stimulation (ventral tegmental area, VTA) subsequent to electrolytic lesions of different PM sites. A total of 13 rats were tested, 11 of these at bilateral VTA stimulation sites. Overall, rewarding effectiveness was reduced in five rats and pontentiated in four. The presence and magnitude of the effects were site-, current- and time-dependent, and ranged from 0.1 to 0.4 log10 unit shifts in reward magnitude, with most effects falling below 0.3 log10 units. Generally, these effects became apparent approximately two weeks after the lesion. In addition to these effects, PM lesions placed on or close to the midline also produced small transient reductions in rewarding effectiveness immediately after the lesion, an effect that disappeared within three days. Conversely, lateral PM lesions were associated either with no immediate effects or with small transient potentiations of reward. The finding that lesions of the PM placed on the midline, just off the midline or laterally all altered the rewarding effectiveness of VTA stimulation suggests that the reward-relevant circuitry is distributed diffusely throughout the PM.