Mesencephalic substrate of reward: axonal connections.

The behavioral version of the collision technique was used to study the existence of axonal linkage between reward-relevant sites in the ventral tegmental area (VTA) and posterior mesencephalon (PM) in six rats trained to self-administer trains of electrical brain stimulation. The combined use of fixed and moveable stimulation electrodes allowed us to carry out collision tests at a total of 46 different combinations of VTA-PM sites, and collision-like effects were observed at 24 of these. Stimulation of the VTA and the most caudal PM sites generally resulted in collision curves that were characterized by a single increase in paired-pulse effectiveness (E-values), whereas recovery in those collision curves obtained from stimulation of the VTA and more rostral PM sites was generally slower, and often characterized by a double rise. Despite little variability in interelectrode distances (1.0-3.8 mm), collision intervals varied widely, ranging from 1.5 to 7.3 msec. Recovery from refractoriness (initial 25%) was also estimated and ranged from 0.7 to 1.0 msec, resulting in conduction-time estimates of 0.7-6.3 msec. The lack of correspondence between interelectrode distances and conduction times suggests the presence of axonal branching. Results of this study constitute the first behavioral evidence of a reward-relevant axonal link between the VTA and the PM. In addition, the finding that in one animal the anterior electrode was located within the posterior portion of the lateral hypothalamus (LH) suggests that the reward-relevant axonal bundle linking the LH and VTA may extend as far back as the caudal regions of the PM.