MESENCEPHALIC SUBSTRATE OF REWARD - AXONAL CONNECTIONS

The behavioral version of the collision technique was used to study th e existence of axonal linkage between reward-relevant sites in the ven tral tegmental area (VTA) and posterior mesencephalon (PM) in six rats trained to self-administer trains of electrical brain stimulation. Th e 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 the se. Stimulation of the VTA and the most caudal PM sites generally resu lted in collision curves that were characterized by a single increase in paired-pulse effectiveness (E-values), whereas recovery in those co llision curves obtained from stimulation of the VTA and more rostral P M 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. Reco very from refractoriness (initial 25%) was also estimated and ranged f rom 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 o f this study constitute the first behavioral evidence of a reward-rele vant axonal link between the VTA and the PM. In addition, the finding that in one animal the anterior electrode was located within the poste rior 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.