Mesencephalic substrate of reward: possible role for lateral pontine tegmental cells

The present study was aimed at determining whether cells located in the lateral pontine tegmentum could constitute part of the neural circuitry that mediates the rewarding effect of mesencephalic electrical brain stimulation. Single action potentials were recorded from lateral pontine tegmental cells in urethane-anesthetized rats following antidromic activation from the ventral tegmental area and/or posterior mesencephalon. A total of 445 cells were recorded in 13 animals and of these, 44 were antidromically driven from the ventral tegmental area (n=13 ipsi-, n=5 contralateral), the posterior mesencephalon (n=8 ipsi-, n=5 midline), or from both sites (n=13). The occurrence of collision between ortho- and antidromic action potentials triggered by concurrent stimulation of both sites is consistent with psychophysical data obtained previously in behaving animals, and likewise suggests that the two sites are linked by uninterrupted axons. In five of the cells that were driven from both sites, the inter-electrode conduction time exceeded the difference in latencies, suggesting that stimulation of the ventral tegmental area and posterior mesencephalon triggered action potentials in different axonal branches of the same cell. Estimates of the end of the absolute refractory period ranged from 0.44 to 1.6 ms (ventral tegmental area) and from 0.3 to 2.0 ms (posterior mesencephalon), times that overlap with behaviorally derived estimates for mesencephalic reward-relevant neurons. These results suggest that cells originating in the lateral pontine tegmentum might constitute part of the directly-stimulated substrate responsible for the rewarding effect of mesencephalic electrical brain stimulation.