Localization of reward-relevant neurons in the pontine tegmentum: a moveable electrode mapping study.

Monopolar moveable stimulation electrodes were implanted in male adult rats in order to map the reward substrate in the pontine tegmentum. Electrodes were implanted 6 mm below the surface of the skull and subsequently lowered by steps of 0.16 or 0.32 mm. Each bar press in a Skinner box delivered a train (0.4 s in duration) of cathodal rectangular pulses of fixed intensity (200 microA) and width (0.1 ms). Self-stimulation was recorded from zero to maximum performance by varying the number of pulses per train. The rewarding effectiveness of the stimulation at each positive site was inferred by determining the frequency threshold. Out of 476 sites that were sampled, 137 supported self-stimulation. Eighty-one percent of the positive sites (111 out of 137) were located within 1 mm of the midline. Of the 181 sites that were sampled in the region posterior to the caudal end of the dorsal raphe, only 9 sites (less than 5%) supported self-stimulation. These results suggest that the majority of neurons that constitute the brainstem reward substrate either originate from and/or terminate in the rostral pons.