Electrophysiological Responses of Rostral Versus Caudal Ventral Tegmental Neurons

Abstract

Accumulating evidence suggests that neurons of the ventral tegmental area (VTA) are highly correlated with event salience. Different regions of the VTA have different projection sites, yet little is known about the VTA response across its rostro-caudal axis. High-density recording methods were used to monitor cells from the rostral and caudal VTA of adult male rats as they ran for food reward. Reward magnitude was unexpectedly changed during recording sessions. We confirm the sensitivity of VTA neurons to reward and reward magnitude. Furthermore, we observed three 'classes' of VTA neurons: 1) neurons with gradual firing rate changes following reward delivery; 2) neurons with steep changes; and 3) neurons with no response to reward. Within these categories, neurons exhibited one of two different waveforms. While the broad responses to reward delivery were present in both VTA regions, the rostral VTA had a significantly higher proportion of cells with steep response characteristics. Finally, rostral VTA neurons with no response to reward exhibited firing rate changes when reward contingencies were altered. Caudal VTA neurons did not show this property. These results suggest that the VTA exhibits heterogeneous responses to reward which may have important implications for processing in structures receiving differential VTA output.