The suprachiasmatic nucleus (SCN) is a circadian pacemaker that synchronizes a number of vital physiological and behavioral processes. Although a great deal of research has focused on input pathways to the SCN and on the central clock itself, relatively little is known about SCN output signaling pathways. The ventral tegmental area (VTA) has been extensively studied for its influence in motivated learning, and more recently for a potential role in arousal and sleep-wake regulation. Possible circadian influences on VTA neurons have received little attention, in part because the SCN does not directly project to the VTA. However, data from our lab indicate that VTA neurons exhibit diurnal fluctuations in impulse activity (Luo and Aston-Jones, SFN 2004). We hypothesized that diurnal fluctuations in VTA activity are due to indirect afferent input from the SCN. To test this idea, the retrograde transneuronal tracer Pseudorabies virus (PRV; Bartha strain) was microinjected into the VTA. A time-course analysis of ...