PROJECT SUMMARY Adolescence is a vulnerable period for initiating substance use and abuse, during which time sleep and circadian rhythm disruptions are pervasive. It is increasingly recognized that sleep and circadian rhythm causally, and powerfully regulate reward processing, but the underlying mechanisms remain poorly understood. Could sleep and circadian rhythm traits be related to reward circuit function? Whether and how do sleep and circadian disruptions lead to increased vulnerability for substance use in adolescents? The central hypothesis of the Center application is that adolescent development acts on underlying sleep and circadian traits to modify homeostatic sleep drive, circadian phase, and circadian alignment, which in turn impact cortico-limbic functions critical to substance use risk (e.g., reward and cognitive control). It is further hypothesized that specific manipulations of sleep and circadian rhythms during adolescence will affect reward responsivity and cognitive control in either positive or negative directions. This research project (Project 5) will focus on rodent models to determine the cellular and synaptic mechanisms within the cortico-limbic circuit through which sleep and circadian disruptions alter reward processing. Specifically, the nucleus accumbens (NAc) is a reward-processing “hub” in the ventral striatum which is sensitive to both sleep and circadian disruptions. For example, acute sleep deprivation reduces glutamate release at medial prefrontal cortex-to-NAc medium spiny principal neurons (MSNs) synapses;; chronic sleep fragmentation increases cholinergic neural activity in the NAc (preliminary results);; robust diurnal fluctuations in AMPA receptor (AMPAR) levels and intrinsic membrane excitability are also observed in the NAc MSNs, and circadian gene mutation in the NAc leads to altered AMPAR transmission in MSNs. Together, these results suggest that the NAc may represent a converging site for sleep and circadian rhythm to regulate reward processing. Accordingly, Project 5 will test the hypothesis that sleep and circadian rhythm target aspects of NAc synaptic transmission and neural modulation to regulate reward-motivated behaviors. Thus, Aim 1 will use genetically diverse outbred rats to determine whether naturally occurring “early” and “late” chronotypes are associated with different diurnal variation of AMPAR transmission in NAc MSNs. This aim will accommodate molecular (Project 3) and behavioral (Project 4) characterizations of these rats. Aim 2 will determine whether circadian disruptions without changes in sleep alter the diurnal variation of membrane excitability and/or postsynaptic AMPAR levels in the NAc MSNs. Aim 3 will determine the effects of acute and chronic sleep restrictions on ade...