PROJECT SUMMARY Dopaminergic and serotonergic systems of the brain are associated with many psychiatric conditions including addiction and depression. However, there has been less investigation regarding how these systems dynamically interact to modulate changes in motivation following adverse events in early life and how they regulate responses leading to recovery and gain of function. For example, clinical and preclinical findings that exposure to exogenous cannabinoids such as Δ9-tetrahydrocannabinol (THC, the main psychoactive component of marijuana) during adolescence, leads to motivational deficits similar to those caused by stress that are consistent with compromised affective states such as depression. Exciting preliminary work from our laboratory shows that serotonin (5HT) release in the ventral tegmental area (VTA) excites dopamine (DA) neurons, enhances phasic DA release in the nucleus accumbens (NAc) and is reinforcing. Our preliminary data also suggests that potential counter-motivational effects of adolescent THC can arise from a decrease in the probability of neurotransmitter release from 5HT terminals in the VTA caused by activation of CB1 receptors on these axons. Here, we propose two experiments to investigate the functional role of CB1 receptors on 5HT terminals in the VTA. First, we will control CB1 receptor expression via cre-lox recombination approaches involving cell-type specific loss of function and rescue to isolate the role of CB1 receptors on VTA 5HT terminals to probe cue-related dopaminergic encoding of motivated behavior (aim 1). Next, we will determine if chronic adolescent exposure to THC reduces motivation and its dopaminergic correlates in adulthood and whether these maladaptive effects require CB1 receptors expressed on 5HT terminals in the VTA (aim 2). The present study will incorporate a combination of behavior, anatomy, electrophysiology, fast-scan cyclic voltammetry, virally-mediated gene transfer and optogenetics. In addition, experiments will include males and females and will be replicated in adult animals. Specific genetic control of CB1 receptors on 5HT afferents to VTA DA neurons and optogenetic interrogation of this circuit, will allow explicit tests of current hypotheses of endocannabinoid function in motivation and the consequences of its dysfunction following THC exposure in adolescence. Thus, by investigating 5HT terminal/DA interactions with CB1 receptor signaling, the present proposal makes use of tools not yet applied to these questions to respond to NIDA's mission and to generate new insights on therapeutic strategies for the treatment of conditions involving compromised motivation.