Abstract Drug addiction is a prominent medical and social problem with no effective treatments. The malfunction of the nucleus accumbens (NAc) has been identified to critically contribute to the development of addiction and related behaviors. To understand the drug-induced functional alterations in the NAc contributing to addiction, extensive efforts have been spent investigating NAc medium spiny neurons (MSNs), which constitute over 90% of NAc neurons and directly mediate the output of the NAc. In contrast fast-spiking interneurons (FSIs) have largely been ignored. While FSIs only constitute a very small portion of NAc neurons (<1%), our preliminary results show that FSIs within the NAc provide robust inhibition to MSNs in response to specific excitatory inputs, gating MSN activity and in turn shaping the functional output of the NAc. This regulation allows FSIs the capacity to greatly influence behaviors mediated by the NAc, including those associated with addiction. The objective of this application is to determine the role of FSIs in mediating addiction-related behaviors, using a clinically relevant rodent model of cue-induced cocaine seeking. Our preliminary studies have found that upon exposure to cocaine-associated cues following abstinence from cocaine self- administration, FSIs display a transient increase in activity while MSNs are simultaneously suppressed. This timing-contingent cue-induced activity pattern and other results lead us to hypothesize!that the local FSI-MSN connection dictates the output of NAc by controlling the activation of MSNs to promote cue-induced cocaine seeking after abstinence. We will test this hypothesis by pursuing two aims. Our Aim 1 will thoroughly characterize the cellular and circuit properties of FSI-mediated regulation of MSNs within the NAc to understand the mechanistic and functional dynamics of this circuit in gating the functional output of the NAc to influence behavior. Our Aim 2 will directly test the hypothesis that cue-induced FSI activity gates MSNs within the NA to promote cue-induced cocaine seeking after withdrawal from cocaine self-administration. The expected outcomes will provide a new conceptual basis in understanding how information is integrated and computed within the NAc to shape addiction-related behaviors by introducing FSIs as critical components within the NAc circuit. This information may provide new avenues for the development of novel therapeutic strategies that target this small neural population for the treatment of addiction. As such, this proposal is highly relevant to the mission of NIDA and NIH.