Biological sex has been pinpointed as significant biological factor impacting the prevalence and prognosis of Substance Use Disorder (SUD). Despite the overwhelming utilization of male subjects in SUD research, epidemiological evidence shows that women are the most susceptible population. At the hub of female vulnerability to SUD is significant dysfunction in the mesolimbic dopamine system connecting the ventral tegmental area (VTA) to the nucleus accumbens (NAc). Importantly, an important characteristic of dopamine release from axon terminals in the NAc is that it is rapidly modulated by local regulatory microcircuit mechanisms independent of somatic activity in the VTA. In the NAc, dopamine is released in tonic (slow and regular) and phasic (short, burst/spikes) patterns that are subject to heavy modulation by cholinergic (ChAT) interneurons signaling through α4β2*-containing nicotinic acetylcholine receptors (nAChRs) located directly on dopamine terminals. Previous work suggests that ChAT regulation of dopamine release through nAChRs is fundamentally different between males and females, yet the processes and mechanisms that underlie these sex differences are largely unknown. In this project, we took a multifaceted approach to determine sex-dependent neurochemical mechanisms that underlie ChAT regulation of dopamine release through nAChRs in male, naturally cycling and ovariectomized (OVX) female mice. Using ex vivo fast scan cyclic voltammetry (FSCV) paired with pharmacological applications, we found that ChAT regulation of dopamine release through α4β2*-nAChRs is not present in female mice under most conditions. Deficits in nAChR modulation of dopamine release in intact females were not affected by the estrous cycle; however, they were rescued by ovariectomy – indicating that ovarian hormones play significant a role in this process. Critically, we find that 17β−Estradiol (E2) increases dopamine release acutely, an effect that is blocked by antagonism of α4β2*-nAChRs. Additionally, we observed reduced nAChR agonist effects on dopamine release in females; which is what would be expected for desensitized receptors. Finally, Gq-DREADD behavior studies revealed that male mice learned at a faster rate than intact females when ChAT interneurons were activated. Overall, we show that circulating ovarian hormones, regardless of the hormone cycle, alter the ability of α4β2*-nAChRs on dopamine terminals to modulate dopamine release in the NAc. This suggests that sex differences in ChAT regulation of dopamine neurotransmission underlies sex-dependent differentiation in reward learning. Moving forward, it will be critical to directly link these sex differences to reward learning for the development of sex-specific pharmacotherapies to treat SUD.
Funding for this project was provided by startup funds from Vanderbilt University School of Medicine (E.S.C.), as well as funding from the National Institutes of Health (NIH). Funds from the National Institute on Drug Abuse (NIDA) DA042111 (E.S.C.), the Brain and Behavior Research Foundation (E.S.C.), the Vanderbilt University Academic Pathways Postdoctoral Research Fellowship (L.J.B.) and the NIH / NIDA (1K99DA052641) MOSAIC K99/R00 (L.J.B.) also supported this work.
