Sex differences in cholinergic regulation of dopamine release mechanisms underlying reward learning

Abstract

Substance use disorder (SUD) is a chronic, recurring brain disease characterized by significant dysfunction in reward-seeking behavior. SUD research focused on understanding the neural mechanisms in the brain's reward circuitry has overwhelmingly utilized male subjects, even though epidemiological evidence shows that women are the most susceptible population. The neural control of reward is dependent on tonic/phasic dopamine release in the NAc that are subject to heavy modulation by cholinergic interneurons (ChATs) signaling through nicotinic acetylcholine receptors (nAChRs) located directly on dopamine terminals; a process that is essential to encoding information about environmental reward predictive cues. Importantly, dopamine release in the NAc can be elicited completely independent of action potential firing from the soma – through activation of α4β2*-containing nAChRs located on dopamine terminals - highlighting the critical role that this local microcircuit plays in controlling dopamine neurotransmission. Previous work suggests that ChAT control of reward learning and regulation of dopamine release through nAChRs is fundamentally different between males and females, yet the reward information encoding processes and mechanisms that underlie sex differences in reward learning are largely unknown. To understand how the neural circuits that underlie reward learning are differentially modulated between males and females, we took a multifaceted approach to outlining sex differences in ChAT regulation of tonic/phasic dopamine release mechanisms through nAChR modulation in the NAc. Using fast-scan cyclic voltammetry paired with pharmacology we measured the potentiating effect of 17β-estradiol with and without application of the selective α4β2-nAChR antagonist, Dhβe, on sub-second tonic/phasic dopamine release in NAc slices from male and naturally cycling female mice. First, we found that 17β-estradiol enhanced tonic dopamine release in both sexes with greater biological effects in males. Additionally, 17β-estradiol potentiated dopamine release in response to phasic stimulation patterns in males with no significant effect in females. To investigate how ovarian activity affects the ability of 17β-estradiol to potentiate tonic/phasic dopamine release, ovariectomized females were utilized, showing a similar potentiating effect of 17β-estradiol on tonic/phasic dopamine release as seen in the males. Finally, blocking α4β2*-nAChRs with Dhβe reduced the potentiating effects of 17β-estradiol on dopamine release in males and ovariectomized females, but had no effect in female mice with intact ovaries. This data suggests that circulating ovarian hormones alter the effect of α4β2*-nAChRs on dopamine terminals to regulate dopamine release in the NAc. 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.