Sex Differences and the Role of the Serotonin System in Early Life Stress-Induced Ethanol-Motivated Behaviors in Adulthood

Abstract

Alcohol is a psychoactive substance with dependence-producing properties that has been used by many cultures in human society for millennia. Among people 20-39 years old, 13.5% of total deaths are attributed to alcohol, and harmful use is a causal factor in more than 200 diseases and injury conditions. Current pharmacological treatments of alcohol use disorder mainly target the positive reinforcing properties of alcohol or increase sensitivity to the adverse effects of alcohol. However, these medications have limited efficacy in reducing alcohol use and maintaining sobriety. The majority of these treatments stem from data conducted solely in males and fail to target the psychological distress underlying alcohol motivation in those assigned female at birth and individuals with adverse childhood experiences who are more apt to drink to relieve negative affective states. The dorsal raphe nucleus (DRN)-serotonin (5-HT) system has an important role in mood regulation, is sexually dimorphic, vulnerable to stress-induced perturbations during development, and shows an inverse relationship with ethanol (EtOH) consumption. These characteristics make the 5-HT system an ideal target to investigate the mechanisms underlying aberrant alcohol consumption and drinking despite negative consequences, especially in at-risk populations. Our guiding hypothesis was that DRN 5-HT neurotransmission would underlie the sex-dependent effects of early life stress on EtOH intake and consumption despite negative consequences. To test this hypothesis, we first investigated the effects of sex and early life stress on EtOH-motivated behaviors in rats during adulthood. Next, we examined the role of the DRN 5-HT system in sex- and stress-dependent EtOH-motivated behaviors. Adolescent social isolation stress (SIS) was used as a translational model of early life stress. Male and female rats were group housed or isolated postweaning before undergoing voluntary EtOH consumption (i.e., homecage drinking, self-administration (SA), footshock-punished SA) paradigms in adulthood to test the effects of sex and stress on EtOH consumption and consumption despite negative consequences. Consistent with our hypothesis, behavioral results indicated that SIS increased homecage EtOH preference and consumption, as well as responding during SA, particularly in females. Females also showed more punishment-resistant responding for EtOH than males. To complement these behavioral findings, we employed a combination of electrophysiological, functional neuroanatomy, and chemogenetic strategies to explore the role of the DRN 5-HT system in sex- and stress-dependent EtOH-motivated behaviors. Both SIS and EtOH exposure induced hypofunction of 5-HT neurons, particularly in females, paralleling SIS-induced increases in EtOH-motivated behaviors. Chemogenetic strategies expanded upon the sex- and stress-dependent nature of this relationship. Chemogenetic activation of DRN 5-HT neurons in Tph2-iCre rats reduced responding for both natural and EtOH reward and elevated punished responding for EtOH, indicating a causal connection between DRN 5-HT signaling and acute responding to rewards and punishment. Collectively, our findings reveal an inverse relationship between EtOH intake and 5-HT neurotransmission, further implicating EtOH consumption as an important negative reinforcer to “normalize” SIS-induced hypofunction of the DRN 5-HT system, particularly in stress-sensitive females. Furthermore, our conclusions highlight the DRN 5-HT system as a potential pharmacotherapeutic target to treat aberrant alcohol consumption as a means to “self-medicate” hypofunction of the 5-HT system in at-risk populations.