Baseline Lateral Habenula Firing Rate Negatively Correlates with Increase in Ethanol Intake over Days in Adult Long Evans Rats

While many adults consume alcohol, yet only some individuals are at a risk to develop alcohol use disorder (AUD). Variability in the risk for alcohol abuse is multifactorial and includes differences in behavioral and neuronal traits. The lateral habenula (LHb) has been shown to mediate aversive state-related behavioral responses. Interestingly, in both humans and rodents, depression-like symptoms are associated with high LHb activity. Additionally, there is a high co-morbidity between major depressive disorder and AUD. However, LHb lesions in rodents increase ethanol intake over time. Thus, we wanted to determine how baseline LHb activity correlates with ethanol intake over days. Specifically, we wanted to test whether individual variation in baseline LHb activity in ethanol-naive rats is related to home-cage ethanol drinking patterns. Hence, in this study, we determined the correlation between individual variability in baseline LHb neural activity, the negative-affective state-related ultrasonic vocalizations (USVs; 22-28 kHz), and the extent of ethanol intake over days. We first surgically implanted a unilateral 16-wire electrode array in the LHb of adult male Long Evans rats (n=11). Following recovery from surgery, rats were placed in sound-insulated chambers for two hours, where they were free to explore while we simultaneously recorded neuronal signals from their LHb and the USVs emitted by them. Next, these rats underwent an intermittent access to ethanol (IAE) paradigm, where they received 20% ethanol for 24 hours on alternate days (Monday-Wednesday-Friday) and ad-libitum water in their home cages for four weeks. The change in ethanol intake over days differed between rats, with some rats escalating their ethanol intake in the four weeks, while other rats showing no meaningful change in ethanol intake over days as compared to the first session. We found a significant negative correlation between average baseline LHb firing rates in rats and the changes in ethanol intake in the first week of IAE. Specifically, rats with higher baseline LHb firing rats, unlike rats with lower baseline firing rates, did not escalate their ethanol intake from the first session to the second and fourth ethanol sessions of the IAE paradigm. We also found a moderate positive correlation between the number of 22-28 kHz USVs and the average LHb firing rates of these rats. These results indicate that higher baseline LHb neuronal activity in normal adult ethanol naive rats is associated with decreased motivation to seek and consume ethanol during the early stages of ethanol consumption.