Different patterns of acute and chronic alcohol activation of the mesopontine tegmentum across cell-type, sub-region, and sex

Abstract

Alcohol is the most widely abused illicit substance, and rates of Alcohol Use Disorder (AUD) diagnosis have been increasing over the last 10 years. Available therapies have a wide range of side-effects and low efficacy in clinical populations; therefore, the neurobiology of alcohol addiction needs to be better understood to inform more effective treatments. Substance Use Disorder (SUD) is a multi-faceted condition impacting a wide range of neural circuits, and the effects of drugs of abuse on the neurological system varies between the class of drug, history of drug use, and behavioral symptoms of various stages of the addiction cycle. Therefore, a refined understanding of alcohol’s mechanisms of action across the various stages of addiction symptomology, and how underlying biological variables such as sex contribute to alcohol mechanism would provide valuable insight into more effective strategy for treatment intervention.All drugs of abuse including alcohol modulates the dopamine neurons in the ventral tegmental area (VTA) and cause neurobiological dysregulation of the mesolimbic reward circuitry that produces addiction-related behaviors. Voluntary alcohol consumption increases endogenous acetylcholine release in the VTA and dopamine release into Nucleus Accumbens (NAc). Blocking cholinergic signaling within the VTA using cholinergic receptor antagonists reduces dopamine release in the NAc and reduces alcohol consumption, suggesting that cholinergic signaling with the VTA regulates dopamine-mediated alcohol reward processes. The only source of acetylcholine in the VTA is from the mesopontine tegmentum (MPT), which consists of two adjacent nuclei the pedunculopontine tegmentum (PPN) and laterodorsal tegmentum (LDT). The cholinergic and glutamatergic neurons of the MPT project to the VTA and modulate dopamine burst firing, and the MPT contains reward-responsive neural populations. Direct stimulation of the cholinergic and glutamatergic MPT neurons is reinforcing, and the modulation of the MPT regulates the intake and subjective reward of drugs of abuse such as nicotine, cocaine, and morphine. However, the role of the MPT on alcohol-related behaviors is understudied. Pharmacological inhibition of MPT reduces alcohol seeking in rats, and MPT lesions block the subjective reward of alcohol in a condition place preference (CPP). Therefore, the MPT is involved in alcohol-mediated reward processing, but the exact cell-type and sub-region contributions impacted by alcohol mechanisms of action are not known. Therefore, the purpose of my thesis work was to characterize the cell-type patterns of activation across acute and chronic alcohol exposure between alcohol dose, MPT sub-region, and between sex. We investigated the effect of acute and chronic ethanol administration on cholinergic and glutamatergic neuron activation in the PPN and LDT in male and female mice. We show that ethanol activates neurons of the PPN and not the LDT in male mice. Acute 4 g/kg and chronic 15 daily injections of 2 g/kg ethanol induced Fos expression in cholinergic and glutamatergic PPN neurons in male mice, whereas ethanol did not increase cholinergic and glutamatergic neuronal activation in the LDT. In contrast, acute or chronic ethanol at either dose or duration had no effect on the activation of cholinergic or glutamatergic neurons in the MPT of female mice. Female mice had higher level of activation in cholinergic neurons compared with males after saline treatment. We also found a population of co-labeled cholinergic and glutamatergic neurons in the PPN and LDT which were highly active in the saline- and ethanol-treated groups in both sexes. These findings illustrate the complex differential effects of ethanol across dose, time point, MPT subregion and sex.