Genetic Differences in Dorsal Hippocampus Acetylcholinesterase Activity Predict Contextual Fear Learning Across Inbred Mouse Strains

Interstrain differences in adolescent fear conditioning after acute alcohol exposure

Adolescent sensitivity to alcohol is a predictor of continued alcohol use and misuse later in life. Thus, it is important to understand the many factors that can impact alcohol sensitivity. Data from our laboratory suggested that susceptibility to alcohol-associated contextual fear learning deficits varied among adolescent and adult mice from two mouse strains. To investigate the extent of genetic background's influences on adolescent learning after alcohol exposure, we examined how 9 inbred mouse strains differed in vulnerability to alcohol-induced contextual and cued fear conditioning deficits. We demonstrated significant strain- and sex-dependent effects of acute alcohol exposure on adolescent fear learning, with alcohol having most pronounced effects on contextual fear learning. Female adolescents were more susceptible than males to alcohol-induced impairments in contextual, but not cued, fear learning, independent of genetic background. Heritability for contextual and cued fear learning after alcohol exposure was estimated to be 31 % and 18 %, respectively. Learning data were compared to Blood Ethanol Concentrations (BEC) to assess whether strain differences in alcohol metabolism contributed to strain differences in learning after alcohol exposure. There were no clear relationships between BEC and learning outcomes, suggesting that strains differed in learning outcomes for reasons other than strain differences in alcohol metabolism. Genetic analyses revealed polymorphisms across strains in notable genes, such as Chrna7, a promising genetic candidate for susceptibility to alcohol-induced fear conditioning deficits. These results are the first to demonstrate the impact of genetic background on alcohol-associated fear learning deficits during adolescence and suggest that the mechanisms underlying this sensitivity are distinct from alcohol metabolism.

Loss of GABA co-transmission from cholinergic neurons impairs behaviors related to hippocampal, striatal, and medial prefrontal cortex functions

Introduction: Altered signaling or function of acetylcholine (ACh) has been reported in various neurological diseases, including Alzheimer's disease, Tourette syndrome, epilepsy among others. Many neurons that release ACh also co-transmit the neurotransmitter gamma-aminobutyrate (GABA) at synapses in the hippocampus, striatum, substantia nigra, and medial prefrontal cortex (mPFC). Although ACh transmission is crucial for higher brain functions such as learning and memory, the role of co-transmitted GABA from ACh neurons in brain function remains unknown. Thus, the overarching goal of this study was to investigate how a systemic loss of GABA co-transmission from ACh neurons affected the behavioral performance of mice. Methods: To do this, we used a conditional knock-out mouse of the vesicular GABA transporter (vGAT) crossed with the ChAT-Cre driver line to selectively ablate GABA co-transmission at ACh synapses. In a comprehensive series of standardized behavioral assays, we compared Cre-negative control mice with Cre-positive vGAT knock-out mice of both sexes. Results: Loss of GABA co-transmission from ACh neurons did not disrupt the animal's sociability, motor skills or sensation. However, in the absence of GABA co-transmission, we found significant alterations in social, spatial and fear memory as well as a reduced reliance on striatum-dependent response strategies in a T-maze. In addition, male conditional knockout (CKO) mice showed increased locomotion. Discussion: Taken together, the loss of GABA co-transmission leads to deficits in higher brain functions and behaviors. Therefore, we propose that ACh/GABA co-transmission modulates neural circuitry involved in the affected behaviors.