Chronic Alcohol Dysregulates Glutamatergic Function in the Basolateral Amygdala in a Projection- and Sex-Specific Manner

Chronic intermittent ethanol (CIE) produces alcohol dependence, facilitates anxiety-like behavior, and increases post-CIE alcohol intake. The basolateral amygdala (BLA) is one of several brain regions that regulates anxiety-like behavior and alcohol intake through downstream projections. The BLA receives information from two distinct input pathways. Afferents from medial structures like the thalamus and prefrontal cortex enter the BLA through the stria terminalis whereas lateral cortical structures like the anterior insula cortex enter the BLA through the external capsule. CIE induces input- and sex-specific adaptations to glutamatergic function in the BLA. Previous studies sampled neurons throughout the BLA, but did not distinguish between projection-specific populations. The current study investigated BLA neurons that project to the NAC (BLA-NAC neurons) or the BNST (BLA-BNST neurons) as representative "reward" and "aversion" BLA neurons, and showed that CIE alters glutamatergic function and excitability in a projection- and sex-specific manner. CIE increases glutamate release from stria terminalis inputs only onto BLA-BNST neurons. At external capsule synapses, CIE increases postsynaptic glutamatergic function in male BLA-NAC neurons and female BLA-BNST neurons. Subsequent experiments demonstrated that CIE enhanced the excitability of male BLA-NAC neurons and BLA-BNST neurons in both sexes when glutamatergic but not GABAergic function was intact. Thus, CIE-mediated increased glutamatergic function facilitates hyperexcitability in male BLA-NAC neurons and BLA-BNST neurons of both sexes.

Alcohol availability during withdrawal gates the impact of alcohol vapor exposure on responses to alcohol cues

Background: Chronic intermittent ethanol (CIE) vapor inhalation is a widely used model of alcohol dependence, but the impact of CIE on cue-elicited alcohol seeking is not well understood. Here, we assessed the effects of CIE on alcohol-seeking elicited by previously learned cues, and on acquisition of new cue-alcohol associations. Methods: In Experiment 1, male and female Long Evans rats were first trained in a discriminative stimulus (DS) task, in which one auditory cue (the DS) predicts the availability of 15% ethanol and a control cue (the NS) predicts nothing. Rats then underwent CIE or served as controls. Subsets of each group received access to oral ethanol twice a week during acute withdrawal. After CIE, rats were presented with the DS and NS cues under extinction and retraining conditions to determine whether they would alter their responses to these cues. In Experiment 2, rats underwent CIE prior to training in the DS task. We also assessed alcohol consumption, aversion resistant drinking, somatic withdrawal symptoms, and behavior in an open field. Results: We found that CIE enhanced behavioral responses to previously learned alcohol cues, but only in rats that received access to alcohol during acute withdrawal. CIE disrupted cue responses in rats that did not. When CIE occurred before cue learning, male rats were slower to develop cue responses and less likely to enter the alcohol port, even though they had received alcohol during acute withdrawal. We also found that CIE increased alcohol consumption and aversion-resistant drinking in male but not female rats. Conclusions: These results suggest that CIE alone does not potentiate the motivational value of alcohol cues, but that an increase in cue responses requires the potentiation of the value of alcohol during acute withdrawal. Further, under some conditions CIE may disrupt responses to previously learned and subsequently acquired alcohol cues.

Chronic intermittent ethanol promotes ventral subiculum hyperexcitability via increases in extrinsic basolateral amygdala input and local network activity

The hippocampus, particularly its ventral domain, can promote negative affective states (i.e. stress and anxiety) that play an integral role in the development and persistence of alcohol use disorder (AUD). The ventral hippocampus (vHC) receives strong excitatory input from the basolateral amygdala (BLA) and the BLA-vHC projection bidirectionally modulates anxiety-like behaviors. However, no studies have examined the effects of chronic alcohol on the BLA-vHC circuit. In the present study, we used ex vivo electrophysiology in conjunction with optogenetic approaches to examine the effects of chronic intermittent ethanol exposure (CIE), a well-established rodent model of AUD, on the BLA-vHC projection and putative intrinsic vHC synaptic plasticity. We discovered prominent BLA innervation in the subicular region of the vHC (vSub). CIE led to an overall increase in the excitatory/inhibitory balance, an increase in AMPA/NMDA ratios but no change in paired-pulse ratios, consistent with a postsynaptic increase in excitability in the BLA-vSub circuit. CIE treatment also led to an increase in intrinsic network excitability in the vSub. Overall, our findings suggest a hyperexcitable state in BLA-vSub specific inputs as well as intrinsic inputs to vSub pyramidal neurons which may contribute to the negative affective behaviors associated with CIE.