Research Component 3: Frontolimbic Circuitry, Behavioral Flexibility, and Adolescent Alcohol History
Research Component 3: Frontolimbic Circuitry, Behavioral Flexibility, and Adolescent Alcohol History
Primary Investigator: Dr. Charlotte Boettiger
Co-Investigator(s): Dr. Donita Robinson
Most alcohol-related harm costs are due to binge-drinking1, which is prevalent among adolescents. In 2020, a third of those ages 18-212, 17% of 12th graders, 10% of 10th graders, and 5% of 8th graders report binge drinking in the past two weeks3. Binge alcohol exposure in this population is of particular concern because the adolescent brain is vulnerable to alcohol insult, leading to persistent atypical neurocognitive function4. We previously found that adolescent binge drinking in people and adolescent intermittent ethanol (AIE) exposure in rats reduces behavioral flexibility and alters resting-state functional connectivity MRI (fcMRI) among prefrontal and subcortical control circuits in adulthood5,6. Moreover, AIE increases density of perineuronal nets (PNN) that encapsulate parvalbumin (PV+) GABAergic interneurons in the prefrontal cortex (PFC), which regulate the synchrony of PFC outputs. Behavioral flexibility relies on intact PFC function7,8, and reduced behavioral flexibility in rodents has been associated with decreased GABA in the PFC9,10, which may skew the balance between excitation and inhibition, or excitatory and inhibitory (E/I) balance. Our Center colleagues have shown that chronic ethanol exposure alters E/I balance in the rodent PFC11,12, but few studies to date have tested whether the reduced behavioral flexibility associated with AIE is exacerbated by adult binge exposure or reflects a skewed E/I balance. Our translational preliminary data indicate that behavioral flexibility in adult humans is associated with two putative indices of E/I balance in the PFC: the ratio of GABA and glutamate/glutamine (Glx) measured by magnetic resonance spectroscopy (MRS)13, and the steepness of the EEG power spectral (1/f) slope14. We also found that modulating the human dorsolateral PFC (dlPFC) via bilateral transcranial alternating current stimulation (tACS) shifts behavioral flexibility15. We hypothesize that adolescent binge alcohol promotes a generally excitatory shift in the balance between E/I signaling within control circuits, which increases habitual action selection and vulnerability to alcohol insult in adulthood. We further hypothesize that such excitatory shifts are associated with altered fcMRI and neurochemistry (e.g., PNN, PV) of control hubs, including the dlPFC (or rodent analog, prelimbic cortex, PL) and anterior insula (aIC). We will test these hypotheses with three interrelated aims, using both human and rodent subjects.
Aim 1: Test whether skewed excitatory/inhibitory indices in control circuits mediate interacting effects of adolescent and adult binge alcohol exposure on habitual action selection. We will collect MRS and EEG measures of E/I balance, and measure behavioral flexibility using the Hidden Association Between Images Task (HABIT) in adult humans (ages 22-50). We will recruit the subjects into three groups – those reporting no adolescent binge drinking and current World Health Organization (WHO) risk drinking levels of either 0-1 (low risk) or 3-4 (high risk), and those reporting adolescent binge drinking and current WHO risk level 3-4 (high risk) – to test whether adolescent binge drinking magnifies differences in E/I balance associated with adult alcohol misuse. We predict that adolescent and adult binge history will have interacting effects on E/I balance indices and behavioral flexibility and that E/I balance indices will mediate the relationship between alcohol misuse and behavioral flexibility. As a complementary approach, we will use rats exposed to AIE and/or adult chronic intermittent ethanol (CIE) to test the hypothesis that increased density of PNN around PV+ neurons mediates the effect of ethanol exposure on reversal learning, with greatest effects in AIE+CIE rats.
Aim 2: Test the hypothesis that changes in excitatory/inhibitory indices in control circuitry mediate changes in behavioral flexibility. We propose to test a causal relationship between E/I balance and behavioral flexibility in two complementary ways. First, in the human subjects from Aim 1, we will test whether 10Hz-tACS to bilateral dlPFC alters habitual action selection in the HABIT Test in proportion to its effects on the dlPFC 1/f EEG slope and/or the MRS-derived GABA/Glx ratio. Second, we will use a bidirectional, chemogenetic approach in rats to test the hypothesis that activation of GABAergic PV+ neurons in the PL will improve reversal learning in binge alcohol-exposed rats. We predict that in both rats and humans, changes in indices of E/I balance induced by these manipulations will inversely associate with changes in habitual response selection.
Aim 3: Collaborative Aim: Test whether excitatory/inhibitory signaling measures in fronto-limbic circuitry of rats and humans correlate with altered functional MRI connectivity. This collaborative aim expressly focuses on human-rodent translational measures. First, we will collect fcMRI measures from the humans tested in Aims 1 and 2, who will have a range of binge alcohol history, providing a valuable translational resource for this P60. Thus, we will work with each Component (CP) and the Scientific Resource Core (SRC) to evaluate fcMRI among specific ROI in light of each CP’s hypotheses. Second, we will partner with CP2 and CP4 to measure prefrontal MRS-derived GABA/Glx ratio and fcMRI in rats via the SRC. This study will assess (1) whether AIE exacerbates CIE reductions in fronto-limbic connectivity and (2) whether animal models of binge ethanol exposure produce changes in GABA/Glx ratio that are consistent with those observed in humans engaged in risky drinking. Third, in collaboration with CP5, we will use slice electrophysiology to measure E/I balance in PV+ and pyramidal neurons in the PL and aIC, bridging multiple CPs of the P60.
CP3 uses an explicitly translational approach to identify enduring consequences of binge alcohol exposure on behavioral flexibility and to delineate the neural mediators of these consequences.