Binge ethanol drinking associated with sex-dependent plasticity of neurons in the insula that project to the bed nucleus of the stria terminalis

Retrieval of an ethanol-conditioned taste aversion promotes GABAergic plasticity in the insular cortex

Blunted sensitivity to ethanol's aversive effects can increase motivation to consume ethanol; yet, the neurobiological circuits responsible for encoding these aversive properties are not fully understood. Plasticity in cells projecting from the insular cortex (IC) to the basolateral amygdala (BLA) is critical for taste aversion learning and retrieval, suggesting this circuit's potential involvement in modulating the aversive properties of ethanol. Here, we tested the hypothesis that GABAergic activity onto IC-BLA projections would be facilitated following the retrieval of an ethanol-conditioned taste aversion (CTA). Consistent with this hypothesis, frequency of mIPSCs was increased following retrieval of an ethanol-CTA across cell layers in IC-BLA projection neurons. This increase in GABAergic plasticity occurred in both a circuit-specific and learning-dependent manner. Additionally, local inhibitory inputs onto layer 2/3 IC-BLA projection neurons were greater in number and strength following ethanol-CTA. Finally, DREADD-mediated inhibition of IC parvalbumin-expressing cells blunted the retrieval of ethanol-CTA in male, but not female, mice. Collectively, this work implicates a circuit-specific and learning-dependent increase in GABAergic tone following retrieval of an ethanol-CTA, thereby advancing our understanding of how the aversive effects of ethanol are encoded in the brain.

Neural Activity in the Anterior Insula at Drinking Onset and Licking Relates to Compulsion-Like Alcohol Consumption

Much remains unknown about the etiology of compulsion-like alcohol drinking, where consumption persists despite adverse consequences. The role of the anterior insula (AIC) in emotion, motivation, and interoception makes this brain region a likely candidate to drive challenge-resistant behavior, including compulsive drinking. Indeed, subcortical projections from the AIC promote compulsion-like intake in rats and are recruited in heavy-drinking humans during compulsion for alcohol, highlighting the importance of and need for more information about AIC activity patterns that support aversion-resistant responding. Single-unit activity was recorded in the AIC from 15 male rats during alcohol-only and compulsion-like consumption. We found three sustained firing phenotypes, sustained-increase, sustained-decrease, and drinking-onset cells, as well as several firing patterns synchronized with licking. While many AIC neurons had session-long activity changes, only neurons with firing increases at drinking onset had greater activity under compulsion-like conditions. Further, only cells with persistent firing increases maintained activity during pauses in licking, suggesting roles in maintaining drive for alcohol during breaks. AIC firing was not elevated during saccharin drinking, similar to lack of effect of AIC inhibition on sweet fluid intake in many studies. In addition, we observed subsecond changes in AIC neural activity tightly entrained to licking. One lick-synched firing pattern (determined for all licks in a session) predicted compulsion-like drinking, while a separate lick-associated pattern correlated with greater consumption across alcohol intake conditions. Collectively, these data provide a more integrated model for the role of AIC firing in compulsion-like drinking, with important relevance for how the AIC promotes sustained motivated responding more generally.

Forced Abstinence from Volitional Ethanol Intake Drives a Vulnerable Period of Hyperexcitability in BNST-Projecting Insular Cortex Neurons

The insular cortex (IC) integrates sensory and interoceptive cues to inform downstream circuitry executing adaptive behavioral responses. The IC communicates with areas involved canonically in stress and motivation. IC projections govern stress and ethanol recruitment of bed nucleus of the stria terminalis (BNST) activity necessary for the emergence of negative affective behaviors during alcohol abstinence. Here, we assess the impact of the chronic drinking forced abstinence (CDFA) volitional home cage ethanol intake paradigm on synaptic and excitable properties of IC neurons that project to the BNST (IC→BNST). Using whole-cell patch-clamp electrophysiology, we investigated IC→BNST circuitry 24 h or 2 weeks following forced abstinence (FA) in female C57BL6/J mice. We find that IC→BNST cells are transiently more excitable following acute ethanol withdrawal. In contrast, in vivo ethanol exposure via intraperitoneal injection, ex vivo via ethanol wash, and acute FA from a natural reward (sucrose) all failed to alter excitability. In situ hybridization studies revealed that at 24 h post FA BK channel mRNA expression is reduced in IC. Further, pharmacological inhibition of BK channels mimicked the 24 h FA phenotype, while BK activation was able to decrease AP firing in control and 24 h FA subjects. All together these data suggest a novel mechanism of homeostatic plasticity that occurs in the IC→BNST circuitry following chronic drinking.

Neural Circuitries and Alcohol Use Disorder: Cutting Corners in the Cycle

An implicit tenet of the alcohol use disorder (AUD) research field is that knowledge of how alcohol interacts with the brain is critical to the development of an understanding of vulnerability to AUD and treatment approaches. Gaining this understanding requires the mapping of brain function critical to specific components of this heterogeneous disorder. Early approaches in humans and animal models focused on the determination of specific brain regions sensitive to alcohol action and their participation in AUD-relevant behaviors. Broadly speaking, this research has focused on three domains, Binge/Intoxication, Negative Affect/Withdrawal, and Preoccupation/Anticipation, with a number of regions identified as participating in each. With the generational advances in technologies that the field of neuroscience has undergone over the last two decades, this focus has shifted to a circuit-based analysis. A wealth of new data has sharpened the field's focus on the specific roles of the interconnectivity of multiple brain regions in AUD and AUD-relevant behaviors, as well as demonstrating that the three major domains described above have much fuzzier edges than originally thought.In this chapter, we very briefly review brain regions previously implicated in aspects of AUD-relevant behavior from animal model research. Next, we move to a more in-depth overview of circuit-based approaches, and the utilization of these approaches in current AUD research.

Whole-brain connectivity to the bed nucleus of the stria terminalis calretinin-expressing interneurons in male mice

The bed nucleus of the stria terminalis (BNST) is a neuropeptide-enriched brain region that modulates a wide variety of emotional behaviours and states, including stress, anxiety, reward and social interaction. The BNST consists of diverse subregions and neuronal ensembles; however, because of the high molecular heterogeneity within BNST neurons, the mechanisms through which the BNST regulates distinct emotional behaviours remain largely unclear. Prior studies have identified BNST calretinin (CR)-expressing neurons, which lack neuropeptides. Here, employing virus-based cell-type-specific retrograde and anterograde tracing systems, we mapped the whole-brain monosynaptic inputs and axonal projections of BNST CR-expressing neurons in male mice. We found that BNST CR-expressing neurons received inputs mainly from the amygdalopiriform transition area, central amygdala and hippocampus and moderately from the medial preoptic area, basolateral amygdala, paraventricular thalamus and lateral hypothalamus. Within the BNST, plenty of input neurons were primarily located in the oval and interfascicular subregions. Furthermore, numerous BNST CR-expressing neuronal boutons were observed within the BNST but not in other brain regions, thus suggesting that these neurons are a type of interneuron. These results will help further elucidate the neuronal circuits underlying the elaborate and distinct functions of the BNST.

Insula Dynorphin and Kappa Opioid Receptor Systems Regulate Alcohol Drinking in a Sex-Specific Manner in Mice

Alcohol use disorder is complex and multifaceted, involving the coordination of multiple signaling systems across numerous brain regions. Previous work has indicated that both the insular cortex and dynorphin (DYN)/kappa opioid receptor (KOR) systems contribute to excessive alcohol use. More recently, we identified a microcircuit in the medial aspect of the insular cortex that signals through DYN/KOR. Here, we explored the role of insula DYN/KOR circuit components on alcohol intake in a long-term intermittent access (IA) procedure. Using a combination of conditional knock-out strategies and site-directed pharmacology, we discovered distinct and sex-specific roles for insula DYN and KOR in alcohol drinking and related behavior. Our findings show that insula DYN deletion blocked escalated consumption and decreased the overall intake of and preference for alcohol in male and female mice. This effect was specific to alcohol in male mice, as DYN deletion did not impact sucrose intake. Further, insula KOR antagonism reduced alcohol intake and preference during the early phase of IA in male mice only. Alcohol consumption was not affected by insula KOR knockout in either sex. In addition, we found that long-term IA decreased the intrinsic excitability of DYN and deep layer pyramidal neurons (DLPNs) in the insula of male mice. Excitatory synaptic transmission was also impacted by IA, as it drove an increase in excitatory synaptic drive in both DYN neurons and DLPNs. Combined, our findings suggest there is a dynamic interplay between excessive alcohol consumption and insula DYN/KOR microcircuitry.SIGNIFICANCE STATEMENT The insular cortex is a complex region that serves as an integratory hub for sensory inputs. In our previous work, we identified a microcircuit in the insula that signals through the kappa opioid receptor (KOR) and its endogenous ligand dynorphin (DYN). Both the insula and DYN/KOR systems have been implicated in excessive alcohol use and alcohol use disorder (AUD). Here, we use converging approaches to determine how insula DYN/KOR microcircuit components contribute to escalated alcohol consumption. Our findings show that insula DYN/KOR systems regulate distinct phases of alcohol consumption in a sex-specific manner, which may contribute to the progression to AUD.

Dopamine D2 receptors in the bed nucleus of the stria terminalis modulate alcohol-related behaviors

Dysregulation of the dopamine (DA) system is a hallmark of substance abuse disorders, including alcohol use disorder (AUD). Of the DA receptor subtypes, the DA D2 receptors (D2Rs) play a key role in the reinforcing effects of alcohol. D2Rs are expressed in numerous brain regions associated with the regulation of appetitive behaviors. One such region is the bed nucleus of the stria terminalis (BNST), which has been linked to the development and maintenance of AUD. Recently, we identified alcohol withdrawal-related neuroadaptations in the periaqueductal gray/dorsal raphe to BNST DA circuit in male mice. However, the role of D2R-expressing BNST neurons in voluntary alcohol consumption is not well characterized. In this study, we used a CRISPR-Cas9-based viral approach, to selectively reduce the expression of D2Rs in BNST VGAT neurons and interrogated the impact of BNST D2Rs in alcohol-related behaviors. In male mice, reduced D2R expression potentiated the stimulatory effects of alcohol and increased voluntary consumption of 20% w/v alcohol in a two-bottle choice intermittent access paradigm. This effect was not specific to alcohol, as D2R deletion also increased sucrose intake in male mice. Interestingly, cell-specific deletion of BNST D2Rs in female mice did not alter alcohol-related behaviors but lowered the threshold for mechanical pain sensitivity. Collectively, our findings suggest a role for postsynaptic BNST D2Rs in the modulation of sex-specific behavioral responses to alcohol and sucrose.

Sexually dimorphic role for insular perineuronal nets in aversion-resistant alcohol consumption

Compulsive alcohol drinking is a key symptom of alcohol use disorder (AUD) that is particularly resistant to treatment. An understanding of the biological factors that underly compulsive drinking will allow for the development of new therapeutic targets for AUD. One animal model of compulsive alcohol drinking involves the addition of bitter-tasting quinine to an ethanol solution and measuring the willingness of the animal to consume ethanol despite the aversive taste. Previous studies have demonstrated that this type of aversion-resistant drinking is modulated in the insular cortex of male mice by specialized condensed extracellular matrix known as perineuronal nets (PNNs), which form a lattice-like structure around parvalbumin-expressing neurons in the cortex. Several laboratories have shown that female mice exhibit higher levels of aversion-resistant ethanol intake, but the role of PNNs in females in this behavior has not been examined. Here we compared PNNs in the insula of male and female mice and determined if disrupting PNNs in female mice would alter aversion-resistant ethanol intake. PNNs were visualized in the insula by fluorescent labeling with Wisteria floribunda agglutinin (WFA) and disrupted in the insula by microinjecting chondroitinase ABC, an enzyme that digests the chondroitin sulfate glycosaminoglycan component of PNNs. Mice were tested for aversion-resistant ethanol consumption by the addition of sequentially increasing concentrations of quinine to the ethanol in a two-bottle choice drinking in the dark procedure. PNN staining intensity was higher in the insula of female compared to male mice, suggesting that PNNs in females might contribute to elevated aversion-resistant drinking. However, disruption of PNNs had limited effect on aversion-resistant drinking in females. In addition, activation of the insula during aversion-resistant drinking, as measured by c-fos immunohistochemistry, was lower in female mice than in males. Taken together, these results suggest that neural mechanisms underlying aversion-resistant ethanol consumption differ in males and females.

Chronic Ethanol Exposure Modulates Periaqueductal Gray to Extended Amygdala Dopamine Circuit

The bed nucleus of the stria terminalis (BNST) is a component of the extended amygdala that regulates motivated behavior and affective states and plays an integral role in the development of alcohol-use disorder (AUD). The dorsal subdivision of the BNST (dBNST) receives dense dopaminergic input from the ventrolateral periaqueductal gray (vlPAG)/dorsal raphe (DR). To date, no studies have examined the effects of chronic alcohol on this circuit. Here, we used chronic intermittent ethanol exposure (CIE), a well-established rodent model of AUD, to functionally interrogate the vlPAG/DR-BNST dopamine (DA) circuit during acute withdrawal. We selectively targeted vlPAG/DR^DA neurons in tyrosine hydroxylase-expressing transgenic adult male mice. Using ex vivo electrophysiology, we found hyperexcitability of vlPAG/DR^DA neurons in CIE-treated mice. Further, using optogenetic approaches to target vlPAG/DR^DA terminals in the dBNST, we revealed a CIE-mediated shift in the vlPAG/DR-driven excitatory-inhibitory (E/I) ratio to a hyperexcitable state in dBNST. Additionally, to quantify the effect of CIE on endogenous DA signaling, we coupled optogenetics with fast-scan cyclic voltammetry to measure pathway-specific DA release in dBNST. CIE-treated mice had significantly reduced signal half-life, suggestive of faster clearance of DA signaling. CIE treatment also altered the ratio of vlPAG/DR^DA-driven cellular inhibition and excitation of a subset of dBNST neurons. Overall, our findings suggest a dysregulation of vlPAG/DR to BNST dopamine circuit, which may contribute to pathophysiological phenotypes associated with AUD.SIGNIFICANCE STATEMENT The dorsal bed nucleus of the stria terminalis (dBNST) is highly implicated in the pathophysiology of alcohol-use disorder and receives dopaminergic inputs from ventrolateral periaqueductal gray/dorsal raphe regions (vlPAG/DR). The present study highlights the plasticity within the vlPAG/DR to dBNST dopamine (DA) circuit during acute withdrawal from chronic ethanol exposure. More specifically, our data reveal that chronic ethanol strengthens vlPAG/DR-dBNST glutamatergic transmission while altering both DA transmission and dopamine-mediated cellular inhibition of dBNST neurons. The net result is a shift toward a hyperexcitable state in dBNST activity. Together, our findings suggest chronic ethanol may promote withdrawal-related plasticity by dysregulating the vlPAG/DR-dBNST DA circuit.

Sexually Dimorphic Role for Insular Perineuronal Nets in Aversion-Resistant Ethanol Consumption

Compulsive alcohol drinking is a key symptom of alcohol use disorder (AUD) that is particularly resistant to treatment. An understanding of the biological factors that underly compulsive drinking will allow for the development of new therapeutic targets for AUD. One animal model of compulsive alcohol drinking involves the addition of bitter-tasting quinine to an ethanol solution and measuring the willingness of the animal to consume ethanol despite the aversive taste. Previous studies have demonstrated that this type of aversion-resistant drinking is modulated in the insular cortex of male mice by specialized condensed extracellular matrix known as perineuronal nets (PNNs), which form a lattice-like structure around parvalbumin-expressing neurons in the cortex. Several laboratories have shown that female mice exhibit higher levels of aversion-resistant ethanol intake but the role of PNNs in females in this behavior has not been examined. Here we compared PNNs in the insula of male and female mice and determined if disrupting PNNs in female mice would alter aversion-resistant ethanol intake. PNNs were visualized in the insula by fluorescent labeling with Wisteria floribunda agglutinin (WFA) and disrupted in the insula by microinjecting chondroitinase ABC, an enzyme that digests the chondroitin sulfate glycosaminoglycan component of PNNs. Mice were tested for aversion-resistant ethanol consumption by the addition of sequentially increasing concentrations of quinine to the ethanol in a two-bottle choice drinking in the dark procedure. PNN staining intensity was higher in the insula of female compared to male mice, suggesting that PNNs in females might contribute to elevated aversion-resistant drinking. However, disruption of PNNs had limited effect on aversion-resistant drinking in females. In addition, activation of the insula during aversion-resistant drinking, as measured by c-fos immunohistochemistry, was lower in female mice than in males. Taken together, these results suggest that neural mechanisms underlying aversion-resistant ethanol consumption differ in males and females.

Better living through understanding the insula: Why subregions can make all the difference

Insula function is considered critical for many motivated behaviors, with proposed functions ranging from attention, behavioral control, emotional regulation, goal-directed and aversion-resistant responding. Further, the insula is implicated in many neuropsychiatric conditions including substance abuse. More recently, multiple insula subregions have been distinguished based on anatomy, connectivity, and functional contributions. Generally, posterior insula is thought to encode more somatosensory inputs, which integrate with limbic/emotional information in middle insula, that in turn integrate with cognitive processes in anterior insula. Together, these regions provide rapid interoceptive information about the current or predicted situation, facilitating autonomic recruitment and quick, flexible action. Here, we seek to create a robust foundation from which to understand potential subregion differences, and provide direction for future studies. We address subregion differences across humans and rodents, so that the latter's mechanistic interventions can best mesh with clinical relevance of human conditions. We first consider the insula's suggested roles in humans, then compare subregional studies, and finally describe rodent work. One primary goal is to encourage precision in describing insula subregions, since imprecision (e.g. including both posterior and anterior studies when describing insula work) does a disservice to a larger understanding of insula contributions. Additionally, we note that specific task details can greatly impact recruitment of various subregions, requiring care and nuance in design and interpretation of studies. Nonetheless, the central ethological importance of the insula makes continued research to uncover mechanistic, mood, and behavioral contributions of paramount importance and interest. This article is part of the special Issue on 'Neurocircuitry Modulating Drug and Alcohol Abuse'.