β-Endorphin and sex differentially modulate the response to EtOH in a site-specific manner

Sensitivity to the initial rewarding effects of alcohol: Influence of age, sex, and β-endorphin

BACKGROUND

Alcohol use disorders (AUDs) are widespread, devastating and complex. About 20% of people who consume alcohol develop problem use, accounting for over 5% of worldwide deaths. While numerous animal models have facilitated understanding of the consequences of excessive drinking, translational models allow for experimental manipulation of factors thought to contribute to AUD liability.

METHODS

We employ a single-exposure conditioned place preference assay (SE-CPP) to investigate the influence of age, sex and the opioid peptide β-endorphin (bE) on the initial rewarding effects of ethanol, a strong predictor of AUDs. Adolescent (PND28-35) and adult (PND70-90) male and female, control C57BL/6J and bE-deficient mice were tested following a single injection of 1.5 g/kg of ethanol. Following the SE-CPP test, animals were deeply anesthetized, sacrificed, and perfused, and the brains were subsequently sectioned at 40 microns and processed for immunohistochemical localization of c-fos. One-sample, two-tailed t-tests were used to assess drug preference or aversion and the locomotor effects of alcohol.

RESULTS

In general, adults were more sensitive to the effects of alcohol than adolescents, and outcomes depended on sex and bE. For example, among females, adolescents were stimulated by the drug, but insensitive to locomotor effects as adults, while among males, adolescents were insensitive and adults sedated. Wild-type adolescents of both sexes failed to evince initial subjective reward from the drug, but bE-deficient adolescents, and all adult subjects, preferred a context once associated with ethanol over one that had been paired with saline. c-fos immunoreactivity in multiple brain regions was attenuated in bE-deficient animals, though influences of both sex and bE grew stronger with age.

CONCLUSIONS

This study demonstrates the utility of the SE-CPP paradigm for elucidating factors that contribute to the liability for AUDs, and supports the growing body of research that shows that sensitivity to the rewarding effects of alcohol changes during the course of development. Our results also suggest that developmental contributions are sex-dependent, and may also depend on the influence of endogenous opioid signaling.

β-Endorphin influences sedative and ataxic effects of alcohol

Beta-endorphin (β-E) is an opioid peptide linked to the behavioral effects of ethanol. For example, β-E provides negative feedback to inhibit the hypothalamic-pituitary-adrenal (HPA) stress axis, and neuroadaptation of this system to ethanol may facilitate sex differences in disordered drinking. Locomotor sensitivity to ethanol may also influence the risk for addiction; however, the role of β-E in psychomotor effects of ethanol is not fully understood. We examined the role of β-E and sex on locomotor effects of ethanol using adult male and female wild-type C57BL/6J and β-E deficient B6.129S2-Pomctm1Low/J mice in a parallel rod floor apparatus following 0.75 or 2.0 g/kg ethanol. Beginning 15 min after intraperitoneal injection, we recorded foot slips, distance traveled, slips per meter, first instance of immobility, and total time spent off-balance (lying on the floor) over 15 min, and collected blood for analysis of ethanol concentration 60 min after injection. Overall, β-E deficient mice were more sedated and ataxic following ethanol; at the lower dose they slipped more frequently and had a higher rate of slips per meter traveled. At the higher dose, β-E deficient mice were predominantly sedated, slipping less frequently, and traveling less, as well as spending more time off-balance and becoming immobile sooner. Genotype interacted with sex in that male β-E deficient mice slipped more frequently than their female counterparts, suggesting that β-E may elicit sex-dependent effects of ethanol-induced ataxia. Blood ethanol concentration did not differ between any group, suggesting that behavioral differences result from altered sensitivity to ethanol. Our data support the contention that β-E modulates the locomotor effects of ethanol and may influence ataxia in a sex-dependent manner. These findings help elucidate the role of β-E in diverging behavioral responses to ethanol and may aid the development of targeted treatments for alcohol use disorders.

Age, β-endorphin, and sex dependent effects of maternal separation on locomotor activity, anxiety-like behavior, and alcohol reward

Introduction

Childhood adversity is pervasive and linked to numerous disadvantages in adulthood, including physical health problems, mental illness, and substance use disorders. Initial sensitivity to the rewarding effects of alcohol predicts the risk of developing an alcohol use disorder, and may be linked to developmental stress. The opioid peptide β-endorphin (β-E) regulates the stress response and is also implicated in the risk for excessive alcohol consumption.

Methods

We explored the influence of β-E in an animal model of early life adversity using controlled maternal separation by evaluating changes in locomotor activity, anxiety-like behavior, and the initial rewarding effects of alcohol in a single exposure conditioned place preference paradigm in control C57BL/6J and β-E deficient β-E +/+ 0.129S2-Pomc tm1Low/J; β-E −/− mice. Maternal separation (MS) occurred for 3 h each day from post-natal days (PND) 5–18 in approximately half the subjects.

Results

Maternal interactions increased following the separation protocol equally in both genotypes. MS and control subjects were tested as adolescents (PND 26–32) or adults (PND 58–72); the effects of MS were generally more pronounced in older subjects. Adults were more active than adolescents in the open field, and MS decreased activity in adolescent mice but increased it in adults. The increase in adult activity as a result of early life stress depended on both β-E and sex. β-E also influenced the effect of maternal separation on anxiety-like behavior in the Elevated Plus Maze. MS promoted rewarding effects of alcohol in male β-E deficient mice of either age, but had no effect in other groups.

Discussion

Taken together, these results suggest that the effects of MS develop over time and are β-E and sex dependent and may aid understanding of how individual differences influence the impact of adverse childhood experiences.

Influence of early-life adversity on responses to acute and chronic ethanol in female mice

BACKGROUND

Stressful early-life experiences increase the risk of developing an alcohol use disorder. We previously found that male C57BL/6J mice reared under limited bedding and nesting (LBN) conditions, a model of early-life adversity, escalate their ethanol intake in limited-access two-bottle choice (2BC) sessions faster than control (CTL)-reared counterparts when exposed to chronic intermittent ethanol (CIE) vapor inhalation. However, the alcohol consumption of female littermates was not affected by LBN or CIE. In the present study, we sought to determine whether this phenotype reflected a general insensitivity of female mice to the influence of early-life stress on alcohol responses.

METHODS

In a first experiment, CTL and LBN females with a history of 2BC combined or not with CIE were tested in affective and nociceptive assays during withdrawal. In a second group of CTL and LBN females, we examined ethanol-induced antinociception, sedation, plasma clearance, and c-Fos induction.

RESULTS

In females withdrawn from chronic 2BC, CIE increased digging, reduced grooming, and increased immobility in the tail suspension test regardless of early-life history. In contrast, LBN rearing lowered mechanical nociceptive thresholds regardless of CIE exposure. In females acutely treated with ethanol, LBN rearing facilitated antinociception and delayed the onset of sedation without influencing ethanol clearance rate or c-Fos induction in the paraventricular nucleus of the hypothalamus, paraventricular nucleus of the thalamus, central nucleus of the amygdala, or auditory cortex.

CONCLUSION

CIE withdrawal produced multiple indices of negative affect in C57BL/6J females, suggesting that their motivation to consume alcohol may differ from air-exposed counterparts despite equivalent intake. Contrasted with our previous findings in males, LBN-induced mechanical hyperalgesia in chronic alcohol drinkers was specific to females. Lower nociceptive thresholds combined with increased sensitivity to the acute antinociceptive effect of ethanol may contribute to reinforcing ethanol consumption in LBN females but are not sufficient to increase their intake.

Ethanol-induced conditioned taste aversion and associated neural activation in male rats: Impact of age and adolescent intermittent ethanol exposure

Individuals that initiate alcohol use at younger ages and binge drink during adolescence are more susceptible to developing alcohol use disorder. Adolescents are relatively insensitive to the aversive effects of alcohol and tend to consume significantly more alcohol per occasion than adults, an effect that is conserved in rodent models. Adolescent typical insensitivity to the aversive effects of alcohol may promote greater alcohol intake by attenuating internal cues that curb its consumption. Attenuated sensitivity to the aversive effects of alcohol is also retained into adulthood following protracted abstinence from adolescent intermittent ethanol (AIE) exposure. Despite these effects, much remains unknown regarding the neural contributors. In the present study, we used a conditioned taste aversion (CTA) paradigm to investigate neuronal activation in late-developing forebrain structures of male adolescents and adult cFos-LacZ transgenic rats as well as in AIE adults following consumption of 0.9% sodium chloride previously paired with an intraperitoneal injection of 0, 1.5 or 2.5 g/kg of ethanol. Adults that were non-manipulated or received water exposure during adolescence showed CTA to both ethanol doses, whereas adolescents displayed CTA only to the 2.5 g/kg ethanol dose. Adults who experienced AIE did not show CTA. Adults displayed increased neuronal activation indexed via number of β-galactosidase positive (β-gal+) cells in the prefrontal and insular cortex that was absent in adolescents, whereas adolescents but not adults had a reduced number of β-gal+ cells in the central amygdala. Adults also displayed greater cortical-insular functional connectivity than adolescents as well as insular-amygdalar and prefrontal cortex-accumbens core functional connectivity. Like adolescents, adults previously exposed to AIE displayed reduced prefrontal-insular cortex and prefrontal-accumbal core functional connectivity. Taken together, these results suggest that attenuated sensitivity to the aversive effects of ethanol is related to a loss of an insular-prefrontal cortex-accumbens core circuit.