Sex differences in affective states and association with voluntary ethanol intake in Sprague-Dawley rats

Ceftriaxone alters the gut microbiome composition and reduces alcohol intake in male and female Sprague-Dawley rats

Ceftriaxone is an antibiotic that increases central nervous system (CNS) protein expression of the glutamate transporters GLT-1 and xCT and ameliorates pathological behaviors in rodent models of neurological disease and substance use disorder. However, little ceftriaxone passes through the blood-brain-barrier, the CNS binding partner of ceftriaxone is unknown, and ceftriaxone does not consistently upregulate GLT-1 and xCT in cell culture. Ceftriaxone alters the gut microbiome composition in rodents and humans, and the microbiome-gut-brain axis regulates drug-seeking. Thus, here we test the hypothesis that ceftriaxone reduces alcohol intake while ameliorating alcohol-induced disruption of the gut microbiome composition. Male and female Sprague-Dawley rats received intermittent access to alcohol (IAA) while controls received access to only water. Following 17 IAA sessions, ceftriaxone/vehicle treatment was given for 5 days. Analysis of the gut microbiome composition was assessed by 16S rRNA gene amplicon sequencing conducted on fecal pellets collected prior to and after alcohol consumption and following ceftriaxone treatment. Male rats displayed escalated alcohol intake and preference over the course of the 17 sessions; however, total alcohol intake did not differ between the sexes. Ceftriaxone reduced alcohol intake and preference in male and female rats. While alcohol affected a diverse set of amplicon sequencing variants (ASV), ceftriaxone markedly reduced the diversity of microbial communities reflected by a blooming of the Enterococcaceae family. The remaining effects of ceftriaxone, however, encompassed families both affected and unaffected by prior alcohol drinking and highlight the Ruminococcaceae and Muribaculaceae families as bidirectionally modulated by alcohol and ceftriaxone. Altogether, our study confirms that ceftriaxone reduces alcohol intake in rats and partially reverses alcohol-induced dysbiosis.

Divergent Population-Specific Effects of Chronic Ethanol Exposure on Excitability and Inhibitory Transmission in Male and Female Rat Central Amygdala

The central nucleus of the amygdala (CeA) is implicated in alcohol use disorder (AUD) and AUD-associated plasticity. The CeA is a primarily GABAergic nucleus that is subdivided into lateral and medial compartments with genetically diverse subpopulations. GABAA receptors are heteromeric pentamers with subunits conferring distinct physiological characteristics. GABAA receptor signaling in the CeA has been implicated in ethanol-associated plasticity; however, population-specific changes in inhibitory signaling and subunit expression remain unclear. Here, we combined electrophysiology with single-cell gene expression analysis of population markers and GABAA receptor subunits to examine population-specific changes in inhibitory control in male and female rats following chronic ethanol exposure. We found that chronic ethanol exposure and withdrawal produced global changes in GABAA receptor subunit expression at the transcript and protein levels, increased excitability in female CeA neurons, and increased inhibitory synaptic transmission in male CeA neurons. When we examined CeA neurons at the single-cell level we found heterogenous populations, as previously reported. We observed ethanol-induced increases in excitability only in somatostatin neurons in the CeA of females, decreases in excitability only in the protein kinase C delta (PKCd) population in males, and ethanol-induced increases in inhibitory transmission in male PKCd and calbindin 2-expressing CeA neurons. There were no population-specific differences in GABAA receptor (Gabr) subunits in males but reduced GabrA5 expression in female somatostatin neurons. Collectively, these findings suggest that defined CeA populations display differential ethanol sensitivity in males and females, which may play a role in sex differences in vulnerability to AUD or expression of AUD pathology.SIGNIFICANCE STATEMENT The CeA is involved in the effects of ethanol in the brain; however, the population-specific changes in CeA activity remain unclear. We used recordings of CeA neuronal activity and single-cell gene expression to examine population-specific changes in inhibitory control in male and female rats following chronic ethanol exposure and found sex- and population-specific effects of chronic ethanol exposure and withdrawal. Specifically, female CeA neurons displayed increased excitability in the somatostatin CeA population, whereas male CeA neurons displayed increased inhibitory control in both PKCd and calbindin populations and decreased excitability in the PKCd population. These findings identify CeA populations that display differential sensitivity to ethanol exposure, which may contribute to sex differences in vulnerability to alcohol use disorder.

Genetics and epigenetics: paternal adolescent ethanol consumption in serotonin transporter knock-out rats and offspring sensitivity to ethanol

RATIONALE

Alcohol use disorder (AUD) is shown to have an overall heritability of around 50%. One of the genes associated with AUD is SLC6A4 (solute carrier family 6 member A4) which codes for the serotonin transporter (SERT). The study looked at serotonin dysfunction on ethanol consumption in adolescents and the subsequent intergenerational effects of drinking by using a rat model: SERT^+/+ (regular functioning), SERT^+/- (50% transporter reduction) and SERT^-/- (complete reduction).

OBJECTIVES

We investigated sex and genotype differences in ethanol consumption in SERT knock-out Wistar rats (F0) followed by studying behaviour in the offspring (F1) of the male drinkers to assess effects of paternal alcohol consumption.

METHODS

An intermittent access two-bottle choice paradigm (IA2BC) was used to yield ethanol drinking behaviour in F0 adolescent Wistar rats. The highest drinking males were mated to alcohol-naive females and their offspring were compared with controls. Drinking behaviour (IA2BC) and ethanol-induced motor coordination effects (via rotarod) were measured in the F1s.

RESULTS

F0 drinking saw no SERT genotype differences in males. However, females consumed higher volumes of ethanol compared to males, with SERT^-/- females showing the highest intake. A clearer genotype effect was seen in the F1 animals, with reduction in SERT activity leading to enhanced ethanol intake in both sexes. Importantly, paternal exposure to ethanol significantly reduced the ethanol induced motor side effects in offspring, independent of sex and genotype.

CONCLUSIONS

These indicate a difference in the way genetic factors may act across sexes and suggest the involvement of epigenetic mechanisms in the intergenerational effects of alcohol.

A critical review of front-loading: A maladaptive drinking pattern driven by alcohol's rewarding effects

Front-loading is a drinking pattern in which alcohol intake is skewed toward the onset of reward access. This phenomenon has been reported across several different alcohol self-administration protocols in a wide variety of species, including humans. The hypothesis of the current review is that front-loading emerges in response to the rewarding effects of alcohol and can be used to measure the motivation to consume alcohol. Alternative or additional hypotheses that we consider and contrast with the main hypothesis are that: (1) front-loading is directed at overcoming behavioral and/or metabolic tolerance and (2) front-loading is driven by negative reinforcement. Evidence for each of these explanations is reviewed. We also consider how front-loading has been evaluated statistically in previous research and make recommendations for defining this intake pattern in future studies. Because front-loading may predict long-term maladaptive alcohol drinking patterns leading to the development of alcohol use disorder (AUD), several future directions are proposed to elucidate the relationship between front-loading and AUD.

Sex-related differences in pattern of ethanol drinking under the intermittent-access model and its impact on exploratory and anxiety-like behavior in Long-Evans rats

BACKGROUND

While men in the United States consume more alcohol than women, rates of drinking are converging. Nevertheless, females remain underrepresented in preclinical alcohol research. Here, we examined rats' sex-related differences in patterns of ethanol (EtOH) drinking and the effects of this drinking on exploratory and anxiety-like behavior.

METHODS

Adult male and female Long-Evans rats were given 20% ethanol under the intermittent-access two-bottle-choice paradigm. Their intake was measured daily for the first 7 weeks. During the eighth week, intake was measured over the 24 h of daily access. During the ninth week, they, along with EtOH-naive controls, were tested prior to daily access in a novel chamber, light-dark box, and hole board apparatus. During the tenth week, blood ethanol concentration (BEC) was assessed after 30 to 40 min of access.

RESULTS

Females overall demonstrated higher ethanol intake and preference across all access weeks than males, although only half of females drank significantly more than males. Across 24 h of daily access, both sexes had their highest intake in the first 30 min and their lowest in the middle of the light phase of the light/dark cycle. Despite their greater ethanol intake, females did not show significantly different BECs than males. In behavioral tests, females showed less vertical time in a novel activity chamber, more movement between chambers in a light-dark box, and more nose pokes in a hole-board apparatus than males. While a history of ethanol drinking led to a trend for lower vertical time in the activity chamber and greater chamber entries in the light-dark box, the effects were not sex-dependent.

CONCLUSIONS

These results suggest that female and male rats could both be tested for acute effects of ethanol after 30 min of daily access, but that nuanced considerations are needed in the design of these experiments and the interpretation of their findings.