Individual differences in voluntary alcohol consumption are associated with conditioned fear in the fear incubation model

Impairments in expression of devaluation in a Pavlovian goal-tracking task, but not a free operant devaluation task, after fentanyl exposure in female rats

In laboratory animals, there are numerous demonstrations that past exposure to drugs of abuse can lead to devaluation impairments weeks after the final drug exposure, with the majority of these demonstrations examining effects of exposure to psychostimulants. There has been minimal investigation into whether prior exposure to opiates can lead to devaluation impairments. Here, we first trained female rats that two separate cuelights predicted two different foods and measured Pavlovian goal-tracking responses (Experiment 1) or trained female rats to press two levers to earn two different foods and measured this operant response (Experiment 2). In both experiments, we subsequently gave the rats injections of fentanyl twice daily for 6 days, and then tested rats for conditioned responses after satiation on one of the foods 48-h after the final injection. We found that rats were impaired in the expression of devaluation in the Pavlovian task after fentanyl exposure, but were unimpaired in the expression of devaluation in the operant task. The pattern of results is most consistent with an impairment in lateral orbitofrontal cortex function, but additional research is needed to determine the neurobiological cause of this pattern of results.

Cannabidiol tempers alcohol intake and neuroendocrine and behavioural correlates in alcohol binge drinking adolescent rats. Focus on calcitonin gene-related peptide's brain levels

Alcohol binge drinking is common among adolescents and may challenge the signalling systems that process affective stimuli, including calcitonin gene-related peptide (CGRP) signalling. Here, we employed a rat model of adolescent binge drinking to evaluate reward-, social- and aversion-related behaviour, glucocorticoid output and CGRP levels in affect-related brain regions. As a potential rescue, the effect of the phytocannabinoid cannabidiol was explored. Adolescent male rats underwent the intermittent 20% alcohol two-bottle choice paradigm; at the binge day (BD) and the 24 h withdrawal day (WD), we assessed CGRP expression in medial prefrontal cortex (mPFC), nucleus accumbens (NAc), amygdala, hypothalamus and brainstem; in addition, we evaluated sucrose preference, social motivation and drive, nociceptive response, and serum corticosterone levels. Cannabidiol (40 mg/kg, i.p.) was administered before each drinking session, and its effect was measured on the above-mentioned readouts. At BD and WD, rats displayed decreased CGRP expression in mPFC, NAc and amygdala; increased CGRP levels in the brainstem; increased response to rewarding- and nociceptive stimuli and decreased social drive; reduced serum corticosterone levels. Cannabidiol reduced alcohol consumption and preference; normalised the abnormal corticolimbic CGRP expression, and the reward and aversion-related hyper-responsivity, as well as glucocorticoid levels in alcohol binge-like drinking rats. Overall, CGRP can represent both a mediator and a target of alcohol binge-like drinking and provides a further piece in the intricate puzzle of alcohol-induced behavioural and neuroendocrine sequelae. CBD shows promising effects in limiting adolescent alcohol binge drinking and rebalancing the bio-behavioural abnormalities.

Voluntary alcohol consumption during distinct phases of adolescence differentially alters adult fear acquisition, extinction and renewal in male and female rats

Alcohol use during adolescence coincides with elevated risks of stress-related impairment in adults, particularly via disrupted developmental trajectories of vulnerable corticolimbic and mesolimbic systems involved in fear processing. Prior work has investigated the impact of binge-like alcohol consumption on adult fear and stress, but less is known about whether voluntarily consumed alcohol imparts differential effects based on adolescence phases and biological sex. Here, adolescent male and female Long Evans rats were granted daily access to alcohol (15%) during either early (Early-EtOH; P25-45) or late adolescence (Late-EtOH; P45-55) using a modified drinking-in-the-dark design. Upon adulthood (P75-80), rats were exposed to a three-context (ABC) fear renewal procedure. We found that male and female Early-EtOH rats showed faster acquisition of fear but less freezing during early phases of extinction and throughout fear renewal. In the extinction period specifically, Early-EtOH rats showed normal levels of freezing in the presence of fear-associated cues, but abnormally low freezing immediately after cue offset, suggesting a key disruption in contextual processing and/or novelty seeking brought by early adolescent binge consumption. While the effects of alcohol were most pronounced in the Early-EtOH rats (particularly in females), Late-EtOH rats displayed some changes in fear behavior including slower fear acquisition, faster extinction, and reduced renewal compared with controls, but primarily in males. Our results suggest that early adolescence in males and females and, to a lesser extent, late adolescence in males is a particularly vulnerable period wherein alcohol use can promote stress-related dysfunction in adulthood. Furthermore, our results provide multiple bases for future research focused on developmental correlates of alcohol mediated disruption in the brain.

Voluntary alcohol consumption during distinct phases of adolescence differentially alters adult fear acquisition, extinction and renewal in male and female rats

Alcohol use during adolescence coincides with elevated risks of stress-related impairment in adults, particularly via disrupted developmental trajectories of vulnerable corticolimbic and mesolimbic systems involved in fear processing. Prior work has investigated the impact of binge-like alcohol consumption on adult fear and stress, but less is known about whether voluntarily consumed alcohol imparts differential effects based on adolescence phases and biological sex. Here, adolescent male and female Long Evans rats were granted daily access to alcohol (15%) during either early (Early-EtOH; P25-45) or late adolescence (Late-EtOH; P45-55) using a modified drinking-in-the-dark design. Upon adulthood (P75-80), rats were exposed to a three-context (ABC) fear renewal procedure. We found that male and female Early-EtOH rats showed faster acquisition of fear but less freezing during early phases of extinction and throughout fear renewal. In the extinction period specifically, Early-EtOH rats showed normal levels of freezing in the presence of fear-associated cues, but abnormally low freezing immediately after cue offset, suggesting a key disruption in contextual processing and/or novelty seeking brought by early adolescent binge consumption. While the effects of alcohol were most pronounced in the Early-EtOH rats (particularly in females), Late-EtOH rats displayed some changes in fear behavior including slower fear acquisition, faster extinction, and reduced renewal compared with controls, but primarily in males. Our results suggest that early adolescence in males and females and, to a lesser extent, late adolescence in males is a particularly vulnerable period wherein alcohol use can promote stress-related dysfunction in adulthood. Furthermore, our results provide multiple bases for future research focused on developmental correlates of alcohol mediated disruption in the brain.

Extended operant training increases infralimbic and prelimbic cortex Fos regardless of fear conditioning experience

Extended fear training can lead to initially low fear expression that grows over time, termed fear incubation. Conversely, a single fear conditioning session typically results in high fear initially that is sustained over time. Fear expression decreases across extended training, suggesting that a fear extinction-like process might be responsible for low fear observed soon after training. Because of the prominent role medial prefrontal cortex (mPFC) plays in fear conditioning and extinction, we decided to examine Fos expression resulting from a cued fear retrieval test to gain insight into possible mechanisms involved in extended training fear incubation. Male Long-Evans rats received 1 or 10 days of tone-shock pairings or tone-only exposure (while lever-pressing for food). Two days after the end of fear training, rats received a cued fear test, with perfusions timed to visualize Fos expression during test. As expected, the limited fear conditioning group exhibited higher fear in the test than any of the other groups (as measured with conditioned suppression of lever-pressing). Interestingly, we found that extended training animals (whether they received tone-shock pairings or tone-only exposure) expressed higher levels of Fos in both prelimbic and infralimbic cortices than limited training animals. There was no association between fear expression and mPFC Fos expression. These results suggest we may have visualized Fos expression related to operant overtraining rather than conditioned fear related processes. Further research is needed to determine the neurobiological basis of extended training fear incubation and to determine processes represented by the pattern of Fos expression we observed.

Alcohol Consumption during Adulthood Does Not Impair Later Go/No-Go Reversal Learning in Male Rats

Reversal learning tasks are used to model flexible decision-making in laboratory animals, and exposure to drugs of abuse can cause long-term impairments in reversal learning. However, the long-term effects of alcohol on reversal learning have varied. We evaluated whether six weeks of voluntary alcohol consumption through chronic intermittent alcohol access (elevated by food restriction) in adult male rats would impair rats in a go/no-go reversal learning task when tested at an interval beyond acute withdrawal. In our go/no-go task, rats were reinforced for pressing one lever or withholding from pressing another lever, and the identities of the two levers were switched twice (once rats reached an accuracy criterion). We found no evidence that prior alcohol consumption altered discrimination or reversal learning in our task. This replicates previous patterns from our laboratory that higher alcohol consumption in food-restricted rats did not impair discrimination or reversal learning in a different go/no-go task and that alcohol consumption in free-fed adolescent/early adult rats did not impair go/no-go discrimination or reversal learning in the same task. It is unclear whether this represents an insensitivity of this task to alcohol exposure generally or whether an alcohol exposure procedure that leads to higher blood ethanol concentration (BEC) levels would impair learning. More research is needed to investigate these possibilities.

Pre-training naltrexone increases conditioned fear learning independent of adolescent alcohol consumption history

Our previous research has shown a relationship between low voluntary alcohol consumption and high conditioned fear in male Long Evans rats. Here, we examined whether differences in the endogenous opioid systems might be responsible for these differences. Rats received 6 weeks of chronic intermittent to 20% alcohol (v/v) or water-only from PND 26-66. Based on their consumption during the last 2 weeks of alcohol access, the alcohol-access rats were divided into high drinking (>2.5 g/kg/24-h) or low drinking (<2 g/kg/24-h). Rats were then given injections of the preferential mu opioid receptor antagonist naltrexone (1 mg/kg, s.c.) or the selective kappa opioid receptor antagonist LY2456302 (10 mg/kg, s.c.) prior to fear conditioning and were then tested for conditioned fear 2 days later. Pre-training naltrexone increased conditioned suppression of lever-pressing during training and testing, with no differences between high versus low alcohol drinkers or between water-only versus alcohol access groups (averaged across drinking levels). There was no effect of LY2456302 on conditioned fear in any comparison. We also found no differences between high and low alcohol drinkers and no reliable effect of prior alcohol access (averaged across drinking levels) on conditioned fear. Our experiment replicates and extends previous demonstrations that a preferential MOR antagonist can increase fear learning using conditioned suppression of lever-pressing as a fear measure. However, additional research is needed to determine the cause of the differences in conditioned fear that we previously observed (as they were not observed in the current experiments).

Dose-dependent effects of alcohol injections on omission-contingency learning have an inverted-U pattern

Previous examinations of the long-term effects of alcohol exposure on omission-contingency learning have produced mixed results across different age or sex groups, with evidence for faster learning or no effect. However, none of these experiments made comparisons using the same exposure-dose across the age/sex groups. Here, we exposed rats to 6 weeks of alcohol injections (3 days/week, 1.75 or 3.5 g/kg/24-h, i.p. broken up into 2 injections/day) in adolescent/early adult males or females (PND27-66) or adult males (PND62-101). We then tested the rats in autoshaping and omission-contingency tasks. In contrast to our hypotheses, the low 1.75-g/kg/24-h dose led to slower omission learning and the higher 3.5-g/kg/24-h dose had no effect. There were no age- or sex-differences in omission learning. Additionally, during autoshaping training, rats exposed in adolescence/early adulthood had a faster shift to sign-tracking in their sign-tracking/goal-tracking ratios than rats exposed in adulthood, with no consistent effect of alcohol exposure or sex-differences. Our results suggest complex effects of alcohol on the neural substrates of omission-contingency learning at different doses, which will require future investigation.

Incubation of food craving in rats: A review

Incubation of food craving is an abstinence-dependent increase in responding for reward-paired cues. Incubation of craving was first reported for rats responding for cocaine-paired cues, and later generalized to several drugs of abuse and for food. Incubation of drug and food craving has been reported in clinical studies as well. Incubation of food craving by rats has been reported for standard chow as well as for high fat and sucrose reinforcers. Parametric and other evaluations of the incubation of food craving reveal manipulations that reduce incubation, including environmental enrichment and pharmacological manipulation of dopamine, glutamate, and endogenous opiates. Several brain regions are likely involved in the effect, including mesolimbic terminals and the central nucleus of the amygdala. Further study of the incubation of food craving could facilitate development of treatments for cravings that precede relapse characteristic of drug and food addictions.

Voluntary alcohol access during adolescence/early adulthood, but not during adulthood, causes faster omission contingency learning

In omission contingency training, rodents learn to suppress their natural tendency to approach or touch a reward-predictive cue (termed "autoshaping" or "sign-tracking" responses) if the approach/touching responses lead to the omission of the reward. Previous research has shown that high levels of alcohol exposure (through alcohol vapor exposure) or adolescent alcohol consumption (with some versions of the omission contingency task) can lead to faster omission contingency learning. However, the alcohol exposure procedures and/or omission contingency task parameters differed between these different demonstrations. It was unclear whether the same voluntary alcohol consumption procedures during adolescence/early adulthood and/or adulthood would lead to faster omission contingency learning in one or both age groups. Here, rats received 6 weeks of chronic intermittent access to 20% alcohol or water from PND 26-66 (adolescence/early adulthood in Exp. 1) or PND 68-108 (adulthood in Exp. 2) and began behavioral training (autoshaping training followed by omission contingency training) several days later. We found no evidence that alcohol access at either age altered the number of trials with a sign-tracking response on the levers during autoshaping training. However, alcohol access during adolescence/early adulthood, but not during adulthood, led to faster learning to withhold responding on the lever under omission contingencies during the subsequent phase. We also found no evidence that the level of alcohol consumption was correlated with sign-tracking behavior in the autoshaping phase or with the suppression of lever-pressing during the omission contingency phase. Our results suggest that adolescent/early adult rats have increased vulnerability, compared with adults, to some long-term behavioral effects of voluntary alcohol consumption.