Subsecond fear discrimination in rats: adult impairment in adolescent heavy alcohol drinkers

A fear conditioned cue orchestrates a suite of behaviors in rats

Pavlovian fear conditioning has been extensively used to study the behavioral and neural basis of defensive systems. In a typical procedure, a cue is paired with foot shock, and subsequent cue presentation elicits freezing, a behavior theoretically linked to predator detection. Studies have since shown a fear conditioned cue can elicit locomotion, a behavior that - in addition to jumping, and rearing - is theoretically linked to imminent or occurring predation. A criticism of studies observing fear conditioned cue-elicited locomotion is that responding is non-associative. We gave rats Pavlovian fear discrimination over a baseline of reward seeking. TTL-triggered cameras captured 5 behavior frames/s around cue presentation. Experiment 1 examined the emergence of danger-specific behaviors over fear acquisition. Experiment 2 examined the expression of danger-specific behaviors in fear extinction. In total, we scored 112,000 frames for nine discrete behavior categories. Temporal ethograms show that during acquisition, a fear conditioned cue suppresses reward seeking and elicits freezing, but also elicits locomotion, jumping, and rearing - all of which are maximal when foot shock is imminent. During extinction, a fear conditioned cue most prominently suppresses reward seeking, and elicits locomotion that is timed to shock delivery. The independent expression of these behaviors in both experiments reveals a fear conditioned cue to orchestrate a temporally organized suite of behaviors.

The effects of voluntary adolescent alcohol consumption on alcohol taste reactivity in Long Evans rats

RATIONALE

The relationship between age, ethanol intake, and the hedonic value of ethanol is key to understanding the motivation to consume ethanol.

OBJECTIVE

It is uncertain whether ethanol drinking during adolescence changes ethanol's hedonic value into adulthood.

METHODS

The current study compared voluntary intermittent ethanol consumption (IAE; 2-bottle choice; 20%v/v) among adolescent and adult Long-Evans rats to examine the effects of age and IAE on taste reactivity in adulthood. For taste reactivity, orally infused fluids included water, ethanol (5, 20, and 40%v/v), and sucrose (0.01, 0.1, 1M).

RESULTS

IAE results indicate that adolescents drank more ethanol during IAE but had a lower rate of change in ethanol consumption across time than adults due to initially high adolescent drinking. During taste reactivity testing for ethanol, IAE rats had greater hedonic responding, less aversive responding, and a more positive relationship between hedonic responses and ethanol concentration than water-receiving control rats. Hedonic responses had positive, while aversive responses had negative relationships with ethanol concentration and total ethanol consumed during IAE. Adolescent+IAE rats displayed less hedonic and more aversive responses to ethanol than Adult+IAE rats. Sucrose responding was unrelated to ethanol consumption.

CONCLUSIONS

These results suggest that ethanol consumption influences the future hedonic and aversive value of ethanol in a way that makes ethanol more palatable with greater prior consumption. However, it appears that those drinking ethanol as adolescents may be more resistant to this palatability shift than those first drinking as adults, suggesting different mechanisms of vulnerability to consumption escalation for adolescents and adults.

Foot shock facilitates reward seeking in an experience-dependent manner

Animals organize reward seeking around aversive events. An abundance of research shows that foot shock, as well as a shock-associated cue, can elicit freezing and suppress reward seeking. Yet, there is evidence that experience can flip the effect of foot shock to facilitate reward seeking. Here we examined cue suppression, foot shock suppression and foot shock facilitation of reward seeking in a single behavioural setting. Male Long Evans rats received fear discrimination consisting of danger, uncertainty, and safety cues. Discrimination took place over a baseline of rewarded nose poking. With limited experience (1-2 sessions), all cues and foot shock suppressed reward seeking. With continued experience (10-16 sessions), suppression became specific to shock-associated cues, foot shock briefly suppressed, then facilitated reward seeking. Our results provide a means of assessing positive properties of foot shock, and may provide insight into maladaptive behaviour around aversive events.

Early adolescent adversity alters periaqueductal gray/dorsal raphe threat responding in adult female rats

Early adolescent adversity increases adult risk for anxiety disorders. The ventrolateral periaqueductal gray (vlPAG) and neighboring dorsal raphe (DR) are integral to threat prediction, and are responsive to acute stressors. Here, we tested the hypothesis that early adolescent adversity reshapes vlPAG/DR threat-related cue activity and threat probability signaling. Female, Long Evans rats experienced a battery of adverse adolescent experiences (n = 12), while controls did not (n = 8). Single-unit activity was recorded 50 + days following the final adverse experience, when the adult rats received fear discrimination consisting of danger, uncertainty and safety cues. Despite achieving fear discrimination that was equivalent to controls, vlPAG/DR threat responding was altered in adverse-experienced rats. Early adolescent adversity resulted in a greater proportion of cue-responsive neurons. Cue-excited neurons showed greater increases in firing and cue-inhibited neurons showed greater decreases. Even more, early adversity reduced flexible, threat probability signaling by cue-excited neurons and promoted more rigid, fear output signaling by cue-inhibited neurons. The results reveal long-lasting changes in vlPAG/DR threat responding resulting from early adolescent adversity.

Anterior Cingulate Cortex Signals Attention in a Social Paradigm that Manipulates Reward and Shock

The ability to recognize emotions in others and adapt one's behavior accordingly is critical for functioning in any social context. This ability is impaired in several psychiatric disorders, such as autism and psychopathy. Recent work has identified the anterior cingulate cortex (ACC) among other brain regions involved in this process. Neural recording studies have shown that neurons in ACC are modulated by reward or shock when delivered to a conspecific and when experienced first-hand. Because previous studies do not vary reward and shock within the same experiment, it has been unclear whether the observed activity reflects how much attention is being paid to outcomes delivered to a conspecific or the valence associated with those stimuli. To address this issue, we recorded from ACC as rats performed a Pavlovian task that predicted whether reward, shock, or nothing would be delivered to the rat being recorded from or a conspecific located in the opposite chamber. Consistent with previous reports, we found that the firing of ACC neurons was modulated by aversive stimuli delivered to the recording rat and their conspecific. Activity of some of these neurons genuinely reflected outcome identity (i.e., reward or shock); however, the population of neurons as a whole responded similarly for both reward and shock, as well as for cues that predicted their occurrence (i.e., reward > neutral and shock > neutral; attention). These results suggest that ACC can process information about outcomes (i.e., identity and recipient) in the service of promoting attention in some social contexts.

The Nucleus Accumbens Core is Necessary to Scale Fear to Degree of Threat

Fear is adaptive when the level of the response rapidly scales to degree of threat. Using a discrimination procedure consisting of danger, uncertainty, and safety cues, we have found rapid fear scaling (within 2 s of cue presentation) in male rats. Here, we examined a possible role for the nucleus accumbens core (NAcc) in the acquisition and expression of fear scaling. In experiment 1, male Long-Evans rats received bilateral sham or neurotoxic NAcc lesions, recovered, and underwent fear discrimination. NAcc-lesioned rats were generally impaired in scaling fear to degree of threat, and specifically impaired in rapid uncertainty-safety discrimination. In experiment 2, male Long-Evans rats received NAcc transduction with halorhodopsin (Halo) or a control fluorophore. After fear scaling was established, the NAcc was illuminated during cue or control periods. NAcc-Halo rats receiving cue illumination were specifically impaired in rapid uncertainty-safety discrimination. The results reveal a general role for the NAcc in scaling fear to degree of threat, and a specific role in rapid discrimination of uncertain threat and safety.SIGNIFICANCE STATEMENT Rapidly discriminating cues for threat and safety is essential for survival and impaired threat-safety discrimination is a hallmark of stress and anxiety disorders. In two experiments, we induced nucleus accumbens core (NAcc) dysfunction in rats receiving fear discrimination consisting of cues for danger, uncertainty, and safety. Permanent NAcc dysfunction, via neurotoxic lesion, generally disrupted the ability to scale fear to degree of threat, and specifically impaired one component of scaling: rapid discrimination of uncertain threat and safety. Reversible NAcc dysfunction, via optogenetic inhibition, specifically impaired rapid discrimination of uncertain threat and safety. The results reveal that the NAcc is essential to scale fear to degree of threat, and is a plausible source of dysfunction in stress and anxiety disorders.

Cue-inhibited ventrolateral periaqueductal gray neurons signal fear output and threat probability in male rats

The ventrolateral periaqueductal gray (vlPAG) is proposed to mediate fear responses to imminent danger. Previously we reported that vlPAG neurons showing short-latency increases in firing to a danger cue - the presumed neural substrate for fear output - signal threat probability in male rats (Wright et al., 2019). Here, we scrutinize the activity vlPAG neurons that decrease firing to danger. One cue-inhibited population flipped danger activity from early inhibition to late excitation: a poor neural substrate for fear output, but a better substrate for threat timing. A second population showed differential firing with greatest inhibition to danger, less to uncertainty and no inhibition to safety. The pattern of differential firing reflected the pattern of fear output, and was observed throughout cue presentation. The results reveal an expected vlPAG signal for fear output in an unexpected, cue-inhibited population.

The ventrolateral periaqueductal grey updates fear via positive prediction error

Aversive, positive prediction error (+PE) provides a mechanism to update and increase future fear to uncertain threat predictors. The ventrolateral periaqueductal grey (vlPAG) has been offered as a neural locus for +PE computation. Yet, a causal demonstration of vlPAG +PE activity to update fear remains elusive. We devised a fear discrimination procedure in which a danger cue predicts shock deterministically and an uncertainty cue predicts shock probabilistically, requiring prediction errors to achieve an appropriate fear response. Recording vlPAG single-unit activity during fear discrimination in Long-Evans rats, we reveal activity related to shock is consistent with +PE and updates subsequent fear to uncertainty at the trial level. We further demonstrate that vlPAG inhibition during shock selectively decreases future fear to uncertainty, but not danger, and temporal emergence of this effect is consistent with single-unit activity. These findings provide causal evidence that vlPAG +PE is necessary for fear updating.

Voluntary ethanol consumption during early social isolation and responding for ethanol in adulthood

Little is known about the influence of rearing environments concurrent with voluntary intermittent access to ethanol on subsequent adult ethanol-related behaviors. Previous research has shown that adult rats reared in post-weaning, social isolation conditions (IC) respond more for operant ethanol compared to laboratory standard conditions (SC). Ethanol-exposed adolescents tend to consume more ethanol in adulthood than rats exposed as adults. The current study examined voluntary ethanol consumption during adolescence between IC and SC rats, subsequent operant responding for ethanol, and extinction of responding in the same rats as adults. Differences in ethanol metabolism may alter the amount of reward value per unit of ethanol consumed. Therefore, the current study also examined blood ethanol concentrations (BEC) between IC rats and SC rats. Ethanol-naïve Long-Evans rats arrived in the lab at postnatal day (PND) 21 and were separated into either IC or SC where they remained for the duration of the experiments. On PND 27, rats received intermittent access to 20% ethanol (3 days/week) for 4 or 6 weeks. Rats in the 6-week cohort were then trained to lever press for 20% ethanol in 30-min sessions followed by extinction. A separate cohort was reared in IC or SC, injected with 1.5 or 3.0 g/kg of ethanol (intraperitoneally [i.p.]), followed by BEC measurement. Overall, IC rats had higher ethanol preference and consumption during adolescence/early adulthood. IC and SC rats did not differ in their rates of operant responding for ethanol, and SC rats responded more than IC rats during extinction. There were no differences in BEC between IC and SC rats. These findings highlight the importance of the environment during rat adolescent development with isolation conditions increasing binge-like drinking and ethanol preference after 3-4 weeks without differences in metabolism as a potential factor. Additionally, the findings indicate that intermittent adolescent access to ethanol may change typical differences in operant responding patterns between IC and SC rats in adulthood.

Chemogenetic stimulation of the infralimbic cortex reverses alcohol-induced fear memory overgeneralization

Post-traumatic stress disorder (PTSD) and alcohol use disorder (AUD) are often comorbid. Drinking tends to increase following trauma, which may exacerbate PTSD-related symptoms. Despite a clear relationship between excessive alcohol use and PTSD, how alcohol impacts the expression of traumatic fear remains unclear. This study aims to determine the neurobehavioral impact of chronic alcohol (ethanol; EtOH) on the expression of established fear memories in C57BL/6 N mice. We show that chronic EtOH selectively augments cued fear memory generalization and impairs fear extinction retrieval, leaving the expression of the original cued response intact. Immunohistochemistry for Arc/arg3.1 (Arc) revealed EtOH-induced decreases in Arc expression in the infralimbic cortex (IL) and basolateral amygdala complex (BLA) that were associated with cued fear memory overgeneralization. Chemogenetic stimulation of IL pyramidal neurons reversed EtOH-driven fear memory overgeneralization, identifying a role for the IL in cued fear memory precision. Considering the modulatory influence of the IL over conditioned fear expression, these data suggest a model whereby chronic EtOH-driven neuroadaptations in the IL promote fear memory overgeneralization. These findings provide new mechanistic insight into how excessive alcohol use, following exposure to a traumatic event, can exacerbate symptoms of traumatic fear.

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

Previous research in male Long Evans rats has shown a relationship between low voluntary alcohol consumption and high conditioned fear after a single training session. Here, we determined whether chronic intermittent access (CIA) to alcohol during adolescence/early adulthood or during adulthood would alter or be associated with auditory-cued conditioned fear levels using an extended training fear incubation procedure. This training procedure leads to low fear soon after training that grows over one month. Rats received 6 weeks of CIA to 20% alcohol or water from PND 26-66. Ten or eleven days later, the rats began behavioral testing that included 10 sessions of tone-shock pairings. Rats then received 4 weeks of CIA exposure during the 1-month fear incubation period and were tested for conditioned fear 6 days after the end of alcohol access. We found no evidence that voluntary alcohol consumption during adolescence/early adulthood or adulthood altered fear expression. However, we found that rats that consumed more alcohol during early adulthood (PND 54-66) had lower fear than low-consumption rats on day 1 of conditioned fear training and in the day 2 and 1-month tests. This extends associations we previously found between individual differences in alcohol consumption and conditioned fear to a different fear conditioning procedure. Combined with our previous data that show that the rate of instrumental extinction is associated with both alcohol consumption and conditioned fear, these data provide further support for the generality and reliability of a pair of phenotypes that encompass a wide variety of learning traits.

Ventrolateral periaqueductal gray neurons prioritize threat probability over fear output

Faced with potential harm, individuals must estimate the probability of threat and initiate an appropriate fear response. In the prevailing view, threat probability estimates are relayed to the ventrolateral periaqueductal gray (vlPAG) to organize fear output. A straightforward prediction is that vlPAG single-unit activity reflects fear output, invariant of threat probability. We recorded vlPAG single-unit activity in male, Long Evans rats undergoing fear discrimination. Three 10 s auditory cues predicted unique foot shock probabilities: danger (p=1.00), uncertainty (p=0.375) and safety (p=0.00). Fear output was measured by suppression of reward seeking over the entire cue and in one-second cue intervals. Cued fear non-linearly scaled to threat probability and cue-responsive vlPAG single-units scaled their firing on one of two timescales: at onset or ramping toward shock delivery. VlPAG onset activity reflected threat probability, invariant of fear output, while ramping activity reflected both signals with threat probability prioritized.

Early adolescent adversity inflates threat estimation in females and promotes alcohol use initiation in both sexes

Childhood adversity is associated with exaggerated threat processing and earlier alcohol use initiation. Conclusive links remain elusive, as childhood adversity typically co-occurs with detrimental socioeconomic factors, and its impact is likely moderated by biological sex. To unravel the complex relationships among childhood adversity, sex, threat estimation, and alcohol use initiation, we exposed female and male Long-Evans rats to early adolescent adversity (EAA). In adulthood, >50 days following the last adverse experience, threat estimation was assessed using a novel fear discrimination procedure in which cues predict a unique probability of footshock: danger (p = 1.00), uncertainty (p = .25), and safety (p = .00). Alcohol use initiation was assessed using voluntary access to 20% ethanol, >90 days following the last adverse experience. During development, EAA slowed body weight gain in both females and males. In adulthood, EAA selectively inflated female threat estimation, exaggerating fear to uncertainty and safety, but promoted alcohol use initiation across sexes. Meaningful relationships between threat estimation and alcohol use initiation were not observed, underscoring the independent effects of EAA. Results isolate the contribution of EAA to adult threat estimation, alcohol use initiation, and reveal moderation by biological sex. (PsycINFO Database Record

Lateral orbitofrontal cortex partitions mechanisms for fear regulation and alcohol consumption

Anxiety disorders and alcohol use disorder are highly comorbid, yet identifying neural dysfunction driving comorbidity has been challenging. Lateral orbitofrontal cortex (lOFC) dysfunction has been independently observed in each disorder. Here we tested the hypothesis that the lOFC is essential to partition mechanisms for fear regulation and alcohol consumption. Specifically, the capacity to regulate fear and the propensity to consume alcohol are unrelated when lOFC is intact, but become linked through lOFC dysfunction. Male Long Evans rats received bilateral, neurotoxic lOFC lesions or sham surgery. Fear regulation was determined by establishing discrimination to danger, uncertainty, and safety cues then shifting the shock probability of the uncertainty cue. Alcohol consumption was assessed through voluntary, intermittent access to 20% ethanol. The neurotoxic lesion approach ensured lOFC dysfunction spanned testing in fear regulation and alcohol consumption. LOFC-lesioned rats demonstrated maladaptive fear generalization during probability shifts, inverting normal prediction error assignment, and subsequently consumed more alcohol. Most novel, fear regulation and alcohol consumption were inextricably linked only in lOFC-lesioned rats: extreme fear regulation predicted excessive alcohol consumption. The results reveal the lOFC is essential to partition mechanisms for fear regulation and alcohol consumption and uncover a plausible source of neural dysfunction contributing to comorbid anxiety disorders and alcohol use disorder.

Individual differences in conditioned fear are associated with levels of adolescent/early adult alcohol consumption and instrumental extinction

Previous research has shown a relationship between alcohol exposure and conditioned fear, but the nature of this relationship remains unclear. We determined whether chronic intermittent access to alcohol during adolescence and early adulthood would alter or be associated with the level of conditioned fear to an auditory cue in male Long Evans rats. Rats received 6 weeks of chronic intermittent access to 20% alcohol or water from PND 26-66 and began behavioral testing 10 days later. We found no evidence that voluntary alcohol consumption altered fear. However, we found that rats that consumed more alcohol had lower fear, as measured with conditioned suppression of lever-pressing and conditioned freezing to an auditory cue. We have previously shown that higher levels of alcohol consumption are correlated with faster instrumental extinction learning. Therefore, we determined whether instrumental extinction would be directly associated with conditioned fear in rats never given alcohol access. As predicted, we found that rats that exhibited faster instrumental extinction also exhibited lower conditioned fear, as measured with conditioned suppression of lever-pressing and conditioned freezing. Our results suggest that at least part of the relationship between alcohol consumption levels and fear learning differences may be due to pre-existing individual differences. In addition, our finding that conditioned fear and instrumental extinction abilities (both separately associated with alcohol consumption levels) are associated with each other suggests that alcohol consumption levels may be a marker that can distinguish two separate phenotypes that encompass a wide variety of learning traits.

Adolescent Alcohol Drinking Renders Adult Drinking BLA-Dependent: BLA Hyper-Activity as Contributor to Comorbid Alcohol Use Disorder and Anxiety Disorders

Adolescent alcohol drinking increases the risk for alcohol-use disorder in adulthood. Yet, the changes in adult neural function resulting from adolescent alcohol drinking remain poorly understood. We hypothesized that adolescent alcohol drinking alters basolateral amygdala (BLA) function, making alcohol drinking BLA-dependent in adulthood. Male, Long Evans rats were given voluntary, intermittent access to alcohol (20% ethanol) or a bitter, isocaloric control solution, across adolescence. Half of the rats in each group received neurotoxic BLA lesions. In adulthood, all rats were given voluntary, intermittent access to alcohol. BLA lesions reduced adult alcohol drinking in rats receiving adolescent access to alcohol, but not in rats receiving adolescent access to the control solution. The effect of the BLA lesion was most apparent in high alcohol drinking adolescent rats. The BLA is essential for fear learning and is hyper-active in anxiety disorders. The results are consistent with adolescent heavy alcohol drinking inducing BLA hyper-activity, providing a neural mechanism for comorbid alcohol use disorder and anxiety disorders.