The effects of restraint stress on voluntary ethanol consumption in rats

Maternal Separation Stress Affects Voluntary Ethanol Intake in a Sex Dependent Manner

Maternal separation (MS) stress is a predictive animal model for evaluating the effects of early stress exposure on alcohol use disorders (AUD). The extended amygdala (AMY) is a complex circuit involved in both stress- and ethanol-related responses. We hypothesized that MS stress may increase ethanol consumption in adulthood, as well as augment neuronal activity in extended AMY, in a sex-dependent manner. We aimed to investigate the influence of MS stress on the ethanol consumption of male and female mice, and the involvement of extended amygdala sub-nuclei in this process. The C57BL/6J pups were subjected to 180min of MS, from postnatal day (PND) 1 to 14. The control group was left undisturbed. On PND 45, mice (n=28) in cages were exposed to a bottle containing 20% ethanol (w/v) for 4h during the dark period of the light-dark cycle, for 3weeks. Afterward, mice underwent ethanol self-administration training in operant chambers under fixed ratio (FR) schedule. Then, subjects were tested under 2h sessions of a progressive-ratio (PR) schedule of reinforcement (the last ratio achieved was considered the breaking point), and at the end, a 4h session of FR schedule (binge-intake). An immunohistochemistry assay for Fos protein was performed in Nucleus Accumbens (NAcc), Bed Nucleus of Stria Terminalis (BNST), and AMY. Our results showed that in the third week of training, the female MS group consumed more ethanol than the respective control group. The MS group presented increased breakpoint parameters. Female control group and male MS group were more resistant to bitter quinine taste. Increased Fos-immunoreactive neurons (Fos-IR) were observed in the central nucleus of AMY, but not in NAcc nor BNST in male maternal-separated mice. Maternal separation stress may influence ethanol intake in adulthood, and it is dependent on the sex and reinforcement protocol.

Age-related effects of chronic restraint stress on ethanol drinking, ethanol-induced sedation, and on basal and stress-induced anxiety response

Adolescents are sensitive to the anxiolytic effect of ethanol, and evidence suggests that they may be more sensitive to stress than adults. Relatively little is known, however, about age-related differences in stress modulation of ethanol drinking or stress modulation of ethanol-induced sedation and hypnosis. We observed that chronic restraint stress transiently exacerbated free-choice ethanol drinking in adolescent, but not in adult, rats. Restraint stress altered exploration patterns of a light-dark box apparatus in adolescents and adults. Stressed animals spent significantly more time in the white area of the maze and made significantly more transfers between compartments than their non-stressed peers. Behavioral response to acute stress, on the other hand, was modulated by prior restraint stress only in adults. Adolescents, unlike adults, exhibited ethanol-induced motor stimulation in an open field. Stress increased the duration of loss of the righting reflex after a high ethanol dose, yet this effect was similar at both ages. Ethanol-induced sleep time was much higher in adult than in adolescent rats, yet stress diminished ethanol-induced sleep time only in adults. The study indicates age-related differences that may increase the risk for initiation and escalation in alcohol drinking.

Cognitive bias in rats is not influenced by oxytocin

The effect of oxytocin on cognitive bias was investigated in rats in a modified conditioned place preference paradigm. Fifteen male rats were trained to discriminate between two different cue combinations, one paired with palatable foods (reward training), and the other paired with unpalatable food (aversive training). Next, their reactions to two ambiguous cue combinations were evaluated and their latency to contact the goal pot recorded. Rats were injected with either oxytocin (OT) or saline with the prediction that rats administered OT would display a shorter average latency to approach on ambiguous trials. There was no significant difference between latencies to approach on ambiguous trials compared to reward trials, but the rats were significantly slower on the aversive compared to the ambiguous conditions. Oxytocin did not affect approach time; however, it was unclear, after follow-up testing, whether the OT doses tested were sufficient to produce the desired effects on cognitive bias. Future research should consider this possibility.

Oral self-administration of EtOH: sex-dependent modulation by running wheel access in C57BL/6J mice

BACKGROUND

The effects of stress, including neuroendocrine and behavioral sequelae aimed at maintaining homeostasis, are associated with increased alcohol consumption. Because both stress and drinking are multifactorial, the mechanisms underlying the relationship are difficult to elucidate. We therefore employed an animal model investigating the influence of blocked access to a running wheel on drinking in C57BL/6J (B6) mice.

METHODS

In the first experiment, naïve, adult male and female subjects were individually housed for 2 weeks with 24-hour access to a running wheel and 12% ethanol (EtOH) in a 2-bottle, free choice paradigm. After determining baseline consumption and preference, experimental subjects had the running wheel placed in a locked position for 6 hours, and the EtOH bottle was removed during the first half of this period. Two subsequent experiments, again in adult, naïve B6 mice, examined the influence of locked running wheels on self-administration of 20% EtOH in a limited access paradigm, and blood EtOH concentrations (BECs) were determined on the final day of this protocol.

RESULTS

In all 3 studies, using both between- and within-subject analyses, females showed transient yet reliable increases in alcohol drinking during blocked access to a rotating activity, while drinking in male mice was largely insensitive to this manipulation, although both sexes showed appreciable BECs (>130 mg/dl in females and 80 mg/dl in males) following a 2-hour EtOH access period.

CONCLUSIONS

These data add to a burgeoning literature suggesting that the factors contributing to excessive alcohol use differ between males and females and that females may be especially sensitive to the influence of wheel manipulation. Elucidating the sex-dependent mechanisms mediating differences in alcohol sensitivity and response is critical to understanding the causes of alcoholism and in developing effective treatments and interventions.

Central effects of ethanol interact with endogenous mu-opioid activity to control isolation-induced analgesia in maternally separated infant rats

Endogenous opioid activity plays an important role in ethanol consumption and reinforcement in infant rats. Opioid systems are also involved in mediation and regulation of stress responses. Social isolation is a stressful experience for preweanling rats and changes the effects of ethanol through opioid-dependent mechanisms. The present study assessed effects of intracisternal (i.c.) administration of a selective mu-opioid antagonist (CTOP) and i.p. administration of a nonspecific opioid antagonist (naloxone) on voluntary intake and behavior in socially isolated 12-day-old (P12) pups treated with 0.5 g/kg ethanol. Voluntary intake of 0.1% saccharin or water, locomotion, rearing activity, paw licking and grooming were assessed during short-term isolation from littermates (STSI; 8-min duration). Thermal nociceptive reactivity was measured before and after this intake test, with normalized differences between pre- and post-test latencies of paw withdrawal from a hot plate (49°C) used as an index of isolation-induced analgesia (IIA). Results indicated several effects of social isolation and ethanol mediated through the mu-opioid system. Effects of low dose ethanol (0.5 g/kg) and voluntary consumption of saccharin interacted with endogenous mu-opioid activity associated with STSI. Blockade of mu-opioid receptors on saccharin consumption and paw licking-grooming affected intoxicated animals. Low dose ethanol and ingestion of saccharin blunted effects of CTOP on rearing behavior and nociceptive reactivity. Central injections of CTOP stimulated paw licking and grooming dependent on ethanol dose and type of fluid ingested. Ethanol selectively increased saccharin intake during STSI in females, naloxone and CTOP blocked ethanol-mediated enhancement of saccharin intake. We suggest that enhancement of saccharin intake by ethanol during STSI is the product of synergism between isolation-induced mu-opioid activity that increases the pup's sensitivity to appetitive taste stimulation and the anxiolytic effects of 0.5 g/kg ethanol that decreases behaviors otherwise competing with independent ingestive activity.

Transient increase in alcohol self-administration following a period of chronic exposure to corticosterone

Stressful life events and chronic stressors have been associated with escalations in alcohol drinking. Stress exposure leads to the secretion of glucocorticoids (cortisol in the human; corticosterone (CORT) in the rodent). To model a period of heightened elevations in CORT, the present work assessed the effects of chronic exposure to the stress hormone CORT on alcohol self-administration. Male Long Evans rats were trained to self-administer a sweetened alcohol solution (2% sucrose/15% alcohol) resulting in moderate levels of daily alcohol intake (0.5-0.7 g/kg). Following stable baseline operant self-administration, rats received CORT in the drinking water for 7 days. A transient increase in alcohol self-administration was observed on the first self-administration session following CORT exposure, and behavior returned to control levels by the second session. Control experiments determined that this increase in alcohol self-administration was specific to alcohol, unrelated to general motor activation, and functionally dissociated from decreased CORT levels at the time of testing. These results indicate that repeated exposure to heightened levels of stress hormone (e.g., as may be experienced during stressful episodes) has the potential to lead to exacerbated alcohol intake in low to moderate drinkers. Given that maladaptive drinking patterns, such as escalated alcohol drinking following stressful episodes, have the potential to put an individual at risk for future drinking disorders, utilization of this model will be important for examination of neuroadaptations that occur as a consequence of CORT exposure in order to better understand escalated drinking following stressful episodes in nondependent individuals.

Alcohol administration blocks stress-induced impairments in memory and anxiety, and alters hippocampal neurotransmitter receptor expression in male rats

Chronic exposure to stress has many deleterious effects on behavior, which can often lead to self-medication with anxiolytics, antidepressants, or alcohol. We determined the effects of alcohol administration following a stressor on established behavioral, physiological, and neural responses to stress. Male Sprague-Dawley rats received: No alcohol/No stress (CON), Alcohol alone (ALC), Stress alone (STR), or Stress plus Alcohol (STR+ALC). For seven consecutive days, two cohorts received an oral dose of 2.0 g/kg of either 20% ethanol or saline. In Cohort 1, behavioral testing began after the final treatment (day-8). Memory was tested using the object recognition (OR) and Y-maze, anxiety on the plus maze, and depression on the forced swim task. Memory on OR and Y-maze tasks was impaired in the ALC and STR groups. This deficit was reversed in the STR+ALC group, which performed not differently from the CON group. Stress alone was associated with increased anxiety, which was alleviated with alcohol treatment. No treatment effects were found in the forced swim task. In Cohort 2, hippocampal GABAα4 was upregulated in the STR+ALC group and GluN2B was upregulated in the ALC and STR+ALC groups. The STR+ALC group in Cohort 1 showed enhanced corticosterone levels after forced swim. The STR+ALC group in Cohort 2 showed increased corticosterone levels on day-1 of treatment and a habituation by day-7. In conclusion, this study found a reversal of stress-induced deficits in cognition and anxiety when alcohol was given post-stress, and changes in neurotransmitter receptor expression may contribute to these behavioral effects.

Alterations in ethanol seeking and self-administration following yohimbine in selectively bred alcohol-preferring (P) and high alcohol drinking (HAD-2) rats

Evidence suggests that stress increases alcohol drinking and promotes relapse in humans. Animal models that assess related behaviors include the sipper tube ethanol self-administration and the stress-induced reinstatement paradigms. While selectively bred for the same high-ethanol-drinking behavior, alcohol-preferring P rats appear to show greater sensitivity to ethanol reinforcement than high-alcohol-drinking HAD rats. The present experiment tested the effects of the pharmacological stressor, yohimbine, on the motivation to seek and consume ethanol implementing a combined sipper tube/reinstatement model using male P and HAD-2 rats. Following training to self-administer ethanol using the sipper tube procedure, rats were tested for the effects of yohimbine (0.625-2.5 mg/kg) on ethanol drinking. Subsequently, rats were tested for the effects of 1.25 mg/kg yohimbine on reinstatement of ethanol seeking. Yohimbine (0.625 and 1.25 mg/kg) increased ethanol self-administration, and the latter dose also decreased latency to complete the response requirement. Yohimbine elicited reinstatement of ethanol seeking in both lines. HAD-2 rats drank more ethanol, but showed similar responding on the ethanol-associated lever compared to P rats. These findings extend both the reinstatement and sipper tube models and justify further exploration of this unique combined paradigm. Despite prior evidence suggesting that P rats are more motivated to seek and consume ethanol, differences in these behaviors between P and HAD-2 rats were not systematic in the present experiment. Further investigation may elucidate whether either selected line may be more sensitive than other selectively bred or outbred rats to stress-related changes in ethanol's reinforcing effects.

The interaction of chronic restraint stress and voluntary alcohol intake: effects on spatial memory in male rats

Alcohol consumption and exposure to stressful life events activate similar neural pathways and thus result in several comparable physiological and behavioral effects. Alcoholics in treatment claim that life stressors are the leading cause of continued drinking or relapse. However, few studies have investigated the interactive effects of stress and alcohol on cognitive behavior. The effects of restraint stress, alcohol, and stress in combination with alcohol were examined on a spatial memory test, the object placement (OP) task. In addition, intake levels were measured to determine if stress altered general consumption of alcohol. Male Sprague-Dawley rats were assigned to one of four conditions: no alcohol/no stress control (CON), stress alone (STR), alcohol alone (ALC), and STR+alcohol (STR+ALC). Following each restraint stress bout, the STR+ALC and the ALC groups were given access to 8% alcohol for 1h using the two-bottle choice limited access paradigm. As predicted, the STR+ALC group significantly increased alcohol consumption, while the ALC group had consistent drinking over the 10-day treatment. On the OP task, STR and ALC groups performed at chance levels, whereas the CON and STR+ALC groups significantly discriminated between objects in the new and old locations. These data show that stress increases alcohol intake and the intake of alcohol is associated with reduction of the stress-induced impairment of spatial memory. The data have important implications for the development of alcohol abuse and its treatment.

The role of social isolation in ethanol effects on the preweanling rat

The present experiments investigated the effects of acute ethanol exposure on voluntary intake of 0.1% saccharin or water as well as behavioral and nociceptive reactivity in 12-day-old (P12) rats exposed to differing levels of isolation. The effects of ethanol emerged only during short-term social isolation (STSI) with different patterns observed in males and females and in pups exposed to saccharin or water. The 0.5g/kg ethanol dose selectively increased saccharin intake in females, decreased rearing activity in males and attenuated isolation-induced analgesia (IIA) in all water-exposed pups. Ingestion of saccharin decreased IIA, and the 0.5g/kg ethanol dose further reduced IIA. The 1.0g/kg ethanol dose, administered either intragastrically or intraparentionally, also decreased IIA in P12 females, but not in P9 pups. A significant correlation between voluntary saccharin intake and baseline nociceptive reactivity was revealed in saline injected animals, saccharin intake was inversely correlated with behavioral activation and latency of reaction to noxious heat after 0.5g/kg ethanol in females. The 0.5g/kg ethanol dose did not affect plasma corticosterone (CORT) measured 5h after maternal separation or 20min after ethanol injection. Female pups CORT level was inversely correlated with magnitude of IIA that accompanied the first episode of STSI (pretest isolation) 1.5-2h before CORT measurement. The present findings suggest that the anxiolytic properties of ethanol are responsible for enhancement of saccharin intake during STSI. Furthermore, differential reactivity of P12 males and females to STSI plays an important role in ethanol effects observed at this age.

Bidirectional interactions between acute psychosocial stress and acute intravenous alcohol in healthy men

BACKGROUND

The biological mechanisms by which acute stress increases alcohol consumption are unclear. One potential mechanism is that stress acts by altering the pharmacological and subjective effects of alcohol. Acute stress produces a cascade of physiological and psychological effects, each with a distinctive time course. In this study, we investigated whether different phases of response to an acute stress alter the subjective effects of intravenous alcohol, by administering the drug at 2 different times after the stress.

METHODS

Healthy men (n = 25) participated in 2 sessions: 1 with the Trier Social Stress Test and the other with a nonstressful control task, each followed by infusions of intravenous alcohol (targeting 40 mg% in 5 minutes) and placebo. One group of participants received alcohol within 1 minute of completing the tasks (Alc0, n = 11), followed by placebo 30 minutes later. In the other group (Alc30, n = 14), the order of alcohol and placebo infusions was reversed. Subjective effects (i.e., anxiety, stimulation, want more) and physiological measures (heart rate, blood pressure, salivary cortisol) were measured before and at repeated intervals after the tasks and infusions.

RESULTS

Stress did not change the subjective effects of alcohol in either group. However, when individual differences in alcohol responses were considered, stress differentially altered the stimulant-like and sedative effects of alcohol. Among individuals who exhibited predominantly stimulant responses to alcohol in the nonstressful condition, stress decreased the stimulant-like effects of alcohol and "wanting more." By contrast, among participants who did not report stimulation after alcohol in the control session, stress decreased the sedative effects and increased "want more." In addition, alcohol administered immediately after the Trier Social Stress Test dampened cortisol responses yet prolonged negative subjective responses to the stress.

CONCLUSIONS

These findings demonstrate that there are bidirectional relationships between alcohol and stress. Alcohol influences responses to stress, and stress changes reactions to alcohol, depending on an individual's pattern of response to alcohol. This study highlights the fact that stress-alcohol interactions vary among individual drinkers, suggesting that the effects of stress on motivation to drink alcohol may also differ between individuals.

Endocannabinoid-mediated synaptic plasticity and addiction-related behavior

Endogenous cannabinoids (eCBs) are retrograde messengers that provide feedback inhibition of both excitatory and inhibitory transmission in brain through the activation of presynaptic CB₁ receptors. Substantial evidence indicates that eCBs mediate various forms of short- and long-term plasticity in brain regions involved in the etiology of addiction. The present review provides an overview of the mechanisms through which eCBs mediate various forms of synaptic plasticity and discusses evidence that eCB-mediated plasticity is disrupted following exposure to a variety of abused substances that differ substantially in pharmacodynamic mechanism including alcohol, psychostimulants and cannabinoids. The possible involvement of dysregulated eCB signaling in maladaptive behaviors that evolve over long-term drug exposure is also discussed, with a particular focus on altered behavioral responses to drug exposure, deficient extinction of drug-related memories, increased drug craving and relapse, heightened stress sensitivity and persistent affective disruption (anxiety and depression).

Neuropeptide Y (NPY)-induced reductions in alcohol intake during continuous access and following alcohol deprivation are not altered by restraint stress in alcohol-preferring (P) rats

Administration of neuropeptide Y (NPY) reduces anxiety-like behavior and alcohol intake in alcohol-preferring rats. The present experiment examined whether the effects of NPY on alcohol drinking are modulated by stress exposure during continuous access or following ethanol deprivation. Female P rats underwent 6 weeks of continuous access to 15% v/v ethanol and water prior to intracerebroventricular (ICV) cannula implantation. Deprived rats underwent two cycles of 5 days of ethanol exposure followed by 2 days of ethanol deprivation, while non-deprived rats had uninterrupted access to ethanol. Stressed rats in both ethanol access groups were exposed to restraint stress for 1h 4-6h after ethanol was removed from the deprived group in both cycles. ICV infusions of 5.0 μg NPY or aCSF were administered 48 h following the deprivation/stress procedure, after which ethanol was returned. Rats showed increased ethanol intake following ethanol deprivation compared to non-deprived controls. Food and water intake were increased, while ethanol intake was decreased, in rats infused with NPY. Stress did not increase ethanol intake or alter the response to NPY. Although no stress effects were found, the present experiment replicates previous findings regarding the effectiveness of NPY in reducing ethanol consumption. Future studies aimed at determining the extent to which stress may affect relapse to ethanol drinking and response to NPY would benefit from implementing different stress paradigms and varying the pattern of ethanol access.

Voluntary alcohol consumption alters stress-induced changes in dopamine-2 receptor binding in Wistar-Kyoto rat brain

The Wistar-Kyoto (WKY) rat has been proposed as an animal model of depressive behavior and exhibits hyper-responsiveness to stressful stimulation when compared to other rat strains. We have demonstrated that WKY rats consume 200% more alcohol under naïve conditions as compared to their outbred counterparts, Wistar (WIS) rats. The present study was designed to understand the influence of stress and alcohol consumption on central dopamine type-2 (D2) receptor sites in these two behaviorally distinct rat strains. The first part of this study examined the effects of chronic stress on alcohol consumption, while the second part examined the binding of [(125)I]-Iodosulpiride to D2 receptors in control, stressed or stress and alcohol co-treated WKY compared to WIS rats. Exposure to chronic stress led to an increase in the amount of alcohol consumed by both rat strains, with WKY rats consuming significantly more alcohol than WIS rats with or without stress exposure. Quantitative autoradiography experiments showed that chronic stress increased D2 receptor binding in the caudate putamen (CPu), nucleus accumbens (NAc), substantia nigra (SN) and ventral tegmental area (VTA) of WKY rats, and reduced receptor binding in the CPu and SN of WIS rats. Compared to the stressed animals, WKY rats co-treated with stress and alcohol demonstrated a reduction in D2 receptor sites in the cell body regions (SN and VTA), while WIS rats showed no changes in receptor binding. The observed changes in D2 receptor sites may indicate altered DA neurotransmission following stress and alcohol exposure. Since stressed WKY rats consumed more alcohol, it is possible that consumption of alcohol reverses the stress-induced D2 receptor alterations in the cell body regions, suggestive of a self medicating phenotype.

Drug challenges reveal differences in mediation of stress facilitation of voluntary alcohol drinking and withdrawal-induced anxiety in alcohol-preferring P rats

BACKGROUND

There is controversy over whether exposure to stress precipitates relapse and/or increases alcohol (ethanol) intake. Our laboratory has demonstrated that repeated stress prior to withdrawal from a brief forced exposure to alcohol results in withdrawal-induced anxiety-like behavior. Because anxiety is often regarded as a precipitating factor in relapsing alcoholics, we decided to examine the consequences of stressing alcohol-preferring P rats on both voluntary alcohol drinking and withdrawal-induced anxiety.

METHODS

P rats were subjected to 3 cycles of 5 days of voluntary alcohol drinking and 2 days of deprivation. Restraint stress (60 min) was applied to some animals during the first and second deprivations/withdrawals (at 4 h). Drugs (flumazenil, buspirone, SB242,084, CP154,526, CRA1000, naloxone, haloperidol, olanzapine, naloxone, and haloperidol) were given to some rats 30 min prior to restraint stress.

RESULTS

Stressed, deprived P rats exhibited both a longer duration of elevated alcohol drinking and anxiety-like behavior in the social interaction test upon withdrawal after the third cycle of voluntary alcohol drinking. When given prior to each of the restraint stresses, the benzodiazepine receptor antagonist flumazenil (5 mg/kg), the corticotrophin releasing factor receptor antagonists CRA1000 (3 mg/kg) and CP154,526 (10 mg/kg), the serotonin 5-HT(1A) receptor partial agonist buspirone (0.6 mg/kg), and the mixed 5-HT(2C)/D2 receptor antagonist olanzapine were effective in reducing the increased duration of elevated alcohol drinking and the withdrawal-induced anxiety-like behavior. In contrast, while the opiate receptor antagonist naloxone (20 mg/kg), the 5-HT(2C) receptor antagonist SB242084 (3 mg/kg), and the dopamine receptor antagonist haloperidol (0.1 mg/kg) also reduced drinking, they did not significantly alter anxiety like behavior.

CONCLUSION

These results suggest that stress-induced facilitation of alcohol drinking and withdrawal-induced anxiety-like behavior in P rats may be closely but imperfectly linked.