Effects of amphetamine exposure during adolescence on behavior and prelimbic cortex neuron activity in adulthood.

Repeated exposure to psychostimulants during adolescence produces long-lasting changes in behavior that may be mediated by disrupted development of the mesocorticolimbic dopamine system. Here, we tested this hypothesis by assessing the effects of amphetamine (AMPH) and dopamine receptor-selective drugs on behavior and neuron activity in the prelimbic region of the medial prefrontal cortex (PFC). Adolescent male, Sprague-Dawley rats were given saline or 3 mg/kg AMPH between postnatal day (P) 27 and P45. In Experiment 1, locomotor behavior was assessed during adulthood following challenges with a dopamine D1 (SKF 82958) or D2 (quinpirole) receptor-selective agonist. In Experiment 2, pre-exposed rats were challenged during adulthood with AMPH and a D1 (SKF 83566) or D2 (eticlopride) receptor-selective antagonist. In Experiment 3, the activity of putative pyramidal cells in the prelimbic cortex was recorded as rats behaved in an open-field arena before and after challenge injections with AMPH and one of the antagonists. We found that compared to controls, adolescent pre-exposed rats were more sensitive to the stimulant effects of AMPH and the dopamine receptor agonists, as well as to the ability of the antagonists to reverse AMPH-induced stereotypy. Prelimbic neurons from AMPH pre-exposed rats were also more likely to respond to an AMPH challenge in adulthood, primarily by reducing their activity, and the antagonists reversed these effects. Our results suggest that exposure to AMPH during adolescence leads to enduring adaptations in the mesocorticolimbic dopamine system that likely mediate heightened response to the drug during adulthood.

Sex differences in adolescent ethanol drinking to behavioral intoxication.

Rodent models have been especially useful for investigating adolescent ethanol exposure. However, there is a paucity of studies examining sex differences in behavioral intoxication from adolescent ethanol drinking. Here, we used an ethanol drinking model to investigate if adolescent rats of both sexes readily drink ethanol to measurable behavioral intoxication, indicated by increased impulsive action and motor incoordination. Beginning on postnatal day (P) 28, male and female Long-Evans rats were given 30-min access to a solution of sucrose (20%) or sweetened ethanol (20% sucrose +15% ethanol) every other day until P60 and once after 2 weeks of forced abstinence (on P75). On alternate (nondrinking) days, rats were reinforced with a food pellet for making a cued nosepoke response. Beginning on P56, rats were tested in this task after drinking sessions to assess ethanol-induced changes in impulsive action, defined as premature responding prior to cue presentation. Motor coordination was assessed before and after drinking sessions using an incline plane test. Adolescent male and female rats readily consumed ethanol to behavioral intoxication, measured as reduced motor coordination. Following forced abstinence, females displayed greater ethanol-induced impulsive action. These studies provide evidence for sex differences in behavioral intoxication following adolescent ethanol drinking.

Effects of ß-hydroxy-ß-methyl butyrate on working memory and cognitive flexibility in an animal model of aging.

OBJECTIVES: Normal aging results in cognitive decline and nutritional interventions have been suggested as potential approaches for mitigating these deficits. Here, we used rats to investigate the effects of short- and long-term dietary supplementation with the leucine metabolite ß-hydroxy-ß-methyl butyrate (HMB) on working memory and cognitive flexibility. METHODS: Beginning ∼12 months of age, male and female Long-Evans rats were given twice daily access to sipper tubes containing calcium HMB (450 mg/kg) or vehicle (285 mg/kg calcium lactate) in a sucrose solution (20% w/v). Supplementation continued for 1 or 7 months (middle- and old-age (OA) groups, respectively) before testing began. Working memory was assessed by requiring rats to respond on a previously sampled lever following various delays. Cognitive flexibility was assessed by training rats to earn food according to a visual strategy and then, once acquired, shifting to an egocentric response strategy. RESULTS: Treatment with HMB improved working memory performance in middle-age (MA) males and OA rats of both sexes. In the cognitive flexibility task, there was a significant age-dependent deficit in acquisition of the visual strategy that was not apparent in OA males treated with HMB. Furthermore, HMB ameliorated an apparent deficit in visual strategy acquisition in MA females. DISCUSSION: Together, these findings suggest that daily nutritional supplementation with HMB facilitates learning and improves working memory performance. As such, HMB supplementation may mitigate age-related cognitive deficits and may therefore be an effective tool to combat this undesirable feature of the aging process.

Age-dependent effects of repeated amphetamine exposure on working memory in rats.

Cognitive dysfunction is a hallmark of chronic psychostimulant misuse. Adolescents may have heightened risk of developing drug-induced deficits because their brains are already undergoing widespread changes in anatomy and function as a normal part of development. To address this hypothesis, we performed two sets of experiments where adolescent and young adult rats were pre-exposed to saline or amphetamine (1 or 3mg/kg) and subsequently tested in a prefrontal cortex (PFC)-sensitive working memory task. A total of ten injections of AMPH or saline (in control rats) were given every other day over the course of 19 days. After rats reached adulthood (>90 days old), cognitive performance was assessed using operant-based delayed matching-to-position (DMTP) and delayed nonmatching-to-position (DNMTP) tasks. DNMTP was also assessed following challenges with amphetamine (0.3-1.25mg/kg), and ketamine (5.0-10mg/kg). In experiment one, we also measured the locomotor response following the first and tenth pre-exposure to amphetamine and after an amphetamine challenge given at the conclusion of operant testing. Compared to adult-exposed groups, adolescents were less sensitive to the psychomotor effects of amphetamine. However, they were more vulnerable to exposure-induced cognitive impairments. For example, adolescent-exposed rats displayed delay-dependent deficits in accuracy, increased sensitivity to proactive interference, and required more training to reach criterion. Drug challenges produced deficits in DNMTP performance, but these were not dependent on pre-exposure group. These studies demonstrate age of exposure-dependent effects of amphetamine on cognition in a PFC-sensitive task, suggesting a heightened sensitivity of adolescents to amphetamine-induced neuroplasticity.

Sex differences in the effects of ethanol pre-exposure during adolescence on ethanol-induced conditioned taste aversion in adult rats.

Alcohol use, which typically begins during adolescence and differs between males and females, is influenced by both the rewarding and aversive properties of the drug. One way adolescent alcohol use may modulate later consumption is by reducing alcohol's aversive properties. Here, we used a conditioned taste aversion (CTA) paradigm to determine if pre-exposure to alcohol (ethanol) during adolescence would attenuate ethanol-induced CTA assessed in adulthood in a sex-dependent manner. Male and female Long-Evans rats were given intraperitoneal (i.p.) injections of saline or 3.0g/kg ethanol in a binge-like pattern during postnatal days (PD) 35-45. In adulthood (>PD 100), rats were given access to 0.1% saccharin, followed by saline or ethanol (1.0 or 1.5g/kg, i.p.), over four conditioning sessions. We found sex differences in ethanol-induced CTA, with males developing a more robust aversion earlier in conditioning. Sex differences in the effects of pre-exposure were also evident: males, but not females, showed an attenuated CTA in adulthood following ethanol pre-exposure, which occurred approximately nine weeks earlier. Taken together, these findings indicate that males are more sensitive to the aversive properties of ethanol than females. In addition, the ability of pre-exposure to the ethanol US to attenuate CTA is enhanced in males compared to females.

The effects of pre-pubertal gonadectomy and binge-like ethanol exposure during adolescence on ethanol drinking in adult male and female rats.

The pubertal surge in gonadal hormones that occurs during adolescence may impact the long-term effects of early alcohol exposure and sex differences in drinking behavior in adulthood. We investigated this hypothesis by performing sham or gonadectomy surgeries in Long-Evans rats around post-natal day (P) 20. From P35-45, males and females were given saline or 3.0 g/kg ethanol using a binge-like model of exposure (8 injections total). As adults (P100), they were trained to self-administer ethanol via a sucrose-fading procedure and then given access to different unsweetened concentrations (5-20%, w/v) for 5 days/concentration. We found that during adolescence, ethanol-induced intoxication was similar in males and females that underwent sham surgery. In gonadectomized males and females, however, the level of intoxication was greater following the last injection compared to the first. During adulthood, females drank more sucrose per body weight than males and binge-like exposure to ethanol reduced sucrose consumption in both sexes. These effects were not seen in gonadectomized rats. Ethanol consumption was higher in saline-exposed females compared to males, with gonadectomy reversing this sex difference by increasing consumption in males and decreasing it in females. Exposure to ethanol during adolescence augmented ethanol consumption in both sexes, but this effect was statistically significant only in gonadectomized females. Together, these results support a role for gonadal hormones during puberty in the short- and long-term effects of ethanol on behavior and in the development of sex differences in consummatory behavior during adulthood.

Disparate cocaine-induced locomotion as a predictor of choice behavior in rats trained in a delay-discounting task.

Heightened impulsivity and differential sensitivity to a drug's behavioral effects are traits that, individually, have been associated with chronic drug use and dependence. Here, we used an animal model to test whether individual differences in cocaine-induced activity are predictive of impulsive choice behavior. Adult, male Sprague-Dawley rats were given cocaine (10mg/kg, i.p.) and classified into low or high cocaine responders (LCRs or HCRs, respectively) based on their locomotor response in an open-field arena. Rats were then trained in a delay-discounting task that offers a choice between immediately delivered, but smaller reinforcements, or larger reinforcements that are delivered after a delay. We also examined the effects of amphetamine (AMPH; 0.3-1.0mg/kg) and the 5-HT1A agonist 8-OH-DPAT (0.3-1.0mg/kg) on delay-discounting. Lastly, all rats were retested in the open-field to determine if phenotypes were stable. We observed baseline differences in choice behavior between the groups, with HCRs behaving more impulsively (i.e., choosing the small reinforcement) compared to LCRs. AMPH decreased choice of the large reinforcement in LCRs, but did not alter choice in HCRs. Impulsive choice was increased in both phenotypes following 8-OH-DPAT, with LCRs exhibiting changes across a wider range of delays. When cocaine-induced open-field behavior was retested, responses in LCRs were similar whereas HCRs showed evidence of tolerance. Our results suggest that differential sensitivity to cocaine-induced locomotion is predictive of impulsivity and the potential neurobiological differences in LCRs and HCRs may provide insight into mechanisms contributing to vulnerability for chronic drug use and/or dependence.

Time-dependent effects of amphetamine on feeding in rats.

Following administration of a moderate dose of amphetamine, rats appear to pass through a sequence of physiological/psychological states, including stimulant and depressant states. The present research evaluated whether these states could be inferred from time-dependent changes in feeding-related measures. Male rats were housed in individual stations (light-dark 12-12 h, free access to water) where, at 3-h intervals, they could respond for food for 1 h. The work requirement was fixed ratio 1, and each lever press produced six 94-mg food pellets. When the pattern of responding for food stabilized across the light-dark cycle, a series of 6 or 7 tests was run. During each test, rats received a saline treatment (1.0 ml/kg, subcutaneously) followed by a 48-h monitoring period, and then they received an amphetamine treatment (2.0 mg/kg, subcutaneously) followed by a 72-h monitoring period. Different groups were treated at either light onset or light offset. Lever presses and head-in-feeding-bin responses were monitored throughout these tests. Administration of amphetamine at light onset and at light offset produced cumulative food intake functions having four regions: post-treatment hours 1-6 (hypophagia), 7-12 (normal intake), 13-27 (hypophagia), and 28 and beyond (normal intake). The sequence, duration, and quality of the amphetamine-induced changes in food intake resembled those formerly seen in cue state and activity, and provided further evidence of a transient withdrawal state 20-24 h post-amphetamine treatment.