The influence of postweaning housing conditions on drug-induced conditioned taste aversion

Memantine and SKF82958 but not an enriched environment modulate naloxone-precipitated morphine abstinence syndrome without affecting hippocampal tPA mRNA levels in rats

There is accumulating evidence supporting the involvement of tissue-plasminogen activator (tPA) in the mechanisms underlying the effects of morphine and an enriched environment. This study was designed to investigate possible interactive roles of the glutamatergic and the dopaminergic systems regarding hippocampal tPA in the neurobiology of morphine dependence. For this purpose, Wistar albino rats, housed in either a standard- (SE) or an enriched environment (EE) were implanted subcutaneously with morphine (150 mg base) or placebo pellets. Behavioral and somatic signs of morphine abstinence precipitated by an opioid-receptor antagonist naloxone (1 mg/kg, i.p.) 72 h after the pellet implantation were observed individually for 15 min in all groups. Memantine (10 mg/kg i.p.), an antagonist of N-methyl-D-aspartic acid class of glutamatergic receptor-subtype decreased teeth-chattering, ptosis, diarrhea and the loss of body weight. SKF82958 (1 mg/kg, i.p.), a dopamine D1-receptor agonist decreased jumping and ptosis but increased rearing and loss of body weight. On the other hand, co-administration of SKF82958 with memantine prevented some of their effects that occur when administered alone at the same doses. Furthermore, the EE did not change the intensity of morphine abstinence. The level of hippocampal tPA mRNA was found to be lower in the SE morphine abstinence group than in the placebo group and close to the EE morphine abstinence group, whereas there was no significant alteration of its level in the memantine or SKF82958 groups. These findings suggest that the interaction between the glutamatergic and the dopaminergic systems may be an important component of the neurobiology of morphine dependence, and the role of tPA in this interaction should be further investigated.

Brief environmental enrichment elicits metaplasticity on the insular cortex in vivo and reduces the strength of conditioned taste aversion

Environmental enrichment (EE) is known to improve memory and cognition and modulate the impact of aversive stimuli in animals, promoting the development of resilience to stressful situations. Likewise, it is known that EE can modulate synaptic plasticity as is the case of long-term potentiation (LTP). These findings have been described initially in ex vivo preparations, suggesting that the effects of EE are the result of an early modification of the synaptic excitability and transmission. In this regard, it is known that metaplasticity refers to the persistent modification, by previous activity, in the ability to induce synaptic plasticity. Our previous studies have shown that prior training in conditioned taste aversion (CTA) prevents the subsequent induction of LTP in the projection from the basolateral nucleus of the amygdala (Bla) to the insular cortex (IC) in vivo. In addition, we have shown that CTA extinction allows the induction but not the maintenance of IC-LTP of the Bla-IC pathway. Recently, we also showed that prior exposure to environmental enrichment for three weeks reduces the strength of CTA, restoring the brain-derived neurotrophic factor (BDNF) levels in the IC. The present study aimed to analyze the effects of brief exposure to an enriched environment on the strength of aversive memory, as well as on the in vivo IC-LTP. To do so, adult rats were exposed for seven days to an EE, either before CTA training or LTP induction in the Bla-IC pathway. Our results demonstrate that a seven-day exposure to an enriched environment attenuates the aversive response to a strong CTA and allows the induction but not the maintenance of LTP in the insular cortex. These findings provide evidence that metaplastic regulation in a neocortical region takes part in the mechanisms through which brief exposure to enriched environments attenuates an aversive response.

Age-dependent effects of tobacco smoke and nicotine on cognition and the brain: A systematic review of the human and animal literature comparing adolescents and adults

Cigarette smoking is often initiated during adolescence and an earlier age of onset is associated with worse health outcomes later in life. Paradoxically, the transition towards adulthood also marks the potential for recovery, as the majority of adolescents are able to quit smoking when adulthood emerges. This systematic review aimed to evaluate the evidence from both human and animal studies for the differential impact of adolescent versus adult repeated and long-term tobacco and nicotine exposure on cognitive and brain outcomes. The limited human studies and more extensive yet heterogeneous animal studies, provide preliminary evidence of heightened fear learning, anxiety-related behaviour, reward processing, nicotinic acetylcholinergic receptors expression, dopamine expression and serotonin functioning after adolescent compared to adult exposure. Effects of nicotine or tobacco use on impulsivity were comparable across age groups. These findings provide novel insights into the mechanisms underlying adolescents' vulnerability to tobacco and nicotine. Future research is needed to translate animal to human findings, with a focus on directly linking a broader spectrum of brain and behavioural outcomes.

Caffeine-based flavor preference conditioning in the rat

The massive consumption of caffeine-containing beverages has prompted many studies involving human participants that have obtained caffeine-based increases in liking for a flavor. However, few studies have succeeded in obtaining caffeine-based flavor preference learning in rats. The main aim of the present study was to examine the conditions under which such learning can be detected. Three experiments differed mainly in terms of the base solution to which caffeine was added. Using a base of maltodextrin and saccharin, Experiment 1 found modest increases in flavor preferences in both food- and fluid-restricted rats. Experiment 2 found a strong caffeine-based flavor preference when water, but not saccharin, was used as the base. Whereas the first two experiments used a within-subject design, in which one flavor was paired with caffeine and a second flavor was not, Experiment 3 used a between-subject design with fluid-restricted rats given almond-flavored water containing caffeine in the Paired condition but not in the Unpaired condition; caffeine-based flavor preference learning was again found. In Experiments 1 and 2 post-conditioning exposure to the flavor alone produced a decrease in preference. In summary, the main achievements of this study were to extend the conditions under which caffeine-based flavor preferences can be detected in rats and demonstrate that such learned preferences are subject to extinction.

Age differences in conditioned place preferences and taste aversions to nicotine

Adolescents and adults differ in their behavioral sensitivities to drugs of abuse, including nicotine. Studies have shown that both rewarding and aversive properties of drugs of abuse can exist concomitantly. The present study investigated the ontogeny of these opposing qualities across a range of doses using a combined conditioned taste aversion and place preference paradigm in pair-housed rats that were not deprived of food or water. Results indicated that adolescents were more sensitive to the rewarding properties of nicotine than adults. In contrast, although all doses produced a taste aversion at both ages in the same rats, the aversion was weaker at lower than high doses in adolescents whereas adults showed strong aversion at all doses, suggesting modest attenuation in nicotine's aversive properties among adolescents relative to adults. Thus, attenuated aversive and accented appetitive sensitivities of adolescents to nicotine can be experienced simultaneously in the same animals. © 2016 Wiley Periodicals, Inc. Dev Psychobiol 58: 660-666, 2016.

Pre-pubertal gonadectomy and the social consequences of acute ethanol in adolescent male and female rats

It has previously been shown that pre-pubertal or adult gonadectomy (GX) increases ethanol intake in male rats. This study examined whether this sex-selective increase reflects a GX-induced maintenance in males of more adolescent-typical responsiveness to ethanol characterized by enhanced sensitivity to positive (e.g., socially facilitating) and a decreased sensitivity to adverse (e.g., socially inhibitory) effects of ethanol. Male and female Sprague-Dawley rats were pre-pubertally GX, sham (SH)-operated, or non-manipulated (NM) at postnatal day (P) 25. During the late adolescent transition into adulthood (P48 - baseline day), rats were given a saline injection, placed alone into a familiar test apparatus for 30min and then exposed for 10min to an unfamiliar partner of the same age and sex. On the following day (P49), similar testing occurred after administration of 0.5, 0.75, 1.0 or 1.25g/kg ethanol. At baseline, GX males and females displayed higher levels of social activity (especially adolescent-typical play and contact behavior) than SH and NM animals, with GX females displaying greater social activity than GX males. Neither males nor females demonstrated social facilitation at lower ethanol doses, regardless of hormonal status. Whereas the social inhibitory effects of higher doses of ethanol were similar across groups among females, SH males were less sensitive than both GX and NM males to ethanol-induced social inhibition. These results suggest that enhanced ethanol intake in GX males is not related to alterations in sensitivity to ethanol's social inhibitory effects. GX, however, results in retention of adolescent-typical social behaviors, with older GX adolescent rats resembling early adolescents in exhibiting elevated social activity-particularly play and contact behavior.

Effects of an early handling-like procedure and individual housing on anxiety-like behavior in adult C57BL/6J and DBA/2J mice

Manipulations of rearing conditions have been used to examine the effects of early experience on adult behavior with varying results. Evidence suggests that postnatal days (PND) 15-21 are a time of particular susceptibility to environmental influences on anxiety-like behavior in mice. To examine this, we subjected C57BL/6J and DBA/2J mice to an early handling-like procedure. Pups were separated from dams from PND 12-20 for 30 minutes daily or received standard care. On PND 21, pups were weaned and either individually- or group-housed. On PND 60, anxiety-like behavior was examined on the elevated zero-maze. Although individually-housed animals took longer to enter an open quadrant of the maze, they spent more time in the open than group-housed animals. Additionally, we observed a trend of reduced anxiety-like behavior in C57BL/6J, but not DBA/2J mice that underwent the handling-like procedure.

Effects of environmental enrichment on sensitivity to mu, kappa, and mixed-action opioids in female rats

Several studies report that environmental enrichment enhances sensitivity to opioid receptor agonists in male rats. Very few studies have examined the effects of enrichment in female rats, and thus it is not clear whether females are similarly sensitive to these effects. Consequently, the purpose of the present study was to examine the effects of environmental enrichment on sensitivity to representative mu, kappa, and mixed-action opioids in female rats. Following a protocol established in males, females were obtained at weaning and randomly assigned to two groups immediately upon arrival: isolated rats were housed individually with no visual or tactile contact with other rats; enriched rats were housed in groups of four in large cages and given various novel objects on a regular basis. After 6 weeks under these conditions, the antinociceptive effects of mu (morphine, levorphanol), kappa (spiradoline, U69,593), and mixed-action (buprenorphine, butorphanol) opioids were examined in a warm-water, tail-withdrawal procedure. All the opioids examined produced dose-dependent increases in antinociception; however, no differences in opioid sensitivity were observed between the two groups. To determine whether these findings were consistent across behavioral endpoints, the antidiuretic effects of representative mu opioids, and the diuretic effects of representative kappa opioids, were examined in female rats reared under isolated or enriched conditions for 10 weeks. Similar to that seen in the antinociceptive experiment, no significant differences in opioid sensitivity were observed between groups. These data indicate that environmental enrichment does not alter sensitivity to the effects of opioid receptor agonists in female rats, and suggest that females may respond differently to environmental enrichment than males.

Early maternal separation and sex differences in the aversive effects of amphetamine in adult rats

Early life stress increases vulnerability to psychostimulant abuse, sometimes in a sex-dependent manner. These effects are presumably produced by modulation of neurobiological reward mechanisms; however, drug intake is also influenced by sensitivity to the drug's aversive properties, and little is known about the effects of early life stress on stimulant aversion. To assess this possibility, Sprague-Dawley rat litters experienced daily separation from the dam for 3 h (MS) or 15 min (H) on post-natal days 2-14; control litters (AFR) experienced twice-weekly routine animal facility care only. As adults, the animals were tested for conditioned taste aversions (CTA) induced by 1.5, 2.0 or 3.0 mg/kg d-amphetamine administered intraperitoneally over three acquisition trials followed by six drug-free extinction trials (n=7-8/sex/dose/rearing group). All groups acquired significant aversions compared to their vehicle-treated controls, but differential rearing had no effect on CTA acquisition or extinction. Collapsed across rearing groups, females exhibited significantly stronger aversions and slower extinction than their male counterparts at the low 1.5 mg/kg dose, and unlike the males, failed to fully extinguish relative to their vehicle controls at 1.5 and 2.0 mg/kg. These data underscore the importance of sex differences in assays of stimulant abuse liability, and further support the hypothesis that the effects of post-natal stress on the reinforcing efficacy of psychostimulants may be predominantly due to modulation of reward mechanisms.

Biological research on drug abuse and addiction in Hispanics: current status and future directions

Impressive progress has been made in the understanding of biological contributions to drug abuse and addiction. An area that has only recently begun to receive attention is potential ethnic and racial differences in biological systems that contribute to, or protect from, problem drug use. This article reviews recent research on drug abuse and addiction in Hispanics in which biological questions have been addressed, including work on genes, gene products (proteins), physiology and pharmacotherapy. Taken together, work to date suggests that there are both similarities and differences between Hispanics and other ethnic groups in biological factors related to drug abuse and addiction. Although the results are intriguing, relatively few studies have been done, and those that have been done have often been inconclusive due to low numbers of Hispanic subjects. Moreover, studies have often failed to recognize the complexity and heterogeneity of Hispanic populations in the United States and around the world. After reviewing the current status of the field, recommendations are given for future research in both humans and relevant animal models that will lead to a better understanding of drug abuse and addiction in Hispanics.

Caffeine as a performance-enhancing drug in rats: sex, dose, housing, and task considerations

Past animal studies of the performance-enhancing properties of stimulant drugs, such as caffeine, may have suffered from a number of procedural and ethical problems. For example. the housing condition of the animals was often not taken into consideration. As well, endurance tests, such as the forced swim task, sometimes involved ethically (and procedurally) questionable interference with natural swimming behaviour. Some of the manipulations, such as attaching a weight to the swimming animal's tail to increase the difficulty of the task and using mortality as a dependent variable, seem grotesque, even unnecessary. In this experiment, the performance-enhancing effects of caffeine in a modified forced swim task and a dominance task were evaluated using male and female rats as subjects (N=60), housed in either enriched or isolated environments. Analysis indicated that rats respond to caffeine as an interactive function of sex, housing, dose, and task characteristics. It was concluded that performance-enhancing properties of stimulant drugs may be the result of a complex interplay of variables, making simple generalizations questionable.

Differences in locomotor response to an inescapable novel environment predict sensitivity to aversive effects of amphetamine

Differences in locomotor response to an inescapable novel environment have previously been shown to predict sensitivity to amphetamine reward, where high responders (HR), compared to low responders (LR), showed greater initial sensitivity to amphetamine self-administration. The present experiments sought to extend these findings and assessed the relationship between locomotor response to an inescapable novel environment and conditioned taste aversion (CTA) with amphetamine and lithium chloride (LiCl). Male Sprague-Dawley rats were tested for their locomotor response to an inescapable novel environment and divided into high (HR) or low (LR) responders, based on whether their locomotor scores were above or below the median activity level of the subject sample. After several days, the animals were tested in a CTA procedure and conditioned with either amphetamine or lithium chloride. Compared to HR rats, LR rats showed greater sensitivity to amphetamine CTA at the doses tested. In contrast, the results with LiCl showed no relationship between locomotor response to an inescapable novel environment and CTA. Taken together, the present results suggest that LR, compared to HR, rats show less sensitivity to the rewarding effects of amphetamine because they are more sensitive to aversive effects of amphetamine, as reflected in CTA. In contrast, HR rats display less sensitivity to aversive effects of amphetamine, which may explain their greater propensity to self-administer amphetamine.