Psychobiological risk factors for vulnerability to psychostimulants in human adolescents and animal models

Delay aversion but preference for large and rare rewards in two choice tasks: implications for the measurement of self-control parameters

Background

Impulsivity is defined as intolerance/aversion to waiting for reward. In intolerance-to-delay (ID) protocols, animals must choose between small/soon (SS) versus large/late (LL) rewards. In the probabilistic discount (PD) protocols, animals are faced with choice between small/sure (SS) versus large/luck-linked (LLL) rewards. It has been suggested that PD protocols also measure impulsivity, however, a clear dissociation has been reported between delay and probability discounting.

Results

Wistar adolescent rats (30- to 46-day-old) were tested using either protocol in drug-free state. In the ID protocol, animals showed a marked shift from LL to SS reward when delay increased, and this despite adverse consequences on the total amount of food obtained. In the PD protocol, animals developed a stable preference for LLL reward, and maintained it even when SS and LLL options were predicted and demonstrated to become indifferent. We demonstrate a clear dissociation between these two protocols. In the ID task, the aversion to delay was anti-economical and reflected impulsivity. In the PD task, preference for large reward was maintained despite its uncertain delivery, suggesting a strong attraction for unitary rewards of great magnitude.

Conclusion

Uncertain delivery generated no aversion, when compared to delays producing an equivalent level of large-reward rarefaction. The PD task is suggested not to reflect impulsive behavior, and to generate patterns of choice that rather resemble the features of gambling. In summary, present data do indicate the need to interpret choice behavior in ID and PD protocols differently.

Prevalence and profile of methamphetamine users in adolescents at a juvenile classification home

The objective of the present study was to investigate the prevalence and risk factors of methamphetamine use in adolescents at a juvenile classification home. The present subjects were 1362 adolescents (1172 male and 190 female) who had been admitted to the Nagoya Juvenile Classification Home. The participants were divided into two groups, a methamphetamine user group and a control group, based on history of methamphetamine use. The presence of methamphetamine use was analyzed in terms of gender, age, number of admissions, violence (types of crime), history of psychiatric treatment, family history (crime, drug misuse and/or alcohol-related disorder), and experience of being abused by their parents or by the persons who were responsible for raising them. The prevalence of methamphetamine use was 6.8% (93/1362). Multivariate logistic regression analyses indicated that gender (female; odds ratio [OR]: 8.1; 95% confidence interval [CI]: 4.6-14.3), age (OR: 1.8, 95%CI: 1.5-2.1), number of admissions (>2, OR: 2.9, 95%CI: 1.8-4.8), violence (OR: 0.4, 95%CI: 0.2-0.7), history of psychiatric treatment (OR: 8.7, 95%CI: 4.0-19.0), and family history of drug misuse (OR: 4.0, 95%CI: 1.6-9.6) were all significantly associated with methamphetamine use. Approximately 7% of participants used methamphetamine. Female gender was a risk factor. Higher age and multiple admissions suggest the persistency and repetition of delinquency. Methamphetamine users were less violent than control subjects. Psychosocial environment (family history of drug misuse) and psychiatric problems (history of psychiatric treatment) were also related to methamphetamine use.

MDMA in humans: factors which affect the neuropsychobiological profiles of recreational ecstasy users, the integrative role of bioenergetic stress

Many recreational ecstasy/MDMA users display neuropsychobiological deficits, whereas others remain problem free. This review will investigate some of the drug and non-drug factors which influence the occurrence of these deficits. Acute and chronic MDMA usage are both important. Intensive use within a session is often associated with more problems. In term of lifetime usage, novice users generally remain unimpaired, whereas most heavy users report memory or other psychobiological problems which they attribute to ecstasy. These complaints are confirmed by objective deficits in working memory, attention, frontal-executive, and episodic memory tasks. Psychobiological deficits include disturbed sleep, sexual dysfunction, reduced immuno-competence, and increased oxidative stress. Further MDMA-related factors which may contribute to these changes, include acute and chronic tolerance, and drug dependence. Around 90ñ95% of ecstasy/MDMA users also take cannabis, and this can independently contribute to the adverse neuropsychobiological pro.les; although in some situations the acute co-use of these two drugs may be interactive rather than additive, since cannabis has relaxant and hypothermic properties. Alcohol, nicotine, amphetamine, and other drugs, can also affect the psychobiological pro.les of ecstasy polydrug users in complex ways. Pure MDMA users are rare but they have been shown to display significant neurocognitive deficits. Psychiatric aspects are debated in the context of the diathesis-stress model. Here the stressor of ecstasy polydrug drug use, interacts with various predisposition factors (genetic, neurochemical, personality), to determine the psychiatric outcome. Recreational MDMA is typically taken in hot and crowded dances/raves. Prolonged dancing, feeling hot, and raised body temperature, can also be associated with more psychobiological problems. This is consistent with the animal literature, where high ambient temperature and other metabolic stimulants boost the acute effects of MDMA, and cause greater serotonergic neurotoxicity. In conclusion, the neuropsychobiological effects of MDMA are modulated by a wide range of drug and non-drug factors. These multiple influences are integrated within a bioenergetic stress model, where factors which heighten acute metabolic distress lead to more neuropsychobiological problems.

Methylphenidate treatment induces oxidative stress in young rat brain

Methylphenidate (MPH) is frequently prescribed for the treatment of attention deficit/hyperactivity disorder. Psychostimulants can cause long-lasting neurochemical and behavioral adaptations. Here, we evaluated oxidative damage in the rat brain and the differential age-dependent response to MPH after acute and chronic exposure. We investigated the oxidative damage, assessed by the thiobarbituric acid reactive species (TBARS), and the protein carbonyl assays in cerebellum, prefrontal cortex, hippocampus, striatum, and cerebral cortex of young (25 days old) and adult (60 days old) male Wistar rats after acute and chronic exposure to MPH. Chronic MPH-treated young rats presented a dose-dependent increase in TBARS content and protein carbonyls formation in specific rat brain regions. In the acute exposure, only MPH highest dose increased lipid peroxidation in the hippocampus. No difference in protein carbonylation was observed among groups in all structures analyzed. In adult rats, we did not find oxidative damage in both acute and chronic treatment. Chronic exposure to MPH in induces oxidative damage in young rat brain, differentially from chronic exposure during adulthood. These findings highlight the need for further research to improve understanding of MPH effects on developing nervous system and the potential consequences in adulthood resulting from early-life drug exposure.

The role of age, genotype, sex, and route of acute and chronic administration of methylphenidate: A review of its locomotor effects

Children with attention deficit hyperactivity disorder (ADHD) are treated for extended periods of time with the psychostimulant methylphenidate (MPD). The psychostimulants cocaine, amphetamine, and MPD exhibit similar structural configuration and pharmacological profile. The consequence of the long-term use of psychostimulants such as MPD as treatment for ADHD in the developing brain of children is unknown. Repeated treatment with psychostimulants has been shown to elicit adverse effects in behavior, such as dependence, paranoia, schizophrenia, and behavioral sensitization. Behavioral sensitization and cross-sensitization between two drugs are used as experimental markers to determine the potential of a drug to develop dependence/addiction. Although there are many reviews written about behavioral sensitization involving psychostimulants, scarcely any have focused specifically on MPD-elicited behavioral sensitization and cross-sensitization with other psychostimulants. Moreover, the response to MPD and the expression of ADHD vary among females and males and among different populations due to genetic variability. Since the interpretation and synthesis of the data reported are controversial, this review focuses on the adverse effects of MPD and the role of age, sex, and genetic composition on the acute and chronic effects of MPD, such as MPD-elicited behavioral sensitization and cross-sensitization with amphetamine in animal models. Animal models of drug-induced locomotor stimulation, particularly locomotor sensitization, can be used to understand the mechanisms underlying human drug-induced dependence.

Fear, risk assessment, and playfulness in the juvenile rat

The effects of predatory odors on play were assessed in juvenile rats. When rats were exposed directly to a collar previously worn by a cat, play was abolished and remained suppressed for up to 6 days. Providing rats with an opportunity to hide did not alter cat odor's ability to reduce their play. Rat play was also suppressed shortly after they were exposed to cat odor in their home cage, and a substantial amount of risk assessment behavior was present up to 24 hr later. Trimethylthiazoline, a component found in fox feces, only reduced play during exposure. These data suggest that predatory odor-induced reductions in play may provide a useful model for gaining insight into the consequences of fear and anxiety in young animals.

Effect of stress on the voluntary intake of a sweetened ethanol solution in pair-housed adolescent and adult rats

BACKGROUND

Data regarding the effects of stressors on ethanol intake are mixed. Previous experiments reporting greater voluntary intake of ethanol in adolescent than adult rats have examined intake in isolate-housed animals. Given that the stress of isolate housing may differ ontogenetically as well as confound interpretation of other stressor effects, the present study examined stressor/ethanol interactions among pair-housed adolescent and adult rats.

METHODS

Sprague-Dawley male rats were implanted with identification tags that allowed individual monitoring of home cage intake of water and either a 10% (v/v) ethanol solution containing 0.1% (w/v) saccharin or saccharin alone over a 14-day access period. Animals were given zero, one, or eight daily 15-min footshock sessions, with shock-induced freezing and pre-, post-, and recovery corticosterone levels determined on the first and last footshock exposure days. After the access period, withdrawal was assessed with a plus maze, and tolerance to ethanol-induced loss of righting reflex was examined.

RESULTS

Nonstressed adolescents drank considerably more sweetened ethanol than did adults, with chronic stress suppressing this adolescent consumption. Ethanol access in adolescents disrupted within-session adaptation to footshock in terms of freezing behavior, although no such disruption was evident at either age when indexed hormonally. Despite relatively high ethanol intakes (up to 6 g/kg/day in the adolescents), no evidence for withdrawal-associated anxiogenesis emerged. Evidence for tolerance was mixed and, to the extent that it was present, was metabolic in nature.

CONCLUSIONS

Previous reports of heightened voluntary ethanol intake among adolescent rats are not a function of isolate stress but are evident in pair-housed animals. Adolescents were more sensitive to ethanol/stress interactions than were adults, with the elevated ethanol intake of pair-housed adolescents selectively disrupted by chronic stress, a stress-induced disruption not evident in adults. Likewise, ethanol disrupted behavioral adaptation to the footshock stressor among adolescents but not adults.

Enhanced behavioral response to repeated-dose cocaine in adolescent rats

RATIONALE

Most lifelong drug addiction in humans originates during adolescence. Important structural and functional changes in the brain occur during adolescence, but there has been little direct study of how this impacts on drug abuse vulnerability. An emerging literature suggests that adolescents exhibit different behavioral responses to single doses of several addictive drugs, including ethanol, amphetamine, and cocaine. However, few studies have explored behavioral responses to the repeated dosing that is characteristic of human abuse of these substances.

OBJECTIVES

We have investigated age-related behavioral responses to acute "binge" cocaine treatment between adults and adolescents.

RESULTS

Adolescent rats displayed an exaggerated behavioral response to cocaine administered in two different binge patterns. Total locomotion after cocaine administration was the same in adolescents and adults. However, adolescent rats engaged in more intense stereotypic behaviors, including paw treading, head weaving, and focused sniffing than adult rats. These differences were observable following a modest dose of cocaine and became more robust following subsequent doses within a binge. Cocaine [corrected] brain levels were not significantly different between age groups during any of the exposure sessions.

CONCLUSIONS

These findings suggest that equivalent tissue concentrations of cocaine produce a greater behavioral response in young rats, and that adolescent animals display an apparent form of intrabinge sensitization.

Age-specific behavioral responses to psychostimulants in mice

This study investigated the influence of age on the behavioral responses elicited by psychostimulants in male CD-1 mice. Behavioral activity including locomotion and stereotypy was measured following acute or repeated administration of cocaine, methylphenidate, amphetamine or saline to postweanling (24 days old), periadolescent (33 days old) and adult (60 days old) mice. Postweanling mice exhibited less total and ambulatory activity than periadolescent mice following a single acute injection of cocaine (20 or 30 and 30 mg/kg, respectively). Further, postweanling mice showed less total activity than both periadolescent and adult mice at a dose of 10 mg/kg methylphenidate. Less stereotypy was also seen in postweanling mice when compared to adolescent mice after 30 mg/kg amphetamine. Seven daily injections of cocaine resulted in a heightened behavioral response on day 7 as compared to day 1, indicative of behavioral sensitization in adult and periadolescent, but not postweanling mice. Repeated methylphenidate resulted in increased total activity in adult, but not periadolescent or postweanling mice. None of the animals were sensitized to the behavioral activating effects of amphetamine. The magnitude of behavioral response and the development of sensitization were dependent upon the age of the animal and the agent tested.

Cocaine-induced locomotor activity is increased by prior handling in adolescent but not adult female rats

Adolescence is a period of transition that is associated with increased levels of stress and a heightened propensity to initiate drug use. Neuronal development is still occurring during this transitional period, which includes the continued development of the dopamine system during the adolescent period. In the present study, the effects of pre-test handling on cocaine-induced locomotor activity were investigated among female adolescent and young adult rats upon presentation to a novel environment. On postnatal days (PND) 41-44 and 56-59 animals were handled (b.i.d.) in the colony room for 3 min. On PND 45 or PND 60, animals were removed from the colony room, weighed, and administered an acute injection of either cocaine or saline and presented to a novel environment where behavior was recorded for 30 min. Adolescent females (PND 45) that were handled prior to cocaine administration demonstrated elevated levels of cocaine-induced activity relative to their age-matched non-handled counterparts and also to their handled-adult counterparts. In contrast, among non-handled animals, young adults (PND 60) exhibited elevated drug-induced locomotion at several time points during the trial. Non-handled adolescent animals demonstrated the previously described "hyporesponsive" behavioral profile relative to their non-handled adult counterparts. The results from the present experiment indicate that adolescent animals may be more sensitive to basic laboratory manipulations such as pre-test handling, and care must be taken when utilizing adolescent animals in behavioral testing. Handling appears to be a sensitive manipulation in elucidating differences in cocaine-induced behavioral activation between ages.

Exposure to chronic stress increases the locomotor response to cocaine and the basal levels of corticosterone in adolescent rats

Repeated exposure to stress results in augmentation in the locomotor response to psychostimulant drugs. We investigated the locomotor response to a novel environment or cocaine [10 mg/kg, intraperitoneally (i.p.)] and basal corticosterone levels in male adolescent rats exposed to chronic restraint or variable stress. Animals in the chronic restraint group were restrained for 1 hour daily. The chronic variable stress protocol consisted of exposure to different stressors twice a day in random order. Chronic restraint and variable stress regimens began simultaneously on postnatal day (P) 25 and were applied for 10 days. During this period the control group was left undisturbed except for cleaning the cages. Three days after the last exposure to stress, cocaine- and novelty-induced locomotion were recorded in an activity cage. Plasma corticosterone levels were determined in a subset of stress and control animals. Exposure to both chronic restraint and variable stress increased cocaine-induced locomotion and basal corticosterone plasma levels, while no change was observed in the response to a novel environment. Moreover, rats exposed to variable stress displayed the greatest locomotor response following a challenge dose with cocaine when compared to control and chronic restraint stress groups. This observation indicates that the stress regimen is relevant to the degree of stress-induced sensitization to cocaine.

Behavioural and neuroendocrine effects of cannabinoids in critical developmental periods

The present article focuses on psychoneuroendocrine effects of cannabinoids in developing animals, with special emphasis on the perinatal, periweanling and periadolescent periods. We describe and discuss published data dealing with acute and long-term effects of exposure to cannabinoid agonists in such critical periods. Human studies have demonstrated that the consumption of marijuana by women during pregnancy affects the neurobehavioural development of their children. Investigations using animal models provide useful information for a better understanding of the long-lasting deleterious consequences of cannabis exposure during pregnancy and lactation. The increasing use of cannabis among adolescents and its associated public health problems have led to a parallel increase in basic research on appropriate animal models. Chronic administration of cannabinoid agonists during the periadolescent period causes persistent behavioural alterations in adult animals. Some of these alterations may be related to a possible increased risk of psychosis and other neuropsychiatric disorders in early onset cannabis users.

The differential impact of risk factors on illicit drug involvement in females

BACKGROUND

Initiation of drug use and progression to abuse/dependence involve complex pathways. Potential risk factors may correlate with initiation or progression or both. Are there risk factors that associate with illicit drug use or illicit drug abuse/dependence? Is the magnitude of the association the same for use and abuse/dependence? Does this pattern of association differ across categories of drugs?

METHODS

We used data from female-female adult twins to assess the association of 26 putative risk factors with use and abuse/dependence of six illicit psychoactive drugs. Drug involvement was represented by independent dichotomous outcomes and by a single ordinal variable. Odds ratios were obtained by logistic regression and a continuation ratio was used to test the magnitude of association.

RESULTS

Factors associate in similar patterns with different drug categories. Some associated factors interact only with initiation while others relate with both stages. There is a stronger association of significant socio-demographic factors with drug use while the psychiatric diagnoses are more strongly associated with progression to abuse/dependence.

CONCLUSIONS

Risk factors may be use-specific, abuse/dependence-specific or common to use and abuse/dependence. The trend of associations is similar across different illicit drugs. This suggests complex, interacting pathways that determine drug habits in individuals. These results are hypothesis-generating and future studies of causal relationships may draw from the outcomes presented in these analyses.

Stimulant actions in rodents: implications for attention-deficit/hyperactivity disorder treatment and potential substance abuse

Most evidence supports the continued use of stimulants as the best available pharmacotherapy for the treatment of children with attention-deficit/hyperactivity disorder (ADHD), but little is known about possible enduring behavioral and neuroadaptational consequences of long-term stimulant exposure. Although a variety of preclinical studies, particularly those using methylphenidate (MP), have attempted to address these issues, most of these studies have used procedures that might not adequately simulate clinical treatment conditions, and results have not been entirely consistent. In particular, the rationale for selection of MP doses that simulate clinical exposure has not been well defined. We suggest that the use of more appropriate treatment conditions, including doses that result in plasma drug levels comparable to therapeutic levels, will provide a more accurate model for adequately assessing the therapeutic mechanisms and potential long-term consequences of stimulant psychotherapy in the treatment of ADHD.

Differential behavioral and neuroendocrine effects of repeated nicotine in adolescent and adult rats

Despite the high prevalence of tobacco abuse among adolescents, the neurobiology of nicotine addiction has been studied mainly in adult animals. Repeated administration of this drug to adult rats induces behavioral sensitization. Nicotine activates the HPA axis in adult rats as measured by drug-induced increases in ACTH and corticosterone. Both behavioral sensitization and corticosterone are implicated in drug addiction. We examined the expression of behavioral sensitization induced by nicotine as well as the changes in corticosterone levels after repeated injections of nicotine in adolescent and adult animals. Adolescent and adult rats received subcutaneous (s.c.) injections of saline or 0.4 mg/kg of nicotine once daily for 7 days. Three days after the last injection animals were challenged with saline or nicotine (0.4 mg/kg; s.c.). Nicotine-induced locomotion was recorded in an activity cage. Trunk blood samples were collected in a subset of adolescent and adult rats and plasma corticosterone levels were determined by radioimmunoassay. Adult, but not adolescent, rats expressed behavioral sensitization. Pretreatment with nicotine abolished corticosterone-activating effect of this drug only in adult animals, indicating the development of tolerance at this age. Our results provide evidence that adolescent rats exposed to repeated nicotine display behavioral and neuroendocrine adaptations distinct from that observed in adult animals.

The distribution of cocaine in mice differs by age and strain

Few studies have examined the influence of the age and the strain of mouse on the pharmacokinetics of psychostimulants, or the role of pharmacokinetics in age-related differences in drug responses. The present study compared concentrations of cocaine, and its metabolite, benzoylecgonine (BZE), in the blood and brain of early (P35) and later (P42) periadolescent and adult (P63) CD-1 and C57BL/6 male mice 15 min after acute intraperitoneal injection of cocaine (20 mg/kg). Brain levels of cocaine and BZE after seven daily cocaine injections in CD-1 and C57BL/6 mice beginning on P35 and on P63 were also measured. P35 periadolescents of both strains had lower blood cocaine levels than did the adults, but only C57BL/6 periadolescents had lower brain cocaine levels than the adults. C57BL/6 mice of both ages had higher blood cocaine levels than did the corresponding CD-1 mice. Concomitant with lower cocaine levels, periadolescent CD-1 mice had higher blood BZE levels than the adults, suggesting that periadolescents may metabolize cocaine faster. Brain cocaine levels in P42 C57BL/6 mice were similar to those of adults. Cocaine-induced activity did not differ between periadolescent and adult CD-1 mice after a single injection of cocaine, whereas periadolescent C57BL/6 mice had lower activity levels than did the adults after a single cocaine injection. Periadolescent CD-1 mice exhibited higher levels of locomotor activity following cocaine injection than did periadolescent C57BL/6 mice. Following chronic cocaine administration, cocaine and BZE levels in the brains of periadolescent and adult mice did not differ from each other in either strain. However, brain cocaine levels at both ages were lower in CD-1 mice than in C57BL/6 mice. In conclusion, the age and the strain of mouse significantly affect the levels of cocaine obtained in brain and blood following acute administration. Our data are consistent with the notion that CD-1 and C57BL/6 mice metabolize cocaine faster during the early periadolescent period than as adults. Furthermore, potentially important strain differences between CD-1 and C57BL/6 mice were noted in cocaine levels following acute and chronic cocaine administration, and in locomotor activity following acute cocaine administration.

Effects of acute bupropion administration on locomotor activity in adolescent and adult mice

Responses to some psychoactive substances seem to differ between adolescents and adults. Bupropion, an antidepressant which is also used for smoking cessation, induces a dose-dependent increase in locomotor activity in adult mice, although its behavioral actions in adolescents have not been evaluated. In the present study the effects of acute bupropion administration (5, 10, 15 and 20 mg/kg) on locomotor activity were examined in early adolescent (postnatal day (pnd): 29-31 days), late adolescent (pnd: 47-49 days) and adult (pnd > 70 days) male NMRI mice, using an infrared photocell system. Locomotion was recorded for a total period of 90 min. Results indicated that there were significant differences in motor activity counts between the three ages evaluated, with late adolescents being more active than early adolescents. Bupropion (at doses 20, 15 and 10 mg/kg) induced a significant increase in locomotion, but there was no significant interaction between age and treatment. This suggests that the locomotor-stimulating effects of bupropion can be observed at different ages (early adolescence, late adolescence and adulthood), although the detailed analysis of the temporal course of locomotion changes induced by different bupropion doses reflected some differences between ages. The lowest dose (5 mg/kg) failed to induce hyperactivity in either adolescent or adult mice.

Handling alters cocaine-induced activity in adolescent but not adult male rats

The developmental period of adolescence is one that is characterized by increased levels of stress and vulnerability to drugs. Pre-test handling is an experimental manipulation that is used to acclimate animals prior to behavioral testing and exposure to a novel environment. Therefore, the present study was conducted in order to address the issue of pre-test handling of adolescent and adult male rats on subsequent cocaine-induced locomotor activity upon presentation to a novel environment. On days one through four, postnatal day (PND) 41-44 or PND 56-59, respectively, animals were handled b.i.d. for three minutes. On the fifth day, PND 45 or PND 60, animals were administered 30 mg/kg/ip cocaine or saline and immediately placed in a novel environment where locomotor activity was measured for 30 minutes. Cocaine increased locomotor activity similarly in all non-handled animals, regardless of age. Interestingly, adolescent animals expressed a differential effect when handled prior to an acute cocaine administration. Specifically, handling increased cocaine-induced locomotor activity in adolescent but not adult animals. These findings indicate that adolescent males that have been acclimated to the handling procedure experience significantly more behavioral reactivity than do adults to a high dose of cocaine upon exposure to a novel environment.

Aspects of spatial memory and behavioral disinhibition in Tg2576 transgenic mice as a model of Alzheimer’s disease

Transgenic mouse models of Alzheimer's disease (AD) have been recently advanced. Tg2576 mice have been shown to develop progressive beta-amyloid (Abeta) neuritic plaques and exhibit impairment of cognitive function. The aim of this study was a better characterization of different aspects of spatial memory performance of transgenic mice, observed at a time when levels of soluble Abeta are elevated and Abeta neuritc plaques start to appear. A general elevation of basal locomotory activity in the home cage was found in Tg2576 mice, which also exhibited an impairment of spontaneous alternation in the Y-maze test. Tg2576 mice were not flexible upon changes in the schedule and failed to codify spatially the testing environment. Consistently, a deficit of spatial memory was also observed when mice were assessed for levels of reactivity to spatial change in the modified open-field test with objects. Compared to controls, Tg2576 mice also exhibited an increased number of explorative approaches to the different objects, and failed to discriminate the displacement of the object. Consistently with the hypothesis of increased disinhibition, a differential behavioural response to the plus-maze paradigm was exhibited by Tg2576 mice. Results clearly indicate that Tg2576 mice are characterized by a number of specific behavioral cognitive alterations, compatible with Alzheimer's disease (AD), which make them a suitable animal model for testing of novel anti-AD drugs.

Beneficial effects of enriched environment on adolescent rats from stressed pregnancies

The capacity of an early environmental intervention to normalize the behavioural and immunological dysfunctions produced by a stressed pregnancy was investigated. Pregnant Sprague-Dawley rats underwent three 45-min sessions per day of prenatal restraint stress (PS) on gestation days 11-21, and their offspring were assigned to either an enriched-environment or standard living cages throughout adolescence [postnatal days (pnd) 22-43]. Juvenile rats from stressed pregnancies had a prominent depression of affiliative/playful behaviour and of basal circulating CD4 T lymphocytes, CD8 T lymphocytes and T4/T8 ratio. They also showed increased emotionality and spleen and brain frontal cortex levels of pro-inflammatory interleoukin-1beta (IL-1beta) cytokine. A more marked response to cyclophosphamide (CPA: two 2 mg/kg IP injections) induced immunosuppression was also found in prenatal stressed rats. Enriched housing increased the amount of time adolescent PS rats spent in positive species-typical behaviours (i.e. play behaviour), reduced emotionality and reverted most of immunological alterations. In addition to its effects in PS rats, enriched housing increased anti-inflammatory IL-2 and reduced pro-inflammatory IL-1beta production by activated splenocytes, also producing a marked alleviation of CPA-induced immune depression. In the brain, enriched housing increased IL-1beta values in hypothalamus, while slightly normalizing these values in the frontal cortex from PS rats. This is a first indication that an environmental intervention, such as enriched housing, during adolescence can beneficially affect basal immune parameters and rats response to both early stress and drug-induced immunosuppression.

MAO-B knockout mice exhibit deficient habituation of locomotor activity but normal nicotine intake

Low levels of monoamine oxidase-B (MAO-B) activity, such as those observed in smokers, are also associated with behavioral traits such as a heightened responsiveness to novelty. However, the exact mechanism by which low MAO-B activity influences smoking and heightened responsiveness to novelty is still poorly understood. We used MAO-B knockout (KO) mice to test the hypothesis that MAO-B concomitantly affects locomotor responses in a novel inescapable open field and nicotine intake. Male wild-type (WT) and MAO-B KO mice were placed in an inescapable open field and their horizontal locomotor activity was measured for 30 min per day for 5 days. MAO-B KO mice exhibited impaired within-session habituation of locomotor activity, as compared to WT mice. Separate groups of male WT and MAO-B KO mice were individually housed in their home cages with two water bottles. One of the bottles contained tap water and the other contained nicotine (0, 3.125, 6.25, 12.5, 25, 50 or 100 micro g/ml). The total amount of water and nicotine solution consumed was measured every three days for 16 days. MAO-B KO mice and WT mice consumed equal amounts of nicotine and exhibited comparable concentration-dependent nicotine preference and aversion over a period of 16 days. The data suggest that the absence of MAO-B impairs the ability of mice to habituate in the inescapable environment, but does not alter their nicotine intake.

Adolescent exposure to cannabinoids induces long-lasting changes in the response to drugs of abuse of rat midbrain dopamine neurons

BACKGROUND

Recent studies have raised concerns about subtle long-lasting neurobiological changes that might be triggered by exposure to Cannabis derivatives, especially in a critical phase of brain maturation, such as puberty. The mesolimbic dopamine (DA) system, involved in the processing of drug-induced reward, is a locus of action of cannabinoids and endocannabinoids. Thus, we compared the effects of repeated cannabinoid administration in adolescent and adult rats on DA neuronal functions and responses to drugs of abuse.

METHODS

Single-unit extracellular recordings from antidromically identified mesoaccumbens DA neurons and from their target cells in the nucleus accumbens were carried out in urethane-anesthetized rats. Animals were pretreated during adolescence or adulthood, for 3 days, with the cannabinoid agonist WIN55212.2 (WIN) or vehicle and allowed a 2-week interval.

RESULTS

In cannabinoid-administered rats, DA neurons were significantly less responsive to the stimulating action of WIN, regardless of the age of pretreatment; however, in the adolescent group, but not in the adult, long-lasting cross-tolerance developed to morphine, cocaine, and amphetamine.

CONCLUSIONS

Our study suggests that an enduring form of neuronal adaptation occurs in DA neurons after subchronic cannabinoid intake at a young age, affecting subsequent responses to drugs of abuse.

Sex differences in voluntary oral nicotine consumption by adolescent mice: a dose-response experiment

Recent studies with adolescent rodents offer valuable information regarding the neurochemical and behavioral effects of adolescent nicotine exposure. One hundred twenty-one male and 125 female adolescent (35 days of age) C57BL/6J mice were tested for voluntary nicotine consumption by providing 24-h access to both saccharin-only (SAC) and one of six nicotine-containing solutions [10, 25, 50, 75, 100, 200 ug (-)-freebase nicotine/ml in 2% SAC] in the home cage for 7 days. Although males and females drank similar volumes (ml) of nicotine, the female mice consumed more nicotine adjusted for body weight (mg/kg) and as a percentage of total fluid intake than did the male mice. In contrast, there was no sex difference in overall serum cotinine levels (adjusted for liver weight). For all mice, nicotine consumption and serum cotinine levels increased in a dose-dependent manner, and the volume of nicotine intake (ml), percent nicotine intake, and nicotine dosage (mg/kg) on the last day of the experiment were positively correlated with cotinine levels. Cotinine levels were inversely related to body weight only for females. Sex differences in nicotine consumption, but not in cotinine levels, suggest sex differences in pharmacokinetic processes that may contribute to oral nicotine consumption behavior during periadolescence.

Prenatal stress affects 3,4-methylenedioxymethamphetamine pharmacokinetics and drug-induced motor alterations in adolescent female rats

We examined the influence of prenatal stress on 3,4-methylenedioxymethamphetamine (MDMA, 5 mg/kg p.o.) pharmacokinetics in adolescent female SD rats (30 days). Our results indicate that the metabolic rate of MDMA was higher in the prenatal stress group than in the control group. Moreover, MDMA-induced motor alterations were increased in prenatally stressed rats. These findings provide evidence that (i) prenatal stress increases sensitivity to MDMA, (ii) these effects are already detectable at the adolescent stage and (iii) early differences in metabolism may play a role in the behavioural changes associated with this drug of abuse.

Environmental enrichment during adolescence reverses the effects of prenatal stress on play behaviour and HPA axis reactivity in rats

Prenatal stress (PS) can produce profound and long-lasting perturbations of individual adaptive capacities, which in turn can result in an increased proneness to behavioural disorders. Indeed, in PS rats there is evidence of impaired social play behaviour, disturbances in a variety of circadian rhythms, enhanced anxiety and increased hypothalamic-pituitary-adrenal (HPA) axis reactivity. This study was designed to experimentally investigate the degree of reversibility of PS-induced disturbances of social play and HPA reactivity by assessing the effect of the enrichment of the physical environment on PS rats during periadolescence. PS subjects showed a reduced expression of social play behaviour and a prolonged corticosterone secretion in response to restraint stress, but both these effects were markedly reversed following environmental enrichment. Interestingly, the enrichment procedure increased social behaviour but had no effect on corticosterone secretion in nonstressed animals, indicating a differential impact of the postnatal environment as a function of prenatal background. As a whole, results clearly indicate that rats prenatally exposed to stress can benefit during periadolescence from the modulatory effects of an enriched environment. Moreover, they confirm that PS may well represent a suitable animal model for the design and testing of new therapeutic strategies for behavioural disorders produced by early insults.

Chronic nicotine differentially alters cocaine-induced locomotor activity in adolescent vs. adult male and female rats

Tobacco use is prevalent in the adolescent population. It is a major concern because tobacco is highly addictive and has also been linked to illicit drug use. There is not much research, however, on the interaction between nicotine and other stimulant drugs in animal models of early adolescence. This study examined the effects of chronic nicotine alone and on cocaine-stimulated activity in male and female periadolescent rats compared to male and female adult rats. During the seven-day nicotine pretreatment period, nicotine increased locomotor activity in all groups compared to vehicle controls. Male and female adult rats and female periadolescent rats developed sensitization to the locomotor-activating effects of nicotine over the 7-day treatment period, while male periadolescent rats did not. All groups treated with nicotine, however, exhibited sensitization to nicotine-induced repetitive motion over the 7-day nicotine treatment period. On day 8, male periadolescent rats pretreated with nicotine were more markedly sensitized to the locomotor-activating effects of cocaine than male adult rats, while female rats pretreated with nicotine were not sensitized to cocaine. In contrast, male and female periadolescent rats, but not adult rats, had increased amounts of repetitive beam breaks induced by cocaine after nicotine pretreatment. Overall, it appears that cross-sensitization to cocaine is greater in periadolescent than in adult rats, and that males are more sensitized than females. Thus, it may be that nicotine use during adolescence carries a greater risk than during adulthood and that male adolescents may be particularly vulnerable to the risk of cocaine abuse after nicotine use. This information should be taken into account so as to help us better understand the development of drug addiction in adolescents compared to adults.

Characterizing the ontogeny of ethanol-associated increases in corticosterone

The following experiments were conducted as adjuncts to previous work in an effort to characterize the ontogenetic profile of the elevations in corticosterone after ethanol challenge. In Experiment 1, female and male Sprague-Dawley rats were administered intraperitoneally either a 1.5- or a 4.5-g/kg dose of ethanol on postnatal day (PND) 16, 26, 36, or 56. Blood samples were collected at 40, 80, or 160 min after ethanol injection and analyzed by means of radioimmunoassay for corticosterone levels and correlated with brain alcohol levels (BrALs) determined from brain samples collected at the same time intervals. In Experiment 2, the ethanol dose was varied ontogenetically to equate functional impairment across age, with the use of intraperitoneal doses of ethanol of 3.2, 2.6, or 2.2 g/kg, to induce equivalent amounts of ethanol-induced motor impairment in infant (PND 22), adolescent (PND 28), or adult (PND 60) rats, respectively. Animals were tested on a swim task 15 min after injection, with blood and brain samples collected immediately after the swim and analyzed for corticosterone levels and BrALs as in Experiment 1. Reminiscent of previous reports of an age-related increase in sensitivity to the hypnotic and motor-impairing effects of ethanol, the corticosterone response to an ethanol challenge increased at least through adolescence, with sex differences emerging by PND 26 and becoming more pronounced in adulthood. To the extent that corticosterone release is involved in the reinforcing effects of drugs, ontogenetic differences in the response of the hypothalamic-adrenal-pituitary (HPA) axis to ethanol could contribute to the excessive alcohol consumption often observed during adolescence.

The spontaneously hypertensive-rat as an animal model of ADHD: evidence for impulsive and non-impulsive subpopulations

Attention-deficit hyperactivity disorder (ADHD) is a neuropsychiatric syndrome, affecting human infants and adolescents. Two main behavioural features are reported: (1). impaired attention and (2). an impulsive-hyperactive behavioural trait. The latter has been studied in a series of experiments, using the spontaneously hypertensive-rat (SHR) strain (which is regarded as a validated animal model for ADHD) in operant tasks. Food-restricted SHRs and their Wistar-Kyoto (WKY) controls were tested during adolescence (i.e. post-natal days 30-45), in operant chambers provided with two nose-poking holes. Nose-poking in one hole (H1) resulted in the immediate delivery of a small amount of food, whereas nose-poking in the other hole (H5) delivered a larger amount of food after a delay, which was increased progressively each day (0-100 s). As expected, all animals showed a shift in preference from the large (H5) to the immediate (H1) reinforcer as the delay length increased. Impulsivity can be measured by the steepness of this preference-delay curve. The two strains differed in home-cage circadian activity, SHRs being more active than WKYs at several time-points. During the test for impulsivity, inter-individual differences were completely absent in the WKY strain, whereas a huge inter-individual variability was evident for SHRs. On the basis of the median value of average hole-preference, we found an 'impulsive' SHR subgroup, with a very quick shift towards the H1 hole, and a flat-slope ('non-impulsive') SHR subgroup, with little or no shift. The impulsive subpopulation also presented reduced noradrenaline levels in both cingulated and medial-frontal cortex, as well as reduced serotonin turnover in the latter. Also, cannabinoid CB1 receptor density resulted significantly lower in the prefrontal cortex of impulsive SHRs, when compared to both the non-impulsive subgroup and control WKYs. Interestingly, acute administration of a cannabinoid agonist (WIN 55,212, 2 mg/kg s.c.) normalized the impulsive behavioural profile, without any effect on WKY rats. Thus, two distinct subpopulations, differing for impulsive behaviour and specific neurochemical parameters, were evidenced within adolescent SHRs. These results support the notion that a reduced cortical density of cannabinoid CB1 receptors is associated with enhanced impulsivity. This behavioural trait can be positively modulated by administration of a cannabinoid agonist. Present results confirm and extend previous literature, indicating that adolescent SHRs represent a suitable animal model for the preclinical investigation of the early-onset ADHD syndrome.

Genetics and drug use as a complex phenotype

Drug use is a complex behavior influenced by multiple biological, family, and sociocultural factors. The concurrent use/misuse of multiple drugs is often seen and drug use also co-occurs with other psychiatric conditions. Behavior and molecular genetic studies support an important posited role of genes in drug use. This posited genetic risk does not appear to be conferred by one or two major genes manifesting large effects, but rather by a number of genes manifesting smaller effects. Genetic factors explain, on average, only about half of the total variability in drug use, with the remaining variability influenced by environmental factors. Also, genetic risk may be differentially expressed in the presence vs. absence of particular environmental conditions. Thus, investigation of environmental factors and their interaction with genetic risk is a necessary component of genetic research. While the full potential of genetic investigations for the prevention of drug misuse has yet to be realized, an example of the impact of risk factor modification under various conditions of gene-environment interaction is provided, and the implications for use of genetic information in drug-misuse prevention are discussed. The multifactorial nature of drug use necessitates coordinated investigation from multiple disciplines and timely dissemination of scientific findings. In addition, this work demands adherence to the highest standards of confidentiality and ethical use of genetic information to best inform future prevention efforts.

Novelty-seeking behavior and stress-induced locomotion in rats of juvenile period differentially related to morphine place conditioning in their adulthood

The relationship between motor responses in a novel environment and susceptibility to place conditioning effect of psychostimulants has been reported in adult rats. However, it is in question whether this correlation could be generalized to motor activity in rats of juvenile period and place conditioning effect in their adulthood for narcotic morphine. In the present study, we tested locomotor activity in an arena open-field and the subsequent novelty-seeking behavior after adaptation process in juvenile rats (P42) and morphine (2 mg/kg) place conditioning effect 56 days later in the same rats' adulthood (P98). Our results showed that rats with high response to novelty (HRN) spent more prolonged duration in the drug-paired compartment in the place conditioning test compared with their low response counterparts (LRN), with the latter group no salient change on this measure. Moreover, rats with high response to the open-field test (HRS) expressed equally elevated duration in drug-paired side relative to their low response counterparts (LRS). The present research demonstrated that novelty-seeking behavior and locomotor activity in the open-field in rats of juvenile period differentially related to morphine place conditioning in their adulthood, with slow acquisition of morphine place conditioning effect in LRN animals.

Methylphenidate treatment during pre- and periadolescence alters behavioral responses to emotional stimuli at adulthood

BACKGROUND

Methylphenidate (MPH) is a psychomotor stimulant medication widely used for the treatment of attention-deficit/hyperactivity disorder (ADHD). Given the extent of prescribed use of MPH, and because MPH interacts with the same brain pathways activated by drugs of abuse, most research has focused on assessing MPH's potential to alter an individual's risk for adult drug addiction. Data examining other potential long-term behavioral consequences of early MPH administration are lacking, however.

METHODS

We investigated the long-term behavioral consequences of chronic administration of MPH (2.0 mg/kg) during pre- and periadolescent development in adult rats by assessing their behavioral reactivity to a variety of emotional stimuli.

RESULTS

The MPH-treated animals were significantly less responsive to natural rewards such as sucrose, novelty-induced activity, and sex compared with vehicle-treated control animals. In contrast, MPH-treated animals were significantly more sensitive to stressful situations, showed increased anxiety-like behaviors, and had enhanced plasma levels of corticosterone.

CONCLUSIONS

Chronic exposure to MPH during development leads to decreased sensitivity to rewarding stimuli and results in enhanced responsivity to aversive situations. These results highlight the need for further research to improve understanding of the effects of stimulants on the developing nervous system and the potential enduring effects resulting from early-life drug exposure.

Adolescent exposure to methylphenidate alters the activity of rat midbrain dopamine neurons

BACKGROUND

Methylphenidate is commonly used to treat children and adolescents with attention-deficit/hyperactivity disorder. A health concern is its long-term effects with respect to later stimulant exposure. We reported that repeated exposure to a low dose of methylphenidate during adolescence increases self-administration of a low, typically nonreinforcing dose of cocaine in adult rats. We also showed that enhanced vulnerability to cocaine is associated with elevated impulse and bursting activity of midbrain dopamine neurons in drug-naïve adult rats and might constitute a substrate critically associated with abuse liability. Thus we sought to determine whether repeated exposure to low-dose methylphenidate in adolescence alters dopamine neuronal excitability in adulthood.

METHODS

After 3-day and 2-week withdrawal from repeated low-dose adolescent exposure to methylphenidate, we used extracellular single-unit recording in chloral hydrate-anesthetized rats to determine basal firing and bursting activity of midbrain dopamine neurons and dopamine autoreceptor sensitivity to the D2-class direct receptor agonist quinpirole.

RESULTS

Dopamine neuronal impulse activity was increased after 3 days and decreased after 2 weeks' withdrawal from methylphenidate given in adolescence. No difference between groups was evident with respect to autoreceptor sensitivity to quinpirole.

CONCLUSIONS

Adolescent exposure to methylphenidate induces neuronal changes associated with increased addiction liability in rats.

Compulsive use of dopamine replacement therapy in Parkinson's disease: reward systems gone awry?

Dopamine replacement therapy (DRT) is the most effective treatment for Parkinson's disease (PD); it provides substantial benefit for most patients, extends independence, and increases survival. A few patients with PD, however, take increasing quantities of medication far beyond those required to treat their motor disabilities. These patients demand rapid drug escalation and continue to request more DRT despite the emergence of increasingly severe drug-induced motor complications and harmful behavioural consequences. In this article we detail the features of compulsive DRT-seeking and intake in PD, in relation to theories of compulsive drug use.

"Laughing" rats and the evolutionary antecedents of human joy?

Paul MacLean's concept of epistemics-the neuroscientific study of subjective experience-requires animal brain research that can be related to predictions concerning the internal experiences of humans. Especially robust relationships come from studies of the emotional/affective processes that arise from subcortical brain systems shared by all mammals. Recent affective neuroscience research has yielded the discovery of play- and tickle-induced ultrasonic vocalization patterns ( approximately 50-kHz chirps) in rats may have more than a passing resemblance to primitive human laughter. In this paper, we summarize a dozen reasons for the working hypothesis that such rat vocalizations reflect a type of positive affect that may have evolutionary relations to the joyfulness of human childhood laughter commonly accompanying social play. The neurobiological nature of human laughter is discussed, and the relevance of such ludic processes for understanding clinical disorders such as attention deficit hyperactivity disorders (ADHD), addictive urges and mood imbalances are discussed.

Elevated levels of impulsivity and reduced place conditioning with d-amphetamine: two behavioral features of adolescence in mice

Human adolescents may have experience with easily available psychoactive drugs. Impulsivity and/or peculiarities in reward systems may play a role. These variables were studied in adolescent (Postnatal Day [PND] 30-49) and adult (PND > 60) CD-1 mice. In Experiment 1 (impulsivity), food-restricted mice were tested in operant chambers with 2 nose-poking holes that delivered 1 food pellet immediately or 5 pellets after a delay, respectively. Delay length was increased over days (0-100 s). Adolescent mice showed a shift to the left in the intolerance-delay curve, as well as enhanced demanding when nose-poking was not reinforced. In Experiment 2 (place conditioning with d-amphetamine at 0.0. 1.0, 2.0, 3.3, or 5.0 mg/kg for 3 days), adolescent mice showed no reliable evidence of place conditioning when compared with adults. Hence, 2 main features of adolescence were elevated impulsivity and restlessness, and low (or absent) rewarding efficacy of amphetamine.

Differential associations of sex and D2 dopamine receptor (DRD2) genotype with negative affect and other substance abuse risk markers in children of alcoholics

Children of alcoholics have increased risk for substance abuse problems. Self-medication of negative affect may be one developmental path to future substance abuse. Because the 146 young (adolescent) children of alcoholics in the current sample had not used enough abused substances to study substance use directly, the relation of substance abuse risk markers to negative affect was assessed. Because the D2 dopamine receptor (DRD2) A1 allele has been associated with alcoholism and other substance use disorders, negative affect, measured by the Beck Depression Inventory (BDI), was determined in four groups of children: boys and girls with the A1+ allele (A1A1 and A1A2 genotypes) and with the A1- allele (A2A2 genotype). The other risk markers were stress, low amplitude of the P300 evoked potential, poor visuospatial functioning, novelty seeking (NS), and harm avoidance (HA). Stress was correlated with BDI scores in all groups. In contrast, low P300 was associated with BDI scores only in boys with the A1+ allele (P = .04), NS was associated with BDI scores only in girls with the A1+ allele (P = .02), and HA was associated with BDI scores only in boys with the A1- allele (P = .01). In addition, boys with the A1+ allele had lower BDI (P = .05) and HA (P = .005) scores than the respective scores for boys with the A1- allele. Girls with the A1- allele had lower HA scores compared with scores for boys with the A1- allele (P = .02). Girls with the A1+ allele had lower visuospatial functioning than that of boys with the A1+ allele (P<.001). Results indicate that both sex and DRD2 genotype modify associations between negative affect and other substance abuse risk markers.

Evidence for enhanced neurobehavioral vulnerability to nicotine during periadolescence in rats

Epidemiological studies indicate that there is an increased likelihood for the development of nicotine addiction when cigarette smoking starts early during adolescence. These observations suggest that adolescence could be a "critical" ontogenetic period, during which drugs of abuse have distinct effects responsible for the development of dependence later in life. We compared the long-term behavioral and molecular effects of repeated nicotine treatment during either periadolescence or postadolescence in rats. It was found that exposure to nicotine during periadolescence, but not a similar exposure in the postadolescent period, increased the intravenous self-administration of nicotine and the expression of distinct subunits of the ligand-gated acetylcholine receptor in adult animals. Both these changes indicated an increased sensitivity to the addictive properties of nicotine. In conclusion, adolescence seems to be a critical developmental period, characterized by enhanced neurobehavioral vulnerability to nicotine.

Dietary restriction and nicotine can reduce anxiety in female rats

Anxiety may play a role in the initiation of smoking and there is evidence to suggest that sex and age may predetermine responses to nicotine. At present, the greatest increase in smoking is in women and it is often accompanied by dieting. The purpose of the present study was to investigate how the impact of dietary restriction might modify the effects of nicotine in female adult and adolescent rats. The effects of nicotine in the elevated plus-maze test of anxiety were compared in free-feeding animals and those subjected to dietary restriction that reduced body weight to 85% of free-feeding weight. In nondeprived adult females, nicotine (0.05-0.5 mg/kg, s.c.) reduced the percentage of time spent on the open arms, indicating anxiogenic effects. However, the effects of nicotine were dramatically changed in food-restricted adult females and 0.05 mg/kg had a striking anxiolytic effect. No significant effects of nicotine were found in the adolescent female rats, suggesting a role of circulating sex hormones in modulating nicotine's effects on anxiety. However, in the adolescent females, dietary restriction significantly increased the percentages of time spent and entries onto the open arms, without changing closed arm entries, indicating an anxiolytic effect. These results raise the important possibility that, in prepubertal girls, dietary restriction may have anxiolytic effects and this might contribute to the onset of anorexia. Circulating female hormones reduce this effect, but in adult females the combination of dietary restriction and nicotine may have important anxiolytic effects that impact on the initiation of regular smoking.

Developmental neurocircuitry of motivation in adolescence: a critical period of addiction vulnerability

OBJECTIVE

Epidemiological studies indicate that experimentation with addictive drugs and onset of addictive disorders is primarily concentrated in adolescence and young adulthood. The authors describe basic and clinical data supporting adolescent neurodevelopment as a biologically critical period of greater vulnerability for experimentation with substances and acquisition of substance use disorders.

METHOD

The authors reviewed recent literature regarding neurocircuitry underlying motivation, impulsivity, and addiction, with a focus on studies investigating adolescent neurodevelopment.

RESULTS

Adolescent neurodevelopment occurs in brain regions associated with motivation, impulsivity, and addiction. Adolescent impulsivity and/or novelty seeking as a transitional trait behavior can be explained in part by maturational changes in frontal cortical and subcortical monoaminergic systems. These developmental processes may advantageously promote learning drives for adaptation to adult roles but may also confer greater vulnerability to the addictive actions of drugs.

CONCLUSIONS

An exploration of developmental changes in neurocircuitry involved in impulse control has significant implications for understanding adolescent behavior, addiction vulnerability, and the prevention of addiction in adolescence and adulthood.

Animal models of periadolescent substance abuse

Use of addictive substances by human juveniles and adolescents is common, including use to intoxication and progression from one drug to other drugs. Until quite recently, animal studies have not addressed the periadolescent period as a time of special vulnerability to substance abuse. For ethanol (EtOH), the most studied drug in periadolescent animals, it has become clear that effects of alcohol are similar in some ways, but different in others, compared to the effects seen in adult animals. Sparse data suggest that this conclusion may apply to other drugs as well. Recent work in rats indicates that periadolescent substance use may disrupt normal pubertal development, and may induce stronger effects on systems subserving plasticity and cognition than in adults. Animal data also indicate that some drugs may produce altered subsequent sensitivity to the same or a different drug, changes in adult cognitive capabilities, and even CNS damage. It is now clear that there can be no presumption that all studies of abusable substances carried out in adult animals will generalize readily to periadolescents. Some data suggest that continuing developmental changes in receptor expression may underlie age-related changes in drug effects. However, the biological characteristics of periadolescence which predispose toward consumption to intoxication, and the mechanisms underlying drug progression and persisting drug effects, still remain to be well defined.

Intrauterine position has long-term influence on brain mu-opioid receptor density and behaviour in mice

In multiparous rodents, a naturally occurring variation in degree of exposure to sex steroids during the prenatal phase of sexual differentiation derives from the in-utero proximity to opposite sex foetuses. So far, the studies on intrauterine position (IUP) phenomenon have mostly focused on traits relating to reproduction and behaviour, while its influence on neurochemical substrates and pharmacological response has been largely unexplored. We investigated possible variations in the function and the profile of expression of the mu-opioid receptor system in three groups of adult mice from known IUP: 2M mice (located between two males), 0M (between two females), and 1M (between a male and a female). Autoradiographic study revealed in female mice that proximity to at least a male in utero (1M and 2M position) resulted associated at adulthood with an increased density of midbrain mu-opioid receptors. Behavioural observations were conducted following injection with the specific mu-opioid agonist Fentanyl (at 0, 0.01 or 0.05 mg/kg IP). A drug-conditioned place preference test confirmed that 1M and 2M subjects were also more sensitive to the rewarding effects of the drug, since mice spent significantly more time in the drug-paired compartment than 0M subjects. In a hot-plate test, 2M subjects showed levels of drug-induced analgesia that were much higher than other IUP groups. No reliable differences were observed between the IUP groups for locomotor activity upon drug treatment. Overall, these data indicate for the first time that the organisation of the mu-opioid receptor system in the brain, as well as a differential vulnerability to abuse of opiate drugs can be modulated by epigenetic variables such as the prenatal in utero contiguity to male foetuses.

Adolescent and adult male rats differ in sensitivity to nicotine's activity effects

More than 90% of cigarette smokers begin smoking during adolescence, suggesting that adolescents may be particularly vulnerable to nicotine's effects. This experiment examined: (1) nicotine's acute effects on locomotion in adolescent and adult male Sprague-Dawley rats (Drug Phase I); (2) the effects of age of initial nicotine exposure on locomotion when nicotine was not administered (Interim Phase); and (3) the effects of age of initial nicotine exposure on later responses to nicotine (Drug Phase II). In Drug Phase I, animals were administered 0, 0.01, 0.10, 0.50, or 1.0 mg/kg nicotine sc for 12 days and horizontal activity was measured daily. During the Interim Phase, activity was measured but nicotine was not administered. During Drug Phase II, animals were administered the same nicotine dosages as in Drug Phase I for 12 days, and activity was measured daily. Drug Phase I revealed dose-response differences between adolescents and adults such that adolescents exhibited peak activity at both the 0.50- and 1.0-mg/kg dosages, but adults exhibited peak activity at the 0.50-mg/kg dosage. Initial nicotine exposure in adolescence (0.50 and 1.0 mg/kg), but not in adulthood, resulted in hyperactivity in adulthood in the absence of nicotine (Interim Phase). Reexposure to nicotine when all animals were adults (Drug Phase II) revealed that initial nicotine exposure in adolescence compared to adulthood resulted in dose-response differences in adulthood similar to those in Drug Phase I. In addition, animals initially exposed in adolescence exhibited sensitization to nicotine's activity-increasing effects in adulthood. These findings suggest that there are age differences in nicotine sensitivity that could predispose individuals initially exposed to nicotine in adolescence to long-term smoking.

Ontogenesis of behavioral sensitization and conditioned place preference induced by psychostimulants in laboratory rodents

The present review deals mainly with the ontogenesis of two important phenomena involved in vulnerability to several neuropsychiatric disorders, namely with drug-induced sensitization (both contextual and non-contextual) and with conditioned place preference. The term 'infancy' covers the first three postnatal weeks during development in rats and mice. Conversely, the term 'adolescence' may cover the whole postnatal period ranging from weaning (PND 21) to adulthood (at least PND 60) or specifically the period around the onset of puberty (animals aged 33-44 days). Recent studies in rats demonstrated that the establishment of a context-dependent sensitization appears during the first (for repeated drug administration) or during the second (for a single drug administration) postnatal week. However, the memory of drug-context association is transient in developing pups (lasting one or two days following the drug pretreatment). The long-term retention of drug-context associations matures progressively, and is complete by the third week of postnatal life. Finally, those mechanisms responsible for an adult-like profile of context-independent pharmacological sensitization appear later during ontogenesis, being mature by the fourth week of postnatal life. Another set of experiments extended this ontogenetic characterization by comparing adolescent and adult mice. When compared to the latter, the former subjects exhibit a greater amphetamine-induced locomotor sensitization, almost no sensitization of aversive stereotyped behaviors, and a less marked place conditioning. The strength of the drug-induced place conditioning was also directly compared with the unconditioned novelty-seeking drive. In conclusion, neonatal rats are able to show a relatively short-lasting retention of sensitized drug effects (short-term sensitization), whereas the ability to exhibit relatively long-lasting sensitized effects matures progressively during infancy (long-term sensitization). On the other hand, adolescent mice show a reduced sensitization of drug-induced psychotic symptoms, together with a more marked sensitization of arousing and euphorigenic properties of the drug and a reduced incentive memory of its hedonic effects. These age-related changes do imply very different degrees of vulnerability to drug addiction and several other neuropsychiatric disorders.

Parallel developmental trajectories of sensation seeking and regular substance use in adolescents

This study applied piecewise latent growth modeling to longitudinal survey data from 2 different samples of adolescents (N=1,002 and N=1,206) to examine the hypothesis that development of sensation seeking in middle school would predict development of substance use (cigarettes, alcohol, and marijuana) in middle school and high school. Results showed that sensation seeking had strong predictive value for both concurrent and distal marijuana and alcohol use in both samples; however, initial level of sensation seeking predicted initial level of cigarette use during high school in 1 sample only. White participants scored consistently higher on both initial level and rate of increase in sensation seeking than did participants of other ethnicities. Advantages of this methodology are discussed in the context of substance use research.

Risk-taking behavior in adolescent mice: psychobiological determinants and early epigenetic influence

Epidemiological research has emphasized that adolescence is associated with some temperamental and behavioral traits that are typical of this age and that might substantially contribute to both psychological and psychobiological vulnerability. The contribution of the important developmental rearrangements in neurobiological and neuroendocrinological processes has received surprisingly little investigation. The present review summarizes recent work in animal models, indicating that adolescent rodents exhibit marked peculiarities in their spontaneous behavioral repertoire. When compared to adults, adolescents show an unbalanced and 'extremes-oriented' behavior, consisting of an increased novelty seeking, together with decreased novelty-induced stress and anxiety, an increased risk-taking behavior in the plus-maze, as well as elevated levels of impulsivity and restlessness. Age-related discontinuities in the function of monoaminergic systems, which are a main target of abused drugs, can perhaps account for such a profile. In particular, a peculiar function within reward-related dopaminergic brain pathways actually seems to underlie the search for novel and rewarding sensations, as well as changes in the magnitude of psychostimulant effects. The role played by early epigenetic factors in the shaping of novelty-seeking behavior of adolescent and adult rodents are also reviewed. Two examples are considered, namely, subtle variations in the hormonal milieu as a function of intrauterine position and precocious or delayed maturation of nutritional independence as a function of changes in time of weaning. As for spontaneous drug consumption, a prominent vulnerability to the oral intake of nicotine during early adolescence is reported. In conclusion, adolescence in rodents may represent a suitable animal model with enough face- and construct-validity. Actually, this model is able to show behavioral features that resemble those found in human adolescents, including vulnerability to the consumption of psychoactive drugs.

Unconditioned and conditioned factors contribute to the 'reinstatement' of cocaine place conditioning following extinction in C57BL/6 mice

Relapse to drug use following prolonged periods of abstinence results, in part, from the ability of contextual cues paired previously with self-administered drug to elicit drug craving and -seeking behavior. Given the popularity of the mouse for the genetic analysis of drug-induced behaviors, a place conditioning model of drug-seeking behavior was used to examine the ability of cocaine (COC) to reinstate extinguished conditioned reward in mice. In a series of experiments, COC place conditioning was produced in male C57BL/6 (B6) mice by four pairings of COC (15 or 25 mg/kg, IP) with the non-preferred compartment of a two-compartment place conditioning apparatus. Following a post-conditioning test (Post-Test), place conditioning was extinguished by repeated testing. The mice were then challenged with one of five COC doses (0, 5, 10, 15 or 25 mg/kg, IP) and allowed free access to both environments. Following extinction, COC injections reinstated place conditioning to 100% or greater, relative to the Post-Test. In a control experiment, mice received either COC or SAL paired with non-preferred compartment and were then challenged with either COC (15 mg/kg, IP) or SAL on the Post-Test. COC-conditioned, but not SAL-conditioned, mice exhibited place conditioning when tested in a COC-free state. Interestingly, COC injection on the Post-Test elicited an increase in approach behavior in both SAL- and COC-conditioned mice and this increase was equivalent to that produced by COC conditioning alone. No direct relationships were observed between the magnitude of place conditioning and either COC-induced or -conditioned locomotor hyperactivity in the non-preferred compartment. Thus, at least two independent processes appear to underlie the ability of a COC injection to elicit approach behavior towards the non-preferred compartment of a biased place conditioning apparatus in mice-reactivation of the conditioned incentive motivational properties of COC-paired cues and elicitation of unconditioned behavioral disinhibition. One or both of these processes sensitizes with the passage of time, increasing the propensity of B6 mice to approach non-preferred environments upon COC re-administration.

Effects of the social conditions of housing through testing on cocaine-induced contextual sensitisation and conditioned locomotion in C57BL/6J mice

The potential differential effects of isolated and collective housing through the testing phase on sensitisation to cocaine-induced locomotion, the subsequent conditioned locomotion and the context-dependent expression of sensitisation were examined in C57Bl/6J male mice. Sensitisation was first generated in mice receiving seven once-daily subcutaneous injections of either saline or 10 mg/kg cocaine, before being placed in a testing chamber (singly) or in their home cage. On Day 8, mice were tested for conditioned locomotion (under saline). On Day 12, after three daily sessions of reinstatement of sensitisation, they were tested for contextual sensitisation (under cocaine). Whereas little or no effect of housing on the development of sensitisation was found, postsensitisation conditioned activity was significantly greater in isolation-kept mice (in comparison with the group-kept animals) and the likelihood of inducing a context-dependent expression of sensitisation was greater in grouped-housed mice. The results indicate that some of the aspects of contextual sensitisation can be influenced by the social conditions of lodging, not only when these conditions start several weeks before or during previous developmental periods of animals life (as previously published), but also when social isolation or social grouping are initiated after sexual maturity and applied through the period of treatments and testing. Moreover, the differential effect of social conditions of lodging on conditioned activity and contextual expression of sensitisation disagrees with the excitatory conditioning account of contextual sensitisation.

Exposure of adolescent rats to oral methylphenidate: preferential effects on extracellular norepinephrine and absence of sensitization and cross-sensitization to methamphetamine

Methylphenidate (MP) (ritalin) is widely used in the treatment of children and adolescents with attention deficit hyperactivity disorder, but little is known about therapeutic mechanisms or about possible consequences of long-term exposure. To more closely simulate the clinical use of the drug, we orally administered MP to adolescent rats during the dark-active phase of the circadian cycle at doses (0.75-3.0 mg/kg) below threshold for locomotor activation. We found that doses in this range increased extracellular norepinephrine in hippocampus without affecting dopamine in nucleus accumbens. These results suggest that norepinephrine systems may play an important role in the therapeutic action of this drug. To examine one potential consequence of long-term exposure to MP, i.e., the development of locomotor sensitization, an adaptational change that has been implicated in drug abuse liability, animals received three daily oral administrations of these doses of MP for up to 4 weeks through adolescence. The animals were then challenged with methamphetamine (0.5 mg/kg). We found that the behavioral response to MP did not change during the course of chronic treatment and that MP-pretreated animals did not exhibit a sensitized locomotor response to the methamphetamine challenge. We propose that, to the extent that this treatment protocol more closely reflects clinical exposure patterns, the relative insensitivity of accumbens dopamine to the acute administration of these MP doses, and the corresponding absence of evidence for the development of locomotor sensitization, supports one clinical view that there is little abuse liability associated with low dose, long-term MP treatment.

Acute and carryover effects in mice of MDMA ("ecstasy") administration during periadolescence

In spite of the increasing evidence concerning its neurotoxicity, young human individuals are often involved in the recreational use of amphetamine-type stimulants such as 3,4-methylenedioxymethamphetamine (MDMA or "ecstasy"). A study aimed to investigate short- and long-term consequences of a repeated and intermittent MDMA administration (0, 5 or 10 mg/kg i.p., 3 days treatment history) was conducted in mice. Mice were injected at different phases in development, namely at early (28 days old), middle (38 days old) or late (52 days old) adolescence. When assessed for nociceptive response, a dose-dependent analgesia was found in middle and late adolescent mice. Carryover consequences of previous MDMA treatment were then investigated at adulthood (80 days old). In a social interaction test, levels of environment exploration and social behaviour resulted markedly increased in drug-free state as a function of drug exposure during development, whereas others behaviours were reduced. MDMA challenge (5-mg/kg dose) produced the expected hyperactivity, as well as a marked increment of hypothalamic serotonin (5-hydroxyhyptamine, 5-HT) levels. Mice treated chronically with MDMA during middle and late adolescence were associated with important reductions of the indoleamine. As a whole, these results indicate a differential long-term vulnerability to behavioural and neurotoxicant effects of MDMA as a function of the developmental stage of exposure.