Amphetamine- and nicotine-induced cross-sensitization in adolescent rats persists until adulthood

The co-use of nicotine and prescription psychostimulants: A review of their behavioral and neuropharmacological interactions

BACKGROUND

Nicotine is commonly co-used with other psychostimulants. These high co-use rates have prompted much research on interactions between nicotine and psychostimulant drugs. These studies range from examination of illicitly used psychostimulants such as cocaine and methamphetamine to prescription psychostimulants used to treat attention deficit hyperactivity disorder (ADHD) such as methylphenidate (Ritalin™) and d-amphetamine (active ingredient of Adderall™). However, previous reviews largely focus on nicotine interactions with illicitly used psychostimulants with sparse mention of prescription psychostimulants. The currently available epidemiological and laboratory research, however, suggests high co-use between nicotine and prescription psychostimulants, and that these drugs interact to modulate use liability of either drug. The present review synthesizes epidemiological and experimental human and pre-clinical research assessing the behavioral and neuropharmacological interactions between nicotine and prescription psychostimulants that may contribute to high nicotine-prescription psychostimulant co-use.

METHODS

We searched databases for literature investigating acute and chronic nicotine and prescription psychostimulant interactions. Inclusion criteria were that participants/subjects had to experience nicotine and a prescription psychostimulant compound at least once in the study, in addition to assessment of their interaction.

RESULTS AND CONCLUSIONS

Nicotine clearly interacts with d-amphetamine and methylphenidate in a variety of behavioral tasks and neurochemical assays assessing co-use liability across preclinical, clinical, and epidemiological research. The currently available research suggests research gaps examining these interactions in women/female rodents, in consideration of ADHD symptoms, and how prescription psychostimulant exposure influences later nicotine-related outcomes. Nicotine has been less widely studied with alternative ADHD pharmacotherapy bupropion, but we also discuss this research.

Age- and Sex-Dependent Nicotine Pretreatment Effects on the Enhancement of Methamphetamine Self-Administration in Sprague-Dawley Rats

INTRODUCTION

Initiation of tobacco products typically occurs in adolescence. Adolescence is a critical period in development where the maturation of brain neurocircuitry is vulnerable to nicotine. Nicotine-containing products and psychostimulants, such as methamphetamine (METH), are often co-abused. Rodent studies have shown that nicotine exposure in early adolescence increases subsequent drug intake and reward. Given the exponential increase in e-cigarette use among adolescents, there is a pressing need to understand whether adolescent nicotine exposure impacts concurrent increased methamphetamine use. The objective of this study is to evaluate age, sex, and longitudinal effects of nicotine pretreatment on methamphetamine reinforcement.

METHODS

Male and female Sprague-Dawley rats were pretreated with a sub-chronic, low dose nicotine (2x, 30 μg/kg/0.1 mL, intravenous) or saline during early adolescence (postnatal days 28-31) or adulthood (postnatal days 86-89). Following nicotine pretreatment, on postnatal day 32 or postnatal day 90, animals underwent operant intravenous self-administration for methamphetamine (20 μg/kg/inf) over a 2-hour period for five consecutive days.

RESULTS

Early adolescent nicotine exposure enhances intravenous methamphetamine self-administration in male, but not female adolescents. Male adult rats self-administer methamphetamine over the 5-day testing period, independent of nicotine exposure. In contrast, nicotine exposure increases methamphetamine self-administration in female adults during the later sessions of the 5-day testing period.

CONCLUSIONS

Taken together, our data highlight age- and sex-dependent effects of low dose, sub-chronic nicotine pretreatment on subsequent intravenous methamphetamine self-administration.

IMPLICATIONS

A majority of polysubstance users begin smoking before the age of 18. Mounting evidence highlights adolescent susceptibility to nicotine exposure on brain and behavior. With the escalation in nicotine-containing products and stimulant use among adolescents, it is important to identify the consequences from adolescent nicotine use, including polysubstance use. Our study provides evidence that adolescent nicotine exposure enhances subsequent methamphetamine use, with important sex- and age-dependent effects.

Effects of nicotine exposure on oral methamphetamine self-administration, extinction, and drug-primed reinstatement in adolescent male and female rats

BACKGROUND

Adolescent nicotine exposure increases methamphetamine (MA) intake in adult male rats; however, little is known about how nicotine affects MA self-administration during the adolescent period. Therefore, we assessed whether exposing rats to nicotine during early or late adolescence affects oral MA self-administration.

METHODS

146 male and female rats were treated with saline or nicotine (0.16 or 0.64 mg/kg) from postnatal day (PD) 25-PD 34 (the early exposure phase) and/or PD 35-PD 55 (the late exposure phase). Rats began an oral MA self-administration procedure on PD 35.

RESULTS

Only the sex variable, but not nicotine, affected sucrose and MA acquisition, as female rats had more nose pokes than males during training. On the test sessions, female rats exposed to nicotine (0.64 mg/kg) in the early exposure phase had more active nose pokes than saline-treated female rats or nicotine-treated male rats. Rats exposed to nicotine (0.16 mg/kg) in the late exposure phase had fewer active nose pokes during testing than rats exposed to saline. Nose poke responding during extinction was not altered by nicotine exposure, but administering nicotine (0.16 or 0.64 mg/kg) to male rats in the early exposure phase did decrease nose pokes during the drug-primed reinstatement session.

CONCLUSIONS

Our results show that adolescent female rats are more sensitive to the reinforcing effects of oral sucrose and MA than adolescent males, and that preadolescent nicotine exposure enhances oral MA self-administration in female rats. These findings suggest that preteen nicotine use may increase vulnerability to later MA abuse in teenage girls.

Behavioral cross-sensitization between testosterone and fenproporex in adolescent and adult rats

The abuse of psychoactive drugs is considered a global health problem. During the last years, a relevant number of studies have investigated the relationship between anabolic-androgenic steroids (AAS) and other psychoactive drugs. AAS, such as testosterone, can cause a dependence syndrome that shares many features with the classical dependence to psychoactive substances. Pre-clinical evidence shows that there are interactions between testosterone and psychoactive drugs, such as cocaine. However, few studies have been performed to investigate the effect of repeated testosterone treatment on behavioral effects of amphetamine derivatives, such as fenproporex. The purpose of the present study was to investigate the effects of repeated testosterone administration on fenproporex-induced locomotor activity in adolescent and adult rats. Adolescent male Wistar rats were injected with testosterone (10 mg/kg sc for 10 days). After 3 days, animals received an acute injection of fenproporex (3.0 mg/kg ip) and the locomotor activity was recorded during 40 min. Thirty days later, the same animals received the same treatment with testosterone followed by a fenproporex challenge injection as described above. Our results demonstrated that repeated testosterone induced behavioral sensitization to fenproporex in adolescent but not in adult rats. These findings suggest that repeated AAS treatment might increase the dependence vulnerability to amphetamine and its derivatives in adolescent rats.

Making Sense of: Sensitization in Schizophrenia

Sensitization is defined as a process whereby repeated intermittent exposure to a given stimulus results in an enhanced response at subsequent exposures. Next to robust findings of an increased dopamine synthesis capacity in schizophrenia, empirical data and neuroimaging studies support the notion that the mesolimbic dopamine system of patients with schizophrenia is more reactive compared with healthy controls. These studies led to the conceptualization of schizophrenia as a state of endogenous sensitization, as stronger behavioral response and increased dopamine release after amphetamine administration or exposure to stress have been observed in patients with schizophrenia. These findings have also been integrated into the neurodevelopmental model of the disorder, which assumes that vulnerable neuronal circuits undergo progressive changes during puberty and young adulthood that lead to manifest psychosis. Rodent and human studies have made an attempt to identify the exact mechanisms of sensitization of the dopaminergic system and its association with psychosis. Doing so, several epigenetic and molecular alterations associated with dopamine release, neuroplasticity, and cellular energy metabolism have been discovered. Future research aims at targeting these key proteins associated with sensitization in schizophrenia to enhance the knowledge of the pathophysiology of the illness and pave the way for an improved treatment or even prevention of this severe psychiatric disorder.

Mechanisms and genetic factors underlying co-use of nicotine and alcohol or other drugs of abuse

Concurrent use of tobacco and alcohol or psychostimulants represents a major public health concern, with use of one substance influencing consumption of the other. Co-abuse of these drugs leads to substantial negative health outcomes, reduced cessation, and high economic costs, but the underlying mechanisms are poorly understood. Epidemiological data suggest that tobacco use during adolescence plays a particularly significant role. Adolescence is a sensitive period of development marked by major neurobiological maturation of brain regions critical for reward processing, learning and memory, and executive function. Nicotine exposure during this time produces a unique and long-lasting vulnerability to subsequent substance use, likely via actions at cholinergic, dopaminergic, and serotonergic systems. In this review, we discuss recent clinical and preclinical data examining the genetic factors and mechanisms underlying co-use of nicotine and alcohol or cocaine and amphetamines. We evaluate the critical role of nicotinic acetylcholine receptors throughout, and emphasize the dearth of preclinical studies assessing concurrent drug exposure. We stress important age and sex differences in drug responses, and highlight a brief, low-dose nicotine exposure paradigm that may better model early use of tobacco products. The escalating use of e-cigarettes among youth necessitates a closer look at the consequences of early adolescent nicotine exposure on subsequent alcohol and drug abuse.

Age-contingent influence over accumbal neurotransmission and the locomotor stimulatory response to acute and repeated administration of nicotine in Wistar rats

Nicotine addiction is one of the leading contributors to the global burden of disease, and early onset smokers report a more severe addiction with lower chance of cessation than those with a late onset. Preclinical research supports an age-dependent component to the rewarding and reinforcing properties of nicotine, and the aim of this study was to define behavioral adaptations and changes in accumbal neurotransmission that arise over 15 days of intermittent nicotine treatment (0.36 mg/kg/day) in rats of three different ages (5 weeks, 10 weeks, 36 weeks old). Repeated treatment increased the locomotor stimulatory response to nicotine in all age groups, but significantly faster in the two younger groups. In addition, nicotine decreased rearing activity in a way that sustained even after repeated administration in aged rats but not in the younger age groups. Electrophysiological field potential recordings revealed a decline in input/output function in the nucleus accumbens (NAc) of animals intermittently treated with nicotine starting at 5 weeks of age, but not in older animals. In drug naïve rats, acute administration of nicotine modulated both accumbal dopamine output and excitatory transmission in a partially age-dependent manner. Fifteen days of intermittent nicotine treatment did not alter the acute effect displayed by nicotine on dopamine levels or evoked field potentials. The data presented here show that both acute and repeated nicotine administration modulates accumbal neurotransmission and behavior in an age-contingent manner and that these age-dependent differences could reflect important neurobiological underpinnings associated with the increased vulnerability for nicotine-addiction in adolescents.

Emergence of sex differences in the development of substance use and abuse during adolescence

Substance use and abuse begin during adolescence. Male and female adolescent humans initiate use at comparable rates, but males increase use faster. In adulthood, more men than women use and abuse addictive drugs. However, some women progress more rapidly from initiation of use to entry into treatment. In animal models, adolescent males and females consume addictive drugs similarly. However, reproductively mature females acquire self-administration faster, and in some models, escalate use more. Sex/gender differences exist in neurobiologic factors mediating both reinforcement (dopamine, opioids) and aversiveness (CRF, dynorphin), as well as intrinsic factors (personality, psychiatric co-morbidities) and extrinsic factors (history of abuse, environment especially peers and family) which influence the progression from initial use to abuse. Many of these important differences emerge during adolescence, and are moderated by sexual differentiation of the brain. Estradiol effects which enhance both dopaminergic and CRF-mediated processes contribute to the female vulnerability to substance use and abuse. Testosterone enhances impulsivity and sensation seeking in both males and females. Several protective factors in females also influence initiation and progression of substance use including hormonal changes of pregnancy as well as greater capacity for self-regulation and lower peak levels of impulsivity/sensation seeking. Same sex peers represent a risk factor more for males than females during adolescence, while romantic partners increase risk for women during this developmental epoch. In summary, biologic factors, psychiatric co-morbidities as well as personality and environment present sex/gender-specific risks as adolescents begin to initiate substance use.

Cigarette Smoking During Substance Use Disorder Treatment: Secondary Outcomes from a National Drug Abuse Treatment Clinical Trials Network study

INTRODUCTION

The majority of patients enrolled in treatment for substance use disorders (SUDs) also use tobacco. Many will continue to use tobacco even during abstinence from other drugs and alcohol, often leading to smoking-related illnesses. Despite this, little research has been conducted to assess the influence of being a smoker on SUD treatment outcomes and changes in smoking during a treatment episode.

METHODS

In this secondary analysis, cigarette smoking was evaluated in participants completing outpatient SUD treatment as part of a multi-site study conducted by the National Drug Abuse Treatment Clinical Trials Network. Analyses included the assessment of changes in smoking and nicotine dependence via the Fagerström Test for Nicotine Dependence during the 12-week study among all smokers (aim #1), specifically among those in the experimental treatment group (aim #2), and the moderating effect of being a smoker on treatment outcomes (aim #3).

RESULTS

Participants generally did not reduce or quit smoking throughout the course of the study. Among a sub-set of participants with higher baseline nicotine dependence scores randomized to the control arm, scores at the end of treatment were lower compared to the experimental arm, though measures of smoking quantity did not appear to decrease. Further, being a smoker was associated with poorer treatment outcomes compared to non-smokers enrolled in the trial.

CONCLUSIONS

This study provides evidence that patients enrolled in community-based SUD treatment continue to smoke, even when abstaining from drugs and alcohol. These results add to the growing literature encouraging the implementation of targeted, evidence-based interventions to promote abstinence from tobacco among SUD treatment patients.

Adolescent brain maturation and smoking: what we know and where we're headed

Smoking is a leading cause of mortality and morbidity worldwide. Smoking initiation often occurs during adolescence. This paper reviews and synthesizes adolescent development and nicotine dependence literatures to provide an account of adolescent smoking from onset to compulsive use. We extend neurobiological models of adolescent risk-taking, that focus on the interplay between incentive processing and cognitive control brain systems, through incorporating psychosocial and contextual factors specific to smoking, to suggest that adolescents are more vulnerable than adults to cigarette use generally, but that individual differences exist placing some adolescents at increased risk for smoking. Upon smoking, adolescents are more likely to continue smoking due to the increased positive effects induced by nicotine during this period. Continued use during adolescence, may be best understood as reflecting drug-related changes to neural systems underlying incentive processing and cognitive control, resulting in decision-making that is biased towards continued smoking. Persistent changes following nicotine exposure that may underlie continued dependence are described. We highlight ways that interventions may benefit from a consideration of cognitive-neuroscience findings.

Nicotine exposure beginning in adolescence enhances the acquisition of methamphetamine self-administration, but not methamphetamine-primed reinstatement in male rats

BACKGROUND

Nicotine is commonly abused in adolescence and is believed to be a "gateway" to other drugs of abuse [e.g., methamphetamine (METH)]. The relationship between early nicotine exposure and later METH use is complicated because the majority of juvenile smokers continue to use cigarettes into adulthood. Thus, the present investigation examined the individual and combined contribution of adolescent and adult nicotine exposure on METH self-administration.

METHODS

Forty-three male rats were pretreated with saline or nicotine (0.16 or 0.64 mg/kg, SC) from postnatal day (PD) 35-50. On PD 51, subjects were split into the following groups: SAL-SAL, 0.16-0.16, 0.16-SAL, 0.64-0.64, and 0.64-SAL. Rats were then trained to lever press for METH (0.05 mg/kg) for seven days on an FR1 and seven days on an FR3 reinforcement schedule. After acquisition training, rats underwent 14 days of extinction and were then tested for METH-induced primed reinstatement (1.0mg/kg, IP).

RESULTS

Data showed that rats receiving continuous injections of the low dose of nicotine (0.16-0.16) obtained more METH infusions versus the control group (SAL-SAL) on an FR1 and FR3 schedule. In addition, rats on the FR3 schedule that received a low dose of nicotine during the adolescent period only (0.16-SAL) had more METH intake than the control group (SAL-SAL). Interestingly, the high dose of nicotine exposure had no effect on METH intake and neither nicotine dose altered METH seeking behavior.

CONCLUSIONS

Low dose exposure to nicotine during adolescence enhances the reinforcing effects of METH, while heavier exposure has no effect on METH intake.

The effects of paradoxical sleep deprivation on amphetamine-induced behavioral sensitization in adult and adolescent mice

Drug-induced behavioral sensitization (BS), paradoxical sleep deprivation (PSD) and adolescence in rodents are associated with changes in the mesolimbic dopaminergic system. We compared the effects of PSD on amphetamine-induced BS in adult and adolescent mice. Adult (90 days old) and adolescent (45 days old) Swiss mice were subjected to PSD for 48h. Immediately after PSD, mice received saline or 2.0mg/kg amphetamine intraperitoneally (i.p.), and their locomotion was quantified in activity chambers. Seven days later, all the animals were challenged with 2.0mg/kg amphetamine i.p., and their locomotion was quantified again. Acute amphetamine enhanced locomotion in both adult and adolescent mice, but BS was observed only in adolescent mice. Immediately after its termination, PSD decreased locomotion of both saline- and amphetamine-treated adolescent mice. Seven days later, previous PSD potentiated both the acute stimulatory effect of amphetamine and its sensitization in adolescent mice. In adult animals, previous PSD revealed BS. Our data suggest that adolescent mice are more vulnerable to both the immediate and long-term effects of PSD on amphetamine-induced locomotion. Because drug-induced BS in rodents shares neuroplastic changes with drug craving in humans, our findings also suggest that both adolescence and PSD could facilitate craving-related mechanisms in amphetamine abuse.

Nicotine administration in adolescence reprograms the subsequent response to nicotine treatment and withdrawal in adulthood: sex-selective effects on cerebrocortical serotonergic function

Nicotine exposure in adolescence produces lasting changes in subsequent behavioral responses to addictive agents. We gave nicotine to adolescent rats (postnatal days PN30-47), simulating plasma levels in smokers, and then examined the subsequent effects of nicotine given again in adulthood (PN90-107), focusing on cerebrocortical serotonin levels and utilization (turnover) as an index of presynaptic activity of circuits involved in emotional state. Our evaluations encompassed responses during the period of adult nicotine treatment (PN105) and withdrawal (PN110, PN120, PN130), as well as long-term changes (PN180). In males, prior exposure to nicotine in adolescence greatly augmented the increase in serotonin turnover evoked by nicotine given in adulthood, an interaction that was further exacerbated during withdrawal. The effect was sufficiently large that it led to significant depletion of serotonin stores, an effect that was not seen with nicotine given alone in either adolescence or adulthood. In females, adolescent nicotine exposure blunted or delayed the spike in serotonin turnover evoked by withdrawal from adult nicotine treatment, a totally different effect from the interaction seen in males. Combined with earlier work showing persistent dysregulation of serotonin receptor expression and receptor coupling, the present results indicate that adolescent nicotine exposure reprograms future responses of 5HT systems to nicotine, changes that may contribute to life-long vulnerability to relapse and re-addiction.

Short-term development of behavioral sensitization to amphetamine requires N-methyl-D-aspartate- and nicotinic-dependent mechanisms in the nucleus accumbens

Repeated administration of psychostimulant drugs, such as amphetamine, induces an enhanced behavioral response to subsequent drug challenge. This behavioral sensitization is proposed to model the increased drug craving observed in human psychostimulant abusers. Current thinking is that the ventral tegmental area, but not the nucleus accumbens, plays a critical role in the development of behavioral sensitization. Here, we report that the concomitant blockade of glutamatergic and nicotinic ionotropic receptors in the core of the nucleus accumbens blocks the development of behavioral sensitization to amphetamine and further abolishes the increase in extracellular dopamine release induced by amphetamine in the nucleus accumbens. These findings demonstrate that the development of behavioral sensitization to amphetamine depends, in addition to the well-known role of the ventral tegmental area, on glutamatergic and nicotinic-dependent mechanisms in the core of the nucleus accumbens and further indicate that the dopaminergic mesolimbic pathway must be viewed as a single coordinated system of critical importance in the development of behavioral sensitization to psychostimulant drugs.

Effects of environmental enrichment on nicotine-induced sensitization and cross-sensitization to d-amphetamine in rats

INTRODUCTION

Research indicates that adolescent nicotine exposure may predispose individuals to use other psychostimulants later in adulthood, offering support for the incentive-sensitization theory of addiction. Preclinical studies testing the incentive-sensitization theory show that repeated nicotine exposure in adolescent rats can lead to an increased sensitivity to the motor stimulant effects of nicotine and other psychostimulants in adulthood. Although previous nicotine exposure can increase sensitivity to stimulant drugs, rats raised in enriched conditions (EC) show, decreased sensitivity to psychostimulant drugs compared to rats raised in isolation conditions (IC).

METHODS

We examined whether nicotine sensitization or cross-sensitization to d-amphetamine induced by adolescent nicotine exposure is altered by exposure to environmental enrichment. Adolescent EC and IC male rats received subcutaneous (s.c.) injections of saline or 0.4mg/kg of nicotine once daily for seven days. Thirty-five days following the last nicotine injection EC and IC animals were challenged with saline, nicotine (0.2 or 0.4mg/kg) or d-amphetamine (0.5 or 1.0mg/kg).

RESULTS

EC rats failed to show nicotine sensitization at either nicotine dose tested while IC rats showed nicotine sensitization following the 0.4mg/kg nicotine dose. EC rats also failed to show nicotine-induced cross-sensitization to the 0.5mg/kg dose of d-amphetamine while IC rats displayed cross-sensitization. However, EC rats did exhibit nicotine-induced cross-sensitization to the 1.0mg/kg dose of d-amphetamine.

CONCLUSION

These findings indicate that environmental enrichment can alter the ability of adolescent nicotine exposure to induce sensitization and cross-sensitization in adulthood and may be used as a protectant factor against adolescent nicotine exposure.

Neuronal nicotinic acetylcholine receptors: neuroplastic changes underlying alcohol and nicotine addictions

Addictive drugs can activate systems involved in normal reward-related learning, creating long-lasting memories of the drug's reinforcing effects and the environmental cues surrounding the experience. These memories significantly contribute to the maintenance of compulsive drug use as well as cue-induced relapse which can occur even after long periods of abstinence. Synaptic plasticity is thought to be a prominent molecular mechanism underlying drug-induced learning and memories. Ethanol and nicotine are both widely abused drugs that share a common molecular target in the brain, the neuronal nicotinic acetylcholine receptors (nAChRs). The nAChRs are ligand-gated ion channels that are vastly distributed throughout the brain and play a key role in synaptic neurotransmission. In this review, we will delineate the role of nAChRs in the development of ethanol and nicotine addiction. We will characterize both ethanol and nicotine's effects on nAChR-mediated synaptic transmission and plasticity in several key brain areas that are important for addiction. Finally, we will discuss some of the behavioral outcomes of drug-induced synaptic plasticity in animal models. An understanding of the molecular and cellular changes that occur following administration of ethanol and nicotine will lead to better therapeutic strategies.

Tobacco use and psychiatric comorbidity among adolescents in substance abuse treatment

Adolescents in substance abuse treatment have approximately four times higher rates of tobacco use compared with adolescents in the general population, yet many substance abuse treatment programs do not provide tobacco cessation interventions. This study examined change in tobacco use among 151 adolescents in state-funded substance abuse treatment from intake to 12-month follow-up in relation to psychiatric comorbidity and substance use. Most adolescents (67.5%) used tobacco at intake and follow-up. Having a diagnosis of a mood, anxiety, or behavioral disorder was significantly associated with change in tobacco use. Individuals with a psychiatric disorder were less likely to be nonusers of tobacco than to be individuals who continued tobacco use (adjusted odds ratio [OR] = 0.153, 95% confidence interval [CI] = 0.040-0.587, p < .01), and they were less likely to have initiated tobacco use at follow-up than individuals who continued tobacco use (adjusted OR = 0.320, 95% CI = 0.105-0.970, p < .05). Contrary to the hypothesis, alcohol use and drug use during the 12-month follow-up was not significantly related to change in tobacco use. Adolescents in substance abuse treatment with comorbid psychiatric disorders may be particularly vulnerable to continuing tobacco use.

Effects of simultaneous exposure to stress and nicotine on nicotine-induced locomotor activation in adolescent and adult rats

Preclinical studies have shown that repeated stress experiences can result in an increase in the locomotor response to the subsequent administration of drugs of abuse, a phenomenon that has been termed behavioral cross-sensitization. Behavioral sensitization reflects neuroadaptive processes associated with drug addiction and drug-induced psychosis. Although cross-sensitization between stress- and drug-induced locomotor activity has been clearly demonstrated in adult rats, few studies have evaluated this phenomenon in adolescent rats. In the present study, we determined if the simultaneous exposure to stress and nicotine was capable of inducing behavioral sensitization to nicotine in adolescent and adult rats. To this end, adolescent (postnatal day (P) 28-37) and adult (P60-67) rats received nicotine (0.4 mg/kg, sc) or saline (0.9% NaCl, sc) and were immediately subjected to restraint stress for 2 h once a day for 7 days. The control group for stress was undisturbed following nicotine or saline injections. Three days after the last exposure to stress and nicotine, rats were challenged with a single dose of nicotine (0.4 mg/kg, sc) or saline and nicotine-induced locomotion was then recorded for 30 min. In adolescent rats, nicotine caused behavioral sensitization only in animals that were simultaneously exposed to stress, while in adult rats nicotine promoted sensitization independently of stress exposure. These findings demonstrate that adolescent rats are more vulnerable to the effects of stress on behavioral sensitization to nicotine than adult rats.

Stress-induced cross-sensitization to amphetamine is related to changes in the dopaminergic system

Repeated stress engenders behavioral sensitization. The mesolimbic dopamine system is critically involved in drug-induced behavioral sensitization. In the present study we examined the differences between adolescent and adult rats in stress-induced behavioral sensitization to amphetamine and changes in dopamine (DA) and its metabolite levels in the mesolimbic system. Adolescent or adult rats were restrained for 2 h, once a day, for 7 days. Three days after the last exposure to stress, the animals were challenged with saline or amphetamine (1.0 mg/kg i.p.) and amphetamine-induced locomotion was recorded for 40 min. Immediately after the behavioral tests, rats were decapitated and the nucleus accumbens (NAcc), ventral tegmental area (VTA) and amygdala (AM) were removed to measure tissue levels of DA and its metabolites by HPLC. Exposure to repeated restraint stress promoted behavioral sensitization to amphetamine in both adult and adolescent rats. In adult rats, amphetamine administration increased DA levels in both the stress and control groups in the NAcc and VTA. In adolescent rats, amphetamine increased DA levels in the NAcc in rats exposed to stress. Furthermore, in the AM of adolescent rats in the control group, amphetamine increased the DA levels; however, amphetamine reduced this neurotransmitter in the rats that were exposed to stress. No alteration was observed in the dopamine metabolite levels. Therefore, stress promoted behavioral sensitization to amphetamine and this may be related to changes in DA levels in the mesolimbic system. These changes appear to be dependent on ontogeny.

Residual performance impairments in adult rats trained on an object discrimination task subsequent to cocaine administration during adolescence

The present study was conducted to determine whether cognitive impairments in adult rats treated with cocaine during adolescence demonstrated in previous investigations extend to tests of object discrimination learning. Accordingly, 30-day-old male Long-Evans rats were injected subcutaneously with either 10 or 20 mg/kg cocaine or received control injections of saline for 7-8 consecutive days. An extended abstinence period was then introduced (mean = 70.7 ± 9.8 days) before subjects, who were now young adults (mean = 106.3 ± 10.2 days old), were assessed for acquisition of a two-choice object discrimination task. Using a correctional learning procedure conducted in a water maze, subjects were trained (eight trials per day for 10 days) to approach one of two multi-dimentional 'junk' objects. Although all animals acquired the discrimination to a reasonable extent, cocaine-treated subjects exhibited lower percentages of correct choices over the course of training (10 mg/kg = 59.6 ± 7.2% and 20 mg/kg = 59.4 ± 4.9%) relative to the saline control group (67.5 ± 4.9%). Further analyses revealed that saline-treated subjects acquired proficient discrimination performance earlier during the course of training, achieving an approximate 72% performance rate after only 3 days of training. This was in contrast to the two cocaine-treated groups needing 7 days of training to achieve comparable levels of performance. In addition, saline-treated subjects required significantly fewer trials (20.8 ± 8.9) than either cocaine-treated group (10 mg/kg = 52.2 ± 11.9 and 20 mg/kg = 63.3 ± 8.7) to reach an 87.5% correct response criterion (i.e. 7-correct-out-of-8-consecutive-trials) and performed at a higher above-chance level (13.5%) than either cocaine-treated group (3.6% and 5.3% for the 10 and 20 mg/kg cocaine groups, respectively). These findings demonstrate the existence of cognitive impairments in adulthood subsequent to cocaine exposure during adolescence despite a prolonged drug-free interval. Speculation regarding the neurobiological basis for this effect, especially with regard to alterations to prefrontal circuitry, is provided.

The sensitizing effect of acute nicotine on amphetamine-stimulated behavior and dopamine efflux requires activation of β2 subunit-containing nicotinic acetylcholine receptors and glutamate N-methyl-D-aspartate receptors

Nicotine has been demonstrated to enhance the subsequent use of illicit drugs in animals and humans. We previously demonstrated in female, Holtzman rats that one low dose of nicotine will potentiate locomotor activity and dopamine (DA) efflux in response to a subsequent low dose of d-amphetamine (AMPH) given 1-4 h later. In the present study, we show this also occurs in male rats and characterize the receptors required for the rapid sensitizing effect of nicotine on AMPH-stimulated locomotor behavior and AMPH-induced DA efflux. Pretreatment of male, Holtzman rats with a low dose (0.1 mg/kg, i.p.) of nicotine 2-4 h before a challenge with AMPH (0.32 mg/kg, i.p.) enhanced locomotor behavior as compared to saline pretreatment. Dihydro-β-erythroidine (DHβE), a relatively selective antagonist at β2 subunit-containing (β2∗) nicotinic acetylcholine receptors (nAChR), but not methyllycaconitine (MLA), a relatively selective antagonist at α7 nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor activity. Pretreatment with varenicline, a partial agonist selective for β2∗ nAChRs, blocked the sensitizing effect of nicotine on AMPH-stimulated locomotor behavior. Nicotine pretreatment sensitized AMPH-induced DA overflow in slices from ventral (nucleus accumbens, NAc), but not dorsal striatum as compared to saline-pretreated rats. Nicotine sensitization of the DA overflow was blocked by DHβE. Pretreatment with the glutamate N-methyl-D-aspartate (NMDA) receptor antagonist (+)-MK-801 (0.1 mg/kg, s.c.) 30 min before nicotine blocked sensitization of both locomotion and DA overflow in response to AMPH challenge. These results demonstrate that activation of the β2∗ nAChRs and NMDA receptors are required for the rapid sensitizing effect of nicotine on AMPH actions. This article is part of a Special Issue entitled 'Trends in neuropharmacology: in memory of Erminio Costa'.