Behavioral and neurochemical responses to cocaine in periadolescent and adult rats

Female and male rats in late adolescence differ from adults in amphetamine-induced locomotor activity, but not in conditioned place preference for amphetamine

Rodent models display differences in drug-induced behaviour between prepubertal/young adolescents and adults that parallel developmental differences in people; however, little is known as to when the transition to 'adultlike' behaviour occurs. We investigated the differences in locomotor and reward responses to amphetamines in male and female rats in late adolescence and compared them with corresponding adult responses. Long-Evans rats were tested for locomotor activity and conditioned place preference (CPP) for amphetamine (0.25, 0.5 or 1.0 mg/kg), beginning at 45 or 69 days of age. Adolescent female rats moved less to the first injection of amphetamine compared with adult female rats irrespective of dose, whereas adolescent male rats did not differ from adults. Adolescent female rats significantly increased locomotor activity in response to subsequent injections of amphetamine at all three doses, whereas such sensitization was only found at the highest dose for adult female and male rats. No effect of repeated injections at any dose was observed in adolescent male rats. No age differences were observed in CPP, but female rats showed greater CPP during the dioestrous than during the oestrous phase of the cycle. These data suggest that differences in neural systems underlying some behavioural responses to amphetamine continue to mature postpubertally into late adolescence in a sex-specific manner.

Age- and sex-dependent amphetamine self-administration in rats

RATIONALE

Recreational drug use peaks in the developmental stage of adolescence, and exposure to drugs during adolescence may predict drug dependence in adulthood. Nevertheless, adolescent drug vulnerability is not widely studied in animal models of drug intake, and very few studies have investigated sex differences in drug-related behavior during adolescence.

OBJECTIVES

We compared patterns of intravenous (i.v.) amphetamine self-administration among adolescent vs adult, male vs female Sprague-Dawley rats on a fixed ratio (FR) followed by a progressive ratio (PR) schedule of reinforcement.

MATERIALS AND METHODS

After surgical implantation of i.v. catheters, adolescent [postnatal day (P) 35-52] and adult (P90-106) male and female rats were allowed to acquire lever-pressing behavior reinforced by either 0.025 or 0.05 mg/kg/0.1-ml amphetamine infusions over 14 daily 2-h sessions on an FR1 schedule (n = 9-12 per age-, sex-, and dose-group). Subsequently, responding maintained by 0.0125 or 0.05 mg/kg per infusion amphetamine in 4-h sessions on a PR schedule was tested.

RESULTS

Adolescent rats acquired amphetamine self-administration faster than adults, reached a higher number of infusions, and took more amphetamine than their adult counterparts during the acquisition phase, although age differences varied by dose. In PR testing, young adult males earned fewer infusions than older adult males, whereas young adult females earned more infusions than their older adult counterparts, and more than age-matched males.

CONCLUSION

These results suggest that i.v. amphetamine self-administration in rats is a useful model to investigate the potential neurochemical and endocrine bases for age and sex differences in vulnerability to behavioral reinforcement by amphetamine.

Gestational exposure to nicotine and monoamine oxidase inhibitors influences cocaine-induced locomotion in adolescent rats

RATIONALE

Many pregnant women continue to smoke, despite a strong association between maternal smoking and neurobehavioral deficits in the offspring. Although gestational nicotine (GN) treatment in rodents is used as the primary animal model of maternal smoking, tobacco smoke contains more than 4,000 constituents, including monoamine oxidase inhibitors (MAOIs).

OBJECTIVES

The aim of this study was to determine whether there are interactions between the effects of gestational exposure to nicotine and MAOIs on cocaine-induced locomotor sensitization in adolescent rats.

MATERIALS AND METHODS

Pregnant rats were implanted on day 4 of gestation with osmotic minipumps delivering saline, nicotine (3 mg/kg per day), the MAOIs clorgyline and deprenyl (1 and 0.25 mg/kg per day, respectively), or nicotine/clorgyline/deprenyl (GMN). Adolescent female offspring were tested for cocaine-induced locomotor sensitization. Animals were treated with saline or cocaine (5 or 15 mg/kg, intraperitoneally) daily from postnatal (P) days 32-36 and challenged with cocaine (15 mg/kg) on P51 (day 20).

RESULTS

Group differences were observed in chronic but not acute effects of cocaine. Whereas gestational MAOI treatment, with or without nicotine, increased ambulatory response to cocaine on day 5, the opposite was found for vertical activity. Different adaptive responses were observed on cocaine challenge day. GNM animals exhibited enhanced locomotor activity in the cocaine-associated environment before cocaine challenge on day 20. In contrast, only GN animals exhibited significant locomotor sensitization to the cocaine challenge.

CONCLUSIONS

Gestational nicotine and MAOIs both influence brain development. Such interactions may sensitize adolescents to drug abuse and should be considered in animal models of maternal smoking.

Cocaine self-administration improves performance in a highly demanding water maze task

RATIONALE

Long-term potentiation (LTP) is considered to be a cellular substrate of learning and memory. Indeed, the involvement of LTP-like mechanisms in spatial learning has consistently been demonstrated in the Morris water maze test. We have previously shown that hippocampal LTP in Lewis rats was modulated by cocaine self-administration, although the performance of cocaine-self-administered rats in the Morris water maze was not altered.

OBJECTIVE

Given that the ease of the task previously used could have masked any possible effects of the cocaine-induced LTP enhancement on spatial learning, a new and more difficult water maze task was devised to address this issue.

MATERIALS AND METHODS

Animals self-administered cocaine (1 mg/kg) or saline under a fixed ratio 1 schedule of reinforcement for 22 days. Spatial learning was assessed in a difficult water maze task (four sessions, two trials per session with a 90-min intertrial interval), and spatial memory was also evaluated 48 h after training (a 90-s test). Additionally, reversal learning and perseverance were also studied.

RESULTS

There was a reduced latency in finding the hidden platform during training, as well as improved memory of the platform location in cocaine-self-administered rats with respect to animals that self-administered saline. No differences were observed in reversal learning or perseverance between groups.

CONCLUSIONS

Our data suggest that cocaine self-administration facilitates learning and memory in the water maze test only when animals are submitted to highly demanding tasks, involving working memory or consolidation-like processes during the intertrial interval.

Differential effects of self-administered cocaine in adolescent and adult rats on stimulus-reward learning

RATIONALE

Adult cocaine addicts, abstinent at the time of testing, show a variety of neurocognitive impairments. Less clear is whether there are differences in the degree of impairment if cocaine use is initiated during adolescence rather than adulthood.

OBJECTIVES

Using a preclinical model, we evaluated if stimulus-reward learning was impacted differently in rats exposed to cocaine during adolescence (beginning on postnatal day 37) vs adulthood (beginning on postnatal days 74-79) and then tested after a drug-free period.

MATERIALS AND METHODS

A yoked-triad design of intravenous cocaine self-administration in adult (n = 8 triads) and adolescent (n = 8 triads) rats was used. Sets of three animals either contingently self-administered cocaine or received cocaine or saline in a noncontingent manner. Rats self-administering 1-mg/kg doses of cocaine responded under a fixed-ratio 5, timeout 20-s schedule of reinforcement. After 18 2-h drug or saline sessions, all rats (now adults) began the drug-free period in their home environments. Testing in a stimulus-reward learning task (conditioned cue preference) began 19 days later.

RESULTS

Self-administration behavior was similar in adolescent and adult rats. Lever responses were not significantly different, and both age groups averaged approximately 20 infusions per session. Rats contingently self-administering cocaine or passively exposed to cocaine during adulthood showed stimulus-reward learning deficits in the conditioned cue preference task. Rats exposed to contingent or noncontingent cocaine during adolescence had normal learning, showing strong preferences for a Froot Loops-paired cue.

CONCLUSIONS

These findings suggest that adolescents are insensitive to cocaine-induced impairment of learning related to amygdala memory system functioning.

Protracted 'pro-addictive' phenotype produced in mice by pre-adolescent phenylpropanolamine

For decades, the sympathomimetic phenylpropanolamine (PPA; +/- -norepinephrine) was an active ingredient found in popular children's over-the-counter (OTC) cold, cough, and allergy medications. To examine the possibility that pre-adolescent PPA exposure may induce neuroadaptations that influence behavioral and neurochemical responding to cocaine, C57BL/6J mice were pretreated with PPA (0-40 mg/kg) during postnatal days 21-31. The behavioral and neurochemical responses to acute and repeated cocaine (4 x 15 mg/kg) were then assessed in adulthood when the mice were 10 weeks of age. Whereas pre-adolescent PPA exposure did not influence the acute locomotor response to 15 mg/kg cocaine, PPA pre-exposure dose-dependently enhanced the expression of cocaine-induced place conditioning, reduced the expression of locomotor sensitization, but did not influence cocaine-induced stereotypy. Pre-adolescent PPA exposure completely prevented the capacity of cocaine to elevate extracellular levels of catecholamines in the nucleus accumbens, but facilitated the development of cocaine-induced glutamate sensitization. Neither acute nor repeated cocaine altered extracellular GABA levels in the accumbens of control mice; however, 15 mg/kg cocaine lowered GABA levels by approximately 40% in PPA pretreated mice and this effect showed tolerance with repeated cocaine administration. These data provide the first evidence that early exposure to an OTC compound produces protracted effects upon cocaine-induced changes in nucleus accumbens neurotransmission that may contribute to a 'pro-addictive' phenotype in adulthood.

Acquisition of i.v. cocaine self-administration in adolescent and adult male rats selectively bred for high and low saccharin intake

Adolescence and excessive intake of saccharin have each been previously associated with enhanced vulnerability to drug abuse. In the present study, we focused on the relationship between these two factors using male adolescent and adult rats selectively bred for high (HiS) and low (LoS) levels of saccharin intake. On postnatal day 25 (adolescents) or 150 (adults), rats were implanted with an intravenous catheter and trained to self-administer cocaine (0.4 mg/kg) using an autoshaping procedure that consisted of two 6-h sessions. In the first 6 h, rats were given non-contingent cocaine infusions at random intervals 10 times per hour, and during the second 6-h session, rats were allowed to self-administer cocaine under a fixed ratio 1 (FR 1) lever-response contingency. Acquisition was defined as a total of at least 250 infusions over 5 consecutive days, and rats were given 30 days to meet the acquisition criterion. Subsequently, saccharin phenotype scores were determined by comparing 24-h saccharin and water consumption in two-bottle tests to verify HiS/LoS status. Adolescent LoS rats had a faster rate of acquisition of cocaine self-administration than adult LoS rats; however, adolescent and adult HiS rats acquired at the same rate. Both HiS and LoS adolescents had significantly higher saccharin phenotype scores than HiS and LoS adults, respectively. Additionally, saccharin score was negatively correlated with the number of days to meet the acquisition criterion for cocaine self-administration, but this was mostly accounted for by the HiS adolescents. These results suggest that during adolescence, compared with adulthood, rats have both an increased avidity for sweets and vulnerability to initiate drug abuse.

Chronic ethanol exposure during adolescence increases basal dopamine in the nucleus accumbens septi during adulthood

BACKGROUND

In humans, adolescent exposure to alcohol is associated with the onset of adult alcohol dependency and suggests that early use potentiates vulnerability to addiction. The aim of the present study was to address whether chronic ethanol exposure during adolescence would alter nucleus accumbens septi (NAcc) dopamine (DA) levels in the adult brain.

METHODS

Rats were injected daily from postnatal day (PND) 30 to 50 with either 0.75 g/kg/i.p. ethanol or saline followed by an ethanol-abstinent period from PND 51 to 65. Changes in extracellular DA levels in the anterior NAcc shell were measured via the no net flux (NNF) paradigm.

RESULTS

Extracellular DA levels were greater in rats chronically treated with ethanol during adolescence (6.5 nM DA) in comparison with saline-exposed controls (3.6 nM DA). There were no differences in extraction fraction (E(d)), an indirect measure of DA reuptake, between ethanol-treated (87%) and nontreated (68%) rats.

CONCLUSIONS

Together these findings suggest that changes in extracellular DA may be an underlying physiological mechanism in adolescent vulnerability to the rewarding properties of ethanol.

Low dose nicotine treatment during early adolescence increases subsequent cocaine reward

Adolescence is a critical period for the initiation of drug use, starting with tobacco and alcohol and progressing to marijuana and other illicit drugs. These findings have led to the suggestion that tobacco and alcohol are 'gateway' drugs that sensitize maturing reward pathways to the effects of illicit substances such as cocaine. To test this hypothesis, we have examined whether low-dose nicotine pretreatment alters acquisition of cocaine self-administration in adolescents more than in adults. Male and female Sprague-Dawley rats, aged postnatal day (P) 28 or P86, were given two daily intravenous injections of nicotine (0.03 mg/kg/0.1 ml) or saline for 4 days. At P32 and P90, rats were placed in self-administration chambers and tested for acquisition of cocaine (0.2 or 0.5 mg/kg/inj) for 5 days. Data were collapsed across cocaine dose and sex since there was no significant effect of these variables. Adolescent rats pretreated with nicotine exhibited significantly greater cocaine-reinforced responding as compared to saline controls or adults (p<0.01). This drug pretreatment effect did not generalize to all rewards, since nicotine did not increase responding for sucrose pellets in adolescents. These findings provide evidence that the adolescent brain is uniquely vulnerable to the effects of nicotine on subsequent drug reward.