Adolescents differ from adults in cocaine conditioned place preference and cocaine-induced dopamine in the nucleus accumbens septi

Abstract

In humans, adolescent exposure to illicit drugs predicts the onset of adult drug abuse and suggests that early drug use potentiates vulnerability to drug addiction. Cocaine conditioned place preferences were measured in early adolescent [postnatal day (PND) 35], late adolescent (PND 45) and young adult (PND 60) rats by injecting either 0, 5 or 20 mg/kg cocaine and conditioning them to environmental cues. Cocaine preferences were found for all ages at the high dose. PND 35s were the only age group to have a preference at the low dose. To address whether age-related differences in cocaine place preferences were related to differences in the mesolimbic dopaminergic system, we measured extracellular dopamine levels in the nucleus accumbens septi of PND 35, PND 45 and PND 60 rats via quantitative microdialysis under transient conditions. Rats were injected daily with either 5 mg/kg/ip or saline for 4 days and surgically implanted with a microdialysis probe aimed at the nucleus accumbens. Rats were perfused with either 0, 1, 10 or 40 nM dopamine and the extracellular dopamine concentration was measured. Our results show that adolescents differ from adults in basal dopamine. All cocaine treated rats, regardless of age, showed a significant increase in dopamine over baseline in response to a cocaine challenge. Additionally, there were age-related differences in the extraction fraction (E_d), an indirect measure of dopamine reuptake. Together these findings suggest ontogenetic differences in extracellular dopamine and dopamine reuptake and that these differences may provide an explanation for adolescent vulnerability to addiction.

Introduction

Animal models of drug use have recently been extended to investigate the impact of drugs on adolescent behavior and brain functioning. Specifically, conditioned place preference provides a measure of drug reward by assessing an animal's ability to associate drug-induced effects with environmental cues. In adult rats, a cocaine place preference is effectively established for 20 mg/kg cocaine (Spyraki et al., 1982, Bardo et al., 1986). In young rodents, the expression of cocaine place conditioning has been shown for both pre-adolescent and adolescent rats (Laviola et al., 1992, Pruitt et al., 1995, Campbell et al., 2000, Schramm-Sapyta et al., 2004). Given that clinical studies have shown that early drug use, such as during the adolescent period, is highly associated with chronic and more severe drug use as an adult (Estroff et al., 1989, Anthony and Petronis, 1995, Clark et al., 1998), it would be expected that preclinical studies would show that adolescents find cocaine more rewarding than adult comparisons. However, similar responses for place conditioning have been found for adolescent and adult rats at high cocaine doses (Campbell et al., 2000, Schramm-Sapyta et al., 2004) suggesting that adolescents are just as vulnerable as adults to the rewarding properties of cocaine. Other drugs, such as nicotine and alcohol, have elicited greater place preferences in adolescents relative to adult rats (Vastola et al., 2002, Belluzzi et al., 2004, Philpot et al., 2003, Torrella et al., 2004), while the opposite was found for amphetamine (Adriani and Laviola, 2003) with greater place preferences in adults relative to adolescent rats. Indeed, it should be noted that a wide variety of place conditioning protocols were used in these experiments and the various age differences for each drug type may differ for several methodological reasons such as adolescent age, species, apparatus, design, pharmacological drug action and or drug dose. Together, these data suggest that the place conditioning paradigm is a sufficient measure of drug reward for adolescent and adult rats and that this particular paradigm is sensitive enough to measure differences in drug reward for several drugs of abuse.

Neurochemical research in adolescence has focused on the development of the mesolimbic pathway, a dopaminergic projection from the ventral tegmental area to the nucleus accumbens (Dahlstrom and Fuxe, 1964). A fine tuning of neuronal anatomy occurs during adolescence including a general heightened neural activity (Chugani et al., 1987), and changes in receptor density. During adolescence, mesolimbic dopamine receptors and dopamine transporters overproduce and subsequently prune back excess dopamine receptors (Lidow et al., 1991, Teicher et al., 1995, Tarazi et al., 1998a, Tarazi et al., 1998b, Tarazi et al., 1999). Age-related differences in dopamine degradation were reported including an adult specific cocaine-induced dopamine transporter upregulation that was not expressed in adolescents (Collins and Izenwasser, 2002) and age differences in enzymatic degradation of dopamine in the striatum (Nakano and Mizuno, 1996) and the nucleus accumbens (Philpot and Kirstein, 2004). These findings suggest that the mesolimbic dopamine system undergoes marked change during adolescence.

Conventional in vivo microdialysis has been used to measure dopamine; however, recovery of dopamine by the microdialysis probe is influenced by degradation processes such as reuptake and metabolism (Smith and Justice, 1994). A variant of this procedure, quantitative microdialysis under transient conditions, measures the effects of cocaine on extracellular dopamine and dopamine recovery in adult rats (Olson and Justice, 1993). In this procedure, each rat is perfused with a single concentration of dopamine afterwhich perfusate and brain dopamine concentrations equilibrate and cocaine is systemically administered. Basal and cocaine-induced extracellular dopamine are calculated (dopamine concentration in the perfusate – dopamine concentration in the sample; C_in − C_out) and the net difference is averaged and plotted against C_in to form a linear regression line. The x-intercept represents the extracellular dopamine concentration while the slope of the line represents the extraction fraction (E_d) and provides a measure of dopamine recovery and an indirect measure of dopamine reuptake. Quantitative microdialysis studies of repeated cocaine pretreatment have reported ∼ 400% increase in extracellular dopamine after a subsequent cocaine challenge injection (Chefer and Shippenberg, 2002). Decreased E_d has been observed after systemic acute or repeated cocaine and also in response to locally administered cocaine suggesting that the E_d is directly influenced by changes in dopamine reuptake (Olson and Justice, 1993, Smith and Justice, 1994, Chefer and Shippenberg, 2002). To date, no one has used quantitative microdialysis to measure cocaine-induced extracellular dopamine in the nucleus accumbens of adolescent rats. Given that clinical studies have shown that early drug use is highly associated with chronic and more severe drug use as an adult (Estroff et al., 1989, Anthony and Petronis, 1995, Clark et al., 1998) and similar responses for place conditioning have been found for adolescent and adult rats at high cocaine doses (Campbell et al., 2000, Schramm-Sapyta et al., 2004) it was hypothesized that low doses of cocaine would be more rewarding for adolescent than for adult rats. The present study investigated preferences for low and high doses of cocaine in early adolescent, late adolescent and young adult rats using the place conditioning paradigm. It was also hypothesized that adolescent and adult rats would have a different dopaminergic response to repeated cocaine treatment. Quantitative microdialysis under transient conditions was used to quantify cocaine-induced dopamine and E_d in the nucleus accumbens of early adolescent, late adolescent and adult rats.