Kimberly A. Badanich

Alcohol during adolescence selectively alters immediate and long-term behavior and neurochemistry

Alcohol use increases across adolescence and is a concern in the United States. In humans, males and females consume different amounts of alcohol depending on the age of initiation, and the long-term consequences of early ethanol consumption are not readily understood. The purpose of our work was to better understand the immediate and long-term impact of ethanol exposure during adolescence and the effects it can have on behavior and dopaminergic responsivity. We have assessed sex differences in voluntary ethanol consumption during adolescence and adulthood and the influence of binge ethanol exposure during adolescence. We have observed that males are sensitive to passive social influences that mediate voluntary ethanol consumption, and early ethanol exposure induces long-term changes in responsivity to ethanol in adulthood. Exposure to moderate doses of ethanol during adolescence produced alterations in dopamine in the nucleus accumbens septi during adolescence and later in adulthood. Taken together, all of these data indicate that the adolescent brain is sensitive to the impact of early ethanol exposure during this critical developmental period.

Effects of chronic intermittent ethanol exposure on orbitofrontal and medial prefrontal cortex-dependent behaviors in mice.

In humans, stroke or trauma-induced damage to the orbitofrontal cortex (OFC) or medial prefrontal cortex (mPFC) results in impaired cognitive flexibility. Alcoholics also exhibit similar deficits in cognitive flexibility, suggesting that the OFC and mPFC are susceptible to alcohol-induced dysfunction. The present experiments investigated this issue using an attention set-shifting assay in ethanol dependent adult male C57BL/6J mice. Ethanol dependence was induced by exposing mice to repeated cycles of chronic intermittent ethanol (CIE) vapor inhalation. Behavioral testing was conducted 72 hours or 10 days following CIE exposure to determine whether ethanol-induced changes in OFC-dependent (reversal learning) and mPFC-dependent (set-shifting) behaviors are long lasting. During early ethanol abstinence (72 hrs), CIE mice showed reduced reversal learning performance as compared to controls. Reversal learning deficits were revealed as greater number of trials to criterion, more errors made, and a greater difficulty in performing a reversal learning task relative to baseline performance. Furthermore, the magnitude of the impairment was greater during reversal of a simple discrimination rather than reversal of an intra-dimensional shift. Reversal learning deficits were no longer present when mice were tested 10 days after CIE exposure, suggesting that ethanol-induced changes in OFC function can recover. Unexpectedly, performance on the set-shifting task was not impaired during abstinence from ethanol. These data suggest reversal learning, but not attention set-shifting, is transiently disrupted during short-term abstinence from CIE. Given that reversal learning requires an intact OFC, these findings support the idea that the OFC may be vulnerable to the cognitive impairing actions of ethanol.

Developmental Differences in Nicotine Place Conditioning

Abstract: To understand the motivations and implications of the prevalence of smoking, studies have compared the behavioral effects of nicotine, the psychoactive drug in tobacco, in adolescent and adult animals. The present study used a biased three‐chambered conditioned‐place preference procedure without prior habituation to examine the potential rewarding and anxiolytic effects of nicotine across adolescence and adulthood to assess the presence of age‐dependent differences in response to nicotine.

Ethanol Reduces Neuronal Excitability of Lateral Orbitofrontal Cortex Neurons Via a Glycine Receptor Dependent Mechanism

Trauma-induced damage to the orbitofrontal cortex (OFC) often results in behavioral inflexibility and impaired judgment. Human alcoholics exhibit similar cognitive deficits suggesting that OFC neurons are susceptible to alcohol-induced dysfunction. A previous study from this laboratory examined OFC mediated cognitive behaviors in mice and showed that behavioral flexibility during a reversal learning discrimination task was reduced in alcohol-dependent mice. Despite these intriguing findings, the actions of alcohol on OFC neuron function are unknown. To address this issue, slices containing the lateral OFC (lOFC) were prepared from adult C57BL/6J mice and whole-cell patch clamp electrophysiology was used to characterize the effects of ethanol (EtOH) on neuronal function. EtOH (66 mM) had no effect on AMPA-mediated EPSCs but decreased those mediated by NMDA receptors. EtOH (11–66 mM) also decreased current-evoked spike firing and this was accompanied by a decrease in input resistance and a modest hyperpolarization. EtOH inhibition of spike firing was prevented by the GABAA antagonist picrotoxin, but EtOH had no effect on evoked or spontaneous GABA IPSCs. EtOH increased the holding current of voltage-clamped neurons and this action was blocked by picrotoxin but not the more selective GABAA antagonist biccuculine. The glycine receptor antagonist strychnine also prevented EtOH’s effect on holding current and spike firing, and western blotting revealed the presence of glycine receptors in lOFC. Overall, these results suggest that acutely, EtOH may reduce lOFC function via a glycine receptor dependent process and this may trigger neuroadaptive mechanisms that contribute to the impairment of OFC-dependent behaviors in alcohol-dependent subjects.

Early adolescents show enhanced acute cocaine-induced locomotor activity in comparison to late adolescent and adult rats.

Initiation of drug use during adolescence is associated with an increased probability to develop a drug addiction. The present study examined dose-response effects of cocaine (0, 5, 10, or 20 mg/kg, i.p.) on locomotor activity in early adolescent (postnatal day (PND) 35), late adolescent (PND 45), and young adults (PND 60) by measuring total distance moved (TDM) and frequency of start-stops. In response to 20 mg/kg cocaine, early adolescents showed the greatest cocaine-induced increase in TDM in comparison to late adolescent and adult rats. At this same dose, early adolescents showed the greatest cocaine-induced attenuation of start-stops relative to older rats. Results suggest that early adolescents engage in more cocaine-induced locomotor activity and less stationary behavior indicating that early adolescents are more sensitive to locomotor activating effects of high dose cocaine than older rats.

Effects of MDMA (“ecstasy”) during adolescence on place conditioning and hippocampal neurogenesis

The use of 3,4,methylenedioxymethamphetamine (MDMA), the active agent in ecstasy, during adolescence is widespread yet the effects on adolescent behavior and brain development are unknown. The aim of the present study was 1) to evaluate effects of MDMA in adolescent rats using the conditioned place preference (CPP) paradigm to measure MDMA-induced reward and 2) assess effects of MDMA administration on cellular proliferation, survival and neurogenesis in the dentate gyrus of the hippocampus. During the adolescent period, MDMA CPP was measured in adolescents [postnatal day (PND) 28-39] by training rats to associate 1.25, 2.5, 5.0mg/kg MDMA or saline administration with environmental cues. After CPP ended, bromodeoxyuridine (BrdU) was injected and rats were euthanized either 24h (to evaluate cell proliferation) or 2 weeks (to assess neurogenesis) after the last MDMA injection. Adolescents expressed a CPP for 2.5mg/kg MDMA. Repeated exposure to 5.0mg/kg MDMA during adolescence increased cell proliferation, yet diminished neurogenesis, an effect that was replicated using flow cytometry. These findings suggest differential dose effects of adolescent MDMA exposure on reward related behaviors and hippocampal neurogenesis.

NR2B‐deficient mice are more sensitive to the locomotor stimulant and depressant effects of ethanol

The NR2B subunit of N‐methyl d‐aspartate glutamate receptors influences pharmacological properties and confers greater sensitivity to the modulatory effects of ethanol. This study examined behavioral responses to acute ethanol in a conditional knockout mouse model that allowed for a delayed genetic deletion of the NR2B subunit to avoid mouse lethality. Mice lacking the NR2B gene (knockout) were produced by mating NR2B[f/f] mice with CAMKIIa‐driven tTA transgenic mice and the tetO‐CRE transgenic mice. Adult male and female offspring representing each of the resultant genotypes (knockout, CAM, CRE and wildtype mice) were tested for open‐field locomotor activity following acute low‐ and high‐dose ethanol challenge as well as loss of righting reflex. Findings indicate that male and female mice lacking the NR2B subunit exhibited greater overall activity in comparison to other genotypes during the baseline locomotor activity test. NR2B knockout mice exhibited an exaggerated stimulant response to 1.5 g/kg (i.p.) and an exaggerated depressant response to 3.0 g/kg (i.p.) ethanol challenge. In addition, NR2B knockout mice slept longer following a high dose of ethanol (4.0 g/kg, i.p.). To evaluate pharmacokinetics, clearance rates of ethanol (1.5, 4.0 g/kg, i.p.) were measured and showed that female NR2B knockouts had a faster rate of metabolism only at the higher ethanol dose. Western blot analyses confirmed significant reduction in NR2B expression in the forebrain of knockout mice. Collectively, these data indicate that the NR2B subunit of the N‐methyl d‐aspartate glutamate receptor is involved in regulating low‐dose stimulant effects of ethanol and the depressant/hypnotic effects of ethanol.

Nicotine Administration Significantly Alters Accumbal Dopamine in the Adult but Not in the Adolescent Rat

Abstract: Many drug‐dependent adults began using drugs during adolescence. In fact, adolescent drug users are more likely to become drug‐dependent adults than those abstaining from drug use until after the age of 18. Because of this, recent research has begun to investigate the consequences of adolescent drug use. Specifically, research has begun to focus on the behavioral effects of drugs on the developing brain and the development of drug addiction. The present study examined the responsiveness of the mesolimbic dopamine (DA) pathway during development through the use of in vivo microdialysis. Specificall, it was determined whether nicotine‐induced accumbal DA release differs between adolescent and adult rats. To assess nicotines effects across age, animals received acute or repeated nicotine at early adolescence (postnatal day (PND) 35), late adolescence (PND 45), or young adulthood (PND 60). Findings suggest that there are significant differences between adolescent and adult animals in their dopaminergic response to nicotine. Adult animals had an enhanced DA response to acute nicotine challenge, an effect absent in adolescence. Additionally, this nicotine‐induced increase in adults was not apparent after repeated nicotine treatment. These results provide insight into how the adolescent brain responds to nicotine and may also provide evidence as to how prolonged nicotine use affects normal brain development and responsiveness.

Cocaine-Induced Reinstatement of a Conditioned Place Preference in Developing Rats: Involvement of the D2 Receptor

Reinstatement of conditioned place preferences have been used to investigate physiological mechanisms mediating drug-seeking behavior in adolescent and adult rodents; however, it is still unclear how psychostimulant exposure during adolescence affects neuron communication and whether these changes would elicit enhanced drug-seeking behavior later in adulthood. The present study determined whether the effects of intra-ventral tegmental area (VTA) or intra-nucleus accumbens septi (NAcc) dopamine (DA) D2 receptor antagonist infusions would block (or potentiate) cocaine-induced reinstatement of conditioned place preferences. Adolescent rats (postnatal day (PND 28–39)) were trained to express a cocaine place preference. The involvement of D2 receptors on cocaine-induced reinstatement was determined by intra-VTA or intra-NAcc infusion of the DA D2 receptor antagonist sulpiride (100 μM) during a cocaine-primed reinstatement test (10 mg/kg cocaine, i.p.). Infusion of sulpiride into the VTA but not the NAcc blocked reinstatement of conditioned place preference. These data suggest intrinsic compensatory mechanisms in the mesolimbic DA pathway mediate responsivity to cocaine-induced reinstatement of a conditioned place preference during development.

Localization of stereotaxic coordinates for the ventral tegmental area in early adolescent, mid-adolescent and adult rats.

The ventral tegmental area (VTA) is a brain region implicated in drug addiction and related behaviors; however, little research has been conducted examining the role of the VTA in these processes in adolescent rats. Understanding the development of the VTA is imperative for elucidating mechanisms mediating adolescent vulnerability to drug addiction. The purpose of the present study was to define stereotaxic coordinates for the VTA in developing rats. Early adolescent [postnatal day (PND) 28], mid-adolescent (PND 35), or adult male rats (PND 70) were surgically implanted with a guide cannula aimed at the VTA. Adult coordinates, (P: -3.5, L: +1.0, V: -8.5 mm from Bregma) were used as a baseline and guided localization of VTA coordinates for early and mid-adolescent rats. After recovery, dye was injected via a microdialysis probe and brains were removed, sliced, and stained for histological verification of cannula placement in the VTA. Data suggest VTA coordinates in adolescents differ significantly from adult rats. These findings imply that it is imperative to consider anatomical differences in the development of the VTA when comparing the neurochemical effects of abused drugs in adolescent and adult rats.