Clyde W. Hodge

Adolescent cortical development : A critical period of vulnerability for addiction

Cortical growth and remodeling continues from birth through youth and adolescence to stable adult levels changing slowly into senescence. There are critical periods of cortical development when specific experiences drive major synaptic rearrangements and learning that only occur during the critical period. For example, visual cortex is characterized by a critical period of plasticity involved in establishing visual acuity. Adolescence is defined by characteristic behaviors that include high levels of risk taking, exploration, novelty and sensation seeking, social interaction and play behaviors. In addition, adolescence is the final period of development of the adult during which talents, reasoning and complex adult behaviors mature. This maturation of behaviors corresponds with periods of marked changes in neurogenesis, cortical synaptic remodeling, neurotransmitter receptors and transporters, as well as major changes in hormones. Frontal cortical development is later in adolescence and likely contributes to refinement of reasoning, goal and priority setting, impulse control and evaluating long and short term rewards. Adolescent humans have high levels of binge drinking and experimentation with other drugs. This review presents findings supporting adolescence as a critical period of cortical development important for establishing life long adult characteristics that are disrupted by alcohol and drug use.

GABAA receptor alpha-1 subunit deletion alters receptor subtype assembly, pharmacological and behavioral responses to benzodiazepines and zolpidem

Potentiation of GABA(A) receptor activation through allosteric benzodiazepine (BZ) sites produces the anxiolytic, anticonvulsant and sedative/hypnotic effects of BZs. Using a mouse line lacking alpha1 subunit expression, we investigated the contribution of the alpha1 subunit to GABA(A) receptor pharmacology, function and related behaviors in response to BZ site agonists. Competitive [(3)H]flunitrazepam binding experiments using the Type I BZ site agonist, zolpidem, and the Type I and II BZ site non-specific agonist, diazepam, demonstrated the complete loss of Type I BZ binding sites in alpha1(-/-) mice and a compensatory increase in Type II BZ binding sites (41+/-6%, P<0.002). Chloride uptake analysis in alpha1(-/-) mice revealed an increase (108+/-10%, P<0.001) in the efficacy (E(max)) of flunitrazepam while the EC(50) of zolpidem was increased 495+/-26% (alpha1(+/+): 184+/-56 nM; alpha1(-/-): 1096+/-279 nM, P<0.01). An anxiolytic effect of diazepam was detected in both alpha1(+/+) and alpha1(-/-) mice as measured on the elevated plus maze; however, alpha1(-/-) mice exhibited a greater percentage of open arm entries and percentage of open arm time following 0.6 mg/kg diazepam. Furthermore, alpha1(-/-) mice were more sensitive to the motor impairing/sedative effects of diazepam (1-10 mg/kg) as measured by locomotor activity in the open field. Knockout mice were insensitive to the anticonvulsant effect of diazepam (1-15 mg/kg, P<0.001). The hypnotic effect of zolpidem (60 mg/kg) was reduced by 66% (P<0.001) in alpha1(-/-) mice as measured by loss of righting reflex while the effect of diazepam (33 mg/kg) was increased 57% in alpha1(-/-) mice (P<0.05). These studies demonstrate that compensatory adaptations in GABA(A) receptor subunit expression result in subunit substitution and assembly of functional receptors. Such adaptations reveal important relationships between subunit expression, receptor function and behavioral responses.

CRF-1 antagonist and CRF-2 agonist decrease binge-like ethanol drinking in c57bl/6j mice independent of the HPA axis

Recent evidence suggests that corticotropin-releasing factor (CRF) receptor (CRFR) signaling is involved in modulating binge-like ethanol consumption in C57BL/6J mice. In this report, a series of experiments were performed to further characterize the role of CRFR signaling in binge-like ethanol consumption. The role of central CRFR signaling was assessed with intracerebroventricular (i.c.v.) infusion of the nonselective CRFR antagonist, α-helical CRF9–41 (0, 1, 5, 10 μg/1 μl). The contribution of central CRF type 2 receptor (CRF2R) signaling was assessed with i.c.v. infusion of the selective CRF2R agonist, urocortin (Ucn) 3 (0, 0.05, 0.1, or 0.5 μg/1 μl). The role of the hypothalamic–pituitary–adrenal (HPA) axis was assessed by pretreating mice with intraperitoneal (i.p.) injection of (1) the corticosterone synthesis inhibitor, metyrapone (0, 50, 100, 150 mg/kg) or (2) the glucocorticoid receptor antagonist, mifepristone (0, 25, 50 mg/kg), and (3) by using radioimmunoassay to determine whether binge-like ethanol intake influenced plasma corticosterone levels. Finally, we determined whether the ability of the CRF1R antagonist, CP-154,526 (CP; 0, 10, 15 mg/kg, i.p.), to blunt binge-like drinking required normal HPA axis signaling by comparing the effectiveness of CP in adrenalectomized (ADX) and normal mice. Results showed that i.c.v. infusion of a 1 μg dose of α-helical CRF9–41 significantly attenuated binge-like ethanol consumption relative to vehicle treatment, and i.c.v. infusion of Ucn 3 dose-dependently blunted binge-like drinking. On the other hand, metyrapone nonselectively reduced both ethanol and sucrose consumption, mifepristone did not alter ethanol drinking, and binge-like drinking did not correlate with plasma corticosterone levels. Finally, i.p. injection of CP significantly attenuated binge-like ethanol intake in both ADX and normal mice. Together, these results suggest that binge-like ethanol intake in C57BL/6J mice is modulated by CRF1R and CRF2R signaling, such that blockade of CRF1R or activation of CRF2R effectively reduces excessive ethanol intake. Furthermore, normal HPA axis signaling is not necessary to achieve binge-like drinking behavior.

Metabotropic Glutamate Receptor 5 Activity in the Nucleus Accumbens Is Required for the Maintenance of Ethanol Self-Administration in a Rat Genetic Model of High Alcohol Intake

BACKGROUND Systemic modulation of Group I and II metabotropic glutamate receptors (mGluRs) regulate ethanol self-administration in a variety of animal models. Although these receptors are expressed in reward-related brain regions, the anatomical specificity of their functional involvement in ethanol self-administration remains to be characterized. This study sought to evaluate the functional role of Group I (mGluR5) and Group II (mGluR2/3) in mesocorticolimbic brain regions in ethanol self-administration. METHODS Alcohol-preferring (P) rats, a genetic model of high alcohol drinking, were trained to self-administer ethanol (15% v/v) versus water in operant conditioning chambers. Effects of brain site-specific infusion of the mGluR5 antagonist 2-methyl-6-(phenylethynyl)pyridine hydrochloride (MPEP) and the mGluR2/3 agonist were then assessed on the maintenance of self-administration. RESULTS Microinjection of the mGluR5 antagonist MPEP in the nucleus accumbens reduced ethanol self-administration at a dose that did not alter locomotor activity. By contrast, infusion of the mGluR2/3 agonist LY379268 in the nucleus accumbens reduced self-administration and produced nonspecific reductions in locomotor activity. The mGluR5 involvement showed anatomical specificity as evidenced by lack of effect of MPEP infusion in the dorsomedial caudate or medial prefrontal cortex on ethanol self-administration. To determine reinforcer specificity, P-rats were trained to self-administer sucrose (.4% w/v) versus water, and effects of intra-accumbens MPEP were tested. The MPEP did not alter sucrose self-administration or motor behavior. CONCLUSIONS These results suggest that mGluR5 activity specifically in the nucleus accumbens is required for the maintenance of ethanol self-administration in individuals with genetic risk for high alcohol consumption.

Cue-induced reinstatement of alcohol-seeking behavior is associated with increased ERK1/2 phosphorylation in specific limbic brain regions: blockade by the mGluR5 antagonist MPEP.

Relapse to alcohol use after periods of abstinence is a hallmark behavioral pathology of alcoholism and a major clinical problem. Emerging evidence indicates that metabotropic glutamate receptor 5 (mGluR5) antagonists attenuate relapse to alcohol-seeking behavior but the molecular mechanisms of this potential therapeutic effect remain unexplored. The extracellular signal-regulated kinase (ERK1/2) pathway is downstream of mGluR5 and has been implicated in addiction. We sought to determine if cue-induced reinstatement of alcohol-seeking behavior, and its reduction by an mGluR5 antagonist, is associated with changes in ERK1/2 activation in reward-related limbic brain regions. Selectively-bred alcohol-preferring (P) rats were trained to lever press on a concurrent schedule of alcohol (15% v/v) vs. water reinforcement. Following 9 days of extinction, rats were given an additional extinction trial or injected with the mGluR5 antagonist MPEP (0, 1, 3, or 10mg/kg) and tested for cue-induced reinstatement. Brains were removed 90-min later from the rats in the extinction and MPEP (0 or 10mg/kg) conditions for analysis of p-ERK1/2, total ERK1/2, and p-ERK5 immunoreactivity (IR). Cue-induced reinstatement of alcohol-seeking behavior was associated with a three to five-fold increase in p-ERK1/2 IR in the basolateral amygdala and nucleus accumbens shell. MPEP administration blocked both the relapse-like behavior and increase in p-ERK1/2 IR. p-ERK1/2 IR in the central amygdala and NAcb core was dissociated with the relapse-like behavior and the pharmacological effect of mGluR5 blockade. No changes in total ERK or p-ERK5 were observed. These results suggest that exposure to cues previously associated with alcohol self-administration is sufficient to produce concomitant increases in relapse-like behavior and ERK1/2 activation in specific limbic brain regions. Pharmacological compounds, such as mGluR5 antagonists, that reduce cue-induced ERK1/2 activation may be useful for treatment of relapse in alcoholics that is triggered by exposure to environmental events.

Abstinence following Alcohol Drinking Produces Depression-Like Behavior and Reduced Hippocampal Neurogenesis in Mice

Alcoholism and depression show high degrees of comorbidity. Clinical evidence also indicates that depression that emerges during abstinence from chronic alcohol use has a greater negative impact on relapse than pre-existing depression. Although no single neurobiological mechanism can account for the behavioral pathologies associated with these devastating disorders, converging evidence suggests that aspects of both alcoholism and depression are linked to reductions in hippocampal neurogenesis. Here, we report results from a novel preclinical behavioral model showing that abstinence from voluntary alcohol drinking leads to the emergence of depression-like behavior and reductions in neurogenesis. C57BL/6J mice were allowed to self-administer ethanol (10% v/v) vs H2O in the home cage for 28 days. Alcohol was then removed for 1 or 14 days, and mice were tested in the forced swim test to measure depression-like behavior. After 14 days, but not 1 day of abstinence from alcohol drinking, mice showed a significant increase in depression-like behavior. The significant increase in depression-like behavior during abstinence was associated with a reduction in proliferating cell nuclear antigen (PCNA) and doublecortin (DCX) immunoreactivity in the dentate gyrus of the hippocampus indicating that both the number of proliferating neural progenitor cells (NPC) and immature neurons were reduced, respectively. The number of NPCs that were labeled with bromo-deoxyuridine (BrdU) at the beginning of alcohol exposure was not altered indicating that survival of NPCs is not linked to abstinence-induced depression. Chronic treatment (14 days) with the antidepressant desipramine during abstinence prevented both the emergence of depression-like behavior and the reduction in hippocampal neurogenesis indicating that abstinence-induced depression is associated with structural plasticity in the hippocampus. Overall, the results of this study support the conclusion that profound functional (ie behavioral) and structural changes occur during abstinence from alcohol use and suggest that antidepressant treatment may alleviate some of these pathological neurobehavioral adaptations.

Effects of acute acamprosate and homotaurine on ethanol intake and ethanol-stimulated mesolimbic dopamine release

The purpose of the present study was to determine the acute effects of the anticraving compound acamprosate (calcium acetylhomotaurinate) and the closely related compound homotaurine on ethanol intake and ethanol-stimulated dopamine release in the nucleus accumbens. Male rats were treated with acamprosate (200 or 400 mg/kg intraperitoneally, i.p.) or homotaurine (10, 50, or 100 mg/kg i.p.) 15 min prior to access to 10% ethanol and water for 1 h in a two-bottle choice restricted access paradigm. A separate group of rats was implanted with microdialysis probes in the nucleus accumbens and given an acute injection of ethanol (1.5 g/kg i.p.) that was preceded by saline, acamprosate, or homotaurine. Acamprosate and homotaurine dose-dependently reduced ethanol intake and preference. These compounds also delayed or suppressed ethanol-stimulated increases in nucleus accumbens dopamine release, suggesting that acamprosate and homotaurine may reduce ethanol intake by interfering with the ability of ethanol to activate the mesolimbic dopamine reward system.

Ethanol preexposure increases ethanol self-administration in C57BL/6J and DBA/2J mice

Genetic variables are thought to interact with environmental factors, such as alcohol exposure history, to produce individual differences in alcohol abuse and alcoholism. The objective of this study was to test the potential interaction between genetic predisposition to consume alcohol and alcohol pretreatment on subsequent self-administration. To accomplish this goal, four groups of mice from the ethanol-avoiding DBA/2J (D2) and ethanol-preferring C57BL/6J (B6) inbred strains were exposed to saline, acute ethanol (2 g/kg), or chronic intermittent ethanol (1 or 2 g/kg) intraperitoneal (i.p.) injections. Locomotor activity was monitored after each injection. After preexposure, animals were given a two-bottle choice test with various concentrations of ethanol/sucrose vs. sucrose or ethanol vs. water for 4 days at each concentration. Then, all animals were challenged with a 2.0 g/kg ethanol i.p. injection and locomotor activity was assessed. Acute and chronic ethanol pretreatment increased locomotor activity in response to a challenge dose of ethanol (2 g/kg) in D2 mice but had no effect on B6 mice. Prior exposure to ethanol altered the amount of ethanol consumed in a mouse strain-dependent manner. D2 mice showed a positive relationship between ethanol intake and dose or duration of ethanol preexposure. B6 mice preexposed to ethanol consumed more ethanol than naive animals, independent of dose or duration of exposure. During the last phase of self-administration testing, D2 mice exposed to chronic ethanol (2 g/kg) consumed as much ethanol as B6 from the same pretreatment condition. After a history of ethanol self-administration, saline control mice from the D2 strain showed equal locomotor activation as compared to D2 mice that were pretreated with ethanol injections. B6 mice showed no change in locomotor activity after ethanol self-administration or injection. These results demonstrate that genetic predisposition to avoid alcohol (D2 mice) can be modified by a history of preexposure and that a predisposition to prefer alcohol (B6 mice) may be also amenable to influence by drug history. In general, the results of this study suggest that genetic factors may interact with previous exposure to ethanol to modify ethanol self-administration.

Intra-amygdala infusion of the NPY Y1 receptor antagonist BIBP 3226 attenuates operant ethanol self-administration.

BACKGROUND Neuropeptide Y (NPY) is the most abundant and widely distributed peptide in the mammalian central nervous system. Evidence suggests that NPY transmission at Y1 receptors may regulate alcohol self-administration in rodent models. The purpose of the present study was to test the involvement of NPY Y1 receptors in the amygdala in the reinforcing effects of alcohol. METHODS Long-Evans rats were trained to self-administer ethanol (10% v/v) vs. water on a concurrent FR-1 schedule of reinforcement using a sucrose fading procedure. After a 1 month baseline period, bilateral injector cannulae were surgically implanted to terminate 1 mm dorsal to the central nucleus of the amygdala. Daily (Monday through Friday) operant self-administration sessions were conducted for 6 months after surgery. Then, the effects of intra-amygdala infusion of the high-affinity nonpeptide NPY Y1 receptor antagonist BIBP 3226 (1, 10, or 20 microMg) were determined on parameters of operant alcohol self-administration. RESULTS Intra-amygdala administration of 10 microM or 20 microM BIBP 3226 decreased total alcohol-reinforced responding and dose of self-administered ethanol (g/kg) without significantly altering total water responses or intake compared with vehicle control. Response onset was unaffected. Analysis of the temporal pattern of ethanol- and water-reinforced responding showed that BIBP 3226 decreased cumulative ethanol-reinforced responding during the 30 to 60 min period of the sessions. Water-reinforced responses were increased by the low dose of BIBP 3226 (1 microM) during the 50 to 60 min period. CONCLUSIONS Results from this study indicate that alcohol-reinforced responding is reduced by acute blockade of NPY Y1 receptors in the amygdala of rats with a long-term history of alcohol self-administration. These data are consistent with the hypothesis that alcohol self-administration is maintained by NPY neurotransmission at Y1 receptors in the central nucleus of the amygdala.

Suppression of heavy drinking and alcohol seeking by a selective ALDH-2 inhibitor

BACKGROUND Inherited human aldehyde dehydrogenase 2 (ALDH-2) deficiency reduces the risk for alcoholism. Kudzu plants and extracts have been used for 1,000 years in traditional Chinese medicine to treat alcoholism. Kudzu contains daidzin, which inhibits ALDH-2 and suppresses heavy drinking in rodents. Decreased drinking due to ALDH-2 inhibition is attributed to aversive properties of acetaldehyde accumulated during alcohol consumption. However, daidzin can reduce drinking in some rodents without necessarily increasing acetaldehyde. Therefore, a selective ALDH-2 inhibitor might affect other metabolic factors involved in regulating drinking. METHODS Aldehyde dehydrogenase 2 inhibitors were synthesized based on the co-crystal structure of ALDH-2 and daidzin. We tested the efficacy of a highly selective reversible ALDH-2 inhibitor, CVT-10216, in models of moderate and high alcohol drinking rats. We studied 2-bottle choice and deprivation-induced drinking paradigms in Fawn Hooded (FH) rats, operant self-administration in Long Evans (LE), FH, and inbred P (iP) rats and in cue-induced reinstatement in iP rats. We also assayed blood acetaldehyde levels as well as dopamine (DA) release in the nucleus accumbens (NAc) and tested possible rewarding/aversive effects of the inhibitor in a conditioned place preference (CPP) paradigm. RESULTS CVT-10216 increases acetaldehyde after alcohol gavage and inhibits 2-bottle choice alcohol intake in heavy drinking rodents, including deprivation-induced drinking. Moreover, CVT-10216 also prevents operant self-administration and eliminates cue-induced reinstatement of alcohol seeking even when alcohol is not available (i.e., no acetaldehyde). Alcohol stimulates DA release in the NAc, which is thought to contribute to increased drinking and relapse in alcoholism. CVT-10216 prevents alcohol-induced increases in NAc DA without changing basal levels. CVT-10216 does not show rewarding or aversive properties in the CPP paradigm at therapeutic doses. CONCLUSION Our findings suggest that selective reversible ALDH-2 inhibitors may have therapeutic potential to reduce excessive drinking and to suppress relapse in abstinent alcoholics.