Regulation of glutamate signaling in the extended amygdala by adolescent alcohol exposure.

Adolescence is a critical period for brain development and behavioral maturation, marked by increased risk-taking behavior and the initiation of drug use. There are significant changes in gray matter volume and pruning of synapses along with a shift in excitatory to inhibitory balance which marks the maturation of cognition and decision-making. Because of ongoing brain development, adolescents are particularly sensitive to the detrimental effects of drugs, including alcohol, which can cause long-lasting consequences into adulthood. The extended amygdala is a region critically implicated in withdrawal and negative affect such as anxiety and depression. As negative affective disorders develop during adolescence, the effects of adolescent alcohol exposure on extended amygdala circuitry needs further inquiry. Here we aim to provide a framework to discuss the existing literature on the extended amygdala, the neuroadaptations which result from alcohol use, and the intersection of factors which contribute to the long-lasting effects of this exposure.

Adolescent alcohol exposure produces sex differences in negative affect-like behavior and group I mGluR BNST plasticity.

Adolescent alcohol exposure increases the risk of developing alcohol use disorders (AUDs), yet the mechanisms responsible for this vulnerability remain largely unknown. One potential target for alcohol-induced changes is the circuitry that modulates negative affect and stress, two sexually dependent drivers of alcohol relapse. The bed nucleus of the stria terminalis (BNST) is a sexually dimorphic region that critically regulates negative affective- and stress-induced relapse. Group I metabotropic glutamate receptors (mGluR) are a target of interest due to their regulation of stress, anxiety behaviors, and BNST plasticity. The current studies investigate sex-dependent sensitivity to the effects of adolescent intermittent ethanol vapor exposure (AIE) on negative affect during acute and protracted alcohol withdrawal and following stress in adulthood. This work also assessed whether BNST group I mGluR-mediated long-term depression (LTD) was disrupted at these timepoints. During acute withdrawal, AIE altered LTD induced by the group I mGluR antagonist DHPG in females, but not males. During adulthood, stress unmasked persistent changes in DHPG-induced LTD and behavior that were not present under basal conditions. Females with an AIE history demonstrated enhanced negative affective-like behavior in the novelty-induced hypophagia test following restraint stress-a phenotype that could be blocked with systemic mGluR5 allosteric antagonism via MTEP. Conversely, males with an AIE history demonstrated elevated freezing in a contextual fear conditioning paradigm. These studies demonstrate long-lasting, sex-dependent phenotypes produced by AIE and suggest pharmaceutical interventions for alcohol use and comorbid disorders may be more effective if designed with sex differences in mind.

Knockdown of BNST GluN2B-containing NMDA receptors mimics the actions of ketamine on novelty-induced hypophagia.

Administration of a single low dose of the N-methyl-D-aspartate (NMDA) receptor antagonist ketamine has been demonstrated to elicit long-lasting antidepressant effects in humans with depression, as well as in rodent models of depression. Although pharmacological studies have implicated the GluN2B subunit of the NMDA receptor in these effects, drugs targeting this subunit have off-target actions, and systemic administration of these compounds does not allow for delineation of specific brain regions involved. In this study, we assessed the role of GluN2B in the bed nucleus of the stria terminalis (BNST) in novelty-induced hypophagia (NIH) in mice. First, we verified that ketamine, as well as the GluN2B antagonist Ro25-6981, decreased the latency to consume food in a novel environment in a version of the NIH test. We then hypothesized that GluN2B-containing receptors within the BNST may be a target of systemic ketamine and contribute to behavioral effects. Through the combination of a GluN2B-floxed mouse line and stereotaxic delivery of lentiviral Cre recombinase, we found that targeted knockdown of this subunit within the BNST mimicked the reduction in affective behavior observed with systemic ketamine or Ro25-6981 in the NIH test. These data suggest a role for GluN2B-containing NMDARs within the BNST in the affective effects of systemic ketamine.

Developmental changes in the acute ethanol sensitivity of glutamatergic and GABAergic transmission in the BNST.

Glutamatergic and GABAergic transmission undergo significant changes during adolescence. Receptors for both of these transmitters (NMDAR, and GABAA) are known to be key targets for the acute effects of ethanol in adults. The current study set out to investigate the acute effects of ethanol on both NMDAR-mediated excitatory transmission and GABAergic inhibitory transmission within the bed nucleus of the stria terminalis (BNST) across age. The BNST is an area of the brain implicated in the negative reinforcing properties associated with alcohol dependence, and the BNST plays a critical role in stress-induced relapse. Therefore, assessing the developmental regulation of ethanol sensitivity in this key brain region is important to understanding the progression of ethanol dependence. To do this, whole-cell recordings of isolated NMDAR-evoked excitatory postsynaptic currents (eEPSCs) or evoked GABAergic inhibitory postsynaptic currents (eIPSCs) were performed on BNST neurons in slices from 4- or 8-week-old male C57BL/6J mice. Ethanol (50 mm) produced greater inhibition of NMDAR-eEPSCs in adolescent mice than in adult mice. This enhanced sensitivity in adolescence was not a result of shifts in function of the GluN2B subunit of the NMDAR, measured by Ro25-6981 inhibition and decay kinetics measured across age. Adolescent mice also exhibited greater ethanol sensitivity of GABAergic transmission, as ethanol (50 mm) enhanced eIPSCs in the BNST of adolescent but not adult mice. Collectively, this work illustrates that a moderate dose of ethanol produces greater inhibition of transmission in the BNST (through greater excitatory inhibition and enhancement of inhibitory transmission) in adolescents compared to adults. Given the role of the BNST in alcohol dependence, these developmental changes in acute ethanol sensitivity could accelerate neuroadaptations that result from chronic ethanol use during the critical period of adolescence.

Absence of Ca2+-stimulated adenylyl cyclases leads to reduced synaptic plasticity and impaired experience-dependent fear memory.

Ca(2+)-stimulated adenylyl cyclase (AC) 1 and 8 are two genes that have been shown to play critical roles in fear memory. AC1 and AC8 couple neuronal activity and intracellular Ca(2+) increases to the production of cyclic adenosine monophosphate and are localized synaptically, suggesting that Ca(2+)-stimulated ACs may modulate synaptic plasticity. Here, we first established that Ca(2+)-stimulated ACs modulate protein markers of synaptic activity at baseline and after learning. Primary hippocampal cell cultures showed that AC1/AC8 double-knockout (DKO) mice have reduced SV2, a synaptic vesicle protein, abundance along their dendritic processes, and this reduction can be rescued through lentivirus delivery of AC8 to the DKO cells. Additionally, phospho-synapsin, a protein implicated in the regulation of neurotransmitter release at the synapse, is decreased in vivo 1 h after conditioned fear (CF) training in DKO mice. Importantly, additional experiments showed that long-term potentiation deficits present in DKO mice are rescued by acutely replacing AC8 in the forebrain, further supporting the idea that Ca(2+)-stimulated AC activity is a crucial modulator of synaptic plasticity. Previous studies have demonstrated that memory is continually modulated by gene-environment interactions. The last set of experiments evaluated the effects of knocking out AC1 and AC8 genes on experience-dependent changes in CF memory. We showed that the strength of CF memory in wild-type mice is determined by previous environment, minimal or enriched, whereas memory in DKO mice is unaffected. Thus, overall these results show that AC1 and AC8 modulate markers of synaptic activity and help integrate environmental information to modulate fear memory.

Temperament related to early-onset substance use: test of a developmental model.

We tested a theoretical model of early-onset substance (tobacco, alcohol, and marijuana) use. A sample of 1,810 public school students was surveyed in sixth grade (M age 11.5 years) and seventh grade. Temperament dimensions were related to substance use, and structural modeling analyses showed indirect effects through self-control constructs. Good self-control had a path to higher academic competence and had direct effects to less peer use and less adolescent substance use; poor self-control had a path to more adolescent life events and more deviant peer affiliations. Academic competence and life events had indirect effects to adolescent substance use, through peer affiliations. Findings from self-report data were corroborated by independent teacher ratings. Effects were also noted for family variables and demographic characteristics. Implications of epigenetic theory for prevention research are discussed.

Time perspective and early-onset substance use: a model based on stress-coping theory.

This research tested the relation of time perspective to early-onset substance use (tobacco, alcohol, and marijuana) with a sample of 454 elementary school students with a mean age of 11.8 years. An adaptation of the Zimbardo Time Perspective Inventory (P. G. Zimbardo & J. N. Boyd, 1999) was administered with measures derived from stress-coping theory. Independent effects showed future orientation inversely related to substance use and present orientation positively related to substance use. Structural modeling analysis indicated that the relation of time perspective measures to substance use was indirect, mediated through behavioral coping and anger coping. Proximal factors for substance use were negative affect, peer substance use, and resistance efficacy. Results are discussed with respect to epigenetic models and the role of executive functions in self-control ability.

Coping dimensions, life stress, and adolescent substance use: a latent growth analysis.

The relation of seven coping dimensions to substance (tobacco, alcohol, marijuana) use was tested with a sample of 1,668 participants assessed at mean age 12.5 years and two yearly follow-ups. An associative latent-growth model showed one index of engagement (behavioral coping) to be inversely related to initial level of adolescent use and growth over time in peer use. Three indices of disengagement (anger coping, helpless coping, and hangout coping) were positively related to initial levels of peer use and adolescent use and to growth in adolescent use. Life stress was positively related to initial levels for peer use and adolescent use and to growth in adolescent use. Moderation tests indicated that effects of coping were significantly greater at higher level of stress; behavioral coping buffered the effects of disengagement. Effects of life stress were greater for girls than for boys. Results are discussed with reference to mechanisms of coping-substance use relationships.

Family risk factors and adolescent substance use: moderation effects for temperament dimensions.

This research tested for moderation in the relation of family risk factors (parent-child conflict, family life events, and parental substance use) to adolescent substance use (tobacco, alcohol, and marijuana). A sample of 1,810 participants was surveyed at the mean age of 11.5 years and followed with 2 yearly assessments. Temperament dimensions were assessed with the Revised Dimensions of Temperament Survey and the Emotionality, Activity, and Sociability Inventory. Multiple-group latent growth analyses indicated moderation occurred through (a) alteration of effects of parental variables on the adolescent substance use intercept and on the peer substance use intercept and slope and (b) alteration of the effect of the peer substance use intercept on the adolescent substance use slope. The impact of parental risk factors was decreased among participants with higher task attentional orientation and positive emotionality (resilience effect) and was increased among participants with higher activity level and negative emotionality (vulnerability effect). Results from self-report data were corroborated by independent teacher reports.

Temperament and adolescent substance use: an epigenetic approach to risk and protection.

We outline an epigenetic approach to understanding the relation between simple dispositional characteristics and complex problem behaviors, with a focus on adolescent substance use. Epigenetic theory predicts that effects of temperament are mediated through self-control and risk-taking tendency, isomorphic attributes that are based in temperament but represent developmental elaborations of these characteristics. We describe how the research program has confirmed predictions from epigenetic theory, addressed additional questions embodied in the theory, and clarified the multiple pathways from temperament characteristics to life stress, social relationships, and motives for substance use. In a final section, we discuss implications for the study of problem behavior and psychopathology.