Exploring regulation and function of dopamine D3 receptors in alcohol use disorder. A PET [11C]-(+)-PHNO study.

Preclinical studies support an important role of dopamine D3 receptors (DRD3s) in alcohol use disorder (AUD). In animals, voluntary alcohol consumption increases DRD3 expression, and pharmacological blockade of DRD3s attenuates alcohol self-administration and reinstatement of alcohol seeking. However, these findings have yet to be translated in humans. This study used positron emission tomography (PET) and [11C]-(+)-PHNO to compare receptor levels in several dopamine D2 receptor (DRD2) and DRD3 regions of interest between AUD subjects in early abstinence (n = 17; 6.59 ± 4.14 days of abstinence) and healthy controls (n = 18). We recruited non-treatment seeking subjects meeting DSM-5 criteria for AUD. We examined the relationship between DRD2/3 levels and both alcohol craving and alcohol motivation/wanting, using a cue reactivity procedure and an intravenous alcohol self-administration (IVASA) paradigm, respectively. [11C]-(+)-PHNO binding levels in AUD subjects were significantly lower than binding in HCs when looking at all DRD2/3 ROIs jointly (Wilk's Λ = .58, F(6,28) =3.33, p = 0.013, η2p = 0.42), however there were no region-specific differences. Binding values demonstrate -12.3% and -16.1% lower [11C]-(+)-PHNO binding in the SMST and SN respectively, though these differences did not withstand Bonferroni corrections. There was a positive association between [11C]-(+)-PHNO binding in the SN (almost exclusively reflective of DRD3) and alpha (lower values reflect higher alcohol demand) in the APT after Bonferroni corrections (r = 0.66, p = 0.0080). This demonstrates that AUD subjects with lower DRD3 levels in the SN exhibit increased demand for alcohol. These results replicate previous findings demonstrating reduced DRD2/3 levels while also supporting a lack of DRD3 upregulation and potential downregulation in early abstinent AUD. Furthermore, the finding that binding in the SN is associated with alcohol demand warrants further examination.

Effects of cannabidiol on alcohol-related outcomes: A review of preclinical and human research.

Increased access to medicinal and recreational cannabis will be accompanied by greater exposure to its chemical constituents, including Δ9-tetrahydrocannabinol (Δ9-THC) and cannabidiol (CBD), the primary nonpsychoactive compound. Increasing attention has focused on CBD, in part, due to its potential therapeutic properties. Relatively little is known about how CBD might interact with other commonly used drugs. While a number of studies have explored the influence of cannabis or Δ9-THC on alcohol consumption and treatment outcomes, few have examined the effects of CBD on alcohol-related outcomes. This article reviews preclinical and human studies examining the effects of CBD administration on alcohol responses. Preliminary preclinical results suggest that CBD can attenuate alcohol consumption and potentially protect against certain harmful effects of alcohol, such as liver and brain damage. Also reviewed herein are the few existing studies involving CBD and alcohol coadministration in humans. The paucity of such studies precludes any definitive conclusions relating to CBD-alcohol interactions. Effects of CBD on alcohol use and potential therapeutic implications for alcohol use disorder are discussed. (PsycINFO Database Record (c) 2019 APA, all rights reserved).

Behavioural sensitization to alcohol: Bridging the gap between preclinical research and human models.

Alcohol use disorder (AUD) is an increasingly prevalent disorder that contributes significantly to the global burden of disease. According to the incentive sensitization theory (IST) of addiction, repeated alcohol exposure produces persistent neuroadaptations that promote the craving, relapse, and drug-taking characteristic of addiction. Critical to the IST model is the prediction that stimulant or hedonic drug effects become more pronounced with repeated exposure (i.e., sensitization). While there is an extensive body of preclinical alcohol sensitization research, there have been few studies examining this aspect of the incentive sensitization model in human alcohol research. In particular, developmental studies assessing sensitization over time in humans are lacking, due largely to ethical considerations precluding alcohol administration in alcohol-naïve individuals, and the lack of a reliable protocol for the prospective measurement of sensitization. The lack of translation between preclinical and human models of alcohol sensitization presents a significant barrier to further understanding the relevance of IST to the development and progression of AUD. The present review discusses how the gap between preclinical and clinical alcohol sensitization research can be bridged and how animal studies can inform human alcohol sensitization research.

A role for nucleus accumbens glutamate in the expression but not the induction of behavioural sensitization to ethanol.

Mechanisms underlying differential sensitivity to behavioural sensitization to ethanol (EtOH) remain poorly understood, although accumulating evidence suggests a role for glutamatergic processes in the ventral striatum. Efforts to address this issue can benefit from the well-documented fact that in any given cohort, some of the mice (High sensitized; HS) show robust sensitization, while others (Low sensitized; LS) show little, if any, sensitization. Here, we examined whether this variability might be differentially associated with nucleus accumbens (NAc) glutamate processes. Male DBA mice received 5 EtOH (2.2g/kg) or saline injections twice a week and were challenged with EtOH (1.8g/kg) 2 weeks after injection 5. When an EtOH challenge was administered 2 weeks following the induction of sensitization, HS, but not LS, mice showed a robust increase in glutamate levels (67%, P<0.01) as measured by in vivo microdialysis. In a separate cohort, the mGlu2/3 agonist LY354740 (10mg/kg), given prior to the EtOH challenge, abolished the expression of sensitization. To ascertain whether enhanced release could also be observed during the induction of sensitization, glutamate levels were measured after the 1st and 5th EtOH injection and were found to be unchanged in HS mice, although briefly elevated in LS mice at injection 5. To further assess possible glutamate involvement during the induction of sensitization, sensitizing EtOH injections were co-administered with NMDAR antagonists. At the doses used, MK-801 (0.25mg/kg) and CGS 19755 (10mg/kg) blocked the expression of sensitization, but did not significantly interfere with the development of EtOH sensitization. Within the limitations of the present design, the results suggest an important role for EtOH-induced glutamate release in the NAc when sensitization is well established, but not necessarily during the development of sensitization.

A Review of Developmental Considerations in Human Laboratory Alcohol Research.

PURPOSE OF REVIEW: Human laboratory studies involving alcohol administration have generated critical knowledge about individual differences in risk for alcohol use disorder (AUD), but have primarily involved adult populations and cross-sectional research designs. Ethical constraints have largely precluded human laboratory alcohol research in adolescence, and prospective studies have been rare. This paper provides an overview of developmental considerations in human laboratory alcohol research, with a focus on studies conducted with youth. RECENT FINDINGS: Recent human laboratory studies from Europe and Canada have examined aspects of alcohol response during late adolescence, while recent survey studies from the United States have highlighted methods for circumventing alcohol administration in studies of adolescents. SUMMARY: Across several decades of research, exceedingly few laboratory studies have examined developmental differences in alcohol responses or utilized prospective designs. Efforts to prioritize prospective research would further clarify the role of alcohol sensitivity traits as predictors or markers of AUD onset and progression.

Localized brain differences in Arc expression between mice showing low vs. high propensity to ethanol sensitization.

Behavioral sensitization to ethanol (EtOH) manifests as a progressive and enduring increase in locomotor activity with repeated drug exposure. However, not all mice sensitize to EtOH and the neuronal mechanisms mediating vulnerability and resistance to EtOH sensitization remain unclear. We examined regional brain expression of the immediate early gene activity-regulated cytoskeleton-associated protein (Arc) in order to identify brain areas in which neuroplastic changes may contribute to the development and expression of EtOH sensitization. Male DBA/2J mice received 5 biweekly injections of EtOH (2.2g/kg, i.p.) or saline (SAL). They were categorized as high- (HS) or low-sensitized (LS) on the basis of final locomotor activity scores. In both LS and HS mice sacrificed after the last sensitization injection, Arc expression was decreased throughout the brain in comparison to SAL animals. A similar pattern was seen in mice sacrificed after an EtOH challenge two weeks after the last sensitization injection. However in this cohort, Arc expression was significantly increased in the central amygdala (CeA) in LS mice and in SAL mice receiving EtOH for the first time. No significant increases in Arc expression were seen in brains of sensitized (HS) animals. These results indicate an acute EtOH challenge results in different patterns of Arc expression in brains of LS, HS, and SAL mice. The dramatic increases in Arc expression in the CeA in LS and SAL mice showing little or no behavioral activation suggests that neural activity in this region may serve to inhibit the stimulant effects of EtOH. The observation that HS mice do not show increases in Arc expression with an EtOH challenge suggests the possibility that increased tolerance to the Arc-inducing effects of EtOH may be a factor in behavioral sensitization.

Changes in dendritic spine density in the nucleus accumbens do not underlie ethanol sensitization.

Behavioral sensitization to various drugs of abuse has been shown to change dendritic spine density and/or morphology of nucleus accumbens (NAc) medium spiny neurons, an effect seen across drug classes. However, is it not known whether behavioral sensitization to ethanol (EtOH) is also associated with structural changes in this region. Here we compared dendritic spine density and morphology between mice showing High vs. Low levels of EtOH sensitization and found that high levels of EtOH sensitization were not associated with changes in dendritic spine density or spine type. Unexpectedly, however, a significant increase in the density of stubby-type spines was seen in mice that were resistant to sensitization. Since the presence of this spine type has been associated with long-term depression and cognitive/learning deficits this may explain why these mice fail to sensitize and why they show poor performance in conditioning tasks, as previously shown. A possible causal role for structural plasticity in behavioral sensitization to various drugs has been debated. In the case of EtOH sensitization, our results suggest that drug-induced changes in structural plasticity in the accumbens neurons may not be the cause of sensitized behavior.

Effects of high-frequency stimulation of the nucleus accumbens on the development and expression of ethanol sensitization in mice.

The behavioural effects of high-frequency electrical stimulation (HFS) are often similar to the effects of lesions, with the advantage of being reversible. The present study examined the effects of HFS of the nucleus accumbens (NAc), an area that has been shown to be important for sensitization to several psychostimulants, on the development and expression of EtOH sensitization. Male DBA/2 mice received five biweekly injections of EtOH (2.2 g/kg, intraperitoneally) or saline (SAL) immediately before assessments of locomotor activity (LMA). For some of the mice, each EtOH or SAL injection was preceded by 2 h of bilateral NAc HFS, whereas the remaining mice received no stimulation. Seven days after the last injection, LMA was again measured after the mice received a challenge dose of EtOH (1.8 g/kg, intraperitoneally) or SAL, either preceded or not preceded by 2 h of HFS. Mice receiving NAc HFS before EtOH injections during the sensitization period showed progressive increases in LMA that were not different from the LMA scores of EtOH-injected mice which had received no HFS. However, when the latter group was subsequently challenged after receiving HFS, a strong suppression of LMA was observed, in comparison with their own previous LMA scores (-72%) and compared with EtOH-sensitized groups challenged in the absence of HFS (-70%). A separate cohort of mice that were surgically implanted but not stimulated showed a robust EtOH sensitization response that did not differ from that of EtOH-treated mice without electrodes, demonstrating that HFS behavioural effects were not merely a result of the presence of electrodes in the NAc. These results suggest that NAc HFS may have different effects at different stages of the EtOH sensitization process, specifically suppressing expression, but not the development of EtOH sensitization. This pattern of distinct effects of NAc manipulations on different aspects of sensitization is similar to what has been reported for other drugs of abuse, suggesting a commonality of mechanisms. Our findings also suggest that the sensitization may provide a useful paradigm for the investigation of mechanisms of clinical effectiveness of HFS in humans.

Dopamine D3 receptor is necessary for ethanol consumption: an approach with buspirone.

Mesolimbic dopamine (DA) controls drug- and alcohol-seeking behavior, but the role of specific DA receptor subtypes is unclear. We tested the hypothesis that D3R gene deletion or the D3R pharmacological blockade inhibits ethanol preference in mice. D3R-deficient mice (D3R(-/-)) and their wild-type (WT) littermates, treated or not with the D3R antagonists SB277011A and U99194A, were tested in a long-term free choice ethanol-drinking (two-bottle choice) and in a binge-like ethanol-drinking paradigm (drinking in the dark, DID). The selectivity of the D3R antagonists was further assessed by molecular modeling. Ethanol intake was negligible in D3R(-/-) and robust in WT both in the two-bottle choice and DID paradigms. Treatment with D3R antagonists inhibited ethanol intake in WT but was ineffective in D3R(-/-) mice. Ethanol intake increased the expression of RACK1 and brain-derived neurotrophic factor (BDNF) in both WT and D3R(-/-); in WT there was also a robust overexpression of D3R. Thus, increased expression of D3R associated with activation of RACK1/BDNF seems to operate as a reinforcing mechanism in voluntary ethanol intake. Indeed, blockade of the BDNF pathway by the TrkB selective antagonist ANA-12 reversed chronic stable ethanol intake and strongly decreased the striatal expression of D3R. Finally, we evaluated buspirone, an approved drug for anxiety disorders endowed with D3R antagonist activity (confirmed by molecular modeling analysis), that resulted effective in inhibiting ethanol intake. Thus, DA signaling via D3R is essential for ethanol-related reward and consumption and may represent a novel therapeutic target for weaning.

Altered NMDA receptor subunit gene expression in brains of mice showing high vs. low sensitization to ethanol.

Repeated administration of ethanol (EtOH) in mice leads to behavioural sensitization, a progressive increase in locomotor activity. Since not all mice sensitize equally to EtOH, the objective of the present study was to determine whether variability in this response is associated with altered subunit gene expression of the N-methyl-d-aspartate receptor (NMDAR), a primary target of EtOH. We examined NR1, NR2A, and NR2B expression throughout the brain during the development phase of EtOH sensitization, as well as after a 14 day withdrawal period. Male DBA/2J mice received 5-6 injections of EtOH (2.2g/kg, i.p.) or saline (SAL) and were categorized as high- (HS) or low-sensitized (LS) on the basis of locomotor activity scores after the final injection. NMDAR subunits were analyzed by in situ hybridization in brains removed either immediately following the final EtOH injection or 14 days thereafter. At the end of development phase, LS mice showed increased NR2A expression in several brain areas compared to saline controls. LS animals also had greater NR1 expression in the nucleus accumbens core (+11%, p=0.05) and shell (+14%, p=0.04) compared to HS mice, and increased NR2B expression in hippocampal CA1 (+20%, p=0.05) relative to saline-treated animals. High-sensitized mice showed increased NR2A expression in the bed nucleus of the stria terminalis when compared to controls (+54%, p=0.02). No differences in gene expression between the treatment groups were seen 14 days after the final injection. These findings suggest that region-specific NMDAR subunits may play an important role in the variability associated with the induction of EtOH sensitization. Low-sensitized mice may be more sensitive to the NMDAR inhibitory effects of EtOH, with the NR1 and NR2A subunits potentially playing a key role in the failure to sensitize upon repeated EtOH exposure.

Susceptibility to ethanol sensitization is differentially associated with changes in pCREB, trkB and BDNF mRNA expression in the mouse brain.

Repeated exposure to ethanol in mice induces behavioural sensitization, a progressive increase in locomotor activity that is common to drugs of abuse. Not all mice however show sensitization to ethanol. The goal of the present study was to examine whether variability in the sensitization response to ethanol (EtOH) is differentially associated with regional brain changes in specific molecular markers associated with neuroplasticity, namely BDNF and its receptor trkB, and levels of phosphorylated cyclic AMP-regulated element-binding protein (pCREB), 14 days after withdrawal from chronic, intermittent EtOH exposure. Male DBA/2NCrl mice received 7 biweekly EtOH (2.2g/kg, i.p.) or saline (SAL) injections and were classified as Sensitized or Non-sensitized on the basis of final locomotor activity (LMA) scores. Brains were removed two weeks later for immunohistochemical and in situ hybridization analyses. Compared to SAL-treated and Non-sensitized mice, Sensitized animals showed a higher number of pCREB-immunoreactive cells in the nucleus accumbens shell (+68% and +50%, respectively) and in the bed nucleus of the stria terminalis (+61% and 46%, respectively), whereas SAL and Non-sensitized groups did not differ from each other. A different pattern was seen when BDNF and trkB mRNA levels were analyzed in the same groups. Non-sensitized mice displayed lower BDNF mRNA in several brain areas and significantly lower trkB levels throughout the brain when compared to either the Sensitized or to SAL groups, which did not differ from each other. These results indicate that sensitization to EtOH is differentially associated with increased pCREB levels in specific brain areas. The observed decrease in BDNF and trkB mRNA in the Non-sensitized group suggests the possibility that EtOH may have neurotoxic effects in a subpopulation of mice, which might in turn prevent the development of behavioural sensitization. The lack of a difference in BDNF and trkB mRNA expression between Sensitized and SAL mice suggests that EtOH sensitization may be mediated by mechanisms different from those mediating sensitization to other psychostimulants.