The GlyT1-inhibitor Org 24598 facilitates the alcohol deprivation abolishing and dopamine elevating effects of bupropion + varenicline in rats.

Alcohol Use Disorder (AUD) is a relapsing brain disorder that involves perturbations of brain dopamine (DA) systems, and combined treatment with varenicline + bupropion produces additive effects on accumbal DA output and abolishes the alcohol deprivation effect (ADE) in rats. Also, direct and indirect glycine receptor (GlyR) agonists raise basal DA, attenuate alcohol-induced DA release in the nucleus Accumbens (nAc) and reduce alcohol consumption in rats. This study in rats examines whether the GlyT1-inhibitor Org 24598, an indirect GlyR agonist, enhances the ADE-reducing and DA elevating action of the combined administration of varenicline + bupropion in lower doses than previously applied. Effects on voluntary alcohol consumption, the ADE and extracellular levels of glycine and DA in nAc were examined following treatment with Org 24598 6 and 9 mg/kg i.p., bupropion 3.75 mg/kg i.p. and varenicline 1.5 mg/kg s.c., in monotherapy or combined, using a two-bottle, free-choice alcohol consumption paradigm with an ADE paradigm, and in vivo microdialysis in male Wistar rats. Notably, all treatment regimens appeared to abolish the ADE but only the effect produced by the triple combination (Org24598 + varenicline + bupropion) was significant compared to vehicle. Hence, addition of Org 24598 may enhance the ADE-reducing action of varenicline + bupropion and appears to allow for a dose reduction of bupropion. Treatment with Org 24598 raised accumbal glycine levels but did not significantly alter DA output in monotherapy. Varenicline + bupropion produced a substantial elevation in accumbal DA output that was slightly enhanced following addition of Org 24598. Conceivably, the blockade of the ADE is achieved by the triple combination enhancing accumbal DA transmission in complementary ways, thereby alleviating a hypothesized hypodopaminergia and negative reinforcement to drink. Ultimately, combining an indirect or direct GlyR agonist with varenicline + bupropion may constitute a new pharmacological treatment principle for AUD, although further refinement in dosing and evaluation of other glycinergic compounds are warranted.

Free-access intravenous alcohol self-administration in social drinkers and individuals with alcohol use disorder: Evaluation of relationships with phosphatidylethanol and self-reported alcohol consumption.

BACKGROUND: The free-access (FA) intravenous alcohol self-administration (IV-ASA) paradigm is an experimental approach that can identify modulators of alcohol consumption in humans. Moreover, the outcome measures of IV-ASA paradigms are associated with self-reported alcohol intake using the timeline follow-back method (TLFB). To evaluate how FA IV-ASA reflects drinking in real life, we examined the relationship between an objective marker of recent alcohol intake, phosphatidylethanol in blood (B-PEth), and TLFB and measures obtained during IV-ASA in individuals with alcohol use disorder (AUD) and social drinkers (SD). We also explored the associations between these measures and gut-brain peptides involved in AUD pathophysiology. METHODS: Thirty-eight participants completed a laboratory session in which they self-administered alcohol intravenously. The safety limit was 200 mg%, and main outcomes were mean and peak breath alcohol concentrations (BrAC). Blood samples were drawn prior to IV-ASA and subjective alcohol effects were rated during the experiment. RESULTS: The study sample comprised 24 SD and 14 participants with DSM-5 mild AUD. Although BrACs were not associated with B-PEth or TLFB in the full sample or AUD subgroup, there was an association with TLFB in SD. In both subgroups, BrACs were associated with alcohol craving but with differential timing. Total ghrelin levels were higher in AUD participants than in SD. CONCLUSIONS: No associations between B-PEth levels and achieved BrACs were observed in the mild AUD group, the SD group, or the full sample. The ability for FA IV-ASA to reflect recent drinking was confirmed only for TLFB in SD, whereas there were no associations within the smaller subsample of participants with mild AUD or in the full sample. Further studies that include a larger AUD sample are warranted. The association of BrACs with craving for alcohol suggests that the IV-ASA method may be useful for assessing interventions that target craving. This could be explored by using the FA IV-ASA model to evaluate the effects on craving of approved pharmacotherapies for AUD.

The glycine-containing dipeptide leucine-glycine raises accumbal dopamine levels in a subpopulation of rats presenting a lower endogenous dopamine tone.

Interventions that elevate glycine levels and target the glycine receptor (GlyR) in the nucleus Accumbens (nAc) reduce ethanol intake in rats, supposedly by acting on the brain reward system via increased basal and attenuated ethanol-induced nAc dopamine release. Glycine transport across the blood brain barrier (BBB) appears inefficient, but glycine-containing dipeptides elevate whole brain tissue dopamine levels in mice. This study explores whether treatment with the glycine-containing dipeptides leucine-glycine (Leu-Gly) and glycine-leucine (Gly-Leu) by means of a hypothesized, facilitated BBB passage, alter nAc glycine and dopamine levels and locomotor activity in two rodent models. The acute effects of Leu-Gly and Gly-Leu (1-1000 mg/kg, i.p.) alone or Leu-Gly in combination with ethanol on locomotion in male NMRI mice were examined in locomotor activity boxes. Striatal and brainstem slices were obtained for ex vivo HPLC analyses of tissue levels of glycine and dopamine. Furthermore, the effects of Leu-Gly i.p. (1-1000 mg/kg) on glycine and dopamine output in the nAc were examined using in vivo microdialysis coupled to HPLC in freely moving male Wistar rats. Leu-Gly and Gly-Leu did not significantly alter locomotion, ethanol-induced hyperlocomotor activity or tissue levels of glycine or dopamine, apart from Gly-Leu 10 mg/kg that slightly raised nAc dopamine. Microdialysis revealed no significant alterations in nAc glycine or dopamine levels when regarding all rats as a homogenous group. In a subgroup of rats defined as dopamine responders, a significant elevation of nAc dopamine (20%) was seen following Leu-Gly 10-1000 mg/kg i.p, and this group of animals presented lower baseline dopamine levels compared to dopamine non-responders. To conclude, peripheral injection of glycine-containing dipeptides appears inefficient in elevating central glycine levels but raises accumbal dopamine levels in a subgroup of rats with a lower endogenous dopamine tone. The tentative relationship between dopamine baseline and ensuing response to glycinergic treatment and presumptive direct interactions between glycine-containing dipeptides and the GlyR bear insights for refinement of the glycinergic treatment concept for alcohol use disorder (AUD).

Effects of systemic glycine on accumbal glycine and dopamine levels and ethanol intake in male Wistar rats.

Approved medications for alcohol use disorder (AUD) display modest effect sizes. Pharmacotherapy aimed at the mechanism(s) by which ethanol activates the dopamine reward pathway may offer improved outcomes. Basal and ethanol-induced accumbal dopamine release in the rat involve glycine receptors (GlyR) in the nucleus accumbens (nAc). Glycine transporter 1 (GlyT-1) inhibitors, which raise extracellular glycine levels, have repeatedly been shown to decrease ethanol intake in the rat. To further explore the rational for elevating glycine levels in the treatment of AUD, this study examined accumbal extracellular glycine and dopamine levels and voluntary ethanol intake and preference in the rat, after systemic treatment with glycine. The effects of three different doses of glycine i.p. on accumbal glycine and dopamine levels were examined using in vivo microdialysis in Wistar rats. In addition, the effects of the intermediate dose of glycine on voluntary ethanol intake and preference were examined in a limited access two-bottle ethanol/water model in the rat. Systemic glycine treatment increased accumbal glycine levels in a dose-related manner, whereas accumbal dopamine levels were elevated in a subpopulation of animals, defined as dopamine responders. Ethanol intake and preference decreased after systemic glycine treatment. These results give further support to the concept of elevating central glycine levels to reduce ethanol intake and indicate that targeting the glycinergic system may represent a pharmacologic treatment principle for AUD.