Glycinergic compounds for modulating accumbal dopamine levels and alcohol intake

Abstract

Alcohol use disorder (AUD) is a brain disorder that substantially contributes to the global burden of disease. The positive reinforcing properties of alcohol involve phasic dopamine (DA) release in the nucleus accumbens (nAc), a key structure of the brain reward circuitry. In AUD, neuroadaptations within this circuitry compromise its function, yielding a hypothesized hypodopaminergic state that constitutes a neurobiological correlate of anhedonia and perpetuates the disorder through negative reinforcement. Inhibitory glycine receptors (GlyRs) in the nAc regulate both alcohol- induced and basal accumbal DA levels; glycine acts as an endogenous GlyR agonist, whereas alcohol functions as a positive allosteric modulator. While intra-accumbal glycine perfusion and systemic glycine transporter 1 (GlyT1) inhibition elevate basal DA levels and reduce alcohol intake in rats, improved strategies for glycinergic treatment remain to be explored. The aim of this thesis was to investigate different glycinergic treatment approaches in modulating accumbal DA levels and voluntary alcohol intake. To this end, behavioral paradigms modeling voluntary alcohol consumption was combined with in vivo microdialysis in freely moving Wistar rats. Systemic glycine increased extracellular glycine levels in the nAc and, in a subgroup of rats, elevated basal DA levels. In another group of rats, alcohol intake was reduced, thus presenting effects similar to those produced by glycine perfused into the nAc. Glycine- containing dipeptides, hypothesized to facilitate central glycine availability, produced modest DA elevations in a subpopulation of animals but did not consistently increase extracellular glycine levels. Combining an irreversible GlyT1-inhibitor with compounds manipulating mesolimbic DA output at threshold doses yielded additive effects on basal DA levels. While all active treatments abolished the alcohol deprivation effect (ADE), no further reductions in behavior reflecting impaired control of alcohol intake could be discerned with the doses used. Immunohistochemistry confirmed the presence of glycine transporter 2 (GlyT2)-expressing fibers in the nAc. Partial, but not full GlyT2 inhibition, modestly elevated accumbal DA levels and attenuated the ADE without detectable changes in extracellular glycine. Combined GlyT1 and GlyT2 inhibition did not produce additive neurochemical effects; however, a reversible GlyT1-inhibitor robustly elevated basal DA levels, indicating that extrasynaptic rather than synaptic GlyRs near GlyT2 regulate basal DA. Altogether, these findings support the notion that differential glycinergic treatment strategies can elevate basal accumbal DA levels and reduce alcohol intake in rats. Ultimately, this work may contribute to the development of novel pharmacological approaches aimed at counteracting compromised mesolimbic DA function and targeting negative reinforcement in AUD.