A Role for Glycine Transporter-2 Inhibitors in Regulating Accumbal Dopamine Levels and Voluntary Alcohol Intake in Rats.

BACKGROUND: Glycinergic neurotransmission has been suggested as a tentative pharmacological treatment target for Alcohol Use Disorder (AUD), a devastating brain disease with limited treatment options that involves reduced dopamine (DA) activity in the nucleus Accumbens (nAc) and loss of control of alcohol intake. Glycine receptors (GlyRs) in the nAc, a pivotal part of the brain reward circuitry, are involved in regulating basal and alcohol-induced accumbal DA levels as well as voluntary alcohol intake in rats. METHODS: This study in rats examined if inhibition of the glycine transporter 2 (GlyT-2, SLC6A5), present within glycinergic synapses, alters (1) accumbal DA and glycine content as measured by in vivo microdialysis coupled to HPLC and (2) the alcohol deprivation effect (ADE) in a two-bottle-choice alcohol consumption paradigm. Finally, (3) the presence of GlyT2-expressing fibers and GlyRs in the nAc was examined by immunohistochemistry (IHC). RESULTS: In brief, the two partial GlyT2-inhibitors N-Arachidonyl-Glycine (NAGly, 10 mg/kg i.p.) and Opiranserin (20 mg/kg i.p.), but not the full GlyT2-inhibitor Org 25543, slightly elevated accumbal DA levels, while no effects on extrasynaptic glycine levels were observed. The reversible GlyT1-inhibitor Bitopertin (10 mg/kg p.o.) produced a pronounced glycine and DA elevation but combined GlyT1- and GlyT2 inhibition did not yield any statistically significant additive effects. In the alcohol consumption study, NAGly abolished the ADE as compared to placebo treatment. Finally, histology confirmed the presence of GlyT2 and GlyR in the nAc, although with a higher signal for GlyT2 in, for example, septal and basal forebrain regions. CONCLUSIONS: Overall, this study indicates that synaptic GlyRs are involved in regulating nAc DA levels and alcohol intake in rats, but probably to a lesser extent than extrasynaptic GlyRs. Further studies that specifically target GlyT2 in, for example, the nAc and septal regions and that utilize refined techniques for measuring synaptic glycine levels are warranted to better characterize the role of glycinergic innervation in the reward circuitry and for regulating alcohol intake.