Timing-dependent reduction in ethanol sedation and drinking preference by NMDA receptor co-agonist d-serine

Abstract

NMDA receptors become a major contributor to acute ethanol intoxication effects at high concentrations as ethanol binds to a unique site on the receptor and inhibits glutamatergic activity in multiple brain areas. Although a convincing body of literature exists on the ability of NMDA receptor antagonists to mimic and worsen cellular and behavioral ethanol effects, receptor agonists have been less well-studied. In addition to a primary agonist site for glutamate, the NMDA receptor contains a separate co-agonist site that responds to endogenous amino acids glycine and d-serine. d-serine is both selective for this co-agonist site and potent in boosting NMDA dependent activity even after systemic administration. In this study, we hypothesized that exogenous d-serine might ameliorate some acute ethanol behaviors by opposing NMDA receptor inhibition. We injected adult male C57 mice with a high concentration of d-serine at various time windows relative to ethanol administration and monitored sedation, motor coordination and voluntary ethanol drinking. d-serine (2.7 g/kg, ip) prolonged latency to a loss of righting reflex (LoRR) and shortened LoRR duration when given 15 min before ethanol (3 g/kg) but not when it was injected with or shortly after ethanol. Blood samples taken at sedative recovery and at fixed time intervals revealed no effect of d-serine on ethanol concentration but an ethanol-induced decrease in l-serine and glycine content was prevented by acute d-serine pre-administration. d-serine had no effect on ethanol-induced (2 g/kg) rotarod deficits in young adult animals but independently and interactively degraded motor performance in a subset of older mice. Finally, a week-long series of daily ip injections resulted in a 50% decrease in free choice ethanol preference for d-serine treated animals compared to saline-injected controls in a two-bottle choice experiment.