Transiently increased glutamate cycling in rat PFC is associated with rapid onset of antidepressant-like effects

Several drugs have recently been reported to induce rapid antidepressant effects in clinical trials and rodent models. Although the cellular mechanisms involved remain unclear, reports suggest that increased glutamate transmission contributes to these effects. Here, we demonstrate that the antidepressant-like efficacy of three unique drugs, with reported rapid onset antidepressant properties, is coupled with a rapid transient rise in glutamate cycling in the medial prefronal cortex (mPFC) of awake rats as measured by ex vivo ¹H-[¹³C]-nuclear magnetic resonance spectroscopy. Rats were acutely pretreated by intraperitoneal injection with a single dose of ketamine (1, 3, 10, 30 and 80 mg kg^-1), Ro 25-6981 (1, 3 and 10 mg kg^-1), scopolamine (5, 25 and 100 μg kg^-1) or vehicle (controls). At fixed times after drug injection, animals received an intravenous infusion of [1,6-¹³C₂]glucose for 8 min to enrich the amino-acid pools of the brain with ¹³C, followed by rapid euthanasia. The mPFC was dissected, extracted with ethanol and metabolite ¹³C enrichments were measured. We found a clear dose-dependent effect of ketamine and Ro 25-6981 on behavior and the percentage of ¹³C enrichment of glutamate, glutamine and GABA (γ-aminobutyric acid). Further, we also found an effect of scopolamine on both cycling and behavior. These studies demonstrate that three pharmacologically distinct classes of drugs, clinically related through their reported rapid antidepressant actions, share the common ability to rapidly stimulate glutamate cycling at doses pertinent for their antidepressant-like efficacy. We conclude that increased cycling precedes the antidepressant action at behaviorally effective doses and suggest that the rapid change in cycling could be used to predict efficacy of novel agents or identify doses with antidepressant activity.