Evidence for Tonic Control by the GABAA Receptor of Extracellular D-Serine Concentrations in the Medial Prefrontal Cortex of Rodents

Endogenous D-serine is a putative dominant co-agonist for the N-methyl-D-aspartate glutamate receptor (NMDAR) in the mammalian forebrain. Although the NMDAR regulates the higher order brain functions by interacting with various neurotransmitter systems, the possible interactions between D-serine and an extra-glutamatergic system largely remain elusive. For the first time, we show in the rat and mouse using an in vivo microdialysis technique that the extracellular D-serine concentrations are under tonic increasing control by a major inhibitory transmitter, GABA, via the GABAA (GABAAR) in the medial prefrontal cortex (mPFC). Thus, an intra-mPFC infusion of a selective GABAAR antagonist, bicuculline (BIC), caused a concentration-dependent and reversible decrease in the extracellular levels of D-serine in the rat mPFC without affecting those of another intrinsic NMDAR coagonist, glycine and an NMDAR agonist, L-glutamate. The decreasing effects of BIC were eliminated by co-infusion of a selective GABAA agonist, muscimol (MUS) and were mimicked by a GABAA antagonist, gabazine (GBZ). In contrast, selective blockade of the GABAB or homomeric ρGABAA (formerly GABAC) receptor by saclofen or (1,2,5,6-tetrahydropyridin-4-yl)-methylphosphinic acid (TPMPA), respectively, failed to downregulate the prefrontal extracellular D-serine levels. Moreover, the local BIC application attenuated the ability of NMDA given to the mPFC to increase the cortical extracellular concentrations of taurine, indicating the hypofunction of the NMDAR. Finally, in the mouse mPFC, the reduction of the extracellular D-serine levels by a local injection of BIC into the prefrontal portion was replicated, and was precluded by inhibition of the neuronal or glial activity by co-local injection with tetrodotoxin (TTX) or fluorocitrate (Fluo), respectively. These findings suggest that the GABAAR-mediated regulation of the D-serine signaling may exert fine-tuning of the NMDAR function and require both neuronal and glial activities in the mammalian mPFC.