Glutamate NMDA receptors containing GluN2C subunit relay the reward signal of the ventral tegmental area upon dorsal raphe stimulation

Background: Glutamate relays the reward signal from the dorsal raphe (DR) to ventral tegmental area (VTA) dopamine (DA) neurons. However, its role is complex and not clearly understood. We measured NMDA receptors subunits expression in limbic brain areas and studied the effects of VTA down-regulation of GluN2C subunit-containing NMDA receptor on the reward signal that arises from DR electrical stimulation. Methods: Via qPCR, we identified the relative composition of the different GluN2 NMDA receptors subunits in several brain areas. Then we used fluorescent in situ hybridization (FISH) to evaluate the colocalization of Grin2c and Tyrosine hydroxylase (Th) mRNA in VTA neurons. To assess the role of GluN2C in reward pursuit; we downregulated this receptor using small interfering RNA (siRNA) in rats self-stimulating for electrical pulses delivered to the DR. To delineate further the specific role of GluN2C in relaying the reward signal, we pharmacologically altered the function of VTA NMDA receptors by either bilaterally microinjecting the NMDA receptor antagonist PPPA or the glutamate transporter inhibitor (GLT-1), Dhk. Results: We identified that GluN2C is the most abundant NMDA receptor subunit expressed in the VTA. FISH revealed that a substantial number of TH+ neurons colocalize with Grin2C. The siRNA manipulation produced a selective down-regulation of GluN2C and a significant reduction in reward pursuit. Interestingly, PPPA and DHK respectively enhanced and suppressed reward pursuit, but only in rats that received the non-active RNA sequence. Conclusion: The present results suggest that VTA Glutamate neurotransmission relays the reward signal initiated by DR stimulation by acting on GluN2C NMDA receptors.