GABA-glutamate corelease is a mechanism for state-dependent neurotransmission

Ventral tegmental area neurons projecting to lateral habenula (LHb) corelease the main inhibitory and excitatory transmitters, GABA and glutamate (VTA^GG). Yet the functional role of this synchronous signal remains unclear. We hypothesized that the sign of VTA^GG action depends on postsynaptic state in LHb. Ex vivo, activating VTA^GG terminals evoked excitatory and inhibitory responses in LHb that varied with postsynaptic membrane potential. In vivo, VTA^GG inputs drove net inhibition and supported positive reinforcement that was dependent on GABA, but not glutamate, release. Using chemogenetics to bidirectionally modulate LHb, we found that LHb hyperpolarization shifted VTA^GG effects toward excitation, abolishing positive reinforcement, whereas LHb depolarization enhanced net inhibition and positive reinforcement. Thus, the activity state of LHb neurons dictates whether GABA-glutamate corelease is functionally inhibitory or excitatory and can reverse the motivational valence of VTA^GG input, supporting a homeostatic role for GABA-glutamate cotransmission with broad implications for disorders of imbalanced motivation.