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 (VTAGG). Yet the functional role of this synchronous signal remains unclear. We hypothesized that the sign of VTAGG action depends on postsynaptic state in LHb. Ex vivo, activating VTAGG terminals evoked excitatory and inhibitory responses in LHb that varied with postsynaptic membrane potential. In vivo, VTAGG 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 VTAGG 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 VTAGG input, supporting a homeostatic role for GABA-glutamate cotransmission with broad implications for disorders of imbalanced motivation.