Differential Subcellular Distribution and Release Dynamics of Co-transmitted Cholinergic and GABAergic Synaptic Inputs Modifies Dopaminergic Neuronal Excitability.

We identified three types of monosynaptic cholinergic inputs spatially arranged onto medial substantia nigra dopaminergic neurons in male and female mice: co-transmitted acetylcholine (ACh)/GABA, GABA only, and ACh only. There was a predominant GABA-only conductance along lateral dendrites and soma-centered ACh/GABA co-transmission. In response to repeated stimulation the GABA conductance found on lateral dendrites decremented less than the proximally located GABA conductance, and was more effective at inhibiting action potentials. While soma-localized ACh/GABA co-transmission showed depression of the GABA component with repeated stimulation, ACh-mediated nicotinic responses were largely maintained. We investigated whether this differential change in inhibitory/excitatory inputs leads to altered neuronal excitability. We found that a depolarizing current or glutamate preceded by co-transmitted ACh/GABA was more effective in eliciting an action potential compared to current, glutamate, or ACh/GABA alone. This enhanced excitability was abolished with nicotinic receptor inhibitors, and modulated by T- and L-type calcium channels; thus, establishing that activity of multiple classes of ion channels integrate to shape neuronal excitability.Our lab has previously discovered a population of substantia nigra (SN) dopaminegic neurons (DA) that receive co-transmitted ACh and GABA. This study used subcellular optogenetic stimulation of cholinergic presynaptic terminals to map the functional ACh and GABA synaptic inputs across the somatodendritic extent of SN DA neurons. We determined spatially clustered GABA only inputs on the lateral dendrites while co-transmitted ACh and GABA clustered close to the soma. We have shown that the action of GABA and ACh in co-transmission spatially clustered near the soma play a critical role in enhancing glutamate-mediated neuronal excitability through the activation of T-type and L-type voltage-gated calcium channels.