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. Author manuscript; available in PMC: 2018 Jul 10.
Published in final edited form as: Curr Biol. 2017 Jun 22;27(13):1900–1914.e4. doi: 10.1016/j.cub.2017.05.084

Figure 2. Medial SNc expressed mainly cholinergic mediated GABAergic neurotransmission and biphasic GABAergic and nAChR currents including coreleased GABA and ACh responses.

Figure 2

(A, B) Blue light activation of cholinergic fibers in the medial SNc elicited a fast IPSC mediated by presynaptic nicotinic receptors (Vh = −20 mV). The disynaptic (indirect) response was blocked by both GABAA receptor and nAChR antagonists. (C) Monosynaptic (direct) GABA currents held at different potentials in the presence of nAChR antagonists and its corresponding I–V plot (D). (E) Bicuculline sensitive monosynaptic (direct) GABAA current (Vh= −20 mV) and nAChR antagonist cocktail (MEC, MLA and DHβE) sensitive nicotinic current (Vh= −70 mV) recorded in the same DA neuron, indicating corelease of ACh and GABA. The lack of inhibition of GABAA currents with nAChR inhibitors suggests release of GABA from cholinergic neurons. (F) Direct monosynaptic GABAA currents were significantly reduced in ChAT-ChR2-VGAT KO mice as compared to ChAT-ChR2-VGAT WT. No differences were found in blue light evoked disynaptic (indirect) GABAA currents between ChAT-ChR2-VGAT WT and ChAT-ChR2-VGAT KO mice. (G) Traces of biphasic direct and indirect GABAA mediated IPSCs with their corresponding nAChR currents. (H) Onset latencies for direct and indirect IPSCs over different blue light stimulation intensities. (I) The mean latency of current onset for nAChR currents was significantly greater than that of direct GABAergic currents, while the mean latency of current onset for nAChR currents iwas significantly less than that of indirect GABAergic currents. (J) Further evidence that coreleased nAChR EPSCs and GABAergic IPSCs are monosynaptic since 4-AP was able to restore the EPSC and IPSC following abolishment with TTX. (K) Meanwhile, indirect GABA IPSCs were disynaptic since 4-AP was unable to rescue the elimination of the response with TTX. (L) Injection of a AAV floxed ChR2-YFP into the PPT of ChAT-cre mice resulted in blue light evoked indirect GABAergic IPSCs that were reversibly inhibited by nAChR inhibitors. The PPT injected mouse also expressed coreleased direct GABAergic IPSCs and nAChR EPSCs that were blocked by GABAzine and nAChR inhibitors. See also Figures S1–S3, and Tables S1 and S2.