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. Author manuscript; available in PMC: 2019 Aug 22.
Published in final edited form as: Neuron. 2018 Jul 12;99(4):754–767.e4. doi: 10.1016/j.neuron.2018.06.032

Figure 5. Melanopsin increases the intrinsic excitability of M4 cells from scotopic to photopic light levels.

Figure 5.

(A) Z-stack, confocal image of an M4 cell filled with Neurobiotin (Nb, green) in retina co-immunolabeled for ChAT (magenta). Bottom panel shows a Z-projected view of the same cell in the orthogonal plane. (B) Whole-cell recording of WT M4 cells in response to a 150 pA current injection in darkness (left panels) and in dim (9 log quanta/cm2/s, top) or bright (12 log quanta/cm2/s, bottom) background illumination (right panels). (C) Firing rate plotted as a function of current injected in WT M4 cells at background illumination from 9 to 12 log quanta/cm2/s. Average dark firing rates for all cells is plotted in black. Spike output was higher in background illumination than in darkness at all light levels tested. (D) Whole-cell current clamp recording of Gq-DREADD-expressing M4 cell in an Opn4Cre/Cre retina exposed to 100 pM CNO in the dark in the presence of synaptic blockers. CNO application did not elicit depolarization, mimicking lack of depolarization in dim background light (dim light data re-plotted from Figure 4C). (E) Whole-cell recording of melanopsin null (Opn4Cre/Cre) M4 cells infected with Gq-DREADDs in response to a 100 pA current injection in darkness before (black) and after either 100 pM (light green, top) 10 nM (dark green, bottom) CNO application. (F) Firing rate plotted as a function of current injected into melanopsin null (Opn4Cre/Cre) M4 cells infected with Gq-DREADDs in the absence (black) and then presence of either 100 pM (light green) or 10 nM (dark green) CNO. Acute activation of the Gq pathway in Opn4−/− M4 cells in darkness results in increases in excitability identical to those seen in WT M4 cells in background light. All data are mean ± SEM. Recordings were made in synaptic blockers and cells were initially held at a subthreshold Vm of ~−75mV. Measurements were made from single cells in darkness and then in a single background light intensity or following application of CNO. ChAT: choline acetyl transferase. CNO: clozapine-N-oxide.