< 0.0001, 74.1% at 100Hz, p > 0.05; n= 3-6) (Fig. 2D). Therefore, endogenous cholinergic activity under the present experimental conditions does not produce the high-pass filtering elicited by bath application of nicotine. To confirm whether CINs follow high frequency patterns of optical stimulation, we performed intracellular electrophysiological recordings. Under whole-cell current clamp, optical stimulation of CINs at 10 Hz elicited a corresponding sequence of action potentials at 10 Hz (Fig. 2E). However, optical stimulation at 30 Hz elicited a sequence of action potentials where only the first action potential reached full amplitude (Fig. 2F), suggesting that CINs optimally follow optical stimulation at a rate of ~10 Hz. However, CIN responsiveness is limited at higher stimulation frequencies (although this may also result from ChR2 channel dynamics).
