that when the conditions for endogenous ACh release are met, a frequency-dependent limiting factor for ACh/DA release exists, most probably resulting from activation of DA or/and ACh auto-receptors. Next, we monitored DA levels following different patterns of optical stimulation to determine the optimal range of CIN activity resulting in enhanced DA release. When compared to single pulse stimulation (n=17), analysis of T80 values (see supplemental materials) of DA concentration versus time curves showed a significant T80 increase following stimulation at 5Hz (p < 0.0001, n= 15) and at 10Hz (p < 0.0001, n=16) but not at 30 Hz (n=10) (Fig. 2C, top). DA peak values were also significantly different when triggered by 5Hz (p < 0.05) and 10Hz (p < 0.001), when compared to single pulse stimulation (Fig. 2C, bottom) This suggests that CIN firing rates between ~5-10 Hz (frequencies well within the normal range of CIN firing) preferentially control release mechanisms. Application of TTX completely abolished DA release triggered by optical activation of CINs with a single pulse or 10 Hz trains (n=2; data not shown), confirming that the control of CIN activity over DA release is action potential-dependent.
