Because of cholinergic receptor agonists modulate the release of DA, we hypothesized that changes in CIN firing rate should correspondingly enhance DA release. To test this, we measured DA following application of different patterns of CIN optical stimulation. First, we monitored DA levels in response to paired pulse optical stimulations at different intervals. When two pulses of blue light (10 mW, 4 msec duration) were delivered at an interval of 2 sec, the second pulse triggered a second peak of DA, which was 8.3 ± 1.6 % (n=4), of the first peak (Fig 2A). Subsequent tests showed lessened reduction in the second peak down of 16 ± 2.1% at 5 s intervals (n=4), 31.5 ± 3.1% at 10 s (n=4), 66.0 ± 3.0% at 30 s (n=4) and 84.75 ± 1.1% at 60 sec interval (n=4) (Fig. 2A,B). This suggests 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
