Fig. 2. Effects of COMT inhibition and DAT blockade on evoked dopamine release in the NAc.

A Schematic of recording and stimulating electrodes in NAc core and VTA, respectively (see Figure S1 for precise locations). B Illustration of features of dopamine transients quantified during analysis. C Schematic of experiment structure. Drug injections are indicated below the timeline and time points of interest above it. D An example recording made in the NAc core. The pseudocolour plot shows the recorded current as a function of the applied potential of the triangular waveform (y axis) over an 11 s period (x axis); timing and duration of stimulation indicated by thick black bar. The trace above the pseudocolour plot shows the extracted current attributable to dopamine release as a function of time. The cyclic voltammogram (current versus applied voltage; boxed inset) during the peak of the signal, 1.5 s after the time of stimulation, is consistent with dopamine release. E Evoked dopamine release in NAc in animals given the COMT inhibitor as drug 1 (red) compared to release in those given vehicle as drug 1 (black) 85 min after the first injection. Release is normalized to the average pre-drug baseline peak height (equivalent to 100% on the y axis), binned over 15 min centered on the time point of interest, and presented as mean ± SEM across animals within each drug group. Timing and duration of stimulation indicated by thick black bar. F As in E, but comparing release in animals given the DAT blocker as drug 2 (blue) with release in those given vehicle as drug 2 (black) 30 min after the second injection. G–J Influence of COMT inhibition or DAT blockade on dopamine release kinetics, measured as a change from pre-drug baseline. G Quantification of the peak height of evoked dopamine release in NAc following administration of the COMT inhibitor. Left: peak height at the same time point shown in E. Each animal’s data is shown individually and is normalized to its average pre-drug baseline peak height (equivalent to 100% on the y axis). Box plots show median and 25th and 75th percentiles; whiskers extend from the minimum to maximum value. Right: normalized peak height (mean ± SEM for each drug group) of dopamine transients induced by VTA stimulation, recorded every 5 min over the 90 min following the first injection in animals that received the COMT inhibitor compared with those that received vehicle as drug 1. H As in G, but comparing data from animals that received the DAT blocker with data from those that received vehicle as drug 2; right-hand plot shows peak height over the 90 min following the second injection. I As in the left-hand plot of G, but showing the quantification of the latency from stimulation to peak (left) and of the decay from the peak to T50 (right). Decay constant data are shown on a log₁₀ scale for clarity. J As in I, but for the DAT blocker. Statistical analysis: COMT inhibition did not alter the size or kinetics of evoked release (all F < 2.4, p > 0.14; 95% CI for difference between tolcapone and vehicle: release: −27.22 to 23.84; latency: −15.99 to 8.45; decay: −18.3 to 45.38). DAT blockade increased the size of evoked release compared to vehicle (GBR-12909 x time interaction: F_1,19 = 18.1, p = 0.0004; main effect of GBR-12909: F_1,19 = 7.8, p = 0.011; main effect of time: F_1,19 = 11.5, p = 0.003). Post-hoc tests confirmed significantly greater evoked release following administration of the DAT blocker than vehicle (p = 0.002, 95% CI for difference: 26.32 to 104.66). DAT blockade also slowed the latency to peak compared to vehicle (GBR-12909 × time interaction: F_1,19 = 12.6, p = 0.002; main effect of GBR-12909: F_1,19 = 7.8, p = 0.011; main effect of time: F_1,19 = 6.3, p = 0.021; 95% CI for difference between GBR-12909 and vehicle: 12.59 to 48.58). In addition, there was an interactive effect of the DAT blocker and time on the rate of decay from peak (F_1,19 = 6.5, p = 0.020). Post-hoc tests confirmed the selective effect of the DAT blocker on the latency to peak (p = 0.002) and showed a trend level effect of DAT blockade on post-peak kinetics compared to vehicle (p = 0.078, 95% CI for difference −82.15 to 4.78), with a difference in the rate of decay in the DAT blockade group before and after receiving the drug (p = 0.007) that was not seen in the vehicle group (p = 0.565).