Role of vesicle pools in action potential pattern-dependent dopamine overflow in rat striatum in vivo

Action potential (AP) patterns and dopamine (DA) release are known to correlate with rewarding behaviors, but how codes of AP bursts translate into DA release in vivo remains elusive. Here, a given AP pattern was defined by four codes, termed total AP number, frequency, number of AP bursts, and interburst time [N, f, b, i]. The 'burst effect' was calculated by the ratio (γ) of DA overflow by multiple bursts to that of a single burst when total AP number was fixed. By stimulating the medial forebrain bundle using AP codes at either physiological (20 Hz) or supraphysiological (80 Hz) frequencies, we found that DA was released from two kinetically distinct vesicle pools, the fast-releasable pool (FRP) and prolonged-releasable pool (PRP), in striatal dopaminergic terminals in vivo. We examined the effects of vesicle pools on AP-pattern dependent DA overflow and found, with given 'burst codes' [b = 8, i = 0.5 s], a large total AP number [N = 768, f = 80 Hz] produced a facilitating burst-effect (γ[b8/b1] = 126 ± 3%), while a small total AP number [N = 96, 80 Hz] triggered a depressing-burst-effect (γ[b8/b1] = 29 ± 4%). Furthermore, we found that the PRP (but not the FRP) predominantly contributed to the facilitating-burst-effect and the FRP played an important role in the depressing-burst effect. Thus, our results suggest that striatal DA release captures pre-synaptic AP pattern information through different releasable pools.