One critical difference between current injection and a laser pulse is the number of neurons GDC-0941 order activated: the laser beam will synchronously activate a population of ChIs and/or their axons owing to the extensive overlapping arborization of ChI axons and dendrites (Contant et al., 1996). These data therefore suggest that ChI-driven DA release occurs during synchronization of activity in ChIs. The requirement for synchronization was confirmed by showing that laser stimuli that minimize synchrony in ChIs did not evoke DA release. To achieve this, we recruited activity gradually in a population of ChIs by slowly ramping laser

intensity during continuous exposure until threshold for spiking was reached in a given recorded ChI. Using this protocol, outcome on activity in each ChI was variable (e.g., threshold intensity, see variation in spike frequency in Figure 2C, n = 6) and this protocol did not evoke DA release (Figure 2C, n = 6). Multiple spikes in a given ChI per se did not preclude DA release

since longer duration laser pulses above threshold that evoked burst firing in ChIs were accompanied by DA release (Figure 2D, n = 11). These data show that synchronous check details recruitment of activity in a population of ChIs and/or axons evokes DA release. We also noted that multiple action potentials in a given ChI induced by long laser pulses did not evoke more DA release than a single action potential (compare Figures 2D and 2B), suggesting that ChI-driven DA release does not convey frequency information from individual ChIs. This weak relationship between frequency and DA release is also seen with striatal electrical stimulation when DA axons and ChIs are simultaneously depolarized (Rice and Cragg, 2004 and Zhang and Sulzer, 2004), but not with stimulation of medial forebrain bundle when DA axons are activated (Chergui et al., 1994). These observations suggest that ChI-driven DA release does not report frequency and see more moreover that it may limit how frequency information in ascending

DA axons is transduced into DA release. We therefore explored the relationships between frequency of activation and DA transmission during activation of ChIs only, DA axons only, or both in combination. Trains of four laser pulses at a range of frequencies in ChR2-expressing ChAT-Cre striatum reliably evoked four action potentials in ChIs at corresponding frequencies (Figure 3A), but the consequent DA release was invariant, reaching only DA levels seen with a single light pulse (and single action potentials) (Figures 3B and 3D, n = 8). This refractoriness (or depression) of rerelease after release by single synchronized spikes in ChIs was therefore not due to spike attenuation in ChIs (and was also not due to activation of mAChRs or D2 receptors causing ACh terminal inhibition, data not shown). These data show that ChI-driven DA release is not a direct readout of the frequency of activity in a given ChI.