Study of frequency-dependent relations between CIN stimulation and DA levels showed a clear paired-pulse depression, suggesting strong mechanisms of presynaptic control of release at either, or both, CIN and DA neuron terminals. Although this has been described separately at DA and ACh synapses, more detailed studies are necessary to demonstrate how interactions between these two sites of release interact into determining final DA levels. Moreover, we report that sustained optical stimulation of CINs does not mimic the nicotine-dependent high-pass filtering of electrically evoked DA release (Exley and Cragg, 2008). Together, these results point to a crucial role of mAChR activation in limiting the effects of persistent endogenous ACh activity on nAChRs. This feedback mechanism is absent under the effect of nicotine, which promotes desensitization of nAChRs, thought to be the main mechanism underlying nicotine-evoked high-pass filtering of DA release (Rice and Cragg, 2004; Exley and Cragg, 2008). In support of this notion, we confirmed that β2-containing nAChRs mediate ACh-evoked release of DA, and that mAChRs play a predominant role in limiting endogenous ACh release, because ACh-evoked DA release is enhanced (albeit modestly) following blockade of mAChRs.
