This is something I've been hearing about for some time and is now on-line in Neuron from Stachniac, Ghosh and Sternson wherein they nicely demonstrate that hM4Di induces presynaptic silencing (via an unknown but likely interesting mechanism). They also report an hM4D-neurexin variant which targets to axons. We also have such a beast which may have i mproved surface expression and transport which may be similarly useful and is now available to all who wish to use it. I see the Sternson variant is now available via ADDGENE. This will be a highly useful reagent for spatio-temporal control of synaptic transmission. I note that Steve Mahler from Gary Aston-Jones' lab has also reported in Nature Neuroscience what appears to be a similar observation related to presynaptic dopamine release via local infusion of CNO.