Modulation of direct pathway striatal projection neurons by muscarinic M₄-type receptors.

Models of basal ganglia (BG) function posit a dynamic balance between two classes of striatal projection neurons (SPNs): direct pathway neurons (dSPNs) that facilitate movements, and indirect pathway neurons (iSPNs) that repress movement execution. Two main modulatory transmitters regulate the output of these neurons: dopamine (DA) and acetylcholine (ACh). dSPNs express D1-type DA, M1-and M4-type ACh receptors, while iSPNs express D2-type DA and M1-type ACh receptors. Actions of M1-, D1-, and D2-receptors have been extensively reported, but we still ignore most actions of muscarinic M4-type receptors. Here, we used whole-cell recordings in acutely dissociated neurons, pharmacological tools such as mamba-toxins, and BAC D(1 or 2)-eGFP transgenic mice to show that activation of M4-type receptors with bath applied muscarine enhances Ca(2+)-currents through CaV1-channels in dSPNs and not in iSPNs. This action increases excitability of dSPNs after both direct current injection and synaptically driven stimulation. The increases in Ca(2+)-current and excitability were blocked specifically by mamba toxin-3, suggesting mediation via M4-type receptors. M4-receptor activation also increased network activity of dSPNs but not of iSPNs as seen with calcium-imaging techniques. Moreover, actions of D1-type and M4-type receptors may add to produce a larger enhancement of excitability of dSPNs or, paradoxically, oppose each other depending on the order of their activation. Possible implications of these findings are discussed.