ACh activation of M1 receptors elevates MSN excitability by promoting the closure of several potassium channels, including Kv7 (KCNQ) [38], and Kir2 [27], on the postsynaptic side. Unlike Kv7 and SK channels, which are active only near spike threshold, Kir2 channels are constitutively active. Kir2 channels are dendritically positioned in MSNs as in many other types of neurons [27]. Three Kir2 channel subunits are abundant in MSNs. Kir2.1 subunits in MSNs are largely restricted along dendritic shafts. By contrast, Kir2.3 subunits are found primarily in spines. Kir2 channels mediate the hyperpolarization common to MSNs at rest. ACh modulates Kir2 conductance, by reducing its opening [27], profoundly altering dendritic input resistance - a key factor controlling dendritic excitability and synaptic integration.
