Identification of subtypes of muscarinic receptors that regulate Ca ²⁺ and K ⁺ channel activity in sympathetic neurons

Many different G protein-coupled receptors modulate the activity of Ca ²⁺ and K ⁺ channels in a variety of neuronal types. There are five known subtypes (M ₁ -M ₅ ) of muscarinic acetylcholine receptors. Knockout mice lacking the M ₁ , M ₂ , or M ₄ subtypes are studied to determine which receptors mediate modulation of voltage-gated Ca ²⁺ channels in mouse sympathetic neurons. In these cells, muscarinic agonists modulate N- and L-type Ca ²⁺ channels and the M-type K ⁺ channel through two distinct, G-protein mediated pathways. The fast and voltage-dependent pathway is lacking in the M ₂ receptor knockout mice. The slow and voltage-independent pathway is absent in the M ₁ receptor knockout mice. Neither pathway is affected in the M ₄ receptor knockout mice. Muscarinic modulation of the M current is absent in the M ₁ receptor knockout mice, and can be reconstituted in a heterologous expression system using cloned channels and M ₁ receptors. Our results using knockout mice are compared with pharmacological data in the rat.