Assignment of muscarinic receptor subtypes mediating G-protein modulation of Ca ²⁺ channels by using knockout mice

Abstract

There are five known subtypes of muscarinic receptors (M 1 -M 5 ). We have used knockout mice lacking the M 1 , M 2 , or M 4 receptors to determine which subtypes mediate modulation of voltage-gated Ca 2؉ channels in mouse sympathetic neurons. Muscarinic agonists modulate N-and L-type Ca 2؉ channels in these neurons through two distinct Gprotein-mediated mechanisms. One pathway is fast and membrane-delimited and inhibits N-and P͞Q-type channels by shifting their activation to more depolarized potentials. The other is slow and voltage-independent and uses a diffusible cytoplasmic messenger to inhibit both Ca 2؉ channel types. Using patch-clamp methods on acutely dissociated sympathetic neurons, we isolated each pathway by pharmacological and kinetic means and found that each one is nearly absent in a particular knockout mouse. The fast and voltage-dependent pathway is lacking in the M 2 receptor knockout mice; the slow and voltage-independent pathway is absent from the M 1 receptor knockout mice; and neither pathway is affected in the M 4 receptor knockout mice. The knockout effects are clean and are apparently not accompanied by compensatory changes in other muscarinic receptors.

Towards a muscarinic hypothesis of schizophrenia

“…63,64 However, the M 5 -subtype may be relevant to schizophrenia as it is located in the brainstem and midbrain, where it has an effect on dopamine release. 65 Based on their functional activity, muscarinic receptors can be subdivided into two groups (M 1 , M 3 [66][67][68][69] A better understanding of the physiological role of the different subtypes of the muscarinic receptors has been gained from the study of knockout animals that lack one or more of these receptors; [70][71][72][73][74][75] for a review see Bymaster et al 76 ). Depending on the muscarinic receptor subtype involved, cholinergic activation can have different effects on the peripheral and central nervous function.…”

Role of the M1 receptor in regulating circadian rhythms

“…Up-regulation of M 2 , M 3 , or M 4 receptors was not detected in the hippocampus [22]. Deficits of function reported include loss of the muscarinic receptor-dependent M-current K ϩ channel activity in sympathetic ganglion neurons, reduced susceptibility to pilocarpine-induced cortical seizure [22], and disabled N-and L-type Ca 2 ϩ -mediated neurotransmitter release in superior cervical ganglion [23]. Prelimary studies [24] suggest that the M 1 KO mouse as well as transgenic mice with other muscarinic receptor deficits offer important models for critically evaluating cholinergic effects on the circadian clock.…”

Characterization of a CNS penetrant, selective M1muscarinic receptor agonist, 77‐LH‐28‐1

“…There are five mAChR subtypes (M 1 -M 5 ; Alexander et al, 2007): M 1 , M 3 and M 5 receptor subtypes couple predominantly through G q/11 proteins to mobilize intracellular calcium (Caulfield and Birdsall, 1998) and regulate multiple ion channel conductances, including the tonically active, voltage-dependent M current (Hamilton et al, 1997) as well as both N-and P/Q-type Ca 2 þ conductances (Shapiro et al, 1999). In contrast, M 2 and M 4 receptor subtypes couple predominantly through G i/o proteins to inhibit adenylyl cyclase and decrease intracellular cAMP levels (Caulfield and Birdsall, 1998).…”