Desensitizing Function of Calcium Mobilized by the Postsynaptic Neuronal-Type Nicotinic Acetylcholine Receptors at the Neuromuscular Junction

Neuronal-type nicotinic acetylcholine receptors (N-nAChR) are co-localized with muscle-type (M-)nAChR in the postjunctional endplate membrane of adult skeletal muscle fibers. The postsynaptic desensitizing functions of the N-nAChR at the neuromuscular junction and at single skeletal muscle cells have been investigated using aequorin luminescence and fluorescence confocal imaging. A biphasic elevation of local intracellular Ca2+ is elicited by prolonged nicotinic action at the mouse muscle endplates. The contractile fast and noncontractile slow Ca2+ components are operated by postsynaptic M- and colocalized N-type nAChR, respectively. We have named the latter slow one RAMIC (receptor-activity modulating intracellular Ca2+). The N-nAChR are activated by nicotine and choline, and RAMIC are antagonized by methyllycaconitine and dihydro-β-erythroidine. Neuromuscular functions may be regulated by a dual nAChR system to maintain the normal postsynaptic excitability. Certain N-nAChR may be also endowed with the same functional role in the central nervous system.