Slow channel congenital myasthenic syndrome is a rare muscle disease resulting in muscle weakness and the degradation of muscle endplates. The condition is caused by single point mutations in muscle nicotinic ACh receptor subunits. One such mutation, εL78P, is responsible for a mild form of the disease (Croxen et al. 2002) and has been investigated at the single-channel level to elucidate the effects of the mutation on channel kinetics. Single-channel recordings were made in the cell-attached configuration of HEK293 cells transfected transiently with cDNA encoding enhanced green fluorescent protein, human α, β, δ, and either wild-type ε or εL78P nicotinic receptor subunits. The receptor with εL78P produced longer bursts of openings than wild-type channels. For both types of channel the burst length distribution could be fitted with four exponential components (100 nM ACh, -100 mV, resolution 25-35 µs). The synaptic current decay depends mainly on the slowest component (τ₄) of the distribution (Wyllie et al. 1998). For wild-type channels (n = 5, mean ± S.E.M.) τ₄ = 8.4 ± 3.0 ms, for εL78P channels (n = 5) τ₄ = 54.6 ± 6.8 ms. To determine the effect of the εL78P mutation, the rate constants in a scheme with two binding sites and singly liganded openings (Colquhoun & Sakmann, 1985) were estimated from single-channel data. The likelihood of the entire sequence of open and shut times was maximised with exact allowance for missed brief events (Colquhoun et al. 1996; HJCFIT program). With these methods the total dissociation rate, diliganded channel closing rate (α₂) and diliganded channel opening rate (β₂) can be measured accurately. For the wild-type channels (n = 5) total dissociation rate was 15700 ± 887 s^-1, α₂ = 1540 ± 148 s^-1 and β₂ = 44900 ± 5040 s^-1. For εL78P channels (n = 5) total dissociation rate was 4680 ± 1120 s^-1, α₂ = 780 ± 131 s^-1 and β₂ = 63700 ± 6870 s^-1. Thus the effect of the εL78P mutation is to slow dissociation of ACh 3.4-fold, and to decrease α₂. Combined with a slight increase in β₂ this results in a 2.8-fold increase in the efficacy, defined as β₂/α₂.

This work was supported by the MRC and The Wellcome Trust.