An acetylcholine receptor lacking both γ and ε subunits mediates transmission in zebrafish slow muscle synapses

Fast and slow skeletal muscle types in larval zebrafish can be distinguished by a fivefold difference in the time course of their synaptic decay. Single-channel recordings indicate that this difference is conferred through kinetically distinct nicotinic acetylcholine receptor (AChR) isoforms. The underlying basis for this distinction was explored by cloning zebrafish muscle AChR subunit cDNAs and expressing them in Xenopus laevis oocytes. Measurements of single-channel conductance and mean open burst duration assigned alpha(2)beta delta epsilon to fast muscle synaptic current. Contrary to expectations, receptors composed of only alpha beta delta subunits (presumed to be alpha(2)beta delta(2) receptors) recapitulated the kinetics and conductance of slow muscle single-channel currents. Additional evidence in support of gamma/epsilon-less receptors as mediators of slow muscle synapses was reflected in the inward current rectification of heterologously expressed alpha(2)beta delta(2) receptors, a property normally associated with neuronal-type nicotinic receptors. Similar rectification was reflected in both single-channel and synaptic currents in slow muscle, distinguishing them from fast muscle. The final evidence for alpha(2)beta delta(2) receptors in slow muscle was provided by our ability to convert fast muscle synaptic currents to those of slow muscle by knocking down. subunit expression in vivo. Thus, for the first time, muscle synaptic function can be ascribed to a receptor isoform that is composed of only three different subunits. The unique functional features offered by the alpha(2)beta delta(2) receptor likely play a central role in mediating the persistent contractions characteristic to this muscle type.