Cholinesterase is associated with the basal lamina at the neuromuscular junction

Abstract

THE cholinesterase (ChE) of skeletal muscle is concentrated at the neuromuscular junction, where it hydrolyses acetylcholine released from the nerve terminal^1–5. In frog muscle treated with a histochemical stain for ChE, enzyme activity is demonstrable at all terminal branches of the neuromuscular junction (Fig. 1a), with the reaction product occupying the synaptic cleft between nerve and muscle (Fig. 1d)^1,6. A particulate enzyme, ChE was assumed to be integral to the postsynaptic plasma membrane² until Hall and Kelly⁷ showed that mild protease treatment of intact muscles released active ChE into the medium without apparent damage to the plasma membrane. One possibility raised by this result is that ChE is associated with the basal lamina (BL; sometimes called basement membrane) that runs through the synaptic cleft⁸. This idea was supported by the finding that a subunit of ChE from electric organs of fishes and rays has several properties in common with collagen^9–11, a protein known to be a principal component of BLs¹². Here, we present direct evidence that junctional ChE is associated with the BL of the synaptic cleft in skeletal muscle.