Destabilization of junctional ACh receptors by a reinnervating frog motor nerve.

The distribution of ACh receptors (AChRs) in neuromuscular junctions of the frog cutaneous pectoris muscle has been investigated at various times after denervation, as the motor nerve regenerates to reinnervate the junctions. Original extent of synaptic gutter is judged by the distribution of ACh esterase, which is compared with the staining pattern of rhodamine-labelled alpha-bungarotoxin. It is found that within 8-10 days after the beginning of reinnervation, many junctions show loss of AChR from large stretches of original gutter, in contrast to other regions, which still have a high density of receptors. This loss of receptor does not happen if the muscle remains denervated. It is suggested that the innervated portions of the junctions retain AChR, while the receptors in uninnervated portions are destabilized and rapidly lost. These findings indicate a powerful role for the nerve in eliminating both extrajunctional receptors and junctional receptors that are not stabilized by the direct action of an overlying nerve. One can imagine that a similar removal of AChR or other important synaptic molecules from gutters that are innervated later or by a terminal of lower efficacy might account for the competitive elimination of certain inputs and retention of others during development of reinnervation.