Abandoned synaptic sites in muscles of normal adult frog.

In previous investigations light microscopic cholinesterase (ChE) deposits without any nerve were found adjacent to normally occupied parts of a frog neuromuscular synapse [4, 9]. After identifying one such site at the light microscopic level ultrathin sections were cut and viewed with an electron microscope. ChE reaction product-filled secondary, clefts were observed in the region studied but a nerve was invariably missing. From this it is concluded that these loci are former synaptic sites from which nerve and Schwann cell have retracted, When incubating muscles with fluorescence-labelled alpha-bungarotoxin, all 23 abandoned sites found in 6 muscles remained bare of visible amounts of label. This indicate that receptor molecules eventually disappear from the synaptic membrane after retraction of the nerve and Schwann cell. No information as to the underlying time schedule of nerve retraction, turnover of ChE and alpha-bungarotoxin binding sites was obtained. Taken together with the evidence for synapse new formation in untreated frog muscles obtained previously [9, 11] the present observations indicate some ongoing remodeling of frog neuromuscular junction.