The distribution of acetylcholine receptors in the normal and denervated neuromuscular junction of the frog

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Abstract
A combined light and electron microscopic investigation was performed to study the distribution and fate of clusters of horseradish peroxidase (HRP)-labelled or rhodamine-labelled alpha-bungarotoxin binding sites in normal and denervated cutaneus pectoris muscles of the frog. After staining for axon and cholinesterase (ChE) it appears that, in normal muscles, binding sites for rhodamine alpha-bungarotoxin are strictly confined to those parts of the synaptic gutter occupied by the nerve terminal. Binding sites are missing in axon-abandoned gutters or gutters occupied only by the Schwann cell. Similarly, in partially occupied gutters, HRP-toxin binding sites are confined to the parts of the muscle fibre membrane apposed to the nerve; they are missing at axon-free lateral parts at which secondary clefts and ChE are present. These observations suggest that junctional acetylcholine (ACh) receptor clusters are strictly controlled by the nerve. Thirty-five days after denervation, receptor density was apparently reduced in some parts of the gutters while other parts of the same gutters showed high receptor density. In addition, the length of gutter totally devoid of receptor clusters increased from an average fraction of 9% in control muscles to 14% in denervated muscles. Loss of receptors occurred both at Schwann cell-free and at Schwann cell-occupied gutters. In muscles denervated for 500–750 days, no toxin binding sites could be detected in the junctional membrane, whereas ChE was still present. Schwann cells had apparently abandoned some gutters but were present at others. Upon arrival of spontaneously reinnervating axons, muscle fibres accumulate ACh receptor clusters at the junctional membrane. Patches intensely labelled with alpha-bungarotoxin and associated with ChE reaction product were found near former junctions in long-term denervated muscles. It is concluded that after long-term denervation the muscle fibre cannot maintain junctional ACh receptor clusters by itself. In normally innervated muscles, receptor clusters are actively maintained by nerve-borne factors near the transmitter release sites.