A study of desensitization of acetylcholine receptors using nerve‐released transmitter in the frog

Publisher Website
Google Scholar
Abstract
Desensitization of acetylcholine (ACh) receptors was studied at the frog neuromuscular junction under voltage clamp. ACh was applied directly to junctional receptors by stimulating the motor nerve with trains of impulses. End-plate currents (EPC) were used to estimate the total number of channel openings by the junctional ACh receptors, and miniature end-plate currents (MEPC) were used to measure changes in post-synaptic sensitivity. Under the conditions of these experiments the changes in MEPC amplitudes were post-synaptic in origin and thus provided a measure of desensitization. When the acetylcholinesterase was inhibited with DFP, neostigmine or collagenase treatment to prolong the duration of the nerve-released ACh in the synaptic cleft, desensitization developed during repetitive stimulation of 1000 impulses at 5-33 impulses/s and then recovered after the conditioning trains, with a time constant of about 25 s. When the acetylcholinesterase was active so that the duration of ACh in the synaptic cleft resulting from each nerve impulse was brief (< 300 .mu.s), desensitization developed in response to 300-500 pairs of nerve stimuli if the interval between the impulses of each pair was 25 ms or less. When the interval was 30 ms or greater, measurable desensitization did not occur, even if the total number of channel openings was many times greater than in the experiments with shorter intervals or inhibited esterase where desensitization readily occurred. The desensitization observed to pairs of impulses was enhanced by chlorpromazine and decreased when the post-synaptic membrane was depolarized, properties similar to those described previously for desensitization to bath and iontophoretic application of ACh. Apparently desensitization to nerve-released transmitter is not a simple consequence of receptor activation, is not due to blockade of the open receptor channels by ACh, and does not result from ACh binding directly to desensitized receptors with a resulting shift in the receptor population towards the desensitized state. The desensitization observed to nerve-released transmitter may be a 2-step process with both steps initiated by ACh. In the 1st step ACh converts some receptors into a desensitizable state which has an apparent lifetime of < 30 ms; in the 2nd step ACh desensitizes the desensitizable state.