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

Abstract

SUMMARY1. Desensitization of acetylcholine (ACh) receptors was studied at the frog neuromuscular junction under voltage clamp.2. ACh was applied directly to junctional receptors by stimulating the motor nerve with trains of impulses. End-plate currents (e.p.c.s) were used to estimate the total number of channel openings by the junctional ACh receptors, and miniature end-plate currents (m.e.p.c.s) were used to measure changes in post-synaptic sensitivity. Under the conditions of these experiments the changes in m.e.p.c. amplitudes were shown to be post-synaptic in origin and thus provided a measure of desensitization.3. When the acetylcholinesterase was inhibited with diisopropylfluorophosphate, 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/sec and then recovered after the conditioning trains, with a time constant of about 25 sec.4. When the acetylcholinesterase was active so that the duration of ACh in the synaptic cleft resulting from each nerve impulse was brief( < 300 gsec), desensitization developed in response to 300-500 pairs of nerve stimuli if the interval between the impulses of each pair was 25 msec or less. When the interval was 30 msec or greater, however, 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.5. 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 ionophoretic application of ACh.6. These results indicate that 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.7. We suggest that the desensitization observed to nerve-released transmitter is a two-step process with both steps initiated by ACh. In the first step ACh converts some receptors into a desensitizable state which has an apparent lifetime of less than 30 msec; in the second step ACh desensitizes the desensitizable state. pmY 316 K. L. MAGLEBY AND B. S. PALLOTTA

Desensitization Mechanism in Prokaryotic Ligand-gated Ion Channel

“…We find that lidocaine not only blocks GLIC but also slows desensitization, suggestive of a "foot-in-the-door" type of mechanism seen for quaternary ammonium compounds in several channels (70). In a way this result was somewhat surprising in that lidocaine has been shown to enhance desensitization of nAChR (71). Differences in the pore architecture might underlie a variation in the allosteric effects of the blocker.…”

Augmentation Increases Vesicular Release Probability in the Presence of Masking Depression at the Frog Neuromuscular Junction

“…Electrode placement at the endplates was determined by inserting the electrode along a muscle fiber until maximal EPP amplitude was achieved (Fatt and Katz, 1951). Results from previous studies indicate that postsynaptic sensitivity would be expected to remain constant for the conditions of our experiments (Magleby and Pallotta, 1981;. Thus, the changes in EPP amplitude in our experiments reflect changes in transmitter release.…”

The Number of Components of Enhancement Contributing to Short-Term Synaptic Plasticity at the Neuromuscular Synapse during Patterned Nerve Stimulation Progressively Decreases As Basal Release Probability Is Increased from Low to Normal Levels by Changing Extracellular Ca2+

“…Such surface recording gives a good measure of average intracellular response (Magleby, 1973a). Under the conditions of our experiments, where EPP amplitudes are typically a few millivolts or less because of low quantal content or the presence of curare, nonlinear summation of EPP amplitudes (McLachlan and Martin, 1981) is minimal and quantal size (miniature EPP amplitude) and postsynaptic sensitivity remain constant during a stimulation train (Magleby and Pallotta, 1981;Zengel and Sosa, 1994). Hence, observed changes in EPP amplitudes (synaptic strength) and STP in our experiments are presynaptic in origin, arising from a change in the number of vesicles whose contents are released by each nerve impulse.…”