Variability of transmitter quanta released during incorporation of a false transmitter into cholinergic nerve terminals

Abstract

1. Electrophysiological techniques have been used to investigate the variability of quanta released during various stages of the incorporation of acetylmonoethylcholine (AMECh) into the transmitter store at the neuromuscular junction.2. Stimulation of the rat phrenic nerve-diaphragm preparation at 3 Hz for 15-25 min reduced the time constant of decay tau(m.e.p.c.) of miniature end-plate currents from 1.61 +/- 0.08 (mean +/- S.E. of mean) msec to 1.32 +/- 0.09 msec; this represents 45% replacement of acetylcholine (ACh) by AMECh. Total replacement of ACh was achieved by a further 30-60 min stimulation which reduced tau(m.e.p.c.) to 0.93 +/- 0.06-msec.3. At the intermediate stage of exchange, the coefficient of variation c.v.(tau) of tau(m.e.p.c.) increased from the control value of 0.094 +/- 0.005 to 0.154 +/- 0.012. The latter value was less than the value (0.29) calculated for two distinct populations of quanta (i.e. pure ACh and pure AMECh quanta). Moreover, frequency distribution histograms of tau(m.e.p.c.) did not reveal a bimodal distribution. Total replacement of ACh by AMECh achieved by the second period of stimulation decreased c.v.(tau) towards the control value.4. Pre-incubation of muscles with MECh (to replace any stored choline) or stimulation in the presence of a cholinesterase inhibitor (to prevent ACh destruction and hence re-uptake of choline) did not produce any greater degree of heterogeneity than was seen in the absence of these procedures.5. The increase in c.v.(tau) observed at the intermediate stage of replacement disappeared spontaneously with a half-time of about 1 hr at 20 degrees C.6. These results are discussed in terms of the vesicle hypothesis.