Quantal analysis of transmitter release at an inhibitory synapse in the central nervous system of the leech.

Abstract

The quantal nature of transmitter release was analysed at central inhibitory synapses in the leech [Hirudo medicinalis] nervous system between an interneuron (HN) and a motoneuron (HE) that regulate heartbeat. Ganglia were bathed in leech Ringer fluid containing 20 mM-Mg and 1.8 mM-Ca and the membrane of the presynaptic HN interneuron was hyperpolarized by current injection. Under these conditions successive inhibitory potentials in the HE motoneuron, evoked by impulses in the HN interneuron, showed striking fluctuations in amplitude. Assuming a Poisson distribution of the IPSP [inhibitory postsynaptic potentials] and estimating the number of failures from the amplitude histograms of the observed responses, the mean size of the quantal unit was estimated as 0.25 .+-. 0.015 mV (SE of mean, n = 26). When m, the mean number of quanta released per trial, was varied by changing the membrane potential of the presynaptic HN cell (Nicholls et Wallace, 1978), the experimentally observed amplitude distributions could be predicted by the Poisson theory. An independent estimate of the unit size was obtained by noise analysis. A long subthreshold depolarizing pulse applied to the presynaptic HN interneuron evoked a sustained hyperpolarization of the HE motoneuron, apparently caused by an increase in the rate of on-going release of quanta by the HN cell terminals. From the mean change in membrane potential and the increase in variance, the size of the unit was calculated as 0.21 .+-. 0.039 mV (SE of mean, n = 11). For 10 pairs of cells an estimate of unit amplitude was made both from the Poisson analysis and the analysis of variance, again with good agreement. For these cells, the estimated unit sizes were 0.24 .+-. 0.023 mV (SE of mean, n = 10) from the failures of 0.21 .+-. 0.043 mV (SE of mean, n = 10) from the noise. A similar analysis was made of the inhibitory synaptic potentials evoked in one HN interneuron by stimulation of its contralateral homologue. Transmission appeared to be quantal; the mean unit amplitude from Poisson analysis was 0.31 .+-. 0.022 mV (SE of mean, n = 19) and from the noise 0.29 .+-. 0.027 mV (SE of mean, n = 3). Transmitter was released from the terminals of the HN interneuron in quantal units that evoked miniature IPSP of about 0.25 mV in the post-synaptic cells. Modulation of transmission produced by variation in the presynaptic resting potential and during presynaptic inhibition resulted from changes in the mean number of quanta released by each impulse.