Dual mode of junctional transmission at synapses between single primary afferent fibres and motoneurones in the amphibian

Abstract

The isolated hemisected frog spinal cord was used to examine the effects of changes in ionic composition of perfusing medium on the intracellularly recorded EPSP [excitatory post-synaptic potential], produced in single motoneurons by direct stimulation of individual dorsal root fibers through a separate intracellular microelectrode. The monosynaptic single-fiber EPSP usually reveals 2 distinct components. The early component is resistant to replacement of external Ca2+ by Mn2+, the later is reversibly abolished. The junction between primary afferent fiber and motoneuron apparently provides joint electrical and chemical transmission. The average amplitude of the unitary electrical EPSP varies at different junctions from 25-430 .mu.V, mean 124 .+-. 17 .mu.V (n = 50). It is relatively stable and changes its amplitude only with changes in the height of presynaptic spike. The individual amplitudes observed in a given cell usually have a normal distribution, suggesting that any variability in electrical responses is entirely due to noise. The average amplitude of the single-fiber chemically mediated EPSP varies from < 20 .mu.V to 1.7 mV, mean 222 .+-. 33 .mu.V (n = 71). A positive correlation was found between the amplitudes of chemical and electrical EPSP recorded from different motoneurons. Chemical EPSP evoked by consecutive impulses in a single presynaptic fiber show statistical fluctuations in amplitude. The fluctuations occur in quantal steps in a manner described by binomial or Poisson statistics. Only in a few cases the deviation from stochastic distribution can be attributed to variable invasion of nerve impulses into the terminal region. The amplitude of the unit EPSP varies between 33-104 .mu.V, mean 66.4 .+-. 4.3 .mu.V (n = 19). The mean quantum content (m) varies from < 1 to > 10. The m is reversibly reduced by Ca2+ lack and by Mn2+ until the chemically mediated response fluctuates according to a Poisson distribution with the unit EPSP equivalent to the single quantum of transmitter. When paired or repetitive stimuli are applied to the presynaptic fiber the facilitation of the chemically mediated unitary EPSP can be observed, as well as the post-tetanic potentiation. The amplitude of the electrical EPSP remains unchanged during the period of post-tetanic potentiation; this indicates that the latter is attributable to some change restricted to the specific presynaptic mechanism responsible for the transmitter release, but not to changes in presynaptic spike height.