[Paired facilitation of the summated extracellular synaptic potentials of individual retino-tectal fibers in the frog].

The paired-pulse facilitation of the EEG quanta (the extracellular monosynaptic PSPS of all synapses of one axon) was examined in layer F of the frog tectum at normal and at increased concentrations of external Ca or Mg ions. The maximum values of facilitation were observed with intervals between stimuli of about 5 ms. Under normal conditions the amount of maximal facilitation was different for separate EEG quanta and varied from 1.4 to 2.4. Distribution of the EEG quanta by the amount of maximal facilitation had two modes at the values fmax = 1.65 and 1.95. The time course of facilitation could be approximated by two exponential components with the time constants of decays tau 1 = 5-6 ms and tau 2 = 140-150 ms for more facilitated EEG quanta, and tau 1 = 6-8 ms and tau 2 = 60-70 ms for the others. Nonequal degree of facilitation of separate EEG quanta and experiments with increased external Ca2+ and Mg2+ concentrations have suggested the dependence of paired-pulse facilitation on the quantum content of the transmitter released in retinotectal synapses. Two types of terminal arbors of axons whose synapses differ in quantum content are supposed to exist in the tectum layer F.