POSTSYNAPTIC CURRENTS AT THE HATCHETFISH GIANT FIBER SYNAPSE

Abstract

Postsynaptic currents at the hatchetfish giant synapse were evoked in the giant fiber from an individual synapse, 10 (mu)m in diameter, and were voltage clamped.;The relation of the peak current in the psc to voltage was curvilinear, the slope being greater for more negative potentials, with no deviation from isopotentiality occuring during voltage steps in the relevant range of potentials. The peak psc during inward current at the onset of a clamping pulse was the same as during the late current, although the evoked currents were much larger than the peak current of the psc. The nonlinear reaction between peak psc and voltage may arise from voltage dependence of the channel closing rate without rectification in the single channel conductance.;Near the giant fiber resting potential (-90 mV) the postsynaptic current (PSC) had a rise time of 80 (+OR-) 10 (mu)s (n = 4) and decayed in two phases: a fast and a slow component both of which could be approximated by exponential decays. The time constant of decay of early component was 250 (+OR-) 70 (mu)s (n = 4) and the time constant of decay of the late component was 750 (+OR-) 200 (mu)s (n = 4). The early component showed little voltage dependence. The late component was voltage dependent, becoming faster at depolarizing potentials and merging with the early component near the reversal potential (-15 mV). Miniature postsynaptic currents (mpsc) had amplitudes up to 15 (mu)V and rise times as fast as 60 (mu)s at -90 mV. The decay of the mpsc was comparable to that of the psc but the noise prevented identification of two components of decay.;The two phased decay of the postsynaptic current indicates that at potentials below the reversal potential it must be described by at least two rate determining steps. Models which can account for the observed postsynaptic currents include (1) kinetic schemes in which an intermediate state is conductive and (2) models in which the removal of transmitter is one of the rate determining steps and (3) the presence of two independent populations of receptors.