Physiology of a bidirectional, excitatory, chemical synapse

Neurons of the motor nerve net of the jellyfish Cyanea are connected by chemical synapses that, from their ultrastructure, appear to be bidirectional chemical synapses. These synapses were examined physiologically, by recording intracellularly from synaptically connected cells, with the whole cell configuration of the patch-clamp recording technique. Subthreshold depolarizations produced neither small voltage responses indicative of electrical coupling, nor unitary depolarizations suggestive of excitatory postsynaptic potentials (EPSP). Synaptic transmission was affected only when the presynaptic cell was depolarized above spike threshold. The synaptic delay was slightly less than 1 ms at room temperature. The postsynaptic response was initially suprathreshold, resulting in an action potential, but with time this gave way to a large 60 mV amplitude EPSP that did not produce action potentials. The amplitude of the EPSP was directly related to the postsynaptic membrane potential and extrapolated to a reversal potential close to zero mV. Reversal of the EPSP was never observed, even in the presence of intracellular tetrathylammonium (TEA). The relationship between presynaptic depolarization and postsynaptic response was difficult to examine in normal conditions, but in the presence of extracellular lidocaine, which blocked the Na+ and K+ channels in these membranes, a distinct relationship was apparent. The synapse was physiologically nonpolarized and conducted equally well in either direction with a constant synaptic delay.