Magnesium-resistant excitatory synaptic potentials in the leech Retzius cell.

Postsynaptic potentials (PSPs) recorded from leech Retzius cells in response to stimulation of interganglionic connective could not be reversed by soma depolarization or abolished by 40 mM Mg ion, nor could input resistance changes be detected during them. Alteration of external Cl and K over a tenfold range provided no clear evidence that the PSPs involved a conductance change to either ion. The method of extrapolation yielded an apparent PSP equilibrium potential of about -20 mV. The steep portion of the relationship between Retzius cell action potential amplitude and membrane potential extrapolated to an apparent reversal potential of -13 mV. It is likely that the connective-to-Retzius cell PSPs were principally electrical events. Their apparent reversal potentials could have been in the range associated with chemical synapses because they traversed an electrical synapse with a variable coupling resistance, or because the polarizing currents, passing "backwards" across electrical synapses, changed the amplitude of the presynaptic action potentials.