DMSO effects on synaptic facilitation and calcium dependence in bullfrog sympathetic ganglion

Synaptic transmission in the bullfrog sympathetic ganglion was studied by means of extra- and intracellular recordings. DMSO (3-10%) caused a single orthodromic stimulus to generate a brief burst of repetitive postganglionic discharges. DMSO also partially reversed a preexisting transmission failure in low Ca2+ medium. Ganglia were exposed to gradual reductions in extracellular Ca2+, in the absence and in the presence of DMSO. The recorded amplitude of the postganglionic compound action potential (CAP) was plotted as a function of Ca2+ concentration. In the absence of DMSO transmission failed progressively as Ca2+ was reduced from 1.8 to 0.47 mM but DMSO (3% and 10%) shifted the curve of transmission failure to the left (lower Ca2+ concentration). DMSO inhibits ganglion cholinesterase activity, but this is not the mechanism of its facilitatory effect on Ca2+ entry, since physostigmine did not shift the curve of transmission failure in low Ca2+ to the left. The data suggest that DMSO maintained transmitter release in low Ca2+ by a direct, nonspecific enhancement of Ca2+ influx into the presynaptic nerve terminal.