Abstract
The effects of Ni++, Co++, Zn++, Mg++ and Sr++ on synaptic transmission and electrical properties of the ganglion cell were examined by extracellular and intracellular recording in isolated bullfrog sympathetic ganglia. CaCl2 (1.8 mM) was always present in the Ringer's solution, to which the test divalent cation was then added. The linear parts of the concentration-transmission block curves were in the ranges: 0.3 to 1 mM (Co++, Zn++, Ni++) and 3 to 10 mM (Mg++, Sr++). Mg++ block was not purely synaptic, as it also depressed preganglionic axonal conduction. Intracellular recordings showed that orthodromic transmission block could be reversed by increasing stimulus repetition rates 3- or 10-fold. Antidromic spike amplitudes were significantly reduced by 10 and 26 mM Mg++ and 10 mM Co++, but significantly increased by 1 mM Co++ and 10 mM Sr++. All tested concentrations of Co++, Mg++ and Sr++ raised the current threshold for direct stimulation, and all but Sr++ reduced resting membrane potential. Sr++ was also unique in causing a high-frequency discharge of miniature synaptic potentials. The data emphasize that potency order for transmission block parallels that for depression of overshoot in the "Ca++ membrane" of barnacle muscle and that Sr++ and Mg++, despite equivalent synaptic blocking potencies, differ strikingly in their effects on electrical properties of the ganglion cell and on the frequency of miniature synaptic potentials.
Footnotes
- Received December 29, 1972.
- Accepted April 4, 1973.
- © 1972 by The Williams & Wilkins Co.
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