| Abstract: | Dependence of the inhibitory action of adenosine on the extracellular composite EPSP on the concentrations of Mg and Ca cations in the medium was investigated in isolated slices of rat hippocampusin vitro. Extracellular EPSPs were derived in the region of apical dendrites of pyramidal cells in area CA1 during stimulation of Schaffer's collaterals. The blocking action of bivalent cations (an increase in Mg++ or a decrease in Ca++) developed almost five times more slowly than the action of adenosine. An increase in the external Mg++ concentration potentiated whereas a decrease weakened the inhibitory action of adenosine. Ca++ ions had the opposite effect. Antagonistic relations were exhibited between Mg++ and Ca++ ions. Analysis of dose-response curves for adenosine showed that during a simultaneous increase in the extracellular Ca++ and decrease in Mg++ concentrations, not only was the maximal effect of adenosine reduced, but so also was its binding constant with the receptor. The results suggest that antagonism between Ca cations and adenosine is mixed in character — both competitive and noncompetitive. The possible mechanism of the inhibitory action of adenosine on synaptic transmission and the role of bivalent cations in this process are discussed.A. A. Bogomolets Institute of Physiology, Academy of Sciences of the Ukrainian SSR, Kiev. Translated from Neirofiziologiya, Vol. 16, No. 4, pp. 532–539, July–August, 1984. |
