Analysis of Combined Action of Antagonists of Excitatory and Inhibitory Amino Acids on Motoneuron Postsynaptic Potentials of the Frog Rana ridibunda

The effect of blockers of excitatory and inhibitory amino acid receptors on postsynaptic potentials (PSP) evoked by activation of three synaptic inputs of the lumbar motoneuron (stimulation of the dorsal root, reticular formation, ventral and lateral columns) was studied on preparation of the isolated spinal cord of the frog Rana ridibunda. It has been shown that sensitivity of PSP to antagonists differs in different motoneurons, in the same motoneuron at activation of different inputs, and in the same input in different PSP components. It has been found that many descendent (DC) PSPs resistant to kynurenate or CNQX [1] were inhibited by blockers of inhibitory receptors. In this case the early component of DC-PSP varied considerably by amplitude and changed its polarity from positive to negative on the background of a low transmembrane depolarizing current. These changes were absent under conditions of replacement of chlorine ion by sulfate in the perfusion solution or treatment of the spinal cord with a blocker of inhibitory amino acids. All this allows suggesting that these DC-PSPs or their components were inhibitory. A part of PSPs resistant to kynurenate and CNQX were also resistant to the blockers of inhibitory amino acids (strychnine, picrotoxin, and bicuculline). In some cases, as a result of treatment with convulsants, the same blockers of excitatory receptors inhibited the initially resistant PSPs.