After-hyperpolarizations produced in frog motoneurons by excitatory amino acid analogues

After-hyperpolarizations (AHPs) produced in frog motoneurons by applications of the excitatory amino acid analogues quisqualate (QUIS), N-methyl-D-aspartate (NMDA), and kainate (KA) were studied in the isolated hemisected frog spinal cord using sucrose gap techniques. AHPs were present following 98% of QUIS-induced depolarizations, but were seen in only 35% and 15% of NMDA- and KA-evoked responses respectively. AHPs produced by QUIS are produced both by direct effects of QUIS on motoneuron membranes and by indirect effects mediated through a synaptic process involving interneurons. Thus, application of Mg2+, Mn2+, or tetrodotoxin (TTX) in concentrations sufficient to block synaptic transmission and interneuronal firing, reduced, but did not abolish the AHPs produced by QUIS. In contrast, NMDA- and KA-AHPs appear to be entirely mediated by indirect means as block of synaptic transmission and interneuronal firing eliminated AHPs produced by these substances. Exposure of the cord to Mn2+ after addition of TTX did not affect the size of QUIS-AHPs. In the presence of TTX, QUIS-AHPs were reduced or completely blocked by addition of dinitrophenol (DNP) and sodium cyanide, by dihydro-ouabain, by removal of K+ from the superfusate, by cooling, and by replacement of 50% of the external Na+ with Li+. The results suggest that the QUIS-AHPs are largely the result of the direct effect of the excitatory amino acid agonist on motoneuron membranes and is caused by activation of an electrogenic Na+ pump. AHPs following depolarizations evoked by NMDA and KA are presumably the result of indirect actions of these latter analogues on interneurons.