Inhibitory Regulation of Responses of Glutamate Receptors in Frog Motoneurons

Experiments on isolated frog spinal cords (Rana ridibunda) with intracellular recording of potentials from lumbar motoneurons were performed to study interactions between the effects of exogenous excitatory (glutamate and its agonists NMDA, AMPA, kainate) and inhibitory (GABA, glycine) amino acids. Application of glycine and GABA induced reductions in responses to application of glutamate to a mean of 45.8 ± 2.9% (n = 12) and 67.8 ± 3.9% (n = 16), responses to AMPA to 13.9 ± 4.3% (n = 9) and 32.1 ± 8.3% (n = 12), responses to KA to 36.8 ± 8.2% (n = 7) and 48.0 ± 11.8% (n = 6), responses to NMDA to 7.7 ± 3.5% (n = 9) and 18.1 ± 3.8% (n = 14) of control baseline levels respectively. Similar patterns were seen on sequential application of excitatory amino acids (EAA) after glycine and GABA and on application of a mixture of GABA and glycine with an EAA. The specific glycine receptor antagonist strychnine blocked the effect of suppression of EAA responses on the background of glycine but not GABA administration, while the GABAA receptor blocker gabazine decreased the effect of suppression of EAA responses induced by application of GABA. These results point to a difference in the inhibitory influences on excitatory reactions mediated by different types of glutamate receptors and particularly the predominance of suppression of excitatory reactions mediated by NMDA receptors and the relatively weak suppression of responses mediated by kainate receptors. The inhibitory effect of glycine was about twice that of GABA (at equal concentrations).