Contributions of Glycine and GABAA Receptors to the Generation of Inhibitory Postsynaptic Potentials in Frog Spinal Cord Motoneurons

Abstract

Intracellular recording from lumbar motoneurons in isolated spinal cord preparations from the frog Rana ridibunda was used to study the contributions of glycine and GABAA receptors to the generation of inhibitory postsynaptic potentials (IPSP) induced by microstimulation of fibers close to these motoneurons. IPSP were identified by blockade of excitatory synaptic transmission using kynurenate, CNQX, and AP-5 and by reversion of polarity on injection of a depolarizing current (1–10 nA) via the microelectrode. The selective glycine receptor antagonist strychnine at 1–5 μM decreased IPSP amplitude in all the motoneurons studied (by an average factor of 4.7), while the GABAA receptor antagonist bicuculline at 50–70 mM decreased the amplitude (by an average factor of 1.6) in only some motoneurons, while no decrease occurred in others. Sequential application of strychnine and bicuculline completely blocked IPSP. These data support the view that postsynaptic inhibition in frog motoneurons is mediated mainly by glycine and to a lesser extent by GABAA receptors. These latter are probably partially extrasynaptic.