THE EFFECTS OF POTASSIUM CHANNEL BLOCKING-AGENTS ON THE RESPONSES OF INVITRO FROG MOTONEURONES TO GLUTAMATE AND OTHER EXCITATORY AMINO-ACIDS - AN INTRACELLULAR STUDY

Intracellular recordings were obtained from motoneurons of the slice preparation of the in vitro frog spinal cord to assess the participation of K+ channels in the generation of amino acid-induced excitation. The amino acids were applied by fast superfusion and at low temperature to reduce their uptake processes. Intracellular Cs+, bath-applied tetraethylammonium [TEA] or 4-aminopyridine [4-AP] prolonged the spike and blocked its afterhyperpolarizations. TEA and 4-AP enhanced the synaptic activation of motoneurons which could be abolished by including Mn2+ in the superfusion fluid. Mn2+ reduced the duration of action potentials already prolonged by 4-AP but appeared to cause plateaus to develop on TEA-prolonged spikes. The hypothesis that amino acids depolarize motoneurons by closing K+ channels was tested by observing the effects of K+ channel blocking agents on amino acid responses. Neither TEA nor 4-AP reduced the excitatory depolarizations evoked by glutamate, N-methyl-D-asparate, quisqualate or DL-homocysteate. Intracellular Cs+ blocked responses to amino acids irrespective of their receptor preference, an action which has been interpreted as nonspecific.