Dual‐component amino‐acid‐mediated synaptic potentials: excitatory drive for swimming in Xenopus embryos.

Abstract

The neuronal basis of the excitation received by motoneurons during swimming in curarized Xenopus embryos was investigated. Extracellular stimulation of axons in the fiber tracts of the spinal cord were used to evoke unitary excitatory post-synaptic potentials (p.s.p.s.) in motoneurons. The p.s.p.s. had a rise time of 3-5 ms and a long falling phase lasting up to 200 ms. These potentials consist of 2 components: a fast p.s.p. which is insensitive to 50 .mu.M-(.+-.)-2-amino-5-phosphonovaleric acid (APV) but is blocked by 2 mM-cis-2,3-piperidine dicarboxylic acid (PDA) and is therefore probably mediated by kainate/quisqualate receptors, and a slow p.s.p. which is blocked by both APV and PDA and is therefore probably mediated by N-methyl-D-aspartate (NMDA) receptors. Paired intracellular recordings from motoneurons and interneurons have revealed a class of spinal cord interneuron which makes descending excitatory amino-acid-dependent synapses onto motoneurons and commissural interneurons. The p.s.p.s. evoked by intracellular stimulation of these excitatory interneurons consist of fast and slow components identical in shape and pharmacological properties to those of the extracellularly evoked potentials. One neuron may, therefore, be able to release a transmitter which activates both NMDA and non-NMDA receptors on the same post-synaptic neuron generating fast and slow post-synaptic potentials. The excitatory interneurons play an important role in the generation of the swimming pattern in the curarized Xenopus embryo. Like motoneurons, they fire once per swimming cycle in phase with the ipsilateral motoneurons and receive a background excitation during swimming that is excitatory amino acid mediated. They are therefore part of the swimming rhythm generator. The temporal summation of the extracelluarly evoked p.s.p.s. shows that these excitatory interneurons are sufficient to generate the excitatory drive received by motoneurons during swimming.