Characterization and Function of Spinal Excitatory Interneurons with Commissural Projections in Xenopus laevis embryos

The multipolar somata of dorsolateral commissural (dlc) interneurons (Roberts and Clarke, 1982) lie in a superficial dorsolateral position in the spinal cord of Xenopus laevis embryos. By applying horseradish peroxidase to one‐half of the 100 μm diameter spinal cord, these neurons have been backfilled. Their dendritic branching pattern, commissural axonal projection and distribution near the time of hatching is described. Using Lucifer yellow‐filled microelectrodes a population of sensory interneurons with dlc morphology has been identified. They have multipolar somata in a dorsolateral superficial position, obliquely projecting dendrites and a ventral commissural axon. They receive presumed monosynaptic excitation in response to electrical stimulation of sensory neurites in the skin on the same side as the soma. During fictive swimming activity in curarized embryos the die interneurons are rhythmically inhibited in time with ventral root discharge on the same side. Die interneurons can fire multiple impulses and can turn on fictive swimming when stimulated by intracellular current injection. Skin stimulation is followed by excitatory postsynaptic potentials (EPSPs) in contralateral ventral rhythmic neurons. These EPSPs are reduced by the application of NMDA receptor antagonist. We conclude that dlc interneurons are excited by primary skin afferent Rohon ‐ Beard neurons, carry sensory information across the spinal cord to excite neurons on the opposite side by release of an excitatory amino acid transmitter and participate in reflexes and in the initiation of swimming.