Identification of an Interneuronal Population that Mediates Recurrent Inhibition of Motoneurons in the Developing Chick Spinal Cord

Studies on the development of synaptic specificity, embryonic activity, and neuronal specification in the spinal cord have all been limited by the absence of a functionally identified interneuron class (defined by its unique set of connections). Here, we identify an interneuron population in the embryonic chick spinal cord that appears to be the avian equivalent of the mammalian Renshaw cell (R-interneurons). These cells receive monosynaptic nicotinic, cholinergic input from motoneuron recurrent collaterals. They make predominately GABAergic connections back onto motoneurons and to other R-interneurons but project rarely to other spinal interneurons. The similarity between the connections of the developing R-interneuron, shortly after circuit formation, and the mature mammalian Renshaw cell raises the possibility that R-interneuronal connections are formed precisely from the onset. Using a newly developed optical approach, we identified the location of R-interneurons in a column, dorsomedial to the motor nucleus. Functional characterization of the R-interneuron population provides the basis for analyses that have so far only been possible for motoneurons.