Direct evidence for postsynaptic inhibition in the embryonic chick spinal cord

Motoneurons were recorded intracellularly in the isolated perfused spinal cord of 10 - 16-day chick embryos. Inhibitory postsynaptic potentials (IPSPs) were present in motoneurones of all ages studied and could be evoked by both ventral white column and dorsal root stimulation. IPSPs produced by orthodromic stimulation displayed many features of mature vertebrate motoneuronal IPSPs including the chloride dependence and sensitivity to currents passed through the cell membrane. Strychnine and chloride-free solution produced marked disinhibitory effects in the spinal cord indicating the presence of inhibitory synapses in interneuronal circuits of at least 11-day and older embryos. Possible sources of descending inhibitory influences on motoneurones and some functional aspects are discussed. The results support the hypothesis that the inhibition starts in the embryonic chick spinal cord rather early, before the 10th day of development.