The development of sensorimotor synaptic connections in the lumbosacral cord of the chick embryo.

We have examined the development of synaptic connections between afferents and motoneurons in the lumbosacral spinal cord of the chick embryo between stages 28 and 39. The central projection of afferents was visualized following injection of dorsal root ganglia with HRP. Afferent fibers first entered the dorsal gray matter between stages 29 and 31. They grew in a ventrolateral direction, reaching motoneuron dendrites by stage 32. Quantitative analysis of axon numbers suggested that individual axons did not begin to branch extensively until they approached the lateral motor column at stage 36. Connectivity between afferents and motoneurons was assessed by stimulating dorsal roots or nerves supplying the femorotibialis muscle and recording the resulting motoneuron synaptic potentials intracellularly or from the cut ventral roots. At stages 37-39, low-intensity stimulation produced a short-latency positive potential that was followed at higher stimulus currents by slower positive potentials. All of these potentials were abolished in solutions that block chemical synaptic transmission (zero Ca2+/2 mM Mn2+). The early potential, which includes the monosynaptic EPSP produced by muscle afferents, persisted in the presence of the N-methyl-D-aspartate antagonist, 2-amino-5-phosphonovaleric acid (APV), but was largely eliminated by the more general excitatory amino acid antagonist, kynurenic acid. Therefore, in the chick, as in other species, a glutamate-like transmitter appears to be released at the synapses between muscle afferents and motoneurons. The APV-resistant potential was reduced in amplitude during bath application of the glycine and GABA antagonists, strychnine and picrotoxin, suggesting that it was composed of depolarizing inhibitory as well as excitatory components at these stages. The monosynaptic EPSP could be recorded in ventral roots as early as stages 32-33, when muscle afferents first grew into the vicinity of motoneuron dendrites. The EPSP in these young embryos was unaffected by picrotoxin and strychnine, but responded to APV and kynurenate in a manner similar to that at later stages. Between stages 28 and 32, only long-latency, slowly rising potentials could be evoked in the ventral roots by afferent activation. These potentials were abolished by superfusion with zero Ca2+/2 mM Mn2+, APV, or kynurenic acid, and could be revealed before stage 31 only by removing Mg2+ from the bath.(ABSTRACT TRUNCATED AT 400 WORDS)