Multiple-site optical recording reveals embryonic organization of synapticnetworks in the chick spinal cord

We examined embryonic expression of postsynaptic potentials in stages 26-31 (E5 to E7) chick spinal cord slices. Slow optical signals related to the p ostsynaptic potentials which were evoked by electrical stimulation of affer ent fibers were identified in the dorsal grey matter and the ventral motone uronal area. In cervical spinal cord (C13) preparations, the dorsal slow si gnal appeared from stage 28 (E6), whilst the ventral slow signal was recogn ized from stage 29. At stages 26 and 27 (E5), no slow signal was observed i n either the dorsal or ventral regions. On the other hand, in lumbosacral s pinal cord (LS5) preparations, the dorsal, as well as ventral, slow signals appeared from stage 29; at stage 28 no slow signal was detected in the dor sal or ventral regions. These results suggest that there are differences in the ontogenetic expression of synaptic functions between the dorsal and ve ntral regions, and between the cervical and lumbosacral spinal cords. In em bryos older than stage 29, removal of Mg2+ from the bathing solution marked ly enhanced the amplitude and incidence of the ventral slow signal. In addi tion, in C13 preparations at stage 28, removal of Mg2+ elicited small slow signals in the ventral region in which no synaptic response was evoked in n ormal Ringer's solution. The slow signals induced in the Mg2+-free solution were blocked by 2-amino-5-phosphonovaleric acid (APV), showing that they a re attributable to N-methyl- d-aspartate (NMDA) receptors. These results su ggest that functional synaptic connections via polysynaptic pathways are al ready generated on motoneurons, but are suppressed by a Mg2+ block on the N MDA receptors at developmental stages when synaptic transmission from the p rimary afferents to the dorsal interneurons is initially expressed in the d orsal region.