SODIUM-CHANNELS, GABA-A RECEPTORS, AND GLUTAMATE RECEPTORS DEVELOP SEQUENTIALLY ON EMBRYONIC RAT SPINAL-CORD CELLS

It is not well understood when during embryonic development the elemen ts of a cell's responsiveness first appear, nor the factors controllin g their appearance. A strategy to approach this issue is to determine which aspects of neuronal development are highly stereotyped in presen ce, timing, or pattern across a variety of cell types, and which are m ore diversified by cell type, region, or other parameters. We have use d a fluorescent potentiometric oxonol dye in conjunction with a digita l video imaging system to record the emergence and distribution of spe cific forms of excitability in dissociated embryonic rat spinal cord c ells. We studied the expression of responses to veratridine, a sodium channel activator; muscimol, a GABA(A) receptor agonist; and kainic ac id, an agonist at a class of glutamate receptors. Responses were consi stently detectable in a percentage of cells dissociated from the earli est age examined, embryonic day 13, and increased progressively in lat er ages. Cells were examined from four regions, with cervical-lumbosac ral and ventrodorsal distinctions. In the population of cells from eac h region, functional sodium channels appeared prior to GABA(A) recepto rs, which in turn emerged prior to kainate-activated glutamate recepto rs. This pattern was common to all spinal cord regions and revealed ve ntrodorsal and rostrocaudal gradients reflecting the known pattern of spinal cord neurogenesis. Analysis of the individual cell responses in dicated that the stereotypical pattern of sequential channel developme nt occurs individually on most cells in each region.