Sodium channels, GABAA receptors, and glutamate receptors develop sequentially on embryonic rat spinal cord cells

It is not well understood when during embryonic development the elements of a cell's responsiveness first appear, nor the factors controlling their appearance. A strategy to approach this issue is to determine which aspects of neuronal development are highly stereotyped in presence, timing, or pattern across a variety of cell types, and which are more diversified by cell type, region, or other parameters. We have used a fluorescent potentiometric oxonol dye in conjunction with a digital video imaging system to record the emergence and distribution of specific forms of excitability in dissociated embryonic rat spinal cord cells. We studied the expression of responses to veratridine, a sodium channel activator; muscimol, a GABAA receptor agonist; and kainic acid, an agonist at a class of glutamate receptors. Responses were consistently detectable in a percentage of cells dissociated from the earliest age examined, embryonic day 13, and increased progressively in later ages. Cells were examined from four regions, with cervical-lumbosacral and ventrodorsal distinctions. In the population of cells from each region, functional sodium channels appeared prior to GABAA receptors, which in turn emerged prior to kainate-activated glutamate receptors. This pattern was common to all spinal cord regions and revealed ventrodorsal and rostrocaudal gradients reflecting the known pattern of spinal cord neurogenesis. Analysis of the individual cell responses indicated that the stereotypical pattern of sequential channel development occurs individually on most cells in each region.