Specific frequencies of spontaneous Ca2+ transients upregulate GAD 67 transcripts in embryonic spinal neurons

Spontaneous Ca2+ transients expressed prior to synaptogenesis regulate the developmental appearance of GABA in cultured Xenopus spinal neurons. We fin d that glutamic acid decarboxylase (GAD) immunoreactivity is also Ca2+-depe ndent and parallels the appearance of GABA. We show that. xGAD 67 transcrip ts first appear in the embryonic spinal cord during the period in which the se Ca2+ spikes are generated, in a pattern that is temporally and spatially appropriate to account for differentiation of GABAergic interneurons. RNas e protection and competitive quantitative RT-PCR demonstrate that transcrip t levels are approximately threefold greater when neurons are cultured in t he presence of extracellular Ca2+ that permits generation of transients tha n when cultured in its absence. The frequency of spontaneous Ca2+ spikes pl ays a crucial role in the regulation of transcripts, since reimposition of Ca2+ transients at the frequency generated in cultured neurons rescues norm al expression. We conclude that naturally occurring low frequencies of thes e Ca2+ transients regulate levels of xGAD 67 mRNA in differentiating neuron s.