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GABA (gamma-amino butyric acid), a fast-acting synaptic transmitter in the mature CNS, is synthesized from glutamate by GAD (glutamic acid decarboxylase). We have developed an ultrasensitive PCR technique to quantify the expression of GAD-related mRNAs during the development of the rat cervical spinal cord and have localized them using in situ hybridization. GAD65, GAD67, and an alternatively spliced variant of GAD67, EP10, were quantified each day from embryonic (E) day 11 through E21, and at postnatal days 0, 7, 14, and adult. GAD65 and GAD67 mRNAs were detected at E11 and increased exponentially over three orders of magnitude during embryonic development, then declined approximately threefold in the first-2 postnatal weeks. While the exponential growth phase coincided with the progressive appearance of GAD67 in situ signals in both the ventral and dorsal cord, the postnatal decline coincided with the virtual disappearance of expression in the ventral region. EP10 expression was prominent in the embryo, then declined markedly together with the mRNA encoding the neuroepithelial stem cell marker, nestin. The concerted appearance of GAD-related mRNAs paralleled transcripts encoding neuronal markers (light and heavy neurofilaments) and also closely correlated with the expression of GABA, mRNAs encoding GABAA receptor subunits, and depolarizing responses to GABA. We have used the results on GAD-related mRNA expressions to formulate a simple, minimal mathematical model that accounts for their kinetics in terms of positive and negative feedback loops.
Somogyi et al. (Sat,) studied this question.