In situ hybridization to localize mRNA encoding the neurotransmitter synthetic enzyme glutamate decarboxylase in mouse cerebellum.

Glutamate decarboxylase (GAD; EC 4.1.1.15) is responsible for the synthesis of the neurotransmitter gamma-aminobutyric acid (GABA). We have used a cDNA sequence encoding GAD to produce a single-stranded RNA hybridization probe for GAD mRNA. This probe detects GAD mRNA in individual cells in sections of mouse cerebellum. The specificity of in situ hybridization with this probe rests on four criteria: the distribution of labeled cells matched the results we and others obtain with GAD immunohistochemistry (Purkinje, Golgi II, stellate, and basket neurons were labeled, whereas granule cells and glia were not); a negative control probe having a sequence identical to GAD mRNA did not specifically label any cerebellar cells; prior treatment of the sections with RNase abolished specific labeling; the labeling showed the melting behavior typical of nucleic acid hybrids. Translation of GAD mRNA is apparently restricted to neuronal cell bodies since GAD mRNA was detectable in neuronal perikarya but not in terminals. Also, the choice of GABA as a neurotransmitter appears to be made at the level of transcription since granule neurons did not contain detectable GAD mRNA. The level of GAD mRNA varied among the classes of neurons as well as from cell to cell within each neuron type.