Expression of glutamic acid decarboxylase mRNA in normal and monocularly deprived cat visual cortex

Neurons expressing glutamic acid decarboxylase (GAD) mRNA were localized by in situ hybridization in normal and monocularly deprived cat visual cortex by using single-stranded RNA probes transcribed from cDNAs cloned in vectors with the T3 and T7 RNA polymerase promoters. In Northern blot analyses, these RNA probes identified 2 forms of GAD mRNA, one of which is approximately 200 bases longer than the other which has previously been identified. The distribution of neurons containing GAD mRNA was compared with the distribution of immunocytochemically identified GABA neurons and in both cases the highest density of labeled neurons was found in layers II, III, and upper VI. All other cellular layers contained a homogeneous, but lower density of labeled cells. Cells expressing GAD mRNA outnumbered GABA immunostained neurons by approximately 10%, but colocalization of GAD mRNA with GABA immunocytochemistry revealed that the two methodologies were detecting the same neuronal population. To determine whether decreased retinal activity affected the levels of GAD mRNA in adult cats, neurons containing GAD mRNA were counted in normal and monocularly deprived visual cortex. However, the number of cells expressing GAD mRNA did not change following monocular deprivation.