Photoreceptor degeneration and loss of immunoreactive GABA in the Abyssinian cat retina.

GABA (gamma-amino butyric acid) and its synthesizing enzyme, GAD (glutamate decarboxylase; EC 4.1.1.15) were localized in the retina of Abyssinian cats homozygous for a recessively inherited retinal degenerative disorder which in several respects is similar to the human disease, retinitis pigmentosa. Clinically normal mongrel cats and heterozygous Abyssinian cats were studied for comparison. The GABA and GAD immunoreactive neurons of the heterozygous or young homozygous (clinically unaffected animals) had the same distribution and morphology as normal mongrel European type cats. The neuronal GABA immunoreactivity in both the inner and outer parts of the retina gradually disappeared in the course of the disease, with little or no loss of GAD immunoreactive neurons. Early in the disease, the changes were most severe in patches in the mid periphery of the eye and then spread both centrally and peripherally. Loss of photoreceptors was a prerequisite for the loss of GABA immunoreactivity. The observations show that retinal changes are not limited to the photoreceptors. The GABA loss is not likely to be due to a loss of neurons, because of the persistence of GAD immunoreactive neurons.