Quantitative visualization of γ-aminobutyric acid receptors in hippocampus and area dentata demonstrated by [ 3 H]muscimol autoradiography

Abstract

Muscimol, a potent GABA agonist, was used in a radioactively labeled form for the quantitative localization of GABA receptors in the rat''s hippocampus (CA1 to CA4) and area dentata. [3H]Muscimol was injected directly in vivo or used in the incubation medium of tissue slices, and the tissues were then fixed and prepared for autoradiography. [3H]Muscimol-bound GABA receptors are weakly though evenly distributed over the fimbria of the fornix. There was a laminar distribution in CA1 to CA4 and the area dentata, with an increasing density of the GABA receptors in that order. The lowest density was found in the alveus of CA1 and CA2 and the highest in the stratum granulosum of the area dentata. The greatest density was found in the neuropil between granule cells, in which are found dendrites and the basket-like plexuses of the inhibitory GABA-containing local circuit neurons. The molecular layers of the area dentata, CA1, and CA2 also have a high density of GABA receptors, indicating a probable distribution over the dendrites of granule and pyramidal cells. The laminar distribution of GABA receptors in the hippocampus and area dentata is similar to the distribution of the GABA-synthesizing enzyme, glutamate decarboxylase and of GABA previously published. Neurons with label of various density are found in the polymorphic cell layer of the area dentata, in the stratum radiatum of CA3, and in CA4. These are posibly the GABA-containing basket local circuit neurons.