Subpopulations of GABA neurons in the dentate gyrus express high levels of the ?1 subunit of the GABAA receptor

The alpha 1 subunit of the gamma-aminobutyric acid (GABA)A receptor is highly expressed in a subgroup of neurons in the hippocampal formation. The distribution and chemical identities of these neurons in the dentate gyrus have been studied with double-labeling in situ hybridization and immunohistochemical methods. Double labeling for the alpha 1 subunit and glutamate decarboxylase 65 (GAD65) mRNAs indicated that virtually all neurons in the dentate gyrus that are heavily labeled for the alpha 1 subunit are GABA neurons. However, many GAD65 mRNA-labeled neurons in the hilus do not contain high levels of the alpha 1 subunit mRNA and protein. Studies were thus conducted to determine if the somatostatin neurons of the hilus were part of the alpha 1 subunit-labeled group. Double labeling for the alpha 1 subunit and pre-prosomatostatin mRNAs demonstrated virtually no co-localization of these mRNAs in hilar neurons. Thus, the strongly labeled alpha 1 mRNA-containing neurons and the somatostatin neurons constitute two distinct populations of hilar GABA neurons. Double labeling for the alpha 1 subunit polypeptide and its mRNA with immunohistochemical and in situ hybridization methods demonstrated directly that neurons of the dentate gyrus that express high levels of the alpha 1 subunit mRNA are the same neurons that show extensive labeling for the alpha 1 subunit along their somal and dendritic surfaces. The high levels of alpha 1 subunit expression in some populations of GABA neurons could be related to prominent disinhibitory functions of these neurons.