THE GAMMA-2 SUBUNIT OF THE GABA(A) RECEPTOR IS CONCENTRATED IN SYNAPTIC JUNCTIONS CONTAINING THE ALPHA-1 AND BETA-2 3 SUBUNITS IN HIPPOCAMPUS, CEREBELLUM AND GLOBUS-PALLIDUS/

The gamma 2 subunit is necessary for the expression of the full benzod iazepine pharmacology of GABA(A) receptors and is one of the major sub units in the brain. In order to determine the location of channels con taining the gamma 2 subunit in relation to GABA-releasing terminals on the surface of neurons, a new polyclonal antipeptide antiserum was de veloped to the gamma 2 subunit and used in high resolution, postembedd ing, immunoelectron-microscopic procedures. Dual immunogold labelling of the same section for two subunits, and up to three sections of the same synapse reacted for different subunits, were used to characterize the subunit composition of synaptic receptors. The gamma 2 subunit wa s present in type 2, ''symmetrical'' synapses in each of the brain are as studied, with the exception of the granule cell layer of the cerebe llum. The gamma 2 subunit was frequently co-localized in the same syna ptic junction with the alpha 1 and beta 2/3 subunits. The immunolabell ing of synapses was coincident with the junctional membrane specializa tion of the active zone. Immunolabelling for the receptor often occurr ed in multiple clusters in the synapses. In the hippocampus, the gamma 2 subunit was present in basket cell synapses on the somata and proxi mal dendrites and in axo-axonic cell synapses on the axon initial segm ent of pyramidal and granule cells. Some synapses on the dendrites of GABAergic interneurones were densely labelled for the gamma 2, alpha 1 and beta 2/3 subunits. In the cerebellum, the gamma 2 subunit was pre sent in both distal and proximal Purkinje cell dendritic synapses esta blished by stellate and basket cells, respectively. On the soma of Pur kinje cells, basket cell synapses were only weakly labelled. Synapses on interneuron dendrites were more densely labelled for the gamma 2, a lpha 1 and beta 2/3 subunits than synapses on Purkinje or granule cell s. Although immunoperoxidase and immunofluorescence methods show an ab undance of the gamma 2 subunit in granule cells, the labelling of Golg i synapses was much weaker with the immunogold method than that of the other cell types. In the globus pallidus, many type 2 synapses were l abelled for the gamma 2 subunit together with alpha 1 and beta 2/3 sub units. The results show that gamma 2 subunit-containing receptor chann els are highly concentrated in GABAergic synapses that also contain th e alpha 1 and beta 2/3 subunits. Channels containing the gamma 2 subun it are expressed in synapses on functionally distinct domains of the s ame neuron receiving GABA from different presynaptic sources. There ar e quantitative differences in the density of GABA(A) receptors at syna pses on different cell types in the same brain area. Copyright (C) 199 6 Elsevier Science Ltd.