Composition of the GABA(A) receptors of retinal dopaminergic neurons

Transgenic technology, single-cell RT-PCR, and immunocytochemistry were com bined to investigate the composition of the GABA(A) receptors of dopaminerg ic (interplexiform) amacrine (DA) cells. A mouse line was used in which the se neurons were labeled with human placental alkaline phosphatase and could therefore be identified in vitro after dissociation of the retina. We perf ormed single-cell RT-PCR on the isolated cells and showed that (1) DA cells contained the messages for alpha 1, alpha 3, alpha 4, beta 1, beta 3, gamm a 1, gamma 2(S), and gamma 2(L) subunits; (2) this transcript repertory did not change on dissociation of the retina and throughout the time required for cell harvesting; and (3) all DA cells contained the entire transcript r epertory. Immunocytochemistry with subunit-specific antibodies showed that all subunits were expressed and appeared homogeneously distributed througho ut the cell membrane at a low concentration. In addition, with the exceptio n of alpha 4, the subunits formed clusters at the surface of the dendrites and on the inner pole of the cell body. Because of their size, shape, and t opographic coincidence with GABAergic endings, the clusters were interprete d as postsynaptic active zones containing GABA(A) receptors. The compositio n of the synaptic receptors was not uniform: clusters distributed throughou t the dendritic tree contained alpha 3, beta 3, and, less frequently, beta 1 subunits, whereas clusters containing the alpha 1 subunit were confined t o large dendrites. Therefore, DA cells possess at least two types of GABA(A ) receptors localized in different synapses. Furthermore, they exhibit mult iple extrasynaptic GABA(A) receptors.