GABA(C) receptors are localized with microtubule-associated protein 1B in mammalian cone photoreceptors

Protein MAP1B was recently reported to link GABA(C) receptors to the cytosk eleton at neuronal synapses. This interaction was demonstrated in the mamma lian retina, where GABA(C) receptors were thought to be exclusively express ed in bipolar cells. Our previous studies on cultured photoreceptors sugges ted however the presence of GABA(C) receptors in cones. To further investig ate GABA(C) receptor expression in cones, we measured GABA responses in mam malian photoreceptors in situ, and we examined the distribution of the rece ptor and that of protein MAP1B in the mammalian outer retina. Photoreceptor s were recorded from flat-mounted retinas of retinal degeneration mice at a n age when the retina becomes cone-dominated after rod cell death. GABA(A) and GABA(C)-gated currents were produced only in cones but not rods. Record ing freshly dissociated retinal cells from wild-type C57 mice confirmed the presence of GABA(A) and GABA(C) receptors in cones. Immunohistochemical la beling of mouse and rat retinal sections localized GABA(C) receptors to con e terminals that were identified by peanut agglutinin lectin staining. As e xpected from previous studies on bipolar cells, the punctate immunostaining was not restricted to cone terminals in the outer plexiform layer. MAP1B i mmunolabeling was obtained in rat and pig retinas and was similarly found i n cone terminals identified by the peanut agglutinin lectin staining. These results provide physiological and histological evidence that cones receive a GABA feedback in the mammalian retina and are consistent with the notion that protein MAP1B links GABA(C) receptors to the cytoskeleton at postsyna ptic sites.