Characterization of secretagogin-immunoreactive amacrine cells in marmoset retina

The retina contains at least 30 different types of amacrine cells but not many are well characterized. In the present study the calcium-binding protein secretagogin was localized in a population of regular and displaced amacrine cells in the retina of the common marmoset Callithrix jacchus. Irrespective of their soma location, the dendrites of secretagogin amacrine cells occupy strata 2, 3, and 4 of the inner plexiform layer, between the two bands formed by cholinergic amacrine cells. Segretagogin amacrine cells are also immunopositive to antibodies against glutamic acid decarboxylase, suggesting that they use γ-aminobutyric acid (GABA) as their neurotransmitter. The spatial density of secretagogin amacrine cells decreases from a peak of about 400 cells/mm(2) near 1 mm eccentricity to less than 100 cells/mm(2) in peripheral retina; these densities account for about 1% of amacrine cells in the inner nuclear layer and for up to 27% of displaced amacrine cells. The cell bodies form a regular mosaic, suggesting that they constitute a single amacrine cell population. Secretagogin cells have varicose dendrites, which are decorated with small spines. Intracellular injection of DiI into secretagogin cells revealed an average dendritic field diameter of 170 μm and an average coverage factor of 3.2. In summary, secretagogin cells in marmoset retina are medium-field amacrine cells that share their stratification pattern with narrow-field amacrine cells and their neurotransmitter with wide-field amacrine cells. They may mediate spatial inhibition spanning the centralmost (on and off) bands of the inner plexiform layer.