A new look at calretinin-immunoreactive amacrine cell types in the monkey retina

Abstract

We have examined amacrine cells that are calretinin-immunoreactive (-IR) in the macaque monkey retina with the aim of classifying them into morphological and functional subtypes. There are calretinin-IR cells in the fovea and throughout the retina. Their highest density is reached at 1.0 mm from the foveal pit (10,500 cells/mm2) and falls to 2,600/mm2 by 10 mm of eccentricity. Nearest-neighbor statistics for the calretinin-IR cell body distribution indicate a nonregular pattern, with a regularity index of 1.4–1.6. There is an increase or “bump” of cell density 3.5–4.0 mm from the foveal pit, corresponding to the rod photoreceptor density peak. Based on morphological differences, there appear to be three types of amacrine cell that are calretinin-IR. To determine the types, we doubly immunolabeled retinas, from fovea to periphery, for calretinin-IR in combination with other calcium binding proteins and inhibitory amino acid neurotransmitters. Labeling with parvalbumin and calretinin antibodies indicated that 70% of the amacrine cells were solely calretinin-IR, and 30% contained parvalbumin-IR as well. In the same way, 70% of the calretinin-IR amacrine cells colocalized calbindin, but 30% were only calretinin-IR. Among the calretinin/calbindin-colocalized cells, there were small-field and wide-field types. Double labeling with antibodies to calretinin and γ-aminobutyric acid (GABA) and to calretinin and glycine revealed the majority to be glycine-IR, but some were GABA-IR. The glycine-IR population consists mainly of AII amacrine cell types, but clearly another non-AII type is involved. The non-AII glycine-IR population resembles a small- to medium-field diffuse type. The calretinin-IR wide-field type is GABAergic and corresponds to an A19 type. The central, rod-free, fovea contains the calretinin-IR, non-AII glycine-IR type and the calretinin-IR, GABAergic type only. To learn more concerning the circuitry of the calretinin/glycine-IR, non-AII amacrine cell type in isolation from AII amacrine cells, we concentrated on the rod-free fovea, where AII amacrine cells are absent. We performed a serial section electron microscopy (EM) study on four calretinin-IR cells. They were involved with cone pathway circuitry. They got input from ON and OFF midget bipolar cells, reciprocated synapses to these bipolar cells, and provided synapses to ON-center ganglion cells. Thus we have obtained new information on a cone pathway amacrine cell of the central monkey fovea that is involved in the midget system.