IMMUNOCYTOCHEMICAL CHARACTERIZATION AND SPATIAL-DISTRIBUTION OF MIDGET BIPOLAR CELLS IN THE MACAQUE MONKEY RETINA

Midget bipolar cells form the first distinct step in the parvocellular (P-) pathway of the primate visual system, and are the major determin ant of the receptive field properties of colour selective midget gangl ion cells. This paper describes the sampling properties of the midget bipolar cell population and relates this to the processing of chromati c information in the P-pathway. Immunocytochemical markers were used t o label midget bipolar cells so that their spatial density could be co mpared with that of cones and ganglion cells. Sections through macaque monkey retinae were immunostained with antibodies against cholecystok inin (CCK), and recoverin. In CCK-labelled sections, in addition to bl ue cone bipolar cells, numerous thin bipolar cell dendrites, which cou ld be associated with individual cone pedicles are stained. CCK-immuno reactive midget bipolar cells are found throughout the retina. A diffe rent population of midget bipolar cells is revealed in recoverin-label led sections. Based on a comparison with midget bipolar cells in Golgi -stained retinae we propose that ON-midget (invaginating) bipolars are immunoreactive for CCK and confirm that OFF-midget (flat) bipolar cel ls are immunoreactive for recoverin [Milam, Dacey and Dizhoor (1993) V isual Neuroscience, 10 1-12]. The density of recoverin labelled midget bipolars matches the cone density to an eccentricity of about 10 mm; from there outwards it drops to 60% of the cone density. This suggests convergence of several cones to individual midget bipolar cells in Pe ripheral retina; We conclude that midget bipolar cells are present thr oughout the entire primate retina, and could, in peripheral as well as in central retina, provide chromatically specific input to the P-path way.