Localization of immunoreactive cholecystokinin precursor to amacrine cells and bipolar cells of the macaque monkey retina.
We used antisera that recognized precursors of the neuropeptide cholecystokinin extended at the carboxyl terminus in an immunocytochemical study of the macaque retina. A subpopulation of bipolar cells with long, obliquely oriented dendrites was labeled. Their axons terminated exclusively in the fifth stratum of the inner plexiform layer, where they contacted processes of amacrine and ganglion cells. Based on their morphology, these cells appeared to be the type that contacts short-wavelength cones selectively. Two types of amacrine cells were also labeled, and processes from both types formed dense plexuses in the second and fourth strata of the inner plexiform layer. The majority of their synaptic connections were with other amacrine cells, but they had more contacts with bipolar cell axons and retinal ganglion cell dendrites than any other peptidergic cells in the macaque retina. We studied extracts of macaque retina with gel-filtration chromatography and radioimmunoassays to confirm our immunohistochemical results. We found cholecystokinin octapeptide and other immunoreactive forms that were amidated at their carboxyl termini and were therefore likely to be biologically active. Unlike most other regions of the CNS, however, the retina had relatively low concentrations of amidated forms, and forms with extended carboxyl termini that are presumably their precursors were far more abundant. These findings suggest that the rate of cholecystokinin synthesis in the retina is quite high, as we would expect if the peptide were found in tonically active neurons.