Cholinergic neurons in the rabbit retina: dendritic branching and ultrastructural connectivity

The structure and synaptic connectivity of rabbit retinal cholinergic neurons have been studied with an immunocytochemical technique for the localization of choline acetyltransferase (ChAT). Cholinergic processes ramified in two narrow strata within the inner plexiform layer; viewed in the plane of the retina, each immunoreactive stratum took the form of a polygonal meshwork with openings of about 20 microns. There was frequently an extensive matching of the patterns of the two strata, such that openings in one meshwork lay directly over openings of similar size in the other meshwork. It is hypothesized that the pattern of branching of these cholinergic processes is a reflection of the branching pattern of ganglion cell dendrites in the cholinergic strata. The proximal cholinergic stratum was examined ultrastructurally, and had the following characteristics: (1) ChAT-immunoreactive processes made large numbers of synaptic contacts with ganglion cell dendrites; often there were many such ganglion cell dendrites running past each other at various angles within the plane of the cholinergic stratum; (2) cholinergic processes were never observed presynaptic to any other type of inner plexiform process; (3) the principal input onto cholinergic processes was provided by bipolar cell axon terminals; (4) ganglion cell dendrites within the cholinergic stratum also received direct bipolar input; and (5) unidentified (i.e. non-cholinergic and probably non-GABAergic) amacrine cell processes were often found that were presynaptic to these same ganglion cell dendrites, and that also formed reciprocal contacts with bipolar axon terminals within these synaptic complexes.