Choline acetyltransferase is found in terminals of horizontal cells that label with GABA, nitric oxide synthase and calcium binding proteins in the turtle retina

In this study, we discriminated the various types of horizontal cell in the turtle retina on their content of neuroactive substances. Double label imm unocytochemistry was performed on sectioned and wholemount retina using ant isera to neural- and endothelial-nitric oxide synthase (nNOS, and eNOS), ca lretinin (CR), calbindin (CB), gamma-aminobutyric acid (GABA) and choline a cetyltransferase (ChAT). H1 cells and their axon terminals label with CR, C B and GABA. Only H1 axon terminals label with eNOS. H2 cells contain CB, CR , nNOS and GABA maybe in their dendrites. H3 cells label only with nNOS. Th e localization of nNOS in the H2 and H3 cells is a novel finding. None of t hese antibodies labels H4 cells. The photoreceptor subtypes have been diffe rentiated by different intensity of labeling with CB. The accessory member of the double cone is less intensely labeled with CB than the principal mem ber and rods and blue cones do not appear to label at all. ChAT-IR is locat ed in terminal boutons of H1 and H2 horizontal cells and H1 axon terminals and these boutons contact rods and all spectral types of cones. Clearly, GA BA is present in H1 horizontal cells and may be used in neurotransmission b etween horizontal cells and possibly for feedback pathways to photoreceptor s. The evidence of nNOS immunoreactivity in H2 and H3 horizontal cells, com bined with available physiological evidence, suggests that NO may be involv ed in electrical coupling and/or modulation of synaptic input to these type s of cells. Furthermore, our results raise the possibility that cholinergic synaptic transmission may occur from horizontal cell processes to photorec eptors in the outer plexiform layer of the turtle retina. (C) 2000 Elsevier Science B.V. All rights reserved.