SYNAPTIC CIRCUITRY OF SEROTONIN-SYNTHESIZING AND SEROTONIN-ACCUMULATING AMACRINE CELLS IN THE RETINA OF THE CANE TOAD, BUFO-MARINUS

The synaptic connections of amacrine cells synthesizing or accumulatin g serotonin in the retina of the cane toad, Bufo marinus, were studied by using preembedding double-labeling electron-microscopic immunocyto chemistry. The binding sites of an anti-serotonin antibody were reveal ed by the diaminobenzidine reaction, whilst a colloidal gold-conjugate d secondary antibody was used to detect an antibody to phenylalanine h ydroxylase. Since the latter antibody recognizes tryptophan 5-hydroxyl ase, one of the synthesizing enzymes for serotonin, as well as tyrosin e hydroxylase, the rate-limiting enzyme for catecholamine synthesis, t he double labeling of the present study enabled us to identify three g roups of labeled profiles at the ultrastructural level. The profiles o f serotonin-synthesizing amacrine cells contained both diaminobenzidin e reaction product and colloidal gold particles, whilst those of serot onin-accumulating and dopaminergic amacrine cells contained only diami nobenzidine reaction product or colloidal gold particles, respectively . The synapses of serotonin-synthesizing or serotonin-accumulating ama crine cells were distributed all through the inner plexiform layer of the retina. The profiles of serotonin-synthesizing amacrine cells pred ominantly received synapses from, and made synapses onto, unlabeled am acrine cell dendrites. They also received synapses from, and made syna pses onto, bipolar cell terminals. They also made synapses onto presum ed ganglion cell dendrites. However, the profiles of serotonin-accumul ating cells made synapses only with unlabeled amacrine cell processes. There were close contacts between the profiles of serotonin-synthesiz ing and serotonin-accumulating amacrine cells. No synaptic relationshi ps were observed between dopaminergic and serotonin-synthesizing or se rotonin-accumulating amacrine cells. These observations indicate that serotonin-synthesizing amacrine cells and serotonin-accumulating amacr ine cells in the retina of Bufo marinus are involved in different micr ocircuits in visual information processing.