Synaptic organization of the dopaminergic neurons in the rabbit retina

Abstract

AbstractA number of substances were tested for their ability to label amine‐accumulating neurons in the rabbit retina after fixation with OsO4 or glutaraldehyde and OsO4. Useful results were obtained with 5,6‐dihydroxytryptamine (5,6‐DHT) and 6‐hydroxydopamine (6‐HDA). Labelled processes were characterized by small (40–50 mm) pleomorphic synaptic vesicles containing electron‐dense cores, and at times by swelling of mitochondria and by increased electron density of membranes and cytoplasm. Fluorescence microscopy showed that 5,6‐DHT labelled both dopaminergic and indoleamine‐accumulating neurons. In most experiments, therefore, the indoleamine‐accumulating neurons were removed with 5,7‐dihydroxytryptamine. In such retinas the dopaminergic processes labelled by 5,6‐DHT were found to make synapses of the conventional type, characterized by an accumulation of synaptic vesicles on the presumed presynaptic side and some aggregation of material on the cytoplasmic side of the synaptic membranes and within the synaptic cleft. The dopaminergic processes were found to contact each other and also non‐dopaminergic amacrine cells and their processes. Conventional synapses onto dopaminergic processes were observed from both labelled and unlabelled amacrine processes. The input from labelled neurons was observed on varicose dopaminergic processes whereas input from non‐labelled elements was found on the intervaricose parts of the dopaminergic processes. No Contacts of dopaminergic processes with bipolar or ganglion cells were observed. Injections of 6‐HDA gave the same results, although this drug gave less distinct labelling which made the observations less decisive than with 5,6‐DHT. In retinas treated with 5,6‐DHT alone (i.e., in which the indoleamine‐accumulating neurons remained) numerous processes were observed which were both pre‐ and postsynaptic to bipolar terminals. These observations suggest that the indoleamine‐accumulating processes synapse with bipolar cells.The results show that the dopaminergic neurons form a network involving only amacrine cells, suggesting a regulatory function for them. By analogy with the dopaminergic interplexiform cells of the goldfish retina, it is suggested that the dopaminergic neurons in the rabbit may regulate lateral inhibitory effects mediated by amacrine cells. Furthermore, the finding that the dopaminergic and indoleamine‐accumulating cells apparently have a different synaptic organization suggests that it is appropriate to categorize amacrine cells according to their transmitter content as well as their morphology.