Journal of Neuroscience, Vol 8, 3427-3450, Copyright © 1988 by Society for Neuroscience
Serotonergic and serotonin-accumulating neurons in the goldfish retina
RE Marc, WL Liu, K Scholz and JF Muller
Sensory Sciences Center, University of Texas Graduate School of Biomedical Sciences, Houston.
Autoradiography of goldfish retinas incubated in micromolar levels of
3H-serotonin displayed 3 kinds of labeled somas in the inner nuclear layer:
S1 amacrine cells with heavy labeling, large somas, and a sparse
distribution (approximately 93/mm2); S2 amacrine cells with moderate
labeling, smaller somas, and a denser distribution (approximately 500/mm2);
and a subset of bipolar cells with light labeling, small somas, and a very
dense distribution (approximately 4000/mm2). Serotonin-like
immunoreactivity was observed only in S1 amacrine cells and their synaptic
terminals. Radiolabeled terminals in the inner plexiform layer formed 4
strata that were differentially assigned to the 3 cell types. S1 amacrine
cells arborized in sublayers 1 and 5, received inputs from type a1 bipolar
cells and amacrine cells, and made synapses on other amacrine cells, type
a1 bipolar cells and unidentified processes. Thus, S1 amacrine cells seem
to receive significant input from "off-center" pathways. S2 amacrine cells
arborized in sublayer 3 and made synapses onto amacrine cells. Labeled
bipolar cell terminals were exclusively located in sublayer 2 and were
identified as type a2 mixed rod-cone bipolar cells. We conclude that the S1
amacrine cell is truly serotonergic and that radiolabeling of S2 amacrine
cells and type a2 bipolar cells is due to cross-specificity for another
carrier or processes unrelated to their neurochemical identities. These
observations partially reconcile many previous observations on the types,
numbers, and synaptologies of teleost retinal neurons identified by
different markers for indoleaminergic transmission.