Connections of indoleamine-accumulating cells in the rabbit retina.

To study the connections of the neurons of the rabbit retina that accumulate indoleamines, we injected 5,7-dihydroxytryptamine into the vitreous body. It accumulated within a subset of amacrine cells and could be visualized there by aldehyde-induced fluorescence. The fluorescent labeling was photo-converted to an insoluble, osmiophilic product by irradiation in the presence of diaminobenzidine, and the tissue was examined by electron microscopy. Preservation of the structure of the tissue after photoconversion was satisfactory and the dendrites of the indoleamine-accumulating cells could easily be identified. They form a dense plexus near the junction of the inner plexiform and ganglion cell layers, where they exhibit large synaptic endings that occupy a substantial fraction of the surface of rod bipolar terminals. The dendrites of the indoleamine-accumulating cells receive input from rod bipolars at dyad synapses, where the other postsynaptic partner is a dendrite of a narrow-field, bistratified amacrine cell; in addition, they receive amacrine cell input throughout the inner plexiform layer. The only outputs we observed are reciprocal synapses onto the rod bipolar endings. Thus, these amacrine cells appear to exert an important effect on the transmission of scotopic information through the retina.

5,7-Dihydroxytryptamine identifies living dopaminergic neurons in mesencephalic cultures.

The autofluorescent serotonin analogue 5,7-dihydroxytryptamine (5,7-DHT) was used to identify living catecholaminergic neurons in monolayer cultures derived from the embryonic rat mesencephalon. A high correlation between 5,7-DHT accumulation and aldehyde-induced catecholamine fluorescence as well as tyrosine hydroxylase but not dopamine-beta-hydroxylase or phenylethanolamine-N-methyltransferase immunoreactivity was found. This indicates that these cells were dopamine-containing neurons. Whole-cell patch recordings showed that all mesencephalic neurons had resting membrane potentials of -50 mV or greater and input resistances ranging between 200 and 700 M omega and exhibited spontaneous action potentials and postsynaptic potentials. The duration of the action potential of the dopamine-containing neurons was characteristically longer than that of the non-dopamine-containing mesencephalic cells. In some dopamine-containing neurons, repolarization of the action potential was clearly biphasic, and the slow phase of repolarization was reversibly blocked by local application of Cd2+ or Co2+. This "shoulder" in the action potential was never observed in non-dopamine-containing neurons, where Cd2+ or Co2+ application was always without effect. It is concluded that 5,7-DHT can be used to identify living dopamine-containing neurons in dissociated mesencephalic cultures and these neurons express distinct electrical properties.

Direct histochemical localisation of 5,7-dihydroxytryptamine and the uptake of serotonin by a subpopulation of GABA neurones in the rabbit retina.

The same cells in both the intact retina and retinal cultures of the rabbit retina take up exogenous serotonin or 5,7-dihydroxytryptamine. Both substances can be localised by immunohistochemistry using a monoclonal antibody directed against serotonin. The 5,7-dihydroxytryptamine can also be revealed directly in both fixed and living tissues using appropriate U.V. light. Some of the cells in the intact retina and retinal cultures which take up 5,7-dihydroxytryptamine also stain positively for the immunohistochemical localisation of gamma-aminobutyric acid (GABA). Cell counts in the cultures show that 80% of all 5,7-dihydroxytryptamine-positive cells stain positively for GABA, while 20% of the GABA-immunoreactive cells also accumulate 5,7-dihydroxytryptamine. These results demonstrate unambiguously that a subpopulation of GABA cells has the capacity to take up exogenous 5,7-dihydroxytryptamine and therefore utilise GABA and, probably, serotonin. Findings in favour of the 'co-occurrence' of GABA and serotonin come from studies where tissues were exposed to radioactive GABA and unlabelled serotonin so that the dual localisation of these substances by autoradiography and immunohistochemistry could be followed. It could be shown both in cultures and in intact retinal pieces that certain cells take up GABA and serotonin while other cells take up either GABA or serotonin.

Dihydroxytryptamines as tools to study the neurobiology of serotonin.

The neurotoxins 5,6- and 5,7-dihydroxytryptamine are accepted tools for "chemical degeneration" of serotonergic (5-HT) axons in the CNS (for reviews, see [11, 12, 15, 20] ). Optimum application of these substances requires knowledge of their chemical properties, disposition in the biophase and mechanism of action. Current knowledge and concepts on this issue are described and results of recent studies utilizing 5,7-DHT uptake as a tool for localizing 5-HT neurons neuroanatomically are reviewed.