Arguments against 5-hydroxytryptamine as neurotransmitter in the rabbit retina.

The indoleamine accumulating retinal neurons are visualized only by uptake of exogenously applied indoleamines but lack fluorescence in the normal retina. These neurons have now been studied in the rabbit after pretreatment with L-tryptophan and a monoamine oxidase inhibitor with an improved histofluorescence method based on perfusion with formaldehyde-glyoxylic acid pH 4.5 containing high amounts of magnesium. The method does not, however, reveal any indoleamine fluorescence in the retina. Also, p-chloroamphetamine has no toxic effect on the neurons. Selective inhibitors of the uptake of 5-hydroxytryptamine have a lower inhibitory effect in the retina than in the hypothalamus. A substance other than 5-hydroxytryptamine is therefore suggested to be the transmitter of the indoleamine accumulating neurons.