Presence and distribution of histaminergic components in rat and bovine retina

The presence of histamine and its related enzymes, histidine decarboxylase and histamine N-methyltransferase and the subcellular distribution of the amine and of H(1)-receptors were studied in the retina of two mammalian species. Histamine is present in rat and bovine retinas in concentrations (113 +/- 10 and 72 +/- 9 ng/g wet tissue, respectively) similar to those found in the brain. Histological examination and release experiments with Compound 48/80 performed in rat retina indicate a non mast cell location for the amine. Histidine decarboxylase and histamine N-methyltransferase activities in rat and bovine retinas were also comparable to those found in brain cortex suggesting that histamine can be synthesized and catabolyzed in situ. Subcellular fractionation of bovine retina showed that both the amine and H(1)-receptors are concentrated in particulate fractions where small sized synaptosomes sediment, presumably derived from horizontal and amacrine cells. These results are in agreement with a neurotransmitter or neuromodulator role for histamine in cells of the retinal inner nuclear layer.