Histamine levels and activities of histamine metabolizing enzymes in mammalian ocular tissues: comparison between day and night.

Iris, ciliary body, choroid and retina of the cow, rabbit and man contain histamine; the retinas are less rich in histamine (55-135 ng/g tissue) than the uvea (209-3073 ng/g tissue). The tissues show no diamine oxidase activity, no or marginal histidine decarboxylase activity, and well expressed histamine-N-methyltransferase activity. The killing of the rabbits at noon, light-midnight and dark-midnight revealed some day-night and light-dark dependent variations in both histamine content and histamine-N-methyltransferase activity in the iris-ciliary body, choroid and retina. It is suggested that histamine in mammalian eye may be of some functional importance.