Histamine in the rabbit eye: distribution, synthesis, catabolism, and changes by light stimulation.

Histamine (HI) is present in all studied ocular (retina, choroid, optic nerve) and brain structures of the albino rabbit. HI levels in neural eye elements (retina and optic nerve) are relatively low, i.e. 70-140 ng/g tissue, and comparable with those found in the brain; a typical vascular tissue, i.e. choroid, contained approximately 10 times more HI. Histadine decarboxylase (HD) activity was found only in brain tissue, while histamine-methyltransferase (HMT) activity was present in all the eye and brain structures. Light stimulation produced changes in HI content only in the retina (decrease) and in the optic nerve (increase).