Immunohistochemical demonstration of neurotensin and tyrosine hydroxylase in iris nerves of the rabbit eye.

In previous studies we have provided evidence that intracameral administration of neurotensin (NT), an endogenous tridecapeptide, produces strong miosis in the rabbit. The presence of NT immunoreactivity was investigated in rabbit iris whole mounts by light microscopic immunohistochemistry, and its distribution in the iris compared to that, of tyrosine hydroxylase (TH). A few scattered NT-positive cell bodies were localized in the dilator muscle. Both, the NT cell bodies and processes appeared parallel to the muscle cells. Extensive branching of NT-containing cell processes was observed in connection with the sphincter muscle. These NT-positive fibers formed a dense, randomly oriented network throughout the sphincter muscle cells. The distribution of TH immunoreactivity was similar to that of NT-positive cell processes, except that no TH-positive cell bodies were detected in any of the iris structures examined. Moderate branching of TH-positive fibers was observed in the dilator and sphincter iris muscles. These findings provide neuroanatomical support for an important role of NT in pupillary physiology. Its similar topographical distribution with TH suggests that NT and dopamine may be co-localized, as it has already been described in brain.