Serotonin-accumulating cells in the iris-ciliary body and cornea of various species

Serotonin was biochemically shown to be present in the iris-ciliary body of the frog, pigeon, goldfish and guinea-pig eyes at a concentration of between 55 and 95 ng per g. The aqueous fluid, in contrast, had no measurable serotonin, though a small amount of dopamine was present. Immunohistochemistry of the iris-ciliary body and cornea of all species failed to demonstrate the localization of serotonin. However, when tissues were first incubated with exogenous serotonin and then processed immunohistochemically for the localization of serotonin, positive staining was observed. Serotonin-accumulating fibres were present in the corneal stroma of the frog, pigeon and guinea-pig but not in the goldfish. In this species only a few unidentified non-neural cells in the corneal epithelium took up exogenous serotonin. The evidence of serotonin-accumulating fibres in the frog, rat or goldfish iris-ciliary body complex was not conclusive. This contrasted with the pigeon iris-ciliary body where there were some delicate fibres in the epithelium layers which take up exogenous serotonin. It is however, in the guinea-pig iris-ciliary body that the serotonin-accumulating cells are most numerous with fibres being situated in the muscular and epithelial areas. The distribution of tyrosine-hydroxylase immunoreactivity and serotonin-accumulating fibres in the guinea-pig iris-ciliary body was similar. The rat tissue also demonstrated a perfuse distribution of tyrosine-hydroxylase-positive fibres but lacked clear serotonin-accumulating fibres. The serotonin-accumulating fibres, therefore, do not give an indication of catecholaminergic fibres in all species.