IMMUNOCYTOCHEMICAL LOCALIZATION OF HYDROXYINDOLE-O-METHYLTRANSFERASE IN PINEAL PHOTORECEPTOR CELLS OF SEVERAL FISH SPECIES

Melatonin is an internal ''Zeitgeber,'' involved in the timing and con trol of a number of rhythmic functions and behaviours. Its synthesisin g cells remain to be identified in the fish pineal. The last step in t he melatonin biosynthetic pathway is catalysed by the enzyme hydroxyin dole-O-methyltransferase. An affinity-purified antibody, directed agai nst chicken pineal hydroxyindole-O-methyltransferase, was used in the present study to identify the melatonin synthesising cells in four fis h species: a primitive chondrostean (sturgeon), a saltwater teleost (d orado), and two freshwater teleosts (pike, trout). Western blot immuno labeling of pike and trout pineal proteins revealed a single band at 3 8 KDa, which corresponds to the known molecular weight of the enzyme i n bovine, rat, and chicken pineal. Regardless of the species, a specif ic immunocytochemical labeling, visualised by means of the peroxidase- antiperoxidase method, was exclusively associated with the photorecept or cells. These results provide evidence that photoreceptors of the fi sh pineal are responsible for the biosynthesis of 5-methoxyindoles, in cluding melatonin. In the pike, reactions were less intense in the dis tal portion of the pineal vesicle than in the other regions of the org an. It is questioned whether this might be related to the existence of a germinative zone, generating new photoreceptor cells in this distal portion. Hydroxyindole-O-methyltransferase has been previously demons trated in mammalian pinealocytes, and modified photoreceptors of the a vian pineal. It is now demonstrated in pineal photoreceptors of a prim itive fish and of more evolved saltwater and freshwater fish. The resu lts strengthen the view that these cells are related through phylogeny and that their well conserved melatoninergic function appears early i n the course of evolution. (C) 1994 Wiley-Liss, Inc.