J. Falcon et al., IMMUNOCYTOCHEMICAL LOCALIZATION OF HYDROXYINDOLE-O-METHYLTRANSFERASE IN PINEAL PHOTORECEPTOR CELLS OF SEVERAL FISH SPECIES, Journal of comparative neurology, 341(4), 1994, pp. 559-566
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.