Teleosts lack a hypophyseal portal system and hence neurohormones are carried by nerve
fibers from the preoptic region to the pituitary. The various cell types in the teleost
pituitary are organized in discrete domains. Fish possess two gonadotropins (GtH)
similar to FSH and LH in other vertebrates; they are heterodimeric hormones that consist
of a common alpha subunit non-covalently associated with a hormone-specific beta subunit.
In recent years the availability of molecular cloning techniques allowed the isolation
of the genes coding for the GtH subunits in 56 fish species representing at least
14 teleost orders. Advanced molecular engineering provides the technology to produce
recombinant GtHs from isolated cDNAs. Various expression systems have been used for
the production of recombinant proteins. Recombinant fish GtHs were produced for carp,
seabream, channel and African catfish, goldfish, eel, tilapia, zebrafish, Manchurian
trout and Orange-spotted grouper. The hypothalamus in fishes exerts its regulation
on the release of the GtHs via several neurohormones such as GnRH, dopamine, GABA,
PACAP, IGF-I, norepinephrine, NPY, kisspeptin, leptin and ghrelin. In addition, gonadal
steroids and peptides exert their effects on the gonadotropins either directly or
via the hypothalamus. All these are discussed in detail in this review. In mammals,
the biological activities of FSH and LH are directed to different gonadal target cells
through the cell-specific expression of the FSH receptor (FSHR) and LH receptor (LHR),
respectively, and the interaction between each gonadotropin-receptor couple is highly
selective. In contrast, the bioactivity of fish gonadotropins seems to be less specific
as a result of promiscuous hormone-receptor interactions, while FSHR expression in
Leydig cells explains the strong steroidogenic activity of FSH in certain fish species.
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