Structure and function of gonadotropin-releasing hormone in the Thai catfish, Clarias macrocephalus
Date
2018-07-06
Authors
Ngamvongchon, Somsri
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Abstract
Two forms of gonadotropin-releasing hormone (GnRH) were
extracted from brain-pituitary tissues of two species of
Thai catfish, Clarias inacrocephalus and C. batrachus.
The peptides were detected using high-performance liquid
chromatography (HPLC) and radioimmunoassay (RIA), The
amino acid sequences of both forms were determined using
Edman degradation. One form of GnRH in the brain-pituitary
tissues of the Thai catfish was novel, whereas
the second form of GnRH was identical to chicken GnRH-XI.
The presence of the N-terminal pGlu residue in both
peptides was established by digestion with pyroglutamyl
aminopeptidase. In addition, catfish GnRH-I was studied
by mass spectrometry.
The localization of these two peptides was determined
to be in the discrete brain areas and in the pituitary of
female and male catfish, C. macrocaphalus, using
heterologous and homologous radioimmunoassays. Initially
a heterologous RIA was used with mammalian GnRH as
iodinated tracer and standard, and an antiserum made
against salmon GnRH. Catfish GnRH-I (novel form) was
found in most areas of the female and male brain with the
highest content and concentration in the female pituitary
and in the male hypothalamus,, Catfish GnRH-II (chicken
GnRH-II) was found with the highest content in the female
Cerebellum and highest concentration in the pituitary,
catfish GnRH-II (chicken GnRH-II) was found with the
highest content and concentration for males in the same
area, hypothalamus. Additionally, a homologous RIA was
used with catfish GnRH-II (chicken GnRH-II) as iodinated
tracer and standard, and an antiserum prepared against
chicken GnRH-II. Catfish GnRH-II was detected with the
highest content and concentration in the cerebellum of
both sexes. These values are higher than the results
obtained in the heterologous assay. The location of
catfish GnRH-I suggests that it plays a role in
regulating the release of gonadotropin from the pituitary
since the high content and concentration of this
immunoreactive GnRH are detected in the hypothalamus and
pituitary gland. In contrast, catfish GnRH-II may act as
a neurotransmitter in the catfish brain, in particular in
tha cerebellum where a high content and concentration of
immunoreactive GnRH are detected.
Physiological in vivo studies indicate that catfish
GnRH-II is more effective than catfish GnRH-I and other
forms of GnRH such as mammalian and dogfish GnRH for
induction of ovulation in catfish, C. macrocephalus.
Eight GnRH analogs had varying potencies for the
induction of ovulation, but the most effective forms were
two forms of catfish GnRH-II (chicken GnRH-II) modified
in positions six and ten. In vitro studies found that
catfish GnRH-I not only causes the release of
gonadotropin but also the release of growth hormone in a
dose-dependent manner.
The primary structures of the two catfish GnRH peptides
are important for understanding the evolution of this
family peptide. The novel catfish GnRH shows that only
positions 5, 7 and 8 vary in the GnRH molecule in jawed
vertebrates, whereas catfish GnRH-II provides direct
evidence that the structure of this GnRH is conserved in
teleosts.
Description
Keywords
Luteinizing hormone releasing hormone, Gonadotropin, Catfishes