tellogenesisin females, the pituitary concentrations was found that stGTH II was more potent than
of both gonadotropins increase, with the stGTH I stGTH I in inducing germinal vesicle breakdown,
being the predominant gonadotropin in the pitui- GVBD (Moberg et al. 1991b). This effect. in con-
tary. Plasma concentration of stGTH I also increas- junction with the increased pituitary and plasma
es during vitellogenesis, similar to the increases of concentrations of stGTH II prior to final ovarian
GTH I reported during the same state of develop- maturation, suggests that stGTH II gonadotropin
ment in salmonids (Suzuki et al. 1988). These in- regulates the final reproductive events leading to
creases in the pituitary and plasma concentrations spawning.
of stGTH I suggest that this hormone regulates the In males, similar changes in the secretion of
onset of vitellogenesis as has been observed in trout stGTHs occur during their annual reproductive cy-
where GTHIstimulates the uptake of vitellogenin cle. In the winter during the meiotic state of sper-
by the developing oocytes (Tyler et al. 1991).While matogenesis, the mature white sturgeon male has
it is not known whether stGTH I has a similar role, greater amounts of stGTH I than stGTH II in the
we have observed that female sturgeon in the previ- pituitary. If GnRHa is administered during sperma-
tellogenic state will not sequester circulating vitel- togenesis, stGTH I is also released in greater
logenin, even in the presence of elevated concentra- amounts than stGTH II consistent with stGTH I
tions of plasma estrogen and vitellogenin (Moberg being responsible for regulating the meiotic phase
et al. 1991a), when the concentrations of stGTH I of spermatogenesis. In the spring, during spermia-
are low in both the pituitary and plasma. Further- tion. the pituitary concentration of stGTH II ex-
more, the administration of a gonadotropin releas- ceeds the levels of stGTH Iand the GnRHa-in-
ing hormone analog, GnRHa, will not stimulate the duced release of this gonadotropin is maximal, im-
secretion of the gonadotropin in these females even plying that stGTH II may be responsible for con-
though GnRHa administration will stimulate the trolling spermiation. In the summer, during
secretion of stGTH I in mature male sturgeon reproductive quiescence, relative low amounts of
(Moberg & Doroshov 1992) and induce ovulation both stGTHs are released following the administra-
and spermiation in ripe fish (Doroshov & Lutes tion of GnRHa, indicating that the regulation of the
1984, Fujii et al. 1991, Goncharov et al. 1991). These neuroendocrine axis is modulated by such environ-
findings are consistent with the hypothesized role mental factors as season (Moberg et al. 1995).
of stGTH I in initiating ovarian development and In some species of teleosts, the neurotransmitter
the onset of vitellogenesis. dopamine acts as an endogenous inhibitor of
Priortothe onset of final ovarian maturation and GnRH-induced pituitary secretion of GTH (Peter
ovulation, pituitary concentration of stGTH II rises et al. 1991). A comparable effect has been found in
sharply to a level greater than 100 μg mg-1inthe pre- sturgeon (Pavlick 1995). Administration of dopa-
ovulatory white sturgeon female (Moberg et al. mine to mature white sturgeon males effectively in-
1995). It is during this period of reproduction that hibited the GnRHa-induced elevation of both
stGTH II becomes the predominant gonadotropin stGTHs inblood plasma as well as spermiation,
in the pituitary. Comparable changes in gonadotro- while the administration of the dopamine antag-
pins have also been observed during the reproduc- onist pimozide potentiated stGTH I and stGTH II
tive cycle of salmonids (Swanson et al. 1989). As was release when used in combination with GnRHa.
observed for GTH II in salmonids (Suzuki et al. These results indicate a dopaminergic inhibition of
1988), the plasma concentration of stGTH II in GnRH action in white sturgeon, similar to what has
white sturgeon is not elevated until the time of final been observed in several teleost species.
gonadal maturation and spawning, when it increas- Little is known about the feedback effects of the
es to more than 20 ng ml–1(Moberget al. 1995). We gonadal steroids on the regulation of the hypotha -
have also observed a functional difference between lamic-pituitary-gonadal axis of sturgeon. From the
the stGTHs in an in vitro oocyte maturation bioas- onset of the meiotic phase in gametogenesis, both
say using ripe ovarian follicles of white sturgeon. It sexes in sturgeon exhibit high concentrations of
martin jones
(Martin Jones)
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