cultured Siberian sturgeon,A. baerii(Akimova et
al, 1979, Williot et al. 1991).
Neuroendocrine control
Knowledge of the neuroendocrine regulation of re-
production in Acipenseriformes remains inade-
quate, but our recent observations on cultured
white sturgeon appear to indicate general similarity
of regulatory mechanisms with those of teleosts.
Based on these preliminary observations, we pro-
pose a model for the neuroendocrine regulation
of reproduction in white sturgeon (Figure 6).
We believe the gonadotropin-releasing hormone
(GnRH), and possibly the neurotransmitter dopa-
mine (DA) interact on the pituitary gonadotropes
to regulate the synthesis and release of two putative
gonadotropins. The gonadotropins control gonadal
development and gamete release, and stimulate the
synthesis of gonadal steroids, androgen (A) and es-
trogen (E). The gonadal steroids, especially testos-
terone: appear to feedback on the pituitary, and
possibly the hypothalamus, to influence the synthe-
sis and secretion of gonadotropins. The function of
the hypothalamic-pituitary-gonadal axis is further
modulated by a number of exogenous and endoge-
nous factors, including season, body size and age of
fish, as previously discussed. The proposed model
has been developed from the following data.
The brains of white,A. transmontanus,and Rus-
sian sturgeon,A. gueldenstaedtii,contain the mam-
malian form of GnRH (mGnRH) and a small amount
of chicken GnRH II (Sherwood et al. 1991, Lescheid
et al. 1995). High concentrations of mGnRH in
brains of mature Russian sturgeon caught during
their spawning migrations, as well as its localization
in the forebrain of Siberian sturgeon (Lepretre et
al. 1993), suggest that this form of GnRH is respons-
ible for pituitary gonadotropin release (Lescheid et
al. 1995). High sensitivity of sturgeon pituitary go-
nadotropes to exogenous stimulation by mGnRH
analog (D-Ala^6 , Pro^9 N-Et]GnRH, has been dem-
onstrated by experiments on spawning induction in
European sturgeon species (Goncharov et al. 1991)
and studies on the effect of GnRHa injections on
plasma concentrations of gonadotropins in stellate
Figure 6.Hypothetical neuroendocrine reproductive axis of cul-
tured whitesturgeonbased on current studies (see text for de-
scription).sturgeon,A. stellatus(Barannikova & Bukovskaya
1990), and white sturgeon (Moberg et al. 1995).
The presence of a dual gonadotropin control of
gonadal development in sturgeon is being investi-
gated. Gonadotropic hormone was first purified
from pituitary glands of stellate sturgeon, and it was
initially believed that Acipenseriformes possessed
only a single form of GTH that regulated all aspects
of reproductive development (Burzawa-Gerard et
al. 1975a, 1975b). Recently, two potential gonado-
tropins have been isolated from Russian sturgeon
pituitaries, designated sturgeon gonadotropin I, or
stGTH I, and sturgeon gonadotropin II, or stGTH
II (Moberg et al. 1995). Based on physiological evi-
dence, these two gonadotropins appear tobe func-
tional analogs of the salmonid gonadotropins GTH
I and GTH II (Kawauchi et al. 1989, Swanson et al.
1989), possessing many of the same biological func-
tions. Like GTH I in salmonids, stGTH I appears to
induce and maintain follicular development and vi-
tellogenesis, while stGTH II is instrumental in in-
ducing ovarian maturation and ovulation (Moberg
et al. 1995).
Plasma levels of both stGTH I and stGTH II are
low(<1â2ng mlâ1) in reproductively immature stur-
geon. With the onset of meiosis in males and vi-