50
TA. Merchant et al.
experiment and increased growth rates at the warm temperature (Table 2) were
often observed in the absence of significantly increased serum GH levels
(Fig. 2). In addition, an influence of photoperiod on serum GH levels similar
to
that observed for growth was not observed. Blood samples were taken from
the fish only at the end of the experiment, and it is possible that sampling
should have been done more frequently to provide a more accurate assessment
of
the
influence of constant photoperiods and temperatures on serum GH levels
in
the goldfish°
Earlier, we hypothesized that photoperiod was the major factor regulat—
ing seasonal changes in circulating GH levels (Marchant and Peter, 1985).
Results
from this
study indicate that temperature can also influence serum GH
levels but it is not clear how this observation relates
to
the seasonal
changes in GH levels. Although the experiments described above have demon—
strated important and interesting effects of photoperiod, temperature and
season
on
growth rates and circulating levels of GH, additional work is
required to clarify the role of each of these environmental variables in
determining the seasonal pattern of GH secretion in the goldfish, and conse—
quently, the role of these changes in GH in determining the seasonal pattern
of growth. These results also indicate that the relationship of circulating
GH levels
to
growth rates is not direct; high GH levels are associated with
high growth rates, but increased growth rates can occur in the presence of
relatively low circulating levels of GH. Many factors other than GH
undoubtedly influence growth in teleosts and future studies will help to pro—
vide a greater understanding of the relationship between environment, GH and
growth in teleosts.
THE NEURAL REGULATION OF GH SECRETION
In mammals, the release of GH from the somatotrophs is regulated by
hypophysiotropic releasing and release—inhibiting peptides secreted by the
hypothalamus (Arimura and Culler, 1985). The structure of the mammalian GH
release—inhibiting factor somatostatin (SRIF) has been known for some time
and the GH releasing factor (CRF) has also been recently isolated and identi—
fied from mammalian tissues
(Arimura and Culler, 1985). The secretion of GH
and, consequently, the profile of circulating levels of GH are a result of
the
integration of SRIF and GRF release from the hypothalamus°
There have been relatively few comparative studies concerning the hypo—
thalamic regulation of GH secretion in nonmammalian species, and, as a
result, little is known about the neural control of GH release in teleost
species. Previous studies in a limited number of teleosts have provided evi—
dence for
the existence of an inhibitory hypothalamic influence on GH secretion
(for review: Ball, 1981). SRIF—like immunoreactive material has been
detected in the brain and pituitary of a wide variety of teleost species
including the goldfish (Kah g
1982; Olivereau
1984) and the
common
carp
(Olivereau et al., 1984). Confirmation of the presence of a
SRIF—like molecule in piscine tissues has come from the peptide sequence
determination of SRIF isolated from two teleost species, the channel catfish
Ictalurus punctata (Andrews and Dixon, 1981; Oyama gt al., 1980) and the
anglerfish Ègphiug americanus (Hobart gg êl., 1980). In both teleost species,
two
distinct somatostatin molecules are present, one of which is structurally
identical to mammalian SRIF, demonstrating a very high degree of conservation
of
the structure of
the SRIF molecule throughout vertebrate evolution.
The presence of SRIF
in teleost tissues has lead to the suggestion that
SRIF may
have a role in the hypothalamic regulation of GH secretion. Evidence
supporting this has been provided by a study in which synthetic mammalian
TA. Merchant et al.
experiment and increased growth rates at the warm temperature (Table 2) were
often observed in the absence of significantly increased serum GH levels
(Fig. 2). In addition, an influence of photoperiod on serum GH levels similar
to
that observed for growth was not observed. Blood samples were taken from
the fish only at the end of the experiment, and it is possible that sampling
should have been done more frequently to provide a more accurate assessment
of
the
influence of constant photoperiods and temperatures on serum GH levels
in
the goldfish°
Earlier, we hypothesized that photoperiod was the major factor regulat—
ing seasonal changes in circulating GH levels (Marchant and Peter, 1985).
Results
from this
study indicate that temperature can also influence serum GH
levels but it is not clear how this observation relates
to
the seasonal
changes in GH levels. Although the experiments described above have demon—
strated important and interesting effects of photoperiod, temperature and
season
on
growth rates and circulating levels of GH, additional work is
required to clarify the role of each of these environmental variables in
determining the seasonal pattern of GH secretion in the goldfish, and conse—
quently, the role of these changes in GH in determining the seasonal pattern
of growth. These results also indicate that the relationship of circulating
GH levels
to
growth rates is not direct; high GH levels are associated with
high growth rates, but increased growth rates can occur in the presence of
relatively low circulating levels of GH. Many factors other than GH
undoubtedly influence growth in teleosts and future studies will help to pro—
vide a greater understanding of the relationship between environment, GH and
growth in teleosts.
THE NEURAL REGULATION OF GH SECRETION
In mammals, the release of GH from the somatotrophs is regulated by
hypophysiotropic releasing and release—inhibiting peptides secreted by the
hypothalamus (Arimura and Culler, 1985). The structure of the mammalian GH
release—inhibiting factor somatostatin (SRIF) has been known for some time
and the GH releasing factor (CRF) has also been recently isolated and identi—
fied from mammalian tissues
(Arimura and Culler, 1985). The secretion of GH
and, consequently, the profile of circulating levels of GH are a result of
the
integration of SRIF and GRF release from the hypothalamus°
There have been relatively few comparative studies concerning the hypo—
thalamic regulation of GH secretion in nonmammalian species, and, as a
result, little is known about the neural control of GH release in teleost
species. Previous studies in a limited number of teleosts have provided evi—
dence for
the existence of an inhibitory hypothalamic influence on GH secretion
(for review: Ball, 1981). SRIF—like immunoreactive material has been
detected in the brain and pituitary of a wide variety of teleost species
including the goldfish (Kah g
1982; Olivereau
1984) and the
common
carp
(Olivereau et al., 1984). Confirmation of the presence of a
SRIF—like molecule in piscine tissues has come from the peptide sequence
determination of SRIF isolated from two teleost species, the channel catfish
Ictalurus punctata (Andrews and Dixon, 1981; Oyama gt al., 1980) and the
anglerfish Ègphiug americanus (Hobart gg êl., 1980). In both teleost species,
two
distinct somatostatin molecules are present, one of which is structurally
identical to mammalian SRIF, demonstrating a very high degree of conservation
of
the structure of
the SRIF molecule throughout vertebrate evolution.
The presence of SRIF
in teleost tissues has lead to the suggestion that
SRIF may
have a role in the hypothalamic regulation of GH secretion. Evidence
supporting this has been provided by a study in which synthetic mammalian