Comfm! of ovuiaïion
147
DISCUSSION
The present results demonstrate that ovulation in common carp can be
induced by manipulation of environmental conditions and, in the absence of
appropriate environmental conditionsy by the injection of LHRH—A in
combination with drugs that block the synthesis or action of dopamine°
The
introduction of males and floating vegetation, as a spawning
substrate9
to
sexually mature female common carp results in a pronounced
surge in serum GtH
levels and ovulation when water temperature exeeds 20°C
l)°
Both males
and vegetation were found to be necessary for
spontaneous ovulation (unpublished data)a
Water
temperature seems to be a
permissive factor for spontaneous ovulation of common carp; ovulation will
not
occur
when water
temperature is maintained below about 18°C°
This
is
somewhat
different
from the
situation
in goldfish where floating vegetation
is
the primary cue for Spontaneous ovulation (Stacey et al° 1979a,b), and a
low frequency of spontaneous ovulation will occur at ÏËmperatures as low as
12°C (Ro Peterî unpublished results)o
Administration of LHRH—A alone
failed
to
induce ovulation in common
carp,
silver carp and loach°
The
lack of an effect of LHRH—A on ovulation
in common carp
results
from a failure
to
stimulate an increase in serum GtH
levels
of
similar magnitude to that found in fish which ovulate
spontaneouslya
The
limited
effectiveness
of LHRH—A in these
species may
also be a result
of
the predominant influence of dopamine inhibition on the
control of GtH releasea
In
this
regard, the dopamine receptor antagonist
PIM caused a marked potentiation of the action of LHRH—A on GtH release in
common
carp
2), and the combined injections of PIM and LHRH-A induced
ovulation in common carp
(Table 1 and 2) and silver carp (Table 3)«
In
addition, the present results demonstrate that injection RES to block
catecholamine
synthesis also potentiates the effect of LHRH—A on GtH release
and induces
ovulation in common carp
(Fig° 2; Table 3), and ovulation in
loach (Table 5)“
The present results on common carp confirm the earlier
studies
by Billard et
(1983) indicating that PIM and LHRH-A combined are
effective for
inducing—ovulation°
Unlike
studies
on
common
carp9
LHRH—A alone stimulates a prolonged
increase in serum GtH levels and ovulation in bream (Fig° 3; Table 4)c
Combined injections of PIM and LHRH—A result in a greater increase in serum
GtH levels than LHRH—A alone and a decreased time interval to ovulation°
These data suggest that although dopamine inhibition is involved in the
regulation of GtH secretion in bream, it plays a less important role in
comparison to the influence of dopamine in common carp.
The results
of
the present studies also suggest that there may be major
differences in pituitary responsiveness to PIM and LHRH—A in common carp and
goldfish°
Studies
on
goldfish held at 18—20°C have shown that b0th PIM
alone and LHRH—A alone stimulate large increases in serum GtH levels;
furthermore, combined injections of PIM and LHRH—A result in much higher
serum GtH levels
than reported here for common carp (see Sokolowska et Îl°
1984; 1985a,,b)°
In addition, PIM together with LHRH-A represenE
&
highly
effective means of inducing ovulation in goldfish held at 18—20 Cu
In
the
Present
studies combined injections of PIM and LHRH—A failed to induce
OVulation in common carp held at 18°5—1905°C, but were effective at h1gher
Unlike the apparent insensitivitY Of common
t°
PIM and
LHRH—A when temperature is less than 20°C» PIM and LHRH_A W111 1nduce
ovulation in goldfish held at 12°C (Chang and Peter, 19833)”
147
DISCUSSION
The present results demonstrate that ovulation in common carp can be
induced by manipulation of environmental conditions and, in the absence of
appropriate environmental conditionsy by the injection of LHRH—A in
combination with drugs that block the synthesis or action of dopamine°
The
introduction of males and floating vegetation, as a spawning
substrate9
to
sexually mature female common carp results in a pronounced
surge in serum GtH
levels and ovulation when water temperature exeeds 20°C
l)°
Both males
and vegetation were found to be necessary for
spontaneous ovulation (unpublished data)a
Water
temperature seems to be a
permissive factor for spontaneous ovulation of common carp; ovulation will
not
occur
when water
temperature is maintained below about 18°C°
This
is
somewhat
different
from the
situation
in goldfish where floating vegetation
is
the primary cue for Spontaneous ovulation (Stacey et al° 1979a,b), and a
low frequency of spontaneous ovulation will occur at ÏËmperatures as low as
12°C (Ro Peterî unpublished results)o
Administration of LHRH—A alone
failed
to
induce ovulation in common
carp,
silver carp and loach°
The
lack of an effect of LHRH—A on ovulation
in common carp
results
from a failure
to
stimulate an increase in serum GtH
levels
of
similar magnitude to that found in fish which ovulate
spontaneouslya
The
limited
effectiveness
of LHRH—A in these
species may
also be a result
of
the predominant influence of dopamine inhibition on the
control of GtH releasea
In
this
regard, the dopamine receptor antagonist
PIM caused a marked potentiation of the action of LHRH—A on GtH release in
common
carp
2), and the combined injections of PIM and LHRH-A induced
ovulation in common carp
(Table 1 and 2) and silver carp (Table 3)«
In
addition, the present results demonstrate that injection RES to block
catecholamine
synthesis also potentiates the effect of LHRH—A on GtH release
and induces
ovulation in common carp
(Fig° 2; Table 3), and ovulation in
loach (Table 5)“
The present results on common carp confirm the earlier
studies
by Billard et
(1983) indicating that PIM and LHRH-A combined are
effective for
inducing—ovulation°
Unlike
studies
on
common
carp9
LHRH—A alone stimulates a prolonged
increase in serum GtH levels and ovulation in bream (Fig° 3; Table 4)c
Combined injections of PIM and LHRH—A result in a greater increase in serum
GtH levels than LHRH—A alone and a decreased time interval to ovulation°
These data suggest that although dopamine inhibition is involved in the
regulation of GtH secretion in bream, it plays a less important role in
comparison to the influence of dopamine in common carp.
The results
of
the present studies also suggest that there may be major
differences in pituitary responsiveness to PIM and LHRH—A in common carp and
goldfish°
Studies
on
goldfish held at 18—20°C have shown that b0th PIM
alone and LHRH—A alone stimulate large increases in serum GtH levels;
furthermore, combined injections of PIM and LHRH—A result in much higher
serum GtH levels
than reported here for common carp (see Sokolowska et Îl°
1984; 1985a,,b)°
In addition, PIM together with LHRH-A represenE
&
highly
effective means of inducing ovulation in goldfish held at 18—20 Cu
In
the
Present
studies combined injections of PIM and LHRH—A failed to induce
OVulation in common carp held at 18°5—1905°C, but were effective at h1gher
Unlike the apparent insensitivitY Of common
t°
PIM and
LHRH—A when temperature is less than 20°C» PIM and LHRH_A W111 1nduce
ovulation in goldfish held at 12°C (Chang and Peter, 19833)”