Biology Reference
In-Depth Information
2.3.1 Dopamine
In
Manduca
, dopamine is the only biogenic amine present in the CA
(
Granger et al., 1996
) and is detected also in the brain (
Goodman &
Granger, 2005
).
In vitro
studies show that dopamine stimulates JH produc-
tion by the CA in the early fifth instar, but inhibits its production on day 3
fifth instar (
Granger et al., 1996
).
In
Bombyx
, dopamine levels in the hemolymph increase after the last
larval ecdysis (
Noguchi & Hayakawa, 2001
); therefore, it is possible that
dopamine is responsible for the decline of JH synthesis in the early fifth instar
stadium (
Kaneko & Hiruma, 2007
).
In vitro
studies showed that dopamine
had little effect on JH synthetic activity by the CA in fourth stage larvae,
but it suppressed JH synthesis in the fifth stage at concentrations higher than
5
m
M(
Fig. 3.2
B and C). This strong suppressive action was observed shortly
after the last larval ecdysis and the sensitivity of the CA to dopamine declined
with development. Dopamine is produced by neurosecretory cells in the
brain and transferred to the CA through nerve axons in fifth stage
Bombyx
larvae (
Fig. 3.2
A). Thus, it is thought that both dopamine from the brain and
hemolymph dopamine may act directly on the CA to suppress JH synthesis
(
Fig. 3.4
B).
Dopamine receptors have been isolated in insects and are classified
as D1 and D2 types (
Mustard, Beggs, & Mercer, 2005
); the
Bombyx
D1
receptor has two isoforms:
BmDopR1
and
BmDopR2
(
Mitsumasu et al.,
2008
). As mentioned above, dopamine suppresses JH synthesis by the
CA of fifth instar larvae
in vitro
. Yet addition of a D1 receptor-specific
antagonist, R(
)-SCH23390, inhibits this suppressive action (
Fig. 3.2
D),
suggesting that dopamine action is through the D1 receptor. The
expression patterns of
BmDopR1
and
BmDopR2
in the CA show similar
profiles (
Fig. 3.2
E), with both exhibiting a sharp peak shortly before the last
larval ecdysis, but the amount of
BmDopR2
mRNA was much higher than
that of
BmDopR1
, especially just after ecdysis when only trace amounts
of DopR1 mRNA are seen, although DopR2 transcripts are present at
noticeably higher levels (
Fig. 3.2
E). Thus, most likely the suppressive
action of JH biosynthesis by dopamine is through the arthropod-type
BmDopR2 receptor (
Fig. 3.4
B). The reason dopamine has little effect
on the CA in the fourth instar larvae is unknown. Presumably, other
factors such as cofactor(s) are required for dopamine action and fourth
instar larvae lack these factors. Unlike 20E action in the fifth instar stadium,
dopamine does not affect the expressions of any of the JH synthetic
enzymes
รพ
(data not
shown). How dopamine regulates
JH synthesis
is
