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Pharmacology of octopamine- dopamine- and 5-hydroxytryptamine-stimulated cyclic amp accumulation in the corpus cardiacum of the american cockroach Periplaneta americana L.

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Archives of Insect Biochemistry and Physiology 5:119-128 (1987)
Pharmacology of Octopamine-, Dopamine-,
and 5-Hydroxytryptamine-Stimulated Cyclic
AMP Accumulation in the Corpus Cardiacum
of the American Cockroach, Periplaneta
americana L.
JohnW.D. Gole, Gregory L. Om, and Roger G.H. Downer
Department of Biology, University of Waterloo, Waterloo, Ontario, Canada
A variety of potential receptor agonists were incubated with isolated corpora
cardiaca from the American cockroach Periplaneta americana t o determine
their effects o n cyclic AMP production in this gland. Octopamine, dopamine,
and 5-hydroxytryptamine elevated cyclic AMP levels in a dose-dependent
manner with estimated K, values of 15.8, 1.7, and 1.1 pM, respectively, and
their stimulation was found t o be additive. Several vertebrate receptor
antagonists were tested against the three amines and a preliminary
pharmacological profile developed.
Key words: biogenic amine, amine receptor, adenylate cyclase
INTRODUCTION
Octopamine mediates several important physiological processes in insects
[l] including the regulation of (neuro)hormonal release from the CC.* The
release of AKH from the glandular lobes of the CC in locusts is regulated by
axons of NCCII [2,3] through the mediation of an octopamine-sensitive
adenylate cyclase located on the postsynaptic membrane of the hormonecontaining cells [4,5]. In the American cockroach, the release of hypertrehalosemic hormone(s) from CC is influenced by adrenergic components of
NCCII [6] and electrical stimulation of the nerve results in elevated levels of
cyclic AMP and release of hypertrehalosemic factor(s) from the CC [7J. Cyclic
*Abbreviations: CC = corpus cardiacum; AKH = adipokinetic hormone; NCCll = nervi
corporis cardiacurn 11; GTR = glucose-trehaloseringers; IBMX = 3-isobutyl-I-methylxanthine;
DCDM = N’-demethylchlordimeform.
Received August 5,1986; accepted December 8,1986.
Address reprint requests to Dr. R.G.H. Downer, Department of Biology, University of Waterloo, Waterloo, Ontario, Canada N2L 3C1.
0 1987 Alan R. Liss, Inc.
120
Cole, Orr, and Downer
AMP levels in the CC are also increased by octopamine, dopamine, and 5three putative neurotransmitters that have been
hydroxytryptamine
identified in this tissue [8,9]. However, octopamine is the only monoamine
Thus the release of this
that elicits release of a hypertrehalosemic factor
(these) hormone(s) appears to be under the control of octopamine, whereas
dopamine and 5-hydroxytryptamine may regulate the release of other
(neuro)hormones. The present study was undertaken to confirm the existence of three separate monoaminergic receptors in the corpus cardiacum of
Peeriplaneta ameericana and to provide a pharmacological characterization of
octopamine-, dopamine-, and 5-hydroxytryptamine-stimulatedcyclic AMP
accumulation.
[n,
[n.
MATERIALS AND METHODS
Experimental Animals
Adult male cockroaches were taken from a colony of Peeriplaneta ameericuna
L. maintained under standard conditions in this laboratory [lo]. Insects were
taken between 1and 5 months after the final molt and isolated 24 h prior to
dissection.
Incubation of Corpora Cardiaca
The CC was dissected free of adhering tissue and placed in 1.5 ml microcentrifuge tubes containing 100 pl physiological saline containing glucose and
trehalose [11,12] for 30 min at room temperature. The incubation medium
was then removed and replaced with 100 pl fresh GTR containing the test
compound and 0.5 mM IBMX. Following a further 10 min incubation (aminestimulated increases in cyclic AMP were linear with respect to time for 10
min), the reaction was terminated by the addition of 0.5 m10.4 N perchloric
acid. Following perchloric acid treatment, the solution was brought to pH 6.2
with 2.5 M potassium bicarbonate and centrifuged at 2,OOOg for 5 min. A 100
pl aliquot of the supernatant was used to determine the concentration of
cyclic AMP.
In experiments involving blocking agents, the 30 min preincubation was
followed by a 5 min incubation in fresh GTR containing the antagonist. This
solution was removed and replaced with GTR containing both the agonist
and antagonist, in addition to IBMX, and this was allowed to incubate for 10
min before the reaction was terminated with perchloric acid. Calcium-free
incubation conditions were maintained by using GTR with no added CaC12
for the 30 min preincubation and by including 2.0 mM ethyleneglycol-bis-(Paminoethyl ether) in the CaC12-free GTR containing the test compound.
Measurement of Cyclic AMP Production
The measurement of cyclic AMP was performed by radioimmunoassay
using a kit obtained from New England Nuclear Research Products (Boston,
MA). This system utilizes the method of Steiner et al. [13] as modified by
Cailla et al. [14].
Amine-Stimulated CAMP Accumulation in Corpus Cardiacum
121
Chemicals
N’-Demethylchlordimeform was generously provided by Dr. I. Orchard,
University of Toronto. The following compounds were also received as gifts
from the indicated suppliers: flupenthixol (Lundbeck and Co., Copenhagen,
Denmark), cyproheptadine (Merck, Sharp and Dohm, Kirkland, Quebec,
Canada), promethazine (Poulenc Ltd., Montreal, Quebec, Canada), phentolamine (Ciba-Geigy, Dorvall, Quebec, Canada). Mianserin and butaclamol
were purchased from Research Biochemicals Inc. (Wayland, MA), forskolin
from Calbiochem (San Diego, CA), and all other reagents from Sigma Chemical Co. (St. Louis, MO).
Determination of
and Ki Values
The IC50value was defined as the concentration of antagonist required to
cause a 50% inhibition of agonist-mediated production of cyclic AMP. The
Ki, or inhibitor constant, refers to the dissociation constant of the receptorinhibitor complex, assuming that competitive inhibition is occurring. The Ki
was calculated from the equation Ki = IC50/(l + SIK,), where S is the
concentration of agonist used and K, is the concentration of agonist causing
half-maximal cyclic AMP production, and IC50was determined as previously
described [15].
RESULTS
Effect of Potential Agonists on Cyclic AMP Production
Data summarized in Table 1indicate that octopamine, dopamine, and 5hydroxytryptamine cause statistically significant increases in cyclic AMP levels (4.9-fold, 3.7-fold, and 2.7-fold, respectively). The absence of calcium
from the incubation medium does not affect the octopamine-stimulated increase in cyclic AMP levels. These data, although not conclusive, suggest
that octopamine is acting through a receptor that is coupled to adenylate
cyclase rather than effecting the release of an endogenous substance that in
turn causes increased cyclic AMP levels. Ergonovine maleate, which is a
dopamine agonist/5-hydroxytryptamineantagonist causes a 2.6-fold increase
in cyclic AMP levels, whereas ergotamine tartrate, a dopamine agonist and
antagonist of 5-hydroxytryptamine and a-adrenergic receptors is without
effect. The formamidine DCDM elevates cyclic AMP levels (1.9-fold) and the
diterpene forskolin causes the largest increase observed (22-fold over
controls).
Octopamine, dopamine, and 5-hydroxytryptamine increase cyclic AMP
levels in a dose-dependent manner (Fig. l), with octopamine eliciting a
maximal 13-fold increase in cyclic AMP production, whereas the maximal
production caused by dopamine and 5-hydroxytryptamine is about 9-fold
greater than that of the control. The estimated K, values (50% of maximal
response) for octopamine, dopamine, and 5-hydroxytryptamine are 15.8, 1.7,
and 1.1pM, respectively. Ergonovine maleate also causes a dose-dependent
increase in cyclic AMP with an estimated K, of 23.0 pM and a maximal
production that is 3.3-fold greater than that of the control. Additivity studies
122
Gole, Orr, and Downer
TABLE 1. Effect of Various Potential Receptor Agonists on CAMPProduction in CC of
Peridaneta americana
Treatmenta
Octopamine
Octopamine (-Ca2+)
Dopamine
5-Hydroxytryptamine
Synephrine
Ergonovine maleate
DCDM
Forskolin (0.1 mM)
Phenylephrine
Isoproterenol
Apomorphine
Tyramine
Norepinephrine
Clonidine
0-Phenylethylamine
Phenylpropanolamine
Norphenylephrine
Ergotamine tartrate
Hydroxymandelic acid
Ephedrine
CAMPincrease
(pmollminlCC)
0.171
0.165
0.120
0.077
0.118
0.070
0.040
0.926
-0.008
-0.014
-0.008
-0.003
0.005
0.012
-0.005
-0.011
0.013
0.005
-0.012
-0.004
f 0.019 (14)b,*
f 0.029 (5)*
f 0.019 (5)*
f 0.021 (5)*
f 0.021 (5)*
f 0.078 (5)*
f 0.007 (5)*
f 0.055 (5)*
f 0.004 (10)
f 0.003 (10)
*f 0.002
(5)
0.006 (5)
0.008 (5)
f 0.008 (5)
f 0.008 (5)
f 0.002 (5)
& 0.011 (4)
f 0.008 (4)
f 0.003 (4)
0.004 (4)
aTreatment concentrations were 10 pM unless otherwise stated.
bValues indicate mean f SEM for the number of determinations in parenthesis.
*Values significantly different from control values; P = 0.01, Dunnett’s test [control value =
0.044 f 0.003 (14) pmollminlCC].
using octopamine, dopamine, and 5-hydroxytryptamine at maximally effective concentrations of each agonist demonstrate that the cyclic AMP production is completely additive (Table 2), suggesting that these compounds are
working through separate receptor populations.
Effects of Antagonists on the Octopamine-, Dopamine-, and 5Hydroxytryptamine-Mediated Increases in Cyclic AMP
To distinguish further between the various amine responses, a variety of
vertebrate receptor antagonists were tested for their effectiveness in inhibiting the increase in cyclic AMP levels. The most effective inhibitor of the
octopamine-mediated response is the a-adrenergic antagonist and 5-hydroxytryptamine inhibitor mianserin. Gramine, an inhibitor of 5-hydroxytryptamine, phentolamine, a general a-adrenergic antagonist, and cyproheptadine, a 5-hydroxytryptaminelhistamineantagonist, are approximately 10fold less potent. Promethazine, a phenothiazine derivative with antihistamine properties, the antipsychotic drugs flupenthixol and +butaclamol, which
block dopamine receptors, and the P-adrenergic blocking agent propranolol
are poor antagonists of the octopamine-mediated response (Table 3).
The dopamine-response is blocked most effectively by cis-flupenthixol,
with gramine, +butaclamol, and mianserin being less effective (1.3-, 1.9-,
and 6.6-fold, respectively; Table 4). The 5-hydroxytryptamine-mediatedincrease in cyclic AMP levels is most effectively blocked by cyproheptadine.
Amine-Stimulated cAMP Accumulation in Corpus Cardiacum
123
0.5
-
04.
\
c
.
E
;Y"
03.
0
-E
w
v)
9
U
02.
V
L
a
I
Q,
01.
-
0.0
1 ~ - 7
10-6
10-5
10-4
10-3
10-2
AMINE CONCENTRATION (MI
Fig. 1. Effect of octoparnine (V),dopamine (O), and 5-hydroxytryptarnine (m) on cAMP
production in incubated cockroach CC. Each point represents the mean f SEM for five to 15
determinations. Cyclic AMP production in control incubations was 0.031 f 0.001 pmol/min/
CC (mean of three experiments).
TABLE 2. Additivity of Octopamine-, Dopamine-, and 5-Hydroxytryptamine-Mediated
Increases in cAMP in CC of Periplaneta americana
Treatmenta
Octopamine
Dopamine
5-Hydroxytryptamine
Octopamine + dopamine
Dopamine + 5-hydroxytryptamine
Octopamine + dopamine + 5-hydroxytryptamine
cAMP increaseb
(pmoliminlCC)
0.351 f 0.040 (15)'
0.314 f 0.028 (15)
0.311 & 0.035 (10)
0.674 f 0.092 (5)
0.634 f 0.049 (5)
1.046 & 0.174 (5)
aAll treatments were 0.1 mM.
bControl value was 0.031 k 0.005 (9).
CValuesindicate mean f SEM for the number of determinations in parenthesis.
Mianserin is 39-fold less effective, whereas the other compounds tested are
relatively poor antagonists (Table 5).
DISCUSSION
The present study provides the first pharmacological characterization of
amine-stimulated cyclic AMP accumulation in the CC. Incubation of potential
receptor agonists with this tissue (Table 1)indicates pharmacological similarities with other insect systems [16-221. The cardiotonic diterpene forskolin
enhances adenylate cyclase activity in a variety of intact and broken tissue
124
Cole, Orr, and Downer
TABLE 3. Effect of Receptor Antagonists on the Octopamine-Mediated Increases in
cAMP in CC of Periplaneta americana
Antagonist
Mianserin
Gramine
Phentolamine
Cyproheptadine
cis-Flupenthixol
Promethazine
Propranolol
-Butaclamol
trans-Flupenthixol
+Butaclamol
IC50 (PM)'
0.45
2.82
5.01
6.92
17.80
105.00
120.00
Ki (PM)
0.37
2.32
4.13
5.70
14.70
86.50
98.90
% Inhibition with
0.1 mM antagonist
100.0
100.0
99.8
78.8
60.2
49.1
45.0
32.0
24.2
15.0
Toncentration of octopamine used was 5.0 pM.
TABLE 4. Effect of Receptor Antagonists on Dopamine-Mediated Increases i n cAMP i n
CC of Peridaneta americana
Antagonist
cis-Flupenthixol
Gramine
+Butaclamol
Mianserin
Promethazine
Phentolamine
Propranolol
Cyproheptadine
-Butaclamol
trans-Flupenthixol
IC50 bMIa
Ki (PM)
1.60
2.14
3.16
10.60
20.00
42.20
236.00
310.00
-
0.89
1.19
1.76
5.91
11.20
23.50
132.00
173.00
-
-
-
Yo Inhibition with
0.1 mM antagonist
100.0
86.6
82.6
92.5
89.8
58.1
37.6
1.0
24.2
6.2
aConcentration of dopamine was 5.0 pM.
TABLE 5. Effect of Receptor Antagonists on 5-Hydroxytryptamine-Mediated Increases in
cAMP in CC of Periplaneta americana
Antagonist
Cyproheptadine
Mianserin
cis-Flupenthixol
+Butaclamol
Promethazine
Phentolamine
Gramine
trans-Flupenthixol
Propranolol
-Butaclamol
IC50 (PM)'
Ki (PM)
0.10
0.40
15.80
37.20
58.90
70.80
321.00
616.00
3.79
8.93
14.10
17.00
77.10
148.00
-
-
-
-
~
aConcentration of 5-hydroxytryptamine was 5.0 pM.
-
% Inhibition with
0.1 mM antagonist
96.2
91.2
80.1
72.7
53.8
38.5
31.6
20.1
10.0
3.4
Amine-Stimulated CAMPAccumulation in Corpus Cardiacum
125
preparations [23] by activation of the catalytic subunit of adenylate cyclase.
Forskolin-enhancement of cyclic AMP production by the cockroach CC suggests similarities between insect and other eukaryotic adenylate cyclases.
The increase in cyclic AMP resulting from DCDM treatment confirms the
proposal that formamidine pesticides may exert some of their effects through
interaction with octopamine-sensitivereceptors coupled to adenylate cyclase
[24-27]. Formamidines also interact to a lesser degree with dopamine- and 5hydroxytryptamine-sensitive adenylate cyclase [16], so some of the effects of
these pesticides may result from interaction with receptors other than those
that are specific for octopamine.
Synephrine (n-methyloctopamine), which has not been detected in insects, probably stimulates cyclic AMP production in the cockroach CC by
interacting with octopamine receptors [21,22]. The other phenylethylamine
derivatives tested are typically poor agonists of insect adenylate cyclase (K,
values greater than 10-fold those of octopamine), since substitution at either
the /3-hydroxyl or p-hydroxyl group on the octopamine molecule substantially reduces potency [21,22].
In general, the data presented in Table 3 confirm earlier studies on insect
octopamine receptors in that the a-adrenergic antagonist phentolamine and
the 5-hydroxytryptaminelhistamine inhibitor cyproheptadine are effective
antagonists of the octopamine-stimulated cyclic AMP increase, whereas /3adrenergic blockers, such as propranolol, are ineffective [1,20-22,28,29]. In
addition, gramine, a 5-hydroxytryptamine antagonist, which is a potent
inhibitor of octopamine-sensitive receptors in locust CC 1291, was also effective in cockroach CC (Table 3). For cockroach CC (Table 3), nerve cord [16],
and hemocytes [22], the most potent inhibitor of the octopamine response is
mianserin. Nerve cord octopamine-sensitive adenylate cyclase is only 1-3fold less sensitive to blockage by phentolamine, gramine, and cyproheptadine than mianserin [lo], whereas these compounds are 11-100-fold less
effective than mianserin in CC (Table 3) and hemocytes [22]. Therefore, based
on the equal sensitivity to mianserin, phentolamine, gramine, and cyproheptadine, the octopamine-sensitive receptors of the nerve cord appear to be
pharmacologically distinct from those present in CC and hemocytes. The
octopamine receptors associated with CC and hemocytes can be further
differentiated on the basis of their response to gramine, cyproheptadine, and
promethazine. Gramine and cyproheptadine are better antagonists in the CC
than in hemocytes relative to mianserin, whereas the reverse is true of
promethazine. In all three cockroach tissues, the dopamine blockers flupenthixol and butaclamol are relatively weak antagonists of the octopamine
response.
A classification of multiple octopamine-receptor subtypes has been proposed by Evans [28] in the locust extensor-tibiae muscle. In this classification,
the antagonists chlorpromazine and metochlopramide are used to distinguish between octopamine-1 and octopamine-2 receptors. However, in cockroach hemocytes, metochlopramide and chlorpromazine are equally poor
antagonists [22]; therefore, although the agonist profile approximates that of
locust octopamine-2 receptors, the antagonist profile is not supportive. The
classification proposed by Evans [28] is based on physiological responses
126
Cole, Orr, and Downer
(myogenic rhythm and twitch tension) rather than effects on adenylate cyclase, as in the study of cockroach hemocytes [22], and pharmacological
characterization of the octopamine-sensitive adenylate cyclase of locust tibiae
muscle indicates poor correlation with the agonist profile developed on the
basis of twitch tension [30]. Therefore, pharmacological classification of the
octopamine receptor(s) may vary according to the particular response being
studied. More specific antagonists for octopamine may be required before a
complete classification is possible.
The dopamine-stimulated increase in cyclic AMP in cockroach CC (Table
4) is sensitive to inhibition by vertebrate dopamine antagonists. Thus the
most effective compound is cis-flupenthixol with +butaclamol being only 2fold less effective, whereas the inactive isomers of these compounds are poor
antagonists. Phentolamine and cyproheptadine are relatively poor antagonists of the dopamine response in cockroach CC (Table 4), which is in
agreement with previous studies in locust salivary glands [31] and cockroach
brain [21] and nerve cord [16].
The ergot-alkaloid ergonovine maleate causes a dose-dependent increase
in cyclic AMP (Fig. 2), and additivity studies indicate that it is additive with
the octopamine response but nonadditive with dopamine (unpublished observations). This suggests a possible interaction with dopamine receptors. In
vertebrates, ergonovine acts as a dopaminergic agonist at both D1 and D2
receptors, whereas another ergot-alkaloid, ergotamine, is more effective as a
D2 agonist [32]. The lack of response to ergotamine tartrate and the stimulatory effects of ergonovine maleate on cockroach CC cyclic AMP levels (Table
1)suggest not only similarities between vertebrate and cockroach dopamine
1
-
.
.
-
0.10.
V
0
.s
E
0.08.
e
Ln
-:
006.
W
In
5K
V
z
0
0.04.
002.
0.OoJ
1o-’
10-6
10-5
10-4
10-3
ERGONOVINE-MALEATE CONCENTRATION (M)
Fig. 2. Effect of ergonovine maleate on cyclic AMP production in cockroach CC. Points
SEM for five determinations. Cyclic AMP production in control
represent the mean
incubations was 0.029 f 0.004 prnol/rnin/CC.
Amine-Stimulated CAMPAccumulation in Corpus Cardiacum
127
receptors but also indicate that CC dopamine receptors may be pharmacologically similar to vertebrate D1 receptors.
The 5-hydroxytryptamine-stimulatedincrease in cyclic AMP in cockroach
CC is the most sensitive to inhibition by cyproheptadine (Table 5). In nerve
cord, cis-flupenthixol is approximately 30-fold more effective than cyproheptadine, followed by gramine, which is 2-fold more effective than cyproheptadine [16]. By contrast, gramine is a poor antagonist of CC 5-hydroxytryptamine response (Table 5). Therefore, based on the relative potency to
cyproheptadine cis-flupenthixol and gramine, the 5-hydroxytryptamine-sensitive receptors of cockroach CC and nerve cord appear to be pharmacologically distinct. This may be analogous to the situation that prevails in vertebrate
neural tissue in which distinct adenylate cyclase-coupled 5-hydroxytryptamine receptor subtypes have been proposed [33]. Comparison with other
studies of 5-hydroxytryptamine pharmacology in insects is difficult because
of the limited number of compounds used. The 5-hydroxytryptamine-sensitive adenylate cyclase of cockroach nerve cord [16,34] and CC (Fig. 1)and
that present in the ventral nerve cord of Manduca sexta [35] have a K, of
approximately 1pM, similar to that of many vertebrate 5-hydroxytryptamine
receptors [36]. Furthermore, compounds such as cyproheptadine and mianserin, which are known to block vertebrate 5-hydroxytryptamine receptors
[37], are also antagonists of cockroach 5-hydroxytryptamine-sensitive adenylate cyclase (Table 5) 116,341. A more detailed pharmacologic study, including ligand-binding studies of insect 5-hydroxytryptamine receptors, is
necessary before meaningful comparisons can be made.
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cockroaches, octopamine, cardiacus, cyclic, dopamine, periplaneta, corpus, pharmacology, amp, hydroxytryptamine, american, stimulate, accumulation
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