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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics
JPET 287:815–823, 1998
Vol. 287, No. 3
Printed in U.S.A.
Delta-Opioid Ligands Reverse Alfentanil-Induced Respiratory
Depression but Not Antinociception1
YING-FU SU, ROBERT W. MCNUTT and KWEN-JEN CHANG
Departments of Anesthesiology and Pharmacology (Y.-F.S., K.-J.C.), Duke University Medical Center, Durham, Division of Chemistry (R.W.M.),
Glaxo Wellcome Co., Research Triangle Park, and Delta Pharmaceuticals, Inc. (K.-J.C.), Research Triangle Park, North Carolina
Accepted for publication May 5, 1998
This paper is available online at http://www.jpet.org
Respiratory depression is the most serious side effect associated with the use of opioid analgesics (Reisine and Pasternak, 1996). All opioids in clinical use today produce respiratory depression at therapeutic doses. The antinociceptive
effect of depression is generally thought to be mediated by
the mu receptor subtype. Early experimental evidence from
animals seems to point to the possibility that both mu and
delta receptors are linked to the respiratory action of opioids
(McGilliard and Takemori, 1978; Ward and Takemori, 1983;
Pazos and Florez, 1984; Yeadon and Kitchen, 1990). A direct
approach by micro-application of mu or delta drugs into sensitive areas of the central nervous system confirmed the
respiratory depressant effect of the opioids. The i.c.v. adminReceived for publication December 26, 1997.
1
This work was supported in part by United States Public Health Service
Grants DA04240 and DA08362 from the National Institute on Drug Abuse.
R)-a-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide. In addition, intracerebroventricular
delivery of a number of peptide delta ligands cyclic[ DPen2,5]enkephalin, deltorpnin II and H-Tyr-Tic(c)[CH2NH]PhePhe-OH also produced the same differential reversal of hypercapnia without affecting antinociception. Thus, both the traditional
delta agonists and antagonists are able to reverse the alfentanilinduced hypercapnia without affecting antinociception. The reversal of alfentanil-induced hypercapnia by these delta ligands was
antagonized by a novel synthetic delta antagonist cis-4-(a-(4-((Z)2-butenyl)-3,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N,Ndiethylbenzamide. We propose that in this experimental respiration model, the delta antagonists naltrindole and H-TyrTic(c)[CH2NH]Phe-Phe-OH behave like delta agonists with low
but sufficient intrinsic activities to reverse alfentanil-induced hypercapnia in rats. The results suggest that a function of the delta
receptor is to modulate or counteract the respiratory depression
induced by the mu receptor.
istration of beta-endorphin, met-enkephalin, [D-Ala2, MePhe4,
Met(O)5-ol]enkephalin, [D-Ala2, D-Leu5]enkephalin and dermorphin produced respiratory depression (Holaday, 1982; Haddad
et al., 1982; Pazos and Florez, 1984; Feuerstein and Faden,
1983). Respiratory depression was believed to be associated
with the inhibition of spontaneous discharge activity of neurons
in the rostrodorsal surface of the pons, which was proposed to
regulate basic rhythmicity that controls respiratory frequency.
Such inhibition was demonstrated for morphine, met-enkephalin and [D-Ser2, Leu5]enkephalin-Thr suggesting that both mu
and delta receptors might be involved in the respiratory depression (Hurle et al., 1985; Morin-Surun et al., 1984). The nucleus
tractus solitarius and the nucleus ambiguus are also known to
mediate opioid-induced respiratory depression (Hassen et al.,
1982, 1984). However, the neural interactions among these
nuclei and other brain-stem bulbo-pontine respiratory centers
and the complex respiratory regulation involving opioid recep-
ABBREVIATIONS: BNTX, 7-benzylidenenaltrexone; (1)BW373U86, (1)-4-((a-R)-a-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)N,N-diethylbenzamide; CTOP, D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH2; Del II, deltorphin II, Tyr-D-Met-Phe-Glu-Val-Val-Gly; DPDPE, cyclic[DPen2,5]enkephalin; DPI2505, cis-4-(a-(4-((Z)-2-butenyl)-3,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide; MPE, maximal percent effect; NTI, naltrindole; NTB, naltriben; TIPP(c), H-Tyr-Tic(c)[CH2NH]Phe-Phe-OH; i.c.v., intracerebroventricular.
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ABSTRACT
Evidence suggests both opioid mu and delta receptors may participate in the regulation of respiration at different central nervous
system sites. In the past, the overlapping receptor specificity of
various opioid drugs has made it difficult to dissect the receptor
subtype-specific activities involved in respiratory regulation. The
new family of delta receptor selective agents such as cyclic[DPen2,5]enkephalin, deltorphins, (1)-4-((a-R)-a-((2S,5R)-4-allyl-2,5dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide,
naltrindole and H-Tyr-Tic(c)[CH2NH]Phe-Phe-OH have now made
it feasible to more clearly define the role of delta receptors in
respiratory control. In a series of experiments we observed that
systemic infusion of rats with the highly mu receptor-specific
opioid alfentanil induced antinociception and hypercapnia, and
both of these effects were antagonized by the mu antagonist
D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH2. However, peripheral administration of the delta receptor antagonist naltrindole reverses
the hypercapnia but not the antinociceptive activity of alfentanil.
This differential effect of naltrindole on antinociception and hypercapnia could also be produced with the delta agonist (1)-4-((a-
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Su et al.
delta ligands suggest that delta receptors play a significant
role in respiratory regulation.
Materials and Methods
Materials. DPDPE and Del II were purchased from Peninsula
Laboratories, Inc. (Belmont, CA). NTI, NTB, 7-benzylidenenaltrexone were synthesized according to the published method of Portoghese et al. (1991). TIPP(c) was a gift from Dr. P.W. Schiller (Clinical
Research Institute of Montreal, Quebec, Canada). Alfentanil was
purchased from Janssen Pharmaceuticals (Titusville, NJ). The active isomer (1)BW373U86 was synthesized by Bishop and McNutt
(1995). All other chemicals were reagent grade and purchased from
Sigma Chemical Co. (St. Louis, MO). Buffers and solutions were
prepared with deionized-distilled water.
Experimental procedures. Male Sprague Dawley rats (275–325
g) were used. Rats were anesthetized with 3% halothane in a mixture
of 30% O2 and 70% N2O. After the rat was anesthetized, halothane
was reduced to 0.5%. The femoral artery and vein were cannulated
with PE 50 tubing for drug injection and blood sampling. After
surgery, the anesthetic gases were removed and the rat was allowed
to rest in a plastic restrainer for 60 min to establish the baseline
value of blood gas. Alfentanil or saline vehicle was infused via the
venous line and blood samples were collected via the arterial line. A
second venous line was also cannulated for bolus drug injection if
needed.
For i.c.v. administration, rats were anesthetized with Nembutal (5
mg/kg, i.p.). A stainless steel guide cannula was stereotaxically inserted into the right lateral ventricle and fixed with instant glue
(Eastman 910 adhesive, Eastman Kodak, Rochester, NY). Coordinates for the lateral ventricle are AP 5 20.8 mm and L 5 1.2 mm.
After the operation the rat was allowed to rest for 3 days with food
and water ad libitum. On the day of experiment, the rat was anesthetized with anesthetic gas mixture as described above and an
arterial catheter placed as described. After catheterization, the rat
was placed in a plastic restrainer with a 30-g cannula (7.5 mm)
inserted through the guide i.c.v. cannula. The injection cannula was
connected by a polyethylene tubing to a Hamilton micro-syringe
(Hamilton Company, Reno, NV). The rat was rested for 1 hr to
recover from the anesthetic gases. The experiment was started with
alfentanil or saline i.v. infusion. Drugs were then injected i.c.v. over
a 30-sec period in a volume of 10 to 20 ml. Blood samples were taken
and the tail-flick test performed at the indicated times.
Arterial blood was withdrawn into a syringe prewetted with heparin. The syringes were capped immediately and were kept on ice
and analyzed within 10 min with a blood gas analyzer (model 1306
PH/Gas Analyzer, Inst. Lab). The blood exposed to air at the tip of
syringe was expelled and the blood was mixed by gentle inversion
and an aliquot of 0.1 ml was injected into the blood gas analyzer. The
volume of blood taken each time was 0.3 ml which allowed multiple
determinations. The pH and partial pressure of O2 and CO2 (pO2 and
pCO2, mm Hg) are measured.
The antinociceptive assay was the standard tail-flick test and
antinociception was expressed as MPE as described previously
(Wong et al., 1992). Briefly, the tail flick test was performed with rats
in a restrainer and the tail was subjected to radiant heat from a high
intensity light source. The intensity of the lamp was adjusted so that
all control rats have a mean latency response of approximately 4 sec.
This value was used as the baseline response time. An automatic
cut-off setting at 10 sec prevents unnecessary tissue damage. After
drug treatment, the latency response time, in seconds, was converted
and expressed as MPE (MPE 5 response time 2 baseline time/cut-off
time 2 baseline time, expressed in percent). Responses that fell
below the baseline or above the cut-off time were assigned values of
0 and 100%, respectively.
In all figures presented, each data point is the average of values
from six rats. The S.E.s are indicated by error bars.
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tors are still not fully understood (see reviews Mueller et al.,
1982; Shook et al., 1990).
In addition, opioids might have a stimulatory effect on
respiration. Microinjection of low doses of [D-Ala2,
5
2
4
5
D-Leu ]enkephalin, [D-Ala , MePhe , Met(O) -ol]enkephalin
or morphine into selective areas of the brain stem of anesthetized rats increased respiratory frequency but reduced
tidal volume (Hassen et al., 1982; Hurle et al., 1985). Morphine induced an increase of instantaneous minute ventilation when injected intracisternally into dogs (Haddad et al.,
1984; Schaeffer and Haddad, 1985). At lower doses mu 1
agonists demorphin and analog Tyr-D-Arg2-Phe-sarcosine
stimulate respiration (Paakkari et al., 1990, 1993). Szeto and
colleagues also found stimulatory effects of opioids on respiration; the fetal breathing movement of lamb was stimulated
by lower doses of DPDPE and deltrophin I (Cheng et al.,
1992, 1993a, b). The involvement of the kappa receptor in
respiratory control seems to be minimal (Pfeiffer et al., 1983;
Butelman et al., 1993).
The literature summarized above seems to suggest that
both mu and delta receptors are involved in the respiratory
depressant effect of opioids, but the mechanism has yet to be
delineated. One of the difficulties was the fact that opioid
compounds used in the past have relatively poor receptor
selectivity. [D-Ala2, D-Leu5]enkephalin exhibits almost equal
affinity for mu and delta receptors in both receptor binding
and isolated tissue studies (Mosberg et al., 1983). The newer
delta-specific agent DPDPE has a delta-selectivity of about
1000-fold. The antagonist NTI has a delta receptor selectivity
of about 100-fold (Portoghese, 1991). TIPP(c) is the most
selective delta antagonist; its selectivity for delta receptors is
10,000-fold over other receptors (Schiller et al., 1993). The
poor receptor selectivity of delta drugs used in the past studies may have contributed to the disparate results concerning
the delta receptor’s role in respiration. Other factors such as
route of administration, animal species used and the pharmacodynamics of the drugs studied are also important variables in animal studies. The lack of delta-specific drugs that
can penetrate the blood-brain barrier also hinders the research in this regard.
The first nonpeptide delta selective agonist, BW373U86
(Chang et al., 1993) is a potent analgesic that produces antinociceptive effect in mouse tail-flick and writhing tests
when administered intrathecally (Wild et al., 1993). This
action is clearly mediated through the delta receptor, because
the effect is abolished by delta antagonists NTI and
ICI174,864. BW373U86 makes it possible to delineate analgesia and respiratory depression mediated by peripherally
administered delta-specific drugs. This compound could be
the prototype of potentially useful nonpeptide delta agonists.
The highly selective delta agonist Del II (Erspamer et al.,
1989) is also a useful tool for studies of delta receptor-specific
effects. Del II was reported to posses $104 selectivity at delta
sites over mu sites (mouse vas deferense compared to guinea
pig ileum; Erspamer et al., 1989). The synthesis of the delta
receptor selective antagonists NTI and TIPP(c) (Portoghese
et al., 1988, 1991; Schiller et al., 1993) have now made the
study of in vivo delta receptor activities feasible.
We report the effects of highly selective delta receptor
ligands on systemic alfentanil-induced respiratory depression in conscious rats. The pharmacological characteristics of
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1998
Respiration and Delta-Opioid Receptors
Results
ated through mu receptors. Therefore, mu antagonists
should readily block the alfentanil-induced antinociception
and respiratory depression effects described above (increases
in pCO2 and decreases in pO2 and pH). This was tested by
using the highly specific mu receptor antagonist, CTOP.
CTOP reversed the alfentanil induced antinociceptive effect
as well as the elevated pCO2 level in a dose-dependent manner (fig. 2). In this experiment, CTOP was delivered by i.c.v.
because it is a peptide drug. Over the dose range tested,
CTOP produced a similar extent of reversal of both the antinociceptive and the respiratory effects induced by alfentanil. Given the high selectivity of CTOP for mu receptors,
this result suggests that alfentanil-induced respiratory depression is mediated by mu receptors. CTOP injection also
reversed pO2 and pH with time courses similar to that of
pCO2 (data not shown). Bolus injection of CTOP by itself did
not produce antinociception nor affect blood gas parameters
(data not shown).
Antagonism of alfentanil-induced hypercapnia by
NTI. We found that the delta receptor antagonist NTI also
reversed the alfentanil-induced pCO2 increase in rats. The
effective dose by bolus i.v. route is in the range of 0.1 to 0.5
mg/kg (fig. 3). In contrast to mu antagonist CTOP, the antinociceptive effect of alfentanil was not affected by NTI even
at the highest dose tested of 0.5 mg/kg (fig. 3), which was
sufficient to reverse the hypercapnia almost to control level.
In fact, the lowest dose of NTI that affected MPE was 4
mg/kg, eight times the dose that effectively antagonized the
respiratory depression effect (data not shown). In addition,
delta antagonists NTB and BNTX also produced similar results on blood gas parameters at doses of 0.2 and 0.5 mg/kg,
Fig. 1. Time course of changes in MPE, pCO2, pO2 and pH induced by alfentanil. Rats with indwelling catheter were infused with alfentanil at 3, 6
or 9 mg/kg/min via femoral artery as described in “Materials and Methods.” The doses of each compound are indicated. At the time points indicated,
the tail-flick test was performed, and a blood sample was taken and analyzed by blood gas analyzer.
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Alfentanil infusion and respiratory depression in
rats. In animals, administration of alfentanil by various
routes produces respiratory depression in addition to antinociception. In conscious rats under systemic i.v. infusion of
alfentanil, antinociception was quickly induced. Infusion
rates of 3, 6 or 9 mg/kg/min induced antinociception, as measured in MPE, that reached a steady state in 10 min (fig. 1).
Infusion rates of 6 or 9 mg/kg/min achieved maximal antinociception; infusion at 3 mg/kg/min produced about 70% MPE.
Termination of infusion brought the MPE down to basal level
within 15 to 30 min, indicating rapid pharmacological metabolism of the drug. Arterial blood samples taken at various
time points of alfentanil infusion were analyzed for pCO2,
pO2 and pH. The data show that alfentanil induces a rapid
rise in arterial pCO2 and concomitant drop in pO2 and pH.
Infusion at 3 mg/kg/min affected pCO2 only slightly, and the
MPE never reached 100%. At infusion rates of 6 or 9 mg/kg/
min the MPE for antinociception reached 100% at 10 min and
was sustained at a maximal level throughout the infusion
period: in contrast, the pCO2 takes 30 min to reach a steady
level. Because the effects of alfentanil on MPE and blood gas
parameters reach a peak at 6 mg/kg/min, this infusion rate
was chosen for the rest of the studies. At this dose, the
maximum increase of pCO2 is about 60%, increasing from a
resting value of approximately 35 to 56 mmHg. Infusion with
saline vehicle showed no significant change in the blood gas
parameters tested (data not shown).
Antagonism by opioid mu antagonist CTOP on alfentanil effects. It is generally accepted that both antinociception and respiratory depression induced by opioids are medi-
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respectively (fig. 4). At these doses no effect on antinociception was observed with these drugs (data not shown). These
antagonists alone did not produce antinociception or changes
in blood gas parameters.
Delta agonist (1)BW373U86 also reversed the hypercapnia effect of alfentanil. (1)BW373U86 is a new synthetic nonpeptide delta agonist with unique antinociceptive
properties (Chang et al., 1993). In vitro tissue studies showed
that the drug has a good selectivity for delta receptors (700x).
We discovered that (1)BW373U86, as with the antagonist
NTI described above, also has a potent effect in reversing the
respiratory depressant activity of alfentanil. At doses of 0.1,
0.2 and 0.5 mg/kg, bolus i.v. injection of (1)BW373U86 rapidly reversed the alfentanil-induced pCO2 increase without
affecting the antinociceptive activity of alfentanil (fig. 5).
(1)BW373U86 also abrogates the changes of pO2 and pH
produced by alfentanil infusion. The reversal of respiratory
depression was nearly complete at the dose of 0.5 mg/kg. At
a higher alfentanil infusion rate of 9 mg/min/kg, the pCO2
increase can also be reversed by increasing (1)BW373U86 to
1 mg/kg (data not shown). The reversal effect of
(1)BW373U86 can also be demonstrated in morphine-induced respiratory depression (data not shown), suggesting
that the effect of (1)BW373U86 is likely to be general against
respiratory depression induced by all mu opiate drugs.
It has been previously reported that BW373U86 administered systemically or peripherally has no significant antinociceptive effect by itself in rats (Chang et al., 1993). We also
found that in the absence of alfentanil infusion, i.v. injection
Fig. 3. Effects of delta antagonist NTI on MPE and pCO2 responses in
alfentanil-infused rats. Different doses of NTI were injected via femoral
artery. NTI injection was carried out 30 min after initiating alfentanil
infusion (6 mg/kg/min). At the time points indicated, the tail-flick test was
performed, and a blood sample was taken and analyzed by blood gas
analyzer.
Fig. 4. Effects of delta antagonists NTI, NTB and BNTX on pCO2 response in alfentanil-infused rats. NTI, NTB or BNTX was injected via
femoral artery. Injection was carried out 30 min after initiating alfentanil
infusion (6 mg/kg/min). The doses of each compound are indicated. At the
time points indicated, the tail-flick test was performed, and a blood
sample was taken and analyzed by blood gas analyzer.
of (1)BW373U86 alone did not produce any significant
change in the antinociceptive response or pCO2 values during a 60-min time course after bolus injection of 1 mg/kg of
the drug (fig. 6).
Opioid delta ligands produce the same effects by
i.c.v. route. To further explore the effects of other delta
ligands, drugs were injected in small volumes into the lateral
ventricle of rats under constant alfentanil infusion. Similar
to results obtained by peripheral injection, the i.c.v. administration of microgram quantities of either delta agonists Del
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Fig. 2. Effects of mu antagonist CTOP on MPE and pCO2 responses in
alfentanil-infused rats. After 30 min of alfentanil infusion (6 mg/kg/min),
CTOP was injected via catheter into the lateral ventricle as described in
“Materials and Methods.” At the time points indicated, the tail-flick test
was performed, and a blood sample was taken and analyzed by blood gas
analyzer.
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1998
Respiration and Delta-Opioid Receptors
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Fig. 6. Effects of (1)BW373U86 on MPE and blood pCO2 in control rats.
Conscious rats with no alfentanil infusion were injected intravenously
with (1)BW373U86 (1 mg/kg). At the time points indicated, the tail-flick
test was performed, and a blood sample was taken and analyzed by blood
gas analyzer.
II, DPDPE, (1)BW373U86 or delta antagonists NTI and
TIPP(c) rapidly reversed the respiratory depression induced
by systemic alfentanil. At a dose of 10 mg, (1)BW373U86
decreased pCO2 to near basal level in 15 min. Del II, DPDPE
and TIPP(c) also produced similar effects at 5 to 20 mg (fig.
7). In all cases the MPE was not affected by i.c.v. injection of
these drugs (data not shown). In separate experiments, i.c.v.
injection of microgram amounts of these drugs into salineinfused rats (controls) did not produce any significant
changes in the blood gas levels or in antinociception, under
the same experimental conditions. The effective dose of
(1)BW373U86 is 10 to 30 mg/kg or 0.1 to 0.5 mg/kg, for i.c.v.
or i.v. administration, respectively.
The respiratory effects of delta-ligands are reversed
by DPI2505. It is a paradox that both delta agonists and
antagonists, administered peripherally or centrally, produced a similar pattern of reversal of alfentanil-induced hypercapnia. Further screening of other novel delta compounds
by this experimental method led to the discovery of an unique
new synthetic compound DPI2505 that has a chemical structure of DPI2505 (fig. 8). This compound displayed antagonist
activity against delta opioids in both receptor binding and in
vitro tissue assays. The affinity of DPI2505 for delta receptors in the binding assay was estimated to be approximately
1.5 nM, and its selectivity for delta receptors over mu receptors was 400-fold. The detailed activities of this compound
will be published separately (K-J Chang, unpublished data).
In our experimental respiration model, i.v. administration of
DPI2505 produced no measurable effect on blood gas parameters or antinociception in rats infused with saline (data not
shown). In alfentanil-infused rats it also did not produce significant effects on MPE or pCO2, but it antagonized the respiratory depression-reversal effect produced by (1)BW373U86 (fig.
9). In this experiment, bolus i.v. doses of DPI2505 delivered 10
min after bolus i.v. administration of (1)BW373U86 effectively
reversed the effect of (1)BW373U86 in a dose-dependent manner. Other blood gas parameters were similarly reversed (data
not shown).
Furthermore, DPI2505 was able to antagonize the hypercapnia reversal effect of all delta ligands described above.
Bolus i.v. injected DPI2505 antagonized the effect of peripherally administered NTI and i.c.v. administered Del II,
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Fig. 5. Effects of (1)BW373U86 on MPE and pCO2 responses in alfentanil-infused rats. (1)BW373U86 was injected at 0.1, 0.2 or 0.5 mg/kg via femoral
artery. Injection was carried out 30 min after the initiating alfentanil infusion (6 mg/kg/min). At the time points indicated, the tail-flick test was
performed, and a blood sample was taken and analyzed by blood gas analyzer.
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Vol. 287
Fig. 8. Molecular structure of DPI2505. The chemical formula of DPI2505
is cis-4-(a-(4-((Z)-2-butenyl)-3,5-dimethyl-1-piperazinyl)-3-hydroxybenzyl)-N,N-diethylbenzamide.
DPDPE and TIPP(c) (fig. 10). The DPI2505 antagonism is
rapid and long lasting, and in all cases there was no significant effect on antinociception. Thus, the new delta ligand
DPI2505 exhibited an activity completely opposite to other
known delta drugs, and it antagonized the respiratory effects
of other delta agonist and antagonist ligands in this experimental protocol.
Discussion
The nature of the respiratory depressant effect of opioids is
yet to be fully understood. Although the literature is replete
with observations on the respiratory depressant effect of
various mu opioids the precise mechanism remains elusive.
Fig. 9. Reversal of pCO2 effect of (1)BW373U86 by DPI2505 in alfentanil-infused rats. Conscious rats infused with alfentanil (6 mg/kg/min)
received injections i.v. at 25 min with (1)BW373U86 or DPI2505 (8
mg/kg). In the (1)BW373U86 treated rats, an additional injection of
DPI2505 (8 mg/kg i.v.) was done 10 min later as shown. At the time points
indicated, the tail-flick test was performed, and a blood sample was taken
and analyzed by blood gas analyzer.
The role of delta or kappa receptors in respiratory control is
even less well understood. This study showed that mu receptors in the central nervous system mediate the respiratory
response of opioid stimulation. This is demonstrated by the
respiratory effect of alfentanil infusion and the antagonistic
action of CTOP on alfentanil-induced antinociception and
hypercapnia (figs. 1 and 2). CTOP is a high affinity mu ligand
with mu to delta selectivity of 2000x (Pelton et al., 1986;
Hawkins et al., 1989). It is clear that both antinociception
and hypercapnia are antagonized in parallel by CTOP with
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Fig. 7. Effects of delta opioids delivered by i.c.v. on blood pCO2 responses in alfentanil-infused rats. After 30 min of alfentanil infusion (6 mg/kg/min),
(1)BW373U86, DPDPE, Del II and TIPP were injected via a catheter into the lateral ventricle as described in “Materials and Methods.” Doses
administered are indicated. At the time points indicated, the tail-flick test was performed, and a blood sample was taken and analyzed by blood gas
analyzer.
1998
Respiration and Delta-Opioid Receptors
821
no significant difference in the dosage requirement. This
suggests that both the antinociception and the respiratory
depression are brought about by stimulation of mu receptors.
The fact that delta agonist (1)BW373U86 and other delta
agonists tested have no direct effect on respiration (fig. 6)
further supports the notion that mu receptors play a significant role in mediating opioid-induced respiratory depression. It has been suggested by Pasternak (1986) that respiratory depression is mediated by the mu 2 receptor subtype.
Because CTOP is not able to differentiate between mu 1 and
mu 2 activities, it is not clear which subtype is more important in the respiratory actions of opioids. Whether opioids
affect respiration by directly interacting with mu receptors in
the central respiration centers or indirectly through other
neurotransmitter interactions is still not known (Bonham,
1995).
The effect of delta antagonists is quite different. When
administered peripherally, all three nonpeptide delta antagonists NTI, NTB and BNTX produced reversal of alfentanilinduced hypercapnia at doses that do not affect antinociception (figs. 3 and 4). Moreover, by i.c.v. administration, the
highly selective peptide antagonist TIPP(c) also produced
the same effect (fig. 7). It should be noted that these delta
antagonists reverse the already depressed respiratory effects
of alfentanil; however, by themselves they showed no direct
effect on respiration, either stimulatory or inhibitory (data
not shown). NTI has a delta receptor selectivity of about
100-fold (Portoghese, 1991), and TIPP(c) is the most selective
delta antagonist with a selectivity for delta receptors of
10,000-fold over other opioid receptors (Schiller et al., 1993).
With these degrees of receptor selectivity it is thus argued
that the observed hypercapnia-reversal effect is mediated
through delta receptor antagonism. Freye et al. (1991, 1992)
observed that NTI and NTB reversed sufentanil-induced respiratory depression in conscious mongrel dogs. Based on the
potent effect of these antagonists they proposed that the
delta receptor plays an important role in respiratory regulation and the respiratory depression induced by mu ligands is
mediated by interaction with delta receptors. This is the
simplest model to explain the delta antagonists’ effect we
observe in this report. However, this model is insufficient to
explain the data on delta agonists.
The effect of delta agonists is unexpected. In this experimental model, delta agonists also reverse the alfentanil-induced hypercapnia but not antinociception when administered by either i.v. or i.c.v. route. There is no apparent
difference between the hypercapnia-reversal effects of the
putative delta 1 agonist DPDPE and the delta 2 agonist Del
II. At the dose of 10 mg i.c.v., these agonists did not produce
measurable antinociception. This is possibly the first observation on the reversal effect of delta agonists on opioidinduced respiratory depression. The strikingly similar effect
of delta agonists and antagonists is difficult to explain with a
simple receptor model, and complex models invoking stimulatory or inhibitory feedback loops have to be constructed to
accommodate the data. It is thus apparent that the involvement of opioid receptors in the physiological regulation of
respiration is complex and multifaceted, involving complex
Downloaded from jpet.aspetjournals.org at ASPET Journals on June 18, 2017
Fig. 10. Reversal of pCO2 effect of delta opioids by DPI2505 in alfentanil-infused rats. Conscious rats were continuously infused with alfentanil (6
mg/kg/min). Delta ligands were injected i.v. at 25 min, followed 5 min later by DPI2505 at doses indicated. At the time points indicated, the tail-flick
test was performed, and a blood sample was taken and analyzed by blood gas analyzer.
822
Su et al.
Acknowledgments
The excellent technical assistance of Ms. Catherine Pan is gratefully acknowledged.
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Another important point is the lack of respiratory effects
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The action of DPI2505 is unique in this system. Although it
is inactive by itself on the alfentanil-induced antinociception
and hypercapnia, it behaves like an “antagonist” in blocking
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If this conclusion is true, our results suggest that a function
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Send reprint requests to: Dr. Y.-F. Su, Department of Medicine, Box 3942,
Duke University Medical Center, Durham, NC 27710.
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