0022-3565/00/2922-0538$03.00/0
THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 2000 by The American Society for Pharmacology and Experimental Therapeutics
JPET 292:538–544, 2000
Vol. 292, No. 2
Printed in U.S.A.
Effects of Spinal Cholecystokinin Receptor Antagonists on
Morphine Antinociception in a Model of Visceral Pain in the
Rat1
ANN E. FRIEDRICH and GERALD F. GEBHART
Department of Pharmacology, University of Iowa College of Medicine, Bowen Science Building, Iowa City, Iowa
Accepted for publication October 14, 1999
This paper is available online at http://www.jpet.org
Originally discovered as a gut protein, cholecystokinin
(CCK) has been implicated in a wide variety of physiological
functions, including the modulation of nociceptive transmission (Stanfa et al., 1994). Anatomical studies have revealed a
striking overlap in the distribution of endogenous CCK and
opioid peptides and receptors, particularly within areas involved in pain processing, such as the intralaminar nuclei of
the thalamus, periaqueductal gray matter, and superficial
lamina of the spinal cord (Gall et al., 1987). Such findings
provide anatomical evidence for a functional relationship
between the two neurotransmitter systems.
Results from behavioral studies suggest CCK has antiopioid actions that are pronociceptive (Faris et al., 1983; O’Neill
et al., 1989). CCK attenuates opioid-induced antinociception
(Kellstein et al., 1991), whereas CCK receptor antagonists
enhance the antinociceptive effect of morphine and endogenous opioid peptides (Watkins et al., 1985a; Singh et al.,
1996). Thus, CCK appears to modulate pain transmission by
antagonizing opioid antinociception. Under physiological
conditions, very little spinal CCK is released; however, preReceived for publication July 23, 1999.
1
Supported by Grants NS 199121 and F31 DA 05852 (to A.E.F.).
L-365,260, a specific CCKB receptor antagonist, dose dependently increased morphine’s antinociceptive effects in vehicletreated rats but had no effect in rats with TNBS-induced colonic
inflammation. L-364,718, a specific CCKA receptor antagonist,
had no effect on morphine antinociception in either vehicletreated or TNBS-treated rats. These data indicate that CCK,
acting at the CCKB receptor, is involved in modulating morphine antinociception following a noxious visceral stimulus.
However, CCK receptor antagonists no longer enhance morphine antinociception after instillation of intracolonic TNBS,
suggesting that visceral inflammation may lead to a reduction in
spinal CCK release.
vious studies suggest morphine accelerates the release of
CCK (Zhou et al., 1993). In support, CCK receptor antagonists administered alone have no antinociceptive effect; CCK
receptor antagonists produce antinociception only in the
presence of opioids (Watkins et al., 1985b).
Most research on the effects of CCK in nociceptive transmission has focused on modulation of cutaneous pain. Yet
visceral pain, particularly chronic visceral pain, is a major
clinical concern, and mechanisms involved in pain transmission from cutaneous and deep structures are most likely
distinct (Gebhart, 1995). Persistent pain of the viscera, such
as arises from inflammatory bowel disease, is a challenging
medical problem. It has been estimated that 50 to 100 of
every 100,000 people in the United States suffer from an
inflammatory bowel disease (de Dombal, 1986). The neural
mechanisms influencing the perception of pain in such inflammatory conditions appear to differ from pain arising
from healthy tissues. In animals with inflammatory conditions, the antinociceptive effect of morphine is reported to be
enhanced by as much as 30-fold (Stanfa et al., 1992). CCK
receptor antagonists were shown to have little to no effect on
morphine antinociception in a model of cutaneous inflammation, yet CCK still attenuated morphine antinociception
ABBREVIATIONS: CCK, cholecystokinin; CRD, colorectal distension; i.th., intrathecal; EMG, electromyographic; TNBS, 2,4,6-trinitrobenzenesulfonic acid; VMR, visceromotor response; DMSO, dimethyl sulfoxide; MPO, myeloperoxidase; AUC, area under the curve.
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ABSTRACT
The objective of the present study was to determine the effects
of spinal cholecystokinin (CCK) receptor antagonists on morphine antinociception in a model of visceral nociception, colorectal distension, in rats with chronic colonic inflammation and
vehicle-treated controls. Three to five days after intracolonic
instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS), an enhanced visceromotor response to all pressures of colorectal
distension (10 – 80 mm Hg) was evident. The ED50 of intrathecal
morphine (0.93 ␮g) in vehicle-treated rats produced significantly greater antinociception in TNBS-treated rats. Intrathecal
proglumide, a nonselective CCK receptor antagonist, dose dependently enhanced the antinociceptive effect of morphine in
vehicle-treated rats, but not in TNBS-treated rats. Similarly,
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Spinal CCK Is Antiopioid in Visceral Nociception
Materials and Methods
Animals and Surgical Preparation. Experiments were conducted in male Sprague-Dawley rats (Harlan, Indianapolis, IN)
weighing 400 to 425 g. All protocols were approved by The University
of Iowa Animal Care and Use Committee. Rats were deeply anesthetized with an i.p. injection of sodium pentobarbital (50 mg/kg,
Nembutal; Abbott Laboratories, Abbott Park, IL). A small incision
was made in the atlanto-occipital membrane to allow insertion of an
intrathecal (i.th.) catheter (PE-10 tubing) into the subarachnoid
space of the spinal cord. The catheter was 8.5 cm in length and
terminated at the level of the lumbosacral spinal cord, the termination site of colonic pelvic nerve afferent input. At the end of each
experiment, Fast Green dye was injected into the catheter and postmortem examination of the spinal cord confirmed the location of the
distal end of the catheter. Electromyographic (EMG) electrodes (Teflon-coated, 40-gauge stainless steel wires; Cooner Wire, Chatsworth, CA) were sutured into the external oblique musculature, just
superior to the inguinal ligament. The electrode leads were tunneled
s.c. and externalized with the i.th. catheter at the nape of the neck.
Rats were allowed 4 to 7 days for recovery.
Experimental Protocol. A model of inflammatory bowel disease
was established with intracolonic instillation of 2,4,6-trinitrobenzenesulfonic acid (TNBS) (30 mg/ml; Sigma Chemical Co., St. Louis,
MO) in halothane-anesthetized rats (Morris et al., 1989; Duchmann
et al., 1996). The TNBS was administered in a 50% ethanol vehicle
(1.0 ml/rat); ethanol was used to break the mucosal barrier that
normally protects the colon. Control rats received an equal volume of
50% ethanol/saline. Inflammation is maximal 3 to 5 days after TNBS
instillation in this model (Morris et al., 1989), and all animals were
tested at this time.
CRD was used as the model of visceral nociception in TNBStreated and ethanol/saline-treated rats. Animals were allowed 2 h to
acclimate to the testing room. As described in detail in Gebhart and
Sengupta (1996), flexible tygon plastic tubing was inserted into a
latex balloon (6 –7 cm in length), with the end of the balloon securely
tied to the tube. The balloon was coated with Surgilube (E. Fougera
and Co., Melville, NY) and inserted intra-anally into the descending
colon such that the end of the balloon was 1 cm into the rectum. This
assembly was held in place by taping the balloon catheter to the base
of the tail. Rats were fully awake and placed inside a restraining
glove during testing. CRD was produced by pressure-controlled air
inflation of the balloon. The catheter was connected to a pressure
control device (Bioengineering, University of Iowa, Iowa City, IA)
that regulated inflation of the balloon and provided a measure of
intracolonic pressure.
The visceromotor response (VMR), comprised of contraction of the
peritoneal musculature, was quantified from EMG activity recorded
from the electrodes implanted in the external oblique musculature.
The EMG signal was amplified (10,000⫻, 300 –1000 Hz; A-M Systems, Everett, WA) and filtered (200-Hz high pass, 4-pole butterworth; graphic equalizer, Yamaha). The intracolonic pressure and
EMG signals were digitized at 100 Hz (DT280; Data Translation,
Marlboro, MA) and recorded with a computer program written in
A-SYST.
The intracolonic balloon was inflated with staircase increases in
pressure. Each pressure step lasted 10 s, and the pressure was
increased incrementally by steps of 10 mm Hg, from 0 to 80 mm Hg
(Burton and Gebhart, 1998). All staircase distensions were administered at 5-min intervals. Figure 1 represents a typical VMR to CRD
with the staircase paradigm. Four staircase distensions were given
to establish the baseline response before either TNBS or the ethanol/
saline vehicle was instilled into the colon. Three to five days later,
another four staircase distensions were given immediately before
i.th. drug administration. After drug administration, the EMG response at each 10 mm Hg step from 0 to 80 mm Hg was compared
with the baseline response at the respective pressure in the same
pre-TNBS or pre-ethanol/saline animal. EMG activity from the first
5 s of each pressure step was quantified and taken for data analysis
(shaded columns in Fig. 1).
Drugs. The ED50 of morphine was determined with a cumulative
dosing paradigm (Fig. 2): four staircase distensions were administered 0 (immediately after), 5, 10, and 15 min after i.th. morphine
(dissolved in 5 ␮l of saline; followed by a 10-␮l saline flush). The next
dose of morphine was administered at 19 min followed by another
four staircase distensions, and so forth. The ED50 was defined as the
dose that reduced the magnitude of the VMR to 50% of the predrug
response. The approximate ED50 dose of 1.0 ␮g of morphine was used
in all subsequent experiments.
In other experiments, the effects of pretreatment with proglumide
(Sigma Chemical Co.), a nonselective CCK receptor antagonist;
L-364,718, a specific CCKA receptor antagonist; or L-365,260, a
specific CCKB receptor antagonist (both from Merck, Sharpe &
Dohme, Terlings Park, Harlow, Essex, UK) on morphine antinociception were assessed. After four predrug staircase distensions, a
CCK receptor antagonist was administered i.th. Proglumide was
dissolved in 10% dimethyl sulfoxide (DMSO)/saline; L-364,718 and
L-365,260 were dissolved in 20% DMSO/saline. All drugs were delivered in 5-␮l volumes, followed by a 10-␮l flush of saline. Ten
minutes after administration of the CCK receptor antagonist, the
ED50 dose of i.th. morphine (1 ␮g/5 ␮l saline) was administered. A
staircase distension was given every 5 min thereafter until a return
to predrug responsiveness was observed.
The effects of CCK receptor antagonists alone or morphine alone
were determined in both ethanol/saline-treated and TNBS-treated
Fig. 1. Typical VMR to staircase increases in CRD pressure from 0 to 80
mm Hg. The top panel represents the integrated EMG activity recorded
from the external oblique musculature. The bottom panel represents
intracolonic pressure. Shaded regions denote the first 5 s of each pressure
step from which the responses to CRD were quantified.
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(Stanfa and Dickenson, 1993). These findings suggest that
following inflammation, a decrease in CCK activity may allow an increase in antinociceptive potency of morphine (Ossipov et al., 1994).
The objective of the present study was to determine
whether spinal CCK is involved in modulating nociceptive
transmission from the viscera. We addressed this issue by
examining the effect of CCK receptor antagonists on the
potency of morphine in a model of visceral nociception, colorectal distension (CRD) (Ness and Gebhart, 1988). We repeated the experiments in a model of inflammatory bowel
disease to determine the effects of chronic colonic inflammation on CCK receptor antagonist-morphine interactions. Finally, we confirmed which CCK receptor subtype, CCKA or
CCKB, is involved in modulating nociceptive transmission
from the viscera at the level of the spinal cord. Preliminary
reports of some of these data have appeared in abstract form
(Friedrich and Gebhart, 1997).
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Friedrich and Gebhart
Vol. 292
Fig. 2. Dose-response relationship of cumulative doses of i.th. morphine.
Data are illustrated as percentages of control (means ⫾ S.E.).
Results
Effects of Intracolonic TNBS on CRD. Three to five
days after instillation of intracolonic TNBS, the magnitude of
the VMR to CRD was significantly greater in TNBS-treated
rats than in vehicle-treated controls (F ⫽ 117.3, P ⬍ .05) (Fig.
3).
Effects of Morphine on CRD. In six naive, noninflamed
rats, the ED50 dose of morphine given i.th. was determined to
be 0.93 ␮g (0.27–1.53, 95% CI) (Fig. 2). In subsequent exper-
Fig. 3. Effect of TNBS-induced inflammation on the VMR to colorectal
distension. The VMR to distending pressures from 0 to 80 mm Hg were
compared in rats 3 to 5 days after instillation of intracolonic TNBS or
ethanol/saline vehicle. Data are expressed as means ⫾ S.E.
iments, an approximate ED50 dose of 1.0 ␮g of morphine was
used and found to have a significantly greater antinociceptive effect on TNBS-treated animals than on vehicle-treated
controls. As shown in Fig. 4, the peak antinociceptive effect of
morphine occurred ⬃40 min after i.th. morphine administration. Because the maximum VMR usually occurred in response to the 60 mm Hg stimulus, this pressure was chosen
to represent the effects of morphine and CCK receptor antagonists on a noxious visceral stimulus. Morphine (1.0 ␮g)
reduced the response to 60 mm Hg CRD to 62 ⫾ 7.9% of the
pre-ethanol/saline baseline response (Fig. 4A), and to 24 ⫾
4.8% of the pre-TNBS baseline response (Fig. 4B, P ⬍ .05).
When expressed as AUC (Fig. 4C), the effect of 1.0 ␮g i.th.
morphine on the VMR to 60 mm Hg distension was 2.5-fold
greater in TNBS-treated rats (1609 versus 4060, P ⬍ .05).
Effects of Proglumide on Morphine Antinociception.
Proglumide dose dependently increased the duration and
maximum effect of morphine in vehicle-treated rats (Fig. 4A).
In the absence of proglumide, the peak antinociceptive effect
of morphine occurred at 40 min postinjection and lasted ⬃80
min. Pretreatment with i.th. proglumide enhanced morphine’s antinociceptive effect in vehicle-treated but not
TNBS-treated rats. Because time of peak effect, duration of
effect, and magnitude of effect of morphine all changed in the
presence of proglumide, an AUC was used for analysis of data
(Fig. 4C). In vehicle-treated rats, pretreatment with proglumide (0.01, 0.1, or 1.0 ␮g) dose dependently enhanced the
effect of 1.0 ␮g of morphine from 1609 ⫾ 146.1 to 1784 ⫾
123.5 (P ⬎ .5), 2835 ⫾ 352.3 (P ⬍ .05), and 3117 ⫾ 402.6 (P ⬍
.05), respectively. No change in response to CRD was observed following administration of proglumide (not illustrated) or DMSO vehicle alone.
The antinociceptive effect of morphine in TNBS-treated
rats was significantly greater than produced by the same
1.0-␮g i.th. dose in vehicle-treated rats, but pretreatment
with proglumide provided no further enhancement of effect
(Fig. 4B). Concerned about a possible “floor effect,” we thus
repeated the experiment in TNBS-treated rats with 0.6 ␮g of
i.th. morphine (the approximate ED30) in the presence of the
same doses of proglumide. Proglumide did not enhance the
antinociceptive effects of this lower dose of morphine (Fig. 5).
Effects of Specific CCKA and CCKB Receptor Antagonists on Morphine Analgesia. To elucidate which CCK
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animals. Stimulus-response functions were generated, and the data
were normalized as a percentage of the response to CRD in the
pre-TNBS or pre-ethanol/saline animal. The response to CRD after
injection of vehicle (10 or 20% DMSO in saline) also was determined.
The time of onset, peak effect, and duration of effect on the VMR was
observed and compared in animals that received only morphine
versus those given CCK receptor antagonist before morphine. Additionally, differences between vehicle-treated rats and rats with
chronic colonic inflammation were determined.
Assessment of Inflammation. To quantify the extent of TNBSinduced inflammation, colonic myeloperoxidase (MPO) activity in
TNBS-treated and ethanol/saline-treated rats was determined. MPO
is an enzyme found predominantly in azurophilic granules of polymorphonuclear leukocytes (neutrophils). Neutrophil infiltration is a
characteristic feature of inflammation, and greater MPO activity in
tissue represents increased neutrophil infiltration into inflamed tissue (Krawisz et al., 1984; Mullane et al., 1985). The MPO assay was
performed in distension-naive rats 4 days after intracolonic instillation of either TNBS or ethanol/saline. Approximately 15 mm of the
distal colon was removed and weighed. The tissue was then suspended in 0.5% hexadecyltrimethylammonium bromide in 50 mM
potassium phosphate buffer and homogenized for 10 min. The homogenate was freeze-thawed three times, centrifuged, and the MPO
activity of the supernatant was determined. The supernatant was
mixed with a dye solution containing o-dianisidine dihydrochloride
and 0.0005% hydrogen peroxide. One unit of MPO activity was
defined as that converting 1 ␮mol of hydrogen peroxide to water in 1
min. The change in absorbance at 460 nm was determined spectrophotometrically.
Data Analysis. All experiments were repeated in at least six
animals, and all data are presented as means ⫾ S.E. Data were
analyzed with unpaired t tests and repeated-measures ANOVA (with
GraphPad, Prism, and Excel computer programs), where applicable.
P ⬍ .05 was considered statistically significant in all cases. The ED50
dose of morphine was defined as the dose required to attenuate the
VMR to 50% of the predrug response. The ED50 dose (and 95%
confidence interval) of morphine was calculated using a computer
curve-fitting program (GraphPad, Prism). Area under the curve
(AUC) was calculated with an Excel computer program.
2000
Spinal CCK Is Antiopioid in Visceral Nociception
541
Fig. 4. Effects of the nonselective CCK receptor antagonist proglumide
(pro) on morphine (mor) antinociception. The VMR to 60 mm Hg CRD was
determined in (A) ethanol/saline vehicle-treated rats or (B) TNBS-treated
rats. The VMR to 60 mm Hg CRD before either TNBS or ethanol/saline
vehicle was instilled into the colon was averaged and used as the 100%
control (pre). The response to 60 mm Hg CRD was measured following
i.th. mor alone or mor after i.th. pretreatment with pro 3 to 5 days after
intracolonic TNBS or vehicle. The E represent the responses in either
TNBS-treated or vehicle-treated rats following i.th. administration of
vehicle (10% DMSO in saline). Data are expressed as percentages of
control (means ⫾ S.E.). C, AUC from (A) and (B) were determined for mor
and mor plus pro in both ethanol/saline vehicle-treated rats and TNBStreated rats.
receptor is involved in modulating spinal opioid antinociception, CRD was performed in animals following administration of morphine after pretreatment with specific CCKA or
CCKB receptor antagonists (L-364,718 and L-365,260, respectively). Administration of the specific CCKA receptor antagonist did not enhance morphine antinociception. Figure 6
shows the VMR to 60 mm Hg pressure of distension in ethanol/saline vehicle-treated and TNBS-treated rats following
morphine alone or morphine plus the CCKA receptor antagonist (0.001, 0.01, and 1.0 ␮g). The antinociceptive effect of
morphine (1.0 ␮g) was again significantly greater in TNBStreated rats compared with vehicle-treated rats (P ⬍ .05).
There was no significant effect of the CCKA receptor antagonist on morphine-produced antinociception in vehicletreated or TNBS-treated rats (Fig. 6C).
The specific CCKB receptor antagonist L-365,260, however, dose dependently enhanced morphine antinociception
in vehicle-treated animals. Figure 7 depicts the VMR to 60
mm Hg CRD after administration of morphine and after
pretreatment with the CCKB receptor antagonist (0.01, 0.1,
and 1.0 ng) in vehicle-treated and TNBS-treated animals.
L-365,260 significantly enhanced the antinociceptive effects
of morphine (time of peak effect, magnitude, and duration) in
vehicle-treated rats (Fig. 7C), but not in TNBS-treated rats.
The effects of the CCKB receptor antagonist on morphine
antinociception were indistinguishable from the effects of
proglumide on morphine antinociception (compare Figs. 4C
and 7C). CCKA or CCKB receptor antagonists alone did not
affect the VMR to CRD.
Assessment of TNBS-Induced Inflammation. TNBStreated rats developed diarrhea after TNBS instillation, and
colons from TNBS-treated rats appeared thickened and reddened. No differences were observed between ethanol/salinetreated rats and naive rats. To further assess the extent of
inflammation caused by TNBS, colonic MPO activity was
determined in both TNBS-treated and vehicle-treated rats.
MPO activity in vehicle-treated rats was significantly less
than MPO activity in TNBS-treated rats (0.01 ⫾ 0.01 versus
25.3 ⫾ 8.2 U MPO/g wet weight of colon, respectively, P ⬍
.05).
Discussion
The present study demonstrates that spinal CCK is involved in modulating spinal opioid antinociception following
a noxious visceral stimulus. Intrathecal CCK receptor antagonists, both nonspecific and the specific CCKB receptor antagonist, significantly enhanced morphine antinociception in
a dose-dependent manner. These results are consistent with
studies on the effects of spinal CCK in models of cutaneous
nociception (Watkins et al., 1985b; Stanfa and Dickenson,
1993). Proglumide was effective in the CRD model of visceral
nociception in dose ranges similar to those reported to be
effective in models of cutaneous nociception. Although mechanisms involved in pain modulation of visceral and cutaneous structures are most likely distinct, we have shown that
spinal CCK modulates the transmission of noxious visceral
input similarly to what has been documented for cutaneous
stimulation.
The development of selective CCK receptor antagonists led
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Fig. 5. Effects of proglumide (pro), a nonselective CCK receptor antagonist, on the antinociceptive effects of the approximate ED30 dose of
morphine (mor; 0.6 ␮g) in TNBS-treated rats. Intrathecal mor was administered alone or following i.th. proglumide (pro), and the VMR to 60
mm Hg CRD was determined. The VMR to 60 mm Hg CRD before TNBS
was instilled into the colon was averaged and used as the 100% control.
The E represent the responses in TNBS-treated rats following i.th. administration of 10% DMSO vehicle. Data are expressed as percentages of
control (means ⫾ S.E.).
542
Friedrich and Gebhart
to the classification of two CCK receptor subtypes, CCKA and
CCKB. The CCK receptor antagonist L-364,718 (also known
as devazepide) is 150 times more selective for the CCKA
receptor than the CCKB receptor, whereas L-365,260 is ⬃50
times more selective for the CCKB receptor (Hughes et al.,
1990). CCKA receptors are predominantly found in the periphery, whereas CCKB receptors are principally located in
the central nervous system (Moran et al., 1986; Wank et al.,
1992). Previous studies suggest that interactions between
CCK receptor antagonists and opioids are primarily mediated via CCKB receptors (Valverde et al., 1994). For example,
Vanderah et al. (1994) showed that pretreatment with i.c.v.
CCKB receptor antisense oligonucleotide in mice enhanced
the antinociceptive potency of i.c.v. morphine. We report
herein that the CCKA receptor antagonist L-364,718 had no
effect on the antinociceptive potency of morphine. However,
the CCKB receptor antagonist L-365,260 enhanced the antinociceptive effect of morphine in a manner similar to proglumide, the nonspecific CCK receptor antagonist, confirm-
Fig. 7. Effects of the CCKB receptor antagonist L-365,260 (L-365) on
morphine (mor) antinociception. Responses to 60 mm Hg CRD were
determined in (A) ethanol/saline vehicle-treated rats or (B) TNBS-treated
rats. The VMR to 60 mm Hg CRD before either TNBS or vehicle was
instilled into the colon was averaged and used as the 100% control. The
response to 60 mm Hg CRD was measured following i.th. mor alone or
mor after i.th. pretreatment with L-365 3 to 5 days after intracolonic
TNBS or vehicle. The E represent the responses in either TNBS-treated
or vehicle-treated rats following i.th. administration of 20% DMSO vehicle. Data are expressed as percentages of control (means ⫾ S.E.). C, AUC
from (A) and (B) were determined for mor and mor plus L-365 in both
ethanol/saline vehicle-treated rats and TNBS-treated rats.
ing that the CCK receptor associated with modulation of
spinal visceral nociception is the CCKB receptor.
The mechanism whereby CCK attenuates morphine antinociception is not known; however, two major theories predominate. Following CCK receptor activation, Ca2⫹ may be
mobilized from intracellular stores and facilitate neurotransmitter release (Wang et al., 1992). This would oppose the
inhibition of Ca2⫹ channels and concurrent reduction in
transmitter release observed after opioid receptor activation
(Piros et al., 1995). Alternatively, CCK may attenuate morphine antinociception by inhibiting the release and/or availability of endogenous enkephalins (Ossipov et al., 1994; Vanderah et al., 1996; Skinner et al., 1997), which act principally
at the ␦-opioid receptor. Because an additive interaction has
been observed between ␮- and ␦-opioid receptor agonists
(Barrett and Vaught, 1982), a decrease in availability of
endogenous enkephalins following morphine administration
and subsequent CCK release would lessen the overall opioid
antinociceptive effect.
Neural mechanisms involved in nociceptive transmission
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Fig. 6. Effects of the specific CCKA receptor antagonist L-364,718 (L-364)
on morphine (mor) antinociception. Responses to 60 mm Hg CRD were
determined in (A) ethanol/saline vehicle-treated rats or (B) TNBS-treated
rats. The VMR to 60 mm Hg CRD before either TNBS or vehicle was
instilled into the colon was averaged and used as the 100% control. The
response to 60 mm Hg CRD was measured following i.th. mor alone or
mor after i.th. pretreatment with L-364 3 to 5 days after intracolonic
TNBS or vehicle. The unfilled circles represent the responses in either
TNBS-treated or vehicle-treated rats following i.th. administration of
20% DMSO vehicle. Data are expressed as percentages of control
(means ⫾ S.E.). C, AUC from (A) and (B) were determined for mor and
mor plus L-364 in both ethanol/saline vehicle-treated rats and TNBStreated rats.
Vol. 292
2000
543
sponse to spinal opioids is not known. However, preliminary
results suggest that supraspinal sites are necessary for release of spinal CCK following i.th. morphine administration
(Friedrich and Gebhart, 1998).
The physiological basis for release of endogenous CCK in
response to opioids is unknown. Such endogenous antianalgesic mechanisms may function to return an animal to a
basal state of pain responsivity after endogenous or exogenous opiate-induced analgesia. However, morphine tolerance
may result from this compensatory increase in activity of
endogenous CCK systems after repetitive opiate administration. Previous studies have shown that administration of
CCK receptor antagonists in addition to morphine can enhance the antinociceptive effects of morphine as well as prevent morphine tolerance (Watkins et al., 1984; IdanpaanHeikkila et al., 1997). A decrease in CCK release or
availability following inflammation, however, may work to
enhance the antinociceptive effect of opioids during a more
chronic pain state.
Acknowledgments
We thank Michael Burcham for preparation of the graphics and
Susan Birely for secretarial assistance.
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TNBS-treated rats following i.th. morphine administration,
suggesting a central consequence of peripheral, colonic inflammation.
The increase in morphine potency observed following
chronic visceral inflammation was not further enhanced by
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enhance morphine antinociception. CCK, however, still attenuates morphine antinociception in the presence of inflammation. Stanfa and Dickenson (1993) propose that this enhancement of spinal morphine potency during inflammation
may be explained by a reduction in spinal CCK release by
morphine. The source(s) of the spinal CCK released in re-
Spinal CCK Is Antiopioid in Visceral Nociception
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Send reprint requests to: A. E. Friedrich, Department of Pharmacology,
University of Iowa College of Medicine, Bowen Science Building, Iowa City, IA
52242-1109. E-mail: ann-friedrich@uiowa.edu
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