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The Journal of Neuroscience, October 1, 2001, 21(19):7788-7792
Potentiation of Opioid Analgesia in Dopamine2
Receptor Knock-Out Mice: Evidence for a Tonically Active Anti-Opioid
System
Michael A.
King1,
Sheri
Bradshaw2,
Albert H.
Chang1,
John E.
Pintar2, and
Gavril W.
Pasternak1
1 Laboratory of Molecular Neuropharmacology, Memorial
Sloan-Kettering Cancer Center, New York, New York 10021, and
2 Department of Neuroscience and Cell Biology, Robert Wood
Johnson Medical School, University of Medicine and Dentistry of New
Jersey, Piscataway, New Jersey 08854
 |
ABSTRACT |
Dopamine systems are intimately involved with opioid actions.
Pharmacological studies suggest an important modulatory effect of
dopamine and its receptors on opioid analgesia. We have now examined
these interactions in a knock-out model in which the dopamine2 (D2) receptor has
been disrupted. Loss of D2 receptors enhances,
in a dose-dependent manner, the analgesic actions of the µ analgesic morphine, the 1 agonist U50,488H and the
3 analgesic naloxone benzoylhydrazone. The responses to
the opioid analgesic [D-Pen2,D-Pen5]enkephalin
were unaffected in the knock-out animals. Loss of D2
receptors also potentiated spinal orphanin FQ/nociceptin analgesia. Antisense studies using a probe targeting the D2 receptor
revealed results similar to those observed in the knock-out model. The modulatory actions of D2 receptors were independent of receptor systems because the agonist (+)-pentazocine lowered opioid
analgesia in all mice, including the D2 knock-out group.
Thus, dopamine D2 receptors represent an additional,
significant modulatory system that inhibits analgesic responses to µ and opioids.
Key words:
analgesia; dopamine; dopamine receptor; D2 receptor; knock-out; antisense; anti-opioid; nociception; analgesic
 |
INTRODUCTION |
Dopamine-opioid interactions have
been widely studied. Anatomically, opioid and dopamine systems are
closely related (Khachaturian and Watson, 1982 ) and their interactions
are functionally significant. Neurochemically, opioids influence
dopamine release (Wood et al., 1980 ; Wood, 1983 ; Wood and Pasternak,
1983 ) and therefore prolactin release (Wood et al., 1980 ; Spiegel et
al., 1982 ; Wood and Pasternak, 1983 ; Brent and Bunn, 1994 ). Chronic
haloperidol upregulates enkephalin levels (Hong et al., 1978 ) and
dopamine agonists upregulate the expression of µ opioid receptor mRNA
(Azaryan et al., 1996 ), whereas opioid treatment downregulates
[3H]spiperone binding (Brent and Bunn,
1994 ).
The dopamine system also influences opioid behaviors. The study of
dopamine systems in opioid anti-nociception goes back 30 years (Calcutt
et al., 1971 ; Tulunay et al., 1975 ; Rodgers, 1977 ; McGilliard and
Takemori, 1979 ). These early studies reported lowered opioid analgesia
after activation of dopamine systems and enhanced analgesia with
dopamine receptor antagonists. More recent studies support these
initial observations, including some looking at specific
dopamine2 (D2) receptor
drugs. The dopamine D2 receptor agonist
quinpirole (Kamei and Saitoh, 1996 ) and the dopamine precursor L-3,4-dihydroxyphenylalanine (Kunihara et al., 1993 )
both lower the analgesic activity of morphine. The
D2 antagonist ( )-sulpiride potentiates the
analgesic actions of the µ-selective opioid sulfentanil, whereas the
D1 receptor antagonist SCH23390 was without
effect (Rooney and Sewell, 1989 ), implying that a tonically active
D2 receptor system downregulates opioid analgesia.
However, others report that activation of dopamine systems can
facilitate analgesic systems (Bodnar and Nicotera, 1982 ). The D2 receptor agonist RU24926 is analgesic in mice
and the response is blocked by both D2 receptor
antagonists and the opioid antagonist naloxone (Michael-Titus et al.,
1990 ; Suaudeau and Costentin, 1995 ). In the formalin test, a
D2 antagonist diminished morphine analgesia,
whereas the D2 agonist quinpirole elicited
analgesia (Morgan and Franklin, 1991 ), an action quite different from
that seen by others using a thermal paradigm (Kamei and Saitoh, 1996 ). Thus, dopamine has complex effects on opioid systems and is able to
facilitate and/or inhibit opioid analgesia.
Opioid analgesia also is modulated by a tonically active anti-opioid
1 receptor system that can be activated by the
1 agonist (+)-pentazocine and blocked by
haloperidol (Chien and Pasternak, 1993 , 1994 , 1995a ,b ). The existence
of this system has been confirmed using antisense approaches against
the cloned receptor in both mice and rats (Pan et al., 1998 ; Mei
and Pasternak, 2001 ). Although the actions of haloperidol on opioid
analgesia clearly involve receptors, the possibility of an
additional role for dopamine D2 receptors remains
based on the high affinity of haloperidol for both
1 and D2 receptors.
Knock-out strategies offer a unique approach toward defining the roles
of specific proteins in behavior. For example, a
D2 knock-out mouse demonstrated the importance of
D2 dopamine receptors in the rewarding behavior
of morphine (Maldonado et al., 1997 ). To explore the relationship of
dopamine D2 receptors to
1 receptors and their modulation of opioid
analgesia we have examined the effects of disruption of
D2 receptors on opioid analgesia.
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MATERIALS AND METHODS |
Morphine sulfate, morphine-6 -glucuronide (M6G),
[D-Pen2,D-Pen5]enkephalin
(DPDPE), and U50,488H were gifts from the Research Technology Branch of
the National Institute on Drug Abuse (Bethesda, MD).
(+)-Pentazocine was a gift from Sanofi-Winthrop (New York, NY).
( )-Sulpiride was purchased from Sigma (St. Louis, MO).
Naloxone benzoylhydrazone (NalBzoH) was synthesized
as described previously (Luke et al., 1988 ). Clonidine was
purchased from Research Biochemicals International (Natick,
MA). Halothane was purchased from Halocarbon Laboratory (Hackensack,
NJ). Orphanin FQ/nociceptin (OFQ/N) was synthesized by the Core
Facility at Memorial Sloan-Kettering Cancer Center and purified
by HPLC; its structure was verified by mass spectroscopy.
Male Crl:CD-1(ICR)BR mice (24-32 gm) were purchased from
Charles River Laboratories (Raleigh, VA). The mutated mice were
generated as described previously (Jung et al., 1999 ). Mice were
generated from heterozygous matings of mice derived from a cross of
C57BL6/J × 129/SwEv and were maintained on a 12 hr light/dark
cycle with food and water available ad libitum. The drugs
were administered subcutaneously, intracerebroventricularly (Haley and
McCormick, 1957 ), or intrathecally (Hylden and Wilcox, 1980 ). Analgesia
was assessed quantally using the radiant heat tailflick assay with baseline latencies ranging from 2 to 3 sec, as described previously (King et al., 1997a ). A 10 sec cutoff was imposed to minimize tissue
damage. Analgesia was defined quantally as a doubling or greater of the
baseline latency for the individual mouse. Group comparisons were
performed using Fisher's exact test. A modification of the Litchfield
and Wilcoxon method was used to determine ED50 values and 95% confidence limits (Tallarida and Murray, 1987 ).
The dopamine D2 receptor antisense
oligodeoxynucleotide sequence was based on the mouse
D2 receptor sequence (Montmayeur et al., 1991 ;
Guiramand et al., 1995 ). The oligodeoxynucleotide was synthesized by
Midland Certified Reagent Co. (Midland, TX), purified in our
laboratory, and dissolved in 0.9% saline. Antisense A (GGT TGG CTC TGA
AAG CTC GGC) corresponds to nucleotides 755-775. The mismatch
oligodeoxynucleotide (GGT GTG CTC TAG AAG TCC
GGC) is based on antisense A and differs only in the sequence of six
bases (underlined). Male Crl:CD-1(ICR)BR mice received antisense
A (5.0 µg/2.0 µl, i.c.v.) on days 1, 3, and 5 and were
tested for analgesia on day 6, as described previously (Standifer et
al., 1994 ; King et al., 1997a ).
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RESULTS |
First, we explored the role of D2 dopamine
receptors in modulating opioid analgesia by examining the effects of
the dopamine D2 antagonist sulpiride on morphine
analgesia. A low dose of morphine was used to facilitate our ability to
detect an increased analgesic response. Sulpiride enhanced morphine
analgesia in wild-type mice, increasing the response from only 10% to
60% (Fig. 1). In contrast, sulpiride had
no effect on morphine analgesia in the knock-out mice, suggesting that
its potentiation of analgesia in the wild-type mice reflected dopamine
D2 receptor blockade.

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Figure 1.
Sulpiride effect on morphine analgesia.
Dopamine2 receptor knock-out mice (n 10) received morphine (2 mg/kg, s.c.) and ( )-sulpiride (10 mg/kg,
s.c.). Analgesia was assessed 30 min later. ( )-Sulpiride
significantly potentiated morphine analgesia in the wild-type animals
(*p < 0.03) but not in the homozygous
animals.
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These initial experiments showed that morphine alone was more potent in
the knock-out mice than in the wild-type group (Fig. 1). To explore
this observation further, we examined the analgesic actions of various
opioids in wild-type, heterozygous, and knock-out mice in which the
D2 receptor had been disrupted. Supraspinally, the D2 receptor knock-out mice were significantly
more sensitive to the µ drugs morphine and M6G, the
1 analgesic U50,488H, and the
3 drug NalBzoH (Fig.
2A). The heterozygotes
gave intermediate responses. In contrast, the drug DPDPE given
supraspinally had similar activities in the wild-type and knock-out
groups. Supraspinal OFQ/N had little effect in any of the
groups. Spinally, we observed a slightly different pattern (Fig.
2B). Morphine, M6G, and U50,488H still produced far
greater responses in the knock-out mice, whereas DPDPE analgesia was
unaffected. However, OFQ/N analgesia at the spinal level was
significantly enhanced in the knock-out mice.

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Figure 2.
Effects of centrally administered opioid
analgesics in D2 knock-out mice. A, Groups
of mice (n 10) received the indicated drugs
[morphine (233 ng, i.c.v.), M6G (4 ng, i.c.v.), DPDPE (4 µg,
i.c.v.), U50,488H (25 µg, i.c.v.), or NalBzoH (15 µg, i.c.v.)]
supraspinally, and analgesia was assessed by tailflick assay. Results
are the percentage of mice that were analgesic, defined quantally as a
doubling or greater of their baseline values; *p < 0.01. B, Groups of mice (n 10)
received morphine (233 ng, i.t.), M6G (4 ng, i.t.), DPDPE (0.3 µg,
i.t.), U50,488H (25 µg, i.t.), NalBzoH (15 µg, i.t.), or OFQ (5 µg, i.t.) spinally, and analgesia was assessed by tailflick assay.
Results are the percentage of mice that were analgesic, defined
quantally as a doubling or greater of their baseline values;
*p < 0.01.
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Systemic administration gave results that were similar to those seen
with central injections (Fig. 3).
Dose-response curves with morphine, M6G, U50,488H, and NalBzoH all
revealed significant shifts to the left in the knock-out groups. The
sensitivity of the knock-out mice to morphine and M6G analgesia was
enhanced more than twofold, but even greater effects were observed with the drugs. The response curve for the 1
analgesic U50,488H was shifted 12-fold to the left and the curve for
the 3 agent NalBzoH was shifted fivefold
(Table 1).

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Figure 3.
Effects of systemic opioid analgesics in
D2 knock-out mice. Cumulative dose-response curves were
generated in groups of mice (n 10) for morphine,
M6G, U50,488H, or NalBzoH. All mice in the group received the lowest
drug dose, and analgesia was assessed 30 min later. Animals that were
not analgesic at the first dose then received a second dose and were
tested 30 min afterward. This same procedure was then repeated until
all mice were analgesic. The ED50 values (95% confidence
limits) are presented in Table 1.
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To ensure that the effects seen in the knock-out mice were attributable
to the loss of the targeted protein and not to more generalized
developmental changes secondary to the loss of the D2 receptor, we also used an antisense approach
in adult mice. D2 antisense approaches have been
widely reported in the literature (Weiss et al., 1993 , 1997 ; Zhang and
Creese, 1993 ; Creese and Tepper, 1998 ). An antisense
oligodeoxynucleotide given supraspinally and targeting the
D2 receptor enhanced the analgesic actions of morphine, M6G, U50,488H, and NalBzoH (Fig.
4). As in the knock-out mice, DPDPE
analgesia was unchanged. The effect was specific because a mismatch
probe with minor sequence changes was inactive.

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Figure 4.
Effect of dopamine D2 receptor
antisense on opioid analgesia. Groups of CD-1 mice
(n 20) received saline or the indicated
oligodeoxynucleotide (5 µg, i.c.v.) on days 1, 3, and 5. On day 6 all
mice were tested with morphine (3 mg/kg, s.c.), M6G (2.5 mg/kg, s.c.),
DPDPE (4 µg, i.c.v.), U50,488H (2 mg/kg, s.c.), or NalBzoH (30 mg/kg, s.c.), and analgesia was assessed quantally. Antisense A
significantly potentiated morphine, M6G, U50,488H, and NalBzoH
analgesia; *p < 0.01; **p < 0.001.
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(+)-Pentazocine reverses opioid analgesia despite its inability to bind
to opioid receptors, an action that has been attributed to activation
of 1 receptors (Chien and Pasternak, 1993 ,
1994 ). To explore the relationship between the and
D2 systems, we subsequently examined the effects
of the ligand (+)-pentazocine in the D2 knock-out animals. (+)-Pentazocine (5 mg/kg, s.c.) lowered morphine analgesia in all three groups of mice (Fig.
5A). In all cases, the actions
of (+)-pentazocine were blocked by the concurrent administration of
haloperidol. Although haloperidol binds to both and dopamine
D2 receptors with similar high affinities, its continued activity in the knock-out group lacking dopamine
D2 receptors confirms a receptor mechanism of
action in this model.

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Figure 5.
(+)-Pentazocine and haloperidol modulation of
morphine analgesia. A, Groups of mice
(n 10) received morphine (5 mg/kg, s.c.) and
(+)-pentazocine (3 mg/kg, s.c.) alone, (+)-pentazocine (3 mg/kg, s.c.)
with haloperidol (0.1 mg/kg, s.c.), or nothing. Analgesia was assessed
30 min later. (+)-Pentazocine significantly lowered the analgesic
response in all three groups (*p < 0.01).
B, Groups of mice (n 14) received
morphine (2 mg/kg, s.c.) alone or with haloperidol (0.1 mg/kg, s.c.).
Haloperidol significantly increased the analgesic responses in the
heterozygotes (p < 0.003) and in the
knock-out mice (*p < 0.003;
**p < 0.02).
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Finally, we examined the effects of haloperidol directly on morphine
analgesia using a low dose of the opioid (Fig. 5B).
Haloperidol enhanced the morphine response in all three groups, with
the most significant effects observed in the heterozygotes and
the knock-out mice. This observation demonstrated that the
D2 receptor knock-out mice retained a tonically
active anti-opioid system and showed conclusively that the
anti-opioid D2 and systems are distinct.
 |
DISCUSSION |
The modulation of the opioid analgesia by other transmitters is
quite complex. A number of transmitters decrease the sensitivity of
animals to opioid analgesics, including receptor agonists (Chien
and Pasternak, 1993 , 1994 , 1995a ,b ), orphanin FQ/nociceptin (Grisel et
al., 1996 ; Mogil et al., 1996 ; Tian et al., 1997 ; King et al., 1998 ),
cholecystokinin (Faris et al., 1983 ; Cesselin, 1995 ; Nichols et
al., 1995 ; Xu et al., 1996b ), and neuropeptide FF (Cesselin,
1995 ; Roumy and Zajac, 1998 ). Traditional pharmacological studies
suggested a similar anti-opioid activity of dopamine
D2 receptors (Rooney and Sewell, 1989 ; Kunihara
et al., 1993 ; Kamei and Saitoh, 1996 ), a concept which is supported by
the current study.
Disruption of D2 receptors potentiated opioid
analgesia, but not all opioid systems were affected similarly. µ Analgesia was enhanced both spinally and supraspinally, but the
greatest effects were seen with the drugs. Analgesia is often
not as easily demonstrated as µ analgesia, perhaps because of
activity of anti-opioid systems. This was clearly shown in studies
looking at the effects of receptors for which antagonism of sites enhanced analgesics more prominently than µ drugs and even
accounted for some of the differences in sensitivity of mouse strains
to these drugs (Chien and Pasternak, 1993 , 1994 , 1995b ; King et al.,
1997a ; Pan et al., 1998 ). Because many clinical analgesics have activity, these findings raise the possibility that concurrent use of
D2 antagonists with these drugs might increase
their utility in pain management.
Disrupting the D2 receptor had little effect on
analgesia, emphasizing the differences among the opioid analgesic
systems. The pharmacology of OFQ/N supraspinally is quite complex, with hyperalgesic (Meunier et al., 1995 ; Reinscheid et al., 1995 ), anti-opioid (Grisel et al., 1996 ; Mogil et al., 1996 ; King et al.,
1998 ), and analgesic activities (Rossi et al., 1996 , 1997 ) depending on
the paradigm, dose, time course, and even strain of mouse. Supraspinal
OFQ/N analgesia was not seen in these studies. Previously, we were able
to detect significant supraspinal OFQ/N analgesia only in conjunction
with haloperidol (Rossi et al., 1996 ). The inability to detect
appreciable OFQ/N analgesia in the D2 knock-out mice would suggest that
the actions of haloperidol in these previous studies were attributable
to the blockade of 1 and not
D2 receptors. Several studies have documented a
more robust analgesic activity of OFQ/N given spinally than
supraspinally (Rossi et al., 1996 , 1997 ; Xu et al., 1996a ; King et al.,
1997b ). In the current studies we confirmed the presence of a potent
OFQ/N analgesia after intrathecal administration that was markedly
enhanced in the D2 receptor knock-out mice.
The anatomical site of the physiological interactions between the
D2 receptor and opioid systems remains unclear.
Many regions contain both D2 and opioid
receptors, such as lamina I within the spinal cord and a variety of
supraspinal structures (Khachaturian and Watson, 1982 ; Curran and
Watson, 1995 ; van Dijken et al., 1996 ; Khan et al., 1998 ), raising the
possibility of direct D2 receptor-opioid
interactions. However, there is no evidence that the
D2 receptor system acts directly on circuits
containing opioid neurons, leaving open the possibility that this
system modulates analgesia through intermediary neuronal circuits or
possibly even at higher levels of sensory integration. There are some
indications that supraspinal µ and opioid receptors
are downregulated supraspinally in the knock-out mice, whereas
1 opioid receptors and nociceptin/orphanin FQ
(NOP1 or ORL1) receptors
are upregulated both supraspinally and spinally (I. Kitchen and J. E. Pintar, unpublished observations). However, these changes are quite
modest and their significance remains to be demonstrated.
In knock-out mice, behavioral changes may simply reflect the loss of
the targeted protein, but the question of compensatory developmental
changes also must be considered. Antisense approaches can avoid some of
these problems. Although the degree of downregulation is often limited,
the technique can be applied to adult animals, eliminating potential
developmental effects. Antisense studies have been used effectively
against D2 receptors in the past (Weiss et al.,
1993 , 1997 ; Zhang and Creese, 1993 ; Creese and Tepper, 1998 ). In our
studies, antisense treatment gave results that were indistinguishable
from those seen in the knock-out model. Downregulation of
D2 receptors supraspinally potentiated µ and
analgesia without affecting systems. The similar findings in
both the knock-out and antisense paradigms imply that actions are
attributable to a loss of the D2 receptor itself
and are not a result of compensatory developmental changes in the
knock-out mice.
The 1 receptor system also modulates opioid
activity. Indeed, the differences in opioid sensitivity
among some strains result from varying tonic levels of
1 receptor activity (Chien and Pasternak, 1993 , 1994 ). Haloperidol has been used extensively to explore the
effects of systems. However, haloperidol has high affinity for both
D2 and receptor systems, making conclusions
difficult. The knock-out mice provided a model to explore the
relationship between the two systems. The persistent
ability of the 1 agonist (+)-pentazocine to
lower morphine analgesia in the D2 knock-out mice
demonstrated the continued presence of the 1
receptor system in these animals. In addition, the reversal of this
action by haloperidol confirmed its activity as a
1 antagonist. Thus, the anti-opioid
1 system is independent of the dopamine
D2 receptor system.
In conclusion, the modulation of opioid analgesia by other
neurotransmitter systems is complex. Many systems facilitate opioid actions, whereas others are inhibitory. The current studies in mice
with a disruption of the dopamine D2 receptor
reveal an important modulatory role for this dopamine that may be used
clinically in the management of pain.
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FOOTNOTES |
Received March 1, 2001; revised July 17, 2001; accepted July 18, 2001.
This work was supported by National Institute on Drug Abuse Grants
DA08622 (J.E.P.); DA07241, DA02615, and DA00220 (G.W.P.); and
T32DA07274 (M.A.K.) and by National Cancer Institute Core Grant CA08748
to Memorial Sloan-Kettering Cancer Center.
Correspondence should be addressed to Dr. Gavril Pasternak, Memorial
Sloan-Kettering Cancer Center, 1275 York Avenue, New York, NY 10021. E-mail: pasterng{at}mskcc.org.
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