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Vol. 53, Issue 3, 381-415, September 2001
Department of Psychology, McGill University, Montreal, Quebec Canada (J.S.M.); and Laboratory of Molecular Neuropharmacology, Memorial Sloan-Kettering Cancer Center, New York, New York (G.W.P.)
Abstract
I. Introduction
II. Molecular Biology of the Orphanin FQ/Nociceptin Receptor
A. Cloning NOP1 and Its Gene
B. Alternatively Splicing NOP1
C. Molecular Modifications of NOP1
III. Receptor Binding
IV. NOP1 Receptor Heterogeneity
V. Orphanin FQ/Nociceptin
A. Structure of Orphanin FQ/Nociceptin
B. Orphanin FQ/Nociceptin Analogs and Antagonists
C. Orphanin FQ/Nociceptin Precursors and Their Processing
VI. Anatomy of Orphanin FQ/Nociceptin and Its Receptor
VII. Range of Effects of Orphanin FQ/Nociceptin
VIII. Effects of Orphanin FQ/Nociceptin on Pain
A. Effects of Supraspinally Administered Orphanin FQ/Nociceptin
B. Effects of Spinally Administered Orphanin FQ/Nociceptin
C. Effects of Peripherally Administered Orphanin FQ/Nociceptin
D. Reconciling the Literature
1. Noxious Stimulus Modality.
2. Robustness of Various Phenomena.
3. Influence of Stress.
4. Organismic Factors: Species, Strain, and Sex Differences.
5. Dose Dependence.
6. Opioid Tone.
E. Effects of Other NOP1 Receptor Agonists
F. Phenotypes of Knockout Mice
G. Effects of NOP1 Down-Regulation or Blockade
1. Antisense Studies.
2. Pharmacological Antagonists.
H. Mechanisms of Orphanin FQ/Nociceptin Actions: Ubiquitous Cellular Inhibition as a Unifying Hypothesis?
IX. Effects of Related Peptides on Pain
A. Nocistatin
B. Orphanin FQ/Nociceptin 2
X. Involvement of NOP1 in Other Central Nervous System-Mediated Behaviors
A. Locomotor Activity and Reward
B. Anxiety, Fear, and Stress
C. Tolerance and Dependence
D. Learning and Memory
E. Feeding
XI. Conclusions and Future Directions
Acknowledgments
References
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Abstract |
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The isolation of an opioid receptor-related clone soon led to the isolation and characterization of a new neuropeptide, termed orphanin FQ or nociceptin (OFQ/N). This heptadecapeptide binds to the NOP1 (previously termed ORL1) receptor with exceedingly high affinity, but does not interact directly with classical opioid receptors. Functionally, the actions of OFQ/N are diverse and intriguing. Most work has focused upon pain mechanisms, where OFQ/N has potent anti-analgesic actions supraspinally and analgesic actions spinally. Other OFQ/N activities are less clear. The diversity of responses might reflect NOP1 receptor heterogeneity, but this remains to be established. The actions of this neurochemical system may also be uniquely dependent on contextual factors, both genetic and environmental. This review will address the molecular biology and behavioral pharmacology of OFQ/N and its receptor.
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I. Introduction |
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The use of molecular biological approaches has led to
extraordinary advances in our understanding of opioid action in the last decade. Soon after the cloning of the
opioid peptide receptor (DOP1, originally termed DOR-1) (Evans et al.,
1992
; Kieffer et al., 1992
), a number of laboratories identified clones
corresponding to the µ opioid peptide (MOP1,
originally termed MOR-1) and
opioid peptide
(KOP1, originally termed KOR-1) receptors. A
meeting held in Washington, DC, as a tribute to the memory of Dr.
William Martin,2
documented these advances in opioid receptor pharmacology (Uhl et al.,
1994
). At this meeting, several laboratories first described a fourth
receptor clone closely homologous to the traditional opioid receptors
(Table 1). These clones were isolated
from a number of species and were typical G-protein-coupled receptors with the expected predicted seven transmembrane domains. These novel
clones displayed approximately 50% identity with the traditional opioid receptors overall, with the transmembrane regions showing even
higher homologies of up to 80%.
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Despite their close homology to the other opioid receptors, the novel
clones were difficult to characterize and were considered by many to be
an orphan receptor. Few opioids labeled these novel clones, and their
affinities were markedly lower than those seen with the cloned opioid
receptors. Functionally, the ability of opioids to modulate adenylate
cyclase with these clones also was markedly limited (Mollereau et al.,
1994
; Pan et al., 1995
). Several early papers uncovered a close
relationship between this clone and the
3
receptor but concluded that they were not identical (Pan et al., 1994
,
1995
). The evidence for a relationship between them came from several
lines of investigation. A monoclonal antibody capable of neutralizing
3 opioid binding in brain tissue and
3 analgesia in vivo recognized the expressed
receptor generated through in vitro translation in Western blot
analysis. Furthermore, in antisense mapping studies a number of
antisense probes directed against the second and third coding exons of
the murine clone (KOR-3) blocked the analgesic activity of
3 analgesic naloxone benzoylhydrazone in mice
without affecting the analgesic actions of traditional µ,
, and
1 drugs. Yet, the inability of a range of
antisense probes targeting the first coding exon and the markedly different binding profile of the new clone indicated that the novel
clone was not identical to the
3 receptor. The
differences between the two were further documented with the
identification of the endogenous ligand for this receptor, a
heptadecapeptide termed orphanin FQ or nociceptin
(OFQ/N3) (Fig.
1). Despite an exceedingly high affinity
for the cloned receptor, OFQ/N does not compete binding to the
traditional µ opioid receptors.
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The nomenclature in this field is confusing, particularly for the
receptor. In the early papers each laboratory used its own nomenclature
for the receptor (Table 1), but as the years went by the term ORL1
gained favor. Recently, it has been suggested that all the opioid
receptor family of receptors be renamed, with the term
NOP1 (nociceptin/orphanin FQ peptide) receptor
referring to the ORL1 clone in all species. Similarly, the ligand is
known as nociceptin or orphanin FQ, having been isolated by two groups independently (Meunier et al., 1995
; Reinscheid et al., 1995
). Nociceptin was chosen by one group to denote its presumed
pronociceptive activity. The term orphanin FQ refers to its affinity
for the "orphan" opioid receptor, while the F and Q refer to the
first and last amino acids, phenylalanine and glutamine. Neither term predominates and most laboratories use the two together, as is done in
this review: orphanin FQ/nociceptin, or OFQ/N.
This field has burgeoned enormously since the cloning of the receptor
and the identification of its peptides. Aspects of this field have been
reviewed previously (Henderson and McKnight, 1997
; Meunier, 1997
;
Civelli et al., 1998
; Darland et al., 1998
; Taylor and Dickenson, 1998
;
Zaki and Evans, 1998
; Yamamoto et al., 1999
; also see a special issue
of the journal Peptides, volume 21, number 7, 2000). The
current review will focus rather comprehensively upon the behavioral
pharmacology of OFQ/N, with an attempt to understand it from the
molecular perspective.
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II. Molecular Biology of the Orphanin FQ/Nociceptin Receptor |
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A. Cloning NOP1 and Its Gene
NOP1 was cloned from various species by a
number of laboratories at about the same time.
NOP1 is a typical G-protein-coupled receptor with
seven predicted transmembrane domains (Fig.
2A) and is localized to murine chromosome
2 (Chen et al., 1994
; Nishi et al., 1994
). It was readily detected by
Northern analysis, where a major band of 3.4 kb was detected in mice
(Pan et al., 1995
). Several laboratories found a similar band in rats
at approximately 3.4 kb, as well as additional bands (Chen et al.,
1994
; Wang et al., 1994
; Lachowicz et al., 1995
). One group reported
additional bands of 7.5 and 10 kb (Chen et al., 1994
), another observed
a single additional band of approximately 7.6 kb (Lachowicz et al., 1995
), and a third group reported two additional bands of 13 and 23 kb
(Wang et al., 1994
). The significance of these additional larger bands
is not clear, particularly with the differences noted among groups.
However, the differing ratios of these larger bands to the 3.4-kb band
among regions raising interesting questions regarding regional
processing (Wang et al., 1994
; Lachowicz et al., 1995
).
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Southern analysis from a number of groups implies a single copy of the
gene for the NOP1 receptor, which is termed
Oprl1. The murine NOP1 gene structure
was elucidated soon after the initial reports of the receptor (Pan et
al., 1996b
). The receptor has three coding exons, similar to the other
opioid receptors. The first coding exon yields the amino terminus and
the first transmembrane domain (Fig. 2A). The second coding exon is
responsible for the next three transmembrane domains. The splice site
between the second and third coding exons is located in the second
extracellular loop, and the third coding exon is responsible for the
remainder of the protein, including the last three transmembrane
domains and the intracellular carboxyl tail (Fig. 2A). The binding
pocket has been proposed to comprise several of the transmembrane
domains (Topham et al., 1998
; Mouledous et al., 2000
). The initial gene structure identified five exons, with a noncoding exon preceding and
following the three coding exons (Fig. 2B) (Pan et al., 1996b
). More
recent work has identified two mini-exons between the first and second
coding exons that are alternatively spliced for a total of seven. The
numbering of the exons has changed, as new ones have been uncovered. In
the current review, exon 2 corresponds to the first coding exon of the
original clone, exon 5 to the second coding exon and exon 6 to the last one.
B. Alternatively Splicing NOP1
Like other members of the opioid receptor family,
NOP1 undergoes alternative splicing. The first
variant, NOP1d, was identified in lymphocytes and
contains a 15-bp deletion from the 3' end of the first coding exon
(exon 2) (Halford et al., 1995
; Wick et al., 1995
). Similar variants
were obtained from mouse (Pan et al., 1998b
), rat (Wick et al., 1994
;
Xie et al., 1999
), and human (Peluso et al., 1998
) brains. An intron
retention variant, NOP1e, containing the intron
between the second and third coding exons (exons 5 and 6) was reported
in mice (Pan et al., 1998b
), rats (Chen et al., 1994
; Xie et al., 1999
,
2000
), and human brain (Xie et al., 1999
). The initial report in rats
found an 84-bp insertion that could be translated through to generate a
full-length receptor. The mouse version, however, had only 81 bp, and
the presence of a stop codon prevented translation of the last three
transmembrane domains (Pan et al., 1998a
). A more recent report in rats
found a similar 81-bp insertion with a stop codon (Xie et al., 1999
). It is not clear which of the two rat variants predominates, but this is
an important issue since one has the potential of being a functional
variant whereas the other does not.
An additional three NOP1 variants have been
described that contain mini-exons located between the first and second
coding exons (exons 2 and 5) (Fig. 2B) (Xie et al., 1999
).
NOP1a contains a 34-bp insertion (exon 3) between
the first two coding exons. Due to a frameshift, it gives a predicted
stop codon in exon 5, yielding a truncated protein lacking the seven
transmembrane domains typically associated with G-protein-coupled
receptors. NOP1c contains a different mini-exon
insertion of 139 bp (exon 4) and predicts a truncated protein due to a
frameshift and a predicted stop-codon. The other variant,
NOP1b, shows a different splice site within exon
4, the same mini-exon as NOP1c, and contains only
the 3' portion of the mini-exon (98 bp). Like the others, it also gives a predicted truncated protein. The significance of these truncated proteins is still not fully understood. Clearly, they do not function like traditional G-protein-coupled receptors. Yet, this does not necessarily imply that they have no functional significance. It is
interesting that similar truncated variants have been reported for all
the other opioid receptor genes.
C. Molecular Modifications of NOP1
Although NOP1 itself has little affinity for
traditional opioids, it can be converted into an opioid-like receptor
either by simple mutations or by generating chimeras. In the rat
NOP1, a series of mutations within the
transmembrane regions that had little effect upon the affinity of OFQ/N
itself markedly transformed the affinity of the receptor up to 50-fold
for dynorphin A and several of its analogs, although the mutants still
did not show appreciable affinity for
-endorphin (Meng et al.,
1996
). These mutations were dispersed throughout the protein, including
an A213K mutation in TM5, a triple mutation VQV276-278IHI in TM6, and
a T302I mutation near the top of TM7. When the T302I and VQV276-278IHI mutations were combined, the affinity of dynorphin A increased even
further, with a Ki under 1 nM.
Chimeras also illustrate the close relationship between the
NOP1 receptor and the traditional opioid
receptors. Chimeras combining the NOP1 and
KOP1 (originally termed KOR-1) receptors were
able to maintain high affinity for OFQ/N and dynorphin A (Lapalu et al., 1998
; Mollereau et al., 1999
). The first coding exon of the NOP1 receptor encodes the first transmembrane
domain, just as in the traditional opioid receptors. Exchanging the
first coding exon of the NOP1 receptor with the
first exon of the µ opioid receptor MOP1
(originally termed MOR-1) or the
opioid receptor DOP1 (originally termed DOR-1) did not
appreciably affect the affinity of the receptor for OFQ/N, but it did
enhance the affinity of the
3 ligand naloxone
benzoylhydrazone (NalBzoH) approximately 5-fold and diminished the
affinity of the truncated OFQ/N derivative OFQ/N(1-11) (Pan et al.,
1996c
). More interesting, however, the exchange with the first exon of
DOP1 converted NalBzoH from an agonist into an
antagonist without changing its affinity. Opiates such as morphine
still did not show appreciable affinity for any of the chimeras.
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III. Receptor Binding |
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Structurally, the NOP1 receptor has the
anticipated seven transmembrane domains expected from members of the
G-protein-coupled class of receptors. NOP1
binding is sensitive to sodium and divalent cations (Ardati et al.,
1997
), as first described with opioid receptors (Pert and Snyder, 1973
;
Pasternak et al., 1975a
,b
; Wilson et al., 1975
), and OFQ/N and its
analogs modulate guanosine 5'-3-O-(thio)triphosphate binding in a pertussis toxin-sensitive manner (Reinscheid et al., 1995
,
1996
; Knoflach et al., 1996
; Shimohira et al., 1997
; Sim and Childers,
1997
; Meis and Pape, 1998
; Narita et al., 1999
). The first description
of OFQ/N binding to NOP1 utilized an iodinated analog, 125I-[Tyr14]OFQ/N
(Reinscheid et al., 1995
). Although many laboratories have used this
ligand, others have used 3H-OFQ/N (Dooley and
Houghten, 1996
), which yields results virtually identical to those of
the iodinated ligand (Ardati et al., 1997
). More recently, a novel
radioligand for the NOP1 receptor,
3H-ac-RYYRWK-NH2, was
reported (Thomsen et al., 2000
).
Most groups found a high affinity of OFQ/N for the transfected
NOP1 receptor, typically around 50 pM, although
there is a moderately wide range of values that likely reflect
differences in assay techniques, buffers, and cell lines (Dooley and
Houghten, 2000
) and ligand stability (Quigley et al., 2000
). Yet, there is good agreement regarding the specificity of the labeling, which clearly distinguished the NOP1 receptor from
traditional opioid receptors. Of a wide variety of opiates, only
NalBzoH showed a significant affinity for the site
(Ki = 310 nM; Table
2), and even this was far less than its
potency at opioid sites (Ki < 10 nM).
Morphine and other alkaloids have Ki
values well above 1000 nM.
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The initial characterization of NOP1 binding
studies examined the interactions of OFQ/N and its analogs with the
cloned receptor. Subsequent studies on brain tissue have yielded more
diverse results. Although several laboratories found
KD values in brain similar to those in
transfected cell lines (Albrecht et al., 1998
; Nicholson et al., 1998
;
Thomsen et al., 2000
), a number of other groups report far lower
affinities (Dooley and Houghten, 1996
, 2000
; Wu et al., 1997
; Mathis et
al., 1998
, 1999
). Wide variations of binding levels also were reported.
For example, reports of binding in rat cortex range from 22 fmol/mg of
protein (Thomsen et al., 2000
) to 291 fmol/mg of protein (Albrecht et
al., 1998
). The reasons underlying these differences are not clear.
However, there are a number of variables that may play a role that
include the choice of radioligand and binding conditions (Dooley and
Houghten, 2000
).
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IV. NOP1 Receptor Heterogeneity |
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A major question regarding the NOP1 receptor involves binding site heterogeneity. Evidence raising the possibility of multiple OFQ/N receptors comes from both pharmacological and binding sources. As noted earlier, the Oprl1 gene that encodes the NOP1 receptor undergoes alternative splicing, but none of the additional variants identified to date encode a full-length receptor. Although the combined evidence for multiple classes of OFQ/N binding sites is suggestive, none of the studies provides conclusive evidence for NOP1 receptor heterogeneity. Yet, it is worthwhile reviewing the evidence.
The behavioral pharmacology of OFQ/N is reviewed in detail in
subsequent sections. However, several issues played a major role in
exploring the possibility of multiple OFQ/N receptors. As discussed
below, OFQ/N reportedly has both hyperalgesic and analgesic activities.
The hyperalgesic activity was insensitive to opioid antagonists, as
expected based upon their poor affinity for the OFQ/N binding sites.
However, several laboratories have observed that opioid antagonists
reverse the analgesic responses of OFQ/N (Rossi et al., 1996b
, 1997
,
1998b
; Jhamandas et al., 1998
; Kolesnikov and Pasternak, 1999
). Many
assumed that OFQ/N activated a neural circuit with a downstream opioid
link, which could be blocked by the antagonists, and this is still a
consideration. However, the opioid antagonist Win44,441 blocks the
inhibition of cAMP accumulation produced by OFQ/N in mouse brain
(Mathis et al., 1997
). Since this assay directly measures the effects of the receptor coupled to cyclase in membrane fragments, it is difficult to envision how the antagonist could act other than directly
blocking the receptor activated by OFQ/N. Thus, this OFQ/N action in
brain membranes argues for an opioid antagonist-sensitive OFQ/N receptor.
Antisense mapping studies also differentiated between OFQ/N analgesia
and hyperalgesia. Whereas probes targeting the second and third coding
exons of the NOP1 receptor down-regulated OFQ/N and NalBzoH analgesia (King et al., 1997
; Rossi et al., 1997
), a probe
targeting the first coding exon was ineffective. Yet, the same
antisense probe based upon the first coding exon blocked the
hyperalgesia and anti-opioid activity of OFQ/N, whereas the antisense
probes targeting exons 2 and/or 3 that were active against OFQ/N
analgesia had no effect against hyperalgesia (King et al., 1997
; Rossi
et al., 1997
, 1998
). Although these pharmacological assays are
suggestive, behavioral approaches have many potential subtleties and
alternative explanations. Thus, they do not provide conclusive evidence
for multiple OFQ/N receptors.
Binding studies also suggest binding site heterogeneity. Early studies
with 125I-[Tyr14]OFQ/N in
mouse brain revealed curvilinear Scatchard plots, suggestive of sites
of differing affinity (Mathis et al., 1997
). However, this does not
necessarily imply different receptors. In
NOP1-transfected HEK293 cells with high levels of
expression, 3H-OFQ/N binding also yielded
biphasic Scatchard plots, presumably reflecting two conformations of
the receptor (Ardati et al., 1997
). Alternatively, curvilinear
Scatchard plots can result from radioligand degradation. Saturation
studies alone cannot distinguish among these possibilities and must be
interpreted in conjunction with other types of studies. However, other
approaches also implied NOP1 receptor binding heterogeneity.
OFQ/N(1-11) is a truncated peptide derived from OFQ/N. In vivo, it is
functionally active, eliciting analgesia (Rossi et al., 1997
) and
inhibiting cAMP accumulation in brain membranes (Mathis et al., 1997
).
These actions were not anticipated based upon its very poor affinity
for the cloned NOP1 receptor in binding assays. To more accurately assess the possibility of a novel OFQ/N(1-11) binding site, tyrosine-containing analogs were generated that could be
iodinated and used to examine binding sites directly (Mathis et al.,
1998
), as done earlier with OFQ/N itself (Reinscheid et al., 1995
). Of
the analogs, [Tyr10]OFQ/N(1-11) proved most
valuable. Like OFQ/N(1-11),
[Tyr10]OFQ/N(1-11) is analgesic when given
supraspinally in mice and it competes
125I-[Tyr14]OFQ/N binding
in mouse brain more potently (Ki = 79 nM) than OFQ/N(1-11) itself (Ki = 262 nM). Iodinating the analog to
iodo[Tyr10]OFQ/N(1-11) further enhanced its
potency (Ki = 39 nM).
In mouse brain,
125I-[Tyr10]OFQ/N(1-11)
labeling strongly suggested a novel binding site distinct from the
binding of
125I-[Tyr14]OFQ/N.
Binding parameters of
125I-[Tyr10]OFQ/N(1-11)
revealed an affinity (KD) of 0.24 nM,
which is over 100-fold lower than its
Ki against
125I-[Tyr14]OFQ/N binding
in mouse brain and more than 10-fold lower than its
KD determined in CHO cells transfected
with the NOP1 receptor. Furthermore, in brain it
displayed a Bmax of only 43 fmol/mg of protein, which is far fewer sites than observed in companion assays with 125I-[Tyr14]OFQ/N
(Table 3) or from the literature (Dooley
and Houghten, 1996
; Albrecht et al., 1998
; Nicholson et al., 1998
). It
also is interesting that the capacity of the
125I-[Tyr10]OFQ/N(1-11)
site is similar to the higher affinity
(KD = 4 pM) site observed in mouse
brain for
125I-[Tyr14]OFQ/N. A
possible association of the two is further suggested by saturation
studies with
125I-[Tyr14]OFQ/N in
which the inclusion of OFQ/N(1-11) appeared to selectively reduce the
higher affinity binding component of
125I-[Tyr14]OFQ/N.
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The difference in selectivity between OFQ/N(1-11) and the standard
OFQ/N radioligands was quite revealing (Mathis et al., 1999
). OFQ/N and
its analogs labeled the
125I-[Tyr10]OFQ/N(1-11)
site with very high affinity, confirming its classification as an OFQ/N
site. The affinity of OFQ/N(1-11) increases about 30-fold, with its
Ki dropping to only 8.7 nM. It is
interesting, however, that the affinity of OFQ/N(1-7) is unchanged and
remains quite poor.
As previously noted,
125I-[Tyr14]OFQ/N binding
is insensitive to opioids. In contrast,
125I-[Tyr10]OFQ/N(1-11)
binding is competed by a wide variety of opioid ligands. Although the
affinities of most of the opioids examined remain lower than against
traditional opioid receptors, a number of compounds showed high
affinity for this site (Table 4). Among
the opiates, NalBzoH was the most impressive. In brain membranes, its
affinity against
125I-[Tyr10]OFQ/N(1-11)
binding (Ki = 3.9 nM) is similar to
that seen with traditional opioid binding sites and almost 100-fold
greater than 125I-[Tyr14]OFQ/N
binding. The affinity of fentanyl is increased over 100-fold against
the OFQ/N(1-11) site. Some opioid peptides also show high affinity for
the
125I-[Tyr10]OFQ/N(1-11)
site, particularly dynorphin A and
-neoendorphin. Indeed, dynorphin
A labels the
125I-[Tyr10]OFQ/N(1-11)
site as potently as µ and
opioid receptors.
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Together, along with the dramatic anatomical differences between 125I-[Tyr14]OFQ/N and 125I-[Tyr10]OFQ/N(1-11) binding (see below), these studies suggest the possibility of OFQ/N receptor heterogeneity. If multiple OFQ/N sites exist, they may correspond to splice variants of NOP1 receptor. Alternatively, they might correspond to post-translational modifications of the receptors, result from modulation of the receptor by additional proteins, or be expressed by a totally different gene. However, without more definitive biochemical evidence, OFQ/N binding site heterogeneity remains tentative.
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V. Orphanin FQ/Nociceptin |
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A. Structure of Orphanin FQ/Nociceptin
The unusual properties of the "orphan opioid receptor" soon
led to the identification of an endogenous peptide, termed orphanin FQ
(Reinscheid et al., 1995
) or nociceptin (Meunier et al., 1995
) (OFQ/N),
that labeled the cloned receptor with very high affinity (Fig. 1).
OFQ/N is a heptadecapeptide with some interesting structural homologies
to the classical opioid peptide dynorphin A (Fig. 1). Both peptides are
comprised of 17 amino acids bounded by pairs of basic amino acids
important in their production from their precursors. Furthermore, both
have internal pairs of basic amino acids, raising the possibility of
further processing. The opioid peptides share a YGGF motif, where the
fifth amino acid is either leucine or methionine. The amino terminus of
OFQ/N is a phenylalanine instead of a tyrosine, followed by GGF.
Finally, both peptides contain the same last two amino acids at the
carboxyl terminus. Despite these similarities, the peptides are
functionally quite distinct. OFQ/N has no appreciable affinity for any
of the opioid receptors. Alanine scanning reveals the importance of the
amino acids in positions 1, 2, 4, and 8 (Dooley and Houghten, 1996
). Of
these, the phenylalanine in position 1 is particularly important in
establishing the selectivity of binding since replacing it with a
tyrosine yields analogs with far greater affinity at traditional opioid
receptors, although the [Tyr1] analog still can
induce naloxone-insensitive actions presumably mediated through
NOP1 receptors (Champion and Kadowitz, 1997a
,b
). The basic structure can be modified and even truncated at its carboxyl
terminus without major loss of activity, but the initial FGGF motif is
required for activity (Guerrini et al., 1997
).
B. Orphanin FQ/Nociceptin Analogs and Antagonists
Full descriptions of all the structure-activity relationships of
OFQ/N is beyond the scope of this review. However, several analogs are
important. The first analog, [Tyr14]OFQ/N, was
developed to enable the detection of receptor binding and has been
particularly important (Reinscheid et al., 1995
). Replacing the leucine
at position 14 yielded a peptide that could be iodinated and still
maintain affinity for the receptor similar to the parent compound
(Reinscheid et al., 1995
). This analog has proven extremely valuable in
the characterization of the receptor in both transfected cell lines and
in the brain.
OFQ/N has two pairs of basic amino acids within its structure, raising
the possibility of further processing to yield OFQ/N(1-11) and
OFQ/N(1-7). Although there are studies showing the activity of these
peptides and suggesting that their pharmacology may differ from that of
OFQ/N itself (Rossi et al., 1997
), as described below, the
physiological significance of these truncated peptides has not been
fully established. The possibility that the truncated peptides also
might be relevant led to the development of a tyrosine-containing analog of OFQ/N(1-11) suitable for iodination (Mathis et al., 1998
).
Analogs were synthesized with tyrosine at positions 1, 10, or 11. The
placement of tyrosine at position 1 lowered its affinity against
NOP1 binding in transfected cells, but enhanced its potency against the traditional opioid receptors.
[Tyr11]OFQ(1-11) and its iodinated version,
iodo[Tyr11]OFQ/N(1-11), on the other hand,
were devoid of activity against traditional opioid receptors and more
potent against NOP1 binding in transfected cells
than OFQ/N(1-11) itself. Both
[Tyr11]OFQ(1-11) and
iodo[Tyr11]OFQ/N(1-11) were pharmacologically
active, eliciting analgesia in mice. As discussed earlier, these agents
have proven valuable in binding studies.
The evaluation of the pharmacology of OFQ/N was hindered
for a number of years by the lack of an effective antagonist. The first
proposed antagonist,
[Phe1
(CH2-NH)Gly2]-nociceptin(1-13)-NH2,
was subsequently found to be a partial agonist, with many groups
observing OFQ/N-like actions in a variety of models. A recently
described small molecule antagonist, J-113397 (1-[3R,4R)-1-cyclooctylmethyl-3-hydroxymethyl-4-piperidyl]-3-ethyl-1,3-dihydro-2H-benzimidazol-2-one) (Kawamoto et al., 1999
; Ozaki et al., 2000a
,b
), has proven valuable in
a number of models (see Section
VIII.G.2.).
C. Orphanin FQ/Nociceptin Precursors and Their Processing
The OFQ/N sequence contains pairs of basic amino acids that might
imply additional processing of the peptide to OFQ/N(1-11) and/or
OFQ/N(1-7). Both of these truncated peptides are functionally active
when administered in vivo (Rossi et al., 1997
), producing analgesia
that is reversed by opioid antagonists. Neither peptide shows
appreciable affinity for any of the traditional opioid receptors, but
OFQ/N(1-11) does label cloned NOP1 receptors
moderately well (Ki = 55 nM), although
its affinity still is far lower than OFQ/N itself. OFQ/N(1-7) does not
compete with binding to the NOP1 receptor at
doses as high as 1 µM. The true significance of these peptides remains to be demonstrated.
Like most neuropeptides, OFQ/N is generated from a larger precursor
peptide, prepro-OFQ/N (ppOFQ/N) that has been cloned from mouse, rat,
and human (Fig. 3) (Meunier et al., 1995
;
Pan et al., 1996a
; Reinscheid et al., 2000
) and that has been localized
in man to chromosome 8 (8p21) (Mollereau et al., 1996
). Overall, there
is high interspecies homology, with 80% identity among the three
organisms. Within the precursor, there are several additional peptides
suggested by the presence of pairs of basic amino acids. Nocistatin has
been examined most extensively (see Section IX.A.). Nocistatin possesses analgesic actions and presumably acts through a
distinct receptor since it has no appreciable affinity for any of the
traditional opioid receptors or NOP1. It is
interesting that the nocistatin sequence shows the most variability of
the putative peptides within ppOFQ/N among species. The mouse version is the longest, containing 41 amino acids, whereas the rat peptide has
35 and the human form only 30. The mouse sequence has an interesting DAEPGA motif that is repeated three times. The rat form has a similar
double repeat, but the human form does not. The differences between the
species rests primarily over the length of this repeat, with the human
form lacking 10 of the amino acids of the mouse version at this
location.
|
Another peptide was predicted from the sequence of ppOFQ/N based upon
the presence of pairs of basic amino acids suggesting sites of peptide
processing. Orphanin FQ2 is a heptadecapeptide, like OFQ/N and
dynorphin A, with a phenylalanine (F) and glutamine (Q) at the first
and last position, leading to its name, OFQ2 (also called NocII; and
hereinafter called OFQ2/NocII). The placement of OFQ2/NocII within
ppOFQ/N is interesting in that OFQ2/NocII is immediately downstream
from OFQ, much like dynorphin B is immediately downstream of dynorphin
A in preprodynorphin. When administered centrally, OFQ2/NocII is
pharmacologically active, raising the possibility that it is
physiologically relevant (Rossi et al., 1998a
; Florin et al., 1999
)
(see Section IX.B.). A longer peptide containing the
OFQ2/NocII sequence at its amino terminus, ppOFQ/N(180-187), has been
described and it also is functionally active in mice (Mathis et al.,
2001
). It is still an open question as to whether ppOFQ/N(180-187) is
active itself or whether it is further processed to OFQ2/NocII.
| |
VI. Anatomy of Orphanin FQ/Nociceptin and Its Receptor |
|---|
|
|
|---|
The regional distribution of OFQ/N and the
NOP1 receptor have been well described (Bunzow et
al., 1994
; Fukuda et al., 1994
; Mollereau et al., 1994
; Wick et al.,
1994
; Lachowicz et al., 1995
; Nothacker et al., 1996
; Riedl et al.,
1996
; Houtani et al., 2000
; Neal et al., 1999a
,b
; Letchworth et al.,
2000
; O'Donnell et al., 2001
). These series of publications provide
detailed descriptions of the distribution of the
NOP1 receptor mRNA and binding in the brain which
are beyond the scope of this review. Overall, they report a good
correlation between receptor binding distributions and those seen with
in situ hybridization. Regions with NOP1 receptor binding typically express NOP1 mRNA as well,
although the levels of mRNA and binding do not always match very
closely. Regions with high levels of NOP1
binding/mRNA include the cortex, anterior olfactory nucleus, lateral
septum, hypothalamus, hippocampus, amygdala, central gray, pontine
nuclei, interpeduncular nucleus, substantia nigra, raphe complex, locus
coeruleus, and spinal cord. The distribution patterns have suggested
the involvement of the NOP1 receptor system in
"motor and balance control, reinforcement and reward, nociception,
the stress response, sexual behavior, aggression and autonomic control
of physiological processes" (Neal et al., 1999a
).
The distribution of OFQ/N also has been well described in the
literature (Dickenson, 1996
; Riedl et al., 1996
; Kummer and Fischer,
1997
; Mitsuma et al., 1998
; Neal et al., 1999b
; Houtani et al., 2000
;
O'Donnell et al., 2001
). In brief, the localization of OFQ/N
corresponds reasonably well with the NOP1
receptor. As with the receptor, OFQ/N immunoreactivity and mRNA levels
detected using in situ hybridization are closely correlated. OFQ/N is
found in lateral septum, hypothalamus, ventral forebrain, claustrum, mammillary bodies, amygdala, hippocampus, thalamus, medial habenula, ventral tegmentum, substantia nigra, central gray, interpeduncular nucleus, locus coeruleus, raphe complex, solitary nucleus, nucleus ambiguous, caudal spinal trigeminal nucleus, and reticular formation, as well the ventral and dorsal horns of the spinal cord (Neal et al.,
1999b
).
The distribution of
125I-[Tyr10]OFQ/N(1-11)
in the brain also is distinct autoradiographically (Fig.
4) (Letchworth et al., 2000
). The
distribution of
125I-[Tyr14]OFQ/N binding
was described earlier.
125I-[Tyr10]OFQ/N(1-11)
binding also shows intense labeling of the cortex, but far lower levels
of labeling in deeper structures. Compared with
125I-[Tyr14]OFQ/N,
125I-[Tyr10]OFQ/N(1-11)
labeling is far less intense in the olfactory tubercle, nucleus
accumbens, striatum, lateral and medial septum, hypothalamus, as well
as a number of brain stem structures such as the periaqueductal gray,
medial raphe, and locus coeruleus.
|
| |
VII. Range of Effects of Orphanin FQ/Nociceptin |
|---|
|
|
|---|
Befitting its particularly wide distribution in the nervous system (see above), there are a myriad of proposed functional roles for OFQ/N. Receiving by far the most attention is the involvement of this peptide in the mediation and modulation of pain in the supraspinal, spinal, and peripheral compartments of the nervous system. Related proposed functions for OFQ/N include roles in opiate tolerance, dependence/withdrawal, and adaptive responses to anxiety and stress. However, studies based on direct injection of the peptide, measurement of peptide levels, administration of antagonist/antisense compounds and/or the evaluation of the phenotype of transgenic "knockout" mutants have implicated OFQ/N in the mediation of biological phenomena ranging from learning and memory to hearing to water balance to reproductive physiology. A list of OFQ/N-associated systems-level phenomena is presented in Table 5. Some of the more well studied and noteworthy phenomena will be discussed presently, starting with pain processing.
|
| |
VIII. Effects of Orphanin FQ/Nociceptin on Pain |
|---|
|
|
|---|
A. Effects of Supraspinally Administered Orphanin FQ/Nociceptin
The first in vivo action of OFQ/N reported by both its discoverers
was a reduction in latency to respond to noxious thermal stimuli on the
tail-flick (Reinscheid et al., 1995
) and hot-plate tests (Meunier et
al., 1995
) after supraspinal (intracerebroventricular) injection in the
mouse. Both groups interpreted these data as reflective of a
hyperalgesic action; i.e., a decrease in nociceptive threshold
(increase in nociceptive sensitivity) produced by the peptide. This was
very much a surprise, since classical opioids, with the possible
exception of dynorphin (see Caudle and Mannes, 2000
), produce analgesic
and/or antihyperalgesic effects (see Pasternak, 1993
). The apparent
hyperalgesia produced by supraspinal OFQ/N inspired Meunier and
colleagues (1995)
to dub the peptide nociceptin.
Exogenous administration of an endogenous compound is not an ideal method for gleaning its true physiological role. When injected intracerebroventricularly, OFQ/N will be widely dispersed throughout the ventricular system, possibly affecting populations of ORL1 receptors that would not normally be activated by endogenously released peptide. Tissue levels are dependent upon diffusion of the agent from the cerebrospinal fluid into the brain, which results in a decreasing gradient of drug concentrations in deeper structures. The drug even can diffuse to spinal sites, particularly with high injection volumes. This makes it difficult to judge the concentration of peptide in relevant brain loci and thereby assess whether its concentration is appropriate or grossly supraphysiological. Finally, this approach entirely ignores contextual elements accompanying OFQ/N release under usual circumstances. Nonetheless, in the absence of an ORL1 antagonist and with the vast majority of the studies reviewed herein conducted before any such antagonist was available, direct injection of OFQ/N was one of only a handful of feasible experimental approaches.
In contrast to the conclusions from the initial descriptions of OFQ/N,
we now recognize that there is no widely accepted "role" of OFQ/N
in supraspinal pain-modulatory circuits. In fact, even the effects of
supraspinal projection of OFQ/N on nociceptive sensitivity remain
highly contentious. As detailed in Table
6, reports in the literature have
suggested six different "effects" of supraspinal OFQ/N on
nociception: 1) hyperalgesia, 2) analgesia, 3) hyperalgesia followed by
analgesia, 4) neither hyperalgesia nor analgesia, 5) anti-analgesia but
not hyperalgesia, and 6) anti-analgesia plus hyperalgesia. The
only uncontested observation is the anti-analgesic activity of OFQ/N,
first documented in 1996 (Mogil et al., 1996a
)
|
OFQ/N blocks analgesia from a wide variety of exogenous and
endogenous opioid compounds. Since OFQ/N has negligible affinity for
any of the traditional opioid receptors, it must act through neural
circuits as a "functional antagonist", rather than through direct
molecular interactions with opioid receptors. Given
intracerebroventricularly, OFQ/N can reverse and/or prevent analgesia
from drugs acting at supraspinal µ-opioid receptors, including
morphine (Grisel et al., 1996
; Mogil et al., 1996a
; Tian et al., 1997b
;
Zhu et al., 1997
; Calo' et al., 1998
; King et al., 1998
; Lutfy et al.,
1999
; Citterio et al., 2000
), DAMGO (Mogil et al., 1996b
), fentanyl (Zhu et al., 1998
), acetorphan (Suaudeau et al., 1998
), endomorphin-1 (Wang et al., 1999a
,c
), and morphine-6
-glucuronide (King et al., 1998
). It has similar effects against supraspinal
-opioid agonists, like DPDPE (Mogil et al., 1996b
; King et al., 1998
) and DSLET (Zhu et
al., 1998
; Wang et al., 1999a
),
1-opioid
agonists like U50,488 (Mogil et al., 1996b
; Zhu et al., 1998
; Wang et
al., 1999a
), and dynorphin A (Citterio et al., 2000
), and the
3-opioid agonist, naloxone benzoylhydrazone
(King et al., 1998
).
Direct injections of OFQ/N into specific brain loci also induce
anti-analgesic actions. OFQ/N placed into the periaqueductal gray (PAG)
blocks morphine analgesia (Morgan et al., 1997
) and its administration
into the rostral ventromedial medulla (RVM) reverses DAMGO analgesia
(Heinricher et al., 1997
; Pan et al., 2000
) (see Section
VIII.H.). Importantly, OFQ/N also blocks analgesia from endogenous
opioid-mediated manipulations, including electroacupuncture (Zhu et
al., 1996
; Tian et al., 1997a
; Zhang et al., 1997
) and mild stressors
(Mogil et al., 1996a
; Suaudeau et al., 1998
; Rizzi et al., 2001
). The
latter phenomenon may be responsible for much of the confusion
surrounding the actions of OFQ/N (see Section VII.D.3.).
The anti-opioid effect of OFQ/N against morphine analgesia is
long-lasting, persisting for up to 4 to 6 h (Candeletti and Ferri,
2000
). Repeated OFQ/N dosing induces tolerance, with a decreasing
response over time (Lutfy et al., 1999
). Although it is tempting to
only assume a functional interaction between OFQ/N and other members of
the opioid gene family, the anti-analgesic actions of this peptide are
by no means restricted to opioid analgesia. OFQ/N equally efficaciously
blocks analgesia from the
2-adrenergic receptor agonist, clonidine (King et al., 1998
), the
GABAB receptor agonist, baclofen (Citterio et
al., 2000
), and naloxone-insensitive forms of swim stress (Rizzi et
al., 2001
).
This ability to block non-opioid analgesia sets OFQ/N apart from other
known functional anti-opioid peptides, including adrenocorticotrophic hormone (ACTH), cholecystokinin (CCK), dynorphin, FMRFamide (and its
analogs),
-melanocyte-stimulating hormone (
-MSH),
MIF-1/Tyr-MIF-1, neurotensin- and tyrosine-releasing hormone
(Rothman, 1992
), and
1 receptor systems (e.g.,
Chien and Pasternak, 1993
). Anti-opioid systems are thought to play
important roles in a number of pain-relevant phenomena, including the
mediation of individual differences in analgesic sensitivity (Chien and
Pasternak, 1993
; Tang et al., 1997
), the induction of tolerance and
dependence (Rothman, 1992
) (see Section X.D.), and in
plastic changes underlying neuropathic pain (Wiesenfeld-Hallin et al.,
1997
). Elucidation of the precise actions of OFQ/N vis-à-vis
these other anti-opioid peptides will be a major research challenge for
the future.
These anti-analgesic actions of OFQ/N are the most robust activities
observed following supraspinal administration, having been seen by all
groups examining this question. The two contentious issues, regarding
OFQ/N actions, that remain involve direct analgesia and hyperalgesia.
In one study, for example, higher OFQ/N doses induced analgesia in
mice, although this action is not easily detected in all strains (Rossi
et al., 1997
). In this study, an initial hyperalgesic response was
followed by analgesia. The analgesic response was reversed by opioid
antagonists, but the hyperalgesic actions were not. Indeed, the
biphasic hyperalgesic/analgesic activity seen with OFQ/N alone reverted
to only a monophasic hyperalgesia in the presence of the opioid
antagonist. Others, of course, see neither hyperalgesia or analgesia.
Factors relevant to interpreting the conflicting data presented in
Table 6 are discussed below.
A final comment concerns not the effect of OFQ/N on pain, but the
effect of pain on OFQ/N. A recent study by Rosen and colleagues (2000)
examined OFQ/N-like immunoreactivity in various nociception-related brain areas 2 weeks after the induction of a neuropathic state using
Bennett and Xie's (1988)
surgical model or a carrageenan inflammatory
model. Both injuries increased OFQ/N levels in the cingulate cortex,
and carrageenan increased levels also in the hypothalamus and the
dorsal horn of the spinal cord. OFQ/N levels did not change in the PAG
or RVM (in contrast to levels of dynorphin B and
met-enkephalin-Arg-Phe), prompting the authors to conclude that OFQ/N
is involved in the modulation of ascending nociceptive transmission pathways rather than descending nociceptive
modulation pathways (but see Section VIII.H.). OFQ/N has
also been identified in human cerebrospinal fluid but not at higher
levels in women with ongoing labor pain compared with those presenting
for elective Caesarean section (Brooks et al., 1998
). Thus, any
clinical relevance of supraspinal OFQ/N remains to be demonstrated.
B. Effects of Spinally Administered Orphanin FQ/Nociceptin
Although the seminal investigations of OFQ/N featured supraspinal
administration of the peptide, opioids play an equally crucial role in
pain modulation in the spinal level (see Yaksh, 1999
). Although OFQ/N
injected intrathecally (10 nmol, i.t.) was initially reported to have
no effect on thermal nociception (Reinscheid et al., 1995
), a
subsequent study reported a trend (p = 0.053) toward
enhanced morphine analgesia by intrathecal OFQ/N (Grisel et al., 1996
)
followed by additional support for spinal OFQ/N analgesia (Xu et al.,
1996
; King et al., 1997
). The situation has become more complicated
since then, as shown in Table 7. Strikingly low OFQ/N doses spinally produce spontaneous pain, as
evidenced by caudally directed scratching, biting, and licking (SBL)
behaviors, and hypersensitivity to thermal and mechanical stimuli.
These SBL behaviors are reminiscent of those elicited by substance P
and N-methyl-D-aspartate (NMDA) and
are eliminated by pretreatment with morphine and neurokinin-1
(NK1) receptor antagonists, but not neurokinin-2
(NK2) or NMDA receptor antagonists (Sakurada et
al., 1999b
). At higher OFQ/N doses, a number of laboratories have
observed analgesic and anti-hyperalgesic/anti-allodynic effects. However, some have been unable to demonstrate OFQ/N analgesia at
presumably effective doses. Still others have demonstrated anti-analgesic effects reminiscent of supraspinal peptide, alone or in
combination with hyperalgesia (see Table 7).
|
Despite the many contradictions in the established literature to date,
most reviewers have concluded that the dominant spinal action of high
doses of OFQ/N is inhibitory
congruent with the findings of all
electrophysiological studies
producing behavioral analgesia and/or
anti-hyperalgesia/anti-allodynia (Henderson and McKnight, 1997
;
Meunier, 1997
; Harrison and Grandy, 2000
; Xu et al., 2000
). Wang and
colleagues (1996)
have arrived at the same conclusion for the
trigeminal system. Assuming that spinal OFQ/N is analgesic, the
potential role of classical opioid receptors remains a further
unresolved issue. Of the eight studies looking at the effects of opioid
antagonists on OFQ/N analgesia, only two reported a reversal (King et
al., 1997
; Hao and Ogawa, 1998
). In another study, repeated
administration of spinal OFQ/N resulted in the development of tolerance
to the peptide's analgesic effects and cross-tolerance to morphine
(Jhamandas et al., 1998
). This finding, however, is directly
contradicted by yet another study that found no cross-tolerance (Hao et
al., 1997
).
Nociception-relevant elements in the spinal cord undergo anatomical and
functional alterations after peripheral nerve injury or inflammation
and this plasticity is thought to be important in producing and
maintaining chronic pain states (Woolf, 1983
). OFQ/N appears to be no
exception. OFQ/N levels and binding increase in the dorsal horn of the
spinal cord after inflammation (Jia et al., 1998
; Rosen et al., 2000
).
In one study, this increase was bilateral, but restricted to the
superficial laminae (I and II) of the cord (Jia et al., 1998
).
Inflammation also induces expression of the prepro-OFQ/N gene in the
dorsal root ganglion, although the increased synthesis of OFQ/N was
quite short-lived (<6 h) (Andoh et al., 1997
). In contrast to
dynorphin, which was increased in the dorsal horn after a Bennett model
nerve injury, OFQ/N levels in this study trended lower, although the
decrease did not achieve statistical significance. These findings are
hard to reconcile with data demonstrating that high-dose OFQ/N's
depressive effect on the flexor reflex is decreased in inflamed rats
and increased somewhat in nerve injured rats (Abdulla and Smith, 1998
; also see Hao et al., 1998
; Xu et al., 1999a
). This pattern of functional changes is exactly opposite to that of µ opioids, which exhibit increased efficacy in inflammatory states (Stanfa and Dickenson, 1995
) and greatly reduced efficacy against neuropathic pain
(Arner and Meyerson, 1988
). As pointed out by Xu and colleagues (1999a)
, however, the effectiveness of exogenously applied OFQ/N is
primarily determined by the status of NOP1
receptors, not endogenous peptide levels. No data have thus far been
collected as to whether NOP1 receptors are
altered after injury.
C. Effects of Peripherally Administered Orphanin FQ/Nociceptin
In addition to their effects in the CNS, opioids can produce
analgesia in the periphery, especially in the presence of inflammation (Stein et al., 1990
; Kolesnikov et al., 1996
). This fact, along with
the ability of OFQ/N to affect transmitter release in the peripheral
nervous system (see Giuliani et al., 2000
), suggests that OFQ/N may
modulate nociception directly at the site of pain and/or injury. A
small number of studies have investigated this possibility, again with
somewhat conflicting results. Two elegant studies by Inoue and
colleagues (1998
, 1999
) demonstrated the ability of OFQ/N at remarkably
low doses, up to 10,000-fold lower than substance P and 1000-fold lower
than bradykinin, to elicit the nociceptive flexor reflex after
intraplantar injection into the hindpaw. This effect appears to be
secondary to local substance P release in the paw, since the phenomenon
can be blocked by inhibition of transmitter release by botulinum toxin,
depletion of substance P by capsaicin, by NK1
receptor antagonists, and is abolished in tachykinin-1 gene knockout
mice (Inoue et al., 1998
). In the second study, however, a higher OFQ/N
dose (1 nmol) was analgesic, producing a complete blockade of substance
P-induced flexor reflexes and SBL (Inoue et al., 1999
) (see
Section VIII.D.5.). Another group, also using higher doses,
demonstrated the analgesic efficacy of OFQ/N applied subcutaneously to
the tail (Kolesnikov and Pasternak, 1999
). This analgesia was
naloxone-reversible, but insensitive to antagonism by either µ- or
-specific antagonists.
The modulatory effects of OFQ/N on rat knee joint afferents were very
recently studied by McDougall et al. (2000)
. They found a sensitizing
effect of OFQ/N in normal joints, and a desensitizing effect during
hyper-rotation in acutely inflamed knees. Interestingly, both these
effects may be explained by the OFQ/N-substance P interactions described above (Inoue et al., 1998
; Lecci et al., 2000b
). However, Kumar and colleagues (1999)
were unable to demonstrate
[3H]OFQ/N binding in human synovial joint fluid
or tissue. OFQ/N has been implicated in fibromyalgia where female
sufferers display decreased plasma levels of the peptide (Anderberg et
al., 1998
).
D. Reconciling the Literature
The preceding descriptions of OFQ/N effects on nociceptive phenomena at the supraspinal, spinal, and peripheral levels (see Tables 2 and 3) illustrate the considerable uncertainty that still surrounds the simplest of questions: What are the actions of OFQ/N when injected? The next sections will address a number of factors that may be relevant to reconciling the divergent results found in the literature and thus to illuminating the endogenous role of OFQ/N.
1. Noxious Stimulus Modality.
There is a large literature
demonstrating differential processing of different types of pain by
neurochemically distinct circuitries (for reviews, see Mogil et al.,
1996c
, 1999b
). It is possible, therefore, that activities of OFQ/N may
be dependent upon the nociceptive assay used. The majority of the
studies to date have used thermal assays (tail-flick/withdrawal,
hot-plate tests) (Tables 2 and 3), which is to be expected since these
assays are easily performed and commonly used in the opioid field.
Supraspinal OFQ/N anti-analgesia is a robust response and has been
demonstrated against thermal, electrical, and chemical assays, of
varying durations (acute to chronic). Spinal hyperalgesia/allodynia and
analgesia have been demonstrated against thermal, chemical, and
mechanical assays. Even some of the less common findings, such as
spinal anti-analgesia or anti-hyperalgesia/anti-allodynia, have been observed using multiple nociceptive assays. Overall, there does not
appear to be strong evidence at the present time for modality-specific effects of the pronociceptive actions of OFQ/N.
2. Robustness of Various Phenomena.
Not all of the reported
phenomena are equally robust. For example, of the 16 studies reporting
supraspinal hyperalgesia listed in Table 6, half featured latency
decreases, or formalin rating increases, of <40% compared with
baseline and/or vehicle values. By contrast, virtually all studies
reporting anti-opioid analgesic actions of OFQ/N demonstrated a
complete blockade of even profound analgesia. Also, the supraspinal
hyperalgesic actions of OFQ/N are quite transient compared with the
anti-analgesic actions, with the former lasting only 15 to 30 min in
virtually all cases. One can easily point to degradation of the peptide
as an explanation of transient effects, but such degradation does not
prevent long-lasting anti-analgesic actions (see especially Candeletti
and Ferri, 2000
). Particularly weak is the phenomenon of supraspinal
OFQ/N analgesia defined as a quantal doubling of the baseline
tail-flick latency, which can be demonstrated in only 40% of CD-1 mice
and was not seen in two other strains (see Section
VIII.D.3.) (Rossi et al., 1996b
, 1997
). It should be noted that
blockade of the anti-opioid
1 receptor system
with the
receptor antagonist, haloperidol, dramatically enhanced
the analgesic actions of OFQ/N and its fragments in all strains tested
(Rossi et al., 1997
). Supraspinal OFQ/N analgesia does seem to be more
robust in the rat (Rossi et al., 1998b
).
3. Influence of Stress.
The original investigations of the
supraspinal OFQ/N quantified the effect of the peptide relative to a
control group receiving an isovolumetric injection of vehicle (Meunier
et al., 1995
; Reinscheid et al., 1995
). Although a reasonable control,
it alone is not sufficient since mice receiving intracerebroventricular
injections are not at "baseline". This is particularly evident when
dealing with nociceptive assays capable of detecting stress analgesia. Employing either "no injection" and/or preinjection baseline
control groups, depending on the nociceptive assay, Mogil and
colleagues (1996a)
demonstrated that the apparent hyperalgesia noted
previously could be explained by the reversal of stress-induced
analgesia by OFQ/N. In these studies, the apparent hyperalgesia
compared with the vehicle group was actually reversal of stress-induced analgesia related to the injection when compared with the no-injection group (Mogil et al., 1996a
).