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2
Mechanisms of Pain
T
his chapter provides an analysis of the differences between nocicep-
tion and pain, on the basis of the anatomy of the peripheral and cen-
tral nervous systems and the role of nociceptors in pain perception. It
includes discussion of the concept of persistent pain and presents informa-
tion on the embryologic origins of pain. Finally it addresses the modulatory
role of anxiety, fear, and stress on pain.
NOCICEPTION OR PAIN
Before discussing the anatomical and physiological bases for the gener-
ation of pain, it is important to reiterate the difference between nociception
and pain. Nociception refers to the peripheral and central nervous system
(CNS) processing of information about the internal or external environment,
as generated by the activation of nociceptors. Typically, noxious stimuli,
including tissue injury, activate nociceptors that are present in peripheral
structures and that transmit information to the spinal cord dorsal horn or its
trigeminal homologue, the nucleus caudalis. From there, the information
continues to the brainstem and ultimately the cerebral cortex, where the
perception of pain is generated (Figure 2-1).
Pain is a product of higher brain center processing, whereas nocicep-
tion can occur in the absence of pain. For example, the spinal cord of an
individual who suffered a complete spinal cord transection can still process
information transmitted by nociceptors, but because the information cannot
be transmitted beyond the transection stimulus-evoked pain is unlikely (see
Chapter 1 for additional discussion).
33
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34 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
PAIN
Cortex
Decorticate
Thalamus
Decerebrate
NOCICEPTION
Brainstem
Spinal
Spinal cord
Primary afferent
nociceptors
FIGURE 2-1 Anatomical distribution of nociception and pain. This figure sche-
matizes the major neuroanatomical structures that differentiate nociception and
pain, an understanding of which is essential for studies in which the animals may
experience pain. Nociception refers to the process through which information about
peripheral stimuli is transmitted by primary afferent nociceptors to the spinal cord,
brainstem, thalamus, and subcorticalR01499
structures. In contrast, the experience of pain
can result only when there is activity of 2-1 revised
Figure thalamocortical networks (represented in the
dark shaded box at the top) that process the information conveyed by pathways of
nociception. The magnitude of pain is determined to a great extent by the strength
of descending inhibitory and facilitatory controls (in the lighter shaded boxes) that
originate throughout the neuraxis and regulate the processing of ascending nocicep-
tive messages. The figure also illustrates several important surgical preparations used
to study nociceptive processing under conditions in which different parts of the brain
are disconnected from afferent nociceptive input. Thus, transection of the spinal
cord produces a “spinal” preparation. Decerebrate preparation entails transection of
the brain between the midbrain (at the level of the colliculi) and the thalamus. In the
decorticate preparation, connections from the thalamus to the cortex are severed. In
all of these conditions, information generated by the activity of nociceptors located
below the level of transection is unlikely to reach structures above the transection.
No evidence exists at present that hormonal or other nonneural mechanisms are
able to “bypass” the transection to access the brain and evoke a pain perception.
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3
MECHANISMS OF PAIN
The distinction between nociception and pain is also important for
behavioral studies in which an understanding of pain mechanisms is the
ultimate goal. Many behavioral tests involve assessment of reflex responses
to noxious stimuli, typically applied at threshold or just suprathreshold
intensities (such as heating of the tail or the hindpaw) to incite a brief with-
drawal of the tail (e.g., in the tail flick test) or paw. These are principally
tests of nociceptive processing because stimulus duration is limited by the
animal’s response (e.g., a nociceptive withdrawal reflex). On the other
hand, the endpoints of more complex behaviors (e.g., those involved in
operant tests) are presumed to involve supraspinal areas of the brain and as
such are tests of both nociception and pain. In that respect, operant tests in
which animals perform a particular behavior (e.g., press a bar) to escape a
stimulus provide information about both nociceptive processing and pain
(see also Box 1-4 in Chapter 1).
Mechanisms of Nociception and Pain
Nociceptors
The anatomical basis for the generation of momentary pain is very well
understood (Basbaum and Jessell 2000). Nociceptors are unusual neurons
because they have a cell body with a peripheral axon and terminal (ending)
that responds to the stimulus and a central branch that carries the informa-
tion into the CNS. Briefly, there are two major classes of nociceptors that
respond to different modalities of noxious stimuli.
The largest group of nociceptors is associated with unmyelinated axons,
also called C-fibers, that conduct slowly and that respond to noxious ther-
mal, mechanical, or chemical stimulation. Proteins in the membrane of
these nociceptors transduce natural thermal, mechanical, or chemical stim-
ulus energy into electrical impulses, which in turn are propagated along
the peripheral and central axon of the nociceptor into the CNS (the spinal
cord for the body and the trigeminal nucleus for the head). Importantly,
biochemical and molecular analysis of the nociceptor has identified many
of the transducer molecules that are activated by noxious stimuli, such as
TRPV1, which responds to noxious heat, reduced pH as occurs in inflam-
mation, and the chemical capsaicin. Another channel, TRPM8, responds to
cold (Julius and Basbaum 2001). Many of these molecules are targets for
therapeutic intervention in clinical pain conditions.
The second major nociceptor population is associated with thinly
myelinated axons (A-delta fibers). These nociceptors conduct more rapidly
than do unmyelinated C-fibers and likely convey “fast” (or sharp) momen-
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36 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
tary pain, as opposed to slow, diffuse pain, which is transmitted by the
C-fibers.1
There is yet one more category of nociceptors characterized by unique
properties. “Sleeping” or “silent” nociceptors are typically unresponsive to
noxious intensities of mechanical stimulation except at extreme ranges of
intensity. Although silent nociceptors are difficult to activate within the nor-
mal range of noxious stimulus intensities, after tissue insult these nocicep-
tors “wake up” in response to endogenous chemical mediators associated
with tissue injury. Silent nociceptors are typically associated with increased
spontaneous activity and responsiveness to noxious and even innocuous
stimulus intensities.
Spontaneous activity in nociceptors (whether A-delta, C-, or silent)
is undesirable and pain producing; moreover, awakening silent nocicep-
tors creates essentially new, additional nociceptive input to the CNS. All
nociceptors have the capacity to sensitize. When they become more eas-
ily excitable (i.e., the threshold for activation is lowered), hyperalgesia
(an increased response to a noxious stimulus) with or without allodynia
develops and normally innocuous stimuli may provoke pain, thus directly
affecting animal welfare. The consequences of such activities are discussed
below in the section on persistent pain.
The Central Nerous System
The central branch of the nociceptor terminates in the dorsal horn of the
spinal cord (or its trigeminal homologue in the brainstem), where it makes
synaptic connections with a complex array of neurons that play different
roles in nociceptive processing and pain. Some interneurons make connec-
tions with motor neurons that generate nociceptive withdrawal reflexes.
Output neurons of the spinal cord, on the other hand, project rostrally and
transmit the nociceptive message to the brainstem reticular formation and
thalamus. Among the ascending pathways arising from the spinal cord (and
its trigeminal homologue) are the spinothalamic and spinoreticulothalamic
tracts, as well as the spinoparabrachial-amygdala pathway, which provides
more direct access to limbic emotional circuits in the brain (via the amyg-
dala) (Basbaum and Jessell 2000). Note that there is not a unitary pathway
for generation of the affective component of the pain experience. Rather it
is likely that different aspects of the nociceptive message are conveyed via
different pathways and widely distributed to the cerebral cortex from the
reticular formation, thalamus, and amygdala.
1 Whereas virtually all nociceptors are A-delta and C-fibers, not all A-delta and C-fibers are
nociceptors. It is thus both inaccurate and incorrect to generically refer to C-fibers as “pain”
fibers.
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MECHANISMS OF PAIN
Until recently, remarkably little was understood about the cortical
mechanisms that underlie the perception of pain. Although electrophysi-
ological studies have demonstrated that some neurons in the cortex respond
to noxious stimuli, the extent to which this response represents or even
correlates with pain was not clear. The development of powerful imaging
methods, however, has provided critical information about the cortical pro-
cessing of pain-related information (e.g., Apkarian et al. 2005; Bingel and
Tracey 2008; Tracey and Mantyh 2007) and revealed that pain is not pro-
cessed in a single area of the brain. Rather, the activity of different regions of
the cortex underlies various features of the pain percept and cognitive recall
for responses or emotional reactions. This information comes largely from
human studies, in which a verbal correlate of pain perception is possible.
For example, activity in the somatosensory cortices (S1 and S2) correlates
best with the sensory-discriminative properties of the stimulus (e.g., loca-
tion and intensity), and the affective components of the pain experience
correlate with activity in the anterior cingulate gyrus and the insular cortex.
Unfortunately, the activity of these regions cannot be used as a biomarker
for pain, as it can also be generated by conditions that are clearly not pain-
ful (for additional discussion see Chapter 1).
Further Comments on the Distinction between Nociception and Pain
An unusual model to investigate the brain circuitry involved in noci-
ception and pain was developed at the beginning of the 20th century
by Charles Sherrington (1906), who appreciated early on the distinction
between nociception and pain. Use of a “decerebrate preparation” (cer eau
isolé) in laboratory animal research was more common years ago, but it
remains useful for recording the activity of spinal cord or brainstem neurons
under conditions not compromised by anesthetics or analgesics. With the
animals under deep general anesthesia, the procedure involves transection
of the brainstem at the level of the midbrain (typically between the inferior
and superior colliculi), after which the rostral part of the brain (particularly
subcortical structures and the cortex) no longer receives direct neuronal
input from the spinal cord or brainstem trigeminal structures and a state of
permanent unconsciousness is induced.
Using the decerebrate preparation, Woodworth and Sherrington (1904)
illustrated the essential contribution of the cortex to the perception of pain
and defined the “pseudaffective” reflex. In response to a noxious stimulus,
this reflex corresponds to a remarkable behavioral repertoire, even including
occasional vocalization, due to the fact that its pathways are coordinated
at spinal and supraspinal brainstem levels below the midbrain transection
(i.e., it is a spino-bulbo-spinal reflex; Woodworth and Sherrington 1904).
Despite the behaviors observed, no pain is experienced. In fact, the AVMA
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38 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
Guidelines on Euthanasia state that “for pain to be experienced, the cerebral
cortex and subcortical structures must be functional” (AVMA 2007, p. 2).
The pseudaffective reflex is useful in animal studies that investigate neurons
of the spinal cord without the influence of anesthesia (e.g., the decerebrate
animal preparation). It should be noted that decerebrate preparations are
necessarily nonsurvival experiments; Silverman and colleagues (2005, p.
1) note that an animal that recovers from the anesthesia for this procedure
typically provides research data “for a period of a few hours or a day” after
which it must be euthanized.
Because decerebration severs the connection between the rostral part
of the brain and lower CNS structures, it also eliminates the powerful
modulatory control mechanisms that descend from supraspinal sites. These
descending control mechanisms are predominantly inhibitory and act as a
“brake” on spinal cord neurons and circuits that process nociceptive infor-
mation. Their removal via decerebration leads to enhanced nociceptive
reflexes and spinal neuron responses to nociceptive input. Accordingly,
spinal cord transection often follows decerebration to enable physiological
studies in unanesthetized animals, but it is not a prerequisite of the decer-
ebrate preparation.
Finally, it is important to distinguish the decerebrate from the decorti-
cate preparation. In the latter, only the cerebral cortex is removed, leaving
intact the underlying subcortical structures (i.e., the thalamus, brainstem,
and spinal cord). Because there have been suggestions that under some
conditions pain processing can occur even at the level of the thalamus (e.g.,
Merker 2007), studies of decorticate animals (which these days are rare)
must be performed under general anesthesia.
THE DEVELOPMENT OF PERSISTENT PAIN
The mechanisms that contribute to the development of postoperative/
postprocedural and persistent pain are far more complicated than the rather
simple anatomical and physiological underpinnings of momentary pain. It is
important to appreciate that these types of pain are not merely instances of
momentary pain that do not resolve quickly. Rather, they arise in the con-
text and environment of tissue or nerve injury and involve changes in the
properties not only of nociceptors but also of the circuits that these recep-
tors engage in the spinal cord and at other levels of the neuraxis (Basbaum
and Woolf 1999; Urban and Gebhart 1999; Basbaum and Jessell 2000;
Julius and Basbaum 2001). These changes generally enhance signals in
“pain” transmission circuits, such that innocuous stimuli can evoke behav-
iors indicative of pain (extensive discussion of the sickness syndrome, an
underappreciated postoperative occurrence, is in Chapter 4). As a result of
advances in scientific understanding of these mechanisms, many pharma-
cological treatments for postoperative/-procedural and persistent pain in
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MECHANISMS OF PAIN
humans are directed at interfering with the development and duration of
hyperalgesia and allodynia.
Hyperalgesia is a hallmark of inflammatory pain and is a consequence
of many types of tissue insults (ranging from a skin incision to nerve injury).
It is defined as an increased response to a noxious stimulus and mani-
fests as an increased sensitivity to pain (Treede et al. 1992; Campbell and
Meyer 2006). Because the threshold for response also typically decreases,
sometimes even nonnoxious stimuli can cause pain, a phenomenon called
allodynia.
There are two types of hyperalgesia, primary and secondary, each
associated with different mechanisms. Primary hyperalgesia is character-
ized by increased excitability of nociceptors at the site of the insult (e.g.,
the site of an incision). It occurs most commonly after skin injury, but may
also develop following insults to joints, muscle, or viscera. For example,
when an incision in the skin is examined, the response to stimuli applied
to that site typically increases. Surrounding the site of injury, and often at
sites rather distant from the injury (particularly when joints and especially
the viscera are involved), is an area of increased sensitivity referred to as
the area of secondary hyperalgesia. This is most evident with visceral insult,
where sensations are referred or perceived to arise from overlying structures,
most notably skin. The classic example is myocardial oxygen deficiency
(angina) in which the pain is referred to the shoulder, down the left arm,
and occasionally up to the jaw.
When either primary or secondary hyperalgesia occurs, it is accom-
panied by an increase in the excitability and responses of neurons in the
nervous system. Primary hyperalgesia is largely attributed to an increase in
the excitability of nociceptors (i.e., the peripheral afferent sensory ending
and fiber), whereas secondary hyperalgesia is associated with changes in
the excitability of neurons in the CNS, including the spinal cord and supra-
spinal sites in the brain. Accordingly, primary hyperalgesia is associated
with peripheral sensitization of nociceptors and secondary hyperalgesia
with central sensitization. The terms indicate an increase in the excitability
and responses of peripheral (i.e., nociceptor) and central neurons because
of tissue insult.
Numerous mediators in both the peripheral and central nervous systems
contribute to the processes of sensitization (Basbaum and Jessell 2000;
Basbaum and Woolf 1999; Julius and Basbaum 2001; Treede et al. 1992;
see McMahon et al. 2005 for an overview). At the injury site, primary
hyperalgesia is induced by the release of numerous inflammatory mediators
including the products of cyclooxygenase enzyme activation. The critical
contribution of these enzymes accounts for the beneficial effects of non-
steroidal anti-inflammatory drugs, which, by inhibiting the enzyme, reduce
peripheral sensitization and help alleviate persistent or postoperative/
-procedural pain.
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40 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
Central sensitization is a considerably more complicated process that
can result from changes in the amount of neurotransmitter released from
nociceptor terminals in the spinal cord or brainstem, notably glutamate
and the neuropeptide substance P (Basbaum and Jessell 2000; Basbaum
and Woolf 1999; Woolf 1983); from loss of inhibitory regulation exerted
by inhibitory interneurons in the spinal cord and at supraspinal loci; and
from biochemical changes in the “pain” transmission neurons that increase
their responsiveness to peripheral inputs. It is likely that the pain-alleviating
effects of drugs such as ketamine are partly due to the reduction of central
sensitization produced by the release of glutamate. In contrast, the benefi-
cial effects of anticonvulsants for pain treatment are likely related to their
blockade of neurotransmitter release from primary afferents or the enhance-
ment of inhibitory controls.
The remarkable number of molecules implicated in central sensitization
(whether produced by tissue or nerve injury) may lead to the development
of new pharmacological approaches to managing persistent pain. Of par-
ticular interest is the recent understanding of the contribution of glia to the
process of central sensitization. In fact, there is considerable evidence that
glia, notably microglia and astrocytes, are activated in the setting of nerve
injury and that they are the source of mediators that enhance the central
consequences of nociceptor activity (Thacker et al. 2007; Watkins et al.
2007). For this reason, there are now several pharmaceutical programs for
the development of novel pain therapies that attempt to interfere with the
biochemistry of the “activated” glial cell.
ONTOGENY OF PAIN
Large numbers of developmental neurobiology studies have increased
knowledge of the origin and maturation of nociceptive circuitry and behav-
ior. Importantly, it is now possible to identify subpopulations of sensory neu-
rons, including nociceptors, early in embryonic development, well before
they project to central and peripheral targets (Fitzgerald 2005).
Neurogenesis and subsequent maturation and synaptogenesis of sen-
sory neurons occur in two waves. In rats, outgrowth of myelinated A-delta
fibers from the neuraxis precedes outgrowth of unmyelinated C-fibers.
These processes occur during embryonic days 15 to 17 (E15-17) and 18 to
20 (E18-20) respectively and coincide with the first appearance of reflex
responses to mechanical stimuli (Fitzgerald 2005). A-delta fiber synapses
have been identified in the spinal dorsal horn at E13 in rats, whereas the
terminals of C-fibers do not appear until E18-19 (ibid.). In fact, physiologi-
cal recordings of nociceptive fibers in rat pups during the first few postnatal
days demonstrate responses to noxious chemical, mechanical, and thermal
stimuli that are similar to those of mature C-fibers.
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MECHANISMS OF PAIN
Neonates of multiple species demonstrate exaggerated spinally medi-
ated reflex responses to noxious stimuli compared to adults (see Fitzgerald
2005 and Hathway and Fitzgerald 2008 for reviews). For example, in rat
pups it is not until postnatal day 10 (P10) that these reflexes develop spa-
tial precision; they then achieve adult levels of both spatial and temporal
precision by P21. Nonnoxious tactile stimuli are important for fine-tuning
of nociceptive reflexes during this critical postnatal period. Likewise, matu-
ration of ascending and descending neuronal pathways, at approximately
P10 in rat pups, contributes to the development of mature nociceptive
processing. Hyperalgesia can be documented in rat pups as young as 3
days of age, but it is significantly less prominent, both in magnitude and
duration, at early ages than it is in the adult animal. By approximately 34 to
40 days of age, adult-like hyperalgesia is evident (Jiang and Gebhart 1998).
Taken together, these observations demonstrate the maturation of synaptic
connections in the superficial laminae of the dorsal horn during the first 3
postnatal weeks (Fitzgerald 2005).
Both somatic and visceral tissue insults in the neonate appear to alter
processing of nociceptive inputs in adulthood. Neonatal injury has thus
been associated with either hyperalgesia or hypoalgesia, depending on the
type and severity of injury and the sensory modality tested (Bhutta et al.
2001). Colorectal distension in neonatal rats (P8-12) results in colon hyper-
sensitivity in adults (Al-Chaer et al. 2000). In addition to altered nocicep-
tive processing, repetitive or persistent pain in the neonatal period leads to
changes in brain development, widespread alterations in animal behavior,
and increased vulnerability to stress and anxiety disorders or chronic pain
syndromes (Anand et al. 1999, 2007; Al-Chaer et al. 2000; Bhutta et al.
2001).
Specifically, inflammation produced by repeated injections of complete
Freund’s adjuvant in rat pups (P0, P3, P14) leads to hyperalgesia and last-
ing changes in nociceptive circuitry of the adult dorsal horn (Ruda et al.
2000). Similarly, rat pups that received repeated formalin injections in the
paw developed generalized thermal hypoalgesia as they aged (Bhutta et al.
2001). When noxious formalin stimuli were preceded by morphine anal-
gesia in neonatal rats, hyperalgesia in adulthood was significantly reduced
(ibid.). In other models of persisent pain, rat pups less than 21 days old
did not develop signs of neuropathic pain after nerve injury (Howard et al.
2005).
Whereas a growing number of studies have demonstrated altered pain
processing after neonatal injury in humans, not all outcomes reported are
necessarily applicable to the laboratory animal (e.g., see Grunau and Tu
2007). However, an important conclusion from this body of research is that
untreated neonatal pain can permanently alter sensitivity to pain, consistent
with modulation of primary afferent activation and central sensitization in
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42 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
response to subsequent nociceptive challenges in adulthood. Thus measures
to minimize pain in neonates may reduce alterations in neuronal develop-
ment and long-term sensitivity to sensory stimuli.
MODULATORY INFLUENCES ON PAIN: ANXIETY, FEAR, AND STRESS
As noted above, pain is not merely the appreciation of the presence,
location, and magnitude of nociceptive input but rather a complex event
with an important emotional/affective component. In addition, psychologi-
cal factors can significantly influence the experience of pain (also discussed
in Chapter 1, text and Figure 1-1). For example, fear and anxiety can
enhance responses to and interpretation of pain-producing events (Hunt and
Mantyh 2001; Linton 2000; Morley 1999; Munro 2007; Perkins and Kehlet
2000; Ploghaus et al. 2001). For these reasons, the predisposition of certain
strains of animals or individuals to anxiety should be considered in efforts
to assess the possible contribution of anxiety to the experience of pain
(Ulrich-Lai et al. 2006). In humans, measures to reduce anxiety can reduce
pain—this is true for both behavioral (cognitive) interventions and anxiolytic
drugs (Belzung 2001). Similarly, behavioral interventions to reduce anxiety
in animals can include acclimation to human handlers, training to withstand
some research procedures, socialization and housing with cage mates, or
training and exercise. Reliable and reproducible testing of animals is best
achieved in a situation in which the animal is habituated to the test appara-
tus and the test environment (e.g., light, noise, temperature, humidity).
The extent to which stress is present in normal laboratory situations
should also be considered. There are numerous examples in which expo-
sure to stressors can influence the response to a noxious stimulus. Some-
what paradoxically, the response can manifest as an apparent reduction of
pain, a phenomenon referred to as “stress-induced analgesia” (Amit and
Galina 1986; Keogh and Cochrane 2002; for commentary on how exposure
to a predator reduces nociceptive responses in rats see Lester and Fanselow
1985). Moreover, environmental enrichment may also affect stress-related
nociceptive responses. A recent study reported that C3H mice exposed to
environmental enrichment, which can reduce stress compared with a stan-
dard environment (i.e., standard plastic cages with bedding), reacted more
quickly (i.e., exhibited a shorter freezing time) to electric shock training
than did mice habituated in standard housing conditions. Such an outcome,
possibly due to decreased fearfulness or anxiety, may require more nuanced
staff training in recognizing modulatory influences on painful situations
(Benaroya-Milshtein et al. 2004).
Whether the magnitude of stress experienced in typical laboratory set-
tings is sufficient to significantly alter the perception of pain is difficult to
determine. A priori one would assume that reducing stress is a good objec-
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MECHANISMS OF PAIN
tive for both experimental outcomes and animal welfare, since perturbation
of the latter may lead to stress/distress (see the 2008 NRC report Recognition
and Alleiation of Distress in Laboratory Animals for detailed information on
the effects of stress/distress on animal welfare). The stressors typically used
to evoke stress-induced analgesia are intense and rather unnatural and can
be useful for evaluating pain behavior in response to an applied stimulus.
How data from such studies translate into the normal behavioral repertoire
of animals in a laboratory environment and in other types of experimental
studies remains to be determined. Nevertheless, it is important to keep in
mind the possibility of stress-induced effects when assessing pain in animals
because the absence of response to a noxious stimulus or of pain-indica-
tive behavior may be due to significant stress and misleadingly suggest the
absence of pain. Because pain can be enhanced by anxiety or fear, readers
should consult the discussion of the role of anxiolytics in pain management
in Chapter 4.
CONCLUSIONS AND RECOMMENDATIONS
Pain is not a foregone outcome when an animal is exposed to a nox-
ious stimulus, because, as discussed in Chapter 1, the experience of pain is
informed by the perceptive abilities of the brain.
1. It is critical to appreciate that nociception is not equivalent to pain.
Noxious stimuli trigger several levels of information processing as
the activity of primary afferent nociceptors is conveyed to the spinal
cord and from there to the higher centers of the brain. Neurons at
many levels of the neuraxis respond to noxious stimuli, but that
response does not necessarily indicate or lead to pain. In fact, stud-
ies of animals with transections of the neuraxis at various levels
illustrate that complex responses can be elicited in the absence of
pain (i.e., when the cortex is disconnected from the nociceptive
processing networks).
2. Until better methods (e.g., biomarkers, imaging) are available to
objectively measure pain, behavioral indices and to some extent
extrapolation from the human experience are the best sources of
information and the only methods available to assess pain in labo-
ratory animals (see Chapter 3).
3. Pain is not exclusively associated with noxious stimuli. After some
injuries (e.g., nerve injury), even innocuous stimuli can cause pain,
and repeated exposure to noxious stimuli can lead to sensitization
and enhance responses to subsequent stimuli both innocuous and
noxious.
4. Injury may have long-term consequences to the neural systems
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44 RECOGNITION AND ALLEVIATION OF PAIN IN LABORATORY ANIMALS
that process nociceptive information. This is particularly true of
procedures performed in the neonatal animal, but it may also be
relevant in the adult. This information underscores the importance
of adequate postoperative pain management and to some extent
provides the rationale for preemptive analgesia (see Chapter 4).
Psychological factors also likely contribute to the pain experienced
during and after an injury; their effect is perhaps more difficult to
assess and address in the context of laboratory experiments, but its
recognition is important.
5. Pain represents a cascade of physiological, immunological, cogni-
tive, and behavioral effects that may confound experimental results
in addition to being detrimental to the animals’ welfare.
Finally, and as discussed in Chapter 1, unless not recommended due to
experimental outcomes, relief from pain is an ethical and regulatory obliga-
tion. Further, the committee emphasizes that effective pain management is
scientifically advantageous, as unalleviated pain may adversely influence
scientific projects and research outcomes in a number of ways. The reader
is referred to Box 1-4 of Chapter 1 and to Chapter 4 for an extended discus-
sion of the consequences of unrelieved pain.
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