++
The preceding section emphasized that tissue injury yielded activity
in small primary afferents and that small afferent input resulted
in monosynaptic and polysynaptic excitation of dorsal horn neurons
that projected to the brainstem and higher centers. Importantly,
the pathway appears to preserve several properties of the stimulus,
the anatomic sign (localization), and intensity. Thus, input from
an area of skin might be expected to activate a given population
of spinal neurons that received afferent input from that part of
the body surface, and the intensity of the stimulus was mirrored
either by the specific neuronal population activated (e.g., nociceptive
specific cells) or by the frequency of the discharge (as with the
WDR neurons), or both. This linkage, even in its simplest form,
would be described as the “pain pathway,” as it
reflects the connectivity by which afferent traffic generated by
tissue injury reaches higher centers and the conscious state. This afferent
substrate, in fact, represents only one component of the system
that is essential to the processing of nociceptive input. The excitation
of dorsal horn neurons evoked by small afferent input is subject
to modulation by a number of receptor systems within the spinal
cord. Technically, this modulation may be thought of in terms of
those systems that increase or decrease the efficacy of synaptic
connections of the afferent pathway.
+++
Plasticity of
the Encoding of Persistent Afferent Input
++
Acute activation of small afferents by high-intensity mechanical
or thermal stimuli will result in a clearly defined pain behavior
in humans and animals. This event is believed to be mediated by
the release of the excitatory afferent transmitters outlined earlier
and, consequently, the depolarization of projection neurons. The
magnitude of the response of a dorsal horn neuron, either WDR or nociceptive
specific, is related to the frequency (and identity) of the afferent
input.
++
The frequency of the afferent input is proportional to the magnitude
of the acutely applied stimulus. The organization of this system’s
response to an acute stimulus is thus typically modeled in terms
of a monotonic (linear) relationship between activity in the peripheral
afferent and the activity of neurons that project out of the spinal
cord to the brain.
++
As previously noted, in the face of tissue injury, the afferent
input is characterized by a persistent afferent barrage. As discussed
subsequently, such input reveals the initiation of a variety of
inhibitor and facilitatory processes that lead to a nonlinear increase
in spinal output.
+++
Intrinsic Inhibitory
Processes
++
The activation of dorsal horn neurons by afferent input can be
attenuated by several discrete systems.
+++
Large Afferent
Axon Interactions
++
Dorsal horn WDR neurons can be inhibited by transient activation
of large primary afferents. Such inhibition is mediated by a local
spinal circuit that produces primary afferent depolarization (PAD),
which exerts an inhibitory effect on the terminals of adjacent afferents.
This serves to reduce the amount of neurotransmitter released from
the afferent fibers in response to a fixed input. PAD is mediated
by release from local interneurons in the substantia gelatinosa
of the inhibitory amino acids, γ-aminobutyric acid
(GABA) and glycine.21 In human psychophysical studies,
vibratory stimuli applied to a painful area served to activate large
myelinated fibers and reduced perception of chronic musculoskeletal
pain. Dorsal column stimulation by antidromically activating collaterals
of large primary afferent fibers would also induce PAD, and this
mechanism may account for some of the antinociceptive actions of
dorsal column stimulation.
++
It is important to appreciate that the presence of these small
inhibitory interneurons is important for the ongoing encoding of
large afferent input. Thus, the spinal action of GABAA and
glycine receptor inhibitors will induce a potent tactile allodynia.
These results suggest that the nonaversive characteristics of large
afferent stimulation depend on this ongoing modulation. Studies
in nerve injury–induced pain states have suggested that
there is a loss of glycine or GABAergic inhibition secondary to
the loss of dorsal horn neurons. The reduction in such inhibition
may provide a partial explanation of the potent allodynia that accompanies
such nerve injury states.
+++
Bulbospinal
and Spinal Modulation
++
Considerable evidence indicates that a variety of spinal terminal
systems may serve to attenuate small, afferent-evoked excitation.
Early work demonstrated that activation of bulbospinal pathways
would suppress spinal nociceptive processing and produce a behaviorally
defined analgesia by the release of noradrenaline and the activation
of dorsal horn α2 receptors. Evidence
that small afferent activation could indirectly activate systems
that mediated the spinal release of hormones, such as enkephalin
or noradrenaline, which could act at such modulatory receptors supported
the perspective that nociceptive processing was under a tonic endogenous
inhibition. In spite of the observation that activating these receptors
(e.g., spinal injection of opiate and α2 agonists)
could yield a powerful analgesia by an action pre- and postsynaptic
to the small afferent terminal, the delivery of antagonists for
these receptors has surprisingly modest effects on spontaneous pain
thresholds. This suggests that however potent a modulatory system
these several receptors represent, these endogenous systems are
not in a tonically active mode, and downregulation of small afferent
input is not a pervasive component of the ongoing processing of
input generated by tissue injury.22
+++
Intrinsic Facilitatory
Processes: Repetitive Small Afferent Input
++
WDR neurons in the spinal and medullary dorsal horn display a
stable response to the discrete, periodic activation of afferent
C fibers. However, repetitive stimulation of C (but not A) fibers
at a moderately faster rate results in a progressively facilitated
discharge. This facilitated response is called “wind-up” (see
Fig. 1-9). This condition has three properties:
++
- 1. The conditioning serves to enhance the response of
the dorsal horn neuron to subsequent input, such that a given stimulus
yields a greater response that would otherwise be anticipated from
that stimulus.
- 2. The conditioning of the spinal cord with repetitive small
afferent stimulation has the additional effect of increasing the
receptive field size of the neuron. Thus, afferent input from dermatomal areas
that previously did not activate the WDR neuron being studied now
evokes a prominent response. The anatomic substrate for this increased
receptive field size is believed to reflect the otherwise weak excitatory
input that comes from collaterals of afferents innervating the skin
areas adjacent to the injury. In the face of the induction of a
facilitated state in the specific neuron under study, this otherwise
ineffective excitatory drive becomes adequate to drive depolarization.
Intracellular recording reveals that the facilitated state reflects
a progressive and sustained partial depolarization of the cell,
rendering the membrane increasingly susceptible to afferent input.
- 3. Low-threshold tactile stimulation also becomes increasingly
effective in driving these neurons. This facilitation by repetitive
C fiber input, therefore, increases the subsequent neuronal response to
low-threshold afferent input, and enhances the response generated
by a given noxious afferent input. These mechanisms are discussed
further in the next sections.
+++
Wind-Up and
Central Facilitation
++
In animal studies, WDR neurons in the dorsal horn display a stimulus-dependent
response to discrete activation of afferent C fibers. Repetitive
stimulation of C (but not A) fibers at a moderately faster rate
results in a progressively facilitated discharge.
++
The exaggerated discharge was dubbed wind-up by Mendell23 (Fig.
1-11). Intracellular recording has indicated that the facilitated
state is represented by a progressive and long-sustained partial depolarization
of the cell, rendering the membrane increasingly susceptible to
afferent input.24
++
++
Given the likelihood that WDR discharge frequency contributes
to the encoding of a high-threshold stimulus as aversive, and that
many of these WDR neurons project through the ventrolateral quadrant
of the spinal cord (i.e., spinobulbar or spinothalamic projections),
this augmented response is believed to be an important component
of the pain message. In addition, the conditioning of the afferent
input as described has the added effect of increasing the receptive
field size of the neurons, such that afferent input from dermatomal
areas that previously did not activate the WDR neuron now evokes
a prominent response. Moreover, low-threshold tactile stimulation
also becomes increasingly effective in driving these neurons.
++
This facilitation by repetitive C-fiber input, therefore, increases
the subsequent neuronal response to low-threshold afferent input,
enhances the response generated by a given noxious afferent input,
and increases the size of the receptive field at which a stimulus
can evoke activity in that neuron. Given the likelihood that WDR
discharge frequency is part of the encoding of the intensity of
a high-threshold stimulus, and that many of these WDR neurons project
in the ventrolateral quadrant of the spinal cord (i.e., spinobulbar
projections), this augmented response is believed to be an important
component of the pain message.
++
The enhanced responsiveness reflects an augmented sensitization
of the neuronal membrane leading to an enhanced response for a given
depolarization. The pharmacology leading to this sensitization is
examined in the discussion that follows. The enlarged receptive
field is believed to reflect, in part, on the collateral input from
distal segments (see Fig. 1-6). In the presence of sensitization,
these distal inputs that exert a degree of excitation that is otherwise
inadequate to activate the neuron will become sufficient. This combination
of spinal events results in an apparent increase in the size of
the neuronal receptive field.
+++
Functional Correlates
of Injury-Evoked Central Facilitation
++
Protracted pain states, such as those that may occur with inflamed
or injured tissue (leading to the peripheral release of active factors),
would routinely result in such an augmented afferent drive of the
WDR neuron and, thence, to the ongoing facilitation. Such observations
are consistent with the speculation that the afferent C-fiber burst
may initiate long-lasting events, resulting in changes in spinal
processing that will alter the response to subsequent input. The
preceding observations regarding this dorsal horn system have been
shown to have behavioral consequences. This phenomenon has clear
functional correlates.
++
Studies in animals have shown that the acute injection of an
irritant such as formalin will induce an acute afferent barrage
followed by a prolonged low level of afferent activity. During this
later period after formalin injection, WDR dorsal horn neurons show
an unexpected level of activity, given the modest afferent input
(e.g., a central facilitation). Finally, examination of behavior
has shown that the animal displays an exaggerated response (flinching)
during the second phase, consistent with the activity in dorsal
horn neurons but, again, greater than might be anticipated based on
the afferent traffic25 (Fig. 1-12).
++
++
The role of this afferent-evoked facilitation in the postinjury
pain state cannot be minimized. After tissue injury, in animals
and in humans, inflammation and cellular/vascular injury
lead to the local peripheral release of active factors. Such active
factors will produce a prolonged activation of C fibers that evoke
a facilitated state of processing in WDR neurons, and, thence, an
ongoing facilitation of nociceptive perception. Such observations
are consistent with the speculation that the afferent C-fiber burst
may initiate long-lasting events, resulting in changes in spinal
processing, which will alter the response to subsequent input.
++
The relevance of this C-fiber–evoked facilitation to
humans has been emphasized by psychophysical studies. To observers,
the activation of C fibers by the intradermal injection of capsaicin
will lead to an initial pain state followed for an extended period
of time by a large region of profoundly enhanced mechanical and
thermal sensitivity.26 This phenomenon is referred
to as secondary hyperesthesia (Fig. 1-13). Thus, in humans, following
local injury where C-fibers are similarly activated, there is every
reason to believe that similar processes apply and that important
components of the post-injury pain state are the events consequent
to the afferent barrage and not, strictly speaking, the input present
in the post-injury phase.
++
+++
Pharmacology
of Central Facilitation
++
The pharmacology of this central facilitation suggests that the
wind-up state reflects more than simply the repetitive activation
of a simple excitatory system. The first real demonstration of this unique
pharmacology was presented by showing that the phenomenon of spinal
wind-up was prevented by the spinal delivery of antagonists for
the N-methyl-d-aspartate (NMDA) receptor27 (Fig.
1-14). Importantly, this agent had no effect on acute evoked activity,
but reduced the wind-up. Subsequent behavioral work demonstrated
that such drugs had no effect on acute pain behavior, but reduced
the facilitated states induced after tissue injury.
++
++
Protracted pain states, such as those that may occur with inflamed
or injured tissue (leading to the peripheral release of active factors),
will routinely result in such an augmented afferent drive of the
WDR neuron and, thence, to an ongoing facilitation (such as the
wind-up described in Fig. 1-14, for example). Such observations
are consistent with the speculation that the afferent C-fiber burst
may initiate longer lasting events, resulting in changes in spinal
processing that will alter the response to subsequent input. The
pharmacology of the central facilitation suggests that the state of
central facilitation reflects more than the repetitive activation
of a simple excitatory system. Based on studies examining the spinal
pharmacology of the electrophysiologic and behavioral response to
the postinjury stimulus state, it has become apparent that the arrival
of the first small afferent barrage appears to trigger a cascade
of events that serve to facilitate the response to subsequent peripheral
stimuli.
++
Aspects of the complex pharmacology of central facilitation in
the dorsal horn are presented in Fig. 1-15. The following points
may be made that define components of spinal systems involved in
the postinjury pain state. For review purposes, the cascade may
be thought of in terms of primary and secondary systems.
++
++
The initial activation generated by small afferent input is mediated
by specific primary afferent transmitters. Primary afferent C fibers
release peptide (e.g., substance P, calcitonin gene–related peptide,
and others) and excitatory amino acid (glutamate) products. These
substances evoke excitation in second-order neurons.
+++
Substance P
Receptors
++
The spinal delivery of substance P results in a mild acute “pain
behavior” and a subsequent reduced response latency to
thermal stimuli (thermal hyperalgesia). These effects are believed
to be mediated by neurokinin 1 (NK-1) receptors that are located
in the superficial dorsal horn on second-order neurons. Blockade
of the NK-1 receptor by intrathecal antagonists or downregulation
of NK-1 receptor expression by intrathecal treatment with NK-1 receptor
mRNA antisense has little effect on acute nociceptive thresholds,
but reduces the facilitated state and behaviorally defined hyperalgesia
(as in the second phase of the formalin test) induced by peripheral
injury.29
++
For glutamate, direct monosynaptic excitation of the second-order
neuron is believed to be mediated by the ionotrophic non-NMDA (AMPA)
receptors (i.e., acute primary afferent excitation of dorsal horn
neurons is not mediated by the NMDA receptor). The spinal delivery
of agonists for the alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic
acid and NMDA receptors will evoke a potent spontaneous pain behavior
and a subsequent hyperalgesia and tactile allodynia.30
++
Opiates targeted at the spinal cord serve to block the release
of transmitter from C fibers by acting presynaptically on the terminals
of C fibers to prevent the opening of voltage-sensitive calcium channels
responsible for the depolarization-evoked release of terminal transmitters.
Opioid receptors have been demonstrated to be present on small afferent
terminals, with the highest density of opioid binding present in
the substantia gelatinosa.
++
Following the initial excitation, it is appreciated that the
spinal dorsal horn displays a prominent facilitation of its input-output
function. This central facilitation represents a cascade that is
initiated by the ongoing afferent drive.
++
As indicated earlier, wind-up as evoked by repetitive small afferent
input is diminished by NMDA receptor antagonists. Repetitive small
afferent input (as occurs after tissue injury) will evoke spinal
glutamate release (see Fig. 1-14). Blockade of spinal AMPA receptors
by intrathecal antagonists will elevate acute nociceptive thresholds,
as well as the first and second phase of the formalin test. In contrast,
intrathecal NMDA antagonists have little effect on acute nociception, but
diminish facilitated states of processing. It is appreciated that
the channel associated with the NMDA receptor is blocked by normal
resting physiologic levels of magnesium, and so no change in excitability
of the neurons possessing NMDA receptors can occur until this is
removed (Fig. 1-16). The magnesium block is only removed by a shift
in the membrane voltage toward depolarization. Thus, the binding
of glutamate to the receptor alone is insufficient to activate the
channel. The receptor channel complex is unique because of this
dual requirement: the channel is gated by both ligand binding to
the receptor, and by the membrane voltage. An added degree of complexity is
that glycine is a required coagonist with glutamate for activation
of the receptor, acting at a strychnine-insensitive site closely
associated with the NMDA receptor. Thus, for the NMDA receptor channel
to operate, certain conditions need to be met: the release and binding
of glutamate and the binding of glycine at the strychnine-insensitive
site on the NMDA ionophore are needed, together with a non-NMDA–induced
depolarization to remove the tonic magnesium block. C-fiber stimulation
will induce the release of glutamate but also with excitatory peptides, and
the latter may provide the required depolarization. Hence, as noted
in the discussion that follows, the events mediated by NMDA-receptor
activation occur secondary to an initial conditioning input. Mechanistically,
the NMDA receptor is a Ca2+ ionophore
that, when activated, will lead to a significant increase in intracellular
Ca2+. As emphasized later,
it is in part due to this increase in intracellular Ca2+ that
the cascade initiated by repetitive afferent input is initiated.31
++
++
Cyclooxygenase is found in the spinal dorsal horn and inhibitors,
and prostaglandin-receptor antagonists given intrathecally will
diminish hyperalgesia induced in the postinjury pain state. This
reaction reflects on the important role of prostaglandins released
from the spinal cord. Dialysis of the spinal cord has emphasized
that in the postinjury pain state, there is an increase in prostaglandin
E2 release. Prostanoids will facilitate release of C-fiber
transmitters such as substance P. Consistent with the observation
that NMDA antagonists can block a hyperalgesic state, spinal NMDA
agonists evoke a hyperalgesic state, and this hyperalgesia is blocked
by spinal cyclooxygenase inhibitors. Such observations suggest that
spinal NMDA-receptor occupancy can evoke the release of prostanoids
that, in turn, augments spinal nociceptive processing.32
++
It is currently appreciated that there are two cyclooxygenase
enzymes. The present data emphasizes that the cyclooxygenase-2 (COX-2)
isozyme is constitutively in the spinal cord, and it is COX-2 that
is primarily responsible for the prostaglandin E2 release
generated by small afferent input.
++
An additional interesting variant, suggested in Figure 1-15,
is that COX-2 (and likely other proteins) expression may be elevated
by circulating factors, released by inflammation and septic process,
such as interleukin-1β, tumor necrosis factor-α,
and lipopolysaccharide, that activate spinal astrocytes and microglia.
Such activation may lead to an enhanced expression of a variety
of enzymes (such as COX-2), channels, and receptors. The relevance
of this non-neuronal expression in the central nervous system is
not certain, but these cells may serve as a source of neuraxial prostaglandins.
Such material would likely result in an enhanced release from neuronal
terminals. It suggests an additional and intriguing role played
by the central COX-2 isozyme and its selective inhibitors. Interestingly,
these factors are believed to work on terminals that may lie distant from
the site of synthesis and release. As such, they are referred to
as volume transmitters.33
++
Small dorsal root ganglion cells as well as some postsynaptic
elements contain NO synthase (NOS). NOS inhibitors given intrathecally
will diminish hyperalgesia induced by intrathecal NMDA and by the
postinjury pain state. Increased citrulline (a product of NO formation)
is released from the spinal cord in the postinjury pain state. NO
has been shown to facilitate terminal release of glutamate.34
+++
Kinases and
Phosphorylation
++
Increasing intracellular Ca2+I —through
the inositol 1,4,5-triphosphate (IP3) pathway by neurokinin
receptor or by the influx of Ca2+ through
voltage-gated Ca2+ channels or
ionophores (NMDA receptor)—activates kinases that phosphorylate
and phosphatases that dephosphorylate local proteins. Phosphorylating
enzyme systems consist of several classes of kinases that are distinguished
by the structure and pharmacology of their inhibitors. In the spinal dorsal
horn, mitogen-activated kinases (e.g., MAP kinases), cAMP-dependent
kinase, and camkinase II have been observed in the spinal dorsal
horn and dorsal root ganglia. Protein kinase C (PKC) consists of
a large family of isozymes. Although some or all of the previously
noted phosphorylating enzymes may play a role, the use of inhibitors
for protein kinase A (PKA) and PKC have shown the particular importance
of this family of kinases in regulating spinal facilitation. Many
hyperalgesic states are mediated by a spinal NMDA receptor. The
NMDA receptor is multiply phosphorylated by PKA and PKC. Intrathecal
delivery of PKC inhibitors has been shown stereospecifically to
diminish injury induced-hyperalgesia.35,36
++
After local tissue injury, the components of posttissue injury
pain reflect an increased receptive field and a left shift in the
stimulus response curve for spinal dorsal horn neurons that is evoked initially
and then sustained in part by persistent small afferent input. The
contribution of these changes in spinal function to the behaviorally
relevant nociceptive state is substantiated by comparing the pharmacology
associated with the effects on the behavior of the unanesthetized
animal with the effects of the drugs on the underlying electrophysiology.
Based on such observations, it is possible to formulate a heuristic
picture of the organization of several pharmacologically defined
spinal systems that mediate the response of the animal to a strong
and tissue-injurious stimulus. Thus, repetitive afferent input increases
excitatory amino acid and peptide release from primary afferents
that serve initially to depolarize dorsal horn neurons. Persistent
depolarization serves to increase intracellular calcium, activating
a variety of intracellular enzymes (COX-2, NOS) and various kinases
(PKC). Prostaglandins and nitric oxide are released spinally and
serve acutely to enhance the subsequent release of afferent peptides
and glutamate. Activation of local kinases serves to phosphorylate
membrane receptors and channels. As an example, the NMDA receptor
when phosphorylated displays an enhanced calcium flux (see Fig.
1-5). The role of these system-level changes in spinal nociceptive
processing in “pain behavior” is supported by
the analgesic effects of spinally delivered agents known (1) to
reduce small afferent transmitter release (μ, ∂ opioid,
and α2-adrenergic agonists) and the antihyperalgesic
actions of spinally delivered neurokinin-1 and NMDA-receptor antagonists,
as well as inhibitors of spinal COX-2, NOS, and PKC.
++
It is important to note that the WDR-wind-up studies discussed
earlier are carried out in animals that are under 1 minimum alveolar
concentration anesthesia. The relevance of the observations to the
performance of surgery on volatile “anesthetized” patients
is clear. The implication of the afferent-evoked facilitation is
that it is better to prevent small afferent input than to deal with
its sequelae. This is believed to represent the basis of the consideration
of the use of preemptive analgesics.37