Abstract Pain research has uncovered important neuronal mechanisms that underlie clin- ically relevant pain states such as inﬂammatory and neuropathic pain. Importantly, both the peripheral and the central nociceptive system contribute signiﬁcantly to the generation of pain upon inﬂammation and nerve injury. Peripheral nociceptors are sensitized during inﬂammation, and peripheral nerve ﬁbres develop ectopic discharges upon nerve injury or disease. As a consequence a complex neuronal response is evoked in the spinal cord where neurons become hyperexcitable, and a new balance is set between excitation and inhibition. The spinal processes are signiﬁcantly inﬂuenced by brain stem circuits that inhibit or facil- itate spinal nociceptive processing. Numerous mechanisms are involved in peripheral and central nociceptive processes including rapid functional changes of signalling and long- term regulatory changes such as up-regulation of mediator/receptor systems. Conscious pain is generated by thalamocortical networks that produce both sensory discriminative and affective components of the pain response.
Keywords Nociceptive system · Nociceptors · Inﬂammatory pain · Neuropathic pain · Peripheral sensitization · Central sensitization · Ectopic discharges ·
Descending inhibition · Descending facilitation
Introduction on Pain
Types of Pain
In daily life the sensation pain is speciﬁcally evoked by potential or actual nox- ious (i.e. tissue damaging) stimuli applied to the body such as heat, squeezing a skin fold or over-rotating a joint. The predictable correlation between the noxious stimulus and the pain sensation causes us to avoid behaviour and situations that evoke pain. Pain during disease is different from “normal” pain. It occurs in the absence of external noxious stimuli, during mild stimu- lation or in an unpredictable way. Types of pain have been classiﬁed accord- ing to their pathogenesis, and pain research intends to deﬁne their neuronal mechanisms.
Cervero and Laird (1991) distinguish between three types of pain. Applica- tion of an acute noxious stimulus to normal tissue elicits acute physiological nociceptive pain. It protects tissue from being (further) damaged because with- drawal reﬂexes are usually elicited. Pathophysiological nociceptive pain occurs when the tissue is inﬂamed or injured. It may appear as spontaneous pain (pain in the absence of any intentional stimulation) or as hyperalgesia and/or allodynia. Hyperalgesia is extreme pain intensity felt upon noxious stimula- tion, and allodynia is the sensation of pain elicited by stimuli that are normally below pain threshold. In non-neuropathic pain, some authors include the low- ering of the pain threshold in the term hyperalgesia. While nociceptive pain is elicited by stimulation of the sensory endings in the tissue, pain!neuropathic results from injury or disease of neurons in the peripheral or central nervous system. It does not primarily signal noxious tissue stimulation and often feels abnormal. Its character is often burning or electrical, and it can be persistent or occur in short episodes (e.g. trigeminal neuralgia). It may be combined with hyperalgesia and allodynia. During allodynia even touching the skin can cause
Peripheral and Central Mechanisms of Pain Generation
intense pain. Causes of neuropathic pain are numerous, including axotomy, nerve or plexus damage, metabolic diseases such as diabetes mellitus, or her- pes zoster. Damage to central neurons (e.g. in the thalamus) can cause central neuropathic pain.
This relatively simple classiﬁcation of pain will certainly be modiﬁed for several reasons. First, in many cases pain is not strictly inﬂammatory or neuro-
pathic because neuropathy may involve inﬂammatory components and neuro- pathic components may contribute to inﬂammatory pain states. Second, pain research now addresses other types of pain such as pain during surgery (inci- sional pain), cancer pain, pain during degenerative diseases (e.g. osteoarthri-
tis), or pain in the course of psychiatric diseases. This research will probably lead to a more diversiﬁed classiﬁcation that takes into account general and disease-speciﬁc neuronal mechanisms.
An important aspect is the distinction between acute and chronic pain. Usually pain in patients is called “chronic” when it lasts longer than6 months (Russo and Brose 1998). Chronic pain may result from a chronic disease and
may then actually result form persistent nociceptive processes. More recently the emphasis with chronic pain is being put on its character. In many chronic pain states the causal relationship between nociception and pain is not tight and pain does not reﬂect tissue damage. Rather psychological and social factors
seem to inﬂuence the pain, e.g. in many cases of low back pain (Kendall 1999). Chronic pain may be accompanied by neuroendocrine dysregulation, fatigue, dysphoria, and impaired physical and even mental performance (Chapman
and Gavrin 1999).
The Nociceptive System: An Overview
Nociception is the encoding and processing of noxious stimuli in the nervous system that can be measured with electrophysiological techniques. Neurons involved in nociception form the nociceptive system. Noxious stimuli activate primary nociceptive neurons with “free nerve endings” (Aδ and C ﬁbres, no-
ciceptors) in the peripheral nerve. Most of the nociceptors respond to noxious mechanical (e.g. squeezing the tissue), thermal (heat or cold), and chemical stimuli and are thus polymodal (cf. in Belmonte and Cervero 1996). Nocicep-
tors can also exert efferent functions in the tissue by releasing neuropeptides [substance P (SP), calcitonin gene-related peptide (CGRP)] from their sensory endings. Thereby they induce vasodilatation, plasma extravasation, attraction
of macrophages or degranulation of mast cells, etc. This inﬂammation is called neurogenic inﬂammation (Lynn 1996; Schaible et al. 2005).
Nociceptors project to the spinal cord and form synapses with second order neurons in the grey matter of the dorsal horn. A proportion of second-order
neurons have ascending axons and project to the brain stem or to the thala- mocortical system that produces the conscious pain response upon noxious
stimulation. Other spinal cord neurons are involved in nociceptive motor re- ﬂexes, more complex motor behaviour such as avoidance of movements, and the generation of autonomic reﬂexes that are elicited by noxious stimuli.
Descending tracts reduce or facilitate the spinal nociceptive processing. The descending tracts are formed by pathways that originate from brainstem nuclei (in particular the periaqueductal grey, the rostral ventromedial medulla) and descend in the dorsolateral funiculus of the spinal cord. Descending inhibition is part of an intrinsic antinociceptive system (Fields and Basbaum 1999).
The Peripheral Pain System: Primary Afferent Nociceptors
Responses to Noxious Stimulation of Normal Tissue
Nociceptors of different tissues are assumed to share most of their general properties. However, qualitative and quantitative differences of neurons sup- plying different tissues cannot be ruled out, e.g. the mechanical threshold of nociceptors may be quite different in different tissues because the poten- tially damaging stimuli may be of low (as in the cornea) or higher intensity (in the skin, muscle or joint). Furthermore, evidence was provided that dor- sal root ganglion (DRG) neurons supplying ﬁbres to different tissues differ in their passive and active electrophysiological properties (Gold and Traub 2004). Thus, subtle differences in nociceptor properties may be important for pain mechanisms in different tissues.
In skin, muscle and joint, many Aδ andC ﬁbres have elevated thresholds for mechanical stimuli, thus acting as speciﬁc nociceptors that detect potentially
or actually damaging mechanical stimuli. At least in the skin many nociceptors respond to noxious heat. The heat threshold may be below the frankly nox- ious range but the neurons encode different heat intensities by their response frequency. In some visceral organs such as the bladder, most slow-conducting
ﬁbres have thresholds in the innocuous range and stronger responses in the noxious range, raising the possibility that visceral noxious stimuli are also en- coded by “wide dynamic range neurons” and not only by speciﬁc nociceptors.
In addition, many nociceptors are sensitive to chemical stimuli (chemosensi- tivity). Most of the nociceptors are thus polymodal (cf. Belmonte and Cervero).
Nociceptors are different from afferents subserving other modalities. Most fast-conducting Aβ afferents with corpuscular endings are mechano-receptors that respond vigorously to innocuous mechanical stimuli. Although they may show their strongest response to a noxious stimulus, their discharge pattern does not discriminate innocuous from noxious stimuli. A proportion of Aδ and C ﬁbres are warmth or cold receptors encoding innocuous warm and cold stimuli but not noxious heat and cold.
In addition to polymodal nociceptors, joint, skin and visceral nerves contain Aδ and C ﬁbres that were named silent or initially mechano-insensitive noci- ceptors. These neurons are not activated by noxious mechanical and thermal stimuli in normal tissue. However, they are sensitized during inﬂammation
and then start to respond to mechanical and thermal stimuli (Schaible and Schmidt 1988; Weidner et al. 1999). In humans this class of nociceptors exhibits a particular long-lasting response to algogenic chemicals, and such nocicep- tors are crucial in mediating neurogenic inﬂammation (Ringkamp et al. 2001). Moreover, they play a major role in initiating central sensitization (Kleede et al. 2003). These neurons have distinct axonal biophysical characteristics separating them from polymodal nociceptors (Orstavik et al. 2003; Weidner et al. 1999).
Changes of Neuronal Responses During Inflammation
During inﬂammation the excitation threshold of polymodal nociceptors drops such that even normally innocuous, light stimuli activate them. Noxious stimuli evoke stronger responses than in the non-sensitized state. After sensitization of “pain ﬁbres”, normally non-painful stimuli can cause pain. Cutaneous no- ciceptors are in particular sensitized to thermal stimuli; nociceptors in deep somatic tissue such as joint and muscle show pronounced sensitization to me- chanical stimuli (Campbell and Meyer 2005; Mense 1993; Schaible and Grubb
1993). In addition, during inﬂammation initially mechano-insensitive nerve ﬁbres become mechano-sensitive. This recruitment of silent nociceptors adds
signiﬁcantly to the inﬂammatory nociceptive input to the spinal cord. Resting discharges may be induced or increased in nociceptors because of inﬂamma- tion, providing a continuous afferent barrage into the spinal cord.
Peripheral Neuronal Mechanisms of Neuropathic Pain
In healthy sensory nerve ﬁbres action potentials are generated in the sensory endings upon stimulation of the receptive ﬁeld. Impaired nerve ﬁbres often show pathological ectopic discharges. These action potentials are generated at the site of nerve injury or in the cell body in DRG. The discharge patterns vary from rhythmic ﬁring to intermittent bursts (Han et al. 2000; Liu et al. 2000).
Ectopic discharges occur in Aδ and C ﬁbres and in thick myelinated Aβ ﬁbres. Thus, after nerve injury both low threshold Aβ as well as high threshold Aδ and C ﬁbres may be involved in the generation of pain. Aβ ﬁbres may evoke exaggerated responses in spinal cord neurons that have undergone the process
of central sensitization (see Sect. 3.3.2). Recently, however, it was proposed that pain is not generated by the injured nerve ﬁbres themselves but rather by intact
nerve ﬁbres in the vicinity of injured nerve ﬁbres. After an experimental lesion in the L5 dorsal root, spontaneous action potential discharges were observed in C ﬁbres in the uninjured L4 dorsal root. These ﬁbres may be affected by the process of a Wallerian degeneration (Wu et al. 2001).
Molecular Mechanisms of Activation and Sensitization of Nociceptors
Recent years have witnessed considerable progress in the understanding of molecular events that lead to activation and sensitization of nociceptors. No- ciceptors express ion channels for stimulus transduction and action potential generation, and a large number of receptors for inﬂammatory and other medi- ators (Fig. 1). These receptors are either coupled to ion channels or, more often, activate second messenger systems that inﬂuence ion channels. Sensitization of nociceptors by inﬂammatory mediators is induced within a few minutes. If noxious stimuli or inﬂammatory conditions persist, the expression of ion channels, receptors and mediator substances may change. An up-regulation of excitatory receptors may contribute to the maintenance of pain. Furthermore, some receptors exert trophic inﬂuences on the neurons regulating synthesis of mediators and expression of ion channels and receptors in these cells.
Fig. 1 Model of the sensory ending of a nociceptor showing ion channels for transduction of thermal and mechanical stimuli and action potential generation and metabotropic receptors subserving chemosensitivity
The ﬁrst cloned nociceptive ion channel was the TRPV1 receptor, which is expressed in about 40% of DRG cells. This ion channel is opened by binding of capsaicin, the compound in hot pepper that causes burning pain. In par- ticular, Ca2+ ﬂows through this channel and depolarizes the cell. The TRPV1
receptor is considered one of the transducers of noxious heat because it is opened by heat (>43°C). In TRPV1 knock-out mice, the heat response is not abolished but the mice do not exhibit thermal hyperalgesia during inﬂam- mation, showing the importance of TRPV1 for inﬂammatory hyperalgesia (Caterina et al. 2000; Davis et al. 2000). Up-regulation of TRPV1 transcription during inﬂammation explains longer-lasting heat hypersensitivity (Ji et al.
2002; Wilson-Gering et al. 2005). Following experimental nerve injury and in animal models of diabetic neuropathy, TRPV1 receptor is present on neu- rons that do not normally express TRPV1 (Rashid et al. 2003; Hong and Wi- ley 2005).
The TRPV1 receptor is a member of the TRP (transient receptor protein) family. Other TRP members may be transducers of temperature stimuli in other ranges (Papapoutian et al. 2003). The TRPV2 receptor in nociceptors is thought to be a transducer for extreme heat (threshold >50°C). TRPA1 could be the transducer molecule in nociceptors responding to cold (Peier et al. 2002). It is activated by pungent compounds, e.g. those present in cinnamon oil, mustard oil and ginger (Bandell et al. 2004). By contrast, TRPV3 and/or TRPV4 may be transduction molecules for innocuous warmth in warm receptors, and TRPM8 may transduce cold stimuli in innocuous cold receptors. Although the putative warmth transducer TRPV4 shows some mechano-sensitivity, it is still unclear whether TRPV4 is involved in the transduction of mechanical stimuli (Marchand et al. 2005).
Voltage-Gated Sodium Channels and ASICs
While most voltage-gated Na+ channels are blocked by tetrodotoxin (TTX), many small DRG cells express TTX-resistant (R) Na+ channels (NaV1.8 and NaV1.9) in addition to TTX-sensitive (S) Na+ channels. Both TTX-S and TTX-R Na+ channels contribute to the Na+ inﬂux during the action potential. Inter- estingly, TTX-R Na+ currents are inﬂuenced by inﬂammatory mediators. They
are enhanced e.g. by prostaglandin E2 (PGE2) that sensitizes nociceptors (Mc- Cleskey and Gold 1999). This raises the possibility that TTX-R Na+ channels also play a role in the transduction process of noxious stimuli (Brock et al.
1998). SNS−/− knock-out mice (SNS is a TTX-R Na+ channel) exhibit pro-
nounced mechanical hypoalgesia but only small deﬁcits in the response to thermal stimuli (Akopian et al. 1999).
Acid sensing ion channels (ASICs) are Na+ channels that are opened by low pH. This is of interest because many inﬂammatory exudates exhibit a low pH. Protons directly activate ASICs with subsequent generation of action potentials (Sutherland et al. 2001).
Receptors of Inflammatory Mediators (Chemosensitivity of Nociceptors)
The chemosensitivity of nociceptors allows inﬂammatory and trophic medi- ators to act on these neurons. Sources of inﬂammatory mediators are inﬂam- matory cells and non-neuronal tissue cells. The ﬁeld of chemosensitivity is extremely complicated due to the large numbers of receptors that have been identiﬁed in primary afferent neurons (Gold 2005; Marchand et al. 2005). Re- ceptors that are involved in the activation and sensitization of neurons are either ionotropic (the mediator opens an ion channel) or metabotropic (the mediator activates a second messenger cascade that inﬂuences ion channels and other cell functions). Many receptors are coupled to G proteins, which signal via the production of the second messengers cyclic AMP (cAMP), cyclic guano- sine monophosphate (cGMP), diacylglycerol and phospholipase C. Other re- ceptor subgroups include receptors bearing intrinsic protein tyrosine kinase domains, receptors that associate with cytosolic tyrosine kinases and protein serine/threonine kinases (Gold 2005). Table 1 shows the mediators to which re- ceptors are expressed in sensory neurons (Gold 2005; Marchand et al. 2005). It is beyond the scope of this chapter to describe all the important mediators. Many of the mediators and their receptors will be addressed in the following chapters.
Functions of mediators are several-fold. Some of them activate neurons directly (e.g. the application of bradykinin evokes action potentials by itself ) and/or they sensitize neurons for mechanical, thermal and chemical stimuli (e.g. bradykinin and prostaglandins increase the excitability of neurons so that mechanical stimuli evoke action potentials at a lower threshold than under control conditions). PGE2, for example, activates G protein-coupled EP recep- tors that cause an increase of cellular cAMP. This second messenger activates protein kinase A, and this pathway inﬂuences ion channels in the membrane, leading to an enhanced excitability of the neuron with lowered threshold and increased action potential frequency elicited during suprathreshold stimula- tion. Bradykinin receptors are of great interest because bradykinin activates
numerous Aδ and C ﬁbres and sensitizes them for mechanical and thermal stimuli (Liang et al. 2001). Bradykinin receptor antagonists reverse thermal
hyperalgesia, and Freund’s complete adjuvant induced mechanical hyperalge- sia of the rat knee joints. Some reports suggest that in particular bradykinin B1 receptors are up-regulated in sensory neurons following tissue or nerve in- jury, and that B1 antagonists reduce hyperalgesia. Other authors also found an
up-regulation of B2 receptors during inﬂammation (Banik et al. 2001; Segond von Banchet et al. 2000).
Peripheral and Central Mechanisms of Pain Generation
Table 1 Receptors in subgroups of sensory neurons
Ionotropic receptors for
ATP, H+ (acid-sensitive ion channels, ASICs), glutamate (AMPA, kainate, NMDA
receptors), acetylcholine (nicotinic receptors), serotonin (5-HT3)
Metabotropic receptors for
Acetylcholine, adrenaline, serotonin, dopamine, glutamate, GABA, ATP
Prostanoids (prostaglandin E2 and I2 ), bradykinin, histamine, adenosine, endothelin Neuropeptides (e.g. substance P, calcitonin gene-related peptide, somatostatin, opioids) Proteases (protease-activated receptors, PAR1 and PAR2)
Neurotrophins [tyrosine kinase (Trk) receptors] Glial cell line-derived neurotrophic factor (GDNF)
Inﬂammatory cytokines (non-tyrosine kinase receptors)
While prostaglandins and bradykinin are “classical” inﬂammatory medi- ators, the list of important mediators will be extended by cytokines. Some cytokines such as interleukin (IL)-1β are pro-nociceptive upon application to the tissue (Obreja et al. 2003). It is likely that cytokines play an important role
in both inﬂammatory and neuropathic pain (Marchand et al. 2005; Sommer and Schröder 1995).
Neurotrophins are survival factors during the development of the nervous system, but during inﬂammation of the tissue, the level of nerve growth factor
(NGF) is substantially enhanced. By acting on the tyrosine kinase A (trk A) receptors, NGF increases the synthesis of SP and CGRP in the primary afferents. NGF may also act on mast cells and thereby activate and sensitize sensory
endings by mast cell degranulation (cf. Schaible and Richter 2004).