Neuronal plasticity: increasing the gain in pain

Bradykinin and nerve growth factor release the capsaicin receptor from PtdIns(4,5)P2-mediated inhibition

Tissue injury generates endogenous factors that heighten our sense of pain by increasing the response of sensory nerve endings to noxious stimuli. Bradykinin and nerve growth factor (NGF) are two such pro-algesic agents that activate G-protein-coupled (BK2) and tyrosine kinase (TrkA) receptors, respectively, to stimulate phospholipase C (PLC) signalling pathways in primary afferent neurons. How these actions produce sensitization to physical or chemical stimuli has not been elucidated at the molecular level. Here, we show that bradykinin- or NGF-mediated potentiation of thermal sensitivity in vivo requires expression of VR1, a heat-activated ion channel on sensory neurons. Diminution of plasma membrane phosphatidylinositol-4,5-bisphosphate (PtdIns(4,5)P2) levels through antibody sequestration or PLC-mediated hydrolysis mimics the potentiating effects of bradykinin or NGF at the cellular level. Moreover, recruitment of PLC-gamma to TrkA is essential for NGF-mediated potentiation of channel activity, and biochemical studies suggest that VR1 associates with this complex. These studies delineate a biochemical mechanism through which bradykinin and NGF produce hypersensitivity and might explain how the activation of PLC signalling systems regulates other members of the TRP channel family.

Role of peripheral N-methyl-D-aspartate (NMDA) receptors in visceral nociception in rats

BACKGROUND & AIMS

N-methyl-D-aspartate (NMDA) receptors are ligand-gated ion channels that have an important role in long-term potentiation and memory processing in the central nervous system. The aims in this study were to determine whether NMDA receptors are expressed in the peripheral nervous system and identify their role in mediating behavioral pain responses to colonic distention in the normal gut.

METHODS AND RESULTS

Immunohistochemical localization of the NR1 subunit showed that NMDA receptors are expressed on the cell bodies and peripheral terminals of primary afferent nerves innervating the colon. Dorsal root ganglia neurons retrogradely labeled from the colon in short-term culture responded to addition of NMDA with increased intracellular [Ca2+]. Activation of peripheral NMDA receptors in colonic tissue sections caused Ca2+-dependent release of the proinflammatory neuropeptides, calcitonin gene-related peptide and substance P. Behavioral pain responses to noxious mechanical stimulation were inhibited in a reversible, dose-dependent manner by intravenous administration of memantine, a noncompetitive antagonist of the NMDA receptor. Single fiber recordings of decentralized pelvic nerves showed that colorectal distention responsive afferent nerve activity was inhibited by memantine.

CONCLUSIONS

Peripheral NMDA receptors are important in normal visceral pain transmission, and may provide a novel mechanism for development of peripheral sensitization and visceral hyperalgesia.

The neurobiology of pain: developmental aspects

Invasive procedures that would be painful in children and adults are frequently performed on infants admitted to the neonatal intensive care unit. This article discusses sensory responses to these procedures in the immature nervous system and highlights the fact that, in addition to causing distress and delayed recovery, pain in infancy is also a developmental issue. First, the immaturity of sensory processing within the newborn spinal cord leads to lower thresholds for excitation and sensitization, therefore potentially maximizing the central effects of these tissue-damaging inputs. Second, the plasticity of both peripheral and central sensory connections in the neonatal period means that early damage in infancy can lead to prolonged structural and functional alterations in pain pathways that can last into adult life.

Potentiation of capsaicin receptor activity by metabotropic ATP receptors as a possible mechanism for ATP-evoked pain and hyperalgesia

The capsaicin (vanilloid) receptor, VR1, is a sensory neuron-specific ion channel that serves as a polymodal detector of pain-producing chemical and physical stimuli. It has been proposed that ATP, released from different cell types, initiates the sensation of pain by acting predominantly on nociceptive ionotropic purinoceptors located on sensory nerve terminals. In this study, we examined the effects of extracellular ATP on VR1. In cells expressing VR1, ATP increased the currents evoked by capsaicin or protons through activation of metabotropic P2Y(1) receptors in a protein kinase C-dependent pathway. The involvement of G(q/11)-coupled metabotropic receptors in the potentiation of VR1 response was confirmed in cells expressing both VR1 and M1 muscarinic acetylcholine receptors. In the presence of ATP, the temperature threshold for VR1 activation was reduced from 42 degrees C to 35 degrees C, such that normally nonpainful thermal stimuli (i.e., normal body temperature) were capable of activating VR1. This represents a novel mechanism through which the large amounts of ATP released from damaged cells in response to tissue trauma might trigger the sensation of pain.

Temporal facilitation of the flexor reflex induced by C-fiber activity: comparison between adult and aged rats

We investigated the wind-up phenomenon of the flexor reflex in adult and aged rats. The sural nerve was stimulated at C-fiber strength and reflex activity was recorded from the semitendinosus muscle. The wind-up rate, the increment rate of the C-fiber response (i.e. activity from 100 to 600 ms after stimulation) by successive stimuli (five train pulses), was decreased exponentially with increasing stimulus intervals from 3 to 20 s. The time constant of the decay for the aged rats was 9.2+/-3.2 s (mean+/-SD), which was significantly longer than for the adult rats (6.4+/-2.9 s). The findings indicate that the effects of C-fiber activation on the spinal nociceptive pathways attenuate more slowly in aged rats as compared with adult rats.

Ion channels associated with the ectopic discharges generated after segmental spinal nerve injury in the rat

In an attempt to identify important ion channels contributing to the generation of ectopic discharges, the present study examined the effects of ion channel blockers on ectopic discharges of injured sensory neurons after spinal nerve ligation. The main focus of the study was to examine the effect of the sodium channel blocker, tetrodotoxin (TTX), in order to identify important subtype(s) (i.e. TTX-sensitive and TTX-resistant) of sodium channels that are involved in ectopic discharge generation. In addition, the effects of potassium and calcium channel blockers were also tested for comparison with the results of previous studies. The dorsal root ganglion (DRG) of the injured segment was removed along with the dorsal root (DR) and the spinal nerve 7-14 days after spinal nerve ligation in the rat. The tissue was placed in an in-vitro recording chamber consisting of multiple compartments that were independently perfused with 35 degrees C artificial cerebrospinal fluid (ACSF). Single unit recordings were made from teased DR fibers. Once a spontaneously active unit was found and characterized, ACSF containing a channel blocker was perfused to the DRG, the site where almost all ectopic discharges originate after spinal nerve ligation. All the recorded spontaneously active units were found to be Abeta and Adelta fibers (no C fibers were detected). Perfusion of the DRG with a sodium channel blocker (lidocaine) at a dose much less than that required to block conduction of action potentials, significantly inhibited ectopic discharges in all recorded fibers. In addition, ectopic discharges were inhibited by TTX perfused to the DRG at a dose much lower (average of 22.1 nM) than that required to block TTX-resistant subtypes of sodium channels. The data suggest that TTX-sensitive sodium channels are likely to be involved in the generation of ectopic discharges. The present study also confirmed the results of previous studies on the additional potential roles of potassium and calcium channels, thus suggesting that multiple ion channels are likely to be involved in the generation of ectopic discharges.

Plasticity of motor systems after incomplete spinal cord injury

Although spontaneous regeneration of lesioned fibres is limited in the adult central nervous system, many people that suffer from incomplete spinal cord injuries show significant functional recovery. This recovery process can go on for several years after the injury and probably depends on the reorganization of circuits that have been spared by the lesion. Synaptic plasticity in pre-existing pathways and the formation of new circuits through collateral sprouting of lesioned and unlesioned fibres are important components of this recovery process. These reorganization processes might occur in cortical and subcortical motor centres, in the spinal cord below the lesion, and in the spared fibre tracts that connect these centres. Functional and anatomical evidence exists that spontaneous plasticity can be potentiated by activity, as well as by specific experimental manipulations. These studies prepare the way to a better understanding of rehabilitation treatments and to the development of new approaches to treat spinal cord injury.

Pathophysiologic mechanisms of neuropathic pain

New animal models of peripheral nerve injury have facilitated our understanding of neuropathic pain mechanisms. Nerve injury increases expression and redistribution of newly discovered sodium channels from sensory neuron somata to the injury site; accumulation at both loci contributes to spontaneous ectopic discharge. Large myelinated neurons begin to express nociceptive substances, and their central terminals sprout into nociceptive regions of the dorsal horn. Descending facilitation from the brain stem to the dorsal horn also increases in the setting of nerve injury. These and other mechanisms drive various pathologic states of central sensitization associated with distinct clinical symptoms, such as touch-evoked pain.

Interleukin-1beta-mediated induction of Cox-2 in the CNS contributes to inflammatory pain hypersensitivity

Inflammation causes the induction of cyclooxygenase-2 (Cox-2), leading to the release of prostanoids, which sensitize peripheral nociceptor terminals and produce localized pain hypersensitivity. Peripheral inflammation also generates pain hypersensitivity in neighbouring uninjured tissue (secondary hyperalgesia), because of increased neuronal excitability in the spinal cord (central sensitization), and a syndrome comprising diffuse muscle and joint pain, fever, lethargy and anorexia. Here we show that Cox-2 may be involved in these central nervous system (CNS) responses, by finding a widespread induction of Cox-2 expression in spinal cord neurons and in other regions of the CNS, elevating prostaglandin E2 (PGE2) levels in the cerebrospinal fluid. The major inducer of central Cox-2 upregulation is interleukin-1beta in the CNS, and as basal phospholipase A2 activity in the CNS does not change with peripheral inflammation, Cox-2 levels must regulate central prostanoid production. Intraspinal administration of an interleukin-converting enzyme or Cox-2 inhibitor decreases inflammation-induced central PGE2 levels and mechanical hyperalgesia. Thus, preventing central prostanoid production by inhibiting the interleukin-1beta-mediated induction of Cox-2 in neurons or by inhibiting central Cox-2 activity reduces centrally generated inflammatory pain hypersensitivity.

Representation of acute and persistent pain in the human CNS: potential implications for chemical intolerance

The study of pain may be relevant to the study of chemical intolerance (CI) in many ways. Pain is often reported as a symptom of CI and it is defined as a subjective experience similar to many other symptoms of CI, making its objectification difficult. Furthermore, the CNS plastic changes that underlie the development of persistent pain states and abnormal pain responses may share some similarities with those involved in the sensitization to environmental chemicals. Functional brain imaging studies in humans demonstrate that acute pain evoked by nociceptive stimulation is accompanied by the activation of a widely distributed network of cerebral structures, including the thalamus and the somatosensory, insular, and anterior cingulate cortices. Abnormal activity within these regions has been associated with the experience of pain following damage to the peripheral or central nervous system (neuropathic pain) in a number of clinical populations. In normal individuals, activity within this network is correlated with subjective pain perception, is highly modifiable by cognitive interventions such as hypnosis and attention, and has been associated with emotions. Other cognitive mediators such as expectations can also produce robust changes in pain perception (e.g., in placebo analgesia). These effects likely depend on both higher-order cerebral structures and descending mechanisms modulating spinal nociceptive activity. These psychological processes can be solicited to reduce clinical pain and we speculate that they may further attenuate or promote central mechanisms involved in the transition from acute to persistent pain states. The investigation of central determinants of subjective experience is essential to assess the possibility that higher-order brain/psychological processes modulate and/or mediate the development of persistent pain states. These factors may contribute to the development of symptoms in CI.

Neuronal plasticity and signal transduction in nociceptive neurons: implications for the initiation and maintenance of pathological pain

Pathological pain, consisting of tissue injury-induced inflammatory and nerve injury-induced neuropathic pain, is an expression of neuronal plasticity. One component of this is that the afferent input generated by injury and intense noxious stimuli triggers an increased excitability of nociceptive neurons in the spinal cord. This central sensitization is an activity-dependent functional plasticity that results from activation of different intracellular kinase cascades leading to the phosphorylation of key membrane receptors and channels, increasing synaptic efficacy. Central sensitization is both induced and maintained in a transcription-independent manner. Several different intracellular signal transduction cascades converge on MAPK (mitogen-activated protein kinase), activation of which appears to be a master switch or gate for the regulation of central sensitization. In addition to posttranslational regulation, the MAPK pathway may also regulate long-term pain hypersensitivity, via transcriptional regulation of key gene products. Pharmacological intervention targeted specifically at the signal transduction pathways in nociceptive neurons may provide, therefore, new therapeutic opportunities for pathological pain.

Neurotrophins and asthma

The neurotrophins are a family of peptides that promote survival, growth, and differentiation of neurons. Neurotrophins may also influence the function of nonneuronal cell types, including immune cells. The development and maintenance of asthma is thought to involve the nervous system and the immune system, but the role that neurotrophins play in asthma is unknown. The cellular sources of the neurotrophins include mast cells, lymphocytes, macrophages, epithelial cells, smooth muscle cells, and eosinophils. The activation of neurotrophin receptors in immune cells and neurons involves ligand-induced homodimerization, which leads to activation of intrinsic Trk receptor kinase. The exact consequences of activating these receptors on immune cells is unknown, but rather than having unique actions on immune cells, the neurotrophins appear to act in concert with known immune regulating factors to modulate the maturation, accumulation, proliferation, and activation of immune cells. Neurotrophins can modulate afferent nerve function by stimulating the production of neuropeptides within airway afferent neurons. These neuropeptides may be released from the central terminals of airway afferent neurons, which leads to heightened autonomic reflex activity, and increased reactivity in the airways.