Neurotrophins are a family of structurally similar proteins that regulate the growth, differentiation, function, survival and plasticity of nerve cells. These proteins are expressed in greatest abundance within the nervous system, including target tissues for sensory nerve endings. Neurotrophins produce their effects in responsive cells by interacting with two classes of cell surface receptors: the selective trk receptors (trkA, trkB and trkC) and the non-selective p75 NGF receptors, which do not discriminate between the various neurotrophin proteins. Nerve Growth Factor (NGF) is the prototypic member of the neurotrophin family, having been discovered over 40 years ago. The dysfunction of NGF-mediated signalling has been implicated in disorders such as chronic pain, ALS, Parkinson's disease and stroke. Nerve growth factor is a member of a family of peptides known as the neurotrophins. The NGF levels are elevated in several painful conditions in humans, including arthritis, cystitis, prostatitis and chronic headaches.
As the name implies, interest in this molecule was originally stimulated by its effectiveness in stimulating neurite outgrowth, particularly from neurons of neural crest origin (dorsal root ganglia, sympathetic ganglion cells) in the developing animal. More recently, an important role in neuronal survival has been ascribed to the neurotrophins. Different types of neurons require different neurotrophins, depending on the neurotrophin receptor expressed on their membrane. NGF has been shown to be a critical survival factor for small, peptide-expressing neurons that project into the superficial laminae of the dorsal horn (i.e., putative nociceptive neurons). These neurons have been shown to express the trk-A receptor for which the specific ligand is NGF. These include A-delta high-threshold mechanoreceptors taking on the properties of low-threshold D-hairs, C-fibre polymodal nociceptors losing their heat sensitivity, and the emergence of a class of mechanoreceptors with thresholds intermediate between low and high. Other receptor types (e.g., low-threshold cutaneous afferents) are not sensitive to treatment with anti NGF. Nerve growth factor is critical for the survival and maintenance of sympathetic and sensory neurons. The NGF is the founding member of the neurotrophin family of structurally related secreted proteins that includes brain-derived neurotrophic factor (BDNF), neurotrophin 3 (NT-3) and NT-4. They bind to two types of receptor: a common receptor, p75NTR, which binds all neurotrophins with a similar affinity; and members of the trk family of receptor tyrosine kinases, trkA, trkB and trkC, which bind different neurotrophins. TrkA is the receptor tyrosine kinase for NGF
NGF as a nociceptive agent
The notion that NGF plays a role in hyperalgesia is predictable from the finding that the concentration of this molecule is increased by inflammatory injury. This increase takes place within minutes of the injury in the skin and is observed later in sensory axons and DRG cells, following neuronal uptake and retrograde transport of NGF. The enhanced concentration of NGF in skin is due to its release from keratinocytes following the action of tumour necrosis factor alpha (TNFalpha), as well as interleukins (e.g., IL-1beta) released from macrophages or mast cells in injured tissue. A more direct demonstration of the role of NGF in hyperalgesia has emerged from the sensory changes after NGF injections.
Thermal hyperalgesia has a much more rapid onset, requiring only a few minutes to become measurable. As with mechanical hyperalgesia, the duration is several days. The initial rapid component of heat hyperalgesia has been attributed to a peripheral mechanism, with two cell populations demonstrated to play a role in this action: the mast cells and the autonomic neurons both of which have trkA receptors. The long-latency thermal hyperalgesia, however, is selectively eliminated by the N-methyl-D-aspartate (NMDA) blocker MK-801, presumably via its effect on central sensitization. Recent studies have shown that bradykinin-1 (BK-1) blockers selectively attenuate the late component of thermal hyperalgesia.
Involvement of NGF in hyperalgesia has also been shown in human studies carried out as part of the first phase of clinical trials to assess the safety of NGF .The patients reported muscle ache in response to systemically administered NGF. Subcutaneous injections resulted in local tenderness to touch and heat. There is now extensive evidence that neurotrophins alter the functions of nerve cells that recognize painful stimuli (nociceptors). Specifically, NGF binding to trkA/p75 NGF has been shown to have both acute and long-term effects on nociceptor function. Tissue damage or inflammation induces high levels of NGF secretion. The acute effect of NGF is to stimulate the release of naturally occurring chemicals that increase the sensitivity of nociceptors to pain (e.g., substance P, CGRP, histamine). After this initial phase, the over-stimulation of NGF receptors leads to a remodelling of pain pathways with an increase in the number of nociceptive fibres and pain receptors, such as ion channels. These acute and long term changes in the processing of pain signals and reorganization of the neuronal networks mediated by NGF underlie the chronic pain mechanisms induced by nerve damage or disease. Evidence that NGF has a key role in the generation and potentiation of pain is derived from experimental and clinical studies in a wide variety of acute and chronic pain states.
The NGF is a key element of inflammatory pain. It induces hyperalgesia by up-regulating the transcription of genes encoding receptors, ion channels, and neuropeptides. Acid-sensing ion channel 3 (ASIC3), a depolarizing sodium channel gated by protons during tissue acidosis, is specifically expressed in sensory neurons. It has been associated to cardiac ischemic and inflammatory pains. Low endogenous NGF was responsible for ASIC3 basal expression and high NGF during inflammation increased ASIC3 expression parallel to the development of neuron hyper excitability associated with hyperalgesia. NGF is known to activate numerous signalling pathways through trkA and p75 receptors
In rodents, NGF causes robust, long-lasting mechanical and thermal hyperalgesia following either local or systemic administration. In humans, the ability of NGF to provoke pain became known in clinical studies to explore its potential in the treatment of polyneuropathies. Subcutaneous injection of NGF into the forearm of healthy volunteers produced allodynia and hypersensitivity in the surrounding skin that lasted for up to three weeks, and generalized muscle pain occurred more frequently in subjects who received NGF compared with those who received placebo.
Visceral pain is difficult to manage therefore it remains as a major clinical problem. In animal models of visceral pain, systemic administration of NGF-blocking agents inhibits the behavioural responses elicited by irritants instilled into the colon, stomach and bladder. Congenital insensitivity to pain with anhidrosis is an autosomal recessive disorder that is caused by null mutations in the gene encoding trkA (NTRK1). A recently discovered mutation in the gene that encodes human NGF is also associated with diminished pain perception.
Induction of the expression of NGF is an early event in injured and inflamed tissues, and elevated levels of NGF are sustained in chronic inflammation. These changes in the synthesis of NGF appear to be caused, in part at least, by the action of pro-inflammatory cytokines, many of which induce the synthesis of NGF in several cell types in vitro and in vivo. NGF sensitizes nociceptive neurons directly to several pain-provoking stimuli by causing rapid post-translational changes in the transient receptor potential vanilloid receptor 1 (TRPV1) cation channel and by modulating the expression of genes that influence nociceptor function. The NGF also sensitizes nociceptors indirectly by activating mast cells.
Evidence indicates that trkA receptors are required for the nociceptive actions of NGF, but that p75NTR might also have a role. TrkA receptors are expressed selectively on nociceptive neurons in dorsal root ganglia (DRG). Although p75NTR is expressed on these and most other kinds of sensory neurons in DRG, the demonstration that NGF-induced hyperalgesia occurs in mice that lack p75NTR indicates that the trkA receptor is sufficient to mediate the acute, noxious actions of NGF. Accordingly, K252a (see chemical names), an alkaloid that inhibits trk signalling, attenuates mechanical hypersensitivity in an animal model of pancreatic pain. The p75NTR is important for up regulation of the bradykinin receptor on sensory neurons. This raises the possibility that p75NTR might have a role in pain states in which bradykinin is an important mediator.
The TRPV1 is a cation channel that was identified originally as the capsaicin receptor. It is expressed on polymodal nociceptors and serves as a molecular detector for noxious heat and extra cellular acidification, which occurs in tissue inflammation. When activated, TRPV1 enables the influx of monovalent and divalent cations, predominantly Ca2+, which results in membrane depolarization and the generation of action potentials. In culture, NGF rapidly potentiates the activity of TRPV1 channels in DRG neurons treated with capsaicin. Accordingly, NGF-induced hyperalgesia is inhibited by a PKC-3-selective peptide inhibitor.
Retrograde NGF signalling from the peripheral terminals to the cell bodies of nociceptive neurons enhances the expression of several proteins that further sensitize these neurons and facilitate activation of second-order neurons in the CNS. These include viz., (i) substance P (which acts at central synapses to increase the activity of second-order nociceptive neurons) (ii) the Nav1.8 NaC channel (which is expressed exclusively by nociceptors and is implicated in NGF-induced hyperalgesia) (iii) the acid-sensing ion channel 3 (which is activated by protons in ischaemic and inflammatory pain) (iv) the TRPV1 channel; and (v) BDNF (which is transported to central synapses where it increases nociceptive spinal-reflex excitability). Retrograde signalling by NGF also increases the anterograde transport of TRPV1 from the cell body to the peripheral terminals of nociceptors.
Exposure of isolated mast cells to NGF and lyso phosphatidyl serine (a molecule on the surface of activated platelets), but not to either factor alone, induces the release of 5-hydroxytryptamine (5-HT). This indicates that NGF sensitizes mast cells under conditions of tissue injury and inflammation. In addition to 5-HT, activated mast cells release other pain mediators such as prostaglandins, bradykinin, histamine, ATP, HC and NGF itself, which stimulate nociceptor terminals and potentiates the pain response. This positive-feedback loop is likely to contribute to the pain associated with acute and chronic inflammatory processes.
Conclusion
One interesting outcome of this research has been our increasing understanding of the role of neurotrophins, specifically NGF. Until recently, NGF was thought to be active in development, largely in promoting neurite extension and survival of neurons. With regard to the latter function, it had been demonstrated that NGF is up-regulated in the target at about the time of arrival of the axons in the periphery, when its role in survival begins.
The studies outlined here indicate a more extensive role for NGF by demonstrating that it is up-regulated in adults in response to nociceptive stimuli and is crucial in eliciting the hyperalgesia associated with inflammation.
Interest in a possible therapeutic role for NGF began with a consideration of its potential for arresting or reversing neural damage in degenerating diseases. Unfortunately, its use is complicated by its hyperalgesic potency. However, the understanding that the effects of exogenously administered NGF represent an exaggeration of a normal pathophysiological action affords a new perspective on how the manipulation of NGF might be used clinically to attenuate inflammatory hyperalgesia. Recognition of the dominant role of NGF as a pain mediator in adults provides an unexpected and attractive opportunity to develop a novel class of pain therapeutics. NGF expression and signalling are elevated in injured and inflamed tissue, and in various acute and chronic pain states. Stimulation of NGF receptors on nociceptive sensory neurons initiates and potentiates pain signalling by multiple interactive mechanisms. Blocking the action of NGF provides highly effective pain relief in many animal models of acute and chronic pain. Growing evidence indicates that selective antagonists of NGF provide effective pain relief without the side-effects that are typical of NSAID and opiate drugs and, because of their distinct mechanism of action, selective NGF antagonists might satisfy unmet medical needs in pain relief.
The authors are AN Nagappa, Prakash Meet, Parvataneni Haritha with Pharmacy Practice, MCOPS, Manipal, Karnataka and Pharmacy Group, BITS-Pilani. email anantha1232000@gmail.com