| Allodynia | |
|---|---|
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| Specialty | Neurology |
Allodynia is a condition in which pain is caused by a stimulus that does not normally elicit pain. [1] For example, sunburn can cause temporary allodynia, so that usually painless stimuli, such as wearing clothing or running cold or warm water over it, can be very painful. It is different from hyperalgesia, an exaggerated response from a normally painful stimulus. The term comes from Ancient Greek άλλος (állos) 'other'and οδύνη (odúnē) 'pain'.
There are different kinds or types of allodynia:
Allodynia is a clinical feature of many painful conditions, such as neuropathies, [4] complex regional pain syndrome, postherpetic neuralgia, fibromyalgia, and migraine. Allodynia may also be caused by some populations of stem cells used to treat nerve damage including spinal cord injury. [5]
Mechanoreceptors can influence the output of nociceptors by making connections with the same interneurons, the activation of which can reduce or eliminate the sensation of pain. Another way to modulate the transmission of pain information is via descending fibers from the brain. These fibers act through different interneurons to block the transmission of information from the nociceptors to secondary neurons. [6]
Both of these mechanisms for pain modulation have been implicated in the pathology of allodynia. Several studies suggest that injury to the spinal cord might lead to loss and re-organization of the nociceptors, mechanoreceptors, and interneurons, leading to the transmission of pain information by mechanoreceptors. [7] [8] A different study reported the appearance of descending fibers at the injury site. [9] All of these changes ultimately affect the circuitry inside the spinal cord, and the altered balance of signals probably leads to the intense sensation of pain associated with allodynia.
Different cell types have also been linked to allodynia. For example, there are reports that microglia in the thalamus might contribute to allodynia by changing the properties of the secondary nociceptors. [10] The same effect is achieved in the spinal cord by the recruitment of immune system cells such as monocytes/macrophages and T lymphocytes. [11]
There is a substantial body of evidence that the so-called sensitization of the central nervous system contributes to the emergence of allodynia. Sensitization refers to the increased response of neurons following repetitive stimulation. In addition to repeated activity, the increased levels of certain compounds lead to sensitization. The work of many researchers has led to the elucidation of pathways that can result in neuronal sensitization both in the thalamus and dorsal horns. Both pathways depend on the production of chemokines and other molecules important in the inflammatory response.[ citation needed ]
An important molecule in the thalamus appears to be cysteine-cysteine chemokine ligand 21 (CCL21). The concentration of this chemokine is increased in the ventral posterolateral nucleus of the thalamus, where secondary nociceptive neurons make connections with other neurons. The source of CCL21 is not exactly known, but two possibilities exist. First, it might be made in primary nociceptive neurons and transported to the thalamus. Most likely, neurons intrinsic to the ventral posterolateral nucleus make at least some of it. [10] In any case, CCL21 binds to C-C chemokine receptor type 7 and chemokine receptor CXCR3 receptors on microglia in the thalamus. [12] The physiologic response to the binding is probably the production of prostaglandin E2 (PGE2) by cyclooxygenase 2 (COX-2). [13] Activated microglia making PGE2 can then sensitize nociceptive neurons as manifested by their lowered threshold to pain. [14]
The mechanism responsible for sensitization of the central nervous system at the spinal cord level differs from that in the thalamus. Tumor necrosis factor-alpha (TNF-alpha) and its receptor are the molecules that seem to be responsible for the sensitization of neurons in the dorsal horns of the spinal cord. Macrophages and lymphocytes infiltrate the spinal cord, for example, because of injury, and release TNF-alpha and other pro-inflammatory molecules. [15] TNF-alpha then binds to the TNF receptors expressed on nociceptors, activating the MAPK/NF-kappa B pathways. This leads to the production of more TNF-alpha, its release, and binding to the receptors on the cells that released it (autocrine signalling). [11] This mechanism also explains the perpetuation of sensitization and, thus, allodynia. TNF-alpha might also increase the number of AMPA receptors and decrease the numbers of GABA receptors on the membrane of nociceptors, both of which could change the nociceptors in a way that allows for their easier activation. [16] Another outcome of the increased TNF-alpha is the release of PGE2, with a mechanism and effect similar to the ones in the thalamus. [17]
Numerous compounds alleviate the pain from allodynia. Some are specific for certain types of allodynia while others are general. They include: [18]
The list of compounds that can be used to treat allodynia is even longer than this. For example, many non-steroidal anti-inflammatory drugs, such as naproxen, can inhibit COX-1 and/or COX-2, thus preventing the sensitization of the central nervous system. Another effect of naproxen is the reduction of the responsiveness of mechano- and thermoreceptors to stimuli. [19]
Other compounds act on molecules important for the transmission of an action potential from one neuron to another. Examples of these include interfering with receptors for neurotransmitters or the enzymes that remove neurotransmitters not bound to receptors.
Endocannabinoids are molecules that can relieve pain by modulating nociceptive neurons. When anandamide, an endocannabinoid, is released, pain sensation is reduced. Anandamide is later transported back to the neurons releasing it using transporter enzymes on the plasma membrane, eventually disinhibiting pain perception. However, this re-uptake can be blocked by AM404, elongating the duration of pain inhibition. [20]
In physiology, nociception, also nocioception; from Latin nocere 'to harm/hurt') is the sensory nervous system's process of encoding noxious stimuli. It deals with a series of events and processes required for an organism to receive a painful stimulus, convert it to a molecular signal, and recognize and characterize the signal to trigger an appropriate defensive response.
The grey columns are three regions of the somewhat ridge-shaped mass of grey matter in the spinal cord. These regions present as three columns: the anterior grey column, the posterior grey column, and the lateral grey column, all of which are visible in cross-section of the spinal cord.
A nociceptor is a sensory neuron that responds to damaging or potentially damaging stimuli by sending "possible threat" signals to the spinal cord and the brain. The brain creates the sensation of pain to direct attention to the body part, so the threat can be mitigated; this process is called nociception.
Sensory neurons, also known as afferent neurons, are neurons in the nervous system, that convert a specific type of stimulus, via their receptors, into action potentials or graded receptor potentials. This process is called sensory transduction. The cell bodies of the sensory neurons are located in the dorsal root ganglia of the spinal cord.
Hyperalgesia is an abnormally increased sensitivity to pain, which may be caused by damage to nociceptors or peripheral nerves and can cause hypersensitivity to stimulus. Prostaglandins E and F are largely responsible for sensitizing the nociceptors. Temporary increased sensitivity to pain also occurs as part of sickness behavior, the evolved response to infection.
A dorsal root ganglion is a cluster of neurons in a dorsal root of a spinal nerve. The cell bodies of sensory neurons known as first-order neurons are located in the dorsal root ganglia.
Neuropathic pain is pain caused by a lesion or disease of the somatosensory nervous system. Neuropathic pain may be associated with abnormal sensations called dysesthesia or pain from normally non-painful stimuli (allodynia). It may have continuous and/or episodic (paroxysmal) components. The latter resemble stabbings or electric shocks. Common qualities include burning or coldness, "pins and needles" sensations, numbness and itching.
The neuroimmune system is a system of structures and processes involving the biochemical and electrophysiological interactions between the nervous system and immune system which protect neurons from pathogens. It serves to protect neurons against disease by maintaining selectively permeable barriers, mediating neuroinflammation and wound healing in damaged neurons, and mobilizing host defenses against pathogens.
The chemokine ligand 1 (CXCL1) is a small peptide belonging to the CXC chemokine family that acts as a chemoattractant for several immune cells, especially neutrophils or other non-hematopoietic cells to the site of injury or infection and plays an important role in regulation of immune and inflammatory responses. It was previously called GRO1 oncogene, GROα, neutrophil-activating protein 3 (NAP-3) and melanoma growth stimulating activity, alpha (MGSA-α). CXCL1 was first cloned from a cDNA library of genes induced by platelet-derived growth factor (PDGF) stimulation of BALB/c-3T3 murine embryonic fibroblasts and named "KC" for its location in the nitrocellulose colony hybridization assay. This designation is sometimes erroneously believed to be an acronym and defined as "keratinocytes-derived chemokine". Rat CXCL1 was first reported when NRK-52E cells were stimulated with interleukin-1β (IL-1β) and lipopolysaccharide (LPS) to generate a cytokine that was chemotactic for rat neutrophils, cytokine-induced neutrophil chemoattractant (CINC). In humans, this protein is encoded by the gene CXCL1 and is located on human chromosome 4 among genes for other CXC chemokines.
Quisqualic acid is an agonist of the AMPA, kainate, and group I metabotropic glutamate receptors. It is one of the most potent AMPA receptor agonists known. It causes excitotoxicity and is used in neuroscience to selectively destroy neurons in the brain or spinal cord. Quisqualic acid occurs naturally in the seeds of Quisqualis species.
Group C nerve fibers are one of three classes of nerve fiber in the central nervous system (CNS) and peripheral nervous system (PNS). The C group fibers are unmyelinated and have a small diameter and low conduction velocity, whereas Groups A and B are myelinated. Group C fibers include postganglionic fibers in the autonomic nervous system (ANS), and nerve fibers at the dorsal roots. These fibers carry sensory information.
Transient receptor potential cation channel, subfamily A, member 1, also known as transient receptor potential ankyrin 1, TRPA1, or The Mustard and Wasabi Receptor, is a protein that in humans is encoded by the TRPA1 gene.
Nav1.8 is a sodium ion channel subtype that in humans is encoded by the SCN10A gene.
Mechanosensation is the transduction of mechanical stimuli into neural signals. Mechanosensation provides the basis for the senses of light touch, hearing, proprioception, and pain. Mechanoreceptors found in the skin, called cutaneous mechanoreceptors, are responsible for the sense of touch. Tiny cells in the inner ear, called hair cells, are responsible for hearing and balance. States of neuropathic pain, such as hyperalgesia and allodynia, are also directly related to mechanosensation. A wide array of elements are involved in the process of mechanosensation, many of which are still not fully understood.
The rostral ventromedial medulla (RVM), or ventromedial nucleus of the spinal cord, is a group of neurons located close to the midline on the floor of the medulla oblongata. The rostral ventromedial medulla sends descending inhibitory and excitatory fibers to the dorsal horn spinal cord neurons. There are 3 categories of neurons in the RVM: on-cells, off-cells, and neutral cells. They are characterized by their response to nociceptive input. Off-cells show a transitory decrease in firing rate right before a nociceptive reflex, and are theorized to be inhibitory. Activation of off-cells, either by morphine or by any other means, results in antinociception. On-cells show a burst of activity immediately preceding nociceptive input, and are theorized to be contributing to the excitatory drive. Neutral cells show no response to nociceptive input.
Group A nerve fibers are one of the three classes of nerve fiber as generally classified by Erlanger and Gasser. The other two classes are the group B nerve fibers, and the group C nerve fibers. Group A are heavily myelinated, group B are moderately myelinated, and group C are unmyelinated.
Microglia are the primary immune cells of the central nervous system, similar to peripheral macrophages. They respond to pathogens and injury by changing morphology and migrating to the site of infection/injury, where they destroy pathogens and remove damaged cells.
Lorne Mendell is a neurobiologist currently employed as a distinguished professor in the department of neurobiology and behavior at Stony Brook University in New York. His research focuses primarily on neurotrophins in neonatal and adult mammals, and on the neuroplasticity of the mammalian spinal cord. His research interests lie in other areas including pain, nerve wind-up, and specifically the neurotrophin NT-3. He has contributed to the growing pool of knowledge of axonal development and regeneration of immature and mature neurons. He has been a part of the search for novel treatments for spinal cord injuries and continues to study neurotrophins to determine their effects on neuronal plasticity. He served a term as president of the Society of Neuroscience during 1997–1998.
Sandra M. Garraway is a Canadian-American neuroscientist and assistant professor of physiology in the Department of Physiology at Emory University School of Medicine in Atlanta, Georgia. Garraway is the director of the Emory Multiplex Immunoassay Core (EMIC) where she assists researchers from both academia and industry to perform, analyze, and interpret their multiplexed immunoassays. Garraway studies the neural mechanisms of spinal nociceptive pain after spinal cord injury and as a postdoctoral researcher she discovered roles for both BDNF and ERK2 in pain sensitization and developed novel siRNA technology to inhibit ERK2 as a treatment for pain.
Epigenetics of chronic pain is the study of how epigenetic modifications of genes affect the development and maintenance of chronic pain. Chromatin modifications have been found to affect neural function, such as synaptic plasticity and memory formation, which are important mechanisms of chronic pain. In 2019, 20% of adults dealt with chronic pain. Epigenetics can provide a new perspective on the biological mechanisms and potential treatments of chronic pain.