Molecular mechanisms of peripheral sensitisation, physiological pain and clinical pain
Dr Adam Woo
Research Pain Fellow, UCL Hospitals
This refers to the potential of increased pain perception to a given stimulus after an initial thermal or mechnical injury, for example.
There are a few important receptors involved in the transduction of a pain signal. They are the ion channels transient receptor potential family (especially TRPV1, heat and capsaicin sensitive) and tetrodotoxin-resistant sodium channel (Nav1.8). They are both found on C fibers which are important in the sensitisation process. Acid sensing ion channel (ASIC) and P2X receptors activated by ATP are relevant but probably play a smaller role. Many chemicals cause peripheral sensitisation via these receptors.
Bradykinin is a 9-amino acid peptide chain formed by proteolytic cleavage of its kininogen precursor, high-molecular-weight kininogen. Levels are increased with noxious stimulation. It activates G protein coupled receptors (BK2) which then activates protein kinase C. This increases activity at the ion channel TRPV1 in a G-protein coupled manner. Bradykinin also sensitises TRPV1 receptors through a protein kinase C independent manner, which is thought to be caused by release of PtdIns(4,5)P2 from the TRPV1 receptor. Bradykinin also contributes to vasodilation and increased vascular permeability of the injury site.
Prostaglandins are autocrine and paracrine lipid mediators that act upon neurons to sensitise them. They are formed by the action of cyclooxygenase on arachidonic acid. Prostaglandins, especially PGE2 and PGI (via their receptors EP1 and IP respectively) are thought to activate protein kinases C and A. They can both increase activity at the TRPV1 and Nav1.8 receptors respectively (Moruyama 2005). Hence nociception transmission is enhanced. Interestingly PGE2 may also be involved in the enhancement of activity at the central terminals of primary afferents at he spinal cord.
Substance P is an important neuropeptide in primary afferents, especially in a subset of C fibers. It is released by sensory nerve endings locally by noxious stimuli and also via the axon reflex. It activates the neurokinin-1 receptor. It causes increased vascular permeability, vasodilation and increased synthesis of prostaglandins. In addition calcitonin gene related peptide is another co-transmitter released together with substance P with similar actions.
A drop in pH is an important mediator of sensitisation. The threshold for TRPV1 opening is lowered with this drop in pH, so that even at room temperature activation of the receptor occurs when pH is below 6 (Tominaga cited in Julius and Besbaum 2001). Protons also activate the ASIC directly. They may also produce sustained general depolarisation of sensory neurons by directly activating a non selective cationic current (Bevan cited in Julius and Besbaum 2001).
Neurotrophic factors are produced in peripheral targets of nociceptors such as fibroblasts and mast cells and is overexpressed in tissue injury. Although implicated more in chronic pain, inflammation and tissue damage increase their expression in the acute phase promoting thermal sensitivity. Nerve growth factor is such an example, which sensitise nociceptors to substance P and other noxious stimuli. They also exert long term gene expression changes in nerve cells.
As a response to inflammatory reaction of tissue damage, mast cells are attracted to the site. When activated they release histamines which, via H1 receptors, are important in smooth muscle contraction, vasodilation, pain and separation of endothelial cells (hives). Mast cells also release prostaglandins. Increased blood flow caused by this and other agents encourages immune cells to reach the injury site.
Serotonin is another important mediator released from mast cells and platelets. Serotonin (5-HT), is known to be an algogen capable of directly (via ion channels) or indirectly (via protein phosphorylation) activating nociceptive afferent fibers.
Silent nociceptors, found in both visceral and peripheral targets of afferents neurons are activated by noxious stimuli such as tissue inflammation and are normally dormant and not reactive to innocuous stimuli. They can be recruited as part of tissue injury and contribute to hyperalgesia.
All the above are mediators which take variable time frames to occur. Hence, after an initial period post-injury, pain experience often becomes worse with further stimuli, even innocuous ones. This hyperalgesia and allodynia is commonly seen. The inflamatory ’soup’ all conspire together, with one facilitating the other, to cause peripheral sensitisation.
 Julius D & Besbaum A 2001. Molecular Mechanisms of nociception. Nature, 413, 203-210
 Moriyama T 2005. Sensitization of TRPV1 by EP1 and IP reveals peripheral nociceptive mechanism of prostaglandins. Molecular Pain 1:3 doi:10.1186/1744-8069-1-3
The two main types of primary afferent neurons which respond to pain are the Aδ and C type fibres. These transmit thermal, mechanical and chemical induced pain. Aβ fibres play a smaller part in mechanical and thermal type nociception.
In thermal pain (>45 and <5oC) thermal sensitive fibers alter firing thresholds and are interpreted as noxious (Nociceptor 2007). For mechanical nociception, the perception involves integrated activity of several afferents in the same region involving chemical mediators like 5-HT and ATP. In other chemical nociception, tissue damage or ischaemia release hydrogen ions, histamine and bradykinin which activate C-fibres mainly but also some Aδ fibers. Activation of primary afferents by these stimuli through receptors like TRPV1 and Nav1.8 initiate action potentials which then travel towards the central terminals.
The central terminations are on the dorsal horns neurons of the spinal cord. Dorsal horn neurons are organized into different laminae. Lamina I neurons receive inputs from Aδ and C fibres, and have little projections elsewhere, due to restricted dendritic trees. The neurons here have been characterized by their specific physiological responses. There are ‘nociceptive’ cells which correspond to the three main modalities of nociception. They have been identified to correspond to the first sharp pain and also the second burning indistinct phases of pain. Lamina II (substantia gelatinosa) also receive inputs from Aδ and C fibres. Neurotransmitters are released when the action potentials arrive at the laminae. Glutamate is the main excitatory neurotransmitter but Substance P and CGRP are also important. These dorsal horn neurons then send projections to the central nervous system for pain transmission, including the spinothalamic tract, spinomesenchephalic and spinoparabrachial tracts. The spinothalamic tract project to the thalamus which then sends projections to the somatosensory cortex for pain and temperature sensations.
Besides lamina I, lamina V in the dorsal horn is thought to be important for integration of pain information. They receive C fibres directly, interneurons from other laminae and larger myelinated fibres. The neurons here have been termed wide dynamic range neurons as they respond in a graded manner to innocuous and noxious pain. They are thought of as an integration centre for stimuli within the spinal cord and is dynamic in function (Craig 2003).
Clinical pain, hence, depends partly on the interaction of these neurons. The landmark gate theory alludes to convergent mechanisms in this area. For example, larger fibers, when activated by touch or vibration can activate inhibitory interneurons resulting in decreased signal transmission to higher centres (TENS, rubbing). In addition to peripheral mechanisms, clinical or ‘felt’ pain phenomena of hyperalgesia and allodynia are also the result of central sensitization. Hyperexcitability of dorsal horn neurons (possible WDR neurons in lamina V) can account for mechanical hyperalgesia. Sproutings of new terminals from Aβ fibers to lamina II in the dorsal horn can also account for chronic allodynia. Other mechanisms accounting for central sensitisation include receptor changes centrally, neuronal cell death and second messenger signalling cascade changes.
Ascending tracts to the brain integrate sensory and affective-motivational meaning of noxious stimuli. From these centres, a descending modulation is initiated via centres like the periaqueductal and nucleus raphe magnus. These are known to be important in decreasing pain perception by sending projections to the dorsal horns. Via opioid, 5-HT, noradrenaline and GABA as mediators, primary afferent neuron signals can be attenuated via synapses within the dorsal horn (Fields 2004). Mood, distraction and cognitions may thus augment pain perception via these descending paths.
 Craig AD 2003. Pain mechanisms: labeled line vs convergence in central processing. Annual Review of Neuroscience, 26, 1-30
 Fields H 2004. State dependent opioid control of pain. Nat Rev Neurosci. 5: 565-575