Pharmacology of inflammatory pain: local alteration in receptors and mediators.

Background: Inflammation is commonly associated with hyperalgesia. Ideally, this change should abate once inflammation is resolved, but this is not necessarily the case because phenotypic changes in the tissue can persist, as appears to be the case in post-infectious irritable bowel syndrome. Basically, all primary afferent neurons supplying the gut can be sensitized in response to pro-inflammatory mediators, and the mechanisms whereby hypersensitivity is initiated and maintained are, thus, of prime therapeutic interest.

Experimental And Clinical Findings: There is a multitude of molecular nocisensors that can be responsible for the hypersensitivity of afferent neurons. These entities include: (i) receptors and sensors at the peripheral terminals of afferent neurons that are relevant to stimulus transduction, (ii) ion channels that govern the excitability and conduction properties of afferent neurons, and (iii) transmitters and transmitter receptors that mediate communication between primary afferents and second-order neurons in the spinal cord and brainstem. Persistent increases in the sensory gain may result from changes in the expression of transmitters, receptors or ion channels; changes in the subunit composition and biophysical properties of receptors and ion channels; or changes in the structure, connectivity and survival of afferent neurons. Particular therapeutic potential is attributed to targets that are selectively expressed by afferent neurons and whose number and function are altered in abdominal hypersensitivity.

Conclusion: Emerging targets of therapeutic relevance include distinct members of the transient receptor potential (TRP) channel family (TRPV1, TRPV4, TRPA1), acid-sensing ion channels, protease-activated receptors, corticotropin-releasing factor receptors and sensory neuron-specific sodium channels.