Sensory C-fibres

Nociceptin and orphanin are synonyms for the peptide that acts at an opioid-like receptor. Nociceptin may act by inhibiting tachykinin release from sensory C-fibres, and a clinical trial has started to test its effects on cough. 

The release of some peptides may differ from that of other transmitters, depending on the firing rate of the neurons. The large vesicles needed to store a peptide may need a greater rate of depolarisation for membrane fusion and release of the contents. In the salivary gland the release of vasoactive intestinal polypeptide requires high-frequency stimulation whereas acetylcholine is released by all stimuli. Due to the complexities and problems of access to CNS synapses it is not known if the same occurs here but there is no reason why this should not. In sensory C-fibres a prolonged stimulus appears to be a prerequisite for the release of substance P. 

At present, BP 2.94 (36) is under clinical development in Phase II trials for the treatment of asthma, pneumoallergic diseases, and others. Preclinical studies in rodents clearly displayed anti-inflammatory as well as antinociceptive activity of BP 2.94 (36) given orally at low doses. These effects are mediated by inhibitory H3 receptors located on sensory C-fibres in several different tissues. In particular, capsaicin-induced plasma protein extravasation was dose-dependently inhibited in airways, digestive tract, skin, conjunctiva, urinaiy bladder, nasal mucosa, and dura mater of the rat. In the p-phenylbenzoquinone-induced writhing test in mice, BP 2.94 (36) had a pronounced antinociceptive activity similar to that of acetylsalicylic acid. This effect was significantly abolished by the H3 receptor antagonist thioperamide but not by naloxone. Furthermore, BP 2.94 (36) reduced zymosan-induced edema. This antiinflammatory effect was also abolished by thioperamide [6]. 

The Y2 receptor is the predominant NPY receptor type in the rat brain and such receptors are particularly numerous in the hippocampus (Dumont et al., 1992). Many Y2 receptor-mediated effects have been linked to suppression of transmitter release. For instance, the release of glutamate from terminals synapsing on to rat hippocampal CA1 neurons is inhibited by activation of Y2 receptors. Peripheral Y2 receptors are associated with suppression of transmitter release from sympathetic, parasympathetic and sensory C-fibres. Table 3 summarizes these effects. 

Sensory C-fibres Rat intestine Canine lipocytes Certain blood vessels... 

Sensory nerves are peripheral nerves involved in the detection of exogenous and endogenous stimuli for transmission into the CNS. They can be subdivided into RARs and C-fibres according to their channel characteristics. 

The afferent fibres differ in their conduction velocity and degree of myelination, and can be distinguished by their diameter. The large diameter A S-fibres are myelinated by Schwann cells and hence have a fast conduction velocity. This group of nerve fibres innervates receptors in the dermis and is involved in the transmission of low-threshold, non-noxious information, such as touch. The A5-fibre is less densely myelinated and conveys both non-noxious and noxious sensory information. The unmyelinated C-fibre conveys high-threshold noxious inputs and has the slowest conduction velocity of all three fibre types. 

The arrival of action potentials in the dorsal horn of the spinal cord, carrying the sensory information either from nociceptors in inflammation or generated both from nociceptors and intrinsically after nerve damage, produces a complex response to pain. Densely packed neurons, containing most of the channels, transmitters and receptors found anywhere in the CNS, are present in the zones where the C-fibres terminate... 

Akopian, A.N., Sivilotti, L., Wood, J.N. A tetrodotoxin-resistant sodium channel expressed by C-fibre associated sensory neurons, Nature 1996, 379, 257-262. 

Thi ,DPhe BK [Thi ,DPhe ]-bradykinin. bradykinin-potentiating peptide teprotide. bradykinin potentiator B teprotide. BRADYKININ RECEPTOR AGONISTS act at sites recognizing members and derivatives of the bradykinin family of hormone peptides - kinins - of which bradykinin (BK) and kallidin (lysyl-bradykinin Lys-BK KD) are the main mammalian members. The bradykinin family is distinct from the tachykinin family of peptides, though both have profound hypotensive actions and contract many intestinal and other smooth muscles. Historically, it was noted that the former action was relatively slow-developing, hence the name bradykinin. Notable actions of bradykinin and kallidin are to dilate blood vessels and increase their permeability to plasma proteins, and to stimulate sensory nerve C-fibres. These actions are pro-inflammatory, and reflect the fact that the kinin-formation system is activated in inflammation, and enzymes (kallikreins) form the kinins from blood-borne or tissue precursors (kininogens) on injurious insult. 

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