Nicolson, Trudy Agnes; (2000) Modulation of intracellular calcium by inflammatory mediators in sensory neurones. Doctoral thesis (Ph.D), UCL (University College London).

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Abstract

During inflammation and tissue damage, there are many circulating chemical mediators which contribute to the inflammatory state. Some of these, such as bradykinin, ATP, 5-HT and protons, directly activate sensory neurones by evoking a rise in [Ca2+]i. Histamine is a potent inflammatory mediator and is known to activate sensory neurones to produce the sensation of itch but little is known about its effects on [Ca2+]i in sensory neurones. In this thesis, I have shown that histamine evokes a dose-dependent rise in [Ca2+]i in a sub-population of cultured sensory neurones. This [Ca2+]i response is mediated by the activation of the H1 receptor sub-type as It was blocked by the H1 antagonist, mepyramine, but not by an H2 or an H3 antagonist. This data suggests that the H1 receptor in sensory neurones is coupled to the phospholipase C (PLC) / inositol trisphosphate (IP3) pathway as histamine stimulated a rise in IP3 formation which was prevented by mepyramine. Furthermore, the [Ca2+]i, response was abolished by the PLC Inhibitor, U73122. Other chemical mediators do not directly activate sensory neurones themselves but enhance the sensitivity of these neurones to other Inflammatory mediators. The prostanoids, in particular prostaglandin E2 (PGE2), has been shown to sensitise sensory neurones to bradykinin and is thought to do so via a cyclic AMP (cAMP)-dependent mechanism. I have therefore investigated the interaction between PGE2 and histamine in these neurones. PGE2, I2 and D2, but not PGF2α, enhanced the sensitivity of sensory neurones by Increasing the percentage of neurones that responded to low concentrations of histamine with a rise In [Ca2+]i. Pre-treating the neurones with agents that elevate intracellular cAMP levels such as forskolin and 8-bromo-cAMP produced a similar increase in the number of cells responding to histamine. The PGE2-induced sensitisation to histamine was inhibited when adenylyl cyclase activity was blocked by tetrahydro-furyl-adenine (THFA). This data suggests that the prostanoid-induced sensitisation of sensory neurones is mediated by a cAMP-dependent process. The sensitised Ca2+ response obtained to histamine appears to be due to enhanced Ca2+ influx as removing extracellular Ca2+ or blocking voltage gated Ca2+ channels with lanthanum significantly reduced the percentage of cells responding. When the cells were pre-treated with U73122 in the absence of extracellular Ca2+, no cells responded to histamine. Therefore, I propose that the [Ca2+]i response obtained to low concentrations of histamine alone is mediated by Ca2+ mobilisation from intracellular IP3 sensitive stores, whereas the sensitised Ca2+ response involves a Ca2+ entry pathway.

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