Nociceptors, or pain-sensitive neurones, are unlike other sensory receptors in that their sensitivity increases in response to prolonged or intense painful stimulation. The process of sensitization has obvious protective value for the organism, but can also cause severe disability in chronic inflammatory conditions, such as arthritis. We have investigated some of the mechanisms underlying both short-term sensitization, in response to inflammatory mediators, and long-term sensitization, in which regulation of gene expression may play an important role.
Heat stimuli above about 45°C gate specific heat-sensitive ion channels in the membrane of cultured nociceptive neurones from rat, and the current is sensitized by the potent inflammatory mediator bradykinin (Cesare & McNaughton, 1996). The epsilon isoform of protein kinase C is an essential element of the sensitization pathway (Cesare et al. 1999). Capsaicin, the active extract of chilli peppers, gates a similar current, which is also sensitized by PKC activation. Lowering calcium also has a sensitizing effect. We interpret these results as showing that the heat-sensitive ion channel is sensitized by phosphorylation by PKC epsilon, and that this process is reversed by dephosphorylation by the calcium-sensitive phosphatase calcineurin (Vellani et al. 2001). We also obtained similar results in HEK293 cells expressing the vanilloid receptor, VR1, a molecular substrate for heat and capsaicin detection (Caterina et al. 1997).
Nerve growth factor (NGF) is a potent inflammatory mediator that has both a short-term and a long-term sensitizing effect in vivo (Lewin et al. 1993). We have reproduced the short-term effect of NGF in vitro by examining the sensitizing effect of NGF on the calcium increase induced by a brief application of capsaicin to cultured neonatal mouse sensory neurones. Around 40 % of capsaicin-responsive neurones showed an enhancement of the response to capsaicin on application of NGF. This sensitization was not abolished by inhibitors of the MAP kinase pathway nor of phospholipase C, but was completely blocked by wortmannin, a potent inhibitor of PI3 kinase, showing that it is this last pathway which is responsible for short-term sensitization. The sensitization was also blocked by inhibitors of PKC and of CAM kinase, though the order in which these kinases are activated in the events downstream of PI3 kinase has not as yet been determined.
The long-term effects of NGF in upregulating expression of receptors for bradykinin have also been investigated in cultured adult mouse dorsal root ganglia (DRG). In situ hybridisation demonstrates that B2 mRNA expression is upregulated in small-diameter neurones in response to NGF. Nerve injury in vivo also upregulates B2 expression. The effect of nerve injury can be reproduced in vitro by exposing DRG to a medium conditioned by prior culture with lesioned nerve. The conditioned medium causes B2 upregulation in small-diameter neurones, and NGF is the main factor responsible because the effect can be largely blocked with an antibody to NGF.This work was supported by the BBSRC, the MRC and The Wellcome Trust.
- Caterina, M.J., Schumacher, M.A., Tominaga, M., Rosen, T.A., Levine, J.D. & Julius, D. (1997). Nature 389, 816-824.
Cesare, P., Dekker, L.V., Sardini, A., Parker, P.J. & McNaughton, P.A. (1999). Neuron 23, 617-624.
Cesare, P. & McNaughton, P.A. (1996). Proc. Natl Acad Sci.USA 93, 15435-15439.
Lewin, G.R., Ritter, A.M. & Mendell, L.M. (1993). J. Neurosci. 13, 2136-2148.
Vellani, V., Mapplebeck, S., Moriondo, A., Davis, J.B. & McNaughton, P.A. (2001). J. Physiol. 534, 813-825. abstract