Upregulation of bradykinin B2 receptor expression by neurotrophic factors and nerve injury in mouse sensory neurons

Sensitization to bradykinin B1 and B2 receptor activation in UV-B irradiated human skin

Bradykinin B1 and B2 receptors contribute to nociceptor sensitization under inflammatory conditions. Here, we examined the vascular inflammatory responses and nociceptive effects resulting from activation of B1 and B2 receptors in healthy and UV-B irradiated skin in human volunteers. The B1 receptor agonist des-Arg(10)-Kallidin (10(-6)-10(-3)M) and the B2 receptor agonist bradykinin (10(-9)-10(-4)M) were administered by dermal microdialysis to the ventral thigh. UV-B irradiation was performed 24 h prior to the experiment with the threefold minimum erythemal dose. Pain sensation perceived during the stimulation with the bradykinin receptor agonists was estimated on a numeric scale. Local and axon reflex-induced vasodilations were recorded by laser Doppler imaging. For protein extravasation, total protein content in the dialysate was assessed as a measure of increased endothelial permeability. In normal skin, both B1 and B2 receptor activation dose-dependently evoked pain, vasodilatation and protein extravasation. In UV-B irradiated skin, pain sensation and axon reflex vasodilatation were enhanced by both B1 and B2 agonists, whereas local vasodilatation was increased only following B1 receptor activation. The UV-B irradiation did not enhance B1 and B2 receptor-induced protein extravasation indicating a differential sensitization of the neuronal, but not the vascular response.

Switching of bradykinin-mediated nociception following partial sciatic nerve injury in mice

Bradykinin (BK) is well known as a potent mediator of pain and hyperalgesia. Using a highly sensitive nociception test, we found that intraplantar (i.pl.) injection of BK produced nociceptive hyper-responses in partial sciatic nerve-injured mice, compared with the control sham-operated animals. By use of selective agonists and antagonists, we revealed that BK nociception in sham-operated mice was mediated through B2 receptor, whereas that in injured mice was mediated through B1 receptor. When we examined the activation of extracellular signal-regulated protein kinase (ERK) in dorsal root ganglion (DRG) neurons upon i.pl. injection of BK, phosphorylated ERK was mainly observed in unmyelinated neurons in sham-operated mice, and in case of nerve-injured mice, ERK was mainly activated in myelinated neurons and satellite cells. The B1 receptor agonist, [Lys-des-Arg(9)]-BK also produced nociceptive response and activated ERK only in nerve-injured mice. BK or B1 agonist-induced activation of ERK in DRG neurons of nerve-injured mice was completely blocked by pretreatment with antisense oligodeoxynucleotide (AS-ODN) for B1 receptor. We found that in sham-operated mice mainly B2 receptors were expressed in unmyelinated DRG neurons with a very little presence of B1 receptor. After nerve injury, B2 receptor expression drastically decreased, whereas B1 receptors were newly expressed mainly in myelinated DRG neurons and satellite cells. Finally, BK nociception in sham-operated mice was blocked by AS-ODN for B2 receptors and that in injured mice by AS-ODN for B1 receptors. Altogether, these findings confirm a switching of receptor and fiber subtype for BK nociception after peripheral nerve injury, which might contribute to the pathobiology of neuropathic pain.

Homeobox gene expression in adult dorsal root ganglia during sciatic nerve regeneration: is regeneration a recapitulation of development?

After damage of the sciatic nerve, a regeneration process is initiated. Neurons in the dorsal root ganglion regrow their axons and functional connections. The molecular mechanisms of this neuronal regenerative process have remained elusive, but a relationship with developmental processes has been conceived. This chapter discusses the applicability of the developmental hypothesis of regeneration to the dorsal root ganglion; this hypothesis states that regeneration of dorsal root ganglion neurons is a recapitulation of development. We present data on changes in gene expression upon sciatic nerve damage, and the expression and function of homeobox genes. This class of transcription factors plays a role in neuronal development. Based on these data, it is concluded that the hypothesis does not hold for dorsal root ganglion neurons, and that regeneration-specific mechanisms exist. Cytokines and the associated Jak/STAT (janus kinase/signal transducer and activator of transcription) signal transduction pathway emerge as constituents of a regeneration-specific mechanism. This mechanism may be the basis of pharmacological strategies to stimulate regeneration.

Signalling pathways involved in the sensitisation of mouse nociceptive neurones by nerve growth factor

Nerve growth factor (NGF) causes a rapid sensitisation of nociceptive sensory neurones to painful thermal stimuli owing to an action on the heat and capsaicin receptor TRPV1 (formerly known as VR1). We have developed a new technique to study this rapid sensitisation of TRPV1 by monitoring the effects of NGF on the increase in intracellular calcium concentration ([Ca2+]i) following exposure to capsaicin. Brief applications of capsaicin caused a rise in [Ca2+]i, and NGF was found to enhance this rise in 37 % of capsaicin-responsive neurones within 2 min. Pathways responsible for transducing the sensitisation of TRPV1 by TrkA, the NGF receptor, were characterised by observing the effects of inhibitors of key members of NGF-activated second messenger signalling cascades. Specific inhibitors of the ras/MEK (mitogen-activated protein and extracellular signal-regulated kinases) pathway and of phospholipase C did not abolish the NGF-induced sensitisation, but wortmannin, a specific inhibitor of phosphatidylinositol-3-kinase (PI3K), totally abolished the effect of NGF. Pharmacological blockade of protein kinase C (PKC) or calcium-calmodulin-dependent protein kinase II (CaMK II) activation also prevented NGF-induced sensitisation, while blockade of protein kinase A (PKA) was without effect. These data indicate that the crucial early pathway activated by NGF involves PI3K, while PKC and CaMK II are also involved, probably at subsequent stages of the NGF-activated signalling pathway.