The cAMP Transduction Cascade Mediates the Prostaglandin E₂Enhancement of the Capsaicin-Elicited Current in Rat Sensory Neurons: Whole-Cell and Single-Channel Studies

Abstract

Treatment with proinflammatory prostaglandin E2 (PGE2) produced a transient sensitization of whole-cell currents elicited by the vanilloid capsaicin. The intracellular signaling pathways that mediate the initiation of this PGE2-induced sensitization of the capsaicin-elicited current in rat sensory neurons are not well established. Treatment with either forskolin (100 nM to 10 microM) or membrane-permeant analogs of cAMP, 8-bromo-cAMP (8-Br-cAMP) and chlorphenylthio-cAMP (10 microM to 1 mM), transiently sensitized neuronal responses elicited by capsaicin in a manner analogous to that produced by PGE2. The duration of sensitization was lengthened with increasing concentrations of forskolin; however, higher concentrations of 8-Br-cAMP or chlorphenylthio-cAMP led to a shortening of sensitization. The inactive analog of forskolin, dideoxy-forskolin, had no effect on capsaicin responses. Inclusion of the inhibitor of protein kinase A in the recording pipette completely suppressed the sensitization produced by PGE2 or forskolin. In recordings from membrane patches in the cell-attached configuration, the bath application of capsaicin evoked single-channel currents in which the level of channel activity was concentration-dependent and had an EC50 of 1.4 microM. These single-channel currents evoked by capsaicin exhibited an apparent reversal potential of +4 mV and were blocked by the capsaicin antagonist capsazepine. Exposure of the sensory neuron to either PGE2 or forskolin produced a large and transient increase in the mean channel activity (NPo) elicited by capsaicin, although the unitary conductance remained unaltered. Taken together, these observations suggest that modulation of the capsaicin-gated channel by the cAMP-protein kinase A signaling pathway enhanced the gating of these channels and consequently resulted in the sensitization of the whole-cell currents.

Downregulation of Transient Receptor Potential Melastatin 8 by Protein Kinase C-Mediated Dephosphorylation

“…Protein kinase C (PKC) is known to modulate activity of several ion channels. Potentiation of TRPV1 by PKC and agonists like bradykinin (BK) has been studied extensively (Lopshire and Nicol, 1998;Cesare et al, 1999a,b;Khasar et al, 1999;Premkumar and Ahern, 2000;Numazaki et al, 2002). Here, we demonstrate a functional downregulation of TRPM8 by activation of PKC resulting in inhibition of TRPM8-mediated channel activity and synaptic transmission in contrast to TRPV1, which is upregulated.…”

Phosphorylation of Vanilloid Receptor 1 by Ca2+/Calmodulin-dependent Kinase II Regulates Its Vanilloid Binding

“…The sensitization of VR1 by PKC requires the phosphorylation of VR1 at both Ser-502 and Ser-800 because mutations at both of these putative PKC sites blocks VR1 sensitization (31). Similarly, prostaglandin is also known to sensitize i cap in dorsal root ganglion neurons via the PKA pathway (38). The sensitization of VR1 by PKA requires phosphorylation of all three putative PKA consensus sites, namely, Thr-144, Thr-370, and Ser-502, because mutation of any of these three PKA sites blocks its sensitization to heat (39).…”

“…PKA is known to sensitize VR1, which augments I cap in response to repeated applications of capsaicin (38,39). VR1 has three putative phosphorylation motifs for PKA, namely, Thr-144, Thr-370, and Ser-502, of which Thr-370 and Ser-502 are common to CaMKII sites (1).…”

Interaction between vanilloid receptors and purinergic metabotropic receptors: Pain perception and beyond

“…One way to overcome this would be to sensitize the receptor by phosphorylation. Previous studies have shown that the VR function could be modulated by phosphorylation or dephosphorylation (15,16,26). The findings by Tominaga et al (13) are of significance in that a similar sensitization of VRs could be achieved by stimulating metabotropic receptors that leads to phosphorylation of VRs.…”