The Role of Calcium in the Desensitization of Capsaicin Responses in Rat Dorsal Root Ganglion Neurons

Abstract

Capsaicin (Cap) is a pungent extract of the Capsicum pepper family, which activates nociceptive primary sensory neurons. Inward current and membrane potential responses of cultured neonatal rat dorsal root ganglion neurons to capsaicin were examined using whole-cell and perforated patch recording methods. The responses exhibited strong desensitization operationally classified as acute (diminished response during constant Cap exposure) and tachyphylaxis (diminished response to successive applications of Cap). Both acute desensitization and tachyphylaxis were greatly diminished by reductions in external Ca2+ concentration. Furthermore, chelation of intracellular Ca2+ by addition of either EGTA or bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid to the patch pipette attenuated both forms of desensitization even in normal Ca2+. Release of intracellular Ca2+ by caffeine triggered acute desensitization in the absence of extracellular Ca2+, and barium was found to effectively substitute for calcium in supporting desensitization. Cap activated inward current at an ED50 of 728 nM, exhibiting cooperativity (Hill coefficient, 2.2); however, both forms of desensitization were only weakly dependent on [Cap], suggesting a dissociation between activation of Cap-sensitive channels and desensitization. Removal of ATP and GTP from the intracellular solutions resulted in nearly complete tachyphylaxis even with intracellular Ca2+ buffered to low levels, whereas changes in nucleotide levels did not significantly alter the acute form of desensitization. These data suggest a key role for intracellular Ca2+ in desensitization of Cap responses, perhaps through Ca2+-dependent dephosphorylation at a locus that normally sustains Cap responsiveness via ATP-dependent phosphorylation. It also seems that the signaling mechanisms underlying the two forms of desensitization are not identical in detail.

Caged Vanilloid Ligands for Activation of TRPV1 Receptors by 1- and 2-Photon Excitation

“…These activation kinetics are very fast in light of earlier reports that extracellularly applied capsaicin could evoke inward currents in neurons, but with latency-to-activation of at least a few hundred milliseconds (40,41). The latency has been attributed to the need for extracellular capsaicin to cross the plasma membrane in order to access the TRPV1 ligand-binding site, which is intracellular (42).…”

Effects of piperine, the pungent component of black pepper, at the human vanilloid receptor (TRPV1)

“…As discussed above, it is possible that differences in the native TRPV1 receptor of TG neurones may underlie these differences, although they may also reflect differences in the recording conditions employed. We chose to replace extracellular Ca 2 þ with Ba 2 þ so as to reduce the Ca 2 þ -dependent component of desensitisation (Koplas et al, 1997), and gain a more accurate insight into the agonist-induced desensitisation profile obtained. In terms of its physiology, it is interesting to note that the greater degree of desensitisation produced by piperine may help shape its apparent pungency and will contribute to its long lasting effects versus TRPV1 in the body.…”

Modulation of I A Currents by Capsaicin in Rat Trigeminal Ganglion Neurons

“…In experiments where the effects of capsaicin on I A currents were investigated in CS neurons, our goal was to have calcium enter the cell where it can desensitize vanilloid receptors (Koplas et al 1997) to an extent that the magnitude of this current is sufficiently small so that its contribution to the leak current can be corrected. To ensure that calcium enters the cell during capsaicin application, TG neurons were held at -60 mV.…”