Rapid, Opioid-sensitive Mechanisms Involved in Transient Receptor Potential Vanilloid 1 Sensitization

Abstract

TRPV1 is a nociceptive, Ca2؉ -selective ion channel involved in the development of several painful conditions. Sensitization of TRPV1 responses by cAMP-dependent PKA crucially contributes to the development of inflammatory hyperalgesia. However, the pathways involved in potentiation of TRPV1 responses by cAMP-dependent PKA remain largely unknown. Using HEK cells stably expressing TRPV1 and the opioid receptor, we demonstrated that treatment with the adenylate cyclase activator forskolin significantly increased the multimeric TRPV1 species. Pretreatment with the opioid receptor agonist morphine reversed this increased TRPV1 multimerization. FRET analysis revealed that treatment with forskolin did not cause multimerization of pre-existing TRPV1 monomers on the plasma membrane and that intracellular pools of TRPV1 exist mostly as monomers in this model. This suggests that increased TRPV1 multimerization occurred from an intracellular store of inactive TRPV1 monomers. Treatment with forskolin also caused an increase in TRPV1 expression on the plasma membrane not resulting from increased TRPV1 expression, and this rapid TRPV1 translocation was inhibited by treatment with morphine. Thus, potentiation of TRPV1 responses by cAMPdependent PKA involves plasma membrane insertion of functional TRPV1 multimers formed from an intracellular store of inactive TRPV1 monomers. This potentiation occurs rapidly and can be dynamically modulated by activation of the opioid receptor under conditions where cAMP levels are raised, such as with inflammation. Increased translocation and multimerization of TRPV1 channels provide a cellular mechanism for finetuning of nociceptive responses that allow for rapid modulation of TRPV1 responses independent of transcriptional changes.The ability to sensitize or desensitize painful stimuli is fundamental for survival. In inflammation, sensitization of peripheral nociception contributes to the development of hyperalgesia. Pro-inflammatory mediators including prostaglandins mediate an increase in cellular cAMP levels, which in turn leads to sensitization of nociception as a result of activation of cAMPdependent protein kinase (PKA) 2 (1). Such sensitization may involve the transient receptor potential vanilloid 1 (TRPV1) (2).TRPV1 is a calcium-permeable ion channel that is activated by the prototypical agonist capsaicin, the component that conveys the sensation of "heat" to chili peppers (3). Endogenous TRPV1 activators include lipophilic arachidonic acid metabolites such as N-arachidonoyl-dopamine and 12-hydroperoxyeicosatetraenoic acid (4, 5), but also heat and protons (3). TRPV1 and endovanilloid signaling are implicated in various inflammatory hyperalgesic conditions. The contribution of TRPV1 to inflammatory hyperalgesia has been established through observations that TRPV1 antagonists can dose-dependently reverse both thermal and mechanical inflammatory hyperalgesia (4, 5). In addition, thermal inflammatory hyperalgesia is significantly reduced in TRPV1 knock-out mice (6).Several pro-inflammato...

Modulation of Transient Receptor Vanilloid 1 Activity by Transient Receptor Potential Ankyrin 1

“…Previously, a stabilizing effect of TRPV1 on the membrane expression of TRPA1 was suggested . Internalization of membrane-bound channels can be suppressed by interactions with other proteins including subunits of the channels (Bernstein and Jones, 2007), and functional TRPV1 tetramers can be modulated by cAMP-dependent translocation of TRPV1 monomers from intracellular pools to the cell membrane (Vetter et al, 2008). Again, since neither the affinity nor the number of TRPV1 binding sites changed during TRPA1 activation, our experiments indicate that TRPV1 was not internalized or recruited to the cell membrane from intracellular pools.…”

Heteromeric Heat-sensitive Transient Receptor Potential Channels Exhibit Distinct Temperature and Chemical Response

“…Expression of TRP channels in native tissues is dynamically regulated by both developmental and environmental cues (43)(44)(45). Of particular clinical importance is the induced overexpression of TRPV channels under pathological conditions such as traumatic and diabetic neuropathy, tissue injury, and inflammation (11, 14, 40, 42, 46 -48).…”

Modulation of Peripheral Sensory Neurons by the Immune System: Implications for Pain Therapy

“…In addition, low extracellular pH increases opioid agonist efficacy, presumably by altering the interaction of opioid receptors with G proteins and adenylyl cyclase (Rasenick and Childers, 1989;Selley et al, 1993;Vetter et al, 2006). Under conditions of elevated cAMP/protein kinase A activity (as seen in inflammation), morphine can inhibit TRPV1 translocation to the plasma membrane, which may lead to reduced neuronal excitability (Vetter et al, 2008). All of these mechanisms can contribute to the increased antinociceptive efficacy of opioids in peripheral inflamed tissue.…”