Role of a novel nociceptor autocrine mechanism in chronic pain

Abstract

We have previously shown, in the rat, that neuropathic and inflammatory events produce a neuroplastic change in nociceptor function whereby a subsequent exposure to a proinflammatory mediator (e.g. prostaglandin E2 ; PGE2 ) produces markedly prolonged mechanical hyperalgesia. While the initial approximately 30 min of this prolonged PGE2 hyperalgesia remains PKA-dependent, it subsequently switches to become dependent on protein kinase C epsilon (PKCε). In this study we tested the hypothesis that the delayed onset, PKCε-mediated, component of PGE2 hyperalgesia is generated by the active release of a nucleotide from the peripheral terminal of the primed nociceptor and this nucleotide is then metabolized to produce adenosine, which acts on a Gi-coupled A1 adenosine receptor on the nociceptor to generate PKCε-dependent hyperalgesia. We report that inhibitors of ATP-binding cassette transporters, of ecto-5'-phosphodiesterase and ecto-5'nucleotidase (enzymes involved in the metabolism of cyclic nucleotides to adenosine) and of A1 adenosine receptors each eliminated the late, but not the early, phase of PGE2 -induced hyperalgesia in primed animals. A second model of chronic pain induced by transient attenuation of G-protein-coupled receptor kinase 2, in which the prolongation of PGE2 hyperalgesia is not PKCε-dependent, was not attenuated by inhibitors of any of these mechanisms. Based on these results we propose a contribution of an autocrine mechanism, in the peripheral terminal of the nociceptor, in the hyperalgesic priming model of chronic pain.

Plasma Membrane Mechanisms in a Preclinical Rat Model of Chronic Pain

“…Treatment with ODN against integrin β1 was started 11 days later, at which time ψεRACK-induced mechanical hyperalgesia had resolved (Fig. 1A) and, then, after 3 consecutive daily injections, testing for priming was performed by intradermal injection of the A1 adenosine receptor agonist CPA 15 . Of note, we have previously shown that, although intradermal injection of CPA does not induce changes in the mechanical threshold in non-primed animals, following priming it produces prolonged PKCε-dependent mechanical hyperalgesia 15 .…”

“…1A) and, then, after 3 consecutive daily injections, testing for priming was performed by intradermal injection of the A1 adenosine receptor agonist CPA 15 . Of note, we have previously shown that, although intradermal injection of CPA does not induce changes in the mechanical threshold in non-primed animals, following priming it produces prolonged PKCε-dependent mechanical hyperalgesia 15 . We observed that, when rats previously primed with ψεRACK, 2 weeks before, and treated with ODN-AS or MM for integrin β1 mRNA for 3 days, received injection of CPA on the dorsum of the hind paw, hyperalgesia developed only in the group treated with MM, showing that the expression of priming (i.e., the CPA-induced hyperalgesia) is integrin β1 dependent (Fig.…”

“…Hyperalgesic priming, a model of chronic pain, was produced using a procedure we have used in previous studies 14, 15, 17, 21, 22 ; the selective protein kinase C epsilon (PKCε) activator, ψεRACK (1 μg/5 μL) 19 , was injected intradermally on the dorsum of the hind paw. This induces mechanical hyperalgesia that resolves in 3–5 days, subsequent to which intradermal injection of PGE 2 at the same site produces markedly prolonged, PKCε-dependent, mechanical hyperalgesia 3, 21, 22 .…”

“…The selection of the drug doses used was based on dose–response curves determined in previous studies 15, 18, 20, 21, 27, 29 . The stock solution of PGE 2 (1 μg/μL) was prepared in 10% ethanol, and additional dilutions made with physiological saline (0.9% NaCl), yielding a final concentration of ethanol < 1%.…”

“…In the extracellular space cAMP is metabolized to adenosine by ectonucleotidases, a process dependent on an extracellular matrix (ECM) chondroitin sulfate proteoglycan, versican. Acting at the A1 adenosine receptor on nociceptors, adenosine, which is analgesic in control (non-primed) animals, now couples to protein kinase C epsilon (PKCε to produce prolonged hyperalgesia 15 . In the present study we have pursued plasma membrane mechanisms involved in the prolongation of PGE 2 hyperalgesia in the setting of hyperalgesic priming.…”