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Molecular Pain

, 9:8

First Online: 04 March 2013Received: 28 September 2012Accepted: 25 February 2013

Abstract

BackgroundOpioid receptors regulate a diverse array of physiological functions.
Mu opioid receptor agonists are well-known analgesics for treating acute pain.
In contrast, animal models suggest that chronic pain is more effectively relieved by delta opioid receptor agonists.
A number of studies have shown that chronic pain results in increased function of delta opioid receptors.
This is proposed to result from enhanced trafficking of the delta opioid receptor to the cell membrane induced by persistent tissue injury.
However, recent studies have questioned this mechanism, which has resulted in some uncertainty as to whether delta opioid receptors are indeed upregulated in chronic pain states.
To clarify this question, we have examined the effect of chronic inflammatory pain over time using both an ex vivo measure of delta function: receptor-Ca channel coupling, and an in vivo measure; the relief of chronic pain by a delta opioid receptor agonist.
In addition, as beta-arrestin 2 can regulate delta opioid receptor trafficking and signaling, we have further examined whether deleting this scaffolding and signal transduction molecule alters delta opioid receptor function.

ResultsWe used the Complete Freund’s Adjuvant model of inflammatory pain, and examined the effectiveness of the delta agonist, SNC80, to both inhibit Ca channels in primary afferent neurons and to attenuate mechanical allodynia.
In naïve beta-arrestin 2 wildtype and knockout mice, SNC80 neither significantly inhibited voltage-dependent Ca currents nor produced antinociception.
However, following inflammatory pain, both measures showed a significant and long-lasting enhancement of delta opioid receptor function that persisted for up to 14 days post-injury regardless of genotype.
Furthermore, although this pain model did not alter Ca current density, the contribution of N-type Ca channels to the total current appeared to be regulated by the presence of beta-arrestin 2.

ConclusionsOur results indicate that there is an upregulation of delta opioid receptor function following chronic pain.
This gain of function is reflected in the increased efficacy of a delta agonist in both behavioral and electrophysiological measures.
Overall, this work confirms that delta opioid receptors can be enhanced following tissue injury associated with chronic pain.

KeywordsPrimary afferent SNC80 Delta opioid receptor Chronic pain Dorsal root ganglia Ca channel AbbreviationsSNC80+-4-αR-α-2S,5R-4-Allyl-2,5-di methyl-1-piperazinyl-3-methoxybenzyl-N,N-diethyl benzamide

CFAComplete freund’s adjuvant

VDCCVoltage dependent Ca current

δORDelta opioid receptor

μORMu opioid receptor

Electronic supplementary materialThe online version of this article doi:10.1186-1744-8069-9-8 contains supplementary material, which is available to authorized users.

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Autor: Amynah Pradhan - Monique Smith - Brenna McGuire - Christopher Evans - Wendy Walwyn

Fuente: https://link.springer.com/



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