Chronic Inflammatory Injury Results in Increased Coupling of Delta Opioid Receptors to Voltage-Gated Ca²⁺ Channels

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-Ca2+ 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 Ca2+ 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 Ca2+ 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 Ca2+ current density, the contribution of N-type Ca2+ 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.

Presynaptic inhibition of optogenetically identified VGluT3+ sensory fibres by opioids and baclofen

“…In contrast to MOR activation, DOR and GABA B R activation exhibited uniform effects on all fibres investigated. However, neurotransmitter receptor expression or function might be altered in conditions of chronic pain, and intrathecal DOR agonists may prove useful therapeutic agents for the treatment of pain, if chronic injury or inflammation leads to an increased functional coupling of DORs to voltage-gated calcium channels [37], or to an enhanced plasma membrane expression in distinct fibre populations [3;5;18; but see also 45]. Given that DOR activation was largely ineffective in inhibiting synaptic transmission in naïve animals, such a treatment would most likely result in reduced adverse effects as compared to pharmaceuticals acting on a large number of fibres in the naïve state.…”

Molecular Pharmacology ofδ-Opioid Receptors

“…Tissue injury associated with chronic inflammation increases DOPr function, as evidenced by an enhanced antihyperalgesic effect and an increase in coupling to voltage-dependent d-Opioid Receptor Pharmacology calcium channels. These changes in function were positively correlated with recruitment of DOPr from intracellular stores to the plasma membrane in the spinal cord, neocortex, and sensory neurons Lucido et al, 2005;Gendron et al, 2006;Pradhan et al, 2013). Membrane trafficking in the spinal cord and sensory neurons was also proposed to account for the enhanced potency or appearance of effects produced by DOPr agonists in various brain regions after unilateral hindpaw inflammation (Hylden et al, 1991;Fraser et al, 2000;Hurley and Hammond, 2000;Qiu et al, 2000;Cao et al, 2001).…”

“…Thus, in naïve animals, the highest levels of responsiveness were associated with the use of less selective ligands and the absence of tetraethylammonium in the bath, which was shown to mask the effect DOPr agonists on N-type channels . Concerning neuronal populations that display Cav2 modulation by DOPr, when responses from small versus medium-large size neurons were assessed in parallel, 30% of medium-large neurons versus ,10% of small neurons displayed Ca +2 channel currents responsive to DOPr agonists (Acosta and López, 1999;Wu et al, 2008;Pradhan et al, 2013). Independent of these 666 considerations, responsiveness to DOPr agonists was enhanced in neurons obtained from animals with inflammatory pain or neurons exposed to inflammatory mediators (Pettinger et al, 2013).…”

“…In DRG sensory neurons, the most prominent inhibition by DOPr agonists was on N-type (Cav2.2) channels sensitive to v-conotoxin GVIA, but additional occlusion by v-agatoxin-IVA implied that Cav2.1 channels (P/Q) were also DOPr effectors (Acosta and López, 1999;Khasabova et al, 2004;Wu et al, 2008). The proportion of neurons in primary DRGs or trigeminal neuron cultures that were sensitive to Cav2 channel inhibition by DOPr varied between approximately 75% (Acosta and López, 1999) and none (Walwyn et al, 2005), depending on the agonist used, the recording conditions, the size of neurons, and pre-exposure to stimuli that target DOPr to the membrane (Khasabova et al, 2004;Mittal et al, 2013;Pettinger et al, 2013;Pradhan et al, 2013). Thus, in naïve animals, the highest levels of responsiveness were associated with the use of less selective ligands and the absence of tetraethylammonium in the bath, which was shown to mask the effect DOPr agonists on N-type channels .…”

GRK2 Constitutively Governs Peripheral Delta Opioid Receptor Activity

“…Importantly, BK induces rapid functional competence of DOR antinociception (Patwardhan et al , 2005; Rowan et al , 2009), indicating that peripheral DOR exists naively in a desensitized state. Inflammation-induced DOR analgesic competence at peripherally restrictive doses is known as “priming” (Patwardhan et al , 2005; Rowan et al , 2009; Pradhan et al , 2013), yet a mechanism for this phenomenon remains unknown. Recent work has identified a role for protein kinase C (PKC) (Patwardhan et al , 2005; Rowan et al , 2009), which agrees with work demonstrating that BK activation drives phospholipase C (PLC) activity to stimulate downstream PKC isoforms (Fu et al , 1989; Tippmer et al , 1994; Graness et al , 1997).…”