Neuropathic Pain Induced Alterations in the Opioidergic Modulation of a Descending Pain Facilitatory Area of the Brain

Abstract

Opioids play a major role at descending pain modulation but the effects of neuropathic pain on the brain opioidergic system remain understudied. Since descending facilitation is enhanced during neuropathic pain, we studied the opioidergic modulation of the dorsal reticular nucleus (DRt), a medullary pain facilitatory area, in the spared nerve injury (SNI) model of neuropathic pain. We first performed a series of behavioral experiments in naïve-animals to establish the role of μ-opioid receptor (MOR) in the effects of endogenous and exogenous opioids at the DRt. Specifically, we showed that lentiviral-mediated MOR-knockdown at the DRt increased sensitivity to thermal and mechanical stimuli while the MOR agonist DAMGO induced the opposite effects. Additionally, we showed that MOR-knockdown and the pharmacological blockade of MOR by CTAP at the DRt decreased and inhibited, respectively, the analgesic effects of systemic morphine. Then, we performed in vivo microdialysis to measure enkephalin peptides in the DRt and evaluated MOR expression in the DRt at mRNA, protein and phosphorylated form levels by quantitative real-time PCR and immunohistochemistry, respectively. SNI-animals, compared to sham control, showed higher levels of enkephalin peptides, lower MOR-labeled cells without alterations in MOR mRNA levels, and higher phosphorylated MOR-labeled cells. Finally, we performed behavioral studies in SNI animals to determine the potency of systemic morphine and the effects of the pharmacologic and genetic manipulation of MOR at the DRt. We showed a reduced potency of the antiallodynic effects of systemic morphine in SNI-animals compared to the antinociceptive effects in sham animals. Increasing MOR-cells at the DRt of SNI-animals by lentiviral-mediated MOR-overexpression produced no effects on mechanical allodynia. DAMGO induced anti-allodynia only after MOR-overexpression. These results show that MOR inhibits DRt pain facilitatory actions and that this action contributes to the analgesic effects of systemic opioids. We further show that the inhibitory function of MOR is impaired during neuropathic pain. This is likely due to desensitization and degradation of MOR which are adaptations of the receptor that can be triggered by MOR phosphorylation. Skipping counter-regulatory pathways involved in MOR adaptations might restore the opioidergic inhibition at pain facilitatory areas.

Shift of µ-opioid Receptor Signaling in the Dorsal Reticular Nucleus Is Implicated in Morphine-induced Hyperalgesia in Male Rats

“…µ-opioid receptor is expressed in dorsal reticular nucleus-spinally and nonspinally projecting neurons, 22 and its activation plays a fundamental inhibitory role at the dorsal reticular nucleus, 23 which accounts for the analgesic effects of systemic opioids. 24 However, the effects of sustained opioid treatment on µ-opioid receptor signaling at the dorsal reticular nucleus has never been studied. Here, after establishing the involvement of the dorsal reticular nucleus in a model of opioid-induced hyperalgesia induced by sustained systemic morphine infusion, 14 we explored the effects of sustained morphine on µ-opioid receptor function and signaling at the dorsal reticular nucleus.…”

“…The lentiviral vectors used were produced as previously described. 24 Briefly, the cDNA for the µ-opioid receptor was cloned into a lentiviral transfer vector, inserted in antisense orientation relative to the human synapsin promoter which restricts transgene expression to neurons. 26 This transfer vector, which is the vector for µ-opioid receptor knockdown, also contained an encephalomyocarditis virus internal ribosome entry site, the enhanced green fluorescent protein, and the woodchuck hepatitis virus posttranscriptional regulatory element.…”

“…A guide cannula was implanted into the left dorsal reticular nucleus for pharmacologic experiments following the coordinates, determined according to the rat brain atlas 27 relative to the interaural line (Anterior-Posterior: −6.0 mm; Medial-Lateral: −1.4 mm; Dorsal-Ventral: −1.5 mm), and procedures described previously. 24 Lentiviral Vector Injection. Stereotaxic injections were performed for the injection of the vector for µ-opioid receptor knockdown in morphine-(n = 6) and saline-infused animals (n = 5) or the injection of the control vector in morphine-(n = 6) and saline-infused animals (n = 7) in two rostrocaudal parts of the left dorsal reticular nucleus following the coordinates of the atlas Paxinos and Watson 27 (first injection: Anterior-Posterior: −6.0 mm; Medial-Lateral: −1.4 mm; Dorsal-Ventral: −1.5 mm; second injection: Anterior-Posterior: −6.4 mm; Medial-Lateral: −1.3 mm; Dorsal-Ventral: −1.7 mm), as described previously.…”

Multifunctional Opioid-Derived Hybrids in Neuropathic Pain: Preclinical Evidence, Ideas and Challenges

“…Endogenous opioid system serves an adaptive role for the organism by inducing analgesia when noxious environmental stimuli occur, as the ability to temporarily ignore the pain of the injury heightens the chances of surviving a dangerous event [ 20 ]. This state is reflected in animal NP models by the increased level of endogenous opioids in pain-facilitating spinal and brain areas, where the opioid peptides can induce their inhibitory effects [ 21 , 22 ]. Nevertheless, as the environmentally produced analgesia must be timely suppressed to help modify the behavior once the danger ceases, continued opioid signaling activated in the presence of nerve tissue trauma initiates the homeostatic compensatory mechanisms that eventually counteract the inhibitory effects of opioid activation [ 23 ].…”

“…In human NP patients that suffer either from central or peripheral neuropathic pain, decreases in opioid receptor binding are observed in supraspinal regions: posterior midbrain, medial thalamus and the insular, temporal and prefrontal cortices [ 35 , 36 ]. The reduced supraspinal opioid receptor availability is in all likelihood a direct result of chronic pain itself (and not, for example, pre-existing individual differences), as similar decreased supraspinal MOP receptor expression is observed in animal models of NP [ 21 , 37 ]. The effect is accompanied by altered efficacy of G-protein stimulation within opioid-sensitive CNS structures, and altered expression of DOP and KOP receptor mRNAs, which suggests that neuropathy recruits and modifies several opioidergic circuits [ 38 ].…”