Morphine and its congeners remain a sub-optimal approach to treat pain in sickle cell disease (SCD) due to side effects, risk of addiction, and development of tolerance. To avoid these liabilities associated with use of morphine and mu opioid receptor (MOR) agonist analgesics, we examined the ability of nociceptin receptor (NOPR) ligands to treat pain in SCD. The NOPR and its endogenous ligand nociceptin/orphanin FQ (N/OFQ) belonging to the opioid receptor (OR) and opioid family respectively, are involved in nociceptive signaling. Small molecule NOPR agonists are non-addicting, while providing analgesia in acute and chronic pain models. We used AT-200 ([1-(1-cyclooctylpiperidin-4-yl)-indolin-2-one), which has high binding affinity and agonist efficacy at NOPR and low efficacy, partial agonist activity at MOR, to treat chronic and hypoxia/reoxygenation (H/R)-evoked pain, in sickle HbSS-BERK and control HbAA-BERK mice, expressing sickle and normal human hemoglobin, respectively. AT-200 was injected subcutaneously at a dose of 10 mg/Kg. Equal number of mice were treated with vehicle in parallel. Pain behaviors were analyzed over a period of time in the following paradigms [i] under normoxia after a single dose of AT-200, [ii] after injecting AT-200 following the incitement of H/R, and [iii] AT-200 given everyday for 7 days. For comparison, we used 20 mg/Kg morphine treatment following H/R. Pain measures that were characterized included deep tissue pain by grip force measurement, cutaneous hyperalgesia by mechanical sensitivity to von Frey filaments and thermal sensitivity to heat and cold as described by us for sickle mice (Kohli et al., Blood 2010). Blood flow and dorsal skin temperature were measured using laser doppler velocimetry and infrared thermography, respectively. The measure of mechanical threshold and suprathreshold to a 1.0 g von Frey fiber showed a significant reduction in mechanical sensitivity even 24h after AT-200 treatment as compared to vehicle under normoxia (p<0.05 for threshold and p<0.005 for suprathreshold) in sickle mice. H/R evoked a persistent increase in mechanical hyperalgesia in sickle mice up to 24h and in control mice up to 4h, and both were ameliorated by AT-200. Morphine reduced mechanical hyperalgesia following H/R in sickle mice up to 4h but not 24h and its effect was significantly less than that of AT-200. H/R-incitement increased deep pain for 24h in sickle mice and for 30 min in control mice and AT-200 ameliorated both. Interestingly, morphine significantly blocked deep pain at 30 min following H/R (p<0.005), but remained ineffective thereafter in sickle mice. Since deep pain appears is a characteristic feature of vasoocclusive pain in sickle mice as compared to cutaneous pain, it is likely that AT-200 acts as a superior analgesic with a long-lasting effect to treat sickle-specific vasoocclusive pain, compared to morphine. Heat and cold sensitivity were persistently elevated in sickle mice up to 24h post-H/R but returned to baseline (before H/R) in control mice. Both, AT-200 and morphine inhibited the H/R-induced thermal sensitivity up to 24h, in sickle mice, but the effect of AT-200 was significantly more remarkable than morphine. Blood flow and skin temperature increased significantly following H/R in sickle mice (p<0.05 for both vs baseline before H/R) and were significantly reduced by AT-200 and morphine equally, suggesting that both drugs have a similar acute effect on vascular physiology and inflammation in sickle mice. The enhanced analgesic effectiveness of AT-200 compared to morphine, in sickle mice could be due to a ∼2-fold increase in NOPR expression in the dorsal root ganglion of sickle mice compared to control, observed by qPCR in this study. Chronic pain conditions, including pain in sickle mice, are characterized by reduced MOR expression peripherally and in the spinal cord. However, spinal NOPR expression by qPCR did not show a significant difference between sickle and control mice. AT-200 was effective in treating chronic cutaneous, deep tissue and thermal hyperalgesia even after 7 days of continuous treatment without causing tolerance. Thus, AT-200 provided an analgesic effect in chronic pain and that evoked by H/R, without the development of analgesic tolerance in sickle mice. Targeting NOPR appears to be a promising strategy warranting further development of novel NOPR agonists to treat pain in SCD.
No relevant conflicts of interest to declare.
Asterisk with author names denotes non-ASH members.