Differential effects of specific delta and kappa opioid receptor antagonists on the bidirectional dose-dependent effect of systemic naloxone in arthritic rats, an experimental model of persistent pain

Opiate Antagonists for Chronic Pain: A Review on the Benefits of Low-Dose Naltrexone in Arthritis versus Non-Arthritic Diseases

Chronic pain conditions create major financial and emotional burdens that can be devastating for individuals and society. One primary source of pain is arthritis, a common inflammatory disease of the joints that causes persistent pain in affected people. The main objective of pharmacological treatments for either rheumatoid arthritis (RA) or osteoarthritis (OA) is to reduce pain. Non-steroidal anti-inflammatory drugs, opioids, and opioid antagonists have each been considered in the management of chronic pain in arthritis patients. Naltrexone is an oral-activated opioid antagonist with biphasic dose-dependent pharmacodynamic effects. The molecule acts as a competitive inhibitor of opioid receptors at high doses. However, naltrexone at low doses has been shown to have hormetic effects and provides relief for chronic pain conditions such as fibromyalgia, multiple sclerosis (MS), and inflammatory bowel disorders. Current knowledge of naltrexone suggests that low-dose treatments may be effective in the treatment of pain perception in chronic inflammatory conditions observed in patients with either RA or OA. In this review, we evaluated the therapeutic benefits of low-dose naltrexone (LDN) on arthritis-related pain conditions.

Blockade of opioid receptors in the medullary reticularis nucleus dorsalis, but not the rostral ventromedial medulla, prevents analgesia produced by diffuse noxious inhibitory control in rats with muscle inflammation

Diffuse Noxious Inhibitory Controls (DNIC) involves application of a noxious stimulus outside the testing site to produce analgesia. In human subjects with a variety of chronic pain conditions, DNIC is less effective; however, in animal studies, DNIC is more effective after tissue injury. While opioids are involved in DNIC analgesia, the pathways involved in this opioid-induced analgesia are not clear. The aim of the present study was to test the effectiveness of DNIC in inflammatory muscle pain, and to study which brainstem sites mediate DNIC- analgesia. Rats were injected with 3% carrageenan into their gastrocnemius muscle and responses to cutaneous and muscle stimuli were assessed before and after inflammation, and before and after DNIC induced by noxious heat applied to the tail (45 °C and 47 °C). Naloxone was administered systemically, into rostral ventromedial medulla (RVM), or bilaterally into the medullary reticularis nucleus dorsalis (MdD) prior to the DNIC-conditioning stimuli. DNIC produced a similar analgesic effect in both acute and the chronic phases of inflammation reducing both cutaneous and muscle sensitivity in a dose-dependent manner. Naloxone systemically or microinjected into the MdD prevented DNIC-analgesia, while naloxone into the RVM had no effect on DNIC analgesia. Thus, DNIC analgesia involves activation of opioid receptors in the MdD.

PERSPECTIVE

The current study shows that DNIC activates opioid receptors in the MdD, but not the RVM, to produce analgesia. These data are important for understanding clinical studies on DNIC as well as for potential treatment of chronic pain patients.

Deletion of the CCK2 receptor gene reduces mechanical sensitivity and abolishes the development of hyperalgesia in mononeuropathic mice

Previous studies suggest that cholecystokinin (CCK) is implicated in the modulation of pain sensitivity and the development of neuropathic pain. We used CCK(2) receptor deficient (CCK(2) (-/-)) mice and assessed their mechanical sensitivity using Von Frey filaments, as well as the development and time course of mechanical hyperalgesia in a model of neuropathic pain. We found that CCK(2) (-/-) mice displayed mechanical hyposensitivity, which was reversed to the level of wild-type animals after administration of naloxone (0.1-10 mg/kg). On the other hand, injection of L-365260 (0.01-1 mg/kg), an antagonist of CCK(2) receptors, decreased dose-dependently, mechanical sensitivity in wild-type mice. The mechanism of reduced mechanical sensitivity in CCK(2) (-/-) mice may be explained by changes in interactions between CCK and opioid systems. Indeed, CCK(2) (-/-) mice natively expressed higher levels of lumbar CCK(1), opioid delta and kappa receptors. Next, we found that CCK(2) (-/-) mice did not develop mechanical hyperalgesia in the Bennett's neuropathic pain model. Induction of neuropathy resulted in decrease of lumbar pro-opiomelanocortin (POMC) gene expression in wild-type mice, but increase of POMC expression in CCK(2) (-/-) mice. In addition, induction of neuropathy resulted in further increase of opioid delta receptor in CCK(2) (-/-) mice. Gene expression results indicate up-regulation of opioid system in CCK(2) (-/-) mice, which apparently result in decreased neuropathy score. Our study suggests that not only pain sensitivity, but also mechanical sensitivity and the development of neuropathic pain are regulated by antagonistic interactions between CCK and opioid systems.

Epidural naloxone reduces pruritus and nausea without affecting analgesia by epidural morphine in bupivacaine

PURPOSE

To determine whether epidural naloxone preserved analgesia while minimizing side effects caused by epidural morphine.

METHODS

Eighty patients undergoing combined epidural and general anesthesia for hysterectomy were randomly assigned to one of four groups. All received 2 mg epidural morphine bolus one hour before the end of surgery and a continuous epidural infusion was started containing 4 mg morphine in 100 ml bupivacaine 0.125% with either no naloxone (Group 1, n = 20), 0.083 microg x kg(-1) x hr(-1) of naloxone (Group 2, n = 20), 0.125 microg x kg(-1) x hr(-1) of naloxone (Group 3, n = 20) or 0.167 microg x kg(-1) x hr(-1) of naloxone (Group 4, n = 20). Analgesia and side effects were evaluated by blinded observers.

RESULTS

The combination of epidural morphine and bupivacaine provided good analgesia. Eight hours after the end of surgery, the pain score in the group receiving the highest dose of naloxone was lower than in the control group (VAS 1.2 vs. 2.0, P<0.05) but there was less pruritus in the high-dose naloxone group (itching score 1.3 vs. 1.9, P<0.05). Pain scores were no different in any of the naloxone groups from the control group. Itching was less in both of the higher dose naloxone groups (P<0.05 at 8, 16, and 32 hours). The incidence of vomiting in the control group was 40% vs. 5% for high dose naloxone group (P<0.05).

CONCLUSIONS

Epidural naloxone reduced morphine-induced side effects in dose-dependent fashion without reversal of the analgesic effect.