Is Paradoxical Pain Induced by Sustained Opioid Exposure an Underlying Mechanism of Opioid Antinociceptive Tolerance?

Abstract

Opiates are the primary treatment for pain management in cancer patients reporting moderate to severe pain, and are being increasingly used for non-cancer chronic pain. However, prolonged administration of opiates is associated with significant problems including the development of antinociceptive tolerance, wherein higher doses of the drug are required over time to elicit the same amount of analgesia. High doses of opiates result in serious side effects such as constipation, nausea, vomiting, dizziness, somnolence, and impairment of mental alertness. In addition, sustained exposure to morphine has been shown to result in paradoxical pain in regions unaffected by the initial pain complaint, and which may also result in dose escalation, i.e. ‘analgesic tolerance’. A concept that has been gaining considerable experimental validation is that prolonged use of opioids elicits paradoxical, abnormal pain. This enhanced pain state requires additional opioids to maintain a constant level of antinociception, and consequently may be interpreted as antinociceptive tolerance. Many substances have been shown to block or reverse antinociceptive tolerance. A non-inclusive list of examples of substances reported to block or reverse opioid antinociceptive tolerance include: substance P receptor (NK-1) antagonists, calcitonin gene-related peptide (CGRP) receptor antagonists, nitric oxide (NO) synthase inhibitors, calcium channel blockers, cyclooxygenase (COX) inhibitors, protein kinase C inhibitors, competitive and non-competitive antagonists of the NMDA (N-methyl-D-aspartate) receptor, AMPA (alpha-amino-3-hydroxy-5-methyl-4 isoxazolepropionic acid) antagonists, anti-dynorphin antiserum, and cholecystokinin (CCK) receptor antagonists. Without exception, these substances are also antagonists of pain-enhancing agents. Prolonged opiate administration indeed induces upregulation of substance P (SP) and calcitonin gene-related peptide (CGRP) within sensory fibers in vivo, and this is accompanied by an enhanced release of excitatory neurotransmitters and neuropeptides from primary afferent fibers upon stimulation. The enhanced evoked release of neuropeptides is correlated with the onset of abnormal pain states and opioid antinociceptive tolerance. Importantly, the descending pain modulatory pathway from the brainstem rostral ventromedial medulla (RVM) via the dorsolateral funiculus (DLF) is critical for maintaining the changes observed in the spinal cord, abnormal pain states and antinociceptive tolerance, because animals with lesion of the DLF did not show enhanced evoked neuropeptide release, or develop abnormal pain or antinociceptive tolerance upon sustained exposure to opiates. Microinjection of either lidocaine or a CCK antagonist into the RVM blocked both thermal and touch hypersensitivity as well as antinociceptive tolerance. Thus, prolonged opioid exposure enhances a descending pain facilitatory pathway from the RVM that is mediated at least in part by CCK activity and is essential for the maintenance of antinociceptive tolerance.

Preclinical and Early Clinical Investigations Related to Monoaminergic Pain Modulation

“…Mechanisms of action proposed to be essential for the development of an increased sensitivity to pain resulting from long-term exposure to opiates include enhanced NK1 receptor transmission. 110 Experiments performed previously have shown that sustained morphine exposure increases pronociceptive agent substance P and NK1 receptor expression in the spinal dorsal horn. 110 Moreover, spinal administration of the NK1 receptor antagonist L-732,138 fully reversed thermal hypersensitivity in rats receiving sustained morphine treatment via a subcutaneous morphine-pellet implant.…”

“…110 Experiments performed previously have shown that sustained morphine exposure increases pronociceptive agent substance P and NK1 receptor expression in the spinal dorsal horn. 110 Moreover, spinal administration of the NK1 receptor antagonist L-732,138 fully reversed thermal hypersensitivity in rats receiving sustained morphine treatment via a subcutaneous morphine-pellet implant. 111 Interestingly, morphine-induced hyperalgesia was not observed in NK1 receptor knockout (NK1 Ϫ/Ϫ ) mice, providing evidence for a critical involvement of the NK1 receptor in the manifestation of morphine-induced hyperalgesia.…”

Exploring joint effects of genes and the clinical efficacy of morphine for cancer pain: OPRM1 and COMT gene

“…A known mechanism for this phenomenon is the reduction in the responsiveness of the G-protein coupled opioid receptors (Nestler 1992) leading to either desensitization or downregulation. More recently the concept of paradoxical pain leading to analgesic tolerance has also been proposed (King, Ossipov et al 2005). …”

Activation of TRPV1 Contributes to Morphine Tolerance: Involvement of the Mitogen-Activated Protein Kinase Signaling Pathway

“…The development of opioid tolerance has also been associated with enhanced pain sensitivity such as hyperalgesia in both laboratory and clinical settings (Sjogren et al, 1993;Mao et al, 1994Mao et al, , 2002Vanderah et al, 2000Vanderah et al, , 2001. There is emerging evidence suggesting that opioid tolerance and pathological pain may share common cellular mechanisms (Mao et al, 1995a;Mayer et al, 1999), and the abnormal pain elicited by opioids is a critical factor in the behavioral manifestation of opioid tolerance because the same manipulations that block abnormal pain also block opioid tolerance (King et al, 2005b;Ossipov et al, 2005). Despite considerable progress, the molecular and cellular mechanisms mediating morphine tolerance are not yet completely understood.…”