Morphine inhibits herpetic allodynia through mu-opioid receptors induced in Abeta-fiber neurons

Opioids and Viral Infections: A Double-Edged Sword

Opioids and their receptors have received remarkable attention because they have the ability to alter immune function, which affects disease progression. In vitro and in vivo findings as well as observations in humans indicate that opioids and their receptors positively or negatively affect viral replication and virus-mediated pathology. The present study reviews recent insights in the role of opioids and their receptors in viral infections and discusses possible therapeutic opportunities. This review supports the emerging concept that opioids and their receptors have both favorable and unfavorable effects on viral disease, depending on the type of virus. Targeting of the opioid system is a potential option for developing effective therapies; however caution is required in relation to the beneficial functions of opioid systems.

Mu opioid receptors on primary afferent nav1.8 neurons contribute to opiate-induced analgesia: insight from conditional knockout mice

Opiates are powerful drugs to treat severe pain, and act via mu opioid receptors distributed throughout the nervous system. Their clinical use is hampered by centrally-mediated adverse effects, including nausea or respiratory depression. Here we used a genetic approach to investigate the potential of peripheral mu opioid receptors as targets for pain treatment. We generated conditional knockout (cKO) mice in which mu opioid receptors are deleted specifically in primary afferent Nav1.8-positive neurons. Mutant animals were compared to controls for acute nociception, inflammatory pain, opiate-induced analgesia and constipation. There was a 76% decrease of mu receptor-positive neurons and a 60% reduction of mu-receptor mRNA in dorsal root ganglia of cKO mice. Mutant mice showed normal responses to heat, mechanical, visceral and chemical stimuli, as well as unchanged morphine antinociception and tolerance to antinociception in models of acute pain. Inflammatory pain developed similarly in cKO and controls mice after Complete Freund's Adjuvant. In the inflammation model, however, opiate-induced (morphine, fentanyl and loperamide) analgesia was reduced in mutant mice as compared to controls, and abolished at low doses. Morphine-induced constipation remained intact in cKO mice. We therefore genetically demonstrate for the first time that mu opioid receptors partly mediate opiate analgesia at the level of Nav1.8-positive sensory neurons. In our study, this mechanism operates under conditions of inflammatory pain, but not nociception. Previous pharmacology suggests that peripheral opiates may be clinically useful, and our data further demonstrate that Nav1.8 neuron-associated mu opioid receptors are feasible targets to alleviate some forms of persistent pain.

Reduced analgesic effect of acupuncture-like TENS but not conventional TENS in opioid-treated patients

Evidence from recent animal studies indicates that the analgesic effect of low-frequency transcutaneous electrical nerve stimulation (TENS) is reduced in opioid-tolerant animals. The aim of the present study was to compare the analgesic effect of conventional (high frequency) and acupuncture-like (low frequency) TENS between a group of opioid-treated patients and a group of opioid-naive patients in order to determine if this cross-tolerance effect is also present in humans. Twenty-three chronic pain patients (11 who took opioids and 12 who did not) participated in the study. Participants were assigned in a randomized crossover design to receive alternately conventional and acupuncture-like TENS. There was a significant reduction in pain during and after conventional TENS when compared to baseline for both the opioid and nonopioid group (P < .01). For acupuncture-like TENS however, the analgesic effect of TENS was only observed in the nonopioid group (P < .01), with opioid-treated patients showing no change in pain scores during and after TENS when compared to baseline (P > .09). The reduced analgesic effect of acupuncture-like TENS in opioid-treated patients is coherent with previous animal studies and suggests that conventional TENS should be preferred in patients taking opioids on a regular basis.

PERSPECTIVE

This study shows that patients taking opioids on a regular basis are less susceptible to benefit from acupuncture-like TENS. This phenomenon is probably attributable to the fact that the analgesia induced by acupuncture-like TENS and opioids are mediated by the same receptors (ie, μ opioid receptors).

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).