Exogenous nerve growth factor attenuates opioid-induced inhibition of voltage-activated Ba2+ currents in rat sensory neurons

Effect of transdermal opioids in experimentally induced superficial, deep and hyperalgesic pain

BACKGROUND AND PURPOSE

Chronic pain and hyperalgesia can be difficult to treat with classical opioids acting predominately at the µ-opioid receptor. Buprenorphine and its active metabolite are believed to act through µ-, κ- and δ-receptors and may therefore possess different analgesic and anti-hyperalgesic effects compared with pure µ-receptor agonists, for example, fentanyl. Here, we have compared the analgesic and anti-hyperalgesic effects of buprenorphine and fentanyl.

EXPERIMENTAL APPROACH

Twenty-two healthy volunteers were randomized to treatment with transdermal buprenorphine (20 µg·h(-1), 144 h), fentanyl (25 µg·h(-1), 72 h) or placebo patches in a double-blind, cross-over experimental pain study. The experimental pain tests (phasic pain, sensitization) involved pressure at the tibial bone, cutaneous electrical and thermal stimulation, intramuscular nerve growth factor, UVB light burn injury model and intradermal capsaicin-induced hyperalgesia. Pain testing was carried out at baseline, 24, 48, 72 and 144 h after application of the drugs.

KEY RESULTS

Compared with placebo, buprenorphine, but not fentanyl, significantly attenuated pressure at the tibial bone as well as pressure pain in the primary hyperalgesic area induced by UVB light The two drugs were equipotent and better than placebo against cutaneous thermal pain stimulation), but failed to show significant analgesic effect to cutaneous electrical stimulation, nerve growth factor-induced muscle soreness and to capsaicin-induced hyperalgesia.

CONCLUSIONS AND IMPLICATIONS

Buprenorphine, but not fentanyl, showed analgesic effects against experimentally induced, bone-associated pain and primary hyperalgesia compared with placebo. These tissue- and modality-differentiated properties may reflect the variable effects of opioid drugs observed in individual patients.

Isoflurane inhibits growth but does not cause cell death in hippocampal neural precursor cells grown in culture

BACKGROUND

Isoflurane causes long-term hippocampal-dependent learning deficits in rats despite limited isoflurane-induced hippocampal cell death, raising questions about the causality between isoflurane-induced cell death and isoflurane-induced cognitive function. Neurogenesis in the dentate gyrus is required for hippocampal-dependent learning and thus constitutes a potential alternative mechanism by which cognition can be altered after neonatal anesthesia. The authors tested the hypothesis that isoflurane alters proliferation and differentiation of hippocampal neural progenitor cells.

METHODS

Multipotent neural progenitor cells were isolated from pooled rat hippocampi (postnatal day 2) and grown in culture. These cells were exposed to isoflurane and evaluated for cell death using lactate dehydrogenase release, caspase activity, and immunocytochemistry for nuclear localization of cleaved caspase 3. Growth was assessed by cell counting and BrdU incorporation. Expression of markers of stemness (Sox2) and cell division (Ki67) were determined by quantitative polymerase chain reaction. Cell fate selection was assessed using immunocytochemistry to stain for neuronal and glial markers.

RESULTS

Isoflurane did not change lactate dehydrogenase release, activity of caspase 3/7, or the amount of nuclear cleaved caspase 3. Isoflurane decreased caspase 9 activity, inhibited proliferation, and decreased the proportion of cells in s-phase. messenger ribonucleic acid expression of Sox2 (stem cells) and Ki67 (proliferation) were decreased. Differentiating neural progenitor cells more often select a neuronal fate after isoflurane exposure.

CONCLUSIONS

The authors conclude that isoflurane does not cause cell death, but it does act directly on neural progenitor cells independently of effects on the surrounding brain to decrease proliferation and increase neuronal fate selection. These changes could adversely affect cognition after isoflurane anesthesia.

The roles of nerve growth factor and cholecystokinin in the enhancement of morphine analgesia in a rodent model of central nervous system inflammation

Animal models of inflammatory pain are characterized by the release of inflammatory mediators such as cytokines and neurotrophic factors, and enhanced analgesic sensitivity to opioids. In this study, we examine the mechanisms underlying this effect, in particular the roles of cholecystokinin (CCK) and nerve growth factor (NGF), in an animal model of central nervous system (CNS) inflammation induced by spinal administration of lipopolysaccharide (LPS). Although spinal administration of LY-225910 (25 ng), a CCK-B antagonist, enhanced morphine analgesia in naïve rats, it was unable to do so in LPS-treated animals. Conversely, spinal CCK-8S administration (1 ng) decreased morphine analgesia in LPS-treated rats, but not in naïve animals. Further, spinal anti-NGF (3 microg) was able to reduce morphine analgesia in LPS-treated rats, but not in naïve animals, an effect that was reversed by spinal administration of LY-225910. While CCK-8S concentration was increased in spinal cord extracts of LPS animals as compared to controls, morphine-induced spinal CCK release in the extracellular space, as measured by in-vivo spinal cord microdialysis was inhibited in LPS animals as compared to controls, and this was reversed by anti-NGF pretreatment. Finally, chronic spinal administration of beta-NGF (7 microg/day) for 7 days enhanced spinal morphine analgesia, possibly by mimicking a CNS inflammatory state. We suggest that in intrathecally LPS-treated rats, spinal CCK release is altered resulting in enhanced morphine analgesia, and that this mechanism may be regulated to an important extent by NGF.

The function neutralizing anti-TrkA antibody MNAC13 reduces inflammatory and neuropathic pain

Nerve growth factor (NGF) is involved in pain transduction mechanisms and plays a key role in many persistent pain states, notably those associated with inflammation. On this basis, both the NGF ligand and its receptor TrkA (tyrosine kinase A) represent an eligible target for pain therapy. Although the direct involvement of NGF in pain modulation is well established, the effect of a direct functional block of the TrkA receptor is still unknown. In this study, we have demonstrated that MNAC13, the only anti-TrkA monoclonal antibody for which function neutralizing properties have been clearly shown both in vitro and in vivo, induces analgesia in both inflammatory and neuropathic pain models, with a surprisingly long-lasting effect in the latter. The formalin-evoked pain licking responses are significantly reduced by the MNAC13 antibody in CD1 mice. Remarkably, treatment with the anti-TrkA antibody also produces a significant antiallodynic effect on neuropathic pain: repeated i.p. injections of MNAC13 induce significant functional recovery in mice subjected to sciatic nerve ligation, with effects persisting after administration. Furthermore, a clear synergistic effect is observed when MNAC13 is administered in combination with opioids, at doses that are not efficacious per se. This study represents a direct demonstration that neutralizing antibodies directed against the TrkA receptor may display potent analgesic effects in inflammatory and chronic pain.

Endogenous opiates and behavior: 2004

This paper is the 27th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over 30 years of research. It summarizes papers published during 2004 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, and the roles of these opioid peptides and receptors in pain and analgesia; stress and social status; tolerance and dependence; learning and memory; eating and drinking; alcohol and drugs of abuse; sexual activity and hormones, pregnancy, development and endocrinology; mental illness and mood; seizures and neurologic disorders; electrical-related activity and neurophysiology; general activity and locomotion; gastrointestinal, renal and hepatic functions; cardiovascular responses; respiration and thermoregulation; and immunological responses.

The endogenous opioid system and clinical pain management

The endogenous opioid system is one of the most studied innate pain-relieving systems. This system consists of widely scattered neurons that produce three opioids: beta-endorphin, the met- and leu-enkephalins, and the dynorphins. These opioids act as neurotransmitters and neuromodulators at three major classes of receptors, termed mu, delta, and kappa, and produce analgesia. Like their endogenous counterparts, the opioid drugs, or opiates, act at these same receptors to produce both analgesia and undesirable side effects. This article examines some of the recent findings about the opioid system, including interactions with other neurotransmitters, the location and existence of receptor subtypes, and how this information drives the search for better analgesics. We also consider how an understanding of the opioid system affects clinical responses to opiate administration and what the future may hold for improved pain relief. The goal of this article is to assist clinicians to develop pharmacological interventions that better meet their patient's analgesic needs.