Pretreatment with antiserum against dynorphin, substance P, or cholecystokinin enhances the morphine-produced anti-allodynia in the sciatic nerve ligated mice

The Role of Neurogenic Inflammation in Blood-Brain Barrier Disruption and Development of Cerebral Oedema Following Acute Central Nervous System (CNS) Injury

Acute central nervous system (CNS) injury, encompassing traumatic brain injury (TBI) and stroke, accounts for a significant burden of morbidity and mortality worldwide, largely attributable to the development of cerebral oedema and elevated intracranial pressure (ICP). Despite this, clinical treatments are limited and new therapies are urgently required to improve patient outcomes and survival. Originally characterised in peripheral tissues, such as the skin and lungs as a neurally-elicited inflammatory process that contributes to increased microvascular permeability and tissue swelling, neurogenic inflammation has now been described in acute injury to the brain where it may play a key role in the secondary injury cascades that evolve following both TBI and stroke. In particular, release of the neuropeptides substance P (SP) and calcitonin gene-related peptide (CGRP) appear to be critically involved. In particular, increased SP expression is observed in perivascular tissue following acute CNS injury, with the magnitude of SP release being related to both the frequency and degree of the insult. SP release is associated with profound blood-brain barrier disruption and the subsequent development of vasogenic oedema, as well as neuronal injury and poor functional outcomes. Inhibition of SP through use of a neurokinin 1 (NK1) antagonist is highly beneficial following both TBI and ischaemic stroke in pre-clinical models. The role of CGRP is more unclear, especially with respect to TBI, with both elevations and reductions in CGRP levels reported following trauma. However, a beneficial role has been delineated in stroke, given its potent vasodilatory effects. Thus, modulating neuropeptides represents a novel therapeutic target in the treatment of cerebral oedema following acute CNS injury.

Effect of the Combination of CI-988 and Morphine on Neuropathic Pain after Spinal Cord Injury in Rats

Cholecystokinin is known to be involved in the modulation of nociception and to reduce the efficacy of morphine analgesia. This study investigated the effects of intrathecal administration of morphine and the cholecystokinin type B antagonist CI-988 on below-level neuropathic pain after spinal cord injury in rats. We also examined the interaction of morphine and CI-988 in the antinociceptive effect. Both morphine and CI-988 given individually increased the paw withdrawal threshold to mechanical stimulation in a dose-dependent manner. The combination of ineffective doses of intrathecally administered CI-988 and morphine produced significant analgesic effects and the combination of effective doses resulted in analgesic effects that were greater than the sum of the individual effects of each drug. Thus, morphine showed a synergistic interaction with CI-988 for analgesia of central neuropathic pain.

Sex, pain, and opioids: interdependent influences of sex and pain modality on dynorphin-mediated antinociception in rats

The role of dynorphin A (1-17; Dyn) and its associated kappa opioid receptor (KOR) in nociception represents a longstanding scientific conundrum: Dyn and KOR (Dyn/KOR) have variously been reported to inhibit, facilitate, or have no effect on pain. We investigated whether interactions between sex and pain type (which are usually ignored) influenced Dyn/KOR-mediated antinociception. Blockade of the spinal α(2)-noradrenergic receptor (α(2)-NAR) using yohimbine elicited comparable spinal Dyn release in females and males. Nevertheless, the yohimbine-induced antinociception exhibited sexual dimorphism that depended on the pain test used: in the intraperitoneal acetic acid-induced writhing test, yohimbine produced antinociception only in females, whereas in the intraplantar formalin-induced paw flinch test, antinociception was observed only in males. In females and males, both intrathecal Dyn antibodies and spinal KOR blockade eliminated the yohimbine-induced antinociception, indicating that Dyn/KOR mediated it. However, despite the conditional nature of spinal Dyn/KOR-mediated yohimbine antinociception, both intraplantar formalin and intraperitoneal acetic acid activated spinal Dyn neurons that expressed α(2)-NARs. Moreover, Dyn terminals apposed KOR-expressing spinal nociceptive neurons in both sexes. This similar organization suggests that the sexually dimorphic interdependent effects of sex and pain type may result from the presence of nonfunctional (silent) KORs on nociceptive spinal neurons that are responsive to intraplantar formalin (in females) versus intraperitoneal acetic acid (in males). Our findings that spinal Dyn/KOR-mediated antinociception depends on interactions between sex and pain type underscore the importance of using both sexes and multiple pain models when investigating Dyn/KOR antinociception.

The role of nociceptin and dynorphin in chronic pain: implications of neuro-glial interaction

Nociceptin-opioid peptide (NOP) receptor, also known as opioid receptor like-1 (ORL1), was identified following the cloning of the kappa-opioid peptide (KOP) receptor, and the characterization of these receptors revealed high homology. The endogenous ligand of NOP, nociceptin (NOC), which shares high homology to dynorphin (DYN), was discovered shortly thereafter, and since then, it has been the subject of several investigations. Despite the many advances in our understanding of the involvement of NOC and DYN systems in pain, tolerance and withdrawal, the precise function of these systems has not been fully characterized. Here, we review the recent literature concerning the distribution of the NOC and DYN systems in the central nervous system and the involvement of these systems in nociceptive transmission, especially under chronic pain conditions. We discuss the use of endogenous and exogenous ligands of NOP and KOP receptors in pain perception, as well as the potential utility of NOP ligands in clinical practice for pain management. We also discuss the modulation of opioid effects by NOC and DYN. We emphasize the important role of neuro-glial interactions in the effects of NOC and DYN, focusing on their presence in neuronal and non-neuronal cells and the changes associated with chronic pain conditions. We also present the dynamics of immune and glial regulation of neuronal functions and the importance of this regulation in the roles of NOC and DYN under conditions of neuropathic pain and in the use of drugs that alter these systems for better control of neuropathic pain.

Electroacupuncture attenuates bone-cancer-induced hyperalgesia and inhibits spinal preprodynorphin expression in a rat model

Cancer pain impairs the quality of life of cancer patients, but opioid intervention can cause significant side effects that further decrease quality of life. Although electroacupuncture (EA) has been used to treat cancer pain, its mechanisms are largely unknown. To examine its effects and underlying mechanisms on cancer pain, we injected AT-3.1 prostate cancer cells into the tibia to induce bone cancer in the male Copenhagen rat. The resulting pain was treated with 10Hz/2mA/0.4ms pulse EA for 30min daily at the point equivalent to the human acupoint GB30 (Huantiao) between days 14 and 18 after the injection. For sham control, EA needles were inserted into GB30 without stimulation. Thermal hyperalgesia, a decrease in paw withdrawal latency (PWL) to a noxious thermal stimulus, and mechanical hyperalgesia, a decrease in paw withdrawal pressure threshold (PWPT), was measured at baseline and 20min after the EA treatment. Preprodynorphin mRNA and dynorphin were determined by RT-PCR and immunohistochemistry, respectively. Thermal and mechanical hyperalgesia developed ipsilaterally between days 12 and 18 after cancer cell inoculation. EA significantly (P<0.05) attenuated this hyperalgesia, as shown by increased PWL and PWPT, and inhibited up-regulation of preprodynorphin mRNA and dynorphin compared to sham control. Intrathecal injection of antiserum against dynorphin A (1-17) also significantly inhibited the cancer-induced hyperalgesia. These results suggest that EA alleviates bone cancer pain at least in part by suppressing dynorphin expression, and they support the clinical use of EA in the treatment of cancer pain.

Endogenous opiates and behavior: 2005

This paper is the 28th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2005 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, neurophysiology and transmitter release (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); immunological responses (Section 17).