Inhibition of the N -methyl-d-aspartate receptor unmasks the antinociception of endogenous opioids in the periphery

A synthetic peptide disturbing GluN2A/SHP1 interaction in dorsal root ganglion attenuated pathological pain

Src Homology 2 domain-containing protein tyrosine phosphatase 1 (SHP1) interacts specifically with GluN2A subunit of N-methyl-D-aspartate (NMDA) subtype of glutamate receptors in spinal cord dorsal horn. This molecular interaction is involved in the development of GluN2A-dependent spinal sensitization of nociceptive behaviors. Intrathecal application of a GluN2A-derived polypeptide (short for pep-GluN2A) has been shown to disturb spinal GluN2A/SHP1 interaction and inhibit inflammatory pain. Here we found that SHP1 was also located at dorsal root ganglion (DRG) neurons and formed complexes with GluN2A subunit. Peripheral inflammation activated SHP1 in DRG neurons, which promoted GluN2A tyrosine phosphorylation. The SHP1 binding to GluN2A facilitated the glutamate release from primary afferent fibers and exaggerated nociceptive synaptic transmission onto postsynaptic spinal cord neurons. Our data showed that intradermal application of pep-GluN2A disrupted GluN2A/SHP1 interaction in DRG neurons, attenuated the ability of GluN2A subunit-containing NMDA receptors to regulate the presynaptic glutamate release and more importantly, alleviated the pain hypersensitivity caused by carrageenan, complete Freund's adjuvant and formalin. The neuropathic pain induced by spared nerve injury was also ameliorated by intradermal pep-GluN2A application. These data suggested that disruption of GluN2A/SHP1 interaction in DRG neurons generated an effective analgesic action against pathological pain.

Peripheral group II and III metabotropic glutamate receptors in the knee joint attenuate carrageenan-induced nociceptive behavior in rats

This study sought to evaluate whether peripheral group II and III metabotropic glutamate receptors (mGluRs) in the knee joint have inhibitory effects on carrageenan-induced nociceptive behavior. To this end, changes in weight load and hind paw withdrawal threshold were measured in rats following the administration of specific peripheral group II and III mGluR agonists 30min before (induction phase) and 4h after (maintenance phase) the injection of carrageenan (1%, 50μl). During the induction phase of arthritic pain, a significant recovery of reduced weight load occurred after the administration of 500μM APDC ((2R, 4R)-4-aminopyrrolidine-2,4-dicarboxylate; group II agonist) and 100 and 500μM L-AP4 (l-2-amino-4-phosphonobutylate; group III agonist). Similarly, 100 and 500μM APDC and 500μM L-AP4 significantly reduced mechanical hyperalgesia during the induction phase. In the maintenance phase, APDC at all doses (10, 100 and 500μM) and 100 and 500μM L-AP4 significantly counteracted the reduction in weight load, and APDC and L-AP4 at all doses (10, 100 and 500μM) inhibited mechanical hyperalgesia. The current study suggests that peripheral group II and III mGluRs in the knee joint negatively modulates nociceptive behavior during both the induction and maintenance phases of carrageenan-induced arthritic pain.

Nonopioid effect of β-endorphin

This review presents the generalized literature data and the results of our own research of the nonopioid effect of β-endorphin, an opioid neuropeptide interacting not only with opioid but also with nonopioid (insensitive to the opioid antagonist naloxone) receptors. The roles of the hormone and its receptors in regulation of the immune, nervous, and endocrine systems are discussed. The effect of neuromediator on the immune system mediated by both opioid and nonopioid receptors is considered in detail. The data on distribution and function of the nonopioid β-endorphin receptor in human and animal organisms are presented. All available data on the characteristics of the nonopioid β-endorphin receptor obtained by means of radioligand analysis are given. The discussed information is supposed to extend our conceptions of the role of β-endorphin in mammals and to be of extensive use in medicine and pharmacology.

Glutamate pharmacology and metabolism in peripheral primary afferents: physiological and pathophysiological mechanisms

In addition to using glutamate as a neurotransmitter at central synapses, many primary sensory neurons release glutamate from peripheral terminals. Primary sensory neurons with cell bodies in dorsal root or trigeminal ganglia produce glutaminase, the synthetic enzyme for glutamate, and transport the enzyme in mitochondria to peripheral terminals. Vesicular glutamate transporters fill neurotransmitter vesicles with glutamate and they are shipped to peripheral terminals. Intense noxious stimuli or tissue damage causes glutamate to be released from peripheral afferent nerve terminals and augmented release occurs during acute and chronic inflammation. The site of action for glutamate can be at the autologous or nearby nerve terminals. Peripheral nerve terminals contain both ionotropic and metabotropic excitatory amino acid receptors (EAARs) and activation of these receptors can lower the activation threshold and increase the excitability of primary afferents. Antagonism of EAARs can reduce excitability of activated afferents and produce antinociception in many animal models of acute and chronic pain. Glutamate injected into human skin and muscle causes acute pain. Trauma in humans, such as arthritis, myalgia, and tendonitis, elevates glutamate levels in affected tissues. There is evidence that EAAR antagonism at peripheral sites can provide relief in some chronic pain sufferers.

Endogenous opiates and behavior: 2009

This paper is the 32nd consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2009 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).

Increased basal mechanical pain sensitivity but decreased perceptual wind-up in a human model of relative hypocortisolism

Clinical data have accumulated showing that relative hypocortisolism, which may be regarded as a neuroendocrinological correlate of chronic stress, may be a characteristic of some functional pain syndromes. However, it has not been clarified yet whether deregulations of the hypothalamus-pituitary-adrenal (HPA) axis may directly alter pain perception and thus be causally involved in the pathophysiology of these disorders. To test this hypothesis, we performed a randomized placebo-controlled crossover trial in N=20 healthy drug-free volunteers (median age 24yrs) and analyzed the effects of metyrapone-induced hypocortisolism on quantitatively assessed basal mechanical pain sensitivity (1.5-13m/s impact stimuli), perceptual wind-up (9m/s impact stimuli at 1Hz) and temporal summation of pain elicited by inter-digital web pinching (IWP; 10N pressure stimuli for 2min). Experimentally induced hypocortisolism significantly decreased pain detection thresholds and augmented temporal summation of IWP-induced pain (p<.05). The latter effect was dependent on the relative reduction in cortisol levels, and seemed to rely on a potentiated sensitization and not merely on the observed changes in basal pain sensitivity. Perceptual wind-up by contrast was reduced when cortisol synthesis was blocked (p<.05). This result is reminiscent of findings from animal studies showing a reversal of NMDA receptor activation by glucocorticoid receptor antagonists in neuropathic pain models. Our results speak in favor of a potential causal role of HPA axis alterations in pain chronicity.

Heterodimerization of ORL1 and opioid receptors and its consequences for N-type calcium channel regulation

We have investigated the heterodimerization of ORL1 receptors and classical members of the opioid receptor family. All three classes of opioid receptors could be co-immunoprecipitated with ORL1 receptors from both transfected tsA-201 cell lysate and rat dorsal root ganglia lysate, suggesting that these receptors can form heterodimers. Consistent with this hypothesis, in cells expressing either one of the opioid receptors together with ORL1, prolonged ORL1 receptor activation via nociceptin application resulted in internalization of the opioid receptors. Conversely, mu-, delta-, and kappa-opioid receptor activation with the appropriate ligands triggered the internalization of ORL1. The mu-opioid receptor/ORL1 receptor heterodimers were shown to associate with N-type calcium channels, with activation of mu-opioid receptors triggering N-type channel internalization, but only in the presence of ORL1. Furthermore, the formation of opioid receptor/ORL1 receptor heterodimers attenuated the ORL1 receptor-mediated inhibition of N-type channels, in part because of constitutive opioid receptor activity. Collectively, our data support the existence of heterodimers between ORL1 and classical opioid receptors, with profound implications for effectors such as N-type calcium channels.