Involvement of neuronal nitric oxide synthase in desensitisation of µ-opioid receptors in the rat locus coeruleus

Microglial inflammation modulates opioid analgesic tolerance

As we all know, opioids are the drugs of choice for treating severe pain. However, very often, opioid use leads to tolerance, dependence, and hyperalgesia. Therefore, understanding the mechanisms underlying opioid tolerance and designing strategies for increasing the efficacy of opioids in chronic pain are important areas of research. Microglia are brain macrophages that remove debris and dead cells from the brain and participate in immune defense of the central nervous system during an insult or injury. However, recent studies indicate that microglial activation and generation of proinflammatory molecules (e.g., cytokines, nitric oxide, eicosanoids, etc.) in the brain may contribute to opioid tolerance and other side effects of opioid use. In this review, we will summarize the evidence and possible mechanisms by which proinflammatory molecules produced by activated microglia may antagonize the analgesic effect induced by opioids, and thus, lead to opioid tolerance. We will also delineate specific examples of studies that suggest therapeutic targets to counteract the development of tolerance clinically using suppressors of microglial inflammation.

Synthesis of the Mechanisms of Opioid Tolerance: Do We Still Say NO?

The use of morphine as a first-line agent for moderate-to-severe pain is limited by the development of analgesic tolerance. Initially opioid receptor desensitization in response to repeated stimulation, thought to underpin the establishment of tolerance, was linked to a compensatory increase in adenylate cyclase responsiveness. The subsequent demonstration of cross-talk between N-methyl-D-aspartate (NMDA) glutamate receptors and opioid receptors led to the recognition of a role for nitric oxide (NO), wherein blockade of NO synthesis could prevent tolerance developing. Investigations of the link between NO levels and opioid receptor desensitization implicated a number of events including kinase recruitment and peroxynitrite-mediated protein regulation. Recent experimental advances and the identification of new cellular constituents have expanded the potential signaling candidates to include unexpected, intermediary compounds not previously linked to this process such as zinc, histidine triad nucleotide-binding protein 1 (HINT1), micro-ribonucleic acid (mi-RNA) and regulator of G protein signaling Z (RGSZ). A further complication is a lack of consistency in the protocols used to create tolerance, with some using acute methods measured in minutes to hours and others using days. There is also an emphasis on the cellular changes that are extant only after tolerance has been established. Although a review of the literature demonstrates a lack of spatio-temporal detail, there still appears to be a pivotal role for nitric oxide, as well as both intracellular and intercellular cross-talk. The use of more consistent approaches to verify these underlying mechanism(s) could provide an avenue for targeted drug development to rescue opioid efficacy.

Characterization of functional μ opioid receptor turnover in rat locus coeruleus: an electrophysiological and immunocytochemical study

BACKGROUND AND PURPOSE

Regulation of μ receptor dynamics such as its trafficking is a possible mechanism underlying opioid tolerance that contributes to inefficient recycling of opioid responses. We aimed to characterize the functional turnover of μ receptors in the noradrenergic nucleus locus coeruleus (LC).

EXPERIMENTAL APPROACH

We measured opioid effect by single-unit extracellular recordings of LC neurons from rat brain slices. Immunocytochemical techniques were used to evaluate μ receptor trafficking.

KEY RESULTS

After near-complete, irreversible μ receptor inactivation with β-funaltrexamine (β-FNA), opioid effect spontaneously recovered in a rapid and efficacious manner. In contrast, α₂ -adrenoceptor-mediated effect hardly recovered after receptor inactivation with the irreversible antagonist EEDQ. When the recovery of opioid effect was tested after various inactivating time schedules, we found that the longer the β-FNA pre-exposure, the less efficient and slower the functional μ receptor turnover became. Interestingly, μ receptor turnover was slower when β-FNA challenge was repeated in the same cell, indicating constitutive μ receptor recycling by trafficking from a depletable pool. Double immunocytochemistry confirmed the constitutive nature of μ receptor trafficking from a cytoplasmic compartment. The μ receptor turnover was slowed down when LC neuron calcium- or firing-dependent processes were prevented or vesicular protein trafficking was blocked by a low temperature or transport inhibitor.

CONCLUSIONS AND IMPLICATIONS

Constitutive trafficking of μ receptors from a depletable intracellular pool (endosome) may account for its rapid and efficient functional turnover in the LC. A finely-tuned regulation of μ receptor trafficking and endosomes could explain neuroadaptive plasticity to opioids in the LC.

Effects of kynurenic acid analogue 1 (KYNA-A1) in nitroglycerin-induced hyperalgesia: Targets and anti-migraine mechanisms

Background Trigeminal sensitization represents a major mechanism underlying migraine attacks and their recurrence. Nitroglycerin (NTG) administration provokes spontaneous migraine-like headaches and in rat, an increased sensitivity to the formalin test. Kynurenic acid (KYNA), an endogenous regulator of glutamate activity and its analogues attenuate NTG-induced neuronal activation in the nucleus trigeminalis caudalis (NTC). The anti-hyperalgesic effect of KYNA analogue 1 (KYNA-A1) was investigated on animal models specific for migraine pain. Aim Rats made hyperalgesic by NTG administration underwent the plantar or orofacial formalin tests. The effect of KYNA-A1 was evaluated in terms of nocifensive behavior and of neuronal nitric oxide synthase (nNOS), calcitonin gene-related peptide (CGRP) and cytokines expression in areas involved in trigeminal nociception. Results KYNA-A1 abolished NTG-induced hyperalgesia in both pain models; NTG alone or associated to formalin injection induced an increased mRNA expression of CGRP, nNOS and cytokines in the trigeminal ganglia and central areas, which was reduced by KYNA-A1. Additionally, NTG caused a significant increase in nNOS immunoreactivity in the NTC, which was prevented by KYNA-A1. Conclusion Glutamate activity is likely involved in mediating hyperalgesia in an animal model specific for migraine. Its inhibition by means of a KYNA analogue modulates nNOS, CGRP and cytokines expression at peripheral and central levels.

Interactions of nitric oxide with α2 -adrenoceptors within the locus coeruleus underlie the facilitation of inhibitory avoidance memory by agmatine

BACKGROUND AND PURPOSE

Agmatine, a putative neurotransmitter, plays a vital role in learning and memory. Although it is considered an endogenous ligand of imidazoline receptors, agmatine exhibits high affinity for α-adrenoceptors, NOS and NMDA receptors. These substrates within the locus coeruleus (LC) are critically involved in learning and memory processes.

EXPERIMENTAL APPROACH

The hippocampus and LC of male Wistar rat were stereotaxically cannulated for injection. Effects of agmatine, given i.p. or intra-LC, on acquisition, consolidation and retrieval of inhibitory avoidance (IA) memory were measured. The NO donor S-nitrosoglutathione, non-specific (L-NAME) and specific NOS inhibitors (L-NIL, 7-NI, L-NIO), the α2 -adrenoceptor antagonist (yohimbine) or the corresponding agonist (clonidine) were injected intra-LC before agmatine. Intra-hippocampal injections of the NMDA antagonist, MK-801 (dizocilpine), were used to modify the memory enhancing effects of agmatine, SNG and yohimbine. Expression of tyrosine hydroxylase (TH) and eNOS in the LC was assessed immunohistochemically.

KEY RESULTS

Agmatine (intra-LC or i.p.) facilitated memory retrieval in the IA test. S-nitrosoglutathione potentiated, while L-NAME and L-NIO decreased, these effects of agmatine. L-NIL and 7-NI did not alter the effects of agmatine. Yohimbine potentiated, whereas clonidine attenuated, effects of agmatine within the LC. The effects of agmatine, S-nitrosoglutathione and yohimbine were blocked by intra-hippocampal MK-801. Agmatine increased the population of TH- and eNOS-immunoreactive elements in the LC.

CONCLUSIONS AND IMPLICATIONS

The facilitation of memory retrieval in the IA test by agmatine is probably mediated by interactions between eNOS, NO and noradrenergic pathways in the LC.

Endogenous opiates and behavior: 2014

This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants). This paper is the thirty-seventh consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2014 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 (endogenous opioids and receptors), and the roles of these opioid peptides and receptors in pain and analgesia (pain and analgesia); stress and social status (human studies); tolerance and dependence (opioid mediation of other analgesic responses); learning and memory (stress and social status); eating and drinking (stress-induced analgesia); alcohol and drugs of abuse (emotional responses in opioid-mediated behaviors); sexual activity and hormones, pregnancy, development and endocrinology (opioid involvement in stress response regulation); mental illness and mood (tolerance and dependence); seizures and neurologic disorders (learning and memory); electrical-related activity and neurophysiology (opiates and conditioned place preferences (CPP)); general activity and locomotion (eating and drinking); gastrointestinal, renal and hepatic functions (alcohol and drugs of abuse); cardiovascular responses (opiates and ethanol); respiration and thermoregulation (opiates and THC); and immunological responses (opiates and stimulants).

Contribution of nitric oxide-dependent guanylate cyclase and reactive oxygen species signaling pathways to desensitization of μ-opioid receptors in the rat locus coeruleus

Nitric oxide (NO) is involved in desensitization of μ-opioid receptors (MOR). We used extracellular recordings in vitro to unmask the NO-dependent pathways involved in MOR desensitization in the rat locus coeruleus (LC). Perfusion with ME (3 and 10 μM) concentration-dependently reduced subsequent ME effect, indicative of MOR desensitization. ME (3 μM)-induced desensitization was enhanced by a NO donor (DEA/NO 100 μM), two soluble guanylate cyclase (sGC) activators (A 350619 30 μM and BAY 418543 1 μM) or a cGMP-dependent protein kinase (PKG) activator (8-pCPT-cGMP 30 μM). DEA/NO-induced enhancement was blocked by the sGC inhibitor NS 2028 (10 μM). A 350619 effect was also blocked by NS 2028, but not by the antioxidant Trolox. ME (10 μM)-induced desensitization was blocked by the neuronal NO synthase inhibitor 7-NI (100 μM) and restored by the PKG activator 8-Br-cGMP (100-300 μM). Paradoxically, ME (10 μM)-induced desensitization was not modified by sGC inhibitors (NS 2028 and ODQ), PKG inhibitors (H8 and Rp-8-Br-PET-cGMP) or antioxidant agents (Trolox, U-74389G and melatonin), but it was attenuated by a combination of NS 2028 and Trolox. In conclusion, MOR desensitization in the LC may be mediated or regulated by NO through sGC and reactive oxygen species signaling pathways.