Actions of NMDA and cholinergic receptor antagonists in the rostral ventromedial medulla upon beta-endorphin analgesia elicited from the ventrolateral periaqueductal gray

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Why Do Birds Flock? A Role for Opioids in the Reinforcement of Gregarious Social Interactions

The formation of social groups provides safety and opportunities for individuals to develop and practice important social skills. However, joining a social group does not result in any form of obvious, immediate reinforcement (e.g., it does not result in immediate copulation or a food reward), and individuals often remain in social groups despite agonistic responses from conspecifics. Much is known about neural and endocrine mechanisms underlying the motivation to perform mate- or offspring-directed behaviors. In contrast, relatively little is known about mechanisms underlying affiliative behaviors outside of these primary reproductive contexts. Studies on flocking behavior in songbirds are beginning to fill this knowledge gap. Here we review behavioral evidence that supports the hypothesis that non-sexual affiliative, flocking behaviors are both (1) rewarded by positive social interactions with conspecifics, and (2) reinforced because affiliative contact reduces a negative affective state caused by social isolation. We provide evidence from studies in European starlings, Sturnus vulgaris, that mu opioid receptors in the medial preoptic nucleus (mPOA) play a central role in both reward and the reduction of a negative affective state induced by social interactions in flocks, and discuss potential roles for nonapeptide/opioid interactions and steroid hormones. Finally, we develop the case that non-sexual affiliative social behaviors may be modified by two complementary output pathways from mPOA, with a projection from mPOA to the periaqueductal gray integrating information during social interactions that reduces negative affect and a projection from mPOA to the ventral tegmental area integrating information leading to social approach and reward.

Cholinergic activation affects the acute and chronic antinociceptive effects of morphine

Current studies indicate that the cholinergic and opioid systems interact to modulate pain. In the present work, we investigated the influence of the cholinesterase inhibitors, donepezil (0.5; 1 or 3mg/kg, i.p.) and rivastigmine (0.03; 0.5 or 1mg/kg, i.p.), on the acute antinociceptive effects of morphine (5mg/kg, i.p.) in the hot plate test in mice. Herein, both inhibitors were found to enhance and prolong the analgesic effects of morphine without affecting latencies themselves. In an extension of this work, we determined which cholinergic receptors subtype mediates the enhancement of analgesic effects of morphine, following inhibition of cholinesterases. In this part of the study, scopolamine (0.5mg/kg, i.p.), a muscarinic cholinergic receptors antagonist, but not mecamylamine (3mg/kg, i.p.), a nicotinic cholinergic receptors antagonist, reversed the enhancing effects of donepezil (3mg/kg, i.p.) and rivastigmine (1mg/kg, i.p.) on the morphine antinociception. Moreover, both cholinesterase inhibitors attenuated the development of tolerance to the antinociceptive effects of morphine. In contrast, acute administration of donepezil (3mg/kg, i.p.) or rivastigmine (1mg/kg, i.p.) on the day of expression of tolerance, had no effect on the already developed morphine tolerance. What is more, in both set of experiments, rivastigmine was slightly more potent than donepezil due to the broader inhibitory spectrum of this drug on acetylcholine degradation. Thus, our results suggest that the cholinesterase inhibitors, donepezil and rivastigmine, may be administered with morphine in order to enhance the latter's analgesic effects for the treatment of acute and chronic pain.

Muscarinic pain pharmacology: realizing the promise of novel analgesics by overcoming old challenges

The antinociceptive and analgesic effects of muscarinic receptor ligands in human and nonhuman species have been evident for more than half a century. In this review, we describe the current understanding of the roles of different muscarinic subtypes in pain modulation and their mechanism of action along the pain signaling pathway, including peripheral nociception, spinal cord pain processing, and supraspinal analgesia. Extensive preclinical and clinical validation of these mechanisms points to the development of selective muscarinic agonists as one of the most exciting and promising avenues toward novel pain medications.

Activation of μ-Opioid Receptors Inhibits Synaptic Inputs to Spinally Projecting Rostral Ventromedial Medulla Neurons

The rostral ventromedial medulla (RVM) is a major locus for the descending control of nociception and opioid analgesia. However, it is not clear how opioids affect synaptic inputs to RVM neurons. In this study, we determined the effect of mu-opioid receptor activation on excitatory and inhibitory synaptic transmission in spinally projecting RVM neurons. RVM neurons were retrogradely labeled with a fluorescent tracer injected into the dorsal horn of the spinal cord in rats. Whole-cell voltage-clamp recordings were performed on labeled RVM neurons in brain slices in vitro. The mu-receptor agonist [D-Ala(2),N-Me-Phe(4),Gly(5)-ol]-enkephalin (DAMGO, 1 microM) significantly decreased the amplitude of evoked excitatory postsynaptic currents (EPSCs) in 52% (9 of 17) of labeled cells. DAMGO also significantly reduced the amplitude of evoked inhibitory postsynaptic currents (IPSCs) in 69% (11 of 16) of cells examined. Furthermore, DAMGO significantly decreased the frequency of miniature EPSCs in 55% (15 of 27) of cells and significantly decreased the frequency of miniature IPSCs in all 12 cells studied. Although most EPSCs and IPSCs were mediated by glutamate and GABA, the nicotinic and glycine receptor antagonists attenuated EPSCs and IPSCs, respectively, in some labeled RVM neurons. Immunocytochemical labeling revealed that only 35% of recorded RVM neurons were tryptophan hydroxylase-positive, and 15% cells had GABA immunoreactivity. Thus, this study provides important functional evidence that activation of mu-opioid receptors decreases the release of both excitatory and inhibitory neurotransmitters onto most spinally projecting RVM neurons.

Reciprocal interactions between the amygdala and ventrolateral periaqueductal gray in mediating of Q/N(1-17)-induced analgesia in the rat

The opioid peptide, Orphanin FQ/nociceptin (OFQ/N(1-17))(,) its active fragments, and a related precursor peptide each produce analgesia following microinjection into the amygdala of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala is blocked by amygdala pretreatment of either general, mu, kappa, or delta-opioid antagonists even though OFQ/N(1-17) binds poorly to these receptor subtypes, and the antagonists bind poorly to the ORL-1/KOR-3 receptor. Agonists at mu and kappa opioid receptors as well as beta-endorphin each produce analgesia elicited from the amygdala that is blocked by opioid antagonist pretreatment in the ventrolateral periaqueductal gray (vlPAG) of rats. The present study examined whether pretreatment of general and selective opioid antagonists in the vlPAG blocked OFQ/N(1-17)-induced analgesia on the tail-flick test elicited from the amygdala, and whether pretreatment of general and selective opioid antagonists in the amygdala blocked OFQ/N(1-17)-induced analgesia elicited from the vlPAG of rats. OFQ/N(1-17)-induced analgesia elicited from the amygdala was significantly and markedly reduced following vlPAG pretreatment with a dose range of either naltrexone, beta-funaltrexamine (beta-FNA, mu), nor-binaltorphamine (NBNI, kappa) or naltrindole (NTI, delta). In contrast, opioid antagonists administered into misplaced mesencephalic control placements ventral and lateral to the vlPAG actually enhanced OFQ/N(1-17)-induced analgesia elicited from the amygdala. OFQ/N(1-17)-induced analgesia elicited from the vlPAG was significantly and markedly reduced following amygdala pretreatment with naltrexone and NBNI, to a lesser degree by NTI, and was unaffected by beta-FNA. Yet, opioid antagonists administered into misplaced amygdala control placements were generally ineffective in altering OFQ/N(1-17)-induced analgesia elicited from the vlPAG. Latencies were transiently increased by general, but not selective opioid antagonist treatment alone in the amygdala, but not the vlPAG. These data indicate reciprocal and regional interactions between the amygdala and vlPAG in the mediation of OFQ/N(1-17) by classic opioid receptor subtype antagonists in rats.

Acetylcholine receptors do not mediate the immobilization produced by inhaled anesthetics

Acetylcholine receptors transmit excitatory impulses, are broadly distributed throughout the central nervous system, and are particularly sensitive to the depressant effects of inhaled anesthetics. Thus these receptors are potential mediators of the immobility produced by inhaled anesthetics. We tested this potential in rats by giving intraperitoneal atropine, scopolamine, and mecamylamine to block muscarinic (atropine and scopolamine) and neuronal nicotinic (mecamylamine) acetylcholine receptors. Block with scopolamine (up to 100 mg/kg), atropine (10 mg/kg), mecamylamine (up to 4 mg/kg), or atropine (10 mg/kg) plus mecamylamine (up to 4 mg/kg) did not significantly decrease the isoflurane concentration required to suppress movement to noxious stimulation (minimum alveolar anesthetic concentration). We also gave atropine intrathecally, finding that the infusions that did not cause permanent paralysis produced slight or no decreases in the minimum alveolar anesthetic concentration. We conclude that acetylcholine receptors do not seem to play a role as mediators of immobilization by inhaled anesthetics.

IMPLICATIONS

Inhaled anesthetics produce two crucial effects: amnesia and immobility in the face of noxious stimulation. Block of muscarinic and neuronal nicotinic acetylcholine receptors in rats does not significantly decrease the isoflurane concentration required to suppress movement to stimulation. Thus, acetylcholine receptors do not seem to play a major role as mediators of the immobilization produced by inhaled anesthetics. Their capacity to mediate other effects of inhaled anesthetics (e.g., amnesia) remains to be tested.

Management of progressive pain in a patient with intramedullary chordoma of the spine

OBJECTIVES

The case here presented adequately reflects the difficulties involved in the treatment of pain in patients where the neuropathic component of pain predominates, and shows the different therapeutic steps that may be taken-from surgery and radiotherapy, to the administration of different drugs via the spinal route, to, finally, the presently little-used option of a direct intraventricular access.

CONCLUSIONS

Spinal tumors are infrequent, but pose great difficulties for the management and control of the pain they cause. The utility of the spinal route as an early approach for the provision of adequate analgesia seems clear. However, it also appears to lose efficacy with time, and dose incrementing and/or the addition of drugs that enhance the analgesic action of morphine are not always effective. In such selected cases, the intraventricular route may constitute a useful alternative, allowing improved symptoms control with lower morphine doses, and the use of the system previously implanted for intrathecal spinal infusion.

Evidence for epsilon-opioid receptor-mediated beta-endorphin-induced analgesia

Among the opioid receptors, which have been pharmacologically classified as mu, delta, kappa and epsilon, the existence of the epsilon receptor has been controversial, and this receptor is generally not recognized as a member of the opioid peptide receptor family because it has not been precisely characterized. However, results from pharmacological, physiological and opioid receptor binding studies clearly indicate the presence of epsilon-opioid receptors, which are distinct from mu-, delta- or kappa-opioid receptors. This putative epsilon-opioid receptor is stimulated supraspinally by the endogenous opioid peptide beta-endorphin, which induces the release of Met-enkephalin, which, in turn, acts on spinal delta2-opioid receptors to produce antinociception. In this article, this beta-endorphin-sensitive epsilon-opioid receptor-mediated descending pain control system, which is distinct from that activated by the mu-opioid receptor agonist morphine, is described and the physiological role of the beta-endorphin-mediated system in pain control activated by cold-water swimming and intraplantar injection of formalin is discussed.

Glutamate receptors and nociception: implications for the drug treatment of pain

Evidence from the last several decades indicates that the excitatory amino acid glutamate plays a significant role in nociceptive processing. Glutamate and glutamate receptors are located in areas of the brain, spinal cord and periphery that are involved in pain sensation and transmission. Glutamate acts at several types of receptors, including ionotropic (directly coupled to ion channels) and metabotropic (directly coupled to intracellular second messengers). Ionotropic receptors include those selectively activated by N-methyl-D-aspartate, alpha-amino-3-hydroxy-5-methylisoxazole-4-propionic acid and kainate. Metabotropic glutamate receptors are classified into 3 groups based on sequence homology, signal transduction mechanisms and receptor pharmacology. Glutamate also interacts with the opioid system, and intrathecal or systemic coadministration of glutamate receptor antagonists with opioids may enhance analgesia while reducing the development of opioid tolerance and dependence. The actions of glutamate in the brain seem to be more complex. Activation of glutamate receptors in some brain areas seems to be pronociceptive (e.g. thalamus, trigeminal nucleus), although activation of glutamate receptors in other brain areas seems to be antinociceptive (e.g. periaqueductal grey, ventrolateral medulla). Application of glutamate, or agonists selective for one of the several types of glutamate receptor, to the spinal cord or periphery induces nociceptive behaviours. Inhibition of glutamate release, or of glutamate receptors, in the spinal cord or periphery attenuates both acute and chronic pain in animal models. Similar benefits have been seen in studies involving humans (both patients and volunteers); however, results have been inconsistent. More research is needed to clearly define the role of existing treatment options and explore the possibilities for future drug development.

Endogenous opiates: 1999

This paper is the twenty-second installment of the annual review of research concerning the opiate system. It summarizes papers published during 1999 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; learning, memory, and reward; eating and drinking; alcohol and other drugs of abuse; sexual activity, pregnancy, and development; mental illness and mood; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; gastrointestinal, renal, and hepatic function; cardiovascular responses; respiration and thermoregulation; and immunologic responses.

Supraspinal circuitry mediating opioid antinociception: antagonist and synergy studies in multiple sites

Supraspinal opioid antinociception is mediated by sensitive brain sites capable of supporting this response following microinjection of opioid agonists. These sites include the ventrolateral periaqueductal gray (vIPAG), the rostral ventromedial medulla (RVM), the locus coeruleus and the amygdala. Each of these sites comprise an interconnected anatomical and physiologically relevant system mediating antinociceptive responses through regional interactions. Such interactions have been identified using two pharmacological approaches: (1) the ability of selective antagonists delivered to one site to block antinociception elicited by opioid agonists in a second site, and (2) the presence of synergistic antinociceptive interactions following simultaneous administration of subthreshold doses of opioid agonists into pairs of sites. Thus, the RVM has essential serotonergic, opioid, cholinergic and NMDA synapses that are necessary for the full expression of morphine antinociception elicited from the vIPAG, and the vIPAG has essential opioid synapses that are necessary for the full expression of opioid antinociception elicited from the amygdala. Further, the vIPAG, RVM, locus coeruleus and amygdala interact with each other in synergistically supporting opioid antinociception.

Analysis of sex and gonadectomy differences in beta-endorphin antinociception elicited from the ventrolateral periaqueductal gray in rats

Male rats exhibit significantly greater antinociception following central administration of morphine than female rats. The present study examined potential differences in beta-endorphin (5.2-26 microg) antinociception elicited from the ventrolateral periaqueductal gray in adult sham-operated and gonadectomized male and female rats. Male rats displayed significantly greater peak (30 min) tail-flick latencies across the entire range of beta-endorphin doses administered into the ventrolateral periaqueductal gray than female rats tested during the estrous phase of the estrous cycle. Adult gonadectomy failed to appreciably change the pattern of this effect in either males of females. Thus, antinociception elicited from the ventrolateral periaqueductal gray by beta-endorphin, like morphine, is sensitive to sex differences.