Systematic examination in the rat of brain sites sensitive to the direct application of morphine: observation of differential effects within the periaqueductal gray

Copaifera langsdorffii Desf. tree oleoresin-induced antinociception recruits µ1- and κ -opioid receptors in the ventrolateral columns of the periaqueductal gray matter

Popular medicine has been using oleoresin from several species of copaíba tree for the treatment of various diseases and its clinical administration potentially causes antinociception. Electrical stimulation of ventrolateral (vlPAG) and dorsolateral (dlPAG) columns of the periaqueductal gray matter also causes antinociception. The aim this study was to verify the antinociceptive effect of oleoresin extracted from Copaifera langsdorffii tree and to test the hypothesis that oleoresin-induced antinociception is mediated by µ1- and κ-opioid receptors in the vlPAG and dlPAG. Nociceptive thresholds were determined by the tail-flick test in Wistar rats. The copaíba tree oleoresin was administered at different doses (50, 100 and 200 mg/kg) through the gavage technique. After the specification of the most effective dose of copaíba tree oleoresin (200 mg/kg), rats were pretreated with either the µ1-opioid receptor selective antagonist naloxonazine (at 0.05, 0.5 and 5 µg/ 0.2 µl in vlPAG, and 5 µg/ 0.2 µl in dlPAG) or the κ-opioid receptor selective antagonist nor-binaltorphimine (at 1, 3 and 9 nmol/ 0.2 µl in vlPAG, and 9 nmol/ 0.2 µl in dlPAG). The blockade of µ1 and κ opioid receptors of vlPAG decreased the antinociception produced by copaíba tree oleoresin. However, the blockade of these receptors in dlPAG did not alter copaíba tree oleoresin-induced antinociception. These data suggest that vlPAG µ1 and κ opioid receptors are critically recruited in the antinociceptive effect produced by oleoresin extracted from Copaifera langsdorffii.

In vivo photopharmacology with light-activated opioid drugs

Traditional methods for site-specific drug delivery in the brain are slow, invasive, and difficult to interface with recordings of neural activity. Here, we demonstrate the feasibility and experimental advantages of in vivo photopharmacology using "caged" opioid drugs that are activated in the brain with light after systemic administration in an inactive form. To enable bidirectional manipulations of endogenous opioid receptors in vivo, we developed photoactivatable oxymorphone (PhOX) and photoactivatable naloxone (PhNX), photoactivatable variants of the mu opioid receptor agonist oxymorphone and the antagonist naloxone. Photoactivation of PhOX in multiple brain areas produced local changes in receptor occupancy, brain metabolic activity, neuronal calcium activity, neurochemical signaling, and multiple pain- and reward-related behaviors. Combining PhOX photoactivation with optical recording of extracellular dopamine revealed adaptations in the opioid sensitivity of mesolimbic dopamine circuitry in response to chronic morphine administration. This work establishes a general experimental framework for using in vivo photopharmacology to study the neural basis of drug action.

Cell-type specific molecular architecture for mu opioid receptor function in pain and addiction circuits

Opioids are potent analgesics broadly used for pain management; however, they can produce dangerous side effects including addiction and respiratory depression. These harmful effects have led to an epidemic of opioid abuse and overdose deaths, creating an urgent need for the development of both safer pain medications and treatments for opioid use disorders. Both the analgesic and addictive properties of opioids are mediated by the mu opioid receptor (MOR), making resolution of the cell types and neural circuits responsible for each of the effects of opioids a critical research goal. Single-cell RNA sequencing (scRNA-seq) technology is enabling the identification of MOR-expressing cell types throughout the nervous system, creating new opportunities for mapping distinct opioid effects onto newly discovered cell types. Here, we describe molecularly defined MOR-expressing neuronal cell types throughout the peripheral and central nervous systems and their potential contributions to opioid analgesia and addiction.

How Do Expectations Modulate Pain? A Motivational Perspective

Expectations can profoundly modulate pain experience, during which the periaqueductal gray (PAG) plays a pivotal role. In this article, we focus on motivationally evoked neural activations in cortical and brainstem regions both before and during stimulus administration, as has been demonstrated by experimental studies on pain-modulatory effects of expectations, in the hope of unraveling how the PAG is involved in descending and ascending nociceptive processes. This motivational perspective on expectancy effects on the perception of noxious stimuli sheds new light on psychological and neuronal substrates of pain and its modulation, thus having important research and clinical implications.

Persistent inflammation selectively activates opioid-sensitive phasic-firing neurons within the vlPAG

The ventrolateral periaqueductal gray (vlPAG) is a key brain area within the descending pain modulatory pathway and an important target for opioid-induced analgesia. The vlPAG contains heterogeneous neurons with respect to neurotransmitter content, receptor and channel expression, and in vivo response to noxious stimuli. This study characterizes intrinsic membrane properties of vlPAG neurons to identify neuron types that respond to inflammation and determine whether the pain-responsive neurons are inhibited by opioids. Surveying 382 neurons identified four neuron types with distinct intrinsic firing patterns: Phasic (48%), Tonic (33%), Onset (10%), and Random (9%). Mu-opioid receptor (MOR) expression was determined by the ability of a selective MOR agonist (DAMGO) to activate G protein-coupled inwardly rectifying potassium channel (GIRK) currents. Opioid-sensitive neurons were observed within each neuron type. Opioid sensitivity did not correlate with other intrinsic firing features, including low-threshold spiking that has been previously proposed to identify opioid-sensitive GABAergic neurons in the vlPAG of mice. Complete Freund's adjuvant (CFA)-induced acute inflammation (2 h) had no effect on vlPAG neuron firing patterns. However, persistent inflammation (5-7 days) selectively activated Phasic neurons through a significant reduction in their firing threshold. Opioid-sensitive neurons were strongly activated compared with the opioid-insensitive Phasic neurons. Overall, this study provides a framework to further identify neurons activated by persistent inflammation so that they may be targeted for future pain therapies.NEW & NOTEWORTHY Intrinsic firing properties define four distinct vlPAG neuron populations, and a subset of each population expresses MORs coupled to GIRK channels. Persistent, but not acute, inflammation selectively activates opioid-sensitive Phasic vlPAG neurons. Although the vlPAG is known to contribute to the descending inhibition of pain, the activation of a single physiologically defined neuron type in the presence of persistent inflammation represents a mechanism by which the vlPAG participates in descending facilitation of pain.

In vivo photopharmacology with light-activated opioid drugs

Traditional methods for site-specific drug delivery in the brain are slow, invasive, and difficult to interface with recordings of neural activity. Here, we demonstrate the feasibility and experimental advantages of in vivo photopharmacology using "caged" opioid drugs that are activated in the brain with light after systemic administration in an inactive form. To enable bidirectional manipulations of endogenous opioid receptors in vivo , we developed PhOX and PhNX, photoactivatable variants of the mu opioid receptor agonist oxymorphone and the antagonist naloxone. Photoactivation of PhOX in multiple brain areas produced local changes in receptor occupancy, brain metabolic activity, neuronal calcium activity, neurochemical signaling, and multiple pain- and reward-related behaviors. Combining PhOX photoactivation with optical recording of extracellular dopamine revealed adaptations in the opioid sensitivity of mesolimbic dopamine circuitry during chronic morphine administration. This work establishes a general experimental framework for using in vivo photopharmacology to study the neural basis of drug action.

Highlights

A photoactivatable opioid agonist (PhOX) and antagonist (PhNX) for in vivo photopharmacology. Systemic pro-drug delivery followed by local photoactivation in the brain. In vivo photopharmacology produces behavioral changes within seconds of photostimulation. In vivo photopharmacology enables all-optical pharmacology and physiology.

A Regional and Projection-Specific Role of RGSz1 in the Ventrolateral Periaqueductal Grey in the Modulation of Morphine Reward

Opioid analgesics exert their therapeutic and adverse effects by activating μ opioid receptors (MOPR); however, functional responses to MOPR activation are modulated by distinct signal transduction complexes within the brain. The ventrolateral periaqueductal gray (vlPAG) plays a critical role in modulation of nociception and analgesia, but the exact intracellular pathways associated with opioid responses in this region are not fully understood. We previously showed that knockout of the signal transduction modulator Regulator of G protein Signaling z1 (RGSz1) enhanced analgesic responses to opioids, whereas it decreased the rewarding efficacy of morphine. Here, we applied viral mediated gene transfer methodology and delivered adeno-associated virus (AAV) expressing Cre recombinase to the vlPAG of RGSz1fl\fl mice to demonstrate that downregulation of RGSz1 in this region decreases sensitivity to morphine in the place preference paradigm, under pain-free as well as neuropathic pain states. We also used retrograde viral vectors along with flippase-dependent Cre vectors to conditionally downregulate RGSz1 in vlPAG projections to the ventral tegmental area (VTA) and show that downregulation of RGSz1 prevents the development of place conditioning to low morphine doses. Consistent with the role for RGSz1 as a negative modulator of MOPR activity, RGSz1KO enhances opioid-induced cAMP inhibition in periaqueductal gray (PAG) membranes. Furthermore, using a new generation of bioluminescence resonance energy transfer (BRET) sensors, we demonstrate that RGSz1 modulates Gαz but not other Gαi family subunits and selectively impedes MOPR-mediated Gαz signaling events invoked by morphine and other opioids. Our work highlights a regional and circuit-specific role of the G protein-signaling modulator RGSz1 in morphine reward, providing insights on midbrain intracellular pathways that control addiction-related behaviors. SIGNIFICANCE STATEMENT: This study used advanced genetic mouse models to highlight the role of the signal transduction modulator named RGSz1 in responses to clinically used opioid analgesics. We show that RGSz1 controls the rewarding efficacy of opioids by actions in ventrolateral periaqueductal gray projections to the ventral tegmental area, a key component of the midbrain dopamine pathway. These studies highlight novel mechanisms by which pain-modulating structures control the rewarding efficacy of opioids.

Cellular and circuit diversity determines the impact of endogenous opioids in the descending pain modulatory pathway

The descending pain modulatory pathway exerts important bidirectional control of nociceptive inputs to dampen and/or facilitate the perception of pain. The ventrolateral periaqueductal gray (vlPAG) integrates inputs from many regions associated with the processing of nociceptive, cognitive, and affective components of pain perception, and is a key brain area for opioid action. Opioid receptors are expressed on a subset of vlPAG neurons, as well as on both GABAergic and glutamatergic presynaptic terminals that impinge on vlPAG neurons. Microinjection of opioids into the vlPAG produces analgesia and microinjection of the opioid receptor antagonist naloxone blocks stimulation-mediated analgesia, highlighting the role of endogenous opioid release within this region in the modulation of nociception. Endogenous opioid effects within the vlPAG are complex and likely dependent on specific neuronal circuits activated by acute and chronic pain stimuli. This review is focused on the cellular heterogeneity within vlPAG circuits and highlights gaps in our understanding of endogenous opioid regulation of the descending pain modulatory circuits.

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.

Hyporesponsivity to mu-opioid receptor agonism in the Wistar-Kyoto rat model of altered nociceptive responding associated with negative affective state

ABSTRACT

Chronic pain is often comorbid with anxiety and depression, altering the level of perceived pain, which negatively affects therapeutic outcomes. The role of the endogenous mu-opioid receptor (MOP) system in pain-negative affect interactions and the influence of genetic background thereon are poorly understood. The inbred Wistar-Kyoto (WKY) rat, which mimics aspects of anxiety and depression, displays increased sensitivity (hyperalgesia) to noxious stimuli, compared with Sprague-Dawley (SD) rats. Here, we report that WKY rats are hyporesponsive to the antinociceptive effects of systemically administered MOP agonist morphine in the hot plate and formalin tests, compared with SD counterparts. Equivalent plasma morphine levels in the 2 rat strains suggested that these differences in morphine sensitivity were unlikely to be due to strain-related differences in morphine pharmacokinetics. Although MOP expression in the ventrolateral periaqueductal gray (vlPAG) did not differ between WKY and SD rats, the vlPAG was identified as a key locus for the hyporesponsivity to MOP agonism in WKY rats in the formalin test. Moreover, morphine-induced effects on c-Fos (a marker of neuronal activity) in regions downstream of the vlPAG, namely, the rostral ventromedial medulla and lumbar spinal dorsal horn, were blunted in the WKY rats. Together, these findings suggest that a deficit in the MOP-induced recruitment of the descending inhibitory pain pathway may underlie hyperalgesia to noxious inflammatory pain in the WKY rat strain genetically predisposed to negative affect.

Central Nervous System Targets: Supraspinal Mechanisms of Analgesia

While the acute sensation of pain is protective, signaling the presence of actual or potential bodily harm, its persistence is unpleasant. When pain becomes chronic, it has limited evolutionarily advantage. Despite the differing nature of acute and chronic pain, a common theme is that sufferers seek pain relief. The possibility to medicate pain types as varied as a toothache or postsurgical pain reflects the diverse range of mechanism(s) by which pain-relieving "analgesic" therapies may reduce, eliminate, or prevent pain. Systemic application of an analgesic able to cross the blood-brain barrier can result in pain modulation via interaction with targets at different sites in the central nervous system. A so-called supraspinal mechanism of action indicates manipulation of a brain-defined circuitry. Pre-clinical studies demonstrate that, according to the brain circuitry targeted, varying therapeutic pain-relieving effects may be observed that relate to an impact on, for example, sensory and/or affective qualities of pain. In many cases, this translates to the clinic. Regardless of the brain circuitry manipulated, modulation of brain processing often directly impacts multiple aspects of nociceptive transmission, including spinal neuronal signaling. Consideration of supraspinal mechanisms of analgesia and ensuing pain relief must take into account nonbrain-mediated effects; therefore, in this review, the supraspinally mediated analgesic actions of opioidergic, anti-convulsant, and anti-depressant drugs are discussed. The persistence of poor treatment outcomes and/or side effect profiles of currently used analgesics highlight the need for the development of novel therapeutics or more precise use of available agents. Fully uncovering the complex biology of nociception, as well as currently used analgesic mechanism(s) and site(s) of action, will expedite this process.

Endogenous opioid peptides in the descending pain modulatory circuit

The opioid epidemic has led to a serious examination of the use of opioids for the treatment of pain. Opioid drugs are effective due to the expression of opioid receptors throughout the body. These receptors respond to endogenous opioid peptides that are expressed as polypeptide hormones that are processed by proteolytic cleavage. Endogenous opioids are expressed throughout the peripheral and central nervous system and regulate many different neuronal circuits and functions. One of the key functions of endogenous opioid peptides is to modulate our responses to pain. This review will focus on the descending pain modulatory circuit which consists of the ventrolateral periaqueductal gray (PAG) projections to the rostral ventromedial medulla (RVM). RVM projections modulate incoming nociceptive afferents at the level of the spinal cord. Stimulation within either the PAG or RVM results in analgesia and this circuit has been studied in detail in terms of the actions of exogenous opioids, such as morphine and fentanyl. Further emphasis on understanding the complex regulation of endogenous opioids will help to make rational decisions with regard to the use of opioids for pain. We also include a discussion of the actions of endogenous opioids in the amygdala, an upstream brain structure that has reciprocal connections to the PAG that contribute to the brain's response to pain.

Periaqueductal Gray and Rostromedial Tegmental Inhibitory Afferents to VTA Have Distinct Synaptic Plasticity and Opiate Sensitivity

The ventral tegmental area (VTA) is a major target of addictive drugs and receives multiple GABAergic projections originating outside the VTA. We describe differences in synaptic plasticity and behavior when optogenetically driving two opiate-sensitive GABAergic inputs to the VTA, the rostromedial tegmental nucleus (RMTg), and the periaqueductal gray (PAG). Activation of GABAergic RMTg terminals in the VTA in vivo is aversive, and low-frequency stimulation induces long-term depression in vitro. Low-frequency stimulation of PAG afferents in vitro unexpectedly causes long-term potentiation. Opioid receptor activation profoundly depresses PAG and RMTg inhibitory synapses but prevents synaptic plasticity only at PAG synapses. Activation of the GABAergic PAG terminals in the VTA promotes immobility, and optogenetically-driven immobility is blocked by morphine. Our data reveal the PAG as a source of highly opioid-sensitive GABAergic afferents and support the idea that different GABAergic pathways to the VTA control distinct behaviors.

Restoration of Cyclo-Gly-Pro-induced salivary hyposecretion and submandibular composition by naloxone in mice

Cyclo-Gly-Pro (CGP) attenuates nociception, however its effects on salivary glands remain unclear. In this study, we investigated the acute effects of CGP on salivary flow and composition, and on the submandibular gland composition, compared with morphine. Besides, we characterized the effects of naloxone (a non-selective opioid receptor antagonist) on CGP- and morphine-induced salivary and glandular alterations in mice. After that, in silico analyses were performed to predict the interaction between CGP and opioid receptors. Morphine and CGP significantly reduced salivary flow and total protein concentration of saliva and naloxone restored them to the physiological levels. Morphine and CGP also reduced several infrared vibrational modes (Amide I, 1687-1594cm^-1; Amide II, 1594-1494cm^-1; CH₂/CH₃, 1488-1433cm^-1; C = O, 1432-1365cm^-1; PO₂ asymmetric, 1290-1185cm^-1; PO₂ symmetric, 1135-999cm^-1) and naloxone reverted these alterations. The in silico docking analysis demonstrated the interaction of polar contacts between the CGP and opioid receptor Cys219 residue. Altogether, we showed that salivary hypofunction and glandular changes elicited by CGP may occur through opioid receptor suggesting that the blockage of opioid receptors in superior cervical and submandibular ganglions may be a possible strategy to restore salivary secretion while maintaining antinociceptive action due its effects on the central nervous system.

Dopaminergic mechanisms in periaqueductal gray-mediated antinociception

As important as perceiving pain is the ability to modulate this perception in some contextual salient situations. The periaqueductal gray (PAG) is perhaps the most important site of endogenous pain modulation; however, little is known about dopaminergic mechanisms underlying PAG-mediated antinociception. In this study, we used a pharmacological approach to evaluate this subject. We found that µ-opioid receptor-induced antinociception (DAMGO, 0.3 μg) from PAG was blocked by the coadministration of either D1-like or D2-like dopaminergic antagonists (SCH23390, 2, 4, and 6 μg or raclopride, 2 and 4 μg, respectively) both in the tail-flick and in the mechanical paw-withdrawal test. A selective D2-like receptor agonist (piribedil, 6 and 12 μg into the PAG) induced antinociception in the mechanical paw-withdrawal test, but not in the tail-flick test. This effect was blocked by the coadministration of its selective antagonist (raclopride 4 μg), as well as by either a GABAA agonist (muscimol, 0.1 μg) or an opioid receptor antagonist (naloxone, 0.5 μg). A selective D1-like receptor agonist (SKF38393, 1, 5, and 10 μg into the PAG) induced a poor and transient antinociceptive effect, but when combined with piribedil, a potentiated antinociceptive effect emerged. None of these treatments affected locomotion in the open-field test. These findings suggest that µ-opioid antinociception from the PAG depends on dopamine acting on both D1-like and D2-like receptors. Selective activation of PAG D2-like receptors induces antinociception mediated by supraspinal mechanisms dependent on inhibition of GABAA and activation of opioid neurotransmission.

Yin-and-yang bifurcation of opioidergic circuits for descending analgesia at the midbrain of the mouse

In the descending analgesia pathway, opioids are known to disinhibit the projections from the periaqueductal gray (PAG) to the rostral ventromedial medulla (RVM), leading to suppression of pain signals at the spinal cord level. The locus coeruleus (LC) has been proposed to engage in the descending pathway through noradrenergic inputs to the spinal cord. Nevertheless, how the LC is integrated in the descending analgesia circuit has remained unknown. Here, we show that the opioidergic analgesia pathway is bifurcated in structure and function at the PAG. A knockout as well as a PAG-specific knockdown of phospholipase C β4 (PLCβ4), a signaling molecule for G protein-coupled receptors, enhanced swim stress-induced and morphine-induced analgesia in mice. Immunostaining after simultaneous retrograde labeling from the RVM and the LC revealed two mutually exclusive neuronal populations at the PAG, each projecting either to the LC or the RVM, with PLCβ4 expression only in the PAG-LC projecting cells that provide a direct synaptic input to LC-spinal cord (SC) projection neurons. The PAG-LC projection neurons in wild-type mice turned quiescent in response to opiates, but remained active in the PLCβ4 mutant, suggesting a possibility that an increased adrenergic function induced by the persistent PAG-LC activity underlies the enhanced opioid analgesia in the mutant. Indeed, the enhanced analgesia in the mutant was reversed by blocking α2-noradrenergic receptors. These findings indicate that opioids suppress descending analgesia through the PAG-LC pathway, while enhancing it through the PAG-RVM pathway, i.e., two distinct pathways with opposing effects on opioid analgesia. These results point to a therapeutic target in pain control.

Regulators of G-Protein Signaling (RGS) Proteins Promote Receptor Coupling to G-Protein-Coupled Inwardly Rectifying Potassium (GIRK) Channels

Regulators of G-protein signaling (RGS) proteins negatively modulate presynaptic μ-opioid receptor inhibition of GABA release in the ventrolateral periaqueductal gray (vlPAG). Paradoxically, we find that G-protein-coupled receptor (GPCR) activation of G-protein-gated inwardly rectifying K⁺ channels (GIRKs) in the vlPAG is reduced in an agonist- and receptor-dependent manner in transgenic knock-in mice of either sex expressing mutant RGS-insensitive Gαo proteins. μ-Opioid receptor agonist activation of GIRK currents was reduced for DAMGO and fentanyl but not for [Met⁵]-enkephalin acetate salt hydrate (ME) in the RGS-insensitive heterozygous (Het) mice compared with wild-type mice. The GABA_B agonist baclofen-induced GIRK currents were also reduced in the Het mice. We confirmed the role of Gαo proteins in μ-opioid receptor and GABA_B receptor signaling pathways in wild-type mice using myristoylated peptide inhibitors of Gαo₁ and Gαi_1-3 The results using these inhibitors indicate that receptor activation of GIRK channels is dependent on the preference of the agonist-stimulated receptor for Gαo versus that for Gαi. DAMGO and fentanyl-mediated GIRK currents were reduced in the presence of the Gαo₁ inhibitor, but not the Gαi_1-3 inhibitors. In contrast, the Gαo₁ peptide inhibitor did not affect ME activation of GIRK currents, which is consistent with results in the Het mice, but the Gαi_1-3 inhibitors significantly reduced ME-mediated GIRK currents. Finally, the reduction in GIRK activation in the Het mice plays a role in opioid- and baclofen-mediated spinal antinociception, but not supraspinal antinociception. Thus, our studies indicate that RGS proteins have multiple mechanisms of modulating GPCR signaling that produce negative and positive regulation of signaling depending on the effector.SIGNIFICANCE STATEMENT Regulators of G-protein signaling (RGS) proteins positively modulate GPCR coupling to GIRKs, and this coupling is critical for opioid- and baclofen-mediated spinal antinociception, whereas μ-opioid receptor-mediated supraspinal antinociception depends on presynaptic inhibition that is negatively regulated by RGS proteins. The identification of these opposite roles for RGS proteins has implications for signaling via other GPCRs.

Periaqueductal efferents to dopamine and GABA neurons of the VTA

Neurons in the periaqueductal gray (PAG) modulate threat responses and nociception. Activity in the ventral tegmental area (VTA) on the other hand can cause reinforcement and aversion. While in many situations these behaviors are related, the anatomical substrate of a crosstalk between the PAG and VTA remains poorly understood. Here we describe the anatomical and electrophysiological organization of the VTA-projecting PAG neurons. Using rabies-based, cell type-specific retrograde tracing, we observed that PAG to VTA projection neurons are evenly distributed along the rostro-caudal axis of the PAG, but concentrated in its posterior and ventrolateral segments. Optogenetic projection targeting demonstrated that the PAG-to-VTA pathway is predominantly excitatory and targets similar proportions of Ih-expressing VTA DA and GABA neurons. Taken together, these results set the framework for functional analysis of the interplay between PAG and VTA in the regulation of reward and aversion.

Short-Term Immunosuppression Enhances Long-Term Survival of Bovine Chromaffin Cell Xenografts in Rat Cns

Xenogeneic donors, a largely untapped resource, would solve many of the problems associated with the limited availability of human donor tissue for neural transplantation. Previous work in our laboratory has revealed that xenografts of isolated bovine chromaffin cells survive transplantation into the periaqueductal gray (PAG) of immunosuppressed adult rats. Electron microscopic analysis reveals that graft sites contain healthy chromaffin cells, but do not contain host immune cells typical of graft rejection.

The aim of the current study was to assess the necessary conditions for long-term survival of bovine chromaffin cell xenografts in the central nervous system (CNS). In particular, the need for short-course vs. permanent immunosuppressive therapy with cyclosporine A (CsA) for the long-term survival of grafted bovine chromaffin cells was addressed. Grafts from animals receiving continuous CsA treatment for either 3, 6, or 12 wk contained large clumps of dopamines-β-hydroxylase (DBH) positive cells in contrast to the few surviving cells observed in nonimmunosuppressed animals. In addition, grafts from animals that had CsA treatment terminated at 3 or 6 wk contained similarly large clumps of DBH-positive cells. Furthermore, short-term immunosuppression (3 wk) appeared to enhance the long-term survival of grafted cells, since clumps of DBH staining cells could still be positively identified in the host PAG at least 1 yr after transplantation.

Complete rejection of graft tissue depends on several factors, such as blood–brain barrier integrity, the presence of major histocompatability complex (MHC) antigens in either the host or graft, and the status of the host immune system. By using a suspension of isolated bovine chromaffin cells, potential MHC antigen presenting cells, such as endothelial cells, are eliminated. In addition, CsA treatment may negate the immunologic consequences of increased blood–brain barrier permeability following surgical trauma by attenuating the host cell mediated response. In summary, long-term survival of isolated chromaffin cell xenografts in the rat CNS may be attained by a short-term course of CsA.

Upregulation of fatty acid amide hydrolase in the dorsal periaqueductal gray is associated with neuropathic pain and reduced heart rate in rats

Nerve damage can induce a heightened pain response to noxious stimulation, which is termed hyperalgesia. Pain itself acts as a stressor, initiating autonomic and sensory effects through the dorsal periaqueductal gray (dPAG) to induce both sympathoexcitation and analgesia, which prior studies have shown to be affected by endocannabinoid signaling. The present study addressed the hypothesis that neuropathic pain disrupts autonomic and analgesic regulation by endocannabinoid signaling in the dPAG. Endocannabinoid contents, transcript levels of endocannabinoid signaling components, and catabolic enzyme activity were analyzed in the dPAG of rats at 21 days after painful nerve injury. The responses to two nerve injury models were similar, with two-thirds of animals developing hyperalgesia that was maintained throughout the postinjury period, whereas no sustained change in sensory function was observed in the remaining rats. Anandamide content was lower in the dPAG of rats that developed sustained hyperalgesia, and activity of the catabolic enzyme fatty acid amide hydrolase (FAAH) was higher. Intensity of hyperalgesia was correlated to transcript levels of FAAH and negatively correlated to heart rate and sympathovagal balance. These data suggest that maladaptive endocannabinoid signaling in the dPAG after nerve injury could contribute to chronic neuropathic pain and associated autonomic dysregulation. This study demonstrates that reduced anandamide content and upregulation of FAAH in the dPAG are associated with hyperalgesia and reduced heart rate sustained weeks after nerve injury. These data provide support for the evaluation of FAAH inhibitors for the treatment of chronic neuropathic pain.

Entanglement between thermoregulation and nociception in the rat: the case of morphine

In thermoneutral conditions, rats display cyclic variations of the vasomotion of the tail and paws, the most widely used target organs in current acute or chronic animal models of pain. Systemic morphine elicits their vasoconstriction followed by hyperthermia in a naloxone-reversible and dose-dependent fashion. The dose-response curves were steep with ED50 in the 0.5-1 mg/kg range. Given the pivotal functional role of the rostral ventromedial medulla (RVM) in nociception and the rostral medullary raphe (rMR) in thermoregulation, two largely overlapping brain regions, the RVM/rMR was blocked by muscimol: it suppressed the effects of morphine. "On-" and "off-" neurons recorded in the RVM/rMR are activated and inhibited by thermal nociceptive stimuli, respectively. They are also implicated in regulating the cyclic variations of the vasomotion of the tail and paws seen in thermoneutral conditions. Morphine elicited abrupt inhibition and activation of the firing of on- and off-cells recorded in the RVM/rMR. By using a model that takes into account the power of the radiant heat source, initial skin temperature, core body temperature, and peripheral nerve conduction distance, one can argue that the morphine-induced increase of reaction time is mainly related to the morphine-induced vasoconstriction. This statement was confirmed by analyzing in psychophysical terms the tail-flick response to random variations of noxious radiant heat. Although the increase of a reaction time to radiant heat is generally interpreted in terms of analgesia, the present data question the validity of using such an approach to build a pain index.

Different processing of meningeal and cutaneous pain information in the spinal trigeminal nucleus caudalis

Introduction

Within superficial trigeminal nucleus caudalis (Sp5C) (laminae I/II), meningeal primary afferents project exclusively to lamina I, whereas nociceptive cutaneous ones distribute in both lamina I and outer lamina II. Whether such a relative absence of meningeal inputs to lamina II represents a fundamental difference from cutaneous pathways in the central processing of sensory information is still unknown.

Methods

We recorded extracellular field potentials in the superficial Sp5C of anesthetised rats evoked by electrically stimulating the dura mater, to selectively assess the synaptic transmission between meningeal primary afferents and second-order Sp5C neurons, the first synapse in trigeminovascular pathways. We tested the effect of systemic morphine and local glycinergic and GABAAergic disinhibition.

Results

Meningeal stimulation evokes two negative field potentials in superficial Sp5C. The conduction velocities of the activated primary afferents are within the Aδ- and C-fibre ranges. Systemic morphine specifically suppresses meningeal C-fibre-evoked field potentials, and this effect is reversed by systemic naloxone. Segmental glycinergic or GABAAergic disinhibition strongly potentiates meningeal C-fibre-evoked field potentials but not Aδ-fibre ones. Interestingly, the same segmental disinhibition conversely potentiates cutaneous Aδ-fibre-evoked field potentials and suppresses C-fibre ones.

Conclusion

These findings reveal that the different anatomical organization of meningeal and cutaneous inputs into superficial Sp5C is associated with a different central processing of meningeal and cutaneous pain information within Sp5C. Moreover, they suggest that the potentiation upon local disinhibition of the first synapse in trigeminovascular pathways may contribute to the generation of headache pain.

Relative contribution of the dorsal raphe nucleus and ventrolateral periaqueductal gray to morphine antinociception and tolerance in the rat

The dorsal raphe nucleus (DRN) is embedded in the ventral part of the caudal periaqueductal gray (PAG). Electrical or chemical activation of neurons throughout this region produces antinociception. The objective of this manuscript is to determine whether the ventrolateral PAG and DRN are distinct antinociceptive systems. This hypothesis was tested by determining the antinociceptive potency of microinjecting morphine into each structure (Experiment 1), creating a map of effective microinjection sites that produce antinociception (Experiment 2) and comparing the development of antinociceptive tolerance to repeated microinjections of morphine into the ventrolateral PAG and DRN (Experiment 3). Morphine was more potent following cumulative injections (1.0, 2.2, 4.6 & 10 μg/0.2 μL) into the ventrolateral PAG (D₅₀  = 3.3 μg) compared to the lateral (4.3 μg) or medial DRN (5.8 μg). Antinociception occurred following 94% of the morphine injections into the ventrolateral PAG, whereas only 68.3% and 78.3% of the injections into the lateral and medial aspects of the DRN produced antinociception. Repeated microinjections of morphine into the ventrolateral PAG produced tolerance as indicated by a 528% difference in potency between morphine and saline pretreated rats. In contrast, relatively small changes in potency occurred following repeated microinjections of morphine into the lateral and medial aspects of the DRN (107% and 49%, respectively). These data indicate that the ventrolateral PAG and DRN are distinct antinociceptive structures. Antinociception is greater with injections into the ventrolateral PAG compared to the DRN, but this antinociception disappears rapidly because of the development of tolerance.

Analysis of morphine-induced changes in the activity of periaqueductal gray neurons in the intact rat

Microinjection of morphine into the periaqueductal gray (PAG) produces antinociception. In vitro slice recordings indicate that all PAG neurons are sensitive to morphine either by direct inhibition or indirect disinhibition. We tested the hypothesis that all PAG neurons respond to opioids in vivo by examining the extracellular activity of PAG neurons recorded in lightly anesthetized and awake rats. Spontaneous activity was less than 1Hz in most neurons. Noxious stimuli (heat, pinch) caused an increase in activity in 57% and 75% of the neurons recorded in anesthetized and awake rats, respectively. The same noxious stimuli caused a decrease in activity in only 17% and 6% of neurons recorded in anesthetized and awake rats. Systemic administration of morphine caused approximately equal numbers of neurons to increase, decrease, or show no change in activity in lightly anesthetized rats. In contrast, administration of morphine caused an increase in the activity of 22 of the 27 neurons recorded in awake rats. No change in activity was evident in the remaining five neurons. Changes in activity caused by morphine were surprisingly modest (a median increase from 0.7 to 1.3Hz). The small inconsistent effects of morphine are in stark contrast to the large changes produced by morphine on the activity of rostral ventromedial medulla (RVM) neurons or the widespread inhibition and excitation of PAG neurons treated with opioids in in vitro slice experiments. The relatively modest effects of morphine in the present study suggest that morphine produces antinociception by causing small changes in the activity of many PAG neurons.

Stress induces analgesia via orexin 1 receptor-initiated endocannabinoid/CB1 signaling in the mouse periaqueductal gray

The orexin system consists of orexin A/hypocretin 1 and orexin B/hypocretin 2, and OX1 and OX2 receptors. Our previous electrophysiological study showed that orexin A in the rat ventrolateral periaqueductal gray (vlPAG) induced antinociception via an OX1 receptor-initiated and endocannabinoid-mediated disinhibition mechanism. Here, we further characterized antinociceptive effects of orexins in the mouse vlPAG and investigated whether this mechanism in the vlPAG can contribute to stress-induced analgesia (SIA) in mice. Intra-vlPAG (i.pag.) microinjection of orexin A in the mouse vlPAG increased the hot-plate latency. This effect was mimicked by i.pag. injection of WIN 55,212-2, a CB1 agonist, and antagonized by i.pag. injection of the antagonist of OX1 (SB 334867) or CB1 (AM 251), but not OX2 (TCS-OX2-29) or opioid (naloxone), receptors. [Ala(11), D-Leu(15)]-orexin B (i.pag.), an OX2 selective agonist, also induced antinociception in a manner blocked by i.pag. injection of TCS-OX2-29, but not SB 334867 or AM 251. Mice receiving restraint stress for 30 min showed significantly longer hot-plate latency, more c-Fos-expressing orexin neurons in the lateral hypothalamus and higher orexin levels in the vlPAG than unrestrained mice. Restraint SIA in mice was prevented by i.pag. or intraperitoneal injection of SB 334867 or AM 251, but not TCS-OX2-29 or naloxone. These results suggest that during stress, hypothalamic orexin neurons are activated, releasing orexins into the vlPAG to induce analgesia, possibly via the OX1 receptor-initiated, endocannabinoid-mediated disinhibition mechanism previously reported. Although activating either OX1 or OX2 receptors in the vlPAG can lead to antinociception, only OX1 receptor-initiated antinociception is endocannabinoid-dependent.

Mu opioid receptor stimulation activates c-Jun N-terminal kinase 2 by distinct arrestin-dependent and independent mechanisms

G protein-coupled receptor desensitization is typically mediated by receptor phosphorylation by G protein-coupled receptor kinase (GRK) and subsequent arrestin binding; morphine, however, was previously found to activate a c-Jun N-terminal kinase (JNK)-dependent, GRK/arrestin-independent pathway to produce mu opioid receptor (MOR) inactivation in spinally-mediated, acute anti-nociceptive responses [Melief et al.] [1]. In the current study, we determined that JNK2 was also required for centrally-mediated analgesic tolerance to morphine using the hotplate assay. We compared JNK activation by morphine and fentanyl in JNK1(-/-), JNK2(-/-), JNK3(-/-), and GRK3(-/-) mice and found that both compounds specifically activate JNK2 in vivo; however, fentanyl activation of JNK2 was GRK3-dependent, whereas morphine activation of JNK2 was GRK3-independent. In MOR-GFP expressing HEK293 cells, treatment with either arrestin siRNA, the Src family kinase inhibitor PP2, or the protein kinase C (PKC) inhibitor Gö6976 indicated that morphine activated JNK2 through an arrestin-independent Src- and PKC-dependent mechanism, whereas fentanyl activated JNK2 through a Src-GRK3/arrestin-2-dependent and PKC-independent mechanism. This study resolves distinct ligand-directed mechanisms of JNK activation by mu opioid agonists and understanding ligand-directed signaling at MOR may improve opioid therapeutics.

Functional mu opioid receptor polymorphism (OPRM1 A(118) G) associated with heroin use outcomes in Caucasian males: A pilot study

BACKGROUND

Heroin's analgesic, euphoric and dependence-producing effects are primarily mediated by the mu opioid receptor (MOR). A single gene, OPRM1, encodes the MOR. The functional polymorphism A(118)G, located in exon 1 of the OPRM1 gene, results in anatomically-specific reductions in MOR expression, which may alter an individual's response to heroin. In prior studies 118G (rare allele) carriers demonstrated significantly greater opioid tolerance, overdose vulnerability, and pain sensitivity than 118AA homozygotes. Those findings suggest OPRM1 genotype may impact characteristics of heroin use.

METHODS

The present pilot study characterized the impact of OPRM1 genotype (rs1799971, 118G allele carriers vs. 118AA homozygotes) on heroin-use phenotypes associated with heroin dependence severity in a sample of male, Caucasian chronic heroin users (n = 86).

RESULTS

Results indicate that 118G allele carriers reported significantly more heroin use-related consequences and heroin-quit attempts, and were more likely to have sought treatment for their heroin use than 118AA homozygotes.

CONCLUSIONS

These preliminary findings, consistent with extant data, illustrate a role for OPRM1 allelic variation on heroin use characteristics, and provide support for considering genotype in heroin treatment and relapse prevention.

The σ1 receptor engages the redox-regulated HINT1 protein to bring opioid analgesia under NMDA receptor negative control

AIMS

The in vivo pharmacology of the sigma 1 receptor (σ1R) is certainly complex; however, σ1R antagonists are of therapeutic interest, because they enhance mu-opioid receptor (MOR)-mediated antinociception and reduce neuropathic pain. Thus, we investigated whether the σ1R is involved in the negative control that glutamate N-methyl-d-aspartate acid receptors (NMDARs) exert on opioid antinociception.

RESULTS

The MOR C terminus carries the histidine triad nucleotide-binding protein 1 (HINT1) coupled to the regulator of G-protein signaling RGSZ2-neural nitric oxide synthase assembly. Activated MORs stimulate the production of nitric oxide (NO), and the redox zinc switch RGSZ2 converts this signal into free zinc ions that are required to recruit the redox sensor PKCγ to HINT1 proteins. Then, PKCγ impairs HINT1-RGSZ2 association and enables σ1R-NR1 interaction with MOR-HINT1 complexes to restrain opioid signaling. The inhibition of NOS or the absence of σ1Rs prevents HINT1-PKCγ interaction, and MOR-NMDAR cross-regulation fails. The σ1R antagonists transitorily remove the binding of σ1Rs to NR1 subunits, facilitate the entrance of negative regulators of NMDARs, likely Ca(2+)-CaM, and prevent NR1 interaction with HINT1, thereby impairing the negative feedback of glutamate on opioid analgesia.

INNOVATION

A redox-regulated process situates MOR signaling under NMDAR control, and in this context, the σ1R binds to the cytosolic C terminal region of the NMDAR NR1 subunit.

CONCLUSION

The σ1R antagonists enhance opioid analgesia in naïve mice by releasing MORs from the negative influence of NMDARs, and they also reset antinociception in morphine tolerant animals. Moreover, σ1R antagonists alleviate neuropathic pain, probably by driving the inhibition of up-regulated NMDARs.

Regulators of G-protein-signaling proteins: negative modulators of G-protein-coupled receptor signaling

Regulators of G-protein-signaling (RGS) proteins are a category of intracellular proteins that have an inhibitory effect on the intracellular signaling produced by G-protein-coupled receptors (GPCRs). RGS along with RGS-like proteins switch on through direct contact G-alpha subunits providing a variety of intracellular functions through intracellular signaling. RGS proteins have a common RGS domain that binds to G alpha. RGS proteins accelerate GTPase and thus enhance guanosine triphosphate hydrolysis through the alpha subunit of heterotrimeric G proteins. As a result, they inactivate the G protein and quickly turn off GPCR signaling thus terminating the resulting downstream signals. Activity and subcellular localization of RGS proteins can be changed through covalent molecular changes to the enzyme, differential gene splicing, and processing of the protein. Other roles of RGS proteins have shown them to not be solely committed to being inhibitors but behave more as modulators and integrators of signaling. RGS proteins modulate the duration and kinetics of slow calcium oscillations and rapid phototransduction and ion signaling events. In other cases, RGS proteins integrate G proteins with signaling pathways linked to such diverse cellular responses as cell growth and differentiation, cell motility, and intracellular trafficking. Human and animal studies have revealed that RGS proteins play a vital role in physiology and can be ideal targets for diseases such as those related to addiction where receptor signaling seems continuously switched on.

Enhanced GABAergic synaptic transmission at VLPAG neurons and potent modulation by oxycodone in a bone cancer pain model

BACKGROUND AND PURPOSE

We demonstrated previously that oxycodone has potent antinociceptive effects at supraspinal sites. In this study, we investigated changes in neuronal function and antinociceptive mechanisms of oxycodone at ventrolateral periaqueductal gray (VLPAG) neurons, which are a major site of opioid action, in a femur bone cancer (FBC) model with bone cancer-related pain.

EXPERIMENTAL APPROACH

We characterized the supraspinal antinociceptive profiles of oxycodone and morphine on mechanical hypersensitivity in the FBC model. Based on the disinhibition mechanism underlying supraspinal opioid antinociception, the effects of oxycodone and morphine on GABAA receptor-mediated inhibitory postsynaptic currents (IPSCs) in VLPAG neurons were evaluated in slices from the FBC model.

KEY RESULTS

The supraspinal antinociceptive effects of oxycodone, but not morphine, were abolished by blocking G protein-gated inwardly rectifying potassium1 (Kir 3.1) channels. In slices from the FBC model, GABAergic synaptic transmission at VLPAG neurons was enhanced, as indicated by a leftward shift of the input-output relationship curve of evoked IPSCs, the increased paired-pulse facilitation and the enhancement of miniature IPSC frequency. Following treatment with oxycodone and morphine, IPSCs were reduced in the FBC model, and the inhibition of presynaptic GABA release by oxycodone, but not morphine was enhanced and dependent on Kir 3.1 channels.

CONCLUSION AND IMPLICATIONS

Our results demonstrate that Kir 3.1 channels are important for supraspinal antinociception and presynaptic GABA release inhibition by oxycodone in the FBC model. Enhanced GABAergic synaptic transmission at VLPAG neurons in the FBC model is an important site of supraspinal antinociception by oxycodone via Kir 3.1 channel activation.

Neurochemical properties of BDNF-containing neurons projecting to rostral ventromedial medulla in the ventrolateral periaqueductal gray

The periaqueductal gray (PAG) modulates nociception via a descending pathway that relays in the rostral ventromedial medulla (RVM) and terminates in the spinal cord. Previous behavioral pharmacology and electrophysiological evidence suggests that brain-derived neurotrophic factor (BDNF) plays an important role in descending pain modulation, likely through the PAG-RVM pathway. However, detailed information is still lacking on the distribution of BDNF, activation of BDNF-containing neurons projecting to RVM in the condition of pain, and neurochemical properties of these neurons within the PAG. Through fluorescent in situ hybridization (FISH) and immunofluorescent staining, the homogenous distributions of BDNF mRNA and protein were observed in the four subregions of PAG. Both neurons and astrocytes expressed BDNF, but not microglia. By combining retrograde tracing methods and formalin pain model, there were more BDNF-containing neurons projecting to RVM being activated in the ventrolateral subregion of PAG (vlPAG) than other subregions of PAG. The neurochemical properties of BDNF-containing projection neurons in the vlPAG were investigated. BDNF-containing projection neurons expressed the autoreceptor TrkB in addition to serotonin (5-HT), neurotensin (NT), substance P (SP), calcitonin gene related peptide (CGRP), nitric oxide synthase (NOS), and parvalbumin (PV) but not tyrosine decarboxylase (TH). It is speculated that BDNF released from projection neurons in the vlPAG might participate in the descending pain modulation through enhancing the presynaptic release of other neuroactive substances (NSs) in the RVM.

Free-operant avoidance behavior by rats after reinforcer revaluation using opioid agonists and D-amphetamine

The associative processes that support free-operant instrumental avoidance behavior are still unknown. We used a revaluation procedure to determine whether the performance of an avoidance response is sensitive to the current value of the aversive, negative reinforcer. Rats were trained on an unsignaled, free-operant lever press avoidance paradigm in which each response avoided or escaped shock and produced a 5 s feedback stimulus. The revaluation procedure consisted of noncontingent presentations of the shock in the absence of the lever either paired or unpaired with systemic morphine and in a different cohort with systemic d-amphetamine. Rats were then tested drug free during an extinction test. In both the d-amphetamine and morphine groups, pairing of the drug and shock decreased subsequent avoidance responding during the extinction test, suggesting that avoidance behavior was sensitive to the current incentive value of the aversive negative reinforcer. Experiment 2 used central infusions of D-Ala(2), NMe-Phe(4), Gly-ol(5)]-enkephalin (DAMGO), a mu-opioid receptor agonist, in the periacqueductal gray and nucleus accumbens shell to revalue the shock. Infusions of DAMGO in both regions replicated the effects seen with systemic morphine. These results are the first to demonstrate the impact of revaluation of an aversive reinforcer on avoidance behavior using pharmacological agents, thereby providing potential therapeutic targets for the treatment of avoidance behavior symptomatic of anxiety disorders.

Curvilinear relationships between mu-opioid receptor labeling and undirected song in male European starlings (Sturnus vulgaris)

Female-directed communication in male songbirds has been reasonably well studied; yet, relatively little is known about communication in other social contexts. Songbirds also produce song that is not clearly directed towards another individual (undirected song) when alone or in flocks. Although the precise functions of undirected song may differ across species, this type of song is considered important for flock maintenance, song learning or practice. Past studies show that undirected song is tightly coupled to analgesia and positive affective state, which are both mediated by opioid activity. Furthermore, labeling for the opioid met-enkephalin in the medial preoptic nucleus (POM) correlates positively with undirected song production. We propose that undirected song is facilitated and maintained by opioid receptor activity in the POM and other brain regions involved in affective state, analgesia, and social behavior. To provide insight into this hypothesis, we used immunohistochemistry to examine relationships between undirected song and mu-opioid receptors in male starlings. Polynomial regression analyses revealed significant inverted-U shaped relationships between measures of undirected song and mu-opioid receptor labeling in the POM, medial bed nucleus of the stria terminalis (BSTm), and periaqueductal gray (PAG). These results suggest that low rates of undirected song may stimulate and/or be maintained by mu-opioid receptor activity; however, it may be that sustained levels of mu-opioid receptor activity associated with high rates of undirected song cause mu-opioid receptor down-regulation. The results indicate that mu-opioid receptor activity in POM, BSTm, and PAG may underlie previous links identified between undirected song, analgesia, and affective state.

Evidence for the involvement of descending pain-inhibitory mechanisms in the antinociceptive effect of hecogenin acetate

Hecogenin is a sapogenin present in the leaves of species from the Agave genus, with a wide spectrum of reported pharmacological activities. The present study was undertaken to evaluate whether hecogenin acetate (1) has antinociceptive properties and to determine its mechanism of action. The nociceptive threshold was evaluated using the tail flick test in mice. Mice motor performance was evaluated in a Rotarod test. By using Fos expression as a marker of neural activation, the involvement of the periaqueductal gray in 1-induced antinociception was evaluated. Intraperitoneal administration of 1 (5-40 mg/kg) produced a dose-dependent increase in the tail flick latency time compared to vehicle-treated group (p < 0.01). Mice treated with 1 (40 mg/kg) did not show motor performance alterations. The antinociception of 1 (40 mg/kg) was prevented by naloxone (nonselective opioid receptor antagonist; 5 mg/kg), CTOP (μ-opioid receptor antagonist; 1 mg/kg), nor-BNI (κ-opioid receptor antagonist; 0.5 mg/kg), naltrindole (δ-opioid receptor antagonist; 3 mg/kg), or glibenclamide (ATP-sensitive K(+) channel blocker; 2 mg/kg). Systemic administration of 1 (5-40 mg/kg) increased the number of Fos positive cells in the periaqueductal gray. The present study has demonstrated for the first time that 1 produces consistent antinociception mediated by opioid receptors and endogenous analgesic mechanisms.

CC12, a P450/epoxygenase inhibitor, acts in the rat rostral, ventromedial medulla to attenuate morphine antinociception

Brain cytochrome P450 epoxygenases were recently shown to play an essential role in mediating the pain-relieving properties of morphine. To identify the CNS sites containing the morphine-relevant P450s, the effects of intracerebral (ic) microinjections of the P450 inhibitor CC12 were determined on morphine antinociception in rats. CC12 inhibited morphine antinociception when both drugs were injected into the rostral ventromedial medulla (RVM), but not following co-injections into the periaqueductal gray (PAG) or into the spinal subarachnoid space. In addition, intra-RVM CC12 pretreatment nearly completely blocked the effects of morphine following intracerebroventricular (icv) administration. Although morphine is thought to act in both the PAG and RVM by pre-synaptic inhibition of inhibitory GABAergic transmission, the present findings show that 1) the mechanism of morphine action differs between these two brainstem areas, and 2) P450 activity within the RVM is important for supraspinal morphine antinociception. Characterization of morphine-P450 interactions within RVM circuits will further enhance the understanding of the biochemistry of pain relief.

Mapping of reinforcing and analgesic effects of the mu opioid agonist endomorphin-1 in the ventral midbrain of the rat

INTRODUCTION

Agonists at the mu opioid receptor (MOR) are widely recognized for their effects on reward and pain. Although prior studies have attributed some of these effects to MORs on GABA neurons in the ventral tegmental area (VTA), recent studies have identified a region of particularly strong MOR immunostaining residing caudal to the VTA, in a region denoted the rostromedial tegmental nucleus (RMTg).

METHODS

Hence, we examined whether rats would self-administer small doses (50-250 pmol) of the selective MOR agonist endomorphin-1 (EM1) into the RMTg and adjacent sites. EM1 was chosen due to its short half-life, thus limiting drug spread, and due to its presence endogenously in brain neurons, including some afferents to the RMTg.

RESULTS

The highest rates of EM1 self-administration occurred within 0.5 mm of the RMTg center, in a region roughly 0.8-1.6 mm caudal to the majority of VTA DA neurons. In contrast, self-administration rates were much lower in the adjacent VTA, interpeduncular nucleus, central linear nucleus, or median raphe nucleus. Furthermore, EM1 infusions into the RMTg, but not surrounding regions, produced conditioned place preference, while EM1 infusions into the RMTg but not anterior VTA markedly reduced formalin-induced pain behaviors. EM1 effects were mimicked by infusions of the GABA agonist muscimol into the same region, consistent with EM1 having inhibitory actions on its target neurons.

CONCLUSION

These results implicate a novel brain region in modulating MOR influences on both appetitive and aversive behavior.

Quantitative assessment of nocifensive behavioral responses and the underlying neuronal circuitry

This paper reviews several recently developed animal models that allow a quantitative assessment of the magnitude of nocifensive behavioral responses across a range of noxious stimulus intensities. Models discussed in detail include: (a) the rodent tail flick reflex, and a modification that allows measurement of tail flick magnitude, (b) rat hindlimb flexion withdrawal reflex elicited by noxious thermal stimulation of the paw, and (c) a learned operant response (nose bar press) evoked by noxious thermal stimulation of the rat's tail. These models are discussed in terms of their advantages over previous methods measuring response threshold, their fulfillment of criteria for ideal pain assessment models, and the neuronal circuitry underlying the behavioral response.

Context-dependent links between song production and opioid-mediated analgesia in male European starlings (Sturnus vulgaris)

Little is known about the neural mechanisms that ensure appropriate vocal behaviors within specific social contexts. Male songbirds produce spontaneous (undirected) songs as well as female-directed courtship songs. Opioid neuropeptide activity in specific brain regions is rewarding, at least in mammals, and past studies suggest that the opioid met-enkephalin in such areas is more tightly linked to undirected than female-directed song. Recent data using a song-associated place preference paradigm further suggest that production of undirected but not directed song is tightly linked to intrinsic reward. Opioids have analgesic properties. Therefore, if production of undirected song is closely linked to opioid-mediated reward, the production of undirected but not directed song should be associated with analgesia. Consistent with this prediction, in male starlings we identified a positive correlation between analgesia (decreased reactivity to a hot water bath) and undirected song (in non-breeding season condition males in affiliative flocks) but not female-directed song (in breeding season condition males presented with females). When breeding condition males were divided according to social status, a negative correlation was found in subordinate males (i.e. males that failed to acquire a nest box). These data are consistent with the hypotheses 1) that the production of undirected song is facilitated or maintained by opioids (and/or other neuromodulators that also induce analgesia) and 2) that production of female-directed song is not linked in the same way to release of the same neuromodulators. Results also demonstrate a link between analgesia and song in subordinate individuals lacking a nesting territory within the breeding season. Overall, the findings indicate that distinct neural mechanisms regulate communication in different social contexts and support the working hypothesis that undirected but not directed song is tightly linked to opioid release.

The role of mu-opioid receptor signaling in the dorsolateral periaqueductal gray on conditional and unconditional responding to threatening and aversive stimuli

Here we examined how mu-opioid receptor signaling in the periaqueductal gray (PAG) mediates conditional and unconditional responses to aversive stimuli. The mu-opioid agonist morphine (MOR) and/or the partially mu-selective antagonist naltrexone (NAL) were infused into dorsolateral PAG (dlPAG) during a fear conditioning task, in which rats were trained to fear an auditory conditional stimulus (CS) by pairing it with a unilateral eyelid shock unconditional stimulus (US). During drug-free test sessions, the CS elicited movement suppression responses (indicative of freezing) from trained rats that had not recently encountered the US. In trained rats that had recently encountered the US, the CS elicited flight behavior characterized by turning in the direction away from the eyelid where US delivery was anticipated. Infusions of MOR (30 nmol/side) into dlPAG prior to the test session did not impair CS-evoked movement suppression, but did impair CS-evoked turning behaviors. MOR infusions also reduced baseline motor movement, but US-evoked reflex movements remained largely intact. NAL was infused at two dosages, denoted 1x (26 nmol/side) and 10x (260 nmol/side). Infusions of NAL into dlPAG did not affect CS- or US-evoked behavioral responses at the 1x dosage, but impaired CS-evoked movement suppression at the 10x dosage, both in the presence and absence of MOR. When rats were co-infused with MOR and NAL, MOR-induced effects were not reversed by either dosage of NAL, and some measures of MOR-induced movement suppression were enhanced by NAL at the 1x dosage. Based on these findings, we conclude that mu-opioid receptors in dlPAG may selectively regulate descending supraspinal motor pathways that drive active movement behaviors, and that interactions between MOR and NAL in dlPAG may be more complex than simple competition for binding at the mu receptor.

Reduced expression of the μ opioid receptor in some, but not all, brain regions in mice with OPRM1 A112G

OPRM1 A118G is a common single nucleotide polymorphism (SNP) in the coding region of the human mu opioid receptor (MOPR) gene OPRM1. This SNP is associated with higher morphine doses required for postoperative analgesia as well as a variety of drug addiction phenotypes. A mouse model possessing the equivalent substitution (A112G) in the Oprm1 gene was generated to facilitate mechanistic studies. Mice homozygous for the G112 allele (G/G) displayed lower antinociception to morphine compared with those homozygous for A112 allele (A/A), similar to humans, suggesting that the mice are a good model to further characterize underlying factors contributing to phenotypes associated with this SNP. Here, we compared [³H]DAMGO binding to the MOPR in the brains of A/A and G/G mice using quantitative in vitro autoradiography. A/A mice exhibited higher [³H]DAMGO binding than G/G in the cingulate, motor, and insular cortices, nucleus accumbens core and shell, hypothalamus, thalamus, amygdala, periaqueductal gray, superficial gray of superior colliculus, and ventral tegmental area. No genotype differences were observed in somatosensory cortex, caudate putamen, and hippocampus. When males and females were examined separately, A/A mice showed higher [³H]DAMGO binding than G/G mice in more brain regions in males than in females. Radioligand binding using brain membranes also showed higher [³H]DAMGO binding in the cortex and thalamus in A/A mice than G/G mice but no genotype differences in the caudate putamen or hippocampus. Thus, the A112G SNP is associated with reduced MOPR expression in some, but not all, brain regions, and appears to have some sex differences. The elevated MOPR expression in periaqueductal gray and thalamus in A/A mice are consistent with their higher antinociceptive responses to morphine. The higher MOPR levels in nucleus accumbens and/or ventral tegmental area of A/A mice is consistent with the higher morphine-induced hyperactivity and locomotor sensitization observed in these mice. Thus, these results provide some insights into the observed decreased clinical opioid potency in humans with the A118G SNP.

RGSZ2 binds to the neural nitric oxide synthase PDZ domain to regulate mu-opioid receptor-mediated potentiation of the N-methyl-D-aspartate receptor-calmodulin-dependent protein kinase II pathway

Morphine increases the production of nitric oxide (NO) via the phosphoinositide 3-kinase/Akt/neural nitric oxide synthase (nNOS) pathway. Subsequently, NO enhances N-methyl-D-aspartate receptor (NMDAR)/calmodulin-dependent protein kinase II (CaMKII) cascade, diminishing the strength of morphine-activated Mu-opioid receptor (MOR) signaling. During this process, NO signaling is restricted by the association of nNOS to the MOR.

AIMS

Here, we examined how nNOS/NO signaling is downregulated by the morphine-activated MOR and how this regulation affects antinociception.

RESULTS

Accordingly, we show that the MOR-NMDAR regulatory loop relies on the negative control of nNOS activity exerted by RGSZ2, a protein physically coupled to the MOR. This regulation requires binding of the nNOS N terminal PDZ domain to the RGSZ2 PDZ binding motifs that lie upstream of the RGS box.

INNOVATION

Indeed, in RGSZ2-deficient mice morphine over-stimulates the nNOS/NO/NMDAR/CaMKII pathway, causing analgesic tolerance to develop rapidly. Recovery of RGSZ2 levels or inhibition of nNOS, protein kinase C, NMDAR, or CaMKII function restores MOR signaling and morphine recovers its full analgesic potency.

CONCLUSION

This RGSZ2-dependent regulation of NMDAR activity is relevant to persistent pain disorders associated with heightened NMDAR-mediated glutamate responses and the reduced antinociceptive capacity of opioids.

Antagonism of stimulation-produced analgesia by naloxone and N-methyl-D-aspartate: role of opioid and N-methyl-D-aspartate receptors

The present study aims to investigate the influence of electrical stimulation of periaqueductal gray (PAG) following peripheral nerve injury and its modulation by naloxone and N-methyl-D-aspartate (NMDA). Chronic neuropathic pain was induced by chronic constriction injury of the sciatic nerve, and subsequently a cannula was implanted in the PAG area for the purpose of electrical stimulation and intra-PAG drug administration. Intra-PAG administration of morphine, ketamine, and their combination were found to elicit antinociceptive response on hot-plate test. Electrical stimulation of PAG was also observed to demonstrate decreased pain response on hot-plate test, and this effect was reversed by the administration of naloxone, NMDA, and their combination, when injected into the PAG area. These findings suggest that apart from the opioid receptors, probably NMDA receptors also have a role to play in stimulation-produced analgesia.