Sensitivity to naloxone of the behavioral signs of morphine withdrawal and c-Fos expression in the rat CNS: a quantitative dose-response analysis

Human OPRM1 and murine Oprm1 promoter driven viral constructs for genetic access to μ-opioidergic cell types

With concurrent global epidemics of chronic pain and opioid use disorders, there is a critical need to identify, target and manipulate specific cell populations expressing the mu-opioid receptor (MOR). However, available tools and transgenic models for gaining long-term genetic access to MOR+ neural cell types and circuits involved in modulating pain, analgesia and addiction across species are limited. To address this, we developed a catalog of MOR promoter (MORp) based constructs packaged into adeno-associated viral vectors that drive transgene expression in MOR+ cells. MORp constructs designed from promoter regions upstream of the mouse Oprm1 gene (mMORp) were validated for transduction efficiency and selectivity in endogenous MOR+ neurons in the brain, spinal cord, and periphery of mice, with additional studies revealing robust expression in rats, shrews, and human induced pluripotent stem cell (iPSC)-derived nociceptors. The use of mMORp for in vivo fiber photometry, behavioral chemogenetics, and intersectional genetic strategies is also demonstrated. Lastly, a human designed MORp (hMORp) efficiently transduced macaque cortical OPRM1+ cells. Together, our MORp toolkit provides researchers cell type specific genetic access to target and functionally manipulate mu-opioidergic neurons across a range of vertebrate species and translational models for pain, addiction, and neuropsychiatric disorders.

Two kinds of transcription factors mediate chronic morphine-induced decrease in miR-105 in medial prefrontal cortex of rats

Chronic morphine administration alters gene expression in different brain regions, an effect which may contribute to plastic changes associated with addictive behavior. This change in gene expression is most possibly mediated by addictive drug-induced epigenetic remodeling of gene expression programs. Our previous studies showed that chronic morphine-induced decrease of miR-105 in the medial prefrontal cortex (mPFC) contributed to context-induced retrieval of morphine withdrawal memory. However, how chronic morphine treatment decreases miR-105 in the mPFC still remains unknown. The present study shows that chronic morphine induces addiction-related change in miR-105 in the mPFC via two kinds of transcription factors: the first transcription factor is CREB activated by mu receptors-ERK-p90RSK signaling pathway and the second transcription factor is glucocorticoid receptor (GR), which as a negative transcription factor, mediates chronic morphine-induced decrease in miR-105 in the mPFC of rats.

Extended amygdala, conditioned withdrawal and memory consolidation

Opioid withdrawal can be associated to environmental cues through classical conditioning. Exposure to these cues can precipitate a state of conditioned withdrawal in abstinent subjects, and there are suggestions that conditioned withdrawal can perpetuate the addiction cycle in part by promoting the storage of memories. This review discusses evidence supporting the hypothesis that conditioned withdrawal facilitates memory consolidation by activating a neurocircuitry that involves the extended amygdala. Specifically, the central amygdala, the bed nucleus of the stria terminalis, and the nucleus accumbens shell interact functionally during withdrawal, mediate expression of conditioned responses, and are implicated in memory consolidation. From this perspective, the extended amygdala could be a neural pathway by which drug-seeking behaviour performed during a state of conditioned withdrawal is more likely to become habitual and persistent.

Opioid withdrawal and memory consolidation

It is well established that learning and memory are central to substance dependence. This paper specifically reviews the effect of opioid withdrawal on memory consolidation. Although there is evidence that opioid withdrawal can interfere with initial acquisition and retrieval of older memories, there are several reasons to postulate a facilitatory action on the consolidation of newly acquired memories. In fact, there is substantial evidence that memory consolidation is facilitated by the release of stress hormones, that it requires the activation of the amygdala, of central noradrenergic and cholinergic pathways, and that it involves long-term potentiation. This review highlights evidence that very similar neurobiological processes are involved in opioid withdrawal, and summarizes recent results indicating that naltrexone-precipitated withdrawal enhanced consolidation in rats. From this neurocognitive perspective, therefore, opioid use may escalate during the addiction cycle in part because memories of stimuli and actions experienced during withdrawal are strengthened.

Differential effects of naloxone on rewarding electrical stimulation of the central nucleus of the amygdala and parabrachial complex in a place preference study

The central nucleus of the amygdala (CeA) is considered to be involved in different affective, sensory, regulatory, and acquisition processes. This study analyzed whether electrical stimulation of the PB-CeA system induces preferences in a concurrent place preference (cPP) task, as observed after stimulation of the parabrachial-insular cortex (PB-IC) axis. It also examined whether the rewarding effects are naloxone-dependent. The results show that electrical stimulation of the CeA and external lateral parabrachial subnucleus (LPBe) induces consistent preference behaviors in a cPP task. However, subcutaneous administration of an opiate antagonist (naloxone; 4mg/ml/kg) blocked the rewarding effect of the parabrachial stimulation but not that of the amygdala stimulation. These results are interpreted in the context of multiple brain reward systems that appear to differ both anatomically and neurochemically, notably with respect to the opiate system.

Naloxone can act as an analgesic agent without measurable chronic side effects in mice with a mutant mu-opioid receptor expressed in different sites of pain pathway

Midbrain periaqueductal gray (PAG) and spinal cord dorsal horn are major action sites of opioid analgesics in the pain pathway. Our previous study has shown that opioid antagonists activate MORS196A-CSTA (a mutant of mu-opioid receptor) as full agonists in vitro cell models and naloxone showed antinociceptive effects after the expression of MORS196A-CSTA in the spinal cord in mice. The purpose of this study is to investigate the site-directed antinociceptive effects of naloxone in mice injected with dsAAV-MORS196A-CSTA-EGFP at spinal cord or at periaqueductal gray. MORS196A-CSTA-EGFP was administered to ICR mice using dsAAV as vector. We measured MORS196A-CSTA-EGFP expression by detecting the EGFP visualization with a fluorescence microscope. The antinociceptive effect of naloxone was determined by tail-flick test and hot plate test. Drug rewarding effect was evaluated by the conditioned place preference test. Naloxone (10 mg/kg, s.c.) elicited both supraspinal and spinal antinociceptive responses in mice injected with the virus at PAG while only spinal antinociceptive response was observed in mice injected with virus at dorsal horn region. Chronic naloxone treatment did not induce physical dependence or rewarding effect in mice injected with MORS196A-CSTA-EGFP in spinal cord or PAG. These data suggest that the observed naloxone-induced antinociceptive response is the consequence of the local expression of MORS196A-CSTA at specific sites of pain pathway. Injection of such MOR mutant and the systemic administration of naloxone can be a new strategy in the management of chronic pain without the various side effects associated with the use of morphine.

Differential respiratory control of the upper airway and diaphragm muscles induced by 5-HT1A receptor ligands

BACKGROUND

Serotonin (5-HT) has a role in respiratory function and dysfunction. Although 5-HT affects respiratory drive to both phrenic and cranial motoneurons, relatively little is known about the role of 5-HT receptor subtypes in the control of upper airway muscle (UAM) respiratory activity.

MATERIALS AND METHODS

Here, we performed central injections of 5-HT1A agonist (8-OHDPAT) or antagonist (WAY100635) in anesthetized rats and analyzed changes in the electromyographic activity of several UAM and other cardiorespiratory parameters. We also compared the pattern of Fos expression induced after central injection of a control solution or 8-OHDPAT.

RESULTS

Results showed that 8-OHDPAT induced a robust increase in UAM activity, associated with either tachypnea under volatile anesthesia or bradypnea under liquid anesthesia. Injection of WAY100635 switched off UAM respiratory activity and led to bradypnea, suggesting a tonic excitatory role of endogenous 5-HT1A receptor activation. Co-injection of the agonist and the antagonist blocked the effects produced by each drug alone. Besides drug-induced changes in respiratory frequency, only slight increases in surface of diaphragm bursts were observed. Significant increases in Fos expression after 5-HT1A receptor activation were seen in the nucleus tractus solitarius, nucleus raphe pallidus, parapyramidal region, retrotrapezoid nucleus, lateral parabrachial, and Kölliker-Fuse nuclei. This restricted pattern of Fos expression likely identified the neural substrate responsible for the enhancement of UAM respiratory activity observed after 8-OHDPAT injection.

CONCLUSIONS

These findings suggest an important role for the 5-HT1A receptors in the neural control of upper airway patency and may be relevant to counteract pharyngeal atonia during obstructive sleep apneas.

Forebrain PENK and PDYN gene expression levels in three inbred strains of mice and their relationship to genotype-dependent morphine reward sensitivity

RATIONALE

Vulnerability to drug abuse disorders is determined not only by environmental but also by genetic factors. A body of evidence suggests that endogenous opioid peptide systems may influence rewarding effects of addictive substances, and thus, their individual expression levels may contribute to drug abuse liability.

OBJECTIVES

The aim of our study was to assess whether basal genotype-dependent brain expression of opioid propeptides genes can influence sensitivity to morphine reward.

METHODS

Experiments were performed on inbred mouse strains C57BL/6J, DBA/2J, and SWR/J, which differ markedly in responses to morphine administration: DBA/2J and SWR/J show low and C57BL/6J high sensitivity to opioid reward. Proenkephalin (PENK) and prodynorphin (PDYN) gene expression was measured by in situ hybridization in brain regions implicated in addiction. The influence of the kappa opioid receptor antagonist nor-binaltorphimine (nor-BNI), which attenuates effects of endogenous PDYN-derived peptides, on rewarding actions of morphine was studied using the conditioned place preference (CPP) paradigm.

RESULTS

DBA/2J and SWR/J mice showed higher levels of PDYN and lower levels of PENK messenger RNA in the nucleus accumbens than the C57BL/6J strain. Pretreatment with nor-BNI enhanced morphine-induced CPP in the opioid-insensitive DBA/2J and SWR/J strains.

CONCLUSIONS

Our results demonstrate that inter-strain differences in PENK and PDYN genes expression in the nucleus accumbens parallel sensitivity of the selected mouse strains to rewarding effects of morphine. They suggest that high expression of PDYN may protect against drug abuse by limiting drug-produced reward, which may be due to dynorphin-mediated modulation of dopamine release in the nucleus accumbens.

Anxiolytic-like effects of morphine and buprenorphine in the rat model of fear-potentiated startle: tolerance, cross-tolerance, and blockade by naloxone

RATIONALE

Morphine and buprenorphine have analgesic and anxiolytic-like properties. While their analgesic effects have been well characterized, their anxiolytic-like properties have not.

OBJECTIVES

Effects of acute morphine and buprenorphine on the expression of acoustic fear-potentiated startle (FPS) and naloxone pretreatment were assessed. Effects of chronic morphine and buprenorphine on tolerance, cross-tolerance, and withdrawal were also examined.

MATERIALS AND METHODS

Fear-conditioned rats were given subcutaneous drug treatment immediately before testing for FPS. Experiment 1, rats were administered morphine (0.03, 0.25, 0.63, 2.5, or 10 mg/kg) or buprenorphine (0.004, 0.0075, 0.015, 0.03, or 0.25 mg/kg). Experiment 2, rats were given saline or naloxone (0.5 mg/kg) and 5 min later given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg). Experiment 3, rats received once-daily injections of saline, morphine (10 mg/kg), or buprenorphine (0.25 mg/kg) for 7 days. Immediately before testing, saline-treated rats were given saline, morphine (2.5 mg/kg), or buprenorphine (0.03 mg/kg), morphine-treated rats were given morphine (2.5 mg/kg) or buprenorphine (0.03 mg/kg), and buprenorphine-treated rats were given buprenorphine (0.03 mg/kg) or morphine (2.5 mg/kg). Tolerance and cross-tolerance in analgesia were assessed via the tail-flick test, as were naloxone-precipitated withdrawal.

RESULTS

Morphine and buprenorphine had parallel dose-response curves in blocking FPS, with buprenorphine 40 times more potent than morphine. Naloxone reversed these effects. Morphine and buprenorphine showed tolerance and cross-tolerance in their anxiolytic-like and analgesic effects. Chronic buprenorphine produced less withdrawal than chronic morphine.

CONCLUSIONS

Cross-tolerance between morphine and buprenorphine suggests a common receptor mediating their anxiolytic-like and analgesic effects.

Reactivity and plasticity in the amygdala nuclei during opiate withdrawal conditioning: differential expression of c-fos and arc immediate early genes

Opiate withdrawal leads to the emergence of an aversive state that can be conditioned to a specific environment. Reactivation of these withdrawal memories has been suggested to be involved in relapse to drug-seeking of abstinent opiate addicts. Among the limbic areas that are likely to mediate these features of opiate dependence, amygdala nuclei represent critical neural substrates. Using a conditioned place aversion paradigm (CPA), we have previously shown specific opposite patterns of reactivity in the basolateral (BLA) and the central (CeA) amygdala, when comparing the experience of acute opiate withdrawal with the re-exposure to a withdrawal-paired environment. These data gave clues about the potential mechanisms by which amygdala nuclei may be involved in withdrawal memories. To extend these results, the present study aimed to assess the cellular reactivity and plasticity of amygdala nuclei during the opiate withdrawal conditioning process. For this, we have quantified c-fos and arc expression using in situ hybridization in rats, following each of the three conditioning sessions during CPA, and after re-exposure to the withdrawal-paired environment. BLA output neurons showed an increase in the expression of the plasticity-related arc gene during conditioning that was also increased by re-exposure to the withdrawal-paired environment. Interestingly, the CeA showed an opposite pattern of responding, and the intercalated cell masses (ITC), a possible inhibitory interface between the BLA and CeA, showed a persistent activation of c-fos and arc mRNA. We report here specific c-fos and arc patterns of reactivity in amygdala nuclei during withdrawal conditioning. These findings improve our understanding of the involvement of the amygdala network in the formation and retrieval of withdrawal memories. Plasticity processes within BLA output neurons during conditioning, may participate in increasing the BLA reactivity to conditioned stimuli, which could in turn (by the control of downstream nuclei) reinforce and drive the motivational properties of withdrawal over drug consumption.

Induction of c-Fos and zif268 in the nociceptive amygdala parallel abstinence hyperalgesia in rats briefly exposed to morphine

Opioid-induced analgesia can be followed by spontaneous pain in humans, and hyperalgesia in rodents. In this study, opioid-induced hyperalgesia was measured by the tail-flick test when acute abstinence was precipitated by administering naloxone to drug naive rats that had experienced morphine analgesia for only 30 min. In a further experiment, the drug treatment that previously caused opioid-induced hyperalgesia was found to increase neurons expressing nuclear c-Fos or zif268 proteins in extended amygdalar regions targeted by projections of the ascending spino-parabrachio-amygdaloid nociceptive pathway. Transcription factor induction, however, was not detected in multiple brain regions known to respond in parallel with the same extended amygdalar structures when (1) rats are exposed to interoceptive/physical stressors, or (2) naloxone is used to precipitate abstinence in opioid dependent rats. Surprisingly, in many regions c-Fos induction by morphine was reduced or blocked by naloxone, even though these subjects had also experienced the effects of morphine for 30 min prior to antagonist administration. It is suggested transcription factor induction during opioid hyperalgesia in non-dependent rats could support the induction or consolidation of neural plasticity in nociceptive amygdaloid circuitry previously suggested to function in bi-directional control of pain and expression of pain-related behaviors.

Characterization of neurons in the rat central nucleus of the amygdala: cellular physiology, morphology, and opioid sensitivity

The central nucleus of the amygdala (CeA) orchestrates autonomic and other behavioral and physiological responses to conditioned stimuli that are aversive or elicit fear. As a related CeA function is the expression of hypoalgesia induced by conditioned stimuli or systemic morphine administration, we examined postsynaptic opioid modulation of neurons in each major CeA subdivision. Following electrophysiological recording, biocytin-filled neurons were precisely located in CeA regions identified by chemoarchitecture (enkephalin-immunoreactivity) and cytoarchitecture (DAPI nuclear staining) in fixed adult rat brain slices. This revealed a striking distribution of physiological types, as 92% of neurons in capsular CeA were classified as late-firing, whereas no neurons in the medial CeA were of this class. In contrast, 60% or more of neurons in the lateral and medial CeA were low-threshold bursting neurons. Mu-opioid receptor (MOPR) agonists induced postsynaptic inhibitory potassium currents in 61% of CeA cells, and this ratio was maintained in each subdivision and for each physiological class of neuron. However, MOPR agonists more frequently inhibited bipolar/fusiform cells than triangular or multipolar neurons. A subpopulation of MOPR-expressing neurons were also inhibited by delta opioid receptor agonists, whereas a separate population were inhibited kappa opioid receptors (KOPR). The MOPR agonist DAMGO inhibited 9/9 CeM neurons with projections to the parabrachial nucleus identified by retrograde tracer injection. These data support models of striatopallidal organization that have identified striatal-like and pallidal-like CeA regions. Opioids can directly inhibit output from each subdivision by activating postsynaptic MOPRs or KOPRs on distinct subpopulations of opioid-sensitive neurons.

Cross talk between nitric oxide and ERK1/2 signaling pathway in the spinal cord mediates naloxone-precipitated withdrawal in morphine-dependent rats

Our recent study has shown activation of spinal extracellular signal-regulated kinase-1 and -2 (ERK1/2), a member of the mitogen-activated protein kinase (MAPK) family, contributes to naloxone-precipitated withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats. However, the mechanism and significance of the spinal ERK1/2 activation during morphine dependence and withdrawal remain unknown. In this study, we reported that intrathecal (i.t.) pretreatment with either the non-selective nitric oxide synthase (NOS) inhibitor N(G)-nitro-L-arginine methyl ester (L-NAME), neuronal NOS (nNOS) inhibitor 7-nitro indazole (7-NI), or the inducible NOS (iNOS) inhibitor aminoguanidine (AG), could reduce morphine withdrawal-induced increase of phospho-ERK1/2 (pERK1/2) expression in the rat spinal cord. On the other hand, attenuation of the spinal ERK phosphorylation by the MAPK kinase (MEK) inhibitor U0126 also could inhibit the increase of nNOS and iNOS expression in the spinal cord of morphine withdrawal rats. Inhibitory expression of pERK1/2 by i.t. NOS inhibitor L-NAME, 7-NI or AG and of nNOS and iNOS by i.t. U0126 in the spinal cord were accompanied by decreased scores of morphine withdrawal and the inhibited spinal Fos protein (a maker for neuronal excitation or activation) expression induced by morphine withdrawal. These findings suggest cross talk between nitric oxide (NO) and the ERK1/2 signaling pathway mediates morphine withdrawal and withdrawal-induced spinal neuronal sensitization in morphine-dependent rats.

Inhibition of the spinal phosphoinositide 3-kinase exacerbates morphine withdrawal response

The present study investigates the roles of the spinal phosphoinositide 3-kinase (PI3K) signaling pathway in naloxone-precipitated withdrawal in acute and chronic morphine-dependent mice. There are two principal findings: (1) intrathecal pretreatment with wortmannin or LY294002, two structurally unrelated PI3K inhibitors, produced a dose-dependent increase of naloxone-precipitated withdrawal jumping, which was accompanied by an increased expression of spinal Fos protein in acute and chronic morphine-dependent mice; and (2) the expression of spinal p110gamma, the catalytic subunit PI3K, in the membrane fraction was significantly down-regulated by naloxone-precipitated withdrawal in acute and chronic morphine-dependent mice. This study provides new evidence showing that inactivation of the PI3K signaling pathway in the spinal cord may be involved in the expression of morphine withdrawal.

Modulation of apoptosis in the mouse brain after morphine treatments and morphine withdrawal

We have examined the effects of acute or chronic morphine and naltrexone-precipitated withdrawal on mouse brain apoptotic cell death. The associated changes in the expression of apoptosis regulatory proteins were also analyzed. After a single dose of morphine, no apoptotic cells were detected by TUNEL or active caspase-3 immunocytochemistry. Concurrently, a down-regulation of the proapoptotic proteins FasL and Bad was detected in cortical lysates. On the other hand, the brains of chronic-morphine-treated mice and abstinent mice exhibited scattered apoptotic neurons and astrocytes throughout the brain. This neurotoxic effect was accompanied by up-regulation of the proapoptotic proteins FasL, Fas, and Bad and the active fragments of caspases-8 and -3 in cortical and hippocampal lysates. Abstinent mice also displayed a reduced expression of the antiapoptotic protein Bcl-2. No changes on t-Bid expression were detected under any experimental condition. These results suggest a neurotoxic effect exerted by chronic, but not acute, morphine and its withdrawal by activating both the intrinsic and the extrinsic apoptotic pathways. The possible clinical implications of our findings are discussed.

A telemetric examination of cardiovascular function during the development of, and recovery from, opiate dependence in rats

Rats were subject to daily injections of morphine or saline and were then allowed to spontaneously withdraw from morphine for 4 days. Mean arterial blood pressure (MAP) and heart rate (HR) were recorded continuously, via radiotelemetry, during the development of, and recovery from, opiate dependence. Injections of morphine produced pronounced and prolonged increases in MAP and HR which increased as morphine dose increased. There were also significant increases in MAP during the 19-23 h period after each morphine injection indicating the presence of withdrawal. Spontaneous withdrawal from morphine was associated with a pronounced (20% increase from baseline) and prolonged (72 h) increase in MAP. MAP returned to baseline levels 72-96 h after last morphine exposure. These results show that intermittent injections of morphine, and spontaneous withdrawal from these injections, are associated with profound alterations in cardiovascular function and confirm the usefulness of radiotelemetry for studying opiate dependence.

Sensitization to morphine withdrawal in guinea-pigs

The aim of this study was to determine whether sensitization occurred to morphine withdrawal. Guinea-pigs were treated twice daily with increasing doses of morphine (10-100 mg/kg s.c.) for 3 days followed by injection of morphine 100 mg/kg on the fourth day. Sixty min after the last morphine injection, animals were withdrawn from morphine with naltrexone, 15 mg/kg s.c., and locomotor activity and all other behaviours scored over 90 min. Animals were then rested for 3 days. This procedure was repeated twice over the next 2 weeks. Control animals were treated with saline for the first two treatment cycles. Guinea-pigs subjected to three cycles of morphine withdrawal showed a significant increase in the total number of withdrawal behaviour counts over the 90-min observation period following the third cycle of withdrawal compared with the first and second withdrawal cycles. However, locomotor activity, a major sign of morphine withdrawal in guinea-pigs, was not significantly increased. Fos-LI was markedly increased in the repeatedly withdrawn animals in several brain regions, including amygdala, dorsal striatum, thalamus, ventral tegmental area, and ventrolateral periaqueductal gray area. It is concluded that sensitization to morphine withdrawal occurs in guinea-pigs.

Involvement of the bed nucleus of the stria terminalis activated by the central nucleus of the amygdala in the negative affective component of morphine withdrawal in rats

The central nucleus of the amygdala (Ce) and the bed nucleus of the stria terminalis (BST) are key structures of the extended amygdala, which is suggested to be involved in drug addiction and reward. We have previously reported that the Ce plays a crucial role in the negative affective component of morphine withdrawal. In the present study, we examined the involvement of the neural pathway between the Ce and the BST in the negative affective component of morphine withdrawal in rats. Rats were rendered morphine dependent by s.c. implantation of a 75-mg morphine pellet for 3 days, and morphine withdrawal was precipitated by an i.p. injection of naloxone (0.3 mg/kg). In the place-conditioning paradigm, discrete bilateral excitotoxic lesions of the Ce or the BST significantly reduced naloxone-precipitated morphine withdrawal-induced conditioned place aversion. On the other hand, they had little effect on morphine withdrawal-induced somatic signs. In an immunohistochemical study for c-Fos protein, naloxone-precipitated morphine withdrawal dramatically induced c-Fos-immunoreactive neurons in the capsular part of the Ce, and the lateral and medial divisions of the BST. Bilateral excitotoxic lesion of the Ce reduced the number of morphine withdrawal-induced c-Fos-immunoreactive neurons in the lateral and medial BST, with significant decreases in the posterior, ventral and juxtacapsular parts of lateral division, and anterior part of the medial division, but not in the ventral part of the medial division of the BST. On the other hand, bilateral excitotoxic lesion of the BST had no effect on such c-Fos induction within the capsular part, nor the ventral and medial divisions of the Ce. These results suggest that activation of the BST mediated through the neural pathway from the Ce contributes to the negative affective component of morphine withdrawal.

Facilitation of morphine withdrawal symptoms and morphine-induced conditioned place preference by a glutamate transporter inhibitor DL-threo-beta-benzyloxyaspartate in rats

There is a body of evidence implying the involvement of the central glutamatergic system in morphine dependence. In this study, we examined the effect of intracerebroventricular (i.c.v.) administration of a potent glutamate transporter inhibitor, DL-threo-beta-benzyloxyaspartate (DL-TBOA), on acute morphine-induced antinociception, expression of somatic and negative affective components of morphine withdrawal, and acquisition of morphine-induced conditioned place preference in rats. I.c.v administration of DL-TBOA (10 nmol) to naive rats did not affect the acute antinociceptive effect of morphine. I.c.v. administration of DL-TBOA (10 nmol) to morphine-dependent rats significantly facilitated the expression of naloxone-precipitated somatic signs and conditioned place aversion. DL-TBOA (3 and 10 nmol) significantly facilitated acquisition of morphine-induced conditioned place preference. DL-TBOA itself produced neither conditioned place aversion nor place preference in naive rats. These results suggest that central glutamate transporters play inhibitory roles in the expression of somatic and negative affective components of morphine withdrawal and the reinforcing effect of morphine.

Withdrawal-induced c-Fos expression in the rat centromedial amygdala 24 h following a single morphine exposure

RATIONALE

An opiate antagonist was found to induce motivational withdrawal signs 24 h or even up to 48 h after a single dose of morphine in rats.

OBJECTIVE

The present study was undertaken to determine whether such a withdrawal state would modify the neuronal activity in the brain.

METHODS

A conditioned place aversion was established following a one-trial paradigm in rats undergoing a single exposure to morphine (10 mg/kg) 24 h prior to naloxone administration (0.5 mg/kg). Subsequently, the expression of the protein product of c-fos gene (c-Fos) following naloxone administration was measured within the extended amygdala.

RESULTS

A significant increase in c-Fos immunoreactivity was seen in the centromedial amygdala (CMA), but not in the bed nucleus of the stria terminalis (BST) and the shell (AcbSh) of the nucleus accumbens (Acb) in rats treated with both morphine and naloxone. Further examination of the distribution of c-Fos-positive neurons along the rostrocaudal axis within CMA showed that the positive neurons distributed throughout this brain area and the caudal level of its central division (the central nucleus of the amygdala, CeA) exhibited the most robust labeling.

CONCLUSIONS

Neuronal activity can be increased by naloxone at a dose that produces conditioned place aversion 24 h after a single morphine exposure. CMA, particularly the caudal level of its central division, was of high sensitivity. The current data also suggest a possible involvement of CMA in negative motivational component of precipitated withdrawal from acute morphine dependence.

Effects of anesthetics on hypoglossal nerve discharge and c-Fos expression in brainstem hypoglossal premotor neurons

This study examined the effects of anesthesia on the hypoglossal nerve and diaphragm activities and on c-Fos expression in brainstem hypoglossal premotor neurons (pmXII). Experiments were performed in 71 rats by using halothane inhalation, pentobarbital sodium, or mixtures of alpha-chloralose and urethane or ketamine and xylazine. First, various cardiorespiratory parameters were measured in the rats (n = 31) during both awake and anesthetized conditions. The volatile anesthetic halothane, but not the other anesthetics, was always associated with a strong phasic inspiratory activity in the hypoglossal nerve. Second, a double-immunohistochemical study was performed in awake and anesthetized rats (n = 40) to gauge the level of activity of pmXII neurons. Brainstem pmXII neurons were identified after microiontophoresis of the retrograde tracer Fluoro-Gold in the right hypoglossal motor nucleus. Patterns of c-Fos expression at different brainstem levels were compared in five groups of rats (i.e., awake or anesthetized with halothane, pentobarbital, chloralose-urethane, and ketamine-xylazine). Sections were processed for double detection of c-Fos protein and Fluoro-Gold by using the standard ABC method and a two-color peroxidase technique. Anesthesia with halothane induced the strongest c-Fos expression in a restricted pool of pmXII located in the pons at the level of the Kölliker-Fuse nucleus and the intertrigeminal region. The results demonstrated a major effect of halothane in inducing changes in hypoglossal activity and revealed a differential expression of c-Fos protein in pmXII neurons among groups of anesthetized rats. We suggest that halothane mediates changes in respiratory hypoglossal nerve discharge by altering activity of premotor neurons in the Kölliker-Fuse and intertrigeminal region.

Spinal administration of lipoxygenase inhibitors suppresses behavioural and neurochemical manifestations of naloxone-precipitated opioid withdrawal

1. This study investigated the role of spinal lipoxygenase (LOX) products in the induction and expression of opioid physical dependence using behavioural assessment of withdrawal and immunostaining for CGRP and Fos protein expression in the spinal cord. 2. Administration of escalating doses (5-50 mg kg-1; i.p.) of morphine for 5 days markedly elevated CGRP-like immunoreactivity in the dorsal horn of the rat spinal cord. Naloxone (2 mg kg-1; i.p.) challenge precipitated a robust withdrawal syndrome that depleted CGRP-like immunoreactivity and increased the number of Fos-like immunoreactive neurons in the dorsal horn. 3. Intrathecal administration of NDGA (10, 20 microg), a nonselective LOX inhibitor, AA-861 (1.5, 3 microg), a 5-LOX selective inhibitor, or baicalein (1.4, 2.8 microg), a 12-LOX selective inhibitor, concurrently with systemic morphine for 5 days or as a single injection immediately preceding naloxone challenge, blocked the depletion of CGRP-like immunoreactivity, prevented increase in the number of Fos-like immunoreactive neurons in the dorsal horn, and significantly attenuated the morphine withdrawal syndrome. 4. The results of this study suggest that activity of LOX products, at the spinal level, contributes to the expression of opioid physical dependence, and that this activity may be expressed through increased sensory neuropeptide release.

c-Fos and peptide immunoreactivities in the central extended amygdala of morphine-dependent rats after naloxone-precipitated withdrawal

The central extended amygdala, a forebrain macrostructure, may represent a common substrate for acute drug reward and the dysphoric effects of drug withdrawal. To test its involvement during opiate withdrawal, we studied the distribution of c-Fos immunoreactive neurons, in relation to their neuropeptide content, in brain sections from morphine-dependent or naive rats, killed 90 min after naloxone or saline intraperitoneal injection. Naloxone treatment in naive rats induced a slight increase in c-Fos immunoreactivity in the central amygdaloid nucleus, the lateral bed nucleus of the stria terminalis and the interstitial nucleus of the posterior limb of the anterior commissure. In morphine-dependent rats, naloxone injection significantly increased the number of c-Fos-positive neurons in these structures as well as in the majority of the other central extended amygdala components. Double immunocytochemistry was used to determine the neurochemical nature of c-Fos-positive neurons in the central extended amygdala. Corticotropin-releasing factor- and methionine-enkephakin-immunoreactive neurons displayed c-Fos immunoreactivity in naive rats after naloxone injection, whereas only enkephalinergic neurons were found to be c-Fos positive in morphine-dependent rats after naloxone injection. The possible involvement of the corticotropin-releasing factor system during withdrawal is discussed. These results suggest that the whole central extended amygdala is activated during opiate withdrawal, with a lateral to medial decreasing gradient, and emphasize the role of peptidergic systems in this morphofunctional continuum.

Morphine withdrawal precipitated by specific mu, delta or kappa opioid receptor antagonists: a c-Fos protein study in the rat central nervous system

We have recently shown concurrent changes in behavioural responses and c-Fos protein expression in the central nervous system in both naive and morphine-dependent rats after systemic administration of the opioid antagonist naloxone. However, because naloxone acts on the three major types of opioid receptors, the present study aimed at determining, in the same animals, both changes in behaviour and c-Fos-like immunoreactivity after intravenous injection of selective opioid antagonists, such as mu (beta-funaltrexamine, 10 mg/kg), delta (naltrindole, 4 mg/kg) or kappa (nor-binaltorphimine, 5 mg/kg) opioid receptor antagonists, in naive or morphine-dependent rats. In a first experimental series, only beta-funaltrexamine increased c-Fos expression in the eight central nervous system structures examined, whereas no effect was seen after naltrindole or nor-binaltorphimine administration in naive rats. These results suggest a tonic activity in the endogenous opioid peptides acting on mu opioid receptors in normal rats. A second experimental series in morphine-dependent rats showed that beta-funaltrexamine had the highest potency in the induction of classical signs of morphine withdrawal syndrome, as well as the increase in c-Fos expression in the 22 central nervous system structures studied, suggesting a major role of mu opioid receptors in opioid dependence. However, our results also demonstrated that naltrindole and, to a lesser extent, nor-binaltorphimine were able to induce moderate signs of morphine withdrawal and relatively weak c-Fos protein expression in restricted central nervous system structures. Therefore, delta and kappa opioid receptors may also contribute slightly to opioid dependence.

Effects of excitotoxic lesions of the central or basolateral nucleus of the amygdala on naloxone-precipitated withdrawal-induced conditioned place aversion in morphine-dependent rats

We examined the effects of discrete, bilateral excitotoxic lesions of the central or basolateral nucleus of the amygdala on naloxone-precipitated withdrawal-induced conditioned place aversion in morphine-dependent rats. Lesions of the central nucleus significantly attenuated the conditioned place aversion, while lesions of the basolateral nucleus had little effect. These results suggest that the central nucleus of the amygdala, rather than the basolateral nucleus, plays a crucial role in the negative affective component of morphine abstinence.

Endogenous opiates and behavior: 2001

This paper is the twenty-fourth installment of the annual review of research concerning the opiate system. It summarizes papers published during 2001 that studied the behavioral effects of the opiate peptides and antagonists. The particular topics covered this year include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology(Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Neural correlates of the motivational and somatic components of naloxone-precipitated morphine withdrawal

In morphine-dependent rats, low naloxone doses have been shown to induce conditioned place aversion, which reflects the negative motivational component of opiate withdrawal. In contrast, higher naloxone doses are able to induce a 'full' withdrawal syndrome, including overt somatic signs. The c-fos gene is commonly used as a marker of neuronal reactivity to map the neural substrates that are recruited by various stimuli. Using in situ hybridization, we have analysed in the brain of morphine-dependent rats the effects of acute withdrawal syndrome precipitated by increasing naloxone doses on c-fos mRNA expression. Morphine dependence was induced by subcutaneous implantation of slow-release morphine pellets for 6 days and withdrawal was precipitated by increasing naloxone doses inducing the motivational (7.5 and 15 micro g/kg) and somatic (30 and 120 micro g/kg) components of withdrawal. Our mapping study revealed a dissociation between a set of brain structures (extended amygdala, lateral septal nucleus, basolateral amygdala and field CA1 of the hippocampus) which exhibited c-fos mRNA dose-dependent variations from the lowest naloxone doses, and many other structures (dopaminergic and noradrenergic nuclei, motor striatal areas, hypothalamic nuclei and periaqueductal grey) which were less sensitive and recruited only by the higher doses. In addition, we found opposite dose-dependent variations of c-fos gene expression within the central (increase) and the basolateral (decrease) amygdala after acute morphine withdrawal. Altogether, these results emphasize that limbic structures of the extended amygdala along with the lateral septal nucleus, the basolateral amygdala and CA1 could specifically mediate the negative motivational component of opiate withdrawal.

Involvement of glutamate receptors within the central nucleus of the amygdala in naloxone-precipitated morphine withdrawal-induced conditioned place aversion in rats

Chronic use of morphine leads to physical and psychological dependence. The amygdala is known to be involved in the expression of emotion such as anxiety and fear, and several studies have shown that the central nucleus of the amygdala (CeA) is involved in morphine dependence. In the present study, we investigated the role of glutamate receptors within the CeA in the negative affective component of morphine abstinence by evaluating naloxone-precipitated withdrawal-induced conditioned place aversion (CPA) in morphine-dependent rats. We found that microinjection of the AMPA/kainate-glutamate-receptor antagonist CNQX (30 nmol/side) into the bilateral CeA significantly attenuated the naloxone-precipitated withdrawal-induced CPA, as well as several somatic signs, in morphine-dependent rats, without preference or aversive effects by itself in non-dependent rats. Furthermore, microinjection of the non-competitive NMDA-receptor antagonist MK-801 (30 nmol/side) or competitive NMDA-receptor antagonist D-CPPene (0.01 and 0.1 nmol/side) into the CeA significantly attenuated the naloxone-precipitated morphine withdrawal-induced CPA, but not somatic withdrawal signs. These results suggest that the activation of AMPA /kainate and NMDA receptors within the CeA play a crucial role in the negative affective component of morphine abstinence.

Projections from the nociceptive area of the central nucleus of the amygdala to the forebrain: a PHA-L study in the rat

The lateral capsular division (CeLC) of the central nucleus (Ce) of the amygdala, in the rat, has been shown to be the main terminal area of a spino(trigemino)-parabrachio-amygdaloid nociceptive pathway [Bernard & Besson (1990) J. Neurophysiol. 63, 473-490; Bernard et al. (1992) J. Neurophysiol. 68, 551-569; Bernard et al. (1993) J. Comp. Neurol. 329, 201-229]. The projections to the forebrain from the CeLC and adjacent regions were studied in the rat by using microinjections of Phaseolus vulgaris leucoagglutinin (PHA-L) restricted in subdivisions of the Ce and the basolateral amygdaloid nucleus anterior (BLA). Our data showed that the entire CeLC projects primarily and extensively to the substantia innominata dorsalis (SId). The terminal labelling is especially dense in the caudal aspect of the SId. The other projections of the CeLC in the forebrain were dramatically less dense. They terminate in the bed nucleus of the stria terminalis (BST) and the posterior hypothalamus (pLH). No (or only scarce) other projections were found in the remaining forebrain areas. The Ce lateral division (CeL) and the Ce medial division (CeM), adjacent to the CeLC, also project to the SId with slightly lower density labelling. However, contrary to the case of the CeLC, both the CeL and the CeM extensively project to the ventrolateral subnucleus of the BST (BSTvl) with a few additional terminals found in other regions of the lateral BST. Only the CeM projects densely to both the interstitial nucleus of the posterior limb of the anterior commissure and the caudal most portion of the pLH. The projections of the BLA are totally different from those of the Ce as they terminate in the dorsal striatum, the accumbens nucleus, the olfactory tubercle, the nucleus of olfactory tract and the rostral pole of the cingulate/frontal cortex. This study demonstrates that the major output of the nociceptive spino(trigemino)-parabrachio-CeLC pathway is to the SId. It is suggested that the CeLC-SId pathway could have an important role in anxiety, aversion and genesis of fear in response to noxious stimuli.