Differential regulation of morphine antinociceptive effects by endogenous enkephalinergic system in the forebrain of mice

Regulation of Opioid Receptors by Their Endogenous Opioid Peptides

Activation of μ, δ, and κ opioid receptors by endogenous opioid peptides leads to the regulation of many emotional and physiological responses. The three major endogenous opioid peptides, β-endorphin, enkephalins, and dynorphins result from the processing of three main precursors: proopiomelanocortin, proenkephalin, and prodynorphin. Using a knockout approach, we sought to determine whether the absence of endogenous opioid peptides would affect the expression or activity of opioid receptors in mice lacking either proenkephalin, β-endorphin, or both. Since gene knockout can lead to changes in the levels of peptides generated from related precursors by compensatory mechanisms, we directly measured the levels of Leu-enkephalin and dynorphin-derived peptides in the brain of animals lacking proenkephalin, β-endorphin, or both. We find that whereas the levels of dynorphin-derived peptides were relatively unaltered, the levels of Leu-enkephalin were substantially decreased compared to wild-type mice suggesting that preproenkephalin is the major source of Leu-enkephalin. This data also suggests that the lack of β-endorphin and/or proenkephalin does not lead to a compensatory change in prodynorphin processing. Next, we examined the effect of loss of the endogenous peptides on the regulation of opioid receptor levels and activity in specific regions of the brain. We also compared the receptor levels and activity in males and females and show that the lack of β-endorphin and/or proenkephalin leads to differential modulation of the three opioid receptors in a region- and gender-specific manner. These results suggest that endogenous opioid peptides are important modulators of the expression and activity of opioid receptors in the brain.

Mu and delta opioid receptors play opposite nociceptive and behavioural roles on nerve-injured mice

BACKGROUND AND PURPOSE

Mu and delta opioid receptors(MOP, DOP) contribution to the manifestations of pathological pain is not understood. We used genetic approaches to investigate the opioid mechanisms modulating neuropathic pain and its comorbid manifestations.

EXPERIMENTAL APPROACH

We generated conditional knockout mice with MOP or DOP deletion in sensoryNav1.8-positive neurons (Nav1.8), in GABAergic forebrain neurons (DLX5/6) orconstitutively (CMV). Mutant mice and wild-type littermates were subjected topartial sciatic nerve ligation (PSNL) or sham surgery and their nociception wascompared. Anxiety-, depressivelike behaviour and cognitive performance were also measured. Opioid receptor mRNA expression, microgliosis and astrocytosis were assessed in the dorsalroot ganglia (DRG) and/or the spinal cord (SC).

KEY RESULTS

Constitutive CMV-MOP knockouts after PSNL displayed reduced mechanical allodynia and enhanced heat hyperalgesia. This phenotype was accompanied by increased DOP expression in DRG and SC, and reduced microgliosis and astrocytosis in deep dorsal horn laminae. Conditional MOP knockouts and control mice developed similar hypersensitivity after PSNL, except for anenhanced heat hyperalgesia by DLX5/6-MOP male mice. Neuropathic pain-induced anxiety was aggravated in CMV-MOP and DLX5/6-MOP knockouts. Nerve-injured CMV-DOP mice showed increased mechanical allodynia, whereas Nav1.8-DOP and DLX5/8-DOP mice had partial nociceptive enhancement. CMV-DOP and DLX5/6-DOP mutants showed increased depressive-like behaviour after PSNL.

CONCLUSIONS AND IMPLICATIONS

MOP activity after nerve injury increased anxiety-like responses involving forebrain GABAergic neurons and enhanced mechanical pain sensitivity along with repression of DOP expression and spinal cord gliosis. In contrast, DOP shows a protective function limiting nociceptive and affective manifestations of neuropathic pain.

The globus pallidus as a target for neuropeptides and endocannabinoids participating in central activities

The globus pallidus in the basal ganglia plays an important role in movement regulation. Neuropeptides and endocannabinoids are neuronal signalling molecules that influence the functions of the whole brain. Endocannabinoids, enkephalin, substance P, neurotensin, orexin, somatostatin and pituitary adenylate cyclase-activating polypeptides are richly concentrated in the globus pallidus. Neuropeptides and endocannabinoids exert excitatory or inhibitory effects in the globus pallidus mainly by modulating GABAergic, glutamatergic and dopaminergic neurotransmission, as well as many ionic mechanisms. Pallidal neuropeptides and endocannabinoids are associated with the pathophysiology of a number of neurological disorders, such as Parkinson's disease, Huntington's disease, schizophrenia, and depression. The levels of neuropeptides and endocannabinoids and their receptors in the globus pallidus change in neurological diseases. It has been demonstrated that spontaneous firing activity of globus pallidus neurons is closely related to the manifestations of Parkinson's disease. Therefore, the neuropeptides and endocannabinoids in the globus pallidus may function as potential targets for treatment in some neurological diseases. In this review, we highlight the morphology and function of neuropeptides and endocannabinoids in the globus pallidus and their involvement in neurological diseases.

Spontaneous Cingulate High-Current Spikes Signal Normal and Pathological Pain States

Prominent 7-12 Hz oscillations in frontal cortical networks in rats have been reported. However, the mechanism of generation and the physiological function of this brain rhythm have not yet been clarified. Multichannel extracellular field potentials of the ACC were recorded and analyzed using the current source density method in halothane-anesthetized rats. Spontaneous high-current spikes (HCSs) were localized in the deep part of layer II/III and upper part of layer V of the ACC. The frequency of HCSs in the ACC was 7-12 Hz, with an amplitude of 6.5 ± 0.76 mV/mm² and duration of 55.24 ± 2.43 ms. The power density significantly decreased (84.56 ± 6.93%, p < 0.05, t test) after pinching the hindpaw and significantly increased (149.28 ± 15.96%) after treatment with morphine. The suppressive effect of pinching was reversed by naloxone (0.7 mg/kg, i.p.). HCSs coincided with initiation of the depolarization of cingulate neurons and remained in a depolarized upstate. The occurrence of cingulate HCSs was persistently preceded by a hyperpolarization phase and a burst of multiunit spike activity in the medial dorsal thalamic nucleus. Spontaneous field-potential oscillations changed from 10 Hz to a lower band (i.e., ∼7.5 Hz) when a central poststroke pain condition was induced. The central poststroke pain group had a higher average coherence coefficient compared with the control group. Our results indicate that spontaneous cingulate cortical HCSs could be initiated by thalamocortical synaptic inputs from the medial dorsal thalamic nucleus and maintained by intracortical neuronal upstate mechanisms in physiological and pathological pain states.SIGNIFICANCE STATEMENT This study elucidated the mechanism of generation and physiological function of prominent 7-12 Hz frequency oscillations in frontal cortical networks in rats. Spontaneous cingulate cortical high-current spikes in anesthetized rats could be initiated by thalamocortical synaptic inputs from the medial dorsal thalamic nucleus and maintained by intracortical neuronal upstate mechanisms. Suppression of the anterior cingulate cortex-filtered EEG during noxious stimulation may have resulted from the desynchronization of high-current spikes in the ACC. The enhancement of fast Fourier transform power after a systemic morphine injection suggested that the opioid system may play an important role in synchronizing cingulate cortical neuronal networks. Spontaneous cingulate high-current spikes may also play an important role in thalamocortical dysrhythmia in central poststroke pain.

In vitro and in vivo characterization of the bifunctional μ and δ opioid receptor ligand UFP-505

BACKGROUND AND PURPOSE

Targeting more than one opioid receptor type simultaneously may have analgesic advantages in reducing side-effects. We have evaluated the mixed μ opioid receptor agonist/ δ opioid receptor antagonist UFP-505 in vitro and in vivo.

EXPERIMENTAL APPROACH

We measured receptor density and function in single μ, δ and μ /δ receptor double expression systems. GTPγ35 S binding, cAMP formation and arrestin recruitment were measured. Antinociceptive activity was measured in vivo using tail withdrawal and paw pressure tests following acute and chronic treatment. In some experiments, we collected tissues to measure receptor densities.

KEY RESULTS

UFP-505 bound to μ receptors with full agonist activity and to δ receptors as a low efficacy partial agonist At μ, but not δ receptors, UFP-505 binding recruited arrestin. Unlike morphine, UFP-505 treatment internalized μ receptors and there was some evidence for internalization of δ receptors. Similar data were obtained in a μ /δ receptor double expression system. In rats, acute UFP-505 or morphine, injected intrathecally, was antinociceptive. In tissues harvested from these experiments, μ and δ receptor density was decreased after UFP-505 but not morphine treatment, in agreement with in vitro data. Both morphine and UFP-505 induced significant tolerance.

CONCLUSIONS AND IMPLICATIONS

In this study, UFP-505 behaved as a full agonist at μ receptors with variable activity at δ receptors. This bifunctional compound was antinociceptive in rats after intrathecal administration. In this model, dual targeting provided no advantages in terms of tolerance liability.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Usefulness of knockout mice to clarify the role of the opioid system in chronic pain

Several lines of knockout mice deficient in the genes encoding each component of the endogenous opioid system have been used for decades to clarify the specific role of the different opioid receptors and peptide precursors in many physiopathological conditions. The use of these genetically modified mice has improved our knowledge of the specific involvement of each endogenous opioid component in nociceptive transmission during acute and chronic pain conditions. The present review summarizes the recent advances obtained using these genetic tools in understanding the role of the opioid system in the pathophysiological mechanisms underlying chronic pain. Behavioural data obtained in these chronic pain models are discussed considering the peculiarities of the behavioural phenotype of each line of knockout mice. These studies have identified the crucial role of specific components of the opioid system in different manifestations of chronic pain and have also opened new possible therapeutic approaches, such as the development of opioid compounds simultaneously targeting several opioid receptors. However, several questions still remain open and require further experimental effort to be clarified. The novel genetic tools now available to manipulate specific neuronal populations and precise genome editing in mice will facilitate in a near future the elucidation of the role of each component of the endogenous opioid system in chronic pain.

LINKED ARTICLES

This article is part of a themed section on Emerging Areas of Opioid Pharmacology. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v175.14/issuetoc.

Lhx2 Expression in Postmitotic Cortical Neurons Initiates Assembly of the Thalamocortical Somatosensory Circuit

Cortical neurons must be specified and make the correct connections during development. Here, we examine a mechanism initiating neuronal circuit formation in the barrel cortex, a circuit comprising thalamocortical axons (TCAs) and layer 4 (L4) neurons. When Lhx2 is selectively deleted in postmitotic cortical neurons using conditional knockout (cKO) mice, L4 neurons in the barrel cortex are initially specified but fail to form cellular barrels or develop polarized dendrites. In Lhx2 cKO mice, TCAs from the thalamic ventral posterior nucleus reach the barrel cortex but fail to further arborize to form barrels. Several activity-regulated genes and genes involved in regulating barrel formation are downregulated in the Lhx2 cKO somatosensory cortex. Among them, Btbd3, an activity-regulated gene controlling dendritic development, is a direct downstream target of Lhx2. We find that Lhx2 confers neuronal competency for activity-dependent dendritic development in L4 neurons by inducing the expression of Btbd3.

Endogenous opioidergic dysregulation of pain in fibromyalgia: a PET and fMRI study

Endogenous opioid system dysfunction potentially contributes to chronic pain in fibromyalgia (FM), but it is unknown if this dysfunction is related to established neurobiological markers of hyperalgesia. We previously reported that µ-opioid receptor (MOR) availability was reduced in patients with FM as compared with healthy controls in several pain-processing brain regions. In the present study, we compared pain-evoked functional magnetic resonance imaging with endogenous MOR binding and clinical pain ratings in female opioid-naive patients with FM (n = 18) using whole-brain analyses and regions of interest from our previous research. Within antinociceptive brain regions, including the dorsolateral prefrontal cortex (r = 0.81, P < 0.001) and multiple regions of the anterior cingulate cortex (all r > 0.67; all P < 0.02), reduced MOR availability was associated with decreased pain-evoked neural activity. Additionally, reduced MOR availability was associated with lower brain activation in the nucleus accumbens (r = 0.47, P = 0.050). In many of these regions, pain-evoked activity and MOR binding potential were also associated with lower clinical affective pain ratings. These findings are the first to link endogenous opioid system tone to regional pain-evoked brain activity in a clinical pain population. Our data suggest that dysregulation of the endogenous opioid system in FM could lead to less excitation in antinociceptive brain regions by incoming noxious stimulation, resulting in the hyperalgesia and allodynia commonly observed in this population. We propose a conceptual model of affective pain dysregulation in FM.

Dopamine D1-like Receptors Regulate Constitutive, μ-Opioid Receptor-Mediated Repression of Use-Dependent Synaptic Plasticity in Dorsal Horn Neurons: More Harm than Good?

The current study reports on a synaptic mechanism through which D1-like receptors (D1LRs) modulate spinal nociception and plasticity by regulating activation of the μ-opioid receptor (MOR).D1LR stimulation with agonist SKF 38393 concentration-dependently depressed C-fiber-evoked potentials in rats receiving spinal nerve ligation (SNL), but not in uninjured rats. Depression was prevented by MOR- but not GABA-receptor blockade. Neurons expressing the D1 subtype were immunopositive for met-enkephalin and vesicular glutamate transporter VGLUT2, but not for GABAergic marker vGAT.Nerve ligation was followed by increased immunoreactivity for D1 in synaptic compartment (P3) in dorsal horn homogenates and presynaptic met-enkephalin-containing boutons. SNL led to increased immunoreactivity for met-enkephalin in dorsal horn homogenates, which was dose-dependently attenuated by selective D1LR antagonist SCH 23390. During blockade of either D1R or MOR, low-frequency (0.2 or 3 Hz) stimulation (LFS) to the sciatic nerve induced long-term potentiation (LTP) of C-fiber-evoked potentials, revealing a constituent role of both receptors in repressing afferent-induced synaptic plasticity. LFS consistently induced NMDA receptor-dependent LTP in nerve-injured rats. The ability of MOR both to prevent LTP and to modulate mechanical and thermal pain thresholds in behavioral tests was preserved in nerve-ligated rats that were postoperatively treated with SCH 23390. D1LR priming for 30 min sufficed to disrupt MOR function in otherwise naive rats via a mechanism involving receptor overuse.The current data support that, whereas D1LR-modulated MOR activation is instrumental in antinociception and endogenous repression of synaptic plasticity, this mechanism deteriorates rapidly by sustained use, generating increased vulnerability to afferent input.

SIGNIFICANCE STATEMENT

The current study shows that dopamine D1-like receptors (D1LRs) and μ-opioid receptors (MOR) in the spinal dorsal horn constitutively repress the expression of synaptic long-term potentiation (LTP) of C-fiber-evoked potentials. Anatomical data are provided supporting that the D1 subtype regulates MOR function by modulating met-enkephalin release. Sustained neuropathic pain induced by spinal nerve ligation is accompanied by D1R and met-enkephalin upregulation, acquired D1LR-mediated antinociception, and a loss of endogenous repression of further synaptic plasticity. We show that the ability of MOR to oppose LTP is rapidly impaired by sustained D1LR activation via a mechanism involving sustained MOR activation.

Postnatal maturation of endogenous opioid systems within the periaqueductal grey and spinal dorsal horn of the rat

Significant opioid-dependent changes occur during the fourth postnatal week in supraspinal sites (rostroventral medulla [RVM], periaqueductal grey [PAG]) that are involved in the descending control of spinal excitability via the dorsal horn (DH). Here we report developmentally regulated changes in the opioidergic signalling within the PAG and DH, which further increase our understanding of pain processing during early life. Microinjection of the μ-opioid receptor (MOR) agonist DAMGO (30 ng) into the PAG of Sprague-Dawley rats increased spinal excitability and lowered mechanical threshold to noxious stimuli in postnatal day (P)21 rats, but had inhibitory effects in adults and lacked efficacy in P10 pups. A tonic opioidergic tone within the PAG was revealed in adult rats by intra-PAG microinjection of CTOP (120 ng, MOR antagonist), which lowered mechanical thresholds and increased spinal reflex excitability. Spinal adminstration of DAMGO inhibited spinal excitability in all ages, yet the magnitude of this was greater in younger animals than in adults. The expression of MOR and related peptides were also investigated using TaqMan real-time polymerase chain reaction and immunohistochemistry. We found that pro-opiomelanocortin peaked at P21 in the ventral PAG, and MOR increased significantly in the DH as the animals aged. Enkephalin mRNA transcripts preceded the increase in enkephalin immunoreactive fibres in the superficial dorsal horn from P21 onwards. These results illustrate that profound differences in the endogenous opioidergic signalling system occur throughout postnatal development.

Modulation of brain electroencephalography oscillations by electroacupuncture in a rat model of postincisional pain

The present study aimed to investigate how ongoing brain rhythmical oscillations changed during the postoperative pain and whether electroacupuncture (EA) regulated these brain oscillations when it relieved pain. We established a postincisional pain model of rats with plantar incision to mimic the clinical pathological pain state, tested the analgesic effects of EA, and recorded electroencephalography (EEG) activities before and after the EA application. By analysis of power spectrum and bicoherence of EEG, we found that in rats with postincisional pain, ongoing activities at the delta-frequency band decreased, while activities at theta-, alpha-, and beta-frequency bands increased. EA treatment on these postincisional pain rats decreased the power at high-frequency bands especially at the beta-frequency band and reversed the enhancement of the cross-frequency coupling strength between the beta band and low-frequency bands. After searching for the PubMed, our study is the first time to describe that brain oscillations are correlated with the processing of spontaneous pain information in postincisional pain model of rats, and EA could regulate these brain rhythmical frequency oscillations, including the power and cross-frequency couplings.

Involvement of the opioid and cannabinoid systems in pain control: new insights from knockout studies

The endogenous opioid and cannabinoid systems are involved in the physiological inhibitory control of pain and are of particular interest for the development of therapeutic approaches for pain management. The involvement of these endogenous systems in pain control has been studied from decades by the use of compounds with different affinities for each cannabinoid and opioid receptor or for the different enzymes involved in endocannabinoid and endogenous opioid metabolism. However, the selectivity of these pharmacological tools in vivo has represented an important limitation for these studies. The generation of genetically modified mice with selective mutations in specific components of the endocannabinoid and endogenous opioid system has provided important advances in the identification of the specific contribution of each component of these endogenous systems in the perception of noxious stimuli and the development of pathological pain states. Different lines of constitutive and conditional knockout mice deficient in specific cannabinoid and opioid receptors, specific precursors of the endogenous opioid peptides and the main enzymes involved in endocannabinoid and endogenous opioid degradation are now available. These knockout mice have also been used to evaluate the contribution of each component of the endocannabinoid and opioid system in the antinociceptive effects of cannabinoid and opioid agonists, including those currently used to treat pain in humans. This review summarizes the main advances provided in the last 15 years by the use of these genetic tools in the knowledge of the physiological control of pain and the pharmacology of cannabinoid and opioid compounds for pain management.

Topiramate modulates habituation in migraine: evidences from nociceptive responses elicited by laser evoked potentials

BACKGROUND

Lack of habituation during repetitive stimulation is the most consistent interictal abnormality of cortical information processing observed in migraine. Preventive migraine treatments might act by stabilizing cortical excitability level and thus the habituation to external stimuli.

METHODS

We examined the effects of preventive treatment with topiramate on migraineur's habituation to nociceptive stimulation. Scalp potentials were evoked by Nd-YAP Laser stimulation of the hand dorsum and supraorbital region in 13 patients with migraine without aura (MO) and 15 healthy volunteers (HV). The exam was repeated in MO before and after treatment.

RESULTS

We observed a lack of habituation and lower initial amplitudes in MO compared to HV. These abnormalities reached statistical significance for N1 LEPs component, generated in the secondary somatosensory cortex (SII), but not for N2/P2 complex, generated in the insula and anterior cingulated cortex (ACC). Topiramate normalized the N1 habituation pattern in MO, with a significant correlation between clinical effects and normalization of neurophysiological responses.

CONCLUSIONS

Our results indicate a modulating action of topiramate on cortical processing of sensorial stimuli, mainly regarding the sensory-discriminative component of pain, elaborated by SII, without a significant effect on the affective dimension of pain, in which the ACC has an important role.

Ultrastructural relationship between the mu opioid receptor and its interacting protein, GPR177, in striatal neurons

GPR177, the mammalian ortholog of Drosophila Wntless/Evi/Sprinter, was recently identified as a novel mu-opioid receptor (MOR) interacting protein. GPR177 is a trans-membrane protein pivotal to mediating the secretion of Wnt signaling proteins. Wnt proteins, in turn, are essential in regulating neuronal development, a phenomenon inhibited upon chronic exposure to MOR agonists such as morphine and heroin. We previously showed that GPR177 and MOR are co-localized in the mouse dorsolateral striatum; however, the nature of this interaction was not fully elucidated. Therefore, in the present study, we examined cellular substrates for interactions between GPR177 and MOR using a combined immunogold-silver and peroxidase detection approach in coronal sections in the dorsolateral segment of the striatum. Semi-quantitative analysis of the ultrastructural distribution of GPR177 and MOR in striatal somata and in dendritic processes showed that, of the somata and dendritic processes exhibiting GPR177, 32% contained MOR immunolabeling while for profiles exhibiting MOR, 37% also contained GPR177 immunoreactivity. GPR177-labeled particles were localized predominantly along both the plasma membrane and within the cytoplasm of MOR-labeled dendrites. Somata and dendritic processes that contained both GPR177 and MOR more often received symmetric (inhibitory-type) synapses from unlabeled axon terminals. To further define the phenotype of GPR177 and MOR-containing cellular profiles, triple immunofluorescence detection showed that GPR177 and MOR are localized in neurons containing the opioid peptide, enkephalin, within the dorsolateral striatum. The results provide an anatomical substrate for interactions between MOR and its interacting protein, GPR177, in striatal opioid-containing neurons that may underlie the morphological alterations produced in neurons by chronic opiate use.

Anxiety- and depressive-like responses and c-fos activity in preproenkephalin knockout mice: oversensitivity hypothesis of enkephalin deficit-induced posttraumatic stress disorder

The present study used the preproenkephalin knockout (ppENK) mice to test whether the endogenous enkephalins deficit could facilitate the anxiety- and depressive-like symptoms of posttraumatic stress disorder (PTSD). On Day 1, sixteen wildtype (WT) and sixteen ppENK male mice were given a 3 mA or no footshock treatment for 10 seconds in the footshock apparatus, respectively. On Days 2, 7, and 13, all mice were given situational reminders for 1 min per trial, and the freezing response was assessed. On Day 14, all mice were tested in the open field test, elevated plus maze, light/dark avoidance test, and forced swim test. Two hours after the last test, brain tissues were stained to examine c-fos expression in specific brain areas. The present results showed that the conditioned freezing response was significant for different genotypes (ppENK vs WT). The conditioned freezing effect of the ppENK mice was stronger than those of the WT mice. On Day 14, the ppENK mice showed more anxiety- and depressive-like responses than WT mice. The magnitude of Fos immunolabeling was also significantly greater in the primary motor cortex, bed nucleus of the stria terminalis-lateral division, bed nucleus of the stria terminalis-supracapsular division, paraventricular hypothalamic nucleus-lateral magnocellular part, central nucleus of the amygdala, and basolateral nucleus of the amygdala in ppENK mice compared with WT mice. In summary, animals with an endogenous deficit in enkephalins might be more sensitive to PTSD-like aversive stimuli and elicit stronger anxiety and depressive PTSD symptoms, suggesting an oversensitivity hypothesis of enkephalin deficit-induced PTSD.

Chronic morphine administration delays wound healing by inhibiting immune cell recruitment to the wound site

Patients prescribed morphine for the management of chronic pain, and chronic heroin abusers, often present with complications such as increased susceptibility to opportunistic infections and inadequate healing of wounds. We investigated the effect of morphine on wound-healing events in the presence of an infection in an in vivo murine model that mimics the clinical manifestations seen in opioid user and abuser populations. We show for the first time that in the presence of an inflammatory inducer, lipopolysaccharide, chronic morphine treatment results in a marked decrease in wound closure, compromised wound integrity, and increased bacterial sepsis. Morphine treatment resulted in a significant delay and reduction in both neutrophil and macrophage recruitment to the wound site. The delay and reduction in neutrophil reduction was attributed to altered early expression of keratinocyte derived cytokine and was independent of macrophage inflammatory protein 2 expression, whereas suppression of macrophage infiltration was attributed to suppressed levels of the potent macrophage chemoattractant monocyte chemotactic protein-1. When the effects of chronic morphine on later wound healing events were investigated, a significant suppression in angiogenesis and myofibroblast recruitment were observed in animals that received chronic morphine administration. Taken together, our findings indicate that morphine treatment results in a delay in the recruitment of cellular events following wounding, resulting in a lack of bacterial clearance and delayed wound closure.

Morphine disrupts interleukin-23 (IL-23)/IL-17-mediated pulmonary mucosal host defense against Streptococcus pneumoniae infection

Streptococcus pneumoniae is a pathogen that causes serious respiratory disease and meningitis in the immunocompromised drug abuse population. However, the precise mechanisms by which drug abuse compromises the host immune defense to pulmonary S. pneumoniae infection is not fully understood. Using a well-established murine model of opiate abuse and S. pneumoniae lung infection, we explored the influence of morphine treatment on the interleukin-23 (IL-23)/IL-17 axis and related innate immunity. Impairment of early IL-23/IL-17 production caused by morphine treatment was associated with delayed neutrophil migration and decreased pneumococcal clearance. Furthermore, morphine treatment impaired MyD88-dependent IL-23 production in alveolar macrophages and dendritic cells in response to in vitro S. pneumoniae cell infection. Moreover, morphine treatment significantly inhibited the S. pneumoniae-induced phosphorylation of interferon response factor 3 (IRF3), ATF2, and NF-kappaBp65. T-cell receptor delta (TCRdelta)-deficient mice showed a decrease in IL-17 production and a severely weakened capacity to clear lung S. pneumoniae infection. Finally, morphine treatment resulted in diminished secretion of antimicrobial proteins S100A9 and S100A8/A9 during early stages of S. pneumoniae infection. In conclusion, morphine treatment causes a dysfunction in IL-23-producing dendritic cells and macrophages and IL-17-producing gammadeltaT lymphocytes in response to S. pneumoniae lung infection. This leads to diminished release of antimicrobial S100A8/A9 proteins, compromised neutrophil recruitment, and more-severe infection.

Endogenous opiates and behavior: 2008

This paper is the 31st consecutive installment of the annual review of research concerning the endogenous opioid system. It summarizes papers published during 2008 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

Morphine modulation of pain processing in medial and lateral pain pathways

Background

Despite the wide-spread use of morphine and related opioid agonists in clinic and their powerful analgesic effects, our understanding of the neural mechanisms underlying opioid analgesia at supraspinal levels is quite limited. The present study was designed to investigate the modulative effect of morphine on nociceptive processing in the medial and lateral pain pathways using a multiple single-unit recording technique. Pain evoked neuronal activities were simultaneously recorded from the primary somatosensory cortex (SI), ventral posterolateral thalamus (VPL), anterior cingulate cortex (ACC), and medial dorsal thalamus (MD) with eight-wire microelectrode arrays in awake rats.

Results

The results showed that the noxious heat evoked responses of single neurons in all of the four areas were depressed after systemic injection of 5 mg/kg morphine. The depressive effects of morphine included (i) decreasing the neuronal response magnitude; (ii) reducing the fraction of responding neurons, and (iii) shortening the response duration. In addition, the capability of cortical and thalamic neural ensembles to discriminate noxious from innocuous stimuli was decreased by morphine within both pain pathways. Meanwhile, morphine suppressed the pain-evoked changes in the information flow from medial to lateral pathway and from cortex to thalamus. These effects were completely blocked by pre-treatment with the opiate receptor antagonist naloxone.

Conclusion

These results suggest that morphine exerts analgesic effects through suppressing both sensory and affective dimensions of pain.