Inhibitory Effect Of Morphine Research Articles

Critical interactions between opioid and cannabinoid receptors during tolerance and physical dependence development to opioids in the murine gastrointestinal tract: proof of concept

IntroductionTolerance (TOL) and physical dependence (PD) constitute important limitations of opioid therapy. The aim of our study was to validate research tools to investigate TOL and PD and to characterize the interactions between opioid (OR) and cannabinoid (CB) receptors in these processes in the GI tract.MethodsTOL was assessed through the comparison of morphine ability to inhibit electrically evoked smooth muscles contractility in the mouse ileum that was previously incubated with/without morphine for 1 h. To evaluate the PD, the ileum was incubated with morphine for 10 min, then challenged with naloxone to induce withdrawal response (WR). The OR/CB interactions were evaluated using mixed agonist (PR-38) and AM-251 (CB1 antagonist).ResultsThe inhibitory effect of morphine on ileal contractions was weaker in tissue incubated with this opioid than in tissue incubated without opioid. The opposite was noted for PR-38. In tissues exposed to morphine, but not to PR-38, naloxone induced a WR. The blockage of CB1 receptors with AM-251 before the addition of PR-38 resulted in a naloxone-induced WR.ConclusionThe co-activation of OR and CB reduced development of TOL and PD to opioids in the mouse GI tract and mixed OR/CB agonists are promising alternative to currently used opioid drugs.

Stimulatory and inhibitory effects of morphine on pentylenetetrazol-induced epileptic activity in rat

Aims The present study attempts to evaluate the effects of different doses of morphine on experimental epileptiform activity caused by pentylenetetrazol (PTZ) in rats. Methods Thirty adult male rats were assigned to saline (n = 5), morphine (2, 5, and 10 mg/kg, n = 15), naloxone (1 mg/kg, n = 5), and pre-treated with naloxone+morphine (1 + 10 mg/kg, n = 5) groups. The animals were anesthetized with ketamine + xylazine (80 + 8 mg/kg), and then a bipolar electrode was implanted into the CA1 (AP: −2.76 mm, ML: −1.4 mm and DV: 3 mm). To evaluate the effects of drugs on spike count and their amplitudes by elab amplifier, after drug administration for 25 min, PTZ (80 mg/kg, i.p.) was injected to induce epileptiform activity. Finally, diazepam (10 mg/kg) was used to suppress epileptic activity. Results The results revealed that morphine at a dose of 2 mg/kg decreased, and at doses of 5 and 10 mg/kg had an increasing effect on seizure-like events (SLEs). Nevertheless, morphine at a dose of 10 mg/kg enhanced SLEs significantly (p < 0.01). Naloxone at a dose of 1 mg/kg had no significant effect on the spike count but increased amplitude of them (p < 0.001). Moreover, being pretreatment with naloxone at a dose of 1 mg/kg, the morphine group showed significantly increased in the spike count (p < 0.05). Conclusions Morphine has biphasic effects on PTZ-induced epileptiform activities that way at a low dose has an inhibitory effect, but if the dose is increased, it will intensify the desired event and that the stimulatory effects of morphine appear not to be via opioid receptors.

Supraspinal Opioid Circuits Differentially Modulate Spinal Neuronal Responses in Neuropathic Rats.

The anterior cingulate cortex and central nucleus of the amygdala connect widely with brainstem nuclei involved in descending modulation, including the rostral ventromedial medulla. Endogenous opioids in these circuits participate in pain modulation. The hypothesis was that a differential opioidergic role for the brain nuclei listed in regulation of spinal neuronal responses because separable effects on pain behaviors in awake animals were previously observed. This study utilized in vivo electrophysiology to determine the effects of morphine microinjection into the anterior cingulate cortex, right or left central nucleus of the amygdala, or the rostral ventromedial medulla on spinal wide dynamic range neuronal responses in isoflurane-anesthetized, male Sprague-Dawley rats. Ongoing activity in the ventrobasal thalamus was also measured. In total, 33 spinal nerve ligated and 26 control age- and weight-matched control rats were used. Brainstem morphine reduced neuronal firing to 60-g von Frey stimulation in control rats (to 65 ± 12% of control response (means ± 95% CI), P < 0.001) with a greater inhibition in neuropathic rats (to 53 ± 17% of control response, P < 0.001). Contrasting anterior cingulate cortex morphine had only marginal modulatory effects on spinal neuronal responses with limited variance in effect between control and neuropathic rats. The inhibitory effects of morphine in the central nucleus of the amygdala were dependent on pain state and laterality; only right-side morphine reduced neuronal firing to 60-g stimulation in neuropathic rats (to 65 ± 14% of control response, P = 0.001). In addition, in neuropathic rats elevated ongoing neuronal activity in the ventral posterolateral thalamus was not inhibited by anterior cingulate cortex morphine, in contrast to evoked responses. Cumulatively the data support opioid modulation of evoked responses predominately through a lateralized output from the right amygdala, as well as from the brainstem that is enhanced in injured conditions. Minimal modulation of dorsal horn responses was observed after anterior cingulate cortex opioid administration regardless of injury state.

Disc degeneration induces a mechano-sensitization of disc afferent nerve fibers that associates with low back pain

We aimed to investigate mechano-sensitivity at the afferent nerve fibers projecting to degenerated intervertebral disc (IVD) and nociceptive behaviour in a rat model of low back pain (LBP). Animal model with LBP was established by lumbar 4/5 IVD puncture and nucleus pulposus aspiration. Invivo single nerve recordings (n=121) were introduced to measure discharge frequency at the afferent nerve fiber innervating the IVD during mechanical stimulations (von Frey filament or intradiscal pressure). Nerve growth factor (NGF) expression levels in the IVD (n=20) were assessed by Western blot. LBP-related behaviour (n=22) was assessed by measuring changes in rearing, mechanical paw-withdrawal threshold, and dynamic weight bearing in a freely walking rat. Inhibitory effect of morphine on the neuronal excitability (n=19) and painful behaviour (n=28) was also assessed. Compared to those with sham or naïve IVD, animal group with degenerated IVD displayed the sensitized neuronal responses and painful behaviour, with hyperexcitability of the afferent nerve fibers in any range of mechanical stimulations (von Frey filament stimulation; 1, 2, and 26g; intradiscal pressure, 1,500-3,000mm Hg), strong upregulation of NGF (200-250 % increase), and LBP-like behaviour such as failure of rearing, front limbs-dependent walking pattern, and hypersensitivity in hind-paws. However, the neuronal hyperexcitability and pain behaviour were attenuated after local (30 μM) or systemic (3mgkg-1) morphine administration. Our study suggests that enhanced mechano-sensitivity at the afferent nerve fiber innervating degenerated IVD is deeply correlated with LBP development, which supports the hypothesis that hyperexcited responses at the nerve fibers represent a decisive source of LBP.

Minocycline increases firing rates of accumbal neurons and modifies the effects of morphine on neuronal activity.

Accumulating evidence indicated that minocycline, a glial cell modulator, is able to modify a variety of morphine effects. Here, we investigated minocycline effects on electrical activity of nucleus accumbens (NAc) neurons using single unit recording in urethane-anesthetized rats. In addition, we investigated whether minocycline can modify the effects of morphine on NAc neural activity during reinstatement of morphine-seeking behavior. Minocycline increased the NAc firing activity in intact animals. Electrophysiological recording in morphine-treated animals was performed, following the acquisition of morphine-induced conditioned place preference (5mg/kg, s.c., 3days) and a drug-free extinction period. In acutely minocycline- treated animals, the neurons were recorded for 40minutes following a single injection of either minocycline (50μg/5μl, i.c.v.) or saline. Then a priming dose of morphine (1mg/kg, s.c.) was injected while the recording was continued for an additional 40minutes. Minocycline significantly increased the firing rates of neurons and significantly modified morphine inhibitory effects on NAc neurons. In subchronically minocycline-treated groups, the rats were given daily injections of minocycline (50μg/5μl, i.c.v) during the extinction period. Then, on the reinstatement day, NAc neurons were recorded for 10minutes, the priming dose of morphine was administered and the recording was continued for 45minutes. Our results showed the failure of minocycline to significantly modify the inhibitory effects of morphine. In conclusion, our findings indicated that minocycline modifies morphine-induced decreases in the firing rates of NAc neurons in the reinstatement phase.

Comparison of In Vitro Effects of Opioid Analgesics on Spontaneous Proximal and Distal Colon Contractions in Healthy Rats and Rats with Peritonitis.

The goal of this study was to investigate and compare the effects of opioids on proximal and distal colon contractions in normal rats and rats with peritonitis, with and without the presence of naloxone in the environment. The study was approved by Cumhuriyet University Ethics committee. In this study, 16 Wistar Albino male rats were used. Rats were divided into two groups. Peritonitis was induced using a cecum ligation and perforation method, 24 h before the tissues of rats in the peritonitis group were collected, and sham surgery was performed 24 h before the tissues of rats in the control group were collected. Twenty-four hours after the surgery, rats' organs were harvested and hung in organ baths. Concentration-dependent inhibitory effects of morphine and meperidine on spontaneous intestinal movements were observed. Any differences between the groups were tested using the Kruskal-Wallis test, and any differences between the groups were tested using the Tukey test. No significant difference was observed between the proximal and distal colon smooth muscle contraction responses in both groups after 80 mM Potassium Chloride (KCl) injection (p>0.005). In the peritonitis group, amplitudes and frequencies of spontaneous contractions in proximal and distal colon significantly increased (p<0.05). Drugs decreased the amplitude and frequency responses in the control group (p<0.05). In the peritonitis group, whereas morphine decreased the amplitude and frequency responses in comparison with the control group (p<0.05), meperidine did not cause any significant changes (p>0.05). In both groups, adding naloxone to the organ baths before adding opioids completely blocked the morphine's inhibitory effect on the amplitude and frequency (p<0.05), but it could not completely block the inhibition caused by meperidine. Morphine and meperidine exhibit an inhibitory effect on the intestinal motility in both groups. This effect can be blocked by naloxone completely in morphine, and partially in meperidine.

Differences in the morphine-induced inhibition of small and large intestinal transit: Involvement of central and peripheral μ-opioid receptors in mice

Constipation is the most common side effect of morphine. Morphine acts centrally and on peripheral sites within the enteric nervous system. There are a few comprehensive studies on morphine-induced constipation in the small and large intestine by the activation of central and peripheral μ-opioid receptors. We investigated the differences in the inhibition of the small and large intestinal transit in normal and morphine-tolerant mice. Morphine reduced the geometric center in the fluorescein isothiocyanate–dextran assay and prolonged the bead expulsion time in a dose-dependent manner. The inhibitory effects of morphine were blocked by μ-opioid antagonist β-funaltrexamine, but not by δ- and κ-opioid antagonists. The peripheral opioid receptor antagonist, naloxone methiodide, partially blocked morphine's effect in the small intestine and completely blocked its effect in the large intestine. The intracerebroventricular administration of naloxone significantly reversed the delay of small intestinal transit but did not affect morphine-induced inhibition of large intestinal transit. Naloxone methiodide completely reversed the inhibition of large intestinal transit in normal and morphine-tolerant mice. Naloxone methiodide partially reversed the morphine-induced inhibition of small intestinal transit in normal mice but completely reversed the effects of morphine in tolerant mice. Chronic treatment with morphine results in tolerance to its inhibitory effect on field-stimulated contraction in the isolated small intestine but not in the large intestine. These results suggest that peripheral and central opioid receptors are involved in morphine-induced constipation in the small and large intestine during the early stage of treatment, but the peripheral receptors mainly regulate constipation during long-term morphine treatment.

Mitochondrial events responsible for morphine's cardioprotection against ischemia/reperfusion injury

Morphine may induce cardioprotection by targeting mitochondria, but little is known about the exact mitochondrial events that mediate morphine's protection. We aimed to address the role of the mitochondrial Src tyrosine kinase in morphine's protection. Isolated rat hearts were subjected to 30min ischemia and 2h of reperfusion. Morphine was given before the onset of ischemia. Infarct size and troponin I release were measured to evaluate cardiac injury. Oxidative stress was evaluated by measuring mitochondrial protein carbonylation and mitochondrial ROS generation. HL-1 cells were subjected to simulated ischemia/reperfusion and LDH release and mitochondrial membrane potential (ΔΨm) were measured. Morphine reduced infarct size as well as cardiac troponin I release which were aborted by the selective Src tyrosine kinase inhibitors PP2 and Src-I1. Morphine also attenuated LDH release and prevented a loss of ΔΨm at reperfusion in a Src tyrosine kinase dependent manner in HL-1 cells. However, morphine failed to reduce LDH release in HL-1 cells transfected with Src siRNA. Morphine increased mitochondrial Src phosphorylation at reperfusion and this was abrogated by PP2. Morphine attenuated mitochondrial protein carbonylation and mitochondrial superoxide generation at reperfusion through Src tyrosine kinase. The inhibitory effect of morphine on the mitochondrial complex I activity was reversed by PP2. These data suggest that morphine induces cardioprotection by preventing mitochondrial oxidative stress through mitochondrial Src tyrosine kinase. Inhibition of mitochondrial complex I at reperfusion by Src tyrosine kinase may account for the prevention of mitochondrial oxidative stress by morphine.

Dopamine D3 receptor dysfunction prevents anti-nociceptive effects of morphine in the spinal cord

Dopamine (DA) modulates spinal reflexes, including nociceptive reflexes, in part via the D3 receptor subtype. We have previously shown that mice lacking the functional D3 receptor (D3KO) exhibit decreased paw withdrawal latencies from painful thermal stimuli. Altering the DA system in the CNS, including D1 and D3 receptor systems, reduces the ability of opioids to provide analgesia. Here, we tested if the increased pain sensitivity in D3KO might result from a modified μ-opioid receptor (MOR) function at the spinal cord level. As D1 and D3 receptor subtypes have competing cellular effects and can form heterodimers, we tested if the changes in MOR function may be mediated in D3KO through the functionally intact D1 receptor system. We assessed thermal paw withdrawal latencies in D3KO and wild type (WT) mice before and after systemic treatment with morphine, determined MOR and phosphorylated MOR (p-MOR) protein expression levels in lumbar spinal cords, and tested the functional effects of DA and MOR receptor agonists in the isolated spinal cord. In vivo, a single morphine administration (2 mg/kg) increased withdrawal latencies in WT but not D3KO, and these differential effects were mimicked in vitro, where morphine modulated spinal reflex amplitudes (SRAs) in WT but not D3KO. Total MOR protein expression levels were similar between WT and D3KO, but the ratio of pMOR/total MOR was higher in D3KO. Blocking D3 receptors in the isolated WT cord precluded morphine's inhibitory effects observed under control conditions. Lastly, we observed an increase in D1 receptor protein expression in the lumbar spinal cord of D3KO. Our data suggest that the D3 receptor modulates the MOR system in the spinal cord, and that a dysfunction of the D3 receptor can induce a morphine-resistant state. We propose that the D3KO mouse may serve as a model to study the onset of morphine resistance at the spinal cord level, the primary processing site of the nociceptive pathway.

Effects of intrathecal carbenoxolone treatment on nociception and analgesia in rat.

Gap junctions (GJ) are important in pain signalling at the spinal cord level. The aim of this investigation was to study the effects of GJ on nociception and the analgesic/hyperalgesic effects of morphine following administration of carbenoxolone as a GJ blocker. Male Wistar rats (200-250 g) were divided into three groups: saline i.p., 10 mg/kg and 1 μg/kg i.p. morphine, each with two subgroups. One was treated intrathecally with saline and the other with carben oxolone. Animal experiment. The thermal nociception threshold was measured prior to and after injections using the tail flick test. Chemical nociception assessment was conducted using a 0.05-mL subplantar injection of 2.5% formalin. Both formalin-induced neurogenic and inflammatory nociception were reduced in the [saline i.p./carbenoxolone i.t.] and [morphine 1 μg/kg, i.p./carbenoxolone i.t.] subgroups (p<0.001). The 10 mg/kg i.p. morphine, i.t./carbenoxolone treatment reduced morphine-induced analgesia in the inflammatory phase (p<0.05), while it was ineffective in the neurogenic phase. Carbenoxolone decreased 1 μg/kg i.p. morphine-induced hyperalgesia in the tail flick test (p<0.001). Based on the results, GJ probably play a role in nociception at the spinal cord level. This may be due to the facilitation of inflammatory mediators released from glial cells or the connection between stimulatory interneurons and projection neurons. GJ blocking attenuated morphine-induced analgesia. This may be due to the attenuation of pre/post-synaptic inhibitory effects of morphine at the spinal cord level. As demonstrated by the investigations, GJ are present between inhibitory interneurons. Therefore, we can assume that blockage of GJ between inhibitory interneurons will reduce morphine-induced analgesia at the spinal cord level. However, this requires further investigation.

New insights into mechanisms of opioid inhibitory effects on capsaicin-induced TRPV1 activity during painful diabetic neuropathy

Painful diabetic neuropathy is a disease of the peripheral sensory neuron with impaired opioid responsiveness. Since μ-opioid receptor (MOR) activation can inhibit the transient receptor potential vanilloid 1 (TRPV1) activity in peripherally sensory neurons, this study investigated the mechanisms of impaired opioid inhibitory effects on capsaicin-induced TRPV1 activity in painful diabetic neuropathy. Intravenous injection of streptozotocin (STZ, 45 mg/kg) in Wistar rats led to a degeneration of insulin producing pancreatic β-cells, elevated blood glucose, and mechanical hypersensitivity (allodynia). In these animals, local morphine's inhibitory effects on capsaicin-induced nocifensive behavior as well as on capsaicin-induced TRPV1 current in dorsal root ganglion cells were significantly impaired. These changes were associated with a loss in MOR but not TRPV1 in peripheral sensory neurons. Intrathecal delivery of nerve growth factor in diabetic animals normalized sensory neuron MOR and subsequently rescued morphine's inhibitory effects on capsaicin-induced TRPV1 activity in vivo and in vitro. These findings identify a loss in functional MOR on sensory neurons as a contributing factor for the impaired opioid inhibitory effects on capsaicin-induced TRPV1 activity during advanced STZ-induced diabetes. Moreover, they support growing evidence of a distinct regulation of opioid responsiveness during various painful states of disease (e.g. arthritis, cancer, neuropathy) and may give novel therapeutic incentives.

Morphine Inhibits Migration of Tumor-Infiltrating Leukocytes and Suppresses Angiogenesis Associated with Tumor Growth in Mice

Tumor cells secrete factors that stimulate the migration of peripheral blood leukocytes and enhance tumor progression by affecting angiogenesis. In these studies, we investigated the effect of morphine, a known immunosuppressant, on leukocyte migration and recruitment to conditioned media derived from long-term cultures of mouse Lewis lung carcinoma cells. Our results indicate that morphine treatment reduced the migration and recruitment of tumor-infiltrating leukocytes into Matrigel plugs and polyvinyl alcohol sponges containing conditioned media derived from long-term cultures of mouse Lewis lung carcinoma cells when compared with placebo. A reciprocal increase in peripheral blood leukocytes was observed at the time of plug or sponge removal in morphine-treated mice. Decreased angiogenesis was observed in conditioned media derived from long-term cultures of mouse Lewis lung carcinoma cells Matrigel plugs taken from morphine-treated wild-type mice when compared with placebo but was abolished in morphine-treated μ-opioid receptor knockout mice. In addition, in vitro studies using trans-well and electric cell substrate impedance sensing system studies reveal for the first time morphine's inhibitory effects on leukocyte migration and their ability to transmigrate across an activated endothelial monolayer. Taken together, these studies indicate that morphine treatment can potentially decrease leukocyte transendothelial migration and reduce angiogenesis associated with tumor growth. The use of morphine for cancer pain management may be beneficial through its effects on angiogenesis.

Morphine Activates the E Twenty Six-Like Transcription Factor-1/Serum Response Factor Pathway via Extracellular Signal-Regulated Kinases 1/2 in F11 Cells Derived from Dorsal Root Ganglia Neurons

Morphine-induced signaling via opioid receptors (ORs) in dorsal root ganglia (DRG) neurons, the spinal cord, and various brain regions has been shown to modulate gene activity. Hitherto, little attention has been paid to extracellular signal-regulated kinases-1/2 (ERK-1/2)-mediated activation of the serum response factor (SRF) and ternary complex factors (TCFs) such as the E twenty six-like transcription factor-1 (ELK-1) in this context. Using TCF/SRF-dependent reporter gene constructs, a specific ERK-1/2 inhibitor and a dominant-negative ELK-1 mutant, we show herein that morphine activates ELK-1 via ERK-1/2 in DRG-derived F11 cells endogenously expressing μ and δ ORs. Previous studies with glioma cell lines such as NG108-15 cells attributed morphine-induced gene expression to the activation of the cAMP-responsive element binding protein (CREB). Thus, we also analyzed morphine-dependent activation of CREB in F11 and NG108-15 cells. In contrast to the CREB stimulation found in NG108-15 cells, we observed an inhibitory effect of morphine in F11 cells, indicating cell type-specific regulation of CREB by morphine. To obtain data about putative target genes of morphine-induced ELK-1/SRF activation, we analyzed mRNA levels of 15 ELK-1/SRF-dependent genes in cultured rat DRG neurons and F11 cells. We identified the early growth response protein-4 (EGR-4) as the strongest up-regulated gene in both cell types and observed ELK-1 activity-dependent activation of an EGR-4-driven reporter in F11 cells. Overall, we reveal an important role of ELK-1 for morphine-dependent gene induction in DRG-derived cells and propose that ELK-1 and EGR-4 contribute to the effects of morphine on neuronal plasticity.

Morphine Decreases Bacterial Phagocytosis by Inhibiting Actin Polymerization through cAMP-, Rac-1-, and p38 MAPK-Dependent Mechanisms

Morphine increases the susceptibility to opportunistic infection by attenuating bacterial clearance through inhibition of Fcγ receptor (FcgR)-mediated phagocytosis. Mechanisms by which morphine inhibits this process remain to be investigated. Actin polymerization is essential for FcgR-mediated internalization; therefore, disruption of the signaling mechanisms involved in this process is detrimental to the phagocytic ability of macrophages. To our knowledge, this study is the first to propose the modulation of actin polymerization and upstream signaling effectors [cAMP, Rac1-GTP, and p38 mitogen-activated protein kinase (MAPK)] as key mechanisms by which morphine leads to inhibition of pathogen clearance. Our results indicate that long-term morphine treatment in vitro and in vivo, through activation of the μ-opioid receptor, leads to an increase in intracellular cAMP, activation of protein kinase A, and inhibition of Rac1-GTPase and p38 MAPK, thereby attenuating actin polymerization and reducing membrane ruffling. Furthermore, because of long-term morphine treatment, FcgR-mediated internalization of opsonized dextran beads is also reduced. Morphine's inhibition of Rac1-GTPase activation is abolished in J774 macrophages transfected with constitutively active pcDNA3-EGFP-Rac1-Q61L plasmid. Dibutyryl-cAMP inhibits, whereas H89 restores, activation of Rac-GTPase and abolishes morphine's inhibitory effect, implicating cAMP as the key effector in morphine's modulation of actin polymerization. These findings indicate that long-term morphine treatment, by increasing intracellular cAMP and activating protein kinase A, leads to inhibition of Rac1-GTPase and p38 MAPK, causing attenuation of actin polymerization, FcgR-mediated phagocytosis, and decreased bacterial clearance.

Naloxone-insensitive Inhibitory and Excitatory Effects of Opioid Agonists in the Rat Isolated Uterus

Morphine (3 x 10(-6)-10(-3) M) produced a concentration-dependent inhibition of both spontaneous rhythmic contractions and tonic contraction induced by potassium chloride (KCl) (50 mM) in rat isolated uterus. Uteri at the metoestrus phase were the most sensitive to the inhibitory effect of morphine. Morphine-induced relaxation was characterized by a post-wash tonic contraction. The mu-specific opioid agonist, D-alaglymepheglycol (DAGO) (4 x 10(-8)-1.2 x 10(-5) M) did not affect spontaneous rhythmic contraction but produced a partial inhibition of the KCl depolarized uterus. There was no post-wash contraction following DAGO. In contrast to the inhibition produced by both morphine and DAGO, methionine enkephalin (8 x 10(-7)-2.4 x 10(-5) M) produced a concentration-dependent contraction of the KCl depolarized uterus. Naloxone up to 100 microM, propranolol (10 microM), flurbiprofen (50 microM) and metiamide (10 microM) did not affect either the relaxation produced by morphine and DAGO or the contraction produced by methionine enkephalin. The results showed that the opioid agonists may have non-receptor mediated direct effects on the rat uterus.

Supersensitivity to morphine after chronic sympathetic denervation in guinea-pig colon.

The possible role of opioid systems in the adaptive changes which follow chronic sympathetic denervation in the guinea-pig colon has been studied by comparing the effects of the opioid agonist morphine in control animals and after chronic sympathetic denervation. Supersensitivity to the inhibitory effects of morphine on the peristaltic reflex was observed after chronic sympathetic denervation, while the potency against acetylcholine release was unmodified. Our results suggest that a modification of the opioid system occurs after sympathetic denervation in the guinea-pig colon. Supersensitivity to endogenous opioids at a site different from that regulating acetylcholine release could account for the counter-regulation of intestinal motility after chronic sympathetic denervation.

Chronic morphine administration induces tolerance to suppressive morphine effects in two canonical LPS-induced innate immunity responses in mice. (162.20)

Abstract The immunosuppressive effects of acute morphine administration are well described. However, little is known about chronic morphine treatment actions using different administration schedules. The aim of this work was to study the effects of various morphine administration schemes on peritoneal Tumor Necrosis Factor-α (TNF-α) release and serum corticosterone levels after i.p. injection of bacterial lipopolysaccharide (LPS) in mice. Our results show that morphine per se (0.1-10 mg/kg) had no effects on TNF-α levels. Morphine inhibited LPS-induced TNF-α release but this effect occurred only at high doses (3.1 y 10 mg/kg) and was not dose dependent. Serum corticosterone levels increased after a single LPS challenge and morphine dose-dependently prevented this effect. After repeated administration, tolerance developed to the inhibitory effects of morphine on peritoneal TNF-α release and serum corticosterone levels in response to LPS. These data suggest that morphine inhibitory effects on innate immunity depend on the opioid administration schedule and duration treatment.

Morphine Suppresses Tumor Angiogenesis through a HIF-1α/p38MAPK Pathway

Morphine, a highly potent analgesic agent, is frequently prescribed for moderate to severe cancer pain. In this study, morphine was administered at a clinically relevant analgesic dose to assess tumor cell-induced angiogenesis and subcutaneous tumor growth in nude mice using mouse Lewis lung carcinoma cells (LLCs). Implantation of mice with a continuous slow-release morphine pellet achieved morphine plasma levels within 250-400 ng/ml (measured using a radioimmunoassay, Coat-A-Count Serum Morphine) and was sufficient to significantly reduce tumor cell-induced angiogenesis and tumor growth when compared with placebo treatment. Morphometric analysis for blood vessel formation further confirmed that morphine significantly reduced blood vessel density (P < 0.003), vessel branching (P < 0.05), and vessel length (P < 0.002) when compared with placebo treatment. Morphine's effect was abolished in mice coadministered the classical opioid receptor antagonist, naltrexone, and in mu-opioid receptor knockout mice, supporting the involvement of the classical opioid receptors in vivo. Morphine's inhibitory effect is mediated through the suppression of the hypoxia-induced mitochondrial p38 mitogen-activated protein kinase (MAPK) pathway. Our results suggest that in vitro morphine treatment of LLCs inhibits the hypoxia-induced nuclear translocation of hypoxia-inducible transcription factor 1alpha to reduce vascular endothelial growth factor transcription and secretion, in a manner similar to pharmacological blockade with the p38 MAPK-specific inhibitor, SB203585. These studies indicate that morphine, in addition to its analgesic function, may be exploited for its antiangiogenic potential.

Discharge activities of neurons in the nucleus paragigantocellularis during the development of morphine tolerance and dependence: a single unit study in chronically implanted rats.

The nucleus paragigantocellularis (PGi) has been proposed to play a role in opiate dependence/withdrawal. In the present study, we examined the discharge activity of PGi neurons before and after the development of morphine tolerance/dependence in rats. A multi-wire electrode was chronically implanted in the PGi, which allowed us to monitor the effects of both acute and chronic morphine treatments on the activity of PGi neurons recorded from the same site. We found that acute morphine excited, inhibited or had no effect on 36%, 35% or 29% of PGi neurons (N=556), respectively. After 3 days of continuous morphine infusion, which led to morphine tolerance/dependence, the firing rates of both excitatory and inhibitory PGi neurons returned to pre-morphine treatment levels, indicating that the PGi neurons developed tolerance to both excitatory and inhibitory effects of morphine. Naltrexone-precipitated withdrawal from chronic morphine treatment also induced heterogeneous responses in the PGi. On a site-by-site basis, most of the sites that showed excitatory response to acute morphine exhibited inhibitory response during withdrawal, and all the sites that showed inhibitory response to acute morphine exhibited excitatory response during withdrawal. Correlation analysis further quantitatively showed that PGi neurons' responses to acute morphine and that during withdrawal were inversely correlated with a correlation coefficient of 0.73, suggesting that adaptations in the PGi during the development of morphine dependence share common neural mechanisms with the acute effect of morphine. These results provide new insights into the role of the PGi in the development of morphine tolerance/dependence.

Opioid peptides and opiate alkaloids in immunoregulatory processes

Among the various non-neuronal cell types known to express and utilize neuropeptides, those of the immune system have received much attention in recent years. In particular, comparative studies in vertebrates and invertebrates have shown that endogenous opioid peptides are engaged in receptor mediated autoregulatory immune and neuroendocrine processes. The majority of these immune processes are stimulatory, as determined by their effects on conformational changes indicative of immunocyte activation, cellular motility, and phagocytosis. Endogenous opioid peptides form an effective network of messenger molecules in cooperation with cytokines, opiate alkaloids, and certain regulatory enzymes (neutral endopeptidase 24.11). Peptide-mediated immunostimulatory effects observed in this system are operationally counteracted by the inhibitory effects of morphine and related opiates. Opioid/opiate signaling processes are mediated by several types of receptors with different degrees of selectivity. Among them the recently identified, opioid insensitive µ3 receptor deserves attention on account of its specificity for opiate alkaloids.