Central Mu Opioid Receptors Research Articles

Mu-opioid receptor system modulates responses to vocal bonding and distress signals in humans.

Laughter is a contagious prosocial signal that conveys bonding motivation; adult crying conversely communicates desire for social proximity by signalling distress. Endogenous mu-opioid receptors (MORs) modulate sociability in humans and non-human primates. In this combined PET-fMRI study (n = 17), we tested whether central MOR tone is associated with regional brain responses to social signals of laughter and crying. MOR availability was measured with positron emission tomography (PET) using the high-affinity agonist radioligand [11C]carfentanil. Haemodynamic responses to social laughter and crying vocalizations were measured using functional magnetic resonance imaging (fMRI). Social laughter evoked activation in the auditory cortex, insula, cingulate cortex, amygdala, primary and secondary somatosensory cortex, and primary and secondary motor cortex; crying sounds led to more restricted activation in the auditory cortex and nearby areas. MOR availability was negatively correlated with the haemodynamic responses to social laughter in the primary and secondary somatosensory cortex, primary and secondary motor cortex, posterior insula, posterior cingulate cortex, precuneus, cuneus, temporal gyri and lingual gyrus. For crying-evoked activations, MOR availability was negatively correlated with medial and lateral prefrontal haemodynamic responses. Altogether our findings highlight the role of the MOR system in modulating acute brain responses to both positive and negative social signals. This article is part of the theme issue 'Cracking the laugh code: laughter through the lens of biology, psychology and neuroscience'.

The role of neuronal nitric oxide synthase in cocaine place preference and mu opioid receptor expression in the nucleus accumbens

There is evidence that central mu opioid receptors (MORs) are implicated in several aspects of cocaine addiction, and that MOR expression is elevated by cocaine in vitro and in the nucleus accumbens (NAc) when administered in vivo. To understand the cellular mechanisms involved in regulating MOR expression, this study explored whether neuronal nitric oxide synthase (nNOS) modulates the neurochemical and behavioral effects of acute and repeated cocaine administration. Male Sprague-Dawley rats received a single cocaine injection (20mg/kg, i.p.) in combination with the selective nNOS inhibitor 7-nitroindazole (7-NI) (0, 25, or 50mg/kg, i.p.), and the expression of MOR and nNOS messenger RNA (mRNA) and protein levels in the NAc were measured. In a separate conditioned place preference (CPP) experiment, 7-NI (0, 25, or 50mg/kg, i.p.) was administered prior to cocaine (0 or 20mg/kg, i.p.) conditioning sessions, and levels of MOR and nNOS mRNA and protein in the NAc were measured following CPP test. Acute cocaine administration significantly enhanced nNOS and MOR mRNA and protein expression in the NAc, and this increase in MOR expression was blocked by 7-NI. Furthermore, in 7-NI pre-treated rats, cocaine-induced CPP was not statistically significant and the increase in MOR mRNA expression in the NAc in these animals was attenuated. These findings suggest that nNOS modulates MOR expression following acute cocaine administration, and that cocaine CPP and associated upregulation of MOR expression involve both nNOS-dependent and independent mechanisms. Elucidation of these molecular events may identify useful therapeutic target for cocaine addiction.

In vivo immune interactions of multipotent stromal cells underlie their long-lasting pain-relieving effect

Systemic infusion of bone marrow stromal cells (BMSCs), a major type of multipotent stromal cells, produces pain relief (antihyperalgesia) that lasts for months. However, studies have shown that the majority of BMSCs are trapped in the lungs immediately after intravenous infusion and their survival time in the host is inconsistent with their lengthy antihyperalgesia. Here we show that long-lasting antihyperalgesia produced by BMSCs required their chemotactic factors such as CCL4 and CCR2, the integrations with the monocytes/macrophages population, and BMSC-induced monocyte CXCL1. The activation of central mu-opioid receptors related to CXCL1-CXCR2 signaling plays an important role in BMSC-produced antihyperalgesia. Our findings suggest that the maintenance of antihypergesia can be achieved by immune regulation without actual engraftment of BMSCs. In the capacity of therapeutic use of BMSCs other than structural repair and replacement, more attention should be directed to their role as immune modulators and subsequent alterations in the immune system.

Mu‐Opioid Receptors (MORs) in Pre‐Botzinger Complex (PBC) Are Involved in Fentanyl Switching of Pulmonary C‐Fibers (PCFs)‐Mediated Rapid Shallow Breathing (RSB) into An Apnea in Anesthetized Rats

Our previous study showed that fentanyl (iv), a selective MOR agonist, could switch the PCFs‐mediated RSB into an apnea and that peripheral MORs are necessary for triggering this switch. Here we further tested if central MORs, especially those in the medial nucleus tractus solitarius (mNTS) receiving PCFs’ inputs and the PBC responsible for respiratory rhythmogenesis, were also involved in such switching. Right atrial injection of phenylbiguanide (3–5 μg/kg) was performed to stimulate PCFs in anesthetized rats before and after fentanyl (8 μg/kg, iv) without or with prior central administration of naloxone methiodide (M‐NLX, an opioid receptor antagonist). The central administration was achieved by injecting M‐NLX into the cisterna magna (8 μg/4μl), the mNTS or the PBC (10 mM, 15 nl in each side). Our results showed that phenylbiguanide alone shortened expiratory duration (TE) by 32 ± 6 % (RSB), while it markedly prolonged TE by 5.5‐fold (an apnea) after fentanyl. M‐NLX injected into the cisterna magna or the PBC, but not the mNTS, prevented the fentanyl switching with the TE of RSB slightly but significantly prolonged. This study, along with our previous results, suggests that although peripheral MORs are critical to trigger the fentanyl switching of PCF‐mediated RSB into an apnea, central MORs, especially those in the PBC, are necessary to establish this switching (Supported by ALA RG‐191095‐N and HL 107462).

The potential for μ‐opioid receptor agonists to be anti‐emetic in humans: a review of clinical data

In animal models of vomiting, mu-opioid (MOP, OP(3)) receptors mediate both emesis and anti-emesis. mu-receptors within the blood-brain barrier, mediating anti-emesis, are more rapidly accessible to lipid-soluble mu-opioid receptor agonists such as fentanyl than to morphine, and fentanyl has broad-spectrum anti-emetic effects in a number of species. Whether a similar situation exists in humans is not known. A search was performed for clinical studies comparing the emetic side effects of opioids administered peri-operatively in an attempt to identify differences between morphine and more lipid-soluble mu-receptor-selective agonists such as fentanyl. Overall, the evidence appears to suggest that fentanyl and other phenylpiperidines are associated with less nausea and vomiting than morphine, but not all studies support this, and fentanyl-like drugs are associated with nausea and vomiting per se. Good evidence, however, exists to show that fentanyl and alfentanil do not cause more nausea and vomiting than the ultra fast-acting remifentanil. Because remifentanil is cleared rapidly post-operatively, such trials suggest that the emetic side effects of fentanyl and alfentanil are minimal. The clinical evidence, although limited, is at least consistent with the possibility that central mu-opioid receptors may mediate anti-emesis in humans. It is possible that the role of mu-opioid agonists in anti-emesis may become clearer in the future as a result of the use of peripheral mu-opioid receptor antagonists.

Modulation of Peripheral Opioid Receptors Affects the Concentration of μ-Opioid Receptors in Rat Brain

Radioligand binding assay was used to evaluate characteristics of central mu-opioid receptors after peripheral administration of mu-opioid receptor agonist loperamide and antagonist methylnaloxone. These substances do not cross the blood-brain barrier. Loperamide and methylnaloxone produced opposite effects on the density of mu-opioid receptors in the frontal cortex of rat brain. These data confirm our hypothesis on reciprocal interactions between central and peripheral compartments of the endogenous opioid system.

Seizure activity involved in the up-regulation of BDNF mRNA expression by activation of central Mu opioid receptors

Chemical-induced seizures up-regulated brain-derived neurotrophic factor (BDNF) mRNA expression. Intracerebroventricular (i.c.v.) administration of endogenous opioids preferentially activating μ opioid receptor (MOR) could also increase BDNF mRNA expression. The aim of this study was to determine to what extent i.c.v. administration of synthetic MOR-selective agonists in rats can modulate both seizure activity and up-regulation of BDNF mRNA expression. Effects and potencies of i.c.v. administration of morphine and [ d-Ala 2, N-Me-Phe 4, Gly 5-ol]-enkephalin (DAMGO), were directly investigated by scoring behavioral seizures and measuring BDNF mRNA expression. In addition, effects of the opioid receptor antagonist naloxone and antiepileptic drugs, diazepam, phenobarbital, and valproate, on i.c.v. MOR agonist-induced behavioral seizures and up-regulation of BDNF mRNA expression were determined. A single i.c.v. administration of morphine (10–100 μg) or DAMGO (0.15–1.5 μg) dose-dependently elicited behavioral seizures and increased BDNF mRNA expression in the widespread brain regions. However, s.c. administration of MOR agonists neither produced behavioral seizures nor increased BDNF mRNA expression. Pretreatment with naloxone 1 mg/kg significantly reduced behavioral seizure scores and the up-regulation of BDNF mRNA expression elicited by i.c.v. morphine or DAMGO. Similarly, diazepam 10 mg/kg and phenobarbital 40 mg/kg significantly blocked i.c.v. MOR agonist-induced actions. Pretreatment with valproate 300 mg/kg only attenuated behavioral seizures, but it did not affect morphine-induced increase of BDNF mRNA expression. This study provides supporting evidence that seizure activity plays an important role in the up-regulation of BDNF mRNA expression elicited by central MOR activation and that decreased inhibitory action of GABAergic system through the modulation on GABA receptor synaptic function by central MOR activation is involved in its regulation of BDNF mRNA expression.

Hypothesis on Reciprocal Interactions between the Central and Peripheral Components of the Endogenous Opioid System

A hypothesis on reciprocal interactions between the central and peripheral components of the endogenous opioid system was formulated on the basis of results of our experimental studies and published data. In order to verify this hypothesis, we studied the effects of peripheral administration of loperamide (mu-opioid receptor agonist) and methylnaloxone (opioid receptor antagonist) not penetrating through the blood-brain barrier on the pain sensitivity of rats, morphine-induced analgesia, and formation of morphine analgesia tolerance. Peripheral loperamide and methylnaloxone modulated the central mechanisms of perception of painful stimuli. This fact confirmed the hypothesis on the reciprocal interactions between the central and peripheral compartments of the endogenous opioid system. Methylnaloxone exhibits an antagonistic effect on peripheral mu-opioid receptors, which probably leads to activation of the central mu-opioid receptors and to the development of analgesia. Loperamide activates peripheral, but suppresses the central mu-opioid receptors, which leads to hyperalgesia. Methylnaloxone suppresses morphine-induced analgesia under conditions of morphine activation of the central antinociceptive mechanisms. Peripheral injection of mu-opioid agonist loperamide virtually did not modify the central compartments of the opioid system under conditions of morphine treatment. Methylnaloxone and loperamide partially prevented the development of morphine analgesia tolerance. Hence, the results confirm the hypothesis about the reciprocal interactions between the central and peripheral compartments of the endogenous opioid system. The relationships between the central and peripheral compartments of the opioid system can be more intricate when its function is modulated.

SPINAL µ‐OPIOID RECEPTOR EXPRESSION AND HYPERALGESIA WITH DEXAMETHASONE IN CHRONIC ADJUVANT‐INDUCED ARTHRITIS IN RATS

1. It is known that inflammation influences peripheral and central mu-opioid receptor expression. Previous studies have indicated that glucocorticoids may influence the density of mu-opioid receptors. In the present study, we investigated the fluctuations of spinal mu-opioid receptor expression and hyperalgesia induced by complete Freund's adjuvant (CFA) under long-term administration of dexamethasone. 2. Adjuvant arthritis (AA) was induced by subcutaneous injection of CFA in the right hindpaw of male Wistar rats. Spinal mu-opioid receptor expression was detected by semiquantitative reverse transcription-polymerase chain reaction on Days 6 and 21 following AA induction. Dexamethasone (0.1 mg/kg) was administered intraperitoneally for 21 days. Variations in thermal hyperalgesia were checked by radiant heat on the same days as mu-opioid receptor expression was determined. 3. The results indicated a significant increase in spinal mu-opioid receptor expression on Days 6 and 21 after AA induction compared with the control group. Spinal mu-opioid receptor expression decreased significantly only on Day 21 in the AA + dexamethasone group compared with the AA alone group. Thermal hyperalgesia on Day 6 after AA induction showed a significant increase compared with the control group. Hyperalgesia decreased significantly on Day 21 after AA compared with Day 6. The AA + dexamethasone group showed a significant decrease in hyperalgesia on Day 6 compared with the AA group, but hyperalgesia increased significantly on Day 21 in the AA + dexamethasone group compared with the AA group. 4. The effects of long-term dexamethasone on both spinal mu-opioid receptor expression and hyperalgesia during persistent AA inflammation are time dependent. In addition, the effect of long-term dexamethasone administration on hyperalgesia during persistent arthritis inflammation needs to be investigated further.

Naltrexone in the Treatment of Alcohol Dependence

Naltrexone is an opioid receptor antagonist that has been shown to be effective for maintaining abstinence in alcohol-dependent persons. It is particularly effective in a subset of persons who suffer from high craving, as it reduces craving for alcohol. Family history has been shown to be a predictor of treatment response and, indeed, allelic variation in the mu opioid receptor gene predicts treatment response to naltrexone. The therapeutic effects of naltrexone are mediated by blockade of central mu opioid receptors. The site of action is under investigation but evidence supports a role of mu opioid receptors in the central nucleus of the amygdala, nucleus accumbens, and ventral tegmental area in the therapeutic actions of naltrexone for alcohol dependence. This article reviews the role of the endogenous opioid system in addictive diseases, especially alcoholism and discusses the pharmacological basis for the use of naltrexone in the treatment of alcohol dependence.

(692): Differential changes in mu-opioid receptor (MOR) availability following acupuncture and sham acupuncture therapy in fibromyalgia (FM) patients

Multiple lines of evidence implicate MORs in analgesia following placebo and acupuncture treatments. No study has examined the differential effects of these two interventions on binding of central MORs in chronic pain patients. We investigated the relationship between changes in MOR availability following acupuncture and sham acupuncture and subsequent changes in clinical pain. 18 female FM patients (ages 18-75) were randomized to receive either one acupuncture (n=9) or one sham acupuncture (n=9) treatment.

Prolonged Central μ-Opioid Receptor Occupancy after Single and Repeated Nalmefene Dosing

The opioid antagonist nalmefene offers an alternative to traditional pharmacological treatments for alcoholism. The present study was designed to investigate the relationship between nalmefene plasma concentration and central mu-opioid receptor occupancy after a clinically effective dose (20 mg, orally). Pharmacokinetics and mu-opioid receptor occupancy of nalmefene after single and repeated dosing over 7 days was studied in 12 healthy subjects. Serial blood samples were obtained after both dosings, and pharmacokinetic parameters for nalmefene and main metabolites were determined. Central mu-opioid receptor occupancy of nalmefene was measured with positron emission tomography (PET) and [(11)C]carfentanil at four time points (3, 26, 50, 74 h) after both dosings. Nalmefene was rapidly absorbed in all subjects. The mean t(1/2) of nalmefene was 13.4 h after single and repeated dosing. The accumulation of nalmefene and its main metabolites in plasma during the repeated dosing period was as expected for a drug with linear pharmacokinetics, and steady-state was reached for all analytes. Both nalmefene dosings resulted in a very high occupancy at mu-opioid receptors (87-100%), and the decline in the occupancy was similar after both dosings but clearly slower than the decline in the plasma concentration of nalmefene or metabolites. High nalmefene occupancy (83-100%) persisted at 26 h after the dosings. The prolonged mu-opioid receptor occupancy by nalmefene indicates slow dissociation of the drug from mu-opioid receptors. These results support the rational of administering nalmefene when needed before alcohol drinking, and they additionally suggest that a high mu-opioid receptor occupancy can be maintained when nalmefene is taken once daily.

The Cardiovascular and Renal Effects of the Potent and Highly Selective μ Opioid Agonist [Dmt1]DALDA

The cardiovascular and renal effects of a mu opioid agonist, [Dmt]DALDA, were studied in conscious Sprague-Dawley rats. During the first hour postinjection, [Dmt]DALDA (0.025-250 microg/rat, IV) evoked a dose-dependent diuresis. The dose of 2.5 microg increased urine volume from 1.0 +/- 0.2 to 3.4 +/- 0.3 mL/h (P < 0.001, n = 30), urinary excretion of sodium, potassium, and cGMP, and induced a mild antihypertensive effect. This dose increased cumulative 4-hour urine volume but significantly inhibited sodium and potassium excretions. The renal and cardiovascular effects were abolished by naloxone (4 mg/kg), but not by naloxonazine (35 mg/kg SC), a selective mu-1 receptor antagonist. Pretreatment with 8 mg/kg naloxone methiodide, an opioid antagonist with limited access to the brain, partially inhibited the renal effects of [Dmt]DALDA. Inhibition of nitric oxide synthases with L-NAME (1 mg/kg) had no effect on the renal and cardiovascular actions of [Dmt]DALDA. Plasma ANP and AVP, measured at 20 and 120 minutes after injection, were not altered by 2.5 and 25 microg [Dmt]DALDA. Therefore, [Dmt]DALDA evokes renal and cardiovascular effects that may primarily be mediated by central naloxonazine-insensitive mu opioid receptors (non-mu-1). These findings indicate that the central mu opioid system is involved in the regulatory mechanism of renal handling of sodium and water.

Central effect of mu-opioid agonists on antral motility in conscious rats

Centrally applied opioids delay gastric emptying and inhibit intestinal transit. However, the mechanism of inhibitory effects of central opioids on gastric motility still remains unclear. It also remains unclear which opioid receptor (mu, delta, and kappa) stimulation affects gastric motility. We studied the central effect of opioids on antral motility in conscious rats. A strain gauge transducer was implanted on the gastric antrum to record the circular muscle contractions. The area under the curve of the antral motility, calculated as a motility index, was evaluated before and after the intracerebroventricular (icv) injection of various opioid agonists in each rat. [ d-Ala 2, N-Me-Phe 4, Gly 5-ol] enkephalin (DAMGO, 0.1–10 nmol), a mu-opioid selective agonist, significantly inhibited antral motility in a dose-dependent manner ( n=4). The motility index was significantly decreased to 47.3±10.8% ( n=4) of controls at 20 min after icv injection of DAMGO (1.0 nmol). In contrast, [ d-Pen 2, l-Pen 5] enkephalin (DADLE, 1.0 nmol), a delta-opioid selective agonist, and U50,488 (1.0 nmol), a kappa-opioid selective agonist, had no significant effects on antral motility. Pretreatment with subcutaneous guanethidine (5 mg/kg) and propranolol (1 mg/kg), but not phentolamine (1 mg/kg), significantly antagonized the inhibitory effect of DAMGO (1.0 nmol). Reduced motility index induced by DAMGO (1.0 nmol) was restored from 48.7±3.5% to 88.6±10.9% ( n=5) and 80.4±2.2% ( n=5) by guanethidine and propranolol, respectively. Our findings suggest that central mu-opioid receptor has major inhibitory effects on antral motility in conscious rats. The inhibitory effects of mu-opioid receptors are mediated via sympathetic pathways and beta-adrenoceptors.

Opioid action: Role of central mu opioid receptors in opioid-induced itch in primates

Pruritus (itch sensation) is one of side effects associated with opioid analgesics. The aim of this study was to elucidate the role of opioid receptor types and the site of action in opioid-induced itch in monkeys. Scratching responses after administration of various drugs were counted by observers blinded to conditions. Systemic administration of mu opioid receptor (MOR) agonists (morphine, fentanyl, alfentanil, and remifentanil) evoked scratching dose- and time-dependently. However, a kappa opioid agonist (U-50488H) and a delta opioid agonist (SNC80) did not increase scratching. Intrathecal (i.t.) administration of a peptidic MOR agonist (DAMGO, 0.00032-0.01 mg) potently evoked scratching. However, i.v. administration of DAMGO (0.01-1 mg/kg) did not increase scratching. This large potency difference between i.t. and i.v. routes was also observed with morphine-induced scratching (i.t., 0.0032-0.032 mg; i.v., 0.1-3.2 mg/kg). In addition, systemic administration of an opioid receptor antagonist, naltrexone (0.0032-0.1 mg/kg), dose-dependently attenuated i.v. fentanyl (0.018 mg/kg)- or morphine (1 mg/kg)-induced scratching. However, quaternary naltrexone (0.0032-0.32 mg/kg) did not block either i.v.. fentanyl- or morphine-induced scratching. Moreover, systemic administration of naltrexone (0.0032-0.032 mg/kg) dose-dependently attenuated i.t. morphine (0.032 mg)-induced scratching. In contrast, an antihistamine, diphenhydramine (0.1-10 mg/kg), did not attenuate i.t. morphine-induced scratching. Taken together, these data suggest that MOR, not other opioid receptor types or histamine, mediates scratching evoked by opioid analgesics. More important, this study provides in vivo pharmacological evidence that activation of central MOR plays an important role in opioid-induced itch in primates (Supported by USPHS Grant DA13685).

The role of central mu opioid receptors in opioid-induced itch in primates.

Pruritus (itch sensation) is a significant clinical problem. The aim of this study was to elucidate the roles of opioid receptor types and the site of action in opioid-induced itch in monkeys. Observers who were blinded to the conditions counted scratching after administration of various drugs. Intravenous (i.v.) administration of mu opioid receptor (MOR) agonists (fentanyl, alfentanil, remifentanil, and morphine) evoked scratching in a dose- and time-dependent manner. However, the kappa opioid agonist U-50488H [trans-(+/-)-3,4-dichloro-N-methyl-N-(2-[1-pyrrolidinyl]-cyclohexyl)-benzeneacetamide] and delta opioid agonist SNC80 [(+)-4-[(alphaR)-alpha-[2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl]-3-methoxybenzyl]-N,N-diethylbenzamide] did not increase scratching. Intrathecal (i.t.) administration of peptidic MOR agonist [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAMGO, 0.00032-0.01 mg) evoked scratching, but i.v. DAMGO (0.01-1 mg/kg) did not increase scratching. A similar difference between i.t. and i.v. effectiveness was seen with morphine. Antagonist studies revealed that i.v. administration of an opioid receptor antagonist (naltrexone, 0.0032-0.1 mg/kg) dose dependently attenuated scratching induced by i.v. fentanyl (0.018 mg/kg) or morphine (1 mg/kg). However, a peripherally selective opioid antagonist (quaternary naltrexone, 0.0032-0.32 mg/kg) did not block i.v. fentanyl- or morphine-induced scratching. Moreover, a histamine antagonist (diphenhydramine, 0.1-10 mg/kg), failed to attenuate scratching induced by i.t. morphine (0.032 mg) or i.v. morphine (1 mg/kg). Pretreatment with a selective MOR antagonist (clocinnamox, 0.1 mg/kg), but not kappa or delta opioid antagonists (nor-binaltorphimine or naltrindole), blocked i.t. morphine-induced scratching. Together, these data suggest that MOR, not other opioid receptor types or histamine, mediates scratching evoked by opioid analgesics. More important, this study provides in vivo pharmacological evidence that activation of central MOR plays an important role in opioid-induced itch in primates.

Functional effects of systemically administered agonists and antagonists of mu, delta, and kappa opioid receptor subtypes on body temperature in mice.

We have investigated the roles of peripheral and central mu, delta, and kappa opioid receptors and their subtypes in opioid-induced hypothermia in mice. Measuring rectal temperature after i.p. injection, opioid agonists [morphine, fentanyl, SNC80 ((+)-4-[(alphaR)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)3-methoybenzyl]-N,N-diethylbenzamide), U50,488H ((trans-(dl)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]-benzeneacetamide), and loperamide)] were tested alone or with opioid antagonists [naloxone, beta-funaltrexamine, naloxonazine, naltrindole, 7-benzylidenenaltrexone (BNTX), naltriben, nor-binaltorphimine, 2-(3,4-dichlorophenyl)-N-methyl-N-[(1S)-1-(3-isothiocyanatophenyl)-2-(1-pyrrolidinyl)ethyl]acetamide (DIPPA), and methyl-naltrexone] given 15 min after the agonist. All agonists produced dose-related hypothermia, although at low doses, morphine and U50,488H produced hyperthermia. The effects of morphine and fentanyl were antagonized by naloxone and by the mu(1) antagonist naloxonazine. The delta(2) antagonist naltriben potentiated the hypothermic effect of mu agonists. SNC80-induced hypothermia was blocked by the delta antagonist naltrindole but not by the delta(1) antagonist BNTX. Depending on the dose, the delta(2) antagonist naltriben produced either a potentiation or an attenuation of the effect of SNC80. U50,488H-induced hypothermia was antagonized by the kappa antagonist nor-binaltorphimine but not by acute treatment with the irreversible kappa antagonist DIPPA. The peripherally acting opioid loperamide produced hypothermia that could be blocked by several mu-, delta-, or kappa-selective antagonists as well as the peripherally acting antagonist methyl-naltrexone. Methyl-naltrexone produced a weak potentiation of morphine-, fentanyl-, and U50,488H-induced hypothermia, whereas a significant attenuation of SNC80-induced hypothermia was observed. In conclusion, at high doses, morphine- and fentanyl-induced hypothermia may involve composite action on mu, kappa, and possibly delta opioid receptors after initial activation. In the mediation of delta opioid-induced hypothermia, no clear selectivity between the delta(1) and delta(2) subtypes was defined. The studies provide new evidence that maintenance of the initial effects of agonist/receptor activation vary with the agonist and the receptor. The existence of both central and peripheral components of opioid-induced hypothermia is also emphasized.

Up-regulation of central mu–opioid receptors in a model of hepatic encephalopathy: A potential mechanism for increased sensitivity to morphine in liver failure

Increased GABA-mediated neurotransmission, reported to occur in hepatic encephalopathy (HE), is associated with a decrease in the release of Met–enkephalin and the expression of its coding gene in the brain. Furthermore, patients with cirrhosis and a history of HE exhibit increased sensitivity to the neuroinhibitory effects of morphine. Thus, there is a rationale to study the status of the endogenous opioid system in HE. The aim of this study was to determine whether mu–opioid receptors in the brain are up-regulated in a well characterized model of HE. Binding parameters of mu-opioid receptors were derived by assaying the binding of the opiate agonist [ 3H]-tyr-D-Ala-Gly-N-Methyl-Phe-Gly-ol (DAMGO) to brain membranes from rats with precisely defined stages of HE and control animals. The mean density of mu-opioid receptor sites (Bmax) in rats with stage II, III, and IV HE was 15, 29, and 33% higher, respectively, than the corresponding control value (p<0.01). In addition, the affinity of mu opioid receptors for the agonist (1/Kd) also increased with progression of HE (mean for stage IV HE vs. corresponding control mean, p<0.01). In conclusion, in liver failure, increased density and affinity of central mu–opioid receptors in the brain may: (i) be the basis for the documented increased sensitivity to opiate agonists; and (ii) occur as a consequence of increased GABAergic tone reducing neuronal synthesis and release of opioid agonist peptides.

Different endogenous opioid effects on delta- and mu-receptor subtypes in antral and duodenal motility of conscious dogs.

To evaluate the role of endogenous opioids in regulation of upper gastrointestinal motility in unanesthetized dogs and to differentiate the actions on mu- and delta-opioid receptors, seven strain gauges and five platinum electrodes were chronically implanted at the serosa along the antrum and duodenum and connected to a plug in the neck of the dog. Signals were processed by a Hellige AC amplifier, a rectilinear recorder, and a data aquisition system. A motility index (MI) was calculated and together with the electrical data a contractile activity percentage (CAP) was determined for consecutive 30 min periods. The delta-opioid antagonist ICI 741 864 and the mu-receptor blocker naloxone were injected intraarterially through a chronic Groshong catheter placed in the pyloric region and connected to a subcutaneous port. After a meal of solid food, ICI 174 864 increased motility relative to controls in the antrum averaged over 5 hr by 144.4%+/-26 for the MI and 73%+/-26 for CAP; after naloxone MI increased by 222%+/-60 and CAP 121%+/-76. In the duodenum, ICI 174 864 decreased MI over a range of 57% to 22% (P < 0.05). Naloxone increased MI and CAP significantly after 2.5 hr. We interpreted the results of ICI 174 864 in the duodenum to reflect suppression of a tonic opioid influence at the delta-receptor mainly at the prevertebral ganglion. In the interdigestive state in 56% of the dogs, naloxone delayed the occurrence of phase III of the migrating myoelectric complex (MMC) for up to 370 min, while under ICI 174 864, normal interdigestive cycles were present. Disturbance of the timing of the interdigestive cycles at central mu-opioid receptors may by involved in the effect.

Ablation of pituitary pro-opiomelanocortin (POMC) cells produces alterations in hypothalamic POMC mRNA levels and midbrain mu opioid receptor binding in a conditional transgenic mouse model.

The hypothalamic-pituitary-adrenal (HPA) axis is regulated by stress-related excitatory inputs, and various inhibitory and negative-feedback controls by glucocorticoids and opioids, including pro-opiomelanocortin (POMC)-derived peptides. The role of POMC-derived peptides of pituitary origin in the modulation of brain POMC mRNA expression and opioid receptor binding was investigated using a line of transgenic mice that express a fusion gene composed of the pituitary expression-specific promoter region of the POMC gene driving the herpes simplex viral-1 thymidine kinase (TK). Male adult mice were treated with the antiherpes agent ganciclovir that selectively ablates cells expressing TK. Following treatment, POMC mRNA levels, measured by quantitative solution hybridization/RNase protection assays, were decreased by 48% in the pituitary of the TK+/+ mice, reflecting an expected loss of the pituitary corticotrope POMC cells. This treatment also significantly lowered pituitary beta-endorphin immunoreactivity content and plasma concentrations of corticosterone. In contrast, POMC mRNA levels were increased by 79% in the hypothalamus of the TK+/+ mice with pituitary POMC cell ablation. Binding of [(3)H]DAMGO to mu opioid receptors, as measured by quantitative autoradiography, was significantly reduced in several brain regions including the central grey, median raphe and superficial grey layer of the superior colliculus. These regions are innervated by hypothalamic POMC neurones. No significant differences in binding to either kappa or delta opioid receptors were found in the brain regions studied. These results suggest that POMC-derived peptides of pituitary origin may exert a tonic negative-feedback effect on hypothalamic POMC neurones. In turn, the downregulation of central mu opioid receptors in this model may be mediated through a mechanism related to hypothalamic POMC overexpression.