Irreversible Mu-opioid Receptor Antagonist Research Articles

Agonist‐Dependent Modulation by the Long‐Acting Mu Opioid Receptor Antagonist, Methocinnamox (MCAM)

Opioid overdose is a leading cause of death in the US. Naloxone (NLX), a mu opioid receptor (MOR) competitive antagonist, is the only treatment currently available for reversing opioid overdose. Unfortunately, NLX’s effects are limited by a short duration of action and ease of surmountability by opioid agonists. Methocinnamox (MCAM) is a novel MOR antagonist that has a long duration of action in vivo and holds promise as a better treatment for overdose and perhaps opioid use disorder. In this study, we compared the antagonist properties of MCAM to that of NLX and beta‐funaltrexamine (β‐FNA; an irreversible MOR antagonist). In HEK cells that express human MOR, DAMGO inhibited forskolin‐stimulated cAMP levels in a concentration dependent manner with an EC50 of 10 nM and a maximal inhibition of 40%. Pretreatment (15 min or 2 hrs) with NLX (100 nM, 10 x Ki) shifted the DAMGO concentration response curve (CRC) to the right, 100‐fold, in a fully surmountable manner that was independent of pretreatment time and fully reversed following NLX washout. Pretreatment with β‐FNA (10 nM) reduced the DAMGO maximal response in a time‐dependent, non‐surmountable and irreversible manner with no effect on the potency of DAMGO. By contrast to both β‐FNA and NLX, pretreatment with MCAM (10 nM) reduced the maximal response in a time‐dependent, non‐surmountable, non‐washable, and irreversible manner and shifted the DAMGO CRC to the right 1000‐fold. We hypothesized that this rightward shift in the DAMGO CRC may be due to allosteric properties of MCAM at MOR. A hallmark of allosterism is ligand dependence, so we next tested effects of MCAM, β‐FNA and NLX on a different mu opioid agonist, fentanyl. As expected, pretreatment with β‐FNA reduced the maximal response to fentanyl in a non‐surmountable manner without affecting potency, whereas NLX pretreatment shifted the CRC of fentanyl to the right (decreased the potency) in a manner that was fully surmountable and independent of pretreatment time. By contrast, pretreatment with MCAM reduced the maximal response to fentanyl in a non‐surmountable manner and shifted the CRC to the left (increased potency). Altogether, these data suggest that MCAM may be an irreversible orthosteric antagonist and an allosteric modulator at MOR.Support or Funding InformationSupported by NIH/NIDA RO1 grant DA048214

The antinociceptive effects of ferulic acid on neuropathic pain: involvement of descending monoaminergic system and opioid receptors.

Neuropathic pain can be considered as a form of chronic stress that may share common neuropathological mechanism between pain and stress-related depression and respond to similar treatment. Ferulic acid (FA) is a major active component of angelica sinensis and has been reported to exert antidepressant-like effects; however, it remains unknown whether FA ameliorate chronic constriction injury (CCI)-induced neuropathic pain and the involvement of descending monoaminergic system and opioid receptors. Chronic treatment with FA (20, 40 and 80 mg/kg) ameliorated mechanical allodynia and thermal hyperalgesia in von Frey hair and hot plate tasks, accompanied by increasing spinal noradrenaline (NA) and serotonin (5-HT) levels. Subsequent study suggested that treatment of CCI animals with 40 and 80 mg/kg FA also inhibited spinal MAO-A levels. FA's effects on mechanical allodynia or thermal hyperalgesiawas blocked by 6-hydroxydopamine (6-OHDA) or p-chlorophenylalanine (PCPA) via pharmacological depletion of spinal noradrenaline or serotonin. Moreover, the anti-allodynic action of FA on mechanical stimuli was prevented by pre-treatment with beta2-adrenoceptor antagonist ICI 118,551, or by the delta-opioid receptor antagonist naltrindole. While the anti-hyperalgesia on thermal stimuli induced by FA was blocked by pre-treatment with 5-HT1A receptor antagonist WAY-100635, or with the irreversible mu-opioid receptor antagonist beta-funaltrexamine. These results suggest that the effect of FA on neuropathic pain is potentially mediated via amelioration of the descending monoaminergic system that coupled with spinal beta2- and 5-HT1A receptors and the downstream delta- and mu-opioid receptors differentially.

Ferulic Acid Alleviates Neuropathic Pain: Involvement of Descending Monoaminergic System and Opioid Receptors

Neuropathic pain can be considered as a form of chronic stress that may share common neuropathological mechanism between pain and stress‐related depression and respond to similar treatment. FA is a major active component of angelica sinensis and has been reported to exert antidepressant‐like effects; however, it remains unknown whether resveratrol ameliorate CCI‐induced neuropathic pain. Chronic treatment with FA (10, 20, 40 and 80 mg/kg) twice daily for 3 weeks ameliorated mechanical allodynia and thermal hyperalgesia in von Frey hair and hot plate tasks, accompanied by increasing spinal monoamine levels. Pharmacological depletion of spinal noradrenaline (NA) or serotonin (5‐HT) by 6‐hydroxydopamine (6‐OHDA) or p‐chlorophenylalanine (PCPA) completely blocked FA's effects on mechanical allodynia or thermal hyperalgesia, respectively. The anti‐allodynic action of FA on mechanical stimuli was prevented by co‐treatment with beta2‐adrenoceptor antagonist ICI 118,551, or by co‐treatment with delta‐opioid receptor antagonist naltrindole. Moreover, the anti‐hyperalgesia on thermal stimuli induced by FA was blocked by co‐treatment with 5‐HT1A receptor antagonist WAY‐100635, or with the irreversible mu‐opioid receptor antagonist beta‐funaltrexamine. These results suggest that the effect of FA on neuropathic pain is potentially mediated via modulation of descending monoaminergic system that coupled with spinal b2‐ and 5‐HT1A receptors and the downstream opioid receptors.

Blockade of mu-opioid receptors in the medial thalamus inhibits acquisition, but not expression, of morphine-induced conditioned place preference

The medial thalamus contains abundant mu-opioid receptors and is activated by acute morphine administration. However, the role of the medial thalamus in the rewarding effects of morphine is unclear. The present study examined whether mu-opioid receptors of the medial thalamus influenced the acquisition and expression of morphine-induced conditioned place preference (CPP) in rats. An unbiased apparatus and biased subject assignment were used. Administration of morphine in increasing doses (2 mg/kg, 4 mg/kg, 6 mg/kg, 10 mg/kg, s.c.) was paired with an initially non-preferred chamber and saline administration was paired with an initially preferred chamber. Conditioning trials were conducted twice daily for 4 days. Microinjection of the irreversible mu-opioid receptor antagonist, β-funaltrexamine (5 μg/rat), into the medial thalamus 23 h prior to each morphine conditioning completely blocked the acquisition of CPP. However, microinjection of β-funaltrexamine into the medial thalamus after morphine conditioning trials, but 23 h prior to a test session, had no effect on the expression of CPP. It is concluded that mu-opioid receptors in the rat medial thalamus are involved in the acquisition, but not expression, of morphine-induced CPP.

A Single Exposure to Morphine Induces Long‐Lasting Hyporeactivity of Rat Caudate Putamen Dopaminergic Nerve Terminals

The long-lasting effects of exposure to drugs of abuse on the brain is a central theme in drug addiction research. This study was designed to evaluate whether enduring neurochemical adaptations within caudate putamen can be evoked by a single injection of a high dose of morphine. Rats were pretreated once with 10 mg/kg morphine. Seven days later the effect of another injection of 10 mg/kg morphine on total levels of dopamine (DA), 3,4-dihydroxyphenylacetic acid (DOPAC), and homovanilic acid (HVA) in caudate putamen was assessed in half the pretreated animals. An irreversible mu-opioid receptor antagonist, cloccinamox (C-CAM; 0.1 mg/kg), significantly antagonized the elevation of the HVA/DA ratio, but not the elevation of the DOPAC/DA ratio induced by morphine in the caudate putamen from drug-naive animals. Pretreatment with morphine blunted changes in the HVA/DA ratio induced by another morphine challenge, but it had no effect on the DOPAC/DA ratio within the caudate putamen. Therefore, a single dose of 10 mg/kg morphine hampered nigrostriatal DA release and extraneuronal metabolism, mu-opioid receptor mediated, on another 10 mg/kg morphine challenge. This confirms that the first exposure to morphine does not go without long-lasting neurochemical adaptations.

Pharmacokinetic-pharmacodynamic analysis of the EEG effect of alfentanil in rats following beta-funaltrexamine-induced mu-opioid receptor "knockdown" in vivo.

The objective of this investigation was to determine the influence of pre-treatment with the irreversible mu-opioid receptor antagonist beta-funaltrexamine (beta-FNA) on the pharmacokinetic-pharmacodynamic (PK/PD) relationship of alfentanil in rats. The PK/PD correlation of alfentanil (2 mg x kg(-1) intravenously in 20 min) was determined in chronically instrumented rats using amplitudes in the 0.5-4.5 Hz frequency band of the EEG as pharmacodynamic endpoint. Beta-FNA was administered intravenously (10 mg x kg(-1)) either 35 min or 24 h prior to the PK/PD experiments. Pre-treatment with beta-FNA had no influence on the pharmacokinetics of alfentanil. The in vivo concentration-EEG effect relationships, however, were steeper and shifted towards higher concentrations with no difference between the 35-min and the 24-h pre-treatment groups. Analysis of the data on basis of the operational model agonism revealed that the observed changes could be explained by a 70-80% reduction in alfentanil efficacy in beta-FNA pre-treated rats. This is consistent with results from an in vitro receptor bioassay showing a 40-60% reduction in the number of specific mu-opioid binding sites in the brain. This investigation confirms the validity of a previously postulated mechanism-based PK/PD model for the effect of synthetic opiates in rats.

Ischemic preconditioning in the intact rat heart is mediated by delta1- but not mu- or kappa-opioid receptors.

Our laboratory has previously shown that delta-opioid receptors are involved in the cardioprotective effect of ischemic preconditioning in the rat heart. However, this class of receptors consists of two subtypes, delta1, and delta2, and mu- or kappa-opioid receptors may also exist in the heart. Therefore, the purpose of the present study was to test the hypothesis that ischemic preconditioning is mediated through stimulation of one or both delta-opioid receptor subtypes. Anesthetized, open chest, male Wistar rats were assigned to 1 of 14 groups. All animals were subjected to 30 minutes of occlusion and 2 hours of reperfusion. Ischemic preconditioning was elicited by three 5-minute occlusion periods interspersed with 5 minutes of reperfusion. Two doses of 7-benzylidenenaltrexone (BNTX; 1 and 3 mg/kg i.v.), a selective delta1-opioid receptor antagonist, or naltriben (NTB; 1 and 3 mg/kg i.v.), a selective delta2-opioid receptor antagonist, were given before ischemic preconditioning. To test for a role of mu-opioid receptors, rats were pretreated with beta-funaltrexamine (beta-FNA; 15 mg/kg s.c), an irreversible mu-opioid receptor antagonist, 24 hours before ischemic preconditioning or given the mu-opioid receptor agonist D-Ala,2N-Me-Phe,4glycerol5-enkephalin (DAMGO) as three 5-minute infusions (1, 10, and 100 microg/kg per infusion i.v., respectively) interspersed with 5-minute drug-free periods before the prolonged ischemic and reperfusion periods (lowDAMGO, medDAMGO, and hiDAMGO, respectively). The involvement of kappa-opioid receptors was tested by administering one of two doses of nor-binaltorphimine (nor-BNI; 1 and 5 mg/kg i.v.) before ischemic preconditioning. Infarct size (IS) as a percent of the area at risk (AAR) was measured by triphenyltetrazolium stain. Ischemic preconditioning markedly reduced IS/AAR (14+/-4%, P<.05) compared with control (55+/-4%). NTB, beta-FNA, and nor-BNI were unable to block the cardioprotective effect of ischemic preconditioning. In addition, DAMGO had no effect on IS/AAR. However, the high dose of BNTX (3 mg/kg i.v.) significantly attenuated the cardioprotective effect of ischemic preconditioning (39+/-5%; P<.05 versus control and ischemic preconditioning). These results indicate that delta1-opioid receptors play an important role in the cardioprotective effect of ischemic preconditioning in the rat heart.

Differential effects of delta- and mu-opioid receptor antagonists on the amphetamine-induced increase in extracellular dopamine in striatum and nucleus accumbens.

The specific opioid receptor antagonist naloxone attenuates the behavioral and neurochemical effects of amphetamine. Furthermore, the amphetamine-induced increase in locomotor activity is attenuated by intracisternally administered naltrindole, a selective delta-opioid receptor antagonist, but not by the irreversible mu-opioid receptor antagonist beta-funaltrexamine. Therefore, this research was designed to determine if naltrindole would attenuate the neurochemical response to amphetamine as it did the behavioral response. In vivo microdialysis was used to monitor the change in extracellular concentrations of dopamine in awake rats. Naltrindole (3.0, 10, or 30 micrograms) or vehicle was given 15 min before and beta-funaltrexamine (10 micrograms) or vehicle 24 h before the start of cumulative dosing, intracisternally in a 10-microliters volume, while the rats were lightly anesthetized with methoxyflurane. Cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) followed pretreatment injections at 30-min intervals. Dialysate samples were collected every 10 min from either the striatum or nucleus accumbens and analyzed for dopamine content by HPLC. Amphetamine dose-dependently increased dopamine content in both the striatum and nucleus accumbens, as reported previously. Naltrindole (3.0, 10, and 30 micrograms) significantly reduced the dopamine response to amphetamine in the striatum. In contrast, 30 micrograms of naltrindole did not modify the dopamine response to amphetamine in the nucleus accumbens. On the other hand, beta-funaltrexamine (10 micrograms) had no effect in the striatum but significantly attenuated the amphetamine-induced increase in extracellular dopamine content in the nucleus accumbens. These data suggest that delta-opioid receptors play a relatively larger role than mu-opioid receptors in mediating the amphetamine-induced increase in extracellular dopamine content in the striatum, whereas mu-opioid receptors play a larger role in mediating these effects in the nucleus accumbens.

Endogenous opioids: do they modulate the rat pup's response to social isolation?

Previous studies with several different species have suggested that opioids and their receptors are involved in the mediation of the infant's vocal response to social isolation. In the case of the rat pup, 2 models have been hypothesized to relate opioids and the ultrasonic call emitted during social isolation. One model views the comforting effects of social contact as opioid mediated and the apparent distress of social isolation as analogous to opiate withdrawal. The 2nd model considers social separation as a stressor that recruits endogenous opioids. This article describes 3 experiments that tested both of these models in 7-10-day-old rat pups. In Experiment 1, morphine (0.04-0.40 mg/kg) decreased the rate of isolation calls in a dose-dependent, naloxone-reversible fashion. However, the decrease in calling rate was observed only at doses that decreased locomotor activity. Administration of the reversible opiate antagonist naloxone (0.05-5.0 mg/kg) did not alter the rate of calls during either 2- or 6-min isolation tests at either 24 or 32 degrees C. In Experiment 2, the irreversible mu opioid receptor antagonist beta-funaltrexamine (beta-FNA) was administered into the lateral ventricle of 6-day-old pups. Again, no change in the rate of isolation calls was found, although sensitivity to morphine was markedly decreased, and mu (but not delta or kappa) receptors were decreased in selected brain regions by about 40%. In Experiment 3, in vivo receptor binding was used to directly investigate the availability of mu opioid receptors during social contact and social isolation. Pups injected with 3H-diprenorphine showed relatively high levels of specific in vivo binding that followed the regional pattern of in vitro binding, but no effects of social isolation were apparent in the 5 brain regions assayed. Taken together, the consistent negative results with opiate receptor antagonists, as well as the inability to detect an alteration of in vivo binding, suggest that the mu opioid receptor is not an essential part of the rat pup's vocal response to social separation.

Synthesis and evaluation of 1- and 2-substituted fentanyl analogues for opioid activity.

We synthesized fentanyl analogues that possess key groups common to the opioid peptides to investigate whether or not these two classes of compounds interact with common subsites on opioid receptors. The design of the analogues was based on the possibility of structural analogy between the two aromatic rings of fentanyl and the Tyr1 and Phe4 residues of the opioid peptides. The synthesized compounds showed very weak or no opioid activity as tested in the electrically stimulated longitudinal muscle of the guinea pig ileum or mouse vas deferens preparations. These results, together with those of reported studies, suggest that fentanyl and the opioid peptides interact with different subsites on either mu or sigma receptors. Studies using the irreversible mu opioid receptor antagonist, beta-funaltrexamine, indicate that fentanyl interacts preferentially with mu opioid receptors.