Anatomical and functional correlation of the endomorphins with mu opioid receptor splice variants

Modulation of OPRM1 Alternative Splicing by Morphine and HIV-1 Nef

Clinically used opioids, such as morphine, activate the mu opioid receptor (MOR) encoded by Opioid Receptor Mu 1 (OPRM1) gene. Examination of the opioid receptor genes showed that the human OPRM1 pre-mRNA undergoes extensive alternative splicing events and capable of expressing 21 isoforms. However, characterization of OPRM1 signaling is generalized, and only one isoform (MOR-1) has been extensively studied. Compounding this issue is the increasing significance of intravenous drug abuse in HIV neuropathogenesis. Here, we investigated the molecular impact of morphine and HIV-1 on regulation of OPRM1 pre-mRNA splicing in in vitro and in vivo models. Our results suggested that morphine treatment specifically induces the alternative splicing of MOR-1X isoform among the other isoforms analyzed in neuronal cells. Interestingly, alternative splicing and expression of MOR-1X isoform was also induced in postmortem brain tissues obtained from people with HIV (PWH). Additionally, treatment of control rats with morphine induced alternative splicing of MOR-1X in the brain regions involved in the reward pathways. More interestingly, HIV-1 transgenic (HIV-1Tg) rats, showed an additive induction of MOR-1X isoform with the exposure to morphine. To further assess the possible role of HIV secretory proteins in alternative splicing of OPRM1 gene, we analyzed the impact of HIV-1 Tat, gp120 and Nef proteins on alternative splicing of MOR-1X isoform. While the Tat and gp120 had no visible effects, treatment of neurons with Nef induced MOR-1X alternative splicing that was comparable to treatment with morphine. Altogether, our results suggest that HIV-1 may alter MOR isoform expression with Nef protein by amplifying the rate of MOR-1X alternative splicing induced by morphine.

Interactive Mechanisms of Supraspinal Sites of Opioid Analgesic Action: A Festschrift to Dr. Gavril W. Pasternak

Almost a half century of research has elaborated the discoveries of the central mechanisms governing the analgesic responses of opiates, including their receptors, endogenous peptides, genes and their putative spinal and supraspinal sites of action. One of the central tenets of "gate-control theories of pain" was the activation of descending supraspinal sites by opiate drugs and opioid peptides thereby controlling further noxious input. This review in the Special Issue dedicated to the research of Dr. Gavril Pasternak indicates his contributions to the understanding of supraspinal mediation of opioid analgesic action within the context of the large body of work over this period. This review will examine (a) the relevant supraspinal sites mediating opioid analgesia, (b) the opioid receptor subtypes and opioid peptides involved, (c) supraspinal site analgesic interactions and their underlying neurophysiology, (d) molecular (particularly AS) tools identifying opioid receptor actions, and (e) relevant physiological variables affecting site-specific opioid analgesia. This review will build on classic initial studies, specify the contributions that Gavril Pasternak and his colleagues did in this specific area, and follow through with studies up to the present.

Mu opioids and their receptors: evolution of a concept

Opiates are among the oldest medications available to manage a number of medical problems. Although pain is the current focus, early use initially focused upon the treatment of dysentery. Opium contains high concentrations of both morphine and codeine, along with thebaine, which is used in the synthesis of a number of semisynthetic opioid analgesics. Thus, it is not surprising that new agents were initially based upon the morphine scaffold. The concept of multiple opioid receptors was first suggested almost 50 years ago (Martin, 1967), opening the possibility of new classes of drugs, but the morphine-like agents have remained the mainstay in the medical management of pain. Termed mu, our understanding of these morphine-like agents and their receptors has undergone an evolution in thinking over the past 35 years. Early pharmacological studies identified three major classes of receptors, helped by the discovery of endogenous opioid peptides and receptor subtypes-primarily through the synthesis of novel agents. These chemical biologic approaches were then eclipsed by the molecular biology revolution, which now reveals a complexity of the morphine-like agents and their receptors that had not been previously appreciated.

Epigenetics of µ-opioid receptors: intersection with HIV-1 infection of the central nervous system

The abuse of intravenous drugs, such as heroin, has become a major public health concern due to the increased risk of HIV-1 infection. Opioids such as heroin were originally identified and subsequently abused for their analgesic effects. However, many investigations have found additional effects of opioids, including regulation of the immune system. As such, chronic opioid abuse has been shown to promote HIV-1 pathogenesis and facilitate HIV-1-associated neurocognitive dysfunction. Clinical opioids, such as morphine and methadone, as well as illicit opioids, such as heroin, exert their effects primarily through interactions with the µ-opioid receptor (MOR). However, the mechanisms by which opioids enhance neurocognitive dysfunction through MOR-mediated signaling pathways are not completely understood. New findings in the regulation of MOR expression, particularly epigenetic and transcriptional regulation as well as alternative splicing, sheds new insights into possible mechanisms of HIV-1 and opiate synergy. In this review, we identify mechanisms regulating MOR expression and propose novel mechanisms by which opioids and HIV-1 may modulate this regulation. Additionally, we suggest that differential regulation of newly identified MOR isoforms by opioids and HIV-1 has functional consequence in enhancing HIV-1 neurocognitive dysfunction.

Pleiotropic opioid regulation of spinal endomorphin 2 release and its adaptations to opioid withdrawal are sexually dimorphic

We studied adaptations to acute precipitated opioid withdrawal of spinal μ-opioid receptor (MOR)-coupled regulation of the release of endomorphin 2 (EM2). The release of this highly MOR-selective endogenous opioid from opioid-naive spinal tissue of male rats is subjected to MOR-coupled positive as well as negative modulation via cholera toxin-sensitive G(s) and pertussis toxin-sensitive G(i)/G(o), respectively. The net effect of this concomitant bidirectional modulation is inhibitory. MOR-coupled pleiotropic regulation of EM2 release is retained in opioid-withdrawn spinal tissue of male rats, but the balance of MOR-coupled inhibitory and facilitatory regulation shifted such that facilitatory regulation predominates. Augmented coupling of MOR to G(s) is causally associated with this change. Strikingly, pleiotropic characteristics of MOR-coupled regulation of spinal EM2 release and adaptations thereof to opioid withdrawal are male-specific. In females, MOR-coupled regulation of EM2 release from opioid-naive and -withdrawn spinal tissue does not have a significant G(s)-coupled facilitatory component, and MOR-coupled inhibition of EM2 release persists unabated in withdrawn preparations. The male-specific adaptations to chronic morphine that shift the relative predominance of opposing dual G protein-coupled MOR pathways provides a mechanism for mitigating inhibitory MOR signaling without losing MOR-coupled feedback regulation. These adaptations enable using endogenous EM2 as a substitute for morphine that had been precipitously removed. The sexually dimorphic functionality and regulation of spinal EM2/MOR-coupled signaling suggest the clinical utility of using sex-specific treatments for addiction that harness the activity of endogenous opioids.

Identification and characterization of seven new exon 11-associated splice variants of the rat μ opioid receptor gene, OPRM1

BACKGROUND

The mouse mu opioid receptor (OPRM1) gene undergoes extensive alternative splicing at both the 3'- and 5'-ends of the gene. Previously, several C-terminal variants generated through 3' splicing have been identified in the rat OPRM1 gene. In both mice and humans 5' splicing generates a number of exon 11-containing variants. Studies in an exon 11 knockout mouse suggest the functional importance of these exon 11-associated variants in mediating the analgesic actions of a subset of mu opioids, including morphine-6β-glucuronide (M6G) and heroin, but not others such as morphine and methadone. We now have examined 5' splicing in the rat.

RESULTS

The current studies identified in the rat a homologous exon 11 and seven exon 11-associated variants, suggesting conservation of exon 11 and its associated variants among mouse, rat and human. RT-PCR revealed marked differences in the expression of these variants across several brain regions, implying region-specific mRNA processing of the exon 11-associated variants. Of the seven rat exon 11-associated variants, four encoded the identical protein as found in rMOR-1, two predicted 6 TM variants, and one, rMOR-1H2, generated a novel N-terminal variant in which a stretch of an additional 50 amino acids was present at the N-terminus of the previously established rMOR-1 sequence. When expressed in CHO cells, the presence of the additional 50 amino acids in rMOR-1H2 significantly altered agonist-induced G protein activation with little effect on opioid binding.

CONCLUSION

The identification of the rat exon 11 and its associated variants further demonstrated conservation of 5' splicing in OPRM1 genes among rodents and humans. The functional relevance of these exon 11 associated variants was suggested by the region-specific expression of their mRNAs and the influence of the N-terminal sequence on agonist-induced G protein coupling in the novel N-terminal variant, rMOR-1H2. The importance of the exon 11-associated variants in mice in M6G and heroin analgesia revealed in the exon 11 knockout mouse implies that these analogous rat variants may also play similar roles in rat. The complexity created by alternative splicing of the rat OPRM1 gene may provide important insights of understanding the diverse responses to the various μ opioids seen in rats.

Neurochemical phenotypes of endomorphin-2-containing neurons in vagal nodose neurons of the adult rat

It has been shown that endomorphin-2-like immunoreactive (EM2-LI) neurons in dorsal root ganglion play important roles in regulating somatic information transmission. Although EM2-ergic neurons have been found in nodose ganglion (NG) which is mainly involved in transmitting visceral information into the nucleus tractus solitarii (NTS), the neurochemical phenotypes of EM2-ergic neurons have not yet been investigated. In the present study, immunofluorescent histochemical staining showed that 43.5% of the NG neurons contained EM2 and these neurons were small to medium in size. 15.2%, 27.8%, 74.4% and 25.2% of the EM2-LI NG neurons expressed substance P (SP), calcitonin gene-related peptide (CGRP), nitric oxide synthase (NOS) and vasoactive intestinal peptide (VIP), respectively. In addition, about 90.8% of EM2-LI NG neurons also contained mu-opioid receptor (MOR). EM2/MOR and EM2/SP double-labeled peripheral axons were observed in the vagal trunk. Anterograde tracing combined with immunofluorescent staining showed EM2/MOR and EM2/SP double-labeled vagal afferents in the NTS. EM2/MOR/SP and EM2/MOR/CGRP triple-labeled neurons and axons were observed in the NG. Importantly, at the ultrastructrual level, post-embedding electron microscopy revealed that EM2-LI and SP-LI gold particles coexisted in the same large dense-cored synaptic vesicles in the pre-synaptic button, while MOR-LI gold particles existed on both pre- and post-synaptic membranes in the NTS. These results suggest that EM2 in axon terminals of NG neurons might be involved in visceral information transmission and homeostatic control through modulating the release of other neurotransmitters (such as SP, CGRP, NO, VIP) via pre-synaptic MOR and through post-synaptic mechanisms in the NTS.

Coexpression of the mu-opioid receptor splice variant MOR1C and the vesicular glutamate transporter 2 (VGLUT2) in rat central nervous system

It has been reported that mu-opioid agonists depress glutamate release in some neurons but the specific receptor subtype mediating this effect is unclear. The purpose of the present study was to examine whether a particular mu-opioid receptor (MOR) splice-variant, MOR(1C), is expressed in rat central nervous system (CNS) by terminals expressing the vesicular glutamate transporter2 (VGLUT2), a marker of glutamatergic neurons. Several MOR splice variants have been identified in mice and MOR(1C) appears mainly to be localized to fibers and terminals, from which most neurotransmitter release would be expected. In addition, VGLUT2 has been found in the CNS and antibodies to it are reliable markers for glutamatergic terminals. Using fluorescence immunohistochemistry and confocal microscopy to examine spatial relationships between MOR(1C) and VGLUT2, we found that MOR(1C) and VGLUT2 puncta were widely distributed throughout the rat CNS; moreover, many regions contained terminals that expressed both. Thus, it appears that coexpression of MOR(1C) and VGLUT2 is common in the rat CNS. We hypothesize that activation of MOR(1C) by mu-opioid agonists at some glutamatergic terminals may be a mechanism by which glutamate release is inhibited.

Endomorphin-2 is released from newborn rat primary sensory neurons in a frequency- and calcium-dependent manner

Recent evidence indicates that endomorphins, endogenous mu-opioid receptor (MOR) agonists, modulate synaptic transmission in both somatic and visceral sensory pathways. Here we show that endomorphin-2 (END-2) is expressed in newborn rat dorsal root ganglion (DRG) and nodose-petrosal ganglion complex (NPG) neurons, and rarely co-localizes with brain-derived neurotrophic factor (BDNF). In order to examine activity-dependent release of END-2 from neurons, we established a model using dispersed cultures of DRG and NPG cells activated by patterned electrical field stimulation. To detect release of END-2, we developed a novel rapid capture enzyme-linked immunosorbent assay (ELISA), in which END-2 capture antibody was added to neuronal cultures shortly before their electrical stimulation. The conventional assay was effective at reliably detecting END-2 only when the cells were stimulated in the presence of CTAP, a MOR-selective antagonist. This suggests that the strength of the novel assay is related primarily to rapid capture of released END-2 before it binds to endogenous MORs. Using the rapid capture ELISA, we found that stimulation protocols known to induce plastic changes at sensory synapses were highly effective at releasing END-2. Removal of extracellular calcium or blocking voltage-activated calcium channels significantly reduced the release. Together, our data provide the first evidence that END-2 is expressed by newborn DRG neurons of all sizes found in this age group, and can be released from these, as well as from NPG neurons, in an activity-dependent manner. These results point to END-2 as a likely mediator of activity-dependent plasticity in sensory pathways.

In vitro stability and permeability studies of liposomal delivery systems for a novel lipophilic endomorphin 1 analogue

We have previously shown that the stability and permeability of peptides can be greatly improved by conjugation with lipoamino acids such as 2-aminododecanoic acid (C12Laa). However, the increase in lipophilicity which this conjugation provides can also cause a significant decrease in the compound's water solubility. In this study, we coupled C12Laa to the N-terminus of endomorphin1 (Endo-1, Tyr-Pro-Trp-Phe-NH(2)), and addressed its solubility issue by formulating C12Laa-Endo-1 into phosphatidylcholine liposomes. The aqueous solubility of the lipidic analogue was greatly improved, facilitating the accurate analysis of the compound in in vitro assays. The metabolic stability and in vitro endothelial permeability of the C12Laa-Endo-1 liposomal formulation was assessed using Caco-2 cells, and compared with the formulation of the parent peptide Endo-1. The liposome-encapsulated C12Laa-Endo exhibited significant increases in both stability and permeability. These results suggest that the combination of chemical modification and liposome formulation has great potentials in improving the bioavailability of neuroactive peptides.

Colocalization and shared distribution of endomorphins with substance P, calcitonin gene-related peptide, gamma-aminobutyric acid, and the mu opioid receptor

The endomorphins are endogenous opioids with high affinity and selectivity for the mu opioid receptor (MOR, MOR-1, MOP). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2); EM1) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2); EM2) have been localized to many regions of the central nervous system (CNS), including those that regulate antinociception, autonomic function, and reward. Colocalization or shared distribution (overlap) of two neurotransmitters, or a transmitter and its cognate receptor, may imply an interaction of these elements in the regulation of functions mediated in that region. For example, previous evidence of colocalization of EM2 with substance P (SP), calcitonin gene-related peptide (CGRP), and MOR in primary afferent neurons suggested an interaction of these peptides in pain modulation. We therefore investigated the colocalization of EM1 and EM2 with SP, CGRP, and MOR in other areas of the CNS. EM2 was colocalized with SP and CGRP in the nucleus of the solitary tract (NTS) and with SP, CGRP and MOR in the parabrachial nucleus. Several areas in which EM1 and EM2 showed extensive shared distributions, but no detectable colocalization with other signaling molecules, are also described.

Sex-/Ovarian Steroid-Dependent Release of Endomorphin 2 from Spinal Cord

Mu-opioid receptor (MOR) agonists have been shown to be more potent analgesics in male than female rodents. Regulation of spinal MOR-coupled antinociception by 17beta-estradiol (estrogen, E2) and progesterone (P) is also sexually dimorphic; pregnancy levels of E2/P activate MOR-coupled analgesic pathways in male but not female rats. We hypothesized that the sexual dimorphic characteristics of MOR-coupled antinociception reflects sexual dimorphism in the regulation of the release from spinal cord of the endogenous MOR agonist, endomorphin 2 (EM2). Parameters of spinal EM2 release manifesting sexual dimorphism include its 1) magnitude: in vitro basal and K+-evoked release of EM2 from spinal tissue of male rats is approximately 50% greater than that observed from spinal cord of females; 2) modulation by ovarian sex steroids: E2/P treatment significantly enhanced K+-evoked EM2 release from spinal tissue of males, but not females; and 3) enhancement by opioid receptor blockade: naloxone enhanced stimulated EM2 release from spinal tissue of both males and females, but it augmented basal release from spinal tissue of only males. Enhancement of EM2 release by naloxone reflects negative coupling of MOR to EM2 release and hence its modulation by negative feedback since only activation of MOR, not kappa-or delta-opioid receptors, was able to inhibit evoked EM2 release. These data reveal that the EM2-MOR spinal analgesic system is more robust and "higher gain" in male versus female rodents. These findings could provide a mechanistic rubric for understanding the male female dichotomy in prevalence and intensity of chronic pain syndromes.

Reduced suppression of CO2-induced ventilatory stimulation by endomorphins relative to morphine

Opioids are among the most effective analgesics, but a major limitation for their therapeutic usefulness is their induction of respiratory depression. Endomorphin-1 (EM1), in contrast to several other mu opioids, exhibits a threshold for respiratory depression that is well above its threshold for analgesia. Its effect on sensitivity to CO(2), however, remains unknown. Minute ventilation (V(E)) in 2, 4, and 6% CO(2) was measured before and after systemic administration of EM1, endomorphin-2 (EM2), DAMGO, and morphine in the conscious rat. EM1 and EM2 attenuated the hypercapnic ventilatory response (HCVR) only in high doses, while DAMGO and morphine diminished the HCVR in much lower doses. The ventilatory effects of high doses of all 4 agonists were blocked by the mu-opioid antagonist naloxone (0.4 mg/kg i.v.), but not by the peripherally restricted mu-opioid antagonist, methyl-naloxone (0.4 mg/kg i.v.). It was concluded that the endomorphins attenuated the HCVR only in large doses, well beyond the analgesic threshold, and did so through a centrally mediated mu-opioid mechanism.

Modulation of synaptic transmission in the rat nucleus of the solitary tract by endomorphin-1

Activation of opioid receptors in the periphery and centrally in the brain results in inhibition of gastric and other vagally mediated functions. The aim of this study was to examine the role of the endogenous opioid agonist endomorphin 1 (EM-1) in regulating synaptic transmission within the nucleus tractus solitarius (NTS), an integration site for autonomic functions. We performed whole cell patch-clamp recordings from coronal brain slices of the rat medulla. A subset of the neurons studied was prelabeled with a stomach injection of the transsynaptic retrograde virus expressing EGFP, PRV-152. Solitary tract stimulation resulted in constant latency excitatory postsynaptic currents (EPSCs) that were decreased in amplitude by EM-1 (0.01-10 microM). The paired-pulse ratio was increased with little change in input resistance, suggesting a presynaptic mechanism. Spontaneous EPSCs were decreased in both frequency and amplitude by EM-1, and miniature EPSCs were reduced in frequency but not amplitude, suggesting a presynaptic mechanism for the effect. Spontaneous inhibitory postsynaptic currents (IPSCs) were also reduced in frequency by EM-1, but the effect was blocked by TTX, suggesting activity at receptors on the somata of local inhibitory neurons. Synaptic input arising from local NTS neurons, which were activated by focal photolysis of caged glutamate, was inhibited by EM-1. The actions of EM-1 were similar to those of D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO) and were blocked by naltrexone, D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2 (CTOP), or D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP). These results suggest that EM-1 acts at mu-opioid receptors to modulate viscerosensory input and specific components of local synaptic circuitry in the NTS.

Feeding induced by food deprivation is differentially reduced by opioid receptor antisense oligodeoxynucleotide probes in rats

The increases in food intake following 24 h of food deprivation are reduced by systemic and central administration of general opioid antagonists. The use of selective opioid antagonists revealed that mu-selective antagonists were more effective than kappa-selective antagonists in reducing deprivation-induced intake, whereas delta-selective antagonists were minimally effective. Antisense oligodeoxynucleotide (AS ODN) probes directed against different exons of the mu (MOP), delta (DOP), kappa (KOP) and nociceptin (NOP) opioid peptide receptor genes have been able to differentially alter feeding responses elicited by glucoprivation, lipoprivation and by different opioid peptides and receptor agonists. The present study examined whether lateral ventricular administration of AS ODN probes directed against different exons of the MOP, DOP, KOP or NOP opioid receptor genes altered food intake and body weight changes following 24 h of food deprivation in rats. Deprivation-induced feeding was significantly and maximally reduced by an AS ODN probe directed against exon 2, but not exons 1 or 3 of the KOP gene. This response was also significantly though modestly reduced by AS ODN probes directed against exons 2, 3 or 4 of the MOP gene, exon 1 of the DOP gene, or exon 1 of the NOP gene. Recovery of body weight following postdeprivation food reintroduction was significantly reduced by AS ODN probes directed against either exons 2, 3 or 4 of the MOP gene, exons 1 or 2 of the DOP gene, or exons 1, 2 or 3 of the KOP gene. The parallel patterns in the magnitude of alterations in deprivation-induced feeding by delta antagonists and DOP AS ODN probes on one hand, and by kappa antagonists and KOP AS ODN probes on the other, provide converging and complementary evidence for their relative involvement in this response. The modest reductions by MOP AS ODN probes relative to the more potent reductions induced by mu-selective antagonists suggest that the mu receptor-mediated actions upon deprivation-induced feeding may involve recently-identified splice variants or isoforms of the MOP gene.

The pain of antisense: in vivo application of antisense oligonucleotides for functional genomics in pain and analgesia

As the genomic revolution continues to evolve, there is an increasing demand for efficient and reliable tools for functional characterization of individual gene products. Antisense oligonucleotide-mediated knockdown has been used successfully as a functional genomics tool in animal models of pain and analgesia yet skepticism regarding the validity and utility of antisense technology remains. Contributing to this uncertainty are the lack of systematic studies exploring antisense oligonucleotide use in vivo and the many technical and methodological challenges intrinsic to the method. This article reviews the contributions of antisense oligonucleotide-based studies to the field of pain and analgesia and the general principles of antisense technology. A special emphasis is placed on technical issues surrounding the successful application of antisense oligonucleotides in vivo, including sequence selection, antisense oligonucleotide chemistry, DNA controls, route of administration, uptake, dose-dependence, time-course and adequate evaluation of knockdown.

Endogenous opiates and behavior: 2002

This paper is the twenty-fifth consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2002 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).

Endomorphin-2 axon terminals contact mu-opioid receptor-containing dendrites in trigeminal dorsal horn

The endomorphins represent a novel group of endogenous opioid peptides that have high affinity for the mu-opioid receptor (MOR1). Endomorphin-2 is present in high density in the spinal and trigeminal dorsal horns and is localized to primary afferents. If endomorphin-2 were an endogenous ligand for the MOR1, we would expect to find the receptor at cellular sites in close association with the peptide. We used dual-labeling immunocytochemical methods combined with electron microscopy to determine if a cellular substrate exists for functional interactions between endomorphin-2 and MOR1. We confirmed the localization of endomorphin-2 to unmyelinated axons and axon terminals in the trigeminal dorsal horn. A small proportion of these endomorphin-2 axons contained MOR1, but many of the dendritic targets of endomorphin-2 terminals contained MOR1. Consistent with previous studies, endomorphin-2 was contained primarily in dense core vesicles and MOR1 was located primarily at non-synaptic sites. These morphological characteristics are consistent with the hypothesis that peptides are released extra-synaptically and their receptors may be located at sites distal to the synaptic junction. These anatomical data support the hypothesis that endomorphin-2 is a ligand for MORs in the trigeminal dorsal horn, particularly at postsynaptic sites.

Antinociception by a peripherally administered novel endomorphin-1 analogue containing beta-proline

We previously described a novel endomorphin-1 analogue (Tyr-L-beta-Pro-Trp-Phe-NH(2); Endo1-beta-Pro) more resistant to enzymatic hydrolysis than endomorphin-1 that acts as a mu-opioid receptor agonist. In this study we report that Endo1-beta-Pro, s.c. injected in the mouse, is an effective antinociceptive agent in the tail flick (ED(50)=9.2 mg/kg) and acetic acid-induced abdominal constriction (ED(50)=1.2 mg/kg) tests. Moreover, s.c. Endo1-beta-Pro significantly decreases, in the mouse, the gastrointestinal propulsion measured as transit of an orally administered charcoal meal (ED(50)=10.0 mg/kg). Subcutaneous beta-funaltrexamine or a high dose of the mu(1)-opioid receptor-selective antagonist naloxonazine (50 mg/kg) prevents the antinociceptive and antitransit action of Endo1-beta-Pro; moreover, these effects are partially blocked by i.c.v. naloxone or by i.p. naloxone methiodide, this latter does not readily cross the blood-brain barrier. On the contrary, the kappa-opioid receptor antagonist nor-binaltorphimine or the delta-opioid receptor antagonist naltrindole are ineffective Thus, Endo1-beta-Pro may act, preferentially, through central and peripheral mu(2)-opioid receptors to produce antinociception and to inhibit gastrointestinal transit. Endo1-beta-Pro is among the first endomorphin-1 analogues showing antinociceptive activity after systemic administration. This compound will be extremely useful for exploring the pharmacological profile of endomorphins in vivo and confirms the potential therapeutic interest of endomorphin derivatives as novel analgesic agents.