Endomorphin analog ZH853 shows low reward, tolerance, and affective-motivational signs of withdrawal, while inhibiting opioid withdrawal and seeking

Currently available μ-opioid receptor agonist pharmacotherapies for opioid use disorder possess adverse effects limiting their use and, despite treatment, rates of relapse remain high. We previously showed that endomorphin analog ZH853 had no effect in rodent models that predict abuse liability in humans. Here we extended these findings by examining dependence liability and reinforcing properties in female rats and male rats with previous opioid exposure. The potential use of ZH853 in managing opioid use disorder was evaluated by examining its effect on opioid-seeking behavior and withdrawal. We found that ZH853 did not induce locomotor activation in male and female mice and was not self-administered by female rats. Relative to morphine, ZH853 led to similar somatic signs of withdrawal, but low affective-motivational signs of withdrawal, and absent changes in ventral tegmental area K(+)-Cl(-) co-transporter expression associated with reward dysregulation. The low abuse liability of ZH853 was further supported in oxycodone self-administering male rats, where ZH853 substitution extinguished opioid-seeking behavior. ZH853 priming also did not reinstate morphine conditioned place preference. Lastly, ZH853 inhibited oxycodone-seeking behavior during relapse after forced abstinence and decreased the expression of morphine withdrawal. These findings suggest the potential use of ZH853 as a safer opioid medication for long-term treatment of pain and opioid use disorder.

Discriminative Stimulus and Low Abuse Liability Effects of Novel Endomorphin Analogs Suggest a Potential Treatment Indication for Opioid Use Disorder

Opioid dependence can be difficult to manage using existing pharmacotherapies. A long-acting opioid with low abuse liability that substitutes for a shorter-acting opioid may improve treatment of opioid use disorders (OUDs). We recently characterized an endomorphin (EM) analog (ZH853) that produced a longer duration of antinociception compared with morphine, but did not produce self-administration or several other adverse effects preclinically. Here, we further characterized ZH853 in tests of antinociception, abuse liability, and drug discrimination. A conditioned place preference (CPP) procedure, that included a locomotor activity assessment, was used to test abuse liability in rats. Subsequently, dopamine (DA) cell-somas located in the ventral tegmental area (VTA) from these rats were assessed by size using immunohistochemistry and Stereo Investigator software. A hot-plate antinociception test in male and female mice confirmed central penetration. Morphine-substitution effects of several EM analogs (ZH850, ZH831, and ZH853) were tested in a drug discrimination (DD) procedure in rats. Morphine produced dose-dependent CPP and locomotor sensitization and reduced the size of DA cell somas in VTA, whereas ZH853 did not produce any of these effects relative to control. The antinociceptive effects of ZH853 were μ-receptor selective since β-funaltrexamine antagonized these effects. Rats responded on a morphine-trained lever when injected with ZH831 and ZH853 during DD experiments. The favorable morphine-substitution effects of these EM analogs relative to their low abuse liability indicate promising novel compounds that may improve treatment of OUD. SIGNIFICANCE STATEMENT: In this experiment, we investigated the preclinical effects of novel endomorphin analogs for use as substitution therapies for opioid use disorder, a problem that has contributed to an opioid overdose epidemic. Several endomorphin analogs substituted for morphine without producing adverse effects, including reward behaviors associated with abuse liability. These compounds have the potential to become important additional tools to treat opioid use disorders.

Morphine immunomodulation prolongs inflammatory and postoperative pain while the novel analgesic ZH853 accelerates recovery and protects against latent sensitization

Background

Numerous studies have identified the proinflammatory, pronociceptive effects of morphine which ultimately exacerbate pain. Our novel endomorphin analog ZH853 does not produce proinflammatory effects on its own and gives potent, long-lasting analgesia. This study investigates whether ZH853’s lack of interaction with the neuroimmune system reduces the risk of prolonged pain.

Methods

Adult male Sprague-Dawley rats were subjected to one of two treatment paradigms. Either (1) chronic pain followed by chronic treatment with morphine, ZH853 or vehicle, or (2) chronic drug administered prior to pain induction. Complete Freund’s adjuvant (CFA) was injected or paw incision surgery was performed on the left hind plantar foot pad. Drugs were administered through Alzet osmotic minipumps at a rate of 1 μl/h for 5 days at appropriate doses based on prior experiments. Animals were tested for mechanical allodynia and thermal hyperalgesia using von Frey filaments and the Hargreaves apparatus, respectively. Additionally, several gait parameters were measured using the CatWalk XT. When all animals had recovered from pain, 1 mg/kg of naltrexone was administered to test for development of latent sensitization (LS). A second set of animals was used to investigate dorsal horn inflammation following CFA and drug treatment. ANOVAs were used to assess differences between drug treatment groups.

Results

As expected, morphine increased and prolonged pain in all experiments compared to vehicle treatment. However, ZH853 treatment reduced the overall time spent in pain and the severity of pain scores compared to morphine. ZH853 not only reduced inflammation versus morphine treatment but also, in some instances, acted as an anti-inflammatory drug compared to vehicle treatment. Finally, ZH853 prevented the development of LS while vehicle- and morphine-treated animals showed robust relapse to pain.

Conclusions

ZH853 has a favorable side effect profile versus morphine and provides superior analgesia in a number of pain states. We now know that chronic use of this compound reduces time spent in a chronic pain state, the opposite of common opioids like morphine, and reduces the risk of LS, making ZH853 an excellent candidate for clinical development in humans for inflammatory and postoperative pain.

Novel Endomorphin Analogs Are More Potent and Longer-Lasting Analgesics in Neuropathic, Inflammatory, Postoperative, and Visceral Pain Relative to Morphine

Activation of the mu-opioid receptor provides the gold standard for pain relief, but most opioids used clinically have adverse effects that have contributed to an epidemic of overdose deaths. We recently characterized mu-opioid receptor selective endomorphin (EM) analogs that provide potent antinociception with reduction or absence of a number of side effects of traditionally prescribed opioids including abuse liability, respiratory depression, motor impairment, tolerance, and inflammation. The current study explores the effectiveness of these EM analogs relative to morphine in four major pain models by intrathecal as well as intravenous administration in male Sprague Dawley rats and subcutaneous administration in male CD-1 mice. In the spared nerve injury model of neuropathic pain, mechanical allodynia and mechanical hyperalgesia were assessed with von Frey and Randall-Selitto tests, respectively. In the paw incision model of postoperative pain, von Frey testing was used to assess mechanical allodynia and thermal hyperalgesia was evaluated with Hargreaves testing. In the Complete Freund's Adjuvant model of inflammatory pain, thermal hyperalgesia was assessed using Hargreaves testing. In CD-1 mice, visceral pain was assessed with the acetic acid writhing test. In all cases, EM analogs had equal or greater potency and longer duration of action relative to morphine. The data suggest that EM analogs, particularly analog 4 (ZH853), could provide effective therapy for a diverse spectrum of pain conditions with low risk of adverse side effects compared with currently used opioids such as morphine.

PERSPECTIVE

Novel EM analogs show equal or greater potency and effectiveness relative to morphine in multiple pain models. Together with substantially reduced side effects, including abuse liability, the compounds show promise for addressing the critical need for effective pain relief as well as reducing the opioid overdose epidemic.

Endomorphin analog analgesics with reduced abuse liability, respiratory depression, motor impairment, tolerance, and glial activation relative to morphine

Opioids acting at the mu opioid receptor (MOR) are the most effective analgesics, however adverse side effects severely limit their use. Of particular importance, abuse liability results in major medical, societal, and economic problems, respiratory depression is the cause of fatal overdoses, and tolerance complicates treatment and increases the risk of side effects. Motor and cognitive impairment are especially problematic for older adults. Despite the host of negative side effects, opioids such as morphine are commonly used for acute and chronic pain conditions. Separation of analgesia from unwanted effects has long been an unmet goal of opioid research. Novel MOR agonist structures may prove critical for greater success. Here we tested metabolically stable analogs of the endomorphins, endogenous opioids highly selective for the MOR. Compared to morphine, the analogs showed dramatically improved analgesia-to-side-effect ratios. At doses providing equal or greater antinociception than morphine in the rat, the analogs showed reduced a) respiratory depression, b) impairment of motor coordination, c) tolerance and hyperalgesia, d) glial p38/CGRP/P2X7 receptor signaling, and e) reward/abuse potential in both conditioned place preference and self-administration tests. Differential effects on glial activation indicate a mechanism for the relative lack of side effects by the analogs compared to morphine. The results suggest that endomorphin analogs described here could provide gold standard pain relief mediated by selective MOR activation, but with remarkably safer side effect profiles compared to opioids like morphine.

PPARγ activation blocks development and reduces established neuropathic pain in rats

Peroxisome proliferator-activated receptor gamma (PPARγ) is emerging as a new pharmacotherapeutic target for chronic pain. When oral (3-30 mg/kg/day in chow for 7 wk) or twice-daily intraperitoneal (1-10 mg/kg/day for 2 wk) administration began before spared nerve injury (SNI), pioglitazone, a PPARγ agonist, dose-dependently prevented multiple behavioral signs of somatosensory hypersensitivity. The highest dose of intraperitoneal pioglitazone did not produce ataxia or reductions in transient mechanical and heat nociception, indicating that inhibitory effects on hypersensitivity were not secondary to adverse drug-induced behaviors or antinociception. Inhibitory effects on hypersensitivity persisted at least one week beyond cessation of pioglitazone administration, suggestive of long-lasting effects on gene expression. Blockade of PPARγ with GW9662, an irreversible and selective PPARγ antagonist, dose-dependently reduced the inhibitory effect of pioglitazone on hypersensitivity, indicating a PPARγ-dependent action. Remarkably, a single preemptive injection of pioglitazone 15 min before SNI attenuated hypersensitivity for at least 2 weeks; this was enhanced with a second injection delivered 12 h after SNI. Pioglitazone injections beginning after SNI also reduced hypersensitivity, albeit to a lesser degree than preemptive treatment. Intraperitoneal pioglitazone significantly reduced the nerve injury-induced up-regulation of cd11b, GFAP, and p-p38 in the dorsal horn, indicating a mechanism of action involving spinal microglia and/or astrocyte activation. Oral pioglitazone significantly reduced touch stimulus-evoked phospho-extracellular signal-related kinase (p-ERK) in lamina I-II, indicating a mechanism of action involving inhibition of central sensitization. We conclude that pioglitazone reduces spinal glial and stimulus-evoked p-ERK activation and that PPARγ activation blocks the development of and reduces established neuropathic pain.

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.

A novel method to quantify histochemical changes throughout the mediolateral axis of the substantia gelatinosa after spared nerve injury: characterization with TRPV1 and substance P

Nerve injury dramatically increases or decreases protein expression in the spinal cord dorsal horn. Whether the spatial distribution of these changes is restricted to the central innervation territories of injured nerves or could spread to adjacent territories in the dorsal horn is not understood. To address this question, we developed a simple computer software-assisted method to precisely distinguish and efficiently quantify immunohistochemical staining patterns across the mediolateral axis of the dorsal horn 2 weeks after transection of either the tibial and common peroneal nerves (thus sparing the sural branch, spared nerve injury, [SNI]), the tibial nerve, or the common peroneal and sural nerves. Using thiamine monophosphatase (TMP) histochemistry, we determined that central terminals of the tibial, common peroneal, sural, and posterior cutaneous nerves occupy the medial 35%, medial-central 20%, central-lateral 20%, and lateral 25% of the substantia gelatinosa, respectively. We then used these calculations to show that SNI reduced the expression of SP and TRPV1 immunoreactivity within the tibial and peroneal innervation territories in the L4 dorsal horn, without changing expression in the uninjured, sural sector. We conclude that SNI-induced loss of SP and TRPV1 in central terminals of dorsal horn is restricted to injured fibers. Our new method enables direct comparison of injured and uninjured terminals in the dorsal horn so as to better understand their relative contributions to mechanisms of chronic pain.

PERSPECTIVE

A simple computer software-assisted algorithm was developed to precisely distinguish and efficiently quantify immunohistochemical staining patterns across the mediolateral axis of the dorsal horn after distal sciatic-branch transection. This method will facilitate a better understanding of the relative contribution of injured and uninjured terminals to mechanisms of chronic pain.

Isolation of a heptapeptide Val-Val-Tyr-Pro-Trp-Thr-Gln (valorphin) with some opiate activity

Bovine hypothalamic tissue was extracted and purified by solid phase extraction and several reversed-phase HPLC steps. The amino acid sequence of the purified peptide was determined by Edman degradation to be Val-Val-Tyr-Pro-Trp-Thr-Gln. This was confirmed by comparison of its chromatographic behavior with that of the synthetic peptide, and mass spectrometric analysis resulted in a mass identical to the calculated mass for this peptide. This heptapeptide shows homology with residues 32-38 of the beta-chain of bovine hemoglobin. The peptide inhibited the electrically induced contractions of the guinea pig ileum muscle preparation; this inhibition was reversible by naloxone. It also inhibited the binding of 125I-DAMGO (selective for mu receptors) to rat brain with an IC50 of 10 microM and the binding of 3H-DPDPE (selective for sigma receptors) with an IC50 of 185 microM. With two valines at the N-terminus and some opiate activity, valorphin seems a suitable name for this newly isolated peptide.

Blockade of micro-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, beta-funaltrexamine (5 microg/rat), into the medial thalamus 23 h prior to each morphine conditioning completely blocked the acquisition of CPP. However, microinjection of beta-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.

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.

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.

Mu-opioid receptor is present in dendritic targets of Endomorphin-2 axon terminals in the nuclei of the solitary tract

Endomorphins represent a group of endogenous opioid peptides with high affinity for the mu-opioid receptor. In the brainstem, Endomorphin-2 is found in trigeminal dorsal horn and the nuclei of the solitary tract, suggesting its presence in both nociceptive and visceral primary afferents. If Endomorphin-2 were an endogenous ligand for the mu-opioid receptor, we would expect to find the receptor at cellular sites in close association with the peptide. We used dual-labeling immunocytochemistry combined with electron microscopy to examine interactions between Endomorphin-2-immunoreactive and mu-opioid receptor-immunoreactive profiles within the nuclei of the solitary tract in the rat. Endomorphin-2-immunoreactivity was found primarily in unmyelinated axons and axon terminals in nuclei of the solitary tract and the majority of these terminals contained dense core vesicles. Endomorphin-2-immunoreactive axon terminals often formed asymmetric synapses with dendritic spines lacking mu-opioid receptor-immunoreactivity, but mu-opioid receptor-immunoreactivity was found in many of the larger dendritic targets of Endomorphin-2-immunoreactive terminals. Thus, mu-opioid receptor-immunoreactivity was found in the postsynaptic targets of Endomorphin-2-immunoreactive axon terminals, consistent with the hypothesis that Endomorphin-2 is an endogenous ligand for this receptor within the nuclei of the solitary tract. A small number of Endomorphin-2-immunoreactive somata, dendrites, and axon terminals also contained mu-opioid receptor-immunoreactivity. Cells that contain both the opioid peptide and its receptor may be a substrate for potential autoregulation of nuclei of the solitary tract neurons by opioid ligands. Finally, using tract tracing and confocal microscopy, we found Endomorphin-2-immunoreactivity in a subset of vagal afferents. Together these findings support the hypothesis that Endomorphin-2 is a ligand for the mu-opioid receptor within nuclei of the solitary tract and that the peptide is at least partially derived from primary visceral afferents.

Postnatal development of ectopic sensory fibers containing endomorphin-2 in the white matter of the spinal cord of a transgenic mouse expressing nerve growth factor in oligodendrocytes

Transgenic mice ectopically expressing nerve growth factor in oligodendrocytes have high levels of nerve growth factor immunoreactivity in the white matter of the spinal cord from birth until 2 months of age. The nerve growth factor over-expression leads to the appearance of ectopic substance P containing sensory fibers in the white matter of the spinal cord that persist throughout the life of the animal. These transgenic mice have been found to display hypersensitivity to a thermal stimulus following a sensitizing pinch stimulus known to release endogenous substance P. Surprisingly, this hypersensitivity is completely reversed following the administration of morphine, to the extent that transgenic mice become less sensitive to pain than the wild type mice given morphine. Endomorphin-2, an endogenous opioid peptide, has been found co-localized with substance P in primary sensory fibers in the spinal cord. In this study, we show that the ectopic fibers also express endomorphin-2, and describe the postnatal development of such expression, as detected by immunocytochemistry. We confirmed that endomorphin-2 expression starts later in the postnatal period than substance P. Surprisingly, transgenic animals had delayed appearance of endomorphin-2 in the superficial dorsal horn, compared with wild type, and expressed particularly high levels of endomorphin-2 immunoreactivity in the ectopic fibers from postnatal days 10-30, coinciding with the peak of nerve growth factor expression in oligodendrocytes. Endomorphin-2 immunoreactivity was still readily detected in ectopic fibers of 120-day-old animals. Furthermore, we detected immunoreactivity for the mu-opioid receptor in the ectopic fibers, where it was co-localized with endomorphin-2 immunoreactivity. In the superficial dorsal horn, there were no apparent differences in the distribution and intensity of mu-opioid receptor immunoreactivity between wild type and transgenic animals. Taken together, these data could provide an explanation for the enhanced effect of opioid analgesics in transgenic mice, when compared with control mice, as well as provide the basis for studies of the postnatal development of the hyperalgesia and allodynia demonstrated by these animals.

Internalization and down-regulation of mu opioid receptors by endomorphins and morphine in SH-SY5Y human neuroblastoma cells

The human neuroblastoma cell line, SH-SY5Y, was used to examine the effects of morphine and the endogenous opioid peptides, endomorphin-1 (EM-1) and endomorphin-2 (EM-2), on mu opioid receptor (MOR) internalization and down-regulation. Treatment for 24 h with EM-1, EM-2 or morphine at 100 nM, 1 microM and 10 microM resulted in a dose-dependent down-regulation of mu receptors. Exposure of cells to 10 microM EM-1 for 2.5, 5 and 24 h resulted in a time-dependent down-regulation of mu receptors. Down-regulation of mu receptors by morphine and EM-1 was blocked by treatment with hypertonic sucrose, consistent with an endocytosis-dependent mechanism. Sensitive cell-surface binding studies with a radiolabeled mu antagonist revealed that morphine was able to induce internalization of mu receptors naturally expressed in SH-SY5Y cells. EM-1 produced a more rapid internalization of mu receptors than morphine, but hypertonic sucrose blocked the internalization induced by each of these agonists. This study demonstrates that, like morphine, the endomorphins down-regulate mu opioid receptors in a dose- and time-dependent manner. This study also demonstrates that morphine, as well as EM-1, can induce rapid, endocytosis-dependent internalization of mu opioid receptors in SH-SY5Y cells. These results may help elucidate the ability of mu agonists to regulate the number and responsiveness of their receptors.

Endomorphins exit the brain by a saturable efflux system at the basolateral surface of cerebral endothelial cells

Endomorphin-1 (EM-1) and endomorphin-2 (EM-2) are two highly selective mu-opiate receptor agonists. We recently demonstrated that EM-1 and EM-2 have a saturable transport system from brain-to-blood in vivo. Since the endothelial cells are the main component of the non-fenestrated microvessels of the blood-brain barrier (BBB), we examined whether these endogenous tetrapeptides have a saturable transport system in cultured cerebral endothelial cells. EM-1 and EM-2 binding and transport were studied in a transwell system in which primary mouse endothelial cells were co-cultured with rat glioma cells. We found that binding of both endomorphins was greater on the basolateral than the apical cell surface. Flux of EM-1 and EM-2 occurred predominantly in the basolateral to apical direction, each showing self-inhibition with an excess of the respective endomorphin. Transport was not influenced by the addition of the P-glycoprotein inhibitor, cyclosporin A. Neither the mu-opiate receptor agonist DAMGO nor the delta-opiate receptor agonist DPDPE had any effect on the transport. Thus, the results show that a saturable transport system for EM-1 and EM-2 occurs at the level of endothelial cells of the BBB, and unlike beta-endorphin and morphine, P-glycoprotein is not needed for the brain-to-blood transport. Cross-inhibition of the transport of each endomorphin by the other suggests a shared transport system that is different from mu- or delta-opiate receptors. As endormorphins are mainly produced in the CNS, the presence of the efflux system at the BBB could play an important role in pain modulation and neuroendocrine control.

Analgesic tolerance and cross-tolerance to i.c.v. endomorphin-1, endomorphin-2, and morphine in mice

The present study examined the development of analgesic tolerance to endomorphin-1 (EM1), endomorphin-2 (EM2), and morphine, and cross-tolerance among these drugs. Male Swiss Webster mice were injected i.c.v. with EM1, EM2, morphine, or vehicle once daily for 5 days, and tested for analgesia in the tail flick test. To determine the extent of cross-tolerance, on the sixth day mice from each of the above groups received i.c.v. injections of EM1, EM2, morphine, or vehicle before analgesic testing. The development of tolerance to EM1 and EM2 closely resembled that of morphine. Complete, symmetrical cross-tolerance was observed between all drugs in the study. These results demonstrate a time-course and extent of tolerance similar to morphine, and support a common mechanism of action through the mu-opioid receptor.

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.

Morphological evidence of endomorphin as an agonist for the mu-opioid receptor in the rat spinal cord

Endomorphin 2 is a newly discovered peptide that has high affinity and specificity for the mu-opioid receptor. One criterion for establishing that endomorphin serves as an endogenous agonist for the mu receptor is that it be anatomically distributed in close proximity to that receptor. We tested this idea with a preembedding double immunostaining technique to study synaptic relationships between them. The distributions of both endomorphin 2 and the mu-opioid receptor were similar in the dorsal horn of the cervical spinal cord at the light microscopic level. At the electron microscopic level, axon terminals with dense-cored vesicles containing endomorphin 2-like immunoreactivity were observed making mostly asymmetrical synapses on profiles immunostained for the mu-opioid receptor. The immunostaining for the mu-opioid receptor was found mostly in postsynaptic membranes in profiles having dendrite-like appearance. The results support the idea that endomorphin 2 is an endogenous ligand for the mu-opioid receptor. Furthermore, the results indicate that such a role is mediated at least in part through synaptic relationships.

Electron microscopic examination of the endomorphin 2-like immunoreactive neurons in the rat hypothalamus

Endomorphins are endogenous opioid peptides with high affinity and selectivity for the mu-opioid receptor. In the present study, we examined the morphology of the endomorphin 2-like immunoreactive (EM2-LI) neurons in the hypothalamus at the light and electron microscopic levels. At the light microscopic level, EM2-LI neurons were found mostly distributed in the regions between the dorsomedial and ventromedial hypothalamic nuclei and the region near the third ventricle. At the electron microscopic level, EM2-LI perikarya could be divided into two groups. Type I perikarya contained relatively undeveloped endoplasmic reticulum and Golgi apparatus while type II perikarya contained well-developed rough-surfaced endoplasmic reticulum and Golgi apparatus. Both type I and type II neurons contained numerous EM2-LI dense-cored vesicles. Type II perikarya and dendrites received synapses and showed immunoreactivity in the endoplasmic reticulum and Golgi apparatus. EM2-LI axon terminals formed synapses with both immunonegative and immunopositive dendrites. In some cases, the axon terminals contained both immunonegative and immunopositive dense-cored vesicles. EM2-LI neurons often had synaptic relationships with neurons containing immunonegative dense-cored vesicles. Myelinated axon shafts containing EM2-LI were also found. This first demonstration of the ultrastructure and synaptic relationships of EM2-LI neurons in the hypothalamus provides morphological evidence that suggests (1) endomorphin 2-containing neurons modulate physiological function through synaptic relationships; (2) endomorphin 2 may coexist with other neurotransmitters in the same neurons; and (3) endomorphin 2-containing neurons could modulate other endomorphin 2-containing neurons as well as those containing other neurotransmitters.

Endomorphin-1 and -2 immunoreactive cells in the hypothalamus are labeled by fluoro-gold injections to the ventral tegmental area

Endomorphin-1 and -2 (EM1, EM2) are endogenous opioids with high affinity and selectivity for the mu-opioid receptor. Cells expressing EM-like immunoreactivity (EM-LI) are present in the hypothalamus, and fibers containing EM-LI project to many brain regions, including the ventral tegmental area (VTA). The VTA is one of the most sensitive brain regions for the rewarding and locomotor effects of opioids. It contains mu-opioid receptors, which are thought to mediate gamma-aminobutyric acid-dependent disinhibition of dopamine transmission to the nucleus accumbens. We investigated whether hypothalamic EM-LI cells project to the VTA, where they could play a natural role in this circuitry. The retrograde tracer Fluoro-Gold (FG) was microinjected into the anterior or posterior VTA in rats. Nine days later, colchicine was injected, and 24 hours later, the animals were perfused and processed for fluorescence immunocytochemistry. Numerous FG-labeled cells were detected in the hypothalamus. Both EM1-LI and EM2-LI cells were present in the periventricular nucleus, between the dorsomedial and ventromedial hypothalamus and between the ventromedial and arcuate nuclei. Subpopulations of EM1-LI and EM2-LI cells were labeled by FG. Injections of FG to the anterior and posterior VTA were both effective in producing double-labeled cells, and an anterior-posterior topographical organization between the VTA and hypothalamus was observed. The results support the idea that some endomorphin-containing neurons in the hypothalamus project to the VTA, where they may modulate reward and locomotor circuitry.

Endomorphin-2 immunoreactivity in the cervical dorsal horn of the rat spinal cord at the electron microscopic level

Endomorphin-2 is a newly discovered endogenous opioid peptide with high affinity and selectivity for the micro-opioid receptor, and potent analgesic activity, particularly in the spinal cord. Using immunoelectron microscopy, we examined the ultrastructure of the endomorphin-2-like immunoreactive processes and their synaptic relationships in the spinal cord. Endomorphin-2-like immunopositive dense-cored vesicles were observed in many axon terminals, and, in a few cases, were observed together with immunonegative dense-cored vesicles. Immunopositive axons with or without myelination were also observed. The endomorphin-2-like immunoreactive axon terminals formed synapses with both immunopositive and immunonegative processes. Most synapses were asymmetrical, but symmetrical synapses were also found. Examples of axo-dendritic, axo-somatic and axo-axonic contacts were observed. This first demonstration of the ultrastructure and synaptic relationships of endomorphin-2-like immunoreactive axon terminals in the spinal cord dorsal horn provides morphological evidence that this peptide functions as a transmitter regulating pain processes.

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

The present study characterizes the relationship between the endogenous mu opioid peptides endomorphin-1 (EM-1) and endomorphin-2 (EM-2) and several splice variants of the cloned mu opioid receptor (MOR-1) encoded by the mu opioid receptor gene (Oprm). Confocal laser microscopy revealed that fibers containing EM-2-like immunoreactivity (-LI) were distributed in close apposition to fibers showing MOR-1-LI (exon 4-LI) and to MOR-1C-LI (exons 7/8/9-LI) in the superficial laminae of the lumbar spinal cord. We also observed colocalization of EM-2-LI and MOR-1-LI in a few fibers of lamina II, and colocalization of EM-2-LI and MOR-1C-LI in laminae I-II, and V-VI. To assess the functional relevance of the MOR-1 variants in endomorphin analgesia, we examined the effects of antisense treatments that targeted individual exons within the Oprm1 gene on EM-1 and EM-2 analgesia in the tail flick test. This antisense mapping study implied mu opioid receptor mechanisms for the endomorphins are distinct from those of morphine or morphine-6beta-glucuronide (M6G).

The alpha9/alpha10-containing nicotinic ACh receptor is directly modulated by opioid peptides, endomorphin-1, and dynorphin B, proposed efferent cotransmitters in the inner ear

Opioid peptides have been detected in the auditory and vestibular efferent neurons where they colocalize with the major neurotransmitter, acetylcholine. We investigated the function of opioids to modulate neurotransmission mediated by hair cell's alpha9/alpha10-containing nicotinic acetylcholine receptors (alpha9/alpha10nAChRs). The endogenous opioid peptides, endomorphin-1 (mu agonist) and dynorphin B (kappa agonist), but not a delta agonist [D-Pen2,D-Pen-5]enkephalin, inhibited the acetylcholine-evoked currents in frog saccular hair cells and rat inner hair cells. This inhibition was noncompetitive, voltage-independent, and was accompanied by an acceleration of the rate of current decay. Selective mu- and kappa-opioid receptor antagonists did not block the inhibition, although partial reduction by naloxone was observed. All opioid antagonists tested also reduced the acetylcholine response. Endomorphin-1 and dynorphin B inhibited the acetylcholine-evoked currents in alpha9/alpha10-expressing Xenopus oocytes. Because oocytes lack opioid receptors, it provides strong evidence for the direct interaction of opioid peptides with alpha9/alpha10nAChR.

CONCLUSION

alpha9/alpha10nAChR is a target for modulation by endomorphin-1 and dynorphin B, efferent cotransmitters in the inner ear.

Isolation and distribution of endomorphins in the central nervous system

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2, EM-1) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2, EM-2) have the highest affinity and selectivity for the mu-opioid receptor (MOP-R) of all known mammalian opioids. They were isolated from bovine and human brain, and are structurally distinct from the other endogenous opioids. Both EM-1 and EM-2 have potent antinociceptive activity in a variety of animal models of acute, neuropathic and allodynic pain. They regulate cellular signaling processes in a manner consistent with MOP-R-mediated effects. The EMs are implicated in the natural modulation of pain by extensive data localizing EM-like immunoreactivity (EM-LI) near MOP-Rs in several regions of the nervous system known to regulate pain. These include the primary afferents and their terminals in the spinal cord dorsal horn, where EM-2 is well-positioned to modulate pain in its earliest stages of perception. In a nerve-injury model of chronic pain, a loss of spinal EM2-LI occurs concomitant with the onset of chronic pain. The distribution of the EMs in other areas of the nervous system is consistent with a role in the modulation of diverse functions, including autonomic, neuroendocrine and reward functions as well as modulation of responses to pain and stress. Unlike several other mu opioids, the threshold dose of EM-1 for analgesia is well below that for respiratory depression. In addition, rewarding effects of EM-1 can be separated from analgesic effects. These results indicate a favorable therapeutic profile of EM-1 relative to other mu opioids. Thus, the pharmacology and distribution of EMs provide new avenues both for therapeutic development and for understanding the neurobiology of opioids.

Morphological studies of the endomorphinergic neurons in the central nervous system

Endomorphins (EMs) are newly found endogenous opioid peptides. Both endomorphin-1 (EM-1) and -2 (EM-2) are composed of four amino acids. Their high affinity and specificity for mu-opioid receptors have been confirmed by many physiological and pharmacological studies. In the present minireview, we discuss the distribution and localization of these peptides. While EM-2 is more prevalent in the spinal cord and lower brainstem, EM-1 is more widely and densely distributed throughout the brain than EM-2. We also discuss the possible coexistence of EM with other neurotransmitters. Finally, we introduce some new results regarding the ultrastructure and synaptic relationships of EM-2 obtained by the immunoelectron microscopic method.

Differential antagonism of endomorphin-1 and endomorphin-2 supraspinal antinociception by naloxonazine and 3-methylnaltrexone

To determine if different subtypes of mu-opioid receptors were involved in antinociception induced by endomorphin-1 and endomorphin-2, the effect of pretreatment with various mu-opioid receptor antagonists beta-funaltrexamine, naloxonazine and 3-methylnaltrexone on the inhibition of the paw-withdrawal induced by endomorphin-1 and endomorphin-2 given intracerebroventricularly (i.c.v.) were studied in ddY male mice. The inhibition of the paw-withdrawal induced by i.c.v. administration of endomorphin-1, endomorphin-2 or DAMGO was completely blocked by the pretreatment with a selective mu-opioid receptor antagonist beta-funaltrexamine (40 mg/kg), indicating that the antinociception induced by all these peptides are mediated by the stimulation of mu-opioid receptors. However, naloxonazine, a mu1-opioid receptor antagonist pretreated s.c. for 24h was more effective in blocking the antinociception induced by endomorphin-2, than by endomorphin-1 or DAMGO given i.c.v. Pretreatment with a selective morphine-6 beta-glucuronide blocker 3-methylnaltrexone 0.25mg/kg given s.c. for 25 min or co-administration of 3-methylnaltrexone 2.5 ng given i.c.v. effectively attenuated the antinociception induced by endomorphin-2 given i.c.v. and co-administration of 3-methylnaltrexone shifted the dose-response curves for endomorphin-2 induced antinociception to the right by 4-fold. The administration of 3-methylnaltrexone did not affect the antinociception induced by endomorphin-1 or DAMGO given i.c.v. Our results indicate that the antinociception induced by endomorphin-2 is mediated by the stimulation of subtypes of mu-opioid receptor, which is different from that of mu-opioid receptor subtype stimulation by endomorphin-1 and DAMGO.

Role of nitric oxide in mediating vasodilator responses to opioid peptides in the rat

1. Endomorphins 1 and 2, endogenous ligands for the mu-opioid receptor, and nociceptin (orphanin FQ; OFQ), an endogenous ligand for the ORL1 receptor, have vasodilator activity in the vascular bed of the hindquarters of the rat. In the present study, the role of nitric oxide (NO), vasodilator prostaglandins and the opening of KATP channels in mediating vasodilator responses to these novel agonists was investigated in the rat. 2. Under constant-flow conditions, injections of endomorphins 1 and 2, PL017 ([N-MePhe3,D-Pro4]-morphiceptin), nociceptin and Tyr-D-Ala-Gly-MePhe-Gly(ol)-enkephalin (DAMGO) produced dose-dependent decreases in hindquarters perfusion pressure. Vasodilator responses to endomorphin 1 and 2, acetylcholine and adrenomedullin, were attenuated by the NO synthase inhibitor NG-nitro-L-arginine methyl ester (L-NAME) at a time when vasodilator responses to nociceptin, adrenomedullin and the NO donor diethylamine/NO were not altered. 3. Vasodilator responses to endomorphins 1 and 2, nociceptin, PL017 and DAMGO were not altered after administration of sodium meclofenamate at a time when vasodilator responses to arachidonic acid were reduced significantly or after administration of U-37883A at a time when vasodilator responses to levcromakalim were reduced significantly. 4. The results of these studies indicate that vasodilator responses to endomorphins 1 and 2, PL017 and DAMGO are mediated, in large part, by the release of NO, whereas vasodilator responses to nociceptin are mediated by an L-NAME-insensitive mechanism. Moreover, these results demonstrate that the vasodilator responses to these peptides are not due to the release of vasodilator prostaglandins or the opening of KATP channels in the hindquarters vascular bed of the rat.

Endomorphins, Met-enkephalin, Tyr-MIF-1, and the P-glycoprotein efflux system

The P-glycoprotein (P-gp) transport system, responsible for the efflux of many therapeutic drugs out of the brain, recently has been shown to transport the endogenous brain opiate endorphin. We used P-gp knockout mice (Mdr1a) and their controls to determine where P-gp is involved in the saturable efflux systems of four other endogenous opiate-modulating peptides across the blood-brain barrier (BBB). After injection of endomorphin-1 (Tyr-Pro-Trp-Phe-NH(2)), endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)), Met-enkephalin (Tyr-Gly-Gly-Phe-Met-OH), and Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH(2)) into the lateral ventricle of the mouse brain, residual radioactivity was measured at 0, 2, 5, 10, and 20 min later. The results showed no difference in the disappearance of any of these peptides from the brains of knockout mice compared with their controls. This demonstrates that unlike endorphin and morphine, P-gp does not seem to be required for the brain-to-blood transport of the endomorphins, Met-enkephalin, or Tyr-MIF-1 across the BBB.

Endomorphin1-like immunoreactivity in the limbic system of Syrian hamsters (Mesocricetus auratus)

Recently, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2; EM1), an endogenous peptide that has high affinity and selectivity for the mu-opiate receptor, has been shown to modulate emotional behavior in mice and social behavior in Syrian hamsters. Endomorphin-1 (EM1) is present throughout the central nervous system in rats, mice, and guinea pigs; however, the distribution of EM1 in hamsters has not been described. The purpose of the present study was to investigate the distribution of EM1-like immunoreactivity (EM1L-IR) in the limbic system of Syrian hamsters using immunocytochemistry. Perikarya containing EM1L-IR were present in the anterior area, dorsomedial, ventromedial, periventricular, posterior, and arcuate nuclei of the hypothalamus. Fibers expressing EM1L-IR were present in the nucleus accumbens, caudate putamen, septum, bed nucleus of the stria terminalis, amygdaloid complex, and hypothalamus. The distribution of EM1 suggests a potential endogenous role for this peptide in major processes modulated by opiates, including affective states and social behavior.

Decreases in endomorphin-2-like immunoreactivity concomitant with chronic pain after nerve injury

Nerve injury often leads to chronic, sometimes excruciating, pain. The mechanisms contributing to this syndrome include neurochemical plasticity in neurons involved in the earliest stages of pain transmission. Endomorphin-2 (Tyr-Pro-Phe-Phe-NH(2)) is an endogenous morphine-like substance that binds to the mu-opioid receptor with high affinity and selectivity. Endomorphin-2-like immunoreactivity (LI) is present in the superficial layers of the dorsal horn in the spinal cord and in primary afferents, suggesting a role for this peptide in pain transmission. To determine whether spinal endomorphin-2-LI is altered in an animal model of chronic pain, the left sciatic nerve of Swiss Webster and ICR mice was ligated in a modified Seltzer model of nerve injury. Changes in endomorphin-2-LI were assessed by immunocytochemistry at 2, 4 and 14 days after nerve injury. The side of the spinal cord ipsilateral to the nerve injury exhibited a dramatic decrease in endomorphin-2-LI relative to the contralateral side and to control animals. The change was restricted to the medial dorsal horn in the lumbar segments innervated by the sciatic nerve. Substance P-LI showed a small decrease, while calcitonin gene-related peptide-LI was unchanged. Both thermal hyperalgesia, as evidenced by significantly decreased paw withdrawal latencies, and decreased endomorphin-2-LI were observed within 2 days of injury and were most pronounced at 2 weeks after injury. The decrease in endomorphin-2-LI during the development of chronic pain is consistent with the loss of an inhibitory influence on pain transmission. These results provide the first evidence that reduction of an endogenous opioid in primary afferents is associated with injury-induced chronic pain.

The effects of endomorphin-1 on conditioned defeat in Syrian hamsters (Mesocricetus auratus)

The present study examined the effect of endomorphin-1 (EM1), an endogenous opioid with a high affinity for the mu opiate receptor, on conditioned defeat. Conditioned defeat is a phenomenon in which hamsters that have been defeated subsequently fail to exhibit normal territorial aggression and instead display submissive/defensive behaviors even when paired with a non-aggressive intruder. In experiment 1, animals were placed in the home cage of a larger resident for 15 min and were defeated. After 24 h, animals received a 3-microl injection of EM1 (0.0, 0.3, 3.0, or 10 nmol) into the left lateral cerebral ventricle 5 min before a smaller non-aggressive intruder was placed in the home cage of the experimental animal. In experiment 2, animals were infused with EM1 immediately after the initial defeat and were paired with a non-aggressive intruder 24 h later as in experiment 1. EM1 reduced the duration of submissive/defensive behavior in experiment 1 (P<0.05) but not in experiment 2 (P>0.05). These data support the hypothesis that the highly selective mu receptor agonist endomorphin-1 modulates the expression of conditioned defeat, but provides no support for the hypothesis that endomorphin-1 modulates the consolidation of conditioned defeat.

Differential antinociceptive effects induced by intrathecally administered endomorphin-1 and endomorphin-2 in the mouse

Two highly selective mu-opioid receptor agonists, endomorphin-1 and endomorphin-2, have been identified and postulated to be endogenous ligands for mu-opioid receptors. Intrathecal (i.t.) administration of endomorphin-1 and endomorphin-2 at doses from 0.039 to 5 nmol dose-dependently produced antinociception with the paw-withdrawal test. The paw-withdrawal inhibition rapidly reached its peak at 1 min, rapidly declined and returned to the pre-injection levels in 20 min. The inhibition of the paw-withdrawal responses to endomorphin-1 and endomorphin-2 at a dose of 5 nmol observed at 1 and 5 min after injection was blocked by pretreatment with a non-selective opioid receptor antagonist naloxone (1 mg/kg, s.c.). The antinociceptive effect of endomorphin-2 was more sensitive to the mu (1)-opioid receptor antagonist, naloxonazine than that of endomorphin-1. The endomorphin-2-induced paw-withdrawal inhibition at both 1 and 5 min after injection was blocked by pretreatment with kappa-opioid receptor antagonist nor-binaltorphimine (10 mg/kg, s.c.) or the delta(2)-opioid receptor antagonist naltriben (0.6 mg/kg, s.c.) but not the delta(1)-opioid receptor antagonist 7-benzylidine naltrexone (BNTX) (0.6 mg/kg s.c.). In contrast, the paw-withdrawal inhibition induced by endomorphin-1 observed at both 1 and 5 min after injection was not blocked by naloxonazine (35 mg/kg, s.c.), nor-binaltorphimine (10 mg/kg, s.c.), naltriben (0.6 mg/kg, s.c.) or BNTX (0.6 mg/kg s.c.). The endomorphin-2-induced paw-withdrawal inhibition was blocked by the pretreatment with an antiserum against dynorphin A-(1-17) or [Met(5)]enkephalin, but not by antiserum against dynorphin B-(1-13). Pretreatment with these antisera did not affect the endomorphin-1-induced paw-withdrawal inhibition. Our results indicate that endomorphin-2 given i.t. produces its antinociceptive effects via the stimulation of mu (1)-opioid receptors (naloxonazine-sensitive site) in the spinal cord. The antinociception induced by endomophin-2 contains additional components, which are mediated by the release of dynorphin A-(1-17) and [Met(5)]enkephalin which subsequently act on kappa-opioid receptors and delta(2)-opioid receptors to produce antinociception.

Regional differences in peptide degradation by rat cerebral microvessels: a potential novel regulatory mechanism for communication between blood and brain

The blood-brain barrier (BBB), composed of the microvessels of cerebral capillary endothelial cells, regulates the passage of peptides into the brain in several ways, mainly by saturable transport or passive diffusion. Here we describe an additional mechanism by which this regulatory function can occur. Cerebral microvessels were isolated from different regions of the brain and incubated with the mu-opiate selective endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) or the opiate-modulating Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2), both tetrapeptides selectively tritiated at the Pro. Degradation was determined by HPLC. For both peptides, the metabolism by microvessels from the cerebral cortex was much greater than that by microvessels from the hypothalamus or pons. For endomorphin-1, the least degradation was in the pons; for Tyr-MIF-1 there was no difference in metabolism by microvessels from the pons or hypothalamus. The results show a novel mechanism at the BBB by which the BBB can selectively regulate the activity of different peptides in different regions of the brain.

Saturable brain-to-blood transport of endomorphins

Opiate-modulating tetrapeptides such as tyrosine-melanocyte-stimulating hormone-release inhibiting factor-1 (Tyr-MIF-1; Tyr-Pro-Leu-Gly-NH2) and Tyr-W-MIF-1 (Tyr-Pro-Trp-Gly-NH2) are saturably transported from brain to blood. We examined whether two recently described endogenous opiate tetrapeptides with similar structures, the mu-specific endomorphins, also are transported across the blood-brain barrier (BBB). We found that the efflux rates of endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) were each self-inhibited by an excess of the respective endomorphin, thereby defining saturable transport. Cross-inhibition of the transport of each endomorphin by the other indicated shared transport. By contrast, no inhibition of the efflux of either endomorphin resulted from coadministration of Tyr-MIF-1, indicating that peptide transport system-1 (PTS-1) was not involved. Tyr-W-MIF-1, which is partially transported by PTS-1, significantly (P<0.01) decreased the transport of endomorphin-1 and tended (P=0.051) to decrease the transport of endomorphin-2, consistent with its role as both an opiate and antiopiate. Although involved in modulation of pain, coinjection of calcitonin gene-related peptide or constriction of the sciatic nerve did not appear to inhibit endomorphin efflux. Thus, the results demonstrate the existence of a new efflux system across the BBB which saturably transports endomorphins from brain to blood.

Administration of FGF-1 through transfected cells alleviates MPTP toxicity in mice

The use of genetically modified cells to deliver growth factors has been proposed as a possible treatment for neurodegeneration, including Parkinson's disease. Here we demonstrate that the implantation of fibroblasts genetically modified to secrete fibroblast growth factor-1 (FGF) increased striatal dopamine concentrations in a 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-treated mouse model of Parkinson's disease.

In vitro demonstration of a saturable transport system for leptin across the blood-brain barrier

A saturable blood-to-brain transport system for leptin across the blood-brain barrier (BBB) has been observed in vivo. Since the main component of the non-fenestrated microvessels of the BBB is the endothelial cell, we established an in vitro culture system of these cerebrovascular cells to study leptin transport and to determine whether the self-inhibition of leptin transport characteristic of a saturable system occurs at this level. The results show that 125I-leptin crossed from the luminal to abluminal side of a monolayer of cerebral microvessel cells significantly faster than the albumin and lactalbumin controls. This transport of 125I-leptin across an in vitro BBB was significantly faster than in the opposite direction and was dose-relatedly inhibited by the addition of unlabeled leptin. Thus, the results establish that the saturable transport system for leptin across the BBB occurs at the level of the endothelial cells of the BBB.

Dissociation of analgesic and rewarding effects of endomorphin-1 in rats

The mu-receptor is the primary mediator of the effects of morphine and the endogenous opiates, endomorphin-1 and endomorphin-2. Here we demonstrate a dissociation of the analgesic and rewarding effects of endomorphin-1 in rats. Tail-flick results revealed that endomorphin-1 produced significant analgesic effects within 10-min after injection. However, it failed to show reward properties in the standard 45- min conditioned place preference (CPP) paradigm or in an abbreviated 10-min pairing which paralleled the time frame of the tail-flick findings. Morphine induced both analgesia and reward. Endomorphin-1 therefore is the first mu opiate shown to produce potent analgesia in the absence of reward behavior, and thus may have significant clinical potential.

Differential antagonism of endomorphin-1 and endomorphin-2 spinal antinociception by naloxonazine and 3-methoxynaltrexone

To determine the role of spinal mu-opioid receptor subtypes in antinociception induced by intrathecal (i.t.) injection of endomorphin-1 and -2, we assessed the effects of beta-funaltrexamine (a selective mu-opioid receptor antagonist) naloxonazine (a selective antagonist at the mu(1)-opioid receptor) and a novel receptor antagonist (3-methoxynaltrexone) using the paw-withdrawal test. Antinociception of i.t. endomorphins and [D-Ala(2), MePhe(4), Gly(ol)(5)]enkephalin (DAMGO) was completely reversed by pretreatment with beta-funaltrexamine (40 mg/kg s.c.). Pretreatment with a variety of doses of i.t. or s.c. naloxonazine 24 h before testing antagonized the antinociception of endomorphin-1, -2 and DAMGO. Judging from the ID(50) values of naloxonazine, the antinociceptive effect of endomorphin-2 was more sensitive to naloxonazine than that of endomorphin-1 or DAMGO. The selective morphine-6beta-glucuronide antagonist, 3-methoxynaltrexone, which blocked endomorphin-2-induced antinociception at each dose (0.25 mg/kg s.c. or 2.5 ng i.t.) that was inactive against DAMGO, did not affect endomorphin-1-induced antinociception but shifted the dose-response curve of endomorphin-2 3-fold to the right. These findings may be interpreted as indicative of the existence of a novel mu-opioid receptor subtype in spinal sites, where antinociception of morphine-6beta-glucuronide and endomorphin-2 are antagonized by 3-methoxynaltrexone. The present results suggest that endomorphin-1 and endomorphin-2 may produce antinociception through different subtypes of mu-opioid receptor.

Differential cardiorespiratory effects of endomorphin 1, endomorphin 2, DAMGO, and morphine

The novel endogenous mu-opioid receptor (MOR) agonists endomorphin 1 (EM1) and 2 (EM2) were tested for their cardiorespiratory effects in conscious, freely behaving rats. After systemic (intravenous) administration of EM1, EM2, or the selective MOR agonist DAMGO, analgesia, minute ventilation (V E), heart rate (HR) and mean arterial blood pressure (BP) were measured. The threshold dose for analgesia was similar for all 3 peptides ( approximately 900 nmol/kg). All 3 compounds elicited biphasic V E responses, with marked, short-lived V E depressions (4-6 s) followed by more sustained V E increases (10-12 min). However, compared with responses elicited by EM2 or DAMGO, EM1 decreased V E only at higher doses, and produced greater V E stimulation. Morphine produced a V E decrease, but no subsequent V E increase. EM2 and DAMGO decreased HR and BP, while EM1 decreased HR, but did not decrease BP in conscious rats at doses up to 9,600 nmol/kg. In anesthetized rats, all 3 peptides decreased HR and BP. The decreases in V E, HR, and BP were blocked by the MOR antagonist, naloxone HCI (NIx). Only the HR and BP responses, however, were blocked by naloxone-methiodide (MeNIx), indicating central mediation of V E responses and peripheral mediation of cardiovascular responses. We conclude that MOR-selective compounds vary in their cardiorespiratory response characteristics which could be linked to differential cellular actions. The results support the concept that the analgesic, respiratory, and cardiovascular effects of MOR agonists can be dissociated and that EM1-like compounds could provide the basis for novel, safer analgesics.

Analgesic effects of endomorphin-1 and endomorphin-2 in the formalin test in mice

Two recently isolated peptides, endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), are highly selective micro-opioid receptor agonists with analgesic actions in the tail-flick test. To further assess the analgesic properties of these peptides, the effects of endomorphin-1, endomorphin-2, and morphine were examined in the formalin test. Male Swiss Webster mice were injected i.c.v. with endomorphin-1, endomorphin-2, or morphine (0, 1, 3, 10 microg) 5 min before injection of 20 microl of 5% formalin s.c. into the plantar surface of one hind-paw. The mice were observed for 60 min after formalin injection. Endomorphin-1 and endomorphin-2 produced dose-dependent analgesia that was shorter in duration than for morphine. Increased locomotion was observed after morphine, but not after endomorphin-1 or endomorphin-2. These findings extend previous results and suggest that endomorphins may have therapeutic potential for the treatment of acute pain.

The ontogeny of endomorphin-1- and endomorphin-2-like immunoreactivity in rat brain and spinal cord

Endomorphin-1 and endomorphin-2 are recently described peptides with high affinity and specificity for the mu opioid receptor. They are believed to be the endogenous ligands for that receptor. We describe the maturation of the endomorphins in brain and spinal cord using recently characterized antibodies to each. Endomorphin-1-like immunoreactivity was examined in brain, focusing on the periaqueductal gray of the midbrain and the diagonal band of Broca; endomorphin-2-like immunoreactivity is reported for the medulla and spinal cord. In these regions, and in all other regions studied but not described in this paper, the endomorphins were not seen at birth or at 3 days of age. Staining was present in 7-day-old and older animals. At these early ages, the pattern and density of staining are not fully developed, but appear complete by 21 days of age. The results suggest that both endomorphin-1 and endomorphin-2 develop relatively late compared to other opioid peptides.

Endomorphins: novel endogenous mu-opiate receptor agonists in regions of high mu-opiate receptor density

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2, EM-1) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2, EM-2) are peptides recently isolated from brain that show the highest affinity and selectivity for the mu (morphine) opiate receptor of all the known endogenous opioids. The endomorphins have potent analgesic and gastrointestinal effects. At the cellular level, they activate G-proteins (35S-GTP gamma-S binding) and inhibit calcium currents. Support for their role as endogenous ligands for the mu-opiate receptor includes their localization by radioimmunoassay and immunocytochemistry in central nervous system regions of high mu receptor density. Intense EM-2 immunoreactivity is present in the terminal regions of primary afferent neurons in the dorsal horn of the spinal cord and in the medulla near high densities of mu receptors. Chemical (capsaicin) and surgical (rhizotomy) disruption of nociceptive primary afferent neurons depletes the immunoreactivity, implicating the primary afferents as the source of EM-2. Thus, EM-2 is well-positioned to serve as an endogenous modulator of pain in its earliest stages of perception. In contrast to EM-2, which is more prevalent in the spinal cord and lower brainstem, EM-1 is more widely and densely distributed throughout the brain than EM-2. The distribution is consistent with a role for the peptides in the modulation of diverse functions, including autonomic, neuroendocrine, and reward functions as well as modulation of responses to pain and stress.

Comparison of active conformations of the insectatachykinin/tachykinin and insect kinin/Tyr-W-MIF-1 neuropeptide family pairs

A comparison of solution conformations of active, restricted-conformation analogues of two sequence-similar insect/vertebrate neuropeptide family pairs shed light on the potential existence of molecular evolutionary relationships. Analogues of the locustatachykinins and the mammalian tachykinin substance P, containing a sterically hindered Aib-NMePhe/Tyr residue block, share similar low-energy turn conformations incorporating a cis peptide bond. Conversely, restricted conformation analogues of the insect kinins and the mammalian opiate peptide Tyr-W-MIF-1, with near identical C-terminal tetrapeptide sequences, adopt different conformations. The insect kinins adopt a cisPro 1-4 beta-turn, in which the Phe1 is critical for bioactivity. Tyr-W-MIF-1 prefers a transPro 2-5 turn, and an additional N-terminal Phe severely inhibits mu-opiate receptor binding. Comparisons of the chemical/conformational requirements for receptor interaction are consistent with a distant evolutionary relationship between the insectatachykinins and tachykinins, but not between the insect kinins and Tyr-W-MIF-1. Therefore, analogues of the insect kinins with pest control potential can be readily designed to avoid mammalian interactions.

Passage of interleukin-1-beta across the blood-brain barrier is reduced in aged mice: a possible mechanism for diminished fever in aging

OBJECTIVE

Cytokine signaling is the key to fighting infection. Fever is elicited by the production of inflammatory cytokines, particularly interleukin-1beta (IL-1beta), and the subsequent action of cytokines in the hypothalamus. In old age, the ability to produce fever in response to infection or to peripheral injections of IL-1beta is diminished. Intracerebroventricular injections of IL-1beta can still produce a normal fever response in the aged. A logical hypothesis to explain this discrepancy is that passage of IL-1beta across the blood-brain barrier (BBB) is altered.

METHOD

We used a quantitative in vivo technique, which previously showed a saturable system transporting IL-1beta across the BBB, to investigate the speed at which radiolabeled IL-1beta crosses from blood to brain in mice of widely different ages.

RESULTS

We found that passage of IL-1beta across the BBB was significantly decreased in old (23-month) mice as compared with young (2-month) or middle-aged (12-month) animals. Passage of IL-1beta across the blood-testis barrier was not significantly different among the groups. The passage of radiolabeled albumin across the BBB was not increased in any group, ruling out any disruption of the BBB by IL-1beta.

CONCLUSION

These results provide a mechanism that could help explain why fever production is reduced in old age and suggest an important role for the BBB in regulating immune changes.

Phe(13),Tyr(19)-melanin-concentration hormone and the blood-brain barrier: role of protein binding

Melanin-concentrating hormone (MCH), found both peripherally and centrally, is involved in food ingestion. Although its expression in brain is increased by fasting, it is not known whether it crosses the blood-brain barrier (BBB). Use of the sensitive method of multiple-time regression analysis has shown that almost all of the peptides and polypeptides tested cross the BBB at a rate faster than the vascular marker albumin. With this same method, however, we found that the 19-amino acid 125I-Phe13,Tyr19-MCH did not cross faster than 99mTc-albumin. Several mechanisms were excluded as possible explanations for the slow rate of influx. These included degradation, association with capillary endothelial cells, and transport from brain to blood. When Phe13,Tyr19-MCH was perfused in blood-free buffer, however, it entered the brain significantly faster than albumin. This suggested protein binding as an explanation for the slow rate of influx when the MCH was administered in blood. Protein binding was confirmed by capillary zone electrophoresis, which showed that almost all of the Phe13,Tyr19-MCH added to blood migrated with a large-molecular-weight substance. Sodium dodecyl sulfate-capillary gel electrophoresis of Phe13,Tyr19-MCH in buffer additionally showed that the MCH aggregated as a trimer, a factor not preventing its influx by blood-free perfusion. Thus, the results show that blood-borne Phe13,Tyr19-MCH does not significantly cross the BBB, probably because of its binding to serum proteins.

Maturation of endomorphin-2 in the dorsal horn of the medulla and spinal cord of the rat

The endomorphins are potent and selective endogenous agonists at the mu opioid receptor. We describe here the postnatal ontogeny of endomorphin-2 like immunoreactivity (EM2-LI) in the dorsal horn of the rat medulla and spinal cord. EM2-LI is dense in the superficial lamina of the dorsal horn of the adolescent and adult rat, with fibers also present in skin and dorsal root ganglia. No staining was noted at 3 days of age or younger. Faint and limited staining was noted by 7 days of age. The density and distribution of the immunoreactivity increased with age, reaching an adult-like distribution by 21 days of age. Stress-induced responses mediated by endogenous opioids occur late in development and may be related to the late appearance of endomorphin-2.

Peptide transport system-1 (PTS-1) for Tyr-MIF-1 and Met-enkephalin differs from the receptors for either

Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) and Met-enkephalin share a saturable transport system (peptide transport system-1, PTS-1) across the blood-brain barrier but do not readily bind to each other's receptors. This information allows the unique opportunity to differentiate the transport protein(s) from the receptors for either peptide in brain endothelial cells. PTS-1 was studied in vitro by allowing radiolabeled Tyr-MIF-1 (125I-Tyr-MIF-1) to bind to the solubilized proteins of isolated murine brain microvessels in the presence or absence of potential inhibitors. Sephadex chromatography separated bound from free labeled peptide. The binding was saturable as shown by inhibition with increasing concentrations of unlabeled Tyr-MIF-1. 125I-Tyr-MIF-1 binding was not inhibited by an unrelated peptide or iodo-tyrosine. D-Tyr-MIF-1 had no effect, demonstrating the stereospecificity of the system. Met-enkephalin decreased the binding of 125I-Tyr-MIF-1 to 84.4% of total, whereas Leu-enkephalin was without effect. Agonists for the mu, delta, and kappa opiate receptors did not change the binding, indicating that the proteins which bound to 125I-Tyr-MIF-1 were not endogenous opiate receptors. The results indicate that, in vitro, Tyr-MIF-1 binds to brain microvessel proteins with characteristics similar to PTS-1.

Differential involvement of mu-opioid receptor subtypes in endomorphin-1- and -2-induced antinociception

We investigated the role of mu-opioid receptor subtypes in both endomorphin-1 and endomorphin-2 induced antinociception in mice using supraspinally mediated behavior. With tail pressure as a mechanical noxious stimulus, both intracerebroventricularly (i.c.v.) and intrathecally (i.t.) injected-endomorphins produced potent and significant antinociceptive activity. Antinociception induced by i.t. and i.c.v. injection of endomorphin-1 was not reversed by pretreatment with a selective mu1-opioid receptor antagonist, naloxonazine (35 mg/kg, s.c.). By contrast, antinociception induced by i.t. and i.c.v. endomorphin-2 was significantly decreased by mu1-opioid receptor antagonist. Antinociception of both i.t. and i.c.v. endomorphin-1 and -2 was completely reversed by pretreatment with beta-funaltrexamine (40 mg/kg, s.c.). The results indicate that endomorphins may produce antinociception through the distinct mu1 and mu2 subtypes of mu-opioid receptor.

Tests used to assess the cognitive abilities of aged rats: their relation to each other and to hippocampal morphology and neurotrophin expression

BACKGROUND

Aged rodents have proven to be a useful tool in studying age-related cognitive decline, particularly with regard to hippocampal function. A number of maze tests have been developed to evaluate hippocampal function in aged rodents, including the eight-arm radial maze, Barnes circular platform maze and Morris water maze. To some extent, these mazes have been used interchangeably to evaluate aged animals. Few researchers, however, have examined how performance of individual, aged animals compares in these three mazes.

OBJECTIVE

The purpose of this study was to compare the performances in the three mazes and to examine how such performances are related to each other, to hippocampal morphology and to neurotrophin gene expression.

METHODS

We screened groups of young and old Fisher 344 x Brown Norway rats for general health and physical abilities, tested the animals in the three mazes and examined correlations among performances in the mazes and in screening tests. Hippocampal neuron density and expression of hippocampal neurotrophin mRNAs were also examined and compared with behavior in the three mazes.

RESULTS

Aged animals were found to be impaired in all three mazes and to have lower hippocampal neuron densities compared with young animals, with poor learning behavior significantly correlating with reduced hippocampal neuron density. Differences were observed between performance in the different mazes, but in general the Morris water maze and Barnes circular platform maze were found to give similar results.