Endomorphins decrease heart rate and blood pressure possibly by activating vagal afferents in anesthetized rats

Vagal apnea and hypotension evoked by systemic injection of an antinociceptive analogue of endomorphin-2

PK20M (Dmt-D-Lys-Phe-Phe-OH) is a novel modified endomorphin-2 (EM-2) peptide producing strong dose- and time-dependent antinociceptive activity. Yet its prototype, endogenous EM-2, has been reported to trigger respiratory and vascular effects such as apnea and hypotension. The purpose of this study was to investigate the potency of the PK20M to evoke respiratory and cardiovascular responses in comparison to endogenous endomorphins. The engagement of the vagal pathway and μ opioid receptors in mediation of these responses was investigated. The effects of intravenous injections of PK20M, EM-1, and EM-2 were studied in anaesthetized, spontaneously breathing rats. The main dose-dependent effect of all endomorphins in the intact rats was immediate apnea, blood pressure and heart rate decrease. PK20M produced apnea in at least half of the intact animals in a much smaller dose than EM-1 and EM-2. The effects of all compounds were abrogated by pre-treatment with MNLX, a peripherally acting μ receptor antagonist. Cervical vagotomy eliminated arrest of breathing in the case of each tested compound. Hypotension was reduced by vagi section only after EM-1 and EM-2 administration. Our results demonstrated that apnea and bradycardia caused by systemic injection of all endomorphins were mediated via activation of μ vagal opioid receptors. The hypotension depended on intact vagi nerves only in the case of EM-1 and EM-2, whereas PK20M decreased blood pressure via other mechanisms outside vagal innervation. Modified opioid agonist is more potent in evoking extended hypotension; at the same time, it produces an arrest of breathing less frequently than its prototype EM-2.

Endomorphin-1 prevents lipid accumulation via CD36 down-regulation and modulates cytokines release from human lipid-laden macrophages

CD36 is a scavenger receptor known to play a critical role in the development of atherosclerosis by mediating the uptake of oxidized low-density lipoproteins (oxLDL) by macrophages, thus leading to foam cell formation. It is now generally recognized that the immune system has a pivotal role in the pathogenesis of atherosclerosis, whose progression is determined by ongoing inflammatory reactions. Recently, several studies pointed out that opioid peptides exert anti-inflammatory activities. Therefore the aim of the present study was to evaluate a possible endomorphin-1 (EM-1) immunomodulatory activity on human foam cells. Our results showed that EM-1 reduced Nile Red-stained lipid droplets content, decreased the expression of CD36 receptor and modulated tumor necrosis factor-α (TNF-α) and interferon-γ (IFN-γ) release from lipid-laden macrophages. Furthermore, Naloxone, an opioid receptors antagonist, reverted the anti-atherogenic and anti-inflammatory observed effects of EM-1. These data demonstrated, for the first time, an unprecedented ability of EM-1 to act as a novel modulator for macrophage-to-foam cell transformation, and for inflammatory cytokines profile, suggesting possible novel endomorphin-based anti-atherosclerotic approaches for the prevention and treatment of atherosclerosis.

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.

Endomorphin 1 and endomorphin 2 suppress in vitro antibody formation at ultra-low concentrations: anti-peptide antibodies but not opioid antagonists block the activity

Endomorphin 1 (EM-1) and endomorphin 2 (EM-2) were tested for their capacity to alter immune function. Addition of either of these peptides to murine spleen cells in vitro inhibited antibody formation to sheep red blood cells in a bi-phasic dose dependent manner. Maximal inhibition was achieved at doses in the range of 10(-13) to 10(-15)M. Neither naloxone (general opioid receptor antagonist) nor CTAP (selective mu opioid receptor antagonist) blocked the immunosuppressive effect. To show that there was specificity to the immunosuppressive activity of the peptides, affinity-purified rabbit antibodies were raised against each of the synthetic EM peptides haptenized to KLH and tested for capacity to inhibit immunosuppression. Antibody responses were monitored by a standard solid phase antibody capture ELISA, and antibodies were purified by immunochromatography using the synthetic peptides coupled to a Sepharose 6B resin. Verification of the specificity of affinity-purified antisera was performed by immunodot-blot and solid-phase RIA assays. The antisera specific for both EM-1 and EM-2 neutralized the immunosuppressive effects of their respective peptides in a dose-related manner. Control normal rabbit IgG had no blocking activity on either EM-1 or EM-2. These studies show that the endomorphins are immunomodulatory at ultra-low concentrations, but the data do not support a mechanism involving the mu-opioid receptor.

Cardiovascular responses to intravenous administration of human hemokinin-1 and its truncated form hemokinin-1(4-11) in anesthetized rats

Human hemokinin-1 and its carboxy-terminal fragment human hemokinin-1(4-11) have been recently identified as the members of the tachykinin family. The peripheral cardiovascular effects of these two tachykinin peptides were investigated in anesthetized rats. Lower doses of human hemokinin-1 (0.1-3 nmol/kg) injected intravenously (i.v.) induced depressor response, whereas higher doses (10 and 30 nmol/kg) caused biphasic (depressor and pressor) responses. The depressor response is primarily due to the action on endothelial tachykinin NK(1) receptor to release endothelium-derived relaxing factor (NO) and vagal reflex was absent in this modulation. The pressor response is mediated through the activation of tachykinin NK(1) receptor to release catecholamines from sympathetic ganglia and adrenal medulla. Moreover, human hemokinin-1 injected i.v. produced a dose-dependent tachycardia response along with blood pressure responses and the activation of sympathetic ganglia and adrenal medulla are involved in the tachycardia response. Human hemokinin-1(4-11) only lowered mean arterial pressure dose-dependently (0.1-30 nmol/kg) and the mechanisms involved in the depressor response are similar to that of human hemokinin-1. Additionally, human hemokinin-1(4-11) could also produce tachycardia response dose-dependently and the mechanisms involved in the tachycardia response are similar to that of human hemokinin-1 except that bilateral adrenalectomy could not affect the tachycardia markedly, indicating that the tachycardia induced by human hemokinin-1(4-11) is primarily due to the stimulation of sympathetic ganglia. In a word, to a certain extent, human hemokinin-1(4-11) is the active fragment of human hemokinin-1, however, the differences between human hemokinin-1 and hemokinin-1(4-11) involved in the effects of cardiovascular system suggest that the divergent amino acid residues at the N-terminus of human hemokinin-1 produced different activation properties for tachykinin NK(1) receptor.

Differential cardiovascular effects of synthetic peptides derived from endomorphin-1 in anesthetized rats

Previously, five synthetic peptides derived from endomorphin-1 (Tyr1-Pro2-Trp3-Phe4-NH2, EM-1), including Tyr-D-Ala-Trp-p-Cl-Phe-NH2 (HDAPC), Tyr-D-Ala-Trp-Phe-NH2 (HDADC), Nalpha-amidino-Tyr-D-Ala-Trp-p-Cl-Phe-NH2 (GDAPC), Nalpha-amidino-Tyr-D-Ala-Trp-Phe-NH2 (GDADC) and Nalpha-amidino-Tyr-D-Pro-Gly-Trp-p-Cl-Phe-NH2 (GBDPC), were described to elicit analgesia by subcutaneous administration with enhanced metabolic stabilities. To further our knowledge of the influences of particular modification on the pharmacological activities of EM-1, the present study was undertaken to investigate cardiovascular effects of these peptides in anesthetized rats by intravenous injection. Our results showed that the four D-Ala-containing peptides decreased the systemic arterial pressure (SAP) and heart rate (HR) through a naloxone-sensitive mechanism. Different patterns, potencies and durations of cardiovascular effects were observed among these peptides. When compared to EM-1, the hemodynamic responses to these four tetrapeptides were significantly lower in magnitude but much longer in duration. Surprisingly, intravenous administration of the only pentapeptide GBDPC produced fairly prolonged hypertensive and tachycardiac effects, which was naloxone-insensitive, thus providing evidence that changes in the primary structure of a peptide can profoundly affect its pharmacological activity. Comparisons of the cardiovascular effects between these peptides showed that each modification introduced into EM-1, including N-amidination, chloro-halogenation and unnatural amino acid substitution, played a role in the influence on the cardiovascular regulation of these peptides.

Cardiovascular responses to rat/mouse hemokinin-1, a mammalian tachykinin peptide: systemic study in anesthetized rats

Rat/mouse hemokinin-1 is a mammalian tachykinin peptide whose biological functions have not been well characterized. In the present study, an attempt has been made to investigate the effect and mechanism of action of rat/mouse hemokinin-1 on systemic arterial pressure after intravenous (i.v.) injections in anesthetized rats by comparing it with that of substance P. Our data showed that injection of rat/mouse hemokinin-1 (0.1, 0.3, 1, 3 and 10 nmol/kg) lowered systemic arterial pressure dose-dependently. This effect was significantly blocked by pretreatment with SR140333 (a selective tachykinin NK1 receptor antagonist) and the NO synthase inhibitor L-NAME (Nomega-nitro-L-arginine methyl ester hydrochloride), respectively, but was not affected by bilateral vagotomy or the muscarinic receptor blocker atropine. Compared to rat/mouse hemokinin-1, a dose of 3 nmol/kg of substance P caused biphasic changes in systemic arterial pressure (depressor and pressor responses). The results suggest that the mechanism of the depressor response caused by substance P was similar to rat/mouse hemokinin-1 in that it was inhibited by SR140333 and L-NAME, respectively, but that there was a component of the cardiovascular change induced by rat/mouse hemokinin-1 (but not substance P) that was attenuated by SR48968 (a selective tachykinin NK2 receptor antagonist). The depressor response induced by rat/mouse hemokinin-1 (i.v.) might be explained primarily by the action on endothelial tachykinin NK1 receptors to release endothelium-derived relaxing factor (NO) and this effect was not affected by vagal components. In addition, rat/mouse hemokinin-1 could not induce the pressor response through stimulation of sympathetic ganglion like substance P in anesthetized rats.

The endomorphin system and its evolving neurophysiological role

Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2) and endomorphin-2 (Tyr-Pro-Phe-Phe-NH2) are two endogenous opioid peptides with high affinity and remarkable selectivity for the mu-opioid receptor. The neuroanatomical distribution of endomorphins reflects their potential endogenous role in many major physiological processes, which include perception of pain, responses related to stress, and complex functions such as reward, arousal, and vigilance, as well as autonomic, cognitive, neuroendocrine, and limbic homeostasis. In this review we discuss the biological effects of endomorphin-1 and endomorphin-2 in relation to their distribution in the central and peripheral nervous systems. We describe the relationship between these two mu-opioid receptor-selective peptides and endogenous neurohormones and neurotransmitters. We also evaluate the role of endomorphins from the physiological point of view and report selectively on the most important findings in their pharmacology.

Cardiovascular responses to intrathecal administration of endomorphins in anesthetized rats

Endomorphins (EMs), the endogenous, potent and selective mu-opioid receptor agonists, have been shown to decrease systemic arterial pressure (SAP) in rats after intravenous (i.v.) administration. In the present study, cardiovascular responses to intrathecal (i.t.) injection of EMs were investigated in urethane-anesthetized rats. It is noteworthy that EMs elicited decreases in SAP and heart rate (HR) in a dose-dependent manner; 10-300nmol/kg were injected intrathecally. Furthermore, these vasodepressor and bradycardic effects were significantly antagonized by naloxone (0.5mg/kg, i.t.). Interestingly, i.t. (5mg/kg) or i.v. (50mg/kg) administrations of N(omega)-nitro-l-arginine methylester (l-NAME) attenuated the vasodepressor and bradycardic effects. Moreover, pretreatment of the rats with muscarinic receptor antagonist atropine (2mg/kg, i.v.) and alpha-adrenoceptor antagonist phentolamine (1mg/kg, i.v.) significantly reduced the vasodepressor effects of EMs. Nevertheless, pretreatment with beta-adrenoceptor antagonist propranolol (2mg/kg, i.v.) could only block the bradycardia effects induced by EMs, but had no significant effects on the hypotension. In summary, all the results suggested that i.t. administration of EMs decreased SAP and HR which were possibly mediated by the activation of opioid receptors in the rat spinal cord. In addition, nitric oxide (NO) release in both the spinal cord and in peripheral tissues might regulate the cardiovascular activities of EMs, and the muscarinic receptor and adrenoceptor played an important role in the regulation of the cardiovascular responses to i.t. administration of EMs.

C-terminal amide to alcohol conversion changes the cardiovascular effects of endomorphins in anesthetized rats

Endomorphin1-ol (Tyr-Pro-Trp-Phe-ol, EM1-ol) and endomorphin2-ol (Tyr-Pro-Phe-Phe-ol, EM2-ol), with C-terminal alcohol (-ol) containing, have been shown to exhibit higher affinity and lower intrinsic efficacy in vitro than endomorphins. In the present study, in order to investigate the alterations of systemic hemodynamic effects induced by C-terminal amide to alcohol conversion, responses to intravenous (i.v.) or intracerebroventricular (i.c.v.) injection of EM1-ol, EM2-ol and their parents were compared in the system arterial pressure (SAP) and heart rate (HR) of anesthetized rats. Both EM1-ol and EM2-ol induced dose-related decrease in SAP and HR when injected in doses of 3-100 nmol/kg, i.v. In terms of relative vasodepressor activity, it is interesting to note that EM2-ol was more potent than endomorphin2 [the dose of 25% decrease in SAP (DD25) = 6.01+/-3.19 and 13.99+/-1.56 nmol/kg, i.v., respectively] at a time when responses to EM1-ol were less potent than endomorphin1. Moreover, decreases in SAP in response to EM1-ol and EM2-ol were reduced by naloxone, atropine sulfate, L-NAME and bilateral vagotomy. It indicated that the vasodepressor responses were possibly mediated by a naloxone-sensitive, nitric oxide release, vagus-activated mechanism. It is noteworthy that i.c.v. injections of -ol derivatives produced dose-related decreases in SAP and HR, which were significantly less potent than endomorphins and were attenuated by naloxone and atropine sulfate. In summary, the results of the present study indicated that the C-terminal amide to alcohol conversion produced different effects on the vasodepressor activity of endomorphin1 and endomorphin2 and endowed EM2-ol distinctive hypotension characters in peripheral (i.v.) and central (i.c.v.) tissues. Moreover, these results provided indirect evidence that amidated C-terminus might play an important role in the regulation of the cardiovascular system.

Cardiovascular effects of endomorphins in alloxan-induced diabetic rats

Endomorphins, the endogenous, potent and selective mu-opioid receptor agonists, have been shown to decrease systemic arterial pressure (SAP) in rats. In the present study, responses to endomorphins were investigated in systemic vascular bed of alloxan-induced diabetic rats and in non-diabetic rats. Diabetes was induced by alloxan (220 mg/kg, i.p.) in male Wistar rats. At 4-5 weeks after the onset of diabetes, intravenous injections of endomorphins (1-30 nmol/kg) led to an increase of SAP and heart rate (HR) consistently and dosed-dependently. SAP increased 7.68+/-3.73, 11.19+/-4.55, 21.19+/-2.94 and 27.48+/-6.21% from the baseline at the 1, 3, 10 and 30 nmol/kg dose, respectively, of endomorphin 1 (n=4; p<0.05), and similar changes were observed in response to endomorphin 2. The hypertension could be antagonized markedly by i.p. 2 mg/kg of naloxone. On the other hand, bilateral vagotomy would attenuate the effects of hypertension and diminished the changes of HR in response to endomorphins. With diabetic rats, 6-10 weeks after the induction of diabetes, intravenous injections of endomorphins produced non-dose-related various changes in SAP, such as a single decrease, or a single increase, or biphasic changes characterized by an initial decrease followed by a secondary increase, or no change at all. These results suggest that diabetes may lead to the dysfunction of the cardiovascular system in response to endomorphins. Furthermore, the diabetic rats of 4-5 weeks after alloxan-treatment, the increase in SAP and HR caused by i.v. endomorphins might be explained by a changed effect of vagus and by a naloxone-sensitive 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.

Resistance of cholestatic rats against epinephrine-induced arrhythmia: the role of nitric oxide and endogenous opioids

Short-term ligation of bile duct has been used as a model to study acute cholestasis and is associated with various cardiovascular abnormalities. We examined the role of nitric oxide (NO) and endogenous opioids on epinephrine-induced arrhythmia in 7-day bile duct-ligated (BDL) rats. Six groups of rats, each of which was subdivided into two subgroups (sham-operated and BDL), were examined. First group of animals were chronically treated with normal saline. In the second and third groups, single intraperitoneal administration of N(omega)-nitro-L-arginine methyl ester (L-NAME, 10 mg/kg) or naltrexone (20 mg/kg) was performed 30 min before evaluation of epinephrine-induced arrhythmia. Two groups received chronic administration of low dose (3 mg/kg/day) or high dose (10 mg/kg/day) L-NAME; and the last group was treated chronically with naltrexone (20 mg/kg/day). Chronic drug administration was performed subcutaneously for 6 consecutive days following BDL or sham operation. After induction of arrhythmia by intravenous injection of 10 microg/kg epinephrine, mean arterial pressure and electrocardiogram were recorded for 1 min. Heart rate and mean arterial pressure were significantly lower in BDL rats (P<0.01). Chronic injection of naltrexone increased heart rate and mean arterial pressure in BDL (P<0.05). Chronic low dose L-NAME administration had no effect on baseline hemodynamic parameters. High dose L-NAME injection corrected hypotension in BDL rats, but not bradycardia (P<0.05). Epinephrine induced less arrhythmia in BDL rats (P<0.05). Acute and chronic injection of naltrexone had no effect on the resistance of BDL rats against epinephrine-induced arrhythmia. Although acute L-NAME administration enhanced arrhythmias in sham-operated rats (P<0.001), it had no effect on BDL animals. Chronic injection of low dose or high dose L-NAME, without having any effect on sham-operated animals, increased arrhythmias in BDL rats (P<0.01). This study showed that BDL animals are resistant against epinephrine-induced arrhythmia and this resistance depends on long-term NO overproduction.

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.

Mu-opioid receptors are located postsynaptically and endomorphin-1 inhibits voltage-gated calcium currents in premotor cardiac parasympathetic neurons in the rat nucleus ambiguus

Activation of opioid receptors in the CNS evokes a dramatic decrease in heart rate which is mediated by increases in inhibitory parasympathetic activity to the heart. Injection of opiates into the nucleus ambiguus, where premotor cardiac parasympathetic nucleus ambiguus neurons are located elicits an increase in parasympathetic cardiac activity and bradycardia. However, the mechanisms responsible for altering the activity of premotor cardiac parasympathetic nucleus ambiguus neurons is unknown. This study examined at the electron microscopic level whether premotor cardiac parasympathetic nucleus ambiguus neurons possess postsynaptic opioid receptors and whether mu-opioid receptor agonists alter voltage-gated calcium currents in these neurons. Premotor cardiac parasympathetic nucleus ambiguus neurons were identified in the rat using retrograde fluorescent tracers. One series of experiments utilized dual-labeling immunocytochemical methods combined with electron microscopic analysis to determine if premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. In a second series of experiments whole cell patch clamp methodologies were used to determine whether activation of postsynaptic opioid receptors altered voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons in brainstem slices. The perikarya and 78% of the dendrites of premotor cardiac parasympathetic nucleus ambiguus neurons contain mu-opioid receptors. Voltage-gated calcium currents in premotor cardiac parasympathetic nucleus ambiguus neurons were comprised nearly entirely of omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents. Activation of mu-opioid receptors inhibited these voltage-gated calcium currents and this inhibition was blocked by pretreatment with pertusis toxin. The mu-opioid receptor agonist endomorphin-1, but not the mu-opioid receptor agonist endomorphin-2, inhibited the calcium currents. In summary, mu-opioid receptors are located postsynaptically on premotor cardiac parasympathetic nucleus ambiguus neurons. The mu-opioid receptor agonist endomorphin1 inhibited the omega-agatoxin-sensitive P/Q-type voltage-gated calcium currents in premotor cardiac vagal nucleus ambiguus neurons. This inhibition is mediated via a G-protein mediated pathway which was blocked by pretreatment with pertusis toxin. It is possible that the inhibition of calcium currents may act to indirectly facilitate the activity of premotor cardiac parasympathetic nucleus ambiguus neurons by disinhibition, such as by a reduction in inhibitory calcium activated potassium currents.

Endomorphins and related opioid peptides

Opioid peptides and their G-protein-coupled receptors (delta, kappa, mu) are located in the central nervous system and peripheral tissues. The opioid system has been studied to determine the intrinsic mechanism of modulation of pain and to develop uniquely effective pain-control substances with minimal abuse potential and side effects. Two types of endogenous opioid peptides exist, one containing Try-Gly-Gly-Phe as the message domain (enkephalins, endorphins, dynorphins) and the other containing the Tyr-Pro-Phe/Trp sequence (endomorphins-1 and -2). Endomorphin-1 (Tyr-Pro-Trp-Phe-NH2), which has high mu receptor affinity (Ki = 0.36 nM) and remarkable selectivity (4000- and 15,000-fold preference over the delta and kappa receptors, respectively), was isolated from bovine and human brain. In addition, endomorphin-2 (Tyr-Pro-Phe-Phe-NH2), isolated from the same sources, exhibited high mu receptor affinity (Ki = 0.69 nM) and very high selectivity (13,000- and 7500-fold preference relative to delta and kappa receptors, respectively). Both opioids bind to mu-opioid receptors, thereby activating G-proteins, resulting in regulation of gastrointestinal motility, manifestation of antinociception, and effects on the vascular systems and memory. To develop novel analgesics with less addictive properties, evaluation of the structure-activity relationships of the endomorphins led to the design of more potent and stable analgesics. Opioidmimetics and opioid peptides containing the amino acid sequence of the message domain of endomorphins, Tyr-Pro-Phe/Trp, could exhibit unique binding activity and lead to the development of new therapeutic drugs for controlling pain.

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.

Do endogenous opioids contribute to the bradycardia of rats with obstructive cholestasis?

Endogenous opioids have nitric oxide (NO)-dependent cardiovascular actions. In the light of biological evidence of accumulation of endogenous opioids in cholestasis and also existence of NO-dependent bradycardia in cholestatic subjects, this study was carried out to evaluate the role of endogenous opioids in the generation of bradycardia in a rat model of cholestasis. Male Sprague-Dawley rats were used to induce cholestasis by surgical ligation of the bile duct, with sham-operated animals serving as a control. The animals were divided into six groups which received naltrexone [20 mg/kg/day, subcutaneously (s.c.)], N(G)-L-nitro-arginine methyl ester (L-NAME, 3 mg/kg/day, s.c.), aminoguanidine (200 mg/kg/day, s.c.), L-arginine (200 mg/kg/day, s.c.), naltrexone + L-NAME (20 and 3 mg/kg/day, s.c) or saline. One week after the operation, a lead II electrocardiogram (ECG) was recorded and the spontaneously beating atria of the animals were then isolated and the chronotropic responses to epinephrine evaluated. The plasma L-nitro-tyrosine level and alanine amino transferase and alkaline phosphatase activities were also measured. The heart rate of cholestatic animals was significantly lower than that of control rats in vivo and this bradycardia was corrected with daily adminstration of naltrexone or L-NAME. The basal spontaneous beating rate of atria in cholestatic animals was not significantly different from that of sham-operated animals in vitro. Cholestasis induced a significant decrease in the chronotropic effect of epinephrine. This effect was corrected by daily injection of naltrexone or L-NAME, or concurrent administration of naltrexone + L-NAME, and was not corrected by aminoguanidine. L-arginine had an equivalent effect to L-NAME and increased the chronotropic effect of epinephrine in cholestatic rats but not in control animals. Bile duct ligation increased the plasma activity of liver enzymes as well as the level of L-nitro-tyrosine. L-arginine and naltrexone treatment significantly decreased the elevation of liver enzymes in bile duct-ligated rats. Pretreatment of cholestatic animals with naltrexone or L-NAME decreased the plasma L-nitro-tyrosine level. The results suggest that either prevention of NO overproduction or protection against liver damage is responsible for recovery of bradycardia after naltrexone administration.

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.

Opioid peptides endomorphin-1 and endomorphin-2 in the immune system in humans and in a rodent model of inflammation

Endomorphin (EM)-1 and EM-2 are tetrapeptides with high affinity and selectivity for the micro-opioid receptor. We have utilized specific radioimmunoassays to characterize EM-1 and EM-2 in immune tissues from normal human subjects and from rats with adjuvant arthritis (AA). PBLs from three normal human subjects contained 248, 13, and 303 pg EM-1 per 100 million cells, whereas EM-2 was measured in two subjects at 69 and 588 pg per 100 million cells. In AA rats, EM-1 (but not EM-2) contents in the spleen and thymus were elevated compared with levels in tissues from non-AA controls. EM-1 was detectable in five of eight samples of synovial tissue from inflamed hind paws, whereas EM-2 was detectable in two of eight synovial extracts. Neither EM-1 nor EM-2 were detectable in synovial tissue from non-AA rats. To our knowledge, this is the first report of endomorphins in normal human immune cells. Increased endomorphin expression or uptake in peripheral tissues in a rodent model of chronic inflammation provides potential for endomorphins to selectively modulate chronic inflammation in mammals.

Stress-specific opioid modulation of haemodynamic counter-regulation

1. The haemodynamic and cardiovascular responses to stress, in addition to being under control of the autonomic nervous system, are also under opiate modulation. Our studies have provided evidence for activation of the endogenous opioid system in haemorrhagic shock, sepsis and trauma. Furthermore, we have demonstrated that both central and systemic opiate administration to naïve rats result in marked alterations in haemodynamic responses, which are associated with activation of the sympathetic nervous system. 2. Because of the ubiquitous presence of opiate receptors in both the central nervous system and peripheral tissues, as well as their production and release centrally and peripherally, this facilitates an endocrine as well as a paracrine contribution to modulating vascular responses to stress, either directly or indirectly. Results from previous studies suggest that endogenous opioids are not involved in mediating the lipopolysacharide-induced hypotensive response. 3. In more recent studies, we have examined the role of opiate receptor activation in modulating the haemodynamic and neuroendocrine responses to fixed pressure haemorrhagic shock in conscious unrestrained rats. Using systemic opiate blockade (naltrexone, 15 mg/kg, i.p.) prior to haemorrhage, we have observed that blood loss required to achieve mean arterial blood pressure of 40 mmHg was higher in naltrexone-treated animals than in time-matched saline controls. Interestingly, the haemodynamic modulation exerted by naltrexone cannot be attributed to differences in circulating catecholamine levels. Haemorrhage produced an immediate and progressive increase in circulating adrenaline and noradrenaline levels, reaching values that were 50- and 20-fold higher than basal, respectively. Naltrexone pretreatment did not alter the time-course or magnitude of the rise in circulating levels of catecholamines. 4. These results indicate that endogenous opioid activation contributes to the haemodynamic dishomeostasis associated with blood loss. Our findings suggest stress-specific roles for opiate-sensitive haemodynamic counter-regulatory responses.

Effects of morphine and naloxone on fetal heart rate and movement in the pig

To test the hypothesis that an increasing opioid tonus is involved in decreases in fetal heart rate (FHR) and movement (FM) during late gestation, we studied the effects of intravenous bolus injections of morphine (1 mg) and naloxone (1 mg) on FHR and FM in the fetal pig. Twenty-one fetuses (1 per sow) were catheterized at 90-104 days of gestation (median 100 days). Recordings of FHR (electrocardiograph or Doppler-derived signals) and FM (ultrasonography) were made from 15 min before to 45 min after treatment. Morphine administration significantly decreased FHR, but it increased FHR variation and forelimb movements (LM). LM were clustered, and this stereotyped behavior has never before been observed in any mammalian fetus. Naloxone administration increased gross body movements and FHR without significant changes in FHR variation. It is concluded that FHR and motility are under opioidergic control in the pig fetus. Both morphine and naloxone induce hypermotility, suggesting that naloxone does not act as a pure opioid antagonist in the fetal pig.

Endomorphin-1 and endomorphin-2: pharmacology of the selective endogenous mu-opioid receptor agonists

The recently discovered endogenous opioid peptides, endomorphins-1 and -2, appear to have properties consistent with neurotransmitter/neuromodulator actions in mammals. This review surveys the information gained so far from studies of different aspects of the endomorphins. Thus, the endomorphins have been found unequally in the brain; they are stored in neurons and axon terminals, with a heterogeneous distribution; they are released from synaptosomes by depolarization; they are enzymatically converted by endopeptidases; and they interact specifically and with high affinity with mu-opioid receptors. The most outstanding effect of the endomorphins is their antinociceptive action. This depends on both central and peripheral neurons. Additionally, the endomorphins cause vasodilatation by stimulating nitric oxide release from the endothelium. Their roles in different central and peripheral functions, however, have not been fully clarified yet. From a therapeutic perspective, therefore, they may be conceived at present as potent antinociceptive and vasodilator agents.

High levels of endorphin and related pathologies of veterinary concern. A review

The authors report information about endogenous opioid peptides (EOP), receptors, antagonists and their interference with pain, stress, endocrine and immune system. A relationship between EOP and calcium homeostasis, both at extracellular and intracellular level, has been observed. In vitro, beta-endorphin exerts different actions through calcium channel functionality in epithelial cells. In rat aorta and cerebral cortex: beta-endorphin or Naloxone alternatively influence oocyte maturation through the mu-receptor gene expression and intracellular calcium concentration in granulosa and cumulus cells. Calcium channel block is removed by administrating Naloxone and calcium. In vivo, Naloxone and calcium removes EOP induced apoptosis in granulosa cells; is the most safe therapy in cow's milk fever; allow to remove ovarian follicular cysts. A negative influence of opioids on immune response after vaccination was established; EOP-related metabolic problems in post-partum cows. Abnormal intestinal motility, in which a Ca++ influence is well known, can be removed by Naloxone and calcium administration. Calcium-related function and neuromodulation must be re-evaluated since high level of EOP are involved in many pathologies through their influence on calcium activity. The use of calcium salts and Naloxone offers a safe and supplementary therapeutical possibility, active in any condition of altered endogenous opioids.

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.

Novel opioid peptides endomorphin-1 and endomorphin-2 are present in mammalian immune tissues

Endomorphin (EM)-1 and EM-2 are opioid tetrapeptides, reported within the central nervous system, which have very high specificity and affinity for the mu-opioid receptor. We have used newly developed and well-characterised radioimmunoassays (RIAs) in combination with reversed-phase high-performance liquid chromatography (HPLC) to detect EM-1 and EM-2 immunoreactivity (ir) in rat immune tissues. Endomorphins were detectable in extracts of rat spleen (total EM-1-ir/spleen: 440+/-73 pg, mean+/-SEM, a=group of eight rats; EM-2-ir: 150+/-12 pg) and thymus (EM-1-ir: 152+/-18 pg, mean+/-SEM n=8; EM-2-ir: 156+/-28 pg). EM-2-ir was detectable in extracts of human spleen (338+/-196 pg/g tissue, n=3). Multiple peaks of EM-1-ir and EM-2-ir were observed in rat spleen and thymus extracts, and multiple peaks of EM-2-ir were observed in extracts of human spleen, following reversed-phase HPLC and RIAs. This is the first report of endomorphin immunoreactivity in tissues of the rat and human immune systems.

Pressor effects of orexins injected intracisternally and to rostral ventrolateral medulla of anesthetized rats

Orexin A and B, two recently isolated hypothalamic peptides, have been reported to increase food consumption upon intracerebroventricular injections in rats. In addition to the hypothalamus, orexin A-immunoreactive fibers have been observed in several areas of the medulla that are associated with cardiovascular functions. The present study was undertaken to evaluate the hypothesis that orexins may influence cardiovascular response by interacting with neurons in the medulla. Intracisternal injections of orexins A (0.0056-7.0 nmol) or B (0.028-0.28 nmol) dose dependently increased mean arterial pressure (MAP) by 4-27 mmHg and heart rate (HR) by 26-80 beats/min in urethan-anesthetized rats, with orexin A being more effective in this regard. MAP and HR were not changed by intravenous injection of orexins at higher concentrations. Microinjection of orexin A (14 pmol/50.6 nl) to the rostral ventrolateral medulla, which was confirmed by histological examination, increased MAP and HR. Our results indicate that, in addition to a role in positive feeding behavior, orexins may enhance cardiovascular response via an action on medullary neurons.

Endogenous opiates: 1998

This paper is the twenty-first installment of our annual review of research concerning the opiate system. It summarizes papers published during 1998 that studied the behavioral effects of the opiate peptides and antagonists, excluding the purely analgesic effects, although stress-induced analgesia is included. The specific topics covered this year include stress; tolerance and dependence; eating and drinking; alcohol; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurologic disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunologic responses; and other behaviors.