Respiratory and cardiovascular effects of the mu-opioid receptor agonist [Lys7]dermorphin in awake rats

Analgesic Opioid Ligand Discovery Based on Nonmorphinan Scaffolds Derived from Natural Sources

Strong opioid analgesics, including morphine, are the mainstays for treating moderate to severe acute pain and alleviating chronic cancer pain. However, opioid-related adverse effects, including nausea or vomiting, sedation, respiratory depression, constipation, pruritus (itch), analgesic tolerance, and addiction and abuse liability, are problematic. In addition, the use of opioids to relieve chronic noncancer pain is controversial due to the "opioid crisis" characterized by opioid misuse or abuse and escalating unintentional death rates due to respiratory depression. Hence, considerable research internationally has been aimed at the "Holy Grail" of the opioid analgesic field, namely the discovery of novel and safer opioid analgesics with improved opioid-related adverse effects. In this Perspective, medicinal chemistry strategies are addressed, where structurally diverse nonmorphinan-based opioid ligands derived from natural sources were deployed as lead molecules. The current state of play, clinical or experimental status, and novel opioid ligand discovery approaches are elaborated in the context of retaining analgesia with improved safety and reduced adverse effects, especially addiction liability.

The use of hypercapnic conditions to assess opioid-induced respiratory depression in rats

INTRODUCTION

Whole-body plethysmography (WBP) in unrestrained, non-anesthetized rodents is a preclinical method to assess the respiratory depressant effects of opioids, the leading cause of opioid overdose death in humans. However, low baseline respiration rates under normocapnic conditions (i.e., "floor" effect) can render the measurement of respiratory decreases challenging. We assessed hypercapnia-induced increases in respiration as a strategy to assess opioid-induced decreases in respiration in rats.

METHODS

WBP was used to assess respiration frequency, tidal volume and minute volume in the presence of normocapnic and hypercapnic (8% CO₂) conditions in rats during the rat diurnal period of the light cycle. The mu-opioid receptor agonist fentanyl was administered intravenously, and the hot plate test was used to assess acute antinociception.

RESULTS AND DISCUSSION

Hypercapnia-induced increases in respiratory parameters (frequency, minute volume, and tidal volume) were decreased by fentanyl at doses that did not decrease the same parameters under the normocapnic conditions. These findings show that hypercapnia increases sensitivity to respiratory depressant effects of fentanyl, as compared with assessments during the rat diurnal period when activity and breathing rate are generally low, i.e., there is a floor effect. The current approach is highly sensitive to opioid-induced respiratory depression, and therefore provides a useful method for assessment in a pre-clinical setting.

Activation of peripheral opioid µ-receptors in blood vessel may lower blood pressure in spontaneously hypertensive rats

BACKGROUND/AIMS

The role of opioid receptors in the regulation of vascular function remains unclear. In the current study, we evaluated the ability of loperamide, a peripheral opioid receptor agonist, to regulate blood pressure in spontaneously hypertensive rats (SHRs) and examined the mechanism(s) by which loperamide exerts its effects.

METHODS

In male SHRs, mean arterial pressure (MAP) was measured and hemodynamic analysis was recorded. Additionally, the isometric tension of aortic rings isolated from SHRs was determined.

RESULTS

Loperamide dose-dependently decreased MAP in SHRs but not in the normal group of Wistar-Kyoto rats. This reduction of MAP in conscious SHRs was abolished by the selective opioid μ-receptor antagonist cyprodime, but not by naloxonazine, the μ(1)-opioid receptor antagonist. However, cardiac output was not altered by loperamide in anesthetized SHRs. Moreover, loperamide-induced relaxation in isolated aortic rings precontracted with phenylephrine or vasopressin. This relaxation was abolished by cyprodime, but not by naloxonazine. Loperamide-induced relaxation was also attenuated by glibenclamide, an ATP-sensitive potassium (K(ATP)) channel blocker. Additionally, vasodilatation by loperamide was reduced by an inhibitor of protein kinase A (PKA) and enhanced by an inhibitor of phosphodiesterases.

CONCLUSION

We suggest that loperamide can lower MAP in SHRs via μ(2)-opioid receptor-dependent cAMP-PKA pathway that induces vascular relaxation by opening K(ATP) channels.

Supranodose vagotomy eliminates respiratory depression evoked by dermorphin in anaesthetized rats

The respiratory effects of stimulation of mu-opioid receptors were studied in spontaneously breathing anaesthetized rats that were either neurally intact or subjected to bilateral supranodosal vagotomy. An intravenous dermorphin bolus of 0.5 mg/kg evoked the apnea followed by breathing of reduced rate and compensatory augmentation of tidal volume, which resulted in an invariable minute ventilation. Cardiovascular effects consisted of hypotension and temporary fall in heart rate. In rats initially treated by supranodosal vagotomy, dermorphin did not evoke any respiratory and cardiovascular effects. These results indicate that vagal pathway and the nodose ganglia are involved in dermorphin-induced respiratory depression.

Reversal of morphine, methadone and heroin induced effects in mice by naloxone methiodide

Opioid overdose, which is commonly associated with opioid induced respiratory depression, is a problem with both therapeutic and illicit opioid use. While the central mechanisms involved in the effects of opioids are well described, it has also been suggested that a peripheral component may contribute to the effects observed. This study aimed to further characterise the effects of the peripherally acting naloxone methiodide on the respiratory, analgesic and withdrawal effects produced by various opioid agonists. A comparison of the respiratory and analgesic effects of morphine, methadone and heroin in male Swiss-Albino mice was conducted and respiratory depressive ED(80) doses of each opioid determined. These doses (morphine 9 mg/kg i.p., methadone 7 mg/kg i.p., and heroin 17 mg/kg i.p.) were then used to show that both naloxone (3 mg/kg i.p.) and naloxone methiodide (30-100 mg/kg i.p.) could reverse the respiratory and analgesic effects of these opioid agonists, but only naloxone precipitated withdrawal. Further investigation in female C57BL/6J mice using barometric plethysmography found that both opioid antagonists could reverse methadone induced decreases in respiratory rate and increases in tidal volume. Its effects do not appear to be strain or sex dependent. It was concluded that naloxone methiodide can reverse the respiratory and analgesic actions of a variety of opioid agonists, without inducing opioid withdrawal.

Mu opioid receptors in rat ventral medulla: effects of endomorphin-1 on phrenic nerve activity

Anatomical and in vitro studies suggest that mu opioid receptors (MOR) on pre-Bötzinger complex neurons are responsible for opioid induced respiratory depression (Grey et al., Science 286 (1999) 1566). However, mu opioid agonists injected in vivo, in other regions of the ventral respiratory group (VRG), produce respiratory depression, suggesting that opioids are widely distributed in the VRG. We therefore re-examined the distribution of the MOR in the ventral medulla and found MOR-immunoreactive neurons and terminals in all subdivisions of the VRG. Furthermore, we determined, in rats, the effects of a MOR agonist (endomorphin-1, 10 mM, 60 nl, unilateral), microinjected into different subdivisions of the VRG, on phrenic nerve activity. Endomorphin-1 produced changes in phrenic nerve frequency and amplitude, throughout the VRG. Unexpectedly, endomorphin-1 microinjected into the Bötzinger and pre-Bötzinger complexes consistently increased phrenic nerve frequency. These results support the widespread distribution of MOR in the VRG and also indicate that endomorphin-1, a postulated endogenous ligand, may differentially regulate respiration.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

Anti-inflammatory properties of the mu opioid receptor support its use in the treatment of colon inflammation

The physiologic role of the mu opioid receptor (MOR) in gut nociception, motility, and secretion is well established. To evaluate whether MOR may also be involved in controlling gut inflammation, we first showed that subcutaneous administration of selective peripheral MOR agonists, named DALDA and DAMGO, significantly reduces inflammation in two experimental models of colitis induced by administration of 2,4,6-trinitrobenzene sulfonic acid (TNBS) or peripheral expansion of CD4(+) T cells in mice. This therapeutic effect was almost completely abolished by concomitant administration of the opioid antagonist naloxone. Evidence of a genetic role for MOR in the control of gut inflammation was provided by showing that MOR-deficient mice were highly susceptible to colon inflammation, with a 50% mortality rate occurring 3 days after TNBS administration. The mechanistic basis of these observations suggests that the anti-inflammatory effects of MOR in the colon are mediated through the regulation of cytokine production and T cell proliferation, two important immunologic events required for the development of colon inflammation in mice and patients with inflammatory bowel disease (IBD). These data provide evidence that MOR plays a role in the control of gut inflammation and suggest that MOR agonists might be new therapeutic molecules in IBD.

In vitro and in vivo opioid activity of [DPro(6)]dermorphin, a new dermorphin analogue

To study the effects of inducing stereo-chemical modifications in the structure of dermorphin (DM) so as to improve its mu-opioid receptor affinity and its resistance to C-terminal enzymatic degradation, in the Institute of Molecular Genetics of Moscow, we synthesized a new DM analogue ([DPro(6)]DM) and analyzed the changes induced in the biological activities of DM by substituting the Pro(6) residue with DPro(6). We compared the activity of the new DM analogue and DM in in vitro assays and in in vivo tests of analgesia, thermoregulation, heart rate recordings, and gastrointestinal motility in rats. In the in vitro tests, guinea pig ileum (GPI) and mouse vas deferens (MVD), although the opioid activities of [DPro(6)]DM indicated that the peptide was always less potent than DM, its lower IC(50) ratios (mu/delta) showed that it had higher mu-opioid receptor selectivity. In the in vivo analgesic test, [DPro(6)]DM, when injected intraperitoneally (i.p.) (0.5-5 and 10mg/kg) in rats, had the same antinociceptive efficacy as DM and when injected intranasally (i.n.) (0.005 and 0.02 mg/kg) it induced a more stable and long-lasting analgesia than DM (the AUC was about 91% higher for [DPro(6)]DM than for DM). Moreover, these data confirm that the intranasal route is advantageous for peripheral drug administration. In the heart rate study, [DPro(6)]DM and DM (0.5mg/kg, i.p.), induced a similar, weak bradycardia. The only difference was that [DPro(6)]DM induced a longer-lasting effect than DM. Conversely, in body temperature regulation [DPro(6)]DM induced weaker inhibitory activity than DM (56% of the DM-induced response); it did so only in a cold environment and at the maximal used dose (0.5mg/kg, i.p.) without inducing vasomotor effects. In the gastrointestinal study, [DPro(6)]DM and DM (0.005, 0.05, and 0.5mg/kg, i.p.) significantly slowed upper gastrointestinal transit of a charcoal meal and inhibited colonic propulsion. Comparison of the ED(50) values of [DPro(6)]DM (0.03 mg/kg) and DM (0.009 mg/kg) showed that the DM analogue was about three times less potent than DM in slowing gastrointestinal and colonic transit. In conclusion, all these data overall suggest that structural maneuvering in the Pro(6)-residue of the DM molecule changes its affinity for mu-opioid receptor subtypes and confirms the usefulness of experimental studies involving structural modifications in obtaining new therapeutic agents.

Lack of involvement of mu(1) opioid receptors in dermorphin-induced inhibition of hypoxic and hypercapnic ventilation in rat pups

The effects of dermorphin, a mu-selective opioid agonist, on respiratory responses to altered O(2) and CO(2) during postnatal development were investigated in conscious, unrestrained Wistar rats aged 2-21 days. Respiration was recorded by barometric plethysmography. Dermorphin (4 mg kg(-1)) was administered subcutaneously, and the ventilatory responses to hypoxia (11% O(2), 89% N(2)) in 2-21-day-old pups and hyperoxia (100% O(2)), and hypercapnia (8% CO(2), 92% O(2)) in 2-13-day-old pups were assessed in the presence and absence of the mu(1) receptor antagonist naloxonazine (10 mg kg(-1) s.c.) administered 1 day before testing. Six minutes of hypoxia increased ventilation in all age groups, largely via an increase in frequency. Dermorphin inhibited the ventilatory response to hypoxia, and this inhibition was insensitive to naloxonazine. After 5 min of hyperoxia, ventilation was the same as with air breathing except in the presence of dermorphin, when hyperoxic ventilation was depressed by a naloxonazine-insensitive decrease in frequency. Following this 5 min 100% O(2) exposure, pups were exposed to hypercapnia, and respiratory parameters were measured 5 min later. The ventilatory response to CO(2) was inhibited by dermorphin in a naloxonazine-insensitive manner. There was no evidence for endogenous mu(1) receptor modulation of the ventilatory responses to altered gases in rat pups of any age. Thus, mu opioid-induced inhibition of the hypoxic and hypercapnic responses in young rats does not occur via activation of mu(1) opioid receptors.

Modulation of breathing by mu1 and mu2 opioid receptor stimulation in neonatal and adult rats

Opioid modulation of breathing during postnatal development through to the adult was investigated in the rat. Respiratory frequency, tidal volume and minute volume were recorded in unanesthetized, unrestrained rat pups and adults using barometric plethysmography. Subjects were administered the highly selective mu opioid agonists dermorphin and fentanyl. Fentanyl, which readily crosses the blood-brain barrier, was included to ensure that developmental changes in blood-brain barrier restrictions did not mask some of the dermorphin effects in older neonates. Drugs were administered subcutaneously in neonates and adults, although dermorphin was given by intracerebroventricular route only in adults. In neonates, mu agonist administration caused a gasping-like pattern of breathing, characterized by a marked fall in frequency and a smaller increase in tidal volume. The gasping response was prevented by pre-treatment with the long-acting mu1 antagonist naloxonazine (NALZ). In the presence of NALZ, mu agonists elicited only a small, but significant, reduction in tidal volume. Both dermorphin and fentanyl showed more potent activity in younger pups than in older pups, possibly in the case of dermorphin because of developmental maturation of blood-brain barrier function. In adults, fentanyl and dermorphin both caused a reduction in frequency and minute volume. The response of adults to fentanyl, but not dermorphin, was prevented by NALZ. These results suggest that both mu1 and mu2 receptors contribute to opioid-induced respiratory depression during neonatal and adult life.

Opioidergic modulation of ventilatory response to sustained hypoxia in obese Zucker rats

OBJECTIVE

To determine whether altered central and/or peripheral opioidergic mechanisms contribute to the altered ventilatory response to sustained hypoxia in obese Zucker rats.

RESEARCH METHODS AND PROCEDURES

Eight lean (176 +/- 8 [SEM] g) and eight obese (225 +/- 12 g) Zucker rats were studied at 6 weeks of age. Pulmonary ventilation ((E)), tidal volume (V(T)), and breathing frequency (f) at rest and in response to sustained (30 minutes) hypoxic (10% O(2)) challenges were measured on three separate occasions by the barometric method after the randomized, blinded administration of equal volumes of saline (control), naloxone methiodide (N(M); 5 mg/kg, peripheral opioid antagonist), or naloxone hydrochloride (N(HCl); 5 mg/kg, peripheral and central opioid antagonist).

RESULTS

Administration of N(M) and N(HCl) in lean animals had no effect on (E) either at rest or during 30 minutes of sustained exposure to hypoxia. Similarly, N(M) failed to alter (E) in obese rats. In contrast, N(HCl) significantly (p < 0.05) increased (E) and V(T) both at rest and during 2 to 10 minutes of hypoxic exposure in obese rats. After 20 to 30 minutes of hypoxic exposure, V(T) remained elevated with N(HCl), but the earlier elevation of (E) seemed to be attenuated due to a decrease in f at 20 minutes of exposure to hypoxia.

DISCUSSION

Thus, endogenous opioids modulate both resting (E) and the ventilatory response to sustained hypoxia in obese, but not in lean, Zucker rats by acting specifically on opioid receptors located within the central nervous system.

Effects of the potent analgesic enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan on respiration

We investigated whether the enkephalin-catabolizing enzyme inhibitors RB101 and kelatorphan, which have been shown to be potent analgesics, depress respiration as do opioid analgesics. Ventilation was measured in cats and rodents by the barometric method, in the awake state and during anesthesia. Tissue distribution of the inhibitors was either generalized (RB101, 40-160 mg/kg i.p.), largely restricted by the blood-brain barrier to the periphery (kelatorphan, 0.7-20 mg/kg i.v.), or restricted to the brainstem (i.c.v. injection of RB101 in the fourth ventricle). RB101 did not affect ventilation in any condition tested, and large doses of kelatorphan produced a naloxone-reversible increase in ventilation and breathing frequency. Thus endogenous opioids released during conditions of normal ventilation do not exert any depressant neuromodulatory effect on this function, even when their extracellular concentrations are increased by peptidase inhibitors. The differential effect of these inhibitors on ventilation and nociception is discussed. We conclude that kelatorphan and RB101 are devoid of respiratory-depressant effects and might be interesting pharmacological alternatives to morphine and other opioid agonists.

Pharmacology of amphibian opiate peptides

In 1980 the skin of certain frogs belonging to the genus Phyllomedusinae was found to contain two new peptides that proved to be selective mu-opioid agonists, and named dermorphins. Since 1987 deltorphins, a family of highly selective delta-opioid peptides were identified either by cloning of the cDNA from frog skins or isolation of the peptides. The distinctive feature of opioid peptides is the presence of a naturally occurring D-enantiomer at the second position in their common N-terminal sequence, Tyr-D-Xaa-Phe. The discovery of the amphibian opiate peptides, provided new insights into the functional role of the mu- and delta-opiate systems. It also provided models for novel analgesics with enhanced therapeutic benefits and reduced toxicity.