Endogenous opioid peptides contribute to antinociceptive potency of intrathecal [Dmt1]DALDA

Exploring μ-Opioid Receptor Splice Variants as a Specific Molecular Target for New Analgesics

Since a μ-opioid receptor gene containing multiple exons has been identified, the variety of splice variants for μ-opioid receptors have been reported in various species. Amidino-TAPA and IBNtxA have been discovered as new analgesics with different pharmacological profiles from morphine. These new analgesics show a very potent analgesic effect but do not have dependence liability. Interestingly, these analgesics show the selectivity to the morphine-insensitive μ-opioid receptor splice variants. The splice variants, sensitive to these new analgesics but insensitive to morphine, may be a better molecular target to develop the analgesics without side effects.

Neurokinin-1 receptor-based bivalent drugs in pain management: The journey to nowhere?

Hybrid compounds (also known as chimeras, designed multiple ligands, bivalent compounds) are chemical units where two active components, usually possessing affinity and selectivity for distinct molecular targets, are combined as a single chemical entity. The rationale for using a chimeric approach is well documented as such novel drugs are characterized by their enhanced enzymatic stability and biological activity. This allows their use at lower concentrations, increasing their safety profile, particularly when considering undesirable side effects. In the group of synthetic bivalent compounds, drugs combining pharmacophores having affinities toward opioid and neurokinin-1 receptors have been extensively studied as potential analgesic drugs. Indeed, substance P is known as a major endogenous modulator of nociception both in the peripheral and central nervous systems. Hence, synthetic peptide fragments showing either agonism or antagonism at neurokinin 1 receptor were both assigned with analgesic properties. However, even though preclinical studies designated neurokinin-1 receptor antagonists as promising analgesics, early clinical studies revealed a lack of efficacy in human. Nevertheless, their molecular combination with enkephalin/endomorphin fragments has been considered as a valuable approach to design putatively promising ligands for the treatment of pain. This paper is aimed at summarizing a 20-year journey to the development of potent analgesic hybrid compounds involving an opioid pharmacophore and devoid of unwanted side effects. Additionally, the legitimacy of considering neurokinin-1 receptor ligands in the design of chimeric drugs is discussed.

MEL-N16: A Series of Novel Endomorphin Analogs with Good Analgesic Activity and a Favorable Side Effect Profile

Opioid peptides are neuromodulators that bind to opioid receptors and reduce pain sensitivity. Endomorphins are among the most active endogenous opioid peptides, and they have good affinity and selectivity toward the μ opioid receptor. However, their clinical usage is hindered by their inability to cross the blood-brain barrier and their poor in vivo activity after peripheral injection. In order to overcome these defects, we have designed and synthesized a series of novel endomorphin analogs with multiple site modifications. Radioligand binding, cAMP accumulation, and β-arrestin-2 recruitment assays were employed to determine the activity of synthesized endomorphin analogs toward opioid receptors. The blood-brain barrier permeability and antinociceptive effect of these analogs were determined in several rodent models of acute and persistent pain. In addition, the side effects of the analogs were examined. The radioligand binding assay and functional activity examination indicated that the MEL-N16 series of compounds were more active agonists against μ opioid receptor than were the parent peptides. Notably, the analogs displayed biased downstream signaling toward G-protein pathways over β-arrestin-2 recruitment. The analogs showed highly potent antinociceptive effects in the tested nociceptive models. In comparison with endomorphins, the synthesized analogs were better able to penetrate the blood-brain barrier and exerted their pain regulatory activity in the central nervous system after peripheral injection. These analogs also have lower tendency to cause side effects than morphine does at similar or equal antinociceptive doses. The MEL-N16 compounds have highly potent and efficacious analgesic effects in various pain models with a favorable side effect profile.

Alanine analogues of [D-Trp]CJ-15,208: novel opioid activity profiles and prevention of drug- and stress-induced reinstatement of cocaine-seeking behaviour

BACKGROUND AND PURPOSE

The novel macrocyclic peptide cyclo[Phe-D-Pro-Phe-D-Trp] ([D-Trp]CJ-15,208) exhibits κ opioid (KOP) receptor antagonist activity in both in vitro and in vivo assays. The four alanine analogues of this peptide were synthesized and characterized both in vitro and in vivo to assess the contribution of different amino acid residues to the activity of [D-Trp]CJ-15,208.

EXPERIMENTAL APPROACH

The peptides were synthesized by a combination of solid phase peptide synthesis and cyclization in solution. The analogues were evaluated in vitro in receptor binding and functional assays, and in vivo with mice using a tail-withdrawal assay for antinociceptive and opioid antagonist activity. Mice demonstrating extinction of cocaine conditioned-place preference (CPP) were pretreated with selected analogues to evaluate prevention of stress or cocaine-induced reinstatement of CPP.

KEY RESULTS

The alanine analogues displayed pharmacological profiles in vivo distinctly different from [D-Trp]CJ-15,208. While the analogues exhibited varying opioid receptor affinities and κ and μ opioid receptor antagonist activity in vitro, they produced potent opioid receptor-mediated antinociception (ED50 = 0.28-4.19 nmol, i.c.v.) in vivo. Three of the analogues also displayed KOP receptor antagonist activity in vivo. Pretreatment with an analogue exhibiting both KOP receptor agonist and antagonist activity in vivo prevented both cocaine- and stress-induced reinstatement of cocaine-seeking behaviour in the CPP assay in a time-dependent manner.

CONCLUSIONS AND IMPLICATIONS

These unusual macrocyclic peptides exhibit in vivo opioid activity profiles different from the parent compound and represent novel compounds for potential development as therapeutics for drug abuse and possibly as analgesics.

Effects of opium addiction on level of sensory block in spinal anesthesia with bupivacaine for lower abdomen and limb surgery: a case-control study

BACKGROUND

In clinical practice, the level of sensory block in spinal anesthesia in opium abusers is lower than that in non-abusers because of adaptive changes caused by opium use.

OBJECTIVES

The aim of this study was to investigate the level of sensory block resulting from the intrathecal administration of 0.5% bupivacaine in opium abuser patients undergoing lower extremity and lower abdominal surgeries.

PATIENTS AND METHODS

A total of 100 patients who were candidates of elective lower extremity orthopedic and lower abdominal surgeries were recruited and assigned to two groups based on their history of opium addiction (Case or control). Both groups underwent the same anesthesia procedure and pinprick test was used to assess the level of anesthesia.

RESULTS

No statistically significant difference was observed between groups regarding age, duration of the surgery, and type of surgery. The frequency of addiction was higher in males than in females. The level of sensory block at three minutes was significantly lower in the opium abuser group (P = 0.006). The mean time to achieve T10 sensory block was 10.33 ± 5.79 minutes in the opium abusers and 6.89 ± 3.88 minutes in the controls (P = 0.001). The level of the highest sensory block was lower in the opium abuser group (P = 0.002).

CONCLUSIONS

The findings of this study suggested that after induction of spinal anesthesia with intrathecal administration of bupivacaine, chronic opium abusers would have a lower level of sensory block in comparison with patients without a history of opium abuse.

Effectiveness of local anesthesia with lidocaine in chronic opium abusers

CONTEXT

It had been demonstrated that chronic opium abusers have lower pain thresholds compared with than non-abusers.

AIMS

This study aimed to compare the required dose of lidocaine, a common local anesthetic drug, with regard to the onset of anesthesia in opium abusers with that in non-abusers undergoing similar suturing of minor hand lacerations.

MATERIALS AND METHODS

In this study, 109 opium abusers and 91 non-abusers underwent similar suturing for hand lacerations known as digital block. We explained the patients that their case records would remain confidential and admitting to any history of opioid abuse could assist in alleviating any pain associated with the procedure.

RESULTS

There was no statistically significant difference between the two groups with regard to age, gender, and cause of injury (P > 0.05). We found a significantly longer duration of onset of analgesia in nonopioid abusers (5.42 ± 1.93 min) than that in abusers (10.30 ± 1.79 min) (P < 0.001). In addition, the lidocaine dose administered to abusers (6.67 ± 1.21 ml) was higher (4.07 ± 1.26) as well as statistically significant (P < 0.001) than that in non-abusers.

CONCLUSION

The findings from this study suggest a longer duration of onset of anesthesia and requirement of a higher lidocaine dose in opium abusers compared with nonopioid abuser. Moreover, chronic opium abusers (4-5 years) experienced significantly higher time of onset of anesthesia compared with nonchronic abusers. This outcome may be useful in selecting a more suitable local anesthetic protocol in such patients.

[Dmt(1)]DALDA analogues with enhanced μ opioid agonist potency and with a mixed μ/κ opioid activity profile

Analogues of [Dmt(1)]DALDA (H-Dmt-d-Arg-Phe-Lys-NH2; Dmt=2',6'-dimethyltyrosine), a potent μ opioid agonist peptide with mitochondria-targeted antioxidant activity, were prepared by replacing Phe(3) with various 2',6'-dialkylated Phe analogues, including 2',6'-dimethylphenylalanine (Dmp), 2',4',6'-trimethylphenylalanine (Tmp), 2'-isopropyl-6'-methylphenylalanine (Imp) and 2'-ethyl-6'-methylphenylalanine (Emp), or with the bulky amino acids 3'-(1-naphthyl)alanine (1-Nal), 3'-(2-naphthyl)alanine (2-Nal) or Trp. Several compounds showed significantly increased μ agonist potency, retained μ receptor selectivity and are of interest as drug candidates for neuropathic pain treatment. Surprisingly, the Dmp(3)-, Imp(3)-, Emp(3)- and 1-Nal(3)-containing analogues showed much increased κ receptor binding affinity and had mixed μ/κ properties. In these cases, molecular dynamics studies indicated conformational preorganization of the unbound peptide ligands due to rotational restriction around the C(β)C(γ) bond of the Xxx(3) residue, in correlation with the observed κ receptor binding enhancement. Compounds with a mixed μ/κ opioid activity profile are known to have therapeutic potential for treatment of cocaine abuse.

Involvement of spinal release of α-neo-endorphin on the antinociceptive effect of TAPA

The antinociceptive effect of i.t.-administered Tyr-d-Arg-Phe-β-Ala (TAPA), an N-terminal tetrapeptide analog of dermorphin, was characterized in ddY mice. In the mouse tail-flick test, TAPA administered i.t. produced a potent antinociception. The antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with the κ-opioid receptor antagonist nor-binaltorphimine, as well as by the μ-opioid receptor antagonist β-funaltrexamine and the μ1-opioid receptor antagonist naloxonazine. TAPA-induced antinociception was also significantly suppressed by co-administration of the μ1-opioid receptor antagonist Tyr-d-Pro-Phe-Phe-NH2 (d-Pro(2)-endomorphin-2) but not by co-administration of the μ2-opioid receptor antagonists Tyr-d-Pro-Trp-Phe-NH2 (d-Pro(2)-endomorphin-1) and Tyr-d-Pro-Trp-Gly-NH2 (d-Pro(2)-Tyr-W-MIF-1). In CXBK mice whose μ1-opioid receptors were naturally reduced, the antinociceptive effect of TAPA was markedly suppressed compared to the parental strain C57BL/6ByJ mice. Moreover, the antinociception induced by TAPA was significantly attenuated by i.t. pretreatment with antiserum against the endogenous κ-opioid peptide α-neo-endorphin but not antisera against other endogenous opioid peptides. In prodynorphin-deficient mice, the antinociceptive effect of TAPA was significantly reduced compared to wild-type mice. These results suggest that the spinal antinociception induced by TAPA is mediated in part through the release of α-neo-endorphin in the spinal cord via activation of spinal μ1-opioid receptors.

The effect of adjuvant midazolam compared with fentanyl on the duration of spinal anesthesia with 0.5% bupivacaine in opium abusers

Background

There are a number of adjuvants to be used for local anesthetics in spinal block. The aim of this study was to demonstrate the possible effect of intrathecal midazolam compared with bupivacaine as adjuvants in spinal anesthesia with bupivacaine in chronic opium abuses.

Methods

In a double blind, randomized clinical trial, 90 opium abuser patients undergoing lower limb orthopedic surgery were selected and randomly assigned into 3 groups (30 cases each). The patients received 15 mg plain bupivacaine, or 15 mg bupivacaine plus 25 mcg fentanyl or 15 mg bupivacaine plus 1 mg midazolam, intrathecally.

Results

The duration of anesthesia was much longer in the bupivacaine-midazolam group than the bupivacaine-fentanyl group; both were longer than the plain bupivacaine group (P < 0.05).

Conclusions

Subarachnoid injection of adjuvant midazolam or fentanyl with plain 0.5% bupivacaine in opium abusers in lower limb orthopedic surgery increases the duration of sensory block. Therefore midazolam is more effective than fentanyl in such cases.

Endogenous endomorphin-2 contributes to spinal ĸ-opioid antinociception

BACKGROUND/AIMS

Multiple opioid receptor (OR) types and endogenous opioid peptides exist in the spinal dorsal horn and there may be interactions among these receptor types that involve opioid peptides. In a previous study we observed that antinociceptive effects of the selective κ-opioid receptor (κOR) agonist, U50,488H, was attenuated in μ-opioid receptor (μOR) knockout mice as compared to wild-type mice when administered spinally. This suggests that an interaction between κORs and μORs exits in the spinal cord. The present study was aimed at investigating whether endogenous opioid peptides were involved in such interaction.

METHODS

We examined whether the presence of antibodies to endogenous opioid peptides, endomorphin-2, met-enkephalin and dynorphin A affected the antinociceptive effects of spinal U50,488H in rats. The tail-flick test was used to assess pain thresholds.

RESULTS

The increase in tail-flick latency after spinal U50,488H was attenuated when the rats were pretreated intrathecally with antiserum against endomorphin-2. Pretreatments with antisera against met-enkephalin and dynorphin A had no effect on U50,488H antinociception. The antisera alone did not affect pain threshold.

CONCLUSION

The results suggest that endomorphin-2, an endogenous opioid peptide highly selective to the μOR, plays a role in antinociception induced by κOR activation in the spinal cord.

Unexpected opioid activity profiles of analogues of the novel peptide kappa opioid receptor ligand CJ-15,208

An alanine scan was performed on the novel κ opioid receptor (KOR) peptide ligand CJ-15,208 to determine which residues contribute to the potent in vivo agonist activity observed for the parent peptide. These cyclic tetrapeptides were synthesized by a combination of solid-phase peptide synthesis of the linear precursors, followed by cyclization in solution. Like the parent peptide, each of the analogues exhibited agonist activity and KOR antagonist activity in an antinociceptive assay in vivo. Unlike the parent peptide, the agonist activity of the potent analogues was mediated predominantly, if not exclusively, by μ opioid receptors (MOR). Thus analogues 2 and 4, in which one of the phenylalanine residues was replaced by alanine, exhibited both potent MOR agonist activity and KOR antagonist activity in vivo. These peptides represent novel lead compounds for the development of peptide-based opioid analgesics.

Dermorphin tetrapeptide analogs as potent and long-lasting analgesics with pharmacological profiles distinct from morphine

Dermorphin (Tyr-d-Ala-Phe-Gly-Tyr-Pro-Ser-NH(2)) is a heptapeptide isolated from amphibian skin. With a very high affinity and selectivity for μ-opioid receptors, dermorphin shows an extremely potent antinociceptive effect. The structure-activity relationship studies of dermorphin analogs clearly suggest that the N-terminal tetrapeptide is the minimal sequence for agonistic activity at μ-opioid receptors, and that the replacement of the d-Ala(2) residue with d-Arg(2) makes the tetrapeptides resistant to enzymatic metabolism. At present, only a handful of dermorphin N-terminal tetrapeptide analogs containing d-Arg(2) have been developed. The analogs show potent antinociceptive activity that is greater than that of morphine with various injection routes, and retain high affinity and selectivity for μ-opioid receptors. Interestingly, some analogs show pharmacological profiles that are distinct from the traditional μ-opioid receptor agonists morphine and [d-Ala(2),NMePhe(4),Gly-ol(5)]enkephalin (DAMGO). These analogs stimulate the release of dynorphins through the activation of μ-opioid receptors. The activation of κ-opioid receptors by dynorphins is suggested to reduce the side effects of μ-opioid receptor agonists, e.g., dependence or antinociceptive tolerance. The dermorphin N-terminal tetrapeptide analogs containing d-Arg(2) may provide a new target molecule for developing novel analgesics that have fewer side effects.

Involvement of mouse μ-opioid receptor splice variants in the spinal antinociception induced by the dermorphin tetrapeptide analog amidino-TAPA

The involvement of the mouse μ-opioid receptor (mMOR-1) splice variants in the antinociceptive effect of intrathecally (i.t.) administered N(α)-amidino-Tyr-D-Arg-Phe-β-Ala (amidino-TAPA) and [D-Ala(2),N-MePhe(4),Gly-ol(5)]enkephalin (DAMGO) was investigated in mice by monitoring the recovery from acute antinociceptive tolerance to amidino-TAPA and DAMGO. A single i.t. pretreatment with DAMGO produced an acute antinociceptive tolerance, which peaked at 2h and disappeared within 5h after the pretreatment. In contrast, a single i.t. pretreatment with amidino-TAPA produced an acute antinociceptive tolerance, which disappeared within 3h after the pretreatment. The concomitant i.t. pretreatment with an antisense oligodeoxynucleotide (ODN) for exon-1, exon-12, exon-13 or exon-14 of mMOR-1 maintained the acute antinociceptive tolerance to amidino-TAPA for 24h after the pretreatment. On the other hand, the concomitant i.t. pretreatment with an antisense ODN for exon-1 of mMOR-1, but not an antisense ODN for exon-12, exon-13 or exon-14 of mMOR-1, maintained the acute antinociceptive tolerance to DAMGO for 24h after the pretreatment. The present results suggest that the spinal antinociception of amidino-TAPA is partially mediated through the activation of the amidino-TAPA-sensitive and DAMGO-insensitive mMOR-1 splice variants MOR-1J, MOR-1K and MOR-1L, which contain the sequence encoded by exon-12, exon-13 and exon-14, respectively.

Bi- or multifunctional opioid peptide drugs

Strategies for the design of bi- or multifunctional drugs are reviewed. A distinction is made between bifunctional drugs interacting in a monovalent fashion with two targets and ligands containing two distinct pharmacophores binding in a bivalent mode to the two binding sites in a receptor heterodimer. Arguments are presented to indicate that some of the so-called "bivalent" ligands reported in the literature are unlikely to simultaneously interact with two binding sites. Aspects related to the development of bi- or multifunctional drugs are illustrated with examples from the field of opioid analgesics. The drug-like properties of the tetrapeptide Dmt(1)[DALDA] with triple action as a micro opioid agonist, norepinephrine uptake inhibitor and releaser of endogenous opioid peptides to produce potent spinal analgesia are reviewed. Rationales for the development of opioid peptides with mixed agonist/antagonist profiles as analgesics with reduced side effects are presented. Progress in the development of mixed micro opioid agonist/delta opioid antagonists with low propensity to produce tolerance and physical dependence is reviewed. Efforts to develop bifunctional peptides containing a micro opioid agonist and a cholecystokinin antagonist or an NK1 receptor antagonist as analgesics expected to produce less tolerance and dependence are also reviewed. A strategy to improve the drug-like properties of bifunctional opioid peptide analgesics is presented.

The duration of spinal anesthesia with 5% lidocaine in chronic opium abusers compared with nonabusers

BACKGROUND

It has been demonstrated that chronic opium abusers have lower thresholds for pain. In this study we sought to determine whether chronic opium abuse has any effect on the duration of spinal block by local anesthetics.

METHODS

In a case-controlled study, 50 opium abusers and 50 nonabusers undergoing lower abdomen operations were selected from among the patients admitted to a university hospital for elective surgery. All patients received 100 mg hyperbaric preservative-free 5% lidocaine in dextrose, intrathecally.

RESULTS

The duration of anesthesia was much shorter in the opium abusers (60 +/- 7 min) than in the nonabusers (83 +/- 10 min) (P < 0.0001).

CONCLUSION

The study documents a shortened duration of spinal block in opium abusers.

Morphine can produce analgesia via spinal kappa opioid receptors in the absence of mu opioid receptors

Previous studies have demonstrated the virtual lack of analgesia in mu opioid receptor knockout mice after systemic administration of morphine. Thus, it has been suggested that analgesic actions of morphine are produced via the mu opioid receptor, despite its ability to bind to kappa and delta receptors in vitro. However, it is not clear whether the results of these studies reflect the effect of morphine in the spinal cord. In the present study, we report study of the analgesic actions of spinally-administered morphine and other opioid receptor agonists in mu opioid receptor knockout and wild type mice. Morphine produced a dose-dependent antinociceptive effect in the tail flick test in the knockout mice, although higher doses were needed to produce antinociception than in wild type mice. The antinociceptive effect of morphine was completely blocked by naloxone (a non-selective opioid antagonist) and nor-binaltorphimine (nor-BNI, a selective kappa-opioid receptor antagonist), but not by naltrindole (a selective delta-opioid receptor antagonist). U-50,488H (a selective kappa-opioid receptor agonist) also produced a dose-dependent antinociceptive effect in knockout mice but presented lower analgesic potency in knockout mice than in wild type mice. Analgesic effects of [d-Pen2,d-Pen5]enkephalin (DPDPE, a selective delta-opioid receptor agonist) were observed in wild type mice but abolished in knockout mice. SNC80 (a selective delta-opioid receptor agonist) was not antinociceptive even in wild type mice. The present study demonstrated that morphine can produce thermal antinociception via the kappa opioid receptor in the spinal cord in the absence of the mu opioid receptor. Lower potency of U50,488H in mu opioid receptor knockout mice suggests interaction between kappa and mu opioid receptors at the spinal level.

Opioid peptide-derived analgesics

Two recent developments of opioid peptide-based analgesics are reviewed. The first part of the review discusses the dermorphin-derived, cationic-aromatic tetrapeptide H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, where Dmt indicates 2',6'-dimethyltyrosine), which showed subnanomolar mu receptor binding affinity, extraordinary mu receptor selectivity, and high mu agonist potency in vitro. In vivo, [Dmt(1)]DALDA looked promising as a spinal analgesic because of its extraordinary antinociceptive effect (3000 times more potent than morphine) in the mouse tail-flick assay, long duration of action (4 times longer than morphine), and lack of effect on respiration. Unexpectedly, [Dmt(1)]DALDA also turned out to be a potent and long-acting analgesic in the tail-flick test when given subcutaneously (s.c.), indicating that it is capable of crossing the blood-brain barrier. Furthermore, little or no cross-tolerance was observed with s.c. [Dmt(1)]DALDA in morphine-tolerant mice. The second part of the review concerns the development of mixed mu agonist/delta antagonists that, on the basis of much evidence, are expected to be analgesics with a low propensity to produce tolerance and physical dependence. The prototype pseudopeptide H-Dmt-TicPsi[CH(2)NH]Phe-Phe-NH(2) (DIPP-NH(2)[Psi], where Tic indicates 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid) showed subnanomolar mu and delta receptor binding affinities and the desired mu agonist/delta antagonist profile in vitro. DIPP-NH(2)[Psi] produced a potent analgesic effect after intracerebroventricular administration in the rat tail-flick assay, no physical dependence, and less tolerance than morphine. The results obtained with DIPP-NH(2)[Psi] indicate that mixed mu agonist/delta antagonists look promising as analgesic drug candidates, but compounds with this profile that are systemically active still need to be developed.

Differential respiratory effects of [Dmt1]DALDA and morphine in mice

H-Dmt-D-Arg-Phe-Lys-NH(2) ([Dmt(1)]DALDA, dDAL), a highly selective mu-opioid peptide, produces potent analgesia without respiratory depression after intrathecal administration. Despite carrying 3+ net charge, dDAL is also a potent analgesic after systemic administration. We compared the respiratory effects of dDAL and morphine after subcutaneous administration in mice using whole body plethysmography. Analgesic doses of 3 and 10 times ED(50) were examined. Both drugs dose-dependently decreased respiratory frequency and minute volume in room air. Tidal volume was increased by the lower dose of morphine, while it was decreased by the higher dose of dDAL. The decrease in minute volume by dDAL and morphine was completely reversed by naloxone. No difference in ventilatory response to CO(2) was observed between dDAL and morphine at three times ED(50). Ventilatory response to hypoxia was significantly diminished by dDAL compared to morphine and saline, and this effect of dDAL was naloxone-irreversible. Thus dDAL likely reduces the sensitivity of the peripheral chemoreflex loop through a non-opioid action.

Endogenous opiates and behavior: 2003

This paper is the 26th consecutive installment of the annual review of research concerning the endogenous opioid system, now spanning over a quarter-century of research. It summarizes papers published during 2003 that studied the behavioral effects of molecular, pharmacological and genetic manipulation of opioid peptides, opioid receptors, opioid agonists and opioid antagonists. The particular topics that continue to be covered include the molecular-biochemical effects and neurochemical localization studies of endogenous opioids and their receptors related to behavior (Section 2), and the roles of these opioid peptides and receptors in pain and analgesia (Section 3); stress and social status (Section 4); tolerance and dependence (Section 5); learning and memory (Section 6); eating and drinking (Section 7); alcohol and drugs of abuse (Section 8); sexual activity and hormones, pregnancy, development and endocrinology (Section 9); mental illness and mood (Section 10); seizures and neurologic disorders (Section 11); electrical-related activity and neurophysiology (Section 12); general activity and locomotion (Section 13); gastrointestinal, renal and hepatic functions (Section 14); cardiovascular responses (Section 15); respiration and thermoregulation (Section 16); and immunological responses (Section 17).

The Cardiovascular and Renal Effects of the Potent and Highly Selective μ Opioid Agonist [Dmt1]DALDA

The cardiovascular and renal effects of a mu opioid agonist, [Dmt]DALDA, were studied in conscious Sprague-Dawley rats. During the first hour postinjection, [Dmt]DALDA (0.025-250 microg/rat, IV) evoked a dose-dependent diuresis. The dose of 2.5 microg increased urine volume from 1.0 +/- 0.2 to 3.4 +/- 0.3 mL/h (P < 0.001, n = 30), urinary excretion of sodium, potassium, and cGMP, and induced a mild antihypertensive effect. This dose increased cumulative 4-hour urine volume but significantly inhibited sodium and potassium excretions. The renal and cardiovascular effects were abolished by naloxone (4 mg/kg), but not by naloxonazine (35 mg/kg SC), a selective mu-1 receptor antagonist. Pretreatment with 8 mg/kg naloxone methiodide, an opioid antagonist with limited access to the brain, partially inhibited the renal effects of [Dmt]DALDA. Inhibition of nitric oxide synthases with L-NAME (1 mg/kg) had no effect on the renal and cardiovascular actions of [Dmt]DALDA. Plasma ANP and AVP, measured at 20 and 120 minutes after injection, were not altered by 2.5 and 25 microg [Dmt]DALDA. Therefore, [Dmt]DALDA evokes renal and cardiovascular effects that may primarily be mediated by central naloxonazine-insensitive mu opioid receptors (non-mu-1). These findings indicate that the central mu opioid system is involved in the regulatory mechanism of renal handling of sodium and water.