Regulation of human B lymphocyte activation by opioid peptide hormones. Inhibition of IgG production by opioid receptor class (mu-, kappa-, and delta-) selective agonists

Opioid peptides have been reported by many laboratories to modulate in vitro and in vivo cell-mediated and humoral immune responses. However, less attention has been afforded to the class or classes of opioid receptors involved in these immunomodulatory effects. Previous studies by this laboratory indicated that beta-endorphin and methionine-enkephalin were potent inhibitors of Staphylococcus aureus, Cowen strain I (SAC)-induced IgG production by human B lymphocytes. Results obtained from the present studies indicate that, at pharmacological concentrations, mu-, delta-, and kappa-receptor-selective agonists are potent inhibitors of SAC-induced IgG-secreting cells (IgG-ISC) by human B lymphocytes. Moreover, the suppression of IgG-ISC formation was reversed by mu-, delta-, and kappa-receptor class-selective antagonists, [D'Tic]cTAP, ICI 174,864, and nor-BNI, respectively. These findings are in agreement with other studies showing that more than one class of receptors are involved in opioid peptide-mediated immunoregulation. Additional studies indicated that all three class-selective receptor agonists were found to suppress SAC-induced IL-6 production in intact PBMC cultures. As observed for suppression of IgG-ISC formation, inhibition of IL-6 production was found to be reversed by the appropriate receptor class-selective antagonist. These results support the hypothesis that one mechanism of opioid peptide-mediated inhibition of antibody production is via the down regulation of cytokine synthesis.

Blood-to-central nervous system entry and stability of biphalin, a unique double-enkephalin analog, and its halogenated derivatives

Biphalin (Tyr-D-Ala-Gly-Phe-NH)2 is a unique opioid peptide analog that contains two active enkephalin pharmacophores and is more potent than morphine and etorphine in eliciting analgesia after intrathecal administration. After systemic administration, only a small amount was detected in the brain, but analgesia was observed. Because halogenation of enkephalin analogs has been shown to increase the brain uptake after systemic administration, our research group synthesized both p-[Cl-Phe4,4']biphalin and p-[F-Phe4,4']biphalin. The aim of the present study was to characterize and compare the blood-to-central nervous system (CNS) pharmacokinetics and biological stability of biphalin and related halogenated analogs. The initial screening used an in vitro blood-brain barrier model and identified p-[Cl-Phe4,4')biphalin as the enkephalin analog with the best potential for greater CNS entry. The CNS uptake and stability of biphalin and p-[Cl-Phe4,4']biphalin was examined further using an in situ brain perfusion technique coupled to high-performance liquid chromatography analysis. Both biphalin and its chlorohalogenated analog, were found to significantly enter the CNS through both the blood-brain and blood-cerebrospinal fluid barriers. Chlorohalogenation of biphalin was shown to both improve CNS entry, most likely through an enhancement in lipophilicity, and increase biological stability. This study suggests that incorporation of chlorohalogens at the p-Phe4,4' position is a promising structural modification in the development of biphalin as a successful opioid drug for the clinic.

Clocinnamox antagonism of opioid suppression of schedule-controlled responding in rhesus monkeys

The antagonist effects of clocinnamox were evaluated against opioid agonists, acting at mu, kappa and delta-receptors, in rhesus monkeys (n = 3-4) responding under a fixed-ratio 30 (FR 30) schedule for food delivery. Clocinnamox (0.032-0.1 mg/kg) dose-dependently antagonized fentanyl (0.001-0.32 mg/kg) after either a 3-h or 1-day pretreatment; there was substantial recovery of agonist potency by 1 week after clocinnamox. Etonitazene (0.0001-0.01 mg/kg) was also antagonized by clocinnamox (0.1 mg/kg), but to a lesser extent than fentanyl. The smaller extent of antagonism was not due to the appearance of non mu-opioid response-decreasing effects of etonitazene, since the competitive antagonist quadazocine (0.1 mg/kg) shifted the etonitazene dose-effect curve in the presence of clocinnamox (0.1 mg/kg). Clocinnamox (0.1-0.32 mg/kg) did not antagonize the rate-suppressing effects of the delta-agonist BW373U86 (0.0.01-1.0 mg/kg) or the kappa-agonist U69,593 (0.001-0.032 mg/kg). These results are consistent with previous in vivo and in vitro evidence that characterized clocinnamox as an insurmountable antagonist, with selectivity for mu- over kappa- and delta-receptors.

delta- and mu-opioid receptor mobilization of intracellular calcium in SH-SY5Y human neuroblastoma cells

1. In this study we have investigated delta and mu opioid receptor-mediated elevation of intracellular Ca2+ concentration ([Ca2+]i) in the human neuroblastoma cell line, SH-SY5Y. 2. The Ca(2+)-sensitive dye, fura-2, was used to measure [Ca2+]i in confluent monolayers of SH-SY5Y cells. Neither the delta-opioid agonist, DPDPE ([D-Pen2,5]-enkephalin) nor the mu-opioid agonist, DAMGO (Tyr-D-Ala-Gly-N-Me-Phe-Gly-ol enkephalin) elevated [Ca2+]i when applied alone. However, when either DPDPE or DAMGO was applied in the presence of the cholinoceptor agonist, carbachol (100 nM-1 mM) they evoked an elevation of [Ca2+]i above that caused by carbachol alone. 3. In the presence of 1 microM or 100 microM carbachol, DPDPE elevated [Ca2+]i with an EC50 of 10 nM. The elevation of [Ca2+]i was independent of the concentration of carbachol. The EC50 for DAMGO elevating [Ca2+]i in the presence of 1 microM and 100 microM carbachol was 270 nM and 145 nM respectively. 4. The delta-receptor antagonist, naltrindole (30 nM), blocked the elevations of [Ca2+]i by DPDPE (100 nM) without affecting those caused by DAMGO while the mu-receptor antagonist, CTAP (D-Phe-Cys-Tyr-D-Trp-Arg-Pen-Thr-NH2) (100 nM-1 microM) blocked the elevations of [Ca2+]i caused by DAMGO (1 microM) without affecting those caused by DPDPE. 5. Block of carbachol activation of muscarinic receptors with atropine (10 microM) abolished the elevation of [Ca2+]i by the opioids. The nicotinic receptor antagonist, mecamylamine (10 microM), did not affect the elevations of [Ca2+]i caused by opioids in the presence of carbachol. 6. Muscarinic receptor activation, not a rise in [Ca2+]i, was required to reveal the opioid response. The Ca2+ channel activator, maitotoxin (3 ng ml-1), also elevated [Ca2+]i but subsequent application of opioid in the presence of maitotoxin caused no further changes in [Ca2+]i. 7. The elevations of [Ca2+]i by DPDPE and DAMGO were abolished by pretreatment of the cells with pertussis toxin (200 ng ml-1, 16 h). This treatment did not significantly affect the response of the cells to carbachol. 8. The opioids appeared to elevate [Ca2+]i by mobilizing Ca2+ from intracellular stores. Both DPDPE and DAMGO continued to elevate [Ca2+]i when applied in nominally Ca(2+)-free external buffer or when applied in a buffer containing a cocktail of Ca2+ entry inhibitors. Thapsigargin (100 nM), an agent which discharges intracellular Ca2+ stores, also blocked the opioid elevations of [Ca2+]i. 9. delta and mu Opioids did not appear to mobilize intracellular Ca2+ by modulating the activity of protein kinases. The application of H-89 (10 microM), an inhibitor of protein kinase A, H-7 (100 microM), an inhibitor of protein kinase C, protein kinase A and cyclic GMP-dependent protein kinase, or Bis I, an inhibitor of protein kinase C, did not alter the opioid mobilization of [Ca2+]i. 10. Thus, in SH-SY5Y cells, opioids can mobilize Ca2+ from intracellular stores but they require ongoing muscarinic receptor activation. Opioids do not elevate [Ca2+]i when applied alone.

Opioid infidelity: novel opioid peptides with dual high affinity for delta- and mu-receptors

Deltorphins represent the paragon of delta-opioid-receptor ligands of natural origin, since they exceed the affinity and selectivity of the endogenous enkephalins by orders of magnitude. A series of opioid peptides have been developed in which the change in a single amino acid causes an extraordinary increase in mu-receptor binding while maintaining high affinity for the delta-receptor. The peptides appear to have a similar extended conformation in solution with a type-I beta-turn in the N-terminus region, suggesting that tertiary architecture plays a pivotal role in enabling the peptide to bind indiscriminately to mu- and delta-receptors. These dual-affinity peptide ligands can serve to mask delta- and mu-receptors while mapping kappa-receptors in the nervous system, to provide an understanding of the differences and similarities in the structure of the binding domains of delta- and mu-receptors, and might lead to a comprehensive new regime for the clinical management of acute and chronic pain.

Neurotoxic versus neuroprotective actions of endogenous opioid peptides: implications for treatment of CNS injury

Endogenous opioid systems seem to have both neurodestructive and neuroprotective roles in CNS injury. Whereas mu and kappa 1 receptors appear to mediate neuroprotective actions, kappa 2 receptors may be involved in secondary injury responses. Among the endogenous opioids, dynorphin has marked neurotoxic effects when given intrathecally to rats; when administered in subinjury doses, dynorphin exacerbates the response to brain or spinal cord trauma. Because of the neurotoxic effects of dynorphin, one should employ this compound with great caution in human studies of addiction treatment. It has not been established which endogenous opioids might be protective. Taken together, these observations may suggest novel approaches to the treatment of CNS injury using selective mixed opioid agonist-antagonist compounds such as buprenorphine.

[The role of opioid neuropeptides and adrenocorticotropic hormone in the mechanisms compensating for disordered central nervous system functions during stimulation and after destruction of the limbic structures in the brain of the hedgehog Erinaceus auritus]

On the model of food behaviour of hedgehogs possibilities of disrupted acquired and congenital forms of nervous activity have been studied using opioid neuropeptides, beta-endorphin, met-enkephalin and adrenocorticotropin (ACTH). It was established that preliminary introduction of opioid neuropeptides removed inhibitory effects of stimulation of limbic cortex on conditioned-reflex brain activity. The influence of different neuropeptides on higher nervous activity and behaviour revealed strict tendency to specialization. In particular, at met-enkephalin introduction latent periods of conditioned food-procuring reactions were essentially shortened, fading inhibition was not developed and intersignal, motor and food excitement were increased. At beta-endorphin introduction formation of fading inhibition did ton differ from the one in norm. On the background of ACTH introduction incomplete removing of inhibitory effects of limbic cortex irritation was revealed. Met-enkephalin introduction to animals with broken limbic cortex and basolateral part of amygdala was accompanied by restoration of food-procuring, orientation-research and motor activity. Restoration of acquired forms of nervous activity was uncomplete and transitory. The problem of possibility of neurochemical compensation of disrupted functions in mammalian brain is discussed.

Positive and negative immunomodulation by opioid peptides

The data that follow review part of the existing evidence concerning the neuroimmune functions mediated by opioid peptides, with particular regard to dual immunomodulatory effects. Limited references to substances other than opioid peptides are included, mainly to emphasize the possible similarities in the mediation of neuroimmune interactions by different informational substances, while the interactions directed from the immune to the nervous system have deliberately been omitted.

Biochemical and pharmacological characterization of multiple beta-endorphinergic antinociceptive systems in the rat periaqueductal gray

Based on the differential abilities of the opioid antagonists naltrexone and D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (CTP) to antagonize the antinociceptive action of beta-endorphin and morphine in the rat periaqueducatal gray (PAG), three pharmacologically distinct mechanisms were determined to mediate the antinociceptive effect of beta-endorphin. Two of these mechanisms are unique to beta-endorphin, possess a high affinity for CTP and can be discriminated based on their differential sensitivity to naltrexone. The third mechanism displays characteristics common to that activated by morphine. The results of radioligand binding studies were consistent with these observations. [125I]-beta-Endorphin labeled a population of sites in the PAG which (compared to those labeled by [3H]morphine) displayed a significantly higher affinity for CTP. In addition, a naltrexone-insensitive binding component was identified in the [125I]-beta-endorphin, but not [3H]morphine assays. Furthermore, comparable competitor affinities were determined across assays, suggesting an interaction of the radioligands with common PAG sites. A naltrexone-insensitive component to beta-endorphin antinociception also was identified in studies which evaluated the ability of the antagonist to shift the beta-endorphin dose-response curve. Interestingly, the ability of low doses of CTP and naltrexone to inhibit increasing doses of beta-endorphin was described by a U-shaped dose effect curve. The response to low and high, but not intermediate, doses of beta-endorphin were antagonized by picomole doses of both antagonists. As there was no evidence for allosteric interactions between [125I]-beta-endorphin binding sites in the PAG, it appears that beta-endorphin also may activate pain facilitory mechanisms which counterbalance its overall antinociceptive effect.

Pronociceptive effects of the neuropeptide, nociceptin, in the land snail, Cepaea nemoralis

The peptide, Phe-Gly Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln-OH, recently isolated from rat brain, has been suggested to be an endogenous agonist for an orphan, opioid-like receptor (ORL1). This peptide, called "nociceptin" (or orphanin FQ), has been suggested to have pronociceptive, hyperalgesic functions. The present study examined the effects of nociceptin on aversive thermal (nociceptive) responses in an invertebrate, the land snail, Cepaea nemoralis. Nociceptin had significant, dose-related pro-nociceptive effects in Cepaea, whereas the opioid peptide, dynorphin A, which shares some sequence similarities with nociceptin, had significant antinociceptive effects. The effects of dynorphin were blocked by the kappa-opiate receptor antagonist, nor-binaltorphimine, whereas those of nociceptin were unaffected. Repeated daily administrations of nociceptin resulted in reduced pronociceptive effects, suggestive of the development of tolerance to the hyperalgesic actions of this opioid-related peptide. These findings suggest that the novel peptide, nociceptin, can influence nociceptive responses in the snail, Cepaea, in a manner similar to that indicated for rodents.

Orphanin FQ: receptor binding and analog structure activity relationships in rat brain

A tritiated form of orphanin FQ (a heptadecapeptide also known as Nociceptin) has been prepared. This radioligand (33 Ci/mmole) was used to develop a radioreceptor assay using rat brain homogenates. Binding was observed to be saturable, and analyses of the binding data indicate the presence of a single binding site with a dissociation constant of 5 +/- 1.1 nM and Bmax of 535 +/- 85 fmoles/mg protein. Thirty-four analogues of orphanin FQ, including a complete alanine "scan" of orphanin FQ, and truncation analogues from both the N- and C- terminals were synthesized and tested. The data obtained indicate that the N-terminus plays a more critical role in binding than the C-terminus and that residues 1, 2, 4, and 8 are essential for binding.

Orphanin FQ: a neuropeptide that activates an opioidlike G protein-coupled receptor

A heptadecapeptide was identified and purified from porcine brain tissue as a ligand for an orphan heterotrimeric GTP-binding protein (G protein)-coupled receptor (LC132) that is similar in sequence to opioid receptors. This peptide, orphanin FQ, has a primary structure reminiscent of that of opioid peptides. Nanomolar concentrations of orphanin FQ inhibited forskolin-stimulated adenylyl cyclase activity in cells transfected with LC132. This inhibitory activity was not affected by the addition of opioid ligands, nor did the peptide activate opioid receptors. Orphanin FQ bound to its receptor in a saturable manner and with high affinity. When injected intracerebroventricularly into mice, orphanin FQ caused a decrease in locomotor activity but did not induce analgesia in the hot-plate test. However, the peptide produced hyperalgesia in the tail-flick assay. Thus, orphanin FQ may act as a transmitter in the brain by modulating nociceptive and locomotor behavior.

Role of opioid peptides in the regulation of DNA synthesis in immature rat uterus

The effects of a single dose of naloxone and of [D-Met2,Pro5]enkephalinamide on the DNA synthesis in the uterus of 7, 14 and 21-day-old rat were studied. After [D-Met2,Pro5]enkephalinamide treatment, an age-dependent decrease in in vitro [3H]thymidine incorporation into DNA was observed in all studied age groups. In the 21-day-old age group a reduced rate of DNA synthesis was detected for 12 h after [D-Met2,Pro5]enkephalinamide treatment followed by the return to control values at 24 h. The rate of inhibition was more marked in the younger age groups. The effect was also more pronounced in younger animals. Specific [3H]naloxone binding was detected both in membrane and nuclear fractions of uterine homogenates. While no age-related changes in binding affinities were found, the number of binding sites varied characteristically during development. Our data suggest the novel involvement of opioid peptides and their receptors in the regulation of uterine development.

Opioid modulation of fetal glucose homeostasis: role of receptor subtypes

Opioids have long been known to influence glucose homeostasis in the adult. However, their role in modulating glucose regulation in the fetus is not known. The objectives of this study were to determine the effects of morphine on fetal plasma glucose levels and to ascertain the role of opioid receptor subtypes in fetal glucose homeostasis. The studies were carried out in 38 unanesthetized fetal sheep (123-142 days) (term being approximately 145 days). Intravenous infusion of morphine to the fetus resulted in dual actions on fetal plasma glucose, with hypoglycemia after 1.2 mg/hr (F3,16 = 6.02; P = .006; n = 5) and hyperglycemia after 5.0 mg/hr (F3,16 = 5.58; P = .008; n = 5). Significant increase in plasma lactate concentration also was found after 5.0 mg/hr (F3, 16 = 5.25; P = .010). Both hypoglycemia and hyperglycemia were antagonized by i.v. naloxone, indicating both were mediated by specific opioid receptors. The mu-selective agonist, [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (100 micrograms/hr i.c.v., n = 6), resulted in a significant increase in both plasma glucose (F3,20 = 11.50; P = .001) and lactate (F3,20 = 3.77; P = .007) concentrations. In contrast, the delta-selective agonists, [D-Pen2,D-Pen5]-enkephalin (30 and 100 micrograms/hr i.c.v.) and [D-Ala2]-deltorphin I (0.3 and 1.0 micrograms/hr i.c.v.) had no effect on plasma glucose or lactate levels. Similarly, Dynorphin A(1-13) (160 and 480 micrograms/hr i.c.v.) and U50,488H [trans-(+/-)-3,4-dichloro-N-methyl-[2-(1-pyrrolidinyl)-cyclohexyl] benzeneacetamide] (200 micrograms/hr i.c.v.) also had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid and anti-opioid peptides

The numerous endogenous opioid peptides (beta-endorphin, enkephalins, dynorphins ... ) and the exogenous opioids (such as morphine) exert their effects through the activation of receptors belonging to four main types, mu, delta, kappa and epsilon. Opioidergic neurones and opioid receptors are largely distributed centrally and peripherally. It is thus not surprising that opioids have numerous pharmacological effects and that endogenous opioids are thought to be involved in the physiological control of various functions, among which nociception is particularly emphasized. Some opioid targets may be components of homeostatic systems tending to reduce the effects of opioids. "Anti-opioid" properties have been attributed to various peptides, especially cholecystokinin (CCK), neuropeptide FF (NPFF) and melanocyte inhibiting factor (MIF)-related peptides. In addition, a particular place should be attributed, paradoxically, to opioid peptides themselves among the anti-opioid peptides. These peptides can oppose some of the acute effects of opioids, and a hyperactivation of anti-opioid peptidergic neurones due to the chronic administration of opioids may be involved in the development of opioid tolerance and/or dependence. In fact, CCK, NPFF and the MIF family of peptides have complex properties and can act as opioid-like as well as anti-opioid peptides. Thus, "opioid modulating peptides" would be a better term to designate these peptides, which probably participate, together with the opioid systems, in multiple feed-back loops for the maintenance of homeostasis. "Opioid modulating peptides" have generally been shown to act through the activation of their own receptors. For example, CCK appears to exert its anti-opioid actions mainly through the activation of CCK-B receptors, whereas its opioid-like effects seem to result from the stimulation of CCK-A receptors. However, the partial agonistic properties at opioid receptors of some MIF-related peptides very likely contribute to their ability to modulate the effects of opioids. CCK- and NPFF-related drugs have potential therapeutic interest as adjuncts to opioids for alleviating pain and/or for the treatment of opioid abuse.

Synthesis and opioid activities of [D-Leu-8]Dynorphin(1-8) analogs containing a reduced peptide bond, psi(CH2-NH)

[D-Leu8]Dynorphin(1--8)-NH2 analogs, in which each peptide bond was systematically replaced with a psi(CH2NH) peptide bond, were synthesized by the solid-phase method. The psi(CH2NH) bond was introduced by the Boc-amino acid aldehyde/NaCNBH3 method on a solid support. In the syntheses of the analogs, undesirable double alkylation took place at the sequences of Tyr1 psi(CH2NH)Gly2 and Gly2 psi(CH2NH)Gly3, possibly due to the low steric hindrance of the glycine residue. To suppress the double alkylation, a minimum amount of aldehydes was employed. In the receptor binding assay, the psi(CH2NH) replacement of N-terminal peptide bonds which led to 1 psi 2-(2) and 2 psi 3-analogs (3) resulted in a marked reduction in binding affinities for mu-, delta-, and kappa-opioid receptors, while that of the other peptide bonds afforded analogs with a high kappa-receptor affinity. A 3 psi 4-analogs (4) showed extremely high kappa-receptor selectivity (mu/kappa Ki ratio = 339, delta/kappa Ki ratio = 24104). In the in vitro bioactivity assay (guinea pig ileum assay), 2 showed a very low IC50 ratio (2.0) in the presence and absence of peptidase inhibitors whereas those of other analogs were >27, suggesting that the introduction of the CH2NH isostere at Tyr1-Gly2 greatly enhanced the enzymatic stability of the parent peptide. Furthermore, analogs 2 and 3 showed a very low sensitivity to the inhibitory effect of NaCl plus 5'-guanylylimidodiphosphate on their binding at a kappa-receptor site as compared with the other analogs and the parent peptide. These results suggest that the two analogs (2 and 3) have partial kappa-antagonist properties.

Peptide and nonpeptide ligands for opioid receptors

Morphine and other natural alkaloid opiates have been used in medicine for centuries. Synthesis of analogs of opiate alkaloids and primary structure activity studies have almost a hundred year history. The endogenous opioid peptides, their genetic expression and enzymatic metabolism, have been described. A number of non-peptide and peptide analogs have led to characterization of opioid receptor types (mu, delta, and kappa) and propose their subtypes. Very recently, all types of opioid receptors of different species have been characterized at the molecular level. The progressive studies of the opioid system have allowed introduction of various new types of drugs. In addition, the opioid system, as one of the best characterized, is often used as a model for studies in neurobiology as well as in bioorganic chemistry. Therefore, opioid system is a good example of the tremendous progress in medicinal chemistry, but also an illustrations of the limitations of scientific tools currently used.

Complex avoidance behaviour and its neurochemical regulation in the land snail Cepaea nemoralis

1. In hot plate experiments, the pulmonate land snail Cepaea nemoralis displays a biphasic passive/active avoidance behaviour composed of retraction and subsequent searching mediated by antagonistic muscular systems. The switch, between the behaviours, is under neuronal control. 2. Leu- and met-enkephalin, as well as FMRFamide-antiserum, attenuated the retraction response and potentiated the searching behaviour. Opposite effects were achieved by injection of antisera to the enkephalins. 3. Both retraction and searching behaviours were potentiated by 5-HT. Methysergide antagonized the effects of the enkephalins on the searching behaviour. 4. We conclude that endogenous opioids act antagonistic to FMRFamide in the neuronally controlled switch between passive and active avoidance behaviour.

Structure-activity relationships of dermorphin analogues containing N-substituted amino acids in the 2-position of the peptide sequence

A series of dermorphin analogues containing an N-alkylated amino-acid residue Xaa in the 2-position of the peptide sequence was synthesized (Xaa = N-methylalanine, proline, pipecolic acid, N-methylphenylalanine, 1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid [Tic]). These peptides have the potential of assuming a cis Tyr1-Xaa2 peptide bond. Their in vitro opioid activity profiles were determined in mu- and delta-receptor-representative binding assays and bioassays. Aside from [D-Pro2]dermorphin, all analogues showed high affinity for mu- and/or delta-opioid receptors. Whereas most compounds were found to be full mu-agonists in the guinea pig ileum (GPI) assay, [Tic2]dermorphin (compound 7) was a partial mu-agonist. Replacement of Gly4 in 7 with Phe resulted in an analogue (8) with weak mu-antagonist activity. Furthermore, analogues 7 and 8 both were potent delta-antagonists (Ke = 3-40 nM) against the delta-agonists Leu-enkephalin, DPDPE and deltorphin I in the mouse vas deferens (MVD) assay. Compound 3, containing L-Pro in the 2-position, turned out to be one of the most mu-receptor-selective linear dermorphin analogues reported to date. Low-temperature HPLC experiments using micropellicular octadecyl silica as stationary phase revealed conformational heterogeneity of the dermorphin analogues which was ascribed to cis-trans isomerization around the Tyr1-Xaa2- and Tyr5-Pro6 peptide bonds. In the case of analogue 7 four separate peaks corresponding to the four possible isomers were apparent at -5 degrees C.(ABSTRACT TRUNCATED AT 250 WORDS)

Enhancement of phagocytosis by dynorphin A in mouse peritoneal macrophages

The effects of the opioid peptide dynorphin A (DynA) on phagocytosis in peritoneal macrophages was examined by flow cytometry (FCM). DynA enhanced phagocytosis in a dose-dependent manner. Leucine-enkephalin (Leu-Enk), methionine-enkephalin (Met-Enk), beta-neo-endorphin (beta Neo-End), DynA(9-17) and DynA(13-17) had no such activity. Alpha-Neo-endorphin (alpha Neo-End), dynorphin B (DynB), DynA(1-13) and DynA(6-17) enhanced phagocytosis less effectively than DynA. Naloxone did not inhibit the enhancement of phagocytosis induced by DynA. Unstimulated control phagocytosis was partially suppressed in Ca2+-free EGTA-containing solution and even in this solution DynA enhanced phagocytosis. However, the enhancement by DynA was suppressed in EGTA- and BAPTA-AM-containing Ca2+-free solution. The present study showed that enhancement of phagocytosis by DynA was independent of extracellular Ca2+ ([Ca2+]o) and dependent on intracellular Ca2+ ([Ca2+]i). The present results support DynA being one of the mediators from the nervous system that modulates the immune system.

Endogenous morphine

This review surveys the discovery of endogenous alkaloids in mammals. The formation of morphine in mammalian brain was assumed in 1970. The existence of morphine was demonstrated by radioimmunoassay. Identification of morphine was performed by spectroscopic methods. The isolation of mammalian morphine raises the question of biosynthesis. Recently, it has been shown that the biosynthetic pathway is similar to that that exists in poppy.

Assessment of the discriminative stimulus effects of cocaine in the rat: lack of interaction with opioids

The present study examined the effects of several opioid agonists and antagonists in rats trained to discriminate cocaine (10 mg/kg) from saline in a two-lever, food-reinforced, discrimination task. Neither fentanyl, a mu agonist, nor the delta agonist BW 373U86 elicited cocaine-appropriate responding. Although pretreatment with fentanyl failed to alter the discriminative stimulus effects of low doses of cocaine, cocaine reversed the rate-suppressant effects of fentanyl. Although the kappa agonist U50,488H decreased response rates, it did not substitute for cocaine. Injection of U50,488H in combination with the training dose of cocaine (10 mg/kg) reversed the rate-suppressant effects of U50,488H but failed to affect the cocaine cue. Administration of U50,488H (3 mg/kg), in conjunction with several doses of cocaine, did not shift the cocaine dose-response curve. Naltrindole and naltrexone, delta and mu antagonists respectively, did not block the effects of cocaine. Further, naltrindole did not substitute for the cocaine cue. Complete generalization was observed to the dopamine uptake inhibitor bupropion (30 mg/kg). These results suggest that fentanyl and U50,488H, at doses that purportedly influence mesolimbic dopamine levels, do not alter the discriminative stimulus effects of cocaine. Moreover, activation of delta receptors and blockade of mu and delta receptors are similarly ineffective.

Evidence for the involvement of opioid peptides in phagocytosis, conformation, granulation and aggregation of immunocompetent Lumbricus terrestris amoebocytes

Opioid peptides and their analogs were shown to enhance Lumbricus terrestris amoebocyte phagocytosis, aggregation, granulation and conformation. These actions appear to be concentration dependent with 10(-8) M met-enkephalin and 10(-6) M dynorphin A having the greatest effect on the earthworm amoebocytes (P < 0.05). Interleukin-1 alpha significantly enhanced met-enkephalin mediated earthworm amoebocyte aggregation; leu-enkephalin mediated phagocytosis and aggregation; DAMA mediated phagocytosis; and U69,593 mediated phagocytosis, aggregation and conformation (P < 0.05). Naloxone generally inhibited these activities indicating opioid receptor mediated mechanisms for earthworm amoebocyte function.

Evidence for peptidase activity in the rat intestine

1. Cholinergic contraction was induced in segments of rat jejunum by transmural stimulation (10 Hz, 1 ms for 8 s). The synthetic delta-opiate agonist, DADLE (100 nM), caused a prolonged inhibition of the cholinergic response. 2. The naturally occurring opioid peptides, dynorphin A (1-13) (200 nM), leu-enkephalin (400 nM), met-enkephalin (200 nM) and the synthetic delta-agonist, DSLET (30 nM), also caused large inhibitions in the response. 3. Each of these peptides lost a significant amount of their original activity at 6 min, which was reduced by a mixture of peptidase inhibitors consisting of bestatin (30 microM), thiorphan (10 microM), captopril (10 microM) and L-leucyl-L-leucine (2 mM). 4. The enkephalinase inhibitor, thiorphan (10 microM), significantly lengthened the time at which met-enkephalin was active, but not to the same extent as the mixture of peptidase inhibitors. However, the mixture of peptidase inhibitors did not significantly alter the cholinergic contraction in the absence of opioid peptides. 5. It is concluded that peptidases, including enkephalinase, are present in the rat intestine. However, the model presently described does not release functional amounts of endogenous opioid peptides, nor does it become tolerant to the effect of stimulating its delta-opioid receptors.

Supersensitivity to opioid analgesics following chronic opioid antagonist treatment: relationship to receptor selectivity

The effect of chronic opioid antagonist treatment on the analgesic potency of six opioid agonists was compared to changes in opioid receptor density and the selectivity of each agonist for mu (DAMGO), delta (DPDPE) and kappa (U69,593) opioid receptors. Mice were implanted SC with a 15-mg naltrexone or placebo pellet for 8 days. The pellets were removed and 24 h later, mice were sacrificed and binding studies were conducted, or mice were tested in analgesia (tail-flick) dose-response studies. All six analgesics acted as full agonists for both placebo and naltrexone-treated mice. Naltrexone increased the analgesic potency of methadone, etorphine, fentanyl, meperidine, and oxycodone by 1.9-3.2-fold. The analgesic potency of propoxyphene was not increased significantly (1.3-fold). In saturation binding studies in brain homogenate, naltrexone increased the Bmax of mu, delta, and kappa opioid receptors by 86, 43, and 33%, respectively, without altering Kd. Competition binding studies for each receptor type were conducted in brains from untreated mice, and KIs were determined for each agonist. All agonists had greatest selectivity toward mu compared with delta and kappa receptors. There did not appear to be an obvious relationship between receptor selectivity and the magnitude of supersensitivity. These studies indicate that supersensitivity occurs for a broad range of opioid analgesics following chronic opioid antagonist treatment in the mouse. However, the selectivity of these agonists for mu, delta, and kappa receptors does not appear to correlate with differences in supersensitivity.

Highly potent side chain-main chain cyclized dermorphin-deltorphin analogues: an integrated approach including synthesis, bioassays, NMR spectroscopy and molecular modelling

Our continuing efforts to study structure-activity relationships of peptide opioids have resulted in the synthesis of a series of cyclic opioids related to dermorphins and deltorphins. The biological activities of the compounds have been determined and the conformational analyses carried out using 1H-NMR spectroscopy and molecular modelling. The three compounds in the series Tyr-c[D-Orn-Phe-Ala], Tyr-c[D-Lys-Phe-Ala], and Tyr-c[A2bu-Phe-Ala-Leu] are cyclized via a lactam bridge from the side-chain of the residue at the second position with the carboxyl terminus of each compound. The molecules incorporate 12-, 13- and 14-membered rings, respectively. They include a phenylalanine at the third position which is a distinguishing characteristic of dermorphins and deltorphins. The guinea pig ileum and mouse vas deferens assays show that the compounds are highly active at both mu- and delta-opioid receptors. The compounds are all highly effective antinociceptive agents as measured by the intrathecal rat hot plate test. Conformational analyses of the molecules indicate that they can adopt topochemical arrays required for bioactivity at both mu- and delta-receptors which explains their high activity in both guinea pig ileum and mouse vas deferens in vitro assays. The results support our models for mu- and delta-receptor activity for constrained peptide opioids.

The dopaminergic system modulates the endogenous opioid system in guinea-pig isolated ileal longitudinal muscle

The effects of the dopamine antagonists haloperidol and sultopride were investigated on the twitch response, evoked by 0.1 Hz stimulation of guinea-pig isolated ileal longitudinal muscle, and on the inhibition of the twitch response induced by 10 Hz stimulation (post-tetanic twitch inhibition) and by application of opioids. Both haloperidol and sultopride concentration-dependently inhibited the twitch response, with threshold concentrations of 2 and 50 microM, respectively, and could also shift the concentration-response curve for ACh-contraction to the right in a non-competitive manner. Haloperidol (1 microM) and sultopride (20 microM) increased post-tetanic twitch inhibition and this could be prevented by naloxone (100 nM). Twitch inhibition induced by morphine and dynorphin 1-13 was not affected by haloperidol (1 microM) or sultopride (20 microM). Prazosin (1 microM) and yohimbine (2 microM) did not affect either the twitch response or the post-tetanic twitch inhibition. These results suggest that dopamine receptors are involved in the modulation of the ileal opioid system, in such a manner as to diminish the release of endogenous opioids by tetanic stimulation.

[Opioid receptors in the era of molecular biology: they account for more than 6,000 years of empirical pharmacology]

The medicinal use of opium and of morphine in different cultures and ancient civilizations is described. Research within the past 40 years have demonstrated the existence of brain opiate receptors. Morphine and related opioid analgetic interact at these sites in the nervous system to produce the characteristic pharmacological effects of these drugs. The opiate receptors have structural homologies with a variety of other cell membrane receptors; they activate second messenger-based chemical transduction systems in the cell membrane and are endowed with several regulation mechanisms. These opiate receptors are presumably activated under specific physiological conditions by endogenous ligands (opiopeptins). It is currently thought that morphine mimicks the opiopeptins by interacting with these receptors either at different molecular subsites or with a different mode of action.

Opioid blockade attenuates acquisition and expression of cocaine-induced place preference conditioning in rats

Endogenous opioid systems have been implicated in experimental cocaine addiction. One aspect of this involvement may be the modulation of the motivational properties of cocaine by endogenous opioids. The present study assessed the effect of opioid blockade with naloxone (NLX) on cocaine's motivational properties using the conditioned place preference procedure. Treatment with doses of NLX that did not induce place aversion (0.01-1.0 mg/kg-1, SC), dose-dependently attenuated place preference induced by cocaine (10 or 20 mg/kg-1, IP). This effect of NLX was present when administered during acquisition of cocaine-induced place preference and when administered before expression of cocaine's motivational effects. These data support the notion that the (conditioned) motivational properties of cocaine are modulated through activation of opioid systems by endogenous opioid peptides. Furthermore, it is suggested that an interaction between endogenous opioid systems and dopaminergic systems in the brain might be of importance in the motivational facilitation of experimental cocaine addiction.

Opioid tolerance and dependence in the magnocellular oxytocin system: a physiological mechanism?

At the neurosecretory terminals in the neural lobe, oxytocin secretion is restrained by co-secreted endogenous opioids, which act via kappa-receptors. The co-secreted opioids include products of pro-dynorphin (released by both vasopressin and oxytocin terminals) and proenkephalin (released by oxytocin terminals). In morphine-tolerant rats this opioid mechanism is more effective, but in late pregnancy it is less effective. Opioids also act directly on oxytocin cell bodies, via separate mu- and kappa-receptors, inhibiting excitation by all stimuli tested, and also exert presynaptic and more distal actions on afferent systems. During chronic morphine exposure, tolerance and dependence develop in oxytocin neurones; the former involves reduction in mu-opioid receptor density, while the latter may involve compensatory upregulation of mechanisms regulating Ca2+ influx. In mid-pregnancy, the effectiveness of opioid mechanisms in the neural lobe increases, assisting the accumulation of oxytocin stores in advance of parturition, but by the end of pregnancy the effectiveness of these mechanisms is reduced. At this time, a separate endogenous opioid system, acting via mu-receptors, actively restrains the electrical activity of oxytocin neurones. Release of this endogenous opioid inhibition may contribute to the increase in activity during parturition analogous to that occurring during morphine withdrawal excitation. Central opioid mechanisms retain the ability to control oxytocin neurones during parturition, and can interrupt established parturition by inhibiting oxytocin neurone firing rate in disadvantageous environmental circumstances.

Expression of the conditioned NK cell activity is beta-endorphin dependent

We are interested in identifying the pathways which are responsible for triggering the conditioned enhancement of natural killer (NK) cell activity. Earlier studies have suggested that central opioid(s) are involved in eliciting the expression of the conditioned NK cell activity. The purpose of this study was to identify the central opioid peptides that allow the central nervous system (CNS) to communicate with the immune system. Mediators that activate the efferent pathway of communication between the CNS and immune system was examined by injection of the mediator via the cisterna magna (CM). Conditioning was used as a tool to show that the bi-directional communication between the CNS and the immune system does take place. We found that beta-endorphin but not dynorphin could stimulate NK cell activity, when beta-endorphin or dynorphin was injected into the CM. In addition, when anti-beta-endorphin or anti-dynorphin antibody was injected into the conditioned animals via CM the conditioned response was blocked by anti-beta-endorphin but not by anti-dynorphin antibody. These observations suggest that beta-endorphin appears to be one of the signals that is induced in the brain at the CS recall step of the conditioned response to trigger the elevation of NK cell activity.

Opioid peptide inhibition of endogenous norepinephrine release from the A2 noradrenergic cell group in vitro

We investigated the potential influence of opioid and melanotropic peptides on endogenous norepinephrine (NE) release from the A2 noradrenergic cell group of rats using a static, fixed-volume incubation procedure. Norepinephrine release from slices of caudal dorsomedial medulla (CDMM) was evoked by high potassium concentrations (20 and 60 mM) in a Ca(2+)-dependent and dose-related manner. Treatment with the potent melanotropin agonist [Nle4,D-Phe7] alpha-MSH(NDP-MSH) had no effect on K(+)-induced NE release. In contrast, human beta-endorphin1-31 significantly reduced K(+)-stimulated NE release, but not in the presence of naloxone. The highly-selective mu-opioid agonist [D-Ala2, N-Me-Phe4, Gly-ol5]-enkephalin (DAMGO) also significantly reduced evoked NE release. The inhibitory effect of DAMGO was completely blocked by naloxone. Naloxone alone did not alter evoked NE release. The inhibitory effect of DAMGO was not enhanced by reducing the stimulatory concentration of K+. None of the peptides tested influenced basal NE release. These data indicate that melanotropin receptors do not regulate NE release in CDMM. In contrast, the opioid peptides DAMGO and beta-endorphin inhibit K(+)-stimulated release of endogenous NE. These data suggest a role for mu-opioid receptors in controlling NE release from A2 noradrenergic neurons.

Inhibitory effects of dynorphin-A on norepinephrine uptake by cardiac synaptosomal-mitochondrial fractions

The effect of dynorphin A-(1-13) (Dyn A-(1-13)) on [3H]norepinephrine ([3H]NE) uptake was examined in cardiac synaptosomal-mitochondrial fractions of control rats (Wistar, WR; Wistar-Kyoto, WKY) and spontaneously hypertensive rats (SHR). In adult WR, Dyn A-(1-13) caused naloxone-insensitive dose-dependent inhibition of [3H]NE uptake with an IC50 of 4.0 microM. The nonopioid Dyn A fragments Dyn A-(2-13) and Dyn A-(6-10) displayed 89 and 11% of the potency of Dyn A-(1-13), respectively, whereas Dyn A-(1-8), Leu-enkephalin, and the selective opioid agonists [D-Ala2, N-MePhe4, Glyol5]enkephalin (DAGO, mu), [D-Ser2, Thr6]Leu-enkephalin (DSLET, delta), and U-50488H (kappa) were inactive. The relative potency of various analogues and fragments of Dyn A in inhibiting [3H]NE uptake correlated well (r = 0.96) with their potency in inhibiting binding of [3H]Dyn A-(1-13) to nonopioid sites on cardiac membrane preparations. Dyn A-(1-13) showed the same potency in inhibiting [3H]NE uptake in prehypertensive (4-week-old) SHR as in age-matched WR and WKY. However, at ages 8 and 16 weeks Dyn A-(1-13) was twice as potent in SHR as in WR and WKY. The increased inhibitory potency of Dyn A-(1-13) in 8-week-old SHR was accompanied by a 1.3-fold increase in number of cardiac nonopioid [3H]Dyn A-(1-13) binding sites. Dyn A and related peptides inhibit [3H]NE uptake by cardiac synaptosomes by a nonopioid mechanism. The possible involvement of such a mechanism in development of hypertension in SHR is discussed.

kappa-Opioid receptor-transfected cell lines: modulation of adenylyl cyclase activity following acute and chronic opioid treatments

The opioid receptors mu, delta and kappa have recently been cloned. Here we show that kappa-agonists inhibit adenylyl cyclase activity in Chinese hamster ovary cells stably transfected with rat kappa-opioid receptor cDNA. Chronic exposure of the cells to kappa-agonists did not lead to significant desensitization of the capacity of the agonists to inhibit adenylyl cyclase. On the other hand, withdrawal of the agonist following the chronic treatment led to the phenomenon of supersensitivity ('overshoot') of adenylyl cyclase activity. Both the inhibition of adenylyl cyclase activity by the acute opioid treatment and the chronic agonist-induced supersensitivity are pertussis toxin sensitive, demonstrating involvement of Gi/Go proteins in both processes.

In vivo cleavability of a disulfide-based chimeric opioid peptide in rat brain

Brain delivery of systemically administered neuropeptide drugs may be achieved by the synthesis of chimeric peptides, wherein the peptide is coupled to transport vectors via avidin-biotin technology. The present study focuses on factors that optimize the linkage of drugs to transport vectors. The vector is the OX26 monoclonal antibody to the transferrin receptor, and the model peptide used in these studies is [Lys7]dermorphin (K7DA). The K7DA is monobiotinylated at the epsilon-amino group of the Lys7 residue with either a cleavable linker, e.g., disulfide, using NHS-SS-biotin, or a noncleavable linker, e.g., amide, using NHS-XX-biotin. Disulfide cleavage of the biotinylated derivative yields the desbiotinylated peptide, which is thiolated. Structures of the K7DA analogues were confirmed by secondary ion mass spectrometry. The biotinylated peptides were coupled to a thiol-ether conjugate of the OX26 antibody and either neutral avidin (NLA) or streptavidin. The binding constants (Ki) of the K7DA, the biotinylated K7DA (bio-XX-K7DA), the desbiotinylated K7DA, and the bio-XX-KD7A conjugated to NLA-OX26 were 0.62 +/- 0.14, 1.59 +/- 0.27, 1.24 +/- 0.24, and > 10 nM, respectively, and were determined with a mu-opioid peptide radioreceptor assay. Comparable results were obtained with in vivo tail-flick analgesia testing following intracerebroventricular (icv) injection of opioid chimeric peptides. Reversibility of pharmacologic action of thiolated peptide was demonstrated by icv naloxone administration. The cleavability of the disulfide linker in vivo in rat plasma and brain was assessed with gel filtration HPLC and internal carotid artery perfusion of labeled opioid chimeric peptides.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioidergic inhibition of capsaicin-evoked release of glutamate from rat spinal dorsal horn slices

We investigated the effects of opioid agonists on the capsaicin-evoked release of glutamate from nociceptive primary afferent fibers of the rat (6-8 weeks) using a fluorometric on-line continuous monitoring system for glutamate. In the presence of 0.3 microM tetrodotoxin, the application of 3 microM capsaicin to spinal dorsal horn slices produced an evoked glutamate release (55.9 +/- 4.02 pmol.mg-1 protein, n = 15). DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin; 0.3-10 microM) and morphine (1-30 microM), mu-opioid agonists, produced a concentration-dependent reduction (approximately 85 and approximately 77% reduction, respectively) in the capsaicin (3 microM)-evoked release of glutamate. These inhibitory effects were significantly antagonized by naloxone (1 microM). DPDPE ([D-Pen2,5]enkephalin; 1-10 microM), a delta-opioid agonist, also reduced the capsaicin-evoked release in a concentration-dependent manner (approximately 59% reduction). Naltrindole (1 microM), a selective delta-antagonist, significantly antagonized the inhibitory effect of DPDPE (10 microM). In contrast, neither U-50,488H (1-10 microM) nor U-69,593 (10 microM), kappa-opioid agonists, had any effects on the evoked release of glutamate. These results suggest that mu-, and delta-opioid agonists modulate pain transmission in the spinal dorsal horn, at least in part, by inhibiting the release of glutamate from capsaicin-sensitive primary afferents.

Behavior of rats during chronic activation and blockade of the neostriatal opiate system

The effects of daily microinjections (MI), over of three weeks, bilaterally into the rostral striatum, of morphine, promedol, native leu-enkephalin and its synthetic tetrapeptide analogs were studied in experiments on rats. Naloxone was used as an antagonist. An active avoidance conditioned reflex was developed preliminary in a shuttle box. A decrease in the accuracy of the realization and an increase in the latent period of the reflex were observed after the first MI of morphine and enkephalins. The effect of the most stable aminated ornithine-containing tetrapeptides proved to be the strongest. A search stereotypy and increased motoric activity were recorded in the rats during the development of the chronic effects of the activators of the opiate system. A clear correlation was not found between the motor and conditioned reflex shifts. The blockade of the opiate receptors with naloxone did not lead to substantial changes in behavior. The data obtained confirm the current hypothesis regarding the important role of the enkephalinergic system of the neostriatum in the regulation of complex forms of behavior and its close functional association with the dopaminergic system.

Action of opioid peptides on the rat adrenal cortex: stimulation of steroid secretion through a specific mu opioid receptor

While there have been several studies on the actions of opioid peptides on adrenocortical steroidogenesis, the results of these studies have failed to resolve the question as to whether these peptides exert a direct action on the adrenal cortex. The present studies were designed to address this question directly, using collagenase-dispersed rat zona glomerulosa and zonae fasciculata/reticularis cells incubated in vitro. The results obtained clearly show that the opioid peptides tested (beta-endorphin, Leu-enkephalin, Met-enkephalin, and its long-acting analogue, DALA) all exerted a significant stimulatory effect on aldosterone secretion by zona glomerulosa cells and all, except Leu-enkephalin, stimulated corticosterone secretion by inner zone cells. The response was shown to be inhibited by naloxone. There did not appear to be a significant interaction between the effects of ACTH and the opioid peptides on adrenocortical cells. Studies using specific agonists for opioid receptor subtypes (DAMGO, DPDPE and U-50488H, specific for mu, delta and kappa receptors respectively) showed that the effect of opioid peptides on the zona glomerulosa appeared to be mediated exclusively by mu receptors while the response of inner zone cells was mediated by both mu and, to a lesser extent, kappa receptors. Finally, studies on the second messenger systems activated by the opioid peptides and the receptor agonists showed that these peptides act to increase labelling of inositol trisphosphate, and strongly suggest that, in the rat adrenal cortex, both mu and kappa opioid receptors are linked to the activation of phospholipase C.

Conserved delta-activity in reverse enantiomeric opioid peptide

A reverse enantiomeric peptide has a reversed amino acid sequence with enantiomeric amino acid residues compared with its parent peptide. In most cases the random change of amino acid sequence or chirality might be expected to bring about significant changes in peptide activity. However, the reverse enantiomeric peptides of Leu-enkephalin and Tyr-D-Ala-Gly-Phe-D-Leu (DADLE) have shown affinity for the opioid delta-receptor, but not for mu- or kappa-receptors. This suggests that delta-opioid receptor recognition occurs primarily through interaction with the peptide side chains, since the native opioid peptide and its reverse enantiomer are able to have similar side-chain conformation.

Pharmacokinetic considerations in gastrointestinal motor disorders

Although it has been recognised that alterations in gastrointestinal motility, whether induced by physiological or pathological processes, have significant effects on the pharmacokinetics of orally administered drugs, this subject has received inappropriately little attention. Studies relating to this topic have focused on healthy volunteers and animals and have largely been confined to the effects of single drug doses. There is limited information about the effects of disease on pharmacokinetics under steady-state conditions. Changes in gastrointestinal motility may affect the pharmacokinetics of orally administered drugs by altering the rate of delivery, bioavailability or mucosal absorption of the drug. In general the rate of absorption and time taken to achieve maximal plasma concentrations for well absorbed drugs may be modified by changes in gastrointestinal motility, but overall bioavailability is not usually affected. In these cases the therapeutic and clinical effects of the alteration in pharmacokinetics will, therefore, depend on which parameters are important for the action of the drug. For poorly absorbed drugs both the rate of absorption and bioavailability are likely to be altered by changes in gastrointestinal motility. However, the complex effects of food and disease, as well as the properties and formulation of any drug (solubility, ease of dispersion, delayed release formulation) often make the prediction of the magnitude, or even the direction, of any effect difficult to predict. Drugs with direct effects on gastrointestinal motility may influence their own patterns of absorption. In patients with gastrointestinal motility disorders, drugs administered in a controlled release formulation, or those with poor bioavailability, are most likely to have a poorly predictable therapeutic effect. Care should be taken to ensure that the formulation of the drug, its timing of administration in relation to meals and the use of coadministered drugs optimise, or at least ensure consistent absorption.

Effects of endogenous opioid peptides and their analogs on the activities of hypothalamic arcuate neurons in brain slices from diestrous and ovariectomized rats

Various endogenous opioid peptides and some of their analogs were used in this study to test their effects on the membrane activities of hypothalamic arcuate neurons in brain slices. Both ovariectomized and diestrous rats were used in the study, and freshly prepared brain slices from these animals were used for extracellular single-unit recording studies. All of the opioids exhibited potent inhibitory effects on the firing of arcuate neurons, viz., beta-endorphin inhibited 55% (n = 33), DAGO 62% (n = 21), dynorphin A 55% (n = 11), U50,488 36% (n = 39), Met-enkephalin 35% (n = 54), and DPDPE 50% (n = 8) of tested arcuate neurons from ovariectomized rats. Significantly higher percentage of inhibition was observed in slice preparations from diestrous rats for DAGO 86% (n = 22), and slightly higher for dynorphin A 59% (n = 22) and U50,488 53% (n = 15). Pretreatment with naloxone prevented most of the actions by beta-endorphin and DAGO, and nor-binaltorphimine prevented those by dynorphin A and U50,488. Most of the effects of Met-enkephalin could also be blocked by nor-binaltorphimine (67%, n = 6), but less by naltrindole (25%, n = 8). Naltrindole, however, seemed to be more effective in blocking the action of [D-Pen2,5]-enkephalin (100%, n = 2). In summary, all opioids tested exerted potent inhibitory effects upon the firing of arcuate neurons possibly through multiple opioid receptors, and the presence of ovarian hormones may have an effect on the neuron's responsiveness to opioid acting on mu type receptors.

Endogenous opioids modulate ventilation in the obese Zucker rat

This study evaluated the modulatory role of endogenous opioids on ventilation in young and mature, lean and obese male Zucker rats. Naloxone, an opioid receptor antagonist, and saline (control) were administered subcutaneously to awake rats, and ventilation in air and in response to an hypoxic and an hypercapnic gas challenge measured. In response to naloxone young, obese but not lean rats exhibited a marked increase of ventilation in all three conditions. Older obese Zucker rats that were morbidly obese breathed at a frequency of over 200 breaths per minute and showed only a modest increase of ventilation in response to naloxone. Older lean rats increased ventilation with naloxone only when exposed to hypercapnia. Unlike the stimulatory effects hypoxia and hypercapnia had on ventilation in older, lean rats, the ventilatory responses of the obese, older rats to hypoxia and to hypercapnia were blunted. We conclude that the obese Zucker rat may be a good animal model to assess how chest wall loading and endogenous opioids interact in the development of ventilatory control abnormalities.

Effects of opioid substances on cAMP response to the beta-adrenergic agonist isoproterenol in human mononuclear leukocytes

The effects of different opioid substances on isoproterenol and forskolin-stimulated cyclic AMP (cAMP) intracellular accumulation, and on the binding of 125I-pindodol (IPIN) to beta 2-adrenoceptors were studied in human mononuclear leukocytes (MNL). The opioids used were alpha-endorphin, beta-endorphin, tau-endorphin, DAGO (a mu receptor agonist), dermenkephalin (a delta receptor agonist and morphine. Only morphine was able to increase the cAMP response to isoproterenol. The EC50 of isoproterenol for cAMP accumulation was shifted leftward by morphine; this effect was blocked by naloxone. On the contrary, the cAMP response to forskolin, direct activator of adenylate cyclase, was similar in the control test with respect to the experiments with morphine. The five opioid peptides induced no changes in the dose-response curves with isoproterenol and forskolin. Furthermore, none of the opioids induced changes in the IPIN binding. Our data show that morphine is able to exert a significant enhancement of the response of beta 2-adrenergic receptors to isoproterenol in human mononuclear leukocytes. This effect seems to be mediated by mu opioid receptors and seems to involve G protein.

Endogenous opiates: 1994

This article is the 17th installment of our annual review of research concerning the opiate system. It includes papers published during 1994 involving the behavioral, nonanalgesic, effects of the endogenous opiate peptides. The specific topics covered this year include stress; tolerance and dependence; eating; drinking; gastrointestinal, renal, and hepatic function; mental illness and mood; learning, memory, and reward; cardiovascular responses; respiration and thermoregulation; seizures and other neurological disorders; electrical-related activity; general activity and locomotion; sex, pregnancy, and development; immunological responses; and other behaviors.

An all D-amino acid opioid peptide with central analgesic activity from a combinatorial library

A synthetic combinatorial library containing 52,128,400 D-amino acid hexapeptides was used to identify a ligand for the mu opioid receptor. The peptide, Ac-rfwink-NH2, bears no resemblance to any known opioid peptide. Simulations using molecular dynamics, however, showed that three amino acid moieties have the same spatial orientation as the corresponding pharmacophoric groups of the opioid peptide PLO17. Ac-rfwink-NH2 was shown to be a potent agonist at the mu receptor and induced long-lasting analgesia in mice. Analgesia produced by intraperitoneally administered Ac-rfwink-NH2 was blocked by intracerebroventricular administration of naloxone, demonstrating that this peptide may cross the blood-brain barrier.

[The effect of endogenous opioids in blood pressure regulation and arterial hypertension]

There are numerous hints to the participation of endogenous opioids in the tonic regulation of blood pressure and in pathogenesis and maintenance of essential hypertension. Endogenous mu- and delta-receptor agonists of the central nervous system seem to regulate long-term control of the sympathetic tone in essential hypertension. In the initial stage of hypertension, altered responses to opioid agonists and antagonists could be demonstrated in young spontaneously hypertensive rats. In young, but not in old hypertensive patients alterations of plasma concentrations of endogenous opioids were found. Reactivity and reagibility of blood pressure seem to be regulated rather independently from endogenous opioids, but they may modulate the influence of psychosocial stress on blood pressure.