Endogenous Opioid Signaling in the Mouse Retina Modulates Pupillary Light Reflex

Opioid peptides and their receptors are expressed in the mammalian retina; however, little is known about how they might affect visual processing. The melanopsin-expressing intrinsically photosensitive retinal ganglion cells (ipRGCs), which mediate important non-image-forming visual processes such as the pupillary light reflex (PLR), express β-endorphin-preferring, µ-opioid receptors (MORs). The objective of the present study was to elucidate if opioids, endogenous or exogenous, modulate pupillary light reflex (PLR) via MORs expressed by ipRGCs. MOR-selective agonist [D-Ala², MePhe⁴, Gly-ol⁵]-enkephalin (DAMGO) or antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH₂ (CTAP) was administered via intravitreal injection. PLR was recorded in response to light stimuli of various intensities. DAMGO eliminated PLR evoked by light with intensities below melanopsin activation threshold but not that evoked by bright blue irradiance that activated melanopsin signaling, although in the latter case, DAMGO markedly slowed pupil constriction. CTAP or genetic ablation of MORs in ipRGCs slightly enhanced dim-light-evoked PLR but not that evoked by a bright blue stimulus. Our results suggest that endogenous opioid signaling in the retina contributes to the regulation of PLR. The slowing of bright light-evoked PLR by DAMGO is consistent with the observation that systemically applied opioids accumulate in the vitreous and that patients receiving chronic opioid treatment have slow PLR.

Delta opioid receptor-mediated immunoregulatory role of methionine-enkephalin in freshwater teleost Channa punctatus (Bloch.)

The immunoregulatory role of methionine-enkephalin (Met-enk) is well studied in mammals, but has not been explored in ectotherms despite the fact that this peptide is highly conserved in vertebrates. The present study demonstrates the diverse effects of Met-enk depending on its concentration and specific function of splenic phagocytes in the freshwater fish Channa punctatus. Although Met-enk increased both phagocytic as well as respiratory burst activity, the concentration-related response was opposite to each other. It had the maximum stimulatory effect on phagocytosis at 10(-9)M, while the same concentration was least effective in increasing superoxide production. Similarly, Met-enk at concentrations lower or higher than 10(-9)M was either ineffective or less effective in case of phagocytosis, while highly effective in stimulating superoxide production. On the other hand, concentration-independent inhibitory effect of Met-enk was observed in case of nitrite production. Nonetheless, Met-enk regulated all the functions of phagocyte through opioid receptors since non-specific opioid receptor antagonist naltrexone completely blocked the effect of Met-enk on phagocytosis, superoxide and nitrite production by splenic phagocytes of C. punctatus. Among selective opioid receptor antagonists, delta-opioid receptor antagonist naltrindole completely antagonized the effect of Met-enk on phagocytosis, superoxide and nitrite production, while mu- and kappa-opioid receptor antagonist, CTAP and norbinaltorphimine, respectively, were ineffective in influencing any of the functions. This suggests that Met-enk modulates splenic phagocyte functions in the fish C. punctatus via delta-opioid receptor. This is further substantiated by using highly selective delta-opioid receptor agonist, SNC80.

Protection of endogenous enkephalin catabolism as natural approach to novel analgesic and antidepressant drugs

The most efficient drugs to alleviate severe pain are opioid compounds. However, their chronic use could be associated with serious drawbacks, such as tolerance, respiratory depression and constipation. Therefore, there is a need for compounds able to efficiently alleviate inflammatory and neurogenic pain following chronic treatment. The discovery that the endogenous opioid peptides, enkephalins, are inactivated by two metallopeptidases, neutral endopeptidase and aminopeptidase N, which can be blocked by synthetic dual inhibitors, represents a promising way to develop 'physiological' analgesics devoid of morphine side effects. These dual inhibitors also have antidepressant-like properties through enkephalin-related activation of delta-opioid receptors. This is expected to reduce the emotional component of pain in humans. This article reviews the promising data obtained for future development of a new class of analgesic that could be of major interest in a number of severe and chronic pain syndromes.

Reversal of ongoing thermal hyperalgesia in mice by a recombinant herpesvirus that encodes human preproenkephalin

Herpesvirus-mediated transfer of the human preproenkephalin gene to primary afferent nociceptors prevents phasic thermal allodynia/hyperalgesia in mice. It is not known, however, whether similar viral treatments would reverse ongoing or chronic pain and allodynia/hyperalgesia. To this end, mice were given intrathecal injections of pertussis toxin (PTX), which produces a weeks-long thermal hyperalgesia apparently by uncoupling certain G proteins from inhibitory neurotransmitter receptors. This treatment produced profound thermal hyperalgesia in both Adelta and C-fiber thermonociceptive tests lasting at least 6 weeks. However, treatment of skin surfaces with an enkephalin-encoding herpesvirus, but not control virus or vehicle, completely reversed this hyperalgesia. This profound anti-hyperalgesia was observed for both Adelta- and C-fiber-mediated responses. Interestingly, however, while the anti-hyperalgesic effect of the enkephalin-encoding virus on C-fiber-mediated responses was reversed by intrathecal application of micro or delta opioid antagonists, only delta antagonists reversed the effect of this virus on Adelta hyperalgesia. Thus, virus-mediated delivery of the proenkephalin cDNA reverses thermal hyperalgesia produced by PTX-induced ribosylation of inhibitory G proteins by an opioid-mediated mechanism. These results suggest that herpesvirus vectors encoding analgesic peptides may be useful in attenuating centrally mediated, ongoing neuropathic pain and/or hyperalgesia.

Species-specific effects on hemolymph glucose control by serotonin, dopamine, and L-enkephalin and their inhibitors inSquilla mantis andAstacus leptodactylus (crustacea)

Hemolymph glucose level is controlled by crustacean Hyperglycemic Hormone (cHH) released from the eyestalk neuroendocrine centers under conditions of both physiological and environmental stress. Biogenic amines and enkephalin have been found to mediate the release of several neurohormones from crustacean neuroendocrine tissue. We investigated the effect of serotonin, dopamine, and Leucine-enkephalin in vivo--injected into the stomatopod Squilla mantis and the decapod Astacus leptodactylus--whether increasing or depressing glycemia. Serotonin had a marked effect in elevating glucose level compared with initial values in both species. 5-HT1-like receptors are more involved in mediating serotonin action as co-injected cyproheptadine was a more effective antagonist than ketanserin (5-HT2-like receptor inhibitor). Dopamine injection in intact animals produced a decrease below initial levels of hemolymph glucose. This effect was significantly antagonized by domperidone. No significant effect of both amines occurred in eyestalkless animals. L-enkephalin shows a differential effect: in S. mantis it induced hypoglycemia while in A. leptodactylus it caused an increase of glucose level. Co-injected antagonist naloxone affected the direction of the response. Serotonin appears to provide a major control on glucose mobilization, whereas dopamine and L-enkephalin act as modulators whose plasticity in use or action varies among species.

Enkephalinergic involvement in substantia nigra in the modulation of hypothalamically-induced predatory attack behavior

The present study was carried out in five cats which did not attack the rats spontaneously. Predatory attack on an anaesthetized rat was elicited by electrical stimulation of lateral hypothalamus at a mean current strength of 650 microA. The attack was accompanied by minimal affective display and culminated in neck biting. Microinfusions of DAME (delta-alanine methionine enkephaline) in 500 ng dose in substantia nigra facilitated the predatory attack and there was a significant reduction in the threshold current strength for affective display as well as somatomotor components. Microinfusions of naloxone, an opioid antagonist in 1.0 microg dose when DAME effect was at its peak reversed the facilitatory effects and the threshold returned to the control levels within 10 minutes of naloxone infusion at the same locus. Microinfusions of naloxone alone in similar dosage completely blocked the predatory attack response as indicated by an increase in the threshold current strength for somatomotor as well as affective display components. The somatomotor were completely inhibited and could not be elicited even when the current strength was increased to 1000 microA. Control injections of saline in similar volumes (0.5 microl) failed to produce any response Microinfusions of naloxone in lower dose (250 ng) failed to produce any blocking effect. These findings indicate that hypothalamically elicited predatory attack is facilitated by enkephalinergic mechanisms operating at the midbrain level.

Further evidence for the interaction of mu- and delta-opioid receptors in the antinociceptive effects of the dual inhibitor of enkephalin catabolism, RB101(S). A spinal c-Fos protein study in the rat under carrageenin inflammation

We have previously shown that RB101, a dual inhibitor of enkephalin-degrading enzymes, decreased carrageenin-evoked c-Fos protein expression at the spinal cord level in awake rats. Moreover, we have also shown that c-Fos expression is a useful marker of the possible direct or indirect interactions between neural pathways, such as opioid and cholecystokinin systems. We now investigated the respective roles of the three main types of opioid receptors (mu, delta, or kappa) and their possible interactions, in the depressive effects of RB101 in inflammatory nociceptive conditions induced by intraplantar carrageenin (6 mg/150 microl of saline). We used beta-funaltrexamine (beta-FNA), naltrindole (NTI), and nor-binaltorphimine (BNI) as specific antagonists for mu, delta- and kappa-opioid receptors, respectively. c-Fos protein-immunoreactivity (c-Fos-IR) was evaluated as the number of c-Fos-IR nuclei in the lumbar spinal cord 90 min after carrageenin. c-Fos-IR nuclei were preferentially located in the superficial (I-II) and deep (V-VI) laminae of segments L4-L5 (areas containing numerous neurons responding exclusively, or not, to nociceptive stimuli). RB101(S) (30 mg/kg, i.v.) significantly reduced the total number of carrageenin-evoked c-Fos-IR nuclei (30% reduction, P<0.01). This effect was completely blocked by beta-FNA (10 mg/kg, i.v.), or NTI (1 mg/kg, i.v.). In contrast, BNI (2.5 mg/kg, i.v.) did not reverse the reducing effects of RB101(S) on carrageenin-evoked c-Fos protein expression. These results suggest that functional interactions occur between mu- and delta-opioid receptors in enkephalin-induced antinociceptive effects.

Synthesis and characterization of the first potent inhibitor of yapsin 1. Implications for the study of yapsin-like enzymes

The potent peptidic inhibitor, Y1, of the basic residue-specific yeast aspartyl protease, yapsin 1, was synthesized and characterized. The inhibitor was based on the peptide sequence of a cholecystokinin(13-33) analog that yapsin 1 cleaved with an efficiency of 5.2 x 10(5) m(-1) s(-1) (Olsen, V., Guruprasad, K., Cawley, N. X., Chen, H. C., Blundell, T. L., and Loh, Y. P. (1998) Biochemistry 37, 2768-2777). The apparent K(i) of Y1 for the inhibition of yapsin 1 was determined to be 64.5 nm, and the mechanism is competitive. Y2 was also developed as an analog of Y1 for coupling to agarose beads. The resulting inhibitor-coupled agarose beads were successfully used to purify yapsin 1 to apparent homogeneity from conditioned medium of a yeast expression system. Utilization of this new reagent greatly facilitates the purification of yapsin 1 and should also enable the identification of new yapsin-like enzymes from mammalian and nonmammalian sources. In this regard, Y1 also efficiently inhibited Sap9p, a secreted aspartyl protease from the human pathogen, Candida albicans, which has specificity for basic residues similar to yapsin 1 and might provide the basis for the prevention or control of its virulence. A single-step purification of Sap9p from conditioned medium was also accomplished with the inhibitor column. N-terminal amino acid sequence analysis yielded two sequences indicating that Sap9p is composed of two subunits, designated here as alpha and beta, similar to yapsin 1.

Roles of endogenous opioid peptides in modulation of nocifensive response to formalin

Roles of endogenous opioid peptides and their receptors in modulation of the nocifensive responses to formalin in mice were studied. Mice were pretreated i.c.v. or intrathecally (i.t.) with selective opioid receptor antagonists or intrathecally with antisera against endogenous opioid peptides and the nocifensive licking responses to intraplantar injection of formalin (0.5%, 25 microl) were then observed. Pretreatment with the epsilon-opioid receptor antagonist beta-endorphin(1-27) or the selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-Orn-Thr-Pen-Thr-NH(2) (CTOP) given i.c.v. dose dependently enhanced the second, but not the first phase of the nocifensive response. However, i.c.v. pretreatment with the selective delta-receptor antagonist naltrindole or kappa-receptor antagonist nor-binaltrophimine did not affect the nocifensive responses. Intrathecal pretreatment with selective delta(1)-opioid antagonist 7-benzylidene naltrexamine significantly enhanced both the first and second phases of nocifension. Intrathecal pretreatment with CTOP also increased the second but not the first phase of the nocifension. However, i.t. pretreatment with the selective delta(2)-receptor antagonist naltriben or nor-binaltrophimine did not affect the second phase of the nocifension. Intrathecal pretreatment with antiserum against Leu-enkephalin, Met-enkephalin, or dynorphin A(1-17), but not beta-endorphin, enhanced only the second phase of nocifensive response to formalin. It is concluded that the blockade of epsilon- and mu-receptors, but not delta- or kappa-receptors, at the supraspinal sites enhanced the second phase of formalin-induced nocifension. In the spinal cord, Leu-enkephalin, and to a lesser extent, Met-enkephalin and dynorphin A(1-17) and mu- and delta(1)-opioid receptors, but not delta(2)- or kappa-opioid receptors, are involved in modulating the feedback inhibition of the second phase of formalin-induced nocifension.

The role of the 7B2 CT peptide in the inhibition of prohormone convertase 2 in endocrine cell lines

Prohormone convertase (PC) 2 plays an important role in the processing of neuropeptide precursors via the regulated secretory pathway in neuronal and endocrine tissues. PC2 interacts with 7B2, a neuroendocrine protein that is cleaved to a 21-kDa domain involved in proPC2 maturation and a carboxyl-terminal peptide (CT peptide) that represents a potent inhibitor of PC2 in vitro. A role for the CT peptide as an inhibitor in vivo has not yet been established. To study the involvement of the CT peptide in PC2-mediated cleavages in neuroendocrine cells, we constructed a mutant proenkephalin (PE) expression vector containing PE with its carboxyl-terminal peptide (peptide B) replaced with the 7B2 inhibitory CT peptide. This PECT chimera was stably transfected into two PC2-expressing cell lines, AtT-20/PC2 and Rin cells. Although recombinant PECT proved to be a potent (nM) inhibitor of PC2 in vitro, cellular PC2-mediated cleavages of PE were not inhibited by the PECT chimera, nor was proopiomelanocortin cleavage (as assessed by adrenocorticotropin cleavage to alpha-melanocyte-stimulating hormone) inhibited further than in control cells expressing only the competitive substrate PE. Tests of stimulated secretion showed that both the CT peptide and the PE portion of the chimera were stored in regulated secretory granules of transfected clones. In both AtT-20/PC2 and Rin cells expressing the chimera, the CT peptide was substantially internally hydrolyzed, potentially accounting for the observed lack of inhibition. Taken together, our data suggest that overexpressed CT peptide derived from PECT is unable to inhibit PC2 in mature secretory granules, most likely due to its inactivation by PC2 or by other enzyme(s).

Opioid modulation of calcium current in cultured sensory neurons: mu-modulation of baroreceptor input

We used the whole cell open-patch or perforated-patch technique to characterize mu-opioid modulation of Ca(2+) current (I(Ca)) in nodose sensory neurons and in a specific subpopulation of nodose cells, aortic baroreceptor neurons. The mu-opiate receptor agonist Tyr-D-Ala-Gly-MePhe-Gly-ol enkephalin (DAGO) inhibited I(Ca) in 95% of neonatal [postnatal day (P)1-P3] nodose neurons. To the contrary, only 64% of juvenile cells (P20-P35) and 61% of adult cells (P60-P110) responded to DAGO. DAGO-mediated inhibition of I(Ca) was naloxone sensitive, irreversible in the presence of guanosine 5'-O-(3-thiotriphosphate), absent with guanosine 5'-O-(2-thiodiphosphate), and eliminated with pertussis toxin; DAGO's inhibition of I(Ca) was G protein mediated. Incubation of neurons with omega-conotoxin GVIA eliminated the effect of DAGO in neonatal but not in juvenile cells. In the latter, DAGO reduced 37% of the current remaining in the presence of omega-conotoxin. In the subset of nodose neurons, aortic baroafferents, the effect of DAGO was concentration dependent, with an IC(50) of 1.82 x 10(-8) M. DAGO slowed activation of I(Ca), but activation curves constructed from tail currents were the same with and without DAGO (100 nM). In summary, mu-opiate modulation of I(Ca) in nodose neurons was demonstrated in three age groups, including specifically labeled baroafferents. The demonstration of a mechanism of action of mu-opioids on baroreceptor afferents provides a basis for the attenuation of the baroreflex that occurs at the level of the nucleus tractus solitarii.

Enkephalin inhibits vagal control of heart rate, contractile force and coronary blood flow in the canine heart in vivo

The following studies were conducted to determine if the ability of the intrinsic cardiac opioid, met-enkephalin-arg-phe to interrupt vagal bradycardia can be generalized to include the disruption of vagal effects on atrial contraction and coronary blood flow. Anesthetized dogs were instrumented to measure heart rate and left atrial contractile force or heart rate and coronary blood flow. The response of each variable was recorded at rest and during vagal stimulation. During the evaluation of vagal effects on contractile activity and coronary blood flow, heart rate was maintained constant by electrically pacing the hearts above their resting heart rate. In the first protocol, vagal stimulation reduced both heart rate and atrial contractile force in a frequency dependent fashion. When met-enkephalin-arg-phe (MEAP) was infused systemically for three min at 3 nmol min(-1) kg(-1), there were no observed changes in resting heart rate or atrial contraction. However, when the vagal stimuli were reapplied during the peptide infusion, the previously observed vagal effects on rate and contractile force were reduced in magnitude by one-half to two-thirds. The ability of MEAP to interrupt the vagal control of heart rate and contractile activity involves opiate receptors since the effect was eliminated in both cases by prior opiate receptor blockade with the high affinity antagonist, diprenorphine. In the second protocol, vagal stimulation produced a transient increase in coronary blood flow and an accompanying increase in myocardial oxygen consumption. These effects were reduced by approximately 80% during the systemic infusion of MEAP. A similar increase in coronary blood flow mediated by the direct acting muscarinic agonist, methacholine, was unaltered by the infusion of peptide. In summary, these data suggest that the intrinsic cardiac enkephalin, MEAP, is capable of inhibiting the vagal control of heart rate, contractile force and coronary blood flow and probably does so through a common opiate receptor located prejunctionally on vagal nerve terminals or within nearby parasympathetic ganglia.

Differences in opioidergic inhibition of spinal reflexes and Fos expression evoked by mechanical and chemical noxious stimuli in the decerebrated rabbit

Noxious mechanical and chemical stimuli were applied to the toes of the left hind limb of decerebrated, spinalized rabbits and their effects on a hind limb spinal withdrawal reflex and expression of Fos-like immunoreactivity in the spinal cord were measured. The animals were prepared so as to minimize nociceptive inputs arising from surgery. A single crush stimulus applied with a pair of haemostatic forceps caused long-lasting (c. 20 min) inhibition of reflexes evoked in medial gastrocnemius motoneurons by electrical stimulation of the skin at the heel. Naloxone (0.25 mg/kg i.v.) increased reflexes to more than 1000% of pre-drug controls and reversed crush-evoked inhibition. Mustard oil applied to the toes had no consistent effects on the heel-gastrocnemius reflex before or after naloxone. Both crush and mustard oil stimuli gave rise to unilateral increases in the number of Fos-immunopositive profiles in the superficial dorsal horn of spinal segments L7 and S1. There were significantly more Fos-immunoreactive elements in the central and lateral parts of lamina I of both segments in animals receiving the crush stimulus than there were in animals receiving the mustard oil stimulus. Immunochemical localization of enkephalins in rabbit spinal cord showed a dense network of fibres and terminals in laminae I and II, accompanied by infrequent but distinctly stained neuronal cell bodies. The same pattern, with increased numbers of visible cell bodies, was seen after treatment with colchicine. The present data show that tonic and stimulus-evoked opioidergic inhibition of the heel-gastrocnemius reflex of the rabbit are not epiphenomena of surgical preparation of the hindlimb. Opioid-mediated inhibition of the heel-gastrocnemius withdrawal reflex of the rabbit was evoked by noxious mechanical but not by chemical stimulation of the toes. Of these stimuli, the former gave rise to greater activation of neurons in central and lateral lamina I of segments L7 and S1, the region of termination of afferent fibres from the heel and the location of some enkephalin-positive neuronal cell bodies. Thus, noxious mechanical stimulation of the toes elicits inhibition of the heel-gastrocnemius withdrawal reflex, probably via activation of enkephalinergic neurons in the lateral half of lamina I in the L7 and S1 segments.

[Enkephalin catabolism inhibitors and antalgics of the future: from preclinical research to clinical trials]

It is now well accepted that the pain suppression effect of morphine is related to the interaction of this alkaloid with binding sites located in the central nervous system. The wide distribution of opioid receptors probably accounts for the multiplicity of pharmacological responses elicited by morphine administration, as in addition to its strong analgesic potency morphine induces side effects. Therefore, there is a critical need for new analgesics able to fulfil the gap existing between opioid analgesics and antalgics. These new analgesics could be of major interest in a number of severe pain syndromes. The discovery that the endogenous morphine-like peptides enkephalins are degraded by well-defined metabolic pathways represents a promising outlook for the development of new analgesics. The complete inhibitors of enkephalin catabolism produce their physiological effects by increasing the extracellular levels of endogenous opioid peptides released either tonically or following stimuli-evoked depolarization. Under these conditions, their effects will depend upon the magnitude and duration of the enkephalin release evoked by a particular stimulus, which probably varies in the different enkephalinergic pathways. It is expected that increasing the levels of endogenous opioid peptides would avoid serious drawbacks inasmuch as they appear related to a ubiquitous overstimulation of brain opioid receptors. Some mixed inhibitors of enkephalin degrading enzymes are now undergoing preclinical trials.

Treatment of chronic allodynia in spinally injured rats: effects of intrathecal selective opioid receptor agonists

We examined the effects of intrathecal (i.t.) selective opioid receptor agonists in alleviating mechanical and cold allodynia in spinally injured rats. Both DAMGO ([D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin, a mu-opioid receptor agonist) and DPDPE ([D-Phe2,D-Phe5]-enkephalin, a delta-opioid receptor agonist) dose-dependently relieved the chronic allodynia-like behavior at doses selective for their respective receptors. The anti-allodynic effect of DAMGO and DPDPE was reversed by the selective mu- and delta-opioid receptor antagonists CTOP (D-Phe-Cys-Tyr-D-Trp-Orn-Thr-Pen-Thr-NH2) and naltrindole, respectively. In contrast, the selective kappa-opioid receptor agonist U50488H did not alleviate the allodynia-like behavior, but rather enhanced it. The anti-nociceptive and anti-allodynic effect of i.t. DAMGO was blocked by U50488H. Thus, activation of spinal mu- and delta-, but not kappa-opioid receptors produced anti-allodynic effect in this model of central pain. Drugs which act selectively on opioid receptor subtypes may be useful in managing chronic central pain of spinal cord origin.

Intraventricular insulin reduces the antinociceptive effect of [D-Ala2, NMePhe4, Gly-ol5]enkephalin in mice

The effects of pretreatment with insulin on the antinociception induced by intracerebroventricular (i.c.v.) administration of the mu-opioid receptor agonist [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAMGO) were studied in mice. Intracerebroventricular pretreatment with insulin (1 and 3 mU) for 60 min dose dependently attenuated the antinociception induced by i.c.v. DAMGO (5.6 ng) in mice. Intracerebroventricular pretreatment with a highly selective tyrosine kinase inhibitor, lavendustin A, at doses of 100 and 300 ng for 10 min, dose dependently reversed the antinociceptive effect of DAMGO (5.6 ng) in insulin-treated mice. The antinociceptive effect of DAMGO (5.6 ng, i.c.v.) was significantly reduced in C57BL/KsJ-db/db diabetic mice compared with that in age-matched control (C57BL/KsJ-db/ + + ) mice. When C57BL/KsJ-db/db diabetic mice were pretreated with lavendustin A (300 ng), the antinociceptive effect of DAMGO was significantly increased. These results indicate that tyrosine kinase may be involved in the reduction of DAMGO-induced antinociception by insulin in mice. Furthermore, the attenuation of DAMGO-induced antinociception in C57BL/KsJ-db/db diabetic mice may be due in part to increased tyrosine kinase activity as a result of hyperinsulinemia.

7-Spiroindanyl derivatives of naltrexone and oxymorphone as selective ligands for delta opioid receptors

A series consisting of spiroindanyl (5-7), benzospiroindanyl (8-10), and spiroperinaphthyl (11) derivatives of naltrexone and oxymorphone were synthesized in order to investigate the role of an orthogonal-oriented "address" for delta opioid receptors. All of the ligands exhibited a preference for delta receptors in vitro. The 7-benzospiroindanyl derivative 8 (BSINTX) was the most selective delta opioid receptor antagonist in vitro. In mice BSINTX antagonized the delta 1-selective agonist, [D-Pen2,D-Pen5]enkephalin without significantly affecting the antinociceptive potency of delta 2, mu, and kappa agonists. The results of this study are consistent with an orthogonally-oriented address favoring delta 1 activity.

Central regulation of urine production by a selective mu-opioid agonist, [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin, in rats

We have investigated opioid mechanisms concerning regulation of urine production in the hypothalamic supraoptic nucleus (SON). In this study, the effect of [D-Ala2, N-Me-Phe4, Gly5-ol]-enkephalin (DAMGO), a potent selective mu-opioid agonist, microinjected into the SON of anesthetized hydrated rats, on the urine outflow rate was examined. DAMGO caused a dose-dependent decrease in the urine outflow rate with no significant changes in blood pressure nor heart rate. The ED50 value for the antidiuresis was calculated to be 0.055 nmol from the dose-response curve. The antidiuresis elicited by DAMGO (0.1 nmol) was partially inhibited by intra-SON pre-injection of naloxone (3 nmol), a relatively mu-selective opioid antagonist, and timolol (100 nmol), a beta-adrenoceptor antagonist, but not by intra-SON pre-injection of phenoxybenzamine (20 nmol), an alpha-adrenoceptor antagonist, nor atropine (300 nmol), a muscarinic antagonist. Intravenous injection of d(CH2)5-D-Tyr(Et)VAVP (16.7 micrograms), a vasopressin receptor antagonist, did not influence the DAMGO-induced antidiuresis. These findings suggest that antidiuresis mediated through mu-opioid receptors in the SON involves beta-adrenoceptors in the nuclei, but does not involve an increase in vasopressin release.

RB 101, a purported pro drug inhibitor of enkephalin metabolism, is antinociceptive in pregnant mice

In an earlier study, we demonstrated the enhancement of pregnancy-induced analgesia with an inhibitor of endogenous enkephalin metabolism. The purpose of the present study was to evaluate the antinociceptive effect of another inhibitor of enkephalin metabolism, RB 101, on pregnant mice. Further, since other studies have shown RB 101 to be free of opioid side effects, we examined its effect on respiratory rate. Analgesia was assessed using the hot plate test, and respiratory rate was measured by recording the output from an end-tidal carbon dioxide detector. In pregnant mice, experiments were conducted on Day 17 or Day 18 of pregnancy; mice usually deliver on Day 19. For the hot plate test, animals were tested in the following groups: Group 1, RB 101 150 mg/kg (n = 15); Group 2, RB 101 50 mg/kg (n = 15); Group 3, RB 101 vehicle (n = 15); Group 4, morphine 5 mg/kg (n = 14); and Group 5, RB 101 150 mg/kg + naloxone 5 mg/kg (n = 10). The test was repeated on the second day after delivery in animals in Groups 1 and 3 (given RB 101 150 mg/kg and RB 101 vehicle, respectively). RB 101 150 mg/kg and morphine 5 mg/kg were significantly different (mean percentage of maximum possible effect 30.0 and 37.7, respectively, at 30 min and 41.6 and 32.6, respectively, at 60 min) in their antinociceptive effect in pregnant animals from all other groups. Naloxone, when coadministered with RB 101, prevented the development of antinociception. RB 101 150 mg/kg was not antinociceptive after delivery. Depression of respiratory rate was tested in a separate set of animals in the following groups: Group 1, RB 101 150 mg/kg (n = 16); Group 2, morphine 5 mg/kg (n = 16); Group 3, RB 101 vehicle (n = 15). Morphine 5 mg/kg produced significant depression of respiratory rate at 30 min postinjection when compared with RB 101 150 mg/kg and RB 101 vehicle (mean percent change in respiratory rate was 78.5% compared with 87.7% and 92.4%, respectively, where 100% = no change). These results suggest that drugs such as RB 101 may produce antinociception with minimal effects on respiration.

Effects of some 7-arylidene and 7-heteroarylidene morphinan-6-ones on the antinociceptive activity of [D-Pen2, D-Pen5]enkephalin and [D-Ala2, Glu4]deltorphin II and on multiple opioid receptors

The in vivo and functional effects of several 7-arylidene and 7-heteroarylidene morphinan-6-ones were determined at the mu-, delta-, and kappa-opioid receptors using the guinea pig brain membranes, guinea pig ileum (GPI), and mouse vas deferens (MVD). In vivo effects included the antagonism by these compounds given subcutaneously on the antinociceptive actions of intracerebroventricularly injected [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Ala2, Glu4]deltorphin II (deltorphin II), the highly selective putative delta 1- and delta 2-opioid receptor agonists. Finally, the partition coefficients of these compounds were estimated (CLOGP) and determined experimentally at pH 7.4 in the 1-octanol/water system. Compared with E-7-benzylidenenaltrexone (BNTX), most compounds except for E-7-(4-chlorobenzylidene)naltrexone, were more potent at delta-opioid receptors than at the mu-opioid receptor, whereas, in comparison to the kappa-opioid receptor, the activities of the E-7-arylidene or E-7-heteroarylidene naltrexone derivatives at the delta-receptor were in the following order, where the 7-substituents were: 4-fluorobenzylidene- > benzylidene > 3-pyridylmethylene- > 4-pyridylmethylene- > 1-methyl-2-imidazolylmethylene > 4-chlorobenzylidene. In the MVD preparation, the potencies at the delta-opioid receptor, in comparison to BNTX, were in the following order, where the 7-substituents were: benzylidene = 1-methyl-2-imidazolylmethylene- > 4-fluorobenzylidene- = 3-pyridylmethylene- = 4-pyridylmethylene-. All compounds antagonized delta 1, and delta 2-opioid receptor agonist-induced analgesia. The antagonist potencies at the delta 1-opioid receptor were in the following order, where the 7-substituents were: benzylidene- > 4-chlorobenzylidene- > 4-fluorobenzylidene- > 3-pyridylmethylene- > 1-methyl-2-imidazolymethylene- approximately 4-pyridylmethylene-, whereas at the delta 2-opioid receptor, the order was benzylidene- > 4-chlorobenzylidene- > 4-fluorobenzylidene- > 3-pyridylmethylene- > 1-methyl-2-imidazolymethylene- > 4-pyridylmethylene. In general, all compounds exhibited greater potency at the delta 2- than delta 1-opioid receptor. The computed partition coefficients were, as expected, greater than the apparent log P values, which were determined experimentally. Generally, the lipophilicity values in decreasing order were: 4-chlorobenzylidene- > 4-fluorobenzylidene- > benzylidene > 3-pyridylmethylene- = 4-pyridylmethylene- > 1-methyl-2-imidazolylmethylene-. In general, the benzylidene and 4-pyridylmethylene derivatives, which have medium lipophilicities, were equally effective at the delta 1- and delta 2-receptors; the 3-pyridylmethylene and 1-methyl-2-imidazolylmethylene derivatives had lower lipophilicities and were more selective for the delta 2- than delta 1-receptor; the 4-chlorobenzylidene and 4-fluorobenzylidene derivatives were more lipophilic and had intermediate activity. The plot of pED50 values for the in vivo tests for the delta 1- and delta 2-receptors showed that the two receptors are not independent with respect to this series of compounds.

Neuropeptide FF reverses the effect of mu-opioid on Ca2+ channels in rat spinal ganglion neurones

Single neurones isolated from spinal ganglia of young rats were loaded with Fluo3, a fluorescent indicator of intracellular Ca2+ concentration. In 27 of 47 neurones the depolarization-evoked rise in fluorescence was reduced by 47.8 +/- 3.3% by prior perfusion with DAGO (1 microM, 30 s) a mu-opioid agonist. In 12 neurones an analogue of neuropeptide FF ((1DMe)Y8Fa, 10 nM, 30 min) did not affect the Ca2+ response to depolarization. (1DMe)Y8Fa reversed the effect of DAGO by 63 +/- 8% in seven of these 12 neurones. We conclude that stimulation of neuropeptide FF receptors antagonizes mu-opioid modulation of Ca2+ channels at the cellular level.

Opioid receptor-mediated control of acetylcholine release in human neocortex tissue

The effects of various opioid receptor agonists and antagonists on evoked acetylcholine release were studied in slices of human neocortex prelabelled with [3H]-choline, superfused and depolarized electrically (2 min, 3 Hz, 2 ms, 24 mA) or by K+ (20 mM). The delta-opioid receptor agonist DPDPE and the kappa-opioid receptor agonist U50488 reduced the evoked [3H]-overflow (acetylcholine release) in a concentration-dependent fashion; the delta-opioid receptor antagonist naltrindole and the kappa-opioid receptor antagonist norbinaltorphimine, respectively, antagonized these effects. Application of the mu-opioid receptor agonist DAGO also resulted in an inhibition of acetylcholine release; however, both delta- and kappa-opioid receptor antagonists were able to block this effect. The mu-opioid receptor agonists morphine and (+)-nortilidine had no effect. These results indicate that acetylcholine release in human neocortex is inhibited through delta- and kappa-opioid receptors, but not through mu-opioid receptors. Acetylcholine release was significantly increased by the delta-opioid receptor antagonist naltrindole in the presence of a mixture of peptidase inhibitors providing evidence for a delta-opioid receptor-mediated inhibition of acetylcholine release by endogenous enkephalin. K(+)-evoked acetylcholine release in the presence of TTX was inhibited by U50488, but not by DPDPE, suggesting the presence of kappa-opioid receptors on cholinergic terminals and the localization of delta-receptors on cortical interneurons. Therefore, the potent effect of DPDPE on acetylcholine release is likely to be indirect, by modulation of intrinsic cortical neurons. These interneurons probably do not use GABA as neurotransmitter since both GABAA and GABAB receptor agonists (muscimol and baclofen, respectively) were without effect on acetylcholine release.

Similar decrease in spontaneous morphine abstinence by methadone and RB 101, an inhibitor of enkephalin catabolism

1. The dual inhibitor of enkephalin degrading enzymes, RB 101, is able to block endogenous enkephalin metabolism completely, leading to potent antinociceptive responses potentiated by blockade of CCKB receptors. In this study we have investigated the effects induced by RB 101 given alone, or with the CCKB antagonist, PD-134,308, on a model of spontaneous morphine withdrawal and substitutive maintenance in rats. 2. Animals were chronically treated with morphine for 7 days followed, 36 h after the interruption of drug administration, by a maintenance treatment for 5 days with methadone (2 mg kg-1, i.p.), clonidine (0.025 mg kg-1, i.p.), RB 101 (40 mg kg-1, i.p.), PD-134,308 (3 mg kg-1, i.p.) or a combination of RB 101 plus PD-134,308. Several behavioural observations were made during this period in order to evaluate the acute effects as well as the consequence of chronic maintenance induced on spontaneous withdrawal by the different treatments. 3. Methadone was the most effective compound in decreasing the spontaneous withdrawal syndrome after acute administration. Both, methadone and RB 101 had similar effectiveness in reducing opiate abstinence during the period of substitutive treatment. PD-134,308 did not show any effect when administered alone and did not modify the effect of RB 101. 4. Naloxone (1 mg kg-1, s.c.) failed to precipitate any sign of withdrawal when injected at the end of the chronic maintenance treatment suggesting that, under the present conditions, methadone and RB 101 did not induce significant physical opiate-dependence. 5. The mildness of the side effects induced by chronic RB 101, suggests that systemically active inhibitors of enkephalin catabolism could represent a promising treatment in the maintenance of opiate addicts.

Effects of cold acclimation and central opioid processes on thermoregulation in rats

Two experiments, using centrally administered [D-Ala2-MePhe4-Gly(ol)5]enkephalin (DAMGO), a selective mu-opioid agonist, assessed the thermoregulatory consequences of cold acclimation. Experiment 1 assessed whether cold acclimation influenced DAMGO hyperthermia at room temperature. Sialo-adenectomized rats were implanted with ICV cannulae and IP Mini-Mitters. After 3 weeks of exposure to 5 degrees C (cold acclimation) or 22 degrees C (non-cold acclimation) rats were pretreated with IP naltrexone HCl (2 mg/kg b.wt.) or vehicle (0.15 M saline) and later administered a 5-microliters ICV injection of 0.15 M saline, 0.1, or 1.0 microgram DAMGO. Cold acclimation exerted little effect on core temperature but potentiated DAMGO hyperthermia in a dose-dependent, naltrexone-reversible, activity-independent manner. Experiment 2 assessed the effect these same manipulations exerted on operant escape from a convective source of mild heat (37 degrees C). Duration of heat escape increased with cold acclimation in a naltrexone-resistant manner, yet was not influenced by DAMGO in either non-cold-acclimated or cold-acclimated rats. These findings suggest that two central adaptations occur with cold acclimation: A non-mu-opioid process that increases heat sensitivity and a mu-opioid process that potentiates hyperthermia but fails to alter heat escape due to mu-opioid-mediated analgesia.

Ocular action of an opioid peptide, DPDPE

This study was performed to examine some ocular actions of an opioid agonist. Experiments were performed to evaluate the effects of a delta opioid agonist, DPDPE ([D-pen2, D-pen5]enkephalin (where pen = penicillamine)), on: 1) intraocular pressure (IOP) in rabbits; 2) cAMP accumulation in rabbit iris ciliary bodies (ICBs). Unilateral, topical administration of DPDPE caused bilateral depression of IOP. Intravitreal injection of DPDPE caused a greater IOP decrease than intravitreal injection of NaH2PO4 (vehicle). Topical administration of naloxone partially inhibited the effect of DPDPE on IOP in normal rabbits. In other experiments, DPDPE suppressed both basal and isoproterenol (ISO)-stimulated cAMP accumulation in ICBs. The presence of naltrindole (NTI), a delta receptor antagonist, did not prevent the suppression of cAMP levels by DPDPE. The conclusions drawn from the findings suggest that the lowering of IOP by DPDPE is mediated, in part, by actions at postjunctional (ciliary body) sites and may involve an atypical opioid receptor.

Brain sites involved in mu-opioid receptor-mediated actions: a 2-deoxyglucose study

Brain regions that may be functionally involved in the neuropharmacological actions of mu-opioid agonists have been examined in conscious rats using the quantitative [14C]2-deoxyglucose autoradiographic technique. At 0.5 microgram and 1 microgram intracerebroventricularly the highly selective mu-opioid receptor agonist D-Ala2, MePhe4, Gly-ol5-enkephalin effected statistically significant increases as well as statistically significant decreases in regional glucose utilization: in limbic structures, such as hippocampal formation, medial amygdala and lateral septum, glucose utilization was most prominently increased after D-Ala2, MePhe4, Gly-ol5-enkephalin; glucose utilization was further increased in the lateral habenular nucleus, the hypothalamus, ventromedial nucleus and dorsal raphe; whereas decreases were found in the mamillary body and anterior thalamus. Glucose utilization in structures associated with somatosensory and nociceptive processing was increased in the central gray of the midbrain and decreased in the nucleus gelatinosus. Only increases in glucose utilization were produced by D-Ala2; MePhe4, Gly-ol5-enkephalin in brain regions involved in motor control, including the globus pallidus, the substantia nigra, pars reticulata, the nucleus ruber and the cerebellum, and brain regions involved in visual processing--the visual cortex and superior colliculus deep layer. It is concluded that this pattern of regional changes underlies the mu-opioid receptor-mediated antinociceptive-, epileptogenic-, memory- and mood-modulating actions of mu-opioid agonists.

Systemic and supraspinal, but not spinal, opiates suppress allodynia in a rat neuropathic pain model

The effects of intraperitoneal (I.P.), intracerebroventricular (ICV) and intrathecal (IT) opiates were studied in the rat neuropathic pain model of Kim and Chung. Dose dependent reduction of allodynia was observed after I.P. and ICV morphine, but not after IT morphine, IT or ICV c[D-pen2 D-pen5]enkephalin (DPDPE) (delta agonist), or IT or ICV U50488H (kappa agonist). The effects of ICV morphine were blocked by I.P. naloxone, but not by IT methysergide, phentolamine or 8-sulfophenyltheophylline. Catalepsy (immobility) was observed after IT, ICV and IT morphine but this was not reliably associated with a reduction of allodynia. I.P. and ICV morphine may thus reduce tactile allodynia via supraspinal, but not spinal, mu opioid receptors.

Gene-peptide relationships in the developing rat brain: the response of preproenkephalin mRNA and [Met5]-enkephalin to acute opioid antagonist (naltrexone) exposure

[Met5]-enkephalin, encoded by the preproenkephalin (PPE) gene, serves as a growth factor during brain development in addition to its role as a neurotransmitter. This study examined the relationship of gene and peptide expression in the developing (postnatal day 6) rat brain by disrupting peptide-receptor interaction with either a brief (4-6 h) or continuous opioid receptor blockade using a single injection of 1 or 50 mg/kg naltrexone (NTX), respectively; such perturbations result in growth inhibition or acceleration, respectively. In the caudate putamen, an area that has completed neurogenesis by postnatal day 6 and has an abundance of PPE mRNA and enkephalins in adulthood, NTX did not influence PPE mRNA in either NTX group, or the enkephalin levels in the 1 mg/kg NTX group. [Met5]-enkephalin values in the neostriatum, however, were 67-183% greater than controls in rats given 50 mg/kg NTX, beginning 5 min after drug injection. In the cerebellum, PPE mRNA expression was depressed from 5 min to 4 h in the 1 mg/kg NTX group, and was normal thereafter; mRNA levels in the 50 mg/kg NTX group were markedly subnormal for 24 h. Enkephalin levels were significantly depressed within 5 min of drug injection and remained so for 4 h in the 1 mg/kg NTX group, but were elevated to approximately 135% of control values at 8, 16, and 24 h. Enkephalin levels were not changed in the cerebellum of the 50 mg/kg NTX group, or in the plasma of either NTX group. These data suggest that a single exposure to NTX can affect transcriptional and translational mechanisms related to PPE mRNA and opioid peptide expression in a rapid and sustained manner, and that this treatment elicits a specific pattern of alterations dependent upon the brain region sampled, drug dosage, and/or the duration of opioid receptor blockade. Additionally, our results indicate that the decreased DNA synthesis in external germinal cells occurring after opioid receptor blockade as recorded earlier may be related to an increase in the potent opioid growth factor, [Met5]-enkephalin.

Differential blockade of morphine and morphine-6 beta-glucuronide analgesia by antisense oligodeoxynucleotides directed against MOR-1 and G-protein alpha subunits in rats

An antisense oligodeoxynucleotide directed against the 5'-untranslated region of MOR-1 blocks the analgesic actions of the mu 1 analgesics morphine and [D-Ala2,D-Leu5]enkephalin (DADL) when they are microinjected into the periaqueductal gray. In contrast, morphine-6 beta-glucuronide (M6G) analgesia is unaffected by this treatment. Antisense oligodeoxynucleotides directed against distinct Gi alpha subunits also distinguish between morphine and M6G analgesia. A probe targeting Gi alpha 2 blocks morphine analgesia, as previously reported, but is inactive against M6G analgesia. Conversely, an antisense oligodeoxynucleotide against Gi alpha 1 inhibits M6G analgesia without affecting morphine analgesia. The antisense oligodeoxynucleotide directed against G(o)alpha is ineffective against both compounds. These results confirm the prior association of Gi alpha 2 with morphine analgesia and strongly suggests that M6G acts through a different opioid receptor, as revealed by its insensitivity towards the MOR-1 antisense probe and differential sensitivity towards G-protein alpha subunit antisense oligodeoxynucleotides.

Human delta opioid receptor: a stable cell line for functional studies of opioids

A human delta opioid receptor cDNA clone (pREP10/hDOR) was transfected into Chinese hamster ovary (CHO) cells. The stable cell line expressed a high density of delta opioid receptors (137,000 +/- 21,600 receptors/cell). DPDPE inhibited 90% of the forskolin-stimulated cAMP accumulation in these cells with high potency (EC50 = 1.3 nM). This effect of DPDPE was antagonized by naltrindole. The pseudo-pA2 value (Ke) of 155 pM for naltrindole is consistent with that measured for delta receptor antagonism in the mouse vas deferens. This is the first detailed characterization of DPDPE activity on forskolin-stimulated cAMP accumulation mediated through a human delta opioid receptor. The data support the use of the recombinant cell line for functional studies of opioid drugs.

Delta opioid agonists inhibit proliferation of highlypurified murine CD4+ and CD8+ T-cells

A substantial body of evidence demonstrates that opiates and opioid peptides modulate immune function. The present study used highly purified murine CD4+ and CD8+ T-cells to determine the effects of delta opioid receptor (DOR) agonists on proliferation. Splenic T-cells, obtained from male or female C57BL/6 or CD1 mice, were separated by a fluorescence activated cell sorter. Cells were stimulated to proliferate in serum free medium by cross-linking the T-cell receptor using plate-coated anti-CD3-epsilon, 3H-thymidine uptake was determined at 48 hours. Previous experiments had shown that deltorphin and [D-Ala2]-met-enkephalinamide (DAME), at concentrations from 10(-11) to 10(-7) M, dose dependently inhibited the proliferation of CD4+ and CD8+ T-cells obtained from female C57BL/6 or CD1 mice. Similarly, the experiments herein demonstrate that proliferation of CD4+ T-cells from female CD1 mice was inhibited by 2,5 DPDP-enkephalin (DPDP-E), in direct relation to dose. In contrast, the anti-proliferative response of cells from C57BL/6 mice demonstrated an inverse relationship to dose. At 10(-11) M, the most effective dose of DPDP-E studied, 3H-thymidine uptake was inhibited by 50%. The selective DOR antagonist, naltrindole (10(-12) M), abolished this. DAME was used to compare the effects of DOR agonists on CD8+ T-cells from both strains of female mice. 3H-Thymidine uptake was dose-dependently inhibited to a similar degree in both strains; 10(-7) M DAME maximally reduced proliferation by 70%. DAME had similar effects on both CD8+ and CD4+ T-cells from male mice, and its inhibitory effect was markedly attenuated after 72 hours.(ABSTRACT TRUNCATED AT 250 WORDS)

Effects of the delta-opioid agonist, [D-Pen2,D-Pen5]-enkephalin, on fetal lamb EEG

Opiates are known to exert biphasic effects on level of arousal, with excitation at low doses and depression at higher doses. It has been suggested that this dual excitatory and depressant actions of opiates may be mediated by different receptor subtypes. We have previously shown that activation of mu 1-opioid receptors evoked EEG activation in the fetal lamb. The purpose of the present study was to quantitate the effects of DPDPE, a highly selective delta-opioid agonist, on fetal EEG. When infused ICV (4.6-154 nmol/h), DPDPE elicited dose-dependent activation of fetal EEG, with a reduction in power distribution in the delta (1-4 Hz) band, and an increase in the beta (15-32 Hz) band. This activation was reflected by an increase in the spectral edge frequency. This EEG activation was greatly attenuated at DPDPE doses greater than 154 nmol/h, resulting in a U-shaped dose-response curve. The EEG activation was completely blocked by naloxone or naltrindole (delta antagonist), but not by naloxonazine (mu 1 antagonist). These results indicate that the activation of delta-opioid receptors will evoke EEG activation in the fetal lamb.

Dexamethasone-induced selective inhibition of the central mu opioid receptor: functional in vivo and in vitro evidence in rodents

1. Endogenous corticosteroids and opioids are involved in many functions of the organism, including analgesia, cerebral excitability, stress and others. Therefore, we considered it important to gain information on the functional interaction between corticosteroids and specific opioid receptor subpopulations. 2. We have found that systemic administration (i.p.) of the potent synthetic corticosteroid, dexamethasone, reduced the antinociception induced by the highly selective mu agonist, DAMGO or by less selective mu agonists morphine and beta-endorphin administered i.c.v.. On the contrary dexamethasone exerted little or no influence on the antinociception induced by a delta 1 agonist, DPDPE and a delta 2 agonist deltorphin II. Dexamethasone potentiated the antinociception induced by the kappa agonist, U50,488. 3. In experiments performed in an in vitro model of cerebral excitability in the rat hippocampal slice, dexamethasone strongly prevented both the increase of the duration of the field potential recorded in CA1, and the appearance and number of additional population spikes induced by mu receptor agonists. 4. In both models pretreatment with cycloheximide, a protein synthesis inhibitor, prevented the antagonism by dexamethasone of responses to the mu opioid agonists. 5. Our data indicate that in the rodent brain there is an important functional interaction between the corticosteroid and the opioid systems at least at the mu receptor level, while delta and kappa receptors are modulated in different ways.

[D-Pen2-D-Pen5]enkephalin, a delta opioid agonist, given intracerebroventricularly in the mouse produces antinociception through medication of spinal GABA receptors

Intracerebroventricular (ICV) administration of [D-Pen2-D-Pen5]enkephalin (DPDPE), a delta opioid receptor agonist, activates a descending antinociceptive pathway that inhibits the tail-flick response in mice. Involvement of spinal GABA receptors in this response was studied by giving GABA antagonist intrathecally. First, antinociception produced by intrathecally administered isoguvacine, a GABAA agonist, was inhibited by intrathecal bicuculline (GABA receptor antagonist) or picrotoxin (chloride channel antagonist). Then, antinociception induced by ICV DPDPE was antagonized by intrathecal picrotoxin and bicuculline in a dose-and time-dependent manner. Second, intrathecal administration of 2-hydroxysaclofen, a GABAB antagonist (which inhibited antinociception induced by a GABAB agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA agonist, baclofen, given IT), produced a shift of the dose-response curve for ICV DPDPE to the right. GABAA and B antagonists given together intrathecally produced a greater than additive antagonistic effect against ICV DPDPE-induced antinociception. Thus, the delta agonist action of DPDPE in the brain leads to activation of descending spinal pathways which involve mediation by spinal GABAA and GABAB receptors in the antinociceptive response.

Differential antagonism by MK-801 against antinociception induced by opioid receptor agonists administered supraspinally in mice

Various doses of MK-801 ((+/-)-5-methyl-10,11-dihydro-5H-dibenzo(a,d) cyclohepten-5, 10-imine maleate), a non-competitive N-methyl-D-aspartic acid (NMDA) receptor antagonist (0.001-1 microgram) injected intracerebroventricularly (i.c.v.) alone did not show any antinociceptive effect. MK-801 (0.001-1 microgram i.c.v.) dose dependently attenuated the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered morphine (1 microgram), [D-Pen2, D-Pen5]enkephalin (DPDPE; 10 micrograms), and U50,488H (trans-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)cyclohexyl]benzeoce tamide ) 60 micrograms). However, the inhibition of the tail-flick and hot plate responses induced by i.c.v. administered beta-endorphin (1 microgram) was not changed by i.c.v. administered MK-801. Our results indicate that, at the supraspinal level, NMDA receptors are involved in the production of antinociception induced by supraspinally administered morphine, DPDPE, and U50,488H but not beta-endorphin.

Disruption of target interactions prevents the development of enkephalin immunoreactivity in sympathetic neurons

We compared the development of enkephalin (Enk) expression in normal rats and rats in which target contact was transiently disrupted with 6-hydroxydopamine (6-OHDA). During the first 3 postnatal weeks, there was a striking increase in Enk immunoreactivity (-IR) in superior cervical ganglia (SCG) assayed by radioimmunoassay (RIA). This increase was correlated with the appearance of Enk-IR in postganglionic neurons. In the caudal region of the SCG, the proportion of Enk-IR neurons and their immunoreactivity increased until one-third of the neurons possessed Enk-IR between postnatal days (P) 14 and 21. After P21, the number of Enk-IR neurons and their immunofluorescence decreased. By 6 weeks, only occasional neurons possessed moderate Enk-IR. The increases in Enk-IR were correlated with increased ganglionic proenkephalin A mRNA detected by in situ hybridization. The decrease in IR after P21 was not, however, paralleled by a comparable decrease in proenkephalin A mRNA. To determine whether interactions between SCG neurons and their target tissues influence Enk expression, we disrupted them by treating neonatal rats with a single dose of 6-OHDA at P0. This treatment transiently reduced sympathetic fiber density in the submandibular gland, one target of Enk-IR neurons, over 90%. Two weeks later, the fiber density in glands of treated animals was not different from control. Following 6-OHDA, the concentration of Enk-IR in SCG extracts and the number of Enk-IR neurons and their immunofluorescence intensity failed to increase. SCG from treated rats also contained fewer neurons with proenkephalin A mRNA. In contrast, the content of neuropeptide Y (NPY) and the proportion of NPY-IR neurons were not decreased by 6-OHDA treatment. Our results indicate that the developmental history of Enk expression differs from that of other neuropeptides in rat sympathetic ganglia, suggesting that distinct mechanisms regulate the expression of individual neuropeptides. Further, they provide evidence that target contact during a critical period is important for the induction of Enk.

Beta-funaltrexamine blockade of opioid-induced inhibition of somatostatin secretion from rat stomach

Opioid peptides are potent inhibitors of gastric somatostatin secretion. In the current investigation the effect of mu-opioid receptor blockade on responses to [D-Ala2,N-Me-Phe4,Gly5-ol]enkephalin (DAGO) was studied. Gastric inhibitory polypeptide (GIP; 1 nM) -stimulated secretion of immunoreactive somatostatin was almost completely inhibited by DAGO (1 microM). The mu-receptor antagonists, beta-funaltrexamine and naloxonazine, blocked the effect of DAGO. Pretreatment of rats with beta-funaltrexamine, 24 h prior to perfusion, reduced the percentage inhibition by DAGO from 88.6 +/- 5.2% to 50.7 +/- 9.3%. These studies support the involvement of mu-opioid inhibitory receptors in the regulation of gastric somatostatin secretion.

Antinociceptive response induced by mixed inhibitors of enkephalin catabolism in peripheral inflammation

RB101 (N-((R,S)-2-benzyl-3[(S)(2-amino-4-methylthio)butyl dithio]-1-ox-opropyl)-L-phenylalanine benzyl ester) is a recently developed full inhibitor of the enkephalin-catabolizing enzymes able to cross the blood-brain barrier, whereas RB38A ((R)-3-(N-hydroxycarboxamido-2-benzylpropanoyl)-L-phenylalanine) is as potent as RB101 but almost unable to enter the brain. In this study, we have investigated the effects of systemic administration of morphine, RB101 and RB38A on nociception induced by pressure on inflamed peripheral tissues. Antinociceptive test was performed between 4 and 5 days after injection into the rat left hindpaw of Freund's complete adjuvant to produce localized inflammation. Morphine (1, 2 and 4 mg/kg, i.v.) induced antinociception in inflamed paws at all the doses used, and only at the highest dose in non-inflamed paws. RB101 (10 and 20 mg/kg, i.v.) induced an antinociceptive response only in the inflamed paws. RB38A, also induced an antinociceptive effect in the inflamed paws, but only at the highest dose (20 mg/kg, i.v.). The responses induced by morphine and the inhibitors of enkephalin catabolism were antagonized by the systemic administration of naloxone (1 mg/kg) or methylnaloxonium (2 mg/kg) which acts essentially outside the brain. Central injection (i.c.v.) of methylnaloxonium (2 micrograms) blocked the effect of morphine only in non-inflamed paws, and slightly decreased the response induced by RB101 on inflamed paws. These results indicate that the endogenous opioid peptides, probably enkephalins, are important in the peripheral control of nociception from inflamed tissues.

Antisense oligodeoxynucleotide to a delta-opioid receptor selectively blocks the spinal antinociception induced by delta-, but not mu- or kappa-opioid receptor agonists in the mouse

An antisense oligodeoxynucleotide (A-oligo) to delta-opioid receptor mRNA was utilized to block the expression of mouse delta-opioid receptor in the spinal cord of male ICR mice. Intrathecal treatment with A-oligo (1.6-163 pmol) dose-dependently attenuated the antinociception induced by i.t. administered DPDPE ([D-Pen2,5]enkephalin) or [D-Ala2]deltorphin II, delta-opioid receptor agonist, without affecting the antinociception induced by DAMGO ([D-Ala2-MePhe4,Gly(ol)5]enkephalin) or U50,488H, respective mu- or kappa-opioid receptor agonists. Scrambled sense oligodeoxynucleotide (163 pmol) was ineffective against the tail-flick inhibition induced by DPDPE,[D-Ala2]deltorphin, DAMGO or U50,488H. The studies confirm previous pharmacological studies at the molecular level indicating a distinct delta-opioid receptor for antinociception in the spinal cord.

[L-Ala3]DPDPE: a new enkephalin analog with a unique opioid receptor activity profile. Further evidence of delta-opioid receptor multiplicity

To investigate delta-opioid receptor topography near the 3-position of [D-Pen2,D-Pen5]enkephalin (DPDPE), a series of small-group 3-position analogs of DPDPE have been synthesized and assayed for binding potencies and in vitro biological activities. L-Amino acid substitutions at this position are highly favored over D-amino acid substitutions, with the smallest, [L-Ala3]DPDPE (DPADPE), being the most favored in the series investigated. [L-Ala3]DPDPE is nearly as delta-potent and more delta-selective in both rat brain binding (18 nM vs [3H] [p-ClPhe4]DPDPE and mu/delta = 610) and peripheral bioassays (12 nM in the MVD and GPI/MVD = 4500) when compared to DPDPE (8.5 nM, mu/delta = 73 and 4.1 nM, GPI/MVD = 1800, respectively). Whereas DPDPE is a potent analgesic when given icv, [L-Ala3]DPDPE is only a weak analgesic. However, [L-Ala3]DPDPE has been found to antagonize DPDPE, but not Deltorphin II, in a moderately potent (pA2 = 5.7) and selective fashion in vivo. Thus, [L-Ala3]DPDPE is a fairly potent agonist at peripheral delta receptors and is a moderately potent (mixed) antagonist of delta 1 receptors in the brain. It appears that [L-Ala3]DPDPE does not interact in any significant manner with delta 2 or mu receptors in the brain.

Endogenous dopamine inhibits the release of enkephalin-like immunoreactivity from amacrine cells of the chicken retina in the light

The activity of the enkephalin-immunoreactive (ENSLI) amacrine cells of the chicken retina is low in the light and high in the dark, resulting in parallel increases and decreases in the levels of the enkephalins. In vivo, the selective dopaminergic D1 antagonist SCH23390 increased the activity of the ENSLI amacrine cells in the light (ED50; 20 pmol), but had a much lesser effect in the dark, whereas the selective dopaminergic D2 antagonist sulpiride had effects only at very high concentrations (ED50; 39 nmol). In contrast, the non-selective dopamine agonist ADTN hardly affected the activity of the ENSLI amacrine cells in the light, but markedly reduced their activity in the dark. This pattern of effects suggests that dopamine actively inhibits the ENSLI amacrine cells in the light, but exerts much less inhibitory activity in the dark, consistent with the idea that dopamine is released during the exposure of the retina to light. Thus dopaminergic controls over the ENSLI amacrine cells appear to contribute to the light:dark differences in activity of the ENSLI amacrine cells. Results obtained on the dopaminergic control of enkephalin release in vitro were generally consistent with this model, except that ADTN appeared to stimulate the ENSLI amacrine cells in the dark.

Dynorphin A-(1-13) potently improves the impairment of spontaneous alternation performance induced by the mu-selective opioid receptor agonist DAMGO in mice

The effects of i.c.v. injection of the mu-selective opioid receptor agonist DAMGO and the effects of its combination with the endogenous kappa-opioid receptor agonist dynorphin A-(1-13) on memory processes were examined in mice, using spontaneous alternation performance associated with working memory in a Y-maze. DAMGO (10 and/or 30 ng) impaired spontaneous alternation performance and increased total arm entries, which are considered to reflect locomotor activity. beta-Funaltrexamine (5 micrograms, i.c.v.), a mu-selective opioid receptor antagonist, almost completely antagonized the impairment of alternation performance induced by DAMGO (10 ng). Physostigmine (0.1 mg/kg, i.p.), a cholinesterase inhibitor, improved the DAMGO (10 ng)-induced impairment of alternation performance. Dynorphin A-(1-13) (1, 3 and 10 micrograms, i.c.v.) alone was without significant effects on alternation performance. On the other hand, dynorphin A-(1-13) (3 and 10 micrograms) significantly improved the impairment of spontaneous alternation performance induced by DAMGO (10 ng). The effects of dynorphin A-(1-13) (3 micrograms) on the DAMGO-induced impairment of spontaneous alternation were almost completely reversed by pretreatment with nor-binaltorphimine (4 micrograms, i.c.v.), a kappa-selective opioid receptor antagonist. The present results demonstrate that DAMGO impairs alternation performance by activating mu-opioid receptors, whereas dynorphin A-(1-13) attenuates the DAMGO-induced impairment of alternation performance through the mediation of kappa-opioid receptors. These findings suggest that mu- and kappa-opioid systems are fully involved in memory function and have opposite effects on spontaneous alternation performance as it is reflected by working memory in mice.

Reduction in ATP-sensitive potassium channel-mediated antinociception in diabetic mice

To test our hypothesis that the abnormally low efficacy of mu-opioid agonists in diabetic mice may be due to functional changes in ATP-sensitive potassium channels, we evaluated the effects of cromakalim on the tail-flick latencies in diabetic and non-diabetic mice. Anti nociceptive effects of morphine (10 micrograms, ICV) in diabetic mice were significantly less than that in non-diabetic mice. Morphine-induced antinociception in non-diabetic mice was antagonized by pretreatment with glibenclamide (30 micrograms, ICV), an ATP-sensitive potassium channel blocker. Cromakalim (0.3 and 1 micrograms, ICV) produced significant, dose-dependent antinociception in non-diabetic mice, which was significantly reduced by pretreatment with glibenclamide. However, cromakalim did not markedly affect the tail-flick latencies in diabetic mice, even at higher doses (3 micrograms, ICV). On the other hand, [D-Pen2,5]enkephaline (DPDPE, 5 micrograms, ICV), a selective delta-opioid receptor agonist, produced significant antinociception in both diabetic and non-diabetic mice. Since pretreatment with glibenclamide significantly reduced the antinociceptive effect of DPDPE in non-diabetic mice but not in diabetic mice, delta-opioid receptor-mediated antinociception in diabetic mice may be independent of potassium channels. These results suggest that dysfunction of ATP-sensitive potassium channels may contribute to the demonstrated poor antinociceptive response of diabetic mice to mu-opioid agonists.

Antibodies raised against the N-terminal sequence of delta opioid receptors blocked delta-mediated supraspinal antinociception in mice

A polyclonal antiserum directed against the first 16 amino acids of the N-terminal sequence of the murine delta opioid receptor was raised in rabbits. The intracerebroventricular (i.c.v.) injection to mice of the anti delta receptor IgGs impaired the antinociception produced by DPDPE, [D-Ala2]-Deltorphin II, DADLE and beta-endorphin-(1-31) when studied 24 h later in the tail-flick test. Antinociception produced by morphine and DAMGO was fully expressed in mice undergoing this treatment. The selective delta antagonist ICI 174864 (0.8 nmols/mouse, i.c.v.) significantly reduced the antinociceptive activity of opioids to the extent observed after giving the antibodies. ICI 174864 did not decrease further the antinociception that remained after the anti delta receptor serum. The specific binding displayed by 3 nM [3H]-DPDPE was reduced in membranes pre-incubated with the antiserum, whereas no change could be detected for 0.6 nM [3H]-DAMGO labelling mu receptors. This experimental approach revealed the delta component of opioid-evoked supraspinal antinociception in mice.

The influence of chronic activation and blockade of the dopamine- and enkephalinergic systems of the neostriatum on conditioned reflex behavior and dopamine metabolism in the rat nigrostriatal system

The effects of daily injections, over the course of 14 days, of 45 micrograms of phenamine, 5 micrograms of haloperidol and naloxone, 15 micrograms of leu-enkephalin and its analog, a tetrapeptide, into the rostral neostriatum have been studied. The chronic stimulation of the dopaminergic system of the striatum induced facilitation of the realization of an active avoidance conditioned reflex, and stimulated exploratory stereotypy, while its blockade led to suppression of conditioned reflex activity against the background of a clearcut rigid akinetic syndrome. The microinjections of leu-enkephalin and naloxone did not substantially alter the behavior, but the injection of the tetrapeptide was accompanied by changes in behavior, with symptoms of catalepsy and hyperkinesia. Injections of phenamine and haloperidol were accompanied by a decrease in the content of dopamine in the striatum and an increase in the level of DOPAC; the injections of enkephalin and naloxone induced changes of the reverse order. The possible causes of the noncorrespondence of the behavioral and neurochemical shifts in the presence of a direct chronic pharmacological action on the mediator of the neostriatum.

Characterization and localization of delta opioid binding sites in the bovine pineal gland

Opioid binding sites in the bovine pineal were characterized using the highly selective delta opioid agonist 3H-[D-Pen2,pCl-Phe4,D-Pen5] enkephalin (DPDP(C1)E). Pineal membranes possess a single class of high affinity binding sites for this delta ligand (Kd = 0.26 nM; Bmax = 250 fmol/mg protein). The specific opioid antagonist naloxone dose dependently inhibited 3H-DPDP(C1)E binding, confirming that this ligand is indeed binding to opioid receptors. The delta selective ligands deltorphin and [D-Pen2,5]enkephalin (DPDPE) were much more potent than the mu selective compounds dermorphin and [D-Ala2,MePhe4,Gly5- ol]enkephalin (DAMGO) in inhibiting 3H-DPDP(C1)E binding. These results demonstrate that in bovine pineal membranes, DPDP(C1)E binds to delta opioid sites. Autoradiographic studies showed a uniform distribution of 3H-DPDP(C1)E binding over the bovine pineal in the sections we analyzed. This distribution suggests that delta opioid binding sites are associated with pinealocytes which account for the majority of cell types in the pineal. However, it is not possible to rule out that these receptors may also be associated with other cell types which are present in the bovine pineal. The density and widespread distribution of delta opioid receptors supports the hypothesis that endogenous opioid peptides directly modulate pineal function.

Pharmacological interaction between neuropeptides and pethidine or fentanyl in rat spinal cord

Male Wistar rats were treated with pethidine (PT) or fentanyl (FN) subcutaneously (sc) followed by intrathecal (ith) non analgesic doses of methionine- (MENK) or leucine-enkephalin (LENK), neurotensin, (NT), substance P (SP) or cholecystokinin octapeptide 26-33 (CCK-8). Then the antinociceptive effect was measured during 1 h using tail-immersion test. LENK potentiated strongly PT and FN analgesia. MENK antagonized PT analgesia only transiently 30 min after administration and transiently potentiated FN analgesia. SP and CCK-8 potentiated significantly PT analgesia, whereas NT acted biphasically: increasing and then decreasing PT analgesia. SP, CCK-8 and NT augmented FN analgesia. Naloxone inhibited analgesia elicited by the studied opioids and neuropeptides. These data show that LENK affects similarly the analgesic effects of both studied opioids, whereas MENK acted differently on PT and FN analgesia. This may suggest that individual enkephalins have different pharmacological features when interacting with different analgesics. Also NT interacted differently with pethidine and fentanyl.

Effect of mixed (RB 38A) and selective (RB 38B) inhibitors of enkephalin degrading enzymes on a model of depression in the rat

This is a study of the effects of the endogenous opioid peptides, enkephalins, on learned helplessness, an experimental model of depression in rats. For this purpose, the responses induced by RB 38A, a mixed inhibitor of enkephalin catabolism, and RB 38B, a selective inhibitor of neutral endopeptidase EC 3.4.24.11, were compared with the antidepressive effect induced by imipramine. RB 38A and RB 38B induced an imipramine-like effect in reducing helpless behavior, as illustrated by the decrease in the number of escape failures. According to the different pharmacological potential of both inhibitors to reduce enkephalin metabolism, complete inhibition of enkephalins (RB 38A) produced a higher response than that obtained with a partial inhibitor (RB 38B). On the other hand, naloxone (NLX) was found to facilitate the induction of learned helplessness, and to antagonize the effect of both enkephalin-degrading enzyme inhibitors. These results suggest that modifications in the activity of the endogenous opioid system could take place in this model of depression. The antidepressant-like effects induced by RB 38B, and especially by RB 38A, in the learned helplessness paradigm suggest that new mixed enkephalinase inhibitors, able to cross the blood-brain barrier, could provide a new strategy in the treatment of affective disorders.