Mu and delta receptors: their role in analgesia in the differential effects of opioid peptides on analgesia

Linear and cyclic beta-casomorphin analogues with high analgesic activity

We investigated the antinociceptive efficacy of casomorphin (CM) derivatives using the vocalization test. Male Wistar rats received chronic microcannulae into the right lateral ventricle. One week later we examined the analgesic effect of CM derivatives 10, 30, 60, and 90 min after intraventricular injection (5 microliters). The analgesic effect was calculated as the individual percent increase in the pain threshold and was compared to controls (saline treatment). The substitution of D-lysine and D-ornithine in position 2 in connection with a cyclization through ring closure of the 2 position side chain amino group to the C-terminal glycine-COOH group resulted in high analgesic potency. The substitution of D-Pro4 was without any effect in the ineffective linear derivatives and decreased the effectiveness in the highly effective cyclic derivatives. The cyclic [D-Orn2]CM-5 and the cyclic [D-Lys2]CM-5 are the CM derivatives with the highest antinociceptive activity. The cyclic [D-Orn2]CM-5 is greater than 1000 times more effective than morphine. We conclude, on the basis of studies of receptor binding and in vitro investigations, that mu receptor activity alone is not responsible for the analgesic activity. The delta receptor and possibly also the kappa receptor could modulate the nociceptive effectiveness.

Antinociceptive and motor effects of delta/mu and kappa/mu combinations of intrathecal opioid agonists

Interactions between selective opioid agonists acting at spinal mu-, delta-, and kappa-opioid receptors were evaluated by co-administering a low-antinociceptive dose of the selective delta-agonist, DPDPE, or the selective kappa-agonist, U50,488H, with sequentially increasing doses of the selective mu-agonist, DAMGO, intrathecally. Antinociceptive synergy (i.e., a more than additive antinociceptive effect) was observed with both combinations of opioid agonists tested. The demonstration of antinociceptive synergy suggests that the subtypes of spinal opioid receptors can act, at least in part, through a common neural circuit. Since our measure of antinociception, the Randall-Selitto paw-withdrawal test, is dependent on a normally functioning motor system, we also evaluated the effects of these same combinations of opioid peptides on motor coordination using a rotarod treadmill. A low-antinociceptive dose of DPDPE or U50,488H co-administered intrathecally, with sequentially increasing doses of DAMGO, did not worsen the decrement in rotarod performance observed with the same doses of DAMGO administered as a single agent. In fact, the low-antinociceptive dose of DPDPE significantly attenuated the decrease in rotarod performance produced when the same dose of DAMGO was administered as a single agent. The results of this study suggest that intrathecal combinations of selective mu- with both delta- or kappa-selective opioid agonists can produce antinociceptive synergy without producing an increase in motor side effects.(ABSTRACT TRUNCATED AT 250 WORDS)

Antinociceptive interactions of opioid delta receptor agonists with morphine in mice: supra- and sub-additivity

In this study, the antinociceptive interactions of fixed ratio combinations of intracerebroventricularly (i.c.v.) given morphine and subantinociceptive doses of the delta agonists, [D-Pen2, D-Pen5]enkephalin (DPDPE), [D-Ala2, Glu4]deltorphin (DELT) or [Met5]enkephalin (MET) were examined using the mouse warm water tail flick test. When morphine was coadministered with DPDPE or DELT in a 4:1 and 9:1 mixture, respectively, a synergistic antinociceptive effect was observed. In contrast, when morphine was coadministered with MET in a 1:2 fixed ratio mixture, a subadditive interaction occurred. These results demonstrate both positive and negative modulatory interactions of delta agonists with morphine in an antinociceptive endpoint and that these interactions can be either supra- or subadditive. The data support the concept of a functional interaction between opioid mu and delta receptors and a potential regulatory role for the endogenous ligands of the opioid delta receptor.

Neurochemical studies on the mesolimbic circuitry of antinociception

Previous studies using the technique of microinjection into brain nuclei indicated that the periaqueductal gray (PAG), nucleus accumbens, habenula and amygdala play an essential role in pain modulation and that these nuclei possibly act through a 'mesolimbic neural loop' to exert an analgesic effect, in which Met-enkephalin (MEK) and beta-endorphin (beta-EP) have been implicated as the two major opioid peptides involved in antinociception. In the present study performed in rabbits, intracranial microinjection was supplemented with push-pull perfusion and radioimmunoassay to determine whether the release of enkephalins (ENK) and beta-EP was increased in these nuclei when the putative neural circuit was activated by morphine administered into one of the nuclei. The results showed: (1) microinjection of morphine into the PAG increased the release of ENK and beta-EP in the N. accumbens, and vice versa; (2) microinjection of morphine into the N. accumbens increased the release of ENK and beta-EP in the amygdala, and vice versa; (3) morphine microinjected into the PAG caused an increase in the release of ENK and beta-EP in the amygdala and vice versa, although the release of ENK in PAG was statistically not significant. These results indicate that PAG, N. accumbens and amygdala are connected in a network served by a positive feedback circuitry.

Modulation of mu-mediated antitussive activity in rats by a delta agonist

Effects of selective mu and delta receptor agonists on capsaicin-induced cough reflex in rats were studied. Intracisternal injection (i.cist.) of a selective mu receptor agonist [D-Ala2,Mephe4,Gly-ol5]enkephalin (DAMGO) produced dose-related depression of coughs over the 0.003-0.03 nmol dose range. The antitussive potency of DAMGO was 100-fold more potent than morphine. The antitussive effects of DAMGO and morphine were significantly reduced by naloxone (1 nmol i.cist.). The selective delta receptor agonist, [D-Pen2,D-Pen5]enkephalin (DPDPE), at a dose of 10 nmol (i.cist.), had no significant effect on the number of coughs. When co-administered i.cist., DPDPE (10 nmol) consistently and significantly decreased the antitussive potencies of DAMGO and morphine. The decrease in the antitussive effects of DAMGO and morphine caused by DPDPE were prevented by selective delta receptor antagonist, naltrindole (3 nmol). These results suggest that the antitussive effects of opioids are mediated predominantly by mu receptors, and delta receptors may play an inhibitory role in antitussive processes that are mediated by the mu receptors.

Binding to opioid receptors of enkephalin derivatives taking alpha-helical conformation and its dimer

The opioid receptor binding of [Leu]enkephalin derivatives with extended address segment to the C-terminal was studied. The extension peptide is designed to take an amphiphilic helical structure in order to evaluate effects of helical conformation and membrane affinity of enkephalin moiety of the derivatives on receptor binding. In the delta-receptor-selective binding assay, Tyr-Gly-Gly-Phe-Leu-Lys-Aib-Leu-Aib-OH (1) showed the same affinity as enkephalinamide, whereas in the mu-receptor-selective binding assay, a 7-fold reduction in affinity was observed. On the other hand, Tyr-Gly-Gly-Phe-Leu-(Lys-Aib-Leu-Aib)2-OH (2) showed 51- and 96-fold decreases in affinities for delta- and mu-receptors, respectively, compared with enkephalinamide. The low receptor affinity of derivative 2 is considered due to alpha-helical conformation, which might not be compatible with topological requirements of delta- and mu-receptors. A dimer, Tyr-Gly-Gly-Leu-Phe-(Lys-Aib-Leu-Aib)2-Lys(X)-Aib-OCH3 (X = Tyr-Gly-Gly-Phe-Leu-, (4], showed 2.5- and 3.0-fold increases in affinities respectively for delta- and mu-receptors compared with the monovalent derivative 2, possibly due to cross-linking of neighboring receptors. The Hill plot of the binding of the dimer to bovine brain membranes was composed of two phases, although such a heterogeneity of receptors was not observed in the presence of naloxone or in the binding to NG108-15 cell membranes. These findings indicate the presence of the bivalent-ligand-induced interactions between delta- and mu-receptors in bovine brain membranes.

Central analgesic mechanisms: a review of opioid receptor physiopharmacology and related antinociceptive systems

Clinical applications of these principles, based on the increased understanding of central analgetic mechanisms, are already being undertaken. Not only does the use of intrathecal and epidural opioids have the potential to decrease pain and related morbidity after surgical procedures, but there is at least one study that demonstrates a significant reduction in both major morbidity and mortality in high-risk surgical patients in whom epidural anesthesia and analgesia were used. These principles are also useful for the management of patients undergoing cardiac surgery. Currently, high-dose narcotic anesthesia is the technique of choice for such patients because of the greater hemodynamic stability this anesthetic technique provides. However, breakthrough hypertension and tachycardia still occur, and prolonged postoperative ventilation is a necessary consequence due to the high doses of narcotics that are required. In one study of patients undergoing coronary artery surgery, preoperative administration of clonidine, 5 micrograms/kg, orally, was demonstrated to decrease fentanyl requirements by 45% (110 to 61 micrograms/kg) while producing a similar degree of hemodynamic stability as seen with high-dose fentanyl. Extubation times were not compared, but the significantly lower dosage of fentanyl in the clonidine-treated group would be expected to lead to an earlier extubation. Whether similar potentiation of narcotic effects would be seen with dexmedetomidine, which may also prevent narcotic-induced rigidity, has not been determined, but the clinical application of such synergistic and complementary agents is another consequence of the greater understanding of central analgesic mechanisms, and augurs well for the future.

Interaction of beta-funaltrexamine with [3H]cycloFOXY binding in rat brain: further evidence that beta-FNA alkylates the opioid receptor complex

beta-Funaltrexamine (beta-FNA) is an alkylating derivative of naltrexone. In addition to acting as an irreversible inhibitor of mu-receptor-mediated physiological effects, intracerebroventricular (i.c.v.) administration of beta-FNA to rat attenuates the ability of selective delta receptor antagonists and naloxone to reverse delta receptor-mediated effects. Moreover, recent work demonstrated that i.c.v. administration of beta-FNA alters the conformation of the opioid receptor complex, as inferred by a decrease in the Bmax of the lower affinity [3H][D-ala2,D-leu5]enkephalin binding site. Consistent with the decreased potency of naloxone as an inhibitor of delta receptor mediated effects, beta-FNA doubled the naloxone IC50 for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site. These data collectively support the hypothesis that the opioid receptor complex postulated to mediate mu-delta interactions in vivo is identical to the opioid receptor complex as defined by vitro ligand binding studies. A direct prediction of this hypothesis is that beta-FNA should increase the Kd of antagonists for the mu binding site (mu cx) of the receptor complex. The data reported in this paper demonstrate that beta-FNA doubled the IC50 of the potent narcotic antagonist, 6-desoxy-6 beta-fluoronaltrexone (cycloFOXY) for displacing [3H][D-ala2,D-leu5]enkephalin from its lower affinity binding site, and doubled the Kd of [3H]cycloFOXY for its mu binding site, providing additional data that the mu binding site labeled by [3H]cycloFOXY is the mu binding site of the opioid receptor complex. beta-FNA also altered the kappa binding site labeled by [3H]cycloFOXY, and when administered intrathecally to mice, beta-FNA produced a longlasting antinociception in the acetic acid writhing test.

Probing the opioid receptor complex with (+)-trans-superfit. I. Evidence that [D-Pen2,D-Pen5]enkephalin interacts with high affinity at the delta cx binding site

A variety of data support the existence of an opioid receptor complex composed of distinct but interacting mu cx and delta cx binding sites, where "cx" indicates "in the complex." The ability of subantinociceptive doses of [Leu5]enkephalin and [Met5]enkephalin to potentiate and attenuate morphine-induced antinociception, respectively, is thought to be mediated via their binding to the delta cx binding site. [D-Pen2,D-Pen5]Enkephalin also modulates morphine-induced antinociception, but has very low affinity for the delta cx binding site in vitro. In the present study, membranes were depleted of their delta ncx binding sites by pretreatment with the site-directed acylating agent, (3S,4S)-(+)-trans-N-[1-[2-(4-isothiocyanato)phenyl)-ethyl]-3-methy l-4- piperidyl]-N-phenylpropaneamide hydrochloride, which permits selective labeling of the delta cx binding site with [3H][D-Ala2,D-Leu5]enkephalin. The major findings of this study are that with this preparation of rat brain membranes: a) there are striking differences between the delta cx and mu binding sites; and b) both [D-Pen2,D-Pen5]enkephalin and [D-Pen2,L-Pen5]enkephalin exhibit high affinity for the delta cx binding site.

Opioid-induced respiratory depression and analgesia may be mediated by different subreceptors

Use of selective delta opioid antagonists provide evidence that the delta receptor within the brain seems an integrated part in the mediation of respiratory depression induced by a potent analgesic like fentanyl. Low doses of the delta antagonists RX-8008M (3-6 micrograms/kg) as well as ICI 174,864 (3-6 micrograms/kg) reversed fentanyl-related respiratory depression (arterial blood gases) in the unanesthetized canine. Opioid-induced blockade of afferent sensory nerve volleys (amplitude height of the somatosensory-evoked potential) could be reversed only by a high dose (9 micrograms/kg) of RX-8008M. Depression of amplitude height of the SEP could not be reversed by ICI 174,864 over the whole dose range (3-6-9 micrograms/kg). In comparison, naloxone (1-5-10 micrograms/kg) not only reversed depression of PaO2, it also reversed the blockade of afferent sensory nerve impulses in the low (5-micrograms/kg)-dose range. A highly selective delta antagonist may have a therapeutic value in reversing opioid-related respiratory depression, resulting in little or no attenuation of analgesia.

Development of delta opioid peptides as nonaddicting analgesics

Although much effort has been devoted to opioid research since the identification of enkephalins, understanding of the physiological importance and mechanisms of action of endogenous opioids lags behind understanding of opiate alkaloids such as morphine. In recent years, several novel approaches have been refined with promise for the successful development of the long-awaited nonaddicting analgesics that act at the opioid delta receptor. The present communication reviews these efforts.

Comparison of naloxonazine and beta-funaltrexamine antagonism of mu 1 and mu 2 opioid actions

beta-Funaltrexamine (beta-FNA) irreversibly blocks morphine analgesia, lethality and its inhibition of gastrointestinal transit, confirming that these actions involve mu receptors. In dose-response studies, beta-FNA antagonized all the actions with similar potencies (ID50 values of 12.1, 11.3 and 12.3 mg/kg, respectively). beta-FNA also reduced intra-cerebroventricular and intrathecal DAMGO analgesia equally well (ID50 values of 6.09 and 7.7 mg/kg, respectively). Naloxanazine blocked systemic morphine analgesia (ID50 value 9.5 mg/kg) and supraspinal DAMGO analgesia (ID50 value 6.1 mg/kg) as potently as beta-FNA. However, against spinal DAMGO analgesia, morphine's inhibition of gastro-intestinal transit or lethality, naloxonazine (ID50 values 38.8, 40.7 and 40.9 mg/kg, respectively) was significantly less active than beta-FNA (p less than 0.05). beta-FNA remains a valuable tool in the classification of mu opioid actions. Within the mu category, actions can be defined as either mu 1 (naloxonazine-sensitive) or mu 2 (naloxonazine-insensitive).

Upregulation of the opioid receptor complex by the chronic administration of morphine: a biochemical marker related to the development of tolerance and dependence

Studies conducted after the development of the rapid filtration assay for opiate receptors, and before the recognition of multiple opioid receptors, failed to detect changes in opioid receptors induced by chronic morphine. Recent experiments conducted in our laboratories were designed to examine the hypothesis that only one of several opioid receptor types might be altered by chronic morphine. Using binding surface analysis and irreversible ligands to increase the "resolving power" of the ligand binding assay, the results indicated that chronic morphine increased both the Bmax and Kd of the opioid receptor complex, labeled with either [3H][D-Ala2,D-Leu5]enkephalin, [3H][D-Ala2-MePhe4,Gly-ol5]enkephalin or [3H]6-desoxy-6 beta-fluoronaltreone. In the present study rats were pretreated with drugs known to attenuate the development of tolerance and dependence [the irreversible mu-receptor antagonist, beta-funaltrexamine (beta-FNA), and the inhibitor of tryptophan hydroxylase, para-chlorophenylalanine], prior to subcutaneous implantation of morphine pellets. The results demonstrated that 1) unlike chronic naltrexone, beta-FNA failed to upregulate opioid receptors and 2) both beta-funaltrexamine and PCPA pretreatment attenuated the chronic morphine-induced increase in the Bmax, but not the Kd, of the opioid receptor complex. These results provide evidence that naltrex-one-induced upregulation of the opioid receptor complex might occur indirectly as a consequence of interactions at beta-funaltrexamine-insensitive opioid receptors and that morphine-induced upregulation (increased Bmax) of the opioid receptor complex is a relevant in vitro marker related to the development of tolerance and dependence. These data collectively support the hypothesis that endogenous antiopiate peptides play an important role in the development of tolerance and dependence to morphine.

Novel approaches in the development of new analgesics

A recently developed series of highly selective and systemically active delta-agonists such as Tyr-X-Gly-Phe-Leu-Thr(OtBu), with X = D.Ser (OtBu) in BUBU and X = D.Cys(StBu) in BUBUC, and complete inhibitors of enkephalin metabolism (Kelatorphan, RB 38 A, PC 12) have enabled the major role played by mu-opioid receptors in supraspinal analgesia to be demonstrated. This is in agreement with the results of in vivo mu-receptor occupancy measured by taking into account the cross-reactivity of the delta-ligands for mu-sites. In contrast, mu and delta binding sites seem to act independently to control pain at the spinal level. Strong analgesic effects, especially in arthritic rats, can also be obtained by complete protection of tonically or phasically released endogenous enkephalins with mixed inhibitors such as RB38A. Chronic icv administration of the mu agonist DAGO, led to a severe naloxone precipitated withdrawal syndrome whilst a weak dependence was seen with the delta agonist, DSTBULET or with RB 38 A. Moreover, mixed inhibitors did not induce any significant respiratory depression. All these data emphasize the interest in developing delta-agonists and mixed inhibitors with appropriate bioavailability for clinical evaluation.

Analgesic synergy and improved motor function produced by combinations of mu-delta- and mu-kappa-opioids

This study evaluated the effects of intrathecal administration of a low-analgesic dose of the selective mu-agonist DAMGO co-administered with sequentially increasing doses of either the selective delta-agonist DPDPE or the selective kappa-agonist, U50,488H on mechanical nociceptive thresholds in the rat. Potent analgesic synergy was observed with both combinations. Since an elevation in nociceptive threshold can result from motor deficits, as well as true analgesia, we also evaluated the effects of the combination regimens on motor coordination using a rotarod apparatus. The combination regimens produced significantly less motor deficits than those observed when DPDPE and U50,488H were administered as single agents. These findings of enhanced analgesia with decreased motor side-effects associated with administration of fixed mu/delta or mu/kappa combinations suggest that co-administration of opiates that act at different receptors may constitute a superior approach to the treatment of pain.

Morphine and oxycodone in the management of cancer pain: plasma levels determined by chemical and radioreceptor assays

Morphine and oxycodone were administered to ten patients suffering from severe cancer pain in a double-blind cross-over study. The patients titrated themselves pain-free, first intravenously, using a patient-controlled analgesia device, and then orally. Each titration phase lasted for 48 hours. Blood samples were drawn after 36 hr of each administration phase. The plasma levels of morphine, morphine-6- and morphine-3 glucoronides were determined with high performance liquid chromatography (HPLC), whereas the oxycodone samples were assayed with gas chromatography (GC). Twin samples were analyzed for plasma opioid activity with a radioreceptor assay (RRA) using 3H-dihydromorphine and 3H-naloxone as radioligands. Adequate analgesia was achieved with both morphine and oxycodone. About 30% more oxycodone was needed intravenously, whereas 25% less oxycodone than morphine was consumed orally. There was a good linear correlation between the morphine concentrations measured with HPLC and RRA. The mean morphine-6-glucuronide to morphine concentration ratio was 2.3 after intravenous and 4.6 after oral administration. Results from RRA indicate that oxycodone in vivo is a potent mu-agonist and that at least part of its analgesic action is mediated by active metabolites. In vitro morphine glucuronides enhanced morphine in displacing radioligands from the opioid receptors, thus suggesting their complex interactions in vivo.

Modulation of morphine antinociception by peripheral [Leu5]enkephalin: a synergistic interaction

This study determined whether the modulation of morphine antinociception by [Leu5]enkephalin, a compound which does not produce detectable antinociception when given i.p., was additive or synergistic. Co-administration of graded i.p. doses of morphine and [Leu5]enkephalin 20 min before testing resulted in a progressive leftward and parallel displacement of the i.p. morphine dose-response line. The increase in potency produced by i.p. [Leu5]enkephalin, but not the antinociception of i.p. morphine alone, was blocked by i.c.v. ICI 174,864. The data demonstrate strong and dose-related synergism between the two compounds, supporting the view that (a) peripheral administration of [Leu5]enkephalin can modulate morphine antinociception, apparently at the level of the brain and (b) that the interaction between mu and delta opioid agonists is not the result of simple additive action at the same (i.e., mu) receptor. Further evidence is thus provided for the existence of functional mu-delta interactions.

Kappa- and delta-opioid agonists synergize to produce potent analgesia

This study evaluated the interaction of the analgesic effects of a selective kappa- (U50, 488H) and a selective delta- ([D-Pen2,5]enkephalin, DPDPE) opioid agonist, co-injected intrathecally, using the Randall-Selitto paw-withdrawal test, in the rat. Intrathecal administration of both U50, 488H and DPDPE, as single agents, produced dose-dependent increases in mechanical nociceptive threshold. However, when the dose-response curves for both U50, 488H and DPDPE in the presence of a low-analgesic dose of the other agent were compared with the dose-response curves for the respective agonist administered alone, the curves for the combination regimens were shifted to the left. A statistically significant deviation from parallelism between the dose-response curves of the single versus the combined agents, as well as isobolographic analysis, demonstrates that the simultaneous administration of opioid agonists, the act at kappa- and delta-opioid receptor sites, can produce analgesic synergy.

Comparative pharmacological and biochemical studies between butorphanol and morphine

A number of in vivo and in vitro studies were undertaken to compare the pharmacological and biochemical effects of the partial agonist, butorphanol, with that of morphine. Both compounds were equipotent antinociceptive agents in the rat tail withdrawal test. In the acetic acid writhing test butorphanol had approximately 3.5 times the antiwrithing activity on a molar basis than morphine. In a study of the effects of these compounds on body temperature, butorphanol as well as morphine produced hyperthermia after acute dosing. Additionally, butorphanol produced a profound diuresis and decrease in urine osmolality after acute administration. In contrast, morphine produced an antidiuresis throughout most of the study period with no significant changes in urine osmolality from control. Butorphanol administration had no effect on respiratory rate, while morphine markedly decreased respiratory rate. In in vitro radioligand displacement studies, butorphanol was a potent competitor against 3H-DAGO, 3H-DPDPE, and 3H(-)-EKC binding, exhibiting 3, 10, and 30 times more activity, respectively, than morphine. Both compounds were weak inhibitors of 3H-(+)-SKF 10047 binding, yielding IC50 values of in excess of 1 microM. The results indicate that butorphanol has multiple actions on the opioid receptor system, and shares similarities as well as differences in its mechanism(s) of actions with morphine.

Modulation of mu-mediated antinociception by delta agonists: characterization with antagonists

The functional interactions between supraspinal mu and delta receptors were characterized in the mouse using mu receptor-selective antagonists. The effects of pretreatment with the mu opioid antagonists, beta-funaltrexamine (beta-FNA) and naloxonazine on the modulation of morphine antinociception by the delta agonists [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Ala2,Met5]enkephalinamide (DAMA) were studied. When co-administered in the same i.c.v. injection, a sub-antinociceptive dose of DPDPE consistently and significantly increased the antinociceptive potency of morphine in control animals, while a sub-effective dose of DAMA decreased morphine antinociception; both the respective increase and the decrease of morphine potency by DPDPE and DAMA had been previously shown to be blocked by ICI 174,864, a delta antagonist. Pretreatment of mice with the non-equilibrium mu antagonist beta-FNA 4 h prior to testing, a pretreatment which had no effect on i.c.v. DPDPE or DAMA antinociception, prevented the modulation of morphine antinociception by both DPDPE and DAMA. Pretreatment with the long acting mu 1 antagonist naloxonazine, 24 h prior to testing, failed to affect the modulation of morphine antinociception by either DPDPE or DAMA; such a pretreatment had no effect on the antinociceptive effects of DPDPE or DAMA when given alone. These results provide further support for the concept of a functionally coupled mu-delta receptor complex which is sensitive to antagonism by beta-FNA, but not naloxonazine, and support the notion that subtypes of opioid mu and delta (i.e. complexed and non-complexed) receptors may exist.

Modulation of mu-mediated antinociception by delta agonists in the mouse: selective potentiation of morphine and normorphine by [D-Pen2,D-Pen5]enkephalin

The effect of the delta-selective agonist [D-Pen2,D-Pen5]enkephalin (DPDPE) on the antinociception produced by intracerebroventricular (i.c.v.) administration of the mu agonists morphine, [D-Ala2,NMePhe4,Gly-ol5]enkephalin (DAGO), [NMePhe3,D-Pro4]morphiceptin (PLO17), beta-endorphin, phenazocine, etorphine and sufentanil was studied in mice. Only the antinociceptive effects of morphine and normorphine were modulated by i.c.v. coadministration of a dose of DPDPE which did not produce any significant antinociception alone. Both the morphine and normorphine dose-response lines were displaced to the left in the presence of DPDPE. The delta-selective antagonist ICI174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH) (where Aib is alpha-aminoisobutyric acid) blocked the modulation of morphine antinociception by DPDPE. ICI 174,864 alone failed to produce either a significant increase or decrease of morphine, phenazocine, etorphine or beta-endorphin antinociception. The results of the present study provide support for the hypothesis that the enkephalins may function to modulate antinociception produced at the mu receptor; such modulation may come about via the existence of an opioid mu-delta receptor complex. The mu receptors existing in such a complex may be selectively activated by morphine and normorphine, but not the other mu agonists studied here. Thus, the enkephalins may function both to directly initiate, as well as to modulate, some forms of supraspinal mu receptor-mediated antinociception.

Stress-induced potentiation of morphine-induced analgesia in morphine-tolerant rats

This study was designed to evaluate whether or not rats that were tolerant to the analgesic action of morphine were also tolerant to stress-induced potentiation of morphine-induced analgesia. Rats were trained to drink either solutions of morphine (0.5 mg/ml) or drug-free tap water on a limited access schedule (10 min every 6 hr). The daily intake of morphine averaged 46 mg/kg. Nontolerant and rats tolerant to morphine were tested for morphine-induced analgesia (tail-flick assay), while either unstressed or stressed (i.e. immobilized in Plexiglas cylinders). Morphine produced dose- and time-dependent increases in tail-flick latencies in all groups. Increased sensitivity to analgesia induced by morphine was evident for both nontolerant and tolerant, stressed rats, when compared to their unstressed counterparts. Stress-induced potentiation of morphine-induced analgesia was characterized by dose-related increases in the peak effect and duration of the effect. Stress potentiated the analgesic effect of morphine, comparably in nontolerant (1.7-fold) and tolerant (1.5-fold) rats. Differential tolerance to analgesia induced by morphine and to stress-induced potentiation of morphine-induced analgesia suggests that different mechanisms mediate these two effects.

Chronic administration of morphine and naltrexone up-regulate mu-opioid binding sites labeled by [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin: further evidence for two mu-binding sites

A variety of data support the hypothesis of an opiate receptor complex composed of distinct, yet interacting mu and delta binding sites (termed mu cx and delta cx to indicate binding sites 'in the complex'), in addition to independent mu and delta binding sites, termed mu ncx and delta ncx, to indicate binding sites 'not in the complex'. Ligand binding studies using membranes and slide-mounted sections of rat brain support the hypothesis that the irreversible mu-antagonist beta-funaltrexamine (FNA) selectively alkylates the opiate receptor complex, altering the binding of mu agonists to the mu cx binding site and the binding of [3H][D-Ala2,D-Leu5]enkephalin to the delta cx site. Previous studies demonstrated that the chronic administration of morphine to rats selectively 'upregulates' the opiate receptor complex. In contrast, the chronic administration of naltrexone upregulates several types of opioid receptors, including kappa, the delta ncx binding site, and multiple binding sites labeled by mu agonists. A prediction based upon these observations is that, using [3H][D-Ala2,MePhe4,Gly-ol5]enkephalin to label mu binding sites, chronic morphine should upregulate only the mu cx binding site, whereas chronic naltrexone should additionally up-regulate the mu ncx binding site. In this study we test and confirm this hypothesis, using sensitivity to FNA to define the mu cx binding site. The implications of these data for models of the opioid receptors and the mechanism(s) of tolerance and dependence are discussed.

Potentiation of pentazocine analgesia by low-dose naloxone

The analgesia produced by combinations of low-dose naloxone with pentazocine or morphine was studied in 105 patients with moderately severe postoperative pain after standardized surgery for removal of impacted third molars. Pain intensity was quantified using a visual-analogue scale. To eliminate the release of endogenous opioids produced by the placebo component of open drug administration, all injections were made by a preprogrammed infusion pump. The analgesia produced by pentazocine, an agonist-antagonist opiate-analgesic acting predominantly at the kappa opiate receptor, was potentiated by low-dose naloxone, whereas the analgesia produced by morphine, a mu-agonist, was attenuated by low-dose naloxone. To evaluate whether similar potentiation would be present in an animal model, and specifically, in the absence of diazepam, which patients receive, we performed an analogous experiment in rats in which nociceptive threshold was determined using the Randall-Selitto paw-withdrawal test. The results were completely analogous to the clinical results: pentazocine analgesia was potentiated by low-dose naloxone, whereas morphine analgesia was attenuated by low-dose naloxone. These data demonstrate a novel interaction between opiates, and suggest a rationale for opiate combinations to produce potent analgesia with fewer autonomic side effects and less abuse potential than presently available analgesics.

Decrease in delta-opioid receptor density in rat brain after chronic [D-Ala2,D-Leu5]enkephalin treatment

Chronic treatment of Sprague-Dawley rats with [D-Ala2,D-Leu5]enkephalin (DADLE) resulted in the development of tolerance to the antinociceptive effect of this opioid peptide. When opioid receptor binding was measured, time-dependent decreases in [3H]diprenorphine binding to the P2 membranes prepared from the cortex, midbrain and striatum were observed. Scatchard analysis of the saturation binding data revealed a decrease in Bmax values and no change in the Kd values of [3H]diprenorphine binding to these brain regions, indicative of down-regulation of the receptor. This reduction in the opioid receptor binding activities could be demonstrated to be due to the DADLE effect on the delta-opioid receptors in these brain regions. When [3H]DADLE binding was carried out in the presence of morphiceptin, a significant reduction in the delta-opioid receptor binding was observed in all brain areas tested. mu-Opioid receptor binding decrease was observed only in the striatum after 5 days of DADLE treatment. Additionally, the onset of delta-opioid receptor decrease in the midbrain area was rapid, within 6 h of the initiation of the chronic DADLE treatment. Thus, analogous to previous observations in which chronic etorphine treatment preferentially reduced mu-opioid receptor binding, chronic DADLE treatment preferentially reduced delta-opioid receptor binding activity.

Chronic administration of morphine and naltrexone up-regulate[3H][D-Ala2,D-leu5]enkephalin binding sites by different mechanisms

Previous studies have demonstrated that chronic administration of morphine up-regulated the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin, without producing a detectable alteration in the higher affinity binding site for [3H][D-ala2,D-leu5]enkephalin (Rothman et al., Eur. J. Pharmac. 124: 113-119, 1986). The experiments reported in this paper tested the hypothesis that chronic administration of morphine and naltrexone up-regulated the binding sites for [3H][D-ala2,D-leu5]enkephalin by different mechanisms. Rats were given either morphine or naltrexone chronically. Chronic administration of morphine up-regulated the lower affinity site, while chronic administration of naltrexone up-regulated both the higher and lower affinity binding sites for [3H][D-ala2,D-leu5]enkephalin. Unlike the lower affinity binding site for [3H][D-ala2,D-leu5]enkephalin present in membranes prepared from rats treated with placebo pellets, the lower affinity binding sites which were up-regulated by naltrexone and morphine were partially (naltrexone) or completely (morphine) labile to preincubation for 60 min at 25 degrees C in 50 mM Tris-HCl, pH 7.4, containing 0.4 M NaCl. These data suggest that chronic administration of morphine and naltrexone up-regulate binding sites for [3H][D-ala2,D-leu5]enkephalin through different mechanisms, and that the lower affinity binding sites for [3H][D-ala2, D-leu5]enkephalin which are up-regulated by chronic administration of morphine and naltrexone might differ biochemically from the lower affinity binding sites present in membranes treated with placebo.

Management of pediatric pain with opioid analgesics

We have attempted to dispel many of the myths and misconceptions surrounding the use of narcotic analgesics in the treatment of childhood pain. Our hope is that an improved understanding and the application of effective, safe therapy will minimize the suffering of the child with acute or chronic pain.

Opposite effects of restraint on morphine analgesia and naloxone-induced jumping

It has been demonstrated that the effects of exogenous opiates like morphine could be modified by exposure of an organism to stress, but it is uncertain whether this modification is due to the action of endogenous opioid peptides released by stressful stimuli. The stress of restraint produced an antinociceptive response in mice measured by a latency to escape from a hot plate and, in addition, markedly potentiated analgesia induced by low doses of morphine. Both effects were antagonized by naloxone in a dose-dependent manner. On the other hand, restraint reduced the naloxone-precipitated jumping after single morphine injection. Morphine analgesia and a jumping response were not correlated when tested in two different strains of mice. It is suggested that the enhancement of morphine analgesia by restraint and the reduction in naloxone-induced jumping are mediated via independent mechanisms.

A highly selective ligand for brain delta opiate receptors, a cyclopropyl(E)Phe(4)-enkephalin analog, suppresses mu receptor-mediated thermal analgesia by morphine

[D-Ala(2)(2R,3S)-delta(E)Phe(4)Leu(5)]enkephalin (CP-OH) [delta denoting cyclopropyl; superscript E indicating the E-configuration about the cyclopropane ring], a highly selective opioid ligand for delta receptors in rat brain, but not for those in the mouse vas deferens, was examined for in vivo biological activities by intracerebroventricular administration. CP-OH (5-20 micrograms) showed no analgesic activity in the hot plate (51 degrees C) test using rats. However, it suppressed completely the analgesic effects of intraperitoneally administered morphine (3 mg/kg rat) in a dose-dependent manner. CP-OH showed no binding affinity for brain kappa receptors to which dynorphin, an opioid peptide that inhibits morphine analgesia, binds predominantly. These results suggest that, besides the conventional delta receptors which mediate analgesia, the rat brain contains another delta-like receptor which has a modulatory role to attenuate morphine-induced analgesia mediated through the mu receptors, and that this modulatory receptor does not exist in the mouse vas deferens.

Synergism between the analgesic actions of morphine and pentazocine

In a double-blind, placebo-controlled study the analgesic efficacy of the combination of morphine, a mu-opiate receptor agonist and pentazocine, a kappa-opiate receptor partial-agonist was evaluated. Groups of 20 patients received either vehicle, morphine (2, 4, 8 or 16 mg), pentazocine (15, 30 or 60 mg) or a combination of morphine and pentazocine (2 mg or 4 mg and 15 mg or 30 mg, respectively). The combination of morphine and pentazocine produced a level of analgesia significantly greater than can be accounted for by simple addition of the analgesic effects of each opiate analgesic alone. We propose that the observed synergism between morphine and pentazocine is due to interaction between mu- and kappa-opioidergic components of endogenous opioid-mediated analgesia systems.

Opioid antinociceptive effects of delta-receptor antagonists

The antinociceptive effects of delta opioid receptor antagonists (ICI 154129 and ICI 174864) have been studied using the mouse writhing assay. When administered intracerebroventricularly (ICV), ICI 154129 and ICI 174864 produced dose-related inhibition of writhing with respective ED50's of 97 micrograms/mouse and 1.4 micrograms/mouse. Inhibition of writhing by ICI 174864 (3 micrograms, ICV) was antagonized by subcutaneous (SC) naloxone doses of 0.1 mg/kg and greater. Pretreatment of mice with 80 mg/kg (SC) of beta-funaltrexamine (beta-FNA), an irreversible mu-receptor antagonist, 28 hr before ICV injection of ICI 174864 shifted the dose-effect curve for ICI 174864 to the right (ED50 of 7.3 micrograms/mouse). When administered SC, ICI 174864 inhibited writhing with an ED50 of 8.5 mg/kg. Maximal inhibition occurred 30 min after SC administration and decreased 50% by 2 hr. After beta-FNA pretreatment, doses of ICI 174864 as high as 80 mg/kg (SC) did not inhibit writhing. There was no antinociceptive effect of ICI 174864 in mice chronically maintained on morphine, i.e., chronic morphine produced cross-tolerance to the delta antagonist. These results show that delta-selective receptor antagonists produced antinociception which was related to the mu-receptor, but was probably not a result of direct agonist action.

Morphine-6-glucuronide: analgesic effects and receptor binding profile in rats

The antinociceptive effects of morphine-6-glucuronide (M6G) were examined in two animal models of pain, the tail immersion test (reflex withdrawal to noxious heat) and the formalin test (behavioral response to minor tissue injury). In the tail immersion test, M6G produced an increase in withdrawal latency that rose rapidly between 0.01 and 0.025 ug ICV or 1 and 2 mg/kg SC. A further increase occurred at doses greater than 0.2 ug ICV or 4 mg/kg SC and was associated with marked catalepsy and cyanosis. Naloxone, 0.1 mg/kg SC, shifted the lower component of the dose-effect relation by a factor of 24. In the formalin test, 0.01 ug M6G ICV produced hyperalgesia, while between 0.05 and 0.2 ug ICV, antinociception increased rapidly without toxicity. The dose effect relations for hyperalgesia and antinociception were shifted to the right by factors of 20- and 3-fold, respectively. By comparison, ICV morphine was 60 (formalin test) to 145-200 (tail immersion test) times less potent than M6G. At sub-nanomolar concentrations, M6G enhanced the binding of [3H]-etorphine, [3H]-dihydromorphine and [3H]-naloxone to rat brain membrane receptors by 20-40%. At higher concentrations, M6G displaced each ligand from binding sites, with Ki values of about 30 nM, as compared to morphine Ki values of about 3 nM. The data indicate that the in vivo and in vitro effects of M6G are complex and that M6G may play an important role in analgesia in experimental animals, and by implication, in man.

Morphine-like discriminative stimulus effects of opioid peptides: possible modulatory role of D-Ala2-D-Leu5-enkephalin (DADL) and dynorphin A (1-13)

The role of the different opioid receptors was studied in rats trained to discriminate SC injections of 3.0 mg/kg morphine from saline by tests for generalization to graded doses of morphine and receptor-selective peptides administered into the lateral cerebral ventricle. Dose-dependent morphine-like stimulus effects were produced over a wide range of doses (0.001-30 micrograms), depending upon ligand and animal, by morphine, by the mu-selective peptides DAGO[D-Ala2-NMePhe4-Gly(ol)-enkephalin] and FK33824[D-Ala2,NMePhe4-Met(O)5-(ol)-enkephalin], and by the delta-selective peptide, DADL[D-Ala2,D-Leu5)enkephalin]. The order of relative potency of these substances was: FK33824 greater than DAGO greater than morphine greater than DADL. In contrast, DPLPE[D-Pen2,L-Pen5)enkephalin], which has much greater delta receptor selectivity than does DADL, and dynorphin A(1-13) (0.1-10 micrograms), a kappa-receptor agonist, engendered choice responding appropriate for saline. When 1.0 micrograms DADL, a dose lacking morphine-like discriminative effects, was administered concurrently with SC morphine, the stimulus effects of morphine were potentiated. Concurrent administration of 10 micrograms dynorphin A(1-13) and morphine attenuated the stimulus effects of morphine inconsistently. These results support previous findings that mu-opioid receptors are of primary importance in mediating the morphine-like discriminative effects of opioid peptides. They also suggest that morphine-like discriminative effects can be modulated by other types of opioid receptors.

Chronic morphine upregulates a mu-opiate binding site labeled by [3H]cycloFOXY: a novel opiate antagonist suitable for positron emission tomography

CycloFOXY (17-cyclopropylmethyl-3,14-dihydroxy-4,5-alpha-epoxy-6-beta- fluoromorphinan) is a novel opiate antagonist synthesized as a ligand suitable for in vivo visualization of opiate receptors using positron emission transaxial tomography. In this paper we report that [3H]cycloFOXY labels two distinct opiate binding sites in rat brain membranes, tentatively identified as mu and kappa. Furthermore, chronic administration of morphine results in a selective up-regulation of the mu binding site. The implications of this finding for models of the opioid receptors and the mechanism of the sodium effect are discussed.

Enhanced binding of morphine and nalorphine to opioid delta receptor by glucuronate and sulfate conjugations at the 6-position

Effect of the modification of morphine and nalorphine by glucuronate and sulfate conjugations at the 3- and 6-positions on the binding to opioid receptors was examined in a particulate fraction of rat brain. Competing potencies of both drugs against [3H]morphine and [3H]leucine enkephalin bindings were extremely decreased by either glucuronate or sulfate conjugation at the 3-position. On the other hand, the potencies of morphine and nalorphine against [3H]leucine enkephalin binding were considerably enhanced by the conjugations at the 6-position, whereas the potencies against [3H]morphine binding were decreased. These altered interactions of the conjugates at the 6-position with the two ligands were attributed to their enhanced binding to delta-receptor and reduced binding to mu-receptor by Hill plot and modified Scatchard analysis. Resulted comparable and simultaneous interactions with mu- and delta- receptors were assumed to be a cause of the enhanced mu-receptor-directed analgesia of morphine and elevated same receptor-directed antagonistic effect of nalorphine, which have been found previously in our laboratory.

Opioid delta-receptor involvement in supraspinal and spinal antinociception in mice

The possibility that the opioid delta-receptor mediates antinociception in tests where heat is the noxious stimulus was investigated using highly selective mu- and delta-agonist and -antagonists. Antinociceptive dose-response curves were constructed for mu ([D-Ala2,NMePhe4,Gly-ol]enkephalin, DAGO; morphine) and delta ([D-Pen2,D-Pen5]enkephalin, DPDPE)-agonists in the absence, and in the presence of the mu non-surmountable antagonist, beta-funaltrexamine (beta-FNA) or the delta-antagonist ICI 174,864 (N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH, where Aib is alpha-amino-isobutyric acid). Agonists and ICI 174,864 were given alone in the same intracerebroventricular (i.c.v.) or intrathecal (i.th.) injection to mice 20 min prior to testing in the warm-water (55 degrees C) tail-withdrawal test (+10 min for i.th. DPDPE); beta-FNA was given as a single i.c.v. or i.th. pretreatment dose (20 and 0.01 nM, respectively) 4 h prior to testing. I.c.v. pretreatment with beta-FNA resulted in a rightward displacement of the DAGO and morphine antinociceptive dose-response lines, but failed to displace the i.c.v. DPDPE curve. Similarly, i.th. pretreatment with beta-FNA displaced the i.th. morphine dose-response curve to the right without affecting the i.th. DPDPE antinociceptive dose-response line. ICI 174,864 (1 and 3 micrograms) produced a dose-related antagonism of i.c.v. or i.th. DPDPE, but did not alter the antinociceptive effects of DAGO or morphine given by the same routes. Co-administration of ICI 174,864 (3 micrograms) with i.c.v. morphine in beta-FNA pretreated (but not control) mice resulted in a further rightward displacement of the morphine dose-response line.(ABSTRACT TRUNCATED AT 250 WORDS)

Distribution of opiate receptor subtypes and enkephalin and dynorphin immunoreactivity in the hippocampus of squirrel, guinea pig, rat, and hamster

The distribution of enkephalin and dynorphin immunoreactivity in the hippocampus of four rodent species (gray squirrel, guinea pig, rat, and hamster) is compared with the pattern of opiate receptor subtypes (mu, delta, and kappa). The distribution of opioid peptides is fairly consistent in the anterior hippocampus of these four species. Intense immunoreactivity for dynorphin and enkephalin is found in the hilus of the dentate gyrus and in the mossy fiber system. Occasional immunoreactive processes are seen in the dentate molecular layer and scattered throughout the CA1 and CA3 fields. In the rat and hamster, an additional plexus of enkephalinergic fibers straddles both sides of the hippocampal fissure. Cells immunoreactive for both opioid peptides are located in and just superficial to the dentate granule cell layer. Opiate receptors are variably distributed in these rodent species. In the squirrel, guinea pig, and hamster, mu and kappa binding is dense in the stratum lucidum of CA3 and the molecular layer of the dentate gyrus. In the rat, dense mu and kappa binding is localized within and adjacent to the pyramidal and granule cell layers. Delta receptor patterns show additional species differences. In the rat, the delta distribution is similar to the mu and kappa patterns. In the other species, the delta binding pattern is generally the inverse of the mu/kappa pattern: most areas of the hippocampus are enriched in delta sites, whereas the stratum lucidum and the pyramidal cell layer are receptor-sparse. Thus, the stratum lucidum--site of dense terminations of mossy fibers containing opioid peptides--is characterized by selectively sparse delta receptors in four species and by selectively dense kappa receptors in three species. The three receptor subtypes, taken either individually or together and compared to the peptides, are more variably and more widely distributed throughout the hippocampus and fail to show a correspondence with opioid-peptide-containing terminals. The mismatches suggest that receptor locations and densities are organized without relation to the sites of relevant transmitter release.

Evidence that endogenous enkephalins produce delta-opiate receptor mediated neuronal inhibitions in rat dorsal horn

Kelatorphan, a full inhibitor of aminopeptidases, enkephalinase and dipeptidylaminopeptidase, enzymes which degrade the enkephalins, produced inhibitions (around 50%) of dorsal horn C fibre evoked responses in the rat, following local application. The inhibitions were reversed by the selective delta-opiate receptor antagonists ICI 174,864. Furthermore the inhibitions produced by two doses of Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO), a potent selective mu-opiate receptor agonist, were not altered in the presence of kelatorphan, so demonstrating an additive effect of the mu-agonist and the enzyme inhibitor. The inhibitions produced by the enzyme inhibitor would seem due to an increased availability of endogenous opioids, presumably the enkephalins which act via the delta-opiate receptor.

A comparative autoradiographic study of the distributions of substance P and eledoisin binding sites in rat brain

The relative potencies of tachykinin peptide analogs competing for binding of [125I]Bolton Hunter-conjugated substance P ([125I]BH-SP) or [125I]Bolton Hunter-conjugated eledoisin ([125I]BH-ED) in slide-mounted rat brain sections are very different, indicating the presence of two distinct tachykinin binding sites. The structure-activity profiles resemble those described in peripheral bioassay studies in which two tachykinin receptors have been postulated. Autoradiography of the two iodinated ligands bound with selective and one-site in vitro incubation conditions shows two discrete and distinctly different distribution patterns in brain. Binding sites for [125I]BH-ED are densely distributed in the accessory olfactory bulb, intermediate layers of the cerebral neocortex, portions of the hippocampal CA fields, hypothalamic supraoptic and paraventricular nuclei, central portions of the interpeduncular nucleus, sphenoid nucleus, medial subdivision of the solitary tract complex, and the substantia gelatinosa of the spinal cord. Binding sites for [125I]BH-SP are present in many of these same structures, but the densities and distribution patterns are different. In addition, [125I]BH-SP binds in numerous structures not labeled by [125I]BH-ED. Neither pattern matches the locations of terminations of endogenous tachykinin pathways marked by immunohistochemistry. The results suggest that it would be inappropriate to name brain tachykinin receptors according to the endogenous ligand which binds with highest affinity.

Opiate-induced seizures: a study of mu and delta specific mechanisms

Two groups of experiments were conducted to determine if morphine- and enkephalin-induced seizures are specifically mediated by the mu and delta receptor, respectively. In the first experiments, designed to assess the ontogeny of mu- or delta-seizures, rats from 6 h to 85 days of age received implanted cortical and depth electrodes as well as an indwelling cannula in the lateral ventricle. Various amounts of the mu-receptor agonists, morphine and morphiceptin, and the delta agonists, D-Ala2-D-Leu5-enkephalin (DADL) and Tyr-D-Ser-Gly-Phe-Leu-Thr (DSLET), were then administered intracerebroventricularly (icv) with continuous EEG monitoring. The second experiments entailed use of the nonspecific opiate antagonist, naloxone, as well as the specific delta antagonist, ICI 154,129, against seizures induced by icv-administered morphine, morphiceptin, DADL, or DSLET. Both morphine and morphiceptin produced electrical seizure activity in rats as young as 5 days after birth. The drugs produced similar seizure activity in terms of electrical morphology, observed behavior, ontogeny, threshold dose, and reversibility with small doses of naloxone. In the pharmacologic experiments, icv naloxone blocked all opiate-induced seizures. ICI 154,129 blocked DSLET seizure, had little effect on enkephalin or DADL seizures, and no effect on morphine or morphiceptin seizures. These data indicate that DSLET seizures are delta-specific but that all other opiate-induced seizures studied may involve multiple opiate receptor-mediated mechanisms.

Comparison of two penicillamine-containing enkephalins: mu, not delta, activity produces analgesia

Two penicillamine-containing enkephalin analogs were compared for inhibition of mouse writhing after intracerebroventricular administration. The delta receptor-selective analog was less potent in inhibiting writhing than the non-selective analog. Inhibition of writhing produced by the delta-selective analog was antagonized by very low doses of naloxone, and produced additive analgesic effects with the delta receptor antagonist, ICI 174864. These results suggest that inhibition of writhing was produced by mu receptor agonist activity, and not delta receptor agonist activity.

An assessment of the role of opioid receptors in the response to cannabimimetic drugs

Cannabimimetic drugs have been shown to inhibit adenylate cyclase activity in N18TG2 neuroblastoma cells. This investigation examines the possible role of opioid receptors in the cannabimimetic response. Opioid receptors of the delta subtype were found on N18TG2 membranes using [3H]D-Ala2-D-Leu5-enkephalin. No mu or kappa receptors were detected using selective ligands for these sites. The delta binding affinity and capacity were unaltered by cannabimimetic drugs. To test if cannabimimetic drugs may modulate opioid effector mechanisms, cyclic AMP metabolism was determined in intact cells and in membranes. N18TG2 adenylate cyclase was inhibited by the cannabimimetic drugs delta 9-tetrahydrocannabinol and desacetyllevonantradol, and by the opioid agents morphine, etorphine, and D-Ala2-Met5-enkephalinamide. The opioid inhibition was reversed by naloxone and naltrexone; however, the cannabimimetic response was unaffected. Both cannabimimetic and opioid drugs decreased cyclic AMP accumulation in intact cells, but opioid antagonists blocked the response only to the latter. Thus, cannabimimetic effects are observed even though opioid receptors are blocked by antagonist drugs. The interaction between desacetyllevonantradol and etorphine was neither synergistic nor additive at maximal concentrations, suggesting that these two drugs operate via the same effector mechanism. Other neuronal cell lines having an opioid response were also examined. The cannabimimetic inhibition of cyclic AMP accumulation in NG108-15 neuroblastoma X glioma cells was not as great as the response in N18TG2. N4TG1 neuroblastoma cells did not respond to cannabimimetic drugs under any conditions tested. Thus, the cannabimimetic inhibition of adenylate cyclase is not universally observed, and the efficacy of the cannabimimetic response does not correlate with the efficacy of the opioid response.

Morphine tolerance increases mu-noncompetitive delta binding sites

In light of more recent knowledge concerning endogenous opioid peptides and their multiple opiate receptors, we reevaluated the effects of morphine tolerance on opiate receptor binding parameters. Rats were implanted with morphine or placebo pellets, and [3H][D-Ala2,D-Leu5]enkephalin ([3H]DADL) was used to label brain membranes. Utilizing the technique of binding surface analysis, we observed a selective 47% up-regulation of lower affinity [3H]DADL binding sites (mu-noncompetitive delta binding sites) in morphine pelleted rats. To corroborate these results, we treated brain membranes with the site directed alkylating agent FIT (N-phenyl-N-[1-(2-p-isothiocyanato)phenyl-ethyl)-4-piperidinyl] propanamide), which results in membranes highly enriched with the lower affinity [3H]DADL binding site. Scatchard plots of [3H]DADL binding to FIT-treated membranes also revealed that chronic morphine treatment produced a 60-65% up-regulation of the mu-noncompetitive delta binding site. These data indicate that chronic morphine alters a selective subpopulation of opiate receptors that may play a role in the mechanisms of opiate tolerance and physical dependence.

Dermorphin inhibits spinal nociceptive flexion reflex in humans

Dermorphin (D) is a potent opiate-like peptide isolated from the skin of some species of frogs. Experimental studies in animals indicate that D has a potent antinociceptive effect, while no investigation exists about its analgesic properties in humans. Our study shows that i.v. infusion of 0.16 mg/kg D induces a marked and long-lasting increase in the threshold of nociceptive flexion reflex in healthy volunteers. This effect is also evident in a complete chronic spinal subject, showing that D depresses the nociceptive transmission mainly acting at spinal level. Naloxone, while fully antagonizing the effects of morphine and enkephalin analogue, is able to reverse only partly (ca. 50%) the depressive effect of D on nociceptive spinal reflex. This fact may suggest that D interacts with different spinal opiate receptor populations in inducing analgesia.

Synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes

We report the synthesis and binding properties of specific photoaffinity ligands for mu and delta opioid receptor subtypes. These ligands are derived from DAGO: Tyr-D-Ala-Gly-NMePhe-Gly-ol, a mu selective probe and DTLET: Tyr-D-Thr-Gly-Phe-Leu-Thr, a delta selective probe by modifying the Phe 4 residue. These modifications are: i) a nitro group on the para position of Phe ring as Phe(4 NO2) or Nip, ii) an azido group as Phe(4 N3) or AZ. Pharmacological responses on mouse vas deferens (delta sites) and guinea pig ileum (mu sites), as well as competition experiments with [3H] DAGO and [3H] DTLET on crude rat brain membranes have been performed. The nitro group on the phenyl ring of the Phe residue preserves the affinity and selectivity of each probe: NipDAGO for the mu sites, NipDTLET for the delta ones. However the nitro probes do not appear to be photoactivable by u.v. irradiation. Likewise, azidation of the phenyl ring of the Phe residue does not change the receptor selectivity of each probe, but AZDAGO has less affinity than its parent molecule DAGO, while AZDTLET has more affinity than DTLET. These compounds are photoactivable and provide an efficient tool to characterize and isolate the different receptor subtypes, especially the delta site.