Selective antagonists at the opiate delta-receptor

WIN 44,441: A Stereospecific and Long-Acting Narcotic Antagonist

The opiate antagonist WIN 44,441-3 is a potent, stereospecific antagonist of mu, delta, and kappa opiate receptors. This antagonist activity is of long duration (> 4 h) with no agonist activity being observed. It therefore appears that WIN 44,441-3 will be a useful long-acting opiate antagonist for in vivo studies.

Post-treatment with the novel deltorphin E, a delta2-opioid receptor agonist, increases recovery and survival after severe hemorrhagic shock in behaving rats

Deltorphin E was investigated as a pharmaceutical intervention in the ischemic hemorrhagic model. To monitor the hemodynamic biomarkers mean arterial pressure (MAP) and heart rate (HR) and to facilitate i.v. injections, rats were surgically fitted with femoral artery and vein catheters under anesthesia. After removal of 48% of total blood volume (range, 12-15 mL), posthemorrhage i.v. injections of 5.5-mg/kg deltorphin E were found to significantly (P < 0.05) increase maximum MAP, pulse pressure, and survival after hemorrhage, whereas lactic acid concentration was decreased when compared with saline injections. The results for the 5.5-mg/kg deltorphin E-treated animals versus saline controls showed the following values (expressed as mean +/- SEM): maximum MAP, 58 +/- 7 vs. 35 +/- 9 mmHg, respectively; lactic acid, 6.5 +/- 1.25 vs. 8.9 +/- 0.12 mmol/L, respectively; pulse pressure, 47.9 +/- 0.55 vs. 38.3 +/- 0.44 mmHg, respectively; and at least a fourfold increase in survival, 331 +/- 18 vs. 50 +/- 8 min, respectively. Heart rate in deltorphin E-treated groups was not significantly different from that in saline-treated groups (maximum HR, 396 +/- 40 vs. 425 +/- 94 bpm, respectively). Using logistic analysis, deltorphin E did not significantly alter the baroreflex sensitivity. However, a significant deltorphin E dose-dependent correlation was found between survival time and lactic acid production. Increased pulse pressure was also correlated with survival. Glibenclamide, a potassium-sensitive adenosine triphosphate-sensitive channel blocker, did not interfere with the positive effects of deltorphin E. Only the antagonists tested, known to affect delta(2)-opioid receptors, interfered with the deltorphin E survival benefit after hemorrhage. As a conclusion, deltorphin E is an effective pharmaceutical intervention in severe hemorrhagic shock and, perhaps, in other ischemic shock scenarios when administered after the onset of stress. Therefore, deltorphin E may have clinical potential.

Role of delta-opioid receptor subtypes in anxiety-related behaviors in the elevated plus-maze in rats

RATIONALE

Recent studies have shown that endogenous opioid systems are associated with the regulation of emotional responses. In particular, it has been reported that delta-opioid receptors act naturally to inhibit stress and anxiety.

OBJECTIVE

The present study was designed to examine the possible involvement of opioid delta-receptor subtypes in the anxiety-related behavior in the elevated-plus-maze test.

METHODS

Six-week-old male Lewis rats were used. The total numbers of visits to the closed and open arms and the cumulative time spent and visits in the open arms were determined. Plasma corticosterone levels were measured by enzyme immunoassay.

RESULTS

Naltrindole (NTI), a delta-opioid receptor antagonist (3 mg/kg s.c.), induced a significant decrease in the percentages of time spent and visits in the open arms. Naltriben (NTB), a delta2-opioid receptor antagonist (3 mg/kg s.c.), but not 7-benzylidenenaltrexone, a delta1-opioid receptor antagonist, produced similar anxiety-related behaviors in the elevated plus-maze. These effects of NTI and NTB were antagonized by pretreatment with (+)-4-[(aR)-a-((2S,5R)-4-allyl-2,5-dimethyl-1piperazinyl)-3-methoxybenzyl]-N,N-diethylbenzamide (SNC80), a delta-opioid receptor agonist. Furthermore, after exposure to the elevated plus-maze, the maximal increase in the plasma corticosterone level in NTI-treated rats was clearly higher than that in vehicle-treated rats. However, when NTI and SNC80 were coadministered, higher levels of plasma corticosterone were not seen after exposure to the elevated plus-maze.

CONCLUSION

These results suggest that endogenous delta2-opioid-receptor-mediated systems are involved in the regulation of anxiety-related behaviors and might play a physiologically important role in the regulation of adrenocortical activity.

(2S,3R) beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] is a potent, selective delta-opioid receptor antagonist in mouse brain

The constrained opioid peptide (2S,3R)beta-methyl-2',6'-dimethyltyrosine-L-tetrahydroisoquinoline-3-carboxylic acid [(2S,3R)TMT-L-Tic-OH] exhibits high affinity and selectivity for the delta-opioid receptors (). In the present study, we examined the pharmacological properties of (2S,3R)TMT-L-Tic-OH in mouse brain. A 5'-O-(3-[(35)S]thiotriphosphate) ([(35)S]GTP gamma S) binding assay was used to determine the effect of (2S,3R)TMT-L-Tic-OH on G protein activity in vitro, in mouse brain membranes. delta- (SNC80; (+)-4-[(alpha R)-alpha-((2S,5R)-4-allyl-2,5-dimethyl-1-piperazinyl)-3-methoxy-benzyl]-N,N-diethyl-benzamide) or mu- (DAMGO; [D-Ala(2), Me-Phe(4),Gly(ol)(5)]enkephalin) selective opioid full agonists stimulated [(35)S]GTP gamma S binding in mouse brain membranes 150 +/- 4.5% and 152 +/- 5.7% over the basal level, respectively. (2S,3R)TMT-L-Tic-OH did not influence basal [(35)S]GTP gamma S binding in mouse brain membranes but dose dependently shifted the dose-response curve of SNC80 to the right, with a K(e) value of 3.6 +/- 0.7 nM. In contrast, (2S,3R)TMT-L-Tic-OH had no effect on the dose-response curve of the mu-selective opioid agonist, DAMGO. Warm water (55 degrees C) tail-flick and radiant heat paw-withdrawal tests were used to determine the in vivo nociceptive properties of (2S,3R)TMT-L-Tic-OH in the mouse. Intracerebroventricular injection of (2S,3R)TMT-L-Tic-OH had no significant effect on withdrawal latencies in either nociceptive tests. (2S,3R)TMT-L-Tic-OH (30 nmol/mouse) attenuated deltorphin II- but not DAMGO-mediated antinociception (40 +/- 13 and 100% of maximal possible effect, respectively) when administered intracerebroventricularly 10 min before the agonist. Taken together these results suggest that (2S,3R)TMT-L-Tic-OH is a potent highly selective neutral delta-opioid antagonist in mouse brain.

Chronic agonist treatment converts antagonists into inverse agonists at delta-opioid receptors

In cellular models, chronic exposure to mu-opioid agonists converts antagonists into inverse agonists at mu-receptors. Such adaptations could contribute to the development of tolerance and/or dependence. To determine whether delta-receptors respond similarly, or whether this adaptation is unique for mu-receptors, this study examined the effects of prolonged agonist exposure on the intrinsic activity of several delta-opioid ligands in GH(3) cells expressing delta-receptors. In opioid naive cells, delta-receptors were constitutively active, and a series of delta-ligands displayed a range of intrinsic activities for G protein activation. Chronic treatment with the full delta-agonist [D-Pen(2,5)]-enkephalin reduced the acute ability of [D-Pen(2,5)]-enkephalin to stimulate and the full inverse agonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI-174864) to inhibit G protein activation. In contrast, although naloxone and naltriben exhibited weak partial agonism in opioid naive cells, both ligands acted as full inverse agonists to produce concentration-dependent inhibition of guanosine 5'-O-(3-[(35)S]thio)triphosphate binding after prolonged exposure to [D-Pen(2,5)]-enkephalin or to the partial agonist morphine. This effect was reversed by a neutral delta-antagonist (N,N-bisallyl)-Tyr-Gly-Gly-psi-(CH(2)S)-Phe-Leu-OH (ICI-154129). Finally, as is also characteristic of inverse agonists, naloxone and naltriben demonstrated higher affinities for uncoupled delta-receptors in cells chronically treated with [D-Pen(2,5)]-enkephalin, relative to opioid naive cells. Therefore, this relatively novel adaptation is shared by both mu- and delta-opioid receptors and therefore may serve as an important common mechanism involved the development of tolerance and/or dependence.

Delta- and kappa-opioid receptors in the caudal midline medulla mediate haemorrhage-evoked hypotension

In mammals blood loss can trigger, shock, an abrupt, life-threatening hypotension and bradycardia. In the halothane-anaesthetised rat this response is blocked by inactivation of a discrete, vasodepressor area in the caudal midline medulla (CMM). Haemorrhagic shock is blocked also by systemic or ventricular injections of the opioid antagonist, naloxone. This study investigated, in the halothane anaesthetised rat, the contribution of delta-, kappa- and mu-opioid receptors in the CMM vasodepressor region to haemorrhage-evoked shock (i.e. hypotension and bradycardia) and its recovery. It was found that microinjections into the CMM of the delta-opioid receptor antagonist, naltrindole delayed and attenuated the hypotension and bradycardia evoked by haemorrhage, but did not promote recompensation. In contrast, CMM microinjections of the kappa-opioid receptor antagonist, nor-binaltorphamine, although it did not alter haemorrhage-evoked hypotension and bradycardia, did lead to a rapid restoration of AP, but not HR. CMM microinjections of the mu-opioid receptor antagonist, CTAP had no effect on haemorrhage-evoked shock or recompensation. These data indicate that delta- and kappa- (but not mu-) opioid receptor-mediated events within the CMM contribute to the hypotension and bradycardia evoked by haemorrhage and the effectiveness of naloxone in reversing shock.

ICI 174,864, a selective delta opioid antagonist, reverses the learning impairment produced by [leu]enkephalin

The role of opioid delta receptors in the learning impairment produced by [leu]enkephalin (LE) in one-way active avoidance conditioning was investigated in mice. LE (30 and 100 micrograms/kg) impaired acquisition of the avoidance response, whereas ICI 174,864 (3.0 mg/kg), a selective delta opioid receptor antagonist, enhanced acquisition. The impairment produced by 100 micrograms/kg LE was completely reversed by 1.0 mg/kg ICI 174,864, a dose of the antagonist that by itself had no effect. Control studies provided evidence that the effects of ICI 174,864 and LE on conditioning cannot be explained by performance variables such as alterations in activity levels or footshock sensitivity. The results suggest that opioid delta receptors play an important role in the modulation of learning, and that the effects of LE on avoidance conditioning are mediated by delta receptors.

The role of delta-opioid receptor subtypes in neuropathic pain

A large body of evidence suggests an important role of delta-opioid receptor agonists in antinociception at the level of the spinal cord. Our study was undertaken to analyse the spinal antinociceptive and antiallodynic effects of delta(1)- and delta(2)-opioid receptor agonists and antagonist after their acute and chronic intrathecal administration in a neuropathic pain model in the rat. In rats with a crushed sciatic nerve, the delta(1)-opioid receptor agonist [D-Pen(2), D-Pen(5)]enkephalin (DPDPE, 5-25 microg i.t.) and the delta(2)-opioid receptor agonist deltorphin II (1.5-25 microg i.t.) dose dependently antagonized the cold-water allodynia which developed after sciatic nerve injury. These effects of DPDPE were antagonized by 7-benzylidenenaltrexon (BNTX, 1 microg i.t.) while the effects of deltorphin II were antagonized by 5'naltrindole izotiocyanate (5'NTII, 25 microg i.t.). Both agonists had a dose-dependent, statistically significant effect on the tail-flick latency in two tests, with focused light and cold water. Chronic administration of DPDPE (25 microg i.t.) and deltorphin II (15 microg i.t.) resulted in significant prolongation of the reaction time determined on days 2, 4 and 6 post-injury. In conclusion, our results show an antiallodynic and antinociceptive action of DPDPE and deltorphin II at the spinal cord level, which suggests that both delta-opioid receptor subtypes play a similar role in neuropathic pain. This indicates that not only delta(1)- but also delta(2)-opioid receptor agonists can be regarded as potential drugs for the therapy of neuropathic pain.

Structure-activity relationships of nociceptin and related peptides: comparison with dynorphin A

Nociceptin and its receptor (OP(4)) share sequence homologies with the opioid peptide ligand dynorphin A and its receptor OP(2). Cationic residues in the C-terminal sequence of both peptides seem to be required for selective receptor occupation, but the number and the distribution of these basic residues are different and quite critical. Both receptors are presumably activated by the peptides N-terminal sequence (Xaa-Gly Gly-Phe, where Xaa = Phe or Tyr); however, although OP(4) requires Phe(4) as a determinant pharmacophore, OP(2) requires Tyr(1) as do the other opioid receptors. An extensive structure-activity analysis of the N-terminal tetrapeptide has led to conclude that the presence of aromatic residues in position one and four, preferably Phe, as well as the distance between Phe(1) and Phe(4) are extremely critical for occupation and activation of OP(4) in contrast with other opioid receptors (e.g. OP(1), OP(3), OP(2)). Modification of distance between the side chains of Phe(1) and Phe(4) (as obtained with Nphe(1) substitution in both NC and NC(1-13)-NH(2)) and/or conformational orientation of Phe(1) (as in Phe(1)psi(CH(2)-NH)-Gly(2)) has brought to discovery of pure antagonist ([Nphe(1)]-NC(1-13)-NH(2)) and a partial agonist ([Phe(1) psi(CH(2)-NH)-Gly(2)]-NC(1-13)-NH(2)), which have allowed us to characterize and classify the OP(4) receptor in several species. Thus, although antagonist activities at the OP(4) receptor are obtained by chemical modification of Phe(1)-Gly(2) peptide bond or by a shift of Phe(1) side chain of NC peptides, antagonism at the OP(2) receptor requires the diallylation of the N-terminal amino function, for instance, of dynorphin A. These considerations support the interpretation that the two systems nociceptin/OP(4) and dynorphin A/OP(2) are distinct pharmacological entities that differs in both their active sites (Tyr(1) for Dyn A and Phe(4) for NC) and the number and position of cationic residues in the C-terminal portions of the molecules. The chemical features of novel OP(4) receptor ligands either pseudopeptides obtained by combinatorial library screening or molecules of nonpeptide structure are reported and discussed in comparison with NC and NC related peptides.

Structure-activity studies on nociceptin/orphanin FQ: from full agonist, to partial agonist, to pure antagonist

A heptadecapeptide (Phe-Gly-Gly-Phe-Thr-Gly-Ala-Arg-Lys-Ser-Ala-Arg-Lys-Leu-Ala-Asn-Gln) was identified from rat brain and from porcine brain as a ligand for OP4, a new G-protein coupled receptor that is similar in sequence to opioid receptors. The OP4 receptor is widely expressed in the nervous system where it mediates a broad range of physiological functions. The new peptide, nociceptin (NC), has a primary sequence recalling that of opioid peptides. Despite the homologies (a) of the OP4 receptor with known opioid receptors, especially the OP2 (kappa) receptor, and (b) of NC with opioid peptides, particularly dynorphin A, the two biological systems have different anatomical locations and chemical requirements for activation. NC does not bind to opioid receptors, and mammalian opioid peptides do not interact with the OP4 receptor. The presence of Phe in position 1 and Arg in position 8, appear to be instrumental to exclude NC from interacting with the opioid receptors. Contrary to opioid peptides which strikly require Tyr in position 1, the active core that activates the OP4 appears to be towards the centre of the peptide molecule and includes Phe4. Based on the message/address model, several changes have been made in the N-terminal tetrapeptide Phe-Gly-Gly-Phe (message) and a few also in the C-terminal of the template NC(1-13)-NH2, a fragment that acts as a full agonist both in vitro and in vivo. Subtle changes of the N-terminal sequence, especially at Phe1, led to the discovery of peptide antagonists ([Phe1 psi (CH2-NH)Gly2[-NC(1-13)-NH2 and [Nphe1[-NC(1-13)-NH2). The first compound has been widely used to characterize NC actions in the periphery and in the central nervous system. It has been shown to act mainly as an antagonist outside the brain and as an agonist in the central nervous system. [Nphe1[-NC(1-13)-NH2- on the contrary, acts as antagonist both in the periphery and in the brain. These first peptide prototypes may soon be followed by non-peptide compounds, some of which, are already described in patient literature.

Constitutive activity of the delta-opioid receptor expressed in C6 glioma cells: identification of non-peptide delta-inverse agonists

1. G-protein coupled receptors can exhibit constitutive activity resulting in the formation of active ternary complexes in the absence of an agonist. In this study we have investigated constitutive activity in C6 glioma cells expressing either the cloned delta-(OP1) receptor (C6delta), or the cloned mu-(OP3) opioid receptor (C6mu). 2. Constitutive activity was measured in the absence of Na+ ions to provide an increased signal. The degree of constitutive activity was defined as the level of [35S]-GTPgammaS binding that could be inhibited by pre-treatment with pertussis toxin (PTX). In C6delta cells the level of basal [35S]-GTPgammaS binding was reduced by 51.9+/-6.1 fmols mg-1 protein, whereas in C6mu; and C6 wild-type cells treatment with PTX reduced basal [35S]-GTPgammaS binding by only 10.0+/-3.5 and 8.6+/-3.1 fmols mg-1 protein respectively. 3. The delta-antagonists N, N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH (ICI 174,864), 7-benzylidenenaltrexone (BNTX) and naltriben (NTB), in addition to clocinnamox (C-CAM), acted as delta-opioid receptor inverse agonists. Naloxone, buprenorphine, and naltrindole were neutral antagonists. Furthermore, naltrindole blocked the reduction in [35S]-GTPgammaS binding caused by the inverse agonists. The inverse agonists did not inhibit basal [35S]-GTPgammaS binding in C6mu; or C6 wild-type cell membranes. 4. Competition binding assays in C6delta cell membranes revealed a leftward shift in the displacement curve of [3H]-naltrindole by ICI 174,864 and C-CAM in the presence of NaCl and the GTP analogue, GppNHp. There was no change in the displacement curve for BNTX or NTB under these conditions. 5. These data confirm the presence of constitutive activity associated with the delta-opioid receptor and identify three novel, non-peptide, delta-opioid inverse agonists.

Subtleties of structure-agonist versus antagonist relationships of opioid peptides and peptidomimetics

The development of novel delta opioid antagonists and delta opioid agonists structurally derived from the prototype delta antagonist TIPP (H-Tyr-Tic-Phe-Phe-OH), is reviewed. Both delta antagonists and delta agonists with extraordinary potency and unprecedented delta receptor selectivity were discovered. Some of them are already widely used as pharmacological tools and are also of interest as potential therapeutic agents for use in analgesia. The results of the performed structure-activity studies revealed that the delta antagonist versus delta agonist behavior of this class of compounds depended on very subtle structural differences in diverse locations of the molecule. These observations can be best explained with a receptor model involving a number of different inactive and active receptor conformations.

Antinociceptive effects of isoleucine derivatives of deltorphin I and deltorphin II in rat spinal cord: a search for selectivity of delta receptor subtypes

Deltorphins show a high affinity and selectivity for delta opioid receptors. Analogs of deltorphins with substitution of Val residues with more hydrophobic Ile appear to have a higher in vitro activity and selectivity than parent deltorphins. In our study, changes in the nociceptive threshold after intrathecally injected deltorphin I (DELT I), deltorphin II (DELT II) and their Ile - derivatives (ILE-DELT I and ILE-DELT II, respectively) were investigated in a tail-flick (TF) and a paw pressure (PP) tests. Male Wistars rats (260-350 g) with a chronically implanted catheter in the lumbar enlargement of the spinal cord were used. DELT I and DELT II, injected i.th. in doses of 0.15, 1.5 and 15 microg, increased the TF latency in a dose-dependent manner. The effect of their derivatives was similar, but the action of ILE-DELT II was shorter than that of the parent peptide. In the PP test, the antinociceptive effects of DELT I and their derivative ILE-DELT I were similar, but the effect of a higher dose of ILE-DELT I lasted longer in comparison with the parent peptide. Both DELT II and ILE-DELT II exhibited a low and short-lasting antinociceptive potency in the PP test. The effect of DELT I (1.5 microg) was antagonized by pretreatment with NTI (30 microg), a non-selective delta opioid receptor antagonist, as well as by the delta2 receptor antagonist NTB (3 microg) and the delta1 antagonist BNTX (1 microg) in both those tests used. The antinociceptive effect of DELT II (1.5 microg) was antagonized by pretreatment with NTI (30 microg) and NTB (3 microg) in the TF test, but not in the PP test. In the latter test, the antinociceptive effect of DELT II was potentiated by pretreatment with BNTX (1 microg). The effects of both the derivatives ILE-DELT I and ILE-DELT II were antagonized by NTI (30 microg) in the TF test, and by NTI (30 microg) and NTB (3 microg) in the PP test. Like in the case of the parent peptide, the effect of ILE-DELT II was potentiated by pretreatment with the delta1 antagonist BNTX (1 microg). Summing up, modification of the DELT I and II by substituting Ile for Val residues appears to influence the delta selectivity rather then the potency of the peptides at spinal delta receptors.

Design and solution structure of a partially rigid opioid antagonist lacking the basic center--models of antagonism

To discriminate between two general models of antagonism (participation and allosteric), an opioid antagonist lacking the basic nitrogen of tyramine was designed and characterized. Cyclo-[Tyr(Me)2-Tic-], the diketopiperazine of 2,6-dimethyltyrosyl-1,2,3,4-tetrahydroisoquinoline-3-carboxylic acid, is a partially rigid opioid antagonist; its pA2 (5.8) is one smaller than that of N,N-bisallyl-enkephalin but it has a very high binding affinity (10 nM) and has a delta selectivity (66 with respect to the binding to mu receptors) higher than that of naltrindole. The conformational state of this diketopiperazine, studied under a variety of solvent and temperature conditions by NMR and molecular dynamics, can be described in terms of only three conformers whose relative populations vary widely with solvent. Only one of the three conformers, characterized by a 90 degree arrangement of the aromatic rings of Tyr(Me)2 and Tic similar to those of rigid agonists and of the bioactive conformation of the corresponding linear antagonist, is consistent with the antagonist activity. This finding favors the participation model among the general mechanisms proposed to explain antagonism. Due to the simple composition of the conformational mixture and to the rigidity of the molecule, it is possible to propose a quantitative explanation for the discrepancy between the very high binding affinity (10 nM) and the fairly small in mouse vas deferens value (1.5 microM).

Mu and delta opioid receptors mediate opposite modulations by morphine of the spinal release of cholecystokinin-like material

The possible modulations by morphine and various opioids of the spinal release of cholecystokinin-like material (CCKLM) evoked by 30 mM K+ was studied in vitro, using slices of the dorsal part of the rat lumbar enlargement superfused with an artificial cerebrospinal fluid. Addition of the mu agonist, DAGO (0.1-10 microM), to the perfusing fluid produced a concentration-dependent decrease in the peptide release, which could be prevented by the preferential mu antagonist, naloxone. Complex modulations were induced by the delta agonist, DTLET, as this drug inhibited CCKLM release when added at 10 nM-3 microM to the perfusing fluid, but enhanced it at 10 microM. Both effects were preventable by the delta antagonists naltrindole and ICI 154129, suggesting that delta receptors, possibly of different subtypes, mediated the inhibition and stimulation by DTLET. Morphine also exerted a biphasic effect, as the alkaloid decreased CCKLM release at 0.01-0.1 microM and enhanced it at 10 microM. Morphine-induced inhibition was preventable by naloxone, whereas its stimulatory effect could be blocked by naltrindole and ICI 154129. Although inactive on its own on CCKLM release, the selective kappa 1 agonist U 50488H (1 microM) prevented the inhibitory effects of both DAGO (10 microM) and morphine (0.1 microM), suggesting the existence of interactions between kappa 1 and mu receptors within the dorsal zone of the rat spinal cord. These data indicate that low concentrations of morphine exert an inhibitory influence on spinal CCKergic neurons that depends on the stimulation of mu opioid receptors. The excitatory influence of 10 microM morphine likely results from the simultaneous stimulation of mu, delta and kappa receptors, as the inhibitory effect of mu receptor stimulation can be masked by that of kappa 1 receptors, allowing only the expression of a delta-dependent excitatory effect similar to that induced by 10 microM DTLET.

Opioidergic control of the spinal release of neuropeptides. Possible significance for the analgesic effects of opioids

Several neuropeptides play a key role in the transfer (substance P, calcitonin gene-related peptide, etc) and control (enkephalins, cholecystokinin, etc) of nociceptive messages from primary afferent fibres to spino-thalamic neurones in the dorsal horn of the spinal cord. This first relay in nociceptive pathways has been shown to be a major target for opioids such as analgesic drugs, and the effects of exogenous (mainly morphine) and endogenous opioids on the release of neuropeptides within the dorsal horn are reviewed here for a better understanding of the cellular mechanisms responsible for their antinociceptive action. Complex modulations of the in vitro (from tissue slices) and in vivo (in halothane-anaesthetized rats whose intrathecal space was perfused with an artificial cerebrospinal fluid) release of substance P and calcitonin gene-related peptide by opioids have been reported, depending on the opioid receptor (mu, delta, kappa, and their subtypes) stimulated by these compounds. In particular, the inhibition by delta agonists of substance P release from primary afferent fibres, and that by the concomitant stimulation of mu and kappa receptors of the release of calcitonin gene-related peptide are very probably involved in the analgesic action of specific opioids and morphine at the level of the spinal cord. Furthermore, the negative modulation (through presynaptic opioid autoreceptors) by delta and mu agonists of the spinal release of met-enkephalin, and the complex inhibitory/excitatory influence of delta, mu and kappa receptor ligands on the release of cholecystokinin within the dorsal horn very likely also contribute to the antinociceptive action of these drugs and morphine. The reviewed data strongly support the existence of functional interactions between mu and kappa receptors within the spinal cord, and their key role in the analgesic action of non specific opiates (acting on mu, delta and kappa receptors) such as morphine.

Opioid antagonist diprenorphine microinjected into parabrachial nucleus selectively inhibits vasopressin response to hypovolemic stimuli in the rat

Subcutaneous injection of the potent, nonselective opioid antagonist diprenorphine inhibits the vasopressin response to acute hypovolemia. To determine if this inhibition is due to antagonism of opioid receptors in brain pathways that mediate volume control, we determined the vasopressin response to different stimuli when diprenorphine or other opiates were injected into the cerebral ventricles, the nucleus tractus solitarius (NTS), or the lateral parabrachial nucleus (PBN) of rats. We found that the vasopressin response to hypovolemia was inhibited by injection of diprenorphine into the cerebral ventricles at a dose too low to be effective when given subcutaneously. This response also was inhibited when a 20-fold lower dose of diprenorphine was injected into the PBN but not when it was injected into the NTS. The inhibitory effect of diprenorphine in the PBN was not attributable to a decrease in osmotic or hypovolemic stimulation and did not occur with osmotic or hypotensive stimuli. Injecting the PBN with equimolar doses of the mu antagonist naloxone, the delta antagonist ICI-154,129 or the kappa-1 agonist U-50,488H had no effect on basal or volume-stimulated vasopressin. We conclude that the inhibition of vasopressin by diprenorphine is due partially to action at a novel class of opioid receptors that transmit volume stimuli through the PBN.

Precipitation of morphine withdrawal syndrome in rats by administration of mu-, delta- and kappa-selective opioid antagonists

The acute effects of opioid drugs are generally hypothesized to be mediated by multiple receptors, for which three types of binding sites have been established. In order to evaluate the selective participation of each type of opioid receptor in opiate withdrawal, the opiate withdrawal syndrome, precipitated by the intraventricular acute administration of mu-, delta- and kappa-selective opioid antagonists was investigated. After implantation of the cannula into the lateral ventricle, rats were made physically dependent by subcutaneous insertion of two 75-mg pellets of morphine (base). D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen-Thr-NH2 (CTAP) (5-5000 ng), a mu-selective opioid antagonist, naltrindole (62-2000 ng), a delta-selective antagonist or nor-binaltorphimine (nor-BNI) (600-20,000 ng), a kappa-selective antagonist, were administered 72 hr after implantation of the pellets. All three drugs elicited some signs of morphine withdrawal but they differed in both their potency and their efficacy. The most efficacious and the most potent was CTAP, eliciting 8 of the 14 withdrawal signs at doses of 5-5000 ng. Nor-BNI was less efficacious and less potent, eliciting a significant increase in 5 of the 14 withdrawal signs in a dose range of 600-20,000 ng. Naltrindole was the least potent and least efficacious of the three drugs, eliciting a significant increase of only 2 withdrawal signs after intraventricular administration of 2000 ng. In a second experiment, the withdrawal syndrome was precipitated by the combined administration of CTAP+naltrindole or CTAP+nor-BNI. The severity of withdrawal, obtained with these two combinations, was similar to that observed with CTAP alone. These results support the importance of the mu receptor in the expression of central opiate dependence and suggest a minor role for delta and kappa receptors.

Kappa-/mu-receptor interactions in the opioid control of the in vivo release of substance P-like material from the rat spinal cord

The possible involvement of mu and kappa receptors in the opioid control of the spinal release of substance P-like material was assessed in vivo, in halothane-anaesthetized rats whose intrathecal space was continuously perfused with an artificial cerebrospinal fluid supplemented with various opioid receptor agonists and antagonists. Whereas the intrathecal perfusion with the mu agonist DAGO (10 microM) significantly enhanced (approximately + 50%) the spontaneous release of substance P-like material, that with the kappa agonist U 50488 H (10 microM) produced no change in the peptide outflow. The respective antagonists naloxone (10 microM) for the mu receptors and nor-binaltorphimine (10 microM) for the kappa receptors did not affect the spontaneous release of substance P-like material, indicating that endogenous opioids acting at mu and kappa receptors do not exert a tonic control on substance P-containing neurons in the spinal cord of halothane-anaesthetized rats. However, as expected from the involvement of mu receptors, the stimulatory effect of DAGO on the peptide outflow could be prevented by naloxone but not norbinaltorphimine. Furthermore, instead of an increase with DAGO alone, a significant decrease in the spinal release of substance P-like material was observed upon the intrathecal perfusion with DAGO plus U 50488 H. Additional experiments with the respective mu and kappa antagonists naloxone and nor-binaltorphimine demonstrated that this effect actually resulted from the simultaneous stimulation of mu and kappa receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Opioid control of the in vitro release of cholecystokinin-like material from the rat substantia nigra

Possible interactions between Met-enkephalin and cholecystokinin (CCK)-containing neurons in the rat substantia nigra were investigated by looking for the effects of various opioid receptor ligands and inhibitors of enkephalin-degrading enzymes on the K(+)-evoked overflow of CCK-like material (CCKLM) from substantia nigra slices. The delta-opioid agonists D-Pen2, D-Pen5-enkephalin (50 microM) and Tyr-D-Thr-Gly-Phe-Leu-Thr (DTLET; 3 microM) enhanced, whereas the mu-opioid agonists Tyr-D-Ala-Gly-MePhe-Gly-ol (DAGO; 10 microM) and MePhe3, D-Pro4-morphiceptin (PL 017; 10 microM) decreased, the K(+)-evoked release of CCKLM. By contrast, the kappa-opioid agonist U-50488 H (5 microM) was inactive. The stimulatory effect of DTLET could be prevented by the delta antagonist ICI-154129 (50 microM), but not by the mu antagonist naloxone (1 microM). Conversely, the latter drug, but not ICI-154129, prevented the inhibitory effect of DAGO and PL 017. A significant increase in CCKLM overflow was observed upon tissue superfusion with the peptidase inhibitors kelatorphan or bestatin plus thiorphan. This effect probably resulted from the stimulation of delta-opioid receptors by endogenous enkephalins protected from degradation, because it could be prevented by ICI-154129 (50 microM). Furthermore the peptidase inhibitors did not enhance CCKLM release further when delta-opioid receptors were stimulated directly by DTLET (3 microM). These data indicate that opioids acting on delta and mu receptors may exert an opposite influence, i.e., excitatory and inhibitory, respectively, on CCK-containing neurons in the rat substantia nigra.(ABSTRACT TRUNCATED AT 250 WORDS)

A novel opioid structure which accepts protonated as well as non-protonated nitrogen: a family of pure, delta receptor selective antagonists

Conventional opioids including opioid peptides require an "opioid" nitrogen which exists in protonated state while interacting with the receptor. In the present paper we demonstrate that the Tyr-Pro-Gly-Phe-Leu-Thr hexapeptide sequence accepts N-terminal substituents such as N-t-Boc, N-phenylacetyl and N-diphenylacetyl where the N cannot become protonated, as well as "traditional" substitutions such as N,N-diallyl, where protonation is likely under physiological conditions. The opioid peptides bearing these substituents are pure antagonists of medium affinity (Ke values in the mouse vas deferens bioassay against [Met5]-enkephalin are in the 3 x 10(-7)-4 x 10(-6) M range) with a high delta receptor preference (50-350-fold delta over mu selectivity ratios).

Differential inhibitory/stimulatory modulation of spinal CCK release by mu and delta opioid agonists, and selective blockade of mu-dependent inhibition by kappa receptor stimulation

Opioid-cholecystokinin (CCK) interactions at the spinal level were investigated by looking for possible modulations by various opioid agonists of the release of cholecystokinin-like material (CCKLM) from slices of the dorsal zone of the rat lumbar enlargement. K(+)-evoked CCKLM overflow was reduced by 0.1-10 microM of the mu agonist DAGO or 10 nM to 3 microM of the delta agonist DTLET. By contrast, at a higer concentration (10 microM), the latter drug as well as morphine enhanced CCKLM overflow. Although inactive alone, the kappa opioid agonist U 50488 H (1 microM) prevented the inhibitory effect of DAGO without affecting that of DTLET. These data suggest that an opioid acting through the stimulation of mu, delta and kappa receptors (such as morphine) should produce a net increase in the spinal release of CCK.

An in vitro study on the hippocampal epileptogenic properties of enkephalins and enkephalinase inhibitors in rats

1. The effects of enkephalins and enkephalinase inhibitors were studied in CA1 area in rat hippocampal slices. 2. The data demonstrate a prevalent involvement of mu opiate receptors in the epileptogenic properties of enkephalins. 3. A potentiation of the mu opiate receptor-mediated epileptogenic response by enkephalinase inhibitors has been shown. 4. The results also show an inability to affect basal CA1 field potentials by inhibition of endogenous endopeptidase.

The growth hormone secretory response to fentanyl in rat: an involvement of mu type receptors

Fentanyl, a selective mu opioid receptor agonist, administered intravenously, influences growth hormone secretion in conscious male rats. A dose-response study demonstrated that the maximum growth hormone release was obtained with 10 micrograms/kg while higher doses were less or not effective. MR-2266 (6 mg/kg i.v.), a mu and kappa opioid receptor antagonist, and bremazocine (0.1 mg/kg i.v.) a mu opioid receptor antagonist with kappa agonistic properties, both potently inhibited the growth hormone response to fentanyl (10 micrograms/kg i.v.). In contrast, the effect of fentanyl on growth hormone release was not blocked in rats treated with either ICI-154129 (30 mg/kg i.v. or 150 micrograms/kg intracerebroventricularly a selective delta opioid receptor antagonist, or U-50488 (10 mg/kg i.v.), a specific kappa opioid receptor agonist. These results suggest that opioid receptors of the mu type are involved in the fentanyl-induced growth hormone release.

Single residue modifications of the delta opioid receptor selective peptide, [D-Pen2,D-Pen5]-enkephalin (DPDPE). Correlation of pharmacological effects with structural and conformational features

Six analogs of the highly delta opioid receptor selective, conformationally restricted, cyclic peptide [D-Pen2,D-Pen5]enkephalin, Tyr-D-Pen-Gly-Phe-D-PenOH (DPDPE), were synthesized and evaluated for opioid activity in rat brain receptor binding and mouse vas deferens (MVD) smooth muscle assays. All analogs were single amino acid modifications of DPDPE and employed amino acid substitutions of known effects in linear enkephalin analogs. The effect on binding affinity and MVD potency of each modification within the DPDPE structural framework was consistent with the previous reports on similarly substituted linear analogs. Conformational features of four of the modified DPDPE analogs were examined by 1H NMR spectroscopy and compared with DPDPE. From these studies it was concluded that the observed pharmacological differences with DPDPE displayed by diallyltyrosine1-DPDPE ([DAT1]DPDPE) and phenylglycine4-DPDPE ([Pgl4]DPDPE) are due to structural and/or conformational differences localized near the substituted amino acid. The observed enhanced mu receptor binding affinity of the carboxamide terminal DPDPE-NH2 appears to be founded solely upon electronic differences, the NMR data suggesting indistinguishable conformations. The observation that the alpha-aminoisobutyric acid substituted analog [Aib3]DPDPE displays similar in vitro opioid behavior as DPDPE while apparently assuming a significantly different solution conformation suggests that further detailed conformational analysis of this analog will aid the elucidation of the key structural and conformational features required for action at the delta opioid receptor.

Antagonism of swim-stress-induced antinociception by the delta-opioid receptor antagonist naltrindole in adult and young rats

1. The availability of the non-peptide delta-opioid receptor antagonist naltrindole has provided the possibility for in vivo studies on the function of delta-opioid receptors. We have studied the effects of naltrindole on swim-stress-induced antinociception in adult and neonatal rats. 2. Adult, 25 and 20 day old rats were stressed by warm water (20 degrees C) swimming for 3 min periods and antinociception was assessed by the tail immersion test (50 degrees C). 3. Naltrindole (0.5 and 1 mg kg-1) antagonized swim-stress-induced antinociception in adult and 25 day old rats but in 20 day old rats naltrindole (1 mg kg-1) was without effect. 4. Antinociception induced by the highly mu-opioid receptor selective agonist alfentanil was completely antagonized by naloxone (1 mg kg-1) but virtually unaffected by naltrindole (1 mg kg-1). 5. Neither naloxone nor naltrindole (1 mg kg-1) antagonized swim-stress-induced rises in plasma corticosterone in adult rats at the time of peak antinociception. 6. In conclusion, naltrindole shows in vivo antagonism of opioid-mediated responses. Swim-stress-induced antinociception is mediated through the delta-opioid receptor in 25 day old and adult rats and through the mu-opioid site in 20 day old animals.

Involvement of multiple opiate receptors in opioid kindling

D-Tyr-Ser-Gly-Phe-Leu-Thr (DSLET), beta-endorphin, morphiceptin and morphine were microinjected at 48-h intervals into the amygdala or hippocampus of awake rats in an attempt to identify the opiate receptor types involved in opioid kindling. DSLET, beta-endorphin, morphiceptin and morphine were injected into the lateral ventricle to assess the possibility of kindling seizures by this route. The delta-receptor agonist DSLET effectively kindled convulsions when microinjected into amygdala or ventral hippocampus. The convulsions were suppressed or strongly attenuated by ICI 174,864, a specific antagonist of the delta-receptor, microinjected into the same brain site, but were not affected by ICI 174,864 administered peripherally. When microinjected into amygdala or hippocampus, beta-endorphin and morphiceptin also kindled convulsions, which were antagonized by naloxone but not by ICI 174,864. Morphine evoked EEG epileptiform activity but did not kindle convulsions from limbic brain sites. DSLET occasionally evoked epileptiform spiking and submaximal convulsions when injected into ventricle, and morphiceptin evoked epileptiform spiking only, but tolerance to these effects occurred after repetition of the injections. Thus, convulsions can be kindled by activation of either mu-, delta- or epsilon-receptors when opioids are injected directly into limbic tissue. However, the ability of these compounds to kindle seizures is markedly reduced when they are administered into ventricle. The striking differences between the present results and previous results obtained by peripheral or intraventricular administration of opioid peptides suggest that the route of administration, among other variables, is a crucial factor in assessing the epileptogenic properties of opioid peptides.

The selectivity of the opioid antagonist, naltrindole, for delta-opioid receptors

In the mouse vas deferens, naltrindole gave pKB values of 9.7, 8.3, and 7.5 at the delta-, mu-, and kappa-sites and in binding assays, pIC50 values of 9.6, 7.8 and 7.2 at the same sites. The affinity of naltrindole for the delta binding site was increased in the presence of sodium ions and 5'-guanylylimidophosphate. Naltrindole is, thus, a potent opioid antagonist with marked selectivity for the delta-opioid receptor.

Intrathecal administration of delta receptor agonists in the urethane anesthetized rat provokes an increase in arterial pressure via a non-opioid mechanism

Intrathecal administration of the delta receptor specific agonists Leu5-enkephalin (Leu-Enk; 300 nmol), Met5-enkephalin (Met-Enk; 300 nmol) and [D-Pen2,D-Pen5]enkephalin (DPDPE; 100 nmol) to the T2 or the T9 segment of the rat spinal cord provoked a transient (less than 5 min) increase (15-20 mm Hg) in arterial pressure. DPDPE, but not Leu-Enk or Met-Enk, also significantly increased heart rate by 30-35 bpm. Intravenous administration of 300 nmol of Leu-Enk mimicked the effects observed following intrathecal administration. The hypertensive effect of intrathecal and intravenous Leu-Enk administration was blocked by prior systemic administration (10 mg/kg) of the nicotinic ganglion blocker hexamethonium, suggesting that the effect was mediated via sympathetic activation. The increase in arterial pressure observed following intrathecal Leu-Enk administration was not blocked by either intrathecal (305 nmol) or intravenous (10 mg/kg) administration of the opiate receptor blocker naloxone, although naloxone did block the hypertension provoked by intravenous Leu-Enk administration. Moreover, intrathecal administration of Des-Tyr1-Leu-Enk (300 nmol), an enkephalin fragment devoid of opiate receptor activity, also increased arterial pressure. These results suggest that the hypertension elicited by intrathecal delta agonist administration was not mediated via an opioid mechanism.

The opioid system in neurologic and psychiatric disorders and in their experimental models

Evidence from experimental and clinical studies suggests the involvement of the endogenous opioid system in several neurologic and psychiatric disorders (Alzheimer's, Huntington's and Parkinson's diseases, drug-induced movement disorders, Gilles de la Tourette syndrome, stroke, ischemia, brain and spinal cord injury, epilepsy, schizophrenia and affective disorders). However, its involvement is rather a secondary one, perhaps being a severe consequence of a primary, nonopioid disturbance. Thus, treatment of an opioidergic manifestation of a disorder of nonopioidergic origin is necessarily symptomatic and targets only the restoration of the opioid system; such treatment may be beneficial in ameliorating the clinical symptoms of the disorder.

Biologically active conformation of [Met5]- and [Leu5]enkephalin on delta opioid receptor

Investigations of the conformation of endogenous enkephalins are generally based on structural comparisons of enkephalins with other opiates and on experimental pharmacological studies. Based on such investigations, we now propose a novel model of the biologically active (receptor-bound) conformation of [Met5]- and [Leu5]enkephalin on the delta opioid receptor. The model helps with the design of new opioid analgesics.

Role of central opiate receptor subtypes in the circulatory responses of awake rabbits to graded caval occlusions

1. In unanaesthetized rabbits, haemorrhage was simulated by inflating a cuff placed round the inferior vena cava so that cardiac output fell at a constant rate of approximately 8% of its resting value per minute. The circulatory responses were measured after injections into the fourth ventricle of saline vehicle, selective opioid antagonists, selective opioid agonists, and agonist-antagonist mixtures. Three sets of experiments were done to determine if a specific subtype of opiate receptor within the central nervous system is responsible for the circulatory decompensation that occurs during simulated haemorrhage. 2. In six rabbits the effects of ascending doses of the antagonists naloxone (mu-selective), Mr 2266 (kappa- and mu-selective), ICI 174864 (delta-selective) and nor-binaltorphimine (kappa-selective) were tested. In three rabbits the effects of the antagonist naloxone, the agonists HTyr-D-Ala-Gly-MePhe-NH(CH2)2OH (DAGO, mu-selective), U 50488H (kappa-selective), and [D-Pen2,D-Pen5]-enkephalin (DPDPE, delta-selective), and combinations of these agonists with naloxone were tested. In four rabbits the dose-related effects of DAGO on respiratory, as well as circulatory, functions were examined. 3. After injecting saline vehicle, the circulatory response to simulated haemorrhage had two phases. During the first phase, systemic vascular conductance fell, heart rate rose, and mean arterial pressure fell by only approximately 10 mmHg. A second, decompensatory, phase began when cardiac output had fallen to approximately 50% of its resting level. At this point, there was an abrupt rise in systemic vascular conductance and a fall in mean arterial pressure to less than or equal to 40 mmHg. 4. The lower range of doses of naloxone (3-30 nmol), Mr 2266 (10-100 nmol), ICI 174864 (10-30 nmol), and all doses of nor-binaltorphimine (1-100 nmol), were without effect on the circulatory response to stimulated haemorrhage. Higher doses of naloxone (30-100 nmol), Mr 2266 (100-300 nmol) and ICI 174864 (30-100 nmol) abolished the decompensatory phase. The relative order of antagonist potency was ICI 174864 greater than or equal to naloxone greater than Mr 2266 greater than or equal to nor-binaltorphimine. 5. In the second set of experiments, the critical dose of naloxone necessary to prevent circulatory decompensation during simulated haemorrhage was 30-150 nmol. The delta-agonist DPDPE (50 nmol) did not affect the haemodynamic response to simulated haemorrhage, but it did block the effect of naloxone on the response.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Selective kappa antagonist properties of nor-binaltorphimine in the rat MES seizure model

The opioid antagonist properties of nor-binaltorphimine (nor-BNI; 17,17'-Bis(cyclopropylmethyl)-6,6',7,7'-tetradehydro-4,5:4', 5'-diepoxy-6,6'-(imino) [7,7'-bimorphinan]-3,3',14,14'-tetrol) were evaluated in vivo in the rat maximal electroshock (MES) seizure model. Following s.c. or i.c.v. pretreatment, nor-BNI selectively antagonized the anticonvulsant effects of the kappa opioid U50, 488, significantly increasing its ED50 by 2.3 and 4.5 fold, respectively. In contrast, pretreatment with nor-BNI (s.c. or i.c.v.) failed to antagonize the anticonvulsant effects of the selective mu opioid, DAMGO. At the doses and injection routes used, nor-BNI itself had no apparent effect on overt behavior or MES-induced convulsions. These data support the earlier suggestion that the anticonvulsant effects of U50,488 are mediated by kappa opioid receptors and confirm 1) the selectivity of nor-BNI as a kappa antagonist and 2) its applicability as a pharmacological tool in the differentiation of multiple opioid receptors.

N,N-diallyl-tyrosyl substitution confers antagonist properties on the kappa-selective opioid peptide [D-Pro10]dynorphin A(1-11)

1. In the search for kappa-opioid antagonists, we have designed two N,N-diallyl substituted analogues of the kappa-selective peptide [D-Pro10]dynorphin A (1-11)(DPDYN). In this study, we have examined (i) the binding properties of N,N-diallyl-DPDYN (analogue 1) and N,N-diallyl-[Aib2,3]DPDYN (analogue 2) at the three main types (mu, delta, kappa) of opioid binding sites, (ii) their binding sensitivity to Na+ ions (120 mM NaCl) and guanine nucleotide (50 microM Gpp(NH)p) at mu- and kappa-binding sites and (iii) their biological activity in two pharmacological bioassays specific for mu- and kappa-(guinea-pig ileum) and kappa-(rabbit vas deferens) opioid receptors. 2. Steric hindrance resulting from incorporation of two bulky allyl groups at the tyrosal nitrogen atom greatly altered the binding properties of DPDYN. A dramatic fall in apparent affinity for the three types (mu, delta, kappa) of site as well as selectivity for kappa-sites was observed for the two N,N-diallyl-substituted peptide analogues. 3. At kappa-sites of guinea-pig cerebellum and mu-sites of rabbit cerebellum, N,N-diallyl-substitution led to a complete loss of binding sensitivity to the inhibitory effect of 120 mM NaCl + 50 microM Gpp(NH)p compared to the high sensitivity of DPDYN. This may therefore suggest that the N,N-diallyl-DPDYN analogues are endowed with opioid antagonist properties. 4. No agonist activity of the analogues was observed in guinea-pig myenteric plexus and rabbit vas deferens organ preparations. In contrast, both of the diallyl-substituted peptides displayed similar antagonist properties against the kappa-agonist DPDYN in both preparations. In the guinea-pig ileum, the affinities of the antagonist peptides against the mu-agonist Tyr-D-Ala-Gly-MePhe- NH(CH2)20H(DAGOL) were approximately half that observed against DPDYN. 5. These results show that N,N-diallyl-tyrosyl substitution leads to analogues of DPDYN which act in vitro as pure opioid antagonists and exhibit a reasonable affinity at, but a weak selectivity for, the K-opioid receptors.

Anticonvulsant effects of mu (DAGO) and delta (DPDPE) enkephalins in rats

The effects of highly selective mu and delta opioid peptide agonists were determined in two rat models of experimentally-induced convulsions, the flurothyl threshold test and the maximal electroshock test. Intracerebroventricular injections of the mu selective enkephalin DAGO (0.3-2.2 nmol) resulted in a dose-related protection in both seizure models. Pretreatment with a low dose of naloxone (29 nmol) or the irreversible mu antagonist beta-FNA (21 nmol), but not the delta opioid antagonist ICI 154,129 (50 nmol), antagonized the anticonvulsant actions of DAGO. Intracerebroventricular injections of the delta selective enkephalin DPDPE (70-140 nmol) also resulted in seizure protection. These effects were selectively antagonized by the delta antagonist ICI 174,864 (2.8 nmol), but not by pretreatment with beta-FNA. Thus, using agonists and antagonists highly selective for mu and delta opioid receptors, anticonvulsant actions of enkephalin have been described against chemically- and electrically-induced convulsions in rats.

Selectivity of ligand binding to opioid receptors in brain membranes from the rat, monkey and guinea pig

Conditions for the equilibrium binding to opioid receptor of [3H]sufentanil (mu selective), [3H][D-Pen2,D-Pen5]enkephalin (delta selective), and [3H]U69,593 (kappa selective) were established in membranes from rat brain cerebrum, monkey cortex, or guinea pig cerebellum. The selectivity index of various opioid alkaloids and peptides in binding to the mu, delta, or kappa opioid receptors was expressed as the ratio of their EC50 values in displacing two selective radiolabeled ligands: [3H]sufentanil/[3H](D-Pen2,D-Pen5)enkephalin (selectivity: mu/delta), [3H]sufentanil/[3H]U69,593 (selectivity: mu/kappa), or [3H][D-Pen2,D-Pen5]enkephalin/[3H]U69,593 (selectivity: delta/kappa). High resolution in binding selectivity was observed: in rat brain the mu/delta selectivity for Tyr-D-Ala-Gly-(Me)Phe-Gly-ol and sufentanil were 0.02 and 0.03, whereas for [D-Pen2,D-Pen5]enkephalin and ICI 174,864 they were 1,200 and 998. Compared to mu opiates, the specific binding of delta and kappa agonists was less sensitive to sodium. The results describe a routinely applicable methodological approach for the assessment of selective ligand binding to the mu, delta and kappa opioid receptors in rodent and monkey brain membranes.

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.