Mu-delta opioid interactions. III: Differential antagonism of DPDPE-induced increases in morphine EEG and EEG power spectra by DALCE and naltrindole

In the present study, the effects of DALCE ([D-Ala2,Leu5,Cys6]enkephalin) and naltrindole on DPDPE ([D-Pen2,D-Pen5]enkephalin)-induced increases in morphine EEG and EEG power spectra were assessed. Adult female Sprague-Dawley rats were implanted with cortical EEG electrodes and permanent indwelling ICV and IV cannulae. Rats were pretreated with ICV DALCE at 15.7 nmol, ICV naltrindole at 20 nmol, or ICV sterile water. Rats were then administered ICV DPDPE at 2.5 nmol or ICV sterile water followed, 10 min later, by IV morphine at 3 mg/kg. Morphine-induced changes in EEG global (1-50 Hz) spectral parameters, the duration of morphine-induced high-voltage EEG bursts, the duration of EEG and behavioral excitation, and the latency to onset of slow-wave sleep were assessed. The DALCE pretreatment significantly decreased morphine-induced total spectral power seen in the DPDPE + morphine group. The DALCE pretreatment reversed the effects of DPDPE on the duration of morphine-induced EEG bursts and the duration of EEG and behavioral excitation. The ICV naltrindole, however, had no significant effect on DPDPE-induced increases in morphine EEG, EEG spectral parameters, and behavior. These data, therefore, suggest that DPDPE may be increasing the effects of morphine on EEG through DALCE-sensitive delta opioid receptors associated within the mu-delta opioid receptor complex.

The kappa opioid agonist U-50,488H antagonizes respiratory effects of mu opioid receptor agonists in conscious rats

The interactive effects of mu and kappa opioid receptor agonists on respiratory function were investigated following their i.c.v. injection into conscious rats. The highly selective mu receptor agonist [D-Ala2,N-Methyl-Phe4,Gly-ol] enkephalin (DAMGO; 1.2-10 nmol) and the relatively selective mu agonist morphine (20 and 30 nmol) significantly decreased arterial pH and PO2, and increased arterial PCO2 and blood pressure. Morphine and a low dose of DAMGO (1.2 nmol) also significantly elevated respiratory rate. Heart rate was decreased by DAMGO and, depending upon dose, was either decreased (20 nmol) or increased (30 nmol) by morphine. The selective kappa opioid agonist U-50,488H (200 nmol i.c.v.), which by itself had no significant effect on either respiration or cardiovascular function, dose-dependently antagonized the acidotic, hypoxemic and hypercapnic effects of both DAMGO (2.5 nmol) and morphine (30 nmol). Furthermore, these mu antagonistic properties of U-50,488H were blocked completely after pretreatment with 25 nmol of the highly selective kappa opioid antagonist nor-binaltorphimine. These results indicate that the antagonism of mu opioid respiratory depressant effects by U-50,488H is kappa opioid receptor mediated.

Cocaine-like discriminative stimulus properties of the delta-selective opioid receptor agonist, [D-Pen2,L-Pen5]enkephalin, in the rat

The delta-selective opioid receptor agonist, [D-Pen2,L-Pen5]enkephalin (DPLPE) (10 micrograms i.c.v.), fully generalized to cocaine cue in the rat trained to discriminate 10 mg/kg of cocaine from vehicle. The cocaine-like discriminative stimulus effects of DPLPE were almost completely reversed by naltrindole (56 micrograms i.c.v.), a delta-selective opioid receptor antagonist. In contrast, the mu-selective opioid receptor agonist [D-Ala2,NMePhe4,Gly-ol]enkephalin (DAMGO) (0.03-0.3 micrograms i.c.v.) failed to generalize to cocaine cue. These results suggest that the discriminative stimulus effects of cocaine are mediated through the activation of delta-opioid receptors.

Electrophysiological actions of delta opioids in CA1 of the rat hippocampal slice are mediated by one delta receptor subtype

Various opioid agonists and antagonists were examined for their ability to alter extracellularly and intracellularly recorded CA1 pyramidal cell activity. All opioid agonists tested, with the exception of [D-ala2]deltorphin II, increased primary population spike amplitude. Of these active agonists, all except DPDPE and p-Cl-DPDPE produced secondary population spikes. DSLET and DAMGO, but not DPDPE, reduced the amplitude of the orthodromically stimulated IPSP. Naltrexone antagonized the actions of all agonists tested. The actions of DPDPE and p-Cl-DPDPE, but not those of DSLET, DAMGO or morphine, were antagonized by the delta antagonist naltrindole. Similarly, the delta antagonist ICI-174,864 blocked the actions of DPDPE, but not DSLET or DAMGO. Based on the inactivity of [D-ala2]deltorphin II and the lack of delta antagonist-sensitive actions of DSLET, the data suggest that the delta 1 subtype is the predominant delta subtype in the CA1 region of the hippocampus.

7-Benzylidenenaltrexone (BNTX): a selective delta 1 opioid receptor antagonist in the mouse spinal cord

Recent reports provided evidence that at least two delta opioid receptors may mediate antinociception in mice. In this study, we studied further the involvement of delta opioid receptor subtypes in mediating antinociception at spinal sites in mice using subtype selective agonists and antagonists. The antinociceptive ED50 values (95% C.I.) of i.t. administered DPDPE [(D-Pen2, D-Pen5)enkephalin] (delta 1 receptor agonist) and DELT II [(D-Ala2)deltorphin II] (delta 2 receptor agonist) were 6.3 (5.2-7.6) and 6.4 (5.4-7.7) nmol/mouse, respectively. Administration of BNTX, s.c. increased the antinociceptive ED50 value of DPDPE 5.9-fold whereas that of DELT II was not changed significantly. On the other hand administration of naltriben (NTB, the benzofuran derivative of naltrindole), s.c. increased the antinociceptive ED50 value of DELT II 12.5-fold but did not alter that of DPDPE. Similarly administration of BNTX, i.t. increased the antinociceptive ED50 value of DPDPE 4-fold without altering significantly that of DELT II. NTB given i.t. enlarged the antinociceptive ED50 of DELT II 11-fold without affecting significantly that of DPDPE. BNTX, s.c. did not alter the antinociceptive ED50 values of the mu-agonists, DAMGO [(D-Ala2,MePhe4,Gly-ol5) enkephalin] and morphine or that of the kappa-agonist, U50,488H [trans(+/-)-3,4-dichloro-N-methyl-N-[2-(1- pyrrolidinyl-cyclohexyl]benzeneacetamide] These results demonstrate that BNTX is highly selective for delta 1 opioid receptors at spinal sites. Also, the present data provide for the involvement of both delta 1 and delta 2 opioid receptors in mediating antinociception at spinal sites in mice.

Tyr-MIF-1 and hemorphin can act as opiate agonists as well as antagonists in the guinea pig ileum

The brain peptide Tyr-MIF-1 (Tyr-Pro-Leu-Gly-NH2) was tested for its effects on electrically stimulated contractions in the guinea pig ileum assay. Tyr-MIF-1 acted as an opiate agonist in reducing these contractions. Its IC50 was about 9 microM, and its effects were reversed by naloxone and CTOP. The ability of Tyr-MIF-1 also to antagonize the inhibitory effects of opiates on electrically stimulated contractions was more evident in the ileum removed from a guinea pig tolerant to morphine or after partial inactivation of opiate receptors with beta-CNA. Similar results were observed with hemorphin. The endogenous peptide Tyr-MIF-1 and the blood-derived peptide hemorphin, therefore, can act as agonists as well as antagonists in the guinea pig ileum. The effects as antagonists are best observed in preparations of ileum with reduced receptor reserve (tolerant or beta-CNA treated) and are consistent with the idea that properties of endogenous peptides as opiate antagonists are enhanced in the tolerant state.

Effects of the delta-opioid receptor antagonist naltrindole on antinociceptive responses to selective delta-agonists in post-weanling rats

1. Antagonism, by the selective delta-opioid receptor antagonist naltrindole, of the antinociceptive effects of [D-Pen2, D-Pen5] enkephalin (DPDPE), [D-Ser2, Leu5, Thr6] enkephalin (DSLET) and D-Ala2 deltorphin I (DELT I) has been studied in 25 day old rats. 2. Antinociception was measured by the 50 degrees C tail immersion test following i.p. administration of agonists and/or antagonists. 3. Dose-related antinociception was observed with DPDPE, DSLET and DELT I and ED75 doses were computed (0.66 mg kg-1, 0.65 mg kg-1, 0.032 mg kg-1 respectively) and used for antagonism studies. 4. Naltrindole (0.01 mg kg-1) significantly attenuated the antinociceptive effects of DPDPE and DSLET with 0.1 mg kg-1 producing complete reversal of the effects of the ED75 dose. In contrast, naltrindole at 0.01 and 0.1 mg kg-1 did not alter antinociceptive responses to DELT I. Naltrindole at 1 mg kg-1 significantly attenuated DELT I antinociception. 5. Naloxone (1 mg kg-1) produced equivalent degrees of antagonism of the antinociceptive effects of DPDPE, DSLET and DELT I. ICI 174,864 (1 mg kg-1) also antagonized antinociception with a differential degree of attenuation (DSLET > DPDPE > DELT I). 6. Naltrindole (1 mg kg-1) had no effect on the antinociception induced by the selective mu-agonist alfentanil (60 micrograms kg-1). Naltrindole, naloxone or ICI 174,864 had no effect on nociceptive latencies. 7. The differential antagonism by naltrindole of the effects of three selective delta-agonists suggests delta-receptor heterogeneity.Further, the lower sensitivity of response to DELT I suggests that this agent may exert its antinociceptive effects at a different 6 receptor subtype from DPDPE or DSLET.

Central opioid modulation of breathing dynamics in the fetal lamb: effects of [D-Pen2,D-Pen5]-enkephalin and partial antagonism by naltrindole

The effects of opiates on fetal breathing movements (FBM) have been shown to be complicated, with stimulation at low doses and suppression at higher doses. Recent studies have shown that morphine-induced stimulation of FBM can be blocked by naloxonazine (NALZ), suggesting action at the mu 1 opioid receptor. To examine the role of delta receptors in modulating FBM, the effects of [D-Pen2,D-Pen5]-enkephalin (DPDPE) on breathing dynamics were studied in fetal lambs with chronically implanted diaphragmatic electromyographic electrodes. DPDPE given i.c.v. (4.6-465 nmol/hr) caused significant time and dose-related increases in the number of breaths/hr and the incidence of fetal breathing movements, without significant changes in blood pH, PCO2 or PO2. Higher doses resulted in an attenuation of the responses, with a significant decrease in breaths/hr at 465 nmol/hr. DPDPE also induced a much more continuous and regular breathing pattern. All DPDPE effects were completely abolished by pretreatment with i.v. naloxone, but were unaffected by naloxonazine pretreatment. Naltrindole did not alter the effects of DPDPE on breath number or incidence of FBM, but blocked the effects on continuity and regularity of the breathing pattern. These results demonstrate that DPDPE stimulates breathing activity as well as alters breathing dynamics in the fetal lamb. The differential sensitivity of these two actions to naltrindole suggest that they may be mediated by different delta receptor subtypes, and that the mu 1 receptor is not involved.

Potentiation of the dorsal immobility response following intrastriatal injections of enkephalins

The effects of bilateral intrastriatal injections (1.0 microgram/side) of leucine5- and methionine5-enkephalins and their related nonopiate fragments upon three measures of immobility over a time course were investigated. Both leucine5-enkephalin and des-Tyr1-leucine-enkephalin potentiated the duration of the dorsal immobility response (DIR) 15 min postinjection and over a 1-h time course. On the other hand, methionine5-enkephalin and des-Tyr1-methionine-enkephalin potentiated the duration of the DIR at 5 and 15 min. These enkephalins and their fragments had no effect upon vertical cling and bar catalepsy. In a second study, an SC injection of 4 mg/kg naloxone 15 min prior to the central injections blocked the potentiation of the DIR effects of the enkephalins.

Isolation of a novel tetrapeptide with opiate and antiopiate activity from human brain cortex: Tyr-Pro-Trp-Gly-NH2 (Tyr-W-MIF-1)

A novel tetrapeptide, Tyr-Pro-Trp-Gly-NH2 (Tyr-W-MIF-1), was purified from extracts of frontal cortex of human brain tissue by several consecutive reversed-phase high performance liquid chromatographic steps followed by a radioimmunoassay originally developed for Tyr-Pro-Leu-Gly-NH2 (Tyr-MIF-1). Sequencing, mass spectrometric analysis, and comparison of its chromatographic behavior with that of the synthetic peptide confirmed the structure. Like Tyr-MIF-1, which was previously isolated from human brain tissue, Tyr-W-MIF-1 can inhibit the binding of 3H-DAMGO (selective for mu opiate receptors) to rat brain and can act as an opiate agonist as well as antagonist. Tyr-W-MIF-1 was a more potent opiate agonist than Tyr-MIF-1, the free acid of Tyr-W-MIF-1, and the structurally related hemoglobin-derived opiate peptide hemorphin-4 (Tyr-Pro-Trp-Thr) in the guinea pig ileum. Each of these peptides acted as opiate antagonists on the ileum from morphine-tolerant guinea pigs; the free acid of Tyr-W-MIF-1 was the most potent antagonist in inhibiting the activity of DAMGO. The results demonstrate the presence in human brain of a new member of the Tyr-MIF-1 family of biologically active peptides.

Mu-delta opioid interactions. II: Beta-FNA inhibits DPDPE-induced increases in morphine EEG and EEG spectral power

In the present study, the effects of beta-FNA on DPDPE-induced increases in morphine EEG and EEG power spectra were assessed. Adult female Sprague-Dawley rats were implanted with cortical EEG electrodes and permanent indwelling ICV and IV cannulae. Rats were administered ICV beta-FNA at 20 nmol or ICV sterile water. Then 18-24 h later, rats were administered ICV DPDPE at 2.5 nmol or ICV sterile water followed, 10 min later, by IV morphine at 3 mg/kg. Morphine-induced changes in EEG global (1-50 Hz) spectral parameters, the duration of morphine-induced high voltage EEG bursts, the period of EEG and behavioral excitation, and the latency to onset of slow-wave sleep were statistically analyzed using a one-way analysis of variance. beta-FNA pretreatment significantly decreased morphine-induced total spectral power seen in the DPDPE + morphine group. beta-FNA pretreatment also significantly decreased the duration of morphine-induced EEG bursts, the period of EEG and behavioral excitation, and the latency to onset of slow-wave sleep in the DPDPE + morphine group. These data, therefore, suggest that DPDPE may be increasing the effects of morphine on EEG through delta opioid receptors associated with the mu-delta opioid receptor complex.

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

When co-administered intracisternally, the selective delta-opioid agonist [D-Pen2,5]enkephalin (DPDPE), which had no significant effect on the cough reflex, consistently and significantly decreased the antitussive potencies of kappa-receptor agonists, U-50,488H and U-62,066E. The decrease in the antitussive effects of these kappa-receptor agonists caused by DPDPE were prevented by selective delta receptor antagonist, naltrindole. These results suggest that delta receptors may play an inhibitory role in antitussive processes that are mediated by the kappa-receptors.

Selective impairment of neuroendocrine and hemodynamic responses to a mu-opioid peptide in aged rats

The objective of this study was to determine if there are age-related alterations in hemodynamic and/or neuroendocrine responses to the mu-opioid receptor agonist, [D-Ala2,MePhe4,Gly(ol)5] enkephalin (DAMGO), or corticotropin releasing hormone (CRH) administered centrally. To this end, DAMGO (1-3 nmoles) or CRH (1 nmole) was injected intracerebroventricularly (icv) to freely moving young (6-8 month) and aged (24-26 month) Fischer 344 male rats. Blood pressure, heart rate (HR), and plasma concentrations of norepinephrine (NE), epinephrine (EPI), adrenocorticotropin (ACTH), and prolactin (PRL) were measured over time. Under basal conditions, NE levels were higher and blood pressures were lower in aged rats, whereas there were no significant differences in EPI, ACTH, or PRL levels. The stimulatory effect of DAMGO on blood pressure, HR, and plasma EPI and ACTH was attenuated, but the PRL response was enhanced in aged cohorts. In contrast, there were no age-related differences in the NE responses to DAMGO or CRH nor in CRH-induced increases in EPI or ACTH. The sympathoadrenal and hemodynamic effects of DAMGO were blocked by naloxone in both age groups. These results indicate that alterations in mu-opioid function with age are specific for the opioid system and do not reflect a generalized decline in central regulation of neuroendocrine and cardiovascular function.

Cholecystokinin administered intrathecally selectively antagonizes intracerebroventricular beta-endorphin-induced tail-flick inhibition in the mouse

The effects of sulfated cholecystokinin octapeptide (CCK8s) given intrathecally (i.t.) or intracerebroventricularly (i.c.v.) on inhibition of the tail-flick and paw-licking hot-plate responses induced by beta-endorphin, morphine, D-Ala2-N-Me-Phe4-Gly-ol-Enkephalin (DAMGO) and D-Pen2-D-Pen5-Enkephalin (DPDPE), given i.t. or i.c.v., were studied in male ICR mice. CCK8s (1 ng) given i.t. effectively antagonized inhibition of the tail-flick response induced by i.c.v. administered beta-endorphin (2 micrograms) and DPDPE (10 micrograms) but not morphine (4 micrograms) or DAMGO (0.02 microgram). However, CCK8s given i.t. did not affect inhibition of the hot-plate response induced by any of the opioid agonists. CCK8s (0.2-40 ng) in combination with beta-endorphin (2 micrograms) or morphine (4 micrograms) given i.c.v. did not affect beta-endorphin- or morphine-induced inhibition of the tail-flick and hot-plate responses. CCK8s and its fragments given i.t. attenuated i.c.v. beta-endorphin-induced tail-flick inhibition with different potencies and efficacies. CCK8s was the most potent compound in antagonizing i.c.v. beta-endorphin-induced tail-flick inhibition. The rank order of potencies was CCK8s greater than CCK(27-33) much greater than caerulein. All three compounds were efficacious, whereas CCK(30-33) was not, in antagonizing beta-endorphin-induced tail-flick inhibition. Intrathecal administration of CCK8s (1 ng) significantly attenuated the tail-flick inhibition induced by i.t. beta-endorphin (0.5-1 microgram) and DPDPE (5 micrograms) but not morphine (0.5-1 microgram), DAMGO (5 ng), norepinephrine (5 ng) or serotonin (16 micrograms).(ABSTRACT TRUNCATED AT 250 WORDS)

Characterization of solubilized opioid receptors: reconstitution and uncoupling of guanine nucleotide-sensitive agonist binding

Opioid receptors were solubilized from bovine striatal membranes with the zwitterionic detergent 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate-(CHAPS). High concentrations of NaCl (0.5-1.0 M) were necessary to ensure optimal yields, which ranged from 40 to 50% of membrane-bound receptors. This requirement was found to be specific for sodium, with only lithium able to substitute partially, as previously reported for solubilization with digitonin. Opioid antagonists, but not agonists, were able to bind to soluble receptors with high affinity. High-affinity binding of mu, delta, and kappa agonists was reconstituted following polyethylene glycol precipitation and resuspension of CHAPS extract. Evidence is presented suggesting that this is the result of inclusion of receptors in liposomes. Competition and saturation studies indicate that the three opioid receptor types retain their selectivity and that they exist in the reconstituted CHAPS extract in a ratio (50:15:35) identical to that in the membranes. In reconstituted CHAPS extract, as in membranes, mu-agonist binding was found to be coupled to a guanine nucleotide binding protein (G protein), as demonstrated by the sensitivity of [3H][D-Ala2,N-methyl-Phe4,Gly5-ol]-enkephalin ([3H]DAGO) binding to guanosine 5'-O-(thiotriphosphate) (GTP gamma S). In the reconstituted CHAPS extract, complete and irreversible uncoupling by GTP gamma S was observed, whereas membrane-bound receptors were uncoupled only partially. Treatment with GTP gamma S, at concentrations that uncoupled the mu receptors almost completely, resulted in a fourfold decrease in the Bmax of [3H]DAGO binding with a relatively small change in the KD. Competition experiments showed that the Ki of DAGO against [3H]bremazocine was increased 200-fold.(ABSTRACT TRUNCATED AT 250 WORDS)

Further characterization of alpha N-acetyl beta-endorphin-(1-31) regulatory activity, I: Effect on opioid- and alpha 2-mediated supraspinal antinociception in mice

Picomol doses of the acetylated derivative of beta-endorphin-(1-31), injected intracerebroventricularly (icv) in mice, reduced the analgesic activity of morphine, etorphine and beta-endorphin-(1-31), while the efficiency of DAGO and DADLE in producing analgesia was enhanced. The effects of the delta agonists DPDPE and [D-Ala2]-Deltorphin II were not altered by this treatment. After alpha N-acetyl beta-endorphin-(1-31) injection, morphine antagonized the analgesia of DAGO. The regulatory effect of alpha N-acetyl beta-endorphin-(1-31) was exhibited when giving the peptide both before (up to 24 h) and after the opioids. Naloxone did not prevent or reverse that modulatory activity; moreover, pretreatment with the acetylated peptide did not change the pA2 value displayed by the antagonist at the mu receptor. The antinociceptive activity of the alpha 2-adrenoceptor agonist clonidine was also increased in mice treated with alpha N-acetyl beta-endorphin-(1-31). The reducing activity of alpha N-acetyl beta-endorphin-(1-31) upon morphine- and beta-endorphin-induced analgesia was not exhibited in mice undergoing treatment with pertussis toxin or N-ethylmaleimide, agents known to impair the function of Gi/Go transducer proteins. However, the enhancing activity displayed by this peptide upon DAGO- DADLE and clonidine-evoked antinociception was still manifested. These results confirm and strengthen the idea of alpha N-acetyl beta-endorphin-(1-31) acting as a non-competitive regulator of mu opioid- and alpha 2-adrenoceptor-mediated supraspinal antinociception. A neural substrate acted on by both receptors (likely Gi/Go transducer proteins) appears to be involved in the effects of that neuropeptide.

Lesion of dopamine mesolimbic neurons blocks behavioral effects induced by the endogenous enkephalins but not by a mu-opioid receptor agonist

Lesioning of dopamine neurons by injection of 6-hydroxydopamine into the nucleus accumbens blocked the increased rearing activity measured in the open-field and induced by injection into the ventral tegmental area of: [R)-3-(N-hydroxylcarboxamido-2-benzyl-propanoyl)-L-alanine), kelatorphan (complete inhibitor of enkephalin catabolism) or by (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr(OtBu)): BUBU (selective delta agonist) but not the hypolocomotion evoked by the mu agonist (Tyr-D-Ala-Gly-N(Me)-Phe-Glyol): DAMGO. This suggests the involvement of different neuronal pathways in mu and delta effects.

Selective antagonism by naltrindole of the antinociceptive effects of the delta opioid agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin in the rat

Spinal delta opioid receptors have been proposed to mediate antinociception in the rat on the basis of 1) the efficacy of a small number of agonists; 2) the lack of effect of mu-selective antagonists; and 3) the lack of cross-tolerance with mu-selective agonists. However, direct evidence to support or refute this postulate has not been obtained in the rat due to a lack of suitable delta-selective antagonists. The present study characterized the ability of Naltrindole (NTI, 17-cyclopropylmethyl-6,7-dehydro-4,5 alpha-epoxy-3,14-dihydroxy-6,7-2',3'-indolomorphinan), a recently discovered delta-selective antagonist, to antagonize the antinocieption produced by intrathecal (i.t.) administration of the prototypic delta-selective agonist cyclic[D-penicillamine2-D-penicillamine5]enkephalin (DPDPE) or the mu-selective agonists morphine and [D-Ala2,MePhe4,Gly-ol5] enkephalin (DAMGO) in the rat. Intrathecal coadministration of NTI with DPDPE significantly antagonized the increase in tail-flick latency (TFL) and hot-plate latency (HPL) produced by DPDPE. In the absence of NTI, the ED50 values and 95% CL of DPDPE in the tail-flick and hot-plate tests were 2.8 (1.1-4.7) and 19.5 (13.3-33.7) micrograms, respectively. In the presence of 10 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was unchanged and was increased by 2-fold in the hot-plate test to 35.9 (26.2-60.1) micrograms. In the presence of 30 micrograms of NTI, the ED50 value of DPDPE in the tail-flick test was increased by 5-fold to 14.5 (8.5-24.9) micrograms and its antinociceptive effect in the hot-plate test was antagonized completely.(ABSTRACT TRUNCATED AT 250 WORDS)

[The biochemical antagonism of cholinolytics and cholinomimetics at the level of the opiate system]

The experiments on albino rats with the use of the radioimmunoassay showed that M-cholinoblockers (atropine, amizil, glypine) decrease the contents of enkephalins and beta-endorphin in the brain and blood whereas M-cholinomimetics (arecoline, nicotine, physostigmine) increase the level of opioid neuropeptides. This suggested that between cholinoblockers and cholinomimetics there is not only functional but also biochemical antagonism at the level of the opiate system. In addition, the statement is developed that toxic effects of cholinoblockers and cholinomimetics are largely related to disturbances of metabolism and function of opioid neuropeptides.

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.

Beta-endorphin: a highly selective endogenous opioid agonist for presynaptic mu opioid receptors

In the presence of physiological cations (in Krebs-4-(2-hydroxyethyl)-1- piperazineethanesulfonic acid buffer) at 37 degrees C the Ki value's of beta-endorphin for mu- and delta-opioid receptor binding sites in rat neocortical membranes, labeled with [3H][D-Ala2,MePhe4,Gly- ol5]enkephalin (DAMGO) and [3H][D-Ala2-D-Leu5]enkephalin (in the presence of unlabeled DAMGO), respectively, amounted to about 9 and 22 nM. Surprisingly, a very different selectivity pattern for the endogenous opioid peptide was found when the affinity of beta-endorphin for functional presynaptic opioid receptors was examined. Thus, beta-endorphin strongly inhibited the electrically evoked release of [3H]NE from rat neocortical slices with an IC50 value of about 0.5 nM, whereas [14C] acetylcholine release from neostriatal slices was inhibited with an IC50 value of about 100 nM. On the other hand, the electrically evoked release of [3H]dopamine from striatal slices was not affected by beta-endorphin. The inhibitory effects of DAMGO and beta-endorphin on [3H]NE release from neocortical slices were equally well antagonized by naloxone. Moreover, 10 nM of the highly selective mu-opioid receptor antagonist D-Phe-Cys-Tyr-D-Trp-Arg-Thr-Pen- Thr-NH2 antagonized competitively the inhibitory effect of beta-endorphin on [3H]NE release.(ABSTRACT TRUNCATED AT 250 WORDS)

[Evaluation of the discriminative stimulus effect of an enkephalin analog, EK-399, in the rat]

Four groups of rats were trained to discriminate between the no-drug conditions (saline, s.c.) and the effect of s.c. injection of the novel enkephalin analog Tyr-D-Met (O)-Gly-EtPhe-NHNHCOCH3.AcOH (EK-399, 1 mg/kg), morphine (3 mg/kg), ethylketocyclazocine (EKC, 0.3 mg/kg) or N-allylnormetazocine (NANM, 3 mg/kg) in a two-lever choice, water reinforced procedure. All groups of animals acquired the ability to discriminate EK-399, morphine, EKC or NANM from saline. Naloxone (0.03-0.3 mg/kg, s.c.) completely antagonized the discriminative stimulus effects of EK-399, morphine and EKC, but not that of NANM. In stimulus generalization tests, morphine (10 mg/kg) and buprenorphine (0.03 mg/kg), mu-opioid receptor agonists, completely substituted for EK-399 in groups trained with EK-399, whereas EK-399 (0.1-3 mg/kg) only partially substituted for morphine in rats trained with morphine. EKC (0.01-0.1 mg/kg), a kappa-opioid receptor agonist, partially substituted for EK-399, and EK-399 (0.1-3 mg/kg) partially substituted for EKC. NANM (0.3-10 mg/kg), a sigma-receptor agonist, partially substituted for EK-399, but EK-399 (0.1-3 mg/kg) did not substitute for NANM. These results suggest that the discriminative stimulus effect of EK-399 in rats mainly involves mu-opioid receptor-mediating action and also involves, to a lesser extent, other receptor (probably delta-opioid receptor)-mediating actions.

Differential antagonism of opioid delta antinociception by [D-Ala2,Leu5,Cys6]enkephalin and naltrindole 5'-isothiocyanate: evidence for delta receptor subtypes

The present study has investigated the direct opioid delta receptor-mediated antinociception produced by i.c.v. administration of the highly selective delta agonists, [D-Pen2,D-Pen5]enkephalin (DPDPE) and [D-Ala2]deltorphin II, as well as that of the less delta-selective [D-Ser2,Leu5,Thr6]enkephalin (DSLET), by using two novel nonequilibrium opioid antagonists, [D-Ala2,Leu5,Cys6] enkephalin (DALCE) and naltrindole 5'-isothiocyanate (5'-NTII). At times ranging from 8 to 48 hr after a single i.c.v. pretreatment of mice with 5'-NTII, the antinociceptive effects of [D-Ala2] deltorphin II were significantly antagonized. In contrast, 5'-NTII pretreatment at times between 10 min and 24 hr failed to antagonize the antinociceptive effects of DPDPE. Previous studies have shown that pretreatment with i.c.v. DALCE produces a dose- and time-related antagonism of DPDPE, but not morphine, antinociception. However, pretreatment with i.c.v. DALCE failed to antagonize the antinociceptive effects of [D-Ala2]deltorphin II. Similarly, i.c.v. administration of DSLET produced time- and dose-related antinociception which was partially antagonized by either beta-funaltrexamine (beta-FNA) or by ICI 174,864 (N,N-dialyl-Tyr-Aib-Aib-Phe-Leu-OH), suggesting mixed activity at mu and delta receptors. ICI 174,864 produced essentially complete antagonism of DSLET antinociception in beta-FNA-pretreated mice. Pretreatment with 5'-NTII (at -8 to -48 hr), blocked the antinociception produced by DSLET in control or in beta-FNA-pretreated mice. In contrast, pretreatment with DALCE failed to antagonize the antinociception produced by i.c.v. DSLET in either control or in beta-FNA-pretreated mice.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulation of endogenous calcium and magnesium levels by delta opioid receptors in the rat brain

The effects of the potent agonist (D-Pen2, D-Pen5) enkephalin (DPDPE), a conformationally restricted ligand which is highly selective for delta opioid receptors, were studied on the endogenous levels and regional distributions of selected mono- and divalent cations in rat brain and thoracic spinal cord by means of atomic absorption spectrophotometry. In general, lower Na+, K+, Ca2+, Mg2+ and Mn2+ levels were characteristic of the medulla and spinal cord compared to that of cortices, striatum and hippocampus, while the highest metal ion levels were detected in the olfactory bulb. Sixty minutes after a single dose of 0.2 microgram DPDPE, administered intracerebroventricularly, transient decreases of endogenous Ca2+ and Mg2+ contents in the parietal cortex, hippocampus and striatum were found with no changes observed in the levels of monovalent cations or Mn2+. A time-dependent down-regulation in Ca2+ and Mg2+ content was also demonstrated, with Ca2+ being faster to respond to DPDPE treatment. The action of DPDPE was dose-dependent (0.2-1.0 micrograms) and could be antagonized by a 30 min naloxone pretreatment. Naloxone alone had no effect on the endogenous cation levels. It is concluded that delta opioid receptors may specifically be involved in the regulation of endogenous ion levels and their movements in the central nervous system of rat.

[Cholecystokinin-octapeptide antagonizes the central depressive effect of opioid peptides in rats]

Cholecystokinin-octapeptide (CCK-8) has been shown to antagonize the analgesia produced by opioid peptides. The present study was performed to evaluate its effect on cardiovascular regulatory functions of opioids. Both CCK-8 and opioid peptides were injected intrathecally (ith) in pentobarbital anaesthetized rats. The depressive effects induced by the mu agonist PL017 (5 micrograms), delta agonist DADLE (25 micrograms) and kappa agonist 66A-078 (1 microgram) were antagonized by CCK-8 within a dosage of 10 micrograms in a dose dependent manner. CCK-8 can also partly antagonize the bradycardiac effects induced by PL017, DADLE and 66A-078. The antagonistic effect of CCK-8 on DADLE in MAP could be reversed by pretreatment with CCK receptor antagonist proglumide (100 micrograms). No significant changes in MAP were found following ith administration of CCK-8 0.5-10 micrograms and proglumide 100 micrograms, but a large dose (50 micrograms) of CCK-8 lowered MAP dramatically. The results suggest that within a certain range of dose CCK-8 in spinal cord may play an antagonistic role against opioid effects in the regulation of cardiovascular function and this effect of CCK-8 seems to be mediated by CCK receptor. These results support the hypothesis that CCK-8 may act as an anti-opioid substance in the CNS of the rat.

Interaction between opiate subtype and alpha-2 adrenergic agonists in suppression of noxiously evoked activity of WDR neurons in the spinal dorsal horn

Several studies have demonstrated synergistic antinociception following low-dose administration of morphine and alpha-2 adrenergic agonists at the spinal level. This study was carried out in order to identify the opiate subtypes that are likely to be involved in such synergistic suppression of noxiously evoked activity of wide-dynamic-range (WDR) neurons in the dorsal horn of the spinal cord. We also examined the effect of opiate antagonists and alpha-2 adrenergic antagonists on the suppression produced by opiate or alpha-2 adrenergic agonists. Extracellular activity of single WDR neurons in the spinal dorsal horn, which was evoked by a radiant heat stimulus (51 degrees C), was recorded in decerebrate, spinally transected cats. Agonists were administered spinally and antagonists intravenously. In the synergism study, ineffective doses of the moderately selective mu agonist morphine (25 micrograms), the delta agonist DADL (20 micrograms), and the selective delta agonist DPDPE (30 micrograms), when combined with an ineffective dose of the alpha-2 adrenergic agonist clonidine (5 micrograms) produced significant synergistic suppression of noxiously evoked WDR neuronal activity. However, the ineffective or slightly effective dose of the selective mu agonist DAGO (1 or 1.5 micrograms, respectively) did not show any synergistic action with clonidine. Furthermore, the synergism between morphine and clonidine was reversed by the selective delta antagonist ICI 174,864. We interpret these results to indicate that opiates interact at spinal delta receptors to produce a synergistic suppression of evoked WDR neuronal activity in the presence of spinal clonidine. An alternative explanation is that ICI 174,864 may interact in some way with alpha-adrenergic systems.(ABSTRACT TRUNCATED AT 250 WORDS)

Spinally mediated opioid antidiarrheal effects

To assess the role of opioid receptors in the spinal cord in regulation of functions of the intestinal mucosa in a secretory model, we evaluated the ability of i.t. administered mu (PL017), delta (DPDPE) and kappa (U50,488H) selective opioid agonists to inhibit diarrhea produced in mice by an injection of prostaglandin E2 (PGE2) (200 micrograms/mouse, i.p.). I.t. PL017 and DPDPE inhibited diarrhea in a dose-related fashion. U50,488H had only minimal antidiarrheal effects. The i.t. doses of PL017 and DPDPE required to inhibit diarrhea were higher than the doses required to produce antinociception and inhibit gastrointestinal transit. Spinally administered PL017 and DPDPE were considerably less potent in the diarrhea model than after i.c.v. administration but far more effective than after peripheral (s.c.) dosing. The antidiarrheal effects of spinally administered opioids were antagonized by concurrently administered naloxone. These data indicate that opioid chemosensitive sites in the spinal cord can modulate diarrhea produced by PGE2, and that the receptor specific opioids, PL017 and DPDPE, and to a lesser extent U50,488H, all possess antidiarrheal activity when administered i.t.

Analgesic responses elicited by endogenous enkephalins (protected by mixed peptidase inhibitors) in a variety of morphine-sensitive noxious tests

It has been suggested that the endogenous opioid peptides, methionine and leucine enkephalin, participate only in naloxone-facilitated antinociceptive responses. To reassess this proposal, analgesic effects resulting from complete inhibition of enkephalin metabolism by intracerebroventricular (i.c.v.) administration of the mixed inhibitor RB 38A (R,S)HONHCOCH2CH(CH2 phi)CONHCH(CH2 phi)COOH) were compared to the effects of morphine (i.c.v.) in various assays commonly used to select analgesics: mouse hot plate-test, tail flick test with mice and rats, electrical stimulation of the tail (TES), paw pressure test with rats, and phenylbenzoquinone-induced writhing test with mice. The ED50s of morphine vs. ED50s of RB 38A in the writhing, hot plate (jumping) and tail flick tests with mice were 0.24 nmol vs. 38 nmol, 1 nmol vs. 36 nmol and 3.2 nmol vs. 285 nmol, respectively. RB 38A (ED30 153 nmol) was only 15 times less active in the tail flick test with rats than morphine and only halve as active in the paw pressure test. Noxious TES in rat was very sensitive to the inhibitory action of endogenous opioids protected by RB 38A, particularly the post-vocalization response which was also shown to be alleviated by antidepressants. All the analgesic effects observed were reversed by naloxone. This first direct evidence of analgesia resulting from peptidase inhibition, in the tail flick test with mice and rats, hot plate (paw lick) and TES shows that the pain suppressive effects of endogenous opioid peptides are not restricted to naloxone-facilitated noxious stimuli but occur more generally, in all morphine-sensitive tests. The differential effects of RB 38A in the various assays is likely to be related to the amount of enkephalins released and to the efficiency of peptidase inactivation in particular brain regions implicated in the control of a given nociceptive input. This mechanism could account for the reduction in side-effects compared to those of morphine following chronic administration of RB 38A.

Delta opioid antagonist, naltrindole, selectively blocks analgesia induced by DPDPE but not DAGO or morphine

Initial reports suggest that naltrindole hydrochloride (NTI), a recently developed opioid, acts as a selective delta (delta) antagonist in vivo. Three experiments were conducted in rats to test NTI for its ability to dose-dependently and selectively block the analgesia produced by a delta-selective opioid agonist without affecting analgesia produced by mu (mu) receptor opioid agonists. Intracerebroventricular (ICV) administration of the delta-selective agonist, DPDPE (30 micrograms/rat), and the mu-selective agonist, DAGO (0.3 micrograms/rat), increased paw-lick latency (2-fold relative to baseline) in the hot-plate assay. NTI (0.01-1.0 micrograms/rat, ICV) dose-dependently attenuated DPDPE-induced analgesia (1.0 micrograms reduced paw-lick latency to baseline), but failed to affect DAGO-induced analgesia at any dose tested. A third experiment determined whether the ICV administration of NTI (1.0 micrograms/rat) would attenuate restraint stress-induced potentiation of morphine analgesia as indexed by the tail-flick assay. Rats that underwent 5 days of 1 h restraint stress and nonstressed rats were injected subcutaneously with morphine (1.0-8.0 mg/kg). The magnitude (greater than 2-fold) and duration of morphine-induced analgesia in restrained rats were significantly potentiated compared to nonstressed rats. NTI (1 microgram, ICV) failed to affect the magnitude and duration of morphine-induced analgesia regardless of restraint treatment. Thus, NTI failed to attenuate the analgesia produced by DAGO or morphine (in two assays of antinociception), whereas NTI (0.01-1.0 micrograms, ICV) antagonized dose-dependently DPDPE-induced analgesia. These results support the view that NTI is a selective delta-receptor antagonist in vivo.

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).

Enkephalins modulate inhibitory neuromuscular transmission in circular muscle of human colon via delta-opioid receptors

1. A sucrose-gap technique was used to investigate the neuromodulatory actions of enkephalins on non-adrenergic, non-cholinergic inhibitory junction potentials (IJPs) in the circular muscle of the human large intestine. 2. The native enkephalins, [Leu5]enkephalin (LENK) and [Met5]enkephalin (MENK) caused a concentration-dependent reduction in amplitude of IJPs without a significant effect on the smooth muscle membrane. 3. The actions of LENK and MENK were mimicked by the delta-selective opioid receptor agonists [D-Pen2, D-Pen5]enkephalin (DPDPE) and [D-Ala2, D-Leu5]enkephalin (DADLE). 4. The actions of LENK, MENK and DPDPE were antagonized to similar extents by the delta-selective opioid receptor antagonist ICI 174,864. 5. The mu-selective opioid receptor agonist [D-Ala2, Me Phe, Gly-ol5]enkephalin was approximately 100-fold less potent than any of the native or synthetic enkephalins at reducing the amplitude of the IJP. Dynorphin A and beta-endorphin both had very weak activity. 6. Responses to all of the agonists were inhibited by naloxone. The degree of antagonism of DPDPE or DADLE by naloxone (1 microM) was the same as that of LENK or MENK. 7. Neither MENK nor LENK affected hyperpolarization of the smooth muscle membrane induced by ATP or 5-hydroxytryptamine. Vasoactive intestinal polypeptide (1 pM-1 microM) did not produce any observable responses and this lack of reactivity was not affected by the enkephalins. 8. It is concluded that in the circular muscle of the human colon, LENK and MENK can act on prejunctional delta-opioid receptors to produce inhibition of non-adrenergic, non-cholinergic inhibitory neuromuscular transmission. Possible physiological significance of this prejunctional receptor is discussed.

Evidence for delta-opioid binding and GTP-regulatory proteins in 5-day-old rat brain membranes

The availability of the bispenicillamine enkephalin [3H] [D-Pen2,D-Pen5]enkephalin ([3H]DPDPE) a highly selective ligand for delta-opioid receptors, has made possible a more definitive examination of the ontogeny of this receptor subtype. In this report, the binding characteristics of [3H]DPDPE in 5-day-old neonatal (P-5) and adult rat brain are compared. Analysis of saturation curves as well as homologous displacement data revealed no significant difference in the binding affinity of [3H]DPDPE between P-5 animals and adults. Conversely, the binding capacity increased fivefold during this period. The delta-specificity of the sites was further proven by competition experiments with mu- and delta-selective ligands. Mn2+ (0.5 mM) elevated [3H]DPDPE specific binding by lowering the Kd, whereas 50 microM 5'-guanylylimidodiphosphate inhibited it by decreasing the total number of high-affinity binding sites in both P-5 animals and adults. Pertussis toxin-catalyzed ADP ribosylation experiments revealed the presence of 40-kDa proteins, with a molecular mass corresponding to G protein subunits alpha i/alpha o, as early as 1 h after birth. There was a low, but detectable, basal low-Km GTPase activity in P-5 animals, which increased fivefold during postnatal development. The present report establishes the existence of high-affinity [3H]DPDPE binding as well as GTP-regulatory proteins 5 days after birth. Yet, heterologous competition studies and ionic effects suggest that neonatal binding sites differ from adult receptors. Whether the neonatal sites are newly synthesized, incompletely processed sites or a developmentally programmed isoform remains to be determined.

Antinociceptive activity of glycosidic enkephalin analogues

The antinociceptive activity of two new enkephalin analogues: N1.5-(beta-D-glucopyranosyl)[D-Met2, Pro5]enkephalinamide and N1.5-(beta-D-galactopyranosyl)[D-Met2, Pro5]enkephalinamide was assessed using the tail immersion and paw pressure behavioural tests. Both enkephalin analogues appear to be more active than morphine when injected either into the fourth ventricle or intrathecally; the galactose analogue is more than 5000 times more active than morphine when injected into the fourth ventricle. The analgesic effects produced by the analogues are partially reversed by SC naloxone (0.1 mg/kg) and totally reversed when the dose of naloxone used was 1 mg/kg, suggesting that the analogues act upon more than one type of opiate receptor (mu/delta).

Antinociceptive effects of [D-Ala2]deltorphin II, a highly selective delta agonist in vivo

The present study has characterized the antinociceptive actions of [D-Ala2]deltorphin II following intracerebroventricular (i.c.v.) administration in the mouse tail-flick test. [D-Ala2]deltorphin II produced dose- and time-related antinociception, with maximal effects at +10 min and significant antinociception which lasted for 40-60 min. [D-Ala2]deltorphin II was 13-fold more potent than i.c.v. [D-Pen2, D-Pen5]enkephalin (DPDPE), a second highly selective delta agonist, and approximately equipotent with i.c.v. morphine in producing antinociception. The antinociceptive effects of i.c.v. [D-Ala2]deltorphin II and DPDPE, but not those of morphine, were antagonized by the selective delta antagonist, ICI 174,864. In contrast, pretreatment with the non-equilibrium mu antagonist, beta-funaltrexamine blocked morphine antinociception, but failed to antagonize [D-Ala2]deltorphin II and DPDPE antinociception. These data indicate that [D-Ala2]deltorphin II produced its antinociceptive effects at a supraspinal delta receptor. [D-Ala2]deltorphin II appears to be the most appropriate delta opioid agonist currently available for studies in vivo and support the involvement of delta receptors in supraspinal antinociception.

Interaction of S-activated enkephalin analogs with opiate receptors

Enkephalin analogs containing a thiol activated by a thiomethyl (SCH3)*** or 3-nitro-2-pyridinesulfenyl (Npys) group were synthesized. Incubation of such S-activated enkephalin analogs as [D-Ala2, Leu(CH2S)SCH(3)5]enkephalin or [D-Ala2,Leu(CH2S)Npys5]enkephalin with guinea pig ileum (GPI) resulted in the continuous stimulation of the mu opiate receptors. This sustained GPI-activity was completely reversed with the antagonist naloxone, while subsequent washings elicited again the full enkephalin activity. When GPI showing full enkephalin activity was incubated with 1 mM dithiothreitol, about 70% of the activity was eliminated. Examination of enkephalin analogs containing Cys(Npys) at position 1, 5, or 6 suggested that no other thiols occur near the enkephalin binding site of the mu receptor. From these results, it is considered that only one thiol group exists near the binding site of the mu receptor in GPI. Similar results were also obtained for the mu receptors in mouse vas deferens.

Multi-dimensional analysis of behavior in mice treated with the delta opioid agonists DADL (D-Ala2-D-Leu5-enkephalin) and DPLPE (D-Pen2-L-Pen5-enkephalin)

The effects of intracerebroventricular injection of the delta-selective opioid peptides, DADL (D-Ala2-D-Leu5-enkephalin) and DPLPE (D-Pen2-L-Pen5-enkephalin), on spontaneous locomotor activity were investigated in mice using multi-dimensional behavioral analysis, based upon a capacitance system. The analysers classified the movements into 9 sizes (1/1, 1/2, 1/4, 1/8, 1/16, 1/32, 1/64, 1/128 and 1/256). Specific patterns of behavior were each registered on these sizes of movement. At 1.0 and 3.0 micrograms, DADL produced a significant increase in circling (1/4 size of movements) within 15 min after the start of measurements, while it produced a marked increase in linear locomotion (1/2 size), circling (1/4 size), rearing (1/16 size) and grooming (1/32, 1/64 and 1/128 sizes) within 15-30 min after the start. At 10.0 micrograms, DPLPE decreased linear locomotion (1/1 size) and conversely increased circling behavior (1/4 size) within 15 min after the start, whilst this peptide at 3.0 or 10.0 micrograms, produced a marked increase in linear locomotion (1/2 size), circling (1/4 size) and grooming (1/128 size) within 15-30 min after the start. The behavioral effects induced by DADL (3.0 micrograms) and DPLPE (10.0 micrograms) were completely reversed by naloxone (1.0 and 2.0 mg/kg). These results obtained with DPLPE, a delta-selective peptide and DADL, a less delta-selective peptide, indicate a common pattern of activity which was presumably delta receptor-mediated. However, one component (linear locomotion, at times immediately after administration of the peptide) did clearly differ between these two peptide analogues.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity for opioid receptor subtypes of enkephalin analogues in isolated smooth muscle and in the analgesic effect in mice

Selectivity for opioid receptor subtypes of enkephalin analogues (KK-1, -2, -3 and -4) of Tyr moiety on the N-terminal, and Phe-ol group on the C-terminal, connected with the methylene group (n = 1-4) were examined in isolated smooth muscle preparations and in the analgesic effect in mice. In the longitudinal muscle preparations of guinea pig ileum (GPI), morphine, U-50488H and all the enkephalin analogues inhibited electrically evoked contractions, and the inhibitory effects of morphine, KK-1, KK-2 and KK-3 were antagonized by naloxone with relatively high pA2 values, while that of U-50488H and KK-3 were preferentially antagonized by norbinaltorphimine. In the rabbit vas deferens preparations (RVD), on the other hand, U-50488H, KK-3 and KK-4 showed weak inhibitory effects and the inhibition of U-50488H and KK-3 were antagonized by norbinaltorphimine. By intracerebroventricularly (i.c.v.) injection, enkephalin analogues produced analgesia in the acetic acid (AcOH) writhing test, and the effect of KK-1 and KK-2 as well as morphine was antagonized by 1 mg/kg naloxone, while those of U-50488H and KK-3 were sensitive to 1 mg/kg Mr2266. In conclusion, enkephalin analogues with a short methylene chain between the functional groups, KK-1 and KK-2, mainly exert their effect through opioid mu-receptors, while those of longer chain, KK-3 and KK-4, act through kappa-receptors preferentially, and KK-3 is situated in the alternating point of the selectivity for mu- and kappa-receptors.

Blockade of enkephalinergic and GABAergic mediated locomotion in the nucleus accumbens by muscimol in the ventral pallidum

The mu-opioid agonist, DAGO, and the indirect GABAA antagonist, picrotoxin, produced a dose-dependent increase in horizontal motor activity following injection into the nucleus accumbens that was independent of dopamine release. Injection of muscimol into the ventral pallidum antagonizes the motor stimulant effect of dopamine agonists. It was shown that muscimol abolished the locomotor stimulant effect of DAGO microinjection into the nucleus accumbens, and partially antagonized the effect of picrotoxin.

In vivo release of CCK-8 from the dorsal horn of the rat: inhibition by DAGOL

There is evidence that CCK-8 may interact with opioids and that both systems are probably implicated in pain modulation. In order to elucidate this relationship we sought to examine factors governing the movement of CCK-8 from the spinal cord into the extracellular space. We report that CCK-8 like immunoreactivity, as measured by RIA, is released from the spinal cord of the rat in vivo, following potassium stimulation and by direct activation of high threshold peripheral afferents by stimulation of the sciatic nerve. Also, we show that CCK-8 release is inhibited by the mu-selective opioid receptor agonist DAGOL. Naloxone totally reversed the effect produced by DAGOL, implying an opiate mediated mechanism.

New kelatorphan-related inhibitors of enkephalin metabolism: improved antinociceptive properties

In order to improve the in vivo protection of enkephalins from enzymatic degradation, a new series of inhibitors derived from kelatorphan [HONHCOCH2CH(CH2Ph)CONHCH(CH3)COOH], the first-described complete inhibitor of enkephalin metabolism, were designed by modification of the C-terminal amino acid. The progressive lengthening of the chain of this residue shows that a beta-alanine seems to be the best basic model for the conception of such types of compounds. On the other hand, the methylation of the amide bond, which is well accepted by aminopeptidase N (EC 3.4.11.2) and dipeptidylaminopeptidase, induced a significant loss of affinity for neutral endopeptidase -24.11. Starting from these data, compounds containing a variously substituted beta-alanine residue and corresponding to the general formula HONHCOCH2CH(CH2Ph)CONHCH(R1)CH(R2)COOH were synthesized. All these molecules inhibit neutral endopeptidase -24.11 and dipeptidylaminopeptidase in the nanomolar range, and those containing an aromatic chain (compound 7A, R1 = CH2Ph,R2 = H, and compound 8A, R1 = Ph, R2 = H) inhibit the biologically relevant aminopeptidase N, with IC50's around 10(-8) M. Intracerebroventricular injection in mice of these multienzyme inhibitors produced an efficient and naloxone-reversible analgesic response (hot plate test): compounds 7A and 8A were shown to be more potent than kelatorphan in increasing the jump latency time, in agreement with their in vitro properties, and these new compounds were found to increase the forepaw lick latency, a reflex considered as a typical morphine response.

Delta opioid antagonist, 16-Me cyprenorphine, selectively attenuates conditional fear- and DPDPE-induced analgesia on the formalin test

The effects of 16-Me cyprenorphine (M80) on the antinociception produced by reexposing rats to a chamber associated with footshock (1 mA, 0.75 sec) 24 hr earlier was assessed with the formalin test. In Experiment 1, intracerebroventricular administration of M80 dose-dependently (0.5-8 micrograms) reversed conditional analgesia. Experiment 2 demonstrated that M80 (5 micrograms) had no effect on baseline pain sensitivity, but completely reversed conditional analgesia. Experiment 3 demonstrated that 0.25 micrograms DAGO, 3.5 micrograms DPDPE, and 28 micrograms U50,488H all produced equivalent levels of antinociception on the formalin test. The 5 micrograms dose of M80 completely reversed the antinociception produced by DPDPE but did not influence that produced by DAGO or U50,488H. These data suggest, that at the doses employed, M80 is a selective delta-opioid receptor antagonist and that delta-receptors are involved in conditional fear-induced analgesia.

Enkephalin-degrading enzyme inhibitors. Crucial role of the C-terminal residue on the inhibitory potencies of retro-hydroxamate dipeptides

The retro-inversion of the amide bond in kelatorphan and analogs, the first series of complete inhibitors of enkephalin metabolism, led to compounds highly efficient only against the neutral endopeptidase 24-11 (NEP). In order to increase the recognition of the aminopeptidase N (APN) and dipeptidylaminopeptidase (DAP), without loss of affinity for NEP, the malonyl group of these retro-inhibitors was replaced by diversely substituted succinyl moieties. All the molecules synthesized are highly efficient NEP inhibitors with Ki's in the 0.2-1 nM range, indicating that NEP possesses a relatively large and not very selective S'2 subsite. In contrast, inhibition of DAP activity is crucially dependent on the size and the position of the substituent in the succinyl moiety. Inhibitory potencies in the nanomolar range are obtained with compounds containing a benzyl group in the alpha-position related to the retro amide bond. Finally, a relatively modest inhibition of APN was observed with Ki's in the 0.5-1 microM range for compounds with benzyl or cyclohexyl group in P'2 position. However, these data demonstrate that efficient and complete inhibition of enkephalin degradation can be obtained with hydroxamate dipeptides containing a retro amide bond. The analgesic potency of the most active inhibitors was measured using the hot plate test in mice. Significant antinociceptive responses were obtained but these effects were rather weaker than those expected from the in vitro inhibitory potencies of these compounds on the three enkephalin-degrading enzymes.

Blockade of dopamine receptors reverses the behavioral effects of endogenous enkephalins in the Nucleus caudatus but not in the Nucleus accumbens: differential involvement of delta and mu opioid receptors

We have previously (Daugé et al. 1988) demonstrated that injection of the mu agonist [D-Ala2, MePhe4, Gly-ol5]-enkephalin (DAGO) or the delta agonist [D-Thr2, Leu5]-enkephalyl-Thr6 (DTLET) into the rat Nucleus accumbens (N.Acc.), or Nucleus caudatus (N.Caud.) induced a hypoactivity followed by hyperactivity 150 min later in the case of the mu agonist and a hyperactivity in the case of the delta agonist. Moreover, naloxone reversible delta-type responses were obtained by local infusion of kelatorphan, ([(R)-3(N-hydroxylcarboxamido-2-benzylpropanoyl)-L-alanine]), a complete inhibitor of enkephalin catabolism, suggesting a tonic control of the behavioral activity of rat by the endogenous opioid peptides. In this work, the putative involvement of the dopaminergic system in these behavioral responses was investigated by using the DA antagonist thioproperazine. In the N.Acc., the behavioral effects of kelatorphan or of mu or delta agonists were not altered by thioproperazine-induced blockade of dopamine receptors. In contrast, the hyperactivity produced by DTLET or by kelatorphan in the N.Caud. was reversed by thioproperazine while the time-dependent biphasic effect resulting from DAGO injection remained unaffected by the DA antagonist. This blocking effect of thioproperazine is in agreement with the previously described delta-selective enhancement of the release of newly synthesized DA in the striatum but not in the N.Acc.

Mu but not delta opioid receptor stimulation intensifies kainic acid-induced neurotoxicity in rat hippocampus

In the present study, we compared the effects of the selective mu agonist, [D-Ala2-N-methyl-pHe4-Gly-ol]-enkephalon (DAGO), and the selective delta agonist, [D-Pen2,5]-enkephalin (DPDPE), on kainic acid-induced neurotoxicity in rats. Infusion of kainic acid (0.5 ug/1.5 ul, ic.v.) alone caused pyramidal cell loss predominantly in hippocampal field CA3 with minimal involvement of the CA1 field. Coadministration of DAGO plus kainic acid into the lateral ventricle intensified the extent of degeneration of hippocampal pyramidal cells in the CA1 field. The potentiating effect of DAGO was completely blocked by naltrexone. In contrast, DPDPE had no significant effect on kainic acid-induced neurotoxicity. Thus, activation of mu but not delta receptors intensifies the neurotoxic effects of kainic acid in the hippocampus.

Study on the receptor subtypes mediating the analgesic action of an enkephalin analog, Tyr-D-Met(O)-Gly-EtPhe-NHNHCOCH3.AcOH (EK-399)

The analgesic action of the enkephalin analog EK-399 (Tyr-D-Met(O)-Gly-EtPhe-NHNHCOCH3.AcOH) and the subtypes of the opiate receptors mediating the action were studied. The analgesic effect of subcutaneously injected EK-399 was ten times as potent as that of morphine in the rat tail flick test. EK-399 had a longer latency time and duration time than morphine. The analgesic action of EK-399 injected into the rat spinal subarachnoid space was about 800 (1800 in molar ratio) times as potent as that of morphine in the hot plate test. EK-399 had high affinities for both mu and delta opiate receptors in the rat brain receptor binding assay. The apparent pA2 values with naloxone were 7.65 for morphine and 5.98 for EK-399 in the rat tail flick test; the difference was significant. A cross tolerance between EK-399 and morphine was examined in the rat tail flick test. Although morphine tolerant rats showed no tolerance to EK-399, EK-399 tolerant rats showed a clear tolerance to morphine. These results indicate that EK-399 has a potent and long lasting analgesic effect via opiate receptors in rats. In addition to mu-receptors, delta-receptors may be involved in its analgesic mechanism.

Naloxone increases mucosal fluid secretion in the inflamed and distended feline gall bladder: evidence for a possible protective mechanism by endogenous opioids

1. Endogenous opioid peptides are found in the enteric nervous system in the gastrointestinal tract. The opioid peptide enkephalin, which has anti-secretory action in the small intestine, is also contained in nerves in the gall-bladder wall. 2. In experimental cholecystitis, there is active fluid secretion by the epithelial cells into the gall-bladder lumen, when the intraluminal hydrostatic pressure is low. This fluid secretion to the lumen was abolished by intravenous administration of enkephalin, an effect that was blocked by naloxone pretreatment. The flux of fluid into the lumen was also abolished when the intraluminal hydrostatic pressure was raised to the level initially observed in the inflamed and obstructed gall bladder. Fluid absorption in the normal gall bladder was unaffected by enkephalin. 3. In experimental cholecystitis, naloxone, used as a non-specific antagonist of opiate action, did not affect the gall-bladder mucosal fluid transport observed at a low intraluminal hydrostatic pressure, but it induced fluid secretion when this pressure was high. 4. It is suggested that a raised intraluminal hydrostatic pressure in experimental cholecystitis, which distends the gall bladder, releases endogenous opioids that inhibit active fluid secretion by the gall-bladder epithelial cells. This response may represent a defence mechanism that could be present also in the gastrointestinal tract. In the obstructed and inflamed gall bladder it may prevent progressive distension, ischaemia and perforation of the wall.

Mu opioid antagonist properties of a cyclic somatostatin octapeptide in vivo: identification of mu receptor-related functions

We have shown previously that D-Phe-Cys-Tyr-D-Trp-Lys-Thr-Pen-Thr-NH2 (CTP) produces selective antagonism of mu, but not delta or kappa, opioid receptor-selective ligands in the guinea pig ileum and mouse vas deferens bioassays, and in radioligand binding assays using homogenized rat brains. In the present study we characterized the agonist and opioid antagonist profile of CTP in analgesic (hot-plate test, abdominal stretch test) and in gastrointestinal assays (transit time test) in mice. CTP was a potent antagonist of the supraspinal and spinal analgesic effects of the mu selective agonist [MePhe3, D-Pro4]morphiceptin (PL017) in both assays. The gastrointestinal antitransit actions of PL017 were also antagonized by CTP at both supraspinal and spinal sites. CTP did not alter the effects of the kappa agonist trans-3,4-dichloro-N-methyl-N-(2-(1-pyrolidinyl)cyclohexyl)benz eneacetamine in any test. Surprisingly, CTP also antagonized the analgesia produced by i.c.v. and intrathecal administration of [D-Pen2, D-Pen5]enkephalin (DPDPE), a highly delta selective agonist, in both analgesic tests. Differential antagonism of DPDPE, but not PL017, by the delta selective antagonist N,N-diallyl-Tyr-Aib-Aib-Phe-Leu-OH in the hot-plate test indicates that PL017 and DPDPE may act at separate receptors to produce analgesia (mu and delta, respectively). In contrast, CTP did not reverse the gastrointestinal antitransit effects of intrathecal DPDPE. Schild analysis of the interactions of CTP with supraspinal mu and delta agonists in the hot-plate test indicated that although CTP antagonized PL017 in a competitive fashion (Schild slope = -1.0), the interaction of CTP with DPDPE was not competitive (Schild slope = -0.5).(ABSTRACT TRUNCATED AT 250 WORDS)

Naloxone efficacy in bladder rehabilitation of spinal cord injury patients

Naloxone has been reported to facilitate detrusor activity by blocking the inhibitory action of the enkephalins. We tested the urodynamic effects of intravenous naloxone (0.4 to 0.8 mg.) in 15 patients with spinal cord injury via rapid fill carbon dioxide cystometry and perineal electromyography. Fourteen patients had suprasacral lesions (3 complete), while 1 had an incomplete lower cord lesion. The average interval from injury was 4.2 months. Most of the patients were managed with intermittent catheterization and most had chronic bacteriuria. Initially, 10 patients had detrusor areflexia and 5 had detrusor hyperreflexia (2 with dyssynergia). Of the 15 patients 14 had no neuropathic condition as noted on baseline electromyography. After naloxone injection no significant cystometrographic changes occurred but 11 patients (73 per cent) had decreased electromyographic activity. The vesicourethral pattern of the 2 patients with dyssynergia did not change after naloxone, even though the electromyographic response decreased in both. The patients showed no other effects of the naloxone administration. Contrary to other studies, naloxone seemed to diminish pelvic floor (electromyographic) activity but it did not change bladder (cystometrographic) function during bladder rehabilitation of spinal cord injury patients.