Differentiation of opiate receptors in the brain by the selective development of tolerance

Acquisition of a morris water maze task is impaired during early but not late withdrawal from morphine

Behavioral changes in male Sprague-Dawley rats during early and late withdrawal from morphine were investigated. Morphine-treated subjects (M) were implanted (SC) with osmotic pumps containing 2.0 ml of a 159 mg/ml morphine sulfate solution while control subjects (C) received sham implants. Implants were removed after 7 days. M subjects exhibited a significant decrease in body weight during withdrawal that recovered by 21 days after pump removal. Beginning 1 or 21 days following pump removal, subjects were tested for 8 days in a Morris water maze (MWM) task. M subjects trained in the MWM during early withdrawal exhibited significantly longer escape latencies than C subjects. However, during sequential probe trials, the same subjects exhibited a significant preference for the target quadrant of the maze and executed accurate searches for the escape platform. Though these subjects failed to locate the platform as efficiently as controls during training trials, they learned the location of the escape platform. M rats trained during late withdrawal exhibited no deficits in any measure of MWM performance relative to C subjects. The data suggest that a variety of processes involved in the acquisition and performance of the MWM task are differentially affected during early withdrawal from morphine.

Autoradiographic distribution of receptors to FLFQPQRFamide, a morphine-modulating peptide, in rat central nervous system

The neuropeptide FLFQPQRFamide is a structure related to FMRFamide which is able to inhibit the effects of both endogenous and exogenous opiates. This morphine-modulating activity is mediated via the stimulation of specific FLFQPQRFamide receptors, different from opiate receptors. In vitro quantitative receptor autoradiography was performed on frozen sections of rat central nervous system to characterize binding properties and visualize FLFQPQRFamide receptors using the specific ligand [125I]YLFQPQRFamide, a radio-iodinated analogue of FLFQPQRFamide. [125I]YLFQPQRFamide appeared to interact reversibly with a single class of binding sites (KD = 0.2 nM). The specific binding represented 80% of the total binding at 0.05 nM, the FLFQPQRFamide concentration used in this mapping study. Sites labelled with [125I]YLFQPQRFamide were distributed heterogeneously within the brain and spinal cord. A high density of FLFQPQRFamide binding sites was detected in the most external layers of the dorsal horn of spinal cord and various nuclei of pons and medulla including trigeminal, dorsal tegmental and reticular nuclei. Nucleus of solitary tract, parabrachial, ambiguous and facial nuclei are also intensively labelled. Some structures of mesencephalon and diencephalon exhibited a high density of FLFQPQRFamide binding sites: central gray, raphe nuclei and thalamic nuclei such as parafascicular, laterodorsal, central median, paratenial and paraventricular nuclei. Suprachiasmatic and mammillary nuclei, lateral, posterior and anterior areas of hypothalamus and medial preoptic area exhibited high labelling. FLFQPQRFamide binding sites were also seen in some structures of the dopaminergic meso-cortico-limbic system including ventral tegmental area, cingulate cortex, lateral septum and the head of the caudate-putamen. Dense labelling appeared in the presubiculum of hippocampus. The dissimilar mapping of FLFQPQRFamide and opiate brain receptors confirms our previous pharmacological findings in FLFQPQRFamide binding studies on rat spinal cord membranes, showing that FLFQPQRFamide receptors are different from opiate receptors. There was a good correspondence between localization of binding sites and that of the putative endogenous peptide. Both occur in brain areas previously associated with analgesic action of opiates. However, the mapping of FLFQPQRFamide receptors in the central nervous system suggests that the FLFQPQRFamide system could be implicated in other physiological functions.

Effects of receptor-selective opioids on operant behavior in morphine-treated and untreated rats

Selective cross-tolerance is often used as evidence to differentiate opioid receptor subtypes. We used this strategy to study operant behavioral effects of the opioid peptides, [D-Ala2, NMePhe4, Gly-ol5]enkephalin (DAGO), [D-Pen2.5]-enkephalin (DPDPE) and dynorphin, agonists highly selective for mu, delta, and kappa receptors, respectively. Food-deprived rats were trained to lever-press on a fixed-interval 3-min schedule of food-reinforcement. Time-effect and dose-effect curves were generated for each of the peptides, as well as for morphine, administered ICV, 5 min prior to the 1-h operant session. Experiments were performed on untreated subjects and on subjects receiving chronic infusion of morphine (10 mg/kg/day) from osmotic pumps. In untreated animals, morphine and the mu-selective peptide, DAGO, induced relatively long-lasting dose-related decreases in responding, whereas the non-mu agonists, DPDPE and dynorphin, induced only transient effects: response rates increased at low doses and decreased at high doses. Animals receiving chronic morphine infusion were tolerant to the rate-decreasing effects of morphine and DAGO; ED50s increased by factors of 8 and 6, respectively. There was some evidence of cross-tolerance to DPDPE and of sensitization to dynorphin in the morphine-maintained animals.

Chronic opioids impair acquisition of both radial maze and Y-maze choice escape

Chronic morphine impaired acquisition of two dissimilar behavioral tasks. In the radial maze, the performance of saline-treated and morphine-treated groups diverged with the latter failing to improve despite extensive training. In contrast, rats treated with naltrexone became skilled in the procedure 2-4 times as rapidly as saline controls. Withdrawal of treatment significantly improved performance of morphine-treated rats, with no change for rats treated with saline or naltrexone. When a second group of rats was extensively trained prior to instituting chronic morphine treatment, performance scores were not affected, suggesting that morphine does not impair spatial working memory despite subjective evidence of other gross behavioral effects, such as ataxia. In the Y-maze choice escape task, acquisition of a response strategy was significantly impaired in rats that had been previously treated with morphine for 17-21 days, despite clear indications that morphine-treated rats were sensitive to the aversive stimulus.

Morphine-ethanol interaction on body temperature

1. The interaction between ethanol and morphine on core temperature was investigated in Swiss Webster mice. 2. Morphine (2.5-30 mg/kg) and ethanol (0.5-3.0 mg/g) administered individually resulted in a dose dependent decrease in body temperature. 3. When both drugs were injected simultaneously, body temperature decreased less than it would be expected to if the effects were additive. 4. Naloxone antagonized the hypothermic effect of morphine, but the hypothermic effect of ethanol and that of the combination of morphine plus ethanol was only reversed with high doses of naloxone (10 mg/kg). 5. Individual morphine and ethanol plasma levels were not significantly altered by their concomitant administration. 6. Binding of tritiated naloxone to opiate receptors in mouse brain membrane preparations was unchanged by pretreatment with ethanol (0.5 and 2 mg/g). 7. The interaction between morphine and ethanol was found to be less than additive and not related to pharmacokinetic changes of either drug.

Differential cross-tolerance to mu and kappa opioid agonists in morphine-tolerant rats responding under a schedule of food presentation

If different populations of opioid receptors mediate the actions of mu and kappa opioid agonists, then tolerance induced by the chronic administration of a mu agonist should confer cross-tolerance to other mu agonists but not necessarily to those compounds whose effects are mediated by the kappa receptor. This hypothesis was evaluated in the present investigation by examining the effects of the mu agonists morphine, l-methadone and fentanyl, the kappa agonists U50,488 and bremazocine, and the mixed kappa/mu agonist ethylketocyclazocine in rats responding under a fixed-ratio 30 schedule of food presentation before, during and after exposure to a regimen of chronic morphine administration. For comparison, naloxone was evaluated as a representative mu antagonist and the phenothiazine chlorpromazine as a control drug. During all phases of the experiment, each of these compounds produced dose-related decreases in rate of responding. During the daily administration of 40 mg/kg morphine, tolerance developed to the rate-decreasing effects of morphine, l-methadone and fentanyl, and an enhanced sensitivity to the effects of naloxone. In contrast to the effects obtained with these mu opioids, there was no evidence that chronic morphine administration produced tolerance or enhanced sensitivity to the rate-decreasing effects of U50,488, bremazocine, ethylketocyclazocine and chlorpromazine. The present findings demonstrate that the chronic administration of morphine results in the selective development of tolerance to other mu agonists.(ABSTRACT TRUNCATED AT 250 WORDS)

Differences in physical dependence induced by selective mu or delta opioid agonists and by endogenous enkephalins protected by peptidase inhibitors

The aim of the present study was to investigate if a physical dependence could be induced by chronic activation of the endogenous enkephalinergic system. We have therefore evaluated naloxone-induced withdrawal syndrome in rats after central infusion during 7 days of comparable antinociceptive doses of RB 38 A ((R,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a mixed enkephalin catabolism blocker and of the selective mu, DAGO (Tyr-D-Ala-Gly-(Me)Phe-Gly-ol) and delta, DSTBULET (Tyr-D-Ser(OtBu)-Gly-Phe-Leu-Thr), opioid agonists. The responses were compared to those induced by RB 38 B ((S,S)HONH-CO-CH2-CH(CH2C6H5)-CONH-CH(CH2C6H5)-COOH), a selective inhibitor of the 24.11 neutral endopeptidase (NEP) 'enkephalinase'. DAGO induced a severe withdrawal syndrome evidenced by a large weight loss, hypothermia, jumping, mastication, teeth chattering, diarrhoea, lacrimation and salivation. In contrast, DSTBULET and RB 38 A produced only a moderate physical dependence. Only two signs were statistically different in these two groups: wet dog shakes and temperature. Chronic i.c.v. administration of DAGO, DSTBULET and RB 38 A produced a time-dependent reduction in analgesia, but 120 h after continuous infusion only RB 38 A was able to still induce a significative antinociceptive effect. The present data suggest that even in the drastic conditions used here long-term complete inhibition of enkephalin catabolism induces a weak tolerance and a moderate physical dependence, similar to that produced by delta opioid agonists. This effect was not observed after chronic selective inhibition of NEP by RB 38 B.

Tolerance to the respiratory actions of opiates: withdrawal tolerance and asymmetrical cross-tolerance

Cross-tolerance to the respiratory depression induced by i.c.v. [D-Ala2,D-Leu5]enkephalin (DADLE) and by s.c., i.c.v. and i.v. morphine was studied in anesthetized rats that had been rendered tolerant to s.c. sufentanil (4 micrograms/h per 7 days). Tolerance induced by i.c.v. sufentanil was also compared during withdrawal and its absence. Tolerance was evaluated by estimation of the changes in both the maximum attained reduction of respiratory frequency (Emax) and the area under the time-course curve for the effects (AUC). In contrast to the failure to develop tolerance, as estimated with the Emax, after i.c.v. sufentanil in the absence of abstinence (tolerance index: 1.1), rats displayed significant degrees of cross-tolerance to i.c.v. morphine (3.6), i.c.v. DADLE (4.3) and s.c. morphine (6.6). No tolerance, however, was obtained to i.v. morphine. Tolerance to i.c.v. sufentanil in non-abstinent rats was not present when the Emax was considered but was when AUC was considered. During withdrawal animals showed tolerance as estimated with both the Emax and AUC. These results complement previous findings and show that the appearance of dissimilar degrees of cross-tolerance to different agonists depends not only on their intrinsic activities but also on their pharmacokinetic properties. These observations, along with the critical influence of withdrawal upon the manifestation of tolerance, support the need to distinguish between the development of the tolerance state and the degree of tolerance that can be expressed under specific conditions.

Parker and Radow test of drug withdrawal aversion: opposite effect in rats chronically infused with sufentanil or amphetamine

In rats, cessation of periodic injections of morphine reduces a preference for a palatable saccharin solution presented in a choice with water, and this has been interpreted to reflect withdrawal malaise. We confirmed and examined this "Parker and Radow Model" using subcutaneously implanted osmotic minipumps as the means of drug delivery and the opiate, sufentanil, and the psychostimulant, amphetamine, as the treatment drugs; surgical removal of the pumps was used to initiate withdrawal. Thus, rats withdrawn after 2 weeks exposure to a sufentanil-delivering pump (0.25 microgram/hr) showed a decreased preference for the saccharin and animals exposed to an amphetamine pump (68 micrograms/hr) showed an increased preference, as compared to placebo-exposed controls. This pattern of effects was systematically replicated in new subjects using 4 weeks of treatment and 136 micrograms/hr amphetamine. Since the locomotor increasing and body weight decreasing effects of amphetamine were also demonstrated and the doses of amphetamine and sufentanil were in comparable dose ranges, it was concluded that the Parker and Radow procedure may be a reliable measure of opiate withdrawal, but under similar test and treatment conditions other processes may be operative in amphetamine-treated animals. Problems of measuring motivation of withdrawal, particularly of spontaneous withdrawal, were noted.

Selective changes in mu opioid receptor properties induced by chronic morphine exposure

Chronic infusion of morphine to guinea pigs produced selective changes in mu agonist binding properties in cerebrocortical membrane preparations. Employing the mu-selective opioid agonist [D-Ala2,MePhe4,Gly-ol5]enkephalin (DAMGO) in direct binding studies and in competition of labeled antagonist binding, we found that the major changes were a decrease in the number of sites with high affinity for agonist, a small reduction in total receptor number, and a loss in the ability of guanosine 5'-[gamma-thio]triphosphate to regulate binding. A fraction of high-affinity mu receptors appeared to retain their high affinity for agonist and their sensitivity to guanine nucleotide analogue after the induction of morphine tolerance, possibly because the morphine concentrations achieved in brain were insufficient to uncouple all mu receptors from associated guanine nucleotide-binding regulatory proteins. Some membrane preparations were treated with pertussis toxin, which has been shown to functionally uncouple mu opioid receptors from their effector systems. In these preparations, a single agonist-affinity state of the receptor was observed. The apparent dissociation constant for this affinity state in pertussis toxin-treated membranes was similar to the lower-affinity state observed in preparations from morphine-tolerant animals. In contrast to the changes observed at mu opioid binding sites, no significant changes in agonist affinity or binding density were observed for selective delta or kappa agonists, consistent with the development of selective tolerance at mu receptors.

Evaluation of delta receptor mediation of supraspinal opioid analgesia by in vivo protection against the beta-funaltrexamine antagonist effect

The involvement of delta opioid receptors in supraspinal analgesia was investigated. With this aim, opioids that produced analgesia in the tail immersion test were administered i.c.v. to mice a few minutes before the irreversible antagonist, beta-funaltrexamine (beta-FNA). Protection of the respective analgesic effects from beta-FNA blockade was obtained when evaluated 24 h later. Moreover, mu ligands protected the analgesia evoked by ED50s of morphine, [D-Ala2,N-Me-Phe4,Met-(o)5-ol]enkephalin (FK 33-824), [D-Ala2,N-Me-Phe4,Gly-ol5]enkephalin (DAGO) and human beta-endorphin at doses (ED50s) lower than those required for delta ligands (approximately ED90s) to reach a similar protection. delta Preferential ligands effectively protected the analgesia induced by ED50s of [D-Ala2,D-Leu5]enkephalin (DADLE), [D-Thr2,Leu5]enkephalin-Thr6 (DTLET) and [D-Pen2,D-Pen5]enkephalin (DPDPE) from the beta-FNA-deteriorating effect. FK 33-824 and DAGO also provided good protection of the analgesia elicited by these delta ligands whereas morphine protected much less. Binding studies after i.c.v. injection of beta-FNA showed that its alkylating effect on opioid receptors was restricted to periventricular areas. In PAG, where the mu/delta receptor ratio is about 10, [3H]DADLE specific binding was still present after ED50s of DPDPE, DAGO, morphine and DADLE as protecting agents. [3H]Dihydromorphine [( 3H]DHM) binding was well protected by ED90s of morphine and DAGO, and to a lesser extent by DPDPE and DADLE. These results suggest that delta ligands, after binding to delta receptors, also need to act upon mu receptors to produce high levels of supraspinal analgesia in the tail immersion test.

Unidirectional analgesic cross-tolerance between morphine and levorphanol in the rat

Clinically, patients often demonstrate incomplete cross-tolerance between opiate analgesics. Although dispositional and pharmacokinetic factors may be a factor, our results suggest that differences in selectivity of various opioids for those opioid receptor subtypes involved in analgesia, mu 1, kappa and delta, also play an important role. In binding studies, levorphanol potently labelled all 3 classes whereas morphine was relatively selective for mu sites. Levorphanol infusions yielded tolerance to both morphine and levorphanol while morphine infusions selectively produced tolerance to morphine. This unidirectional tolerance might be due to the selectivity of morphine for mu receptors compared to levorphanol's ability to interact more potently with other relevant receptor subtypes. These observations raise the possibility that the order in which different opioid analgesics are administered may be of clinical significance.

Effects of footshock stress and morphine on natural killer lymphocytes in rats: studies of tolerance and cross-tolerance

Exposure to a form of footshock stress known to cause opioid-mediated analgesia suppresses the cytotoxic activity of natural killer (NK) cells in rats. This suppression is blocked by the opioid antagonist, naltrexone and is mimicked by morphine administration, suggesting mediation by opioid receptors. Supporting this hypothesis, we now report that the morphine-induced suppression of NK activity shows tolerance after 14 daily injections. The NK-suppressive effect of stress, however, shows neither tolerance with repetition nor cross-tolerance in morphine-tolerant rats.

Unequal opiate cross-tolerance to morphine in the locomotor-activation model in the mouse

Dose-response relationships for the opiate-induced stimulation of locomotion in mice were determined for etorphine, fentanyl, levorphanol, lofentanil and morphine, both before and after the subcutaneous implantation of a morphine pellet for 3 days. In non-tolerant mice, the relative potencies of these drugs compared to morphine paralleled those reported for other in vivo models of the actions of opiates. The most potent was lofentanil (ED50 = 3.8 micrograms/kg), which was approx. 6000 times more potent than morphine (ED50 = 24.2 mg/kg). A tolerance-induced 22-fold increase in the ED50 for morphine resulted in a similar cross-tolerance to levorphanol, but there was less cross-tolerance to fentanyl (4-fold), lofentanil (5-fold) and etorphine (7-fold). The smaller cross-tolerances of etorphine and lofentanil to morphine were not accompanied by changes in the levels of [3H]etorphine or [3H]lofentanil in brain, induced by morphine pellets, although the bound fraction of [3H]lofentanil in brain was slightly decreased (11-15%). The implications of the phenomenon of unequal cross-tolerance for the mechanism of tolerance at the opiate mu receptor are discussed.

Pharmacological evidence for a protective role of the endogenous opioid system on electroshock-induced seizures in the mouse

Acute administration of morphine produced a protective effect on electroshock (ECS)-induced seizures in mice, while naloxone and naltrexone decreased ECS seizure threshold. Chronic morphine administration in mice resulted in a decrease of ECS-induced seizure threshold evident within 24 h following the end of drug treatment. This effect disappeared 5 days after the end of chronic morphine treatment. Moreover, tolerance to the anticonvulsant effect of morphine was evident in mice chronically treated with morphine and subjected to ECS 30 min after the last injection of the drug, as well as in mice subjected to ECS 24 h after the end of chronic treatment.

In vivo opiate receptor binding of oripavines to mu, delta and kappa sites in rat brain as determined by an ex vivo labeling method

The relative in vivo receptor affinities of three oripavine drugs given subcutaneously were determined at the mu, delta and kappa type of opiate binding sites in rat brain. The oripavines include the agonist etorphine, the antagonist diprenorphine and the mixed agonist-antagonist buprenorphine. With the use of mu, delta and kappa specific labeling conditions in brain homogenates immediately after sacrifice (ex vivo labeling), the method relies on the assay of those receptor sites that remain unbound in vivo. Because of the slow receptor binding kinetics of the oripavines, little or no dissociation of the in vivo ligand occurs during the ex vivo labeling period. All three drugs displayed lower affinity in vivo at the delta sites relative to mu sites, whereas the kappa affinities were highly variable. Etorphine displayed considerable mu selectivity, while burpenorphine's affinity at the mu and kappa sites was similar. The apparent in vivo binding affinities obtained from the ex vivo labeling approach are compatible with previous results where tracers were applied in vivo. The dramatic differences of the in vivo and in vitro opiate receptor binding properties of the oripavines demonstrate the need for in vivo receptor binding parameters in the analysis of the function of individual receptor types.

Cross-tolerance between morphine- and bombesin-induced inhibition of intestinal transit in rats

Intracerebroventricular (i.c.v.) injection of either morphine or bombesin to rats inhibits intestinal transit of an intraduodenally administered radiochromium marker. In this work, we show that tolerance develops to this effect of bombesin after i.c.v. infusion of the peptide (0.5 micrograms/h for 4 days via an s.c. implanted Alzet 2001 osmotic minipump). Tolerance also develops to the inhibition of intestinal transit associated with i.c.v. morphine after s.c. injections of morphine. Bombesin-induced delay of transit is not attenuated by naltrexone (10 mg/kg, s.c.), a standard narcotic antagonist. Nevertheless, two-way cross-tolerance develops between bombesin and morphine in this system. This is a surprising result since both bombesin and morphine are believed to act on different receptors and cause opposite effects on intestinal motility in rats.

Multiple opioid ligands and receptors in the control of nociception

This paper summarizes the results of recent data characterizing the role of endogenous opioid peptides and opioid receptors in nociception. In addition, evidence is given that antinociception induced by intracerebroventricular injection of opioids into mouse brain is mediated by receptors resembling those mediating the inhibitory action of these substances on the rat vas deferens (putative epsilon-receptors). The endogenous ligands for these receptor are beta-endorphin and the peptides deriving from proenkephalin A.

Activating endogenous opioid systems by electroconvulsive shock or footshock stress inhibits recurrent kindled seizures in rats

Electroconvulsive shock (ECS) significantly decreased the behavioral manifestations of seizures elicited by amygdaloid stimulation in kindled rats. This anticonvulsant effect was significantly reduced by the opiate antagonist, naloxone, and by the development of morphine tolerance. A form of footshock stress known to cause opioid-mediated analgesia had a similar anticonvulsant effect, whereas another form causing non-opioid analgesia did not. These results suggest that the anticonvulsant effects of ECS and stress are mediated by the release of endogenous opioids.

Opiate receptor binding-effect relationship: sufentanil and etorphine produce analgesia at the mu-site with low fractional receptor occupancy

The analgesic activity of the opiate agonists etorphine and sufentanil and the antagonistic effects of diprenorphine and naloxone have been related to the occupancy of 3 classes of opiate binding sites previously defined in vivo in order to establish their pharmacological significance. Sufentanil binds specifically in vivo to the first type of site (site 1), exhibiting approximately 1100-fold selectivity over site 2, whereas etorphine displays approximately 20-fold selectivity for site 1 over site 2. Neither agonist has a measurable affinity to the third type of binding site. The binding data suggest that site 1 is analogous to the mu site previously identified in vitro. Both agonists produce analgesia in the rat tail flick test at the same low fractional occupancy of site 1 (approximately 2% at the ED50) while they display much lower and quite different occupancies at site 2. Both of the opiate antagonists naloxone and diprenorphine reduce the potency of sufentanil and etorphine by a factor of 2 at 50% occupancy of site 1 alone. These results provide strong evidence that these 4 drugs exert their effects by interaction with site 1 (mu sites) which therefore may be regarded as the receptor responsible for analgesic action in this test. The assumption of a direct relationship between antagonistic effect and fractional occupancy appears to be valid for naloxone and diprenorphine at site 1, while the agonists exert their action at a very low fractional occupancy implying a non-linear binding-effect process.

Interaction of naloxone with mu- and delta-opioid agonists on the respiration of rats

The respiratory depression induced by two mu-opiate agonists morphine and Tyr-D-Ala-Gly-N-Me-Phe-Met(O)-ol (FK-33824), and two delta-agonists Tyr-D-Ala-Gly-Phe-D-Leu (DADLE) and Tyr-D-Ser-Gly-Phe-Leu-Thr (D-Ser2-Thr6) was studied in rats by using the intracerebroventricular route. The four opioids caused a dose-dependent depression of respiratory frequency down to apnea but high doses of morphine elicited motor activation and seizure activity. FK-33824 was the most potent, followed by DADLE, D-Ser2-Thr6 and morphine. The in vivo apparent pA2 values were determined for naloxone against FK-33824, DADLE and D-Ser2-Thr6. The pA2 value of naloxone interacting with the mu-agonist FK-33824 was significantly lower than those obtained against the two delta-agonists. It is proposed that different types of opiate receptors are involved in the opiate-induced respiratory depression.

Endogenous opiates: 1981

This paper is the fourth of an annual series reviewing the research concerning the endogenous opiate peptides. This installment covers only work published during 1981 and attempts to provide a comprehensive, but not exhaustive, survey of the area. Previous papers in the series have dealt with research done before 1981. Topics concerning endogenous opiates reviewed here include a delineation of their receptors, their distribution, their precursors and degradation, behavioral effects resulting from their administration, their possible involvement in physiological responses, and their interactions with other peptides and hormones. Due to the burgeoning literature in this field, the comprehensive nature of this review in the future will be limited to considerations of behavioral phenomena related to the endogenous opiates.

Similarities and differences between D-ALA2 MET5 enkephalin amide and morphine in the induction of tolerance to their effects on catalepsy and on dopamine metabolism in the rat brain

Rats were made tolerant to morphine or to DALA, a synthetic analogue of met-enkephalin, by prolonged exposure to these compounds. Tolerance was assessed by evaluating the resistance of the treated rats to present catalepsy after an acute dose of the opiates. Both morphine and DALA induced tolerance and cross-tolerance to the cataleptic effect. Acute administration of morphine and DALA increased the concentration of DOPAC in striatum, limbic area and s.nigra of control rats. This increase was not present when morphine was given acutely to chronically morphine-treated rats, indicating that these animals were tolerant to this effect. Chronically morphine-treated rats given DALA presented partial tolerance to the biochemical effect of the peptide in limbic area and in s.nigra but not in striatum, indicating that only in certain areas was cross-tolerance produced by chronic morphine. When DALA was administered at different doses to chronically DALA treated rats, the peptide induced rise in DA catabolite was similar to that produced in control animals, so clearly there was no tolerance to this biochemical effect. In these animals cross tolerance to morphine's effect on DA metabolism was present in s.nigra but not in the other two areas, indicating that s.nigra is particularly sensitive to opiate-induced tolerance on DA metabolism.

Opioid dependence and cross-dependence in the isolated guinea-pig ileum

The development of opioid dependence and tolerance attributed to selective types of opiate receptors was studied in the isolated ileum of guinea pigs chronically exposed to specific opioids. These investigations were based on reports that in this preparation highly tolerant opiate receptors may coexist with opiate receptors of almost unchanged sensitivity. Thus, the ilea were set up in vitro and tested for tolerance and dependence. Apparently precipitation of the withdrawal contracture, indicating dependence, proved a more sensitive parameter than the phenomenon of tolerance. Maximal dependence was determined at rather low degrees of tolerance (5 to 10 fold). The intensity of the withdrawal contracture failed to increase as opiate tolerance did. Furthermore, the experiments failed to present evidence for the existence of selective dependence at specific opiate receptor types. These findings may suggest multiple adaptational mechanisms upon chronic activation of opiate receptors. One mechanism may be responsible for the development of dependence and a low degree of tolerance, whilst a further increase of tolerance may be associated with changes at the opiate binding site level.

Antinociceptive potencies of beta-casomorphin analogs as compared to their affinities towards mu and delta opiate receptor sites in brain and periphery

beta-Casomorphins and their analogs were tested for their opioid activities in the myenteric plexus longitudinal muscle preparation of the guinea pig ileum (GPI), the isolated mouse vas deferens (MVD), and for their affinities to mu- delta- and kappa- binding sites in rat brain membranes. C-terminal amidation of beta-casomorphin-4 and (-5) increased opioid potency in both organ preparations (GPI, MVD) and affinity to mu-binding sites in brain whereas binding to delta-sites was diminished. These beta-casomorphin-amides displayed a 2-3 times greater naloxone reversible antinociceptive effect than natural beta-casomorphins. Introduction of D-alanine at position 2 in the beta-casomorphin-amides increased potency in the GPI whereas activity in the MVD was only slightly changed. These compounds, however, showed a remarkable increase in binding to delta-sites in brain with an unaffected or slightly increased binding to mu-sites and decreased binding to kappa-sites. D-Ala2-beta-casomorphin-4 and (-5) amides were 10 times more potent antinociceptive agents than corresponding beta-casomorphin-amides. These results suggest firstly, that peripheral delta-receptors in the MVD are not as closely related to delta-binding sites at rat brain membranes as is the case with mu-receptors in the GPI and mu-binding sites, and secondly, in addition to mu-receptors, delta-receptors may be of importance in mediating antinociception.

Effects of taurine on tolerance to [D-Ala2, Met5]enkephalinamide in rats

Effects of taurine on tolerance to [D-Ala2, Met5]enkephalinamide (DAME) were investigated in rats. Tolerance was produced by five intraventricular administrations of DAME (50 microgram) during 3 consecutive days. The magnitude of developed tolerance to DAME was not uniform for each behavioral parameter; tolerance to analgesia effects developed more intensively and rapidly from the repeated injections of the peptide than that to akinesia effects. Pretreatment with taurine (9.5 X 10(-2) M) which was injected in a volume of 10 microliter intraventricularly 10 min prior to every administration of DAME suppressed the development of tolerance to both analgesia and akinesia effects of this peptide, whereas pretreatment with L-leucine at the same concentration did not. Spontaneous locomotor activity was measured for 1 h after the 90-min behavioral observation period was completed. That activity increased with the number of the peptide injections. Taurine pretreatment inhibited the induction of 'hyper'-locomotor activity. These results support the view that taurine may possess an ability to inhibit development of tolerance to morphine-like peptides in rats.

In vivo receptor binding of the opiate partial agonist, buprenorphine, correlated with its agonistic and antagonistic actions

1 In order to gain more insight into the mechanisms behind the actions of opiate partial agonists, an analysis of the dual agonist/antagonist properties of the partial agonist, buprenorphine, was made in conjunction with in vivo binding studies on the drug in the rat. 2 Buprenorphine revealed a bell-shaped dose-response curve for antinociception peaking at approx. 0.5 mg/kg subcutaneously. It antagonized morphine antinociception at doses which normally have agonistic effects and produced maximum antagonistic effects at doses above those having prominent agonistic activity. The withdrawal precipitating potency of buprenorphine as measured in highly morphine-dependent rats was present at doses above those having agonistic activity. The entire dose-response curve for buprenorphine was shifted symmetrically to the right by the opiate antagonist, naltrexone. 3 The dose-dependent occupation of receptors in vivo by buprenorphine seemed to be almost complete over the agonist dosage range; almost no further receptor occupation over the antagonist range was seen. 4 The possibility is discussed that site-to-site receptor interactions leading to cooperativity of effect may be the best explanation of these results.

A selective distribution pattern of different opiate receptors in certain areas of rat brain as revealed by in vitro autoradiography

A differential localization of binding sites in the rat brain for [3H]D-Ala2,Leu5-enkephalin ([3H]DADL) and [3H]etorphine ([3H]Eto) was been demonstrated by an in vitro autoradiography technique. Labeling with [3H]DADL was found predominantly in septum and paraventricular nuclei of hypothalamus, whereas [3H]Eto labeling was primarily observed in the periaqueductal gray and several thalamic nuclei. The distinctive patterns of labeling by the two ligands in certain central nervous system structures suggests the existence of specific neuronal pathways which are preferentially labeled with either [3H]DADL or [3H]Eto.