Buprenorphrine: demonstration of physical dependence liability.

This study was designed to assess the dependence-producing capacity of the opiate partial agonist, buprenorphine. Rats chronically treated with buprenorphine for 4 days showed only very weak signs of withdrawal upon cessation of buprenorphine treatment or upon challenge with naloxone, although complete tolerance had developed to the drug at this time. However, more intense withdrawal could be induced when buprenorphine treatment was followed by substitution treatment with morphine. Even one injection of morphine given 12 h after the last buprenorphine treatment enabled the precipitation of withdrawal with naloxone. Naloxone could not precipitate signs of withdrawal in naive rats treated with this dose of morphine. Thus, contrary to some claims in the literature, buprenorphine, like other opiate agonists and partial agonists, induces dependence. The fact that only few signs of withdrawal are seen in direct dependence tests, probably reflects the slow dissociation of the drug from the receptor - which probably limits the intensity of withdrawal by preventing the rapid uncovery of the receptor upon discontinuance of treatment with the drug or upon injection of an antagonist. In addition, the maximum degree of dependence induced by buprenorphine - in comparison to pure agonists is limited, like that of other partial agonists.

Opiate antagonist--receptor interaction unchanged by acute or chronic opiate treatment.

The apparent pA2 of naloxone, a measure of the apparent receptor affinity for the antagonist, was estimated in naive rats, in rats after acute and various schedules of chronic morphine pretreatment, and in rats during abrupt withdrawal. The results were adjusted to compensate for the amount of pretreatment drug in the rat brains as, otherwise, such a drug could have inflated the apparent pA2 values (see appendix). After this adjustment, a tendency of the apparent pA2 values to be increased after chronic pretreatment disappeared. The apparent pA2 of naloxone in acutely pretreated rats was also unchanged from that in naive rats, while the results in abruptly withdrawn rats were ambiguous. Thus, no change in the affinity of the antagonist for opiate receptors was found and theories suggesting that a conformational change takes place in the opiate receptor during the development of tolerance/dependence could not be supported.

Site of naloxone-precipitated opiate withdrawal dissociates from that at which apomorphine reinitiates this phenomenon.

The site of action of naloxone to induce jumping in morphine tolerant/dependent rats appears to be dissociated from structures where apomorphine initiates its action to reinduce jumping in previously withdrawn animals. These findings suggest that dopaminergic pathways do not directly affect the neuronal circuit involved in withdrawal jumping behavior, but instead exert a facilitatory influence on neurons that become supersensitive during the state of withdrawal. Thus, an increased response to apomorphine during naloxone-precipitated opiate withdrawal does not necessarily imply a specific supersensitivity of the dopaminergic system.

The opiate-like action of tilidine is mediated by metabolites.

The analgesic effect of tilidine in rats is completely antagonized by the narcotic antagonist naloxone. Radioreceptor assays revealed, however, that the main metabolites of tilidine, nortilidine and bisnortilidine, rather than tilidine exhibit affinity to opiate receptors. These findings were confirmed in studies using the electrically stimulated guinea pig ileum and the mouse vas deferens. Chronic tilidine administration to rats caused a considerable degree of physical dependence, which was expected from the ability of the intact animal to metabolize tilidine. In the isolated ileum from chronically morphinized guinea pigs, both nortilidine and bisnortilidine fully substituted for morphine in preventing induction of withdrawal, indicating dependence liability of these metabolites.

Changes in striatal dopamine metabolism during precipitated morphine withdrawal.

Precipitation of withdrawal in morphine tolerant/dependent rats by either naloxone or the partial agonist ZK 48491 caused a significant increase in the contration of striatal DA, which persisted for at least 1 h. During the same time the probenecid-induced accumulation of HVA and DOPAC was reduced in the striatum in relation to probenecid-treated tolerant/dependent controls. 20 min after precipitation of withdrawal by naloxone, the striatal concentration of 3-methoxytyramine was decreased by about 40%, while the activity of the DA metabolizing enzymes, MAO and COMT, remained unchanged. Naloxone-precipitated withdrawal was, further, found to delay the depletion of striatal DA caused by inhibition of synthesis 90 min after alpha-methyl-p-tyrosine treatment. All these results provide evidence for a decreased release of DA from the striatum during precipitated morphine withdrawal.

Activity of the nigro-striatal dopaminergic system during precipitated morphine withdrawal investigated in rats with acute unilateral inactivation of the striatum.

The activity of the striatal dopamine system during precipitated morphine withdrawl was studied in rats using a model in which the striatum was unilaterally inactivated by the local injection of KCl. In naive rats dopamine agonists administered just prior to KCL induced ipsilateral turning or circling, while dopamine antagonists in the same situation caused contralateral turning. Withdrawal precipitated by morphine antagonists in rats made dependent by repeated implantation of morphine pellets induced contralateral circling during unilateral inactivation of the striatum. This contralateral circling was only slightly enhanced by haloperidol, but strongly enhanced by a low dosage of apomorphine as well as by some weak dopamine agonists such as CB 154 or By 101. However, high doses of apomorphine completely reversed the withdrawal-induced contralateral circling into ipsilateral circling. Other dopamine agonists, such as d-amphetamine, L-Dopa and piribedil, did not abolish the withdrawal-induced contralateral circling, however, they caused the appearance of an additional ipsilateral circling. Other types of drugs which are known to intensify withdrawal-induced jumping (desipramine, atropine, caffeine) enhanced contralateral circling. There are also other parallels jumping and contralateral circling induced by withdrawal. The direction of naloxone-induced asymmetric behaviour during acute unilateral inactivation of the striatum suggests that striatal dopaminergic activity is reduced during precipitated withdrawal; the other results reported point to the possiblity that extrastriatal dopaminergic mechanisms or different dopamine receptor types within the striatum are involved.

Tolerance and dependence induced by morphine-like pituitary peptides in rats.

An extract from porcine pituitaries containing peptides with opiate-like activity (endorphins) was investigated for possible tolerance/dependence liability in rats. Repeated intraventricular administration of endorphins induced a degree of tolerance similar to that induced by normorphine. Rats made tolerant to morphine by repeated pellet implantation proved cross-tolerant to the pituitary extract as well as to synthetic methionine-enkephalin. Naloxone given to rats, repeatedly pretreated with endorphins, precipitated a withdrawal syndrome strongly resembling that induced in rats treated with normorphine. Withdrawal precipitated in morphine-dependent rats was suppressed by intraventricularly applied endorphins. The results suggest that identical sites and mechanisms are involved in the development of tolerance/dependence induced by opiates and pituitary endorphins.

Porcine pituitary peptides with opiate-like activity: partial purification and effects in the rat after intraventricular injection.

A peptide material with opiate-like activity in the guinea-pig ileum was extracted from porcine pituitaries using a hot glacial acetic acid extraction method and was partially purified by gel filtration. When injected intraventricularly in rats, these purified peptides induced strong analgesia, catelepsy, respiratory depression and other opiate-like effects, which lasted for several hours.

Sites of action of morphine involved in the development of physical dependence in rats. I. Comparison of precipitated morphine withdrawal after intraperitoneal and intraventricular injection of morphine antagonists.

In rats made dependent on morphine by implantation of morphine pellets, withdrawal, as precipitated by intraventricular injection of morphine antagonists, was compared to withdrawal as precipitated by systemic antagonist application. The results, most clearly those obtained with a hydrophilic compound, diallyl-nor-morphinium-bromide, point to periventricularly located sites of action for the release of most withdrawal signs by antagonists. Jumping, reaching only low levels after i.ventr. injection of levallorphan and nalorphine, was very pronounced when the benzomorphane derivative SH 254, was used. In the case of writhing and diarrhea, the situation is more complicated. Possibly, central as well as peripheral mechanisms are involved in the expression of these signs.

Central serotonergic mechanisms and development of morphine dependence.

The effects of different manipulations of brain serotonin (5-HT) content on the development of morphine dependence were investigated in rats, which were implanted with morphine pellets for 40 days. Serotonin content was decreased by (a) short or long term inhibition of tryptophan hydroxylase with para-chlorophenylalanine (PCPA), (b) by short or long term degeneration of 5-HT containing nerve terminals with 5,6-dihydroxytryptamine or (c) by degeneration of 5-HT containing nerve terminals by lesioning of midbrain raphe nuclei. With all methods used, the frequency of withdrawal jumping was significantly reduced, while other withdrawal signs remained more or less unchanged. Additional administration of 5-HTP to chronically PCPA treated rats did not reverse the PCPA effect. Since chronic reduction of 5-HT level during the whole time of morphine exposure changed withdrawal symptomatology in nearly the same way as did a decrease in 5-HT level during the time of withdrawal only, it is suggested that serotonergic mechanisms are not linked to the basic processes underlying dependence development but that they are only involved in the nervous pathways mediating the expression of some withdrawal signs.

The role of dopamine in withdrawal jumping in morphine dependent rats.

The role of dopamine (DA) in precipitated withdrawal jumping was studied in morphine dependent rats. Pretreatment with various dopaminergic agonists induced a dose-dependent increase in naloxone induced jumping. Pimocide totally blocked the facilitatory effect of piribedil while naloxone induced jumping was not dose-dependently decreased. Biochemical measurements revealed that during precipitated withdrawal the level of DA was elevated and the accumulation of 3,4-dihydroxy-phenylacetic acid (DOPAC) and homovanillic acid (HVA) after probenecid as well as the level of 3-methoxytyramine in the striatum was reduced. Unilateral inactivation of the caudate by local injection of KCl induced contralateral circling during withdrawal. Additional systemic haloperidol pretreatment did not change the direction of circling while additional apomorphine pretreatment changed the direction to ipsilateral and increased the circling rate highly. These latter as well as the biochemical findings strongly suggest an inhibition of striatal DA-mechanisms during withdrawal. The apparent contradiction of these findings to the above finding showing a facilitatroy role of DA-agonists on jumping is discussed.

Comparison of withdrawal precipitating properties of various morphine antagonists and partial agonists in relation to their stereospecific binding to brain homogenates.

In morphine-dependent rats the withdrawal precipitating properties of various morphine antagonists and partial agonists were studied by quantitatively evaluating a variety of different withdrawal signs. A comparison of the dose response curves of the various substances obtained for the different signs revealed marked differences in respect to the lowest effective doses (EDs) necessary to precipitate the withdrawal signs as well as in the maximum frequencies of the signs induced. The "pure" antagonist, naloxone, which was judged very potent according to the ED, precipitated the lowest levels of jumping, whereas certain partial agonists of the benzomorphane type, which were less potent according to the ED, induced very high levels of this sign. These latter compounds, however, failed to precipitate "complete" withdrawal, as evidenced by the nearly complete absence of some of the withdrawal signs. The jumping precipitating potency of the antagonists as judged from the ED was found to be highly correlated to the stereospecific binding of these substances to rat brain homogenate. On the other hand, the ability of the substances to precipitate high levels of jumping was seen to increase, at least within a certain range, with increasing degree of agonistic properties, as indicated by the ratio of stereospecific binding in the presence and absence of sodium.