Effects of repeated Ro 15-1788 administration in benzodiazepine-dependent baboons

European Journal of Pharmacology, 110 (1985) 257-261 Elsevier

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Short communication EFFECTS OF REPEATED Ro 15-1788 ADMINISTRATION IN B E N Z O D I A Z E P I N E - D E P E N D E N T BABOONS RICHARD J. LAMB and ROLAND R. GRIFFITHS * Division of Behavioral Biology, Departments of Psychiatry and Neuroscience, The Johns Hopkins University School of Medicine, Baltimore, Maryland 21205, U.S.A. Received 22 January 1985, accepted 5 February 1985

R.J. LAMB and R.R. GRIFFITHS, Effects of repeated Ro 15-1788 administration in benzodiazepine-dependent baboons, European J. Pharmacol. 110 (1985) 257-261. Administration of the benzodiazepine antagonist, Ro 15-1788, to baboons that were chronically exposed to diazepam or triazolam precipitated withdrawal signs. When Ro 15-1788 was administered repeatedly at one or three day intervals, precipitated withdrawal signs were attenuated. However, these baboons remained tolerant to the sedative effects of the high doses of benzodiazepines to which they were continuously exposed. While tolerance to agonist effects of drugs and development of physical dependence are often thought to be functionally interrelated phenomena, the present results suggest that these may be separable properties of the benzodiazepines. The present results clearly indicate that certain actions of benzodiazepine agonists and antagonists can be independently regulated. Physical dependence

Benzodiazepines

Tolerance

1. Introduction Administration of Ro 15-1788 (flumazepil; ethyl-8-fluoro-5,6-dihydro-5-methyl-6-oxo-4H-imidazo(1,5a)(1,4)benzodiazepine-3-carboxylate) to baboons that have been chronically exposed to benzodiazepines precipitates a withdrawal syndrome, which includes such signs as tremor, retching, vomiting, and abnormal posturing (Lukas and Griffiths, 1982). Ro 15-1788 is thought to competitively antagonize the actions of benzodiazepines (Bonetti et al., 1982; Hunkeler et al., 1981; Mrhler and Richards, 1981; Mrhler et al., 1981; Polc et al., 1981). Thus Ro 15-1788 may precipitate withdrawal signs by decreasing the proportion of benzodiazepine receptors occupied by the agonist. In the course of determining the Ro 15-1788 dose-response curve for producing withdrawal signs, we noticed that after high doses of Ro 15-1788 the effects of lower doses were difficult to

* To whom all correspondenceshould be addressed. 0014-2999/85/$03.30 © 1985 ElsevierSciencePublishers B.V.

Diazepam

Triazolam

Ro 15-1788

replicate. To determine whether tolerance develops to the ability of Ro 15-1788 to precipitate withdrawal signs, we administered Ro 15-1788 at one and three day intervals to baboons receiving continuous intragastric infusions of either diazepam or triazolam.

2. Materials and methods Three male baboons (Papio anubis) weighing 20 to 31 kg were used. They were implanted with intragastric catheters as previously described (Lukas et al., 1982). The catheters were protected by a harness-tether system (Lukas et al., 1982) that allowed the animal virtually unrestricted movement within primate cages (0.8 x 0.9 × 1.2 m high). The baboons had ad libitum access to primate chow and water, and received daily supplements of fresh fruit. Diazepam (20 m g / k g per day; baboons JE, AR) and triazolam (5 m g / k g per day; baboon SA) were suspended with Bio-Serv Suspending Agent

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K ® (4 g/l), and were administered as a continuous intragastric infusion via a peristaltic pump. The baboons received these agonists for at least one month prior to the beginning of these experiments and for at least one month between the series of daily or every third day administration of Ro 15-1788. Baboon AR was exposed to the 3-day series first and the 1-day series second. Baboons JE and SA were exposed to the 1-day series first and the 3-day series second; subsequently, JE was exposed to another 1-day series. Ro 15-1788 (5 mg/kg) was suspended in a vehicle of propylene glycol:ethanol:sterile water (40 : 10 : 50 v / v ) and administered intramuscularly. Following administration of Ro 15-1788 the baboons' behavior was recorded continuously for one hour, with the passage of time noted at one minute intervals. Withdrawal signs previously described (Lukas and Griffiths, 1982) were recorded, as well as any other normal or abnormal behavior that occurred. Abnormal postures included all those previously described. For baboon JE the 'withdrawn/depressed' abnormal posture was not included in the data analysis because there was an unusually high baseline level of this posture in JE's repertoire. Nose rubs were defined as a substantial displacement of the nasus externus from midline, and were distinguished from nose wipes, which involved simply touching the nose.

trol conditions. Under control conditions (i.e., placebo administration or administration of Ro 15-1788 to baboons not treated with agonist) abnormal postures, limb tremors, and vomiting/ retching were never observed and nose rubs were never observed in more than five one-minute intervals. As can be seen in fig. 1 the number of minutes in which abnormal postures, limb tremor, nose rubs, and vomiting/retching were observed declined upon repeated daily administration of Ro 15-1788 in all three baboons. These signs, except for limb-tremor, also declined when Ro 15-1788 was administered at three day intervals (fig. 2). The decline of these withdrawal signs was consistent with the opinions of various observers that the overall severity of the precipitated withdrawal I~ 39 I~ 39 W n-

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syndrome was declining with repeated antagonist administration. By the end of the series of repeated Ro 15-1788 administrations it was difficult to detect any Ro 15-1788 effects. During the repeated administration of Ro 15-1788, baboons remained tolerant to the sedative/muscle relaxant effects of the agonists, as indicated by continued absence of gross sedation, ataxia, lip droop, and prominent loss of muscle tone.

4. Discussion

Several possible explanations exist for the tolerance that develops to the ability of Ro 15-1788 to

precipitate withdrawal signs. While it is possible that this tolerance is dispositional, this seems unlikely since an enormous decrease in the half-life of a rapidly acting drug (such as Ro 15-1788) would be required to reduce its peak effects (Kalant et al., 1971). Tolerance via some sort of learning process (e.g., operant or respondent conditioning) is possible, though also unlikely. This type of tolerance should be long lasting, however, tolerance to Ro 15-1788 lasted less than one month. Some form of habituation (such as that which occurs with an orienting response to noise) cannot, however, be ruled out. Another possibility is that Ro 15-1788 prevents the agonists' occupation of benzodiazepine receptors for sufficient time to reduce the degree of physical dependence (and thereby withdrawal) a n d / o r prevent the reoccurrence of withdrawal signs. However, Ro 15-1788 antagonized the discriminative stimulus effects of benzodiazepines for only five to six hours in the baboon (Ator and Griffiths, 1983) which may reflect the duration of the blockade of benzodiazepine receptors by Ro 15-1788 in the baboon. Thus, when Ro 15-1788 was administered at 1 or 3 day intervals, the agonist should have had 18-66 h of relatively unrestricted access to benzodiazepine receptors. Since previous experiments have shown that 24 h of agonist exposure is sufficient to demonstrate a precipitated withdrawal syndrome in the baboon (Lukas and Griffiths, 1984), explanations other than continuous benzodiazepine receptor blockade are more likely to be correct. Some form of pharmacodynamic adaptation (which w o u l d include processes such as habituation) thus seems the most likely explanation for the tolerance observed. It is important to note that repeated administration of Ro 15-1788 does not simply reinstate the original conditions that existed before the administration of benzodiazepine agonists. Although animals became tolerant to the effects of Ro 151788, they also remained tolerant to the sedative effects of the high doses of agonist to which they were continuously being exposed. Thus, physical dependence (as assessed by precipitated withdrawal signs) and tolerance to the sedative/muscle relaxant effects of agonists are separable properties of the benzodiazepines. This dissociation is not

260 likely to be due to an inability of Ro 15-1788 to interact with the sites responsible for production of sedation and muscle relaxation since Ro 15-1788 has been shown to antagonize the sedative/muscle relaxant effects seen after initial administration of benzodiazepines to baboons tested under identical conditions (Lukas and Griffiths, 1982; 1984). This dissociation is particularly interesting because physical dependence and tolerance to agonist effects are often thought to be functionally interrelated. The phenomena of tolerance to antagonist precipitated withdrawal and dissociation of physical dependence and tolerance to agonist effects may not be unique to benzodiazepines. When opioid antagonists were administered daily to mice implanted with pellets of morphine the withdrawal syndrome initially precipitated became attenuated with repeated antagonist administration (Huidobro et al., 1963; Huidobro and Maggiolo, 1964; 1965). Unfortunately, the effects of morphine during repeated daily opioid antagonist administration were not reported, thus making comparison to the present study limited. However, in another set of studies, when opioid antagonists were administered three times per day the precipitated withdrawal syndrome remained stable (i.e., was not attenuated) with repeated antagonist administration, although the effects of morphine became more pronounced with repeated antagonist administration (Huidobro et al., 1963; Huidobro and Maggiolo, 1964). Thus, in these latter experiments, tolerance to the effects of agonists and physical dependence as measured by precipitated withdrawal were (as in the present experiment with benzodiazepines) separable phenomena. The types of separation were, however, distinctly different [loss of tolerance with no attenuation of withdrawal (Huidobro et al., 1963) vs. attenuation of withdrawal with no change in tolerance (present study)]. Tolerance to an effect of Ro 15-1788 (precipitation of withdrawal signs) and tolerance to an effect of benzodiazepine agonists (sedation) can exist concurrently on some occasions, but not on other occasions. This indicates that the actions of benzodiazepine agonists and antagonists can be independently regulated. Additionally, these changes

in the activity of Ro 15-1788 with repeated administration suggest that the antagonism of the effects of benzodiazepines by Ro 15-1788 may be more complex than simply the competition for a single common receptor site.

Acknowledgements We would like to thank the Upjohn Companyfor the gift of triazolam, and Hoffmann-LaRochefor the gifts of diazepam and Ro 15-1788. We would also like to thank Ms. P. Brinkley, Ms. E. Cook, Ms. B. Doty and Ms. D. Spicer for their expert technical assistance, Ms. D. Sabotka for typing the manuscript, Dr. S.E. Lukas for helpful surgical and scientific assistance, and Drs. N.A. Ator, M.J. Kuhar, S.E. Lukas, D.E. McMillan, and J.D. Roache for their critical reading of the manuscript. This research was supported by National Institute on Drug Abuse grant DA 01147 and Contract 271-80-3178. R.J. Lamb is a recipient of a National Institute on Drug Abuse National Research Service Award DA 05237.

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