Further evidence for a motility substitution test as a tool to detect the narcotic character of new drugs in rats.

Time effects of three doses of codeine, d-propoxyphene, meperidine and dextromethorphan upon locomotor activity have been investigated in naive rats as well as in animals treated chronically with 20 mg/kg of morphine daily. The first three drugs exhibited a motility pattern qualitatively similar to that of morphine; in chronically-treated rats cross-tolerance occurred to acute depressant effects and cross-sensitization to acute excitatory effects. In the case of meperidine however cross-sensitization appeared to be small. Dextromethorphan presented effects similar to morphine only in a very large dose. The present data, together with those of previous work, demonstrate that a locomotor activity substitution test may predict the morphine-like addictive properties of a drug.

Previous treatment with morphine and sensitization to the excitatory actions of opiates: dose-effect relationship.

The stimulatory effect of morphine on locomotor activity has been shown to be largely modified in rats that have been formerly dependent on this drug. In the present study, the relationship between the chronic dose of morphine and the degree of sensitization to the excitatory effect of opiates was investigated. To this end, four groups of rats were treated daily for 30 days with 1.25, 5, 20 and 80 mg/kg of morphine (i.p.) and challenged with morphine (1.25 and 2.5 mg/kg) or fentanyl (40 and 80 micrograms/kg) 1-4 months after ceasing the treatment. Drug-induced hypermotility in post-dependent rats appeared to be linearly related to the dose of the preceding chronic treatment after test doses of both morphine and fentanyl. The results are discussed in terms of a persistent dose-related modification of the neuronal mechanism subserving the excitatory component of the action of opiates; such a modification might offer a neurobiological basis for the fact that "relapse tendencies" for opiates persist for a long time after withdrawal.

Cross-tolerance between morphine and clonidine: a study on motility in rats.

The effects of clonidine on motility were determined in non-dependent, morphine-dependent (a 20 mg/kg dose i.p. for 26 days) and post-dependent rats. In naive animals, clonidine (30-100 micrograms/kg) produced a dose-related suppression of motility. However, when the drug was administered in morphine-dependent rats, it induced slight hyperactivity at small doses and a decrease of activity only at the largest dose (100 micrograms/kg). Thus, tolerance to morphine conferred cross-tolerance to clonidine. On the contrary, the effects of clonidine on motility in post-dependent animals did not differ from those observed in control animals. The results are discussed in terms of similarities and differences between the depressant and excitatory effects of morphine and clonidine.

Cross-sensitization to the excitatory effect of morphine in post-dependent rats.

Time-effects of morphine, methadone, meperidine and pentazocine upon locomotor activity were investigated in naive and in post-dependent rats. Dependence was induced by daily injection of 20 mg/kg (i.p.) of morphine for 30 days. Tests were run starting from 1 month after withdrawal from morphine. Morphine produced a greater increase in activity in post-dependent than in naive rats. Marked cross-sensitization to the excitatory effect occurred with methadone but not with pentazocine. The motility pattern of meperidine was similar in naive and in post-dependent animals. The findings presented here suggest that: different mechanisms underlie the excitatory actions of opiates; the narcotic character of a drug can be detected by challenging this drug in rats previously dependent on morphine.

Sensitivity to the narcotic cue in non-dependent, morphine-dependent and post-dependent rats.

Using a food-reinforced two-lever operant procedure, 12 rats were trained to discriminate 10 mg/kg (i.p.) of morphine from saline. Five animals were given daily non-contingent exposure to morphine (20 mg/kg on saline, or no-test days, and 10 mg/kg on drug days) from the beginning of the experiment; the others received injections of saline. In the morphine generalization tests, the dependent rats showed an increased sensitivity to the narcotic cue as compared with non-dependent animals (ratio of the ED50 values: 2.30). This increased sensitivity was still present 3 months after discontinuing the non-contingent treatment with morphine (ratio of the ED50 values: 1.98). The results of the present study, together with other results reported in the literature, suggest that the experimental procedure plays a role in determining whether tolerance, no tolerance or enhanced sensitivity to the discriminative stimulus properties of narcotics, is observed.

Effects of buprenorphine on motility in chronically morphine treated rats.

The effects of buprenorphine (0.01-0.1 mg/kg) on the activity of nondependent and morphine dependent rats (a 20 mg/kg dose for 28 days) were determined. In naive animals buprenorphine exhibited both depressive and stimulatory actions upon the motility of rats, as repeatedly described for morphine. When buprenorphine was administered to rats chronically treated with morphine, the depressive effect disappeared (cross-tolerance to the inhibitory action); on the contrary the excitatory effect was enhanced (cross-sensitization). The results are discussed in terms of cross-sensitization to the excitatory effects in morphine dependent animals as predictor of morphine-like addictive properties in humans.

Long-term sensitization to the excitatory effects of morphine. A motility study in post-dependent rats.

Motility effects of excitatory and depressant doses of morphine were studied in naïve, dependent (20 mg/kg daily) and post-dependent (about 40 days after withdrawal) rats. Doses of 0.6 and 1.25 mg/kg of morphine produced a greater increase in activity in post-dependent than in naïve rats. A 20-mg/kg dose exerted an excitatory effect in dependent rats instead of the depressant one induced in naïve animals. This excitatory effect was still evident but less pronounced in post-dependent rats. The dose of 1.25 mg/kg was repeatedly challenged in post-dependent (80 mg/kg daily) rats. It was found that the increased excitatory effect of this dose persisted unchanged 160 days after ceasing treatment. The present results demonstrate that several months after the withdrawal of morphine, sensitization still exists to the excitatory effect of the drug while tolerance to the depressant action is slight or no longer present shortly after ceasing treatment.

Rm values and structure-activity relationship of benzodiazepines.

Quantitative structure-activity relationships (QSAR) have been formulated for the activities of a series of benzodiazepines in rats. The lipophilic character of molecules was expressed by means of the chromatographic Rm values which were very well correlated with experimental or calculated log P values. The ideal lipophilic character for activity of benzodiazepines in the exploratory behavior test is not far from that of compounds acting in the central nervous system as unspecific depressant agents. The results of both the conflict and exploratory behavior studies might support the hypothesis of different sites of action for the antianxiety and sedative effects of benzodiazepines.

Time-dose relationships for locomotor activity effects of morphine after acute or repeated treatment.

1. Effects of morphine sulphate (1.25, 2.5, 5, 10, 20 and 40 mg/kg i.p.) on locomotor activity of male rats were observed for 8 h after single doses in non-tolerant rats. The lower three doses had only an excitatory effect, whereas the higher three doses caused initial depression followed by a delayed excitatory effect.2. The same doses of morphine were administered daily for 30 days. No tolerance developed within this time to the excitatory effect. The locomotor excitatory effect of the higher three doses of morphine became progressively more pronounced over treatment periods of 30 days (and 48 days for 20 mg/kg), while the latency to peak activity decreased.3. An explanation of these results is suggested on the basis of two different central drug-receptor interactions affecting motility.

Morphine effects upon discriminated approach and discriminated avoidance in rats: antagonism by naloxone.

The effects of various doses of morphine (5, 10 and 20 mg/kg) alone or in combination with a constant dose of naloxone (1.25 mg/kg) were examined in rats trained on a discriminated approach schedule (in which bar pressing in the presence of a stimulus light produced food), or on a discriminated avoidance (in which the same response produced stimulus-shock termination). Since the performance of rats in the discriminated avoidance varied widely, drug effects were examined separately in groups of good, intermediate or poor performer rats. Comparable patterns of responding in the presence of light were found in the approach group and in the good performer avoidance group. Morphine induced a dose-related decrease of this responding which was identical in both cases. Other effects of morphine were a dose-related increase of escape failures in all the avoidance groups and stimulatory or depressant effects upon bar presses performed during the no light periods. All the effects of morphine were antagonized by naloxone. The data suggest that comparable patterns of responding maintained by different reinforcements can be similarly affected by morphine.

Effects of combined treatment with nortriptiline and lorazepam on conflict behavior and motility of rats.

Lorazepam attenuated the suppressant effects of punishment on the response rate of rats in the multiple schedule of reinforcement devised by Geller and Seifter (1960). Nortriptiline alone was ineffective on punished responses. Both drugs, at certain dose levels, inhibited the nonpunished response. Combined treatment with the two drugs in a dose ratio of 1:20 attenuated the effects of punishment at all dose levels tested. The effects of the combination upon both punished and nonpunished responding was greater than might be accounted for by a simple additive effect of the individual treatments. Lorazepam and nortriptiline both induced a dose-related decrease in the locomotor activity of rats; when given together the 2 drugs antagonized each other. The results give an experimental support to the clinical observations about the usefulness of the benzodiazepine-antidepressant combination in certain depressive illnesses.

Motivational properties of buprenorphine as assessed by place and taste conditioning in rats.

Buprenorphine, a mixed agonist-antagonist opioid with considerable analgesic activity, is currently indicated as a therapeutic agent with low abuse potential. Nevertheless, buprenorphine abuse has been recently reported from some countries. Thus the present experiments were performed to characterize further the motivational properties of buprenorphine in rats. Rewarding and aversive effects were assessed by place preference and taste aversion conditioning, respectively. It was found that buprenorphine (0.025, 0.050, 0.100 mg/kg s.c.) causes a significant increase in the amount of time spent on the conditioned side, but no significant decrease in saccharin consumption. Therefore buprenorphine data are not consistent with the general finding that psychoactive drugs cause rewarding and aversive effects within a similar dose range.

Role of repeated exposure to morphine in determining its affective properties: place and taste conditioning studies in rats.

Male Sprague Dawley rats were injected daily with saline (morphine naive rats) or 20 mg/kg morphine (morphine experienced rats), starting at least 12 days before training. Subsequent place and taste conditioning indicated that 2.5 mg/kg morphine caused a significant increase in the amount of time spent on the least preferred side by morphine experienced but not by morphine naive rats; furthermore, saccharin consumption was markedly decreased and slightly increased by 10-20 mg/kg morphine in naive and experienced rats, respectively. It was concluded that morphine experience enhances the reinforcing efficacy of morphine and broadens the conditions under which the drug is reinforcing; thus it possibly increases morphine abuse potential.

Morphine attenuation of a conditioned emotional response in post-dependent rats.

Rats placed in a test chamber where they had received repetitive shocks the previous day significantly reduced their motor activity; this was taken as indicator of a conditioned emotional response. Morphine attenuated this conditioned suppression of motility, possibly due to a reduction of the anxiety associated with the expectation of the noxious stimuli. Previous morphine dependence (20 mg/kg daily for 26 days) did not modify the effect of the opioid on the conditioned suppression of motility. This fact suggests that the action of morphine on pain anticipatory anxiety is similar in non-dependent and in post-dependent rats. Opioids are considered to produce pain relief in part by decreasing the anticipatory anxiety. The present results thus indicate that this important component of the analgesic action of morphine is unchanged in post-addicts.

Morphine and clonidine oral self-administration: a study in morphine dependent or abstinent rats.

The ability of morphine and of clonidine to support self-administration has been evaluated in morphine dependent or abstinent rats, using an orally reinforced operant technique (F.R. 20). It was found that: Morphine drinking can function as a reinforcer of an operant response both in dependent and abstinent rats. The bitter taste of morphine becomes a secondary reinforcer for rats usually working for the alkaloid. Clonidine supports oral self-administration in morphine abstinent, but not in morphine dependent rats. The data are in line with clinical findings and give a further evidence that oral self administration in rats can be an useful model for the analysis of drug dependence.

Stimulus properties of clonidine in chronically morphine treated rats trained to a morphine-saline discrimination.

In an operant behavior procedure of lever pressing on an FR 10 schedule of food reinforcement, morphine dependent and nondependent rats were trained to respond on a lever on one side of the food tray after a morphine (10 mg/kg i.p.) injection and to respond on a lever on the alternate side after a saline injection. Following discrimination training, in both dependent and nondependent rats saline was generalized to various doses of clonidine (10, 30 and 50 micrograms/kg i.p.). A response inhibition of about 65% was obtained with the highest dose. It was concluded that, even if clonidine can suppress signs of narcotic withdrawal, the internal state induced by morphine in an abstinent rat does differ from the one induced by clonidine in the same animal.

Stimulus-response relationships in a quickly learned escape from shock: effects of morphine.

The relationship between stimulus intensity and analgesic effectiveness of morphine was investigated by means of an operant technique. Various doses of morphine were tested in rats trained to press a bar to escape from shocks of varying intensity. Under control conditions a good linear relationship between the log of the stimulus intensity and the log of the speed to press the lever was found. Morphine showed inhibitory effects upon this escape behavior, which were greater at any given dose with greater intensity of the shock. These effects were dose related, that is, the slopes of the shock-response lines decreased by increasing the dose. The data obtained do not appear to be a consequence of a general depressant effect of the drug upon behavior and are in line with several experimental observations showing that in animals, as well as in humans, the magnitude of the analgesic effect of morphine tends to increase as pain severity increases.

Benzodiazepine effects upon Geller-Seifter conflict test in rats: analysis of individual variability.

The aim of the present study was to investigate in a large group of "drug sophisticated" animals the effect of several doses of oxazepam upon conflict behavior. To this end 43 rats, trained according to the original Geller-Seifter paradigm, were tested with 5 doses (6.25, 12.5, 20.9, 25, and 50 mg/kg IP) of oxazepam. In addition the influence of prior drug experience on the effects of benzodiazepines on punished and unpunished responding was investigated comparing data from the same animals relative to a single oxazepam treatment before and after "drug sophistication." It was found that: (1) after "drug sophistication" oxazepam effect upon the unpunished schedule is decreased, while the disinhibitory action upon punished behavior is increased, unchanged or even decreased; (2) sedative and anticonflict activities of the drug cannot be explained in terms of rate dependency and are independently assessable since, even when unpunished responding is lowered by high doses, the anxiolytic effect is masked in only 27% of the cases; (3) about 20% of the animals appear to be insensitive to the anticonflict effect of oxazepam; (4) the responsiveness to the anxiolytic effect of the drug is related to the shock intensities given during training and to the animal variability under control conditions.

Increased sensitivity to the stimulus properties of morphine in food deprived rats.

Recent research has shown that food deprivation increases opiate self-administration; in this line a first purpose of the present experiments was to determine whether the food deprivation effect could be replicated by the use of place conditioning, an alternative procedure for the study of drug reinforcement. It was found that the conditioned reinforcing properties of morphine (2.5 mg/kg IP) paired cues are greater in food deprived rats both after 1 and 3 conditioning sessions. A second objective of the work was to examine the possibility that food deprivation could also influence the discriminative stimulus properties of opiates. To this end rats trained to discriminate 10 mg/kg IP of morphine from saline were submitted to morphine generalization tests when food deprived or after 15 min supplemental feeding in the home cages. The ED50 value was significantly lower for food deprived (6.09 mg/kg) than for partially satiated (7.79 mg/kg) rats. It was concluded that food deprived rats are mores sensitive to both the reinforcing and the discriminative stimulus properties of morphine.

Time-dependent generalization of morphine stimulus properties to meperidine: antagonism by naloxone.

The time course of the stimulus generalization to morphine by meperidine (20 mg/kg) was determined in rats trained to discriminate morphine (10 mg/kg) from saline in a standard two-lever procedure with food reinforcement. It was found that morphine lever selection following meperidine was a strictly time-dependent phenomenon. Naloxone (0.3 mg/kg) antagonized the stimulus properties of both morphine and meperidine; however, the antagonism was significantly more pronounced against morphine. The results suggest that there may be certain differences in the neuropharmacology of the stimulus properties of morphine and meperidine.