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Cross-sensitization to the excitatory effect of morphine in post-dependent rats
Neuropharmacoiogy
Vol.24,
No.
9, pp.
889-893,
1985
0028-3908/85
Printed in Great Britain. All rights reserved
$3.00
+ 0.00
Copyright 0 1985 Pergamon Press Ltd
CROSS-SENSITIZATION TO THE EXCITATORY EFFECT OF MORPHINE IN POST-DEPENDENT RATS M. BARTOLETTI,M. GAIARDI, C. GUBELLINI,A. BACCHI and M. BABBINI Institute
of Pharmacology,
of Bologna, Via Irnerio 48, I-40126, Italy
University
Summary-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 20mg/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: (1) different mechanisms underlie the excitatory actions of opiates; (2) the narcotic character of a drug can be detected by challenging this drug in rats previously dependent on morphine. Key words: morphine,
methadone,
pentazocine,
motility,
cross-sensitization,
morphine
post-dependent
rats.
It is well known that rapid tolerance develops to the depressant action of morphine on locomotor activity, while sensitization has been found to occur to the stimuIant action of the alkaloid during repeated administrations (Babbini and Davis, 1972; Vasko and Domino, 1978). These changes have been explained assuming that the opiate receptor which subserves behavioura! excitation is distinct from a second one which subserves sedation (La Bella, Pinsky and Havlicek, 1979; Marqais, Bonnet and Costentin, 1981; Brady and Holtzman, 1981b). Chronic treatment could induce an increased sensitivity of the receptor mediating the stimulatory effect and a decreased sensitivity of the receptor mediating the depressant effect (Babbini and Davis, 1972; Babbini, Gaiardi and Bartoletti, 1975; Joyce and Iversen, 1979). Opiate drugs, when substituted for morphine in chronically-treated rats, caused patterns of effects very different from those obtained in naive animals and qualitatively similar to those of morphine. However differences were noted as regard to the quantitative aspect of the changed reactivity of morphinetreated rats to the various narcotic drugs. In fact, some opiates produced both cross-toterance and cross-sensitization and others almost exclusively cross-tolerance or cross-sensitization; this further supports the hypothesis of different receptors for the stimulatory and inhibitory actions (Babbini, Gaiardi and Bartoletti, 1981; Bartoletti, Gaiardi, Gubeilini and Babbini, 1983a). Tolerance to the depressant effect seemed to decline rapidly after discontinuing the chronic treatment (Babbini et al., 1975; Bartoletti, Gaiardi, Gubellini, Bacchi and Babbini, 1983b). On the contrary, the hyperactivity produced by morphine was still
significantly enhanced in post-dependent rats as compared to the naive ones (Nakamura, Ishii and Shimizu, 1978; Brady and Holtzman, 1981a; Bartoletti et al., 1983b). These results would suggest a long-lasting sensitization of the receptor site mediating the excitatory effect. Thus, narcotic drugs which do not yield crosssensitization in morphine-dependent rats should induce, in post-dependent animals, a motility pattern similar to that obtained in the naive ones. On the other hand, drugs which cause an increased excitatory action in morphine-dependent rats should maintain this effect in post-dependent animals. The present work was performed to test these predictions. METHODS
Subjects
The subjects were male Sprague-Dawley rats, weighing about 300 g at the start of the experiment. The animals were housed three to a cage with 12 hr room illumination and constant temperature (22°C). Food and water were continuously available. Drugs
The drugs used were morphine HCI, methadone HCI, meperidine HCl and pentazocine lactate. All were dissolved with saline and injected intraperitoneally in a volume of 2 ml/kg. Apparatus and procedure
Activity was measured using six jiggle cage actometers; the oscillatory movements of the cage were counted by means of solid-state circuits. The ac889
M. BARTOLETTIet al.
890
(condition, dose, time). Single comparisons, doses and times were performed by F tests.
tometers were equipped with holders for food and water. Tests were run during the day and in the dark. The animais were left in the activity cages for 1 hr in order to acclimatize to the apparatus; then they were injected with the test drug and put again into the actometers 15 min later. Motility data were recorded for 6 hr, readings being made at hourly intervals. The effects of narcotic drugs upon non-dependent and post-dependent rats were tested in two groups of 26 and 21 animals respectively. Dependence on morphine was induced by daily injection of 20 mg/kg of morphine for 30 days; non-dependent animals were treated with saline. Tests were run 1 to 4 months after the termination of chronic treatment since previous work (Babbini et al., 1975) demonstrated that the heightened response to morphine in post-dependent animals remained stable for at Last 8 months after withdrawal. Nondependent and post-dependent rats were challenged every 15 days or more with various doses of morphine (5, lo,20 and 40 mg/kg), methadone (2.5,5 and 10 mg/kg), meperidine (10, 20 and 40 mg/kg) and pentazocine (15, 30 and 60 mg/kg). The challenge treatments (a given dose of a given drug) were given randomly over the whole period of 3 months. Each rat received a maximum of 5 treatments. The locomotor baseline activity after injection of saline was recorded in post-dependent as well as in nondependent rats. Statistical
RESULTS
The time-effect curves for morphine in naive and post-dependent rats are compared in Fig. 1. Five milIigrammes/kilogramme of the drug produced both in naive and post-dependent rats a significant initial increase in activity (return to baseiine level at the third hour of recording); at the first hour, a significantly greater excitatory effect was obtained in post-dependent than in naive animals. After lOmg/kg of morphine, naive rats showed a significant increase in motility at the second and third hour of recording; post-dependent animals also exhibited a significant stimulant effect during the first hour. A larger dose of morphine (20 mg/kg) induced in naive and in post-dependent animals a significant and similar depressant effect. However the subsequent excitatory effect, lasting for 3 hr, was significantly greater in post-dependent rats. Also, with 4Omg/kg of morphine, the initial si~i~c~t depressant effect was equal in both groups, but some increase in activity was induced by this dose only in post-dependent rats. The motility pattern produced by methadone in naive and in post-dependent rats is shown in Fig. 2. The smallest dose (2.5 mg/kg) induced in both groups a similar initial significant excitatory effect. By increasing the dose (5 mg/kg) the stimulant effect, at the first hour of recording, was no longer present in naive rats but was evident in post-dependent animals. The return to baseline for motility was observed at the fourth hour. After the largest dose of methadone (10 mg/kg) the same depressant effect was obtained in naive and in post-dependent rats, but only these
anatysis
Data were expressed as differences from the mean corresponding values for saline at each hour of recording. The time-effect curves were compared in the post-dependent and non-dependent rats by an analysis of variance following a three factor design
5 mg/kg
IO mg/
kg
/
/
20 mg/kg
40 mg/kg
* I
I
I
within
I
123456
123456
Hr
Fig. I. Effects of various doses of morphine upon locomotor activity of naive (#---e) and post-dependent rats (O----O). Each point represents the mean of S-10 values. Statistically different from 0 ( x f, statistically different from naive animals (*); P < 0.05.
Cross-sensitization
2.5
mg/kg
to morphine
5mg
/kg
891
lOmg/kg
123456
123456
Hr
Fig. 2. Effects of various doses of methadone upon locomotor activity of naive (+---0) and post-dependent rats (O----O). Each point represents the mean of 612 values. Statistically different from 0 (x ), statistically different from naive animals (*); P < 0.05.
animals showed a delayed significant increase in activity at the second and third hour of recording. Meperidine, at all the doses tested, did not induce any marked increase in activity in either naive or in post-dependent rats. An initial considerable depression of activity was induced by 20 and 40mg of the drug both in naive and post-dependent animals (Fig. 3). Motility data with pentazocine are shown in Fig. 4. Fifteen milligrammes/kilogramme of the drug produced in naive and post-dependent rats similar time-effect curves with an initial significant increase in activity and return to baseline at the third hour of recording. Some differences in the level of activity between naive and post-dependent animals were observed after administration of 30mg/kg of pentazocine, but these differences only approached statistical significance (P = 0.07). The largest dose of
IO mg
pentazocine induced, both in naive and postdependent animals, an initial depression of activity and a delayed significant increase of motility (return to baseline level after 4 hr). DISCUSSION
The data reported in the present work confirm the selective recovery of changes induced by chronic treatment upon the effects of morphine on motility. Tolerance to the depressant effect was no longer present in post-dependent rats; in fact, the hypomotility induced by the largest doses of morphine was similar in naive and post-dependent animals. On the contrary, a persistent hypermotility effect which was greater in post-dependent than in naive animals was observed. These results support the concept of different opiate receptors mediating the motility
ZOmg/kg
/kg
40mgIkg
I
I
123456
,
I
/
I
-ILIIIII 12
3456
123456
Hr
Fig. 3. Effects of various doses of meperidine upon locomotor activity of naive (a----•) and post-dependent rats (O----O). Each point represents the mean of 5-7 values. Statistically different from 0 (x), statistically different from naive animals (*); P < 0.05.
M. BARTOLETII ei al.
892
> 0L
k
2
I
15
mg /kg
123456
30 mg /kg
12
3456
60 mg/kg
123456
Hr
Fig. 4. Effects of various doses of pentazocine upon locomotor activity of naive (0-O) and post-dependent rats (O----G). Each point represents the mean of 5-8 values. Statistically different from 0 (x ), statistically different from naive animals (*); P < 0.05.
effect of morphine and of protracted functional modification in the receptor site, mediating the excitatory effect after chronic narcotic treatment. Methadone, a classical narcotic agonist, behaved like morphine in post-dependent rats. Differences between naive and post-dependent animals were evident after 5 and 10 mg/kg of the drug, but no dose induced, in post-dependent rats, excitatory effects as marked as those caused by morphine. These findings can be related to other data which indicate that: (1) The stimulant effects of methadone are similar to those of morphine but of less magnitude in naive rats (Davis and Brister, 1973; Babbini, Gaiardi and Bartoletti, 1979; present results) as well as in rats treated chronically with the alkaloid (Babbini et al., 1979). (2) The amplification of the excitatory effects upon motility during repeated daily administration occurs with methadone also but the peak of the excitation reached with this drug remains lower than that induced by chronic treatment with morphine (Davis, Hemnani and Pace, 1982). On the whole the findings described here, together with data in the literature, suggest that the efficacy of the excitatory actions of methadone on motility is less than that of morphine. The time-effect curves for meperidine in postdependent rats did not differ from those obtained in naive animals. According to previous results (Bartoletti et al., 1983a), meperidine given to rats receiving morphine chronically showed a complete cross-tolerance, but only a small, if any, sensitization of the excitatory effect. It can be noted that the stimulatory actions of meperidine in naive animals were poor, as also reported by Davis and Brister (1973), even if an increased sensitivity to these actions can be induced by repeated constant dose treatment (Davis et al., 1982). Differences between the stimulant actions of morphine and meperidine have also been observed in other experimental situations. For exam-
ple, it has been reported that meperidine, unlike morphine, does not stimulate food intake in rats (Lowy and Yim, 1983). Meperidine is considered to be an opiate somewhat different from morphine because large doses exert behavioural actions which are not easily antagonized by naloxone and it also has psychotomimetic effects (Harris, 1980). In this regard, the mechanisms underlying the excitatory actions of meperidine upon locomotor activity could be different from those of other classical narcotic analgesics. Pentazocine showed a slight sensitization to the excitatory effect in post-dependent rats. The stimulatory component of the action of pentazocine was evident in naive rats and after repeated daily treatment it became greater than that induced by morphine under the same conditions (Davis et al., 1982). However the acute stimulant action of this agonist-antagonist was not attenuated by large doses of naloxone (Holtzman and Jewett, 1972) indicating that pentazocine also could affect the motility of rats by mechanisms which are in some way different from those invoked for morphine. Nevertheless, the results of the present study do not agree with the marked cross-sensitization to pentazocine obtained in morphine-dependent rats (Babbini et al., 1981). It should be noted that pentazocine appears to possess both sigma and kappa agonist activity (Gilbert and Martin, 1976) and it has been suggested that the kappa depressant effect antagonizes the sigma locomotor-stimulant effect (Iwamoto, 1981). Morphine, on the other hand, exerts weak activity at the kappa receptor (Gilbert and Martin, 1976). Therefore discrepancies between results obtained in dependent and in post-dependent rats could be explained assuming that chronic treatment with morphine decreases the sensitivity of the kappa receptor and induces, in consequence, a further increase of the
Cross-sensitization
motility response to pentazocine In conclusion,
the results
in dependent
of the present
rats. study
performed in rats previously dependent on morphine confirm the previously reported sensiti~tion to the excitatory effect of the alkaloid and demonstrate a cross-sensitization between morphine and some, but not all, opiates. In this regard, it is worth noting that only a
classical narcotic agonist, i.e. methadone, was able to cause a marked gross-sensitization in rats nreviouslv dependent on morphine; thus, a cross-sensitization seems to be indicative of the narcotic character of a drug.
In previous
work
(Babbini
et al.,
1981;
Bartoletti et aI.. 1983a1 it was sueeested that locomotor activity in morphine-dependent rats could be used to perform a substitution test to detect the narcotic character of a drug. In the light of the present results it is proposed that the morphine-like Y”
properties
of a drug
can
also
be determined
by
challenging this drug in rats previously dependent on morphine. ~ck~ow~edgeme~?s-This work was supported by CNR (National Research Council of Italy) on the “Progetto Finalizzato Medicina Preventiva e Riabilitativa, Sottoprogetto Tossicodipendenza’” (contract no. 83.02522.56). REFERENCES Babbini M. and Davis W. M. (1972) Time-dose relabonships for locomotor activity effects of morphine after acute or repeated treatment. Br. J. Pharmac. 46: 2 13-224.
Babbini M., Gaiardi M. and Bartoletti M. (1975) Persistence of chronic morphine effects upon activity in rats 8 months after ceasing the treatment. Neuropharmacoiogy 14: 61 I-614. Babbini M., Gaiardi M. and Bartoletti M. (1979) Dose-time motility effects of methadone when substituted for morphine in chronically treated rats. Pharmac. Res. Commun. 11: 809-816. Babbini M., Gaiardi M. and Bartoletti M. (1981) Effects of pentazocine and nalorphine on motility in chronically morphine treated rats: a potential substitution test to detect the narcotic character of a drug. Neuropharmaeoiogy 20: 49-53.
Bartoletti M.. Gaiardi M.. Gubellini C. and Babbini M. (1983a) Further evidence for a motility substitution test as a tool to detect the narcotic character of new drugs in rats. Neuropharmacology 22: 177-I 8 1.
to morphine
893
Bartoletti M., Gaiardi M., Gubellini C., Bacchi A. and Babbini M. (1983b) Long-term sensitization to the excitatory effects of morphine. A motility study in postdependent rats. ~europ~armacolog~ 22: 1193-f 196. Brady L. S. and Holtzman S. G. (198la) Locomotor activity in morphine-dependent and post-dependent rats. Pharmat. Biochem. Behav. 14: 361-370. Brady L. S. and Holtzman S. G. (1981b) Effects of intraventricular morphine and enkephalins on locomotor activity in nondependent, morphine-dependent and post-
dependent rats. J. Pharmae. exp. Ther. 218: 613-620. Davis W. M. and Brister C. C. (1973) Acute effects of narcotic analgesics on behavioral arousal in the rat. J. Pharmac. Sci. 62: 974-979. Davis W. M., Hemnani K. L. and Pace H. B. (1982) Motility response of rats to chronic constant-dose treatment with narcotics. Pharmac. Biochem. Behav. 17: 489494. Gilbert P. E. and Martin W. R. (1976) The effects of morphineand nalorphine-like drugs in the nondependent, mo~hine-dependent and cyclazocinedependent chronic spinal dog. J. Pharmac. exp. Ther. 1% 66-82.
Harris R. A. (1980) Interactions between narcotic agonists,
partial agonists and antagonists evaluated by schedulecontrolled behavior. J. Pharmac. exp. Ther. 213: 497-503. Holtzman S. G. and Jewett R. E. (1972) Some actions of pentazocine on behavior and brain monoamines in the rat. J. Pharmac. exp. Ther. 181: 346-356. Iwamoto E. T. (1981) Locomotor activity and antinociception after putative mu, kappa and sigma opioid receptor agonists in the rat: influence of dopaminergic agonists and antagonists. J. Pharmac. exp. Ther. 217: 45 l-460. Joyce E. M. and
lversen S. D. (1979) The effects of morphine applied locally to mesencephalic dopamine cell bodies on spontaneous motor activity in the rat. Neurosci.
Lett. 14: 207-212.
LaBella F. S., Pinsky C. and Havlicek V. (1979) Morphine derivatives with diminished opiate receptor potency show enhanced central excitatory activity. Brain Res. 174: 263-27 1. Lowy M. T. and Yim G. K. W. (1983) Stimulation of food intake following opiate agonists in rats but not hamsters. P~yc~o~harmaco1og.y 81: 28-32.
Marcais H., Bonnet J. J. and Costentin J. (1981) Evidence for sedative effects of low doses of morphine in mice involving receptors insensitive to naloxone. Ll@ Sci. 28: 2737-2742. Nakamura H., Ishii K. and Shimizu M. (1979) Some altered responses in rats formerly dependent on morphine. Psy~hopharmaco~ogy 56: 269-277.
Vasko M. R. and Dominio E. F. (1978) Tolerance development to the biphasic effects of morphine on locomotor activity and brain acetylcholine in the rat. J. Pharmac. exp. Ther. 207: 848-858.
Vol.24,
No.
9, pp.
889-893,
1985
0028-3908/85
Printed in Great Britain. All rights reserved
$3.00
+ 0.00
Copyright 0 1985 Pergamon Press Ltd
CROSS-SENSITIZATION TO THE EXCITATORY EFFECT OF MORPHINE IN POST-DEPENDENT RATS M. BARTOLETTI,M. GAIARDI, C. GUBELLINI,A. BACCHI and M. BABBINI Institute
of Pharmacology,
of Bologna, Via Irnerio 48, I-40126, Italy
University
Summary-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 20mg/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: (1) different mechanisms underlie the excitatory actions of opiates; (2) the narcotic character of a drug can be detected by challenging this drug in rats previously dependent on morphine. Key words: morphine,
methadone,
pentazocine,
motility,
cross-sensitization,
morphine
post-dependent
rats.
It is well known that rapid tolerance develops to the depressant action of morphine on locomotor activity, while sensitization has been found to occur to the stimuIant action of the alkaloid during repeated administrations (Babbini and Davis, 1972; Vasko and Domino, 1978). These changes have been explained assuming that the opiate receptor which subserves behavioura! excitation is distinct from a second one which subserves sedation (La Bella, Pinsky and Havlicek, 1979; Marqais, Bonnet and Costentin, 1981; Brady and Holtzman, 1981b). Chronic treatment could induce an increased sensitivity of the receptor mediating the stimulatory effect and a decreased sensitivity of the receptor mediating the depressant effect (Babbini and Davis, 1972; Babbini, Gaiardi and Bartoletti, 1975; Joyce and Iversen, 1979). Opiate drugs, when substituted for morphine in chronically-treated rats, caused patterns of effects very different from those obtained in naive animals and qualitatively similar to those of morphine. However differences were noted as regard to the quantitative aspect of the changed reactivity of morphinetreated rats to the various narcotic drugs. In fact, some opiates produced both cross-toterance and cross-sensitization and others almost exclusively cross-tolerance or cross-sensitization; this further supports the hypothesis of different receptors for the stimulatory and inhibitory actions (Babbini, Gaiardi and Bartoletti, 1981; Bartoletti, Gaiardi, Gubeilini and Babbini, 1983a). Tolerance to the depressant effect seemed to decline rapidly after discontinuing the chronic treatment (Babbini et al., 1975; Bartoletti, Gaiardi, Gubellini, Bacchi and Babbini, 1983b). On the contrary, the hyperactivity produced by morphine was still
significantly enhanced in post-dependent rats as compared to the naive ones (Nakamura, Ishii and Shimizu, 1978; Brady and Holtzman, 1981a; Bartoletti et al., 1983b). These results would suggest a long-lasting sensitization of the receptor site mediating the excitatory effect. Thus, narcotic drugs which do not yield crosssensitization in morphine-dependent rats should induce, in post-dependent animals, a motility pattern similar to that obtained in the naive ones. On the other hand, drugs which cause an increased excitatory action in morphine-dependent rats should maintain this effect in post-dependent animals. The present work was performed to test these predictions. METHODS
Subjects
The subjects were male Sprague-Dawley rats, weighing about 300 g at the start of the experiment. The animals were housed three to a cage with 12 hr room illumination and constant temperature (22°C). Food and water were continuously available. Drugs
The drugs used were morphine HCI, methadone HCI, meperidine HCl and pentazocine lactate. All were dissolved with saline and injected intraperitoneally in a volume of 2 ml/kg. Apparatus and procedure
Activity was measured using six jiggle cage actometers; the oscillatory movements of the cage were counted by means of solid-state circuits. The ac889
M. BARTOLETTIet al.
890
(condition, dose, time). Single comparisons, doses and times were performed by F tests.
tometers were equipped with holders for food and water. Tests were run during the day and in the dark. The animais were left in the activity cages for 1 hr in order to acclimatize to the apparatus; then they were injected with the test drug and put again into the actometers 15 min later. Motility data were recorded for 6 hr, readings being made at hourly intervals. The effects of narcotic drugs upon non-dependent and post-dependent rats were tested in two groups of 26 and 21 animals respectively. Dependence on morphine was induced by daily injection of 20 mg/kg of morphine for 30 days; non-dependent animals were treated with saline. Tests were run 1 to 4 months after the termination of chronic treatment since previous work (Babbini et al., 1975) demonstrated that the heightened response to morphine in post-dependent animals remained stable for at Last 8 months after withdrawal. Nondependent and post-dependent rats were challenged every 15 days or more with various doses of morphine (5, lo,20 and 40 mg/kg), methadone (2.5,5 and 10 mg/kg), meperidine (10, 20 and 40 mg/kg) and pentazocine (15, 30 and 60 mg/kg). The challenge treatments (a given dose of a given drug) were given randomly over the whole period of 3 months. Each rat received a maximum of 5 treatments. The locomotor baseline activity after injection of saline was recorded in post-dependent as well as in nondependent rats. Statistical
RESULTS
The time-effect curves for morphine in naive and post-dependent rats are compared in Fig. 1. Five milIigrammes/kilogramme of the drug produced both in naive and post-dependent rats a significant initial increase in activity (return to baseiine level at the third hour of recording); at the first hour, a significantly greater excitatory effect was obtained in post-dependent than in naive animals. After lOmg/kg of morphine, naive rats showed a significant increase in motility at the second and third hour of recording; post-dependent animals also exhibited a significant stimulant effect during the first hour. A larger dose of morphine (20 mg/kg) induced in naive and in post-dependent animals a significant and similar depressant effect. However the subsequent excitatory effect, lasting for 3 hr, was significantly greater in post-dependent rats. Also, with 4Omg/kg of morphine, the initial si~i~c~t depressant effect was equal in both groups, but some increase in activity was induced by this dose only in post-dependent rats. The motility pattern produced by methadone in naive and in post-dependent rats is shown in Fig. 2. The smallest dose (2.5 mg/kg) induced in both groups a similar initial significant excitatory effect. By increasing the dose (5 mg/kg) the stimulant effect, at the first hour of recording, was no longer present in naive rats but was evident in post-dependent animals. The return to baseline for motility was observed at the fourth hour. After the largest dose of methadone (10 mg/kg) the same depressant effect was obtained in naive and in post-dependent rats, but only these
anatysis
Data were expressed as differences from the mean corresponding values for saline at each hour of recording. The time-effect curves were compared in the post-dependent and non-dependent rats by an analysis of variance following a three factor design
5 mg/kg
IO mg/
kg
/
/
20 mg/kg
40 mg/kg
* I
I
I
within
I
123456
123456
Hr
Fig. I. Effects of various doses of morphine upon locomotor activity of naive (#---e) and post-dependent rats (O----O). Each point represents the mean of S-10 values. Statistically different from 0 ( x f, statistically different from naive animals (*); P < 0.05.
Cross-sensitization
2.5
mg/kg
to morphine
5mg
/kg
891
lOmg/kg
123456
123456
Hr
Fig. 2. Effects of various doses of methadone upon locomotor activity of naive (+---0) and post-dependent rats (O----O). Each point represents the mean of 612 values. Statistically different from 0 (x ), statistically different from naive animals (*); P < 0.05.
animals showed a delayed significant increase in activity at the second and third hour of recording. Meperidine, at all the doses tested, did not induce any marked increase in activity in either naive or in post-dependent rats. An initial considerable depression of activity was induced by 20 and 40mg of the drug both in naive and post-dependent animals (Fig. 3). Motility data with pentazocine are shown in Fig. 4. Fifteen milligrammes/kilogramme of the drug produced in naive and post-dependent rats similar time-effect curves with an initial significant increase in activity and return to baseline at the third hour of recording. Some differences in the level of activity between naive and post-dependent animals were observed after administration of 30mg/kg of pentazocine, but these differences only approached statistical significance (P = 0.07). The largest dose of
IO mg
pentazocine induced, both in naive and postdependent animals, an initial depression of activity and a delayed significant increase of motility (return to baseline level after 4 hr). DISCUSSION
The data reported in the present work confirm the selective recovery of changes induced by chronic treatment upon the effects of morphine on motility. Tolerance to the depressant effect was no longer present in post-dependent rats; in fact, the hypomotility induced by the largest doses of morphine was similar in naive and post-dependent animals. On the contrary, a persistent hypermotility effect which was greater in post-dependent than in naive animals was observed. These results support the concept of different opiate receptors mediating the motility
ZOmg/kg
/kg
40mgIkg
I
I
123456
,
I
/
I
-ILIIIII 12
3456
123456
Hr
Fig. 3. Effects of various doses of meperidine upon locomotor activity of naive (a----•) and post-dependent rats (O----O). Each point represents the mean of 5-7 values. Statistically different from 0 (x), statistically different from naive animals (*); P < 0.05.
M. BARTOLETII ei al.
892
> 0L
k
2
I
15
mg /kg
123456
30 mg /kg
12
3456
60 mg/kg
123456
Hr
Fig. 4. Effects of various doses of pentazocine upon locomotor activity of naive (0-O) and post-dependent rats (O----G). Each point represents the mean of 5-8 values. Statistically different from 0 (x ), statistically different from naive animals (*); P < 0.05.
effect of morphine and of protracted functional modification in the receptor site, mediating the excitatory effect after chronic narcotic treatment. Methadone, a classical narcotic agonist, behaved like morphine in post-dependent rats. Differences between naive and post-dependent animals were evident after 5 and 10 mg/kg of the drug, but no dose induced, in post-dependent rats, excitatory effects as marked as those caused by morphine. These findings can be related to other data which indicate that: (1) The stimulant effects of methadone are similar to those of morphine but of less magnitude in naive rats (Davis and Brister, 1973; Babbini, Gaiardi and Bartoletti, 1979; present results) as well as in rats treated chronically with the alkaloid (Babbini et al., 1979). (2) The amplification of the excitatory effects upon motility during repeated daily administration occurs with methadone also but the peak of the excitation reached with this drug remains lower than that induced by chronic treatment with morphine (Davis, Hemnani and Pace, 1982). On the whole the findings described here, together with data in the literature, suggest that the efficacy of the excitatory actions of methadone on motility is less than that of morphine. The time-effect curves for meperidine in postdependent rats did not differ from those obtained in naive animals. According to previous results (Bartoletti et al., 1983a), meperidine given to rats receiving morphine chronically showed a complete cross-tolerance, but only a small, if any, sensitization of the excitatory effect. It can be noted that the stimulatory actions of meperidine in naive animals were poor, as also reported by Davis and Brister (1973), even if an increased sensitivity to these actions can be induced by repeated constant dose treatment (Davis et al., 1982). Differences between the stimulant actions of morphine and meperidine have also been observed in other experimental situations. For exam-
ple, it has been reported that meperidine, unlike morphine, does not stimulate food intake in rats (Lowy and Yim, 1983). Meperidine is considered to be an opiate somewhat different from morphine because large doses exert behavioural actions which are not easily antagonized by naloxone and it also has psychotomimetic effects (Harris, 1980). In this regard, the mechanisms underlying the excitatory actions of meperidine upon locomotor activity could be different from those of other classical narcotic analgesics. Pentazocine showed a slight sensitization to the excitatory effect in post-dependent rats. The stimulatory component of the action of pentazocine was evident in naive rats and after repeated daily treatment it became greater than that induced by morphine under the same conditions (Davis et al., 1982). However the acute stimulant action of this agonist-antagonist was not attenuated by large doses of naloxone (Holtzman and Jewett, 1972) indicating that pentazocine also could affect the motility of rats by mechanisms which are in some way different from those invoked for morphine. Nevertheless, the results of the present study do not agree with the marked cross-sensitization to pentazocine obtained in morphine-dependent rats (Babbini et al., 1981). It should be noted that pentazocine appears to possess both sigma and kappa agonist activity (Gilbert and Martin, 1976) and it has been suggested that the kappa depressant effect antagonizes the sigma locomotor-stimulant effect (Iwamoto, 1981). Morphine, on the other hand, exerts weak activity at the kappa receptor (Gilbert and Martin, 1976). Therefore discrepancies between results obtained in dependent and in post-dependent rats could be explained assuming that chronic treatment with morphine decreases the sensitivity of the kappa receptor and induces, in consequence, a further increase of the
Cross-sensitization
motility response to pentazocine In conclusion,
the results
in dependent
of the present
rats. study
performed in rats previously dependent on morphine confirm the previously reported sensiti~tion to the excitatory effect of the alkaloid and demonstrate a cross-sensitization between morphine and some, but not all, opiates. In this regard, it is worth noting that only a
classical narcotic agonist, i.e. methadone, was able to cause a marked gross-sensitization in rats nreviouslv dependent on morphine; thus, a cross-sensitization seems to be indicative of the narcotic character of a drug.
In previous
work
(Babbini
et al.,
1981;
Bartoletti et aI.. 1983a1 it was sueeested that locomotor activity in morphine-dependent rats could be used to perform a substitution test to detect the narcotic character of a drug. In the light of the present results it is proposed that the morphine-like Y”
properties
of a drug
can
also
be determined
by
challenging this drug in rats previously dependent on morphine. ~ck~ow~edgeme~?s-This work was supported by CNR (National Research Council of Italy) on the “Progetto Finalizzato Medicina Preventiva e Riabilitativa, Sottoprogetto Tossicodipendenza’” (contract no. 83.02522.56). REFERENCES Babbini M. and Davis W. M. (1972) Time-dose relabonships for locomotor activity effects of morphine after acute or repeated treatment. Br. J. Pharmac. 46: 2 13-224.
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Bartoletti M.. Gaiardi M.. Gubellini C. and Babbini M. (1983a) Further evidence for a motility substitution test as a tool to detect the narcotic character of new drugs in rats. Neuropharmacology 22: 177-I 8 1.
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893
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