European Journal of Pharmacology, 54 (1979) 61--68

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© Elsevier/North-Holland Biomedical Press

THE EFFECT OF DOPAMINERGIC MODIFIERS ON MORPHINE-INDUCED ANALGESIA AND RESPIRATORY DEPRESSION * KIP L. MCGILLIARD ** and A.E. TAKEMORI Department of Pharmacology, University of Minnesota, Minneapolis, Minnesota 55455, U.S.A.

Received 28 July 1978, revised MS received 24 October 1978, accepted 30 October 1978

K.L. McGILLIARD and A.E. TAKEMORI, The effect of dopaminergic modifiers on morphine-induced analgesia and respiratory depression, European J. Pharmacol. 54 (1979) 61--68. The influences of the dopaminergic system on morphine-induced analgesia and respiratory depression were compared using modulators of dopaminergic activity. Blockade of dopaminergic receptors by haloperidol or pimozide produced a potentiation of morphine analgesia, while stimulation of dopaminergic activity by L-dopa methyl ester inhibited morphine analgesia. Morphine-induced depression of respiratory rate was potentiated by haloperidol and inhibited by pimozide or L-dopa methyl ester. These results suggest that the dopaminergic system plays a modulating role in morphine-induced analgesia, but not in morphine-induced respiratory depression. Analgesia

Respiratory depression

Morphine

1. Introduction It is widely believed that the dopaminergic system is involved in morphine analgesia (see reviews by Smith, 1972; Lal, 1975; Takemori, 1976). Most evidence suggests t h a t stimulation of the dopaminergic system inhibits morphine analgesia and depression of the dopaminergic system potentiates morphine analgesia (Contreras and Tamayo, 1966; Major and Pleuvry, 1971; Pleuvry and Tobias, 1971; Calcutt and Spencer, 1971; VanderWende and Spoerlein, 1972, 1973; Nakamura et al., 1973; Eidelberg and Erspamer, 197 5; Tulunay et al., 1975, 1976; Rodgers, 1977). We have recently shown t h a t the receptor

Dopaminergicantagonists

L-Dopa

interactions involved in narcotic-induced respiratory depression differ from those involved in narcotic-induced analgesia (McGilliard and Takemori, 1978a,b). While m a n y studies have been conducted on dopaminergic influences on morphine analgesia, there have been none concerning dopaminergic influences on morphine-induced respiratory depression. In order to further characterize the action of morphine, we have studied the analgesic and respiratory effects of morphine in combination with three modulators of dopaminergic activity, haloperidol, pimozide and L-dihydroxyphenylalanine (L-dopa) methyl ester.

2. Materials and methods * This investigation was supported by U.S. Public Health Service Grant DA-00289. ** Partially supported by U.S. Public Health Service Grant GM0117. Present address: Department of Pharmacology, University of Oregon Health Sciences Center, 3181 S.W. Sam Jackson Park Road, Portland, Oregon 97201, U.S.A.

2.1. A n i m a l s

Male Swiss-Webster mice (Biolab, White Bear Lake, Minn.) weighing between 20 and 30 g were used in all experiments. Each animal was used only once.

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2.2. Respiratory measurements Respiratory rate was measured in unanesthetized mice using a b o d y plethysmograph modified from the method of Alarie (1966). The apparatus has been described previously (McGilliard and Takemori, 1978a). We have previously shown that morphine produces dose-related decreases in respiratory rate, tidal volume and minute volume of mice, and that respiratory rate is a good quantitative index of morphine-induced respiratory depression in this species (McGilliard and Takemori, 1978a). Three control measurements of 20 sec duration were obtained, after which mice were injected with drug s.c. in the scalp without removing them from the plethysmographs. Control respiratory rates averaged 262-+ 2 (S.E.) inspirations/min in = 323). A b o u t 1 mouse in 10 failed to adjust to the apparatus and was rejected. Post-injection rates were expressed as percent of the individual control means. The IDs0 of morphine, defined as the dose of morphine which decreases respiratory rate b y half of the maximal depression of 56% (McGilliard and Takemori, 1978a), was determined 30 min after injection of morphine. At least 19 mice were used to determine each dose--response curve and IDs0.

2.3. Analgesic assay Analgesia was measured by the tail-flick method of D ' A m o u r and Smith (1941) as modified by Hayashi and Takemori (1971}. The animal responses were made quantal by establishing an end point which represented a significant increase in reaction time 30 min after injection of morphine. This end point was an increase in the reaction time of an individual animal of greater than three standard deviations of the control mean reaction time for all animals used in the assay. The usual control mean reaction time was 1.59 + 0.02 (S.E.) sec. At least 24 animals were used to determine each dose--response curve and EDs0. These data, as well as the respiratory

K.L. MCGILLIARD, A.E. TAKEMORI

data, were analyzed by the parallel line assay of Finney (1964) using a CDC 3300 (Control Data Corporation, Minneapolis, Minn.) computer. The EDs0 (or IDs0) of morphine after various treatments with dopaminergic modifiers was considered significantly different from the control EDs0 of morphine if the experimental EDs0 lay outside the 95% confidence interval of the control EDs0 and the control EDs0 lay outside the 95% confidence interval of the experimental EDs0.

2.4. Drugs Morphine sulfate was obtained from Merck and Co., Inc. (Rahway, N.J.). Naloxone hydrochloride was a gift of Endo Laboratories, Inc. (Garden City, N.Y.). Haloperidol and pimozide were gifts of McNeil Laboratories, Inc. (Fort Washington, Pa.) and Janssen Pharmaceutica N.V. (Beerse, Belgium), respectively. L-~-3,4-dihydroxyphenylalanine methyl ester hydrochloride was obtained from Sigma Chemical Company (St. Louis, Mo.). Each of the above drugs was dissolved in saline solution except haloperidol, which was dissolved in a minimum a m o u n t of dilute HC1, and pimozide, which was dissolved in 3% citric acid. The acidic solutions were then partially neutralized with NaOH (final pH 5 . 0 - 6 . 0 ) and diluted to final volume with saline. The injection volume of all solutions was 10 ml/kg b o d y weight.

3. Results

3.1. Effect o f dopaminergic modifiers on tailflick latency and respiratory rate The effects of various dopaminergic modifiers on nociceptive responses of mice are summarized in table 1. Tail-flick reaction times were not altered by haloperidol or by the lower dose of L-dopa methyl ester (100 m g / k g ) 30 rain after s.c. injection. However, 30 min after administration of 0.5 or 2.0 mg/

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MORPHINE AND DOPAMINE INTERACTION TABLE 1 Effect o f dopaminergic modifiers on respiratory rate and tail-flick reaction times in mice. Treatment 1

Respiratory rate (% o f c o n t r o l ) + S.E.

Change in tailflick reaction time (sec + S.E.)

Saline L-Dopa m e t h y l ester (50 mg/kg) L-Dopa m e t h y l ester (100 mg/kg) L-Dopa m e t h y l ester (200 mg/kg) Haloperidol (0.25 mg/kg) Haloperidol (0.5 mg/kg) Haloperidol (1.0 mg/kg) Haloperidol (2.0 mg/kg) Haloperidol (2.0 mg/kg) + n a l o x o n e (1.0 mg/kg) Pimozide (0.25 mg/kg) P i m o z i d e (0.5 mg/kg) P i m o z i d e (2.0 mg/kg)

99.8 99.6 91,2 112,8 92,6 87.5 80.8 70.4 74.8 99.2 95.2 97.5

--0.01 + 0.03

+- 2.8 + 4,6 + 4.5 + 4.7 + 1.8 + 2.6 + 1.4 + 5.6 + 3.8 + 2.6 + 4.3 + 4.0

2 2 2 2 2

--0.02 + 0.07 --0.16 + 0.05 3 +0.03 + 0.05 --0.04 + O.06

+0.11 + 0 . 0 4 3 +0.19 + 0.07 3

I All drugs were administered s.c. 30 rain before testing. 2 Values were significantly different f r o m saline controls (P < 0 . 0 5 ) . 3 Values were significantly different f r o m pre-drug reaction times (P < 0.05, paired t-test).

kg of pimozide, reaction times were significantly increased over pre-drug controls. The higher dose of L-dopa methyl ester (200 mg/ kg) significantly enhanced the nociceptive response. Also summarized in table 1 are the effects of dopaminergic modifiers on the respiratory rate of mice. Haloperidol was found to depress respiration in a dose-dependent manner. 30 min after s.c. administration of doses of 0.5 mg/kg or higher, the respiratory rates of haloperidol-treated mice were significantly lower than those of saline-treated controls. The depression was not blocked by 1.0 mg/kg of naloxone. The highest dose of L-dopa methyl ester tested (200 mg/kg), produced a significant hyperventilation. Respiratory rate was unaltered by pimozide at the doses tested throughout a 2-h testing period.

3.2. Effect o f dopaminergic modifiers on morphine-induced analgesia and respiratory depression The EDs0 and IDs0 values of morphine for analgesia and respiratory depression, respectively, after various treatments axe listed in

table 2. The doses of dopaminergic modifiers used in this study were in the range of those reported by other investigators to influence the analgesic effect of morphine in mice. For each drug, except pimozide, at least one dose--response curve was determined at a dose of modifier which did not alter respiratory rate or tail-flick latency when given alone. The dopaminergic receptor blocking agent, haloperidol, potentiated both morphineinduced respiratory depression and analgesia. At the higher dose of haloperidol the IDs0 of morphine for respiratory depression was reduced to one-ninth, while the EDs0 of morphine for analgesia was reduced to onesixth of control (fig. 1). However, at the lower dose of haloperidol the potentiating effect was more prominent in the tail-flick assay than on respiratory rate. In contrast, pimozide, another dopaminergic receptor blocker, potentiated only the analgesic effect of morphine (fig. 2a). This potentiation was dose-related. On the other hand, pimozide administration resulted in a noncompetitive type of inhibition of morphine-induced respiratory depression (fig. 2b}. When given simultaneously with 2.0 mg/kg

64

K.L. MCGILLIARD, A.E. TAKEMORI

TABLE 2 Effect of dopaminergic modifiers on morphine-induced respiratory depression and analgesia. Treatment l

IDs0 or EDs0 of morphine sulfate (mg/kg) (95% confidence limits) Respiratory rate

Control Haloperidol (0.25 mg/kg) Haloperidol (1.0 mg/kg) Pimozide (0.25 mg/kg) Pimozide (0.5 mg/kg) Pimozide (2.0 mg/kg) L-Dopa methyl ester (100 mg/kg) L-Dopa methyl ester (200 mg/kg)

2.5 1.8 0.3 4.5

Tail-flick analgesia

(1.9--3.3) (1.4--2.4) 2 (0.2--0.4) 2 (3.3--6.4) 2

4.6 (4.1--5.2) 1.9 (1.6--2.1) 2 0.7 (0.6--0.8) 2

3 3

1.4 0.6 8.5 9.2

4.8 (3.5--5.8) 2 11.1 (8.4--14.7) 2

(1.0--1.9) 2 (0.4--0.8) 2 (7.2--10.1) 2 (7.8--10.9) 2

1 All drugs, including morphine, were injected 30 rain before testing. 2 Values were significantly different from controls (P < 0.05). a iDs0 could not be obtained due to a plateauing of the dose--response curve. l

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