effects of morphine on central catecholamine turnover, blood pressure and heart rate in the rat

Naunyn-Schmiedeberg's Arch. Pharmacol. 294, 141 - 147 (1976) Naunyn-Schmiedeberg's

Archives of Pharmacology �9 by Springer-Verlag 1976

Effects of Morphine on Central Catecholamine Turnover, Blood Pressure and Heart Rate in the Rat

C. GOMES*, T. H. SVENSSON, and G. TROLIN

Department of Pharmacology, University of G6teborg, Fack, S-40033 G6teborg 33, Sweden

Summary. In the unanaesthetized rat morphine caused increased dopamine (DA) turnover, unchanged or possibly increased central noradrenaline (NA) turnover (utilization), hypertension and tachycardia. In the anaesthetized rat, brain DA turnover was not affected, whereas the NA-turnover was decelerated, particularly in some brain regions, e.g. cerebral cortex and medulla oblongata, and hypotension and bradycardia was obtained. Both biochemical and cardiovascular effects of morphine were antagonized by naloxone. A very small dose of morphine (1 mg/kg) caused tachycardia also in the anaesthetized rat. De- cerebration just inferior to the inferior colliculus abolished the cardiovascular, excitatory effects of morphine in the conscious rat, but left the circulatory, depressant actions of the drug unchanged.

The morphine-induced cardiovascular effects, particularly the hypotension and bradycardia in the anaesthetized animal, are suggested to be related to, or mediated by, the effects of the drug on brain NA- mechanisms, especially in view of several similarities between morphine and the antihypertensive c~-adrenergic agonist clonidine. Whereas higher brain structures appear important in the excitatory, circulatory effects of morphine, structures below the decerebration level, e.g. medulla oblongata, appear primarily involved in the hypotension and bradycardia obtained in the anaesthetized animal. Possibly, morphine has a diphasic dose-response curve with respect to cardiovascular function and, by inference, on brain noradrenergic mechanisms.

Key words: Morphine - Central catecholamines - Blood pressure - Heart rate.

Send offprint requests to . T. H. Svensson at the above address

* Fellow from the Dept. of Biochemistry and Pharmacology, Escola Paulista de Medicina, S. Paulo, Brasil

INTRODUCTION

Several similarities exist between the effect on brain noradrenergic mechanisms of the narcotic analgesic agent morphine and the antihypertensive, e-adrenergic agonist, clonidine. Recently morphine was found to specifically inhibit the firing rate of brain noradrenaline (NA) containing neurons in locus coeruleus (Korf et al., 1974) similar to clonidine (Svensson et al., 1975). Moreover, both clonidine (Starke and Montel, 1973) and morphine (Montel et al., 1975a, b) inhibit the electrically induced release of NA from brain slices. However, whereas clonidine decelerates the disappearance of central NA after inhibition of its synthesis (And6n et al., 1970), a similar effect was not obtained with morphine in whole brain analysis of unanaesthetized rats (Gunne et al., 1969).

The blood pressure response to morphine appears to be dependent on whether the experimental animal is anaesthetized or not: In the unanaesthetized cat morphine increases blood pressure (Kayaalp and Kaymakgalan, 1966), whereas in the anaesthetized cat and rat a hypotensive response is obtained (Kayaalp and Kaymakgalan, 1966; Evans et al., 1952).

Previous studies indicate that the cardiovascular effects, of some narcotic analgesics, e.g. morphine, are of central origin (Mansour et al., 1970; Laubie et al., 1974; Marmo et al., 1975). In parallel, the cardiovascular effects of clonidine are considered to be centrally mediated, and in all probability related to its interference with brain NA mechanisms (see Trolin, 1975; Henning, 1975).

Against the above background, the present experiments were performed in order to establish the relationship between the effects of morphine on central catecholamine (CA) turnover and on cardiovascular function both in the unanaesthetized and the anaesthetized rat. A few experiments were also de- signed to localize the level within the central nervous

142 Naunyn-Schmiedeberg's Arch. Pharmacol. 294 (1976)

system (CNS), at which the circulatory effects might be elicited. Moreover, since different peripheral adrenergic nerves appear to respond in a different way to morphine (Montel and Starke, 1973), a regional biochemical analysis within the CNS of the effect of morphine on the NA-turnover was also performed.

Parts of these results were presented at the Sym- posium "Chemical Tools in Catecholamine Research" (see Svensson and Trolin, 1975).

MATERIAL AND METHODS

Male Sprague-Dawley rats, 2 0 0 - 2 5 0 g (Anticimex, Stockholm) were used.

Drugs. The following drugs were used: Morphine hydrochloride (10 mg/ml, ACO, Stockholm), chloral hydrate (Kebo AB, Stock- holm), c~-methyl-p-tyrosine methylester (c~-MT) (H44/68, Hfissle AB*, Mdlndal, Sweden), naloxone (Endo Laboratories*, Inc., Brusse!s, Belgium), nialamide (Pfizer AB*, T~iby). Morphine was given in a dose of 10 mg/kg unless otherwise stated.

Circulatory Experiments. Blood pressure and heart frequency were recorded through in-dwelling carotid arterial catheters connected to pressure transducers (Statham P 23 Dc) writing on a Grass Poly- graph. Another catheter was introduced into the external jugular vein and exteriorized with the arterial catheter at the back of the neck (for details see Henning, t969). Heart frequency was triggered on the pulse wave. The rats were kept in small individual cages.

Biochemical Experiments. The effect of morphine on turnover (or utilization) of the catecholamines was studied on the disappearance of NA or dopamine (DA) following inhibition of their synthesis by e-MT (250 mg/kg i.p., 3.5 h before sacrifice) (see Anddn et al., 1969). As a measure of central CA-release by morphine, its effect on the accumulation of the 3-O-methylated metabolites normetanephrine (NM) and methoxytyramine (MT) was studied in rats, pretreated with an inhibitor of monoamine oxidase (nialamide, 200 mg/kg i.p., 3.5 h before sacrifice). Since the 3-O-methylation probably occurs outside the CA-neurons (Carlsson, 1960; Axelrod, 1966), the concentrations of these metabolites probably reflect the amount of released CA. After sacrifice .the brains and spinal cords were rapidly dissected out and homogenized in ice cold perchloric acid. NA and DA were determined after isolation on a single Dowex 50 resin as previously described (Bertler et al., 1958; Atack and Magnusson, 1970; Atack, 1973). NM was analyzed according to Carlsson and Lindqvist (1962), MT according to Carlsson and Waldeck (1964). In some experiments the brains and spinal cords were dissected into six parts before homogenisation: cortex, mesencephalon- diencephalon- striatum, medulla obIongata-pons, cerebellum, thoracic spinal cord and lumbo-sacral spinal cord.

Pretreatments

a) Conscious Rats. Catheters for circulatory experiments were im planted under pentobarbitone anaesthesia (Mebumal | 40 mg/kg i.p.) 1 day before the experiments.

b) Anaesthetized Rats. These animals were treated as the conscious rats up to the time for the experiments. In the circulatory experiments, chloral hydrate (400 mg/kg i.p.) was injected 45 rain before morphine. In the biochemical experiments, chloral hydrate (400 rag/ kg i.p.) was given 10 min before the administration of e-MT and thereafter given at a dose of 50--100 mg/kg i.p. when necessary to maintain anaesthesia.

c) Decerebrated Rats. A hole in the skull was drilled under pentobarbitone anaesthesia immediately caudal to the coronary suture 3 days before the experiments. Catheter implantation and decerebration were performed under halothane anaesthesia (Halothane | Fluothan| Catheters were implanted immediately before a blunt decerebration, which was performed at a level just inferior to the inferior colliculus using a spatula inserted through the drillhole described above. The rats were allowed to recover from the anaesthesia 30 rain before the recordings. After the recordings a11 decerebrated brains were checked and only the results from cor- rectly decerebrated rats were analysed. A few experiments were also performed in chloral hydrate (400 mg/kg i.p.) anaesthetized, decerebrated rats.

Statistics. Tests of significance were performed using one-way analysis of variance for the biochemical experiments and two-way analysis of variance ~br the circulatory experiments (Wirier, 1962). These analyses were followed by t-test, p-Values less than 0.05 were regarded as significant. All significant differences in the circulatory experiments represent differences between basal levels of blood pressure or heart rate before the respective treatments, and these parameters measured in the same animals after the treatments.

RESULTS

A. Circulatory Experiments

1. Conscious Rats. Figure 1

An i.v. injection of morphine increased the blood pressure significantly at 10 and 15 rain after the injection (P < 0.005 and P < 0.05, respectively). Also the heart frequency was significantly increased at 10, 15 and 30 rain (P < 0.05 at all intervals).

Naloxone (0.05 mg/kg i.v.) 15 min before the injection of morphine did not per se change the basal blood pressure or heart frequency but abolished the morphine-induced increase in blood pressure. The increase in heart rate was, however, not antagonized, and persisted at 10, 15 and 30 min after morphine (P < 0.025, P < 0.001 and P < 0.001, respectively).

2. Anaesthetized Rats. Figures 2 and 3

Chloral hydrate (400 mg/kg i.p.) alone produced a fall in blood pressure and a small increase in heart rate (compare Figs. 1 and 3). When given 45 rain after chloral hydrate (Fig. 2), morphine 1 mg/kg i.v. produced a significant increase in blood pressure (P < 0.025 at 30 rain) and heart frequency (P < 0.025 at 30 rain). Also 0.9 ~ NaC1 (0.5 ml i.v.) produced a significant increase in blood pressure (30rain: P < 0.005) but the heart frequency was unaffected. At the dose of 5 mg/kg i.v., morphine produced a shortlasting hypotension (P < 0.005 at I rain). An initial bradycardia was converted into a tachycardia after 30 rain (P < 0.025). The dose of 10 mg/kg produced a sustained depression of both blood pressure and heart frequency (P < 0.001 from 1 - 3 0 min).

C. Gomes et al. : Effects of Morphine on Circulation and Brain Catecholamines 143

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90 Fig. ]. Mean blood pressure (solid lines) and heart frequency (broken lines) of conscious rats. At the time 0 morphine (10 mg/kg i.v.) was injected without (O) (n = 9) or with (at) (n = 6) pretreat- ment with naloxone (0.05 mg/kg i.v. 15 rain before morphine). The S.E_M.s obtained with two-way analysis of variance are 3.211 80- (BP 0), 11.340 (HF 0), 2.036 (BP at) and 10.030 (HF at)

Fig. 2. Mean blood pressure (solid lines) and heart frequency 70 (broken lines) in the rat. The values are changes from the basal level 45 min after chloral hydrate (400 mg/kg i.p.). At the time 0 0.5 ml 0.9% NaCI i.v. (I) , morphine 1 mg/kg i.v. (at), morphine 5 mg/kg i.v. (m) or morphine 10 mg/kg i.v. (1) were injected. Number of experiments are indicated in the figure. The S.E.M.s obtained by two-way analysis of variance are 4.137 (0), 6.946 (O), 2.564 (at), 15.166 (a), 5.514 (11), 12.779 (n), 4.933 (O) and 12.789 (o)

Fig. 3. Mean blood pressure (solid lines) and heart frequency E (broken lines) in rats pretreated with chloralhydrate (400 mg/kg g i.v. 45 rain before the time 0). At the time 0, morphine (10 mg/kg 150- i.v.) was injected without (O) (n = 7) or with (at) (n = 7) pretreat- merit with naloxone (0.05 mg/kg i.v. 15 rain before morphine). The S.E.M.s obtained by two-way analysis of variance are 4.933 140- (BP O), 12.789 (HF 0), 3.286 (BP at) and 18.901 (HF at)

Fig. 4. Mean blood pressure (solid line) and heart frequency 130- (broken line) of unanaesthetized, decerebrated rats (n = 7). De- cerebration was performed under halothane anaesthesia 30 min before the injection of morphine (10 mg/kg i.v.) at the time 0. Thirty minutes after the morphine injection, naloxone (0.05 mg/kg 120- i.v.) was injected. The S.E.M.s obtained by two-way analysis of variance are 2.238 (BP) and 11.692 (HF)

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3. Decerebrated Rats. Figure 4

O n e h o u r a f t e r d e c e r e b r a t i o n the h e a r t f r e q u e n c y was i n c r e a s e d f r o m 3 3 0 - 4 7 1 b e a t s / m i n ( P < 0.001). A l s o the b l o o d p r e s s u r e a p p e a r e d to be i n c r e a s e d f r o m 1 1 7 - 1 3 ] m m H g b u t th is effect w a s n o t s ign i f ican t ( c o m p a r e b a s a l levels in F igs . 1 a n d 4). In these ra t s m o r p h i n e h a d no effect o n b l o o d p re s su re , whi l e the


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Fig. 6. Effects of morphine or morphine plus naloxone on the c~-MT-induced disappearance of whole brain NA and DA. The rats were anaesthetized with chloralhydrate (400 mg/kg i.p. 32/3 h before sacrifice + 50-100 mg/kg i.p. when necessary to maintain the anaesthesia). In all other respects the rats were treated as indicated in the legend to Figure 5. The values are the means of 4 - 6 experiments with their S.E.M. indicated

hear t f requency was significantly reduced (P < 0.001 f rom 1 - 3 0 rain).

W h e n na loxone (0.05 mg/kg i.v.) was injected 30 min after morphine , the heart rate showed an im- mediate increase, while there was no significant change in b lood pressure. One minute after naloxone, the heart rate was not significantly different f rom the basal heart f requency before morph ine (P > 0.05).

B. Biochemical Experiments

1, Conscious Rats

Whole Brain. In the whole brain of the unanaesthet ized rat, morph ine reduced brain DA, both when given alone (P < 0.025) and after c~-MT (Fig. 5, P < 0.005). There was only an insignificant tendency to an increase in N A turnover . Na loxone blocked these effects.

The accumula t ion o f the N A and D A metabolites, N M and MT, after inhibit ion o f m o n o a m i n e oxidase (Table 1) was significantly increased by morph ine (P < 0.01 and P < 0.001, respectively). The increase in M T was greater than that o f N M , Almos t all rats died when the same experiment was per formed in animals anaesthetized with chloral hydrate (400 mg/kg i.p.),

Table 1. Accumulation (pg/g) of the NA and DA metabolites normetanephrine and methoxytyramine in the whole brain of unanaesthetized rats treated with the monoamine oxidase inhibitor nialamide (200 mg/kg i.p. 3.5 h before sacrifice), or nialamide + morphine (morphine 10 mg/kg i.p. 1 h 45 rain after nialamide). Values are means + S.E.M. with the number of determinations within parentheses

Treatment Whole brain

normetanephrine methoxytyramine

Nialamide 0.169 _+ 0.009 (4) 0.077 +_ 0.005 (4) Nialamide + morphine 0.212 _+ 0.,006 (4) 0.173 _+ 0.002 (4)

P < 0.01 ; P < 0.001

2. Anaesthetized Rats

Whole Brain. Figure 6. In these rats, morph ine retarded the disappearance o f N A after synthesis inhibit ion ( P < 0.001). This effect was blocked b y naloxone. D A turnover was not affected by morphine.

b) Different Parts of the Brain. Table 2. After morphine alone, the N A content o f the medulla oblongata- pons and the thoracic spinal cord was significantly increased (P < 0.001 and P < 0.005, respectively). In all brain parts, c~-MT decreased the N A content. Mor - phine retarded the disappearance o f N A significantly


Table 2. Concentrations (btg/g) of noradrenaline in 6 areas of brain and spinal cord of anaesthetized rats (chloralhydrate 400 mg/kg i.p.) pretreated with ~-MT (250 mg/kg i.p. 3 h before sacrifice), morphine (10 mg/kg i.p. 23/4 h and 11/4 h before sacrifice) and/or 0.9 ~ saline. Values are means • S.E.M. with the number of determinations in parentheses. Differences between values within each region, not indicated by a P-value, are insignificant

Treatment Noradrenaline (lig/g)

Brain cortex Mesencephalon- Cerebellum M. oblongata- Thoracic spin. Lumbo-sacral diencephalon- "~ons cord spin. cord striatum

NaCl 0.9~ (a) 0.143 • 0.016 (4) 0.456 4- 0.073 (4) 0.096 • 0.005 (8) 0.346 • 0.014 (7) 0.202 • 0.004 (4) 0.334 • 0.018 (3) Morphine (b) 0.159 • 0.013 (4) 0.431 • 0.010 (4) 0.112 • 0.011 (8) 0.587 • 0.026 (5) 0.245 _+ 0.007 (3) 0.392 • 0.030 (3) c~-MT (c) 0.075 • (4) 0.289 • (4) 0.064• 0.011 (8) 0.233 • (5) 0.117 • (4) 0.203_+0.020 (3) c~-MT + morphine (d) 0.128 • 0.019 (4) 0.269 • 0.023 (4) 0.076 + 0.007 (8) 0.432 • 0.021 (5) 0.095 • 0.025 (4) 0.285 • 0.016 (3)

Significance: a:c P<0.005 a:c P<0.025 a:c P<0.025 a:b P<0.001 a:c P<0.005 a:c P<0.005 b:c P<0.001 a:d P<0.01 b:c P<0.001 a:c P<0.001 a:d P<0.001 b:c P<0.001 c:d P<0.025 b:c P<0.05 b:d P<0.01 a:d P< 0.005 b:c P< 0.001 b:d P< 0.01

b:d P<0.025 b:c P<0.001 b:d P<0.00J c:d P<0.05 b:d P < 0.001 c:d P < 0.001

in the brain cortex (P < 0.01), the medulla oblongata- pons (P < 0.001) and the lumbo-sacral spinal cord (P < 0.05).

D I S C U S S I O N

The finding that morphine caused an increased D A turnover in the unanaesthetized rat, but did not seem to affect brain N A turnover is in agreement with previous reports (e.g. Gunne et al., 1969). However, in the anaesthetized rat a retardation of brain N A turnover (or utilization) was found, whereas DA turnover was not affected. Thus, a biochemical correlate to the previously mentioned morphine-induced inhibition of NA-neuronal impulse flow was obtained (cf. Intro- duction). Although the inhibition of firing rate of locus coeruleus neurones was obtained also in the conscious rat (Kor f et al., 1974), the possibility re- mains that a slightly higher dose of morphine was required in this case, since no dose-response curves were shown. Moreover, the disappearance of CA after synthesis inhibition may be a relatively insensitive method to disclose changes in neuronal impulse flow (see And~n et al., 1969). Interestingly, the retardation of brain N A turnover was different in different regions in the CNS. The effect was pronounced in the cerebral cortex, which is innervated mainly f rom N A neurons originating in the locus coeruleus (see Ungerstedt, 1970), whereas in mesencephalon-diencephalon-stri- a tum no significant retardation was obtained. Thus, it appears as if different NA-neurons in the CNS respond differently to morphine, similar to previous

findings in the peripheral adrenergic system (cf. Intro- duction). Of particular interest is the result in the medulla oblongata-pons, where morphine both retarded the N A disappearance after synthesis inhibition and markedly increased the level of NA. Since a significantly increased concentration of NA was obtained also in the thoracic spinal cord, the N A accumulation in the lower brain stem is probably not mainly located in cell bodies. The finding is reminiscent of that seen in central D A neurons after interruption of impulse flow by axotomy or after gammahydroxybutyr ic acid (Carlsson et al., 1972; And6n et al., 1973; Stock et al., 1973; Walters et al., 1973; Carlsson, 1975) and sug- gests the possibility that the NA-neurons involved may have regulatory mechanisms previously only encountered in the nigro-neostriatal D A pathway, and in contrast to other NA-neurons. Needless to say, further studies are necessary to confirm the finding.

Whereas morphine (10 mg/kg) in the unanaesthetized rat increased blood pressure and heart frequency, the opposite result was obtained with the same dose in the anaesthetized rat, i.e. hypotension and bradycardia. These circulatory depress• effects were sustained and longlasting, and are clearly not secondary to a respiratory depressant action, since they persist even when respiration is maintained arti- ficially (Kayaalp and Kaymak~alan, 1966; Grundy, 1971). Interestingly the hypotensive effect of small doses of clonidine in the anaesthetized rat is not obtained in the conscious animal (Trolin, 1975), underlining the similarities between the circulatory actions of morphine and clonidine.


Thus, in the anaesthetized rat, when brain NA turnover was reduced by morphine, the hypotension and bradycardia was obtained, whereas in the conscious animal, when the whole brain NA turnover was unchanged, no cardiovascular depression appeared. In fact, the turnover of NA in the lower brain stem has actually been found to be increased by morphine (Dahlstr6m et al., 1975) in the unanaesthetized rat, underlining the parallelism between morphine-induced effects on brain NA turnover vs. cardiovascular function. In addition, it should be mentioned that 7-hy- droxybutyric acid, which contrary to e.g. clonidine, accelerated the disappearance of brain NA after synthesis inhibition, caused hypertension and tachycardia in the rat presumably via a central mechanism (Gomes et al., 1976). That morphine in the unanaesthetized rats facilitates central noradrenergic (and dopaminergic) transmission is further supported by our finding of increased concentrations of NM (and MT) following morphine administration to nialamide-pretreated animals, indicating increased CA-release (cf. Meth- ods). The effects of morphine on brain DA are probably not related to its circulatory actions, since avail- able evidence argues against any major role for this amine in cardiovascular function in the rat (Henning and Rubenson, 1970; Trolin, 1975). Since both the morphine-induced effects on brain CA-turnover and on blood pressure in the conscious as well as the anaesthetized rat were antagonized by naloxone, they are probably due to an interaction with specific morphine receptor sites.

The increase in heart rate obtained by the small dose of morphine (1 mg/kg) but not by the saline injection in the anaesthetized rat might imply a diphasic dose-response curve for morphine. However, this hypothesis must await further studies. The blood pressure was unaffected by this small dose of morphine.

Decerebration changed the hypertension and tachycardia obtained by morphine in the intact, conscious rat in the direction of the response to the drug in the intact, anaesthetized animal, i.e. the heart frequency was markedly reduced by morphine and no increase in blood pressure was obtained. Although the basal blood pressure was slightly increased by decerebration, the increase in blood pressure by morphine in the intact, conscious animal surpassed this level, thus arguing for a true blockade by decerebration of the hypertensive effect of morphine. Conse- quently, the cardiovascular, excitatory effects of morphine are probably dependent on the integrity of structures above the transection level. Interestingly, also the hypertensive effects of clonidine (Trolin, 1975) and 7-hydroxybutyrate (unpublished observations) are blocked by decerebration, suggesting that

brain NA mechanisms may be involved also in the mediation of the excitatory, circulatory effects of morphine. Since the effect of morphine in the decerebrated rats was readily antagonized by naloxone, a primary interaction with morphine specific receptors is suggested.

In 3 decerebrated, anaesthetized rats (unpublished data) morphine caused hypotension and bradycardia as in the intact, anaesthetized rats, indicating that these depressive effects of morphine are elicited primarily from structures below the transection level e.g. medulla oblongata, and presumably noradrenergic (cf. above). However, an indirect influence also of higher structures on the circulatory, depressant action of morphine cannot be excluded. For example both decerebration and anaesthesia may interrupt regulatory impulses from higher centers on e.g. medullary neurons, and hence facilitate the depressant action of morphine on the lower structures. Underlining morphine's similarities with clonidine it should be mentioned, that the hypotension and bradycardia seen after clonidine is also obtained in the decerebrated, anaesthetized rat (Henning, personal com- munication).

The possibility that brain NA turnover is affected secondary to morphine-induced changes in blood pressure seems unlikely, since recent experiments show that peripheral sympatholysis by i.v. 6-hydroxy- dopamine in the rat, does not reduce brain NA turnover, in spite of a marked hypotensive effect (unpublished observations).

Thus, a parallelism was found between the cardiovascular effects of morphine and its effects on brain NA turnover, particularly in some regions, e.g. medulla oblongata and cerebral cortex, and the drug shows several similarities with the c~-adrenergic agonist clonidine. However, both the cardiovascular and the biochemical effects of morphine are in all probability due to interaction with specific morphine receptor sites, since both actions were blocked by naloxone. The central NA neurons are likely to be involved in the mediation of the circulatory effects of morphine especially since we recently have found a small dose of yohimbine to antagonize both the circulatory depressant action and the retardation of brain NA turnover induced by morphine (Svensson and Trolin, 1975). The localization of the morphine-induced cardiovascular effects needs further studies, although certain brain regions, e.g. medulla oblongata, seem to be of particular interest. This is so also in view of the medullary localization of the vasomotor center (Alexander, 1946).

Acknowledgements. This work was supported by the Swedish Medical Research Council (projects nos. 4747, 2157, 00155, 2863) and the Medical Faculty, University of G6teborg. C. Gomes was


supported by a grant from the Fundag~.o de Amparo fi Pesquisa do Estado de S. Paulo, S. Paulo, Brasil. For generous supplies of drugs we thank the companies indicated by an asterisk above. Valuable technical assistance was given by Mrs. Brigitta Andersson and Miss Ann-Sofie Jfirnstr6m.

R E F E R E N C E S

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And6n, N.-E., Corrodi, H., Fuxe, K., H6kfelt, B., H6kfelt, T., Rydin, C., Svensson, T. : Evidence for a central noradrenaline receptor stimulation by clonidine. Life Sci. 9, 513-523 (1970)

And6n, N.-E., Magnusson, T., Stock, G. : Effects of drugs influenc- ing monoamine mechanisms on the increase in brain dopamine produced by axotomy or treatment with gammahydroxybutyric acid. Naunyn-Schmiedeberg's Arch. Pharmacol. 278, 363-372 (1973)

Atack, C. V. : The determination of dopamine by a modification of the dihydroxyindole fluorimetric assay. Brit. J. Pharmacol. 48, 699-714 (1973)

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Received January 27 / Accepted April 20, 1976

effects of morphine on central catecholamine turnover, blood pressure and heart rate in the rat

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