SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-l-phenyl-. IH-3-benzazepine HCI, Research Biochemicals Inc., Wayland,. Mass, U.S.A.), reserpine (Sigma Chemical ...
Pharmacology & Toxicohgy 1989, 64, 12-11.
The Involvement of Dopamine D1 and DZ Receptors in the Locomotor Stimulation Produced by ( +)-Amphetamine in Naive and DopamineDepleted Mice S. B. ROSS',D. M. Jacksonz and S. R. Edwards
Department of Pharmacology, The University of Sydney, New South Wales 2006, Australia (Received April 13, 1988; Accepted July 1, 1988) Absrract; The interaction between (+)-amphetamine and dopamine (DA) D I and D2 receptors was investigated. In naive mice, i.e., mice with intact stores of DA, both the selective DI antagonist SCH23390 and the selective D2 antagonist spiperone blocked the locomotor stimulation produced by (+)-amphetamine. The selective D I agonist SKF38393 (6 mg/ kg intraperitoneally) did not produce a consistent dose-dependent effect on the response to (+)-amphetamine in na'ive mice. In mice depleted of DA with reserpine 24 hr before a challenge with (+)-amphetamine, neither SCH23390 nor spiperone were completely effective in blocking (+)-amphetamine. A combination of spiperone plus SCH23390 was, however, more effective than either drug alone, although significant activity remained even after the combination. In mice pretreated with reserpine and various doses of a methyl-p-tyrosine (aMPT, intraperitoneally), the degree of stimulation produced by (+)-amphetamine was dependent on the amount and frequency of aMPT dosage - the higher and more frequent the dose, the more effective the blockade. In these animals, both SKF38393 and the selective D2 agonist quinpirole potentiated the stimulation induced by (+)-amphetamine when the dose of aMPT was not maximal. However, in those animals pretreated with reserpine plus two doses each of 400 mg/kg aMPT, neither SKF38393 nor quinpirole were effective in potentiating (+)-amphetamine. Nevertheless, when SKF38393 and quinpirole were administered simultaneously to these mice, marked locomotor stimulation occurred implying that the pretreatment itself had not rendered the mice incapable of locomotion. The data indicate that (+)-amphetamine produces excitation in na'ive mice by an indirect action on DI and D2 receptors, both of which must be stimulated for activation to occur. In animals depleted of DA with less than optimal doses of aMPT, (+)-amphetamine could still release enough DA to stimulate D I and D2 receptors, as either SKF38393 or quinpirole could potentiate the excitation induced by (+)-amphetamine. However, in mice pretreated with the highest doses of aMPT, (+)-amphetamine was virtually inactive even when combined with either a DI or a D2 agonist, implying a lack of released DA available for receptor stimulation.
(+)-Amphetamine is an indirectly-acting dopamine (DA) receptor agonist which releases DA from the nerve terminals (Randrup & Munkvad 1966; Scheel-Kriiger 1971). It appears that the DA released is from a so-called newly synthesised store, since the tyrosine hydroxylase inhibitor a methyl-p-tyrosine (aMPT) antagonizes the central stimulant effects of (+)-amphetamine, whereas the granule-depleting agent reserpine does not (Scheel-Kruger 1971; Svensson 1970). Thus, (+)-amphetamine causes a pronounced stimulation of the locomotor activity in reserpinised mice, which is blocked by the pretreatment of the animals with aMPT. Its electrophysiological effects are similarly dependent upon the release of newly synthesized DA (Bunney et al. 1973). With the availability of selective DI and D2 agonists and antagonists, it has become clear that both subtypes of the DA receptor play a role in locomotor activity (Gershanik et al. 1983; Arnt 1985). Thus, pretreatment of mice with either reserpine or aMPT reduces the locomotor stimulant effects of the selective Dz agonists quinpirole, RU24926 and bromocriptine (Gershanik et al. 1983; Jackson & Hashizume I Permanent address: CNS Laboratory, Astra Alab AB, Sodertalje, Sweden. To whom all correspondence should be addressed.
1986; Walters et al. 1987). The stimulant effect, however, can be restored by the administration of a selective DI agonist such as SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine).Thus, both subtypes of receptor require stimulation for optimal locomotor activity. With regard to the stimulant effects of (+)-amphetamine, two general questions can be posed. First, which of the DA receptor subtypes are stimulated by the DA released by (+)-amphetamine in na'ive (i.e., DA-intact) mice and in DA-depleted mice? This was examined in the present paper by testing the effect of pretreatment with either the selective DI antagonist SCH23390 or the selective D2 antagonist spiperone, on the response of mice to (+)-amphetamine challenge. Second, if the two subtypes of receptor require different synaptic concentrations of DA to be stimulated because, for example, they have a different affinity for DA (Seeman & Grigoriadis 1987), the lack of response to (+)amphetamine in aMPT-treated mice may be due to an imbalance in the stimulation of the two receptor subtypes, rather than merely reflecting a reduction in synaptic D e concentrations. In order to examine this possibility, reserpinised mice were pretreated with various doses of aMPT followed by (+)-amphetamine in combination with SKF38393 or quinpirole.
13
(+)-AMPHETAMINE AND DI AND Dz RECEPTORS
Materials and Methods 6
General. QS strain mice (2540 g, University of Sydney Animal Farm) were kept under constant temperature (22 f 2") and lighting (6 a.m. to 6 p.m. light) conditions. Free access to food and water was allowed except during locomotor activity and stereotypy measurements. The locomotor activity of groups of 3 mice was measured each 5 min. beginning immediately after the injection of the DA agonists in photocell cages as described by Bailey & Jackson (1978). Each mouse was used on1 once. The raw activity scores were subjected to a transform (&x + 0.51) to normalize the data (Winer 1971) and data analyzed by t-tests or ANOVARs as appropriate.
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Fig. 2. Groups of 3 mice were challenged with SKF38393, 0 mg/kg Chemicals. SKF38393 (2,3,4,5-tetrahydro-7,8-dihydroxy-l-phenylIH-3-benzazepine HCI, Research Biochemicals Inc., Wayland, Mass, U.S.A.), reserpine (Sigma Chemical Company, St. Louis, MO., U.S.A.) and spiperone (Janssen Pharmaceutica*, Belgium) were dissolved in a minimum of glacial acetic acid and diluted with water. aMPT (as the methyl ester hydrochloride, Sigma), quinpirole (LY17155S. trans-(-)-4aR-4a,5,6,7,8,8a,9-octahydro-5-propyl-lH (or 2H)-pyrazolo [3,4-g] quinoline, Eli Lilly and Co.,* Indianapolis, IN, U.S.A.) and (+)-amphetamine (as the sulphate, Smith, Kline and French Labs.*, Philadelphia, U.S.A.) were dissolved in water. SCH23390 ([R (+)-7-chIoro-2,3,4,S-tetrahydro-3-methyl-S-phenylIH-3-benzazepin-8-01], Schering Corp., U.S.A.) was dissolved in 0.01% ascorbic acid solution. All compounds were injected ip, unless otherwise stated, in a volume to body weight of 10 ml/kg. The dose of reserpine was 5 mg/kg subcutaneously 24 hrs before the experiment.
Results Naive mice. (+)-amphetamine produced dose-dependent locomotor stimulation (fig. l), which was completely blocked by either spiperone (0.2 or 0.4 mg/kg) (table 1) or a relatively low dose of SCH23390 (0.1 mg/kg) (fig. 1). SKF38393 did not exert a consistent effect on (+)-amphetamine-induced locomotor stimulation (fig. 2), but partially
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Fig. I. Groups of 3 mice were pretreated with SCH23390, 0 mg/kg (0), 0.02 mg/kg ( A ) or 0.1 mg/kg ( 0 )and 30 min. later challenged with various doses of (+)-amphetamine, and their locomotor activity measured for the following 150 min. The data represent the mean transformed total activityfS.E.M. of 7 to 8 replicates. Data analysis indicated that (+)-amphetamine produced significant stimulation in the control group (F3,74 = 14.67, P
