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R(+)-8-OH-DPAT, a selective 5-HT1A receptor agonist, attenuated amphetamine-induced dopamine synthesis in rat striatum, but not nucleus accumbens or medial prefrontal cortex
Brain Research 872 (2000) 204–207 www.elsevier.com / locate / bres
Short communication
R(1)-8-OH-DPAT, a selective 5-HT 1A receptor agonist, attenuated amphetamine-induced dopamine synthesis in rat striatum, but not nucleus accumbens or medial prefrontal cortex Toshihide Kuroki, Jin Dai, Herbert Y. Meltzer, Junji Ichikawa* The First Floor Laboratory, Psychopharmacology Division, Departments of Psychiatry and Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37212, USA Accepted 18 April 1999
Abstract R(1)-8-OH-DPAT (0.05, but not 0.025, 0.1, 1 mg / kg), a 5-HT 1A receptor agonist, decreased l-3,4-dihydroxyphenylalanine (DOPA) accumulation in rat striatum following NSD-1015, an l-aromatic amino acid decarboxylase inhibitor. Amphetamine (1 mg / kg) increased striatal DOPA accumulation, an effect attenuated by R(1)-8-OH-DPAT (0.05 mg / kg). However, both amphetamine (1 mg / kg) and R(1)-8-OH-DPAT (0.05 mg / kg) decreased cortical DOPA accumulation; there were no additional decreases from their combination. Neither amphetamine (1 mg / kg), R(1)-8-OH-DPAT (0.05 mg / kg), or the combination, significantly affected DOPA accumulation in the nucleus accumbens. The significance of and possible mechanisms for these findings are discussed. 2000 Elsevier Science B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors Topic: Interactions between neurotransmitters Keywords: R(1)-8-OH-DPAT; 5-HT 1A receptor; D-Amphetamine; Dopamine synthesis; Rat
We have recently reported that R(1)-8-hydroxy-2-(di-npropylamino)tetralin (R(1)-8-OH-DPAT, 0.05 mg / kg), a selective serotonin (5-HT) 1A receptor agonist [4], significantly inhibited amphetamine (1 mg / kg)-induced dopamine (DA) release in rat striatum (STR), nucleus accumbens (NAC) [8], and medial prefrontal cortex (mPFC) [14]. These effects were completely antagonized by pretreatment with n-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-n-(2-pyridinyl)cyclohexanecarbox-amide trihydrochloride (WAY100635, 0.1 mg / kg), a selective 5-HT 1A receptor antagonist [7]. Thus, 5-HT 1A receptor agonism may inhibit the ability of amphetamine to increase DA release in these three regions. Amphetamine is believed to selectively release newly synthesized DA at the nerve terminal. Reduction of DA synthesis produced by alpha*Corresponding author. Present address: 1601 23rd Avenue South, Suite 306, The Psychiatric Hospital at Vanderbilt, Nashville, TN 37212, USA. Tel.: 11-615-327-7242; fax: 11-615-322-2522. E-mail address: [email protected] (J. Ichikawa)
methyl-p-tyrosine, a DA synthesis inhibitor, has been reported to inhibit amphetamine-induced DA release in the STR [5]. (6)-8-OH-DPAT has been reported to decrease DA synthesis in the STR, as indicated by the accumulation of l-3,4-dihydroxyphenylalanine (DOPA), following NSD1015 (3-hydroxybenzyl hydrazine dihydrochloride), an laromatic amino acid inhibitor, whereas NAN-190, a 5HT 1A receptor antagonist, increased DA synthesis [11]. Thus, it is postulated that stimulation of 5-HT 1A receptors attenuates the ability of amphetamine to increase DA release in the mPFC, NAC and STR [8,14] via decreasing DA synthesis. The present study was designed to test the hypothesis that R(1)-8-OH-DPAT decreases basal and amphetamine-induced DA synthesis in the STR, NAC and mPFC. Preliminary data have been reported in abstract form [13]. Male Sprague–Dawley albino rats (Zivic-Miller Laboratories, Porterville, PA) weighing 200–250 g were used throughout the study. Rats were housed two or three per cage and maintained in a controlled 12:12 h light:dark
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02437-9
T. Kuroki et al. / Brain Research 872 (2000) 204 – 207
cycle and under constant temperature at 228C, with free access to food and water. R(1)-8-OH-DPAT hydrobromide (RBI, Natick, MA) was subcutaneously (s.c.) administered to rats, 30 min prior to D-amphetamine sulfate (1 mg / kg, Sigma, St. Louis, MO). These two drugs were dissolved in deionized water. The control rats received deionized water (1 ml / kg s.c.). R(1)-8-OH-DPAT was administered to rats, 30 min prior to amphetamine, as has been conducted in our previous studies [8,14]. Also, in accord with the previous report of amphetamine-induced DOPA accumulation [21], NSD-1015 (100 mg / kg, i.p., Sigma) was administered to rats 15 min after amphetamine. Rats were sacrificed 30 min after NSD-1015 (45 min after amphetamine and 75 min after R(1)-8-OHDPAT, respectively) and the brains were quickly removed. The mPFC, NAC (mostly the shell part) and dorsolateral STR tissues were punched out bilaterally from slices coronally dissected with approximately 1 mm thickness; coordinates: A13.2 mm for mPFC, A12.0 mm for NAC and A10.5 mm for STR from bregma, respectively, according to the reference atlas [16]. The procedures applied in these experiments were approved by the Institutional Animal Care and Use Committee of Case Western Reserve University in Cleveland, OH, where we completed the present study. Sample tissues were homogenized in 0.1 M perchloric acid and centrifuged at 95003g for 20 min. Fifty microlitre of each supernatant was applied onto HPLC system with coulometric detection (Model 5100A, ESA, Bedford, MA). DOPA was separated on a reverse phase column (Ultrasphere ODS 5 mm, 4.63250 mm, Beckman, San Roman, CA). The mobile phase consisted of 0.05 M citrate-phosphate buffer (pH 2.8), including 25%
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(v / v) methanol, 50 mg / l sodium octyl sulfate and 10 mM Na 2 EDTA. The tissue pellets were dissolved with 1.0 M NaOH and the protein concentrations were determined by the Lowry’s method [15]. All data were expressed as absolute values of DOPA concentrations (means6S.E.M. pmol / mg protein). The dose effect of R(1)-8-OH-DPAT on striatal DOPA accumulation was subjected to one-way ANOVA followed by the Dunn’s procedure for comparing a control to all means, and other data were analyzed using one-way ANOVA followed by the Fisher’s protected least significant difference post-hoc test. A P,0.05 was considered significant in the present study. Systemic administration of R(1)-8-OH-DPAT (0.025, 0.05, 0.1 and 1 mg / kg s.c.) produced a U-shaped inhibition of DOPA accumulation in the STR (Fig. 1A) (F4,2953.14, P50.03). Also 0.05 mg / kg R(1)-8-OH-DPAT showed a significant decrease in striatal DOPA accumulation (P, 0.01), and 0.025 mg / kg showed a non-significant trend for a decrease (P50.09), respectively, compared with vehicle controls. R(1)-8-OH-DPAT (0.05 mg / kg s.c.) significantly decreased DOPA accumulation in the mPFC (F1,1358.65, P50.01), but had no effect in the NAC, compared with vehicle controls (Fig. 2). DOPA accumulation following amphetamine (1 mg / kg s.c.) was significantly increased up to 156% (F1,1256.03, P50.03) in the STR (Fig. 1B), and decreased to 46% (F1,1156.20, P50.03) in the mPFC (Fig. 2B), of vehicle controls, respectively. R(1)-8-OHDPAT (0.05 mg / kg s.c.), given 30 min prior to amphetamine, significantly attenuated the ability of amphetamine (1 mg / kg s.c.) to increase DOPA accumulation in the STR (F1,1257.96, P50.02), but had no effect on the amphetamine-induced decrease in the mPFC, compared with the
Fig. 1. Effect of R(1)-8-OH-DPAT on basal and amphetamine-induced striatal DOPA accumulation. (A) R(1)-8-OH-DPAT (0.025, 0.05, 0.1 and 1 mg / kg s.c.) produced an U-shaped inhibition of DOPA accumulation. A significant decrease in DOPA accumulation was observed at 0.05 mg / kg of R(1)-8-OH-DPAT, but not at 0.025 mg / kg which showed a non-significant trend of a decrease, compared with vehicle controls (VEH). (B) Amphetamine (AMPH, 1 mg / kg s.c.) significantly increased DOPA accumulation. R(1)-8-OH-DPAT (DPAT, 0.05 mg / kg s.c.), given 30 min prior to amphetamine, significantly attenuated the ability of amphetamine to increase DOPA accumulation, compared with the effect of amphetamine alone. Each bar represents the mean6S.E.M. Number of animals is in the parenthesis. Significant differences are indicated by * P,0.05 or ** P,0.01.
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Fig. 2. Effect of R(1)-8-OH-DPAT on amphetamine-induced DOPA accumulation in the NAC (A) and mPFC (B). R(1)-8-OH-DPAT (DPAT, 0.05 mg / kg s.c.) and amphetamine (1 mg / kg s.c.) by themselves significantly decreased DOPA accumulation in the mPFC, but had no effect in the NAC, compared with vehicle controls (VEH). R(1)-8-OH-DPAT (0.05 mg / kg s.c.), given 30 min prior to amphetamine, had no additional effect on the ability of amphetamine to decrease DOPA accumulation in the mPFC or the inability of amphetamine to alter DOPA accumulation in the NAC, compared with the effect of amphetamine or R(1)-8-OH-DPAT alone. Each bar represents the mean6S.E.M. Number of animals is in the parenthesis. Significant differences are indicated by * P,0.05 or ** P,0.01, compared with vehicle controls (VEH1VEH).
effect of amphetamine alone. Amphetamine (1 mg / kg s.c.) produced a non-significant increase in DOPA accumulation in the NAC (F1,1151.20, P50.08), compared with vehicle controls; this effect was not influenced by R(1)-8OH-DPAT (0.05 mg / kg s.c.) (Fig. 2A). NAN-190 and WAY100635, 5-HT 1A receptor antagonists, have been reported to increase basal DA synthesis in the STR and antagonize the ability of (6)-8-OH-DPAT to decrease striatal DA synthesis [1,11,12]. Thus, the ability of R(1)-8-OH-DPAT, a selective 5-HT 1A receptor agonist, to decrease DA synthesis in the STR is most likely due to stimulation of 5-HT 1A receptors. Systemic administration of amphetamine (3 mg / kg) has been reported to increase striatal DA synthesis at 45 min after injection, followed by a decrease below control levels at 75 and 90 min [21]. Increased DA synthesis in the STR can increase the releasable pool of DA available for amphetamine. Thus, it is important that 0.05 mg / kg of R(1)-8-OH-DPAT in 0.025, 0.05, 0.1 and 1 mg / kg, which produced maximum decrease in striatal DA synthesis, produced maximum inhibition of amphetamine-induced DA release in the STR [8]. The lesser effects of high dose R(1)-8-OH-DPAT on basal DA synthesis and amphetamine-induced DA release in the STR, are unlikely to be relevant to the ability of high dose 8-OH-DPAT (0.225 mg / kg) [18] or R(1)-8-OHDPAT (0.2 mg / kg) [10] to decrease basal DA release in the STR. Mechanisms other than 5-HT 1A receptor stimulation by high dose R(1)-8-OH-DPAT should be considered [8]; e.g. high dose R(1)-8-OH-DPAT inhibits DA reuptake into the nerve terminal [17], which may interfere with end-product inhibition of DA synthesis [6]. (6)-8-OHDPAT may have D 2 receptor agonist properties [2,19,22],
and hence could decrease DA synthesis. However, this is unlikely in the present study since S(2)-sulpiride, a D 2 receptor antagonist, has been reported not to antagonize (6)-8-OH-DPAT-induced reduction of DA synthesis in striatal synaptosome [11]. Furthermore, we have reported that R(1)-8-OH-DPAT had no effect on S(2)-sulpirideinduced DA release in the STR [9]. The results reported here do not support the hypothesis that R(1)-8-OH-DPAT attenuates amphetamine-induced increases in DA synthesis in the mPFC and NAC, and, thereby, attenuates amphetamine-induced DA release in these regions [8,14]. First, amphetamine (1 mg / kg) decreased DA synthesis in the mPFC, but had no significant effect in the NAC, as similarly reported by others [21]. Second, R(1)-8-OH-DPAT (0.05 mg / kg), which by itself decreased DA synthesis in the mPFC, produced no additional effects on the ability of amphetamine to decrease DA synthesis in that region. Finally, neither amphetamine (1 mg / kg), R(1)-8-OH-DPAT (0.05 mg / kg), nor their combination affected DA synthesis in the NAC. Thus, the effect of R(1)-8-OH-DPAT on DA synthesis in the mPFC and NAC is unlikely to contribute to the ability of R(1)-8OH-DPAT to inhibit amphetamine-induced DA release in these regions [8,14]. These results also indicate a dissociation between synthesis and release of DA in the mPFC and NAC with respect to the effect of either amphetamine or R(1)-8-OH-DPAT. For example, 5-HT 1A receptor stimulation by 8-OH-DPAT has been reported to increase basal DA release in the mPFC [3,14,18,20], whereas the present study showed an inhibitory effect of R(1)-8-OH-DPAT on DA synthesis in the same region. It should be noted, however, that the effect of R(1)-8-OH-DPAT on DA
T. Kuroki et al. / Brain Research 872 (2000) 204 – 207
synthesis was determined only at one time point (75 min after R(1)-8-OH-DPAT) in the present study, and that the possibility remains that R(1)-8-OH-DPAT may significantly increase DA synthesis at different time points. In summary, 0.05 mg / kg R(1)-8-OH-DPAT, the dose which produced maximum decrease in striatal DA synthesis among the doses tested (0.025–1 mg / kg), attenuated amphetamine-induced increases in striatal DA synthesis. Since R(1)-8-OH-DPAT (0.05 mg / kg) has been reported to attenuate amphetamine (1 mg / kg)-induced striatal DA release [8], this inhibition may be due, at least in part, to the ability of R(1)-8-OH-DPAT to decrease DA synthesis. However, because R(1)-8-OH-DPAT had no effect on amphetamine-induced DA synthesis in the mPFC and NAC, factors other than DA synthesis may contribute to the ability of R(1)-8-OH-DPAT to inhibit amphetamineinduced DA release in these regions [8,14].
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgements We are grateful to Mr Michael T. Kitchen for his excellent laboratory assistance. This study was supported, in part, by a grant from the Warren Foundation.
[14]
[15]
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Short communication
R(1)-8-OH-DPAT, a selective 5-HT 1A receptor agonist, attenuated amphetamine-induced dopamine synthesis in rat striatum, but not nucleus accumbens or medial prefrontal cortex Toshihide Kuroki, Jin Dai, Herbert Y. Meltzer, Junji Ichikawa* The First Floor Laboratory, Psychopharmacology Division, Departments of Psychiatry and Pharmacology, Vanderbilt University School of Medicine, Nashville, TN 37212, USA Accepted 18 April 1999
Abstract R(1)-8-OH-DPAT (0.05, but not 0.025, 0.1, 1 mg / kg), a 5-HT 1A receptor agonist, decreased l-3,4-dihydroxyphenylalanine (DOPA) accumulation in rat striatum following NSD-1015, an l-aromatic amino acid decarboxylase inhibitor. Amphetamine (1 mg / kg) increased striatal DOPA accumulation, an effect attenuated by R(1)-8-OH-DPAT (0.05 mg / kg). However, both amphetamine (1 mg / kg) and R(1)-8-OH-DPAT (0.05 mg / kg) decreased cortical DOPA accumulation; there were no additional decreases from their combination. Neither amphetamine (1 mg / kg), R(1)-8-OH-DPAT (0.05 mg / kg), or the combination, significantly affected DOPA accumulation in the nucleus accumbens. The significance of and possible mechanisms for these findings are discussed. 2000 Elsevier Science B.V. All rights reserved. Theme: Neurotransmitters, modulators, transporters, and receptors Topic: Interactions between neurotransmitters Keywords: R(1)-8-OH-DPAT; 5-HT 1A receptor; D-Amphetamine; Dopamine synthesis; Rat
We have recently reported that R(1)-8-hydroxy-2-(di-npropylamino)tetralin (R(1)-8-OH-DPAT, 0.05 mg / kg), a selective serotonin (5-HT) 1A receptor agonist [4], significantly inhibited amphetamine (1 mg / kg)-induced dopamine (DA) release in rat striatum (STR), nucleus accumbens (NAC) [8], and medial prefrontal cortex (mPFC) [14]. These effects were completely antagonized by pretreatment with n-(2-[4-(2-methoxyphenyl)-1-piperazinyl]ethyl)-n-(2-pyridinyl)cyclohexanecarbox-amide trihydrochloride (WAY100635, 0.1 mg / kg), a selective 5-HT 1A receptor antagonist [7]. Thus, 5-HT 1A receptor agonism may inhibit the ability of amphetamine to increase DA release in these three regions. Amphetamine is believed to selectively release newly synthesized DA at the nerve terminal. Reduction of DA synthesis produced by alpha*Corresponding author. Present address: 1601 23rd Avenue South, Suite 306, The Psychiatric Hospital at Vanderbilt, Nashville, TN 37212, USA. Tel.: 11-615-327-7242; fax: 11-615-322-2522. E-mail address: [email protected] (J. Ichikawa)
methyl-p-tyrosine, a DA synthesis inhibitor, has been reported to inhibit amphetamine-induced DA release in the STR [5]. (6)-8-OH-DPAT has been reported to decrease DA synthesis in the STR, as indicated by the accumulation of l-3,4-dihydroxyphenylalanine (DOPA), following NSD1015 (3-hydroxybenzyl hydrazine dihydrochloride), an laromatic amino acid inhibitor, whereas NAN-190, a 5HT 1A receptor antagonist, increased DA synthesis [11]. Thus, it is postulated that stimulation of 5-HT 1A receptors attenuates the ability of amphetamine to increase DA release in the mPFC, NAC and STR [8,14] via decreasing DA synthesis. The present study was designed to test the hypothesis that R(1)-8-OH-DPAT decreases basal and amphetamine-induced DA synthesis in the STR, NAC and mPFC. Preliminary data have been reported in abstract form [13]. Male Sprague–Dawley albino rats (Zivic-Miller Laboratories, Porterville, PA) weighing 200–250 g were used throughout the study. Rats were housed two or three per cage and maintained in a controlled 12:12 h light:dark
0006-8993 / 00 / $ – see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S0006-8993( 00 )02437-9
T. Kuroki et al. / Brain Research 872 (2000) 204 – 207
cycle and under constant temperature at 228C, with free access to food and water. R(1)-8-OH-DPAT hydrobromide (RBI, Natick, MA) was subcutaneously (s.c.) administered to rats, 30 min prior to D-amphetamine sulfate (1 mg / kg, Sigma, St. Louis, MO). These two drugs were dissolved in deionized water. The control rats received deionized water (1 ml / kg s.c.). R(1)-8-OH-DPAT was administered to rats, 30 min prior to amphetamine, as has been conducted in our previous studies [8,14]. Also, in accord with the previous report of amphetamine-induced DOPA accumulation [21], NSD-1015 (100 mg / kg, i.p., Sigma) was administered to rats 15 min after amphetamine. Rats were sacrificed 30 min after NSD-1015 (45 min after amphetamine and 75 min after R(1)-8-OHDPAT, respectively) and the brains were quickly removed. The mPFC, NAC (mostly the shell part) and dorsolateral STR tissues were punched out bilaterally from slices coronally dissected with approximately 1 mm thickness; coordinates: A13.2 mm for mPFC, A12.0 mm for NAC and A10.5 mm for STR from bregma, respectively, according to the reference atlas [16]. The procedures applied in these experiments were approved by the Institutional Animal Care and Use Committee of Case Western Reserve University in Cleveland, OH, where we completed the present study. Sample tissues were homogenized in 0.1 M perchloric acid and centrifuged at 95003g for 20 min. Fifty microlitre of each supernatant was applied onto HPLC system with coulometric detection (Model 5100A, ESA, Bedford, MA). DOPA was separated on a reverse phase column (Ultrasphere ODS 5 mm, 4.63250 mm, Beckman, San Roman, CA). The mobile phase consisted of 0.05 M citrate-phosphate buffer (pH 2.8), including 25%
205
(v / v) methanol, 50 mg / l sodium octyl sulfate and 10 mM Na 2 EDTA. The tissue pellets were dissolved with 1.0 M NaOH and the protein concentrations were determined by the Lowry’s method [15]. All data were expressed as absolute values of DOPA concentrations (means6S.E.M. pmol / mg protein). The dose effect of R(1)-8-OH-DPAT on striatal DOPA accumulation was subjected to one-way ANOVA followed by the Dunn’s procedure for comparing a control to all means, and other data were analyzed using one-way ANOVA followed by the Fisher’s protected least significant difference post-hoc test. A P,0.05 was considered significant in the present study. Systemic administration of R(1)-8-OH-DPAT (0.025, 0.05, 0.1 and 1 mg / kg s.c.) produced a U-shaped inhibition of DOPA accumulation in the STR (Fig. 1A) (F4,2953.14, P50.03). Also 0.05 mg / kg R(1)-8-OH-DPAT showed a significant decrease in striatal DOPA accumulation (P, 0.01), and 0.025 mg / kg showed a non-significant trend for a decrease (P50.09), respectively, compared with vehicle controls. R(1)-8-OH-DPAT (0.05 mg / kg s.c.) significantly decreased DOPA accumulation in the mPFC (F1,1358.65, P50.01), but had no effect in the NAC, compared with vehicle controls (Fig. 2). DOPA accumulation following amphetamine (1 mg / kg s.c.) was significantly increased up to 156% (F1,1256.03, P50.03) in the STR (Fig. 1B), and decreased to 46% (F1,1156.20, P50.03) in the mPFC (Fig. 2B), of vehicle controls, respectively. R(1)-8-OHDPAT (0.05 mg / kg s.c.), given 30 min prior to amphetamine, significantly attenuated the ability of amphetamine (1 mg / kg s.c.) to increase DOPA accumulation in the STR (F1,1257.96, P50.02), but had no effect on the amphetamine-induced decrease in the mPFC, compared with the
Fig. 1. Effect of R(1)-8-OH-DPAT on basal and amphetamine-induced striatal DOPA accumulation. (A) R(1)-8-OH-DPAT (0.025, 0.05, 0.1 and 1 mg / kg s.c.) produced an U-shaped inhibition of DOPA accumulation. A significant decrease in DOPA accumulation was observed at 0.05 mg / kg of R(1)-8-OH-DPAT, but not at 0.025 mg / kg which showed a non-significant trend of a decrease, compared with vehicle controls (VEH). (B) Amphetamine (AMPH, 1 mg / kg s.c.) significantly increased DOPA accumulation. R(1)-8-OH-DPAT (DPAT, 0.05 mg / kg s.c.), given 30 min prior to amphetamine, significantly attenuated the ability of amphetamine to increase DOPA accumulation, compared with the effect of amphetamine alone. Each bar represents the mean6S.E.M. Number of animals is in the parenthesis. Significant differences are indicated by * P,0.05 or ** P,0.01.
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Fig. 2. Effect of R(1)-8-OH-DPAT on amphetamine-induced DOPA accumulation in the NAC (A) and mPFC (B). R(1)-8-OH-DPAT (DPAT, 0.05 mg / kg s.c.) and amphetamine (1 mg / kg s.c.) by themselves significantly decreased DOPA accumulation in the mPFC, but had no effect in the NAC, compared with vehicle controls (VEH). R(1)-8-OH-DPAT (0.05 mg / kg s.c.), given 30 min prior to amphetamine, had no additional effect on the ability of amphetamine to decrease DOPA accumulation in the mPFC or the inability of amphetamine to alter DOPA accumulation in the NAC, compared with the effect of amphetamine or R(1)-8-OH-DPAT alone. Each bar represents the mean6S.E.M. Number of animals is in the parenthesis. Significant differences are indicated by * P,0.05 or ** P,0.01, compared with vehicle controls (VEH1VEH).
effect of amphetamine alone. Amphetamine (1 mg / kg s.c.) produced a non-significant increase in DOPA accumulation in the NAC (F1,1151.20, P50.08), compared with vehicle controls; this effect was not influenced by R(1)-8OH-DPAT (0.05 mg / kg s.c.) (Fig. 2A). NAN-190 and WAY100635, 5-HT 1A receptor antagonists, have been reported to increase basal DA synthesis in the STR and antagonize the ability of (6)-8-OH-DPAT to decrease striatal DA synthesis [1,11,12]. Thus, the ability of R(1)-8-OH-DPAT, a selective 5-HT 1A receptor agonist, to decrease DA synthesis in the STR is most likely due to stimulation of 5-HT 1A receptors. Systemic administration of amphetamine (3 mg / kg) has been reported to increase striatal DA synthesis at 45 min after injection, followed by a decrease below control levels at 75 and 90 min [21]. Increased DA synthesis in the STR can increase the releasable pool of DA available for amphetamine. Thus, it is important that 0.05 mg / kg of R(1)-8-OH-DPAT in 0.025, 0.05, 0.1 and 1 mg / kg, which produced maximum decrease in striatal DA synthesis, produced maximum inhibition of amphetamine-induced DA release in the STR [8]. The lesser effects of high dose R(1)-8-OH-DPAT on basal DA synthesis and amphetamine-induced DA release in the STR, are unlikely to be relevant to the ability of high dose 8-OH-DPAT (0.225 mg / kg) [18] or R(1)-8-OHDPAT (0.2 mg / kg) [10] to decrease basal DA release in the STR. Mechanisms other than 5-HT 1A receptor stimulation by high dose R(1)-8-OH-DPAT should be considered [8]; e.g. high dose R(1)-8-OH-DPAT inhibits DA reuptake into the nerve terminal [17], which may interfere with end-product inhibition of DA synthesis [6]. (6)-8-OHDPAT may have D 2 receptor agonist properties [2,19,22],
and hence could decrease DA synthesis. However, this is unlikely in the present study since S(2)-sulpiride, a D 2 receptor antagonist, has been reported not to antagonize (6)-8-OH-DPAT-induced reduction of DA synthesis in striatal synaptosome [11]. Furthermore, we have reported that R(1)-8-OH-DPAT had no effect on S(2)-sulpirideinduced DA release in the STR [9]. The results reported here do not support the hypothesis that R(1)-8-OH-DPAT attenuates amphetamine-induced increases in DA synthesis in the mPFC and NAC, and, thereby, attenuates amphetamine-induced DA release in these regions [8,14]. First, amphetamine (1 mg / kg) decreased DA synthesis in the mPFC, but had no significant effect in the NAC, as similarly reported by others [21]. Second, R(1)-8-OH-DPAT (0.05 mg / kg), which by itself decreased DA synthesis in the mPFC, produced no additional effects on the ability of amphetamine to decrease DA synthesis in that region. Finally, neither amphetamine (1 mg / kg), R(1)-8-OH-DPAT (0.05 mg / kg), nor their combination affected DA synthesis in the NAC. Thus, the effect of R(1)-8-OH-DPAT on DA synthesis in the mPFC and NAC is unlikely to contribute to the ability of R(1)-8OH-DPAT to inhibit amphetamine-induced DA release in these regions [8,14]. These results also indicate a dissociation between synthesis and release of DA in the mPFC and NAC with respect to the effect of either amphetamine or R(1)-8-OH-DPAT. For example, 5-HT 1A receptor stimulation by 8-OH-DPAT has been reported to increase basal DA release in the mPFC [3,14,18,20], whereas the present study showed an inhibitory effect of R(1)-8-OH-DPAT on DA synthesis in the same region. It should be noted, however, that the effect of R(1)-8-OH-DPAT on DA
T. Kuroki et al. / Brain Research 872 (2000) 204 – 207
synthesis was determined only at one time point (75 min after R(1)-8-OH-DPAT) in the present study, and that the possibility remains that R(1)-8-OH-DPAT may significantly increase DA synthesis at different time points. In summary, 0.05 mg / kg R(1)-8-OH-DPAT, the dose which produced maximum decrease in striatal DA synthesis among the doses tested (0.025–1 mg / kg), attenuated amphetamine-induced increases in striatal DA synthesis. Since R(1)-8-OH-DPAT (0.05 mg / kg) has been reported to attenuate amphetamine (1 mg / kg)-induced striatal DA release [8], this inhibition may be due, at least in part, to the ability of R(1)-8-OH-DPAT to decrease DA synthesis. However, because R(1)-8-OH-DPAT had no effect on amphetamine-induced DA synthesis in the mPFC and NAC, factors other than DA synthesis may contribute to the ability of R(1)-8-OH-DPAT to inhibit amphetamineinduced DA release in these regions [8,14].
[8]
[9]
[10]
[11]
[12]
[13]
Acknowledgements We are grateful to Mr Michael T. Kitchen for his excellent laboratory assistance. This study was supported, in part, by a grant from the Warren Foundation.
[14]
[15]
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