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Role of dopamine receptor subtypes in the acquisition of a testosterone conditioned place preference in rats
Neuroscience Letters 282 (2000) 17±20 www.elsevier.com/locate/neulet
Role of dopamine receptor subtypes in the acquisition of a testosterone conditioned place preference in rats Jason P. Schroeder, Mark G. Packard* Department of Psychology, Yale University, 2 Hillhouse Avenue, New Haven, CT 06520, USA Received 10 December 1999; received in revised form 13 January 2000; accepted 13 January 2000
Abstract The present experiments investigated the neurochemical bases of the rewarding properties of testosterone, focusing on the role of dopaminergic function in the acquisition of a testosterone conditioned place preference (CPP). In two experiments for 8 alternating days adult male Long±Evans rats received peripheral injections of testosterone in a watersoluble hydroxypropyl-b-cyclodextrin inclusion complex (0.8 mg/kg) or saline immediately prior to being con®ned for 30 min to one of two compartments of a place preference apparatus. On day 10 the rats were given a 20-min test session and allowed access to all compartments of the apparatus. No hormone was injected prior to the test, and the amount of time spent in each compartment of the apparatus was recorded. In each experiment administration of testosterone was found to induce a CPP. Injections of the mixed D1/D2 receptor antagonist a-¯upenthixol (0.3 mg/kg), the selective D1 antagonist SCH23390 (0.1 mg/kg), or the selective D2 antagonist sulpiride (20 mg/kg), each blocked acquisition of the testosterone CPP. The ®ndings suggest a role for both dopamine D1 and D2 receptor subtypes in the acquisition of testosterone CPP. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Testosterone; Androgen; Reward; Dopamine; SCH23390; Sulpiride; Flupenthixol
The conditioned place preference (CPP) paradigm infers the rewarding properties of a drug treatment by comparing the amount of time spent in an environment which has been paired with the affective consequences of a drug treatment to that in an environment paired with vehicle administration. Drugs with abuse potential in humans such as amphetamine, cocaine, heroin, or diazepam produce conditioned place preferences [4]. In addition, naturally occurring stimuli such as access to food [19], or a receptive female [11] produce conditioned place preferences in male rats. Experiments investigating the neurochemistry of reward have predominately assessed the role of dopaminergic neurotransmission. For example, peripheral injection of the mixed dopamine (i.e. D1/D2) receptor antagonist haloperidol attenuates acquisition of a CPP for food reinforcement [19] and for morphine [17]. Similarly, the mixed dopamine receptor antagonist a-¯upenthixol blocks the acquisition of an amphetamine-CPP [7,10]. Additional work has focused on the role of D1 and D2 dopamine receptor subtypes, and a role for both of these subtypes in place * Corresponding author. Tel.: 11-203-432-4671; fax: 11-203432-7172. E-mail address: [email protected] (M.G. Packard)
conditioning has been demonstrated [20]. For example, the selective D1 receptor antagonist SCH23390 has been found to block acquisition of CPPs for morphine [1,18], nicotine [1], diazepam, [1], and amphetamine [3,7]. The D2 receptor antagonists sulpiride [7] and metclopramide [3,7] have also been found to prevent the acquisition of an amphetamine CPP. Recent work in our laboratory indicates that peripheral administration of the sex steroid hormone testosterone produces a CPP [2,15], suggesting that testosterone possesses rewarding affective properties. In addition, both peripheral and intra-nucleus accumbens injections of a¯upenthixol administered on the test day blocked the expression of testosterone-CPP, indicating that activation of dopamine receptors is necessary in order to express the conditioned rewarding properties of testosterone [15]. In the present experiments we extended these ®ndings by examining the effects of selective D1 and D2 dopamine receptor subtype antagonists on the acquisition of a testosteroneCPP. Subjects were 40 adult male Long±Evans rats (300±375 g). The rats were individually housed in a temperaturecontrolled environment on a 12-h light/dark cycle with the
0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 0) 00 83 9- 9
18
J.P. Schroeder, M.G. Packard / Neuroscience Letters 282 (2000) 17±20
lights on from 7:00 to 19:00 h and had ad libitum access to food and water. The place preference apparatus was the same used as that used in our previous studies examining testosterone-induced reward [14,15]. The apparatus was constructed of wood, and had a plexiglass front wall. It consisted of three different compartments, two of which were identical in size (45 £ 45 £ 30 cm). One compartment was painted black with a ¯oor covered with 12-mm wire mesh. The other compartment was painted white and had wood chips scattered on the ¯oor. These two compartments were separated from each other by a wooden partition. A roofed tunnel painted gray (36 £ 18 £ 20 cm), protruding from the rear of the large compartments, connected the two entrances. All drugs were injected into the intraperitoneal cavity, and injection volume was held constant at 1.0 ml/kg of body weight. Testosterone hydroxypropyl-b-cyclodextrin (HBC) inclusion complex (Research Biochemicals Inc; for discussion of the behavioral use of testosterone in a cyclodextrin preparation in the CPP task, see Refs. [2,15]), HBC, a-¯upenthixol, and the D1 receptor antagonist SCH23390 were all dissolved in physiological saline. The D2 receptor antagonist sulpiride was dissolved in distilled water and 0.1 N HCl, and NaOH was used to attain a pH of 6.5±7.0. The dose of testosterone-HBC (0.8 mg/kg) was chosen based on our previous research demonstrating its ability to produce a conditioned place preference [2,15]. In both experiments, the behavioral procedures took place over a total of 10 days, and were similar to our previous work examining testosterone CPP [2,14,15]. On the ®rst day, subjects were allowed access to all three compartments in a drug free state for a total of 10 min. Four hormone and four saline pairings on alternating days (one pairing per day) followed habituation. Subjects were given their training injections for that day and placed immediately in the pairing compartment for 30 min. Half of the subjects received testosterone-HBC prior to exposure to the black compartment, and HBC-saline prior to exposure to the white compartment. The remaining subjects received testosterone-HBC prior to exposure to the white compartment, and HBC-saline prior to exposure to the black compartment. Half of the subjects received testosterone-HBC on odd numbered days, and half-received HBC-saline on even numbered days. Following the eight training days, animals were given a 20-min test session during which time they were allowed access to all three compartments of the CPP apparatus. Time spent in each of the compartments was recorded. In experiment one subjects received either saline (n 8 per group) or a-¯upenthixol (0.3 mg/kg) 20 min prior to training injections of testosterone-HBC, and saline injections 20 min prior to training injections of saline-HBC. The dose of a-¯upenthixol used (0.3 mg/kg) was chosen due to its ability to block the expression of a testosteroneinduced place preference [15]. On the test-day, subjects were allowed access to all three compartments of the CPP
apparatus in a drug free state. Fig. 1 (top) shows the amount of time spent in each of the large compartments on the testday. A two-way one-repeated measures analysis of variance (ANOVA) with pre-training injections as an independent variable, and paired side vs. unpaired side as a repeated measure revealed a signi®cant interaction [F(1,14) 7.664, P , 0:05]. Post hoc two-tailed paired t-tests revealed that the subjects receiving pre-training injections of saline spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration than on the side which had been paired with HBC-saline administration [t(7) 2.716, P , 0:05], indicating that testosterone produced a CPP. In contrast, rats receiving pre-training injections of a-¯upenthixol did not spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration [t(7) 1.136, n.s.]. These ®ndings indicate a blockade of the acquisition of testosterone-CPP by a-¯upenthixol.
Fig. 1. (Top) Effect of a-¯upenthixol (0.3 mg/kg) on acquisition of a testosterone (0.8 mg/kg) conditioned place preference. Mean time spent (s ^ SEM) during a 20 min hormone-free test session in the previously paired (testosterone) and unpaired (saline) compartments of the place preference apparatus. *Signi®cant place preference. (Bottom) Effect of SCH 23390 (0.1 mg/kg), and sulpiride (20 mg/kg) on acquisition of a testosterone (0.8 mg/kg) conditioned place preference. Mean time spent (s ^ SEM) during a 20 min hormone-free test session in the previously paired (testosterone) and unpaired (saline) compartments of the place preference apparatus. *Signi®cant place preference.
J.P. Schroeder, M.G. Packard / Neuroscience Letters 282 (2000) 17±20
An additional study was conducted to address the possibility that a-¯upenthixol may have conditioned a place aversion rather than simply preventing the acquisition of a testosterone-CPP. Eight subjects received training as above alternating injections of a-¯upenthixol and saline. Analysis of the time spent in the two large compartments on the test day revealed no conditioned place aversion [t(7) 0.365, n.s.]. This result demonstrates that the blockade of the testosterone-CPP was not due to the establishment of an a-¯upenthixol conditioned place aversion. Experiment 2 examined the effects of SCH23390 and sulpiride on acquisition of a testosterone-CPP. Subjects received either saline (n 8 per group), SCH23390 (0.1 mg/kg), or sulpiride (20 mg/kg) 20 min prior to training injections of testosterone-HBC, and saline injections 20 min prior to training injections of saline-HBC. The doses of SCH23390 and sulpiride were chosen based on the ability of a similar dose of SCH23390 and the same dose of sulpiride to block acquisition of an amphetamine-CPP [7]. Fig. 1 (bottom) shows the amount of time spent in each of the large compartments on the test-day. A two-way one-repeated measures ANOVA revealed a signi®cant interaction [F(2,21) 4.827, P , 0:05]. Post hoc two-tailed paired ttests revealed testosterone produced a CPP [t(7) 3.251, P , 0:05]. In contrast, rats receiving pre-training injections of SCH23390 [t(7) 1.209, n.s.] or sulpiride [t(7) 0.820, n.s.] did not spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration, indicating a blockade of the acquisition of testosterone-CPP by both SCH23390 and sulpiride. In both experiments, testosterone administration produced a reliable CPP, providing a replication of the rewarding affective properties of peripherally-injected testosterone observed in previous studies [2,5,15]. Acquisition of testosterone CPP was blocked by injection of the mixed dopamine D1/D2 receptor antagonist a-¯upenthixol, the selective D1 receptor antagonist 23390, and the selective D2 receptor antagonist sulpiride. As a-¯upenthixol binds both D1 and D2 receptors, and did not block acquisition of the testosterone CPP by inducing a place aversion, it is unlikely that an aversive effect mediates the ability of either of the selective DA receptor antagonists to block the testosterone CPP. These ®ndings suggest that activation of D1 and D2 dopamine receptors may mediate the primary rewarding properties of testosterone, mechanisms necessary for the formation of an association between the affective properties of testosterone and environmental stimuli, or possibly both. The contribution of dopamine D1 and D2 receptors to the acquisition of testosterone CPP are strikingly similar to that previously observed for an amphetamine CPP. For example, acquisition of an amphetamine CPP is blocked by a similar dose of the D1 receptor antagonist SCH 23390 as that used in the present experiments [9]. In addition, the D2 receptor antagonist sulpiride blocked acquisition of an amphetamine CPP [7] at the same dose which prevented acquisition of testosterone CPP [7]. Thus, acquisition of testosterone and
19
amphetamine CPP's appear to share a common dopaminergic mechanism. The use of peripheral hormone and drug injections in the present study precludes knowledge of the anatomical site(s) at which the treatments acted. However, ®ndings of recent research are consistent with the hypothesis that the rewarding affective properties of testosterone, and dopaminergic mediation of these effects, involve an in¯uence on the nucleus accumbens. The nucleus accumbens receives a dopaminergic projection originating primarily in the ventral tegmental area [12], and this mesolimbic dopamine pathway has been implicated in the rewarding effects of several drug treatments (e.g. psychostimulants, heroin), as well as naturally occurring stimuli. We have recently observed that intra-accumbens injections of testosterone produce a CPP [14]. In addition, expression of testosterone CPP produced by peripheral hormone injections is blocked by test-day intra-accumbens infusion of the dopamine receptor antagonist a-¯upenthixol [15]. Further research is necessary in order to examine the mechanism(s) by which testosterone induces rewarding affect, as well as the functional signi®cance of the rewarding properties of testosterone. With regard to possible mechanisms, the relatively brief pairing time used in the CPP task (30 min), along with evidence indicating that acquisition of a CPP requires temporal overlap between the treatment's effect and apparatus exposure [4], raises the possibility that a non-genomic action mediates the rewarding affective properties of testosterone. The functional signi®cance of testosterone-induced reward may be related to the role of this hormone in motivational aspects of sexual behavior, a function that may involve an interaction with brain reward systems [2,6], for further discussion see [2,13±16]. Finally, ®ndings indicating overlap in the neuroanatomical site (i.e. nucleus accumbens), and neurochemical mechanisms (i.e. dopaminergic), mediating testosterone reward and those of several drugs of abuse in humans, provides further support for the hypothesis [8], that androgenic anabolic steroids may possess abuse potential in humans. [1] Acquas, E., Carboni, E. and Di Chiara, G., SCH23390 blocks drug-conditioned place-preference and place-aversion: anhedonia (lack of reward) or apathy (lack of motivation) after dopamine-receptor blockade? Psychopharmacology, 99 (1989) 151±155. [2] Alexander, G.M., Packard, M.G. and Hines, M., Testosterone has rewarding affective properties in male rats: implications for the biological bases of sexual motivation. Behav. Neurosci., 108 (1994) 424±428. [3] Benninger, R.J., Hoffman, D.C. and Mazurski, E.J., Receptor subtype-speci®c dopaminergic agents and conditioned behavior. Neurosci. Biobehav. Rev., 13 (1989) 13±122. [4] Carr, G.D., Fibiger, H.C. and Phillips, A.G., Conditioned place preference as a measure of drug reward. In J.M. Leibman and S.J. Cooper (Eds.), Oxford Reviews in Psychopharmacology, Neuropharmacological Basis of Reward, Vol. 1, Oxford University Press, New York, 1989, pp. 265± 319.
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J.P. Schroeder, M.G. Packard / Neuroscience Letters 282 (2000) 17±20
[5] DeBeun, R., Jansen, E., Slangen, J.L. and Van de Poll, N.E., Testosterone as appetitive and discriminative stimulation in rats. Physiol. Behav., 52 (1992) 629±643. [6] Everitt, B.J., Sexual motivation: a neural and behavioral analysis of the mechanisms underlying appetitive and copulatory responses of male rats. Neurosci. Biobehav. Rev., 14 (1990) 217±232. [7] Hiroi, N. and White, N.M., The amphetamine conditioned place preference: differential involvement of dopamine receptor subtypes and two dopaminergic terminal areas. Brain Res., 552 (1991) 141±152. [8] Kashkin, K.B. and Kleber, H.D., Hooked on hormones? An anabolic steroid addiction hypothesis. J. Am. Med. Assoc., 262(22) (1996) 2166±3170. [9] Leone, P. and Di Charia, G., Blockade of D1 receptors by SCH23390 antagonizes morphine- and amphetamineinduced place preference conditioning. Eur. J. Pharmacol., 135 (1987) 251±254. [10] Mackey, W.B. and Van der Kooy, D., Neuroleptics block the positive reinforcing effects of amphetamine but not morphine as measured by place conditioning. Phamacol. Biochem. Behav., 22 (1985) 101±105. [11] Miller, R.L. and Baum, M.J., Naloxone inhibits mating and conditioned place preference for an estrous female in male rats soon after castration. Pharmacol. Biochem. Behav., 26 (1987) 781±789. [12] Moore, R.Y. and Bloom, F.E., Central catecholamine neuron systems: Anatomy and physiology of the dopamine systems. Annu. Rev. Neurosci., 1 (1978) 129±169.
[13] Moses, J., Loucks, J.A., Watson, H.L., Matuszewich, L. and Hull, E.M., Dopaminergic drugs in the nucleus acccumbens: Effects on motor activity sexual motivation and sexual performance. Pharmacol. Biochem. Behav., 51 (1995) 681± 686. [14] Packard, M.G., Cornell, A.H. and Alexander, G.M., Rewarding affective properties of intra-accumbens injections of testosterone. Behav. Neurosci, 111(1) (1997) 219±224. [15] Packard, M.G., Schroeder, J.P. and Alexander, G.M., Expression of testosterone conditioned place preference is blocked by peripheral or intra-accumbens injection of a-¯upenthixol. Horm. Behav., 34 (1998) 39±47. [16] Pfaus, J.G. and Phillips, A.G., Role of dopamine in anticipatory and consummatory aspects of sexual behavior in the male rat. Behav. Neurosci., 105 (1991) 725±741. [17] Schwartz, A.S. and Marchok, P.L., Depression of morphine seeking-behavior by dopamine inhibition. Nature, 248 (1974) 257±258. [18] Shippenberg, T.S. and Herz, A., Place preference conditioning reveals the involvement of D1-dopamine receptors in the motivational properties of m- and l-opioid agonists. Brain Res., 436 (1987) 169±172. [19] Spyraki, C., Fibiger, H.C. and Philips, A.G., Attenuation by haloperidol of place preference conditioning using food reinforcement. Psychopharmacology, 77 (1982) 379±382. [20] White, N.M., Packard, M.G. and Hiroi, N., Place conditioning with dopamine D1 and D2 agonists injected peripherally or into nucleus accumbens. Psychopharmacology, 103 (1991) 271±276.
Role of dopamine receptor subtypes in the acquisition of a testosterone conditioned place preference in rats Jason P. Schroeder, Mark G. Packard* Department of Psychology, Yale University, 2 Hillhouse Avenue, New Haven, CT 06520, USA Received 10 December 1999; received in revised form 13 January 2000; accepted 13 January 2000
Abstract The present experiments investigated the neurochemical bases of the rewarding properties of testosterone, focusing on the role of dopaminergic function in the acquisition of a testosterone conditioned place preference (CPP). In two experiments for 8 alternating days adult male Long±Evans rats received peripheral injections of testosterone in a watersoluble hydroxypropyl-b-cyclodextrin inclusion complex (0.8 mg/kg) or saline immediately prior to being con®ned for 30 min to one of two compartments of a place preference apparatus. On day 10 the rats were given a 20-min test session and allowed access to all compartments of the apparatus. No hormone was injected prior to the test, and the amount of time spent in each compartment of the apparatus was recorded. In each experiment administration of testosterone was found to induce a CPP. Injections of the mixed D1/D2 receptor antagonist a-¯upenthixol (0.3 mg/kg), the selective D1 antagonist SCH23390 (0.1 mg/kg), or the selective D2 antagonist sulpiride (20 mg/kg), each blocked acquisition of the testosterone CPP. The ®ndings suggest a role for both dopamine D1 and D2 receptor subtypes in the acquisition of testosterone CPP. q 2000 Elsevier Science Ireland Ltd. All rights reserved. Keywords: Testosterone; Androgen; Reward; Dopamine; SCH23390; Sulpiride; Flupenthixol
The conditioned place preference (CPP) paradigm infers the rewarding properties of a drug treatment by comparing the amount of time spent in an environment which has been paired with the affective consequences of a drug treatment to that in an environment paired with vehicle administration. Drugs with abuse potential in humans such as amphetamine, cocaine, heroin, or diazepam produce conditioned place preferences [4]. In addition, naturally occurring stimuli such as access to food [19], or a receptive female [11] produce conditioned place preferences in male rats. Experiments investigating the neurochemistry of reward have predominately assessed the role of dopaminergic neurotransmission. For example, peripheral injection of the mixed dopamine (i.e. D1/D2) receptor antagonist haloperidol attenuates acquisition of a CPP for food reinforcement [19] and for morphine [17]. Similarly, the mixed dopamine receptor antagonist a-¯upenthixol blocks the acquisition of an amphetamine-CPP [7,10]. Additional work has focused on the role of D1 and D2 dopamine receptor subtypes, and a role for both of these subtypes in place * Corresponding author. Tel.: 11-203-432-4671; fax: 11-203432-7172. E-mail address: [email protected] (M.G. Packard)
conditioning has been demonstrated [20]. For example, the selective D1 receptor antagonist SCH23390 has been found to block acquisition of CPPs for morphine [1,18], nicotine [1], diazepam, [1], and amphetamine [3,7]. The D2 receptor antagonists sulpiride [7] and metclopramide [3,7] have also been found to prevent the acquisition of an amphetamine CPP. Recent work in our laboratory indicates that peripheral administration of the sex steroid hormone testosterone produces a CPP [2,15], suggesting that testosterone possesses rewarding affective properties. In addition, both peripheral and intra-nucleus accumbens injections of a¯upenthixol administered on the test day blocked the expression of testosterone-CPP, indicating that activation of dopamine receptors is necessary in order to express the conditioned rewarding properties of testosterone [15]. In the present experiments we extended these ®ndings by examining the effects of selective D1 and D2 dopamine receptor subtype antagonists on the acquisition of a testosteroneCPP. Subjects were 40 adult male Long±Evans rats (300±375 g). The rats were individually housed in a temperaturecontrolled environment on a 12-h light/dark cycle with the
0304-3940/00/$ - see front matter q 2000 Elsevier Science Ireland Ltd. All rights reserved. PII: S03 04 - 394 0( 0 0) 00 83 9- 9
18
J.P. Schroeder, M.G. Packard / Neuroscience Letters 282 (2000) 17±20
lights on from 7:00 to 19:00 h and had ad libitum access to food and water. The place preference apparatus was the same used as that used in our previous studies examining testosterone-induced reward [14,15]. The apparatus was constructed of wood, and had a plexiglass front wall. It consisted of three different compartments, two of which were identical in size (45 £ 45 £ 30 cm). One compartment was painted black with a ¯oor covered with 12-mm wire mesh. The other compartment was painted white and had wood chips scattered on the ¯oor. These two compartments were separated from each other by a wooden partition. A roofed tunnel painted gray (36 £ 18 £ 20 cm), protruding from the rear of the large compartments, connected the two entrances. All drugs were injected into the intraperitoneal cavity, and injection volume was held constant at 1.0 ml/kg of body weight. Testosterone hydroxypropyl-b-cyclodextrin (HBC) inclusion complex (Research Biochemicals Inc; for discussion of the behavioral use of testosterone in a cyclodextrin preparation in the CPP task, see Refs. [2,15]), HBC, a-¯upenthixol, and the D1 receptor antagonist SCH23390 were all dissolved in physiological saline. The D2 receptor antagonist sulpiride was dissolved in distilled water and 0.1 N HCl, and NaOH was used to attain a pH of 6.5±7.0. The dose of testosterone-HBC (0.8 mg/kg) was chosen based on our previous research demonstrating its ability to produce a conditioned place preference [2,15]. In both experiments, the behavioral procedures took place over a total of 10 days, and were similar to our previous work examining testosterone CPP [2,14,15]. On the ®rst day, subjects were allowed access to all three compartments in a drug free state for a total of 10 min. Four hormone and four saline pairings on alternating days (one pairing per day) followed habituation. Subjects were given their training injections for that day and placed immediately in the pairing compartment for 30 min. Half of the subjects received testosterone-HBC prior to exposure to the black compartment, and HBC-saline prior to exposure to the white compartment. The remaining subjects received testosterone-HBC prior to exposure to the white compartment, and HBC-saline prior to exposure to the black compartment. Half of the subjects received testosterone-HBC on odd numbered days, and half-received HBC-saline on even numbered days. Following the eight training days, animals were given a 20-min test session during which time they were allowed access to all three compartments of the CPP apparatus. Time spent in each of the compartments was recorded. In experiment one subjects received either saline (n 8 per group) or a-¯upenthixol (0.3 mg/kg) 20 min prior to training injections of testosterone-HBC, and saline injections 20 min prior to training injections of saline-HBC. The dose of a-¯upenthixol used (0.3 mg/kg) was chosen due to its ability to block the expression of a testosteroneinduced place preference [15]. On the test-day, subjects were allowed access to all three compartments of the CPP
apparatus in a drug free state. Fig. 1 (top) shows the amount of time spent in each of the large compartments on the testday. A two-way one-repeated measures analysis of variance (ANOVA) with pre-training injections as an independent variable, and paired side vs. unpaired side as a repeated measure revealed a signi®cant interaction [F(1,14) 7.664, P , 0:05]. Post hoc two-tailed paired t-tests revealed that the subjects receiving pre-training injections of saline spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration than on the side which had been paired with HBC-saline administration [t(7) 2.716, P , 0:05], indicating that testosterone produced a CPP. In contrast, rats receiving pre-training injections of a-¯upenthixol did not spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration [t(7) 1.136, n.s.]. These ®ndings indicate a blockade of the acquisition of testosterone-CPP by a-¯upenthixol.
Fig. 1. (Top) Effect of a-¯upenthixol (0.3 mg/kg) on acquisition of a testosterone (0.8 mg/kg) conditioned place preference. Mean time spent (s ^ SEM) during a 20 min hormone-free test session in the previously paired (testosterone) and unpaired (saline) compartments of the place preference apparatus. *Signi®cant place preference. (Bottom) Effect of SCH 23390 (0.1 mg/kg), and sulpiride (20 mg/kg) on acquisition of a testosterone (0.8 mg/kg) conditioned place preference. Mean time spent (s ^ SEM) during a 20 min hormone-free test session in the previously paired (testosterone) and unpaired (saline) compartments of the place preference apparatus. *Signi®cant place preference.
J.P. Schroeder, M.G. Packard / Neuroscience Letters 282 (2000) 17±20
An additional study was conducted to address the possibility that a-¯upenthixol may have conditioned a place aversion rather than simply preventing the acquisition of a testosterone-CPP. Eight subjects received training as above alternating injections of a-¯upenthixol and saline. Analysis of the time spent in the two large compartments on the test day revealed no conditioned place aversion [t(7) 0.365, n.s.]. This result demonstrates that the blockade of the testosterone-CPP was not due to the establishment of an a-¯upenthixol conditioned place aversion. Experiment 2 examined the effects of SCH23390 and sulpiride on acquisition of a testosterone-CPP. Subjects received either saline (n 8 per group), SCH23390 (0.1 mg/kg), or sulpiride (20 mg/kg) 20 min prior to training injections of testosterone-HBC, and saline injections 20 min prior to training injections of saline-HBC. The doses of SCH23390 and sulpiride were chosen based on the ability of a similar dose of SCH23390 and the same dose of sulpiride to block acquisition of an amphetamine-CPP [7]. Fig. 1 (bottom) shows the amount of time spent in each of the large compartments on the test-day. A two-way one-repeated measures ANOVA revealed a signi®cant interaction [F(2,21) 4.827, P , 0:05]. Post hoc two-tailed paired ttests revealed testosterone produced a CPP [t(7) 3.251, P , 0:05]. In contrast, rats receiving pre-training injections of SCH23390 [t(7) 1.209, n.s.] or sulpiride [t(7) 0.820, n.s.] did not spent signi®cantly more time in the compartment which had been paired with testosterone-HBC administration, indicating a blockade of the acquisition of testosterone-CPP by both SCH23390 and sulpiride. In both experiments, testosterone administration produced a reliable CPP, providing a replication of the rewarding affective properties of peripherally-injected testosterone observed in previous studies [2,5,15]. Acquisition of testosterone CPP was blocked by injection of the mixed dopamine D1/D2 receptor antagonist a-¯upenthixol, the selective D1 receptor antagonist 23390, and the selective D2 receptor antagonist sulpiride. As a-¯upenthixol binds both D1 and D2 receptors, and did not block acquisition of the testosterone CPP by inducing a place aversion, it is unlikely that an aversive effect mediates the ability of either of the selective DA receptor antagonists to block the testosterone CPP. These ®ndings suggest that activation of D1 and D2 dopamine receptors may mediate the primary rewarding properties of testosterone, mechanisms necessary for the formation of an association between the affective properties of testosterone and environmental stimuli, or possibly both. The contribution of dopamine D1 and D2 receptors to the acquisition of testosterone CPP are strikingly similar to that previously observed for an amphetamine CPP. For example, acquisition of an amphetamine CPP is blocked by a similar dose of the D1 receptor antagonist SCH 23390 as that used in the present experiments [9]. In addition, the D2 receptor antagonist sulpiride blocked acquisition of an amphetamine CPP [7] at the same dose which prevented acquisition of testosterone CPP [7]. Thus, acquisition of testosterone and
19
amphetamine CPP's appear to share a common dopaminergic mechanism. The use of peripheral hormone and drug injections in the present study precludes knowledge of the anatomical site(s) at which the treatments acted. However, ®ndings of recent research are consistent with the hypothesis that the rewarding affective properties of testosterone, and dopaminergic mediation of these effects, involve an in¯uence on the nucleus accumbens. The nucleus accumbens receives a dopaminergic projection originating primarily in the ventral tegmental area [12], and this mesolimbic dopamine pathway has been implicated in the rewarding effects of several drug treatments (e.g. psychostimulants, heroin), as well as naturally occurring stimuli. We have recently observed that intra-accumbens injections of testosterone produce a CPP [14]. In addition, expression of testosterone CPP produced by peripheral hormone injections is blocked by test-day intra-accumbens infusion of the dopamine receptor antagonist a-¯upenthixol [15]. Further research is necessary in order to examine the mechanism(s) by which testosterone induces rewarding affect, as well as the functional signi®cance of the rewarding properties of testosterone. With regard to possible mechanisms, the relatively brief pairing time used in the CPP task (30 min), along with evidence indicating that acquisition of a CPP requires temporal overlap between the treatment's effect and apparatus exposure [4], raises the possibility that a non-genomic action mediates the rewarding affective properties of testosterone. The functional signi®cance of testosterone-induced reward may be related to the role of this hormone in motivational aspects of sexual behavior, a function that may involve an interaction with brain reward systems [2,6], for further discussion see [2,13±16]. Finally, ®ndings indicating overlap in the neuroanatomical site (i.e. nucleus accumbens), and neurochemical mechanisms (i.e. dopaminergic), mediating testosterone reward and those of several drugs of abuse in humans, provides further support for the hypothesis [8], that androgenic anabolic steroids may possess abuse potential in humans. [1] Acquas, E., Carboni, E. and Di Chiara, G., SCH23390 blocks drug-conditioned place-preference and place-aversion: anhedonia (lack of reward) or apathy (lack of motivation) after dopamine-receptor blockade? Psychopharmacology, 99 (1989) 151±155. [2] Alexander, G.M., Packard, M.G. and Hines, M., Testosterone has rewarding affective properties in male rats: implications for the biological bases of sexual motivation. Behav. Neurosci., 108 (1994) 424±428. [3] Benninger, R.J., Hoffman, D.C. and Mazurski, E.J., Receptor subtype-speci®c dopaminergic agents and conditioned behavior. Neurosci. Biobehav. Rev., 13 (1989) 13±122. [4] Carr, G.D., Fibiger, H.C. and Phillips, A.G., Conditioned place preference as a measure of drug reward. In J.M. Leibman and S.J. Cooper (Eds.), Oxford Reviews in Psychopharmacology, Neuropharmacological Basis of Reward, Vol. 1, Oxford University Press, New York, 1989, pp. 265± 319.
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