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Short communication
Enhancement of ethanol reward by dopamine D3 receptor blockade
*
Janel M. Boyce , Fred O. Risinger
Department of Behavioral Neuroscience, Portland Alcohol Research Center, Oregon Health Sciences University, 3181 SW Sam Jackson Park Road, Portland, OR 97201-3098, USA
Accepted 2 August 2000
Abstract
This experiment examined the influence of U-99194A, a dopamine D3 receptor antagonist, on ethanol’s rewarding effect in a place conditioning paradigm. Swiss–Webster mice received six pairings of a tactile stimulus with ethanol (2 g / kg, i.p.), U-99194A (20 mg / kg, i.p.) with ethanol, or U-99194A alone. A different stimulus was paired with saline. During conditioning, ethanol or ethanol / U-99194A produced similar increases in locomotor activity. U-99194A alone produced modest increases in activity on some trials. As expected, the 2 g / kg ethanol dose produced a nonsignificant trend towards conditioned place preference. However, U-99194A enhanced the acquisition of ethanol preference, whereas U-99194A alone did not produce either place preference or aversion. The results are consistent with the notion that dopamine D3 systems are important in the response to ethanol and further suggest that D3 receptor blockade increases ethanol reward. 2000 Elsevier Science B.V. All rights reserved.
Theme: Neural basis of behavior
Topic: Drugs of abuse: alcohol, barbiturates, and benzodiazepines
Keywords: Ethanol; U-99194A; Conditioned place preference; Mice; Swiss–Webster
The mesocorticolimbic dopamine system has long been of cocaine’s reinforcing effects [1]. Involvement of D3 advocated as an important neurobiological locus for the receptors in the rewarding properties of ethanol is sug-experience of reward [25]. In particular, attention has often gested by the finding of reduced D3 receptor mRNA levels been drawn to the role of dopaminergic systems in in rats experiencing 9 months free access to ethanol [5]. In modulation of reward or reinforcement associated with addition, D3 agonists have been reported to decrease abused drugs such as ethanol [10,18]. Dopamine receptor ethanol self-administration [3].
subtypes are currently classified as D1-like (D1 and D5) The purpose of the present experiment was to further and D2-like (D2, D3, and D4) [19]. Receptors of the D3 examine the role of dopamine D3 receptor processes in subtype are primarily expressed in reward-related limbic ethanol reward by characterizing the effect of a specific D3 areas such as the nucleus accumbens, leading to interest in receptor antagonist (U-99194A) on acquisition of ethanol-this receptor’s role in drug reinforcement [20]. D3 agonists induced conditioned place preference. Place conditioning such as 7-OH-DPAT inhibit locomotor activity in rats and procedures have been widely used to study the neural mice [21,7], whereas D3 antagonists such as U-99194A substrates of drug reward [23]. In mice, ethanol produces stimulate locomotor activity [2,11]. Both a D3 agonist conditioned place preference depending on dose [17]. (7-OH-DPAT) and antagonist (U-99194A) have been Relatively few studies of dopamine systems and ethanol reported to have rewarding properties demonstrated by the reward have utilized this task, which does not rely on production of conditioned place preference in rats [9]. In measurement of consummatory behaviors. In the present addition, several D3 agonist compounds produce decreases study, a subthreshold dose of ethanol (2 g / kg) was used in in cocaine self-administration, indicating an enhancement order to determine whether pretreatment with U-99194A would enhance the acquisition of ethanol-induced con-ditioned place preference. If decreased ethanol consump-*Corresponding author. Tel.: 11-503-494-2036; fax: 1
1-503-494-tion following D3 agonist treatment reflects a decrease in 6877.
E-mail address: boycej@ohsu.edu (J.M. Boyce). ethanol’s rewarding effect [3], one would expect greater
levels of conditioned preference in mice receiving D3 During conditioning, mice were randomly assigned to antagonist treatment and ethanol. one of three drug treatment groups. Group SE received a Male Swiss–Webster mice were obtained from Simon- 10 ml / kg saline injection (i.p.) 15 min before receiving 2 son (Gilroy, CA) at 7 weeks-of-age and acclimated to the g / kg ethanol (i.p., 20% v / v in saline). Group UE received colony room for 6 days before the experiments began. 20 mg / kg U-99194A (RBI / Sigma, Natick, MA), followed Mice were housed in groups of 4 in polycarbonate cages by 2 g / kg ethanol. Group US received 20 mg / kg U-(27.939.5312.7 cm) with cob bedding. A 12 h light / dark 99194A, followed by saline. Animals were returned to cycle was in effect (lights on at 07.00) and the colony their home cages after the first injection. After the second room was maintained at an ambient temperature of injection subjects were placed in the conditioning cham-21618C. Experiments were conducted during the light bers for a 5 min trial. For each drug treatment group, mice cycle. Lab chow and water were continuously available in were randomly assigned to one of two conditioning the home cage. subgroups (n56–8 / group) and exposed to an unbiased The place conditioning apparatus consisted of eight differential conditioning procedure. On alternate days mice identical acrylic and aluminum chambers (30315315 cm), received drug treatment (CS1days) prior to placement on each enclosed in a ventilated, light and sound attenuating the grid floor (Grid1 subgroup) or the hole floor (Grid2 box (Med Associates ENV-015M; St. Albans, VT). In- subgroup). Mice received saline injections (CS2 days) frared light sources and detectors were positioned opposite prior to placement on the other floor type. Therefore, one each other at 5 cm intervals on the long walls of each complete conditioning trial consisted of a pairing of a chamber, 2.2 cm above the floor surface. Occlusion of the distinctive floor after ethanol exposure and a pairing of a infrared light beams was used both as a measure of different floor with saline. Presentation of CS1 and CS2 locomotor activity and to determine the animal’s position days was counterbalanced for order of presentation. in the chamber. Data were recorded each minute by ANOVA was utilized for analyses of locomotor activity. computer. The floor of each box consisted of interchange- For the floor preference tests, between-group comparisons able halves with one of two distinctive textures: ‘hole’ examined conditioning subgroup for each drug treatment floors were made from perforated stainless steel with 6.4 [8]. The alpha level for all analyses was set at 0.05. mm round holes on 9.5 mm staggered centers; ‘grid’ floors Mean (6S.E.M.) activity counts per minute for each were composed of 2.3 mm stainless-steel rods mounted 6.4 conditioning trial are given in Table 1. SE subjects showed mm apart in Plexiglas rails. This combination of floor higher mean activity counts per minute on CS1trials than types results in equal unconditioned preference in saline- on CS2 trials, indicating ethanol-induced locomotor acti-treated Swiss–Webster mice [16]. vation. UE subjects were also activated on CS1 trials The experimental sequence was as follows: habituation compared to their first CS2 trials, indicating that D3 (one 5 min session), conditioning (eight 5 min sessions), antagonist treatment did not reduce ethanol’s locomotor testing (one 60 min session), conditioning (four 5 min stimulant effect. US subjects showed modest increases in sessions), and testing (one 60 min session). Sessions were activity on several CS1 trials. Activity levels on CS2 conducted daily (5 days / week) with the habituation ses- trials declined in all drug treatment groups.
sion occurring on a Sunday and both test sessions occur- Drug Treatment3Trial Type (CS1/ CS2) analyses of ring on a Thursday. each conditioning trial yielded significant effects of Drug During habituation, all subjects received two saline Treatment (all Fs2,40.8.9, P,0.001) and Drug injections (i.p.) separated by a 15 min interval. After the Treatment3Trial Type (all Fs2,40.14.5, P,0.001). Group first injection, subjects were returned to the home cage. SE showed significant Trial Type effects on each con-After the second injection subjects were immediately ditioning trial (all Fs1,13.31.1, Ps,0.001). Group UE also placed in the conditioning chamber for 5 min on a smooth showed significant Trial Type effects on each conditioning floor covered with white paper. This session was intended trial (all Fs1,12.21.6, Ps,0.001). Group US showed to reduce the novelty and stress associated with handling, significant Trial Type effects on conditioning trials 2, 3, injection, and exposure to the apparatus. and 4 (all Fs1,15.5.2, Ps,0.04) but not on trials 1, 5 and
Table 1
Mean (6S.E.M.) activity counts per minute during place conditioning
Drug treatment
group / trial type Trial 1 Trial 2 Trial 3 Trial 4 Trial 5 Trial 6
SE CS1 114.7(10.2) 106.1(7.7) 114.2(6.1) 94.7(8.4) 103.4(7.5) 105.6(7.9)
CS2 63.4(3.7) 50.4(2.3) 48.3(3.4) 44.5(3.0) 48.9(3.6) 37.5(4.0)
UE CS1 104.5(7.2) 94.0(6.9) 101.7(8.9) 91.4(9.4) 104.6(10.9) 103.3(10.9)
CS2 64.3(4.5) 52.0(4.0) 46.9(4.0) 42.3(3.8) 54.1(2.9) 48.3(4.7)
US CS1 61.7(3.8) 52.0(3.6) 54.6(2.4) 49.4(2.6) 51.3(2.0) 42.9(2.9)
6 (all Fs1,15,2.1, Ps,0.1). Significant effects of Drug ence) in the UE groups (F1,1157.4, P,0.02), but not in the Treatment were seen on each CS1 trial (all Fs2,40.13.7, SE groups (F1,1252.4, P,0.1) or the US groups (F1,145 Ps,0.001). Post hoc Tukey comparisons showed groups 0.1, P,0.8). Analysis of the second test also showed a SE and UE had higher activity levels than group US on all reliable effect of conditioning (F1,37511.1, P,0.002). trials (Ps,0.001). Groups SE and UE had equivalent Between-group comparisons of Conditioning Group in activity on CS1 trials. Significant effects of Drug Treat- each drug treatment showed reliable preference in the UE ment were not seen on CS2trials (Fs2,40,2.0, Ps,0.1). groups (F1,11517.2, P,0.002) but not the SE groups Analysis of activity changes over CS1 conditioning (F1,1251.2, P,0.3) or the US groups (F1,1451.1, P,0.3). sessions yielded significant effects of Drug Treatment Mean (6S.E.M.) activity counts per minute during test 1 (F2, 40532.6, P,0.001), but not Drug Treatment3 were as follows: Test 1: group SE (31.862.4), group UE Conditioning Trial (F10,20050.9, P,0.4). Analysis of CS2 (38.962.5), group US (37.562.2); Test 2: group SE trials showed a significant effect of Conditioning Trial (28.963.0), group UE (35.963.2), group US (34.462.8). (F5,200523.1, P,0.001), but not Drug Treatment (F2,405 Analyses showed no significant differences among the 0.7, P,0.5) or Drug Treatment3Conditioning Trial drug treatment groups during either test session (both (F10,20051.0, P,0.4). Fs2,40,2.4, Ps,0.1).
Fig. 1 depicts the mean (6S.E.M.) seconds per minute Our results indicate that a specific dopamine D3 receptor on the grid floor during preference testing for both antagonist (U-99194A) magnifies ethanol’s rewarding ef-subgroups within each drug treatment condition. The fects measured using the place conditioning paradigm. results of the first test (after four trials) are shown on the This outcome is consistent with an important role of left and the results of the second test (after two additional mesolimbic D3 receptors in the experience of drug-induced trials) are shown on the right. As indicated by the between- reward [20]. As shown previously in this lab, 2 g / kg group difference between the Grid1 and Grid2 sub- ethanol is a sub-threshold dose for conditioning place groups, mice receiving ethanol1U-99194A showed con- preference in Swiss–Webster mice, allowing for observa-ditioned place preference for the floor type paired with tion of drug-induced enhancement of ethanol’s rewarding drug treatment. Ethanol alone or U-99194A alone did not effects [17]. In the present study, the observed enhance-produced significant place conditioning. ment of ethanol-induced conditioned place preference Ethanol Dose3Conditioning Group analysis of the time appears not to be due to a rewarding action of D3 blockade spent on the grid floor type yielded reliable effects of per se, since mice given U-99194A without ethanol Conditioning Group (F1,3756.8, P,0.02). Between-group showed no evidence of place conditioning.
comparisons of Conditioning Group with each drug treat- A variety of dopaminergic agents reduce rodent ethanol ment indicated the significant conditioning (place prefer- self-administration measured in the home cage or in
operant chambers [4,12]. However, dopaminergic agents Acknowledgements
also have locomotor effects which can alter drinking or
responding independently from altering ethanol’s reward- Supported by NIAAA grants AA10520, AA10760, ing motivational effects [6]. In contrast to studies showing AA07468. Pierre Freeman is thanked for assistance with reduced ethanol drinking after D2 antagonist treatment, a data collection.
study using the place conditioning paradigm indicated that dopamine D2 receptor blockade by haloperidol treatment did not alter ethanol reward at doses that reduced ethanol
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