NEUROPSYCHOPHARMACOLOGY 1993-VOL. 8, NO. 3 195

Selective Effects of Low-Dose D2 Antagonism in a Reaction-Time Task in Rats Marianne Amalric, Ph.D., Melissa Berhow, B.S., Ilham Polis, B.S., and George F. Koob, Ph.D.

Operant responses involving a cued discrimination are In contrast, a selective D2 StnSitively disrupted by neuroleptic that block (50, 100, or 200 J1.glkg) induced a dose-dependent dopamine (DA) receptors in the brain; however, it is not increase in the number of incorrect responses (release of dtarwhich DA receptor subtypes may be involved in the lever over the RT limit) associated with an increase theseeffects. The role of Dl or D2 DA receptor in the RT. The results suggest that the I1Itagonists on the execution of a conditioned reaction­ nigrostriatal system, which has previously been shown to time (RT) motor task was investigated in the present be specifically involved in this RT task (Amalric and study. Rats were trained to release a lever after the Koob 1987), appears to be a sensitive site for sensorimotor presentation of a visual cue within a RT limit to be integration, and that the execution of the conditioned RT rrinforced by a food pellet. The Dl receptor antagonist motor task may depend preferentially on the activation of SCH-23390, at doses that significantly decrease the the dopaminergic D2 receptors in this system. behavioral effects of , did not impair performance lNeuropsychopharmacology 8:195-200, 1993] ,/ any dose (5, 10, or 20 J1.glkg) injected subcutaneously.

KEY WORDS: Dopamine Dl receptors; Dopamine D2 02 sites (Kebabian and Calne 1979) on the basis of rrctptors; SCH-23390; Raclopride; Reaction-time task; in vitro physiological and biochemical studies has RAt opened questions as to the functional differences of these two receptors. Thebrain dopam ine (OA) systems have been implicated Most of the neuroleptic drugs are antagonists at ina variety of behaviors including self-administration both receptor subtypes. However, the recently devel­ oipsychoactive drugs or in motor control. Neuroleptic oped OA antagonist, raclopride, has a high affinity for drugsthat block OA receptors in the brain (Wise 1982) the 02 OA receptor and crosses the blood-brain barrier suppressoperant responses reinforced with food, brain fairly easily, reaching its peak concentration at about stimulation, or drugs. Furthermore, dopa­ 15 minutes after intravenous injection (Kohler et al. minergicreceptor blockade or neurotoxic lesions of the 1985). Raclopride has no effect on the activity of ade­ brain DA system also induce motor and cognitive nylate cyclase, indicating the absence of action on the debcits. The subdivision of OA receptors into 01 and 01 receptors (Ogren et al. 1986). On the other hand, the SCH-23390 has a very high affinityfor the OA Fromthe Dep artment of , The ScrippsResearch 01 receptor, as compared to its affinity for the 02 Institute,La Jolla, California; and Laboratoire de Neurobiologie Cel- 1uIaire et Fonctionnelle, CNRS, Marseille, France. receptor (Billard et al. 1984; Hyttel 1983; Hjorth and Address reprintrequests: to M. Amalric, Ph. D., Laboratoirede Neu­ Carlsson 1988); SCH-23390has been shown to suppress robiologieCellulai re et Fonctionnelle, CNRS, 31 chemin J. Aiguier, numerous OA-stimulated behaviors such as stereotypy 13402 Marseille Cedex 9, France. ReceivedNovem ber 18, 1991; revised March 24, 1992 and June 25, or locomotor activity, demonstrating its dopaminergic 1992; accepted June 30, 1992. action (for review, see Clark and White 1987). Using

C 1993 American College of Neuropsychopharmacology Published by Elsevier Science Publishing Co., Inc. 65S Avenue of the Americas, New York, NY 10010 0893-133X/93/$6.00 Amalric et al. NEUROPSYCHOPHARMACOLOGY 1993-VOL. 8, NO.3

specifIc selective compounds for the Dl or the D2 re­ Rf ceptor subtypes, many investigators have suggested Correct trial c.s. that Dl and D2 receptors may interact with each other Delays ( 25. in producing a variety of responses, such as hyperac­ , _ 0 05. tivity, stereotypy, circling, and grooming in rats (Wad­ 075. 1.0 s) dington 1986). u..-vcruP"1 dowo_ _ __ _ Behavioral studies have indicated a role for the RT mesocorticolimbic dopaminergic system in the medi­ Anticipatory trial ation of the pharmacological actions of psychomotor such as or cocaine. Dopamine in the appears to be involved in the reinforcing and locomotor hyperactivity induced by psychomotor stimulants (Koob et al. 1987; Roberts et al. 1980, 1982; Yokel and Wise 1978; Le Moal and Si­ mon 1991). Whether these behavioral effects are medi­ ated through the activation of Dl or D2 DA receptors is still a matter of discussion (Koob et al. 1987; Wool­ RTlimit=7

0.1% hydrochloric acid, and the second group with a Correct l>2receptor antagonist (raclopride at 0, 50, 100, or 200 IIg/kg, dissolved in saline vehicle) subcutaneously in a volume of 0.1 mlllOOg. Thirtyminutes later allanimals were tested in the RT task and their motor performance recorded. The pH of the SCH-23390 solution was read­ justed to 6 to 7 with NaOH 0.1 N before each injection. Within each group, each rat was injected four times with allof the different doses tested, using a Latin-square 60 design to control for potential order effectsof repeated Anticipatory injection. Between each test day (separated by 2 days) all animals were pretreated with a control injection (NaCl 0.9%) and their performance recorded to test for 30 the potential effect of the handling and injection pro­ cedure.

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Data Analysis 40 Long Thenu mber of correct or incorrect (AN, LG) responses asw ell as RTs for correct and long trials were analyzed separately. Each parameter of the performance, mea­ 20 sured before drug treatment (mean of the two control sessions [saline injection] preceding any test day) and ..... during the test day, was subjected to a two-factor anal­ 0 ...... ,..- o 5 10 20 (Ilg/kg) ysisofvariance (ANOVA), with repeated measures on Sch 23390 twofacto rs. The drug groups (doses) constituted the independent factor, the sessions pre- and postdrug Figure 2. Performance of rats in the RT task, after pretreat­ were the dependent factor. Individual means between ment with SCH-23390, injected subcutaneously 30 minutes controland drug values were compared using a paired before the test. A correct trial is a lever release before the RT­ I·test. limit restriction (700 msec after the CS). An AN error is a re­ lease of the lever before the visual CS. An LG error is a re­ lease of the lever after the time-liIriit restriction. RESULTS from an increased number of long trials as shown by Aftertraining, the performance of the animals was sta­ a signifIcant interaction between dose and sessions bilized at a level of mean correct trials by session of 54 (ANOVA: F = 21.34, df = 3,45, P < .01) (Fig. 3). In­ for groupI (SCH-23390) and 52 for group II (raclopride) dividual comparisons of means, using a paired t-test, (ranging from 21 to 82). The mean correct RT averaged showed that the three doses tested signifIcantly in­ 340rosec (group I) and 342msec (group II).During base­ creased the number of long trials, as compared to con­ line, incorrect responses resulted largely from a high trol values. Two of the 16 animals receiving the highest number ofAN responses (mean number of AN per ses­ doses did not reach the criterion of 100 lever presses sion was 36 for group I and 38 for group II) rather than within the session. They stopped pressing the lever af­ from LGres ponses over the RT limit (mean number of ter 60 trials and remained in a state of prostration away LG per session was 10 for both groups with a mean RT from the lever. No signifIcantchange in the correct RTs averaging 1500 msec). (below 700 msec) was observed after raclopride treat­ The pretreatment with the D1 DA receptor an­ ment, except with the lowest dose (50 Ilg/kg). However, tagonist (SCH-23390, 5 to 20 Ilg/kg) did not modify the the RTs of non-rewarded trials (over 700 msec) were motor performance at any dose tested (Fig. 2). The dramatically increased in a dose-dependent manner animals performed the RT task with the same rate of (ANOVA: F = 14.10, df = 3,45, P < .01) as shown in correct trials as during the control sessions (data not Figure 4. The RTs were increased up to 2500 msec for shown). the highest dose of raclopride. In contrast, the pretreatment with the D2 DA receptorantagonist (raclopride, 50 to 200 Ilg/kg) dose­ dependently decreased the number of correct trials. DISCUSSION Analysis of variance revealed a signifIcant dose x pre­ and posttreatment sessions interaction (F = 14.16, The present results show that doses of the D1 recep­ df = 3,45, p< .01). Incorrect responses mainly resulted tor antagonist SCH-23390 that signifIcantlydecrease the 198 M. Amalric et al. NEUROPSYCHOPHARMACOLOGY 1993-VOL. 8, NO.3

Correct 400 60 * *

30

0...u....._L.

60 Anticipatory

Ul 3000 E * * '-' C1i E * pre post � 2000 * t::

40 .9..... u � � 1000 20

O..u....-r-.L.. o 50 100 200 �g/kg o 50 100 200 (�g/kg) Raclopride Raclopride

Figure3. Performance of rats in the RT task after pretreatment Figure 4. Effect of rac10pride pretreatment on RT. Reaction with rac10pride injected subcutaneously 30 minutes before time (in msec ± SEM) was measured from the CS onset to the test. A correct trial is a lever release before the RT-limit the lever release. Correct RTs occurred before the RT-limit restriction (700 msec after the CS). An AN error is a release restriction (top), incorrect RTs occurred after the RT limit. of the lever before the visual CS. An LG error is a release of Asterisks indicate signifIcant difference from control values the lever after the time-limit restriction. Asterisks indicate (p < .05, paired t-test, following signifIcant ANOVA). significantdiff erence from control values(p < .05, paired t-test) following signifIcant ANOV A. results). This latter effectcould explain the behavior of the two animals that did not complete the full session reinforcing effects or the locomotor stimulation induced under the effect of the highest dose of raclopride in the by cocaine (Koob et aI. 1987; Cabib et al. 1991) and present study. However, it should be stressed that the d-amphetamine in rats (Mailman et al. 1984) do not im­ animals are food-deprived in our behavioral conditions pair performance of an appetitive simple RT motor task. and are thus less sensitive to the hypokinetic effect of However, the selective 02 receptor antagonist raclo­ raclopride. pride in doses of 50, 100, and 200 Ilg/kg induced a dose­ The blockade of the 02 receptors by noncataleptic dependent disruption of the same task. This disrup­ doses of raclopride in our behavioral situation results tion was characterized by decreases in the number of in a motor deficit, expressed by an increase in RTs. correct responses that were wholly attributable to in­ However, the hypothesis of a possible reduced "rein­

creases in the time to release the lever after the cue. forcement value II of the food reward cannot be ruled The abilityof raclopride (50 to 300 Ilg/kg) and SCH- out as it could also result in an impairment of the mo­ 23390 (5 to 20 Ilg/kg) to produce catalepsy has been tor performance. Nevertheless, an "extinction-like" re­ tested using the horizontal bar. As previously shown sponse (increase in the number of lever releases fol­ by others, SCH-23390 did not elicit catalepsy at the three lowed by a decrease in performance) wouldbe expect ed doses tested. The EOso to produce catalepsy, calcu­ in this case. This was not observed in the present study. lated by others, was shown to vary between 20 to 80 The change in performance induced by raclopride Ilg/kg (Morelli and Oi Chiara 1985; Meller et al. 1985). resembles closely the deficitsin RT performance previ­ Raclopride at low doses (50 and 100 Ilg/kg) did not pro­ ously observed with the mixed 01/02 antagonist a-flu­ duce catalepsy either, whereas higher doses were found penthixol (Amalric and Koob 1987) and systemic halo­ to be cataIeptogenic (A. Ouagazzal et al., unpublished peridol (unpublished data). Perhaps more importantly, NEUROPSYCHOPHARMACOLOGY 1993-VOL. 8, NO.3 Dopamine Receptor Antagonism in Reaction Time 199

thissame profIle of increases in RT was observed after by dopaminergic activity in the corpus striatum would rothDA depletions of the corpus striatum (Amalric and be dependent on the dose of the drugs employed. Koob1987) and intracaudate infusions of haloperidol Higher doses of SCH-23390 clearly produce pro­ (Amalricand Koob 1989). A similar increase in operant nounced motor disturbances and catalepsy (Ioro et a1. response duration was observed by others after sys­ 1983; Creese and Chen 1985; Hjorth and Carlsson 1988), temicinjections of haloperidol at low doses (Liao and and animals will completely stop responding in any Fowler1990; Fowler et a1. 1986; Meck 1986). These mo­ operant situation. In contrast, lower doses of SCH- tor defIcits in response initiation resemble the symp­ 23390 within the same dose range as tested in the pres­ tomsof Parkinson's disease and presumably reflect an ent study have been found to selectively reverse DA­ impairment of DA transmission within the striatum mediated behaviors, known to involve mesolimbic DA resultingin extrapyramidal motor effects. Thus it ap­ hyperactivity. For example, SCH-23390 (3 to 20 Ilg/kg) pears that systemic injection of low doses of raclopride signifIcantly reduced the stimulant effectof cocaine on caneffect a blockade of D A function sufficient to block locomotion (Cabib et al. 1991) and the hyperlocomotor RTperformance, a task mediated by the nigrostriatal activity following injection in rats with DA system. 6-0HDA lesion of the mesolimbic terminals in the nu­ Thereis some evidence from neurochemical studies cleus accumbens (Amalric et al. 1987; Mailman et al. that D2 antagonists at low doses may preferentially 1984). Furthermore, the rewarding properties of psy­ affectth e nigrostriatal system. It is striking to note that chostimulants (cocaine and d-amphetarnine), whichare tritiatedligands of the same DA antagonists used in the thought to reflect an activation of the mesolimbic DA presentst ud y show that there is a higher relative bind­ system, are also blocked by low doses of SCH-23390 ingof raclopride in the corpus striatum (Kohler et al. (5 to 40 Ilg/kg) (Koob et a1. 1987; Hiroi and White 1991). 1985)and a higher relative binding of SCH-23390 in the The possibility of selectively affectinga given func­ nucleus accumbens in the rat (A. Pert, personal com­ tion of the striatal or mesolimbic DA system by systemic munication). Furthermore, autoradiographic studies injections of a selective receptor subtype antagonist has have also revealed a lateral to medial gradient of D2- important implications for the therapeutic use of these receptor density in the rat striatum (Joyce et a1. 1985), compounds. For example, there is some evidence to which corresponds to anatomical and biochemical suggest that chronic treatment with D1 antagonists differences in the striatum, such as a higher choliner­ does not produce the acute extrapyramidal syndrome gicactivity in the lateral striatum. This acetylcholine­ observed with D2 antagonists (Coffin et al. 1989). Thus rich lateral part of the striatum also receives a cortical at the low end of the dose-effect functions it may be innervation from the sensorimotor cortex. Altogether, possible to selectively access the function of the nigro­ theseres ults suggest a direct relationship between the striatal or meso limbic systems and affect their respec­ � receptors and sensorimotor functions of the stria­ tive therapeutic effects. tumGoyce et al. 1985), although a differencein the rel­ ative density of the D1 and D2 receptors, as demon­ strated by binding studies in differentbrain structures, ACKNOWLEDGMENTS does not necessarily imply a functional role. A preferential involvement of D2 or anta­ This ispublication number7124NP from The Scripps Research gonists inthe expression of nigrostriatal dopaminergic Institute. This research was supported by National Institute for Drug Abuse Grant DA04398 to George F. Koob. Marianne functions isalso supported by behavioral experiments. Amalric was supported by a fellowship from the "Fondation Infusion of a D2 DA receptor , , in­ pour la Recherche Medicale," France. The authors thank tracerebroventricularly into rats with 6-hydroxydopa­ Robert Lintz for excellent technical assistance with computer mine (6-0HDA) lesions of the midbrain DA system programming and electronics. We thank the Molecular and produces more stereotyped behavior than intracere­ Experimental Medicine Word Processing Department for their help in manuscript preparation. broventricular infusions of a D1 agonist, SKF-38393, and conversely intraaccumbens infusions of SKF-38393 weremore effectivethan quinpirole in eliciting locomo­ toractivity in these rats (Breese et al. 1987). Similar REFERENCES results have been seen with microinjections of SKF- 38393 and quinpirole into the ventrolateral striatum Acquas E, Carboni E, Leone P, Di Chiara G (1989): SCH 23390 blocks drug-conditioned place-preference and place­ (Delfs and Kelley 1990). 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