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THE JOURNAL OF PHARMACOLOGY AND EXPERIMENTAL THERAPEUTICS
Copyright © 1998 by The American Society for Pharmacology and Experimental Therapeutics
JPET 284:1015–1025, 1998
Vol. 284, No. 3
Printed in U.S.A.
Discriminative Stimulus Properties of Cocaine: Enhancement
by Monoamine Reuptake Blockers
MARK S. KLEVEN and WOUTER KOEK
Centre de Recherche Pierre Fabre, 17 avenue Jean Moulin, 81106 Castres Cedex France
Accepted for publication November 25, 1997
This paper is available online at http://www.jpet.org
Whereas in vitro studies show that cocaine inhibits reuptake of NE, DA and 5-HT nonselectively (Koe, 1976; Reith
et al., 1986), inhibition of DA reuptake is primarily responsible for cocaine’s subjective effects in humans (Volkow et al.,
1996, 1997). However, pretreatment with 5-HT/NE reuptake
blockers can alter the DS effects of cocaine in rodents and
nonhuman primates (Cunningham and Callahan, 1991;
Spealman, 1993, 1995), reinforcing effects in rodents (Tella,
1995) and subjective effects in humans (Fischman et al.,
1990; Walsh et al., 1994), which suggests that the indirect
effects of cocaine on NE and 5-HT reuptake may be important in vivo. Although there is strong evidence that DA
reuptake blockade plays a necessary and sufficient role (Giros et al., 1996; Silvia et al., 1997), these interaction studies
indicate that NE and 5-HT may be important modulators of
the behavioral effects of cocaine.
Results from several recent drug discrimination studies
show convincingly that monoamine reuptake blockers enhance the behavioral effects of cocaine. Cunningham and
Callahan (1991, 1997) reported that coadministration of the
Received for publication April 25, 1997.
blockers we examined. In contrast, NE reuptake apparently
does not play a strong role, despite the finding that desipramine, talsupram and nortriptyline enhanced the DS effects of
cocaine. However, pretreatment with the alpha-1 adrenergic
antagonist prazosin failed to alter completely the ability of
desipramine to enhance the DS effects of the low training dose
of cocaine, but did produce dose-related decreases in the
cocaine-enhancing effects of the beta adrenergic antagonist
propranolol (10 mg/kg i.p.). These findings suggested that,
under some conditions, NE interactions can modulate the DS
effects of cocaine. In all, the results confirm reports that monoamine reuptake blockers enhance the DS effects of cocaine
and indicate that 5-HT and DA can effectively modulate the DS
effects of cocaine, but suggest that NE interactions may be
relatively less important in the rat.
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ABSTRACT
In this study we examined the ability of compounds varying in
their in vitro potencies as inhibitors of dopamine (DA), norepinephrine (NE) or serotonin (5-HT) reuptake to enhance the
discriminative stimulus (DS) effects of cocaine. Compounds
were administered in combination with cocaine (2.5 mg/kg i.p.)
to rats trained to discriminate a low dose from a high dose of
cocaine (2.5 vs. 10 mg/kg i.p.) in a two-lever, FR10 drug discrimination paradigm. All the monoamine reuptake blockers
produced high-dose-appropriate responding in a dose-related
manner when combined with a low dose of cocaine, but compounds from other pharmacological classes (benztropine, caffeine, diazepam, or 8-hydroxy-2-(di-n-propylamino)tetralin) did
not enhance the DS effects of cocaine. Analysis of the relationship between behavioral and in vitro biochemical potencies
indicated that inhibition of DA and 5-HT transport is responsible
for the cocaine-enhancing effects of the monoamine reuptake
5-HT reuptake inhibitor fluoxetine or the selective NE reuptake inhibitor desipramine produced leftward shifts in
drug-appropriate responding in rats trained to discriminate
cocaine from saline in a two-lever drug discrimination paradigm. Spealman (1995) reported similar effects of the NE
inhibitors talsupram and tomoxetine in squirrel monkeys
trained to discriminate cocaine from saline; however, unlike
results obtained in rats, the selective 5-HT reuptake inhibitor citalopram produced a rightward shift in the cocaine
dose-response function (Spealman, 1993). This difference between species has not been explained adequately; yet taken
in conjunction with findings that 1) fluoxetine does enhance
the DS effects of a lower training dose of cocaine in squirrel
monkeys (Schama et al., 1997), and 2) the rate-increasing
effects of cocaine on schedule-controlled behavior in squirrel
monkeys are attenuated by 5-HT reuptake inhibitors (Spealman, 1993; Howell and Byrd, 1995), monoamine reuptake
blockers are indeed capable of altering behavioral effects of
cocaine in both rats and nonhuman primates.
In addition to interaction experiments, several recent
pharmacological characterizations of the DS effects of low
doses of cocaine indicate that NE may play a more important
ABBREVIATIONS: 5-HT, serotonin; 8-OH-DPAT, (8-hydroxy-2-(di-n-propylamino)tetralin); DA, dopamine; DS, discriminative stimulus; NE, norepinephrine; FR, fixed ratio; FRF, sum of the responses made on either lever before the first reinforcement occurred; GBR12935, (1-[2(diphenylmethoxy)ethyl]-4-(3-phenylpropyl)-piperazine); HDL, high-dose lever; MW, molecular weight; NE, norepinephrine; SKF 81297 (also
designated 6-chloro-APB), (6)-6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine; VTA, ventral tegmental area.
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Methods
Animals. Male Sprague-Dawley rats (Ico: OFA SD (I.O.P.S. Caw)
Iffa Credo, St. Germain sur l’Arbresle, France), weighing between
240 and 260 g at the beginning of the studies, were used. Rats were
housed in individual cages (Iffa Credo, 28 3 21 3 18 cm) with metal
grid floors in air-conditioned rooms (21 6 1°C; relative humidity
60 6 5%) under a 12-hr light-dark cycle (lights on from 7:00 A.M. to
7:00 P.M.). Filtered (0.22 m) water was freely available, but access to
standard laboratory food (A04, Usine d’Alimentation Rationalle, Epinay sur Orge, France) was limited to 10 g per day, except during
weekends when food was freely available between 5:00 P.M. Friday
and 2:00 P.M. Sunday. Experiments were conducted between 9:00
A.M. and 5:00 P.M., Monday through Friday. Animals were cared for
in accordance with guidelines set by the US Department of Health
and Human Services for humane treatment of animals (Guide for the
Care and Use of Laboratory Animals, US DHHS, PHS, National
Institutes of Health publication no. 85–23, revised 1985), and the
experimental protocol (assigned no. 009 by our local regulatory committee) was carried out in accordance with French law and the local
ethical committee guidelines for animal research.
Apparatus. Experiments were conducted in standard operant
conditioning chambers (model E10 –10, Coulbourn Instruments, Lehigh Valley, PA) housed in light- and sound-attenuating enclosures
that were ventilated by a fan, which also produced a masking noise.
Each chamber contained a house light that was mounted above a
food pellet receptacle located between two levers that were situated
2.5 cm above the grid floor. Food pellets (45 mg dustless pellets,
Bioserv, Frenchtown, NJ) were delivered by a pellet dispenser (model E14 –12, Coulbourn Instruments, Lehigh Valley, PA). Scheduling
of reinforcement contingencies, reinforcement delivery and data recording were controlled by a SKED-11 system (State Systems,
Kalamazoo, MI) implemented on a PDP-11 computer (Digital Equipment Corporation, Maynard, MA).
Discrimination procedure. All of the rats used in this study
were trained initially to discriminate cocaine (10 mg/kg i.p.) from
saline in a two-lever, food-reinforced FR10 drug discrimination paradigm by methods identical with those described recently (Koek et
al., 1995; Kleven et al., 1997). Saline or cocaine (10 mg/kg) were
administered 15 min before sessions during which responding on one
of two levers, depending on pretreatment, was reinforced. Discrimination training was continued until fewer than three responses were
made on the injection-inappropriate lever before the first food presentation during ten consecutive sessions (i.e., the FRF was less than
13).
Dose-dose discrimination training. On reaching the salinecocaine training criterion, rats were trained subsequently to discriminate a low dose (2.5 mg/kg) from the 10 mg/kg training dose via
successive training periods wherein low doses of cocaine (0.63 or 2.5
mg/kg) were substituted for saline. That is, after first reaching
criterion performance (FRF , 13 during ten consecutive sessions),
the low training dose was 0.63 mg/kg, and after criterion performance was reached again it was changed to 2.5 mg/kg.
Test sessions were conducted twice per week on Wednesdays
and/or Fridays, although training continued on intervening days.
During test sessions, the lever on which ten responses accumulated
first was defined as the selected lever. After lever selection, the
animal received the first food pellet, and subsequent reinforcement
was made contingent on pressing the selected lever. A test session
ended after 15 min. Testing was postponed to the next scheduled test
day if, on either of the 2 most recent training days, the FRF value
exceeded 15. Also, test data were discarded and the test condition
later retested if the test session was followed by a training session of
which the FRF value exceeded 15.
Drug administration. All drugs were injected in a volume of 1
ml/100 g and doses are expressed as weight of the free base. For
interaction studies in dose-dose trained rats, saline or drug was
injected i.p. 30 min before the session, i.e., 15 min before administration of saline or cocaine (2.5 mg/kg i.p.). The order of treatment
with individual drugs and doses was unsystematic.
Data analysis. Test sessions generated data on two variables: 1)
the selected manipulandum, i.e., saline, drug, high-dose, or low-dose
lever, representing the measure of discriminative responding and 2)
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role than previously believed. That is, NE reuptake blockers
(e.g., desipramine, nisoxetine or talsupram) engender more
complete substitution for the DS effects of a low training dose
in either rats (Terry et al., 1994) or monkeys (Spealman,
1995), in contrast to findings obtained in animals trained
with higher doses (Colpaert et al., 1979; Broadbent et al.,
1991; Baker et al., 1993; Spealman, 1995). Additionally, the
alpha-1 adrenergic antagonist prazosin produced rightward
shifts in cocaine dose-response functions in monkeys trained
to discriminate either low or high doses of cocaine from
saline, although it has not been reported to antagonize the
DS effects of cocaine in rats. It could be that species and/or
training conditions are important determinants of the extent
that NE is involved in the DS effects of cocaine. Nonetheless,
taken together with findings that selective NE inhibitors
enhance the DS effects of cocaine in rats and nonhuman
primates, it is conceivable that NE may play a modulatory
role in both species.
The purpose of our study was to characterize further recent
findings that compounds with 5-HT and/or NE reuptakeblocking properties enhance the DS effects of cocaine (Cunningham and Callahan, 1991; Spealman, 1995; Callahan
and Cunningham, 1997). These initial studies demonstrated clearly that monoamine reuptake inhibitors such
as fluoxetine and desipramine enhance the DS effects of
cocaine, but the relative importance of NE and 5-HT to the
DS effects of cocaine has not been firmly established in the
rat. Inasmuch as cocaine is considerably less potent than
desipramine and fluoxetine at inhibiting 5-HT and NE
reuptake (Koe, 1976; Hyttel, 1982), the fact that high doses
of selective monoamine reuptake inhibitors are needed to
enhance its DS effects (Cunningham and Callahan, 1991)
makes it doubtful that these monoamines are involved
when cocaine is given alone. Furthermore, although considered selective (Fuller, 1993), compounds such as fluoxetine and desipramine do have appreciable activity at
other monoamine reuptake sites (Stanford, 1996). Thus,
both 5-HT and NE reuptake blockade may be involved in
the cocaine-enhancing effects of monoamine reuptake
blockers, although the relative importance of different
monoamines in modulating the DS effects of cocaine cannot be determined readily from the few compounds that
have been examined to date in the rat. In our study the in
vivo potencies of a large variety of monoamine reuptake
inhibitors for enhancing the DS effects of cocaine were
correlated with their potencies obtained from in vitro DA,
NE or 5-HT reuptake-blocking studies. In this study a
dose-dose discrimination procedure was used primarily because we expected compounds that substitute partially in
cocaine-saline-trained animals to engender low-dose lever
selection (Colpaert and Janssen, 1986; Kleven and Koek,
1997), a factor which makes it easier to detect the cocaineenhancing effects of such compounds. We used this procedure recently to demonstrate that beta adrenergic antagonists enhance the DS effects of cocaine (Kleven and Koek,
1997).
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Dose-Dose Cocaine Discrimination
mM; “inactive” inhibitors, IC50 . 1.0 mM were divided by their
corresponding IC50 values for cocaine reported by Koe (1976): 5-HT
(0.85 mM), NE (0.27 mM) and DA (1.7 mM). The resulting grouping
criteria, expressed in terms relative to cocaine, were then applied to
the in vitro potencies shown in table 1.
The DA reuptake blockers GBR 12935 and indatraline were excluded from the multiple regression analysis because their in vivo
potencies were expected to deviate from their extremely high DA
reuptake affinity in vitro (see “Discussion”), a finding that was confirmed by post hoc outlier analysis. For the remaining 16 compounds,
outliers (desipramine and nisoxetine) were identified by use of externally studentized residuals (Snedecor and Cochran, 1967), i.e.,
where residuals are divided by the residual standard deviation obtained from the regression that omits the case (Data Desk, Data
Descriptions Inc., Ithaca, NY). The level of statistical significance
was adjusted to allow for the fact that the largest absolute deviations
were selected (i.e., P.05 5 .05/n, in which n 5 the number of deviations).
Drugs. The drugs used in this study were alaproclate HCl (MW
292.2), GBR 12935 di-HCl (MW 487.5), 6-chloro-APB HCl (MW
370.7), cocaethylene HCl (MW 389.8), 8-OH-DPAT HBr (MW 328.3),
indatraline HCl (MW 328.7), mazindol (MW 284.7), nomifensine
maleate (MW 354.4), nisoxetine HCl (MW 307.8), (2)-propranolol
HCl (MW 295.8) (all from Research Biochemicals Int., Natick, MA);
benztropine mesylate (MW 403.5), bupropion HCl (MW 276.2), desipramine HCl (also designated desmethylimipramine; MW 302.8),
caffeine (MW 194.2), nortriptyline HCl (MW 299.8), prazosin HCl
(MW 419.9), procaine HCl (MW 272.8) (all from Sigma, Fresnes,
France); fluoxetine HCl (MW 345.8) and paroxetine HCl (MW 374.8)
(J.-L. Maurel, Centre de Recherche Pierre Fabre, Castres, France);
imipramine HCl (MW 316.9) (Interchim, Paris, France); citalopram
HBr (MW 405.3) and talsupram HCl (MW 348.0) (Lundbeck A/S,
Copenhagen-Valby, Denmark); methylphenidate HCl (MW 269.9)
(Ciba-Geigy Co., Basle, Switzerland); and cocaine HCl (MW 339.8)
(Coopération Pharmaçeutique Française, Melun, France). All drugs,
with the exception of paroxetine, mazindol and nomifensine, were
dissolved and administered in distilled water; paroxetine was prepared as a suspension in aqueous Tween 80 (2 drops/10 ml distilled
TABLE 1
Potencies of monoamine reuptake blockers in behavioral and biochemical studies
Drug
ED50
Relative Behavioral
Potencya
Obtained
Predicted
Relative in Vitro Potencyb
5-HT
NE
Groupc
DA
5-HT
NE
DA
P
P
P
S
S
M
M
S
M
M
I
M
I
P
P
P
S
P
I
I
I
I
I
I
I
I
M
P
P
M
S
I
S
W
I
I
P
P
P
M
M
W
W
I
I
S
P
S
M
M
S
M
M
I
mmol/kg
5-HT/NE
Paroxetine
Citalopram
Fluoxetine
Alaproclate
Desipramine
Talsupram
Nortriptyline
Imipramine
Nisoxetine
DA/5-HT/NE
Mazindol
Indatraline
Nomifensine
Cocaine
Methylphenidate
GBR-12935
Cocaethylene
Bupropion
Procaine
a
2.2
8.1
11
14
2.3
12
16
20
38
0.39
1.4
1.9
2.5
0.39
2.0
2.8
3.5
6.6
0.49
1.8
1.1
3.4
2.2*
3.2
2.4
2.1
1.2*
1.1
3.7
5.3
5.7
7.8
12
18
24
317
0.20
0.64
0.93
1.0
1.4
2.1
3.2
4.2
55
0.19
0.027*
0.69
1.1
3.2
0.26*
2.0
4.5
39
0.0012
0.0069
0.027
0.34
0.81
3.3
2.3
0.13
1.4
0.40
40
1.7
145
0.0044
0.0036
0.035
0.091
0.029
0.12
0.0018
3.2
1.0
95
0.95
7.0
73
1971d
0.0030
0.0012
0.030
1.0
0.74
3.2d
3.6
6.8
136d
Behavioral potency relative to cocaine for enhancement of the discriminative stimulus effects of 2.5 mg/kg cocaine.
Potencies relative to cocaine were obtained from the literature as described under “Methods.”
P 5 potent; S 5 strong; M 5 moderate; W 5 weak; I 5 “inactive” based on designations as described by Koe (1976).
d
Relative potencies based on binding results.
* Significant deviation (P , .05) from obtained relative behavioral potency.
b
c
19
132
16
35
29
30
12
58
1.7
0.058
0.0032
0.15
1.0
0.76
0.0077
1.1
1.9
83
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the response rate, i.e., the total number of responses made on either
lever during the 15-min session, expressed as a percentage of the
response rate during the most recently preceding saline or low-dose
training session. Selection data were used to calculate the percentage of animals at each treatment condition selecting the 10 mg/kg
cocaine-appropriate lever. Drug effects on this variable were analyzed by use of the Litchfield and Wilcoxon procedure (Tallarida and
Murray, 1987), implemented with the research programming language, RS/1 (Bolt Beranek and Newman Inc., Cambridge, MA), to
estimate ED50 values and 95% confidence limits. When less than two
intermediate effects were observed, 0 and/or 100% effects were transformed by means of Berkson’s adjustment (Hubert, 1984) to permit
the use of the Litchfield and Wilcoxon procedure.
Multiple linear regression of in vitro biochemical and in vivo
behavioral potencies and correlation analysis among behavioral potencies were conducted by use of Statview 4.5 (Abacus Concepts, Inc.,
Berkeley, CA). The in vitro biochemical potencies shown in table 1
were, unless noted otherwise, based on IC50 values from published
monoamine uptake experiments in which cocaine was also studied
(Hyttel, 1982; Hyttel and Larsen, 1985). In vitro data for all of the
compounds, with the exception of cocaethylene, GBR 12935, methylphenidate, procaine and nisoxetine, were obtained from Hyttel
(1982) or Hyttel and Larsen (1985); other sources were used for
potencies of GBR 12935 (Matecka et al., 1996), cocaethylene (J.
Elsworth, personal communication), procaine (Woodward et al.,
1995) and methylphenidate and nisoxetine (Koe, 1976). Because NE
and/or 5-HT IC50 values for GBR 12935 and procaine were not
available, relative potencies were based on binding affinities (Ritz et
al., 1987; Rothman et al., 1993) .
Monoamine uptake inhibitors were grouped according to the classification scheme described by Koe (1976), modified to take into
account the fact that absolute IC50 values can vary widely among
different laboratories (Stanford, 1996). The grouping values for 1)
5-HT and DA: potent inhibitors, IC50 , 0.1 mM; strong inhibitors,
IC50 0.1–1 mM; moderate inhibitors, IC50 1–5 mM; weak inhibitors,
IC50 5–10 mM; “inactive” inhibitors, IC50 . 10 mM and 2) NE: potent
inhibitors, IC50 , 0.01 mM; strong inhibitors, IC50 0.01– 0.1 mM;
moderate inhibitors, IC50 0.1– 0.5 mM; weak inhibitors, IC50 0.5–1.0
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water); and mazindol and nomifensine were dissolved in distilled
water to which a small amount of acetic acid was added and the pH
adjusted to 5 to 7 with 4% NaOH.
Results
Fig. 1. Discriminative stimulus effects of cocaine in rats trained to discriminate a low dose of cocaine (2.5 mg/kg) from a high dose of cocaine (10
mg/kg). Values represent the percentage of animals (n 5 9/dose) selecting
the 10-mg/kg appropriate lever (upper panel) or mean 6 S.E. rate of
responding (lower panel) expressed as a percentage of low-dose training
sessions. The results of the training doses in dose-dose trained rats were
published previously (Kleven and Koek, 1997).
tion of the low training dose of cocaine engendered doserelated increases in HDL selection, with 89 to 100% of the
animals selecting the HDL after administration of the highest doses of each compound. In contrast, the NE reuptake
inhibitor nisoxetine produced only intermediate levels of
HDL-appropriate responding (maximum HDL selection,
50%). With the exception of desipramine, the estimated ED50
values were very similar (i.e., ranging from 3.6 to 5.6 mg/kg,
table 2); desipramine (ED50 5 0.60 mg/kg; 0.21–1.7 mg/kg)
was apparently more potent, i.e., the 95% confidence limits
did not include the ED50 values of any of the remaining
compounds from this group.
Maximal effects on HDL selection engendered by cotreatment with 5-HT and NE reuptake blockers were observed at
doses that also produced apparent decreases in rate of responding (i.e., maximal decreases ranging from 22 6 12 to
60 6 9.1% of low-training-dose control sessions, table 2).
To determine whether NE/5-HT reuptake blockers produced HDL selection in the absence of the administration of
cocaine, saline was administered in combination with the
doses producing maximal %HDL selection. In contrast to
results obtained when 2.5 mg/kg cocaine was coadministered,
HDL selection was not observed in any of the animals treated
with paroxetine, citalopram, fluoxetine and alaproclate in
combination with saline (fig. 1, filled symbols). When saline
was administered in combination with the doses producing
maximal %HDL selection, intermediate levels of HDL selection were observed after talsupram (50%) and nortriptyline
(20%).
Interactions with compounds having DA reuptakeblocking properties. Similar to that found after administration 5-HT and NE selective reuptake blockers, treatment
with a variety of compounds that have DA reuptake-blocking
properties 15 min before administration of the low training
dose of cocaine engendered dose-related increases in HDL
selection (fig. 3, open symbols), with estimated ED50 values
ranging among high (0.32 mg/kg, mazindol), intermediate
(5.8 mg/kg, cocaethylene and bupropion) and low potency (75
mg/kg, procaine; table 2).
In contrast to the effects of NE/5-HT reuptake blockers, all
the compounds having DA reuptake-blocking properties,
with the exception of GBR 12935, bupropion and procaine,
engendered more than 50% HDL selection when administered in combination with saline (fig. 4, closed symbols); GBR
12935, bupropion and procaine did not produce HDL selection in more than 50% of the animals even when administered at doses that decreased the rate of responding to 14 to
60% of control rates. The ED50 values when drugs were
administered in combination with saline were approximately
two to three times higher than when they were administered
in combination with cocaine for all the compounds, with the
exception of mazindol and bupropion, which were approximately 16 and 7 times higher, respectively. Doses of procaine
larger than 80 mg/kg could not be examined because they
produced lethality.
Relationship between in vitro and in vivo potencies.
Table 1 shows the potencies (micromoles per kilogram) of the
monoamine reuptake blockers for enhancing the DS effects of
2.5 mg/kg cocaine in dose-dose trained rats and in vitro
relative potencies obtained from published biochemical studies (see “Methods”). Behavioral potencies relative to cocaine
ranged from approximately five times more potent (mazin-
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Effects of cocaine. The discrimination between 2.5 mg/kg
and 10 mg/kg cocaine was acquired in all 39 animals that
were trained (median sessions to final criterion, excluding
sessions that were used to calculate criterion performance,
53 sessions; semi-interquartile range, 40 –78 sessions). The
average postcriterion accuracy after administration of the
high training dose during training sessions was significantly
greater (P , .0001; paired t test) than after administration of
the low training dose (mean 6 S.E. % correct lever selections 5 96 6 0.68 vs. 88 6 1.2, 10 vs. 2.5 mg/kg training
doses, respectively).
Administration of cocaine during test sessions engendered
dose-related increases in responding on the lever associated
with the 10 mg/kg cocaine dose (fig. 1), whereas saline engendered only low-dose-appropriate lever selection in dosedose trained rats (n 5 9). The estimated ED50 value for
cocaine was 3.7 mg/kg (95% confidence limits, 2.6 –5.2 mg/
kg). Administration of cocaine during test sessions produced
dose-related decreases in the rate of responding expressed as
a percentage of control training sessions (i.e., cocaine 2.5
mg/kg).
Interactions with 5-HT/NE reuptake blockers. Treatment with the relatively selective 5-HT reuptake blockers,
paroxetine, citalopram, fluoxetine and alaproclate, 15 min
before administration of the low training dose of cocaine,
resulted in dose-related increases in HDL selection, with 71
to 100% of the animals selecting the HDL after administration of the highest doses of each compound (fig. 2). The
cocaine-enhancing effects of 5-HT reuptake blockers occurred
over a similar dose range (i.e., ED50 values ranging from 0.74
to 4.0 mg/kg and the 95% confidence limits were overlapping,
table 2).
Similar to that found after administration of selective
5-HT reuptake blockers, treatment with desipramine, talsupram, nortriptyline or imipramine 15 min before administra-
Vol. 284
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Dose-Dose Cocaine Discrimination
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Fig. 2. Serotonin/norepinephrine reuptake blockers
enhance the discriminative stimulus effects of a low
dose of cocaine (2.5 mg/kg, open symbols) in rats
trained to discriminate a low dose of cocaine (2.5 mg/
kg) from a high dose of cocaine (10 mg/kg). Values
represent the percentage of animals (n 5 5–9/dose)
selecting the high-dose lever during 15-min sessions
conducted in rats administered monoamine reuptake
blockers in combination with cocaine (open symbols) or
saline (closed symbols).
1Cocaine (2.5 mg/kg)
Drug
1Saline
Maximum Effect
n
ED50
95% CL
Maximum Effect
Dose
% HDL
n
% Rspa
a
b
c
95% CL
Dose
% HDL
mg/kg
5-HT/NE
Paroxetine
Citalopram
Fluoxetine
Alaproclate
Desipramine
Talsupram
Nortriptyline
Imipramine
Nisoxetine
DA/5-HT/NE
Mazindol
Indatraline
Nomifensine
Cocaine
Methylphenidate
GBR-12935
Cocaethylene
Bupropion
Procaine
ED50
% Rsp
mg/kg
5
7
5
7
9
7
7
7
9
0.74
2.6
3.4
3.7
0.60
3.6
4.2
5.6
10c
0.14–3.7
0.47–15
1.3–9.1
1.4–9.6
0.21–1.7
1.5–8.5
2.1–8.3
n.d.b
100
71
100
100
89
100
100
86
50
24 6 5.3
54 6 7.7
22 6 12
32 6 8.0
60 6 9.1
33 6 9.1
39 6 4.4
30 6 6.7
46 6 10
10
10
10
10
2.5
10
10
10
10
9
5
5
5
5
7
7
7
7
5
5
5
5
5
5
5
5
7
7
0.32
1.1
1.3
1.7
1.8
5.0
5.8
5.8
75
0.16–0.63
0.46–2.5
0.65–2.5
0.62–4.9
0.69–4.8
n.d.
n.d.
2.7–12
18–320
100
100
100
100
100
100
80
100
71
78 6 23
36 6 12
59 6 6.4
46 6 12
54 6 22
22 6 6.5
56 6 16
14 6 9.6
60 6 2.2
0.63
2.5
2.5
10
10
10
10
40
80
5
5
5
5
5
5
5
5
7
Mean 6 S.E. rate of responding expressed as a percentage of preceding low-dose training sessions.
n.d.: 95% confidence limits not determined because of insufficient data.
Estimated: lowest dose producing 50% HDL selection.
dol) to about five times less potent (e.g., bupropion), whereas
the in vitro biochemical potencies relative to cocaine varied
across a considerably wider range. For example, paroxetine is
reportedly about 800 times more potent than cocaine in inhibiting 5-HT reuptake, whereas bupropion and procaine are
73 and 1971 times less potent than cocaine, bupropion and
procaine, respectively. Similarly, reported biochemical potencies for NE reuptake range from greater than 500 times more
potent (e.g., indatraline, mazindol, desipramine, talsupram)
to 145 to 136 less potent than cocaine (e.g., alaproclate and
procaine). Although a wide range of relative potencies in
blocking DA reuptake is seen, all the compounds referred to
in this study as 5-HT/NE reuptake blockers are about 12 to
132 times less potent than cocaine, whereas the remaining
.10
.10
.10
.10
.2.5
.10
.10
.10
.10
5.0
1.7
3.6
4.7
5.5
10c
11
40c
.80
2.6–9.5
0.70–4.1
1.3–10
2.0–11
n.d.
n.d.
5.6–20
n.d.
0
0
0
0
0
0
50
20
0
0
95 6 3.8
45 6 12
82 6 3.5
53 6 9.5
65 6 18
75 6 10
23 6 13
45 6 14
48 6 7.8
75 6 7.7
10
10
10
10
2.5
10
10
10
10
100
100
100
100
100
50
100
50
43
51 6 28
28 6 12
57 6 32
43 6 9.8
36 6 17
1.2 6 0.80
10 6 9.0
060
75 6 10
10
5
10
10
10
40
40
80
80
Downloaded from jpet.aspetjournals.org at ASPET Journals on April 11, 2017
TABLE 2
Effects of monoamine reuptake blockers administered in combination with cocaine (2.5 mg/kg) or saline in rats trained to discriminate 2.5 vs. 10
mg/kg cocaine
compounds exhibit relative potencies ranging from about 400
times more potent (e.g., GBR 12935) to 2 to 83 times less
potent than cocaine (e.g., bupropion and procaine).
With respect to the relative selectivity according to the
scheme of Koe (1976), few of the compounds (citalopram,
alaproclate and bupropion) were classified as “inactive” in
two of the three monoamines; procaine was “inactive” at all
the sites. Among the compounds typically classified as 5-HT
and/or NE reuptake blockers, only citalopram and alaproclate were “inactive,” which indicates that the remaining
compounds in this group would be considered to have activity
as both 5-HT and NE reuptake blockers. Among the compounds considered to have DA reuptake-blocking properties,
considerable variability in selectivity could be observed. Pro-
1020
Kleven and Koek
Vol. 284
Fig. 3. Compounds having dopamine reuptake-blocking properties enhance the discriminative stimulus effects of a low dose of cocaine (2.5 mg/kg, open symbols)
or engender HDL-appropriate responding (closed symbols) in rats trained to discriminate a low dose of cocaine (2.5 mg/kg) from a high dose of cocaine (10 mg/kg).
Details are as in figure 2.
rbppred510
Downloaded from jpet.aspetjournals.org at ASPET Journals on April 11, 2017
caine would be considered “inactive” at all three sites,
whereas, in contrast, indatraline was classified as “potent” at
5-HT, NE and DA reuptake blockade.
To determine the relationship between the relative behavioral and biochemical potencies, a multiple linear regression
was performed with log-transformation of the values shown
in table 1. Analysis of results obtained from 14 of the 18
compounds, excluding the outliers, desipramine, GBR 12935,
indatraline and nisoxetine, yielded a significant multiple R
(0.95, P , .0001) and in vitro potencies in blocking reuptake
of DA (coefficient, 0.31; P , .0005) and 5-HT (coefficient,
0.25; P , .0001) were significant predictor variables. In contrast, in vitro potencies for NE reuptake blockade did not
contribute significantly to the prediction of relative behavioral potency (coefficient, 0.056; P . .25).
As expected from the high coefficient of determination of
the best-fitting equation, the residuals (i.e., differences between the potency values predicted from the multiple regression equation and the experimental potencies) are relatively
small for most compounds examined in this study (see table
1 for predicted relative potencies). Predicted behavioral potencies relative to cocaine (rbppred) were obtained according
to the following equation:
saline 60 min before the session (fig. 4B, left panels), desipramine and the beta adrenergic antagonist propranolol ad-
(0.31zlog~ DArip! 1 0.25 z log~ 5-HTrip! 1 0.056zlog~ NErip! 1 0.049)
where DArip, 5-HTrip and NErip are the in vitro potencies of
the compounds for blocking reuptake of the different monoamines, relative to cocaine, as shown in table 1. Note, however, that the obtained potency of desipramine is smaller
than that predicted from the relationship derived by use of
the remaining compounds, whereas the reverse is true for
nisoxetine, indatraline and GBR 12935. In contrast, the predicted potencies for talsupram and nortriptyline are relatively close to their obtained values, whereas their biochemical profiles closely resemble that of desipramine (see table
1).
Effects of prazosin on the cocaine-enhancing effects
of desipramine and propranolol. In rats treated with
saline, both 60 and 30 min before the session, cocaine produced dose-related increases in HDL selection (fig. 4A, left
panel). In control experiments in animals pretreated with
Fig. 4. Effects of the alpha-1 adrenergic antagonist prazosin on the
ability of desipramine or propranolol to enhance the discriminative stimulus effects of cocaine in rats trained to discriminate a low dose of cocaine
(2.5 mg/kg) from a high dose of cocaine (10 mg/kg). Values represent the
percentage of animals (n 5 7/dose) selecting the high-dose lever. (A, left
panel) rats were treated with saline 60 min (10 ml/kg s.c.) and 30 min (10
ml/kg i.p.) presession and cocaine (2.5–10 mg/kg i.p.) 15 min presession;
(right panel) rats were treated with prazosin (0.16 –10 mg/kg s.c.) 60 min
presession, saline 30 min presession and 10 mg/kg i.p. cocaine 15 min
presession. (B, left panels) rats were treated with saline (10 ml/kg s.c.) 60
min presession; desipramine (0.63–10 mg/kg i.p.) or propranolol (0.63–10
mg/kg i.p.) 30 min presession; and cocaine (2.5 mg/kg i.p.) 15 min presession. (right panels) Rats were treated with prazosin (0.16 –10 mg/kg s.c.)
60 min presession; desipramine or propranolol (10 mg/kg i.p.) 30 min
presession and cocaine (2.5 mg/kg i.p.) 15 min presession.
1998
Dose-Dose Cocaine Discrimination
HDL selection when combined with the low dose of cocaine
(ED50, 0.27 mg/kg; 95% confidence limits, 0.074 – 0.98 mg/kg).
Similar to the HDL selection observed with other cocaineenhancing compounds, the highest doses of caffeine and SKF
81297 did not engender HDL selection when administered in
combination with saline. In contrast to compounds that enhanced the DS effects of the low training dose, the weak DA
reuptake blocker/muscarinic antagonist benztropine, the
benzodiazepine diazepam and the 5-HT1A agonist 8-OHDPAT did not engender HDL selection in more than 25% of
animals treated. All these latter compounds were tested at
doses that decreased the rate of responding to less than 50%
of that observed under low-training-dose sessions.
Discussion
In this study, the discriminative stimulus effects of a relatively low dose of cocaine (2.5 mg/kg) were enhanced dosedependently by pretreatment with compounds having varying in vitro potencies for blocking reuptake of DA, NE and/or
5-HT. Multiple regression analysis of the relationship between behavioral and in vitro potencies indicated that DA
and 5-HT reuptake-blocking properties explain the cocaineenhancing effects of most of the compounds we examined in
this study. In contrast, evidence for the involvement of NE
reuptake-blocking properties was not as convincing, despite
the paradoxical finding that relatively selective NE reuptake
inhibitors (nortriptyline, talsupram and desipramine) effectively enhanced the DS effects of cocaine. However, pretreatment with the alpha-1 adrenergic antagonist prazosin did
not decrease completely the cocaine-enhancing effects of desipramine, which suggested that NE reuptake blockade does
not play a unique role in its ability to enhance the DS effects
of cocaine. The pharmacological specificity of the observed
enhancement of the DS effects of cocaine was demonstrated
by findings that pretreatment with a variety of compounds
from other pharmacological classes (caffeine, benztropine,
8-OH-DPAT and diazepam) did not produce effects similar to
TABLE 3
Results of tests of compounds in combination with saline or the low training dose of cocaine (2.5 mg/kg i.p.) in rats trained to discriminate a low
dose from a high dose (10 mg/kg) of cocaine
Drug 1 Cocainea
Dose
N
Rats Selecting
HDL/Rats
Responding
% HDL Selection
% Low-Dose
Response Rate
(mean 6 S.E.M.)
mg/kg
Benztropine
2.5
10
5
5
1/5
0/0
20
–b
29 6 5.5
060
Caffeine
2.5
10
40
40
5
5
5
5
1/5
3/5
2/5
0/5
20
60
40
0
85 6 14
80 6 24
42 6 7.6
47 6 12
7
7
7
7
1/7
2/6
5/7
1/7
14
33
71
14
84 6 15
64 6 16
45 6 10
80 6 10
5
5
1/5
2/2
20
–
74 6 10
5.6 6 3.5
5
5
1/4
1/4
25
25
26 6 14
19 6 7.9
(caffeine 1 saline)
SKF 81297
(SKF 81297 1 saline)
Diazepam
8-OH-DPAT
a
b
0.04
0.16
0.63
0.63
2.5
10
0.16
0.63
Parentheses refer to data obtained when saline was administered instead of cocaine (2.5 mg/kg).
Percentage HDL not calculated when less than 50% of animals selected a lever.
Downloaded from jpet.aspetjournals.org at ASPET Journals on April 11, 2017
ministered 15 min before the low training dose of cocaine,
engendered dose-related increases in HDL selection, with
maximal effects reaching 80 to 100%. Inasmuch as the 95%
confidence limits overlapped, the potencies of cocaine (ED50,
4.1 mg/kg; 95% confidence limits, 2.5– 6.7) and desipramine
(ED50, 1.7 mg/kg; 95% confidence limits, 0.50 –5.9 mg/kg) did
not differ significantly from those observed under slightly
different treatment conditions (i.e., single or double injections), although the ED50 value for desipramine was apparently higher than that observed previously (0.60 mg/kg; table
2). The cocaine-enhancing potency of propranolol (ED50, 3.5
mg/kg; 95% confidence limits, 1.3–9.0 mg/kg) was similar to
that reported recently (Kleven and Koek, 1997).
Pretreatment with the alpha-1 adrenergic antagonist prazosin (0.16 –2.5 mg/kg s.c.) 60 min before the session did not
decrease HDL selection engendered by either cocaine (10
mg/kg i.p) or the combination of desipramine (10 mg/kg i.p.,
30-min presession) and the low training dose of cocaine (2.5
mg/kg i.p., 15-min presession) in more than 50% of the animals treated (fig. 4, right panels). Doses of prazosin higher
that 10 mg/kg were not examined because of the high response-rate decreasing effects which were already evident in
rats treated with this dose in combination with either cocaine
10 mg/kg (15 6 4.8% of low-training-dose control sessions) or
desipramine 10 mg/kg (6.6 6 3.2%). In contrast to results
obtained in combination with cocaine-enhancing doses of cocaine or desipramine, pretreatment with prazosin engendered dose-related decreases in the cocaine-enhancing effects
of propranolol 10 mg/kg (ED50, 0.81 mg/kg; 95% confidence
limits, 0.31–2.1 mg/kg).
Interactions with other compounds. In addition to
monoamine reuptake blockers, compounds from a variety of
different pharmacological classes were tested in combination
with the low dose of cocaine (table 3). The adenosine antagonist caffeine produced intermediate levels of HDL selection
(i.e., maximal HDL selection of 60%) when administered in
combination with the low dose of cocaine. The D1 dopamine
agonist SKF 81297 engendered dose-related increases in
1021
1022
Kleven and Koek
session length and cumulative dosing procedure may be responsible. Irrespective of these differences, it is clear not only
from our study, but also from previous studies in rats (Cunningham and Callahan, 1991; Callahan and Cunningham,
1997) that 5-HT reuptake inhibition effectively enhances the
DS effects of cocaine in this species.
Because cocaine can be considered a strong inhibitor of
5-HT reuptake (Koe, 1976; Hyttel, 1982), it is conceivable
that this monoamine could be involved in its DS effects. In
favor of this idea is the recent finding that systemic injection
of cocaine (10 mg/kg i.p.) significantly increased extracellular
levels of 5-HT in the nucleus accumbens, an effect that also
can be mimicked by local perfusion of cocaine in the nucleus
accumbens (Teneud et al., 1996). But, 5-HT receptor antagonists do not block the DS effects of cocaine (Meert and
Janssen, 1992; Peltier et al., 1994), and moreover, the selective 5-HT reuptake blockers examined in this study were
generally less potent than cocaine, despite their considerably
higher in vitro potencies. In this context, the in vivo 5-HT
reuptake-blocking properties of cocaine may contribute little
to its DS effects. Even so, it is clear that combined administration of cocaine and 5-HT reuptake blockers influences its
behavioral effects (Walsh and Cunningham, 1997), which
suggests that 5-HT could play a modulatory role.
The model that accounts for the cocaine-enhancing effects
of the majority of the compounds examined in this study fails
to predict accurately the potencies of GBR 12935, indatraline, nisoxetine and desipramine. With respect to indatraline
and members of the 1,4-dialkylpiperazine series of DA reuptake inhibitors (e.g. GBR 12783 and GBR 12909), previous
studies indicate that their in vivo potencies are not consistent
with their ability to block reuptake of DA in vitro. GBR 12935
is less potent behaviorally than cocaine, whereas indatraline
is only slightly more potent than cocaine; however, both of
these compounds are more than 100 times more potent than
cocaine in blocking DA reuptake. For indatraline, pharmacokinetic factors may be largely responsible for this dissociation: it has a slow onset of action and its effects can last for
several days (Rosenzweig-Lipson et al., 1992). But, the unexpectedly low in vivo potency of GBR-related compounds
(Heikkila and Manzino, 1984) remains enigmatic, although it
has been suggested (Rosenzweig-Lipson et al., 1992) that
dispositional factors might explain the low in vivo potency of
GBR 12909. Additionally, it has been proposed that multiple
binding sites or different binding kinetics (Matecka et al.,
1996) can explain the unexpectedly low in vivo potency of
related GBR-related compounds.
The results obtained with nisoxetine and desipramine are
more problematic; however, desipramine was somewhat less
potent in the replication experiment when saline was administered subcutaneously 30 min before desipramine (fig. 4B).
Moreover, as with indatraline, it may be unreasonable to
expect that all the compounds examined in this study should
exhibit their peak effects 30 min after administration. In
contrast to the effects of nisoxetine and desipramine, the
selective reuptake inhibitors talsupram and nortriptyline did
enhance the DS effects of cocaine, although their in vivo
potencies are closely predicted by a model which does not rely
on in vitro NE reuptake-blocking potency. Note that talsupram and nortriptyline could be considered “moderate” inhibitors of 5-HT reuptake (table 1) according to the classification scheme of Koe (1976). Moreover, findings that
Downloaded from jpet.aspetjournals.org at ASPET Journals on April 11, 2017
those found after coadministration of monoamine reuptake
blockers. Altogether these findings establish the pharmacological basis for reported interactions between cocaine and
monoamine reuptake blockers (Cunningham and Callahan,
1991; Spealman, 1993; Callahan and Cunningham, 1995a)
and confirm the idea that 5-HT may be an important modulator of behavioral effects of cocaine in the rat.
The pharmacological specificity of the enhancement of the
DS effects of cocaine is supported by the finding that a
variety of compounds from different pharmacological classes
do not engender HDL selection when combined with the low
dose of cocaine. Compounds that do not have cocaine-like DS
properties [the 5-HT1A agonist 8-OH-DPAT (Callahan and
Cunningham, 1995b), the benzodiazepine agonist diazepam
(Emmett-Oglesby et al., 1983) and the weak DA reuptake
blocker/muscarinic antagonist benztropine (Acri et al., 1996)]
did not produce more than 25% HDL selection when combined with the low training dose of cocaine. Further, we
reported recently (Kleven and Koek, 1997) that neither the
alpha-1 adrenergic agonist cirazoline nor the alpha-2 adrenergic ligands (6)-efaroxan and UK-14304 enhanced the DS
effects of a low dose of cocaine. In contrast to compounds that
were completely ineffective, several drugs from other classes
produced a limited enhancement of the DS effects of cocaine.
For example, caffeine engendered HDL selection along an
inverted U-shaped dose-response function, but it also enhanced the DS effects of cocaine in cocaine-saline-trained
rats (Gauvin et al., 1989,1990; Harland et al., 1989). Similarly, the D1 dopamine agonist SKF 81297 engendered intermediate levels of drug-appropriate responding in cocainesaline-trained rats (Witkin et al., 1991) and, in the present
study, produced dose-related increases in HDL selection
when combined with cocaine. This latter finding agrees with
recently reported results wherein intermediate efficacy D1
dopamine agonists, including SKF 81297, enhanced the DS
effects of cocaine in monkeys (Spealman et al., 1997). Because compounds from a variety of different pharmacological
classes do not engender HDL-appropriate responding, the
results suggested that specific pharmacological mechanisms
are involved in the enhancement of the DS effects of cocaine
by monoamine reuptake blockers.
In addition to evidence supporting the involvement of DA
reuptake blockade in the DS of cocaine (Broadbent et al.,
1991; Baker et al., 1993), the multiple regression analysis
indicated that 5-HT reuptake blockade properties contribute
significantly to the ability of monoamine reuptake blockers to
enhance the DS effects of cocaine. These findings agree with
previous results demonstrating that fluoxetine and citalopram enhanced the DS effects of cocaine in cocaine-salinetrained rats (Cunningham and Callahan, 1991; Callahan and
Cunningham, 1995a) and strongly suggest that in vivo blockade of 5-HT reuptake is responsible for this phenomenon. As
noted above, however, citalopram did not enhance the DS
effects of cocaine in squirrel monkeys. Although it is conceivable that species differences account for this, squirrel monkeys are sensitive to cocaine-enhancing effects of fluoxetine
when a lower training dose is used (Schama et al., 1997).
Therefore, with respect to drug discrimination studies, it is
possible that differences in effective training doses may explain the discrepancies between data obtained in rats and
squirrel monkeys. Alternatively, methodological differences
such as route of administration, differences in onset of action,
Vol. 284
1998
1023
Although the effects of selective NE compounds, with the
exception of beta adrenergic antagonists (Kleven and Koek,
1997) may not be as robust as DA and/or 5-HT compounds,
the finding that the alpha-1 adrenergic antagonist prazosin
reverses the cocaine-enhancing effects of the beta adrenergic
antagonist (2)-propranolol, suggests that NE mechanisms
may indeed modulate the DS effects of cocaine (Spealman,
1995). That beta adrenergic receptors play a role in the DS
effects of cocaine is supported not only by previous findings
that propranolol substitutes in rats trained to discriminate
cocaine from saline (Colpaert et al., 1979), but also by recent
studies showing that propranolol augments unconditioned
and conditioned behavioral effects of cocaine and cocaineinduced increases in extracellular dopamine in the nucleus
accumbens (Harris et al., 1996).
It has been hypothesized that the ability of NE compounds
to modulate or mimic the DS effects of cocaine is mediated by
stimulation of postsynaptic alpha-1 adrenergic receptors in
the VTA (Spealman, 1995). However, several findings are
inconsistent with this hypothesis: 1) the presynaptic alpha-2
adrenergic antagonist efaroxan, which via autoreceptor inhibition (Grenhoff and Svensson, 1989) should augment
postsynaptic NE tone effectively, does not enhance the DS
effects of cocaine, either in squirrel monkeys (Spealman,
1995) or rats (Kleven and Koek, 1997); 2) the alpha-1 adrenergic agonist cirazoline did not enhance the effects of the low
dose of cocaine (Kleven and Koek, 1997); and 3) neurochemical lesioning of noradrenergic fibers innervating the VTA
decreases DA utilization in the prefrontal cortex, but not in
the nucleus accumbens (Herve et al., 1982). Because the DS
effects of cocaine are mediated predominantly by the nucleus
accumbens (Wood and Emmett-Oglesby, 1989; Callahan et
al., 1994), it is likely that noradrenergic control of DA neurons projecting from the VTA to the nucleus accumbens cannot explain interactions between noradrenergic compounds
and the DS effects of cocaine. However, 6-hydroxydopamine
lesions of the locus ceruleus reportedly decrease DA levels in
the nucleus accumbens (Lategan et al., 1990), thus other
pathways or mechanisms may be responsible for the interactions between, for example, beta adrenergic agents and cocaine. Nonetheless, the many inconsistencies indicate that
further studies are needed to explain this phenomenon.
In conclusion, in this study 5-HT and DA reuptake-blocking properties explain the ability of monoamine reuptake
blockers to enhance the DS effects of cocaine. Because complete drug-appropriate responding generally is not obtained
after administration of direct serotonergic agonists in cocaine-saline-trained rats (Callahan and Cunningham, 1995b)
and 5-HT antagonists do not block cocaine-appropriate responding (Peltier et al., 1994; Callahan and Cunningham,
1995b), 5-HT is neither necessary nor sufficient to evoke
cocaine-like DS effects. However, 5-HT reuptake blockers
reliably enhance the DS effects of cocaine when combined
with cocaine. That 5-HT reuptake blockade plays a modulatory role in the DS effects of cocaine agrees with previous
studies (see Cunningham et al., 1995; Walsh and Cunningham, 1997). In contrast to interactions between DA and
5-HT, the involvement of NE reuptake inhibition in the DS
effects of cocaine may be relatively limited. A neuronal mechanism for the interaction between cocaine and catecholaminergic drugs remains to be elucidated, although several independent groups have reported that NE reuptake blockers
Downloaded from jpet.aspetjournals.org at ASPET Journals on April 11, 2017
prazosin did not inhibit completely the cocaine-enhancing
effects of desipramine support the idea that NE reuptake
may not be the only mechanism involved in this phenomenon. Because the model that explains the cocaine-enhancing
effects of most of the compounds examined in this study,
including many that have strong NE reuptake-blocking properties, does not contain a significant NE component, it is
likely that NE reuptake blockade plays a relatively weak role
in modulating the DS effects of cocaine.
Although the absence of cocaine-enhancing effects of some
compounds, such as the NE reuptake blocker nisoxetine
(Hyttel, 1982), may be related to differences in onset of action
or peak effects, the finding that prazosin did not fully antagonize the cocaine-enhancing effects of desipramine suggests
that factors other than NE reuptake blockade play a role.
Whereas these results are consistent with the model, in that
the NE component failed to reach significance, they contradict the idea that alpha-1 adrenergic stimulation plays a
significant role in the stimulant effects of cocaine (Snoddy
and Tessel, 1985; Tessel and Barrett, 1986). In further contradiction, prazosin failed to antagonize the ability of cocaine
(10 mg/kg) to engender HDL selection. However, findings
that prazosin reverses the behavioral effects of psychomotor
stimulants have almost invariably been reported in species
other than the rat (Snoddy and Tessel, 1985; Tessel and
Barrett, 1986; Johanson and Barrett, 1993; Spealman, 1995),
again raising the possibility that species or methodological
differences are important factors in this phenomenon. In
contrast, in the present study, neither the regression model
nor the findings obtained with prazosin support the idea that
NE reuptake blockade is solely responsible for the cocaineenhancing effects of compounds such as desipramine.
Thus, mechanisms other than NE reuptake blockade may
also account for the cocaine-enhancing effects of nortriptyline, desipramine and talsupram in the rat. One such possible mechanism may be that monoamine reuptake blockers
increase brain levels of cocaine (Tella and Goldberg, 1993).
This effect is reportedly short-lived, but it could contribute to
the enhancement of the DS effects of cocaine. A more likely
explanation for the discrepancy between in vitro and in vivo
potencies of NE reuptake inhibitors is that they enhance
cocaine by blocking reuptake of DA by noradrenergic neurons
(Kelly et al., 1985; Izenwasser et al., 1990). This has been
demonstrated in microdialysis studies in prefrontal cortex
(Carboni et al., 1990) and the VTA and nucleus accumbens
(Chen and Reith, 1997) areas that receive noradrenergic
neurons from the locus ceruleus. However, the density of NE
uptake sites in the nucleus accumbens, the region that is
most highly implicated in the DS effects of cocaine, is quite
low relative to DA uptake sites (Li et al., 1996), which suggests that this mechanism may be less important than in
other regions. Nonetheless, the association between nucleus
accumbens DA and NE output after administration of various monoamine reuptake blockers was significantly correlated, although the relationship was reportedly weaker than
in the VTA (Chen and Reith, 1994, 1997). Indeed, desipramine showed the weakest relationship between DA and NE
output (Chen and Reith, 1997), consistent with the report
that acute administration of a dose of desipramine similar to
that used in this study (5 mg/kg i.p.) did not alter extracellular levels of DA in the nucleus accumbens (Nomikos et al.,
1991).
Dose-Dose Cocaine Discrimination
1024
Kleven and Koek
enhance cocaine (Cunningham and Callahan, 1991; Spealman, 1995) and there are clearly other conditions (i.e., species and training dose) where NE has been demonstrated to
play a role in the DS effects of cocaine. The generality or
mechanism of these findings remains to be established,
whereas 5-HT may play a modulatory role in the effects of
cocaine under a larger variety of conditions.
Acknowledgments
The authors thank C. Grevoz-Barret and Y. Cros for technical
assistance and J. Besnard for data management. We also thank J.-L.
Maurel for supplying fluoxetine and paroxetine and Lundbeck A/S
for their generous donation of citalopram and talsupram.
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