Psychopharmacology (2001) 154:213–218 DOI 10.1007/s002130100895

R A P I D C O M M U N I C AT I O N

Anne Dekeyne · Alain Gobert · Loretta Iob Laetitia Cistarelli · Christophe Melon Mark J. Millan

Discriminative stimulus properties of the selective norepinephrine reuptake inhibitor, reboxetine, in rats Received: 15 March 2001 / Accepted: 21 July 2001 / Published online: 11 September 2001 © Springer-Verlag 2001

Abstract Rationale: Although drug discrimination procedures have proven difficult to apply to antidepressant agents, we recently characterized discriminative stimulus properties of the selective serotonin (5-HT) reuptake inhibitor, citalopram, in rats. However, discriminative stimulus properties of selective norepinephrine (NE) reuptake inhibitors remain to be evaluated. Objective: We determined the potential discriminative stimulus properties of the highly selective NE reuptake inhibitor and antidepressant, reboxetine. Methods: Employing a twolever discrimination procedure, rats were trained to discriminate reboxetine (2.5 mg/kg, IP) from saline. In parallel, the influence of reboxetine (2.5 mg/kg) upon dialysate levels of monoamines in frontal cortex and dorsal hippocampus of freely moving rats was determined. Results: After 54±10 training sessions, reboxetine elicited robust stimulus recognition, fully generalizing to itself with an ED50 of 1.2 mg/kg. Two further NE reuptake inhibitors, desipramine (5.3) and maprotiline (1.8), as well as the 5-HT/NE reuptake inhibitor, venlafaxine (1.0), likewise generalized. In contrast, the 5-HT reuptake inhibitors, paroxetine, citalopram and sertraline, and the DA reuptake inhibitors, GBR12935 and bupropion, did not show significant generalization. Reboxetine markedly increased dialysate levels of NE, but not 5-HT, in frontal cortex and hippocampus. Dopamine (DA) levels were also (though less markedly) enhanced in frontal cortex. Conclusion: In parallel with an elevation in extracellular levels of NE, the selective NE reuptake inhibitor, reboxetine, elicits a specific discriminative stimulus in rats.

A. Dekeyne (✉) · A. Gobert · L. Iob · L. Cistarelli · C. Melon M.J. Millan Institut de Recherches Servier, Centre de Recherches de Croissy, Psychopharmacology Department, 125 Chemin de Ronde, 78290 Croissy-sur-Seine, Paris, France e-mail: [email protected] Tel.: +33-1-55722438, Fax: +33-1-55722470

Keywords Drug discrimination · Dialysis · Norepinephrine transporter · Norepinephrine reuptake inhibitor · Depression · Antidepressant

Introduction While drug discrimination procedures have proven invaluable in the characterization of anxiolytics and antipsychotics, comparatively little information is available for antidepressant agents (Goudie and Leathley 1993). Generalized toxic properties of tricyclics, and their perturbation of behaviour, severely compromise their utilization (Schechter 1983; Zhang and Barrett 1991), while discriminative stimulus (DS) properties of the “atypical” antidepressant, mianserin, reflect blockade of muscarinic receptors, an action related to side-effects rather than therapeutic activity (Kelley et al. 1995). Nevertheless, it was recently demonstrated that the selective serotonin (5-HT) reuptake inhibitor (SSRI), citalopram, elicits a DS in rats (Marona-Lewicka and Nichols 1998; Millan et al. 1999a, 1999b). Notably, other SSRIs, such as paroxetine and sertraline, as well as the mixed 5-HT/ norepinephrine (NE) reuptake inhibitor, venlafaxine (Schweizer et al. 1997), potently and fully generalize to citalopram (Millan et al. 1999b). Like SSRIs, drugs preferentially interfering with neuronal reuptake of NE versus 5-HT and dopamine (DA), such as desipramine, possess antidepressant properties (Burke and Preskorn 1995; Owens et al. 1997), and it would similarly appear of importance to characterize their DS properties. However, data in this respect are limited. Although desipramine generated a DS in rats, general toxicity (training could only be completed for approximately half the population commenced) precluded further characterization (Shearman et al. 1978). Further, desipramine interacts with many sites other than NE transporters (Owens et al. 1997). With regard to a further NE reuptake inhibitor, nisoxetine, data are similarly limited inasmuch as cross-generalization was demonstrated with amphetamine but no generalization

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studies were performed with additional NE reuptake inhibitors or with other antidepressant agents: moreover, this work was undertaken in mice (Snoddy and Tessel 1983, 1985). Finally, although it was shown that the tricyclic, NE reuptake inhibitor, (+)-oxaprotiline, generates a DS in rats, this drug, like desipramine, interacts with receptors other than NE reuptake sites and apart from generalization of desipramine and maprotiline, no further data have been reported (Filip et al. 1993). Recently, it was shown that the novel morpholine derivative and antidepressant, reboxetine, behaves as a highly selective and potent inhibitor of NE reuptake both in vitro and in vivo (Riva et al. 1989; Sacchetti et al. 1999; Versiani et al. 2000; Wong et al. 2000; Millan et al. 2001a, 2001b). It appears thus to be an ideal agent for a thorough characterization of DS properties associated with selective inhibition of NE reuptake. The purpose of the present studies was as follows: first, to determine whether reboxetine generates a DS in rats; second, to evaluate whether other NE reuptake inhibitors generalize to the reboxetine DS, as compared to drugs inhibiting 5-HT or DA reuptake; and third, in order to confirm the specificity of the reboxetine DS, to examine the actions of other classes of psychotropic agent: the neuroleptic, haloperidol, and the anxiolytic, diazepam. Finally, in an independent study, the influence of reboxetine upon extracellular levels of NE, 5-HT and DA in the frontal cortex (FCX) and dorsal hippocampus was evaluated. These structures were selected in view of their major implication in the actions of antidepressant agents and since, employing a highly sensitive coulometric detection technique, we have previously demonstrated that it is possible simultaneously to determine levels of 5-HT, NE and (for the FCX) DA in single dialysate samples of these structures (Gobert et al. 1998; Millan et al. 1999b, 2000, 2001b).

Materials and methods

forced with food according to a fixed ratio 10 schedule of reinforcement. Each 15-min daily session (5 days/week) started 15 min after injection. During reboxetine sessions, responses on only one lever were reinforced while, during saline sessions, responses on the other lever were reinforced. “Reboxetine” (R) or “saline” (S) sessions alternated as follows: RSSRS-SRRSSSRSRR-RSRSR-, etc. Correct responding was defined as no more than 13 presses on both levers to obtain the first reinforcement. The discrimination criterion was ten consecutive sessions with correct responding. Thereafter, generalization tests were conducted every Wednesday and Friday, whereas training sessions continued on the other days. Rats were tested only if they showed correct responding on the two preceding training sessions. During testing, responding on the selected lever, i.e. the lever for which ten (not necessarily consecutive) responses were recorded first, was reinforced for the remainder of the 15-min session. Test drugs were administered instead of reboxetine, 15 min before the session.

Dialysis studies The influence of reboxetine upon levels of NE versus 5-HT and DA in single dialysate samples of the FCX, and upon levels of NE versus 5-HT in the hippocampus (DA was undetectable in this structure), was determined as detailed previously (Millan et al. 1999b), employing HPLC plus coulometric detection in freelymoving rats implanted 1 week prior to testing with a guide cannula. Samples were taken every 20 min. Basal DA, NE and 5-HT levels were monitored over 1 h, then reboxetine (2.5 mg/kg, IP) injected, and samples taken for a further 3 h. Changes were expressed relative to basal values. Drugs Drug doses are in terms of the base. Drug discrimination training was undertaken by the IP route in order to avoid potential cutaneous toxicity. As in previous studies in this laboratory, test drugs were dissolved in sterile water and administered SC (to avoid hepatic first-pass metabolism), except maprotiline which (since it is poorly tolerated SC) was given in a suspension IP. Drug sources, salts and structures were as follows: bupropion HCl (Burroughs Wellcome Co., N.C., USA); GBR12935 diHCl (Research Biochemicals International, Natick, Mass., USA); desipramine HCl, haloperidol and maprotiline HCl (Sigma, Chesnes, France); diazepam (HoffmanLa Roche, Basel, Switzerland) and paroxetine HCl (Beecham Pharmaceuticals, Brentford, UK). Citalopram HBr, reboxetine methane sulfonate, sertraline HCl and venlafaxine HCl were synthesized by Servier chemists (G. Lavielle and J.-L. Péglion).

Animals Male Wistar rats (180–200 g body weight upon arrival for drug discrimination and 200–220 g for dialysis studies; Iffa-Credo, l’Arbresle, France) were housed individually in sawdust-lined standard polycarbonate cages with free access to water and, for drug discrimination studies, with restricted access to chow (10–11 g per day) in order to maintain their weight at 80% of freefeeding values. They were kept under a 12-h/12-h light-dark cycle with lights on at 0700 hours. Laboratory temperature was 21±1.0°C and humidity, 60±5%. All animal use procedures conformed to international European ethical standards (86/609-CEE) and the French National Committee (décret 87/848) for the care and use of laboratory animals. Drug discrimination procedure As described previously (Millan et al. 1999b), rats were trained to discriminate reboxetine (2.5 mg/kg, IP) from saline in operant conditioning chambers equipped with two levers. They were rein-

Data analysis In the drug discrimination study, to evaluate the influence of reboxetine upon training, the mean total number of responses (i.e. presses on both levers)±SEM was calculated for the five last “reboxetine” versus “saline” sessions before the first test session. Data recorded during a test session were lever selection and the total number of responses. Lever selection data were expressed as the percentage of rats selecting the drug lever and were compared by a Fisher Exact Probability Test to control values of the preceding saline training session (0% drug lever selection). Effective dose50s (ED50s) plus 95% confidence limits (95% CL) were calculated to estimate drug potency. For description of drug actions, “full” generalization corresponds to 80% or greater “reboxetine” lever selection. The total numbers of responses in the presence of drug were compared by a paired t-test to that obtained during the preceding saline training session. In the dialysis studies, data were analyzed by ANOVA with sampling time as the repeated withinsubject factor.

215 Table 1 Actions of drugs which failed to show significant generalization to reboxetine. SSRI selective serotonin reuptake inhibitors; DARI dopamine reuptake inhibitor; BZD benzodiazepine; n number of rats selecting a lever; N number of tested rats. The total numbers of responses were not significantly modified in any case (P>0.05; paired t-test), as compared to the preceding control (saline) session

Drug

Clas

Citalopram

SSRI

Paroxetine

SSRI

Sertraline

SSRI

GBR12935

DARI

Bupropion

DARI

Diazepam

BZD

Haloperidol

Neuroleptic

Dose (mg/kg, SC) 0.63 2.5 0.16 0.63 0.16 0.63 0.63 10.0 2.5 10.0 0.16 0.63 0.01 0.04 0.16

% Lever selection 40 33 40 0 40 20 33 20 20 20 0 20 0 0 –

Total number of Responses ± SEMs Control session

Test session

1987±246 1626±207 1872±213 1698±179 1712±149 2035±170 1637±170 1786±206 1861±203 1954±249 1725±135 1713±167 1812±177 1788±104 1857±105

1631±118 1656±175 1886±214 1681±165 1877±150 1960±182 1737±127 1824±226 1944±166 2078±125 1828±84 870±264 1671±184 1262±311 –

n/N

5/5 6/6 5/5 5/5 5/5 5/5 6/6 5/5 5/5 5/5 5/5 5/5 5/5 5/5 0/3

Fig. 1 Percent correct responding during acquisition of a discriminative stimulus with reboxetine (2.5 mg/kg, IP)

Results As shown in Fig. 1, acquisition of a DS with reboxetine (2.5 mg/kg, IP) was successful. The mean±SEM number of training sessions required to achieve the discrimination criterion was 54±10 (n=9). The total number of responses during training was slightly but significantly lower (P<0.05 in paired t-test) for reboxetine (1214±141; corresponding to a maximum number of 135 pellets/session) as compared to saline (1741±127; 187 pellets/ session). Reboxetine dose-dependently and fully (100%) generalized to itself with an ED50 (95% CL) of 1.2 (1.0–1.5) mg/kg, and slightly but significantly decreased the total number of responses at all doses (Fig. 2). Desipramine and maprotiline also dose-dependently generalized with ED50s (95% CL) of 5.3 (3.2–0.5) mg/kg and 1.8 (0.7–4.3) mg/kg, respectively, and, at the highest doses tested, they significantly decreased the total number of responses (Fig. 2). Venlafaxine generalized with an ED50 (95% CL) of 1.0 (0.3–2.7) mg/kg, without significantly influencing the total number of responses (Fig. 2). In contrast, citalopram, paroxetine, sertraline, GBR12935 and bupropion did not show significant generalization (P>0.05 in Fisher Exact Probability test), and they did

Fig. 2 Dose-dependent generalization of reboxetine, desipramine, maprotiline and venlafaxine to the discriminative stimulus elicited by reboxetine. Upper panels: percentage of rats selecting the “reboxetine” lever and lower panel: means±SEMs of total number of responses. n=5–9 per value. In the upper panel, asterisks indicate significance of differences to control values (0% “reboxetine” lever selection) in Fisher’s Exact Probability test. *P<0.05. In the lower panel, asterisks indicate significant decreases in the total number of responses (*P<0.05; paired t-test) as compared to the most recent saline-training session

not significantly affect the total number of responses (Table 1). Diazepam and haloperidol similarly did not generalize to reboxetine (P>0.05), even at doses inducing a (non-significant) decrease in the total number of responses (Table 1). For haloperidol, at a higher dose of 0.16, all animals failed to respond (Table 1).

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Fig. 3 Influence of reboxetine (2.5 mg/kg, IP) upon dialysate levels of NE, 5-HT and DA in the frontal cortex (upper panel) and upon dialysate levels of NE and 5-HT in the dorsal hippocampus (lower panel). Data are means±SEMs of levels of NE, 5-HT and DA expressed as a percentage of basal, pre-injection values (=100%). n=5 per value. Asterisks indicate significance of the difference between the drug-treated groups (closed symbols) and the vehicle-treated groups (open symbols). *P<0.05

In the dialysis study, baseline levels were as follows: frontal cortex, NE, 1.79±0.10; 5-HT, 0.84±0.09 and DA, 0.96±0.06 and dorsal hippocampus, NE, 0.52±0.03 and 5-HT, 0.70±0.06 pg/20 µl dialysate. Reboxetine (2.5 mg/kg, IP) elicited a marked and sustained increase in extracellular levels of NE, but not 5-HT, in FCX. Levels of DA were also elevated, though less markedly (Fig. 3). ANOVA (20–180 min) were as follows: NE, F(1,10)=46.6, P<0.01; 5-HT, F(1,9)=3.1, P>0.05 and DA, F(1,10)=227.7, P<0.01. In dorsal hippocampus, reboxetine (2.5 mg/kg, IP) also selectively elevated levels of NE as compared to those of 5-HT (Fig. 3). ANOVA (20–180 min) were as follows: NE, F(1,11)=36.5, P<0.01 and 5-HT, F(1,11)=0.1, P>0.05.

Discussion After 9–13 weeks of training, reboxetine generated a stable DS to which both reboxetine itself and two further selective NE reuptake inhibitors, desipramine and maprotiline (Owens et al. 1997) fully generalized. These data with the highly selective NE reuptake inhibitor, reboxetine (Riva et al. 1989; Sacchetti et al. 1999; Wong

et al. 2000; Millan et al. 2001b) extend and amplify previous studies (Shearman et al. 1978; Snoddy and Tessel 1983, 1985; Filip et al. 1993) of the less selective NE reuptake inhibitors (+)-oxaprotiline, desipramine and nisoxetine (in mice) in demonstrating that blockade of NE reuptake can sustain a robust DS in rats. Further, underpinning the role of NE reuptake sites in the DS elicited by reboxetine, full and dose-dependent generalization to reboxetine was obtained for not only desipramine but also maprotiline, in analogy to its generalization to (+)-oxaprotiline (Filip et al. 1993). Interestingly, the mixed 5-HT/NE reuptake inhibitor, venlafaxine (Schweizer et al. 1997; Millan et al. 2001a, 2001b), similarly generalized to reboxetine, reflecting its interaction with NE transporters. In contrast, several SSRIs (citalopram, sertraline and paroxetine), at doses which fully generalize to citalopram (Millan et al. 1999b), failed to show significant generalization. The above observations, together with the lack of generalization of the DA reuptake inhibitors, GBR12935 and bupropion, at doses which markedly increase dialysate levels of DA in FCX (Millan et al. 2000), underscore the role of NE reuptake inhibition in the DS elicited by reboxetine. The lack of generalization of the anxiolytic, diazepam, and the antipsychotic, haloperidol, further underpins the role of NE transporters in its DS properties. Citalopram elicits a specific DS in rats at a dose selectively elevating levels of 5-HT versus NE and DA in FCX (Marona-Lewicka and Nichols 1998; Millan et al. 1999b), while GBR12935 and bupropion elicit DS via dopaminergic mechanisms (Melia and Spealman 1991; Terry and Katz 1997; Millan et al. 2000). Further, as shown herein, none of these drugs generalizes to reboxetine. In view of this lack of cross-generalization, their DS effects appear to be distinctive. Indeed, the present lack of generalization of SSRIs to reboxetine indicates contrasting interoceptive properties and is consistent with the suggestion, based on clinical studies, that the influence of NE reuptake inhibitors upon mood differs to those of SSRIs (Dubini et al. 1997; Montgomery 1997). Nevertheless, it should be pointed out that the present study was undertaken with a single training dose of reboxetine and previous studies of psychotropic agents, such as the psychostimulant, cocaine, and the antipsychotic, clozapine (Terry et al. 1994; Goudie et al. 1998), indicate that DS properties of drugs may be dosedependent. Further, it remains unclear to what extent DS properties of drugs in rodents and other species correspond to their subjective properties in humans (Kamien et al. 1993). Thus, extrapolation of the present and previous studies (Millan et al. 1999a, 1999b) to observations concerning subjective properties of antidepressant agents in man should be made cautiously. 5-HT2C receptors mediate DS properties of citalopram in rats (Millan et al. 1999a), while 5-HT1A receptors are implicated in DS properties of a further SSRI, LY233708, in pigeons (Wolff and Leander 1999). On the other hand, D1 and D2 receptors are involved in the DS

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properties of bupropion and GBR12935 (Melia and Spealman 1991; Terry and Katz 1997). In a previous study, it was shown that prazosin antagonizes the DS properties of nisoxetine indicative of the participation of α1-adrenoceptors (Snoddy and Tessel 1985) and it will be of considerable interest to examine the role of specific adrenergic receptors in the DS properties of reboxetine. Such studies are currently being initiated. As concerns the total number of responses, during training and generalization testing, there was a mild but significant reduction relative to saline sessions for reboxetine. Further, at high doses generalizing to reboxetine, the NE reuptake inhibitors, maprotiline and desipramine, as well as the 5-HT/NE reuptake inhibitor, venlafaxine, similarly reduced the total number of responses. In the study of nisoxetine drug discrimination in mice, number of responses were not presented (Snoddy and Tessel 1983, 1985). However, in line with the present data, previous studies of rats with other NE reuptake inhibitors have also reported decreases in the total number of responses (Shearman et al. 1978; Filip et al. 1993). The mechanism underlying the decrease in total number of responses with reboxetine and the other NE reuptake inhibitors is unclear, but in separate studies, we have found that reboxetine, desipramine, nisoxetine and maprotiline significantly reduces spontaneous locomotor activity in both rats and mice (Millan et al. 2001a; Brocco et al. 2001). An alternative explanation for the decrease in the number of responses not evoking motor function may relate to the inhibitory influence of NE reuptake inhibitor upon food intake (Gehlert et al. 1998; Rowland et al. 2000). Interestingly, the decrease in the total number of responses was specific to NE reuptake inhibitors and the 5-HT/NE reuptake inhibitor, venlafaxine, inasmuch as SSRIs and DA reuptake inhibitors all failed to modify the total number of responses. As regards the suppression of responses by diazepam and haloperidol, this observation corresponds to their well-established perturbation of motor behaviour (Treit 1991; Jørgensen et al. 1994). In amplification of a recent report (Sacchetti et al. 1999) of the influence of a high dose of reboxetine (15.0 mg/kg, IP) upon frontocortical levels of NE, at a lower dose (2.5 mg/kg, IP) exploited for the present drug discrimination studies, reboxetine markedly elevated dialysate levels of NE in the FCX of freely-moving rats. Reboxetine likewise enhanced extracellular levels of NE in dorsal hippocampus, a further structure implicated in depressive states (Millan et al. 2000). In the study by Sacchetti et al. (1999), a paradoxical (mild) increase in 5-HT levels in the striatum was observed, whereas, herein, in both FCX and hippocampus, the elevation in NE levels was exerted in the absence of an influence upon levels of 5-HT, consistent with the preferential action of reboxetine at NE versus 5-HT transporters in vitro (Riva et al. 1999; Wong et al. 2000). Although reboxetine elevated FCX levels of DA in parallel with those of NE, as discussed elsewhere (Millan et al. 2000), this effect does not reflect an interaction with DA transporters, but rather

the role of NE transporters in clearance of DA in the FCX. Indeed, in contrast to the FCX, reboxetine does not modify DA levels in the nucleus accumbens and striatum (Sacchetti et al. 1999; Millan et al. 2000). In conclusion, the present observations demonstrate that selective blockade of NE transporters by reboxetine sustain a robust DS in rodents. This model comprises a novel tool for characterization of the interoceptive properties and mechanisms of action of NE reuptake inhibitors, and may provide important insights into their influence upon mood and depressive states.

References Brocco M, Dekeyne A, Veiga S, Girardon S, Millan MJ (2001) Induction of locomotion in mice by inhibition of serotonin reuptake: a pharmacological characterization of diverse classes of antidepressant agent. Pharmacol Biochem Behav (in press) Burke MJ, Preskorn SH (1995) Short-term treatment of mood disorders with standard antidepressants. In: Bloom FE, Kupfer DJ (eds) Psychopharmacology: the fourth generation of progress. Raven Press, New York, pp 1053–1066 Dubini A, Bosc M, Polin V (1997) Do noradrenaline and serotonin differentially affect social motivation and behaviour? Eur Neuropsychopharmacol 7:S49–S55 Filip M, Chojnacka-Wojcik E, Przegalinski E (1993) Discriminative stimulus properties of (+)-oxaprotiline in rats. Pol J Pharmacol 45:151–154 Gehlert DR, Dreshfield L, Tinsley F, Benvega MJ, Gleason S, Fuller RW, Wong DT, Hemrick-Luecke SK (1998) The selective norepinephrine reuptake inhibitor, LY368975, reduces food consumption in animal models of feeding. J Pharmacol Exp Ther 287:122–127 Gobert A, Rivet J-M, Audinot V, Newman-Tancredi A, Cistarelli L, Millan MJ (1998) Simultaneous quantification of serotonin, dopamine and noradrenaline levels in single dialysates of freely moving rats reveals a complex pattern of reciprocal autoand heteroreceptor-mediated control of release. Neuroscience 84:413–429 Goudie AJ, Leathley MJ (1993) Drug discrimination assays. In: Shagal A (ed) Behavioural neuroscience. University Press, Oxford, pp 145–167 Goudie AJ, Taylor MAI, Smith JA (1998) Stimulus properties of clozapine and related agents in rats at two clozapine training doses. J Psychopharmacol 12:269 Jørgensen HA, Andreassen OE, Hole K (1994) The relationship between motor effects in rats following acute and chronic haloperidol treatment. Psychopharmacology 116:89–92 Kamien JB, Bickel WK, Hughes JR, Higgins ST, Smith BJ (1993) Drug discrimination by humans compared to nonhumans: current status and future directions. Psychopharmacology 111:259–270 Kelley BM, Porter JH, Varvel SA (1995) Mianserin as a discriminative stimulus in rats: asymmetrical cross-generalization with scopolamine. Psychopharmacology 120:491–493 Marona-Lewicka D, Nichols DE (1998) Drug discrimination studies of the interoceptive cues produced by selective serotonin uptake inhibitors and selective serotonin releasing agents. Psychopharmacology 138:67–75 Melia KF, Spealman RD (1991) Pharmacological characterization of the discriminative-stimulus effects of GBR 12909. J Pharmacol Exp Ther 258:626–632 Millan MJ, Girardon S, Dekeyne A (1999a) 5-HT2C are involved in the discriminative stimulus effects of citalopram in rats. Psychopharmacology 142:432–434 Millan MJ, Gobert A, Girardon S, Dekeyne A (1999b) Citalopram elicits a discriminative stimulus in rats at a dose selectively increasing extracellular levels of serotonin vs. dopamine and noradrenaline. Eur J Pharmacol 364:147–150

218 Millan MJ, Lejeune F, Gobert A (2000) Reciprocal autoreceptor and heteroreceptor control of serotonergic, dopaminergic and noradrenergic transmission in the frontal cortex: relevance to the actions of antidepressant agents. J Psychopharmacol 14: 114–138 Millan MJ, Dekeyne A, Papp M, Drieu La Rochelle C, MacSweeny C, Peglion J-L, Brocco M (2001a) S33005, a novel ligand at both serotonin and norepinephrine transporters: II. Behavioural profile in comparison to venlafaxine, reboxetine, citalopram and clomipramine. J Pharmacol Exp Ther 298:581–591 Millan MJ, Gobert A, Lejeune F, Newman-Tancredi A, Rivet J-M, Auclair A, Peglion J-L (2001b) S33005, a novel ligand at both serotonin and norepinephrine transporters: I. Receptor binding, electrophysiological and neurochemical profile in comparison to venlafaxine, reboxetine, citalopram and clomipramine. J Pharmacol Exp Ther 298:565-580 Montgomery SA (1997) Reboxetine: additional benefits to the depressed patient. J Psychopharmacol 11:S9–S15 Owens MJ, Morgan WN, Plott SJ, Nemeroff CB (1997) Neurotransmitter receptor and transporter binding profile of antidepressants and their metabolites. J Pharmacol Exp Ther 283: 1305–1322 Riva M, Brunello N, Rovescalli AC, Galimbert R, Carfagna N, Carmineti P, Pozzi O, Ricciardi S, Roncucci R, Rossi A, Racagni G (1989) Effect of reboxetine, a new antidepressant drug, on the central noradrenergic system: behavioural and biochemical studies. J Drug Dev 1:243–253 Rowland NE, Marshall M, Roth JD (2000) Comparison of either norepinephrine-uptake inhibitors or phentermine combined with serotonergic agents on food intake in rats. Psychopharmacology 149:77–83 Sacchetti G, Bernini M, Bianchetti A, Parini S, Invernizzi RW, Samanin R (1999) Studies on the acute and chronic effects of reboxetine on extracellular noradrenaline and other monoamines in the rat brain. Br J Pharmacol 128:1332– 1338

Schechter MD (1983) Discriminative stimulus control with imipramine: transfer to other anti-depressants. Pharmacol Biochem Behav 19:751–754 Schweizer E, Thielen RJ, Frazer A (1997) Venlafaxine, a novel antidepressant compound. Exp Opin Invest Drugs 6:65–78 Shearman G, Miksic S, Lal H (1978) Discriminative stimulus properties of desipramine. Neuropharmacology 17:1045–1048 Snoddy AM, Tessel RE (1983) Nisoxetine and amphetamine share discriminative stimulus properties in mice. Pharmacol Biochem Behav 19:205–210 Snoddy AM, Tessel RE (1985) Prazosin: effect on psychomotorstimulant cues and locomotor activity in mice. Eur J Pharmacol 116:221–228 Terry P, Katz JL (1997) Dopaminergic mediation of the discriminative stimulus effects of bupropion in rats. Psychopharmacology 134:201–212 Terry P, Witkin JM, Katz JL (1994) Pharmacological characterization of the novel discriminative stimulus effects of a low dose of cocaine. J Pharmacol Exp Ther 270:1041–1048 Treit D (1991) Anxiolytic effects of benzodiazepines and 5-HT1A agonists: animal models. In: Rodgers RJ, Cooper SJ (eds) 5-HT1A agonists, 5-HT3 antagonists and benzodiazepines: their comparative behavioural pharmacology. Wiley, New York, pp 107–131 Versiani M, Amin M, Chouinard G (2000) Double-blind, placebocontrolled study with reboxetine in inpatients with severe major depressive disorder. Clin Psychopharmacol 20:28–34 Wolff MC, Leander JD (1999) The discriminative stimulus properties of LY233708, a selective serotonin reuptake inhibitor, in the pigeon. Psychopharmacology 146:275–279 Wong EHF, Sonders MS, Amara SG, Tinholt PM, Piercey MFP, Hoffmann WP, Hyslop DK, Franklin S, Porsolt RD, Bonsignon A, Carfagna N, McArthur RA (2000) Reboxetine: a pharmacologically potent, selective, and specific norepinephrine reuptake inhibitor. Biol Psychiatry 47:818–829 Zhang L, Barrett JE (1991) Imipramine as a discriminative stimulus. J Pharmacol Exp Ther 259:1088–1093