Procholinergic and memory enhancing properties of the selective norepinephrine uptake inhibitor atomoxetine

Abstract

Atomoxetine has been approved by the FDA as the first new drug in 30 years for the treatment of attention deficit/hyperactivity disorder (ADHD). As a selective norepinephrine uptake inhibitor and a nonstimulant, atomoxetine has a different mechanism of action from the stimulant drugs used up to now for the treatment of ADHD. Since brain acetylcholine (ACh) has been associated with memory, attention and motivation, processes dysregulated in ADHD, we investigated the effects of atomoxetine on cholinergic neurotransmission. We showed here that, in rats, atomoxetine (0.3–3 mg/kg, i.p.), – increases in vivo extracellular levels of ACh in cortical but not subcortical brain regions. The marked increase of cortical ACh induced by atomoxetine was dependent upon norepinephrine α-1 and/or dopamine D1 receptor activation. We observed similar increases in cortical and hippocampal ACh release with methylphenidate (1 and 3 mg/kg, i.p.) – currently the most commonly prescribed medication for the treatment of ADHD – and with the norepinephrine uptake inhibitor reboxetine (3–30 mg/kg, i.p.). Since drugs that increase cholinergic neurotransmission are used in the treatment of cognitive dysfunction and dementias, we also investigated the effects of atomoxetine on memory tasks. We showed that, consistent with its cortical procholinergic and catecholamine-enhancing profile, atomoxetine (1–3 mg/kg, p.o.) significantly ameliorated performance in the object recognition test and the radial arm-maze test.

Introduction

The norepinephrine (NE) uptake inhibitor atomoxetine constitutes today the only first-line alternative to the psychostimulant drugs, methylphenidate and D-amphetamine for pharmacotherapy of attention deficit/hyperactivity disorder (ADHD). The psychostimulants are currently classified as schedule II drugs of the Controlled Substance Act and have been used for half a century in the treatment of ADHD, the most common psychiatric disorder in children (3–10% prevalence) that often (60% of the cases) persists into adulthood. ADHD is characterized by attention deficits, hyperactivity and impulsivity, and atomoxetine significantly improves these symptoms in children and adults with ADHD.1, 2

In the central nervous system, cholinergic neurons modulate information flow in cortical and subcortical regions implicated in vigilance and cognition. In particular, sustained and selective attention and working memory depend upon the integral function of cortical and hippocampal cholinergic afferents (reviewed in Everitt and Robbins3). Procholinergic drugs such as cholinesterase inhibitors are used in the therapeutics of Alzheimer's disease and other neurodegenerative diseases with cognitive impairment. Recently, the ability of atypical antipsychotics to reduce negative symptoms and to improve performance in cognitive tasks has been associated with the stimulatory effects of these agents on neocortical and hippocampal acetylcholine (ACh) release.4, 5

Although the effects of atomoxetine on DA and NE efflux in the brain have been reported,6 its effects on cholinergic neurotransmission have not been studied. Therefore, we assessed the ability of the compound to enhance ACh efflux in cortical (medial prefrontal cortex and hippocampus) and subcortical (nucleus accumbens) regions by in vivo microdialysis in rats. We also investigated in this species the procognitive potential of the drug in two animal cognition models, the radial-arm maze and the object-recognition tests. We report increases in cortical ACh efflux engendered by atomoxetine and parallel enhancement in memory performance.

Materials and methods

Animals

All studies were performed according to the guidelines set forth by the National Institutes of Health and implemented by the Animal Care and Use Committee of Eli Lilly and Company. Male Wistar or Sprague–Dawley rats (250–300 g, purchased from Harlan Sprague–Dawley, Indianapolis, IN, USA) were used for experiments.

In vivo microdialysis

Surgical procedures

At 2 weeks prior to the microdialysis experiments, the rats were anesthetized with a mixture of chloral hydrate and pentobarbital (170 and 36 mg kg−1 in 30% propylene glycol and 14% ethanol), placed in a stereotaxic apparatus and implanted with a guide cannula (Bioanalytical Systems, West Lafayette, IN, USA (BAS)) in the hippocampus (coordinates AP: −5.2, ML: 5.2, DV: −3.8), medial prefrontal cortex (AP: 3.2, ML: 0.6, DV: −2.2) or nucleus accumbens (AP: 1.6, ML: 1.2, DV: −6.3) according to Paxinos and Watson.7 At 24 h before testing, a 4 mm (hippocampus, prefrontal cortex) or a 2 mm (nucleus accumbens) concentric microdialysis probe (BAS, model BR-4 or BR-2) was inserted through the guide cannula. The actual location of the probes was verified histologically at the end of the experiment.

ACh measurements

ACh determination in dialysates from the different brain regions was performed as described8 with some modifications.9 On the day of the experiment, a modified Ringer's solution (147.0 mM. NaCl, 3.0 mM KCl, 1.3 mM CaCl2, 1.0 mM MgCl2, 1.0 mM Na2HPO4 × 7H2O, 0.2 mM NaH2PO4 × H2O pH=7.25) supplemented with 0.1 μ M neostigmine was perfused at a rate of 2.4 μl/min in the hippocampus, prefrontal cortex or nucleus accumbens. Samples were collected every 15 min and analyzed immediately, on-line, with HPLC coupled to electrochemical detection, with a 150 × 3 mm ACH-3 column (Environmental Sciences Associates (ESA), Inc., MA, USA) maintained at 35°C. The mobile phase (100 mM di-sodium hydrogen phosphate, 2 mM 1-octanesulfonic acid and 50 μl/l of a microbicide reagent (MB, ESA, Inc.); pH 8.0, adjusted with phosphoric acid) was delivered by an HPLC pump (ESA, Inc.) at 0.4 ml/min. The potentiostat used for electrochemical detection (ESA, Inc.; model Coulochem II) was connected with a solid phase reactor for ACh (ESA, Inc.; model ACH-SPR) and with an analytical cell with platinum target (ESA, Inc.; model 5041).

Atomoxetine, reboxetine and methylphenidate (synthesized at Eli Lilly and Company) were dissolved in saline (0.9% NaCl) and injected i.p. at a volume of 1 ml/kg, at the doses indicated. Prazosin and SCH 23390 (Sigma) (dissolved in saline) were administered s.c. at a volume of 1 ml/kg. The doses of atomoxetine, reboxetine and methylphenidate that were used were based on those previously used in in vivo microdialysis studies and are relevant to clinically used doses.6

Data (n=4–7 rats per group) were expressed as multifold change from baseline, which is the average of the five basal values before any manipulation and were analyzed either with one-way, that is, treatment (between subjects variable), two-way, that is, treatment (between subjects variable) × time (within subjects variable) or three-way (treatment 1 × treatment 2 × time) ANOVA followed by Duncan's test. The effects of each of the drugs are presented both over a course of time every 15 min after the injection of the drug as well as overall average effects during the 3-h observation period after the injection of the drug (index of area under curve).

Object recognition test

Each rat was placed in a clear 25 × 25 cm2 Plexiglas observation box with two identical objects, designated A. The rat was allowed to explore for 2 min and the time interacting with the objects (sniffing, gnawing and behavior oriented to an object) was recorded. Behavior oriented to the object distinguishes between accidental sitting, standing on the object or touching the object when passing by, and active interaction/exploration of the object. After a 3-h delay, the rat was returned to the observation box for the test trial. The test box contained a familiar object (object A) and a novel (object B) object. In the test trial, the objects were placed at the exact same position as in the learning trial. The amount of time spent interacting with each object during the 2-min test was recorded. Atomoxetine was administered orally in 5% acacia over a dose range of 0.3, 1, 3 and 10 mg/kg, 1 h before the first trial. Results are expressed as percent time exploring the novel object during the retention test (i.e., tB × 100/(tA+tB), where tA and tB are the time spent during test trial with familiar object A and novel object B, respectively), and were analyzed with an one-way ANOVA followed by Dunnett's test.

Radial arm-maze

The effects of atomoxetine on memory retention were examined in a delayed nonmatch to sample task conducted in an eight-arm radial maze. Well-trained rats were required to recall, after a 7-h delay period, where they received food during the information phase in order to obtain the remaining rewards during the retention phase conducted after a delay period. The apparatus, the training of the rats as well as the information and retention sessions of the test were as previously described.10 Atomoxetine (in 5% acacia) over a dose range of 1, 3 and 10 mg/kg or vehicle was administered orally immediately after the information phase. During the retention phase, an entry into a nonbaited arm or a re-entry into an arm previously visited during this phase of testing was counted as an error. Significance (P<0.05) was determined using a repeated measure ANOVA followed by Dunnett's test. It should be noted that for the behavioral experiments, the oral route of administration was selected for reasons of consistency with previous studies assessing the cognitive profile of diverse reference compounds.

Locomotor activity measures

The effects of atomoxetine, as compared to the stimulant methylphenidate, on locomotor activity were measured with a 20 station Photobeam Activity Systems (San Diego Instruments, San Diego, CA, USA) with seven photocells per station. Rats were placed in the locomotor activity boxes (40.6 × 20.3 × 15.2 cm3) for a habituation period of 20 min. Immediately after habituation, drugs (atomoxetine 1, 3, 10 mg/kg or methylphenidate 3 mg/kg dissolved in saline) or saline were administered i.p. at a volume of 1 ml/kg. Locomotion was assessed for a 60-min period following the injection. Data (n=8 rats per group) are expressed as total ambulations (where ambulation was defined as the sequential breaking of adjacent photobeams) for the entire 60 min period and were analyzed with one-way ANOVA followed by Duncan's post hoc test.

Results

In vivo microdialysis studies

The effects of a single i.p. administration of atomoxetine (0.3, 1 and 3 mg/kg), reboxetine (1, 3, 10 and 30 mg/kg), and methylphenidate (1 and 3 mg/kg) on ACh efflux were assessed by in vivo microdialysis in the medial prefrontal cortex, the hippocampus and the nucleus accumbens in rats. There were no statistically significant differences in basal ACh values among groups receiving vehicle or any of the drugs in any of the regions tested. Basal ACh values (in pmol/15 min sample) were 1.48±0.12 for the medial prefrontal cortex, 1.29±0.2 for the hippocampus and 0.48±0.02 for the nucleus accumbens.

Effects of atomoxetine on ACh efflux

Atomoxetine (0.3, 1 and 3 mg/kg) dose-dependently increased ACh efflux in the prefrontal cortex and the hippocampus but not in the nucleus accumbens as assessed by in vivo microdialysis (Figure 1). In the medial prefrontal cortex (Figure 1a, b), the atomoxetine-induced increases of extracellular ACh were statistically significant at all doses tested. At the highest dose depicted (3 mg/kg), atomoxetine produced a marked (three-fold) and sustained (2 h) peak increase of cortical ACh. A higher dose of atomoxetine (10 mg/kg) did not produce further increases in cortical ACh efflux indicating that maximal efficacy is achieved with the 3 mg/kg dose (data not shown). Atomoxetine also augmented hippocampal extracellular ACh (Figure 1c, d) in a dose-dependent manner, although this increase was significant only for the 1 and 3 mg/kg doses and much less pronounced as compared to the cortex. In contrast, atomoxetine had no effect on ACh concentrations in the nucleus accumbens even at the highest dose tested (Figure 1c).

Figure 1
figure1

Time course (a, c, e) and overall (b, d, f) effects of atomoxetine on ACh efflux in the prefrontal cortex (a, b), the hippocampus (c, d) and the nucleus accumbens (e, f) of the rat. Atomoxetine dose-dependently increases ACh levels in the prefrontal cortex (F(1,12)=11.33, P<0.001; two-way ANOVA for time course effects at the highest dose and F(3,15)=35.56, P<0.001; one-way ANOVA for overall effects) as well as the hippocampus (F(1,70)=2,18, P<0.05; two-way ANOVA for time course effects at the highest dose and F(3,17)=6.61, P<0.01; one-way ANOVA for overall effects) but not in the nucleus accumbens (F(1,48)=0.51, NS; two-way ANOVA for time course effects and F(2,6)=0.839, NS; one-way ANOVA for overall effects). Data (expressed as fold over baseline established prior to any treatment) represent mean±s.e.m. of n=4–6 animals per group. *P<0.05; **P<0.01; ***P<0.001 as compared to the vehicle-treated group. For time course effects (a, c, e) statistical significance as compared to vehicle for each time point is indicated only for the highest dose of the drug for reasons of clarity of the presentation.

Effects of reboxetine and methylphenidate on ACh efflux

A similar dose-dependent increase of cortical and hippocampal ACh was also observed after administration of either the selective NE uptake inhibitor reboxetine (Figure 2) or methylphenidate (Figure 3). In the medial prefrontal cortex at the highest dose tested (30 mg/kg for reboxetine and 3 mg/kg for methylphenidate), the two drugs increased ACh release by three fold (i.e., with the same peak amplitude as atomoxetine). In the hippocampus, both reboxetine and methylphenidate dose-dependently enhanced ACh release, but as for atomoxetine, increases in the hippocampus were less pronounced than those observed in the cortex. In the nucleus accumbens, the highest dose of methylphenidate but not of reboxetine induced a transient and small, but statistically significant, increase in ACh concentration.

Figure 2
figure2

Time course (a, c, e) and overall (b, d, f) effects of reboxetine on ACh efflux in the prefrontal cortex (a, b), the hippocampus (c, d) and the nucleus accumbens (e, f) of the rat. Reboxetine dose-dependently increases ACh levels in the prefrontal cortex (F(1,12)=2.55, P<0.05; two-way ANOVA for time course effects at the highest dose and F(3,20)=15.61, P<0.001; one-way ANOVA for overall effects) as well as the hippocampus (F(1,70)=2.53, P<0.05; two-way ANOVA for time course effects at the highest dose and F(2,11)=14.02, P<0.001; one-way ANOVA for overall effects) but not in the nucleus accumbens (F(1,48)=1.54, NS; two-way ANOVA for time course effects and F(2,6)=0.086, NS; one-way ANOVA for overall effects). Symbols and figure details as in Figure 1.

Figure 3
figure3

Time course (a, c, e) and overall (b, d, f) effects of methylphenidate on ACh efflux in the prefrontal cortex (a, b), the hippocampus (c, d) and the nucleus accumbens (e, f) of the rat. Methylphenidate dose-dependently increases ACh levels in the prefrontal cortex (F(1,12=8.94, P<0.001; two-way ANOVA for time course effects at the highest dose and F(2,14)=11.09, P<0.01; one-way ANOVA for overall effects) as well as the hippocampus (F(1,70)=6.28, P<0.001; two-way ANOVA for time course effects at the highest dose and F(2,10)=16.05, P<0.001; one-way ANOVA for overall effects) and in the nucleus accumbens (F(1,60)=2.45, P<0.05; two-way ANOVA for time course effects and F(2,7)=5.76, P<0.05; one-way ANOVA for overall effects). Symbols and figure details as in Figure 1.

The procholinergic effects of atomoxetine are dependent upon α1-NE and D1-DA receptor activation

As a selective NE uptake inhibitor, atomoxetine increases brain NE levels; atomoxetine has also been shown to increase DA release in the medial prefrontal cortex of the rat.6 We investigated whether NE or DA systems mediate the procholinergic effects of atomoxetine in the medial prefrontal cortex. For this, the α1-NE antagonist prazosin or the D1-DA receptor antagonist SCH 23390 were administered systemically 15 min before the injection of atomoxetine (3 mg/kg). SCH 23390 at 0.3 mg/kg, a dose that by itself did not affect cortical ACh concentrations, completely prevented the increase in ACh efflux in the medial prefrontal cortex induced by atomoxetine (Figure 4). Prazosin (1 mg/kg) that by itself had no effect on cortical ACh concentration, also completely abolished the stimulatory effects of atomoxetine on ACh efflux in the prefrontal cortex (Figure 5).

Figure 4
figure4

The D1 antagonist SCH 23390 abolishes the stimulatory effects of atomoxetine on ACh efflux in the medial prefrontal cortex. SCH 23390 (0.3 mg/kg, s.c.) 15 min before atomoxetine (3 mg/kg) prevented the atomoxetine-induced ACh release (F(1,81)=4.09, P<0.001; three-way ANOVA for time course effects; F(1,18)=41.6, P<0.001; two-way ANOVA for overall effects). Data (expressed as fold increase over baseline established prior to any treatment) represent mean±s.e.m. of n=4–6 animals per group. *P<0.05, *P<0.01 and ***P<0.001 as compared to vehicle-treated animals (filled stars) and versus animals treated with vehicle prior to atomoxetine (open stars), at each time point.

Figure 5
figure5

The α1 antagonist prazosin abolishes the stimulatory effects of atomoxetine on ACh efflux in the medial prefrontal cortex. Prazosin administered (1 mg/kg, s.c.) 15 min before atomoxetine (3 mg/kg) prevented the atomoxetine-induced ACh release (F(1,72)=2.5, P<0.01; three-way ANOVA for time course effects; F(1,15)=46.21, P<0.001; two-way ANOVA for overall effects). Data (expressed as fold over baseline established prior to any treatment) represent mean±s.e.m. of n=4–6 animals per group. *P<0.05, *P<0.01 and ***P<0.001 as compared to vehicle treated animals (filled stars) and versus animals treated with vehicle prior to atomoxetine (open stars), at each time point.

Behavioral tasks

Memory tasks

The procholinergic profile of atomoxetine that we characterize here indicates a potential for this compound to enhance cognition. We therefore tested the ability of atomoxetine to improve memory retention in two distinct tasks that involve neocortical and hippocampal circuits, a spatial delay task performed in a 8-arm radial maze and the object recognition test.

8-arm radial maze

Performance in this task is dependent upon the length of time the information must be retained and is sensitive to amnesics and putative cognitive enhancers.10, 11, 12 When injected immediately after the information phase, atomoxetine at 3 mg/kg decreased the number of errors that occurred during the retention phase conducted 7 h later (Figure 6). However, atomoxetine at 1 and 10 mg/kg did not siginificantly alter performance in this task.

Figure 6
figure6

Atomoxetine improves performance in the 8-radial-arm maze test. Atomoxetine administered immediately after the information phase reduced the average number of errors that occurred during the retention phase conducted after a 7-h delay (F(3,16)=7.518 P<0.05; one-way ANOVA for treatment). Data represent mean±s.e.m. of n=7 animals per group. *P<0.05 for animals treated with atomoxetine versus animals treated with vehicle.

Object recognition

The two-object recognition task is an animal model for testing memory performance and is based on the rat's natural differential exploration of new and familiar objects.13, 14 Given a choice a rat will spend more time interacting with a new object than with a familiar object. The preference for novelty is observed across species and used as an indicator of recognition memory. During the learning trial, there was no difference in the behavior of the animals in the different groups (vehicle- versus atomoxetine-treated) and the time spent exploring the objects was very similar (between 10 and 15 s, for a 2-min trial) among all groups. In the test trial, atomoxetine (1 and 3 mg/kg) significantly improved performance, as evidenced by an increase in preference for the novel object at testing (Figure 7), whereas 0.3 and 10 mg/kg did not significantly alter performance in this task.

Figure 7
figure7

Atomoxetine improves performance in the object recognition test. Rats that received 1 and 3 mg/kg of atomoxetine spent significantly more time exploring the novel object, following the 3-h delay, than did vehicle-treated rats (F(4,39)=5.06, P<0.01; one-way ANOVA for treatment). Data (expressed as percent time exploring the novel object during the retention test) represent mean±s.e.m. of n=8–10 animals per group. *P<0.05 for animals treated with atomoxetine versus animals treated with vehicle prior to the acquisition trial.

Locomotor activity

We next assessed the effects of atomoxetine, as compared to the stimulant methylphenidate, on basal locomotor activity in the rat. We show that atomoxetine had no effect on horizontal locomotor activity at any of the doses tested (1, 3 and 10 mg/kg), whereas, as expected because of its profile as a stimulant, methylphenidate (3 mg/kg) robustly (by three-fold) increased locomotion (Figure 8). The lack of effect of atomoxetine on locomotor activity suggests that nonspecific motor effects of atomoxetine do not confound our results in the radial arm maze and the object recognition test.

Figure 8
figure8

Atomoxetine does not affect spontaneous locomotor activity. Rats that received 1, 3 and 10 mg/kg of atomoxetine did not show alterations in spontaneous locomotor activity. On the contrary, methylphenidate (3 mg/kg) induced a marked hyperlocomotion (F(4,35)=7.4, P<0.001; one-way ANOVA for treatment). Data (expressed as the average number of ambulations during the entire test period, i.e., total number of ambulations/60 min) represent mean±s.e.m. of n=8 animals per group. *P<0.05 for animals treated with methylphenidate versus animals treated with vehicle.

Discussion

Catecholaminergic activity in the cortex and subcortex is central to current pathophysiological models of ADHD. The present results indicate that enhancing cortical ACh efflux could also contribute to the mechanism of action of drugs that effectively treat ADHD.

We indeed show here by in vivo microdialysis that atomoxetine increases ACh efflux and presumably increases cholinergic neurotransmission in cortical regions, the medial prefrontal cortex, in particular. Furthermore, we provide evidence that in addition to atomoxetine, the NE-uptake inhibitor reboxetine, and (as previously reported in Acquas and Fibiger15) methylphenidate stimulate ACh release in this cortical region. In our model, although methylphenidate increases cortical ACh release to about the same extent as pharmacologically equivalent doses of atomoxetine and reboxetine, methylphenidate's stimulatory effects upon ACh efflux were more transient. These results, when compared to the effects of atomoxetine, reboxetine and methylphenidate on cortical DA and NE release that we reported previously,6 show that for each of the above drugs the time course curves for ACh, DA and NE increases overlap to a great extent. Thus, similarly to our observations for ACh efflux, the methylphenidate-induced rise of cortical NE and DA was apparent for only about 1 h while those of atomoxetine and reboxetine were maintained for twice as long.6 The short-lived, methylphenidate-induced DA and ACh increases are predicted by the relatively brief half-life of the compound and may account for the constraint of multiple daily administrations for its efficacy in ADHD patients. Furthermore, methylphenidate but neither atomoxetine nor reboxetine increased ACh efflux in the nucleus accumbens, consistent with and similar to previous findings on DA release in this region.6 It should be noted that D-amphetamine, the other stimulant drug widely used in ADHD therapeutics, also increases ACh efflux in the striatal complex including the nucleus accumbens.16 Thus, atomoxetine and reboxetine are differentiated from the psychostimulants in their lack of increase of striatal and nucleus accumbens DA and ACh levels. The enhancement of striatal and nucleus accumbens DA and ACh efflux by the psychostimulants may contribute to their abuse liability.

We show that the atomoxetine-induced ACh release in the prefrontal cortex is increased in the same dose range as NE and DA.6 Furthermore, our data, along with data from other studies, are consistent with a synchronous α1-NE and D1-DA receptor activation mechanism implicated in cortical ACh release. Thus, there is ample evidence that, in this region, where NE- and DA-afferents converge, NE activity can regulate extracellular DA efflux in multiple ways. These could involve either the NE transporter that removes NE and DA from the synaptic cleft with equipotent affinity17 or NE-heterorereceptors, which could directly control terminal DA activity.18 However, it is quite unlikely that atomoxetine increases cortical ACh release through a serial NE-α1-DA-D1 trans-synaptic circuit. First, α2- rather than α1-NE receptors were shown to modulate cortical DA release18 and antagonism of α1-NE receptors blocked atomoxetine-induced increases in ACh efflux. Second, D1-mediated DA control of cortical ACh release involves distant but not local activation of D1 receptors.19, 20 Furthermore, in our hands blockade of α1-NE receptors by prazosin (1 mg/kg) effectively reversed cortical ACh, but not cortical DA efflux elicited by atomoxetine (data not shown). Alternatively, the atomoxetine-induced ACh release could involve an integrated mechanism operating only when parallel D1-DA and α1-NE signals temporally and spatially coincide. Thus, converging DA and NE systems could regulate cortical ACh release remotely by modulating the firing of cholinergic neurons at the somatodendritic level. Indeed, in the nucleus basalis, where the ACh neurons projecting to the cortex reside, DA and NE afferents have been shown to synapse on cholinergic somata.21 The role of nucleus basalis D1-DA and α1-NE receptors in regulating cortical ACh release and in mediating the procholinergic effects of atomoxetine warrants further investigation.

As mentioned, a procholinergic neurochemical profile has been related to the ability of a number of compounds (marketed or under development) to ameliorate cognitive function in patients, or to improve attention and memory in animal models.14, 22 Specifically, studies with knockout mice have shown that the integrity of cholinergic neurotransmission in particular is necessary for habituation and memory functions,23, 24 as well as for behavioral shifting and temporal organization of action.25 Thus, we investigated the procognitive properties of atomoxetine in two distinct behavioral assays, the 8-arm radial arm maze and the object recognition test, classically used in rodents to screen for compounds that alter learning and memory. Although these behavioral tasks are not specific indicators of cholinergic system tone and their performance measures can be altered by many manipulations, both tests involve cortical (prefrontal and hippocampal) pathways and depend upon cholinergic activity. Thus, in general, cholinergic antagonists (e.g., scopolamine) impair, while procholinergic compounds (in particular acetylcholinesterase inhibitors) improve performance in the object recognition test and in the radial-maze (see, e.g., Ennaceur and Meliani14 and Myhrer26). Consistent to its neurochemical profile and at doses relevant to those that increase cortical monoamine release, atomoxetine increased the time rats spent interacting with the novel object in the object recognition test and decreased errors in spatial pattern recognition in the radial arm maze, suggesting improved memory performance in both tests. The two tasks differ in that the object recognition task depends on the animal's natural exploratory behavior and is nonspatially mediated, whereas the 8-arm radial maze test is a spatial task driven by food reward. In addition, in the object recognition task, atomoxetine was administered prior to the task and thus may affect acquisition, consolidation and retrieval processes. Atomoxetine was administered after the information phase in the 8-arm radial maze task and more likely would affect memory consolidation and less likely retrieval processes due to its relatively short half-life in rats. It should be noted nevertheless that while the increases in monoamine efflux were linear over the dose spectrum studied (see also Bymaster et al.6), the improved cognitive performance was characteristically nonlinear with high doses not showing any activity. Bell-shaped efficacy curves in animal cognitive models have been noted with a number of mechanistically diverse procognitive agents27, 28, 29, 30, 31, 32 and in clinical studies as well.33, 34, 35 This genre of dose–response relationship between neurochemistry and behavior suggests that a fine permissive tuning of cortical monoaminergic neurotransmission probably accounts for the expression of cognitive amelioration.

In conclusion, the evidence we present here indicates a procholinergic profile and a procognitive potential for the novel nonstimulant ADHD medication atomoxetine. In view of the clinically validated value of atomoxetine in the treatment of ADHD, further studies addressing the underlying molecular mechanisms of the procholinergic and procognitive effects of the compound that we demonstrate here are warranted.

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Correspondence to G G Nomikos.

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Tzavara, E., Bymaster, F., Overshiner, C. et al. Procholinergic and memory enhancing properties of the selective norepinephrine uptake inhibitor atomoxetine. Mol Psychiatry 11, 187–195 (2006). https://doi.org/10.1038/sj.mp.4001763

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Keywords

  • ADHD
  • atomoxetine
  • acetylcholine
  • microdialysis
  • reboxetine
  • methylphenidate

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