Differential effects of voluntary exercise and housing density on anxiety-like behavior in C57Bl/6 mice.

The interaction of voluntary exercise and housing density on a) anxiety-like behavior and b) the stimulant effects of methamphetamine in C57Bl/6 mice were evaluated. Upon arrival, mice were housed singly or in pairs, and permitted access to home-cage running wheels or not for 4 weeks. Testing for anxiety-like behavior occurred over the next 3 weeks, one test per week [Elevated-Plus Maze (EPM) → Hyponeophagia (HNP) task → Open-Field (OF) task]. The final, OF task involved an 8-hour session in which mice were permitted to explore the chamber (drug free) during Hours 1-3; given an injection (s.c.) of methamphetamine (1.0 mg/kg) after Hour 3; followed by continued behavioral sampling during Hours 4-8. Several tasks (HNP and OF, but not EPM) consistently showed voluntary exercise induced anxiety-like behavior. In addition, two measures (time in center and time resting in the perimeter) in the OF task revealed that exercise mice compared to controls were more responsive to the anxiogenic effects of methamphetamine. Although pair housing was anxiolytic, it did not ameliorate the anxiogenic effects of voluntary exercise. Taken together, these results, when viewed in tandem with previous studies that utilized a less anxious mouse strain (Swiss Webster), may suggest that voluntary exercise is anxiogenic in an anxiety-prone mouse strain such as C57Bl/6 and highlight the importance of considering mouse strain when evaluating the impact of environmental manipulations on anxiety-like behavior in animal models.

Propranolol blocks the unconditioned and conditioned hyperactive effects of methamphetamine in CD-1 mice.

The present experiment examined the contribution of the ß-adrenergic receptor system in mediating the unconditioned (i.e. pharmacological) and conditioned (i.e. learned) hyperactive effects of methamphetamine. To this end, mice underwent an 8-day conditioning procedure involving two different, alternating session types (chamber and home-cage days). On chamber days (1, 3, 5, and 7), mice were injected (intraperitoneally) with vehicle (dH 2 O) or propranolol (16 or 32 mg/kg) and were injected (subcutaneously) 30 min (min) later by either vehicle (saline; unpaired) or methamphetamine (1.0 mg/kg; paired). On home-cage days (2, 4, 6, and 8), mice were injected (subcutaneously) with either vehicle (saline; paired) or methamphetamine (1.0 mg/kg; unpaired) in their home cages. The test day for conditioned hyperactivity occurred 48 h after the last chamber day. Propranolol dose-dependently blocked the unconditioned and conditioned hyperactive effects of methamphetamine, implicating a role for the ß-adrenergic system in mediating these effects of methamphetamine.

Drug Conditioning and Sensitization in Mice: Importance of Early Voluntary Exercise.

The timing of voluntary exercise relative to drug conditioning is important to its "neuroprotective" effects, though it is unclear whether the voluntary exercise needs to occur temporally contiguous with drug conditioning, or occur during an early, developmental period, but non-contiguous with drug conditioning, for its "neuroprotective" effects. To distinguish between these two ideas, the timing of voluntary exercise relative to drug conditioning on the development and extinction of conditioned hyperactivity, and induction of sensitization was manipulated in the present experiment. Specifically, half of the exercise mice were permitted access to home-cage running wheels for 6 continuous weeks (Exercise-Exercise) whereas the other half of the exercise mice were permitted access to home-cage running wheels only for the first 3  weeks and then had the wheels removed (Exercise-Sedentary). Likewise, half of the sedentary mice had no home-cage running wheels for 6 continuous weeks (Sedentary-Sedentary) whereas the other half of the sedentary mice were permitted access to the home-cage running wheels for the last 3 weeks prior to the acquisition phase (Sedentary-Exercise). Mice received vehicle or methamphetamine (Meth; 1.0 mg/kg; acquisition), followed by saline-alone sessions (extinction) and finally challenged with an escalating Meth-regimen (0.25 → 1.0 mg/kg). While all Meth-paired groups, regardless of exercise regimen, showed conditioned hyperactivity, Exercise-Exercise and Exercise-Sedentary mice were less responsive to chronic Meth exposure and showed slower extinction compared to the other Meth-paired groups. These results suggest an early exercise regimen, during a critical developmental window, protects against the stimulant properties of Meth and simultaneously facilitates contextual learning.

Differential effects of voluntary exercise on development and expression of methamphetamine conditioned hyperactivity and sensitization in mice.

The present experiments determined the effects of voluntary home-cage wheel running on the development (Experiments 1 and 2a) and expression (Experiment 2b) of conditioned hyperactivity and long-term sensitization in male, Swiss-Webster mice. Mice experienced 3 weeks of wheel running (exercise) or not (sedentary) either beginning prior to (Experiments 1 and 2a), or immediately following (Experiment 2b), the acquisition phase. During the acquisition phase, mice (n = 12-15/group) received injections (subcutaneous) of either vehicle (saline) or methamphetamine (0.5 or 1.0 mg/kg, Experiment 1; 1.0 mg/kg, Experiments 2a and 2b) immediately prior to 5 locomotor-activity sessions. The extinction phase began 48 hours (h) (Experiment 1) or 3 weeks (Experiments 2a and 2b) after acquisition and all mice received vehicle injections prior to 4 locomotor-activity sessions. Tests of long-term sensitization occurred 72 h after the last extinction session and involved an escalating, methamphetamine-dose regimen (0.25 âž” 1.0 mg/kg), 1 dose/session for 3 sessions. While pre-acquisition wheel running failed to alter development of conditioned hyperactivity after training with the 0.5 mg/kg methamphetamine dose, it blunted the development of conditioned hyperactivity, and blocked (Experiment 1) or attenuated (Experiment 2a) induction of long-term sensitization after training with the 1.0 mg/kg methamphetamine dose. Furthermore, while post-acquisition wheel running retarded extinction of conditioned hyperactivity, it did not alter expression of conditioned hyperactivity or long-term sensitization (Experiment 2b). Collectively, the results suggest that the impact of voluntary exercise on context-drug associations and long-term sensitization is critically dependent on the timing of exercise relative to drug conditioning.

Voluntary exercise ameliorates anxiogenic effects of acute methamphetamine exposure in Swiss-Webster mice.

BACKGROUND: The present experiment examined the ability of voluntary exercise (i.e., home-cage wheel running; HCWR) to ameliorate anxiety-like behavior associated with acute methamphetamine exposure in male, Swiss-Webster mice. METHODS: Mice were permitted access to home-cage running wheels (Exercise), locked home-cage running wheels or no home-cage running wheels (Sedentary) for 6 weeks and then exposed to different methamphetamine doses (vehicle, 0.25, 0.5, or 1.0 mg/kg) once weekly during an 8 h open-field session for 4 weeks. Group differences in hypothalamic-pituitary-adrenal (HPA) activity also were assessed by weighing adrenal glands. RESULTS: It was found that HCWR ameliorated anxiety-like behavior after an injection of either the 0.5 or 1.0 mg/kg methamphetamine dose. Adrenal weights did not differ between Exercise and Sedentary mice. CONCLUSION: Taken together, these results suggest that voluntary exercise ameliorates the anxiogenic effects of methamphetamine depending on the dose, perhaps via a non-HPA mechanism.

Timing of SCH 23390 administration influences extinction of conditioned hyperactivity in mice.

The precise role of the dopamine subtype-1 (D1) receptor in differentially mediating extinction-related processes (memory retrieval vs. memory reconsolidation) in the conditioned hyperactivity paradigm is unknown. Thus, the present experiments determined the effect of a selective D1 receptor antagonist, SCH 23390, on extinction of conditioned hyperactivity when SCH 23390 was administered immediately after (memory reconsolidation; experiment 1) or before (memory retrieval; experiment 2) extinction sessions. Male, Swiss-Webster mice received subcutaneous injections of methamphetamine (1.0 mg/kg) associated with locomotor activity chambers (paired) or in their home cages (unpaired) during the acquisition phase. Following acquisition, paired and unpaired mice received an intraperitoneal injection of either vehicle (physiological saline) or SCH 23390 (0.0125, 0.025, 0.05 mg/kg) immediately after (experiment 1) or received vehicle or SCH 23390 (0.05 mg/kg) 30 min before (experiment 2), daily extinction sessions. Methamphetamine produced robust conditioned hyperactivity, followed by extinction. Furthermore, SCH 23390 (0.05 mg/kg) blocked expression of conditioned hyperactivity, without nonspecifically impairing locomotor activity, when administered before the extinction session, but did not alter the rate of extinction when administered immediately following the sessions. Taken together, these results suggest that the D1 receptor is involved in memory retrieval, but not memory reconsolidation, processes during extinction of conditioned hyperactivity.

Time-dependent effects of prazosin on the development of methamphetamine conditioned hyperactivity and context-specific sensitization in mice.

The present experiments examined the effects of prazosin, a selective α1-adrenergic receptor antagonist, on the development of methamphetamine conditioned hyperactivity and context-specific sensitization. Mice received an injection of vehicle (distilled water) or prazosin (0.5, 1.0 or 2.0 mg/kg) 30 min prior to a second injection of vehicle (saline) or methamphetamine (1.0 mg/kg) during the conditioning sessions (Experiment 1). Following the conditioning sessions, mice were tested for conditioned hyperactivity and then tested for context-specific sensitization. In subsequent experiments, mice received an injection of vehicle (distilled water) or prazosin (2.0 mg/kg) immediately (Experiment 2) or 24 h (Experiment 3) after the conditioning sessions and then tested for conditioned hyperactivity and context-specific sensitization. Prazosin dose-dependently blocked the development of methamphetamine conditioned hyperactivity and context-specific sensitization when administered prior to the methamphetamine during the conditioning phase; however nonspecific motor impairments also were observed (Experiment 1). Immediate (Experiment 2), but not the 24-h delay (Experiment 3), post-session administration of prazosin attenuated the development of methamphetamine conditioned hyperactivity and context-specific sensitization. Nonspecific motor impairments were not observed in these latter experiments. Collectively, these results suggest that the α1-adrenergic receptor mediates the development of methamphetamine-conditioned hyperactivity and context-specific sensitization, perhaps by altering memory consolidation and/or reconsolidation processes.

Renewal of sucrose-seeking behavior in rats: Role of D(2) dopamine receptors.

The present study characterized renewal of sucrose-seeking behavior in rats (Experiment 1). The role of the dopamine subtype-2 (D(2)) receptors in mediating renewal of sucrose-seeking behavior also was examined (Experiment 2). Rats were trained to respond for sucrose pellets (45mg each) on a fixed-ratio 25 (Experiments 1 and 2) schedule of reinforcement in Context A. Following acquisition, rats underwent extinction and 4 renewal tests in Contexts B and A, respectively. In Experiment 2, rats were pretreated with vehicle or the D(2) dopamine receptor antagonist eticlopride (5, 10, 20, or 40microg/kg) 30min prior to the first renewal test session. A follow-up experiment (Experiment 3) examined the effect of a high eticlopride dose (40microg/kg) on locomotor activity. Renewal of sucrose-seeking behavior persisted for 3 sessions. Eticlopride dose-dependently blocked renewal of sucrose-seeking behavior without suppressing locomotor activity, implicating a role of D(2) dopamine receptors in mediating renewal of sucrose-seeking behavior.

Bupropion differentially alters the aversive, locomotor and rewarding properties of nicotine in CD-1 mice.

The present experiments determined the effects of bupropion on the motivational (aversive and rewarding) and locomotor properties of nicotine in CD-1 mice. Preliminary experiments determined effective nicotine doses (0.1-2.0 mg/kg) to produce a conditioned taste aversion (CTA) or conditioned place preference (CPP; Experiments 1a and 2a, respectively). Mice were administered vehicle or bupropion (1-20 mg/kg) followed by vehicle or nicotine after drinking saccharin during CTA training (Experiment 1b). Mice were administered vehicle or bupropion (1-20 mg/kg) 15 (Experiment 2b) or 30 (Experiment 2c) minutes (min) prior to vehicle or nicotine during CPP training. The two highest nicotine doses produced CTAs and a moderate nicotine dose (0.4 mg/kg) produced a CPP. Bupropion dose-dependently blocked nicotine CTA. For the 15-min pretreatment interval, bupropion dose-dependently increased locomotor activity and produced CPPs when administered alone; whereas for the 30-min pretreatment interval, only the highest bupropion dose increased locomotor activity and produced a CPP. However, bupropion failed to alter nicotine CPP and the co-administration of bupropion and nicotine did not increase locomotor activity more so than when bupropion was administered alone regardless as to the pretreatment interval. Thus, bupropion selectively altered the aversive properties of nicotine in CD-1 mice.

Wistar Kyoto and Wistar rats differ in the affective and locomotor effects of nicotine.

Anhedonia is a characteristic of clinical depression and has been associated with dysfunction of the mesolimbic dopaminergic system, a system also involved in mediating nicotine reward. To further examine the relationship between anhedonia, clinical depression and nicotine reward, the present experiment determined if Wistar Kyoto (WKY) rats, an animal model of clinical depression, differed from Wistar rats in nicotine conditioned place preference (CPP). Strain differences in nicotine-induced changes in locomotor activity also were determined simultaneously. To determine if strain differences were specific to reward-based learning, nicotine or lithium chloride (LiCl) conditioned taste avoidance (CTA) experiments were conducted. Rats received vehicle or nicotine (0.4 or 0.8 mg/kg) during a multi-trial, biased CPP training procedure or received vehicle, nicotine (0.2, 0.4 or 0.8 mg/kg) or lithium chloride (LiCl; 0.0375, 0.075 or 0.15 M) during a multi-trial CTA training procedure. Whereas both nicotine doses (0.4 and 0.8 mg/kg) initially induced hypoactivity, only the moderate nicotine dose (0.4 mg/kg) induced hyperactivity with repeated administration and produced a CPP in Wistar rats. Both nicotine doses failed to alter locomotor activity or produce a CPP in WKY rats. WKY rats also acquired a LiCl CTA more slowly and less robustly compared to Wistar rats. In contrast, nicotine dose-dependently produced a CTA in both strains and WKY rats were more sensitive to the avoidance effects of nicotine compared to Wistar rats. Collectively, these results suggest that WKY rats show deficits in nicotine reward and specific aversive drug stimuli compared to Wistar rats.

Review of the pharmacology and clinical profile of bupropion, an antidepressant and tobacco use cessation agent.

Bupropion hydrochloride ((+/-)-2-tert-butylamino)-3'-chloropropiophenone x HCl) is a nonselective inhibitor of the dopamine transporter (DAT) and the norepinephrine transporter (NET) and is also an antagonist at neuronal nicotinic acetylcholine receptors (nAChRs). In animal models used commonly to screen for antidepressant activity, bupropion shows a positive response. Also using animal models, bupropion has been shown to attenuate nicotine-induced unconditioned behaviors, to share or enhance discriminative stimulus properties of nicotine and to have a complex effect on nicotine self-administration, i.e., low doses augmenting nicotine self-administration and high doses attenuating self-administration. Current studies show that bupropion facilitates the acquisition of nicotine conditioned place preference in rats, further suggesting that bupropion enhances the rewarding properties of nicotine. Bupropion has been shown to attenuate the expression of nicotine withdrawal symptoms in both animal models and human subjects. With respect to relapse, current studies show that bupropion attenuates nicotine-induced reinstatement in rats, but large individual differences are apparent. Clinically, bupropion is used as a treatment for two indications, as an antidepressant, the indication for which it was developed, and as a tobacco use cessation agent. In clinical trials, bupropion is being tested as a candidate treatment for psychostimulant drug abuse, attention-deficit hyperactivity disorder (ADHD) and obesity. Bupropion is available in three bioequivalent oral formulations, immediate release (IR), sustained release (SR), and extended release (XL). Extensive hepatic metabolism of bupropion produces three pharmacologically active metabolites, which may contribute to its clinical profile.

Tolerance does not develop to the decrease in nicotine self-administration produced by repeated bupropion administration.

The atypical antidepressant bupropion has been shown to be an efficacious smoking cessation agent; however, its therapeutic mechanism of action is unknown. To further understand the mechanism by which bupropion reduces smoking, the present study determined the effect of repeated bupropion pretreatment on nicotine self-administration or sucrose-maintained responding. Rats were trained to self-administer intravenous nicotine (0.02 mg/kg/infusion; Experiment 1) or to respond for sucrose pellets (45 mg each; Experiment 2) on a fixed-ratio 5 schedule. Once rats reached stable responding, bupropion (70 mg/kg, subcutaneously) or vehicle was injected 15 min before the session for 14 consecutive sessions. Bupropion acutely decreased both nicotine self-administration and sucrose-maintained responding by approximately 60%-70%. With repeated bupropion pretreatment, however, responding for nicotine decreased completely. In contrast, the bupropion-induced decrease in responding for sucrose following acute administration did not change significantly with repeated bupropion administration. These results suggest that bupropion acquired some specificity with repeated use, decreasing the intake of nicotine and producing an extinction-like pattern in nicotine self-administration. Thus the present results parallel human clinical studies with bupropion demonstrating its smoking cessation properties following repeated treatment. These results indicate that the rat nicotine self-administration paradigm is a useful animal model for assessing smoking cessation pharmacotherapies.

Effect of bupropion on nicotine self-administration in rats.

RATIONALE AND OBJECTIVE: The mechanisms underlying the therapeutic efficacy of bupropion as a smoking cessation agent are unknown. Bupropion inhibits monoamine uptake as well as neuronal nicotinic receptor (nAChR) function. The present study compared effects of bupropion on nicotine self-administration to those of other stimulant drugs (methamphetamine and apomorphine) that lack nAChR activity in order to determine its mechanism of action. To determine the specificity of bupropion-induced changes in nicotine self-administration, the ability of bupropion to alter sucrose-maintained responding or amphetamine self-administration was determined. METHODS: In nicotine and amphetamine self-administration and sucrose-maintained responding experiments, rats responded for nicotine (0.01 or 0.02 mg/kg per infusion, IV), amphetamine (0.2 mg/kg per infusion, IV) and sucrose pellets (45 mg), respectively, on a fixed ratio 5 schedule. Once responding stabilized, rats were pretreated 15 min before the session with bupropion (1-78 mg/kg) or vehicle. The ability of methamphetamine (0.3-3 mg/kg) or apomorphine (0.01-0.2 mg/kg) to alter responding for nicotine (0.02 mg/kg per infusion, IV) was determined. RESULTS: Bupropion produced a biphasic dose-response pattern at both nicotine infusion doses, increasing infusions at low bupropion doses and decreasing infusions at high bupropion doses. Methamphetamine produced a similar biphasic pattern, whereas apomorphine only decreased nicotine infusions at high doses. Bupropion dose-dependently decreased responding for sucrose and amphetamine. CONCLUSIONS: These results suggest that high bupropion doses decrease responding nonspecifically; whereas low bupropion doses selectively increase responding for nicotine. The increase in nicotine self-administration is likely due to inhibition of dopamine and norepinephrine transporters, combined with inhibition of nAChRs.

Reboxetine: attenuation of intravenous nicotine self-administration in rats.

The ability of reboxetine, a selective inhibitor of the norepinephrine transporter and noncompetitive antagonist at neuronal nicotinic receptors, to alter nicotine self-administration in rats was compared with that of mecamylamine, a classical noncompetitive antagonist at nicotinic receptors. The ability of reboxetine to alter sucrose-maintained responding was also examined to assess the specificity of the effect on nicotine self-administration. Rats were trained on a fixed ratio 5 schedule to self-administer nicotine (0.02 mg/kg/infusion i.v.) or to respond for sucrose pellets. Upon reaching a stable baseline, rats were pretreated 15 min before the session with vehicle, reboxetine (racemic), (+)-(S,S)-reboxetine (0.3-30 mg/kg s.c.) or mecamylamine (0.5-4 mg/kg s.c). To assess the effect of repeated administration, reboxetine (5.6 mg/kg) was injected once daily for 14 consecutive sessions before either nicotine self-administration or sucrose-maintained responding. Specificity was further assessed by examining the ability of repeated administration of reboxetine (5.6 mg/kg) to alter nicotine-induced hyperactivity (0.8 mg/kg). Reboxetine, (+)-(S,S)-reboxetine, and mecamylamine dose dependently decreased nicotine self-administration by ~60%, whereas reboxetine and (+)-(S,S)-reboxetine decreased sucrose-maintained responding to a lesser extent (~20%). Repeated administration of reboxetine (5.6 mg/kg) decreased nicotine self-administration and sucrose-maintained responding across the 14 sessions, suggesting that tolerance did not develop to these effects of reboxetine. Additionally, reboxetine did not alter baseline locomotor activity, indicating that the decrease in operant responding for nicotine and sucrose was not the result of a nonspecific decrease in activity. The reboxetine-induced decrease in nicotine self-administration and sucrose-maintained responding may be the result of inhibition of norepinephrine transporters and/or neuronal nicotinic receptor function.

Effects of opioid antagonists on unconditioned and conditioned hyperactivity to morphine.

In a series of experiments, the ability of selective mu- (beta-funaltrexamine, beta-FNA), delta- (naltrindole, nalt) and kappa- (nor-binaltorphimine, nor-BNI) opioid receptor antagonists to attenuate the unconditioned and conditioned hyperactive effects of morphine was examined. For comparison, the nonselective opioid receptor antagonist naloxone (nalx) was also examined. Locomotor activity served as the behavioral measure. Experiment 1 found that doses of 1 and 4, but not 16 mg/kg, of morphine effectively produced conditioned hyperactivity (CH). Experiments 2a-d found that beta-FNA, nalt, nor-BNI and nalx, respectively, attenuated unconditioned morphine-induced hyperactivity. Experiments 3a-c, however, found that none of the selective antagonists, given individually, attenuated CH. In contrast, nalx did attenuate CH (Experiment 3d). Collectively results suggest that the unconditioned and conditioned hyperactive responses to morphine are mediated by different receptor systems and that activation of multiple opioid-receptor subtypes mediate expression of CH.