Repeated exposure of adolescent rats to oral methylphenidate does not induce behavioral sensitization or cross-sensitization to nicotine

Several lines of evidence indicate that the use of stimulant drugs, including methylphenidate (MPD), increases tobacco smoking. This has raised concerns that MPD use during adolescence could facilitate nicotine abuse. Preclinical studies have shown that repeated treatment with an addictive drug produces sensitization to that drug and usually cross-sensitization to other drugs. Behavioral sensitization has been implicated in the development of drug addiction. We examined whether repeated oral MPD administration during adolescence could induce behavioral sensitization to MPD and long-lasting cross-sensitization to nicotine. Adolescent male Wistar rats were treated orally with 10 mg/kg MPD or saline (SAL) from postnatal day (PND) 27 to 33. To evaluate behavioral sensitization to MPD in adolescent rats (PND 39), the SAL pretreated group was subdivided into two groups that received intragastric SAL (1.0 mL/kg) or MPD (10 mg/kg); MPD pretreated rats received MPD (10 mg/kg). Cross-sensitization was evaluated on PND 39 or PND 70 (adulthood). To this end, SAL- and MPD-pretreated groups received subcutaneous injections of SAL (1.0 mL/kg) or nicotine (0.4 mg/kg). All groups had 8 animals. Immediately after injections, locomotor activity was determined. The locomotor response to MPD challenge of MPD-pretreated rats was not significantly different from that of the SAL-pretreated group. Moreover, the locomotor response of MPD-pretreated rats to nicotine challenge was not significantly different from that of the SAL-pretreated group. This lack of sensitization and cross-sensitization suggests that MPD treatment during adolescence does not induce short- or long-term neuroadaptation in rats that could increase sensitivity to MPD or nicotine.

Repeated exposure of adolescent rats to oral methylphenidate does not induce behavioral sensitization or cross-sensitization to nicotine.

Several lines of evidence indicate that the use of stimulant drugs, including methylphenidate (MPD), increases tobacco smoking. This has raised concerns that MPD use during adolescence could facilitate nicotine abuse. Preclinical studies have shown that repeated treatment with an addictive drug produces sensitization to that drug and usually cross-sensitization to other drugs. Behavioral sensitization has been implicated in the development of drug addiction. We examined whether repeated oral MPD administration during adolescence could induce behavioral sensitization to MPD and long-lasting cross-sensitization to nicotine. Adolescent male Wistar rats were treated orally with 10 mg/kg MPD or saline (SAL) from postnatal day (PND) 27 to 33. To evaluate behavioral sensitization to MPD in adolescent rats (PND 39), the SAL pretreated group was subdivided into two groups that received intragastric SAL (1.0 mL/kg) or MPD (10 mg/kg); MPD pretreated rats received MPD (10 mg/kg). Cross-sensitization was evaluated on PND 39 or PND 70 (adulthood). To this end, SAL- and MPD-pretreated groups received subcutaneous injections of SAL (1.0 mL/kg) or nicotine (0.4 mg/kg). All groups had 8 animals. Immediately after injections, locomotor activity was determined. The locomotor response to MPD challenge of MPD-pretreated rats was not significantly different from that of the SAL-pretreated group. Moreover, the locomotor response of MPD-pretreated rats to nicotine challenge was not significantly different from that of the SAL-pretreated group. This lack of sensitization and cross-sensitization suggests that MPD treatment during adolescence does not induce short- or long-term neuroadaptation in rats that could increase sensitivity to MPD or nicotine.

Exposure to chronic stress increases the locomotor response to cocaine and the basal levels of corticosterone in adolescent rats.

Repeated exposure to stress results in augmentation in the locomotor response to psychostimulant drugs. We investigated the locomotor response to a novel environment or cocaine [10 mg/kg, intraperitoneally (i.p.)] and basal corticosterone levels in male adolescent rats exposed to chronic restraint or variable stress. Animals in the chronic restraint group were restrained for 1 hour daily. The chronic variable stress protocol consisted of exposure to different stressors twice a day in random order. Chronic restraint and variable stress regimens began simultaneously on postnatal day (P) 25 and were applied for 10 days. During this period the control group was left undisturbed except for cleaning the cages. Three days after the last exposure to stress, cocaine- and novelty-induced locomotion were recorded in an activity cage. Plasma corticosterone levels were determined in a subset of stress and control animals. Exposure to both chronic restraint and variable stress increased cocaine-induced locomotion and basal corticosterone plasma levels, while no change was observed in the response to a novel environment. Moreover, rats exposed to variable stress displayed the greatest locomotor response following a challenge dose with cocaine when compared to control and chronic restraint stress groups. This observation indicates that the stress regimen is relevant to the degree of stress-induced sensitization to cocaine.

Fencamfamine-induced changes in the parameters of the matching law and the effect of previous exposure to the drug.

The purpose of this study was to assess the effects of the psychomotor stimulant fencamfamine (FCF) on the operant response rate and on the parameters k and R(e) of Herrnstein's hyperbola. In the first experiment, rats were trained on a multiple variable interval (VI) schedule. Each rat was then given two i.p. injections of a randomly assigned FCF dose (0.0, 0.88, 1.75 and 3.5 mg/kg) at 48 h intervals. The overall response rate, the individual VI response rate, and the k and R(e) parameters of the hyperbola equation were calculated. In the second experiment, two groups of animals were trained drug-free on the same schedule. Afterwards, the animals in one group received six i.p. injections of 1.75 mg/kg FCF at 48 h intervals while subjects in the other group received vehicle injections, 15 min before each experimental session. Seven days after the last injection, a challenge dose (0.88 mg/kg) of FCF was administered and performance was measured. FCF increased overall response rate and decreased R(e) without changing k. Previous exposure to FCF did not induce sensitization of these measures. These results indicate that FCF has similar effects on k and R(e) as those produced by other indirect dopaminergic agents, and supports previous findings suggesting FCF as a potential drug of abuse.

Fencamfamine modulates sodium, potassium-ATPase through cyclic AMP and cyclic AMP-dependent protein kinase in rat striatum.

Dopamine (DA) and fencamfamine (FCF) modulatory action on Na,K-ATPase and Mg-ATPase activity were evaluated in rat striatum. DA and FCF induced a decrease in Na,K-ATPase, without affecting Mg-ATPase activity. The effect of FCF was dose-dependent from 10 to 100 microM, with an IC50 of 4.7 x 10(-5) M. Furthermore, the effect of FCF (100 microM) increasing AMPc levels, but not GMPc, was nonadditive with that of DA (10 microM), which is consistent to a common site of action. The 8-bromo-cyclic AMP also induced a specific reduction in the Na,K-ATPase activity. The reduction of Na,K-ATPase induced by FCF (100 microM) was blocked by either SCH 23390 or sulpiride, which are D1 and D2 receptor antagonists. The decrease in striatal NA,K-ATPase activity induced by FCF was blocked by KT 5720, a selective inhibitor of cyclic AMP-dependent protein kinase (PKA), but not by KT 5823, a selective inhibitor of cyclic GMP-dependent protein kinase (PKG). Otherwise, KT 5720 or KT 5823 did not produce any change in Na,K-ATPase or Mg-ATPase activity. These data suggest that FCF reduces Na,K-ATPase activity through cyclic AMP-dependent changes in protein phosphorylation via a PKA mechanism.

Involvement of dopamine receptors in diethylpropion-induced conditioning place preference.

Diethylpropion (DEP) is an amphetamine-like agent used as an anorectic drug. Abuse of DEP has been reported and some restrictions of its use have been recently imposed. The conditioning place preference (CPP) paradigm was used to evaluate the reinforcing properties of DEP in adult male Wistar rats. After initial preferences were determined, animals weighing 250-300 g (N = 7 per group) were conditioned with DEP (10, 15 or 20 mg/kg). Only the dose of 15 mg/kg produced a significant place preference (358 +/- 39 vs 565 +/- 48 s). Pretreatment with the D1 antagonist SCH 23,390 (0.05 mg/kg, s.c.) 10 min before DEP (15 mg/kg, i.p.) blocked DEP-induced CPP (418 +/- 37 vs 389 +/- 31 s) while haloperidol (0.5 mg/kg, i.p.), a D2 antagonist, 15 min before DEP was ineffective in modifying place conditioning produced by DEP (385 +/- 36 vs 536 +/- 41 s). These results suggest that dopamine D1 receptors mediate the reinforcing effect of DEP.

Involvement of dopamine receptors in diethylpropion-induced conditioning place preference

Diethylpropion (DEP) is an amphetamine-like agent used as an anorectic drug. Abuse of DEP has been reported and some restrictions of its use have been recently imposed. The conditioning place preference (CPP) paradigm was used to evaluate the reinforcing properties of DEP in adult male Wistar rats. After initial preferences were determined, animals weighing 250-300 g (N= 7 per group) were conditioned with DEP (10, 15 or 20 mg/kg). Only the dose of 15 mg/kg produced a significant place preference (358 + 39 vs 565 + 48s). Pretreatment with the D1 antagonist SCH 23390 (0.05 mg/kg, sc) 10 min before DEP (15 mg/kg, ip) blocked DEP-induced CPP (418 + 37 vs 389 + 31 s) while haloperidol (0.5 mg/kg, ip), a D2 antagonist, 15 min before DEP was ineffective in modifying place conditioning produced by DEP (385 + 36 vs 536 + 41 s). These results suggest that dopamine D1 receptors mediate the reinforcing effect of DEP.

Repeated administration intensifies the reinforcing effect of fencamfamine in rats.

1. In the present study, we evaluated the role of repeated administration on conditioning place preference (CPP) induced by fencamfamine (FCF) in male rats. 2. Repeated FCF (3.5 mg/kg) or saline once or daily for ten consecutive days enhanced sniffing duration and decreased locomotion and rearing duration. 3. At the 3.5 mg/kg dose, FCF produced a significant place-preference effect. 4. Repeated exposures to FCF intensified its reinforcing properties. 5. These results suggest that repeated FCF administration sensitizes its rewarding effects, as with other addictive substances.

Circadian time-dependent effects of fencamfamine on inhibition of dopamine uptake and release in rat striatal slices.

Fencamfamine (FCF) is a central stimulant that facilitates central dopaminergic transmission through inhibition of dopamine uptake and enhanced release of the transmitter. We evaluated the changes in the inhibition of uptake and the release of striatal [3H]-dopamine at 9:00 and 21:00 h, times corresponding to maximal and minimal behavioral responses to FCF, respectively. Adult male Wistar rats (200-250 g) maintained on a 12-h light/12-h dark cycle (lights on at 7:00 h) were used. In the behavioral experiments the rats (N = 8 for each group) received FCF (3.5 mg/kg, ip) or saline at 9:00 or 21:00 h. Fifteen minutes after treatment the duration of activity (sniffing, rearing and locomotion) was recorded for 120 min. The basal motor activity was higher (28.6 +/- 4.2 vs 8.4 +/- 3.5 s) after saline administration at 21:00 h than at 9:00 h. FCF at a single dose significantly enhanced the basal motor activity (38.3 +/- 4.5 vs 8.4 +/- 3.5 s) and increased the duration of exploratory activity (38.3 +/- 4.5 vs 32.1 +/- 4.6 s) during the light, but not the dark phase. Two other groups of rats (N = 6 for each group) were decapitated at 9:00 and 21:00 h and striata were dissected for dopamine uptake and release assays. The inhibition of uptake and release of [3H]-dopamine were higher at 9:00 than at 21:00 h, suggesting that uptake inhibition and the release properties of FCF undergo daily variation. These data suggest that the circadian time-dependent effects of FCF might be related to a higher susceptibility of dopamine presynaptic terminals to the action of FCF during the light phase which corresponds to the rats' resting period.

Circadian time-dependent effects of fencamfamine on inhibition of dopamine uptake and release in rat striatal slices

Fencamfamine (FCF) is a central stimulant that facilitates central dopaminergic transmission through inhibition of dopamine uptake and enhanced release of the transmitter. We evaluated the changes in the inhibition of uptake and the release of striatal [3H]-dopamine at 9:00 and 21:00 h, times corresponding to maximal and minimal behavioral responses to FCF, respectively. Adult male Wistar rats (200-250 g) maintained on a 12-h light/12-h dark cycle (lights on at 7:00 h) were used. In the behavioral experiments the rats (N = 8 for each group) received FCF (3.5 mg/kg, ip) or saline at 9:00 or 21:00 h. Fifteen minutes after treatment the duration of activity (sniffing, rearing and locomotion) was recorded for 120 min. The basal motor activity was higher (28.6 + 4.2 vs 8.4 + 3.5 s) after saline administration at 21:00 h than at 9:00 h. FCF at a sigle dose significantly enhanced the basal motor activity (38.3 + 4.5 vs 8.4 + 3.5 s) and increased the duration of exploratory activity (38.3 + 4.5 vs 32.1 + 4.6 s) during the light, but not the dark phase. Two other groups of rats (N = 6 for each group) were decapitated at 9:00 and 21:00 h and striata were dissected for dopamine uptake and release assays. The inhibition of uptake and release of [3H]-dopamine were higher at 9:00 than at 21:00 h, suggesting that uptake inhibition and the release properties of FCF undergo daily variation. These data suggest that the circadian time-dependent effects of FCF might be related to a higher susceptibility of dopamine presynaptic terminals to the action of FCF during the light phase which corresponds to the rats' resting period.

The prooxidant effect of 5-aminolevulinic acid in the brain tissue of rats: implications in neuropsychiatric manifestations in porphyrias.

5-Aminolevulinic acid (ALA), a heme precursor accumulated during the clinical expression of acute intermittent porphyria, lead poisoning, and tyrosinosis, has been hypothesized to act as an endogenous source of oxyradicals. We now report oxidative effects on brain tissue of rats submitted to ALA treatment. Upon acute treatment (40 mg/kg body weight) increased total nonheme iron in the cortex (20%) was observed. After prolonged ALA administration (40 mg/kg body weight on alternate days during 2 weeks), the following indicators of oxidative stress were found to be significantly increased: CuZnSOD activity (67%) in total brain homogenate, total iron (68%) and ferritin (71%) in the cortex, ferritin in striatum (44%), protein carbonyls in homogenate of cerebral cortex (threefold) and 45Ca2+ uptake by cortical synaptosomes (45%). In addition, synaptic membranes prepared from whole brain assayed with the radioligand 3H-muscimol, revealed increased Kd values (twofold) of the high-affinity GABAergic receptor binding and formation of protein carbonyl groups, thiobarbituric acid reactive products, and conjugated dienes. In vitro, ALA produced similar effects upon the high affinity 3H-muscimol binding. No apparent alteration of either dopaminergic or serotonergic [3H]-ligand binding was observed. These results argue in favor of ALA-triggered oxidative stress in brain accompanied by iron metabolism alterations and GABAergic receptor damage, which may be implicated in the neuropsychiatric manifestations of the aforementioned porphyrias.

Effect of apomorphine on rat motor activity induced by fencamfamine and other indirect psychostimulant drugs.

We evaluated the effects of low doses of apomorphine on the stimulant behavioral effects induced by amphetamine (2.5 mg/kg), fencamfamine (6.0 mg/kg) and cocaine (15.0 mg/kg). Rats received 0.02 mg/kg of apomorphine (sc) and 30 min later were injected with one of the stimulants. Motor activity including locomotion, rearing and sniffing was quantified in the animals home cages for 60 min at 15-min intervals. All stimulant drugs induced hyperactivity. When apomorphine was administered before cocaine, but not when administered before fencamfamine or amphetamine, distinctive changes occurred. The behavioral pattern resulting from high stimulation was replaced by that related to low stimulation, suggesting that apomorphine induces a transfer in the predominant behavior in cocaine-, and partially in fencamfamine-, but not in amphetamine-treated animals, by decreasing the intensity of the stereotyped effect. While no changes occurred when apomorphine was administered before amphetamine, the fencamfamine group showed intermediate alterations (nonsignificant changes in sniffing but a significant increase in rearing behavior). These results are discussed in terms of the different mechanisms of presynaptic action of the drugs studied.

The behavioral sensitization induced by fencamfamine is not related to plasma drug levels.

Fencamfamine (FCF) is a CNS stimulant that facilitates central dopaminergic transmission primarily through blockade of dopamine uptake. In the present study we evaluated the relationship between plasma FCF concentration and behavioral sensitization effect. Adult male Wistar rats (250-300 g) received FCF (10 mg/kg, ip) or saline once or daily for 10 consecutive days (N = 10 for each group). Blood samples were collected 30 min after injections and plasma FCF was measured by gas chromatography using an electron capture detector. FCF treatment enhanced sniffing duration (16.8 +/- 0.8 vs 26.6 +/- 0.9 s) and decreased rearing behavior (8.2 +/- 0.8 vs 3.7 +/- 0.6 s) when days 1 and 10 of drug administration were compared. Comparison of pair of means by the Student t-test did not show significant differences in plasma FCF concentration (390 +/- 40 vs 420 +/- 11 ng/ml) when blood samples were collected 30 min after acute FCF administration or after daily administration of 10 mg/kg for 10 days. In conclusion, the behavioral sensitization to FCF could not be correlated with plasma drug levels, and changes in the activity of dopaminergic systems should be considered to explain the sensitization to the effect of FCF.

The behavioral sensitization induced by fencamfamine is not related to plasma drug levels

Fencamfamine (FCF) is a CNS stimulant that facilitates central dopaminergic transmission primarily though blockade of dopamine uptake. In the present study we evaluated the relationship between plasma FCF concentration and behavioral sensitization effect. Adult male Wistar rats (250-300 g) received FCF (10 mg/Kg, ip) or saline oince or daily for 10 consecutive days (N = 10 for each group). Blood samples were collected 30 min after injections and plasma FCF was measured by gas chromatography using an electron capture detector. FCF treatment enhanced sniffing duration (16.8 ñ 0.8 vs 26.6 ñ 0.9s) and decreased rearing behavior (8.2 ñ 0.8 vs 3.7 ñ 0.6s) when days 1 and 10 of drug administration were compared. Comparison of pair of means by the Student t-test did not show significant differences in plasma FCF concentration (390 ñ 40 vs 420 ñ 11 ng/ml) when blood samples were collected 30 min after acute FCF administration or after daily administration of 10 mg/Kg for 10 days. In conclusion, the behavioral sensitization to FCF could not be correlated with plasma drug levels, and changes in the activity of dopaminergic systems should be considered to explain the sensitization to the effect of FCF

Reinforcing properties of fencamfamine: involvement of dopamine and opioid receptors.

Fencamfamine (FCF) is a psychostimulant classified as an indirect dopamine agonist. The conditioning place preference (CPP) paradigm was used to investigate the reinforcing properties of FCF. After initial preferences had been determined, animals were conditioned with FCF (1.75, 3.5, or 7.0 mg/kg; IP). Only at the dose of 3.5 mg/kg FCF produced a significant place preference. Pretreatment with SCH23390 (0.05 mg/kg; SC) or naloxone (1.0 mg/kg; SC) 10 min before FCF (3.5 mg/kg, IP) blocked both FCF-induced hyperactivity and CPP. Pretreatment with metoclopramide (10.0 mg/kg; IP) or pimozide (1.0 mg/kg, IP), respectively, 30 min or 4 h before FCF (3.5 mg/kg; IP), which blocked the FCF-induced locomotor activity, failed to influence place conditioning produced by FCF. In conclusion, the present study suggests that dopamine D1 and opioid receptors are related to FCF reinforcing effect, while dopamine D2 subtype receptor was ineffective in modifying FCF-induced CPP.

Effects of fencamfamine withdrawal in rats.

1. The effects produced by discontinuation of long-term treatment with fencamfamine (FCF) were evaluated recording behavioral and body weight changes. 2. 48 hr after withdrawal of FCF rats showed a significant decrease in exploratory behavior when compared to saline-treated ones. 3. Discontinuation of treatment with FCF resulted in a significant increase in body weight on days of drug withdrawal. 4. These results suggest that FCF caused signs of withdrawal similar to other psychostimulant drugs.

Daily variation in plasma concentration of fencamfamine and striatal dopamine receptors in rats.

Fencamfamine (FCF) is a psychostimulant drug classified as an indirect dopamine agonist. In the present study we evaluated the daily variation in plasma FCF concentration and in striatal dopamine receptors. Adult male Wistar rats (250-300 g) maintained on a 12-h light/12-h dark cycle (lights on at 07:00 h) were used. Rats received FCF (10.0 mg/kg, ip) at 09:00, 15:00, 21:00 or 03:00 h and blood samples were collected 30 (N = 6) or 60 (N = 6) min after the injections. Plasma FCF was measured by gas chromatography using an electron capture detector. Two-way ANOVA showed significant differences in FCF concentration when blood samples were collected 30 min after the injection, and the highest value was obtained following injection at 21:00 h. Moreover, at 15:00, 21:00 and 03:00 h, plasma FCF levels were significantly lower 60 min after injection when compared to the 30-min interval. Two other groups of rats (N = 6) were decapitated at 09:00 or 21:00 h and the striata were dissected for the binding assays. The Bmax for [3H]-spiroperidol binding to striatal membranes was higher at 21:00 h, without changes in affinity constant (Kd). In conclusion, plasma FCF levels and dopamine receptors undergo daily variation, a phenomenon that should be considered to explain the circadian time-dependent effects of FCF.

Daily variation in plasma concentration of fencamfamine and striatal dopamine receptors in rats

Fencamfamine (FCF) is a psychostimulant drug classified as an indirect dopamine agonist. In the present study we evaluated the daily variation in plasma FCF concentration and in striatal dopamine receptors. Adult male Wistar rats (250-300 g) maintained on a 12-h light/12-h dark cycle (lights on at 07:00 h) were used. Rats received FCF (10.0 mg/kg, ip) at 09:00, 15:00, 21:00 or 03:00 h and blood samples were collected 30 (N = 6) or 60 (N = 6) min after the injections. Plasma FCF was measured by gas chromatography using an electron capture detector. Two-way ANOVA showed significant differences in FCF concentration when blood samples were collected 30 min after the injection, and the highest value was obtained following injection at 21:00 h. Moreover, at 15:00, 21:00 and 03:00 h, plasma FCF levels were significantly lower 60 min after injection when compared to the 30-min interval. Two other groups of rats (N = 6) were decapitated at 09:00 or 21:00 h and the striata were dissected for the binding assays. The Bmax for [3H]-spiroperidol binding to striatal membranes was higher at 21:00 h, without changes in affinity constant (Kd). In conclusion, plasma FCF levels and dopamine receptors undergo daily variation, a phenomenon that should be considered to explain the circadian time-dependent effects of FCF

Effect of fencamfamine on avoidance performances of rats

The effects of fencamfamine (1.0 and 5.0 mg/kg, ip single dose) on an inhibitory task were studied in rats (N=15 per group).Post-training treatment with fencamfamine (1.0 mg/kg) significantly increased avoidance latency from 23 ñ 3 to 146 ñ 28 and 170 ñ 33 s for training day 1 and day 7, respectively, indicating an enhacement of retention.However, retention was significantly reduced with a high dose of fencamfamine (5.0 mg/kg). These results demonstrate that fencamfamine caused a reproducible dose-related increase and reduction in avoidance latency

Effect of fencamfamine on avoidance performances of rats.

The effects of fencamfamine (1.0 and 5.0 mg/kg, ip, single dose) on an inhibitory task were studied in rats (N = 15 per group). Post-training treatment with fencamfamine (1.0 mg/kg) significantly increased avoidance latency from 23 +/- 3 to 146 +/- 28 and 170 +/- 33 s for training day 1 and day 7, respectively, indicating an enhancement of retention. However, retention was significantly reduced with a high dose of fencamfamine (5.0 mg/kg). These results demonstrate that fencamfamine caused a reproducible dose-related increase and reduction in avoidance latency.