Striatal dopamine metabolism in response to apomorphine: the effects of repeated amphetamine pretreatment

Sensitization and cross-sensitization after chronic treatment with methylphenidate in adolescent Wistar rats

An increasing debate exists about the potential of early exposure to methylphenidate to increase the risk for drug abuse. In addition, little is known about the neurobiological effects of early exposure to methylphenidate. This study was designed to investigate whether chronic treatment with methylphenidate induces behavioral sensitization to subsequent methylphenidate and D-amphetamine challenge in adolescent Wistar rats. Young Wistar rats (P25) were treated with either methylphenidate (1, 2, or 10 mg/kg, intraperitoneally) or saline for 28 days. After 14 days of washout, animals were challenged with methylphenidate 2.5 mg/kg intraperitoneally or D-amphetamine 2 mg/kg intraperitoneally (P67). Locomotor behavior was assessed using the open field test. Rats chronically treated with methylphenidate in the adolescent period showed augmented locomotor sensitization to D-amphetamine but not to methylphenidate in the adult phase. These findings suggest that early exposure do methylphenidate might increase the risk for subsequent D-amphetamine abuse. Further studies focusing on the neurobiological effects of early exposure to methylphenidate are warranted.

Evaluation of reward processes in an animal model of depression

RATIONALE

Anhedonia is a core symptom of major depression. Deficits in reward function, which underlie anhedonia, can be readily assessed in animals. Therefore, anhedonia may serve as an endophenotype for understanding the neural circuitry and molecular pathways underlying depression.

OBJECTIVE

Surprisingly, there is scant knowledge regarding alterations in brain reward function after olfactory bulbectomy (OB), an animal model which results in a behavioural syndrome responsive to chronic antidepressant treatment. Therefore, the present studies aimed to assess reward function after bulbectomy.

MATERIALS AND METHODS

The present study utilized sucrose preference, cocaine-induced hyperlocomotion and intra-cranial self-stimulation (ICSS) responding to examine reward processes in the OB model.

RESULTS

Bulbectomized animals showed a marked preference (>90%) for 0.8% sucrose solution compared with water; similar to the preference exhibited by sham controls. Importantly, there were pronounced deficits in brain reward function, as assessed using ICSS, which lasted 8 days before returning to baseline levels. Furthermore, bulbectomized animals were hyper-responsive to the locomotor stimulating properties of an acute and a repeated cocaine regimen. However, no difference in ICSS facilitation was observed in response to an acute cocaine injection.

CONCLUSIONS

Taken together, these results suggest that bulbectomized rats display alterations in brain reward function, but these changes are not long-lasting and thus, not amenable to investigating the effects of pharmacological interventions. However, given that OB animals are hypersensitive to drugs of abuse, bulbectomy may be an appropriate inducing factor for the development of animal models of co-morbid depression and drug dependence.

Modulation of a 40-kDa catecholamine-regulated protein following D-amphetamine treatment in discrete brain regions

A 40-kDa catecholamine-regulated protein (CRP40) has been demonstrated to be expressed in the central nervous system, and is known to bind to dopamine and related catecholamines. Recently, it has been shown that dopamine D1 receptor antagonist and dopamine D2 receptor antagonist differentially modulated the CRP40 protein in the striatum. In the present study, we examined the effects of the indirect psychostimulant, D-amphetamine, on (CRP40) expression in discrete brain regions. The technique of Western immunoblotting was utilized for quantitation of CRP40 in different experimental paradigms following D-amphetamine treatment. Acute treatment with D-amphetamine (5.0 mg/kg, i.p.) caused no significant change in CRP40 levels in either of the two brain regions studied: striatum and nucleus accumbens. Chronic D-amphetamine administration (2.5 mg/kg, i.p.) significantly increased CRP40 levels in striatum and nucleus accumbens (37.64 +/- 14.57% and 27.86 +/- 8.40%, respectively, P < or = 0.05). Chronic and possibly sensitized D-amphetamine challenged rats (0.5 mg/kg, i.p.) showed a significant increase in CRP40 levels in the nucleus accumbens only (40.49 +/- 15.91%, P < or = 0.05). Although CRP40 has a consensus motif with the 70-kDa heat shock protein (HSP70), levels of HSP70 remained unchanged under identical experimental conditions. The results of this study demonstrate selective modulation of CRP40 by D-amphetamine treatment, without affecting the 70-kDa heat shock protein.

Alterations in extracellular and tissue levels of biogenic amines in rat brain induced by the serotonin2 receptor antagonist, ritanserin

Systemic administration of ritanserin elicited rapid changes in dopamine (DA) and serotonin (5-HT) levels in both dialysate and neuronal tissue extracts. These effects occurred in both a site-selective and a dose-related manner. Increases in extracellular levels of DA and 5-HT in the nucleus accumbens were maximal at 120-140 min after treatment. A dose of 0.63 mg/kg of ritanserin elicited larger and more prolonged increases in extracellular DA and 5-HT levels than did the 0.3 mg/kg dose. By contrast, 0.63 mg/kg of ritanserin elicited no changes in either DA or 5-HT levels with dialysate collected from the striatum. Ritanserin also induced dose-related decreases in tissue levels of DA and 5-HT from the nucleus accumbens. The site specificity of action was again noted in that there were no dose-dependent decreases in tissue levels of DA or 5-HT measured from the striatum. Ritanserin exerted little effect on metabolite levels from either dialysate or tissue extracts. Taken together, these findings show that selective 5-HT2 receptor antagonism modulates DA and 5-HT neurotransmission in a specific manner. These actions appear to involve increased release of DA and 5-HT rather than significant changes in metabolism. These findings add further weight to the importance of 5-HT2 receptor interactions as an important component of antipsychotic activity.

Chronic stress induces strain-dependent sensitization to the behavioral effects of amphetamine in the mouse

Following 10 days of daily restraint stress, sensitization developed to the stimulatory effect of amphetamine on locomotion in DBA/2 but not in C57BL/6 mice tested 24 h after the last stressful experience regardless of their being naive or habituated to the test cages. Saline-injected C57BL/6 mice, however, showed an increase of locomotion 24 h after chronic stress treatment. Chronically stressed mice of the two strains did not exhibit any alteration of dopamine and metabolites (3-4-dihydroxphenylacetic acid, homovanillic acid, and 3-methoxytyramine) levels in the frontal cortex, caudatus putamen, or nucleus accumbens septi, thus ruling out that stress-induced alteration of basal dopamine metabolism affected the behavioral response to amphetamine challenging in DBA/2 mice. Ten daily amphetamine injections (5 mg/kg) did not significantly modify the behavioral response to amphetamine in either strain of mice tested 24 h after the end of the chronic treatment and did not increase locomotion in saline-injected C57BL/6 mice. Finally, chronically stressed hybrids B6D2F1 did not show sensitization to the locomotor effects of amphetamine, suggesting a dominant mode of inheritance in the response to chronic stress of the C57BL/6 strain.

Genotype-dependent effects of chronic stress on apomorphine-induced alterations of striatal and mesolimbic dopamine metabolism

After 10 daily consecutive restraint experiences, DBA/2 (DBA) mice showed an increase of climbing behavior after injection of 0.25 mg/kg of the dopamine (DA) agonist apomorphine (APO), while no changes were observed following vehicle or 1 mg/kg of APO. By contrast, chronically stressed C57BL/6 (C57) mice showed a clear-cut decrease of climbing behavior at the dose of 0.25 mg/kg of APO and a similar, although less pronounced, effect of stress on the behavior of mice injected either with vehicle or with 1 mg/kg APO. The DA agonist at these same doses decreased 3,4-dihydroxyphenylacetic acid (DOPAC), homovanillic acid (HVA) and 3-methoxytyramine (3-MT) concentrations in the caudatus putamen (CP) and nucleus accumbens septi (NAS) of both strains. Higher DOPAC, HVA and 3-MT concentrations were evident in stressed DBA mice receiving 0.25 mg/kg but not 1 mg/kg of APO, in both CP and NAS. Concerning C57 mice, lower concentrations of the 3 metabolites were present at both doses of APO in the NAS of stressed mice in comparison with non-stressed animals, while no significant stress-related effects were evident in the CP. Non-significant differences between control and stressed mice of both strains were evident as regards DA concentrations in CP and NAS. These results suggest that repeated stressful experiences lead to a hyposensitivity of DA presynaptic receptors in DBA mice while they produce a sensitization of mesolimbic DA presynaptic receptors possibly accompanied by down-regulation of postsynaptic DA receptors in the C57 strain.

Subchronic methamphetamine treatment enhances methamphetamine- or cocaine-induced dopamine efflux in vivo

Intracerebral dialysis was used to study the mechanism underlying behavioral sensitization. Rats were divided into two groups: a control group that received intraperitoneal injections of saline and an experimental group that was given methamphetamine (MAP) (4 mg/kg) once a day for 14 days. Seven days after the last injection, dopamine (DA) and its metabolites were measured in striatal dialysates obtained from awake freely moving rats. A challenge injection of MAP (4 mg/kg) caused a marked increase in the extracellular concentrations of DA, and the extent of the increase was significantly greater in MAP-pretreated rats than in the saline-pretreated controls. A challenge injection of cocaine (20 mg/kg) also caused a significantly greater increase in extracellular DA levels in MAP-pretreated rats than in saline-pretreated rats. These results suggest that an enhancement in striatal DA efflux may play an important role in MAP-induced behavioral sensitization and cross-sensitization to cocaine.

Regulation of dopamine release by the autoreceptor after repeated methamphetamine treatment in rats: an intracerebral dialysis study

A group of rats received intraperitoneal injections of methamphetamine (MAP) once a day for 14 days. Seven days after the last injection, the striatal dopamine efflux and its inhibition by a low dose (100 micrograms/kg) of apomorphine were examined by in vivo intracerebral dialysis. The extent of inhibition was not altered after the MAP pretreatment, which does not suggest the presence of subsensitivity of dopamine auto-receptors.

The response of apomorphine administered into the accumbens in rats with bilateral lesions of the nucleus accumbens, induced with 6-hydroxydopamine

Bilateral lesions of the nucleus accumbens, induced with 6-hydroxydopamine, reduced motor activity and produced a 20-35% depletion of the concentrations of dopamine (DA) and its main metabolites 3,4-dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA). Small doses of apomorphine (1-10 ng), injected into the nucleus accumbens of sham-lesioned rats, decreased motor activity, while larger doses (1-10 micrograms) produced hyperactivity. In rats lesioned with 6-hydroxydopamine, apomorphine caused hyperactivity only, and this apomorphine-induced response was more pronounced than in sham-lesioned rats. Large doses of apomorphine decreased, only in sham-lesioned animals, the levels of DOPAC and HVA. These data suggest that the apomorphine-induced hypomotility is mediated by presynaptically located DA receptor systems in the nucleus accumbens, whereas the apomorphine-induced hypermotility is likely to be mediated by postsynaptically located DA receptor systems.

Apomorphine enantiomers' effects on dopamine metabolism: receptor and non-receptor related actions

The enantiomers of apomorphine (APO) inhibited dopamine synthesis in rat striatal synaptosomes, with R(-)-APO being about twice as potent as S(+)-APO. Sulpiride, a DA receptor antagonist, partially antagonized the inhibitory effects of only (-)-APO, suggesting that (-)-APO's, but not (+)-APO's, effects on dopamine synthesis may be at least partially receptor-mediated. The addition of 6-methyl-5,6,7,8-tetrahydrobiopterine (6-MPH4), an artificial cofactor for tyrosine hydroxylase, partially antagonized the inhibitory effects of both enantiomers, being considerably more effective against the (+)enantiomer. These data suggest that the APO enantiomers may directly inhibit enzymes within the synaptosome which regulate dopamine synthesis. Furthermore, investigations measuring DA synthesis rates in synaptosomes that had been pre-incubated with (-)-APO and then washed to remove the (-)-APO in the medium, indicate that (-)-APO may be retained by synaptosomes. Preliminary studies measuring the accumulation of [3H](-)-APO by synaptosomes also suggest that synaptosomes can accumulate APO. Although both APO enantiomers suppressed DA synthesis in vitro, only (-)-APO reduced striatal DA metabolite concentrations in vivo, and this reduction was prevented by haloperidol, a DA receptor antagonist. In addition, 6-MPH4 prevented the decrease in the DA metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) produced by (-)-APO but not the decrease in the DA metabolite homovanillic acid (HVA).

Alterations in dopamine metabolism after chronic administration of haloperidol. Possible role of increased autoreceptor sensitivity

Basal levels of the metabolites of dopamine (DA) were reduced following the chronic administration of haloperidol. The ability of small doses (50 or 100 micrograms/kg) of apomorphine to reduce the concentrations of the metabolite of DA, homovanillic acid (HVA), was also enhanced following the chronic administration of haloperidol. In addition, the accumulation of DA and of the metabolite of DA, 3-methoxytyramine (3-MT), in rats treated with pargyline was reduced following the chronic administration of haloperidol, suggesting that basal turnover and release of DA also may be reduced. These findings are discussed in relation to possible changes in the sensitivity of DA autoreceptors and the activity of DA-containing neurons.