Differential psychostimulant-induced activation of neural circuits in dopamine transporter knockout and wild type mice

Persistence of one-trial cocaine-induced behavioral sensitization in young rats: regional differences in Fos immunoreactivity

RATIONALE

Unlike adult rats, young rats exhibit context-dependent and context-independent behavioral sensitization when assessed after a single pretreatment injection of cocaine.

OBJECTIVE

The purpose of this study was to determine whether: (1) the context-dependent and context-independent sensitization of young rats can be dissociated based on the persistence of the sensitized response and (2) the expression of behavioral sensitization is associated with region-specific increases in Fos immunoreactivity (Fos-IR).

MATERIALS AND METHODS

On postnatal day (PD) 19, rats were injected with either saline or cocaine (30 mg/kg) in a novel test chamber or the home cage. After 1, 3, 5, 7, 14, or 61 abstinence days, rats were challenged with 20 mg/kg cocaine and locomotor activity was measured for 60 min. In a separate experiment, rats pretreated on PD 19 were challenged with cocaine (10-30 mg/kg) on PD 80.

RESULTS

The sensitized responding of young rats persisted for the same length of time (5 days) regardless of whether cocaine pretreatment occurred in a novel environment or the home cage. Behavioral sensitization did not reemerge in adulthood. When assessed after three abstinence days (i.e., on PD 22), acute treatment with cocaine increased Fos-IR in various brain regions, but sensitized responding was associated with elevated Fos expression in only the caudate-putamen (CP) and prefrontal cortex (PFC).

CONCLUSIONS

Persistence of the sensitized response cannot be used to dissociate the one-trial context-dependent and context-independent sensitization of young rats. Fos data indicate that the CP and PFC may be involved in the mediation of short-term behavioral sensitization on PD 22.

Differential sensitivity of the perioculomotor urocortin-containing neurons to ethanol, psychostimulants and stress in mice and rats

The perioculomotor urocortin-containing population of neurons (pIIIu: otherwise known as the non-preganglionic Edinger-Westphal nucleus) is sensitive to alcohol and is involved in the regulation of alcohol intake. A recent study indicated that this brain region is also sensitive to psychostimulants. Since pIIIu has been shown to respond to stress, we investigated how psychostimulant-induced pIIIu activation compares to stress- and ethanol-induced activation, and whether it is independent from a generalized stress response. Several experiments were performed to test how the pIIIu responds to psychostimulants by quantifying the number of Fos immunoreactive nuclei after acute i.p. injections of saline, 10-30 mg/kg cocaine, 5 mg/kg methamphetamine, 5 mg/kg amphetamine, 2.5 g/kg ethanol, 2 h of restraint stress, 10 min of swim stress, or six applications of mild foot shock in male C57BL/6 J mice. We also compared Fos immunoreactivity in pIIIu after acute (20 mg/kg cocaine) and repeated cocaine exposure (7 days of 20 mg/kg cocaine) injections in male C57BL/6 J mice in order to investigate the potential habituation of this response. Finally, we quantified the number of Fos immunoreactive nuclei in pIIIu after administration of saline, 2.5 g/kg ethanol, 20 mg/kg cocaine, or 2 h of restraint stress in male Sprague-Dawley rats. We found that exposure to psychostimulants and ethanol induced significantly higher Fos levels in pIIIu compared to stress in mice. Furthermore, repeated cocaine injections did not decrease Fos immunoreactivity as would be expected if this response were due to stress. In rats, exposure to ethanol, psychostimulant and restraint stress all induced pIIIu Fos immunoreactivity compared to saline-injected controls. In both mice and rats, ethanol- and cocaine-induced Fos immunoreactivity occurred exclusively in urocortin 1-positive, but not in tyrosine hydroxylase-positive, cells. These results provide evidence that the pIIIu Fos-response to psychostimulants is independent of a generalized stress in mice, but not rats. They additionally show that the pIIIu response to stress differs significantly between species.

Atypical antipsychotics clozapine and quetiapine attenuate prepulse inhibition deficits in dopamine transporter knockout mice

Sensorimotor gating disruptions are seen in various psychiatric illnesses with putatively different pathologies, including schizophrenia and bipolar disorder. Interestingly, mice lacking the dopamine (DA) transporter (DAT) gene display markedly increased levels of DA, deficits in sensorimotor gating, and hyperactivity relative to wild-type mice. Atypical antipsychotics are effective treatments of schizophrenia and manic symptoms, presumably in part by antagonizing DA receptors. Here we report that treatment with clozapine (3 mg/kg) or quetiapine (2.5 mg/kg) attenuated prepulse inhibition deficits in male DAT knockout mice. Thus male DAT knockout mice may provide a useful animal model for predicting the efficacy of novel drugs in treating psychiatric illnesses characterized by a dysregulated DA system.

Effects of pharmacological stressors on c-fos and CRF mRNA in mouse brain: relationship to alcohol seeking

A marked heterogeneity exists among stressors in their ability to reinstate alcohol seeking in rats. We have reported that the pharmacological stressor yohimbine, an alpha-2 adrenoceptor antagonist, potently reinstated alcohol seeking, but FG-7142, a benzodiazepine inverse agonist was ineffective. In rats, we determined that yohimbine elicits patterns of brain expression of the mRNAs for c-fos, a marker of neuronal activation, and corticotropin-releasing factor (CRF) a stress-related peptide, distinct from that produced by FG-7142. The purpose of the present experiment is to determine if these differential effects of yohimbine and FG-7142 on regional c-fos and CRF mRNA expression generalize to another animal commonly used in alcohol research, the C57 BL/6J mouse. In comparing the results of the present study to those of our previous one, we found a number of commonalities in the patterns of activation elicited by yohimbine and FG-7142 between the two species, and some notable differences. As we found in the rat, yohimbine selectively increased c-fos mRNA in the mouse NACs, BLA and CeA. Yohimbine increased CRF mRNA only in the mouse PVN, but was without effect on CRF mRNA in extrahypothalamic sites, the BNST and CeA. This differs from what we saw in the rat, where yohimbine increased CRF mRNA in these extrahypothalamic regions, but not the PVN. The selective induction of c-fos in the NACs, BLA and CeA of mice and rats by yohimbine offers further support for the idea that activation of these structures participates in reinstatement induced by such stressors.

The evolution of drug development in schizophrenia: past issues and future opportunities

Schizophrenia is a disease syndrome with major public health implications. The primary advance in pharmacotherapeutics was in 1952 with the introduction of antipsychotic medications (ie, chlorpromazine, dopamine D2 antagonism). Barriers to progress have been substantial, but many will be subject to rapid change based on current knowledge. There are attractive psychopathology indications for drug discovery (eg, impaired cognition and negative symptoms), and drugs with efficacy in these domains may have application across a number of disease classes. These pathologies are observed prior to psychosis raising the possibility of very early intervention and secondary prevention. Success in drug discovery for cognition and negative symptom pathologies may bring forth issues in ethics as the potential for enhancing normal function is explored.

Dopamine transporter mutant mice in experimental neuropharmacology

An opportunity to perform targeted genetic manipulations in mice has provided another dimension for modern pharmacological research. Genetically modified mice have become important tools to investigate functions of previously unexplored proteins, define mechanism of action of new and known pharmacological drugs, and validate novel targets for treatment of human disorders. One of the best examples of such use of genetic models in experimental pharmacology represents investigations involving mice deficient in the gene encoding the dopamine transporter (DAT). The dopamine transporter tightly regulates the extracellular dynamics of dopamine by recapturing released neurotransmitter into the presynaptic terminals, and genetic deletion of this protein results in profound alterations in both the presynaptic homeostasis and the extracellular dynamics of dopamine. By using this model of severe dopaminergic dysregulation, significant progress has been made in defining the major target of psychotropic drugs, understanding the mechanisms of their action, unraveling novel signaling events relevant for dopaminergic transmission, and mapping neuronal pathways involved in dopamine-related behaviors. Furthermore, DAT mutant mice provided an opportunity to model in vivo conditions of extreme dopaminergic dysfunction that could be relevant for human disorders such as ADHD, schizophrenia, and Parkinson's disease and, thus, could serve as test systems for developing novel treatments for these and related disorders.

Functional internal complexity of amygdala: focus on gene activity mapping after behavioral training and drugs of abuse

The amygdala is a heterogeneous brain structure implicated in processing of emotions and storing the emotional aspects of memories. Gene activity markers such as c-Fos have been shown to reflect both neuronal activation and neuronal plasticity. Herein, we analyze the expression patterns of gene activity markers in the amygdala in response to either behavioral training or treatment with drugs of abuse and then we confront the results with data on other approaches to internal complexity of the amygdala. c-Fos has been the most often studied in the amygdala, showing specific expression patterns in response to various treatments, most probably reflecting functional specializations among amygdala subdivisions. In the basolateral amygdala, c-Fos expression appears to be consistent with the proposed role of this nucleus in a plasticity of the current stimulus-value associations. Within the medial part of the central amygdala, c-Fos correlates with acquisition of alimentary/gustatory behaviors. On the other hand, in the lateral subdivision of the central amygdala, c-Fos expression relates to attention and vigilance. In the medial amygdala, c-Fos appears to be evoked by emotional novelty of the experimental situation. The data on the other major subdivisions of the amygdala are scarce. In conclusion, the studies on the gene activity markers, confronted with other approaches involving neuroanatomy, physiology, and the lesion method, have revealed novel aspects of the amygdala, especially pointing to functional heterogeneity of this brain region that does not fit very well into contemporarily active debate on serial versus parallel information processing within the amygdala.

Adolescent maturation of cocaine-sensitive neural mechanisms

Both clinical and animal studies have shown that adolescents undergo a late maturation of the central nervous system, which may underlie adolescent typical behaviors. In particular, decreased behavioral response to cocaine has been found in adolescents as compared to adults. In the present study, cocaine was used as a tool to explore adolescent brain maturation. Juvenile (postnatal day (P) 27), adolescent (P37), and adult (P90) male Sprague-Dawley rats were treated acutely with cocaine (750 microg/kg/injection x 2, i.v.), and c-fos mRNA expression, a marker of neuronal activation, was evaluated by in situ hybridization. Cocaine-induced c-fos mRNA was similar across ages in the dorsal caudate putamen (CPu), nucleus accumbens, and lateral bed nucleus of the stria terminalis. In contrast, there was a diminished response in juvenile/adolescent ventral CPu and in juvenile central nucleus of the amygdala, and an increased response in juvenile/adolescent cortex. Further studies evaluated the mechanism of the late maturation of cocaine response in ventral CPu. No significant age differences were observed in regional dopamine (DA) transporter binding. Although striatal DA content was significantly reduced at P27 as compared to adult, there was no difference between dorsal and ventral subregions. In contrast, basal- and cocaine-induced extracellular DA overflow, as measured by in vivo microdialysis, was lower in juvenile ventral CPu than in the adults. This age difference was not observed in dorsal CPu. These findings suggest that impulse activity in DA afferents to ventral CPu is immature in adolescents. In conclusion, the present study showed that cocaine-sensitive neuronal circuits continue to mature during adolescence.

Dysregulation of gene induction in corticostriatal circuits after repeated methylphenidate treatment in adolescent rats: differential effects on zif 268 and homer 1a

Psychostimulants and other dopamine agonists produce molecular changes in neurons of cortico-basal ganglia-cortical circuits, and such neuronal changes are implicated in behavioural disorders. Methylphenidate, a psychostimulant that causes dopamine overflow (among other effects), alters gene regulation in neurons of the striatum. The present study compared the effects of acute and repeated methylphenidate treatment on cortical and striatal gene regulation in adolescent rats. Changes in the expression of the immediate-early genes zif 268 and homer 1a were mapped in 23 striatal sectors and 22 cortical areas that provide input to these striatal sectors, in order to determine whether specific corticostriatal circuits were affected by these treatments. Acute administration of methylphenidate (5 mg/kg, i.p.) produced modest zif 268 induction in cortical areas. These cortical zif 268 responses were correlated in magnitude with zif 268 induction in functionally related striatal sectors. In contrast, after repeated methylphenidate treatment (10 mg/kg, 7 days), cortical and striatal gene induction were dissociated. In these animals, the methylphenidate challenge (5 mg/kg) produced significantly greater gene induction (zif 268 and homer 1a) in the cortex. This enhanced response was widespread but regionally selective, as it occurred predominantly in premotor, motor and somatosensory cortical areas. At the same time, striatal gene induction was partly suppressed (zif 268) or unchanged (homer 1a). Thus, repeated methylphenidate treatment disrupted the normally coordinated gene activation patterns in cortical and striatal nodes of corticostriatal circuits. This drug-induced dissociation in cortical and striatal functioning was associated with enhanced levels of behavioural stereotypies, suggesting disrupted motor switching function.

Neurobiology of animal models of attention-deficit hyperactivity disorder

Attention-deficit hyperactivity disorder (ADHD) is a heterogeneous, highly heritable, disorder resulting from complex gene-gene and gene-environment interactions. The defining symptoms of hyperactivity, impulsivity and impaired sustained attention are not unique to ADHD. It is therefore not surprising that animals with distinctly different neural defects model the behavioural characteristics of the disorder. Consistent with ADHD being a developmental disorder, animal models are either genetic (spontaneously hypertensive rats (SHR), dopamine transporter (DAT) knock-out mice, SNAP-25 mutant mice, mice expressing a mutant thyroid receptor) or have suffered an insult to the central nervous system during the early stages of development (anoxia, 6-hydroxydopamine). It appears that neural transmission is impaired by either direct disruption of dopaminergic transmission or a more general impairment of neurotransmission that gives rise to compensatory changes in monoaminergic systems that are not sufficient to completely normalize neural function. In general, results obtained with animal studies suggest that dopamine neurons are functionally impaired. However, evidence obtained from some animal models suggests that the noradrenergic and serotonergic neurotransmitter systems may be the target of drugs that ameliorate ADHD symptoms.

The role of the plasmalemmal dopamine and vesicular monoamine transporters in methamphetamine‐induced dopaminergic deficits

Amphetamine (AMPH) and methamphetamine (METH) are members of a collection of phenethylamine psychostimulants that are commonly referred to collectively as "amphetamines." Amphetamines exert their effects, in part, by affecting neuronal dopamine transport. This review thus focuses on the effects of AMPH and METH on the plasmalemmal dopamine transporter and the vesicular monoamine transporter-2 in animal models with a particular emphasis on how these effects, which may vary for the different stereoisomers, contribute to persistent dopaminergic deficits.

Methylphenidate regulates activity regulated cytoskeletal associated but not brain-derived neurotrophic factor gene expression in the developing rat striatum

Methylphenidate (MPH) is a psychostimulant drug used to treat attention deficit hyperactivity disorder in children. To explore the central effects of chronic MPH, we investigated the expression of an effector immediate early gene, activity regulated cytoskeletal associated (arc), and the neurotrophin, brain-derived neurotrophic factor (bdnf) in the brain of immature and adult rats following repeated MPH. Prepubertal (postnatal day (PD) 25-38) and adult (PD 53-66) male rats were injected once daily for: a) 14 days with saline or MPH (2 or 10 mg/kg; s.c.) or b) 13 days with saline followed by a single dose of MPH (2 or 10 mg/kg; s.c.). To determine possible long-term effects of MPH, prepubertal rats were allowed a drug-free period of 4 weeks following the 14 days of treatment, and then were given a challenge dose of MPH. We demonstrated, for the first time, that an acute injection of MPH increased levels of activity-regulated cytoskeletal protein (ARC) and arc mRNA in the prepubertal rat striatum and cingulate/frontal cortex. This response was significantly attenuated by chronic MPH. The desensitization in arc expression observed in prepubertal rats persisted in the adult striatum following a later MPH challenge. In contrast to these data we observed little effect of MPH on bdnf expression. We also developed an effective, non-stressful technique to treat freely moving immature rats with oral MPH. Consistent with the results described above, we observed that oral MPH (7.5 and 10 mg/kg) also increased arc expression in the prepubertal rat striatum. However, unlike the effects of injected MPH, repeated oral MPH (7.5 mg/kg) did not alter the normal arc response. This result raises the important possibility that oral doses of MPH that reproduce clinically relevant blood levels of MPH may not down-regulate gene expression, at least in the short term (14 days). We confirmed, using mass spectrometry, that the oral doses of MPH used in our experiments yielded blood levels within the clinical range observed in children. The novel oral administration paradigm that we describe thus provides a clinically relevant animal model to further explore the effects of chronic drug exposure on central gene expression in the developing rat brain.

Genetic and Structural Analysis of the Basolateral Amygdala Complex in BXD Recombinant Inbred Mice

The amygdala integrates and coordinates emotional and autonomic responses. The genetics that underlie variation in amygdala structure may be coupled to variation in levels of aggression, fear, anxiety, and affiliated behaviors. We systematically quantified the volume and cell populations of the basolateral amygdala complex (BLAc) across 35 BXD recombinant inbred (RI) lines, the parental strains--C57BL/6J (B6) and DBA/2J (D2)--and F1 hybrids (n cases=199, bilateral analysis). Neuron number and volume vary 1.7- to 2-fold among strains (e.g., neuron number ranged from 88,000 to 170,000). Glial and endothelial populations ranged more widely (5- to 8-fold), in part because of higher technical error. A quantitative trait locus (QTL) for the BLAc size is located on chromosome (Chr) 8 near the Large gene. This locus may also influence volume of other regions including hippocampus and cerebellum. Cell populations in the BLAc appear to be modulated more weakly by loci on Chrs 11 and 13. Candidate genes were selected on the basis of correlation with BLAc traits, chromosomal location, single nucleotide polymorphism (SNP) density, and expression patterns in the Allen Brain Atlas. Neurod2, a gene shown to be significant for the formation of the BLAc by knockout studies, is among the candidates genes. Other candidates include Large, and Thra. Responses to drugs of abuse and locomotor activity were the most notable behavioral correlates of the BLAc traits.

Dopamine transporter density and behavioral response to methylphenidate in a hyperlocomotor rat model

Rats exposed prenatally to 5-bromo-2'-deoxyuridine (BrdU-rats) display hyperlocomotive activity, making them a possibly useful animal model for the study of attention deficit hyperactivity disorder (ADHD). Using this model, we investigated dopamine transporter (DAT) density and behavioral outcomes in BrdU-rats, some of which were also administered methylphenidate, a psychostimulant that is widely used for the treatment of ADHD. Pregnant rats were exposed to BrdU from gestational day 9 through 15. In male offspring, DAT densities in different regions of the striatum were quantified at three weeks of age. At seven weeks of age, locomotor, rearing and grooming behaviors were evaluated in an open-field setting, with or without methylphenidate treatment (1 mg/kg or 4 mg/kg). The results revealed no significant changes in striatal DAT densities in BrdU-rats compared with controls. Extreme hyperlocomotion of BrdU-rats was detected in the open-field environment, an effect that was exacerbated following treatment with the lower and higher dose of methylphenidate. Such increase in locomotor activity was observed only with the higher dose in control animals. In summary, degeneration of dopaminergic neurons in the terminal field was not detected in juvenile BrdU-rats, although adult animals displayed hyperactive behavior in a mildly stressful environment as well as hypersensitivity to a psychostimulant that facilitates dopaminergic neurotransmission.

Transgenic mice expressing a human mutant beta1 thyroid receptor are hyperactive, impulsive, and inattentive

Attention deficit hyperactivity disorder (ADHD) is the most commonly diagnosed childhood psychiatric disorder. We have found that a transgenic mouse bearing a human mutant thyroid receptor (TRbeta1) expresses all of the defining symptoms of ADHD--inattention, hyperactivity, and impulsivity--as well as a 'paradoxical' response to methylphenidate (MPH). As with ADHD, the behavioral phenotypes expressed by the TRbeta transgenic mice are dynamic and sensitive to changes in environmental conditions, stress, and reinforcement. TRbeta transgenic mice are euthyroid except for a brief period during postnatal development, but the behavioral phenotypes, elevated dopamine turnover, and paradoxical response to MPH persist into adulthood. Thus, like the vast majority of children with ADHD, the TRbeta transgenic mice exhibit the symptoms of ADHD in the complete absence of thyroid abnormalities. This suggests that even transient perturbations in developmental thyroid homeostasis can have long-lasting behavioral and cognitive consequences, including producing the full spectrum of symptoms of ADHD.

Methylphenidate exerts no neurotoxic, but neuroprotective effects in vitro

Methylphenidate (MPH) is the most common used drug in child and adolescent psychiatry. Despite of this fact, however, little is known about its exact pharmacological mechanisms. Here we investigated the toxic effects of MPH in vitro in human embryonic kidney (HEK-293) cells stably expressing the human dopamine transporter (HEK-hDAT cells) and in cultured rat embryonic (E14.5) mesencephalic cultures. MPH alone (up to 1 mM) affected neither the growth of HEK-hDAT cells nor the survival of dopaminergic (DA) neurons in primary cultures after treatment up to 72 h. No differences in neuronal arborisation or in the density of synapses were detected. 1-methyl-4-phenylpyridinium (MPP(+)) showed no toxic effect in HEK-293 cells, but had significant toxic effects in HEK-hDAT cells and DA neurons. MPH (1 microM - 1 mM) dose-dependently reduced this cytotoxicity in HEK-hDAT cells and primary mesencephalic DA neurons. The presented results show that application of MPH alone does not have any toxic effect on DA cells in vitro. The neurotoxic effects of MPP(+) could be significantly reduced by co-application of MPH, an effect that is most likely explained by MPH blocking the DAT.

Inhibition of methylphenidate-induced gene expression in the striatum by local blockade of D1 dopamine receptors: interhemispheric effects

Psychostimulants change the function of cortico-basal ganglia circuits. Some of these effects are mediated by altered gene regulation in projection neurons of the striatum which participate in these circuits. Psychostimulant-induced changes in gene expression in these neurons are a consequence of excessive stimulation of G-protein-coupled receptors, particularly the D1 dopamine receptor subtype. Recent findings show that the psychostimulant methylphenidate, which causes dopamine overflow in the striatum, produces changes in striatal gene regulation similar, but not identical, to those induced by psychostimulants such as cocaine and amphetamine. We investigated, in rats, the role of striatal D1 receptors in methylphenidate-induced gene expression, by intrastriatal administration of the D1 receptor antagonist SCH-23390. Effects on the expression of two plasticity-related molecules, the transcription factor zif 268 and the synaptic plasticity factor Homer 1a, in the striatum and cortex were assessed. Intrastriatal infusion of SCH-23390 (2-10 microg) attenuated zif 268 and Homer 1a mRNA expression induced by methylphenidate (10 mg/kg, i.p.) in a dose-dependent manner. Moreover, this unilateral SCH-23390 infusion not only inhibited gene induction at the infusion site in the central striatum, but also in distant striatal regions including the nucleus accumbens, as well as throughout the entire contralateral striatum. These results indicate that striatal D1 receptors are critical for gene induction by methylphenidate. Moreover, the ipsilateral and contralateral effects of local SCH-23390 administration suggest that D1 receptor-stimulated striatal output exerts robust control over widespread striatal activities/gene expression via regulation of input to the striatum.

Regulation of psychostimulant-induced preprodynorphin, c-fos and zif/268 messenger RNA expression in the rat dorsal striatum by mu opioid receptor blockade

Several studies have shown that psychostimulants can induce differential immediate early gene and neuropeptide expression in the patch versus matrix compartments of dorsal striatum. The patch compartment contains a high density of mu opioid receptors and activation of these receptors may contribute to psychostimulant-induced gene expression in the patch versus matrix compartments of dorsal striatum. However, the contribution of mu opioid receptor activation to psychostimulant-induced changes in gene expression in the patch compartment of dorsal striatum has not been examined. The current study examined the role of mu opioid receptors in psychostimulant induction of preprodynorphin, c-fos and zif/268 messenger RNA expression in the patch versus matrix compartments of dorsal striatum. Male Sprague-Dawley rats were treated with the mu opioid receptor antagonist, clocinnamox (1 mg/kg, s.c.), 24 h prior to treatment with cocaine (30 mg/kg, i.p.) or methamphetamine (15 mg/kg, s.c.) and sacrificed 45 min or 3 h later. Mu opioid receptor antagonism blocked psychostimulant-induced preprodynorphin messenger RNA expression only in the rostral patch compartment, whereas psychostimulant-induced zif/268 messenger RNA expression in the patch and matrix compartments was attenuated throughout the dorsal striatum. Clocinnamox pretreatment had no effect on stimulant-induced increases in c-fos expression. These data suggest that mu opioid receptor activation plays a specific role in psychostimulant-induced preprodynorphin messenger RNA expression in the rostral patch compartment and zif/268 messenger RNA expression throughout dorsal striatum.

Expression of c-Fos in the mouse Edinger-Westphal nucleus following ethanol administration is not secondary to hypothermia or stress

Restraint stress, lipopolysaccharide (LPS), and ethanol (EtOH) administration have all been found to induce c-Fos in the brain, and to cause hypothermia. The present study was designed to assess whether the c-Fos expression that occurs in the Edinger-Westphal nucleus (EW) after EtOH administration is independent of the hypothermia or any stress effects that occur. To test this, we used restraint stress and LPS in addition to EtOH, and also examined two control areas, the dorsal raphe nucleus (DRN) and the periaqueductal gray (PAG), in addition to EW. Male C57BL6/J mice were used. Groups of mice received intraperitoneal (IP) injections of EtOH (2 g/kg), LPS (600 microg/kg or 50 microg/kg), or saline. A separate group of mice received no injection, but were placed in plastic restrainers for the entirety of the experiment. For all groups, core temperatures were monitored rectally every 30 min for 3 h postinjection, after which, the animals were sacrificed. Then, the number of Fos-positive cells in the brain regions of the EW, DRN, and PAG was quantified. Both EtOH and restraint stress induced a transient hypothermia, where core temperature (Tc) declined immediately and then rose again. Both doses of LPS induced a slower developing, longer lasting hypothermia, while saline had no effect on Tc. Only EtOH induced a significant amount of c-Fos in EW, while both doses of LPS and restraint stress induced c-Fos in DRN, and only restraint stress caused induction in PAG. These data demonstrate that activation of EW after EtOH is unrelated to hypothermia or stress.

Chronic cocaine administration switches corticotropin-releasing factor2 receptor-mediated depression to facilitation of glutamatergic transmission in the lateral septum

Corticotropin-releasing factor (CRF) and urocortin (Ucn I) are endogenous members among a family of CRF-related peptides that activate two different and synaptically localized G-protein-coupled receptors, CRF1 and CRF2. These peptides and their receptors have been implicated in stress responses and stress with cocaine abuse. In this study, we observed significant alterations in excitatory transmission and CRF-related peptide regulation of excitatory transmission in the lateral septum mediolateral nucleus (LSMLN) after chronic cocaine administration. In brain slice recordings from the LSMLN of control (saline-treated) rats, glutamatergic synaptic transmission was facilitated by activation of CRF1 receptors with CRF but was depressed after activation of CRF2 receptors with Ucn I. After acute withdrawal from a chronic cocaine administration regimen, CRF1 activation remained facilitatory, but CRF2 activation facilitated rather than depressed LSMLN EPSCs. These alterations in CRF2 effects occurred through both presynaptic and postsynaptic mechanisms. In saline-treated rats, CRF1 and CRF2 coupled predominantly to protein kinase A signaling pathways, whereas after cocaine withdrawal, protein kinase C activity was more prominent and likely contributed to the CRF2-mediated presynaptic facilitation. Neither CRF nor Ucn I altered monosynaptic GABA(A)-mediated IPSCs before or after chronic cocaine administration, suggesting that loss of GABAA-mediated inhibition could not account for the facilitation. This switch in polarity of Ucn I-mediated neuromodulation, from a negative to positive regulation of excitatory glutamatergic transmission after chronic cocaine administration, could generate an imbalance in the brain reward circuitry associated with the LSMLN.

Animal models of attention-deficit hyperactivity disorder

Although animals cannot be used to study complex human behaviour such as language, they do have similar basic functions. In fact, human disorders that have animal models are better understood than disorders that do not. ADHD is a heterogeneous disorder. The relatively simple nervous systems of rodent models have enabled identification of neurobiological changes that underlie certain aspects of ADHD behaviour. Several animal models of ADHD suggest that the dopaminergic system is functionally impaired. Some animal models have decreased extracellular dopamine concentrations and upregulated postsynaptic dopamine D1 receptors (DRD1) while others have increased extracellular dopamine concentrations. In the latter case, dopamine pathways are suggested to be hyperactive. However, stimulus-evoked release of dopamine is often decreased in these models, which is consistent with impaired dopamine transmission. It is possible that the behavioural characteristics of ADHD result from impaired dopamine modulation of neurotransmission in cortico-striato-thalamo-cortical circuits. There is considerable evidence to suggest that the noradrenergic system is poorly controlled by hypofunctional α₂-autoreceptors in some models, giving rise to inappropriately increased release of norepinephrine. Aspects of ADHD behaviour may result from an imbalance between increased noradrenergic and decreased dopaminergic regulation of neural circuits that involve the prefrontal cortex. Animal models of ADHD also suggest that neural circuits may be altered in the brains of children with ADHD. It is therefore of particular importance to study animal models of the disorder and not normal animals. Evidence obtained from animal models suggests that psychostimulants may not be acting on the dopamine transporter to produce the expected increase in extracellular dopamine concentration in ADHD. There is evidence to suggest that psychostimulants may decrease motor activity by increasing serotonin levels. In addition to providing unique insights into the neurobiology of ADHD, animal models are also being used to test new drugs that can be used to alleviate the symptoms of ADHD.

Methylphenidate differentially regulates c-fos and fosB expression in the developing rat striatum

Methylphenidate (MPH, Ritalin) is a psychostimulant drug used in very young children with attention deficit hyperactivity disorder (ADHD). To explore the central effects of MPH, we compared repeated MPH treatments on c-fos and fosB expression in the striatum of immature and adult rats. Prepubertal (PD25-38) or adult (PD53-66) male rats were treated once daily for: (a) 14 days with either saline or MPH (2 or 10 mg/kg) or (b) 13 days with saline followed by a single dose of MPH (2 or 10 mg/kg) on day 14. To determine long-term effects of MPH, another group of prepubertal rats was allowed a drug-free period of 4 weeks following the initial 14 days of treatment, and received a challenge dose of MPH at adulthood. All rats were sacrificed 2 h post-injection on the final day. Expression of c-fos and fosB was quantified by densitometric analysis of cFOS and FOSB-immunoreactivity (-ir). We demonstrated that FOSB-ir was increased by a single dose of MPH in the prepubertal and adult striatum, and this effect was further elevated by chronic MPH in prepubertal rats, in contrast to the inhibitory effect of MPH (2 and 10 mg/kg) on cFOS-ir. In adult rats, repeated MPH down-regulated cFOS-ir only at the higher dose (10 mg/kg), while fosB expression remained at levels comparable to acute MPH. The reduction in cFOS-ir observed in prepubertal rats given repeated MPH (10 mg/kg) persisted in the adult striatum following MPH challenge at adulthood. Our results suggest that (1) repeated MPH treatment differentially regulates c-fos and fosB expression in the immature and adult brain; (2) MPH-induced changes in gene expression may be enduring, and (3) the immature brain is more sensitive to the stimulant effects of MPH than the adult. Thus, our findings have implications for the long-term use of MPH in ADHD.

Rodent models of attention-deficit/hyperactivity disorder

An ideal animal model should be similar to the disorder it models in terms of etiology, biochemistry, symptomatology, and treatment. Animal models provide several advantages over clinical research: simpler nervous systems, easily interpreted behaviors, genetic homogeneity, easily controlled environment, and a greater variety of interventions. Attention-deficit/hyperactivity disorder (ADHD) is a neurobehavioral disorder of childhood onset that is characterized by inattentiveness, hyperactivity, and impulsiveness. Its diagnosis is behaviorally based; therefore, the validation of an ADHD model must be based in behavior. An ADHD model must mimic the fundamental behavioral characteristics of ADHD (face validity), conform to a theoretical rationale for ADHD (construct validity), and predict aspects of ADHD behavior, genetics, and neurobiology previously uncharted in clinical settings (predictive validity). Spontaneously hypertensive rats (SHR) fulfill many of the validation criteria and compare well with clinical cases of ADHD. Poor performers in the five-choice serial reaction time task and Naples high-excitability rats (NHE) are useful models for attention-deficit disorder. Other animal models either focus on the less important symptom of hyperactivity and might be of limited value in ADHD research or are produced in ways that would not lead to a clinical diagnosis of ADHD in humans, even if ADHD-like behavior is displayed.

Topography of methylphenidate (ritalin)-induced gene regulation in the striatum: differential effects on c-fos, substance P and opioid peptides

Dopamine action alters gene regulation in striatal neurons. Methylphenidate increases extracellular levels of dopamine. We investigated the effects of acute methylphenidate treatment on gene expression in the striatum of adult rats. Molecular changes were mapped in 23 striatal sectors mostly defined by their predominant cortical inputs in order to determine the functional domains affected. Acute administration of 5 and 10 mg/kg (i.p.) of methylphenidate produced robust increases in the expression of the transcription factor c-fos and the neuropeptide substance P. Borderline effects were found with 2 mg/kg, but not with 0.5 mg/kg. For 5 mg/kg, c-fos mRNA levels peaked at 40 min and returned to baseline by 3 h after injection, while substance P mRNA levels peaked at 40-60 min and were back near control levels by 24 h. These molecular changes occurred in most sectors of the caudate-putamen, but were maximal in dorsal sectors that receive sensorimotor and medial agranular cortical inputs, on middle to caudal levels. In rostral and ventral striatal sectors, changes in c-fos and substance P expression were weaker or absent. No effects were seen in the nucleus accumbens, with the exception of c-fos induction in the lateral part of the shell. In contrast to c-fos and substance P, acute methylphenidate treatment had minimal effects on the opioid peptides dynorphin and enkephalin. These results demonstrate that acute methylphenidate alters the expression of c-fos and substance P preferentially in the sensorimotor striatum. These molecular changes are similar, but not identical, to those produced by other psychostimulants.

Methylphenidate regulates c-fos and fosB expression in multiple regions of the immature rat brain

Methylphenidate (Ritalin, MPH) is a common psychostimulant used to treat childhood attention-deficit hyperactivity disorder (ADHD). Little is known about the long-term developmental effects on gene expression and behavior, which may occur with extended MPH use. We reported previously that the striatum is a major target of MPH, consistent with human MRI studies. In the present study, we tested the hypothesis that MPH is likely to have widespread effects in extra-striatal regions of the brain. We used the expression of two immediate early genes, c-fos and fosB, as probes to map the response of the immature rat brain to single (1 day) versus repeated (14 days) MPH treatment (2 or 10 mg/kg; s.c.) from postnatal day 25 to 38. Consistent with previous reports, the striatum is a major target of acute MPH action, as indicated by elevated levels of cFOS-immunoreactivity (-ir). Increases in c-fos expression were also seen in the nucleus accumbens, cingulate/frontal cortex and piriform cortex, and Islands of Calleja. FosB expression was elevated only in the striatum following a single stimulation. Chronic MPH treatment (10 mg/kg/day for 14 days) resulted in an attenuation of c-fos expression in the striatum and Islands of Calleja. However, levels of cFOS-ir remained elevated in the nucleus accumbens and frontal cortex. In contrast to the inhibitory effect of repeated MPH exposure on c-fos expression, FOSB-ir was further elevated in the striatum, and an increase was observed in the cingulate/frontal and piriform cortices. Thus, chronic MPH differentially regulated expression of c-fos and fosB in several brain regions. Our data suggest that MPH may exert its stimulant effects at multiple sites in the immature brain, which has implications for long-term treatment in children.

Methylphenidate (Ritalin) induces Homer 1a and zif 268 expression in specific corticostriatal circuits

Corticostriatal circuits participate in limbic, attentional, motor and other networks, and are implicated in psychostimulant addiction. The psychostimulant methylphenidate is used in the treatment of attention-deficit hyperactivity disorder and for recreational purposes. Recent studies indicate that methylphenidate alters gene expression in striatal neurons. We investigated whether methylphenidate affects gene regulation in specific corticostriatal circuits, by comparing drug-induced molecular changes in different functional domains of the striatum with changes in their cortical input regions. In order to assess the potential functional significance of methylphenidate-induced molecular changes, we examined members of two different classes of plasticity-related molecules, the transcription factor zif 268 and the synaptic plasticity factor Homer 1a. Acute methylphenidate administration in adult rats increased the expression of Homer 1a and zif 268 in both cortex and striatum in a dose-dependent and regionally selective manner. These changes in gene expression occurred after doses of 2 mg/kg (i.p.) and higher, and were highly correlated between cortical regions and their striatal targets. In the cortex, increases were maximal in the medial agranular (premotor) and cingulate cortex, followed by motor and somatosensory cortex, and were minimal in the insular cortex. Correspondingly, in the striatum, increases were most robust in sensorimotor sectors that receive medial agranular input, and were weaker or absent in ventral sectors. The methylphenidate-induced increases in cortical Homer 1a and zif 268 expression were also correlated with increases in striatal substance P and dynorphin expression (direct pathway). Overall, the regional distribution of methylphenidate-induced molecular changes in the striatum was similar to that of changes induced by psychostimulants such as cocaine. These findings demonstrate that methylphenidate affects transcription and synaptic plasticity regulatory proteins in specific corticostriatal circuits, including those implicated in attentional functions and psychostimulant addiction. Such methylphenidate-induced gene regulation may contribute to the therapeutic effects and/or abuse liability of this psychostimulant.

Regulation of affect by the lateral septum: implications for neuropsychiatry

Substantial evidence indicates that the lateral septum (LS) plays a critical role in regulating processes related to mood and motivation. This review presents findings from the basic neuroscience literature and from some clinically oriented research, drawing from behavioral, neuroanatomical, electrophysiological, and molecular studies in support of such a role, and articulates models and hypotheses intended to advance our understanding of these functions. Neuroanatomically, the LS is connected with numerous regions known to regulate affect, such as the hippocampus, amygdala, and hypothalamus. Through its connections with the mesocorticolimbic dopamine system, the LS regulates motivation, both by stimulating the activity of midbrain dopamine neurons and regulating the consequences of this activity on the ventral striatum. Evidence that LS function could impact processes related to schizophrenia and other psychotic spectrum disorders, such as alterations in LS function following administration of antipsychotics and psychotomimetics in animals, will also be presented. The LS can also diminish or enable fear responding when its neural activity is stimulated or inhibited, respectively, perhaps through its projections to the hypothalamus. It also regulates behavioral manifestations of depression, with antidepressants stimulating the activity of LS neurons, and depression-like phenotypes corresponding to blunted activity of LS neurons; serotonin likely plays a key role in modulating these functions by influencing the responsiveness of the LS to hippocampal input. In conclusion, a better understanding of the LS may provide important and useful information in the pursuit of better treatments for a wide range of psychiatric conditions typified by disregulation of affective functions.

Role of the dopamine transporter in the differential cocaine-induced locomotor activation of inbred long-sleep and short-sleep mice

The locomotor-stimulant effects of cocaine, mediated through inhibition of the dopamine transporter (DAT), can be influenced by environmental factors. Previously, we found that following a short exposure to the testing environment, cocaine induces greater locomotor activation in inbred long-sleep (ILS) mice, compared to inbred short-sleep (ISS) mice. In the present study, all animals received prolonged habituation to the testing chambers prior to cocaine injection, and the results were compared with those from our previous study. When mice were tested with saline on day 1 and with either saline or cocaine (10-20 mg/kg) on day 2, we observed significant locomotor stimulation in ILS, but not ISS, mice at all tested doses of cocaine. Thus, prolonged habituation does not alter the differential responsiveness of these two strains of mice to cocaine. We found no strain differences in striatal cocaine levels. However, [3H]WIN 35,428 binding studies showed a lower number of striatal DATs in ILS, compared to ISS, mice. In vivo analysis of striatal DAT activity revealed not only that ILS mice cleared exogenously applied DA more slowly than ISS mice, but also that cocaine (10 mg/kg) decreased DA clearance selectively in ILS mice. Thus, functional differences in striatal DATs between ILS and ISS mice likely contribute to the differential behavioral activation of cocaine in these two mouse strains.

Distinct patterns of Fos expression induced by systemic amphetamine in the striatal complex of C57BL/6JICo and DBA/2JICo inbred strains of mice

Mice from the inbred strains C57BL/6 and DBA/2 are characterized by striking differences in their behavioral response to addictive drugs. We used Fos expression as a tool to reveal strain differences in the postsynaptic effects of amphetamine (AMPH; 2.5 mg/kg) within the nucleus accumbens (NAc) (core and shell) and the dorsal caudate (dorsomedial and dorsolateral). AMPH stimulated Fos expression in all striatal regions of mice from both strains. However, while C57BL/6 showed a higher Fos response than DBA/2 mice in both NAc shell and core, the opposite was true for the dorsolateral caudate. The effects of AMPH were prevented by D1 blockade in all striatal regions of both strains and mimicked by the D1 agonist, SKF82958 (0.1 mg/kg), in both regions of the caudate and in the NAc shell, but not in the core. Our results suggest that the functional heterogeneity of the striatal complex is under genetic control and that this control may implicate DA transmission and corticostriatal interactions.

Dopamine D4 receptors inhibit depolarization-induced [3H]GABA release in the rat subthalamic nucleus

We explored the role of dopamine D4 receptors on [3H]GABA release in the subthalamic nucleus. [3H]GABA release was evoked by high K+ in slices of the nucleus. The selective dopamine D4 receptor agonist PD168,077 (N-[[4-(2-cyanophenyl)-1-piperazynil]methyl]-3-methyl-benzamide) inhibited GABA release with greater potency (EC50=3.2 nM) than quinpirole (EC50=200 nM). SKF 21297 (6-chloro-7,8-dihydroxy-1-phenyl-2,3,4,5-tetrahydro-1H-3-benzazepine hydrobromide), a dopamine D1-like receptor agonist, had no effect. L-745,870 (3-[[4-(4-chlorophenyl)piperazin-1-yl]methyl]-1-1H-pyrollo[2,3-b] pyridine), a selective dopamine D4 receptor antagonist, reverted the quinpirole inhibition with greater potency (IC50=8.7 nM) than that of the dopamine D2/D3 receptor antagonist sulpiride and raclopride (IC50=4804 and 788 nM, respectively). Both methylphenidate and methamphetamine, dopamine reuptake blockers, inhibited by 30% high K(+)-evoked GABA release; the inhibition was blocked by L-745,870. These results show that dopamine D4 receptors modulate GABA release in the subthalamic nucleus. The results would explain how agents that increase interstitial dopamine like methylphenidate and amphethamine might control locomotor hyperactivity seen in disorders of dopamine D4 receptors.

The Type IV phosphodiesterase inhibitor rolipram interferes with drug-induced conditioned place preference but not immediate early gene induction in mice

Behavioural effects of psychostimulant and opiate drugs are mediated in part by cAMP pathways operating in the nucleus accumbens. Degradation of cAMP occurs through the action of phosphodiesterases, such as the Type IV phosphodiesterases (PDE4s) that are found throughout the brain. To examine the potential role of PDE4 in reward-mediated behaviour, we measured the effects of rolipram, a PDE4 selective inhibitor, on cocaine (18 mg/kg i.p.) and morphine (5 mg/kg s.c.) conditioned place preference in Swiss Webster mice. Rolipram (0, 0.2 or 1.0 mg/kg i.p.) given 30 min prior to drug administration dose-dependently reduced conditioning due to both cocaine and morphine. However, rolipram did not affect place preference induced by food, nor did it prevent the expression of a previously established place preference conditioned by cocaine or morphine. In a second experiment, rolipram administered 30 min prior to a single cocaine injection (50 mg/kg i.p.), did not alter cocaine-induced c-Fos expression in the caudate putamen or nucleus accumbens core. However, rolipram, but not cocaine, induced c-Fos in the nucleus accumbens shell. These results indicate that elevation of cAMP in neurons that express PDE4s may attenuate the rewarding properties of cocaine and morphine, but does not alter the cocaine signalling cascade that induces c-Fos expression. Thus, PDE4-mediated regulation of cAMP levels could underlie the establishment of reward valence to abused drugs.

Altered midbrain dopaminergic neurotransmission during development in an animal model of ADHD

To understand the onset and the molecular mechanisms triggering dopaminergic (DA) dysregulation in Attention-Deficit Hyperactivity Disorder (ADHD), we have used the Spontaneously Hypertensive Rats (SHR), the most widely studied animal model for this disease. We have studied the pattern of expression of specific genes involved in DA neuron differentiation, survival and function during postnatal (P) development of the ventral midbrain in SHR males. Our results show that tyrosine hydroxylase and DA transporter gene expression are significantly and transiently reduced in the SHR midbrain during the first month of postnatal development, although with a different kinetic. The other genes analyzed do not show significant variation between SHR and control rats. In addition, high-affinity DA uptake activity is significantly reduced in synaptosomes obtained from the striatum of 1-month-old SHR, when compared to controls. Our data suggest that down-regulation of DA neurotransmission occurs in the midbrain of SHR in a developmentally regulated temporal window during postnatal development, thus strengthening the hypodopaminergic hypothesis in the pathogenesis of ADHD.

Neural Substrates Underlying Impulsivity

Attention deficit hyperactivity disorder (ADHD) is a neuropsychiatric disorder whose three main symptoms are impulsiveness, inattention, and hyperactivity. Although ADHD is an early developmental disorder, it may persist into adulthood, resulting in deficits associated with poor academic performance, frequent job changes, poor and unstable marriages, and increases in motor vehicle accidents. Of the three primary symptoms of ADHD, deficits in impulse control are the most challenging to the social network and the judicial system. While the etiology of ADHD remains unknown, recent work suggests that the central deficits in ADHD may be due to poor response inhibition that is linked to monoamine and prefrontal lobe deficiencies. In the past, preclinical studies designed to understand the lack of impulse control have generally been relegated to studies linked to aggression and drug abuse. With the use of innovative noninvasive techniques, like anatomical and functional magnetic resonance imaging, selective neurochemical and behavioral paradigms have converged with preclinical reports and lend support to the premise that monoaminergic neurotransmitter systems and the cortico-striatal circuitry are essential to impulse control. Furthermore, new emerging data on neural substrates underlying impulsivity have incorporated brain regions involved in reinforcement, reward, and decision making such as the nucleus accumbens, cerebellum, and amygdala. As noninvasive brain imaging, neurochemical, and behavioral approaches are combined, our knowledge of the neural networks underlying impulsivity will hopefully give rise to therapeutic approaches aimed at alleviating this disorder.

Dopamine responsiveness to drugs of abuse: A shell-core investigation in the nucleus accumbens of the mouse

The existence of subterritories within the nucleus accumbens has now been widely supported by histochemical, neurochemical, electrophysiological, as well as morphological and ultrastructural studies and suggest specific afferent and efferent systems involved in different behavioral aspects. Microdialysis studies in the rat have consistently shown that most drugs of abuse increase extracellular dopamine levels preferentially in the shell subregion of the nucleus accumbens. The study of the relative roles of NAc subregions may considerably help our understanding of the neurobiological basis of drug addiction. Accordingly, the aim of the present work was to extend the outcome of rat studies to the mouse species. Five major drugs of abuse were systemically and acutely administered to mice with a microdialysis probe implanted in either the shell or the core. A statistical comparison was performed on data transformed as percentage values of baseline dopamine vs. logarithmic values with baseline dopamine as a covariate. Results show a significant increase in dopamine levels in both the shell and core subregions following cocaine, amphetamine, nicotine, ethanol, and morphine treatments. A difference between shell and core after cocaine, nicotine, and morphine was evident when data were analyzed as percent values of baseline. However, such a shell-core dichotomy became no longer significant when ANOVA was applied on the statistically more appropriate logarithmic transformation of data with baseline as a covariate. The significant baseline differences among groups of mice (dopamine levels in the shell significantly lower compared with dopamine levels in the core) may have compromised, at least in part, the statistical procedure usually applied in microdialysis studies. These findings suggest that a careful evaluation of the data is required when subtle changes in extracellular levels of DA are measured.

Stimulant and reinforcing effects of cocaine in monoamine transporter knockout mice

A large body of evidence supports the hypothesis that the reinforcing effects of cocaine depend on its ability to block the dopamine transporter (DAT), thereby increasing dopamine extracellular concentration within the mesocorticolimbic system. However, the fact that cocaine similarly binds to the serotonin and norepinephrine transporters (SERT and NET, respectively), raises the possibility that modulation of mesocorticolimbic dopaminergic transmission might be achieved through alternate pathways. The successful disruption of the genes coding for the DAT, the SERT and the NET offered ideal tools to determine the extent of the participation of these transporters and respective monoaminergic systems in the reinforcing effects of cocaine. Studies of cocaine-induced motor activation and maintenance of intravenous (i.v.) self-administration in DAT- and in NET-knockout (KO) mice are reviewed here, and discussed in light of new observations obtained from double monoamine transporters KO mice (i.e., DAT-KO/SERT-KO, NET-KO/SERT-KO). The reinforcing potency of cocaine is maintained in the absence of the DAT but decreased in the absence of the NET; its motivational rewarding effect is observed in the absence of the SERT, but not when both DAT and SERT are lacking. Moreover, a dichotomy between cocaine motor activating and reinforcing effects is reported. Such dichotomy is suggestive of independent mechanisms underlying the psychomotor stimulant and reinforcing effects of cocaine. Overall, these studies provide evidence that cocaine dynamically acts at multiple sites through pathways that might be exchangeable under certain circumstances.

Repeated methylphenidate treatment in adolescent rats alters gene regulation in the striatum

Methylphenidate is a psychostimulant which inhibits the dopamine transporter and produces dopamine overflow in the striatum, similar to the effects of cocaine. Excessive dopamine action is often associated with changes in gene expression in dopamine-receptive neurons. Little is known about methylphenidate's effects on gene regulation. We investigated whether a methylphenidate treatment regimen known to produce behavioural changes would alter gene expression in the striatum. Using in situ hybridization histochemistry, we assessed the effects of acute and repeated methylphenidate treatment on the expression of immediate-early genes (c-fos, zif 268) and neuropeptides (dynorphin, substance P, enkephalin) in adolescent rats. Acute methylphenidate treatment (0-10 mg/kg, i.p.) produced a dose-dependent increase in the expression of c-fos and zif 268. These effects were most pronounced in the dorsal striatum at middle to caudal striatal levels, and were found for doses as low as 2 mg/kg. Repeated treatment with methylphenidate (10 mg/kg/day, 7 days) increased the expression of dynorphin, which was highly correlated with the acute immediate-early gene response across different striatal regions. Moreover, after repeated methylphenidate treatment, cocaine-induced expression of c-fos and zif 268, as well as of substance P, was significantly attenuated throughout the striatum. These effects of repeated methylphenidate treatment mirror those produced by repeated treatment with cocaine or other psychostimulants and are considered to reflect drug-induced neuroadaptations. Thus, our findings demonstrate that acute and repeated methylphenidate treatment can produce molecular alterations similar to other psychostimulants.