Elevated serotonin is involved in hyperactivity but not in the paradoxical effect of amphetamine in mice neonatally lesioned with 6-hydroxydopamine

Chronic exposure to metam sodium-based pesticide in mice during adulthood elevated anxiety and depression-like behaviors: Involvement of serotoninergic depletion and gut microbiota dysbiosis

Metam sodium-based pesticide (MS-BP) is widely used in agriculture and public health. We have previously demonstrated that maternal exposure to MS-BP resulted in sensorimotor alterations in mice offspring with long-lasting deficits including anxiety- and depression-like behaviors. Here, we project to verify whether these two neurobehavioral effects occur during adulthood following direct exposure to MS-BP and whether it results in changes in the serotoninergic system and gut microbiota. Our findings showed that chronic exposure to MS-BP increased anxiety- and depression-like behaviors, accompanied by a depletion of serotonin-like neurons within the dorsal raphe nucleus and a reduction in serotoninergic terminals in the infralimbic cortex and the basolateral amygdala. In addition, all MS-BP-exposed animals exhibited a reduced total bacterial number and diversity of gut microbiota. Taken together, our data demonstrated that MS-BP-induced behavioral changes could be related to the impairment of the serotoninergic system and gut microbiota dysbiosis.

Interactions Between the Serotonergic and Other Neurotransmitter Systems in the Basal Ganglia: Role in Parkinson's Disease and Adverse Effects of L-DOPA

Parkinson's disease (PD) is characterized by the progressive loss of dopaminergic neurons in the substantia nigra. However, other non-dopaminergic neuronal systems such as the serotonergic system are also involved. Serotonergic dysfunction is associated with non-motor symptoms and complications, including anxiety, depression, dementia, and sleep disturbances. This pathology reduces patient quality of life. Interaction between the serotonergic and other neurotransmitters systems such as dopamine, noradrenaline, glutamate, and GABA controls the activity of striatal neurons and are particularly interesting for understanding the pathophysiology of PD. Moreover, serotonergic dysfunction also causes motor symptoms. Interestingly, serotonergic neurons play an important role in the effects of L-DOPA in advanced PD stages. Serotonergic terminals can convert L-DOPA to dopamine, which mediates dopamine release as a "false" transmitter. The lack of any autoregulatory feedback control in serotonergic neurons to regulate L-DOPA-derived dopamine release contributes to the appearance of L-DOPA-induced dyskinesia (LID). This mechanism may also be involved in the development of graft-induced dyskinesias (GID), possibly due to the inclusion of serotonin neurons in the grafted tissue. Consistent with this, the administration of serotonergic agonists suppressed LID. In this review article, we summarize the interactions between the serotonergic and other systems. We also discuss the role of the serotonergic system in LID and if therapeutic approaches specifically targeting this system may constitute an effective strategy in PD.

Effects of dopamine and serotonin synthesis inhibitors on the ketamine-, d-amphetamine-, and cocaine-induced locomotor activity of preweanling and adolescent rats: sex differences

The pattern of ketamine-induced locomotor activity varies substantially across ontogeny and according to sex. Although ketamine is classified as an NMDA channel blocker, it appears to stimulate the locomotor activity of both male and female rats via a monoaminergic mechanism. To more precisely determine the neural mechanisms underlying ketamine's actions, male and female preweanling and adolescent rats were pretreated with vehicle, the dopamine (DA) synthesis inhibitor ∝-methyl-_DL-p-tyrosine (AMPT), or the serotonin (5-HT) synthesis inhibitor 4-chloro-_DL-phenylalanine methyl ester hydrochloride (PCPA). After completion of the pretreatment regimen, the locomotor activating effects of saline, ketamine, d-amphetamine, and cocaine were assessed during a 2 h test session. In addition, the ability of AMPT and PCPA to reduce dorsal striatal DA and 5-HT content was measured in male and female preweanling, adolescent, and adult rats. Results showed that AMPT and PCPA reduced, but did not fully attenuate, the ketamine-induced locomotor activity of preweanling rats and female adolescent rats. Ketamine (20 and 40 mg/kg) caused a minimal amount of locomotor activity in male adolescent rats, and this effect was not significantly modified by AMPT or PCPA pretreatment. When compared to ketamine, d-amphetamine and cocaine produced different patterns of locomotor activity across ontogeny; moreover, AMPT and PCPA pretreatment affected psychostimulant- and ketamine-induced locomotion differently. When these results are considered together, it appears that both dopaminergic and serotonergic mechanisms mediate the ketamine-induced locomotor activity of preweanling and female adolescent rats. The dichotomous actions of ketamine relative to the psychostimulants in vehicle-, AMPT-, and PCPA-treated rats, suggests that ketamine modulates DA and 5-HT neurotransmission through an indirect mechanism.

Histamine Modulates Midbrain Dopamine Neuron Differentiation Through the Regulation of Epigenetic Marks

During midbrain development, dopamine neuron differentiation occurs before birth. Epigenetic processes such as DNA methylation and demethylation as well as post-translational modification of histones occur during neurogenesis. Here, we administered histamine (HA) into the brain of E12 embryos in vivo and observed significant lower immunoreactivity of Lmx1a+ and Tyrosine Hydroxylase (TH)+ cells, with parallel decreases in the expression of early (Lmx1a, Msx1) and late (Th) midbrain dopaminergic (mDA) genes. With MeDIP assays we found that HA decreases the percentage of 5-methylcytosine of Pitx3 and Th, without changes in 5-hydroxymethylcytosine. Additionally, HA treatment caused a significant increase in the repressive epigenetic modifications H3K9me3 in Pitx3 and Th, and also more H3K27me3 marks in Th. Furthermore, HA has a long-term effect on the formation of the nigrostriatal and mesolimbic/mesocortical pathways, since it causes a significant decrease in midbrain TH immunoreactivity, as well as alterations in dopaminergic neuronal fibers, and significant lower TH-positive area in the forebrain in whole-mount stainings. These findings suggest that HA diminishes dopaminergic gene transcription by altering several epigenetic components related to DNA and histone modifications, which affects mDA neuron progression during development.

Behavioral responses to anxiogenic tasks in young adult rats with neonatal dopamine depletion

The dopaminergic neural system plays a crucial role in motor regulation as well as regulation of anxiety-related behaviors. Although rats with neonatal dopamine depletion exhibit motor hyperactivity and have been utilized as animal models of attention deficit hyperactivity disorder, characterization of their behavior under anxiogenic conditions is lacking. In the present study, we investigated behavioral responses to anxiogenic stimuli in young adult rats with neonatal dopamine depletion using the open field (OF), elevated plus maze (EPM), and light/dark (L/B) box tests. The OF and EPM tests were performed under low-light and bright-light conditions. The ameliorative effects of pretreatment with methamphetamine (MAP) or atomoxetine (ATX) on abnormal behaviors induced by neonatal dopamine depletion were also assessed. Rats that underwent 6-hydroxydopamine treatment 4 day after birth showed significant increases in motor activity and decreases in anxiety-related behaviors in OF tests under both conditions and in EPM tests under bright-light conditions. Furthermore, rats with neonatal dopamine depletion did not show normal behavioral responsiveness to changes in the intensity of anxiogenic stimuli. Pretreatment with MAP (4 mg/kg) and ATX (1.2 mg/kg/day) ameliorated motor hyperactivity but not abnormal anxiety-related behaviors. These results suggest that the dopaminergic system plays a crucial role in the development of neural networks involved in locomotion as well as in those involved in anxiety-related behavior. The results indicate that the mechanisms underlying the abnormal anxiolytic responses partially differ from those underlying motor hyperactivity.

Manipulation of dopamine metabolism contributes to attenuating innate high locomotor activity in ICR mice

Attention-deficit hyperactivity disorder (ADHD) is defined as attention deficiency, restlessness and distraction. The main characteristics of ADHD are hyperactivity, impulsiveness and carelessness. There is a possibility that these abnormal behaviors, in particular hyperactivity, are derived from abnormal dopamine (DA) neurotransmission. To elucidate the mechanism of high locomotor activity, the relationship between innate activity levels and brain monoamines and amino acids was investigated in this study. Differences in locomotor activity between ICR, C57BL/6J and CBA/N mice were determined using the open field test. Among the three strains, ICR mice showed the greatest amount of locomotor activity. The level of striatal and cerebellar DA was lower in ICR mice than in C57BL/6J mice, while the level of L-tyrosine (L-Tyr), a DA precursor, was higher in ICR mice. These results suggest that the metabolic conversion of L-Tyr to DA is lower in ICR mice than it is in C57BL/6J mice. Next, the effects of intraperitoneal injection of (6R)-5, 6, 7, 8-tetrahydro-l-biopterin dihydrochloride (BH₄) (a co-enzyme for tyrosine hydroxylase) and L-3,4-dihydroxyphenylalanine (L-DOPA) on DA metabolism and behavior in ICR mice were investigated. The DA level in the brain was increased by BH₄ administration, but the increased DA did not influence behavior. However, L-DOPA administration drastically lowered locomotor activity and increased DA concentration in several parts of the brain. The reduced locomotor activity may have been a consequence of the overproduction of DA. In conclusion, the high level of locomotor activity in ICR mice may be explained by a strain-specific DA metabolism.

Functional Interplay between Dopaminergic and Serotonergic Neuronal Systems during Development and Adulthood

The complex integration of neurotransmitter signals in the nervous system contributes to the shaping of behavioral and emotional constitutions throughout development. Imbalance among these signals may result in pathological behaviors and psychiatric illnesses. Therefore, a better understanding of the interplay between neurotransmitter systems holds potential to facilitate therapeutic development. Of particular clinical interest are the dopaminergic and serotonergic systems, as both modulate a broad array of behaviors and emotions and have been implicated in a wide range of affective disorders. Here we review evidence speaking to an interaction between the dopaminergic and serotonergic neuronal systems across development. We highlight data stemming from developmental, functional, and clinical studies, reflecting the importance of this transmonoaminergic interplay.

Involvement of cholinergic system in hyperactivity in dopamine-deficient mice

Dopaminergic systems have been known to be involved in the regulation of locomotor activity and development of psychosis. However, the observations that some Parkinson's disease patients can move effectively under appropriate conditions despite low dopamine levels (eg, kinesia paradoxia) and that several psychotic symptoms are typical antipsychotic resistant and atypical antipsychotic sensitive indicate that other systems beyond the dopaminergic system may also affect locomotor activity and psychosis. The present study showed that dopamine-deficient (DD) mice, which had received daily L-DOPA injections, could move effectively and even be hyperactive 72 h after the last L-DOPA injection when dopamine was almost completely depleted. Such hyperactivity was ameliorated by clozapine but not haloperidol or ziprasidone. Among multiple actions of clozapine, muscarinic acetylcholine (ACh) activation markedly reduced locomotor activity in DD mice. Furthermore, the expression of choline acetyltransferase, an ACh synthase, was reduced and extracellular ACh levels were significantly reduced in DD mice. These results suggest that the cholinergic system, in addition to the dopaminergic system, may be involved in motor control, including hyperactivity and psychosis. The present findings provide additional evidence that the cholinergic system may be targeted for the treatment of Parkinson's disease and psychosis.

Aberrant Monoaminergic System in Thyroid Hormone Receptor-β Deficient Mice as a Model of Attention-Deficit/Hyperactivity Disorder

BACKGROUND

Thyroid hormone receptors are divided into 2 functional types: TRα and TRβ. Thyroid hormone receptors play pivotal roles in the developing brain, and disruption of thyroid hormone receptors can produce permanent behavioral abnormality in animal models and humans.

METHODS

Here we examined behavioralchanges, regional monoamine metabolism, and expression of epigenetic modulatory proteins, including acetylated histone H3 and histone deacetylase, in the developing brain of TRα-disrupted (TRα (0/0) ) and TRβ-deficient (TRβ (-/-) ) mice. Tissue concentrations of dopamine, serotonin (5-hydroxytryptamine) and their metabolites in the mesocorticolimbic pathway were measured.

RESULTS

TRβ (-/-) mice, a model of attention-deficit/hyperactivity disorder, showed significantly high exploratory activity and reduced habituation, whereas TRα (0/0) mice showed normal exploratory activity. The biochemical profiles of dopamine and 5-hydroxytryptamine showed significantly low dopamine metabolic rates in the caudate putamen and nucleus accumbens and overall low 5-hydroxytryptamine metabolic rates in TRβ (-/-) mice, but not in TRα (0/0) mice. Furthermore, the expression of acetylated histone H3 was low in the dorsal raphe of TRβ (-/-) mice, and histone deacetylase 2/3 proteins were widely increased in the mesolimbic system.

CONCLUSIONS

These findings suggest that TRβ deficiency causes dysfunction of the monoaminergic system, accompanied by epigenetic disruption during the brain maturation process.

Metabolism of amino acids differs in the brains of Djungarian hamster (P. sungorus) and Roborovskii hamster (P. roborovskii)

Djungarian hamster (P. sungorus) and Roborovskii hamster (P. roborovskii) belong to the same genus of phodopus. Roborovskii hamster shows high locomotor activity and low level of dopamine (DA) in the brain. Administration of L-tyrosine, a precursor of DA, decreases locomotor activity in Roborovskii hamsters. However, the amino acid metabolism in relation to the hyperactivity is not yet well known. In the present study, L- and D-amino acid concentrations in the brain, liver, and plasma in Djungarian and Roborovskii hamsters were investigated during day and night times to explain the possible difference in hyperactivity between them. Most of the examined amino acids were higher in the night time when hamsters are active compared to those in day time. L- and D-tyrosine concentrations were higher in the liver of Roborovskii hamsters than in Djungarian hamsters. Furthermore, brain concentration of D-tyrosine was higher in the Roborovskii than in Djungarian hamsters, but no significant difference was observed for L-tyrosine concentrations between the two species. These results suggest that the conversion of L-tyrosine to D-tyrosine in the brain of Roborovskii hamster may be higher than in Djungarian hamster, which may cause low DA concentration and hyperactivity in Roborovskii hamster. On the other hand, L- and D-serine, which are known as sedative factors, were lower in Roborovskii hamsters than Djungarian hamster. These results suggest that species-specific regulation in amino acid metabolism may contribute to hyperactivity in Roborovskii hamsters.

Reinforcement, dopamine and rodent models in drug development for ADHD

Attention deficit hyperactivity disorder (ADHD) presents special challenges for drug development. Current treatment with psychostimulants and nonstimulants is effective, but their mechanism of action beyond the cellular level is incompletely understood. We review evidence suggesting that altered reinforcement mechanisms are a fundamental characteristic of ADHD. We show that a deficit in the transfer of dopamine signals from established positive reinforcers to cues that predict such reinforcers may underlie these altered reinforcement mechanisms, and in turn explain key symptoms of ADHD. We argue that the neural substrates controlling the excitation and inhibition of dopamine neurons during the transfer process are a promising target for future drug development. There is a need to develop animal models and behavioral paradigms that can be used to experimentally investigate these mechanisms and their effects on sensitivity to reinforcement. More specific and selective targeting of drug development may be possible through this approach.

Altered serotonergic function may partially account for behavioral endophenotypes in steroid sulfatase-deficient mice

The X-linked gene STS encodes the steroid hormone-modulating enzyme steroid sulfatase. Loss-of-function of STS, and variation within the gene, have been associated with vulnerability to developing attention deficit hyperactivity disorder (ADHD), a neurodevelopmental condition characterized by inattention, severe impulsivity, hyperactivity, and motivational deficits. ADHD is commonly comorbid with a variety of disorders, including obsessive-compulsive disorder. The neurobiological role of steroid sulfatase, and therefore its potential role in ADHD and associated comorbidities, is currently poorly understood. The 39,X(Y)*O mouse, which lacks the Sts gene, exhibits several behavioral abnormalities relevant to ADHD including inattention and hyperactivity. Here, we show that, unexpectedly, 39,X(Y)*O mice achieve higher ratios than wild-type mice on a progressive ratio (PR) task thought to index motivation, but that there is no difference between the two groups on a behavioral task thought to index compulsivity (marble burying). High performance liquid chromatography analysis of monoamine levels in wild type and 39,X(Y)*O brain tissue regions (the frontal cortex, striatum, thalamus, hippocampus, and cerebellum) revealed significantly higher levels of 5-hydroxytryptamine (5-HT) in the striatum and hippocampus of 39,X(Y)*O mice. Significant correlations between hippocampal 5-HT levels and PR performance, and between striatal 5-HT levels and locomotor activity strongly implicate regionally-specific perturbations of the 5-HT system as a neurobiological candidate for behavioral differences between 40,XY and 39,X(Y)*O mice. These data suggest that inactivating mutations and functional variants within STS might exert their influence on ADHD vulnerability, and disorder endophenotypes through modulation of the serotonergic system.

Animal models to guide clinical drug development in ADHD: lost in translation?

We review strategies for developing animal models for examining and selecting compounds with potential therapeutic benefit in attention-deficit hyperactivity disorder (ADHD). ADHD is a behavioural disorder of unknown aetiology and pathophysiology. Current understanding suggests that genetic factors play an important role in the aetiology of ADHD. The involvement of dopaminergic and noradrenergic systems in the pathophysiology of ADHD is probable. We review the clinical features of ADHD including inattention, hyperactivity and impulsivity and how these are operationalized for laboratory study. Measures of temporal discounting (but not premature responding) appear to predict known drug effects well (treatment validity). Open-field measures of overactivity commonly used do not have treatment validity in human populations. A number of animal models have been proposed that simulate the symptoms of ADHD. The most commonly used are the spontaneously hypertensive rat (SHR) and the 6-hydroxydopamine-lesioned (6-OHDA) animals. To date, however, the SHR lacks treatment validity, and the effects of drugs on symptoms of impulsivity and inattention have not been studied extensively in 6-OHDA-lesioned animals. At the present stage of development, there are no in vivo models of proven effectiveness for examining and selecting compounds with potential therapeutic benefit in ADHD. However, temporal discounting is an emerging theme in theories of ADHD, and there is good evidence of increased value of delayed reward following treatment with stimulant drugs. Therefore, operant behaviour paradigms that measure the effects of drugs in situations of delayed reinforcement, whether in normal rats or selected models, show promise for the future.

Rodent models of ADHD

The neonatal 6-OHDA-lesioned rat, coloboma mouse, DAT-KO mouse, and spontaneously hypertensive rat (SHR) models all bear a phenotypic resemblance to ADHD in that they express hyperactivity, inattention, and/or impulsivity. The models also illustrate the heterogeneity of ADHD: the initial cause (chemical depletion or genetic abnormality) of the ADHD-like behaviors is different for each model. Neurochemical and behavioral studies of the models indicate aberrations in monoaminergic neurotransmission. Hyperdopaminergic neurotransmission is implicated in the abnormal behavior of all models. Norepinephrine has a dual enhancing/inhibitory role in ADHD symptoms, and serotonin acts to inhibit abnormal dopamine and norepinephrine signaling. It is unlikely that symptoms arise from a single neurotransmitter dysfunction. Rather, studies of animal models of ADHD suggest that symptoms develop through the complex interactions of monoaminergic neurotransmitter systems.

Chronic L-tyrosine alters the locomotor activity and brain monoamine levels in Roborovskii hamsters

The Roborovskii hamster (Phodopus roborovskii) has high locomotor activity (hyperactivity) and low dopamine levels in the brain compared with the congeneric Djungarian hamster (Phodopus sungorus). To clarify the efficacy of dietary l-tyrosine in ameliorating signs of hyperactivity, we investigated the effects of chronic administration of l-tyrosine, the primary precursor of dopamine, on locomotor activity and brain monoamine levels in Roborovskii hamsters. Chronic supplementation of l-tyrosine had no effect on locomotor activity in the open field, but did decrease locomotor activity in the home cage. Tyrosine increased dopamine and norepinephrine turnover rates and decreased in serotonin turnover rate in the brain. These findings suggest that long-term feeding of l-tyrosine may be effective in ameliorating signs of hyperactivity.

Reward-seeking and discrimination deficits displayed by hypodopaminergic mice are prevented in mice lacking dopamine D4 receptors

The dopamine D4 receptor (D4R) is predominantly expressed in the prefrontal cortex, a brain area that integrates motor, rewarding, and cognitive information. Because participation of D4Rs in executive learning is largely unknown, we challenged D4R knockout mice (Drd4(-/-)) and their wild-type (WT) littermates, neonatally treated with 6-hydroxydopamine (6-OHDA; icv) or vehicle in two operant learning paradigms. A continuous reinforcement task, in which one food-pellet was delivered after every lever press, showed that 6-OHDA-treated mice (hypodopaminergic) WT mice pressed the reinforcing lever at much lower rates than normodopaminergic WT mice. In contrast, Drd4(-/-) mice displayed increased lever pressing rates, regardless of their dopamine content. In another study, mice were trained to solve an operant two-choice task in which a first showing lever was coupled to the delivery of one food pellet only after a second lever emerged. Interval between presentation of both levers was initially 12 s and progressively shortened to 6, 2, and finally 0.5 s. Normodopaminergic WT mice obtained a pellet reward in more than 75% of the trials at 12, 6, and 2 s, whereas hypodopaminergic WT mice were severely impaired to select the reward-paired lever. Absence of D4Rs was not detrimental in this task. Moreover, hypodopaminergic Drd4(-/-) mice were as efficient as their normodopaminergic Drd4(-/-) siblings in selecting the reward-paired lever. In summary, hypodopaminergic mice exhibit severe impairments to retrieve rewards in two operant positive reinforcement tasks, but these deleterious effects are totally prevented in the absence of functional D4Rs.

L-DOPA attenuates hyperactivity of Roborovskii hamsters

The Roborovskii hamster (Phodopus roborovskii) has been shown to have high locomotor activity (hyperactivity) and low dopamine concentrations in the brain. We hypothesized that low brain dopamine concentrations play a role in the pathogenesis of hyperactivity. In this study, therefore, we investigated the effects of L-DOPA (L-3,4-dihydroxyphenylalanine), the precursor of dopamine, on the locomotor activity of Roborovskii hamster to verify the above hypothesis. An open field test was employed to measure the locomotor activity. Administration of L-DOPA dose-dependently decreased locomotor activity including distance of path and time spent moving. L-DOPA increased the brain concentration of dopamine and its metabolite 3,4-dihydroxyphenylacetic acid. Concurrently, L-DOPA caused increase of norepinephrine, decrease of serotonin, and atypical alteration of their metabolite concentrations. These findings mainly suggest that in Roborovskii hamsters, a low level of brain dopamine neurotransmission is one of the reasons for hyperactivity, and hyperactivity can be attenuated by L-DOPA.

The extracellular signal-regulated kinase pathway contributes to the control of behavioral excitement

The extracellular signal-regulated kinase (ERK) pathway mediates neuronal plasticity in the CNS. The mood stabilizers lithium and valproate activate the ERK pathway in prefrontal cortex and hippocampus and potentiate ERK pathway-mediated neurite growth, neuronal survival and hippocampal neurogenesis. Here, we examined the role of the ERK pathway in behavioral plasticity related to facets of bipolar disorder. Mice with ERK1 ablation acquired reduced phosphorylation of RSK1, an ERK substrate, in prefrontal cortex and striatum, but not in hippocampus or cerebellum, indicating the ablation-induced brain region-specific ERK signaling deficits. ERK1 ablation produced a behavioral excitement profile similar to that induced by psychostimulants. The profile is characterized by hyperactivity, enhanced goal-directed activity and increased pleasure-related activity with potential harmful consequence. ERK1-ablated mice were hyperactive in multiple tests and resistant to behavioral despair in the forced swim test. These mice displayed more home-cage voluntary wheel running activities, rearings in a large arena and open-arm visits in an elevated plus maze. Treatments with valproate and olanzapine, but not lithium reduced baseline activities in ERK1-ablated mice. All three treatments attenuated amphetamine-induced hyperactivity in ablated mice. These data indicate a profound involvement of ERK1 signaling in behavioral excitement and in the behavioral action of antimanic agents. The extent to which ERK pathway perturbation contributes to the susceptibility, mood switch mechanism(s) and symptom pathophysiology of bipolar disorder requires further investigation. Whether there is a shared mechanism through which mood stabilizers produce their clinical actions on mood, thought and behavioral symptoms of mania also requires further investigation.

Dopamine-dependent periadolescent maturation of corticostriatal functional connectivity in mouse

Altered corticostriatal information processing associated with early dopamine systems dysfunction may contribute to attention deficit/hyperactivity disorder (ADHD). Mice with neonatal dopamine-depleting lesions exhibit hyperactivity that wanes after puberty and is reduced by psychostimulants, reminiscent of some aspects of ADHD. To assess whether the maturation of corticostriatal functional connectivity is altered by early dopamine depletion, we examined preadolescent and postadolescent urethane-anesthetized mice with or without dopamine-depleting lesions. Specifically, we assessed (1) synchronization between striatal neuron discharges and oscillations in frontal cortex field potentials and (2) striatal neuron responses to frontal cortex stimulation. In adult control mice striatal neurons were less spontaneously active, less responsive to cortical stimulation, and more temporally tuned to cortical rhythms than in infants. Striatal neurons from hyperlocomotor mice required more current to respond to cortical input and were less phase locked to ongoing oscillations, resulting in fewer neurons responding to refined cortical commands. By adulthood some electrophysiological deficits waned together with hyperlocomotion, but striatal spontaneous activity remained substantially elevated. Moreover, dopamine-depleted animals showing normal locomotor scores exhibited normal corticostriatal synchronization, suggesting that the lesion allows, but is not sufficient, for the emergence of corticostriatal changes and hyperactivity. Although amphetamine normalized corticostriatal tuning in hyperlocomotor mice, it reduced horizontal activity in dopamine-depleted animals regardless of their locomotor phenotype, suggesting that amphetamine modified locomotion through a parallel mechanism, rather than that modified by dopamine depletion. In summary, functional maturation of striatal activity continues after infancy, and early dopamine depletion delays the maturation of core functional capacities of the corticostriatal system.

Memory, mood, dopamine, and serotonin in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-lesioned mouse model of basal ganglia injury

The 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned mouse serves as a model of basal ganglia injury and Parkinson's disease. The present study investigated the effects of MPTP-induced lesioning on associative memory, conditioned fear, and affective behavior. Male C57BL/6 mice were administered saline or MPTP and separate groups were evaluated at either 7 or 30 days post-lesioning. In the social transmission of food preference test, mice showed a significant decrease in preference for familiar food 30 days post-MPTP compared to controls. Mice at both 7 and 30 days post-MPTP lesioning had increased fear extinction compared to controls. High Performance Liquid Chromatography analysis of tissues homogenates showed dopamine and serotonin were depleted in the striatum, frontal cortex, and amygdala. No changes in anxiety or depression were detected by the tail suspension, sucrose preference, light-dark preference, or hole-board tests. In conclusion, acute MPTP lesioning regimen in mice causes impairments in associative memory and conditioned fear, no mood changes, and depletion of dopamine and serotonin throughout the brain.

Neonatal 6-hydroxydopamine lesions of the frontal cortex in rats: persisting effects on locomotor activity, learning and nicotine self-administration

Dopaminergic innervation of the frontal cortex in adults is important for a variety of cognitive functions and behavioral control. However, the role of frontal cortical dopaminergic innervation for neurobehavioral development has received little attention. In the current study, rats were given dopaminergic lesions in the frontal cortex with local micro-infusions of 6-hydroxydopamine (6-OHDA) at 1 week of age. The long-term behavioral effects of neonatal frontal cortical 6-OHDA lesions were assessed in a series of tests of locomotor activity, spatial learning and memory, and i.v. nicotine self-administration. In addition, neurochemical indices were assessed with tissue homogenization and HPLC in the frontal cortex, striatum, and nucleus accumbens of neonatal and adult rats after neonatal 6-OHDA lesions. In neonatal rats, frontal 6-OHDA lesions as intended caused a significant reduction in frontal cortical dopamine without effects on frontal cortical 5-HT and norepinephrine. The frontal cortical dopamine depletion increased 5-HT and norepinephrine levels in the nucleus accumbens. Locomotor activity assessment during adulthood in the figure-8 maze showed that lesioned male rats were hyperactive relative to sham-lesioned males. Locomotor activity of female rats was not significantly affected by the neonatal frontal 6-OHDA lesion. Learning and memory in the radial-arm maze was also affected by neonatal frontal 6-OHDA lesions. There was a general trend toward impaired performance in early maze acquisition and a paradoxical improvement at the end of cognitive testing. Nicotine self-administration showed significant lesion x sex interactions. The sex difference in nicotine self-administration with females self-administering significantly more nicotine than males was reversed by neonatal 6-OHDA frontal cortical lesions. Neurochemical studies in adult rats showed that frontal cortical dopamine and DOPAC levels significantly correlated with nicotine self-administration in the 6-OHDA-lesioned animals but not in the controls. Frontal cortical 5-HT and 5HIAA showed inverse correlations with nicotine self-administration in the 6-OHDA-lesioned animals but not in the controls. These results show that interfering with normal dopamine innervation of the frontal cortex during early postnatal development has persisting behavioral effects, which are sex-specific.

Alterations in serotonin signalling are involved in the hyperactivity of Pitx3-deficient mice

Pitx3 deficiency in mice causes a dramatic loss of dopaminergic neurones located in the substantia nigra pars compacta during development. This early disruption of the nigrostriatal pathway in Pitx3-deficient mice is characterized by increased spontaneous home-cage activity levels during the habitual sleep phase of these animals. These findings are reminiscent of the spontaneous hyperactivity in mice neonatally lesioned with 6-hydroxydopamine, which is caused by an extensive serotonergic hyperinnervation of the striatum. The present study investigated whether an imbalance between dopamine (DA) and serotonin (5-HT) signalling is involved in the behavioural phenotype of Pitx3-deficient mice. Serotonergic hyperinnervation was demonstrated by increased [3H]-citalopram autoradiographic binding specifically in the dorsal striatum of adult Pitx3-deficient mice, indicating alterations in 5-HT transporter levels that correlated to DA dysfunction in Pitx3 deficiency. In addition, stimulus-induced release of DA and 5-HT indicated an altered balance between these neurotransmitters in the dorsal striatum of Pitx3-/- mice. To determine whether the increased 5-HT signalling was involved in the spontaneous hyperactivity during the light phase observed in Pitx3 deficiency, we treated Pitx3-deficient and control mice with the selective irreversible tryptophan hydroxylase inhibitor p-chlorophenylalanine to decrease 5-HT levels. Reduction of 5-HT levels in Pitx3-deficient mice decreased their locomotor activity to normal levels, whereas the same treatment increased the locomotor activity levels of control mice. Taken together, our results indicate alterations in 5-HT signalling in Pitx3-deficient mice that underlie their spontaneous hyperactivity.

Persistent neurochemical and behavioral abnormalities in adulthood despite early iron supplementation for perinatal iron deficiency anemia in rats

BACKGROUND

Iron deficiency anemia (IDA) has been associated with altered cognitive, motor, and social-emotional outcomes in human infants. We recently reported that rats with chronic perinatal IDA, had altered regional brain iron, monoamines, and sensorimotor skill emergence during early development.

OBJECTIVE

To examine the long-term consequences of chronic perinatal IDA on behavior, brain iron and monoamine systems after dietary iron treatment in rats.

METHODS

Sixty dams were randomly assigned to iron-sufficient (CN) or low-iron (EID) diets during gestation and lactation. Thereafter, all offspring were fed the iron-sufficient diet, assessed for hematology and behavior after weaning and into adulthood and for brain measures as adults (regional brain iron, monoamines, dopamine and serotonin transporters, and dopamine receptor). Behavioral assessments included sensorimotor function, general activity, response to novelty, spatial alternation, and spatial water maze performance.

RESULTS

Hematology and growth were similar for EID and CN rats by postnatal day 35. In adulthood, EID thalamic iron content was lower. Monoamines, dopamine transporter, and dopamine receptor concentrations did not differ from CN. EID serotonin transporter concentration was reduced in striatum and related regions. EID rats had persisting sensorimotor deficits (delayed vibrissae-evoked forelimb placing, longer sticker removal time, and more imperfect grooming chains), were more hesitant in novel settings, and had poorer spatial water maze performance than CN. General activity and spatial alternation were similar for EID and CN.

CONCLUSION

Rats that had chronic perinatal IDA showed behavioral impairments that suggest persistent striatal dopamine and hippocampal dysfunction despite normalization of hematology, growth and most brain measures.