Prenatal Amphetamine Exposure Effects on Dopaminergic Receptors and Transporter in Postnatal Rats

We investigated the influence of prenatal amphetamine exposure (PAE) on dopamine (DA) receptors, and dopamine transporter (DAT) in various striatal and limbic subregions and locomotor activity induced by novel environmental conditions and amphetamine at two postnatal ages, 35 days old (prepubertal) and 60 days old (postpubertal). Experiments were carried out on pregnant female Sprague–Dawley rats, which were daily injected with either d-amphetamine sulfate (1 mg/kg) or saline solution (0.9%) for 11 days, from gestation day 11–21. In PAE rats compared to control we found the following: at pre-pubertal age, an enhancement of DA D1 in the dorsolateral area of the caudate-putamen (CPu), CPu-ventral and shell of the nucleus accumbens (NAcc) with a decrement of the DA D3 receptors in NAcc, olfactory tubercle (OT), and the islands of Calleja (IoC); whereas at postpubertal age, an increase in the levels of DAT in the NAcc and fundus of the CPu, and OT along with a decrease in the expression of DA D2 receptors only in the NAcc shell were found in PAE rats compared to control. In addition, amphetamine induces a marked decrease in locomotor activity at postpubertal age in rats with PAE. These results suggest a differential effect of amphetamines on the DAT mechanism of the nervous system during embryonic development of animals with implications in behavior and drug addictions at adulthood age.     

Amphetamine-induced locomotion and gene expression are altered in BDNF heterozygous mice

Administration of amphetamine overstimulates medium spiny neurons (MSNs) by releasing dopamine and glutamate from afferents in the striatum. However, these afferents also release brain-derived neurotrophic factor (BDNF) that protects striatal MSNs from overstimulation. Intriguingly, all three neurochemicals increase opioid gene expression in MSNs. In contrast, striatal opioid expression is less in naive BDNF heterozygous (BDNF(+/-)) vs. wild-type (WT) mice. This study was designed to determine whether partial genetic depletion of BDNF influences the behavioral and molecular response to an acute amphetamine injection. An acute injection of amphetamine [5 mg/kg, intraperitoneal (i.p.)] or saline was administered to WT and BDNF(+/-) mice. WT and BDNF(+/-) mice exhibited similar locomotor activity during habituation, whereas BDNF(+/-) mice exhibited more prolonged locomotor activation during the third hour after injection of amphetamine. Three hours after amphetamine injection, there was an increase of preprodynorphin mRNA in the caudate putamen and nucleus accumbens (Acb) and dopamine D(3) receptor mRNA levels were increased in the Acb of BDNF(+/-) and WT mice. Striatal/cortical trkB and BDNF, and mesencephalic tyrosine hydroxylase mRNA levels were only increased in WT mice. These results indicate that BDNF modifies the locomotor responses of mice to acute amphetamine and differentially regulates amphetamine-induced gene expression.     

Role of medial prefrontal cortex dopamine in age differences in response to amphetamine in rats: Locomotor activity after intra-mPFC injections of dopaminergic ligands

Changes in medial prefrontal cortex (mPFC) dopamine receptor expression and in mPFC projections to the nucleus accumbens in adolescence suggest that there may be age differences in the regulation of drug‐related behavior by the mPFC. The age‐specific role of prelimbic D1 dopamine receptors on amphetamine‐induced locomotor activity was investigated. In experiment 1, rats aged postnatal day 30 (P30), P45, and P75, corresponding to early and late adolescence and adulthood, were given an injection of D1 and D2 antagonists into the prelimbic mPFC before a systemic injection of 1.5 mg/kg of amphetamine and locomotor activity was recorded. In experiment 2, effects of intra‐prelimbic injections of a D1 agonist and antagonist on locomotor activity produced by a lower dose (0.5 mg/kg) of amphetamine were investigated. D2 receptor antagonist did not alter amphetamine‐induced activity, whereas the D1 receptor antagonist reduced activity produced by 1.5 mg/kg of amphetamine more in P30 than in P45 and P75 rats. In addition, D1 agonist enhanced the locomotor activating effects of 0.5 mg/kg of amphetamine in adolescent rats and decreased activity in adult rats. These results suggest that insufficient activation of mPFC D1 receptors may underlie the reduced activity at the low dose of amphetamine in early adolescent compared to adult rats. © 2011 Wiley Periodicals, Inc. Develop Neurobiol, 2012     

Evidence for the involvement of ERbeta and RGS9-2 in 17-beta estradiol enhancement of amphetamine-induced place preference behavior

Estrogen enhances dopamine-mediated behaviors, which make women and female rats more sensitive to the effects of the psychostimulant drugs, cocaine and amphetamine. How cocaine and amphetamine elicit more robust behavioral responses in females remains unclear, but studies have shown that the Regulator of G-protein Signaling 9-2 (RGS9-2) protein is an important modulator of the behavioral responses to these drugs. Previously, we reported that 17-beta estradiol reduced RGS9-2 mRNA expression in the shell of the nucleus accumbens, but not the core. The present studies were designed to further evaluate the involvement of RGS9-2 in estradiol enhancement of amphetamine-induced place preference behavior and to examine which estrogen receptor subtype mediates the effect of estradiol. Female Sprague-Dawley rats were ovariectomized and treated for 14 days with an inert vehicle or 17-beta estradiol (by Silastic implant or injection [80 microg/kg]). 17-beta-Estradiol-treated female rats had enhanced amphetamine-induced conditioned place preference behavior compared to vehicle-treated, ovariectomized female rats. In situ hybridization histochemistry and Western blotting identified an inverse relationship between RGS9-2 protein expression in the nucleus accumbens shell and the hormonal enhancement of amphetamine-induced place preference behavior. A similar relationship was not found between place preference behavior and RGS9-2 expression in the accumbens core. Moreover, treatment of ovariectomized female rats with the selective estrogen receptor-beta agonist, diarylpropionitrile (1 mg/kg), for 2 weeks also facilitated amphetamine-induced place preference behavior and selectively reduced nucleus accumbens shell RGS9-2 protein expression. These data provide insight into a potential mechanism by which estrogen and/or sex modulate mesoaccumbal dopamine receptor signaling and possibly, addictive behaviors.     

Long-term effects of prenatal stress on dopamine and glutamate receptors in adult rat brain

Prenatal stress greatly influences the ability of an individual to manage stressful events in adulthood. Such vulnerability may result from abnormalities in the development and integration of forebrain dopaminergic and glutamatergic projections during the prenatal period. In this study, we assessed the effects of prenatal stress on the expression of selective dopamine and glutamate receptor subtypes in the adult offsprings of rats subjected to repeated restraint stress during the last week of pregnancy. Dopamine D₂-like receptors increased in dorsal frontal cortex (DFC), medial prefrontal cortex (MPC), hippocampal CA₁ region and core region of nucleus accumbens (NAc) of prenatally stressed rats compared to control subjects. Glutamate NMDA receptors increased in MPC, DFC, hippocampal CA₁, medial caudate-putamen, as well as in shell and core regions of NAc. Group III metabotropic glutamate receptors increased in MPC and DFC of prenatally stressed rats, but remained unchanged in all other regions examined. These results indicate that stress suffered during the gestational period has long lasting effects that extend into the adulthood of prenatally stressed offsprings. Changes in dopamine and glutamate receptor subtype levels in different forebrain regions of adult rats suggest that the development and formation of the corticostriatal and corticolimbic pathways may be permanently altered as a result of stress suffered prenatally. Maldevelopment of these pathways may provide a neurobiological substrate for the development of schizophrenia and other idiopathic psychotic disorders.     

Differential Effects of δ- and μ-Opioid Receptor Antagonists on the Amphetamine-Induced Increase in Extracellular Dopamine in Striatum and Nucleus Accumbens

Abstract: The specific opioid receptor antagonist naloxone attenuates the behavioral and neurochemical effects of amphetamine. Furthermore, the amphetamine-induced increase in locomotor activity is attenuated by intracisternally administered naltrindole, a selective δ-opioid receptor antagonist, but not by the irreversible μ-opioid receptor antagonist β-funaltrexamine. Therefore, this research was designed to determine if naltrindole would attenuate the neurochemical response to amphetamine as it did the behavioral response. In vivo microdialysis was used to monitor the change in extracellular concentrations of dopamine in awake rats. Naltrindole (3.0, 10, or 30 µg) or vehicle was given 15 min before and β-funaltrexamine (10 µg) or vehicle 24 h before the start of cumulative dosing, intracisternally in a 10-µl volume, while the rats were lightly anesthetized with methoxyflurane. Cumulative doses of subcutaneous d-amphetamine (0.0, 0.1, 0.4, 1.6, and 6.4 mg/kg) followed pretreatment injections at 30-min intervals. Dialysate samples were collected every 10 min from either the striatum or nucleus accumbens and analyzed for dopamine content by HPLC. Amphetamine dose-dependently increased dopamine content in both the striatum and nucleus accumbens, as reported previously. Naltrindole (3.0, 10, and 30 µg) significantly reduced the dopamine response to amphetamine in the striatum. In contrast, 30 µg of naltrindole did not modify the dopamine response to amphetamine in the nucleus accumbens. On the other hand, β-funaltrexamine (10 µg) had no effect in the striatum but significantly attenuated the amphetamine-induced increase in extracellular dopamine content in the nucleus accumbens. These data suggest that δ-opioid receptors play a relatively larger role than μ-opioid receptors in mediating the amphetamine-induced increase in extracellular dopamine content in the striatum, whereas μ-opioid receptors play a larger role in mediating these effects in the nucleus accumbens.     

Adenosine and dopamine receptor antagonist binding in the rat ventral and dorsal striatum: lack of changes after a neonatal bilateral lesion of the ventral hippocampus.

There is experimental evidence from radioligand binding experiments for the existence of strong antagonistic interactions between different subtypes of adenosine and dopamine receptors in the striatum, mainly between adenosine A1 and dopamine D1 and between adenosine A2A and dopamine D2 receptors. These interactions seem to be more powerful in the ventral compared to the dorsal striatum, which might have some implications for the treatment of schizophrenia. The binding characteristics of different dopamine and adenosine receptor subtypes were analysed in the different striatal compartments (dorsolateral striatum and shell and core of the nucleus accumbens), by performing saturation experiments with the dopamine D1 receptor antagonist [125I]SCH-23982, the dopamine D2-3 receptor antagonist [3H]raclopride, the adenosine A1 receptor antagonist [3H]DPCPX and the adenosine A2A receptor antagonist [3H]SCH 58261. The experiments were also performed in rats with a neonatal bilateral lesion of the ventral hippocampus (VH), a possible animal model of schizophrenia. Both dopamine D2-3 and adenosine A2A receptors follow a similar pattern, with a lower density of receptors (40%) in the shell of the nucleus accumbens compared with the dorsolateral caudate-putamen. A lower density of adenosine A1 receptors (20%) was also found in the shell of the nucleus accumbens compared with the caudate-putamen. On the other hand, dopamine D1 receptors showed a similar density in the different striatal compartments. Therefore, differences in receptor densities cannot explain the stronger interactions between adenosine and dopamine receptors found in the ventral, compared to the dorsal striatum. No statistical differences in the binding characteristics of any of the different adenosine and dopamine receptor antagonists used were found between sham-operated and VH-lesioned rats.     

Behavioral responses to pups in males with different reproductive experiences are associated with changes in central OT,TH and OTR,D1R,D2R mRNA expression in mandarin voles

Male rodents behave differently toward pups because of different sexual and/or paternal experiences; however, the mechanisms underlying these responses are not well understood. Using socially monogamous mandarin voles (Microtus mandarinus) we investigated the behavioral responses of males with different reproductive experiences (virgin males, paired males and new fathers) to new born pups. Central levels of neuropeptide oxytocin (OT), tyrosine hydroxylase (TH), as well as oxytocin receptor (OTR), dopamine 1-type receptor (D1R) and dopamine 2-type receptor (D2R) mRNA expression in the nucleus accumbens and medial amygdala were also measured in these males. Our data showed that new fathers exhibited more approaching behavior and contained more OT-immunoreactive and TH-immunoreactive neurons. In addition to increased OTR mRNA expression in the nucleus accumbens and medial amygdala, new fathers had higher D1R and D2R mRNA expression in the nucleus accumbens, and less D1R and D2R mRNA expression in the medial amygdala than paired males. These results demonstrate that males with different reproductive experiences display different behavioral responses to pups and that these differences are associated with altered OT and dopamine, and their receptors in specific brain regions.     

Prenatal Inflammation-Induced Hypoferremia Alters Dopamine Function in the Adult Offspring in Rat: Relevance for Schizophrenia

Maternal infection during pregnancy has been associated with increased incidence of schizophrenia in the adult offspring. Mechanistically, this has been partially attributed to neurodevelopmental disruption of the dopamine neurons, as a consequence of exacerbated maternal immunity. In the present study we sought to target hypoferremia, a cytokine-induced reduction of serum non-heme iron, which is common to all types of infections. Adequate iron supply to the fetus is fundamental for the development of the mesencephalic dopamine neurons and disruption of this following maternal infection can affect the offspring''s dopamine function. Using a rat model of localized injury induced by turpentine, which triggers the innate immune response and inflammation, we investigated the effects of maternal iron supplementation on the offspring''s dopamine function by assessing behavioral responses to acute and repeated administration of the dopamine indirect agonist, amphetamine. In addition we measured protein levels of tyrosine hydroxylase, and tissue levels of dopamine and its metabolites, in ventral tegmental area, susbtantia nigra, nucleus accumbens, dorsal striatum and medial prefrontal cortex. Offspring of turpentine-treated mothers exhibited greater responses to a single amphetamine injection and enhanced behavioral sensitization following repeated exposure to this drug, when compared to control offspring. These behavioral changes were accompanied by increased baseline levels of tyrosine hydroxylase, dopamine and its metabolites, selectively in the nucleus accumbens. Both, the behavioral and neurochemical changes were prevented by maternal iron supplementation. Localized prenatal inflammation induced a deregulation in iron homeostasis, which resulted in fundamental alterations in dopamine function and behavioral alterations in the adult offspring. These changes are characteristic of schizophrenia symptoms in humans.     

Prenatal Morphine Exposure Differentially Alters Addictive and Emotional Behavior in Adolescent and Adult Rats in a Sex-Specific Manner

The effects of prenatal opioid exposure in adult animals has been widely studied, but little is known about the effects of prenatal opioid on adolescents. Most of the risk behaviors associated with drug abuse are initiated during adolescence. The developmental state of the adolescent brain makes it vulnerable to initiate drug use and susceptible to drug-induced brain changes. In this study, pregnant rats were subcutaneously injected with an increasing dose of morphine (5 mg/kg, 7 mg/kg, 10 mg/kg) for 9 days since the gestation day 11. The effects of prenatal morphine (PNM) on learning and memory, anxiety- and depressive- like behavior, morphine induced conditioned place preference (CPP) as well as locomotor sensitization were tested in both adolescent and adult rats. The results showed that: (1) PNM decreased anxiety-like behavior in both adolescent and adult female rats, but not males; (2) PNM decreased depressive-like behavior in adolescent but increased depressive -like behavior in adult females; (3) PNM increased low dose morphine induced locomotor sensitization in females; (4) PNM decreased tyrosine hydroxylase (TH) expression in the prefrontal cortex but decreased dopamine D1 receptor expression in the nucleus-accumbens (NAc) in female rats. These results suggested that PNM altered the emotional and addictive behavior mainly in female rats, with female rats being less anxiety and depressive during adolescence, but more depressive in adult, and more sensitive to low dose morphine induced locomotor activity sensitization, which might be mediated in part by the differential expression of the TH, dopamine D1 receptors in the female brain.

     

The Dopamine D1-D2 Receptor Heteromer Localizes in Dynorphin/Enkephalin Neurons: INCREASED HIGH AFFINITY STATE FOLLOWING AMPHETAMINE AND IN SCHIZOPHRENIA*

The distribution and function of neurons coexpressing the dopamine D1 and D2 receptors in the basal ganglia and mesolimbic system are unknown. We found a subset of medium spiny neurons coexpressing D1 and D2 receptors in varying densities throughout the basal ganglia, with the highest incidence in nucleus accumbens and globus pallidus and the lowest incidence in caudate putamen. These receptors formed D1-D2 receptor heteromers that were localized to cell bodies and presynaptic terminals. In rats, selective activation of D1-D2 heteromers increased grooming behavior and attenuated AMPA receptor GluR1 phosphorylation by calcium/calmodulin kinase IIα in nucleus accumbens, implying a role in reward pathways. D1-D2 heteromer sensitivity and functional activity was up-regulated in rat striatum by chronic amphetamine treatment and in globus pallidus from schizophrenia patients, indicating that the dopamine D1-D2 heteromer may contribute to psychopathologies of drug abuse, schizophrenia, or other disorders involving elevated dopamine transmission.     

Morphine-induced increase in D-1 receptor regulated signal transduction in rat striatal neurons and its facilitation by glucocorticoid receptor activation: Possible role in behavioral sensitization

One month (but not 1–3 days) after intermittent morphine administration, the hyperresponsiveness of rats toward the locomotor effects of morphine and amphetamine was associated with an increase in dopamine (DA) D-1 receptor-stimulated adenylyl cyclase activity and enhanced steady state levels of preprodynorphin gene expression in slices of the caudate/putamen and nucleus accumbens. Such an enduring increase in postsynaptic D-1 receptor efficacy also occurred in cultured γ-aminobutyric acid (GABA) neurons of the striatum obtained from rats prenatally treated with morphine. Interestingly, in vitro glucocorticoid receptor activation in these cultured striatal neurons by corticosterone potentiated this neuroadaptive effect of prior in vivo morphine exposure. Since activation of glucocorticoid receptors by corticosterone did not affect D-1 receptor functioning in cultured neurons of saline-pretreated rats, prior intermittent exposure to morphine (somehow) appears to induce a long-lasting state of corticosterone hyperresponsiveness in striatal neurons. Therefore, DA-sensitive striatal GABA neurons may represent common neuronal substrates acted upon by morphine and corticosterone. We hypothesize that the delayed occurrence of these long-lasting morphine-induced neuroadaptive effects in GABA/dynorphin neurons of the striatum is involved in the enduring nature of behavioral sensitization to drugs of abuse and cross-sensitization to stressors. Special issue dedicated to Dr. Eric J. Simon.     

Prenatal stress increases anxiety related behavior and alters cerebral lateralization of dopamine activity

Effects of unpredictable (random) prenatal stress on the level of anxiety and cerebral lateralization of dopamine turnover rates were studied in rats. The observation of a decrease in the amount of time spent in the open arms of a "plus-maze" supported earlier findings of an increased fearfulness to stressful situations in the offspring in adulthood. We also observed elevated rates of dopamine turnover in the right prefrontal cortex and reduced dopamine activity in the right nucleus accumbens and left corpus striatum of the prenatally stressed animals. This resulted in directional shifts of left-right differences in dopamine activity in all 3 areas. These findings indicate that prenatal stress induces permanent alterations in dopaminergic activity and in cerebral asymmetry. We suggest that the changes in cerebral lateralization of dopamine function may underly the increase in reactivity to anxiety-provoking situations in prenatally stressed offspring.     

Altered dopamine signaling in naturally occurring maternal neglect

Background

Child neglect is the most common form of child maltreatment, yet the biological basis of maternal neglect is poorly understood and a rodent model is lacking.

Methodology/Principal Findings

The current study characterizes a population of mice (MaD1) which naturally exhibit maternal neglect (little or no care of offspring) at an average rate of 17% per generation. We identified a set of risk factors that can predict future neglect of offspring, including decreased self-grooming and elevated activity. At the time of neglect, neglectful mothers swam significantly more in a forced swim test relative to nurturing mothers. Cross-fostered offspring raised by neglectful mothers in turn exhibit increased expression of risk factors for maternal neglect and decreased maternal care as adults, suggestive of possible epigenetic contributions to neglect. Unexpectedly, offspring from neglectful mothers elicited maternal neglect from cross-fostered nurturing mothers, suggesting that factors regulating neglect are not solely within the mother. To identify a neurological pathway underlying maternal neglect, we examined brain activity in neglectful and nurturing mice. c-Fos expression was significantly elevated in neglectful relative to nurturing mothers in the CNS, particularly within dopamine associated areas, such as the zona incerta (ZI), ventral tegmental area (VTA), and nucleus accumbens. Phosphorylated tyrosine hydroxylase (a marker for dopamine production) was significantly elevated in ZI and higher in VTA (although not significantly) in neglectful mice. Tyrosine hydroxylase levels were unaltered, suggesting a dysregulation of dopamine activity rather than cell number. Phosphorylation of DARPP-32, a marker for dopamine D1-like receptor activation, was elevated within nucleus accumbens and caudate-putamen in neglectful versus nurturing dams.

Conclusions/Significance

These findings suggest that atypical dopamine activity within the maternal brain, especially within regions involved in reward, is involved in naturally occurring neglect and that MaD1 mice are a useful model for understanding the basis of naturally occurring neglect.     

Effect of gonadal steroids and gamma-aminobutyric acid on LH release and dopamine expression and activity in the zona incerta in rats

A dopaminergic system in the zona incerta stimulates LH release and may mediate the positive feedback effects of the gonadal steroids on LH release. In this study the mechanisms by which steroids might increase dopamine activity in the zona incerta were investigated. In addition, experiments were conducted to determine whether the inhibitory effects of gamma-aminobutyric acid (GABA) on LH release in the zona incerta are due to suppression of dopamine activity in this area or conversely whether the stimulatory effects of dopamine on LH release are due to suppression of a tonic inhibitory GABAergic system. Ovariectomized rats were treated s.c. with oil, 5 micrograms oestradiol benzoate or 5 micrograms oestradiol benzoate followed 48 h later by 0.5 mg progesterone, and killed 54 h after the oestradiol benzoate injection. At this time the LH concentrations were suppressed in the oestradiol benzoate group and increased in the group treated with oestradiol benzoate and progesterone. The ratio of tyrosine hydroxylase:beta-actin mRNA in the zona incerta was significantly increased by the oestradiol benzoate treatment, but the addition of progesterone resulted in values similar to those in the control group. At the same time, the progesterone treatment increased tyrosine hydroxylase activity in the zona incerta as indicated by an increase in L-dihydroxyphenylalanine (L-DOPA) accumulation after 100 mg 3-hydroxybenzylhydrazine hydrochloric acid (NSD1015) kg-1 and an increase in dopamine release as indicated by a increase in dihydroxyphenylacetic acid (DOPAC) concentrations (one of the major metabolites of dopamine). Ovariectomized rats treated with oestradiol benzoate plus progesterone were also injected i.p. with 75 mg gamma-acetylenic GABA kg-1 (a GABA transaminase inhibitor) to increase GABA concentrations in the brain. This treatment had no effect on the ratio of tyrosine hydroxylase:beta-actin mRNA but decreased L-DOPA accumulation and DOPAC concentrations in the zona incerta, indicating a post-translational inhibition of dopamine synthesis and release. Treatment of ovariectomized rats with oestradiol benzoate followed by 100 mg L-DOPA i.p. to increase dopamine concentrations in the whole brain had no effect on glutamic acid decarboxylase mRNA expression in the zona incerta, although it increased the glutamic acid decarboxylase:beta-actin mRNA ratio in other hypothalamic areas (that is, the medical preoptic area, ventromedial nucleus and arcuate nucleus). In conclusion, the steroids act to increase dopamine activity in different ways: oestrogen increases tyrosine hydroxylase mRNA expression and progesterone acts after translation to increase tyrosine hydroxylase activity and dopamine release (as indicated by increases in DOPAC concentrations). This latter effect may be due to progesterone removing a tonic GABAergic inhibition from the dopaminergic system.     

The distribution of a dopamine D2 receptor mRNA in rat brain

Based on the recently reported sequence of a dopamine D2 receptor cloned from rat brain, we prepared a series of cDNA probes to determine the distribution of mRNA encoding this receptor. Within the forebrain, D2 receptor mRNA is abundant in the caudate-putamen, accumbens nucleus and olfactory tubercle. Moderate to low levels of mRNA are observed in the medial habenular nucleus, diagonal band, lateral septal nucleus, claustrum, dorsal endopiriform nucleus, and entorhinal cortex. In the mesencephalon, D2 receptor mRNA is abundant within the substantia nigra, pars compacta, and the ventral tegmental area. Comparison of the distribution of the mRNA and ligand binding indicates that both presynaptic and postsynaptic D2 receptors of the nigrostriatal, mesolimbic and mesocortical pathways are derived from the same mRNA.     

Neuroadaptations to hyperdopaminergia in dopamine D3 receptor-deficient mice

The dopamine D3 receptor (D3R) has been implicated in schizophrenia, drug addiction, depression and Parkinson's disease. The D3R is localized post-synaptically on nucleus accumbens neurons, but is also an autoreceptor on dopaminergic neurons in the mesencephalon. Its functional role as autoreceptor is highly debated, but supported by the elevated basal extracellular dopamine levels found in D3R-deficient mice. To investigate the functional role of the D3R in vivo, we used mice with a targeted disruption of the D3R gene. We found a higher basal level of grooming in D3R-deficient mice, compared to their wild-type littermates. This behavior, which is under the control of D1R stimulation, may be related to an increased dopaminergic tone, since no changes in the gene expression of dopamine D1 and D2 receptors were noticed in the striatum of these mice. D3R-deficient mice displayed other neuroadaptive changes, including decreased tyrosine hydroxylase, increased dopamine transporter mRNAs and increased dopamine reuptake in striatum. The level of tyrosine hydroxylase protein was unchanged in the striatum, as preprodynorphin and preproenkephalin gene expressions. All the changes identified in D3R-deficient mice cannot explain hyperdopaminergia, but, on the contrary, tend to attenuate this phenotype. These results support a distinct role for D2R and D3R as autoreceptors: the D2R is the release-regulating and firing rate-regulating autoreceptor, whereas the D3R may control basal dopamine levels in the striatum, by an unknown mechanism, which does not involve regulation of dopamine transporters or tyrosine hydroxylase. This hyperdopaminergia phenotype of D3R-deficient mice may explain their hyperactivity to drug-paired environmental cues.     

The usefulness of the spontaneously hypertensive rat to model attention-deficit/hyperactivity disorder (ADHD) may be explained by the differential expression of dopamine-related genes in the brain

Spontaneously hypertensive rats (SHR) are considered to represent a genetic animal model for attention-deficit hyperactivity disorder (ADHD). In the present studies, we compared the locomotor activity, learning and memory functions of juvenile male SHR, with age- and gender-matched genetic control Wistar-Kyoto rats (WKY). In addition, we investigated potential differences in brain morphology by magnetic resonance imaging (MRI). In other complimentary studies of the central nervous system, we used real-time PCR to examine the levels of several dopaminergic-related genes, including those coding for the five major subtypes of dopamine receptor (D1, D2, D3, D4 and D5), those coding for enzymes responsible for synthesizing tyrosine hydroxylase and dopamine-beta-hydroxylase, and those coding for the dopamine transporter. Our data revealed that SHR were more active than WKY in the open field (OF) test. Also, SHR appeared less attentive, exhibiting inhibition deficit, but in the absence of memory deficits relative to spatial learning. The MRI studies revealed that SHR had a significantly smaller vermis cerebelli and caudate-putamen (CPu), and there was also a significantly lower level of dopamine D4 receptor gene expression and protein synthesis in the prefrontal cortex (PFC) of SHR. However, there were no significant differences between the expression of other dopaminergic-related genes in the midbrain, prefrontal cortex, temporal cortex, striatum, or amygdala of SHR and WKY. The data are similar to the situation seen in ADHD patients, relative to normal volunteers, and it is possible that the hypo-dopaminergic state involves a down regulation of dopamine D4 receptors, rather than a general down-regulation of catecholamine synthesis. In conclusion, the molecular and behavioural data that we obtained provide new information that may be relevant to understanding ADHD in man.