Repeated toluene exposure leads to neuroadaptation in dopamine release mechanisms within the nucleus accumbens core.

BACKGROUND: Intentionally inhaling volatile organic solvent like toluene for its intoxicating effects continues to be a public health concern. While repeated abuse of toluene has deleterious behavioral and health effects, little is known about the actions of toluene on the dopaminergic neurotransmitter system within the central nervous system. METHOD: The present study employed complementary neurochemical techniques of slice fast-scan cyclic voltammetry (FSCV) and in vivo microdialysis, to assess dopamine (DA) dynamics immediately after repeated exposure to 2000- or 4000-ppm toluene. DA D3 autoreceptor functionality, measured by FSCV with pharmacological manipulations and brain tissue content analysis with high performance liquid chromatography, were also used to account for the changes in the DA dynamics. RESULTS: Toluene-exposed mice had decreased stimulated DA release only in the nucleus accumbens core immediately after seven days of repeated exposure. DA uptake was decreased in the core only after 2000-ppm exposure. The differences in stimulated DA release were not attributed to alterations in intraneuronal DA levels as measured by tissue content analysis. Basal extracellular DA levels were not significantly different between the air- and toluene-treated mice. However, following an additional toluene exposure, mice had elevated extracellular DA levels in the nucleus accumbens during recovery. This potentiation in extracellular accumbal DA levels was further heightened following potassium stimulation. The accumbal DA D3 autoreceptor function did not appear to play a role as a potential mediator for these differences. CONCLUSION: Our FSCV and microdialysis results suggest a neuroadaptation in DA release mechanics within the nucleus accumbens, but the exact neuronal mechanism of toluene's impact remains elusive.

Developmental pesticide exposure reproduces features of attention deficit hyperactivity disorder.

Attention-deficit hyperactivity disorder (ADHD) is estimated to affect 8-12% of school-age children worldwide. ADHD is a complex disorder with significant genetic contributions. However, no single gene has been linked to a significant percentage of cases, suggesting that environmental factors may contribute to ADHD. Here, we used behavioral, molecular, and neurochemical techniques to characterize the effects of developmental exposure to the pyrethroid pesticide deltamethrin. We also used epidemiologic methods to determine whether there is an association between pyrethroid exposure and diagnosis of ADHD. Mice exposed to the pyrethroid pesticide deltamethrin during development exhibit several features reminiscent of ADHD, including elevated dopamine transporter (DAT) levels, hyperactivity, working memory and attention deficits, and impulsive-like behavior. Increased DAT and D1 dopamine receptor levels appear to be responsible for the behavioral deficits. Epidemiologic data reveal that children aged 6-15 with detectable levels of pyrethroid metabolites in their urine were more than twice as likely to be diagnosed with ADHD. Our epidemiologic finding, combined with the recapitulation of ADHD behavior in pesticide-treated mice, provides a mechanistic basis to suggest that developmental pyrethroid exposure is a risk factor for ADHD.

Deficits in behavioral sensitization and dopaminergic responses to methamphetamine in adenylyl cyclase 1/8-deficient mice.

The cAMP/protein kinase A pathway regulates methamphetamine (METH)-induced neuroplasticity underlying behavioral sensitization. We hypothesize that adenylyl cyclases (AC) 1/8 mediate these neuroplastic events and associated striatal dopamine regulation. Locomotor responses to METH (1 and 5 mg/kg) and striatal dopamine function were evaluated in mice lacking AC 1/8 (DKO) and wild-type (WT) mice. Only 5 mg/kg METH induced an acute locomotor response in DKO mice, which was significantly attenuated versus WT controls. DKO mice showed a marked attenuation in the development and expression of METH-induced behavioral sensitization across doses relative to WT controls. While basal and acute METH (5 mg/kg)-evoked accumbal dialysate dopamine levels were similar between genotypes, saline-treated DKO mice showed elevated tissue content of dopamine and homovanillic acid in the dorsal striatum (DS), reflecting dysregulated dopamine homeostasis and/or metabolism. Significant reductions in DS dopamine levels were observed in METH-sensitized DKO mice compared to saline-treated controls, an effect not observed in WT mice. Notably, saline-treated DKO mice had significantly increased phosphorylated Dopamine- and cAMP-regulated phosphoprotein levels, which were not further augmented following METH sensitization, as observed in WT mice. These data indicate that AC 1/8 are critical to mechanisms subserving drug-induced behavioral sensitization and mediate nigrostriatal pathway METH sensitivity. Calcium/calmodulin-stimulated adenylyl cyclase (AC) isoforms 1 and 8 were studied for their involvement in the adaptive neurobehavioral responses to methamphetamine. AC 1/8 double knockout (DKO) mice showed heightened basal locomotor activity and dorsal striatal dopamine responsivity. Conversely, methamphetamine-induced locomotor activity was attenuated in DKO mice, accompanied by reductions in dopamine and HVA content and impaired DARPP-32 activation. These findings indicate AC 1/8 signaling regulates the sensitivity of the nigrostriatal pathway subserving stimulant and neuroadaptive sensitizing effects of methamphetamine. 3-MT, 3-methoxytyramine; Ca(2+), calcium; CaM, calmodulin; cdk5; cyclin-dependent kinase 5; DA, dopamine; DARPP-32, dopamine- and cAMP-regulated phosphoprotein; D1R, dopamine D1 receptor; HVA, homovanillic acid; PKA, protein kinase A.

Simultaneous detection of monoamine and purine molecules using high-performance liquid chromatography with a boron-doped diamond electrode.

A recently available boron-doped diamond (BDD) working electrode has been developed for use with high-performance liquid chromatography (HPLC) to aid in the detection of molecules with high redox potentials. In this work, we developed a method using a commercially available BDD working electrode for detecting neurotransmitters from two different families with large oxidation potential differences, namely, dopamine (DA) and adenosine (Ado). Hydrodynamic voltammograms were constructed for DA and Ado, and the optimal potentials for the detection of DA and Ado were determined to be +740 and +1200 mV versus a palladium reference electrode, respectively. A working potential of +840 mV was chosen, and the detection range achieved with the BDD electrode for DA and Ado was from low nanomolar to high millimolar levels. To determine the practical function of the BDD electrode, tissue content was analyzed for seven monoamine and two purine molecules, which were resolved in a single run in less than 28 min. Our results demonstrate that the BDD electrode is sensitive and robust enough to detect monoamine and purine molecules from frontal cortex and striatal mouse samples. Using a BDD electrode opens the possibility of exploring multiple classes of neurotransmitters in a single run using electrochemical detection to probe their interactions.

Aberrant striatal dopamine transmitter dynamics in brain-derived neurotrophic factor-deficient mice.

Brain-derived neurotrophic factor (BDNF) modulates the synaptic transmission of several monoaminergic neuronal systems, including forebrain dopamine-containing neurons. Recent evidence shows a strong correlation between neuropsychiatric disorders and BDNF hypofunction. The aim of the present study was to characterize the effect of low endogenous levels of BDNF on dopamine system function in the caudate-putamen using heterozygous BDNF (BDNF(+/-) ) mice. Apparent extracellular dopamine levels in the caudate-putamen, determined by quantitative microdialysis, were significantly elevated in BDNF(+/-) mice compared with wildtype controls (12 vs. 5 nM, respectively). BDNF(+/-) mice also had a potentiated increase in dopamine levels following potassium (120 mM)-stimulation (10-fold) relative to wildtype controls (6-fold). Slice fast-scan cyclic voltammetry revealed that BDNF(+/-) mice had reductions in both electrically evoked dopamine release and dopamine uptake rates in the caudate-putamen. Superfusion of BDNF led to partial recovery of the electrically stimulated dopamine release response in BDNF(+/-) mice. Conversely, tissue accumulation of L-3,4-dihydroxyphenylalanine, extracellular levels of dopamine metabolites, and spontaneous locomotor activity were unaltered. Together, this study indicates that endogenous BDNF influences dopamine system homeostasis by regulating the release and uptake dynamics of pre-synaptic dopamine transmission.

Effects of chronic alcohol exposure on dopamine uptake in rat nucleus accumbens and caudate putamen.

RATIONALE: Existing data strongly suggest that alcohol affects dopamine (DA) neurotransmission in the brain. However, many questions remain about the effects of alcohol on the delicate equilibrium between such neurochemical processes as DA release and uptake. Dysregulation of these processes in the mesolimbic and nigrostriatal systems after chronic alcohol ingestion could be a neuroadaptation contributing to dependence. OBJECTIVES: In the present study, we have employed an alcohol vapor inhalation model to characterize the effects of chronic alcohol exposure on DA dynamics in rat nucleus accumbens (NAc) and caudate putamen (CP) using fast-scan cyclic voltammetry (FSCV) in brain slices. This method provides a unique view of real-time, spatially resolved changes in DA concentration. RESULTS: We found that chronic alcohol exposure enhanced DA uptake rates in rat NAc and CP. These changes would have the effect of down-regulating extracellular DA levels, presumably a compensatory effect related to increased DA release by repeated alcohol exposure. The sensitivity of terminal release-regulating DA autoreceptors was not different in alcohol-exposed rats compared with alcohol-naïve animals. CONCLUSIONS: The DA uptake changes after chronic alcohol exposure documented here using FSCV may be associated with a compensatory response of the DA system aimed at decreasing DA signaling. Alterations in autoreceptor function may require relatively long lasting alcohol exposure.

Toluene's effects on activity and extracellular dopamine in the mouse are altered by GABAA antagonism.

The abuse of inhalants like toluene continues to be widespread around the world, especially among children and teenagers. Despite its frequency of misuse, the dynamics between dopamine (DA) and gamma-aminobutyric acid (GABA) in response to toluene exposure remains unclear. To further decipher toluene's actions, we used a dynamic exposure system in combination with microdialysis to examine in vivo the effects of acutely inhaled toluene on DA release within the mouse caudate putamen (CPu). Results show that toluene inhalation produced increases in DA levels and locomotor activity. In mice that were pretreated with the GABAA antagonist, bicuculline, there was no change in the locomotor response during toluene but activity was potentiated following toluene exposure. Bicuculline pretreatment increased extracellular DA levels during toluene exposure, suggesting that DA and GABA-releasing neuron interaction may play a role in the rewarding properties of toluene.

Dopamine synthesis in alcohol drinking-prone and -resistant mouse strains.

Alcoholism is a prevalent and debilitating neuropsychiatric disease, and much effort has been aimed at elucidating the neurobiological mechanisms underlying maladaptive alcohol drinking in an effort to design rational treatment strategies. In preclinical literature, the use of inbred mouse lines has allowed for the examination of ethanol effects across vulnerable and resistant phenotypes. C57BL/6J mice consistently show higher rates of ethanol drinking compared to most mouse strains. Conversely, DBA/2J mice display low rates of ethanol consumption. Given that the reinforcing and rewarding effects of ethanol are thought to be in part mediated by its actions on dopamine neurotransmission, we hypothesized that alcohol-preferring C57BL/6J and alcohol-avoiding DBA/2J mice would display basal differences in dopamine system function. By administering an L-aromatic acid decarboxylase inhibitor and measuring L-Dopa accumulation via high-performance liquid chromatography as a measure of tyrosine hydroxylase activity, we found no difference in dopamine synthesis between mouse strains in the midbrain, dorsal striatum, or ventral striatum. However, we did find that quinpirole-induced inhibition of dopamine synthesis was greater in the ventral striatum of C57BL/6J mice, suggesting increased presynaptic D2-type dopamine autoreceptor sensitivity. To determine whether dopamine synthesis or autoreceptor sensitivity was altered by a history of ethanol, we exposed C57BL/6J mice to one or two weekly cycles of chronic intermittent ethanol (CIE) exposure and withdrawal. We found that there was an attenuation of baseline dopamine synthesis in the ventral striatum after two cycles of CIE. Finally, we examined tissue content of dopamine and dopamine metabolites across recombinant inbred mice bred from a C57BL/6J × DBA/2J cross (BXD). We found that low dopaminergic activity, as indicated by high dopamine/metabolite ratios, was positively correlated with drinking. Together, these findings show differential autoreceptor effects on dopamine synthesis between C57BL/6J and DBA/2J mice, and suggest that decreased dopaminergic activity is associated with excessive drinking.

Impaired Brain Dopamine and Serotonin Release and Uptake in Wistar Rats Following Treatment with Carboplatin.

Chemotherapy-induced cognitive impairment, known also as "chemobrain", is a medical complication of cancer treatment that is characterized by a general decline in cognition affecting visual and verbal memory, attention, complex problem solving skills, and motor function. It is estimated that one-third of patients who undergo chemotherapy treatment will experience cognitive impairment. Alterations in the release and uptake of dopamine and serotonin, central nervous system neurotransmitters that play important roles in cognition, could potentially contribute to impaired intellectual performance in those impacted by chemobrain. To investigate how chemotherapy treatment affects these systems, fast-scan cyclic voltammetry (FSCV) at carbon-fiber microelectrodes was used to measure dopamine and serotonin release and uptake in coronal brain slices containing the striatum and dorsal raphe nucleus, respectively. Measurements were taken from rats treated weekly with selected doses of carboplatin and from control rats treated with saline. Modeling the stimulated dopamine release plots revealed an impairment of dopamine release per stimulus pulse (80% of saline control at 5 mg/kg and 58% at 20 mg/kg) after 4 weeks of carboplatin treatment. Moreover, Vmax, the maximum uptake rate of dopamine, was also decreased (55% of saline control at 5 mg/kg and 57% at 20 mg/kg). Nevertheless, overall dopamine content, measured in striatal brain lysates by high performance liquid chromatography, and reserve pool dopamine, measured by FSCV after pharmacological manipulation, did not significantly change, suggesting that chemotherapy treatment selectively impairs the dopamine release and uptake processes. Similarly, serotonin release upon electrical stimulation was impaired (45% of saline control at 20 mg/kg). Measurements of spatial learning discrimination were taken throughout the treatment period and carboplatin was found to alter cognition. These studies support the need for additional neurochemical and behavioral analyses to identify the underlying mechanisms of chemotherapy-induced cognitive disorders.

Dopamine and opioid systems interact within the nucleus accumbens to maintain monogamous pair bonds.

Prairie vole breeder pairs form monogamous pair bonds, which are maintained through the expression of selective aggression toward novel conspecifics. Here, we utilize behavioral and anatomical techniques to extend the current understanding of neural mechanisms that mediate pair bond maintenance. For both sexes, we show that pair bonding up-regulates mRNA expression for genes encoding D1-like dopamine (DA) receptors and dynorphin as well as enhances stimulated DA release within the nucleus accumbens (NAc). We next show that D1-like receptor regulation of selective aggression is mediated through downstream activation of kappa-opioid receptors (KORs) and that activation of these receptors mediates social avoidance. Finally, we also identified sex-specific alterations in KOR binding density within the NAc shell of paired males and demonstrate that this alteration contributes to the neuroprotective effect of pair bonding against drug reward. Together, these findings suggest motivational and valence processing systems interact to mediate the maintenance of social bonds.

Local effects of acute ethanol on dopamine neurotransmission in the ventral striatum in C57BL/6 mice.

In this study, fast-scan cyclic voltammetry in brain slices was used to evaluate the effects of acute ethanol on dopamine terminal release and uptake in the nucleus accumbens of C57BL/6 mice. We found that pharmacologically relevant concentrations of ethanol (20 and 100 mM) did not alter electrically evoked dopamine release, while the highest concentration (200 mM) significantly decreased release (approximately 45%). No significant changes were observed in the rate of dopamine uptake after ethanol (20, 100 or 200 mM). In addition, it was established that a moderate dose (2 g/kg, i.p.) of ethanol did not alter the rate of dopamine synthesis, measured as L-dihydroxyphenylalanine (L-DOPA) accumulation. However, a high dose (5 g/kg, i.p.) of ethanol significantly increased the levels of L-DOPA to 60% above the control value. These data are consistent with earlier findings obtained in brain slices from rats; dopamine release, but not clearance, is affected by acute ethanol.

Diphenylpyraline, a histamine H1 receptor antagonist, has psychostimulant properties.

Diphenylpyraline hydrochloride (DPP) is used clinically as an antihistamine drug, but its neurobiological effects are not completely understood. Voltammetry and microdialysis were used to investigate potential actions of DPP on the dopamine system. Voltammetric monitoring of dopamine signals in mouse nucleus accumbens slices showed that DPP (10 microM) markedly inhibited dopamine uptake. There was a 20-fold increase in apparent Km for dopamine uptake, while Vmax was unchanged. Microdialysis experiments demonstrated that DPP (5 mg/kg, i.p.) elevated extracellular dopamine levels (approximately 200%) in mouse nucleus accumbens. DPP (5 and 10 mg/kg) also induced locomotor activation. All of the effects of DPP were comparable with those of cocaine. Taken together, these results indicate that DPP acts as a competitive dopamine transporter inhibitor similar to cocaine.

Striatal dopamine dynamics in mice following acute and repeated toluene exposure.

RATIONALE: The abused inhalant toluene has potent behavioral effects, but only recently has progress been made in understanding the neurochemical actions that mediate the action of toluene in the brain. Available evidence suggests that toluene inhalation alters dopamine (DA) neurotransmission, but toluene's mechanism of action is unknown. OBJECTIVE: The present study evaluated the effect of acute and repeated toluene inhalation (0, 2,000, or 4,000 ppm) on locomotor activity as well as striatal DA release and uptake using slice fast-scan cyclic voltammetry. RESULTS: Acutely, 2,000 and 4,000 ppm toluene increased locomotor activity, while neurochemically only 4,000 ppm toluene potentiated electrically evoked DA release across the caudate-putamen and the nucleus accumbens. Repeated administration of toluene resulted in sensitization to toluene's locomotor activity effects. Brain slices obtained from mice repeatedly exposed to toluene demonstrated no difference in stimulated DA release in the caudate-putamen as compared to control animals. Repeated exposure to 2,000 and 4,000 ppm toluene caused a concentration-dependent decrease of 25-50 % in evoked DA release in the nucleus accumbens core and shell relative to air-exposed mice. CONCLUSIONS: These voltammetric neurochemical findings following repeated toluene exposure suggest that there may be a compensatory downregulation of the DA system. Acute or repeated toluene exposure had no effect on the DA uptake kinetics. Taken together, these results demonstrate that acute toluene inhalation potentiates DA release, while repeated toluene exposure attenuates DA release in the nucleus accumbens only.

Exploring the relationship between serotonin and brain-derived neurotrophic factor: analysis of BDNF protein and extraneuronal 5-HT in mice with reduced serotonin transporter or BDNF expression.

Serotonin (5-HT) has been proposed to promote neuronal plasticity during the treatment of mood and anxiety disorders and following neurodegenerative insult by altering the expression of critical genes including brain-derived neurotrophic factor (BDNF). In this study, mice with constitutive reductions in the serotonin transporter (SERT) or BDNF were investigated to further assess the functional relationship between serotonin neurotransmission and BDNF expression. Using a modified extraction procedure and a commercial enzyme-linked immunosorbant assay, 50% decreases in BDNF protein in hippocampus, frontal cortex and brain stem were confirmed in 4-month-old mice lacking one copy of the BDNF gene (BDNF(+/-)). By contrast, 4-month-old male and female mice with partial (SERT(+/-)) or complete (SERT(-/-)) reductions in SERT expression showed no differences in BDNF protein levels compared to SERT(+/+) mice, although male SERT knockout mice of all genotypes had higher BDNF levels in hippocampus, frontal cortex, and brain stem than female animals. Microdialysis also was performed in BDNF(+/-) mice. In addition to other phenotypic aspects suggestive of altered serotonin neurotransmission, BDNF(+/-) mice show accelerated age-related degeneration of 5-HT forebrain innervation. Nevertheless, extracellular 5-HT levels determined by zero net flux microdialysis were similar between BDNF(+/+) and BDNF(+/-) mice in striatum and frontal cortex at 8-12 months of age. These data illustrate that a 50% decrease in BDNF does not appear to be sufficient to cause measurable changes in basal extracellular 5-HT concentrations and, furthermore, that constitutive reductions in SERT expression are not associated with altered BDNF protein levels at the ages and in the brain regions examined in this study.

Gene dose-dependent alterations in extraneuronal serotonin but not dopamine in mice with reduced serotonin transporter expression.

Serotonin (5-HT) plays an integral regulatory role in mood, anxiety, cognition, appetite and aggressive behavior. Many therapeutic and illicit drugs that modulate these functions act at the serotonin transporter (SERT), thus a mouse model with reduced transporter expression was created to further investigate the effects of differential serotonin reuptake. In the present study, in vivo microdialysis was used to determine homeostatic alterations in extracellular 5-HT levels in unanesthetized SERT knockout mice. SERT(-/-) mice had significantly higher levels of basal dialysate 5-HT than SERT(+/+) mice in striatum and frontal cortex. In addition, although gene-specific increases in 5-HT were evident, neuroadaptive alterations in dialysate dopamine levels were not detected in striatum. Zero net flux microdialysis was utilized to further investigate alterations in extracellular 5-HT. Using this method, a gene dose-dependent increase in extraneuronal 5-HT was observed in striatum (2.8 +/- 1, 9.4 +/- 1 and 18 +/- 3 nM) and frontal cortex (1.4 +/- 0.4, 3.5 +/- 0.9 and 14 +/- 1 nM) in SERT(+/+), SERT(+/-) and SERT(-/-) mice, respectively. Potassium stimulation revealed greater depolarization-induced increases in striatal 5-HT but not dopamine in SERT(-/-) mice. Furthermore, dialysate 5-hydroxyindoleacetic acid (5-HIAA) levels were reduced in striatum in a gene dose-dependent manner, while DOPAC was unchanged in SERT knockout mice. Finally, determination of monoamine oxidase (MAO) activity revealed no significant differences in KM or Vmax of type-A or type-B isozymes indicating that alterations in SERT expression do not cause adaptive changes in the activities of these key catabolic enzymes. Overall, these results demonstrate that constitutive reductions in SERT are associated with increases in 5-HT in the extracellular signaling space in the absence of changes in dopamine neurochemistry. Furthermore, use of zero net flux microdialysis appears warranted in investigations of serotonergic synaptic function where modest changes in extracellular 5-HT are thought to occur in response to altered uptake.

Potentiated striatal dopamine release leads to hyperdopaminergia in female brain-derived neurotrophic factor heterozygous mice.

The goal of this study was to determine whether a reduction in brain-derived neurotrophic factor (BDNF) levels in female mice leads to dopaminergic system dysregulation. Through a series of in vivo brain microdialysis and slice voltammetry experiments, we discerned that female BDNF heterozygous (BDNF(+/-)) mice are hyperdopaminergic, similar to their male BDNF(+/-) counterparts. Zero-net flux microdialysis results showed that female BDNF(+/-) mice had increased striatal extracellular dopamine levels, while stimulated regional release by high potassium concentrations potentiated dopamine release through vesicular-mediated depolarization. Using the complementary technique of fast scan cyclic voltammetry, electrical stimulation evoked greater dopamine release in the female BDNF(+/-) mice, whereas dopamine uptake remained unchanged relative to that of female wildtype mice. Following psychostimulant methamphetamine administration, female BDNF(+/-) mice showed potentiated dopamine release compared to their wildtype counterparts. Taken together, these dopamine release impairments in female mice appear to result in a hyperdopaminergic phenotype without concomitant alterations in dopamine uptake.

Probing the ability of presynaptic tyrosine kinase receptors to regulate striatal dopamine dynamics.

Brain-derived neurotrophic factor (BDNF) modulates the synaptic transmission of several monoaminergic neuronal systems. Molecular techniques using synapatosomes in previous studies have suggested that BDNF's receptor, tyrosine kinases (Trk), can quickly regulate dopamine release and transporter dynamics. Our main objective in this study is to determine whether slice fast scan cyclic voltammetry can be used to investigate the role of the TrkB receptor on dopamine release and uptake processes in the caudate-putamen. Fast scan cyclic voltammetry measured dopamine release and uptake rates in the presence of BDNF, or its agonist 7,8-dihydroxyflavone, or a TrkB inhibitor K252a. Superfusion of BDNF led to partial recovery of the electrically stimulated dopamine release response in BDNF(+/-) mice which is blunted compared to wildtype mice, with no effect in wildtype mice. Conversely, infusion of 7,8-dihydroxyflavone increased electrically stimulated dopamine release in wildtype mice with no difference in BDNF(+/-) mice. Overall, BDNF and 7,8-dihydroxyflavone had no effect on dopamine uptake rates. Concentrations greater than 3 µM 7,8-dihydroxyflavone affected dopamine uptake rates in BDNF(+/-) mice only. To demonstrate that BDNF and 7,8-dihydroxyflavone modulate dopamine release by activating the TrkB receptor, both genotypes were pretreated with K252a. K252a was able to block BDNF and 7,8-DHF induced increases during stimulated dopamine release in BDNF(+/-) and wildtype mice, respectively. Fast scan cyclic voltammetry demonstrates that acute TrkB activation potentiates dopamine release in both genotypes.

Greater ethanol-induced locomotor activation in DBA/2J versus C57BL/6J mice is not predicted by presynaptic striatal dopamine dynamics.

A large body of research has aimed to determine the neurochemical factors driving differential sensitivity to ethanol between individuals in an attempt to find predictors of ethanol abuse vulnerability. Here we find that the locomotor activating effects of ethanol are markedly greater in DBA/2J compared to C57BL/6J mice, although it is unclear as to what neurochemical differences between strains mediate this behavior. Dopamine elevations in the nucleus accumbens and caudate-putamen regulate locomotor behavior for most drugs, including ethanol; thus, we aimed to determine if differences in these regions predict strain differences in ethanol-induced locomotor activity. Previous studies suggest that ethanol interacts with the dopamine transporter, potentially mediating its locomotor activating effects; however, we found that ethanol had no effects on dopamine uptake in either strain. Ex vivo voltammetry allows for the determination of ethanol effects on presynaptic dopamine terminals, independent of drug-induced changes in firing rates of afferent inputs from either dopamine neurons or other neurotransmitter systems. However, differences in striatal dopamine dynamics did not predict the locomotor-activating effects of ethanol, since the inhibitory effects of ethanol on dopamine release were similar between strains. There were differences in presynaptic dopamine function between strains, with faster dopamine clearance in the caudate-putamen of DBA/2J mice; however, it is unclear how this difference relates to locomotor behavior. Because of the role of the dopamine system in reinforcement and reward learning, differences in dopamine signaling between the strains could have implications for addiction-related behaviors that extend beyond ethanol effects in the striatum.

Presynaptic dopamine dynamics in striatal brain slices with fast-scan cyclic voltammetry.

Extensive research has focused on the neurotransmitter dopamine because of its importance in the mechanism of action of drugs of abuse (e.g. cocaine and amphetamine), the role it plays in psychiatric illnesses (e.g. schizophrenia and Attention Deficit Hyperactivity Disorder), and its involvement in degenerative disorders like Parkinson's and Huntington's disease. Under normal physiological conditions, dopamine is known to regulate locomotor activity, cognition, learning, emotional affect, and neuroendocrine hormone secretion. One of the largest densities of dopamine neurons is within the striatum, which can be divided in two distinct neuroanatomical regions known as the nucleus accumbens and the caudate-putamen. The objective is to illustrate a general protocol for slice fast-scan cyclic voltammetry (FSCV) within the mouse striatum. FSCV is a well-defined electrochemical technique providing the opportunity to measure dopamine release and uptake in real time in discrete brain regions. Carbon fiber microelectrodes (diameter of ~ 7 µm) are used in FSCV to detect dopamine oxidation. The analytical advantage of using FSCV to detect dopamine is its enhanced temporal resolution of 100 milliseconds and spatial resolution of less than ten microns, providing complementary information to in vivo microdialysis.

Conserved dorsal-ventral gradient of dopamine release and uptake rate in mice, rats and rhesus macaques.

Although the vast majority of research on the dopamine system has been performed in rodents, and it is assumed that this work will inform us about the human condition, there have been very few direct comparisons of presynaptic dopamine terminal function across multiple species. Because it is difficult to query rapid sub-second dopamine signaling in humans using voltammetric methods, we chose to compare dopamine signals across multiple striatal subregions in slices from C57BL/6J mice, Sprague-Dawley rats and rhesus macaques. We found a dorsal to ventral gradient of dopamine uptake rates with highest levels in the dorsal striatum and lowest levels in the nucleus accumbens shell, which is conserved across species. In addition to uptake rates, there was also a dorsal to ventral, high to low, gradient in the magnitude of stimulated DA release observed in monkeys, mice, and rats. These data demonstrate that there is considerable functional homology across striatal regions in non-human primates and rodents, lending support to the use of rodents as model systems to study dopamine-related circuitry and disorders that are clinically relevant to the human population.