Amphetamine-sensitized animals show a marked increase in dopamine D2 high receptors occupied by endogenous dopamine,even in the absence of acute challenges

While a range of dopamine D(2)-related behaviors are exaggerated in amphetamine-sensitized animals, studies of the dopamine D(2) receptor have reported either no change or a decrease in dopamine D(2) receptor density--especially when measured using radioraclopride. We hypothesized that a decrease in D(2) receptors may actually be "apparent" and that these receptors may still be present, but are noncompetitively "occupied" by endogenous dopamine. Animals sensitized to amphetamine (and their saline controls) were examined 4 weeks after their last injection. We first measured the [(3)H]raclopride binding in vivo, and observed that sensitized animals showed a 29% lower level of raclopride binding in vivo, suggesting an apparently lower level of dopamine D(2) receptors. To assess the reason for this we examined the density of receptors (using Scatchard analysis in vitro) measured by [(3)H]raclopride in the presence and absence of guanilylimidodiphosphate. This guanine nucleotide converts the dopamine-bound high-affinity state of D(2) receptors into low-affinity states, thereby making measurable the absolute density of the sites. As previously reported, the amphetamine-sensitized animals showed a 31% lower number of D(2) receptors in conventional binding (B(max) 15.6 vs. 22.7 pmol/g). However, with the addition of guanilylimidodiphosphate there was an equalization of both groups (B(max) 25.9 vs. 25.6 pmol/g), revealing an additional 10.3 pmol/g in the sensitized animals, but only 2.9 pmol/g in saline controls. There were no changes in the dissociation constant of [(3)H]raclopride for the receptors. The nearly four-fold increase of dopamine D(2) receptors in the high-affinity state occupied by dopamine may explain why amphetamine-sensitized animals show almost an order of magnitude greater response to dopamine-releasing challenges or dopamine agonists, even though the absolute receptor number is unchanged and the "apparent" receptor number is decreased.     

Insular dopamine D2 receptors and novelty seeking personality in Parkinson's disease.

Novelty seeking is a temperament trait characterized by impulsiveness and exploratory behavior. Dopamine has been suggested to be the primary neurotransmitter modulator of novelty seeking, and in young healthy subjects, a correlation between increased novelty seeking and decreased insular cortical dopamine D2 receptor availability has been reported. The proposed link between dopamine deficiency and reduction in novelty seeking in Parkinson's disease is controversial. The present study examined whether a link between insular D2 receptor availability and novelty seeking can be replicated in Parkinson's disease patients. [11C]FLB 457 positron emission tomography imaging was carried out in 28 patients with Parkinson's disease, and the data were analyzed using voxel-based statistical analysis. The results demonstrated a negative correlation between the novelty seeking score and the dopamine D2 availability bilaterally in the insular cortex (corrected P=0.001; r=-0.74 [right hemisphere]; r=-0.66 [left hemisphere]). The results provide further support for a relationship between novelty seeking and insular D2 receptors. They indicate that the association is cross-cultural, independent of age, and unaffected by dopaminergic degeneration.     

PET radiotracer development for imaging high‐affinity state of dopamine D2 and D3 receptors: Binding studies of fluorine‐18 labeled aminotetralins in rodents

Imaging the high‐affinity, functional state (HA) of dopamine D2 and D3 receptors has been pursued in PET imaging studies of various brain functions. We report further evaluation of ¹⁸F‐5‐OH‐FPPAT, and the newer ¹⁸F‐5‐OH‐FHXPAT and ¹⁸F‐7‐OH‐FHXPAT. Syntheses of ¹⁸F‐5‐OH‐FHXPAT and ¹⁸F‐7‐OH‐FHXPAT were improved by modifications of our previously reported procedures. Brain slices and brain homogenates from male Sprague‐Dawley rats were used with the 3 radiotracers (74‐111 kBq/cc). Competition with dopamine (1–100 nM) and Gpp(NH)p (10–50 µM) were carried out to demonstrate binding to dopamine D2 and D3 HA‐states and binding kinetics of ¹⁸F‐5‐OH‐FPPAT measured. Ex vivo brain slice autoradiography was carried out on rats administered with ¹⁸F‐5‐OH‐FHXPAT to ascertain HA‐state binding. PET/CT imaging in rats and wild type (WT) and D2 knock‐out mice were carried out using ¹⁸F‐7‐OH‐FHXPAT (2‐37 MBq). Striatum was clearly visualized by the three radiotracers in brain slices and dopamine displaced more than 80% of binding, with dissociation rate in homogenates of 2.2 × 10^?2 min^?1 for ¹⁸F‐5‐OH‐FPPAT. Treatment with Gpp(NH)p significantly reduced 50–80% striatal binding with faster dissociation rates (5.0 × 10^?2 min^?1), suggesting HA‐state binding of ¹⁸F‐5‐OH‐FPPAT and ¹⁸F‐5‐OH‐FHXPAT. Striatal binding of ¹⁸F‐5‐OH‐FHXPAT in ex vivo brain slices were sensitive to Gpp(NH)p, suggesting HA‐state binding in vivo. PET binding ratios of ¹⁸F‐7‐OH‐FHXPAT in rat brain were ventral striatum/cerebellum = 2.09 and dorsal striatum/cerebellum = 1.65; similar binding ratios were found in the D2 WT mice. These results suggest that in vivo PET measures of agonists in the brain at least in part reflect binding to the membrane‐bound HA‐state of the dopamine receptor.     

Antiparkinsonian effects of the novel D3/D2 dopamine receptor agonist, S32504, in MPTP-lesioned marmosets: Mediation by D2, not D3, dopamine receptors.

L-dopa remains the most common treatment for Parkinson's disease. However, there is considerable interest in D3/D2 receptor agonists such as the novel agent S32504, since they exert antiparkinsonian properties in the absence of dyskinesia. An important question concerns the roles of D2 vs. D3 receptors, an issue we addressed with the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned nonhuman primate model of Parkinson's disease. In L-dopa-primed animals, S32504 (0.16-2.5 mg/kg p.o.) dose-dependently enhanced locomotor activity. This action was abolished by the D2 antagonist, L741,626 (2.5 mg/kg), but potentiated by the D3 antagonist, S33084 (0.63 mg/kg). Both antagonists were inactive alone. In drug-naive animals, a maximally effective dose of S32504 (2.5 mg/kg p.o.) displayed pronounced antiparkinsonian properties from the third day of administration, and its actions were expressed rapidly and durably. Thus, on day 33, antiparkinsonian properties of S32504 were apparent within 5 minutes and present for > 4 hours. Moreover, they were associated with neither wearing off nor significant dyskinesia. In conclusion, the novel D3/D2 agonist S32504 may offer advantages over L-dopa in the treatment of newly diagnosed parkinsonian patients. Its actions are expressed primarily by activation of D2, not D3, receptors.     

Increase of dopamine D2(High) receptors in the striatum of rats sensitized to caffeine motor effects

It has been previously demonstrated how rats can develop behavioral dopamine supersensitivity after long-term administration of caffeine. Since behavioral dopamine supersensitivity in rats is usually accompanied by an elevation in striatal dopamine D2(High) receptors, we examined whether alterations in D2(High) receptors occurred in the striatum of rats administered caffeine according to a regimen capable of eliciting behavioral dopamine supersensitivity (15 mg/kg i.p. every other day for 14 days). An increase of 126% in striatal D2(High) receptors was found in caffeine-sensitized rats. This marked elevation in D2(High) receptors may account for the caffeine-induced behavioral dopamine supersensitivity and may help elucidate the interactions between caffeine and dopamine neurotransmission.     

Increased dopamine D2(High) receptors in amphetamine-sensitized rats, measured by the agonist [(3)H](+)PHNO

Repeated injections of amphetamine causes animals to become sensitized and supersensitive to DA. Previous work showed that the striata from such sensitized rats revealed a 3.5-fold increase in the density of D2(High) DA receptors, as measured by the guanine-nucleotide-sensitive component of [(3)H]raclopride binding. The present study was done to confirm these earlier findings by different methods and different ligands. The striata from amphetamine-sensitized rats showed an increase of 2.2-fold in the density of guanine-nucleotide-sensitive D2 receptors labeled by saturation experiments with [(3)H](+)PHNO. The proportion of D2(High) receptors was also found to increase 2.5-fold using the method of competition between DA and [(3)H]domperidone. The overall 2.2-3.5-fold increase of DA D2(High) receptors may explain why amphetamine-sensitized animals are much more sensitive to DA agonists, even though the total density of D2 receptors may apparently be unchanged or even decreased.     

D1 dopamine receptor--the search for a function: a critical evaluation of the D1/D2 dopamine receptor classification and its functional implications

The present review focuses on the hypothesized D1/D2 dopamine (DA) receptor classification, originally based on the form of receptor coupling to adenylate cyclase activity. The pharmacological effects of compounds exhibiting putative selective agonist or antagonist profiles at those DA receptors positively coupled to adenylate cyclase activity (D1 DA receptors) are extensively reviewed. Comparisons are made with the effects of putative selective D2 DA receptor agonists and antagonists, and on the basis of this work, the DA receptor classification is critically evaluated. A variety of biochemical, behavioral, and electrophysiological evidence is presented which supports the view that D1 and D2 DA receptors can interact in both an opposing and synergistic fashion. Particular attention is focused on the possibility that D1 receptor stimulation is required to enable the expression of certain D2 receptor-mediated effects, and the functional consequences of this form of interaction are considered. A hypothetical model is presented which considers how both the opposing and enabling forms of interaction between D1 and D2 DA receptors can control behavioral expression. Finally, the clinical relevance of this work is discussed and the potential use of selective D1 receptor agonists and antagonists in the treatment of psychotic states and Parkinson's disease is considered.     

Dopamine D2/3 receptor occupancy of apomorphine in the nonhuman primate brain—A comparative PET study with [11C]raclopride and [11C]MNPA

Binding studies in vitro have demonstrated that the dopamine D₂ receptor may exist in two affinity states for agonists. The high affinity state is thought to represent the functional state of the receptor and proportions might alter during disease. In vitro studies further indicate that agonists induce measurable D₂ receptor occupancy at clinically relevant concentrations but only when measured at the high affinity state. Recently developed PET‐radioligands, such as [¹¹C]MNPA, have now made it possible to directly study agonist binding in vivo. The aim of this study was to compare the inhibition by apomorphine of agonist and antagonist radioligand binding to D_2/3 receptors in vivo. A total of 36 PET measurements were performed with the D_2/3 antagonist [¹¹C]raclopride or the D_2/3 agonist [¹¹C]MNPA in two cynomolgus monkeys. On each study day, a baseline measurement was followed by two consecutive pretreatment studies with rising doses of apomorphine (0.01, 0.05, 0.15, 0.5, 1.0, and 3.0 mg/kg). Binding potential (BP_ND) values were calculated for the striatum with cerebellum as reference region. Apomorphine inhibited [¹¹C]raclopride and [¹¹C]MNPA binding in a dose‐dependent manner and to a similar extent. ID₅₀ and K_i values were 0.26 mg/kg and 29 ng/ml for [¹¹C]raclopride and 0.50 mg/kg and 31 ng/ml for [¹¹C]MNPA. The present observations do not support the existence of two affinity states in vivo. It might thus be speculated that all D_2/3 receptors are in the high affinity state at in vivo conditions. Synapse 63:378–389, 2009. © 2009 Wiley‐Liss, Inc.     

Impact of pramipexole on the onset of levodopa‐related dyskinesias

Dyskinesias are a major complication of dopaminergic therapy in the long‐term treatment of Parkinson's disease. In the CALM‐PD trial, subjects were initially randomized to levodopa or pramipexole and could later add levodopa if needed. After adjusting for disease duration and daily levodopa dosage, the incidence of dyskinesias after initiating levodopa was not significantly different among subjects initially randomized to levodopa and those initially randomized to pramipexole. © 2007 Movement Disorder Society     

Switching from ergot to nonergot dopamine agonists in Parkinson's disease: a clinical series and five-drug dose conversion table.

Of 99 patients on ergot-derived dopamine agonists informed about possible long-term side effects, switching to a nonergot was undertaken in 88 (89%). There were adverse events in 26%. After 11 months, 82% were on their switch agonist and 93% were on any agonist. Switching dopamine agonists is feasible in this population.     

Problematic gambling on dopamine agonists: Not such a rarity.

Excessive gambling is recognized with dopamine agonist therapy, but the prevalence is unknown. We assessed the prevalence of excess gambling by specific prospective enquiry in Parkinson's disease patients attending six West Scotland movement disorder clinics. Of 388 patients taking anti-Parkinson medication, 17 (4.4%) developed pathological gambling, all of whom were prescribed dopamine agonists. Thus, 8% of patients taking dopamine agonists had pathological gambling. Pathological gambling is not uncommon, and patients should be made aware of this potential adverse effect.     

An open‐label conversion study of pramipexole to ropinirole prolonged release in Parkinson's disease

Ropinirole prolonged release (PR) is a once daily oral dopamine agonist approved for the treatment of Parkinson's disease (PD). The goal of this 4 week, open‐label study was to determine the most effective conversion ratio with the fewest adverse effects (AEs) when switching from pramipexole to ropinirole PR. Sixty patients with PD taking pramipexole were converted overnight to ropinirole PR at ratios of 1:3, 1:4, or 1:5 such that 20 consecutive subjects were enrolled in each group. Ropinirole PR dose adjustments were allowed to maintain efficacy or to reduce AEs. An overnight switch from pramipexole to ropinirole PR was found to be well tolerated and AEs were typical for a dopamine agonist. The most common AEs were worsening of PD symptoms, dizziness, somnolence, and nausea, the majority of which resolved after dose adjustments. Thirteen subjects discontinued ropinirole PR before 4 weeks. These subjects were taking a significantly greater dose of pramipexole, the majority greater than 4 mg/day, and tended to have longer disease durations. A conversion ratio of 1 mg of pramipexole to 4 mg of ropinirole PR resulted in the fewest discontinuations of ropinirole PR, the fewest dose adjustments and the largest percentage of subjects that preferred ropinirole PR. © 2009 Movement Disorder Society     

Effect of dopamine agonists on fatigue and somnolence in Parkinson's disease.

Dopamine agonists are becoming increasingly important in the treatment of early and advanced symptoms of Parkinson disease (PD). Although dopamine agonists are known to increase somnolence, their effect on fatigue and the relationship between fatigue and somnolence have not been investigated thoroughly. The objective of this study is to quantitatively measure fatigue in patients with PD treated with dopamine agonists and to correlate fatigue with somnolence. Fifteen patients with PD (mean age, 60.6 +/- 9.7 years; mean disease duration, 4.1 +/- 1.9 years) underwent a continuous (30-second) motor task using four muscle groups before and after 3 months of treatment with dopamine agonists. A fatigue index, defined as the decay of maximal force during continuous exercise, was calculated. Findings were compared with 15 healthy, age-matched control subjects. Patients also completed the Multidimensional Fatigue Inventory (MFI), the Epworth Sleepiness Scale (ESS), and the Hamilton Depression Scale at the same time points. Mean fatigue index (FI) before treatment was significantly higher in PD patients than controls (31.37% +/- 3.81% vs. 23.39% +/- 3.03%, P < 0.001). There was no significant between-group difference after 3 months of treatment. There was no difference in FI of the more affected side before and after treatment (33.33% +/- 6.18% vs. 34.08% +/- 5.43%, P > 0.1). No significant change in MFI scores were noted after treatment, although scores on the ESS increased significantly (6.6 +/- 2.63 vs. 11.7 +/- 5.16; P < 0.05). Fatigue is prevalent in patients with PD but is not influenced by dopamine agonists. Somnolence cannot be attributed to the increase in fatigability and apparently involves a different mechanism.     

Extrastriatal dopamine D(2) receptor binding in Huntington's disease

Huntington's disease (HD) is a neurodegenerative disorder, primarily affecting medium spiny neurones in the striatum. The density of striatal dopamine D(2) receptors is reduced in HD but there is little known about this biomarker in brain regions outside the striatum. The primary objective of this study was to compare extrastriatal dopamine D(2) receptor binding, in age-matched control subjects and patients with HD. All subjects were examined using a high-resolution positron emission tomography system and the high-affinity dopamine D(2) receptor radioligand [(11) C]FLB 457. A ROI based analysis was used with an atrophy correction method. Dopamine D(2) receptor binding potential was reduced in the striatum of patients with HD. Unlike the striatum, dopamine D(2) receptor binding in thalamic and cortical subregions was not significantly different from that in control subjects. A partial least square regression analysis which included binding potential values from all investigated cortical and subcortical regions revealed a significant model separating patients from controls, conclusively dependent on differences in striatal binding of the radioligand. Some clinical assessments correlated with striatal dopamine D(2) receptor binding, including severity of chorea and cognitive test performance. Hence, the present study demonstrates that dopamine D(2) receptors extrinsic to the striatum are well preserved in early to mid stage patients with HD. This observation may have implication for the development of therapy for HD.     

Combination of dopamine transporter and D2 receptor SPECT in the diagnostic evaluation of PD, MSA, and PSP.

It is often difficult to differentiate clinically between Parkinson's disease (PD), multiple system atrophy (MSA), and progressive supranuclear palsy (PSP).The objective of this work was to investigate whether combined pre‐ and postsynaptic dopaminergic single photon emission computed tomography (SPECT) scanning can reliably demonstrate changes in the nigrostriatal dopaminergic system and help differentiate between normal controls, PD, MSA, and PSP patients. We performed SPECT evaluation of the dopamine transporter (DAT) and dopamine D2 receptors (D2). SPECT scans using [¹²³I]β‐CIT (for DAT) and [¹²³I]IBF (for D2) were performed in 18 patients with PD (12 dopa‐naïve and 6 on levodopa and/or dopamine agonists), 7 with MSA of the striatonigral degeneration type, 6 with PSP, and 29 normal controls. Antiparkinsonian drugs were withheld for at least 12 hours before the scans. DAT and D2 binding potentials (Rv = V₃/V₂) were measured for caudate, anterior, and posterior putamen on the sides ipsilateral and contralateral to the worst motor symptoms. DAT binding in the posterior putamen was markedly reduced in all patients. However, D2 binding in posterior putamen was significantly increased in dopa‐untreated PD, being greater than the normal range in 4 of 12 (33%), and it was significantly reduced in MSA, being below the normal range in 5 of 7 (71%). None of the patients with PD showed reduced D2 binding below the normal range in posterior putamen. The degree of DAT binding could not discriminate between the patient groups. The ratio of posterior putamen to caudate percentage D2 Rv compared with the controls showed an opposite pattern between PD or PSP and MSA; the caudate was greater in 16 of 18 with PD and 6 of 6 with PSP, whereas caudate was less in 5 of 7 with MSA. These findings suggest that DAT SPECT may be useful in differentiating parkinsonism from controls and D2 SPECT in further differentiating MSA from Parkinson's disease and possibly PSP. © 2002 Movement Disorder Society.     

Increased affinity of dopamine for D(2) -like versus D(1) -like receptors. Relevance for volume transmission in interpreting PET findings

Evidence indicates that dopamine (DA) mainly acts as a volume transmission (VT) transmitter through its release into the extracellular fluid where the D(1) -like and D(2) -like receptors are predominantly extrasynaptic. It was therefore of interest to compare the affinities of the two major families of DA receptors. [(3)H] raclopride /DA and [(3)H] SCH23390/DA competition assays compared the affinity of DA at D(2) -like and D(1) -like receptors in rat dorsal striatal membrane preparations as well as in membrane preparations from CHO cell lines stably transfected with human D(2L) and D(1) receptors. The IC(50) values of DA at D(2) -like receptors in dorsal striatal membranes and CHO cell membranes were markedly and significantly reduced compared with the IC(50) values of DA at D(1) -like receptors. These IC(50) values reflect differences in both the high and low affinity states. The K(iH) value for DA at [(3)H] raclopride-labeled D(2) -like receptors in dorsal striatum was 12 nM, and this can help explain PET findings that amphetamine-induced increases in DA release can produce an up to 50% decrease of [(11)C] raclopride binding in the dorsal striatum in vivo. These combined results give indications for the existence of striatal D(2) -like receptor-mediated DA VT at the local circuit level in vivo. The demonstration of a K(iH) value of 183 nM for DA at D(1) antagonist-labeled D(1) -like receptors instead gives a likely explanation for the failure of a reduction of D(1) -like receptor binding after amphetamine-induced DA release in PET studies using the D(1) -like antagonist radioligands [(11)C] SCH23390 and [(11)C] NNC. It seems difficult to evaluate the role of the extrasynaptic D(1) receptors in VT in vivo with the PET radioligands available for this receptor.     

Upregulation of dopamine D2 receptors in dopaminergic drug-naive patients with Parkin gene mutations.

Medicated patients with Parkinsonism and parkin gene mutations have been reported to show a significant decrease in striatal dopamine D2 receptors (D2R) in comparison to medicated idiopathic Parkinson's disease (IPD) patients with similar age and disease severity. The aim of this study was to verify whether the genetic defect per se is responsible for this decrease. We have studied with [11C]raclopride (RAC) positron emission tomography (PET) in a group of 14 sporadic patients with parkin-linked Parkinsonism, 6 of whom had never received levodopa or dopamine agonists. The remaining 8 patients had been treated with levodopa for at least 5 years. Presynaptic striatal [18F]dopa storage was not significantly different between these two groups of patients. In untreated parkin-positive patients, significant putaminal increases in RAC-binding potential (BP) were found in comparison to an age-matched healthy control group by using a classical region of interest approach and statistical parametric mapping. In contrast, levodopa-treated parkin-positive patients showed significant decreases in RAC-BP in the caudate and putamen when compared to an age-matched healthy control group. The RAC PET findings revealed that striatal D2R upregulation occurs in dopaminergic drug-naive parkin-positive patients, in a similar fashion to the upregulation reported in drug-naive IPD. D2R downregulation observed in medicated parkin-positive patients, therefore, is not caused primarily by the genetic defect itself. Parkin-positive patients appear to have a greater susceptibility to the exposure to dopaminergic medication than IPD patients, which in turn might be an indirect effect of their genetic mutation.     

Dopamine D2 receptor TaqIA and TaqIB polymorphisms in Parkinson's disease.

In a case control study, we examined the association of DRD2 Taq1A and Taq1B polymorphisms and risk of PD, and evaluated the strength of linkage disequilibrium of the polymorphisms. The Taq1A and Taq1B polymorphisms were in strong linkage disequilibrium. There was, however, no significant association of the two polymorphisms with PD.     

Dopamine β‐hydroxylase‐deficient mice have normal densities of D2 dopamine receptors in the high‐affinity state based on in vivo PET imaging and in vitro radioligand binding

In vitro, D₂ dopamine receptors (DAR) can exist in low‐ and high‐affinity states for agonists and increases of D₂ receptors in high‐affinity state have been proposed to underlie DA receptor supersensitivity in vivo. Deletion of the gene for dopamine β‐hydroxylase (DBH) causes mice to become hypersensitive to the effects of psychostimulants, and in vitro radioligand binding results suggest an increased percentage of D₂ receptors in a high‐affinity state. To determine whether DBH knockout mice display an increase of high‐affinity state D₂ receptors in vivo, we scanned DBH knockout and control mice with the agonist PET radioligand [¹¹C]MNPA, which is thought to bind preferentially to the high‐affinity state of the D₂ receptor. In addition, we performed in vitro binding experiments on striatal homogenates with [³H]methylspiperone to measure B_max values and the percentages of high‐ and low‐affinity states of the D₂ receptor. We found that the in vivo striatal binding of [¹¹C]MNPA was similar in DBH knockout mice and heterozygous controls and the in vitro B_max values and percentages of D₂ receptors in the high‐affinity state, were not significantly different between these two groups. In summary, our results suggest that DBH knockout mice have normal levels of D₂ receptors in the high‐affinity state and that additional mechanisms contribute to their behavioral sensitivity to psychostimulants. Synapse 64:699–703, 2010. © 2010 Wiley‐Liss, Inc.     

Methamphetamine-induced loss of striatal dopamine innervation in BDNF heterozygote mice does not further reduce D3 receptor concentrations

Depletion of dopamine (DA) reduces D(3) receptor number, but D(3) receptor expression is also regulated by brain-derived neurotrophic factor (BDNF). We took advantage of transgenic heterozygous BDNF mutant mice (+/-) to determine if reduced BDNF and loss of DA fibers produced by methamphetamine were additive in their impact on D(3) receptor number. We assessed selective markers of the dopaminergic system including caudate-putamen DA concentrations and quantitative autoradiographic measurement of tyrosine hydroxylase (TH) levels, DA transporter (DAT), and DA D(3) receptor binding between vehicle and methamphetamine-treated BDNF +/- and their wildtype (WT) littermate control mice. Caudate-putamen DA concentrations, TH and DAT levels were significantly reduced following methamphetamine treatment in both WT and BDNF +/- mice. The extent of methamphetamine-induced reduction in TH and DAT was greater for the WT than BDNF +/- mice and DAT levels were also decreased to a greater extent in nucleus accumbens of WT as compared to BDNF +/- mice. Lower D(3) receptor existed in caudate-putamen and nucleus accumbens in BDNF +/- mice and these differences were not affected by methamphetamine treatment. Taken together, these results not only substantiate the importance of BDNF in controlling D(3) receptor expression, but also indicate that a methamphetamine-induced depletion of DA fibers fails to produce an additive effect with lowered BDNF for control of D(3) receptor expression. In addition, the reduction of D(3) receptor expression is associated with a decreased neurotoxic response to methamphetamine in BDNF +/- mice.