Quantitative autoradiographic localization of the dopamine transport complex in the rat brain: use of a highly selective radioligand: [3H]GBR 12935

Involvement of the endogenous opioid system in the beneficial influence of physical exercise on amphetamine-induced addiction parameters

Psychostimulant drugs addiction is a chronic public health problem and individuals remain susceptible to relapses increasing public expenses even after withdrawal and treatment. Our research group has focused on finding new therapies to be employed in drug addiction treatment, suggesting the physical exercise as a promising tool. This way, it is necessary to know the mechanisms involved in the beneficial influences of physical exercise observing the pathway that could be explored in drug addiction treatment. Male Wistar rats were conditioned with amphetamine (AMPH) following the conditioned place preference (CPP) protocol and subsequently submitted to swimming for 5 weeks (1 h per day, 5 days per week). Half of the animals were injected with Naloxone (0.3 mg/mL/kg body weight, i.p.) 5 min prior each physical exercise day. After AMPH-CPP re-exposure, our outcomes showed that physical exercise, in addition to minimizing the relapse behavior in the CPP, it increased D1R, D2R and DAT in the Ventral Tegmental Area (VTA), but not in the Nucleus accumbens (NAc). Interestingly, while naloxone inhibited the partial beneficial influence of the exercise on drug-relapse behavior, exercise-induced changes in the dopaminergic system were not observed in the group administered with naloxone as well. Based on these evidences, besides reinforcing the beneficial influence of the physical exercise on AMPH-induced drug addiction, we propose the involvement of endogenous opioid system activation, not as a single one, but as a possible mechanism of action resulting from the physical activity practice, thus characterizing an important therapeutic approach, which may contribute to drug withdrawal consequently preventing relapse.

Quantitative autoradiography of ligands for dopamine receptors and transporters in brain of Göttingen minipig: comparison with results in vivo

The pig has been used as animal model for positron emission tomography (PET) studies of dopamine (DA) receptors and pharmacological perturbations of DA neurotransmission. However, the binding properties of DA receptors and transporters in pig brain have not been characterized in vitro. Therefore, the saturation binding parameters of [3H]SCH 23390 for DA D1 receptors and [3H]raclopride for DA D2/3 receptors were measured by quantitative autoradiography in cryostat sections from brain of groups of 8 week old and adult female Göttingen minipigs. The magnitudes of Bmax and Kd for these ligands were similar in young and old pigs, and were close to those reported for rat and human brain. Furthermore, gradients in the concentrations of D1 and D2/3 sites in striatum measured in vitro agreed with earlier findings in PET studies. However, the dopamine transporter (DAT) ligand [3H]GBR12935 did not bind in pig brain cryostat sections. Whereas the tropane derivative [125I]RTI-55 labeled serotonin transporters (serotonin transporter (SERT)) in pig brain, use of the same ligand under conditions specific for DAT, revealed a pattern of binding similar to that observed for SERT conditions. Parallel studies revealed the presence of DAT in rat and ferret brain. The distribution volume (Vd) of the selective DAT ligand [11C]NS2214 ([11C]Brasofensine) was mapped in groups of normal and MPTP-lesioned Göttingen miniature pigs. The in vivo pattern of Vd matched the distribution of SERT in vitro, and did not differ between the normal pigs and the lesioned animals with documented 60% DA depletions. However, the pattern of specific binding of the selective noradrenaline transporter ligand (S,S)-[11C]MeNER in a single Landrace pig showed that, of the three monoamine transporters, only DAT could not be detected in pig brain. We conclude that the pig is a suitable model for PET studies of DA D1 and D2/3 binding sites, which are fully developed on the eighth postnatal week. However, well-characterized piperazine and tropane radioligands failed to recognize DAT in pig brain; the two tropane radioligands lacked pharmacological specificity for DAT and SERT in pig brain in vitro and in vivo.

A new positron emission tomography imaging agent for the serotonin transporter: synthesis, pharmacological characterization, and kinetic analysis of [11C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine ([11C]AFM)

The synthesis, radiolabeling, and in vitro and in vivo evaluation of a new positron emission tomography (PET) radioligand for the serotonin transporter (SERT), [(11)C]2-[2-(dimethylaminomethyl)phenylthio]-5-fluoromethylphenylamine ([(11)C]AFM) is reported. AFM was prepared from 4-chloro-3-nitrobenzyl acetate and thiosalicylic acid in a five-step synthetic sequence. In binding studies in vitro with cloned human transporters, AFM displayed high binding affinity (Ki 1.04 nmol/L for hSERT) and good selectivity (Ki 664 nmol/L for hNET and >10,000 nmol/L for hDAT) for SERT. The radiolabled compound [(11)C]AFM was prepared in 30-37 minutes from its monomethylamine precursor by reaction with high specific activity [(11)C]iodomethane. Radiochemical yield was 12.3 +/- 8.1% based on [(11)C]iodomethane and specific activity was 1733 +/- 428 Ci/mmol at end of synthesis (EOS, n = 14). Radiochemical and chemical purity of the final product was >97%. Biodistribution studies in rats indicated that [(11)C]AFM entered the brain readily and localized in regions known to contain high concentrations of SERT, with high specific to nonspecific binding ratios. Furthermore, binding of [(11)C]AFM in SERT-rich regions was blocked by the cold compound AFM and the selective serotonin reuptake inhibitor citalopram but not by the selective norepinephrine reuptake inhibitor nisoxetine or the selective dopamine reuptake inhibitor GBR 12935. At 30 minutes after injection, >95% of the brain activity corresponded to the parent compound, indicating the absence of radiolabeled metabolites in the rat brain. PET imaging experiments in baboons showed a brain distribution pattern of [(11)C]AFM consistent with the regional concentrations of SERT, with the highest levels of radioactivity detected in the midbrain and thalamus, moderate levels in the hippocampus and striatum, and the low levels in the cortical regions. Pretreatment of the baboons with citalopram (4 and 6 mg/kg, intravenously) reduced regional brain distribution volumes to low and homogeneous levels, thus underlining the binding specificity of [(11)C]AFM for SERT in vivo. Analysis of blood samples indicated a fast metabolism of the radioligand into more hydrophilic components, as well as the absence of radiolabeled lipophilic metabolites. Regional time-activity curves were analyzed with kinetic and graphical analysis methods using the arterial concentrations as input function. Both methods returned similar kinetic parameters and documented high specific to nonspecific equilibrium coefficients (V(3)") for [(11)C]AFM. Identical V(3)" values were also derived with the simple reference tissue method, indicating that quantification of SERT with [(11)C]AFM can be achieved without arterial blood sampling. In summary, [(11)C]AFM appears to be an excellent PET radioligand for the visualization and reliable quantification of SERT in vivo.

The somatodendritic release of dopamine in the ventral tegmental area and its regulation by afferent transmitter systems

The release of dopamine in the ventral tegmental area (VTA) plays an important role in the autoinhibition of the dopamine neurons of the mesocorticolimbic system through the activation of somatodendritic dopamine D2 autoreceptors. Accordingly, the intra-VTA application of dopamine D2 receptor agonists reduces the firing rate and release of dopamine in the VTA, and this control appears to possess a tonic nature because the corresponding antagonists enhance the somatodendritic release of the transmitter. In addition, the release of dopamine in the VTA is increased by potassium or veratridine depolarization and abolished by tetrodotoxin and calcium omission. Overall, it appears that the somatodendritic release of dopamine is consistently lower than that in nerve endings. Apart from intrinsic dopaminergic mechanisms, other transmitter systems such as serotonin, noradrenaline, acetylcholine, GABA and glutamate play a role in the control of the activity of dopaminergic neurons of the VTA, although the final action depends on the particular receptor involved as well as the neuronal type where it is localized. Given the involvement of the mesocorticolimbic dopaminergic systems in the pathogenesis of severe neuropsychiatric disorders such as schizophrenia, the knowledge of the factors that regulate the release of dopamine in the VTA could provide new insight into the ethiogenesis of the disease as well as its implication on the mechanisms of action of therapeutic drugs.

Intrastriatal Dopamine-rich Implants Reverse the Increase of Dopamine D2 Receptor mRNA Levels Caused by Lesion of the Nigrostriatal Pathway: A Quantitative In Situ Hybridization Study

Changes in striatal dopamine D2 receptor mRNA levels provoked by unilateral 6-hydroxydopamine-induced lesion of the nigrostriatal dopamine pathway were studied by in situ hybridization. The influence of embryonic dopaminergic neurons implanted into the dopamine-depleted striatum on the lesion-induced changes was also examined. Changes in D2 mRNA levels were compared with changes in D2 receptor densities measured in the same animals by receptor autoradiography using [3H]spiperone or [3H]SDZ 205-501 as ligands. The distribution of D2 mRNA in the striatum of control animals closely paralleled that of the D2 receptor itself, as assessed by autoradiography, and the highest density of D2 mRNA occurred in the lateral part of the striatum. One month after lesion, levels of D2 mRNA were 34% higher in the dorsolateral part of the dopamine-depleted striatum than in the corresponding region of the contralateral control striatum. D2 receptor density in this region was increased by 40% relative to the control level. No significant increases could be measured in the medial part of the striatum. The increases in the lateral part were similar at 7 months post-lesion; however, at this time the increase in both D2 mRNA and receptor levels had spread to the medial part of the striatum as well. In the graft-bearing striatum levels of both D2 mRNA and D2 receptors reverted to control levels. This study shows that the post-lesion increase in striatal dopamine receptor and mRNA level is a biphasic phenomenon with a late-occurring component in the medial striatum. It also shows that once the increase in striatal D2 receptor gene expression is accomplished, it is maintained unchanged for long periods, similar to that of D2 receptor levels themselves. Moreover, grafts of embryonic dopaminergic neurons are able to modulate the expression of the dopamine D2 receptor gene.

Localization of ionotropic glutamate receptors in caudate-putamen and nucleus accumbens septi of rat brain: comparison of NMDA, AMPA, and kainate receptors

Changes in binding of selective radioligands at NMDA ([3H]MK-801), AMPA ([3H]CNQX), and kainate ([3H]kainic acid) glutamate (GLU) ionotropic receptors in rat caudate-putamen (CPu) and nucleus accumbens (NAc) were examined by quantitative autoradiography following: 1) unilateral surgical ablation of frontal cerebral cortex to remove descending corticostriatal GLU projections, 2) unilateral injection of kainic acid (KA) into CPu or NAc to degenerate local intrinsic neurons, or 3) unilateral injections of 6-hydroxydopamine (6-OH-DA) into substantia nigra to degenerate ascending nigrostriatal dopamine (DA) projections. Cortical ablation significantly decreased NMDA receptor binding in ipsilateral medial CPu (20%), and NAc (16%), similar to previously reported losses of DA D4 receptors. KA lesions produced large losses of NMDA receptor labeling in CPu and NAc (both by 52%), AMPA (41% and 45%, respectively), and kainate receptors (40% and 45%, respectively) that were similar to the loss of D2 receptors in CPu and NAc after KA injections. Nigral 6-OH-DA lesions yielded smaller but significant losses in NMDA (17%), AMPA (12%), and kainate (11%) receptor binding in CPu. The results indicate that most NMDA, AMPA, and kainate receptors in rat CPu and NAc occur on intrinsic postsynaptic neurons. Also, some NMDA, but not AMPA or kainate, receptors are also found on corticostriatal projections in association with D4 receptors; these may, respectively, represent excitatory presynaptic NMDA autoreceptors and inhibitory D4 heteroceptors that regulate GLU release from corticostriatal axons in medial CPu and NAc. Conversely, the loss of all three GLU receptor subtypes after lesioning DA neurons supports their role as excitatory heteroceptors promoting DA release from nigrostriatal neurons.

Localization of dopamine receptor subtypes in corpus striatum and nucleus accumbens septi of rat brain: comparison of D1-, D2- and D4-like receptors

Changes in D1-, D2- and D4-like dopamine receptor binding in rat brain were examined by quantitative autoradiography following: (i) unilateral surgical ablation of frontal cerebral cortex to remove descending projections to corpus striatum and nucleus accumbens, (ii) unilateral injections of kainic acid into corpus striatum or nucleus accumbens to degenerate local intrinsic neurons, (iii) unilateral injections of 6-hydroxydopamine into substantia nigra to degenerate ascending dopamine projections. Rats were killed one week after lesioning, with contralateral tissue controls. Radioligands were: [3H]SCH-23390 for D1-like (D1/D5) receptors, [3H]nemonapride alone for D2-like (D2/D3/D4) receptors, and [3H]nemonapride with 300 nM S[-]-raclopride and other masking agents for D4-like receptors (identified by blockade with D4 selective L-745,870). Frontal cerebral cortex ablation did not alter D1- or D2-like receptor density, but D4-like binding decreased significantly in both corpus striatum (18%) and nucleus accumbens (23%). Kainic acid markedly reduced D1-like (75% and 84%) and D2-like binding (44% and 52%), with smaller D4-like losses (28% and 27%) in corpus striatum and nucleus accumbens, respectively. Nigral 6-hydroxydopamine lesions (verified by autoradiographic loss of dopamine transporters labelled with [3H]GBR-12935) did not significantly change D1-, D2-, or D4-like binding in the corpus striatum. These results suggest that the majority of D1-, and D2-like, and a smaller portion of D4-like receptors in corpus striatum and nucleus accumbens arise on intrinsic postsynaptic neurons, and that some D4-like, but neither D1- nor D2-like, receptors are found on presynaptic corticostriatal afferents.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

Immunogold localization of the dopamine transporter: an ultrastructural study of the rat ventral tegmental area

The dopamine transporter (DAT) plays an important role in the plasmalemmal reuptake of dopamine and, thus, in the termination of normal dopaminergic neurotransmission. DAT is also a major binding site for cocaine and other stimulants, the psychoactive effects of which are associated primarily with the inhibition of dopamine reuptake within mesocorticolimbic dopaminergic neurons. We used electron microscopy with an anti-peptide antiserum directed against the N-terminal domain of DAT to determine the subcellular localization of this transporter in the rat ventral tegmental area (VTA), the region that contains the cell bodies and dendrites of these dopaminergic neurons. We show that in the VTA, almost 95% of the DAT immunogold-labeled profiles are neuronal perikarya and dendrites, and the remainder are unmyelinated axons. Within perikarya and large proximal dendrites, almost all of the DAT immunogold particles are associated with intracellular membranes, including saccules of Golgi and cytoplasmic tubulovesicles. In contrast, within medium- to small-diameter dendrites and unmyelinated axons, most of the DAT gold particles are located on plasma membranes. In dually labeled tissue, peroxidase reaction product for the catecholamine-synthesizing enzyme tyrosine hydroxylase is present in DAT-immunoreactive profiles. These findings suggest that intermediate and distal dendrites are both the primary sites of dopamine reuptake and the principal targets of cocaine and related psychostimulants within dopaminergic neurons in the VTA.

Localization and quantification of the dopamine transporter: comparison of [3H]WIN 35,428 and [125I]RTI-55

Transport into the presynaptic terminal by the dopamine transporter is the primary mechanism for removing dopamine from the synaptic cleft. This transporter is a specific marker for dopamine terminals and is a primary site for CNS actions of cocaine. Several radioligands have been developed for analysis of the dopamine transporter. The ligands vary in affinity and specificity, leading to differences in reported transporter density in brain regions. We compared two of the most commonly used ligands, [3H]WIN 35,428 and [125I]RTI-55, analyzing the localization and density of sites in the rat brain using serial sections and quantitative autoradiography. Citalopram at 50 nmol/l was used to block [125I]RTI-55 binding to serotonin transport sites. Transporter density was highest in the striatum and both ligands labeled equivalent numbers of sites, with lateral to medial and anterior to posterior gradients. In most areas the density of sites measured with the two ligands was similar. However, [125I]RTI-55 binding was significantly higher than [3H]WIN 35,428 binding in the substantia nigra zona compacta, ventral tegmental area, subthalamic nucleus and a number of other subcortical nuclear groups while [3H]WIN 35,428 binding was higher in lateral striatum and in olfactory tubercle. These differences could reflect different forms of the transporter, perhaps due to post-translational modifications, and they may provide a basis for differential pharmacological regulation of transporter function in discrete brain regions and disease states.

Effects of GBR 12909 on locomotor activity and dopamine turnover in mice: comparison with apomorphine

The effects of GBR 12909 1-[2-[bis(4-fluorophenyl)methoxy]-ethyl]-4- [3-phenylpropyl]piperazine, a very potent and selective dopamine uptake inhibitor, and apomorphine, a dopamine receptor agonist, alone and in combination were investigated on locomotor activity and dopamine turnover in discrete brain regions of mice. The levels of dopamine and its metabolites were examined 40 min after the administration of GBR 12909 and/or apomorphine, when the effects of the drugs on locomotor activity were approximately at a peak. GBR 12909 (10 mg/kg i.p.) reversed a low dose of apomorphine (0.05 mg/kg s.c.)-induced suppression in locomotor activity and significantly increased this activity. Despite the dramatic change in the behavior, GBR 12909 did not influence the decrease in 3,4-dihydroxyphenylacetic acid (DOPAC)/dopamine ratio (which is one of the indications of transmitter turnover) induced by a low dose of apomorphine in the nucleus accumbens and striatum. In contrast, GBR 12909 did not enhance the high-dose apomorphine (2 mg/kg s.c.)-induced hyperlocomotion, and did not modify the larger decrease in dopamine turnover produced by the high dose of apomorphine in the frontal cortex, nucleus accumbens and striatum. This suggests that postsynaptic dopamine receptors may reach maximum stimulation at a high dose of apomorphine. These results indicate that a behavioral change induced via stimulation of postsynaptic dopamine receptors does not necessarily lead to an alteration in dopamine turnover.

The role of excitatory amino acids in experimental models of Parkinson's disease

The aim of this article was to review the recent literature on the role of excitatory amino acids in Parkinson's disease and in animal equivalents of parkinsonian symptoms. Effects of NMDA and AMPA antagonists on the reserpine-induced akinesia, catalepsy and rigidity, on the neuroleptic-induced catalepsy, on the turning behaviour of 6-OHDA-lesioned rats, as well as on the parkinsonian symptoms evoked by MPTP in monkeys were analysed. Moreover, the role of NMDA antagonists in Parkinson's disease was discussed. Data concerning the protective influence of these drugs on degenerative properties of methamphetamine, MPTP and 6-OHDOPA were also presented. On the basis of the above findings, the following conclusions may be drawn: (1) disturbances in the glutamatergic transmission in various brain structures seem to play a significant role in the development of symptoms of Parkinson's disease; (2) the NMDA-receptor blocking component may make a substantial contribution to the therapeutic effect of antiparkinsonian drugs; a similar contribution of AMPA-receptor blocking component has not been sufficiently documented, so far; (3) compounds blocking NMDA receptors may possibly prevent the development of Parkinson's disease; this presumption needs, however further studies; (4) side effects of NMDA receptor antagonists may be a limiting factor in the use of these compounds in humans.

Effect of chronic estradiol and progesterone treatments of ovariectomized rats on brain dopamine uptake sites

Dopamine released from brain nerve terminals is mainly removed from the synaptic cleft by an uptake mechanism. Despite their functional importance, modulation of the dopamine uptake sites is still not well known. Steroid hormones were shown to modulate brain dopamine transmission. The aim of this study was thus to investigate in ovariectomized rats the effects of 17 beta-estradiol and progesterone treatments on brain dopamine uptake sites. Treatments consisted of 17 beta-estradiol (10 micrograms/0.2 ml), progesterone (0.72 mg/0.2 ml), 17 beta-estradiol + progesterone, or the vehicle (0.3% gelatin in saline solution) twice daily for 2 weeks. The steroid treatments left the affinity of [3H]GBR 12935 binding to striatal homogenates unchanged (ovariectomized rats, 0.823 +/- 0.028 nM), whereas the density was increased by these steroids alone or in combination to a similar extent of 16-23%. Chronic treatment of ovariectomized rats with 17 beta-estradiol, progesterone, or their combination increased to the same extent and uniformly [3H]-GBR 12935 binding in the striatum as measured by autoradiography; the increase was similar in the substantia nigra pars compacta, whereas no steroid effect was observed in the nucleus accumbens and in the substantia nigra pars reticulata. In summary, chronic exposure to 17 beta-estradiol and/or progesterone increased dopamine uptake site density in the nigrostriatal dopaminergic system, whereas the nucleus accumbens and the substantia nigra pars reticulata were unaffected.

A pharmacological comparison of [3H]GBR12935 binding to rodent striatal and kidney homogenates: binding to dopamine transporters?

Binding of [3H]GBR12935 to homogenates of mouse and rat striatum and kidney was studied. [3H]GBR12935 bound to both tissue preparations with high affinity (mouse striatum Kd = 2.4 +/- 0.4 nM, n = 4; mouse kidney Kd = 3.8 +/- 0.9 nM, n = 4), in a saturable (striatal Bmax = 1.5 +/- 0.4 pmol/mg protein; kidney Bmax = 4.9 +/- 0.5 pmol/mg protein) and reversible manner. Saturation experiments revealed the presence of a single class of high affinity binding sites in both tissues of both species. Mouse kidney appeared to possess a greater density of [3H]GBR12935 binding sites than the striatum while the reverse situation prevailed for the rat. Although two dopamine uptake inhibitors, namely GBR12909 and benztropine, displaced [3H]GBR12935 binding from striatal and kidney homogenates with a similar affinity in both tissues of these species, unlabelled mazindol, (+/-)cocaine, nomifensine and amfonelic acid were significantly (P < 0.001-0.02) more potent inhibitors of [3H]GBR12935 binding in the striatum than in the kidney. While the pharmacological profile of [3H]GBR12935 binding in the rodent striatum compared well with that of the dopamine transporter reported previously, the pharmacology in the kidney was considerably different to that in the striatum. GBR12909 (1-30 mg/kg, i.p.), a close analog of GBR12935, induced significant antidiuretic and antinatriuretic effects in spontaneously hypertensive rats. These data suggest that while [3H]GBR12935 labels the dopamine uptake sites in the brain, it does not appear to label similar sites in the kidney. The mechanism of action of GBR12909 on sodium and water excretion remains to be determined.

Autoradiographic localization of dopamine uptake sites in the rat brain with3H-GBR 12935

The regional distribution of dopamine (DA) uptake sites in the rat brain has been studied by quantitative autoradiography using [3H]GBR 12935 as a ligand. The binding of [3H]GBR 12935 to striatal sections was saturable and of high affinity (Kd = 1.6 nM); it occurred at a single population of sites and possessed the pharmacological features of the DA uptake sites. The highest densities of [3H]GBR 12935 binding sites were found in the caudate-putamen, nucleus accumbens, olfactory tubercle, ventral tegmental area and substantia nigra (especially in the pars compacta). Moderate levels of [3H]GBR 12935 binding were observed in globus pallidus, thalamus, hypothalamus, hippocampus, amygdala (basolateral nucleus) and prefrontal and singular cortices. This regional distribution of [3H]GBR 12935 binding closely correlated with the reported distribution of dopaminergic nerve terminals. The topographical distribution of [3H]GBR 12935 has also been studied in detail in striatal subregions and this distribution was compared, using quantitative TH immunoreactivity, to the density of striatal dopaminergic nerve terminals. There is good overlapping between these two regional distributions, the highest density of both markers was found in the lateral part of the striatum and a similar rostro-caudal gradient has been observed. A dopaminergic denervation caused a complete loss of [3H]GBR 12935 in basal ganglia ipsilateral to the lesion.

Intrastriatal dopamine-rich implants reverse the changes in dopamine D2 receptor densities caused by 6-hydroxydopamine lesion of the nigrostriatal pathway in rats: an autoradiographic study

The aim of the present study was to test whether intrastriatal implants of embryonic dopaminergic neurons are able to normalize the lesion-induced hypersensitivity of striatal dopaminergic receptors. The ascending dopaminergic pathway of adult rats was unilaterally lesioned using 6-hydroxydopamine. Three weeks later a cell suspension obtained from the mesencephali of ED 14 rat embryos was implanted into the denervated striatum. Rotational responses to dopaminergic agonists were tested five months after implantation. One month later animals were killed and striatal dopaminergic receptor densities were quantified using autoradiography, the dopaminergic reinnervation of the host striatum being visualized with [3H]GBR 12935, a ligand labelling dopamine uptake sites. The lesion induced a behavioural hypersensitivity to dopaminergic agonists and lesioned animals displayed a strong rotation contralateral to the lesion in response to a test dose of the D1 agonist compound SKF 38393 (2.5 mg/kg) or of the D2 agonist LY 171555 (0.15 mg/kg). These responses were completely abolished by the graft. The normal distribution of D1 and D2 dopaminergic receptors in the rat striatum was similar to that described previously. Seven months after the lesion of the nigrostriatal dopaminergic pathway, the density of D1 receptors was not significantly affected while the density of D2 receptors was increased by about 25-50%. The implantation of embryonic dopaminergic neurons into the denervated striatum led to a slight decrease of D1 receptor densities and to a reversal of the lesion-induced increase of striatal dopaminergic D2 receptors six months later. Moreover, this reversal concerned not only the reinnervated striatal region but also extended into non-reinnervated areas of the striatum. It is concluded that grafts of embryonic dopaminergic neurons can normalize the density of dopaminergic D2 receptors.

Characteristics of [3H]GBR 12935 binding in the human and rat frontal cortex

Binding characteristics of the selective dopamine uptake inhibitor [3H]GBR 12935 have been described for the striatum but not for the frontal cortex. We have developed assay conditions for quantifying [3H]GBR 12935 binding in the frontal cortex. In both the rat and human frontal cortex, the assay required four times more tissue (8 mg/ml) than in the striatum (2 mg/ml). [3H]GBR 12935 binding in the frontal is complex, as it involves multiple binding sites. The high-affinity binding site is sodium dependent and is inhibited by sodium. In human but not in rat frontal cortex, addition of K+ reversed the sodium inhibition. The pharmacological profile of the high-affinity [3H]GBR 12935 binding site is consistent with that of the dopamine transporter, because drugs with the most selective dopamine reuptake blocking activities are the most potent displacers of [3H]GBR 12935 binding. There is a positive correlation between the rat and human inhibitory constants, a finding indicating that there are similar pharmacological profiles across at least these two species. Rats with a 6-hydroxydopamine lesion had a 47% decrease in number of [3H]GBR 12935 binding sites, a result indicating that at least a portion of these sites had been on presynaptic dopamine terminals.

Quantitative autoradiography of the dopamine uptake complex in rat brain using [3H]GBR 12935: binding characteristics

The dopamine uptake complex was examined in the rat central nervous system using [3H]GBR 12935 and in vitro quantitative autoradiography to determine all binding data. [3H]GBR 12935 labels two unique binding sites, the dopamine uptake complex and a piperazine acceptor site. These two sites differ in their pharmacologic properties, anatomical distributions, densities, and response to lesions. Using appropriate binding conditions, [3H]GBR 12935 can be used to specifically label the dopamine uptake complex. [3H]GBR 12935 labeled a single binding site with characteristics of the dopamine uptake complex when mazindol (25 microM) was used as a blank. The specific binding and autoradiographic appearance of [3H]GBR 12935 to the dopamine uptake complex was improved by including trans-flupentixol (0.75 microM) to displace binding to a previously described piperazine acceptor site, recently determined to be a site on cytochrome P450IID1. Binding was saturable and reversible to the dopamine uptake complex. The equilibrium dissociation constant (1.4 +/- 0.7 nM), maximal number of binding sites (6.0 +/- 1.3 pmol/mg protein), and Hill coefficient (1.1 +/- 0.1) of [3H]GBR 12935 in rat striatum using mazindol to define non-specific binding was not significantly altered by the inclusion of trans-flupentixol (0.75 microM). Using GBR 12909 as a blank produced a greater maximal number of binding sites (8.4 +/- 2.3 pmol/mg protein), but no significant difference in the equilibrium dissociation constant (1.6 +/- 0.3 nM) or Hill coefficient (1.1 +/- 0.1). A series of drugs that bind to the dopamine uptake complex displaced [3H]GBR 12935 in a rank order consistent with other binding and behavioral studies of this complex. The rank order of these drugs was GBR 12909 greater than mazindol greater than nomifensine greater than benztropine greater than desipramine greater than amphetamine greater than dopamine; all these drugs displayed a Hill coefficient near one and were best modeled as a single site. Cocaine and WIN 35,428 (a cocaine congener) were unique in their competition for [3H]GBR 12935 binding, displaying biphasic curves, low Hill coefficients, and were best modeled as two site fits. Lesioning of the dopaminergic median forebrain bundle resulted in a dramatic loss of the dopamine uptake complex in the striatum, nucleus accumbens, olfactory tubercle, and substantia nigra. Other dopaminergic projection areas were decreased to a lesser extent. Striatal ibotenate lesions did not decrease the density of the dopamine uptake complex, despite a large decrease in the dopamine D1 receptor.(ABSTRACT TRUNCATED AT 400 WORDS)

Imaging the dopamine uptake site with ex vivo [18F]GBR 13119 binding autoradiography in rat brain

We studied the binding of [18F]GBR 13119 (1-[[(4-[18F]fluorophenyl) (phenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine) to rat brain with autoradiography after intravenous injection. The rank order of binding was dorsal striatum greater than nucleus accumbens = olfactory tubercle greater than substantia nigra = ventral tegmental area greater than other areas. Binding was blocked by prior injection of dopamine uptake blockers but not by injection of dopamine receptor antagonists or drugs that bind to the dialkylpiperazine site. Unilateral 6-hydroxy-dopamine lesions of dopamine neurons caused a marked decrease in striatal and nigral binding on the side of the lesion. We conclude that intravenous injection of [18F]GBR 13119 provides a useful marker of presynaptic dopamine uptake sites.

The dopamine transporter and cytochrome P45OIID1 (debrisoquine 4-hydroxylase) in brain: resolution and identification of two distinct [3H]GBR-12935 binding proteins

Two [3H]GBR-12935 binding proteins, identified as the dopamine transporter and cytochrome P45OIID1, were solubilized in digitonin from canine striatal membranes, and were resolved following wheat germ agglutinin (WGA)-lectin column chromatography. Protein adsorbed to and specifically eluted from WGA-lectin with N-acetylglucosamine displayed saturable, high affinity (KD approximately 3 nM), and sodium-dependent binding of [3H]GBR-12935, which was inhibited in a concentration-dependent and stereoselective manner by dopamine uptake blockers and substrates with a pharmacological profile indicative of the dopamine uptake site. Protein not adsorbed to WGA-lectin also bound [3H]-GBR-12935 with high affinity (approximately 7 nM), in a sodium-independent manner, and was insensitive to classical dopamine uptake blockers and substrates such as mazindol or dopamine, corresponding to the so-called "piperazine acceptor" site seen in native membranes. [3H]GBR-12935 binding to this latter protein was, however, inhibited by various compounds with a pharmacological profile indicative of a form of cytochrome P450 designated P45OIID1 (debrisoquine/sparteine monooxygenase) with the following rank order of inhibitory potency: GBR-12909 greater than budipine greater than alpha-lobeline greater than quinidine greater than alpha flupenthixol greater than SKF-525A greater than sparteine greater than quinine. Ki values obtained for inhibition of [3H]-GBR-12935 binding to neuronal WGA passthrough fractions by these drugs correlate well with their respective Ki values for liver P45OIID1 activity. Western blotting and immunoprecipitation analysis with rabbit anti-rat P45OIID1 antibody also supported the identity of the mazindol-insensitive [3H]GBR-12935 binding site (or piperazine acceptor site) as P45OIID1. Furthermore, a [3H]GBR-12935 binding protein with pharmacological and immunological characteristics similar to those of P45OIID1 was solubilized from both bovine and human liver membranes, and GBR-12909 was found to be a potent competitive inhibitor (Ki approximately 100 nM) of sparteine monooxygenase activity in human liver microsomes. These data clearly indicate that [3H]GBR-12935 and its analogs display similar affinities for both the dopamine transporter and neuronal P45OIID1, and that this radioligand may be a useful probe of P45OIID1 activity in brain and liver. The exact molecular and functional association (if any) between these two distinct binding protein populations remains to be established; however, it is tempting to speculate that P45OIID1 is involved in the catabolism and processing of neurotransmitters subsequent to their reuptake into target cells.

Dopamine high-affinity transport site topography in rat brain: major differences between dorsal and ventral striatum

Investigations were conducted to determine the topography of the high-affinity dopamine uptake process within the rat striatum. [3H]Dopamine uptake into crude synaptosomes prepared from micropunch samples was found to be two- to three-fold higher in dorsal caudate-putamen relative to nucleus accumbens septi. In contrast, the concentrations of dopamine in the two regions were equivalent. The recognition site associated with high-affinity dopamine uptake was labeled using [3H]mazindol, and the binding of this ligand was also found to be two- to three-fold higher in homogenates from dorsal caudate-putamen samples relative to nucleus accumbens septi. Regional differences in uptake of [3H]dopamine or binding of [3H]mazindol were shown to be due to variations in Vmax or Bmax, not to differences in apparent affinity. Autoradiography of [3H]mazindol binding in rat striatum revealed a decreasing density of the site along the dorsal-to-ventral axis, with the highest binding occurring in the dorsolateral caudate-putamen, lower binding in the ventral caudate-putamen, and lowest levels in the septal pole of the nucleus accumbens septi. Quantification showed that the extent of this gradient was two-fold. Further autoradiographic studies revealed less striatal heterogeneity in the pattern of binding of [3H]ketanserin, another radioligand associated with the striatal dopaminergic innervation but not linked to the dopamine uptake process of the plasma membrane. The findings suggest that the dopaminergic fibers of the ventral striatum, especially the medial nucleus accumbens septi, may be relatively lacking in their capacity for dopamine uptake following its release. This organization may result in regional differences in the time-course of of extraneuronal dopamine following transmitter release and may render the dopamine-containing terminals of the ventral striatum less susceptible to the degenerative influences of neurotoxins that are incorporated by the high-affinity dopamine uptake process.

Catecholamine uptake sites in mouse brain: distribution determined by quantitative [3H]mazindol autoradiography

Because of the importance of the mouse brain catecholamine system in the study of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and because little information is available concerning the chemical neuroanatomy of the mouse, catecholamine uptake sites were mapped in C57 black mouse brain using [3H]mazindol autoradiography. Displacement studies with known dopamine (DA) and noradrenaline (NA) uptake blockers showed that binding in the striatum was entirely to DA uptake sites, while binding in the locus coeruleus was to NA uptake sites only. By using the selective noradrenergic uptake blocker desmethylimipramine (DMI), a complete map of both DA and NA uptake sites was generated. The mesostriatal DA system was the most clearly labelled and uptake sites were seen better in striatal terminals than the substantia nigra. Within the noradrenergic system, highest binding levels were seen over the locus coeruleus, although it was unclear whether these uptake sites were on cell bodies or terminals from the lateral tegmental noradrenergic system. These maps of the catecholamine uptake system in mouse brain provide a baseline for study of newly discovered neurotoxins and ageing processes.

Localization of the dopamine uptake complex using [3H]N-[1-(2-benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) in rat brain

[3H]N-[1-(2-Benzo(b)thiophenyl)cyclohexyl]piperidine ([3H]BTCP) is a novel phencyclidine derivative with considerable selectivity for dopamine uptake sites. [3H]BTCP was used to label dopamine uptake sites in vitro, in rat brain, and the regions containing these sites were visualized with an autoradiographic technique. The binding was found to be highest in the striatum, where greater than 90% of binding was specific. Furthermore, 6-hydroxydopamine lesions of the nigrostriatal pathway obliterated striatal [3H]BTCP binding ipsilaterally, whereas ibotenic acid injection into the caudate-putamen failed to significantly reduce [3H]BTCP binding in that structure. These results indicate that [3H]BTCP labels dopamine uptake sites in mammalian brain and that it can be employed for autoradiographic studies of this transport complex.

In vivo and in vitro autoradiographic labelling of central dopaminergic systems with [3H]GBR12783 in rodents

Labelling of central dopaminergic systems with the dopamine uptake inhibitor [3H]GBR12783 was performed in vivo in mice by i.v. injection of a tracer dose of the ligand (40 microCi) 1 h before sacrifice. Autoradiographic pictures obtained from these experiments were quantified. The data were compared to those obtained by in vitro labelling of rat brain slices with this ligand. Significant specific labelling was observed both in vivo and in vitro in the striatum, the nucleus accumbens, the olfactory tubercle and the substantia nigra.

Differentiation of dopamine overflow and uptake processes in the extracellular fluid of the rat caudate nucleus with fast-scan in vivo voltammetry

Stimulated overflow of dopamine (DA) into the extracellular fluid of the rat caudate nucleus was measured with fast-scan cyclic voltammetry. DA concentrations were sampled in less than 10 ms at 100-ms intervals with a Nafion-coated, carbon-fiber microelectrode. Overflow of DA was induced by electrical stimulation of the medial forebrain bundle with 300-microA pulses of various duration and frequency. Stimulated overflow was measured as a function of stimulus duration before and after administration of benztropine, bupropion, and amphetamine. These results were correlated with simulated curves based on a simple uptake/overflow model. The observed overflow was assumed to be a function of [DA]p, the concentration of DA which overflows per stimulus pulse, and the kinetics of cellular uptake of DA. Correlation of experimental with stimulated results was obtained at the 95% confidence limit for the duration studies; however, it was not possible to distinguish between the effects of pharmacological agents on uptake and overflow. In contrast, modulation of stimulus frequency did permit such distinction. Simulations of an increase in [DA]p fit results following dihydroxyphenylalanine methyl ester at 95% confidence limits, whereas an equivalent change in the apparent Km did not fit. An increase in the apparent value of Km correlated with results obtained at different frequencies following nomifensine and bupropion administration at the 95% confidence limit, whereas an equivalent increase in [DA]p did not fit. The effects of GBR 12909 best correlated with an increase in the DA available for overflow.

The D1 dopamine receptor in the rat brain: quantitative autoradiographic localization using an iodinated ligand

The distribution of dopamine D1 receptors in the rat, labeled with [125I]SCH 23982, was studied using a quantitative in-vitro light-microscopic autoradiographic method. The binding of [125I]SCH 23982 to slide-mounted tissue sections and membrane preparations of prefrontal cortex was saturable, specific and of high affinity. Scatchard analysis revealed a Kd of 1.15 +/- 0.47 nM and Bmax of 8.76 +/- 0.34 fmol/mg tissue in prefrontal cortex membranes and a Kd of 1.27 +/- 0.14 nM and Bmax of 67.6 +/- 3.75 fmol/mg tissue in slide-mounted tissue sections at the level of the striatum. [125I]SCH 23982 was found to predominantly label D1 receptors, but a small fraction of the binding was to serotonin receptors. D1 receptors were found throughout the forebrain and were concentrated in the substantia nigra pars reticulata, accumbens nucleus, caudate putamen, entopeduncular nucleus, olfactory tubercle and the major island of Calleja. [125I]SCH 23982 binding to serotonin receptors was concentrated in the cortices, dorsal raphe, central gray, anterior hypothalamic area and the molecular cell layer of the cerebellum. Knowledge of the distribution of D1 receptors may increase our understanding of the role of D1 receptors in central nervous system dopaminergic function. Furthermore, data on the potential sites of interaction of [125I]SCH 23982 with serotonin receptors may help to understand the complex physiology and pharmacology of the primarily D1 selective compound.

Synthesis of [18F]GBR13119, a presynaptic dopamine uptake antagonist

[18F]GBR13119 (1-[(4-[18F]fluorophenyl)-(phenyl)methoxy]ethyl-4-(3-phenylpropyl) piperazine) has been prepared in no carrier added form by a four-step synthesis from [18F]fluoride. Isolated yields are 7-10% (uncorrected) in a synthesis time of 120 min. The product is obtained in high specific activity (greater than 1000 Ci/mmol) and high radiochemical purity (greater than 99%) without chromatographic purification. Small amounts of chemical impurity, identified as the nitro-substituted analog by independent synthesis, can be removed by HPLC. [18F]GBR13119 is proposed as a new radiotracer for the presynaptic dopamine uptake system.

[3H]GBR 12935 binding in vivo in mouse brain: labelling of a piperazine acceptor site

The binding of the selective dopamine uptake inhibitor [3H]GBR 12935 was studied in vivo in mouse brain. The binding was reversible with t1/2 = 80 min. The localisation of [3H]GBR 12935 binding and of dopaminergic receptors did not overlap. The binding of [3H]GBR 12935 was distributed almost uniformly throughout the brain. Also, the in vitro inhibition of dopamine uptake and the inhibition of in vivo [3H]GBR 12935 binding did not correlate when a series of relevant reference compounds was used. The potencies of various dopamine uptake inhibitors to induce stereotyped behavior did not correspond to the inhibitory potencies in the [3H]GBR 12935 binding assay. In conclusion, [3H]GBR 12935 labels in vivo a site which is different from the dopamine uptake complex. We have recently obtained results for the in vitro binding of [3H]GBR 12935 that indicate that this site could be a piperazine acceptor site.