Cloning and expression of a cocaine-sensitive rat dopamine transporter

Endogenously released dopamine inhibits the binding of dopaminergic PET and SPECT ligands in superfused rat striatal slices

Pharmacologically induced changes in synaptic levels of dopamine (DA) have been found, in some studies, to affect the in vivo binding of dopaminergic radioligands. In the present study we used a superfused brain slice preparation to examine the effect of synaptically released dopamine on the binding of some commonly used PET and SPECT radioligands under more controlled conditions than those present in vivo. The release of DA was evoked by electrical stimulation of striatal slices and the sensitivity of binding of the D1 receptor ligand, [3H]SCH 23390, the D2 receptor ligands [3H]raclopride and [123I]epidepride, and the DA uptake transporter ligands, [3H]WIN 35,428 and [123I]RTI-55, to the frequency of stimulation examined. Most affected by stimulation was the specific binding of [3H]SCH 23390, which was fully inhibited at 2.5 Hz. This was followed by [3H]raclopride and [123I]epidepride, respectively, the binding of the latter showing only a 50% reduction at the highest frequency of 10 Hz. [3H]WIN 35,428 and [123I]RTI-55 binding was unaffected by stimulation. The effects of stimulation on [3H]raclopride binding were prevented by reserpine pretreatment of the rat, when combined with inclusion of the dopamine synthesis inhibitor, alpha-methyl-p-tyrosine, in the superfusate medium. We conclude that, in brain slices, the binding of D1 and D2 receptor ligands but not that of DA uptake transporter ligands is readily inhibited by DA released into the synaptic cleft. Brain slices may prove to be a useful model system for the investigation of factors affecting competition between radioligand binding and endogenous neurotransmitters.

Dopamine transporter immunohistochemistry in median eminence, amygdala, and other areas of the rat brain

In an extension of our previous work, an antibody directed against a peptide from the N-terminal region of DAT was used to localize specific dopamine transporter immunoreactivity (DAT-IR) in several regions of rat brain. Apparent axons and varicosities were found in the zona incerta, external layer of the median eminence, various nuclei of the amygdala, the cortex-amygdala transition zone, and in periglomerular regions in the olfactory bulb. Apparent stained neuronal perikarya and dendrites were observed in the arcuate nucleus and olfactory bulb. These regions are known to have dopaminergic neurons and innervations, although there was not a perfect correspondence between DAT-IR and the known distribution of dopaminergic neurons. A possible explanation is that different dopamine containing cell groups express different levels of DAT mRNA and protein, as we have previously shown. Also in the tuberoinfundibular neurons, for example, DAT-IR was preferentially localized to distal axons in the median eminence, suggesting intracellular compartmentation.

Distribution and sites of synthesis of NTT4, an orphan member of the Na+/Cl(-)-dependent neurotransmitter transporter family, in the rat CNS

The distribution and sites of synthesis in rat CNS of NTT4, a novel orphan member of the Na+/Cl(-)-dependent neurotransmitter transporter family, were determined by immunohistochemistry and hybridization histochemistry. Antibodies raised against recombinant fusion proteins, corresponding to residues of NTT4, and 35S-labelled oligodeoxyribonucleotide probes, were used to delineate the cellular distribution of the transporter at the protein and mRNA levels. High levels of immunoreactivity (mainly in the neuropil) were found in the olfactory bulb, cerebral cortex, striatum, hippocampus, thalamus, substantia nigra, pontine nuclei, cerebellum and spinal cord. The lowest levels were associated with the lateral hypothalamic area and deep mesencephalic nuclei. In situ hybridization signals correlated well with the immunoreactivity, and demonstrated a widespread distribution of NTT4 transcripts exclusively in neurons. NTT4 transcripts appeared widely codistributed with the N-methyl-D-aspartate receptor subunit 1 (1-4b), i.e. spliced variants characterized by a common 5' 63 bp insertion. These results indicate that the transporter was associated with neuronal processes in specific glutamate innervated CNS regions. Although the substrate transported by NTT4 remains unknown, our findings suggest a possible role for this carrier protein in glutamate/glycine neurotransmission.

Biology of membrane transport proteins

Membrane transporter proteins are encoded by numerous genes that can be classified into several superfamilies, on the basis of sequence identity and biological function. Prominent examples include facilitative transporters, the secondary active symporters and antiporters driven by ion gradients, and active ABC (ATP binding cassette) transporters involved in multiple-drug resistance and targeting of antigenic peptides to MHC Class I molecules. Transported substrates range from nutrients and ions to a broad variety of drugs, peptides and proteins. Deleterious mutations of transporter genes may lead to genetic diseases or loss of cell viability. Transporter structure, function and regulation, genetic factors, and pharmaceutical implications are summarized in this review.

Regional quantification of dopamine transporter mRNA in rat brain using a ribonuclease protection assay

We describe the regional distribution of dopamine transporter (DAT) mRNA in selected regions of rat brain using a highly sensitive and specific nuclease protection assay. This method determines the absolute quantity of mRNA expressed in the brain regions surveyed. DAT mRNA level varied widely between brain regions, and was detected only in cell body regions of the major dopaminergic pathways. Highest expression was seen in substantia nigra/ventral tegmentum (SN/VTA). Lower but detectable expression of a protected mRNA of the expected size was observed within hypothalamus. Expression could not be detected by this method in other brain regions studied. Our results indicate that this method is sufficiently sensitive to allow study of mRNA expression in individual animals.

[3H]Nisoxetine binding sites in the cat brain: an autoradiographic study

The binding of [3H]nisoxetine, a selective inhibitor of the high-affinity noradrenaline uptake sites, was studied on frontal frozen sections of the cat brain. The highest densities in autoradiographic signal were observed in the nucleus locus coeruleus and its ascending pathways, in the area postrema and in the dorsal part of the inferior olive, the pontine nuclei, the raphe nuclei, the colliculi, the periventricular and lateral areas of the hypothalamus, the suprachiasmatic nucleus, the nucleus accumbens and the olfactory bulb. A moderately high concentration of binding sites was observed in the hippocampal formation, especially in the molecular layer of Ammon's horn, in the superficial layers of the entorhinal cortex and in the indusium griseum. Binding sites were visualized in all the subdivisions of the neocortex. The highest density of binding was generally detected in the outer edge of the superficial layer I. In some cortical areas, especially in the visual cortex, labeling with a prevalent laminar distribution in the superficial layers I-III and in the deep layers V-VI was clearly observed. Moderate to low densities of binding sites were seen in most other areas of the brain except in the white matter, the caudate nucleus and putamen, which were devoid of labeling. Overall these findings indicate a good correlation between the distribution of [3H]nisoxetine binding sites and the noradrenergic systems. Furthermore, data suggest that in several areas, high-affinity noradrenaline reuptake mechanisms could play an important role in local interactions between the noradrenergic system and the other monoaminergic systems.

Dopamine transporter immunoreactivity in rat brain

The dopamine transporter (DAT) is a primary site for the action of cocaine in inducing euphoria. Its action is necessary for the selectivities of dopaminergic neurotoxins that provide the best current experimental models of Parkinson's disease. In the present report, rat dopamine transporter-like immunoreactivity (iDAT) was assessed by immunohistochemistry using newly developed polyclonal antisera raised against conjugated peptides corresponding to sequences found in the dopamine transporter's carboxy- and amino-termini. Dense iDAT was observed in patterns consistent with neural processes and terminals in the striatum, nucleus accumbens, olfactory tubercle, nigrostriatal bundle, and lateral habenula. Perikarya in the substantia nigra pars compacta were immunostained with moderate intensity using one of two immunohistochemical methods, while scattered ventral tegmental area perikarya were stained with somewhat less intensity. Immunoreactive neuronal processes with axonal and dendritic morphologies were stained in the substantia nigra and the paranigral and parabrachialis pigmentosus nuclei of the ventral tegmental area, while sparser processes were noted more medially in the ventral tegmental area. Neuronal processes were found in several laminae in the cingulate cortex, with notable fiber densities in the superficial aspects of lamina I and laminae II/III. The intensities of immunoreactivities in striatum and cerebral cortex were dramatically attenuated ipsilateral to nigrostriatal bundle 6-hydroxydopamine lesions. Specificity of immunostaining was supported by agreement of the results using sera directed against two distinct DAT segments, studies with preimmune and preadsorbed sera and studies of the extracted protein. These antisera identify and reveal details of the distribution of DAT immunoreactivity in rat brain and display variations in levels of DAT expression of likely functional significance.

Rat mesencephalic neuronal cells cultured for different periods as a model of dopamine transporter ontogenesis

Ventral mesencephalic neurons contained only low-affinity and sodium-independent binding sites of [3H]WIN 35,428 (marker of dopamine transporter) during the first 10 d in primary cultures. These sites were present in cytosol, and they are not very probably related to dopamine transporter. After 12 d in culture, membrane-bound, high-affinity, and sodium-dependent [3H]WIN 35,428 binding sites were detected. In membranes prepared from cells 14 d in culture, cocaine displaced [3H]WIN 35,428 binding with similar potency to that in striatal membranes of adult rat brain. The high-affinity [3H]WIN 35,428 binding sites in mesencephalic neuronal cell cultures are very probably related to dopamine transporter. The development of high-affinity [3H]WIN 35,428 binding sites in neurons cultured for different time periods could be a useful model of dopamine transporter ontogenesis.

Distinct pharmacological regulation of evoked dopamine efflux in the amygdala and striatum of the rat in vivo

The pharmacological regulation of evoked extracellular dopamine was compared in the basolateral amygdaloid nucleus (BAN) and caudate-putamen (CP) of the urethane-anesthetized rat. The effects of drugs, which alter dopamine uptake, release or degradation, were examined. Dopamine efflux was elicited by electrical stimulation of ascending dopamine fibers and was monitored by fast-scan cyclic voltammetry at Nafion-coated, carbon-fiber microelectrodes. Dopamine uptake inhibitors, nomifensine (25 mg/kg) and cocaine (20 mg/kg), and the dopamine receptor antagonist, haloperidol (0.5 mg/kg), robustly increased evoked extracellular dopamine in the CP. In sharp contrast, these drugs were much less effective in the BAN. The relative potencies of the uptake inhibitors varied between the two regions. Nomifensine was more potent than cocaine in the CP, whereas cocaine was more potent that nomifensine in the BAN. The monoamine oxidase inhibitor, pargyline (75 mg/kg), and the catechol-O-methyltransferase (COMT) inhibitor, Ro 40-7592 (40 mg/kg), had small or negligible effects in either region. No electrochemical evidence was found for the formation of 3-methoxytyramine, the dopamine metabolite formed by the action of COMT on released dopamine, on the time scale of the measurements in control or after pharmacological manipulation of the degradative enzymes for dopamine. The conclusions reached are: (1) potent mechanisms for uptake and autoreceptor inhibition of release, which exist in the CP to tightly control the concentration of extracellular dopamine, are considerably weaker in the BAN; (2) the extracellular clearance of evoked dopamine in the BAN and CP is the result of cellular uptake and not degradation; and (3) these results support the view that the pharmacological regulation of extracellular dopamine is regionally distinct in the brain.

Mammalian brain-specific L-proline transporter. Neuronal localization of mRNA and enrichment of transporter protein in synaptic plasma membranes

The expression of a high affinity Na(+)- (and Cl-) dependent L-proline transporter (PROT) in subpopulations of putative glutamatergic pathways in rat brain raises the possibility of a specific physiological role(s) for this carrier in excitatory neurotransmission (Fremeau, R. T., Jr., Caron, M. G., and Blakely, R. D. (1992) Neuron 8, 915-926). However, the biochemical properties and regional, cellular, and subcellular distribution of the PROT protein have yet to be elucidated. Here, we document the brain-specific expression and neuronal localization of rat PROT mRNA. We also report the first identification and partial biochemical characterization of the mammalian brain PROT protein. An affinity-purified antipeptide antibody was produced that specifically recognized a single 68-kDa PROT protein on immunoblots of rat and human brain tissues. Deglycosylation of rat hippocampal membranes with peptide-N-glycosidase F reduced the apparent molecular mass of the native PROT protein from 68 to 53 kDa, the size of the primary PROT translation product determined by in vitro translation of the rat PROT cDNA in the absence of microsomes. Subcellular fractionation studies demonstrated that the PROT protein was enriched in synaptic plasma membranes but absent from postsynaptic densities. A differential distribution of PROT mRNA and protein was observed in rat striatum, suggesting that the transporter protein is synthesized in neuronal cell bodies in the cortex and exported to axon terminals in the caudate putamen. These findings warrant the consideration of a novel presynaptic regulatory role for this transporter in excitatory synaptic transmission.

Human and mouse dopamine transporter genes: conservation of 5'-flanking sequence elements and gene structures

Synaptic reaccumulation of the neurotransmitter dopamine is mediated by the dopamine transporter (DAT), a member of the family of twelve transmembrane domain, sodium- and chloride-dependent neurotransmitter transporters. Several DAT features, including its exclusive expression in dopaminergic neurons, implication in cocaine action, and prominent role in the mechanisms of Parkinsonism-inducing neurotoxins, make understanding of the DAT gene of interest. Isolation and characterization of the human and mouse DAT genes has allowed elucidation of similarities between each and other members of this transporter gene family. Sequences 5' to transcriptional start sites contain G-C rich, TATA-less, CAAT-less regions with striking conservation between human and mouse gene flanking regions. These studies suggest sequence elements that are candidates to contribute to the dopamine transporter's dopaminergic cell-specific expression.

Regional distribution and developmental variation of the glycine transporters GLYT1 and GLYT2 in the rat CNS

The high-affinity glycine transporter in neurons and glial cells is the primary means of inactivating synaptic glycine. Previous molecular cloning studies have indicated heterogeneity of glycine transporters in the CNS. Here the distribution of glycine transporter GLYT1 and GLYT2 transcripts and proteins in different regions and developmental stages of the rat brain were analysed by Northern, Western and in situ hybridization techniques. Sequence-specific riboprobes and two specific antibodies raised against fusion proteins were used, containing either 76 or 193 amino acids of the C or N terminus of the GLYT1 and GLYT2 transporters respectively. High levels of GLYT1 transcripts were found in the spinal cord, brainstem and cerebellum, and moderate levels in forebrain regions such as the cortex or hippocampus. GLYT2 transcripts are restricted to the spinal cord, brainstem and cerebellum. The onset of both GLYT1 and GLYT2 expression in the brainstem occurred in late fetal life, and full expression of these proteins was observed before weaning. There was a stepwise increase in the levels of mRNA and protein for these two transporters, reaching a maximum by the second postnatal week, followed by a slight decrease until adult values were reached by the fourth postnatal week. These data reveal interesting parallelism between the distribution of different glycine transporters and glycine receptor subunits, and suggest discrete roles for distinct glycine transporters.

The role of N-glycosylation in the targeting and activity of the GLYT1 glycine transporter

To elucidate the role of N-glycosylation in the function of the high affinity glycine transporter GLYT1, we have investigated the effect of the glycosylation inhibitor tunicamycin as well as the effect of the disruption of the putative glycosylation sites by site-directed mutagenesis. SDS-polyacrylamide gel electrophoresis of proteins from GLYT1-transfected COS cells reveals a major band of 80-100 kDa and a minor one of 57 kDa. Treatment with tunicamycin produces a 40% inhibition in transport activity and a decrease in the intensity of the 80-100-kDa band, whereas the 57-kDa band decreases in size to yield a 47-kDa protein corresponding to the unglycosylated form of the transporter. Simultaneous mutation of Asn-169, Asn-172, Asn-182, and Asn-188 to Gln also produces the 47-kDa form of the protein, indicating that there are no additional sites for N-glycosylation. Progressive mutation of the potential glycosylation sites produces a progressive decrease in transport activity and in size of the protein, indicating that the four putative glycosylation sites are actually glycosylated. N-Glycosylation of the GLYT1 is not indispensable for the transport activity itself, as demonstrated by enzymatic deglycosylation of the transporter. Analysis of surface proteins by biotinylation and by immunofluorescence demonstrates that a significant portion of the unglycosylated GLYT1 mutant remains in the intracellular compartment. This suggests that the carbohydrate moiety of glycine transporter GLYT1 is necessary for the proper trafficking of the protein to the plasma membrane.

Effects of dopaminergic and glutamatergic receptor antagonists on the acquisition and expression of cocaine conditioning place preference

A balanced conditioning place preference (CPP) paradigm was used to study the role of dopamine D1 and D2 and glutamatergic NMDA and AMPA/kainate receptors on the acquisition and expression of cocaine place conditioning. The D1 receptor antagonist SCH 23390 (0.1-0.2 mg/kg i.p.), administered before cocaine during the training phase, significantly blocked the establishment of place conditioning (acquisition) but had no effect when administered before testing for place preference in the absence of cocaine (expression). Similar results were obtained with the non-competitive NMDA receptor antagonist MK-801 (0.1-0.5 mg/kg i.p.). The D2 receptor antagonist (-)-sulpiride (50-100 mg/kg i.p.) had no effect on either acquisition or expression of cocaine CPP. The AMPA/kainate receptor antagonist DNQX, administered intracerebroventricularly (0.2-3 micrograms/10 microliters), blocked cocaine CPP when given before testing but not when given before cocaine during the training trials. The results suggest that dopaminergic D1 (but not D2) and glutamatergic NMDA receptors are involved in the primary rewarding properties of cocaine (as assessed by the establishment of CPP) whereas the AMPA/kainate receptors are important only for the behaviour elicited by the stimuli previously associated with the drug action (CPP expression). The implications for the treatment of cocaine craving and relapse are discussed.

Regional brain uptake and pharmacokinetics of [123I]N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane esters in baboons

Four N-omega-fluoroalkyl-2 beta-carboxy-3 beta-(4-iodophenyl)nortropane ester (beta-CIT-FE), N-fluoropropyl, methyl ester (beta-CIT-FP), N-fluoroethyl, isopropyl ester (IP-beta-CIT-FE), and N-fluoropropyl, isopropyl ester (IP-beta-CIT-FP)] were labeled with 125I and evaluated in baboons by dynamic SPECT regional brain imaging, measurement of pharmacokinetics in arterial plasma, and whole body imaging. The labeled tracers were prepared by iododestannylation of the corresponding 4-(trimethylstannyl)phenyl compounds in radiochemical yield 63-96% and radiochemical purity > 96%. Regional SPECT brain imaging was carried out over a period of 5 h with a Strichman 810X Brain Imager to assess regional uptake in the striatum and midbrain compared to reference regions in the occipital cortex and cerebellum; arterial blood samples were taken for analysis of metabolites by solvent extraction and HPLC. The methyl esters showed higher total and specific peak uptake in the striatum than the isopropyl esters. Midbrain uptake was uniformly lower than striatal uptake and washed out more rapidly. beta-CIT-FE had more rapid striatal kinetics than beta-CIT-FP, with specific striatal washout rates of 10-14 vs 4-6% peak/h. Biodistribution of [123I] beta-CIT-FE and [123I] beta-CIT-FP measured by whole body conjugate imaging demonstrated major uptake in the brain, liver, and GI tract, with excretion occurring through both the renal and hepatobiliary routes. Absorbed radiation does estimates based on the MIRD schema indicated highest dose rates to the urinary bladder wall and lower large intestine wall (0.7 and 0.6 rad/mCi for [123I] beta-CIT-FE and 0.7 and 0.9 rad/mCi for [123I]beta-CIT-FP, respectively).(ABSTRACT TRUNCATED AT 250 WORDS)

Colocalization of dopamine and serotonin in the rat pituitary gland and in the nuclei innervating it

The nerve terminals in the intermediate and posterior lobes of the rat pituitary gland are reported to show colocalization of serotonin and tyrosine hydroxylase. This study examined the extent of this colocalization in the pituitary gland and in the nuclei considered to project to the pituitary. In the intermediate lobe, two types of nerve fibers were encountered, one containing serotonin (5-HT-IR) and tyrosine hydroxylase (TH-IR) immunoreactivities and the other showing 5-HT-IR only. Instead, there was no colocalization in the posterior lobe. In the hypothalamus, colchicine treatment with L-tryptophan and pargyline injections resulted in 5-HT-IR in some neurons in the dorsomedial, periventricular and arcuate nuclei, some of which in the arcuate and periventricular nuclei were also TH-IR. In the raphe nuclei no colocalization of 5-HT-IR and TH-IR was observed. Catecholamine neurotoxin, 6-hydroxydopamine, abolished the 5-HT-IR and dramatically reduced the TH-IR in the intermediate lobe nerve fibers. Both effects were prevented by cocaine, a monoamine uptake inhibitor, but not by fluoxetine, a specific serotonin uptake inhibitor. Serotonin neurotoxin p-chloroamphetamine (PCA) had no effect on intermediate lobe fibers, although it caused complete disappearance of 5-HT-IR from the posterior lobe nerve fibers. This effect was prevented by fluoxetine. Our results indicate, that colocalization of serotonin and TH observed in the intermediate lobe occurs both in the nerve terminals within the lobe and in some nuclei that innervate it. Furthermore, drug treatments suggest that serotonin in the intermediate lobe is localized in catecholaminergic fibers, which do not posses a specific serotonin uptake mechanism.

Molecular cloning of an orphan transporter. A new member of the neurotransmitter transporter family

A complementary DNA clone predicted to encode a novel transporter was isolated from rat brain and the localization of its mRNA was examined. The cDNA, designated rB21a, predicts a protein with 12 putative transmembrane domains that exhibits significant sequence homology with neurotransmitter transporters. Expression studies have not yet identified the endogenous substrate for this transporter, but the presence of rB21a mRNA within the leptomeninges of the brain suggests the transporter may regulate CSF levels of its substrate. The cloning of rB21a provides the means to determine its physiological functions and the potential to design novel, transporter-based therapeutic agents for neurological and psychiatric disorders.

Cellular expression of glycine transporter 2 messenger RNA exclusively in rat hindbrain and spinal cord

High-affinity transporters mediate the removal of released neurotransmitters from synapses, thereby terminating their synaptic action. A novel glycine transporter has recently been cloned from a rat brain complementary DNA library. In this study we examined, by means of in situ hybridization with 35S-labelled oligodeoxynucleotide probes, the distribution of messenger RNAs encoding glycine transporter 2 in the rat CNS. Moreover, adjacent series of sections were labelled with [3H]strychnine to reveal the regional distribution of strychnine-sensitive glycine receptors. A very discrete pattern of distribution of the transcripts was found exclusively at the level of the brainstem/cerebellum and spinal cord. In the cerebellum, Golgi cells in the granule cell layer as well as a subpopulation of neurons in the interposed nuclei were consistently labelled. In the brainstem, where the bulk of the labelling was concentrated, several nuclei showed a high level of transcript expression, including the superior olivary complex, nucleus of the trapezoid body and the ventral nucleus of the lateral lemniscus. In the spinal cord, many neurons throughout all layers were labelled, including putative Renshaw cells and a few large neurons at the border of layers 7 and 9. No labelled cells were detected at the levels of the fore- and midbrain. The distribution of glycine transporter 2 messenger RNA-containing cell bodies was very different to that of other glycine transporter messenger RNAs (glycine transporter 1a and glycine transporter 1b), but similar to that of known glycine-immunoreactive neurons and correlated very well with that of strychnine-sensitive glycine receptors in most CNS regions except cerebellum. Our results show that glycine transporter 2 (but not glycine transporter 1) in the brainstem, spinal cord and cerebellum is probably involved in the reuptake of glycine from synapses containing classical strychnine-sensitive glycine receptors. Our findings also suggest that glycine acts as a neurotransmitter in cerebellar Golgi neurons. Whether the synaptic concentration of glycine, as co-agonist at NMDA receptors, is regulated (if at all) by transaminase activity or by a glycine transporter (GLYT1a?) distinct from that described here is not yet known.

Alterations in dopamine uptake sites and D1 and D2 receptors in cats symptomatic for and recovered from experimental parkinsonism

The administration of the neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) to adult cats severely disrupts the dopaminergic innervation of the striatum. Animals display a parkinson-like syndrome, consisting of akinesia, bradykinesia, postural instability, and rigidity, which spontaneously recovers by 4-6 weeks after the last administration of MPTP. In this study we used quantitative receptor autoradiography to examine changes in DA uptake sites and DA receptors in the basal ganglia of normal, and symptomatic and recovered MPTP-treated cats. Consistent with the destruction of the nigrostriatal DA pathway, there was a severe loss of DA uptake sites, labeled with [3H]-mazindol, in the caudate nucleus (64-82%), nucleus accumbens (44%), putamen (63%), and substantia nigra pars compacta (SNc, 53%) of symptomatic cats. Following behavioral recovery, there were no significant changes in DA uptake site density. Significant increases of [3H]-SCH 23390 binding to D1 DA receptors were observed in the dorsal caudate (> 24%; P < 0.05) of symptomatic cats and in all regions of the caudate-putamen (> 30%; P < 0.05) of recovered animals. [3H]-SCH 23390 binding in the substantia nigra pars reticulata was half of that in the striatum and showed no changes in symptomatic or recovered animals. No alterations in the binding of [125I]-epidepride to D2 receptors was observed in any region of the striatum in either symptomatic or recovered animals. [125I]-Epidepride binding in the SNc was decreased by > 36% (P < 0.05) following MPTP treatment. These data show that cats made parkinsonian by MPTP exposure have a significant decrease in the number of DA reuptake sites throughout the striatum and that recovery of sensorimotor function in these animals is not correlated with an increase in the number of striatal reuptake sites. Behavioral recovery, however, does seem to be correlated with a general elevation of D1 receptors throughout the striatal complex. The present data also show that direct correlations between changes in DA receptor regulation after a large DA depleting lesion and behavioral deficits or recovery from those deficits are difficult and that the relationships between DA receptors/transporters and behavior require further study.

Chimeric dopamine-norepinephrine transporters delineate structural domains influencing selectivity for catecholamines and 1-methyl-4-phenylpyridinium

The dopamine (DA) and norephinephrine (NE) transporters demonstrate important differences in their selectivity for catecholamines and the parkinsonism-inducing neurotoxin 1-methyl-4-phenylpyridinium (MPP+), yet their primary sequences and predicted topology are strikingly similar. To delineate discrete structural domains contributing to pharmacologic and kinetic differences between the DA and NE transporters, a series of recombinant chimeras was generated by a restriction site-independent method and expressed in mammalian cells. Functional analyses of the chimeras delineate two discrete regions spanning the first through the third transmembrane domains (TM1-3) and TM10-11 that contribute to differences in their apparent affinities for DA, NE, and MPP+. These studies also suggest that TM2-3 of the DA transporter have a role in selectively increasing the rate of DA uptake as compared with NE. TM4-8 of the DA transporter may influence the relative rate with which MPP+ is taken up into cells and could contribute to its selective toxicity in neurons expressing the DA transporter. These structure-function studies using chimeras of members of the superfamily of Na(+)- and Cl(-)-dependent transporters provide a framework for identifying the specific structural or regulatory determinants contributing to substrate recognition and translocation by the DA and NE transporters.

A putative model of the dopamine transporter

A three-dimensional model of the human dopamine transporter was constructed by molecular modeling techniques from its amino acid sequence, based on sequence analysis of this and 9 other transporter proteins. The model has 12 membrane spanning alpha-helices arranged in two 7-helical bundles, loops between helices and amino- and carboxy termini. The molecular electrostatic potentials were mainly negative at the synaptic side and positive in the cytoplasmic domains of the transporter model, strongly positive in some of the transmembrane domains, and strongly negative in other transmembrane domains. The model suggests specific binding sites for dopamine and cocaine, a functional role for chloride ions, and accounts for known structure-activity relationships of cocaine analogs at the dopamine transporter.

Pharmacological characterization of serotonin synthesis and uptake suggest a false transmitter role for serotonin in the pituitary intermediate lobe

A subpopulation of nerve fibers in the rat pituitary intermediate lobe (IL) have been shown to exhibit colocalization of serotonin (5-HT-IR) and tyrosine hydroxylase immunoreactivities and they are sensitive to neurotoxins specific to catecholamine neurons. This study was set out to examine the uptake and synthesis mechanisms of serotonin in these fibers. We developed an in vitro technique in which the neurointermediate lobe explants were incubated (14 and 48 h) in the presence of various drugs and serotonin was subsequently visualized by immunohistochemistry. Control incubation in the presence of serotonin (10(-6) M) resulted in a rich plexus of 5-HT-IR fibers in both posterior and intermediate lobes. Fluoxetine and citalopram (10(-6) M and 10(-5) M), inhibitors of 5-HT transporter, did not affect 5-HT-IR in the IL fibers, unless they were used in concentrations high enough (10(-4) M and 10(-3) M) to block unspecifically a number of monoamine transporters. The same applied for desipramine (10(-5)-10(-7) M), an inhibitor of the noradrenaline transporter. However, cocaine (10(-5)-10(-6) M) blocked serotonin uptake into these terminals, suggesting that serotonin uptake occurs through a dopamine transporter. Incubation of the IL in presence of L-tryptophan (10(-4) M) did not result in 5-HT-IR in the IL fibers showing colocalization of 5-HT-IR and tyrosine hydroxylase, which suggests that these fibers do not synthesize serotonin. The present results suggest that serotonin is taken up into the IL terminals by a dopamine transporter and is not synthesized in them, at least in vitro.(ABSTRACT TRUNCATED AT 250 WORDS)

Developmentally regulated glycosylation of dopamine transporter

The dopamine transporter (DAT) in rat striatum was examined during postnatal development and aging after photolabeling with [125I]DEEP. The DAT-[125I]DEEP protein complex from adult rats (2 months) appeared as a broad diffuse band in SDS-PAGE gels with average apparent molecular mass of about 80,000 Da as previously found. However, the molecular mass was lower at birth (day 0) and at postnatal ages 4 and 14 days. In aged rats (104 weeks), the molecular mass was slightly higher than that found in young adults (60 days). In binding experiments with [3H]BTCP, there were age-related differences in Kd and Bmax with decreases in both Kd and Bmax found in aged rats. Treatment of photolabeled membranes with neuraminidase caused a reduction in DAT molecular mass, but age-related differences were maintained. Treatment with N-glycanase greatly reduced or eliminated the age-related differences. Several DAT peptide-specific polyclonal antibodies immunoprecipitated DAT-[125I]DEEP protein complex at different developmental ages. Taken together, these results suggest differential glycosylation of rat DAT occurs during postnatal development and aging; the increase is due to increases in the N-linked sugars rather than changes in either sialic acid content or the polypeptide.

The dopamine transporter and dopamine D2 receptor messenger RNAs are differentially expressed in limbic- and motor-related subpopulations of human mesencephalic neurons

Dysfunction of dopamine neural systems is hypothesized to underlie neuropsychiatric disorders and psychostimulant drug abuse. At least three dopamine systems have been characterized in the brain-nigrostriatal, mesolimbic, and mesocortical. Abnormalities of nigrostriatal dopamine neurons cause motor impairment leading to Parkinson's disease, whereas dysfunction of mesolimbic and mesocortical dopamine neurons are most implicated in psychotic disorders such as schizophrenia and in drug addition. One of the primary neural sites of action of potent antipsychotic agents and psychostimulant drugs of abuse are dopamine receptors and dopamine transporters which, respectively, mediate the induction and termination of dopamine's actions. Very limited information is, however, available about which particular set of dopaminergic cells in the human brain actually express the genes for these dopamine-specific proteins. In this study, we observed that the dopamine transporter and D2 receptor messenger RNAs are differentially expressed within the human mesencephalon: highest expression in ventral subpopulations of the substantia nigra pars compacta neurons with lowest expression in the mesolimbic/mesocortical ventral tegmental area and retrorubral cell groups. These findings suggest that motor- and limbic-related mesencephalic neurons in the human brain differ in the degree of dopamine transporter and D2 receptor gene expression.

The molecular cloning and expression of a human synaptic vesicle amine transporter that suppresses MPP+ toxicity

A synaptic vesicle amine transporter cDNA, termed hSVAT, has been isolated by the reverse transcription and polymerase chain reaction (PCR) technique from human substantia nigra and subsequent screening of a human substantia nigra library. The hSVAT sequence obtained is highly homologous to the rat SVAT sequence (92% homology) and is essentially identical to the human sequence identified recently by Surratt and colleagues [33]. This labelled hSVAT cDNA detected a single band (approximately 5.0 kb) when used as a probe for Northern analysis of human nigral RNA extract. In situ hybridization studies using hSVAT specific antisense oligonucleotides showed a strong hybridization signal concentrated over the cells of the substantia nigra pars compacta. This cDNA sequence when expressed in chinese hamster ovary (CHO) cells conferred resistance to MPP+ the toxic metabolite of MPTP and cells containing it accumulated dopamine.

Phorbol esters alter functions of the expressed dopamine transporter

Recent elucidation of the amino acid sequences of the neurotransmitter transporters reveals several consensus sequences for phosphorylation by kinases including protein kinase C. Protein kinase C activation did modulate the function of the rat dopamine transporter expressed in COS cells. Cell treatment with the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) reduced the affinity of binding of the radiolabeled cocaine analog [3H](-)-2 beta-carbomethoxy-3 beta-(4-fluorophenyl)tropane (WIN 35,428) without affecting its Bmax. The uptake of [3H]dopamine was reduced by treatment with PMA in a staurosporine-sensitive manner. Kinetic analysis revealed that the inhibitory effect of PMA on [3H]dopamine uptake was due to reduced uptake velocity and a small reduction of affinity for Na+, without changed affinity for dopamine. 1-Oleoyl-2-acetyl-sn-glycerol (OAG) mimicked these actions of PMA. These results demonstrate that activation of protein kinase C alters dopamine transporter functions in both ligand recognition and substrate translocation. These phosphorylation phenomena in vitro suggest the possibility that phosphorylation could modulate the activity of this important dopaminergic synaptic regulator under physiological conditions.

Cocaine and GBR 12783 recognize nonidentical, overlapping binding domains on the dopamine neuronal carrier

In incubation medium containing Na+ as the only cation, the specific binding of [3H]cocaine to a membrane preparation obtained from rat striatum reached a maximal level for 10 mM Na+, whereas higher concentrations decreased its affinity. The specific binding of [3H]cocaine was inhibited monophasically by GBR 12783, mazindol, nomifensine and substrates of the transporter; in saturation experiments, GBR 12783 competitively blocked the [3H]cocaine specific binding and vice versa. Treatment of the striatal membranes with N-ethylmaleimide resulted in a concentration-dependent reduction of the specific binding of [3H]GBR 12783 (1-[2-(diphenylmethoxy)ethyl]4-(3-phenyl-2-[1-3H]propenyl)-piperaz ine) which was significantly more marked than that of the specific binding of [3H]cocaine, the nonspecific binding of [3H]cocaine being measured with either cocaine or dopamine. Addition of substrates or pure uptake inhibitors to the treatment medium afforded protection against the N-ethylmaleimide-induced reduction in both bindings. In particular, cocaine offered protection for [3H]GBR 12783 binding and vice versa. All results are consistent with a model in which pure uptake blockers and substrates recognize nonidentical but overlapping binding domains on the neuronal carrier of dopamine.

In vivo voltammetric measurement of evoked extracellular dopamine in the rat basolateral amygdaloid nucleus

1. The in vivo measurement of evoked extracellular dopamine was established in the basolateral amygdaloid nucleus (BAN) using fast-scan cyclic voltammetry at carbon-fibre microelectrodes. 2. The identification of evoked extracellular dopamine in the BAN was based on anatomical, electrochemical and pharmacological criteria. Electrochemical and pharmacological evidence indicated that the species was a catecholamine. Mesencephalic sites eliciting overflow and amygdaloid sites supporting overflow correlated well with the mesoamygdaloid dopamine innervation. 3. Marked differences in the dynamics and magnitude of evoked dopamine overflow were observed in the BAN, caudate-putamen and amygdalo-striatal transition area. The results underscore the importance of making spatially resolved measurements of extracellular dopamine in the amygdala. 4. Mesoamygdaloid dopamine neurons have similar release characteristics as mesostriatal dopamine neurons but share with mesoprefrontal cortical dopamine neurons the ability to use a greater percentage of intraneuronal dopamine stores for release.

A functional superfamily of sodium/solute symporters

Eleven families of sodium/solute symporters are defined based on their degrees of sequence similarities, and the protein members of these families are characterized in terms of their solute and cation specificities, their sizes, their topological features, their evolutionary relationships, and their relative degrees and regions of sequence conservation. In some cases, particularly where site-specific mutagenesis analyses have provided functional information about specific proteins, multiple alignments of members of the relevant families are presented, and the degrees of conservation of the mutated residues are evaluated. Signature sequences for several of the eleven families are presented to facilitate identification of new members of these families as they become sequenced. Phylogenetic tree construction reveals the evolutionary relationships between members of each family. One of these families is shown to belong to the previously defined major facilitator superfamily. The other ten families do not show sufficient sequence similarity with each other or with other identified transport protein families to establish homology between them. This study serves to clarify structural, functional and evolutionary relationships among eleven distinct families of functionally related transport proteins.

Monoamine uptake in insect synaptosomal preparations

Biochemical studies of mammalian synaptosomal nerve fractions indicate the existence of multiple transporter proteins important for the termination of synaptic transmission by each of several monoamines. In insects, however, data on monoamine uptake has been limited to the study of whole tissue preparations, making it unclear whether neuronal (as opposed to glial) uptake is a significant mechanism in the insect. The present experiments elucidate the difficulties that have limited the use of insect synaptosomal preparations for characterizing amine reuptake. Key procedural improvements, including the utilization of carrier protein for tracer separation and the use of receptor antagonists to decrease non-specific membrane binding are described. With these and other modifications, reproducible sodium-dependent and cocaine-inhibitable dopamine and octopamine uptake are described in synaptosomal-containing preparations from insect brain and ganglia. These studies therefore support the existence of specific Na(+)-dependent uptake mechanisms in insect neurons.

Characterization of dopamine transporter and locomotor effects of cocaine, GBR 12909, epidepride, and SCH 23390 in C57BL and DBA mice

C57BL/6 and DBA/2 mice were used to examine genetic differences in locomotor activating effects of acute cocaine administration and to determine whether differences were mediated by dopaminergic systems. C57BL/6 mice were less activated than DBA/2 mice at 5 and 10 min after 10 and 15 mg/kg cocaine. HPLC analysis showed equivalent brain cocaine concentrations in the two strains at 5 and 10 min after 10, 15, or 20 mg/kg doses. The selective dopamine uptake inhibitor, GBR 12909, at 5 and 7.5 mg/kg, produced greater locomotor activation in DBA/2 mice than in C57BL/6 mice. However, binding studies with the selective dopamine uptake ligand [3H]GBR 12935, revealed no between-strain difference in Kd or Bmax in caudate putamen (CP) or nucleus accumbens (NA) membranes. Competition assays using unlabeled dopamine to compete for [3H]GBR 12935 binding in CP or NA membranes showed no between-strain difference by brain region. The specific D1 or D2 antagonists, SCH 23390 or epidepride, respectively, produced dose-dependent decreases in locomotor activity but there were no between-strain differences. However, epidepride, at a dose of 0.003 mg/kg, completely reversed cocaine-induced (15 mg/kg) activation in both strains. These findings show that C57BL/6 and DBA/2 mice differ in dopamine-related behaviors and suggest that dopaminergic processes may mediate genetic differences in cocaine sensitivity.

PC12 variants deficient in norepinephrine transporter mRNA have wild type activities of several other related transporters

Wild type PC12 pheochromocytoma cells express a Na(+)-dependent norepinephrine transporter that operates in the uptake of catecholamines. In addition to the previously described Na(+)-dependent system A for the uptake of alpha-amino-isobutyric acid and system Gly for glycine, we have identified two other Na(+)-dependent transporter systems for amino acid uptake in these cells: 1) system beta for beta-alanine and taurine; and 2) a system for creatine. Uptake of alpha-amino-isobutyric acid, glycine, beta-alanine, and creatine is not affected in some PC12 variants that were previously shown to be deficient in catecholamine uptake and to have decreased levels of norepinephrine transporter mRNA. We have isolated two PC12 cDNA clones that are essentially identical in sequence to recently reported cDNAs for rat brain taurine and creatine transporters, respectively, and a third cDNA that appears to code for a novel transporter. mRNAs for these three transporters are present at wild type levels in those variants that express no or little norepinephrine transporter mRNA. These results support the notion that the expression of catecholamine reuptake transporters may be particularly susceptible to down-regulation.

Similar properties of fetal and adult amine transporters in the rat brain

A variety of drug classes, including psychomotor stimulants and antidepressants, interact with monoamine transporters in order to exert their effects. Although these transporters have been extensively characterized in the adult brain, little is known about uptake mechanisms in the fetal system. High affinity dopamine (DA) and serotonin (5-HT) uptake in the striatum and frontal cortex, respectively, were examined in rat fetuses (embryonic day 20; E-20). These results were then compared to uptake in adult rat synaptosomal preparations of the same regions. The data indicate that the fetal (E-20) uptake mechanism is sodium-dependent. Furthermore, the potency of various agents to inhibit transporter function was assessed. These drugs produced a concentration-dependent inhibition of uptake, and the resulting IC50 values were not significantly different from those obtained in the adult preparations. Our results provide evidence that the affinity of monoamine uptake inhibitors for fetal (E-20) DA and 5-HT transporters is similar to that observed with adult transporters. This observation has broad implications when considering neuronal development and in utero exposure to drugs that exert their effects through these transporters.

Intraventricular insulin increases dopamine transporter mRNA in rat VTA/substantia nigra

The hormone insulin can down-regulate the function and synthesis of the re-uptake transporter for norepinephrine (NET) in vivo and in vitro. In the present study we tested whether this action of insulin is generalized to another member of the catecholamine transporter family. We determined the level of dopamine transporter (DAT) mRNA expression in the ventral tegmental area (VTA)/substantia nigra compacta (SNc) of rats which were chronically treated with vehicle or insulin via the third cerebral ventricle (i.c.v.). DAT mRNA was significantly elevated in the VTA/SNc of rats treated with insulin, as compared with levels in vehicle-treated rats. This is in contrast to our previous observation that i.c.v. insulin decreases NET mRNA in the rat locus coeruleus, and suggests that insulin may have differential and specific modulatory effects on CNS catecholaminergic pathways.

'Choline/orphan V8-2-1/creatine transporter' mRNA is expressed in nervous, renal and gastrointestinal systems

Several cDNAs with substantial sequence homologies to members of the neurotransmitter transporter gene family currently remain 'orphan' transporters, without clearly-identified substrates. We were concerned that a cDNA 'V8-2-1' isolated from a ventral midbrain cDNA library in this laboratory and a virtually-identical cDNA 'CHOT1' reported by Mayser et al. [J. Neurochem., 20 (1973) 581-593] might represent such an orphan. Despite initial reports that it could mediate some choline uptake; neither CHOT1 nor V8-2-1 was demonstrated to confer pharmacologically appropriate choline uptake not already present in either Xenopus oocytes or COS cells. Determination of the regional and tissue-specific distribution of mRNA hybridizing with V8-2-1 cDNA was undertaken to aid in identifying its function. Examination of the distribution of V8-2-1 expression reveals several novel features of this transporter gene family member's distribution, including several features that add to current evidence suggesting that the clone may not encode the classical pharmacologically-defined, hemicholinium-3 sensitive high affinity transporter of cholinergic neurons. These data fit with and extend recent data that suggest that this cDNA represents creatine transporter, and provide initial documentation of its regional distribution in brain.

O-526, a piperidine analog of GBR 12909, retains high affinity for the dopamine transporter in monkey caudate-putamen

1,4-Disubstituted piperazines such as GBR 12909 (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine) and GBR 12935 (1-[2-(bis(phenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperazine) are among the most potent and selective ligands for the dopamine transporter in brain. However, they also bind to a widely distributed "piperazine acceptor site". In order to assess whether the piperazine moiety of GBR 12909 is critical for binding to the dopamine transporter, two piperidine analogs of GBR 12909, one with the nitrogen proximal to the diphenylmethyloxy moiety (O-549, (1-[2-[bis(4-fluorophenyl)methoxy]ethyl]-4-(3-phenylpropyl)piperidine), the other distal to this moiety (O-526, 4-[2-[bis(4-fluorophenyl)methoxy]ethyl]-1- (3-phenylpropyl)piperidine) were synthesized. The affinities of O-526, O-549 and GBR 12909 for the dopamine and serotonin transporters in monkey caudate-putamen (Macaca fascicularis) were evaluated using [3H]WIN 35,428 ([3H]CFT, 2 beta-carbomethoxy-3 beta-4-(fluorophenyl)tropane) or [3H]GBR 12935 and [3H]citalopram to label the dopamine and serotonin transporters, respectively. O-526 inhibited [3H]WIN 35,428 binding with potencies that were similar to GBR 12909 (IC50: 24.9 +/- 3.23 vs. 22.0 +/- 2.2 nM, respectively) whereas O-549 was 24 times less potent than O-526 (IC50: 595 +/- 148 nM). The selectivity of GBR 12909 for the dopamine over the serotonin transporter was retained by O-526 but not by O-549 which was relatively non-selective. These results indicate that only one of the two nitrogens on the basic GBR structure is needed for high affinity binding to the dopamine transporter.(ABSTRACT TRUNCATED AT 250 WORDS)

Regulatory and molecular aspects of mammalian amino acid transport

Images

Dopamine transporter messenger RNA in Parkinson's disease and control substantia nigra neurons

Dopamine transporter messenger RNA (mRNA) expression was assessed by in situ hybridization over individual pigmented neurons from the substantia nigra pars compacta in midbrain sections from 7 parkinsonian and 7 age-matched, neurologically normal patients. In the normal control brains, high levels of expression of dopamine transporter mRNA were noted over pigmented neurons in the substantia nigra pars compacta; neurons in the adjacent nucleus paranigralis of the ventral tegmental area displayed less hybridization. Nigra compacta neurons surviving in brains of patients with Parkinson's disease displayed only 57% of the dopamine transporter mRNA hybridization intensity displayed by nigral neurons in normal control brains. The disease-related decrease in the apparent level of dopamine transporter mRNA expression in remaining neurons could reflect neuronal dysfunction. Conceivably, it might also reflect differential vulnerability of those neurons that initially expressed higher levels of this transporter to the insult of parkinsonism.

Nitric oxide inhibits [3H]dopamine uptake

Sodium nitroprusside (SNP), a generator of nitric oxide (NO), decreased [3H]dopamine uptake into rat striatal synaptosomal preparations in a dose-, time- and temperature-dependent fashion. Reduced hemoglobin, a substance that binds NO, prevented the SNP-induced decrease in uptake. Potassium ferri- and ferrocyanide, compounds similar to SNP that do not produce NO, were without effect on uptake. SNP inhibited [3H]dopamine uptake in synaptosomes from nucleus accumbens and olfactory tubercle as well but with a lower potency. SNP inhibited [3H]serotonin and [3H]glutamate uptake but had no effect on [3H]norepinephrine uptake. S-Nitroso-N-acetylpenicillamine (SNAP), another generator of NO, had effects similar to those of SNP. The SNP-induced decrease in [3H]dopamine uptake was due to a Vmax decrease at 100 microM SNP and to both a Vmax and Km change at 300 microM SNP. Depletion of calcium by omission of calcium from buffers and addition of EGTA increased the potency of SNP in inhibiting uptake. There was no change in [3H]WIN 35428 binding to the dopamine transporter with doses of SNP that inhibited uptake. These data indicate that NO can decrease [3H]dopamine transporter function.