Photoaffinity labeling of the monoamine transporter of bovine chromaffin granules and other monoamine storage vesicles using 7-azido-8-[125I]iodoketanserin

Vesicular monoamine transporters: structure-function, pharmacology, and medicinal chemistry

Vesicular monoamine transporters (VMAT) are responsible for the uptake of cytosolic monoamines into synaptic vesicles in monoaminergic neurons. Two closely related VMATs with distinct pharmacological properties and tissue distributions have been characterized. VMAT1 is preferentially expressed in neuroendocrine cells and VMAT2 is primarily expressed in the CNS. The neurotoxicity and addictive properties of various psychostimulants have been attributed, at least partly, to their interference with VMAT2 functions. The quantitative assessment of the VMAT2 density by PET scanning has been clinically useful for early diagnosis and monitoring of the progression of Parkinson's and Alzheimer's diseases and drug addiction. The classical VMAT2 inhibitor, tetrabenazine, has long been used for the treatment of chorea associated with Huntington's disease in the United Kingdom, Canada, and Australia, and recently approved in the United States. The VMAT2 imaging may also be useful for exploiting the onset of diabetes mellitus, as VMAT2 is also expressed in the β-cells of the pancreas. VMAT1 gene SLC18A1 is a locus with strong evidence of linkage with schizophrenia and, thus, the polymorphic forms of the VMAT1 gene may confer susceptibility to schizophrenia. This review summarizes the current understanding of the structure-function relationships of VMAT2, and the role of VMAT2 on addiction and psychostimulant-induced neurotoxicity, and the therapeutic and diagnostic applications of specific VMAT2 ligands. The evidence for the linkage of VMAT1 gene with schizophrenia and bipolar disorder I is also discussed.

Synthesis and evaluation of 2-amino-dihydrotetrabenzine derivatives as probes for imaging vesicular monoamine transporter-2

A novel series of analogs of 2-amino-dihydrotetrabenazine derivatives, 4-6, targeting the vesicular monoamine transporter have been prepared. In vitro binding was carried out in tissue homogenates prepared from rat striatal tissue homogenates with both [(125)I]-iodovinyl-TBZ and [(3)H]DTBZ. There was a good correlation (r(2)=0.925) between the affinities of the different compounds for [(125)I]-iodovinyl-TBZ and [(3)H]-DTBZ binding. Compound 5 exhibited a better affinity for the vesicular monoamine transporter (K(i)=8.68+/-1.26 nM and 7.01+/-0.07 nM, respectively), which may be a good lead compound for further structural modification to develop useful probes for VMAT2.

Protective actions of the vesicular monoamine transporter 2 (VMAT2) in monoaminergic neurons

Vesicular monoamine transporters (VMATs) are responsible for the packaging of neurotransmitters such as dopamine, serotonin, norepinephrine, and epinephrine into synaptic vesicles. These proteins evolved from precursors in the major facilitator superfamily of transporters and are among the members of the toxin extruding antiporter family. While the primary function of VMATs is to sequester neurotransmitters within vesicles, they can also translocate toxicants away from cytosolic sites of action. In the case of dopamine, this dual role of VMAT2 is combined-dopamine is more readily oxidized in the cytosol where it can cause oxidative stress so packaging into vesicles serves two purposes: neurotransmission and neuroprotection. Furthermore, the deleterious effects of exogenous toxicants on dopamine neurons, such as MPTP, can be attenuated by VMAT2 activity. The active metabolite of MPTP can be kept within vesicles and prevented from disrupting mitochondrial function thereby sparing the dopamine neuron. The highly addictive drug methamphetamine is also neurotoxic to dopamine neurons by using dopamine itself to destroy the axon terminals. Methamphetamine interferes with vesicular sequestration and increases the production of dopamine, escalating the amount in the cytosol and leading to oxidative damage of terminal components. Vesicular transport seems to resist this process by sequestering much of the excess dopamine, which is illustrated by the enhanced methamphetamine neurotoxicity in VMAT2-deficient mice. It is increasingly evident that VMAT2 provides neuroprotection from both endogenous and exogenous toxicants and that while VMAT2 has been adapted by eukaryotes for synaptic transmission, it is derived from phylogenetically ancient proteins that originally evolved for the purpose of cellular protection.

Specific derivatization of the vesicle monoamine transporter with novel carrier-free radioiodinated reserpine and tetrabenazine photoaffinity labels

Two iodophenylazide derivatives of reserpine and one iodophenylazide derivative of tetrabenazine have been synthesized and characterized as photoaffinity labels of the vesicle monoamine transporter (VMAT2). These compounds are 18-O-[3-(3'-iodo-4'-azidophenyl)-propionyl]methyl reserpate (AIPPMER), 18-O-[N-(3'-iodo-4'-azidophenethyl)glycyl]methyl reserpate (IAPEGlyMER), and 2-N-[(3'-iodo-4'-azidophenyl)-propionyl]tetrabenazine (TBZ-AIPP). Inhibition of [3H]dopamine uptake into purified chromaffin granule ghosts showed IC50 values of approximately 37 nM for reserpine, 83 nM for AIPPMER, 200 nM for IAPEGlyMER, and 2.1 microM for TBZ-AIPP. Carrier-free radioiodinated [125I]IAPEGlyMER and [125I]TBZ-AIPP were synthesized and used to photoaffinity label chromaffin granule membranes. Sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis showed specific [125I]IAPEGlyMER labeling of a polypeptide that migrated as a broad band (approximately 55-90 kDa), with the majority of the label located between 70 and 80 kDa. The labeling by [125I]IAPEGlyMER was blocked by 100 nM reserpine, 10 microM tetrabenazine, 1 mM serotonin, and 10 mM (-)-norepinephrine and dopamine. Analysis of [125I]TBZ-AIPP-labeled chromaffin granule membranes by SDS-PAGE and autoradiography demonstrated specific labeling of a similar polypeptide, which was blocked by 1 microM reserpine and 10 microM tetrabenazine. Incubation of [125I]TBZ-AIPP-photolabeled chromaffin granule membranes in the presence of the glycosidase N-glycanase shifted the apparent molecular weight of VMAT2 to approximately 51 kDa. These data indicate that [125I]IAPEGlyMER and [125I]TBZ-AIPP are effective photoaffinity labels for VMAT2.

Vesicular monoamine transporter 2: role as a novel target for drug development

In the central nervous system, vesicular monoamine transporter 2 (VMAT2) is the only transporter that moves cytoplasmic dopamine (DA) into synaptic vesicles for storage and subsequent exocytotic release. Pharmacologically enhancing DA sequestration by VMAT2, and thus preventing the oxidation of DA in the cytoplasm, may be a strategy for treating diseases such as Parkinson's disease. VMAT2 may also be a novel target for the development of treatments for psychostimulant abuse. This review summarizes the possible role of VMAT2 as a therapeutic target, VMAT2 ligands reported in the literature, and the structure-activity relationship of these ligands, including tetrabenazine analogs, ketanserin analogs, lobeline analogs, and 3-amine-2-phenylpropene analogs. The molecular structure of VMAT2 and its relevance to ligand binding are briefly discussed.

Regulation of rat brain vesicular monoamine transporter by chronic treatment with ovarian hormones

Ovarian steroids play an important role in neuroregulation and in the pathophysiology of various neuropsychiatric disorders. Most of the studies focused on the impact of gonadal steroids on post-synaptic receptors and plasma membrane transporters. In the present study, we evaluated the effect of chronic treatment with ovarian steroids on the expression of rat brain vesicular monoamine transporter (VMAT2). Ovariectomized rats were treated for 21 days with estradiol, progesterone or both. VMAT2 gene expression was assessed on the protein level by high affinity [3H]dihydrotetrabenazine ([3H]TBZOH) binding using autoradiography and on the mRNA level by in situ hybridization. Progesterone administration led to a decrease in [3H]TBZOH binding in the middle striatum and in the nucleus accumbens and to a parallel decrease in VMAT2 mRNA level in the substantia nigra pars compacta and dorsal raphè nuclei. Chronic estradiol treatment reduced VMAT2 mRNA level in the dorsal raphè and [3H]TBZOH binding in middle part of the striatum and nucleus accumbens but did not affect VMAT2 mRNA level in the substantia nigra pars compacta. Simultaneous administration of both ovarian steroids did not modulate VMAT2 mRNA in the substantia nigra pars compacta as well as [3H]TBZOH binding in the striatum or the nucleus accumbens but reduced VMAT2 mRNA level in the dorsal raphè. It appears that ovarian steroids may play a crucial role in the regulation of VMAT2 gene expression in the dopamine and serotonin systems. This modulatory activity may be relevant to synaptic and neuronal plasticity as well as to the molecular and cellular pathophysiology of gender-specific neuropsychiatric disorders.

High-efficiency expression and characterization of the synaptic-vesicle monoamine transporter from baculovirus-infected insect cells

The full-length cDNA for the rat synaptic-vesicle monoamine transporter (VMAT2) containing a C-terminal polyhistidine epitope has been engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) insect cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed at levels of 7.8x10(6) transporters per cell, as assessed by [3H]dihydrotetrabenazine binding. A 1l culture of infected cells produced approx. 15 nmol (900 microg) of transporter. rVMAT2 expressed in the Sf9 cells bound [3H]dihydrotetrabenazine with a KD of 31.2 nM and a Bmax of 19.9 pmol/mg. Two polypeptides of 55 and 63 kDa were identified using the photolabel, 7-azido-8-[125I]iodoketanserin ([125I]AZIK). Photoaffinity labelling of rVMAT2 by 1 nM [125I]AZIK was protectable by 10 microM tetrabenazine and 10 microM 7-aminoketanserin. Digitonin-solubilized VMAT2 was purified to greater than 95% homogeneity using immobilized Ni2+-affinity chromatography, followed by lectin (Concanavalin A) chromatography. The purified transporter migrates as a single broad band with a molecular mass of approx. 63kDa, as analyzed by SDS/PAGE. The purified transporter retained the ability to bind ligands ([125I]AZIK and [3H]dihydrotetrabenazine). The purified VMAT2 bound [3H]dihydrotetrabenazine with a KD of 86.2 nM. As is the case with the monoamine transporter from bovine chromaffin granule membranes, purified VMAT2 is covalently modified by dicyclohexylcarbodi-imide (DCCD) and is specifically labelled by [14C]DCCD. This labelling is inhibited by tetrabenazine and ketanserin. These data indicate that VMAT2 can be overexpressed using the baculovirus expression system and purified.

Peptide mapping of the [125I]Iodoazidoketanserin and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine binding sites for the synaptic vesicle monoamine transporter

The full-length cDNA for the rat recombinant synaptic vesicle monoamine transporter (rVMAT2) containing a COOH-terminal polyhistidine epitope was engineered into baculovirus DNA for expression in Spodoptera frugiperda (Sf9) cells. Using this recombinant baculovirus and cultured Sf9 cells, rVMAT2 has been expressed to high levels and purified to >95% homogeneity using immobilized Ni2+-affinity chromatography followed by lectin (concanavalin A) chromatography. Purified transporter was photolabeled using [125I]-7-azido-8-iodoketanserin ([125I]AZIK) and [125I]2-N-[(3'-iodo-4'-azidophenyl)propionyl]tetrabenazine ([125I]TBZ-AIPP). Both [125I]AZIK and [125I]TBZ-AIPP photoaffinity labeling of purified rVMAT2 were protectable by 10 microM tetrabenazine (TBZ), 10 microM 7-aminoketanserin, and 1 mM concentrations of the transporter substrates dopamine, norepinephrine, and serotonin. Radiolabeled peptides were generated using enzymatic and chemical methods, purified using sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and NH2-terminal microsequenced. Radiosequencing of [125I]AZIK-labeled rVMAT2 indicated derivatization of Lys-20 in the NH2 terminus, just prior to putative transmembrane domain 1 (TMD1). [125I]TBZ-AIPP derivatized a segment of rVMAT2 between Gly-408 and Cys-431 in TMD10 and 11. These data implicate juxtaposition of TMD1 and 10/11.

SDS-resistant aggregation of membrane proteins: application to the purification of the vesicular monoamine transporter

The vesicular monoamine transporter, which catalyses a H+/ monoamine antiport in monoaminergic vesicle membrane, is a very hydrophobic intrinsic membrane protein. After solubilization, this protein was found to have a high tendency to aggregate, as shown by SDS/PAGE, especially when samples were boiled in the classical Laemmli buffer before electrophoresis. This behavior was analysed in some detail. The aggregation was promoted by high temperatures, organic solvents and acidic pH, suggesting that it resulted from the unfolding of structure remaining in SDS. The aggregates were very stable and could be dissociated only by suspension in anhydrous trifluoroacetic acid. This SDS-resistant aggregation behaviour was shared by very few intrinsic proteins of the chromaffin granule membrane. Consequently, a purification procedure was based on this property. A detergent extract of chromaffin granule membranes enriched in monoamine transporter was heated and the aggregates were isolated by size-exclusion HPLC in SDS. The aggregates, containing the transporter, were dissociated in the presence of trifluoroacetic acid and analysed on the same HPLC column. This strategy might be of general interest for the purification of membrane proteins that exhibit SDS-resistant aggregation.

From synapse to vesicle: the reuptake and storage of biogenic amine neurotransmitters

Biogenic amine transport systems in the presynaptic plasma membrane and the synaptic vesicle provide a mechanism for rapidly terminating the action of released transmitters and for recycling neurotransmitters. Alterations in the activity of these transporters, either by endogenous regulatory mechanisms or by drugs, affect the regulation of synaptic transmitter levels. For drugs such as antidepressants and stimulants that interact with these transport systems, the therapeutic and behavioral consequences are profound. Now that the cDNAs encoding the transporters have been isolated, we can expect rapid progress in understanding how the individual proteins work at the molecular level to couple ion gradients to the reuptake and storage of biogenic amine neurotransmitters.

Reserpine causes differential changes in the mRNA levels of chromogranin B, secretogranin II, carboxypeptidase H, alpha-amidating monooxygenase, the vesicular amine transporter and of synaptin/synaptophysin in rat brain

Brains of rats treated with a high dose of reserpine were analyzed by in situ hybridization. The mRNA levels of several components of large dense core and small synaptic vesicles were determined. After drug treatment the secretogranin II message was elevated in the parvocellular neurons of the paraventricular nucleus, in the zona incerta, dorsal raphe, locus coeruleus and in the nucleus tractus solitarius. The levels of chromogranin B mRNA were increased in the dorsal raphe and in the substantia nigra compacta. In control animals messages for synaptin/synaptophysin could be found in most of the nuclei investigated, that of the vesicular amine transporter was only detectable in substantia nigra compacta, the dorsal raphe and the locus coeruleus whereas those of carboxypeptidase H and the alpha-amidating monooxygenase could only be determined in the paraventricular nucleus. All these messages were not changed after reserpine. We conclude that the chromogranin B/secretogranin II messages are regulated concomitantly with various neuropeptides. They represent useful general markers to identify stimulated neurons. Our results are consistent with the concept that stimulation of neurons leads to an increased synthesis of secretory peptides and consequently to large dense core vesicles filled with higher quanta of peptides.

Selective storage of acetylcholine, but not catecholamines, in neuroendocrine synaptic-like microvesicles of early endosomal origin

We have defined, in the neuroendocrine cell line PC12, the catecholamine- and acetylcholine-storing organelles in the context of the biogenesis of secretory granules and synaptic-like microvesicles (SLMVs). SLMVs were found to originate directly from early endosomes. Both early endosomes and SLMVs exhibited uptake and storage of biosynthetic acetylcholine. Surprisingly, however, despite the presence of a reserpine-sensitive vesicular amine transporter in early endosomes, SLMVs lacked detectable uptake and storage of catecholamines. This was confined to two populations of mature secretory granules, referred to as small and large mature secretory granules, which both derived from immature secretory granules. Our result show that PC12 cells lack small dense core vesicles, i.e., the catecholamine-storing, but secretory protein-lacking, vesicles found in sympathetic neurons and imply that the biogenesis of these vesicles requires the expression of a distinct type of vesicular amine transporter and/or a change in endosomal protein sorting.

Expression cloning of a reserpine-sensitive vesicular monoamine transporter

A cDNA for a rat vesicular monoamine transporter, designated MAT, was isolated by expression cloning in a mammalian cell line (CV-1). The cDNA sequence predicts a protein of 515 amino acids with 12 putative membrane-spanning domains. The characteristics of [3H]serotonin accumulation by CV-1 cells expressing the cDNA clone suggested sequestration by an intracellular compartment. In cells permeabilized with digitonin, uptake was ATP dependent with an apparent Km of 1.3 microM. Uptake was abolished by the proton-translocating ionophore carbonylcyanide p-trifluoromethoxyphenylhydrazone and with tri-(n-butyl)tin, an inhibitor of the vacuolar H(+)-ATPase. The rank order of potency to inhibit uptake was reserpine > tetrabenazine > serotonin > dopamine > norepinephrine > epinephrine. Direct comparison of [3H]monoamine uptake indicated that serotonin was the preferred substrate. Photolabeling of membranes prepared from CV-1 cells expressing MAT with 7-azido-8-[125I]iodoketanserin revealed a predominant tetrabenazine-sensitive photolabeled glycoprotein with an apparent molecular mass of approximately 75 kDa. The mRNA that encodes MAT was present specifically in monoamine-containing cells of the locus coeruleus, substantia nigra, and raphe nucleus of rat brain, each of which expresses a unique plasma membrane reuptake transporter. The MAT cDNA clone defines a vesicular monoamine transporter representing a distinct class of neurotransmitter transport molecules.

Characterization and purification of the monoamine transporter of bovine chromaffin granules

The monoamine transporter of the chromaffin granule membranes can be specifically labeled by the photoaffinity reagent 7-azido-8-[125I]iodoketanserin. The characteristics of the labeled protein have been investigated. Two-dimensional gel electrophoresis of the labeled membranes indicated a MW of about 70,000 and an isoelectric point ranging from 3.8 to 4.6. No clear protein spot was associated with the radioactive material, which migrated between glycoproteins GPII and GPIV. The diffuse aspect of the radioactive material indicated a heterogeneity, which was not modified after a second electrophoresis. This heterogeneity was, at least partially, due to glycosylation of the transporter; neuraminidase treatment increased the protein pI up to 6.3, whereas digestion with N-glycopeptidase markedly decreased the apparent MW, from 70,000 to 50,000. SDS-polyacrylamide gel electrophoresis showed that, at low acrylamide concentrations, the labeled material migrated more rapidly than predicted from the mobility of the markers of molecular weight, a behavior which indicated a marked hydrophobicity of the transporter. The labeled protein was purified to homogeneity by a combination of chromatography on DEAE-cellulose at pH 4.5, on immobilized wheat germ agglutinin, and on hydroxylapatite in the presence of SDS. During this purification, the specific radioactivity was increased by a factor of 300-500, with a yield of 10-20%.

Analysis of the interaction of reserpine with actin by the photoaffinity labelling method

The interaction of reserpine with one of the cytoskeletal proteins, actin, was analyzed by the photoaffinity labelling method using [3H]reserpine. Reserpine bound sufficiently to G- or oligomeric actin, but hardly to F-actin under the same experimental conditions. This result could be explained if reserpine binds to a specific region of the G-actin molecule that is responsible for actin-actin interactions. It is concordant with this idea that [3H]reserpine bound only to specific proteolytic fragments of actin. When reserpine was mixed with crude extracts of two kinds of tissues, chicken gizzard smooth muscle and bovine adrenal medulla, it bound to the 42 kDa protein of sodium dodecyl sulfate-polyacrylamide gel electrophoresis in both cases. Its molecular size and abundance suggest strongly that this 42 kDa protein is actin. The binding of reserpine to actin was inhibited by dopamine in a dose-dependent manner. These results suggest that actin could be one of the target molecules of reserpine.

Binding of [3H]dihydrotetrabenazine and [125I]azidoiodoketanserin photoaffinity labeling of the monoamine transporter of platelet 5-HT organelles

The carrier for 5-hydroxytryptamine (5-HT) of the 5-HT storage organelles of blood platelets was characterized by [3H]dihydrotetrabenazine binding and [125I]azidoiodokentanserin photoaffinity labeling. [3H]Dihydrotetrabenazine bound with high affinity to membrane preparations from different animal species. The [3H]dihydrotetrabenazine Bmax value was about 10-fold higher in rabbit (9.4 +/- 1.3 pmol/mg protein) than in human, rat and guinea-pig preparations (Bmax values = 1.1 +/- 0.2, 1.2 +/- 0.1 and 0.52 +/- 0.06 pmol/mg protein, respectively). After rabbit platelet subcellular fractionation, [3H]dihydrotetrabenazine binding was highly enriched in the fraction corresponding to pure 5-HT organelles, whereas ligand binding was much lower in the other subcellular fractions. Conversely, [3H]paroxetine binding sites were more concentrated in the lower density fractions, with no binding to the 5-HT granules. In competition experiments, [3H]dihydrotetrabenazine binding to human platelet membranes and rabbit platelet 5-HT organelles was markedly inhibited by the benzo[a]quinolizine derivatives, tetrabenazine and Ro 4-1284, and by ketanserin. In isolated rabbit platelet 5-HT organelles, reserpine showed a relatively high IC50 (930 nM), but the presence of ATP increased its potency about 10-fold. Paroxetine, methysergide and carrier substrates had little or no effect. After photoaffinity labeling of rabbit 5-HT granules with [125I]azidoiodoketanserin, the radioactivity was incorporated into several polypeptides. The presence of Ro 4-1284, reserpine and ketanserin prevented the labeling of a polypeptide of 85 kDa. The data obtained suggest that this protein represents a component of the granular carrier which binds [3H]dihydrotetrabenazine.

Subcellular distribution of acetylcholinesterase forms in chromaffin cells. Do chromaffin granules contain a specific secretory acetylcholinesterase?

The presence of acetylcholinesterase (AChE) in chromaffin granules has been controversial for a long time. We therefore undertook a study of AChE molecular forms in chromaffin cells and of their distribution during subcellular fractionation. We characterized four main AChE forms, three amphiphilic forms (Ga1, Ga2 and Ga4), and one non-amphiphilic form (Gna4). Each form shows the same molecular characteristics (sedimentation, electrophoretic migration, lectin interactions) in the different subcellular fractions. All forms are glycosylated and seem to possess both N-linked and O-linked carbohydrate chains. There are differences in the structure of the glycans carried by the different forms, as indicated by their interaction with some lectins. Glycophosphatidylinositol-specific phospholipases C converted the Ga2 form, but not the other amphiphilic forms, into non-amphiphilic derivatives. The distinct patterns of AChE molecular forms observed in various subcellular compartments indicate the existence of an active sorting process. Gna4 was concentrated in fractions of high density, containing chromaffin granules. We obtained evidence for the existence of a lighter fraction also containing chromogranin A, tetrabenazine-binding sites and Gna4 AChE, which may correspond to immature, incompletely loaded granules or to partially emptied granules. The distribution of Gna4 during subcellular fractionation suggested that this form is largely, but not exclusively, contained in chromaffin granules, the membranes of which may contain low levels of the three amphiphilic forms.

Human platelet dense granules: improved isolation and preliminary characterization of [3H]-serotonin uptake and tetrabenazine-displaceable [3H]-ketanserin binding

An improved method for the isolation of human platelet dense granules was developed. A good yield (45%) of highly enriched (69-fold, based on serotonin content) dense granules was obtained after mild sonication and Percoll gradient centrifugation. The method has facilitated characterization of the granule, permitting the first report of Km and Vmax values for [3H]-serotonin uptake, as well as the first determination of Kd and Bmax values for tetrabenazine-displaceable [3H]-ketanserin binding, in the human platelet dense granule. The rates and affinities (Vmax 1.45 nmol/mg/min, Km 0.93 uM) of [3H]-serotonin uptake were similar to those previously reported for porcine dense granules. Tetrabenazine-displaceable [3H]-ketanserin binding was observed with a Kd (9.4 nM) similar to, and a Bmax (5.4 pmol/mg) approximately 10-fold lower than, that previously seen in bovine chromaffin granules.

Quantitative autoradiography of the rat brain vesicular monoamine transporter using the binding of [3H]dihydrotetrabenazine and 7-amino-8-[125I]iodoketanserin

The binding of [3H]dihydrotetrabenazine, a specific ligand of the monoamine transporter present on serotonin and catecholamine synaptic vesicles, was studied on rat brain sections. The characteristics of binding (Kd = 5.0 nM, k1 = 0.13 x 10(6) M-1 s-1; k-1 = 0.66 x 10(-3) s-1) were similar to those previously observed on tissue homogenates. The rostrocaudal topographical distribution of dihydrotetrabenazine binding sites was analysed by quantitative autoradiography. High labelling was observed in regions richly innervated by monoaminergic systems: dopamine in the striatum and olfactory tubercles, noradrenaline in the striatal fissure and in the paraventricular and dorsomedial hypothalamus and serotonin in the lateral septum, islands of Calleja and suprachiasmatic nucleus. Cell bodies were also labelled in the substantia nigra and ventral tegmental area (dopamine), in locus coeruleus (noradrenaline) and in raphe nucleus (serotonin). The pituitary gland (particularly the neural lobe) and the pineal gland were also labelled. Low labelling was observed in various areas of the cerebral cortex and in the cerebellum. Unilateral 6-hydroxydopamine lesion of the substantia nigra dramatically reduced [3H]dihydrotetrabenazine labelling in the ipsilateral striatum. Moreover, ketanserin has recently been shown to possess a nanomolar affinity for the vesicular monoamine transporter, and autoradiographic localization of brain monoaminergic synaptic vesicles was also obtained by means of the derivative 7-amino-8-[125I]iodoketanserin in the presence of 5-hydroxytryptamine2 and alpha 1 antagonists, although the non-specific labelling was higher than with [3H]dihydrotetrabenazine. It is concluded that [3H]dihydrotetrabenazine may represent a valuable monoaminergic marker in in vitro autoradiographic studies.