Dual incorporation of photoaffinity ligands on dopamine transporters implicates proximity of labeled domains.

We have recently developed novel high-affinity blockers for the dopamine transporter (DAT) by carrying out structure-activity studies of GBR 12909 molecule piperidine analogs. To investigate the molecular basis of binding of these compounds in comparison to known sites of action of GBR 12909, cocaine, and benztropine analogs, we developed a piperidine-based photoaffinity label [(125)I]4-[2-(diphenylmethoxy)ethyl]-1-[(4-azido- 3-iodophenyl)methyl]-piperidine [(125)I]AD-96-129), and used proteolysis and epitope-specific immunoprecipitation to identify the protein domains that interact with the ligand. [(125)I]AD-96-129 became incorporated into two different regions of the DAT primary sequence, an N-terminal site containing transmembrane domains (TMs) 1 to 2, and a second site containing TMs 4 to 6. Both of these regions have been identified previously as sites involved in the binding of other DAT photoaffinity labels. However, in contrast to the previously characterized ligands that showed nearly complete specificity in their binding site incorporation, [(125)I]AD-96-129 became incorporated into both sites at comparable levels. These results suggest that the two domains may be in close three-dimensional proximity and contribute to binding of multiple uptake blockers. We also found that DATs labeled with [(125)I]AD-96-129 or other photoaffinity labels displayed distinctive sensitivities to proteolysis of a site in the second extracellular loop, with protease resistance related to the extent of ligand incorporation in the TM4 to 6 region. These differences in protease sensitivity may indicate the relative proximity of the ligands to the protease site or reflect antagonist-induced conformational changes in the loop related to transport inhibition.

Design, synthesis, and characterization of a novel, 4-[2-(diphenylmethoxy)ethyl]-1-benzyl piperidine-based, dopamine transporter photoaffinity label.

The dopamine transporter (DAT) has been implicated strongly in cocaine's reinforcing effects. Many derivatives of piperidine analogs of GBR 12909 have been developed and were found to be quite potent and selective for the DAT. In this regard, most of these derivatives were found to be much more selective for the DAT than conventional GBR compounds e.g. GBR 12909 when their selectivity was compared with the serotonin transporter (SERT). A brief structure-activity relationship (SAR) study has been carried out in the development of a novel photoaffinity ligand which illustrated the effect of the presence of a sterically bulky iodine atom next to the azido group in activity and selectivity for the DAT. This SAR study also led to the development of the compound 4 which is one of the most potent and selective blockers for the DAT known today. The photoaffinity ligand [125I]AD-96-129 was incorporated into the DAT molecule as was demonstrated by immunoprecipitation with serum 16 which is specific for DAT. This photolabeling was antagonized by DAT-specific blockers and was unaffected by specific SERT and norepinephrine transporter (NET) blockers indicating interaction of this novel ligand with the DAT.

Quantification of delta-opioid receptors in human brain with N1'-([11C]methyl) naltrindole and positron emission tomography.

The regional binding of N1'-([11C]methyl)naltrindole (MeNTI), a selective delta-opioid antagonist, was studied in healthy human subjects with positron emission tomography (PET). After the bolus intravenous administration of high specific activity [11C]MeNTI, PET was performed over 90 minutes. Arterial plasma samples were obtained during the scanning period and assayed for the presence of radiolabeled metabolites. The data were analyzed with various kinetic (two- and three-compartment models, Patlak graphical analysis) and nonkinetic (apparent volume of distribution and activity at a late scanning time) approaches. This tracer showed irreversible binding characteristics during the scanning period used. The results of the analyses also were compared with the density and distribution of delta-opioid receptors in the human brain in vitro. Additionally, computer simulations were performed to assess the effects of changes in receptor binding and tracer transport changes on the perceived binding parameters obtained with the models. A constrained three-compartment kinetic model was demonstrated to be superior to other quantification models for the description of MeNTI kinetics and quantification of delta receptor binding in the human brain with 11C-labeled MeNTI.

Differential binding of tropane-based photoaffinity ligands on the dopamine transporter.

Benztropine and its analogs are tropane ring-containing dopamine uptake inhibitors that produce behavioral effects markedly different from cocaine and other dopamine transporter blockers. We investigated the benztropine binding site on dopamine transporters by covalently attaching a benztropine-based photoaffinity ligand, [125I]N-[n-butyl-4-(4"'-azido-3"'-iodophenyl)]-4', 4"-difluoro-3alpha-(diphenylmethoxy)tropane ([125I]GA II 34), to the protein, followed by proteolytic and immunological peptide mapping. The maps were compared with those obtained for dopamine transporters photoaffinity labeled with a GBR 12935 analog, [125I]1-[2-(diphenylmethoxy)ethyl]-4-[2-(4-azido-3-iodophenyl)ethy l]p iperazine ([125I]DEEP), and a cocaine analog, [125I]3beta-(p-chlorophenyl)tropane-2beta-carboxylic acid, 4'-azido-3'-iodophenylethyl ester ([125I]RTI 82), which have been shown previously to interact with different regions of the primary sequence of the protein. [125I]GA II 34 became incorporated in a membrane-bound, 14 kDa fragment predicted to contain transmembrane domains 1 and 2. This is the same region of the protein that binds [125I]DEEP, whereas the binding site for [125I]RTI 82 occurs closer to the C terminal in a domain containing transmembrane helices 4-7. Thus, although benztropine and cocaine both contain tropane rings, their binding sites are distinct, suggesting that dopamine transport inhibition may occur by different mechanisms. These results support previously derived structure-activity relationships suggesting that benztropine and cocaine analogs bind to different domains on the dopamine transporter. These differing molecular interactions may lead to the distinctive behavioral profiles of these compounds in animal models of drug abuse and indicate promise for the development of benztropine-based molecules for cocaine substitution therapies.

Effects of extracellular acetylcholine on muscarinic receptor binding assessed by [125I]dexetimide and a simple probe.

New pharmacologic approaches to enhance brain cholinergic function focus on increasing intrasynaptic acetylcholine. We examined the usefulness of a simple probe and [125I]dexetimide to evaluate in vivo the effects of extracellular acetylcholine on muscarinic receptor binding in the mouse brain. After radiotracer injection continuous time/activity curves were generated over 330 min. [125I]Dexetimide reached a plateau at 90 min post-injection. To increase extracellular acetylcholine, the anticholinesterase physostigmine was administered at 120 min, producing a reversible decrease in [125I]dexetimide specific binding (23%) for 30 min. These findings demonstrate that dynamic changes in extracellular acetylcholine can be evaluated by displacement of [125I]dexetimide binding in vivo using a simple probe system.

Selective in vivo binding of [3H]naltriben to delta-opioid receptors in mouse brain.

Naltriben (NTB) is a selective antagonist for the putative delta2-opioid receptor. We have determined the regional kinetics and pharmacological profile of [3H]naltriben in vivo in mouse brain. After i.v. administration to CD1 mice, [3H]naltriben uptake and retention were high in striatum, cortical regions and olfactory tubercles, and low in superior colliculi and cerebellum. Robust rank order correlation was found between [3H]naltriben uptake in discrete brain regions and prior delta-opioid receptor binding determinations in vitro and in vivo. [3H]Naltriben binding in vivo was saturable, and was blocked by the delta-opioid receptor antagonist naltrindole, but not by the mu-opioid receptor antagonist cyprodime or the K-opioid receptor agonist (trans)-(+/-)-3,4-dichloro-N-methyl-N-[2-(1-pyrrolidinyl)-cyclohexyl]ben zeneacetamide mesylate (U50,488H). (E)-7-Benzylidenenaltrexone (BNTX), a selective antagonist for the putative delta1-opioid receptor, was 9.6- to 12.9-fold less potent than naltriben as an inhibitor of [3H]naltriben binding. Thus, the sites labeled by [3H]naltriben in vivo may correspond to the delta2-opioid receptor subtype. Such assignment is not definitive, particularly considering the 4-fold higher brain uptake of naltriben as compared to (E)-7-benzylidenenaltrexone. Moreover, the regional distribution of [3H]naltriben in brains from CXB-7/BY (CXBK) mice, a strain that shows supraspinal delta1- but not delta2-opioid receptor agonist effects, was quite similar to that found for CD1 mice.

Autoradiographic and SPECT imaging of cerebral opioid receptors with an iodine-123 labeled analogue of diprenorphine.

The feasibility of imaging cerebral opioid receptors by single photon emission computed tomography (SPECT) has been established in baboon using a novel analog of diprenorphine (DPN) radiolabeled with iodine-123. The radioligand, [123I]-O-IA-DPN (C6-O-[123I]iodoallyl-DPN), was prepared in good yield (80%) with high radiochemical purity (>97%) and high specific radioactivity (>2,400 mCi/micromol). In ex vivo autoradiographic studies, with and without naltrexone blockade, [123I]-O-IA-DPN specifically labeled opioid receptors throughout the mouse brain. Nonmetabolized radioligand accounted for >90% of the signal observed in extracts of whole mouse brain. SPECT imaging trials showed that [123I]-O-IA-DPN selectively localized in regions of baboon brain known to have high densities of opioid receptors, such as striatum, thalamus, and temporal cortex. A much lower level of radioligand uptake and retention was noted for cerebellum, a region with few opioid binding sites. Pretreatment with naltrexone (6.5 pmol/kg) blocked [123I]-O-IA-DPN binding in all brain regions. Using naltrexone blockade to define the nonspecific component for a given region of interest, total to nonspecific binding ratios increased linearly (r > or = 0.98) over the SPECT study with maximal values for striatum (9.8), thalamus (7.1), and temporal cortex (6.9) reached at the last time point investigated (3.5 h). Specific binding for these regions, assessed as the difference between regional SPECT activity for the control and blocked states, proved irreversible over the observation period. By the end of the time course, specific [123I]-O-IA-DPN binding was >85% of total radioactivity in regions rich in opioid receptors and 62% of total radioactivity in cerebellum. The aggregate data are consistent with visualization of multiple opioid receptor types. Thus, [123I]-O-IA-DPN should prove useful for SPECT studies within the constraints imposed by a lack of innate selectivity for a single type of brain opioid receptor.

N-substituted phenyltropanes as in vivo binding ligands for rapid imaging studies of the dopamine transporter.

Variously substituted phenyltropanes are proven as superb binding ligands for the dopamine transporter (DAT). In this study, we examine four N-substituted phenyltropanes which are derivatives of RTI-55 as in vivo binding ligands in mice. In this series, the methyl group on the nitrogen was replaced by a propyl (RTI-310), an allyl (RTI-311), a butyl (RTI-312), or a fluoropropyl (RTI-313) group. The in vitro binding potencies of these compounds at rat striatal DAT varied somewhat but were about 1 nM. While these compounds did not display marked selectivity for the dopamine transporter, they were more selective than RTI-55. Injection of the radiolabeled compound into mice resulted in striatal-to-cerebellar ratios that varied from about 4.5-6.5. The ratios peaked most rapidly for RTI-311 and RTI-313, at about 20 min. Pharmacological inhibition studies indicated that these compounds were binding to DATs in the striatum, as expected. These findings suggest that some compounds of this type may be excellent in vivo binding ligands for rapid imaging studies of the DAT.

Imaging of delta- and mu-opioid receptors in temporal lobe epilepsy by positron emission tomography.

The involvement of opioid neurotransmitter systems in seizure mechanisms is well documented. In previous positron emission tomography (PET) studies in patients with unilateral temporal lobe epilepsy, we have found evidence for differential regulation of the opioid-receptor subtypes. The present study extends our previous observations to delta-opioid receptors by using the delta-receptor-selective antagonist [11C]methylnaltrindole ([11C]MeNTI). Paired measurements of delta- and mu-opioid receptor binding and metabolic activity were performed with PET using [11C]MeNTI and [11C]carfentanil ([11C]CFN) and [18F]fluorodeoxyglucose ([18F]FDG), respectively. Binding of [11C]MeNTI and [11C]CFN increased and [18F]FDG uptake decreased in the temporal cortex (TC) ipsilateral to the focus. Decreases in [18F]FDG uptake were more widespread regionally than were increases in opioid receptors. Increases in the delta- and mu-receptor binding showed different regional patterns. Increases in mu-receptor binding were confined to the middle aspect of the inferior TC, whereas binding of delta receptors increased in the mid-inferior TC and anterior aspect of the middle and superior TC. The increase in delta receptors suggests their anticonvulsant action, as previously shown for the delta-receptor subtype, whereas the different regional pattern of receptor alterations suggest the distinct roles of different opioid-receptor subtypes in seizure phenomena.

Imaging of delta opioid receptors in human brain by N1'-([11C]methyl)naltrindole and PET.

Recently, we have developed the positron emitting radiotracer N1'-([11C]methyl)naltrindole ([11C]MeNTI) and demonstrated its high selectivity for delta opioid receptors in the mouse brain [Lever et al. (1992) Eur. J. Pharmacol., 216:449-450]. In the present study, we examined the selectivity of [11C]MeNTI for the delta opioid receptor in the human brain, using positron emission tomography (PET). The regional kinetics and distribution as well as the pharmacology confirmed the selectivity of [11C]MeNTI for delta opioid receptor in the human brain. First, the regional kinetics of [11C]MeNTI are in accordance with the density of the delta opioid receptor. Rapid washout in receptor-poor areas and prolonged retention in receptor-rich areas were observed. Second, the regional distribution of [11C]MeNTI correlated well (r = 0.91) with the in vitro distribution of delta opioid sites but not with mu or kappa site densities (r < or = 0.008 or r < or = 0.014, respectively). [11C]MeNTI binding was highest in regions of the neocortex (insular, parietal, frontal, cingulate, and occipital), caudate nucleus, and putamen. Binding was intermediate in the amygdala and lowest in the cerebellum and thalamus. Third, studies using the competitive antagonist naltrexone demonstrated the inhibition of [11C]MeNTI binding. Naltrexone inhibition of [11C]MeNTI binding was most effective in delta receptor-rich regions, and its inhibitory potency correlated well (r = 0.88) with the regional distribution of delta opioid sites. [11C]MeNTI is the first radioligand which selectively labels delta opioid receptors in vivo in the human brain following systemic administration. The availability of [11C]MeNTI will enable a receptor specific analysis of the role of [11C]MeNTI receptors in normal and abnormal human brain.

9-[(3-[18F]-fluoro-1-hydroxy-2-propoxy)methyl]guanine ([18F]-FHPG): a potential imaging agent of viral infection and gene therapy using PET.

A no-carrier-added synthesis of 9-[(3-[18F]-fluoro-1-hydroxy-2-propoxy)methyl]-guanine ([18F]-FHPG) is reported. The 9-[(1,3-dihydroxy-2-propoxy)methyl)guanine (DHPG) was converted to 9-[N2,O-bis(methoxytrityl)-3-(tosyl)-2-propoxy-methyl]guanine by treatment with methoxytrityl chloride followed by tosylation. The tosylate was reacted with [18F]-KF in the presence of kryptofix 2.2.2. to produce the 3-fluoro-N2-O-bis-(methoxytrityl) derivative. Removal of the methoxytrityl protecting groups by acid hydrolysis produced [18F]-FHPG. The labeled product was purified by HPLC on a reverse-phase C18 column, and eluted in 9 min with a mobile phase of 5% acetonitrile in water. The radiochemical yield was 7-17%, with an average of 10% in 10 runs (corrected for decay to EOB). The radiochemical purity was > 99%, and specific activities with an average of 526 mCi/mumol were obtained. The synthesis time was 70-80 min, including HPLC purification and determination of radiochemical purity and specific activity.

Synthesis and in vivo studies of a selective ligand for the dopamine transporter: 3 beta-(4-[125I]iodophenyl) tropan-2 beta-carboxylic acid isopropyl ester ([125I]RTI-121).

A selective ligand for the dopamine transporter 3 beta-(4-iodophenyl)tropan-2 beta-carboxylic acid isopropyl ester (RTI-121) has been labeled with iodine-125 by electrophilic radioiododestannylation. The [125I]RTI-121 was obtained in good yield (86 +/- 7%, n = 3) with high radiochemical purity (> 99%) and specific radioactivity (1210-1950 mCi/mumol). After i.v. administration of [125I]RTI-121 to mice, the rank order of regional brain tissue radioactivity (striatum > olfactory tubercles > > cortex, hippocampus, thalamus, hypothalamus, cerebellum) was consistent with dopamine transporter labeling. Specific in vivo binding in striatum and olfactory tubercles was saturable, and was blocked by the dopamine transporter ligands GBR 12,909 and (+/-)-nomifensine. By contrast, binding was not reduced by paroxetine, a serotonin transporter inhibitor, or desipramine, a norepinephrine transporter inhibitor. A variety of additional drugs having high affinities for recognition sites other than the neuronal dopamine transporter also had no effect. The [125I]RTI-121 binding in striatum and olfactory tubercles was inhibited by d-amphetamine in dose-dependent fashion. Nonmetabolized radioligand represents 85% of the signal observed in extracts of whole mouse brain. Thus, [125I]RTI-121 is readily prepared, and is a useful tracer for dopamine transporter studies in vivo.

Assessing neuroreceptor occupancy by continuous infusion of carbon-11 labeled radioligands.

In carbon-11 labeled radiotracer studies of neuroreceptors in the brain, a state of practical equilibrium, monitored with a simple dual-probe detector system, was achieved by injection of a bolus of tracer followed by a continuous infusion of the tracer. After investigating several different bolus-to-infusion activity ratios, we observed that a practical equilibrium state could be achieved with a slope of 0.02+/-0.001 cpsxkg/microCi/min for [11C]carfentanil studies in normal human beings. The usefulness of this approach to assess neuroreceptor occupancy was demonstrated in two cases.

In vivo labeling of delta opioid receptors in mouse brain by [3H]benzylidenenaltrexone, a ligand selective for the delta 1 subtype.

(E)-7-Benzylidenenaltrexone (BNTX) is a selective ligand for the putative delta 1 (delta 1) opioid receptor. To explore the feasibility of labeling delta 1 sites in vivo; we determined the cerebral distribution of radioactivity after systemic administration of [3H]BNTX to CD1 mice. Uptake was highest in striatum and lowest in cerebellum throughout the 4 hr time course. Specific radioligand binding, approximated as the difference in radioactivity concentrations between striatum and cerebellum, peaked at 0.32 percent injected dose/g at 30 min and comprised a modest 23% of total striatal radioactivity. For seven brain regions, radioactivity concentrations correlated with delta site densities known from prior in vitro studies (rS = 0.79, p = 0.03), and also with the uptake of N1'-([11C]methyl)naltrindole in vivo (rS = 0.78, p = 0.04) in mice. Specific binding in striatum, olfactory tubercles and cortical regions was saturable by BNTX, and was inhibited stereoselectively by the optical isomers of naloxone. Naltrindole and naltriben (NTB), delta antagonists, blocked 65-99% of [3H]BNTX specific binding at a dosage of 5.0 mumol/kg. Similar doses of the mu antagonist cyprodime, or the kappa agonist U50,488H, did not inhibit binding. Adjusted for the four-fold greater brain penetration of NTB relative to BNTX, dose-response studies suggested that delta 1 selective BNTX (ED50 = 1.51 mumol/kg) was 50% more potent than delta 2 selective NTB (ED50 = 0.56 mumol/kg) in blocking specific [3H]BNTX binding in striatum. In CXBK mice, a strain with functional delta 1 but not delta 2 receptors in antinociceptive assays, radioligand uptake and distribution proved similar to that in CD1 mice. In sum, [3H]BNTX labels murine delta opioid receptors in vivo with a low extent of specific binding. The data is consistent with, but not conclusive for, selective labeling of the delta 1 subtype.

Synthesis of radioiodinated naltrindole analogues: ligands for studies of delta opioid receptors.

Analogues of naltrindole and N1'-methylnaltrindole having radioiodine in the 7'-position of the indole ring have been prepared for evaluation as delta opioid receptor ligands. The no-carrier-added radiosyntheses were conducted by Cu(I) assisted nucleophilic exchange of radioiodide for bromide under reducing conditions at 190 degrees C. A combination of HPLC and solid-phase extraction gave the 125I- or 123I-labeled products in satisfactory yields (47%) with high radiochemical purities (> 98%) and high specific activities (125I: 43-68 GBq/mumol, 1155-1833 mCi/mumol; 123I: > 92 GBq/mumol, 2500 mCi/mumol).

Comparison of [11C]diprenorphine and [11C]carfentanil in vivo binding to opiate receptors in man using a dual detector system.

A simple dual detector coincidence system was used to measure the binding of [11C]carfentanil and [11C]diprenorphine to opiate receptors in normal volunteers before and after the administration of naloxone. Total radioactivity without naloxone and the ratio of total/non-specific radioactivity was 2 times greater for [11C]diprenorphine than [11C]carfentanil. The dose of naloxone required to maximally block specific [11C]diprenorphine binding was 10 times that for [11C]carfentanil, indicating that [11C]diprenorphine labels opiate receptor subtypes in addition to mu opiate receptors.

Assessment of brain muscarinic acetylcholinergic receptors in living mice using a simple probe, [125I]-4-iododexetimide and [125I]-4-iodolevetimide.

This study describes assessment of brain muscarinic acetylcholinergic receptors in living mice using a single-crystal radiation detection system, the high-affinity antagonist [125I]-4-iododexetimide, and the inactive enantiomer [125I]-4-iodolevetimide. Kinetics of radioligand binding, as well as perturbation by atropine displacement, can be determined using this simple probe technique.

Repeated administration of MDMA causes transient down-regulation of serotonin 5-HT2 receptors.

The present study examined short- and long-term effects of MDMA (3,4-methylene-dioxymethamphetamine) on serotonin (5-HT2 and 5-HT1c) receptors in the brain of the rat. N1-Methyl-2-[125I]lysergic acid diethylamide ([125I]MIL) was used to label these receptors in vitro and in vivo. The usefulness of [125I]MIL for in vivo detection of changes in 5-HT2 receptors was confirmed in preliminary experiments in which rats were treated chronically with mianserin (5 mg/kg, once daily for 10 days). Decreases in specific in vivo binding of [125I]MIL, after treatment with mianserin were found to be of the same magnitude as those determined by others, using in vitro methods. The MDMA (8 doses; 5-20 mg/kg each) was administered to rats over a period of 4 days. At various times after administration of the last dose of MDMA, the binding of [125I]MIL was measured. Acutely, treatment with MDMA (20 mg/kg) reduced specific in vivo binding of [125I]MIL in all regions of brain studied. For example, in the frontal cortex, specific binding of [125I]MIL was decreased by 80% at 6 hr and by 62% at 24 hr, after cessation of treatment with MDMA. Twenty-one days after administration of MDMA however, the number of binding sites for [125I]MIL was back to control levels. Reductions in in vivo binding of [125I]MIL in frontal cortex were dependent on the dose of MDMA injected and were associated with decreases in the number of binding sites for [125I]MIL (Bmax values) in tissue homogenates of the same area. Autoradiographic studies of MDMA-treated rats confirmed the decreased density of 5-HT2 receptors and also suggested that the 5-HT1c receptor of the choroid plexus was not affected. These results indicate that repeated administration of MDMA caused transient down-regulation of 5-HT2 receptors in the brain of the rat. Further, they demonstrated that [125I]MIL is a suitable radioligand for labeling 5-HT2 receptors, both in vitro and in vivo. Once labeled with an appropriate radionuclide for SPECT (single photon emission computed tomography) or PET (positron emission tomography), MIL should prove useful for monitoring changes in the density of serotonin receptors in the living mammalian brain.

Uptake of 3-hydroxykynurenine measured in rat brain slices and in a neuronal cell line.

The uptake of 3-hydroxykynurenine (3HK), a tryptophan metabolite with reported convulsant and cytotoxic properties, has been investigated in a neuronally derived hybrid cell line and in tissue slices prepared from rat brain. In both systems, the observed uptake was temperature-dependent and inhibited in the presence of large neutral amino acids. The apparent Km and Vmax determined for 3HK uptake into N18-RE-105 cells were 1.65 mM and 25.5 nmol/(min x mg protein), respectively. The uptake of 3HK into rat brain slices could be resolved into two components on the basis of their requirements for sodium. Kinetic analyses performed using hippocampal slices revealed a Km of 1.1 mM and Vmax of 18.8 nmol/(h x mg protein) for the sodium-independent process and a Km of 4.8 mM and Vmax of 54.5 nmol/(h x mg protein) for the sodium-dependent process. While sodium-dependent uptake was abolished following treatment with metabolic inhibitors, sodium-independent uptake was only slightly impaired. Sodium-independent uptake was inhibited in the presence of the non-metabolizable amino acids, aminoisobutyric acid (AIB) and aminobicyclo(2,2,1)heptane-2-carboxylic acid (BCH), but not by N-methylated amino acid substrates. Sodium-dependent uptake was insensitive to AIB and was completely abolished by BCH. These results indicate that an uptake process for 3HK is present in the mammalian brain, and suggest that the sodium-dependent component of 3HK transport may be mediated by a system which has not previously been described in CNS tissue.

In vivo binding of N1'-([11C]methyl)naltrindole to delta-opioid receptors in mouse brain.

The regional distribution of N1'-([11C]methyl)naltrindole ([11C]MeNTI) in vivo in mouse brain correlates with established delta opioid receptor densities in vitro. [11C]MeNTI binding is blocked by naltrindole, a delta antagonist, but not by cyprodime, a mu antagonist, of by (+/-)-U50,488, a kappa agonist. Thus, [11C]MeNTI selectively labels central delta opioid receptors in vivo in mouse, and shows promise for positron emission tomography studies of delta sites in human brain.