No abstract

Converging lines of evidence indicate that elevations in synaptic dopamine levels play a pivotal role in the reinforcing effects of cocaine, which are associated with its abuse liability. This evidence has led to the exploration of dopamine receptor blockers as pharmacotherapy for cocaine addiction. While neither D1 nor D2 receptor antagonists have proven effective, medications acting at two other potential targets, D3 and D4 receptors, have yet to be explored for this indication in the clinic. Buspirone, a 5-HT1A partial agonist approved for the treatment of anxiety, has been reported to also bind with high affinity to D3 and D4 receptors. In view of this biochemical profile, the present research was conducted to examine both the functional effects of buspirone on these receptors and, in non-human primates, its ability to modify the reinforcing effects of i.v. cocaine in a behaviourally selective manner. Radioligand binding studies confirmed that buspirone binds with high affinity to recombinant human D3 and D4 receptors (~98 and ~29 nM respectively). Live cell functional assays also revealed that buspirone, and its metabolites, function as antagonists at both D3 and D4 receptors. In behavioural studies, doses of buspirone that had inconsistent effects on food-maintained responding (0.1 or 0.3 mg/kg i.m.) produced a marked downward shift in the dose–effect function for cocaine-maintained behaviour, reflecting substantial decreases in self-administration of one or more unit doses of i.v. cocaine in each subject. These results support the further evaluation of buspirone as a candidate medication for the management of cocaine addiction.

Rapid treatment (1 min) of rat striatal synaptosomes with low-dose amphetamine increases surface expression of the dopamine transporter (DAT). Using mouse neuroblastoma N2A cells, stably transfected with green fluorescent protein-DAT, we demonstrate the realtime substrate-induced rapid trafficking of DAT to the plasma membrane using total internal reflection fluorescence microscopy (TIRFM). Both the physiological substrate, dopamine, and amphetamine began to increase surface DAT within 10 s of drug addition and steadily increased surface DAT until removal 2 min later. The substrate-induced rise in surface DAT was dose-dependent, was blocked by cocaine, and abated after drug removal. Although individual vesicle fusion was not visually detectable, exocytosis of DAT was blocked using both tetanus neurotoxin and botulinum neurotoxin C to cleave soluble N-ethylmaleimide-sensitive factor attachment protein receptor (SNARE) proteins. Notably, the dopamine-induced increase in surface DAT was cocaine-sensitive but D 2 -receptor independent. TIRFM data were confirmed in human DAT-N2A cells using biotinylation, and similar effects were detected in rat striatal synaptosomes. A specific inhibitor of protein kinase C-␤ blocked the substrate-mediated increase in surface DAT in both DAT-N2A cells and rat striatal synaptosomes. These data demonstrate that the physiological substrate, dopamine, and amphetamine rapidly increase the trafficking of DAT to the surface by a mechanism dependent on SNARE proteins and protein kinase C-␤ but independent of dopamine D 2 receptor activation. Importantly, this study suggests that the reuptake system is poised to rapidly increase its function during dopamine secretion to tightly regulate dopaminergic neurotransmission.

N-(3-fluoro-4-(4-(2,3-dichloro- or 2-methoxyphenyl)piperazine-1-yl)-butyl)-aryl carboxamides were prepared and evaluated for binding and function at dopamine D3 (D3R) and D2 receptors (D2R). In this series, we discovered some of the most D3R selective compounds reported to date, (e.g. 8d and 8j >1000-fold D3R-selective over D2R.) In addition, chimeric receptor studies further identified the second extracellular (E2) loop as an important contributor to D3R binding selectivity. Further, compounds lacking the carbonyl group in the amide linker were synthesized and while these amine-linked analogues bound with similar affinities to the amides at D2R, this modification dramatically reduced binding affinities at D3R by >100-fold (e.g. D3RKi for 15b = 393 v. for 8j = 2.6 nM) resulting in compounds with significantly reduced D3R selectivity. This study supports a pivotal role for the D3R E2 loop and the carbonyl group in the 4-phenylpiperazine class of compounds and further reveals a point of separation between structure-activity relationships at D3R and D2R.

The dopamine transporter (DAT) is a key mediator of dopaminergic neurotransmission and a major target for amphetamine. We found previously that protein kinase C (PKC) ␤ regulates amphetamine-mediated dopamine efflux. Here, using PKC␤ wild-type (WT) and knockout (KO) mice, we report a novel role for PKC␤ in amphetamine-induced regulation of DAT trafficking and activity. PKC␤ KO mice have less striatal surface DAT, [ 3 H]dopamine uptake, and amphetamine-stimulated dopamine efflux, yet higher novelty-induced locomotor activity than WT mice. Although a short exposure (Յ90 s) to amphetamine rapidly increases striatal surface DAT and [3 H]dopamine uptake in WT mice, this treatment decreases surface DAT and [3 H]dopamine uptake in KO mice. Increases in surface DAT and [ 3 H]dopamine uptake are not evident in KO mice until a longer exposure (60 min) to amphetamine, by which time WT mice exhibit decreased surface DAT and dopamine uptake. The slowness of amphetamine-induced striatal DAT trafficking in PKC␤ KO mice was mimicked by the use of a specific PKC␤ inhibitor, LY379196, in WT mice. Furthermore, PKC␤ KO mice exhibit reduced locomotor responsiveness to amphetamine compared with WT, which could be explained by reduced surface DAT and delayed amphetamine-induced DAT trafficking in KO mice. Our results indicate that PKC␤ is crucial for proper trafficking of DAT to the surface and for functioning of DAT and amphetamine signaling, providing new insight into the role of PKC␤ as an important regulator of dopaminergic homeostasis.The dopamine transporter (DAT) is an integral membrane protein containing 12 putative transmembrane domains and cytosolic amino and carboxyl termini. It belongs to a superfamily of Na ϩ /Cl Ϫ -dependent neurotransmitter transporters. Dopaminergic transmission in the synapse is primarily terminated by removal of dopamine (DA) from presynaptic nerve terminals via DAT. DAT is also a primary target of psychostimulants such as amphetamine (AMPH). As a substrate, AMPH elicits reverse transport of DA through DAT, a mechanism underlying AMPH addiction. Dysfunctional DAT signaling is also implicated in various other neurodegenerative and psychiatric disorders such as Parkinson's disease, attention deficit/hyperactive disorder, and schizophrenia.DAT trafficking and activity are regulated by a complicated protein network involving multiple protein kinases and DAT-interacting proteins (Torres, 2006), among which protein kinase C (PKC) is the most characterized. Longer term activation of PKC (Ͼ20 min) by phorbol 12-myristate 13-acetate, a general PKC activator, leads to increased DAT endocytosis and reduced DAT recycling, resulting in reduced surface DAT expression and activity in rat synaptosomes and cultured cells (Daniels and Amara, 1999;Melikian and Buckley, 1999;Loder and Melikian, 2003). In accordance, PKC inhibitors block phorbol 12-myristate 13-acetate-induced DAT internalization (Daniels and Amara, 1999;Melikian and Buckley, 1999;Chang et al., 2001). An endocytic motif in the C terminus of DAT t...

The dopamine transporter (DAT) is a primary determinant of the concentration of dopamine in the synapse and is involved in a number of psychiatric and neurological diseases. The transporter actively takes up its physiological substrate, dopamine, when it is on the surface of the plasmalemmal membrane, but the concentration of DAT in the membrane is highly regulated by substrate. Substrates initially, and very rapidly, recruit more DAT into the membrane for greater function, but continued presence of substrate downregulates the activity of DAT and even membrane DAT content. This biphasic regulation is orchestrated by numerous signal transduction mechanisms, including a palette of protein kinases. Understanding the mechanisms of rapid regulation of DAT could provide new therapeutic strategies to improve transporter function and modulate responses to its more notorious substrates, amphetamine and methamphetamine. Keywords amphetamine; endocytosis; PKCβ; recycling; trafficking Dopamine (DA), a monoaminergic neurotransmitter, is involved in the control of movement, reward, motivation, emotion, learning, and pituitary and hypothalamic function. The DA transporter (DAT), which removes DA from the synapse through reuptake, is crucial for the regulation of dopaminergic signaling and homeostasis in many areas of the brain. DAT (SLC6A3) belongs to the SLC6 Na + /Cl − -dependent transporter family, which also consists of serotonin transporter (SERT) and norepinephrine transporter (NET). DAT tightly controls the duration and strength of dopaminergic neurotransmission at the synapse by taking up extraneuronal DA into presynaptic terminals, thus terminating DA action at pre-and postsynaptic sites [1]. Dysfunctional dopaminergic signaling related to DAT has been implicated NIH Public Access Author ManuscriptFuture Neurol. Author manuscript; available in PMC 2010 November 1. Published in final edited form as:Future Neurol. 2010 January 1; 5(1): 123. doi:10.2217/fnl.09.76. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscript in multiple neurodegenerative and psychiatric disorders, including Parkinson's disease, schizophrenia, bipolar disorder, attention-deficit/hyperactivity disorder and drug addiction [2][3][4]. Recently, the molecular basis of infantile parkinsonism-dystonia was traced to a homozygous loss-of-function mutation in DAT [5]. Furthermore, the expression density of DAT strongly correlates with the extent of DA cell loss in Parkinson's disease [6]. Coding variants of DAT have been identified in some patients with attention-deficit/hyperactivity disorder and bipolar disorder [7]. DAT is a target of abused psychostimulants, such as methamphetamine and cocaine, and therapeutic drugs, such as amphetamine (AMPH; Dexedrine and Adderall) and methylphenidate (Ritalin), as well as the antidepressant and antismoking drug, bupropion (Wellbutrin and Zyban).The primary function of DAT is to remove DA from the synaptic cleft; the newly taken-up DA then re-enters the synaptic vesicle through the vesicu...

ABSTRACT-ol]enkephalin (DAMGO) was completely blocked by overnight treatment with 100 ng/ml PTX. Treatment for 4 h with lower concentrations led to a PTX-dependent reduction in the maximal effect of DAMGO but no alteration in the potency of DAMGO or morphine nor in the relative maximal effect (relative efficacy) of the partial agonists morphine and buprenorphine compared with the full agonist DAMGO. Using PTX-insensitive G␣ mutants in which the PTXsensitive cysteine was replaced with isoleucine, the potency for a series of -opioid agonists was highest in cells expressing G␣ i3 and G␣ o and lowest with G␣ i1 and G␣ i2 , with no significant change in the order of potency, namely, etorphine Ͼ Ͼ endomorphin-1 ϭ DAMGO ϭ endomorphin-2 ϭ fentanyl ϭ morphine Ͼ Ͼ meperidine. The order of agonist relative efficacy, etorphine ϭ DAMGO ϭ endomorphin-1 ϭ endomorphin-2 ϭ fentanyl Ն morphine Ն meperidine Ͼ buprenorphine Ն nalbuphine, was also the same across all of the PTX-insensitive G␣ i/o subtypes. Highest relative efficacy to stimulate [ 35 S]GTP␥S binding was seen with G␣ i3 . Consequently, reported observations of agonist-directed trafficking at -opioid receptors most likely involve non-PTX-sensitive G␣ protein mechanisms.

The D3 dopamine receptor represents an important target in drug addiction in that reducing receptor activity may attenuate the self-administration of drugs and/or disrupt drug or cue-induced relapse. Medicinal chemistry efforts have led to the development of D3 preferring antagonists and partial agonists that are >100-fold selective vs. the closely related D2 receptor, as best exemplified by extended-length 4-phenylpiperazine derivatives. Based on the D3 receptor crystal structure, these molecules are known to dock to two sites on the receptor where the 4-phenylpiperazine moiety binds to the orthosteric site and an extended aryl amide moiety docks to a secondary binding pocket. The bivalent nature of the receptor binding of these compounds is believed to contribute to their D3 selectivity. In this study, we examined if such compounds might also be “bitopic” such that their aryl amide moieties act as allosteric modulators to further enhance the affinities of the full-length molecules for the receptor. First, we deconstructed several extended-length D3-selective ligands into fragments, termed “synthons”, representing either orthosteric or secondary aryl amide pharmacophores and investigated their effects on D3 receptor binding and function. The orthosteric synthons were found to inhibit radioligand binding and to antagonize dopamine activation of the D3 receptor, albeit with lower affinities than the full-length compounds. Notably, the aryl amide-based synthons had no effect on the affinities or potencies of the orthosteric synthons, nor did they have any effect on receptor activation by dopamine. Additionally, pharmacological investigation of the full-length D3-selective antagonists revealed that these compounds interacted with the D3 receptor in a purely competitive manner. Our data further support that the 4-phenylpiperazine D3-selective antagonists are bivalent and that their enhanced affinity for the D3 receptor is due to binding at both the orthosteric site as well as a secondary binding pocket. Importantly, however, their interactions at the secondary site do not allosterically modulate their binding to the orthosteric site.