Cocaine binding site

Madras, B.K., Fahey, M.A., Bergman, J., Canfield, D.R., and Spealman, R.D., Effects of cocaine and related drugs in nonhuman primates. I. [3H]Cocaine binding sites in caudate-putamen, J. Pharmacol. Exp. Ther., 251, 131, 1989. 

Boja, J.W., Markham, L., Patel, A., Uhl, G., and Kuhar, M.J., Expression of a single dopamine transporter cDNA can confer two cocaine binding sites, Neuroreport, 3, 247, 1992. 

Fig. 1. Scheme for RNA aptamer identification targeting cocaine-binding sites on the nicotinic acetylcholine receptor (nAchR). 

Fowler, Joanna S., Nora D. Volkow, Alfred P. Wolf, Stephen L. Dewey, David J. Schlyer, Robert R. Macgregor, Robert Hitzemann, Jean Logan, Bernard Ben-driem, S. John Gatley, and David Christman. 1989. "Mapping Cocaine Binding Sites in Human and Baboon Brain in Vivo." Synapse 4 371-77. 

Figure 1.1 The dopamine transporter terminates the action of released dopamine by transport back into the presynaptic neuron. Dopamine transport occurs with the binding of one molecule of dopamine, one chloride ion, and two sodium ions to the transporter the transporter then translocates from the outside of the neuronal membrane into the inside of the neuron.22 Cocaine appears to bind to the sodium ion binding site. This changes the conformation of the chloride ion binding site thus dopamine transport does not occur. This blockade of dopamine transport potentiates dopaminergic neurotransmission and may be the basis for the rewarding effects of cocaine.

Kitayama, S., Shimada, S., Xu, H., Markham, L., Donovan, D.M., and Uhl, G.R., Dopamine transporter site-directed mutations differentially alter substrate transport and cocaine binding, Proc. Natl. Acad. Sci. U.S.A., 89, 7782, 1992. 

Kennedy, L.T. and Hanbauer, I., Sodium-sensitive cocaine binding to rat striatal membrane possible relationship to dopamine uptake sites, J. Neurochem., 41, 172, 1983. 

Schoemaker, H., Pimoule, C., Arbilla, S., Scatton, B., Javoy-Agid, R, and Langer, S.Z., Sodium dependent pH]cocaine binding associated with dopamine uptake sites in the rat striatum and human putamen decrease after dopamine denervation and in Parkinson s disease, Naunyn-Schmiedeberg s Arch. Pharmacol., 329, 227, 1985. 

Letchworth S.R., Nader M.A., Smith H.R., Friedman D.P., Porrino LJ. Progression of changes in dopamine transporter binding site density as a result of cocaine self-administration in rhesus monkeys. J. Neurosci. 21 2799, 2001. 

Little K., McLaughlin D., Zhang L. et al. Brain dopamine transporter messenger RNA and binding sites in cocaine users. Arch. Gen. Psychiatry. 55 793, 1998. 

Little K., Kirkman J., Carroll F., Clark T., Duncan G. Cocaine use increases [3H]WIN 35, 428 binding sites in human striatum. Brain Res. 628 17, 1993. 

Many neurotransmitters are inactivated by a combination of enzymic and non-enzymic methods. The monoamines - dopamine, noradrenaline and serotonin (5-HT) - are actively transported back from the synaptic cleft into the cytoplasm of the presynaptic neuron. This process utilises specialised proteins called transporters, or carriers. The monoamine binds to the transporter and is then carried across the plasma membrane it is thus transported back into the cellular cytoplasm. A number of psychotropic drugs selectively or non-selectively inhibit this reuptake process. They compete with the monoamines for the available binding sites on the transporter, so slowing the removal of the neurotransmitter from the synaptic cleft. The overall result is prolonged stimulation of the receptor. The tricyclic antidepressant imipramine inhibits the transport of both noradrenaline and 5-HT. While the selective noradrenaline reuptake inhibitor reboxetine and the selective serotonin reuptake inhibitor fluoxetine block the noradrenaline transporter (NAT) and serotonin transporter (SERT), respectively. Cocaine non-selectively blocks both the NAT and dopamine transporter (DAT) whereas the smoking cessation facilitator and antidepressant bupropion is a more selective DAT inhibitor. 

Fig. 3. Evidence for an endogenous Zn2+-binding site in the DAT but not in the NET. (A) Zn2+ inhibition of [3H]dopamine uptake in COS-7 cells transiently expressing hDAT (filled circles) or hNET (open circles). (B) Effect of Zn2+ on binding of the cocaine analog [3H]WIN 35,428 to hDAT (filled circles) and hNET (open circles). Values are percent of control ([3H]dopamine uptake or [3H] WIN 35,428 binding in the absence of Zn2+) expressed as means S.E. of 3-5 experiments performed in triplicate.

Interestingly, we have recently identified a mutation of a tyrosine in the third intracellular loop of the hDAT that causes a major alteration in the conformational equilibrium of the transport cycle, and thus as such is comparable to mutants on G protein-coupled receptors causing constitutive isomerization of the receptor to the active state (66). Most importantly, this conclusion is based on the observation that mutation of the tyrosine completely reverts the effect of Zn2+ at the endogenous Zn2+ binding site in the hDAT (50,51) from potent inhibition of transport to potent stimulation of transport (Fig. 6). In the absence of Zn2+, transport capacity is reduced to less than 1% of that observed for the wild-type, however, the presence of Zn2+ in only micromolar concentrations causes a close to 30-fold increase in uptake (66). Moreover, it is found that the apparent affinities for cocaine and several other inhibitors are substantially decreased, whereas the apparent affinities for substrates are markedly increased (66). Notably, the decrease in apparent cocaine affinity was around 150-fold and thus to date the most dramatic alteration in cocaine affinity reported upon mutation of a single residue in the monoamine transporters (66). 

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