A multisubstrate kinetic mechanism of dopamine transport in the nucleus accumbens and its inhibition by cocaine.

Kinetic studies of dopamine transport into suspensions of nucleus accumbens (NAcc) and effects of Na+ and Cl- as cosubstrates were performed using rotating disk electrode voltammetry. To mimic chemical neurotransmission, dopamine was added as a rapid pulse, and transporter-mediated clearance of dopamine was evaluated kinetically. This paradigm was shown to approximate a zero trans entry transport experiment. Dopamine was taken up with apparent Km and Vmax values of 1.3 microM and 375 pmol/s/g wet weight, respectively. Transport exhibited apparent trans acceleration. Substitution of Na+ with choline or Cl- with isethionate reduced dopamine transport with reaction orders of two and unity, respectively, accompanied by reductions in Vmax with no changes in Km. Apparent K(Na) and K(Cl) values were 70.0 and 92.1 mM, respectively. Dopamine transport in NAcc was found to follow a partially random, sequential mechanism in which dopamine and Na+ bind randomly to the transporter followed by binding of Cl- before transport. Cocaine inhibited dopamine transport and the influences of the other substrates allosterically with an overall Ki of 0.30 microM. Thus, the general kinetic mechanism of the transport of dopamine in the NAcc is identical to that previously reported by this laboratory for dopamine transport in the striatum. However, the dopamine transporter in the NAcc is more tightly regulated by Na+, possesses a higher kinetic turnover rate, is four times more sensitive to cocaine than the striatal transporter, and exhibits cocaine inhibition independent of [substrate]. These findings suggest that cocaine modulates chemical signaling in NAcc differently than in striatum, providing down-regulation of function irrespective of [substrate], thereby enhancing dopaminergic signaling more robustly in the NAcc than in the striatum.

Monomeric ferric heme peptide derivatives: model systems for hemoproteins.

Electron paramagnetic resonance spectra of a number of ferric heme peptide derivatives, in aqueous-detergent and various aqueous-alcohol solvent mixtures, have been obtained using samples in the concentration range 0.1-1.0 mM. Some of these were clearly monomeric, homogeneous, mixed-ligand adducts, entirely suitable for use as model systems for hemoprotein spectroscopic studies. As anticipated, the measured EPR parameters were largely independent of solvent environment. Surprisingly, micellar preparations of ferric heme undecapeptide in mildly alkaline solution showed no evidence for the formation of a hydroxide adduct, contrary to a previous report [S. Mazumdar, O. K. Medhi and S. Mitra, Inorg. Chem. 30 700 (1991)].

Individual locomotor response to novelty predicts selective alterations in D1 and D2 receptors and mRNAs.

Rats that have a greater locomotor response to novelty (high responders, HR) have differences in measures of presynaptic dopamine transmission compared to low responders (LR) to a novel environment, including altered dopamine release and behavioral response to indirect dopamine agonists. This study examined the role of three dopamine terminal fields, the nucleus accumbens, striatum, and medial prefrontal cortex, in differences between HR and LR. In the first experiment, dopamine was infused directly into the nucleus accumbens (0, 3, 10, and 30 micrograms/side) or the striatum (0, 10, 30, and 100 micrograms/side). HR showed a greater behavioral response to both the 3 and 30 micrograms/side doses infused into the nucleus accumbens compared to LR. No differences between HR and LR were revealed by dopamine infusion into the striatum. In the second experiment, radioligand binding assays were performed to determine if differences exist between high and low responder rats in the Bmax and/or KD of radiolabeled antagonist ligands for the dopamine D1 and/or D2 receptors. There were fewer D2 binding sites in the nucleus accumbens and fewer sites in the striatum in HR compared to LR. High responders showed a greater Bmax for D1 binding sites in the nucleus accumbens than LR. No differences in number of binding sites for D1 receptors were observed between HR and LR in the striatum. No differences between HR and LR in D2 or D1 receptor binding were observed in the medial prefrontal cortex. There were no differences in KD for any of the dopamine receptors in the regions examined.(ABSTRACT TRUNCATED AT 250 WORDS)