Chemical evidence that catecholamines are transported across the chromaffin granule membrane as noncationic species

Bioenergetics of neurotransmitter transport

Neurotransmitter transporters are essential components in the recycling of neurotransmitters released during neuronal activity. These transporters are the targets for important drugs affecting mood and behavior. They fall into at least four gene families, two encoding proteins in the plasma membrane and two in the synaptic vesicle membrane, although the known vesicular transporters have not all been cloned. Each of these transporters works by coupling the downhill movement of small ions such as Na+, Cl-, K+, and H+ to the uphill transport of neurotransmitter. Plasma membrane transporters move the transmitter into the cytoplasm by cotransport with Na+. Many transporters also couple Cl- cotransport to transmitter influx and these all belong to the NaCl-coupled family, although within the family the coupling stoichiometry can vary. Transporters for glutamate couple influx of this excitatory amino acid to Na+ and H+ influx and K+ efflux. Transporters in synaptic vesicles couple H+ efflux to neurotransmitter transport from the cytoplasm to the vesicle lumen.

Identification of residues involved in substrate recognition by a vesicular monoamine transporter

To identify the residues involved in substrate recognition by recently cloned vesicular monoamine transporters (VMAT1 and VMAT2), we have mutagenized the conserved residues in a cytoplasmic loop between transmembrane domains two and three of VMAT2. Although studies of related bacterial antibiotic resistance proteins indicate an important functional role for this region, we found no effect of these mutations on VMAT2 activity. However, replacement of aspartate 33 in the first predicted transmembrane domain with an asparagine (D33N) eliminates transport. D33N shows normal levels of expression and normal binding at equilibrium to the potent inhibitor reserpine. However, in contrast to wild-type VMAT2, serotonin inhibits reserpine binding to D33N very poorly, indicating a specific defect in substrate recognition. Replacement of three serine residues in transmembrane domain three with alanine (Stmd3A) shows a similarly selective but even more profound defect in substrate recognition. The results suggest that by analogy to receptors and plasma membrane transporters for monoamines, the cationic amino group of the ligand interacts with an asparte in the first transmembrane domain of VMAT2 and hydroxyl groups on the catechol or indole ring interact with a group of serines in the third transmembrane domain. Importantly, D33N and Stmd3A retain coupling to the proton electrochemical gradient as measured by the delta microH(+)-induced acceleration of reserpine binding. This indicates that substrate recognition can be separated from coupling to the driving force.

Uptake of norepinephrine and related catecholamines by cultured chromaffin cells: characterization of cocaine-sensitive and -insensitive plasma membrane transport sites

Norepinephrine and its closely related analogues, dopamine and epinephrine, are transported into chromaffin cells in culture by two distinct types of sites on the plasma membrane: one is sensitive to cocaine while the other is not. The cocaine-sensitive site has a high affinity for catecholamines and depends on sodium in the medium. The apparent Km for norepinephrine uptake by the cocaine-sensitive site is 5.8 microM when determined in the presence of 118 mM NaCl, obtained using nonlinear least-square curve fitting. Detailed kinetic analysis has also shown cocaine to be a competitive inhibitor of norepinephrine uptake with an apparent Ki of ca. 1 microM. This site is blocked by a series of tricyclic antidepressant drugs with relative potencies characteristic of norepinephrine transport sites in neurons. In contrast, the cocaine-insensitive site(s) have a low affinity for norepinephrine (apparent Km, approximately 88 microM) and are also able to transport catecholamine analogues such as dimethyl-epinephrine and isoproterenol, which have bulky groups attached to the amine moiety. Transport of norepinephrine at both sites is blocked by low temperature, by mitochondrial uncouplers, and by other metabolic inhibitors. Both of these transport sites in the chromaffin cell plasma membrane, therefore, appear to be different from the well-characterized catecholamine transport sites in the chromaffin granule membrane on the basis of substrate specificity and their sensitivity to inhibitors.

Mechanism of transport and storage of neurotransmitters

This review will focus on the bioenergetics, mechanism, and molecular basis of neurotransmitter transport. As indicated in the next section, these processes play an important role in the overall process of synaptic transmission. During the last few years, direct evidence has been obtained that these processes are coupled chemiosmotically, i.e., the accumulation of neurotransmitters is driven by ion gradients. Two types of neurotransmitter transport systems have been identified: sodium-coupled systems located in the synaptic plasma membrane of nerves (and sometimes in the plasma membrane of glial cells) and proton-coupled systems which are part of the membrane of intracellular storage organelles. From a bioenergetic point of view, the sodium-coupled systems are especially interesting, since it has recently been discovered that many systems require other ions in addition to sodium. It has now been demonstrated in several cases that, besides sodium ions, these additional ions, such as chloride and potassium, serve as additional coupling ions. These systems will be reviewed here in considerable detail with emphasis on the role of the additional ions. In the second part of the review we shall focus on neurotransmitter transport into storage organelles. Although both sodium and proton coupled systems have been reviewed in the past, there has been a shift from a kinetic and thermodynamic to a biochemical approach. In fact, a few transporters have been identified and functionally reconstituted. These developments have of course been incorporated in this review.