Kinetic analysis of developmental changes in vesicular monoamine transporter-2 function

The vesicular monoamine transporter-2 (VMAT-2) sequesters cytoplasmic dopamine (DA) into vesicles and may thus protect neurons from DA-associated oxidative damage. VMAT-2 function, as assessed by measuring [H-3]DA uptake, is less in adolescent when compared with young adult rats and VMAT-2 susceptibility to methamphetamine also changes with age. Thus, developmental alterations in VMAT-2 function warrant further investigation. The current study extends these findings by showing that the initial velocities of both DA uptake and methamphetamine-induced DA efflux are less in adolescent postnatal day (PND 38-42) vs. young adult (PND 8892) rats as assessed in nonmembrane associated (presumably cytoplasmic) vesicles purified from rat striatal synaptosomes. The decrease in DA uptake velocities is due to a decrease in the V-max of DA uptake with no change in the Km. The density of kinetically active VMAT-2 and VMAT-2 immunoreactivity are less in adolescent vs. young adult rats while both the turnover number (2.4-2.8 s(-1)) and rate constant for the association of DA with VMAT-2 (similar to 1 x 10(7) M-1 s(-1)) are similar in these age groups. These results suggest that the kinetics of DA binding and translocation across the membrane are unaltered in the vesicles of PND 38-42 vs. PND 88-92 rats. However, decreased VMAT-2 density in PND 38-42 rats reduces V-max, which in turn lowers DA uptake.