Impaired activity of the extraneuronal monoamine transporter system known as uptake-2 in Orct3/Slc22a3-deficient mice

Two uptake systems that control the extracellular concentrations of release d monoamine neurotransmitters such as noradrenaline and adrenaline have bee n described. Uptake-1 is present at presynaptic nerve endings, whereas upta ke-2 is extraneuronal and has been identified in myocardium and vascular an d nonvascular smooth muscle cells. The gene encoding the uptake-2 transport er has recently been identified in humans (EMT), rats (OCT3), and mice (Orc t3/Slc22a3). To generate an in vivo model for uptake-2, we have inactivated the mouse Orct3 gene. Homozygous mutant mice are viable and fertile with n o obvious physiological defect nd also show no significant imbalance of nor adrenaline or dopamine. However, Orct3-null mice show an impaired uptake-2 activity as measured by accumulation of intravenously administered [H-3]MPP + (1-methyl-4-phenylpyridinium). A 72% reduction in MPP+ levels was measure d in hearts of both male and female Orct3 mutant mice. No significant diffe rences between wild-type and mutant mice were found in any other adult orga n or in plasma. When [H-3]MPP+ was injected into pregnant females, a threef old-reduced MPP+ accumulation was observed in homozygous mutant embryos but not in their placentas or amniotic fluid. These data show that Orct3 is th e principal component for uptake-2 function in the adult heart and identify the placenta as a novel site of action of uptake-2 that acts at the fetopl acental interface.