The reverse transport of DA, what physiological significance?

It is well established that midbrain dopamine neurons innervating the striatum, release their neurotransmitter through an exocytotic process triggered by the neural firing and involving a transient calcium entry in the terminals. Long ago, it had been proposed, however, that another mechanism of release could co-exist with classical exocytosis, involving the reverse-transport of the cytosolic amine by the carrier, ordinarily responsible for uptake function. This atypical mode of release could be evoked directly at the preterminal level by multiple environmental endogenous factors involving transient alterations of the sodium gradient. It cannot be excluded that this mode of release participates in the firing-induced release. In contrast with the classical exocytosis of a preformed DA pool, the reverse-transport of DA requires simultaneous alterations of intraterminal amine metabolism including synthesis and displacement from storage compartment. The concept of a reverse-transport of dopamine is coming from the observations that releasing substances, such as amphetamine-related molecules, actually induce this type of transport. A large set of arguments advocates that reverse-transport plays a role in the maintenance of basal extracellular DA concentration in striatum. It was also often evoked in physiopathological situations including ischemia, neurodegenerative processes, etc. The most recent studies suggest that this release could occur mainly outside the synapses, and thus could constitute a major feature in the paracrine transmission, sometimes evoked for DA.