Oligodendrocytes, myelin-forming glial cells of the central nervous system, are vulnerable to damage in a variety of neurologic diseases. Much is known of primary myelin injury, which occurs in settings of genetic dysmyelination or demyelinating disease. There is growing awareness that oligodendrocytes are also targets of injury in acute ischemia. Recognition of oligodendrocyte damage in animal models of ischemia requires attention to their distinct histologic features or use of specific immunocytochemical markers. Like neurons, oligodendrocytes are highly sensitive to injury by oxidative stress, excitatory amino acids, trophic factor deprivation, and activation of apoptotic pathways. Understanding mechanisms of oligodendrocyte death may suggest new therapeutic strategies to preserve or restore white matter function and structure after ischemic insults.

Summary:Oligodendrocytes, myelin-forming glial cells of the central nervous system, are vulnerable to damage in a variety of neurologic diseases. Much is known of primary myelin injury, which occurs in settings of genetic dysmyelination or demyelinating disease. There is growing awareness that oligodendrocytes are also targets of injury in acute ischemia. Recognition of oligodendrocyte damage in animal models of ischemia requires attention to their distinct histologic features or use of specific immunocytochemical markers. Like neurons, oligodendrocytes are highly sensitive to injury by oxidative stress, excitatory amino acids, trophic factor deprivation, and activation of apoptotic pathways. Understanding mechanisms of oligodendrocyte death may suggest new therapeutic strategies to preserve or restore white matter function and structure after ischemic insults.

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Summary Amphetamines modify the brain and alter behavior through mechanisms generally attributed to their ability to regulate extracellular dopamine concentrations. However, the actions of amphetamine are also linked to adaptations in glutamatergic signaling. We report here that when amphetamine enters dopamine neurons through the dopamine transporter, it stimulates endocytosis of an excitatory amino acid transporter, EAAT3, in dopamine neurons in vitro and in vivo. Consistent with this decrease in surface EAAT3, amphetamine potentiates excitatory synaptic responses in dopamine neurons. We also show that the process of internalization is dynamin- and Rho-mediated and requires a unique sequence in the cytosolic C-terminus of EAAT3. Introduction of a peptide based on this motif into dopamine neurons blocks the effects of amphetamine on EAAT3 internalization and its action on excitatory responses. These data indicate that the internalization of EAAT3 triggered by amphetamine increases glutamatergic signaling and thus contributes to the effects of amphetamine on neurotransmission.

Acute amphetamine (AMPH) exposure elevates extracellular dopamine through a variety of mechanisms that include inhibition of dopamine reuptake, depletion of vesicular stores, and facilitation of dopamine efflux across the plasma membrane. Recent work has shown that the DAT substrate AMPH, unlike cocaine and other nontransported blockers, can also stimulate endocytosis of the plasma membrane dopamine transporter (DAT). Here, we show that when AMPH enters the cytoplasm it rapidly stimulates DAT internalization through a dynamin-dependent, clathrin-independent process. This effect, which can be observed in transfected cells, cultured dopamine neurons, and midbrain slices, is mediated by activation of the small GTPase RhoA. Inhibition of RhoA activity with C3 exotoxin or a dominant-negative RhoA blocks AMPHinduced DAT internalization. These actions depend on AMPH entry into the cell and are blocked by the DAT inhibitor cocaine. AMPH also stimulates cAMP accumulation and PKA-dependent inactivation of RhoA, thus providing a mechanism whereby PKA-and RhoA-dependent signaling pathways can interact to regulate the timing and robustness of AMPH's effects on DAT internalization. Consistent with this model, the activation of D1/D5 receptors that couple to PKA in dopamine neurons antagonizes RhoA activation, DAT internalization, and hyperlocomotion observed in mice after AMPH treatment. These observations support the existence of an unanticipated intracellular target that mediates the effects of AMPH on RhoA and cAMP signaling and suggest new pathways to target to disrupt AMPH action.oth cocaine and amphetamine (AMPH) are psychostimulants that induce euphoria and hyperactivity by increasing extracellular dopamine. Despite the apparent commonalities in the effects and mechanism of action of the two drugs, amphetamine exposure has behavioral, neuroadaptive, and neurotoxic consequences not associated with cocaine use (1-5). These dissimilarities likely stem from several differences in the mechanism of action of the two drugs. Amphetamines, unlike cocaine and other nontransported inhibitors of the dopamine transporter (DAT), are transporter substrates that compete with dopamine for transport, enter dopamine neurons, enhance efflux of dopamine, and also stimulate internalization of the DAT from the cell surface (6, 7). To better understand how amphetamines mediate their effects within the cell, we undertook a series of studies to establish the cellular pathways and signaling mechanisms that underlie the endocytosis of the DAT triggered by amphetamines.AMPH-mediated DAT internalization has been shown to be dynamin-dependent (6). Dynamin-dependence is a key feature of clathrin-mediated endocytosis, and, indeed, previous reports of clathrin-dependent constitutive cycling (8) and PKC-mediated internalization (9) of DAT make it reasonable to consider that the same machinery would be involved in AMPH-mediated internalization. A secondary pathway of dynamin-mediated internalization of membrane-localized proteins involves small GTPases o...