Crystallographic, computational and functional analyses of LeuT have revealed details of the molecular architecture of Na+-coupled transporters and the mechanistic nature of ion/substrate coupling, but the conformational changes that support a functional transport cycle have yet to be described fully. We have used site-directed spin labeling and EPR analysis to capture the dynamics of LeuT in the region of the extracellular vestibule associated with the binding of Na+ and Leu. The results outline the Na+-dependent formation of a dynamic outward-facing intermediate that exposes the primary substrate binding site, and the conformational changes that occlude this binding site upon subsequent binding of the Leu substrate. Furthermore, the binding of the transport inhibitors Trp, clomipramine and octyl-glucoside is shown to induce structural changes that distinguish the resulting inhibited conformation from the Na+/Leu bound state.

Summary Structural plasticity in the adult brain is essential for adaptive behavior. We have found a remarkable anatomical plasticity in the basal ganglia of adult mice that is regulated by dopamine D2 receptors (D2Rs). By modulating neuronal excitability, striatal D2Rs bi-directionally control the density of direct pathway collaterals in the globus pallidus that bridge the direct pathway with the functionally opposing indirect pathway. An increase in bridging collaterals is associated with enhanced inhibition of pallidal neurons in vivo and disrupted locomotor activation after optogenetic stimulation of the direct pathway. Remarkably, chronic blockade with haloperidol, an antipsychotic medication used to treat schizophrenia decreases the extent of bridging collaterals and rescues the locomotor imbalance. These findings identify a role for bridging collaterals in regulating the concerted balance of striatal output, and may have important implications for understanding schizophrenia, a disease involving excessive activation of striatal D2Rs that is treated with D2R blockers.

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