No abstract

Summary Developmental alterations of excitatory synapses are implicated in autism spectrum disorders (ASDs). Here, we report increased dendritic spine density with reduced developmental spine pruning in layer V pyramidal neurons in postmortem ASD temporal lobe. These spine deficits correlate with hyperactivated mTOR and impaired autophagy. In Tsc2+/- ASD mice where mTOR is constitutively overactive, we observed postnatal spine pruning defects, blockade of autophagy, and ASD-like social behaviors. The mTOR inhibitor rapamycin corrected ASD-like behaviors and spine pruning defects in Tsc2+/ mice, but not in Atg7CKO neuronal autophagy deficient mice or Tsc2+/-:Atg7CKO double mutants. Neuronal autophagy furthermore enabled spine elimination with no effects on spine formation. Our findings suggest that mTOR regulated autophagy is required for developmental spine pruning, and activation of neuronal autophagy corrects synaptic pathology and social behavior deficits in ASD models with hyperactivated mTOR.

The trace amine para-tyramine is structurally and functionally related to the amphetamines and the biogenic amine neurotransmitters. It is currently thought that the biological activities elicited by trace amines such as p-tyramine and the psychostimulant amphetamines are manifestations of their ability to inhibit the clearance of extracellular transmitter and/or stimulate the efflux of transmitter from intracellular stores. Here we report the discovery and pharmacological characterization of a rat G protein-coupled receptor that stimulates the production of cAMP when exposed to the trace amines p-tyramine, beta-phenethylamine, tryptamine, and octopamine. An extensive pharmacological survey revealed that psychostimulant and hallucinogenic amphetamines, numerous ergoline derivatives, adrenergic ligands, and 3-methylated metabolites of the catecholamine neurotransmitters are also good agonists at the rat trace amine receptor 1 (rTAR1). These results suggest that the trace amines and catecholamine metabolites may serve as the endogenous ligands of a novel intercellular signaling system found widely throughout the vertebrate brain and periphery. Furthermore, the discovery that amphetamines, including 3,4-methylenedioxymethamphetamine (MDMA; "ecstasy"), are potent rTAR1 agonists suggests that the effects of these widely used drugs may be mediated in part by this receptor as well as their previously characterized targets, the neurotransmitter transporter proteins.

Electrophysiological and pharmacological studies of a cloned human dopamine transporter (hDAT) were undertaken to investigate the mechanisms of transporter function and the actions of drugs at this target. Using two-electrode voltage-clamp techniques with hDAT-expressing Xenopus laevis oocytes, we show that hDAT can be considered electrogenic by two criteria. (1) Uptake of hDAT substrates gives rise to a pharmacologically appropriate "transport-associated" current. (2) The velocity of DA uptake measured in oocytes clamped at various membrane potentials was voltage-dependent, increasing with hyperpolarization. Concurrent measurement of transport-associated current and substrate flux in individual oocytes revealed that charge movement during substrate translocation was greater than would be expected for a transport mechanism with fixed stoichiometry of 2 Na+ and 1 Cl- per DA+ molecule. In addition to the transport-associated current, hDAT also mediates a constitutive leak current, the voltage and ionic dependencies of which differ markedly from those of the transport-associated current. Ion substitution experiments suggest that alkali cations and protons are carried by the hDAT leak conductance. In contrast to the transport-associated functions, the leak does not require Na+ or Cl-, and DAT ligands readily interact with the transporter even in the absence of these ions. The currents that hDAT mediates provide a functional assay that readily distinguishes the modes of action of amphetamine-like "DA-releasing" drugs from cocaine-like translocation blockers. In addition, the voltage dependence of DA uptake suggests a mechanism through which presynaptic DA autoreceptor activation may accelerate the termination of dopaminergic neurotransmission in vivo.