Postsynaptic TRPV1 triggers cell type-specific long-term depression in the nucleus accumbens.

Synaptic modifications in the nucleus accumbens (NAc) are important for adaptive and pathological reward-dependent learning. Medium spiny neurons (MSNs), the major cell type in the NAc, participate in two parallel circuits that subserve distinct behavioral functions, yet little is known about differences in their electrophysiological and synaptic properties. Using bacterial artificial chromosome transgenic mice, we found that synaptic activation of group I metabotropic glutamate receptors in NAc MSNs in the indirect, but not direct, pathway led to the production of endocannabinoids, which activated presynaptic CB1 receptors to trigger endocannabinoid-mediated long-term depression (eCB-LTD) as well as postsynaptic transient receptor potential vanilloid 1 (TRPV1) channels to trigger a form of LTD resulting from endocytosis of AMPA receptors. These results reveal a previously unknown action of TRPV1 channels and indicate that the postsynaptic generation of endocannabinoids can modulate synaptic strength in a cell type-specific fashion by activating distinct pre- and postsynaptic targets.

A selective activity-dependent requirement for dynamin 1 in synaptic vesicle endocytosis.

Dynamin 1 is a neuron-specific guanosine triphosphatase thought to be critically required for the fission reaction of synaptic vesicle endocytosis. Unexpectedly, mice lacking dynamin 1 were able to form functional synapses, even though their postnatal viability was limited. However, during spontaneous network activity, branched, tubular plasma membrane invaginations accumulated, capped by clathrin-coated pits, in synapses of dynamin 1-knockout mice. Synaptic vesicle endocytosis was severely impaired during strong exogenous stimulation but resumed efficiently when the stimulus was terminated. Thus, dynamin 1-independent mechanisms can support limited synaptic vesicle endocytosis, but dynamin 1 is needed during high levels of neuronal activity.

Interactions between glutamate transporters and metabotropic glutamate receptors at excitatory synapses in the cerebellar cortex.

Five glutamate transporter genes have been identified; two of these (EAAT3 and EAAT4) are expressed in neurons and are predominantly confined to the membranes of cell bodies and dendrites. At an ultrastructural level, glutamate transporters have been shown to surround excitatory synapses in hippocampus and cerebellum [J. Neurosci. 18 (1998) 3606; J. Comp. Neurol. 418 (2000) 255]. This pattern of localization overlaps the well-described perisynaptic distribution of Group I metabotropic glutamate receptors or mGluRs [Neuron 11 (1993) 771; J. Chem. Neuroanat. 13 (1997) 77]. Both of the principal excitatory synaptic inputs to cerebellar Purkinje neurons, the parallel fiber (PF) and climbing fiber (CF) synapses, express mGluR-dependent forms of synaptic plasticity [Nat. Neurosci. 4 (2001) 467]. Prompted by the colocalization of postsynaptic glutamate transporters and mGluRs, we have examined whether glutamate uptake limits mGluR-mediated signals and mGluR-dependent forms of plasticity at PF and CF synapses in cerebellar slices. We find that, at PF and, surprisingly also at CF synapses, mGluR activation generates a slow synaptic current and triggers intracellular calcium release. At both PF and CF synapses, mGluR responses are strongly limited by glutamate transporters under resting conditions and are facilitated by short trains of stimuli. Nearly every Purkinje neuron expresses an mGluR-mediated synaptic current upon inhibition of glutamate transport. Global applications of glutamate achieved by photolysis of chemically caged glutamate yield similar results and argue that the colocalized transporters can effectively limit glutamate access to the mGluRs even in the face of such a large amount of transmitter. We hypothesize that neuronal glutamate transporters and Group I mGluRs located in the perisynaptic space interact to sense and then regulate the amount of glutamate escaping excitatory synapses. This hypothesis is currently being tested using electrophysiological methods and the introduction of optically tagged glutamate transporter proteins. In the brain, synaptic signals are terminated mainly by neurotransmitter transporters. Families of genes encoding transporters for the major neurotransmitters (dopamine, GABA, glutamate, glycine, norepinephrine and 5-HT) have been identified. Although transporters serve as targets for important classes of therapeutic drugs (e.g. selective serotonin reuptake inhibitors) and drugs of abuse (amphetamine, cocaine), little is known about how they operate at a molecular level or contribute to synaptic transmission.

Isolation of glutamate transport-coupled charge flux and estimation of glutamate uptake at the climbing fiber-Purkinje cell synapse.

Excitatory amino acid transporters (EAATs) located on neurons and glia are responsible for limiting extracellular glutamate concentrations, but specific contributions made by neuronal and glial EAATs have not been determined. At climbing fiber to Purkinje cell (PC) synapses in cerebellum, a fraction of released glutamate is rapidly bound and inactivated by neuronal EAATs located on postsynaptic PCs. Because transport involves a stoichiometric movement of ions and is electrogenic, postsynaptic currents mediated by EAATs should permit precise calculation of the amount of postsynaptic glutamate uptake. However, this is possible only if a stoichiometric EAAT current can be isolated from all other contaminating signals. We used synaptic stimulation and photolysis of caged glutamate to characterize the current in PCs that is resistant to high concentrations of glutamate receptor antagonists. Some of this response is inhibited by the high-affinity EAAT antagonist TBOA (dl-threo-beta-benzyloxyaspartic acid), whereas the remaining current shows properties inconsistent with glutamate transport. By subtracting this residual non-EAAT current from the response recorded in glutamate receptor antagonists, we have obtained an estimate of postsynaptic uptake near physiological temperature. Analysis of such synaptic EAAT currents suggests that, on average, postsynaptic EAATs take up approximately 1,300,000 glutamate molecules in response to a single climbing fiber action potential.

Glycine transporters not only take out the garbage, they recycle.

Two articles in the current issue of Neuron examine the consequences of deleting the two genes that encode glycine transporters. Interestingly, loss of glial transporters enhances while loss of presynaptic neuronal transporters reduces glycinergic transmission. These two opposing phenotypes resemble distinct human diseases characterized by dysfunction in glycinergic signaling.