Glutamatergic neurotransmission is controlled by presynaptic metabotropic glutamate receptors (mGluRs). A subdomain in the intracellular carboxyl-terminal tail of group III mGluRs binds calmodulin and heterotrimeric guanosine triphosphate-binding protein (G protein) betagamma subunits in a mutually exclusive manner. Mutations interfering with calmodulin binding and calmodulin antagonists inhibit G protein-mediated modulation of ionic currents by mGluR 7. Calmodulin antagonists also prevent inhibition of excitatory neurotransmission via presynaptic mGluRs. These results reveal a novel mechanism of presynaptic modulation in which Ca(2+)-calmodulin is required to release G protein betagamma subunits from the C-tail of group III mGluRs in order to mediate glutamatergic autoinhibition.

Background: hSERT is a neurotransmitter transporter driven by ion gradients with electroneutral stoichiometry but rheogenic properties.Results: hSERT displays coupled and uncoupled currents. The uncoupled current depends on internal K+.Conclusion: The conducting state of hSERT is in equilibrium with an inward facing K+-bound state.Significance: This study provides a framework for exploring transporter-associated currents.

Amphetamine and related substances induce dopamine release. According to a traditional explanation, this dopamine release occurs in exchange for amphetamine by means of the dopamine transporter (DAT). We tested this hypothesis in human embryonic kidney 293 cells stably transfected with the human DAT by measuring the uptake of dopamine, tyramine, and d‐ and l‐amphetamine as well as substrate‐induced release of preloaded N‐methyl‐4‐[3H]phenylpyridinium ([3H]MPP+). The uptake of substrates was sodium‐dependent and was inhibited by ouabain and cocaine, which also prevented substrate‐induced release of MPP+. Patch‐clamp recordings revealed that all four substrates elicited voltage‐dependent inward currents (on top of constitutive leak currents) that were prevented by cocaine. Whereas individual substrates had similar affinities in release, uptake, and patch‐clamp experiments, maximal effects displayed remarkable differences. Hence, maximal effects in release and current induction were ∼25% higher for d‐amphetamine as compared with the other substrates. By contrast, dopamine was the most efficacious substrate in uptake experiments, with its maximal initial uptake rate exceeding those of amphetamine and tyramine by factors of 20 and 4, respectively. Our experiments indicate a poor correlation between substrate‐induced release and the transport of substrates, whereas the ability of substrates to induce currents correlates well with their releasing action.

ATP is a fast transmitter in sympathetic ganglia and at the sympathoeffector junction. In primary cultures of dissociated rat superior cervical ganglion neurons, ATP elicits noradrenaline release in an entirely Ca2+-dependent manner. Nevertheless, ATP-evoked noradrenaline release was only partially reduced (by approximately 50%) when either Na+ or Ca2+ channels were blocked, which indicates that ATP receptors themselves mediated transmembrane Ca2+ entry. An "axonal" preparation was obtained by removing ganglia from explant cultures, which left a network of neurites behind; immunostaining for axonal and dendritic markers revealed that all of these neurites were axons. In this preparation, ATP raised intraaxonal Ca2+ and triggered noradrenaline release, and these actions were not altered when Ca2+ channels were blocked by Cd2+. Hence, Ca2+-permeable ATP-gated ion channels, i.e., P2X purinoceptors, are located at presynaptic sites and directly mediate Ca2+-dependent transmitter release. These presynaptic P2X receptors displayed a rank order of agonist potency of ATP >/= 2-methylthio-ATP > ATPgammaS >> alpha,beta-methylene-ATP approximately beta,gamma-methylene-L-ATP and were blocked by suramin or PPADS. ATP, 2-methylthio-ATP, and ATPgammaS also evoked inward currents measured at neuronal somata, but there these agonists were equipotent. Hence, presynaptic P2X receptors resemble the cloned P2X2 subtype, but they appear to differ from somatodendritic P2X receptors in terms of agonist sensitivity. Suramin reduced depolarization-evoked noradrenaline release by up to 20%, when autoinhibitory mechanisms were inactivated by pertussis toxin. These results indicate that presynaptic P2X purinoceptors mediate a positive, whereas G-protein-coupled P2Y purinoceptors mediate a negative, feedback modulation of sympathetic transmitter release.

The release of transmitters at sympathoeffector junctions is not constant, but subject to modulation by a plethora of different mechanisms. In this respect, presynaptic receptors located on the sympathetic axon terminals are of utmost importance, because they are activated by exogenous agonists and by endogenous neurotransmitters. In the latter case, the transmitters that activate the presynaptic receptors of a nerve terminal may be released either from the very same nerve ending or from a different axon terminal, and the receptors involved are auto- and heteroreceptors, respectively. In terms of their structural and functional features, receptors of sympathetic axon terminals can be categorized as either ionotropic (transmitter-gated ion channels) or metabotropic (most commonly G protein-coupled) receptors. This review summarizes results on more than 30 different metabotropic and four different ionotropic receptors that have been found to control the amount of transmitter being released from sympathetic neurons. Each of these receptors may not only stimulate, facilitate, and reduce sympathetic transmitter release, respectively, but also interact with the functions of other receptors present on the same axonal varicosity. This provides a multitude of mechanisms that regulate the amount of sympathetic transmitter output. Accordingly, a sophisticated cross-talk within and between extra- and intracellular signals is integrated at axon terminals to adapt the strength of sympathoeffector transmission to a given situation. This will not only determine the function of the sympathetic nervous system in health and disease, but also therapeutic and untoward effects of drugs that bind to the presynaptic receptors in sympathetically innervated tissues.

Somatostatin is one of the major peptides in interneurons of the hippocampus. It is believed to play a role in memory formation and to reduce the susceptibility of the hippocampus to seizurelike activity. However, at the cellular level, the actions of somatostatin on hippocampal neurons are still controversial, ranging from inhibition to excitation. In the present study, we measured autaptic currents of hippocampal neurons isolated in singleneuron microcultures. Somatostatin and the analogous peptides seglitide and octreotide reduced glutamatergic, but not GABAergic, autaptic currents via pertussis toxin-sensitive G-proteins. This effect was observed whether autaptic currents were mediated by NMDA or non-NMDA glutamate receptors. Furthermore, somatostatin did not affect currents evoked by the direct application of glutamate, but reduced the frequency of spontaneously occurring excitatory autaptic currents. These results show that presynaptic somatostatin receptors of the SRIF 1 family inhibit glutamate release at hippocampal synapses. Somatostatin, seglitide, and octreotide also reduced the frequency of miniature excitatory postsynaptic currents in mass cultures without affecting their amplitudes. In addition, all three agonists inhibited voltage-activated Ca 2ϩ currents at neuronal somata, but failed to alter K ϩ currents, effects that were also abolished by pertussis toxin. Thus, presynaptic somatostatin receptors in the hippocampus selectively inhibit excitatory transmission via G-proteins of the G i /G o family and through at least two separate mechanisms, the modulation of Ca 2ϩ channels and an effect downstream of Ca 2ϩ entry. This presynaptic inhibition by somatostatin may provide a basis for its reportedly anticonvulsive action.

Mammalian Staufen2 (Stau2) is a member of the double-stranded RNA-binding protein family. Its expression is largely restricted to the brain. It is thought to play a role in the delivery of RNA to dendrites of polarized neurons. To investigate the function of Stau2 in mature neurons, we interfered with Stau2 expression by RNA interference (RNAi). Mature neurons lacking Stau2 displayed a significant reduction in the number of dendritic spines and an increase in filopodia-like structures. The number of PSD95-positive synapses and miniature excitatory postsynaptic currents were markedly reduced in Stau2 down-regulated neurons. Akin effects were caused by overexpression of dominant-negative Stau2. The observed phenotype could be rescued by overexpression of two RNAi cleavage-resistant Stau2 isoforms. In situ hybridization revealed reduced expression levels of β-actin mRNA and fewer dendritic β-actin mRNPs in Stau2 down-regulated neurons. Thus, our data suggest an important role for Stau2 in the formation and maintenance of dendritic spines of hippocampal neurons.

Background: Ibogaine is a noncompetitive inhibitor of SERT that stabilizes the transporter in an inward-open conformation.Results: Ibogaine binds to a site accessible from the cell exterior that does not overlap with the substrate-binding site.Conclusion: Ibogaine binds to a novel binding site on SERT and DAT.Significance: This study provides a mechanistic understanding of an unique inhibitor of SERT and DAT.