The absence of VGLUT3 predisposes to cocaine abuse by increasing dopamine and glutamate signaling in the nucleus accumbens.

Tonically active cholinergic interneurons (TANs) from the nucleus accumbens (NAc) are centrally involved in reward behavior. TANs express a vesicular glutamate transporter referred to as VGLUT3 and thus use both acetylcholine and glutamate as neurotransmitters. The respective roles of each transmitter in the regulation of reward and addiction are still unknown. In this study, we showed that disruption of the gene that encodes VGLUT3 (Slc17a8) markedly increased cocaine self-administration in mice. Concomitantly, the amount of dopamine (DA) release was strongly augmented in the NAc of VGLUT3(-/-) mice because of a lack of signaling by metabotropic glutamate receptors. Furthermore, dendritic spines and glutamatergic synaptic transmission on medium spiny neurons were increased in the NAc of VGLUT3(-/-) mice. Increased DA and glutamate signaling in the NAc are hallmarks of addiction. Our study shows that TANs use glutamate to reduce DA release and decrease reinforcing properties of cocaine in mice. Interestingly, we also observed an increased frequency of rare variations in SLC17A8 in a cohort of severe drug abusers compared with controls. Our findings identify VGLUT3 as an unexpected regulator of drug abuse.

Characterization of a Trypanosoma cruzi antigen with homology to intracellular mammalian lectins.

Two cDNAs, isolated from a Trypanosoma cruzi amastigote library immunoscreened with sera from patients with Chagas disease, encode proteins with sequence homology to eukaryotic components of the cellular sorting and recycling machinery. These proteins, denominated TcAGL, present an N-terminal lectin domain and a C-terminal region containing repetitive amino acids and a poly-glutamine tract. They are products of polymorphic alleles of a single copy gene constitutively expressed during the parasite life cycle. Polyclonal antibodies obtained from mice immunized with the recombinant antigen recognize proteins with apparent molecular weight ranging from 95 to 120 kDa in cell lysates from all three life stages and in various strains of the parasite. Sera from Chagas disease patients recognize the recombinant antigen in ELISA and immunoprecipitation assays but not in Western blot assays under denaturing conditions. Consistent with its proposed role in the glycoprotein secreting pathway, immunofluorescence analyses and expression of a green fluorescent protein-tagged TcAGL protein indicate a sub-cellular localization in the vicinity of the flagellar pocket membrane and the Golgi complex of the parasite.

Investigation of the modulation of glutamate release by sodium channels using neurotoxins.

The modulation of neurotransmitter release by calcium channels is well established, yet, sodium channels were regarded mainly as charge carriers. Many lines of evidence suggest a more fine-tuning role played by sodium channels. Using rat cerebrocortical isolated nerve endings (synaptosomes) and two toxins that have separate sites of action on sodium channels and provoke distinct changes in channel kinetics, we were able to show that depending on the rate of increase in channel conductance, the outcome in terms of neurotransmitter release and calcium channel types coupled to that event are different. Mainly, our study focused on veratridine, an alkaloid from lilaceous plants that binds to sodium channel toxin site 2, and tityustoxin, a toxin purified from the venom of the Brazilian yellow scorpion Tityus serrulatus that binds to site 3. Veratridine induces a slower increase in intrasynaptosomal sodium and calcium concentrations, slower depolarization, delayed exocytosis and a slower and predominantly calcium-independent glutamate release, when compared to tityustoxin.Thus, we have used these two toxins to investigate the events that start with sodium entry and culminate with the release of glutamate in isolated nerve endings (synaptosomes) from rat cerebral cortex. With that in mind we measured intrasynaptosomal free sodium concentration [Na(+)](i), intrasynaptosomal free calcium concentration [Ca(2+)](i), membrane potential, exocytosis and glutamate release using fluorescent probes.

Release of gamma-[(3)H]aminobutyric acid in rat brain cortical slices by alpha-scorpion toxin.

In this paper, the effect of the alpha-scorpion toxin tityustoxin (TsTX) in the release of gamma-[(3)H]aminobutyric acid ([(3)H]GABA) from rat brain cortical slices is described. The TsTX-stimulatory effect on the release of [(3)H]GABA was dependent on incubation time and TsTX concentration, having an EC(50) of 0.33 microM. Tetrodotoxin (TTX) completely inhibited the TsTX action on [(3)H]GABA release. The scorpion toxin effect was calcium-dependent and involves P/Q calcium channels. beta-Alanine also induces the release of [(3)H]GABA that was not inhibited by TTX but was additive in the presence of TsTX. The data suggest a neuronal origin for the release of [(3)H]GABA by TsTX.

Translocation of protein kinase C by halothane in cholinergic cells.

Protein kinase C (PKC) is a signal transducing enzyme that is an important regulator of multiple physiologic processes and a potential molecular target for volatile anaesthetic actions. However, the effects of these agents on PKC activity are not yet fully understood. Volatile anaesthetics increase intracellular calcium concentration ([Ca(2+)](i)) in a variety of cells, thus their effects on PKC activity may be indirect due to [Ca(2+)](i) increase. Alternatively, the anaesthetics could directly stimulate PKC activity. In order to distinguish these two possibilities in intact cells, we used a fully functional green fluorescent protein conjugated PKCbetaII (GFP-PKCbetaII) and confocal microscopy to evaluate the dynamic redistribution of PKC in living SN56 cells, a cholinergic cell line, in response to halothane. Halothane induced PKC translocation in SN56 cells transfected with GFP-PKCbetaII. This effect was not suppressed by dantrolene, a drug that blocks halothane-induced Ca(2+) release from intracellular stores in these cells. These findings indicate that halothane induces PKC translocation in SN56 cells independently of its ability to release calcium from internal stores.

Expression of a functional recombinant Phoneutria nigriventer toxin active on K+ channels.

PnTx3-1 is a peptide isolated from the venom of the spider Phoneutria nigriventer that specifically inhibits A-type K(+) currents (I(A)) in GH(3) cells. Here we used a bacterial expression system to produce an NH(2)-extended mutant of PnTx3-1 (ISEF-PnTx3-1) and tested whether the toxin is functional. The recombinant toxin was purified from bacterial extracts by a combination of affinity and ion-exchange chromatography. The recombinant toxin blocked A-type K(+) currents in GH(3) cells in a fashion similar to that observed with the wild-type toxin purified from the spider venom. These results suggest that recombinant cDNA methods provide a novel source for the production of functional Phoneutria toxins. The recombinant ISEF-PnTx3-1 should be useful for further understanding of the role of A-type K(+) currents in biological processes.

Sleep pattern and learning in knockdown mice with reduced cholinergic neurotransmission.

Impaired cholinergic neurotransmission can affect memory formation and influence sleep-wake cycles (SWC). In the present study, we describe the SWC in mice with a deficient vesicular acetylcholine transporter (VAChT) system, previously characterized as presenting reduced acetylcholine release and cognitive and behavioral dysfunctions. Continuous, chronic ECoG and EMG recordings were used to evaluate the SWC pattern during light and dark phases in VAChT knockdown heterozygous (VAChT-KDHET, n=7) and wild-type (WT, n=7) mice. SWC were evaluated for sleep efficiency, total amount and mean duration of slow-wave, intermediate and paradoxical sleep, as well as the number of awakenings from sleep. After recording SWC, contextual fear-conditioning tests were used as an acetylcholine-dependent learning paradigm. The results showed that sleep efficiency in VAChT-KDHET animals was similar to that of WT mice, but that the SWC was more fragmented. Fragmentation was characterized by an increase in the number of awakenings, mainly during intermediate sleep. VAChT-KDHET animals performed poorly in the contextual fear-conditioning paradigm (mean freezing time: 34.4±3.1 and 44.5±3.3 s for WT and VAChT-KDHET animals, respectively), which was followed by a 45% reduction in the number of paradoxical sleep episodes after the training session. Taken together, the results show that reduced cholinergic transmission led to sleep fragmentation and learning impairment. We discuss the results on the basis of cholinergic plasticity and its relevance to sleep homeostasis. We suggest that VAChT-KDHET mice could be a useful model to test cholinergic drugs used to treat sleep dysfunction in neurodegenerative disorders.

Expression of a recombinant Phoneutria toxin active in calcium channels.

PnTx3-4 is a toxin isolated from the venom of the spider Phoneutria nigriventer that blocks N-, P/Q-, and R-type voltage-gated calcium channels and has great potential for clinical applications. In this report we used the SUMO system to express large amounts of recombinant PnTx3-4 peptide, which was found in both soluble and insoluble fractions of bacterial extracts. We purified the recombinant toxin from both fractions and showed that the recombinant peptide showed biological activity similar to the native PnTx3-4. In silico analysis of the primary sequence of PnTx3-4 indicated that the peptide conforms to all the criteria of a knottin scaffold. Additionally, circular dichroism spectrum analysis of the recombinant PnTx3-4 predicted that the toxin structure is composed of approximately 53% turns/unordered, 31% α-helix and 16% ß-strand, which is consistent with predicted model of the PnTx3-4 knottin scaffold available at the knottin database (http://knottin.cbs.cnrs.fr). These studies provide the basis for future large scale production and structure-function investigation of PnTx3-4.

VAChT knock-down mice show normal prepulse inhibition but disrupted long-term habituation.

The neurotransmitter acetylcholine (ACh) plays a crucial role in both the central and peripheral nervous system. Central cholinergic transmission is important for cognitive functions and cholinergic disruptions have been associated with different neural disorders. We here tested the role of cholinergic transmission in basic cognitive functions, i.e. in prepulse inhibition (PPI) and short-term habituation (STH) as well as long-term habituation (LTH) of startle using mice with a 65% knockdown (KD) of the vesicular ACh transporter (VAChT). These mice are slow in refilling cholinergic synaptic transmitter vesicles, leading to a reduced cholinergic tone. Prepulse inhibition has been assumed to be mediated by cholinergic projections from the midbrain to the reticular formation. Surprisingly, PPI and STH were normal in these mice, whereas LTH was disrupted. This disruption could be rescued by pre-testing injections of the ACh esterase inhibitor galantamine, but not by post-testing injections. The lack of a PPI deficit might be because of the fact that VAChT KD mice show disruptions mainly in prolonged cholinergic activity, therefore the transient activation by prepulse processing might not be sufficient to deplete synaptic vesicles. The disruption of LTH indicates that the latter depends on a tonic cholinergic inhibition. Future experiments will address which cholinergic cell group is responsible for this effect.

Reduced expression of the vesicular acetylcholine transporter causes learning deficits in mice.

Storage of acetylcholine in synaptic vesicles plays a key role in maintaining cholinergic function. Here we used mice with a targeted mutation in the vesicular acetylcholine transporter (VAChT) gene that reduces transporter expression by 40% to investigate cognitive processing under conditions of VAChT deficiency. Motor skill learning in the rotarod revealed that VAChT mutant mice were slower to learn this task, but once they reached maximum performance they were indistinguishable from wild-type mice. Interestingly, motor skill performance maintenance after 10 days was unaffected in these mutant mice. We also tested whether reduced VAChT levels affected learning in an object recognition memory task. We found that VAChT mutant mice presented a deficit in memory encoding necessary for the temporal order version of the object recognition memory, but showed no alteration in spatial working memory, or spatial memory in general when tested in the Morris water maze test. The memory deficit in object recognition memory observed in VAChT mutant mice could be reversed by cholinesterase inhibitors, suggesting that learning deficits caused by reduced VAChT expression can be ameliorated by restoring ACh levels in the synapse. These data indicate an important role for cholinergic tone in motor learning and object recognition memory.

Sleep pattern and learning in knockdown mice with reduced cholinergic neurotransmission

Impaired cholinergic neurotransmission can affect memory formation and influence sleep-wake cycles (SWC). In the present study, we describe the SWC in mice with a deficient vesicular acetylcholine transporter (VAChT) system, previously characterized as presenting reduced acetylcholine release and cognitive and behavioral dysfunctions. Continuous, chronic ECoG and EMG recordings were used to evaluate the SWC pattern during light and dark phases in VAChT knockdown heterozygous (VAChT-KDHET, n=7) and wild-type (WT, n=7) mice. SWC were evaluated for sleep efficiency, total amount and mean duration of slow-wave, intermediate and paradoxical sleep, as well as the number of awakenings from sleep. After recording SWC, contextual fear-conditioning tests were used as an acetylcholine-dependent learning paradigm. The results showed that sleep efficiency in VAChT-KDHET animals was similar to that of WT mice, but that the SWC was more fragmented. Fragmentation was characterized by an increase in the number of awakenings, mainly during intermediate sleep. VAChT-KDHET animals performed poorly in the contextual fear-conditioning paradigm (mean freezing time: 34.4±3.1 and 44.5±3.3 s for WT and VAChT-KDHET animals, respectively), which was followed by a 45% reduction in the number of paradoxical sleep episodes after the training session. Taken together, the results show that reduced cholinergic transmission led to sleep fragmentation and learning impairment. We discuss the results on the basis of cholinergic plasticity and its relevance to sleep homeostasis. We suggest that VAChT-KDHET mice could be a useful model to test cholinergic drugs used to treat sleep dysfunction in neurodegenerative disorders.

The hemicholinium-3 sensitive high affinity choline transporter is internalized by clathrin-mediated endocytosis and is present in endosomes and synaptic vesicles.

Synthesis of acetylcholine depends on the plasma membrane uptake of choline by a high affinity choline transporter (CHT1). Choline uptake is regulated by nerve impulses and trafficking of an intracellular pool of CHT1 to the plasma membrane may be important for this regulation. We have generated a hemagglutinin (HA) epitope tagged CHT1 to investigate the organelles involved with intracellular trafficking of this protein. Expression of CHT1-HA in HEK 293 cells establishes Na+-dependent, hemicholinium-3 sensitive high-affinity choline transport activity. Confocal microscopy reveals that CHT1-HA is found predominantly in intracellular organelles in three different cell lines. Importantly, CHT1-HA seems to be continuously cycling between the plasma membrane and endocytic organelles via a constitutive clathrin-mediated endocytic pathway. In a neuronal cell line, CHT1-HA colocalizes with the early endocytic marker green fluorescent protein (GFP)-Rab 5 and with two markers of synaptic-like vesicles, VAMP-myc and GFP-VAChT, suggesting that in cultured cells CHT1 is present mainly in organelles of endocytic origin. Subcellular fractionation and immunoisolation of organelles from rat brain indicate that CHT1 is present in synaptic vesicles. We propose that intracellular CHT1 can be recruited during stimulation to increase choline uptake in nerve terminals.

Use of fluorescent probes to follow membrane traffic in nerve terminals

Optical tracers in conjunction with fluorescence microscopy have become widely used to follow the movement of synaptic vesicles in nerve terminals. The present review discusses the use of these optical methods to understand the regulation of exocytosis and endocytosis of synaptic vesicles. The maintenance of neurotransmission depends on the constant recycling of synaptic vesicles and important insights have been gained by visualization of vesicles with the vital dye FM1-43. A number of questions related to the control of recycling of synaptic vesicles by prolonged stimulation and the role of calcium to control membrane internalization are now being addressed. It is expected that optical monitoring of presynaptic activity coupled to appropriate genetic models will contribute to the understanding of membrane traffic in synaptic terminals.