Silencing neurotransmission with membrane-tethered toxins.

At synaptic terminals, high voltage-activated Ca(v)2.1 and Ca(v)2.2 calcium channels have an essential and joint role in coupling the presynaptic action potential to neurotransmitter release. Here we show that membrane-tethered toxins allowed cell-autonomous blockade of each channel individually or simultaneously in mouse neurons in vivo. We report optimized constitutive, inducible and Cre recombinase-dependent lentiviral vectors encoding fluorescent recombinant toxins, and we also validated the toxin-based strategy in a transgenic mouse model. Toxins delivered by lentiviral vectors selectively inhibited the dopaminergic nigrostriatal pathway, and transgenic mice with targeted expression in nociceptive peripheral neurons displayed long-lasting suppression of chronic pain. Optimized tethered toxins are tools for cell-specific and temporal manipulation of ion channel-mediated activities in vivo, including blockade of neurotransmitter release.

Cross-reactivity of acid-sensing ion channel and Na⁺-H⁺ exchanger antagonists with nicotinic acetylcholine receptors.

Nicotinic acetylcholine receptors (nAChRs) are widely distributed throughout the mammalian central and peripheral nervous systems, where they contribute to neuronal excitability and synaptic communication. It has been reported that nAChRs are modulated by BK channels and that BK channels, in turn, are inhibited by acid-sensing ion channels (ASICs). Here we investigate the possible functional interaction between these channels in medial habenula (MHb) neurones. We report that selective antagonists of large-conductance calcium-activated potassium channels and ASIC1a channels, paxilline and psalmotoxin 1, respectively, did not induce detectable changes in nicotine-evoked currents. In contrast, the non-selective ASIC and Na(+)-H(+) exchanger (NHE1) antagonists, amiloride and its analogues, suppressed nicotine-evoked responses in MHb neurones of wild-type and ASIC2 null mice, excluding a possible involvement of ASIC2 in the nAChR inhibition by amiloride. Zoniporide, a more selective inhibitor of NHE1, reversibly inhibited α3ß4-, α7- and α4-containing (*) nAChRs in Xenopus oocytes and in brain slices, as well as in PS120 cells deficient in NHE1 and virally transduced with nAChRs, suggesting a generalized effect of zoniporide in most neuronal nAChR subtypes. Independently from nAChR antagonism, zoniporide profoundly blocked synaptic transmission onto MHb neurones without affecting glutamatergic and GABA receptors. Taken together, these results indicate that amiloride and zoniporide, which are clinically used to treat hypertension and cardiovascular disease, have an inhibitory effect on neuronal nAChRs when used experimentally at high doses. The possible cross-reactivity of these compounds with nAChRs in vivo will require further investigation.

An in vivo tethered toxin approach for the cell-autonomous inactivation of voltage-gated sodium channel currents in nociceptors.

Understanding information flow in sensory pathways requires cell-selective approaches to manipulate the activity of defined neurones. Primary afferent nociceptors, which detect painful stimuli, are enriched in specific voltage-gated sodium channel (VGSC) subtypes. Toxins derived from venomous animals can be used to dissect the contributions of particular ion currents to cell physiology. Here we have used a transgenic approach to target a membrane-tethered isoform of the conotoxin MrVIa (t-MrVIa) only to nociceptive neurones in mice. T-MrVIa transgenic mice show a 44 +/- 7% reduction of tetrodotoxin-resistant (TTX-R) VGSC current densities. This inhibition is permanent, reversible and does not result in functional upregulation of TTX-sensitive (TTX-S) VGSCs, voltage-gated calcium channels (VGCCs) or transient receptor potential (TRP) channels present in nociceptive neurones. As a consequence of the reduction of TTX-R VGSC currents, t-MrVIa transgenic mice display decreased inflammatory mechanical hypersensitivity, cold pain insensitivity and reduced firing of cutaneous C-fibres sensitive to noxious cold temperatures. These data validate the use of genetically encoded t-toxins as a powerful tool to manipulate VGSCs in specific cell types within the mammalian nervous system. This novel genetic methodology can be used for circuit mapping and has the key advantage that it enables the dissection of the contribution of specific ionic currents to neuronal function and to behaviour.

Cell-specific synaptic plasticity induced by network oscillations.

Gamma rhythms are known to contribute to the process of memory encoding. However, little is known about the underlying mechanisms at the molecular, cellular and network levels. Using local field potential recording in awake behaving mice and concomitant field potential and whole-cell recordings in slice preparations we found that gamma rhythms lead to activity-dependent modification of hippocampal networks, including alterations in sharp wave-ripple complexes. Network plasticity, expressed as long-lasting increases in sharp wave-associated synaptic currents, exhibits enhanced excitatory synaptic strength in pyramidal cells that is induced postsynaptically and depends on metabotropic glutamate receptor-5 activation. In sharp contrast, alteration of inhibitory synaptic strength is independent of postsynaptic activation and less pronounced. Further, we found a cell type-specific, directionally biased synaptic plasticity of two major types of GABAergic cells, parvalbumin- and cholecystokinin-expressing interneurons. Thus, we propose that gamma frequency oscillations represent a network state that introduces long-lasting synaptic plasticity in a cell-specific manner.

Habenular expression of rare missense variants of the ß4 nicotinic receptor subunit alters nicotine consumption.

The CHRNA5-CHRNA3-CHRNB4 gene cluster, encoding the α5, α3, and ß4 nicotinic acetylcholine receptor (nAChR) subunits, has been linked to nicotine dependence. The habenulo-interpeduncular (Hb-IPN) tract is particularly enriched in α3ß4 nAChRs. We recently showed that modulation of these receptors in the medial habenula (MHb) in mice altered nicotine consumption. Given that ß4 is rate-limiting for receptor activity and that single nucleotide polymorphisms (SNPs) in CHRNB4 have been linked to altered risk of nicotine dependence in humans, we were interested in determining the contribution of allelic variants of ß4 to nicotine receptor activity in the MHb. We screened for missense SNPs that had allele frequencies >0.0005 and introduced the corresponding substitutions in Chrnb4. Fourteen variants were analyzed by co-expression with α3. We found that ß4A90I and ß4T374I variants, previously shown to associate with reduced risk of smoking, and an additional variant ß4D447Y, significantly increased nicotine-evoked current amplitudes, while ß4R348C, the mutation most frequently encountered in sporadic amyotrophic lateral sclerosis (sALS), showed reduced nicotine currents. We employed lentiviruses to express ß4 or ß4 variants in the MHb. Immunoprecipitation studies confirmed that ß4 lentiviral-mediated expression leads to specific upregulation of α3ß4 but not ß2 nAChRs in the Mhb. Mice injected with the ß4-containing virus showed pronounced aversion to nicotine as previously observed in transgenic Tabac mice overexpressing Chrnb4 at endogenous sites including the MHb. Habenular expression of the ß4 gain-of-function allele T374I also resulted in strong aversion, while transduction with the ß4 loss-of function allele R348C failed to induce nicotine aversion. Altogether, these data confirm the critical role of habenular ß4 in nicotine consumption, and identify specific SNPs in CHRNB4 that modify nicotine-elicited currents and alter nicotine consumption in mice.

The biochemical anatomy of cortical inhibitory synapses.

Classical electron microscopic studies of the mammalian brain revealed two major classes of synapses, distinguished by the presence of a large postsynaptic density (PSD) exclusively at type 1, excitatory synapses. Biochemical studies of the PSD have established the paradigm of the synapse as a complex signal-processing machine that controls synaptic plasticity. We report here the results of a proteomic analysis of type 2, inhibitory synaptic complexes isolated by affinity purification from the cerebral cortex. We show that these synaptic complexes contain a variety of neurotransmitter receptors, neural cell-scaffolding and adhesion molecules, but that they are entirely lacking in cell signaling proteins. This fundamental distinction between the functions of type 1 and type 2 synapses in the nervous system has far reaching implications for models of synaptic plasticity, rapid adaptations in neural circuits, and homeostatic mechanisms controlling the balance of excitation and inhibition in the mature brain.

Aversion to nicotine is regulated by the balanced activity of ß4 and α5 nicotinic receptor subunits in the medial habenula.

Nicotine dependence is linked to single nucleotide polymorphisms in the CHRNB4-CHRNA3-CHRNA5 gene cluster encoding the α3ß4α5 nicotinic acetylcholine receptor (nAChR). Here we show that the ß4 subunit is rate limiting for receptor activity, and that current increase by ß4 is maximally competed by one of the most frequent variants associated with tobacco usage (D398N in α5). We identify a ß4-specific residue (S435), mapping to the intracellular vestibule of the α3ß4α5 receptor in close proximity to α5 D398N, that is essential for its ability to increase currents. Transgenic mice with targeted overexpression of Chrnb4 to endogenous sites display a strong aversion to nicotine that can be reversed by viral-mediated expression of the α5 D398N variant in the medial habenula (MHb). Thus, this study both provides insights into α3ß4α5 receptor-mediated mechanisms contributing to nicotine consumption, and identifies the MHb as a critical element in the circuitry controlling nicotine-dependent phenotypes.

Electrophysiological and synaptic characterization of transplanted neurons in adult rat motor cortex.

Lesions in specific areas of the rat motor cortex generate deficits related to fine movement performance affecting the forelimb. We have previously shown that transplants of embryonic frontal cortex ameliorate these motor deficits. Amelioration has been associated with a functional integration of the transplant due to the connections established between the host brain and the graft. In the current investigation, the electrophysiological properties of the transplanted cells and the connections both intra-transplant and with the adjacent host cortex are analyzed. For this purpose, adult rats with a motor cortical lesion plus a fetal cortical graft were used. Neurons in the transplant were recorded using sharp electrodes or whole-cell recordings in brain slices. Application of intracellular depolarizing pulses showed two patterns of cell firing: regular and burst spiking. Postsynaptic responses evoked by both, intra-transplant and adjacent host cortex stimulation were mediated by glutamic acid acting on non-NMDA and NMDA receptors, and were modulated by both cholinergic and GABAergic drugs. In some cells, supra-threshold intra-transplant stimulation generated an epileptiform-like discharge, suggesting an imbalance between excitatory and inhibitory synapses. As expected, immunohistochemistry for cholinergic and GABAergic markers confirmed the electrophysiological results. Thus we show electrophysiological and immunohistochemical evidence supporting the functional development and integration of grafted cells into the host neocortex of adult animals.

Glutamatergic synaptic depression by synthetic amyloid beta-peptide in the medial septum.

The medial septum/diagonal band region, which participates in learning and memory processes via its cholinergic and GABAergic projection to the hippocampus, is one of the structures affected by beta amyloid (betaA) deposition in Alzheimer's disease (AD). The acute effects of betaA (25-35 and 1-40) on action potential generation and glutamatergic synaptic transmission in slices of the medial septal area of the rat brain were studied using current and patch-clamp techniques. The betaA mechanism of action through M1 muscarinic receptors and voltage-dependent calcium channels was also addressed. Excitatory evoked responses decreased (30-60%) in amplitude after betaA (2 microM) perfusion in 70% of recorded cells. However, the firing properties were unaltered at the same concentration. This depression was irreversible in most cases, and was not prevented or reversed by nicotine (5 microM). In addition, the results obtained using a paired-pulse protocol support pre- and postsynaptic actions of the peptide. The betaA effect was blocked by calcicludine (50 nM), a selective antagonist of L-type calcium channels, and also by blocking muscarinic receptors with atropine (5 muM) or pirenzepine (1 microM), a more specific M1-receptor blocker. We show that in the medial septal area this oligomeric peptide acts through calcium channels and muscarinic receptors. As blocking any of these pathways blocks the betaA effects, we propose a joint action through both mechanisms. These results may contribute to a better understanding of the pathophysiology at the onset of AD. This understanding will be required for the development of new therapeutic agents.