PubMed Health⌕ Search

Biomedical subjects

Louis-Eric Trudeau

Publications and source records attributed to Louis-Eric Trudeau.

At least 19 recordsLinked to original sources

Coordinated action of NSF and PKC regulates GABAB receptor signaling efficacy.

The obligatory heterodimerization of the GABAB receptor (GBR) raises fundamental questions about molecular mechanisms controlling its signaling efficacy. Here, we show that NEM sensitive fusion (NSF) protein interacts directly with the GBR heterodimer both in rat brain synaptosomes and in CHO cells, forming a ternary complex that can be regulated by agonist stimulation. Inhibition of NSF binding with a peptide derived from GBR2 (TAT-Pep-27) did not affect basal signaling activity but almost completely abolished agonist-promoted GBR desensitization in both CHO cells and hippocampal slices. Taken with the role of PKC in the desensitization process, our observation that TAT-Pep-27 prevented both agonist-promoted recruitment of PKC and receptor phosphorylation suggests that NSF is a priming factor required for GBR desensitization. Given that GBR desensitization does not involve receptor internalization, the NSF/PKC coordinated action revealed herein suggests that NSF can regulate GPCR signalling efficacy independently of its role in membrane trafficking. The functional interaction between three bona fide regulators of neurotransmitter release, such as GBR, NSF and PKC, could shed new light on the modulation of presynaptic GBR action.

Amino Acid Sequence↗

Expression of D2 receptor isoforms in cultured neurons reveals equipotent autoreceptor function.

Alternative splicing of the dopamine D2 receptor gene produces two distinct isoforms referred to as D2long (D2L) and D2short (D2S). In mesencephalic dopamine neurons, inhibition of the firing rate through activation of somatodendritic D2 receptors and blockade of neurotransmitter release through stimulation of terminal D2 receptors represent major roles of D2 autoreceptors. Recently, data obtained from D2L-deficient mice suggested that D2S acts as the preferential D2 autoreceptor. In the present study, we investigate whether this D2 isoform-specific autoreceptor function is linked to differences in the subcellular localization and/or signaling properties of the D2S and D2L using mesencephalic neurons transfected with enhanced green fluorescent protein (EGFP)-tagged receptors. Our results show that EGFP-tagged D2S and D2L are localized to the axonal and somatodendritic compartments of mesencephalic neurons. In addition, we demonstrate that EGFP-tagged D2S and D2L regulate cellular excitability, neurotransmitter release and basal levels of intracellular calcium with similar effectiveness. Overall, our morphological and electrophysiological studies suggest that the major D2 autoreceptor function attributed to D2S is likely explained by the predominant expression of this isoform in dopamine neurons rather than by distinct subcellular localization and signaling properties of D2S and D2L.

Action Potentials↗

Impact of basic FGF expression in astrocytes on dopamine neuron synaptic function and development.

Behavioural sensitization to amphetamine (AMPH) requires action of the drug in the ventral midbrain where dopamine (DA) neurons are located. In vivo studies suggest that AMPH sensitization requires enhanced expression of basic fibroblast growth factor (bFGF) in the nucleus of midbrain astrocytes. One idea is that the AMPH-induced increase in bFGF expression in astrocytes leads to enhanced secretion of this peptide and to long-term plasticity in DA neurons. To study directly the effects of astrocytic expression of bFGF on DA neurons, we established a cell-culture model of mesencephalic astrocytes and DA neurons. Immunolabelling showed that even in the absence of a pharmacological stimulus, the majority of mesencephalic astrocytes in culture express bFGF at a nuclear level. Arguing against the idea that bFGF was secreted, bFGF was undetectable in the extracellular medium (below 10 pg/mL). However, supplementing culture medium with exogenous bFGF at standard concentrations (20 ng/mL) led to a dramatic change in the morphology of astrocytes, increased spontaneous DA release, and inhibited synapse formation by individual DA neurons. RNA interference (siRNA) against bFGF mRNA, caused a reduction in DA release but produced no change in synaptic development. Together these data demonstrate that under basal conditions (in the absence of a pharmacological stimulus such as amphetamine) bFGF is not secreted even though there is abundant nuclear expression in astrocytes. The effects of bFGF seen here on DA neurons are thus likely to be mediated through more indirect glial-neuronal interactions, leading to enhanced DA release without a necessary change in synapse number.

Animals↗

Basal somatodendritic dopamine release requires snare proteins.

Dopaminergic neurons have the capacity to release dopamine not only from their axon terminals, but also from their somatodendritic compartment. The actual mechanism of somatodendritic dopamine release has remained controversial. Here we established for the first time a rat primary neuron culture model to investigate this phenomenon and use it to study the mechanism under conditions of non-stimulated spontaneous firing (1-2 Hz). We found that we can selectively measure somatodendritic dopamine release by lowering extracellular calcium to 0.5 mm, thus confirming the previously established differential calcium sensitivity of somatodendritic and terminal release. Dopamine release measured under these conditions was dependent on firing activity and independent of reverse transport through the plasma membrane. We found that treatment with botulinum neurotoxins A and B strongly reduced somatodendritic dopamine release, thus demonstrating the requirement for SNARE proteins SNAP-25 and synaptobrevin. Our work is the first to provide such direct and unambiguous evidence for the involvement of an exocytotic mechanism in basal spontaneous somatodendritic dopamine release.

Action Potentials↗

The role of neurotensin in central nervous system pathophysiology: what is the evidence?

The peptide neurotensin has been studied for more than 30 years. Although it is widely distributed in the central and peripheral nervous systems, neurotensin has been more intensely studied with regard to its interactions with the central dopamine system. A number of claims have been made regarding its possible implication in many diseases of the central nervous system, including schizophrenia. In this review, we describe briefly the basic biology of this neuropeptide, and then we consider the strengths and the weaknesses of the data that suggest a role for neurotensin in schizophrenia, drug abuse, Parkinson's disease, pain, central control of blood pressure, eating disorders, cancer, neurodegenerative disorders and inflammation.

Animals↗

The 21-aminosteroid U74389G prevents the down-regulation and decrease in activity of CYP1A1, 1A2 and 3A6 induced by an inflammatory reaction.

In vivo, the 21-aminosteroid U74389G prevents the decrease in cytochrome P450 (P450) activity produced by a turpentine-induced inflammatory reaction (TIIR). To investigate the underlying mechanism of action, four groups of rabbits were used, controls receiving or not U74389G, and rabbits with the inflammatory reaction receiving or not U74389G. Hepatocytes were isolated 48h later and incubated for 4 and 24h with the serum of the rabbits. In vivo, the TIIR diminished CYP1A1/2 and 3A6 expression, and enhanced hepatic malondialdehyde (MDA) and nitric oxide (NO*) concentrations (p<0.05). U74389G prevented the increase in MDA, as well as the decrease in CYP1A1/2 amounts and activity, but increased CYP3A6 expression by 40% (p<0.05). In vitro, compared with serum from control rabbits (S(CONT)), incubation of serum from rabbits with TIIR (S(TIIR)) for 4 and 24h with hepatocytes from rabbits with TIIR (H(TIIR)) reduced CYP1A2 and CYP3A6 activity (p<0.05) and increased the formation of NO* and MDA. In rabbits with TIIR pretreated with U74389G, the S(TIIR+U) failed to reduce CYP1A2 activity or to increase MDA, although increased NO* and further reduced CYP3A6 activity. On the other hand, in hepatocytes harvested from rabbits with TIIR pretreated with U74389G, S(TIIR) did not decrease CYP1A2 activity and did not enhance MDA, but still increased NO*. In vitro, the reduction of CYP1A2 and CYP3A6 activity by S(TIIR) is not associated to NF-kappaB activation. In conclusion, U74389G prevents CYP1A1/2 down-regulation and decrease in activity by a double mechanism: hindering the release of serum mediators and by averting intracellular events, effect possibly associated with its antioxidant activity. On the other hand, U74389G up-regulates CYP3A6 but inhibits its catalytic activity.

Animals↗

Nestin-expressing neural stem cells identified in the scar following myocardial infarction.

Nerve fiber innervation of the scar following myocardial damage may have occurred either via the growth of pre-existing fibers and/or the mobilization of neural stem cells. The present study examined whether neural stem cells were recruited to the infarct region of the rat heart following coronary artery ligation. The neural stem cell marker nestin was detected in the infarct region of 1-week post-myocardial infarct (MI) male rats and cultured scar-derived neural-like cells. By contrast, nestin staining was undetected in either scar myofibroblasts or cardiac myocytes residing in the non-infarcted left ventricle. Reactive astrocytes were isolated from the infarct region and characterized by the co-expression of nestin, glial fibrillary acidic protein, and vimentin. Specific staining of oligodendrocytes and neurons was also detected in the infarct region and cultured scar-derived neural-like cells. Furthermore, neurofilament-M positive fibers were identified in the scar and tyrosine hydroxylase immunoreactivity was observed in peripherin-positive neurons. Neurite formation was induced in PC12 cells treated with the conditioned-media of primary passage scar-derived cells, highlighting the synthesis and secretion of neurotrophic factors. Nerve growth factor (NGF) and brain-derived neurotrophic factor were detected in myofibroblasts and neural cells, and both cell types expressed the NGF receptors trkA and p75. These data highlight the novel observation that neural stem cells were recruited to the infarct region of the damaged rat heart and may contribute in part to nerve fiber growth and subsequent innervation of the scar.

Animals↗

M3-like muscarinic receptors mediate Ca2+ influx in rat mesencephalic GABAergic neurones through a protein kinase C-dependent mechanism.

GABAergic neurones in the mesencephalon are important regulators of dopamine neurones. Cholinergic projections from mesopontine nuclei preferentially synapse onto these GABAergic neurones, thus suggesting that ACh can regulate dopamine neurones indirectly by modulating GABAergic interneurones. Muscarinic receptors mediate excitation of these interneurones through a Ca(2+)-dependent mechanism. Using a mesencephalic primary culture model, we show here that muscarine (10 microM) increases intracellular Ca2+ concentrations ([Ca2+]i) in GABAergic interneurones. Compatible with previous anatomical data, our pharmacological studies further suggest that the M3 receptor is the primary mediator of this increase. The rise in [Ca2+]i induced by muscarine was not activity-dependent but required influx of Ca2+ from the extracellular medium. Consistent with the known coupling of the M3 receptor to PKC, the effect of muscarine was blocked by bisindolylmaleimide, a selective PKC antagonist. The effect of muscarine was inhibited by SKF 96365 and verapamil, drugs known to block non-selective cationic channels such as those formed by transient receptor potential (TRPC) proteins. Finally, GABAergic neurones were found to be immunopositive for TRPC1, 3, 5 and 6. Taken together, these results suggest that the Ca(2+)-dependent regulation of mesencephalic GABAergic neurones by muscarinic receptors requires activation of some receptor-operated Ca2+ channels through a PKC-dependent mechanism.

Analysis of Variance↗

Normal biogenesis and cycling of empty synaptic vesicles in dopamine neurons of vesicular monoamine transporter 2 knockout mice.

The neuronal isoform of vesicular monoamine transporter, VMAT2, is responsible for packaging dopamine and other monoamines into synaptic vesicles and thereby plays an essential role in dopamine neurotransmission. Dopamine neurons in mice lacking VMAT2 are unable to store or release dopamine from their synaptic vesicles. To determine how VMAT2-mediated filling influences synaptic vesicle morphology and function, we examined dopamine terminals from VMAT2 knockout mice. In contrast to the abnormalities reported in glutamatergic terminals of mice lacking VGLUT1, the corresponding vesicular transporter for glutamate, we found that the ultrastructure of dopamine terminals and synaptic vesicles in VMAT2 knockout mice were indistinguishable from wild type. Using the activity-dependent dyes FM1-43 and FM2-10, we also found that synaptic vesicles in dopamine neurons lacking VMAT2 undergo endocytosis and exocytosis with kinetics identical to those seen in wild-type neurons. Together, these results demonstrate that dopamine synaptic vesicle biogenesis and cycling are independent of vesicle filling with transmitter. By demonstrating that such empty synaptic vesicles can cycle at the nerve terminal, our study suggests that physiological changes in VMAT2 levels or trafficking at the synapse may regulate dopamine release by altering the ratio of fillable-to-empty synaptic vesicles, as both continue to cycle in response to neural activity.

Animals↗

Role of calcium in neurotensin-evoked enhancement in firing in mesencephalic dopamine neurons.

Neurotensin (NT) increases neurotransmission within the mesolimbic dopamine system by enhancing the firing rate of dopaminergic (DAergic) neurons and by acting at the nerve terminal level. The signal transduction pathways involved in these effects have not been characterized, but NT receptors are coupled to the phospholipase C pathway and Ca(2+) mobilization. However, an enhancement of intracellular Ca(2+) concentration ([Ca(2+)](i)) evoked by NT in DAergic neurons has yet to be demonstrated. Furthermore, the hypothesis that the excitatory effects of NT in DAergic neurons are Ca(2+) dependent is currently untested. In whole-cell recording experiments, DAergic neurons in culture were identified by their selective ability to express a cell-specific green fluorescent protein reporter construct. These experiments confirmed that NT increases firing rate in cultured DAergic neurons. This effect was Ca(2+) dependent because it was blocked by intracellular dialysis with BAPTA. Using Ca(2+) imaging, we showed that NT caused a rapid increase in [Ca(2+)](i) in DAergic neurons. Most of the Ca(2+) originated from the extracellular medium. NT-induced excitation and Ca(2+) influx were blocked by SR48692, an antagonist of the type 1 NT receptor. Blocking IP(3) receptors using heparin prevented the excitatory effect of NT. Moreover, Zn(2+) and SKF96365 both blocked the excitatory effect of NT, suggesting that nonselective cationic conductances are involved. Finally, although NT can also induce a rise in [Ca(2+)](i) in astrocytes, we find that NT-evoked excitation of DAergic neurons can occur independently of astrocyte activation.

Animals↗

Regulation of rat mesencephalic GABAergic neurones through muscarinic receptors.

Central dopamine neurones are involved in regulating cognitive and motor processes. Most of these neurones are located in the ventral mesencephalon where they receive abundant intrinsic and extrinsic GABAergic input. Cholinergic neurones, originating from mesopontine nuclei, project profusely in the mesencephalon where they preferentially synapse onto local GABAergic neurones. The physiological role of this cholinergic innervation of GABAergic neurones remains to be determined, but these observations raise the hypothesis that ACh may regulate dopamine neurones indirectly through GABAergic interneurones. Using a mesencephalic primary culture model, we studied the impact of cholinergic agonists on mesencephalic GABAergic neurones. ACh increased the frequency of spontaneous IPSCs (151 +/- 49%). Selective activation of muscarinic receptors increased the firing rate of isolated GABAergic neurones by 67 +/- 13%. The enhancement in firing rate was Ca(2+) dependent since inclusion of BAPTA in the pipette blocked it, actually revealing a decrease in firing rate accompanied by membrane hyperpolarization. This inhibitory action was prevented by tertiapin, a blocker of GIRK-type K(+) channels. In addition to its excitatory somatodendritic effect, activation of muscarinic receptors also acted presynaptically, inhibiting the amplitude of unitary GABAergic synaptic currents. Both the enhancement in spontaneous IPSC frequency and presynaptic inhibition were abolished by 4-DAMP (100 nm), a preferential M3 muscarinic receptor antagonist. The presence of M3-like receptors on mesencephalic GABAergic neurones was confirmed by immunocytochemistry. Taken together, these results demonstrate that mesencephalic GABAergic neurones can be regulated directly through muscarinic receptors. Our findings provide new data that should be helpful in better understanding the influence of local GABAergic neurones during cholinergic activation of mesencephalic circuits.

Acetylcholine↗

Postsynaptic injection of calcium-independent phospholipase A2 inhibitors selectively increases AMPA receptor-mediated synaptic transmission.

The calcium-independent form of phospholipase A2 (iPLA2), an enzyme known to generate arachidonic acid (AA), was recently identified as the predominant constitutive phospholipase in the hippocampus. The present study shows that the iPLA2 inhibitor bromoenol lactone, when introduced into hippocampal CA1 pyramidal cells through a patch pipette, generated a dose-dependent increase in the amplitude of alpha-amino-3-hydroxy-5-methylisoxazole-4-propionate (AMPA) receptor-mediated excitatory postsynaptic currents (EPSCs). The iPLA2 inhibitor by itself interfered with neither paired pulse facilitation nor N-methyl-D-aspartate (NMDA) receptor-mediated EPSCs, suggesting that its influence on synaptic transmission is postsynaptic in origin and specific to the AMPA subtype of glutamate receptors. Comparable results were obtained with palmitoyl trifluoromethyl ketone, a second structurally distinct iPLA2 inhibitor. The ability of iPLA2 inhibitors to increase AMPA receptor-mediated currents was also reproduced by MK-866, an inhibitor recognized to interfere with the generation of 5-lipoxygenase by-products of AA. At the biochemical level, we found that AMPA, but not NMDA glutamate receptor subunits, were upregulated in rat brain sections pre-incubated with the iPLA2 inhibitors. Collectively, these results provide the first experimental evidence that constitutive iPLA2 and/or its metabolites play an important role in the postsynaptic modulation of neurotransmission in CA1 pyramidal cells of the hippocampus.

Animals↗

Dopamine neurons in culture express VGLUT2 explaining their capacity to release glutamate at synapses in addition to dopamine.

Dopamine neurons have been suggested to use glutamate as a cotransmitter. To identify the basis of such a phenotype, we have examined the expression of the three recently identified vesicular glutamate transporters (VGLUT1-3) in postnatal rat dopamine neurons in culture. We found that the majority of isolated dopamine neurons express VGLUT2, but not VGLUT1 or 3. In comparison, serotonin neurons express only VGLUT3. Single-cell RT-PCR experiments confirmed the presence of VGLUT2 mRNA in dopamine neurons. Arguing for phenotypic heterogeneity among axon terminals, we find that only a proportion of terminals established by dopamine neurons are VGLUT2-positive. Taken together, our results provide a basis for the ability of dopamine neurons to release glutamate as a cotransmitter. A detailed analysis of the conditions under which DA neurons gain or loose a glutamatergic phenotype may provide novel insight into pathophysiological processes that underlie diseases such as schizophrenia, Parkinson's disease and drug dependence.

Amino Acid Transport Systems, Acidic↗

Glutamate co-transmission as an emerging concept in monoamine neuron function.

Converging research efforts over the last 4 decades have established beyond a doubt that many, if not most, neurons release more than 1 neurotransmitter. Although much attention has been paid to the co-release of small-molecule neurotransmitters with neuropeptides, a number of examples of co-release of 2 small-molecule neurotransmitters have now been described. It has been suggested recently that monoamine neurons use glutamate as a co-transmitter. First, both serotonin (5-HT) and dopamine (DA) neurons in culture establish functional glutamatergic synapses in addition to classic terminals that release 5-HT or DA. Second, immunocytochemical work has provided evidence for the presence of neurotransmitter pools of glutamate in DA, 5-HT and noradrenergic neurons. Third, the recent cloning of 3 vesicular glutamate transporters (VGLUT1-3) has led to the discovery that noradrenergic neurons contain VGLUT2 mRNA, whereas 5-HT neurons contain VGLUT3 mRNA. Finally, although VGLUT2 mRNA does not appear to be abundant in DA neurons in the adult brain, DA neurons cultured from neonatal animals express VGLUT2, suggesting that these neurons may have the capacity to express this protein under specific conditions. Taken together with recent work describing the capacity of neurons to change neurotransmitter phenotype during development or in an activity-dependent manner, the finding of glutamate co-transmission in monoamine neurons may lead to significant revisions of current physiologic models of monoamine neuron function. In addition, the possible role of glutamate co-release in physiopathologic models of diseases that implicate central monoamine pathways, such as schizophrenia, must now be seriously considered.

Biogenic Monoamines↗

Perturbation of synaptic vesicle delivery during neurotransmitter release triggered independently of calcium influx.

Although much evidence suggests that calcium (Ca(2+)) usually triggers synaptic vesicle exocytosis and neurotransmitter release, the role of Ca(2+) in vesicle endocytosis and in the delivery of fusion-competent vesicles (i.e. mobilisation and/or priming) in nerve terminals remains unclear. To address this issue, we have studied synaptic vesicle dynamics in cultured rat neurones under conditions where neurotransmitter release is triggered independently of Ca(2+) using the secretagogue Ruthenium Red (RR). Using a prolonged stimulation protocol, we find that RR causes a rapid increase in neurotransmitter release followed by a gradually decrementing response. In contrast, when release is triggered by moderate membrane depolarisation caused by saline containing 18 mM K(+), release is sustained. These observations suggest that when release is triggered independently of a rise in Ca(2+), endocytosis or vesicle mobilisation/priming are perturbed. Using FM2-10, a fluorescent indicator of synaptic vesicle cycling, we find that neurotransmitter release triggered by RR is accompanied by both uptake and release of this dye, thereby suggesting that vesicle endocytosis is not blocked. To evaluate whether synaptic vesicle mobilisation/priming is perturbed in the absence of a rise in Ca(2+), we compared the kinetics of FM2-10 loss during prolonged stimulation. While 18 mM K(+) induced gradual and continuous dye loss, RR only induced substantial dye loss during the first minute of stimulation. In the presence of low concentrations of the Ca(2+) ionophore ionomycin, release caused by RR was prolonged. Taken together, these results provide evidence suggesting that, although a rise in intraterminal Ca(2+) is not required for endocytosis, it is essential for the continuous delivery of fusion-competent vesicles and to maintain neurotransmitter release during prolonged stimulation.

Animals↗

Presynaptic mu-opioid receptors regulate a late step of the secretory process in rat ventral tegmental area GABAergic neurons.

Gamma-aminobutyric acid (GABA)-containing interneurons of the ventral tegmental area (VTA) regulate the activity of dopaminergic neurons. These GABAergic interneurons are known to be innervated by synaptic terminals containing enkephalin, an endogenous ligand of mu-opioid receptors. Bath application of mu-opioid receptor agonists inhibits the activity of VTA GABAergic neurons but the mechanism whereby mu-opioid receptors regulate synaptic GABA release from these neurons has not been directly identified. Using cultured VTA neurons we have confirmed that mu-opioid receptor agonists inhibit synaptic GABA release. DAMGO, a selective mu-opioid receptor agonist, had four distinct effects on GABAergic IPSCs: (1) it inhibited the frequency and amplitude of spontaneous IPSCs (sIPSCs), (2) it reduced the amplitude of IPSCs evoked by single action potentials, (3) it inhibited the frequency, but not the amplitude of miniature IPSCs (mIPSCs), and (4) DAMGO inhibited mIPSCs evoked by ionomycin, a Ca(2+) ionophore. The inhibition of action potential-evoked IPSCs and of spontaneous and ionomycin-evoked mIPSCs by DAMGO was prevented by the K(+) channel blocker, 4-aminopyridine (4-AP). In conclusion, our work shows that one of the mechanisms through which mu-opioid receptors inhibit GABA release by VTA neurons is through inhibition of the secretory process at the nerve terminal level. In addition, considering that ionomycin stimulates exocytosis through a mechanism that should be insensitive to membrane polarization, our experiments with 4-AP suggest that K(+) channels are implicated in the inhibition of the efficacy of the secretory process by mu-opioid receptors.

Animals↗

Beta-lactamase protein fragment complementation assays as in vivo and in vitro sensors of protein protein interactions.

We have previously described a strategy for detecting protein protein interactions based on protein interaction assisted folding of rationally designed fragments of enzymes. We call this strategy the protein fragment complementation assay (PCA). Here we describe PCAs based on the enzyme TEM-1 beta-lactamase (EC: 3.5.2.6), which include simple colorimetric in vitro assays using the cephalosporin nitrocefin and assays in intact cells using the fluorescent substrate CCF2/AM (ref. 6). Constitutive protein protein interactions of the GCN4 leucine zippers and of apoptotic proteins Bcl2 and Bad, and the homodimerization of Smad3, were tested in an in vitro assay using cell lysates. With the same in vitro assay, we also demonstrate interactions of protein kinase PKB with substrate Bad. The in vitro assay is facile and amenable to high-throughput modes of screening with signal-to-background ratios in the range of 10:1 to 250:1, which is superior to other PCAs developed to date. Furthermore, we show that the in vitro assay can be used for quantitative analysis of a small molecule induced protein interaction, the rapamycin-induced interaction of FKBP and yeast FRB (the FKBP-rapamycin binding domain of TOR (target of rapamycin)). The assay reproduces the known dissociation constant and number of sites for this interaction. The combination of in vitro colorimetric and in vivo fluorescence assays of beta-lactamase in mammalian cells suggests a wide variety of sensitive and high-throughput large-scale applications, including in vitro protein array analysis of protein protein or enzyme protein interactions and in vivo applications such as clonal selection for cells expressing interacting protein partners.

3-Phosphoinositide-Dependent Protein Kinases↗