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SUMMARYRegulation of synaptic neurotransmitter receptor content is a fundamental mechanism for tuning synaptic efficacy during experience-dependent plasticity and behavioral adaptation. However, experimental approaches to track and modify receptor movements in integrated experimental systems are limited. Exploiting AMPA-type glutamate receptors (AMPAR) as a model, we generated a knock-in mouse expressing the biotin acceptor peptide (AP) tag on the GluA2 extracellular N-terminus. Cell-specific introduction of biotin ligase allows the use of monovalent or tetravalent avidin variants to respectively monitor or manipulate the surface mobility of endogenous AMPAR containing biotinylated AP-GluA2 in neuronal subsets. AMPAR immobilization precluded the expression of long-term potentiation and formation of contextual fear memory, allowing for target-specific control of the expression of synaptic plasticity and animal behavior. The AP tag knock-in model offers unprecedented access to resolve and control the spatiotemporal dynamics of endogenous receptors, and opens new avenues to study the molecular mechanisms of synaptic plasticity and learning.

Many neurons concurrently and/or differentially release multiple neurotransmitter substances to selectively modulate the activity of distinct postsynaptic targets within a network. However, the molecular mechanisms that produce synaptic heterogeneity by regulating the cotransmitter release characteristics of individual presynaptic terminals remain poorly defined. In particular, we know little about the regulation of neuropeptide corelease, despite the fact that they mediate synaptic transmission, plasticity and neuromodulation. Here, we report that an identified Lymnaea neuron selectively releases its classical small molecule and peptide neurotransmitters, acetylcholine and FMRFamide-derived neuropeptides, to differentially influence the activity of distinct postsynaptic targets that coordinate cardiorespiratory behaviour. Using a combination of electrophysiological, molecular, and pharmacological approaches, we found that neuropeptide cotransmitter release was regulated by cross-talk between extrinsic neurotrophic factor signaling and target-specific retrograde arachidonic acid signaling, which converged on modulation of glycogen synthase kinase 3. In this context, we identified a novel role for the Lymnaea synaptophysin homologue as a specific and synapse-delimited inhibitory regulator of peptide neurotransmitter release. This study is among the first to define the cellular and molecular mechanisms underlying the differential release of cotransmitter substances from individual presynaptic terminals, which allow for context-dependent tuning and plasticity of the synaptic networks underlying patterned motor behaviour.

In the central nervous system (CNS), cholinergic transmission induces synaptic plasticity that is required for learning and memory. However, our understanding of the development and maintenance of cholinergic circuits is limited, as the factors regulating the expression and clustering of neuronal nicotinic acetylcholine receptors (nAChRs) remain poorly defined. Recent studies from our group have implicated calpain-dependent proteolytic fragments of menin, the product of the MEN1 tumor suppressor gene, in coordinating the transcription and synaptic clustering of nAChRs in invertebrate central neurons. Here, we sought to determine whether an analogous cholinergic mechanism underlies menin’s synaptogenic function in the vertebrate CNS. Our data from mouse primary hippocampal cultures demonstrate that menin and its calpain-dependent C-terminal fragment (C-menin) regulate the subunit-specific transcription and synaptic clustering of neuronal nAChRs, respectively. MEN1 knockdown decreased nAChR α5 subunit expression, the clustering of α7 subunit-containing nAChRs at glutamatergic presynaptic terminals, and nicotine-induced presynaptic facilitation. Moreover, the number and function of glutamatergic synapses was unaffected by MEN1 knockdown, indicating that the synaptogenic actions of menin are specific to cholinergic regulation. Taken together, our results suggest that the influence of menin on synapse formation and synaptic plasticity occur via modulation of nAChR channel subunit composition and functional clustering.

International audience; Lattice light sheet (LLS) fluorescence microscopy is a powerful recent technique for in vivo imaging of single and multi-cellular samples at very high spatio-temporal resolutions. We built a LLS microscope in which we added a photo-stimulation path to perform all-optical neurophysiological studies in rodent hippocampal brain slices. Thanks to the photo-stimulation path we could achieve fluorescence recovery after photobleaching (FRAP) or glutamate uncaging at spatially and temporally controlled regions of interest. Several fluorescence labelling protocols were employed depending on the imaged structure. Sub-micrometric neuronal elements such as spines or dendritic vesicles could be imaged down to ~20 µm below the surface. We demonstrate the performances of LLS in several ongoing studies: measurement of AMPA receptor surface diffusion at single spines, vesicular transport in dendrites, spontaneous and stimulated local calcium activity in neurons and astrocytes.

All functions of the nervous system are contingent upon the precise organization of neuronal connections that are initially patterned during development, and then continually modified throughout life. Determining the mechanisms that specify the formation and functional modulation of synaptic circuitry are critical to advancing both our fundamental understanding of the nervous system as well as the various neurodevelopmental, neurological, neuropsychiatric, and neurodegenerative disorders that are met in clinical practice when these processes go awry. Defining the cellular and molecular mechanisms underlying nervous system development, function, and pathology has proven challenging, due mainly to the complexity of the vertebrate brain. Simple model system approaches with invertebrate preparations, on the other hand, have played pivotal roles in elucidating the fundamental mechanisms underlying the formation and plasticity of individual synapses, and the contributions of individual neurons and their synaptic connections that underlie a variety of behaviors, and learning and memory. In this Review, we discuss the experimental utility of the invertebrate mollusc Lymnaea stagnalis, with a particular emphasis on in vitro cell culture, semi-intact and in vivo preparations, which enable molecular and electrophysiological identification of the cellular and molecular mechanisms governing the formation, plasticity, and specificity of individual synapses at a single-neuron or single-synapse resolution.

Synapse formation and plasticity depend on nuclear transcription and site-specific protein targeting, but the molecular mechanisms that coordinate these steps have not been well defined. The MEN1 tumor suppressor gene, which encodes the protein menin, is known to induce synapse formation and plasticity in the CNS. This synaptogenic function has been conserved across evolution, however the underlying molecular mechanisms remain unidentified. Here, using central neurons from the invertebrate Lymnaea stagnalis, we demonstrate that menin coordinates subunit-specific transcriptional regulation and synaptic clustering of nicotinic acetylcholine receptors (nAChR) during neurotrophic factor (NTF)-dependent excitatory synaptogenesis, via two proteolytic fragments generated by calpain cleavage. Whereas menin is largely regarded as a nuclear protein, our data demonstrate a novel cytoplasmic function at central synapses. Furthermore, this study identifies a novel synaptogenic mechanism in which a single gene product coordinates the nuclear transcription and postsynaptic targeting of neurotransmitter receptors through distinct molecular functions of differentially localized proteolytic fragments.

Depression is a debilitating mental disorder, and selective serotonin reuptake inhibitors (SSRIs) constitute the first-line antidepressant treatment choice for the clinical management of this illness; however, the mechanisms underlying their therapeutic actions and side effects remain poorly understood. Here, we compared the effects of two SSRIs, fluoxetine and citalopram, on synaptic connectivity and the efficacy of cholinergic synaptic transmission between identified presynaptic and postsynaptic neurons from the mollusc Lymnaea. The in vitro paired cells were exposed to clinically relevant concentrations of the two SSRIs under chronic and acute experimental conditions, and the incidence of synapse formation and the efficacy of synaptic transmission were tested electrophysiologically and with fluorescent Ca(2+) imaging. We demonstrate that chronic exposure to fluoxetine, but not to citalopram, inhibits synapse formation and reduces synaptic strength, and that these effects are reversible following prolonged drug washout. At the structural level, we demonstrate that fluoxetine, but not citalopram, prevents the expression and localization of the presynaptic protein synaptophysin. Acute exposure to fluoxetine substantially reduced synaptic transmission and synaptic plasticity (post-tetanic potentiation) in established synapses, whereas citalopram reduced synaptic transmission, but not short-term synaptic plasticity. We further demonstrate that fluoxetine, but not citalopram, directly inhibits voltage-gated Ca(2+) currents in the presynaptic neuron, as well as postsynaptic responsiveness to exogenously applied neurotransmitter. This study provides the first direct evidence that fluoxetine and citalopram exert characteristic, non-specific side effects that are unrelated to their function as SSRIs, and that fluoxetine is more detrimental to synaptic physiology and structure than citalopram.

Current treatment regimes for a variety of mental disorders involve various selective serotonin reuptake inhibitors such as Fluoxetine (Prozac). Although these drugs may 'manage' the patient better, there has not been a significant change in the treatment paradigm over the years and neither have the outcomes improved. There is also considerable debate as to the effectiveness of various selective serotonin reuptake inhibitors and their potential side-effects on neuronal architecture and function. In this study, using mammalian cortical neurons, a dorsal root ganglia cell line (F11 cells) and identified Lymnaea stagnalis neurons, we provide the first direct and unequivocal evidence that clinically relevant concentrations of Fluoxetine induce growth cone collapse and neurite retraction of both serotonergic and non-serotonergic neurons alike in a dose-dependent manner. Using intracellular recordings and calcium imaging techniques, we further demonstrate that the mechanism underlying Fluoxetine-induced effects on neurite retraction from Lymnaea neurons may involve lowering of intracellular calcium and a subsequent retardation of growth cone cytoskeleton. Using soma-soma synapses between identified presynaptic and postsynaptic Lymnaea neurons, we provide further direct evidence that clinically used concentrations of Fluoxetine also block synaptic transmission and synapse formation between cholinergic neurons. Our study raises alarms over potentially devastating side-effects of this antidepressant drug on neurite outgrowth and synapse formation in a developing/regenerating brain. Our data also demonstrate that drugs such as Fluoxetine may not just affect communication between serotonergic neurons but that the detrimental effects are widespread and involve neurons of various phenotypes from both vertebrate and invertebrate species.

Major depressive disorder (MDD) is a debilitating and often recurrent psychiatric illness that impacts the lives of millions. Approximately 20 percent of individuals will experience at least one depressive episode during their lives, and the high rates of chronic morbidity (depressive episodes typically last for several months) and mortality (owing to suicide) attributed to this disorder constitute one of the largest global economic disease burdens (Kessler et al., 2005). Despite the considerable burden of depression, our understanding of its pathophysiology and the therapeutic mechanisms of action of antidepressant drugs remains incomplete. In fact, much of what we currently understand about the neuropathology of depression is derived from the observed effects of known antidepressants on the brain. This situation is confounded by the fact that currently available antidepressant drugs are of limited clinical efficacy – the antidepressant response typically takes several weeks to develop, and a substantial proportion of patients do not experience a significant improvement in depressive symptoms, or full clinical remission. Furthermore, because antidepressant efficacy varies significantly as a function of symptom severity, the benefit of antidepressant therapy may be limited or nonexistent in patients with mild to moderate depressive symptoms, which represent the majority of clinical cases (Fournier et al., 2010). When the inadequate efficacy profiles of antidepressant drugs are weighed against our limited understanding of depression, the therapeutic action of antidepressants, and the potential negative side effects of these drugs arising from nonspecific actions, a difficult question arises: do the benefits of antidepressant pharmacotherapy outweigh the risks? In this chapter, we explore the development of the current theories of depression etiology, the hindrances of current antidepressant therapeutics, and how the poor understanding of the mechanism of antidepressant action contributes to a haphazard interpretation of the benefits and consequences arising from their non-specific interactions. Through the use of a simple single-synapse experimental approach, we have gained insights into the impact of non-specific antidepressant actions on neuronal function, and our findings will be discussed in this context. Finally, we will explore prospective developments in the field of depression pharmacotherapy that may hold the potential to revolutionize the way we understand and treat depression.