Presynaptic Connections Research Articles

Cell Type-Specific Circuit Mapping Reveals the Presynaptic Connectivity of Developing Cortical Circuits.

Mapping cortical connectivity in the developing mammalian brain has been an intractable problem, in part because it has not been possible to analyze connectivity with cell subtype precision. Our study systematically targets the presynaptic connections of discrete neuronal subtypes in both the mature and developing cerebral cortex. We analyzed the connections that Cajal-Retzius cells make and receive, and found that these cells receive inputs from deeper-layer excitatory neurons and inhibitory interneurons in the first postnatal week. We assessed the inputs onto inhibitory interneurons and excitatory projection neurons, the major two types of neurons in the cortex, and found that excitatory inputs are refined by the fourth week of development, whereas local inhibitory inputs increase during this postnatal period.

Effects of homeostatic constraints on associative memory storage and synaptic connectivity of cortical circuits.

The impact of learning and long-term memory storage on synaptic connectivity is not completely understood. In this study, we examine the effects of associative learning on synaptic connectivity in adult cortical circuits by hypothesizing that these circuits function in a steady-state, in which the memory capacity of a circuit is maximal and learning must be accompanied by forgetting. Steady-state circuits should be characterized by unique connectivity features. To uncover such features we developed a biologically constrained, exactly solvable model of associative memory storage. The model is applicable to networks of multiple excitatory and inhibitory neuron classes and can account for homeostatic constraints on the number and the overall weight of functional connections received by each neuron. The results show that in spite of a large number of neuron classes, functional connections between potentially connected cells are realized with less than 50% probability if the presynaptic cell is excitatory and generally a much greater probability if it is inhibitory. We also find that constraining the overall weight of presynaptic connections leads to Gaussian connection weight distributions that are truncated at zero. In contrast, constraining the total number of functional presynaptic connections leads to non-Gaussian distributions, in which weak connections are absent. These theoretical predictions are compared with a large dataset of published experimental studies reporting amplitudes of unitary postsynaptic potentials and probabilities of connections between various classes of excitatory and inhibitory neurons in the cerebellum, neocortex, and hippocampus.

Experience-dependent plasticity of adult-born neuron connectivity

In contrast to most areas of the adult brain, the dentate gyrus (DG) of the hippocampus is endowed with the capability to generate new neurons life-long. While recent evidence suggests that these adult-born neurons exert specialized functions in information processing compared to pre-existing DG granule neurons, to which extent the establishment of their evolving connectivity may be regulated by experience has been elusive. We recently demonstrated that environmental enrichment (EE) induces a surprising input-specific reorganization of the presynaptic connectivity of adult-born neurons, and that this form of structural plasticity appears to large degree confined to a defined period of few weeks shortly after their generation. Here, I briefly discuss how these findings may uncover a previously unknown layer of complexity in the processes regulating the synaptic integration of adult-born neurons and propose that their circuit incorporation within the pre-existing hippocampal network is not prefigured but rather modulated by specific experiences.

Altered synapses in a Drosophila model of Alzheimer's disease

Alzheimer’s disease (AD) is an age-related neurodegenerative disease characterized by memory loss and decreased synaptic function. Advances in transgenic animal models of AD have facilitated our understanding of this disorder, and have aided in the development, speed and efficiency of testing potential therapeutics. Recently, we have described the characterization of a novel model of AD in the fruit fly, Drosophila melanogaster, where we expressed the human AD-associated proteins APP and BACE in the central nervous system of the fly. Here we describe synaptic defects in the larval neuromuscular junction (NMJ) in this model. Our results indicate that expression of human APP and BACE at the larval NMJ leads to defective larval locomotion behavior, decreased presynaptic connections, altered mitochondrial localization in presynaptic motor neurons and decreased postsynaptic protein levels. Treating larvae expressing APP and BACE with the γ-secretase inhibitor L-685,458 suppresses the behavioral defects as well as the pre- and postsynaptic defects. We suggest that this model will be useful to assess and model the synaptic dysfunction normally associated with AD, and will also serve as a powerful in vivo tool for rapid testing of potential therapeutics for AD.

Retrograde monosynaptic tracing reveals the temporal evolution of inputs onto new neurons in the adult dentate gyrus and olfactory bulb

Identifying the connectome of adult-generated neurons is essential for understanding how the preexisting circuitry is refined by neurogenesis. Changes in the pattern of connectivity are likely to control the differentiation process of newly generated neurons and exert an important influence on their unique capacity to contribute to information processing. Using a monosynaptic rabies virus-based tracing technique, we studied the evolving presynaptic connectivity of adult-generated neurons in the dentate gyrus (DG) of the hippocampus and olfactory bulb (OB) during the first weeks of their life. In both neurogenic zones, adult-generated neurons first receive local connections from multiple types of GABAergic interneurons before long-range projections become established, such as those originating from cortical areas. Interestingly, despite fundamental similarities in the overall pattern of evolution of presynaptic connectivity, there were notable differences with regard to the development of cortical projections: although DG granule neuron input originating from the entorhinal cortex could be traced starting only from 3 to 5 wk on, newly generated neurons in the OB received input from the anterior olfactory nucleus and piriform cortex already by the second week. This early glutamatergic input onto newly generated interneurons in the OB was matched in time by the equally early innervations of DG granule neurons by glutamatergic mossy cells. The development of connectivity revealed by our study may suggest common principles for incorporating newly generated neurons into a preexisting circuit.

Burst initiation and propagation in cortical cultures requires an inhomogeneous connectivity distribution and synaptic rescaling

Dissociated cortical cultures grown on microelectrode arrays (MEA) have been established as a useful biological model in the analysis of network dynamics. Their network dynamics are dominated by periods of strongly synchronized spiking, termed 'bursting', whose role is currently not understood. It has been demonstrated that bursts have different motifs and contain structure, refuting the possibility that they are merely chaotic activity. Of particular interest are the conditions required for bursting to initiate and propagate throughout the entire network. Within cultures, initiation sites can be well localized, even under varying experimental conditions such as cell density and network size. Propagation waves display fairly regular patterns of neuron recruitment within the network burst. However, in order to minimize bursting and promote closed-loop communication with the disassociated culture, it is of interest to understand what conditions are necessary for bursting to arise. Interestingly, the propagation of bursts has been observed to be faster than can be accounted for by only local connectivity. While paired intracellular recordings have revealed some clues as to the local structure and short range connectivity, they were unable to clarify the contribution of long-range connectivity of neurons to burst propagation. Additionally, pharmacological studies which result in freezing of synaptic plasticity and impaired cell migration have demonstrated that modifications to connectivity can greatly alter the pattern of burst propagation. As network structure has been established to strongly affect dynamics, we investigate the contribution of network features that can account for experimentally observed burst initiation and propagation patterns. Specifically, we examine the contribution of long-range connections within a 2D model network of spiking neurons representing a mature dissociated cortical culture. We previously demonstrated with networks of rate-based units that within clustered topologies, the relative number of long-range connection to cluster size greatly affects the robustness of the network to noise and ability to sustain activity [1], while including highly recurrent activity is required for fast ignition of activity from low-level background activity. Here, we extend these networks to a population of Integrate and Fire neurons and chart the effect of introducing inhomogeneities and a non-uniform distribution of connections, specifically the numbers and location of post-synaptic connections of each neuron. By driving the network with low levels of background activity, we observe the emergence of burst initiation sites and chart how different cluster configurations act to alter their location while simultaneously inspecting the velocity of burst propagation as it spreads throughout the network. Then, in keeping with recent evidence of synaptic reorganization in cortical cultures [2], we compensated the synaptic strength of neurons that received fewer presynaptic connections and noted how this increased the likelihood of inducing bursting. We compare burst profiles obtained in these simulations against their biological equivalents to identify the necessary topological features required.

Cortical representations of olfactory input by trans-synaptic tracing

In the mouse, each class of olfactory receptor neurons expressing a given odorant receptor has convergent axonal projections to two specific glomeruli in the olfactory bulb, thereby creating an odour map. However, it is unclear how this map is represented in the olfactory cortex. Here we combine rabies-virus-dependent retrograde mono-trans-synaptic labelling with genetics to control the location, number and type of 'starter' cortical neurons, from which we trace their presynaptic neurons. We find that individual cortical neurons receive input from multiple mitral cells representing broadly distributed glomeruli. Different cortical areas represent the olfactory bulb input differently. For example, the cortical amygdala preferentially receives dorsal olfactory bulb input, whereas the piriform cortex samples the whole olfactory bulb without obvious bias. These differences probably reflect different functions of these cortical areas in mediating innate odour preference or associative memory. The trans-synaptic labelling method described here should be widely applicable to mapping connections throughout the mouse nervous system.

Causal networks in the rat barrel cortex provide a signature of stimulus encoding

Identifying stimulus-dependent cortical circuits is a fundamental goal in systems neuroscience. Graph-theoretic techniques provide an elegant tool to infer such circuitry. Recently, we have proposed to use Dynamic Bayesian Networks (DBN) as an efficient graphical model that explains the observed spike train data in local cortical circuits. A unique advantage of DBNs over other metrics of neuronal connectivity (such as crosscorrelograms) is its ability to explain the temporally precise activity of an observed neuron taking into account the activity of the entire observed population. We recorded simultaneously the ensemble neural activity using 32-channel microelectrode arrays from Layer V of the primary somatosensory (barrel) cortex in 3 anesthetized rats. An average population size of 20±7 single units/rat was identified. Responses were recorded during stimulation of individual whiskers (3 whiskers/rat). Each whisker was horizontally deflected with a displacement of 80 μm for 100 ms at 1 Hz frequency. Spike trains within 100 ms of stimulus onset were analyzed using DBN. To rule out the existence of spurious connections inferred by co-modulation of firing rates, a jittered version of the spike trains was formed in which each spike was randomly jittered in a uniform interval of [-5,5] ms. Common connections inferred in the original and jittered data were eliminated. Results demonstrate that stable stimulus-driven network states were obtained, with 22% of the inferred connections appearing in more than 35% of the trials. To examine the validity of these results, we computed the correlation between the response latency of each neuron and the number of inferred pre-synaptic/post-synaptic connections for each stimulated whisker. In a causal network context, we hypothesized that pre-synaptic neurons that fire before other post-synaptic neurons in the observed population should have shorter response latency reminiscent of a “signaling mechanism” to other neurons about stimulus presence. Therefore, these neurons were expected to have more post-synaptic connections than their large response latency counterparts. Correlation between the response latency and the number of post-synaptic (pre-synaptic) connections was -0.63 (+0.61) consistent with our hypothesis. Finally, we used 90% of the trials to obtain a template network model for each whisker stimulated. We then decoded the spike trains in the remaining 10% of the trials to determine the corresponding whisker identity. A decoding accuracy of 97% was obtained indicating that the obtained causal networks are stimulus-dependent in which the population acts cooperatively rather than independently to encode the underlying stimulus.

Netrin Participates in the Development of Retinotectal Synaptic Connectivity by Modulating Axon Arborization and Synapse Formation in the Developing Brain

Netrin has been implicated in retinal ganglion cell (RGC) axon pathfinding in a number of species. In Xenopus laevis, RGC axons reaching their target in the optic tectum can be repelled by a netrin-1 gradient in vitro, suggesting that netrin may also function in wiring events that follow successful axon pathfinding. Here, we examined the contribution of netrin to RGC axon arborization and synapse formation at the target. Time-lapse confocal microscopy imaging of individual RGC axons coexpressing GFP-synaptobrevin and DsRed in the intact Xenopus brain demonstrated a role for deleted in colorectal cancer (DCC)-mediated netrin signaling. Microinjection of netrin-1 into the tectum induced a rapid and transient increase in presynaptic site addition that resulted in higher presynaptic site density over a 24 h observation period. Moreover, netrin induced dynamic axon branching, increasing branch addition and retraction; a behavior that ultimately increased total branch number. In contrast, microinjection of DCC function-blocking antibodies prevented the increase in presynaptic site number normally observed in control axons as well as the associated increase in branch number and axon arbor growth. Dynamic analysis of axon arbors demonstrated that the effects of anti-DCC on axon morphology and presynaptic connectivity were attributable to a specific decrease in new synapse and branch additions, without affecting the stability of existing synapses and branches. Together, these results indicate that, in the absence of DCC signaling, RGC axons fail to branch and differentiate, and support a novel role for netrin in later phases of retinotectal development.

Traitement des règles graduelles au moyen des systèmes hybrides neuro-symboliques

This work belongs to the field of hybrid systems for Artificial Intelligence (Al). It concerns the study of gradual rules, which makes it possible to represent correlations and modulation relations between variables. We propose a set of characteristics to identify these gradual rules, and a classification into direct rules and modulation rules. In neurobiology, pre-synaptic neuronal connections lead to gradual processing and modulation of cognitive information. We propose in the field of connectionism the use of Sigma-Pi connections to allow gradual processing in AI systems. In order to represent as well as possible the modulation processes between the inputs of a network, we have created a new type of connection, Asymmetric Sigma-Pi (ASP) units. These models have been implemented within a pre-existing hybrid neuro-symbolic system, the INSS system. The new hybrid system thus obtained, INSS-Gradual, allows the learning of bases of examples containing gradual modulation relations. ASP units facilitate the extraction of gradual rules from a neural network.

The GTPase dMiro Is Required for Axonal Transport of Mitochondria to Drosophila Synapses

We have identified EMS-induced mutations in Drosophila Miro (dMiro), an atypical mitochondrial GTPase that is orthologous to human Miro (hMiro). Mutant dmiro animals exhibit defects in locomotion and die prematurely. Mitochondria in dmiro mutant muscles and neurons are abnormally distributed. Instead of being transported into axons and dendrites, mitochondria accumulate in parallel rows in neuronal somata. Mutant neuromuscular junctions (NMJs) lack presynaptic mitochondria, but neurotransmitter release and acute Ca2+ buffering is only impaired during prolonged stimulation. Neuronal, but not muscular, expression of dMiro in dmiro mutants restored viability, transport of mitochondria to NMJs, the structure of synaptic boutons, the organization of presynaptic microtubules, and the size of postsynaptic muscles. In addition, gain of dMiro function causes an abnormal accumulation of mitochondria in distal synaptic boutons of NMJs. Together, our findings suggest that dMiro is required for controlling anterograde transport of mitochondria and their proper distribution within nerve terminals.

Magnetic stimulation of the central and peripheral nervous systems.

Since 1985, when the technique of transcranial magnetic stimulation (TMS) was first developed, a wide range of applications in healthy and diseased subjects has been described. Comprehension of the physiological basis of motor control and cortical function has been improved. Modifications of the basic technique of measuring central motor conduction time (CMCT) have included measurement of the cortical silent period, paired stimulation in a conditioning test paradigm, repetitive transcranial magnetic stimulation (rTMS), and peristimulus time histograms (PSTH). These methods allow dissection of central motor excitatory versus inhibitory interplay on the cortical motor neuron and its presynaptic connections at the spinal cord, and have proven to be powerful investigational techniques. TMS can be used to assess upper and lower motor neuron dysfunction, monitor the effects of many pharmacological agents, predict stroke outcome, document the plasticity of the motor system, and assess its maturation and the effects of aging, as well as perform intraoperative monitoring. The recent use of rTMS in the treatment of depression and movement disorders is novel, and opens the way for other potential therapeutic applications.

Correction for Shepherd et al., Three-Dimensional Structure and Composition of CA3→CA1 Axons in Rat Hippocampal Slices: Implications for Presynaptic Connectivity and Compartmentalization

Correction for Shepherd et al., Three-Dimensional Structure and Composition of CA3→CA1 Axons in Rat Hippocampal Slices: Implications for Presynaptic Connectivity and Compartmentalization

Three-Dimensional Structure and Composition of CA3→CA1 Axons in Rat Hippocampal Slices: Implications for Presynaptic Connectivity and Compartmentalization

Physiological studies of CA3-->CA1 synaptic transmission and plasticity have revealed both pre- and postsynaptic effects. Understanding the extent to which individual presynaptic axonal boutons could provide local compartments for control of synaptic efficacy and microconnectivity requires knowledge of their three-dimensional morphology and composition. In hippocampal slices, serial electron microscopy was used to examine a nearly homogeneous population of CA3-->CA1 axons in the middle of stratum radiatum of area CA1. The locations of postsynaptic densities (PSDs), vesicles, and mitochondria were determined along 75 axon segments (9.1 +/- 2.0 micrometer in length). Synapses, defined by the colocalization of PSDs and vesicles, occurred on average at 2.7 micrometer intervals along the axons. Most varicosities (68%) had one PSD, 19% had 2-4 PSDs, and 13% had none. Synaptic vesicles occurred in 90% of the varicosities. One-half (53%) of the varicosities lacked mitochondria, raising questions about their regulation of ATP and Ca2+, and 8% of varicosities contained only mitochondria. Eleven axons were reconstructed fully. The varicosities were oblong and varied greatly in both length (1.1 +/- 0.7 micrometer) and volume (0.13 +/- 0.14 micrometer 3), whereas the intervaricosity shafts were narrow, tubular, and similar in diameter (0.17 +/- 0.04 micrometer) but variable in length (1.4 +/- 1.2 micrometer). The narrow axonal shafts resemble dendritic spine necks and thus could promote biochemical compartmentalization of individual axonal varicosities. The findings raise the intriguing possibility of localized differences in metabolism and connectivity among different axons, varicosities, and synapses.

Foreign Connections are Formed In Vitro by Aplysia Californica Interneurone L10 and Its In Vivo Followers and Non-Followers

Aplysia californica interneurone L10 forms a set of presynaptic connections with many postsynaptic 'follower' cells in the abdominal ganglion. These followers do not connect back to L10. The present study tests whether the direction and sign of these connections are obligatory and are reconstructed when neuronal processes regenerate in vitro. L10 was co-cultured with one of six different followers and two non-followers. 1. In vitro connections that preserve the sign of those formed in vivo were made by L10 onto neurones L11, L12 and L13. The connections consisted of inhibitory postsynaptic potentials (IPSPs) with characteristic fast and slow components. 2. In vitro connections that did not preserve the sign of connections found in vivo were made by L10 onto R15, R16 and L7. Neurones R15 and R16 receive excitatory inputs from L10 in vivo and L7 receives a dual-action input in vivo, with inhibition followed by excitation. A purely inhibitory connection from L10 was formed in vitro onto all these cells. 3. Connections that have never been observed in vivo in terms of both direction and sign were formed in vitro. Followers L7, L11, L12, L13 and R16 and non-follower L14A formed novel connections onto L10. All these connections were inhibitory and some were strong. For example, IPSPs with a magnitude of 20 mV were observed in L10 following a single action potential in L13. Our results show that identified Aplysia neurones can form stereotyped specific connections in vitro. The specificity is different from that in the intact ganglion. The ubiquity of novel connections suggests that restrictions imposed on synaptogenesis in the animal are distinct from those regulating synapse formation in culture.

Modulatory actions of neurotransmitters.

A relatively shorttime ago the individual neuron was viewed as functioning by means of the simple summation of brief inhibitory or excitatory events. The complexity of the nervous system was the outcome largely of the connections between neurons. Recent studies have uncovered a new set of phenomena that indicate that relatively complex information processing may occur at the level of the individual neuron. For example, rather than producing additive effects, synaptic inputs can produce multiplicative effects that serve to alter the gain of the system. In addition, synaptic inputs may be able to alter specific classes of other inputs selectively. One could envision that each class of synaptic input to a cell could be selectively depressed or enhanced by a corresponding modulatory input. Since the modulatory actions can be transmitted intracellularly via second messengers, an extensive array of presynaptic connections may be unnecessary. It remains to be determined which of the modulatory phenomena currently reviewed are functionally important and which are only pharmacological or experimental curiosities. Are there any common attributes of the various synaptic phenomena that have been termed modulatory? The most common features of modulatory synaptic effects are long-duration of action and contingent action. Contingent action refers to the property that modulatory transmitters often have little or no effects in themselves, but instead they alter the effects of other events. Long-duration and contingent action endow modulatory effects with properties ideally suited to the control of behavioral modulations such as learning, motivational state, arousal, and sensitization. While there is no necessary connection between behavioral modulation and neural modulation, the available evidence from invertebrates suggests that there often is a connection. It is likely that examples of neuromodulation in vertebrates also will be tied to behavioral modulation.

Short-term synchronization of intercostal motoneurone activity.

1. The hypothesis is advanced that the joint occurrence of unitary excitatory post-synaptic potentials e.p.s.p.s) evoked in motoneurones by branches of common stem pre-synaptic fibres causes short-term synchronization of their discharge during the rising phases of the unitary e.p.s.p.s. 2. This hypothesis was tested using the pre- and post-stimulus time (PPST) histogram to detect synchronized firing among groups of intercostal motoneurones discharging in response to their natural synaptic drives. 3. Motor nerve action potentials were recorded monophasically from nerve filaments of the external intercostal muscles of anaesthetized, paralysed cats maintained on artificial ventilation. 4. Computer methods were used to measure peak spike amplitude, spike amplitude, spike interval and filament identification for simultaneous recordings from four filaments. The spike amplitude histograms were derived for each filament and groups of spikes were selected for analysis. 5. With spikes of one group designated as 'stimuli' (occurring at zero time) and those of a second as 'response' the PPST histogram was computed with different time bin widths. 6. With bin widths of 100 and 10 msec the central respiratory periodicity was apparent in the PPST histogram. With 1.0 msec bins the PPST histogram showed a narrow central peak extending to +/- 3.0 msec at its base. This 'short-term synchronization' supports the hypothesis of joint firing due to common presynaptic connectivity. 7. It was shown that detection of short-term synchronization was critically dependent on a sufficient quantity of data but that provided a simple criterion of adequate counts per bin in the PPST histogram was met, short-term synchronization could be detected between intercostal motoneurones of the same and adjacent segments.

Shortage of Nurses

<h3>ABSTRACT</h3> Retrograde monosynaptic tracing using glycoprotein-deleted rabies virus is an important component of the toolkit for investigation of neural circuit structure and connectivity. It allows for the identification of first-order presynaptic connections to cell populations of interest across both the central and peripheral nervous system, helping to decipher the complex connectivity patterns of neural networks that give rise to brain function. Despite its utility, the efficiency with which genetically modified rabies virus spreads retrogradely across synapses remains uncertain. While past studies have revealed conditions that can increase or decrease the numbers of presynaptic cells labeled, it is unknown what proportion of total inputs to a starter cell of interest are labeled. It is also unknown whether synapses that are more proximal or distal to the cell body are labeled with different efficiencies. Here we use a new rabies virus construct that allows for the simultaneous labeling of pre and postsynaptic specializations to quantify efficiency of spread at the synaptic level in mouse primary visual cortex. We demonstrate that with typical conditions about 40% of first-order presynaptic excitatory inputs are labeled. We show that using matched tracing conditions there is similar efficiency of spread from excitatory or inhibitory starter cell types. Furthermore, we find no difference in the efficiency of labeling of excitatory inputs to postsynaptic sites at different subcellular locations.