Erratum to: Journal of Computational Neuroscience, DOI 10.1007/s10827-010-0303-y and DOI 10.1007/s10827-010-0304-x

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Modulation of GABAA receptor-mediated synaptic transmission by Zn2+ at a dentate gyrus circuit

Zinc (ionic form Zn2+) is a common trace element in the forebrain, and is especially enriched in the hippocampus, a brain structure important for learning and memory. A large amount of vesicular Zn2+ which is thought to be released upon presynaptic depolarisation is found at synapses formed by the axons of dentate granule cells (GCs), known as mossy fibres (MFs). Zn2+ inhibits NMDA and GABAA receptors (NMDAR and GABAAR) at mono-synaptic inputs between MFs and CA3 pyramidal neurons but its role in synaptic integration in the dentate gyrus remains elusive. Whole-cell recordings were obtained from GCs held in voltage-clamp in acute rat hippocampal slices. One tungsten electrode was positioned in stratum lucidum (SL) of CA3b to activate MFs and another in stratum granulosum (SG) to directly stimulate dentate interneurons. Evoked synaptic currents were blocked by superfusion of the GABAAR antagonist bicuculline implying that they were mediated by GABAARs. In contrast, the AMPA/kainate receptor antagonist NBQX abolished SL evoked inhibitory postsynaptic currents (IPSCs) but had little effect on IPSCs evoked by SG stimulation. Similarly, the group 2 metabotropic receptor agonist DCG-IV depressed SL but not SG evoked IPSCs. These results imply a poly-synaptic inhibitory feedback projection from CA3 to the dentate gyrus via recurrent MFs, and a mono-synaptic input from dentate interneurons to GCs. Zn2+ reversibly depressed evoked IPSCs whereas superfusion of different Zn2+ chelators had the opposite enhancing effect. Blocking T-type Ca2+ channels abolished the effect of Zn2+ chelators. When recording from dentate basket cells, Zn2+ chelation increased spike width, decreased spike threshold, enhanced NMDAR-mediated excitatory postsynaptic currents (EPSCs) and facilitated T–type Ca2+ currents. Finally, chelation of Zn2+ narrowed the time window for integration of perforant path inputs and facilitated GC spiking. Together, the results demonstrate that Zn2+ modulates MF–interneuron–GC communication and thus regulates information transfer to dentate and hippocampal networks

IST Austria Thesis

CA3 pyramidal neurons are thought to pay a key role in memory storage and pattern completion by activity-dependent synaptic plasticity between CA3-CA3 recurrent excitatory synapses. To examine the induction rules of synaptic plasticity at CA3-CA3 synapses, we performed whole-cell patch-clamp recordings in acute hippocampal slices from rats (postnatal 21-24 days) at room temperature. Compound excitatory postsynaptic potentials (ESPSs) were recorded by tract stimulation in stratum oriens in the presence of 10 µM gabazine. High-frequency stimulation (HFS) induced N-methyl-D-aspartate (NMDA) receptor-dependent long-term potentiation (LTP). Although LTP by HFS did not requier postsynaptic spikes, it was blocked by Na+-channel blockers suggesting that local active processes (e.g.) dendritic spikes) may contribute to LTP induction without requirement of a somatic action potential (AP). We next examined the properties of spike timing-dependent plasticity (STDP) at CA3-CA3 synapses. Unexpectedly, low-frequency pairing of EPSPs and backpropagated action potentialy (bAPs) induced LTP, independent of temporal order. The STDP curve was symmetric and broad, with a half-width of ~150 ms. Consistent with these specific STDP induction properties, post-presynaptic sequences led to a supralinear summation of spine [Ca2+] transients. Furthermore, in autoassociative network models, storage and recall was substantially more robust with symmetric than with asymmetric STDP rules. In conclusion, we found associative forms of LTP at CA3-CA3 recurrent collateral synapses with distinct induction rules. LTP induced by HFS may be associated with dendritic spikes. In contrast, low frequency pairing of pre- and postsynaptic activity induced LTP only if EPSP-AP were temporally very close. Together, these induction mechanisms of synaptiic plasticity may contribute to memory storage in the CA3-CA3 microcircuit at different ranges of activity

IST Austria Thesis

Distinguishing between similar experiences is achieved by the brain in a process called pattern separation. In the hippocampus, pattern separation reduces the interference of memories and increases the storage capacity by decorrelating similar inputs patterns of neuronal activity into non-overlapping output firing patterns. Winners-take-all (WTA) mechanism is a theoretical model for pattern separation in which a "winner" cell suppresses the activity of the neighboring neurons through feedback inhibition. However, if the network properties of the dentate gyrus support WTA as a biologically conceivable model remains unknown. Here, we showed that the connectivity rules of PV+interneurons and their synaptic properties are optimizedfor efficient pattern separation. We found using multiple whole-cell in vitrorecordings that PV+interneurons mainly connect to granule cells (GC) through lateral inhibition, a form of feedback inhibition in which a GC inhibits other GCs but not itself through the activation of PV+interneurons. Thus, lateral inhibition between GC–PV+interneurons was ~10 times more abundant than recurrent connections. Furthermore, the GC–PV+interneuron connectivity was more spatially confined but less abundant than PV+interneurons–GC connectivity, leading to an asymmetrical distribution of excitatory and inhibitory connectivity. Our network model of the dentate gyrus with incorporated real connectivity rules efficiently decorrelates neuronal activity patterns using WTA as the primary mechanism. This process relied on lateral inhibition, fast-signaling properties of PV+interneurons and the asymmetrical distribution of excitatory and inhibitory connectivity. Finally, we found that silencing the activity of PV+interneurons in vivoleads to acute deficits in discrimination between similar environments, suggesting that PV+interneuron networks are necessary for behavioral relevant computations. Our results demonstrate that PV+interneurons possess unique connectivity and fast signaling properties that confer to the dentate gyrus network properties that allow the emergence of pattern separation. Thus, our results contribute to the knowledge of how specific forms of network organization underlie sophisticated types of information processing

Tonic inhibition originates from synapses close to the soma

AbstractCentral neurons are subject to a tonic barrage of randomly occurring spontaneous inhibitory events (mIPSCs) resulting from the action potential-independent release of γ-aminobutyric acid (GABA). Do the terminals making synapses onto somatic versus dendritic sites, which arise from specific populations of interneurons, differ in their ability to generate mIPSCs? We have combined the techniques of whole-cell patch-clamp recording and computational simulation to demonstrate that in granule cells of the dentate gyrus, most of the action potential-independent inhibition taking place as mIPSCs originates from proximal sites. Indeed, removal of the bulk (>50%) of the dendritic tree did not change the characteristics of mIPSCs. These results are consistent with a functional segregation of GABAergic terminals synapsing at proximal versus distal portions of central neurons. Thus, proximal GABAergic terminals are responsible for tonic inhibition targeted at the soma

Memories along the longitudinal axis of a rodent hippocampus: acquisition and consolidation of variants of a spatial task

The mammalian hippocampus is a structure of the brain believed to be essential in learning and memory processes. A current controversy concerns whether it is involved in one unique memory process or is responsible for several related but dissociable functions. And irrespective of the function! s) there is controversy concerning its role in processes of memory consolidation. This thesis, divided in two parts, addresses these two issues.Part I: Published studies have suggested that hippocampal involvement in spatial memory acquisition is restricted to the septal (or dorsal) part of the structure, a result that supports the idea that the septal and temporal (ventral) parts of the structure have different functions. In the first part of this thesis I explore further the possibility of functional dissociations along the septotemporal axis of the hippocampus, including the importance of commissural projections. Partial lesions are made to the septal or temporal parts of the rat hippocampus, on one side or both. The behavioural essay involves acquisition of a spatial task (variants of reference memory in the watermaze). Although the original septal versus temporal dissociation is replicated, variations of the task protocol (number of days and trials per day of training) reveal that the temporal hippocampus can also support spatial memory. Learning can be attained with as little as 30% of the hippocampus spared. The results support the idea that the hippocampus is responsible for a unique process to which the projections to the septal and temporal parts, as well as the commissural associations, contribute differently. This contribution could be dependent on the training protocol.Part II: It is well established that damage to the hippocampus, across different species, can result in graded retrograde amnesia. This has be taken by some to imply a role in the consolidation, as well as the acquisition, of memories. The second part of the thesis describes a series of collaborative experiments in which the involvement of the hippocampus in acquisition, consolidation and retrieval of spatial memories is explored. Using an AMPA receptor antagonist the septal part of the hippocampus is temporarily inactivated during acquisition, retrieval or during the memory retention interval of a watermaze reference memory task. The results reveal the hippocampus is involved in all three memory processes when animals are tested 16 days after the end of acquisition. However it is believed that once a memory has been consolidated, its retrieval can occur independently of the hippocampus.Animal experiments suggesting this involved lesions of the septal hippocampus only. In work reported in this thesis, lesions to the septal or the whole hippocampus are made at different times (1 day or 6 weeks) after acquisition. Using a novel memory testing protocol, the temporal 30% of the hippocampus was found to be sufficient in the retrieval of this memory in a time-independent manner. Animals given lesions to the whole of the structure could not be reminded of what they had leamt earlier at either interval. The results suggest that the whole hippocampus is necessary for the consolidation of memories acquired with an intact hippocampus and that at least part of the hippocampus is necessary for retrieval of memories. The results obtained in part two of the thesis could be dependent on the training and testing protocol as well as on the navigational aspects of the task

Signalling properties at single synapses and within the interneuronal network in the CA1 region of the rodent hippocampus

Understanding how the complexity of connections among the neurons in the brain is established and modified in an experience- and activity-dependent way is a challenging task of Neuroscience. Although in the last decades many progresses have been made in characterising the basic mechanisms of synaptic transmission, a full comprehension of how information is transferred and processed by neurons has not been fully achieved. In the present study, theoretical tools and patch clamp experiments were used to further investigate synaptic transmission, focusing on quantal transmission at single synapses and on different types of signalling at the level of a particular interneuronal network in the CA1 area of the rodent hippocampus. The simultaneous release of more than one vesicle from an individual presynaptic active zone is a typical mechanism that can affect the strength and reliability of synaptic transmission. At many central synapses, however, release caused by a single presynaptic action potential is limited to one vesicle (univesicular release). The likelihood of multivesicular release at a particular synapse has been tied to release probability (Pr), and whether it can occur at Schaffer collateral\u2013CA1 synapses, at which Pr ranges widely, is controversial. In contrast with previous findings, proofs of multivesicular release at this synapse have been recently obtained at late developmental stages; however, in the case of newborn hippocampus, it is still difficult to find strong evidence in one direction or another. In order to address this point, in the first part of this study a simple and general stochastic model of synaptic release has been developed and analytically solved. The model solution gives analytical mathematical expressions relating basic quantal parameters with average values of quantities that can be measured experimentally. Comparison of these quantities with the experimental measures allows to determine the most probable values of the quantal parameters and to discriminate the univesicular from the multivesicular mode of glutamate release. The model has been validated with data previously collected at glutamatergic CA3-CA1 synapses in the hippocampus from newborn (P1-P5 old) rats. The results strongly support a multivesicular type of release process requiring a variable pool of immediately releasable vesicles. Moreover, computing quantities that are functions of the model parameters, the mean amplitude of the synaptic response to the release of a single vesicle (Q) was estimated to be 5-10 pA, in very good agreement with experimental findings. In addition, a multivesicular type of release was supported by various experimental evidences: a high variability of the amplitude of successes, with a coefficient of variation ranging from 0.12 to 0.73; an average potency ratio a2/a1 between the second and first response to a pair of stimuli bigger than 1; and changes in the potency of the synaptic response to the first stimulus when the release probability was modified by increasing or decreasing the extracellular calcium concentration. This work indicates that at glutamatergic CA3-CA1 synapses of the neonatal rat hippocampus a single action potential may induce the release of more than one vesicle from the same release site. In a more systemic approach to the analysis of communication between neurons, it is interesting to investigate more complex, network interactions. GABAergic interneurons constitute a heterogeneous group of cells which exert a powerful control on network excitability and are responsible for the oscillatory behaviour crucial for information processing in the brain. They have been differently classified according to their morphological, neurochemical and physiological characteristics. In the second part of this study, whole cell patch clamp recordings were used to further characterize, in transgenic mice expressing EGFP in a subpopulation of GABAergic interneurons containing somatostatin (GIN mice), the functional properties of EGFPpositive cells in stratum oriens of the CA1 region of the hippocampus, in slice cultures obtained from P8 old animals. These cells showed passive and active membrane properties similar to those found in stratum oriens interneurons projecting to stratum lacunosum-moleculare. Moreover, they exhibited different firing patterns which were maintained upon membrane depolarization: irregular (48%), regular (30%) and clustered (22%). Paired recordings from EGFP-positive cells often revealed electrical coupling (47% of the cases), which was abolished by carbenoxolone (200 mM). On average, the coupling coefficient was 0.21 \ub1 0.07. When electrical coupling was particularly strong it acted as a powerful low-pass filter, thus contributing to alter the output of individual cells. The dynamic interaction between cells with various firing patterns may differently control GABAergic signalling, leading, as suggested by simulation data, to a wide range of interneuronal communication. In additional paired recordings of a presynaptic EGFP positive interneuron and a postsynaptic principal cell, trains of action potentials in interneurons rarely evoked GABAergic postsynaptic currents (3/45 pairs) with small amplitude and slow kinetics, and that at 20 Hz exhibited short-term depression. In contrast, excitatory connections between principal cells and EGFP-positive interneurons were found more often (17/55 pairs) and exhibited a frequency and use-dependent facilitation, particularly in the gamma band. In conclusion, it appears that EGFP-positive interneurons in stratum oriens of GIN mice constitute a heterogeneous population of cells interconnected via electrical synapses, exhibiting particular features in their chemical and electrical synaptic signalling. Moreover, the dynamic interaction between these interneurons may differentially affect target cells and neuronal communication within the hippocampal network