Differences in paired-pulse inhibition and facilitation in the dentate gyrus and CA3 field between dorsal and ventral rat hippocampus

Avoidance and escape conditioning adjust adult neurogenesis to conserve a fit hippocampus in adult male rodents

In this study, the connection between cognitive behaviors and the adult rodent hippocampus was investigated. Recording field potentials at performant pathway (PP)-hippocampal dentate gyrus (DG) synapses in transverse slices from the dorsal (d), intermediate (i), and ventral (v) hippocampus showed differences in paired-pulse responses and long-term potentiation in rats. The Barnes maze (BM) and passive avoidance (PA) tests indicated a decrease in escape latency and step-through latency in both rats and mice over training days. A decrease in the use of random or sequential strategy while an increase in the use of direct strategy to search for an escape box occurred in both groups. Evaluation of the levels of neurogenesis markers (Ki67 and BrdU/NeuN) by immunofluorescence assay in the dDG, iDG, and vDG revealed a long-axis disparity in the hippocampal dentate baseline cell proliferation and exposure to the BM and PA task changed the profile of baseline cell proliferation along the DG in both rats and mice. Also, these learning experiences changed the profile of BrdU+ /NeuN+ cells along the DG of rats. Quantitation of hippocampal BDNF protein levels using ELISA exhibited no changes in BDNF levels due to learning experiences in rats. We demonstrate that PP-DG synaptic efficacy and neurogenesis are organized along a gradient. Avoidance and escape conditioning themselves are sufficient to change and calibrate adult neurogenesis along the hippocampal long axis in rodents. Further research will be required to determine the precise mechanisms underlying the role of experience-derived neuroplasticity in cognitive function and decline.

A distinct impact of repeated morphine exposure on synaptic plasticity at Schaffer collateral-CA1, temporoammonic-CA1, and perforant pathway-dentate gyrus synapses along the longitudinal axis of the hippocampus

We aimed to study how morphine affects synaptic transmission in the dentate gyrus and CA1 regions along the hippocampal long axis. For this, recording and measuring of field excitatory postsynaptic potentials (fEPSPs) were utilized to test the effects of repeated morphine exposure on paired-pulse evoked responses and long-term potentiation (LTP) at Schaffer collateral-CA1 (Sch-CA1), temporoammonic-CA1 (TA-CA1) and perforant pathway-dentate gyrus (PP-DG) synapses in transverse slices from the dorsal (DH), intermediate (IH), and ventral (VH) hippocampus in adult male rats. After repeated morphine exposure, the expression of opioid receptors and the α1 and α5 GABA_A subunits were also examined. We found that repeated morphine exposure blunt the difference between the DH and the VH in their basal levels of synaptic transmission at Sch-CA1 synapses that were seen in the control groups. Significant paired-pulse facilitation of excitatory synaptic transmission was observed at Sch-CA1 synapses in slices taken from all three hippocampal segments as well as at PP-DG synapses in slices taken from the VH segment in the morphine-treated groups as compared to the control groups. Interestingly, significant paired-pulse inhibition of excitatory synaptic transmission was observed at TA-CA1 synapses in the DH slices from the morphine-treated group as compared to the control group. While primed-burst stimulation (a protocol reflecting normal neuronal firing) induced a robust LTP in hippocampal subfields in all control groups, resulting in a decaying LTP at TA-CA1 synapses in the VH slices and at PP-DG synapses in both the IH and VH slices taken from the morphine-treated rats. In the DH of morphine-treated rats, we found increased levels of the mRNAs encoding the α1 and α5 GABA_A subunits as compared to the control group. Taken together, these findings suggest the potential mechanisms through which repeated morphine exposure causes differential changes in circuit excitability and synaptic plasticity in the dentate gyrus and CA1 regions along the hippocampal long axis.

Reducing glutamic acid decarboxylase in the dorsal dentate gyrus attenuates juvenile stress induced emotional and cognitive deficits

A high degree of regional, temporal and molecular specificity is evident in the regulation of GABAergic signaling in stress-responsive circuitry, hampering the use of systemic GABAergic modulators for the treatment of stress-related psychopathology. Here we investigated the effectiveness of local intervention with the GABA synthetic enzymes GAD65 and GAD67 in the dorsal dentate gyrus (dDG) vs ventral DG (vDG) to alleviate anxiety-like behavior and stress-induced symptoms in the rat. We induced shRNA-mediated knock down of either GAD65 or GAD67 with lentiviral vectors microinjected into the dDG or vDG of young adult male rats and examined anxiety behavior, learning and memory performance. Subsequently we tested whether reducing GAD65 expression in the dDG would also confer resilience against juvenile stress-induced behavioral and physiological symptoms in adulthood. While knock down of either isoform in the vDG increased anxiety levels in the open field and the elevated plus maze tests, the knock down of GAD65, but not GAD67, in the dDG conferred a significant reduction in anxiety levels. Strikingly, this manipulation also attenuated juvenile stress evoked anxiety behavior, cognitive and synaptic plasticity impairments. Local GABAergic circuitry in the DG plays an important and highly region-specific role in control of emotional behavior and stress responding. Reduction of GAD65 expression in the dDG appears to provide resilience to juvenile stress-induced emotional and cognitive deficits, opening a new direction towards addressing a significant risk factor for developing stress and trauma-related psychopathologies later in life.

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GAD67/65 in the dorsal/ventral dentate gyrus differentially modulate anxiety.

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Reduced GAD65 expression in the dorsal dentate gyrus supports stress resilience.

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The dorsal dentate gyrus plays a key role in stress resilience.

Behavioral profiling reveals an enhancement of dentate gyrus paired pulse inhibition in a rat model of PTSD

We recently introduced behavioral profiling as a translational approach to increase the validity of animal models of posttraumatic stress disorder (PTSD). Behavioral profiling utilizes the response of a 'normal population' of control animals and compares the performance of animals with a history of traumatic stress in different behavioral tests that can capture PTSD-like symptoms. Thus, affected, PTSD-like individuals can be subdivided from resilient trauma-exposed animals. While in our recent study we focused mainly on tests for activity and anxiety, we now expand the behavioral tests battery and include also fear memory and extinction tasks as well as a spatial object recognition test in our behavioral profiling approach. Utilizing underwater trauma as the traumatic event, we found that only a small subset of animals exposed to underwater trauma showed lasting increases in anxiety-like behavior and heightened emotional memory formation. Adding juvenile stress as a model for childhood adversity increased the prevalence of such affected animals and furthermore and induced additional cognitive deficits in a subgroup of such emotionally affected individuals. In addition, multiple affected individual rats displayed increased local circuit activity in the dorsal dentate gyrus, as measured in vivo with paired pulse protocols in anesthetized animals. Together, our findings highlight behavioral profiling, refined by including multiple behavioral tests, as a valid tool to identify PTSD-like vs. resilient individual animals and further suggest that enhanced local inhibition in specific circuits of the dorsal dentate gyrus may be associated with the observed symptoms.

Hippocampal epileptogenesis in autoimmune encephalitis

OBJECTIVE

Autoantibody-mediated forms of encephalitis (AE) include neurological disorders characterized by subacute memory loss, movement disorders, and, often, frequent, focal epileptic seizures. Yet, the electrophysiological effects of these autoantibodies on neuronal function have received little attention. In this study, we assessed the effects of CSF containing autoantibodies on intrinsic and extrinsic properties of hippocampal neurons, to define their epileptogenic potential.

METHODS

We compared the effects of CSF containing leucine-rich glioma inactivated 1 (LGI1), contactin-associated protein-like 2 (CASPR2), and γ-aminobutyric acid receptor B (GABA_B R) antibodies on ex vivo electrophysiological parameters after stereotactic hippocampal inoculation into mice. Whole-cell patch-clamp and extracellular recordings from CA1 pyramidal neurons and CA3-CA1 field recordings in ex vivo murine brain slices were used to study neuronal function.

RESULTS

By comparison to control CSF, AE CSFs increased the probability of glutamate release from CA3 neurons. In addition, LGI1- and CASPR2 antibodies containing CSFs induced epileptiform activity at a population level following Schaffer collateral stimulation. CASPR2 antibody containing CSF was also associated with higher spontaneous firing of CA1 pyramidal neurons. On the contrary, GABA_B R antibody containing CSF did not elicit changes in intrinsic neuronal activity and field potentials.

INTERPRETATION

Using patient CSF, we have demonstrated that the AE-associated antibodies against LGI1 and CASPR2 are able to increase hippocampal CA1 neuron excitability, facilitating epileptiform activity. These findings provide in vivo pathogenic insights into neuronal dysfunction in these conditions.

Translating striatal activity from brain slice to whole animal neurophysiology: A guide for neuroscience research integrating diverse levels of analysis

An important goal of this review is highlighting research in neuroscience as examples of multilevel functional and anatomical analyses addressing basic science issues and applying results to the understanding of diverse disorders. The research of Dr. Michael Levine, a leader in neuroscience, exemplifies this approach by uncovering fundamental properties of basal ganglia function and translating these findings to clinical applications. The review focuses on neurophysiological research connecting results from in vitro and in vivo recordings. A second goal is to utilize these research connections to produce novel, accurate descriptions for corticostriatal processing involved in varied, complex functions. Medium spiny neurons in striatum act as integrators combining input with baseline activity creating motivational "events." Basic research on corticostriatal synapses is described and links developed to issues with clinical relevance such as inhibitory gating, self-injurious behavior, and relative reward valuation. Work is highlighted on dopamine-glutamate interactions. Individual medium spiny neurons express both D₁ and D₂ receptors and encode information in a bivalent manner depending upon the mix of receptors involved. Current work on neurophysiology of reward processing has taken advantage of these basic approaches at the cellular and molecular levels. Future directions in studying physiology of reward processing and action sequencing could profit by incorporating the divergent ways dopamine modulates incoming neurochemical signals. Primary investigators leading research teams should mirror Mike Levine's efforts in "climbing the mountain" of scientific inquiry by performing analyses at different levels of inquiry, integrating the findings, and building comprehensive answers to problems unsolvable without this bold approach.

Sensory gating in bilayer amorphous carbon memristors

Multi-state amorphous carbon-based memory devices have been developed that exhibit both bipolar and unipolar resistive switching behaviour. These modes of operation were implemented independently to access multiple resistance states, enabling higher memory density than conventional binary non-volatile memory technologies. The switching characteristics have been further utilised to study synaptic computational functions that could be implemented in artificial neural networks. Notably, paired-pulse inhibition (PPI) is observed at bio-realistic timescales (<100 ms). Devices displaying this rich synaptic behaviour could function as robust stand-alone synapse-inspired memory or be applied as filters for specialised neuromorphic circuits and sensors.

Architecture of the Mouse Brain Synaptome

Synapses are found in vast numbers in the brain and contain complex proteomes. We developed genetic labeling and imaging methods to examine synaptic proteins in individual excitatory synapses across all regions of the mouse brain. Synapse catalogs were generated from the molecular and morphological features of a billion synapses. Each synapse subtype showed a unique anatomical distribution, and each brain region showed a distinct signature of synapse subtypes. Whole-brain synaptome cartography revealed spatial architecture from dendritic to global systems levels and previously unknown anatomical features. Synaptome mapping of circuits showed correspondence between synapse diversity and structural and functional connectomes. Behaviorally relevant patterns of neuronal activity trigger spatiotemporal postsynaptic responses sensitive to the structure of synaptome maps. Areas controlling higher cognitive function contain the greatest synapse diversity, and mutations causing cognitive disorders reorganized synaptome maps. Synaptome technology and resources have wide-ranging application in studies of the normal and diseased brain.

Less means more: The magnitude of synaptic plasticity along the hippocampal dorso-ventral axis is inversely related to the expression levels of plasticity-related neurotransmitter receptors

The dorsoventral axis of the hippocampus exhibits functional differentiations with regard to (spatial Vs emotional) learning and information retention (rapid encoding Vs long-term storage), as well as its sensitivity to neuromodulation and information received from extrahippocampal structures. The mechanisms that underlie these differentiations remain unclear. Here, we explored neurotransmitter receptor expression along the dorsoventral hippocampal axis and compared hippocampal synaptic plasticity in the CA1 region of the dorsal (DH), intermediate (IH) and ventral hippocampi (VH). We observed a very distinct gradient of expression of the N-methyl-D-aspartate receptor GluN2B subunit in the Stratum radiatum (DH< IH< VH). A similar distribution gradient (DH< IH< VH) was evident in the hippocampus for GluN1, the metabotropic glutamate receptors mGlu1 and mGlu2/3, GABAB and the dopamine-D1 receptor. GABAA exhibited the opposite expression relationship (DH > IH > VH). Neurotransmitter release probability was lowest in DH. Surprisingly, identical afferent stimulation conditions resulted in hippocampal synaptic plasticity that was the most robust in the DH, compared with IH and VH. These data suggest that differences in hippocampal information processing and synaptic plasticity along the dorsoventral axis may relate to specific differences in the expression of plasticity-related neurotransmitter receptors. This gradient may support the fine-tuning and specificity of hippocampal synaptic encoding.

A gradient of frequency-dependent synaptic properties along the longitudinal hippocampal axis

Background

The hippocampus is a functionally heterogeneous brain structure and specializations of the intrinsic neuronal network may crucially support the functional segregation along the longitudinal axis of the hippocampus. Short-term synaptic plasticity plays fundamental roles in information processing and may be importantly involved in diversifying the properties of local neuronal network along the hippocampus long axis. Therefore, we aimed to examine the properties of the cornu ammonis 1 (CA1) synapses along the entire dorsoventral axis of the rat hippocampus using field excitatory postsynaptic potentials from transverse rat hippocampal slices and a frequency stimulation paradigm.

Results

Applying a ten-pulse stimulus train at frequencies from 0.1 to 100 Hz to the Schaffer collaterals we found a gradually diversified pattern of frequency-dependent synaptic effects along the dorsoventral hippocampus axis. The first conditioned response was facilitated along the whole hippocampus for stimulus frequencies 10–40 Hz. However, steady-state responses or averaged responses generally ranged from maximum synaptic facilitation in the most dorsal segment of the hippocampus to maximum synaptic depression in the most ventral segment of the hippocampus. In particular, dorsal synapses facilitated for stimulus frequency up to 50 Hz while they depressed at higher frequencies (75–100 Hz). Facilitation at dorsal synapses was maximal at stimulus frequency of 20 Hz. On the contrary, the most ventral synapses showed depression regardless of the stimulus frequency, only displaying a transient facilitation at the beginning of 10–50 Hz stimulation. Importantly, the synapses in the medial hippocampus displayed a transitory behavior. Finally, as a whole the hippocampal synapses maximally facilitated at 20 Hz and increasingly depressed at 50–100 Hz.

Conclusion

The short-term synaptic dynamics change gradually along the hippocampal long axis in a frequency-dependent fashion conveying distinct properties of information processing to successive segments of the structure, thereby crucially supporting functional segregation along the dorsoventral axis of the hippocampus.

Distinct Properties of Long-Term Potentiation in the Dentate Gyrus along the Dorsoventral Axis: Influence of Age and Inhibition

The hippocampus is important for spatial navigation, episodic memory and affective behaviour. Increasing evidence suggests that these multiple functions are accomplished by different segments along the dorsal-ventral (septal-temporal) axis. Long-term potentiation (LTP), the best-investigated cellular correlate of learning and memory, has distinct properties along this axis in the CA1 region, but so far, little is known about longitudinal differences in dentate gyrus (DG). Therefore, here we examined potential dorsoventral differences in DG-LTP using in vitro multi-electrode array recordings. In young mice, we found higher basal synaptic transmission in the dorsal DG, while the LTP magnitude markedly increased towards the ventral pole. Strikingly, these differences were greatly reduced in slices from middle-aged mice. Short-term plasticity, evaluated by paired-pulse ratios, was similar across groups. Recordings in the presence and absence of GABA_A-receptor blocker picrotoxin suggested a higher inhibitory tone in the ventral DG of young mice, confirmed by an increased frequency of miniature inhibitory postsynaptic currents. Our findings support the view that the hippocampus contains discrete functional domains along its dorsoventral axis and demonstrate that these are subject to age-dependent changes. Since these characteristics are presumably conserved in the human hippocampus, our findings have important clinical implications for hippocampus- and age-related disorders.

The L-type voltage-dependent calcium channel long-term potentiation is higher in the dorsal compared with the ventral associational/commissural CA3 hippocampal synapses

The diversification between dorsal (DH) and ventral (VH) hippocampus includes the different ability to support NMDA receptor-dependent long-term synaptic potentiation (LTP). In this study, we assessed the ability of associational/commissural connections in the CA3 hippocampal field to show NMDA receptor-independent LTP. We found that high-frequency stimulation under blockade of NMDA receptors induced greater LTP in DH (40.7±8.5%) than in VH (17.1±4.6%). The blocker of L-type voltage-dependent calcium channels (VDCC) nifedipine prevented the induction of LTP. We hypothesize that the different ability for VDCC-LTP between DH and VH might have important implications in the memory-related information processing performed by the circuits of the two hippocampal segments.