Asynchronous ripple oscillations between left and right hippocampi during slow-wave sleep

Impaired functional connectivity of the hippocampus in translational murine models of NMDA-receptor antibody associated neuropsychiatric pathology

Decreased hippocampal connectivity and disruption of functional networks are established resting-state functional MRI (rs-fMRI) features that are associated with neuropsychiatric symptom severity in human anti-N-methyl-D-aspartate receptor (NMDAR) encephalitis. However, the underlying pathophysiology of NMDAR encephalitis remains poorly understood. Application of patient-derived monoclonal antibodies against the NR1 (GluN1) subunit of the NMDAR now allows for the translational investigation of functional connectivity in experimental murine NMDAR antibody disease models with neurodevelopmental disorders. Using rs-fMRI, we studied functional connectivity alterations in (1) adult C57BL/6 J mice that were intrathecally injected with a recombinant human NR1 antibody over 14 days (n = 10) and in (2) a newly established mouse model with in utero exposure to a human recombinant NR1 antibody (NR1-offspring) at the age of (2a) 8 weeks (n = 15) and (2b) 10 months (n = 14). Adult NR1-antibody injected mice showed impaired functional connectivity within the left hippocampus compared to controls, resembling impaired connectivity patterns observed in human NMDAR encephalitis patients. Similarly, NR1-offspring showed significantly reduced functional connectivity in the hippocampus after 8 weeks, and impaired connectivity in the hippocampus was likewise observed in NR1-offspring at the age of 10 months. We successfully reproduced functional connectivity changes within the hippocampus in different experimental murine systems that were previously observed in human NMDAR encephalitis patients. Translational application of this method within a combined imaging and histopathological framework will allow future experimental studies to identify the underlying biological mechanisms and may eventually facilitate non-invasive monitoring of disease activity and treatment responses in autoimmune encephalitis.

Uncorrelated bilateral cortical input becomes timed across hippocampal subfields for long waves whereas gamma waves are largely ipsilateral

The role of interhemispheric connections along successive segments of cortico-hippocampal circuits is poorly understood. We aimed to obtain a global picture of spontaneous transfer of activity during non-theta states across several nodes of the bilateral circuit in anesthetized rats. Spatial discrimination techniques applied to bilateral laminar field potentials (FP) across the CA1/Dentate Gyrus provided simultaneous left and right readouts in five FP generators that reflect activity in specific hippocampal afferents and associative pathways. We used a battery of correlation and coherence analyses to extract complementary aspects at different time scales and frequency bands. FP generators exhibited varying bilateral correlation that was high in CA1 and low in the Dentate Gyrus. The submillisecond delays indicate coordination but not support for synaptic dependence of one side on another. The time and frequency characteristics of bilateral coupling were specific to each generator. The Schaffer generator was strongly bilaterally coherent for both sharp waves and gamma waves, although the latter maintained poor amplitude co-variation. The lacunosum-moleculare generator was composed of up to three spatially overlapping activities, and globally maintained high bilateral coherence for long but not short (gamma) waves. These two CA1 generators showed no ipsilateral relationship in any frequency band. In the Dentate Gyrus, strong bilateral coherence was observed only for input from the medial entorhinal areas, while those from the lateral entorhinal areas were largely asymmetric, for both alpha and gamma waves. Granger causality testing showed strong bidirectional relationships between all homonymous bilateral generators except the lateral entorhinal input and a local generator in the Dentate Gyrus. It also revealed few significant relationships between ipsilateral generators, most notably the anticipation of lateral entorhinal cortex toward all others. Thus, with the notable exception of the lateral entorhinal areas, there is a marked interhemispheric coherence primarily for slow envelopes of activity, but not for pulse-like gamma waves, except in the Schafer segment. The results are consistent with essentially different streams of activity entering from and returning to the cortex on each side, with slow waves reflecting times of increased activity exchange between hemispheres and fast waves generally reflecting ipsilateral processing.

Hippocampal responses to electrical stimulation of the major input pathways are modulated by dentate spikes

Dentate gyrus (DG) is important for pattern separation and spatial memory, and it is thought to gate information flow to the downstream hippocampal subregions. Dentate spikes (DSs) are high-amplitude, fast, positive local-field potential events taking place in the DG during immobility and sleep, and they have been connected to memory consolidation in rodents. DSs are a result of signaling from the entorhinal cortex (EC) to the DG, and they suppress firing of pyramidal cells in the CA3 and CA1. To study the effects of DSs to signaling in the hippocampal tri-synaptic loop, we electrically stimulated the afferent fibers of the DG, CA3, and CA1 in adult male Sprague-Dawley rats at different delays from DSs. Responses to stimulation were increased in the EC-DG synapse during DSs, and the effect was amplified after theta-burst stimulation. We concluded that DSs strengthen the excitatory signal from the EC to the DG, which is reinforced by synapse potentiation and increased excitability of granule cells after theta-burst stimulation. This signal boosting may function in enhancing plastic changes in the DG-CA3 synapse. As responses in the CA3 and CA1 remained unaffected by the DS, the DS-contingent silencing of pyramidal cells seems to be a result of a decrease in excitatory input rather than a decrease in the excitability of the pyramidal cells themselves. In addition, we found that the DSs occur asynchronously in the left and right hippocampi, giving novel evidence of lateralization of the rodent hippocampus.

Lateralization of CA1 assemblies in the absence of CA3 input

In the hippocampal circuit CA3 input plays a critical role in the organization of CA1 population activity, both during learning and sleep. While integrated spatial representations have been observed across the two hemispheres of CA1, these regions lack direct connectivity and thus the circuitry responsible remains largely unexplored. Here we investigate the role of CA3 in organizing bilateral CA1 activity by blocking synaptic transmission at CA3 terminals through the inducible transgenic expression of tetanus toxin. Although the properties of single place cells in CA1 were comparable bilaterally, we find a decrease of ripple synchronization between left and right CA1 after silencing CA3. Further, during both exploration and rest, CA1 neuronal ensemble activity is less coordinated across hemispheres. This included degradation of the replay of previously explored spatial paths in CA1 during rest, consistent with the idea that CA3 bilateral projections integrate activity between left and right hemispheres and orchestrate bilateral hippocampal coding.

Bilaterally projecting CA3 inputs may be crucial for integrating the left and right CA1 during memory but this has not been directly examined. Here, the authors show that projections from bilateral CA3 play a key role in the cross-hemispheric coordination of CA1 spatial coding.

Behavioral state-dependent lateralization of dorsal dentate gyrus c-Fos expression in mice

Hemispheric lateralization is a fundamental organizing principle of nervous systems across taxonomic groups with bilateral symmetry. The mammalian hippocampus is lateralized anatomically, physiologically, and chemically; however, functional asymmetries are not yet well understood. Imaging studies in humans have implicated the left and right hippocampus in specialized processing. However, it is not clear if lateralized activity occurs in the rodent hippocampus. c-Fos imaging in animals provides a measure of neuronal activity with a resolution at the level of single cells. The aim of the present study was to determine whether lateralized activity-dependent c-Fos expression occurs in the rodent hippocampus. To understand functional lateralization of hippocampal processing, we compared interhemispheric expression of c-Fos in the dentate gyrus (DG), a structure involved in encoding new experiences, in mice that ran on a wheel, encoded a novel object, or remained in home cages. We found that wheel running (WR) induced the greatest amount of DG c-Fos expression in both hemispheres, with no difference between hemispheres. Object exploration (OB) resulted in left-lateralized DG c-Fos expression, whereas control (CON) mice were not lateralized. We then sought to determine whether differential consideration of hemispheres might influence the conclusions of a study by simulating common cell quantitation methods. We found that different approaches led to different conclusions. These data demonstrate lateralization of neuronal activity in the mouse DG corresponding to the experience of the animal and show that differentially considering hemisphere leads to alternative conclusions.

Bilateral Synchronization of Hippocampal Early Sharp Waves in Neonatal Rats

In the neonatal rodent hippocampus, the first and predominant pattern of correlated neuronal network activity is early sharp waves (eSPWs). Whether and how eSPWs are organized bilaterally remains unknown. Here, using simultaneous silicone probe recordings from the left and right hippocampus in neonatal rats in vivo we found that eSPWs are highly synchronized bilaterally with nearly zero time lag between the two sides. The amplitudes of eSPWs in the left and right hippocampi were also highly correlated. eSPWs also supported bilateral synchronization of multiple unit activity (MUA). We suggest that bilateral correlated activity supported by synchronized eSPWs participates in the formation of bilateral connections in the hippocampal system.

Connectivity and network state-dependent recruitment of long-range VIP-GABAergic neurons in the mouse hippocampus

GABAergic interneurons in the hippocampus provide for local and long-distance coordination of neurons in functionally connected areas. Vasoactive intestinal peptide-expressing (VIP+) interneurons occupy a distinct niche in circuitry as many of them specialize in innervating GABAergic cells, thus providing network disinhibition. In the CA1 hippocampus, VIP+ interneuron-selective cells target local interneurons. Here, we discover a type of VIP+ neuron whose axon innervates CA1 and also projects to the subiculum (VIP-LRPs). VIP-LRPs show specific molecular properties and target interneurons within the CA1 area but both interneurons and pyramidal cells within subiculum. They are interconnected through gap junctions but demonstrate sparse spike coupling in vitro. In awake mice, VIP-LRPs decrease their activity during theta-run epochs and are more active during quiet wakefulness but not coupled to sharp-wave ripples. Together, the data provide evidence for VIP interneuron molecular diversity and functional specialization in controlling cell ensembles along the hippocampo-subicular axis.