Optogenetic Activation of Septal Glutamatergic Neurons Drive Hippocampal Theta Rhythms

Paradoxical effects of optogenetic stimulation in mesial temporal lobe epilepsy

OBJECTIVE

To establish the effects induced by long-term, unilateral stimulation of parvalbumin (PV)-positive interneurons on seizures, interictal spikes, and high-frequency oscillations (80-500Hz) occurring after pilocarpine-induced status epilepticus (SE)-a proven model of mesial temporal lobe epilepsy (MTLE)-in transgenic mice expressing or not expressing ChR2.

METHODS

PV-ChR2 (n = 6) and PV-Cre (n = 6) mice were treated with pilocarpine to induce SE. Three hours after SE onset, unilateral optogenetic stimulation (450nm, 25mW, 20-millisecond pulses delivered at 8Hz for 30 seconds every 2 minutes) of CA3 PV-positive interneurons was implemented for 14 continuous days in both groups.

RESULTS

Rates of seizures (p < 0.01), interictal spikes (p < 0.001), and interictal spikes with fast ripples (250-500Hz) (p < 0.001) were lower in PV-ChR2 than in PV-Cre mice. Ripples (80-200Hz) occurring outside of interictal spikes had higher rates in the PV-ChR2 group (p < 0.01), whereas isolated fast ripples had lower rates (p < 0.01). However, seizure probability was higher during optogenetic stimulation in PV-ChR2 compared to PV-Cre animals (p < 0.05).

INTERPRETATION

Our findings show that the unilateral activation of CA3 PV-positive interneurons exerts anti-ictogenic effects associated with decreased rates of interictal spikes and fast ripples in this MTLE model. However, PV-positive interneuron stimulation can paradoxically trigger seizures in epileptic animals, supporting the notion that γ-aminobutyric acid type A signaling can also initiate ictogenesis. ANN NEUROL 2019;86:714-728.

The Role of Hierarchical Dynamical Functions in Coding for Episodic Memory and Cognition

Behavioral research in human verbal memory function led to the initial definition of episodic memory and semantic memory. A complete model of the neural mechanisms of episodic memory must include the capacity to encode and mentally reconstruct everything that humans can recall from their experience. This article proposes new model features necessary to address the complexity of episodic memory encoding and recall in the context of broader cognition and the functional properties of neurons that could contribute to this broader scope of memory. Many episodic memory models represent individual snapshots of the world with a sequence of vectors, but a full model must represent complex functions encoding and retrieving the relations between multiple stimulus features across space and time on multiple hierarchical scales. Episodic memory involves not only the space and time of an agent experiencing events within an episode but also features shown in neurophysiological data such as coding of speed, direction, boundaries, and objects. Episodic memory includes not only a spatio-temporal trajectory of a single agent but also segments of spatio-temporal trajectories for other agents and objects encountered in the environment consistent with data on encoding the position and angle of sensory features of objects and boundaries. We will discuss potential interactions of episodic memory circuits in the hippocampus and entorhinal cortex with distributed neocortical circuits that must represent all features of human cognition.

A Machine Learning Approach to Characterize the Modulation of the Hippocampal Rhythms Via Optogenetic Stimulation of the Medial Septum

The medial septum (MS) is a potential target for modulating hippocampal activity. However, given the multiple cell types involved, the changes in hippocampal neural activity induced by MS stimulation have not yet been fully characterized. We combined MS optogenetic stimulation with local field potential (LFP) recordings from the hippocampus and leveraged machine learning techniques to explore how activating or inhibiting multiple MS neuronal subpopulations using different optical stimulation parameters affects hippocampal LFP biomarkers. First, of the seven different optogenetic viral vectors used for modulating different neuronal subpopulations, only two induced a substantial change in hippocampal LFP. Second, we found hippocampal low-gamma band to be most effectively modulated by the stimulation. Third, the hippocampal biomarkers were sensitive to the optogenetic virus type and the stimulation frequency, establishing those parameters as the critical ones for the regulation of hippocampal biomarker activity. Last, we built a Gaussian process regression model to describe the relationship between stimulation parameters and activity of the biomarker as well as to identify the optimal parameters for biomarker modulation. This new machine learning approach can further our understanding of the effects of neural stimulation and guide the selection of optimal parameters for neural control.

Septal GABA and Glutamate Neurons Express RXFP3 mRNA and Depletion of Septal RXFP3 Impaired Spatial Search Strategy and Long-Term Reference Memory in Adult Mice

Relaxin-3 is a highly conserved neuropeptide abundantly expressed in neurons of the nucleus incertus (NI), which project to nodes of the septohippocampal system (SHS) including the medial septum/diagonal band of Broca (MS/DB) and dorsal hippocampus, as well as to limbic circuits. High densities of the Gi/o-protein-coupled receptor for relaxin-3, known as relaxin-family peptide-3 receptor (RXFP3) are expressed throughout the SHS, further suggesting a role for relaxin-3/RXFP3 signaling in modulating learning and memory processes that occur within these networks. Therefore, this study sought to gain further anatomical and functional insights into relaxin-3/RXFP3 signaling in the mouse MS/DB. Using Cre/LoxP recombination methods, we assessed locomotion, exploratory behavior, and spatial learning and long-term reference memory in adult C57BL/6J Rxfp3 loxP/loxP mice with targeted depletion of Rxfp3 in the MS/DB. Following prior injection of an AAV(1/2)-Cre-IRES-eGFP vector into the MS/DB to delete/deplete Rxfp3 mRNA/RXFP3 protein, mice tested in a Morris water maze (MWM) displayed an impairment in allocentric spatial learning during acquisition, as well as an impairment in long-term reference memory on probe day. However, RXFP3-depleted and control mice displayed similar motor activity in a locomotor cell and exploratory behavior in a large open-field (LOF) test. A quantitative characterization using multiplex, fluorescent in situ hybridization (ISH) identified a high level of co-localization of Rxfp3 mRNA and vesicular GABA transporter (vGAT) mRNA in MS and DB neurons (~87% and ~95% co-expression, respectively). Rxfp3 mRNA was also detected, to a correspondingly lesser extent, in vesicular glutamate transporter 2 (vGlut2) mRNA-containing neurons in MS and DB (~13% and ~5% co-expression, respectively). Similarly, a qualitative assessment of the MS/DB region, identified Rxfp3 mRNA in neurons that expressed parvalbumin (PV) mRNA (reflecting hippocampally-projecting GABA neurons), whereas choline acetyltransferase mRNA-positive (acetylcholine) neurons lacked Rxfp3 mRNA. These data are consistent with a qualitative immunohistochemical analysis that revealed relaxin-3-immunoreactive nerve fibers in close apposition with PV-immunoreactive neurons in the MS/DB. Together these studies suggest relaxin-3/RXFP3 signaling in the MS/DB plays a role in modulating specific learning and long-term memory associated behaviors in adult mice via effects on GABAergic neuron populations known for their involvement in modulating hippocampal theta rhythm and associated cognitive processes.

The Firing Rate Speed Code of Entorhinal Speed Cells Differs across Behaviorally Relevant Time Scales and Does Not Depend on Medial Septum Inputs

The firing rate of speed cells, a dedicated subpopulation of neurons in the medial entorhinal cortex (MEC), is correlated with running speed. This correlation has been interpreted as a speed code used in various computational models for path integration. These models consider firing rate to be linearly tuned by running speed in real-time. However, estimation of firing rates requires integration of spiking events over time, setting constraints on the temporal accuracy of the proposed speed code. We therefore tested whether the proposed speed code by firing rate is accurate at short time scales using data obtained from open-field recordings in male rats and mice. We applied a novel filtering approach differentiating between speed codes at multiple time scales ranging from deciseconds to minutes. In addition, we determined the optimal integration time window for firing-rate estimation using a general likelihood framework and calculated the integration time window that maximizes the correlation between firing rate and running speed. Data show that these time windows are on the order of seconds, setting constraints on real-time speed coding by firing rate. We further show that optogenetic inhibition of either cholinergic, GABAergic, or glutamatergic neurons in the medial septum/diagonal band of Broca does not affect modulation of firing rates by running speed at each time scale tested. These results are relevant for models of path integration and for our understanding of how behavioral activity states may modulate firing rates and likely information processing in the MEC.SIGNIFICANCE STATEMENT Path integration is the most basic form of navigation relying on self-motion cues. Models of path integration use medial septum/diagonal band of Broca (MSDB)-dependent MEC grid-cell firing patterns as the neurophysiological substrate of path integration. These models use a linear speed code by firing rate, but do not consider temporal constraints of integration over time for firing-rate estimation. We show that firing-rate estimation for speed cells requires integration over seconds. Using optogenetics, we show that modulation of firing rates by running speed is independent of MSDB inputs. These results enhance our understanding of path integration mechanisms and the role of the MSDB for information processing in the MEC.

Social isolation impairs the persistence of social recognition memory by disturbing the glutamatergic tonus and the olfactory bulb-dorsal hippocampus coupling

The absence of companion may jeopardize mental health in social animals. Here, we tested the hypothesis that social isolation impairs social recognition memory by altering the excitability and the dialog between the olfactory bulb (OB) and the dorsal hippocampus (dHIP). Adult male Swiss mice were kept grouped (GH) or isolated (SI) for 7 days. Social memory (LTM) was evaluated using social recognition test. SI increased glutamate release in the OB, while decreased in the dHIP. Blocking AMPA and NMDA receptors into the OB or activating AMPA into the dHIP rescued LTM in SI mice, suggesting a cause-effect relationship between glutamate levels and LTM impairment. Additionally, during memory retrieval, phase-amplitude coupling between OB and dHIP decreased in SI mice. Our results indicate that SI impaired the glutamatergic signaling and the normal communication between OB and HIP, compromising the persistence of social memory.

Long-Term Potentiation and Excitability in the Hippocampus Are Modulated Differently by θ Rhythm

Oscillations in the brain facilitate neural processing and cognitive functions. This study investigated the dependence of long-term potentiation (LTP), a neural correlate of memory, on the phase of the hippocampal θ rhythm, a prominent brain oscillation. Multichannel field potentials and current source-sinks were analyzed in hippocampal CA1 of adult male rats under urethane anesthesia. A single burst (five pulses at 200 Hz) stimulation of stratum oriens (OR) induced LTP of the basal dendritic excitatory sink (ES), which was maximal when the burst was delivered at ∼340° and ∼160° of the distal dendritic θ rhythm. Apical dendritic sink evoked by stratum radiatum (RAD) stimulation also showed biphasic maxima at ∼30° and ∼210° of the distal dendritic θ rhythm, about 50° phase delay to basal dendritic LTP. By contrast, maximal population spike (PS) excitability, following single-pulse excitation of the basal or mid-apical dendrites, occurred at a θ phase of ∼140°, and maximal basal dendritic ES occurred at ∼20°; γ (30–57 Hz) activity recorded in CA1 RAD had maximal power at ∼300° of the distal dendritic θ rhythm, different from the phases of maximal LTP. LTP induced during the rising θ phase was NMDA receptor sensitive. It is suggested that the θ phase modulation of CA1 PS excitability is mainly provided by θ-rhythmic proximal inhibition, while dendritic LTP is also modulated by dendritic inhibition and excitation, specific to basal and apical dendrites. In summary, basal and apical dendritic synaptic plasticity and spike excitability are facilitated at different θ phases in a compartmental fashion.

Transforming Sensory Cues into Aversive Emotion via Septal-Habenular Pathway

Emotions evoked by environmental cues are important for animal survival and life quality. However, neural circuits responsible for transforming sensory signals to aversive emotion and behavioral avoidance remain unclear. Here, we found that medial septum (MS) mediates aversion induced by both auditory and somatosensory stimuli. Ablation of glutamatergic or GABAergic MS neurons results in impaired or strengthened aversion, respectively. Optogenetic activation of the two cell types results in place avoidance and preference, respectively. Cell-type-specific screening reveals that glutamatergic MS projections to the lateral habenula (LHb) are responsible for the induction of aversion, which can be antagonized by GABAergic MS projections to LHb. Additionally, the sensory-induced place avoidance is facilitated by enhanced locomotion mediated by glutamatergic MS projections to the preoptic area. Thus, MS can transmit innately aversive signals via a bottom-up multimodal sensory pathway and produce concurrent emotional and motional effects, allowing animals to efficiently avoid unfavorable environments.

Review: Revisiting the human cholinergic nucleus of the diagonal band of Broca

Although the nucleus of the vertical limb of the diagonal band of Broca (nvlDBB) is the second largest cholinergic nucleus in the basal forebrain, after the nucleus basalis of Meynert, it has not generally been a focus for studies of neurodegenerative disorders. However, the nvlDBB has an important projection to the hippocampus and discrete lesions of the rostral basal forebrain have been shown to disrupt retrieval memory function, a major deficit seen in patients with Lewy body disorders. One reason for its neglect is that the anatomical boundaries of the nvlDBB are ill defined and this area of the brain is not part of routine diagnostic sampling protocols. We have reviewed the history and anatomy of the nvlDBB and now propose guidelines for distinguishing nvlDBB from other neighbouring cholinergic cell groups for standardizing future clinicopathological work. Thorough review of the literature regarding neurodegenerative conditions reveals inconsistent results in terms of cholinergic neuronal loss within the nvlDBB. This is likely to be due to the use of variable neuronal inclusion criteria and omission of cholinergic immunohistochemical markers. Extrapolating from those studies showing a significant nvlDBB neuronal loss in Lewy body dementia, we propose an anatomical and functional connection between the cholinergic component of the nvlDBB (Ch2) and the CA2 subfield in the hippocampus which may be especially vulnerable in Lewy body disorders.

Medial septum modulates hippocampal gamma activity and prepulse inhibition in an N-methyl-d-aspartate receptor antagonist model of schizophrenia

We reviewed the participation of the septohippocampal system in an animal model of schizophrenia that was acutely induced by systemic injection of an N-methyl-d-aspartate (NMDA) receptor antagonist such as phencyclidine, MK-801 and ketamine. The NMDA receptor antagonist-induced model of schizophrenia is characterized by behavioral and electrophysiological disruptions, including a decrease in prepulse inhibition of the acoustic startle response (PPI), hyperlocomotion, decrease in gating of hippocampal auditory evoked potentials and robust increase in hippocampal gamma (30-100Hz) oscillations. Similar disruptions were also induced by a single electrographic seizure in the hippocampus. The behavioral and electrophysiological disruptions induced by an NMDA receptor antagonist can be reduced by inactivation or lesion of GABAergic neurons in the medial septum, deep brain stimulation of the medial septum or nucleus accumbens, or positive modulation of GABA_B receptors. Our results suggest a close association between high-amplitude hippocampal gamma oscillations and psychosis-relevant behaviors including PPI loss, behavioral hyperactivity and loss in auditory gating. Abnormal electrophysiology suggests a disruption of somatic and apical dendritic inhibition in the hippocampus, resulting in distorted sensory integration, and impaired cognitive and memory processing. The hippocampus is suggested to be a hub in a brain network that participates in psychosis-relevant behaviors, through its direct projection to the nucleus accumbens, or through indirect connections via the entorhinal, cingulate and prefrontal cortices.

Optogenetic Entrainment of Hippocampal Theta Oscillations in Behaving Mice

Extensive data on relationships of neural network oscillations to behavior and organization of neuronal discharge across brain regions call for new tools to selectively manipulate brain rhythms. Here we describe an approach combining projection-specific optogenetics with extracellular electrophysiology for high-fidelity control of hippocampal theta oscillations (5-10 Hz) in behaving mice. The specificity of the optogenetic entrainment is achieved by targeting channelrhodopsin-2 (ChR2) to the GABAergic population of medial septal cells, crucially involved in the generation of hippocampal theta oscillations, and a local synchronized activation of a subset of inhibitory septal afferents in the hippocampus. The efficacy of the optogenetic rhythm control is verified by a simultaneous monitoring of the local field potential (LFP) across lamina of the CA1 area and/or of neuronal discharge. Using this readily implementable preparation we show efficacy of various optogenetic stimulation protocols for induction of theta oscillations and for the manipulation of their frequency and regularity. Finally, a combination of the theta rhythm control with projection-specific inhibition addresses the readout of particular aspects of the hippocampal synchronization by efferent regions.

Hippocampal Neural Circuits Respond to Optogenetic Pacing of Theta Frequencies by Generating Accelerated Oscillation Frequencies

Biological oscillations can be controlled by a small population of rhythmic pacemaker cells, or in the brain, they also can emerge from complex cellular and circuit-level interactions. Whether and how these mechanisms are combined to give rise to oscillatory patterns that govern cognitive function are not well understood. For example, the activity of hippocampal networks is temporally coordinated by a 7- to 9-Hz local field potential (LFP) theta rhythm, yet many individual cells decouple from the LFP frequency to oscillate at frequencies ∼1 Hz higher. To better understand the network interactions that produce these complex oscillatory patterns, we asked whether the relative frequency difference between LFP and individual cells is retained when the LFP frequency is perturbed experimentally. We found that rhythmic optogenetic stimulation of medial septal GABAergic neurons controlled the hippocampal LFP frequency outside of the endogenous theta range, even during behavioral states when endogenous mechanisms would otherwise have generated 7- to 9-Hz theta oscillations. While the LFP frequency matched the optogenetically induced stimulation frequency, the oscillation frequency of individual hippocampal cells remained broadly distributed, and in a subset of cells including interneurons, it was accelerated beyond the new base LFP frequency. The inputs from septal GABAergic neurons to the hippocampus, therefore, do not appear to directly control the cellular oscillation frequency but rather engage cellular and circuit mechanisms that accelerate the rhythmicity of individual cells. Thus, theta oscillations are an example of cortical oscillations that combine inputs from a subcortical pacemaker with local computations to generate complex oscillatory patterns that support cognitive functions.

Neural mechanisms of navigation involving interactions of cortical and subcortical structures

Animals must perform spatial navigation for a range of different behaviors, including selection of trajectories toward goal locations and foraging for food sources. To serve this function, a number of different brain regions play a role in coding different dimensions of sensory input important for spatial behavior, including the entorhinal cortex, the retrosplenial cortex, the hippocampus, and the medial septum. This article will review data concerning the coding of the spatial aspects of animal behavior, including location of the animal within an environment, the speed of movement, the trajectory of movement, the direction of the head in the environment, and the position of barriers and objects both relative to the animal's head direction (egocentric) and relative to the layout of the environment (allocentric). The mechanisms for coding these important spatial representations are not yet fully understood but could involve mechanisms including integration of self-motion information or coding of location based on the angle of sensory features in the environment. We will review available data and theories about the mechanisms for coding of spatial representations. The computation of different aspects of spatial representation from available sensory input requires complex cortical processing mechanisms for transformation from egocentric to allocentric coordinates that will only be understood through a combination of neurophysiological studies and computational modeling.

Dual-transmitter systems regulating arousal, attention, learning and memory

An array of neuromodulators, including monoamines and neuropeptides, regulate most behavioural and physiological traits. In the past decade, dramatic progress has been made in mapping neuromodulatory circuits, in analysing circuit dynamics, and interrogating circuit function using pharmacogenetic, optogenetic and imaging methods This review will focus on several distinct neural networks (acetylcholine/GABA/glutamate; histamine/GABA; orexin/glutamate; and relaxin-3/GABA) that originate from neural hubs that regulate wakefulness and related attentional and cognitive processes, and highlight approaches that have identified dual transmitter roles in these behavioural functions. Modulation of these different neural networks might be effective treatments of diseases related to arousal/sleep dysfunction and of cognitive dysfunction in psychiatric and neurodegenerative disorders.

A Non-canonical Reticular-Limbic Central Auditory Pathway via Medial Septum Contributes to Fear Conditioning

In the mammalian brain, auditory information is known to be processed along a central ascending pathway leading to auditory cortex (AC). Whether there exist any major pathways beyond this canonical auditory neuraxis remains unclear. In awake mice, we found that auditory responses in entorhinal cortex (EC) cannot be explained by a previously proposed relay from AC based on response properties. By combining anatomical tracing and optogenetic/pharmacological manipulations, we discovered that EC received auditory input primarily from the medial septum (MS), rather than AC. A previously uncharacterized auditory pathway was then revealed: it branched from the cochlear nucleus, and via caudal pontine reticular nucleus, pontine central gray, and MS, reached EC. Neurons along this non-canonical auditory pathway responded selectively to high-intensity broadband noise, but not pure tones. Disruption of the pathway resulted in an impairment of specifically noise-cued fear conditioning. This reticular-limbic pathway may thus function in processing aversive acoustic signals.

GABAergic Neurons in the Rat Medial Septal Complex Express Relaxin-3 Receptor (RXFP3) mRNA

The medial septum (MS) complex modulates hippocampal function and related behaviors. Septohippocampal projections promote and control different forms of hippocampal synchronization. Specifically, GABAergic and cholinergic projections targeting the hippocampal formation from the MS provide bursting discharges to promote theta rhythm, or tonic activity to promote gamma oscillations. In turn, the MS is targeted by ascending projections from the hypothalamus and brainstem. One of these projections arises from the nucleus incertus in the pontine tegmentum, which contains GABA neurons that co-express the neuropeptide relaxin-3 (Rln3). Both stimulation of the nucleus incertus and septal infusion of Rln3 receptor agonist peptides promotes hippocampal theta rhythm. The G_i/o-protein-coupled receptor, relaxin-family peptide receptor 3 (RXFP3), is the cognate receptor for Rln3 and identification of the transmitter phenotype of neurons expressing RXFP3 in the septohippocampal system can provide further insights into the role of Rln3 transmission in the promotion of septohippocampal theta rhythm. Therefore, we used RNAscope multiplex in situ hybridization to characterize the septal neurons expressing Rxfp3 mRNA in the rat. Our results demonstrate that Rxfp3 mRNA is abundantly expressed in vesicular GABA transporter (vGAT) mRNA- and parvalbumin (PV) mRNA-positive GABA neurons in MS, whereas ChAT mRNA-positive acetylcholine neurons lack Rxfp3 mRNA. Approximately 75% of Rxfp3 mRNA-positive neurons expressed vGAT mRNA (and 22% were PV mRNA-positive), while the remaining 25% expressed Rxfp3 mRNA only, consistent with a potential glutamatergic phenotype. Similar proportions were observed in the posterior septum. The occurrence of RXFP3 in PV-positive GABAergic neurons gives support to a role for the Rln3-RXFP3 system in septohippocampal theta rhythm.

Septo-hippocampal interaction

The septo–hippocampal pathway adjusts CA1 network excitability to different behavioral states and is crucially involved in theta rhythmogenesis. In the medial septum, cholinergic, glutamatergic and GABAergic neurons form a highly interconnected local network. Neurons of these three classes project to glutamatergic pyramidal neurons and different subsets of GABAergic neurons in the hippocampal CA1 region. From there, GABAergic neurons project back to the medial septum and form a feedback loop between the two remote brain areas. In vivo, the firing of GABAergic medial septal neurons is theta modulated, while theta modulation is not observed in cholinergic neurons. One prominent feature of glutamatergic neurons is the correlation of their firing rates to the animals running speed. The cellular diversity, the high local interconnectivity and different activity patterns of medial septal neurons during different behaviors complicate the functional dissection of this network. New technical advances help to define specific functions of individual cell classes. In this review, we seek to highlight recent findings and elucidate functional implications of the septo-hippocampal connectivity on the microcircuit scale.

Appetite suppressive role of medial septal glutamatergic neurons

Feeding behavior is controlled by diverse neurons and neural circuits primarily concentrated in the hypothalamus and hindbrain in mammals. In this study, by using chemo/optogenetic techniques along with feeding assays, we investigate how neurons within the medial septal complex (MSc), a brain area implicated in emotion and cognition, contribute to food intake. We find that chemo/optogenetic activation of MSc glutamatergic neurons profoundly reduces food intake during both light and dark periods of the rodent light cycle. Furthermore, we find that selective activation of MSc glutamatergic projections in paraventricular hypothalamus (PVH) reduces food intake, suggesting that MSc glutamatergic neurons suppress feeding by activating downstream neurons in the PVH. Open-field behavioral assays reveal that these neurons do not overtly affect anxiety levels and locomotion. Collectively, our findings demonstrate that septal glutamatergic neurons exert anorexigenic effects by projecting to the PVH without affecting anxiety and physical activities.

Behavior-Dependent Activity and Synaptic Organization of Septo-hippocampal GABAergic Neurons Selectively Targeting the Hippocampal CA3 Area

Rhythmic medial septal (MS) GABAergic input coordinates cortical theta oscillations. However, the rules of innervation of cortical cells and regions by diverse septal neurons are unknown. We report a specialized population of septal GABAergic neurons, the Teevra cells, selectively innervating the hippocampal CA3 area bypassing CA1, CA2, and the dentate gyrus. Parvalbumin-immunopositive Teevra cells show the highest rhythmicity among MS neurons and fire with short burst duration (median, 38 ms) preferentially at the trough of both CA1 theta and slow irregular oscillations, coincident with highest hippocampal excitability. Teevra cells synaptically target GABAergic axo-axonic and some CCK interneurons in restricted septo-temporal CA3 segments. The rhythmicity of their firing decreases from septal to temporal termination of individual axons. We hypothesize that Teevra neurons coordinate oscillatory activity across the septo-temporal axis, phasing the firing of specific CA3 interneurons, thereby contributing to the selection of pyramidal cell assemblies at the theta trough via disinhibition. VIDEO ABSTRACT.

Modulating medial septal cholinergic activity reduces medial entorhinal theta frequency without affecting speed or grid coding

Medial septal inputs to the hippocampal system are crucial for aspects of temporal and spatial processing, such as theta oscillations and grid cell firing. However, the precise contributions of the medial septum’s cholinergic neurones to these functions remain unknown. Here, we recorded neuronal firing and local field potentials from the medial entorhinal cortex of freely foraging mice, while modulating the excitability of medial septal cholinergic neurones. Alteration of cholinergic activity produced a reduction in the frequency of theta oscillations, without affecting the slope of the non-linear theta frequency vs running speed relationship observed. Modifying septal cholinergic tone in this way also led mice to exhibit behaviours associated with novelty or anxiety. However, grid cell firing patterns were unaffected, concordant with an absence of change in the slopes of the theta frequency and firing rate speed signals thought to be used by grid cells.

Untangling a Cholinergic Pathway from Wakefulness to Memory

Acetylcholine is a major modulator of learning and memory, and its availability varies across the sleep-wake cycle. In this issue of Neuron, Papouin et al. (2017) describe a D-serine-dependent pathway involving astroglia by which the transmitter tunes the hippocampus toward memory encoding during wakefulness.

Predicting the stimulation effectiveness using pre-stimulation neural states via optogenetic activation of the medial septum glutamatergic neurons modulating the hippocampal neural activity

In this study, we explored the role of pre-stimulation neural states on the effectiveness of optogenetic stimulation. Optogenetic stimulation was applied to the medial septum glutamatergic neurons to modulate the hippocampal neural activity in a rat tetanus toxin seizure model. The hippocampal local field potential was recorded using a multi electrode array in an awake and behaving rat. Optical stimulation with a 465nm light source was applied at 35Hz in a 20 seconds off / 20 seconds on pattern with simultaneous recording from the hippocampus. Both the baseline and the stimulation period recordings were divided into 2 second segments and used for the further analysis. In the first experiment, a support vector machine (SVM) model classified the neural states by using spectral features between 0 and 50Hz. 447 out of 545 segments (82.02%) were correctly labeled as `Baseline' while only 326 out of 544 (59.93%) segments from the stimulation period were correctly labeled as `Stimulation.' As the ratio of mislabels is significantly higher for the stimulation period (chi-squared, p<;0.01), we concluded that the stimulation was not always effective. In the second experiment, an SVM model predicted the stimulation effectiveness using the spectral features of the pre-stimulation segments. The classification result shows that 63.7% of the pre-stimulation segments correctly predicted the stimulation effectiveness. These findings suggest that the prediction of the stimulation effectiveness may improve the stimulation efficacy by implementing a state-based stimulation protocol.

Astrocytic signaling supports hippocampal-prefrontal theta synchronization and cognitive function

Astrocytes interact with neurons at the cellular level through modulation of synaptic formation, maturation, and function, but the impact of such interaction into behavior remains unclear. Here, we studied the dominant negative SNARE (dnSNARE) mouse model to dissect the role of astrocyte-derived signaling in corticolimbic circuits, with implications for cognitive processing. We found that the blockade of gliotransmitter release in astrocytes triggers a critical desynchronization of neural theta oscillations between dorsal hippocampus and prefrontal cortex. Moreover, we found a strong cognitive impairment in tasks depending on this network. Importantly, the supplementation with d-serine completely restores hippocampal-prefrontal theta synchronization and rescues the spatial memory and long-term memory of dnSNARE mice. We provide here novel evidence of long distance network modulation by astrocytes, with direct implications to cognitive function.

Models of spatial and temporal dimensions of memory

Episodic memory involves coding of the spatial location and time of individual events. Coding of space and time is also relevant to working memory, spatial navigation, and the disambiguation of overlapping memory representations. Neurophysiological data demonstrate that neuronal activity codes the current, past and future location of an animal as well as temporal intervals within a task. Models have addressed how neural coding of space and time for memory function could arise, with both dimensions coded by the same neurons. Neural coding could depend upon network oscillatory and attractor dynamics as well as modulation of neuronal intrinsic properties. These models are relevant to the coding of space and time involving structures including the hippocampus, entorhinal cortex, retrosplenial cortex, striatum and parahippocampal gyrus, which have been implicated in both animal and human studies.

Intrinsic membrane properties and cholinergic modulation of mouse basal forebrain glutamatergic neurons in vitro

The basal forebrain (BF) controls sleep-wake cycles, attention and reward processing. Compared to cholinergic and GABAergic neurons, BF glutamatergic neurons are less well understood, due to difficulties in identification. Here, we use vesicular glutamate transporter 2 (vGluT2)-tdTomato mice, expressing a red fluorescent protein (tdTomato) in the major group of BF glutamatergic neurons (vGluT2+) to characterize their intrinsic electrical properties and cholinergic modulation. Whole-cell, patch-clamp recordings were made from vGluT2+ neurons in coronal BF slices. Most BF vGluT2+ neurons were small/medium sized (<20µm), exhibited moderately sized H-currents and had a maximal firing frequency of ∼50Hz. However, vGluT2+ neurons in dorsal BF (ventral pallidum) had larger H-currents and a higher maximal firing rate (83Hz). A subset of BF vGluT2+ neurons exhibited burst/cluster firing. Most vGluT2+ neurons had low-threshold calcium spikes/currents. vGluT2+ neurons located in ventromedial regions of BF (in or adjacent to the horizontal limb of the diagonal band) were strongly hyperpolarized by the cholinergic agonist, carbachol, a finding apparently in conflict with their increased discharge during wakefulness/REM sleep and hypothesized role in wake-promotion. In contrast, most vGluT2+ neurons located in lateral BF (magnocellular preoptic area) or dorsal BF did not respond to carbachol. Our results suggest that BF glutamatergic neurons are heterogeneous and have morphological, electrical and pharmacological properties which distinguish them from BF cholinergic and GABAergic neurons. A subset of vGluT2+ neurons, possibly those neurons which project to reward-related areas such as the habenula, are hyperpolarized by cholinergic inputs, which may cause phasic inhibition during reward-related events.

The menagerie of the basal forebrain: how many (neural) species are there, what do they look like, how do they behave and who talks to whom?

The diverse cell-types of the basal forebrain control sleep-wake states, cortical activity and reward processing. Large, slow-firing, cholinergic neurons suppress cortical delta activity and promote cortical plasticity in response to reinforcers. Large, fast-firing, cortically-projecting GABAergic neurons promote wakefulness and fast cortical activity. In particular, parvalbumin/GABAergic neurons promote neocortical gamma band activity. Conversely, excitation of slower-firing somatostatin/GABAergic neurons promotes sleep through inhibition of cortically-projecting neurons. Activation of glutamatergic neurons promotes wakefulness, likely by exciting other cortically-projecting neurons. Similarly, cholinergic neurons indirectly promote wakefulness by excitation of wake-promoting, cortically-projecting GABAergic neurons and/or inhibition of sleep-promoting somatostatin/GABAergic neurons. Both glia and neurons increase the levels of adenosine during prolonged wakefulness. Adenosine presynaptically inhibits glutamatergic inputs to wake-promoting cholinergic and GABAergic/parvalbumin neurons, promoting sleep.

Neural changes in Alzheimer's disease from circuit to molecule: Perspective of optogenetics

Alzheimer's disease (AD), as a crucial neurodegenerative disorder, affects neural activities at many levels. Synaptic plasticity and neural circuits are most susceptible in AD, but the detailed mechanism is unclear. Optogenetic tools provide unprecedented spatio-temporal specificity to stimulate specific neural circuits or synaptic molecules to reveal the precise function of normal brain and mechanism of deficits in AD models. Furthermore, using optogenetics to stimulate neurons can rescue learning and memory loss caused by AD. It also has possibility to use light to control the Neurotransmitter receptors and their downstream signal pathway. These technical methods have broad therapeutic application prospect.

Common oscillatory mechanisms across multiple memory systems

The cortex, hippocampus, and striatum support dissociable forms of memory. While each of these regions contains specialized circuitry supporting their respective functions, all structure their activities across time with delta, theta, and gamma rhythms. We review how these oscillations are generated and how they coordinate distinct memory systems during encoding, consolidation, and retrieval. First, gamma oscillations occur in all regions and coordinate local spiking, compressing it into short population bursts. Second, gamma oscillations are modulated by delta and theta oscillations. Third, oscillatory dynamics in these memory systems can operate in either a 'slow' or 'fast' mode. The slow mode happens during slow-wave sleep (SWS) and is characterized by large irregular activity in the hippocampus and delta oscillations in cortical and striatal circuits. The fast mode occurs during active waking and REM and is characterized by theta oscillations in the hippocampus and its targets, along with gamma oscillations in the rest of cortex. In waking, the fast mode is associated with the efficacious encoding and retrieval of declarative and procedural memories. Theta and gamma oscillations have the similar relationships with encoding and retrieval across multiple forms of memory and brain regions, despite regional differences in microcircuitry and information content. Differences in the oscillatory coordination of memory systems during sleep might explain why the consolidation of some forms of memory is sensitive to SWS, while others depend on REM. In particular, theta oscillations appear to support the consolidation of certain types of procedural memories during REM, while delta oscillations during SWS seem to promote declarative and procedural memories.

Distribution, physiology and pharmacology of relaxin-3/RXFP3 systems in brain

Relaxin-3 is a member of a superfamily of structurally-related peptides that includes relaxin and insulin-like peptide hormones. Soon after the discovery of the relaxin-3 gene, relaxin-3 was identified as an abundant neuropeptide in brain with a distinctive topographical distribution within a small number of GABAergic neuron populations that is well conserved across species. Relaxin-3 is thought to exert its biological actions through a single class-A GPCR - relaxin-family peptide receptor 3 (RXFP3). Class-A comprises GPCRs for relaxin-3 and insulin-like peptide-5 and other peptides such as orexin and the monoamine transmitters. The RXFP3 receptor is selectively activated by relaxin-3, whereas insulin-like peptide-5 is the cognate ligand for the related RXFP4 receptor. Anatomical and pharmacological evidence obtained over the last decade supports a function of relaxin-3/RXFP3 systems in modulating responses to stress, anxiety-related and motivated behaviours, circadian rhythms, and learning and memory. Electrophysiological studies have identified the ability of RXFP3 agonists to directly hyperpolarise thalamic neurons in vitro, but there are no reports of direct cell signalling effects in vivo. This article provides an overview of earlier studies and highlights more recent research that implicates relaxin-3/RXFP3 neural network signalling in the integration of arousal, motivation, emotion and related cognition, and that has begun to identify the associated neural substrates and mechanisms. Future research directions to better elucidate the connectivity and function of different relaxin-3 neuron populations and their RXFP3-positive target neurons in major experimental species and humans are also identified.

LINKED ARTICLES

This article is part of a themed section on Recent Progress in the Understanding of Relaxin Family Peptides and their Receptors. To view the other articles in this section visit http://onlinelibrary.wiley.com/doi/10.1111/bph.v174.10/issuetoc.

Potential roles of cholinergic modulation in the neural coding of location and movement speed

Behavioral data suggest that cholinergic modulation may play a role in certain aspects of spatial memory, and neurophysiological data demonstrate neurons that fire in response to spatial dimensions, including grid cells and place cells that respond on the basis of location and running speed. These neurons show firing responses that depend upon the visual configuration of the environment, due to coding in visually-responsive regions of the neocortex. This review focuses on the physiological effects of acetylcholine that may influence the sensory coding of spatial dimensions relevant to behavior. In particular, the local circuit effects of acetylcholine within the cortex regulate the influence of sensory input relative to internal memory representations via presynaptic inhibition of excitatory and inhibitory synaptic transmission, and the modulation of intrinsic currents in cortical excitatory and inhibitory neurons. In addition, circuit effects of acetylcholine regulate the dynamics of cortical circuits including oscillations at theta and gamma frequencies. These effects of acetylcholine on local circuits and network dynamics could underlie the role of acetylcholine in coding of spatial information for the performance of spatial memory tasks.

Microcircuits in respiratory rhythm generation: commonalities with other rhythm generating networks and evolutionary perspectives

Rhythmicity is critical for the generation of rhythmic behaviors and higher brain functions. This review discusses common mechanisms of rhythm generation, including the role of synaptic inhibition and excitation, with a focus on the mammalian respiratory network. This network generates three phases of breathing and is highly integrated with brain regions associated with numerous non-ventilatory behaviors. We hypothesize that during evolution multiple rhythmogenic microcircuits were recruited to accommodate the generation of each breathing phase. While these microcircuits relied primarily on excitatory mechanisms, synaptic inhibition became increasingly important to coordinate the different microcircuits and to integrate breathing into a rich behavioral repertoire that links breathing to sensory processing, arousal, and emotions as well as learning and memory.

The forebrain medial septal region and nociception

The forebrain medial septum, which is an integral part of the septo-hippocampal network, is implicated in sensorimotor integration, fear and anxiety, and spatial learning and memory. A body of evidence also suggests that the septal region affects experimental pain. Indeed, some explorations in humans have raised the possibility that the region may modulate clinical pain as well. This review explores the evidence that implicates the medial septum in nociception and suggests that non-overlapping circuits in the region facilitate acute nociceptive behaviors and defensive behaviors that reflect affect and cognitive appraisal, especially in relation to persistent nociception. In line with a role in nociception, the region modulates nociceptive responses in the neuraxis, including the hippocampus and the anterior cingulate cortex. The aforementioned forebrain regions have also been implicated in persistent/long-lasting nociception. The review also weighs the effects of the medial septum on nociception vis-à-vis the known roles of the region and emphasizes the fact that the region is a part of network of forebrain structures which have been long associated with reward, cognition and affect-motivation and are now implicated in persistent/long-lasting nociception.

Nucleus incertus promotes cortical desynchronization and behavioral arousal

Arousal and vigilance are essential for survival and relevant regulatory neural circuits lie within the brainstem, hypothalamus and forebrain. The nucleus incertus (NI) is a distinct site within the pontine periventricular gray, containing a substantial population of GABAergic neurons with long-range, ascending projections. Existing neuroanatomical data and functional studies in anesthetized rats, suggest the NI is a central component of a midline behavioral control network well positioned to modulate arousal, vigilance and exploratory navigation, yet none of these roles have been established experimentally. We used a chemogenetic approach-clozapine-N-oxide (CNO) activation of virally delivered excitatory hM3Dq-DREADDs-to activate the NI in rats and examined the behavioral and physiological effects, relative to effects in naïve rats and appropriate viral-treated controls. hM3Dq activation by CNO resulted in long-lasting depolarization of NI neurons with action potentials, in vitro. Peripheral injection of CNO significantly increased c-Fos immunoreactivity in the NI and promoted cortical electroencephalograph (EEG) desynchronization. These brain changes were associated with heightened arousal, and increased locomotor activity in the homecage and in a novel environment. Furthermore, NI activation altered responses in a fear conditioning paradigm, reflected by increased head-scanning, vigilant behaviors during conditioned fear recall. These findings provide direct evidence that the NI promotes general arousal via a broad behavioral activation circuit and support early hypotheses, based on its connectivity, that the NI is a modulator of cognition and attention, and emotional and motivated behaviors.

Central relaxin-3 receptor (RXFP3) activation increases ERK phosphorylation in septal cholinergic neurons and impairs spatial working memory

The medial septum/diagonal band (MS/DB) is a relay region connecting the hypothalamus and brainstem with the hippocampus, and both the MS/DB and dorsal/ventral hippocampus receive strong topographic GABA/peptidergic projections from the nucleus incertus of the pontine tegmentum. The neuropeptide relaxin-3, released by these neurons, is the cognate ligand for a G_i/o-protein-coupled receptor, RXFP3, which is highly expressed within the MS/DB, and both cholinergic and GABAergic neurons in this region of rat brain receive relaxin-3 positive terminals/boutons. Comprehensive in vitro studies have demonstrated that the cell signaling pathways altered by RXFP3 stimulation, include inhibition of forskolin-activated cAMP levels and activation of ERK phosphorylation. In this study we investigated whether intracerebroventricular (icv) injection of RXFP3-A2, a selective relaxin-3 receptor agonist, altered ERK phosphorylation levels in the MS/DB of adult male rats. We subsequently assessed the neurochemical phenotype of phosphorylated (p) ERK-positive neurons in MS/DB after icv RXFP3-A2 administration by dual-label immunostaining for pERK and neuronal markers for cholinergic and GABAergic neurons. Central RXFP3-A2 injection significantly increased levels of pERK immunoreactivity (IR) in MS/DB at 20 and 90 min post-injection, compared to vehicle and naive levels. In addition, RXFP3-A2 increased the number of cells expressing pERK-IR in the MS/DB at 90 (but not 20) min post-injection in cholinergic (but not GABAergic) neurons, which also expressed putative RXFP3-IR. Moreover, icv injection of RXFP3-A2 impaired alternation in a delayed spontaneous T-maze test of spatial working memory. The presence of RXFP3-like IR and the RXFP3-related activation of the MAPK/ERK pathway in MS/DB cholinergic neurons identifies them as a key target of ascending relaxin-3 projections with implications for the acute and chronic modulation of cholinergic neuron activity and function by relaxin-3/RXFP3 signaling.