Ventral hippocampal OLM cells control type 2 theta oscillations and response to predator odor

Electroencephalography Biomarkers of α5-GABA Positive Allosteric Modulators in Rodents

Background

Reduced cortical inhibition mediated by GABA (gamma-aminobutyric acid) is reported in depression, anxiety disorders, and aging. A novel positive allosteric modulator that specifically targets the α5-GABAA receptor subunit (α5-PAM), ligand GL-II-73 shows anxiolytic, antidepressant, and procognitive effects without the common side effects associated with nonspecific modulation by benzodiazepines such as diazepam, thus suggesting novel therapeutic potential. However, it is unknown whether α5-PAM has detectable signatures in clinically relevant brain electroencephalography (EEG).

Methods

We analyzed EEG in 10 freely moving rats at baseline and following injections of α5-PAM (GL-II-73) and diazepam.

Results

We showed that α5-PAM specifically decreased theta peak power, whereas diazepam shifted peak power from high to low theta while increasing beta and gamma power. EEG decomposition showed that these effects were periodic and corresponded to changes in theta oscillation event duration.

Conclusions

Thus, our study shows that α5-PAM has robust and distinct EEG biomarkers in rodents, indicating that EEG could enable noninvasive monitoring of α5-PAM treatment efficacy.

Mental exploration of future choices during immobility theta oscillations

Mental exploration enables flexible evaluation of potential future choices, guiding decision-making without requiring direct real-world iterations. Although the hippocampus is known to be active while imagining the future, the precise mechanisms that support mental exploration of future choices remain unclear. In the hippocampus, the theta rhythm (4-12 Hz) is prevalent during movement and supports memory coding during real-world exploration by organizing neuronal activity patterns into short virtual path segments (theta sequences) around the rat's location. We observed these theta-related neural activity patterns during movement in a hippocampus-dependent working memory task and also, unexpectedly, theta oscillations and theta-related neural activity during immobility. Compared to standard theta sequences during movement, theta sequences during immobility differed in that they occurred at a shifted theta phase and preferentially represented remote locations, in particular the next choice in the working memory task. Coding for future locations was also observed during awake sharp wave ripple, but these short-lasting events occurred rarely and were biased toward frequently visited locations. Therefore, our findings suggest that recurring bouts of theta oscillations during immobility, which are also observed in primates and humans, support the cognitive demands of mental exploration in the hippocampal network and facilitate ongoing predictions of future choices.

Breathing Modulates Network Activity in Frontal Brain Regions during Anxiety

Anxiety elicits various physiological responses, including changes in the respiratory rate and neuronal activity within specific brain regions such as the medial prefrontal cortex (mPFC). Previous research suggests that the olfactory bulb (OB) modulates the mPFC through respiration-coupled oscillations (RCOs), which have been linked to fear-related freezing behavior. Nevertheless, the impact of breathing on frontal brain networks during other negative emotional responses, such as anxiety-related states characterized by higher breathing rates, remains unclear. To address this, we subjected rats to the elevated plus maze (EPM) paradigm while simultaneously recording respiration and local field potentials in the OB and mPFC. Our findings demonstrate distinct respiratory patterns during EPM exploration: slower breathing frequencies prevailed in the closed arms, whereas faster frequencies were observed in the open arms, independent of locomotor activity, indicating that anxiety-like states are associated with increased respiratory rates. Additionally, we identified RCOs at different frequencies, mirroring the bimodal distribution of respiratory frequencies. RCOs exhibited higher power during open-arm exploration, when they showed greater coherence with breathing at faster frequencies. Furthermore, we confirmed that nasal respiration drives RCOs in frontal brain regions and found a stronger effect during faster breathing. Interestingly, we observed that the frequency of prefrontal gamma oscillations modulated by respiration increased with breathing frequency. Overall, our study provides evidence for a significant influence of breathing on prefrontal cortex networks during anxious states, shedding light on the complex interplay between respiratory physiology and emotional processing.

Mystery of gamma wave stimulation in brain disorders

Neuronal oscillations refer to rhythmic and periodic fluctuations of electrical activity in the central nervous system that arise from the cellular properties of diverse neuronal populations and their interactions. Specifically, gamma oscillations play a crucial role in governing the connectivity between distinct brain regions, which are essential in perception, motor control, memory, and emotions. In this context, we recapitulate various current stimulation methods to induce gamma entrainment. These methods include sensory stimulation, optogenetic modulation, photobiomodulation, and transcranial electrical or magnetic stimulation. Simultaneously, we explore the association between abnormal gamma oscillations and central nervous system disorders such as Alzheimer's disease, Parkinson's disease, stroke, schizophrenia, and autism spectrum disorders. Evidence suggests that gamma entrainment-inducing stimulation methods offer notable neuroprotection, although somewhat controversial. This review comprehensively discusses the functional role of gamma oscillations in higher-order brain activities from both physiological and pathological perspectives, emphasizing gamma entrainment as a potential therapeutic approach for neuropsychiatric disorders. Additionally, we discuss future opportunities and challenges in implementing such strategies.

Ventral hippocampal interneurons govern extinction and relapse of contextual associations

Contextual memories are critical for survival but must be extinguished when new conditions render them nonproductive. By most accounts, extinction forms a new memory that competes with the original association for control over behavior, but the underlying circuit mechanisms remain largely enigmatic. Here, we demonstrate that extinction of contextual fear conditioning recruits somatostatin interneurons (SST-INs) in the ventral hippocampus. Correspondingly, real-time activity of SST-INs correlates with transitions between immobility and movement, signaling exit from defensive freezing bouts. Optogenetic manipulation of SST-INs but not parvalbumin interneurons (PV-INs) elicits bidirectional changes in freezing that are specific to the context in which extinction was acquired. Finally, similar effects were obtained following extinction of sucrose-based appetitive conditioning, in which SST-IN inhibition triggers relapse to reward seeking. These data suggest that ventral hippocampal SST-INs play a fundamental role in extinction that is independent of affective valence and may be related to their disruption of spontaneous emotional responses.

O-GlcNAcylation of circadian clock protein Bmal1 impairs cognitive function in diabetic mice

Neuronal damage in the hippocampus induced by high glucose has been shown to promote the onset and development of cognitive impairment in diabetes, but the underlying molecular mechanism remains unclear. Guided by single-cell RNA sequencing, we here report that high glucose increases O-GlcNAcylation of Bmal1 in hippocampal neurons. This glycosylation promotes the binding of Clock to Bmal1, resulting in the expression of transcription factor Bhlhe41 and its target Dnajb4. Upregulated Dnajb4 in turn leads to ubiquitination and degradation of the mitochondrial Na + /Ca2+ exchanger NCLX, thereby inducing mitochondrial calcium overload that causes neuronal damage and cognitive impairment in mice. Notably, Bhlhe41 downregulation or treatment with a short peptide that specifically blocks O-GlcNAcylation of Bmal1 on Ser424 mitigated these adverse effects in diabetic mouse models. These data highlight the crucial role of O-GlcNAcylation in circadian clock gene expression and may facilitate the design of targeted therapies for diabetes-associated cognitive impairment.

Somatostatin: Linking Cognition and Alzheimer Disease to Therapeutic Targeting

Over 4 decades of research support the link between Alzheimer disease (AD) and somatostatin [somatotropin-releasing inhibitory factor (SRIF)]. SRIF and SRIF-expressing neurons play an essential role in brain function, modulating hippocampal activity and memory formation. Loss of SRIF and SRIF-expressing neurons in the brain rests at the center of a series of interdependent pathological events driven by amyloid-β peptide (Aβ), culminating in cognitive decline and dementia. The connection between the SRIF and AD further extends to the neuropsychiatric symptoms, seizure activity, and inflammation, whereas preclinical AD investigations show SRIF or SRIF receptor agonist administration capable of enhancing cognition. SRIF receptor subtype-4 activation in particular presents unique attributes, with the potential to mitigate learning and memory decline, reduce comorbid symptoms, and enhance enzymatic degradation of Aβ in the brain. Here, we review the links between SRIF and AD along with the therapeutic implications. SIGNIFICANCE STATEMENT: Somatostatin and somatostatin-expressing neurons in the brain are extensively involved in cognition. Loss of somatostatin and somatostatin-expressing neurons in Alzheimer disease rests at the center of a series of interdependent pathological events contributing to cognitive decline and dementia. Targeting somatostatin-mediated processes has significant therapeutic potential for the treatment of Alzheimer disease.

Distinct ventral hippocampal inhibitory microcircuits regulating anxiety and fear behaviors

In emotion research, anxiety and fear have always been interconnected, sharing overlapping brain structures and neural circuitry. Recent investigations, however, have unveiled parallel long-range projection pathways originating from the ventral hippocampus, shedding light on their distinct roles in anxiety and fear. Yet, the mechanisms governing the emergence of projection-specific activity patterns to mediate different negative emotions remain elusive. Here, we show a division of labor in local GABAergic inhibitory microcircuits of the ventral hippocampus, orchestrating the activity of subpopulations of pyramidal neurons to shape anxiety and fear behaviors in mice. These findings offer a comprehensive insight into how distinct inhibitory microcircuits are dynamically engaged to encode different emotional states.

Altered firing output of VIP interneurons and early dysfunctions in CA1 hippocampal circuits in the 3xTg mouse model of Alzheimer's disease

Alzheimer's disease (AD) leads to progressive memory decline, and alterations in hippocampal function are among the earliest pathological features observed in human and animal studies. GABAergic interneurons (INs) within the hippocampus coordinate network activity, among which type 3 interneuron-specific (I-S3) cells expressing vasoactive intestinal polypeptide and calretinin play a crucial role. These cells provide primarily disinhibition to principal excitatory cells (PCs) in the hippocampal CA1 region, regulating incoming inputs and memory formation. However, it remains unclear whether AD pathology induces changes in the activity of I-S3 cells, impacting the hippocampal network motifs. Here, using young adult 3xTg-AD mice, we found that while the density and morphology of I-S3 cells remain unaffected, there were significant changes in their firing output. Specifically, I-S3 cells displayed elongated action potentials and decreased firing rates, which was associated with a reduced inhibition of CA1 INs and their higher recruitment during spatial decision-making and object exploration tasks. Furthermore, the activation of CA1 PCs was also impacted, signifying early disruptions in CA1 network functionality. These findings suggest that altered firing patterns of I-S3 cells might initiate early-stage dysfunction in hippocampal CA1 circuits, potentially influencing the progression of AD pathology.

Loss of mGlu5 receptors in somatostatin-expressing neurons alters negative emotional states

Subtype 5 metabotropic glutamate receptors (mGlu5) are known to play an important role in regulating cognitive, social and valence systems. However, it remains largely unknown at which circuits and neuronal types mGlu5 act to influence these behavioral domains. Altered tissue- or cell-specific expression or function of mGlu5 has been proposed to contribute to the exacerbation of neuropsychiatric disorders. Here, we examined how these receptors regulate the activity of somatostatin-expressing (SST+) neurons, as well as their influence on behavior and brain rhythmic activity. Loss of mGlu5 in SST+ neurons elicited excitatory synaptic dysfunction in a region and sex-specific manner together with a range of emotional imbalances including diminished social novelty preference, reduced anxiety-like behavior and decreased freezing during retrieval of fear memories. In addition, the absence of mGlu5 in SST+ neurons during fear processing impaired theta frequency oscillatory activity in the medial prefrontal cortex and ventral hippocampus. These findings reveal a critical role of mGlu5 in controlling SST+ neurons excitability necessary for regulating negative emotional states.

Understanding OLM interneurons: Characterization, circuitry, and significance in memory and navigation

Understanding the intricate mechanisms underlying memory formation and retention relies on unraveling how the hippocampus, a structure fundamental for memory acquisition, is organized. Within the complex hippocampal network, interneurons play a crucial role in orchestrating memory processes. Among these interneurons, Oriens-Lacunosum Moleculare (OLM) cells emerge as key regulators, governing the flow of information to CA1 pyramidal cells. In this review, we explore OLM interneurons in detail, describing their mechanisms and effects on memory processing, particularly in spatial and contextual memory tasks. Our aim is to provide a detailed understanding of how OLM interneurons contribute to the dynamic landscape of memory formation and retrieval.

Exploring the Effect of Arsenic-Containing Hydrocarbon on the Bidirectional Synaptic Plasticity of the Dorsal Hippocampus

Arsenic-containing hydrocarbons (AsHCs) are common in marine organisms. However, there is little research on their effects on the central nervous system's advanced activities, such as cognition. Bidirectional synaptic plasticity dynamically regulates cognition through the balance of long-term potentiation (LTP) and long-term depression (LTD). However, the effects of AsHCs on bidirectional synaptic plasticity and the underlying molecular mechanisms remain unexplored. This study provides the first evidence that 15 μg As L-1 AsHC 360 enhances bidirectional synaptic plasticity, occurring during the maintenance phase rather than the baseline phase. Further calcium gradient experiments hypothesize that AsHC 360 may enhance bidirectional synaptic plasticity by affecting calcium ion levels. The enhancement of bidirectional synaptic plasticity by 15 μg As L-1 AsHC 360 holds significant implications in improving cognitive function, treating neuro-psychiatric disorders, promoting neural recovery, and enhancing brain adaptability.

Phase locking of hippocampal CA3 neurons to distal CA1 theta oscillations selectively predicts memory performance

How the coordination of neuronal spiking and brain rhythms between hippocampal subregions supports memory function remains elusive. We studied the interregional coordination of CA3 neuronal spiking with CA1 theta oscillations by recording electrophysiological signals along the proximodistal axis of the hippocampus in rats that were performing a high-memory-demand recognition memory task adapted from humans. We found that CA3 population spiking occurs preferentially at the peak of distal CA1 theta oscillations when memory was tested but only when previously encountered stimuli were presented. In addition, decoding analyses revealed that only population cell firing of proximal CA3 together with that of distal CA1 can predict performance at test in the present non-spatial task. Overall, our work demonstrates an important role for the synchronization of CA3 neuronal activity with CA1 theta oscillations during memory testing.

Olfactory bulb-medial prefrontal cortex theta synchronization is associated with anxiety

Anxiety is among the most fundamental mammalian behaviors. Despite the physiological and pathological importance, its underlying neural mechanisms remain poorly understood. Here, we recorded the activity of olfactory bulb (OB) and medial prefrontal cortex (mPFC) of rats, which are critical structures to brain's emotional processing network, while exploring different anxiogenic environments. Our results show that presence in anxiogenic contexts increases the OB and mPFC regional theta activities. Also, these local activity changes are associated with enhanced OB-mPFC theta power- and phase-based functional connectivity as well as OB-to-mPFC information transfer. Interestingly, these effects are more prominent in the unsafe zones of the anxiogenic environments, compared to safer zones. This consistent trend of changes in diverse behavioral environments as well as local and long-range neural activity features suggest that the dynamics of OB-mPFC circuit theta oscillations might underlie different types of anxiety behaviors, with possible implications for anxiety disorders.

Alternate-day fasting ameliorated anxiety-like behavior in high-fat diet-induced obese mice

Alternate-day fasting (ADF) has been reported to reduce body weight, neuroinflammation, and oxidative stress damage. However, it is not known whether ADF affects obesity-induced anxiety-like behavior. Here, male C57BL/6 mice were given an alternate fasting and high-fat diet (HFD) or standard chow diet (SD) every other day for 16 or 5 weeks. After the intervention, the degree of anxiety of the mice was evaluated by the open field test (OFT) and the elevated plus maze (EPM) test. Pathological changes in the hippocampus, the expression of Sirt1 and its downstream protein monoamine oxidase A (MAO-A) in the hippocampus, and the expression of 5-hydroxytryptamine (5-HT) were detected. Compared with HFD-fed mice, HFD-fed mice subjected to ADF for 16 weeks had a lower body weight but more brown adipose tissue (BAT), less anxiety behavior, and less pathological damage in the hippocampus, and lower expression of Sirt1 and MAO-A protein and higher 5-HT levels in the hippocampus could be observed. In addition, we noted that long-term ADF intervention could cause anxiety-like behavior in SD mice. Next, we changed the intervention time to 5 weeks. The results showed that short-term ADF intervention could reduce the body weight and increase the BAT mass of SD mice, but it did not affect anxiety. These results indicated that long-term ADF ameliorated obesity-induced anxiety-like behavior and hippocampal damage, but caused anxiety in normal-weight mice. Short-term ADF did not produce adverse emotional reactions in normal-weight mice. Here, we might provide new ideas for the treatment of obesity-induced anxiety.

The role of the hippocampus in the consolidation of emotional memories during sleep

Episodic memory relies on the hippocampus, a heterogeneous brain region with distinct functions. Spatial representations in the dorsal hippocampus (dHPC) are crucial for contextual memory, while the ventral hippocampus (vHPC) is more involved in emotional processing. Here, we review the literature in rodents highlighting the anatomical and functional properties of the hippocampus along its dorsoventral axis that underlie its role in contextual and emotional memory encoding, consolidation, and retrieval. We propose that the coordination between the dorsal and vHPC through theta oscillations during rapid eye movement (REM) sleep, and through sharp-wave ripples during non-REM (NREM) sleep, might facilitate the transfer of contextual information for integration with valence-related processing in other structures of the network. Further investigation into the physiology of the vHPC and its connections with other brain areas is needed to deepen the current understanding of emotional memory consolidation during sleep.

Thalamic nucleus reuniens coordinates prefrontal-hippocampal synchrony to suppress extinguished fear

Traumatic events result in vivid and enduring fear memories. Suppressing the retrieval of these memories is central to behavioral therapies for pathological fear. The medial prefrontal cortex (mPFC) and hippocampus (HPC) have been implicated in retrieval suppression, but how mPFC-HPC activity is coordinated during extinction retrieval is unclear. Here we show that after extinction training, coherent theta oscillations (6-9 Hz) in the HPC and mPFC are correlated with the suppression of conditioned freezing in male and female rats. Inactivation of the nucleus reuniens (RE), a thalamic hub interconnecting the mPFC and HPC, reduces extinction-related Fos expression in both the mPFC and HPC, dampens mPFC-HPC theta coherence, and impairs extinction retrieval. Conversely, theta-paced optogenetic stimulation of RE augments fear suppression and reduces relapse of extinguished fear. Collectively, these results demonstrate a role for RE in coordinating mPFC-HPC interactions to suppress fear memories after extinction.

The medial septum controls hippocampal supra-theta oscillations

Hippocampal theta oscillations orchestrate faster beta-to-gamma oscillations facilitating the segmentation of neural representations during navigation and episodic memory. Supra-theta rhythms of hippocampal CA1 are coordinated by local interactions as well as inputs from the entorhinal cortex (EC) and CA3 inputs. However, theta-nested gamma-band activity in the medial septum (MS) suggests that the MS may control supra-theta CA1 oscillations. To address this, we performed multi-electrode recordings of MS and CA1 activity in rodents and found that MS neuron firing showed strong phase-coupling to theta-nested supra-theta episodes and predicted changes in CA1 beta-to-gamma oscillations on a cycle-by-cycle basis. Unique coupling patterns of anatomically defined MS cell types suggested that indirect MS-to-CA1 pathways via the EC and CA3 mediate distinct CA1 gamma-band oscillations. Optogenetic activation of MS parvalbumin-expressing neurons elicited theta-nested beta-to-gamma oscillations in CA1. Thus, the MS orchestrates hippocampal network activity at multiple temporal scales to mediate memory encoding and retrieval.

The alpha2 nicotinic acetylcholine receptor, a subunit with unique and selective expression in inhibitory interneurons associated with principal cells

Nicotinic acetylcholine receptors (nAChRs) play crucial roles in various human disorders, with the α7, α4, α6, and α3-containing nAChR subtypes extensively studied in relation to conditions such as Alzheimer's disease, Parkinson's disease, nicotine dependence, mood disorders, and stress disorders. In contrast, the α2-nAChR subunit has received less attention due to its more restricted expression and the scarcity of specific agonists and antagonists for studying its function. Nevertheless, recent research has shed light on the unique expression pattern of the Chrna2 gene, which encodes the α2-nAChR subunit, and its involvement in distinct populations of inhibitory interneurons. This review highlights the structure, pharmacology, localization, function, and disease associations of α2-containing nAChRs and points to the unique expression pattern of the Chrna2 gene and its role in different inhibitory interneuron populations. These populations, including the oriens lacunosum moleculare (OLM) cells in the hippocampus, Martinotti cells in the neocortex, and Renshaw cells in the spinal cord, share common features and contribute to recurrent inhibitory microcircuits. Thus, the α2-nAChR subunit's unique expression pattern in specific interneuron populations and its role in recurrent inhibitory microcircuits highlight its importance in various physiological processes. Further research is necessary to uncover the comprehensive functionality of α2-containing nAChRs, delineate their specific contributions to neuronal circuits, and investigate their potential as therapeutic targets for related disorders.

Bidirectional plasticity of GABAergic tonic inhibition in hippocampal somatostatin- and parvalbumin-containing interneurons

GABA_A receptors present in extrasynaptic areas mediate tonic inhibition in hippocampal neurons regulating the performance of neural networks. In this study, we investigated the effect of NMDA-induced plasticity on tonic inhibition in somatostatin- and parvalbumin-containing interneurons. Using pharmacological methods and transgenic mice (SST-Cre/PV-Cre x Ai14), we induced the plasticity of GABAergic transmission in somatostatin- and parvalbumin-containing interneurons by a brief (3 min) application of NMDA. In the whole-cell patch-clamp configuration, we measured tonic currents enhanced by specific agonists (etomidate or gaboxadol). Furthermore, in both the control and NMDA-treated groups, we examined to what extent these changes depend on the regulation of distinct subtypes of GABA_A receptors. Tonic conductance in the somatostatin-containing (SST+) interneurons is enhanced after NMDA application, and the observed effect is associated with an increased content of α5-containing GABA_ARs. Both fast-spiking and non-fast-spiking parvalbumin-positive (PV+) cells showed a reduction of tonic inhibition after plasticity induction. This effect was accompanied in both PV+ interneuron types by a strongly reduced proportion of δ-subunit-containing GABA_ARs and a relatively small increase in currents mediated by α5-containing GABA_ARs. Both somatostatin- and parvalbumin-containing interneurons show cell type-dependent and opposite sign plasticity of tonic inhibition. The underlying mechanisms depend on the cell-specific balance of plastic changes in the contents of α5 and δ subunit-containing GABA_ARs.

Hippocampus: Molecular, Cellular, and Circuit Features in Anxiety

Anxiety disorders are currently a major psychiatric and social problem, the mechanisms of which have been only partially elucidated. The hippocampus serves as a major target of stress mediators and is closely related to anxiety modulation. Yet so far, its complex anatomy has been a challenge for research on the mechanisms of anxiety regulation. Recent advances in imaging, virus tracking, and optogenetics/chemogenetics have permitted elucidation of the activity, connectivity, and function of specific cell types within the hippocampus and its connected brain regions, providing mechanistic insights into the elaborate organization of the hippocampal circuitry underlying anxiety. Studies of hippocampal neurotransmitter systems, including glutamatergic, GABAergic, cholinergic, dopaminergic, and serotonergic systems, have contributed to the interpretation of the underlying neural mechanisms of anxiety. Neuropeptides and neuroinflammatory factors are also involved in anxiety modulation. This review comprehensively summarizes the hippocampal mechanisms associated with anxiety modulation, based on molecular, cellular, and circuit properties, to provide tailored targets for future anxiety treatment.

Direct Enhancement Effect of Hippocampal Cholinergic Neurostimulating Peptide on Cholinergic Activity in the Hippocampus

The cholinergic efferent network from the medial septal nucleus to the hippocampus is crucial for learning and memory. This study aimed to clarify whether hippocampal cholinergic neurostimulating peptide (HCNP) has a rescue function in the cholinergic dysfunction of HCNP precursor protein (HCNP-pp) conditional knockout (cKO). Chemically synthesized HCNP or a vehicle were continuously administered into the cerebral ventricle of HCNP-pp cKO mice and littermate floxed (control) mice for two weeks via osmotic pumps. We immunohistochemically measured the cholinergic axon volume in the stratum oriens and functionally evaluated the local field potential in the CA1. Furthermore, choline acetyltransferase (ChAT) and nerve growth factor (NGF) receptor (TrkA and p75NTR) abundances were quantified in wild-type (WT) mice administered HCNP or the vehicle. As a result, HCNP administration morphologically increased the cholinergic axonal volume and electrophysiological theta power in HCNP-pp cKO and control mice. Following the administration of HCNP to WT mice, TrkA and p75NTR levels also decreased significantly. These data suggest that extrinsic HCNP may compensate for the reduced cholinergic axonal volume and theta power in HCNP-pp cKO mice. HCNP may function complementarily to NGF in the cholinergic network in vivo. HCNP may represent a therapeutic candidate for neurological diseases with cholinergic dysfunction, e.g., Alzheimer's disease and Lewy body dementia.

Theta oscillations represent collective dynamics of multineuronal membrane potentials of murine hippocampal pyramidal cells

Theta (θ) oscillations are one of the characteristic local field potentials (LFPs) in the hippocampus that emerge during spatial navigation, exploratory sniffing, and rapid eye movement sleep. LFPs are thought to summarize multineuronal events, including synaptic currents and action potentials. However, no in vivo study to date has directly interrelated θ oscillations with the membrane potentials (Vm) of multiple neurons, and it remains unclear whether LFPs can be predicted from multineuronal Vms. Here, we simultaneously patch-clamp up to three CA1 pyramidal neurons in awake or anesthetized mice and find that the temporal evolution of the power and frequency of θ oscillations in Vms (θ_Vms) are weakly but significantly correlate with LFP θ oscillations (θ_LFP) such that a deep neural network could predict the θ_LFP waveforms based on the θ_Vm traces of three neurons. Therefore, individual neurons are loosely interdependent to ensure freedom of activity, but they partially share information to collectively produce θ_LFP.

110 μm thin endo-microscope for deep-brain in vivo observations of neuronal connectivity, activity and blood flow dynamics

Light-based in-vivo brain imaging relies on light transport over large distances of highly scattering tissues. Scattering gradually reduces imaging contrast and resolution, making it difficult to reach structures at greater depths even with the use of multiphoton techniques. To reach deeper, minimally invasive endo-microscopy techniques have been established. These most commonly exploit graded-index rod lenses and enable a variety of modalities in head-fixed and freely moving animals. A recently proposed alternative is the use of holographic control of light transport through multimode optical fibres promising much less traumatic application and superior imaging performance. We present a 110 μm thin laser-scanning endo-microscope based on this prospect, enabling in-vivo volumetric imaging throughout the whole depth of the mouse brain. The instrument is equipped with multi-wavelength detection and three-dimensional random access options, and it performs at lateral resolution below 1 μm. We showcase various modes of its application through the observations of fluorescently labelled neurones, their processes and blood vessels. Finally, we demonstrate how to exploit the instrument to monitor calcium signalling of neurones and to measure blood flow velocity in individual vessels at high speeds.

Pathophysiology of nAChRs: limbic circuits and related disorders

Human epidemiological studies have identified links between nicotine intake and stress disorders, including anxiety, depression and PTSD. Here we review the clinical evidence for activation and desensitization of nicotinic acetylcholine receptors (nAChRs) relevant to affective disorders. We go on to describe clinical and preclinical pharmacological studies suggesting that nAChR function may be involved in the etiology of anxiety and depressive disorders, may be relevant targets for medication development, and may contribute to the antidepressant efficacy of non-nicotinic therapeutics. We then review what is known about nAChR function in a subset of limbic system areas (amygdala, hippocampus and prefrontal cortex), and how this contributes to stress-relevant behaviors in preclinical models that may be relevant to human affective disorders. Taken together, the preclinical and clinical literature point to a clear role for ACh signaling through nAChRs in regulation of behavioral responses to stress. Disruption of nAChR homeostasis is likely to contribute to the psychopathology observed in anxiety and depressive disorders. Targeting specific nAChRs may therefore be a strategy for medication development to treat these disorders or to augment the efficacy of current therapeutics.

Theta-gamma phase amplitude coupling in a hippocampal CA1 microcircuit

Phase amplitude coupling (PAC) between slow and fast oscillations is found throughout the brain and plays important functional roles. Its neural origin remains unclear. Experimental findings are often puzzling and sometimes contradictory. Most computational models rely on pairs of pacemaker neurons or neural populations tuned at different frequencies to produce PAC. Here, using a data-driven model of a hippocampal microcircuit, we demonstrate that PAC can naturally emerge from a single feedback mechanism involving an inhibitory and excitatory neuron population, which interplay to generate theta frequency periodic bursts of higher frequency gamma. The model suggests the conditions under which a CA1 microcircuit can operate to elicit theta-gamma PAC, and highlights the modulatory role of OLM and PVBC cells, recurrent connectivity, and short term synaptic plasticity. Surprisingly, the results suggest the experimentally testable prediction that the generation of the slow population oscillation requires the fast one and cannot occur without it.

Cortical regulation of two-stage rapid eye movement sleep

Rapid eye movement (REM) sleep is a sleep state characterized by skeletal muscle paralysis and cerebral cortical activation. Yet, global cortical dynamics and their role in regulating REM sleep remain unclear. Here we show that in mice, REM sleep is accompanied by highly patterned cortical activity waves, with the retrosplenial cortex (RSC) as a major initiation site. Two-photon imaging of layer 2/3 pyramidal neurons of the RSC revealed two distinct patterns of population activities during REM sleep. These activities encoded two sequential REM sleep substages, characterized by contrasting facial movement and autonomic activity and by distinguishable electroencephalogram theta oscillations. Closed-loop optogenetic inactivation of RSC during REM sleep altered cortical activity dynamics and shortened REM sleep duration via inhibition of the REM substage transition. These results highlight an important role for the RSC in dictating cortical dynamics and regulating REM sleep progression.

Control of Theta Oscillatory Activity Underlying Fear Expression by mGlu5 Receptors

Metabotropic glutamate 5 receptors (mGlu₅) are thought to play an important role in mediating emotional information processing. In particular, negative allosteric modulators (NAMs) of mGlu₅ have received a lot of attention as potential novel treatments for several neuropsychiatric diseases, including anxiety-related disorders. The aim of this study was to assess the influence of pre- and post-training mGlu₅ inactivation in cued fear conditioned mice on neuronal oscillatory activity during fear retrieval. For this study we used the recently developed mGlu₅ NAM Alloswicth-1 administered systemically. Injection of Alloswicth-1 before, but not after, fear conditioning resulted in a significant decrease in freezing upon fear retrieval. Mice injected with Alloswicth-1 pre-training were also implanted with recording microelectrodes into both the medial prefrontal cortex (mPFC) and ventral hippocampus (vHPC). The recordings revealed a reduction in theta rhythmic activity (4-12 Hz) in both the mPFC and vHPC during fear retrieval. These results indicate that inhibition of mGlu₅ signaling alters local oscillatory activity in principal components of the fear brain network underlying a reduced response to a predicted threat.

Prefrontal and hippocampal theta rhythm show anxiolytic-like changes during periaqueductal-elicited "panic" in rats

Anxiety and panic are both elicited by threat and co-occur clinically. But, at the neural level, anxiety appears to inhibit the generation of panic; and vice versa. Anxiety and panic are thought to engage more anterior (a) and mid-posterior (m) parts of the periaqueductal gray (PAG), respectively. Anxiety also engages the hippocampus and medial prefrontal cortex. Here, we tested if mPAG but not aPAG stimulation would suppress prefrontal and hippocampal theta rhythm as do anxiolytic drugs. Twelve male rats with implanted electrodes were stimulated alternately (30 s interval) in the left PAG or right reticular formation (reticularis pontis oralis [RPO]-as a positive control) with recording in the left prelimbic cortex and left and right hippocampus. PAG stimulation was set to produce freezing and RPO to produce 7-8 Hz theta rhythm before tests lasting 10 min on each of 5 days. mPAG stimulation decreased, and aPAG increased, theta power at all sites during elicited freezing. mPAG, but not aPAG, stimulation decreased prefrontal theta frequency. Stimulation did not substantially change circuit dynamics (pairwise phase consistency and partial directed coherence). Together with previous reports, our data suggest that panic- and anxiety-control systems are mutually inhibitory, and neural separation of anxiety and panic extends down to the aPAG and mPAG, respectively. Our findings are consistent with recent proposals that fear and anxiety are controlled by parallel neural hierarchies extending from PAG to the prefrontal cortex.

Recurring Cholinergic Inputs Induce Local Hippocampal Plasticity through Feedforward Disinhibition

The CA1 pyramidal neurons are embedded in an intricate local circuitry that contains a variety of interneurons. The roles these interneurons play in the regulation of the excitatory synaptic plasticity remains largely understudied. Recent experiments showed that recurring cholinergic activation of α7 nACh receptors expressed in oriens-lacunosum-moleculare (OLMα2) interneurons can directly induce LTP in Schaffer collateral (SC)-CA1 synapses. Here, we pair in vitro studies with biophysically based modeling to uncover the underlying mechanisms. According to our model, α7 nAChR activation increases OLM GABAergic activity. This results in the inhibition of the fast-spiking interneurons that provide feedforward inhibition onto CA1 pyramidal neurons. This disinhibition, paired with tightly timed SC stimulation, can induce potentiation at the excitatory synapses of CA1 pyramidal neurons. Our work details the role of cholinergic modulation in disinhibition-induced hippocampal plasticity. It relates the timing of cholinergic pairing found experimentally in previous studies with the timing between disinhibition and hippocampal stimulation necessary to induce potentiation and suggests the dynamics of the involved interneurons play a crucial role in determining this timing.Significance StatementWe use a combination of experiments and mechanistic modeling to uncover the key role for cholinergic neuromodulation of feedforward disinhibitory circuits in regulating hippocampal plasticity. We found that cholinergic activation of α7 nAChR on α7 nACh receptors expressed in oriens-lacunosum-moleculare interneurons, when tightly paired with stimulation of the Schaffer collaterals, can cancel feedforward inhibition onto CA1 pyramidal cells, enabling the potentiation of the SC-CA1 synapse. Our work details how cholinergic action on GABAergic interneurons can tightly regulate the excitability and plasticity of the hippocampal network, unraveling the intricate interplay of the hierarchal inhibitory circuitry and cholinergic neuromodulation as a mechanism for hippocampal plasticity.

Huygens synchronization of medial septal pacemaker neurons generates hippocampal theta oscillation

Episodic learning and memory retrieval are dependent on hippocampal theta oscillation, thought to rely on the GABAergic network of the medial septum (MS). To test how this network achieves theta synchrony, we recorded MS neurons and hippocampal local field potential simultaneously in anesthetized and awake mice and rats. We show that MS pacemakers synchronize their individual rhythmicity frequencies, akin to coupled pendulum clocks as observed by Huygens. We optogenetically identified them as parvalbumin-expressing GABAergic neurons, while MS glutamatergic neurons provide tonic excitation sufficient to induce theta. In accordance, waxing and waning tonic excitation is sufficient to toggle between theta and non-theta states in a network model of single-compartment inhibitory pacemaker neurons. These results provide experimental and theoretical support to a frequency-synchronization mechanism for pacing hippocampal theta, which may serve as an inspirational prototype for synchronization processes in the central nervous system from Nematoda to Arthropoda to Chordate and Vertebrate phyla.

Neural Oscillations in Aversively Motivated Behavior

Fear and anxiety-based disorders are highly debilitating and among the most prevalent psychiatric disorders. These disorders are associated with abnormal network oscillations in the brain, yet a comprehensive understanding of the role of network oscillations in the regulation of aversively motivated behavior is lacking. In this review, we examine the oscillatory correlates of fear and anxiety with a particular focus on rhythms in the theta and gamma-range. First, we describe neural oscillations and their link to neural function by detailing the role of well-studied theta and gamma rhythms to spatial and memory functions of the hippocampus. We then describe how theta and gamma oscillations act to synchronize brain structures to guide adaptive fear and anxiety-like behavior. In short, that hippocampal network oscillations act to integrate spatial information with motivationally salient information from the amygdala during states of anxiety before routing this information via theta oscillations to appropriate target regions, such as the prefrontal cortex. Moreover, theta and gamma oscillations develop in the amygdala and neocortical areas during the encoding of fear memories, and interregional synchronization reflects the retrieval of both recent and remotely encoded fear memories. Finally, we argue that the thalamic nucleus reuniens represents a key node synchronizing prefrontal-hippocampal theta dynamics for the retrieval of episodic extinction memories in the hippocampus.

GABAergic interneurons expressing the α2 nicotinic receptor subunit are functionally integrated in the striatal microcircuit

The interactions between the striatal cholinergic and GABAergic systems are crucial in shaping reward-related behavior and reinforcement learning; however, the synaptic pathways mediating them are largely unknown. Here, we use Chrna2-Cre mice to characterize striatal interneurons (INs) expressing the nicotinic α2 receptor subunit. Using triple patch-clamp recordings combined with optogenetic stimulations, we characterize the electrophysiological, morphological, and synaptic properties of striatal Chrna2-INs. Striatal Chrna2-INs have diverse electrophysiological properties, distinct from their counterparts in other brain regions, including the hippocampus and neocortex. Unlike in other regions, most striatal Chrna2-INs are fast-spiking INs expressing parvalbumin. Striatal Chrna2-INs are intricately integrated in the striatal microcircuit, forming inhibitory synaptic connections with striatal projection neurons and INs, including other Chrna2-INs. They receive excitatory inputs from primary motor cortex mediated by both AMPA and NMDA receptors. A subpopulation of Chrna2-INs responds to nicotinic input, suggesting reciprocal interactions between this GABAergic interneuron population and striatal cholinergic synapses.

Cholinergic Regulation of Hippocampal Theta Rhythm

Cholinergic regulation of hippocampal theta rhythm has been proposed as one of the central mechanisms underlying hippocampal functions including spatial memory encoding. However, cholinergic transmission has been traditionally associated with atropine-sensitive type II hippocampal theta oscillations that occur during alert immobility or in urethane-anesthetized animals. The role of cholinergic regulation of type I theta oscillations in behaving animals is much less clear. Recent studies strongly suggest that both cholinergic muscarinic and nicotinic receptors do actively regulate type I hippocampal theta oscillations and thus provide the cholinergic mechanism for theta-associated hippocampal learning. Septal cholinergic activation can regulate hippocampal circuit and theta expression either through direct septohippocampal cholinergic projections, or through septal glutamatergic and GABAergic neurons, that can precisely entrain hippocampal theta rhythmicity.

α7 nicotinic acetylcholine receptors in the hippocampal circuit: taming complexity

Cholinergic innervation of the hippocampus uses the neurotransmitter acetylcholine (ACh) to coordinate neuronal circuit activity while simultaneously influencing the function of non-neuronal cell types. The α7 nicotinic ACh receptor (nAChR) subtype is highly expressed throughout the hippocampus, has the highest calcium permeability compared with other subtypes of nAChRs, and is of high therapeutic interest due to its association with a variety of neurological disorders and neurodegenerative diseases. In this review, we synthesize research describing α7 nAChR properties, function, and relationship to cognitive dysfunction within the hippocampal circuit and highlight approaches to help improve therapeutic development.

A circuit of COCH neurons encodes social-stress-induced anxiety via MTF1 activation of Cacna1h

The hippocampus is a temporal lobe structure critical for cognition, such as learning, memory, and attention, as well as emotional responses. Hippocampal dysfunction can lead to persistent anxiety and/or depression. However, how millions of neurons in the hippocampus are molecularly and structurally organized to engage their divergent functions remains unknown. Here, we genetically target a subset of neurons expressing the coagulation factor c homolog (COCH) gene. COCH-expressing neurons or COCH neurons are topographically segregated in the distal region of the ventral CA3 hippocampus and express Mtf1 and Cacna1h. MTF1 activation of Cacna1h transcription in COCH neurons encodes the ability of COCH neurons to burst action potentials and cause social-stress-induced anxiety-like behaviors by synapsing directly with a subset of GABAergic inhibitory neurons in the lateral septum. Together, this study provides a molecular and circuitry-based framework for understanding how COCH neurons in the hippocampus are assembled to engage social behavior.

Frequency modulation of entorhinal cortex neuronal activity drives distinct frequency-dependent states of brain-wide dynamics

Human neuroimaging studies have shown that, during cognitive processing, the brain undergoes dynamic transitions between multiple, frequency-tuned states of activity. Although different states may emerge from distinct sources of neural activity, it remains unclear whether single-area neuronal spiking can also drive multiple dynamic states. In mice, we ask whether frequency modulation of the entorhinal cortex activity causes dynamic states to emerge and whether these states respond to distinct stimulation frequencies. Using hidden Markov modeling, we perform unsupervised detection of transient states in mouse brain-wide fMRI fluctuations induced via optogenetic frequency modulation of excitatory neurons. We unveil the existence of multiple, frequency-dependent dynamic states, invisible through standard static fMRI analyses. These states are linked to different anatomical circuits and disrupted in a frequency-dependent fashion in a transgenic model of cognitive disease directly related to entorhinal cortex dysfunction. These findings provide cross-scale insight into basic neuronal mechanisms that may underpin flexibility in brain-wide dynamics.

Deciphering how interneuron specific 3 cells control oriens lacunosum-moleculare cells to contribute to circuit function

The wide diversity of inhibitory cells across the brain makes them suitable to contribute to network dynamics in specialized fashions. However, the contributions of a particular inhibitory cell type in a behaving animal are challenging to untangle as one needs to both record cellular activities and identify the cell type being recorded. Thus, using computational modeling and theory to predict and hypothesize cell-specific contributions is desirable. Here, we examine potential contributions of interneuron-specific 3 (I-S3) cells - an inhibitory interneuron found in CA1 hippocampus that only targets other inhibitory interneurons - during simulated theta rhythms. We use previously developed multi-compartment models of oriens lacunosum-moleculare (OLM) cells, the main target of I-S3 cells, and explore how I-S3 cell inputs during in vitro and in vivo scenarios contribute to theta. We find that I-S3 cells suppress OLM cell spiking, rather than engender its spiking via post-inhibitory rebound mechanisms, and contribute to theta frequency spike resonance during simulated in vivo scenarios. To elicit recruitment similar to in vitro experiments, inclusion of disinhibited pyramidal cell inputs is necessary, implying that I-S3 cell firing broadens the window for pyramidal cell disinhibition. Using in vivo virtual networks, we show that I-S3 cells contribute to a sharpening of OLM cell recruitment at theta frequencies. Further, shifting the timing of I-S3 cell spiking due to external modulation shifts the timing of the OLM cell firing and thus disinhibitory windows. We propose a specialized contribution of I-S3 cells to create temporally precise coordination of modulation pathways.

Illuminating Neural Circuits in Alzheimer's Disease

Alzheimer's disease (AD) is the most common neurodegenerative disorder and there is currently no cure. Neural circuit dysfunction is the fundamental mechanism underlying the learning and memory deficits in patients with AD. Therefore, it is important to understand the structural features and mechanisms underlying the deregulated circuits during AD progression, by which new tools for intervention can be developed. Here, we briefly summarize the most recently established cutting-edge experimental approaches and key techniques that enable neural circuit tracing and manipulation of their activity. We also discuss the advantages and limitations of these approaches. Finally, we review the applications of these techniques in the discovery of circuit mechanisms underlying β-amyloid and tau pathologies during AD progression, and as well as the strategies for targeted AD treatments.

The Role of the Medial Septum-Associated Networks in Controlling Locomotion and Motivation to Move

The Medial Septum and diagonal Band of Broca (MSDB) was initially studied for its role in locomotion. However, the last several decades were focussed on its intriguing function in theta rhythm generation. Early studies relied on electrical stimulation, lesions and pharmacological manipulation, and reported an inconclusive picture regarding the role of the MSDB circuits. Recent studies using more specific methodologies have started to elucidate the differential role of the MSDB's specific cell populations in controlling both theta rhythm and behaviour. In particular, a novel theory is emerging showing that different MSDB's cell populations project to different brain regions and control distinct aspects of behaviour. While the majority of these behaviours involve movement, increasing evidence suggests that MSDB-related networks govern the motivational aspect of actions, rather than locomotion per se. Here, we review the literature that links MSDB, theta activity, and locomotion and propose open questions, future directions, and methods that could be employed to elucidate the diverse roles of the MSDB-associated networks.

The Oscillatory Profile Induced by the Anxiogenic Drug FG-7142 in the Amygdala-Hippocampal Network Is Reversed by Infralimbic Deep Brain Stimulation: Relevance for Mood Disorders

Anxiety and depression exhibit high comorbidity and share the alteration of the amygdala–hippocampal–prefrontal network, playing different roles in the ventral and dorsal hippocampi. Deep brain stimulation of the infralimbic cortex in rodents or the human equivalent—the subgenual cingulate cortex—constitutes a fast antidepressant treatment. The aim of this work was: (1) to describe the oscillatory profile in a rodent model of anxiety, and (2) to deepen the therapeutic basis of infralimbic deep brain stimulation in mood disorders. First, the anxiogenic drug FG-7142 was administered to anaesthetized rats to characterize neural oscillations within the amygdala and the dorsoventral axis of the hippocampus. Next, deep brain stimulation was applied. FG-7142 administration drastically reduced the slow waves, increasing delta, low theta, and beta oscillations in the network. Moreover, FG-7142 altered communication in these bands in selective subnetworks. Deep brain stimulation of the infralimbic cortex reversed most of these FG-7142 effects. Cross-frequency coupling was also inversely modified by FG-7142 and by deep brain stimulation. Our study demonstrates that the hyperactivated amygdala–hippocampal network associated with the anxiogenic drug exhibits an oscillatory fingerprint. The study contributes to comprehending the neurobiological basis of anxiety and the effects of infralimbic deep brain stimulation.

Theta-Range Oscillations in Stress-Induced Mental Disorders as an Oscillotherapeutic Target

Emotional behavior and psychological disorders are expressed through coordinated interactions across multiple brain regions. Brain electrophysiological signals are composed of diverse neuronal oscillations, representing cell-level to region-level neuronal activity patterns, and serve as a biomarker of mental disorders. Here, we review recent observations from rodents demonstrating how neuronal oscillations in the hippocampus, amygdala, and prefrontal cortex are engaged in emotional behavior and altered by psychiatric changes such as anxiety and depression. In particular, we focus mainly on theta-range (4–12 Hz) oscillations, including several distinct oscillations in this frequency range. We then discuss therapeutic possibilities related to controlling such mental disease-related neuronal oscillations to ameliorate psychiatric symptoms and disorders in rodents and humans.

Hippocampal Interneuronal α7 nAChRs Modulate Theta Oscillations in Freely Moving Mice

Muscarinic acetylcholine receptors (mAChRs) are critically involved in hippocampal theta generation, but much less is known about the role of nicotinic AChRs (nAChRs). Here we provide evidence that α7 nAChRs expressed on interneurons, particularly those in oriens lacunosum moleculare (OLM), also regulate hippocampal theta generation. Local hippocampal infusion of a selective α7 nAChR antagonist significantly reduces hippocampal theta power and impairs Y-maze spontaneous alternation performance in freely moving mice. By knocking out receptors in different neuronal subpopulations, we find that α7 nAChRs expressed in OLM interneurons regulate theta generation. Our in vitro slice studies indicate that α7 nAChR activation increases OLM neuron activity that, in turn, enhances pyramidal cell excitatory postsynaptic currents (EPSCs). Our study also suggests that mAChR activation promotes transient theta generation, while α7 nAChR activation facilitates future theta generation by similar stimulations, revealing a complex mechanism whereby cholinergic signaling modulates different aspects of hippocampal theta oscillations through different receptor subtypes.

Cortical disinhibitory circuits: cell types, connectivity and function

The concept of a dynamic excitation/inhibition balance tuned by circuit disinhibition, which can shape information flow during complex behavioral tasks, has arisen as an important and conserved information-processing motif. In cortical circuits, different subtypes of GABAergic inhibitory interneurons are connected to each other, offering an anatomical foundation for disinhibitory processes. Moreover, a subpopulation of GABAergic cells that express vasoactive intestinal polypeptide (VIP) preferentially innervates inhibitory interneurons, highlighting their central role in disinhibitory modulation. We discuss inhibitory neuron subtypes involved in disinhibition, with a focus on local circuits and long-range synaptic connections that drive disinhibitory function. We highlight multiple layers of disinhibition across cortical circuits that regulate behavior and serve to maintain an excitation/inhibition balance.

Large-Scale and Multiscale Networks in the Rodent Brain during Novelty Exploration

Neural activity is coordinated across multiple spatial and temporal scales, and these patterns of coordination are implicated in both healthy and impaired cognitive operations. However, empirical cross-scale investigations are relatively infrequent, because of limited data availability and to the difficulty of analyzing rich multivariate datasets. Here, we applied frequency-resolved multivariate source-separation analyses to characterize a large-scale dataset comprising spiking and local field potential (LFP) activity recorded simultaneously in three brain regions (prefrontal cortex, parietal cortex, hippocampus) in freely-moving mice. We identified a constellation of multidimensional, inter-regional networks across a range of frequencies (2-200 Hz). These networks were reproducible within animals across different recording sessions, but varied across different animals, suggesting individual variability in network architecture. The theta band (∼4-10 Hz) networks had several prominent features, including roughly equal contribution from all regions and strong inter-network synchronization. Overall, these findings demonstrate a multidimensional landscape of large-scale functional activations of cortical networks operating across multiple spatial, spectral, and temporal scales during open-field exploration.

Essential requirement of complex number for oscillatory phenomenon in intracellular trafficking process

Intracellular protein trafficking processes consisting of three intracellular states are described by three differential equations. To solve the equations, a quadratic equation is required, and its roots are generally real or complex. The purpose of the present study is to clarify the meanings of roots of real and complex numbers. To clarify the point, we define that: 1) 'k I ' is the insertion rate from an insertion state trafficking to the plasma membrane state; 2) 'k E ', the endocytotic rate from the plasma membrane state trafficking to a recycling state; 3) 'k R ', the recycling rate from the recycling state trafficking to the insertion state. Amounts of proteins in three states are expressed as α e lt + β e mt + γ with α , β , γ  = constant and l and m are roots of a quadratic equation,r 2 + k I + k E + k R r + k I k E + k I k R + k E k R = 0 . When l and m are realk I 2 + k E 2 + k R 2 > 2 k I k E + k E k R + k R k I , amounts of proteins in three states shows no oscillatory change but a monotonic change after a transient increase (or decrease); when l and m are complexk I 2 + k E 2 + k R 2 < 2 k I k E + k E k R + k R k I , amounts of proteins in three states are expressed as α e lt + β e mt + γ = 2 g 2 + h 2 sin b t + σ e at + γ ( α , β , l , m  = complex and γ , a , b , g , h , σ  = real: α , β  = conjugate each other; l , m  = conjugate each other), showing an oscillatory change with time. The frequency of oscillatory change appearance is evaluated to be 60% at random combinations of three trafficking rates,k I ,k E andk R . The present study indicates that complex numbers have an essentially important meaning in appearance of oscillatory phenomena in bodily and cellular function.

A Hypothesis for Theta Rhythm Frequency Control in CA1 Microcircuits

Computational models of neural circuits with varying levels of biophysical detail have been generated in pursuit of an underlying mechanism explaining the ubiquitous hippocampal theta rhythm. However, within the theta rhythm are at least two types with distinct frequencies associated with different behavioral states, an aspect that must be considered in pursuit of these mechanistic explanations. Here, using our previously developed excitatory-inhibitory network models that generate theta rhythms, we investigate the robustness of theta generation to intrinsic neuronal variability by building a database of heterogeneous excitatory cells and implementing them in our microcircuit model. We specifically investigate the impact of three key "building block" features of the excitatory cell model that underlie our model design: these cells' rheobase, their capacity for post-inhibitory rebound, and their spike-frequency adaptation. We show that theta rhythms at various frequencies can arise dependent upon the combination of these building block features, and we find that the speed of these oscillations are dependent upon the excitatory cells' response to inhibitory drive, as encapsulated by their phase response curves. Taken together, these findings support a hypothesis for theta frequency control that includes two aspects: (i) an internal mechanism that stems from the building block features of excitatory cell dynamics; (ii) an external mechanism that we describe as "inhibition-based tuning" of excitatory cell firing. We propose that these mechanisms control theta rhythm frequencies and underlie their robustness.

Lateral septum as a nexus for mood, motivation, and movement

The lateral septum (LS) has been implicated in a wide variety of functions, including emotional, motivational, and spatial behavior, and the LS may regulate interactions between the hippocampus and other regions that mediate goal directed behavior. In this review, we suggest that the lateral septum incorporates movement into the evaluation of environmental context with respect to motivation, anxiety, and reward to output an 'integrated movement value signal'. Specifically, hippocampally-derived contextual information may be combined with reinforcement or motivational information in the LS to inform task-relevant decisions. We will discuss how movement is represented in the LS and the literature on the LS's involvement in mood and motivation. We will then connect these results to LS movement-related literature and hypotheses about the role of the lateral septum. We suggest that the LS may communicate a movement-scaled reward signal via changes in place-, movement-, and reward-related firing, and that the LS should be considered a fundamental node of affect and locomotor pathways in the brain.

Using Cortical Neuron Markers to Target Cells in the Dorsal Cochlear Nucleus

The dorsal cochlear nucleus (DCN) is a region of particular interest for auditory and tinnitus research. However, lack of useful genetic markers for in vivo manipulations hinders elucidation of the DCN contribution to tinnitus pathophysiology. This work assesses whether adeno-associated viral vectors (AAV) containing the calcium/calmodulin-dependent protein kinase 2α (CaMKIIα) promoter and a mouse line of nicotinic acetylcholine receptor α2 subunit (Chrna2)-Cre can target specific DCN populations. We found that CaMKIIα cannot be used to target excitatory fusiform DCN neurons as labeled cells showed diverse morphology indicating they belong to different classes of DCN neurons. Light stimulation after driving Channelrhodopsin2 (ChR2) by the CaMKIIα promoter generated spikes in some units but firing rate decreased when light stimulation coincided with sound. Expression and activation of CaMKIIα-eArchaerhodopsin3.0 in the DCN produced inhibition in some units but sound-driven spikes were delayed by concomitant light stimulation. We explored the existence of Cre+ cells in the DCN of Chrna2-Cre mice by hydrogel embedding technique (CLARITY). There were almost no Cre+ cell bodies in the DCN; however, we identified profuse projections arising from the ventral cochlear nucleus (VCN). Anterograde labeling in the VCN revealed projections to the ipsilateral superior olive and contralateral medial nucleus of the trapezoid body (MNTB; bushy cells), and a second bundle terminating in the DCN, suggesting the latter to be excitatory Chrna2+ T-stellate cells. Exciting Chrna2+ cells increased DCN firing. This work shows that cortical molecular tools may be useful for manipulating the DCN especially for tinnitus studies.