The calcium-binding protein parvalbumin is an established marker for a subset of cortical inhibitory interneurons with similar biophysical features and connectivity. However, parvalbumin is also expressed in a population of excitatory layer 5 cells in neocortex with specific sub-cortical projection targets. Parvalbumin may thus also in excitatory cells identify subclasses with distinct connectivity and function. Here, we investigate parvalbumin expression in excitatory neurons of the hippocampal formation and whether it delineates neurons with specific features. We report parvalbumin-expressing glutamatergic cells in the dorsal distal subiculum, which separate into two subclasses: neurons in deep layers, which specifically project to the antero-ventral thalamus, and neurons in superficial layers, which project to the mammillary bodies. Contrary to adjacent pyramidal cells parvalbumin-positive neurons are non-bursting and display typical morphology. Functionally, the projections diverge from classical driver/modulator subdivisions. Parvalbumin thus marks two sub-types of subicular projection neurons with high target specificity and unique functional features.

In a recent article published in Cell,1 Chowdhury and colleagues apply advanced single-cell mRNA profiling to identify a previously unknown strictly peptidergic neuronal population in the dorsal raphe nucleus(DRN).Prompted by pathway-enriched genes,the authors show that these cholecystokinin(CCK)-expressing neurons integrate a broad spectrum of food intake-and satiation-related signals and play a robust role in regulating meal sizes during homoeostatic eating(Fig.1).

Empirical evidence suggests that heat exposure reduces food intake. However, the neurocircuit architecture and the signalling mechanisms that form an associative interface between sensory and metabolic modalities remain unknown, despite primary thermoceptive neurons in the pontine parabrachial nucleus becoming well characterized1. Tanycytes are a specialized cell type along the wall of the third ventricle2 that bidirectionally transport hormones and signalling molecules between the brain's parenchyma and ventricular system3-8. Here we show that tanycytes are activated upon acute thermal challenge and are necessary to reduce food intake afterwards. Virus-mediated gene manipulation and circuit mapping showed that thermosensing glutamatergic neurons of the parabrachial nucleus innervate tanycytes either directly or through second-order hypothalamic neurons. Heat-dependent Fos expression in tanycytes suggested their ability to produce signalling molecules, including vascular endothelial growth factor A (VEGFA). Instead of discharging VEGFA into the cerebrospinal fluid for a systemic effect, VEGFA was released along the parenchymal processes of tanycytes in the arcuate nucleus. VEGFA then increased the spike threshold of Flt1-expressing dopamine and agouti-related peptide (Agrp)-containing neurons, thus priming net anorexigenic output. Indeed, both acute heat and the chemogenetic activation of glutamatergic parabrachial neurons at thermoneutrality reduced food intake for hours, in a manner that is sensitive to both Vegfa loss-of-function and blockage of vesicle-associated membrane protein 2 (VAMP2)-dependent exocytosis from tanycytes. Overall, we define a multimodal neurocircuit in which tanycytes link parabrachial sensory relay to the long-term enforcement of a metabolic code.

Cholecystokinin-expressing interneurons (CCKIs) are hypothesized to shape pyramidal cell-firing patterns and regulate network oscillations and related network state transitions. To directly probe their role in the CA1 region, we silenced their activity using optogenetic and chemogenetic tools in mice. Opto-tagged CCKIs revealed a heterogeneous population, and their optogenetic silencing triggered wide disinhibitory network changes affecting both pyramidal cells and other interneurons. CCKI silencing enhanced pyramidal cell burst firing and altered the temporal coding of place cells: theta phase precession was disrupted, whereas sequence reactivation was enhanced. Chemogenetic CCKI silencing did not alter the acquisition of spatial reference memories on the Morris water maze but enhanced the recall of contextual fear memories and enabled selective recall when similar environments were tested. This work suggests the key involvement of CCKIs in the control of place-cell temporal coding and the formation of contextual memories.

Neurotoxic amyloid-β (Aβ) peptides cause neurodegeneration in Alzheimer’s disease (AD) patients’ brains. They are released upon proteolytic processing of the amyloid precursor protein (APP) extracellularly at the β-secretase site and intramembranously at the γ-secretase site. Several AD mouse models were developed to conduct respective research in vivo. Most of these classical models overexpress human APP with mutations driving AD-associated pathogenic APP processing. However, the resulting pattern of Aβ species in the mouse brains differs from those observed in AD patients’ brains. Particularly mutations proximal to the β-secretase cleavage site (e.g., the so-called Swedish APP (APPswe) fostering Aβ1-x formation) lead to artificial Aβ production, as N-terminally truncated Aβ peptides are hardly present in these mouse brains. Meprin β is an alternative β-secretase upregulated in brains of AD patients and capable of generating N-terminally truncated Aβ2-x peptides. Therefore, we aimed to generate a mouse model for the production of so far underestimated Aβ2-x peptides by conditionally overexpressing meprin β in astrocytes. We chose astrocytes as meprin β was detected in this cell type in close proximity to Aβ plaques in AD patients’ brains. The meprin β-overexpressing mice showed elevated amyloidogenic APP processing detected with a newly generated neo-epitope-specific antibody. Furthermore, we observed elevated Aβ production from endogenous APP as well as AD-related behavior changes (hyperlocomotion and deficits in spatial memory). The novel mouse model as well as the established tools and methods will be helpful to further characterize APP cleavage and the impact of different Aβ species in future studies.

AbstractThe rapidly expanding field of biomimetics emulates biological principles, distilled through evolutionary time, to solve problems in technology, medicine and material science. Information processing in neuronal circuitry of biological models may offer guidelines for future highly efficient computing systems exhibiting, as for example, low power consumption accompanied by excellent pattern recognition capabilities. Here, we consider the visual information processing in the nervous system of the box jellyfish Tripedalia cystophora as template for pattern recognition hardware to operate self-guiding robotic vehicles or automated driving assistants. LTspice XVII simulations of coupled relaxation-type oscillators, based on programmable unijunction transistors (PUTs), enabled the emulation of basal visual functionalities of the T. cystophora central nervous system. The transfer of such simulations in real hardware circuits including possible limitations are discussed.

The hippocampal formation is one of the best studied brain regions for spatial and mnemonic representations. These representations have been reported to differ in their properties for individual hippocampal subregions. One approach that allows the detection of neuronal representations is immediate early gene imaging, which relies on the visualization of genomic responses of activated neuronal populations, so called engrams. This method permits the within-animal comparison of neuronal representations across different subregions. In this work, we have used compartmental analysis of temporal activity by fluorescence in-situ hybridisation (catFISH) of the immediate early gene zif268/erg1 to compare neuronal representations between subdivisions of the dentate gyrus and CA3 upon exploration of different contexts. Our findings give an account of subregion-specific ensemble sizes. We confirm previous results regarding disambiguation abilities in dentate gyrus and CA3 but in addition report novel findings: Although ensemble sizes in the lower blade of the dentate gyrus are significantly smaller than in the upper blade both blades are responsive to environmental change. Beyond this, we show significant differences in the representation of familiar and novel environments along the longitudinal axis of dorsal CA3 and most interestingly between CA3 regions of both hemispheres.

Working memory requires maintenance of and executive control over task-relevant information on a timescale of seconds. Spatial working memory depends on interactions between hippocampus, for the representation of space, and prefrontal cortex, for executive control. A monosynaptic hippocampal projection to the prefrontal cortex has been proposed to serve this interaction. However, connectivity and inactivation experiments indicate a critical role of the nucleus reuniens in hippocampal-prefrontal communication. We have investigated the dynamics of oscillatory coherence throughout the prefrontal-hippocampal-reuniens network in a touchscreen-based working memory task. We found that coherence at distinct frequencies evolved depending on phase and difficulty of the task. During choice, the reuniens did not participate in enhanced prefrontal-hippocampal theta but in gamma coherence. Strikingly, the reuniens was strongly embedded in performance-related increases in beta coherence, suggesting the execution of top-down control. In addition, we show that during working memory maintenance the prefrontal-hippocampal-reuniens network displays performance-related delay activity.

The lateral septum (LS) has been implicated in the regulation of locomotion. Nevertheless, the neurons synchronizing LS activity with the brain’s clock in the suprachiasmatic nucleus (SCN) remain unknown. By interrogating the molecular, anatomical and physiological heterogeneity of dopamine neurons of the periventricular nucleus (PeVN; A14 catecholaminergic group), we find that Th + / Dat1 + cells from its anterior subdivision innervate the LS in mice. These dopamine neurons receive dense neuropeptidergic innervation from the SCN. Reciprocal viral tracing in combination with optogenetic stimulation ex vivo identified somatostatin-containing neurons in the LS as preferred synaptic targets of extrahypothalamic A14 efferents. In vivo chemogenetic manipulation of anterior A14 neurons impacted locomotion. Moreover, chemogenetic inhibition of dopamine output from the anterior PeVN normalized amphetamine-induced hyperlocomotion, particularly during sedentary periods. Cumulatively, our findings identify a hypothalamic locus for the diurnal control of locomotion and pinpoint a midbrain-independent cellular target of psychostimulants.

Systemic drug application is the main approach in epilepsy treatment. However, the central nervous system (CNS) is a challenging target for drug delivery as the blood-brain barrier (BBB) restricts the transfer of drugs into the brain. Accordingly, there is a general interest in developing new therapeutic strategies to improve CNS drug accessibility. Intrathecal administration of antiseizure drugs (ASDs) e.g. via pumps or advanced materials could be a possible approach to bypass the BBB and increase the availability of neuroactive compounds in the CNS. The aim of this study was the evaluation of intracerebroventricular (i.c.v.) compared to systemic drug application in generalized epilepsy. The i.c.v. administration of the established ASD ethosuximide (ETX) in Genetic Absence Epilepsy Rats from Strasbourg (GAERS) caused a robust and dose-dependent reduction of spike-wave discharges (SWDs) without causing obvious behavioral abnormalities. Additionally, we could show that i.c.v. treatment with ETX is significantly more effective in seizure suppression than systemic treatment with the same dose. The localized application resulted in reduced systemic drug exposure compared to standard systemic ETX therapy. The tracing of dye distribution throughout the CNS supported the view that i.c.v. applied drugs cross into brain tissue surrounding the ventricles but largely remain restricted to the site of injection. Our data suggest that intrathecal application represents a possible route for the treatment in generalized epilepsy through direct drug penetration from CSF into brain tissue.

Although the general architecture of the hippocampus is similar along its longitudinal axis, recent studies have revealed prominent differences in molecular, anatomical and functional criteria suggesting a division into different sub-circuits along its rostro-caudal extent. Owing to differential connectivity and function the most fundamental distinction is made between the dorsal and the ventral hippocampus, which are preferentially involved in spatial and emotional processing, respectively. Accordingly, in vivo work regarding spatial memory formation has focused on the dorsal hippocampus. In contrast, electro-physiological in vitro recordings have been preferentially performed on intermediate-ventral hippocampus, largely motivated by factors like slice viability and circuit integrity. To allow for direct correlation of in vivo data on spatial processing with in vitro data we have adapted previous sectioning methods to obtain highly viable transverse brain slices from the dorsal-intermediate hippocampus for long-term recordings of principal cells and interneurons in the dentate gyrus. As spatial behavior is routinely analyzed in adult mice, we have combined this transversal slicing procedure with the use of protective solutions to enhance viability of brain tissue from mature animals. We use this approach for mice of about 3 months of age. The method offers a good alternative to the coronal preparation which is frequently used for in vitro studies on dorsal hippocampus. We compare these two preparations in terms of quality of recordings and preservation of morphological features of recorded neurons.

The treatment of refractory epilepsy via closed-loop implantable devices that act on seizures either by drug release or electrostimulation is a highly attractive option. For such implantable medical devices, efficient and low energy consumption, small size, and efficient processing architectures are essential. To meet these requirements, epileptic seizure detection by analysis and classification of brain signals with a convolutional neural network (CNN) is an attractive approach. This work presents a CNN for epileptic seizure detection capable of running on an ultra-low-power microprocessor. The CNN is implemented and optimized in MATLAB. In addition, the CNN is also implemented on a GAP8 microprocessor with RISC-V architecture. The training, optimization, and evaluation of the proposed CNN are based on the CHB-MIT dataset. The CNN reaches a median sensitivity of 90% and a very high specificity over 99% corresponding to a median false positive rate of 6.8 s per hour. After implementation of the CNN on the microcontroller, a sensitivity of 85% is reached. The classification of 1 s of EEG data takes t=35 ms and consumes an average power of P≈140 μW. The proposed detector outperforms related approaches in terms of power consumption by a factor of 6. The universal applicability of the proposed CNN based detector is verified with recording of epileptic rats. This results enable the design of future medical devices for epilepsy treatment.

Purkinje cells are the only output cell of the cerebellar cortex. Their spatiotemporal activity is controlled by molecular layer interneurons (MLIs) through GABAA receptor-mediated inhibition. Recently, it has been reported that the cerebellar cortex is required for consolidation of conditioned fear responses during fear memory formation. Although the relevance of MLIs during fear memory formation is currently not known, it has been shown that synapses made between MLIs and Purkinje cells exhibit long term plasticity following fear conditioning. The present study examined the role of cerebellar MLIs in the formation of fear memory using a genetically-altered mouse line (PC-∆γ2) in which GABAA receptor-mediated signaling at MLI to Purkinje cell synapses was functionally removed. We found that neither acquisition nor recall of fear memories to tone and context were altered after removal of MLI-mediated inhibition.

The lateral habenula (LHb) is hyperactive in depression, and thus potentiating inhibition of this structure makes an interesting target for future antidepressant therapies. However, the circuit mechanisms mediating inhibitory signalling within the LHb are not well-known. We addressed this issue by studying LHb neurons expressing either parvalbumin (PV) or somatostatin (SOM), two markers of particular sub-classes of neocortical inhibitory neurons. Here, we find that both PV and SOM are expressed by physiologically distinct sub-classes. Furthermore, we describe multiple sources of inhibitory input to the LHb arising from both local PV-positive neurons, from PV-positive neurons in the medial dorsal thalamic nucleus, and from SOM-positive neurons in the ventral pallidum. These findings hence provide new insight into inhibitory control within the LHb, and highlight that this structure is more neuronally diverse than previously thought.

The nucleus reuniens (RE) is part of the midline thalamus and one of the major sources of thalamic inputs to the hippocampal formation and the medial prefrontal cortex. However, it not only sends strong efferents to these areas but is also heavily innervated by both brain regions. Based on its connectivity and supported by functional studies the RE has been suggested to represent a major hub in reciprocal hippocampal-prefrontal communication. Indeed, inactivation studies have demonstrated that this nucleus is particularly important for cognitive behaviors which depend on prefrontal-hippocampal communication, such as working memory or memory consolidation. However, besides its central role in mediating hippocampal-prefrontal communication, the RE is target of a multitude of other cortical and subcortical afferents, which likely modulate its function. So far, however, studies that have systematically investigated the afferents of the RE have only been performed in rats. Because of the unique role of the mouse as a genetically accessible model system for mammalian brain circuit analysis we have mapped the afferent connectivity of the mouse RE using retrograde Fluoro-Gold tracing. Comparison with similar data from rats indicated a very high level of similarity in prefrontal and hippocampal afferents but some differences in afferent connectivity with other brain regions. In particular, our results suggest interspecies differences regarding the integration of the RE in circuits of fear, aversion, and defense.

Dendritic spines control synaptic transmission and plasticity by augmenting post-synaptic potentials and providing biochemical compartmentalization. In principal cells, spines cover the dendritic tree at high densities, receive the overwhelming majority of excitatory inputs, and undergo experience-dependent structural re-organization. Although GABAergic interneurons have long been considered to be devoid of spines, a number of studies have reported the sparse existence of spines in interneurons. However, little is known about their organization or function at the cellular and network level. Here, we show that a subset of hippocampal parvalbumin-positive interneurons forms numerous dendritic spines with highly variable densities and input-selective organization. These spines form in areas with reduced perineuronal net sheathing, predispose for plastic changes in protein expression, and show input-specific re-organization after behavioral experience.

Abstract: Understanding the contribution of transmitter systems in behavioural pharmacology has a long tradition. Multiple techniques such as transmitter specific lesions, but also localised administration of pharmacological toxins including agonists and antagonists of selected receptors have been applied. More recently, modern genetic tools have permitted cell-type selective interferences, for example by expression of light sensitive channels followed by optogenetic stimulation in behaviourally meaningful settings or by engineered channels termed DREADDS which respond to peripherally administered drugs. We here took a similar approach and employed a Cre recombinase dependent viral delivery system (adenoassociated virus, AAV) to express tetanus toxin light chain (TeLc) and thus block neural transmission specifically in parvalbumin-positive neurons of the limbic and infralimbic prefrontal circuitry.