Activity-dependent changes in the firing properties of neocortical fast-spiking interneurons in the absence of large changes in gene expression

Fast-spiking parvalbumin-positive interneurons in brain physiology and Alzheimer's disease

Fast-spiking parvalbumin (PV) interneurons are inhibitory interneurons with unique morphological and functional properties that allow them to precisely control local circuitry, brain networks and memory processing. Since the discovery in 1987 that PV is expressed in a subset of fast-spiking GABAergic inhibitory neurons, our knowledge of the complex molecular and physiological properties of these cells has been expanding. In this review, we highlight the specific properties of PV neurons that allow them to fire at high frequency and with high reliability, enabling them to control network oscillations and shape the encoding, consolidation and retrieval of memories. We next discuss multiple studies reporting PV neuron impairment as a critical step in neuronal network dysfunction and cognitive decline in mouse models of Alzheimer's disease (AD). Finally, we propose potential mechanisms underlying PV neuron dysfunction in AD and we argue that early changes in PV neuron activity could be a causal step in AD-associated network and memory impairment and a significant contributor to disease pathogenesis.

Loss of Grin2a causes a transient delay in the electrophysiological maturation of hippocampal parvalbumin interneurons

N-methyl-D-aspartate receptors (NMDARs) are ligand-gated ionotropic glutamate receptors that mediate a calcium-permeable component to fast excitatory neurotransmission. NMDARs are heterotetrameric assemblies of two obligate GluN1 subunits (GRIN1) and two GluN2 subunits (GRIN2A-GRIN2D). Sequencing data shows that 43% (297/679) of all currently known NMDAR disease-associated genetic variants are within the GRIN2A gene, which encodes the GluN2A subunit. Here, we show that unlike missense GRIN2A variants, individuals affected with disease-associated null GRIN2A variants demonstrate a transient period of seizure susceptibility that begins during infancy and diminishes near adolescence. We show increased circuit excitability and CA1 pyramidal cell output in juvenile mice of both Grin2a+/- and Grin2a-/- mice. These alterations in somatic spiking are not due to global upregulation of most Grin genes (including Grin2b). Deeper evaluation of the developing CA1 circuit led us to uncover age- and Grin2a gene dosing-dependent transient delays in the electrophysiological maturation programs of parvalbumin (PV) interneurons. We report that Grin2a+/+ mice reach PV cell electrophysiological maturation between the neonatal and juvenile neurodevelopmental timepoints, with Grin2a+/- mice not reaching PV cell electrophysiological maturation until preadolescence, and Grin2a-/- mice not reaching PV cell electrophysiological maturation until adulthood. Overall, these data may represent a molecular mechanism describing the transient nature of seizure susceptibility in disease-associated null GRIN2A patients.

The plasticitome of cortical interneurons

Hebb postulated that, to store information in the brain, assemblies of excitatory neurons coding for a percept are bound together via associative long-term synaptic plasticity. In this view, it is unclear what role, if any, is carried out by inhibitory interneurons. Indeed, some have argued that inhibitory interneurons are not plastic. Yet numerous recent studies have demonstrated that, similar to excitatory neurons, inhibitory interneurons also undergo long-term plasticity. Here, we discuss the many diverse forms of long-term plasticity that are found at inputs to and outputs from several types of cortical inhibitory interneuron, including their plasticity of intrinsic excitability and their homeostatic plasticity. We explain key plasticity terminology, highlight key interneuron plasticity mechanisms, extract overarching principles and point out implications for healthy brain functionality as well as for neuropathology. We introduce the concept of the plasticitome - the synaptic plasticity counterpart to the genome or the connectome - as well as nomenclature and definitions for dealing with this rich diversity of plasticity. We argue that the great diversity of interneuron plasticity rules is best understood at the circuit level, for example as a way of elucidating how the credit-assignment problem is solved in deep biological neural networks.

Altered excitatory and decreased inhibitory transmission in the prefrontal cortex of male mice with early developmental disruption to the ventral hippocampus

Ventral hippocampal (vHPC)-prefrontal cortical (PFC) pathway dysfunction is a core neuroimaging feature of schizophrenia. However, mechanisms underlying impaired connectivity within this pathway remain poorly understood. The vHPC has direct projections to the PFC that help shape its maturation. Here, we wanted to investigate the effects of early developmental vHPC perturbations on long-term functional PFC organization. Using whole-cell recordings to assess PFC cellular activity in transgenic male mouse lines, we show early developmental disconnection of vHPC inputs, by excitotoxic lesion or cell-specific ablations, impairs pyramidal cell firing output and produces a persistent increase in excitatory and decrease in inhibitory synaptic inputs onto pyramidal cells. We show this effect is specific to excitatory vHPC projection cell ablation. We further identify PV-interneurons as a source of deficit in inhibitory transmission. We find PV-interneurons are reduced in density, show a reduced ability to sustain high-frequency firing, and show deficits in excitatory inputs that emerge over time. We additionally show differences in vulnerabilities to early developmental vHPC disconnection, wherein PFC PV-interneurons but not pyramidal cells show deficits in NMDA receptor-mediated current. Our results highlight mechanisms by which the PFC adapts to early developmental vHPC perturbations, providing insights into schizophrenia circuit pathology.

Gabaergic Interneurons in Early Brain Development: Conducting and Orchestrated by Cortical Network Activity

Throughout early phases of brain development, the two main neural signaling mechanisms—excitation and inhibition—are dynamically sculpted in the neocortex to establish primary functions. Despite its relatively late formation and persistent developmental changes, the GABAergic system promotes the ordered shaping of neuronal circuits at the structural and functional levels. Within this frame, interneurons participate first in spontaneous and later in sensory-evoked activity patterns that precede cortical functions of the mature brain. Upon their subcortical generation, interneurons in the embryonic brain must first orderly migrate to and settle in respective target layers before they can actively engage in cortical network activity. During this process, changes at the molecular and synaptic level of interneurons allow not only their coordinated formation but also the pruning of connections as well as excitatory and inhibitory synapses. At the postsynaptic site, the shift of GABAergic signaling from an excitatory towards an inhibitory response is required to enable synchronization within cortical networks. Concomitantly, the progressive specification of different interneuron subtypes endows the neocortex with distinct local cortical circuits and region-specific modulation of neuronal firing. Finally, the apoptotic process further refines neuronal populations by constantly maintaining a controlled ratio of inhibitory and excitatory neurons. Interestingly, many of these fundamental and complex processes are influenced—if not directly controlled—by electrical activity. Interneurons on the subcellular, cellular, and network level are affected by high frequency patterns, such as spindle burst and gamma oscillations in rodents and delta brushes in humans. Conversely, the maturation of interneuron structure and function on each of these scales feeds back and contributes to the generation of cortical activity patterns that are essential for the proper peri- and postnatal development. Overall, a more precise description of the conducting role of interneurons in terms of how they contribute to specific activity patterns—as well as how specific activity patterns impinge on their maturation as orchestra members—will lead to a better understanding of the physiological and pathophysiological development and function of the nervous system.

Downregulation of kainate receptors regulating GABAergic transmission in amygdala after early life stress is associated with anxiety-like behavior in rodents

Early life stress (ELS) is a well-characterized risk factor for mood and anxiety disorders. GABAergic microcircuits in the amygdala are critically implicated in anxiety; however, whether their function is altered after ELS is not known. Here we identify a novel mechanism by which kainate receptors (KARs) modulate feedforward inhibition in the lateral amygdala (LA) and show that this mechanism is downregulated after ELS induced by maternal separation (MS). Specifically, we show that in control rats but not after MS, endogenous activity of GluK1 subunit containing KARs disinhibit LA principal neurons during activation of cortical afferents. GluK1 antagonism attenuated excitability of parvalbumin (PV)-expressing interneurons, resulting in loss of PV-dependent inhibitory control and an increase in firing of somatostatin-expressing interneurons. Inactivation of Grik1 expression locally in the adult amygdala reduced ongoing GABAergic transmission and was sufficient to produce a mild anxiety-like behavioral phenotype. Interestingly, MS and GluK1-dependent phenotypes showed similar gender specificity, being detectable in male but not female rodents. Our data identify a novel KAR-dependent mechanism for cell-type and projection-specific functional modulation of the LA GABAergic microcircuit and suggest that the loss of GluK1 KAR function contributes to anxiogenesis after ELS.

Regulation of Perineuronal Nets in the Adult Cortex by the Activity of the Cortical Network

Perineuronal net (PNN) accumulation around parvalbumin-expressing (PV) inhibitory interneurons marks the closure of critical periods of high plasticity, whereas PNN removal reinstates juvenile plasticity in the adult cortex. Using targeted chemogenetic in vivo approaches in the adult mouse visual cortex, we found that transient inhibition of PV interneurons, through metabotropic or ionotropic chemogenetic tools, induced PNN regression. EEG recordings indicated that inhibition of PV interneurons did not elicit unbalanced network excitation. Likewise, inhibition of local excitatory neurons also induced PNN regression, whereas chemogenetic excitation of either PV or excitatory neurons did not reduce the PNN. We also observed that chemogenetically inhibited PV interneurons exhibited reduced PNN compared with their untransduced neighbors, and confirmed that single PV interneurons express multiple genes enabling individual regulation of their own PNN density. Our results indicate that PNN density is regulated in the adult cortex by local changes of network activity that can be triggered by modulation of PV interneurons. PNN regulation may provide adult cortical circuits with an activity-dependent mechanism to control their local remodeling.SIGNIFICANCE STATEMENT The perineuronal net is an extracellular matrix, which accumulates around individual parvalbumin-expressing inhibitory neurons during postnatal development, and is seen as a barrier that prevents plasticity of neuronal circuits in the adult cerebral cortex. We found that transiently inhibiting parvalbumin-expressing or excitatory cortical neurons triggers a local decrease of perineuronal net density. Our results indicate that perineuronal nets are regulated in the adult cortex depending on the activity of local microcircuits. These findings uncover an activity-dependent mechanism by which adult cortical circuits may locally control their plasticity.

Experience-regulated molecular mechanisms in cortical GABAergic interneurons: from cellular functions to control over circuit plasticity

Experience-induced changes in GABAergic interneurons (INs) are thought to control the plasticity of neural circuits in the developing and adult cortex. However, it remains poorly understood how experience and the ensuing neuronal activity alter the properties and connectivity of specific IN subtypes and how these cellular changes, in turn, control the plasticity of cortical circuits. Here, I discuss recent experimental and theoretical studies that point to specific experience-induced changes in select IN subtypes as central regulators of plasticity in the cortex. In particular, I focus on the recent identification of several experience-regulated secreted molecules that modulate specific sets of synapses in IN subtypes. I argue that elucidating these molecular mechanisms will allow us to test experimentally the predictions made by theoretical models about the plasticity functions of specific IN subtypes.

GluN2D-mediated excitatory drive onto medial prefrontal cortical PV+ fast-spiking inhibitory interneurons

Deficits in fast-spiking inhibitory interneurons (FSINs) within the dorsolateral prefrontal cortex (dlPFC) are hypothesized to underlie cognitive impairment associated with schizophrenia. Though representing a minority of interneurons, this key cell type coordinates broad neural network gamma-frequency oscillations, associated with cognition and cognitive flexibility. Here we report expression of GluN2D mRNA selectively in parvalbumin positive cells of human postmortem dlPFC tissue, but not pyramidal neurons, with little to no GluN2C expression in either cell type. In acute murine mPFC slices the GluN2C/D selective positive allosteric modulator (PAM), CIQ(+), increased the intrinsic excitability as well as enhanced NMDAR-mediated EPSCs onto FSINs. This increase in intrinsic excitability with GluN2C/D PAM was also observed in the Dlx 5/6+/- FSIN developmental deficit model with reported FSIN hypoexcitability. Together these data speak to selective modulation of FSINs by a GluN2D PAM, providing a potential mechanism to counter the FSIN-deficit seen in schizophrenia.

A repeated molecular architecture across thalamic pathways

The thalamus is the central communication hub of the forebrain and provides the cerebral cortex with inputs from sensory organs, subcortical systems and the cortex itself. Multiple thalamic regions send convergent information to each cortical region, but the organizational logic of thalamic projections has remained elusive. Through comprehensive transcriptional analyses of retrogradely labeled thalamic neurons in adult mice, we identify three major profiles of thalamic pathways. These profiles exist along a continuum that is repeated across all major projection systems, such as those for vision, motor control and cognition. The largest component of gene expression variation in the mouse thalamus is topographically organized, with features conserved in humans. Transcriptional differences between these thalamic neuronal identities are tied to cellular features that are critical for function, such as axonal morphology and membrane properties. Molecular profiling therefore reveals covariation in the properties of thalamic pathways serving all major input modalities and output targets, thus establishing a molecular framework for understanding the thalamus.

Enriched Environment Shortens the Duration of Action Potentials in Cerebellar Granule Cells

Environmental enrichment for rodents is known to enhance motor performance. Structural and molecular changes have been reported to be coupled with an enriched environment, but functional alterations of single neurons remain elusive. Here, we compared mice raised under control conditions and an enriched environment. We tested the motor performance on a rotarod and subsequently performed whole-cell patch-clamp recordings in cerebellar slices focusing on granule cells of lobule IX, which is known to receive vestibular input. Mice raised in an enriched environment were able to remain on an accelerating rotarod for a longer period of time. Electrophysiological analyses revealed normal passive properties of granule cells and a functional adaptation to the enriched environment, manifested in faster action potentials (APs) with a higher depolarized voltage threshold and larger AP overshoot. Furthermore, the maximal firing frequency of APs was higher in mice raised in an enriched environment. These data show that enriched environment causes specific alterations in the biophysical properties of neurons. Furthermore, we speculate that the ability of cerebellar granule cells to generate higher firing frequencies improves motor performance.

NMDA-receptor inhibition and oxidative stress during hippocampal maturation differentially alter parvalbumin expression and gamma-band activity

Dysfunction of parvalbumin (PV)-expressing interneurons is thought to underlie the alterations of gamma-band oscillations observed in schizophrenia. Although the pathomechanisms of this disease remain unclear, oxidative stress induced by NMDA receptor (NMDAR) hypofunction and decreased glutathione (GSH) synthesizing capacity have been shown to lead to PV-loss and aberrant oscillatory activity. However, the individual contributions of NMDAR-inhibition and GSH-depletion to the developmental alterations observed in schizophrenia are largely unknown. We therefore investigated each condition in isolation using hippocampal slice cultures wherein interneuron maturation occurs entirely in vitro. Although both treatments caused oxidative stress, NMDAR-inhibition led to an immediate reduction in gamma oscillation frequency and a delayed loss of PV. In contrast, GSH-depletion immediately decreased PV expression and increased power, without affecting frequency. Hence, although disturbances of PV-expression and gamma oscillations coexist in schizophrenia, they can arise from separate pathological processes.

Rapid Disinhibition by Adjustment of PV Intrinsic Excitability during Whisker Map Plasticity in Mouse S1

Rapid plasticity of layer (L) 2/3 inhibitory circuits is an early step in sensory cortical map plasticity, but its cellular basis is unclear. We show that, in mice of either sex, 1 d whisker deprivation drives the rapid loss of L4-evoked feedforward inhibition and more modest loss of feedforward excitation in L2/3 pyramidal (PYR) cells, increasing the excitation-inhibition conductance ratio. Rapid disinhibition was due to reduced L4-evoked spiking by L2/3 parvalbumin (PV) interneurons, caused by reduced PV intrinsic excitability. This included elevated PV spike threshold, which is associated with an increase in low-threshold, voltage-activated delayed rectifier (presumed Kv1) and A-type potassium currents. Excitatory synaptic input and unitary inhibitory output of PV cells were unaffected. Functionally, the loss of feedforward inhibition and excitation was precisely coordinated in L2/3 PYR cells, so that peak feedforward synaptic depolarization remained stable. Thus, the rapid plasticity of PV intrinsic excitability offsets early weakening of excitatory circuits to homeostatically stabilize synaptic potentials in PYR cells of sensory cortex.SIGNIFICANCE STATEMENT Inhibitory circuits in cerebral cortex are highly plastic, but the cellular mechanisms and functional importance of this plasticity are incompletely understood. We show that brief (1 d) sensory deprivation rapidly weakens parvalbumin (PV) inhibitory circuits by reducing the intrinsic excitability of PV neurons. This involved a rapid increase in voltage-gated potassium conductances that control near-threshold spiking excitability. Functionally, the loss of PV-mediated feedforward inhibition in L2/3 pyramidal cells was precisely balanced with the separate loss of feedforward excitation, resulting in a net homeostatic stabilization of synaptic potentials. Thus, rapid plasticity of PV intrinsic excitability implements network-level homeostasis to stabilize synaptic potentials in sensory cortex.

Multiple shared mechanisms for homeostatic plasticity in rodent somatosensory and visual cortex

We compare the circuit and cellular mechanisms for homeostatic plasticity that have been discovered in rodent somatosensory (S1) and visual (V1) cortex. Both areas use similar mechanisms to restore mean firing rate after sensory deprivation. Two time scales of homeostasis are evident, with distinct mechanisms. Slow homeostasis occurs over several days, and is mediated by homeostatic synaptic scaling in excitatory networks and, in some cases, homeostatic adjustment of pyramidal cell intrinsic excitability. Fast homeostasis occurs within less than 1 day, and is mediated by rapid disinhibition, implemented by activity-dependent plasticity in parvalbumin interneuron circuits. These processes interact with Hebbian synaptic plasticity to maintain cortical firing rates during learned adjustments in sensory representations.This article is part of the themed issue 'Integrating Hebbian and homeostatic plasticity'.

Inhibitory interneurons and their circuit motifs in the many layers of the barrel cortex

Recent years have seen substantial progress in studying the structural and functional properties of GABAergic interneurons and their roles in the neuronal networks of barrel cortex. Although GABAergic interneurons represent only about 12% of the total number of neocortical neurons, they are extremely diverse with respect to their structural and functional properties. It has become clear that barrel cortex interneurons not only serve the maintenance of an appropriate excitation/inhibition balance but also are directly involved in sensory processing. In this review we present different interneuron types and their axonal projection pattern framework in the context of the laminar and columnar organization of the barrel cortex. The main focus is here on the most prominent interneuron types, i.e. basket cells, chandelier cells, Martinotti cells, bipolar/bitufted cells and neurogliaform cells, but interneurons with more unusual axonal domains will also be mentioned. We describe their developmental origin, their classification with respect to molecular, morphological and intrinsic membrane and synaptic properties. Most importantly, we will highlight the most prominent circuit motifs these interneurons are involved in and in which way they serve feed-forward inhibition, feedback inhibition and disinhibition. Finally, this will be put into context to their functional roles in sensory signal perception and processing in the whisker system and beyond.

Distinct Physiological Maturation of Parvalbumin-Positive Neuron Subtypes in Mouse Prefrontal Cortex

Parvalbumin-positive (PV⁺) neurons control the timing of pyramidal cell output in cortical neuron networks. In the prefrontal cortex (PFC), PV⁺ neuron activity is involved in cognitive function, suggesting that PV⁺ neuron maturation is critical for cognitive development. The two major PV⁺ neuron subtypes found in the PFC, chandelier cells (ChCs) and basket cells (BCs), are thought to play different roles in cortical circuits, but the trajectories of their physiological maturation have not been compared. Using two separate mouse lines, we found that in the mature PFC, both ChCs and BCs are abundant in superficial layer 2, but only BCs are present in deeper laminar locations. This distinctive laminar distribution was observed by postnatal day 12 (P12), when we first identified ChCs by the presence of axon cartridges. Electrophysiology analysis of excitatory synapse development, starting at P12, showed that excitatory drive remains low throughout development in ChCs, but increases rapidly before puberty in BCs, with an earlier time course in deeper-layer BCs. Consistent with a role of excitatory synaptic drive in the maturation of PV⁺ neuron firing properties, the fast-spiking phenotype showed different maturation trajectories between ChCs and BCs, and between superficial versus deep-layer BCs. ChC and BC maturation was nearly completed, via different trajectories, before the onset of puberty. These findings suggest that ChC and BC maturation may contribute differentially to the emergence of cognitive function, primarily during prepubertal development.SIGNIFICANCE STATEMENT Parvalbumin-positive (PV⁺) neurons tightly control pyramidal cell output. Thus PV⁺ neuron maturation in the prefrontal cortex (PFC) is crucial for cognitive development. However, the relative physiological maturation of the two major subtypes of PV⁺ neurons, chandelier cells (ChCs) and basket cells (BCs), has not been determined. We assessed the maturation of ChCs and BCs in different layers of the mouse PFC, and found that, from early postnatal age, ChCs and BCs differ in laminar location. Excitatory synapses and fast-spiking properties matured before the onset of puberty in both cell types, but following cell type-specific developmental trajectories. Hence, the physiological maturation of ChCs and BCs may contribute to the emergence of cognitive function differentially, and predominantly during prepubertal development.

Genetic and activity-dependent mechanisms underlying interneuron diversity

The proper construction of neural circuits requires the generation of diverse cell types, their distribution to defined regions, and their specific and appropriate wiring. A major objective in neurobiology has been to understand the molecular determinants that link neural birth to terminal specification and functional connectivity, a task that is especially daunting in the case of cortical interneurons. Considerable evidence supports the idea that an interplay of intrinsic and environmental signalling is crucial to the sequential steps of interneuron specification, including migration, selection of a settling position, morphogenesis and synaptogenesis. However, when and how these influences merge to support the appropriate terminal differentiation of different classes of interneurons remains uncertain. In this Review, we discuss a wealth of recent findings that have advanced our understanding of the developmental mechanisms that contribute to the diversification of interneurons and suggest areas of particular promise for further investigation.

Development and long-term integration of MGE-lineage cortical interneurons in the heterochronic environment

Interneuron precursors transplanted into visual cortex induce network plasticity during their heterochronic maturation. Such plasticity can have a significant impact on the function of the animal and is normally present only during a brief critical period in early postnatal development. Elucidating the synaptic and physiological properties of interneuron precursors as they mature is key to understanding how long-term circuit changes are induced by transplants. We studied the development of transplant-derived interneurons and compared it to endogenously developing interneurons (those that are born and develop in the same animal) at parallel developmental time points, using patch-clamp recordings in acute cortical slices. We found that transplant-derived interneurons develop into fast-spiking and non-fast-spiking neurons characteristic of the medial ganglionic eminence (MGE) lineage. Transplant-derived interneurons matured more rapidly than endogenously developing interneurons, as shown by more hyperpolarized membrane potentials, smaller input resistances, and narrower action potentials at a juvenile age. In addition, transplant-derived fast-spiking interneurons have more quickly saturating input-output relationships and lower maximal firing rates in adulthood, indicating a possible divergence in function. Transplant-derived interneurons both form inhibitory synapses onto host excitatory neurons and receive excitatory synapses from host pyramidal cells. Unitary connection properties are similar to those of host interneurons. These transplant-derived interneurons, however, were less densely functionally connected onto host pyramidal cells than were host interneurons and received fewer spontaneous excitatory inputs from host cells. These findings suggest that many physiological characteristics of interneurons are autonomously determined, while some factors impacting their circuit function may be influenced by the environment in which they develop.NEW & NOTEWORTHY Transplanting embryonic interneurons into older brains induces a period of plasticity in the recipient animal. We find that these interneurons develop typical fast-spiking and non-fast-spiking phenotypes by the end of adolescence. However, the input-output characteristics of transplant-derived neurons diverged from endogenously developing interneurons during adulthood, and they showed lower connection rates to local pyramidal cells at all time points. This suggests a unique and ongoing role of transplant-derived interneurons in host circuits, enabling interneuron transplant therapies.

Firing Frequency Maxima of Fast-Spiking Neurons in Human, Monkey, and Mouse Neocortex

Cortical fast-spiking (FS) neurons generate high-frequency action potentials (APs) without apparent frequency accommodation, thus providing fast and precise inhibition. However, the maximal firing frequency that they can reach, particularly in primate neocortex, remains unclear. Here, by recording in human, monkey, and mouse neocortical slices, we revealed that FS neurons in human association cortices (mostly temporal) could generate APs at a maximal mean frequency (F_mean) of 338 Hz and a maximal instantaneous frequency (F_inst) of 453 Hz, and they increase with age. The maximal firing frequency of FS neurons in the association cortices (frontal and temporal) of monkey was even higher (F_mean 450 Hz, F_inst 611 Hz), whereas in the association cortex (entorhinal) of mouse it was much lower (F_mean 215 Hz, F_inst 342 Hz). Moreover, FS neurons in mouse primary visual cortex (V1) could fire at higher frequencies (F_mean 415 Hz, F_inst 582 Hz) than those in association cortex. We further validated our in vitro data by examining spikes of putative FS neurons in behaving monkey and mouse. Together, our results demonstrate that the maximal firing frequency of FS neurons varies between species and cortical areas.

Cell type-specific transcriptomics of hypothalamic energy-sensing neuron responses to weight-loss

Molecular and cellular processes in neurons are critical for sensing and responding to energy deficit states, such as during weight-loss. Agouti related protein (AGRP)-expressing neurons are a key hypothalamic population that is activated during energy deficit and increases appetite and weight-gain. Cell type-specific transcriptomics can be used to identify pathways that counteract weight-loss, and here we report high-quality gene expression profiles of AGRP neurons from well-fed and food-deprived young adult mice. For comparison, we also analyzed Proopiomelanocortin (POMC)-expressing neurons, an intermingled population that suppresses appetite and body weight. We find that AGRP neurons are considerably more sensitive to energy deficit than POMC neurons. Furthermore, we identify cell type-specific pathways involving endoplasmic reticulum-stress, circadian signaling, ion channels, neuropeptides, and receptors. Combined with methods to validate and manipulate these pathways, this resource greatly expands molecular insight into neuronal regulation of body weight, and may be useful for devising therapeutic strategies for obesity and eating disorders.

DOI:http://dx.doi.org/10.7554/eLife.09800.001

Humans and other animals must get adequate nutrition in order to survive. As a result, the body has several systems that work side by side to maintain a healthy body weight and ensure that enough food gets eaten to provide the energy that the body needs. Problems with these systems can contribute towards obesity and other eating disorders.

Certain types of cells in the brain play important roles in controlling weight and appetite, although the genes and cellular mechanisms that underlie these abilities are not well understood. When an animal is deprived of food, so-called AGRP neurons produce molecules that increase appetite and make it easier to gain weight. These neurons also go through structural changes and increase their electrical activity during weight loss. Another group of cells, called the POMC neurons, becomes less active when an animal is deprived of energy.

Using a technique called cell type-specific transcriptomics, Henry, Sugino et al. have now revealed that the expression of hundreds of genes in AGRP and POMC neurons changes depending on whether mice are well fed or food deprived. Food deprivation also affects more genes in AGRP neurons than has been seen in other types of brain cell, and the AGRP neurons are also more sensitive to a change in food intake than POMC neurons.

In the future, this gene expression data and knowledge of the pathways affected by the genes could help researchers to develop new treatments for obesity and other disorders that affect appetite. Henry, Sugino et al. then mapped how these changes in gene expression trigger molecular “pathways” in the neurons that alter how the cells work. These affect many parts of the cells, including ion channels, transcription factors, receptors, and secreted proteins. In addition, food deprivation activated pathways in AGRP neurons that protect the cells from damage and death caused by elevated neuron activity and also triggered signaling pathways that increase body weight.

In the future, this gene expression data and knowledge of the pathways affected by the genes could help researchers to develop new treatments for obesity and other disorders that affect appetite.

DOI:http://dx.doi.org/10.7554/eLife.09800.002

Cortical hypoplasia and ventriculomegaly of p73-deficient mice: Developmental and adult analysis

Trp73, a member of the p53 gene family, plays a crucial role in neural development. We describe two main phenotypic variants of p73 deficiency in the brain, a severe one characterized by massive apoptosis in the cortex leading to early postnatal death and a milder, non-/low-apoptosis one in which 50% of pups may reach adulthood using an intensive-care breeding protocol. Both variants display the core triad of p73 deficiency: cortical hypoplasia, hippocampal malformations, and ventriculomegaly. We studied the development of the neocortex in p73 KO mice from early embryonic life into advanced age (25 months). Already at E14.5, the incipient cortical plate of the p73 KO brains showed a reduced width. Examination of adult neocortex revealed a generalized, nonprogressive reduction by 10-20%. Area-specific architectonic landmarks and lamination were preserved in all cortical areas. The surviving adult animals had moderate ventricular distension, whereas pups of the early lethal phenotypic variant showed severe ventriculomegaly. Ependymal cells of wild-type ventricles strongly express p73 and are particularly vulnerable to p73 deficiency. Ependymal denudation by apoptosis and reduction of ependymal cilia were already evident in young mice, with complete absence of cilia in older animals. Loss of p73 function in the ependyma may thus be one determining factor for chronic hydrocephalus, which leads to atrophy of subcortical structures (striatum, septum, amygdala). p73 Is thus involved in a variety of CNS activities ranging from embryonic regulation of brain size to the control of cerebrospinal fluid homeostasis in the adult brain via maintenance of the ependyma.

Auditory map reorganization and pitch discrimination in adult rats chronically exposed to low-level ambient noise

Behavioral adaption to a changing environment is critical for an animal's survival. How well the brain can modify its functional properties based on experience essentially defines the limits of behavioral adaptation. In adult animals the extent to which experience shapes brain function has not been fully explored. Moreover, the perceptual consequences of experience-induced changes in the brains of adults remain unknown. Here we show that the tonotopic map in the primary auditory cortex of adult rats living with low-level ambient noise underwent a dramatic reorganization. Behaviorally, chronic noise-exposure impaired fine, but not coarse pitch discrimination. When tested in a noisy environment, the noise-exposed rats performed as well as in a quiet environment whereas the control rats performed poorly. This suggests that noise-exposed animals had adapted to living in a noisy environment. Behavioral pattern analyses revealed that stress or distraction engendered by the noisy background could not account for the poor performance of the control rats in a noisy environment. A reorganized auditory map may therefore have served as the neural substrate for the consistent performance of the noise-exposed rats in a noisy environment.

The limits of brain determinacy

The genes do not control everything that happens in a cell or an organism, because thermally induced molecular movements and conformation changes are beyond genetic control. The importance of uncontrolled events has been argued from the differences between isogenic organisms reared in virtually identical environments, but these might alternatively be attributed to subtle, undetected differences in the environment. The present review focuses on the uncontrolled events themselves in the context of the developing brain. These are considered at cellular and circuit levels because even if cellular physiology was perfectly controlled by the genes (which it is not), the interactions between different cells might still be uncoordinated. A further complication is that the brain contains mechanisms that buffer noise and others that amplify it. The final resultant of the battle between these contrary mechanisms is that developmental stochasticity is sufficiently low to make neurobehavioural defects uncommon, but a chance component of neural development remains. Thus, our brains and behaviour are not entirely determined by a combination of genes-plus-environment.

Recent advances in molecular technologies and their application in pathogen detection in foods with particular reference to yersinia

Yersinia enterocolitica is an important zoonotic pathogen that can cause yersiniosis in humans and animals. Food has been suggested to be the main source of yersiniosis. It is critical for the researchers to be able to detect Yersinia or any other foodborne pathogen with increased sensitivity and specificity, as well as in real-time, in the case of a foodborne disease outbreak. Conventional detection methods are known to be labor intensive, time consuming, or expensive. On the other hand, more sensitive molecular-based detection methods like next generation sequencing, microarray, and many others are capable of providing faster results. DNA testing is now possible on a single molecule, and high-throughput analysis allows multiple detection reactions to be performed at once, thus allowing a range of characteristics to be rapidly and simultaneously determined. Despite better detection efficiencies, results derived using molecular biology methods can be affected by the various food matrixes. With the improvements in sample preparation, data analysis, and testing procedures, molecular detection techniques will likely continue to simplify and increase the speed of detection while simultaneously improving the sensitivity and specificity for tracking pathogens in food matrices.