Subtypes of GABAergic neurons project axons in the neocortex

Distinct electrophysiological properties of long-range GABAergic and glutamatergic neurons from the lateral amygdala to the auditory cortex of the mouse

Differentiating between auditory signals of various emotional significance plays a crucial role in an individual's ability to thrive and excel in social interactions and in survival. Multiple approaches, including anatomical studies, electrophysiological investigations, imaging techniques, optogenetics and chemogenetics, have confirmed that the auditory cortex (AC) impacts fear-related behaviours driven by auditory stimuli by conveying auditory information to the lateral amygdala (LA) through long-range excitatory glutamatergic and GABAergic connections. In addition, the LA provides glutamatergic projections to the AC which are important to fear memory expression and are modified by associative fear learning. Here we test the hypothesis that the LA also sends long-range direct inhibitory inputs to the cortex. To address this fundamental question, we used anatomical and electrophysiological approaches, allowing us to directly assess the nature of GABAergic inputs from the LA to the AC in the mouse. Our findings elucidate the existence of a long-range inhibitory pathway from the LA to the AC (LAC) via parvalbumin-expressing (LAC-Parv) and somatostatin-expressing (LAC-SOM) neurons. This research identifies distinct electrophysiological properties for genetically defined long-range GABAergic neurons involved in the communication between the LA and the cortex (LAC-Parv inhibitory projections → AC neurons; LAC-Som inhibitory projections → AC neurons) within the lateral amygdala cortical network. KEY POINTS: The mouse auditory cortex receives inputs from the lateral amygdala. Retrograde viral tracing techniques allowed us to identify two previously undescribed lateral amygdala to auditory cortex (LAC) GABAergic projecting neurons. Extensive electrophysiological, morphological and anatomical characterization of LAC neurons is provided here, demonstrating key differences in the three populations. This study paves the way for a better understanding of the growing complexity of the cortico-amygdala-cortico circuit.

Functional topography of the corpus callosum as revealed by fMRI and behavioural studies of control subjects and patients with callosal resection

The concept of a topographical map of the corpus callosum (CC), the main interhemispheric commissure, has emerged from human lesion studies and from anatomical tracing investigations in other mammals. Over the last few years, a rising number of researchers have been reporting functional magnetic resonance imaging (fMRI) activation in also the CC. This short review summarizes the functional and behavioral studies performed in groups of healthy subjects and in patients undergone to partial or total callosal resection, and it is focused on the work conducted by the authors. Functional data have been collected by diffusion tensor imaging and tractography (DTI and DTT) and functional magnetic resonance imaging (fMRI), both techniques allowing to expand and refine our knowledge of the commissure. Neuropsychological test were also administered, and simple behavioral task, as imitation perspective and mental rotation ability, were analyzed. These researches added new insight on the topographic organization of the human CC. By combining DTT and fMRI it was possible to observe that the callosal crossing points of interhemispheric fibers connecting homologous primary sensory cortices, correspond to the CC sites where the fMRI activation elicited by peripheral stimulation was detected. In addition, CC activation during imitation and mental rotation performance was also reported. These studies demonstrated the presence of specific callosal fiber tracts that cross the commissure in the genu, body, and splenium, at sites showing fMRI activation, consistently with cortical activated areas. Altogether, these findings lend further support to the notion that the CC displays a functional topographic organization, also related to specific behavior.

Long-range GABAergic projections contribute to cortical feedback control of sensory processing

In the olfactory system, the olfactory cortex sends glutamatergic projections back to the first stage of olfactory processing, the olfactory bulb (OB). Such corticofugal excitatory circuits - a canonical circuit motif described in all sensory systems- dynamically adjust early sensory processing. Here, we uncover a corticofugal inhibitory feedback to OB, originating from a subpopulation of GABAergic neurons in the anterior olfactory cortex and innervating both local and output OB neurons. In vivo imaging and network modeling showed that optogenetic activation of cortical GABAergic projections drives a net subtractive inhibition of both spontaneous and odor-evoked activity in local as well as output neurons. In output neurons, stimulation of cortical GABAergic feedback enhances separation of population odor responses in tufted cells, but not mitral cells. Targeted pharmacogenetic silencing of cortical GABAergic axon terminals impaired discrimination of similar odor mixtures. Thus, corticofugal GABAergic projections represent an additional circuit motif in cortical feedback control of sensory processing.

Whole-Brain Direct Inputs to and Axonal Projections from Excitatory and Inhibitory Neurons in the Mouse Primary Auditory Area

Neurons in the primary auditory area (AUDp) innervate multiple brain regions with long-range projections while receiving informative inputs for diverse functions. However, the brain-wide connections of these neurons have not been comprehensively investigated. Here, we simultaneously applied virus-based anterograde and retrograde tracing, labeled the connections of excitatory and inhibitory neurons in the mouse AUDp, and acquired whole-brain information using a dual-channel fluorescence micro-optical sectioning tomography system. Quantified results showed that the two types of neurons received inputs with similar patterns but sent heterogeneous projections to downstream regions. In the isocortex, functionally different areas consistently sent feedback-dominated projections to these neurons, with concomitant laterally-dominated projections from the sensory and limbic cortices to inhibitory neurons. In subcortical regions, the dorsal and medial parts of the non-lemniscal auditory thalamus (AT) were reciprocally connected to the AUDp, while the ventral part contained the most fibers of passage from the excitatory neurons and barely sent projections back, indicating the regional heterogeneity of the AUDp-AT circuit. Our results reveal details of the whole-brain network and provide new insights for further physiological and functional studies of the AUDp.

Long-Range GABAergic Projections of Cortical Origin in Brain Function

The study of long-range GABAergic projections has traditionally been focused on those with subcortical origin. In the last few years, cortical GABAergic neurons have been shown to not only mediate local inhibition, but also extend long-range axons to remote cortical and subcortical areas. In this review, we delineate the different types of long-range GABAergic neurons (LRGNs) that have been reported to arise from the hippocampus and neocortex, paying attention to the anatomical and functional circuits they form to understand their role in behavior. Although cortical LRGNs are similar to their interneuron and subcortical counterparts, they comprise distinct populations that show specific patterns of cortico-cortical and cortico-fugal connectivity. Functionally, cortical LRGNs likely induce timed disinhibition in target regions to synchronize network activity. Thus, LRGNs are emerging as a new element of cortical output, acting in concert with long-range excitatory projections to shape brain function in health and disease.

VIP-Expressing GABAergic Neurons: Disinhibitory vs. Inhibitory Motif and Its Role in Communication Across Neocortical Areas

GABAergic neurons play a crucial role in shaping cortical activity. Even though GABAergic neurons constitute a small fraction of cortical neurons, their peculiar morphology and functional properties make them an intriguing and challenging task to study. Here, we review the basic anatomical features, the circuit properties, and the possible role in the relevant behavioral task of a subclass of GABAergic neurons that express vasoactive intestinal polypeptide (VIP). These studies were performed using transgenic mice in which the VIP-expressing neurons can be recognized using fluorescent proteins and optogenetic manipulation to control (or regulate) their electrical activity. Cortical VIP-expressing neurons are more abundant in superficial cortical layers than other cortical layers, where they are mainly studied. Optogenetic and paired recordings performed in ex vivo cortical preparations show that VIP-expressing neurons mainly exert their inhibitory effect onto somatostatin-expressing (SOM) inhibitory neurons, leading to a disinhibitory effect onto excitatory pyramidal neurons. However, this subclass of GABAergic neurons also releases neurotransmitters onto other GABAergic and non-GABAergic neurons, suggesting other possible circuit roles than a disinhibitory effect. The heterogeneity of VIP-expressing neurons also suggests their involvement and recruitment during different functions via the inhibition/disinhibition of GABAergic and non-GABAergic neurons locally and distally, depending on the specific local circuit in which they are embedded, with potential effects on the behavioral states of the animal. Although VIP-expressing neurons represent only a tiny fraction of GABAergic inhibitory neurons in the cortex, these neurons’ selective activation/inactivation could produce a relevant behavioral effect in the animal. Regardless of the increasing finding and discoveries on this subclass of GABAergic neurons, there is still a lot of missing information, and more studies should be done to unveil their role at the circuit and behavior level in different cortical layers and across different neocortical areas.

Diverse inhibitory projections from the cerebellar interposed nucleus

The cerebellum consists of parallel circuit modules that contribute to diverse behaviors, spanning motor to cognitive. Recent work employing cell-type-specific tracing has identified circumscribed output channels of the cerebellar nuclei (CbN) that could confer tight functional specificity. These studies have largely focused on excitatory projections of the CbN, however, leaving open the question of whether inhibitory neurons also constitute multiple output modules. We mapped output and input patterns to intersectionally restricted cell types of the interposed and adjacent interstitial nuclei in mice. In contrast to the widespread assumption of primarily excitatory outputs and restricted inferior olive-targeting inhibitory output, we found that inhibitory neurons from this region ramified widely within the brainstem, targeting both motor- and sensory-related nuclei, distinct from excitatory output targets. Despite differences in output targeting, monosynaptic rabies tracing revealed largely shared afferents to both cell classes. We discuss the potential novel functional roles for inhibitory outputs in the context of cerebellar theory.

Corticofugal VIP Gabaergic Projection Neurons in the Mouse Auditory and Motor Cortex

Anatomical and physiological studies have described the cortex as a six-layer structure that receives, elaborates, and sends out information exclusively as excitatory output to cortical and subcortical regions. This concept has increasingly been challenged by several anatomical and functional studies that showed that direct inhibitory cortical outputs are also a common feature of the sensory and motor cortices. Similar to their excitatory counterparts, subsets of Somatostatin- and Parvalbumin-expressing neurons have been shown to innervate distal targets like the sensory and motor striatum and the contralateral cortex. However, no evidence of long-range VIP-expressing neurons, the third major class of GABAergic cortical inhibitory neurons, has been shown in such cortical regions. Here, using anatomical anterograde and retrograde viral tracing, we tested the hypothesis that VIP-expressing neurons of the mouse auditory and motor cortices can also send long-range projections to cortical and subcortical areas. We were able to demonstrate, for the first time, that VIP-expressing neurons of the auditory cortex can reach not only the contralateral auditory cortex and the ipsilateral striatum and amygdala, as shown for Somatostatin- and Parvalbumin-expressing long-range neurons, but also the medial geniculate body and both superior and inferior colliculus. We also demonstrate that VIP-expressing neurons of the motor cortex send long-range GABAergic projections to the dorsal striatum and contralateral cortex. Because of its presence in two such disparate cortical areas, this would suggest that the long-range VIP projection is likely a general feature of the cortex’s network.

Characterization and Stage-Dependent Lineage Analysis of Intermediate Progenitors of Cortical GABAergic Interneurons

Intermediate progenitors of both excitatory and inhibitory neurons, which can replenish neurons in the adult brain, were recently identified. However, the generation of intermediate progenitors of GABAergic inhibitory neurons (IPGNs) has not been studied in detail. Here, we characterized the spatiotemporal distribution of IPGNs in mouse cerebral cortex. IPGNs generated neurons during both embryonic and postnatal stages, but the embryonic IPGNs were more proliferative. Our lineage tracing analyses showed that the embryonically proliferating IPGNs tended to localize to the superficial layers rather than the deep cortical layers at 3 weeks after birth. We also found that embryonic IPGNs derived from the medial and caudal ganglionic eminence (CGE) but more than half of the embryonic IPGNs were derived from the CGE and broadly distributed in the cerebral cortex. Taken together, our data indicate that the broadly located IPGNs during embryonic and postnatal stages exhibit a different proliferative property and layer distribution.

Neuronal Circuits in Barrel Cortex for Whisker Sensory Perception

The array of whiskers on the snout provides rodents with tactile sensory information relating to the size, shape and texture of objects in their immediate environment. Rodents can use their whiskers to detect stimuli, distinguish textures, locate objects and navigate. Important aspects of whisker sensation are thought to result from neuronal computations in the whisker somatosensory cortex (wS1). Each whisker is individually represented in the somatotopic map of wS1 by an anatomical unit named a 'barrel' (hence also called barrel cortex). This allows precise investigation of sensory processing in the context of a well-defined map. Here, we first review the signaling pathways from the whiskers to wS1, and then discuss current understanding of the various types of excitatory and inhibitory neurons present within wS1. Different classes of cells can be defined according to anatomical, electrophysiological and molecular features. The synaptic connectivity of neurons within local wS1 microcircuits, as well as their long-range interactions and the impact of neuromodulators, are beginning to be understood. Recent technological progress has allowed cell-type-specific connectivity to be related to cell-type-specific activity during whisker-related behaviors. An important goal for future research is to obtain a causal and mechanistic understanding of how selected aspects of tactile sensory information are processed by specific types of neurons in the synaptically connected neuronal networks of wS1 and signaled to downstream brain areas, thus contributing to sensory-guided decision-making.

Analysis of pallial/cortical interneurons in key vertebrate models of Testudines, Anurans and Polypteriform fishes

The organization of the pallial derivatives across vertebrates follows a comparable elementary arrangement, although not all of them possess a layered cortical structure as sophisticated as the cerebral cortex of mammals. However, its expansion along evolution has only been possible by the development and coevolution of the cellular networks formed by excitatory neurons and inhibitory interneurons. Thus, the comparative analysis of interneuron types in vertebrate models of key evolutionary significance will provide important information, due to the extraordinary anatomical sophistication of their interneuron systems with simpler behavioral implications. Particularly in mammals, the main consensus for classifying interneuron types is based on non-overlapping markers, which do not form a single population, but consist of several distinct classes of inhibitory cells showing co-expression of other markers. In our study, we analyzed immunohistochemically the expression of the main markers like somatostatin (SOM), parvalbumin (PV), calretinin (CR), calbindin (CB), neuropeptide Y (NPY) and/or nitric oxide synthase (NOS) at the pallial regions of three different models of Osteichthyes. First, we selected two tetrapods, one amniote from the genus Pseudemys belonging to the order Testudine, at the base of the amniote diversification and with a three-layered simple cortex, and the Anuran Xenopus laevis, an anamniote tetrapod with a non-layered evaginated pallium, and finally the order Polypteriform, a small fish group at the base of the actinopterygian diversification with an everted telencephalon. SOM was the most conserved interneuron type in terms of its distribution and co-expression with other markers such as CR, in contrast to PV, which showed a different pattern between the models analyzed. In addition, the SOM expression supports a homological relationship between the medial pallial derivatives in all the models. CR and CB expressions in the tetrapods were observed, particularly, CR expressing cells were detected in the medial and the dorsal pallial derivatives, in contrast to CB, which appeared only in discrete scattered populations. However, the pallium of Polypteriforms fishes was almost devoid of CR cells, in contrast to the important number of CB cells observed in all the pallial regions. The NPY immunoreactivity was detected in all the pallial domains of all the models, as well as cells coexpressing CR. Finally, the pallial nitrergic expression was also conserved, which allows to postulate the homological relationships between the ventropallial and the amygdaloid derivatives. In summary, even in basal pallial models the neurochemically characterized interneurons indicate that their first appearance took place before the common ancestor of amniotes. Thus, our results suggest a shared pattern of interneuron types in the pallium of all Osteichthyes.

Auditory Long-Range Parvalbumin Cortico-Striatal Neurons

Previous studies have shown that cortico-striatal pathways link auditory signals to action-selection and reward-learning behavior through excitatory projections. Only recently it has been demonstrated that long-range GABAergic cortico-striatal somatostatin-expressing neurons in the auditory cortex project to the dorsal striatum, and functionally inhibit the main projecting neuronal population, the spiny projecting neuron. Here we tested the hypothesis that parvalbumin-expressing neurons of the auditory cortex can also send long-range projections to the auditory striatum. To address this fundamental question, we took advantage of viral and non-viral anatomical tracing approaches to identify cortico-striatal parvalbumin neurons (CS-Parv inhibitory projections → auditory striatum). Here, we describe their anatomical distribution in the auditory cortex and determine the anatomical and electrophysiological properties of layer 5 CS-Parv neurons. We also analyzed their characteristic voltage-dependent membrane potential gamma oscillation, showing that intrinsic membrane mechanisms generate them. The inherent membrane mechanisms can also trigger intermittent and irregular bursts (stuttering) of the action potential in response to steps of depolarizing current pulses.

Excitation of Cortical nNOS/NK1R Neurons by Hypocretin 1 is Independent of Sleep Homeostasis

We have proposed that cortical nNOS/NK1R interneurons have a role in sleep homeostasis. The hypocretins (orexins) are wake-promoting neuropeptides and hypocretin/orexin (Hcrt) neurons project to the cortex. Hcrt peptides affect deep layer cortical neurons, and Hcrt receptor 1 (Hcrtr1; Ox1r) mRNA is expressed in cortical nNOS/NK1R cells. Therefore, we investigated whether Hcrt neuron stimulation affects cingulate cortex nNOS/NK1R neurons. Bath application of HCRT1/orexin-A evoked an inward current and membrane depolarization in most nNOS/NK1R cells which persisted in tetrodotoxin; optogenetic stimulation of Hcrt terminals expressing channelrhodopsin-2 confirmed these results, and pharmacological studies determined that HCRTR1 mediated these responses. Single-cell RT-PCR found Hcrtr1 mRNA in 31% of nNOS/NK1R cells without any Hcrtr2 mRNA expression; immunohistochemical studies of Hcrtr1-EGFP mice confirmed that a minority of nNOS/NK1R cells express HCRTR1. When Hcrt neurons degenerated in orexin-tTA;TetO DTA mice, the increased EEG delta power during NREM sleep produced in response to 4 h sleep deprivation and c-FOS expression in cortical nNOS/NK1R cells during recovery sleep were indistinguishable from that of controls. We conclude that Hcrt excitatory input to these deep layer cells is mediated through HCRTR1 but is unlikely to be involved in the putative role of cortical nNOS/NK1R neurons in sleep homeostasis.

Glutamatergic neurons in the medial prefrontal cortex mediate the formation and retrieval of cocaine-associated memories in mice

In drug addiction, environmental stimuli previously associated with cocaine use readily elicit cocaine-associated memories, which persist long after abstinence and trigger cocaine craving and consumption. Although previous studies suggest that the medial prefrontal cortex (mPFC) is involved in the expression of cocaine-addictive behaviors, it remains unclear whether excitatory and inhibitory neurons in the mPFC are causally related to the formation and retrieval of cocaine-associated memories. To address this issue, we used the designer receptors exclusively activated by designer drugs (DREADD) technology combined with a cocaine-induced conditioned place preference (CPP) paradigm. We suppressed mPFC neuronal activity in a cell-type- and timing-dependent manner. C57BL/6J wild-type mice received bilateral intra-mPFC infusion of an adeno-associated virus (AAV) expressing inhibitory DREADD (hM4Di) under the control of CaMKII promotor to selectively suppress mPFC pyramidal neurons. GAD67-Cre mice received bilateral intra-mPFC infusion of a Cre-dependent AAV expressing hM4Di to specifically silence GABAergic neurons. Chemogenetic suppression of mPFC pyramidal neurons significantly attenuated both the acquisition and expression of cocaine CPP, while suppression of mPFC GABAergic neurons affected neither the acquisition nor expression of cocaine CPP. Moreover, chemogenetic inhibition of mPFC glutamatergic neurons did not affect the acquisition and expression of lithium chloride-induced conditioned place aversion. These results suggest that the activation of glutamatergic, but not GABAergic, neurons in the mPFC mediates both the formation and retrieval of cocaine-associated memories.

Novel long-range inhibitory nNOS-expressing hippocampal cells

The hippocampus, a brain region that is important for spatial navigation and episodic memory, benefits from a rich diversity of neuronal cell-types. Through the use of an intersectional genetic viral vector approach in mice, we report novel hippocampal neurons which we refer to as LINCs, as they are long-range inhibitory neuronal nitric oxide synthase (nNOS)-expressing cells. LINCs project to several extrahippocampal regions including the tenia tecta, diagonal band, and retromammillary nucleus, but also broadly target local CA1 cells. LINCs are thus both interneurons and projection neurons. LINCs display regular spiking non-pyramidal firing patterns, are primarily located in the stratum oriens or pyramidale, have sparsely spiny dendrites, and do not typically express somatostatin, VIP, or the muscarinic acetylcholine receptor M2. We further demonstrate that LINCs can strongly influence hippocampal function and oscillations, including interregional coherence. The identification and characterization of these novel cells advances our basic understanding of both hippocampal circuitry and neuronal diversity.

Distribution of neuronal structures immunoreactive for parvalbumin in the midcingulate cortex of the rabbit

The rabbit midcingulate cortex that enclosed four cortical areas was immunohistochemically studied using a calcium-binding protein, parvalbumin, as a neurochemical marker. The distribution of parvalbumin-immunopositive somata and fibres was similar across all four areas, where they were present mainly in layers 2/3 and 5. However, there were a slightly greater number of the immunopositive structures in the two ventral areas than the two dorsal areas. Similarity in the distribution of parvalbumin-immunopositive structures across the four areas suggests that neurons expressing parvalbumin may be involved in similar functions across the constituent areas of the rabbit midcingulate cortex.

A Non-Canonical Cortico-Amygdala Inhibitory Loop

Discriminating between auditory signals of different affective value is critical for the survival and success of social interaction of an individual. Anatomical, electrophysiological, imaging, and optogenetics approaches have established that the auditory cortex (AC) by providing auditory information to the lateral amygdala (LA) via long-range excitatory glutamatergic projections has an impact on sound-driven aversive/fear behavior. Here we test the hypothesis that the LA also receives GABAergic projections from the cortex. We addressed this fundamental question by taking advantage of optogenetics, anatomical, and electrophysiology approaches and directly examining the functional effects of cortical GABAergic inputs to LA neurons of the mouse (male/female) AC. We found that the cortex, via cortico-lateral-amygdala somatostatin neurons (CLA-SOM), has a direct inhibitory influence on the output of the LA principal neurons. Our results define a CLA long-range inhibitory circuit (CLA-SOM inhibitory projections → LA principal neurons) underlying the control of spike timing/generation in LA and LA-AC projecting neurons, and attributes a specific function to a genetically defined type of cortical long-range GABAergic neurons in CLA communication.SIGNIFICANCE STATEMENT It is very well established that cortical auditory inputs to the lateral amygdala are exclusively excitatory and that cortico-amygdala neuronal activity has been shown to be involved in sound-driven aversive/fear behavior. Here, for the first time, we show that the lateral amygdala receives long-range GABAergic projection from the auditory cortex and these form direct monosynaptic inhibitory connections onto lateral amygdala principal neurons. Our results define a cellular basis for direct inhibitory communication from auditory cortex to the lateral amygdala, suggesting that the timing and ratio of excitation and inhibition, two opposing forces in the mammalian cerebral cortex, can dynamically affect the output of the lateral amygdala, providing a general mechanism for fear/aversive behavior driven by auditory stimuli.

Cortical nNOS/NK1 Receptor Neurons are Regulated by Cholinergic Projections From the Basal Forebrain

Cholinergic (ACh) basal forebrain (BF) neurons are active during wakefulness and rapid eye movement (REM) sleep and are involved in sleep homeostasis. We have previously shown in adult animals that cortical neurons that express neuronal nitric oxide synthase (nNOS) and the receptor for Substance P (NK1R) are activated during non-REM (NREM) sleep in proportion to homeostatic sleep drive. Here, we show that BF neurons modulate cortical nNOS/NK1R cells. In vitro optogenetic stimulation of BF terminals both activated and inhibited nNOS/NK1R neurons. Pharmacological studies revealed cholinergic responses mediated by postsynaptic activation of muscarinic receptors (mAChRs; M3R > M2/4R > M1R) and that presynaptic M3R and M2R activation reduced glutamatergic input onto nNOS/NK1R neurons whereas nicotinic receptor (nAChR)-mediated responses of nNOS/NK1R neurons were mixed. Cholinergic responses of nNOS/NK1R neurons were largely unaffected by prolonged wakefulness. ACh release, including from BF cells, appears to largely excite cortical nNOS/NK1R cells while reducing glutamatergic inputs onto these neurons. We propose that cholinergic signaling onto cortical nNOS/NK1R neurons may contribute to the regulation of cortical activity across arousal states, but that this response is likely independent of the role of these neurons in sleep homeostasis.

Cortical Circuits of Callosal GABAergic Neurons

Anatomical studies have shown that the majority of callosal axons are glutamatergic. However, a small proportion of callosal axons are also immunoreactive for glutamic acid decarboxylase, an enzyme required for gamma-aminobutyric acid (GABA) synthesis and a specific marker for GABAergic neurons. Here, we test the hypothesis that corticocortical parvalbumin-expressing (CC-Parv) neurons connect the two hemispheres of multiple cortical areas, project through the corpus callosum, and are a functional part of the local cortical circuit. Our investigation of this hypothesis takes advantage of viral tracing and optogenetics to determine the anatomical and electrophysiological properties of CC-Parv neurons of the mouse auditory, visual, and motor cortices. We found a direct inhibitory pathway made up of parvalbumin-expressing (Parv) neurons which connects corresponding cortical areas (CC-Parv neurons → contralateral cortex). Like other Parv cortical neurons, these neurons provide local inhibition onto nearby pyramidal neurons and receive thalamocortical input. These results demonstrate a previously unknown long-range inhibitory circuit arising from a genetically defined type of GABAergic neuron that is engaged in interhemispheric communication.

GABA neurons in the ventral tegmental area regulate non-rapid eye movement sleep in mice

Sleep/wakefulness cycle is regulated by coordinated interactions between sleep- and wakefulness-regulating neural circuitry. However, the detailed mechanism is far from understood. Here, we found that glutamic acid decarboxylase 67-positive GABAergic neurons in the ventral tegmental area (VTA_Gad67+) are a key regulator of non-rapid eye movement (NREM) sleep in mice. VTA_Gad67+ project to multiple brain areas implicated in sleep/wakefulness regulation such as the lateral hypothalamus (LH). Chemogenetic activation of VTA_Gad67+ promoted NREM sleep with higher delta power whereas optogenetic inhibition of these induced prompt arousal from NREM sleep, even under highly somnolescent conditions, but not from REM sleep. VTA_Gad67+ showed the highest activity in NREM sleep and the lowest activity in REM sleep. Moreover, VTA_Gad67+ directly innervated and inhibited wake-promoting orexin/hypocretin neurons by releasing GABA. As such, optogenetic activation of VTA_Gad67+ terminals in the LH promoted NREM sleep. Taken together, we revealed that VTA_Gad67+ play an important role in the regulation of NREM sleep.

Dissociating orexin-dependent and -independent functions of orexin neurons using novel Orexin-Flp knock-in mice

Uninterrupted arousal is important for survival during threatening situations. Activation of orexin/hypocretin neurons is implicated in sustained arousal. However, orexin neurons produce and release orexin as well as several co-transmitters including dynorphin and glutamate. To disambiguate orexin-dependent and -independent physiological functions of orexin neurons, we generated a novel Orexin-flippase (Flp) knock-in mouse line. Crossing with Flp-reporter or Cre-expressing mice showed gene expression exclusively in orexin neurons. Histological studies confirmed that orexin was knock-out in homozygous mice. Orexin neurons without orexin showed altered electrophysiological properties, as well as received decreased glutamatergic inputs. Selective chemogenetic activation revealed that both orexin and co-transmitters functioned to increase wakefulness, however, orexin was indispensable to promote sustained arousal. Surprisingly, such activation increased the total time spent in cataplexy. Taken together, orexin is essential to maintain basic membrane properties and input-output computation of orexin neurons, as well as to exert awake-sustaining aptitude of orexin neurons.

Layer 5 Callosal Parvalbumin-Expressing Neurons: A Distinct Functional Group of GABAergic Neurons

Previous studies have shown that parvalbumin-expressing neurons (CC-Parv neurons) connect the two hemispheres of motor and sensory areas via the corpus callosum, and are a functional part of the cortical circuit. Here we test the hypothesis that layer 5 CC-Parv neurons possess anatomical and molecular mechanisms which dampen excitability and modulate the gating of interhemispheric inhibition. In order to investigate this hypothesis we use viral tracing to determine the anatomical and electrophysiological properties of layer 5 CC-Parv and parvalbumin-expressing (Parv) neurons of the mouse auditory cortex (AC). Here we show that layer 5 CC-Parv neurons had larger dendritic fields characterized by longer dendrites that branched farther from the soma, whereas layer 5 Parv neurons had smaller dendritic fields characterized by shorter dendrites that branched nearer to the soma. The layer 5 CC-Parv neurons are characterized by delayed action potential (AP) responses to threshold currents, lower firing rates, and lower instantaneous frequencies compared to the layer 5 Parv neurons. Kv1.1 containing K⁺ channels are the main source of the AP repolarization of the layer 5 CC-Parv and have a major role in determining both the spike delayed response, firing rate and instantaneous frequency of these neurons.

Rubrocerebellar Feedback Loop Isolates the Interposed Nucleus as an Independent Processor of Corollary Discharge Information in Mice

Understanding cerebellar contributions to motor coordination requires deeper insight into how the output structures of the cerebellum, the cerebellar nuclei, integrate their inputs and influence downstream motor pathways. The magnocellular red nucleus (RNm), a brainstem premotor structure, is a major target of the interposed nucleus (IN), and has also been described in previous studies to send feedback collaterals to the cerebellum. Because such a pathway is in a key position to provide motor efferent information to the cerebellum, satisfying predictions about the use of corollary discharge in cerebellar computations, we studied it in mice of both sexes. Using anterograde viral tracing, we show that innervation of cerebellum by rubrospinal neuron collaterals is remarkably selective for the IN compared with the cerebellar cortex. Optogenetic activation of the pathway in acute mouse brain slices drove IN activity despite small amplitude synaptic currents, suggesting an active role in IN information processing. Monosynaptic transsynaptic rabies tracing indicated the pathway contacts multiple cell types within the IN. By contrast, IN inputs to the RNm targeted a region that lacked inhibitory neurons. Optogenetic drive of IN inputs to the RNm revealed strong, direct excitation but no inhibition of RNm neurons. Together, these data indicate that the cerebellar nuclei are under afferent control independent of the cerebellar cortex, potentially diversifying its roles in motor control.SIGNIFICANCE STATEMENT The common assumption that all cerebellar mossy fibers uniformly collateralize to the cerebellar nuclei and cortex underlies classic models of convergent Purkinje influence on cerebellar output. Specifically, mossy fibers are thought to both directly excite nuclear neurons and drive polysynaptic feedforward inhibition via Purkinje neurons, setting up a fundamental computational unit. Here we present data that challenge this rule. A dedicated cerebellar nuclear afferent comprised of feedback collaterals from premotor rubrospinal neurons can directly modulate IN output independent of Purkinje cell modulation. In contrast to the IN-RNm pathway, the RNm-IN feedback pathway targets multiple cell types, potentially influencing both motor output pathways and nucleo-olivary feedback.

Substance P NK1 receptor in the rat corpus callosum during postnatal development

INTRODUCTION

The expression of substance P (SP) receptor (neurokinin 1, NK1) was studied in the rat corpus callosum (cc) from postnatal day 0 (the first 24 hr from birth, P0) to P30.

METHODS

We used immunocytochemistry to study the presence of intracallosal NK1-immunopositive neurons (NK1_IP-n) during cc development.

RESULTS

NK1_IP-n first appeared on P5. Their number increased significantly between P5 and P10, it remained almost constant between P10 and P15, then declined slightly until P30. The size of intracallosal NK1_IP-n increased constantly from P5 (102.3 μm²) to P30 (262.07 μm²). From P5 onward, their distribution pattern was adult-like, that is, they were more numerous in the lateral and intermediate parts of the cc, and declined to few or none approaching the midline. At P5, intracallosal NK1_IP-n had a predominantly round cell bodies with primary dendrites of different thickness from which originated thinner secondary branches. Between P10 and P15, dendrites were longer and more thickly branched, and displayed several varicosities as well as short, thin appendages. Between P20 and P30, NK1_IP-n were qualitatively indistinguishable from those of adult animals and could be classified as bipolar (fusiform and rectangular), round-polygonal, and pyramidal (triangular-pyriform).

CONCLUSIONS

Number of NK1_IP-n increase between P5 and P10, then declines, but unlike other intracallosal neurons, NK1_IP-n make up a significant population in the adult cc. These findings suggest that NK1_IP-n may be involved in the myelination of callosal axons, could play an important role in their pathfinding. Since they are also found in adult rat cc, it is likely that their role changes during lifetime.

An inhibitory corticostriatal pathway

Anatomical and physiological studies have led to the assumption that the dorsal striatum receives exclusively excitatory afferents from the cortex. Here we test the hypothesis that the dorsal striatum receives also GABAergic projections from the cortex. We addressed this fundamental question by taking advantage of optogenetics and directly examining the functional effects of cortical GABAergic inputs to spiny projection neurons (SPNs) of the mouse auditory and motor cortex. We found that the cortex, via corticostriatal somatostatin neurons (CS-SOM), has a direct inhibitory influence on the output of the striatum SPNs. Our results describe a corticostriatal long-range inhibitory circuit (CS-SOM inhibitory projections → striatal SPNs) underlying the control of spike timing/generation in SPNs and attributes a specific function to a genetically defined type of cortical interneuron in corticostriatal communication.

DOI:http://dx.doi.org/10.7554/eLife.15890.001

The striatum is located beneath the cerebral cortex, where it contributes to processes including learning and movement. The Spanish anatomist Ramon y Cajal, working in the early 20^th century, was the first to observe individual neurons extending from the cortex to the striatum. Cajal published drawings of these neurons in his now celebrated anatomical papers, but knew little about their properties.

In the 1980s, advances in techniques for labeling individual cells made it possible to study these neurons in detail. The results suggested that the pathways are exclusively excitatory: that is, the cortical neurons always increase the activity of their partners in the striatum. However, this result made it difficult to explain why electrically stimulating the cortex can sometimes reduce or inhibit the activity of the striatum. To reconcile these facts, most people assumed that inhibition must occur when excitatory cortical neurons activate networks of inhibitory cells within the striatum itself.

Rock et al. now challenge this view by providing anatomical and physiological evidence for the existence of long-range inhibitory pathways from the cortex to the striatum in the mouse brain. These inhibitory neurons project from the auditory and motor regions of the cortex, and contain a substance called somatostatin. These neurons form connections with a specific type of striatal neuron called medium spiny neurons, which in turn project to other brain regions outside the striatum. The inhibitory cortical neurons can alter the activity of the medium spiny neurons, and can therefore directly control the output of the striatum.

The discovery that the striatum receives both excitatory and inhibitory inputs from cortex suggests that the timing and relative strength of these inputs can affect the activity of the striatum. Future experiments should examine whether this is a general mechanism by which sensory stimuli can influence the processes controlled by the striatum, such as movement.

DOI:http://dx.doi.org/10.7554/eLife.15890.002

NPY+-, but not PV+- GABAergic neurons mediated long-range inhibition from infra- to prelimbic cortex

Anxiety disorders are thought to reflect deficits in the regulation of fear memories. While the amygdala has long been considered a site of storage of fear memories, newer findings suggest that the prefrontal cortex (PFC) is essential in the regulation of amygdala-dependent memories and fear expression. Here, activation of the prelimbic cortex (PrL) enhances the expression of fear, while an elevated activity in the infralimbic cortex (IL) enhances fear extinction. Despite the presence of these facts, we still know very little about the synaptic interconnectivity within the PFC. The aim of the present study was to investigate the inhibitory circuits between prelimbic and IL using morphological and electrophysiological methods. Our immunohistochemical analysis revealed that the distribution of PV(+)- and NPY(+)-GABAergic neurons was strikingly different within the PFC. In addition, we provided the first experimental evidence that the pyramidal neurons in the PrL received a direct inhibitory input mediated by bipolar NPY(+)-GABAergic projection neurons in the IL. Deletion of the anxiety-related neuroligin 2 gene caused a decrease of this direct synaptic inhibition that originated from the IL. Thus, our data suggested that activation of the IL might not only directly activate the corresponding downstream anxiolytic pathway, but also suppress the PrL-related anxiogenic pathway and thus could differentially bias the regulation of fear expression and extinction.

The Neurobiology of Sleep and Wakefulness

Cortical electroencephalographic activity arises from corticothalamocortical interactions, modulated by wake-promoting monoaminergic and cholinergic input. These wake-promoting systems are regulated by hypothalamic hypocretin/orexins, while GABAergic sleep-promoting nuclei are found in the preoptic area, brainstem and lateral hypothalamus. Although pontine acetylcholine is critical for REM sleep, hypothalamic melanin-concentrating hormone/GABAergic cells may "gate" REM sleep. Daily sleep-wake rhythms arise from interactions between a hypothalamic circadian pacemaker and a sleep homeostat whose anatomical locus has yet to be conclusively defined. Control of sleep and wakefulness involves multiple systems, each of which presents vulnerability to sleep/wake dysfunction that may predispose to physical and/or neuropsychiatric disorders.

Corticofugal GABAergic projection neurons in the mouse frontal cortex

Cortical projection neurons are classified by hodology in corticocortical, commissural and corticofugal subtypes. Although cortical projection neurons had been regarded as only glutamatergic neurons, recently corticocortical GABAergic projection neurons has been also reported in several species. Here, we demonstrate corticofugal GABAergic projection neurons in the mouse frontal cortex. We employed viral-vector-mediated anterograde tracing, classical retrograde tracing, and immunohistochemistry to characterize neocortical GABAergic projection neurons. Injections of the Cre-dependent adeno-associated virus into glutamate decarboxylase 67 (GAD67)-Cre knock-in mice revealed neocortical GABAergic projections widely to the forebrain, including the cerebral cortices, caudate putamen (CPu), ventral pallidum (VP), lateral globus pallidus (LGP), nucleus accumbens, and olfactory tubercle (Tu). Minor GABAergic projections were also found in the mediodorsal thalamic nucleus, diagonal band of Broca, medial globus pallidus, substantial nigra, and dorsal raphe nucleus. Retrograde tracing studies also demonstrated corticofugal GABAergic projection neurons in the mouse frontal cortex. Further immunohistochemical screening with neurochemical markers revealed the majority of corticostriatal GABAergic projection neurons were positive for somatostatin (SS)-immunoreactivity. In contrast, corticothalamic GABAergic projection neurons were not identified by representative neurochemical markers for GABAergic neurons. These findings suggest that corticofugal GABAergic projection neurons are heterogeneous in terms of their neurochemical properties and target nuclei, and provide axonal innervations mainly to the nuclei in the basal ganglia.

Identification and Characterization of GABAergic Projection Neurons from Ventral Hippocampus to Amygdala

GABAergic local circuit neurons are critical for the network activity and functional interaction of the amygdala and hippocampus. Previously, we obtained evidence for a GABAergic contribution to the hippocampal projection into the basolateral amygdala. Using fluorogold retrograde labeling, we now demonstrate that this projection indeed has a prominent GABAergic component comprising 17% of the GABAergic neurons in the ventral hippocampus. A majority of the identified GABAergic projection neurons are located in the stratum oriens of area CA1, but cells are also found in the stratum pyramidale and stratum radiatum. We could detect the expression of different markers of interneuron subpopulations, including parvalbumin and calbindin, somatostatin, neuropeptide Y, and cholecystokinin in such retrogradely labeled GABA neurons. Thus GABAergic projection neurons to the amygdala comprise a neurochemically heterogeneous group of cells from different interneuron populations, well situated to control network activity patterns in the amygdalo-hippocampal system.

Building blocks of the cerebral cortex: from development to the dish

Since Ramon y Cajal's examination of the cellular makeup of the cerebral cortex, it has been appreciated that this tissue exhibits some of the greatest degrees of cellular heterogeneity in the entire nervous system. This intricate structure emerges during a well-choreographed developmental process. Here, we review current classifications of the cellular constituents of the cerebral cortex and examine how these building blocks are forged during development. We also look at how basic developmental features underlying cortex formation in vivo have been applied to protocols aimed at generating cortical tissue in vitro.

Functional topography of the corpus callosum investigated by DTI and fMRI

This short review examines the most recent functional studies of the topographic organization of the human corpus callosum, the main interhemispheric commissure. After a brief description of its anatomy, development, microstructure, and function, it examines and discusses the latest findings obtained using diffusion tensor imaging (DTI) and tractography (DTT) and functional magnetic resonance imaging (fMRI), three recently developed imaging techniques that have significantly expanded and refined our knowledge of the commissure. While DTI and DTT have been providing insights into its microstructure, integrity and level of myelination, fMRI has been the key technique in documenting the activation of white matter fibers, particularly in the corpus callosum. By combining DTT and fMRI it has been possible to describe the trajectory of the callosal fibers interconnecting the primary olfactory, gustatory, motor, somatic sensory, auditory and visual cortices at sites where the activation elicited by peripheral stimulation was detected by fMRI. These studies have demonstrated the presence of callosal fiber tracts that cross the commissure at the level of the genu, body, and splenium, at sites showing fMRI activation. Altogether such findings lend further support to the notion that the corpus callosum displays a functional topographic organization that can be explored with fMRI.

Intracallosal neuronal nitric oxide synthase neurons colocalize with neurokinin 1 substance P receptor in the rat

The corpus callosum (cc) contains nitric oxide (NO)-producing neurons. Because NO is a potent vasodilator, these neurons could translate neuronal signals into vascular responses that can be detected by functional brain imaging. Substance P (SP), one of the most widely expressed peptides in the CNS, also produces vasomotor responses by inducing calcium release from intracellular stores through its preferred neurokinin 1 (NK1) receptor, thus inducing NO production via activation of neuronal NO synthase (nNOS). Single- and double-labeling experiments were performed to establish whether NK1-immunopositive neurons (NK1IP -n) are found in the rat cc and the extent of NK1 colocalization with nNOS. NK1IP -n were seen to constitute a large neuronal population in the cc and had a distribution similar to that of nNOSIP neurons (nNOSIP -n). NK1IP -n were numerous in the lateral cc and gradually decreased in the more medial portions, where they were few or absent. Intracallosal NK1IP -n and their dendritic trees were intensely labeled, allowing classification into four morphological types: bipolar, round, polygonal, and pyramidal. Confocal microscopic examination demonstrated that nearly all NK1IP -n contained nNOS (96.43%) and that 84.59% of nNOSIP -n co-expressed NK1. These data suggest that the majority of intracallosal neurons can release NO as a result of the action of SP. A small proportion of nNOSIP -n does not contain NK1 and is not activated by SP; these neurons may release NO via alternative mechanisms. The possible mechanisms by which intracallosal neurons release NO are also reviewed.

A class of GABAergic neurons in the prefrontal cortex sends long-range projections to the nucleus accumbens and elicits acute avoidance behavior

GABAergic projections from the neocortex to subcortical structures have been poorly characterized. Using Dlxi12b-Cre mice, we found anatomical evidence for GABAergic neurons that project from the mouse medial prefrontal cortex (mPFC) to multiple subcortical targets. We used a combination of patch-clamp electrophysiology, optogenetics, and pharmacology to confirm that Dlxi12b-labeled projections from the mPFC to the nucleus accumbens (NAcc) release GABA and do not corelease glutamate. Furthermore, optogenetic stimulation of these GABAergic projections from mPFC to NAcc induces avoidance behavior in a real-time place preference task, suggesting that these long-range projecting GABAergic neurons can transmit aversive signals. Finally, we found evidence for heterogeneous histochemical and/or electrophysiological properties of long-range projecting GABAergic neurons in the mPFC. Some of these neurons were labeled in parvalbumin-Cre and vasoactive intestinal peptide-Cre mice. We also used a novel intersectional targeting strategy to label GABAergic neurons in the mPFC that project to NAcc and found that these neurons have fast-spiking properties and express parvalbumin. These results define possible functions and properties for a class of long-range projecting GABAergic neurons in the neocortex.

Substance P Activates Ca2+-Permeable Nonselective Cation Channels through a Phosphatidylcholine-Specific Phospholipase C Signaling Pathway in nNOS-Expressing GABAergic Neurons in Visual Cortex

To understand the functions of the neocortex, it is essential to characterize the properties of neurons constituting cortical circuits. Here, we focused on a distinct group of GABAergic neurons that are defined by a specific colocalization of intense labeling for both neuronal nitric oxide synthase (nNOS) and substance P (SP) receptor [neurokinin 1 (NK1) receptors]. We investigated the mechanisms of the SP actions on these neurons in visual cortical slices obtained from young glutamate decarboxylase 67-green fluorescent protein knock-in mice. Bath application of SP induced a nonselective cation current leading to depolarization that was inhibited by the NK1 antagonists in nNOS-immunopositive neurons. Ruthenium red and La(3+), transient receptor potential (TRP) channel blockers, suppressed the SP-induced current. The SP-induced current was mediated by G proteins and suppressed by D609, an inhibitor of phosphatidylcholine-specific phospholipase C (PC-PLC), but not by inhibitors of phosphatidylinositol-specific PLC, adenylate cyclase or Src tyrosine kinases. Ca(2+) imaging experiments under voltage clamp showed that SP induced a rise in intracellular Ca(2+) that was abolished by removal of extracellular Ca(2+) but not by depletion of intracellular Ca(2+) stores. These results suggest that SP regulates nNOS neurons by activating TRP-like Ca(2+)-permeable nonselective cation channels through a PC-PLC-dependent signaling pathway.

Genetic dissection of GABAergic neural circuits in mouse neocortex

Diverse and flexible cortical functions rely on the ability of neural circuits to perform multiple types of neuronal computations. GABAergic inhibitory interneurons significantly contribute to this task by regulating the balance of activity, synaptic integration, spiking, synchrony, and oscillation in a neural ensemble. GABAergic interneurons display a high degree of cellular diversity in morphology, physiology, connectivity, and gene expression. A considerable number of subtypes of GABAergic interneurons diversify modes of cortical inhibition, enabling various types of information processing in the cortex. Thus, comprehensively understanding fate specification, circuit assembly, and physiological function of GABAergic interneurons is a key to elucidate the principles of cortical wiring and function. Recent advances in genetically encoded molecular tools have made a breakthrough to systematically study cortical circuitry at the molecular, cellular, circuit, and whole animal levels. However, the biggest obstacle to fully applying the power of these to analysis of GABAergic circuits was that there were no efficient and reliable methods to express them in subtypes of GABAergic interneurons. Here, I first summarize cortical interneuron diversity and current understanding of mechanisms, by which distinct classes of GABAergic interneurons are generated. I then review recent development in genetically encoded molecular tools for neural circuit research, and genetic targeting of GABAergic interneuron subtypes, particularly focusing on our recent effort to develop and characterize Cre/CreER knockin lines. Finally, I highlight recent success in genetic targeting of chandelier cells, the most unique and distinct GABAergic interneuron subtype, and discuss what kind of questions need to be addressed to understand development and function of cortical inhibitory circuits.

GABAergic projection neurons route selective olfactory inputs to specific higher-order neurons

We characterize an inhibitory circuit motif in the Drosophila olfactory system, parallel inhibition, which differs from feedforward or feedback inhibition. Excitatory and GABAergic inhibitory projection neurons (ePNs and iPNs) each receive input from antennal lobe glomeruli and send parallel output to the lateral horn, a higher center implicated in regulating innate olfactory behavior. Ca(2+) imaging of specific lateral horn neurons as an olfactory readout revealed that iPNs selectively suppressed food-related odor responses, but spared signal transmission from pheromone channels. Coapplying food odorant did not affect pheromone signal transmission, suggesting that the differential effects likely result from connection specificity of iPNs, rather than a generalized inhibitory tone. Ca(2+) responses in the ePN axon terminals show no detectable suppression by iPNs, arguing against presynaptic inhibition as a primary mechanism. The parallel inhibition motif may provide specificity in inhibition to funnel specific olfactory information, such as food and pheromone, into distinct downstream circuits.

A role for cortical nNOS/NK1 neurons in coupling homeostatic sleep drive to EEG slow wave activity

Although the neural circuitry underlying homeostatic sleep regulation is little understood, cortical neurons immunoreactive for neuronal nitric oxide synthase (nNOS) and the neurokinin-1 receptor (NK1) have been proposed to be involved in this physiological process. By systematically manipulating the durations of sleep deprivation and subsequent recovery sleep, we show that activation of cortical nNOS/NK1 neurons is directly related to non-rapid eye movement (NREM) sleep time, NREM bout duration, and EEG δ power during NREM sleep, an index of preexisting homeostatic sleep drive. Conversely, nNOS knockout mice show reduced NREM sleep time, shorter NREM bouts, and decreased power in the low δ range during NREM sleep, despite constitutively elevated sleep drive. Cortical NK1 neurons are still activated in response to sleep deprivation in these mice but, in the absence of nNOS, they are unable to up-regulate NREM δ power appropriately. These findings support the hypothesis that cortical nNOS/NK1 neurons translate homeostatic sleep drive into up-regulation of NREM δ power through an NO-dependent mechanism.

Neuronal nicotinic receptors in sleep-related epilepsy: studies in integrative biology

Although Mendelian diseases are rare, when considered one by one, overall they constitute a significant social burden. Besides the medical aspects, they propose us one of the most general biological problems. Given the simplest physiological perturbation of an organism, that is, a single gene mutation, how do its effects percolate through the hierarchical biological levels to determine the pathogenesis? And how robust is the physiological system to this perturbation? To solve these problems, the study of genetic epilepsies caused by mutant ion channels presents special advantages, as it can exploit the full range of modern experimental methods. These allow to extend the functional analysis from single channels to whole brains. An instructive example is autosomal dominant nocturnal frontal lobe epilepsy (ADNFLE), which can be caused by mutations in neuronal nicotinic acetylcholine receptors. In vitro, such mutations often produce hyperfunctional receptors, at least in heterozygous condition. However, understanding how this leads to sleep-related frontal epilepsy is all but straightforward. Several available animal models are helping us to determine the effects of ADNFLE mutations on the mammalian brain. Because of the complexity of the cholinergic regulation in both developing and mature brains, several pathogenic mechanisms are possible, which also present different therapeutic implications.

Neuronal nitric oxide synthase expressing neurons: a journey from birth to neuronal circuits

Nitric oxide (NO) is an important signaling molecule crucial for many physiological processes such as synaptic plasticity, vasomotricity, and inflammation. Neuronal nitric oxide synthase (nNOS) is the enzyme responsible for the synthesis of NO by neurons. In the juvenile and mature hippocampus and neocortex nNOS is primarily expressed by subpopulations of GABAergic interneurons. Over the past two decades, many advances have been achieved in the characterization of neocortical and hippocampal nNOS expressing neurons. In this review, we summarize past and present studies that have characterized the electrophysiological, morphological, molecular, and synaptic properties of these neurons. We also discuss recent studies that have shed light on the developmental origins and specification of GABAergic neurons with specific attention to neocortical and hippocampal nNOS expressing GABAergic neurons. Finally, we summarize the roles of NO and nNOS-expressing inhibitory neurons.

Multiple embryonic origins of nitric oxide synthase-expressing GABAergic neurons of the neocortex

CORTICAL GABAERGIC INTERNEURONS IN RODENTS ORIGINATE IN THREE SUBCORTICAL REGIONS: the medial ganglionic eminence (MGE), the lateral/caudal ganglionic eminence (LGE/CGE), and the preoptic area (POA). Each of these neuroepithelial precursor domains contributes different interneuron subtypes to the cortex. Neuronal NOS (nNOS)-expressing neurons represent a heterogenous population of cortical interneurons. We examined the development of these cells in the mouse embryonic cortex and their abundance and distribution in adult animals. Using genetic lineage tracing in transgenic mice we find that nNOS type I cells originate only in the MGE whereas type II cells have a triple origin in the MGE, LGE/CGE, and POA. The two populations are born at different times during development, occupy different layers in the adult cortex and have distinct neurochemical profiles. nNOS neurons are more numerous in the adult cortex than previously reported and constitute a significant proportion of the cortical interneuron population. Our data suggest that the heterogeneity of nNOS neurons in the cortex can be attributed to their multiple embryonic origins which likely impose distinct genetic specification programs.

DNA methyltransferases1 (DNMT1) and 3a (DNMT3a) colocalize with GAD67-positive neurons in the GAD67-GFP mouse brain

DNA methylation is an epigenetic regulatory mechanism commonly associated with transcriptional silencing. DNA methyltransferases (DNMTs) are a family of related proteins that both catalyze the de novo formation of 5-methylcytosine and maintain these methylation marks in cell-specific patterns in virtually all mitotic cells of the body. In the adult brain, methylation occurs in progenitor cells of the neurogenic zones and in postmitotic neurons. Of the DNMTs, DNMT1 and DNMT3a are most highly expressed in postmitotic neurons. While it has been commonly thought all postmitotic neurons and glia express DNMTs at comparable levels, the coexpression of selected DNMTs with markers of distinct neurotransmitter phenotypes has not been previously examined in detail in the mouse. To this end, we analyzed the expression of DNMT1 and DNMT3a along with GAD67 in the brains of the glutamic acid decarboxylase67-enhanced green fluorescent protein (GAD67-GFP) knockin mice. After first confirming that GFP-immunopositive neurons were also GAD67-positive, we showed that in the motor cortex, piriform cortex, striatum, CA1 region of the hippocampus, dentate gyrus, and basolateral amygdala (BLA), GFP immunofluorescence coincided with the signal corresponding to DNMT1 and DNMT3a. A detailed examination of cortical neurons, showed that ≈30% of NeuN-immunopositive neurons were also DNMT1-positive. These data do not exclude the expression of DNMT1 or DNMT3a in glutamatergic neurons and glia. However, they suggest that their expression is low compared with the levels present in GABAergic neurons.

Characterization of Type I and Type II nNOS-Expressing Interneurons in the Barrel Cortex of Mouse

In the neocortex, neuronal nitric oxide (NO) synthase (nNOS) is essentially expressed in two classes of GABAergic neurons: type I neurons displaying high levels of expression and type II neurons displaying weaker expression. Using immunocytochemistry in mice expressing GFP under the control of the glutamic acid decarboxylase 67k (GAD67) promoter, we studied the distribution of type I and type II neurons in the barrel cortex and their expression of parvalbumin (PV), somatostatin (SOM), and vasoactive intestinal peptide (VIP). We found that type I neurons were predominantly located in deeper layers and expressed SOM (91.5%) while type II neurons were concentrated in layer II/III and VI and expressed PV (17.7%), SOM (18.7%), and VIP (10.2%). We then characterized neurons expressing nNOS mRNA (n = 42 cells) ex vivo, using whole-cell recordings coupled to single-cell reverse transcription-PCR and biocytin labeling. Unsupervised cluster analysis of this sample disclosed four classes. One cluster (n = 7) corresponded to large, deep layer neurons, displaying a high expression of SOM (85.7%) and was thus very likely to correspond to type I neurons. The three other clusters were identified as putative type II cells and corresponded to neurogliaform-like interneurons (n = 19), deep layer neurons expressing PV or SOM (n = 9), and neurons expressing VIP (n = 7). Finally, we performed nNOS immunohistochemistry on mouse lines in which GFP labeling revealed the expression of two specific developmental genes (Lhx6 and 5-HT_3A). We found that type I neurons expressed Lhx6 but never 5-HT_3A, indicating that they originate in the medial ganglionic eminence (MGE). Type II neurons expressed Lhx6 (63%) and 5-HT_3A (34.4%) supporting their derivation either from the MGE or from the caudal ganglionic eminence (CGE) and the entopeduncular and dorsal preoptic areas. Together, our results in the barrel cortex of mouse support the view that type I neurons form a specific class of SOM-expressing neurons while type II neurons comprise at least three classes.

Cortical nNOS neurons co-express the NK1 receptor and are depolarized by Substance P in multiple mammalian species

We have previously demonstrated that Type I neuronal nitric oxide synthase (nNOS)-expressing neurons are sleep-active in the cortex of mice, rats, and hamsters. These neurons are known to be GABAergic, to express Neuropeptide Y (NPY) and, in rats, to co-express the Substance P (SP) receptor NK1, suggesting a possible role for SP in sleep/wake regulation. To evaluate the degree of co-expression of nNOS and NK1 in the cortex among mammals, we used double immunofluorescence for nNOS and NK1 and determined the anatomical distribution in mouse, rat, and squirrel monkey cortex. Type I nNOS neurons co-expressed NK1 in all three species although the anatomical distribution within the cortex was species-specific. We then performed in vitro patch clamp recordings in cortical neurons in mouse and rat slices using the SP conjugate tetramethylrhodamine-SP (TMR-SP) to identify NK1-expressing cells and evaluated the effects of SP on these neurons. Bath application of SP (0.03–1 μM) resulted in a sustained increase in firing rate of these neurons; depolarization persisted in the presence of tetrodotoxin. These results suggest a conserved role for SP in the regulation of cortical sleep-active neurons in mammals.

Association of type I neurons positive for NADPH-diaphorase with blood vessels in the adult monkey corpus callosum

Sagittal sections through the corpus callosum of adult macaque monkeys (n = 7) reveal a subpopulation of neurons positive for NADPH-diaphorase (NADPHd). These are sparsely distributed, with 2–12 neurons scored over the anterior two-thirds of the callosum (about 14 mm). Neurons are densely labeled, type 1; but on the basis of soma and dendritic morphology, these neurons exhibit distinct heterogeneity. In one subpopulation, the cell body is narrowly attenuated (7–10 μm in width). These have bipolar dendrites, extending 300–800 μm from the cell body. One or both of the dendrites is often closely associated with blood vessels and tends to be aligned dorso-ventral, perpendicular to the body of the callosum. Another subpopulation of neurons has a larger soma (typically, 15 μm × 20 μm) and more multipolar dendrites, which are not as obviously associated with blood vessels. White matter neurons positive for NADPHd have previously been observed as a transient population, most numerous during development, in the human corpus callosum, as well as in that of other species. Their persistence in the corpus callosum of adult macaques and their close association with blood vessels has not previously been reported and is suggestive of roles other than axon guidance.

5-HT(3A) receptor-bearing white matter interstitial GABAergic interneurons are functionally integrated into cortical and subcortical networks

In addition to axons and surrounding glial cells, the corpus callosum also contains interstitial neurons that constitute a heterogeneous cell population. There is growing anatomical evidence that white matter interstitial cells (WMICs) comprise GABAergic interneurons, but so far there is little functional evidence regarding their connectivity. The scarcity of these cells has hampered electrophysiological studies. We overcame this hindrance by taking recourse to transgenic mice in which distinct WMICs expressed enhanced green fluorescence protein (EGFP). The neuronal phenotype of the EGFP-labeled WMICs was confirmed by their NeuN positivity. The GABAergic phenotype could be established based on vasoactive intestinal peptide and calretinin expression and was further supported by a firing pattern typical for interneurons. Axons and dendrites of many EGFP-labeled WMICs extended to the cortex, hippocampus, and striatum. Patch-clamp recordings in acute slices showed that they receive excitatory and inhibitory input from cortical and subcortical structures. Moreover, paired recordings revealed that EGFP-labeled WMICs inhibit principal cells of the adjacent cortex, thus providing unequivocal functional evidence for their GABAergic phenotype and demonstrating that they are functionally integrated into neuronal networks.

Mechanisms of inhibition within the telencephalon: "where the wild things are"

In this review, we first provide a historical perspective of inhibitory signaling from the discovery of inhibition through to our present understanding of the diversity and mechanisms by which GABAergic interneuron populations function in different parts of the telencephalon. This is followed by a summary of the mechanisms of inhibition in the CNS. With this as a starting point, we provide an overview describing the variations in the subtypes and origins of inhibitory interneurons within the pallial and subpallial divisions of the telencephalon, with a focus on the hippocampus, somatosensory, paleo/piriform cortex, striatum, and various amygdala nuclei. Strikingly, we observe that marked variations exist in the origin and numerical balance between GABAergic interneurons and the principal cell populations in distinct regions of the telencephalon. Finally we speculate regarding the attractiveness and challenges of establishing a unifying nomenclature to describe inhibitory neuron diversity throughout the telencephalon.

Pioneer GABA cells comprise a subpopulation of hub neurons in the developing hippocampus

Connectivity in the developing hippocampus displays a functional organization particularly effective in supporting network synchronization, as it includes superconnected hub neurons. We have previously shown that hub network function is supported by a subpopulation of GABA neurons. However, it is unclear whether hub cells are only transiently present or later develop into distinctive subclasses of interneurons. These questions are difficult to assess given the heterogeneity of the GABA neurons and the poor early expression of markers. To circumvent this conundrum, we used "genetic fate mapping" that allows for the selective labeling of GABA neurons based on their place and time of origin. We show that early-generated GABA cells form a subpopulation of hub neurons, characterized by an exceptionally widespread axonal arborization and the ability to single-handedly impact network dynamics when stimulated. Pioneer hub neurons remain into adulthood, when they acquire the classical markers of long-range projecting GABA neurons.