New insights into the classification and nomenclature of cortical GABAergic interneurons.

A systematic classification and accepted nomenclature of neuron types is much needed but is currently lacking. This article describes a possible taxonomical solution for classifying GABAergic interneurons of the cerebral cortex based on a novel, web-based interactive system that allows experts to classify neurons with pre-determined criteria. Using Bayesian analysis and clustering algorithms on the resulting data, we investigated the suitability of several anatomical terms and neuron names for cortical GABAergic interneurons. Moreover, we show that supervised classification models could automatically categorize interneurons in agreement with experts' assignments. These results demonstrate a practical and objective approach to the naming, characterization and classification of neurons based on community consensus.

Layer 6 corticothalamic neurons activate a cortical output layer, layer 5a.

Layer 6 corticothalamic neurons are thought to modulate incoming sensory information via their intracortical axons targeting the major thalamorecipient layer of the neocortex, layer 4, and via their long-range feedback projections to primary sensory thalamic nuclei. However, anatomical reconstructions of individual layer 6 corticothalamic (L6 CT) neurons include examples with axonal processes ramifying within layer 5, and the relative input of the overall population of L6 CT neurons to layers 4 and 5 is not well understood. We compared the synaptic impact of L6 CT cells on neurons in layers 4 and 5. We found that the axons of L6 CT neurons densely ramified within layer 5a in both visual and somatosensory cortices of the mouse. Optogenetic activation of corticothalamic neurons generated large EPSPs in pyramidal neurons in layer 5a. In contrast, excitatory neurons in layer 4 exhibited weak excitation or disynaptic inhibition. Fast-spiking parvalbumin-positive cells in both layer 5a and layer 4 were also strongly activated by L6 CT neurons. The overall effect of L6 CT activation was to suppress layer 4 while eliciting action potentials in layer 5a pyramidal neurons. Together, our data indicate that L6 CT neurons strongly activate an output layer of the cortex.

Intracortical circuits of pyramidal neurons reflect their long-range axonal targets.

Cortical columns generate separate streams of information that are distributed to numerous cortical and subcortical brain regions. We asked whether local intracortical circuits reflect these different processing streams by testing whether the intracortical connectivity among pyramidal neurons reflects their long-range axonal targets. We recorded simultaneously from up to four retrogradely labelled pyramidal neurons that projected to the superior colliculus, the contralateral striatum or the contralateral cortex to assess their synaptic connectivity. Here we show that the probability of synaptic connection depends on the functional identities of both the presynaptic and postsynaptic neurons. We first found that the frequency of monosynaptic connections among corticostriatal pyramidal neurons is significantly higher than among corticocortical or corticotectal pyramidal neurons. We then show that the probability of feed-forward connections from corticocortical neurons to corticotectal neurons is approximately three- to fourfold higher than the probability of monosynaptic connections among corticocortical or corticotectal cells. Moreover, we found that the average axodendritic overlap of the presynaptic and postsynaptic pyramidal neurons could not fully explain the differences in connection probability that we observed. The selective synaptic interactions we describe demonstrate that the organization of local networks of pyramidal cells reflects the long-range targets of both the presynaptic and postsynaptic neurons.

Mechanisms generating dual-component nicotinic EPSCs in cortical interneurons.

Activation of cortical nicotinic receptors by cholinergic axons from the basal forebrain (BF) significantly impacts cortical function, and the loss of nicotinic receptors is a hallmark of aging and neurodegenerative disease. We have previously shown that stimulation of BF axons generates a fast α7 and a slow non-α7 receptor-dependent response in cortical interneurons. However, the synaptic mechanisms that underlie this dual-component nicotinic response remain unclear. Here, we report that fast α7 receptor-mediated EPSCs in the mouse cortex are highly variable and insensitive to perturbations of acetylcholinesterase (AChE), while slow non-α7 receptor-mediated EPSCs are reliable and highly sensitive to AChE activity. Based on these data, we propose that the fast and slow nicotinic responses reflect differences in synaptic structure between cholinergic varicosities activating α7 and non-α7 classes of nicotinic receptors.

Prolonged disynaptic inhibition in the cortex mediated by slow, non-α7 nicotinic excitation of a specific subset of cortical interneurons.

Cholinergic activation of nicotinic receptors in the cortex plays a critical role in arousal, attention, and learning. Here we demonstrate that cholinergic axons from the basal forebrain of mice excite a specific subset of cortical interneurons via a remarkably slow, non-α7 nicotinic receptor-mediated conductance. In turn, these inhibitory cells generate a delayed and prolonged wave of disynaptic inhibition in neighboring cortical neurons, altering the spatiotemporal pattern of inhibition in cortical circuits.

Petilla terminology: nomenclature of features of GABAergic interneurons of the cerebral cortex.

Neuroscience produces a vast amount of data from an enormous diversity of neurons. A neuronal classification system is essential to organize such data and the knowledge that is derived from them. Classification depends on the unequivocal identification of the features that distinguish one type of neuron from another. The problems inherent in this are particularly acute when studying cortical interneurons. To tackle this, we convened a representative group of researchers to agree on a set of terms to describe the anatomical, physiological and molecular features of GABAergic interneurons of the cerebral cortex. The resulting terminology might provide a stepping stone towards a future classification of these complex and heterogeneous cells. Consistent adoption will be important for the success of such an initiative, and we also encourage the active involvement of the broader scientific community in the dynamic evolution of this project.

Synaptogenesis of electrical and GABAergic synapses of fast-spiking inhibitory neurons in the neocortex.

Parvalbumin-expressing fast-spiking (FS) cells are interconnected via GABAergic and electrical synapses and represent a major class of inhibitory interneurons in the neocortex. Synaptic connections among FS cells are critical for regulating network oscillations in the mature neocortex. However, it is unclear whether synaptic connections among FS interneurons also play a central role in the generation of patterned neuronal activity in the immature brain, which is thought to underlie the formation of neocortical circuits. Here, we investigated the developmental time course of synaptogenesis of FS cell in mouse visual cortex. In layer 5/6 (L5/6), we recorded from two or three FS and/or pyramidal (PYR) neurons to study the development of electrical and chemical synaptic interactions from postnatal day 3 (P3) to P18. We detected no evidence for functional connectivity for FS-FS or FS-PYR pairs at P3-P4. However, by P5-P6, we found that 20% of FS pairs were electrically coupled, and 24% of pairs were connected via GABAergic synapses; by P15-P18, 42% of FS pairs had established functional electrical synapses, and 47% of FS pairs were connected via GABAergic synapses. FS cell GABAergic inhibition of pyramidal cells showed a similar developmental time line, but no electrical coupling was detected for FS-PYR pairs. We found that synaptogenesis of electrical and GABAergic connections of FS cells takes place in the same period. Together, our results suggest that chemical and electrical connections among FS cells can contribute to patterned neocortical activity only by the end of the first postnatal week.

Motivation and Engagement during Visually Guided Behavior.

Animal behavior is motivated by internal drives, such as thirst and hunger, generated in hypothalamic neurons that project widely to many brain areas. We find that water-restricted mice maintain stable, high-level contrast sensitivity and brief reaction time while performing a visual task, but then abruptly stop and become disengaged. Mice consume a significant amount of water when freely provided in their home cage immediately after the task, indicating that disengagement does not reflect cessation of thirst. Neuronal responses of V1 neurons are reduced in the disengaged state, but pupil diameter does not decrease, suggesting that animals' reduced level of arousal does not drive the transition to disengagement. Our findings indicate that satiation level alone does not have an instructive role in visually guided behavior and suggest that animals' behavior is governed by cost-benefit analysis that can override thirst signals.

D1-like dopamine receptor activation modulates GABAergic inhibition but not electrical coupling between neocortical fast-spiking interneurons.

Dopamine, acting through D(1) receptors, is thought to play an important role in cognitive functions of the frontal cortex such as working memory. D(1) receptors are widely expressed in fast-spiking (FS) interneurons, a prominent class of inhibitory cells that exert a powerful control of neuronal firing through proximal synapses on their postsynaptic targets. FS cells are extensively mutually interconnected by both GABA(A) receptor-mediated synapses and gap junction-mediated electrical synapses, and networks of FS cells play a crucial role in the generation of rhythmic synchronous activity. Although recent studies have documented the effects of dopamine modulation of neocortical synaptic connections among excitatory cells and between excitatory and various inhibitory cells, the effects of dopamine receptor activation on GABAergic and electrical interactions among FS cells is not known. To resolve this, we recorded from pairs of FS cells in the infragranular layers of mouse neocortical slices and tested the effects of D(1)-like (D(1)/D(5)) receptor activation on these connections. We found that D(1)-like receptor activation modulated GABAergic but not electrical connections between them. A D(1)-like receptor agonist preserved the strength of electrical coupling but reduced the amplitude of IPSPs and IPSCs between FS cells. Our results suggest that D(1)-like receptor activation has synapse-specific effects within networks of FS cells, with potential implications for the generation of rhythmic activity in the neocortex.

Cannabinoid sensitivity and synaptic properties of 2 GABAergic networks in the neocortex.

Distinct networks of gamma-aminobutyric acidergic interneurons connected by electrical synapses can promote different patterns of activity in the neocortex. Cannabinoids affect memory and cognition by powerfully modulating a subset of inhibitory synapses; however, very little is known about the synaptic properties of the cannabinoid receptor-expressing neurons (CB(1)-positive irregular spiking [CB(1)-IS]) in the neocortex. Using paired recordings in neocortical slices, we 1st report here that synapses of CB(1)-IS cells, but not synapses of fast-spiking (FS) cells, are suppressed by release of endocannabinoids from pyramidal neurons. CB(1)-IS synapses were characterized by a very high failure rate that contrasted with the high reliability of FS synapses. Furthermore, CB(1)-IS cells received excitatory inputs less frequently compared with FS cells and made significantly less frequent inhibitory contacts onto local pyramids. These distinct synaptic properties together with their characteristic irregular firing suggest that CB(1)-IS cells play different role in neocortical function than that of FS cells. Thus, whereas the synaptic properties of FS cells can allow them to impose high-frequency rhythmic oscillatory activity, those of CB(1)-IS cells may lead to disruption of fast rhythmic oscillations. Our results suggest that activity-dependent release of cannabinoids, by blocking CB(1)-IS synapses, may alter the role of inhibition in neocortical circuits.

Correlation of Synaptic Inputs in the Visual Cortex of Awake, Behaving Mice.

The subthreshold mechanisms that underlie neuronal correlations in awake animals are poorly understood. Here, we perform dual whole-cell recordings in the visual cortex (V1) of awake mice to investigate membrane potential (Vm) correlations between upper-layer sensory neurons. We find that the membrane potentials of neighboring neurons display large, correlated fluctuations during quiet wakefulness, including pairs of cells with disparate tuning properties. These fluctuations are driven by correlated barrages of excitation followed closely by inhibition (∼5-ms lag). During visual stimulation, low-frequency activity is diminished, and coherent high-frequency oscillations appear, even for non-preferred stimuli. These oscillations are generated by alternating excitatory and inhibitory inputs at a similar lag. The temporal sequence of depolarization for pairs of neurons is conserved during both spontaneous- and visually-evoked activity, suggesting a stereotyped flow of activation that may function to produce temporally precise "windows of opportunity" for additional synaptic inputs.

Feature-Specific Organization of Feedback Pathways in Mouse Visual Cortex.

Higher and lower cortical areas in the visual hierarchy are reciprocally connected [1]. Although much is known about how feedforward pathways shape receptive field properties of visual neurons, relatively little is known about the role of feedback pathways in visual processing. Feedback pathways are thought to carry top-down signals, including information about context (e.g., figure-ground segmentation and surround suppression) [2-5], and feedback has been demonstrated to sharpen orientation tuning of neurons in the primary visual cortex (V1) [6, 7]. However, the response characteristics of feedback neurons themselves and how feedback shapes V1 neurons' tuning for other features, such as spatial frequency (SF), remain largely unknown. Here, using a retrograde virus, targeted electrophysiological recordings, and optogenetic manipulations, we show that putatively feedback neurons in layer 5 (hereafter "L5 feedback") in higher visual areas, AL (anterolateral area) and PM (posteromedial area), display distinct visual properties in awake head-fixed mice. AL L5 feedback neurons prefer significantly lower SF (mean: 0.04 cycles per degree [cpd]) compared to PM L5 feedback neurons (0.15 cpd). Importantly, silencing AL L5 feedback reduced visual responses of V1 neurons preferring low SF (mean change in firing rate: -8.0%), whereas silencing PM L5 feedback suppressed responses of high-SF-preferring V1 neurons (-20.4%). These findings suggest that feedback connections from higher visual areas convey distinctly tuned visual inputs to V1 that serve to boost V1 neurons' responses to SF. Such like-to-like functional organization may represent an important feature of feedback pathways in sensory systems and in the nervous system in general.

Electrical synapses define networks of neocortical GABAergic neurons.

Recent work using paired recording has provided a direct demonstration of functional electrical synapses between neocortical neurons of both juvenile and adult animals. Electrical synapses have been found among GABAergic interneurons but not pyramidal cells. Interestingly, necortical electrical synapses almost exclusively connect GABAergic neurons belonging to the same class. So far, at least five different neocortical networks defined by extensive and selective electrical coupling have been studied in the neocortex. These results could provide important clues to the understanding of functional cortical circuitry.

Synaptic interactions of late-spiking neocortical neurons in layer 1.

Layer 1 of the neocortex is an important zone in which synaptic integration of inputs originating from a variety of cerebral regions is thought to take place. Layer 1 does not contain pyramidal cells, and several histochemical studies have suggested that most layer 1 neurons are GABAergic. However, although layer 1 neurons could be an important source of inhibition in this layer, the synaptic action of these neurons and the identity of their postsynaptic targets are unknown. We studied the physiological properties and synaptic interactions of a class of cells within layer 1 called late-spiking (LS) cells. The dendrites and axons of layer 1 LS cells were confined primarily to layer 1. Using paired recording, we showed that LS cells formed GABAergic connections with other LS cells as well as with non-LS cells in layer 1 and with pyramidal cells in layer 2/3. We also found that layer 2/3 pyramidal neurons provide excitatory inputs to LS cells. It has been suggested previously that GABAergic neurons belonging to the same class in the cortex are electrically coupled. In agreement with that hypothesis, we found that LS cells were interconnected by electrical coupling (83%), whereas electrical coupling between LS cells and non-LS cells was infrequent (2%). Thus, we provide evidence showing that a group of GABAergic neurons within layer 1 are specifically interconnected by electrical coupling and can provide significant inhibitory inputs to neurons in layer 1 and to distal dendrites of pyramidal cells.

Electrical coupling among irregular-spiking GABAergic interneurons expressing cannabinoid receptors.

Anatomical studies have shown that the G-protein-coupled cannabinoid receptor-1 (CB1) is selectively expressed in a subset of GABAergic interneurons. It has been proposed that these cells regulate rhythmic activity and play a key role mediating the cognitive actions of marijuana and endogenous cannabinoids. However, the physiology, anatomy, and synaptic connectivity of neocortical CB1-expressing interneurons remain poorly studied. We identified a population of CB1-expressing interneurons in layer II/III in mouse neocortical slices. These cells were multipolar or bitufted, had a widely extending axon, and exhibited a characteristic pattern of irregular spiking (IS) in response to current injection. CB1-expressing-IS (CB1-IS) cells were inhibitory, establishing GABAA receptor-mediated synapses onto pyramidal cells and other CB1-IS cells. Recently, electrical coupling among other classes of cortical interneurons has been shown to contribute to the generation of rhythmic synchronous activity in the neocortex. We therefore tested whether CB1-IS interneurons are interconnected via electrical synapses using paired recordings. We found that 90% (19 of 21 pairs) of simultaneously recorded pairs of CB1-IS cells were electrically coupled. The average coupling coefficient was 6%. Signaling through electrical synapses promoted coordinated firing among CB1-IS cells. Together, our results identify a population of electrically coupled CB1-IS GABAergic interneurons in the neocortex that share a unique morphology and a characteristic pattern of irregular spiking in response to current injection. The synaptic interactions of these cells may play an important role mediating the cognitive actions of cannabinoids and regulating coherent neocortical activity.

Controlling brain states.

Neurons in mouse V1 increase their response to visual stimulation during locomotion. In this issue of Neuron, Lee et al. (2014) show that subthreshold optogenetic stimulation of a brainstem locomotion area can mimic the effect of locomotion on sensory processing.

Nicotinic modulation of cortical circuits.

The ascending cholinergic neuromodulatory system sends projections throughout cortex and has been shown to play an important role in a number of cognitive functions including arousal, working memory, and attention. However, despite a wealth of behavioral and anatomical data, understanding how cholinergic synapses modulate cortical function has been limited by the inability to selectively activate cholinergic axons. Now, with the development of optogenetic tools and cell-type specific Cre-driver mouse lines, it has become possible to stimulate cholinergic axons from the basal forebrain (BF) and probe cholinergic synapses in the cortex for the first time. Here we review recent work studying the cell-type specificity of nicotinic signaling in the cortex, synaptic mechanisms mediating cholinergic transmission, and the potential functional role of nicotinic modulation.

Subthreshold mechanisms underlying state-dependent modulation of visual responses.

The processing of sensory information varies widely across behavioral states. However, little is known about how behavioral states modulate the intracellular activity of cortical neurons to effect changes in sensory responses. Here, we performed whole-cell recordings from neurons in upper-layer primary visual cortex of awake mice during locomotion and quiet wakefulness. We found that the signal-to-noise ratio for sensory responses was improved during locomotion by two mechanisms: (1) a decrease in membrane potential variability leading to a reduction in background firing rates and (2) an enhancement in the amplitude and reliability of visually evoked subthreshold responses mediated by an increase in total conductance and a depolarization of the stimulus-evoked reversal potential. Consistent with the enhanced signal-to-noise ratio for visual responses during locomotion, we demonstrate that performance is improved in a visual detection task during this behavioral state.

Noradrenaline enhances signal-to-noise ratio of inhibitory inputs in the dorsal cochlear nucleus.

What are the mechanisms that enhance the response to behaviorally relevant external stimuli? In this issue of Neuron, Kuo and Trussell show that in the dorsal cochlear nucleus, noradrenaline functions to simultaneously reduce spontaneous inhibitory inputs while increasing evoked inhibition.