Two networks of electrically coupled inhibitory neurons in neocortex

Inhibitory interneurons are critical to sensory transformations, plasticity and synchronous activity in the neocortex. There are many types of inhibitory neurons, but their synaptic organization is poorly understood. Here we describe two functionally distinct inhibitory networks comprising either fast-spiking (FS) or low-threshold spiking (LTS) neurons. Paired-cell recordings showed that inhibitory neurons of the same type were strongly interconnected by electrical synapses, but electrical synapses between different inhibitory cell types were rare. The electrical synapses were strong enough to synchronize spikes in coupled interneurons. Inhibitory chemical synapses were also common between FS cells, and between FS and LTS cells, but LTS cells rarely inhibited one another. Thalamocortical synapses, which convey sensory information to the cortex, specifically and strongly excited only the FS cell network. The electrical and chemical synaptic connections of different types of inhibitory neurons are specific, and may allow each inhibitory network to function independently.     

Potentiation of cortical inhibition by visual deprivation

The fine-tuning of circuits in sensory cortex requires sensory experience during an early critical period. Visual deprivation during the critical period has catastrophic effects on visual function, including loss of visual responsiveness to the deprived eye, reduced visual acuity, and loss of tuning to many stimulus characteristics. These changes occur faster than the remodelling of thalamocortical axons, but the intracortical plasticity mechanisms that underlie them are incompletely understood. Long-term depression of excitatory intracortical synapses has been proposed as a general candidate mechanism for the loss of cortical responsiveness after visual deprivation. Alternatively (or in addition), the decreased ability of the deprived eye to activate cortical neurons could be due to enhanced intracortical inhibition. Here we show that visual deprivation leaves excitatory connections in layer 4 (the primary input layer to cortex) unaffected, but markedly potentiates inhibitory feedback between fast-spiking basket cells (FS cells) and star pyramidal neurons (star pyramids). Further, a previously undescribed form of long-term potentiation of inhibition (LTPi) could be induced at synapses from FS cells to star pyramids, and was occluded by previous visual deprivation. These data suggest that potentiation of inhibition is a major cellular mechanism underlying the deprivation-induced degradation of visual function, and that this form of LTPi is important in fine-tuning cortical circuitry in response to visual experience.     

A network of fast-spiking cells in the neocortex connected by electrical synapses

Encoding of information in the cortex is thought to depend on synchronous firing of cortical neurons. Inhibitory neurons are known to be critical in the coordination of cortical activity, but how interaction among inhibitory cells promotes synchrony is not well understood. To address this issue directly, we have recorded simultaneously from pairs of fast-spiking (FS) cells, a type of gamma-aminobutyric acid (GABA)-containing neocortical interneuron. Here we report a high occurrence of electrical coupling among FS cells. Electrical synapses were not found among pyramidal neurons or between FS cells and other cortical cells. Some FS cells were interconnected by both electrical and GABAergic synapses. We show that communication through electrical synapses allows excitatory signalling among inhibitory cells and promotes their synchronous spiking. These results indicate that electrical synapses establish a network of fast-spiking cells in the neocortex which may play a key role in coordinating cortical activity.     

Endogenous cannabinoids mediate retrograde signalling at hippocampal synapses

Marijuana affects brain function primarily by activating the G-protein-coupled cannabinoid receptor-1 (CB1), which is expressed throughout the brain at high levels. Two endogenous lipids, anandamide and 2-arachidonylglycerol (2-AG), have been identified as CB1 ligands. Depolarized hippocampal neurons rapidly release both anandamide and 2-AG in a Ca2+-dependent manner. In the hippocampus, CB1 is expressed mainly by GABA (gamma-aminobutyric acid)-mediated inhibitory interneurons, where CB1 clusters on the axon terminal. A synthetic CB1 agonist depresses GABA release from hippocampal slices. These findings indicate that the function of endogenous cannabinoids released by depolarized hippocampal neurons might be to downregulate GABA release. Here we show that the transient suppression of GABA-mediated transmission that follows depolarization of hippocampal pyramidal neurons is mediated by retrograde signalling through release of endogenous cannabinoids. Signalling by the endocannabinoid system thus represents a mechanism by which neurons can communicate backwards across synapses to modulate their inputs.     

A disinhibitory microcircuit for associative fear learning in the auditory cortex

Learning causes a change in how information is processed by neuronal circuits. Whereas synaptic plasticity, an important cellular mechanism, has been studied in great detail, we know much less about how learning is implemented at the level of neuronal circuits and, in particular, how interactions between distinct types of neurons within local networks contribute to the process of learning. Here we show that acquisition of associative fear memories depends on the recruitment of a disinhibitory microcircuit in the mouse auditory cortex. Fear-conditioning-associated disinhibition in auditory cortex is driven by foot-shock-mediated cholinergic activation of layer 1 interneurons, in turn generating inhibition of layer 2/3 parvalbumin-positive interneurons. Importantly, pharmacological or optogenetic block of pyramidal neuron disinhibition abolishes fear learning. Together, these data demonstrate that stimulus convergence in the auditory cortex is necessary for associative fear learning to complex tones, define the circuit elements mediating this convergence and suggest that layer-1-mediated disinhibition is an important mechanism underlying learning and information processing in neocortical circuits.     

Function of identified interneurons in the leech elucidated using neural networks trained by back-propagation

Mechanical stimulation of the body surface of the leech causes a localized withdrawal from dorsal, ventral and lateral stimuli. The pathways from sensory to motor neurons in the reflex include at least one interneuron. We have identified a subset of interneurons contributing to the reflex by intracellular recording, and our analysis of interneuron input and output connections suggests a network in which most interneurons respond to more than one sensory input, most have effects on all motor neurons and in which each form of the behaviour is produced by appropriate and inappropriate effects of many interneurons. To determine whether interneurons of this type can account for the behaviour, or whether additional types are required, model networks were trained by back-propagation to reproduce the physiologically determined input-output function of the reflex. Quantitative comparisons of model and actual connection strengths show that model interneurons are similar to real ones. Consequently, the identified subset of interneurons could control local bending as part of a distributed processing network in which each form of the behaviour is produced by the appropriate and inappropriate effects of many interneurons.     

A topographic map of recruitment in spinal cord

Animals move over a range of speeds by using rhythmic networks of neurons located in the spinal cord. Here we use electrophysiology and in vivo imaging in larval zebrafish (Danio rerio) to reveal a systematic relationship between the location of a spinal neuron and the minimal swimming frequency at which the neuron is active. Ventral motor neurons and excitatory interneurons are rhythmically active at the lowest swimming frequencies, with increasingly more dorsal excitatory neurons engaged as swimming frequency rises. Inhibitory interneurons follow the opposite pattern. These inverted patterns of recruitment are independent of cell soma size among interneurons, but may be partly explained by concomitant dorso-ventral gradients in input resistance. Laser ablations of ventral, but not dorsal, excitatory interneurons perturb slow movements, supporting a behavioural role for the topography. Our results reveal an unexpected pattern of organization within zebrafish spinal cord that underlies the production of movements of varying speeds.     

Acetylcholine inhibits identified interneurons in the cat lateral geniculate nucleus

The transmission of visual information from retina to cortex through the dorsal lateral geniculate nucleus (LGNd) is controlled by non-retinal inputs. Enhanced visually evoked responses in cat LGNd relay cells during periods of increased alertness have been attributed in large part to increased rate of acetylcholine (ACh) release by fibres ascending from the brainstem reticular formation. ACh can modulate geniculate visual responses in vivo, but comparatively little is known about the underlying ionic mechanisms of these cholinergic actions. Although direct excitation of LGNd relay neurons has been shown in vitro, the situation is complicated because cholinergic axons form numerous and complex synapses not only with relay cells, but also with inhibitory interneurons, and electrical activation of the brainstem cholinergic neurons reduces inhibitory postsynaptic potentials in the LGNd. We report here that morphologically characterized interneurons in the cat LGNd possess distinctive electrophysiological properties in comparison with those of relay cells and are inhibited by ACh through a muscarinic receptor-mediated increase in potassium conductance. Together the direct excitation of relay cells and inhibition of intrageniculate interneurons allow the ascending cholinergic system to exert a powerful facilitatory influence over the transfer of visual information to the cerebral cortex.     

Activation of specific interneurons improves V1 feature selectivity and visual perception

Inhibitory interneurons are essential components of the neural circuits underlying various brain functions. In the neocortex, a large diversity of GABA (γ-aminobutyric acid) interneurons has been identified on the basis of their morphology, molecular markers, biophysical properties and innervation pattern. However, how the activity of each subtype of interneurons contributes to sensory processing remains unclear. Here we show that optogenetic activation of parvalbumin-positive (PV+) interneurons in the mouse primary visual cortex (V1) sharpens neuronal feature selectivity and improves perceptual discrimination. Using multichannel recording with silicon probes and channelrhodopsin-2 (ChR2)-mediated optical activation, we found that increased spiking of PV+ interneurons markedly sharpened orientation tuning and enhanced direction selectivity of nearby neurons. These effects were caused by the activation of inhibitory neurons rather than a decreased spiking of excitatory neurons, as archaerhodopsin-3 (Arch)-mediated optical silencing of calcium/calmodulin-dependent protein kinase IIα (CAMKIIα)-positive excitatory neurons caused no significant change in V1 stimulus selectivity. Moreover, the improved selectivity specifically required PV+ neuron activation, as activating somatostatin or vasointestinal peptide interneurons had no significant effect. Notably, PV+ neuron activation in awake mice caused a significant improvement in their orientation discrimination, mirroring the sharpened V1 orientation tuning. Together, these results provide the first demonstration that visual coding and perception can be improved by increased spiking of a specific subtype of cortical inhibitory interneurons.     

Turning on and off recurrent balanced cortical activity

The vast majority of synaptic connections onto neurons in the cerebral cortex arise from other cortical neurons, both excitatory and inhibitory, forming local and distant 'recurrent' networks. Although this is a basic theme of cortical organization, its study has been limited largely to theoretical investigations, which predict that local recurrent networks show a proportionality or balance between recurrent excitation and inhibition, allowing the generation of stable periods of activity. This recurrent activity might underlie such diverse operations as short-term memory, the modulation of neuronal excitability with attention, and the generation of spontaneous activity during sleep. Here we show that local cortical circuits do indeed operate through a proportional balance of excitation and inhibition generated through local recurrent connections, and that the operation of such circuits can generate self-sustaining activity that can be turned on and off by synaptic inputs. These results confirm the long-hypothesized role of recurrent activity as a basic operation of the cerebral cortex.     

An intrinsic mechanism of corticogenesis from embryonic stem cells

The cerebral cortex develops through the coordinated generation of dozens of neuronal subtypes, but the mechanisms involved remain unclear. Here we show that mouse embryonic stem cells, cultured without any morphogen but in the presence of a sonic hedgehog inhibitor, recapitulate in vitro the major milestones of cortical development, leading to the sequential generation of a diverse repertoire of neurons that display most salient features of genuine cortical pyramidal neurons. When grafted into the cerebral cortex, these neurons develop patterns of axonal projections corresponding to a wide range of cortical layers, but also to highly specific cortical areas, in particular visual and limbic areas, thereby demonstrating that the identity of a cortical area can be specified without any influence from the brain. The discovery of intrinsic corticogenesis sheds new light on the mechanisms of neuronal specification, and opens new avenues for the modelling and treatment of brain diseases.     

Glutamate induces de novo growth of functional spines in developing cortex

Mature cortical pyramidal neurons receive excitatory inputs onto small protrusions emanating from their dendrites called spines. Spines undergo activity-dependent remodelling, stabilization and pruning during development, and similar structural changes can be triggered by learning and changes in sensory experiences. However, the biochemical triggers and mechanisms of de novo spine formation in the developing brain and the functional significance of new spines to neuronal connectivity are largely unknown. Here we develop an approach to induce and monitor de novo spine formation in real time using combined two-photon laser-scanning microscopy and two-photon laser uncaging of glutamate. Our data demonstrate that, in mouse cortical layer 2/3 pyramidal neurons, glutamate is sufficient to trigger de novo spine growth from the dendrite shaft in a location-specific manner. We find that glutamate-induced spinogenesis requires opening of NMDARs (N-methyl-D-aspartate-type glutamate receptors) and activation of protein kinase A (PKA) but is independent of calcium-calmodulin-dependent kinase II (CaMKII) and tyrosine kinase receptor B (TrkB) receptors. Furthermore, newly formed spines express glutamate receptors and are rapidly functional such that they transduce presynaptic activity into postsynaptic signals. Together, our data demonstrate that early neural connectivity is shaped by activity in a spatially precise manner and that nascent dendrite spines are rapidly functionally incorporated into cortical circuits.     

Dissecting a circuit for olfactory behaviour in Caenorhabditis elegans

Although many properties of the nervous system are shared among animals and systems, it is not known whether different neuronal circuits use common strategies to guide behaviour. Here we characterize information processing by Caenorhabditis elegans olfactory neurons (AWC) and interneurons (AIB and AIY) that control food- and odour-evoked behaviours. Using calcium imaging and mutations that affect specific neuronal connections, we show that AWC neurons are activated by odour removal and activate the AIB interneurons through AMPA-type glutamate receptors. The level of calcium in AIB interneurons is elevated for several minutes after odour removal, a neuronal correlate to the prolonged behavioural response to odour withdrawal. The AWC neuron inhibits AIY interneurons through glutamate-gated chloride channels; odour presentation relieves this inhibition and results in activation of AIY interneurons. The opposite regulation of AIY and AIB interneurons generates a coordinated behavioural response. Information processing by this circuit resembles information flow from vertebrate photoreceptors to 'OFF' bipolar and 'ON' bipolar neurons, indicating a conserved or convergent strategy for sensory information processing.     

NMDA spikes in basal dendrites of cortical pyramidal neurons

Basal dendrites are a major target for synaptic inputs innervating cortical pyramidal neurons. At present little is known about signal processing in these fine dendrites. Here we show that coactivation of clustered neighbouring basal inputs initiated local dendritic spikes, which resulted in a 5.9 +/- 1.5 mV (peak) and 64.4 +/- 19.8 ms (half-width) cable-filtered voltage change at the soma that amplified the somatic voltage response by 226 +/- 46%. These spikes were accompanied by large calcium transients restricted to the activated dendritic segment. In contrast to conventional sodium or calcium spikes, these spikes were mediated mostly by NMDA (N-methyl-D-aspartate) receptor channels, which contributed at least 80% of the total charge. The ionic mechanism of these NMDA spikes may allow 'dynamic spike-initiation zones', set by the spatial distribution of glutamate pre-bound to NMDA receptors, which in turn would depend on recent and ongoing activity in the cortical network. In addition, NMDA spikes may serve as a powerful mechanism for modification of the cortical network by inducing long-term strengthening of co-activated neighbouring inputs.     

Cortex-restricted disruption of NMDAR1 impairs neuronal patterns in the barrel cortex

In the rodent primary somatosensory cortex, the configuration of whiskers and sinus hairs on the snout and of receptor-dense zones on the paws is topographically represented as discrete modules of layer IV granule cells (barrels) and thalamocortical afferent terminals. The role of neural activity, particularly activity mediated by NMDARs (N-methyl-D-aspartate receptors), in patterning of the somatosensory cortex has been a subject of debate. We have generated mice in which deletion of the NMDAR1 (NR1) gene is restricted to excitatory cortical neurons, and here we show that sensory periphery-related patterns develop normally in the brainstem and thalamic somatosensory relay stations of these mice. In the somatosensory cortex, thalamocortical afferents corresponding to large whiskers form patterns and display critical period plasticity, but their patterning is not as distinct as that seen in the cortex of normal mice. Other thalamocortical patterns corresponding to sinus hairs and digits are mostly absent. The cellular aggregates known as barrels and barrel boundaries do not develop even at sites where thalamocortical afferents cluster. Our findings indicate that cortical NMDARs are essential for the aggregation of layer IV cells into barrels and for development of the full complement of thalamocortical patterns.     

Topography and synaptic shaping of direction selectivity in primary auditory cortex

The direction of frequency-modulated (FM) sweeps is an important temporal cue in animal and human communication. FM direction-selective neurons are found in the primary auditory cortex (A1), but their topography and the mechanisms underlying their selectivity remain largely unknown. Here we report that in the rat A1, direction selectivity is topographically ordered in parallel with characteristic frequency (CF): low CF neurons preferred upward sweeps, whereas high CF neurons preferred downward sweeps. The asymmetry of 'inhibitory sidebands', suppressive regions flanking the tonal receptive field (TRF) of the spike response, also co-varied with CF. In vivo whole-cell recordings showed that the direction selectivity already present in the synaptic inputs was enhanced by cortical synaptic inhibition, which suppressed the synaptic excitation of the non-preferred direction more than that of the preferred. The excitatory and inhibitory synaptic TRFs had identical spectral tuning, but with inhibition delayed relative to excitation. The spectral asymmetry of the synaptic TRFs co-varied with CF, as had direction selectivity and sideband asymmetry, and thus suggested a synaptic mechanism for the shaping of FM direction selectivity and its topographic ordering.     

Brain-state- and cell-type-specific firing of hippocampal interneurons in vivo

Neural-network oscillations at distinct frequencies have been implicated in the encoding, consolidation and retrieval of information in the hippocampus. Some GABA (gamma-aminobutyric acid)-containing interneurons fire phase-locked to theta oscillations (4-8 Hz) or to sharp-wave-associated ripple oscillations (120-200 Hz), which represent different behavioural states. Interneurons also entrain pyramidal cells in vitro. The large diversity of interneurons poses the question of whether they have specific roles in shaping distinct network activities in vivo. Here we report that three distinct interneuron types--basket, axo-axonic and oriens-lacunosum-moleculare cells--visualized and defined by synaptic connectivity as well as by neurochemical markers, contribute differentially to theta and ripple oscillations in anaesthetized rats. The firing patterns of individual cells of the same class are remarkably stereotyped and provide unique signatures for each class. We conclude that the diversity of interneurons, innervating distinct domains of pyramidal cells, emerged to coordinate the activity of pyramidal cells in a temporally distinct and brain-state-dependent manner.