The mouse cortico-basal ganglia-thalamic network.

The cortico-basal ganglia-thalamo-cortical loop is one of the fundamental network motifs in the brain. Revealing its structural and functional organization is critical to understanding cognition, sensorimotor behaviour, and the natural history of many neurological and neuropsychiatric disorders. Classically, this network is conceptualized to contain three information channels: motor, limbic and associative1-4. Yet this three-channel view cannot explain the myriad functions of the basal ganglia. We previously subdivided the dorsal striatum into 29 functional domains on the basis of the topography of inputs from the entire cortex5. Here we map the multi-synaptic output pathways of these striatal domains through the globus pallidus external part (GPe), substantia nigra reticular part (SNr), thalamic nuclei and cortex. Accordingly, we identify 14 SNr and 36 GPe domains and a direct cortico-SNr projection. The striatonigral direct pathway displays a greater convergence of striatal inputs than the more parallel striatopallidal indirect pathway, although direct and indirect pathways originating from the same striatal domain ultimately converge onto the same postsynaptic SNr neurons. Following the SNr outputs, we delineate six domains in the parafascicular and ventromedial thalamic nuclei. Subsequently, we identify six parallel cortico-basal ganglia-thalamic subnetworks that sequentially transduce specific subsets of cortical information through every elemental node of the cortico-basal ganglia-thalamic loop. Thalamic domains relay this output back to the originating corticostriatal neurons of each subnetwork in a bona fide closed loop.

Balanced Enhancements of Synaptic Excitation and Inhibition Underlie Developmental Maturation of Receptive Fields in the Mouse Visual Cortex.

Neurons in the developing visual cortex undergo progressive functional maturation as indicated by the refinement of their visual feature selectivity. However, changes of the synaptic architecture underlying the maturation of spatial visual receptive fields (RFs) per se remain largely unclear. Here, loose-patch as well as single-unit recordings in layer 4 of mouse primary visual cortex (V1) of both sexes revealed that RF development following an eye-opening period is marked by an increased proportion of cortical neurons with spatially defined RFs, together with the increased signal-to-noise ratio of spiking responses. By exploring excitatory and inhibitory synaptic RFs with whole-cell voltage-clamp recordings, we observed a balanced enhancement of both synaptic excitation and inhibition, and while the excitatory subfield size remains relatively constant during development, the inhibitory subfield is broadened. This balanced developmental strengthening of excitatory and inhibitory synaptic inputs results in enhanced visual responses, and with a reduction of spontaneous firing rate, contributes to the maturation of visual cortical RFs. Visual deprivation by dark rearing impedes the normal strengthening of excitatory inputs but leaves the apparently normal enhancement of inhibition while preventing the broadening of the inhibitory subfield, leading to weakened RF responses and a reduced fraction of neurons exhibiting a clear RF, compared with normally reared animals. Our data demonstrate that an experience-dependent and coordinated maturation of excitatory and inhibitory circuits underlie the functional development of visual cortical RFs.SIGNIFICANCE STATEMENT The organization of synaptic RFs is a fundamental determinant of feature selectivity functions in the cortex. However, how changes of excitatory and inhibitory synaptic inputs lead to the functional maturation of visual RFs during cortical development remains not well understood. In layer 4 of mouse V1, we show that a coordinated, balanced enhancement of synaptic excitation and inhibition contributes to the developmental maturation of spatially defined visual RFs. Visual deprivation by dark rearing partially interferes with this process, resulting in a relatively more dominant inhibitory tone and a reduced fraction of neurons exhibiting clear RFs at the spike level. These data provide an unprecedented understanding of the functional development of visual cortical RFs at the synaptic level.

Synaptic Specificity and Application of Anterograde Transsynaptic AAV for Probing Neural Circuitry.

Revealing the organization and function of neural circuits is greatly facilitated by viral tools that spread transsynaptically. Adeno-associated virus (AAV) exhibits anterograde transneuronal transport, however, the synaptic specificity of this spread and its broad application within a diverse set of circuits remains to be explored. Here, using anatomic, functional, and molecular approaches, we provide evidence for the preferential transport of AAV1 to postsynaptically connected neurons and reveal its spread is strongly dependent on synaptic transmitter release. In addition to glutamatergic pathways, AAV1 also spreads through GABAergic synapses to both excitatory and inhibitory cell types. We observed little or no transport, however, through neuromodulatory projections (e.g., serotonergic, cholinergic, and noradrenergic). In addition, we found that AAV1 can be transported through long-distance descending projections from various brain regions to effectively transduce spinal cord neurons. Combined with newly designed intersectional and sparse labeling strategies, AAV1 can be applied within a wide variety of pathways to categorize neurons according to their input sources, morphology, and molecular identities. These properties make AAV1 a promising anterograde transsynaptic tool for establishing a comprehensive cell-atlas of the brain, although its capacity for retrograde transport currently limits its use to unidirectional circuits.SIGNIFICANCE STATEMENT The discovery of anterograde transneuronal spread of AAV1 generates great promise for its application as a unique tool for manipulating input-defined cell populations and mapping their outputs. However, several outstanding questions remain for anterograde transsynaptic approaches in the field: (1) whether AAV1 spreads exclusively or specifically to synaptically connected neurons, and (2) how broad its application could be in various types of neural circuits in the brain. This study provides several lines of evidence in terms of anatomy, functional innervation, and underlying mechanisms, to strongly support that AAV1 anterograde transneuronal spread is highly synapse specific. In addition, several potentially important applications of transsynaptic AAV1 in probing neural circuits are described.

Sensory- and Motor-Related Responses of Layer 1 Neurons in the Mouse Visual Cortex.

Cortical layer 1 (L1) contains a sparse and molecularly distinct population of inhibitory interneurons. Their location makes them ideally suited for affecting computations involving long-range corticocortical and subcortical inputs, yet their response properties remain largely unexplored. Here we attempt to characterize some of the functional properties of these neurons in the primary visual cortex of awake mice. We find that the strongest driver of L1 neuron activity is locomotion, with at least half of L1 neurons displaying locomotion-related activity. Visual responses are present in a similar fraction of neurons, but these responses are weaker and frequently suppressive. We also find that ∼43% of L1 neurons respond to noise stimuli and at least 14% respond to whisker touch, with these two populations being statistically independent. Finally, we find that 45% of L1 neurons have generally weak responses correlated with whisking activity. Overall, the spatial distributions of modality-specific responses were more or less random. Our work helps to establish the basic sensory- and motor-related responses of L1 interneurons, revealing several previously unreported characteristics.SIGNIFICANCE STATEMENT Cortical processing even in primary sensory areas is strongly influenced by nonlocal corticocortical and neuromodulatory inputs. Many of these inputs are known to converge onto layer 1 where they target not only distal dendrites of pyramidal neurons but also a sparse population of inhibitory neurons. Previous studies have suggested that layer 1 neurons may play a crucial role in mediating the effects of these long-range projections, but the different types of inputs have mostly been studied in isolation. Here, we take a closer look at the response properties of layer 1 neurons in mouse visual cortex, examining both their visual properties, likely caused by direct thalamocortical inputs, and other sensory and motor properties, likely reflecting corticocortical and neuromodulatory inputs.

Selective Strengthening of Intracortical Excitatory Input Leads to Receptive Field Refinement during Auditory Cortical Development.

In the primary auditory cortex (A1) of rats, refinement of excitatory input to layer (L)4 neurons contributes to the sharpening of their frequency selectivity during postnatal development. L4 neurons receive both feedforward thalamocortical and recurrent intracortical inputs, but how potential developmental changes of each component can account for the sharpening of excitatory input tuning remains unclear. By combining in vivo whole-cell recording and pharmacological silencing of cortical spiking in young rats of both sexes, we examined developmental changes at three hierarchical stages: output of auditory thalamic neurons, thalamocortical input and recurrent excitatory input to an A1 L4 neuron. In the thalamus, the tonotopic map matured with an expanded range of frequency representations, while the frequency tuning of output responses was unchanged. On the other hand, the tuning shape of both thalamocortical and intracortical excitatory inputs to a L4 neuron became sharpened. In particular, the intracortical input became better tuned than thalamocortical excitation. Moreover, the weight of intracortical excitation around the optimal frequency was selectively strengthened, resulting in a dominant role of intracortical excitation in defining the total excitatory input tuning. Our modeling work further demonstrates that the frequency-selective strengthening of local recurrent excitatory connections plays a major role in the refinement of excitatory input tuning of L4 neurons.SIGNIFICANCE STATEMENT During postnatal development, sensory cortex undergoes functional refinement, through which the size of sensory receptive field is reduced. In the rat primary auditory cortex, such refinement in layer (L)4 is mainly attributed to improved selectivity of excitatory input a L4 neuron receives. In this study, we further examined three stages along the hierarchical neural pathway where excitatory input refinement might occur. We found that developmental refinement takes place at both thalamocortical and intracortical circuit levels, but not at the thalamic output level. Together with modeling results, we revealed that the optimal-frequency-selective strengthening of intracortical excitation plays a dominant role in the refinement of excitatory input tuning.

Synaptic Mechanisms for Bandwidth Tuning in Awake Mouse Primary Auditory Cortex.

Spatial size tuning in the visual cortex has been considered as an important neuronal functional property for sensory perception. However, an analogous mechanism in the auditory system has remained controversial. In the present study, cell-attached recordings in the primary auditory cortex (A1) of awake mice revealed that excitatory neurons can be categorized into three types according to their bandwidth tuning profiles in response to band-passed noise (BPN) stimuli: nonmonotonic (NM), flat, and monotonic, with the latter two considered as non-tuned for bandwidth. The prevalence of bandwidth-tuned (i.e., NM) neurons increases significantly from layer 4 to layer 2/3. With sequential cell-attached and whole-cell voltage-clamp recordings from the same neurons, we found that the bandwidth preference of excitatory neurons is largely determined by the excitatory synaptic input they receive, and that the bandwidth selectivity is further enhanced by flatly tuned inhibition observed in all cells. The latter can be attributed at least partially to the flat tuning of parvalbumin inhibitory neurons. The tuning of auditory cortical neurons for bandwidth of BPN may contribute to the processing of complex sounds.

Sparse Representation in Awake Auditory Cortex: Cell-type Dependence, Synaptic Mechanisms, Developmental Emergence, and Modulation.

Sparse representation is considered an important coding strategy for cortical processing in various sensory modalities. It remains unclear how cortical sparseness arises and is being regulated. Here, unbiased recordings from primary auditory cortex of awake adult mice revealed salient sparseness in layer (L)2/3, with a majority of excitatory neurons exhibiting no increased spiking in response to each of sound types tested. Sparse representation was not observed in parvalbumin (PV) inhibitory neurons. The nonresponding neurons did receive auditory-evoked synaptic inputs, marked by weaker excitation and lower excitation/inhibition (E/I) ratios than responding cells. Sparse representation arises during development in an experience-dependent manner, accompanied by differential changes of excitatory input strength and a transition from unimodal to bimodal distribution of E/I ratios. Sparseness level could be reduced by suppressing PV or L1 inhibitory neurons. Thus, sparse representation may be dynamically regulated via modulating E/I balance, optimizing cortical representation of the external sensory world.

Spatial Asymmetry and Short-Term Suppression Underlie Direction Selectivity of Synaptic Excitation in the Mouse Visual Cortex.

Direction selectivity (DS) of neuronal responses is fundamental for motion detection. With in vivo whole-cell voltage-clamp recordings from layer (L)4 neurons in the mouse visual cortex, we observed a strong correlation between DS and spatial asymmetry in the distribution of excitatory input strengths. This raises an interesting possibility that the latter may contribute to DS. The preferred direction of excitatory input was found from the stronger to weaker side of its spatial receptive field. A simple linear summation of asymmetrically distributed excitatory responses to stationary flash stimuli however failed to predict the correct directionality: it at best resulted in weak DS with preferred direction opposite to what was observed experimentally. Further studies with sequential 2 flash-bar stimulation revealed a short-term suppression of excitatory input evoked by the late bar. More importantly, the level of the suppression positively correlated with the relative amplitude of the early-bar response. Implementing this amplitude-dependent suppressive interaction can successfully predict DS of excitatory input. Our results suggest that via nonlinear temporal interactions, the spatial asymmetry can be transformed into differential temporal integration of inputs under opposite directional movements. This mechanism may contribute to the DS of excitatory inputs to L4 neurons.

Thalamocortical Innervation Pattern in Mouse Auditory and Visual Cortex: Laminar and Cell-Type Specificity.

Despite many previous studies, the functional innervation pattern of thalamic axons and their target specificity remains to be investigated thoroughly. Here, in primary auditory cortical slices, we examined thalamic innervation patterns for excitatory and different types of inhibitory neurons across laminae, by optogenetically stimulating axons from the medial geniculate body. We found that excitatory cells and parvalbumin (PV)-expressing inhibitory neurons across layer 2/3 (L2/3) to L6 are directly innervated by thalamic projections, with the strongest innervation occurring in L4. The innervation of PV neurons is stronger than that of excitatory neurons in the same layer, with a relatively constant ratio between their innervation strengths across layers. For somatostatin and vasoactive intestinal peptide inhibitory neurons, essentially only L4 neurons were innervated by thalamic axons and the innervation was much weaker compared with excitatory and PV cells. In addition, more than half of inhibitory neurons in L1 were innervated, relatively strongly, by thalamic axons. Similar innervation patterns were also observed in the primary visual cortex. Thus, thalamic information can be processed independently and differentially by different cortical layers, in addition to the generally thought hierarchical processing starting from L4. This parallel processing is likely shaped by feedforward inhibition from PV neurons in each individual lamina, and may extend the computation power of sensory cortices.

Synaptic Basis for Differential Orientation Selectivity between Complex and Simple Cells in Mouse Visual Cortex.

In the primary visual cortex (V1), orientation-selective neurons can be categorized into simple and complex cells primarily based on their receptive field (RF) structures. In mouse V1, although previous studies have examined the excitatory/inhibitory interplay underlying orientation selectivity (OS) of simple cells, the synaptic bases for that of complex cells have remained obscure. Here, by combining in vivo loose-patch and whole-cell recordings, we found that complex cells, identified by their overlapping on/off subfields, had significantly weaker OS than simple cells at both spiking and subthreshold membrane potential response levels. Voltage-clamp recordings further revealed that although excitatory inputs to complex and simple cells exhibited a similar degree of OS, inhibition in complex cells was more narrowly tuned than excitation, whereas in simple cells inhibition was more broadly tuned than excitation. The differential inhibitory tuning can primarily account for the difference in OS between complex and simple cells. Interestingly, the differential synaptic tuning correlated well with the spatial organization of synaptic input: the inhibitory visual RF in complex cells was more elongated in shape than its excitatory counterpart and also was more elongated than that in simple cells. Together, our results demonstrate that OS of complex and simple cells is differentially shaped by cortical inhibition based on its orientation tuning profile relative to excitation, which is contributed at least partially by the spatial organization of RFs of presynaptic inhibitory neurons. SIGNIFICANCE STATEMENT: Simple and complex cells, two classes of principal neurons in the primary visual cortex (V1), are generally thought to be equally selective for orientation. In mouse V1, we report that complex cells, identified by their overlapping on/off subfields, has significantly weaker orientation selectivity (OS) than simple cells. This can be primarily attributed to the differential tuning selectivity of inhibitory synaptic input: inhibition in complex cells is more narrowly tuned than excitation, whereas in simple cells inhibition is more broadly tuned than excitation. In addition, there is a good correlation between inhibitory tuning selectivity and the spatial organization of inhibitory inputs. These complex and simple cells with differential degree of OS may provide functionally distinct signals to different downstream targets.

Fine-tuning of pre-balanced excitation and inhibition during auditory cortical development.

Functional receptive fields of neurons in sensory cortices undergo progressive refinement during development. Such refinement may be attributed to the pruning of non-optimal excitatory inputs, reshaping of the excitatory tuning profile through modifying the strengths of individual inputs, or strengthening of cortical inhibition. These models have not been directly tested because of the technical difficulties in assaying the spatiotemporal patterns of functional synaptic inputs during development. Here we apply in vivo whole-cell voltage-clamp recordings to the recipient layer 4 neurons in the rat primary auditory cortex (A1) to determine the developmental changes in the frequency-intensity tonal receptive fields (TRFs) of their excitatory and inhibitory inputs. Surprisingly, we observe co-tuned excitation and inhibition immediately after the onset of hearing, suggesting that a tripartite thalamocortical circuit with relatively strong feedforward inhibition is formed independently of auditory experience. The frequency ranges of tone-driven excitatory and inhibitory inputs first expand within a few days of the onset of hearing and then persist into adulthood. The latter phase is accompanied by a sharpening of the excitatory but not inhibitory frequency tuning profile, which results in relatively broader inhibitory tuning in adult A1 neurons. Thus the development of cortical synaptic TRFs after the onset of hearing is marked by a slight breakdown of previously formed excitation-inhibition balance. Our results suggest that functional refinement of cortical TRFs does not require a selective pruning of inputs, but may depend more on a fine adjustment of excitatory input strengths.

Strengthening of Direction Selectivity by Broadly Tuned and Spatiotemporally Slightly Offset Inhibition in Mouse Visual Cortex.

Direction selectivity (DS) of neuronal responses is fundamental for motion detection. How the integration of synaptic excitation and inhibition contributes to DS however remains not well-understood. Here, in vivo whole-cell voltage-clamp recordings in mouse primary visual cortex (V1) revealed that layer 4 simple cells received direction-tuned excitatory inputs but barely tuned inhibitory inputs under drifting-bar stimulation. Excitation and inhibition exhibited differential temporal offsets under movements of opposite directions: excitation peaked earlier than inhibition at the preferred direction, and vice versa at the null direction. This could be attributed to a small spatial mismatch between overlapping excitatory and inhibitory receptive fields: the distribution of excitatory input strengths was skewed and the skewness was strongly correlated with the DS of excitatory input, whereas that of inhibitory input strengths was spatially symmetric. Neural modeling revealed that the relatively stronger inhibition under null directional movements, as well as the specific spatial-temporal offsets between excitation and inhibition, allowed inhibition to enhance the DS of output responses by suppressing the null response more effectively than the preferred response. Our data demonstrate that while tuned excitatory input provides the basis for DS in mouse V1, the largely untuned and spatiotemporally offset inhibition contributes importantly to sharpening of DS.

Differential Receptive Field Properties of Parvalbumin and Somatostatin Inhibitory Neurons in Mouse Auditory Cortex.

Cortical inhibitory circuits play important roles in shaping sensory processing. In auditory cortex, however, functional properties of genetically identified inhibitory neurons are poorly characterized. By two-photon imaging-guided recordings, we specifically targeted 2 major types of cortical inhibitory neuron, parvalbumin (PV) and somatostatin (SOM) expressing neurons, in superficial layers of mouse auditory cortex. We found that PV cells exhibited broader tonal receptive fields with lower intensity thresholds and stronger tone-evoked spike responses compared with SOM neurons. The latter exhibited similar frequency selectivity as excitatory neurons. The broader/weaker frequency tuning of PV neurons was attributed to a broader range of synaptic inputs and stronger subthreshold responses elicited, which resulted in a higher efficiency in the conversion of input to output. In addition, onsets of both the input and spike responses of SOM neurons were significantly delayed compared with PV and excitatory cells. Our results suggest that PV and SOM neurons engage in auditory cortical circuits in different manners: while PV neurons may provide broadly tuned feedforward inhibition for a rapid control of ascending inputs to excitatory neurons, the delayed and more selective inhibition from SOM neurons may provide a specific modulation of feedback inputs on their distal dendrites.

A feedforward inhibitory circuit mediates lateral refinement of sensory representation in upper layer 2/3 of mouse primary auditory cortex.

Sensory information undergoes ordered and coordinated processing across cortical layers. Whereas cortical layer (L) 4 faithfully acquires thalamic information, the superficial layers appear well staged for more refined processing of L4-relayed signals to generate corticocortical outputs. However, the specific role of superficial layer processing and how it is specified by local synaptic circuits remains not well understood. Here, in the mouse primary auditory cortex, we showed that upper L2/3 circuits play a crucial role in refining functional selectivity of excitatory neurons by sharpening auditory tonal receptive fields and enhancing contrast of frequency representation. This refinement is mediated by synaptic inhibition being more broadly recruited than excitation, with the inhibition predominantly originating from interneurons in the same cortical layer. By comparing the onsets of synaptic inputs as well as of spiking responses of different types of neuron, we found that the broadly tuned, fast responding inhibition observed in excitatory cells can be primarily attributed to feedforward inhibition originating from parvalbumin (PV)-positive neurons, whereas somatostatin (SOM)-positive interneurons respond much later compared with the onset of inhibitory inputs to excitatory neurons. We propose that the feedforward circuit-mediated inhibition from PV neurons, which has an analogous function to lateral inhibition, enables upper L2/3 excitatory neurons to rapidly refine auditory representation.

Synaptic mechanisms for generating temporal diversity of auditory representation in the dorsal cochlear nucleus.

In central auditory pathways, neurons exhibit a great diversity of temporal discharge patterns, which may contribute to the parallel processing of auditory signals. How such response diversity emerges in the central auditory circuits remains unclear. Here, we investigated whether synaptic mechanisms can contribute to the generation of the temporal response diversity at the first stage along the central auditory neuraxis. By in vivo whole-cell voltage-clamp recording in the dorsal cochlear nucleus of rats, we revealed excitatory and inhibitory synaptic inputs underlying three different firing patterns of fusiform/pyramidal neurons in response to auditory stimuli: "primary-like," "pauser," and "buildup" patterns. We found that primary-like neurons received strong, fast-rising excitation, whereas pauser and buildup neurons received accumulating excitation with a relatively weak fast-rising phase, followed by a slow-rising phase. Pauser neurons received stronger fast-rising excitation than buildup cells. On the other hand, inhibitory inputs to the three types of cells exhibited similar temporal patterns, all with a strong fast-rising phase. Dynamic-clamp recordings demonstrated that the differential temporal patterns of excitation could primarily account for the different discharge patterns. In addition, discharge pattern in a single neuron varied in a stimulus-dependent manner, which could be attributed to the modulation of excitation/inhibition balance by different stimuli. Further examination of excitatory inputs to vertical/tuberculoventral and cartwheel cells suggested that fast-rising and accumulating excitation might be conveyed by auditory nerve and parallel fibers, respectively. A differential summation of excitatory inputs from the two sources may thus contribute to the generation of response diversity.

Neurite outgrowth inhibitor Nogo-A establishes spatial segregation and extent of oligodendrocyte myelination.

A requisite component of nervous system development is the achievement of cellular recognition and spatial segregation through competition-based refinement mechanisms. Competition for available axon space by myelinating oligodendrocytes ensures that all relevant CNS axons are myelinated properly. To ascertain the nature of this competition, we generated a transgenic mouse with sparsely labeled oligodendrocytes and establish that individual oligodendrocytes occupying similar axon tracts can greatly vary the number and lengths of their myelin internodes. Here we show that intercellular interactions between competing oligodendroglia influence the number and length of myelin internodes, referred to as myelinogenic potential, and identify the amino-terminal region of Nogo-A, expressed by oligodendroglia, as necessary and sufficient to inhibit this process. Exuberant and expansive myelination/remyelination is detected in the absence of Nogo during development and after demyelination, suggesting that spatial segregation and myelin extent is limited by microenvironmental inhibition. We demonstrate a unique physiological role for Nogo-A in the precise myelination of the developing CNS. Maximizing the myelinogenic potential of oligodendrocytes may offer an effective strategy for repair in future therapies for demyelination.

At the interface of sensory and motor dysfunctions and Alzheimer's disease.

Recent evidence indicates that sensory and motor changes may precede the cognitive symptoms of Alzheimer's disease (AD) by several years and may signify increased risk of developing AD. Traditionally, sensory and motor dysfunctions in aging and AD have been studied separately. To ascertain the evidence supporting the relationship between age-related changes in sensory and motor systems and the development of AD and to facilitate communication between several disciplines, the National Institute on Aging held an exploratory workshop titled "Sensory and Motor Dysfunctions in Aging and AD." The scientific sessions of the workshop focused on age-related and neuropathologic changes in the olfactory, visual, auditory, and motor systems, followed by extensive discussion and hypothesis generation related to the possible links among sensory, cognitive, and motor domains in aging and AD. Based on the data presented and discussed at this workshop, it is clear that sensory and motor regions of the central nervous system are affected by AD pathology and that interventions targeting amelioration of sensory-motor deficits in AD may enhance patient function as AD progresses.

Synaptic mechanisms underlying functional dichotomy between intrinsic-bursting and regular-spiking neurons in auditory cortical layer 5.

Corticofugal projections from the primary auditory cortex (A1) have been shown to play a role in modulating subcortical processing. However, functional properties of the corticofugal neurons and their synaptic circuitry mechanisms remain unclear. In this study, we performed in vivo whole-cell recordings from layer 5 (L5) pyramidal neurons in the rat A1 and found two distinct neuronal classes according to their functional properties. Intrinsic-bursting (IB) neurons, the L5 corticofugal neurons, exhibited early and rather unselective spike responses to a wide range of frequencies. The exceptionally broad spectral tuning of IB neurons was attributable to their broad excitatory inputs with long temporal durations and inhibitory inputs being more narrowly tuned than excitatory inputs. This uncommon pattern of excitatory-inhibitory interplay was attributed initially to a broad thalamocortical convergence onto IB neurons, which also receive temporally prolonged intracortical excitatory input as well as feedforward inhibitory input at least partially from more narrowly tuned fast-spiking inhibitory neurons. In contrast, regular-spiking neurons, which are mainly corticocortical, exhibited sharp frequency tuning similar to L4 pyramidal cells, underlying which are well-matched purely intracortical excitation and inhibition. The functional dichotomy among L5 pyramidal neurons suggests two distinct processing streams. The spectrally and temporally broad synaptic integration in IB neurons may ensure robust feedback signals to facilitate subcortical function and plasticity in a general manner.

Slit/Robo signaling mediates spatial positioning of spiral ganglion neurons during development of cochlear innervation.

During the development of periphery auditory circuits, spiral ganglion neurons (SGNs) extend their neurites to innervate cochlear hair cells (HCs) with their soma aggregated into a cluster spatially segregated from the cochlear sensory epithelium. The molecular mechanisms underlying this spatial patterning remain unclear. In this study, in situ hybridization in the mouse cochlea suggests that Slit2 and its receptor, Robo1/2, exhibit apparently complementary expression patterns in the spiral ganglion and its nearby region, the spiral limbus. In Slit2 and Robo1/2 mutants, the spatial restriction of SGNs was disrupted. Mispositioned SGNs were found to scatter in the space between the cochlear epithelium and the main body of spiral ganglion, and the neurites of mispositioned SGNs were misrouted and failed to innervate HCs. Furthermore, in Robo1/2 mutants, SGNs were displaced toward the cochlear epithelium as an entirety. Examination of different embryonic stages in the mutants revealed that the mispositioning of SGNs was due to a progressive displacement to ectopic locations after their initial normal settlement at an earlier stage. Our results suggest that Slit/Robo signaling imposes a restriction force on SGNs to ensure their precise positioning for correct SGN-HC innervations.