The local and long-range input landscape of inhibitory neurons in mouse auditory cortex.

Roughly 20% of the neurons in the mouse cortex are inhibitory interneurons (INs). Of these, the three major subtypes are parvalbumin (PV), somatostatin (SST), and vasoactive intestinal polypeptide (VIP) expressing neurons. We used monosynaptic rabies tracing to compare the presynaptic input landscape onto these three IN subtypes in the mouse primary auditory cortex (A1). We compared both local patterns of monosynaptic inputs as well as long-range input patterns. The local monosynaptic input landscape to SST neurons was more widespread as compared to PV and VIP neurons. The brain-wide input landscape was rich and heterogeneous with >40 brain regions connecting to all the three INs subtypes from both hemispheres. The general pattern of the long-range input landscape was similar among the groups of INs. Nevertheless, a few differences could be identified. At low resolution, the proportion of local versus long-range inputs was smaller for PV neurons. At mesoscale resolution, we found fewer inputs from temporal association area to VIP INs, and more inputs to SST neurons from basal forebrain and lateral amygdala. Our work can be used as a resource for a quantitative comparison of the location and level of inputs impinging onto discrete populations of neurons in mouse A1.

NDNF interneurons in layer 1 gain-modulate whole cortical columns according to an animal's behavioral state.

Processing of sensory information in neural circuits is modulated by an animal's behavioral state, but the underlying cellular mechanisms are not well understood. Focusing on the mouse visual cortex, here we analyze the role of GABAergic interneurons that are located in layer 1 and express Ndnf (L1 NDNF INs) in the state-dependent control over sensory processing. We find that the ongoing and sensory-evoked activity of L1 NDNF INs is strongly enhanced when an animal is aroused and that L1 NDNF INs gain-modulate local excitatory neurons selectively during high-arousal states by inhibiting their apical dendrites while disinhibiting their somata via Parvalbumin-expressing interneurons. Because active NDNF INs are evenly spread in L1 and can affect excitatory neurons across all cortical layers, this indicates that the state-dependent activation of L1 NDNF INs and the subsequent shift of inhibition in excitatory neurons toward their apical dendrites gain-modulate sensory processing in whole cortical columns.

Auditory Corticothalamic Neurons Are Recruited by Motor Preparatory Inputs.

Corticothalamic (CT) neurons comprise the largest component of the descending sensory corticofugal pathway, but their contributions to brain function and behavior remain an unsolved mystery. To address the hypothesis that layer 6 (L6) CTs may be activated by extra-sensory inputs prior to anticipated sounds, we performed optogenetically targeted single-unit recordings and two-photon imaging of Ntsr1-Cre+ L6 CT neurons in the primary auditory cortex (A1) while mice were engaged in an active listening task. We found that L6 CTs and other L6 units began spiking hundreds of milliseconds prior to orofacial movements linked to sound presentation and reward, but not to other movements such as locomotion, which were not linked to an explicit behavioral task. Rabies tracing of monosynaptic inputs to A1 L6 CT neurons revealed a narrow strip of cholinergic and non-cholinergic projection neurons in the external globus pallidus, suggesting a potential source of motor-related input. These findings identify new pathways and local circuits for motor modulation of sound processing and suggest a new role for CT neurons in active sensing.

The Temporal Association Cortex Plays a Key Role in Auditory-Driven Maternal Plasticity.

Mother-infant bonding develops rapidly following parturition and is accompanied by changes in sensory perception and behavior. Here, we study how ultrasonic vocalizations (USVs) are represented in the brain of mothers. Using a mouse line that allows temporally controlled genetic access to active neurons, we find that the temporal association cortex (TeA) in mothers exhibits robust USV responses. Rabies tracing from USV-responsive neurons reveals extensive subcortical and cortical inputs into TeA. A particularly dominant cortical source of inputs is the primary auditory cortex (A1), suggesting strong A1-to-TeA connectivity. Chemogenetic silencing of USV-responsive neurons in TeA impairs auditory-driven maternal preference in a pup-retrieval assay. Furthermore, dense extracellular recordings from awake mice reveal changes of both single-neuron and population responses to USVs in TeA, improving discriminability of pup calls in mothers compared with naive females. These data indicate that TeA plays a key role in encoding and perceiving pup cries during motherhood.

Barrel cortex VIP/ChAT interneurons suppress sensory responses in vivo.

Cortical interneurons expressing vasoactive intestinal polypeptide (VIP) and choline acetyltransferase (ChAT) are sparsely distributed throughout the neocortex, constituting only 0.5% of its neuronal population. The co-expression of VIP and ChAT suggests that these VIP/ChAT interneurons (VChIs) can release both γ-aminobutyric acid (GABA) and acetylcholine (ACh). In vitro physiological studies quantified the response properties and local connectivity patterns of the VChIs; however, the function of VChIs has not been explored in vivo. To study the role of VChIs in cortical network dynamics and their long-range connectivity pattern, we used in vivo electrophysiology and rabies virus tracing in the barrel cortex of mice. We found that VChIs have a low spontaneous spiking rate (approximately 1 spike/s) in the barrel cortex of anesthetized mice; nevertheless, they responded with higher fidelity to whisker stimulation than the neighboring layer 2/3 pyramidal neurons (Pyrs). Analysis of long-range inputs to VChIs with monosynaptic rabies virus tracing revealed that direct thalamic projections are a significant input source to these cells. Optogenetic activation of VChIs in the barrel cortex of awake mice suppresses the sensory responses of excitatory neurons in intermediate amplitudes of whisker deflections while increasing the evoked spike latency. The effect of VChI activation on the response was similar for both high-whisking (HW) and low-whisking (LW) conditions. Our findings demonstrate that, despite their sparsity, VChIs can effectively modulate sensory processing in the cortical microcircuit.

The Pre-synaptic Landscape of Mitral/Tufted Cells of the Main Olfactory Bulb.

In olfaction, all volatile odor information is tunneled through the main olfactory bulb (OB). Odor information is then processed before it is transferred to higher brain centers. Odor processing in the OB is carried out by numerous local inhibitory circuits and modulated by top-down input. Top-down modulation of OB function has been shown to act via interneurons but evidence also exists for its direct impact onto the principle mitral and tufted cells (M/Ts). Here, we used monosynaptic rabies trans-synaptic tracing from the OB to map and quantify the local and top-down pre-synaptic landscape of M/Ts and local inhibitory interneurons. We found that M/Ts receive a significant amount of top-down inputs from various brain regions that match qualitatively but not quantitatively those that synapse onto local inhibitory inter-neurons. These results show that M/Ts are direct targets of top-down inputs.

The Claustrum Supports Resilience to Distraction.

A barrage of information constantly assaults our senses, of which only a fraction is relevant at any given point in time. However, the neural circuitry supporting the suppression of irrelevant sensory distractors is not completely understood. The claustrum, a circuit hub with vast cortical connectivity, is an intriguing brain structure, whose restrictive anatomy, thin and elongated, has precluded functional investigation. Here, we describe the use of Egr2-CRE mice to access genetically defined claustral neurons. Utilizing conditional viruses for anterograde axonal labeling and retrograde trans-synaptic tracing, we validated this transgenic model for accessing the claustrum and extended the known repertoire of claustral input/output connectivity. Addressing the function of the claustrum, we inactivated CLEgr2+ neurons, chronically as well as acutely, in mice performing an automated two-alternative forced-choice behavioral task. Strikingly, inhibition of CLEgr2+ neurons did not significantly impact task performance under varying delay times and cue durations, but revealed a selective role for the claustrum in supporting performance in the presence of an irrelevant auditory distractor. Further investigation of behavior, in the naturalistic maternal pup-retrieval task, replicated the result of sensitization to an auditory distractor following inhibition of CLEgr2+ neurons. Initiating investigation into the underlying mechanism, we found that activation of CLEgr2+ neurons modulated cortical sensory processing, suppressing tone representation in the auditory cortex. This functional study, utilizing selective genetic access, implicates the claustrum in supporting resilience to distraction, a fundamental aspect of attention.

Genetic tagging of active neurons in auditory cortex reveals maternal plasticity of coding ultrasonic vocalizations.

Cortical neurons are often functionally heterogeneous even for molecularly defined subtypes. In sensory cortices, physiological responses to natural stimuli can be sparse and vary widely even for neighboring neurons. It is thus difficult to parse out circuits that encode specific stimuli for further experimentation. Here, we report the development of a Cre-reporter mouse that allows recombination for cellular labeling and genetic manipulation, and use it with an activity-dependent Fos-CreERT2 driver to identify functionally active circuits in the auditory cortex. In vivo targeted patch recordings validate our method for neurons responding to physiologically relevant natural sounds such as pup wriggling calls and ultrasonic vocalizations (USVs). Using this system to investigate cortical responses in postpartum mothers, we find a transient recruitment of neurons highly responsive to USVs. This subpopulation of neurons has distinct physiological properties that improve the coding efficiency for pup USV calls, implicating it as a unique signature in parental plasticity.

Semaphorin 4D/Plexin-B1-mediated M-Ras GAP activity regulates actin-based dendrite remodeling through Lamellipodin.

Semaphorins have been identified as repulsive guidance molecules in the developing nervous system. We recently reported that the semaphorin 4D (Sema4D) receptor Plexin-B1 induces repulsion in axon and dendrites by functioning as a GTPase-activating protein (GAP) for R-Ras and M-Ras, respectively. In axons, Sema4D stimulation induces growth cone collapse, and downregulation of R-Ras activity by Plexin-B1-mediated GAP activity is required for the action. Axonal R-Ras GAP activity downregulates phosphatidylinositol 3-kinase signaling pathway, and thereby induces inactivation of a microtubule assembly promoter protein, CRMP-2. However, in contrast to the well studied roles of semaphorins and plexins in axonal guidance, signaling molecules linking M-Ras GAP to dendritic cytoskeleton remain obscure. Here we identified an Ena/VASP ligand, Lamellipodin (Lpd), as a novel effector of M-Ras in dendrites. Lpd was expressed in F-actin-rich distal dendritic processes and was required for both basal and M-Ras-mediated dendrite development. Subcellular fractionation showed M-Ras-dependent membrane translocation of Lpd, which was suppressed by Sema4D. Furthermore, the Ena/VASP-binding region within Lpd was required for dendrite development, and its membrane targeting was sufficient to overcome the Sema4D-mediated reduction of dendritic outgrowth and disappearance of F-actin from distal dendrites. Furthermore, in utero electroporation experiments also indicated that regulation of the M-Ras-Lpd system by the GAP activity of Plexin is involved in the normal development of cortical dendrites in vivo. Overall, our study sheds light on how repulsive guidance molecules regulate actin cytoskeleton in dendrites, revealing a novel mechanism that the M-Ras-Lpd system regulates actin-based dendrite remodeling by Sema/Plexin in rats or mice of either sex.