Parallel Inhibitory and Excitatory Trigemino-Facial Feedback Circuitry for Reflexive Vibrissa Movement

Where Top-Down Meets Bottom-Up: Cell-Type Specific Connectivity Map of the Whisker System

Sensorimotor computation integrates bottom-up world state information with top-down knowledge and task goals to form action plans. In the rodent whisker system, a prime model of active sensing, evidence shows neuromodulatory neurotransmitters shape whisker control, affecting whisking frequency and amplitude. Since neuromodulatory neurotransmitters are mostly released from subcortical nuclei and have long-range projections that reach the rest of the central nervous system, mapping the circuits of top-down neuromodulatory control of sensorimotor nuclei will help to systematically address the mechanisms of active sensing. Therefore, we developed a neuroinformatic target discovery pipeline to mine the Allen Institute's Mouse Brain Connectivity Atlas. Using network connectivity analysis, we identified new putative connections along the whisker system and anatomically confirmed the existence of 42 previously unknown monosynaptic connections. Using this data, we updated the sensorimotor connectivity map of the mouse whisker system and developed the first cell-type-specific map of the network. The map includes 157 projections across 18 principal nuclei of the whisker system and neuromodulatory neurotransmitter-releasing. Performing a graph network analysis of this connectome, we identified cell-type specific hubs, sources, and sinks, provided anatomical evidence for monosynaptic inhibitory projections into all stages of the ascending pathway, and showed that neuromodulatory projections improve network-wide connectivity. These results argue that beyond the modulatory chemical contributions to information processing and transfer in the whisker system, the circuit connectivity features of the neuromodulatory networks position them as nodes of sensory and motor integration.

Neuroanatomical frameworks for volitional control of breathing and orofacial behaviors

Breathing is the only vital function that can be volitionally controlled. However, a detailed understanding how volitional (cortical) motor commands can transform vital breathing activity into adaptive breathing patterns that accommodate orofacial behaviors such as swallowing, vocalization or sniffing remains to be developed. Recent neuroanatomical tract tracing studies have identified patterns and origins of descending forebrain projections that target brain nuclei involved in laryngeal adductor function which is critically involved in orofacial behavior. These nuclei include the midbrain periaqueductal gray and nuclei of the respiratory rhythm and pattern generating network in the brainstem, specifically including the pontine Kölliker-Fuse nucleus and the pre-Bötzinger complex in the medulla oblongata. This review discusses the functional implications of the forebrain-brainstem anatomical connectivity that could underlie the volitional control and coordination of orofacial behaviors with breathing.

Interareal Synaptic Inputs Underlying Whisking-Related Activity in the Primary Somatosensory Barrel Cortex

Body movements influence brain-wide neuronal activities. In the sensory cortex, thalamocortical bottom-up inputs and motor-sensory top-down inputs are thought to affect the dynamics of membrane potentials (Vm ) of neurons and change their processing of sensory information during movements. However, direct perturbation of the axons projecting to the sensory cortex from other remote areas during movements has remained unassessed, and therefore the interareal circuits generating motor-related signals in sensory cortices remain unclear. Using a Gi/o -coupled opsin, eOPN3, we here inhibited interareal signals incoming to the whisker primary somatosensory barrel cortex (wS1) of awake male mice and tested their effects on whisking-related changes in neuronal activities in wS1. Spontaneous whisking in air induced the changes in spike rates of a subset of wS1 neurons, which were accompanied by depolarization and substantial reduction of slow-wave oscillatory fluctuations of Vm Despite an extensive innervation, inhibition of inputs from the whisker primary motor cortex (wM1) to wS1 did not alter the spike rates and Vm dynamics of wS1 neurons during whisking. In contrast, inhibition of axons from the whisker-related thalamus (wTLM) and the whisker secondary somatosensory cortex (wS2) to wS1 largely attenuated the whisking-related supra- and sub-threshold Vm dynamics of wS1 neurons. Notably, silencing inputs from wTLM markedly decreased the modulation depth of whisking phase-tuned neurons in wS1, while inhibiting wS2 inputs did not impact the whisking variable tuning of wS1 neurons. Thus, sensorimotor integration in wS1 during spontaneous whisking is predominantly facilitated by direct synaptic inputs from wTLM and wS2 rather than from wM1.

In Vivo Whole-Cell Recording from the Mouse Brain

Measuring the membrane potential dynamics of neurons offers a comprehensive understanding of the molecular and cellular mechanisms that form their spiking activity, thus playing a crucial role in unraveling the mechanistic processes governing brain function. Techniques for intracellular recordings of membrane potentials pioneered in the 1940s have witnessed significant advancements since their inception. Among these, whole-cell patch-clamp recording has emerged as a leading method for measuring neuronal membrane potentials due to its high stability and broad applicability ranging from cultured cells to brain slices and even behaving animals. This chapter provides a detailed protocol to acquire stable whole-cell recordings from neurons in the cerebral cortex of awake, head-restrained mice. Significant enhancements to our protocol include implanting a metal head-post using adhesive resin cement and preparing a recording pipette with a long shank for targeting deeper brain regions. This protocol, once implemented, enables whole-cell recordings up to 2.5 mM beneath the cortical surface.

A change in behavioral state switches the pattern of motor output that underlies rhythmic head and orofacial movements

The breathing rhythm serves as a reference that paces orofacial motor actions and orchestrates active sensing. Past work has reported that pacing occurs solely at a fixed phase relative to sniffing. We re-evaluated this constraint as a function of exploratory behavior. Allocentric and egocentric rotations of the head and the electromyogenic activity of the motoneurons for head and orofacial movements were recorded in free-ranging rats as they searched for food. We found that a change in state from foraging to rearing is accompanied by a large phase shift in muscular activation relative to sniffing, and a concurrent change in the frequency of sniffing, so that pacing now occurs at one of the two phases. Further, head turning is biased such that an animal gathers a novel sample of its environment upon inhalation. In total, the coordination of active sensing has a previously unrealized computational complexity. This can emerge from hindbrain circuits with fixed architecture and credible synaptic time delays.

Neurocircuitry underlying the antidepressant effect of retrograde facial botulinum toxin in mice

BACKGROUNDS

Botulinum toxin type A (BoNT/A) is extensively applied in spasticity and dystonia as it cleaves synaptosome-associated protein 25 (SNAP25) in the presynaptic terminals, thereby inhibiting neurotransmission. An increasing number of randomized clinical trials have suggested that glabellar BoNT/A injection improves depressive symptoms in patients with major depressive disorder (MDD). However, the underlying neuronal circuitry of BoNT/A-regulated depression remains largely uncharacterized.

RESULTS

Here, we modeled MDD using mice subjected to chronic restraint stress (CRS). By pre-injecting BoNT/A into the unilateral whisker intrinsic musculature (WIM), and performing behavioral testing, we showed that pre-injection of BoNT/A attenuated despair- and anhedonia-like phenotypes in CRS mice. By applying immunostaining of BoNT/A-cleaved SNAP25 (cl.SNAP25₁₉₇), subcellular spatial localization of SNAP25 with markers of cholinergic neurons (ChAT) and post-synaptic membrane (PSD95), and injection of monosynaptic retrograde tracer CTB-488-mixed BoNT/A to label the primary nucleus of the WIM, we demonstrated that BoNT/A axonal retrograde transported to the soma of whisker-innervating facial motoneurons (wFMNs) and subsequent transcytosis to synaptic terminals of second-order neurons induced central effects. Furthermore, using transsynaptic retrograde and monosynaptic antegrade viral neural circuit tracing with c-Fos brain mapping and co-staining of neural markers, we observed that the CRS-induced expression of c-Fos and CaMKII double-positive neurons in the ventrolateral periaqueductal grey (vlPAG), which sent afferents to wFMNs, was down-regulated 3 weeks after BoNT/A facial pre-administration. Strikingly, the repeated and targeted silencing of the wFMNs-projecting CaMKII-positive neurons in vlPAG with a chemogenetic approach via stereotactic injection of recombinant adeno-associated virus into specific brain regions of CRS mice mimicked the antidepressant-like action of BoNT/A pre-treatment. Conversely, repeated chemogenetic activation of this potential subpopulation counteracted the BoNT/A-improved significant antidepressant behavior.

CONCLUSION

We reported for the first time that BoNT/A inhibited the wFMNs-projecting vlPAG excitatory neurons through axonal retrograde transport and cell-to-cell transcytosis from the injected location of the WIM to regulate depressive-like phenotypes of CRS mice. For the limited and the reversibility of side effects, BoNT/A has substantial advantages and potential application in MDD.

Theory of hierarchically organized neuronal oscillator dynamics that mediate rodent rhythmic whisking

Rodents explore their environment through coordinated orofacial motor actions, including whisking. Whisking can free-run via an oscillator of inhibitory neurons in the medulla and can be paced by breathing. Yet, the mechanics of the whisking oscillator and its interaction with breathing remain to be understood. We formulate and solve a hierarchical model of the whisking circuit. The first whisk within a breathing cycle is generated by inhalation, which resets a vibrissa oscillator circuit, while subsequent whisks are derived from the oscillator circuit. Our model posits, consistent with experiment, that there are two subpopulations of oscillator neurons. Stronger connections between the subpopulations support rhythmicity, while connections within each subpopulation induce variable spike timing that enhances the dynamic range of rhythm generation. Calculated cycle-to-cycle changes in whisking are consistent with experiment. Our model provides a computational framework to support longstanding observations of concurrent autonomous and driven rhythmic motor actions that comprise behaviors.

The whisking oscillator circuit

Central oscillators are primordial neural circuits that generate and control rhythmic movements1,2. Mechanistic understanding of these circuits requires genetic identification of the oscillator neurons and their synaptic connections to enable targeted electrophysiological recording and causal manipulation during behaviours. However, such targeting remains a challenge with mammalian systems. Here we delimit the oscillator circuit that drives rhythmic whisking-a motor action that is central to foraging and active sensing in rodents3,4. We found that the whisking oscillator consists of parvalbumin-expressing inhibitory neurons located in the vibrissa intermediate reticular nucleus (vIRtPV) in the brainstem. vIRtPV neurons receive descending excitatory inputs and form recurrent inhibitory connections among themselves. Silencing vIRtPV neurons eliminated rhythmic whisking and resulted in sustained vibrissae protraction. In vivo recording of opto-tagged vIRtPV neurons in awake mice showed that these cells spike tonically when animals are at rest, and transition to rhythmic bursting at the onset of whisking, suggesting that rhythm generation is probably the result of network dynamics, as opposed to intrinsic cellular properties. Notably, ablating inhibitory synaptic inputs to vIRtPV neurons quenched their rhythmic bursting, impaired the tonic-to-bursting transition and abolished regular whisking. Thus, the whisking oscillator is an all-inhibitory network and recurrent synaptic inhibition has a key role in its rhythmogenesis.

Lateral habenula glutamatergic neurons projecting to the dorsal raphe nucleus promote aggressive arousal in mice

The dorsal raphe nucleus (DRN) is known to control aggressive behavior in mice. Here, we found that glutamatergic projections from the lateral habenula (LHb) to the DRN were activated in male mice that experienced pre-exposure to a rival male mouse ("social instigation") resulting in heightened intermale aggression. Both chemogenetic and optogenetic suppression of the LHb-DRN projection blocked heightened aggression after social instigation in male mice. In contrast, inhibition of this pathway did not affect basal levels of aggressive behavior, suggesting that the activity of the LHb-DRN projection is not necessary for the expression of species-typical aggressive behavior, but required for the increase of aggressive behavior resulting from social instigation. Anatomical analysis showed that LHb neurons synapse on non-serotonergic DRN neurons that project to the ventral tegmental area (VTA), and optogenetic activation of the DRN-VTA projection increased aggressive behaviors. Our results demonstrate that the LHb glutamatergic inputs to the DRN promote aggressive arousal induced by social instigation, which contributes to aggressive behavior by activating VTA-projecting non-serotonergic DRN neurons as one of its potential targets.

Scalable Thousand Channel Penetrating Microneedle Arrays on Flex for Multimodal and Large Area Coverage BrainMachine Interfaces

The Utah array powers cutting-edge projects for restoration of neurological function, such as BrainGate, but the underlying electrode technology has itself advanced little in the last three decades. Here, advanced dual-side lithographic microfabrication processes is exploited to demonstrate a 1024-channel penetrating silicon microneedle array (SiMNA) that is scalable in its recording capabilities and cortical coverage and is suitable for clinical translation. The SiMNA is the first penetrating microneedle array with a flexible backing that affords compliancy to brain movements. In addition, the SiMNA is optically transparent permitting simultaneous optical and electrophysiological interrogation of neuronal activity. The SiMNA is used to demonstrate reliable recordings of spontaneous and evoked field potentials and of single unit activity in chronically implanted mice for up to 196 days in response to optogenetic and to whisker air-puff stimuli. Significantly, the 1024-channel SiMNA establishes detailed spatiotemporal mapping of broadband brain activity in rats. This novel scalable and biocompatible SiMNA with its multimodal capability and sensitivity to broadband brain activity will accelerate the progress in fundamental neurophysiological investigations and establishes a new milestone for penetrating and large area coverage microelectrode arrays for brain-machine interfaces.

A method for ultrafast tissue clearing that preserves fluorescence for multimodal and longitudinal brain imaging

Background

Tissue-clearing techniques have recently been developed to make tissues transparent for three-dimensional (3D) imaging at different scales, including single-cell resolution. However, current tissue-clearing workflows have several disadvantages, including complex protocols, time-consuming application, and fluorescence quenching. Additionally, they can be used mainly for clearing larger-volume samples, preventing wide and easy applicability in conventional experimental approaches. In this study, we aimed to develop a versatile, fast, and convenient method for clearing thin and semi-thick samples, which can be used for three-dimensional imaging of experimental or even clinical samples.

Results

We developed an alkaline solution (AKS) containing a combination of 2,2′-thiodiethanol (TDE), DMSO, D-sorbitol, and Tris for tissue clearing, as the alkaline environment is suitable for maintaining the fluorescence of most commonly used fluorescence protein GFP and its variants, and tested its clearing effect on samples from mice and human brains. We assessed the clearing speed, the preservation of fluorescence protein and dyes, and the imaging depth and quality. The results showed that AKS treatment rapidly cleared 300-μm-thick brain slices and 1-mm-thick slices from different organs within 5 min and 1 h, respectively. Moreover, AKS was compatible with a variety of fluorescence proteins and dyes. Most importantly, AKS enhanced the fluorescence of YFP, in contrast to the majority of existing tissue-clearing methods which reduce the fluorescence intensity of fluorescent proteins. Using AKS, we performed long-time high-resolution imaging of weak fluorescent protein-labelled tissues, long-distance fibre tracking, larger-scale 3D imaging and cell counting of the entire brain area, neural circuit tracing, 3D neuromorphic reconstruction, and 3D histopathology imaging.

Conclusions

AKS can be used for simple and rapid clearing of samples from mice and human brains and is widely compatible with a variety of fluorescent dyes. Therefore, AKS has great potential to be used as a broad tissue-clearing reagent for biological optical imaging, especially for time-sensitive experiments.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12915-022-01275-6.

A vibrissa pathway that activates the limbic system

Vibrissa sensory inputs play a central role in driving rodent behavior. These inputs transit through the sensory trigeminal nuclei, which give rise to the ascending lemniscal and paralemniscal pathways. While lemniscal projections are somatotopically mapped from brainstem to cortex, those of the paralemniscal pathway are more widely distributed. Yet the extent and topography of paralemniscal projections are unknown, along with the potential role of these projections in controlling behavior. Here, we used viral tracers to map paralemniscal projections. We find that this pathway broadcasts vibrissa-based sensory signals to brainstem regions that are involved in the regulation of autonomic functions and to forebrain regions that are involved in the expression of emotional reactions. We further provide evidence that GABAergic cells of the Kölliker-Fuse nucleus gate trigeminal sensory input in the paralemniscal pathway via a mechanism of presynaptic or extrasynaptic inhibition.

Interactions of Whisking and Touch Signals in the Rat Brainstem

Perception is an active process, requiring the integration of both proprioceptive and exteroceptive information. In the rat's vibrissal system, a classical model for active sensing, the relative contribution of the two information streams was previously studied at the peripheral, thalamic, and cortical levels. Contributions of brainstem neurons were only indirectly inferred for some trigeminal nuclei according to their thalamic projections. The current work addressed this knowledge gap by performing the first comparative study of the encoding of proprioceptive whisking and exteroceptive touch signals in the oralis (SpVo), interpolaris (SpVi), and paratrigeminal (Pa5) brainstem nuclei. We used artificial whisking in anesthetized male rats, which allows a systematic analysis of the relative contribution of the proprioceptive and exteroceptive information streams along the ascending pathways in the absence of motor or cognitive top-down modulations. We found that (1) neurons in the rostral and caudal parts of the SpVi convey whisking and touch information, respectively, as predicted by their thalamic projections; (2) neurons in the SpVo encode both whisking and (primarily) touch information; and (3) neurons of the Pa5 encode a complex combination of whisking and touch information. In particular, the Pa5 contains a relatively large fraction of neurons that are inhibited by active touch, a response observed so far only in the thalamus. Overall, our systematic characterization of afferent responses to active touch in the trigeminal brainstem approves the hypothesized functions of SpVi neurons and presents evidence that SpVo and Pa5 neurons are involved in the processing of active vibrissal touch.SIGNIFICANCE STATEMENT The present work constitutes the first comparative study of the encoding of proprioceptive (whisking) and exteroceptive (touch) information in the rat's brainstem trigeminal nuclei, the first stage of vibrissal processing in the CNS. It shows that (1) as expected, the rostral and caudal interpolaris neurons convey primarily whisking and touch information, respectively; (2) the oralis nucleus, whose function was previously unknown, encodes both whisking and (primarily) touch touch information; (3) a subtractive computation, reported at the thalamic level, already occurs at the brainstem level; and (4) a novel afferent pathway probably ascends via the paratrigeminal nucleus, encoding both proprioceptive and exteroceptive information.

Functional Convergence of Autonomic and Sensorimotor Processing in the Lateral Cerebellum

The cerebellum is involved in the control of voluntary and autonomic rhythmic behaviors, yet it is unclear to what extent it coordinates these in concert. We studied Purkinje cell activity during unperturbed and perturbed respiration in lobules simplex, crus 1, and crus 2. During unperturbed (eupneic) respiration, complex spike and simple spike activity encode the phase of ongoing sensorimotor processing. In contrast, when the respiratory cycle is perturbed by whisker stimulation, mice concomitantly protract their whiskers and advance their inspiration in a phase-dependent manner, preceded by increased simple spike activity. This phase advancement of respiration in response to whisker stimulation can be mimicked by optogenetic stimulation of Purkinje cells and prevented by cell-specific genetic modification of their AMPA receptors, hampering increased simple spike firing. Thus, the impact of Purkinje cell activity on respiratory control is context and phase dependent, highlighting a coordinating role for the cerebellar hemispheres in aligning autonomic and sensorimotor behaviors.

•

During unperturbed respiration, Purkinje cells signal ongoing sensorimotor processing

•

After perturbation, mice advance their simple spike activity, whisking, and inspiration

•

Altering simple spike activity affects the impact of whisker stimulation on respiration

•

Cerebellar coordination of autonomic and sensorimotor behaviors is context dependent

An automated homecage system for multiwhisker detection and discrimination learning in mice

Automated, homecage behavioral training for rodents has many advantages: it is low stress, requires little interaction with the experimenter, and can be easily manipulated to adapt to different experimental conditions. We have developed an inexpensive, Arduino-based, homecage training apparatus for sensory association training in freely-moving mice using multiwhisker air current stimulation coupled to a water reward. Animals learn this task readily, within 1–2 days of training, and performance progressively improves with training. We examined the parameters that regulate task acquisition using different stimulus intensities, directions, and reward valence. Learning was assessed by comparing anticipatory licking for the stimulus compared to the no-stimulus (blank) trials. At high stimulus intensities (>9 psi), animals showed markedly less participation in the task. Conversely, very weak air current intensities (1–2 psi) were not sufficient to generate rapid learning behavior. At intermediate stimulus intensities (5–6 psi), a majority of mice learned that the multiwhisker stimulus predicted the water reward after 24–48 hrs of training. Both exposure to isoflurane and lack of whiskers decreased animals’ ability to learn the task. Following training at an intermediate stimulus intensity, mice were able to transfer learning behavior when exposed to a lower stimulus intensity, an indicator of perceptual learning. Mice learned to discriminate between two directions of stimulation rapidly and accurately, even when the angular distance between the stimuli was <15 degrees. Switching the reward to a more desirable reward, aspartame, had little effect on learning trajectory. Our results show that a tactile association task in an automated homecage environment can be monitored by anticipatory licking to reveal rapid and progressive behavioral change. These Arduino-based, automated mouse cages enable high-throughput training that facilitate analysis of large numbers of genetically modified mice with targeted manipulations of neural activity.

Bidirectional learning in upbound and downbound microzones of the cerebellum

Over the past several decades, theories about cerebellar learning have evolved. A relatively simple view that highlighted the contribution of one major form of heterosynaptic plasticity to cerebellar motor learning has given way to a plethora of perspectives that suggest that many different forms of synaptic and non-synaptic plasticity, acting at various sites, can control multiple types of learning behaviour. However, there still seem to be contradictions between the various hypotheses with regard to the mechanisms underlying cerebellar learning. The challenge is therefore to reconcile these different views and unite them into a single overall concept. Here I review our current understanding of the changes in the activity of cerebellar Purkinje cells in different 'microzones' during various forms of learning. I describe an emerging model that indicates that the activity of each microzone is bound to either increase or decrease during the initial stages of learning, depending on the directional and temporal demands of its downstream circuitry and the behaviour involved.

Predictive whisker kinematics reveal context-dependent sensorimotor strategies

Animals actively move their sensory organs in order to acquire sensory information. Some rodents, such as mice and rats, employ cyclic scanning motions of their facial whiskers to explore their proximal surrounding, a behavior known as whisking. Here, we investigated the contingency of whisking kinematics on the animal's behavioral context that arises from both internal processes (attention and expectations) and external constraints (available sensory and motor degrees of freedom). We recorded rat whisking at high temporal resolution in 2 experimental contexts-freely moving or head-fixed-and 2 spatial sensory configurations-a single row or 3 caudal whiskers on each side of the snout. We found that rapid sensorimotor twitches, called pumps, occurring during free-air whisking carry information about the rat's upcoming exploratory direction, as demonstrated by the ability of these pumps to predict consequent head and body locomotion. Specifically, pump behavior during both voluntary motionlessness and imposed head fixation exposed a backward redistribution of sensorimotor exploratory resources. Further, head-fixed rats employed a wide range of whisking profiles to compensate for the loss of head- and body-motor degrees of freedom. Finally, changing the number of intact vibrissae available to a rat resulted in an alteration of whisking strategy consistent with the rat actively reallocating its remaining resources. In sum, this work shows that rats adapt their active exploratory behavior in a homeostatic attempt to preserve sensorimotor coverage under changing environmental conditions and changing sensory capacities, including those imposed by various laboratory conditions.

Sensorimotor integration in the whisker somatosensory brain stem trigeminal loop

The rodent's vibrissal system is a useful model system for studying sensorimotor integration in perception. This integration determines the way in which sensory information is acquired by sensory organs and the motor commands that control them. The initial instance of sensorimotor integration in the whisker somatosensory system is implemented in the brain stem loop and may be essential to the way rodents explore and sense their environment. To examine the nature of these sensorimotor interactions, we recorded from lightly anesthetized rats in vivo and brain stem slices in vitro and isolated specific parts of this loop. We found that motor feedback to the vibrissal pad serves as a dynamic gain controller that controls the response of first-order sensory neurons by increasing and decreasing sensitivity to lower and higher tactile stimulus magnitudes, respectively. This delicate mechanism is mediated through tactile stimulus magnitude-dependent motor feedback. Conversely, tactile inputs affect the motor whisking output through influences on the rhythmic whisking circuitry, thus changing whisking kinetics. Similarly, tactile influences also modify the whisking amplitude through synaptic and intrinsic neuronal interaction in the facial nucleus, resulting in facilitation or suppression of whisking amplitude. These results point to the vast range of mechanisms underlying sensorimotor integration in the brain stem loop.NEW & NOTEWORTHY Sensorimotor integration is a process in which sensory and motor information is combined to control the flow of sensory information, as well as to adjust the motor system output. We found in the rodent's whisker somatosensory system mutual influences between tactile inputs and motor output, in which motor neurons control the flow of sensory information depending on their magnitude. Conversely, sensory information can control the magnitude and kinetics of whisker movement.

Orofacial Movements Involve Parallel Corticobulbar Projections from Motor Cortex to Trigeminal Premotor Nuclei

How do neurons in orofacial motor cortex (MCtx) orchestrate behaviors? We show that focal activation of MCtx corticobulbar neurons evokes behaviorally relevant concurrent movements of the forelimb, jaw, nose, and vibrissae. The projections from different locations in MCtx form gradients of boutons across premotor nuclei spinal trigeminal pars oralis (SpVO) and interpolaris rostralis (SpVIr). Furthermore, retrograde viral tracing from muscles that control orofacial actions shows that these premotor nuclei segregate their outputs. In the most dramatic case, both SpVO and SpVIr are premotor to forelimb and vibrissa muscles, while only SpVO is premotor to jaw muscles. Functional confirmation of the superimposed control by MCtx was obtained through selective optogenetic activation of corticobulbar neurons on the basis of their preferential projections to SpVO versus SpVIr. We conclude that neighboring projection neurons in orofacial MCtx form parallel pathways to distinct pools of trigeminal premotor neurons that coordinate motor actions into a behavior.

Mechanical coupling through the skin affects whisker movements and tactile information encoding

Rats use their whiskers to extract sensory information from their environment. While exploring, they analyze peripheral stimuli distributed over several whiskers. Previous studies have reported cross-whisker integration of information at several levels of the neuronal pathways from whisker follicles to the somatosensory cortex. In the present study, we investigated the possible coupling between whiskers at a preneuronal level, transmitted by the skin and muscles between follicles. First, we quantified the movement induced on one whisker by deflecting another whisker. Our results show significant mechanical coupling, predominantly when a given whisker's caudal neighbor in the same row is deflected. The magnitude of the effect was correlated with the diameter of the deflected whisker. In addition to changes in whisker angle, we observed curvature changes when the whisker shaft was constrained distally from the base. Second, we found that trigeminal ganglion neurons innervating a given whisker follicle fire action potentials in response to high-magnitude deflections of an adjacent whisker. This functional coupling also shows a bias toward the caudal neighbor located in the same row. Finally, we designed a two-whisker biomechanical model to investigate transmission of forces across follicles. Analysis of the whisker-follicle contact forces suggests that activation of mechanoreceptors in the ring sinus region could account for our electrophysiological results. The model can fully explain the observed caudal bias by the gradient in whisker diameter, with possible contribution of the intrinsic muscles connecting follicles. Overall, our study demonstrates the functional relevance of mechanical coupling on early information processing in the whisker system.NEW & NOTEWORTHY Rodents explore their environment actively by touching objects with their whiskers. A major challenge is to understand how sensory inputs from different whiskers are merged together to form a coherent tactile percept. We demonstrate that external sensory events on one whisker can influence the position of another whisker and, importantly, that they can trigger the activity of mechanoreceptors at its base. This cross-whisker interaction occurs pre-neuronally, through mechanical transmission of forces in the skin.

Sensorimotor processing in the rodent barrel cortex

Tactile sensory information from facial whiskers provides nocturnal tunnel-dwelling rodents, including mice and rats, with important spatial and textural information about their immediate surroundings. Whiskers are moved back and forth to scan the environment (whisking), and touch signals from each whisker evoke sparse patterns of neuronal activity in whisker-related primary somatosensory cortex (wS1; barrel cortex). Whisking is accompanied by desynchronized brain states and cell-type-specific changes in spontaneous and evoked neuronal activity. Tactile information, including object texture and location, appears to be computed in wS1 through integration of motor and sensory signals. wS1 also directly controls whisker movements and contributes to learned, whisker-dependent, goal-directed behaviours. The cell-type-specific neuronal circuitry in wS1 that contributes to whisker sensory perception is beginning to be defined.

Quasiperiodic rhythms of the inferior olive

Inferior olivary activity causes both short-term and long-term changes in cerebellar output underlying motor performance and motor learning. Many of its neurons engage in coherent subthreshold oscillations and are extensively coupled via gap junctions. Studies in reduced preparations suggest that these properties promote rhythmic, synchronized output. However, the interaction of these properties with torrential synaptic inputs in awake behaving animals is not well understood. Here we combine electrophysiological recordings in awake mice with a realistic tissue-scale computational model of the inferior olive to study the relative impact of intrinsic and extrinsic mechanisms governing its activity. Our data and model suggest that if subthreshold oscillations are present in the awake state, the period of these oscillations will be transient and variable. Accordingly, by using different temporal patterns of sensory stimulation, we found that complex spike rhythmicity was readily evoked but limited to short intervals of no more than a few hundred milliseconds and that the periodicity of this rhythmic activity was not fixed but dynamically related to the synaptic input to the inferior olive as well as to motor output. In contrast, in the long-term, the average olivary spiking activity was not affected by the strength and duration of the sensory stimulation, while the level of gap junctional coupling determined the stiffness of the rhythmic activity in the olivary network during its dynamic response to sensory modulation. Thus, interactions between intrinsic properties and extrinsic inputs can explain the variations of spiking activity of olivary neurons, providing a temporal framework for the creation of both the short-term and long-term changes in cerebellar output.

Activity of the inferior olive, transmitted via climbing fibers to the cerebellum, regulates initiation and amplitude of movements, signals unexpected sensory feedback, and directs cerebellar learning. It is characterized by widespread subthreshold oscillations and synchronization promoted by strong electrotonic coupling. In brain slices, subthreshold oscillations gate which inputs can be transmitted by inferior olivary neurons and which will not—dependent on the phase of the oscillation. We tested whether the subthreshold oscillations had a measurable impact on temporal patterning of climbing fiber activity in intact, awake mice. We did so by recording neural activity of the postsynaptic Purkinje cells, in which complex spike firing faithfully represents climbing fiber activity. For short intervals (<300 ms) many Purkinje cells showed spontaneously rhythmic complex spike activity. However, our experiments designed to evoke conditional responses indicated that complex spikes are not predominantly predicated on stimulus history. Our realistic network model of the inferior olive explains the experimental observations via continuous phase modulations of the subthreshold oscillations under the influence of synaptic fluctuations. We conclude that complex spike activity emerges from a quasiperiodic rhythm that is stabilized by electrotonic coupling between its dendrites, yet dynamically influenced by the status of their synaptic inputs.

Diverse tuning underlies sparse activity in layer 2/3 vibrissal cortex of awake mice

KEY POINTS

Sparse population activity is a common feature observed across cortical areas, yet the implications for sensory coding are not clear. We recorded single neuron activity in the vibrissal somatosensory cortex of awake head-fixed mice using the cell-attached technique. Unlike the anaesthetised condition, in awake mice a high-velocity, piezo-controlled whisker deflection excited only a small fraction of neurons. Manual probing of whiskers revealed that the majority of these silent neurons could be activated by specific forms of whisker-object contact. Our results suggest that sparse coding in vibrissal cortex may be due to high dimensionality of the stimulus space and narrow tuning of individual neurons.

ABSTRACT

It is widely reported that superficial layers of the somatosensory cortex exhibit sparse firing. This sparseness could reflect weak feedforward sensory inputs that are not sufficient to generate action potentials in these layers. Alternatively, sparseness might reflect tuning to unknown or higher-level complex features that are not fully explored in the stimulus space. Here, we examined these hypotheses by applying a range of vibrotactile and manual vibrissal stimuli in awake, head-fixed mice while performing loose-seal cell-attached recordings from the vibrissal primary somatosensory (vS1) cortex. A high-velocity stimulus delivered by a piezo-electric actuator evoked activity in a small fraction of regular spiking supragranular neurons (23%) in the awake condition. However, a majority of the supragranular regular spiking neurons (84%) were driven by manual stimulation of whiskers. Our results suggest that most neurons in the superficial layers of vS1 cortex contribute to coding in the awake condition when neurons may encounter their preferred feature(s) during whisker-object interactions.

Potentiation of cerebellar Purkinje cells facilitates whisker reflex adaptation through increased simple spike activity

Cerebellar plasticity underlies motor learning. However, how the cerebellum operates to enable learned changes in motor output is largely unknown. We developed a sensory-driven adaptation protocol for reflexive whisker protraction and recorded Purkinje cell activity from crus 1 and 2 of awake mice. Before training, simple spikes of individual Purkinje cells correlated during reflexive protraction with the whisker position without lead or lag. After training, simple spikes and whisker protractions were both enhanced with the spiking activity now leading behavioral responses. Neuronal and behavioral changes did not occur in two cell-specific mouse models with impaired long-term potentiation at their parallel fiber to Purkinje cell synapses. Consistent with cerebellar plasticity rules, increased simple spike activity was prominent in cells with low complex spike response probability. Thus, potentiation at parallel fiber to Purkinje cell synapses may contribute to reflex adaptation and enable expression of cerebellar learning through increases in simple spike activity.

Rodents use their whiskers to explore the world around them. When the whiskers touch an object, it triggers involuntary movements of the whiskers called whisker reflexes. Experiencing the same sensory stimulus multiple times enables rodents to fine-tune these reflexes, e.g., by making their movements larger or smaller. This type of learning is often referred to as motor learning.

A part of the brain called cerebellum controls motor learning. It contains some of the largest neurons in the nervous system, the Purkinje cells. Each Purkinje cell receives input from thousands of extensions of small neurons, known as parallel fibers. It is thought that decreasing the strength of the connections between parallel fibers and Purkinje cells can help mammals learn new movements. This is the case in a type of learning called Pavlovian conditioning. It takes its name from the Russian scientist, Pavlov, who showed that dogs can learn to salivate in response to a bell signaling food. Pavlovian conditioning enables animals to optimize their responses to sensory stimuli.

But Romano et al. now show that increasing the strength of connections between parallel fibers and Purkinje cells can also support learning. To trigger reflexive whisker movements, a machine blew puffs of air onto the whiskers of awake mice. After repeated exposure to the air puffs, the mice increased the size of their whisker reflexes. At the same time, their Purkinje cells became more active and the connections between Purkinje cells and parallel fibers grew stronger. Artificially increasing Purkinje cell activity triggered the same changes in whisker reflexes as the air puffs themselves.

Textbooks still report that only weakening of connections within the cerebellum enables animals to learn and modify movements. The data obtained by Romano al. thus paint a new picture of how the cerebellum works in the context of whisker learning. They show that strengthening these connections can also support movement-related learning.

Circuits in the rodent brainstem that control whisking in concert with other orofacial motor actions

The world view of rodents is largely determined by sensation on two length scales. One is within the animal's peri-personal space; sensorimotor control on this scale involves active movements of the nose, tongue, head, and vibrissa, along with sniffing to determine olfactory clues. The second scale involves the detection of more distant space through vision and audition; these detection processes also impact repositioning of the head, eyes, and ears. Here we focus on orofacial motor actions, primarily vibrissa-based touch but including nose twitching, head bobbing, and licking, that control sensation at short, peri-personal distances. The orofacial nuclei for control of the motor plants, as well as primary and secondary sensory nuclei associated with these motor actions, lie within the hindbrain. The current data support three themes: First, the position of the sensors is determined by the summation of two drive signals, i.e., a fast rhythmic component and an evolving orienting component. Second, the rhythmic component is coordinated across all orofacial motor actions and is phase-locked to sniffing as the animal explores. Reverse engineering reveals that the preBötzinger inspiratory complex provides the reset to the relevant premotor oscillators. Third, direct feedback from somatosensory trigeminal nuclei can rapidly alter motion of the sensors. This feedback is disynaptic and can be tuned by high-level inputs. A holistic model for the coordination of orofacial motor actions into behaviors will encompass feedback pathways through the midbrain and forebrain, as well as hindbrain areas.

A craniofacial-specific monosynaptic circuit enables heightened affective pain

Humans often rank craniofacial pain as more severe than body pain. Evidence suggests that a stimulus of the same intensity induces stronger pain in the face than in the body. However, the underlying neural circuitry for the differential processing of facial versus bodily pain remains unknown. Interestingly, the lateral parabrachial nucleus (PBL), a critical node in the affective pain circuit, is activated more strongly by noxious stimulation of the face than of the hindpaw. Using a novel activity-dependent technology called CANE developed in our laboratory, we identified and selectively labeled noxious-stimulus-activated PBL neurons and performed comprehensive anatomical input-output mapping. Surprisingly, we uncovered a hitherto uncharacterized monosynaptic connection between cranial sensory neurons and the PBL-nociceptive neurons. Optogenetic activation of this monosynaptic craniofacial-to-PBL projection induced robust escape and avoidance behaviors and stress calls, whereas optogenetic silencing specifically reduced facial nociception. The monosynaptic circuit revealed here provides a neural substrate for heightened craniofacial affective pain.