Optogenetic Determination of Dynamic and Cell-Type-Specific Inhibitory Reversal Potentials

The reversal potential refers to the membrane potential at which the net current flow through a channel reverses direction. The reversal potential is determined by transmembrane ion gradients and, in turn, determines how the channel's activity will affect the membrane potential. Traditional investigation into the reversal potential of inhibitory ligand-gated ion channels (EInh) has relied upon the activation of endogenous receptors, such as the GABA-A receptor (GABAAR). There are, however, challenges associated with activating endogenous receptors, including agonist delivery, isolating channel responses, and the effects of receptor saturation and desensitization. Here, we demonstrate the utility of using a light-gated anion channel, stGtACR2, to probe EInh in the rodent brain. Using mice of both sexes, we demonstrate that the properties of this optically activated channel make it a suitable proxy for studying GABAAR receptor-mediated inhibition. We validate this agonist-independent optogenetic strategy in vitro and in vivo and further show how it can accurately capture differences in EInh dynamics following manipulations of endogenous ion fluxes. This allows us to explore distinct resting EInh differences across genetically defined neuronal subpopulations. Using this approach to challenge ion homeostasis mechanisms in neurons, we uncover cell-specific EInh dynamics that are supported by the differential expression of endogenous ion handling mechanisms. Our findings therefore establish an effective optical strategy for revealing novel aspects of inhibitory reversal potentials and thereby expand the repertoire of optogenetics.

Cortical cellular encoding of thermotactile integration

Recent evidence suggests that primary sensory cortical regions play a role in the integration of information from multiple sensory modalities. How primary cortical neurons integrate different sources of sensory information is unclear, partly because non-primary sensory input to a cortical sensory region is often weak or modulatory. To address this question, we take advantage of the robust representation of thermal (cooling) and tactile stimuli in mouse forelimb primary somatosensory cortex (fS1). Using a thermotactile detection task, we show that the perception of threshold-level cool or tactile information is enhanced when they are presented simultaneously, compared with presentation alone. To investigate the cortical cellular correlates of thermotactile integration, we performed in vivo extracellular recordings from fS1 in awake resting and anesthetized mice during unimodal and bimodal stimulation of the forepaw. Unimodal stimulation evoked thermal- or tactile- specific excitatory and inhibitory responses of fS1 neurons. The most prominent features of combined thermotactile stimulation are the recruitment of unimodally silent fS1 neurons, non-linear integration features, and response dynamics that favor longer response durations with additional spikes. Together, we identify quantitative and qualitative changes in cortical encoding that may underlie the improvement in perception of thermotactile surfaces during haptic exploration.

The spatial representation of temperature in the thalamus

Although distinct thalamic nuclei encode sensory information for almost all sensory modalities, the existence of a thalamic representation of temperature is debated and the role of the thalamus in thermal perception remains unclear. To address this, we used high-density electrophysiological recordings across mouse forepaw somatosensory thalamus, and identified an anterior and a posterior representation of temperature that spans three thalamic nuclei. These parallel representations show fundamental differences in the cellular encoding of temperature that reflect their cortical output targets, with the anterior representation encoding cool only and the posterior both cool and warm. Moreover, their inactivation profoundly altered thermal perception. Together our data identifies a novel posterior thalamic representation of temperature and a principal role of the thalamus in thermal perception.

Sensory Schwann cells set perceptual thresholds for touch and selectively regulate mechanical nociception

Previous work identified nociceptive Schwann cells that can initiate pain. Consistent with the existence of inherently mechanosensitive sensory Schwann cells, we found that in mice, the mechanosensory function of almost all nociceptors, including those signaling fast pain, were dependent on sensory Schwann cells. In polymodal nociceptors, sensory Schwann cells signal mechanical, but not cold or heat pain. Terminal Schwann cells also surround mechanoreceptor nerve-endings within the Meissner's corpuscle and in hair follicle lanceolate endings that both signal vibrotactile touch. Within Meissner´s corpuscles, two molecularly and functionally distinct sensory Schwann cells positive for Sox10 and Sox2 differentially modulate rapidly adapting mechanoreceptor function. Using optogenetics we show that Meissner's corpuscle Schwann cells are necessary for the perception of low threshold vibrotactile stimuli. These results show that sensory Schwann cells within diverse glio-neural mechanosensory end-organs are sensors for mechanical pain as well as necessary for touch perception.

Immediate reuse of patch-clamp pipettes after ultrasonic cleaning

The patch-clamp technique has revolutionized neurophysiology by allowing to study single neuronal excitability, synaptic connectivity, morphology, and the transcriptomic profile. However, the throughput in recordings is limited because of the manual replacement of patch-pipettes after each attempt which are often also unsuccessful. This has been overcome by automated cleaning the tips in detergent solutions, allowing to reuse the pipette for further recordings. Here, we developed a novel method of automated cleaning by sonicating the tips within the bath solution wherein the cells are placed, reducing the risk of contaminating the bath solution or internal solution of the recording pipette by any detergent and avoiding the necessity of a separate chamber for cleaning. We showed that the patch-pipettes can be used consecutively at least ten times and that the cleaning process does not negatively impact neither the brain slices nor other patched neurons. This method, combined with automated patch-clamp, highly improves the throughput for single and especially multiple recordings.

Brain-wide connectivity map of mouse thermosensory cortices

In the thermal system, skin cooling is represented in the primary somatosensory cortex (S1) and the posterior insular cortex (pIC). Whether S1 and pIC are nodes in anatomically separate or overlapping thermal sensorimotor pathways is unclear, as the brain-wide connectivity of the thermal system has not been mapped. We address this using functionally targeted, dual injections of anterograde viruses or retrograde tracers into the forelimb representation of S1 (fS1) and pIC (fpIC). Our data show that inputs to fS1 and fpIC originate from separate neuronal populations, supporting the existence of parallel input pathways. Outputs from fS1 and fpIC are more widespread than their inputs, sharing a number of cortical and subcortical targets. While, axonal projections were separable, they were more overlapping than the clusters of input cells. In both fS1 and fpIC circuits, there was a high degree of reciprocal connectivity with thalamic and cortical regions, but unidirectional output to the midbrain and hindbrain. Notably, fpIC showed connectivity with regions associated with thermal processing. Together, these data indicate that cutaneous thermal information is routed to the cortex via parallel circuits and is forwarded to overlapping downstream regions for the binding of somatosensory percepts and integration with ongoing behavior.

A medullary centre for lapping in mice

It has long been known that orofacial movements for feeding can be triggered, coordinated, and often rhythmically organized at the level of the brainstem, without input from higher centers. We uncover two nuclei that can organize the movements for ingesting fluids in mice. These neuronal groups, IRt^Phox2b and Peri5^Atoh1, are marked by expression of the pan-autonomic homeobox gene Phox2b and are located, respectively, in the intermediate reticular formation of the medulla and around the motor nucleus of the trigeminal nerve. They are premotor to all jaw-opening and tongue muscles. Stimulation of either, in awake animals, opens the jaw, while IRt^Phox2b alone also protracts the tongue. Moreover, stationary stimulation of IRt^Phox2b entrains a rhythmic alternation of tongue protraction and retraction, synchronized with jaw opening and closing, that mimics lapping. Finally, fiber photometric recordings show that IRt^Phox2b is active during volitional lapping. Our study identifies one of the subcortical nuclei underpinning a stereotyped feeding behavior.

USH2A is a Meissner's corpuscle protein necessary for normal vibration sensing in mice and humans

Fingertip mechanoreceptors comprise sensory neuron endings together with specialized skin cells that form the end-organ. Exquisitely sensitive, vibration-sensing neurons are associated with Meissner's corpuscles in the skin. In the present study, we found that USH2A, a transmembrane protein with a very large extracellular domain, was found in terminal Schwann cells within Meissner's corpuscles. Pathogenic USH2A mutations cause Usher's syndrome, associated with hearing loss and visual impairment. We show that patients with biallelic pathogenic USH2A mutations also have clear and specific impairments in vibrotactile touch perception, as do mutant mice lacking USH2A. Forepaw rapidly adapting mechanoreceptors innervating Meissner's corpuscles, recorded from Ush2a^-/- mice, showed large reductions in vibration sensitivity. However, the USH2A protein was not found in sensory neurons. Thus, loss of USH2A in corpuscular end-organs reduced mechanoreceptor sensitivity as well as vibration perception. Thus, a tether-like protein is required to facilitate detection of small-amplitude vibrations essential for the perception of fine-grained tactile surfaces.

The Sensory Coding of Warm Perception

Humans detect skin temperature changes that are perceived as warm or cool. Like humans, mice report forepaw skin warming with perceptual thresholds of less than 1°C and do not confuse warm with cool. We identify two populations of polymodal C-fibers that signal warm. Warm excites one population, whereas it suppresses the ongoing cool-driven firing of the other. In the absence of the thermosensitive TRPM2 or TRPV1 ion channels, warm perception was blunted, but not abolished. In addition, trpv1:trpa1:trpm3^−/− triple-mutant mice that cannot sense noxious heat detected skin warming, albeit with reduced sensitivity. In contrast, loss or local pharmacological silencing of the cool-driven TRPM8 channel abolished the ability to detect warm. Our data are not reconcilable with a labeled line model for warm perception, with receptors firing only in response to warm stimuli, but instead support a conserved dual sensory model to unambiguously detect skin warming in vertebrates.

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Mice, like humans, perceive forepaw warming (≥1°C) and discriminate warm from cool

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Warm-activated and warm-silenced polymodal C-fibers both signal forepaw warming

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Mice lacking the cool-sensitive ion channel TRPM8 are unable to perceive warm

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The inability to perceive warm is associated with loss of warm-silenced C-fibers

Precisely Timed Nicotinic Activation Drives SST Inhibition in Neocortical Circuits

Sleep, waking, locomotion, and attention are associated with cell-type-specific changes in neocortical activity. The effect of brain state on circuit output requires understanding of how neuromodulators influence specific neuronal classes and their synapses, with normal patterns of neuromodulator release from endogenous sources. We investigated the state-dependent modulation of a ubiquitous feedforward inhibitory motif in mouse sensory cortex, local pyramidal (Pyr) inputs onto somatostatin (SST)-expressing interneurons. Paired whole-cell recordings in acute brain slices and in vivo showed that Pyr-to-SST synapses are remarkably weak, with failure rates approaching 80%. Pharmacological screening revealed that cholinergic agonists uniquely enhance synaptic efficacy. Brief, optogenetically gated acetylcholine release dramatically enhanced Pyr-to-SST input, via nicotinic receptors and presynaptic PKA signaling. Importantly, endogenous acetylcholine release preferentially activated nicotinic, not muscarinic, receptors, thus differentiating drug effects from endogenous neurotransmission. Brain state- and synapse-specific unmasking of synapses may be a powerful way to functionally rewire cortical circuits dependent on behavioral demands.

Multiple Two-Photon Targeted Whole-Cell Patch-Clamp Recordings From Monosynaptically Connected Neurons in vivo

Although we know a great deal about monosynaptic connectivity, transmission and integration in the mammalian nervous system from in vitro studies, very little is known in vivo. This is partly because it is technically difficult to evoke action potentials and simultaneously record small amplitude subthreshold responses in closely (<150 μm) located pairs of neurons. To address this, we have developed in vivo two-photon targeted multiple (2–4) whole-cell patch clamp recordings of nearby neurons in superficial cortical layers 1–3. Here, we describe a step-by-step guide to this approach in the anesthetized mouse primary somatosensory cortex, including: the design of the setup, surgery, preparation of pipettes, targeting and acquisition of multiple whole-cell recordings, as well as in vivo and post hoc histology. The procedure takes ~4 h from start of surgery to end of recording and allows examinations both into the electrophysiological features of unitary excitatory and inhibitory monosynaptic inputs during different brain states as well as the synaptic mechanisms of correlated neuronal activity.

The Cortical States of Wakefulness

Cortical neurons process information on a background of spontaneous, ongoing activity with distinct spatiotemporal profiles defining different cortical states. During wakefulness, cortical states alter constantly in relation to behavioral context, attentional level or general motor activity. In this review article, we will discuss our current understanding of cortical states in awake rodents, how they are controlled, their impact on sensory processing, and highlight areas for future research. A common observation in awake rodents is the rapid change in spontaneous cortical activity from high-amplitude, low-frequency (LF) fluctuations, when animals are quiet, to faster and smaller fluctuations when animals are active. This transition is typically thought of as a change in global brain state but recent work has shown variation in cortical states across regions, indicating the presence of a fine spatial scale control system. In sensory areas, the cortical state change is mediated by at least two convergent inputs, one from the thalamus and the other from cholinergic inputs in the basal forebrain. Cortical states have a major impact on the balance of activity between specific subtypes of neurons, on the synchronization between nearby neurons, as well as the functional coupling between distant cortical areas. This reorganization of the activity of cortical networks strongly affects sensory processing. Thus cortical states provide a dynamic control system for the moment-by-moment regulation of cortical processing.

Single synaptic inputs drive high-precision action potentials in parvalbumin expressing GABA-ergic cortical neurons in vivo

A defining feature of cortical layer 2/3 excitatory neurons is their sparse activity, often firing in singlets of action potentials. Local inhibitory neurons are thought to play a major role in regulating sparseness, but which cell types are recruited by single excitatory synaptic inputs is unknown. Using multiple, targeted, in vivo whole-cell recordings, we show that single _uEPSPs have little effect on the firing rates of excitatory neurons and somatostatin-expressing GABA-ergic inhibitory neurons but evoke precisely timed action potentials in parvalbumin-expressing inhibitory neurons. Despite a _uEPSP decay time of 7.8 ms, the evoked action potentials were almost completely restricted to the _uEPSP rising phase (~0.5 ms). Evoked parvalbumin-expressing neuron action potentials go on to inhibit the local excitatory network, thus providing a pathway for single spike evoked disynaptic inhibition which may enforce sparse and precisely timed cortical signaling.

Parvalbumin-Expressing GABAergic Neurons in Primary Motor Cortex Signal Reaching

The control of targeted reaching is thought to be shaped by distinct subtypes of local GABAergic inhibitory neurons in primary forelimb motor cortex (M1). However, little is known about their action potential firing dynamics during reaching. To address this, we recorded the activity of parvalbumin-expressing (PV+) GABAergic neurons identified from a larger population of fast-spiking units and putative excitatory regular-spiking units in layer 5 of the mouse forelimb M1 during an M1-dependent, sensory-triggered reaching task. PV+ neurons showed short latency responses to the acoustic cue and vibrotactile trigger stimulus and an increase in firing at reaching onset that scaled with the amplitude of reaching. Unexpectedly, PV+ neurons fired before regular-spiking units at reach onset and showed high overall firing rates during both sensory-triggered and spontaneous reaches. Our data suggest that increasing M1 PV+ neuron firing rates may play a role in the initiation of voluntary reaching.

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Extracellular recordings from layer 5 M1 PV+ neurons during sensory-triggered reaching

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Task-related sensory- and motor-evoked responses in forelimb M1 neurons

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PV+ neuron firing rates are positively correlated with reaching amplitude

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PV+ neurons are activated before regular-spiking units at reaching onset

Regulation of body weight and energy homeostasis by neuronal cell adhesion molecule 1

Susceptibility to obesity is linked to genes regulating neurotransmission, pancreatic β-cell function, and energy homeostasis. Genome-wide association studies identified an association between body mass index and two loci near Cell Adhesion Molecule1 (CADM1) and Cell Adhesion Molecule2 (CADM2), genes encoding membrane proteins mediating synaptic assembly. We show these respective risk variants associate with increased CADM1 and CADM2 expression in the hypothalamus of human subjects. Expression of both genes is elevated in obese mice and induction of Cadm1 in excitatory neurons facilitated weight gain while exacerbating energy expenditure. Loss of Cadm1 protected mice from obesity and tract-tracing analysis revealed Cadm1-positive innervation of POMC neurons via afferent projections originating from beyond the arcuate nucleus. Reducing Cadm1 expression in the hypothalamus and hippocampus promoted a negative energy balance and weight loss. These data identify roles for Cadm1-mediated neuronal input in weight regulation and provide insight into the central pathways contributing to human obesity.

In Vivo Monosynaptic Excitatory Transmission between Layer 2 Cortical Pyramidal Neurons

Little is known about the properties of monosynaptic connections between identified neurons in vivo. We made multiple (two to four) two-photon targeted whole-cell recordings from neighboring layer 2 mouse somatosensory barrel cortex pyramidal neurons in vivo to investigate excitatory monosynaptic transmission in the hyperpolarized downstate. We report that pyramidal neurons form a sparsely connected (6.7% connectivity) network with an overrepresentation of bidirectional connections. The majority of unitary excitatory postsynaptic potentials were small in amplitude (<0.5 mV), with a small minority >1 mV. The coefficient of variation (CV = 0.74) could largely be explained by the presence of synaptic failures (22%). Both the CV and failure rates were reduced with increasing amplitude. The mean paired-pulse ratio was 1.15 and positively correlated with the CV. Our approach will help bridge the gap between connectivity and function and allow investigations into the impact of brain state on monosynaptic transmission and integration.

Matching Cell Type to Function in Cortical Circuits

In this issue of Neuron, Pinto and Dan (2015) performed single-cell calcium imaging in the mouse dorsomedial prefrontal cortex to reveal correlated, cell-type-specific responses in three major GABA-ergic interneuron subtypes during a goal-directed sensory discrimination task.

Keeping an eye on cortical states

Membrane potential recordings in awake mice have correlated cortical state with locomotion and whisker movements. In this issue of Neuron, Reimer et al. (2014) now reveal that pupil dilation in stationary mice equally signals a change in cortical state and an enhancement of visual processing.

Experimental evidence for sparse firing in the neocortex

The advent of unbiased recording and imaging techniques to evaluate firing activity across neocortical neurons has revealed substantial heterogeneity in response properties in vivo, and that a minority of neurons are responsible for the majority of spikes. Despite the computational advantages to sparsely firing populations, experimental data defining the fraction of responsive neurons and the range of firing rates have not been synthesized. Here we review data about the distribution of activity across neuronal populations in primary sensory cortex. Overall, the firing output of granular and infragranular layers is highest. Although subthreshold activity across supragranular neurons is decidedly non-sparse, spikes are much less frequent and some cells are silent. Superficial layers of the cortex may employ specific cell and circuit mechanisms to increase sparseness.

Thalamic control of cortical states

We investigated the impact of thalamus on ongoing cortical activity in the awake, behaving mouse. We demonstrate that the desynchronized cortical state during active behavior is driven by a centrally generated increase in thalamic action potential firing, which can also be mimicked by optogenetic stimulation of the thalamus. The thalamus therefore is key in controlling cortical states.

Coherent phasic excitation during hippocampal ripples

High-frequency hippocampal network oscillations, or "ripples," are thought to be involved in episodic memory. According to current theories, memory traces are represented by assemblies of principal neurons that are activated during ripple-associated network states. Here we performed in vivo and in vitro experiments to investigate the synaptic mechanisms during ripples. We discovered postsynaptic currents that are phase-locked to ripples and coherent among even distant CA1 pyramidal neurons. These fast currents are consistent with excitatory postsynaptic currents (EPSCs) as they are observed at the equilibrium potential of Cl(-), and they display kinetics characteristic of EPSCs. Furthermore, they survived after intracellular blockade of GABAergic transmission and are effective to regulate the timing of action potentials. In addition, our data show a progressive synchronization of phasic excitation and inhibition during the course of ripples. Together, our results demonstrate the presence of phasic excitation during ripples reflecting an exquisite temporal coordination of assemblies of active pyramidal cells.

An embedded subnetwork of highly active neurons in the neocortex

VIDEO ABSTRACT

Unbiased methods to assess the firing activity of individual neurons in the neocortex have revealed that a large proportion of cells fire at extremely low rates (<0.1 Hz), both in their spontaneous and evoked activity. Thus, firing in neocortical networks appears to be dominated by a small population of highly active neurons. Here, we use a fosGFP transgenic mouse to examine the properties of cells with a recent history of elevated activity. FosGFP-expressing layer 2/3 pyramidal cells fired at higher rates compared to fosGFP(-) neurons, both in vivo and in vitro. Elevated activity could be attributed to increased excitatory and decreased inhibitory drive to fosGFP(+) neurons. Paired-cell recordings indicated that fosGFP(+) neurons had a greater likelihood of being connected to each other. These findings indicate that highly active, interconnected neuronal ensembles are present in the neocortex and suggest these cells may play a role in the encoding of sensory information.

Facilitating sensory responses in developing mouse somatosensory barrel cortex

The sensory responses in the barrel cortex of mice aged postnatal day (P)7-P12 evoked by a single whisker deflection are smaller in amplitude and spread over a smaller area than those measured in P13-P21 mice. However, repetitive 10-Hz stimulation or paired pulse whisker stimulation in P7-P12 mice evoked facilitating sensory responses, contrasting with the depressing sensory responses observed in P13-P21 mice. This facilitation occurred during an interval ranging 300-1,000 ms after the first stimulus and was measured using whole cell recordings, voltage-sensitive dye imaging, and calcium-sensitive dye imaging. The facilitated responses were not only larger in amplitude but also propagated over a larger cortical area. The facilitation could be blocked by local application of pharmacological agents reducing cortical excitability. Local cortical microstimulation could substitute for the first whisker stimulus to produce a facilitated sensory response. The enhanced sensory responses evoked by repetitive sensory stimuli in P7-P12 mice may contribute to the activity-dependent specification of the developing cortical circuits. In addition, the facilitating sensory responses allow long integration times for sensory processing compatible with the slow behavior of mice during early postnatal development.

New insights into corollary discharges mediated by identified neural pathways

Sensory systems respond not only to stimuli from the environment but also to cues generated by an animal's own behaviour. This leads to problems of sensory processing because self-generated information can occur at the same time as external sensory information. However, in motor regions of the CNS corollary discharges are generated during behaviour. These signals are not used to generate movements directly but, instead, interact with the processing of self-generated sensory signals. Corollary discharges transiently modulate self-generated sensory responses and can prevent self-induced desensitization or help distinguish between self-generated and externally generated sensory information. Here, we review recent work that has identified corollary discharge pathways at different levels of the CNS in vertebrates and invertebrates.

The cellular basis of a corollary discharge

How do animals discriminate self-generated from external stimuli during behavior and prevent desensitization of their sensory pathways? A fundamental concept in neuroscience states that neural signals, termed corollary discharges or efference copies, are forwarded from motor to sensory areas. Neurons mediating these signals have proved difficult to identify. We show that a single, multisegmental interneuron is responsible for the pre- and postsynaptic inhibition of auditory neurons in singing crickets (Gryllus bimaculatus). Therefore, this neuron represents a corollary discharge interneuron that provides a neuronal basis for the central control of sensory responses.

Corollary discharge inhibition and audition in the stridulating cricket

The romantic notion of crickets singing on a warm summer's evening is quickly dispelled when one comes ear to ear with a stridulating male. Remarkably, stridulating male crickets are able to hear sounds from the environment despite generating a 100 db song (Heiligenberg 1969; Jones and Dambach 1973). This review summarises recent work examining how they achieve this feat of sensory processing. While the responsiveness of the crickets' peripheral auditory system (tympanic membrane, tympanic nerve, state of the acoustic spiracle) is maintained during sound production, central auditory neurons are inhibited by a feedforward corollary discharge signal precisely timed to coincide with the auditory neurons' maximum response to self-generated sound. In this way, the corollary discharge inhibition prevents desensitisation of the crickets' auditory pathway during sound production.

Auditory orientation in crickets: pattern recognition controls reactive steering

Many groups of insects are specialists in exploiting sensory cues to locate food resources or conspecifics. To achieve orientation, bees and ants analyze the polarization pattern of the sky, male moths orient along the females' odor plume, and cicadas, grasshoppers, and crickets use acoustic signals to locate singing conspecifics. In comparison with olfactory and visual orientation, where learning is involved, auditory processing underlying orientation in insects appears to be more hardwired and genetically determined. In each of these examples, however, orientation requires a recognition process identifying the crucial sensory pattern to interact with a localization process directing the animal's locomotor activity. Here, we characterize this interaction. Using a sensitive trackball system, we show that, during cricket auditory behavior, the recognition process that is tuned toward the species-specific song pattern controls the amplitude of auditory evoked steering responses. Females perform small reactive steering movements toward any sound patterns. Hearing the male's calling song increases the gain of auditory steering within 2-5 s, and the animals even steer toward nonattractive sound patterns inserted into the species-specific pattern. This gain control mechanism in the auditory-to-motor pathway allows crickets to pursue species-specific sound patterns temporarily corrupted by environmental factors and may reflect the organization of recognition and localization networks in insects.

Mechanisms underlying phonotactic steering in the cricket Gryllus bimaculatus revealed with a fast trackball system

Phonotactic steering behaviour of the cricket G. bimaculatus was analysed with a new highly sensitive trackball system providing a spatial and temporal resolution of 127 microm and 0.3 ms, respectively. Orientation to artificial calling songs started at 45 dB SPL, it increased up to 75 dB SPL and then saturated. When exposed to two identical patterns of different intensity, crickets significantly steered towards the louder sound pattern, whenever the intensity difference was greater than 1 dB. Bilateral latency differences in sound presentation did not always cause clear orientation towards the leading side. The overall walking direction depended on the number of sound pulses perceived from the left or right side with the animals turning towards the side providing the larger number of pulses. The recordings demonstrated rapid changes in walking direction performed even during a chirp. These rapid steering responses occurred with a latency of 55-60 ms, well before the central nervous system had time to evaluate the temporal structure of a whole chirp. When every other sound pulse was presented from opposite directions, the crickets followed the temporal pattern of sound presentation and rapidly steered towards the left and right side. Steering towards individual sound pulses does not agree with the proposal that crickets analyse the quality of sound patterns and then steer towards the better pattern. Rather, these experiments suggest that fast steering to single sound pulses determines the lateral deviation of the animals and that complex auditory orientation emerges from this simple mechanism of auditory steering.

Complex auditory behaviour emerges from simple reactive steering

The recognition and localization of sound signals is fundamental to acoustic communication. Complex neural mechanisms are thought to underlie the processing of species-specific sound patterns even in animals with simple auditory pathways. In female crickets, which orient towards the male's calling song, current models propose pattern recognition mechanisms based on the temporal structure of the song. Furthermore, it is thought that localization is achieved by comparing the output of the left and right recognition networks, which then directs the female to the pattern that most closely resembles the species-specific song. Here we show, using a highly sensitive method for measuring the movements of female crickets, that when walking and flying each sound pulse of the communication signal releases a rapid steering response. Thus auditory orientation emerges from reactive motor responses to individual sound pulses. Although the reactive motor responses are not based on the song structure, a pattern recognition process may modulate the gain of the responses on a longer timescale. These findings are relevant to concepts of insect auditory behaviour and to the development of biologically inspired robots performing cricket-like auditory orientation.

Temporal pattern recognition based on instantaneous spike rate coding in a simple auditory system

Auditory pattern recognition by the CNS is a fundamental process in acoustic communication. Because crickets communicate with stereotyped patterns of constant frequency syllables, they are established models to investigate the neuronal mechanisms of auditory pattern recognition. Here we provide evidence that for the neural processing of amplitude-modulated sounds, the instantaneous spike rate rather than the time-averaged neural activity is the appropriate coding principle by comparing both coding parameters in a thoracic interneuron (Omega neuron ON1) of the cricket (Gryllus bimaculatus) auditory system. When stimulated with different temporal sound patterns, the analysis of the instantaneous spike rate demonstrates that the neuron acts as a low-pass filter for syllable patterns. The instantaneous spike rate is low at high syllable rates, but prominent peaks in the instantaneous spike rate are generated as the syllable rate resembles that of the species-specific pattern. The occurrence and repetition rate of these peaks in the neuronal discharge are sufficient to explain temporal filtering in the cricket auditory pathway as they closely match the tuning of phonotactic behavior to different sound patterns. Thus temporal filtering or "pattern recognition" occurs at an early stage in the auditory pathway.

Corollary discharge inhibition of ascending auditory neurons in the stridulating cricket

Acoustically communicating animals are able to process external acoustic stimuli despite generating intense sounds during vocalization. We have examined how the crickets' ascending auditory pathway copes with self-generated, intense auditory signals (chirps) during singing (stridulation). We made intracellular recordings from two identified ascending auditory interneurons, ascending neuron 1 (AN1) and ascending neuron 2 (AN2), during pharmacologically elicited sonorous (two-winged), silent (one-winged), and fictive (isolated CNS) stridulation. During sonorous chirps, AN1 responded with bursts of spikes, whereas AN2 was inhibited and rarely spiked. Low-amplitude hyperpolarizing potentials were recorded in AN1 and AN2 during silent chirps. The potentials were also present during fictive chirps. Therefore, they were the result of a centrally generated corollary discharge from the stridulatory motor network. The spiking response of AN1 and AN2 to acoustic stimuli was inhibited during silent and fictive chirps. The maximum period of inhibition occurred in phase with the maximum spiking response to self-generated sound in a sonorously stridulating cricket. In some experiments (30%) depolarizing potentials were recorded during silent chirps. Reafferent feedback elicited by wing movement was probably responsible for the depolarizing potentials. In addition, two other sources of inhibition were present in AN1: (1) IPSPs were elicited by stimulation with 12.5 kHz stimuli and (2) a long-lasting hyperpolarization followed spiking responses to 4.5 kHz stimuli. The hyperpolarization desensitized the response of AN1 to subsequent quieter stimuli. Therefore, the corollary discharge will reduce desensitization by suppressing the response of AN1 to self-generated sounds.

A corollary discharge mechanism modulates central auditory processing in singing crickets

Crickets communicate using loud (100 dB SPL) sound signals that could adversely affect their own auditory system. To examine how they cope with this self-generated acoustic stimulation, intracellular recordings were made from auditory afferent neurons and an identified auditory interneuron-the Omega 1 neuron (ON1)-during pharmacologically elicited singing (stridulation). During sonorous stridulation, the auditory afferents and ON1 responded with bursts of spikes to the crickets' own song. When the crickets were stridulating silently, after one wing had been removed, only a few spikes were recorded in the afferents and ON1. Primary afferent depolarizations (PADs) occurred in the terminals of the auditory afferents, and inhibitory postsynaptic potentials (IPSPs) were apparent in ON1. The PADs and IPSPs were composed of many summed, small-amplitude potentials that occurred at a rate of about 230 Hz. The PADs and the IPSPs started during the closing wing movement and peaked in amplitude during the subsequent opening wing movement. As a consequence, during silent stridulation, ON1's response to acoustic stimuli was maximally inhibited during wing opening. Inhibition coincides with the time when ON1 would otherwise be most strongly excited by self-generated sounds in a sonorously stridulating cricket. The PADs and the IPSPs persisted in fictively stridulating crickets whose ventral nerve cord had been isolated from muscles and sense organs. This strongly suggests that the inhibition of the auditory pathway is the result of a corollary discharge from the stridulation motor network. The central inhibition was mimicked by hyperpolarizing current injection into ON1 while it was responding to a 100 dB SPL sound pulse. This suppressed its spiking response to the acoustic stimulus and maintained its response to subsequent, quieter stimuli. The corollary discharge therefore prevents auditory desensitization in stridulating crickets and allows the animals to respond to external acoustic signals during the production of calling song.

A corollary discharge maintains auditory sensitivity during sound production

Speaking and singing present the auditory system of the caller with two fundamental problems: discriminating between self-generated and external auditory signals and preventing desensitization. In humans and many other vertebrates, auditory neurons in the brain are inhibited during vocalization but little is known about the nature of the inhibition. Here we show, using intracellular recordings of auditory neurons in the singing cricket, that presynaptic inhibition of auditory afferents and postsynaptic inhibition of an identified auditory interneuron occur in phase with the song pattern. Presynaptic and postsynaptic inhibition persist in a fictively singing, isolated cricket central nervous system and are therefore the result of a corollary discharge from the singing motor network. Mimicking inhibition in the interneuron by injecting hyperpolarizing current suppresses its spiking response to a 100-dB sound pressure level (SPL) acoustic stimulus and maintains its response to subsequent, quieter stimuli. Inhibition by the corollary discharge reduces the neural response to self-generated sound and protects the cricket's auditory pathway from self-induced desensitization.