Triple electroantennography captures the range and spatial arrangement of olfactory sensory neuron response on an insect antenna.

BACKGROUND: Electroantennography (EAG) is a basic neuroscientific tool that is widely used to measure olfactory responses in insects. It is particularly adapted to probing the olfactory systems of non-model insect species in chemical ecology and evolutionary biology. As currently practiced, EAG measures have varying degrees of correlation with olfactory responses, especially for insects whose olfactory sensory neurons (OSNs) are arranged in zones on the antennae. This limitation was shown to be partly due to the fact that only a single antennal position was recorded. NEW METHODS: We describe a setup using triple electroantennography (EAG3), whereby three antennal positions are recorded simultaneously. The spatial arrangement of the electrodes ensures the mechanical stability of the assembly. The EAG3 detector was coupled to a gas chromatograph (GC-EAD3), customized using a Dean's switch to improve the EAG signals by chopper modulation. EAG3 signals were analysed through a current point model to estimate olfactory responses across the antenna. RESULTS: Recordings were performed on Tephritidae and Drosophila species, which have antennae of different shapes and sizes. We confirmed that the spatio-temporal pattern of antennal activation was stimulus dependent and allowed us to quantify the antennal olfactory response. COMPARISONS WITH EXISTING METHOD: Compared to typical single-probe EAG, we show that EAG3 improves response quantification and increases the range of compounds for which a sensory response is detected. CONCLUSIONS: Our EAG3 setup is an original low-cost and easy-to-use method. It offers a useful bridge between comprehensive neurophysiological investigations and the broader themes explored in chemical ecology.

Few Sensory Cues Differentiate Host and Dead-End Trap Plant for the Sugarcane Spotted Borer Chilo sacchariphagus (Lepidoptera: Crambidae).

The use of Erianthus arundinaceus as a trap plant in association with sugarcane reduces populations of the spotted sugarcane stalk borer Chilo sacchariphagus (Bojer) (Lepidoptera: Crambidae). This grass acts as a dead-end trap crop because it is the preferred plant for oviposition relative to sugarcane, and it precludes larval development. We explored the chemical mechanisms involved in host choice by C. sacchariphagus. We showed that the insect's antennal receptors are particularly sensitive to the shared compounds found in the volatile emissions produced by sugarcane and E. arundinaceus. In accordance with their phylogenetic proximity, the two plant species share many physicochemical properties, which suggests that C. sacchariphagus has few sensory cues to differentiate between the two. The terpene (E)-ß-ocimene is constitutively emitted by E. arundinaceus, but not by sugarcane. It elicits an electroantennographic response and behavioral responses from female C. sacchariphagus in Y-tube bioassays. Our study suggests that the sensory confusion between host plants, combined with a marginal sensory difference orienting the choice of an egg-laying site, constitutes a mechanism that is relevant to trap cropping. Systems based on this type of mechanism could provide long-term protection for crops vulnerable to insect pests.

Moth olfactory receptor neurons adjust their encoding efficiency to temporal statistics of pheromone fluctuations.

The efficient coding hypothesis predicts that sensory neurons adjust their coding resources to optimally represent the stimulus statistics of their environment. To test this prediction in the moth olfactory system, we have developed a stimulation protocol that mimics the natural temporal structure within a turbulent pheromone plume. We report that responses of antennal olfactory receptor neurons to pheromone encounters follow the temporal fluctuations in such a way that the most frequent stimulus timescales are encoded with maximum accuracy. We also observe that the average coding precision of the neurons adjusted to the stimulus-timescale statistics at a given distance from the pheromone source is higher than if the same encoding model is applied at a shorter, non-matching, distance. Finally, the coding accuracy profile and the stimulus-timescale distribution are related in the manner predicted by the information theory for the many-to-one convergence scenario of the moth peripheral sensory system.

Current Source Density Analysis of Electroantennogram Recordings: A Tool for Mapping the Olfactory Response in an Insect Antenna.

The set of chemosensory receptors expressed by the olfactory receptor neurons lying in an insect's antennae and maxillary palps define the ability of this insect to perceive the volatile chemicals of its environment. The main two electrophysiological methods of antennal recordings for studying the range of chemicals that activate chemosensory receptors have limitations. Single-sensillum recording (SSR) samples a subset of olfactory receptor neurons and therefore does not reveal the full capacity of an insect to perceive an odor. Electroantennography (EAG), even if less resolutive than SSRs, is sometimes preferred since it samples the activity of a large number of the olfactory receptor neurons. But, at least in flies, the amplitude of the EAG signal is not directly correlated with the degree of sensitivity of the insect to the olfactory compound. Such dual methodology was also used to study mammalian brains, and the current source density (CSD) analysis was developed to bridge the gap between the cellular and the population recordings. This paper details the use of a similar approach adapted to the study of olfactory responses within insects with bulbous antennae. The EAG was recorded at multiple antennal positions and the CSD that generates the EAG potentials were estimated. The method measures the activation of olfactory receptor neurons (ORNs) across the antennae and thus it quantifies the olfactory sensitivity of the insect. It allows a rapid mapping of olfactory responses and thus can be used to guide further SSRs or to determine that two chemicals are detected by independent ORNs. This study further explored biases resulting from a limited number of recording positions or from an approximation of the antennal geometry that should be considered for interpreting the CSD maps. It also shows that the CSD analysis of EAGs is compatible with a gas chromatograph stimulator for analyzing the response to complex odors. Finally, I discuss the origin of the EAG signal in light of the CSD theory.

Olfactory coding in the turbulent realm.

Long-distance olfactory search behaviors depend on odor detection dynamics. Due to turbulence, olfactory signals travel as bursts of variable concentration and spacing and are characterized by long-tail distributions of odor/no-odor events, challenging the computing capacities of olfactory systems. How animals encode complex olfactory scenes to track the plume far from the source remains unclear. Here we focus on the coding of the plume temporal dynamics in moths. We compare responses of olfactory receptor neurons (ORNs) and antennal lobe projection neurons (PNs) to sequences of pheromone stimuli either with white-noise patterns or with realistic turbulent temporal structures simulating a large range of distances (8 to 64 m) from the odor source. For the first time, we analyze what information is extracted by the olfactory system at large distances from the source. Neuronal responses are analyzed using linear-nonlinear models fitted with white-noise stimuli and used for predicting responses to turbulent stimuli. We found that neuronal firing rate is less correlated with the dynamic odor time course when distance to the source increases because of improper coding during long odor and no-odor events that characterize large distances. Rapid adaptation during long puffs does not preclude however the detection of puff transitions in PNs. Individual PNs but not individual ORNs encode the onset and offset of odor puffs for any temporal structure of stimuli. A higher spontaneous firing rate coupled to an inhibition phase at the end of PN responses contributes to this coding property. This allows PNs to decode the temporal structure of the odor plume at any distance to the source, an essential piece of information moths can use in their tracking behavior.

Current source density mapping of antennal sensory selectivity reveals conserved olfactory systems between tephritids and Drosophila.

Ecological specialization of insects involves the functional and morphological reshaping of olfactory systems. Little is known about the degree to which insect sensitivity to odorant compounds is conserved between genera, tribes, or families. Here we compared the olfactory systems of six tephritid fruit fly species spanning two tribes and the distantly related Drosophila melanogaster at molecular, functional, and morphological levels. Olfaction in these flies is mediated by a set of olfactory receptors (ORs) expressed in different functional classes of neurons located in distinct antennal regions. We performed a phylogenetic analysis that revealed both family-specific OR genes and putative orthologous OR genes between tephritids and Drosophila. With respect to function, we then used a current source density (CSD) analysis to map activity across antennae. Functional maps mirrored the intrinsic structure of antennae observed with scanning electron microscopy. Together, the results revealed partial conservation of the olfactory systems between tephritids and Drosophila. We also demonstrate that the mapping of olfactory responses is necessary to decipher antennal sensory selectivity to olfactory compounds. CSD analysis can be easily applied to map antennae of other species and therefore enables the rapid deriving of olfactory maps and the reconstructing of the target organisms' history of evolution.

Whisker row deprivation affects the flow of sensory information through rat barrel cortex.

Whisker trimming causes substantial reorganization of neuronal response properties in barrel cortex. However, little is known about experience-dependent rerouting of sensory processing following sensory deprivation. To address this, we performed in vivo intracellular recordings from layers 2/3 (L2/3), layer 4 (L4), layer 5 regular-spiking (L5RS), and L5 intrinsically bursting (L5IB) neurons and measured their multiwhisker receptive field at the level of spiking activity, membrane potential, and synaptic conductance before and after sensory deprivation. We used Chernoff information to quantify the "sensory information" contained in the firing patterns of cells in response to spared and deprived whisker stimulation. In the control condition, information for flanking-row and same-row whiskers decreased in the order L4, L2/3, L5IB, L5RS. However, after whisker-row deprivation, spared flanking-row whisker information was reordered to L4, L5RS, L5IB, L2/3. Sensory information from the trimmed whiskers was reduced and delayed in L2/3 and L5IB neurons, whereas sensory information from spared whiskers was increased and advanced in L4 and L5RS neurons. Sensory information from spared whiskers was increased in L5IB neurons without a latency change. L5RS cells exhibited the largest changes in sensory information content through an atypical plasticity combining a significant decrease in spontaneous activity and an increase in a short-latency excitatory conductance. NEW & NOTEWORTHY: Sensory cortical plasticity is usually quantified by changes in evoked firing rate. In this study we quantified plasticity by changes in sensory detection performance using Chernoff information and receiver operating characteristic analysis. We found that whisker deprivation causes a change in information flow within the cortical layers and that layer 5 regular-spiking cells, despite showing only a small potentiation of short-latency input, show the greatest increase in information content for the spared input partly by decreasing their spontaneous activity.

Firing and intrinsic properties of antennal lobe neurons in the Noctuid moth Agrotis ipsilon.

The antennal lobe (AL) of the Noctuid moth Agrotis ipsilon has emerged as an excellent model for studying olfactory processing and its plasticity in the central nervous system. Odor-evoked responses of AL neurons and input-to-output transformations involved in pheromone processing are well characterized in this species. However, the intrinsic electrical properties responsible of the firing of AL neurons are poorly known. To this end, patch-clamp recordings in current- and voltage-clamp mode from neurons located in the two main clusters of cell bodies in the ALs were combined with intracellular staining on A. ipsilon males. Staining indicated that the lateral cluster (LC) is composed of 85% of local neurons (LNs) and 15% of projection neurons (PNs). The medial cluster (MC) contains only PNs. Action potentials were readily recorded from the soma in LNs and PNs located in the LC but not from PNs in the MC where recordings showed small or no action potentials. In the LC, the spontaneous activity of about 20% of the LNs presented irregular bursts while being more regular in PNs. We also identified a small population of LNs lacking voltage-gated Na(+) currents and generating spikelets. We focused on the firing properties of LNs since in about 60% of LNs, but not in PNs, action potentials were followed by depolarizing afterpotentials (DAPs). These DAPs could generate a second action potential, so that the activity was composed of action potential doublets. DAPs depended on voltage, Ca(2+)-channels and possibly on Ca(2+)-activated non-specific cationic channels. During steady state current injection, DAPs occurred after each action potential and did not require high-frequency firing. The amplitude of DAPs increased when the interspike interval was small, typically within bursts, likely arising from a Ca(2+) build up. DAPs were more often found in bursting than in non-bursting LNs but do not support bursting activity. DAPs and spike doublets also occurred during odor-evoked activity suggesting that they can mediate olfactory integration in the AL.

Regular spiking and intrinsic bursting pyramidal cells show orthogonal forms of experience-dependent plasticity in layer V of barrel cortex.

Most functional plasticity studies in the cortex have focused on layers (L) II/III and IV, whereas relatively little is known of LV. Structural measurements of dendritic spines in vivo suggest some specialization among LV cell subtypes. We therefore studied experience-dependent plasticity in the barrel cortex using intracellular recordings to distinguish regular spiking (RS) and intrinsic bursting (IB) subtypes. Postsynaptic potentials and suprathreshold responses in vivo revealed a remarkable dichotomy in RS and IB cell plasticity; spared whisker potentiation occurred in IB but not RS cells while deprived whisker depression occurred in RS but not IB cells. Similar RS/IB differences were found in the LII/III to V connections in brain slices. Modeling studies showed that subthreshold changes predicted the suprathreshold changes. These studies demonstrate the major functional partition of plasticity within a single cortical layer and reveal the LII/III to LV connection as a major excitatory locus of cortical plasticity.

Spatial structure of multiwhisker receptive fields in the barrel cortex is stimulus dependent.

The tactile sensations mediated by the whisker-trigeminal system allow rodents to efficiently detect and discriminate objects. These capabilities rely strongly on the temporal and spatial structure of whisker deflections. Subthreshold but also spiking receptive fields in the barrel cortex encompass a large number of vibrissae, and it seems likely that the functional properties of these multiwhisker receptive fields reflect the multiple-whisker interactions encountered by the animal during exploration of its environment. The aim of this study was to examine the dependence of the spatial structure of cortical receptive fields on stimulus parameters. Using a newly developed 24-whisker stimulation matrix, we applied a forward correlation analysis of spiking activity to randomized whisker deflections (sparse noise) to characterize the receptive fields that result from caudal and rostral directions of whisker deflection. We observed that the functionally determined principal whisker, the whisker eliciting the strongest response with the shortest latency, differed according to the direction of whisker deflection. Thus, for a given neuron, maximal responses to opposite directions of whisker deflections could be spatially separated. This spatial separation resulted in a displacement of the center of mass between the rostral and caudal subfields and was accompanied by differences between response latencies in rostral and caudal directions of whisker deflection. Such direction-dependent receptive field organization was observed in every cortical layer. We conclude that the spatial structure of receptive fields in the barrel cortex is not an intrinsic property of the neuron but depends on the properties of sensory input.

The Matrix: a new tool for probing the whisker-to-barrel system with natural stimuli.

The whisker to barrel system in rodents has become one of the major models for the study of sensory processing. Several tens of whiskers (or vibrissae) are distributed in a regular manner on both sides of the snout. Many tactile discrimination tasks using this system need multiple contacts with more than one whisker to be solved. With the aim of mimicking those multi-whisker stimuli during electrophysiological recordings, we developed a novel mechanical stimulator composed of 24 independent multi-directional piezoelectric benders adapted to the five rows and the five caudal arcs of the rat whisker pad. The most widely used technology for producing mechanical deflections of the whiskers is based on piezoelectric benders that display a non-linear behavior when driven with high frequency input commands and, if not compensated, show high unwanted ringing at particular resonance frequencies. If not corrected, this non-linear behavior precludes the application of high frequency deflections and the study of cortical responses to behaviorally relevant stimuli. To cope with the ringing problem, a mechanical and a software based solutions have been developed. With these corrections, the upper bound of the linear range of the bender is increased to 1 kHz. This new device allows the controlled delivery of large scale natural patterns of whisker deflections characterized by rapid high frequency vibrations of multiple whiskers.

Spike-timing-dependent plasticity in the intact brain: counteracting spurious spike coincidences.

A computationally rich algorithm of synaptic plasticity has been proposed based on the experimental observation that the sign and amplitude of the change in synaptic weight is dictated by the temporal order and temporal contiguity between pre- and postsynaptic activities. For more than a decade, this spike-timing-dependent plasticity (STDP) has been studied mainly in brain slices of different brain structures and cultured neurons. Although not yet compelling, evidences for the STDP rule in the intact brain, including primary sensory cortices, have been provided lastly. From insects to mammals, the presentation of precisely timed sensory inputs drives synaptic and functional plasticity in the intact central nervous system, with similar timing requirements than the in vitro defined STDP rule. The convergent evolution of this plasticity rule in species belonging to so distant phylogenic groups points to the efficiency of STDP, as a mechanism for modifying synaptic weights, as the basis of activity-dependent development, learning and memory. In spite of the ubiquity of STDP phenomena, a number of significant variations of the rule are observed in different structures, neuronal types and even synapses on the same neuron, as well as between in vitro and in vivo conditions. In addition, the state of the neuronal network, its ongoing activity and the activation of ascending neuromodulatory systems in different behavioral conditions have dramatic consequences on the expression of spike-timing-dependent synaptic plasticity, and should be further explored.

Emergent properties of tactile scenes selectively activate barrel cortex neurons.

Rats discriminate objects by scanning their surface with the facial vibrissae, producing spatiotemporally complex sequences of tactile contacts. The way in which the somatosensory cortex responds to these complex multivibrissal stimuli has not been explored. It is unclear yet whether contextual information from across the entire whisker pad influences cortical responses. Here, we delivered tactile stimuli to the rat vibrissae using a new 24 whisker stimulator. We tested sequences of rostrocaudal whisker deflections that generate multivibrissal motion patterns in different directions across the mystacial pad, allowing to disambiguate local from global sensory integration. Unitary electrophysiological recordings from different layers of the barrel cortex showed that a majority of neurons has direction selectivity for the multivibrissal stimulus. The selectivity resulted from nonlinear integration of responses across the mystacial pad. Our results indicate that the system extracts collective properties of a tactile scene.

Spike timing-dependent synaptic depression in the in vivo barrel cortex of the rat.

Spike timing-dependent plasticity (STDP) is a computationally powerful form of plasticity in which synapses are strengthened or weakened according to the temporal order and precise millisecond-scale delay between presynaptic and postsynaptic spiking activity. STDP is readily observed in vitro, but evidence for STDP in vivo is scarce. Here, we studied spike timing-dependent synaptic depression in single putative pyramidal neurons of the rat primary somatosensory cortex (S1) in vivo, using two techniques. First, we recorded extracellularly from layer 2/3 (L2/3) and L5 neurons, and paired spontaneous action potentials (postsynaptic spikes) with subsequent subthreshold deflection of one whisker (to drive presynaptic afferents to the recorded neuron) to produce "post-leading-pre" spike pairings at known delays. Short delay pairings (<17 ms) resulted in a significant decrease of the extracellular spiking response specific to the paired whisker, consistent with spike timing-dependent synaptic depression. Second, in whole-cell recordings from neurons in L2/3, we paired postsynaptic spikes elicited by direct-current injection with subthreshold whisker deflection to drive presynaptic afferents to the recorded neuron at precise temporal delays. Post-leading-pre pairing (<33 ms delay) decreased the slope and amplitude of the PSP evoked by the paired whisker, whereas "pre-leading-post" delays failed to produce depression, and sometimes produced potentiation of whisker-evoked PSPs. These results demonstrate that spike timing-dependent synaptic depression occurs in S1 in vivo, and is therefore a plausible plasticity mechanism in the sensory cortex.

Spatiotemporal characteristics of neuronal sensory integration in the barrel cortex of the rat.

In primary sensory cortices, neuronal responses to a stimulus presented as part of a rapid sequence often differ from responses to an isolated stimulus. It has been reported that sequential stimulation of two whiskers produces facilitatory modulations of barrel cortex neuronal responses. These results are at odds with the well-known suppressive interaction that has been usually described. Herein, we have examined the dependency of response modulation on the spatiotemporal pattern of stimulation by varying the spatial arrangement of the deflected vibrissae, the temporal frequency of stimulation, and the time interval between whisker deflections. Extracellular recordings were made from primary somatosensory cortex of anesthetized rats. Two contralateral whiskers were stimulated at 0.5 and 8 Hz at intervals ranging from 0 to +/-30 ms. Response interactions were assessed during stimulation of the principal and adjacent whiskers, first from the same row and second from the same arc. When tested at 0.5 Hz, 59% of single units showed a statistically significant suppressive interaction, whereas response facilitation was found in only 6% of cells. In contrast, at 8 Hz, a significant supralinear summation was observed in 19% of the cells, particularly for stimulations along an arc rather than along a row. Multi-unit recordings showed similar results. These observations indicate that most of the interactions in the barrel cortex during two-whisker stimulation are suppressive. However, facilitation can be revealed when stimuli are applied at a physiological frequency and could be the basis for internal representations of the spatiotemporal pattern of the stimulus.