Odor sampling strategies in mice with genetically altered olfactory responses

Peripheral sensory cells and the central neuronal circuits that monitor environmental changes to drive behaviors should be adapted to match the behaviorally relevant kinetics of incoming stimuli, be it the detection of sound frequencies, the speed of moving objects or local temperature changes. Detection of odorants begins with the activation of olfactory receptor neurons in the nasal cavity following inhalation of air and airborne odorants carried therein. Thus, olfactory receptor neurons are stimulated in a rhythmic and repeated fashion that is determined by the breathing or sniffing frequency that can be controlled and altered by the animal. This raises the question of how the response kinetics of olfactory receptor neurons are matched to the imposed stimulation frequency and if, vice versa, the kinetics of olfactory receptor neuron responses determine the sniffing frequency. We addressed this question by using a mouse model that lacks the K+-dependent Na+/Ca2+ exchanger 4 (NCKX4), which results in markedly slowed response termination of olfactory receptor neuron responses and hence changes the temporal response kinetics of these neurons. We monitored sniffing behaviors of freely moving wildtype and NCKX4 knockout mice while they performed olfactory Go/NoGo discrimination tasks. Knockout mice performed with similar or, surprisingly, better accuracy compared to wildtype mice, but chose, depending on the task, different odorant sampling durations depending on the behavioral demands of the odorant identification task. Similarly, depending on the demands of the behavioral task, knockout mice displayed a lower basal breathing frequency prior to odorant sampling, a possible mechanism to increase the dynamic range for changes in sniffing frequency during odorant sampling. Overall, changes in sniffing behavior between wildtype and NCKX4 knockout mice were subtle, suggesting that, at least for the particular odorant-driven task we used, slowed response termination of the odorant-induced receptor neuron response either has a limited detrimental effect on odorant-driven behavior or mice are able to compensate via an as yet unknown mechanism.

Salty surfaces deter feeding in a blood-sucking disease vector

Salts are essential nutrients required for many physiological processes, and deficient or excessive salt results in adverse health problems. Taste is the ultimate sensory modality involved in resource quality assessment, resulting in acceptance or rejection. Here, we show that detection of high-salt substrates by a salt-sensitive antennal gustatory receptor neuron, S1-GRN, results in feeding avoidance in the hematophagous bug Rhodnius prolixus. Knock-down of two antennal-expressed amiloride-sensitive pickpocket channel receptors (PPKs; RproPPK014276 and RproPPK28) using RNA interference, prevents avoidance of bugs to high-salt substrates. Tracing antennal GRNs to the central nervous system reveals the antennal lobes as a gustatory processing center. The identification of the gustatory basis of high-salt detection in a blood feeder provides novel targets to prevent biting and feeding, as well as to promote substrate avoidance in a relevant disease vector.Significance StatementDetection of aversive gustatory stimuli induces avoidance responses in animals. Avoidance acquires particular interest if it reduces the biting rates of blood-feeding insects of medical relevance. Here we describe the molecular and physiological basis of high-salt detection in the blood-sucking disease vector Rhodnius prolixus. We show that detection of high-salt substrates through two PPK receptors expressed in an antennal gustatory receptor neuron produces feeding avoidance. Understanding these gustatory-driven aversive responses allows the hitherto overlooked use of gustatory molecules as a complement to known olfactory repellents.

Analysis of a Signal Transmission in a Pair of Izhikevich Coupled Neurons

Individual neuron models give a comprehensive explanation of the behavior of the electrical potential of cell membranes. These models were and are a source of constant analysis to understand the functioning of, mainly, the complexity of the brain. In this work, using the Izhikevich model, we propose, analyze and characterize the transmission of a signal between two neurons unidirectionally coupled. Two possible states were characterized (sub-threshold and over-threshold) depending on the values of the signal amplitude, as well also the relationship between the transmitted and received signal taking into account the coupling. Furthermore, the activation of the emitting neuron (its transition from a resting state to spiking state) and the transmission to the receptor neuron were analyzed by adding white noise to the system.

Multi-glomerular projection of single olfactory receptor neurons is conserved among amphibians

Individual receptor neurons in the peripheral olfactory organ extend long axons into the olfactory bulb forming synapses with projection neurons in spherical neuropil regions, called glomeruli. Generally, odor map formation and odor processing in all vertebrates is based on the assumption that receptor neuron axons exclusively connect to a single glomerulus without any axonal branching. We comparatively tested this hypothesis in multiple fish and amphibian species by applying sparse cell electroporation to trace single olfactory receptor neuron axons. Sea lamprey (jawless fish) and zebrafish (bony fish) support the unbranched axon concept, with 94% of axons terminating in single glomeruli. Contrastingly, axonal projections of the axolotl (salamander) branch extensively before entering up to six distinct glomeruli. Receptor neuron axons labeled in frog species (Pipidae, Bufonidae, Hylidae and Dendrobatidae) predominantly bifurcate before entering a glomerulus and 59% and 50% connect to multiple glomeruli in larval and post-metamorphotic animals, respectively. Independent of developmental stage, lifestyle and adaptations to specific habitats, it seems to be a common feature of amphibian olfactory receptor neuron axons to frequently bifurcate and connect to multiple glomeruli. Our study challenges the unbranched axon concept as a universal vertebrate feature and it is conceivable that also later diverging vertebrates deviate from it. We propose that this unusual wiring logic evolved around the divergence of the terrestrial tetrapod lineage from its aquatic ancestors and could be the basis of an alternative way of odor processing.Abstract Figure

Suppression of ciliary movements by a hypertonic stress in the newt olfactory receptor neuron

Olfactory receptor neurons isolated from the newt maintain a high activity of the ciliary beat. A cilium of neuron is so unique that only little is known about regulatory factors for its beat frequency. We examined the olfactory receptor neuron immersed in various extracellular media under the video-enhanced differential interference contrast microscope. The activation of voltage-gated Ca2+ channels by K+ depolarization or by application of Ca2+ to membrane-permeabilized olfactory cells did not affect the ciliary movement, suggesting that Ca2+ influx through the cell membrane has no direct effect on the movement. However, when an extracellular medium contained NaCl or sucrose at concentrations only 30% higher than normal levels, ciliary movement was greatly and reversibly suppressed. In contrast, a hypotonic solution of such a solute did not change the ciliary movement. The hypertonic solutions had no effect when applied to permeabilized cells. Suction of the cell membrane with a patch pipette easily suppressed the ciliary movement in an isotonic medium. Application of positive pressure inside the cell through the same patch pipette eliminated the suppressive effect. From these findings, we concluded that the hypertonic stress suppressed the ciliary movement not by disabling the motor proteins, microtubules, or their associates in the cilia, but rather by modifying the chemical environment for the motor proteins. The ciliary motility of the olfactory receptor cell is directly sensitive to the external environment, namely, the air or water on the nasal epithelium, depending on lifestyle of the animal.