Processing of Intraoral Olfactory and Gustatory Signals in the Gustatory Cortex of Awake Rats.

The integration of gustatory and olfactory information is essential to the perception of flavor. Human neuroimaging experiments have pointed to the gustatory cortex (GC) as one of the areas involved in mediating flavor perception. Although GC's involvement in encoding the chemical identity and hedonic value of taste stimuli is well studied, it is unknown how single GC neurons process olfactory stimuli emanating from the mouth. In this study, we relied on multielectrode recordings to investigate how single GC neurons respond to intraorally delivered tastants and tasteless odorants dissolved in water and whether/how these two modalities converge in the same neurons. We found that GC neurons could either be unimodal, responding exclusively to taste (taste-only) or odor (odor-only), or bimodal, responding to both gustatory and olfactory stimuli. Odor responses were confirmed to result from retronasal olfaction: monitoring respiration revealed that exhalation preceded odor-evoked activity and reversible inactivation of olfactory receptors in the nasal epithelium significantly reduced responses to intraoral odorants but not to tastants. Analysis of bimodal neurons revealed that they encode palatability significantly better than the unimodal taste-only group. Bimodal neurons exhibited similar responses to palatable tastants and odorants dissolved in water. This result suggested that odorized water could be palatable. This interpretation was further supported with a brief access task, where rats avoided consuming aversive taste stimuli and consumed the palatable tastants and dissolved odorants. These results demonstrate the convergence of the chemosensory components of flavor onto single GC neurons and provide evidence for the integration of flavor with palatability coding. SIGNIFICANCE STATEMENT: Food perception and choice depend upon the concurrent processing of olfactory and gustatory signals from the mouth. The primary gustatory cortex has been proposed to integrate chemosensory stimuli; however, no study has examined the single-unit responses to intraoral odorant presentation. Here we found that neurons in gustatory cortex can respond either exclusively to tastants, exclusively to odorants, or to both (bimodal). Several differences exist between these groups' responses; notably, bimodal neurons code palatability significantly better than unimodal neurons. This group of neurons might represent a substrate for how odorants gain the quality of tastants.

Olfaction in dentistry.

Practitioners of oral medicine frequently encounter patients with complaints of taste disturbance. While some such complaints represent pathological processes specific to the gustatory system, per se, this is rarely the case. Unless taste-bud mediated qualities such as sweet, sour, bitter, salty, umami, chalky, or metallic are involved, 'taste' dysfunction inevitably reflects damage to the sense of smell. Such 'taste' sensations as chicken, chocolate, coffee, raspberry, steak sauce, pizza, and hamburger are dependent upon stimulation of the olfactory receptors via the nasopharynx during deglutition. In this paper, we briefly review the anatomy, physiology, and pathophysiology of the olfactory system, along with means for clinically assessing its function. The prevalence, etiology, and nature of olfactory disorders commonly encountered in the dental clinic are addressed, along with approaches to therapy and patient management.

Sniffing out new data and hypotheses on the form, function, and evolution of the echinopluteus post-oral vibratile lobe.

The performance requirements of ciliary band feeding explain the convoluted forms of many marine invertebrate larvae. Convolutions increase surface area and therefore feeding rates per unit body volume. We review recent advances in morphology, neural development, and behavior at settlement of the echinoid Lytechinus pictus and provide new ultrastructural and expression data on larvae of its congener, L. variegatus. Larvae of the echinometrid Colobocentrotus atratus contain neurons identified by their expression of nitric oxide synthase (NOS), indicating that this character is not unique to Lytechinus. We hypothesize that in some echinoids the convoluted shape of the post-oral vibratile lobe (POVL) covaries with the distribution of identified sensory neurons to enable olfaction during settlement. An analysis of variation in structural elaboration of the post-oral transverse ciliary band (PTB) within Echinoida and in feeding larvae of other echinoderm classes indicates that only echinoids, but not all echinoids, possess this novel character; larvae that do are distributed heterogeneously within the class. In recognition of this specialized function for the POVL and surrounding ectoderm, and because it is lobate and grows toward the mouth, we propose naming this structure the adoral lobe.

Chemosensory Responses to the Repellent Nepeta Essential Oil and Its Major Component Nepetalactone by Aedes aegypti (Diptera: Culicidae), a Vector of Zika Virus.

Nepeta essential oil (Neo; catnip) and its major component, nepetalactone, have long been known to repel insects including mosquitoes. However, the neural mechanisms through which these repellents are detected by mosquitoes, including the yellow fever mosquito Aedes aegypti (L.), an important vector of Zika virus, were poorly understood. Here we show that Neo volatiles activate olfactory receptor neurons within the basiconic sensilla on the maxillary palps of female Ae. aegypti. A gustatory receptor neuron sensitive to the feeding deterrent quinine and housed within sensilla on the labella of females was activated by both Neo and nepetalactone. Activity of a second gustatory receptor neuron sensitive to the feeding stimulant sucrose was suppressed by both repellents. Our results provide neural pathways for the reported spatial repellency and feeding deterrence of these repellents. A better understanding of the neural input through which female mosquitoes make decisions to feed will facilitate design of new repellents and management strategies involving their use.

Olfactory bulb recovery following reversible deafferentation with repeated detergent application in the adult zebrafish.

The neuroplasticity and regenerative properties of the olfactory system make it a useful model for studying the ability of the nervous system to recover from damage. We have developed a novel method for examining the effects of long-term deafferentation and regeneration of the olfactory organ and resulting influence on the olfactory bulb in adult zebrafish. To test the hypothesis that repeated damage to the olfactory epithelium causes reduced olfactory bulb afferent input and cessation of treatment allows recovery, we chronically ablated the olfactory organ every 2-3 days for 3 weeks with the detergent Triton X-100 while another group was allowed 3 weeks of recovery following treatment. Animals receiving chronic treatment showed severe morphological disruption of the olfactory organ, although small pockets of epithelium remained. These pockets were labeled by anti-calretinin, indicating the presence of mature olfactory sensory neurons (OSNs). Following a recovery period, the epithelium was more extensive and neuronal labeling increased, with three different morphologies of sensory neurons observed. Repeated peripheral exposure to Triton X-100 also affected the olfactory bulb. Bulb volumes and anti-tyrosine hydroxylase-like immunoreactivity, which is an indicator of afferent activity, were diminished in the olfactory bulb of the chronically treated group compared to the control side. In the recovery group, there was little difference in bulb volume or antibody staining. These results suggest that repeated, long-term nasal irrigation with Triton X-100 eliminates a substantial number of mature OSNs and reduces afferent input to the olfactory bulb. It also appears that these effects are reversible and regeneration will occur in both the peripheral olfactory organ and the olfactory bulb when given time to recover following cessation of treatment. We report here a new method that allows observation not only of the effects of deafferentation on the olfactory bulb but also the effects of reinnervation.

An experimental biomimetic platform for artificial olfaction.

Artificial olfactory systems have been studied for the last two decades mainly from the point of view of the features of olfactory neuron receptor fields. Other fundamental olfaction properties have only been episodically considered in artificial systems. As a result, current artificial olfactory systems are mostly intended as instruments and are of poor benefit for biologists who may need tools to model and test olfactory models. Herewith, we show how a simple experimental approach can be used to account for several phenomena observed in olfaction. An artificial epithelium is formed as a disordered distributed layer of broadly selective color indicators dispersed in a transparent polymer layer. The whole epithelium is probed with colored light, imaged with a digital camera and the olfactory response upon exposure to an odor is the change of the multispectral image. The pixels are treated as olfactory receptor neurons, whose optical properties are used to build a convergence classifier into a number of mathematically defined artificial glomeruli. A non-homogenous exposure of the test structure to the odours gives rise to a time and spatial dependence of the response of the different glomeruli strikingly similar to patterns observed in the olfactory bulb. The model seems to mimic both the formation of glomeruli, the zonal nature of olfactory epithelium, and the spatio-temporal signal patterns at the glomeruli level. This platform is able to provide a readily available test vehicle for chemists developing optical indicators for chemical sensing purposes and for biologists to test models of olfactory system organization.

Olfactory responses in a gustatory organ of the malaria vector mosquito Anopheles gambiae.

The proboscis is an important head appendage in insects that has primarily been thought to process gustatory information during food intake. Indeed, in Drosophila and other insects in which they have been identified, most gustatory receptors are expressed in proboscis neurons. Our previous characterization of the expression of AgOR7, a highly conserved odorant receptor (OR) of the Afrotropical malaria vector mosquito Anopheles gambiae in the labellum at the tip of the proboscis was suggestive of a potential olfactory function in this mosquito appendage. To test this hypothesis, we used electrophysiological recording and neuronal tracing, and carried out a molecular characterization of candidate OR expression in the labellum of A. gambiae. These studies have uncovered a set of labial olfactory responses to a small spectrum of human-related odorants, such as isovaleric acid, butylamine, and several ketones and oxocarboxylic acids. Molecular analyses indicated that at least 24 conventional OR genes are expressed throughout the proboscis. Furthermore, to more fully examine AgOR expression within this tissue, we characterized the AgOR profile within a single labial olfactory sensillum. This study provides compelling data to support the hypothesis that a cryptic set of olfactory neurons that respond to a small set of odorants are present in the mouth parts of hematophagous mosquitoes. This result is consistent with an important role for the labellum in the close-range discrimination of bloodmeal hosts that directly impacts the ability of A. gambiae to transmit malaria and other diseases.

Activation and inhibition of the transduction process in silkmoth olfactory receptor neurons.

Electrophysiological responses of olfactory receptor neurons in both male and female silkmoths (Bombyx mori) were investigated. In both sexes, the G-protein activator sodium fluoride and 1,2-dioctanoyl-sn-glycerol, a membrane-permeable analog of the protein kinase C activator diacylglycerol, elicited nerve impulse responses similar to those elicited by weak continuous stimulation with odorants. Therefore, G(q)-proteins and diacylglycerol-activated ion channels seem to be involved in the transduction process in both pheromone-sensitive neurons in males and general odorant-sensitive neurons in females. Decyl-thio-trifluoro-propanone is known to inhibit electrophysiological responses of male moths to pheromones, but has no effect in females. Application of this inhibitor reduced the frequency, but not the amplitude of elementary receptor potentials. It had no inhibitory effect on nerve impulse responses elicited by sodium fluoride or 1,2-dioctanoyl-sn-glycerol. This supports the idea that decyl-thio-trifluoro-propanone acts on a prior step of the transduction cascade, e.g. on the pheromone receptor molecules. General odorants, such as (+/-)-linalool and 1-heptanol, excite olfactory receptor neurons in females, but inhibit the pheromone-sensitive neurons in males. Both (+/-)-linalool and 1-heptanol inhibited the responses of male neurons elicited by sodium fluoride or 1,2-dioctanoyl-sn-glycerol. (+/-)-Linalool reduced the amplitude of elementary receptor potentials. In contrast to decyl-thio-trifluoro-propanone, (+/-)-linalool and 1-heptanol seem to interfere with later processes of the transduction cascade, possibly the opening of ion channels.