Brigitte Palouzier-Paulignan

Olfaction Under Metabolic Influences

Recently published work and emerging research efforts have suggested that the olfactory system is intimately linked with the endocrine systems that regulate or modify energy balance. Although much attention has been focused on the parallels between taste transduction and neuroendocrine controls of digestion due to the novel discovery of taste receptors and molecular components shared by the tongue and gut, the equivalent body of knowledge that has accumulated for the olfactory system, has largely been overlooked. During regular cycles of food intake or disorders of endocrine function, olfaction is modulated in response to changing levels of various molecules, such as ghrelin, orexins, neuropeptide Y, insulin, leptin, and cholecystokinin. In view of the worldwide health concern regarding the rising incidence of diabetes, obesity, and related metabolic disorders, we present a comprehensive review that addresses the current knowledge of hormonal modulation of olfactory perception and how disruption of hormonal signaling in the olfactory system can affect energy homeostasis.

Modulation of Spontaneous and Odorant-Evoked Activity of Rat Olfactory Sensory Neurons by Two Anorectic Peptides, Insulin and Leptin

In mammals, the sense of smell is modulated by the status of satiety, which is mainly signaled by blood-circulating peptide hormones. However, the underlying mechanisms linking olfaction and food intake are poorly understood. Here we investigated the effects of two anorectic peptides, insulin and leptin, on the functional properties of olfactory sensory neurons (OSNs). Using patch-clamp recordings, we analyzed the spontaneous activity of rat OSNs in an in vitro intact epithelium preparation. Bath perfusion of insulin and leptin significantly increased the spontaneous firing frequency in 91.7% (n = 24) and 75.0% (n = 24) of the cells, respectively. When the activity was electrically evoked, both peptides shortened the latency to the first action potential by approximately 25% and decreased the interspike intervals by approximately 13%. While insulin and leptin enhanced the electrical excitability of OSNs in the absence of odorants, they surprisingly reduced the odorant-induced activity in the olfactory epithelium. Insulin and leptin decreased the peak amplitudes of isoamyl acetate-induced electroolfactogram (EOG) signals to 46 and 38%, respectively. When measured in individual cells by patch-clamp recordings, insulin and leptin decreased odorant-induced transduction currents and receptor potentials. Therefore by increasing the spontaneous activity but reducing the odorant-induced activity of OSNs, an elevated insulin and leptin level (such as after a meal) may result in a decreased global signal-to-noise ratio in the olfactory epithelium, which matches the smell ability to the satiety status.

5-Hydroxytryptamine action in the rat olfactory bulb: in vitro electrophysiological patch-clamp recordings of juxtaglomerular and mitral cells.

The olfactory bulb, first relay of olfactory pathways, is densely innervated by serotoninergic centrifugal fibers originating from the raphe nuclei. Although serotonin innervation was reported to be involved in olfactory learning in mammals, the action of this neurotransmitter on its putative cellular targets has been never described through unitary recordings. This lack of data initiated the present study where the effects of 5HT on juxtaglomerular and mitral cells are analyzed using whole-cell recordings on olfactory bulb slices. Serotonin depolarizes 34% of 525 JG cells. A multivariate statistical analysis of juxtaglomerular cells characteristics shows that the serotonin responsive cell group can be individualized regarding their tonic discharge-mode in response to a direct current injection, their lower expression of hyperpolarization-activated cation current and their low membrane capacities. The use of ion channel blockers and ramp voltage protocol indicate that serotoninergic depolarization of juxtaglomerular cells may be due to a nonselective cation current with a reversal potential of -44 mV. Pharmacological tests with serotonin receptor antagonists and agonists reveal that 5HT action on juxtaglomerular cells would be mainly mediated by 5HT2C receptors. In mitral cells, serotonin acts on 49.1% of the 242 tested cells, inducing two types of responses. A first subset of mitral cells (26.8%, n=65) were hyperpolarized by serotonin. This response would be indirect and mediated by action of GABA on GABAA receptors since it was antagonized by bicuculline. The involved GABAergic neurons are hypothesized to be juxtaglomerular and granular cells, on which serotonin would act mainly via 5HT2C and via 5HT2A receptors respectively. The second subset of mitral cells (22.3%, n=54) were directly depolarized by serotonin acting through 5HT2A receptors. Our data on serotonin action on juxtaglomerular cells and mitral cells reveal a part of functional mechanisms whereby serotonin can act on olfactory bulb network. This is expected to enrich the understanding of its determining role in olfactory learning.

Action of vasopressin on hypoglossal motoneurones of the rat: presynaptic and postsynaptic effects

The distribution of vasopressin binding sites in the hypoglossal nucleus of newborn rats was determined using autoradiography on film and a radioiodinated vasopressor antagonist. These sites predominated in the ventromedial and dorsal divisions of the nucleus. The effect of vasopressin on hypoglossal neurones was studied in brainstem slices of newborn animals, using the single-electrode voltage-clamp technique. Vasopressin, at 0.1-0.5 microM, generated a sustained inward current in a majority of neurones, an action which was mediated by V1-type receptors. Antidromic activation or morphological characterization of biocytin-labelled neurones indicate that part of the vasopressin-sensitive cells were motoneurones. When synaptic transmission was blocked by perfusing the preparation with a low-calcium/high-magnesium solution, the average vasopressin current decreased by 65%; and following TTX treatment, the peptide current decreased by 55%. In contrast, in a low-calcium solution, i.e., under conditions of reduced synaptic transmission but of increased neuronal excitability, the vasopressin current was not significantly altered. These results may be interpreted by assuming that the action of vasopressin is in part postsynaptic and in part presynaptic, the latter effect probably depending upon action potential propagation. Current-voltage relations suggest that the postsynaptic effect of vasopressin was due to the induction of a non-inactivating inward current, reversing in polarity at around -15 mV. The data raise the possibility that, in young animals, endogenous vasopressin may modulate the activity of hypoglossal motoneurones.

GABAB receptor-mediated inhibition of mitral/tufted cell activity in the rat olfactory bulb: A whole-cell patch-clamp study in vitro

GABA, the major inhibitory neurotransmitter involved in information processing in the olfactory bulb, is hypothesized to act through GABA(B) receptors by depressing primary neurotransmitter release at the level of olfactory nerve axon endings. The present study was designed to analyze GABA(B) receptor-mediated inhibition mechanisms by performing whole-cell patch-clamp recordings of mitral/tufted cell activity in the rat in vitro. To do so, GABA(B) receptor-mediated action was mimicked by baclofen and antagonized by saclofen. Our protocol led us to provide an original description of GABA(B) receptor-mediated inhibition exerted on mitral/tufted cells. First, their spontaneous activity was shown to be drastically abolished by baclofen. Second, their responses to olfactory nerve electrical stimulation were graded by GABA(B) receptor-mediated inhibition. Indeed, this inhibition may be described as inducing effects ranked from a slight increase in response latency to a complete response suppression.Altogether, our results corroborate the hypothesis of a presynaptic extrasynaptic GABA(B) receptor-mediated inhibition influencing mitral/tufted cell olfactory nerve responsivity. However, the involvement of postsynaptic receptors, with different properties or with different anatomical locations, cannot be ruled out, particularly in the control of spontaneous activity. In conclusion, we underline that, in the vertebrate olfactory bulb, GABA(B) receptor-mediated action appears to contribute to make mitral/tufted cell responses more salient by reducing their resting activity.

Insulin modulates network activity in olfactory bulb slices: impact on odour processing

Olfactory function is largely under metabolic influence. Insulin, one of the major players between food intake and energy balance, is known to act at both central and peripheral levels. The present study assesses the action of insulin in olfactory bulb slices by using patch‐clamp recordings in young rats. The results show that insulin can alter both spontaneous and olfactory nerve‐induced firing activities in most of the main ouput neurons, this action being differentially exerted in two opposite directions. A mathematical model demonstrates that insulin, by acting in this way, could impact odour detection and discrimination mechanisms. Such an impact could be hypothesized as being exerted according to pertinent ecological characteristics, such as the alimentary/ethological valence of odour.

Somatic Acetylcholine Release in Rabbit Nodose Ganglion

In the rabbit, as in various other species, the presence of a cholinergic vagal afferent contingent has been demonstrated previously using biochemical and immunohistological approaches at the nodose ganglion level, where vagal afferent cell bodies are located. This structure is completely devoid of synaptic contacts. In the present study, somatic acetylcholine release is demonstrated on different types of in vitro rabbit nodose ganglion preparations (fragments of nodose tissue or isolated cell bodies) using chemiluminescent detection. Acetylcholine endogenous content was measured and was shown to be greater in the right nodose ganglion compared to the left. This difference was also observed when spontaneous and potassium chloride‐evoked acetylcholine release was measured in extracellular fluid after a 15‐min incubation of nodose ganglion fragments. Calcium removal totally blocked this somatic release. A kinetic study of acetylcholine release was also performed by placing the samples (nodose ganglion fragments or isolated cell bodies) directly in front of the photomultiplier, allowing the direct monitoring of (acetylcholine + choline) and choline effluxes. The net acetylcholine release was then deduced by subtraction. Identical kinetics was obtained with the two different nodose ganglion preparations used. This somatic release is calcium‐dependent. The occurrence of acetylcholine release at the nodose ganglion level is discussed in comparison with the events occurring in the cholinergic nerve endings. These mechanisms could be implicated in the premodulation of the vagal afferent messages conveyed from the periphery to the central nervous system.

Modulation of olfactory sensitivity and glucose-sensing by the feeding state in obese Zucker rats

The Zucker fa/fa rat has been widely used as an animal model to study obesity, since it recapitulates most of its behavioral and metabolic dysfunctions, such as hyperphagia, hyperglycemia and insulin resistance. Although it is well established that olfaction is under nutritional and hormonal influences, little is known about the impact of metabolic dysfunctions on olfactory performances and glucose-sensing in the olfactory system of the obese Zucker rat. In the present study, using a behavioral paradigm based on a conditioned olfactory aversion, we have shown that both obese and lean Zucker rats have a better olfactory sensitivity when they are fasted than when they are satiated. Interestingly, the obese Zucker rats displayed a higher olfactory sensitivity than their lean controls. By investigating the molecular mechanisms involved in glucose-sensing in the olfactory system, we demonstrated that sodium-coupled glucose transporters 1 (SGLT1) and insulin dependent glucose transporters 4 (GLUT4) are both expressed in the olfactory bulb (OB). By comparing the expression of GLUT4 and SGLT1 in OB of obese and lean Zucker rats, we found that only SGLT1 is regulated in genotype-dependent manner. Next, we used glucose oxidase biosensors to simultaneously measure in vivo the extracellular fluid glucose concentrations ([Gluc]ECF) in the OB and the cortex. Under metabolic steady state, we have determined that the OB contained twice the amount of glucose found in the cortex. In both regions, the [Gluc]ECF was 2 fold higher in obese rats compared to their lean controls. Under induced dynamic glycemia conditions, insulin injection produced a greater decrease of [Gluc]ECF in the OB than in the cortex. Glucose injection did not affect OB [Gluc]ECF in Zucker fa/fa rats. In conclusion, these results emphasize the importance of glucose for the OB network function and provide strong arguments towards establishing the OB glucose-sensing as a key factor for sensory olfactory processing.

Choline uptake in cholinergic nodose cell bodies

Isolated living cell bodies were obtained by mechanical and enzymatic dissociation from adult rabbit nodose ganglion followed by separation of fibres and cells using a Percoll gradient. A purification yield of 45% was measured. Based on previous results obtained in whole ganglion and showing the presence of cholinergic cell bodies among the afferent fibres of the vagus nerve, this preparation was used to study choline uptake by neuron cell somata. Cholinergic cells counted after choline acetyltransferase immunohistological staining showed a stained population of 2.9% among the isolated population. Two [3H]choline uptake mechanisms were detected at the cell body level. The first, with Km1 = 7 microM and Vm1 = 200 pmol/h per ganglion is sodium dependent, related to acetylcholine synthesis (43%) and has an IC50 with hemicholinium-3 equal to 50 microM. The second, with Km2 = 54 microM and Vm2 = 2235 pmol/h per ganglion is sodium independent, poorly associated to acetylcholine synthesis (12%) and exhibits an IC50 of 2 microM with hemicholinium-3. Except for their sensitivity to hemicholinium-3, the high and low affinity choline uptake mechanisms observed at the somatic level have, respectively, the same characteristics as the high and low affinity mechanisms described at the synaptic level. Their physiological role, their opposed sensitivity to hemicholinium-3 compared to the synaptic uptake systems and the relation between the somatic high affinity choline transport and an acetylcholine somatic release are discussed.

Nutrient Sensing: Another Chemosensitivity of the Olfactory System

Olfaction is a major sensory modality involved in real time perception of the chemical composition of the external environment. Olfaction favors anticipation and rapid adaptation of behavioral responses necessary for animal survival. Furthermore, recent studies have demonstrated that there is a direct action of metabolic peptides on the olfactory network. Orexigenic peptides such as ghrelin and orexin increase olfactory sensitivity, which in turn, is decreased by anorexigenic hormones such as insulin and leptin. In addition to peptides, nutrients can play a key role on neuronal activity. Very little is known about nutrient sensing in olfactory areas. Nutrients, such as carbohydrates, amino acids, and lipids, could play a key role in modulating olfactory sensitivity to adjust feeding behavior according to metabolic need. Here we summarize recent findings on nutrient-sensing neurons in olfactory areas and delineate the limits of our knowledge on this topic. The present review opens new lines of investigations on the relationship between olfaction and food intake, which could contribute to determining the etiology of metabolic disorders.