Hirac Gurden

The endocannabinoid system controls food intake via olfactory processes

Hunger arouses sensory perception, eventually leading to an increase in food intake, but the underlying mechanisms remain poorly understood. We found that cannabinoid type-1 (CB1) receptors promote food intake in fasted mice by increasing odor detection. CB1 receptors were abundantly expressed on axon terminals of centrifugal cortical glutamatergic neurons that project to inhibitory granule cells of the main olfactory bulb (MOB). Local pharmacological and genetic manipulations revealed that endocannabinoids and exogenous cannabinoids increased odor detection and food intake in fasted mice by decreasing excitatory drive from olfactory cortex areas to the MOB. Consistently, cannabinoid agonists dampened in vivo optogenetically stimulated excitatory transmission in the same circuit. Our data indicate that cortical feedback projections to the MOB crucially regulate food intake via CB1 receptor signaling, linking the feeling of hunger to stronger odor processing. Thus, CB1 receptor–dependent control of cortical feedback projections in olfactory circuits couples internal states to perception and behavior.

Alteration of sensory-evoked metabolic and oscillatory activities in the olfactory bulb of GLAST-deficient mice

Astrocytes are key cellular elements in both the tripartite synapse and the neurovascular unit. To fulfill this dual role in synaptic activity and metabolism, they express a panel of receptors and transporters that sense glutamate. Among them, the GLT-1 and GLAST transporters are known to regulate extracellular glutamate concentrations at excitatory synapses and consequently modulate glutamate receptor signaling. These major uptake systems are also involved in energy supply to neurons. However, the functional role of GLAST in concurrent regulation of metabolic and neuronal activity is currently unknown. We took advantage of the attractive structural and functional features of the main olfactory bulb to explore the impact of GLAST on sensory information processing while probing both glutamate uptake and neuronal activity in glomeruli and deeper cellular layers, respectively. Using odor-evoked 2-deoxyglucose imaging and local field potential recordings in GLAST knockout mice, we show in vivo that deletion of GLAST alters both glucose uptake and neuronal oscillations in olfactory bulb networks.

Rapid increase in PKA activity during long-term potentiation in the hippocampal afferent fibre system to the prefrontal cortex in vivo

The purpose of the present study was to examine whether cAMP‐dependent protein kinase (PKA) was implicated in the process of long‐term potentiation (LTP) in the hippocampal afferent fibre system to the prefrontal cortex in vivo. Using a biochemical approach, we measured PKA activity at different times after induction of LTP. We show that there is an NMDA receptor‐dependent increase in PKA activity in the prefrontal cortex, only at five minutes after LTP induction. These data demonstrate a role of PKA in the induction and not the expression of cortical LTP and suggest that if PKA is involved in the late phase of LTP, it does not appear to be a persistent activation.

Anesthetic regimes modulate the temporal dynamics of local field potential in the mouse olfactory bulb.

Anesthetized preparations have been widely used to study odor-induced temporal dynamics in the olfactory bulb. Although numerous recent data of single-cell recording or imaging in the olfactory bulb have employed ketamine cocktails, their effects on networks activities are still poorly understood, and odor-induced oscillations of the local field potential have not been characterized under these anesthetics. Our study aimed at describing the impact of two ketamine cocktails on oscillations and comparing them to awake condition. Anesthesia was induced by injection of a cocktail of ketamine, an antagonist of the N-methyl-d-aspartate receptors, combined with one agonist of α2-adrenergic receptors, xylazine (low affinity) or medetomidine (high affinity). Spontaneous and odor-induced activities were examined in anesthetized and awake conditions, in the same mice chronically implanted with an electrode in the main olfactory bulb. The overall dynamic pattern of oscillations under the two ketamine cocktails resembles that of the awake state. Ongoing activity is characterized by gamma bursts (>60 Hz) locked on respiration and beta (15-40 Hz) power increases during odor stimulation. However, anesthesia decreases local field potential power and leads to a strong frequency shift of gamma oscillations from 60-90 Hz to 100-130 Hz. We conclude that similarities between oscillations in anesthetized and awake states make cocktails of ketamine with one α2-agonist suitable for the recordings of local field potential to study processing in the early stages of the olfactory system.

Multispectral reflectance imaging of brain activation in rodents: methodological study of the differential path length estimations and first in vivo recordings in the rat olfactory bulb

Dynamic maps of relative changes in blood volume and oxygenation following brain activation are obtained using multispectral reflectance imaging. The technique relies on optical absorption modifications linked to hemodynamic changes. The relative variation of hemodynamic parameters can be quantified using the modified Beer-Lambert Law if changes in reflected light intensities are recorded at two wavelengths or more and the differential path length (DP) is known. The DP is the mean path length in tissues of backscattered photons and varies with wavelength. It is usually estimated using Monte Carlo simulations in simplified semi-infinite homogeneous geometries. Here we consider the use of multilayered models of the somatosensory cortex (SsC) and olfactory bulb (OB), which are common physiological models of brain activation. Simulations demonstrate that specific DP estimation is required for SsC and OB, specifically for wavelengths above 600 nm. They validate the hypothesis of a constant path length during activation and show the need for specific DP if imaging is performed in a thinned-skull preparation. The first multispectral reflectance imaging data recorded in vivo during OB activation are presented, and the influence of DP on the hemodynamic parameters and the pattern of oxymetric changes in the activated OB are discussed.

In vivo detection of excitotoxicity by manganese-enhanced MRI: Comparison with physiological stimulation

Manganese‐enhanced MRI (MEMRI) is a powerful technique for the in vivo monitoring of brain function in animals. Manganese enters into cells through calcium channels, i.e., voltage‐gated calcium channels and activated glutamate receptors (e.g., N‐methyl‐D‐aspartate receptors). N‐methyl‐D‐aspartate receptors are activated both in normal physiological and pathophysiological conditions. Consistent with these mechanisms, we showed that in the olfactory bulb, the MEMRI signal strongly increases when excitotoxic mechanisms are induced by an administration of a N‐methyl‐D‐aspartate receptor agonist, quinolinate. We found that the intensity of the MEMRI signal in excitotoxic conditions is similar to the odor‐evoked signal in normal physiological conditions. Finally, we showed that the dynamics of the MEMRI signal are determined by the early phase of manganese in the olfactory bulb. Overall, these data show that, in addition to physiological studies, MEMRI can be used as an in vivo method to follow‐up the dynamics of excitotoxic events. Magn Reson Med, 2012.

Odor-Induced Neuronal Rhythms in the Olfactory Bulb Are Profoundly Modified in ob/ob Obese Mice

Leptin, the product of the Ob(Lep) gene, is a peptide hormone that plays a major role in maintaining the balance between food intake and energy expenditure. In the brain, leptin receptors are expressed by hypothalamic cells but also in the olfactory bulb, the first central structure coding for odors, suggesting a precise function of this hormone in odor-evoked activities. Although olfaction plays a key role in feeding behavior, the ability of the olfactory bulb to integrate the energy-related signal leptin is still missing. Therefore, we studied the fate of odor-induced activity in the olfactory bulb in the genetic context of leptin deficiency using the obese ob/ob mice. By means of an odor discrimination task with concomitant local field potential recordings, we showed that ob/ob mice perform better than wild-type (WT) mice in the early stage of the task. This behavioral gain of function was associated in parallel with profound changes in neuronal oscillations in the olfactory bulb. The distribution of the peaks in the gamma frequency range was shifted toward higher frequencies in ob/ob mice compared to WT mice before learning. More notably, beta oscillatory activity, which has been shown previously to be correlated with olfactory discrimination learning, was longer and stronger in expert ob/ob mice after learning. Since oscillations in the olfactory bulb emerge from mitral to granule cell interactions, our results suggest that cellular dynamics in the olfactory bulb are deeply modified in ob/ob mice in the context of olfactory learning.

Multispectral imaging of the olfactory bulb activation: influence of realistic differential pathlength correction factors on the derivation of oxygenation and total hemoglobin concentration maps

In vivo multispectral reflectance imaging has been extensively used in the somatosensory cortex (SsC) in anesthetized rodents to collect intrinsic signal during activation and derive hemodynamics signals time courses. So far it has never been applied to the Olfactory Bulb (OB), although this structure is particularly well suited to the optical study of brain activation due to the its well defined organization, the ability to physiologically activate it with odorants, and the low depth of the activated layers. To obtain hemodynamics parameters from reflectance variations data, it is necessary to take into account a corrective factor called Differential Pathlength (DP). It is routinely estimated using Monte Carlo simulations, modeling photons propagation in simplified infinite geometry tissue models. The first goal of our study was to evaluate the influence of more realistic layered geometries and optical properties on the calculation of DP and ultimately on the estimation of the hemodynamics parameters. Since many valuable results have been obtained previously by others in the SSc, for the purpose of validation and comparison we performed Monte Carlo simulations in both the SSC and the OB. We verified the assumption of constant DP during activation by varying the hemoglobin oxygen saturation, total hemoglobin concentration and we also studied the effect of a superficial bone layer on DP estimation for OB. The simulations show the importance of defining a finite multilayer model instead of the coarse infinite monolayer model, especially for the SSc, and demonstrate the need to perform DP calculation for each structure taking into account their anatomofunctional properties. The second goal of the study was to validate in vivo multispectral imaging for the study of hemodynamics in the OB during activation. First results are presented and discussed.

Development of a multi-exposure speckle imaging for mice brain imaging

In the last decade, Laser Speckle Contrast Imaging (LSCI) has been proposed and validated for imaging cerebral blood flow at the rodent brain surface in vivo. The technique relies on the calculation of the spatial speckle contrast, which is related to the velocity of scatterers (red blood cells). The implementation of the technique requires a partial craniotomy so that the brain tissues of interest can be illuminated with a laser diode. However, the studies of changes in the microcirculation during disease progression or treatment require longitudinal studies (i.e. imaging is done repeatedly over weeks or even months). Practically, the less invasive way to obtain such data is to image through the thinned skull without a craniotomy. However the presence of static scatterers (skull) will affect the speckle calculation and produce a bias in the estimation of the microcirculation changes. An extension to LSCI, termed Multi-Exposure Speckle Imaging (MESI) was proposed and validated a few years ago that address these limitations. It relies on a model of the speckle contrast as a function of the exposure time and the proportion of static scatterers. Here, we used MESI with the aim of repeatedly imaging the olfactory bulb of mice models of obesity. First, we have developed a MESI set up which was characterized on microfluidic flow phantoms with different flow-rates and channel diameters to simulate blood flow in animal model characteristics. Second, we show that MESI can discriminate flows in the presence of static scatterers and it can measure flow changes consistently. Finally we provide an in vivo validation of the technique in mice with and without a craniotomy.

Wide-field speckle imaging and two-photon microscopy for the investigation of cerebral blood flow in vivo in mice models of obesity

Vascular activity is necessary to provide suitable energy supply for cellular activity in the brain. Obesity, has become an important health and social issue worldwide. Yet, very little is known regarding morphological and functional vascular changes in the brain in obese patients. The purpose of our study is to evaluate the influence of this pathology on blood flow, vasodilation and vasoconstriction at rest and during sensory stimulation, in normal and obese mice. In order to obtain dynamic and quantitative maps of vascular activity over wide field of cortical tissues in anesthetized mice brain, we have developed a multi-exposure speckle imaging (MESI) system. MESI relies on the sequential recording of speckle images of the brain tissues illuminated with coherent light for increasing durations. For each of these multi exposure images the local speckle contrast is derived. This contrast is assumed to be related to the velocity of scatterers (red blood cells). The acquisition of speckle contrast for different expositions time allows discriminating the contribution of static and moving scatterers to the speckle pattern. Therefore, it allows mapping the blood flow changes over large cortical areas. Blood flow response to sensory activation was studied by imaging the olfactory bulb during olfactory stimulation trials. Data obtained in wild -type and high fat diet obesity model mice are presented showing a different hemodynamic response to olfactory stimulation.