Multisensory learning between odor and sound enhances beta oscillations.

Multisensory interactions are essential to make sense of the environment by transforming the mosaic of sensory inputs received by the organism into a unified perception. Brain rhythms allow coherent processing within areas or between distant brain regions and could thus be instrumental in functionally connecting remote brain areas in the context of multisensory interactions. Still, odor and sound processing relate to two sensory systems with specific anatomofunctional characteristics. How does the brain handle their association? Rats were challenged to discriminate between unisensory stimulation (odor or sound) and the multisensory combination of both. During learning, we observed a progressive establishment of high power beta oscillations (15-35 Hz) spanning on the olfactory bulb, the piriform cortex and the perirhinal cortex, but not the primary auditory cortex. In the piriform cortex, beta oscillations power was higher in the multisensory condition compared to the presentation of the odor alone. Furthermore, in the olfactory structures, the sound alone was able to elicit a beta oscillatory response. These findings emphasize the functional differences between olfactory and auditory cortices and reveal that beta oscillations contribute to the memory formation of the multisensory association.

Functional ultrasound imaging reveals different odor-evoked patterns of vascular activity in the main olfactory bulb and the anterior piriform cortex.

Topographic representation of the outside world is a key feature of sensory systems, but so far it has been difficult to define how the activity pattern of the olfactory information is distributed at successive stages in the olfactory system. We studied odor-evoked activation patterns in the main olfactory bulb and the anterior piriform cortex of rats using functional ultrasound (fUS) imaging. fUS imaging is based on the use of ultrafast ultrasound scanners and detects variations in the local blood volume during brain activation. It makes deep brain imaging of ventral structures, such as the piriform cortex, possible. Stimulation with two different odors (hexanal and pentylacetate) induced the activation of odor-specific zones that were spatially segregated in the main olfactory bulb. Interestingly, the same odorants triggered the activation of the entire anterior piriform cortex, in all layers, with no distinguishable odor-specific areas detected in the power Doppler images. These fUS imaging results confirm the spatial distribution of odor-evoked activity in the main olfactory bulb, and furthermore, they reveal the absence of such a distribution in the anterior piriform cortex at the macroscopic scale in vivo.

Visualizing odor representation in the brain: a review of imaging techniques for the mapping of sensory activity in the olfactory glomeruli.

The brain transforms clues from the external world, the sensory stimuli, into activities in neuroglial networks. These circuits are activated in specialized sensory cortices where specific functional modules are responsible for the spatiotemporal coding of the stimulus. A major challenge in the neuroscience field has been to image the spatial distribution and follow the temporal dynamics of the activation of such large populations in vivo. Functional imaging techniques developed in the last 30 years have enabled researchers to solve this critical issue, and are reviewed here. These techniques utilize sources of contrast of radioisotopic, magnetic and optical origins and exploit two major families of signals to image sensory activity: the first class uses sources linked to cellular energy metabolism and hemodynamics, while the second involves exogenous indicators of neuronal activity. The whole panel of imaging techniques has fostered the functional exploration of the olfactory bulb which is one of the most studied sensory structures. We summarize the major results obtained using these techniques that describe the spatial and temporal activity patterns in the olfactory glomeruli, the first relay of olfactory information processing in the main olfactory bulb. We conclude this review by describing promising technical developments in optical imaging and future directions in the study of olfactory spatiotemporal coding.

A method based on Monte Carlo simulations and voxelized anatomical atlases to evaluate and correct uncertainties on radiotracer accumulation quantitation in beta microprobe studies in the rat brain.

The beta-microprobe is a simple and versatile technique complementary to small animal positron emission tomography (PET). It relies on local measurements of the concentration of positron-labeled molecules. So far, it has been successfully used in anesthetized rats for pharmacokinetics experiments and for the study of brain energetic metabolism. However, the ability of the technique to provide accurate quantitative measurements using (18)F, (11)C and (15)O tracers is likely to suffer from the contribution of 511 keV gamma rays background to the signal and from the contribution of positrons from brain loci surrounding the locus of interest. The aim of the present paper is to provide a method of evaluating several parameters, which are supposed to affect the quantification of recordings performed in vivo with this methodology. We have developed realistic voxelized phantoms of the rat whole body and brain, and used them as input geometries for Monte Carlo simulations of previous beta-microprobe reports. In the context of realistic experiments (binding of (11)C-Raclopride to D2 dopaminergic receptors in the striatum; local glucose metabolic rate measurement with (18)F-FDG and H(2)O(15) blood flow measurements in the somatosensory cortex), we have calculated the detection efficiencies and corresponding contribution of 511 keV gammas from peripheral organs accumulation. We confirmed that the 511 keV gammas background does not impair quantification. To evaluate the contribution of positrons from adjacent structures, we have developed beta-Assistant, a program based on a rat brain voxelized atlas and matrices of local detection efficiencies calculated by Monte Carlo simulations for several probe geometries. This program was used to calculate the 'apparent sensitivity' of the probe for each brain structure included in the detection volume. For a given localization of a probe within the brain, this allows us to quantify the different sources of beta signal. Finally, since stereotaxic accuracy is crucial for quantification in most microprobe studies, the influence of stereotaxic positioning error was studied for several realistic experiments in favorable and unfavorable experimental situations (binding of (11)C-Raclopride to D2 dopaminergic receptors in the striatum; binding of (18)F-MPPF to 5HT1A receptors in the dorsal raphe nucleus).

Simultaneous in vivo magnetic resonance imaging and radioactive measurements with the beta-MicroProbe.

PURPOSE: Multimodal instrumentation is a new technical approach allowing simultaneous and complementary in vivo recordings of complementary biological parameters. To elucidate further the physiopathological mechanisms in intact small animal models, especially for brain studies, a challenging issue is the actual coupling of magnetic resonance imaging (MRI) techniques with positron emission tomography (PET): it has been shown that running the technology for radioactive imaging in a magnet alters the spatiotemporal performance of both modalities. Thus, we propose an alternative coupling of techniques that uses the beta-MicroProbe instead of PET for local measurements of radioactivity coupled with MRI. METHODS: We simultaneously recorded local radioactivity due to [(18)F]MPPF (a 5-HT(1A) receptor PET radiotracer) binding in the hippocampus with the beta-MicroProbe and carried out anatomical MRI in the same anaesthetised rat. RESULTS: The comparison of [(18)F]MPPF kinetics obtained from animals in a magnet with kinetics from a control group outside the magnet allowed us to determine the stability of tracer biokinetic measurements over time in the magnet. We were thus able to show that the beta-MicroProbe reliably measures radioactivity in rat brains under an intense magnetic field of 7 Tesla. CONCLUSION: The biological validation of a beta-MicroProbe/MRI dual system reported here opens up a wide range of future multimodal approaches for functional and pharmacological measurements by the probe combined with various magnetic resonance technologies, including anatomical MRI, functional MRI and MR spectroscopy.

Essential role of D1 but not D2 receptors in the NMDA receptor-dependent long-term potentiation at hippocampal-prefrontal cortex synapses in vivo.

An intact mesocortical dopaminergic (DA) input to the prefrontal cortex (PFC) has been reported to be necessary for long-term potentiation (LTP) to occur at hippocampal-prefrontal cortex synapses. Here, we investigated the role of D1 and D2 receptors in this NMDA receptor-dependent LTP. Local infusion of the D1 agonist SKF81297 at an optimal dose induced a sustained enhancement of hippocampal-PFC LTP, whereas the D1 antagonist SCH23390 caused a dose-related impairment of its induction. The D1 agonist effect was mimicked by infusion of a low dose of the adenylyl cyclase activator forskolin, whereas LTP was severely attenuated with a protein kinase A inhibitor, Rp-cAMPS. To further assess the complex interplay between DA and NMDA receptors, changes in extracellular DA levels in the PFC were estimated during LTP, and a significant increase was observed immediately after tetanus. Taken together, these data suggest that D1 but not D2 receptors are crucial for the DA control of the NMDA receptor-mediated synaptic response on a specific excitatory input to the PFC. The interactions of these receptors may play a crucial role in the storage and transfer of hippocampal information in the PFC.

Integrity of the mesocortical dopaminergic system is necessary for complete expression of in vivo hippocampal-prefrontal cortex long-term potentiation.

The prefrontal cortex receives dopaminergic inputs from the ventral tegmental area and excitatory inputs from the hippocampus. Both afferent pathways target in close proximity dendritic spines of pyramidal cells in layer V-VI of the prefrontal cortex. In view of the prominent role of dopamine in cognitive functions we examined the effects of ventral tegmental area stimulation on the induction of long-term potentiation in the hippocampal-prefrontal cortex pathway of anesthetized rats. Stimulation of the ventral tegmental area at a frequency known to evoke dopamine overflow in the prefrontal cortex produces a long-lasting enhancement of the magnitude of the hippocampal-prefrontal cortex long-term potentiation. The role of dopamine was further examined by investigating the effects of prefrontocortical dopamine depletion induced by an electrolytic ventral tegmental area lesion. A significant correlation (r = 0.8; P < 0.001; n = 14) was obtained between cortical dopamine levels and cortical long-term potentiation amplitude, a depletion of more than 50% of cortical levels corresponding to a dramatic decrease in hippocampal-prefrontal cortex long-term potentiation. However, a recovery to normal long-term potentiation was observed 1 h after tetanic stimulation. In contrast to the effects on long-term potentiation, ventral tegmental area stimulation, when applied at low or high frequency, decreases the amplitude of the hippocampal-prefrontal cortex postsynaptic synaptic response. The present study demonstrates the importance of the integrity of the mesocortical dopaminergic system for long-term potentiation to occur in the hippocampal-prefrontal cortex pathway and suggests a frequency-dependent effect of dopamine on hippocampal-prefrontal cortex transmission.

Calretinin expression as a critical component in the control of dentate gyrus long-term potentiation induction in mice.

We have recently reported that mice homozygous (Cr-/-) for a null mutation in the calretinin gene have impaired long-term potentiation (LTP) induction in the dentate gyrus (S. Schurmans et al. (1997) Proc. Natl. Acad. Sci. USA, 94, 10415 ). Here, we investigated dentate LTP induction in mice heterozygous (Cr+/-) for the same mutation. Despite the presence of calretinin in neurons of these mice, although at reduced levels as compared with normal mice, LTP induction in dentate gyrus was totally impaired. Spatial memory and learning were found unaffected in Cr+/- mice, such as in Cr-/- mice. Altogether, our results suggest that calretinin is a critical component in the control of dentate synaptic plasticity in mice, and that levels of calretinin higher than those observed in Cr+/- mice are required to induce LTP in this area. The possible mechanisms leading to the absence of correlation between gene dosage and biological effects are discussed.

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.

Impaired long-term potentiation induction in dentate gyrus of calretinin-deficient mice.

Calretinin (Cr) is a Ca2+ binding protein present in various populations of neurons distributed in the central and peripheral nervous systems. We have generated Cr-deficient (Cr-/-) mice by gene targeting and have investigated the associated phenotype. Cr-/- mice were viable, and a large number of morphological, biochemical, and behavioral parameters were found unaffected. In the normal mouse hippocampus, Cr is expressed in a widely distributed subset of GABAergic interneurons and in hilar mossy cells of the dentate gyrus. Because both types of cells are part of local pathways innervating dentate granule cells and/or pyramidal neurons, we have explored in Cr-/- mice the synaptic transmission between the perforant pathway and granule cells and at the Schaffer commissural input to CA1 pyramidal neurons. Cr-/- mice showed no alteration in basal synaptic transmission, but long-term potentiation (LTP) was impaired in the dentate gyrus. Normal LTP could be restored in the presence of the GABAA receptor antagonist bicuculline, suggesting that in Cr-/- dentate gyrus an excess of gamma-aminobutyric acid (GABA) release interferes with LTP induction. Synaptic transmission and LTP were normal in CA1 area, which contains only few Cr-positive GABAergic interneurons. Cr-/- mice performed normally in spatial memory task. These results suggest that expression of Cr contributes to the control of synaptic plasticity in mouse dentate gyrus by indirectly regulating the activity of GABAergic interneurons, and that Cr-/- mice represent a useful tool to understand the role of dentate LTP in learning and memory.