There has been recently a renewed interest in using Autofluorescence imaging (AF) of NADH and flavoproteins (Fp) to map brain activity in cortical areas. The recording of these cellular signals provides complementary information to intrinsic optical imaging based on hemodynamic changes. However, which of NADH or Fp is the best candidate for AF functional imaging is not established, and the temporal profile of AF signals is not fully understood. To bring new theoretical insights into these questions, Monte Carlo simulations of AF signals were carried out in realistic models of the rat somatosensory cortex and olfactory bulb. We show that AF signals depend on the structural and physiological features of the brain area considered and are sensitive to changes in blood flow and volume induced by sensory activation. In addition, we demonstrate the feasibility of both NADHAF and Fp-AF in the olfactory bulb.
In the brain, sensory stimulation activates distributed populations of neurons among functional modules which participate to the coding of the stimulus. Functional optical imaging techniques are advantageous to visualize the activation of these modules in sensory cortices with high spatial resolution. In this context, endogenous optical signals that arise from molecular mechanisms linked to neuroenergetics are valuable sources of contrast to record spatial maps of sensory stimuli over wide fields in the rodent brain.Here, we present two techniques based on changes of endogenous optical properties of the brain tissue during activation. First the intrinsic optical signals (IOS) are produced by a local alteration in red light reflectance due to: (i) absorption by changes in blood oxygenation level and blood volume (ii) photon scattering. The use of in vivo IOS to record spatial maps started in the mid 1980's with the observation of optical maps of whisker barrels in the rat and the orientation columns in the cat visual cortex The olfactory system is of central importance for the survival of the vast majority of living species because it allows efficient detection and identification of chemical substances in the environment (food, predators). The OB is the first relay of olfactory information processing in the brain. It receives afferent projections from the olfactory primary sensory neurons that detect volatile odorant molecules. Each sensory neuron expresses only one type of odorant receptor and neurons carrying the same type of receptor send their nerve processes to the same welldefined microregions of ˜100μm 3 constituted of discrete neuropil, the olfactory glomerulus (Fig. 1). In the last decade, IOS imaging has fostered the functional exploration of the OB 5, 6, 7 which has become one of the most studied sensory structures. The mapping of OB activity with FAS imaging has not been performed yet.Here, we show the successive steps of an efficient protocol for IOS and FAS imaging to map odor-evoked activities in the mouse OB. Video LinkThe video component of this article can be found at https://www.jove.com/video/3336/ Protocol 1. Preparing the animal for imaging (in accordance with European recommendations for care and use of laboratory animals, 86/609/EEC directive)1. 6 to 8 weeks old C57BL6 male mice are anesthetized with a cocktail of ketamine (10mg/kg) and xylazine (100mg/kg) injected intraperitonealy. Surgery begins when the mouse no longer responds to hindpaw pinch. During the entire experiment the animal is placed on a heating pad. Body temperature is continuously monitored and maintained at 37°C. Depth of anesthesia is maintained throughout surgery and imaging session by checking out the absence of limb withdraw. A subcutaneous injection of 20% of the initial anesthetic cocktail is otherwise administered. 2. Using clippers, remove the hair from the scalp. Clean the exposed skin from residual hair by using sterile gauze soaked with saline. 3. Place the mouse in the stereotaxic frame. The snout has ...
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