Matt Wachowiak

Representation of Odorants by Receptor Neuron Input to the Mouse Olfactory Bulb

To visualize odorant representations by receptor neuron input to the mouse olfactory bulb, we loaded receptor neurons with calcium-sensitive dye and imaged odorant-evoked responses from their axon terminals. Fluorescence increases reflected activation of receptor neuron populations converging onto individual glomeruli. We report several findings. First, five glomeruli were identifiable across animals based on their location and odorant responsiveness; all five showed complex response specificities. Second, maps of input were chemotopically organized at near-threshold concentrations but, at moderate concentrations, involved many widely distributed glomeruli. Third, the dynamic range of input to a glomerulus was greater than that reported for individual receptor neurons. Finally, odorant activation slopes could differ across glomeruli, and for different odorants activating the same glomerulus. These results imply a high degree of complexity in odorant representations at the level of olfactory bulb input.

Imaging membrane potential with voltage-sensitive dyes

Membrane potential can be measured optically using a variety of molecular probes. These measurements can be useful in studying function at the level of an individual cell, for determining how groups of neurons generate a behavior, and for studying the correlated behavior of populations of neurons. Examples of the three kinds of measurements are presented. The signals obtained from these measurements are generally small. Methodological considerations necessary to optimize the resulting signal-to-noise ratio are discussed.

What the nose tells the brain about odors

Optical recording using calcium sensitive dyes was used to measure the input to the olfactory bulb from the nose. Because all of the receptor neurons projecting to one glomerulus in the bulb express the same receptor protein, the signal from each glomerulus represents the response properties of a single receptor protein. Individual receptors responded to a variety of odorants. In the mouse, the number of responding glomeruli increased with increasing odorant concentration. In the turtle the input maps for different odorant concentrations were similar; the signals were concentration invariant.

4 – Voltage and Calcium Imaging of Brain Activity: Examples from the Turtle and the Mouse

This chapter discusses measurements of population signals with examples from two in vivo preparations: the olfactory bulb of the turtle and of the mouse. It begins with a general discussion of optical recording methods, including the choice of dyes, light sources, optics, cameras, and minimizing noise; and is followed by a more detailed description of voltage-sensitive dye measurements in the turtle and calcium dye measurements in the mouse. Because the light-measuring apparatus is already reasonably optimized, any improvement in the sensitivity of the optical measurements of neuron activity would need to come from the development of better dyes and/or investigating signals from additional optical properties of the dyes. However, because one of the chromophores must be hydrophobic and does not penetrate into brain tissue, it has not been possible to measure signals with a fast pair of dyes in intact tissues. An important new direction is the development of methods for neuron-type-specific staining. Three quite different approaches have been tried. First, the use of retrograde staining procedures has recently been investigated in the embryonic chick and lamprey spinal cords. The second approach is based on the use of cell-type-specific staining. Third approach is to construct a genetically encoded combination of a potassium channel and green fluorescent protein.