Showing that activating and blocking activity have reciprocal phenotypes also strengthens the implication that the neurons are critical decision points for the behavior. Using a very effective blocker helps when identifying subtle neuronal contributions to a behavior—one DAPT mouse does not want to miss a phenotype because the effector expression level was below effective threshold. Finally, acute blockers are often more useful than constitutively acting ones. The options for

manipulating neural activity are varied and effective but there is always room for improvement. For example, an acutely inducible and reversible electrical blocker of neural activity would be a valuable addition to the arsenal of tools for manipulating neural activity. Some neurons may be able to release both a canonical neurotransmitter and a peptide; it would be advantageous to be able

to selectively block each type of release. Finally, there is no blocker of electrical transmission through gap junctions that are encoded by innexin genes in Drosophila, making it more challenging to identify the roles that these connections play in adult brain function. Since the brain acts as an interconnected network, a particular class of neurons may contribute to many behaviors, and their role may be affected by the action of neighboring neurons. Genetic PF-01367338 in vivo targeting methods can direct the expression of fluorescent reporters of neural activity so that relevant neurons can be observed in action Electron transport chain to see how they respond to controlled sensory stimuli or during different behaviors. Recording neuronal activity aids in identification of neurons whose activity is correlated with sensory stimuli, and enables the study of how neurons encode and transform the input signals they receive. This section will discuss these reagents. Optical techniques that use changes in fluorescence to measure neuronal activity are a powerful way to identify neurons that respond to particular sensory stimuli or whose activity correlates with specific behaviors. They

are essential for neural circuit analysis, i.e., how activity in neurons encodes information. When a neuron fires an action potential there is a large local increase in calcium concentration that can be detected by genetically encoded calcium indicators (GECIs) that can be targeted to neurons of interest. Most GECIs use a calcium binding peptide to trigger either circularization of a single split fluorophore (GCaMP) ( Wang et al., 2003) or energy transfer (FRET) between two fluorophores (Cameleon, Camgaroo, and TN-XXL) ( Fiala et al., 2002, Yu et al., 2003 and Mank et al., 2008). Ratiometric imaging is advantageous in preparations that undergo movement because the baseline fluorescence serves as a reference and the change in wavelength shows the change in neural activity.