Methods for simultaneous electrophysiology and optogenetics in vivo

Introduction For hundreds of years, humanity has tried to understand the nervous system. Since the pioneering studies of Galvani (Galvani et al., 1791) in electrophysiology, in which he developed the theory of electrical excitation of neurons while studying the frog muscles, great advancement in techniques, methods and knowledge have brought us closer to understanding the mechanisms governing neuronal activity. Optogenetics (OG), a method to control neuronal activity by light, was revolutionized a decade ago in Karl Deisseroth's laboratory (Deisseroth et al., 2006). This approach had a huge impact on neuroscience by enabling the manipulation of specific types of neurons in space and time. Since it has been introduced, thousands of papers using OG have been published. At the core of OG, a light-sensitive molecule, opsin, is coupled to an ion channel or pump and, upon exposure to light, ion flow through the channel or pump changes the membrane potential. Many new optogenetic tools have been developed and include various types of light-activated channels and ion pumps that are sensitive to different wavelengths across the whole visible spectrum (Hegemann and Möglich, 2011). Before OG, neurons were activated mainly by the use of electrical stimulation or pharmacology. The use of electrical stimulation is non-specific and affects many neurons and other cells close to the stimulating electrode, while pharmacology is more specific but has very low time resolution. When using OG, only the neurons expressing the light-sensitive proteins are activated with millisecond resolution (Boyden et al., 2005). Since the expression can be regulated by a certain promotor or enhancer of choice, only the neurons in which the gene of choice is expressed will respond to the light. In this way, OG enables us to selectively manipulate subpopulations of cells based on genetic markers. Since it has been introduced, OG has been combined with electrophysiology in order to evaluate the effects on neuronal activity. Since the field of electrophysiology spans many different preparations, from tissue culture to behaving animals, different technical approaches had to be incorporated or invented when combined with OG. This chapter reviews the current methods for simultaneous OG and electrophysiology (SOGEP) in vivo. Depending on the electrophysiological requirements, different probes were developed, mainly for extracellular, but also for intracellular, recordings.