In Vivo Population Imaging of Dendritic Integration in Neocortex

Translating the advances seen recently in recording from neurons in vivo to dendritic recordings presents special difficulties. In vivo two-photon imaging of dendrites was achieved over a decade ago and is still the method of choice for recording from small dendritic compartments in single neurons but has proven more difficult to apply to many dendrites simultaneously or to awake, freely moving preparations. An alternative that can be applied to layer 5 neocortical pyramidal neurons is the use of a fiber-optic method combined with bolus loading of fluorescent calcium indicator. This method takes advantage of the fact that the apical dendrites of these neurons are long (~1 mm) and always project in the same manner towards the cortical surface, thus allowing the fluorescence in these dendrites to be isolated from all other filled structures. Here we describe the details of this approach (also called the “periscope” method) and give examples of the kinds of recordings that can be made with this approach.