Inhibitory interneurons are considered to be the controlling units of neural networks,
despite their sparse number and unique morphological characteristics compared with
excitatory pyramidal cells. Although pyramidal cell dendrites have been shown to display
local regenerative events-dendritic spikes (dSpikes)-evoked by artificially patterned
stimulation of synaptic inputs, no such studies exist for interneurons or for spontaneous
events. In addition, imaging techniques have yet to attain the required spatial and
temporal resolution for the detection of spontaneously occurring events that trigger
dSpikes. Here we describe a high-resolution 3D two-photon laser scanning method (Roller
Coaster Scanning) capable of imaging long dendritic segments resolving individual
spines and inputs with a temporal resolution of a few milliseconds. By using this
technique, we found that local, NMDA receptor-dependent dSpikes can be observed in
hippocampal CA1 stratum radiatum interneurons during spontaneous network activities
in vitro. These NMDA spikes appear when approximately 10 spatially clustered inputs
arrive synchronously and trigger supralinear integration in dynamic interaction zones.
In contrast to the one-to-one relationship between computational subunits and dendritic
branches described in pyramidal cells, here we show that interneurons have relatively
small ( approximately 14 mum) sliding interaction zones. Our data suggest a unique
principle as to how interneurons integrate synaptic information by local dSpikes.
