Using holographic illumination to study synaptic signal integration at individual dendritic spines

The physiology of individual synapses on dendritic spines is important because isolated and widely distributed spines are active during sensory information processing in vivo. We used acute cortical brain slices from the rat to investigate synaptic signal integration at the level of individual synapses on dendritic spines in layer 5 pyramidal neurons. We monitored subthreshold synaptic signals using a combination of voltage-sensitive dye recordings, patch-electrode somatic recordings, and 2-photon glutamate uncaging with holographic illumination. We describe the capabilities and limitations of this approach and demonstrate that imaging with an organic voltage-sensitive dye is presently a unique way to monitor temporal summation of uncaging evoked quantal excitatory synaptic potentials locally, at the site of origin on thin basal dendrites. The results indicated that the ability of repetitive activation of individual excitatory synapses on spines to influence the electrical signaling in individual neurons is strictly limited to subthreshold responses. We show that the underlying mechanisms, which control the temporal summation of excitatory synaptic potential in single synapses, can now be investigated with described methodology.