Persistent decrease in dendritic excitability following paired stimulation.

<p>(<b>A</b>)EYFP-ChR2 expressing hippocampal neuron is stimulated with ∼28 um light spot (blue circle) applied on the proximal dendrites coupled with somatically induced spike. The insert shows the detection protocol (top) where 2 s photostimulation was used to determine dendritic excitability and stimulation protocol (bottom) where the same 2 s photostimulation of proximal dendrite is coupled with a spike (<b>B</b>) Schematics of the optical setup. Patterned ChR2 stimulation is achieved by a 470 nm LED is collimated by a lens (CL) and directed using a mirror (M) to Digital Micromirror Device (DMD). The light reflected from DMD is collimated and scaled using a lens system (L) and coupled into the microscope via a beam splitter (BS). (<b>C</b>) An example trace for 2 s report photo depolarization before (black trace) and after (blue trace) the treatment. When no stimulation was applied (top) no change in dendritic excitability is observed, however after paired stimulation (bottom) excitability decreased (<b>D</b>) Dendritic excitability prior to paired stimulation, shows a steady basal level of dendritic excitability as assessed by measuring the peak magnitude of the ChR2-induced photo current. After paired stimulation the current decreases by 13% for 2 s report as compared to 0.74% for no stimulation. (<b>E</b>) Only stimulation by paired APs and dendritic depolarization causes DED = 12.6% (p<0.05)). Controls, where no stimulation takes place or either APs or light alone are delivered show no significant DED (p>0.05 in all cases). (<b>F</b>) There is no change in spike-current relationship between stimulated (□) and unstimulated neurons (o) or neurons stimulated only with APs (▴) indicating no change in intrinsic excitability. The excitability was measured by applying depolarizing current pulses (2 s in duration) in 0.1-nA increments. For each depolarizing step, the number of evoked action potentials was counted and plotted against injected current amplitude. Inset shows a representative current clamp trace.