Figure 1.
Frequency-dependent amplification in basal dendrites as shown using paired-pulse stimulation. A, Experimental setup. Whole-cell voltage-clamp recordings were performed from the soma. The cell was loaded with OGB-1 (200 μm) and was visualized using fluorescence confocal microscopy. Paired-pulse synaptic stimulation was performed via extracellular theta electrode placed under visual control near selected basal dendrites. B, Voltage responses to paired-pulse stimulations with different interstimulus intervals (ISI of 20–500 ms). Top traces were recorded in control conditions, and bottom traces were recorded in the presence of the NMDAR blocker APV (100 μm). Stimulus intensity was gradually changed at each ISI and presented in color coding (see inset). For clarity, only the lowest stimulus intensity that produced NMDA spike is shown for each ISI. Note that spikes were elicited preferentially by the second stimulus when ISIs were between 20 and 200 ms. For larger ISIs (500 ms), spikes were evoked at the first pulse. Addition of APV abolished all regenerative responses. C, Surface plot of the peak somatic EPSP amplitude evoked for all ISIs and stimulating intensities for the cell shown in B. Note the raise in voltage threshold for NMDA spike initiation as the ISI increased. D, Peak EPSP amplitude is plotted as a function of the stimulus intensity for the cell shown in B. Two ISIs (black, 20 ms; dotted line, 200 ms) and single pulse (gray) are presented. Data are shown as average ± SD. E, Summary plot of PPR as a function of ISIs (n = 10) for subthreshold responses (black), stimulations that triggered NMDA spike (red) and synaptic stimulation in the presence of APV (blue). To prevent spurious PPR, the average and SD were calculated in logarithmic scale. Note that, for stimulations that triggered NMDA spikes, strong facilitation is evident for ISIs smaller than 200 ms and paired-pulse depression for larger ISIs. The asterisks note statistical significance. F, Sodium and calcium blockers do not affect the PPR for all ISIs tested. PPR ± SD in the presence of somatic TTX or intracellular QX-314 (magenta; n = 8), the voltage-gated calcium channel blockers nifedipine and nickel (purple; n = 4), and CNQX (gray; n = 10). For clarity, only trials in which NMDA spike was present in either pulse were taken into account. The application of CNQX resulted in extremely high PPR at the short ISIs as the blocker markedly decreased the first EPSP without affecting spike amplitude.