Figure 4. bAPs propagate to low ΔCa_AP dendrites in the absence of EPSPs.

(A) Calcium-dependent fluorescence transients evoked by bAPs, uEPSPs, and bAP/uEPSP pairings in the presence of 10 µM NBQX. (B) Comparison of peak calcium influx in spines evoked by bAP, uEPSP, and bAP/uEPSP pairing, in the presence of 10 µM NBQX. Inset: whole-cell recordings from glutamate uncaging in the presence of NBQX. Calcium influx evoked by pairing was significantly higher than by the uEPSP alone, t-test: N = 12/8/2, t = 4.01, p = 5.8 × 10^–4. (C) Maximum z-projection of Alexa 594 fluorescence from a L2/3 pyramidal cell with a whole-cell recording in the soma and consecutive loose-patch recordings in two branches. Insets: red fluorescence overlaid on differential interference contrast image of loose-patch configuration for each dendrite. The full apical dendritic morphology was not imaged. (D) Calcium-dependent fluorescence transients evoked by bAPs in each dendrite from panel C, recorded during loose-patch recordings. (E) Electrical signals evoked by bAPs measured with dendritic loose-patch recordings in each dendrite from panel C. Inset: expanded trace from gray shaded region. (F) Somatic whole-cell recording of bAPs during each loose-patch recording from panel C. Inset: expanded trace from gray shaded region. (G) Average calcium-dependent fluorescence transients evoked by bAPs in high (black) and low (blue) ΔCa_AP branches during loose-patch recordings. (H) Average electrical signal evoked by bAPs in high (black) and low (blue) ΔCa_AP branches measured with loose-patch recordings. (I) Comparison of peak calcium influx and peak electrical signal evoked in each dendrite by bAPs. Patches indicate selection of high (gray) and low (blue) ΔCa_AP branches used in panels G and H (N = 19/15/9).

Figure 4—figure supplement 1. Loose-patch recording properties.

(A) Comparison of total resistance of loose-patch recordings and amplitude of the dendritic electrical signals. The total resistance includes the pipette resistance and the seal resistance, which was measured after acquiring a loose-patch seal on a target dendrite using a small current step. (B) Comparison of the seal resistance and the amplitude of the dendritic electrical signals. Seal resistance was computed by subtracting the loose-patch resistance (measured after acquiring a loose-patch seal) from the pipette resistance (measured with the pipette in the bath solution). Note: several unmeasured noise sources prohibit inference of intracellular bAP amplitude based on loose-patch recordings. We are primarily measuring capacitive currents, $I_{cap}=C\frac{dV}{dt}$ , but we cannot directly measure dendritic capacitance, which may differ across dendritic branches. Also, the amplitude of the recording is also proportional to the true seal resistance (Figure 4D); however, our loose-patch recordings used large pipettes (pipette resistance ≈8 MΩ) and the total patch resistance only increased by ∼10 MΩ (Figure 4C), so estimates of the seal resistance are likely to be contaminated by the neuropil.