Frequency dependence of CA3 spike phase response arising from h-current properties

Abstract

The phase of firing of hippocampal neurons during theta oscillations encodes spatial information. Moreover, the spike phase response to synaptic inputs in individual cells depends on the expression of the hyperpolarization-activated mixed cation current (I h ), which differs between CA3 and CA1 pyramidal neurons. Here, we compared the phase response of these two cell types, as well as their intrinsic membrane properties. We found that both CA3 and CA1 pyramidal neurons show a voltage sag in response to negative current steps but that this voltage sag is significantly smaller in CA3 cells. Moreover, CA3 pyramidal neurons have less prominent resonance properties compared to CA1 pyramidal neurons. This is consistent with differential expression of I h by the two cell types. Despite their distinct intrinsic membrane properties, both CA3 and CA1 pyramidal neurons displayed bidirectional spike phase control by excitatory conductance inputs during theta oscillations. In particular, excitatory inputs delivered at the descending phase of a dynamic clamp-induced membrane potential oscillation delayed the subsequent spike by nearly 50 mrad. The effect was shown to be mediated by I h and was counteracted by increasing inhibitory conductance driving the membrane potential oscillation. Using our experimental data to feed a computational model, we showed that differences in I h between CA3 and CA1 pyramidal neurons could predict frequency-dependent differences in phase response properties between these cell types. We confirmed experimentally such frequency-dependent spike phase control in CA3 neurons. Therefore, a decrease in theta frequency, which is observed in intact animals during novelty, might switch the CA3 spike phase response from unidirectional to bidirectional and thereby promote encoding of the new context.

Keywords: CA1; CA3; Ih; hippocampus; phase response; resonance; theta oscillation.

Figure 1

Modeling of I_h kinetics. (A) The activation time constants of h-channel conductance for CA3 pyramidal neuron model and CA1 pyramidal neuron model. (B) Voltage response to −100 pA current steps (bottom) in CA3 and CA1 neuron models. CA3 and CA1 neuron models were fit to the voltage response recorded in CA3 and CA1 pyramidal neurons to −100 pA current steps in vitro.

Figure 2

Frequency preference of CA3 and CA1 pyramidal neurons. (A,B) Representative ZAP recordings of CA3 and CA1 cells, respectively. Voltage responses (upper traces) to a sinusoidal current of increasing frequency (0–20 Hz, middle traces) were recorded and relative impedances (lower traces) were calculated. A peak in the impedance curve indicates a frequency preference of the cell. Representative impedance magnitudes of CA3 (C) and CA1 (D) pyramidal neurons at −60, −70 and −80 mV holding potentials. Some cells in CA3 did not show detectable frequency preference (dotted line) and a frequency preference could not be detected. (E) Average frequency preference of CA3 cells with resonance peak above 1 Hz (n = 7) and all CA1 cells (n = 12). Preferred frequency increased at more negative holding potentials and was significantly greater in CA1 than in CA3 cells (^***p < 0.001, One-Way ANOVA with Bonferroni post-hoc test).

Figure 3

Comparable PRCs measured at the soma of mouse CA3 and CA1 pyramidal neurons. (A,B) Representative traces, in CA3 and CA1 respectively, of spike phase shift in response to excitatory input (aEPSG) delivered at different phases of a theta oscillation imposed by dynamic clamp. When aEPSG was delivered at ascending phases, the following spike was advanced compared to the average spike phase over the four preceding cycles (top traces). In contrast, when aEPSG was delivered at descending phases, the following spike was delayed compared to the control spike phase (bottom traces). The spike phase shifts (boxes) are enlarged in the shaded insets. (C) Spike time response curve: plot of spike time shift as a function of aEPSG phase relative to the imposed theta oscillation. (D) Average spike time response induced by aEPSGs at five descending and ascending phases. Neurons in both CA3 and CA1 showed significant delay after excitatory input at the descending phase of theta oscillation. (E) Polar graph of the phase response curve. Data are shown as mean ± SEM, n = 32 for CA3 and 11 for CA1. ^***p < 0.001, one-sample mean angle test; ns: Watson–Williams test; rad, radian; asc, ascending phases; desc, descending phases.

Figure 4

Effect of oscillation strength on the phase response curve. (A) Spike time response curve of CA1 pyramidal neurons measured for strong (3 nS) and weak (1 nS) induced oscillation. Note differences in the input phase for maximum spike time advance as well as the magnitude of time advance. (B) Spikes have a tendency to be more advanced and delayed by aEPSGs delivered at 0.5π rad of the ascending and descending slope of 1 nS than 3 nS-induced oscillation respectively. ^*p < 0.05, ^***p < 0.001, Watson–Williams test. (C) Polar representation of the phase response curves in (A). (D) Maximal spike phase advance occurs significantly earlier with excitatory inputs during oscillations with smaller maximum conductance in CA1 pyramidal neurons. Data are shown as mean ± SEM, n = 11 for 3 nS and n = 5 for 1 nS oscillatory inhibition; ^***p < 0.001, Watson–Williams test; rad, radian; asc, ascending phases; desc, descending phases.

Figure 5

Frequency dependence of spike phase control recorded at 5 Hz and 4 Hz. (A) Representative responses of a CA1 neuron (top traces) to inputs optimized for modulating I_h during induced oscillation (bottom traces) at 5 Hz (left panel) and 4 Hz (right panel). The spike phase shifts (boxes) are enlarged in the shaded insets. (B) Same as in (A) but from a CA3 neuron with a resonance peak > 1 Hz. (C) Average phase responses of CA1 and CA3 pyramidal neurons for both a 5 and 4 Hz oscillation. In CA1, a depolarizing step delayed the following spike, whereas a hyperpolarizing step advanced the following spike compared to the control condition at both oscillation frequencies. CA3 neurons, however, even with a resonance peak > 1 Hz, failed to show phase delay and advance with stimulation during descending phases of an ongoing 5 Hz oscillation. The spike phase delay is partially rescued when the frequency of the imposed oscillation is lowered to 4 Hz (n = 4). Data are shown as mean ± SEM. ^*p < 0.05, ^**p < 0.001, ^***p < 0.0001, Moore's test; rad, radian.

Figure 6

Spike phase control in computational models of hippocampal CA1 and CA3 pyramidal neurons. (A) Voltage response of CA1 neuron model (top traces) in response to excitatory and inhibitory perturbation of an oscillatory input (bottom traces) at 5 Hz (left panel) or 4 Hz (right panel). The voltage response around the control spike is expanded to display spike phase responses. The spike phase shifts (boxes) are enlarged in the shaded insets. (B) Same as in (A) but in CA3 neuron model with h-channel kinetics adjusted to reproduce the voltage sag seen in CA3 neurons. (C) Summary spike phase changes elicited in CA1 and CA3 neuron model by excitatory and inhibitory step inputs during 5 and 4 Hz oscillation. rad, radian.