Spike-time precision and network synchrony are controlled by the homeostatic regulation of the D-type potassium current

Abstract

Homeostatic plasticity of neuronal intrinsic excitability (HPIE) operates to maintain networks within physiological bounds in response to chronic changes in activity. Classically, this form of plasticity adjusts the output firing level of the neuron through the regulation of voltage-gated ion channels. Ion channels also determine spike timing in individual neurons by shaping subthreshold synaptic and intrinsic potentials. Thus, an intriguing hypothesis is that HPIE can also regulate network synchronization. We show here that the dendrotoxin-sensitive D-type K+ current (ID) disrupts the precision of AP generation in CA3 pyramidal neurons and may, in turn, limit network synchronization. The reduced precision is mediated by the sequence of outward ID followed by inward Na+ current. The homeostatic downregulation of ID increases both spike-time precision and the propensity for synchronization in iteratively constructed networks in vitro. Thus, network synchronization is adjusted in area CA3 through activity-dependent remodeling of ID.

Figure 1.

CA3 pyramidal neurons in organotypic slice culture express an outward K⁺ conductance that creates a ramp-and-delay phenotype and reduces AP precision. A, Pharmacological profile of the ramp-and-delay phenotype in CA3 pyramidal neurons. Left, Example of reduced ramp-and-delay phenotype induced by low concentration of 4-AP (30 μm). Note the decrease in the delay to the first spike in the presence of 4-AP (46% of the control). Right, Summary of the normalized delay to the first spike measured in the presence of 20–30 μm 4-AP (squares), 50–100 nm DTX-I (triangles) and 5 mm TEA (circles). B, Pharmacological isolation of the D-type current. Left, Family of leak-subtracted currents evoked by voltage commands to −40/−20 mV from a holding potential of −70 mV, in control (full current; a) and in the presence of 30 μm 4-AP (b). The 4-AP-sensitive current (I _D) is isolated by subtracting currents (a–b). Right, I/V plot of the pharmacologically isolated D-type current. C, First spike precision is enhanced in CA3 neurons by pharmacological blockade of I _D. Left, Example current-clamp recording showing that DTX-I improves AP precision. Histograms below the raw traces show the distribution of the time of the first AP (bin width, 5 ms). Right, Pooled data comparing the SD of the first AP onset time in control and in the presence of 20–30 μm 4-AP or 50–100 nm DTX-I. D, Depolarization rate of the first spike is accelerated when I _D is pharmacologically blocked. Left, The rate of depolarization of the first spike (dashed rectangle) was measured in the same neurons before (control) and after blockade of I _D (DTX-I). Middle, Time expansion showing the effect of DTX-I on the rate of depolarization measured 10 ms before the first spike. Right, Pooled data showing the changes in depolarization rate (dV/dt) induced by the blockade of I _D with 4-AP (squares) or DTX-I (triangles).

Figure 2.

I _D controls spike time precision in response to synaptic-like input. A, Example current-clamp traces of APs evoked by stimulation with dEPSCs, showing significantly more jitter in AP generation in control (left) versus DTX-I (right). Histograms below the raw traces show the distribution of the spike time. Right, Pooled data comparing the CV of the first AP onset time in control and in the presence of 30 μm 4-AP or DTX-I. B, Blocking I _D increases the derivative of the membrane potential (dV/dt) before the AP. Left, Representative traces. Right, Group data. C, Improvement of precision quantified by δdV/dt is inversely correlated with the variation in voltage trajectory.

Figure 3.

Outward–inward sequence in CA3 pyramidal neurons. A, Sequence of outward–inward currents evoked by voltage steps in a CA3 pyramidal neuron. Left column, Current-clamp recording. Middle and right columns, Same neuron recorded in voltage clamp. In control conditions, the neuron displayed a ramp and delay and a transient outward current that precedes te inward current, eventually leading to an action current (truncated). Note the reduction in both the ramp-and-delay phenotype and the amplitude of the outward current, in the presence of 30 μm 4-AP. B, Sequence of outward–inward currents evoked by simulated EPSP voltage commands. Left, Voltage inactivation. The holding potential is −70 mV in control (black traces) and −50 mV for the test condition (gray traces). Note the sequence of outward followed by inward currents and the reduction of the amplitude of the outward current. Bottom graph, Pooled data of peak current before and after voltage inactivation (Inact). Cont, Control. Middle, Effects of 30 μm 4-AP. Right, Effects of DTX-I. Note the decrease in the outward currents that unmasks the presence of an inward current.

Figure 4.

Stochastic Hodgkin-Huxley model of a CA3 pyramidal neuron reproduces the experimental observations. A, Example model response to a DC current step showing the I _D-dependent delay firing phenotype (left, control). The delay in firing is reduced when I _D (middle, No I _D) but not I _A (right, No I _A) is removed from the model. B, Example model response (n = 5 trials) to a synaptic-like EPSC waveform with (left, control) and without (middle, No I _D) I _D and without I _A (right, No I _A). C, Expanded view (horizontal gray bars in E) showing the reduction in the jitter of AP generation in response to an EPSC when I _D but not I _A is removed. The histogram below shows the binned AP time for the three conditions (n = 100 trials). D, Average CV (same runs as in B and C) is reduced when I _D but not I _A is removed (left; n = 100 trials). The membrane potential trajectory (dV/dt) just before AP generation increases after removal of I _D and is relatively unchanged on removal of I _A (center). Finally, removal of I _D but not I _A reduces the variation in the voltage threshold of the AP (right; SD V _thresh).

Figure 5.

Activity deprivation reduces a DTX-sensitive current in CA3 neurons. A, Left, Example current-clamp traces recorded in CA3 pyramidal neurons in control (black) and in Ky-treated (red) cultures. Right, Average data across groups. Ky-treated cells (red; n = 13) show an increase in excitability compared with controls (black; n = 14). B, Left, Example voltage-clamp responses showing the reduction of the Kv currents after activity deprivation by Ky treatment. Right, Pooled I/V plot showing the reduction of the peak (top plot) and sustained (bottom plot) currents measured in control versus treated cells. C, DTX-I increases the excitability of CA3 pyramidal neurons in control cultures (top row) but not in Ky-treated cultures (bottom row). Left, Representative voltage traces before (control/treated) and after application of DTX-I (control + DTX-I/treated + DTX-I). Right, Average data across groups. Note the large shift induced by DTX-I in control but not in treated cells.

Figure 6.

Spike-time precision in response to synaptic-like input is enhanced in Ky-treated neurons. A, Comparison of spike time precision (CV) from control and Ky-treated neurons (*p < 0.05). B, Effect of D-type potassium blocker 4-AP/DTX-I on spike-time precision (top) and voltage-trajectory dV/dt (bottom; ns, p > 0.4).

Figure 7.

Activity-deprived slices synchronize in earlier layers of a hybrid network model. A, Schematic showing the construction of a hybrid network. The network is seeded with a modeled layer firing random APs at 10 Hz (left column, layer 0). Layer 0 makes random connections with the next layer (right column, layer 1). Each neuron in layer 1 is stimulated with a current, which is the summation of a random selection of the input from layer 0. B, Example spike raster plots showing the development of synchrony across neurons in successive layers. Treated slices develop more synchronization beginning with layer 2. C, Expanded plot of spike times in the boxed region of the fourth layer (B) showing more synchrony across neurons in treated versus control. D, The central peak in the normalized CCH histogram (left) increased after activity deprivation (red). The plot of the peak CCH versus layer with Sigmoid fits (right; n = 7; for fourth, p < 0.05) is shown.

Figure 8.

Voltage inactivation of Kv currents increases the development of network synchrony. A, Left, Example traces of the response of fourth layer neurons in control (−70 mV) and voltage inactivation (−51 mV). Right, Normalized CCH as a function of layer for matched control versus voltage-inactivation networks. B, Spike raster plots of neurons in the fourth layer showing an increase in synchrony over time for control and a decrease in synchrony over time for voltage inactivation (Inact). C, Left, Sliding CCH (200 ms window) showing the evolution of synchrony through time. The control layer (black) develops synchrony with time, and the voltage-inactivated layer (red) starts with maximal synchrony and decreases. Right, Slope of sliding CCH as a function of layer, showing the evolution of an increase in slope for control (black) and a decrease in voltage-inactivated layer (red).

Figure 9.

Two rules by which outward K⁺ current can control AP precision. As described by Fricker and Miles (2000), in the left column the sequence of inward followed by outward current produces low AP jitter because the outward current shortens the EPSP decay and prevents late AP generation. Reducing outward K⁺ current in this case will introduce more jitter. In thr right column, the sequence of outward followed by inward current introduces AP jitter by delaying the first spike and reducing the membrane potential trajectory before the spike. Reducing outward K⁺ current in this case improves AP precision.