Voltage- and site-dependent control of the somatic impact of dendritic IPSPs

Abstract

Inhibitory interneurons target specific subcellular compartments of cortical pyramidal neurons, where location-specific interactions of IPSPs with voltage-activated ion channels are likely to influence the inhibitory control of neuronal output. To investigate this, we simulated IPSPs as a conductance source at sites across the somato-apical dendritic axis (up to 750 microm) of neocortical layer 5 pyramidal neurons. Analysis revealed that the electrotonic architecture of cortical pyramidal neurons is highly voltage dependent, resulting in a significant site-dependent disparity between the amplitude, kinetics, and dendro-somatic attenuation of IPSPs generated from depolarized (-50 mV) and hyperpolarized (-80 mV) membrane potentials. At the soma, the time course of IPSPs evoked from depolarized potentials was greatest when generated from proximal dendritic sites and decreased as events were generated more distally, whereas the somatic time course of IPSPs evoked from hyperpolarized potentials was independent of the dendritic site of generation. This behavior resulted from the concerted actions of axo-somatic sodium channels that increased the duration of proximal dendritic IPSPs generated at depolarized potentials and distal dendritic hyperpolarization-activated channels that mediated site independence of somatic IPSP time course at hyperpolarized potentials. Functionally, this voltage-dependent control of IPSPs shaped the spatial and temporal profile of inhibition of axonal action potential firing and dendritic spike generation. Together, these findings demonstrate that the somatic impact of dendritic IPSPs is highly voltage dependent and controlled by classes of ion channels differentially distributed across axodendritic domains, directly revealing site-dependent inhibitory synaptic processing in cortical pyramidal neurons.

Figure 1.

Site- and voltage-dependent control of dIPSP amplitude. A, Simultaneous recording of local dendritic (top traces) and somatic (bottom traces) dIPSPs generated at the indicated sites from dendritic membrane potentials of -50 and -80 mV. The middle traces show the injected IPSC. The inset shows the somatic recording of dIPSPs generated at 750 μm from the soma at higher magnification. B, C, Pooled data demonstrating the site and voltage dependence of local (B) and somatic (C) dIPSP amplitude. Somatic points represent mean ± SD (n = 5). Note the site-dependent increase in the local amplitude of IPSPs generated from membrane potentials of -50 mV (○). D, Pooled data describing the voltage dependence of the dendro-somatic attenuation of dIPSPs. E, Ratio of the dendro-somatic attenuation of dIPSPs generated from membrane potentials of -50 and -80 mV. Note the 1.7-fold increase in the dendro-somatic attenuation of dIPSPs generated from hyperpolarized membrane potentials. Data were fit with single exponential relationship [B (-80 mV), C, D] or a linear regression [B (-50 mV), E].

Figure 2.

Kinetics of dIPSPs at site of generation. A, Voltage-dependent transformation of the local time course of dIPSPs. Voltage signals (top traces) were recorded and current-injected (bottom traces) with independent pipettes (inset, photomicrograph). Note the prolonged time course of proximally generated IPSPs generated from relatively depolarized membrane potentials. Membrane potentials have been offset for illustration. B, Pooled data illustrating the site-dependent transformation of the rise time (10-90%) of dIPSPs generated from membrane potentials of -50 mV (○) and -80 mV (•). C, Pooled analysis of the time course (measured at half-amplitude) of dIPSPs generated from membrane potentials of -50 mV (○) and -80 mV (•). Data in B and C were fit with single exponentials; somatic points represent mean ± SD (n = 5). D, Representative records quantifying the site- and voltage-dependent transformation of the time course (at half-amplitude) of dIPSPs. The right trace (↝) represent proximal dendritically generated stimulus-evoked IPSPs. Gray scale is used to delineate data sets.

Figure 3.

Somatic kinetics of dendritically generated dIPSPs. A, Somatic recording of dIPSPs generated from the indicated dendritic sites at local dendritic membrane potentials of -50 and -80 mV. Note the somatic time course of dIPSPs generated from proximal dendritic sites is prolonged at depolarized membrane potentials. B, Pooled analysis of the site and voltage dependence of the somatic rise time (10-90%) of dendritically generated dIPSPs. Lines represent single exponential fits, and somatic points represent mean ± SD (n = 5). C, Site dependence of the somatic time course (measured at half-amplitude) of dendritically generated dIPSPs evoked from the indicated membrane potentials. Note from local dendritic membrane potentials of -50 mV the somatic time course of dIPSPs decreases in a distance-dependent manner, whereas from local dendritic membrane potentials of -60 and -80 mV, the somatic time course is relatively site independent. Lines represent linear regression. D, Same data as C, plotted as a function of the somatic membrane potential achieved by setting the dendritic membrane potential to -50 mV (○) or -80 mV ( ). Lines represent linear regression.

Figure 4.

Properties of IPSPs simulated as ideal current sources. A, Dendritic recordings of artificial (gray) and dIPSPs generated at the indicated sites, by IPSC-shaped ideal current sources or conductance changes, respectively, from a membrane potential of -60 mV. B, Pooled analysis of the site-dependent increase in local ( /•) and decrease in somatic (○) amplitude of IPSPs generated with an ideal current source (left) or conductance source (right). C, Comparison of the somatic kinetics after peak scaling of artificial IPSPs generated at dendritic (gray; 680 μm) and somatic sites under control and after I_H channel blockade with ZD 7288. The gray figures indicate the dendro-somatic attenuation of dendritic IPSPs. D, Relationship between somatic IPSP time course (half-width) of artificial IPSPs generated under control (○) and after I_H channel blockade ( ). Lines represent a linear regression (control) and a single exponential (ZD 7288). E, Pooled data showing the site dependence of the somatic rise time (10-90%) of artificial IPSPs under control (○) and following I_H channel blockade ( ). Lines represent a single exponential (control) and a linear regression (ZD 7288). Somatic points in B, D, and E represent mean ± SD.

Figure 5.

Dendritic I_H and axo-somatic I_NAP shape IPSP time course. A, Simultaneous dendritic (top traces) and somatic (bottom traces) recording of dIPSPs generated from dendritic membrane potentials of -50 and -80 mV at proximal (150 μm) and distal (600 μm) dendritic sites under control (black), after the blockade of sodium channels with TTX (1 μm; dark gray) and the coblockade of sodium and I_H channels with TTX and ZD 7288 (50 μm; light gray). B, Site dependence of the somatic time course (half-width) of dendritic dIPSPs generated from -50 (○) and -80 mV (•) under the three experimental conditions. Note the coblockade of sodium and I_H channels abolishes the voltage dependence of the somatic time course of dIPSPs. The insets show pooled analysis of percentage changes in the somatic amplitude of dIPSPs at -50 and -80 mV after the application of TTX (middle; relative to control) and the coapplication of TTX and ZD 7288 (right; relative to TTX alone).

Figure 6.

Activation of dendritic I_H by IPSP waveforms. A, Local dendritic voltage-clamp (V_H, -50 mV), leak subtracted currents generated in response to IPSP-shaped waveforms under control (top traces) and after the application of ZD 7288 (50 μm; middle traces). Voltage control was monitored with a second dendritic pipette (bottom traces) in current-clamp mode. The experimental arrangement is shown in the inset. B, ZD 7288-sensitive, nonleak subtracted, time-dependent currents generated in response to -40 mV voltage steps (top traces). The bottom traces show the membrane potential achieved during voltage steps. C, Pooled data demonstrating the voltage dependence of I_H activity generated during IPSP waveforms (all recordings >500 μm from the soma); points represent mean ± SEM. The line represents an exponential fit to the data. The inset shows the percentage voltage error between the clamp command voltage and recorded voltage responses under current clamp; the number of tested neurons is indicated.

Figure 7.

Efficacy of dendritic inhibition. A, Superposed records (n = 100) of action potential firing evoked by somatic injection of random EPSC waveforms is strongly inhibited by somatic (top traces), but not distal dendritic (670 μm), dIPSPs (middle traces). B, Efficacy of inhibition of axonal action potentials by dIPSPs generated at somatic and distal dendritic sites. Peristimulus time histograms of action potential firing rate per 10 msec bin illustrate pooled results for dIPSPs generated from somatic (n = 6) and distal dendritic sites (> 500 μm from the soma; n = 4). The average time course of dIPSCs is shown in gray. Horizontal gray lines indicate the average firing rate before dIPSC onset. C, Site dependence of the efficacy of inhibition. Points represent the difference between pre-dIPSC mean and nadir firing rates for individual neurons. Data for somatic sites are shown as mean ± SD (n = 6). The line represents an exponential fit to the data.

Figure 8.

Voltage and site dependence of dendritic inhibition. A, Overlain records of single action potential firing evoked by short somatic current steps, offset in time by 10 msec, recorded at somatic (left), proximal (middle), and distal (right) dendritic sites. dIPSPs, generated at the indicated sites, suppress axonal action potential initiation over a time window defined by the site of dIPSP generation (compare panels horizontally) and membrane potential (compare panels vertically). The inset shows the effects of dIPSPs on the waveform of backpropagating action potentials. Somatic action potentials have been clipped. B, Summary plots describing the relationship between the time window of inhibition and site of dIPSP generation for local membrane potentials of -55, -60, and -65 mV. Timing = 0 represents coincidence of somatic current step and dIPSC onset. Note from the most hyperpolarized membrane potential dendritic inhibition is relatively ineffective.

Figure 9.

Voltage-dependent inhibition of dendritic spike initiation. A, Tiled records of dendritic spike firing evoked by short dendritic current steps (from -55 mV), offset in time by 10 msec, recorded simultaneously at dendritic (left) and somatic (right) sites. dIPSPs generated at the dendritic site sculpt and suppress dendritic spike generation and consequent axonal action potential initiation. Timing = 0 represents coincidence of the dendritic current step and dIPSC onset. B, Comparison of the time window for inhibition of normalized axonal spike output by dIPSPs. Open symbols show the time window for suppression of axonal spike generation by somatically generated dIPSPs. The gray symbols show the time window for suppression of axonal action potential firing, generated as a consequence of the forward propagation of dendritic spikes, by dendritic dIPSPs. Data are shown for three membrane potentials (-55, -60, and -65 mV). Points represent normalized mean ± SEM. Note the time window for inhibition of neuronal output is significantly longer for somatically generated dIPSPs only at the most depolarized membrane potential tested.