Ionic current correlations are ubiquitous across phyla

Abstract

Ionic currents, whether measured as conductance amplitude or as ion channel transcript numbers, can vary many-fold within a population of identified neurons. In invertebrate neuronal types multiple currents can be seen to vary while at the same time their magnitudes are correlated. These conductance amplitude correlations are thought to reflect a tight homeostasis of cellular excitability that enhances the robustness and stability of neuronal activity over long stretches of time. Although such ionic conductance correlations are well documented in invertebrates, they have not been reported in vertebrates. Here we demonstrate with two examples, identified mouse hippocampal granule cells (GCs) and cholinergic basal forebrain neurons, that the correlation of ionic conductance amplitudes between different ionic currents also exists in vertebrates, and we argue that it is a ubiquitous phenomenon expressed by many species across phyla. We further demonstrate that in dentate gyrus GCs these conductance correlations are likely regulated in a circadian manner. This is reminiscent of the known conductance regulation by neuromodulators in crustaceans. However, in GCs we observe a more nuanced regulation, where for some conductance pairs the correlations are completely eliminated while for others the correlation is quantitatively modified but not obliterated.

Figure 1

Several ionic currents can simultaneously be measured in mouse hippocampal dentate gyrus (DG) granule cells (GCs). I_Kd, I_Na, I_Kir and I_leak were measured with whole cell patch clamp in identified GCs. (a) Typical current traces from which inward rectifier (I_Kir) and leak conductances were derived. (b) Sample I-V curve for leak-subtracted I_Kir showing E_Kir and g_Kir (slope) measurements. (c) Examples of delayed rectifier (I_Kd) and early TTX-sensitive inward current (I_Na). (d) Sample I-V curve of TTX-sensitive Na current. In a and c, top traces show currents; bottom (gray) traces show the pipette potentials at which the currents were measured (−50 mV for I_leak, −120 mV for I_Kir, −20 mV for I_Kd, and +15 mV for I_Na). Synapses were blocked with APV, CNQX and bicuculline. Arrowheads show 0 nA (top traces) and −40 mV (bottom traces).

Figure 2

Ionic conductance correlations in hippocampal GCs of 4 month-old mice. g_Kd, g_Na, and g_Kir are plotted against each other for data recorded at the end-of-day (ZT0, red) and end-of-night (ZT12, black) of a 12 h light-dark cycle. Only these conductance pairs showed significant correlations either at ZT0 or ZT12, shown as Pearson-moment correlation coefficients and their statistical significance. Regression lines are also shown in each panel. The dashed line in b indicates that statistical significance of the correlation is lost after adjustment for multiple comparisons. Means and SD bars for each conductance are shown to the right and top of the plots in a and b. **P<0.001, ns not significant (t-Student tests, see Table 1).

Figure 3

Correlations are not observed when leak conductance in hippocampal GCs is considered: g_Na, g_Kir and g_Kd are plotted against g_leak for data recorded at ZT0 (red) and ZT12 (black) of a 12 h light-dark cycle. None of these conductance pairs showed significant correlations either at ZT0 or ZT12, shown as Pearson-moment correlation coefficients and their statistical significance.

Figure 4

Ionic currents and conductance correlations in adult mouse BFCs. (a) Raw leak-subtracted I_Kd and I_A (black traces). Voltage steps used to elicit the currents are shown in gray below the currents. (b) g_Kd and g_A of all cells recorded are plotted against each other. (c) g_A vs g_h. (d) g_Kd vs g_h. Pearson product-moment correlation coefficients (ρ) and statistical significance (P) are shown in b, c and d; regression line is shown in b.

Figure 5

Ionic conductance correlations in PD neurons from adult crab STG. g_A is a transient K⁺ conductance, g_HTK is a high-threshold K⁺ current composed of a delayed rectifier current plus a much larger Ca⁺⁺-dependent current, and g_H is a hyperpolarization-activated monovalent cation inward current. Only the correlation lines (minus symbols) of data originally described in Fig. 2 of Temporal et al. are shown for clarity (please see methods therein). Control data (red lines) were obtained from ganglia fully exposed to the natural neuromodulatory environment of the STG; Decentralized data (black lines) were obtained from ganglia whose neuromodulatory input was removed 24 hrs prior to the recordings. The dashed lines in b and c indicate that the correlations are not statistical significance (P > 0.05) under those conditions.