The calcium-activated slow AHP: cutting through the Gordian knot

Abstract

The phenomenon known as the slow afterhyperpolarization (sAHP) was originally described more than 30 years ago in pyramidal cells as a slow, Ca(2+)-dependent afterpotential controlling spike frequency adaptation. Subsequent work showed that similar sAHPs were widely expressed in the brain and were mediated by a Ca(2+)-activated potassium current that was voltage-independent, insensitive to most potassium channel blockers, and strongly modulated by neurotransmitters. However, the molecular basis for this current has remained poorly understood. The sAHP was initially imagined to reflect the activation of a potassium channel directly gated by Ca(2+) but recent studies have begun to question this idea. The sAHP is distinct from the Ca(2+)-dependent fast and medium AHPs in that it appears to sense cytoplasmic [Ca(2+)](i) and recent evidence implicates proteins of the neuronal calcium sensor (NCS) family as diffusible cytoplasmic Ca(2+) sensors for the sAHP. Translocation of Ca(2+)-bound sensor to the plasma membrane would then be an intermediate step between Ca(2+) and the sAHP channels. Parallel studies strongly suggest that the sAHP current is carried by different potassium channel types depending on the cell type. Finally, the sAHP current is dependent on membrane PtdIns(4,5)P(2) and Ca(2+) appears to gate this current by increasing PtdIns(4,5)P(2) levels. Because membrane PtdIns(4,5)P(2) is essential for the activity of many potassium channels, these finding have led us to hypothesize that the sAHP reflects a transient Ca(2+)-induced increase in the local availability of PtdIns(4,5)P(2) which then activates a variety of potassium channels. If this view is correct, the sAHP current would not represent a unitary ionic current but the embodiment of a generalized potassium channel gating mechanism. This model can potentially explain the cardinal features of the sAHP, including its cellular heterogeneity, slow kinetics, dependence on cytoplasmic [Ca(2+)], high temperature-dependence, and modulation.

Keywords: Ca2+-activated afterhyperpolarization; KCNQ; PtdIns(4,5)P2; neuromodulation; pyramidal cell; sAHP.

Figure 1

The slow sAHP and underlying current (I_sAHP) in neocortical pyramidal neurons from somatosensory cortex. (A) The three AHPs. A single action potential (AP) was elicited with a 10 ms suprathreshold intracellular current injection (spikes truncated by digitization to emphasize afterpotentials). Note the notch following AP repolarization (the fast AHP: fAHP) and subsequent medium AHP (mAHP). (B) Data from the same cell as in (A), except 10 APs were elicited with 10 ms suprathreshold current injections [@ 50 Hz, Panels A and B are redrawn from results presented in Pineda et al. (1998)]. Following the train of APs, two AHP components are evident: the mAHP is the main determinant of the initial peak response. A much slower decaying (τ > 1 s) slow AHP (sAHP) follows (spikes truncated by digitization to emphasize afterpotentials). (C) The sAHP elicited by 1 s repetitive firing is reduced in the presence of the β-agonist isoproterenol [10 μ M: modified from Figure 5A in Abel et al. (2004)]. (D) Tail currents were elicited following voltage steps from −70 mV to 0 mV for different durations. Following the 20 ms step (black trace), only I_fAHP and I_mAHP were observed upon return to −70 mV. The longer, 150 ms step (red trace) elicited both an initial I_mAHP and I_sAHP. Note the slow time to peak (the peak occurs well after the voltage step) and slow decay of the sAHP [τ > 1 s: modified from Figure 7B in Abel et al. (2004)]. (E) In a different cell, the I_sAHP tail current following a 200 ms voltage step to 0 mV and then returning to −70 mV was blocked by 10 μ M isoproterenol, isolating I_mAHP [modified from Figure 7C in Abel et al. (2004)]. (F) Reversal potential for I_sAHP. Tail currents were elicited by a 200 ms step to +10 mV and amplitudes were measured upon return to various potentials. I_mAHP was measured as the peak response and I_sAHP was measured at 500 ms after the peak, when I_mAHP had completely decayed [modified from Figure 8A in Abel et al. (2004)]. (G) Plots of I_AHP amplitudes from data in (F). Extrapolated reversal potentials approximated E_K, as determined by the Nernst equation [E_K = −102 mV: modified from Figure 8B in Abel et al. (2004)]. (H) Plot of isolated I_mAHP vs. bulk cytoplasmic [Ca²⁺]_i. Since the underlying SK channels respond to a sub-membrane microdomain of [Ca²⁺], the dose-response relationship is distorted [data from eight cells; modified from Figure 10C in Abel et al. (2004)]. (I) Plot of isolated I_sAHP vs. bulk cytoplasmic [Ca²⁺]_i. Note the sigmoidal dose-response curve indicating response to a “well-mixed” bulk [Ca²⁺]_i [data from five cells; estimated K_D = ~200 nM, Hill coefficient ~4.5: modified from Figure 9C in Abel et al. (2004)]. Panels A and B were from layer 5A of somatosensory cortex. Panels C–I were from layer 2/3.

Figure 2

Expression of hippocalcin and the phosphatidylinositol 4-phosphate 5-kinase (PIP5K) regulate I_sAHP. (A) Expression of wild type hippocalcin in hippocampal pyramidal cells in primary culture greatly enhances the amplitude of I_sAHP. This enhancement is not seen with the G2A mutant, which cannot be myristoylated, thus pointing to an essential role for the translocation of hippocalcin to the plasma membrane. ^*Indicates p < 0.001. Redrawn from data in Figure 4 of Tzingounis et al. (2007). (B) Expression of PIP5K enhances the apparent ability of calcium to elicit I_sAHP. In this experiment calcium influx was titrated using depolarizing steps of increasing duration. Under control conditions I_sAHP is activated in a graded manner by depolarizing steps ranging from 10 to 100 ms. In contrast, in cells transfected with PIP5K I_sAHP is activated by much shorter steps. Redrawn from data in Figure 7 of Villalobos et al. (2011).

Figure 3

Mechanism for the activation of the sAHP as proposed in this review. Global increases in cytosolic calcium lead to activation of diffusible neuronal calcium sensors (NCS: hippocalcin, neurocalcin δ). Binding of calcium to NCS exposes a previously sequestered myristoyl moiety allowing NCS to bind to the plasma membrane. Binding of NCS to plasma membrane leads to a transient increase in PtdIns(4,5)P₂ levels and subsequent activation of the sAHP.