Age-related enhancement of the slow outward calcium-activated potassium current in hippocampal CA1 pyramidal neurons in vitro

Abstract

Aging is associated with learning deficits and a decrease in neuronal excitability, reflected by an enhanced post-burst afterhyperpolarization (AHP), in CA1 hippocampal pyramidal neurons. To identify the current(s) underlying the AHP altered in aging neurons, whole-cell voltage-clamp recording experiments were performed in hippocampal slices from young and aging rabbits. Similar to previous reports, aging neurons were found to rest at more hyperpolarized potentials and have larger AHPs than young neurons. Given that compounds that reduce the slow outward calcium-activated potassium current (sI(AHP)), a major constituent of the AHP, also facilitate learning in aging animals, the sI(AHP) was pharmacologically isolated and characterized. Aging neurons were found to have an enhanced sI(AHP,) the amplitude of which was significantly correlated to the amplitude of the AHP (r = 0.63; p < 0.001). Thus, an enhanced sI(AHP) contributes to the enhanced AHP in aging. No differences were found in the membrane resistance, capacitance, or kinetic and voltage-dependent properties of the sI(AHP). Because enhanced AHP in aging neurons has been hypothesized to be secondary to an enhanced Ca2+ influx via the voltage-gated L-type Ca2+ channels, we further examined the sI(AHP) in the presence of an L-type Ca2+ channel blocker, nimodipine (10 microm). Nimodipine caused quantitatively greater reductions in the sI(AHP) in aging neurons than in young neurons; however, the residual sI(AHP) was still significantly larger in aging neurons than in young neurons. Our data, in conjunction with previous studies showing a correlation between the AHP and learning, suggest that the enhancement of the sI(AHP) in aging is a mechanism that contributes to age-related learning deficits.

Fig. 1.

Current-clamp recordings showing an aging-related enhancement of the post-burst AHP. A, Voltage traces showing representative AHPs from young and aging neurons.B, Mean AHP amplitude was greater in aging neurons than in young neurons (mean ± SEM; unpaired t test; *p < 0.05).

Fig. 2.

Voltage-clamp recordings showing an age-related enhancement of the sI_AHP. A, Representative sI_AHP tail currents from young and aging neurons, underlain with the voltage-clamp protocol for evoking the sI_AHP tail current.B, The sI_AHP amplitude is greater in aging neurons than in young neurons (mean ± SEM; unpaired t test; **p < 0.01).C, Frequency distribution of the sI_AHP (1 sec amplitude) in young and aging neurons.

Fig. 3.

Ca²⁺ influx and sI_AHP. A, An example of the sI_AHP in response to depolarizing voltage steps from −50 to −10 mV, in increments of 10 mV. For leak subtraction, 2 mV step potentials were used. Note that although increasing the amplitude of the voltage pulse increased the amplitude of the sI_AHP, no significant sI_AHP was observed until an obvious Ca²⁺ transient was elicited by the step potentials.B, Increasing the pulse duration also increased the amplitude of the sI_AHP. The sI_AHP was activated with depolarizing steps from −50 to −30 mV with varying durations. Voltage steps with longer durations allowed for more Ca²⁺ influx, thereby increasing the sI_AHP.

Fig. 4.

Voltage independence and reversal potential of the AHP tail current. A, The protocol for measuring the voltage dependence of the sI_AHP tail current. The sI_AHP tail current was evoked by a 100 msec, 50 mV pulse from a holding potential of −55 mV. After the voltage pulse, membrane potential was stepped to various test potentials for 2 sec before returning to the −55 mV holding potential. For leak subtraction, the neurons were stepped to the test potentials for 2 sec without the 100 msec, 50 mV prepulse to elicit the sI_AHP tail current. B, Representative currents evoked from the voltage-dependence protocol.C, The reversal potential was obtained by linear extrapolation of the voltage versus sI_AHPamplitude at 1 sec to the voltage axis. The reversal potential of the sI_AHP in this example was −90 mV. Similar to previous reports, the sI_AHP conductance did not show any voltage dependence at either age (young:n = 7; aging: n = 11). The test potentials did not alter the decay rate of the current, and amplitude of the sI_AHP measured at 1 sec after the various test potentials (labeled as 3s and denoted with an arrow) remained the same.

Fig. 5.

The effect of L-type Ca²⁺influx on the sI_AHP. Shown are representative current traces from young and aging neurons before and after bath applications of nimodipine.

Fig. 6.

A, Bath application of nimodipine caused a comparable percentage (25–30%) of reduction in the sI_AHP of young and aging neurons.B, The amount of reductions in the sI_AHP (1 sec amplitude) and the AHP current (integrated area) was significantly larger in aging than in young neurons (unpaired t test; *p < 0.05 and **p < 0.001, respectively;n = 31 for aging; n = 25 for young). C, After eliminating the contribution of the L-type Ca²⁺ influx on the sI_AHP with bath-applied nimodipine, the residual sI_AHP and AHP current were still significantly larger in the aging neurons than in the young neurons (unpaired t test; *p<0.05).