Aging changes in voltage-gated calcium currents in hippocampal CA1 neurons

Abstract

Previous current-clamp studies in rat hippocampal slice CA1 neurons have found aging-related increases in long-lasting calcium (Ca)-dependent and Ca-mediated potentials. These changes could reflect an increase in Ca influx through voltage-gated Ca channels but also could reflect a change in potassium currents. Moreover, if altered Ca influx is involved, it is nuclear whether it arises from generally increased Ca channel activity, lower threshold, or reduced inactivation. To analyze the basis for altered Ca potentials, whole-cell voltage-clamp studies of CA1 hippocampal neurons were performed in nondissociated hippocampal slices of adult (3- to 5-month-old) and aged (25- to 26-month-old) rats. An aging-related increase was found in high-threshold Ca and barium (Ba) currents, particularly in the less variable, slowly inactivating (late) current at the end of a depolarization step. Input resistance of neurons did not differ between age groups. In steady-state inactivation and repetitive-pulse protocols, inactivation of Ca and Ba currents was not reduced and, in some cases, was slightly greater in aged neurons, apparently because of larger inward current. The current blocked by nimodipine was greater in aged neurons, indicating that some of the aging increase was in L-type currents. These results indicate that whole-cell Ca currents are increased with aging in CA1 neurons, apparently attributable to greater channel activity rather than to reduced inactivation. The elevated Ca influx seems likely to play a role in impaired function and enhanced susceptibility to neurotoxic influences.

Fig. 1.

Voltage-response records of a Cs-loaded CA1 hippocampal neuron from a young animal. A, Intracellular current injection induces a burst of Na action potentials. Note that 2 m CsCl in the pipette blocks the AHP. B, Blocking Na action potentials with TTX unmasks a sharp Ca spike followed by a slow lower amplitude (“hump”) phase lasting >200 msec. C, Additional block of repolarizing K conductances with TEA prevents repolarization of the sharp spike component resulting in a long Ca action potential plateau at near-maximum amplitude (∼2 sec). All records are from the same cell and were recorded at a holding potential of −70 mV; 400 msec horizontal scale bar applies toC only.

Fig. 2.

Aging effects on Ca action potentials.A, Representative examples of Ca spike potentials from CA1 neurons of adult (top) and aged (bottom) animals. The third traceillustrates the intracellular constant current pulse (40 msec) used to trigger the Ca spike. The “hump” or slow plateau phase is consistently larger and prolonged in spikes from aged neurons. However, peak amplitude of the fast spike phase is not different with aging.B, Mean ± SEM. Effects of repetitive stimulation (2 Hz train, 40 msec pulses) in 28 adult and 13 aged rat neurons on the inactivation of Ca spike duration. Relative inactivation was unchanged or slightly greater in the aged rat neurons, possibly reflecting larger Ca influx.

Fig. 3.

Voltage-clamp efficacy and voltage dependence of Ca currents. A, Traces acquired during a similar voltage step amplitude (40 mV) from different holding potentials. Theupper left trace reflects a large rapidly inactivating current elicited from −70 mV, whereas the actual voltage trace obtained during that depolarization (bottom left) shows a sharp deviation from the imposed voltage at the peak of the current (arrow). The efficacy of the voltage clamp can be improved by holding the cell at −40 mV, which inactivates much of the current (upper right). The voltage control during the pulse (bottom right) is improved substantially. A prominent long tail current followed the depolarization induced current (see text) but was not assessed in these studies. B, Mean ± SEM for peak currents of a subset of neurons in each age group (adult, n = 6 neurons; aged,n = 5 neurons). Cells were held at −80 mV and stepped to +15 mV in increments of 5 mV. All points are not plotted.

Fig. 4.

Averaged traces of Ca currents activated by 200 msec depolarizing voltage steps to 0 mV from increasingly positive holding potentials (steady-state inactivation protocol).A, Traces shown are averages of the current responses of CA1 neurons from young-adult (n = 13) and aged (n = 9) animals. The voltage protocols (bottom) are schematic representations.B, Mean ± SEM for the peak currents for the cells averaged in A. C, Mean ± SEM for late current measures for the cells shown in A. At increasingly positive voltages, the rapidly inactivating component decreased relatively more than the late current measured at the end of the pulse. Currents from aged neurons were larger than currents from young-adult neurons, although the differences decreased with increasing inactivation at higher voltages. Both a significant main effect of age and a significant interaction between age and holding potential (reflecting the disappearance of the aging effect at more positive, inactivating voltages) were found.

Fig. 5.

A, Representative examples of Ca currents elicited by repetitive 3 Hz depolarizing voltage steps to 0 mV from a holding potential of −40 mV from a CA1 neuron of a young-adult animal (top) and an aged animal (bottom). Voltage steps (actual) shown in the lowest trace ofA are from the aged cell shown above. Currents were obtained for five 200 msec pulses given at 3 Hz. Holding potentials of −40 mV reduced the rapidly inactivating component and allowed more accurate measures of the slowly inactivating Ca current.B, Mean ± SEM of peak (a) and late (b) current measured in the protocol shown inA. The charge carrier was 2 mm Ca. Both peak and late Ca currents were significantly larger in neurons from aged (n = 22) than from young adult (n = 25) animals, but a significant interaction was found between age and repetitive pulses only for peak current (a). C, Mean ± SEM of peak (a) and late (b) current in neurons (n = 12 adult and 11 aged neurons) with Ba (2 mm) substituted for Ca as the charge carrier. The aging difference still is apparent, but the overall rate of inactivation is slowed. However, a significant interaction between age and repetitive pulses was present for both peak and late currents, possibly reflecting greater inactivation in the aged group.