Elevated postsynaptic [Ca2+]i and L-type calcium channel activity in aged hippocampal neurons: relationship to impaired synaptic plasticity - PubMed (original) (raw)

Elevated postsynaptic [Ca2+]i and L-type calcium channel activity in aged hippocampal neurons: relationship to impaired synaptic plasticity

O Thibault et al. J Neurosci. 2001.

Abstract

Considerable evidence supports a Ca(2+) dysregulation hypothesis of brain aging and Alzheimer's disease. However, it is still not known whether (1) intracellular [Ca(2+)](i) is altered in aged brain neurons during synaptically activated neuronal activity; (2) altered [Ca(2+)](i) is directly correlated with impaired neuronal plasticity; or (3) the previously observed age-related increase in L-type voltage-sensitive Ca(2+) channel (L-VSCC) density in hippocampal neurons is sufficient to impair synaptic plasticity. Here, we used confocal microscopy to image [Ca(2+)](i) in single CA1 neurons in hippocampal slices of young-adult and aged rats during repetitive synaptic activation. Simultaneously, we recorded intracellular EPSP frequency facilitation (FF), a form of short-term synaptic plasticity that is impaired with aging and inversely correlated with cognitive function. Resting [Ca(2+)](i) did not differ clearly with age. Greater elevation of somatic [Ca(2+)](i) and greater depression of FF developed in aged neurons during 20 sec trains of 7 Hz synaptic activation, but only if the activation triggered repetitive action potentials for several seconds. Elevated [Ca(2+)](i) and FF also were negatively correlated in individual aged neurons. In addition, the selective L-VSCC agonist Bay K8644 increased the afterhyperpolarization and mimicked the depressive effects of aging on FF in young-adult neurons. Thus, during physiologically relevant firing patterns in aging neurons, postsynaptic Ca(2+) elevation is closely associated with altered neuronal plasticity. Moreover, selectively increasing postsynaptic L-VSCC activity, as occurs in aging, negatively regulated a form of short-term plasticity that enhances synaptic throughput. Together, the results elucidate novel processes that may contribute to impaired cognitive function in aging.

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Figures

Fig. 1.

Subthreshold frequency facilitation. Representative intracellular voltage traces recorded in a young-adult (A) and an aged (B) CA1 neuron during 7 Hz of synaptic activation below threshold for a Na+ action potential. The first second (left) and the last second (right) of a 20 sec train of repetitive activation are shown. The first EPSP of the first second record is the baseline EPSP. The baseline EPSP (arrow, #) is delimited by two_dotted lines_, extended across the width of the figure to highlight the degree of facilitation during the 20 sec train. Measurement of EPSP facilitation during the train is illustrated by the second arrow (##) and two dashed lines. An initial hyperpolarization accompanies repetitive activation but partly decays with EPSP facilitation by the end of the train (see Materials and Methods). Note that EPSPs during the train are measured from the hyperpolarized membrane potential. C, Mean ± SEM of the percentage of facilitation averaged in 1 sec segments during the 20 sec train (n = 11 young-adult and 11 aged neurons). No significant differences were seen between the two age groups. For simultaneous imaging in the same cells, Ca2+ transients were acquired during the first 2 sec and the last 2 sec of the simulation train (horizontal bar insets).

Fig. 2.

Suprathreshold frequency facilitation. Representative traces from a young-adult (A) and an aged (B) CA1 neuron during the first and last sec of 7 Hz of synaptic stimulation above threshold for a Na+ action potential. In the young-adult neuron, the baseline EPSP (#, dotted line) was facilitated rapidly and then depressed slightly over the 20 sec train. In the aged neuron, the initial degree of facilitation (##, dashed lines) was similar, but the EPSP depressed nearly to baseline by the end of the stimulation train (FF last sec). C, Mean ± SEM data (n = 15 young-adult and 16 aged neurons). *Significant age difference (p < 0.05); *> significant age difference at this point and all points thereafter. Imaging transients were acquired during the first 2 sec and the last 2 sec of the simulation train (bar insets). Action potentials are truncated for illustrative purposes.

Fig. 3.

A, Representative pseudocolor Indo-1 ratiometric images acquired from the somata of a young-adult (top) and an aged (bottom) CA1 pyramidal neuron in hippocampal slices during repetitive suprathreshold stimulation. Images were acquired at rest (left), as well as during the first 2 sec of the 7 Hz stimulation (FFearly, middle) and the last 2 sec of stimulation (FFlate, right). Note the substantially greater elevation of [Ca2+]i in the aged neuron (A, bottom) at FFlate. Electrophysiologic traces above each image are simultaneous current-clamp recordings from the same cells demonstrating comparable action potential generation and similar responses in the two cells.B, Mean ± SEM results for measures of [Ca2+]i obtained at rest, during the first 2 sec, and during the last 2 sec of the 20 sec trains of the 7 Hz synaptic activation. Left, Same cells as shown in Figure 1_C_, studied during the subthreshold stimulation protocol (n = 11 per group).Right, Same cells shown in Figure 2_C_during the suprathreshold stimulation protocol (n = 15 young-adult and 16 aged neurons). Significant aging differences were found for [Ca2+]i in the suprathreshold but not in the subthreshold protocol. *p < 0.05 by ANOVA and _post hoc_analysis.

Fig. 4.

Correlations between mean EPSP amplitude and [Ca2+]i during the last 2 sec period (FFlate) of the 20 sec train of suprathreshold stimulation in young-adult (A) and aged (B) neurons (open and_filled diamonds_, respectively). A significant inverse correlation was seen in aged neurons (r = −0.54;p < 0.05) but not in young-adult neurons. In both panels, the thick line shows the regression line and the thin curved lines delineate 95% confidence limits.

Fig. 5.

Representative example of Ca2+imaging with the visible wavelength indicator Calcium Orange.A, Montage of three frames of a young-adult CA1 neuron including soma and apical dendritic shaft. B, Example of %Δ_F/F_ in different segments of a young-adult neuron in which an action potential was generated on each pulse of the 20 sec suprathreshold 7 Hz stimulation train. As reported previously by others, the most rapid Ca2+ changes appeared in the proximal and middle regions of the apical dendrite.

Fig. 6.

Representative examples of AHPs recorded in slices from young-adult animals. A, Superimposed AHPs were triggered from a holding potential of −60 mV at 5 min after break-in and after an additional 15 min of 0.1% DMSO bath perfusion. Three action potentials were generated on each depolarizing pulse. No differences were noted with DMSO perfusion. B, A similar recording paradigm and superimposition of AHP traces obtained before and after 15 min of 1 μ

Bay K8644 bath perfusion (larger trace). The same number of action potentials was triggered before and after drug or vehicle. Bay K8644 nearly doubled the AHP (Table 2). Action potentials are truncated for illustrative purposes.

Fig. 7.

Representative voltage traces of the first and last seconds of repetitive FF stimulation in young-adult neurons recorded with or without Bay K8644 exposure. A, After 15 min of exposure to DMSO alone, EPSP facilitation was well-maintained throughout the duration of the train. B, In contrast, after 15 min of Bay K8644 exposure, strong EPSP depression was seen by the end of the stimulation protocol along with significant membrane depolarization (Table 2). To offset the Bay K8644-induced depolarization during FF, some cells were hyperpolarized during the stimulation train. Despite hyperpolarization, Bay K8644 still significantly depressed FF (data not shown). C, Mean ± SEM for EPSP frequency facilitation during 7 Hz of stimulation in DMSO control neurons or Bay K8644-treated neurons. Bay K8644 significantly depressed FF after the fourth second of stimulation compared with DMSO control conditions. *> Bay K8644 and vehicle significantly different (p < 0.05) at this point and each point thereafter.

References

1. Alford S, Frenguelli BG, Schofield JG, Collingridge L. Characterization of Ca2+ signals induced in hippocampal CA1 neurons by the synaptic activation of NMDA receptors. J Physiol (Lond) 1993;469:693–716. - PMC - PubMed
1. Alger BE, Teyler TJ. Potassium and short-term response plasticity in the hippocampal slice. Brain Res. 1978;159:239–242. - PubMed
1. Andersen P, Lomo T. Mode of control of hippocampal pyramidal cell discharges. In: Whalen RE, Thompson RF, Verzeano M, Weinberger NM, editors. The neural control of behavior. Academic; New York: 1970. pp. 3–25.
1. Applegate MD, Landfield PW. Synaptic vesicle redistribution during hippocampal frequency potentiation and depression in young and aged rats. J Neurosci. 1988;8:1096–1111. - PMC - PubMed
1. Bach ME, Barad M, Son H, Zhuo M, Lu YF, Shih R, Mansuy I, Hawkins RD, Kandel ER. Age-related defects in spatial memory are correlated with defects in the late phase of hippocampal long-term potentiation in vitro and are attenuated by drugs that enhance the cAMP signaling pathway. Proc Natl Acad Sci USA. 1999;96:5280–5285. - PMC - PubMed

Elevated postsynaptic [Ca2+]i and L-type calcium channel activity in aged hippocampal neurons: relationship to impaired synaptic plasticity - PubMed (original) (raw)