Enhanced long-term potentiation during aging is masked by processes involving intracellular calcium stores

The contribution of Ca(2+) release from intracellular Ca(2+) stores (ICS) for regulation of synaptic plasticity thresholds during aging was investigated in hippocampal slices of old (22-24 mo) and young adult (5-8 mo) male Fischer 344 rats. Inhibition of Ca(2+)-induced Ca(2+) release by thapsigargin, cyclopiazonic acid (CPA), or ryanodine during pattern stimulation near the threshold for synaptic modification (5 Hz, 900 pulses) selectively induced long-term potentiation (LTP) to CA1 Schaffer collateral synapses of old rats. Increased synaptic strength was specific to test pathways and blocked by AP-5. Intracellular recordings demonstrated that ICS plays a role in the augmentation of the afterhyperpolarization (AHP) in old rats. The decrease in the AHP by ICS inhibition was reversed by the L-channel agonist, Bay K8644. Under conditions of ICS inhibition and a Bay K8644-mediated enhancement of the AHP, pattern stimulation failed to induce LTP, consistent with the idea that the AHP amplitude shapes the threshold for LTP induction. Finally, ICS inhibition was associated with an increase in the N-methyl-d-aspartate (NMDA) receptor component of synaptic transmission in old animals. This increase in the synaptic response was blocked by the calcineurin inhibitor FK506. The results reveal an age-related increase in susceptibility to LTP-induction that is normally inhibited by ICS and suggest that the age-related shift in Ca(2+) regulation and Ca(2+)-dependent synaptic plasticity is coupled to changes in cell excitability and NMDA receptor function through ICS.     

Redox sensitive calcium stores underlie enhanced after hyperpolarization of aged neurons: role for ryanodine receptor mediated calcium signaling

A decrease in the excitability of CA1 pyramidal neurons contributes to the age related decrease in hippocampal function and memory decline. Decreased neuronal excitability in aged neurons can be observed as an increase in the Ca(2+)- activated K(+)- mediated post burst afterhyperpolarization (AHP). In this study, we demonstrate that the slow component of AHP (sAHP) in aged CA1 neurons (aged-sAHP) is decreased ～50% by application of the reducing agent dithiothreitol (DTT). The DTT-mediated decrease in the sAHP was age specific, such that it was observed in CA1 pyramidal neurons of aged (20-25 mo), but not young (6-9 mo) F344 rats. The effect of DTT on the aged-sAHP was blocked following depletion of intracellular Ca(2+) stores (ICS) by thapsigargin or blockade of ryanodine receptors (RyRs) by ryanodine, suggesting that the age-related increase in the sAHP was due to release of Ca(2+) from ICS through redox sensitive RyRs. The DTT-mediated decrease in the aged-sAHP was not blocked by inhibition of L-type voltage gated Ca(2+) channels (L-type VGCC), inhibition of Ser/Thr kinases, or inhibition of the large conductance BK potassium channels. The results add support to the idea that a shift in the intracellular redox state contributes to Ca(2+) dysregulation during aging.     

Calcineurin as a potential contributor in estradiol regulation of hippocampal synaptic function

Estradiol influences Ca(2+) regulation and Ca(2+)-dependent synaptic plasticity, suggesting estrogenic effects on Ca(2+)-dependent enzymes that regulate synaptic plasticity may mediate hormonal influences on cognition. In ovariectomized female rats, injections of estradiol benzoate (EB, 10 microg) reduced hippocampal cytosolic activity of serine/threonine protein phosphatases, calcineurin and protein phosphatase 1 (PP1). The decreased activity was rapid and recovered substantially over a 24-h period. Decreased calcineurin activity was associated with a decreased level of calcineurin in the cytosol. In contrast, expression of PP1 was not altered suggesting that the level of calcineurin activity regulated PP1 activity. EB application to hippocampal slices rapidly decreased cytosolic phosphatase activity, which was not blocked by the estrogen receptor antagonist, ICI 182780. Decreased phosphatase activity was associated with an increase in CA3-CA1 synaptic transmission. In addition, EB application shifted synaptic plasticity, blocking the induction of long-term depression and facilitating the establishment of long-term potentiation. The reduction in calcineurin activity and shift in synaptic plasticity were mimicked to a lesser extent by 17-alpha-estradiol. From these results we suggest that EB can act to rapidly influence Ca(2+) signaling pathways including the activity of Ca(2+)-regulated phosphatases involved in synaptic plasticity.     

Electrophysiological roles of L-type channels in different classes of guinea pig sympathetic neuron.

The electrophysiological consequences of blocking Ca(2+) entry through L-type Ca(2+) channels have been examined in phasic (Ph), tonic (T), and long-afterhyperpolarizing (LAH) neurons of intact guinea pig sympathetic ganglia isolated in vitro. Block of Ca(2+) entry with Co(2+) or Cd(2+) depolarized T and LAH neurons, reduced action potential (AP) amplitude in Ph and LAH neurons, and increased AP half-width in Ph neurons. The afterhyperpolarization (AHP) and underlying Ca(2+)-dependent K(+) conductances (gKCa1 and gKCa2) were reduced markedly in all classes. Addition of 10 microM nifedipine increased input resistance in LAH neurons, raised AP threshold in Ph and LAH neurons, and caused a small increase in AP half-width in Ph neurons. AHP amplitude and the amplitude and decay time constant of gKCa1 were reduced by nifedipine in all classes; the slower conductance, gKCa2, which underlies the prolonged AHP in LAH neurons, was reduced by 40%. Surprisingly, AHP half-width was lengthened by nifedipine in a proportion of neurons in all classes; despite this, neuron excitability was increased during a maintained depolarization. Nifedipine's effects on AHP half-width were not mimicked by 2 mM Cs(+) or 2 mM anthracene-9-carboxylic acid, a blocker of Cl(-) channels, and it did not modify transient outward currents of the A or D types. The effects of 100 microM Ni(2+) differed from those of nifedipine. Thus in Ph neurons, Ca(2+) entry through L-type channels during a single action potential contributes to activation of K(+) conductances involved in both the AP and AHP, whereas in T and LAH neurons, it acts only on gKCa1 and gKCa2. These results differ from the results in rat superior cervical ganglion neurons, in which L-type channels are selectively coupled to BK channels, and in hippocampal neurons, in which L-type channels are selectively coupled to SK channels. We conclude that the sources of Ca(2+) for activating the various Ca(2+)-activated K(+) conductances are distinct in different types of neuron.     

Diversity of channels involved in Ca(2+) activation of K(+) channels during the prolonged AHP in guinea-pig sympathetic neurons

The types of Ca(2+)-dependent K(+) channel involved in the prolonged afterhyperpolarization (AHP) in a subgroup of sympathetic neurons have been investigated in guinea pig celiac ganglia in vitro. The conductance underlying the prolonged AHP (gKCa2) was reduced to a variable extent in 100 nM apamin, an antagonist of SK-type Ca(2+)-dependent K(+) channels, and by about 55% in 20 nM iberiotoxin, an antagonist of BK-type Ca(2+)-dependent K(+) channels. The reductions in gKCa2 amplitude by apamin and iberiotoxin were not additive, and a resistant component with an amplitude of nearly 50% of control remained. These data imply that, as well as apamin- and iberiotoxin-sensitive channels, other unknown Ca(2+)-dependent K(+) channels participate in gKCa2. The resistant component of gKCa2 was not abolished by 0.5-10 mM tetraethylammonium, 1 mM 4-aminopyridine, or 5 mM glibenclamide. We also investigated which voltage-gated channels admitted Ca(2+) for the generation of gKCa2. Blockade of Ca(2+) entry through L-type Ca(2+) channels has previously been shown to reduce gKCa2 by about 40%. Blockade of N-type Ca(2+) channels (with 100 nM omega-conotoxin GVIA) and P-type Ca(2+) channels (with 40 nM omega-agatoxin IVA) each reduced the amplitude of gKCa2 by about 35%. Thus Ca(2+) influx through multiple types of voltage-gated Ca(2+) channel can activate the intracellular mechanisms that generate gKCa2. The slow time course of gKCa2 may be explained if activation of multiple K(+) channels results from Ca(2+) influx triggering a kinetically invariant release of Ca(2+) from intracellular stores located close to the membrane.     

Dendritic voltage-gated ion channels regulate the action potential firing mode of hippocampal CA1 pyramidal neurons.

The role of dendritic voltage-gated ion channels in the generation of action potential bursting was investigated using whole cell patch-clamp recordings from the soma and dendrites of CA1 pyramidal neurons located in hippocampal slices of adult rats. Under control conditions somatic current injections evoked single action potentials that were associated with an afterhyperpolarization (AHP). After localized application of 4-aminopyridine (4-AP) to the distal apical dendritic arborization, the same current injections resulted in the generation of an afterdepolarization (ADP) and multiple action potentials. This burst firing was not observed after localized application of 4-AP to the soma/proximal dendrites. The dendritic 4-AP application allowed large-amplitude Na(+)-dependent action potentials, which were prolonged in duration, to backpropagate into the distal apical dendrites. No change in action potential backpropagation was seen with proximal 4-AP application. Both the ADP and action potential bursting could be inhibited by the bath application of nonspecific concentrations of divalent Ca(2+) channel blockers (NiCl and CdCl). Ca(2+) channel blockade also reduced the dendritic action potential duration without significantly affecting spike amplitude. Low concentrations of TTX (10-50 nM) also reduced the ability of the CA1 neurons to fire in the busting mode. This effect was found to be the result of an inhibition of backpropagating dendritic action potentials and could be overcome through the coordinated injection of transient, large-amplitude depolarizing current into the dendrite. Dendritic current injections were able to restore the burst firing mode (represented as a large ADP) even in the presence of high concentrations of TTX (300-500 microM). These data suggest the role of dendritic Na(+) channels in bursting is to allow somatic/axonal action potentials to backpropagate into the dendrites where they then activate dendritic Ca(2+) channels. Although it appears that most Ca(2+) channel subtypes are important in burst generation, blockade of T- and R-type Ca(2+) channels by NiCl (75 microM) inhibited action potential bursting to a greater extent than L-channel (10 microM nimodipine) or N-, P/Q-type (1 microM omega-conotoxin MVIIC) Ca(2+) channel blockade. This suggest that the Ni-sensitive voltage-gated Ca(2+) channels have the most important role in action potential burst generation. In summary, these data suggest that the activation of dendritic voltage-gated Ca(2+) channels, by large-amplitude backpropagating spikes, provides a prolonged inward current that is capable of generating an ADP and burst of multiple action potentials in the soma of CA1 pyramidal neurons. Dendritic voltage-gated ion channels profoundly regulate the processing and storage of incoming information in CA1 pyramidal neurons by modulating the action potential firing mode from single spiking to burst firing.     

Differential dendritic Ca2+ signalling in young and mature hippocampal granule cells

Neuronal activity is critically important for development and plasticity of dendrites, axons and synaptic connections. Although Ca(2+) is an important signal molecule for these processes, not much is known about the regulation of the dendritic Ca(2+) concentration in developing neurons. Here we used confocal Ca(2+) imaging to investigate dendritic Ca(2+) signalling in young and mature hippocampal granule cells, identified by the expression of the immature neuronal marker polysialated neural cell adhesion molecule (PSA-NCAM). Using the Ca(2+)-sensitive fluorescent dye OGB-5N, we found that both young and mature granule cells showed large action-potential evoked dendritic Ca(2+) transients with similar amplitude of approximately 200 nm, indicating active backpropagation of action potentials. However, the decay of the dendritic Ca(2+) concentration back to baseline values was substantially different with a decay time constant of 550 ms in young versus 130 ms in mature cells, leading to a more efficient temporal summation of Ca(2+) signals during theta-frequency stimulation in the young neurons. Comparison of the peak Ca(2+) concentration and the decay measured with different Ca(2+) indicators (OGB-5N, OGB-1) in the two populations of neurons revealed that the young cells had an approximately 3 times smaller endogenous Ca(2+)-binding ratio ( approximately 75 versus approximately 220) and an approximately 10 times slower Ca(2+) extrusion rate ( approximately 170 s(-1) versus approximately 1800 s(-1)). These data suggest that the large dendritic Ca(2+) signals due to low buffer capacity and slow extrusion rates in young granule cells may contribute to the activity-dependent growth and plasticity of dendrites and new synaptic connections. This will finally support differentiation and integration of young neurons into the hippocampal network.     

Hippocampal 'zipper' slice studies reveal a necessary role for calcineurin in the increased activity of L-type Ca(2+) channels with aging

Previous studies have shown that inhibition of the Ca(2+)-/calmodulin-dependent protein phosphatase calcineurin (CN) blocks L-type voltage sensitive Ca(2+) channel (L-VSCC) activity in cultured hippocampal neurons. However, it is not known whether CN contributes to the increase in hippocampal L-VSCC activity that occurs with aging in at least some mammalian species. It is also unclear whether CN's necessary role in VSCC activity is simply permissive or is directly enhancing. To resolve these questions, we used partially dissociated hippocampal "zipper" slices to conduct cell-attached patch recording and RT-PCR on largely intact single neurons from young-adult, mid-aged, and aged rats. Further, we tested for direct CN enhancement of L-VSCCs using virally mediated infection of cultured neurons with an activated form of CN. Similar to previous work, L-VSCC activity was elevated in CA1 neurons of mid-aged and aged rats relative to young adults. The CN inhibitor, FK-506 (5muM) completely blocked the aging-related increase in VSCC activity, reducing the activity level in aged rat neurons to that in younger rat neurons. However, aging was not associated with an increase in neuronal CN mRNA expression, nor was CN expression correlated with VSCC activity. Delivery of activated CN to primary hippocampal cultures induced an increase in neuronal L-VSCC activity but did not elevate L-VSCC protein levels. Together, the results provide the first evidence that CN activity, but not increased expression, plays a selective and necessary role in the aging-related increase in available L-VSCCs, possibly by direct activation. Thus, these studies point to altered CN function as a novel and potentially key factor in aging-dependent neuronal Ca(2+) dysregulation.     

Activity-dependent increase of the AHP amplitude in T sensory neurons of the leech

We identified a new form of activity-dependent modulation of the afterhyperpolarization (AHP) in tactile (T) sensory neurons of the leech Hirudo medicinalis. Repetitive intracellular stimulation with 30 trains of depolarizing impulses at 15-s inter-stimulus interval (ISI) led to an increase of the AHP amplitude (~60% of the control). The enhancement of AHP lasted for >/=15 min. The AHP increase was also elicited when a T neuron was activated by repetitive stimulation of its receptive field. The ISI was a critical parameter for the induction and maintenance of AHP enhancement. ISI duration had to fit within a time window with the upper limit of 20 s to make the training effective to induce an enhancement of the AHP amplitude. After recovery from potentiation, AHP amplitude could be enhanced once again by delivering another training session. The increase of AHP amplitude persisted in high Mg(2+) saline, suggesting an intrinsic cellular mechanism for its induction. Previous investigations reported that AHP of leech T neurons was mainly due to the activity of the Na(+)/K(+) ATPase and to a Ca(2+)-dependent K(+) current (I(K/Ca)). In addition, it has been demonstrated that serotonin (5HT) reduces AHP amplitude through the inhibition of the Na(+)/K(+) ATPase. By blocking the I(K/Ca) with pharmacological agents, such as cadmium and apamin, we still observed an increase of the AHP amplitude after repetitive stimulation, whereas 5HT application completely inhibited the AHP increment. These data indicate that the Na(+)/K(+) ATPase is involved in the induction and maintenance of the AHP increase after repetitive stimulation. Moreover, the AHP increase was affected by the level of serotonin in the CNS. Finally, the increase of the AHP amplitude produced a lasting depression of the synaptic connection between two T neurons, suggesting that this activity-dependent phenomenon might be involved in short-term plasticity associated with learning processes.     

Aging effects on the limits and stability of long-term synaptic potentiation and depression in rat hippocampal area CA1

Altered hippocampal synaptic plasticity may underlie age-related memory impairment. In acute hippocampal slices from aged (22-24 mo) and young adult (1-12 mo) male Brown Norway rats, extracellular excitatory postsynaptic field potentials were recorded in CA1 stratum radiatum evoked by Schaffer collateral stimulation. We used enhanced Ca(2+) to Mg(2+) ratio and paired-pulse stimulation protocol to induce maximum changes in the synaptic plasticity. Six episodes of theta-burst stimulation (TBS) or nine episodes of paired low-frequency stimulation (pLFS) were used to generate asymptotic long-term potentiation (LTP) and long-term depression (LTD), respectively. In addition, long-term depotentiation (LTdeP) or de-depression (LTdeD) from maximal LTP and LTD were examined using two episodes of pLFS or TBS. Multiple episodes of TBS or pLFS produced significant LTP or LTD in aged and young adult rats; this was not different between age groups. Moreover, there was no significant difference in the amount of LTdeP or LTdeD between aged and young adult rats. Our results show no age differences in the asymptotic magnitude of LTP or LTD, rate of synaptic modifications, development rates, reversal, or decay after postconditioning. Thus impairment of the basic synaptic mechanisms responsible for expression of these forms of plasticity is not likely to account for decline in memory function within this age range.     

L-type calcium channel blockade modifies anesthetic actions on aged hippocampal neurons

Previous studies in our laboratory demonstrated a reversal of anesthetic actions on aged neurons by decreasing extracellular [Ca(2+)] in hippocampal slices. Such maneuver indirectly attenuated Ca(2+) influx, hence decreased exogenous intraneuronal Ca(2+) loads during neuronal activity and consequently improved intracellular Ca(2+) concentration homeostasis. Therefore, in the present study we hypothesized that decreasing exogenous Ca(2+) loads by blocking voltage-gated calcium influx in aged neurons would oppose isoflurane actions. Conversely, increasing endogenous Ca(2+) loads by suppressing calcium efflux during forced reversal of Na(+)/Ca(2+) exchanger function would enhance anesthetic effects. Hippocampal slices were prepared from young (2-4 months) and old (24-26 months) Fischer 344 rats. Isoflurane depressed the evoked dendritic field excitatory postsynaptic potentials by approximately 45% in slices taken from old animals. However, application of isoflurane in addition with CoCl(2) or nifedipine opposed the anesthetic actions, which then depressed the evoked dendritic field postsynaptic potentials by only 15%. Selective blockade of the N-type and P/Q-type calcium channels with omega-conotoxin GVIA and omega-conotoxin MVIIC respectively caused rapid but partial depression of synaptic transmission in slices taken from old Fischer 344 rats. However, isoflurane actions in these aged slices were not affected compared with slices perfused only with normal artificial cerebrospinal fluid. Young and aged slices were then exposed to a low sodium perfusate that forces the Na(+)/Ca(2+) exchanger protein into a reverse mode, thus increasing intracellular Ca(2+) concentration. Isoflurane actions under such conditions were profoundly potentiated in aged slices but were not altered in young hippocampi. The current results show that in aged central neurons, selectively blocking L-type calcium channels opposes anesthetic-induced depression of excitatory synaptic transmission. On the contrary, increasing calcium loads in aged neurons potentiates these actions.     

Action potential throughput in aged rat hippocampal neurons: regulation by selective forms of hyperpolarization

At hippocampal synapses, repetitive synaptic stimulation (RSS) in the theta frequency range (3–12 Hz) is associated with robust EPSP frequency facilitation (FF) and consequently, enhanced action potential (spike) generation and throughput. A complex, synaptically induced hyperpolarization (SIHP) is also triggered by synaptic activation, and a Ca²⁺-dependent afterhyperpolarization (AHP) is triggered above spike threshold. With aging, the AHP is increased and impairs intracellular spike generation, at least in accommodation protocols. However, little is known about how these aging changes interact to affect spike generation at physiological frequencies of RSS, or if the SIHP also is modified in aging. Here we performed the first tests of the net impact of these excitatory and inhibitory aging changes on spike generation during RSS. We report that during RSS at spike threshold (1) spike throughput is well sustained at theta frequencies in young and aged neurons; (2) an interposed AHP dampens spike generation, particularly in aged neurons and at higher frequencies; (3) compared to the AHP, the SIHP does not exert an equivalent inhibitory effect on spike throughput; and (4) in contrast to the AHP, the SIHP is reduced with aging. Together, these results are consistent with a model in which the source of the hyperpolarization is important in determining hippocampal spike throughput within the theta frequency range.     

Relationships between calcium and pH in the regulation of the slow afterhyperpolarization in cultured rat hippocampal neurons

The Ca(2+)-dependent slow afterhyperpolarization (AHP) is an important determinant of neuronal excitability. Although it is established that modest changes in extracellular pH (pH(o)) modulate the slow AHP, the relative contributions of changes in the priming Ca(2+) signal and intracellular pH (pH(i)) to this effect remain poorly defined. To gain a better understanding of the modulation of the slow AHP by changes in pH(o), we performed simultaneous recordings of intracellular free calcium concentration ([Ca(2+)](i)), pH(i), and the slow AHP in cultured rat hippocampal neurons coloaded with the Ca(2+)- and pH-sensitive fluorophores fura-2 and SNARF-5F, respectively, and whole cell patch-clamped using the perforated patch technique. Decreasing pH(o) from 7.2 to 6.5 lowered pH(i), reduced the magnitude of depolarization-evoked [Ca(2+)](i) transients, and inhibited the subsequent slow AHP; opposite effects were observed when pH(o) was increased from 7.2 to 7.5. Although decreases and increases in pH(i) (at a constant pH(o)) reduced and augmented, respectively, the slow AHP in the absence of marked changes in preceding [Ca(2+)](i) transients, the inhibition of the slow AHP by decreases in pH(o) was correlated with low pH(o)-dependent reductions in [Ca(2+)](i) transients rather than the decreases in pH(i) that accompanied the decreases in pH(o). In contrast, high pH(o)-induced increases in the slow AHP were correlated with the accompanying increases in pH(i) rather than high pH(o)-dependent increases in [Ca(2+)](i) transients. The results indicate that changes in pH(o) modulate the slow AHP in a manner that depends on the direction of the pH(o) change and substantiate a role for changes in pH(i) in modulating the slow AHP during changes in pH(o).     

The effects of isoproterenol on abnormal electrical and contractile activity and diastolic calcium are attenuated in myocytes from aged Fischer 344 rats

We investigated whether the age-related decrease in sensitivity of the heart to catecholamines was accompanied by changes in Ca(2+) homeostasis and abnormal electrical and contractile activity caused by beta-adrenergic receptor (beta-AR) stimulation. Ventricular myocytes were isolated from young adult (3 months) and aged (24 months) male Fischer 344 rats. Unloaded cell shortening was measured in field-stimulated myocytes (2Hz, 37 degrees C); membrane currents and action potentials were measured with microelectrodes. Contractile responses to the non-selective beta-AR agonist, isoproterenol were significantly decreased in aged myocytes compared to younger myocytes and aged myocytes were less sensitive to isoproterenol. In contrast, Ca(2+) transients measured simultaneously with contractions were similar between groups. Isoproterenol increased sarcoplasmic reticulum Ca(2+) stores in both groups, but the increase was larger in aged cells. However, signs of Ca(2+) overload induced by isoproterenol were reduced with age. Diastolic Ca(2+) accumulation, contracture and the incidences of transient inward current, oscillatory afterpotentials (OAPs), aftertransients and aftercontractions induced by isoproterenol also were reduced with age. These results demonstrate that aged myocytes exhibit fewer signs of Ca(2+) overload in response to isoproterenol than young adult myocytes. These age-related changes in intracellular Ca(2+) may protect the aging heart against induction of arrhythmias initiated by OAPs.(1).     

Up-regulation of alpha1D Ca2+ channel subunit mRNA expression in the hippocampus of aged F344 rats

There is growing evidence that alterations in calcium (Ca2+) homeostasis may play a role in processes of brain aging and neurodegeneration. There also is evidence that some of the altered Ca2+ homeostasis in hippocampal neurons may arise from an increased density of L-type voltage sensitive Ca2+ channels (L-VSCC). In the present studies, we tested the possibility that previously observed increases in functional L-VSCC with aging might be related to up-regulated gene/mRNA expression for Ca2+ channel subunits. A significant aging-related increase in mRNA content for the alpha1D subunit of the L-type VSCC was observed in hippocampus of aged F344 rats (25 months old) relative to young (4 months old) and middle-aged animals (13 months old), as assessed by both in situ hybridization analyses (densitometry and grain density) and ribonuclease protection assay (RPA). In RPA analyses, the alpha1C subunit mRNA also showed a significant increase in 25-month-old rats. No age changes were seen in mRNA for the beta1b subunit of VSCC or for GAPDH, a standard control. The clearest increases in alpha1D mRNA expression were observed in subfield CA1, with little or no change seen in dentate gyrus. Although these results alone do not demonstrate that mRNA/gene expression changes contribute directly to changes in functional Ca2+ channels, they clearly fulfill an important prediction of that hypothesis. Therefore, these studies may have important implications for the role of gene expression in aging-dependent alterations in brain Ca2+ homeostasis.     

Relationships between intracellular calcium and afterhyperpolarizations in neocortical pyramidal neurons

We examined the effects of recent discharge activity on [Ca2+]i in neocortical pyramidal cells. Our data confirm and extend the observation that there is a linear relationship between plateau [Ca2+]i and firing frequency in soma and proximal apical dendrites. The rise in [Ca2+] activates K+ channels underlying the afterhyperpolarization (AHP), which consists of 2 Ca(2+)-dependent components: the medium AHP (mAHP) and the slow AHP (sAHP). The mAHP is blocked by apamin, indicating involvement of SK-type Ca(2+)-dependent K+ channels. The identity of the apamin-insensitive sAHP channel is unknown. We compared the sAHP and the mAHP with regard to: 1) number and frequency of spikes versus AHP amplitude; 2) number and frequency of spikes versus [Ca2+]i; 3) IAHP versus [Ca2+]i. Our data suggest that sAHP channels require an elevation of [Ca2+]i in the cytoplasm, rather than at the membrane, consistent with a role for a cytoplasmic intermediate between Ca2+ and the K+ channels. The mAHP channels appear to respond to a restricted Ca2+ domain.     

Rescue of cognitive aging by long-lasting environmental enrichment exposure initiated before median lifespan

The rescue of cognitive function through environmental enrichment (EE) during aging has been extensively documented. However, the age at onset, the duration of EE, and the cerebral mechanisms required to obtain the greatest benefits still remain to be determined. We have recently shown that EE applied for 3 mo after the median lifespan, i.e., the age at which 50% of the population is still alive (from 17 to 20 mo in NMRI mice), failed to prevent cognitive deficits in senescent animals. In the present study, mice were exposed to EE prior to the median lifespan, and for a longer total duration (from 14 to 20 mo), before the assessment of memory performance and the electrophysiological properties of hippocampal neuronal networks. The EE prevented memory deficits and reduced anxiety as the animal aged. Moreover, EE attenuated the age-related impairment of basal glutamatergic neurotransmission in CA1 hippocampal slices, and reversed the decrease in isolated N-methyl-D-Aspartate receptor (NMDA-R)-dependent synaptic potentials. Surprisingly, EE did not prevent the age-related alteration of theta-burst-induced long-term potentiation (LTP). This study therefore suggests that EE needs to be initiated before the age corresponding to the median lifespan and/or required long duration (> 3 mo) to have an effect on cognitive aging. In addition, we show that EE probably acts through theta-burst-independent mechanisms of synaptic plasticity.     

Altered dendritic integration in hippocampal granule cells of spatial learning-impaired aged rats

Glutamatergic transmission at central synapses undergoes activity-dependent and developmental changes. In the hippocampal dentate gyrus, the non-N-methyl d-aspartate (NMDA) receptor component of field excitatory postsynaptic potentials (fEPSPs) increases with age in Fischer-344 rats. This effect may not depend on the animal's activity or experience but could be part of the developmental process. Age-dependent differences in synaptic transmission at the perforant path-granule cell synapse may be caused by changes in non-NMDA and NMDA receptor-mediated currents. To test this hypothesis, we compared whole cell excitatory postsynaptic currents (EPSCs) in dentate granule cells evoked by perforant path stimulation in young (3-4 mo) and aged (22-27 mo) Fischer-344 rats using a Cs+-based intracellular solution. Aged animals as a group showed spatial learning and memory deficits in the Morris water maze. Using whole cell recordings, slope conductances of both non-NMDA and NMDA EPSCs at holding potentials -10 to +50 mV were significantly reduced in aged animals and the non-NMDA/NMDA ratio in aged animals was found to be significantly smaller than in young animals. In contrast, we detected no differences in basic electrophysiological parameters, or absolute amplitudes of non-NMDA and NMDA EPSCs. Extracellular Cs+ increased the fEPSP in young slices to a greater degree than was found in the aged slices, while it increased population spikes to a greater degree in the aged rats. Our results not only provide evidence for reduced glutamatergic synaptic responses in Fischer-344 rats but also point to differential changes in Cs+-sensitive dendritic conductances, such as Ih or inwardly rectifying potassium currents, during aging.     

Reduced serine racemase expression contributes to age-related deficits in hippocampal cognitive function

To gain insight into the contribution of d-serine to impaired cognitive aging, we compared the metabolic pathway and content of the amino acid as well as d-serine-dependent synaptic transmission and plasticity in the hippocampus of young and old rats of the Wistar and Lou/C/Jall strains. Wistar rats display cognitive impairments with aging that are not found in the latter strain, which is therefore considered a model of healthy aging. Both mRNA and protein levels of serine racemase, the d-serine synthesizing enzyme, were decreased in the hippocampus but not in the cerebral cortex or cerebellum of aged Wistar rats, whereas the expression of d-amino acid oxidase, which degrades the amino acid, was not affected. Consequently, hippocampal levels of endogenous d-serine were significantly lower. In contrast, serine racemase expression and d-serine levels were not altered in the hippocampus of aged Lou/C/Jall rats. Ex vivo electrophysiological recordings in hippocampal slices showed a marked reduction in N-methyl-d-aspartate-receptor (NMDA-R)-mediated synaptic potentials and theta-burst-induced long-term potentiation (LTP) in the CA1 area of aged Wistar rats, which were restored by exogenous d-serine. In contrast, NMDA-R activation, LTP induction and responses to d-serine were not altered in aged Lou/C/Jall rats.These results further strengthen the notion that the serine racemase-dependent pathway is a prime target of hippocampus-dependent cognitive deficits with aging. Understanding the processes that specifically affect serine racemase during aging could thus provide key insights into the treatment of memory deficits in the elderly.