Wendy W. Wu

Age-related biophysical alterations of hippocampal pyramidal neurons: Implications for learning and memory

Normal brain aging is associated with deficits in learning and memory. The hippocampus, a structure critical for proper learning and memory functions, is frequently implicated in aging-related learning deficits. There are many reports of learning-related changes in hippocampal pyramidal neurons from animals that were trained in hippocampus-dependent learning paradigms. One consistent finding in hippocampal pyramidal neurons is a learning-related increase in postsynaptic neuronal excitability, resulting from a reduction in the postburst afterhyperpolarization (AHP). The hippocampus, as well as the ability to acquire hippocampus-dependent tasks, is particularly affected by aging. Correspondingly, hippocampal neurons also display an age-related decrease in excitability, resulting from an enhanced AHP. The correlation between neuronal excitability and learning ability strongly suggests that changes in the AHP are critically involved in learning and age-related learning deficits. Additional support for this argument comes from in vitro studies that examined the effect of compounds that facilitated learning in aging animals on the properties of CA1 pyramidal neurons. Many of these compounds increased the excitability of CA1 pyramidal neurons by reducing the AHP. Subsequent voltage-clamp recordings showed that AHP reduction by these compounds mainly reflects the reduction of two of its currents, the I(AHP) and the sI(AHP). Conversely, age-related AHP enhancements primarily impact the I(AHP) and the sI(AHP). Given that the I(AHP) accounts for a small portion of the total AHP, and that the sI(AHP) is the AHP current that most critically modulates neuronal excitability, changes in neuronal excitability seen in learning and in aging are predominantly caused by changes in the sI(AHP). The fact that the sI(AHP) receives neuromodulation from many transmitter systems important for learning and sensitive to aging lends further support for its role in age-related learning deficits. In this article, we review: (1) two hippocampus-dependent learning tasks, trace eyeblink conditioning and Morris water maze training, that are used extensively in our laboratory to examine learning and aging-related learning deficits; (2) aging-related changes in several important neurotransmitter systems, and how the these changes impact learning and memory functions during aging; and (3) changes in the AHP and the sI(AHP) in hippocampal pyramidal neurons in relation to compromised neurotransmission, as well as to learning, in aging animals. The correlations between a reduction in the sI(AHP) in learning, and an enhancement in the sI(AHP) in aging provide compelling evidence that this current plays a critical role in cognitive functions, and further suggest that the key modulators of the AHP are good candidates for future therapeutic interventions in age-related neurodegenerative diseases.

Biophysical alterations of hippocampal pyramidal neurons in learning, ageing and Alzheimer's disease

A series of behavioral, electrophysiological, and molecular biochemical experiments are reviewed indicating that when animals learn hippocampus-dependent tasks, output neurons in the CA1 and CA3 hippocampal subfields show reductions in the slow, post-burst afterhyperpolarization (AHP). The slow AHP is mediated by an apamin-insensitive calcium-activated potassium current. A reduction in the slow AHP makes hippocampal neurons more excitable and facilitates NMDA receptor-mediated response and temporal summation. During normal aging and in a mouse model of Alzheimers disease (AD), the slow AHP is increased, making neurons less excitable and making learning more difficult. The subgroup of aging animals that are able to learn demonstrates the capacity to increase neuronal excitability by reducing the size of the slow AHP. Similarly, in a mouse model of AD, mice that are able to learn normally after a genetic alteration have a normal capacity for increasing hippocampal neuron excitability by reducing their slow AHP. We suggest that reduction in the slow AHP is basic to learning in young and aging animals. Inability to modulate the slow AHP contributes to learning deficits that occur during aging and early stages of AD.

Coupling of L-Type Ca2+ Channels to Kv7/KCNQ Channels Creates a Novel, Activity-Dependent, Homeostatic Intrinsic Plasticity

Experience-dependent modification in the electrical properties of central neurons is a form of intrinsic plasticity that occurs during development and has been observed following behavioral learning. We report a novel form of intrinsic plasticity in hippocampal CA1 pyramidal neurons mediated by the KV7/KCNQ and CaV1/L-type Ca2+ channels. Enhancing Ca2+ influx with a conditioning spike train (30 Hz, 3 s) potentiated the KV7/KCNQ channel function and led to a long-lasting, activity-dependent increase in spike frequency adaptation-a gradual reduction in the firing frequency in response to sustained excitation. These effects were abolished by specific blockers for CaV1/L-type Ca2+ channels, KV7/KCNQ channels, and protein kinase A (PKA). Considering the widespread expression of these two channel types, the influence of Ca2+ influx and subsequent activation of PKA on KV7/KCNQ channels may represent a generalized principle in fine tuning the output of central neurons that promotes stability in firing-an example of homeostatic regulation of intrinsic membrane excitability.

Ovarian Hormone Deficiency Reduces Intrinsic Excitability and Abolishes Acute Estrogen Sensitivity in Hippocampal CA1 Pyramidal Neurons

Premature and uncompensated loss of ovarian hormones following ovariectomy (OVX) elevates the risks of cognitive impairment and dementia. These risks are prevented with estrogen (E2)-containing hormone replacement therapy initiated shortly following OVX but not after substantial delay. Currently, the cellular bases underlying these clinical findings are unknown. At the cellular level, intrinsic membrane properties regulate the efficiency of synaptic inputs to initiate output action potentials (APs), thereby affecting neuronal communication, hence cognitive processing. This study tested the hypothesis that in CA1 pyramidal neurons, intrinsic membrane properties and their acute regulation by E2 require ovarian hormones for maintenance. Whole-cell current-clamp recordings were performed on neurons from ∼7-month-old OVX rats that experienced either short-term (10 d, control OVX) or long-term (5 months, OVXLT) ovarian hormone deficiency. The results reveal that long-term hormone deficiency reduced intrinsic membrane excitability (IE) as measured by the number of evoked APs and firing duration for a given current injection. This was accompanied by AP broadening, an increased slow afterhyperpolarization (sAHP), and faster accumulation of NaV channel inactivation during repetitive firing. In the control OVX neurons, E2 acutely increased IE and reduced the sAHP. In contrast, acute regulation of IE by E2 was absent in the OVXLT neurons. Since the degree of IE of hippocampal pyramidal neurons is positively related with hippocampus-dependent learning ability, and modulation of IE is observed following successful learning, these findings provide a framework for understanding hormone deficiency-related cognitive impairment and the critical window for therapy initiation.

IK1 channels do not contribute to the slow afterhyperpolarization in pyramidal neurons

In pyramidal neurons such as hippocampal area CA1 and basolateral amygdala, a slow afterhyperpolarization (sAHP) follows a burst of action potentials, which is a powerful regulator of neuronal excitability. The sAHP amplitude increases with aging and may underlie age related memory decline. The sAHP is due to a Ca2+-dependent, voltage-independent K+ conductance, the molecular identity of which has remained elusive until a recent report suggested the Ca2+-activated K+ channel, IK1 (KCNN4) as the sAHP channel in CA1 pyramidal neurons. The signature pharmacology of IK1, blockade by TRAM-34, was reported for the sAHP and underlying current. We have examined the sAHP and find no evidence that TRAM-34 affects either the current underling the sAHP or excitability of CA1 or basolateral amygdala pyramidal neurons. In addition, CA1 pyramidal neurons from IK1 null mice exhibit a characteristic sAHP current. Our results indicate that IK1 channels do not mediate the sAHP in pyramidal neurons. DOI: http://dx.doi.org/10.7554/eLife.11206.001

Autonomic dysfunction with mutations in the gene that encodes methyl-CpG-binding protein 2: Insights into Rett syndrome

Rett syndrome (RTT) is an autism spectrum disorder with an incidence of ~1:10,000 females (reviewed in Bird, 2008; Chahrour et al., 2007; Francke, 2006). Affected individuals are apparently normal at birth. Between 6-18 months of age, however, RTT patients begin to exhibit deceleration of head growth, replacement of purposeful hand movements with stereotypic hand wringing, loss of speech, social withdrawal and other autistic features. RTT is caused by loss of function mutations in the gene that encodes methyl-CpG-binding protein 2 (Mecp2) (Amir et al., 1999), a transcriptional repressor that targets genes essential for neuronal survival, dendritic growth, synaptogenesis, and activity dependent plasticity. MECP2 is X-linked, and males die soon after birth. Included in the RTT phenotype are cardiorespiratory disorders involving the autonomic nervous system. The respiratory disorders, including the roles of bioaminergic and brain derived neurotrophic factor (BDNF) signaling in the respiratory pathophysiology of RTT have been recently reviewed (Bissonnette et al., 2007a; Ogier et al., 2008; Katz et al., 2009). Here we will cover the work on RTT regarding respiration that has appeared since 2009 as well as cardiovascular abnormalities.

Developmental profile of SK2 channel expression and function in CA1 neurons

We investigated the temporal and spatial expression of SK2 in the developing mouse hippocampus using molecular and biochemical techniques, quantitative immunogold electron microscopy, and electrophysiology. The mRNA encoding SK2 was expressed in the developing and adult hippocampus. Western blotting and immunohistochemistry showed that SK2 protein increased with age. This was accompanied by a shift in subcellular localization. Early in development (P5), SK2 was predominantly localized to the endoplasmic reticulum in the pyramidal cell layer. But by P30 SK2 was almost exclusively expressed in the dendrites and spines. The level of SK2 at the postsynaptic density (PSD) also increased during development. In the adult, SK2 expression on the spine plasma membrane showed a proximal‐to‐distal gradient. Consistent with this redistribution and gradient of SK2, the selective SK channel blocker apamin increased evoked excitatory postsynaptic potentials (EPSPs) only in CA1 pyramidal neurons from mice older than P15. However, the effect of apamin on EPSPs was not different between synapses in proximal or distal stratum radiatum or stratum lacunosum‐moleculare in adult. These results show a developmental increase and gradient in SK2‐containing channel surface expression that underlie their influence on neurotransmission, and that may contribute to increased memory acquisition during early development.

Galantamine increases excitability of CA1 hippocampal pyramidal neurons.

Galantamine is a third generation cholinesterase inhibitor and an allosteric potentiating ligand of nicotinic acetylcholine receptors. It enhances learning in aging rabbits and alleviates cognitive deficits observed in patients with Alzheimers disease. We examined galantamines effect on CA1 neurons from hippocampal slices of young and aging rabbits using current-clamp, intracellular recording techniques. Galantamine (10-200 microM) dose-dependently reduced the postburst afterhyperpolarization and the spike-frequency accommodation of CA1 neurons from both young and aging animals. These reductions were partially, but significantly, reversed by the addition of the muscarinic receptor antagonist, atropine (1 microM), to the perfusate. In contrast, the nicotinic acetylcholine receptor antagonist, alpha-bungarotoxin (10 nM), had no effect; i.e. alpha-bungarotoxin did not reverse the afterhyperpolarization and accommodation reductions. The allosteric potentiating ligand effect was examined by stimulating the Schaffer collateral and measuring the excitatory postsynaptic potentials for 30 min during bath application of galantamine. Galantamine (200 microM) significantly enhanced the excitatory postsynaptic potential amplitude and area over time. These effects were blocked by 10 nM alpha-bungarotoxin, supporting a role for galantamine as an allosteric potentiating ligand. We did not observe a facilitation of the excitatory postsynaptic potentials with 1 microM galantamine. However, when the excitatory postsynaptic potential was pharmacologically isolated by adding 10 microM gabazine (GABA(A) receptor antagonist) to the perfusate, 1 microM galantamine potentiated the subthreshold excitatory postsynaptic potentials into action potentials. We propose that the learning enhancement observed in aging animals and the alleviation of cognitive deficits associated with Alzheimers disease after galantamine treatment may in part be due to the enhanced function of both nicotinic and muscarinic excitatory transmission on hippocampal pyramidal neurons.

Ovarian Hormone Loss Impairs Excitatory Synaptic Transmission at Hippocampal CA3–CA1 Synapses

Premature and long-term ovarian hormone loss following ovariectomy (OVX) is associated with cognitive impairment. This condition is prevented by estradiol (E2) therapy when initiated shortly following OVX but not after substantial delay. To determine whether these clinical findings are correlated with changes in synaptic functions, we used adult OVX rats to evaluate the consequences of short-term (7–10 d, OVXControl) and long-term (∼5 months, OVXLT) ovarian hormone loss, as well as subsequent in vivo E2 treatment, on excitatory synaptic transmission at the hippocampal CA3–CA1 synapses important for learning and memory. The results show that ovarian hormone loss was associated with a marked decrease in synaptic strength. E2 treatment increased synaptic strength in OVXControl but not OVXLT rats, demonstrating a change in the efficacy for E2 5 months following OVX. E2 also had a more rapid effect: within minutes of bath application, E2 acutely increased synaptic strength in all groups except OVXLT rats that did not receive in vivo E2 treatment. E2s acute effect was mediated postsynaptically, and required Ca2+ influx through the voltage-gated Ca2+ channels. Despite E2s acute effect, synaptic strength of OVXLT rats remained significantly lower than that of OVXControl rats. Thus, changes in CA3–CA1 synaptic transmission associated with ovarian hormone loss cannot be fully reversed with delayed E2 treatment. Given that synaptic strength at CA3–CA1 synapses is related to the ability to learn hippocampus-dependent tasks, these findings provide additional insights for understanding cognitive impairment-associated long-term ovarian hormone loss and ineffectiveness for delayed E2 treatment to maintain cognitive functions.

Apamin boosting of synaptic potentials in CaV2.3 R-type Ca2+ channel null mice

SK2- and KV4.2-containing K+ channels modulate evoked synaptic potentials in CA1 pyramidal neurons. Each is coupled to a distinct Ca2+ source that provides Ca2+-dependent feedback regulation to limit AMPA receptor (AMPAR)- and NMDA receptor (NMDAR)-mediated postsynaptic depolarization. SK2-containing channels are activated by Ca2+ entry through NMDARs, whereas KV4.2-containing channel availability is increased by Ca2+ entry through SNX-482 (SNX) sensitive CaV2.3 R-type Ca2+ channels. Recent studies have challenged the functional coupling between NMDARs and SK2-containing channels, suggesting that synaptic SK2-containing channels are instead activated by Ca2+ entry through R-type Ca2+ channels. Furthermore, SNX has been implicated to have off target affects, which would challenge the proposed coupling between R-type Ca2+ channels and KV4.2-containing K+ channels. To reconcile these conflicting results, we evaluated the effect of SK channel blocker apamin and R-type Ca2+ channel blocker SNX on evoked excitatory postsynaptic potentials (EPSPs) in CA1 pyramidal neurons from CaV2.3 null mice. The results show that in the absence of CaV2.3 channels, apamin application still boosted EPSPs. The boosting effect of CaV2.3 channel blockers on EPSPs observed in neurons from wild type mice was not observed in neurons from CaV2.3 null mice. These data are consistent with a model in which SK2-containing channels are functionally coupled to NMDARs and KV4.2-containing channels to CaV2.3 channels to provide negative feedback regulation of EPSPs in the spines of CA1 pyramidal neurons.