Thomas J. O'Dell

Enhanced long-term potentiation and impaired learning in mice with mutant postsynaptic density-95 protein

Specific patterns of neuronal firing induce changes in synaptic strength that may contribute to learning and memory. If the postsynaptic NMDA (N-methyl-D-aspartate) receptors are blocked, long-term potentiation (LTP) and long-term depression (LTD) of synaptic transmission and the learning of spatial information are prevented. The NMDA receptor can bind a protein known as postsynaptic density-95 (PSD-95), which may regulate the localization of and/or signalling by the receptor. In mutant mice lacking PSD-95, the frequency function of NMDA-dependent LTP and LTD is shifted to produce strikingly enhanced LTP at different frequencies of synaptic stimulation. In keeping with neural-network models that incorporate bidirectional learning rules, this frequency shift is accompanied by severely impaired spatial learning. Synaptic NMDA-receptor currents, subunit expression, localization and synaptic morphology are all unaffected in the mutant mice. PSD-95 thus appears to be important in coupling the NMDA receptor to pathways that control bidirectional synaptic plasticity and learning.

Functional screening of 2 Mb of human chromosome 21q22.2 in transgenic mice implicates minibrain in learning defects associated with Down syndrome

Using Down syndrome as a model for complex trait analysis, we sought to identify loci from chromosome 21q22.2 which, when present in an extra dose, contribute to learning abnormalities. We generated low-copy-number transgenic mice, containing four different yeast artificial chromosomes (YACs) that together cover approximately 2 megabases (Mb) of contiguous DNA from 21q22,2. We subjected independent lines derived from each of these YAC transgenes to a series of behavioural and learning assays. Two of the four YACs caused defects in learning and memory in the transgenic animals, while the other two YACs had no effect. The most severe defects were caused by a 570-kb YAC; the interval responsible for these defects was narrowed to a 180-kb critical region as a consequence of YAC fragmentation. This region contains the human homologue of a Drosophila gene, minibrain, and strongly implicates it in learning defects associated with Down syndrome.

Activity-Dependent β-Adrenergic Modulation of Low Frequency Stimulation Induced LTP in the Hippocampal CA1 Region

Abstract β-Adrenergic receptor activation has a central role in the enhancement of memory formation that occurs during heightened states of emotional arousal. Although β-adrenergic receptor activation may enhance memory formation by modulating long-term potentiation (LTP), a candidate synaptic mechanism involved in memory formation, the cellular basis of this modulation is not fully understood. Here, we report that, in the CA1 region of the hippocampus, β-adrenergic receptor activation selectively enables the induction of LTP during long trains of 5 Hz synaptic stimulation. Protein phosphatase inhibitors mimic the effects of β-adrenergic receptor activation on 5 Hz stimulation–induced LTP, suggesting that activation of noradrenergic systems during emotional arousal may enhance memory formation by inhibiting protein phosphatases that normally oppose the induction of LTP.

Phosphatidylinositol 3-Kinase Regulates the Induction of Long-Term Potentiation through Extracellular Signal-Related Kinase-Independent Mechanisms

Inhibitors of both phosphatidylinositol-3-kinase (PI3-kinase) and MAPK/ERK (mitogen-activate protein kinase/extracellular signal-related kinase) activation inhibit NMDA receptor-dependent long-term potentiation (LTP). PI3-kinase inhibitors also block activation of ERK by NMDA receptor stimulation, suggesting that PI3-kinase inhibitors block LTP because PI3-kinase is an essential upstream regulator of ERK activation. To examine this hypothesis, we investigated the effects of PI3-kinase inhibitors on ERK activation and LTP induction in the CA1 region of mouse hippocampal slices. Consistent with the notion that ERK activation by NMDA receptor stimulation is PI3-kinase dependent, the PI3-kinase inhibitor wortmannin partially inhibited ERK2 activation induced by bath application of NMDA and strongly suppressed ERK2 activation by high-frequency synaptic stimulation. PI3-kinase and MEK (MAP kinase kinase) inhibitors had very different effects on LTP, however. Both types of inhibitors suppressed LTP induced by theta-frequency trains of synaptic stimulation, but only PI3-kinase inhibitors suppressed the induction of LTP by high-frequency stimulation or low-frequency stimulation paired with postsynaptic depolarization. Concentrations of PI3-kinase inhibitors that inhibited LTP when present during high-frequency stimulation had no effect on potentiated synapses when applied after high-frequency stimulation, suggesting that PI3-kinase is specifically involved in the induction of LTP. Finally, we found that LTP induced by theta-frequency stimulation was MEK inhibitor insensitive but still PI3-kinase dependent in hippocampal slices from PSD-95 (postsynaptic density-95) mutant mice. Together, our results indicate that the role of PI3-kinase in LTP is not limited to its role as an upstream regulator of MAPK signaling but also includes signaling through ERK-independent pathways that regulate LTP induction.

Postsynaptic Complex Spike Bursting Enables the Induction of LTP by Theta Frequency Synaptic Stimulation

Long-term potentiation (LTP), a persistent enhancement of synaptic transmission that may be involved in some forms of learning and memory, is induced at excitatory synapses in the CA1 region of the hippocampus by coincident presynaptic and postsynaptic activity. Although action potentials back-propagating into dendrites of hippocampal pyramidal cells provide sufficient postsynaptic activity to induce LTP under somein vitro conditions, it is not known whether LTP can be induced by patterns of postsynaptic action potential firing that occur in these cells in vivo. Here we report that a characteristic in vivo pattern of action potential generation in CA1 pyramidal cells known as the complex spike burst enables the induction of LTP during theta frequency synaptic stimulation in the CA1 region of hippocampal slices maintainedin vitro. Our results suggest that complex spike bursting may have an important role in synaptic processes involved in learning and memory formation, perhaps by producing a highly sensitive postsynaptic state during which even low frequencies of presynaptic activity can induce LTP.

Hippocampal Long-Term Potentiation Is Normal in Heme Oxygenase-2 Mutant Mice

We have generated mice deficient in HO-2, the major cerebral isoform of heme oxygenase, in order to assess the potential role of carbon monoxide as a retrograde messenger in hippocampal LTP. Cerebral HO catalytic activity was markedly reduced in the HO-2 mutant mice, yet no differences were found between wild types and mutants in gross neuroanatomical structure, in basal hippocampal synaptic transmission, or in the amount of potentiation produced by various LTP induction protocols. Furthermore, zinc protoporphyrin IX, an inhibitor of HO, had nearly identical inhibitory effects on LTP in wild-type and HO-2 mutant hippocampal slices. Our data indicate that carbon monoxide produced endogenously by HO is unlikely to be a neuromodulator required for LTP in the hippocampus.

Synapse to nucleus signaling during long-term synaptic plasticity; a role for the classical active nuclear import pathway.

The requirement for transcription during long-lasting plasticity indicates that signals generated at the synapse must be transported to the nucleus. We have investigated whether the classical active nuclear import pathway mediates intracellular retrograde signal transport in Aplysia sensory neurons and rodent hippocampal neurons. We found that importins localize to distal neuronal processes, including synaptic compartments, where they are well positioned to mediate synapse to nucleus signaling. In Aplysia, stimuli known to produce long-lasting but not short-lasting facilitation triggered importin nuclear translocation. In hippocampal neurons, NMDA receptor activation but not depolarization induced importin nuclear translocation. We further showed that LTP-inducing stimuli recruited active nuclear import in hippocampal slices. Together with our finding that long-term facilitation of Aplysia sensory-motor synapses required active nuclear import, our results indicate that regulation of the active nuclear import pathway plays a critical role in transporting synaptically generated signals into the nucleus during learning-related forms of plasticity.

TRPA1 channels are regulators of astrocyte basal calcium levels and long-term potentiation via constitutive D-serine release.

Astrocytes are found throughout the brain where they make extensive contacts with neurons and synapses. Astrocytes are known to display intracellular Ca2+ signals and release signaling molecules such as d-serine into the extracellular space. However, the role(s) of astrocyte Ca2+ signals in hippocampal long-term potentiation (LTP), a form of synaptic plasticity involved in learning and memory, remains unclear. Here, we explored a recently discovered novel TRPA1 channel-mediated transmembrane Ca2+ flux pathway in astrocytes. Specifically, we determined whether block or genetic deletion of TRPA1 channels affected LTP of Schaffer collateral to CA1 pyramidal neuron synapses. Using pharmacology, TRPA1−/− mice, imaging, electrophysiology, and d-serine biosensors, our data indicate that astrocyte TRPA1 channels contribute to basal Ca2+ levels and are required for constitutive d-serine release into the extracellular space, which contributes to NMDA receptor-dependent LTP. The findings have broad relevance for the study of astrocyte–neuron interactions by demonstrating how TRPA1 channel-mediated fluxes contribute to astrocyte basal Ca2+ levels and neuronal function via constitutive d-serine release.

Expression of the Circadian Clock Gene Period2 in the Hippocampus: Possible Implications for Synaptic Plasticity and Learned Behaviour

Genes responsible for generating circadian oscillations are expressed in a variety of brain regions not typically associated with circadian timing. The functions of this clock gene expression are largely unknown, and in the present study we sought to explore the role of the Per2 (Period 2) gene in hippocampal physiology and learned behaviour. We found that PER2 protein is highly expressed in hippocampal pyramidal cell layers and that the expression of both protein and mRNA varies with a circadian rhythm. The peaks of these rhythms occur in the late night or early morning and are almost 180° out-of-phase with the expression rhythms measured from the suprachiasmatic nucleus of the same animals. The rhythms in Per2 expression are autonomous as they are present in isolated hippocampal slices maintained in culture. Physiologically, Per2-mutant mice exhibit abnormal long-term potentiation. The underlying mechanism is suggested by the finding that levels of phosphorylated cAMP-response-element-binding protein, but not phosphorylated extracellular-signal-regulated kinase, are reduced in hippocampal tissue from mutant mice. Finally, Per2-mutant mice exhibit deficits in the recall of trace, but not cued, fear conditioning. Taken together, these results provide evidence that hippocampal cells contain an autonomous circadian clock. Furthermore, the clock gene Per2 may play a role in the regulation of long-term potentiation and in the recall of some forms of learned behaviour.

Synapse-Associated Protein 102/dlgh3 Couples the NMDA Receptor to Specific Plasticity Pathways and Learning Strategies

Understanding the mechanisms whereby information encoded within patterns of action potentials is deciphered by neurons is central to cognitive psychology. The multiprotein complexes formed by NMDA receptors linked to synaptic membrane-associated guanylate kinase (MAGUK) proteins including synapse-associated protein 102 (SAP102) and other associated proteins are instrumental in these processes. Although humans with mutations in SAP102 show mental retardation, the physiological and biochemical mechanisms involved are unknown. Using SAP102 knock-out mice, we found specific impairments in synaptic plasticity induced by selective frequencies of stimulation that also required extracellular signal-regulated kinase signaling. This was paralleled by inflexibility and impairment in spatial learning. Improvement in spatial learning performance occurred with extra training despite continued use of a suboptimal search strategy, and, in a separate nonspatial task, the mutants again deployed a different strategy. Double-mutant analysis of postsynaptic density-95 and SAP102 mutants indicate overlapping and specific functions of the two MAGUKs. These in vivo data support the model that specific MAGUK proteins couple the NMDA receptor to distinct downstream signaling pathways. This provides a mechanism for discriminating patterns of synaptic activity that lead to long-lasting changes in synaptic strength as well as distinct aspects of cognition in the mammalian nervous system.