Postsynaptic expression of a new calcium pathway in hippocampal CA3 neurons and its influence on mossy fiber long-term potentiation

Age matters

The age of an experimental animal can be a critical variable, yet age matters are often overlooked within neuroscience. Many studies make use of young animals, without considering possible differences between immature and mature subjects. This is especially problematic when attempting to model traits or diseases that do not emerge until adulthood. In this commentary we discuss the reasons for this apparent bias in age of experimental animals, and illustrate the problem with a systematic review of published articles on long-term potentiation. Additionally, we review the developmental stages of a rat and discuss the difficulty of using the weight of an animal as a predictor of its age. Finally, we provide original data from our laboratory and review published data to emphasize that development is an ongoing process that does not end with puberty. Developmental changes can be quantitative in nature, involving gradual changes, rapid switches, or inverted U-shaped curves. Changes can also be qualitative. Thus, phenomena that appear to be unitary may be governed by different mechanisms at different ages. We conclude that selection of the age of the animals may be critically important in the design and interpretation of neurobiological studies.

Calcium-permeable AMPA receptors appear in cortical neurons after traumatic mechanical injury and contribute to neuronal fate

Traumatic brain injury (TBI) is one of the most disabling injuries in the population, with 1.5 million Americans new cases each year and 5.3 million Americans overall requiring long-term daily care as a result of their injuries. One critical aspect in developing effective treatments for TBI is determining if new, specific receptor populations emerge in the early phase after injury that can subsequently be targeted to reduce neuronal death after injury. One specific glutamate receptor subtype, the calcium-permeable AMPA receptor (CP-AMPAR), is becoming increasingly recognized for its role in physiological and pathophysiological processes. Although present in relatively low levels in the mature brain, recent studies show that CP-AMPARs can appear following ischemic brain injury or status epilepticus, and the mechanisms that regulate the appearance of these receptors include alterations in transcription, RNA editing, and receptor trafficking. In this report, we use an in vitro model of TBI to show a gradual appearance of CP-AMPARs four hours following injury to cortical neurons. Moreover, the appearance of these receptors is mediated by the phosphorylation of CaMKIIalpha following injury. Selectively blocking CP-AMPARs after mechanical injury leads to a significant reduction in the cell death that occurs 24 h following injury in untreated controls, and is similar in protection offered by broad-spectrum NMDA and AMPA receptor antagonists. These data point to a potentially new and more targeted therapeutic approach for treating TBI.

TEA-induced long-term potentiation at hippocampal mossy fiber-CA3 synapses: characteristics of its induction and expression

Potassium ion channel blockade by tetraethylammonium (TEA) reportedly induces long-term potentiation (LTP) at hippocampal mossy fiber (MF)-CA3 synapses, but the characteristics of induction, expression, and modulation of the LTP remain unclear. In the present study, these features of TEA-induced LTP at MF-CA3 synapses were electrophysiologically examined using rat hippocampal slices. Synaptic responses recorded from MF-CA3 synapses were enhanced long-term by TEA application even under the blockade of NMDA receptors with D-AP5, whereas selective pharmacological blockade of T-type voltage-dependent calcium channels (VDCCs) strongly inhibited TEA-induced LTP. Decrease of the paired-pulse facilitation ratio after LTP induction by TEA suggests the involvement of increased neurotransmitter release probability from MF terminals as LTP expression. The facilitative modulation of MF-CA3 LTP by GABA(A) receptor activation reported previously was reversed when bumetanide, a blocker of Na(+)-K(+)-Cl(-) co-transporters (NKCCs), was applied, suggesting that the region-specific modulation of TEA-induced LTP by GABAergic inputs at MF-CA3 synapses is due to the dominance of NKCC action at MF terminals.

Synaptic plasticity at hippocampal mossy fibre synapses

The dentate gyrus provides the main input to the hippocampus. Information reaches the CA3 region through mossy fibre synapses made by dentate granule cell axons. Synaptic plasticity at the mossy fibre-pyramidal cell synapse is unusual for several reasons, including low basal release probability, pronounced frequency facilitation and a lack of N-methyl-D-aspartate receptor involvement in long-term potentiation. In the past few years, some of the mechanisms underlying the peculiar features of mossy fibre synapses have been elucidated. Here we describe recent work from several laboratories on the various forms of synaptic plasticity at hippocampal mossy fibre synapses. We conclude that these contacts have just begun to reveal their many secrets.

Input- and subunit-specific AMPA receptor trafficking underlying long-term potentiation at hippocampal CA3 synapses

Hippocampal CA3 pyramidal neurons receive synaptic inputs from both mossy fibres (MFs) and associational fibres (AFs). Long-term potentiation (LTP) at these synapses differs in its induction sites and N-methyl-D-aspartate receptor (NMDAR) dependence. Most evidence favours the presynaptic and postsynaptic mechanisms for induction of MF LTP and AF LTP, respectively. This implies that molecular and functional properties differ between MF and AF synapses at both presynaptic and postsynaptic sites. In this study, we focused on the difference in the postsynaptic trafficking of alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptors (AMPARs) between these synapses. To trace the subunit-specific trafficking of AMPARs at each synapse, GluR1 and GluR2 subunits were introduced into CA3 pyramidal neurons in hippocampal organotypic cultures using the Sindbis viral expression system. The electrophysiologically-tagged GluR2 AMPARs, produced by the viral-mediated transfer of the unedited form of GluR2 (GluR2Q), were inserted into both MF and AF postsynaptic sites in a neuronal activity-independent manner. Endogenous Ca(2+)-impermeable AMPARs at these synapses were replaced with exogenous Ca(2+)-permeable receptors, and Ca(2+) influx via the newly expressed postsynaptic AMPARs induced NMDAR-independent LTP at AF synapses. In contrast, no GluR1 AMPAR produced by the gene transfer was constitutively incorporated into AF postsynaptic sites, and only a small amount into MF postsynaptic sites. The synaptic trafficking of GluR1 AMPARs was triggered by the activity of Ca(2+)/calmodulin-dependent kinase II or high-frequency stimulation to induce LTP at AF synapses, but not at MF synapses. These results indicate that MF and AF postsynaptic sites possess distinct properties for AMPAR trafficking in CA3 pyramidal neurons.

Local protein synthesis and GABAB receptors regulate the reversibility of long-term potentiation at murine hippocampal mossy fibre-CA3 synapses

Reversal of long-term potentiation (LTP) by long trains of low-frequency stimulation is generally referred to as depotentiation. One of the intriguing aspects of depotentiation is that the magnitude of depotentiation is inversely proportional to the time lag of depotentiation stimulation following LTP induction. Although the mechanisms underlying depotentiation have been widely explored, the factors that regulate the susceptibility of LTP to depotentiation stimulation remain largely unclear. We now report that multiple trains of high-frequency stimulation provide immediate synaptic resistance to depotentiation stimulation at the mossy fibre-CA3 synapses. The synaptic resistance to depotentiation stimulation depends on the amount of synaptic stimulation used to induce LTP; it is prevented by protein synthesis inhibitors and is input specific. In contrast, neither the transection of mossy fibre axons near granule cell somata nor the application of RNA synthesis inhibitors influences synaptic resistance to depotentiation stimulation. We also provide evidence that the induction of depotentiation is regulated by GABA(B) receptors. Application of a GABA(B) receptor antagonist significantly promoted the synaptic resistance to depotentiation stimulation, whereas inhibition of GABA transport delayed the onset of this synaptic resistance. These results suggest that local protein synthesis is required for the development of synaptic resistance to depotentiation stimulation, whereas the activation of GABA(B) receptors promotes the susceptibility to depotentiation stimulation. These two factors may crucially regulate the reversal and stability of long-term information storage.

Glutamate release increases during mossy-CA3 LTP but not during Schaffer-CA1 LTP

Abstract It is still a matter of dispute whether the expression of hippocampal long-term potentiation (LTP) is due to enhanced transmitter release or enhanced postsynaptic sensitivity. Recently we developed a novel method to monitor synaptically released glutamate. In this method, brain slice preparations are stained with a voltage-sensitive dye RH155 which preferentially stains glial cells, and synaptically induced glial depolarization (SIGD) are optically detected in the presence of the blockers for ionotropic glutamate receptors. We have previously shown that SIGD is due to uptake of synaptically released glutamate by glial glutamate transporters. Here we applied this method to examine change in glutamate release during hippocampal LTP. To examine mossy-CA3 LTP, stimulating electrodes were placed in dentate gyrus and tetanic stimulation was delivered in the presence of 50 micro m APV. The amplitude of SIGD after inducing LTP was significantly greater than that in control experiments in which tetanus was not delivered. The amplitude of SIGD after inducing LTP by a brief (3-5 min) application of 50 micro m forskolin was also significantly greater than that in control experiments. At the Schaffer-CA1 synapse, the change in the amplitude of SIGD during LTP induced either by 100 Hz tetanus LTP or 200 Hz tetanus was not significantly greater than that of control experiments. These results provide evidence for increased glutamate release from the presynaptic terminals as the expression mechanism for both tetanus-induced and forskolin-induced LTP at mossy-CA3 synapses, and evidence supporting a postsynaptic expression mechanism at Schaffer-CA1 synapses.

Isoform-specific distribution of the plasma membrane Ca2+ ATPase in the rat brain

Regulation of cytoplasmic calcium is crucial both for proper neuronal function and cell survival. The concentration of Ca2+ in cytoplasm of a neuron at rest is 10,000 times lower than in the extracellular space, pointing to the importance of the transporters that extrude intracellular Ca2+. The family of plasma membrane calcium-dependent ATPases (PMCAs) represent a major component of the Ca2+ regulatory system. However, little information is available on the regional and cellular distribution of these calcium pumps. We used immunohistochemistry to investigate the distribution of each of the four PMCA isoforms (PMCA1-4) in the rat brain. Each isoform exhibited a remarkably precise and distinct pattern of distribution. In many cases, PMCA isoforms in a single brain structure were differentially expressed within different classes of neurons, and within different subcellular compartments. These data show that each isoform is independently organized and suggest that PMCAs may play a more complex role in calcium homeostasis than generally recognized.

A learning and memory area in the octopus brain manifests a vertebrate-like long-term potentiation

Cellular mechanisms underlying learning and memory were investigated in the octopus using a brain slice preparation of the vertical lobe, an area of the octopus brain involved in learning and memory. Field potential recordings revealed long-term potentiation (LTP) of glutamatergic synaptic field potentials similar to that in vertebrates. These findings suggest that convergent evolution has led to the selection of similar activity-dependent synaptic processes that mediate complex forms of learning and memory in vertebrates and invertebrates.

Overexpression of Ca2+-permeable AMPA receptor promotes delayed cell death of hippocampal CA1 neurons following transient forebrain ischemia

To examine the role of Ca(2+) entry through AMPA receptors in the pathogenesis of the ischemia-induced cell death of hippocampal neurons, we delivered cDNA of Q/R site-unedited form (GluR2Q) of AMPA receptor subunit GluR2 in the hippocampus by using an HVJ-liposome-mediated gene transfer technique. Two days prior to transient forebrain ischemia, we injected an HVJ-liposome containing cDNA of the GluR2Q-myc fusion gene into a rat unilateral hippocampus. In the absence of ischemic insult, overexpression of Ca(2+)-permeable GluR2Q did not cause any neurodegeneration in the cDNA-injected hippocampus. In ischemic rats, overexpression of Ca(2+)-permeable GluR2Q markedly promoted ischemic cell death of CA1 pyramidal neurons, while complete rescue of CA1 pyramidal neurons from ischemic damage occurred in the hippocampal hemisphere opposite the GluR2Q expression. Overexpression of the Q/R-site edited form (GluR2R) of subunit GluR2 did not affect the ischemia-induced damage of CA1 pyramidal neurons. From these results, we suggest that the Ca(2+)-permeability of AMPA receptors does not have a direct contribution to glutamate receptor-mediated neurotoxicity but has a promotive action in the evolution of ischemia-induced neurodegeneration of vulnerable neurons.