Development of NMDA receptor-channel complex and L-type calcium channels in mouse hippocampus

Development of dendritic tonic GABAergic inhibition regulates excitability and plasticity in CA1 pyramidal neurons

Synaptic plasticity rules change during development: while hippocampal synapses can be potentiated by a single action potential pairing protocol in young neurons, mature neurons require burst firing to induce synaptic potentiation. An essential component for spike timing-dependent plasticity is the backpropagating action potential (BAP). BAP along the dendrites can be modulated by morphology and ion channel composition, both of which change during late postnatal development. However, it is unclear whether these dendritic changes can explain the developmental changes in synaptic plasticity induction rules. Here, we show that tonic GABAergic inhibition regulates dendritic action potential backpropagation in adolescent, but not preadolescent, CA1 pyramidal neurons. These developmental changes in tonic inhibition also altered the induction threshold for spike timing-dependent plasticity in adolescent neurons. This GABAergic regulatory effect on backpropagation is restricted to distal regions of apical dendrites (>200 μm) and mediated by α5-containing GABA(A) receptors. Direct dendritic recordings demonstrate α5-mediated tonic GABA(A) currents in adolescent neurons which can modulate BAPs. These developmental modulations in dendritic excitability could not be explained by concurrent changes in dendritic morphology. To explain our data, model simulations propose a distally increasing or localized distal expression of dendritic α5 tonic inhibition in mature neurons. Overall, our results demonstrate that dendritic integration and plasticity in more mature dendrites are significantly altered by tonic α5 inhibition in a dendritic region-specific and developmentally regulated manner.

Immunoexcitotoxicity as a central mechanism in chronic traumatic encephalopathy-A unifying hypothesis

Some individuals suffering from mild traumatic brain injuries, especially repetitive mild concussions, are thought to develop a slowly progressive encephalopathy characterized by a number of the neuropathological elements shared with various neurodegenerative diseases. A central pathological mechanism explaining the development of progressive neurodegeneration in this subset of individuals has not been elucidated. Yet, a large number of studies indicate that a process called immunoexcitotoxicity may be playing a central role in many neurodegenerative diseases including chronic traumatic encephalopathy (CTE). The term immunoexcitotoxicity was first coined by the lead author to explain the evolving pathological and neurodevelopmental changes in autism and the Gulf War Syndrome, but it can be applied to a number of neurodegenerative disorders. The interaction between immune receptors within the central nervous system (CNS) and excitatory glutamate receptors trigger a series of events, such as extensive reactive oxygen species/reactive nitrogen species generation, accumulation of lipid peroxidation products, and prostaglandin activation, which then leads to dendritic retraction, synaptic injury, damage to microtubules, and mitochondrial suppression. In this paper, we discuss the mechanism of immunoexcitotoxicity and its link to each of the pathophysiological and neurochemical events previously described with CTE, with special emphasis on the observed accumulation of hyperphosphorylated tau.

Control of CA3 output by feedforward inhibition despite developmental changes in the excitation-inhibition balance

In somatosensory cortex, the relative balance of excitation and inhibition determines how effectively feedforward inhibition enforces the temporal fidelity of action potentials. Within the CA3 region of the hippocampus, glutamatergic mossy fiber (MF) synapses onto CA3 pyramidal cells (PCs) provide strong monosynaptic excitation that exhibit prominent facilitation during repetitive activity. We demonstrate in the juvenile CA3 that MF-driven polysynaptic IPSCs facilitate to maintain a fixed EPSC-IPSC ratio during short-term plasticity. In contrast, in young adult mice this MF-driven polysynaptic inhibitory input can facilitate or depress in response to short trains of activity. Transgenic mice lacking the feedback inhibitory loop continue to exhibit both facilitating and depressing polysynaptic IPSCs, indicating that this robust inhibition is not caused by the secondary engagement of feedback inhibition. Surprisingly, eliminating MF-driven inhibition onto CA3 pyramidal cells by blockade of GABA(A) receptors did not lead to a loss of temporal precision of the first action potential observed after a stimulus but triggered in many cases a long excitatory plateau potential capable of triggering repetitive action potential firing. These observations indicate that, unlike other regions of the brain, the temporal precision of single MF-driven action potentials is dictated primarily by the kinetics of MF EPSPs, not feedforward inhibition. Instead, feedforward inhibition provides a robust regulation of CA3 PC excitability across development to prevent excessive depolarization by the monosynaptic EPSP and multiple action potential firings.

Burst firing induces postsynaptic LTD at developing mossy fibre-CA3 pyramid synapses

Synaptic development is an activity-dependent process utilizing coordinated network activity to drive synaptogenesis and subsequent refinement of immature connections. Hippocampal CA3 pyramidal neurons (PYRs) exhibit intense burst firing (BF) early in development, concomitant with the period of mossy fibre (MF) development. However, whether developing MF-PYR synapses utilize PYR BF to promote MF synapse maturation remains unknown. Recently, we demonstrated that transient tonic depolarization of postsynaptic PYRs induces a persistent postsynaptic form of long-term depression (depolarization-induced long-term depression, DiLTD) at immature MF-PYR synapses. DiLTD induction is NMDAR independent but does require postsynaptic Ca(2+) influx through L-type voltage gated Ca(2+) channels (L-VGCCs), and is expressed as a reduction in AMPAR function through the loss of GluR2-lacking AMPARs present at immature MF-PYR synapses. Here we examined whether more physiologically relevant phasic L-VGCC activation by PYR action potential (AP) BF activity patterns can trigger DiLTD. Using combined electrophysiological and Ca(2+) imaging approaches we demonstrate that PYR BF effectively drives L-VGCC activation and that brief periods of repetitive PYR BF, produced by direct current injection or intrinsic network activity induces NMDAR-independent LTD by promoting Ca(2+) influx through the activated L-VGCCs. This BF induced LTD, just like DiLTD, is specific for developing MF-PYR synapses, is PICK1 dependent, and is expressed postsynaptically. Our results demonstrate that DiLTD can be induced by phasic L-VGCC activation driven by PYR BF, suggesting the engagement of natural PYR network activity patterns for MF synapse maturation.

Developmental expression of Ca2+-permeable AMPA receptors underlies depolarization-induced long-term depression at mossy fiber CA3 pyramid synapses

Many central excitatory synapses undergo developmental alterations in the molecular and biophysical characteristics of postsynaptic ionotropic glutamate receptors via changes in subunit composition. Concerning AMPA receptors (AMPARs), glutamate receptor 2 subunit (GluR2)-containing, Ca2+-impermeable AMPARs (CI-AMPARs) prevail at synapses between mature principal neurons; however, accumulating evidence indicates that GluR2-lacking, Ca2+-permeable AMPARs (CP-AMPARs) contribute at these synapses early in development. Here, we used a combination of imaging and electrophysiological recording techniques to investigate potential roles for CP-AMPARs at developing hippocampal mossy fiber-CA3 pyramidal cell (MF-PYR) synapses. We found that transmission at nascent MF-PYR synapses is mediated by a mixed population of CP- and CI-AMPARs as evidenced by polyamine-dependent inwardly rectifying current-voltage (I-V) relationships, and partial philanthotoxin sensitivity of synaptic events. CP-AMPAR expression at MF-PYR synapses is transient, being limited to the first 3 postnatal weeks. Moreover, the expression of CP-AMPARs is regulated by the PDZ (postsynaptic density-95/Discs large/zona occludens-1) domain-containing protein interacting with C kinase 1 (PICK1), because MF-PYR synapses in young PICK1 knock-out mice are philanthotoxin insensitive with linear I-V relationships. Strikingly, MF-PYR transmission via CP-AMPARs is selectively depressed during depolarization-induced long-term depression (DiLTD), a postsynaptic form of MF-PYR plasticity observed only at young MF-PYR synapses. The selective depression of CP-AMPARs during DiLTD was evident as a loss of postsynaptic CP-AMPAR-mediated Ca2+ transients in PYR spines and reduced rectification of MF-PYR synaptic currents. Preferential targeting of CP-AMPARs during DiLTD is further supported by a lack of DiLTD in young PICK1 knock-out mice. Together, these findings indicate that the transient participation of CP-AMPARs at young MF-PYR synapses dictates the developmental window to observe DiLTD.

Partial reversal of the effect of maternal care on cognitive function through environmental enrichment

Maternal care influences hippocampal development in the rat. The offspring of mothers that exhibit increased levels of pup licking/grooming and arched-back nursing (High LG-ABN mothers) show increased hippocampal N-methyl-D-aspartate (NMDA) receptor binding and enhanced hippocampal-dependent spatial learning. In these studies we examined whether environmental enrichment from days 22-70 of life might reverse the effects of low maternal care. Environmental enrichment eliminated the differences between the offspring of High and Low LG-ABN mothers in both Morris water maze learning and object recognition. However, enrichment did not reverse the effect of maternal care on long-term potentiation in the dentate gyrus or on hippocampal NMDA receptor binding. In contrast, peripubertal enrichment did reverse the effects of maternal care on hippocampal alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid receptor binding. These findings provide evidence for the reversal of the effects of reduced maternal investment in early life on cognitive function in adulthood. Such effects might involve compensatory changes associated with peripubertal enrichment.

Differential involvement of L-type calcium channels in epileptogenesis of rat hippocampal slices during ontogenesis

Organic calcium channel antagonists block epileptiform activity in adult tissue, suggesting an essential role of L-type channels in epileptogenesis in the mature CNS. By contrast, this remains doubtful for neonatal tissue, as the density of calcium channels changes markedly with ontogenesis. The paper addresses this question by exploring the antiepileptic efficacy of the L-type calcium channel blockers verapamil and nifedipine in low-Mg(2+)-epilepsy in rat hippocampal slices of different postnatal (PN) ages. Field (CA3, CA1) and membrane potentials (CA3) were recorded. Washout of Mg(2+) induced epileptiform potentials, which were blocked age-dependently: Verapamil suppressed activity in all preparations of PN1-5 and PN13-30+, but only in 70% of PN6-12. Nifedipine depressed activity in >75% of slices of PN13-30+, but only in 33% of PN1-12. The findings indicate a role of L-type calcium channels in epileptogenesis from PN13 onwards, with phenylalkylamine-sensitive calcium channels also being involved during PN1-5.

Ontogeny of ethanol-induced locomotor activity and hypothermia differences in selectively bred FAST and SLOW mice

The replicate lines of selectively bred FAST and SLOW mice differ in locomotor response to 2 g/kg ethanol (EtOH). FAST mice show enhanced locomotion; SLOW mice exhibit no change or locomotor depression. Little is known about the responses of FAST and SLOW mice to EtOH during development. We assessed the locomotor responses of FAST and SLOW mice at postnatal days (P) 10, 15, 30, and 60. A genetically correlated response, EtOH-induced hypothermia, was also investigated. Although all animals demonstrated their respective selection phenotypes in adulthood, developing FAST mice exhibited ethanol stimulation by P15 (replicate 1) or P30 (replicate 2). At these ages, responses of FAST mice differed from those of SLOW. The stimulant response in FAST mice was adult-like at P30. EtOH-induced hypothermia was seen in SLOW mice by P15. These data suggest that sensitivity to the locomotor stimulant effects of EtOH changes during postnatal development, and may mirror developmental profiles for certain neurotransmitter systems.

Spatiotemporal changes in hippocampal NMDA receptor binding as a consequence of trimethyltin neurotoxicity in the rat

In the present study we examined the presumable changes in the distribution of N-methyl-D-aspartate (NMDA) receptors in the hippocampus of rat exposed to a potent neurotoxic drug, trimethyltin (TMT). Using in vitro receptor binding autoradiography, [3H]MK801 labelling was determined at 7, 14, 21, 30 and 60 days after treatment with TMT (single dose of 8 mg/kg, i.p.) in various hippocampal areas thought to be affected by the neurotoxin. At 21-60 days after exposure, a decrease in receptor binding was observed in CA1 hippocampal subfield (10-20%, P< 0.05). A reduction in binding density also occurred in CA4/ CA3c, where labelling vanished completely at longer times. In the molecular layer (ML) of the dentate gyrus (DG), however, 16-37% (P<0.05) increase in receptor binding was found at 14-60 days postexposure. These results suggest that exposure to TMT leads to an altered topography of NMDA receptor density sites in the rat hippocampus. Dynamics of the reduction in receptor binding in CA4/CA3c and CA1 followed the development of the well-known degenerative effects induced by the neurotoxin. In contrast, the enhanced binding density in the ML of the DG may be a part of a mechanism of plastic response of granule cells to denervation/reinnervation.

Hypoxia-tolerant neonatal CA1 neurons: relationship of survival to evoked glutamate release and glutamate receptor-mediated calcium changes in hippocampal slices

Neurons in the neonatal mammalian brain survive greater degrees of hypoxic stress than those in the mature brain. To investigate how developmental changes in glutamate receptor-mediated neurotoxicity contribute to this difference, we measured hypoxia-evoked glutamate release, glutamate receptor contribution to hypoxia-evoked intracellular calcium changes, and survival of hypoxia-/ischemia-sensitive CA1 neurons in rat hippocampus. Glutamate release was measured by a fluorescence assay, calcium changes in CA1 neurons with fura-2, and cell viability using Nissl and fluorescence staining with calcein-AM/ethidium homodimer, all in 300-micron thick hippocampal slices from 3-30 post-natal day (PND) rats. Glutamate released from PND 3-7 slices during hypoxia (PO2 = 5 mmHg) was only one third that of PND 18-22 slices. In PND 3-7 slices, survival of CA1 neurons after 5 min of hypoxia and 6 h of recovery was significantly greater than in PND 18-22 slices (viability indices 0.60 and 0.28, respectively, (p < 0.05). Five min of anoxia significantly altered Nissl staining pattern and morphology of CA1 neurons in PND 18-22 but not PND 3-7 slices. Hypoxia (PO2 = 5 mm Hg) caused three to five times greater increases in [Ca2+]i in PND 18-22 slices than in PND 3-7 slices (p < 0.001). During re-oxygenation, [Ca2+]i returned to baseline in PND 3-7 slices, but remained elevated in PND 18-22 slices. Glutamate receptor-mediated calcium changes in CA1 during hypoxia were 33% and 62% of the total calcium change in PND 3-7 and PND 18-22 CA1, respectively. We conclude that survival of CA1 neurons in PND 3-7 slices following hypoxic stress is associated with smaller increases and enhanced recovery of [Ca2+]i, less accumulation of glutamate, and less glutamate receptor-mediated calcium influx than in PND 18-22 slices.

Postnatal development of zinc-containing cells and neuropil in the hippocampal region of the mouse

The present study describes the postnatal development of zinc-containing boutons and their neurons of origin in the hippocampal region of the mouse. Ages investigated for the development of zinc-containing neuropil were postnatal days 0 (P0), P3, P7, P11, P15, P21, and P28. For zinc-containing cell bodies P7, P15, P21, and P28 were studied. In the area dentata, zinc-containing neuropil appeared first by P3 adjacent to the suprapyramidal limb of the granule cell layer and extended later toward the infrapyramidal limb. By P15, inter- and intralaminar gradients corresponded to those seen in adult animals. The appearance of labeled granule cells followed closely, although temporally delayed, the pattern of granule cell neurogenesis. All granule cells were labeled by P28. In the hippocampus proper, zinc-containing neuropil was seen by P0, but staining of the incipient mossy fiber zone was first visible by P3. Staining pattern and intensity developed gradually until they reached their mature appearance by P15. The distribution of labeled cells was identical to that seen in mature animals by P7 in CA3, but first by P21 in CA1. In the subiculum, neuropil staining first appeared proximally by P7, included all of this area by P11, and appeared mature by P21. A few labeled cells were seen in the proximal subiculum at all ages at which labeled cells were present in CA1. Labeled cells which extended further distally became first visible by P21. Their number and labeling intensity reached mature levels by P28. In the presubiculum, retrosplenial area 29e, and parasubiculum, neuropil staining first appeared by P3. The retrosplenial area 29e could be distinguished by P11. This area and the presubiculum reached their adult appearance by P21. This occurred first by P28 in the parasubiculum due to the late maturation of the parasubiculum a. Labeled cells were first seen by P7 in layer III of the presubiculum and by P15 in the retrosplenial area 29e and the parasubiculum. Cell labeling appeared mature by the same times as the neuropil staining. In the entorhinal areas a very light neuropil stain was apparent in the deeper layers by P0. A distinct rise in staining intensity was first observed by P7 in layers I-III. Thereafter, mature characteristics developed gradually and were attained by P21. Cell labeling was not seen in the medial entorhinal area. A few labeled cells were apparent by P7 in the lateral entorhinal area. After a slight increase by P15, numerous labeled cells were found in layer II and layer VI by P21. Their distribution and labeling intensity appeared mature by P28. Zinc-containing cells appear to represent cells formed late in the course of neurogenesis in all areas aside from the lateral entorhinal area. As far as intrinsic connections are concerned, it is the development of projections from this subset of neurons which is monitored in this study. We suggest that the appearance of zinc may contribute via its different effects on N-methyl-D-aspartate (NMDA) and non-NMDA glutamate receptors to the end of a developmental phase that is permissive to changes in synaptic efficacy. Species differences and alternative functions of zinc are considered.

Effects of glutamate application on the rhythm of low magnesium-induced epileptiform activity in hippocampal slices of guinea-pigs

The extracellular concentration of glutamate has previously been reported to increase to more than 10-fold the basal level during seizure activity. In the present study, we tested whether localized increases in extracellular glutamate concentration influence the rhythm of epileptiform discharges in the low-magnesium epilepsy model. In hippocampal slices of guinea-pigs, epileptiform activity was induced by omission of magnesium from the bath fluid. Glutamate and its subreceptor agonists N-methyl-D-aspartate (NMDA) and alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) were ejected into different strata of the CA3 and CA1 regions using microiontophoretic and micropressure application. Glutamate, NMDA and AMPA applied to the CA3 region, but not to the CA1 region, induced a short-lasting increase in epileptiform discharge frequency, often followed by a transient reduction. The effect was most pronounced with application into the stratum lacunosum-moleculare of the CA3 region and could only be evoked in slices exceeding 400 microns in thickness. The effects on the rhythm of epileptiform discharges induced by NMDA and AMPA were blocked by their specific receptor antagonists. They were not influenced by application of GABAA and GABAB receptor antagonists. Changes in somatic membrane potential of CA3 pyramidal neurons did not correlate with changes in the rhythm of epileptiform discharges elicited in this region. The transient suppression of epileptiform discharges that followed the increase in discharge frequency was abolished by an adenosine A1 receptor antagonist. We propose that localized increases in extracellular glutamate concentration modify the rhythm of epileptiform discharges due to changes in neuronal network activity.

Hypoxia and brain development

Hypoxia threatens brain function during the entire life-span starting from early fetal age up to senescence. This review compares the short-term, long-term and life-spanning effects of fetal chronic hypoxia and neonatal anoxia on several behavioural paradigms including novelty-induced spontaneous and learning behaviours. Furthermore, it reveals that perinatal hypoxia is an additional threat to neurodegeneration and decline of cognitive and other behaviours during the aging process. Prenatal hypoxia evokes a temporary delay of ingrowth of cholinergic and serotonergic fibres into the hippocampus and neocortex, and causes an enhanced neurodegeneration of 5-HT-ir axons during aging. Neonatal anoxia suppresses hippocampal ChAT activity and up-regulates muscarinic receptor sites for 3H-QNB and 3H-pirenzepine binding in the hippocampus in the early postnatal age. The altered development of axonal arborization and pre- and postsynaptic cholinergic functions may be an important underlying mechanism to explain the behavioural deficits. As far as the cellular mechanisms of perinatal hypoxia is concerned, our primary aim was to study the putative importance of Ca2+ homeostasis of developing neurons by means of pharmacological interventions and by measuring the development of immunoexpression of Ca(2+)-binding proteins. We assessed that nimodipine, an L-type calcium channel blocker, prevented or attenuated the adverse behavioural and neurochemical effects of perinatal hypoxias, while it enhanced the early postnatal development of ir-Ca(2+)-binding proteins. The results are discussed in the context of different related research areas on brain development and hypoxia and ischaemia.

Ischemia and lesion induced imbalances in cortical function

Cortical structures are often critically affected by ischemic and traumatic lesions which may cause transient or permanent functional disturbances. These disorders consist of changes in the membrane properties of single cells and alterations in synaptic network interactions within and between cortical areas including large-scale reorganizations in the representation of the peripheral input. Prominent functional modifications consisting of massive membrane depolarizations, suppression of intracortical inhibitory synaptic mechanisms and enhancement of excitatory synaptic transmission can be observed within a few minutes following the onset of cortical hypoxia or ischemia and probably represent the trigger signals for the induction of neuronal hyperexcitability, irreversible cellular dysfunction and cell death. Pharmacological manipulation of these early events may therefore be the most effective approach to control ischemia and lesion induced disturbances and to attenuate long-term neurological deficits. The complexity of secondary structural and functional alterations in cortical and subcortical structures demands an early and powerful intervention before neuronal damage expands to intact regions. The unsatisfactory clinical experience with calcium and N-methyl-D-aspartate antagonists suggests that this result might be achieved with compounds that show a broad spectrum of actions at different ligand-activated receptors, voltage-dependent channels and that also act at the vascular system. Whether the same therapy strategies developed for the treatment of ischemic injury in the adult brain may be applied for the immature cortex is questionable, since young cortical networks with a high degree of synaptic plasticity reveal a different response pattern to hypoxic and ischemic insults. Age-dependent molecular biological, morphological and physiological parameters contribute to an enhanced susceptibility of the immature brain to these noxae during early ontogenesis and have to be investigated in more detail for the development of adequate clinical therapy.

Embryonic hypothalamic expression of functional glutamate receptors

Glutamate can play a number of roles in the developing brain, including modulation of gene expression, cell motility, neurite growth and neuronal survival, all critical for the final organization and function of the mature brain. These functions are dependent on the early expression of glutamate receptors and on glutamate release in developing neurons. This subject has received little attention in the hypothalamus, despite glutamate's critical role as an excitatory transmitter in hypothalamic control of circadian rhythms, endocrine secretion, temperature regulation, and autonomic control. A total of 10,922 rat hypothalamic neurons were studied with digital Ca2+ imaging with the ratiometric dye fura-2 to examine their responses to glutamate receptor agonists and antagonists during embryonic development and maturation in vitro. Functional glutamate receptors were found very early in development (embryonic day 15-E15) with both Ca2+ imaging and with patch clamp recording. This is a time when the hypothalamus is beginning to undergo neurogenesis. Ca2+ responses from N-methyl-D-aspartate receptors developed later than those from non-N-methyl-D-aspartate ionotropic receptors that responded to kainate and alpha-amino-3-hydroxy-5-methyl-4-isoxazoleproprionate. The responses of immature E15 cells after one day in vitro were compared with more mature cells after six days in vitro to examine the response to repeated 3 min applications of 100 microM kainate (n = 108). Immature cells showed similar Ca2+ rises (+232nM Ca2+) with each kainate stimulation. In contrast, more mature cells showed an initial Ca2+ rise of 307 nM, with the second rise only to 147 nM above the initial baseline. Immature cells more quickly returned to their pre-kainate baseline than did older cells. The expression of metabotropic glutamate receptors was studied with the selective agonist trans-1-amino-cyclopentyl-1,3-dicarboxylic acid and with glutamate stimulation in the absence of extracellular Ca2+ and presence of 1 mM EGTA. After five days in vitro. E16 astrocytes showed a greater response than did neurons to conditions that would activate the metabotropic glutamate receptor. A dramatic increase in the percentage of cells that responded to N-methyl-D-aspartate was found after only a few days in culture. Only a small number of E15 cells studied on the day of culture (4% of 694 cells) showed a response to 100 microM N-methyl-D-aspartate. Thirty-eight percent of 120 E18 cells cultured for one day in vitro showed an N-methyl-D-aspartate response.(ABSTRACT TRUNCATED AT 400 WORDS)