Action potential-induced dendritic calcium dynamics correlated with synaptic plasticity in developing hippocampal pyramidal cells

Disruption of sphingomyelin synthase 2 gene alleviates cognitive impairment in a mouse model of Alzheimer's disease

The membrane raft accommodates the key enzymes synthesizing amyloid β (Aβ). One of the two characteristic components of the membrane raft, cholesterol, is well known to promote the key enzymes that produce amyloid-β (Aβ) and exacerbate Alzheimer's disease (AD) pathogenesis. Given that the raft is a physicochemical platform for the sound functioning of embedded bioactive proteins, the other major lipid component sphingomyelin may also be involved in AD. Here we knocked out the sphingomyelin synthase 2 gene (SMS2) in 3xTg AD model mice by hybridization, yielding SMS2KO mice (4S mice). The novel object recognition test in 9/10-month-old 4S mice showed that cognitive impairment in 3xTg mice was alleviated by SMS2KO, though performance in the Morris water maze (MWM) was not improved. The tail suspension test detected a depressive trait in 4S mice, which may have hindered the manifestation of performance in the wet, stressful environment of MWM. In the hippocampal CA1, hyperexcitability in 3xTg was also found alleviated by SMS2KO. In the hippocampal dentate gyrus of 4S mice, the number of neurons positive with intracellular Aβ or its precursor proteins, the hallmark of young 3xTg mice, is reduced to one-third, suggesting an SMS2KO-led suppression of syntheses of those peptides in the dentate gyrus. Although we previously reported that large-conductance calcium-activated potassium (BK) channels are suppressed in 3xTg mice and their recovery relates to cognitive amelioration, no changes occurred by hybridization. Sphingomyelin in the membrane raft may serve as a novel target for AD drugs.

Cofilin Activation Is Temporally Associated with the Cessation of Growth in the Developing Hippocampus

Dendritic extension and synaptogenesis proceed at high rates in rat hippocampus during early postnatal life but markedly slow during the third week of development. The reasons for the latter, fundamental event are poorly understood. Here, we report that levels of phosphorylated (inactive) cofilin, an actin depolymerizing factor, decrease by 90% from postnatal days (pnds) 10 to 21. During the same period, levels of total and phosphorylated Arp2, which nucleates actin branches, increase. A search for elements that could explain the switch from inactive to active cofilin identified reductions in β1 integrin, TrkB, and LIM domain kinase 2b, upstream proteins that promote cofilin phosphorylation. Moreover, levels of slingshot 3, which dephosphorylates cofilin, increase during the period in which growth slows. Consistent with the cofilin results, in situ phalloidin labeling of F-actin demonstrated that spines and dendrites contained high levels of dynamic actin filaments during Week 2, but these fell dramatically by pnd 21. The results suggest that the change from inactive to constitutively active cofilin leads to a loss of dynamic actin filaments needed for process extension and thus the termination of spine formation and synaptogenesis. The relevance of these events to the emergence of memory-related synaptic plasticity is described.

Stressful learning paradigm precludes manifestation of cognitive ability in sphingomyelin synthase-2 knockout mice

Sphingomyelin synthases (SMSs) are enzymes converting ceramide to sphingomyelin. The behavioral phenotype attributed to their disruption has not been well described. We examined learning ability and hippocampal synaptic plasticity in mice deficient in SMS2 (SMS2 KO). In context-dependent fear learning and novel object recognition test, no difference in learning ability was detected in SMS2 KO and wild-type (WT) mice. By contrast, achievement of the Morris water maze (MWM) test was deteriorated in SMS2 KO mice. In the hippocampal CA1, while the basic synaptic transmission was normal, both short- and long-term synaptic plasticity was moderately suppressed. We interpret that the MWM test taking place in wet environment may represent learning paradigm under more stressful condition than those performed in dry conditions, and that the learning ability of SMS2 KO mice failed to manifest itself fully in stressful situations. In agreement, forced swimming induced depression-like behavior more easily in SMS2 KO mice. Mass spectrometry suggested a slightly altered species distribution of ceramide in the hippocampus of SMS2 KO mice. These findings support the proposal that altered synthesis of ceramide, which is the substrate of SMS2 and therefore expected to be modified in SMS2 KO mice, is associated with depression-like tendency in animal models and depressive disorder in humans.

LTP enhances synaptogenesis in the developing hippocampus

In adult hippocampus, long-term potentiation (LTP) produces synapse enlargement while preventing the formation of new small dendritic spines. Here, we tested how LTP affects structural synaptic plasticity in hippocampal area CA1 of Long-Evans rats at postnatal day 15 (P15). P15 is an age of robust synaptogenesis when less than 35% of dendritic spines have formed. We hypothesized that LTP might therefore have a different effect on synapse structure than in adults. Theta-burst stimulation (TBS) was used to induce LTP at one site and control stimulation was delivered at an independent site, both within s. radiatum of the same hippocampal slice. Slices were rapidly fixed at 5, 30, and 120 min after TBS, and processed for analysis by three-dimensional reconstruction from serial section electron microscopy (3DEM). All findings were compared to hippocampus that was perfusion-fixed (PF) in vivo at P15. Excitatory and inhibitory synapses on dendritic spines and shafts were distinguished from synaptic precursors, including filopodia and surface specializations. The potentiated response plateaued between 5 and 30 min and remained potentiated prior to fixation. TBS resulted in more small spines relative to PF by 30 min. This TBS-related spine increase lasted 120 min, hence, there were substantially more small spines with LTP than in the control or PF conditions. In contrast, control test pulses resulted in spine loss relative to PF by 120 min, but not earlier. The findings provide accurate new measurements of spine and synapse densities and sizes. The added or lost spines had small synapses, took time to form or disappear, and did not result in elevated potentiation or depression at 120 min. Thus, at P15 the spines formed following TBS, or lost with control stimulation, appear to be functionally silent. With TBS, existing synapses were awakened and then new spines formed as potential substrates for subsequent plasticity.

Improvement of spatial learning by facilitating large-conductance calcium-activated potassium channel with transcranial magnetic stimulation in Alzheimer's disease model mice

Transcranial magnetic stimulation (TMS) is fragmentarily reported to be beneficial to Alzheimer's patients. Its underlying mechanism was investigated. TMS was applied at 1, 10 or 15 Hz daily for 4 weeks to young Alzheimer's disease model mice (3xTg), in which intracellular soluble amyloid-β is notably accumulated. Hippocampal long-term potentiation (LTP) was tested after behavior. TMS ameliorated spatial learning deficits and enhanced LTP in the same frequency-dependent manner. Activity of the large conductance calcium-activated potassium (Big-K; BK) channels was suppressed in 3xTg mice and recovered by TMS frequency-dependently. These suppression and recovery were accompanied by increase and decrease in cortical excitability, respectively. TMS frequency-dependently enhanced the expression of the activity-dependently expressed scaffold protein Homer1a, which turned out to enhance BK channel activity. Isopimaric acid, an activator of the BK channel, magnified LTP. Amyloid-β lowering was detected after TMS in 3xTg mice. In 3xTg mice with Homer1a knocked out, amyloid-β lowering was not detected, though the TMS effects on BK channel and LTP remained. We concluded that TMS facilitates BK channels both Homer1a-dependently and -independently, thereby enhancing hippocampal LTP and decreasing cortical excitability. Reduced excitability contributed to amyloid-β lowering. A cascade of these correlated processes, triggered by TMS, was likely to improve learning in 3xTg mice.

Supralinear dendritic Ca(2+) signalling in young developing CA1 pyramidal cells

Although Ca(2+) is critically important in activity-dependent neuronal development, not much is known about the regulation of dendritic Ca(2+) signals in developing neurons. Here, we used ratiometric Ca(2+) imaging to investigate dendritic Ca(2+) signalling in rat hippocampal pyramidal cells during the first 1-4 weeks of postnatal development. We show that active dendritic backpropagation of Nav channel-dependent action potentials (APs) evoked already large dendritic Ca(2+) transients in animals aged 1 week with amplitudes of ∼150 nm, similar to the amplitudes of ∼160 nM seen in animals aged 4 weeks. Although the AP-evoked dendritic Ca(2+) load increased about four times during the first 4 weeks, the peak amplitude of free Ca(2+) concentration was balanced by a four-fold increase in Ca(2+) buffer capacity κs (∼70 vs. ∼280). Furthermore, Ca(2+) extrusion rates increased with postnatal development, leading to a slower decay time course (∼0.2 s vs. ∼0.1 s) and more effective temporal summation of Ca(2+) signals in young cells. Most importantly, during prolonged theta-burst stimulation dendritic Ca(2+) signals were up to three times larger in cells at 1 week than at 4 weeks of age and much larger than predicted by linear summation, which is attributable to an activity-dependent slow-down of Ca(2+) extrusion. As Ca(2+) influx is four-fold smaller in young cells, the larger Ca(2+) signals are generated using four times less ATP consumption. Taken together, the data suggest that active backpropagations regulate dendritic Ca(2+) signals during early postnatal development. Remarkably, during prolonged AP firing, Ca(2+) signals are several times larger in young than in mature cells as a result of activity-dependent regulation of Ca(2+) extrusion rates.

A nonlinear cable framework for bidirectional synaptic plasticity

Finding the rules underlying how axons of cortical neurons form neural circuits and modify their corresponding synaptic strength is the still subject of intense research. Experiments have shown that internal calcium concentration, and both the precise timing and temporal order of pre and postsynaptic action potentials, are important constituents governing whether the strength of a synapse located on the dendrite is increased or decreased. In particular, previous investigations focusing on spike timing-dependent plasticity (STDP) have typically observed an asymmetric temporal window governing changes in synaptic efficacy. Such a temporal window emphasizes that if a presynaptic spike, arriving at the synaptic terminal, precedes the generation of a postsynaptic action potential, then the synapse is potentiated; however if the temporal order is reversed, then depression occurs. Furthermore, recent experimental studies have now demonstrated that the temporal window also depends on the dendritic location of the synapse. Specifically, it was shown that in distal regions of the apical dendrite, the magnitude of potentiation was smaller and the window for depression was broader, when compared to observations from the proximal region of the dendrite. To date, the underlying mechanism(s) for such a distance-dependent effect is (are) currently unknown. Here, using the ionic cable theory framework in conjunction with the standard calcium based plasticity model, we show for the first time that such distance-dependent inhomogeneities in the temporal learning window for STDP can be largely explained by both the spatial and active properties of the dendrite.

Developmental regulation of the late phase of long-term potentiation (L-LTP) and metaplasticity in hippocampal area CA1 of the rat

Long-term potentiation (LTP) is a form of synaptic plasticity thought to underlie memory; thus knowing its developmental profile is fundamental to understanding function. Like memory, LTP has multiple phases with distinct timing and mechanisms. The late phase of LTP (L-LTP), lasting longer than 3 h, is protein synthesis dependent and involves changes in the structure and content of dendritic spines, the major sites of excitatory synapses. In previous work, tetanic stimulation first produced L-LTP at postnatal day 15 (P15) in area CA1 of rat hippocampus. Here we used a more robust induction paradigm involving theta-burst stimulation (TBS) in acute slices and found the developmental onset of L-LTP to be 3 days earlier at P12. In contrast, at P8-11, TBS only reversed the synaptic depression that occurs from test-pulse stimulation in developing (P8-15) hippocampus. A second bout of TBS delivered 30-180 min later produced L-LTP at P10-11 but not at P8-9 and enhanced L-LTP at P12-15. Both the developmental onset and the enhanced L-LTP produced by repeated bouts of TBS were blocked by the N-methyl-d-aspartate receptor antagonist dl-2-amino-5-phosphonovaleric acid. Thus the developmental onset age is P12 for L-LTP induced by the more robust and perhaps more naturalistic TBS induction paradigm. Metaplasticity produced by repeated bouts of TBS is developmentally regulated, advancing the capacity for L-LTP from P12 to P10, but not to younger ages. Together these findings provide a new basis from which to investigate mechanisms that regulate the developmental onset of this important form of synaptic plasticity.

Suppression of a neocortical potassium channel activity by intracellular amyloid-β and its rescue with Homer1a

It is proposed that intracellular amyloid-β (Aβ), before extracellular plaque formation, triggers cognitive deficits in Alzheimer disease (AD). Here we report how intracellular Aβ affects neuronal properties. This was done by injecting Aβ protein into rat and mouse neocortical pyramidal cells through whole-cell patch pipettes and by using 3xTg AD model mice, in which intracellular Aβ is accumulated innately. In rats, intracellular application of a mixed Aβ(1-42) preparation containing both oligomers and monomers, but not a monomeric preparation of Aβ(1-40), broadened spike width and augmented Ca(2+) influx via voltage-dependent Ca(2+) channels in neocortical neurons. Both effects were mimicked and occluded by charybdotoxin, a blocker of large-conductance Ca(2+)-activated K(+) (BK) channels, and blocked by isopimaric acid, a BK channel opener. Surprisingly, augmented Ca(2+) influx was caused by elongated spike duration, but not attributable to direct Ca(2+) channel modulation by Aβ(1-42). The Aβ(1-42)-induced spike broadening was blocked by electroconvulsive shock (ECS), which we previously showed to facilitate BK channel opening via expression of the scaffold protein Homer1a. In young 3xTg and wild mice, we confirmed spike broadening by Aβ(1-42), which was again mimicked and occluded by charybdotoxin and blocked by ECS. In Homer1a knock-out mice, ECS failed to block the Aβ(1-42) effect. Single-channel recording on BK channels supported these results. These findings suggest that the suppression of BK channels by intracellular Aβ(1-42) is a possible key mechanism for early dysfunction in the AD brain, which may be counteracted by activity-dependent expression of Homer1a.

Age-dependent glutamate induction of synaptic plasticity in cultured hippocampal neurons

A common denominator for the induction of morphological and functional plasticity in cultured hippocampal neurons involves the activation of excitatory synapses. We now demonstrate massive morphological plasticity in mature cultured hippocampal neurons caused by a brief exposure to glutamate. This plasticity involves a slow, 70%-80% increase in spine cross-section area associated with a significant reduction in the width of dendrites. These changes are age dependent and expressed only in cells >18 d in vitro (DIV). Activation of both NMDARs and AMPARs as well as a sustained rise of internal calcium levels are necessary for induction of this plasticity. On the other hand, blockade of network activity or mGluRs does not abolish the observed morphological plasticity. Electrophysiologically, a brief exposure to glutamate induces an increase in the magnitude of EPSCs evoked between pairs of neurons, as well as in mEPSC frequency and amplitude, in mature but not young cultures. These results demonstrate an age-dependent, rapid and robust morphological and functional change in cultured central neurons that may contribute to their ability to express long term synaptic plasticity.

Blocking L-type calcium channels enhances long-term depression induced by low-frequency stimulation at hippocampal CA1 synapses

Specific contributions of voltage-dependent calcium channels (VDCCs) to induction of long-term depression (LTD) have not been thoroughly elucidated. The present study examined roles of T- and L-type VDCCs in N-methyl-D-aspartate (NMDA) receptor-dependent LTD induced at several different levels of synaptic activation (0.5- to 10-Hz presynaptic stimulations) at Schaffer collateral-CA1 synapses in rat hippocampal slices. Blockade of T-type VDCCs with nickel ions failed to change LTD magnitude at all levels of stimulation. However, blockade of L-type VDCCs reduced LTD in response to stimulation at 1 and 2 Hz and, conversely, enhanced LTD at a lower frequency (0.5 Hz). The enhancement of 0.5-Hz LTD under L-type VDCC blockade was shown pharmacologically to depend on NMDA receptors (NMDARs) and intracellular Ca(2+) release. Calcium imaging revealed that contribution of L-type VDCC-mediated calcium influx to the total calcium increase was greater during 0.5-Hz stimulation than during 1.0-Hz stimulation. This finding, combined with the reported suppression of NMDARs mediated by L-type VDCCs, may be relevant to the present enhancement of 0.5-Hz LTD due to L-type VDCC blockade.

Homer 1a enhances spike-induced calcium influx via L-type calcium channels in neocortex pyramidal cells

The scaffold protein family Homer/Vesl serves to couple surface receptors or channels with endoplasmic calcium release channels. Homer 1a/Vesl-1S is regarded as regulating such coupling in an activity-dependent manner. The present calcium photometry and electrophysiological measurement revealed that Homer 1a up-regulates voltage-dependent calcium channels (VDCCs), depending on inositol-1,4,5-trisphosphate (IP3) receptors (IP3Rs). In rat neocortex pyramidal cells, intracellular injection by diffusion from the patch pipette (referred to as 'infusion') of Homer 1a protein enhanced spike-induced calcium increase, depending on both the protein concentration and spike frequency. Induction of this enhancement was disrupted by blockers of key molecules of the mGluR-IP3 signalling pathway, including metabotropic glutamate receptors (mGluRs), phospholipase C and IP3Rs. However, infusion of IP3 failed to mimic the effect of Homer 1a, suggesting requirement for a second Homer 1a-mediated signalling as well as the mGluR-IP3 signalling. In contrast to the induction, maintenance of this enhancement was independent of the mGluR-IP3 signalling, taking the form of augmented calcium influx via L-type VDCCs. Presumably due to the VDCC up-regulation, threshold currents for calcium spikes were reduced. Given that Homer 1a induction is thought to down-regulate neural excitability and hence somatic spike firing, this facilitation of calcium spikes concomitant with such attenuated firing may well have a critical impact on bi-directional synaptic plasticity.

Postnatal development differentially affects voltage-activated calcium currents in respiratory rhythmic versus nonrhythmic neurons of the pre-Bötzinger complex

The mammalian respiratory network reorganizes during early postnatal life. We characterized the postnatal developmental changes of calcium currents in neurons of the pre-Bötzinger complex (pBC), the presumed site for respiratory rhythm generation. The pBC contains not only respiratory rhythmic (R) but also nonrhythmic neurons (nR). Both types of neurons express low- and high-voltage-activated (LVA and HVA) calcium currents. This raises the interesting issue: do calcium currents of the two co-localized neuron types have similar developmental profiles? To address this issue, we used the whole cell patch-clamp technique to compare in transverse slices of mice LVA and HVA calcium current amplitudes of the two neuron populations (R and nR) during the first and second postnatal week (P0-P16). The amplitude of HVA currents did not significantly change in R pBC-neurons (P0-P16), but it significantly increased in nR pBC-neurons during P8-P16. The dehydropyridine (DHP)-sensitive current amplitudes did not significantly change during the early postnatal development, suggesting that the observed amplitude changes in nR pBC-neurons are caused by (DHP) insensitive calcium currents. The ratio between HVA calcium current amplitudes dramatically changed during early postnatal development: At P0-P3, current amplitudes were significantly larger in R pBC-neurons, whereas at P8-P16, current amplitudes were significantly larger in nR pBC-neurons. Our results suggest that calcium currents in pBC neurons are differentially altered during postnatal development and that R pBC-neurons have fully expressed calcium currents early during postnatal development. This may be critical for stable respiratory rhythm generation in the underlying rhythm generating network.

Facilitation of L-type Ca2+ channels in dendritic spines by activation of beta2 adrenergic receptors

We studied the contribution of L-type Ca2+ channels to action potential-evoked Ca2+ influx in dendritic spines of CA1 pyramidal neurons and the modulation of these channels by the beta2 adrenergic receptor. Backpropagating action potentials (bAPs) (three at 50 Hz) were evoked by brief somatic current injections, and Ca2+ transients were recorded in proximal basal dendrites and associated spines. The R- and T-type Ca2+ channel blocker NiCl2 (100 microm) significantly reduced Ca2+ transients in both spines and their parent dendrites (approximately 50%), suggesting that these channels are the major source of bAP-evoked Ca2+ influx in these structures. The L-type Ca2+ channel blockers nimodipine and nifedipine (both 10 microm) reduced spine Ca2+ transients by approximately 10%, whereas the L-type Ca2+ channel activators FPL 64176 (2,5-dimethyl-4-[2-(phenylmethyl)benzoyl]-1H-pyrrole-3-carboxylic acid methylester) and Bay K 8644 ((+/-)-1,4-dihydro-2,6-dimethyl-5-nitro-4-[2-(trifluoromethyl)-phenyl]-3-pyridine carboxylic acid methyl ester) (both 10 microm) significantly enhanced the spine Ca2+ transients by 40-50%. Activation of beta2 adrenergic receptors with salbutamol (40 microm) or formoterol (5 microm) resulted in significant enhancements of the spine (40-50%) but not dendritic Ca2+ transients. This increase was prevented when L-type Ca2+ channels were blocked with nimodipine (10 microm) or when cAMP-dependent protein kinase A (PKA) was inhibited with KT5720 (3 microm), Rp-cAMPS (Rp-adenosine cyclic 3',5'-phosphorothioate) (100 microm), or PKI (100 microm). The above data suggest that L-type Ca2+ channels are functionally present in dendritic spines of CA1 pyramidal neurons, contribute to spine Ca2+ influx, and can be modulated by the beta2 adrenergic receptor through PKA in a highly compartmentalized manner.

Frequency-dependent requirement for calcium store-operated mechanisms in induction of homosynaptic long-term depression at hippocampus CA1 synapses

For induction of long-term depression (LTD), mechanisms dependent on N-methyl-D-aspartate receptors (NMDARs) and on intracellular calcium stores have been separately known. How these two mechanisms coexist at the same synapses is not clear. Here, induction of LTD at hippocampal Schaffer collateral-to-CA1 pyramidal cell synapses was shown to depend on NMDARs throughout the theoretically predicted activation range for LTD induction. With stimulation at 1 Hz, the largest LTD was induced in a store-independent manner. With stimulation at 0.5 and 2.0 Hz the induced LTD was much smaller, and dependence on calcium stores appeared. Under caffeine application, an enlarged LTD was induced with 0.5 Hz stimulation. Postsynaptic blockade of ryanodine receptors prevented this caffeine-induced enhancement of LTD. It is therefore suggested that calcium release from calcium stores facilitated by caffeine contributed to the LTD enhancement, and that the caffeine effect was exerted on the postsynaptic side. Induction of this enhanced LTD was resistant to NMDAR blockade. We thus propose that the store-dependent mechanism for LTD induction is dormant at the centre of the theoretically predicted activation range for LTD induction, but operates at the fringes of this activation range, with its contribution more emphasized when ample calcium release occurs.

Synaptically activated Cl- accumulation responsible for depolarizing GABAergic responses in mature hippocampal neurons

It is known that GABA, a major inhibitory transmitter in the CNS, acts as an excitatory (or depolarizing) transmitter transiently after intense GABAA receptor activation in adult brains. The depolarizing effect is considered to be dependent on two GABAA receptor-permeable anions, chloride (Cl-) and bicarbonate (HCO3-). However, little is known about their spatial and temporal profiles during the GABAergic depolarization in postsynaptic neurons. In the present study, we show that the amplitude of synaptically induced depolarizing response was correlated with intracellular Cl- accumulation in the soma of mature hippocampal CA1 pyramidal cells, by using whole cell patch-clamp recording and Cl- imaging technique with a Cl- indicator 6-methoxy-N-ethylquinolinium iodide (MEQ). The synaptically activated Cl- accumulation was mediated dominantly through GABAA receptors. Basket cells, a subclass of fast-spiking interneurons innervating the somatic portion of the pyramidal cells, actually fired at high frequency during the Cl- accumulation accompanying the depolarizing responses. These results suggest synaptically activated GABAA-mediated Cl- accumulation may play a critical role in generation of an excitatory GABAergic response in the mature pyramidal cells receiving intense synaptic inputs. This may be the first demonstration of microscopic visualization of intracellular Cl- accumulation during synaptic activation.

Glutamatergic Propagation of GABAergic Seizure-Like Afterdischarge in the Hippocampus In Vitro

Previous investigations have suggested that GABA may act actively as an excitatory mediator in the generation of seizure-like (ictal) or interictal epileptiform activity in several experimental models of temporal lobe epilepsy. However, it remains to be known whether or not such GABAergic excitation may participate in seizure propagation into neighboring cortical regions. In our in vitro study using mature rat hippocampal slices, we examined the cellular mechanism underlying synchronous propagation of seizure-like afterdischarge in the CA1 region, which is driven by depolarizing GABAergic transmission, into the adjacent subiculum region. Tetanically induced seizure-like afterdischarge was always preceded by a GABAergic, slow posttetanic depolarization in the pyramidal cells of the original seizure-generating region. In contrast, the slow posttetanic depolarization was no longer observed in the subicular pyramidal cells when the afterdischarge was induced in the CA1 region. Surgical cutting of axonal pathways through the stratum oriens and the alveus between the CA1 and the subiculum region abolished the CA1-generated afterdischarge in the subicular pyramidal cells. Intracellular loading of fluoride ions, a GABAA receptor blocker, into single subicular pyramidal cells had no inhibitory effect on the CA1-generated afterdischarge in the pyramidal cells. Furthermore, the CA1-generated afterdischarge in the subicular pyramidal cells was largely depressed by local application of glutamate receptor antagonists to the subiculum region during afterdischarge generation. The present results indicate that the excitatory GABAergic generation of seizure-like activity seems to be restricted to epileptogenic foci of origin in the seizure-like epilepsy model in vitro.

Excitatory GABA input directly drives seizure-like rhythmic synchronization in mature hippocampal CA1 pyramidal cells

GABA, which generally mediates inhibitory synaptic transmissions, occasionally acts as an excitatory transmitter through intense GABA(A) receptor activation even in adult animals. The excitatory effect results from alterations in the gradients of chloride, bicarbonate, and potassium ions, but its functional role still remains a mystery. Here we show that such GABAergic excitation participates in the expression of seizure-like rhythmic synchronization (afterdischarge) in the mature hippocampal CA1 region. Seizure-like afterdischarge was induced by high-frequency synaptic stimulation in the rat hippocampal CA1-isolated slice preparations. The hippocampal afterdischarge was completely blocked by selective antagonists of ionotropic glutamate receptors or of GABA(A) receptor, and also by gap-junction inhibitors. In the CA1 pyramidal cells, oscillatory depolarizing responses during the afterdischarge were largely dependent on chloride conductance, and their reversal potentials (average -38 mV) were very close to those of exogenously applied GABAergic responses. Moreover, intracellular loading of the GABA(A) receptor blocker fluoride abolished the oscillatory responses in the pyramidal cells. Finally, the GABAergic excitation-driven afterdischarge has not been inducible until the second postnatal week. Thus, excitatory GABAergic transmission seems to play an active functional role in the generation of adult hippocampal afterdischarge, in cooperation with glutamatergic transmissions and possible gap junctional communications. Our findings may elucidate the cellular mechanism of neuronal synchronization during seizure activity in temporal lobe epilepsy.

A distinct form of calcium release down-regulates membrane excitability in neocortical pyramidal cells

We reported a novel type of calcium release from inositol-1,4,5-trisphosphate (IP(3))-sensitive calcium stores synergistically induced by muscarinic acetylcholine receptor (mAchR)-mediated increase in IP(3) and action potential-induced calcium influx (IP(3)-assisted calcium-induced calcium release, IP(3)-assisted CICR). To clarify its functional significance, the effects of IP(3)-assisted CICR on spike-frequency adaptation were examined in layer II/III neurons from rat visual cortex slices. IP(3)-assisted CICR was enabled with a high concentration of the mAchR agonist carbachol (10 microM). The magnitude of this CICR was the more augmented at higher firing frequencies. With 10 microM carbachol, spike-frequency adaptation was reduced for spike trains at 'low' firing frequencies (6-10 Hz), but was rather enhanced at 'high' firing rates (16-22 Hz): excitability was down-regulated at 'high' frequencies. With 1 microM carbachol, by contrast, IP(3)-assisted CICR failed to occur, and spike-frequency adaptation was always reduced at any spike frequencies. Intracellular injection of the IP(3) receptor blocker heparin prevented both the mAchR-mediated occurrence of IP(3)-assisted CICR and enhancement of spike-frequency adaptation with 10 microM carbachol. Both of these mAchR-mediated effects were reproduced by intracellular IP(3) injection, and were shown to be associated with each other by simultaneous recordings of membrane potential and intracellular calcium increase. We propose that IP(3)-assisted CICR offers a novel way to protect these cortical neurons from hyperexcitability and presumably from excitotoxic cell death.

Emergence of a functional coupling between inositol-1,4,5-trisphosphate receptors and calcium channels in developing neocortical neurons

Cortical pyramidal neurons are considered to be less excitable in the immature cortex than in adults. Our previous report revealed that a negative feedback regulation of membrane excitability is highly correlated with a novel form of calcium release from inositol-1,4,5-trisphosphate (IP(3))-sensitive calcium stores (IP(3)-assisted calcium-induced calcium release) in neocortical pyramidal neurons under muscarinic cholinergic activation. As a step to understand the ground for the low membrane excitability in immature tissue, we examined development of IP(3)-assisted calcium-induced calcium release. In visual cortex neurons from 'juvenile' rats (2-3 weeks of age), an enhancement of spike-frequency adaptation occurred at high spike-frequencies (16-22 Hz), whereas the reduction was observed at low frequencies (6-10 Hz). IP(3)-assisted calcium-induced calcium release occurred at the higher frequencies only. In 'early' postnatal tissue (1 week of age), by contrast, at neither high nor low frequencies did this form of calcium release occur, and muscarinic cholinergic activation always induced a reduction of spike-frequency adaptation at any spike-frequencies. The mechanism for the failure of induction of IP(3)-assisted calcium-induced calcium release in 'early' postnatal tissue was investigated. Both an ample supply of calcium influx, elicited by higher frequency spike trains, and a supplementary injection of IP(3) through whole-cell pipets, combined together or applied alone, failed to enable IP(3)-assisted calcium-induced calcium release in 'early' postnatal tissue. Muscarinic cholinergic activation alone induced a conventional IP(3)-induced calcium release similar to that observed in neurons from 'juvenile' tissue. Together, it is most likely that functional IP(3)Rs and calcium channels are already present and functional, but are not yet adequately assembled to allow IP(3)-assisted calcium-induced calcium release in cortical pyramidal neurons from rats of 1 week old.

Changes in calcium signaling during postembryonic dendritic growth in Manduca sexta

Activity-dependent Ca(2+) influx plays crucial roles in adult and developing nervous systems through its influence on signal processing, synaptic plasticity, and neuronal differentiation. The responses to internal Ca(2+) elevations vary depending on the spatial distribution of Ca(2+) accumulation in different cell compartments. In this study, the mechanisms and the distribution of Ca(2+) accumulation are addressed by in situ Ca(2+) imaging of an identified insect motoneuron, MN5, at critical stages of postembryonic life. During metamorphosis of Manduca sexta, MN5 undergoes extensive dendritic regression followed by regrowth. The time course, amplitude, and distribution of Ca(2+) accumulation within MN5 change during development. During the initial stage of rapid dendritic growth and branching, dendritic growth cones are present, and voltage-dependent Ca(2+) currents are small. At this stage, activity-induced elevations of internal Ca(2+) are largest in the distal dendrites, suggesting that the density of voltage-gated Ca(2+) channels is highest in these regions. Later phases of dendritic growth are accompanied by the transient occurrence of prominent Ca(2+) spikes. Single Ca(2+) spikes cause robust Ca(2+) influx of similar amplitudes and time courses in all central compartments of MN5. The resting Ca(2+) levels also increase during development. Ca(2+)-induced Ca(2+) release from intracellular stores did not contribute to the elevations measured at either stage, although Ca(2+) stores are present in the dendrites. These developmental changes of the internal Ca(2+) signaling are consistent with a regulatory role for activity-dependent Ca(2+) influx in postembryonic dendritic growth.

Distance-dependent Ni(2+)-sensitivity of synaptic plasticity in apical dendrites of hippocampal CA1 pyramidal cells

Low concentration of Ni(2+), a T- and R-type voltage-dependent calcium channel (VDCC) blocker, is known to inhibit the induction of long-term potentiation (LTP) in the hippocampal CA1 pyramidal cells. These VDCCs are distributed more abundantly at the distal area of the apical dendrite than at the proximal dendritic area or soma. Therefore we investigated the relationship between the Ni(2+)-sensitivity of LTP induction and the synaptic location along the apical dendrite. Field potential recordings revealed that 25 microM Ni(2+) hardly influenced LTP at the proximal dendritic area (50 microM distant from the somata). In contrast, the same concentration of Ni(2+) inhibited the LTP induction mildly at the middle dendritic area (150 microM) and strongly at the distal dendritic area (250 microM). Ni(2+) did not significantly affect either the synaptic transmission at the distal dendrite or the burst-firing ability at the soma. However, synaptically evoked population spikes recorded near the somata were slightly reduced by Ni(2+) application, probably owing to occlusion of dendritic excitatory postsynaptic potential (EPSP) amplification. Even when the stimulating intensity was strengthened sufficiently to overcome such a reduction in spike generation during LTP induction, the magnitude of distal LTP was not significantly recovered from the Ni(2+)-dependent inhibition. These results suggest that Ni(2+) may inhibit the induction of distal LTP directly by blocking calcium influx through T- and/or R-type VDCCs. The differentially distributed calcium channels may play a critical role in the induction of LTP at dendritic synapses of the hippocampal pyramidal cells.

Subcellular distribution of high-voltage-activated calcium channel subtypes in rat globus pallidus neurons

Globus pallidus (GP) neurons receive dense inhibitory synaptic inputs interspersed with sparse excitatory inputs distributed across the entire extent of their somata and dendrites. Yet, despite this predominance of inhibitory influence, GP neurons fire at a high tonic rate, suggesting that intrinsic properties play an important role in determining the physiological characteristics of these neurons. High-voltage-activated (HVA) calcium channels represent an important class of conductances that plays roles in controlling neurotransmitter release, postsynaptic excitability, and intracellular calcium signaling. To better understand the intrinsic properties of GP neurons, we examined the subcellular localization of HVA calcium channels by using immunocytochemistry at the electron microscopic level. Peroxidase labeling with antibodies against P/Q-, N-, and R-type HVA calcium channels demonstrated the presence of these channels in both proximal and distal dendrites of GP neurons. P/Q-, N-, and R-type channels were also found in presynaptic terminals, whereas L-type channels were found exclusively postsynaptically in neuronal elements. Immunogold labeling demonstrated that, although the density of intracellular L-type calcium channel labeling remains constant throughout the proximal-distal extent of the dendritic tree of GP neurons, the density of plasma membrane-bound channels is greater in distal dendrites. The finding of HVA calcium channels distributed throughout the whole dendritic tree of GP neurons indicates that these channels may interact with synaptic inputs to allow rich processing possibilities for GP neuron dendrites. Furthermore, the finding of a greater density of plasma membrane-bound L-type channels in distal dendrites expands the view that L-type channels are important only in somatic and proximal locations.

N-(2-Chloroethyl)-N-ethyl-2-bromobenzylamine reduces intracellular calcium response to noradrenaline in rat visual cortex

Using the fluorescent indicator Fura-2, we investigated the effects of N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine (DSP-4), a noradrenergic neurotoxin, on intracellular calcium responses to noradrenaline, N-methyl-D-aspartate, and carbamylcholine chloride in brain slices of the rat visual cortex. Noradrenergic depletion in the visual cortex of young rats was induced by DSP-4, and its selectivity was confirmed by two different methods, i.e., immunostaining with anti-dopamine-beta-hydroxylase antibody and biochemical analysis by high-performance liquid chromatography. The treatment with DSP-4 (25 mg/kg i.p., x2) caused disruption of noradrenergic fibers throughout all cortical layers, and reduced the content of noradrenaline to 6.4% of that in the normal control. In the normal cortex, bath-applied noradrenaline (100 microM) increased the intracellular calcium to 123% of the control in terms of the F(340)/F(380) ratio of Fura-2 fluorescence. Quantitative analysis of the F(340)/F(380) ratio was performed in layers II to IV, since the increase was mainly observed in these layers. The intracellular calcium response to noradrenaline was significantly (P<0.0001) reduced in the DSP-4-treated animals to 63.2% of that in the normal control. The response to N-methyl-D-aspartate (100 microM) was also reduced, whereas the response to carbamylcholine chloride, a muscarinic cholinergic agonist (100 microM), was not affected by the DSP-4 treatment. From these findings we suggest that noradrenergic denervation by DSP-4 reduces the intracellular calcium response to noradrenaline through changes in the intracellular signal transduction.

Differentiation of electrical excitability in motoneurons

Investigation of the differentiation of electrical properties of motoneurons has been stimulated by the importance of these neurons for embryonic behavior and facilitated by their experimental accessibility. In this review, we examine the development of different patterns of excitability and their functions, and discuss the emergence of repetitive firing and localization of ion channels in axons and dendrites. Finally, we summarize studies of the role of extrinsic factors in differentiation. These changes associated with differentiation of young motoneurons may presage those occurring later in the context of plasticity in the mature nervous system.

Possible regulatory role of dendritic spikes in induction of long-term potentiation at hippocampal schaffer collateral-CA1 synapses

The amplitude of backpropagating action potentials (BAPs) is attenuated, either activity- or neurotransmitter-dependently in the apical dendrite of hippocampal pyramidal neurons. To test the possibility that this BAP attenuation may contribute to regulating the inducibility of long-term potentiation (LTP), BAPs evoked by theta-burst stimulation (TBS), a standard protocol for LTP induction, to apical dendrite synapses were subjected to perturbation by conditioning stimuli to basal dendrite synapses. During this conditioned TBS (cTBS), the amplitude of BAPs was noticeably attenuated, but that of somatic action potentials was not. In the distal dendrite area, cTBS-induced LTP was much smaller than that induced by TBS. By contrast, no difference was observed between TBS- and cTBS-induced LTP in the proximal dendrite area. These findings suggest that the activity-dependent attenuation of BAPs, propagating along the apical dendrite, may serve to regulate hippocampal synaptic plasticity.

Intracellular calcium releases facilitate induction of long-term depression

In visual cortex layer II/III pyramidal neurons, long-term depression (LTD) of synaptic currents was induced by a combination of the intracellular increase of inositol-1,4,5-trisphosphate (IP(3)), achieved by photolyzing caged-IP(3), and the tetanization to nearby cortex, but not by either of these two procedures alone. A facilitatory role of an IP(3)-induced calcium release in LTD is suggested.

Natural patterns of activity and long-term synaptic plasticity

Long-term potentiation (LTP) of synaptic transmission is traditionally elicited by massively synchronous, high-frequency inputs, which rarely occur naturally. Recent in vitro experiments have revealed that both LTP and long-term depression (LTD) can arise by appropriately pairing weak synaptic inputs with action potentials in the postsynaptic cell. This discovery has generated new insights into the conditions under which synaptic modification may occur in pyramidal neurons in vivo. First, it has been shown that the temporal order of the synaptic input and the postsynaptic spike within a narrow temporal window determines whether LTP or LTD is elicited, according to a temporally asymmetric Hebbian learning rule. Second, backpropagating action potentials are able to serve as a global signal for synaptic plasticity in a neuron compared with local associative interactions between synaptic inputs on dendrites. Third, a specific temporal pattern of activity--postsynaptic bursting--accompanies synaptic potentiation in adults.

An IP3-assisted form of Ca2+-induced Ca2+ release in neocortical neurons

Calcium increases induced by single action potentials in rat visual cortex layer II/III pyramidal neurons were shown to be augmented by muscarinic acetylcholine receptor (mAchR) stimulation. This augmentation was drastically reduced by intracellular injection of heparin but not ruthenium red, therefore involving inositol-1,4,5-trisphosphate (IP3)-sensitive rather than ryanodine-sensitive calcium stores. Only the calcium increase induced by the second or later spike of a spike train, but not that induced by the first spike, was augmented, indicating the requirement of both spike-induced calcium increase and mAchR activation. The calcium store depletor thapsigargin abolished this augmentation use-dependently. These findings suggest a neocortical occurrence of calcium-induced calcium release from IP3-sensitive calcium stores that have been sensitized beforehand by IP3 through mAchR-mediated mechanisms.