Pyramidal Cell Somata Research Articles

Regulation of neuronal input transformations by tunable dendritic inhibition

Transforming synaptic input into action potential output is a fundamental function of neurons. The pattern of action potential output from principal cells of the mammalian hippocampus encodes spatial and nonspatial information, but the cellular and circuit mechanisms by which neurons transform their synaptic input into a given output are unknown. Using a combination of optical activation and cell type-specific pharmacogenetic silencing in vitro, we found that dendritic inhibition is the primary regulator of input-output transformations in mouse hippocampal CA1 pyramidal cells, and acts by gating the dendritic electrogenesis driving burst spiking. Dendrite-targeting interneurons are themselves modulated by interneurons targeting pyramidal cell somata, providing a synaptic substrate for tuning pyramidal cell output through interactions in the local inhibitory network. These results provide evidence for a division of labor in cortical circuits, where distinct computational functions are implemented by subtypes of local inhibitory neurons.

Convergence of parvalbumin and type 1 cannabinoid receptor positive perisomatic inputs onto hippocampal pyramidal cells

Event Abstract Back to Event Convergence of parvalbumin and type 1 cannabinoid receptor positive perisomatic inputs onto hippocampal pyramidal cells V. T. Takács1*, T. F. Freund1 and A. I. Gulyás1 1 Hungarian Academy of Sciences, Dept. of Cellular and Network Neurobiology, Institute of Experimental Medicine, Hungary The soma and proximal dendrites of cortical pyramidal cells are innervated by two functionally distinct types of perisomatic interneurons: the parvalbumin (PV)- and the cholecystokinin-containing, type 1 cannabinoid receptor (CB1) positive basket cells. Here we estimated the proportion of PV- and CB1-positive basket cell inputs in mice by reconstructing parts of hippocampal CA1 pyramidal cell somata and the contacting boutons from serial electron microscopic sections using a new double staining method that gives distinguishable signal at the electron microscopic level. All but 1% of the somatic boutons were labeled by one of the markers. The number of PV- and CB1-positive boutons/soma was ~35 and ~23, respectively. Next, we have measured some of the ultrastructural parameters of 3D reconstructed immunogold-labeled PV- and CB1 positive boutons. The volume occupied by mitochondria was significantly larger in PV-positive boutons. Both types formed more than one synapses often on the same soma or an additional soma or a dendrite. PV-positive boutons possessed small, macular shaped active zones. Synapses of CB1-positive boutons formed a complex structure with multiple small and large irregular shaped active zones. The total synaptic areas/bouton was significantly larger in the case of CB1-positive boutons. Our results show that the physiological differences of the two basket cell types are mirrored by morphological diversity of their boutons. Acknowledgements Support: NKTH-OTKA CNK 77793, NIH DA11322. Keywords: Molecular and cellular neurobiology, Neuroscience Conference: 13th Conference of the Hungarian Neuroscience Society (MITT), Budapest, Hungary, 20 Jan - 22 Jan, 2011. Presentation Type: Abstract Topic: Molecular and cellular neurobiology Citation: Takács VT, Freund TF and Gulyás AI (2011). Convergence of parvalbumin and type 1 cannabinoid receptor positive perisomatic inputs onto hippocampal pyramidal cells. Front. Neurosci. Conference Abstract: 13th Conference of the Hungarian Neuroscience Society (MITT). doi: 10.3389/conf.fnins.2011.84.00051 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Mar 2011; Published Online: 23 Mar 2011. * Correspondence: Dr. V. T Takács, Hungarian Academy of Sciences, Dept. of Cellular and Network Neurobiology, Institute of Experimental Medicine, Budapest, Hungary, viragt@koki.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers V. T Takács T. F Freund A. I Gulyás Google V. T Takács T. F Freund A. I Gulyás Google Scholar V. T Takács T. F Freund A. I Gulyás PubMed V. T Takács T. F Freund A. I Gulyás Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

A unique distribution pattern of the Kv4.2 channels on hippocampal CA1 pyramidal cells is revealed by freeze-fracture replica labeling

Event Abstract Back to Event A unique distribution pattern of the Kv4.2 channels on hippocampal CA1 pyramidal cells is revealed by freeze-fracture replica labeling K. M. Kerti1, A. Lőrincz1 and Z. Nusser1* 1 Hungarian Academy of Sciences, Institute of Experimental Medicine, Hungary A-current flowing through Kv4.2 voltage-gated K+ channels plays a critical role in controlling pyramidal cell (PC) excitability. Electrophysiological recordings have demonstrated a 6-fold increase in A-type K+ current density along the somato-dendritic axis of hippocampal CA1 PCs. However, the underlying channel distribution and densities remained unknown. Here we examined the subcellular distribution and densities of Kv4.2 subunits in CA1 PC somata and 11 distinct dendritic compartments using a highly sensitive immunogold method (SDS-digested freeze-fracture replica labeling). Only a moderate (only 50%) increase in Kv4.2 immunogold density was observed along the proximo-distal axis of apical dendrites in the stratum radiatum with a slight decrease in stratum lacunosum-moleculare. This was characteristic to all dendritic compartments, including main apical dendrites, oblique dendrites and dendritic spines. In addition to the somato-dendritic localization, the Kv4.2 subunit was also found in axon terminals at a 4-fold lower density. Our results demonstrate a novel distribution pattern of the Kv4.2 subunit along the axo-somato-dendritic surface of CA1 PCs and suggest that the strong increase in the A-current density cannot be solely explained by a corresponding increase in channel number. Keywords: Molecular and cellular neurobiology, Neuroscience Conference: 13th Conference of the Hungarian Neuroscience Society (MITT), Budapest, Hungary, 20 Jan - 22 Jan, 2011. Presentation Type: Abstract Topic: Molecular and cellular neurobiology Citation: Kerti KM, Lőrincz A and Nusser Z (2011). A unique distribution pattern of the Kv4.2 channels on hippocampal CA1 pyramidal cells is revealed by freeze-fracture replica labeling. Front. Neurosci. Conference Abstract: 13th Conference of the Hungarian Neuroscience Society (MITT). doi: 10.3389/conf.fnins.2011.84.00153 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Mar 2011; Published Online: 23 Mar 2011. * Correspondence: Dr. Z. Nusser, Hungarian Academy of Sciences, Institute of Experimental Medicine, Budapest, Hungary, nusser@koki.hu Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers K. M Kerti A. Lőrincz Z. Nusser Google K. M Kerti A. Lőrincz Z. Nusser Google Scholar K. M Kerti A. Lőrincz Z. Nusser PubMed K. M Kerti A. Lőrincz Z. Nusser Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

GABAergic complex basket formations in the human neocortex

Certain GABAergic interneurons in the cerebral cortex, basket cells, establish multiple connections with cell bodies that typically outline the somata and proximal dendrites of pyramidal cells. During studies into the distribution of the vesicular GABA transporter (VGAT) in the human cerebral cortex, we were struck by the presence of a very dense, pericellular arrangement of multiple VGAT-immunoreactive (-ir) terminals in certain cortical areas. We called these terminals "Complex basket formations" (Cbk-formations) to distinguish them from the simpler and more typical pericellular GABAergic innervations of most cortical neurons. Here we examined the distribution of these VGAT-ir Cbk-formations in various cortical areas, including the somatosensory (area 3b), visual (areas 17 and 18), motor (area 4), associative frontal (dorsolateral areas 9, 10, 45, 46, and orbital areas 11, 12, 13, 14, 47), associative temporal (areas 20, 21, 22, and 38), and limbic cingulate areas (areas 24, 32). Furthermore, we used dual or triple staining techniques to study the chemical nature of the innervated cells. We found that VGAT-ir Cbk-formations were most frequently found in area 4 followed by areas 3b, 13, and 18. In addition, they were mostly observed in layer III, except in area 17, where they were most dense in layer IV. We also found that 70% of the innervated neurons were pyramidal cells, while the remaining 30% were multipolar cells. Most of these multipolar cells expressed the calcium-binding protein parvalbumin and the lectin Vicia villosa agglutinin.

From Dendrite to Soma: Dynamic Routing of Inhibition by Complementary Interneuron Microcircuits in Olfactory Cortex

Diverse inhibitory pathways shape cortical information processing; however, the relevant interneurons recruited by sensory stimuli and how they impact principal cells are unclear. Here we show that two major interneuron circuits govern dynamic inhibition in space and time within the olfactory cortex. Dendritic-targeting layer 1 interneurons receive strong input from the olfactory bulb and govern early-onset feedforward inhibition. However, this circuit is only transiently engaged during bursts of olfactory bulb input. In contrast, somatic-targeting layer 3 interneurons, recruited exclusively by recurrent excitation from pyramidal cells, produce late-onset feedback inhibition. Our results reveal two complementary interneuron circuits enforcing widespread inhibition, which shifts from the apical dendrites to somata of pyramidal cells during bursts of sensory input.

Two forms of feedback inhibition determine the dynamical state of a small hippocampal network

Pyramidal cells in the hippocampus are part of a small neuronal network that performs computations on external input. The network consists of principal cells and various forms of feedback inhibition. Experimental evidence indicates at least two functionally distinct inhibitory feedback loops in the CA3 area of the hippocampus: (1) a loop in which O-LM interneurons project to the distal dendrites of pyramidal cells with synapses that have slow kinetics, and (2) a loop in which basket interneurons project to the somata of pyramidal cells with synapses that have fast kinetics. There is an interconnection between the two loops in the form of O-LM to basket interneuron inhibition and the configuration is further complicated by the presence of distinct propagation delays and short-term facilitation and depression of certain synapses in the two basic loops. In this study we investigated the consequences of various configurations of the circuit and modulations of the components of inhibition for the computation that the network can perform on its input. Gaussian noise was used as the input to the dendrite of the pyramidal cell and evoked two types of events: spikes or bursts. The event-triggered average (ETA) and the event-triggered covariance (ETC) were determined and the inter-event-intervals between spikes and bursts were analyzed. The ETA and ETC on the pyramidal cell show that this model behaves in first approximation as an activity integrator: with sufficient positive input, bursts as well as spikes are evoked. Which of the two is determined by the input just after the (first) spike: positive input results in a burst; negative input results in a spike. Stronger feedback inhibition, in the slow as well as in the fast loop, increases the event rate of the pyramidal cell. For a single input and large propagation delays, the interaction between the two feedback loops is not of great importance. The consequences of the presence of the slow and/or fast feedback inhibitory loop, with or without facilitation and depression, were analyzed in relation to synapse strength. Facilitation and depression are most relevant when their recovery time constant is of the same order as the mean inter-event interval. Short-term depression can stop activity in the fast loop after several fast spikes and can switch the network to a different state, thus functioning as a kind of 'brake' on the fast inhibitory feedback loop. Thus inhibition and the details of the microcircuit organization play an important role in the information processing of the small neuronal circuit.

Activity-Dependent Control of Neuronal Output by Local and Global Dendritic Spike Attenuation

Neurons possess elaborate dendritic arbors which receive and integrate excitatory synaptic signals. Individual dendritic subbranches exhibit local membrane potential supralinearities, termed dendritic spikes, which control transfer of local synaptic input to the soma. Here, we show that dendritic spikes in CA1 pyramidal cells are strongly regulated by specific types of prior input. While input in the linear range is without effect, supralinear input inhibits subsequent spikes, causing them to attenuate and ultimately fail due to dendritic Na(+) channel inactivation. This mechanism acts locally within the boundaries of the input branch. If an input is sufficiently strong to trigger axonal action potentials, their back-propagation into the dendritic tree causes a widespread global reduction in dendritic excitability which is prominent after firing patterns occurring in vivo. Together, these mechanisms control the capability of individual dendritic branches to trigger somatic action potential output. They are invoked at frequencies encountered during learning, and impose limits on the storage and retrieval rates of information encoded as branch excitability.

Looking After Polysialic Acid

Many aspects of brain function are modulated by neuronal activity; in some cases, the effects of activity are restricted to particular developmental stages, or critical periods. For instance, loss of visual input from one eye early in life leads to a shift in the responsiveness of neurons in the visual cortex toward the functional eye. The onset and time course of this critical period for ocular dominance plasticity is influenced by the maturation of GABAergic innervation, which is itself regulated by visual input and neuronal activity. But how? Di Cristo et al . found that polysialic acid [PSA, a homopolymer of sialic acid that attaches to neural cell adhesion molecules (NCAMs) and modulates intercellular bonds] and NCAMs were abundant in the neonatal mouse visual cortex but that PSA underwent a steep decline after eye opening. Pups reared in the dark showed increased PSA compared with pups reared under conventional light/dark cycles, and pharmacological suppression of visual signals suppressed the decline. In organotypic cortical cultures, enzymatic removal of PSA with endoneuramidase led to premature maturation of the innervation of pyramidal cell somata by GABAergic basket interneurons. Similarly, endoneuramidase injection into the visual cortex promoted both perisomatic GABAergic innervation and the frequency of miniature inhibitory postsynaptic currents. Moreover, it stimulated the early onset of the critical period for ocular dominance plasticity. Thus, the authors conclude that maturation of GABAergic inhibition--and thereby the initiation of ocular dominance plasticity--is regulated by an activity-dependent decrease in PSA. G. Di Cristo, B. Chattopadhyaya, S. J. Kuhlman, Y. Fu, M.-C. Bélanger, C. Z. Wu, U. Rutishauser, L. Maffei, Z. J. Huang, Activity-dependent PSA expression regulates inhibitory maturation and onset of critical period plasticity. Nat. Neurosci . 10 , 1569-1577 (2007). [PubMed]

Sensitivity of coherent oscillations in rat hippocampus to AC electric fields

The sensitivity of brain tissue to weak extracellular electric fields is important in assessing potential public health risks of extremely low frequency (ELF) fields, and potential roles of endogenous fields in brain function. Here we determine the effect of applied electric fields on membrane potentials and coherent network oscillations. Applied DC electric fields change transmembrane potentials in CA3 pyramidal cell somata by 0.18 mV per V m(-1) applied. AC sinusoidal electric fields have smaller effects on transmembrane potentials: sensitivity drops as an exponential decay function of frequency. At 50 and 60 Hz it is approximately 0.4 that for DC fields. Effects of fields of < or = 16 V m(-1) peak-to-peak (p-p) did not outlast application. Kainic acid (100 nm) induced coherent network oscillations in the beta and gamma bands (15-100 Hz). Applied fields of > or = 6 V m(-1) p-p (2.1 V m(-1) r.m.s.) shifted the gamma peak in the power spectrum to centre on the applied field frequency or a subharmonic. Statistically significant effects on the timing of pyramidal cell firing within the oscillation appeared at distinct thresholds: at 50 Hz, 1 V m(-1) p-p (354 mV m(-1) r.m.s.) had statistically significant effects in 71% of slices, and 0.5 V m(-1) p-p (177 mV m(-1) r.m.s.) in 20%. These threshold fields are consistent with current environmental guidelines. They correspond to changes in somatic potential of approximately 70 microV, below membrane potential noise levels for neurons, demonstrating the emergent properties of neuronal networks can be more sensitive than measurable effects in single neurons.

Lifespan Alterations of Basal Dendritic Trees of Pyramidal Neurons in the Human Prefrontal Cortex: A Layer-Specific Pattern

The postnatal development and lifespan alterations in basal dendrites of large layer IIIC and layer V pyramidal neurons were quantitatively studied. Both classes of neurons were characterized by rapid dendritic growth during the first postnatal months. At birth, layer V pyramidal neurons had larger and more complex dendritic trees than those of layer IIIC; however, at 1 postnatal month both classes of neurons displayed a similar extent of dendritic outgrowth. In addition, after a more than year-long "dormant" period of only fine dendritic rearrangement, layer IIIC pyramidal neurons displayed a second period of dendritic growth, starting at the end of the second year and continuing in the third year. During that period, the dendritic tree of layer IIIC pyramidal neurons became more extensive than that of layer V pyramidal neurons. Thus, layer IIIC pyramidal neurons appear to show a biphasic pattern of postnatal dendritic development. Furthermore, the childhood period was characterized by transient increase in size of pyramidal cell somata, which was more pronounced for neurons in layer IIIC. These structural changes occurred during both the period of rapid cognitive development in preschool children and the period of protracted cognitive maturation during the childhood, puberty, and adolescence.

Cholecystokinin inhibits endocannabinoid-sensitive hippocampal IPSPs and stimulates others

Cholecystokinin (CCK) is the most abundant neuropeptide in the central nervous system. In the hippocampal CA1 region, CCK is co-localized with GABA in a subset of interneurons that synapse on pyramidal cell somata and apical dendrites. CCK-containing interneurons also uniquely express a high level of the cannabinoid receptor, CB1, and mediate the retrograde signaling process called DSI. Reported effects of CCK on inhibitory post-synaptic potentials (IPSPs) in hippocampus are inconsistent, and include both increases and decreases in activity. Hippocampal interneurons are very heterogeneous, and these results could be reconciled if CCK affected different interneurons in different ways. To test this prediction, we used sharp microelectrode recordings from pyramidal cells with ionotropic glutamate receptors blocked, and investigated the effects of CCK on pharmacologically distinct groups of IPSPs during long-term recordings. We find that CCK, acting via the CCK2 receptor, increases some IPSPs and decreases others, and most significantly, that the affected IPSPs can be classified into two groups by their pharmacological properties. IPSPs that are increased by carbachol (CCh-sIPSPs), are depressed by CCK, ω-conotoxin GVIA, and endocannabinoids. IPSPs that are enhanced by CCK (CCK-sIPSPs) are blocked by ω-agatoxin IVA, and are unaffected by carbachol or endocannabinoids. Interestingly, a CCK2 antagonist enhances CCh-sIPSPs, suggesting normally they may be partially suppressed by endogenous CCK. In summary, our data are compatible with the hypothesis that CCK has opposite actions on sIPSPs that originate from functionally distinct interneurons.

Extrasynaptic αβ subunit GABAA receptors on rat hippocampal pyramidal neurons

Extrasynaptic GABAA receptors that are tonically activated by ambient GABA are important for controlling neuronal excitability. In hippocampal pyramidal neurons, the subunit composition of these extrasynaptic receptors may include α5βγ and/or α4βδ subunits. Our present studies reveal that a component of the tonic current in the hippocampus is highly sensitive to inhibition by Zn2+. This component is probably not mediated by either α5βγ or α4βδ receptors, but might be explained by the presence of αβ isoforms. Using patch‐clamp recording from pyramidal neurons, a small tonic current measured in the absence of exogenous GABA exhibited both high and low sensitivity to Zn2+ inhibition (IC50 values, 1.89 and 223 μm, respectively). Using low nanomolar and micromolar GABA concentrations to replicate tonic currents, we identified two components that are mediated by benzodiazepine‐sensitive and ‐insensitive receptors. The latter indicated that extrasynaptic GABAA receptors exist that are devoid of γ2 subunits. To distinguish whether the benzodiazepine‐insensitive receptors were αβ or αβδ isoforms, we used single‐channel recording. Expressing recombinant α1β3γ2, α5β3γ2, α4β3δ and α1β3 receptors in human embryonic kidney (HEK) or mouse fibroblast (Ltk) cells, revealed similar openings with high main conductances (∼25–28 pS) for γ2 or δ subunit‐containing receptors whereas αβ receptors were characterized by a lower main conductance state (∼11 pS). Recording from pyramidal cell somata revealed a similar range of channel conductances, indicative of a mixture of GABAA receptors in the extrasynaptic membrane. The lowest conductance state (∼11 pS) was the most sensitive to Zn2+ inhibition in accord with the presence of αβ receptors. This receptor type is estimated to account for up to 10% of all extrasynaptic GABAA receptors on hippocampal pyramidal neurons.

Postsynaptic targets of somatostatin‐containing interneurons in the rat basolateral amygdala

The basolateral amygdala contains several subpopulations of inhibitory interneurons that can be distinguished on the basis of their content of calcium-binding proteins or peptides. Although previous studies have shown that interneuronal subpopulations containing parvalbumin (PV) or vasoactive intestinal peptide (VIP) innervate distinct postsynaptic domains of pyramidal cells as well as other interneurons, very little is known about the synaptic outputs of the interneuronal subpopulation that expresses somatostatin (SOM). The present study utilized dual-labeling immunocytochemical techniques at the light and electron microscopic levels to analyze the innervation of pyramidal cells, PV+ interneurons, and VIP+ interneurons in the anterior basolateral amygdalar nucleus (BLa) by SOM+ axon terminals. Pyramidal cell somata and dendrites were selectively labeled with antibodies to calcium/calmodulin-dependent protein kinase II (CaMK); previous studies have shown that the vast majority of dendritic spines, whether CAMK+ or not, arise from pyramidal cells. Almost all SOM+ axon terminals formed symmetrical synapses. The main postsynaptic targets of SOM+ terminals were small-caliber CaMK+ dendrites and dendritic spines, some of which were CaMK+. These SOM+ synapses with dendrites were often in close proximity to asymmetrical (excitatory) synapses to these same structures formed by unlabeled terminals. Few SOM+ terminals formed synapses with CaMK+ pyramidal cell somata or large-caliber (proximal) dendrites. Likewise, only 15% of SOM+ terminals formed synapses with PV+, VIP+, or SOM+ interneurons. These findings suggest that inhibitory inputs from SOM+ interneurons may interact with excitatory inputs to pyramidal cell distal dendrites in the BLa. These interactions might affect synaptic plasticity related to emotional learning.

Differential Expression ofIAChannel Subunits Kv4.2 and Kv4.3 in Mouse Visual Cortical Neurons and Synapses

In cortical neurons, pore-forming alpha-subunits of the Kv4 subfamily underlie the fast transient outward K+ current (I(A)). Considerable evidence has accumulated demonstrating specific roles for I(A) channels in the generation of individual action potentials and in the regulation of repetitive firing. Although I(A) channels are thought to play a role in synaptic processing, little is known about the cell type- and synapse-specific distribution of these channels in cortical circuits. Here, we used immunolabeling with specific antibodies against Kv4.2 and Kv4.3, in combination with GABA immunogold staining, to determine the cellular, subcellular, and synaptic localization of Kv4 channels in the primary visual cortex of mice, in which subsets of pyramidal cells express yellow fluorescent protein. The results show that both Kv4.2 and Kv4.3 are concentrated in layer 1, the bottom of layer 2/3, and in layers 4 and 5/6. In all layers, clusters of Kv4.2 and Kv4.3 immunoreactivity are evident in the membranes of the somata, dendrites, and spines of pyramidal cells and GABAergic interneurons. Electron microscopic analyses revealed that Kv4.2 and Kv4.3 clusters in pyramidal cells and interneurons are excluded from putative excitatory synapses, whereas postsynaptic membranes at GABAergic synapses often contain Kv4.2 and Kv4.3. The presence of Kv4 channels at GABAergic synapses would be expected to weaken inhibition during dendritic depolarization by backpropagating action potentials. The extrasynaptic localization of Kv4 channels near excitatory synapses, in contrast, should stabilize synaptic excitation during dendritic depolarization. Thus, the synapse-specific distribution of Kv4 channels functions to optimize dendritic excitation and the association between presynaptic and postsynaptic activity.

Differences between the scaling of miniature IPSCs and EPSCs recorded in the dendrites of CA1 mouse pyramidal neurons

Anatomical studies have described inhibitory synaptic contacts on apical dendrites, and an abundant number of GABAergic synapses on the somata and proximal dendrites of CA1 pyramidal cells of the hippocampus. The number of inhibitory contacts decreases dramatically with distance from the soma, but the local electrophysiological characterization of these synapses at their site of origin in the dendrites is missing. We directly recorded dendritic GABA receptor-mediated inhibitory synaptic events in adult mouse hippocampal CA1 pyramidal neurons and compared them to excitatory synaptic currents recorded at the same sites. Miniature GABAergic events were evoked using localized application of a hyperosmotic solution to the apical dendrites in the vicinity of the dendritic whole-cell recording pipette. Glutamatergic synaptic events were blocked by kynurenic acid, leaving picrotoxin-sensitive IPSCs. We measured the amplitude and kinetic properties of mIPSCs at the soma and at three different dendritic locations. The amplitude of mIPSCs recorded at the various sites was similar along the somato-dendritic axis. The rise- and decay-times of local mIPSCs were also independent of the location of the synapses. The frequency of mIPSCs was 5 Hz at the soma, in contrast to < 0.5 Hz at dendritic sites, which could be increased to 10-20 Hz and 6-10 Hz, respectively, by our hyperosmotic stimulation protocol. Miniature glutamatergic events were evoked with the same protocol after blocking inhibitory synapses by bicucculine. The measured amplitudes increased along the somato-dendritic axis proportionally with their distance from the soma. The measured kinetic properties were independent of location. Consistent with the idea that IPSCs may have a restricted local effect in the dendrites, our data show a lack of distance-dependent scaling of miniature inhibitory synaptic events, in contrast to the scaling of excitatory events recorded at the same sites.

Transient ischemia-induced changes of interleukin-2 and its receptor β immunoreactivity and levels in the gerbil hippocampal CA1 region

Interlukin-2 (IL-2) is an important cytokine in the brain: IL-2 and its receptors are involved with inflammatory processes. Chronological changes in IL-2 level in serum, and IL-2 and its receptor (IL-2 receptor β, IL-2Rβ) immunoreactivities and levels were examined in the hippocampal CA1 region after transient forebrain ischemia in gerbils. IL-2 level in serum significantly decreased 12 h after ischemia/reperfusion. IL-2 immunoreactivity was detected in the somata of pyramidal cells in sham-operated group. At 15 min after ischemia, IL-2 immunoreactivity was shown in non-pyramidal cells as well as pyramidal cells. One day after ischemia, IL-2 immunoreactivity was lowest, and IL-2 immunoreactivity is shown in non-pyramidal cells from 2 days after ischemia. Four days after ischemia, IL-2 immunoreactivity was shown in dying pyramidal cells. IL-2Rβ immunoreactivity in the sham-operated and 15 min–3 min post-ischemic groups is detected in the cell membrane of pyramidal cells. From 3 h after ischemia, IL-2Rβ immunoreactivity is found in cytoplasm and nuclei, but not in cell membrane. IL-2Rβ immunoreactivity decreases from 6 h after ischemia and is shown mainly in non-pyramidal cells from 3 days after ischemia. The data of Western blot analyses for IL-2 and IL-2Rβ was similar to the immunohistochemical data. IL-2 infusion into cerebrospinal fluid did not protect hippocampal neurons from ischemic damage. These results suggest that IL-2 and IL-2Rβ show malfunction from 3 h after ischemia, and exogenous IL-2 does not protect ischemic neuronal damage.

High-voltage-activated Ca 2+ currents show similar patterns of expression in stellate and pyramidal cells from rat entorhinal cortex layer II

High-voltage-activated (HVA) Ca 2+ currents were studied in acutely isolated neurons from rat entorhinal cortex (EC) layer II. Stellate and pyramidal cells, the two main neuronal types of this structure, were visually identified based on morphological criteria. HVA currents were recorded by applying the whole-cell, patch-clamp technique, using 5-mM Ba 2+ as the charge carrier. In both neuronal types, the amplitude of total HVA Ba 2+ currents ( I Bas) showed a significant tendency to increase with postnatal age in the time window considered [postnatal day 15 (P15) to P28–29]. At P20–P29, when I Ba expression reached stable levels, I Ba density per unit of membrane area was not different in stellate versus pyramidal cells. The same was also observed when Ca 2+, instead of Ba 2+, was used as the charge carrier. The pharmacological current subtypes composing total HVA currents were characterized using selective blockers. Again, no significant differences were found between stellate and pyramidal cells with respect to the total–current fractions attributable to specific pharmacological Ca 2+ channel subtypes. In both cell types, ∼52–55% of total I Bas was abolished by the L-type channel blocker, nifedipine (10 μM), ∼23–30% by the N-type channel blocker, ω-conotoxin GVIA (1 μM), ∼22–24% by the P/Q-type channel blocker, ω-agatoxin IVA (100 nM), and ∼11–13% remained unblocked (R-type current) after simultaneous application of L-, N-, and P/Q-type channel blockers. The Ca v 2.3 (α 1E) channel blocker, SNX-482 (100 nM), abolished ∼57–62% of total R-type current. We conclude that HVA Ca 2+ currents are expressed according to similar patterns in the somata and proximal dendrites of stellate and pyramidal cells of rat EC layer II.

Metabotropic Glutamate Receptor 8-Expressing Nerve Terminals Target Subsets of GABAergic Neurons in the Hippocampus

Presynaptic metabotropic glutamate receptors (mGluRs) show a highly selective expression and subcellular location in nerve terminals modulating neurotransmitter release. We have demonstrated that alternatively spliced variants of mGluR8, mGluR8a and mGluR8b, have an overlapping distribution in the hippocampus, and besides perforant path terminals, they are expressed in the presynaptic active zone of boutons making synapses selectively with several types of GABAergic interneurons, primarily in the stratum oriens. Boutons labeled for mGluR8 formed either type I or type II synapses, and the latter were GABAergic. Some mGluR8-positive boutons also expressed mGluR7 or vasoactive intestinal polypeptide. Interneurons strongly immunopositive for the muscarinic M2 or the mGlu1 receptors were the primary targets of mGluR8-containing terminals in the stratum oriens, but only neurochemically distinct subsets were innervated by mGluR8-enriched terminals. The majority of M2-positive neurons were mGluR8 innervated, but a minority, which expresses somatostatin, was not. Rare neurons coexpressing calretinin and M2 were consistently targeted by mGluR8-positive boutons. In vivo recording and labeling of an mGluR8-decorated and strongly M2-positive interneuron revealed a trilaminar cell with complex spike bursts during theta oscillations and strong discharge during sharp wave/ripple events. The trilaminar cell had a large projection from the CA1 area to the subiculum and a preferential innervation of interneurons in the CA1 area in addition to pyramidal cell somata and dendrites. The postsynaptic interneuron type-specific expression of the high-efficacy presynaptic mGluR8 in both putative glutamatergic and in identified GABAergic terminals predicts a role in adjusting the activity of interneurons depending on the level of network activity.

Phenotypic and Biochemical Analyses of BACE1- and BACE2-deficient Mice

Beta-secretase (BACE1) is the rate-limiting protease for the generation of the amyloid beta-peptide (Abeta) in Alzheimer disease. Mice in which the bace1 gene is inactivated are reported to be healthy. However, the presence of a homologous gene encoding BACE2 raises the possibility of compensatory mechanisms. Therefore, we have generated bace1, bace2, and double knockout mice. We report here that BACE1 mice display a complex phenotype. A variable but significant number of BACE1 offspring died in the first weeks after birth. The surviving mice remained smaller than their littermate controls and presented a hyperactive behavior. Electrophysiologically, subtle alterations in the steady-state inactivation of voltage-gated sodium channels in BACE1-deficient neurons were observed. In contrast, bace2 knockout mice displayed an overall healthy phenotype. However, a combined deficiency of BACE2 and BACE1 enhanced the bace1-/- lethality phenotype. At the biochemical level, we have confirmed that BACE1 deficiency results in an almost complete block of Abeta generation in neurons, but not in glia. As glia are 10 times more abundant in brain compared with neurons, our data indicate that BACE2 could indeed contribute to Abeta generation in the brains of Alzheimer disease and, in particular, Down syndrome patients. In conclusion, our data challenge the general idea of BACE1 as a safe drug target and call for some caution when claiming that no major side effects should be expected from blocking BACE1 activity.

Regulation of IPSP Theta Rhythm by Muscarinic Receptors and Endocannabinoids in Hippocampus

Theta rhythms are behaviorally relevant electrical oscillations in the mammalian brain, particularly the hippocampus. In many cases, theta oscillations are shaped by inhibitory postsynaptic potentials (IPSPs) that are driven by glutamatergic and/or cholinergic inputs. Here we show that hippocampal theta rhythm IPSPs induced in the CA1 region by muscarinic acetylcholine receptors independent of all glutamate receptors can be briefly interrupted by action potential-induced, retrograde endocannabinoid release. Theta IPSPs can be recorded in CA1 pyramidal cell somata surgically isolated from CA3, subiculum, and even from their own apical dendrites. These results suggest that perisomatic-targeting interneurons whose output is subject to inhibition by endocannabinoids are the likely source of theta IPSPs. Interneurons having these properties include the cholecystokinin-containing cells. Simultaneous recordings from pyramidal cell pairs reveal synchronous theta-frequency IPSPs in neighboring pyramidal cells, suggesting that these IPSPs may help entrain or modulate small groups of pyramidal cells.