Ivy cells: a population of nitric-oxide-producing, slow-spiking GABAergic neurons and their involvement in hippocampal network activity

Imaging inhibitory synaptic potentials using voltage sensitive dyes

Studies of the spatio-temporal distribution of inhibitory postsynaptic potentials (IPSPs) in a neuron have been limited by the spatial information that can be obtained by electrode recordings. We describe a method that overcomes these limitations by imaging IPSPs with voltage-sensitive dyes. CA1 hippocampal pyramidal neurons from brain slices were loaded with the voltage-sensitive dye JPW-1114 from a somatic patch electrode in whole-cell configuration. After removal of the patch electrode, we found that neurons recover their physiological intracellular chloride concentration. Using an improved voltage-imaging technique, dendritic GABAergic IPSPs as small as 1 mV could be resolved optically from multiple sites with spatial averaging. We analyzed the sensitivity of the technique, in relation to its spatial resolution. We monitored the origin and the spread of IPSPs originating in different areas of the apical dendrite and reconstructed their spatial distribution. We achieved a clear discrimination of IPSPs from the dendrites and from the axon. This study indicates that voltage imaging is a uniquely suited approach for the investigation of several fundamental aspects of inhibitory synaptic transmission that require spatial information.

Granule cells in the CA3 area

A fundamental property of neuronal networks in Ammon's horn is that each area comprises a single glutamatergic cell population and various types of GABAergic neurons. Here we describe an exception to this rule, in the form of granule cells that reside within the CA3 area and function as glutamatergic nonprincipal cells with distinct properties. CA3 granule cells in normal, healthy rats, similarly to dentate gyrus granule cells, coexpressed calbindin and the homeobox protein Prox1. However, CA3 granule cells were located outside of the dentate gyrus, often hundreds of micrometers from the hilar border, in the lucidum and radiatum layers. CA3 granule cells were present in numbers that were comparable to the rarer GABAergic neuronal subtypes, and their somato-dendritic morphology, intrinsic properties, and perforant path inputs were similar to those of dentate gyrus granule cells. CA3 granule cell axons displayed giant mossy fiber terminals with filopodial extensions, demonstrating that not all mossy fibers originate from the dentate gyrus. Somatic paired recordings revealed that CA3 granule cells innervated CA3 pyramidal and GABAergic cells similarly to conventional mossy fiber synapses. However, CA3 granule cells were distinct in the specific organization of their GABAergic inputs. They received GABAergic synapses from cholecystokinin-expressing mossy fiber-associated cells that did not innervate the dentate granule cell layer, and these synapses demonstrated unusually strong activity-dependent endocannabinoid-mediated inhibition of GABA release. These results indicate that granule cells in the CA3 constitute a glutamatergic, nonprincipal neuronal subtype that is integrated into the CA3 synaptic network.

Distinctive properties of CXC chemokine receptor 4-expressing Cajal-Retzius cells versus GABAergic interneurons of the postnatal hippocampus

The CXC chemokine receptor 4 (CXCR4) for the chemokine (C-X-C motif) ligand 12/stromal cell-derived factor-1 alpha (CXCL12/SDF-1 alpha) is highly expressed in the postnatal CA1 stratum lacunosum-moleculare. However, both the network events triggered by SDF-1 alpha in this microcircuit and the cellular targets of this chemokine remain virtually unexplored. Here, we have studied SDF-1 alpha-mediated neuromodulation of the stratum lacunosum-moleculare by directly comparing the properties of CXCR4-expressing Cajal-Retzius cells vs. CXCR4-non-expressing interneurons, and by recording the electrophysiological effects caused by application of SDF-1 alpha on either cell type. We demonstrate that SDF-1 alpha dramatically reduces spontaneous firing in Cajal-Retzius cells via hyerpolarization, and that cessation of firing is prevented by the CXCR4-specific antagonist AMD3100. In contrast, no effects on the excitability of interneurons of the same layer were observed following exposure to the chemokine. We also provide evidence that, despite the expression of functional glutamate receptors, Cajal-Retzius cells are integrated in the synaptic network of the stratum lacunosum-moleculare via excitatory GABAergic input. Furthermore, we show that the axons of Cajal-Retzius cells target specifically the stratum lacunosum-moleculare and the dentate gyrus, but lack postsynaptic specializations opposite to their axonal varicosities. These results, taken together with our observation that SDF-1 alpha reduces evoked field responses at the entorhinal cortex-CA1 synapse, suggest that Cajal-Retzius cells produce a diffuse output that may impact information processing of stratum lacunosum-moleculare. We propose that pathological alterations of local levels of SDF-1 alpha or CXCR4 expression may affect the functions of an important hippocampal microcircuit.

Further characterization of sleep-active neuronal nitric oxide synthase neurons in the mouse brain

We recently demonstrated that Fos is induced in a subpopulation of cortical neuronal nitric oxide synthase (nNOS)-immunoreactive neurons in three rodent species both during spontaneous sleep (SS) and recovery sleep (RS) after a period of sleep deprivation (SD); the proportion of cortical Fos(+)/nNOS neurons was significantly correlated with non-REM (NREM) sleep delta energy. The present study was undertaken to evaluate the specificity of this state-dependent activation of cortical nNOS cells. The percentage of nNOS neurons that expressed Fos during SD and RS was determined in nine subcortical brain regions and the cortex of the mouse brain; a significantly greater proportion of Fos(+)/nNOS neurons was observed during RS only in the cortex and in none of the nine subcortical regions. The proportion of calretinin-, calbindin- and parvalbumin-immunoreactive cortical interneurons that expressed Fos during SD and RS was also determined. In contrast to cortical nNOS neurons, a higher percentage of Fos(+)/calbindin neurons was found during SD than RS; there were no differences in the proportions of Fos-expressing parvalbumin or calretinin neurons between these conditions. Since the nNOS and calretinin cortical interneuron populations overlap extensively in the mouse brain, triple-labeling with these two phenotypic markers and Fos was undertaken in mice from the RS group to determine which combination of markers could best identify the rare "sleep-active" cortical interneuron population. The proportions of both Fos(+)/nNOS neurons and Fos(+)/nNOS/calretinin neurons far exceeded the proportion of Fos(+)/calretinin neurons during RS, but the proportions of these two cell types were not significantly different during RS. Thus, functional activation of nNOS neurons during sleep appears to be restricted to the cerebral cortex and cortical nNOS cells and nNOS/calretinin cells collectively define a cortical interneuron population that is activated during sleep.

Expression of COUP-TFII nuclear receptor in restricted GABAergic neuronal populations in the adult rat hippocampus

The COUP-TFII nuclear receptor, also known as NR2F2, is expressed in the developing ventral telencephalon and modulates the tangential migration of a set of subpallial neuronal progenitors during forebrain development. Little information is available about its expression patterns in the adult brain. We have identified the cell populations expressing COUP-TFII and the contribution of some of them to network activity in vivo. Expression of COUP-TFII by hippocampal pyramidal and dentate granule cells, as well as neurons in the neocortex, formed a gradient increasing from undetectable in the dorsal to very strong in the ventral sectors. In the dorsal hippocampal CA1 area, COUP-TFII was restricted to GABAergic interneurons and expressed in several, largely nonoverlapping neuronal populations. Immunoreactivity was present in calretinin-, neuronal nitric oxide synthase-, and reelin-expressing cells, as well as in subsets of cholecystokinin- or calbindin-expressing or radiatum-retrohippocampally projecting GABAergic cells, but not in parvalbumin- and/or somatostatin-expressing interneurons. In vivo recording and juxtacellular labeling of COUP-TFII-expressing cells revealed neurogliaform cells, basket cells in stratum radiatum and tachykinin-expressing radiatum dentate innervating interneurons, identified by their axodendritic distributions. They showed cell type-selective phase-locked firing to the theta rhythm but no activation during sharp wave/ripple oscillations. These basket cells in stratum radiatum and neurogliaform cells fired at the peak of theta oscillations detected extracellularly in stratum pyramidale, unlike previously reported ivy cells, which fired at the trough. The characterization of COUP-TFII-expressing neurons suggests that this developmentally important transcription factor plays cell type-specific role(s) in the adult hippocampus.

Common origins of hippocampal Ivy and nitric oxide synthase expressing neurogliaform cells

GABAergic interneurons critically regulate cortical computation through exquisite spatiotemporal control over excitatory networks. Precision of this inhibitory control requires a remarkable diversity within interneuron populations that is largely specified during embryogenesis. Although interneurons expressing the neuronal isoform of nitric oxide synthase (nNOS) constitute the largest hippocampal interneuron cohort their origin and specification remain unknown. Thus, as neurogliaform cells (NGC) and Ivy cells (IvC) represent the main nNOS(+) interneurons, we investigated their developmental origins. Although considered distinct interneuron subtypes, NGCs and IvCs exhibited similar neurochemical and electrophysiological signatures, including NPY expression and late spiking. Moreover, lineage analyses, including loss-of-function experiments and inducible fate-mapping, indicated that nNOS(+) IvCs and NGCs are both derived from medial ganglionic eminence (MGE) progenitors under control of the transcription factor Nkx2-1. Surprisingly, a subset of NGCs lacking nNOS arises from caudal ganglionic eminence (CGE) progenitors. Thus, while nNOS(+) NGCs and IvCs arise from MGE progenitors, a CGE origin distinguishes a discrete population of nNOS(-) NGCs.

Collagen XIX is expressed by interneurons and contributes to the formation of hippocampal synapses

Extracellular matrix (ECM) molecules contribute to the formation and maintenance of synapses in the mammalian nervous system. We previously discovered a family of nonfibrillar collagens that organize synaptic differentiation at the neuromuscular junction (NMJ). Although many NMJ-organizing cues contribute to central nervous system (CNS) synaptogenesis, whether similar roles for collagens exist at central synapses remained unclear. In the present study we discovered that col19a1, the gene encoding nonfibrillar collagen XIX, is expressed by subsets of hippocampal neurons. Colocalization with the interneuron-specific enzyme glutamate decarboxylase 67 (Gad67), but not other cell-type-specific markers, suggests that hippocampal expression of col19a1 is restricted to interneurons. However, not all hippocampal interneurons express col19a1 mRNA; subsets of neuropeptide Y (NPY)-, somatostatin (Som)-, and calbindin (Calb)-immunoreactive interneurons express col19a1, but those containing parvalbumin (Parv) or calretinin (Calr) do not. To assess whether collagen XIX is required for the normal formation of hippocampal synapses, we examined synaptic morphology and composition in targeted mouse mutants lacking collagen XIX. We show here that subsets of synaptotagmin 2 (Syt2)-containing hippocampal nerve terminals appear malformed in the absence of collagen XIX. The presence of Syt2 in inhibitory hippocampal synapses, the altered distribution of Gad67 in collagen XIX-deficient subiculum, and abnormal levels of gephyrin in collagen XIX-deficient hippocampal extracts all suggest inhibitory synapses are affected by the loss of collagen XIX. Together, these data not only reveal that collagen XIX is expressed by central neurons, but show for the first time that a nonfibrillar collagen is necessary for the formation of hippocampal synapses.

Amnestic concentrations of etomidate modulate GABAA,slow synaptic inhibition in hippocampus

BACKGROUND

Gamma-aminobutyric acid type A (GABA(A)) receptor-mediated inhibition in the central nervous system exists in two forms: phasic (inhibitory postsynaptic currents, IPSCs) and tonic (nonsynaptic). Phasic inhibition is further subdivided into fast (GABA(A,fast)) and slow (GABA(A,slow)) IPSCs. By virtue of its dendritic location and kinetics, GABA(A,slow) has been proposed to control synaptic plasticity and memory. Etomidate is a nonbarbiturate, intravenous anesthetic that selectively modulates GABA(A) receptors and produces amnesia at low doses in vivo. This study tested whether correspondingly low concentrations of etomidate in vitro alter GABA(A,fast) and GABA(A,slow) phasic inhibition.

METHODS

Electrophysiological recordings were obtained from hippocampal slices prepared from postnatal day 3-8 mice and maintained in organotypic culture for 10-14 days. Etomidate was applied at concentrations corresponding to one-half to four times the half maximal effective concentration that impairs hippocampus-dependent learning and memory--i.e., 0.125-1.0 microm.

RESULTS

Etomidate 0.25 microm (the half maximal effective concentration) doubled the time constant of decay of GABA(A,slow) IPSCs, but it had no detectable effect on GABA(A,fast) IPSCs. Higher concentrations of etomidate had stronger effects on both types of phasic inhibition: 0.5 and 1 microm etomidate prolonged the time constant of decay by 310% and 410% for GABA(A,slow) and by 25% and 78% for GABA(A,fast). Concentrations of etomidate up to 1 microm had no significant effects on the amplitudes of either GABA(A,fast) or GABA(A,slow) IPSCs.

CONCLUSIONS

At concentrations that impair hippocampus-dependent memory, etomidate modulates GABA(A,slow) more strongly than GABA(A,fast) IPSCs. Effects of etomidate on GABA(A,slow) IPSCs may contribute to etomidate-induced amnesia.

Input-specific GABAergic signaling to newborn neurons in adult dentate gyrus

Adult neurogenesis is the multistage process of generating neurons from adult neural stem cells. Accumulating evidence indicates that GABAergic depolarization is an important regulator of this process. Here, we examined GABAergic signaling to newly generated granule cells (GCs) of the adult mouse dentate gyrus. We show that the first synaptic currents in newborn GCs are generated by activation of GABA(A) receptors by GABA with a spatiotemporal profile suggestive of transmitter spillover. However, the GABAergic response is not attributable to spillover from surrounding perisomatic synapses. Rather, our results suggest that slow synaptic responses in newborn GCs are generated by dedicated inputs that produce a relatively low concentration of GABA at postsynaptic receptors, similar to slow IPSCs in mature GCs. This form of synaptic signaling drives robust phasic depolarization of newborn GCs when the interneuron network is synchronously active, revealing a potential mechanism that translates hippocampal activity into regulation of adult neurogenesis via synaptic release of GABA.

Altered GABAA,slow inhibition and network oscillations in mice lacking the GABAA receptor beta3 subunit

Phasic GABAergic inhibition in hippocampus and neocortex falls into two kinetically distinct categories, GABA(A,fast) and GABA(A,slow). In hippocampal area CA1, GABA(A,fast) is generally believed to underlie gamma oscillations, whereas the contribution of GABA(A,slow) to hippocampal rhythms has been speculative. Hypothesizing that GABA(A) receptors containing the beta(3) subunit contribute to GABA(A,slow) inhibition and that slow inhibitory synapses control excitability as well as contribute to network rhythms, we investigated the consequences of this subunit's absence on synaptic inhibition and network function. In pyramidal neurons of GABA(A) receptor beta(3) subunit-deficient (beta(3)(-/-)) mice, spontaneous GABA(A,slow) inhibitory postsynaptic currents (IPSCs) were much less frequent, and evoked GABA(A,slow) currents were much smaller than in wild-type mice. Fittingly, long-lasting recurrent inhibition of population spikes was less powerful in the mutant, indicating that receptors containing beta(3) subunits contribute substantially to GABA(A,slow) currents in pyramidal neurons. By contrast, slow inhibitory control of GABA(A,fast)-producing interneurons was unaffected in beta(3)(-/-) mice. In vivo hippocampal network activity was markedly different in the two genotypes. In beta(3)(-/-) mice, epileptiform activity was observed, and theta oscillations were weaker, slower, less regular and less well coordinated across laminae compared with wild-type mice, whereas gamma oscillations were weaker and faster. The amplitude modulation of gamma oscillations at theta frequency ("nesting") was preserved but was less well coordinated with theta oscillations. With the caveat that seizure-induced changes in inhibitory circuits might have contributed to the changes observed in the mutant animals, our results point to a strong contribution of beta(3) subunits to slow GABAergic inhibition onto pyramidal neurons but not onto GABA(A,fast) -producing interneurons and support different roles for these slow inhibitory synapses in the generation and coordination of hippocampal network rhythms.

GABAergic neurons of the medial septum lead the hippocampal network during theta activity

Information processing in the hippocampus critically relies on its reciprocal interaction with the medial septum (MS). Synchronization of the septo-hippocampal system was demonstrated during both major hippocampal activity states, the regular theta rhythm and the large amplitude irregular activity. Previous experimental and modeling data suggest that the MS provides rhythmic drive to the hippocampus, and hippocampo-septal feedback synchronizes septal pacemaker units. However, this view has recently been questioned based on the possibility of intrahippocampal theta genesis. Previously, we identified putative pacemaker neurons expressing parvalbumin (PV) and/or the pacemaker hyperpolarization-activated and cyclic nucleotide-gated nonselective cation channel (HCN) in the MS. In this study, by analyzing the temporal relationship of activity between the PV/HCN-containing medial septal neurons and hippocampal local field potential, we aimed to uncover whether the sequence of events during theta formation supports the classic view of septal drive or the challenging theory of hippocampal pacing of theta. Importantly, by implementing a circular statistical method, a temporal lead of these septal neurons over the hippocampus was observed on the course of theta synchronization. Moreover, the activity of putative hippocampal interneurons also preceded hippocampal local field theta, but by a shorter time period compared with PV/HCN-containing septal neurons. Using the concept of mutual information, the action potential series of PV/HCN-containing neurons shared higher amount of information with hippocampal field oscillation than PV/HCN-immunonegative cells. Thus, a pacemaker neuron population of the MS leads hippocampal activity, presumably via the synchronization of hippocampal interneurons.

Coupling of mesoscopic brain oscillations: recent advances in analytical and theoretical perspectives

Oscillatory brain activities have been traditionally studied in the context of how oscillations at a single frequency recorded from a single area could reveal functional insights. Recent advances in methodology used in signal analysis have revealed that cross-frequency coupling, within or between functional related areas, is more informative in determining the possible roles played by brain oscillations. In this review, we begin by describing the cellular basis of oscillatory field potentials and its theorized as well as demonstrated role in brain function. The recent development of mathematical tools that allow the investigation of cross-frequency and cross-area oscillation coupling will be presented and discussed in the context of recent advances in oscillation research based on animal data. Particularly, some pitfalls and caveats of methods currently available are discussed. Data generated from the application of examined techniques are integrated back into the theoretical framework regarding the functional role of brain oscillations. We suggest that the coupling of oscillatory activities at different frequencies between brain regions is crucial for understanding the brain from a functional ensemble perspective. Effort should be directed to elucidate how cross-frequency and area coupling are modulated and controlled. To achieve this, only the correct application of analytical tools may shed light on the intricacies of information representation, generation, binding, encoding, storage and retrieval in the brain.

Rapid neocortical dynamics: cellular and network mechanisms

The highly interconnected local and large-scale networks of the neocortical sheet rapidly and dynamically modulate their functional connectivity according to behavioral demands. This basic operating principle of the neocortex is mediated by the continuously changing flow of excitatory and inhibitory synaptic barrages that not only control participation of neurons in networks but also define the networks themselves. The rapid control of neuronal responsiveness via synaptic bombardment is a fundamental property of cortical dynamics that may provide the basis of diverse behaviors, including sensory perception, motor integration, working memory, and attention.

Functional specificity of mossy fiber innervation of GABAergic cells in the hippocampus

One million mossy fibers in the rat provide individually sparse but functionally important synaptic connections between the dentate gyrus and hippocampus. Although the majority of mossy fiber targets are GABAergic cells, the functional organization of the feedforward GABAergic machinery modulating the interactions of granule cells and CA3 pyramidal cells are not yet understood. We used mossy fiber bouton to GABA neuron paired recordings in the CA3 to demonstrate that mossy fibers provide cell type-specific innervation to distinct GABAergic neurons with specialized intra- and extrahippocampal outputs. Our results show that mossy fibers contact the perisomatically projecting fast-spiking and regular-spiking basket cells, in addition to the dendritically projecting ivy cells, and the septum-projecting spiny stratum lucidum cells. Monosynaptic mossy fiber inputs to fast-spiking basket cells and spiny stratum lucidum cells were found to be numerous, but they were small in amplitude and displayed low transmission probabilities. In contrast, regular-spiking basket cells and ivy cells were less likely to be innervated by mossy fibers, but the amplitudes of mossy fiber EPSCs were large and the transmission probabilities were high. The dependence of the numbers and strengths of the mossy fiber inputs to CA3 GABAergic cells on the postsynaptic cell type was correlated with the frequency of the background synaptic events, so that cells with weak but numerous mossy fiber inputs received high rates of spontaneous synaptic events. Together, these results reveal the diverse components and high degree of functional specificity of the GABAergic cellular machinery underlying the dentate gyrus-CA3 interface.

Classification of NPY-expressing neocortical interneurons

Neuropeptide Y (NPY) is an abundant neuropeptide of the neocortex involved in numerous physiological and pathological processes. Because of the large electrophysiological, molecular, and morphological diversity of NPY-expressing neurons their precise identity remains unclear. To define distinct populations of NPY neurons we characterized, in acute slices of rat barrel cortex, 200 cortical neurons of layers I-IV by means of whole-cell patch-clamp recordings, biocytin labeling, and single-cell reverse transcriptase-PCR designed to probe for the expression of well established molecular markers for cortical neurons. To classify reliably cortical NPY neurons, we used and compared different unsupervised clustering algorithms based on laminar location and electrophysiological and molecular properties. These classification schemes confirmed that NPY neurons are nearly exclusively GABAergic and consistently disclosed three main types of NPY-expressing interneurons. (1) Neurogliaform-like neurons exhibiting a dense axonal arbor, were the most frequent and superficial, and substantially expressed the neuronal isoform of nitric oxide synthase. (2) Martinotti-like cells characterized by an ascending axon ramifying in layer I coexpressed somatostatin and were the most excitable type. (3) Among fast-spiking and parvalbumin-positive basket cells, NPY expression was correlated with pronounced spike latency. By clarifying the diversity of cortical NPY neurons, this study establishes a basis for future investigations aiming at elucidating their physiological roles.

A putatively novel form of spontaneous coordination in neural activity

We simultaneously recorded local field potentials from three sites along the olfactory-entorhinal axis in rats lightly anesthetized with isoflurane, as part of another experiment. While analyzing the initial data from that experiment with spectrograms, we discovered a potentially novel form of correlated neural activity, with near-simultaneous occurrence across the three widely separated brain sites. After validating their existence further, we named these events Synchronous Frequency Bursts (SFBs). Here we report our initial investigations into their properties and their potential functional significance. In Experiment 1, we found that SFBs have highly regular properties, consisting of brief (approximately 250 ms), high amplitude bursts of LFP energy spanning frequency ranges from the delta band (1-4 Hz) to at least the low gamma band (30-50 Hz). SFBs occurred almost simultaneously across recording sites, usually with onsets <25 ms apart, and there was no clear pattern of temporal leading or lagging among the sites. While the SFBs had fairly typical, exponentially decaying power spectral density plots, their coherence structure was unusual, with high peaks in several narrow frequency ranges and little coherence in other bands. In Experiment 2, we found that SFBs occurred far more often under light anesthesia than deeper anesthetic states, and were especially prevalent as the animals regained consciousness. Finally, in Experiment 3 we showed that SFBs occur simultaneously at a significant rate across brain sites from putatively different functional subsystems--olfactory versus motor pathways. We suggest that SFBs do not carry information per se, but rather, play a role in coordinating activity in different frequency bands, potentially brain-wide, as animals progress from sleep or anesthesia toward full consciousness.

Identification of distinct telencephalic progenitor pools for neuronal diversity in the amygdala

The development of the amygdala, a central structure of the limbic system, remains poorly understood. We found that two spatially distinct and early-specified telencephalic progenitor pools marked by the homeodomain transcription factor Dbx1 are major sources of neuronal cell diversity in the mature mouse amygdala. We found that Dbx1-positive cells of the ventral pallium generate the excitatory neurons of the basolateral complex and cortical amygdala nuclei. Moreover, Dbx1-derived cells comprise a previously unknown migratory stream that emanates from the preoptic area (POA), a ventral telencephalic domain adjacent to the diencephalic border. The Dbx1-positive, POA-derived population migrated specifically to the amygdala and, as defined by both immunochemical and electrophysiological criteria, generated a unique subclass of inhibitory neurons in the medial amygdala nucleus. Thus, this POA-derived population represents a previously unknown progenitor pool dedicated to the limbic system.

GABAA receptor alpha5 subunits contribute to GABAA,slow synaptic inhibition in mouse hippocampus

gamma-Aminobutyric acid type A (GABA(A)) receptor alpha5 subunits, which are heavily expressed in the hippocampus, are potential drug targets for improving cognitive function. They are found at synaptic and extrasynaptic sites and have been shown to mediate tonic inhibition in pyramidal neurons. We tested the hypothesis that alpha5 subunits also contribute to synaptic inhibition by measuring the effect of diazepam (DZ) on spontaneous and stimulus-evoked inhibitory postsynaptic currents (IPSCs) in genetically modified mice carrying a point mutation in the alpha5 subunit (alpha5-H105R) that renders those receptors insensitive to benzodiazepines. In wild type mice, DZ (1 microM) increased the amplitude of spontaneous IPSCs (sIPSCs) and stimulus-evoked GABA(A,slow) IPSCs (eIPSCs) and prolonged the decay of GABA(A,fast) sIPSCs. In alpha5-mutant mice, DZ increased the amplitude of a small-amplitude subset of sIPSCs (<50 pA) and eIPSCs (<300 pA) GABA(A,slow) and prolonged the decay of GABA(A,fast) sIPSCs, but failed to increase the amplitude of larger sIPSCs and eIPSCs GABA(A,slow). These results indicate that alpha5 subunits contribute to a large-amplitude subset of GABA(A,slow) synapses and implicate these synapses in modulation of cognitive function by drugs that target alpha5 subunits.

GABA(B) receptor modulation of feedforward inhibition through hippocampal neurogliaform cells

Feedforward inhibition of neurons is a fundamental component of information flow control in the brain. We studied the roles played by neurogliaform cells (NGFCs) of stratum lacunosum moleculare of the hippocampus in providing feedforward inhibition to CA1 pyramidal cells. We recorded from synaptically coupled pairs of anatomically identified NGFCs and CA1 pyramidal cells and found that, strikingly, a single presynaptic action potential evoked a biphasic unitary IPSC (uIPSC), consisting of two distinct components mediated by GABA(A) and GABA(B) receptors. A GABA(B) receptor-mediated unitary response has not previously been observed in hippocampal excitatory neurons. The decay of the GABA(A) receptor-mediated response was slow (time constant = 50 ms), and was tightly regulated by presynaptic GABA(B) receptors. Surprisingly, the GABA(B) receptor ligands baclofen and (2S)-3-{[(1S)-1-(3,4-dichlorophenyl)ethyl]amino-2-hydroxypropyl}(phenylmethyl)phosphinic acid (CGP55845), while affecting the NGFC-mediated uIPSCs, had no effect on action potential-evoked presynaptic Ca2+ signals monitored in individual axonal boutons of NGFCs with two-photon microscopy. In contrast, baclofen clearly depressed presynaptic Ca2+ transients in non-NGF interneurons. Changes in extracellular Ca2+ concentration that mimicked the effects of baclofen or CGP55845 on uIPSCs significantly altered presynaptic Ca2+ transients. Electrophysiological data suggest that GABA(B) receptors expressed by NGFCs contribute to the dynamic control of the excitatory input to CA1 pyramidal neurons from the temporoammonic path. The NGFC-CA1 pyramidal cell connection therefore provides a unique and subtle mechanism to shape the integration time domain for signals arriving via a major excitatory input to CA1 pyramidal cells.

Identification of a population of sleep-active cerebral cortex neurons

The presence of large-amplitude, slow waves in the EEG is a primary characteristic that distinguishes cerebral activity during sleep from that which occurs during wakefulness. Although sleep-active neurons have been identified in other brain areas, neurons that are specifically activated during slow-wave sleep have not previously been described in the cerebral cortex. We have identified a population of cells in the cortex that is activated during sleep in three mammalian species. These cortical neurons are a subset of GABAergic interneurons that express neuronal NOS (nNOS). Because Fos expression in these sleep-active, nNOS-immunoreactive (nNOS-ir) neurons parallels changes in the intensity of slow-wave activity in the EEG, and these neurons are innvervated by neurotransmitter systems previously implicated in sleep/wake control, cortical nNOS-ir neurons may be part of the neurobiological substrate that underlies homeostatic sleep regulation.

Neuronal diversity and temporal dynamics: the unity of hippocampal circuit operations

In the cerebral cortex, diverse types of neurons form intricate circuits and cooperate in time for the processing and storage of information. Recent advances reveal a spatiotemporal division of labor in cortical circuits, as exemplified in the CA1 hippocampal area. In particular, distinct GABAergic (gamma-aminobutyric acid-releasing) cell types subdivide the surface of pyramidal cells and act in discrete time windows, either on the same or on different subcellular compartments. They also interact with glutamatergic pyramidal cell inputs in a domain-specific manner and support synaptic temporal dynamics, network oscillations, selection of cell assemblies, and the implementation of brain states. The spatiotemporal specializations in cortical circuits reveal that cellular diversity and temporal dynamics coemerged during evolution, providing a basis for cognitive behavior.