Slower spontaneous excitatory postsynaptic currents in spiny versus aspiny hilar neurons

Increased excitability of dentate gyrus mossy cells occurs early in life in the Tg2576 model of Alzheimer's disease

INTRODUCTION

Hyperexcitability in Alzheimer's disease (AD) emerge early and contribute to disease progression. The dentate gyrus (DG) is implicated in hyperexcitability in AD. We hypothesized that mossy cells (MCs), regulators of DG excitability, contribute to early hyperexcitability in AD. Indeed, MCs generate hyperexcitability in epilepsy.

METHODS

Using the Tg2576 model and WT mice (∼1month-old), we compared MCs electrophysiologically, assessed c-Fos activity marker, Aβ expression and mice performance in a hippocampal-dependent memory task.

RESULTS

Tg2576 MCs exhibit increased spontaneous excitatory events and decreased inhibitory currents, increasing the charge transfer excitation/inhibition ratio. Tg2576 MC intrinsic excitability was enhanced, and showed higher c-Fos, intracellular Aβ expression, and axon sprouting. Granule cells only showed changes in synaptic properties, without intrinsic changes. The effects occurred before a memory task is affected.

DISCUSSION

Early electrophysiological and morphological alterations in Tg2576 MCs are consistent with enhanced excitability, suggesting an early role in DG hyperexcitability and AD pathophysiology.

HIGHLIGHTS

∘ MCs from 1 month-old Tg2576 mice had increased spontaneous excitatory synaptic input. ∘ Tg2576 MCs had reduced spontaneous inhibitory synaptic input. ∘ Several intrinsic properties were abnormal in Tg2576 MCs. ∘ Tg2576 GCs had enhanced synaptic excitation but no changes in intrinsic properties. ∘ Tg2576 MCs exhibited high c-Fos expression, soluble Aβ and axonal sprouting.

Advances in understanding hilar mossy cells of the dentate gyrus

Hilar mossy cells (MCs) of the dentate gyrus (DG) distinguish the DG from other hippocampal subfields (CA1-3) because there are two glutamatergic cell types in the DG rather than one. Thus, in the DG, the main cell types include glutamatergic granule cells (GCs) and MCs, whereas in CA1-3, the only glutamatergic cell type is the pyramidal cell. In contrast to GCs, MCs are different in morphology, intrinsic electrophysiological properties, afferent input and axonal projections, so their function is likely to be very different from GCs. Why are MCs necessary to the DG? In past studies, the answer has been unclear because MCs not only excite GCs directly but also inhibit them disynaptically, by exciting GABAergic neurons that project to GCs. Results of new studies are discussed that shed light on this issue. These studies take advantage of recently available transgenic mice with Cre recombinase expression mostly in MCs and techniques such as optogenetics and DREADDs (designer receptors exclusively activated by designer drugs). The recent studies also address in vivo behavioral functions of MCs. Some of the results support past hypotheses whereas others suggest new conceptualizations of how the MCs contribute to DG circuitry and function. While substantial progess has been made, additional research is still needed to clarify the characteristics and functions of these unique cells.

The enigmatic mossy cell of the dentate gyrus

Mossy cells comprise a large fraction of the cells in the hippocampal dentate gyrus, suggesting that their function in this region is important. They are vulnerable to ischaemia, traumatic brain injury and seizures, and their loss could contribute to dentate gyrus dysfunction in such conditions. Mossy cell function has been unclear because these cells innervate both glutamatergic and GABAergic neurons within the dentate gyrus, contributing to a complex circuitry. It has also been difficult to directly and selectively manipulate mossy cells to study their function. In light of the new data generated using methods to preferentially eliminate or activate mossy cells in mice, it is timely to ask whether mossy cells have become any less enigmatic than they were in the past.

Hilar mossy cell circuitry controlling dentate granule cell excitability

Glutamatergic hilar mossy cells of the dentate gyrus can either excite or inhibit distant granule cells, depending on whether their direct excitatory projections to granule cells or their projections to local inhibitory interneurons dominate. However, it remains controversial whether the net effect of mossy cell loss is granule cell excitation or inhibition. Clarifying this controversy has particular relevance to temporal lobe epilepsy, which is marked by dentate granule cell hyperexcitability and extensive loss of dentate hilar mossy cells. Two diametrically opposed hypotheses have been advanced to explain this granule cell hyperexcitability—the “dormant basket cell” and the “irritable mossy cell” hypotheses. The “dormant basket cell” hypothesis proposes that mossy cells normally exert a net inhibitory effect on granule cells and therefore their loss causes dentate granule cell hyperexcitability. The “irritable mossy cell” hypothesis takes the opposite view that mossy cells normally excite granule cells and that the surviving mossy cells in epilepsy increase their activity, causing granule cell excitation. The inability to eliminate mossy cells selectively has made it difficult to test these two opposing hypotheses. To this end, we developed a transgenic toxin-mediated, mossy cell-ablation mouse line. Using these mutants, we demonstrated that the extensive elimination of hilar mossy cells causes granule cell hyperexcitability, although the mossy cell loss observed appeared insufficient to cause clinical epilepsy. In this review, we focus on this topic and also suggest that different interneuron populations may mediate mossy cell-induced translamellar lateral inhibition and intralamellar recurrent inhibition. These unique local circuits in the dentate hilar region may be centrally involved in the functional organization of the dentate gyrus.

Hilar mossy cells of the dentate gyrus: a historical perspective

The circuitry of the dentate gyrus (DG) of the hippocampus is unique compared to other hippocampal subfields because there are two glutamatergic principal cells instead of one: granule cells, which are the vast majority of the cells in the DG, and the so-called “mossy cells.” The distinctive appearance of mossy cells, the extensive divergence of their axons, and their vulnerability to excitotoxicity relative to granule cells has led to a great deal of interest in mossy cells. Nevertheless, there is no consensus about the normal functions of mossy cells and the implications of their vulnerability. There even seems to be some ambiguity about exactly what mossy cells are. Here we review initial studies of mossy cells, characteristics that define them, and suggest a practical definition to allow investigators to distinguish mossy cells from other hilar neurons even if all morphological and physiological information is unavailable due to technical limitations of their experiments. In addition, hypotheses are discussed about the role of mossy cells in the DG network, reasons for their vulnerability and their implications for disease.

The FGIN period: electrophysiological studies

This historical review of the electrophysiology laboratory complemented the activity of the various research teams at the Fidia Georgetown Institute for the Neurosciences and it was the fulfillment of Dr. Erminio Costa's dream to be able to study the inhibitory and excitatory synapse in the central nervous system. These studies were facilitated by the development of the patch clamp technique that allows the functional testing of several of the biochemical and pharmacological hypotheses. The studies described here were the results of the hard work of all the collaborators involved in the projects that will never forget the passionate and stimulating discussion with Dr Costa during and after the development of these projects.

Cholinergic and glutamatergic spontaneous and evoked excitatory postsynaptic currents in optic lobe neurons of cuttlefish, Sepia officinalis

Spontaneous excitatory postsynaptic currents (sEPSCs) were recorded from two different classes of neurons in the optic lobes of the cuttlefish brain and their synaptic activities analyzed and compared. The cell types were as follows: efferent centrifugal neurons, with cell bodies in the inner granule layer and axons projecting to the retina, and interneurons local to the medulla. For both neuronal groups, the sEPSCs reversal potentials were around 0 mV and there were no significant differences in their mean amplitude and rise times. However, the sEPSCs from the centrifugal neurons had a significantly higher frequency and faster decay time constant than those recorded from the medulla. Tetrodotoxin (TTX) reduced the mean frequency of the sEPSCs from both the medulla and centrifugal neurons by 69.66 +/- 4.05% and 57.80 +/- 3.87%, respectively, implying that more than half of these excitatory synaptic inputs were due to action potential-mediated release of neurotransmitter. Pharmacological examination revealed that the centrifugal neurons were driven by spontaneous synaptic inputs mediated by glutamatergic and cholinergic receptors, because co-application of the glutamate antagonist kynurenic acid (KYNA) and the nicotinic antagonist mecamylamine hydrochloride (MCM) resulted in complete blockade of these excitatory inputs. For the medulla neurons, the synaptic inputs were driven by glutamate and other transmitters yet to be identified. Evoked EPSCs (eEPSCs) were recorded from both types of neurons by stimulating the appropriate optic nerve bundles; in centrifugal neurons, the eEPSCs were blocked by co-application of KYNA and MCM, whereas in the medulla neurons, KYNA alone either totally or partially blocked the eEPSCs.

Ionic currents underlying rhythmic bursting of ventral mossy cells in the developing mouse dentate gyrus

The electrophysiological properties of mossy cells were examined in developing mouse hippocampal slices using whole-cell patch-clamp techniques, with particular reference to the dorsoventral difference. Dorsal mossy cells exhibited a higher spontaneous excitatory postsynaptic potential (EPSP) frequency and larger maximal EPSP amplitude than ventral mossy cells. On the other hand, the blockade of synaptic inputs with glutamatergic and GABAergic antagonists disclosed a remarkable dorsoventral difference in the intrinsic activity: none (0/27) of the dorsal mossy cells showed intrinsic bursting, whereas the majority (35/47) of the ventral mossy cells exhibited intrinsic rhythmic bursting. To characterize the ionic currents underlying the rhythmic bursting of mossy cells, we used somatic voltage-clamp recordings in the subthreshold voltage range. Ventral bursting cells possessed both hyperpolarization-activated current (Ih) and persistent sodium current (INaP), whereas dorsal and ventral nonbursting cells possessed Ih but no INaP. Blockade of Ih with cesium did not affect the intrinsic bursting of ventral mossy cells. In contrast, the blockade of INaP with tetrodotoxin or phenytoin established a stable subthreshold membrane potential in ventral bursting cells. The current-voltage curve of ventral bursting cells showed a region of tetrodotoxin-sensitive negative slope conductance between -55 mV and a spike threshold ( approximately -45 mV). On the other hand, no subthreshold calcium conductances played a significant role in the intrinsic bursting of ventral mossy cells. These observations demonstrate the heterogeneous electrophysiological properties of hilar mossy cells, and suggest that the subthreshold INaP plays a major role in the intrinsic rhythmic bursting of ventral mossy cells.

Three classes of pyramidal neurons in layer V of rat perirhinal cortex

Whole-cell recordings from 140 pyramidal neurons in layer V of rat perirhinal cortex (PR) revealed three distinct firing patterns: regular spiking (RS, 76%), burst spiking (BS, 9%), and late spiking (LS, 14%). LS neurons have not previously been reported in layer V of any cortical region. LS cells in layer V of PR exhibited delays of up to 12 s from onset of a depolarizing current step to spike threshold, followed by sustained firing. In contrast, pyramidal cells in layer V of other cortical regions contain only RS and BS cells. Within PR, the percentage of LS neurons in layer V differs markedly from what we previously observed in layers II/III (50% LS) and VI (90% LS). Morphologically, BS neurons in layer V of PR had thick primary apical dendrites that terminated in a tuft within layer I, whereas RS and LS cells had relatively thin primary apicals that terminated either diffusely or in a layer I tuft. At holding potentials near rest, PR neurons exhibited small (approximately 15 pA), inward, spontaneous postsynaptic currents (PSCs) that were indistinguishable among the three cell types. Currents evoked by minimal stimulation of layer I were about 2.8 times larger than the spontaneous PSCs. Evoked currents had unusually long onset latencies with little variation in latency, consistent with monosynaptic responses evoked by stimulation of unmyelinated fibers. The prevalence of LS cells in combination with the long-latency monosynaptically evoked PSCs suggested that PR is not a region of rapid throughput. This is consistent with anatomical data suggesting that PR is a higher-level association cortex. These data further advance an emerging picture of PR as a cortical region with a unique distribution of cell types different from other cortical regions.

Quantal release of glutamate generates pure kainate and mixed AMPA/kainate EPSCs in hippocampal neurons

The relative contribution of kainate receptors to ongoing glutamatergic activity is at present unknown. We report the presence of spontaneous, miniature, and minimal stimulation-evoked excitatory postsynaptic currents (EPSCs) that are mediated solely by kainate receptors (EPSC(kainate)) or by both AMPA and kainate receptors (EPSC(AMPA/kainate)). EPSC(kainate) and EPSC(AMPA/kainate) are selectively enriched in CA1 interneurons and mossy fibers synapses of CA3 pyramidal neurons, respectively. In CA1 interneurons, the decay time constant of EPSC(kainate) (circa 10 ms) is comparable to values obtained in heterologous expression systems. In both hippocampal neurons, the quantal release of glutamate generates kainate receptor-mediated EPSCs that provide as much as half of the total glutamatergic current. Kainate receptors are, therefore, key players of the ongoing glutamatergic transmission in the hippocampus.

Functional characteristics of non-NMDA-type ionotropic glutamate receptor channels in AII amacrine cells in rat retina

The properties of non-NMDA glutamate receptor channels in AII amacrine cells were studied by patch-clamp recording from rat retinal slices. Application of AMPA or kainate to intact cells evoked currents with no apparent desensitization (EC50 of 118 microM for AMPA and 169 microM for kainate). Application of AMPA to patches evoked desensitizing responses with an EC50 of 217 and 88 microM for the peak and steady-state responses, respectively. Kainate-evoked responses of patches displayed no desensitization (EC50 = 162 microM). Cyclothiazide strongly potentiated AMPA-evoked responses and the AMPA-receptor antagonist GYKI 53655 inhibited both AMPA- and kainate-evoked responses (IC50 = 0.5-1.7 microM). Pre-equilibration with GYKI 53655 completely blocked the response to kainate and pretreatment with concanavalin A did not unmask a response mediated by kainate receptors. AMPA- and kainate-evoked currents reversed close to 0 mV. AMPA-evoked peak and steady-state response components in patches displayed linear and outwardly rectifying I-V relations with an RI (ratio of the slope conductances at +40 mV and -60 mV) of 0.96 +/- 0.11 and 5.6 +/- 1.3, respectively. AMPA-evoked currents displayed a voltage-dependent relaxation after steps to positive or negative membrane potentials, indicating that the outward rectification of the steady-state response is caused by a voltage-dependent kinetic parameter of channel gating. Under bi-ionic conditions ([Ca2+](out) = 30 mM, [Cs+)(in) = 171 mM), the reversal potentials of AMPA- and kainate-evoked currents indicated channels with significant Ca2+ permeability (P(Ca)/P(Cs) = 1.9-2.1). Stationary noise analysis indicated that kainate activated channels with an apparent chord conductance of approximately 9 pS. Non-stationary noise analysis indicated that AMPA and glutamate activated channels with apparent chord conductances of approximately 9, approximately 15, approximately 23 and approximately 38 pS. Discrete single-channel gating corresponding to chord conductances of approximately 23 pS could be directly observed in some responses. Thus, our results indicate expression of high-affinity, voltage-sensitive AMPA receptors with significant Ca2+ permeability and relatively large single-channel chord conductances in AII amacrine cells.

Molecular and functional analysis of hyperpolarization-activated pacemaker channels in the hippocampus after entorhinal cortex lesion

Differential display of hippocampal tissue after entorhinal cortex lesion (ECL) revealed decreases in mRNA encoding the neuronal hyperpolarization-activated, cyclic nucleotide-gated channel HCN1. In situ hybridization confirmed that hippocampal transcripts of HCN1, but not HCN2/3/4, are down-regulated after ECL. Expression recovered at approximately 21 days after lesion (dal). Immunohistochemistry demonstrated a corresponding regulation of HCN1 protein expression in CA1-CA3 dendrites, hilar mossy cells and interneurons, and granule cells. Patch-clamp recordings in the early phase after lesion from mossy cells and hilar interneurons revealed an increase in the fast time constant of current activation and a profound negative shift in voltage activation of Ih. Whereas current activation recovered at 30 dal, the voltage activation remained hyperpolarized in mossy cells and hilar interneurons. Granule cells, however, were devoid of any detectable somatic Ih currents. Hence, denervation of the hippocampus decreases HCN1 and concomitantly the Ih activity in hilar neurons, and the recovery of h-current activation kinetics occurs parallel to postlesion sprouting.

Survival of dentate hilar mossy cells after pilocarpine-induced seizures and their synchronized burst discharges with area CA3 pyramidal cells

The clinical and basic literature suggest that hilar cells of the dentate gyrus are damaged after seizures, particularly prolonged and repetitive seizures. Of the cell types within the hilus, it appears that the mossy cell is one of the most vulnerable. Nevertheless, hilar neurons which resemble mossy cells appear in some published reports of animal models of epilepsy, and in some cases of human temporal lobe epilepsy. Therefore, mossy cells may not always be killed after severe, repeated seizures. However, mossy cell survival in these studies was not completely clear because the methods did allow discrimination between mossy cells and other hilar cell types. Furthermore, whether surviving mossy cells might have altered physiology after seizures was not examined. Therefore, intracellular recording and intracellular dye injection were used to characterize hilar cells in hippocampal slices from pilocarpine-treated rats that had status epilepticus and recurrent seizures ('epileptic' rats). For comparison, mossy cells were also recorded from age-matched, saline-injected controls, and pilocarpine-treated rats that failed to develop status epilepticus. Numerous hilar cells with the morphology, axon projection, and membrane properties of mossy cells were recorded in all three experimental groups. Thus, mossy cells can survive severe seizures, and those that survive retain many of their normal characteristics. However, mossy cells from epileptic tissue were distinct from mossy cells of control rats in that they generated spontaneous and evoked epileptiform burst discharges. Area CA3 pyramidal cells also exhibited spontaneous and evoked bursts. Simultaneous intracellular recordings from mossy cells and pyramidal cells demonstrated that their burst discharges were synchronized, with pyramidal cell discharges typically beginning first. From these data we suggest that hilar mossy cells can survive status epilepticus and chronic seizures. The fact that mossy cells have epileptiform bursts, and that they are synchronized with area CA3, suggest a previously unappreciated substrate for hyperexcitability in this animal model.

Unusual target selectivity of perisomatic inhibitory cells in the hilar region of the rat hippocampus

Perisomatic inhibitory innervation of all neuron types profoundly affects their firing characteristics and vulnerability. In this study we examined the postsynaptic targets of perisomatic inhibitory cells in the hilar region of the dentate gyrus where the proportion of potential target cells (excitatory mossy cells and inhibitory interneurons) is approximately equal. Both cholecystokinin (CCK)- and parvalbumin-immunoreactive basket cells formed multiple contacts on the somata and proximal dendrites of mossy cells. Unexpectedly, however, perisomatic inhibitory terminals arriving from these cell types largely ignored hilar GABAergic cell populations. Eighty-ninety percent of various GABAergic neurons including other CCK-containing basket cells received no input from CCK-positive terminals. Parvalbumin-containing cells sometimes innervated each other but avoided 75% of other GABAergic cells. Overall, a single mossy cell received 40 times more CCK-immunoreactive terminals and 15 times more parvalbumin-positive terminals onto its soma than the cell body of an average hilar GABAergic cell. In contrast to the pronounced target selectivity in the hilar region, CCK- and parvalbumin-positive neurons innervated each other via collaterals in stratum granulosum and moleculare. Our observations indicate that the inhibitory control in the hilar region is qualitatively different from other cortical areas at both the network level and the level of single neurons. The paucity of perisomatic innervation of hilar interneurons should have profound consequences on their action potential generation and on their ensemble behavior. These findings may help explain the unique physiological patterns observed in the hilus and the selective vulnerability of the hilar cell population in various pathophysiological conditions.

GABAergic cells are the major postsynaptic targets of mossy fibers in the rat hippocampus

Dentate granule cells communicate with their postsynaptic targets by three distinct terminal types. These include the large mossy terminals, filopodial extensions of the mossy terminals, and smaller en passant synaptic varicosities. We examined the postsynaptic targets of mossy fibers by combining in vivo intracellular labeling of granule cells, immunocytochemistry, and electron microscopy. Single granule cells formed large, complex "mossy" synapses on 11-15 CA3 pyramidal cells and 7-12 hilar mossy cells. In contrast, GABAergic interneurons, identified with immunostaining for substance P-receptor, parvalbumin, and mGluR1a-receptor, were selectively innervated by very thin (filopodial) extensions of the mossy terminals and by small en passant boutons in both the hilar and CA3 regions. These terminals formed single, often perforated, asymmetric synapses on the cell bodies, dendrites, and spines of GABAergic interneurons. The number of filopodial extensions and small terminals was 10 times larger than the number of mossy terminals. These findings show that in contrast to cortical pyramidal neurons, (1) granule cells developed distinct types of terminals to affect interneurons and pyramidal cells and (2) they innervated more inhibitory than excitatory cells. These findings may explain the physiological observations that increased activity of granule cells suppresses the overall excitability of the CA3 recurrent system and may form the structural basis of the target-dependent regulation of glutamate release in the mossy fiber system.

Specialized electrophysiological properties of anatomically identified neurons in the hilar region of the rat fascia dentata

Because of their strategic position between the granule cell and pyramidal cell layers, neurons of the hilar region of the hippocampal formation are likely to play an important role in the information processing between the entorhinal cortex and the hippocampus proper. Here we present an electrophysiological characterization of anatomically identified neurons in the fascia dentata as studied using patch-pipette recordings and subsequent biocytin-staining of neurons in slices. The resting potential, input resistance (RN), membrane time constant (taum), "sag" in hyperpolarizing responses, maximum firing rate during a 1-s current pulse, spike width, and fast and slow afterhyperpolarizations (AHPs) were determined for several different types of hilar neurons. Basket cells had a dense axonal plexus almost exclusively within the granule cell layer and were distinguishable by their low RN, short taum, lack of sag, and rapid firing rates. Dentate granule cells also lacked sag and were identifiable by their higher RN, longer taum, and lower firing rates than basket cells. Mossy cells had extensive axon collaterals within the hilus and a few long-range collaterals to the inner molecular layer and CA3c and were characterized physiologically by small fast and slow AHPs. Spiny and aspiny hilar interneurons projected primarily either to the inner or outer segment of the molecular layer and had a dense intrahilar axonal plexus, terminating onto somata within the hilus and CA3c. Physiologically, spiny hilar interneurons generally had higher RN values than mossy cells and a smaller slow AHP than aspiny interneurons. The specialized physiological properties of different classes of hilar neurons are likely to be important determinants of their functional operation within the hippocampal circuitry.

Interneurons of the dentate-hilus border of the rat dentate gyrus: morphological and electrophysiological heterogeneity

Interneurons located near the border of the dentate granule cell layer and the hilus were studied in hippocampal slices using whole-cell current clamp and biocytin staining. Because these interneurons exhibit both morphological and electrophysiological diversity, we asked whether passive electrotonic parameters or repetitive firing behavior correlated with axonal distribution. Each interneuron was distinguished by a preferred axonal distribution in the molecular layer or granule cell layer, and four groups could be discerned, the axons of which arborized in (1) the granule cell layer, (2) the inner molecular layer, (3) the outer molecular layer, and (4) diffusely in the molecular layer. In our sample, interneurons with axons arborizing diffusely in the molecular layer were most frequent, and those with axons restricted to the granule cell layer were least frequent. Resting potential, input resistance, time constant, electrotonic length, and spike frequency adaptation (SFA) were not significantly different among the four groups, and the variability in SFA between cells with similar axonal distributions was striking. Clear differences in action potential morphology and afterhyperpolarizations, however, emerged when nonadapting interneurons were compared with those exhibiting SFA. Interneurons exhibiting SFA had characteristically broader spikes, progressive slowing of action potential repolarization during repetitive firing, and slow afterhyperpolarizations that distinguished them from nonadapting interneurons. We propose that the variability in repetitive firing behavior and morphology exhibited by each of these interneurons makes each interneuron unique and may provide a high level of fine tuning of inhibitory control critical to information processing in the dentate.

Effects of serotonin through serotonin1A and serotonin4 receptors on inhibition in the guinea-pig dentate gyrus in vitro

The role of serotonin1A and serotonin4 receptors in the modulation of synaptic inhibition in the dentate gyrus of guinea-pig hippocampal slices was studied. The effects of serotonin (5-hydroxytryptamine) on hilar neurons and on inhibitory postsynaptic potentials in granule cells were compared using intracellular recording in the presence of glutamatergic receptor antagonists. On the basis of electrophysiological properties hilar neurons were classified as type I neurons (presumably inhibitory) and type II neurons (presumably excitatory). Serotonin hyperpolarized a proportion of type I hilar neurons (60%) and decreased their input resistance through activation of a K+-conductance. This effect was mediated by serotonin1A receptors since it was mimicked by the selective serotonin1A receptor agonist (+/-)-8-hydroxy-dipropylaminotetralin hydrobromide and blocked by the selective serotonin1A receptor antagonist (+) WAY 100135. In some type I hilar neurons (40%) neither serotonin nor (+/-)-8-hydroxydipropylaminotetralin hydrobromide induced a membrane hyperpolarization. Instead, serotonin induced an excitatory response, depolarizing the cells and blocking the slow afterhyperpolarization. Similar effects were seen in all hilar neurons after blockade of serotonin1A receptors. They were mimicked by the serotonin4 receptor agonist zacopride. Serotonin induced either decreases or increases in the frequency of spontaneous GABA(A) receptor-mediated inhibitory postsynaptic potentials in granule cells via activation of serotonin1A and of serotonin4 receptors, respectively. 4-aminopyridine-evoked GABA(B) receptor-mediated inhibitory postsynaptic potentials were inhibited by serotonin via activation of serotonin1A receptors. However, after blockade of serotonin1A receptors, serotonin increased the frequency of GABA(B)-inhibitory postsynaptic potentials through the activation of serotonin4 receptors. We conclude that a proportion of inhibitory neurons in the dentate area does not express serotonin1A receptors and is excited by serotonin. Other inhibitory neurons express serotonin1A receptors and are inhibited by serotonin.

Conditions required for polysynaptic excitation of dentate granule cells by area CA3 pyramidal cells in rat hippocampal slices

Under control conditions, stimulation of area CA3 pyramidal cells in slices can produce inhibitory postsynaptic potentials in granule cells by a polysynaptic pathway that is likely to involve hilar neurons [Muller W. and Misgeld U. (1990) J. Neurophysiol. 64, 46-56; Muller W. and Misgeld U. (1991) J. Neurophysiol. 65, 141-147; Scharfman H. E. (1993) Neurosci. Lett. 156, 61-66; Scharfman H. F. (1994) Neurosci. Lett. 168, 29-33]. When slices are disinhibited, excitatory postsynaptic potentials occur after the same stimulus [Sharfman H. E. (1994) J. Neurosci. 14, 6041-6057]. The excitatory postsynaptic potentials are likely to be mediated by pyramidal cells that innervate hilar mossy cells, which in turn innervate granule cells. [Scharfman H. F. (1994) J. Neurosci 14, 6041-6057]. These pathways are potentially important, because they could provide positive or negative feedback from area CA3 to the dentate gyrus. However, it is not clear when the CA3-mossy cell-granule cell excitatory pathway operates, because to date it has only been described in detail when GABA(A) receptors are blocked throughout the entire slice [Scharfman H. E. (1994) J. Neurosci 14, 6041-6057]. Furthermore, the monosynaptic excitatory synaptic connections between these cells have only been observed in the presence of bicuculline [Scharfman H. F. (1994) J. Neurophysiol. 72, 2167-2180; Scharfman H. E. (1995) J. Neurophysiol. 74, 179-194]. Yet in vivo data suggest that a CA3-mossy cell-granule cell excitatory pathway may be active under some physiological conditions, because granule cells discharge in association with sharp wave population bursts of CA3 [Ylinen A., et al. (1995) Hippocampus 5, 78-90]. To address whether the CA3-mossy cell-granule cell pathway occurs without global disinhibition of the slice, and where in the network disinhibition may be required, the effects of area CA3 stimulation on granule cells was examined after focal application of the GABAA receptor antagonist bicuculline to restricted areas of hippocampal slices. A micropipette containing 1 mM bicuculline was placed transiently either (i) in the area CA3 cell layer, (ii) the granule cell layer, (iii) the hilus, or (iv) more than one site in succession. If a small segment of the CA3 pyramidal cell layer or the hilus was disinhibited, or bicuculline was applied to both regions, area CA3 stimulation still evoked inhibitory postsynaptic potentials in granule cells. In fact, inhibitory postsynaptic potentials were enhanced under these conditions, probably because excitation of inhibitory cells was increased. When bicuculline was applied just to the area near an impaled granule cell, all inhibitory postsynaptic potentials evoked in that cell were blocked, but no underlying excitatory postsynaptic potential was uncovered. If bicuculline was applied focally to either area CA3 or the hilus and the impaled granule cell, CA3 stimulation subsequently evoked excitatory postsynaptic potentials in that granule cell, presumably because excitatory neurons innervating granule cells were disinhibited while the effects of inhibitory cells on granule cells were blocked. Excitatory postsynaptic potentials were produced without bicuculline application in three of seven cells, simply by stimulating the fimbria repetitively. Thus, if bicuculline is applied to different sites in the slice, different effects occur on the inhibitory postsynaptic potentials of granule cells that are evoked by a fimbria stimulus. If bicuculline is applied to both the granule cell soma and either area CA3 or the hilus, inhibitory postsynaptic potentials are reduced, and reveal that excitatory postsynaptic potentials can be produced by the same stimulus. (ABSTRACT TRUNCATED)

Physiological properties of anatomically identified basket and bistratified cells in the CA1 area of the rat hippocampus in vitro

Basket and bistratified cells form two anatomically distinct classes of GABAergic local-circuit neurons in the CA1 region of the rat hippocampus. A physiological comparison was made of intracellularly recorded basket (n = 13) and bistratified neurons (n = 6), all of which had been anatomically defined by their efferent target profile (Halasy et al., 1996). Basket cells had an average resting membrane potential of -64.2 +/- 7.2 vs. -69.2 +/- 4.6 mV in bistratified cells. The latter had considerably higher mean input resistances (60.2 +/- 42.1 vs. 31.3 +/- 10.9 M Ohms) and longer membrane time constants (18.6 +/- 8.1 vs. 9.8 +/- 4.5 ms) than basket cells. Differences were also apparent in the duration of action potentials, those of basket cells being 364 +/- 77 and those of bistratified cells being 527 +/- 138 microseconds at half-amplitude. Action potentials were generally followed by prominent, fast after-hyperpolarizing potentials which in basket cells were 13.5 +/- 6.7 mV in amplitude vs. 10.5 +/- 5.1 in bistratified cells. The differences in membrane time constant, resting membrane potential, and action potential duration reached statistical significance (P < 0.05). Extracellular stimulation of Schaffer collateral/commissural afferents elicited short-latency excitatory postsynaptic potentials (EPSPs) in both cell types. The average 10-90% rise time and duration (at half-amplitude) of subthreshold EPSPs in basket cells were 1.9 +/- 0.5 and 10.7 +/- 5.6 ms, compared to 3.3 +/- 1.3 and 20.1 +/- 9.7 ms in bistratified cells, the difference in EPSP rise times being statistically significant. Basket and bistratified EPSPs were highly sensitive to a bath applied antagonist of non-N-methyl-D-aspartate (NMDA) receptors, whereas the remaining slow-rise EPSP could be abolished by an NMDA receptor antagonist. Increasing stimulation intensity elicited biphasic inhibitory postsynaptic potentials (IPSPs) in both basket and bistratified cells. In conclusion, basket and bistratified cells in the CA1 area show prominent differences in several of their membrane and firing properties. Both cell classes are activated by Schaffer collateral/commissural axons in a feedforward manner and receive inhibitory input from other, as yet unidentified, local-circuit neurons.

High-resolution immunogold localization of AMPA type glutamate receptor subunits at synaptic and non-synaptic sites in rat hippocampus

The cellular and subcellular localization of the GluRA, GluRB/C and GluRD subunits of the alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionate (AMPA) type glutamate receptor was determined in the rat hippocampus using polyclonal antipeptide antibodies in immunoperoxidase and immunogold procedures. For the localization of the GluRD subunit a new polyclonal antiserum was developed using the C-terminal sequence of the protein (residues 869-881), conjugated to carrier protein and absorbed to colloidal gold for immunization. The purified antibodies immunoprecipitated about 25% of 3[H]AMPA binding activity from the hippocampus, cerebellum or whole brain, but very little from neocortex. These antibodies did not precipitate a significant amount of 3[H]kainate binding activity. The antibodies also recognize the GluRD subunit, but not the other AMPA receptor subunits, when expressed in transfected COS-7 cells and only when permeabilized with detergent, indicating an intracellular epitope. All subunits were enriched in the neuropil of the dendritic layers of the hippocampus and in the molecular layer of the dentate gyrus. The cellular distribution of the GluRD subunit was studied more extensively. The strata radiatum, oriens and the dentate molecular layer were more strongly immunoreactive than the stratum lacunosum moleculare, the stratum lucidum and the hilus. However, in the stratum lucidum of the CA3 area and in the hilus the weakly reacting dendrites were surrounded by immunopositive rosettes, shown in subsequent electron microscopic studies to correspond to complex dendritic spines. In the stratum radiatum, the weakly reacting apical dendrites contrasted with the surrounding intensely stained neuropil. The cell bodies of pyramidal and granule cells were moderately reactive. Some non-principal cells and their dendrites in the pyramidal cell layer and in the alveus also reacted very strongly for the GluRD subunit. At the subcellular level, silver intensified immunogold particles for the GluRA, GluRB/C and GluRD subunits were present at type 1 synaptic membrane specializations on dendritic spines of pyramidal cells throughout all layers of the CA1 and CA3 areas. The most densely labelled synapses tended to be on the largest spines and many smaller spines remained unlabelled. Immunoparticle density at type 1 synapses on dendritic shafts of some non-principal cells was consistently higher than at labelled synapses of dendritic spines of pyramidal cells. Synapses established between dendritic spines and mossy fibre terminals, were immunoreactive for all studied subunits in stratum lucidum of the CA3 area. The postembedding immunogold method revealed that the AMPA type receptors are concentrated within the main body of the anatomically defined type 1 (asymmetrical) synaptic junction. Often only a part of the membrane specialization showed clustered immunoparticles. There was a sharp decrease in immunoreactive receptor density at the edge of the synaptic specialization. Immunolabelling was consistently demonstrated at extrasynaptic sites on dendrites, dendritic spines and somata. The results demonstrate that the GluRA, B/C and D subunits of the AMPA type glutamate receptor are present in many of the glutamatergic synapses formed by the entorhinal, CA3 pyramidal and mossy fibre terminals. Some interneurons have a higher density of AMPA type receptors in their asymmetrical afferent synapses than pyramidal cells. This may contribute to a lower activation threshold of interneurons as compared to principal cells by the same afferents in the hippocampal formation.

Adrenergic modulation of hilar neuron activity and granule cell inhibition in the guinea-pig hippocampal slice

To study the effects of norepinephrine on synaptic inhibition in the dentate gyrus, intracellular recordings were made from hilar neurons in the guinea-pig hippocampal slice. The effects of norepinephrine on hilar neurons were compared with changes in the frequency of spontaneous inhibitory postsynaptic potentials recorded from granule cells. Hilar neurons comprised two electrophysiologically distinct groups: type I hilar neurons displayed a pronounced single spike afterhyperpolarization and little spike frequency accommodation, type II hilar neurons had small afterhyperpolarizations and pronounced spike frequency accommodation. The majority of recordings were from type I hilar neurons which are presumably inhibitory to granule cells. In most instances, effects of norepinephrine (2-10 microM) on hilar neurons could be mimicked by the beta-adrenergic agonist isoproterenol (0.1-1 microM). Isoproterenol induced a slight depolarization, blocked a slow afterhyperpolarization and, in type II neurons, reduced spike frequency accommodation. These effects were associated with an increase in the spontaneous discharge rate and an enhancement of spontaneous excitatory and inhibitory postsynaptic potentials. In accordance, isoproterenol and norepinephrine increased the frequency of inhibitory postsynaptic potentials in granule cells. In the presence of the non-N-methyl-D-aspartate receptor antagonist 6-cyano-7-nitroquinoxaline-2,3-dione and the N-methyl-D-aspartate receptor antagonist CGP 37849, isoproterenol and norepinephrine also increased the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells. Under this experimental condition, however, norepinephrine reduced the discharge rate of type I hilar neurons through an effect on alpha-receptors. In the presence of GABAA receptor blockers, norepinephrine increased the frequency of spontaneously occurring K(+)-dependent inhibitory postsynaptic potentials in granule cells. Accordingly, the frequency of burst discharges in type I hilar neurons was increased. We suggest that the discrepancy in the effect of norepinephrine on the discharge rate of presumed inhibitory hilar neurons and the frequency of Cl- -dependent inhibitory postsynaptic potentials in granule cells results from a direct effect of norepinephrine on GABAergic terminals because norepinephrine also enhanced the frequency of tetrodotoxin-resistant inhibitory postsynaptic potentials in granule cells. Thus, the net effect of synaptically released norepinephrine on synaptic inhibition in the dentate gyrus will be determined by opposing actions of alpha- versus beta-receptor stimulation at the synapse on hilar neurons.

A physiological role for GABAB receptors and the effects of baclofen in the mammalian central nervous system

The inhibitory neurotransmitter GABA acts in the mammalian brain through two different receptor classes: GABAA and GABAB receptors. GABAB receptors differ fundamentally from GABAA receptors in that they require a G-protein. GABAB receptors are located pre- and/or post-synaptically, and are coupled to various K+ and Ca2+ channels presumably through both a membrane delimited pathway and a pathway involving second messengers. Baclofen, a selective GABAB receptor agonist, as well as GABA itself have pre- and post-synaptic effects. Pre-synaptic effects comprise the reduction of the release of excitatory and inhibitory transmitters. GABAergic receptors on GABAergic terminals may regulate GABA release, however, in most instances spontaneous inhibitory synaptic activity is not modulated by endogenous GABA. Post-synaptic GABAB receptor-mediated inhibition is likely to occur through a membrane delimited pathway activating K+ channels, while baclofen, in some neurons, may activate K+ channels through a second messenger pathway involving arachidonic acid. Some, but not all GABAB receptor-gated K+ channels have the typical properties of those G-protein-activated K+ channels which are also gated by other endogenous ligands of the brain. New, high affinity GABAB antagonists are now available, and some pharmacological evidence points to a receptor heterogeneity. The pharmacological distinction of receptor subtypes, however, has to await final support from a characterization of the molecular structure. The function importance of post-synaptic GABAB receptors is highlighted by a segregation of GABAA and GABAB synapses in the mammalian brain.

Ca(2+)-permeable AMPA and NMDA receptor channels in basket cells of rat hippocampal dentate gyrus

1. Glutamate receptor (GluR) channels were studied in basket cells in the dentate gyrus of rat hippocampal slices. Basket cells were identified by their location, dendritic morphology and high frequency of action potentials generated during sustained current injection. 2. Dual-component currents were activated by fast application of glutamate to outside-out membrane patches isolated from basket cell somata (10 microM glycine, no external Mg2+). The fast component was selectively blocked by 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), the slow component by D-2-amino-5-phosphonopentanoic acid (D-AP5). This suggests that the two components were mediated by alpha-amino-3- hydroxy-5-methyl-4-isoxazolepropionate receptor (AMPAR)/kainate receptor and N-methyl-D-aspartate receptor (NMDAR) channels, respectively. The mean ratio of the peak current of the NMDAR component to that of the AMPAR/kainate receptor component was 0.22 (1 ms pulses of 10 mM glutamate). 3. The AMPAR/kainate receptor component, which was studied in isolation in the presence of D-AP5, was identified as AMPAR mediated on the basis of the preferential activation by AMPA as compared with kainate, the weak desensitization of kainate-activated currents, the cross-desensitization between AMPA and kainate, and the reduction of desensitization by cyclothiazide. 4. Deactivation of basket cell AMPARs following 1 ms pulses of glutamate occurred with a time constant (tau) of 1.2 +/- 0.1 ms (mean +/- S.E.M.). During 100 ms glutamate pulses AMPARs desensitized with a tau of 3.7 +/- 0.2ms. 5. The peak current-voltage (I-V) relation of AMPAR-mediated currents in Na(+)-rich extracellular solution showed a reversal potential of -4.0 +/- 2.6 mV and was characterized by a a doubly rectifying shape. The conductance of single AMPAR channels was estimated as 22.6 +/- 1.6 pS using non-stationary fluctuation analysis. AMPARs expressed in hippocampal basket cells were highly Ca2+ permeable (PCa/PK = 1.79). 6. NMDARs in hippocampal basket cells were studied in isolation in the presence of CNQX. Deactivation of NMDARs activated by glutamate pulses occurred bi-exponentially with mean tau values of 266 +/- 23 ms (76%) and 2620 +/- 383 ms (24%). 7. The peak I-V relation of the NMDAR-mediated component in Na(+)-rich extracellular solution showed a reversal potential of 1.5 +/- 0.6 mV and a region of negative slope at negative membrane potentials in the presence of external Mg2+, due to voltage-dependent block by these ions. The conductance of single NMDAR channels in the main open state was 50.2 +/- 1.8 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Structural determinants of allosteric regulation in alternatively spliced AMPA receptors

The flip and flop splice variants of AMPA receptors show strikingly different sensitivity to allosteric regulation by cyclothiazide; heteromers assembled from GluR-A and GluR-B also exhibit splice variant-dependent differences in efficacy for activation by glutamate and kainate. The sensitivity for attenuation of desensitization by cyclothiazide for homomeric GluR-A was solely dependent upon exchange of Ser-750 (flip) and Asn-750 (flop), and was unaffected by mutagenesis of other divergent residues. In contrast, substantial alteration of the relative efficacy of glutamate versus kainate required mutation of multiple residues in the flip/flop region. Modulation by cyclothiazide was abolished by mutation of Ser-750 to Gin, the residue found at the homologous site in kainate-preferring subunits, whereas introduction of Ser at this site in GluR6 imparted sensitivity to cyclothiazide.

Electrophysiological diversity of pyramidal-shaped neurons at the granule cell layer/hilus border of the rat dentate gyrus recorded in vitro

In the rat dentate gyrus, pyramidal-shaped cells located on the border of the granule cell layer and the hilus are one of the most common types of gamma-aminobutyric acid (GABA)-immunoreactive neurons. This study describes their electrophysiological characteristics. Membrane properties, patterns of discharge, and synaptic responses were recorded intracellularly from these cells in hippocampal slices. Each cell was identified as pyramidal-shaped by injecting the marker Neurobiotin intracellularly (n = 17). In several respects the membrane properties of the sampled cells were similar to "fast-spiking" cells (putative inhibitory interneurons) that have been described in other areas of the hippocampus. For example, input resistance was high (mean 91.3 megohms), the membrane time constant was short (mean 7.7 ms), and there was a large afterhyperpolarization following a single action potential (mean 10.5 mV at resting potential). However, the action potentials of most pyramidal-shaped cells were not as brief (mean 1.2 ms total duration) as those of most previously described fast-spiking cells. Many pyramidal-shaped neurons had strong spike frequency adaptation relative to other fast-spiking cells. Although these latter two characteristics were apparent in the majority of the sampled cells, there were exceptional pyramidal-shaped neurons with fast action potentials and weak adaptation, demonstrating the electrophysiological variability of pyramidal-shaped cells. Responses to outer molecular layer stimulation were composed primarily of excitatory postsynaptic potentials (EPSPs) rather than inhibitory postsynaptic potentials (IPSPs), and were usually small (EPSPs evoked at threshold were often less than 2 mV), and brief (less than 30 ms). There was variability, because in a few cells EPSPs evoked at threshold were much larger. However, regardless of EPSP amplitude, suprathreshold stimulation (up to 4 times the threshold stimulus strength) rarely evoked more than one action potential in any cell. The results suggest that stimulation of perforant path axons produces limited excitatory synaptic responses in pyramidal-shaped neurons. This may be one of the reasons why they are relatively resistant to prolonged perforant path stimulation. The pyramidal-shaped neurons located at the base of the granule cell layer have been associated historically with a basket plexus around granule cell somata, and have been called pyramidal "basket" cells. However, basket-like endings were rare and axon collaterals outside the granule cell layer as the outer molecular layer and the central hilus, and antidromic action potentials could be recorded in some cells in response to weak stimulation of these areas. Taken together with the electrophysiological variability, the results indicate that these cells are physiologically heterogeneous.

Excitatory synaptic connections onto rat hippocampal inhibitory cells may involve a single transmitter release site

1. Whole-cell tight-seal records of excitatory postsynaptic currents (EPSCs) were made from inhibitory cells in the CA3 region of thin hippocampal slices. We tested the hypothesis that excitatory synaptic connections made on inhibitory cells involve few transmitter release sites. 2. EPSCs impinging on inhibitory cells had a time to peak of 0.4-3.8 ms and an amplitude of 8-90 pA at a holding potential of -60 mV. They were suppressed by the excitatory amino acid antagonists 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) and DL-2-amino-5-phosphonovaleric acid (APV). 3. Addition of tetrodotoxin (TTX) and Co2+ to the external solution reduced the frequency of EPSCs from 0.90 to 0.25 s-1 (n = 24 cells). In the majority of cells EPSC amplitude distributions were not significantly changed. 4. Increasing Ca2+ and reducing Mg2+ in the external solution, in order to enhance the probability of transmitter release, did not change EPSC amplitude distributions. In contrast, amplitude histograms for IPSCs recorded from pyramidal cells were shifted to higher mean values in this solution. 5. EPSCs were elicited in inhibitory cells by electrical stimulation via a glass pipette placed near to pyramidal cells in stratum pyramidale. EPSCs elicited by weak stimuli had similar amplitude distributions to excitatory synaptic events recorded in the presence of TTX and Co2+. 6. These findings suggest excitatory synaptic connections made with CA3 inhibitory cells involve few or possibly just one transmitter release site.

Muscarinic amplification of fast excitation in hilar neurones and inhibition in granule cells in the guinea-pig hippocampus

1. Effects of the cholinergic agonist, carbachol (CCh), or the acetylcholinesterase inhibitor, eserine, on presumed inhibitory hilar neurones and on inhibition in granule cells were studied by intracellular recording in guinea-pig hippocampal slices. 2. CCh (1-5 microM) strongly enhanced the discharge activity of hilar neurones and spontaneous and evoked IPSPs in granule cells. 3. Eserine, in an atropine-sensitive manner, increased the excitability of hilar neurones through effects on membrane properties and on excitatory synaptic barrage. EPSPs readily triggered long-duration burst discharges. In granule cells, the amplitudes of evoked GABAA and GABAB receptor-mediated IPSPs were enhanced. 4. In the presence of eserine and antagonists for glutamatergic and GABAergic synaptic transmission, train stimulation evoked atropine-sensitive slow EPSPs. In contrast to those in granule cells, slow EPSPs in hilar neurones were invariably preceded by a strong burst-after-hyperpolarization. 5. We suggest that acetylcholine, released from septo-hippocampal fibres, amplifies fast synaptic excitation of inhibitory hilar neurones and inhibition of granule cells. In the dentate area, muscarinic receptor-mediated effects are faster than anticipated from the time course of the slow EPSP.

Block of glutamate transporters potentiates postsynaptic excitation

We have studied the effects of blockers of glutamate transporters on excitatory synaptic transmission to determine whether transporters increase the clearance rate of transmitter from the synaptic cleft on the millisecond time scale. The transporter blockers Li+ and THA increased the amplitude, but not the decay time, of spontaneous miniature AMPA receptor EPSCs recorded at 34 degrees C but not 24 degrees C. Evoked AMPA receptor EPSCs were similarly affected by THA. The rapidly dissociating AMPA receptor competitive antagonist PDA inhibited evoked AMPA receptor EPSCs less in the presence of THA at both temperatures, implying that transporter blockade slows clearance. We suggest that transporters speed glutamate clearance mainly by binding glutamate and that AMPA receptors are not saturated by synaptically released glutamate at 34 degrees C.

Effects of serotonin on hilar neurons and granule cell inhibition in the guinea pig hippocampal slice

Intracellular recordings in guinea pig hippocampal slices were used to study the effects of serotonin (5-HT) on presumed inhibitory hilar neurons and on postsynaptic inhibition of granule cells. 5-HT applied by the bath hyperpolarized only 50% of the hilar neurons tested but all CA3 neurons and granule cells, presumably by activating a K-conductance. The bath application of 4-aminopyridine (4-AP, 50 microM) induced burst discharge activity in hilar neurons and giant inhibitory postsynaptic potentials (IPSPs) in granule cells consisting of a Cl- and K-component. 5-HT (5-10 microM) reversibly blocked the K-component of giant IPSPs in granule cells, but not their Cl-component. In the majority of hilar neurons 5-HT increased the frequency of 4-AP induced burst discharges even when hilar neurons were hyperpolarized. Only in a few hilar neurons 5-HT blocked 4-AP induced burst discharges. We conclude that the changes in burst discharge pattern of hilar neurons correspond with the differential effect of 5-HT on Cl- and K-mediated inhibition of granule cells.

Different glutamate receptor channels mediate fast excitatory synaptic currents in inhibitory and excitatory cortical neurons

Spontaneous excitatory postsynaptic currents (sEPSCs) and responses to rapid application of glutamate were recorded in excitatory spiny, pyramidal neurons and compared with those recorded in inhibitory aspiny interneurons. The sEPSC decay time constant was faster in aspiny interneurons (2.5 ms) compared with pyramidal neurons (4.6 ms). The decay time constant in response to a brief application (1 ms) of glutamate (10 mM) in patches excised from pyramidal and aspiny interneurons were similar (1.9 and 2.7 ms, respectively). However, the rate of desensitization was faster in patches from interneurons compared with pyramidal neurons (3.4 and 12.0 ms, respectively). In addition, single-channel conductance was larger in aspiny interneurons (27 pS) compared with pyramidal neurons (9 pS). These results indicate that pyramidal neurons and aspiny interneurons express different non-N-methyl-D-aspartate receptors and that selective desensitization of interneuron receptors may contribute to depression of inhibition.

The behavior of mossy cells of the rat dentate gyrus during theta oscillations in vivo

Intracellular current clamp recordings were obtained from mossy cells (n = 6, identified by intracellular injection of biocytin) of the dorsal dentate gyrus from rats under ketamine-xylazine anesthesia. During electroencephalographic theta rhythm (4-6 Hz), recorded with a macroelectrode placed in the contralateral dorsal hippocampus near the fissure, mossy cells displayed intracellular membrane potential oscillations at low frequencies (4-6 Hz) which appeared to be phase locked to the electroencephalographic theta rhythm. The frequency of the intracellular theta rhythm was independent of the membrane potential. However, the phase difference between the intracellular and the electroencephalographic theta rhythms as well as the amplitude of the intracellular theta oscillations were voltage-dependent. These findings are consistent with the hypothesis that rhythmic GABAA receptor-mediated inhibitory postsynaptic potentials contribute to the genesis of the intracellular theta rhythm. Indeed, mossy cells displayed an early, fast inhibitory postsynaptic potential in response to electrical stimulation of the entorhinal cortex, which most likely represents a GABAA receptor-mediated event, indicating that mossy cells possess functional GABAA receptors. At the resting membrane potential, mossy cells did not fire at each cycle of the electroencephalographic theta rhythm but fired only rarely (< 1 Hz). However, when they did fire they did so preferentially in phase with the peak positivity of the electroencephalographic theta rhythm. Reconstruction of two mossy cells with axonal projections to the inner molecular layer showed that the spatial extent of the influence such weakly discharging mossy cells may have on other dentate gyrus neurons during theta oscillations can be several millimeters in the septotemporal direction. In conclusion, these findings show that mossy cells of the rat hilus during ketamine-xylazine anesthesia participate in theta oscillations of the hippocampal formation, during which their low-frequency firing may contribute to the phase-locking of a large number of spatially distributed postsynaptic neurons with postsynaptic sites in the inner molecular layer of the dentate gyrus.

Characteristics of spontaneous and evoked EPSPs recorded from dentate spiny hilar cells in rat hippocampal slices

1. Excitation of the spiny subtype of hilar neurons in the fascia dentata was characterized by intracellular recording from hilar cells in hippocampal slices. Stimulation of the outer molecular layer was used to activate the perforant path. Evoked responses were examined, as well as the large spontaneous excitatory potentials that are a distinctive characteristic of spiny hilar cells. 2. Excitatory potentials that occurred spontaneously, as well as those that occurred in response to outer molecular layer stimulation, were similar among the cells that were sampled, regardless of morphological variations such as the presence or absence of thorny excrescences. Spontaneous and evoked excitatory postsynaptic potentials (EPSPs) were complex depolarizations that often had several discrete peaks. Spontaneous EPSPs increased in amplitude slightly with hyperpolarization, and evoked EPSPs clearly increased with hyperpolarization. 3. Applications of selective antagonists of excitatory amino acid receptors were used to determine which excitatory amino acid receptor mediates EPSPs of these cells. 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) was used to block the receptor subtype selective for the agonists alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) and kainic acid (the "AMPA/kainate" receptor). 2-amino-5-phosphonovaleric acid (APV) was used to block receptors specific for the agonist N-methyl-D-aspartate (NMDA; the "NMDA" receptor). Perfusion with CNQX (5-25 microM) completely blocked all spontaneous and evoked excitation, even when activity was examined at relatively depolarized membrane potentials and a low concentration of extracellular magnesium (0.5 mM) was used. Under these conditions, APV (25-50 microM) had no detectable effect on spontaneous activity but did increase the stimulus strength required to elicit responses to outer molecular layer stimulation. 4. When extracellular magnesium was lowered to 0 mM (nominally), there was strong evidence for a contribution of NMDA receptors to spontaneous and evoked EPSPs. Thus, when cells were perfused with 0 mM extracellular magnesium and 5 microM CNQX, spontaneous depolarizations were present and EPSPs could be triggered by stimulation of the outer molecular layer. Both the spontaneous and evoked EPSPs were blocked by 25 microM APV. 5. Because gamma-aminobutyric acid (GABA)A receptors can cause depolarizations in hippocampal neurons, the GABAA receptor antagonist bicuculline was used to determine whether some of the EPSPs were mediated by GABAergic neurons that are normally activated by spontaneous release of excitatory amino acids. Bicuculline (5-25 microM) had no effect on spontaneous depolarizations, and led to an enhancement of evoked depolarizations. Therefore it does not appear that GABAA receptor-mediated depolarizations contribute to hilar cell depolarizations.(ABSTRACT TRUNCATED AT 400 WORDS)

Glutamatergic and GABAergic synaptic currents in ganglion cells from isolated retinae of pigmented rats during postnatal development

This study was aimed at characterizing the earliest phases of synaptogenesis in the mammalian retina. Spontaneous activity of ganglion cells in the isolated superfused retina was used as an indicator for the functionality of synaptic connections. Retinal ganglion neurons (RGNs) were identified by location of their somata in the ganglion cell layer (GCL) and by their ability to generate large (> 500 pA) voltage-activated sodium currents. Spontaneous spiking was found in many RGNs prior to cell perfusion. Between postnatal day (P) 1 and 18, a total of 195 RGNs was tested for light-induced currents, conductance changes in response to exogenous glutamate (Glu) and gamma-aminobutyric acid (GABA), and depolarizing or hyperpolarizing synaptic activity. The vast majority of the material was derived from RGNs at day P5. Whole-cell ion currents were always sampled at somatic sites, using either conventional or perforated patch whole-cell recordings. On day P5, 5% of tested RGNs (n = 73) were already responsive to light stimulation. A higher percentage of cells (23%, n = 187) generated spontaneous depolarizing currents that were regarded as glutamatergic excitatory postsynaptic currents (EPSCs), since (1) they were blocked by Glu antagonists, (2) they conformed to the Na+/Cs+ equilibrium potential, (3) and they displayed a time course characteristic of glutamatergic EPSCs. The mean EPSC amplitude was 19.0 pA (S.D. 11.83 pA). Amplitude distributions were fitted by multiple Gaussian equations rendering a quantal size of 6.6 to 9.1 pA at a holding voltage (Vh) of -70 mV (driving force about 70 mV). Spontaneous EPSCs were never observed under condition of Ca(2+)-free solutions, but they persisted in the presence of tetrodotoxin. Bath application of quisqualate (500 microM) consistently increased EPSC frequencies. In contrast to the relatively high percentage of RGNs generating spontaneous EPSCs, very few RGNs at P5 (3%, n = 187) displayed inhibitory postsynaptic currents (IPSCs), although by that time all tested RGNs (n = 14) were responsive to both exogenous Glu and GABA. These results indicate that in the postnatal rat retina development of excitatory synapses precedes the maturation of inhibitory afferents. Excitatory inputs to RGNs were to some extent functional before the animals opened their eyes. Glutamatergic synaptic activity may, thus, play an important role in shaping visual connections in the absence of visual experience.

Kinetic properties of NMDA receptor-mediated synaptic currents in rat hippocampal pyramidal cells versus interneurones

1. Whole-cell tight-seal recordings were obtained from visually identified pyramidal cells (PCs) and interneurones (INs) in the CA1 field of thin hippocampal slices from 13- to 23-day-old rats. The INs sampled were classified according to their location either in the molecular layer (M-INs) or in the oriens layer and alveus (OA-INs). PCs and INs differed in their mode of firing when depolarized by a prolonged current pulse. Whereas PCs fired a single action potential, most INs responded with non-accommodating high frequency spike firing. 2. In the presence of 1 microM tetrodotoxin (TTX), bath application of either 50 microM L-glutamate with 10 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) or 2.5 microM N-methyl-D-aspartate (NMDA), induced a similar conductance increase in PCs and INs that was completely blocked by 200 microM DL-2-amino-5-phosphonovaleric acid (APV). The NMDA receptor-mediated currents reversed around 4 mV and exhibited an area of negative slope conductance at potentials more negative than -20 to -30 mV in the presence of 1-2 mM Mg2+. 3. Dual-component excitatory postsynaptic currents (EPSCs) were evoked in PCs and INs by stimulating afferent fibres close to the neurone. The NMDA receptor-mediated component of the EPSCs (NMDA EPSC) was isolated by adding 10 microM CNQX to block non-NMDA receptors. The NMDA EPSCs in all cell types reversed around 1.5 mV and were abolished by 50 microM APV. 4. In saline containing 1 mM Mg2+, the peak current-voltage (I-V) relationship of NMDA EPSCs in PCs and INs showed an area of negative slope conductance at voltages more negative than -20 to -30 mV. In nominally Mg(2+)-free saline, the peak I-V relation was linear over a much wider voltage range in both cell types. 5. The 10-90% rise times of NMDA EPSCs at -60 mV ranged from 4.5 to 16 ms in PCs (mean 8.7 ms; n = 25) and in M-INs (mean 9.1 ms; n = 10). Their decay could be best fitted with the sum of two exponentials. The decay of NMDA EPSCs in PCs was significantly slower than that recorded in INs. The average fast (tau f) and slow (tau s) time constants of decay were, respectively, 66.5 and 353.9 ms in PCs, and 34.4 and 212.5 ms in M-INs.(ABSTRACT TRUNCATED AT 400 WORDS)

Activation and desensitization of glutamate-activated channels mediating fast excitatory synaptic currents in the visual cortex

Brief glutamate applications to membrane patches, excised from neurons in the rat visual cortex, were used to assess the role of desensitization in determining the AMPA/kainate receptor-mediated excitatory postsynaptic current (EPSC) time course. A brief (1 ms) application of glutamate (1-10 mM) produced a response that mimicked the time course of miniature EPSCs (mEPSCs). Direct evidence is presented that the rate of onset of desensitization is much slower than the decay rate of the response to a brief application of glutamate, implying that the decay of mEPSCs reflects channel closure into a state readily available for reactivation. Rapid application of glutamate combined with nonstationary variance analysis provided an estimate of the single-channel conductance and open probability, allowing an approximation of the number of available channels at a single synaptic site.