Cell type-specific changes in spontaneous and minimally evoked excitatory synaptic activity in hippocampal CA1 interneurons of kainate-treated rats

The epileptiform activity in the kainic acid (KA) model of epilepsy arises from complex changes in excitation and inhibition. To assess the involvement of excitatory drive onto inhibitory interneurons in this epileptiform activity, we examined changes in spontaneous and minimally evoked excitatory post-synaptic currents (sEPSCs and eEPSCs) in CA1 interneurons in stratum oriens/alveus (O/A) and stratum radiatum (RAD) in rat hippocampal slices after KA treatment. The frequency and amplitude of sEPSCs and the amplitude of eEPSCs were unchanged in O/A interneurons, but the EPSC kinetics were significantly slower. These changes appear to be due to altered kinetics and voltage-dependent properties of the NMDA component of EPSCs in O/A interneurons. In contrast, sEPSCs and eEPSCs in RAD interneurons did not change after KA treatment. The distinct changes in excitatory synaptic activity in interneurons differentially involved in feedback (O/A) versus feedforward (RAD) inhibition suggest a cell type-specific reorganization of excitatory synapses after KA treatment. These modifications in excitatory input to interneurons could contribute to the maintenance of inhibition of CA1 pyramidal cells after KA treatment, or may also create network conditions favourable to epileptiform activity.     

Integration of the CA2 region in the hippocampal network during epileptogenesis

The CA2 pyramidal cells are mostly resistant to cell death in mesial temporal lobe epilepsy (MTLE) with hippocampal sclerosis, but they are aberrantly integrated into the epileptic hippocampal network via mossy fiber sprouting. Furthermore, they show increased excitability in vitro in hippocampal slices obtained from human MTLE specimens or animal epilepsy models. Although these changes promote CA2 to contribute to epileptic activity (EA) in vivo, the role of CA2 in the epileptic network within and beyond the sclerotic hippocampus is still unclear. We used the intrahippocampal kainate mouse model for MTLE, which recapitulates most features of the human disease including pharmacoresistant epileptic seizures and hippocampal sclerosis, with preservation of dentate gyrus (DG) granule cells and CA2 pyramidal cells. In vivo recordings with electrodes in CA2 and the DG showed that EA occurs at high coincidence between the ipsilateral DG and CA2 and current source density analysis of silicon probe recordings in dorsal ipsilateral CA2 revealed CA2 as a local source of EA. Cell-specific viral tracing in Amigo2-icreERT2 mice confirmed the preservation of the axonal projection from ipsilateral CA2 pyramidal cells to contralateral CA2 under epileptic conditions and indeed, EA propagated from ipsi- to contralateral CA2 with increasing likelihood with time after KA injection, but always at lower intensity than within the ipsilateral hippocampus. Furthermore, we show that CA2 presents with local theta oscillations and like the DG, shows a pathological reduction of theta frequency already from 2 days after KA onward. The early changes in activity might be facilitated by the loss of glutamic acid decarboxylase 67 (Gad67) mRNA-expressing interneurons directly after the initial status epilepticus in ipsi- but not contralateral CA2. Together, our data highlight CA2 as an active player in the epileptic network and with its contralateral connections as one possible router of aberrant activity.     

Dysfunction of hippocampal interneurons in epilepsy

Gamma-amino-butyric acid（GABA）-containing interneurons are crucial to both development and function of the brain. Down-regulation of GABAergic inhibition may result in the generation of epileptiform activity. Loss, axonal sprouting, and dysfunction of interneurons are regarded as mechanisms involved in epileptogenesis. Recent evidence suggests that network connectivity and the properties of interneurons are responsible for excitatory-inhibitory neuronal circuits. The balance between excitation and inhibition in CA1 neuronal circuitry is considerably altered during epileptic changes. This review discusses interneuron diversity, the causes of interneuron dysfunction in epilepsy, and the possibility of using GABAergic neuronal progenitors for the treatment of epilepsy.     

Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the "dormant basket cell" hypothesis and its possible relevance to temporal lobe epilepsy.

The relationship between an episode of status epilepticus, the resulting hippocampal pathology, and the subsequent development of pathophysiological changes possibly relevant to human epilepsy was explored using the experimental epilepsy model of perforant path stimulation in the rat. Granule cell hyperexcitability and decreased feedforward and feedback inhibition were evident immediately after 24 hours of intermittent perforant path stimulation and persisted relatively unchanged for more than 1 year. All of the pathophysiological changes induced by perforant path stimulation were replicated in normal animals by a subconvulsive dose of bicuculline, suggesting that the permanent "epileptiform" abnormalities produced by sustained perforant path stimulation may be due to decreased GABA-mediated inhibition. Granule cell pathophysiology was seen only in animals that exhibited a loss of adjacent dentate hilar mossy cells and hilar somatostatin/neuropeptide Y-immunoreactive neurons. GABA-immunoreactive dentate basket cells survived despite the extensive loss of adjacent hilar neurons. However, parvalbumin immunoreactivity, present normally in a subpopulation of GABA-immunoreactive dentate basket cells, was absent on the stimulated side. Whether this represents decreased parvalbumin synthesis in surviving basket cells or a loss of a specific subset of inhibitory cells is unclear. Hyperexcitability and decreased paired-pulse inhibition in response to ipsilateral perforant path stimulation were also present in the CA1 pyramidal cell layer on the previously stimulated side, despite minimal damage to CA1 pyramidal cells or interneurons. The possibility that CA1 inhibitory neurons were hypofunctional or "dormant" due to a loss of excitatory input to inhibitory cells from damaged CA3 pyramidal cells was tested by stimulating the contralateral perforant path in order to activate the same CA1 basket cells via different inputs. Contralateral stimulation evoked CA1 pyramidal cell paired-pulse inhibition immediately in the previously stimulated hippocampus. Thus, we propose the "dormant basket cell" hypothesis, which implies that despite malfunction, inhibitory systems remain intact in "epileptic" tissue and are capable of functioning if appropriately activated.     

Hippocampal interneurons expressing glutamic acid decarboxylase and calcium-binding proteins decrease with aging in Fischer 344 rats

Aging leads to alterations in the function and plasticity of hippocampal circuitry in addition to behavioral changes. To identify critical alterations in the substrate for inhibitory circuitry as a function of aging, we evaluated the numbers of hippocampal interneurons that were positive for glutamic acid decarboxylase and those that expressed calcium-binding proteins (parvalbumin, calbindin, and calretinin) in young adult (4–5 months old) and aged (23–25 months old) male Fischer 344 rats. Both the overall interneuron population and specific subpopulations of interneurons demonstrated a commensurate decline in numbers throughout the hippocampus with aging. Interneurons positive for glutamic acid decarboxylase were significantly depleted in the stratum radiatum of CA1, the strata oriens, radiatum and pyramidale of CA3, the dentate molecular layer, and the dentate hilus. Parvalbumin interneurons showed significant reductions in the strata oriens and pyramidale of CA1, the stratum pyramidale of CA3, and the dentate hilus. The reductions in calbindin interneurons were more pronounced than other calcium-binding protein-positive interneurons and were highly significant in the strata oriens and radiatum of both CA1 and CA3 subfields and in the dentate hilus. Calretinin interneurons were decreased significantly in the strata oriens and radiatum of CA3, in the dentate granule cell and molecular layers, and in the dentate hilus. However, the relative ratio of parvalbumin-, calbindin-, and calretinin-positive interneurons compared with glutamic acid decarboxylase-positive interneurons remained constant with aging, suggesting actual loss of interneurons expressing calcium-binding proteins with age. This loss contrasts with the reported preservation of pyramidal neurons with aging in the hippocampus. Functional decreases in inhibitory drive throughout the hippocampus may occur due to this loss, particularly alterations in the processing of feed-forward information through the hippocampus. In addition, such a profound alteration in interneuron number will likely alter inhibitory control of excitability and neuronal synchrony with behavioral states. J. Comp. Neurol. 394:252–269, 1998. © 1998 Wiley-Liss, Inc.     

Differential expression of GABA and glutamate-receptor subunits and enzymes involved in GABA metabolism between electrophysiologically identified hippocampal CA1 pyramidal cells and interneurons

PURPOSE: The balance between synaptic excitation and inhibition within the hippocampus is critical for maintaining normal hippocampal function. Even mild reduction in inhibition or enhancement of excitation can produce seizures. Synaptic excitation is produced by pyramidal cells and granule cells, whereas inhibition is produced by a smaller number of interneurons. To understand how two subpopulations of these excitatory and inhibitory neurons are regulated at the molecular level, we analyzed specific mRNA expression profiles for receptors that are significantly involved in synaptic transmission and in the synthesis and storage of the principal inhibitory neurotransmitter, gamma-aminobutyric acid (GABA). Our hypothesis was that differences in gene expression between inhibitory and excitatory neurons in the rat hippocampus might point to specific new targets for seizure pharmacotherapy. METHODS: We combined the techniques of (a) whole-cell patch clamping in rat hippocampal slices, (b) biocytin staining for cell identification, (c) single-cell mRNA amplification, and (d) small-scale cDNA microarray analysis to allow us to obtain expression profiles for candidate genes from identified CA1 pyramidal neurons and interneurons. Electrophysiologic and morphologic data and expression profiles were obtained from 12 stratum pyramidale and seven stratum radiatum cells. RESULTS: Presumed inhibitory neurons expressed significantly more GAD65, GAD67, vGAT, GABA(A)-receptor alpha3, and N-methyl-d-aspartate (NMDA)-receptor IIB mRNA, and presumed excitatory neurons expressed more GABA(A)-receptor alpha1, and NMDA-receptor I mRNA. CONCLUSIONS: Differential expression of candidate neurotransmitter-receptor subunits distinguished CA1 pyramidal neurons from interneurons. These differences may indicate potential new targets for altering the balance of inhibition and excitation in the treatment of epilepsy.     

Specific inhibitory synapses shift the balance from feedforward to feedback inhibition of hippocampal CA1 pyramidal cells

Feedforward and feedback inhibition are two fundamental modes of operation widespread in the nervous system. We have functionally identified synaptic connections between rat CA1 hippocampal interneurons of the stratum oriens (SO) and interneurons of the stratum lacunosum moleculare (SLM), which can act as feedback and feedforward interneurons, respectively. The unitary inhibitory postsynaptic currents (uIPSCs) detected with K-gluconate-based patch solution at −50 mV had an amplitude of 20.0 ± 2.0 pA, rise time 2.2 ± 0.2 ms, decay time 25 ± 2.2 ms, jitter 1.4 ± 0.2 ms (average ± SEM, n  = 39), and were abolished by the γ-aminobutyric acid (GABA)_A receptor antagonist 2-(3-carboxypropyl)-3-amino-6-methoxyphenyl-pyridazinium bromide (SR 95531). Post hoc anatomical characterization revealed that all but one of the identified presynaptic neurons were oriens-lacunosum moleculare (O-LM) cells, whereas the postsynaptic neurons were highly heterogeneous, including neurogliaform ( n  = 4), basket ( n  = 4), Schaffer collateral-associated ( n  = 10) and perforant path-associated ( n  = 9) cells. We investigated the short-term plasticity expressed at these synapses, and found that stimulation at 10–40 Hz resulted in short-term depression of uIPSCs. This short-term plasticity was determined by presynaptic factors and was not target-cell specific. We found that the feedforward inhibition elicited by the direct cortical input including the perforant path onto CA1 pyramidal cells was modulated through the inhibitory synapses we have characterized. Our data show that the inhibitory synapses between interneurons of the SO and SLM shift the balance between feedback and feedforward inhibition onto CA1 pyramidal neurons.     

Action of tachykinins in the rat hippocampus: modulation of inhibitory synaptic transmission

Substance P and other neuropeptides of the tachykinin family can powerfully excite CA1 hippocampal interneurons present in the CA1 region. In the present work we show that, by exciting hippocampal interneurons, tachykinins can indirectly inhibit pyramidal neurons. We found that tachykinins caused a decrease in the inhibitory synaptic current interval and an increase in the inhibitory synaptic current amplitude in almost all pyramidal neurons tested. This effect was tetrodotoxin sensitive. Tachykinins did not alter the frequency or amplitude of miniature inhibitory synaptic currents and were without effect on evoked inhibitory synaptic currents. Thus, these neuropeptides acted at the somatodendritic membrane of GABAergic interneurons, rather than at their axon terminals. The effect of substance P on spontaneous inhibitory synaptic currents could be mimicked by a selective agonist of NK1 receptors, but not by selective agonists of NK2 and NK3 receptors. It was suppressed by an NK1 receptor antagonist. In CA1 interneurons located in stratum radiatum, substance P generated a sustained tetrodotoxin-insensitive inward current or induced membrane depolarization and action potential firing. This direct excitatory action was mediated by NK1 receptors. Current-voltage relationships indicate that the net tachykinin-evoked current reversed in polarity at or near the K+ equilibrium potential, suggesting that a suppression of a resting K+ conductance was involved. By increasing the excitability of CA1 GABAergic interneurons, tachykinins can powerfully facilitate the inhibitory synaptic input to pyramidal neurons. This indirect inhibition could play a role in regulating short-term and/or long-term synaptic plasticity, promoting neuronal circuit synchronization or, in some physiopathological situations, influencing epileptogenesis.     

WONOEP appraisal: Optogenetic tools to suppress seizures and explore the mechanisms of epileptogenesis

Optogenetics is a novel technology that combines optics and genetics by optical control of microbial opsins, targeted to living cell membranes. The versatility and the electrophysiologic characteristics of the light‐sensitive ion‐channels channelrhodopsin‐2 (ChR2), halorhodopsin (NpHR), and the light‐sensitive proton pump archaerhodopsin‐3 (Arch) make these optogenetic tools potent candidates in controlling neuronal firing in models of epilepsy and in providing insights into the physiology and pathology of neuronal network organization and synchronization. Opsins allow selective activation of excitatory neurons and inhibitory interneurons, or subclasses of interneurons, to study their activity patterns in distinct brain‐states in vivo and to dissect their role in generation of synchrony and seizures. The influence of gliotransmission on epileptic network function is another topic of great interest that can be further explored by using light‐activated Gq protein–coupled opsins for selective activation of astrocytes. The ever‐growing optogenetic toolbox can also be combined with emerging techniques that have greatly expanded our ability to record specific subtypes of cortical and hippocampal neurons in awake behaving animals such as juxtacellular recording and two‐photon guided whole‐cell recording, to identify the specific subtypes of neurons that are altered in epileptic networks. Finally, optogenetic tools allow rapid and reversible suppression of epileptic electroencephalography (EEG) activity upon photoactivation. This review outlines the most recent advances achieved with optogenetic techniques in the field of epilepsy by summarizing the presentations contributed to the 13th ILAE WONOEP meeting held in the Laurentian Mountains, Quebec, in June 2013.     

Bicarbonate-dependent depolarizing potentials in pyramidal cells and interneurons during epileptiform activity

Experimental and theoretical evidence indicates that GABAergic neurotransmission is fundamental for the synchronization of neuronal activity. In particular, the role of GABA in epileptiform activity has received increased attention due to, among others, the fact that the GABA-mediated potentials can be depolarizing, and hence excitatory, in some circumstances. Evidence is presented here that bicarbonate efflux via GABAA receptors in interneurons and pyramidal cells of the CA1 hippocampal area contribute to depolarizing GABAA-mediated potentials in an in vitro nonpharmacological seizure-like model of status epilepticus. Seizure-like and interictal activity was evoked in rat horizontal hippocampal slices using a superfusing solution with low magnesium concentration (0.5-0.9 mm). Whole-cell recordings from stratum oriens-alveus interneurons and CA1 pyramidal cells revealed that, during epileptiform activity, some GABAA-mediated potentials were depolarizing, and were suppressed by the carbonic anhydrase inhibitor ethoxyzolamide as well as by the GABAA-receptor antagonist bicuculline. These observations indicate that the depolarizing potentials observed during epileptiform activity reflect both glutamatergic and GABAA-receptor-mediated activity, and adds further support for the important role of GABAergic interneurons in promoting long-range synchronization.     

Spontane Netzwerkaktivität in chronisch epileptischem Hirngewebe des Menschen

One of the important questions when investigating chronically epileptic human surgery specimens is whether this tissue, which generated synchronous network activity in patients as reflected in epilepsy-specific discharges in the EEG, will continue to do so in vitro. To address this issue, bioelectrical recordings were performed in human temporal slices resected from therapyresistant patients during epilepsy surgery using extra- and intracellular recording techniques as well as voltage sensitive dyes. In these experiments human epileptic tissue also displays spontaneous sharp waves in vitro, which on the intracellular level were reflected by polyphasic, generally hyperpolarizing postsynaptic potentials. These were carried by both excitatory and inhibitory synaptic processes. Grouped action potentials were observed as populations spikes; however, the neuronal networks required to generate these discharges appeared to be very small. Initiation of epileptic discharges often occurred in supragranular layers with multiple foci. The degree of synchronization within the initiating focus was low, as revealed by voltage-sensitivedye imaging. Increasing epileptogeity experimentally, ictaform activity with massive synchronization was also initiated in small foci of 100–300 μm diameter. In conclusion, multiple, spontaneously active foci exist in chronically epileptic tissue, and seizures may result from a synchronization of these foci.     

Decrease in inhibition in dentate granule cells from patients with medial temporal lobe epilepsy

Alterations in synaptic inhibition are associated with epileptiform activity in several acute animal models; however, it is not clear if there are changes in inhibition in chronically epileptic tissue. We have used intracellular recordings from granule cells of patients with temporal lobe epilepsy to determine whether synaptic inhibition is compromised. Two groups of patients with medial temporal lobe epilepsy were used, those with medial temporal lobe sclerosis (MTLE), and those with extrahippocampal masses (MaTLE) where the cell loss and synaptic reorganization that characterize MTLE are not seen. Although the level of tonic inhibition at the somata was not significantly different in the two patient groups, there was a reduction in the conductance of polysynaptic perforant path–evoked fast and slow inhibitory postsynaptic potentials (IPSPs) (53% and 66%, respectively). We found that there was a comparable decrease in the monosynaptic IPSP conductances examined in the presence of glutamatergic antagonists as that seen for the polysynaptically evoked IPSPs. These data suggest that the decrease in inhibition seen in normal artificial cerebrospinal fluid in MTLE granule cells cannot be solely explained by a decrease in excitatory input onto inhibitory interneurons and may reflect changes at the interneuron–granule cells synapse or in the number of specific inhibitory interneurons. Ann Neurol 1999;45:92–99     

Reversible inactivation of the medial septum: selective effects on the spontaneous unit activity of different hippocampal cell types

The contribution of septal afferents to spontaneous hippocampal single unit activity was examined by reversibly inactivating the medial septal nucleus using microinjections of the local anethetic lidocaine. Septal inactivation reduced spontaneous firing of cells in stratum granulosum and in the hilar/CA3 region for periods of up to about 15 min. The firing rates of CA1 complex-spike (pyramidal) cells, however, were not changed, although CA1 theta cells (inhibitory interneurons) exhibited a significant reduction in spontaneous rate. One interpretation of this pattern of results is that the output of CA1 pyramidal cells is maintained roughly constant in spite of reduced input from CA3 because of a proportional reduction in feedforward inhibition. This interpretation is consistent with Marr's²² formulation of the manner in which the hippocampus implements distributed associative memory. Alternatively, afferents to CA1 originating from regions other than CA3 may play a larger role in regulating CA1 output than previously assumed.     

Selective loss and axonal sprouting of GABAergic interneurons in the sclerotic hippocampus induced by LiCl-pilocarpine

In this study, we performed immunohistochemistry for somatostatin (SS), neuropeptide Y (NPY), and parvalbumin (PV) in LiCl-pilocarpine-treated rats to observe quantitative changes and axonal sprouting of GABAergic interneurons in the hippocampus, especially in the sclerotic hippocampus. Fluoro-Jade B (FJB) was performed to detect the specific degeneration of GABAergic interneurons. Compared with age-matched control rats, there were fewer SS/NPY/PV-immunoreactive (IR) interneurons in the hilus of the sclerotic hippocampus in pilocarpine-treated rats; hilar dentritic inhibitory interneurons were most vulnerable. FJB stain revealed degeneration was evident at 2 months after status epilepticus. Some SS-IR and NPY-IR interneurons were also stained for FJB, but there was no evidence of degeneration of PV-IR interneurons. Axonal sprouting of GABAergic interneurons was present in the hippocampus of epileptic rats, and a dramatic increase of SS-IR fibers was observed throughout all layers of CA1 region in the sclerotic hippocampus. These results confirm selective loss and degeneration of a specific subset of GABAergic interneurons in specific subfields of the hippocampus. Axonal sprouting of inhibitory GABAergic interneurons, especially numerous increase of SS-IR neutrophils within CA1 region of the sclerotic hippocampus, may constitute the aberrant inhibitory circum and play a significant role in the generation and compensation of temporal lobe epilepsy.     

Inhibition in kainate-lesioned hyperexcitable hippocampi: physiologic, autoradiographic, and immunocytochemical observations

Following kainate lesions of hippocampal subfield CA3, the remaining CA 1 pyramidal cells become hyperexcitable. This lesion is of interest because, morphologically, it resembles the damage often seen in cases of temporal lobe epilepsy; it may provide insight into the consequences of such cell loss in humans. The hyperexcitability in CA 1 is associated with a loss of both early and late IPSPs. At long postlesion latencies (2-4 months) inhibition shows partial recovery and the hyperexcitability subsides. The intent of the present work was to determine if alterations in CA 1 excitability and functional inhibition postlesion are correlated with changes in morphologic and physiologic indicators of inhibitory interneuron function or with alterations in binding sites for inhibitory transmitters. Using GAD immunocytochemistry, we found no acute or chronic lesion-induced decrease in numbers of CA 1 interneurons or in qualitative characteristics of the pericellular distribution of their terminals in CA 1 stratum pyramidale. Intracellular recordings from identified cells in CA 1 indicated that putative interneurons were viable in hyperexcitable tissue. It was further observed that "recovery" in tissue studied 2-4 months postlesion primarily involved the early IPSP; the late IPSP failed to reappear. Quantitative in vitro autoradiographic analysis of 3H-flunitrazepam--a marker for the early IPSP associated GABAA receptor complex--indicated that hyperexcitability was associated with an increase in GABAA receptor number in CA 1; receptor binding returned to normal at long postlesion latencies as the early IPSP returned and hyperexcitability subsided. Finally, hyperexcitable pyramidal cells were found to retain their responsivity to exogenously applied GABA. These data indicate that much of the cellular machinery necessary for inhibition is retained in CA 1, despite lesion-induced hyperexcitability. We suggest that the acute loss of the IPSP after kainate lesion is due to a transient disconnection between inhibitory and excitatory elements in CA 1 and/or to a loss of normal afferent drive from CA3 onto some CA 1 interneurons. We further suggest that incomplete recovery can be explained by abnormalities that occur as neuroplastic rearrangements in response to deafferentation of CA 1. The relevance of these studies to human hippocampal necrosis and to other models of focal epilepsy is discussed.     

Mitochondrial complex I deficiency in the epileptic focus of patients with temporal lobe epilepsy

Mitochondria are cellular organelles crucial for energy supply and calcium homeostasis in neuronal cells, and their dysfunction causes seizure activity in some rare human epilepsies. To directly test whether mitochondrial respiratory chain enzymes are abnormal in the most common form of chronic epilepsy, temporal lobe epilepsy (TLE), living human brain specimens from 57 epileptic patients and 2 nonepileptic controls were investigated. In TLE patients with a hippocampal epileptic focus, we demonstrated a specific deficiency of complex I of the mitochondrial respiratory chain in the hippocampal CA3 region. In contrast, TLE patients with a parahippocampal epileptic focus showed reduced complex I activity only in parahippocampal tissue. Inhibitor titrations of the maximal respiration rate of intact human brain slices revealed that the observed reduction in complex I activity is sufficient to affect the adenosine triphosphate production rate. The abnormal complex I activity in the hippocampal CA3 region was paralleled by increased succinate dehydrogenase staining of neurons and marked ultrastructural abnormalities of mitochondria. Therefore, mitochondrial dysfunction is suggested to be specific for the epileptic focus and may constitute a pathomechanism contributing to altered excitability and selective neuronal vulnerability in TLE.     

Somatostatin-immunoreactive interneurons contribute to lateral inhibitory circuits in the dentate gyrus of control and epileptic rats.

Lateral inhibition, a feature of neuronal circuitry that enhances signaling specificity, has been demonstrated in the rat dentate gyrus. However, neither the underlying neuronal circuits, nor the ways in which these circuits are altered in temporal lobe epilepsy, are completely understood. This study examines the potential contribution of one class of inhibitory interneurons to lateral inhibitory circuits in the dentate gyrus of both control and epileptic rats. The retrograde tracer wheat germ ag-glutinin-apo-horse radish peroxidase-gold (WGA-apo-HRP-gold) was injected into the septal dentate gyrus. Neurons double-labeled for glutamic acid decarboxylase (GAD) and the retrograde tracer are concentrated in the hilus and may contribute to lateral inhibition. Neurons double-labeled for somatostatin and the retrograde tracer account for at least 28% of GAD-positive neurons with axon projections appropriate for generating lateral inhibition in control rats. Despite an overall loss of somatostatin-expressing cells in epileptic animals, the number of somatostatin-positive interneurons with axon projections appropriate for generating lateral inhibition is similar to that seen in controls. These findings suggest that somatostatinergic interneurons participate in lateral inhibitory circuits in the dentate gyrus of both control and epileptic rats, and that surviving somatostatinergic interneurons might sprout new axon collaterals in epileptic animals.     

Entorhinal cortical innervation of parvalbumin-containing neurons (basket and chandelier cells) in the rat ammon's horn

Physiological data suggest that in the CA1–CA3 hippocampal areas of rats, entorhinal cortical efferents directly influence the activity of interneurons, in addition to pyramidal cells. To verify this hypothesis, the following experiments were performed: 1) light microscopic double-immunostaining for parvalbumin and the anterograde tracer Phaseolus vulgaris-leucoagglutinin injected into the entorhinal cortex; 2) light and electron microscopic analysis of cleaved spectrin-immunostained (i.e., degenerating axons and boutons) hippocampal sections following entorhinal cortex lesion; and 3) an electron microscopic study of parvalbumin-immunostained hippocampal sections after entorhinal cortex lesion. The results demonstrate that in the stratum lacunosum-moleculare of the CA1 and CA3 regions, entorhinal cortical axons form asymmetric synaptic contacts on parvalbumin-containing dendritic shafts. In the stratum lacunosum-moleculare, parvalbumin-immunoreactive dendrites represent processes of GABAergic, inhibitory basket and chandelier cells; these interneurons innervate the perisomatic area and axon initial segments of pyramidal cells, respectively. A feed-forward activation of these neurons by the entorhinal input may explain the strong, short-latency inhibition of pyramidal cells. © 1996 Wiley-Liss, Inc.     

Cell-type specific GABA synaptic transmission and activity-dependent plasticity in rat hippocampal stratum radiatum interneurons

Abstract In hippocampal pyramidal cells, the efficacy of synaptic transmission at gamma-aminobutyric acid (GABA)ergic synapses, is modulated by activity. However, whether such plasticity occurs at inhibitory synapses on interneurons remains largely unknown. Using whole-cell voltage-clamp recordings of inhibitory postsynaptic currents (IPSCs) in Sprague-Dawley rat hippocampal slices, we examined whether GABA synapses of stratum radiatum interneurons were affected by stimulation protocols known to alter efficacy at inhibitory synapses of CA1 pyramidal cells. Monosynaptically evoked IPSCs (eIPSCs) exhibited different properties with significantly faster kinetics, higher coefficients of variation, a current-voltage (I-V) relationship shifted to depolarized values and a smaller paired-pulse depression, in interneurons than in pyramidal cells. GABA synapses on interneurons also showed a different capacity for plasticity. Indeed, theta-burst stimulation induced a long-term potentiation of eIPSCs in both cell types, but the induction mechanisms differed in interneurons, as it was not affected by antagonists of GABAB receptors and group I/II metabotropic glutamate receptors (mGluRs). Furthermore, 100-Hz tetanization selectively elicited a short-term depression of eIPSCs in pyramidal cells. A postsynaptic depolarization produced a transient suppression of eIPSCs (depolarization-induced suppression of inhibition) in pyramidal cells but not in interneurons. Spontaneous IPSCs were similarly reduced following depolarization in pyramidal cells, but not in interneurons. These results indicate that GABA synapses of stratum radiatum interneurons exhibit different properties and capacity for activity-dependent plasticity than those of pyramidal cells. This cell-type specific mode of transmission and adaptive regulation of GABA synapses may contribute to hippocampal plasticity and functions.