Interneurons of the hippocampus

Quantitative analysis of GABAergic neurons in the mouse hippocampus, with optical disector using confocal laser scanning microscope

The numerical densities (NDs) of glutamic acid decarboxylase (GAD) 67 immunoreactive (IR) neurons in the mouse hippocampus were estimated according to the optical disector method using a confocal laser scanning microscope (CLSM), and the cell sizes of disector-counted neurons were measured. Particularly, we focused on the dorsoventral differences of the NDs and cell sizes in individual subdivisions and layers. The NDs of GAD67-IR neurons were larger at the ventral level than at the dorsal level in most subdivisions and layers, except in the stratum pyramidale (SP) of the CA1 region and stratum radiatum (SR) of the CA3 region. In the whole hippocampus, the ND of GAD67-IR neurons was 5.7+/-0.2x103/mm3 at the dorsal level, and 7.3+/-0.3x103/mm3 at the ventral level. The laminar differences showed that the NDs of GAD67-IR neurons in the principal cell layers were generally larger than those in the dendritic layers in each subdivision. The ND of GAD67-IR neurons was largest in the SP of the CA1 region at the dorsal level (13.5+/-0.9x103/mm3), and smallest in the molecular layer (ML) of the dentate gyrus (DG) at the dorsal level (1.7+/-0.2x103/mm3). The mean cell sizes of GAD67-IR neurons also showed prominent dorsoventral and laminar differences. In the CA3 region, the mean cell size of GAD67-IR neurons was smaller at the dorsal level than at the ventral level, while in the DG, it was larger at the dorsal level than at the ventral level. On the other hand, the mean cell size of GAD67-IR neurons in the CA1 region showed no significant dorsoventral difference. In the whole hippocampus, the mean cell size of GAD67-IR neurons was slightly smaller at the dorsal level (somatic profile area 149.2+/-2.5 microm2) than at the ventral level (154.2+/-2.9 microm2). The laminar differences showed that the mean cell sizes of GAD67-IR neurons in the principal cell layers were generally larger than those in the dendritic layers in each subdivision. The mean cell size of GAD67-IR neurons was largest in the SP of the CA3 region at the ventral level (180.7+/-8.7 microm2), and smallest in the stratum lacunosum-moleculare (SLM) of the CA3 region at the dorsal level (115.9+/-7.9 microm2). The cell size distributions in individual layers revealed that GAD67-IR neurons were roughly classified into two subgroups. The composition of these subgroups suggested the heterogeneity of GAD67-IR neurons in the mouse hippocampus in view of cell size

Slow oscillations (</=1 Hz) mediated by GABAergic interneuronal networks in rat hippocampus

Perfusion of rat brain slices with low millimole CsCl elicits slow oscillations of </=1 Hz in hippocampal CA1 pyramidal neurons. These oscillations are GABAA receptor-mediated hyperpolarizations that permit a coherent fire-pause pattern in a population of CA1 neurons. They can persist without the activation of ionotropic glutamate receptors but require adenosine-dependent inhibition of glutamate transmission. In response to external Cs+, multiple interneurons in the CA1 region display rhythmic discharges that correlate with the slow oscillations in CA1 pyramidal neurons. The interneuronal discharges arise spontaneously from the resting potential, and their rhythmicity is regulated by periodic, GABAA receptor-mediated hyperpolarizations. In addition, interneurons show periodic partial spikes and neurobiotin coupling, and applications of known gap junctional uncouplers interrupt the Cs+-induced slow rhythm in both CA1 pyramidal neurons and interneurons. We propose that these slow oscillations originate from a GABAergic interneuronal network that interacts through reciprocal inhibition and possibly gap junctional connection.

Cell-specific alterations in synaptic properties of hippocampal CA1 interneurons after kainate treatment

Cell-specific alterations in synaptic properties of hippocampal CA1 interneurons after kainate treatment. J. Neurophysiol. 80: 2836-2847, 1998. Hippocampal sclerosis and hyperexcitability are neuropathological features of human temporal lobe epilepsy that are reproduced in the kainic acid (KA) model of epilepsy in rats. To assess directly the role of inhibitory interneurons in the KA model, the membrane and synaptic properties of interneurons located in 1) stratum oriens near the alveus (O/A) and 2) at the border of stratum radiatum and stratum lacunosum-moleculare (LM), as well as those of pyramidal cells, were examined with whole cell recordings in slices of control and KA-lesioned rats. In current-clamp recordings, intrinsic cell properties such as action potential amplitude and duration, amplitude of fast and medium duration afterhyperpolarizations, membrane time constant, and input resistance were generally unchanged in all cell types after KA treatment. In voltage-clamp recordings, the amplitude and conductance of pharmacologically isolated excitatory postsynaptic currents (EPSCs) were significantly reduced in LM interneurons of KA-treated animals but were not significantly changed in O/A and pyramidal cells. The rise time of EPSCs was not significantly changed in any cell type after KA treatment. In contrast, the decay time constant of EPSCs was significantly faster in O/A interneurons of KA-treated rats but was unchanged in LM and pyramidal cells. The amplitude and conductance of pharmacologically isolated gamma-aminobutyric acid-A (GABAA) inhibitory postsynaptic currents (IPSCs) were not significantly changed in any cell type of KA-treated rats. The rise time and decay time constant of GABAA IPSCs were significantly faster in pyramidal cells of KA-treated rats but were not significantly changed in O/A and LM interneurons. These results suggest that complex alterations in synaptic currents occur in specific subpopulations of inhibitory interneurons in the CA1 region after KA lesions. A reduction of evoked excitatory drive onto inhibitory cells located at the border of stratum radiatum and stratum lacunosum-moleculare may contribute to disinhibition and polysynaptic epileptiform activity in the CA1 region. Compensatory changes, involving excitatory synaptic transmission on other interneuron subtypes and inhibitory synaptic transmission on pyramidal cells, may also take place and contribute to the residual, functional monosynaptic inhibition observed in principal cells after KA treatment.

Pharmacological modulation of GABA(A) receptor-mediated postsynaptic potentials in the CA1 region of the rat hippocampus

It is unclear whether GABA(A) receptor-mediated hyperpolarizing and depolarizing synaptic potentials (IPSP(A)s and DPSP(A)s, respectively) are evoked by (a) the same populations of GABAergic interneurones and (b) exhibit similar regulation by allosteric modulators of GABA(A) receptor function. We have attempted to address these questions by investigating the effects of (a) known agonists for presynaptic receptors on GABAergic terminals, and (b) a range of GABA(A) receptor ligands, on each response. The GABA uptake inhibitor NNC 05-711 (10 microM) enhanced whereas bicuculline (10 microM) inhibited both IPSP(A)s and DPSP(A)s. (-)-Baclofen (5 microM), [D-Ala2,N-Me-Phe4,Gly5-ol]-enkephalin (DAGO; 0.5 microM), and carbachol (10 microM) caused substantial depressions (up to 99%) of DPSP(A)s that were reversed by CGP 55845A (1 microM), naloxone (10 microM) and atropine (5 microM), respectively. In contrast, 2-chloroadenosine (CADO; 10 microM) only slightly depressed DPSP(A)s. Quantitatively, the effect of each agonist was similar to that reported for IPSP(A)s. The neurosteroid ORG 21465 (1 - 10 microM), the anaesthetic propofol (50-500 microM), the barbiturate pentobarbitone (100-300 microM) and zinc (50 microM) all enhanced DPSP(A)s and IPSP(A)s. The benzodiazepine (BZ) agonist flunitrazepam (10-50 microM) and inverse agonist DMCM (1 microM) caused a respective enhancement and inhibition of both IPSP(A)s and DPSP(A)s. The BZomega1 site agonist zolpidem (10-30 microM) produced similar effects to flunitrazepam. The anticonvulsant loreclezole (1-100 microM) did not affect either response. These data demonstrate that similar populations of inhibitory interneurones can generate both IPSP(A)s and DPSP(A)s by activating GABA(A) receptors that are subject to similar allosteric modulation.

Residual granule cells can maintain susceptibility of CA3 pyramidal cells to kainate-induced epileptiform discharges

Slices of adult rat hippocampus made from animals exposed neonatally to X-ray irradiation were studied with electrophysiological techniques. A single dose of 6 Gy irradiation of the pup's head significantly but unevenly reduced the number of granule cells in the dentate gyrus. A larger reduction was detected in the septal than in the temporal hippocampus. The number of hilar cells decreased also. Effects of irradiation were confirmed with histological techniques. Field potential responses to mossy fiber stimulation in the pyramidal layer of the CA3 subfield was smaller in irradiated than in normal rats. Superfusion of the slices with kainic acid (KA, 300-500 nM) induced spontaneously recurrent paroxysmal activity (SRPA) in about 40% of irradiated slices in contrast with nearly 90% of slices cut from nonirradiated rats. Intracellular recordings from CA3 pyramidal cells in irradiated rats revealed recurrent bursts of action potentials on top of large depolarizing waves after KA application. Cells impaled in slices from the septal half of hippocampus of irradiated rats failed more often to respond with bursts to KA than cells in slices cut from the temporal half. Removal of mossy fiber input can therefore reduce KA induced hyperexcitability of CA3 pyramidal cells, but quantitative factors such as proportional loss of granule and hilar cells may explain the considerable differences found among cells and slices. Removal of 80% of granule cells reduces hyperexcitability consistently, while SRPA can be found in slices where as much as 50% of granule cells are missing. Intracellular findings suggest that failures of detection of SRPA following KA application to hippocampal slices of irradiated rats does not necessarily mean that CA3 pyramidal cells are no longer responding to KA with epileptiform bursting.

GABAA currents in immature dentate gyrus granule cells

We used whole cell patch clamp and gramicidin perforated patch recordings in hippocampal slices to study gamma-aminobutyric acid (GABA) currents in granule cells (GCs) from juvenile rat dentate gyrus (DG). GCs are generated postnatally and asynchronously such that they can be detected at different stages of their maturation in DG within the first month. In contrast, inhibitory interneurons are generated embryonically, and their circuitry is well developed even as their target GCs and GC excitatory connections are still being formed. In this study, two GABA currents evoked in GCs by medial perforant path stimulation are compared. The first, pharmacologically isolated by glutamate receptor blockade, is the product of direct activation of GABA interneurons with monosynaptic input to the recorded GC (monosynaptic GABAA). Monosynaptic GABAA displays slight outward rectification of its current-voltage relation, is 97% eliminated by 10 microM bicuculline and coincides temporally with the excitatory components of GC postsynaptic currents as has been described for GABAA currents in other brain regions. The second is a novel GABA response that is detectable in 10 microM bicuculline and is present on GCs only at the earliest stages of their maturation. Unlike monosynaptic GABAA, this transient GABA is eliminated by glutamate receptor blockade and hence is likely to be generated by interneurons activated via an intervening glutamatergic synapse (polysynaptically). It is predominantly chloride mediated, has a relative bicarbonate/chloride permeability ratio of 26%, and is unchanged by bath-applied saclofen and strychnine or by intracellular calcium chelation. It is 97% antagonized by 100 microM picrotoxin and 99% antagonized by 100 microM bicuculline. This current is thus a relatively bicuculline (BMI)-resistant GABAA current (BMIR-GABAA). Compared with monosynaptic GABAA, BMIR-GABAA has a later peak, slower time course of decay, and marked outward rectification. Its reversal potential is 7-8 mV depolarized to that of monosynaptic GABAA whether recorded in whole cell or with gramicidin perforated patch to preserve native internal chloride concentration. Together these data may suggest that BMIR-GABAA is evoked by an anatomically segregated population of interneurons activating a unique, developmentally regulated GABAA receptor. Further, the transient nature of this current coupled with its temporal characteristics that preclude overlap with the excitatory components of the synaptic response are consistent with a role that is trophic or signaling rather than primarily inhibitory.

Voltage imaging of epileptiform activity in slices from rat piriform cortex: onset and propagation

The piriform cortex is a temporal lobe structure with a very high seizure susceptibility. To investigate the spatiotemporal characteristics of epileptiform activity, slices of piriform cortex were examined by imaging electrical activity with a voltage-sensitive fluorescent dye. Discharge activity was studied for different sites of stimulation and different planes of slicing along the anterior-posterior axis. Epileptiform behavior was elicited either by disinhibition with a gamma-aminobutyric acid-A receptor antagonist or by induction with a transient period of spontaneous bursting in low-chloride medium. Control activity recorded with fluorescent dye had the same pharmacological and temporal characteristics as control activity reported previously with microelectrodes. Simultaneous optical and extracellular microelectrode recordings of epileptiform discharges showed the same duration, latency, and all-or-none character as described previously with microelectrodes. Under all conditions examined, threshold electrical stimulation applied throughout the piriform cortex evoked all-or-none epileptiform discharges originating in a site that included the endopiriform nucleus, a previously identified site of discharge onset. In induced slices, but not disinhibited slices, the site of onset also included layer VI of the adjoining agranular insular cortex and perirhinal cortex, in slices from anterior and posterior piriform cortex, respectively. These locations had not been identified previously as sites of discharge onset. Thus like the endopiriform nucleus, the deep agranular insular cortex and perirhinal cortex have a very low seizure threshold. Additional subtle differences were noted between the induced and disinhibited models of epileptogenesis. Velocity was determined for discharges after onset, as they propagated outward to the overlying piriform cortex. Propagation in other directions was examined as well. In most cases, velocities were below that for action potential conduction, suggesting that recurrent excitation and/or ephaptic interactions play a role in discharge propagation. Future investigations of the cellular and organizational properties of regions identified in this study should help clarify the neurobiological basis of high seizure susceptibility.

Laminar difference in GABA uptake and GAT-1 expression in rat CA1

1. The axonal plexus of most hippocampal interneurons is restricted to certain strata within the target region. This lamination suggests a possible functional heterogeneity of inhibitory synapses between different interneurons and CA1 pyramidal cells. 2. We therefore compared inhibitory postsynaptic potentials (IPSPs) and currents (IPSCs) in CA1 pyramidal cells, which were evoked from two stimulation sites (stratum oriens and stratum radiatum). Stimulation in stratum oriens yielded faster decaying IPSPs and IPSCs than stimulation in stratum radiatum. 3. IPSP and IPSC kinetics were regulated by GABA uptake in both layers as indicated by the prolongation of the signals under tiagabine, a GAT-1 (neuronal GABA plasma membrane transporter)-specific GABA-uptake blocker. However, the effect of tiagabine was significantly more pronounced following stimulation in stratum radiatum than in stratum oriens (prolongation of IPSC half-decay time by 167 vs. 115 %, respectively). 4. In situ hybridization with antisense mRNA for the GABA-synthesizing enzyme glutamate decarboxylase (GAD65/67) and the GABA transporter GAT-1 showed that the proportion of interneurons expressing GAT-1 was lower in stratum oriens than in stratum radiatum/lacunosum-moleculare. 5. From these functional and molecular data we conclude that the regulation of IPSP and IPSC kinetics in CA1 pyramidal cells by neuronal GABA uptake differs between layers. Our findings suggest that this laminar difference is caused by a lower expression of GAT-1 in interneurons in stratum oriens than in stratum radiatum/lacunosum-moleculare.

Corticotrophin-releasing factor produces a long-lasting enhancement of synaptic efficacy in the hippocampus

We have previously demonstrated that intra-hippocampal injection of corticotrophin-releasing factor improved memory retention of an inhibitory avoidance learning in rats; while the electrophysiological effects corticotrophin-releasing factor produces on hippocampal neurons are largely uncharacterized. In the present study, we found that corticotrophin-releasing factor injected into the dentate gyrus of hippocampus produced a dose-dependent and long-lasting enhancement in synaptic efficacy of these neurons, as measured by an increase in the amplitude and slope of population excitatory postsynaptic potentials, as well as the amplitude of population spike. The onset of corticotrophin-releasing factor-induced potentiation was slow. It was observed approximately 40-60 min after corticotrophin-releasing factor administration and lasted for more than 5 h. This effect of corticotrophin-releasing factor was blocked by pretreatment with the cyclase-adenosine-3,5-monophosphate (cAMP) inhibitor Rp-adenosine-3,5-cyclic monophosphothiolate triethylamine (Rp-cAMPS) and partially blocked by the N-methyl-D-aspartate receptor antagonist MK-801. Further, pretreatment with corticotrophin-releasing factor receptor antagonist dose-dependently diminished tetanization-induced long-term potentiation, and corticotrophin-releasing factor and tetanic stimuli had an additive effect on hippocampal neuron excitation. Moreover, direct injection of corticotrophin-releasing factor increased cAMP level in the dentate gyrus. These results together suggest that corticotrophin-releasing factor-induced potentiation simulates the late phase of tetanization-induced long-term potentiation and cAMP seems to be the messenger mediating this effect. Moreover, corticotrophin-releasing factor-induced potentiation and long-term potentiation may share some similar mechanisms, and corticotrophin-releasing factor is probably involved in the neural circuits underlying long-term potentiation. Thus, corticotrophin-releasing factor may play an important role in modulating synaptic plasticity in the hippocampus.

AMPA receptor subunits are differentially expressed in parvalbumin- and calretinin-positive neurons of the rat hippocampus

Recent studies suggest a functional diversity of native alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate-type glutamate receptor channels (AMPARs). In several types of interneurons, AMPARs are characterized by higher Ca2+ permeability and faster kinetics than AMPARs in principal cells. We studied the expression profile of AMPAR subunits in the hippocampal parvalbumin (PV)- and calretinin (CR)-positive cells, which represent different populations of non-principal cells. To this end, non-radioactive in situ hybridization with AMPAR subunit specific cRNAs was combined with immunocytochemistry for PV or CR. Double-immunolabelling using antibodies against AMPAR subunits and PV or CR was also performed. PV-containing neurons represent a fairly homogeneous population of cells expressing high levels of GluR-A and GluR-D mRNAs, moderate levels of GluR-C and low levels of GluR-B mRNAs in all the examined regions of hippocampus. The vast majority of CR-containing cells have a much lower expression of GluR-A, -C and -D mRNA than PV-positive neurons, although similarly featuring low levels of GluR-B mRNA. Only a subpopulation of CR-containing cells, the spiny neurons of the dentate gyrus and CA3 region of the hippocampus were characterized by a strong expression of GluR-A and -D subunit mRNAs. The differential pattern found for the AMPAR subunit mRNA expression was confirmed by immunocytochemistry at protein level. Despite the common feature of low GluR-B subunit expression, PV- and CR-containing interneurons differ with respect to the density and combination of their expressed AMPAR subunits. The different combination of subunits might subserve different properties of the AMPA channels featured by these cell types, with implications for the functioning of the hippocampal network.

Synaptic potentials mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors in rat hippocampal interneurons

Exogenous application of acetylcholine elicits inward currents in hippocampal interneurons that are mediated via alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors, but synaptic responses mediated via such receptors have never been reported in mammalian brain. In the present study, EPSCs were evoked in hippocampal interneurons in rat brain slices by electrical stimulation and were recorded by using whole-cell voltage-clamp techniques. Nicotinic EPSCs were isolated pharmacologically, using antagonists to block other known types of ligand-gated ion channels, and then were tested with either alpha-bungarotoxin or methyllycaconitine, which are selective antagonists for nicotinic acetylcholine receptors that contain the alpha7 receptor subunit. Each antagonist proved highly effective at reducing the remaining synaptic current. Evoked alpha7-mediated nicotinic EPSCs also were desensitized by superfusion with 1 microM nicotine, had extrapolated reversal potentials near 0 mV, and showed strong inward rectification at positive potentials. In several interneurons, methyllycaconitine-sensitive spontaneous EPSCs also were observed that exhibited a biphasic decay rate very similar to that of the alpha7-mediated evoked response. These studies provide the first demonstration of a functional cholinergic synapse in the mammalian brain, in which the primary postsynaptic receptors are alpha-bungarotoxin-sensitive nicotinic acetylcholine receptors.

Nicotine-evoked nitric oxide release in the rat hippocampal slice

The effects of cholinergic agonists on nitric oxide (NO) release in hippocampal slices from male Sprague-Dawley rats were investigated using electrochemical recording procedures using Nafion and O-phenylenediamine-treated carbon fiber microelectrodes. These microelectrodes are highly selective for NO versus other interferents. Acetylcholine (Ach) with neostigmine, or nicotine was delivered by pressure ejection from pipettes placed within 300 microm of the NO sensors. Both Ach and nicotine produced NO signals ranging from 0.04 to 2.14 microM in the CA1, CA3, and dentate gyrus of the rat hippocampus that lasted for 2-5 min. The Ach responses were not antagonized by the muscarinic antagonist atropine. However, nicotine-evoked responses were partially antagonized by alpha-bungarotoxin, a finding consistent with alpha7-nicotinic cholinergic receptors being involved with the effects of nicotine. These data support the hypothesis that nicotine is capable of evoking long lasting NO release in the hippocampus.

Regional and cellular patterns of reelin mRNA expression in the forebrain of the developing and adult mouse

The reelin gene encodes an extracellular protein that is crucial for neuronal migration in laminated brain regions. To gain insights into the functions of Reelin, we performed high-resolution in situ hybridization analyses to determine the pattern of reelin expression in the developing forebrain of the mouse. We also performed double-labeling studies with several markers, including calcium-binding proteins, GAD65/67, and neuropeptides, to characterize the neuronal subsets that express reelin transcripts. reelin expression was detected at embryonic day 10 and later in the forebrain, with a distribution that is consistent with the prosomeric model of forebrain regionalization. In the diencephalon, expression was restricted to transverse and longitudinal domains that delineated boundaries between neuromeres. During embryogenesis, reelin was detected in the cerebral cortex in Cajal-Retzius cells but not in the GABAergic neurons of layer I. At prenatal stages, reelin was also expressed in the olfactory bulb, and striatum and in restricted nuclei in the ventral telencephalon, hypothalamus, thalamus, and pretectum. At postnatal stages, reelin transcripts gradually disappeared from Cajal-Retzius cells, at the same time as they appeared in subsets of GABAergic neurons distributed throughout neocortical and hippocampal layers. In other telencephalic and diencephalic regions, reelin expression decreased steadily during the postnatal period. In the adult, there was prominent expression in the olfactory bulb and cerebral cortex, where it was restricted to subsets of GABAergic interneurons that co-expressed calbindin, calretinin, neuropeptide Y, and somatostatin. This complex pattern of cellular and regional expression is consistent with Reelin having multiple roles in brain development and adult brain function.

Opioid inhibition of hippocampal interneurons via modulation of potassium and hyperpolarization-activated cation (Ih) currents

The actions of mu- and delta-opioid agonists (DAMGO and DPDPE, respectively) on GABAergic interneurons in stratum oriens of area CA1 of the hippocampus were examined by using whole-cell voltage-clamp recordings in brain slices. Both agonists consistently generated outward currents of similar magnitude (15-20 pA) in the majority of cells. However, under control conditions, current-voltage (I/V) relationships revealed that only a small number of these cells (3 of 77) demonstrated clear increases in membrane conductance, associated with the activation of the potassium current known as Girk. These interneurons also exhibited a slowly activating, inwardly rectifying current known as Ih on hyperpolarizing step commands. Ih was blocked by the extracellular application of cesium (3-9 mM) or ZD 7288 (10-100 microM) but was insensitive to barium (1-2 mM). In an effort to determine whether the holding current changes were attributable to the modulation of Girk and/or Ih, we used known blockers of these ion channels (barium or cesium and ZD 7288, respectively). Extracellular application of cesium (3-9 mM) or ZD 7288 (25-100 microM) blocked Ih and significantly reduced the opioid-induced outward currents by 58%. Under these conditions the opioid agonists activated a potassium current with characteristics similar to Girk. Similarly, during barium (1-2 mM) application the opioid-induced outward currents were reduced by 46%, and a clear reduction in Ih and the whole-cell conductance was revealed. These data suggest that the opioids can modulate both Ih and Girk in the same population of stratum oriens interneurons and that the modulation of these ion channels can contribute to the inhibition of interneuron activity in the hippocampus.

TrkB and TrkC signaling are required for maturation and synaptogenesis of hippocampal connections

Recent studies have suggested a role for neurotrophins in the growth and refinement of neural connections, in dendritic growth, and in activity-dependent adult plasticity. To unravel the role of endogenous neurotrophins in the development of neural connections in the CNS, we studied the ontogeny of hippocampal afferents in trkB (-/-) and trkC (-/-) mice. Injections of lipophilic tracers in the entorhinal cortex and hippocampus of newborn mutant mice showed that the ingrowth of entorhinal and commissural/associational afferents to the hippocampus was not affected by these mutations. Similarly, injections of biocytin in postnatal mutant mice (P10-P16) did not reveal major differences in the topographic patterns of hippocampal connections. In contrast, quantification of biocytin-filled axons showed that commissural and entorhinal afferents have a reduced number of axon collaterals (21-49%) and decreased densities of axonal varicosities (8-17%) in both trkB (-/-) and trkC (-/-) mice. In addition, electron microscopic analyses showed that trkB (-/-) and trkC (-/-) mice have lower densities of synaptic contacts and important structural alterations of presynaptic boutons, such as decreased density of synaptic vesicles. Finally, immunocytochemical studies revealed a reduced expression of the synaptic-associated proteins responsible for synaptic vesicle exocytosis and neurotransmitter release (v-SNAREs and t-SNAREs), especially in trkB (-/-) mice. We conclude that neither trkB nor trkC genes are essential for the ingrowth or layer-specific targeting of hippocampal connections, although the lack of these receptors results in reduced axonal arborization and synaptic density, which indicates a role for TrkB and TrkC receptors in the developmental regulation of synaptic inputs in the CNS in vivo. The data also suggest that the genes encoding for synaptic proteins may be targets of TrkB and TrkC signaling pathways.

Modulation by substance P of synaptic transmission in the mouse hippocampal slice

The modulatory action of substance P on synaptic transmission of CA1 neurons was studied using intra- or extracellular recording from the mouse hippocampal slice preparation. Bath-applied substance P (2-4 microM) or the selective NK1 receptor agonist substance P methylester (SPME, 10 nM-5 microM) depressed field potentials (recorded from stratum pyramidale) evoked by focal stimulation of Schaffer collaterals. This effect was apparently mediated via NK1 receptors since it was completely blocked by the selective NK1 antagonist SR 140333. The field potential depression by SPME was significantly reduced in the presence of bicuculline. Intracellular recording from CA1 pyramidal neurons showed that evoked excitatory postsynaptic potentials (EPSPs) and evoked inhibitory postsynaptic potentials (IPSPs) were similarly depressed by SPME, which at the same time increased the frequency of spontaneous GABAergic events and reduced that of spontaneous glutamatergic events. The effects of SPME on spontaneous and evoked IPSPs were prevented by the ionotropic glutamate receptor blocker kynurenic acid. In tetrodotoxin (TTX) solution, no change in either the frequency of spontaneous GABAergic and glutamatergic events or in the amplitude of responses of pyramidal neurons to 4 microM alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or 10 microM N-methyl-D-aspartate (NMDA) was observed. On the same cells, SPME produced minimal changes in passive membrane properties unable to account for the main effects on synaptic transmission. The present data indicate that SPME exerted its action on CA1 pyramidal neurons via a complex network mechanism, which is hypothesized to involve facilitation of a subset of GABAergic neurons with widely distributed connections to excitatory and inhibitory cells in the CA1 area.

GluR5 kainate receptor activation in interneurons increases tonic inhibition of pyramidal cells

We studied the modulation of GABAergic inhibition by glutamate and kainate acting on GluR5-containing kainate receptors in the CA1 hippocampal region. Glutamate, kainate or ATPA, a selective agonist of GluR5-containing receptors, generates an inward current in inhibitory interneurons and cause repetitive action potential firing. This results in a massive increase of tonic GABAergic inhibition in the somata and apical dendrites of pyramidal neurons. These effects are prevented by the GluR5 antagonist LY 293558. Electrical stimulation of excitatory afferents generates kainate receptor-mediated excitatory postsynaptic currents (EPSCs) and action potentials in identified interneurons that project to the dendrites and somata of pyramidal neurons. Therefore glutamate acting on kainate receptors containing the GluR5 subunit may provide a protective mechanism against hyperexcitability.

Giant depolarizing potentials: the septal pole of the hippocampus paces the activity of the developing intact septohippocampal complex in vitro

In neonatal hippocampal slices, recurrent spontaneous giant depolarizing potentials (GDPs) provide neuronal synchronized firing and Ca2+ oscillations. To investigate the possible role of GDPs in the synchronization of neuronal activity in intact neonatal limbic structures, we used multiple simultaneous electrophysiological recordings in the recently described preparation of intact neonatal septohippocampal complex in vitro. Combined whole-cell (in single or pairs of cells) and extracellular field recordings (one to five simultaneous recording sites) from the CA3 hippocampal region and various parts of the septum indicated that spontaneous GDPs, which can be initiated anywhere along the longitudinal hippocampal axis, are most often initiated in the septal poles of hippocampus and propagate to medial septum and temporal poles of both hippocampi simultaneously. GDPs were abolished in the medial septum but not in the hippocampus after surgical separation of both structures, suggesting hippocampal origin of GDPs. The preferential septotemporal orientation of GDP propagation observed in the intact hippocampus was associated with a corresponding gradient of GDP frequency in isolated portions of hippocampus. Accordingly, most GDPs propagated in the septotemporal direction in both septal and temporal hippocampal isolated halves, and whereas GDP frequency remained similar in the septal part of hippocampus after its surgical isolation, it progressively decreased in more temporally isolated portions of the hippocampus. Because GDPs provide most of the synaptic drive of neonatal neurons, they may modulate the development of neuronal connections in the immature limbic system.

Altered hippocampal kainate-receptor mRNA levels in temporal lobe epilepsy patients

This study determined whether hippocampal kainate (KA) receptor mRNA levels were increased or decreased in temporal lobe epilepsy patients compared with nonseizure autopsies. Hippocampal sclerosis (HS; n = 17), nonsclerosis (non-HS; n = 11), and autopsy hippocampi (n = 9) were studied for KA1-2 and GluR5-7 mRNA levels using semiquantitative in situ hybridization techniques, along with neuron densities. Compared with autopsy hippocampi, HS and non-HS cases showed decreased GluR5 and GluR6 hybridization densities per CA2 and/or CA3 pyramid. Furthermore, HS patients demonstrated increased KA2 and GluR5 hybridization densities per granule cell compared with autopsy hippocampi. These findings indicate that chronic temporal lobe seizures were associated with differential changes in hippocampal KA1-2 and GluR5-7 hybridization densities that vary by subfield and pathology group. In temporal lobe epilepsy patients, these results support the hypothesis that pyramidal cell GluR5 and GluR6 mRNA levels are decreased as a consequence of seizures, and in HS patients granule cell KA2 and GluR5 mRNA levels are increased in association with aberrant fascia dentata mossy fiber sprouting and/or hippocampal neuronal loss.

Interneurones are not so dormant in temporal lobe epilepsy: a critical reappraisal of the dormant basket cell hypothesis

One axiom at the basis of epilepsy research is that there exists an imbalance between excitation and inhibition. This abnormality can be achieved by an increase of excitation on principal cells, a decreased inhibition (i.e. disinhibition) or both. This review focuses on dysfunction of inhibition, and in particular on the 'dormant basket cell hypothesis'. This hypothesis states that, (1) interneurones are functionally disconnected from excitatory afferents, resulting in hyperexcitability of principal neurones and loss of paired pulse inhibition, (2) when properly activated, interneurones can still perform their task, i.e. suppress epileptiform activity and restore paired pulse inhibition. The aim of this review is to discuss the evidence in support of the 'dormant basket cell hypothesis'. We will first discuss the rationale underlying the hypothesis and the criteria needed to validate the hypothesis. We will then show that, (1) the key experimental data offered in support of the hypothesis (Bekenstein and Lothman, 1993. Dormancy of inhibitory interneurones in a model of temporal lobe epilepsy. Science 259, 97-100; Sloviter, 1991. Permanently altered hippocampal structure, excitability, and inhibition after experimental status epilepticus in the rat: the 'dormant basket cell' hypothesis and its relevance to temporal lobe epilepsy. Hippocampus 1, 41-66) are difficult to interpret, and (2) recent recordings from interneurones in epileptic tissue argue against the hypothesis. The 'dormant basket cell hypothesis' is then discussed in the broader context of disinhibition.

Protective effects of cortistatin (CST-14) against kainate-induced neurotoxicity in rat brain

Cortistatin (CST-14) is a recently discovered endogenous peptide which shares similarity to somatostatin and binds to somatostatin receptors. In this study, we show that CST-14 exhibits anticonvulsive and neuroprotective effects in rats. Injection of rats with kainic acid (KA; 10 mg/kg; i.p.) generated a strong seizure activity which was attenuated by the i.c.v. application of 1 and 10 nmol CST-14 when given 10 min before KA. Moreover, 3 days after KA injection, a marked loss of neurons in cortex and hippocampus of rats was observed which was inhibited by pretreatment with CST-14. An immunohistochemical analysis using specific antibodies revealed that KA reduced immunoactive sst2A and sst3 somatostatin receptors in the hippocampus-an effect which was largely prevented by pretreatment with CST-14. Superfusion of hippocampal slices with CST-14 also reduced the stimulated release of 3H-d-aspartate. We conclude that CST-14 exerts neuroprotective effects by binding to somatostatin receptors which in turn leads to a reduced release of excitotoxic neurotransmitters.

Functionally distinct groups of interneurons identified during rhythmic carbachol oscillations in hippocampus in vitro

During distinct behavioral states, the hippocampus exhibits characteristic rhythmic electrical activity. Evidence in vivo suggests that both principal pyramidal cells and GABAergic interneurons participate in generating oscillations. We found that during rhythmic oscillations in area CA3, functionally distinct classes of interneurons could be identified, although all recorded interneurons had similar dendritic and axonal arbors. One group of interneurons was powerfully excited by CA3 pyramidal cells, whereas two other interneuron groups were relatively unaffected by pyramidal cell firing. One of these groups of interneurons was potently inhibited by other local interneurons during the pyramidal cell bursts. Our findings emphasize that morphologically similar cells are wired together very differently within the local circuit. The classes of hippocampal interneurons we have tentatively defined may be used during distinct behavioral states to switch the local network from one oscillatory state to another.

Calcium-dependent spike-frequency accommodation in hippocampal CA3 nonpyramidal neurons

Interneurons of the hippocampal formation are traditionally identified electrophysiologically as those cells that fire trains of weakly accommodating action potentials in response to depolarizing current injection. We studied the firing properties of nonpyramidal neurons in the five substrata of the CA3b region of hippocampus. With the use of whole cell recording methods we found that nonpyramidal neurons fired in a range from weak to strong spike-frequency accommodation (SFA) that was calcium dependent. Slow afterhyperpolarizations were not associated with strong SFA. In addition a subset of interneurons ( approximately 20%) fired with an irregular firing pattern that was generally calcium independent. These results suggest a calcium-dependent mechanism for SFA in nonpyramidal neurons that is distinct from pyramidal cells and further demonstrates the heterogeneity of hippocampal interneurons.

The dynamics of synchronized neurotransmitter release determined from compound spontaneous IPSCs in rat dentate granule neurones in vitro

1. The properties of GABAA receptor-mediated spontaneous IPSCs generated in hippocampal dentate granule neurones were analysed using whole-cell voltage-clamp techniques in order to explore the functional consequences of the low number (6-12) and close proximity of synaptic contacts made by single GABAergic interneurones. 2. Spontaneous IPSCs (sIPSCs) occurred with a frequency of 14.0 +/- 9.1 Hz (n = 31) and revealed a multi-modal positively skewed amplitude distribution (39.0 +/- 19.8 pA, median values). 3. The variance of 10-90% rise times and decay kinetics between IPSCs decreased with increasing peak amplitude. Larger amplitude events had significantly faster rise times, consistent with their site of generation being proximal to the soma. The decay kinetics of sIPSCs did not significantly change with amplitude. 4. Large amplitude sIPSCs occurred singularly or in discrete bursts, repeated regularly at low frequency. The rising phase of such sIPSCs were multi-phasic, composed of clear step-like inflections that were not a product of noise. The variability between the rising phase of individual sIPSCs was quantified by calculating their standard deviation, which produced fast rising (0.22 +/- 0.05 ms time to peak, n = 16) functions with half-widths of 0.38 +/- 0.10 ms, which declined to plateaux. 5. Computer simulations demonstrated that IPSCs with properties similar to those recorded experimentally could be generated by the linear summation of groups of temporally dispersed component events. Standard deviation functions of the rising phase of simulated IPSCs accurately described distributions of the temporal dispersion of unitary components. 6. The GABA uptake inhibitor (R)-N[4,4-bis(3-methyl-2-thienyl)but-3-enl-yl] nipecotic acid (tiagabine) (10 microM, n = 12) significantly prolonged the decay of mIPSCs (6.5 +/- 0.8 to 8.7 +/- 1.0 ms, median values) and sIPSCs (6.2 +/- 0.4 to 7.3 +/- 1.2 ms, median values), but failed to alter the frequency of occurrence, 10-90% rise times or peak amplitude of events. The application of flurazepam (30 microM, n = 7; 50 microM, n = 4) prolonged the decay of sIPSCs regardless of their amplitude. 7. These data indicate that sIPSCs are formed by the summation of unitary components that occur asynchronously and that GABA released from multiple sites has independent post-synaptic actions.

Involvement of distinct pioneer neurons in the formation of layer-specific connections in the hippocampus

During neural development, specific recognition molecules provide the cues necessary for the formation of initial projection maps, which are reshaped later in development. In some systems, guiding cues for axonal pathfinding and target selection are provided by specific cells that are present only at critical times. For instance, the floor plate guides commissural axons in the spinal cord, and the subplate is involved in the formation of thalamocortical connections. Here we study the development of entorhinal and commissural connections to the murine hippocampus, which in the adult terminate in nonoverlapping layers. We show that two groups of pioneer neurons, Cajal-Retzius cells and GABAergic neurons, form layer-specific scaffolds that overlap with distinct hippocampal afferents at embryonic and early postnatal stages. Furthermore, at postnatal day 0 (P0)-P5, before the dendrites of pyramidal neurons develop, these pioneer neurons are preferential synaptic targets for hippocampal afferents. Birthdating analysis using 5'-bromodeoxyuridine (BrdU) pulses showed that most such early-generated neurons disappear at late postnatal stages, most likely by cell death. Together with previous studies, these findings indicate that distinct pioneer neurons are involved in the guidance and targeting of different hippocampal afferents.

A model of hippocampal memory encoding and retrieval: GABAergic control of synaptic plasticity

The current view of the role of GABAergic interneurones in cortical-network function has shifted from one of merely dampening neuronal activity to that of an active role in information processing. In this review, we explore a potential role of hippocampal GABAergic interneurones in providing spatial and temporal conditions for modifications of synaptic weights during hippocampus-dependent memory processes. We argue that knowledge of spatiotemporal activity patterns in distinct classes of interneurone is essential to understanding the cellular mechanisms underlying learning and memory.

Properties of gamma-frequency oscillations initiated by propagating population bursts in retrohippocampal regions of rat brain slices

1. In the hippocampal formation in vivo, brief periods of gamma-frequency activity follow population bursts called sharp waves. The approximately 200 Hz activity of the sharp wave itself may serve to enhance synaptic connections and the approximately 40 Hz gamma activity has been offered as a mechanism for solving the 'binding' problem. We describe epochs of gamma-frequency activity which follow population spikes evoked by low frequency repetitive extracellular stimuli in retrohippocampal neurons of horizontal rat brain slices. 2. gamma-Frequency activity recorded intracellularly from deep layer neurons of entorhinal cortex, presubiculum and parasubiculum consisted of one action potential correlated with each of the three to five gamma cycles recorded with a proximate field potential electrode. A minority of cells exhibited only sub-threshold gamma-frequency membrane potential oscillations (ranging from 5 to 10 mV). No cells fired more than one spike per gamma cycle under any conditions. 3. The range of synchrony varied from individual cells which showed gamma-frequency firing without corresponding oscillations in close field recordings to field potential recordings of oscillations which were well correlated across regions. The lead or lag between any two retrohippocampal regions was in the direction of the conduction delay for the primary population spike, but typically was less, and approached zero milliseconds for some cycles in most cells. The level of synchrony was stable for particular stimulating conditions (intensity, stimulation rate, stimulus location). 4. The duration of the period of gamma activity had the duration of a slow depolarizing potential which was mediated by NMDA receptor activation. NMDA receptor antagonists or low concentrations of AMPA receptor antagonists reduced the duration of, or completely abolished the slow potential, thereby eliminating the gamma portion of the evoked response. 5. gamma-Frequency firing was eliminated by the GABAA receptor antagonist picrotoxin but small (< 5 mV) membrane potential oscillations remained after focal picrotoxin applications, and these exhibited the voltage dependence of EPSPs. Bath application of thiopental lowered the frequency of gamma oscillations, confirming the involvement of GABAA receptors. 6. The GABAB receptor antagonist 2-hydroxy-saclofen appeared to enhance the gamma activity by increasing the duration of the gamma epoch and increasing the amplitude of individual gamma cycles in field potential recordings. These saclofen-induced cycles were, however, less well synchronized across regions. 7. We show that synchronous gamma (40-100 Hz) activity follows population bursts by deep layer retrohippocampal neurons in undrugged slices from rat brain. Responses from medial entorhinal, parasubicular or presubicular cells were not distinguishable. These events can be initiated by a propagating population spike. We suggest that an NMDA receptor mediated depolarization enables the network of deep layer retrohippocampal neurons to oscillate by providing a sustained excitation, the duration of which determines the duration of the gamma episode. gamma-Frequency firing is primarily the result of GABAA receptor dependent inhibition during this period of sustained depolarization. Recurrent excitation appears to be inconsequential for principal cell firing, but may contribute to interneuron firing.

Reliability and state dependence of pyramidal cell-interneuron synapses in the hippocampus: an ensemble approach in the behaving rat

Spike transmission probability between pyramidal cells and interneurons in the CA1 pyramidal layer was investigated in the behaving rat by the simultaneous recording of neuronal ensembles. Population synchrony was strongest during sharp wave (SPW) bursts. However, the increase was three times larger for pyramidal cells than for interneurons. The contribution of single pyramidal cells to the discharge of interneurons was often large (up to 0.6 probability), as assessed by the presence of significant (<3 ms) peaks in the cross-correlogram. Complex-spike bursts were more effective than single spikes. Single cell contribution was higher between SPW bursts than during SPWs or theta activity. Hence, single pyramidal cells can reliably discharge interneurons, and the probability of spike transmission is behavior dependent.

Age-dependent local modulation of hippocampal-evoked responses to perforant path stimulation

Local modulation of hippocampal-evoked responses to perforant path stimulation was studied by leaking drugs from the recording pipette placed in the dentate gyrus of anesthetized young (3 months old), aging (17 months old) and old (28 months old) rats. In old rats, the excitatory postsynaptic potential (EPSP) slope was much reduced compared to young and aging rats. The population spike (PS) size was similar in all age groups. Bicuculline caused a marked increase in PS size relative to population EPSP, and reversed the response to the second pulse in a paired-pulse paradigm from inhibition to facilitation. The effect of bicuculline was only slightly reduced in old rats. The 5-HT1a agonist 8-OH-DPAT potentiated PSs in the dentate gyrus, while not affecting paired-pulse inhibition. The effect of 8-OH-DPAT was slightly reduced in old rats. Carbachol, a cholinergic agonist, reversed paired-pulse inhibition into facilitation in the young brain, but not in aging and old rats. These results demonstrate that age affects differentially the action of biogenic amines on hippocampal reactivity to afferent stimulation.

Development of a transient increase in recurrent inhibition and paired-pulse facilitation in hippocampal CA1 region

Paired-pulse recurrent inhibition (RI) of population spike (PS) and facilitation (PPF) of field excitatory postsynaptic potential (EPSP) were studied in the CA1 region of hippocampal slices taken from Wistar rats aged from 9 days to 16 months. The comparison of three different paired-pulse protocols revealed the antidromic-orthodromic (A-O) stimulation as the most reliable in quantifying the strength of fast (peaking at 10 ms) and slow (peaking at 200 ms) components of recurrent inhibition. Fast RI, present but weak at 9 days, progressively increased to reach its maximal strength at 30 days, declining in adult (2 m) and middle-aged (16 m) animals. Slow RI was replaced by facilitation at 9 days while it was absent at 15 days. It reached adult values at 30 days. A reduction of the test response at interpulse interval (IPI) of 2-4 ms was strong in developing and adult animals, but was significantly decreased in 16 m. At maximal stimulation PPF was expressed as an enhancement of the slow rather than the fast phase of the EPSP and was particularly strong with a prominent N-methyl-D-aspartate dependent component. A very characteristic selectivity for a prominent PPF at stimulation frequency of 5 Hz appeared first at the 18th day and increased gradually to reach a maximum at the 30th day, after which it declined to very low values in middle-aged animals. A similar developmental pattern was observed in slices taken from rats reared in complete darkness, suggesting a strong innate origin. The ability of hippocampal circuits for plastic gating of information appears to be transiently enhanced at the completion of the first postnatal month as it can be exercised at a wider part of the frequency spectrum, with maximal inhibition and potentiation especially at the frequency of theta rhythm.

Differentially interconnected networks of GABAergic interneurons in the visual cortex of the cat

Networks of GABAergic neurons have been implicated in neuronal population synchronization. To define the extent of cellular interconnections, we determined the effect, number, and subcellular distribution of synapses between putative GABAergic neurons in layers II-IV of the cat visual cortex using paired intracellular recordings in vitro followed by correlated light and electron microscopy. All neurons having interneuronal electrophysiological properties were classified by their postsynaptic target profile and were identified as basket (BC; n = 6), dendrite-targeting (DTC; n = 1), and double bouquet (DBC; n = 2) cells. In four out of five anatomically fully recovered and reconstructed cell pairs, synaptic connections were found to be reciprocal. Generally BCs established synaptic junctions closer (21 +/- 20 micron) to postsynaptic somata than did DBCs (43 +/- 19 micron; p < 0.01). The unitary number of synapses (n values, 10, 7, and 20) in each of three BC-to-BC pairs was higher than that in three BC-to-DBC (n values, 1, 2, and 2) and three DBC-to-BC (n values, 1, 4, and 4) connections (p < 0.05). A BC innervated a DTC through two synaptic junctions. Unitary postsynaptic effects mediated by five BCs could be recorded in two BCs, two DBCs, and a DTC. The BCs elicited short-duration fast IPSPs, similar to those mediated by GABAA receptors. At a membrane potential of -55.0 +/- 6.4 mV, unitary IPSPs (n = 5) had a mean amplitude of 919 +/- 863 microV. Postsynaptic response failures were absent when an IPSP was mediated by several release sites. Thus, distinct GABAergic interneurons form reciprocally interconnected networks. The strength of innervation and the proximal placement of synapses suggest a prominent role for BCs in governing the activity of intracortical GABAergic networks in layers II-IV.

Synaptic connectivity of distinct hilar interneuron subpopulations

Dual intracellular recordings of hilar interneurons and CA3 pyramidal cells were performed in transverse slices of guinea pig hippocampus in the presence of the convulsant compound 4-aminopyridine (4-AP) and ionotropic glutamate receptor antagonists. Under these conditions, interneurons burst fire synchronously, producing synchronized inhibitory postsynaptic potentials (sIPSPs) in pyramidal cells. Three different hilar interneuron subpopulations that contributed to the sIPSP were identified based on their projection properties and morphology. These three types were pyramidal-like stellate interneurons, spheroid interneurons, and oviform interneurons. Physiologically, pyramidal-like stellate interneurons could be differentiated from the other interneuron subpopulations because they generated short synchronized bursts of action potentials coincident with the hyperpolarizing and depolarizing gamma-aminobutyric acid-A (GABAA)-mediated inhibitory postsynaptic potentials (IPSPs) recorded in pyramidal cells. The bursts in pyramidal-like stellate cells were abolished by theGABAA-receptor blocker, bicuculline. In contrast, spheroid interneurons of the dentate-hilus (D-H) border and oviform hilar interneurons exhibited prolonged bicuculline-resistant bursts that occurred coincident with the GABAB pyramidal cell sIPSPs. Pyramidal-like stellate interneurons likely did not contribute to the generation of synchronized GABAB responses in hippocampal pyramidal cells. Spheroid interneurons were unique among these subpopulations of interneurons in that the bicuculline-resistant bursts in spheroid interneurons were sustained by a synaptic depolarization that persisted in the presence of antagonists of ionotropic glutamate, GABAA and GABAB receptors [6-cyano-7-nitroquinoxaline-2,3-dione, 20 microM; 3-3(2-carboxipiperazine-4-yl)propyl-1-phosphonate, 20 microM; bicuculline, 10-15 microM; CGP 55845A, 20 microM]. This novel depolarizing potential reversed between -30 and 0 mV. No noticeable synaptic depolarization sustaining burst firing could be isolated in oviform interneurons, suggesting that firing in this interneuron subpopulation was synchronized by nonsynaptic mechanisms. The results of the present study indicate that the hilar inhibitory circuit is composed of at least three different subpopulations of interneurons, distinguishable by their morphological characteristics and synaptic inputs and outputs. These findings give further support to the hypothesis that there are distinct populations of interneurons producing GABAA and GABAB responses with defined functional roles within the hippocampal inhibitory circuit. Notably, we found that spheroid interneurons were unique among the hilar interneurons studied, in that the synchronized bursts observed in these cells are sustained by a novel ionotropic glutamate and GABA receptor-independent synaptic depolarization.

Phasic boosting of medial perforant path-evoked granule cell output time-locked to spontaneous dentate EEG spikes in awake rats

Dentate spikes (DSs) are positive-going field potential transients that occur intermittently in the hilar region of the dentate gyrus during alert wakefulness and slow-wave sleep. The function of dentate spikes is unknown; they have been suggested to be triggered by perforant path input and are associated with firing of hilar interneurons and inhibition of CA3 pyramidal cells. Here we investigated the effect of DSs on medial perforant path (MPP)-granule cell-evoked transmission in freely moving rats. The MPP was stimulated selectively in the angular bundle while evoked field potentials and the EEG were recorded with a vertical multielectrode array in the dentate gyrus. DSs were identified readily on the basis of their characteristic voltage-versus-depth profile, amplitude, duration, and state dependency. Using on-line detection of the DS peak, the timing of MPP stimulation relative to single DSs was controlled. DS-triggered evoked responses were compared with conventional, manually evoked responses in still-alert wakefulness (awake immobility) and, in some cases, slow-wave sleep. Input-output curves were obtained with stimulation on the positive DS peak (0 delay) and at delays of 50, 100, and 500 ms. Stimulation on the peak DS was associated with a significant increase in the population spike amplitude, a reduction in population spike latency, and a decrease in the field excitatory postsynaptic potential (fEPSP) slope, relative to manual stimulation. Granule cell excitability was enhanced markedly during DSs, as indicated by a mean 93% increase in the population spike amplitude and a leftward shift in the fEPSP-spike relation. Maximum effects occurred at the DS peak, and lasted between 50 and 100 ms. Paired-pulse inhibition of the population spike was unaffected, indicating intact recurrent inhibition during DSs. The results demonstrate enhancement of perforant path-evoked granule cell output time-locked to DSs. DSs therefore may function to intermittently boost excitatory transmission in the entorhinal cortex-dentate gyrus-CA3 circuit. Such a mechanism may be important in the natural induction of long-term potentiation in the dentate gyrus and CA3 regions.

Neocortex in the hippocampus: an anatomical and functional study of CA1 heterotopias after prenatal treatment with methylazoxymethanol in rats

Migration disorders cause neurons to differentiate in an abnormal heterotopic position. Although significant insights have been gained into the etiology of these disorders, very little is known about the anatomy of heterotopias. We have studied heterotopic masses arising in the hippocampal CA1 region after prenatal treatment with methylazoxymethanol (MAM) in rats. Heterotopic cells were phenotypically similar to neocortical supragranular neurons and exhibited the same temporal profile of migration and neurogenesis. However, they did not express molecules characteristic of CA1 neurons such as the limbic-associated membrane protein. Horseradish peroxidase injections in heterotopia demonstrated labeled fibers not only in the neocortex and white matter but also in the CA1 stratum radiatum and stratum lacunosum. To study the pathophysiological consequences of this connectivity, we compared the effects of neocortical and limbic seizures on the expression of Fos protein and on cell death in MAM animals. After metrazol-induced seizures, Fos-positive cells were present in CA1 heterotopias, the only hippocampal region to be activated with the neocortex. By contrast, kainic acid-induced seizures caused a prominent delayed cell death in limbic regions and in CA1 heterotopias. Together, these results suggest that neocortical heterotopias in the CA1 region are integrated in both the hippocampal and neocortical circuitry.

Sustained long term potentiation and anxiety in mice lacking the Mas protooncogene

The Mas protooncogene is a maternally imprinted gene encoding an orphan G protein-coupled receptor expressed mainly in forebrain and testis. Here, we provide evidence for a function of Mas in the central nervous system. Targeted disruption of the Mas protooncogene leads to an increased durability of long term potentiation in the dentate gyrus, without affecting hippocampal morphology, basal synaptic transmission, and presynaptic function. In addition, Mas-/- mice show alterations in the onset of depotentiation. The permissive influence of Mas ablation on hippocampal synaptic plasticity is paralleled by behavioral changes. While spatial learning in the Morris water maze is not significantly influenced, Mas-deficient animals display an increased anxiety as assessed in the elevated-plus maze. Thus, Mas is an important modulating factor in the electrophysiology of the hippocampus and is involved in behavioral pathways in the adult brain.

GABA(B) modulation improves sequence disambiguation in computational models of hippocampal region CA3

Computational models of hippocampal region CA3 were used to study the role of theta rhythm in storage and retrieval of temporal sequences of neuronal activity patterns. Retrieval of multiple overlapping temporal sequences requires a mechanism for disambiguation, e.g., for choosing between two sequences with the same starting pattern but different final patterns (forked sequences). Modulatory input to the hippocampus from the medial septum may enhance the disambiguation of pattern sequences by causing phasic changes in the relative strength of afferent input and recurrent excitation. In the models, the strength of recurrent synaptic transmission is modulated by activation of GABA(B) receptors. Theta frequency inputs from the medial septum cause oscillations in the levels of GABA in the model, producing phasic changes in the strength of synaptic potentials during a theta cycle similar to those observed experimentally (Wyble et al., Soc Neurosci Abstr 1997;23: 197.7). These phasic changes in GABA(B) suppression improve sequence disambiguation in the simulations, as previously shown with analysis of a simpler model (Sohal and Hasselmo, Neural Comp 1998;10:889-902). In addition, tonic changes in levels of cholinergic modulation enhance the storage of forked sequences by preventing a strong influence of recurrent synapses during storage.

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.

The pro-convulsant actions of corticotropin-releasing hormone in the hippocampus of infant rats

Whole-cell patch-clamp and extracellular field recordings were obtained from 450-microns-thick brain slices of infant rats (10-13 days postnatal) to determine the actions of corticotropin-releasing hormone on glutamate- and GABA-mediated synaptic transmission in the hippocampus. Synthetic corticotropin-releasing hormone (0.15 microM) reversibly increased the excitability of hippocampal pyramidal cells, as determined by the increase in the amplitude of the CA1 population spikes evoked by stimulation of the Schaffer collateral pathway. This increase in population spike amplitude could be prevented by the corticotropin-releasing hormone receptor antagonist alpha-helical (9-41)-corticotropin-releasing hormone (10 microM). Whole-cell patch-clamp recordings revealed that, in the presence of blockers of fast excitatory and inhibitory synaptic transmission, corticotropin-releasing hormone caused only a small (1-2 mV) depolarization of the resting membrane potential in CA3 pyramidal cells, and it did not significantly alter the input resistance. However, corticotropin-releasing hormone, in addition to decreasing the slow afterhyperpolarization, caused an increase in the number of action potentials per burst evoked by depolarizing current pulses. Corticotropin-releasing hormone did not significantly change the frequency, amplitude or kinetics of miniature excitatory postsynaptic currents. However, it increased the frequency of the spontaneous excitatory postsynaptic currents in CA3 pyramidal cells, without altering their amplitude and single exponential rise and decay time constants. Corticotropin-releasing hormone did not change the amplitude of the pharmacologically isolated (i.e. recorded in the presence of GABAA receptor antagonist bicuculline) excitatory postsynaptic currents in CA3 and CA1 pyramidal cells evoked by stimulation of the mossy fibers and the Schaffer collaterals, respectively. Current-clamp recordings in bicuculline-containing medium showed that, in the presence of corticotropin-releasing hormone, mossy fiber stimulation leads to large, synchronized, polysynaptically-evoked bursts of action potentials in CA3 pyramidal cells. In addition, the peptide caused a small, reversible decrease in the amplitude of the pharmacologically isolated (i.e. recorded in the presence of glutamate receptor antagonists) evoked inhibitory postsynaptic currents in CA3 pyramidal cells, but it did not significantly alter the frequency, amplitude, rise and decay time constants of spontaneous or miniature inhibitory postsynaptic currents. These data demonstrate that corticotropin-releasing hormone, an endogenous neuropeptide whose intracerebroventricular infusion results in seizure activity in immature rats, has diverse effects in the hippocampus which may contribute to epileptogenesis. It is proposed that the net effect of corticotropin-releasing hormone is a preferential amplification of those incoming excitatory signals which are strong enough to reach firing threshold in at least a subpopulation of CA3 cells. These findings suggest that the actions of corticotropin-releasing hormone on neuronal excitability in the immature hippocampus may play a role in human developmental epilepsies.

Immunocytochemical Mapping of an RDL-Like GABA Receptor Subunit and of GABA in Brain Structures Related to Learning and Memory in the Cricket Acheta domesticus

The distribution of putative RDL-like GABA receptors and of γ-aminobutyric acid (GABA) in the brain of the adult house cricket Acheta domesticus was studied using specific antisera. Special attention was given to brain structures known to be related to learning and memory. The main immunostaining for the RDL-like GABA receptor was observed in mushroom bodies, in particular the upper part of mushroom body peduncle and the two arms of the posterior calyx. Weaker immunostaining was detected in the distal part of the peduncle and in the α and β lobes. The dorso- and ventrolateral protocerebrum neuropils appeared rich in RDL-like GABA receptors. Staining was also detected in the glomeruli of the antennal lobe, as well as in the ellipsoid body of the central complex. Many neurons clustered in groups exhibit GABA-like immunoreactivity. Tracts that were strongly immunostained innervated both the calyces and the lobes of mushroom bodies. The glomeruli of the antennal lobe, the ellipsoid body, as well as neuropils of the dorso- and ventrolateral protocerebrum were also rich in GABA-like immuno- reactivity. The data demonstrated a good correlation between the distribution of the GABA-like and of the RDL-like GABA receptor immunoreactivity. The prominent distribution of RDL-like GABA receptor subunits, in particular areas of mushroom bodies and antennal lobes, underlines the importance of inhibitory signals in information processing in these major integrative centers of the insect brain.

The K+ channel, Kv2.1, is apposed to astrocytic processes and is associated with inhibitory postsynaptic membranes in hippocampal and cortical principal neurons and inhibitory interneurons

A variety of voltage-gated ion channels are expressed on principal cell dendrites and have been proposed to play a pivotal role in the regulation of dendritic excitability. Previous studies at the light microscopic level demonstrated that the K+ channel subunit Kv2.1 expression was polarized to the cell soma and dendrites of principal neurons throughout the central nervous system. Here, using double immunostaining we now show that Kv2.1 protein is similarly expressed in the majority of cortical and hippocampal parvalbumin, calbindin and somatostatin-containing inhibitory interneurons. At the electron microscopic level Kv2.1 immunoreactivity was primarily observed on the plasma membrane of the somata and proximal dendrites of both principal neurons and inhibitory interneurons; expression was low on smaller dendritic branches, and absent on axons and presynaptic terminals. Kv2.1 subunit expression was highly concentrated on the cell surface membrane immediately facing astrocytic processes. Kv2.1 expression was also concentrated in specific cytoplasmic compartments and on the subsurface cisterns underlying the plasma membrane facing astrocytes. In addition, Kv2.1 subunit immunoreactivity was associated with postsynaptic densities of a fraction of inhibitory symmetric synapses; while expression at asymmetric synapses was rare. These data demonstrate that channels formed by Kv2.1 subunits are uniquely positioned on the soma and principal dendrites of both pyramidal cells and inhibitory interneurons at sites immediately adjacent to astrocytic processes. This close apposition to astrocytes will ensure a rapid removal and limit the influence of K+ released into the extracellular space. This expression pattern suggests that channels formed by Kv2.1 are poised to provide a role in the regulation of neuronal dendritic excitability.

Salient features of synaptic organisation in the cerebral cortex

The neuronal and synaptic organisation of the cerebral cortex appears exceedingly complex, and the definition of a basic cortical circuit in terms of defined classes of cells and connections is necessary to facilitate progress of its analysis. During the last two decades quantitative studies of the synaptic connectivity of identified cortical neurones and their molecular dissection revealed a number of general rules that apply to all areas of cortex. In this review, first the precise location of postsynaptic GABA and glutamate receptors is examined at cortical synapses, in order to define the site of synaptic interactions. It is argued that, due to the exclusion of G protein-coupled receptors from the postsynaptic density, the presence of extrasynaptic receptors and the molecular compartmentalisation of the postsynaptic membrane, the synapse should include membrane areas beyond the membrane specialisation. Subsequently, the following organisational principles are examined: 1. The cerebral cortex consists of: (i) a large population of principal neurones reciprocally connected to the thalamus and to each other via axon collaterals releasing excitatory amino acids, and, (ii) a smaller population of mainly local circuit GABAergic neurones. 2. Differential reciprocal connections are also formed amongst GABAergic neurones. 3. All extrinsic and intracortical glutamatergic pathways terminate on both the principal and the GABAergic neurones, differentially weighted according to the pathway. 4. Synapses of multiple sets of glutamatergic and GABAergic afferents subdivide the surface of cortical neurones and are often co-aligned on the dendritic domain. 5. A unique feature of the cortex is the GABAergic axo-axonic cell, influencing principal cells through GABAA receptors at synapses located exclusively on the axon initial segment. The analysis of these salient features of connectivity has revealed a remarkably selective array of connections, yet a highly adaptable design of the basic circuit emerges when comparisons are made between cortical areas or layers. The basic circuit is most obvious in the hippocampus where a relatively homogeneous set of spatially aligned principal cells allows an easy visualization of the organisational rules. Those principles which have been examined in the isocortex proved to be identical or very similar. In the isocortex, the basic circuit, scaled to specific requirements, is repeated in each layer. As multiple sets of output neurones evolved, requiring subtly different needs for their inputs, the basic circuit may be superimposed several times in the same layer. Tangential intralaminar connections in both the hippocampus and isocortex also connect output neurones with similar properties, as best seen in the patchy connections in the isocortex. The additional radial superposition of several laminae of distinct sets of output neurones, each representing and supported by its basic circuit, requires a co-ordination of their activity that is mediated by highly selective interlaminar connections, involving both the GABAergic and the excitatory amino acid releasing neurones. The remarkable specificity in the geometry of cells and the selectivity in placement of neurotransmitter receptors and synapses on their surface, strongly suggest a predominant role for time in the coding of information, but this does not exclude an important role also for the rate of action potential discharge in cortical representation of information.

Enkephalin-containing interneurons are specialized to innervate other interneurons in the hippocampal CA1 region of the rat and guinea-pig

Enkephalins are known to have a profound effect on hippocampal inhibition, but the possible endogenous source of these neuropeptides, and their relationship to inhibitory interneurons is still to be identified. In the present study we analysed the morphological characteristics of met-enkephalin-immunoreactive cells in the CA1 region of the rat and guinea-pig hippocampus, their coexistence with other neuronal markers and their target selectivity at the light and electron microscopic levels. Several interneurons in all subfields of the hippocampus were found to be immunoreactive for met-enkephalin. In the guinea-pig, fibres arising from immunoreactive interneurons were seen to form a plexus in the stratum oriens/alveus border zone, and basket-like arrays of boutons on both enkephalin-immunoreactive and immunonegative cell bodies in all strata. Immunoreactive boutons always established symmetric synaptic contacts on somata and dendritic shafts. Enkephalin-immunoreactive cells co-localized GABA, vasoactive intestinal polypeptide and calretinin. Postembedding immunogold staining for GABA showed that all the analysed enkephalin-immunoreactive boutons contacted GABAergic postsynaptic structures. In double-immunostained sections, enkephalin-positive axons were seen to innervate calbindin D28k-, somatostatin-, calretinin- and vasoactive intestinal polypeptideimmunoreactive cells with multiple contacts. Based on these characteristics, enkephalin-containing cells in the hippocampus are classified as interneurons specialized to innervate other interneurons, and represent a subset of vasoactive intestinal polypeptide- and calretinin-containing cells. The striking match of ligand and receptor distribution in the case of enkephalin-mediated interneuronal communication suggests that this neuropeptide may play an important role in the synchronization and timing of inhibition involved in rhythmic network activities of the hippocampus.

Dentate gyrus basket cell GABAA receptors are blocked by Zn2+ via changes of their desensitization kinetics: an in situ patch-clamp and single-cell PCR study

Although GABA type A receptors (GABAARs) in principal cells have been studied in detail, there is only limited information about GABAARs in interneurons. We have used the patch-clamp technique in acute rat hippocampal slices in combination with single-cell PCR to determine kinetic, pharmacological, and structural properties of dentate gyrus basket cell GABAARs. Application of 1 mM GABA (100 msec) to nucleated patches via a piezo-driven fast application device resulted in a current with a fast rise and a marked biexponential decay (time constants 2.4 and 61.8 msec). This decay could be attributed to strong receptor desensitization. Dose-response curves for the peak and the slow component yielded EC50 values of 139 and 24 microM, respectively. Zn2+ caused a marked blocking effect on both the peak and the slow component via a noncompetitive mechanism (IC50 values of 8 and 16 microM). This led to an acceleration of the slow component as well as a prolongation of recovery from desensitization. Zn2+ sensitivity was suggested to depend on the absence of gamma-subunits in GABAARs. To test this hypothesis we performed single-cell reverse transcription PCR that revealed primarily the presence of alpha2-, beta2-, beta3-, gamma1-, and gamma2-subunit mRNAs. In addition, flunitrazepam increased the receptor affinity for its agonist, indicating the presence of functional benzodiazepine binding sites, i.e., gamma-subunits. Thus, additional factors seem to co-determine the Zn2+ sensitivity of native GABAARs. The modulatory effects of Zn2+ on GABAAR desensitization suggest direct influences on synaptic integration via changes in inhibition and shunting at GABAergic synapses.

Induction of calcitonin gene-related peptide-like immunoreactivity in hippocampal neurons following ischemia: a putative regional modulator of the CNS injury/immune response

Calcitonin gene-related peptide (CGRP) is a potent vasodilator and immune cell modulator. In two studies within the hippocampal formation (HF), CGRP-like immunoreactivity (CGRP-LI) was increased in the inner molecular layer of the dentate gyrus after adrenalectomy and in mossy cells after colchicine-induced destruction of granule neurons. Given the increase in CGRP-LI following damage to the granule cell region of the HF, we investigated another trauma model, ischemia, that targeted different areas of the HF, CA1 region, and subiculum to ascertain the regional expression of this peptide after insult. Following ischemia, light microscopic evaluation showed CGRP-LI in basket cell-like neuronal perikarya within the dorsal subiculum and CA1 region of the hippocampus and in varicose fibers within the CA2 region of the hippocampus. Control rats rarely expressed CGRP-LI within neurons in these regions. In ischemic brains, double-labeled immunocytochemistry with antibodies to various neural markers demonstrated co-localization of CGRP-LI primarily within surviving subicular and CA1 cells resembling interneurons containing parvalbumin-LI or calbindin-LI. Electron microscopic analysis of the CA1 region from ischemic brains showed that CGRP-LI was contained in terminals with numerous small synaptic vesicles that formed symmetric synapses with perikarya and large dendrites of pyramidal cells, some of which were degenerating. Collectively, the data from this study and our previous study indicate that damage induces CGRP-LI expression in interneurons and nonprincipal cells in the area of damage, and we hypothesize that CGRP expression in surviving neurons within damage-related regions of the hippocampus is likely to be an important, and possibly a protective, component of the response of the nervous system to injury.

Pro-epileptic changes in synaptic function can be accompanied by pro-epileptic changes in neuronal excitability

Repetitive sensory input, stroboscopic lights or repeated sounds can induce epileptic seizures in susceptible individuals. In order to understand the process we have to consider multiple factors. The output of a set of neurones is determined by the amount of excitatory synaptic input, the degree of positive feedback and their inherent electrical excitability, which can be modified by synaptic inhibition. Recent research has shown that it is possible to separate these phenomena, and that they do not always behave in unison.

The absence of a major Ca2+ signaling pathway in GABAergic neurons of the hippocampus

The Ca2+/calmodulin-dependent protein phosphatase 2B or calcineurin (CN) participates in several Ca2+-dependent signal transduction cascades and, thus, contributes to the short and long term regulation of neuronal excitability. By using a specific antibody to CN, we demonstrate its absence from hippocampal interneurons and illustrate a physiological consequence of such CN deficiency. Consistent with the lack of CN in interneurons as detected by immunocytochemistry, the CN inhibitors FK-506 or okadaic acid significantly prolonged N-methyl-D-aspartate channel openings recorded in the cell-attached mode in hippocampal principal cells but not those recorded in interneurons. Interneurons were also devoid of Ca2+/calmodulin-dependent protein kinase IIalpha, yet many of their nuclei contained the cyclic AMP-responsive element binding protein. On the basis of the CN and Ca2+/calmodulin-dependent protein kinase IIalpha deficiency of interneurons, entirely different biochemical mechanisms are expected to govern Ca2+-dependent neuronal plasticity in interneurons versus principal cells.