Excitatory inputs to CA1 interneurons show selective synaptic dynamics

The dynamic properties of synapses between neurons in the hippocampal CA1 area are important for the frequency-dependent signal transfer of the network. We have examined the synaptic dynamics of excitatory inputs to CA1 interneurons and pyramidal cells using whole cell voltage-clamp recordings. The CA1 network was activated using extracellular stimulation electrodes at the Schaffer collaterals (feedforward activation) or at the Alveus (activation of the feedback loop). The dynamic properties of input from the Schaffer collaterals to CA1 interneurons (basket and bistratified cells) were different from the synaptic dynamics of input from the Alveus. Synaptic input from the Schaffer collaterals to CA1 interneurons showed facilitation for most frequencies. After 10 stimuli the synaptic response reached a plateau level that was approximately 150% of the first response in the train. In contrast, the plateau levels of Alveus inputs to interneurons were not different from the first responses for frequencies 相似文献   

Mu-opioid receptors facilitate the propagation of excitatory activity in rat hippocampal area CA1 by disinhibition of all anatomical layers

Hippocampal mu-opioid receptors (MORs) have been implicated in memory formation associated with opiate drug abuse. MORs modulate hippocampal synaptic plasticity acutely, when chronically activated, and during drug withdrawal. At the network level, MORs increase excitability in area CA1 by disinhibiting pyramidal cells. The precise inhibitory interneuron subtypes affected by MOR activation are unknown; however, not all subtypes are inhibited, and specific interneuron subtypes have been shown to preferentially express MORs. Here we investigate, using voltage-sensitive dye imaging in brain slices, the effect of MOR activation on the patterns of inhibition and on the propagation of excitatory activity in rat hippocampal CA1. MOR activation augments excitatory activity evoked by stimulating inputs in stratum oriens [i.e., Schaffer collateral and commissural pathway (SCC) and antidromic], stratum radiatum (i.e., SCC), and stratum lacunosum-moleculare (SLM; i.e., perforant path and thalamus). The augmented excitatory activity is further facilitated as it propagates through the CA1 network. This was observed as a proportionately larger increase in amplitudes of excitatory activity at sites distal from where the activity was evoked. This facilitation was observed for excitatory activity propagating from all three stimulation sites. The augmentation and facilitation were prevented by GABAA receptor antagonists (bicuculline, 30 microM), but not by GABAB receptor antagonists (CGP 55845, 10 microM). Furthermore, MOR activation inhibited IPSPs in all layers of area CA1. These findings suggest that MOR-induced suppression of GABA release onto GABAA receptors augments all inputs to CA1 pyramidal cells and facilitates the propagation of excitatory activity through the network of area CA1.     

Estrogen up-regulates estrogen receptor alpha and synaptophysin in slice cultures of rat hippocampus

Previous studies have shown that estrogen application increases the density of synaptic input and the number of spines on CA1 pyramidal neurons. Here, we have investigated whether Schaffer collaterals to CA1 pyramidal cells are involved in this estrogen-induced synaptogenesis on CA1 pyramidal neurons. To this end, we studied estrogen-induced expression of both estrogen receptor (ER) subtypes (ERalpha and ERbeta) together with the presynaptic marker synaptophysin in the rat hippocampus. In tissue sections as well as in slice cultures mRNA expression of ERalpha, ERbeta and synaptophysin was higher in CA3 than in CA1, and mRNA expression and immunoreactivity for both ER subtypes were found in both principal cells and interneurons. By using quantitative image analysis we found stronger nuclear immunoreactivity for ERalpha in CA3 than in CA1. In slice cultures, supplementation of the medium with 10(-8) M estradiol led to an increase of nuclear immunoreactivity for ERalpha, but not for ERbeta, which was accompanied by a dramatic up-regulation of synaptophysin immunoreactivity in stratum radiatum of CA1. Together these findings indicate that estrogen effects on hippocampal neurons are more pronounced in CA3 than in CA1 and that ER activation in CA3 neurons leads to an up-regulation of a presynaptic marker protein in the axons of these cells, the Schaffer collaterals. We conclude that estradiol-induced spine formation on CA1 pyramidal cells may be mediated presynaptically, very likely by activation of ERalpha in CA3 pyramidal cells, followed by an increase in Schaffer collateral synapses.     

A non-alpha7 nicotinic acetylcholine receptor modulates excitatory input to hippocampal CA1 interneurons

The nicotinic acetylcholine receptor (nAChR), particularly the alpha7 subtype, has received profound attention for its role in modifying excitatory postsynaptic currents (EPSCs) in hippocampal pyramidal neurons as well as in neurons from other brain regions. Here, we tested the possibility that an nAChR could affect EPSCs in the interneurons of rat hippocampal slices. Using whole-cell patch-clamp technique on CA1 stratum radiatum interneurons and U-tube application of agents, we show that nicotinic agonists enhance EPSC frequency in interneurons. Among the agents tested, cytisine and mecamylamine were the most effective agonist and antagonist, respectively, suggesting a role for alpha3beta4-containing nAChRs in the modulation of interneuron EPSCs. Ligands selective for the alpha7 nAChR had very little or no effect on interneuron EPSCs. Low concentrations of nicotine also enhanced EPSC frequency, implicating the involvement of non-alpha7 nAChRs in controlling interneuron excitability in smokers. We conclude that nAChR-dependent EPSC modulation in the hippocampus is both subtype- and neuron-specific and that a non-alpha7 nAChR, presumably alpha3beta4, controls glutamate transmission to CA1 interneurons.     

Long-term change in synaptic transmission in CA3 circuits followed by spontaneous rhythmic activity in rat hippocampal slices

The relevance of long-term potentiation (LTP) at excitatory synapses in CA3 circuits to generation of spontaneous epileptiform bursts in CA3 was investigated using rat hippocampal slices. CA3 pyramidal cells were antidromically stimulated through Schaffer collaterals. Evoked field potentials were extracellularly recorded from the stratum pyramidale and the stratum radiatum in CA3. Therefore, field potentials reflecting recurrent excitatory post-synaptic potentials (EPSPs) and inhibitory post-synaptic potentials (IPSPs) were positive at the stratum pyramidale and negative at the stratum radiatum. First, we tested how the amplitude of the evoked field potentials depends on a γ-aminobutyric acid (GABA_A) antagonist. Both of the positive and negative field potential peaks reduced in the medium containing penicillin (2 mM) or bicuculline (20 μM). This suggests that unmasked EPSPs due to suppression of IPSPs do not result in an increase in the evoked potentials. Second, CA3 pyramidal cells were antidromically stimulated by tetanic stimulation of Schaffer collaterals in order to induce LTP at synapses in CA3 circuits. Both of the positive and negative field potentials increased, suggesting that recurrent EPSPs were enhanced by tetanic stimulation. Induction of LTP at recurrent excitatory synapses was followed by spontaneous epileptiform bursts which persisted throughout experiments (1.5 h), while LTP of afferent synaptic potential evoked by hilar test stimulation was not induced. These results suggest that LTP at the afferent synapses is not necessary to spontaneous epileptiform bursts in CA3, but LTP at excitatory synapses between CA3 pyramidal cells contribute to spontaneous epileptiform bursts.     

Layer-specific processing of excitatory signals in CA1 interneurons depends on postsynaptic M₂ muscarinic receptors

The hippocampus receives a diffuse cholinergic innervation which acts on pre- and postsynaptic sites to modulate neurotransmission and excitability of pyramidal cells and interneurons in an intricate fashion. As one missing piece in this puzzle, we explored how muscarinic receptor activation modulates the somatodendritic processing of glutamatergic input in CA1 interneurons. We performed whole-cell recordings from visually identified interneurons of stratum radiatum (SR) and stratum oriens (SO) and examined the effects of the cholinergic agonist carbachol (CCh) on EPSP-like waveforms evoked by brief glutamate pulses onto their proximal dendrites. In SO interneurons, CCh consistently reduced glutamate-induced postsynaptic potentials (GPSPs) in control rat and mice, but not in M₂ muscarinic receptor knockout mice. By contrast, the overwhelming majority of interneurons recorded in SR of control and M₂ receptor-deficient hippocampi exhibited muscarinic enhancement of GPSPs. Interestingly, the non-responding interneurons were strictly confined to the SR subfield closest to the subiculum. Our data suggest that postsynaptic modulation by acetylcholine of excitatory input onto CA1 interneurons occurs in a stratum-specific fashion, which is determined by the absence or presence of M₂ receptors in their (somato-)dendritic compartments. Thus cholinergic projections might be capable of recalibrating synaptic weights in different inhibitory circuits of the CA1 region.     

Electrophysiological and pharmacological characterization of perforant path synapses in CA1: mediation by glutamate receptors.

1. With the use of hippocampal slices from adult rats, we studied monosynaptic potentials in CA1 evoked by stimulating either the perforant pathway or the Schaffer collaterals. Excision of region CA3 and the dentate gyrus prevented polysynaptic excitation of CA1 and facilitated interpretation of the extracellular potentials. 2. Laminar profiles distinguished the population excitatory postsynaptic potentials (pEPSPs) in CA1 evoked by stimulating the Schaffer collaterals and the perforant path. Stimulating the perforant path evoked short-latency negative-going pEPSPs in s. lacunosum-moleculare of CA1 and positive-going pEPSPs in s. radiatum. Stimulating the Schaffer collaterals evoked negative-going pEPSPs in s. radiatum. 3. A pharmacological manipulation also distinguished the two pathways in CA1. The selective GABAB agonist baclofen greatly decreased the slope of pEPSPs evoked by stimulating the Schaffer collaterals but did not decrease the slope of pEPSPs evoked by stimulating the perforant path. 4. Combined bath application of the glutamate receptor antagonists 6,7-dinitroquinoxaline-2,3-dione (DNQX) and 2-amino-5-phosphonopentanoic acid (APV) abolished the negative-going pEPSPs in s. lacunosum-moleculare evoked by stimulating the perforant pathway. This application of DNQX and APV revealed a positive-going field potential that was blocked by bath application of the gamma-aminobutyric acid (GABA) receptor antagonists picrotoxin or bicuculline. 5. Although the glutamate-mediated component of the response evoked by stimulating the perforant path was apparently excitatory, we never observed a population spike in s. pyramidal evoked by stimulating the perforant path.(ABSTRACT TRUNCATED AT 250 WORDS)     

Lesions to Schaffer collaterals prevent ischemic death of CA1 pyramidal cells

The contribution of excitatory inputs to CA1 pyramidal cell death after ischemia was examined using rats with unilateral destruction of CA3 pyramidal cells. Intracerebroventricular injection of L-alpha-kainic acid (KA) was performed before the induction of transient forebrain ischemia. Five days after ischemic insult, pyramidal cells and L-glutamate binding sites in the CA1 region ipsilateral to the KA injection were preserved in spite of neuronal necrosis and a significant decrease in L-glutamate receptor density in the contralateral CA1 region, indicating the critical role of Schaffer collaterals in delayed neuronal death.     

Patterned activity in stratum lacunosum moleculare inhibits CA1 pyramidal neuron firing

CA1 pyramidal cells are the primary output neurons of the hippocampus, carrying information about the result of hippocampal network processing to the subiculum and entorhinal cortex (EC) and thence out to the rest of the brain. The primary excitatory drive to the CA1 pyramidal cells comes via the Schaffer collateral (SC) projection from area CA3. There is also a direct projection from EC to stratum lacunosum-moleculare (SLM) of CA1, an input well positioned to modulate information flow through the hippocampus. High-frequency stimulation in SLM evokes an inhibition sufficiently strong to prevent CA1 pyramidal cells from spiking in response to SC input, a phenomenon we refer to as spike-blocking. We characterized the spike-blocking efficacy of burst stimulation (10 stimuli at 100 Hz) in SLM and found that it is greatest at approximately 300-600 ms after the burst, consistent with the time course of the slow GABA(B) signaling pathway. Spike-blocking efficacy increases in potency with the number of SLM stimuli in a burst, but also decreases with repeated presentations of SLM bursts. Spike-blocking was eliminated in the presence of GABA(B) antagonists. We have identified a candidate population of interneurons in SLM and distal stratum radiatum (SR) that may mediate this spike-blocking effect. We conclude that the output of CA1 pyramidal cells, and hence the hippocampus, is modulated in an input pattern-dependent manner by activation of the direct pathway from EC.     

Synaptic reorganization of calbindin-positive neurons in the human hippocampal CA1 region in temporal lobe epilepsy

The distribution, morphology, synaptic coverage and postsynaptic targets of calbindin-containing interneurons and afferent pathways have been analyzed in the control and epileptic CA1 region of the human hippocampus. Numerous calbindin-positive interneurons are preserved even in the strongly sclerotic CA1 region. The morphology of individual cells is altered: the cell body and dendrites become spiny, the radially oriented dendrites disappear, and are replaced by a large number of curved, distorted dendrites. Even in the non-sclerotic epileptic samples, where pyramidal cells are present and calbindin-immunoreactive interneurons seem to be unchanged, some modifications could be observed at the electron microscopic level: they received more inhibitory synaptic input, and the calbindin-positive excitatory afferents - presumably derived from the CA1, the CA2 and/or the dentate gyrus - are sprouted. In the strongly sclerotic tissue, with the death of pyramidal cells, calbindin-positive terminals (belonging to interneurons and the remaining excitatory afferents) change their targets. Our data suggest that an intense synaptic reorganization takes place in the epileptic CA1 region, even in the non-sclerotic tissue, before the death of considerable numbers of pyramidal cells. Calbindin-positive interneurons participate in this reorganization: they show plastic changes in response to epilepsy. The enhanced inhibition of inhibitory interneurons may result in the disinhibition of pyramidal cells or in an abnormal synchrony in the output region of the hippocampus.     

Slow feedback inhibition in the Ca3 area of the rat hippocampus by synergistic synaptic activation of mGluR1 and mGluR5

Interneurons are critical in regulating the excitability of principal cells in neuronal circuits, thereby modulating the output of neuronal networks. We investigated synaptically evoked inhibitory responses in CA3 pyramidal cells mediated by metabotropic glutamate receptors (mGluRs) expressed somatodendritically by interneurons. Although pharmacological activation of mGluRs in interneurons has been shown to enhance their excitability, the inability to record mGluR-mediated synaptic responses has precluded detailed characterization of mGluR function in hippocampal interneurons. We found that a single extracellular pulse in CA3 stratum pyramidale was sufficient to induce disynaptic inhibitory responses mediated by postsynaptic mGluRs of the interneurons in CA3 pyramidal cells of hippocampal slice cultures. The disynaptic inhibitory response followed a short-latency monosynaptic inhibitory response, and was observed at stimulus intensities evoking half-maximal monosynaptic IPSCs. Synergistic activation of mGluR1 and mGluR5 was required to induce the full inhibitory response. When recordings were obtained from interneurons in CA3 stratum radiatum or stratum oriens, a single extracellular stimulus induced a slow inward cationic current with a time course corresponding to the slow inhibitory response measured in pyramidal cells. DCG IV, a group II mGluR agonist, which specifically blocks synaptic transmission through mossy fibres, had no effect on mGluR-mediated synaptic responses in interneurons, suggesting that feed-forward inhibition via mossy fibres is not involved. Thus, postsynaptic mGluR1 and mGluR5 in hippocampal interneurons cooperatively mediate slow feedback inhibition of CA3 pyramidal cells. This mechanism may allow interneurons to monitor activity levels from populations of neighbouring principal cells to adapt inhibitory tone to the state of the network.     

Interneuron loss reduces dendritic inhibition and GABA release in hippocampus of aged rats

Aging is associated with impairments in learning and memory and a greater incidence of limbic seizures. These changes in the aged brain have been associated with increased excitability of hippocampal pyramidal cells caused by a reduced number of gamma-aminobutyric acid-ergic (GABAergic) interneurons. To better understand these issues, we performed cell counts of GABAergic interneurons and examined GABA efflux and GABAergic inhibition in area CA1 of the hippocampus of young (3-5 months) and aged (26-30 months) rats. Aging significantly reduced high K⁺/Ca²⁺-evoked GABA, but not glutamate efflux in area CA1. Immunostaining revealed a significant loss of GABAergic interneurons, but not inhibitory boutons in stratum oriens and stratum lacunosum moleculare. Somatostatin-immunoreactive oriens-lacunosum moleculare (O-LM) cells, but not parvalbumin-containing interneurons were selectively lost. Oriens-lacunosum moleculare cells project to distal dendrites of CA1 pyramidal cells, providing dendritic inhibition. Accordingly, inhibition of dendritic input to CA1 from entorhinal cortex was selectively reduced. These findings suggest that the age-dependent loss of interneurons impairs dendritic inhibition and dysregulates entorhinal cortical input to CA1, potentially contributing to cognitive impairment and seizures.     

Selective induction of metabotropic glutamate receptor 1- and metabotropic glutamate receptor 5-dependent chemical long-term potentiation at oriens/alveus interneuron synapses of mouse hippocampus

Synaptic plasticity in inhibitory interneurons is essential to maintain a proper equilibrium between excitation and inhibition in hippocampal network. Recent studies showed that theta-burst-induced long-term potentiation (LTP) at excitatory synapses of oriens/alveus (O/A) interneurons in CA1 hippocampal region required the activation of metabotropic glutamate receptor (mGluR) 1. However these interneurons also express mGluR5 and the contribution of this receptor subtype in interneuron synaptic plasticity remains unexplored. We combined pharmacological and transgenic approaches to examine the relative contribution of mGluR1/5 in LTP at excitatory synapses on O/A interneurons. Bath-application of the selective mGluR1/5 agonist (s)-3,5-dihydroxyphenylglycine (DHPG) induced LTP of compound excitatory postsynaptic potentials. DHPG-induced LTP was not prevented by application of either mGluR1 or mGluR5 antagonists, was still present in mGluR1 knockout mice, but was blocked by co-application of both antagonists. These results indicate that LTP can be induced at O/A interneuron synapses by either mGluR1 or mGluR5 activation. As previously reported for mGluR1-dependent LTP, the mGluR5-dependent LTP was independent of N-methyl-d-aspartate receptors. Pairing DHPG application with postsynaptic depolarization induced mGluR1- and mGluR5-dependent LTP of minimally-evoked excitatory postsynaptic currents, which were composed of calcium-permeable AMPA receptor and presynaptically modulated by group II mGluRs, hence confirming that both forms of LTP occurred directly at interneuron excitatory synapses. These findings uncover a new mGluR5-dependent form of LTP at O/A interneuron synapses and indicate that activation of mGluR1 or mGluR5 is sufficient to induce LTP at these synapses. Thus, a rich repertoire of adaptive changes may take place at these interneuron synapses to regulate hippocampal feedback inhibition.     

Total number and distribution of inhibitory and excitatory synapses on hippocampal CA1 pyramidal cells

The integrative properties of neurons depend strongly on the number, proportions and distribution of excitatory and inhibitory synaptic inputs they receive. In this study the three-dimensional geometry of dendritic trees and the density of symmetrical and asymmetrical synapses on different cellular compartments of rat hippocampal CA1 area pyramidal cells was measured to calculate the total number and distribution of excitatory and inhibitory inputs on a single cell.A single pyramidal cell has approximately 12,000 microm dendrites and receives around 30,000 excitatory and 1700 inhibitory inputs, of which 40 % are concentrated in the perisomatic region and 20 % on dendrites in the stratum lacunosum-moleculare. The pre- and post-synaptic features suggest that CA1 pyramidal cell dendrites are heterogeneous. Strata radiatum and oriens dendrites are similar and differ from stratum lacunosum-moleculare dendrites. Proximal apical and basal strata radiatum and oriens dendrites are spine-free or sparsely spiny. Distal strata radiatum and oriens dendrites (forming 68.5 % of the pyramidal cells' dendritic tree) are densely spiny; their excitatory inputs terminate exclusively on dendritic spines, while inhibitory inputs target only dendritic shafts. The proportion of inhibitory inputs on distal spiny strata radiatum and oriens dendrites is low ( approximately 3 %). In contrast, proximal dendritic segments receive mostly (70-100 %) inhibitory inputs. Only inhibitory inputs innervate the somata (77-103 per cell) and axon initial segments. Dendrites in the stratum lacunosum-moleculare possess moderate to small amounts of spines. Excitatory synapses on stratum lacunosum-moleculare dendrites are larger than the synapses in other layers, are frequently perforated ( approximately 40 %) and can be located on dendritic shafts. Inhibitory inputs, whose percentage is relatively high ( approximately 14-17 %), also terminate on dendritic spines.Our results indicate that: (i) the highly convergent excitation arriving onto the distal dendrites of pyramidal cells is primarily controlled by proximally located inhibition; (ii) the organization of excitatory and inhibitory inputs in layers receiving Schaffer collateral input (radiatum/oriens) versus perforant path input (lacunosum-moleculare) is significantly different.     

Presynaptic GABAA receptors facilitate spontaneous glutamate release from presynaptic terminals on mechanically dissociated rat CA3 pyramidal neurons

Mossy fiber-derived giant spontaneous miniature excitatory postsynaptic currents have been suggested to be large enough to generate action potentials in postsynaptic CA3 pyramidal neurons. Here we report on the functional roles of presynaptic GABA(A) receptors on excitatory terminals in contributing to spontaneous glutamatergic transmission to CA3 neurons. In mechanically dissociated rat hippocampal CA3 neurons with adherent presynaptic nerve terminals, spontaneous excitatory postsynaptic currents were recorded using conventional whole-cell patch clamp recordings. In most recordings, unusually large spontaneous excitatory postsynaptic currents up to 500 pA were observed. These large spontaneous excitatory postsynaptic currents were highly sensitive to group II metabotropic glutamate receptor activation, and were still observed even after the blockade of voltage-dependent Na(+) or Ca(2+) channels. Exogenously applied muscimol (0.1-3 microM) significantly increased the frequency of spontaneous excitatory postsynaptic currents including the large ones. This facilitatory effect of muscimol was completely inhibited in the presence of 10 microM 6-imino-3-(4-methoxyphenyl)-1(6H)-pyridazinebutanoic acid HBr, a specific GABA(A) receptor antagonist. Pharmacological data suggest that activation of presynaptic GABA(A) receptors directly depolarizes glutamatergic terminals resulting in the facilitation of spontaneous glutamate release. In the current-clamp condition, a subset of large spontaneous excitatory postsynaptic potentials triggered action potentials, and muscimol greatly increased the frequency of spontaneous excitatory postsynaptic potential-triggered action potentials in postsynaptic CA3 pyramidal neurons. The results suggest that presynaptic GABA(A) receptors on glutamatergic terminals play an important role in the excitability of CA3 neurons as well as in the presynaptic modulation of glutamatergic transmission onto hippocampal CA3 neurons.     

Characteristics of structural organization of interneurons in CA 3 region and mossy fibers of hippocampus

The paper reviews the current data on the structural peculiarities of the interneurons of hippocampal area CA 3. Special attention is paid to horizontal spiny interneurons located in the stratum lucidum and forming direct contacts with the system of mossy fibers. Their axons terminate at the proximal portions of dendrites and cell body of pyramidal neurons, occasionally they make contacts with the main cells of the hilus and other hippocampal interneurons. Some portion of axon collaterals is in close vicinity to direct and indirect entorhinal-hippocampal connections, indicating their accommodation to selectively suppress mossy fiber input. The latter form the synapses of various types on the principal cells and on various types of interneurons of area CA 3, indicating different mechanisms of transmitter uptake and presynaptic control of their release.     

Functional relation between interneuron input and population activity in the rat hippocampal cornu ammonis 1 area

Inhibitory interneurons are important components of the cornu ammonis 1 (CA1) network, as they are strategically positioned to control network information transfer. We investigated in detail synaptic input to individual CA1 interneurons (mainly basket and bistratified cells) after the local circuit was activated through the Schaffer-Commissural pathway and related this input to the population activity of the pyramidal cells. Synaptic responses were measured under whole-cell voltage clamp and population activity was determined from local field potentials. The synaptic input that was evoked in CA1 interneurons fell into two distinct groups. Disynaptic input with a long latency always started after the population spike with a mean latency of 3.0+/-0.3 ms (n=22) in respect to the peak of the population spike. This type of synaptic input to the interneurons was causally linked to the occurrence and amplitude of the population spike and most likely driven by CA1 pyramidal cells. Short-latency monosynaptic input occurred 0.8+/-0.2 ms (n=18) before the peak of the population spike. Its timing was strictly linked to the stimulus and showed significantly less jitter than long-latency input. In the absence of a population spike only short-latency input could be observed. Whether an interneuron receives direct monosynaptic Schaffer input or disynaptic input from the pyramidal cell population determines when that interneuron will be recruited in the network after Schaffer collateral stimulation. In addition, we found that the relation between the strength of the synaptic input and the population activity was different for the two types of input. Short-latency monosynaptic input showed large sensitivity to input changes at stimulus intensities that evoked little activity in the pyramidal cell population. In contrast, the amplitude of the long-latency disynaptic input to the interneurons closely reflected the population activity and increased gradually with stimulus intensity. Interneurons receiving the first type of input may expand the input sensitivity of the network, while interneurons receiving the second type could be involved in overall normalization of the output of the CA1 network. Our results underscore the importance of knowledge of the input to an interneuron for the understanding of its inhibitory role in the network.     

Alterations in excitatory and GABAergic inhibitory connections in hippocampal transplants

Solid pieces of embryonic hippocampal tissue were implanted in a cavity formed by aspiration of the fimbria-fornix and the overlying cingulate cortex in adult rats. Six to 8 months after the transplantation, chronic recording electrodes were implanted into the graft and the host hippocampi for the recording of electroencephalogram and unit activity in the freely moving animal. Irregularly occurring sharp waves or electroencephalogram spikes and concurrent synchronous discharge of large groups of neurons dominated the electrical activity of the grafts, in contrast to the situation in normal animals. Light microscopy and GABA immunocytochemistry in the grafts revealed that the three major cell types of the hippocampal formation, i.e. pyramidal neurons, dentate granule cells and GABA-immunoreactive interneurons were present in the hippocampal grafts. At the ultrastructural level, however, significant alterations in connectivity were observed. The most striking finding was the absence or sparse occurrence of synapses on the axon initial segments of pyramidal neurons. The axon initial segments are normally densely covered by GABAergic synapses derived from a specialized type of interneuron, the chandelier or axo-axonic cell. On the other hand, numerous GABA-immunoreactive terminals were found in synaptic contact with somata of pyramidal neurons, suggesting that other types of GABAergic interneurons and their efferent connections may have developed in a normal manner. The cell bodies of pyramidal neurons received, in addition, several asymmetric synapses from GABA-negative terminals. These presumably excitatory synapses are not present on the somata of pyramidal cells in the normally developing hippocampus. We hypothesize that the somatic excitatory synapses originate, at least in part, from the axon collaterals of the neighbouring pyramidal cells in the graft. We suggest that the hyperexcitability of the neuronal circuitry within the graft is due to reduced inhibition (lack of axo-axonic synapses) coupled with increased collateral excitation of the pyramidal neurons.     

NMDA and AMPA receptors contribute to the nicotinic cholinergic excitation of CA1 interneurons in the rat hippocampus

In the hippocampus, glutamatergic inputs to pyramidal neurons and interneurons are modulated by alpha7* and alpha3beta4* nicotinic acetylcholine receptors (nAChRs), respectively, present in glutamatergic neurons. This study examines how nicotinic AMPA, and NMDA receptor nAChR activities are integrated to regulate the excitability of CA1 stratum radiatum (SR) interneurons in rat hippocampal slices. At resting membrane potentials and in the presence of extracellular Mg2+ (1 mM), nicotinic agonists triggered in SR interneurons excitatory postsynaptic currents (EPSCs) that had two components: one mediated by AMPA receptors, and the other by NMDA receptors. As previously shown, nicotinic agonist-triggered EPSCs resulted from glutamate released by activation of alpha3beta4* nAChRs in glutamatergic neurons/fibers synapsing directly onto the neurons under study. The finding that CNQX caused more inhibition of nicotinic agonist-triggered EPSCs than expected from the blockade of postsynaptic AMPA receptors indicated that this nicotinic response also depended on the AMPA receptor activity in the glutamatergic neurons synapsing onto the interneuron under study. Nicotinic agonists always triggered action potentials in CA1 SR interneurons. In most interneurons, these action potentials resulted from activation of somatodendritic AMPA receptors and alpha7* nAChRs. In interneurons expressing somatodendritic alpha4beta2* nAChRs, activation of these receptors caused sufficient membrane depolarization to remove the Mg2+-induced block of somatodendritic NMDA receptors; in these neurons, nicotinic agonist-triggered action potentials were partially dependent on NMDA receptor activation. Removing extracellular Mg2+ or clamping the neuron at positive membrane potentials revealed the existence of a tonic NMDA current in SR interneurons that was unaffected by nAChR activation or inhibition. Thus integration of the activities of nAChRs, NMDA, and AMPA receptors in different compartments of CA1 neurons contributes to the excitability of CA1 SR interneurons.     

GABAB Receptor- and Metabotropic Glutamate Receptor-Dependent Cooperative Long-Term Potentiation of Rat Hippocampal GABAA Synaptic Transmission

Repetitive stimulation of Schaffer collaterals induces activity-dependent changes in the strength of polysynaptic inhibitory postsynaptic potentials (IPSPs) in hippocampal CA1 pyramidal neurons that are dependent on stimulation parameters. In the present study, we investigated the effects of two stimulation patterns, theta-burst stimulation (TBS) and 100 Hz tetani, on pharmacologically isolated monosynaptic GABAergic responses in adult CA1 pyramidal cells. Tetanization with 100 Hz trains transiently depressed both early and late IPSPs, whereas TBS induced long-term potentiation (LTP) of early IPSPs that lasted at least 30 min. Mechanisms mediating this TBS-induced potentiation were examined using whole-cell recordings. The paired-pulse ratio of monosynaptic inhibitory postsynaptic currents (IPSCs) was not affected during LTP, suggesting that presynaptic changes in GABA release are not involved in the potentiation. Bath application of the GABA_B receptor antagonist CGP55845 or the group I/II metabotropic glutamate receptor antagonist E4-CPG inhibited IPSC potentiation. Preventing postsynaptic G-protein activation or Ca²⁺ rise by postsynaptic injection of GDP-β-S or BAPTA, respectively, abolished LTP, indicating a G-protein- and Ca²⁺-dependent induction in this LTP. Finally during paired-recordings, activation of individual interneurons by intracellular TBS elicited solely short-term increases in average unitary IPSCs in pyramidal cells. These results indicate that a stimulation paradigm mimicking the endogenous theta rhythm activates cooperative postsynaptic mechanisms dependent on GABA_BR, mGluR, G-proteins and intracellular Ca²⁺, which lead to a sustained potentiation of GABA_A synaptic transmission in pyramidal cells. GABAergic synapses may therefore contribute to functional synaptic plasticity in adult hippocampus.