Cellular bases of hippocampal EEG in the behaving rat

Rats implanted with recording and stimulating electrodes were trained to run in an activity wheel for a water reward. Unitary discharges and slow activity were recorded by a movable tungsten microelectrode and by fixed electrodes. Single cells were classified according to their spontaneous and evoked response properties as pyramidal cells, granule cells and interneurons. Unit activity, EEG and their interrelations were studied by spectral and spike-triggered averaging methods. Gradual phase-shifts of RSA were observed both in CA1 and the dentate gyrus. Movement-related RSA was correlated with a decrease in firing rate of pyramidal cells and an increase in the firing of both interneurons and granule cells. In the CA1 region pyramidal cells and interneurons fired preferentially on the negative and positive phases of the locally derived RSA, respectively. In the dentate gyrus both granule cells and interneurons discharged mainly on the positive portion of the local RSA waves, about 90° before the CA1 pyramidal cells. Fourier analysis of the spike trains of interneurons and granule cells showed high power at RSA frequency, coherent with the concurrent EEG. Phase relations between discharges of interneurons and RSA remained unchanged following urethane anesthesia. In waking rats, atropine administration resulted in a decreased discharge of interneurons at RSA frequency, and reduced coherence with RSA. Lesions of the septum or the fimbria-fornix abolished RSA and the rhythmic discharges of the interneurons. Isolation of the entorhinal cortex (EC) from its cortical inputs did not change either EEG or neuronal firing. However, in such a preparation atropine completely abolished RSA and related rhythmicity of interneurons. During drinking and immobility but not during walking, sharp waves (SPW) of about 40–100 ms duration appeared in the EEG. SPWs were invariably accompanied by synchronous discharges of several pyramidal cells and interneurons. CA3 pyramidal cells also discharged in synchronous bursts but without local SPWs. Laminar profiles of SPWs and the field potentials evoked by stimulation of Schaf fer collaterals were essentially identical. The behavior-dependent occurrence of SPWs was retained following atropine administration, septal lesion or EC isolation but was lost after fimbria-for-nix-neocortex lesion or following atropine administration in EC isolated rats. In addition to relations to RSA and SPWs, interneurons were phase-locked to the fast EEG pattern (25–70 Hz). This relationship was preserved following lesions of the septum or the fimbria-fornix complex. The above findings allowed us to construct a new model of hippocampal RSA generation based on feed-forward inhibition from the septum and a direct excitation by the entorhinal input. SPWs are suggested to reflect strong synaptic activation of CA1 pyramidal cells via the Schaffer collaterals as a consequence of synchronous discharges of CA3 pyramidal neurons. Fast activity is supposed to reflect synaptic activity close to the somata of pyramidal cells and granule cells.     

3‐Hz subthreshold oscillations of CA2 neurons In vivo

The CA2 region is unique in the hippocampus; it receives direct synaptic innervations from several hypothalamic nuclei and expresses various receptors of neuromodulators, including adenosine, vasopressin, and oxytocin. Furthermore, the CA2 region may have distinct brain functions, such as the control of instinctive and social behaviors; however, little is known about the dynamics of the subthreshold membrane potentials of CA2 neurons in vivo. We conducted whole‐cell current‐clamp recordings from CA2 pyramidal cells in urethane‐anesthetized mice and monitored the intrinsic fluctuations in their membrane potentials. The CA2 pyramidal cells emitted spontaneous action potentials at mean firing rates of ～0.8 Hz. In approximately half of the neurons, the subthreshold membrane potential oscillated at ～3 Hz. In two neurons, we obtained simultaneous recordings of local field potentials from the CA1 stratum radiatum and demonstrated that the 3‐Hz oscillations of CA2 neurons were not correlated with CA1 field potentials. In tetrodotoxin‐perfused acute hippocampal slices, the membrane potentials of CA2 pyramidal cells were not preferentially entrained to 3‐Hz sinusoidal current inputs, which suggest that intracellular 3‐Hz oscillations reflect the neuronal dynamics of the surrounding networks. © 2016 Wiley Periodicals, Inc.     

Synchronization of GABAergic interneuronal networks during seizure-like activity in the rat horizontal hippocampal slice

We studied the contribution of GABAergic (gamma-aminobutyric acid) neurotransmission to epileptiform activity using the horizontal hippocampal rat brain slice. Seizure-like (ictal) activity was evoked in the CA1 area by applying high-frequency trains (80 Hz for 2 s) to the Schaffer collaterals. Whole-cell recordings from stratum oriens-alveus interneurons revealed burst firing with superimposed high-frequency spiking which was synchronous with field events and pyramidal cell firing during ictal activity. On the other hand, interictal interneuronal bursts were synchronous with large-amplitude inhibitory postsynaptic potentials (IPSPs) in pyramidal cells. Excitatory and inhibitory postsynaptic potentials were simultaneously received by pyramidal neurons during the ictal afterdischarge, and were synchronous with interneuronal bursting and field potential ictal events. The GABAA receptor antagonist bicuculline greatly reduced the duration of the ictal activity in the CA1 layer, and evoked rhythmic interictal synchronous bursting of interneurons and pyramidal cells. With intact GABAergic transmission, interictal field potential events were synchronous with large amplitude IPSPs (9.8 +/- 2.4 mV) in CA1 pyramidal cells, and with interneuronal bursting. Simultaneous dual recordings revealed synchronous IPSPs received by widely separated pyramidal neurons during ictal and interictal periods, indicative of widespread interneuronal firing synchrony throughout the hippocampus. CA3 pyramidal neurons fired in synchrony with interictal field potential events recorded in the CA1 layer, and glutamate receptor antagonists abolished interictal interneuronal firing and synchronous large amplitude IPSPs received by CA1 pyramidal cells. These observations provide evidence that the interneuronal network may be entrained in hyperexcitable states by GABAergic and glutamatergic mechanisms.     

Cellular bases of hippocampal EEG in the behaving rat

Rats implanted with recording and stimulating electrodes were trained to run in an activity wheel for a water reward. Unitary discharges and slow activity were recorded by a movable tungsten microelectrode and by fixed electrodes. Single cells were classified according to their spontaneous and evoked response properties as pyramidal cells, granule cells and interneurons. Unit activity, EEG and their interrelations were studied by spectral and spike-triggered averaging methods. Gradual phase-shifts of RSA were observed both in CA1 and the dentate gyrus. Movement-related RSA was correlated with a decrease in firing rate of pyramidal cells and an increase in the firing of both interneurons and granule cells. In the CA1 region pyramidal cells and interneurons fired preferentially on the negative and positive phases of the locally derived RSA, respectively. In the dentate gyrus both granule cells and interneurons discharged mainly on the positive portion of the local RSA waves, about 90 degrees before the CA1 pyramidal cells. Fourier analysis of the spike trains of interneurons and granule cells showed high power at RSA frequency, coherent with the concurrent EEG. Phase relations between discharges of interneurons and RSA remained unchanged following urethane anesthesia. In waking rats, atropine administration resulted in a decreased discharge of interneurons at RSA frequency, and reduced coherence with RSA. Lesions of the septum or the fimbria-fornix abolished RSA and the rhythmic discharges of the interneurons. Isolation of the entorhinal cortex (EC) from its cortical inputs did not change either EEG or neuronal firing. However, in such a preparation atropine completely abolished RSA and related rhythmicity of interneurons. During drinking and immobility but not during walking, sharp waves (SPW) of about 40-100 ms duration appeared in the EEG. SPWs were invariably accompanied by synchronous discharges of several pyramidal cells and interneurons. CA3 pyramidal cells also discharged in synchronous bursts but without local SPWs. Laminar profiles of SPWs and the field potentials evoked by stimulation of Schaffer collaterals were essentially identical. The behavior-dependent occurrence of SPWs was retained following atropine administration, septal lesion or EC isolation but was lost after fimbria-fornix-neocortex lesion or following atropine administration in EC isolated rats. In addition to relations to RSA and SPWs, interneurons were phase-locked to the fast EEG pattern (25-70 Hz). This relationship was preserved following lesions of the septum or the fimbria-fornix complex.(ABSTRACT TRUNCATED AT 400 WORDS)     

Influence of the hippocampus on interneurons of the rat prefrontal cortex

The hippocampus and prefrontal cortex (PFC), two structures implicated in learning and memory processes, are linked by a direct hippocampo-prefrontal pathway. It has been shown that PFC pyramidal cells receive monosynaptic excitatory inputs from the hippocampus and, in this study, we sought to determine the influence of the hippocampus on PFC interneurons in anesthetized rats. Extracellular recordings were coupled to juxtacellular injections of neurobiotin or biotinylated dextran amine to morphologically differentiate interneurons from pyramidal cells. In all cases, the action potentials of labeled interneurons were of shorter duration (< 0.70 ms) than those of identified pyramidal cells (> 0.70 ms). Single pulse stimulation of the hippocampal CA1/subiculum region induced an excitatory response in 70% of recorded interneurons in the prelimbic and medial-orbital areas of the PFC. In contrast to the one to two action potentials generated by pyramidal cells, an important group of interneurons fired a burst of action potentials in response to hippocampal stimulation. A large proportion of these excitatory responses was probably monosynaptic as their latency is consistent with the conduction time of the hippocampo-prefrontal pathway. In addition, when both a pyramidal cell and an interneuron were simultaneously recorded and both responded to stimulation, the interneuron consistently fired before the pyramidal cell. In conclusion, the hippocampus exerts a direct excitatory influence on PFC interneurons and is thus capable of feedforward inhibition of pyramidal cells. Hippocampal output is spatially and temporally focalized via this inhibitory process and consequently could facilitate the synchronization of a specific subset of PFC neurons with hippocampal activity.     

Terminal field and firing selectivity of cholecystokinin-expressing interneurons in the hippocampal CA3 area

Hippocampal oscillations reflect coordinated neuronal activity on many timescales. Distinct types of GABAergic interneuron participate in the coordination of pyramidal cells over different oscillatory cycle phases. In the CA3 area, which generates sharp waves and gamma oscillations, the contribution of identified GABAergic neurons remains to be defined. We have examined the firing of a family of cholecystokinin-expressing interneurons during network oscillations in urethane-anesthetized rats and compared them with firing of CA3 pyramidal cells. The position of the terminals of individual visualized interneurons was highly diverse, selective, and often spatially coaligned with either the entorhinal or the associational inputs to area CA3. The spike timing in relation to theta and gamma oscillations and sharp waves was correlated with the innervated pyramidal cell domain. Basket and dendritic-layer-innervating interneurons receive entorhinal and associational inputs and preferentially fire on the ascending theta phase, when pyramidal cell assemblies emerge. Perforant-path-associated cells, driven by recurrent collaterals of pyramidal cells fire on theta troughs, when established pyramidal cell assemblies are most active. In the CA3 area, slow and fast gamma oscillations occurred on opposite theta oscillation phases. Perforant-path-associated and some COUP-TFII-positive interneurons are strongly coupled to both fast and slow gamma oscillations, but basket and dendritic-layer-innervating cells are weakly coupled to fast gamma oscillations only. During sharp waves, different interneuron types are activated, inhibited, or remain unaffected. We suggest that specialization in pyramidal cell domain and glutamatergic input-specific operations, reflected in the position of GABAergic terminals, is the evolutionary drive underlying the diversity of cholecystokinin-expressing interneurons.     

A model of gamma-frequency network oscillations induced in the rat CA3 region by carbachol in vitro

Carbachol (> 20 microM) and kainate (100 nM) induce, in the in vitro CA3 region, synchronized neuronal population oscillations at approximately 40 Hz having distinctive features: (i) the oscillations persist for hours; (ii) interneurons in kainate fire at 5-20 Hz and their firing is tightly locked to field potential maxima (recorded in s. radiatum); (iii) in contrast, pyramidal cells, in both carbachol and kainate, fire at frequencies as low as 2 Hz, and their firing is less tightly locked to field potentials; (iv) the oscillations require GABAA receptors, AMPA receptors and gap junctions. Using a network of 3072 pyramidal cells and 384 interneurons (each multicompartmental and containing a segment of unmyelinated axon), we employed computer simulations to examine conditions under which network oscillations might occur with the experimentally determined properties. We found that such network oscillations could be generated, robustly, when gap junctions were located between pyramidal cell axons, as suggested to occur based on studies of spontaneous high-frequency (> 100 Hz) network oscillations in the in vitro hippocampus. In the model, pyramidal cell somatic firing was not essential for the oscillations. Critical components of the model are (i) the plexus of pyramidal cell axons, randomly and sparsely interconnected by gap junctions; (ii) glutamate synapses onto interneurons; (iii) synaptic inhibition between interneurons and onto pyramidal cell axons and somata; (iv) a sufficiently high rate of spontaneous action potentials generated in pyramidal cell axons. This model explains the dependence of network oscillations on GABA(A) and AMPA receptors, as well as on gap junctions. Besides the existence of axon-axon gap junctions, the model predicts that many of the pyramidal cell action potentials, during sustained gamma oscillations, are initiated in axons.     

Stratum lacunosum-moleculare interneurons of hippocampal CA1 region. I. Intracellular response characteristics, synaptic responses, and morphology

Stable intracellular recordings were obtained from nonpyramidal cells (interneurons) in stratum lacunosum-moleculare (L-M) of the CA1 region of guinea pig hippocampal slices. The intracellular response characteristics of these interneurons were distinctly different from responses of pyramidal cells and of other interneurons (basket cells and oriens-alveus interneurons). L-M interneurons had a high resting membrane potential (-58 mV), a high input resistance (64 M omega), and a large amplitude (60 mV), relatively long duration (2 msec) action potential. A large afterhyperpolarization (11 mV, 34 msec) followed a single action potential. Most L-M interneurons did not display any spontaneous firing. Lucifer yellow (LY)-filled L-M interneurons showed nonpyramidal morphology. Cells were generally fusiform or multipolar, with aspinous, beaded dendritic processes ramifying in stratum lacunosum-moleculare, radiatum, and (sometimes) oriens. The varicose axon originated from a primary dendrite, projected along stratum lacunosum-moleculare, branched profusely in stratum radiatum, and coursed toward and into stratum pyramidale and occasionally into oriens. Processes of cells with somata in the L-M region of CA1 were not restricted to the CA1 region. The dendritic and axonal processes of some L-M interneurons were seen ascending in stratum lacunosum-moleculare, crossing the hippocampal fissure, and coursing in stratum moleculare of the dentate gyrus. Excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs) were evoked in L-M interneurons from stimulation of major hippocampal afferents. EPSPs were most effectively elicited by stimulation of fiber pathways in transverse slices, whereas IPSPs were predominantly evoked when major pathways were stimulated in longitudinal slices. We have identified a population of interneurons with intracellular response characteristics and morphology distinctly different from previously described pyramidal and nonpyramidal neurons of CA1 region. The possible role of these interneurons in hippocampal circuitry is discussed.     

Synaptic mechanisms of adenosine A2A receptor‐mediated hyperexcitability in the hippocampus

Adenosine inhibits excitatory neurons widely in the brain through adenosine A₁ receptor, but activation of adenosine A_2A receptor (A_2AR) has an opposite effect promoting discharge in neuronal networks. In the hippocampus A_2AR expression level is low, and the receptor's effect on identified neuronal circuits is unknown. Using optogenetic afferent stimulation and whole‐cell recording from identified postsynaptic neurons we show that A_2AR facilitates excitatory glutamatergic Schaffer collateral synapses to CA1 pyramidal cells, but not to GABAergic inhibitory interneurons. In addition, A_2AR enhances GABAergic inhibitory transmission between CA1 area interneurons leading to disinhibition of pyramidal cells. Adenosine A_2AR has no direct modulatory effect on GABAergic synapses to pyramidal cells. As a result adenosine A_2AR activation alters the synaptic excitation ‐ inhibition balance in the CA1 area resulting in increased pyramidal cell discharge to glutamatergic Schaffer collateral stimulation. In line with this, we show that A_2AR promotes synchronous pyramidal cell firing in hyperexcitable conditions where extracellular potassium is elevated or following high‐frequency electrical stimulation. Our results revealed selective synapse‐ and cell type specific adenosine A_2AR effects in hippocampal CA1 area. The uncovered mechanisms help our understanding of A_2AR's facilitatory effect on cortical network activity. © 2014 The Authors Hippocampus Published by Wiley Periodicals, Inc.     

Sustained activation of hippocampal pyramidal cells by 'space clamping' in a running wheel

In contrast to sensory cortical areas of the brain, the relevant physiological inputs to the hippocampus, leading to selective activation of pyramidal cells, are largely unknown. Pyramidal cells are thought to be phasically activated by spatial cues and a variety of sensory and motor stimuli. Here, we used a behavioural ‘space clamp’ method, which involved the confinement of the actively running animal in a defined position in space (running wheel) and kept sensory inputs constant. Twelve percent of the recorded CA1 pyramidal cells were selectively active while the rat was running in the wheel. Cell firing was specific to the direction of running and disappeared after rotating the recording apparatus. The discharge frequency of pyramidal cells and interneurons was sustained as long as the rat ran continuously in the wheel. Furthermore, the discharge frequency of pyramidal cells and interneurons increased with increasing running velocity, even though the frequency of hippocampal theta waves remained constant. The discharge frequency of some ‘wheel-related’ pyramidal cells could increase more than 10-fold between 10 and 100 cm/s, whereas the firing rate of ‘non-wheel’ cells remained constantly low. We hypothesize that: (i) a necessary condition for place-specific discharge of hippocampal pyramidal cells is the presence of theta oscillation; and (ii) relevant stimuli can tonically and selectively activate hippocampal pyramidal cells as long as theta activity is present.     

Firing rate and theta-phase coding by hippocampal pyramidal neurons during 'space clamping'

In the hippocampus, spatial representation of the environment has been suggested to be coded by either the firing rate of pyramidal cell assemblies or the relative timing of the action potentials during the theta EEG cycle. Here, we used a behavioural 'space clamp' method, which involved the confinement of the actively running animal in a defined position in space (running wheel) to examine how 'spatial' and other inputs affect firing rate and timing of hippocampal CA1 pyramidal cells and interneurons. Nineteen per cent of the recorded CA1 pyramidal cells were selectively active while the rat was running in the wheel in a given direction ('wheel' cells). Spatial rotation of the apparatus showed that selective discharge of pyramidal cells in the wheel was under the combined influence of distal and apparatus cues. During steady running, both discharge rate and theta phase were constant. Rotation of the wheel apparatus resulted in a shift of both firing rate and preferred theta phase. The discharge frequency of 'wheel' cells increased threefold (on average) with increasing running velocity. In contrast, change in running speed had relatively little effect on the theta phase-related discharge of 'wheel' cells. Our findings indicate that mechanisms that regulate rate and phase of spikes are overlapping but not necessarily identical.     

An olfactory input to the hippocampus of the cat: Field potential analysis

Hippocampal responses to electrical stimulation of the prepyriform cortex in the cat were studied both in acute experiments under halothane anesthesia and in awake cats with chronically indwelling electrodes. Analysis of field potentials and unit activity indicated the extent to which different hippocampal subareas were activated, the laminar level at which the synaptic action took place and the dynamics of the evoked responses. It was found that: (1) the main generator of evoked responses in the hippocampus upon prepyriform cortex stimulation is localized in the fascia dentata and CA3 (CA1 pyramidal cells, and probably also subiculum cells, are activated but in a lesser degree); (2) the initial synaptic activity takes place at the most distal part of the dendrites of fascia dentata granule cells and CA3 pyramidal cells; and (3) this synaptic activity corresponds to an EPSP that leads to a transient increase in the firing rate of the hippocampal units, which is often followed by a long-lasting decrease in firing rate.We conclude that the pathway from the prepyriform cortex via lateral entorhinal cortex to hippocampal neurons may enable olfactory inputs to effectively excite hippocampal neurons.     

GABAergic contributions to gating,timing, and phase precession of hippocampal neuronal activity during theta oscillations

Successful spatial exploration requires gating, storage, and retrieval of spatial memories in the correct order. The hippocampus is known to play an important role in the temporal organization of spatial information. Temporally ordered spatial memories are encoded and retrieved by the firing rate and phase of hippocampal pyramidal cells and inhibitory interneurons with respect to ongoing network theta oscillations paced by intra‐ and extrahippocampal areas. Much is known about the anatomical, physiological, and molecular characteristics as well as the connectivity and synaptic properties of various cell types in the hippocampal microcircuits, but how these detailed properties of individual neurons give rise to temporal organization of spatial memories remains unclear. We present a model of the hippocampal CA1 microcircuit based on observed biophysical properties of pyramidal cells and six types of inhibitory interneurons: axo‐axonic, basket, bistratistified, neurogliaform, ivy, and oriens lacunosum‐moleculare cells. The model simulates a virtual rat running on a linear track. Excitatory transient inputs come from the entorhinal cortex (EC) and the CA3 Schaffer collaterals and impinge on both the pyramidal cells and inhibitory interneurons, whereas inhibitory inputs from the medial septum impinge only on the inhibitory interneurons. Dopamine operates as a gate‐keeper modulating the spatial memory flow to the PC distal dendrites in a frequency‐dependent manner. A mechanism for spike‐timing‐dependent plasticity in distal and proximal PC dendrites consisting of three calcium detectors, which responds to the instantaneous calcium level and its time course in the dendrite, is used to model the plasticity effects. The model simulates the timing of firing of different hippocampal cell types relative to theta oscillations, and proposes functional roles for the different classes of the hippocampal and septal inhibitory interneurons in the correct ordering of spatial memories as well as in the generation and maintenance of theta phase precession of pyramidal cells (place cells) in CA1. The model leads to a number of experimentally testable predictions that may lead to a better understanding of the biophysical computations in the hippocampus and medial septum. © 2012 Wiley Periodicals, Inc.     

Schaffer-specific local field potentials reflect discrete excitatory events at gamma frequency that may fire postsynaptic hippocampal CA1 units

Information processing and exchange between brain nuclei are made through spike series sent by individual neurons in highly irregular temporal patterns. Synchronization in cell assemblies, proposed as a network language for internal neural representations, still has little experimental support. We use a novel technique to extract pathway-specific local field potentials (LFPs) in the hippocampus to explore the ongoing temporal structure of a single presynaptic input, the CA3 Schaffer pathway, and its contribution to the spontaneous output of CA1 units in anesthetized rat. We found that Schaffer-specific LFPs are composed of a regular succession of pulse-like excitatory packages initiated by spontaneous clustered firing of CA3 pyramidal cells to which individual units contribute variably. A fraction of these packages readily induce firing of CA1 pyramidal cells and interneurons, the so-called Schaffer-driven spikes, revealing the presynaptic origin in the output code of single CA1 units. The output of 70% of CA1 pyramidal neurons contains up to 10% of such spikes. Our results suggest a hierarchical internal operation of the CA3 region based on sequential oscillatory activation of pyramidal cell assemblies whose activity partly gets in the output code at the next station. We conclude that CA1 output may directly reflect the activity of specific ensembles of CA3 neurons. Thus, the fine temporal structure of pathway-specific LFPs, as an accurate readout of the activity of a presynaptic population, is useful in searching for hidden presynaptic code in irregular spikes series of individual neurons and assemblies.     

Stimulation-induced status epilepticus: role of the hippocampal mossy fibers in the seizures and associated neuropathology

A study of seizure activity and neuronal cell death produced by intracerebroventricular kainic acid had suggested that seizures conveyed by the hippocampal mossy fibers are more damaging to CA3 pyramidal cells than seizures conveyed by other pathways. To test this idea, the effects of a unilateral mossy fiber lesion were determined on seizure activity and neuronal degeneration provoked by repetitive electrical stimulation of the hippocampal fimbria in unanesthetized rats. Fimbrial stimulation resulted in self-sustained status epilepticus accompanied by neuronal degeneration in several brain regions, including area CA3 of the hippocampal formation. A unilateral mossy fiber lesion more readily attenuated the electrographic and behavioral seizures provoked by fimbrial stimulation than those provoked by kainic acid. If status epilepticus developed in the presence of a mossy fiber lesion, denervated CA3 pyramidal cells were still destroyed, although similar lesions protect these neurons from kainic acid-induced status epilepticus. Thus the two models of status epilepticus employ somewhat different seizure circuitries and neurodegenerative mechanisms. Seizures which involve the mossy fiber projection are not necessarily more damaging to CA3 pyramidal cells than seizures which do not.     

Functional localization of cannabinoid receptors and endogenous cannabinoid production in distinct neuron populations of the hippocampus

The possible localization of cannabinoid (CB) receptors to glutamatergic and GABAergic synaptic terminals impinging upon GABAergic interneurons in the CA1 region of the rat hippocampus was examined using the electrophysiological measurement of neurotransmitter release in brain slices. Whereas activation of cannabinoid receptors via the application of the cannabinoid agonist WIN55,212-2 significantly and dose-dependently reduced evoked IPSCs recorded from interneurons possessing somata located in the stratum radiatum (S.R.) and stratum oriens (S.O.) lamellae, evoked glutamatergic EPSCs were unaffected in both neuronal populations. However, in agreement with previous reports, WIN55,212-2 significantly reduced EPSCs recorded from CA1 pyramidal neurons. Additional experiments confirmed that the effects of WIN55,212-2 on IPSCs were presynaptic and that they could be blocked by the CB1 receptor antagonist SR141716A. The involvement of endogenous cannabinoids in the presynaptic inhibition of GABA release was also examined in the interneurons and pyramidal cells using a depolarization-induced suppression of inhibition (DSI) paradigm. DSI was observed in CA1 pyramidal neurons under control conditions, and its incidence was greatly increased by the cholinergic agonist carbachol. However, DSI was not observed in the S.R. or S.O. interneuron populations, in either the presence or absence of carbachol. Whereas DSI was not present in these interneurons, the inhibitory inputs to these cells were modulated by the synthetic cannabinoid WIN55,212-2. These data support the hypothesis that cannabinoid receptors are located on inhibitory, but not excitatory, axon terminals impinging upon hippocampal interneurons, and that CA1 pyramidal neurons, and not interneurons, are capable of generating endogenous cannabinoids during prolonged states of depolarization.     

Feed-forward and feed-back activation of the dentate gyrus in vivo during dentate spikes and sharp wave bursts

Intermittently occurring field events, dentate spikes (DS), and sharp waves (SPW) in the hippocampus reflect population synchrony of principal cells and interneurons along the entorhinal cortex-hippocampus axis. We have investigated the cellular-synaptic generation of DSs and SPWs by intracellular recording from granule cells, pyramidal cells, and interneurons in anesthetized rats. The recorded neurons were anatomically identified by intracellular injection of biocytin. Extracellular recording electrodes were placed in the hilus to record field DSs and multiple units and in the CA1 pyramidal cell layer to monitor SPW-associated fast field oscillations (ripples) and unit activity. DSs were associated with large depolarizing potentials in granule cells, but they rarely discharged action potentials. When they were depolarized slightly with intracellular current injection, bursts of action potentials occurred concurrently with extracellularly recorded DSs. Two interneurons in the hilar region were also found to discharge preferentially with DSs. In contrast, CA1 pyramidal cells, recorded extracellularly and intracellularly, were suppressed during DSs. In association with field SPWs, extracellular recordings from the CA1 pyramidal layer and the hilar region revealed synchronous bursting of these cell populations. Intracellular recordings from CA3 and CA1 pyramidal cells, granule cells, and from a single CA3 region interneuron revealed SPW-concurrent depolarizing potentials and action potentials. These findings suggest that granule cells may be discharged anterogradely by entorhinal input or retrogradely by the CA3-mossy cell feedback pathway during DSs and SPWs, respectively. Although both of these intermittent population patterns can activate granule cells, the impact of DSs and SPWs is diametrically opposite on the rest of the hippocampal circuitry. Entorhinal cortex activation of the granule cells during DSs induces a transient decrease in the hippocampal output, whereas during SPW bursts every principal cell population of the hippocampal formation may be recruited into the population event. Hippocampus 7:437–450, 1997. © 1997 Wiley-Liss, Inc.     

Thinking outside the pyramidal cell: unexplored contributions of interneurons and neuropeptide Y to estrogen-induced synapse formation in the hippocampus

Since the first finding that 17beta-estradiol (E) can regulate CA1 pyramidal cell synapse formation, subsequent studies have explored many potential E-dependent mechanisms occurring within CA1 pyramidal cells. Fewer studies have focused on E-dependent processes outside of the pyramidal cell that may influence events activity of the pyramidal cells. This review considers hippocampal interneurons, which can potently regulate the excitability of simultaneously firing pyramidal cells. In particular, we discuss neuropeptide Y (NPY) expression by these interneurons because our published findings show that NPY expression is increased by E in a subset of interneurons which coincidentally exhibit E-regulated increase in GABA synthesis and are uniquely situated anatomically such that they may regulate synaptic activity. Here we review the role of different phenotypes of CA1 interneurons, and we propose a model in which E-stimulated NPY gene expression and the release of NPY by interneurons inhibits glutamate release presynaptically and alters glutamate-dependent synaptic events in the rat hippocampus during adulthood.     

Effects of morphine on hippocampal cells recorded in vitro

Intracellular recordings from CA1 pyramidal cells were obtained in the in vitro hippocampal slice preparation before and after the application of morphine sulfate. Two distinct effects were observed following addition of morphine (1.0 mM) to the bathing media, or application to individual slices through pressure ejection via large tipped pipettes. Within 3–10 min following morphine application CA1 cell membrane potentials began to depolarize (5–10 mV) toward firing threshold. Spontaneous discharge rates were increased and nonactive cells became active. Rhythmic firing at a rate of 5–8 Hz was observed for 30–40 min following drug application. The spontaneous rhythmic firing could be partially reversed by the addition of naloxone (0.1 mM) to the bathing media. The second effect of morphine was to potentiate the development of epileptiform discharges in CA1 cells in response to low frequency orthodromic synaptic stimulation. The findings indicate that morphine alters the excitability of hippocampal pyramidal cells and such changes are likely to be mediated by opiate specific alterations in membrane conductance and/or tonic inhibitory synaptic inputs.