Volume and number of neurons of the human hippocampal formation in normal aging and Alzheimer's disease

In order to observe changes owing to aging and Alzheimer's disease (AD) in the volumes of subdivisions of the hippocampus and the number of neurons of the hippocampal formation, 18 normal brains from subjects who died of nonneurological causes and had no history of long-term illness or dementia (ten of these brains comprised the aged control group) and 13 AD brains were analyzed. An optimized design for sampling, measuring volume by using the Cavalieri principle, and counting the number of neurons by using the optical disector was implemented on 50 μm-thick cresyl-violet sections. The mean total volume of the principal subdivisions of the hippocampal formation (fascia dentata, hilus, CA3-2, CA1, and subiculum) showed a negative correlation with age in normal subjects (r = −0.56, 2P < 0.05), and a 32% mean reduction in the AD group compared with controls (P < 0.001). This finding supports the measurement of the coronal cross-sectional area and the volume of the hippocampal formation in the clinical diagnosis of AD. There was an inverse relationship between the age of normal subjects and the number of neurons in CA1 (r = −0.84 2P < 0.0001) and subiculum (r = −0.49, 2P < 0.05) but not in other subdivisions. Pronounced AD-related reductions in neuron number were found only in the subiculum and the fascia dentata. Compared with controls, both losses represented 23% of neurons (P < 0.05). These results 1) confirm that AD is a qualitatively different process from normal aging and 2) reveal the regional selectivity of neuron loss within the hippocampal formation in aging and AD, which may be relevant to understanding the mechanisms involved in the neuron loss associated with the two processes. J. Comp. Neurol. 379:482–494, 1997. © 1997 Wiley-Liss, Inc.     

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.     

Effects of Glutamate,N-Methyl-d-Aspartate,High Potassium,and Hypoxia on Unit Discharges in CA1 Area of Hippocampal Slices of DBA and C57 Mice

Summary We studied effects of l -glutamate, N-methyl-d -aspartate (NMDA), high K⁺, and hypoxia on spontaneous unit discharges in stratum pyramidale of CA1 region of hippocampal slices in DBA and C57 mice aged 3–4 and 5–6 weeks. Application of l -glutamate (0.5–2.0 mM), NMDA (5–20 μM), high K⁺ (8.5 mM), and a brief period of hypoxia (1 min) to the perfused artificial cerebrospinal fluid (ACSF) all produced different degrees of spontaneous high-frequency discharges from CA1 area of hippocampal slices of both DBA and C57 mice. Two types of responses recorded extracellularly occurred after these manipulations: high-frequency repetitive single spikes and bursts of multiple population spikes. The rate and type of responses from CA1 region of hippocampal slices after these manipulations were different and depended on the strain and age of mice and the nature of manipulations. In general, hippocampal slices from audiogenic seizure-susceptible DBA mice were more sensitive than those from audiogenic seizure-resistant C57 mice, and hippocampal slices from younger animals were more susceptible than those from older ones. Thus, DBA mice aged 3–4 weeks of age were most susceptible and C57 mice aged 5–6 weeks were least susceptible to all these pharmacological, ionic, and hypoxic manipulations. Bursts of multiple population spikes were the most common responses in DBA mice and in younger animals, and repetitive single spikes were the predominant responses in C57 mice and in older animals. In all groups of animals, the average spontaneous discharge rate was highest after l -glutamate perfusion, next highest after NMDA, and lowest after high K⁺ and hypoxia. The latency of the appearance of spontaneous epileptiform activity from CA1 region of hippocampal slices was long (>2 min) after NMDA perfusion and short (<1 min) after l -glutamate, high K⁺ and hypoxia. The duration of the increased spontaneous discharges was short (?1 min) after l -glutamate perfusion, long (>3 min) after high K⁺ and hypoxia, and between short and long after NMDA perfusion. These results suggest that age and strain of animal and nature of stimulus precipitate different patterns of epileptiform activity in CNS.     

Orexin‐A increases the firing activity of hippocampal CA1 neurons through orexin‐1 receptors

Orexins including two peptides, orexin‐A and orexin‐B, are produced in the posterior lateral hypothalamus. Much evidence has indicated that central orexinergic systems play numerous functions including energy metabolism, feeding behavior, sleep/wakefulness, and neuroendocrine and sympathetic activation. Morphological studies have shown that the hippocampal CA1 regions receive orexinergic innervation originating from the hypothalamus. Positive orexin‐1 (OX₁) receptors are detected in the CA1 regions. Previous behavioral studies have shown that microinjection of OX₁ receptor antagonist into the hippocampus impairs acquisition and consolidation of spatial memory. However, up to now, little has been known about the direct electrophysiological effects of orexin‐A on hippocampal CA1 neurons. Employing multibarrel single‐unit extracellular recordings, the present study showed that micropressure administration of orexin‐A significantly increased the spontaneous firing rate from 2.96 ± 0.85 to 8.45 ± 1.86 Hz (P < 0.001) in 15 out of the 23 hippocampal CA1 neurons in male rats. Furthermore, application of the specific OX₁ receptor antagonist SB‐334867 alone significantly decreased the firing rate from 4.02 ± 1.08 to 2.11 ± 0.58 Hz in 7 out of the 17 neurons (P < 0.05), suggesting that endogenous orexinergic systems modulate the firing activity of CA1 neurons. Coapplication of SB‐334867 completely blocked orexin‐A–induced excitation of hippocampal CA1 neurons. The PLC pathway may be involved in activation of OX₁ receptor–induced excitation of CA1 neurons. Taken together, the present study's results suggest that orexin‐A produces excitatory effects on hippocampal neurons via OX₁ receptors. © 2016 Wiley Periodicals, Inc.     

Effect of Kainic Acid on Unit Discharge in CA1 Area of Hippocampal Slice of DBA and C57 Mice

Summary: Spontaneous unit discharges in stratum pyramidale of CA1 area of hippocampal slices from DBA and CS7 mice at different ages were recorded extracellularly. The average rate and amplitude of the spontaneous discharges from CA1 area of hippocampal slices bathed in artificial cerebrospinal fluid (aCSF) were not different between DBA and CS7 mice at either 3–4 or 5–6 weeks of age. Bath application of kainic acid (KA) in concentrations of 0·5–1·0 μM produced different responses in CA1 area from these two strains of mice. In DBA mice at age 3–4 weeks, when they are most susceptible to audiogenic seizures, KA perfusion induced high-frequency repetitive single spikes and bursts of multiple population spikes in CA1 area. Very high-frequency discharges (10-fold higher than most responses) were also observed in 20% of all slices. In audiogenic seizure resistant CS7 mice at age 3–4 weeks, KA perfusion at the same doses induced only the repetitive single spikes. The rate of spontaneous discharges was much lower than that in DBA mice. No burst of multiple population spikes nor very high-frequency responses were recorded in CS7 mice. At age 5–6 weeks, when both DBA and CS7 mice are resistant to audiogenic seizures, the rate of spontaneous discharges recorded from the CAI area during and after KA perfusion was lower than that at age 3–4 weeks, and there was no significant difference between DBA and CS7 mice. Pretreatment of hippocampal slices with AMPA/kainate receptor antagonist CNQX (10 pA4) markedly reduced the rate and amplitude of spontaneous discharges in CA1 area during and after KA perfusion, whereas competitive N-methyl-D-aspartate (NMDA) receptor antagonist D-APS had no effect. These results indicate that the responses in spontaneous discharges recorded extracellularly from stratum pyramidale in CA1 area of hippocampal slices to KA perfusion correlate with susceptibility to audiogenic seizures in DBA and CS7 mice.     

Deterministic and stochastic neuronal contributions to distinct synchronous CA3 network bursts

Computational studies have suggested that stochastic, deterministic, and mixed processes all could be possible determinants of spontaneous, synchronous network bursts. In the present study, using multicellular calcium imaging coupled with fast confocal microscopy, we describe neuronal behavior underlying spontaneous network bursts in developing rat and mouse hippocampal area CA3 networks. Two primary burst types were studied: giant depolarizing potentials (GDPs) and spontaneous interictal bursts recorded in bicuculline, a GABA(A) receptor antagonist. Analysis of the simultaneous behavior of multiple CA3 neurons during synchronous GDPs revealed a repeatable activation order from burst to burst. This was validated using several statistical methods, including high Kendall's coefficient of concordance values for firing order during GDPs, high Pearson's correlations of cellular activation times between burst pairs, and latent class analysis, which revealed a population of 5-6% of CA3 neurons reliably firing very early during GDPs. In contrast, neuronal firing order during interictal bursts appeared homogeneous, with no particular cells repeatedly leading or lagging during these synchronous events. We conclude that GDPs activate via a deterministic mechanism, with distinct, repeatable roles for subsets of neurons during burst generation, while interictal bursts appear to be stochastic events with cells assuming interchangeable roles in the generation of these events.     

The cells of origin of the commissural afferents to the area dentata in the mouse

The hippocampal commissural projection to the area dentata of the mouse was studied using the retrograde horseradish peroxidase (HRP) technique. Small volumes of HRP injected into the molecular layer of the fascia dentata or various subareas of regio inferior of the hippocampus (fields CA3a-c) resulted inlabeled perikarya in the contralateral hippocampus and area dentata. The commissural projection to the fascia dentata was observed to originate exclusively from cells within the hilus fasciae dentatae (CA4) of the contralateral area dentata. There was evidence of a considerable spread of commissural innervation along the septotemporal axis preferentially in the septal direction, confirming earlier observations. In contrast to the septotemporal spread, a sharp homotopic spatial organization was found in the mediolateral direction. For example, injections into the lateral portion of field CA3 (CA3a) resulted in HRP-positive cell bodies only in the contralateral field CA3a. When injections were made which apparently labeled all of the commissural fibers, the HRP reaction product was found in neurons both in the entire regio inferior and as far as the innermost point of the hilus fasciae dentatae; the majority of labeled cells were located in hippocampal subfield CA3c. No labeled cells were observed beyond the tip of the mossy fibers in regio superior.     

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.     

Effect of dimenhydrinate on autonomic activity in humans

The purpose of this study was to examine the effect of dimenhydrinate on resting muscle sympathetic nerve activity (MSNA), the vestibulosympathetic reflex, and the baroreflexes. Sixteen subjects participated in two double-blinded studies that measured mean arterial pressure (MAP), heart rate (HR), and MSNA responses before and after oral administration of dimenhydrinate (100 mg) or a placebo. In study one, 3 min of head-down rotation (HDR) was performed to engage the otolith organs. Dimenhydrinate (n = 10) did not alter resting MSNA, MAP, or HR. HDR increased MSNA before (Δ5 ± 1 bursts/min; P < 0.01) and after (Δ4 ± 1 bursts/min; P < 0.01) drug administration, but these responses were not different from the placebo (n = 6). In study two, 4 min of lower body negative pressure (LBNP) at −30 mmHg was performed. During the third min of LBNP, HDR was performed. MSNA increased during the first 2 min of LBNP before (Δ13 ± 2 bursts/min; P < 0.01) and after (Δ14 ± 2 bursts/min; P < 0.01) dimenhydrinate. HDR combined with LBNP increased MSNA further during the third min of LBNP (Δ18 ± 2 bursts/min before and Δ17 ± 2 bursts/min after dimenhydrinate; P < 0.01). These responses were not significantly different from the placebo. In contrast, HR responses to LBNP during the dimenhydrinate trial were increased when compared to all other trials (Δ5 ± 1 beats/min; P < 0.01). These results indicate that dimenhydrinate augments heart rate responses to baroreceptor unloading, but does not alter resting MSNA, the sympathetic baroreflexes, or the vestibulosympathetic reflex.     

Generators and topography of hippocampal theta (RSA) in the anaesthetized and freely moving rat.

Spontaneous and hypothalamically induced hippocampal rhythmical slow activity (RSA or theta) was studied acutely in rats anaesthetized with urethane or immobilized with D-tubocurarine. Systematic tracking of microelectrodes showed two foci of hippocampal RSA, one located in the basal part (stratum oriens) of CAl (mean amplitude 1 mV) and the other located in stratum moleculare of the dorsal blade of the fascia dentata (mean amplitude 2 mV). The hippocampal RSA recorded from the lower blade of the fascia dentata was always smaller than that found in the upper blade (mean amplitude 1 mV). The whole dorsal hippocampal extent within each generator zone was shown to be in synchrony, and the respective generator zones of both hippocampi were synchronous with one another. A null zone in stratum radiation was found interposed between the two generators and a zone of large amplitude fast activity (30-50 Hz) was localized to the hilus of the fascia dentata. Wave form analysis showed that the RSA recorded from the two generators was approximately 180 degrees out of phase. Amplitude and analysis of phase changes of RSA recorded in brain areas outside of the two generator zones suggested that such activity was due to physical spread from the two generators, with the possible exception of a restricted portion of CA3. The existence of the two generators, 180 degrees out of phase, was demonstrated in freely moving rats. Behavioural observations showed that the two generators were related systematically to concurrent motor behaviour. Preliminary observations suggest that, of the two generators, the one located in CAl may be the more variable.     

Properties of unitary IPSPs evoked by anatomically identified basket cells in the rat hippocampus

Hippocampal pyramidal cells receive GABA-mediated synaptic input from several distinct interneurons. In order to define the effect of perisomatic synapses, intracellular recordings were made with biocytin-containing microelectrodes from synaptically connected inhibitory and pyramidal cell pairs in subfields CA1 and CA3 of the rat hippocampus. Subsequent physiological analysis was restricted to the category of cells, here referred to as basket cells (n= 14), which had an efferent synaptic target profile (n= 282 synaptic contacts) of predominantly somatic (48.2%) and proximal dendritic synapses (45.0%). Electron microscopic analysis revealed that in two instances identified postsynaptic pyramidal cells received a total of 10 and 12 labelled basket cell synapses respectively. At an average membrane potential of -57.8 ± 4.6 mV, unitary inhibitory postsynaptic potentials (IPSPs; n= 24) had a mean amplitude of 450 ± 238 μV, a 10–90% rise time of 4.6 ± 3.2 ms and, measured at half-amplitude, a mean duration of 31.6 ± 18.2 ms. In most instances (n= 19) the IPSP decay could be fitted with a single exponential with a mean time constant of 32.4 ± 18.0 ms. Unitary basket cell-evoked IPSPs fluctuated widely in amplitude, ranging from the level of detectability to <2 mV. The response reversal of IPSPs (n= 5) was extrapolated to be at -74.9 ± 6.0 mV. Averages of unitary IPSPs had a mean calculated conductance of 0.95 ± 0.29 nS, ranging from 0.52 to 1.16 nS. Unitary basket cell IPSPs (n= 3) increased in amplitude by 26.3 ± 19.9% following bath application of the GABA_B receptor antagonist CGP 35845A (1–4 μM), whereas subsequent addition of the GABA^A receptor antagonist bicuculline (10–13 μM) reduced the IPSP amplitude to 13.5 ± 3.1% of the control response. Rapid presynaptic trains of basket cell action potentials resulted in the summation of up to four postsynaptic responses (n= 5). However, any increase in the rate of tonic firing (2- to 10-fold) led to a <50% reduction of the postsynaptic response amplitude. At depolarized membrane potentials, averaged IPSPs could be followed by a distinct depolarizing overshoot or postinhibitory facilitation (n= 4). At firing threshold, pyramidal cells fired postinhibitory rebound-like action potentials, the latter in close temporal overlap with the depolarizing overshoot. In conclusion, hippocampal basket cells have been identified as one source of fast, GABA_A receptor-evoked perisomatic inhibition. Unitary events are mediated by multiple synaptic release sites, thus providing an effective mechanism to avoid total transmission failures.     

Intrinsic bursting of immature CA3 pyramidal neurons and consequent giant depolarizing potentials are driven by a persistent Na+ current and terminated by a slow Ca2+ -activated K+ current

The CA3 area of the mature hippocampus is known for its ability to generate intermittent network activity both in physiological and in pathological conditions. We have recently shown that in the early postnatal period, the intrinsic bursting of interconnected CA3 pyramidal neurons generates network events, which were originally called giant depolarizing potentials (GDPs). The voltage-dependent burst activity of individual pyramidal neurons is promoted by the well-known depolarizing action of endogenous GABA on immature neurons. In the present work, we show that a persistent Na+ current, I-Nap, accounts for the slow regenerative depolarization that triggers the intrinsic bursts in the neonatal rat CA3 pyramidal neurons (postnatal day 3-6), while a slow Ca2+ -activated K+ current, sI-K(Ca), is primarily responsible for the postburst slow afterhyperpolarization and consequent burst termination. In addition, we exploited pharmacological data obtained from intracellular recordings to study the mechanisms involved in network events recorded with field potential recordings. The data as a whole indicate that I-Nap and sI-K(Ca) are involved in the initiation and termination, respectively, of the pyramidal bursts and consequent network events underlying GDPs.     

Induction and transient suppression of long-term potentiation in the peri- and postrhinal cortices following theta-related stimulation of hippocampal field CA1

During behavioral events associated with periods of likely mnemonic processing, CA1 pyramidal cells in rats typically discharge repetitively in either high-frequency bursts (`complex spikes') or single spikes, both of which are tightly phase-locked to the hippocampal theta rhythm. Interestingly, patterned stimulation which mimics the repetitive, learning-related complex spike discharges are optimal for inducing long-term potentiation (LTP) of excitatory field potentials in CA1, and patterned stimulation which mimics the theta-related single action potentials results in a robust and lasting depotentiation at these same synapses. The aim of the present study was to determine the extent to which these physiologically-relevant patterns of hippocampal stimulation have similar effects on synaptic efficacy in the monosynaptic projection from CA1 to the perirhinal and postrhinal cortices (PRh), areas thought to play a prominent role in many forms of learning and memory. Single-pulse stimulation of CA1 evoked a small amplitude, short latency population excitatory postsynaptic potential (EPSP) in the PRh. Theta-burst stimulation (TBS; n=8) delivered to CA1 reliably potentiated the PRh EPSP slope for at least 30 min. Theta-pulse stimulation (TPS; 5 Hz; n=4) delivered to CA1 5 min after TBS substantially but transiently suppressed EPSP slope relative to that of potentiated control preparations. Collectively these data suggest that theta-related patterns of hippocampal activation can reliably induce and transiently suppress LTP in PRh, and are consistent with the notion that behaviorally-relevant, theta-modulated patterns of CA1 unit activity may result in both long- and short-term alterations of synaptic strength within their rhinal cortical targets.     

Bidirectional global spontaneous network activity precedes the canonical unidirectional circuit organization in the developing hippocampus

Spontaneous network activity is believed to sculpt developing neural circuits. Spontaneous giant depolarizing potentials (GDPs) were first identified with single‐cell recordings from rat CA3 pyramidal neurons, but here we identify and characterize a large‐scale spontaneous network activity we term global network activation (GNA) in the developing mouse hippocampal slices, which is measured macroscopically by fast voltage‐sensitive dye imaging. The initiation and propagation of GNA in the mouse is largely GABA‐independent and dominated by glutamatergic transmission via AMPA receptors. Despite the fact that signal propagation in the adult hippocampus is strongly unidirectional through the canonical trisynaptic circuit (dentate gyrus [DG] to CA3 to CA1), spontaneous GNA in the developing hippocampus originates in distal CA3 and propagates both forward to CA1 and backward to DG. Photostimulation‐evoked GNA also shows prominent backward propagation in the developing hippocampus from CA3 to DG. Mouse GNA is strongly correlated to electrophysiological recordings of highly localized single‐cell and local field potential events. Photostimulation mapping of neural circuitry demonstrates that the enhancement of local circuit connections to excitatory pyramidal neurons occurs over the same time course as GNA and reveals the underlying pathways accounting for GNA backward propagation from CA3 to DG. The disappearance of GNA coincides with a transition to the adult‐like unidirectional circuit organization at about 2 weeks of age. Taken together, our findings strongly suggest a critical link between GNA activity and maturation of functional circuit connections in the developing hippocampus. J. Comp. Neurol. 522:2191–2208, 2014. © 2013 Wiley Periodicals, Inc.     

Co-localization of somatostatin mRNA and parvalbumin in the dorsal rat hippocampus after cerebral ischemia

Following transient global ischemia most of the neurons containing somatostatin in the fascia dentata of the dorsal hippocampal formation die, while somatostatinergic neurons in the CA1 region survive. These neurons react to ischemia with a transiently reduced expression of somatostatin mRNA and peptide. We have tested the hypothesis that this selective vulnerability is solely related to those somatostatinergic neurons which do not express the calcium-binding protein parvalbumin. Postischemic changes were studied in rat dorsal hippocampus at 2 and 16 days after 10 min of global cerebral ischemia using a four-vessel occlusion model. We performed a double-staining visualizing the mRNA coding for somatostatin by non-radioactive in situ hybridization and parvalbumin protein by immunocytochemistry. Only 5% of the somatostatinergic cells in the fascia dentata contained parvalbumin. The number of somatostatinergic cells was permanently reduced following ischemia. Among surviving neurons we found cells with and without parvalbumin expression. Thus, expression of parvalbumin is not predictive for survival of somatostatinergic cells in the fascia dentata. In contrast, in CA1, 37% of the somatostatinergic cells contained parvalbumin. These cells were unaffected by the transient ischemic period. The somatostatinergic cells lacking parvalbumin showed transiently reduced mRNA levels at day 2, but recovered to control values at the 16th postischemic day. Thus, expression of the calcium-buffering protein parvalbumin coincides with resistance of somatostatinergic neurons in CA1 to transient effects of ischemia. We conclude that the calcium-buffering capacity of parvalbumin may partially contribute to the protection of somatostatinergic neurons from ischemia in the dorsal hippocampus. However, the survival of somatostatinergic cells without parvalbumin indicates the importance of other factors as well. © 1995 Wiley-Liss, Inc.     

Theta phase precession in hippocampal neuronal populations and the compression of temporal sequences

O'Keefe and Recce [1993] Hippocampus 3:317–330 described an interaction between the hippocampal theta rhythm and the spatial firing of pyramidal cells in the CA1 region of the rat hippocampus: they found that a cell's spike activity advances to earlier phases of the theta cycle as the rat passes through the cell's place field. The present study makes use of large-scale parallel recordings to clarify and extend this finding in several ways: 1) Most CA1 pyramidal cells show maximal activity at the same phase of the theta cycle. Although individual units exhibit deeper modulation, the depth of modulation of CA1 population activity is about 50%. The peak firing of inhibitory interneurons in CA1 occurs about 60° in advance of the peak firing of pyramidal cells, but different interneurons vary widely in their peak phases. 2) The first spikes, as the rat enters a pyramidal cell's place field, come 90°–120° after the phase of maximal pyramidal cell population activity, near the phase where inhibition is least. 3) The phase advance is typically an accelerating, rather than linear, function of position within the place field. 4) These phenomena occur both on linear tracks and in two-dimensional environments where locomotion is not constrained to specific paths. 5) In two-dimensional environments, place-related firing is more spatially specific during the early part of the theta cycle than during the late part. This is also true, to a lesser extent, on a linear track. Thus, spatial selectivity waxes and wanes over the theta cycle. 6) Granule cells of the fascia dentata are also modulated by theta. The depth of modulation for the granule cell population approaches 100%, and the peak activity of the granule cell population comes about 90° earlier in the theta cycle than the peak firing of CA1 pyramidal cells. 7) Granule cells, like pyramidal cells, show robust phase precession. 8) Cross-correlation analysis shows that portions of the temporal sequence of CA1 pyramidal cell place fields are replicated repeatedly within individual theta cycles, in highly compressed form. The compression ratio can be as much as 10:1. These findings indicate that phase precession is a very robust effect, distributed across the entire hippocampal population, and that it is likely to be inherited from the fascia dentata or an earlier stage in the hippocampal circuit, rather than generated intrinsically within CA1. It is hypothesized that the compression of temporal sequences of place fields within individual theta cycles permits the use of long-term potentiation for learning of sequential structure, thereby giving a temporal dimension to hippocampal memory traces. © 1996 Wiley-Liss, Inc.     

Spatial features of the rat hippocampal vascular system

Earlier investigations in this laboratory demonstrated paired internal transverse arteries and veins associated with the rat cornu Ammonis (CA), mapped the vessels, and suggested segmental distribution and drainage patterns. Further studies of the smaller branches and capillaries were required to resolve the presence or absence of structurally segmented or isolated capillary beds common to vertebrate forms having paired intramedullary arteries and veins. Adult Wistar rats were injected with inks to demonstrate the arterial or venous tree or both. Each of the CA3 internal transverse arteries usually supplies numerous small-diameter branches to the adjacent blade of the fascia dentata. Often a major ramus supplies CA1 before the artery branches most profusely near the deepest point of the hippocampal fissure to supply the extrahilar CA3 and portions of the area dentata. Other vessels are located in the subiculum, adjacent regions of CA1, and the area dentata. Branches of the CA3 vessels more nearly parallel the lateral hippocampal axis than either of the other two. Numerous microvascular rami distribute obliquely across the longitudinal and transverse axes. As readily observed from thick, cleared India ink tissue sections, the strata molecularelacumosum and oriens are more vascular than the stratum radiatum where the long axis of capillaries tends to be oriented parallel to the apical dendrites. Clearly a subzone of CA1 stratum pyramidale, but not CA3, is the least vascular CA region. The outer portion of stratum moleculare of the fascia dentata is more vascular than the inner third. Well-developed capillary plexuses exist adjacent to the vascular poor zone of stratum granulosum and CA1. Branches of the internal transverse veins, deep veins, and external veins drain CA3 fields. Evidence of nonexclusive pairing of the internal arteries and veins, multiple systems draining the CA3 field, and the presence of anastomosing capillary beds indicate that structurally isolated segmental microvascular organizations do not exist for CA3 of the rat hippocampus.     

Cellular and field potential properties of epileptogenic hippocampal slices

Epileptogenic activity was induced in hippocampal slices by addition of penicillin (2.0 mM) to the binding medium. Field potential epileptiform events were recorded and single cell bursts studied with intracellular electrodes. Epileptogenic activity was seen in areas CA1 and CA3 of the slice, with bursts in CA3 always leading CA1 bursts; a cut between CA1 and CA3 abolished spontaneous bursting in CA1 but not in CA3. Increased [Mg²⁺] and decreased [Ca²⁺] abolished epileptiform discharge, thus demonstrating its dependence on synaptic activity; burst occurrence was also sensitive to [K⁺]. Measurements of single cell resting potentials, resistance, and time constant in CA1 cells revealed no difference between cells in normal medium and cells made epileptogenic by penicillin. Depolarization shifts in CA1 neurons during epileptogenesis did not behave like ‘giant EPSPs’ but rather were complexes to which depolarizing spike after-potentials, fast prepotentials, and underlying slow depolarizing events all contributed.     

The time of origin of neurons in the hippocampal region of the rhesus monkey

The time of origin of neurons in the hippocampal region was determined in a series of rhesus monkeys, each of which had been exposed to a pulse of tritiated thymidine (³H-TdR) at a different time during ontogeny and sacrificed between the second and fifth month after birth. No heavily labeled cells were found in the hippocampal region of animals exposed to ³H-TdR before embryonic day 33 (E33). Exposure to ³H-TdR given at E36 labels a few neurons in the deepest layers of the entorhinal area, and ³H-TdR given at E38 labels a small number of neurons in all hippocampal subdivisions. Although the first neurons are generated almost simultaneously throughout the hippocampal region, the proliferation ceases at a different time in each subdivision. The last neurons destined for the entorhinal area and presubiculum are generated between E70 and E75, whereas the last parasubicular neurons are generated between E75 and E80. The production of neurons that form the subiculum ends about two weeks earlier, between E56 and E65. Within the hippocampus, genesis of pyramidal cells ends between E70 and E80 in area CA1, between E56 and E65 in area CA2, between E65 and E70 in area CA3, and between E75 and E80 in area CA4. In contrast, the genesis of granule cells of the fascia dentata is considerably prolonged. It continues throughout the second half of gestation, declines steadily in the course of the first postnatal month, and tapers off during the next 2 months. There is a distinct inside-to-outside spatiotemporal gradient in the parahippocampal formation and in the stratum pyramidale of both the subiculum and hippocampus. In contrast, the spatiotemporal pattern of granule cell origin in the dentate gyrus is outside-to-inside. Furthermore, granule cells generated between E36 and E80 are distributed in a distinct suprapyramidal-to-infrapyramidal gradient, whereas those generated at later ages are distributed evenly throughout the fascia dentata. Correlation of the present findings with histological data on hippocampal neurogenesis in the human brain demonstrates that the timing and sequence of developmental events as well as spatiotemporal gradients are similar in both primate species.     

Enhancement of recurrent inhibition by angular bundle kindling is retained in hippocampal slices

Rats were kindled in the right angular bundle. EEG and monosynaptically evoked responses were monitored in the ipsilateral fascia dentata. Although every animal was kindled, Long Term Potentiation (LTP) of monosynaptic responses was observed only in part of the kindling sessions, suggesting that LTP is not required for kindling. Paired pulse inhibition of granule cell discharge was progressively enhanced by kindling. Transverse hippocampal slices (400-700 micron) of fully kindled rats were prepared 1 hour after the last seizure. Field potentials evoked by stimulation appeared completely normal. Spontaneous epileptiform discharges were not observed in control solution, in low [Ca++]0, in high [K+]0 or after high frequency stimulation. Paired pulse inhibition was enhanced in fascia dentata, but not in area CA1. Enhancement of inhibition may be caused by increased activity of inhibitory synapses or by a nonsynaptic hyperpolarizing current. The relation between the absence of spontaneous activity and enhanced inhibition in the fascia dentata is unclear.