Morphological and electrophysiological characteristics of layer V neurons of the rat medial entorhinal cortex

Input from the presubiculum to dendrites of layer-V neurons of the medial entorhinal cortex of the rat

The entorhinal cortex (EC) and the hippocampus are reciprocally connected. Neurons in the superficial layers of EC project to the hippocampus, whereas deep entorhinal layers receive return connections. In the deep layers of EC, pyramidal neurons in layer V possess apical dendrites that ascend towards the cortical surface through layers IIII and II. These dendrites ramify in layer I. By way of their apical dendrites, such layer-V pyramidal cells may be exposed to input destined for the superficial entorhinal neurons. A specific and dense fiber projection that typically ends in superficial entorhinal layers of the medial EC originates in the presubiculum. To investigate whether apical dendrites of deep entorhinal pyramidal neurons indeed receive input from this projection, we injected the anterograde tracer PHA-L in the presubiculum or we lesioned the presubiculum, and we applied in the same experiments the tracer Neurobiotin trade mark pericellularly in layer V of the medial EC of 17 rats. PHA-L labeled presubiculum axons in the superficial layers apposing apical segments of Neurobiotin labeled layer-V cell dendrites were studied with a confocal fluorescence laserscanning microscope. Axons and dendrites were 3D reconstructed from series of confocal images. In cases in which the presubiculum had been lesioned, material was investigated in the electron microscope. At the confocal fluorescence microscope level we found numerous close contacts, i.e. appositions of boutons on labeled presubiculum fibers with identified dendrites of layer-V neurons. In the electron microscope we observed synapses between degenerating axon terminals and spines on dendrites belonging to layer-V neurons. Hence we conclude that layer-V neurons receive synaptic contacts from presubiculum neurons. These findings indicate that entorhinal layer-V neurons have access to information destined for the superficial layers and eventually the hippocampal formation. At the same time, they have access to the hippocampally processed version of that information.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Morphological and numerical analysis of synaptic interactions between neurons in deep and superficial layers of the entorhinal cortex of the rat

Neurons providing connections between the deep and superficial layers of the entorhinal cortex (EC) constitute a pivotal link in the network underlying reverberation and gating of neuronal activity in the entorhinal-hippocampal system. To learn more of these deep-to-superficial neurons and their targets, we applied the tracer Neurobiotin pericellularly in layer V of the medial EC of 12 rats. Labeled axons in the superficial layers were studied with light and electron microscopy, and their synaptic organization recorded. Neurobiotin-labeled layer V neurons displayed "Golgi-like" staining. Two major cell types were distinguished among these neurons: (1) pyramidal neurons with apical spiny dendrites traversing all layers and ramifying in layer I, and (2) horizontal neurons with dendrites confined to the deep layers. Labeled axons ramified profusely in layer III, superficially in layer II and deep in layer I. Analysis of labeled axon terminals in layers I-II and III showed that most synapses (95%) were asymmetrical. Of these synapses, 56% occurred with spines (presumably belonging to principal neurons) and 44% with dendritic shafts (presumably interneurons). A small fraction of the synapses (5%) was of the symmetrical type. Such synapses were mainly seen on dendritic shafts. We found in two sections a symmetrical synapse on a spine. These findings suggest that the deep to superficial projection is mainly excitatory in nature, and that these fibers subserve both excitation and feed-forward inhibition. There is an additional, much weaker, inhibitory component in this projection, which may have a disinhibitory effect on the entorhinal network in the superficial layers.

Ca2+-independent muscarinic excitation of rat medial entorhinal cortex layer V neurons

Cholinergic activation of entorhinal cortex (EC) layer V neurons plays a crucial role in the medial temporal lobe memory system and in the pathophysiology of temporal lobe epilepsy. Here, we demonstrate that muscarinic activation by focal application of carbachol depolarizes EC layer V neurons and induces epileptiform activity in rat brain slices. These seizure-like bursts are associated with a somatic [Ca2+]i increase of 293 +/- 82 nm and are blocked by the glutamate receptor antagonists CNQX and APV. Muscarinic activation did not directly evoke a [Ca2+]i increase, but subthreshold and suprathreshold depolarization did. Functional axon mapping revealed local axon branching as well as axon collaterals ascending to layers II and III. During blockade of ionotropic glutamatergic AMPA and NMDA receptors, carbachol depolarized layer V neurons by +7.5 +/- 3.4 mV. This direct muscarinic depolarization was associated with a conductance increase of 35 +/- 10.3% (+4.3 +/- 1.25 nS). Intracellular buffering of [Ca2+]i changes did not block this depolarization, but prolonged action potential duration and reduced adaptation of action potential firing. The muscarinic depolarization was neither blocked by combining intracellular Ca2+-buffering (EGTA or BAPTA) with non-specific Ca2+-channel inhibition by Ni+ (1 mm), nor by Ba2+ (1 mm) nor during inhibition of the h-current by 2 mm Cs+. In whole-cell patch-clamp recording, reversal of the muscarinic current occurred at about -45 mV and -5 mV with complete substitution of intrapipette K+ with Cs+. Thus, muscarinic depolarization of EC layer V neurons appears to be primarily mediated by Ca2+-independent activation of non-specific cation channels that conduct K+ about three times as well as Na+.

Synaptic activation patterns of the perirhinal-entorhinal inter-connections

Ample neuropsychological evidence supports the role of rhinal cortices in memory. The perirhinal cortex (PRC) represents one of the main conduits for the bi-directional flow of information between the entorhinal-hippocampal network and the cortical mantle, a process essential in memory formation. However, despite anatomical evidence for a robust reciprocal connectivity between the perirhinal and entorhinal cortices, neurophysiological understanding of this circuitry is lacking. We now present the results of a series of electrophysiological experiments in rats that demonstrate robust synaptic activation patterns of the perirhinal-entorhinal inter-connections. First, using silicon multi-electrode arrays placed under visual guidance in vivo we performed current source density (CSD) analysis of lateral entorhinal cortex (LEC) responses to PRC stimulation, which demonstrated a current sink in layers II-III of the LEC with a latency consistent with monosynaptic activation. To further substantiate and extend this conclusion, we developed a PRC-LEC slice preparation where CSD analysis also revealed a current sink in superficial LEC layers in response to PRC stimulation. Importantly, intracellular recording of superficial LEC layer neurons confirmed that they receive a major monosynaptic excitatory input from the PRC. Finally, CSD analysis of the LEC to PRC projection in vivo also allowed us to document robust feedback synaptic activation of PRC neurons to deep LEC layer activation. We conclude that a clear bidirectional pattern of synaptic interactions exists between the PRC and LEC that would support a dynamic flow of information subserving memory function in the temporal lobe.

Visualization of the dendritic arbor of neurons in intact 500 microm thick brain slices

Characterizing the structure and electrophysiological properties of single neurons is essential for understanding how individual cells contribute to the function of neuronal networks. Following intra-cellular recording from neurons in acute brain slices, the structure of the recorded cell has typically been examined by serial sectioning of the tissue slice and then reconstructing the neuron of interest; a labor-intensive and time-consuming process. Here, we have adapted a whole-mount immunohistochemical technique and used it to visualize the dendritic arbor of individual neurons in sections of adult CNS tissue up to 500 microm thick. Permeabilization of the slice and extensive washing allow histochemical reagents to penetrate and be washed from the section, producing limited background staining. Using this method, the cell within the slice can be sectioned optically and reconstructed using the optical sections. We present images of the dendritic trees of neurons in 500 microm thick slices of adult rat entorhinal cortex and hippocampus, labeled either immunohistochemically, or by biocytin injection following whole-cell patch clamp or sharp electrode recordings. The resolution obtained is sufficient to visualize dendritic spines deep within the section. The method is free from artifacts associated with cutting serial sections and is broadly applicable to tasks that require visualization of the fine structure of individual cells in thick slices of CNS tissue.

Graded persistent activity in entorhinal cortex neurons

Working memory represents the ability of the brain to hold externally or internally driven information for relatively short periods of time. Persistent neuronal activity is the elementary process underlying working memory but its cellular basis remains unknown. The most widely accepted hypothesis is that persistent activity is based on synaptic reverberations in recurrent circuits. The entorhinal cortex in the parahippocampal region is crucially involved in the acquisition, consolidation and retrieval of long-term memory traces for which working memory operations are essential. Here we show that individual neurons from layer V of the entorhinal cortex-which link the hippocampus to extensive cortical regions-respond to consecutive stimuli with graded changes in firing frequency that remain stable after each stimulus presentation. In addition, the sustained levels of firing frequency can be either increased or decreased in an input-specific manner. This firing behaviour displays robustness to distractors; it is linked to cholinergic muscarinic receptor activation, and relies on activity-dependent changes of a Ca2+-sensitive cationic current. Such an intrinsic neuronal ability to generate graded persistent activity constitutes an elementary mechanism for working memory.

Three classes of pyramidal neurons in layer V of rat perirhinal cortex

Whole-cell recordings from 140 pyramidal neurons in layer V of rat perirhinal cortex (PR) revealed three distinct firing patterns: regular spiking (RS, 76%), burst spiking (BS, 9%), and late spiking (LS, 14%). LS neurons have not previously been reported in layer V of any cortical region. LS cells in layer V of PR exhibited delays of up to 12 s from onset of a depolarizing current step to spike threshold, followed by sustained firing. In contrast, pyramidal cells in layer V of other cortical regions contain only RS and BS cells. Within PR, the percentage of LS neurons in layer V differs markedly from what we previously observed in layers II/III (50% LS) and VI (90% LS). Morphologically, BS neurons in layer V of PR had thick primary apical dendrites that terminated in a tuft within layer I, whereas RS and LS cells had relatively thin primary apicals that terminated either diffusely or in a layer I tuft. At holding potentials near rest, PR neurons exhibited small (approximately 15 pA), inward, spontaneous postsynaptic currents (PSCs) that were indistinguishable among the three cell types. Currents evoked by minimal stimulation of layer I were about 2.8 times larger than the spontaneous PSCs. Evoked currents had unusually long onset latencies with little variation in latency, consistent with monosynaptic responses evoked by stimulation of unmyelinated fibers. The prevalence of LS cells in combination with the long-latency monosynaptically evoked PSCs suggested that PR is not a region of rapid throughput. This is consistent with anatomical data suggesting that PR is a higher-level association cortex. These data further advance an emerging picture of PR as a cortical region with a unique distribution of cell types different from other cortical regions.

Differential excitability and voltage-dependent Ca2+ signalling in two types of medial entorhinal cortex layer V neurons

The entorhinal cortex (EC) is a key structure in memory formation, relaying sensory information to the hippocampal formation and processed information to the neocortex. EC neurons in the deep layers modulate the transfer of sensory information by the superficial layers and the dentate gyrus, and form the output to the neocortex. Here we characterize two types of EC layer V neurons by their fluorescence morphology, electrophysiology and intracellular Ca2+ signalling using intracellular recording and Ca2+ imaging. Pyramidal neurons show, in response to depolarizing current pulses, regular firing with strong adaptation and a fast and medium afterhyperpolarization (AHP) which are separated by a depolarizing notch and, with hyperpolarizing current injection, a transient sag. Multipolar cells respond to depolarization with delayed firing with very weak adaptation and have no depolarizing notch between fast and medium AHP and no sag with hyperpolarization. The delayed firing was blocked by 30 micro m 4-aminopyridine, indicating mediation by the D-type potassium current. Subthreshold depolarization evoked membrane potential oscillations of 2-5 Hz in both cell types and an increase in [Ca2+]i of 37 nm in pyramidal and 59 nm in multipolar neurons. Repetitive firing at 10 Hz for 30 s increased [Ca2+]i in pyramidal and multipolar neurons by 194 and 295 nm, respectively. Differential temporal firing and Ca2+ signalling suggest specific information processing and synaptic memory storage possibilities in these two layer V cell types of the EC.

Morphological and electrophysiological characteristics of layer V neurons of the rat lateral entorhinal cortex

The intrinsic electrophysiological and morphological properties of lateral entorhinal area (LEA) layer V neurons were investigated by sharp electrode intracellular recording and biocytin labeling in vitro. The morphological analysis revealed that layer V of the LEA contains three distinct subtypes of principal neurons, which were classified as pyramidal, horizontal, and polymorphic neurons. Pyramidal cells were the most abundant subtype (57%) and could be further subdivided into neurons with large, small, and star-like somas. Similarly to pyramidal cells, horizontal neurons (11%) had a prominent apical dendrite. However, their distinctive basal dendritic plexus extended primarily in the horizontal plane. Polymorphic neurons (32%) were characterized by a multipolar dendritic organization. Electrophysiological analysis of neurons in the three categories demonstrated a diversity of electrophysiological profiles within each category and no significant differences between groups. Neurons in the three subgroups could display instantaneous and/or time-dependent inward rectification and different degrees of spike frequency adaptation. None of the recorded cells displayed an intrinsic oscillatory bursting discharge. Many neurons in the three subgroups, however, displayed slow (3.5-14 Hz), sustained, subthreshold membrane potential oscillations. The morphological and electrophysiological diversity of principal neurons in the LEA parallels that previously reported for the medial entorhinal area and suggests that, with respect to the deep layers, similar information processing is performed across the mediolateral extent of the entorhinal cortex. Layer V of the entorhinal cortex may undertake very complex operations beyond acting as a relay station of hippocampal processed information to the neocortex.

A comparison of the firing properties of putative excitatory and inhibitory neurons from CA1 and the entorhinal cortex

The superficial layers of the entorhinal cortex (EC) provide the majority of the neocortical input to the hippocampus, and the deep layers of the EC receive the majority of neocortically bound hippocampal outputs. To characterize information transmission through the hippocampal and EC circuitry, we recorded simultaneously from neurons in the superficial EC, the CA1 region of hippocampus, and the deep EC while rodents ran for food reward in two environments. Spike waveform analysis allowed us to classify units as fast-spiking (FS) putative inhibitory cells or putative excitatory (PE) cells. PE and FS units' firing were often strongly correlated at short time scales, suggesting the presence a monosynaptic connection from the PE to FS units. EC PE units, unlike those found in CA1, showed little or no tendency to fire in bursts. We also found that the firing of FS and PE units from all regions was modulated by the approximately 8 Hz theta rhythm, although the firing of deep EC FS units tended to be less strongly modulated than that of the other types of units. When we examined the spatial specificity of FS units, we determined that FS units in all three regions showed low specificity. At the same time, retrospective coding, in which firing rates were related to past position, was present in FS units from all three regions and deep EC FS units often fired in a "path equivalent" manner in that they were active in physically different, but behaviorally related positions both within and across environments. Our results suggest that while the firing of FS units from CA1 and the EC show similarly low levels of position specificity, FS units from each region differ from one another in that they mirrored the associated PE units in terms of their tendency to show more complex positional firing properties like retrospective coding and path equivalence.

Persistent sodium channel activity mediates subthreshold membrane potential oscillations and low-threshold spikes in rat entorhinal cortex layer V neurons

Entorhinal cortex layer V occupies a critical position in temporal lobe circuitry since, on the one hand, it serves as the main conduit for the flow of information out of the hippocampal formation back to the neocortex and, on the other, it closes a hippocampal-entorhinal loop by projecting upon the superficial cell layers that give rise to the perforant path. Recent in vitro electrophysiological studies have shown that rat entorhinal cortex layer V cells are endowed with the ability to generate subthreshold oscillations and all-or-none, low-threshold depolarizing potentials. In the present study, by applying current-clamp, voltage-clamp and single-channel recording techniques in rat slices and dissociated neurons, we investigated whether entorhinal cortex layer V cells express a persistent sodium current and sustained sodium channel activity to evaluate the contribution of this activity to the subthreshold behavior of the cells. Sharp-electrode recording in slices demonstrated that layer V cells display tetrodotoxin-sensitive inward rectification in the depolarizing direction, suggesting that a persistent sodium current is present in the cells. Subthreshold oscillations and low-threshold regenerative events were also abolished by tetrodotoxin, suggesting that their generation also requires the activation of such a low-threshold sodium current. The presence of a persistent sodium current was confirmed in whole-cell voltage-clamp experiments, which revealed that its activation "threshold" was negative by about 10mV to that of the transient sodium current. Furthermore, stationary noise analysis and cell-attached, patch-clamp recordings indicated that whole-cell persistent sodium currents were mediated by persistent sodium channel activity, consisting of relatively high-conductance ( approximately 18pS) sustained openings. The presence of a persistent sodium current in entorhinal cortex layer V cells can cause the generation of oscillatory behavior, bursting activity and sustained discharge; this might be implicated in the encoding of memories in which the entorhinal cortex participates but, under pathological situations, may also contribute to epileptogenesis and neurodegeneration.