Membrane properties of identified lateral and medial perforant pathway projection neurons

Sleep and wake in a model of the thalamocortical system with Martinotti cells

The mechanisms leading to the alternation between active (UP) and silent (DOWN) states during sleep slow waves (SWs) remain poorly understood. Previous models have explained the transition to the DOWN state by a progressive failure of excitation because of the build-up of adaptation currents or synaptic depression. However, these models are at odds with recent studies suggesting a role for presynaptic inhibition by Martinotti cells (MaCs) in generating SWs. Here, we update a classical large-scale model of sleep SWs to include MaCs and propose a different mechanism for the generation of SWs. In the wake mode, the network exhibits irregular and selective activity with low firing rates (FRs). Following an increase in the strength of background inputs and a modulation of synaptic strength and potassium leak potential mimicking the reduced effect of acetylcholine during sleep, the network enters a sleep-like regime in which local increases of network activity trigger bursts of MaC activity, resulting in strong disfacilitation of the local network via presynaptic GABAB1a -type inhibition. This model replicates findings on slow wave activity (SWA) during sleep that challenge previous models, including low and skewed FRs that are comparable between the wake and sleep modes, higher synchrony of transitions to DOWN states than to UP states, the possibility of triggering SWs by optogenetic stimulation of MaCs, and the local dependence of SWA on synaptic strength. Overall, this work points to a role for presynaptic inhibition by MaCs in the generation of DOWN states during sleep.

Activation of Dopamine D2 Receptors Alleviates Neuronal Hyperexcitability in the Lateral Entorhinal Cortex via Inhibition of HCN Current in a Rat Model of Chronic Inflammatory Pain

Functional changes in synaptic transmission from the lateral entorhinal cortex to the dentate gyrus (LEC-DG) are considered responsible for the chronification of pain. However, the underlying alterations in fan cells, which are the predominant neurons in the LEC that project to the DG, remain elusive. Here, we investigated possible mechanisms using a rat model of complete Freund's adjuvant (CFA)-induced inflammatory pain. We found a substantial increase in hyperpolarization-activated/cyclic nucleotide-gated currents (Ih), which led to the hyperexcitability of LEC fan cells of CFA slices. This phenomenon was attenuated in CFA slices by activating dopamine D2, but not D1, receptors. Chemogenetic activation of the ventral tegmental area -LEC projection had a D2 receptor-dependent analgesic effect. Intra-LEC microinjection of a D2 receptor agonist also suppressed CFA-induced behavioral hypersensitivity, and this effect was attenuated by pre-activation of the Ih. Our findings suggest that down-regulating the excitability of LEC fan cells through activation of the dopamine D2 receptor may be a strategy for treating chronic inflammatory pain.

High Pressure and [Ca 2+ ] Produce an Inverse Modulation of Synaptic Input Strength and Network Excitability in the Rat Dentate Gyrus

Hyperbaric environments induce the high pressure neurological syndrome (HPNS) characterized by hyperexcitability of the central nervous system (CNS) and memory impairment. Human divers and other animals experience the HPNS at pressures beyond 1.1 MPa. High pressure depresses synaptic transmission and alters its dynamics in various animal models. Medial perforant path (MPP) synapses connecting the medial entorhinal cortex with the hippocampal formation are suppressed by 50% at 10.1MPa. Reduction of synaptic inputs is paradoxically associated with enhanced ability of dentate gyrus (DG)’ granule cells (GCs) to generate spikes at high pressure. This mechanism allows MPP inputs to elicit standard GC outputs at 0.1–25 Hz frequencies under hyperbaric conditions. An increased postsynaptic gain of MPP inputs probably allows diving animals to perform in hyperbaric environments, but makes them vulnerable to high intensity/frequency stimuli producing hyperexcitability. Increasing extracellular Ca²⁺ ([Ca²⁺]_o) partially reverted pressure-mediated depression of MPP inputs and increased MPP’s low-pass filter properties. We postulated that raising [Ca²⁺]_o in addition to increase synaptic inputs may reduce network excitability in the DG potentially improving its function and reducing sensitivity to high intensity and pathologic stimuli. For this matter, we activated the MPP with single and 50 Hz frequency stimuli that simulated physiologic and deleterious conditions, while assessing the GC’s output under various conditions of pressure and [Ca²⁺]_o. Our results reveal that the pressure and [Ca²⁺]_o produce an inverse modulation on synaptic input strength and network excitability. These coincident phenomena suggest a potential general mechanism of networks that adjusts gain as an inverse function of synaptic inputs’ strength. Such mechanism may serve for adaptation to variable pressure and other physiological and pathological conditions and may explain the increased sensitivity to strong sensory stimulation suffered by human deep-divers and cetaceans under hyperbaric conditions.

Input integration around the dendritic branches in hippocampal dentate granule cells

Recent studies have shown that the dendrites of several neurons are not simple translators but are crucial facilitators of excitatory postsynaptic potential (EPSP) propagation and summation of synaptic inputs to compensate for inherent voltage attenuation. Granule cells (GCs)are located at the gateway for valuable information arriving at the hippocampus from the entorhinal cortex. However, the underlying mechanisms of information integration along the dendrites of GCs in the hippocampus are still unclear. In this study, we investigated the input integration around dendritic branches of GCs in the rat hippocampus. We applied differential spatiotemporal stimulations to the dendrites using a high-speed glutamate-uncaging laser. Our results showed that when two sites close to and equidistant from a branching point were simultaneously stimulated, a nonlinear summation of EPSPs was observed at the soma. In addition, nonlinear summation (facilitation) depended on the stimulus location and was significantly blocked by the application of a voltage-dependent Ca(2+) channel antagonist. These findings suggest that the nonlinear summation of EPSPs around the dendritic branches of hippocampal GCs is a result of voltage-dependent Ca(2+) channel activation and may play a crucial role in the integration of input information.

Firing properties of entorhinal cortex neurons and early alterations in an Alzheimer's disease transgenic model

The entorhinal cortex (EC) is divided into medial (MEC) and lateral (LEC) anatomical areas, and layer II neurons of these two regions project to granule cells of the dentate gyrus through the medial and lateral perforant pathways (MPP and LPP), respectively. Stellate cells (SCs) represent the main neurons constituting the MPP inputs, while fan cells (FCs) represent the main LPP inputs. Here, we first characterized the excitability properties of SCs and FCs in adult wild-type (WT) mouse brain. Our data indicate that, during sustained depolarization, action potentials (APs) generated by SCs exhibit increased fast afterhyperpolarization and overshoot, making them able to fire at higher frequencies and to exhibit higher spike frequency adaptation (SFA) than FCs. Since the EC is one of the earliest brain regions affected during Alzheimer's disease (AD) progression, we compared SCs and FCs firing in 4-month-old WT and transgenic Tg2576 mice, a well-established AD mouse model. Tg2576-SCs displayed a slight increase in firing frequency during mild depolarization but otherwise normal excitability properties during higher stimulations. On the contrary, Tg2576-FCs exhibited a decreased firing frequency during mild and higher depolarizations, as well as an increased SFA. Our data identify the FCs as a neuronal population particularly sensitive to early pathological effects of chronic accumulation of APP-derived peptides, as it occurs in Tg2576 mice. As FCs represent the major input of sensory information to the hippocampus during memory acquisition, early alterations in their excitability profile could significantly contribute to the onset of cognitive decline in AD.

Cellular properties of principal neurons in the rat entorhinal cortex. I. The lateral entorhinal cortex

The lateral entorhinal cortex (LEC) provides a major cortical input to the hippocampal formation, equaling that of the medial entorhinal cortex (MEC). To understand the functional contributions made by LEC, basic knowledge of individual neurons, in the context of the intrinsic network, is needed. The aim of this study is to compare physiological and morphological properties of principal neurons in different LEC layers in postnatal rats. Using in vitro whole cell current-clamp recordings from up to four post hoc morphologically identified neurons simultaneously, we established that principal neurons show layer specific physiological and morphological properties, similar to those reported previously in adults. Principal neurons in L(ayer) I, LII, and LIII have the majority of their dendrites and axonal collaterals alone in superficial layers. LV contains mainly pyramidal neurons with dendrites and axons extending throughout all layers. A minority of LV and all principal neurons in LVI are neurons with dendrites confined to deep layers and axons in superficial and deep layers. Physiologically, input resistances and time constants of LII neurons are lower and shorter, respectively, than those observed in LV neurons. Fifty-four percent of LII neurons have sag potentials, resonance properties, and rebounds at the offset of hyperpolarizing current injection, whereas LIII and LVI neurons do not have any of these. LV neurons show prominent spike-frequency adaptation and a decrease in spike amplitudes in response to strong depolarization. Despite the well-developed interlaminar communication in LEC, the laminar differences in the biophysical and morphological properties of neurons suggest that their in vivo firing patterns and functions differ, similar to what is known for neurons in different MEC layers.

Dopaminergic suppression of synaptic transmission in the lateral entorhinal cortex

Dopaminergic projections to the superficial layers of the lateral entorhinal cortex can modulate the strength of olfactory inputs to the region. We have found that low concentrations of dopamine facilitate field EPSPs in the entorhinal cortex, and that higher concentrations of dopamine suppress synaptic responses. Here, we have used whole-cell current clamp recordings from layer II neurons to determine the mechanisms of the suppression. Dopamine (10 to 50 μM) hyperpolarized membrane potential and reversibly suppressed the amplitude of EPSPs evoked by layer I stimulation. Both AMPA- and NMDA-mediated components were suppressed, and paired-pulse facilitation was also enhanced indicating that the suppression is mediated largely by reduced glutamate release. Blockade of D₂-like receptors greatly reduced the suppression of EPSPs. Dopamine also lowered input resistance, and reduced the number of action potentials evoked by depolarizing current steps. The drop in input resistance was mediated by activation of D₁-like receptors, and was prevented by blocking K⁺ channels with TEA. The dopaminergic suppression of synaptic transmission is therefore mediated by a D₂ receptor-dependent reduction in transmitter release, and a D₁ receptor-dependent increase in a K⁺ conductance. This suppression of EPSPs may dampen the strength of sensory inputs during periods of elevated mesocortical dopamine activity.

Development of theta rhythmicity in entorhinal stellate cells of the juvenile rat

Mature stellate cells of the rat medial entorhinal cortex (EC), layer II, exhibit subthreshold membrane potential oscillations (MPOs) at theta frequencies (4-12 Hz) in vitro. We find that MPOs appear between postnatal days 14 (P14) and 18 (P18) but show little further change by day 28+ (P28-P32). To identify the factors responsible, we examined the electrical responses of developing stellate cells, paying attention to two currents thought necessary for mature oscillation: the h current I(h), which provides the slow rectification required for resonance; and a persistent sodium current I(NaP), which provides amplification of resonance. Responses to injected current revealed that P14 cells were often nonresonant with a relatively high resistance. Densities of I(h) and I(NaP) both rose by about 50% from P14 to P18. However, I(h) levels fell to intermediate values by P28+. Given the nonrobust trend in I(h) expression and a previously demonstrated potency of even low levels of I(h) to sustain oscillation, we propose that resonance and MPOs are limited at P14 more by low levels of I(NaP) than of I(h). The relative importance of I(NaP) for the development of MPOs is supported by simulations of a conductance-based model, which also suggest that general shunt conductance may influence the precise age when MPOs appear. In addition to our physiological study, we analyzed spine densities at P14, P18, and P28+ and found a vigorous synaptogenesis across the whole period. Our data predict that functions that rely on theta rhythmicity in the hippocampal network are limited until at least P18.

The perforant path: projections from the entorhinal cortex to the dentate gyrus

This paper provides a comprehensive description of the organization of projections from the entorhinal cortex to the dentate gyrus, which together with projections to other subfields of the hippocampal formation form the so-called perforant pathway. To this end, data that are primarily from anatomical studies in the rat will be summarized, complimented with comparative data from other species. The analysis of the organization of any of the connections of the hippocampus, including that of the entorhinal cortex to the dentate gyrus, is severely hampered because of the complex three-dimensional shape of the hippocampus. In particular in rodents, but to a lesser extent also in primates, all traditional planes of sectioning will result in sections that at some point or another do not cut through the hippocampus at an angle that is perpendicular to its long axis. To amend this, we will describe own unpublished tracing data obtained in the rat with the use of the so-called extended preparation. A number of issues will be addressed. First, data will be summarized which will clarify the laminar origin of the perforant pathway within the entorhinal cortex. Second, we will discuss whether or not a radial organization, along the proximo-distal dendritic axis of granule cells, characterizes the entorhinal-dentate projection. Third, we will discuss whether this projection is governed by any transverse organization, and fourth, we will focus on the organization along the longitudinal axis. Finally, the synaptic organization and the contralateral entorhinal-dentate projection will be described briefly. Taken together, the available data suggest that the projection from the entorhinal cortex to the dentate gyrus is a fairly well conserved connection, present in all species studied, exhibiting a grossly similar organization.

Dopamine has bidirectional effects on synaptic responses to cortical inputs in layer II of the lateral entorhinal cortex

Dopaminergic modulation of neuronal function has been extensively studied in the prefrontal cortex, but much less is known about its effects on glutamate-mediated synaptic transmission in the entorhinal cortex. The mesocortical dopamine system innervates the superficial layers of the lateral entorhinal cortex and may therefore modulate sensory inputs to this area. In awake rats, systemic administration of the dopamine reuptake inhibitor GBR12909 (10 mg/kg, ip) enhanced extracellular dopamine levels in the entorhinal cortex and significantly facilitated field excitatory postsynaptic potentials (fEPSPs) in layer II evoked by piriform cortex stimulation. An analysis of the receptor subtypes involved in the facilitation of evoked fEPSPs was conducted using horizontal slices of lateral entorhinal cortex in vitro. The effects of 15-min bath application of dopamine on synaptic responses were bidirectional and concentration dependent. Synaptic responses were enhanced by 10 microM dopamine and suppressed by concentrations of 50 and 100 microM. The D(1)-receptor antagonist SCH23390 (50 microM) blocked the significant facilitation of synaptic responses induced by 10 microM dopamine and the D(2)-receptor antagonist sulpiride (50 microM) prevented the suppression of fEPSPs observed with higher concentrations of dopamine. We propose here that dopamine release in the lateral entorhinal cortex, acting through D(1) receptors, can lead to an enhancement of the salience of sensory representations carried to this region from adjacent sensory cortices.

Morphological and electrophysiological properties of lateral entorhinal cortex layers II and III principal neurons

The intrinsic electrophysiology and morphology of neurons from layers II and III of the lateral entorhinal cortex (EC) was investigated in a rat brain slice preparation by intracellular recording and biocytin labeling. Morphologically, we distinguished three groups of layer II principal neurons. The most numerous group included cells with multiple radiating dendrites that spread over layers II and I in a fan-like fashion. While morphologically "fan" neurons were similar to the "stellate" cells of the medial EC, electrophysiologically the fan cells lacked the persistent rhythmic subthreshold oscillations and the very pronounced time-dependent inward rectification typical of the stellate cells. The second group consisted of pyramidal cells that manifested regular spike firing and had a more negative resting potential and a longer spike duration than the fan cells. In the third group we included all those neurons that had diverse multipolar appearances distinct from the fan cells. Neurons in this group had electrophysiological profiles intermediate between those of the fan and pyramidal cells. All neurons recorded in layer III were pyramidal in shape with a basal dendritic tree that could extend into layer V and an axon that could also give off collaterals into layer V. Electrophysiologically, layer III pyramidal cells were very similar to those of layer II. On the basis of these and other data we suggest that in different EC regions layer II neurons may be conducting more input-dependent specialized processing, while cells from layer III may perform a more global or generalized function.

Hippocampus-mediated activation of superficial and deep layer neurons in the medial entorhinal cortex of the isolated guinea pig brain

The entorhinal cortex (EC) is regarded as the structure that regulates information flow to and from the hippocampus. It is commonly assumed that superficial and deep EC neurons project to and receive from the hippocampal formation, respectively. Anatomical evidences suggest that both the hippocampal output and deep EC neurons also project to superficial EC layers. To functionally characterize the interlaminar synaptic EC circuit entrained the by hippocampal output, we performed simultaneous intracellular recordings and laminar profile analysis in the medial EC (m-EC) of the in vitro isolated guinea pig brain after polysynaptic hippocampal activation by lateral olfactory tract (LOT) stimulation. Optical imaging of voltage-generated signals confirmed that the LOT-evoked hippocampus-mediated response is restricted to the m-EC. The hippocampal output generated an extracellular current sink in layers V-VI, coupled with an EPSP in deep neurons. Deep neuron firing was terminated by a biphasic IPSP. The earliest response observed in superficial layer neurons was characterized by a feedforward IPSP of circa 100 ms (-69 +/- 1.3 mV reversal potential) abolished by local application of 1 mm bicuculline. The feedforward IPSP was followed by a delayed EPSP blocked by AP-5 (100 microM), presumably mediated by deep-to-superficial m-EC connections. Our findings demonstrate that superficial m-EC cells are inhibited by the hippocampal output via a feedforward pathway that prevents activity reverberation in the hippocampal-EC-hippocampal loop. We propose that such inhibition could serve as a protective mechanism to prevent epileptic hyperexcitability.

Modeling Sleep and Wakefulness in the Thalamocortical System

When the brain goes from wakefulness to sleep, cortical neurons begin to undergo slow oscillations in their membrane potential that are synchronized by thalamocortical circuits and reflected in EEG slow waves. To provide a self-consistent account of the transition from wakefulness to sleep and of the generation of sleep slow waves, we have constructed a large-scale computer model that encompasses portions of two visual areas and associated thalamic and reticular thalamic nuclei. Thousands of model neurons, incorporating several intrinsic currents, are interconnected with millions of thalamocortical, corticothalamic, and both intra- and interareal corticocortical connections. In the waking mode, the model exhibits irregular spontaneous firing and selective responses to visual stimuli. In the sleep mode, neuromodulatory changes lead to slow oscillations that closely resemble those observed in vivo and in vitro. A systematic exploration of the effects of intrinsic currents and network parameters on the initiation, maintenance, and termination of slow oscillations shows the following. 1) An increase in potassium leak conductances is sufficient to trigger the transition from wakefulness to sleep. 2) The activation of persistent sodium currents is sufficient to initiate the up-state of the slow oscillation. 3) A combination of intrinsic and synaptic currents is sufficient to maintain the up-state. 4) Depolarization-activated potassium currents and synaptic depression terminate the up-state. 5) Corticocortical connections synchronize the slow oscillation. The model is the first to integrate intrinsic neuronal properties with detailed thalamocortical anatomy and reproduce neural activity patterns in both wakefulness and sleep, thereby providing a powerful tool to investigate the role of sleep in information transmission and plasticity.

Rapid deletion of mossy cells does not result in a hyperexcitable dentate gyrus: implications for epileptogenesis

Loss of cells from the hilus of the dentate gyrus is a major histological hallmark of human temporal lobe epilepsy. Hilar mossy cells, in particular, are thought to show dramatic numerical reductions in pathological conditions, and one prominent theory of epileptogenesis is based on the assumption that mossy cell loss directly results in granule cell hyperexcitability. However, whether it is the disappearance of hilar mossy cells from the dentate gyrus circuitry after various insults or the subsequent synaptic-cellular alterations (e.g., reactive axonal sprouting) that lead to dentate hyperexcitability has not been rigorously tested, because of the lack of available techniques to rapidly remove specific classes of nonprincipal cells from neuronal networks. We developed a fast, cell-specific ablation technique that allowed the targeted lesioning of either mossy cells or GABAergic interneurons in horizontal as well as axial (longitudinal) slices of the hippocampus. The results demonstrate that mossy cell deletion consistently decreased the excitability of granule cells to perforant path stimulation both within and outside of the lamella where the mossy cell ablation took place. In contrast, ablation of interneurons caused the expected increase in excitability, and control aspirations of the hilar neuropil or of interneurons in the presence of GABA receptor blockers caused no alteration in granule cell excitability. These data do not support the hypothesis that loss of mossy cells from the dentate hilus after seizures or traumatic brain injury directly results in hyperexcitability.

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.

Reduced inhibition and increased output of layer II neurons in the medial entorhinal cortex in a model of temporal lobe epilepsy

Temporal lobe epilepsy is the most common type of epilepsy in adults, and its underlying mechanisms are unclear. To investigate how the medial entorhinal cortex might contribute to temporal lobe epilepsy, we evaluated the histology and electrophysiology of slices from rats 3-7 d after an epileptogenic injury (pilocarpine-induced status epilepticus). Nissl staining, NeuN immunocytochemistry, and in situ hybridization for GAD65 mRNA were used to verify the preferential loss of glutamatergic neurons and the relative sparing of GABAergic interneurons in layer III. From slices adjacent to those that were used for anatomy, we obtained whole-cell patch recordings from layer II medial entorhinal cortical neurons. Recordings under current-clamp conditions revealed similar intrinsic electrophysiological properties (resting membrane potential, input resistance, single spike, and repetitive firing properties) to those of controls. Spontaneous IPSCs were less frequent (68% of controls), smaller in amplitude (57%), and transferred less charge (51%) than in controls. However, the frequency, amplitude, and rise time of miniature IPSCs were normal. These findings suggest that after epileptogenic injuries the layer II entorhinal cortical neurons receive less GABA(A) receptor-mediated synaptic input because presynaptic inhibitory interneurons become less active. To investigate the possible consequences of reduced spontaneous inhibitory input to layer II neurons, we recorded field potentials in the dentate gyrus, their major synaptic target. At 5 d after pilocarpine-induced status epilepticus the spontaneous field potentials recorded in vivo were over three times more frequent than in controls. These findings suggest that an epileptogenic injury reduces inhibition of layer II neurons and results in excessive synaptic input to the dentate gyrus.

Input and output stations of the entorhinal cortex: superficial vs. deep layers or lateral vs. medial divisions?

Based on the results of recent electrophysiological and anatomical studies, we argue that the classical division of the entorhinal cortex (EC) into a superficial layer input station and deep layer output station is no longer tenable. We point out that the anatomical data suggest that the medial and lateral divisions of EC are separate, and recent studies of the propagation of signals originating in the lateral olfactory tract and perirhinal cortex to the EC [J. Neurophysiol. 83 (2000) 1924-1931; Biella and de Curtis, 2000) indicate that the lateral division is the input station, and the medial division the output station for information processed in the hippocampus and subiculum.