Electrophysiological properties of dentate granule cells in mouse hippocampal slices maintained in vitro

Intrinsic neurophysiological properties of hilar ectopic and normotopic dentate granule cells in human temporal lobe epilepsy and a rat model

Hilar ectopic dentate granule cells (DGCs) are a salient feature of aberrant plasticity in human temporal lobe epilepsy (TLE) and most rodent models of the disease. Recent evidence from rodent TLE models suggests that hilar ectopic DGCs contribute to hyperexcitability within the epileptic hippocampal network. Here we investigate the intrinsic excitability of DGCs from humans with TLE and the rat pilocarpine TLE model with the objective of comparing the neurophysiology of hilar ectopic DGCs to their normotopic counterparts in the granule cell layer (GCL). We recorded from 36 GCL and 7 hilar DGCs from human TLE tissue. Compared with GCL DGCs, hilar DGCs in patient tissue exhibited lower action potential (AP) firing rates, more depolarized AP threshold, and differed in single AP waveform, consistent with an overall decrease in excitability. To evaluate the intrinsic neurophysiology of hilar ectopic DGCs, we made recordings from retrovirus-birthdated, adult-born DGCs 2-4 mo after pilocarpine-induced status epilepticus or sham treatment in rats. Hilar DGCs from epileptic rats exhibited higher AP firing rates than normotopic DGCs from epileptic or control animals. They also displayed more depolarized resting membrane potential and wider AP waveforms, indicating an overall increase in excitability. The contrasting findings between disease and disease model may reflect differences between the late-stage disease tissue available from human surgical specimens and the earlier disease stage examined in the rat TLE model. These data represent the first neurophysiological characterization of ectopic DGCs from human hippocampus and prospectively birthdated ectopic DGCs in a rodent TLE model.

Inhibition and interneuron distribution in the dentate gyrus of p35 knockout mice

The p35 knockout (p35-/-) mouse is an animal model of temporal lobe epilepsy that recapitulates key neuroanatomic abnormalities-granule cell dispersion and mossy fiber sprouting-observed in the hippocampal formation of humans, as well as spontaneous seizure activity. It is a useful model in which to study the relationship between the abnormal neuronal structure and seizure activity to further our understanding of cortical dysplasia in epileptogenesis. Our previous work using this mouse model characterized the anatomic features of the dentate granule cells and the functional implications of these abnormalities on increased recurrent excitation. These data also suggested that there might be compromised inhibition in this animal model. We pursued this possibility, focusing our investigation on inhibitory circuitry. In preliminary investigations using neuroanatomic tools (immunocytochemistry, camera lucida reconstructions of individually labeled interneurons, and electron microscopy) combined with intracellular electrophysiology, we observed no significant reduction in the number of symmetric versus asymmetric synaptic contacts on dentate granule cell somata, and no statistically significant changes in evoked early or late inhibition. Although there were some abnormalities in the morphology/distribution of inhibitory interneurons (as well as a larger population of dentate granule cells) of the dentate gyrus, overall inhibition in the p35 knockout mouse appeared to be largely intact.

Modest increase in extracellular potassium unmasks effect of recurrent mossy fiber growth

The recurrent mossy fiber pathway of the dentate gyrus expands dramatically in many persons with temporal lobe epilepsy. The new connections among granule cells provide a novel mechanism of synchronization that could enhance the participation of these cells in seizures. Despite the presence of robust recurrent mossy fiber growth, orthodromic or antidromic activation of granule cells usually does not evoke repetitive discharge. This study tested the ability of modestly elevated [K(+)](o), reduced GABA(A) receptor-mediated inhibition and frequency facilitation to unmask the effect of recurrent excitation. Transverse slices of the caudal hippocampal formation were prepared from pilocarpine-treated rats that either had or had not developed status epilepticus with subsequent recurrent mossy fiber growth. During superfusion with standard medium (3.5 mM K(+)), antidromic stimulation of the mossy fibers evoked epileptiform activity in 14% of slices with recurrent mossy fiber growth. This value increased to approximately 50% when [K(+)](o) was raised to either 4.75 or 6 mM. Addition of bicuculline (3 or 30 microM) to the superfusion medium did not enhance the probability of evoking epileptiform activity but did increase the magnitude of epileptiform discharge if such activity was already present. (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (1 microM), which selectively activates type II metabotropic glutamate receptors present on mossy fiber terminals, strongly depressed epileptiform responses. This result implies a critical role for the recurrent mossy fiber pathway. No enhancement of the epileptiform discharge occurred during repetitive antidromic stimulation at frequencies of 0.2, 1, or 10 Hz. In fact, antidromically evoked epileptiform activity became progressively attenuated during a 10-Hz train. Antidromic stimulation of the mossy fibers never evoked epileptiform activity in slices from control rats under any condition tested. These results indicate that even modest changes in [K(+)](o) dramatically affect granule cell epileptiform activity supported by the recurrent mossy fiber pathway. A small increase in [K(+)](o) reduces the amount of recurrent mossy fiber growth required to synchronize granule cell discharge. Block of GABA(A) receptor-mediated inhibition is less efficacious and frequency facilitation may not be a significant factor.

Synaptic transmission in the rat dentate gyrus after adrenalectomy

Granule cells in the rat hippocampal dentate gyrus contain intracellular receptors for the adrenal hormone corticosterone. Activation of these receptors seems essential for granule cell viability, since removal of the adrenal gland (adrenalectomy) results within three days in apoptotic-like degeneration of granule cells. In the present study we used extracellular in vitro recording methods to study the synaptic transmission in the dentate gyrus of adrenalectomized animals, in sham-operated controls and adrenalectomized rats treated with a low dose of corticosterone. We found that particularly three days after adrenalectomy orthodromic field responses in the dentate gyrus were reduced in amplitude. Corticosterone-treated rats did not show this impairment of synaptic transmission. Antidromically-evoked field responses were also reduced after adrenalectomy, which indicates that postsynaptic cell properties rather than signal transduction in the synapses are under steroid control. Responses to paired pulse stimulation were only marginally affected, suggesting that interneuronal networks may be less affected by the hormones than the principal cells. These electrophysiological data indicate that adrenalectomy induced apoptotic-like degeneration in the hippocampal dentate gyrus is clearly associated with impaired processing of incoming information.

Physiological unmasking of new glutamatergic pathways in the dentate gyrus of hippocampal slices from kainate-induced epileptic rats

In humans with temporal lobe epilepsy and kainate-treated rats, the mossy fibers of the dentate granule cells send collateral axons into the inner molecular layer. Prior investigations on kainate-treated rats demonstrated that abnormal hilar-evoked events can occasionally be observed in slices with mossy fiber sprouting when gamma-aminobutyric acid-A (GABAA)-mediated inhibition is blocked with bicuculline. However, these abnormalities were observed infrequently, and it was unknown whether these rats were epileptic. Wuarin and Dudek reported that in slices from kainate-induced epileptic rats (3-13 mo after treatment), hilar stimulation evoked abnormal events in most slices with mossy fiber sprouting exposed simultaneously to bicuculline and elevated extracellular potassium concentration [K+]o. Using the same rats, extracellular recordings were obtained from granule cells in hippocampal slices to determine whether 1) hilar stimulation could evoke abnormal events in slices with sprouting in normal artificial cerebrospinal fluid (ACSF), 2) adding only bicuculline could unmask hilar-evoked abnormalities and glutamate-receptor antagonists could block these events, and 3) increasing only [K+]o could unmask these abnormalities. In normal ACSF, hilar stimulation evoked abnormal field potentials in 27% of slices with sprouting versus controls without sprouting (i.e., saline-treated or only 2-4 days after kainate treatment). In bicuculline (10 microM) alone, hilar stimulation triggered prolonged field potentials in 84% of slices with sprouting, but not in slices from the two control groups. Addition of the N-methyl-D-aspartate (NMDA) receptor antagonist, DL-2-amino-5-phosphonopentanoic acid (AP5), either blocked the bursts or reduced their probability of occurrence. The alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionate (AMPA)/kainate receptor antagonist, 6,7-dinitroquinoxaline-2,3-dione (DNQX), always eliminated the epileptiform bursts. In kainate-treated rats with sprouting, but not in saline-treated controls, abnormal hilar-evoked responses were also revealed in 6-9 mM [K+]o. Additionally, 63% of slices with sprouting generated spontaneous bursts lasting 1-40 s in ACSF containing 9 mm [K+]o; similar bursts were not observed in controls. These results indicate that 1) mossy fiber sprouting is associated with new glutamatergic pathways, and although NMDA receptors are important for propagation through these circuits, AMPA receptor activation is crucial, 2) modest elevations of [K+]o, in a range that would have relatively little effect on granule cells, can unmask these new excitatory circuits and generate epileptiform bursts, and 3) this new circuitry underlies an increased electrographic seizure susceptibility when inhibition is depressed or membrane excitability is increased.

Novel glutamate- and GABA-independent synaptic depolarization in granule cells of guinea-pig hippocampus

1. Dual intracellular recordings of granule cells, hilar interneurons and CA3 pyramidal cells were performed in transverse slices of guinea-pig hippocampus. At resting membrane potential, in the presence of 4-aminopyridine, ionotropic glutamate receptor antagonists and the GABAA receptor antagonist bicuculline, granule cells showed spontaneous, large amplitude depolarizations correlated with synchronous bursting activity of interneurons. 2. Under these conditions, pyramidal cells exhibited large amplitude monophasic GABAB inhibitory postsynaptic potentials (IPSPs) synchronous with the GABAergic interneuron burst discharges. The granule cells also received a GABAB input, which was evident only when the neurons were depolarized by DC injection. The GABAB receptor antagonist CGP 55,845A (CGP) blocked the GABAB IPSPs in both pyramidal cells and granule cells; however, the depolarizing potential in granule cells was unaffected by the drug. 3. The granule cells depolarization in the presence of CGP was monophasic and exhibited linear voltage dependence with a reversal potential around -40 mV, suggesting that it was generated by a synaptic input activating a mixed cationic current. 4. The granule cell depolarization was abolished following the addition of tetrodotoxin to the bath. In addition, perfusing the slice with a low Ca(2+)-containing solution (0.5 mM Ca(2+)-10 mM Mg2+) also abolished the granule cell depolarization, confirming the synaptic origin of the event. 5. (S)-Methyl-4-carboxyphenylglycine, L-(+)-2-amino-3-phosphonopropionic acid, propranolol and atropine did not affect the granule cell depolarization, indicating that metabotropic glutamate receptors, beta-adrenergic receptors and muscarinic cholinergic receptors were not involved in generating the granule cell depolarizing synaptic response. 6. These findings indicate that, in the absence of both glutamatergic and GABAergic inputs, synchronous interneuronal activity can produce a depolarizing synaptic response in granule cells. The neurochemical responsible for the depolarization is currently under investigation.

Single-channel currents trigger action potentials in small cultured hippocampal neurons

Spontaneous neuronal impulse activity appears to play a key role in some neural processes, such as the normal establishment of interneuronal connections during development. In addition, spontaneous impulses may be essential for the functional operation of neuronal networks. Mechanisms of spontaneous non-pacemaker impulse generation are, however, not well known. In this work, spontaneous electrical activity in small cultured hippocampal neurons from rat was studied with tight-seal recording techniques. The results demonstrate that spontaneous individual openings of single ion channels can trigger impulse generation in these high-resistance cells. First, impulses recorded in the whole-cell mode were apparently induced by spontaneous plateau-potential events showing the characteristics expected from individual openings and closures of ion channels. Second, patch-clamp recordings in the cell-attached configuration showed that openings of single ion channels in the patch membrane could trigger cellular impulses, detected as biphasic current deflections. These findings suggest that the random gating of ion channel molecules can be used as a mechanism for stochastic triggering of spontaneous impulses in mammalian central neurons.

Comparison between the membrane and synaptic properties of human and rodent dentate granule cells

We have compared the cellular and synaptic properties of rodent dentate granule cells with those of humans. The human tissue was obtained from neurosurgical procedures which necessitated removal of the hippocampus for treatment of extra-hippocampal tumors which presented clinically with seizures. The hippocampi studied here were neuroanatomically similar to autopsy controls. The present studies have demonstrated that there are few differences between rodent and human granule cells as regards either their membrane properties or their synaptic physiology and pharmacology. The differences we noted were (1) less spike frequency adaptation in the human relative to rodent cells; and (2) perforant path stimulation reliably elicited both feedforward and feedback inhibition in the rodent cells, while in the human tissue feedback inhibition appeared to predominate. It is unclear if these changes are due to the seizure experience or if they represent true species differences.

Electrophysiology of dentate granule cells after kainate-induced synaptic reorganization of the mossy fibers

Morphological data from humans with temporal lobe epilepsy and from animal models of epilepsy suggest that seizure-induced damage to dentate hilar neurons causes granule cells to sprout new axon collaterals that innervate other granule cells. This aberrant projection has been suggested to be an anatomical substrate for epileptogenesis. This hypothesis was tested in the present study with intra- and extracellular recordings from granule cells in hippocampal slices removed from rats 1-4 months after kainate treatment. In this animal model, hippocampal cell loss leads to sprouting of mossy fiber axons from the granule cells into the inner molecular layer of the dentate gyrus. Unexpectedly, when slices with mossy fiber sprouting were examined in normal medium, extracellular stimulation of the hilus or perforant path evoked relatively normal responses. However, in the presence of the GABAA-receptor antagonist, bicuculline, low-intensity hilar stimulation evoked delayed bursts of action potentials in about one-quarter of the slices. In one-third of the bicuculline-treated slices with mossy fiber sprouting, spontaneous bursts of synchronous spikes were superimposed on slow negative field potentials. Slices from normal rats or kainate-treated rats without mossy fiber sprouting never showed delayed bursts to weak hilar stimulation or spontaneous bursts in bicuculline. These data suggest that new local excitatory circuits may be suppressed normally, and then emerge functionally when synaptic inhibition is blocked. Therefore, after repeated seizures and excitotoxic damage in the hippocampus, synaptic reorganization of the mossy fibers is consistently associated with normal responses; however, in some preparations, the mossy fibers may form functional recurrent excitatory connections, but synaptic inhibition appears to mask these potentially epileptogenic alterations.

Impulses and resting membrane properties of small cultured rat hippocampal neurons

1. The impulses and resting membrane parameters of small (soma diameter less than 10 microM) cultured hippocampal neurons from rat embryos were studied with the tight-seal whole-cell recording technique. 2. Mean resting potential was -47 mV, mean input resistance 3.3 G omega, mean capacitance 11 pF, and mean time constant 33 ms. 3. Rectangular suprathreshold current steps elicited regenerative potential responses. The amplitude and time course of the responses were clearly stimulus dependent: stronger current steps caused impulses of larger amplitude. 4. The current threshold was very low: rheobase current was less than 15 pA. 5. The potential response depended on the preceding holding potential, responses from more negative potentials showing sharper peaks than those from more positive potentials. 6. Spontaneous impulses with pre-potentials similar to synaptically induced events were recorded from several cells. The amplitude of the spontaneous impulses varied similarly to that of the stimulus-induced responses.

Effects of ethanol on the excitability of hippocampal granule neurons

The effects of low dose ethanol treatment on neuronal firing threshold were studied in dentate granule neurons using intracellular recording. A higher threshold for orthodromic activation and lower threshold for somatic activation were observed in 50 mM of ethanol, based on an examination of the excitatory postsynaptic potentials, strength duration and repetitive firing properties. Experiments involving blockade of glutamate-receptor subtypes suggest that an N-methyl-D-aspartate (NMDA) receptor mediated mechanism is implicated in this action of ethanol.

Reconstruction of hippocampal granule cell electrophysiology by computer simulation

A model of the hippocampal granule cells was created that closely approximated most of the measured intracellular responses from a neuron under a variety of stimulus conditions. This model suggests that: (1) A simple, four-conductance model can account for most of the intracellular behavior of these neurons. (2) The repolarization mechanism in granule cells may be different from that in squid axons. A weak potassium conductance may be present in hippocampal granule neurons, which simultaneously give rise to a small, passive depolarizing afterpotential. (3) The strength duration properties may assist in identifying the electronic and sodium channel properties with short stimulus pulse widths. (4) Repetitive firing responses are highly dependent on the cell's recent history of activation and the regulation of the slow potassium conductance and calcium dynamics. (5) The anodic break response is probably not a property of typical granule cells. Through thorough and precise comparison of experimental and model responses, computer simulations can help assembling channel information into verifiable models that accurately reproduce intracellular data.

Graded action potentials in small cultured rat hippocampal neurons

The 'all-or-nothing' principle has been central to neurobiology since the beginning of this century. We here demonstrate that action potentials in small cultured neurons from the hippocampus of rat embryos clearly depend on stimulus strength and thus deviate from this principle. We also demonstrate that similar stimulus-dependent action potentials can be predicted from computations based on voltage-clamp measurements. The findings suggest that amplitude modulation of the action potential may be a principle for information processing in the mammalian nervous system.

In vivo intracellular analysis of rat dentate granule cells

In vivo intracellular recordings and dentate EEG were obtained in urethanized-curarized rats. Granule cells (GCs) were identified by antidromic activation as well as by intracellular staining with Lucifer yellow (LY). GCs fired spikes which, in 43.6% of the cases, had brief post-hyperpolarization. Slow spikes were recorded at hyperpolarized levels during the rebound of hyperpolarizing pulses or during inhibitory postsynaptic potentials (IPSPs). Medial septal nucleus or perforant pathway stimulation evoked an excitatory postsynaptic potential (EPSP)-IPSP sequence. During theta (theta) rhythm, the membrane potential of 80.3% of the GCs showed rhythmic sine-like waves of up to 15 mV at a theta frequency which were phase-locked with extracellular theta. GCs were classified into 3 types: type 1 (67.1%) showed intracellular theta and rhythmic firing; type 2 (13.2%) revealed intracellular theta and random firing, but spikes tended to occur at a preferred phase of the dentate theta; and type 3 (19.7%) had neither intracellular theta nor rhythmic firing. Intracellular theta amplitude was wider during injection of the hyperpolarizing current and narrower during depolarizing ones, indicating that rhythmic EPSPs contribute to theta genesis. Intracellular theta was unaffected by Cl- or Cs+ diffusion, suggesting that IPSP is not essential to theta genesis.

Development of ionic conductances in neurons of the inferior olive in the rat: an in vitro study

Neurons of the inferior olive of the rat were studied at different stages of their postnatal (PN) development by using the current clamp technique in slices maintained in vitro. Antidromic and synaptic activation of inferior olivary neurons could be achieved in preparations as young as PN day 2. Neurons at this age already exhibited a variety of ionic conductances which included fast sodium-dependent spikes, high-threshold and low-threshold calcium spikes, potassium-dependent currents, Ca-dependent after-hyperpolarizing potentials (AHPS), and both instantaneous and time-dependent inward rectification at hyperpolarized levels of membrane potential. The two types of Ca-dependent responses recorded in olivary neurons during the first postnatal week were graded with the magnitude of the depolarization imposed on the cells. Furthermore, the high-threshold Ca spikes were only clearly observed during this early period when K conductances were depressed by the injection of caesium into the cells or by bath application of 4-aminopyridine. In contrast, the high-threshold Ca spikes could be obtained without suppression of K currents and were all-or-none in character in some neurons after PN day 8 and in all neurons after PN day 11. The observations suggest that the balance between K and Ca currents changes throughout maturation and is largely in favour of the K current until about the end of the first PN week. At all ages studied, the low-threshold Ca spikes were much less sensitive to the Ca channel blocker cadmium than were the high-threshold Ca spikes. Finally, spontaneous, regular oscillations of the membrane potential were observed for the first time at PN day 16 and were only commonly observed after PN day 19, suggesting a late development of electrotonic coupling between olivary neurons.

Action potential repolarization and a fast after-hyperpolarization in rat hippocampal pyramidal cells

1. The repolarization of the action potential, and a fast after-hyperpolarization (a.h.p.) were studied in CA1 pyramidal cells (n = 76) in rat hippocampal slices (28-37 degrees C). Single spikes were elicited by brief (1-3 ms) current pulses, at membrane potentials close to rest (-60 to -70 mV). 2. Each action potential was followed by four after-potentials: (a) the fast a.h.p., lasting 2-5 ms; (b) an after-depolarization; (c) a medium a.h.p., (50-100 ms); and (d) a slow a.h.p. (1-2 s). Both the fast a.h.p. and the slow a.h.p. (but not the medium a.h.p.) were inhibited by Ca2+-free medium or Ca2+-channel blockers (Co2+, Mn2+ or Cd2+); but tetraethylammonium (TEA; 0.5-2 nM) blocked only the fast a.h.p., and noradrenaline (2-5 microM) only the slow a.h.p. This suggests that two Ca2+-activated K+ currents were involved: a fast, TEA-sensitive one (IC) underlying the fast a.h.p., and a slow noradrenaline-sensitive one (IAHP) underlying the slow a.h.p. 3. Like the fast a.h.p., spike repolarization seems to depend on a Ca2+-dependent K+ current of the fast, TEA-sensitive kind (IC). The repolarization was slowed by Ca2+-free medium, Co2+, Mn2+, Cd2+, or TEA, but not by noradrenaline. Charybdotoxin (CTX; 30 nM), a scorpion toxin which blocks the large-conductance Ca2+-activated K+ channel in muscle, had a similar effect to TEA. The effects of TEA and Cd2+ (or Mn2+) showed mutual occlusion. Raising the external K+ concentration reduced the fast a.h.p. and slowed the spike repolarization, whereas Cl- loading of the cell was ineffective. 4. The transient K+ current, IA, seems also to contribute to spike repolarization, because: (a) 4-aminopyridine (4-AP; 0.1 mM), which blocks IA, slowed the spike repolarization; (b) depolarizing pre-pulses, which inactivate IA, had a similar effect; (c) hyperpolarizing pre-pulses speeded up the spike repolarization; (d) the effects of 4-AP and pre-pulses persisted during Ca2+ blockade (like IA); and (e) depolarizing pre-pulses reduced the effect of 4-AP. 5. Pre-pulses or 4-AP broadened the spike less, and in a different manner, than Ca2+-free medium, Cd2+, Co2+, Mn2+, TEA or CTX. The former broadening was uniform, with little effect on the fast a.h.p., whereas the latter affected mostly the last two-thirds of the spike repolarization and abolished the fast a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological studies on the postnatal development of intracerebellar nuclei neurons in rat cerebellar slices maintained in vitro. II. Membrane conductances

The development of membrane conductances of intracerebellar nuclei neurons was studied in the rat since birth up to the weaning period by the use of thick sagittal cerebellar slices maintained in vitro. Mature nuclear neurons express fast sodium and slowly inactivating sodium conductances, as well as calcium conductances. As early as birth, fast sodium and calcium conductances appear well developed whereas slowly inactivating sodium conductances mature within the first postnatal week.

Intrasomatic and intradendritic recordings of plateau potentials in slices of the dentate gyrus maintained in vitro

Intrasomatic and intradendritic recordings were performed in slices of the dentate gyrus maintained in vitro. When barium ions (2.4 mM) were substituted for calcium ions in the perfusing medium, plateau potentials appeared with an amplitude of 20-40 mV which lasted from 40 ms to more than one min; during these plateau potentials, the input membrane resistance was decreased. In the soma, plateau potentials were also observed in a medium containing barium ions + tetrodotoxin (0.3 or 0.6 microM); whereas, in the dendrites, the barium-induced plateau potentials were abolished after addition of tetrodotoxin to the barium containing perfusion. The somatic plateau potentials had a duration which appeared to be dependent on the stimulus frequency. After being in contact with the barium-tetrodotoxin solution for a long period, the soma membrane potential was observed to jump between two relatively stable levels: a resting state and a depolarized state. In conclusion, calcium conductances appear to be present both at the soma and the dendrites of dentate granule cells; however, at the dendritic level, it appears that, when sodium channel permeability is blocked by tetrodotoxin, there is insufficient inward current to support the generation of action potentials.