In vitro electrophysiology of rat subicular bursting neurons

Intracellular recordings were used to study the electrophysiological properties of rat subicular neurons in a brain slice preparation in vitro. Cells were classified as bursting neurons (n = 102) based on the firing pattern induced by depolarizing current pulses. The bursting response recorded at resting membrane potential (−66.1 ± 6.2 mV, mean ± SD n = 94) was made up of a cluster of fast action potentials riding on a slow depolarization and was followed by an afterhyperpolarization. Tonic firing occurred at a membrane potential of approximately −55 mV. A burst also occurred upon termination of a hyperpolarizing current pulse. Tetrodotoxin (TTX, 1 μM) blocked the burst and decreased or abolished the underlying slow depolarization. These effects were not induced by the concomitant application of the Ca²⁺ channel blockers Co²⁺ (2 mM) and Cd²⁺ (1 mM). Subicular bursting neurons displayed voltage- and time-dependent inward rectifications of the membrane during depolarizing and hyperpolarizing current pulses. The inward rectification in the depolarizing direction was abolished by TTX, while that in the hyperpolarizing direction was blocked by extracellular Cs⁺ (3 mM), but not modified by Ba²⁺ (0.5–1 mM), TTX, or Co²⁺ and Cd²⁺. Tetraethylammonium (10 mM)-sensitive, outward rectification became apparent in the presence of TTX. These results suggest that neurons in the rat subiculum can display voltage-dependent bursts of action potentials as well as membrane rectification in the depolarizing and hyperpolarizing directions. These results also indicate that activation of a voltage-gated Na⁺ conductance may be instrumental in the initiation of bursting activity. Hippocampus 7:48–57, 1997. © 1997 Wiley-Liss, Inc.     

Low-threshold calcium spikes in hypothalamic neurons recorded near the paraventricular nucleus in vitro

From guinea-pig hypothalamic slices, intracellular studies demonstrate the existence of neurons responding to depolarizing current pulses by bursts of fast spikes riding on slow depolarizing potentials, when activated at the resting potential or from hyperpolarized levels (44 cells). Slow depolarizing potentials have a mean amplitude of 17.6 mV and a mean duration of 65.2 msec. They are also produced at the termination of hyperpolarizing current pulses. The ionic basis for these slow potentials have been investigated. Fast spikes constituting the burst discharge are blocked by TTX but the slow component is unaffected, being blocked by Co++ and enhanced by Ba++. Taken together, these results show that the slow depolarizing potentials are generated by a low-threshold Ca++ conductance which is de-inactivated by membrane hyperpolarization. When the neurons are spontaneously active, they exhibit bursts arising from slow depolarizing potentials reminiscent of those evoked by direct stimulation. They also show longer episodes of repetitive firing. Twelve neurons were intracellularly stained and were found in the periphery of the paraventricular nucleus (PVN), in close proximity to the groups of neurophysin-positive neuroendocrine neurons present in the lateral part of this nucleus. Injected neurons have the morphology of reticular cells, judging by their few multipolar, rectilinear and sparsely branched dendrites. Some of their processes are directed towards PVN. Because of their intrinsic electrophysiological properties and their possible relationships with PVN, the population of cells described in the present study may play a role in functions relating to the PVN.     

Membrane Properties of Physiologically Classified Rat Dorsal Horn Neurons In Vitro: Correlation with Cutaneous Sensory Afferent Input

Dorsal horn neurons in the young rat spinal cord-hindlimb preparation were physiologically classified as wide dynamic range (WDR), nociceptive specific (NS) or low threshold (LT) according to their excitatory responses to low and high intensity mechanical stimuli applied to the hindlimb skin. Two additional types were classified: neurons displaying only sub-threshold excitations (SUB) and neurons displaying inhibitory events (INH), such as inhibitory post-synaptic potentials or interruption of spontaneous spiking following cutaneous stimulation. Direct intracellular current injection revealed four different patterns of spiking behaviour: group A neurons were characterized by tonic firing in response to depolarizing current pulses; group B neurons were strongly phasic, producing only one spike at the beginning of the pulse; group A-B neurons generated an early unsustained (< 300 ms) burst of spikes; and group C neurons exhibited anomalous rectification in response to hyperpolarizing current which was followed by a voltage-dependent rebound excitation. A statistically significant ( P ≤ 0.01) association existed between a neuron's physiological classification and its electrophysiological profile. The majority of WDR neurons responded with tonic firing and were assigned to group A, while NS neurons were strongly represented in group A-B. All INH neurons were assigned to group C. LT neurons were distributed between groups A and A-B, and SUB neurons were distributed between groups A and B. These data indicate, firstly, that dorsal horn neurons possess heterogeneous membrane properties and, secondly, that a relationship exists between a neuron's biophysical profile and its excitatory or inhibitory response to peripheral cutaneous afferent stimulation. The implications for dorsal horn somatosensory processing are discussed.     

Electrophysiological properties and cholinergic responses of rat ventral oral pontine reticular neurons in vitro

In order to characterize the electrophysiological properties of morphologically identified neurons of the ventral part of the oral pontine reticular (vRPO) nucleus and the effects of cholinergic agonists on them, intracellular recordings were obtained from 45 cells in a rat brain-slice preparation. Intracellular staining was performed with 2% biocytin in potassium acetate (1 M)-filled micropipettes. Results demonstrated the presence of two types of vRPO neurons. Type I cells (n=12, 24%) were characterized by a break with a decrease of the depolarizing slope following hyperpolarizing pulses which delayed the return to the resting V_m and subsequent spike-firing. The delay was antagonized by 4-AP (200–500 μM) which specifically blocks the transient outward K⁺-mediated current I_A. Type II neurons (n=38, 76%) displayed a typical depolarizing sag during hyperpolarizing current pulses which was blocked by Cs⁺. This behavior is characteristic of the hyperpolarization-activated current I_Q. These two neuronal types displayed different morphological features. Most type I and II cells (100 and 73.7%, respectively) were depolarized by acetylcholine (1–15 μM), carbachol (0.5–1 μM) and muscarine (1–10 μM) through the activation of post-synaptic muscarinic receptors. The remaining type II cells (26.3%) were hyperpolarized (1–10 min, 3–15 mV) through the activation of post-synaptic muscarinic receptors. Results are consistent with the hypothesis that the vRPO could be a neuronal target of Cch in eliciting paradoxical sleep because most of its neurons are activated by muscarinic agonists. © 1979 Elsevier Science B.V. All rights reserved.     

Repetitive firing properties in subpopulations of the chick Edinger Westphal nucleus.

The avian Edinger Westphal nucleus, through the ciliary ganglion, controls accommodation, iris constriction, and blood flow through the choroid. In live brainstem slices, the nucleus is easily identifiable as an olive-shaped cluster of neurons dorsal to the oculomotor nerve and nucleus. Intracellular recordings from neurons in the nucleus identified two classes of responses to sustained (300 to 500 ms) injections of depolarizing current. One set of cells fired action potentials for the duration of the pulse while a second set of cells fired action potentials only transiently, during the first 50 to 100 ms, after which they remained silent regardless of the size of the depolarization. Intracellular recordings followed by injections of the fluorescent dye lucifer yellow revealed that repetitively firing cells were located in the lateral half of the nucleus while non-repetitively or transiently firing cells were located in the medial half. These locations correspond to different Edinger Westphal subdivisions which have distinct inputs and target populations. The varying firing patterns are discussed with reference to the known functions of the subdivisions in which they occur. Replacement of calcium by magnesium in the extracellular medium had no effect on the number of action potentials fired by non-repetitively firing cells, suggesting that a calcium-activated potassium current is not responsible for suppressing repetitive firing in these cells. In contrast, in repetitively firing cells removal of extracellular calcium increased the frequency of action potential discharge and decreased the amplitude of afterhyperpolarizations following single action potentials. Addition of cadmium to the bath medium had similar effects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Electrophysiological properties and their modulation by norepinephrine in the ambiguus neurons of the guinea pig

Electrophysiological properties of guinea pig ambiguus (AMB) neurons were studied in a brainstem slice preparation. During subthreshold depolarization AMB neurons displayed an early slow depolarization and a late outward rectification both of which were blocked by replacing Ca²⁺ with Co²⁺ in the extracellular solution. AMB neurons showed hyperpolarizing inward rectification which was blocked by extracellular Cs⁺ and is likely caused by the activation of Ih. In 58% (n = 49) of AMB neurons spike firing was restricted to the early phase of a long-lasting depolarizing current injection (phasic firing). The remaining AMB neurons showed repetitive firing throughout the depolarization (tonic firing). A Ca²⁺-mediated K⁺ current (I_K(Ca)) caused an afterhyperpolarization that followed both single and repetitive spike firing. I_K(Ca) also controlled the firing pattern in both types of firing, especially in the phasic firing. Norepinephrine (NE) blocked both the hyperpolarizing inward rectification and the Ca²⁺-dependent AHP. These effects of NE were antagonized by propranolol. It is proposed that the blockade of I_K(Ca) and Ih contribute to the improvement of the ‘signal-to-noise ratio’ by NE in AMB neurons.     

Self-inhibition alters firing patterns of neurons in aplysia buccal ganglia

The functional consequences of cholinergic self-inhibitory synaptic potentials (SISPs) upon firing patterns were examined in pairs of electronically coupled neurons of Aplysia buccal ganglia. In each neuron, the size of the peak SISP current decrements exponentially with increased number of previous conditioning action potentials (APs).To determine the effect of SISPs on the firing patterns of each cell, AP trains elicited by constant-current steps with the SISP intact were compared to those with the SISP blocked by curare. The SISP prolonged initial interspike intervals, providing an early supplement to accomodation, and produced a 75% increase in the sensitivity of firing frequency vs injected current plots for the first ISI. Firing rates were more regular in the presence of the SISP. However, the efficacy of the SISP, like the size of the underlying current, decrements with repetition.SISP effects were also studied in electronically coupled pairs of self-inhibitory neurons. Although the SISP altered the shape of the hyperpolarizing component of coupling potentials, DC coupling between the neurons was unaffected. Firing synchrony in coupled pairs stimulated with long DC pulses was assessed with cross-correlation histograms. In 60 mM Ca²⁺, the SISP sharpens the central peak of synchrony and deepens the flanking troughs, increasing the probability of synchronous firing within± 4msec by 76%. The major determinants of synchrony were found to be common input, SISP-dependent regularity of firing, and the depolarizing phase of the coupling potential, rather than the SISP-enhanced hyperpolarizing phase.     

In vivo Intracellular Recording of Neurons in the Supraoptic Nucleus of the Rat Hypothalamus

Intracellular recordings were made from cells in the hypothalamic supraoptic nucleus in the urethane-anaesthetized male rat using the ventral surgical approach. Impalements lasted from 5 min to 1 h and recorded cells had an input resistance of 55 to 170 megohms. Spikes of over 50 mV were recorded from 14 cells which could be antidromically activated by stimulation of the neural stalk. The spikes showed a hyperpolarizing afterpotential and the broadening characteristic of rapidly firing magnocellular neurons, which recovered rapidly (<200 ms). When depolarized, the cells showed evidence of a transient potassium current. Recurrent synaptic coupling between the recorded cell and adjacent cells would be expected to alter the hyperpolarizing afterpotential of an antidromic spike as compared with a spontaneous spike; no perceptible difference in the waveforms of the different types of spike could be detected in 11 spontaneously active cells. Application of just subthreshold stimuli to the neural stalk did not evoke depolarizing or hyperpolarizing potentials. Suprathreshold shocks to the neural stalk, when the antidromic spike was prevented by collision, also had no discernible effect on membrane potential. Thus intracellular recordings from magnocellular neurons in vivo revealed electrophysiological properties similar to those seen in vitro. No evidence for synaptic interconnection between magnocellular neurons was found in male rats.     

Intracellular activity of medullary respiratory neurons

Records of intracellular activity were obtained from respiratory neurons in the lateral reticular formation of the medulla. The membrane resistance of these neurons was generally higher than that of spinal cord motoneurons. This observation corresponds to the smaller size of neurons found in this region of the medulla. Only early expiratory neurons showed a declining frequency during the period when the underlying slow depolarization and apparent firing level were increasing. In other types, especially inspiratory neurons with an augmenting frequency, the slow membrane depolarization and frequency usually changed in parallel. Frequency adaptation to depolarizing pulses of constant current occurred in early expiratory neurons having the adapting discharge pattern during spontaneous respiration and in some cells that normally had a constant discharge frequency. Distinct postsynaptic potentials with regular firing patterns typical of respiratory neurons were seen in several cells. When they occurred during the cell's active phase, they were clearly excitatory and often triggered action potentials. Those occurring during the silent phase although not clearly hyperpolarizing were probably inhibitory and a manifestation of reciprocal innervation. It was concluded that adaptation limits discharge in early expiratory neurons, and probably in some inspiratory and expiratory neurons that have a constant discharge frequency, but not in inspiratory neurons with an augmenting discharge pattern. Since firing of all respiratory neurons was underlain by synaptic activity, it is likely that cells were brought to threshold by summation of excitatory input from cells simultaneously active.     

Evidence for a Slowly Inactivating K+ Current in Prefrontal Cortical Cells

The in vitro slice preparation of rat prefrontal cortical cells was used to analyse the presence and characteristics of a slowly inactivating outward current and its effect on the delayed integration of synaptic inputs. Pyramidal cells were identified as regular firing or bursting cells. In a fraction of these cells a depolarizing current pulse to –40 mV from a holding potential of –95 mV evoked the fast outward I_A current followed by a slower outward current (I_Ks) which inactivated slowly during the 3-s pulse. This slowly inactivating outward current was completely inactivated at holding potentials near –40 mV and was fully deinactivated by large hyperpolarizing pulses of 1 s duration. It was sensitive to micromolar concentrations of 4-aminopyridine and to 10 mM tetraethylammonium. In current clamp experiments, when the cells were maintained at –80 mV, they responded to subliminal depolarizing current pulses by a slow rising depolarization which reached threshold for spike firing after a delay of several seconds. This delay was considerably reduced either by maintaining the cell at less hyperpolarized potentials or by bath application of 40 μM 4-aminopyridine, or by repeated application of depolarizing pulses. The inactivation of I_Ks by the last procedure also led to plateau depolarization of the cell. These results suggest that the activation of the slowly inactivating outward current I_Ks can shunt excitatory inputs, preventing the cell from reaching spike threshold as long as it is not largely inactivated.     

Firing pattern of neurons in the rostral and ventral part of nucleus reticularis thalami during EEG spindles

The firing pattern of neurons in the rostral and ventral part of nucleus reticularis thalami during cortical EEG spindles was investigated in unanesthetized encéphale isolé cats. Spontaneous spindles as well as those induced by a single thalamic shock were accompanied by an increase in discharge frequency in 97% of the neurons in the rostral pole of the nucleus. In most cases the enhanced firing rate was tonically sustained throughout the duration of the spindles, although phasic bursts at EEG wave frequency were sometimes superimposed on the tonic cellular activation. Suppression of triggered spindles by conditioning fast-frequency stimulation in the mesencephalic reticular formation also abolished the rostral reticularis response. Intracellular recordings revealed a depolarizing shift of small amplitude which was sustained throughout the duration of triggered spindies. The majority of neurons in the ventral part of nucleus reticularis, in contrast, underwent a prolonged hyperpolarizing shift in membrane potential during cortical spindles, sustained for as long as 2 sec and interrupted by depolarizing waves. Both the prolonged membrane hyperpolarization and the depolarizing waves were reduced during the intracellular passage of polarizing currents, suggesting that the former was an inhibitory postsynaptic potential and the latter were disinhibitory potentials. Since neurons in dorsal thalamic nuclei, to which the reticularis axons project, are hyperpolarized concomitantly with cortical spindles, the results are viewed as being in agreement with the hypothesis that, during EEG spindles, neurons in the rostral pole but not in the ventral part of nucleus reticularis exert a tonic inhibitory influence on cells throughout the thalamus.     

Acute effect of carbamazepine on corticothalamic 5–9‐Hz and thalamocortical spindle (10–16‐Hz) oscillations in the rat

A major side effect of carbamazepine (CBZ), a drug used to treat neurological and neuropsychiatric disorders, is drowsiness, a state characterized by increased slow‐wave oscillations with the emergence of sleep spindles in the electroencephalogram (EEG). We conducted cortical EEG and thalamic cellular recordings in freely moving or lightly anesthetized rats to explore the impact of CBZ within the intact corticothalamic (CT)–thalamocortical (TC) network, more specifically on CT 5–9‐Hz and TC spindle (10–16‐Hz) oscillations. Two to three successive 5–9‐Hz waves were followed by a spindle in the cortical EEG. A single systemic injection of CBZ (20 mg/kg) induced a significant increase in the power of EEG 5–9‐Hz oscillations and spindles. Intracellular recordings of glutamatergic TC neurons revealed 5–9‐Hz depolarizing wave–hyperpolarizing wave sequences prolonged by robust, rhythmic spindle‐frequency hyperpolarizing waves. This hybrid sequence occurred during a slow hyperpolarizing trough, and was at least 10 times more frequent under the CBZ condition than under the control condition. The hyperpolarizing waves reversed at approximately ?70 mV, and became depolarizing when recorded with KCl‐filled intracellular micropipettes, indicating that they were GABA_A receptor‐mediated potentials. In neurons of the GABAergic thalamic reticular nucleus, the principal source of TC GABAergic inputs, CBZ augmented both the number and the duration of sequences of rhythmic spindle‐frequency bursts of action potentials. This indicates that these GABAergic neurons are responsible for the generation of at least the spindle‐frequency hyperpolarizing waves in TC neurons. In conclusion, CBZ potentiates GABA_A receptor‐mediated TC spindle oscillations. Furthermore, we propose that CT 5–9‐Hz waves can trigger TC spindles.     

Voltage-gated currents in identified rat olfactory receptor neurons

Whole-cell recording techniques were used to characterize voltage-gated membrane currents in neonatal rat olfactory receptor neurons (ORNs) in cell culture. Mature ORNs were identified in culture by their characteristic bipolar morphology, by retrograde labeling techniques, and by olfactory marker protein (OMP) immunoreactivity. ORNs did not have spontaneous activity, but fired action potentials to depolarizing current pulses. Action potentials were blocked by tetrodotoxin (TTX), which contrasts with the TTX-resistant action potentials in salamander olfactory receptor cells (e.g., Firestein and Werblin, 1987). Prolonged, suprathreshold current pulses evoked only a single action potential; however, repetitive firing up to 35 Hz could be elicited by a series of brief depolarizing pulses. Under voltage clamp, the TTX-sensitive sodium current had activation and inactivation properties similar to other excitable cells. In TTX and 20 mM barium, sustained inward current were evoked by voltage steps positive to -30 mV. This current was blocked by Cd (100 microM) and by nifedipine (IC50 = 368 nM) consistent with L-type calcium channels in other neurons. No T-type calcium current was observed. Voltage steps positive to -20 mV also evoked an outward current that did not inactivate during 100-msec depolarizations. Tail current analysis of this current was consistent with a selective potassium conductance. The outward current was blocked by external tetraethylammonium but was unaffected by Cd or 4-aminopyridine (4-AP) or by removal of external calcium. A transient outward current was not observed. The 3 voltage-dependent conductances in cultured rat ORNs appear to be sufficient for 2 essential functions: action potential generation and transmitter release. As a single odorant-activated channel can trigger an action potential (e.g., Lynch and Barry, 1989), the repetitive firing seen with brief depolarizing pulses suggests that ORNs do not integrate sensory input, but rather act as high-fidelity relays such that each opening of an odorant-activated channel reaches the olfactory bulb glomeruli as an action potential.     

Electrophysiological and morphological properties of rat motor cortex neurons in vivo.

This paper describes results obtained from intracellular recordings and stainings of motor cortex neurons in the rat in vivo. Rats were anesthetized with phenobarbital. Neurons were intracellularly recorded with micropipettes filled with K+-methylsulphate + 4% HRP in phosphate buffer (pH 7.4). Successful recordings and stainings were obtained from 31 neurons. Intracellular recordings were distinguished as either intrasomatic or intradendritic. Action potentials (APs) recorded from somata were distinguished by their fast hyperpolarizing afterpotential from those recorded within dendrites. Dendritic APs were broader and often followed by an afterdepolarization. The firing patterns elicited by depolarizing current pulses allowed to distinguish 3 groups of neurons. (a) Group A neurons with a moderate firing-rate of up to 17 APs during a 100 ms depolarizing current pulse of 3.5 nA comprised small and large pyramidal cells and one aspiny multipolar neuron, probably a large basket neuron. (b) Group B neurons generated bursts, which either occurred spontaneously or during low intensity current injection. These neurons were classified as small pyramidal neurons and spiny star cells. (c) Group C neurons had a firing rate 3 times as high as group A neurons. These neurons were small aspiny cells with radial dendritic fields, which were classified as local interneurons. Intradendritic recordings were characterized by the occurrence of broad APs, most likely generated within the dendritic tree. Intracellular current injections produced burst-like potentials consisting of several APs with different amplitude and duration. In 3 penetrations of one apical dendrite up to 4 neurons were stained. In these recordings APs activated by intracellular current injection were particularly broad (up to 40 ms). The results suggest that neuronal firing patterns observed in in-vitro neocortical slices are also observed in in-vivo conditions.     

Nitric oxide as intracellular modulator: internal production of NO increases neuronal excitability via modulation of several ionic conductances

Nitric oxide (NO) has been shown to regulate neuronal excitability in the nervous system, but little is known as to whether NO, which is synthesized in certain neurons, also serves functional roles within NO‐producing neurons themselves. We investigated this possibility by using a nitric oxide synthase (NOS)‐expressing neuron, and studied the role of intrinsic NO production on neuronal firing properties in single‐cell culture. B5 neurons of the pond snail Helisoma trivolvis fire spontaneous action potentials (APs), but once the intrinsic activity of NOS was inhibited, neurons became hyperpolarized and were unable to fire evoked APs. These striking long‐term effects could be attributed to intrinsic NO acting on three types of conductances, a persistent sodium current (I_NaP), voltage‐gated Ca currents (I_Ca) and small‐conductance calcium‐activated potassium (SK) channels. We show that NOS inhibitors 7‐nitroindazole and S‐methyl‐l ‐thiocitrulline resulted in a decrease in I_NaP, and that their hyperpolarizing and inhibiting effects on spontaneous spiking were mimicked by the inhibitor of I_NaP, riluzole. Moreover, inhibition of NOS, soluble guanylate cyclase (sGC) or protein kinase G (PKG) attenuated I_Ca, and blocked spontaneous and depolarization‐induced spiking, suggesting that intrinsic NO controlled I_Ca via the sGC/PKG pathway. The SK channel inhibitor apamin partially prevented the hyperpolarization observed after inhibition of NOS, suggesting a downregulation of SK channels by intrinsic NO. Taken together, we describe a novel mechanism by which neurons utilize their self‐produced NO as an intrinsic modulator of neuronal excitability. In B5 neurons, intrinsic NO production is necessary to maintain spontaneous tonic and evoked spiking activity.     

GABAA receptor stimulation blocks NMDA-induced bursting of dopaminergic neurons in vitro by decreasing input resistance

The effects of the GABA_A agonist, isoguvacine, on NMDA-induced burst firing of substantia nigra dopaminergic neurons were studied with intracellular and whole cell recordings in vitro. NMDA application caused the neurons to fire in rhythmic bursts. Although the NMDA-induced bursty firing pattern was insensitive to hyperpolarization by current injection, it was reversibly abolished by the selective GABA_A agonist, isoguvacine. The block of the rhythmic burst pattern by isoguvacine application occurred regardless of whether the chloride reversal potential was hyperpolarizing (E_Cl⁻=−70 mV) or depolarizing (E_Cl⁻=−40 mV). In either case, the input resistance of the dopaminergic neurons was dramatically decreased by application of isoguvacine. It is concluded that GABA_A receptor activation by isoguvacine disrupts NMDA receptor-mediated burst firing by increasing the input conductance and thereby shunting the effects of NMDA acting at a distally located generator of rhythmic burst firing.     

Suppression by topiramate of epileptiform burst discharges in hippocampal CA3 neurons of spontaneously epileptic rat in vitro

Topiramate, a novel antiepileptic drug, inhibits the seizures of spontaneously epileptic rat (SER), a double mutant (zi/zi, tm/tm) which exhibits both tonic convulsion and absence-like seizures from the age of 8-weeks. Hippocampal CA3 pyramidal neurons in SER show a long-lasting depolarization shift with accompanying repetitive firing when a single electrostimulation is delivered to the mossy fibers in vitro. The effects of topiramate on the excitability of CA3 pyramidal neurons in SER were examined to elucidate the mechanism underlying the antiepileptic action. Intracellular recordings were performed in 23 hippocampal slice preparations of 16 SER aged 8–17 weeks. Topiramate (10–100 μM) dose-dependently inhibited the depolarizing shifts with repetitive firing induced by mossy fiber stimulation without affecting the first spike and resting membrane potentials in hippocampal CA3 neurons of SER. Higher dose of topiramate (100 μM) sometimes inhibited the first spike, and decreased excitatory postsynaptic potentials in the SER CA3 neurons. However, topiramate up to 100 μM did not affect the single action potential elicited by the stimulation in the hippocampal CA3 neurons of age-matched Wistar rat devoid of the seizure. Application of topiramate (100 μM) did not significantly affect the firing induced by depolarizing pulse applied in the CA3 neurons of the SER. In addition, topiramate (100 μM) had no effects on the Ca²⁺ spike induced by intracellularly applied depolarizing pulse in the presence of tetrodotoxin and tetraethylammonium. In contrast, a dose-dependent inhibition of depolarization and repetitive firing induced by bath application of glutamate in CA3 pyramidal neurons was obtained with topiramate (10–100 μM). Furthermore, topiramate (100 μM) decreased the number of miniature postsynaptic potential of CA3 pyramidal neurons of SER. In patch clamp whole cell recording using acutely dissociated hippocampal CA3 neurons from SER aged 8-weeks and age-matched normal Wistar rats, there were no remarkable effects on voltage dependent Ca²⁺ current with topiramate up to 300 μM in either animal; the current was completely blocked by Cd²⁺ at a concentration of 1 mM. These findings suggest that topiramate inhibits release of glutamate from the nerve terminals and/or abnormal firing of the CA3 pyramidal neurons of SER by mainly blocking glutamate receptors in the neurons.     

Synaptic excitability of the burst firing neurons in cat sensorimotor cortex in vitro

We have recently reported that the burst firing neurons are found in layer III as well as in layer V of cat sensorimotor cortex in vitro. In the present study, we examined the synaptic excitability of layer III neurons by white matter stimulation and compared with their firing patterns against the current injections through the recording microelectrodes. The firing patterns of layer III neurons were classified into three main classes as in our previous study, i.e., (1) regular spiking (RS), i.e., the tonic firing that often exhibited spike-frequency adaptation, (2) burst-and-single spiking (BS), i.e., the initial bursting followed by tonic firing, (3) repetitive-bursting (RB), the burst firing that recurred at fast frequency. In RS cells, single action potential was superimposed on the largest EPSPs among all cell types analyzed. BS cells also fired single action potential and never exhibited burst firing synaptically. Only in a part of RB cells, synaptic bursting instead of single action potential was evoked on smaller EPSPs. IPSPs could be observed in about 60% of all the recorded RS and BS cells, however, they were observed in only 10% of the RB cells.     

Rebound spiking properties of mouse medial entorhinal cortex neurons in vivo

The medial entorhinal cortex is the gateway between the cortex and hippocampus, and plays a critical role in spatial coding as represented by grid cell activity. In the medial entorhinal cortex, inhibitory circuits are robust, and the presence of the h‐current leads to rebound potentials and rebound spiking in in vitro experiments. It has been hypothesized that these properties, combined with network oscillations, may contribute to grid cell formation. To examine the properties of in vivo rebound spikes, we performed whole‐cell patch‐clamp recordings in medial entorhinal cortex neurons in anaesthetized mice. We injected hyperpolarizing inputs representing inhibitory synaptic inputs along with sinusoidal oscillations and found that hyperpolarizing inputs injected at specific phases of oscillation had a higher probability of inducing subsequent spikes at the peak of the oscillation in some neurons. This effect was prominent in the cells with large sag potential, which is a marker of the h‐current. In addition, larger and longer hyperpolarizing current square‐pulse stimulation resulted in a larger probability of eliciting rebound spikes, though we did not observe a relationship between the amplitude or duration of hyperpolarizing current pulse stimulation and the delay of rebound spikes. Overall these results suggest that rebound spikes are observed in vivo and may play a role in generating grid cell activity in medial entorhinal cortex neurons.     

Propagation of synchronous burst discharges from entorhinal cortex to morphologically and electrophysiologically identified neurons of rat lateral amygdala

Intracellular and field potential recordings were taken from the lateral nucleus of the amygdala in a rat horizontal brain slice preparation that included hippocampal formation. Pyramidal cells comprised the majority of labeled cells (77%). Electrophysiological classification based on hyperpolarizing or depolarizing afterpotentials subdivided both the pyramidal and non-pyramidal cell classes, although pyramidal cells tended to have hyperpolarizing afterpotentials (70%) and non-pyramidal cells tended to have depolarizing afterpotentials (63%). Synchronous population bursts were triggered with single extracellular stimuli in the deep layers of entorhinal cortex. These events propagated from deep layers of entorhinal cortex into the lateral nucleus of the amygdala. Latencies were consistent with a direct entorhinal to amygdala projection. Individual lateral nucleus neurons exhibited responses ranging from a long burst response that included an initial period of 200 Hz firing and a tail of gamma frequency firing lasting over 100 ms (grade 1) to an epsp with no firing (grade 4). Half of pyramidal cells responding to events initiated in entorhinal cortex were found to receive epsps strong enough to trigger firing. Only one stellate neuron fired in response to entorhinal stimulation. Excitatory postsynaptic responses included NMDA and non-NMDA receptor mediated components. We demonstrate that synchronous population events can propagate from entorhinal cortex to the lateral nucleus of the amygdala and that pyramidal neurons of the lateral nucleus are more common targets than stellate neurons. We conclude that other synchronous events such as sharp waves and interictal spikes can spread from entorhinal cortex to amygdala in the same manner.