Membrane currents in hippocampal neurons

Ca2+-inhibited non-inactivating K+ channels in cultured rat hippocampal pyramidal neurones

1. Whole-cell perforated-patch recording from cultured CA1-CA3 pyramidal neurones from neonatal rat hippocampus (20-22 C; [K+]o = 2.5 mM) revealed two previously recorded non-inactivating (sustained) K+ outward currents: a voltage-independent 'leak' current (Ileak) operating at all negative potentials, and, at potentials >= -60 mV, a time- and voltage-dependent 'M-current' (IK(M)). Both were inhibited by 1 mM Ba2+ or 10 microM oxotremorine-M (Oxo-M). In ruptured-patch recording using Ca2+-free pipette solution, Ileak was strongly enhanced, and was inhibited by 1 mM Ba2+ but unaffected by 0.5 mM 4-aminopyridine (4-AP), 1 mM tetraethylammonium (TEA) or 1-10 nM margatoxin. 2. Single channels underlying these currents were sought in cell-attached patch recordings. A single class of channels of conductance approximately 7 pS showing sustained activity at resting potential and above was identified. These normally had a very low open probability (Po < 0. 1), which, however, showed a dramatic and reversible increase (to about 0.9 at approximately 0 mV) following the removal of Ca2+ from the bath. Under these (Ca2+-free) conditions, single-channel Po showed both voltage-dependent and voltage-independent components on patch depolarization from resting potential. The mean activation curve was fitted by a modified Boltzmann equation. When tested, all channels were reversibly inhibited by addition of 10 microM Oxo-M to the bath solution. 3. The channels maintained their high Po in patches excised in inside-out mode into a Ca2+-free internal solution and were strongly inhibited by application of Ca2+ to the inner face of the membrane (IC50 = 122 nM); this inhibition was observed in the absence of MgATP, and therefore was direct and unrelated to channel phosphorylation/dephosphorylation. 4. Channels in patches excised in outside-out mode were blocked by 1 mM Ba2+ but were unaffected by 4-AP or TEA. 5. Channels in cell-attached patches were inhibited after single spikes, yielding inward ensemble currents lasting several hundred milliseconds. This was prevented in Ca2+-free solution, implying that it was due to Ca2+ entry. 6. The properties of these channels (block by internal Ca2+ and external Oxo-M and Ba2+, and the presence of both voltage-dependent and voltage-independent components in their Po/V relationship) show points of resemblance to those expected for channels associated with both Ileak and IK(M) components of the sustained macroscopic currents. For this reason we designate them Ksust ('sustained current') channels. Inhibition of these channels by Ca2+ entry during an action potential may account for some forms of Ca2+-dependent after-depolarization. Their high sensitivity to internal Ca2+ may provide a new, positive feedback mechanism for cell excitation operating at low (near-resting) [Ca2+]i.

Cardiac arrest in rodents: maximal duration compatible with a recovery of neuronal activity

We report here that during a permanent cardiac arrest, rodent brain tissue is "physiologically" preserved in situ in a particular quiescent state. This state is characterized by the absence of electrical activity and by a critical period of 5-6 hr during which brain tissue can be reactivated upon restoration of a simple energy (glucose/oxygen) supply. In rat brain slices prepared 1-6 hr after cardiac arrest and maintained in vitro for several hours, cells with normal morphological features, intrinsic membrane properties, and spontaneous synaptic activity were recorded from various brain regions. In addition to functional membrane channels, these neurons expressed mRNA, as revealed by single-cell reverse transcription-PCR, and could synthesize proteins de novo. Slices prepared after longer delays did not recover. In a guinea pig isolated whole-brain preparation that was cannulated and perfused with oxygenated saline 1-2 hr after cardiac arrest, cell activity and functional long-range synaptic connections could be restored although the electroencephalogram remained isoelectric. Perfusion of the isolated brain with the gamma-aminobutyric acid A receptor antagonist picrotoxin, however, could induce self-sustained temporal lobe epilepsy. Thus, in rodents, the duration of cardiac arrest compatible with a short-term recovery of neuronal activity is much longer than previously expected. The analysis of the parameters that regulate this duration may bring new insights into the prevention of postischemic damages.

Kinetics of muscarinic reduction of IsAHP in hippocampal neurons: effects of acetylcholinesterase inhibitors

The present experiments were designed to elucidate the time frame in which an evoked cholinergic impulse decreases the Ca2+-dependent K+ current (IsAHP) in hippocampal CA1 neurons, and to determine to what extent acetylcholinesterase (AChE) inhibitors enhance the efficacy of the cholinergic impulse. Whole cell voltage-clamp recordings were performed on hippocampal CA1 neurons of rat brain slices and IsAHPs were evoked by constant depolarizing pulses. Cholinergic afferent fibers in stratum oriens were stimulated electrically and the time interval between the afferent stimulus and the depolarizing pulse was varied from 1 to 30 s. In slices perfused with the standard external medium, the afferent stimulus caused a profound decrease in the following IsAHP only when the stimulus preceded the depolarizing pulse by 1-2 s. The stimulus was without effects on the IsAHP when applied >/=5s before the depolarizing pulse. The effects of the afferent stimulus were greatly enhanced in CA1 neurons exposed to the catalytic AChE inhibitors neostigmine, physostigmine, or 9-amino-1,2,3, 4-tetrahydro-acridine. A substantial decrease in the IsAHP was observed even when the stimulus preceded the depolarizing pulse by >/=30 s. However applications of peripheral site AChE inhibitors decamethonium and propidium caused only minor or no enhancement of the IsAHP reduction after the afferent stimulus. We suggest in physiological conditions that muscarinic modulation of ionic conductances of CNS neurons has a limited time course after a cholinergic impulse and that the modulation is greatly enhanced and prolonged when catalytic activities of AChEs are suppressed pharmacologically.

Corticosteroid effects on sodium and calcium currents in acutely dissociated rat CA1 hippocampal neurons

Consequences of corticosteroid receptor activation on voltage-dependent Na+ conductances were studied in acutely dissociated CA1 hippocampal neurons. This preparation was selected because of the compact electrotonic properties of dissociated neurons, allowing reliable voltage-clamp of the large and fast Na+ currents. The Na+ currents were studied in (i) neurons of adrenalectomized animals (no steroid receptors occupied), (ii) neurons from tissue of adrenalectomized rats treated in vitro with corticosterone and the glucocorticoid receptor antagonist RU38486 (selectively occupying the mineralocorticoid receptor), (iii) corticosterone-treated neurons of adrenalectomized animals (occupying both the mineralocorticoid and glucocorticoid receptors) and (iv) neurons of sham-operated animals. Activation and steady-state inactivation properties of the Na+ current recorded in neurons of adrenalectomized animals were slightly shifted (3-5 mV) to hyperpolarized potentials as compared to the Na+ currents from neurons of the other experimental groups. Furthermore, the removal from inactivation of the Na+ current in the group of neurons of adrenalectomized animals was relatively slow. Although small, these effects could influence neuronal properties like action potential generation and accommodation. Under the present experimental conditions, no apparent differences were seen between cells with predominant mineralocorticoid receptor activation and cells where both mineralocorticoid and glucocorticoid receptors were occupied. In contrast to Na+ currents, voltage-dependent Ca2+ currents displayed no steroid-dependent shifts in voltage-dependent properties. However, Ca2+ current amplitudes were increased by approximately 160% in CA1 neurons of adrenalectomized animals as compared to Ca2+ currents from neurons of the other experimental groups. We conclude that corticosteroid receptor activation affects various properties of voltage-dependent Na+ and Ca2+ conductances in CA1 neurons, indicating that the steroid receptors are involved in the modulation of neuronal excitability in these cells.

1S, 3R-ACPD induces a region of negative slope conductance in the steady-state current-voltage relationship of hippocampal pyramidal cells

Synaptic responses mediated by metabotropic glutamate receptors (mGluRs) display a marked voltage-dependent increase in amplitude when neurons are moderately depolarized beyond membrane potential. We have investigated the basis for this apparent nonlinear behavior by activating mGluRs with 1S, 3R-1-aminocyclopentane-1, 3-dicarboxylate (1S, 3R-ACPD; 10 microM) in CA3 pyramidal cells from rat hippocampal slice cultures with the use of the single-electrode voltage-clamp technique. Under control conditions, cells depolarized from resting potential by 10-20 mV responded with delayed outwardly rectifying currents due to activation of voltage- and Ca(2+)-dependent K+ conductances. In contrast, in the continuous presence of 1S, 3R-ACPD, small depolarizations (10-20 mV) induced a delayed inward current. The steady-state current-voltage relationship for this response displayed a region of negative slope conductance at potentials between -55 and -40 mV. The reversal potential of the corresponding 1S,3R-ACPD-sensitive tail currents (-93.0 +/- 2.2 mV, mean +/- SE) was close to the potassium reversal potential, consistent with an mGluR-mediated suppression of K+ current. When external K+ concentration was increased to 8 mM, there was a positive shift in reversal potential to -76.9 +/- 5.1 mV. The depolarization-induced inward current in the presence of 1S,3R-ACPD was blocked by Ba2+ (1 mM). The response was not dependent on changes in intracellular Ca2+ concentration and was insensitive to bath-applied Cs+ (1 mM), ruling out a contribution of Ca(2+)-dependent currents or the inward rectifier lQ. Furthermore, the effect of 1S,3R-ACPD was not mimicked by inhibiting afterhyperpolarizing current and M current with low-Ca2+ saline (0.5 mM Ca2+, 10 mM Mg2+) containing 10 mM tetraethylammonium chloride. A comparison of the responses induced by 1S,3R-ACPD and N-methyl-D-aspartate showed that both induce an inward current with small depolarizations from resting potential but with different kinetics and Mg2+ sensitivity. These results indicate that the suppression of K+ currents in response to activation of mGluRs is markedly voltage dependent, increasing at depolarized potentials and decreasing at hyperpolarized potentials. The negative slope conductance at membrane voltages positive to resting potential may underlie the amplification of mGluR-mediated responses when the membrane potential approaches action potential threshold.

Acute and chronic actions of ethanol on CA1 hippocampal responses to serotonin

The effects of acute or chronic ethanol on serotonin (5-HT)-induced membrane hyperpolarization and inhibition of the slow Ca2(+)-dependent after hyperpolarization (sAHP) were recorded in rat CA1 pyramidal neurons in hippocampal slices using sharp intracellular electrodes. 5-HT (1-100 microM) caused concentration-dependent hyperpolarization of the membrane that was not altered by simultaneous 30 mM ethanol treatment, but blunted by 10 microM buspirone, a weak 5-HT1A agonist. 5-HT (1-30 microM) also partially inhibited (approximately 40%) the sAHP following a burst of five or more action potentials. Initially ethanol (30 mM) alone did not alter the sAHP, but over a period of 38 min, a slow increase in amplitude (approximately 40%) was observed. 5-HT-mediated inhibition of the sAHP was significantly greater with ethanol present, regardless of the length of exposure. Pyramidal neurons in hippocampal slices prepared from ethanol-dependent animals showed no obvious signs of withdrawal related hyperexcitability and neither concentration-dependent membrane hyperpolarization nor sAHP inhibition caused by 5-HT were significantly changed from responses in controls. These results suggest that hyperpolarizing responses to 5-HT in hippocampal CA1 pyramidal neurons are functionally resistant to acute or chronic ethanol treatment. 5-HT-mediated inhibition of the sAHP is enhanced by ethanol acutely, but does not show an adaptive change as a result of ethanol dependence.

Tolbutamide blocks Ca(2+)- and voltage-dependent K+ currents of hippocampal Ca1 neurons

In current-clamp recordings with KMeSO4 electrodes (either whole-cell or intracellular), though tolbutamide (0.5-1 mM) did not change the resting potential, it increased both input resistance (by 12 +/- 3.8%) and spontaneous firing, and spikes were evoked by smaller depolarizing pulses. Tolbutamide reduced in a dose-dependent manner both components of post-burst afterhyperpolarizations: IC50 was 0.15 mM for medium afterhyperpolarizations and 0.33 mM for slow afterhyperpolarizations. In whole-cell recordings under voltage-clamp, 0.5-1 mM tolbutamide depressed slow outward currents by 65 +/- 5.3%. The tolbutamide-sensitive current was Ca(2+)-dependent-tolbutamide being ineffective in Mn2+, low Ca(2+)-containing medium-though tolbutamide did not significantly depress high voltage-activated Ca2+ currents. Tolbutamide reduced C-type outward currents by 45 +/- 5.9% and M-type current inward relaxations by 41 +/- 12.9%, as well as Q-type current inward relaxations by 22 +/- 5.7%. Glyburide (10 microM) did not depress afterhyperpolarizations or outward currents, even in recordings with electrodes containing 1 mM guanosine diphosphate. We conclude that the most prominent effects of 0.5-1 mM tolbutamide on CA1 neurons are caused by suppression of Ca(2+)-and voltage-dependent outward currents, including IAHP, IC and IM.

Androgen modulates N-methyl-D-aspartate-mediated depolarization in CA1 hippocampal pyramidal cells

Previously, research elucidating steroid hormone actions in the central nervous system has focused on their role in sexual reproduction and maintaining homeostasis. The hippocampus is a target of steroid modulation and is involved in the development of emotional behavior and memory storage. Area CA1 of the hippocampus contains a high density of androgen receptor (AR) and N-methyl-D-aspartate (NMDA) receptors. NMDA receptors underlie excitatory synaptic transmission and excitotoxicity in CA1 neurons. The effects of AR activation on the neurophysiology of hippocampal pyramidal neurons is unknown. Standard intracellular recording techniques in hippocampal slices were used to investigate the effects of the non-aromatizable androgen, 5-alpha-dihydrotestos-terone-proprionate (DHTP), on CA1 pyramidal cell characteristics and NMDA receptor-mediated responses. Male Sprague-Dawley rats were unoperated, sham-operated (SHAM), gonadectomized (GDX), or gonadectomized with DHTP replacement therapy (GDX + DHTP). Neuronal AR was saturated by DHTP treatment as determined by binding studies and immunocytochemistry. Chronic DHTP treatment increased the action potential duration and decreased the fast afterhyperpolarization (fAHP) amplitude. To test the effect of DHTP on glutamate receptor-mediated responses, hippocampal slices were exposed to increasing concentrations of NMDA. In pyramidal cells from SHAM and GDX-treated animals, 30 microM NMDA induced an irreversible depolarization; the membrane potential of pyramidal cells from GDX + DHTP-treated animals recovered to baseline. The effect of DHTP was time dependent, implicating protein synthetic mechanisms. Our findings demonstrate that androgens can influence pyramidal cell characteristics and neurotransmitter-evoked actions in hippocampal CA1 pyramidal cells.

Protein kinase A-independent modulation of ion channels in the brain by cyclic AMP

Ion channels underlying the electrical activity of neurons can be regulated by neurotransmitters via two basic mechanisms: ligand binding and covalent modification. Whereas neurotransmitters often act by binding directly to ion channels, the intracellular messenger cyclic AMP is thought usually to act indirectly, by activating protein kinase A, which in turn can phosphorylate channel proteins. Here we show that cyclic AMP, and transmitters acting via cyclic AMP, can act in a protein kinase A-independent manner in the brain. In hippocampal pyramidal cells, cyclic AMP and norepinephrine were found to cause a depolarization by enhancing the hyperpolarization-activated mixed cation current, IQ (also called Ih). This effect persisted even after protein kinase A activity was blocked, thus strongly suggesting a kinase-independent action of cyclic AMP. The modulation of this current by ascending monoaminergic fibers from the brainstem is likely to be a widespread mechanism, participating in the state control of the brain during arousal and attention.

Identification and molecular localization of a pH-sensing domain for the inward rectifier potassium channel HIR

Inward rectifier potassium channels are found in the heart and CNS, where they are critical for the modulation and maintenance of cellular excitability. We present evidence that the inward rectifier potassium channel HIR is modulated by extracellular pH in the physiological range. We show that proton-induced changes in HIR single-channel conductance underlie the HIR pH sensitivity seen on the macroscopic level. We used chimeric and mutant channels to localize the molecular determinant of HIR pH sensitivity to a single residue, H117, in the M1-to-H5 linker region. This residue provides a molecular context that allows a titratable group to influence pore properties. We present evidence that this titratable group is one of two cysteines located in the M1-to-H5 and H5-to-M2 linkers.

A spectral analysis of the integration of artificial synaptic potentials in mammalian central neurons

In order to simulate the interaction between synaptic input and intrinsic membrane properties in mammalian central neurons a well-defined current was injected into the neurons through a recording electrode. The stimulus was white noise bandpass filtered at 0.5 and 75 Hz and the power spectra of the responses were calculated. Recordings were obtained from neurons of the neocortex, the hippocampus, the thalamus and the cerebellar cortex. The neurons were either located in newly cut slices from adult guinea pig brains or in 3-10-weeks-old slice cultures from brains of newborn rats. In hippocampal and cortical cells the passive membrane properties dominated the shape of the power spectra. In general, when the average membrane potential was made more positive the power of the response increased. When neurons had active subthreshold responses like delayed rectification, sag-and-hump responses or delayed depolarization there was a depression of the response power at frequencies below 10-20 Hz. The depression was voltage dependent in the same way as the current that produced the active subthreshold response. In thalamic cells with a low-threshold Ca2+ spike (lts) the power of the responses grew in the 3-20-Hz range with hyperpolarization. The spectra of the responses of thalamic neurons had multiple peaks indicating multiple frequencies of resonance. Purkinje cells of the cerebellar cortex have prominent plateau potentials. When these cells were stimulated with the white noise at levels where the plateau potentials could be activated the spectra were dominated by a large peak at the lowest frequencies, i.e., below 5 Hz. Few cells in our data base generated spontaneous membrane potential oscillations. When the current stimulus was injected into such neurons the intrinsic rhythm was unaffected by the input and the power spectrum showed a marked peak at the frequency of the intrinsic oscillations. We conclude that bandpass filtered white noise as simulation of synaptic input is valuable for quantification of how passive and active membrane properties affect synaptic integration. The technique can also provide information on the role of transmitters and modulators in the CNS.

Actions of cromakalim on outward currents of CA1 neurones in hippocampal slices

1. Membrane effects of cromakalim (Crom; 50-300 microM) were examined in CA1 neurones recorded mainly by intracellular, single-electrode voltage-clamping in slices (from Sprague-Dawley rats) kept in an interface chamber at 33 degrees C. 2. In 14 cells held at -63 +/- 3.5 mV, in the presence of tetrodotoxin, kynurenic acid and (in most cases) bicuculline, bath applied Crom produced no consistent change in holding current (-59 +/- 66 pA) or input conductance (GN) (-3.9 +/- 5.2%). 3. Overall there were no significant changes in instantaneous inward rectification or in Q-current inward relaxations. 4. In 18 out of 22 cells, outward currents, evoked by 0.5 s pulses to voltages > -50 and < -20 mV, were depressed by Crom (by 42 +/- 11%, for n = 22). Because this effect was consistently seen in Ca current-blocking media, containing either Mn and low Ca, or Cd (and also carbachol), the K channels depressed by Crom were probably of the delayed rectifier (IDR) type. 5. The Crom-control difference current (ICrom), obtained with slow depolarizing ramps, had a biphasic character, inward in the voltage (V) range > -50 < -20 mV (where outward currents are depressed by Crom) and tending outward for V > or = -20 mV. 6. In 10 out of 11 cells, Crom potentiated a D-like, slowly-inactivating outward current (by 88 +/- 31%, for n = 11). 7 The effects of Crom and of 2 min periods of anoxia were compared in 12 cells: unlike anoxia, Cromproduced no consistent increases in GN; the currents evoked in the same cells by anoxia differed significantly from those evoked by Crom (by 150 +/- 60 pA); the directions of current changes induced byCrom and anoxia respectively were not significantly correlated. Crom strongly depressed anoxic outward currents (by 80 +/- 12%, n = 4).8 Some Crom-induced effects (increases in D-like current and the outward current elicited at V>- 20 mV) were always reversed by tolbutamide (1 mM), but much less consistently by glibenclamide(10-30 microM).9 In conclusion, the effects of Crom, recorded with intracellular electrodes in CA1 neurones in slices,show little resemblance to the effects of activation of ATP-sensitive K channels.

Primary structure and characterization of a small-conductance inwardly rectifying potassium channel from human hippocampus

We have isolated a human hippocampus cDNA that encodes an inwardly rectifying potassium channel, termed HIR (hippocampal inward rectifier), with strong rectification characteristics. Single-channel recordings indicate that the HIR channel has an unusually small conductance (13 pS), distinguishing HIR from other cloned inward rectifiers. RNA blot analyses show that HIR transcripts are present in heart, skeletal muscle, and several different brain regions, including the hippocampus.

Potentiation of a metabotropic glutamatergic response following NMDA receptor activation in rat hippocampus

Interactions between metabotropic glutamate and N-methyl-D-aspartate (NMDA) receptor-mediated responses were investigated in hippocampal CA3 cells using the single electrode voltage-clamp method. Bath application (2.5-10 microM, 30 s) or iontophoresis of 1-amino-cyclopentyl-trans-1S,3R-dicarboxylate (ACPD), a selective agonist for metabotropic glutamate receptors, resulted in an inward current associated with a decrease in membrane conductance. Following transient bath application of NMDA (5-10 microM, 30-60 s), the ACPD-induced inward current was potentiated for a period of up to 25 min (by 61 +/- 8% with bath application, by 32 +/- 15% with iontophoresis). Transient application of NMDA did not result in a potentiation of ionotropic RS-alpha-amino-3-hydroxy-5-methyl-4-isoxazolepropionic acid (AMPA) or metabotropic muscarinic responses. ACPD responses were not potentiated following transient AMPA application. Intracellular buffering of calcium with tetrapotassium bis(O-aminophenoxy)-ethane-N,N,N',N'-tetraacetic acid (BAPTA) prevented potentiation by NMDA in all cells. Bath application of arachidonic acid did not mimic the NMDA-induced potentiation. These results demonstrate that activation of NMDA receptors can specifically induce a long-lasting potentiation of a metabotropic glutamatergic response in hippocampal CA3 pyramidal cells. The characterization of this interaction may contribute to the elucidation of the physiological significance of metabotropic glutamate receptors.

Effects of ammonium ions on synaptic transmission and on responses to quisqualate and N-methyl-D-aspartate in hippocampal CA1 pyramidal neurons in vitro

Effects of NH4Cl on CA1 pyramidal neurons and synaptic transmission were investigated with intracellular recording in fully submerged rat hippocampal slices. Superfusion with 1-4 mM NH4Cl reversibly depolarized the membrane by 15.1 +/- 1.4 mV, reduced the amplitude and broadened the duration of action potentials due to a slower rate of repolarization, without significant change in membrane conductance. When membrane potential was returned to control level by the injection of a steady outward current, action potential amplitude recovered but repolarization remained slow. The extent of depolarization was not dependent on the concentration of NH4Cl between 1 and 4 mM. NH4Cl greatly depressed orthodromic transmission evoked by the stimulation of Schaffer collateral/commissural fibers several minutes after depolarizing the CA1 neuron. Interruption of transmission began with a decrease in excitatory postsynaptic potential (EPSP) amplitude and eventually EPSPs were almost eliminated. When NH4Cl was removed, it took 2-3 min for membrane potential and 10-15 min for transmission to recover. Inward currents induced by bath application of quisqualate acting on alpha-amino-3-hydroxy-5-methyl-4-isoxazole propionic acid (AMPA) receptors were also depressed. In contrast, NH4Cl enhanced N-methyl-D-aspartate (NMDA)-induced currents. This potentiation disappeared in the absence of added Mg2+. A reduction in quisqualate-induced responses provided a possible explanation for the inhibition of excitatory transmission by NH4Cl.

Membrane currents in CA1 pyramidal cells during spreading depression (SD) and SD-like hypoxic depolarization

We used the patch clamp technique in whole-cell configuration to investigate the membrane current and membrane resistance of neurons in rat hippocampal tissue slices during spreading depression (SD) induced by high K+ solution or electrical stimulation and during SD-like depolarization caused by hypoxia. The potential of the patch pipette was referred to an extracellular micropipette electrode to ensure control of the true membrane potential during large shifts of extracellular potential, delta Vo. During both hypoxic and normoxic SD, increase of holding current indicated a large inward current which reached a mean maximum of about 1.75 nA. This virtual inward current started and ended at the same time as the extracellularly recorded negative delta Vo shift, but the trajectories of the two differed. When the membrane was clamped at strongly positive potential, the current during SD was outward. The average apparent reversal potential of the current during SD was near zero but in individual cases varied from -26 mV to + 12 mV. During SD the input resistance decreased on the average to 43% of the resting control value. The decrease of the input resistance was not voltage dependent. The increase of holding current and decrease of resistance occurred with both Cs- and K-gluconate recording pipettes and was not suppressed by 2 mM intracellular QX-314. Voltage-gated currents disappeared during SD; a small, Cs(+)-resistant outward rectifying current was the last to be lost. During recovery, reversal potential and input resistant overshot the control level and then returned to normal within about 5 min. The data are consistent with change of both driving potential and conductance for several ions, but the decrease of overall membrane resistance was less than earlier estimates with other methods had suggested. Normoxic SD and hypoxic SD-like depolarization could not be distinguished by these tests.

Presynaptic mechanism for heterosynaptic, posttetanic depression in area CA1 of rat hippocampus

Conditioning stimulation applied to afferent fibers in stratum radiatum or stratum oriens of hippocampal area CA1 produced heterosynaptic, posttetanic depression (PTD) of excitatory postsynaptic potentials (EPSPs). PTD amounted to a 60-80% reduction of EPSPs and recovered over a 5 min period. Conditioning stimulation also induced a posttetanic hyperpolarization (PTH) averaging 4 mV and decaying over a 1-1.5 min period. PTH was accompanied by a large reduction in input resistance. We sought to determine the pre- or postsynaptic locus of heterosynaptic PTD. Our results suggest that PTD reflects a presynaptic mechanism: (1) PTD was observed for both N-methyl-D-aspartate (NMDA) and non-NMDA receptor mediated EPSPs; (2) Direct depolarization of pyramidal cells, substituted for the synaptic depolarization induced by conditioning stimulation, did not elicit PTD; (3) PTD and PTH were differentially affected by pharmacological and postsynaptic manipulations; (4) Conditioning stimulation depressed responses to pressure applied glutamate, but the magnitude and duration were too small to account for PTD. Since afferent fiber volleys were not depressed following conditioning stimulation, while field EPSPs were, we conclude that conditioning stimulation suppresses synaptic release of glutamate.

Frequency-dependent enhancement of hippocampal inhibition by GABA uptake blockers

The effects of GABA uptake inhibitors, SKF 89976A and SKF 100330A, on recurrent inhibition were studied in the rat hippocampal slice preparation by the antidromic-orthodromic stimulation test. Population spikes evoked orthodromically by stimulation of the stratum radiatum and recorded in the CA1 pyramidal cell body layer were inhibited antidromically by stimulation of the alveus by a single pulse or by a train of pulses, either at low or at high frequency. Low frequency train conditioning produced less inhibition than a single pulse. The uptake blockers had no effect or slightly enhanced the inhibition produced by single stimuli or low frequency trains. High frequency train conditioning produced more and much longer inhibition than a single pulse. This inhibition was further substantially enhanced and prolonged by the drugs. Frequency-dependent enhancement of inhibition may be responsible for suppression of epileptiform discharges by GABA uptake blockers.

An ionic current model for neurons in the rat medial nucleus tractus solitarii receiving sensory afferent input

1. Neurons from a horizontal slice of adult rat brainstem were examined using intracellular recording techniques. Investigations were restricted to a region within the nucleus tractus solitarii, medial to the solitary tract and centred on the obex (mNTS). Previous work has shown this restricted area of the NTS to contain the greatest concentration of aortic afferent baroreceptor terminal fields. Electrical stimulation of the tract elicited short-latency excitatory postsynaptic potentials in all neurons. 2. mNTS neurons were spontaneously active with firing frequencies ranging between 1 and 10 Hz, at resting potentials of -65 to -45 mV. These neurons did not exhibit spontaneous bursting activity. 3. Depolarizing current injection immediately evoked a finite, high-frequency spike discharge which rapidly declined to a lower steady-state level (i.e. spike frequency adaptation, SFA). Increasing depolarizations produced a marked increase in the peak instantaneous frequency but a much smaller increase in the steady-state firing level. 4. Conditioning with a hyperpolarizing prepulse resulted in a prolonged delay of up to 600 ms before the first action potential (i.e. delayed excitation, DE) with an attendant decrease in peak discharge rates. DE was modulated by both the magnitude and duration of the prestimulus hyperpolarization, as well as the magnitude of the depolarizing stimulus. Tetrodotoxin (TTX) eliminated spike discharge but had little effect on the ramp-like membrane depolarization characteristic of DE. 5. We have developed a mathematical model for mNTS neurons to facilitate our understanding of the interplay between the underlying ionic currents. It consists of a comprehensive membrane model of the Hodgkin-Huxley type coupled with a fluid compartment model describing cytoplasmic [Ca2+]i homeostasis. 6. The model suggests that (a) SFA is caused by an increase in [Ca2+]i which activates the outward K+ current, IK,Ca, and (b) DE results from the competitive interaction between the injected depolarizing current and the hyperpolarization-activated transient outward K+ currents, IA and ID. 7. We conclude that our ionic current model is capable of providing biophysical explanations for a number of phenomena associated with brainstem neurons, either during spontaneous activity or in response to patterned injections of current. This model is a potentially useful adjunct for on-going research into the central mechanisms involved in the regulation of both blood pressure and ventilation.

Characteristic firing behavior of cell types in the cardiorespiratory region of the nucleus tractus solitarii of the rat

The present in vitro study was performed to characterize neurons within dorsal regions of the nucleus tractus solitarii (NTS), principally at the level of area postrema, and known to receive inputs predominantly from cardiovascular and respiratory afferents (i.e. cardiorespiratory NTS). This report describes 4 classes of neurons (S1-S4) that were silent at their resting membrane potential and received relatively short (< 3.6 ms) and consistent latency synaptic inputs (+/- 0.4 ms) comprising either an EPSP or EPSP/IPSP sequence following low intensity electrical stimulation of the solitary tract (ts). Intracellular recording with sharp electrodes were used to characterize neuron types based on their different firing response patterns to injection of depolarizing current. S1 cells showed a single action potential; S2 fired repetitively; S3 produced a 2-5 spike burst coincident with the start of the current pulse and S4 neurons showed delayed excitation. Accommodation of firing frequency was seen in S2, S3 and some S4 cells. The voltage dependency of the different discharge patterns of the 4 cell groups was tested by current pulse stimulation at different holding potentials. However, in the majority of cells in any one cell class the firing pattern was qualitatively similar. Based on these findings it is suggested that the different firing characteristics reflect differences in intrinsic membrane properties between neuron classes. Representative examples from each of the defined cell classes were further studied in current and voltage clamp using the whole cell patch technique to define the presence and role of certain ionic currents in the firing response patterns of the 4 cell groups. In the current clamp configuration the firing behavior of S1 neurons (single spiking) was unaltered during exposure to 4-aminopyridine (4-AP; 2 mM), cobalt chloride (Co; 5 mM), norepinephrine (NE; 20 microM) and muscarine chloride (50 microM). It is suggested that the relatively low excitability of this neuron is due a persistent outward current which occurred at -40 mV during depolarizing voltage steps in the voltage clamp configuration. A common characteristic of S2 neurons (repetitively firing) was that they showed accommodation during current injection which was greatly attenuated in the presence of Co or NE. In addition, 4-AP slowed the firing frequency, reduced the afterhyperpolarization and broadened the spike width of S2 cells. Interestingly, the amount of accommodation observed in S2 cells was variable for cells of this class and was proportional to the magnitude of a Co-sensitive inward current present during depolarizing voltage steps between -45 to -5 mV.(ABSTRACT TRUNCATED AT 400 WORDS)

Computational modeling of neuronal dynamics for systems analysis: application to neurons of the cardiorespiratory NTS in the rat

The study constructs computational models of neurons in order to examine the contribution that their response dynamics may make to functional properties at the system level. As described in the accompanying study, neurons in the cardiorespiratory nucleus tractus solitarii (NTS) of the rat were recorded in vitro. When these cells were intracellularly injected with a constant current pulse, spike discharge patterns and subthreshold voltage trajectories were observed that were time- and voltage-dependent. The accompanying manuscript describes these dynamic responses in 4 classes of putative second-order cells that appear to receive direct primary afferent input, and a previous paper described two populations of rhythmically firing interneurons, one of which is intrinsically auto-active. In the present manuscript experimental neuronal voltage response data was collected across a current injection series for the S3 neuron type described in the accompanying study and for the auto-active neuron described previously. Using this data, computational model neurons have been constructed for these two neurons by using membrane ion channels to produce and match the observed neuronal voltage behavior. The channels were those implicated in the dynamic responses observed in the companion study, and include gNafast, gKdr, gKA, gKCa, gKAHP, gKM, gCaT and gCaL. The description of channel kinetics follows the Hodgkin-Huxley form. Different neuronal sources from the literature of channel kinetics were investigated and assembled into a 'channel kinetics library' from which both neuron models were tuned, primarily by adjusting the maximum channel densities, g, and time-dependence of kinetics. Methods are described for tuning the channel kinetics library to match various physiological responses. This approach created neuron models that were able to closely replicate the observed complex voltage and spiking responses of the two very different cardiorespiratory NTS neurons. The interaction of voltage- and calcium-dependent conductances were analyzed for their functional contributions by tuning their kinetics. Specific parameters are given that account for the behavior of each model. Sensitivity analyses by perturbing KCa and KA are shown for both neurons, and I/F curves are presented for the auto-active neuron's stimulated and recorded responses. The potential systems-level functional implications resulting from the different kinetics is demonstrated by driving the S3 model neuron in simulation with the pattern of input produced by model primary baroreceptor afferents. The limitations and significance of this approach are discussed.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiology and burst-firing of rat subicular pyramidal neurons in vitro: a comparison with area CA1

Intracellular recordings were made from subicular and CA1 neurons in slices of the ventral hippocampal and parahippocampal region of the rat. All of the subicular cells that were stained by intracellular injection of biocytin were pyramidal in form. Although most electrophysiological properties were similar in the two areas, in response to depolarising current injection, the majority of subicular cells displayed a distinctive pattern of burst-firing which was rarely seen in CA1 cells. Burst-firing was voltage sensitive but was not abolished by blocking excitatory synaptic transmission, suggesting that it is an intrinsic membrane property of subicular pyramidal cells.

Effects of trifluoperazine on synaptically evoked potentials and membrane properties of CA1 pyramidal neurons of rat hippocampus in situ and in vitro

The effects of trifluoperazine (TFP), a phenothiazine antipsychotic, on hippocampal activity were studied in the CA1 subfield, both in situ and in slices. In the extracellular studies in situ and in vitro, both somatic population spikes and dendritic excitatory postsynaptic potentials (EPSP) fields were depressed reversibly by TFP, applied by microiontophoresis or in the bath (50-100 microM). Similar effects were also seen during iontophoretic applications of sphingosine in situ. Like TFP (at micromolar concentrations) sphingosine is a dual Ca2+/calmodulin-dependent kinase and protein kinase C (PKC) inhibitor. In intracellular recordings from slices, 50-100 microM TFP induced a slow depolarization and a decrease in input resistance (RN), probably through a gamma-aminobutyric acid (GABA)-mediated increase in Cl- conductance (GCl). TFP also reduced the slow afterhyperpolarization (AHP) as well as electrically evoked inhibitory postsynaptic potentials (IPSPs), but EPSPs were augmented in both amplitude and duration. When CA1 neurons were voltage clamped, TFP elicited a corresponding inward current (consistent with depolarization), increased the leak conductance, and enhanced excitatory synaptic currents; whereas inhibitory synaptic currents and high-threshold Ca2+ currents were reduced. In conclusion, these effects of TFP--which cannot be readily explained by its potent antidopamine action--are in keeping with other evidence that both Ca2+/calmodulin-dependent kinase and PKC can modulate GCl-conductance and high-threshold Ca(2+)-conductance, as well as inhibitory and excitatory postsynaptic currents.

Modulation of high-threshold Ca current and spontaneous postsynaptic transient currents by phorbol 12,13-diacetate, 1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7), and monosialoganglioside (GM1) in CA1 pyramidal neurons of rat hippocampus in vitro

Phorbol esters, which activate protein kinase C (PKC), enhance synaptic transmission in the CA1 subfield of hippocampus, both in situ and in vitro. The increase in synaptic transmission could be the consequence of enhanced Ca influx into nerve terminals, and perhaps a more general increase in voltage-dependent Ca currents. The effects of phorbol 12,13-diacetate (PDAc) on the high-voltage activated (HVA) Ca currents, as well as spontaneous transient currents were therefore investigated by intracellular recording in hippocampal slices. PDAc selectively augmented, by 45% +/- 10%, the early peak of the HVA Ca current (but not its sustained component), and also spontaneous inhibitory postsynaptic currents. The inactive phorbol ester, 4 alpha-PDAc, had no comparable effects. The actions of PDAc were reversible on prolonged washing, and they were antagonized by the PKC inhibitors (1-(5-isoquinolinesulfonyl)-2-methyl piperazine (H-7) and monosialoganglioside (GM1). In addition, GM1, which also activates the Ca/calmodulin-dependent kinase, enhanced spontaneous excitatory postsynaptic currents, while inhibiting the IPSCs. It is concluded that activation of PKC increases HVA (probably N-type) Ca current and facilitates ongoing GABAergic IPSCs.

Ionic basis of membrane potential changes induced by anoxia in rat dorsal vagal motoneurones

1. The effects of anoxia on membrane properties of 119 dorsal vagal motoneurones (DVMs) were investigated in an in vitro slice preparation of the rat medulla. 2. Membrane potential was unaffected by anoxia in 11% of DVMs. An hyperpolarization accompanied by a decrease in input resistance occurred in 44% of DVMs; the remaining 45% depolarized with either an increase (60%) or decrease in input resistance (40%). TTX at a concentration of 0.3-1 microM did not significantly affect these responses. 3. Anoxic artificial cerebrospinal fluid (ACSF) containing 20 mM-TEA reversed the response of DVMs that hyperpolarized in standard ACSF to reveal a depolarization of 7.4 +/- 2.1 mV, and increased the anoxic depolarization from 5.0 +/- 0.7 to 8.7 +/- 1.4 mV. 4. Anoxic depolarization was converted to an hyperpolarization of 7.3 +/- 2.1 mV in ACSF containing 5 mM-4-aminopyridine (4-AP) and 1 microM-TTX. A residual depolarization of 4.5 +/- 3.5 mV was then observed in ACSF containing 5 mM-4-AP, 1 microM-TTX and 20 mM-TEA. Anoxic hyperpolarization was increased from 7.8 +/- 1.8 to 10.0 +/- 3.9 mV in 5 mM-4-AP and 1 microM-TTX and converted to a depolarization of 5.3 +/- 4.5 mV in 5 mM-4-AP, 1 microM-TTX and 20 mM-TEA. 5. In anoxic ACSF containing TEA, the action potential width was increased from 0.92 +/- 0.04 to 8.1 +/- 1.1 ms in hyperpolarizing DVMs, and from 0.85 +/- 0.01 to 2.4 +/- 1.0 ms in depolarizing DVMs. The increase in width was prevented by 2-3 mM-Mn2+. 6. The long after-hyperpolarization (AHP) of DVMs, which is contributed to by both an apamin-sensitive IK(Ca) and an apamin, charybdotoxin and TEA insensitive IK(Ca) was decreased in duration from 2.59 +/- 0.14 to 1.94 +/- 0.12 s during anoxia. 7. It is concluded that anoxia enhances the delayed rectifier current (IK(DR)) and an inward current, probably ICa, but suppresses the A currents (IA). In DVMs that hyperpolarize during anoxia, the increase in IK(DR) outweighs the increase in ICa and the decrease in IA. In depolarizing DVMs the decrease in IA and increase in ICa outweight the increase in IK(DR). The change in input resistance is determined by the relative sizes of current enhancement or suppression.

Muscarinic agonists block a late-afterhyperpolarization in medial septum/diagonal band neurons in vitro

Intracellular recordings were made from neurons located in the medial septum (MS), and nucleus of the diagonal band (nDB) from slices of guinea pig brain. These forebrain nuclei contain both cholinergic and noncholinergic neurons that project to the cortex and hippocampus and are involved in many cortical functions. Muscarinic agonists (bethanechol, 2-30 microM) had the specific action to reduce a long-duration afterhyperpolarization (long-AHP) while leaving other shorter duration AHPs intact. Since the long-AHP was observed in both cholinergic and non-cholinergic neurons, muscarinic agonists were not selective for any one cell type. Block of a long-AHP was not associated with a consistent increase in cell excitability and therefore can not fully explain the excitatory actions of acetylcholine (ACh) observed in vivo within the MS/nDB.

Ionic conductance mechanisms contributing to the electrophysiological properties of neurons

Neurons have a multiplicity of ionic conductance mechanisms, the interactions of which determine in part the response of a neuron to chemical and electrical synaptic interactions, firing patterns, excitability, membrane potential and action-potential waveform. Several papers published in the past year have provided important new information on the role that ionic conductance mechanisms play in determining the electrophysiological properties of neurons, and how subtle differences can contribute to considerable variability of response.