Biomedical subjects

W J Wadman

Publications and source records attributed to W J Wadman.

At least 19 recordsLinked to original sources

PubMed2007

Seizure-like afterdischarges simulated in a model neuron.

To explore non-synaptic mechanisms in paroxysmal discharges, we used a computer model of a simplified hippocampal pyramidal cell, surrounded by interstitial space and a "glial-endothelial" buffer system. Ion channels for Na+, K+, Ca2+ and Cl- ion antiport 3Na/Ca, and "active" ion pumps were represented in the neuron membrane. The glia had "leak" conductances and an ion pump. Fluxes, concentration changes and cell swelling were computed. The neuron was stimulated by injecting current. Afterdischarge (AD) followed stimulation if depolarization due to rising interstitial K+ concentration ([K+]o) activated persistent Na+ current (INa.P). AD was either simple or self-regenerating; either regular (tonic) or burst-type (clonic); and always self-limiting. Self-regenerating AD required sufficient INa.P to ensure re-excitation. Burst firing depended on activation of dendritic Ca2+ currents and Ca-dependent K+ current. Varying glial buffer function influenced [K+]o accumulation and afterdischarge duration. Variations in Na+ and K+ currents influenced the threshold and the duration of AD. The data show that high [K+]o and intrinsic membrane currents can produce the feedback of self-regenerating afterdischarges without synaptic input. The simulated discharge resembles neuron behavior during paroxysmal firing in living brain tissue.

Animals↗

PubMed2006

Dual role of calbindin-D28K in vesicular catecholamine release from mouse chromaffin cells.

Calbindin-D(28K) is suggested to play a postsynaptic role in neurotransmission and in the regulation of the intracellular Ca(2+) concentration. However, it is still unclear whether calbindin-D(28K) has a role in the regulation of exocytosis, either as Ca(2+) buffer or as Ca(2+) sensor. Amperometric recordings of catecholamine exocytosis from wild-type and calbindin-D(28K) knockout mouse chromaffin cells reveal a strong reduction in the number of released vesicles, as well as in the amount of neurotransmitter released per fusion event in knockout cells. However, Ca(2+) current recordings and Ca(2+) imaging experiments, including video-rate confocal laser scanning microscopy, revealed that the intracellular Ca(2+) dynamics are remarkably similar in wild-type and knockout cells. The combined results demonstrate that calbindin-D(28K) plays an important and dual role in exocytosis, affecting both release frequency and quantal size, apparently without strong effects on intracellular Ca(2+) dynamics. Consequently, the possibility that calbindin-D(28K) functions not only as a Ca(2+) buffer but also as a modulator of vesicular catecholamine release is discussed.

Adrenal Medulla↗

PubMed2006

Dopamine receptor pharmacology: interactions with serotonin receptors and significance for the aetiology and treatment of schizophrenia.

The classification of dopamine receptors proposed more than two decades ago remains valid today. Based on biochemical and pharmaceutical properties two main classes of dopamine receptors can be distinguished: D(1)-like (D(1), D(5)) and D(2)-like (D(2), D(3), and D(4)) dopamine receptors. Dopamine receptors belong to the class of G protein-coupled receptors and signal to a wide range of membrane bound and intracellular effectors such as ion channels, secondary messenger systems and enzymes. Although the pharmacological properties of ligands for D(1)-like and D(2)-like dopamine receptors are quite different, the number of selective ligands for each of the five receptors subtypes is rather small. Many drugs used to treat neurological and neuropsychiatric disorders like Parkinson's disease, restless leg syndrome and schizophrenia have affinities for dopamine receptors. Such medications are not without limitations so the development of novel (selective or aselective) dopamine receptor ligands is of the utmost importance for improved therapeutic approaches for these diseases. In that respect it is also important to understand how dopamine receptor ligands affect receptor signalling processes such as desensitization, receptor heterodimerization and agonist-receptor trafficking, issues which will be discussed in the present review. Furthermore, attention is paid to interactions of dopamine receptors with serotonin receptors since many drugs used to treat above mentioned disorders of the brain also possess affinities for serotonin receptors. Because of the enormity of this area we have tried to focus more specifically on interactions within the prefrontal cortex where it appears that the serotonergic modulation of dopaminergic function might be very relevant to schizophrenia.

Animals↗

PubMed2004

5-HT2 receptors differentially modulate dopamine-mediated auto-inhibition in A9 and A10 midbrain areas of the rat.

5-HT (20 microM) enhanced dopamine (DA) D2-like receptor mediated reduction of the firing rate of DA neurons in the substantia nigra pars compacta (A9) and ventral tegmental area (A10) in a rat midbrain slice preparation. Quinpirole (30 nM) induced a mean reduction of the firing rate in A9 and A10 DA neurons to 64 +/- 4%, respectively, 71 +/- 5% of the baseline value. Bath application of 5-HT in the presence of quinpirole further reduced the firing rate to 37 +/- 7% in A9 and 33 +/- 13% in A10. The 5-HT2 receptor agonist (+/-)-1-(2,5-dimethoxy-4-iodophenyl)-2-aminopropane hydrochloride (DOI, 500 nM) enhanced quinpirole-induced reduction of firing rate of A10 DA neurons, but not of A9 DA neurons, suggesting that different 5-HT receptor subtypes are involved in modulation of dopamine D2-like receptor mediated inhibition in the two regions. The selective 5-HT2A receptor antagonist MDL100907 and the selective 5-HT2C receptor antagonist SB242084 (50 and 500 nM) both abolished the enhancement of quinpirole-induced reduction by either 5-HT or DOI, suggesting the involvement of direct and indirect (possibly via interneurons) modulation pathways in A10. The involvement of 5-HT and specific 5-HT2 receptors in augmentation of auto-inhibition in A10 could have important implications for our understanding of the mechanism of atypical antipsychotic drug action.

Action Potentials↗

PubMed2004

Sequential activation of microcircuits underlying somatosensory-evoked potentials in rat neocortex.

Evoked cortical field potentials are widely used in neurophysiological studies into cortical functioning, but insight in the underlying neural mechanisms is severely hampered by ambiguities in the interpretation of the field potentials. The present study aimed at identifying the precise relationships between the primary evoked cortical field potential (the positive-negative [P1-N1]response) and the temporal and spatial sequence in which different local cortical micro-circuits are recruited. We electrically stimulated the median nerve and recorded field potentials using a 12-channel depth probe in somatosensory cortex of ketamine anesthetized rats. Current source density analysis was used and a grand average was constructed based on all individual animals taking into account individual differences in cortical layering. Manipulation of stimulus strength, selective averaging of single trial responses, and double-pulse stimulation, were used to help disentangle overlapping dipoles and to determine the sequence of neuronal events. We discriminated three phases in the generation of the P1-N1 wave. In the first phase, specific thalamic afferents depolarize both layer III and layer V pyramidal cells. In the second phase, superficial pyramidal cells are depolarized via supragranular intracortical projections. In the third phase, population spikes are generated in layer Vb pyramidal cells, associated with a distinct fast (approximately 1 ms) sink/source configuration. Axon-collaterals of layer Vb pyramidal cells produce an enhanced activation of the supragranular pyramidal cells in layer I-II, which generates N1.

Algorithms↗

PubMed2003

Functional relation between interneuron input and population activity in the rat hippocampal cornu ammonis 1 area.

Inhibitory interneurons are important components of the cornu ammonis 1 (CA1) network, as they are strategically positioned to control network information transfer. We investigated in detail synaptic input to individual CA1 interneurons (mainly basket and bistratified cells) after the local circuit was activated through the Schaffer-Commissural pathway and related this input to the population activity of the pyramidal cells. Synaptic responses were measured under whole-cell voltage clamp and population activity was determined from local field potentials. The synaptic input that was evoked in CA1 interneurons fell into two distinct groups. Disynaptic input with a long latency always started after the population spike with a mean latency of 3.0+/-0.3 ms (n=22) in respect to the peak of the population spike. This type of synaptic input to the interneurons was causally linked to the occurrence and amplitude of the population spike and most likely driven by CA1 pyramidal cells. Short-latency monosynaptic input occurred 0.8+/-0.2 ms (n=18) before the peak of the population spike. Its timing was strictly linked to the stimulus and showed significantly less jitter than long-latency input. In the absence of a population spike only short-latency input could be observed. Whether an interneuron receives direct monosynaptic Schaffer input or disynaptic input from the pyramidal cell population determines when that interneuron will be recruited in the network after Schaffer collateral stimulation. In addition, we found that the relation between the strength of the synaptic input and the population activity was different for the two types of input. Short-latency monosynaptic input showed large sensitivity to input changes at stimulus intensities that evoked little activity in the pyramidal cell population. In contrast, the amplitude of the long-latency disynaptic input to the interneurons closely reflected the population activity and increased gradually with stimulus intensity. Interneurons receiving the first type of input may expand the input sensitivity of the network, while interneurons receiving the second type could be involved in overall normalization of the output of the CA1 network. Our results underscore the importance of knowledge of the input to an interneuron for the understanding of its inhibitory role in the network.

Animals↗

PubMed2002

Conditions for the triggering of spreading depression studied with computer simulations.

In spite of five decades of study, the biophysics of spreading depression (SD) is incompletely understood. Earlier we have modeled seizures and SD, and we have shown that currents through ion channels normally present in neuron membranes can generate SD-like depolarization. In the present study, we define the conditions for triggering SD and the parameters that influence its course in a model of a hippocampal pyramidal cell with more complete representation of ions and channels than the previous version. "Leak" conductances for Na(+), K(+), and Cl(-) and an ion pump were present in the membrane of the entire cell; fast inactivating voltage dependent conductances for sodium and potassium in the soma; "persistent" conductances in soma and apical dendrite, and K(+)- and voltage-dependent N-methyl-D-aspartate (NMDA)-controlled conductance in the apical dendrite. The neuron was surrounded by restricted interstitial space and by a "glia-endothelium" system of extracellular ion regulation bounded by a membrane having leak conductances and an ion pump. Ion fluxes and concentration changes were continuously computed as well as osmotic cell volume changes. As long as reuptake into the neuron and "buffering" by glia kept pace with K(+) released from the neuron, stimulating current applied to the soma evoked repetitive firing that stopped when stimulation ceased. When glial uptake was reduced, K(+) released from neurons could accumulate in the interstitium and keep the neuron depolarized so that strong depolarizing pulses injected into the soma were followed either by afterdischarge or SD. SD-like depolarization was ignited when depolarization spreading into the apical dendrite, activated persistent Na(+) current and NMDA-controlled current. With membrane parameters constant, varying the injected stimulating current influenced SD onset but neither the depolarization nor the increase in extracellular K(+). Glial "leak" conductance influenced SD duration and SD ignition point. Varying maximal conductances (representing channel density) also influenced SD onset time but not the amplitude of the depolarization. Hypoxia was simulated by turning off the Na-K exchange pump, and this resulted in SD-like depolarization. The results confirm that, once ignited, SD runs an all-or-none trajectory, the level of depolarization is governed by feedback involving ion shifts and glutamate acting on ion channels and not by the number of channels open, and SD is ignited if the net persistent membrane current in the apical dendrites turns inward.

Algorithms↗

PubMed2001

In vitro modulation of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area by antipsychotic drugs.

An in vitro experimental midbrain slice preparation is described which allows simultaneous extracellular recordings of the (spontaneous) electrical activity of dopamine neurons in the rat substantia nigra (SN) and the ventral tegmental area (VTA). Under identical in vitro circumstances the mean firing frequency of the SN dopamine neurons was higher than that of the VTA dopamine neurons (2.1 vs. 1.4Hz). With this slice preparation, modulation of the electrical activity of SN and VTA dopamine neurons by (new) drugs can be quickly determined. Experiments with the selective D2 receptor agonist quinpirole and the selective D2 receptor antagonist (-)-sulpiride indicated that dopamine neurons in the SN and VTA hardly differ in their pharmacological properties for the D2-like (auto)receptor. (-)-Sulpiride and to a lesser extent risperidone induced a small increase in firing rate in SN and VTA neurons, which was reversible upon wash-out. Olanzapine-induced increase in firing rate was persistent in SN and VTA neurons, whereas the clozapine-induced increase in firing rate was only completely recovered upon wash-out in SN neurons. The difference in firing rates of SN and VTA dopamine neurons could have consequences for the effectiveness of dopaminergic drugs acting at the D2-like dopamine (auto)receptor on these neurons.

Action Potentials↗

PubMed2001

Sodium currents in isolated rat CA1 pyramidal and dentate granule neurones in the post-status epilepticus model of epilepsy.

Status epilepticus (SE) was induced in the rat by long-lasting electrical stimulation of the hippocampus. After a latent period of 1 week, spontaneous seizures occurred which increased in frequency and severity in the following weeks, finally culminating after 3 months in a chronic epileptic state. In these animals we determined the properties of voltage-dependent sodium currents in acutely isolated CA1 pyramidal neurones and dentate granule cells using the whole-cell voltage-clamp technique. The conductance of the fast transient sodium current was larger in SE rats (84+/-7 nS versus 56+/-6 nS) but related to a difference in cell size so that the neurones had a similar specific sodium conductance (control: 7.8+/-0.8 nS/pF, SE: 6.7+/-0.8 nS/pF). Current activation and inactivation were characterised by a Boltzmann function. After SE the voltage dependence of activation was shifted to more negative potentials (control: -45.1+/-1.4 mV, SE: -51.5+/-2.9 mV, P<0.05). In combination with a small shift in the voltage dependence of inactivation to more depolarised potentials (control: -68.8+/-2.3 mV, SE: -66.3+/-2.3 mV), it resulted in a window current that was much increased in the SE neurones (median: 64 pA in control, 217 pA in SE, P<0.05). The peak of this window current shifted to more hyperpolarised potentials (control: -44 mV, SE: -50 mV, P<0.05). No differences were found in the sodium currents analysed in dentate granule cells of control and SE animals. The changes observed in CA1 neurones after SE contribute to enhanced excitability in particular when membrane potential is near firing threshold. They can, at least partly, explain the lower threshold for epileptic activity in SE animals. The comparison of CA1 with DG neurones in the same rats demonstrates a differential response in the two cell types that participated in very similar seizure activity.

Animals↗

PubMed2000

Neurotensin attenuates the quinpirole-induced inhibition of the firing rate of dopamine neurons in the rat substantia nigra pars compacta and the ventral tegmental area.

In the present study we describe the excitatory effects of the bioactive peptide neurotensin on the electrical activity of dopamine neurons (simultaneously recorded) in the substantia nigra pars compacta and the ventral tegmental area. The neurotensin fragment (8-13) induced comparable increases in firing rate of the substantia nigra and ventral tegmental area dopamine neurons (EC50 values 30 and 45 nM, respectively). The neurotensin receptor antagonist SR142948A antagonized the excitatory effects of neurotensin fragment (8-13) (pA2 values 8.4 and 8.2, respectively). Furthermore, it was found that a low concentration of neurotensin fragment (8-13) (1 nM) attenuated the inhibition of the firing rate by the selective dopamine D2 receptor agonist quinpirole in both neuron types (e.g., the effect of 0.01 microM quinpirole was reduced by approximately 60% in the presence of 1 nM neurotensin fragment [8-13]). Antagonism of this neurotensin fragment (8-13) effect by SR142948A confirms that neurotensin receptors can reduce the effect of dopamine D2 receptors at the single-cell level. These results are discussed in the light of possible roles for neurotensin in neurological disorders such as Parkinson's disease and schizophrenia.

Action Potentials↗

PubMed2000

Two mechanisms that raise free intracellular calcium in rat hippocampal neurons during hypoosmotic and low NaCl treatment.

Previous studies have shown that exposing hippocampal slices to low osmolarity (pi(o)) or to low extracellular NaCl concentration ([NaCl](o)) enhances synaptic transmission and also causes interstitial calcium ([Ca(2+)](o)) to decrease. Reduction of [Ca(2+)](o) suggests cellular uptake and could explain the potentiation of synaptic transmission. We measured intracellular calcium activity ([Ca(2+)](i)) using fluorescent indicator dyes. In CA1 hippocampal pyramidal neurons in tissue slices, lowering pi(o) by approximately 70 mOsm caused "resting" [Ca(2+)](i) as well as synaptically or directly stimulated transient increases of calcium activity (Delta[Ca(2+)](i)) to transiently decrease and then to increase. In dissociated cells, lowering pi(o) by approximately 70 mOsm caused [Ca(2+)](i) to almost double on average from 83 to 155 nM. The increase of [Ca(2+)](i) was not significantly correlated with hypotonic cell swelling. Isoosmotic (mannitol- or sucrose-substituted) lowering of [NaCl](o), which did not cause cell swelling, also raised [Ca(2+)](i). Substituting NaCl with choline-Cl or Na-methyl-sulfate did not affect [Ca(2+)](i). In neurons bathed in calcium-free medium, lowering pi(o) caused a milder increase of [Ca(2+)](i), which was correlated with cell swelling, but in the absence of external Ca(2+), isotonic lowering of [NaCl](o) triggered only a brief, transient response. We conclude that decrease of extracellular ionic strength (i.e., in both low pi(o) and low [NaCl](o)) causes a net influx of Ca(2+) from the extracellular medium whereas cell swelling, or the increase in membrane tension, is a signal for the release of Ca(2+) from intracellular stores.

Animals↗

PubMed2000

Simulated seizures and spreading depression in a neuron model incorporating interstitial space and ion concentrations.

Sustained inward currents in neuronal membranes underlie tonic-clonic seizure discharges and spreading depression (SD). It is not known whether these currents flow through abnormally operating physiological ion channels or through pathological pathways that are not normally present. We have now used the NEURON simulating environment of Hines, Moore, and Carnevale to model seizure discharges and SD. The geometry and electrotonic properties of the model neuron conformed to a hippocampal pyramidal cell. Voltage-controlled transient and persistent sodium currents (I(Na,T) and I(Na,P)), potassium currents (I(K,DR) and I(K,A)), and N-methyl-D-aspartate (NMDA) receptor-controlled currents (I(NMDA)), were inserted in the appropriate regions of the model cell. The neuron was surrounded by an interstitial space where extracellular potassium and sodium concentration ([K(+)](o) and [Na(+)](o)) could rise or fall. Changes in intra- and extracellular ion concentrations and the resulting shifts in the driving force for ionic currents were continuously computed based on the amount of current flowing through the membrane. A Na-K exchange pump operated to restore ion balances. In addition, extracellular potassium concentration, [K(+)](o), was also controlled by a "glial" uptake function. Parameters were chosen to resemble experimental data. As long as [K(+)](o) was kept within limits by the activity of the Na-K pump and the "glial" uptake, a depolarizing current pulse applied to the cell soma evoked repetitive firing that ceased when the stimulating current stopped. If, however, [K(+)](o) was allowed to rise, then a brief pulse provoked firing that outlasted the stimulus. At the termination of such a burst, the cell hyperpolarized and then slowly depolarized and another burst erupted without outside intervention. Such "clonic" bursting could continue indefinitely maintained by an interplay of the rise and fall of potassium and sodium concentrations with membrane currents and threshold levels. SD-like depolarization could be produced in two ways, 1) by a dendritic NMDA-controlled current. Glutamate was assumed to be released in response to rising [K(+)](o). And 2) by the persistent (i.e., slowly inactivating) Na-current, I(Na,P). When both I(NMDA) and I(Na,P) were present, the two acted synergistically. We conclude that epileptiform neuronal behavior and SD-like depolarization can be generated by the feedback of ion currents that change ion concentrations, which, in turn, influence ion currents and membrane potentials. The normal stability of brain function must depend on the efficient control of ion activities, especially that of [K(+)](o).

Action Potentials↗

PubMed2000

Concentration dependence of bicarbonate-induced calcium current modulation.

High-voltage-activated calcium currents (HVA) of CA1 neurons are prominently attenuated following a switch from HEPES-buffered solution to one buffered with CO(2)/HCO(3)(-). In the present study we investigated whether bicarbonate ions or the dissolved CO(2) induce this alteration in current characteristic. The study was carried out on freshly isolated CA1 neurons using the whole cell patch-clamp technique. Maximal calcium conductance and the mean peak amplitude of the currents showed a concentration-dependent decrease when cells were consecutively bathed in solutions containing increasing amounts of bicarbonate and CO(2). This decrease is best described by the Hill equation, yielding a maximal attenuation of 69%, a half-maximal concentration (EC(50)) of 7.4 mM HCO(3-), and a Hill coefficient of 1.8. In parallel, the potentials of half-maximal activation (V(h,a)) and inactivation (V(h,i)) were linearly shifted in hyperpolarizing direction with a maximal shift, in the 10% CO(2)/37 mM HCO(3)(-) containing solution of 10 +/- 1 mV for V(h,a) (n = 23) and 17 +/- 1.4 mV for V(h,i) (n = 18). When currents were evoked in solutions containing equal concentrations of bicarbonate but different amounts of CO(2), only nonsignificant changes were observed, while marked alterations of the currents were induced when bicarbonate was changed and CO(2) held stable. The experiments suggest that bicarbonate is the modulating agent and not CO(2). This bicarbonate-induced modulation may be of critical relevance for the excitation level of the CNS under pathological situation with altered concentration of this ion, such as hyperventilation and metabolic acidosis.

Animals↗

PubMed1999

Calcium currents in acutely isolated stellate and pyramidal neurons of rat entorhinal cortex.

Calcium currents were studied in morphologically identified pyramidal and stellate neurons acutely isolated from layer II/III of rat entorhinal cortex, using the whole-cell patch-clamp configuration. The peak amplitude of high-voltage activated current (HVA) measured at +10 mV was not different in both neuron populations with 0.94+/-0.08 nA for pyramidal and 1.03+/-0.08 nA for stellate cells. Stellate neurons had a larger capacitance (14.4+/-1. 1 pF) than pyramidal neurons (9.6+/-0.8 pF), indicating a 50% larger cell surface. Most striking was the difference between the current density in stellate (79+/-8 pA/pF) versus pyramidal neurons (113+/-13 pA/pF). The potential of half maximal inactivation was not different: -37+/-2 mV (pyramidals) and -37+/-3 mV (stellates). Half of the cells contained a low-voltage activated calcium current (LVA) with a peak amplitude that was twice as large in stellate as in pyramidal neurons (0.21+/-0.04 nA resp. 0.11+/-0.03 nA; at -50 mV). In contrast to the HVA component, the current density of the LVA component was not different between cell types (13+/-3 pA/pF vs. 13+/-2 pA/pF). This implies that the relative abundance of LVA and HVA currents in stellate and pyramidal neurons is different which could result in different firing characteristics. The potential of half maximal LVA inactivation was -88+/-4 mV (pyramidals) and -85+/-3 mV (stellates). The slope of the voltage dependent steady state inactivation was steeper in stellate (7+/-1 mV) than in pyramidal cells (10+/-2 mV).

Animals↗

PubMed1999

Modulation of sodium currents in rat CA1 neurons by carbamazepine and valproate after kindling epileptogenesis.

PURPOSE: To determine the modulation of sodium currents in hippocampal CA1 neurons by carbamazepine (CBZ) and valproate (VPA), before and after kindling epileptogenesis. METHODS: Voltage-dependent sodium current was measured in isolated hippocampal CA1 neurons, by using the whole-cell voltage-clamp technique. CBZ (15-100 microM) or VPA (0.5-5 mM) was applied by bath perfusion. Cells from fully kindled rats were compared with controls, 1 day and 5 weeks after the tenth generalized seizure. RESULTS: CBZ did not affect sodium current activation but selectively shifted the voltage dependence of steady-state inactivation to more hyperpolarized potentials. One day after the last kindled generalized seizure, the shift induced by 15 microM CBZ was 2.1+/-0.5 mV (mean +/- SEM; n = 20) compared with 4.3+/-0.3 mV (n = 16; p<0.001) in matched controls. The EC50 of the concentration-effect relation was 57+/-6 microM compared with 34+/-2 microM (p<0.01) in controls. Five weeks after kindling, these values had recovered to a level not different from control. VPA induces at a relatively high concentration a similar but smaller shift in voltage dependence of inactivation than does CBZ. After kindling, the shift induced by 2 mM VPA (2.8+/-0.6 mV; n = 19) was not different from controls (3.0+/-0.5 mV; n = 22). The EC50 for VPA was 2.6+/-0.3 mM compared with 2.5+/-0.4 mM in controls. CONCLUSIONS: Both CBZ and VPA selectively modulate the voltage dependence of sodium current steady-state inactivation and as a consequence reduce cellular excitability. The effect of CBZ was reduced immediately after kindling epileptogenesis, apparently by a reduced affinity of its receptor. In contrast, the shift induced by VPA was not different at any stage after kindling epileptogenesis. The change in CBZ sensitivity after kindling is related to epileptic activity rather than to the epileptic state, because it almost completely recovers in a period without seizures.

Animals↗

PubMed1999

Miniature inhibitory postsynaptic currents in CA1 pyramidal neurons after kindling epileptogenesis.

Miniature inhibitory postsynaptic currents (mIPSCs) were measured in CA1 pyramidal neurons from long-term kindled rats (>6 weeks after they reached the stage of generalized seizures) and compared with controls. A large reduction in the number of mIPSCs was observed in a special group of large mIPSCs (amplitude >75 pA). The frequency of mIPSCs in this group was reduced from 0.042 Hz in controls to 0.027 Hz in the kindled animals. The reduction in this group resulted in a highly significant difference in the amplitude distributions. A distinction was made between fast mIPSCs (rise time <2.8 ms) and slow mIPSCs. Fast mIPSCs, which could originate from synapses onto the soma and proximal dendrites, had significantly larger amplitudes than slow mIPSCs, which could originate from more distal synapses (35.4 +/- 1.1 vs. 26.2 +/- 0.4 pA in the kindled group; means +/- SE). The difference in the value of the mean of all amplitudes and frequency of fast and slow mIPSCs did not reach significance when the kindled group was compared with controls. The mIPSC kinetics were not different after kindling, from which we conclude that the receptor properties had not changed. Nonstationary noise analysis of the largest mIPSCs suggested that the single-channel conductance and the number of postsynaptic receptors was similar in the kindled and control groups. Our results suggest a 40-50% reduction in a small fraction of (peri-) somatic synapses with large or complex postsynaptic structure after kindling. This functionally relevant reduction may be related to previously observed loss of a specific class of interneurons. Our findings are consistent with a reduction in inhibitory drive in the CA1 area. Such a reduction could underlie the enhanced seizure susceptibility after kindling epileptogenesis.

Animals↗

PubMed1998

Sustained potassium currents in maturing CA1 hippocampal neurones.

Whole-cell voltage clamp techniques were used to characterize sustained outward currents in maturing (P4 to P48) acutely isolated rat CA1 hippocampal neurones. Sodium removal and signal subtraction were used to isolate a sodium dependent sustained potassium current (IKNa). Calcium blockade (Co2+), sensitivity to a low TEA dose (0.5 mM) and sensitivity to Charibdotoxin (CTX 25 nM) and Iberiotoxin (IbTX 25 nM), in conjunction with signal subtraction, were used to isolate a sustained current with the characteristics of IC (IKCa). IKNa was found in both immature (P4-5) and older (P > 21) cells; this corresponded, respectively, to 56 +/- 5% and 36 +/- 6% of the outward current in younger and older cells. In the course of maturation, the voltage dependence of activation of IKNa shifted to more hyperpolarized values by approximately 20 mV. In the younger cells (P5-18) there was no evidence for sensitivity to CTX or IbTX. In 55 out of 77 older cells we found a component sensitive to CTX, IbTX, 0.5 mM TEA and Co2+.

Animals↗

PubMed1998

Volume changes induced by osmotic stress in freshly isolated rat hippocampal neurons.

The degree to which osmotic stress changes the volume of mammalian central neurons has not previously been determined. We isolated CA1 pyramidal cells and measured cell volume in four different ways. Extracellular osmolarity (pio) was lowered by omitting varying amounts of NaCl and raised by adding mannitol; the extremes of pio tested ranged from 134 to 396 mosm/kg. When pio was reduced, cell swelling varied widely. We distinguished three types of cells according to their response: "yielding cells" whose volume began to increase immediately; "delayed response cells" which swelled after a latent period of 2 min or more; and "resistant cells" whose volume did not change during exposure to hypo-osmotic solution. When pio was raised, most cells shrank slowly, reaching minimal volume in 15-20 min. We observed neither a regulatory volume decrease nor an increase. We conclude that the water permeability of the membrane of hippocampal CA1 pyramidal neurons is low compared to that of other cell types. The mechanical support of the plasma membrane given by the cytoskeleton may contribute to the resistance to swelling and protect neurons against swelling-induced damage.

Animals↗