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S E Fox

Publications and source records attributed to S E Fox.

30 records · Page 2Linked to original sources
PubMed1989

Membrane potential and impedance changes in hippocampal pyramidal cells during theta rhythm.

Intracellular recordings were made from hippocampal pyramidal cells identified by their depths and their responses to commissural stimulation. Recordings were made during spontaneous bouts of hippocampal theta rhythm in urethane anesthetized rats. Membrane potentials (Vm) of pyramidal cells varied with the phase of the theta rhythm, that is, there was an "intracellular theta rhythm". The changes in Vm averaged about 2 mV peak to peak. Averaged intracellular theta waves showed that CA1 pyramids were most depolarized at the time of the positive peak of the extracellular theta rhythm recorded in (and superficial to) the CA1 pyramidal cell layer (CA1 theta). Peak depolarizations for CA3/4 pyramids were more broadly distributed, but occurred mainly in the interval just before the positive peak to just before the negative peak of the CA1 theta. Input impedance minima that were measurable at frequencies as high as 100 Hz occurred at about the same phases of the extracellular theta rhythm as the peak depolarizations (positive-going zero crossing to negative-going zero crossing of the CA1 theta). Such impedance changes imply conductance changes on the soma. The magnitude and localization of the conductance changes suggests that somatic IPSPs make major contributions to the intracellular theta rhythm. The phase relation between the intracellular and extracellular theta rhythms could be reversed by long duration current pulses that depolarized the cells slightly. This implies that either the intracellular theta-related IPSPs are depolarizing potential changes, or that they occur simultaneously with EPSPs. The phase of the intracellular theta rhythm was generally unaffected by long duration hyperpolarizing current pulses. Chloride leakage that reversed the evoked IPSPs usually had no effect on the phase of the intracellular theta rhythm, although in one case it appeared to cause its amplitude to increase.

Animals↗
PubMed1989

Two populations of rhythmically bursting neurons in rat medial septum are revealed by atropine.

1. Previous findings, such as the sensitivity of the hippocampal theta rhythm to cholinergic manipulation, support a "pacemaker" role for the cholinergic cells of the medial septal nucleus and the vertical limb of the nucleus of the diagonal band (MSN-NDB). To explore the mechanism(s) of action of systemic antimuscarinic drugs in eliminating the theta rhythm, recordings of hippocampal EEG and rhythmic MSN-NDB neurons that fired in phase with the hippocampal theta rhythm were taken during the administration of atropine in urethane-anesthetized rats. 2. Twenty-two of 33 rhythmic MSN-NDB cells continued to burst at the theta rhythm frequency after administration of a dose of atropine (25 mg/kg iv) that was sufficient to eliminate the theta rhythm (atropine-resistant cells). The remaining 11 cells lost their rhythmic firing pattern over the same time course as the loss of the theta rhythm (atropine-sensitive cells). 3. Both types of rhythmic MSN-NDB cells could be antidromically driven from the fimbria/fornix with similar latencies (range, 0.5-4.0 ms). The extracellularly recorded spike waveforms were not useful in predicting the atropine sensitivity of a given cell. Atropine-resistant cells frequently had higher firing rates than atropine-sensitive cells, but there was sufficient overlap of the two groups to make this a poor predictor of sensitivity. 4. Cooling the fimbria/fornix reversibly eliminated the hippocampal theta rhythm, but had no effect on 21/25 rhythmic MSN-NDB cells tested. This indicates that the atropine-sensitive MSN-NDB cells do not depend on the periodic output from the hippocampus for their rhythmic firing. Recordings from pairs of rhythmic MSN-NDB cells during cooling and/or atropine administration showed unchanged phase relations at the theta rhythm frequency. In rats in which the septohippocampal system was exposed by aspirating the overlying brain tissue, direct application of atropine (10 mg/ml) to the septal nuclei reversibly eliminated the hippocampal theta rhythm. 5. The rhythmic cells of the MSN-NDB are apparently composed of at least two distinct types, both of which potentially contribute to the production of the theta rhythm in the hippocampus. Elimination of hippocampal theta rhythm after local septal atropine application suggests that the loss of rhythmic activity in the group of atropine-sensitive septal cells is sufficient for the elimination of the theta rhythm. A model of the septohippocampal connections necessary for the theta rhythm is presented.

Animals↗
PubMed1986

Hippocampal theta rhythm and the firing of neurons in walking and urethane anesthetized rats.

Recordings were taken from single neurons in the hippocampus and dentate gyrus of rats during walking and urethane anesthesia. Firing histograms for these cells were constructed as a function of the phase of the concurrent extracellularly recorded hippocampal slow wave theta rhythm. Care was taken to be sure of the site of recording of the theta rhythm and its phase with respect to a reliable reference, so that comparisons of the phases of firing could be made across animals. The firing of most of these neurons is deeply modulated as a function of the phase of the theta rhythm. This is true whether the theta rhythm occurs during walking or during urethane anesthesia, but for some types of cells the mean phases of firing are different in the two types of theta rhythm. During walking, pyramidal cells and interneurons in all hippocampal subregions and dentate granule cells have a maximum probability of firing near the positive peak of the theta rhythm recorded in the outer molecular layer of the dentate (dentate theta). During urethane anesthesia, the maximum firing probability for interneurons in CA1 and for dentate granule cells occurs near the negative peak of the dentate theta, while the phases of maximum firing for pyramidal cells and interneurons in CA3 and CA4 become widely distributed. The phases of maximum firing of pyramidal cells in CA1 are, if anything, more narrowly distributed around the positive peak of the dentate theta during urethane anesthesia than during walking. These differences in the firing of hippocampal cells during walking and urethane anesthesia represent some of the differences in cellular mechanisms distinguishing two kinds of hippocampal theta rhythm.

Action Potentials↗
PubMed1985

Location of membrane conductance changes by analysis of the input impedance of neurons. I. Theory.

If the membrane conductance of a neuron changes, its response to injected current changes. If the change in membrane conductance is restricted to a given subregion of the neuron, that region can be located by analysis of the form of the change in the response of the neuron to current injection. The theoretical basis of this method is rigorously developed in this paper. Location of the membrane conductance change is possible because the higher-frequency components of the injected currents are progressively attenuated by the axial resistance and membrane capacitance of the neuron as they pass from the injection site to electrotonically more distant regions. For the lower-frequency components, this attenuation is less pronounced. Therefore, when a conductance change occurs relatively far from the recording/current-passing electrode, only the lower frequency components of the response are altered, because the higher-frequency components of the current do not even reach that site. When such a conductance change occurs relatively near the electrode, both the lower and the higher frequency components of the response are altered. Treating the neuron as a passive network, the input impedance at a given frequency is simply the voltage response of the neuron at that frequency divided by the current injected at that frequency. This is a complex value, having both magnitude and phase components. The change in the magnitude of the input impedance due to a conductance change occurring distally drops off more rapidly with increasing frequency than that due to a proximal conductance change. In addition, for distal conductance increases the magnitude of the input impedance can increase in the higher range of frequencies. This paradoxical effect is treated in APPENDIX I. For many neurons an estimate of the electrotonic location of a conductance change can be made knowing only the change in input resistance, the change in the magnitude of the input impedance at the characteristic frequency (omega 0 = 1/tau 0), and a reasonable estimate of the total electrotonic length of the neuron (L). The sensitivity of the method depends on the electrotonic length of the neuron. The method is most useful in neurons with dendritic trees longer than approximately 0.5 length constants. The dendritic-to-somatic conductance ratio of the neuron does not appreciably affect the forms of the responses. The time constant merely shifts the frequency range of interest.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1985

Location of membrane conductance changes by analysis of the input impedance of neurons. II. Implementation.

Mathematical modeling has shown that it should be possible to determine the electrotonic location of membrane conductance changes in single neurons by analysis of the associated changes in the magnitude of the alternating-current (AC) input impedance. The form of the plot of change in the magnitude of the input impedance as a function of frequency (delta [Zn(f)]) should differ for changes in membrane conductance located at different electrotonic distances from the recording/current-injection site. Due to the axial resistance and the membrane capacitance, the higher frequencies are attenuated with distance to a greater degree than are the lower frequencies. Thus delta [Zn(f)] should drop to zero more rapidly with increasing frequency for distal than for proximal conductance changes. For distal changes in conductance, the sign of the change in the magnitude of the input impedance can even reverse in the higher frequency range, so that increases in conductance would produce increases in impedance. This effect may explain the paradoxical increases in impedance at 100 Hz reported for motor neuron inhibitory postsynaptic potentials. Sine-wave impedance measurements were made in single embryonic chick spinal neurons in tissue culture, and gamma-aminobutyric acid (GABA) was iontophoretically applied alternately to the soma and to a neurite at a measured distance from the soma. The impedance changes produced by the GABA-induced conductance changes were consistent with the expectations from the mathematical modeling, but the results suggest that the axial resistance of the neurites must be quite high in some cases. Distortions due to microelectrode capacitance and stray capacitances in the input stage of single-electrode bridge amplifiers can make sine-wave impedance measurements impossible. This difficulty was eliminated by modifications to the capacity compensation circuit of an active bridge amplifier. Noise and distortion of several other types can also introduce serious errors. Methods for minimizing such problems are discussed. In spite of its limitations, this method can be of great practical value, because it can give the electrotonic location of spontaneously occurring membrane conductance changes in single neurons even when unitary synaptic potentials cannot be resolved. These methods are currently being applied to hippocampal pyramidal cells in vivo to locate conductance changes during the electroencephalogram (EEG) theta-rhythm in rats. In such laminated structures, the determination of the anatomical source of a group of active synapses can be aided by location of the resultant membrane conductance changes.

Animals↗
PubMed1984

Hippocampal excitability related to the phase of theta rhythm in urethanized rats.

Urethanized rats were studied to determine if excitability in the hippocampus is related to the phase of the theta rhythm. The responses recorded from area CA1 were clearly modulated at the theta frequency. Those recorded from the dentate region were not. The most excitable phase for CA1 differed significantly from that previously reported for walking rats. These findings suggested that the mechanisms for theta production differ for normal and urethanized rats.

Animals↗
PubMed1981

Electrophysiological characteristics of hippocampal complex-spike cells and theta cells.

Stimulating electrodes were chronically implanted in the ventral hippocampal commissure and the entorhinal cortex or angular bundle of rats. Moveable metal microelectrodes which could be passed through the hippocampus were implanted. All hippocampal units were classified s complex-spike cells or theta cells on the basis of the form of their action potentials and their rates of firing in various behaviors. Field potentials and unit firing evoked from the stimulating electrodes were recorded during slow wave sleep. Complex-spike cells (1) could often be antidromically activated in CA3 (it was not attempted in CA1); (2) could only be induced to fire one or two action potentials in response to a single stimulus; (3) had action potentials at the same time as the local population-spike and, in condition-test studies, were depressed when the population-spike was depressed. (The population-spike is presumably the summed synchronous action potentials of pyramidal cells.) Theta cells: (1) were antidromically activated in only one out of 25 cases; (2) usually could fire long bursts of action potentials in response to a sufficiently intense single stimulus; (3) this firing occurred before, during, and after the local orthodromic population-spike. Most complex-spike cells in Ammon's horn must be pyramidal cells (projection cells), and vice versa. The case for theta cells is more difficult. Some are non-pyramidal cells with locally ramifying axons, but at least some are projection cells. The data is consistent with most of them being inhibitory interneurons, but this is not established.

Animals↗
PubMedDate not supplied

The suppressant effect of ethanol, delivered via intrahippocampal microdialysis, on the firing of local pyramidal cells in freely behaving rats.

Intrahippocampal microdialysis was performed on 14 freely behaving rats, and the firing of pyramidal cells within the dialysis area was recorded. In one group of rats, the microdialysis was conducted only with artificial cerebrospinal fluid (ACSF) for 2-4 h. In this control group, the recorded neurons displayed normal firing patterns. In another group, ACSF was replaced for 30-60 min with various concentrations of ethanol to deliver this drug via the microdialysis probe into the cell recording area. Ethanol at the concentration of 5% (w/v) significantly and reversibly suppressed the firing of the recorded neurons. The marked firing rate alterations were not accompanied with apparent changes in the hippocampal EEG activity or the behavior of the rats, indicating localized drug actions. These data demonstrate for the first time that in the physiologically functioning brain, ethanol exerts principally a suppressant effect on the electrical activity of hippocampal pyramidal cells.

Animals↗