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M J Gutnick

Publications and source records attributed to M J Gutnick.

At least 19 recordsLinked to original sources
PubMed2001

Enhanced spontaneous transmitter release is the earliest consequence of neocortical hypoxia that can explain the disruption of normal circuit function.

After the onset of an acute episode of arrested circulation to the brain and consequent cerebral hypoxia, EEG changes and modifications of consciousness ensue within seconds. This in part reflects the rapid effect of hypoxia on the neocortex, where oxygen deprivation leads to impaired neuronal excitability and abnormal synaptic transmission. To identify the cellular mechanisms responsible for the earliest changes in neocortical function and to determine their time course, we have used patch-in-slice recording techniques to investigate the effects of acute hypoxia on the synaptic and intrinsic properties of layer 5 neurons. Coronal slices of mouse somatosensory cortex were maintained at 37 degrees C and challenged with episodes of hypoxia (3-4 min of exposure to 95% N(2), 5% CO(2)). In recordings with cell-attached patch electrodes, activation of ATP-sensitive potassium channels first became detectable 211 +/- 11 sec (range, 185-240 sec; n = 6 patches) after the onset of hypoxia. Similar recording techniques revealed no alterations in the properties of Na(+) currents in the first 4 min after the onset of hypoxia. The earliest hypoxia-induced disturbance was a marked increase in the frequency of spontaneous EPSCs and IPSCs, which began within 15-30 sec of the removal of oxygen. This rapid synaptic effect was not sensitive to TTX and was present in Ca(2+)-free perfusate, indicating that the hypoxia had a direct influence on the vesicular release mechanisms. The incoherent, massive increase in miniature PSCs would be expected to deplete the readily releasable pool of vesicles in cortical terminals, and to thereby markedly distort the neuronal interactions that underlie normal circuit function.

Adenosine Triphosphate↗
PubMed1998

Functionally distinct NMDA receptors mediate horizontal connectivity within layer 4 of mouse barrel cortex.

In sensory areas of neocortex, thalamocortical afferents project primarily onto the spiny stellate neurons of Layer 4. Anatomical evidence indicates that these cells receive most of their excitatory input from other cortical neurons, including other spiny stellate cells. Although this local network must play an important role in sensory processing, little is known about the properties of the neurons and synapses involved. We have produced a slice preparation of mouse barrel cortex that isolates Layer 4. We report that excitatory interaction between spiny stellate neurons is largely via N-methyl-D-aspartate receptors (NMDARs) and that a given neuron contains more than one type of NMDAR, as distinguished by voltage dependence. Thus, spiny stellate cells act as effective integrators of powerful and persistent NMDAR-mediated recurrent excitation.

2-Amino-5-phosphonovalerate↗
PubMed1998

Activation of protein kinase C increases neuronal excitability by regulating persistent Na+ current in mouse neocortical slices.

Effects of the protein kinase C activating phorbol ester, phorbol 12-myristate 13-acetate (PMA), were studied in whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex. PMA was applied intracellularly (100 nM to 1 microM) to restrict its action to the cell under study. In current-clamp recordings, it enhanced neuronal excitability by inducing a 10- to 20-mV decrease in voltage threshold for action-potential generation. Because spike threshold in neocortical neurons critically depends on the properties of persistent Na+ current (INaP), effects of PMA on this current were studied in voltage clamp. After blocking K+ and Ca2+ currents, INaP was revealed by applying slow depolarizing voltage ramps from -70 to 0 mV. Intracellular PMA induced a decrease in INaP at very depolarized membrane potentials. It also shifted activation of INaP in the hyperpolarizing direction, however, such that there was a significant increase in persistent inward current at potentials more negative than -45 mV. When tetrodotoxin (TTX) was added to the bath, blocking INaP and leaving only an outward nonspecific cationic current (Icat), PMA had no apparent effect on responses to voltage ramps. Thus PMA did not affect Icat, and it did not induce any additional current. Intracellular application of the inactive PMA analogue, 4 alpha-PMA, did not affect INaP. The specific protein kinase C inhibitors, chelerythrine (20 microM) and calphostin C (10 microM), blocked the effect of PMA on INaP. The data suggest that PMA enhances neuronal excitability via a protein kinase C-mediated increase in INaP at functionally critical subthreshold voltages. This novel effect would modulate all neuronal functions that are influenced by INaP, including synaptic integration and active backpropagation of action potential from the soma into the dendrites.

Alkaloids↗
PubMed1996

Slow inactivation of Na+ current and slow cumulative spike adaptation in mouse and guinea-pig neocortical neurones in slices.

1. Spike adaptation of neocortical pyramidal neurones was studied with sharp electrode recordings in slices of guinea-pig parietal cortex and whole-cell patch recordings of mouse somatosensory cortex. Repetitive intracellular stimulation with 1 s depolarizing pulses delivered at intervals of < 5 s caused slow, cumulative adaptation of spike firing, which was not associated with a change in resting conductance, and which persisted when Co2+ replaced Ca2+ in the bathing medium. 2. Development of slow cumulative adaptation was associated with a gradual decrease in maximal rates of rise of action potentials, a slowing in the post-spike depolarization towards threshold, and a positive shift in the threshold voltage for the next spike in the train; maximal spike repolarization rates and after-hyperpolarizations were unchanged. 3. The data suggested that slow adaptation reflects use-dependent removal of Na+ channels from the available pool by an inactivation process which is much slower than fast, Hodgkin-Huxley-type inactivation. 4. We therefore studied the properties of Na+ channels in layer II-III mouse neocortical cells using the cell-attached configuration of the patch-in-slice technique. These had a slope conductance of 18 +/- 1 pS and an extrapolated reversal potential of 127 +/- 6 mV above resting potential (Vr) (mean +/- S.E.M.; n = 5). Vr was estimated at -72 +/- 3 mV (n = 8), based on the voltage dependence of the steady-state inactivation (h infinity) curve. 5. Slow inactivation (SI) of Na+ channels had a mono-exponential onset with tau on between 0.86 and 2.33 s (n = 3). Steady-state SI was half-maximal at -43.8 mV and had a slope of 14.4 mV (e-fold)-1. Recovery from a 2 s conditioning pulse was bi-exponential and voltage dependent; the slow time constant ranged between 0.45 and 2.5 s at voltages between-128 and -68 mV. 6. The experimentally determined parameters of SI were adequate to simulate slow cumulative adaptation of spike firing in a single-compartment computer model. 7. Persistent Na+ current, which was recorded in whole-cell configuration during slow voltage ramps (35 mV s-1), also underwent pronounced SI, which was apparent when the ramp was preceded by a prolonged depolarizing pulse.

Action Potentials↗
PubMed1996

Kinetics of slow inactivation of persistent sodium current in layer V neurons of mouse neocortical slices.

1. In whole cell recordings from layer V neurons in slices of mouse somatosensory neocortex, tetrodotoxin (TTX)-sensitive persistent Na+ current (INaP) was studied by blocking K+ currents with intracellular Cs+ and Ca2+ currents with extracellular Cd2+. During slow voltage ramps, INaP began to activate at around -60 mV, and attained a peak at around -25 mV. The peak amplitude of INaP varied widely from cell to cell (range 60-3,160 pA; median 308 pA, n = 77). At potentials more positive than -35 mV, INaP in all cells was superimposed on a large, TTX-resistant outward current. 2. In hybrid clamp experiments, INaP was significantly reduced by a preceding high-frequency train of spikes. 3. INaP underwent pronounced slow inactivation, which was revealed by systematically varying the ramp speed between 233 and 2.33 mV/s, or varying the duration of a depolarizing prepulse between 0.1 and 10 s. 4. Onset of slow inactivation at +20 mV was monoexponential with tau = 2.06 s (n = 17 cells). Recovery from slow inactivation was voltage dependent. It followed a monoexponential time course with tau = 2.31 s (n = 6) at -70 mV and tau = 1.10 s (n = 4) at -90 mV. These values are not significantly different than values previously reported for slow inactivation of fast-inactivating INa. 5. Slow inactivation of neocortical INaP will influence all neuronal functions in which this current plays a role, including spike threshold determination, synaptic integration, and active propagation in dendrites. The kinetics of slow inactivation suggest that it may be a factor not only during extremely intense spiking, but also during periods of "spontaneous" activity.

Anesthetics, Local↗
PubMed1995

Electrical consequences of spine dimensions in a model of a cortical spiny stellate cell completely reconstructed from serial thin sections.

We built a passive compartmental model of a cortical spiny stellate cell from the barrel cortex of the mouse that had been reconstructed in its entirety from electron microscopic analysis of serial thin sections (White and Rock, 1980). Morphological data included dimensions of soma and all five dendrites, neck lengths and head diameters of all 380 spines (a uniform neck diameter of 0.1 micron was assumed), locations of all symmetrical and asymmetrical (axo-spinous) synapses, and locations of all 43 thalamocortical (TC) synapses (as identified from the consequences of a prior thalamic lesion). In the model, unitary excitatory synaptic inputs had a peak conductance change of 0.5 nS at 0.2 msec; conclusions were robust over a wide range of assumed passive-membrane parameters. When recorded at the soma, all unitary EPSPs, which were initiated at the spine heads, were relatively iso-efficient; each produced about 1 mV somatic depolarization regardless of spine location or geometry. However, in the spine heads there was a twentyfold variation in EPSP amplitudes, largely reflecting the variation in spine neck lengths. Synchronous activation of the TC synapses produced a somatic depolarization probably sufficient to fire the neuron; doubling or halving the TC spine neck diameters had only minimal effect on the amplitude of the composite TC-EPSP. As have others, we also conclude that from a somato-centric viewpoint, changes in spine geometry would have relatively little direct influence on amplitudes of EPSPs recorded at the soma, especially for a distributed, synchronously activated input such as the TC pathway. However, consideration of the detailed morphology of an entire neuron indicates that, from a dendro-centric point of view, changes in spine dimension can have a very significant electrical impact on local processing near the sites of input.

Animals↗
PubMed1995

Laminar pattern of synaptic inhibition during convulsive activity induced by 4-aminopyridine in neocortical slices.

1. Epileptiform activity was induced in rat neocortical brain slices by application of a low concentration (10 microM) of 4-aminopyridine (4-AP). In intracellular recordings from regular spiking neurons, the activity was characterized by prolonged, all-or-none depolarizing events, with variable delay to a threshold stimulus. 2. At this concentration, 4-AP had no measurable effect on passive electrical properties or on action-potential characteristics. 3. Paroxysmal responses in neurons of deeper layers differed markedly from those of superficial cells. In deep neurons, responses resembled those generated by neocortical neurons exposed to GABAergic blockers. A low-intensity stimulus to the white matter evoked an excitatory postsynaptic potential (EPSP) that was followed with variable latency by a paroxysmal depolarizing shift that reversed at suprathreshold membrane potentials and upon which superimposed repetitive firing was always evident. By contrast, in superficial (layer II-III) neurons, the same stimulus evoked an EPSP that was followed by a prolonged response whose late component reversed at subthreshold membrane potentials (between -50 and -80 mV). These cells rarely fired more than a single spike throughout the response. 4. Repetitive stimulation at relatively low frequencies (0.3-1 Hz) caused a gradual change in the synchronized responses that was most marked in superficial neurons. The reversal potential of the response shifted toward suprathreshold membrane potentials, and subsequently, superimposed repetitive firing became evident. These changes were not associated with measurable changes in input resistance or membrane potential.(ABSTRACT TRUNCATED AT 250 WORDS)

4-Aminopyridine↗
PubMed1995

Paired-pulse facilitation of IPSCs in slices of immature and mature mouse somatosensory neocortex.

1. Whole cell recordings from layer V neurons of mouse somatosensory cortex were made with the use of a "blind" patch-clamp technique. In slices from immature [postnatal days 6 to 11 (P6-P11)] and juvenile (P18-P21) animals, inhibitory postsynaptic currents (IPSCs) were evoked in all cells by extracellular stimulation at the layer V-VI border. Monosynaptic IPSCs, with latency < 2 ms, were isolated pharmacologically by blockade of ionotropic glutamatergic transmission. IPSCs were blocked by bicuculline methiodide and reversed near the predicted equilibrium potential for Cl-. 2. IPSC characteristics were not different for the two age groups. At 1.5-2 times threshold intensity (0.2 Hz), they fluctuated in amplitude with occasional failures. At -70 or -80 mV, mean amplitudes were -202 +/- 20 (SE) pA and -207 +/- 32 pA for immature (39 cells) and juvenile (13 cells) cortex, respectively. Half rise times were 0.74 +/- 0.03 ms (n = 7 cells) in neonates and 0.67 +/- 0.04 ms (n = 7 cells) in juveniles. Decays were biexponential with tau 1 = 14.8 +/- 1.3 ms and tau 2 = 59.0 +/- 7.4 ms (n = 7 cells) in neonates, and tau 1 = 11.9 +/- 1.1 ms and tau 2 = 55.5 +/- 4.2 ms (n = 7 cells) in juveniles. 3. Pairs of stimuli elicited paired-pulse facilitation (PPF) when delivered at brief interstimulus intervals (ISI), and paired-pulse depression (PPD) at long ISI. PPF, which was evident in 64% of immature cells and 38% of juvenile cells, was maximal (38 +/- 4% greater than the conditioning response) at 20-40 ms. PPD, which was evident in 82% of immature cells and 87% of juvenile cells, was maximal (29 +/- 2% smaller than the conditioning response) by 300 ms. In each age group, some animals showed PPF without PPD.(ABSTRACT TRUNCATED AT 250 WORDS)

Action Potentials↗
PubMed1994

A comparison of synapses onto the somata of intrinsically bursting and regular spiking neurons in layer V of rat SmI cortex.

Regular spiking (RS) and intrinsically bursting (IB) neurons show distinct differences in their inhibitory responses. Under various conditions, the synaptic responses of RS cells display marked inhibitory postsynaptic potentials (IPSPs), whereas the responses of most IB cells do not (Silva et al: Soc Neurosci Abstr 14:883, 1988; Chagnac-Amitai and Connors: J Neurophysiol 61:747, 62:1149, 1989; Connors and Gutnick: TINS 13:99, 1990). This investigation is designed to determine if differences in the inhibitory responses of RS versus IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified by using intracellular recording and labeling, examined with the light microscope, and then serial thin-sectioned prior to examination with the electron microscope. Axonal terminals presynaptic to their somata and proximal dendrites were identified and classified according to criteria described by Peters and coworkers (Peters et al: J Neurocytol 19:584, 1990; Peters and Harriman: J Neurocytol 19:154, 1990; 21:679, 1992). The locations of these boutons were displayed on the surfaces of 3-D reconstructions of the somata and proximal dendrites. The reconstructions were produced directly from the serial thin sections by using a novel, electron microscopic, image-processing computer resource. Our analysis showed no significant difference in the types and concentration of boutons presynaptic to the cell bodies and proximal dendrites of intrinsically bursting versus regular spiking neurons. We conclude that the differences observed in the inhibitory responses of intrinsically bursting versus regular spiking neurons cannot be explained by differences in the concentrations of synapses onto their somata.

Action Potentials↗
PubMed1994

Ca2+ accumulations in dendrites of neocortical pyramidal neurons: an apical band and evidence for two functional compartments.

Apical dendrites constitute a prominent feature of the microcircuitry in the neocortex, yet their function is poorly understood. Using fura-2 imaging of layer 5 pyramidal neurons from slices of rat somatosensory cortex, we have investigated the Ca2+ influx into dendrites under intracellular, antidromic, synaptic, and receptor-agonist stimulation. We find three spatial patterns of Ca2+ accumulations: an apical band in the apical dendrite approximately 500 microns from the soma, an accumulation restricted to the basal dendrites, soma, and proximal apical dendrite, and a combination of both of these. We show that the apical band can be activated antidromically and synaptically and that, under blocked Na+ and K+ conductances, it generates Ca2+ spikes. Thus, the apical band may serve as a dendritic trigger zone for regenerative Ca2+ spikes or as a current amplifier for distal synaptic events. Our results suggest that the distal apical dendrite should be considered a separate functional compartment from the rest of the cell.

Animals↗
PubMed1994

Long-term changes in neocortical activity after chemical kindling with systemic pentylenetetrazole: an in vitro study.

1. Rats were chemically kindled by systemic administration of pentylenetetrazole (PTZ) every 48 h. An initially subthreshold dose that did not elicit a motor response when first applied caused severe epileptiform seizures when the animal was kindled. Once kindled, animals continued to respond to the initially subthreshold dose with a full-blown seizure for > 2 mo, even when regular administration ceased for > or = 1 mo. 2. In neocortical slices taken from kindled rats, low-intensity electrical stimulation evoked generation of prolonged (hundreds of milliseconds) paroxysmal extracellular field potentials and intracellular depolarizing potentials, indicating synchronized activity of large populations of neurons. This hyperexcitability usually appeared as an all-or-none event of variable latency. In a few cases it increased gradually with increasing stimulus intensity. The intensity of the paroxysmal response was greatly enhanced by application of gamma-aminobutyric acid-A (GABAa) receptor blockers to the bath. 3. Intracellular recordings revealed that PTZ-kindled cells differ from normal cells in their higher input resistance (42.4 + 13.6 vs. 26.4 + 9.2 M omega, mean +/- SE). Spikes generated by kindled cells differed significantly from those in normal cells in that they were of longer duration (1.65 + 0.3 vs. 1.40 + 0.15 ms) and had a slower maximal rate of fall (103 + 29.7 vs. 126 + 20.8 volts/s). 4. Injection of the lidocaine derivative QX-314 to the recorded neurons (100 mM) blocked the fast Na+ spikes. Under these conditions slow spikes, probably Ca2+ mediated, were evoked from the soma in neurons from kindled but not from normal cortex. 5. The role of N-methyl-D-aspartate (NMDA) receptors in generating paroxysmal events was evaluated by application of 20 microM 2-amino-5-phosphonovaleric acid, a specific blocker of this glutamate receptor type. Blockage of NMDA receptors cut short the paroxysmal field potentials but did not prevent their generation. Intracellularly recorded paroxysmal responses were also cut short but not abolished by intracellular hyperpolarization. 6. In slices from kindled animals intracellular responses in neurons of deeper layers differed markedly from those of superficial cells. In deep neurons, responses resembled those generated by neocortical neurons exposed to GABAergic blockers. A low-intensity stimulus to the white matter evoked an excitatory postsynaptic potential (EPSP) followed with variable latency by a paroxysmal depolarizing shift that reversed at suprathreshold membrane potentials and on which superimposed repetitive firing was always evident.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗
PubMed1993

Injection of tetanus toxin into the neocortex elicits persistent epileptiform activity but only transient impairment of GABA release.

Focal injection of a minute quantity of tetanus toxin into the rat neocortex induces chronic epileptogenesis. Within a day, spontaneous and stimulus-evoked paroxysmal discharges appear in widespread regions of both hemispheres and this lasts for at least nine months. Tetanus toxin blocks transmitter release, apparently by catalysing the breakdown of synaptobrevin, a synaptic protein. It specifically binds to neuronal membranes but its potent epileptogenic properties have been ascribed to a higher affinity for inhibitory neurons. Following focal injection of tetanus toxin into the hippocampus a long-lasting epileptic syndrome also develops. During the early part of the syndrome GABA release is depressed in slices from the injected side, but not in slices from the contralateral, secondary focus. In the present experiments on neocortex, release of radiolabelled GABA was measured from primary and secondary epileptic foci induced by unilateral focal injection of tetanus toxin into the parietal cortex. By four weeks after the injection, no differences were detected in GABA release from any neocortical site in control or toxin-injected animals, despite the persistence of profound epileptic activity in slices from the latter. At earlier times (1.5 days) after the toxin injection, however, release was significantly depressed in both hemispheres. The results indicate that at first, the toxin induces focal neocortical epileptogenesis by directly impeding GABAergic synaptic transmission but that with time there is a recovery from this initial effect. We propose, as has also been suggested for other models, that the initial epileptogenesis leaves in its wake a long-lasting change in the local functional connectivity, such that the neocortex is rendered permanently epileptic.

Animals↗
PubMed1993

Non-uniform propagation of epileptiform discharge in brain slices of rat neocortex.

In neocortical brain slices of the rat that were exposed to 50 microM picrotoxin, low-intensity stimuli evoked all-or-none epileptiform events that propagated across the slice with an average velocity of 0.07 m/s. Simultaneous recordings from pairs of electrodes, in which one was held in a constant position and the other was systematically advanced across the slice in small steps, revealed that propagation of the synchronous activity was saltatory rather than uniform. Analysis of the propagation pattern showed that local regions (< 1000 microns) of uniform velocity were separated by distinct borders. Within these regions, local propagation velocity was determined by the threshold for synchronous activation of still-smaller (< 200 microns) neuronal aggregates. Although the velocity was sensitive to physiological factors that affect the precise threshold for synchronization, the location of the borderlines between adjacent regions remained unchanged. We propose that these invariant borders reflect the details of local neuronal organization within the slice, and that the pattern of propagation of epileptiform discharge is a manifestation of the intrinsic organization of the neocortex when deprived of afferent input.

Animals↗
PubMed1993

Transient Ca2+ currents in neurons isolated from rat lateral habenula.

1. The properties of the low-voltage-activated transient Ca2+ current (LVA, IT) that underlies rhythmic burst firing in neurons of the lateral habenula (LHb) were examined to further our understanding of mechanisms that promote rhythmogenesis in the CNS. We compared these properties with those of IT in thalamic ventrobasal relay neurons (IVB) and of the more slowly inactivating ITs of thalamic reticular neurons (InRt). 2. Patch-clamp techniques were used to record whole cell Ca2+ currents in LHb cells acutely isolated from rats ranging in age from postnatal days 6 to 34 (P6-P34). The LVA current in LHb (ILHb) had a number of properties similar to those of IVB, including activation threshold (near -65 mV) and voltage-dependent steady-state activation [half-activation voltage (V1/2) = -58.5 mV, slope = 3.4 mV-1] and inactivation (V1/2 = -83.5 mV, slope = 5.0 mV-1) functions. 3. ILHb was characterized by biphasic inactivation, with a fast, voltage-dependent time constant (20-50 ms) similar to that of IVB and a slower, voltage-independent decay phase (time constant approximately 120 ms) that was much more prominent than in IVB. Recovery of ILHb from inactivation was monophasic (time constant, 507 ms at -90 mV), and was slower than for IVB and about the same as for InRt. 4. ILHb was relatively insensitive to equimolar substitution of Ba2+ for Ca2+, in contrast to IVB, which was decreased, and InRt, which was enhanced. 5. In computer simulations, these results could not be accounted for by a mixture of the two previously described IT types (IVB and InRt) in individual LHb cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗
PubMed1993

A novel approach to correlative studies of neuronal structure and function.

We have recently organized and directed a consortium of manufacturers to assemble a unique electron microscopic-video-computer resource that reduces the time and effort required to make 3-dimensional reconstructions of neurons, and to collect and analyze data on synaptic organization by many fold. With the introduction of this system the quantitative study of synapses has entered a new age, characterized by a markedly increased efficiency that allows previously unrealistic studies to be carried out. In this paper, we present data designed to test the hypothesis that pyramidal cell types, identified by their intrinsic firing patterns, display characteristic inhibitory responses and distinctive synaptic patterns. These studies focus on the synaptic connectivity of regular spiking (RS) and intrinsically bursting (IB) neurons. Previous studies have shown that these neurons display distinct differences in their intrinsic membrane properties and in their morphologies as assessed with the light microscope. Under various conditions the synaptic responses of RS cells display marked inhibitory postsynaptic potentials, whereas most IB cells do not. This investigation is designed to determine if differences in the inhibitory responses of RS vs. IB cells are reflected in differences in the concentration of inhibitory synapses onto their somata. RS and IB neurons in rat somatosensory cortex were identified using intracellular recording and labeling, examined with the light microscope, and then serial thin sectioned prior to examination with the electron microscope. Synapses onto their somata and proximal dendrites were identified and plotted onto computer-assisted 3-D reconstructions made from the serial thin sections. Our analysis showed no significant difference in the types and concentration of synapses made onto the cell bodies and proximal dendrites of IB vs. RS neurons. Thus the differences observed in the inhibitory responses of IB vs. RS neurons cannot be explained by differences in the concentrations of synapses onto their somata.

Action Potentials↗
PubMed1993

Stepwise repolarization from Ca2+ plateaus in neocortical pyramidal cells: evidence for nonhomogeneous distribution of HVA Ca2+ channels in dendrites.

Although cortical dendrites have classically been thought of as passive structures, recent evidence suggests that active conductances, including Ca2+ conductance, are also present in the dendritic membrane. To investigate this, we have recorded intracellularly in slices of rat neocortex bathed in 24 mM tetraethylammonium chloride and 1 microM TTX. Under these conditions, pyramidal neurons generated prolonged Ca2+ spikes. In computer simulations, the breakpoint voltage from which the plateau level began to repolarize was closely related to a specific region on the voltage/activation curve of the high-voltage-activated Ca2+ conductance underlying the spike. This modeling result was supported by the experimental observation that substituting Ba2+ for Ca2+ caused a hyperpolarizing shift in breakpoint voltage by 8-10 mV. Often there was stepwise repolarization from the Ca2+ spike to one or more additional plateau levels. In compartmental computer models, this could be simulated by two different mechanisms: (1) the presence of multiple, electrotonically separated sites of Ca2+ spike electrogenesis in the dendritic tree, and (2) the presence of Ca2+ channels with different voltage dependencies in the same compartment. In experiments, brief hyperpolarizing pulses could cut short the high-amplitude plateau without terminating the smaller "steps." This result could be simulated by both computer models. However, only the multicompartmental model could simulate effects of prolonged depolarizing and hyperpolarizing currents on the breakpoint. Thus, the more depolarized the breakpoint, and hence the closer the spike initiation zone to the recording site, the less it was affected by the injected current. In experiments, the ratio of the breakpoint voltages for the different plateau levels was equal to the ratio of the highest repolarization rates. These data indicate that the breakpoint voltage and the time course of repolarization were the same at all the sites of Ca2+ electrogenesis. Our findings provide strong evidence that Ca2+ spike initiation occurs at electrotonically separated "hot spots" in the dendrites, and that voltage dependence of the Ca2+ channels that underlie the spikes is the same at all sites.

Animals↗
PubMed1992

Slow depolarizing afterpotentials in neocortical neurons are sodium and calcium dependent.

Depolarizing afterpotentials (DAPs) were studied in intracellular recordings from neocortical slices bathed in tetrodotoxin (TTX) (1 microM) and tetraethylammonium chloride (TEA) (24 mM), to block voltage-dependent Na+ currents and most K+ currents. The DAP was Ca(2+)-dependent, in that its magnitude varied as a function of the duration of the preceding Ca2+ plateau. It had an apparent reversal potential of between -40 and -5 mV. The DAP was blocked when choline replaced all extracellular Na+; there was a hyperpolarizing shift in apparent reversal potential when extracellular Na+ was lowered. The DAP was blocked by amiloride (1 mM), which also decreased the preceding Ca2+ plateau. The data are consistent with the hypothesis that the DAP is due to electrogenic Na+/Ca2+ exchange.

Animals↗