Xenotransplantation of human intestine into mouse abdomen or subcutaneous tissue: Novel platforms for the study of the human enteric nervous system

BACKGROUND

Current efforts to develop stem cell therapy as a novel treatment for neurointestinal diseases are limited by the unavailability of a model system to study cell transplantation in the human intestine. We propose that xenograft models support enteric nervous system (ENS) development in the fetal human intestine when transplanted into mice subcutaneously or intra-abdominally.

METHODS

Fetal human small and large intestine were grafted onto the small intestinal mesentery and into the subcutaneous tissue of immunodeficient mice for up to 4 months. Intestinal cytoarchitecture and ENS development were studied using immunohistochemistry.

KEY RESULTS

In both abdominal and subcutaneous grafts, the intestine developed normally with formation of mature epithelial and mesenchymal layers. The ENS was patterned in two ganglionated plexuses containing enteric neurons and glia, including cholinergic and nitrergic neuronal subtypes. c-Kit-immunoreactive interstitial cells of Cajal were present in the gut wall.

CONCLUSIONS & INFERENCES

Abdominal xenografts represent a novel model that supports the growth and development of fetal human intestine. This in vivo approach will be a useful method to study maturation of the ENS, the pathophysiology of neurointestinal diseases, and the long-term survival and functional differentiation of neuronal stem cells for the treatment of enteric neuropathies.

Chronic epileptic foci in neocortex: in vivo and in vitro effects of tetanus toxin

Injection of 0.2 - 3.0 ng of tetanus toxin into rat parietal neocortex resulted in permanent (> 7 months) changes in the local circuit properties of this tissue. It caused excessive synchronization of neuronal activity. This was seen as spontaneous paroxysmal field potentials and/or evoked all-or-none population burst discharges. Such activity was recorded widely over the parietal and temporal areas of both the injected and the contralateral hemispheres from as little as 16 h after injection up to the maximum survival time of 7 months. Several observations suggest that the speed with which the hypersynchronous activity spread to the opposite hemisphere reflects transport of the toxin through corticocortical axons, and consequent blockade of synaptic inhibition. However, from what is known of the half life of the peptide in brain, it is unlikely that the persistent, widespread distribution of epileptiform discharge several months after injection was due to the continued presence of toxin. Thus, intracortical application of tetanus toxin provides a good experimental model of chronic focal epilepsies, and raises fundamental questions regarding the long term regulation of local circuit properties in the neocortex.

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.

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.

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.

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.

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.

Hyperexcitability in a model of cortical maldevelopment

The presence of developmental cortical malformations has been associated with the occurrence of epilepsy, and correlative anatomic-clinical electrophysiological studies suggest that microdysgenic lesions may actually initiate epileptiform activity. We have investigated the electrophysiological properties of an animal model of polymicrogyria created by making cortical freeze lesions in rat pups at P0 or P1. Such lesions create microgyri with histological features similar to those of human polymicrogyria. We have determined that there is a focal region of hyperexcitability around the lesion in this rat microgyrus. Field potentials evoked by stimulation within a few millimeters of the microgyrus have characteristics typical of epileptiform activity. This aberrant activity is seen as early as 12 d after the lesion, as well as in animals as old as 118 d. Immunochemical staining for the calcium binding protein, parvalbumin, shows a decrease in neuronal and neuropil staining within the microgyrus. These findings suggest that inhibition might be decreased within the lesion, which may contribute to generation of the adjacent hyperexcitable region. These results demonstrate that this animal model is appropriate for examining the mechanisms contributing to epileptogenesis associated with a cortical malformation.

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)

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.

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)

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.

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)

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.

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.

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.

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.

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)

Regenerative activity in apical dendrites of pyramidal cells in neocortex

In intracellular recordings from three neocortical pyramidal cells in vitro, intracellular dye injection identified the impalement site as the primary trunk of the apical dendrite. Dendritic recordings displayed two types of regenerative events: relatively fast, low-threshold spikes with amplitudes of 12-69 mV, and slower, higher-threshold spikes up to 80 mV in amplitude. This distinctive dendritic firing pattern was also encountered in six recordings without dye-filled electrodes. Fast spike frequency was extremely sensitive to small changes in membrane potential at the recording site. In one recording, the fast spikes were blocked by 1 microM TTX, while slow events were spared. A computational model of a pyramidal cell was constructed to assist in interpreting the recordings. Simulations suggested that the fast spikes were generated primarily by active Na+ conductance concentrated at a distance from the impalement site, probably in the region of the soma. The low threshold of the fast spikes suggested that Na+ channels also exist in the apical dendrites, where they have a relatively low density. The data strongly imply that there are Ca2+ channels in the apical dendrites.

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.

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.

Laminar distribution of neuronal membrane properties in neocortex of normal and reeler mouse

1. Reeler is an autosomal recessive mutation of mice that alters neuronal migration during development, yielding a general inversion of the laminae in the neocortex. We recorded in vitro from slices of normal and reeler neocortex to study the influence of neuron position and shape on membrane properties and synaptic responses. 2. The intrinsic firing patterns, action-potential shapes, resting membrane potentials, input resistances, and evoked excitatory postsynaptic potentials (EPSPs) and inhibitory postsynaptic potentials (IPSPs) did not differ between reelers and controls when data were grouped. 3. The depth distribution of intrinsic firing patterns was inverted in the reeler: intrinsically bursting (IB) neurons were found only in layer 5 in the normal mouse, but they were found exclusively in supragranular layers of the reeler cortex. 4. The spatial distribution of synaptic responses in the reeler was also inverted: very prominent IPSPs were characteristic of upper layer neurons in the normal mouse, but in the reeler similar inhibitory responses were observed predominantly in deep infragranular layers. 5. Dye injections in reeler pyramidal neurons revealed atypical morphologies, including distorted apical dendrites and cell inversion. 6. The data imply that cortical neurons develop the membrane and synaptic properties appropriate to their function, despite being malformed and mispositioned.

Pentylenetetrazole-induced kindling is prevented by prior treatment with cysteamine

We have previously demonstrated that in pentylenetetrazole (PTZ)-kindled rats, cysteamine causes prolonged depression of the kindled state. We now report that administration of cysteamine before or during the kindling process prevents attainment of the kindled state. This effect lasts long after cysteamine administration has ceased, suggesting that depletion or somatostatin may not be the only mechanism underlying cysteamine's effect on kindling. The results also support the likelihood that PTZ kindling primarily effects neocortical rather than limbic structures.

Intrinsic firing patterns of diverse neocortical neurons

Neurons of the neocortex differ dramatically in the patterns of action potentials they generate in response to current steps. Regular-spiking cells adapt strongly during maintained stimuli, whereas fast-spiking cells can sustain very high firing frequencies with little or no adaptation. Intrinsically bursting cells generate clusters of spikes (bursts), either singly or repetitively. These physiological distinctions have morphological correlates. RS and IB cells can be either pyramidal neurons or spiny stellate cells, and thus constitute the excitatory cells of the cortex. FS cells are smooth or sparsely spiny non-pyramidal cells, and are likely to be GABAergic inhibitory interneurons. The different firing properties of neurons in neocortex contribute significantly to its network behavior.

Intracellular Calcium and Control of Burst Generation in Neurons of Guinea-Pig Neocortex in Vitro

Response properties of neurons in brain slices of guinea pig parietal neocortex were examined following intracellular injection of the Ca2+ chelators, EGTA and BAPTA. Although chelator injection did not cause any consistent change in passive membrane properties, it did induce 81% of neurons encountered at all sub-pial depths to become 'bursters', in that just-threshold depolarizing current pulses triggered all-or-none bursts of 2 - 5 fast action potentials. Transition to 'burstiness' was associated with disappearance of an AHP and appearance of a DAP. Although chelator caused a slight increase in steady-state firing rate, marked accommodation persisted. Extracellular Co2+ or Mn2+ had an effect on steady-state firing rate similar to that of the intracellular chelators; however, exposure to these Ca2+ channel blockers also caused steady state depolarization, increased resting input resistance and time constant, and profound spike broadening. This treatment never induced transition to 'burstiness'. Chelator-injected neurons ceased to generate bursts when Ca2+ was replaced by Mn2+ in the Ringer's solution. During exposure to 10-6 M TTX and 20 mM TEA, 50 - 200 msec Ca2+ spikes followed brief depolarizing pulses. As chelator was injected into the cell, there was progressive prolongation of the Ca2+ plateaus, which was associated with slowing of the rate at which membrane resistance gradually recovered following the initial increase in conductance. These findings indicate that under normal conditions, activity-related increases in intracellular Ca2+ activate processes which prevent most neocortical neurons from being bursters. These processes probably include Ca2+-dependent K+ currents, and Ca2+-dependent Ca2+ channel inactivation.

Synchronized neuronal activities in neocortical explant cultures

Intracellular recordings revealed that in neocortical explant cultures prepared on the day of birth and examined 3-6 weeks later, neurons mature and establish complex synaptic relationships that lead to spontaneous and triggered synchronous discharge. The spontaneous synchronous activity took several forms, including periodic generation of epileptiform depolarizing waves, prolonged periods of seizure-like discharge, and periodic, intense barrages of IPSPs. Synchronous depolarizations were associated with a marked increase in membrane conductance. Intracellular injection of currents of varying polarity and intensity affected their amplitudes and polarities without influencing the probability of their occurrence, indicating that the discharge reflected the synchronous activities of a neuronal population. This conclusion was confirmed with simultaneous recordings from pairs of neurons. Effects of the GABAa receptor antagonist, bicuculline, and the NMDA receptor antagonist, 2-aminophosphonvalerate (2APV), were used to assess the contributions of impairment of inhibition and enhancement of excitation to the initiation of synchronous discharge. The frequency with which spontaneous depolarizations were generated in normal medium was markedly reduced by 2APV. Moreover, seizure-like activity was induced by removing Mg++ from the medium, a condition that enhances conductance through NMDA receptor-coupled channels. This behavior was also attenuated by 2APV. Perfusion of bicuculline was potently epileptogenic. 2APV cut short the late, voltage-dependent phase of bicuculline-induced paroxysmal depolarizations, indicating a role of NMDA receptors in generating this component of the wave. Epileptiform activities induced by withdrawal of Mg++ were greatly augmented by bicuculline, indicating that blockade of inhibition was not a prerequisite for seizure-like activity. This conclusion is supported by the finding that in many neurons in untreated cultures, paroxysmal generation of trains of IPSPs was the primary manifestation of spontaneous, synchronous population discharge.

Electrophysiological characteristics of neurons in neocortical explant cultures

We examined the electrophysiological and morphological properties of neocortical neurons maintained in explant cultures prepared from the parietal cortex of newborn Sprague-Dawley rats. After 3-6 weeks in vitro, cultures showed regional differences in cellular density reminiscent of cortical layering, and an abundance of axonal processes. Pyramidal-shaped neurons with spinous dendrites were the dominant elements revealed by Lucifer yellow injections. Intracellular recordings revealed that many electrophysiological properties of neurons in the explants resembled those of neocortical neurons in vivo and in slice preparations. In response to depolarizing current injection, neurons in the explants showed the same three patterns of repetitive firing described in neocortical slices, as well as a similar array of responses. Spontaneous synaptic potentials were recorded from all neurons and complex PSPs were evoked in response to focal extracellular stimulation. GABAa receptors mediated a significant component of the evoked responses. Fifteen of sixty neurons generated action potentials that arose spontaneously from resting potentials. Neurons in many slices generated large, prolonged depolarizing potentials that reflected coordinated synaptic activity within the explants. These results underscore the usefulness of the neocortical explant as a valuable model for studying aspects of the behavior of circuits of cortical neurons.

Low threshold calcium spikes, intrinsic neuronal oscillation and rhythm generation in the CNS

Field potential studies in vivo have shown that many subcortical structures, such as the inferior olivary nucleus, the thalamic nuclei and the lateral habenular nucleus, generate behaviorally relevant rhythms. In each region, intracellular analysis in brain slices has revealed that activation of a transient, low-threshold calcium current plays a pivotal role in rhythmogenesis. The membrane potential of each individual neuron may oscillate rhythmically as a result of interplay between this current and other inward and outward voltage and calcium-dependent currents. Synchronization of this oscillatory single-cell activity, through mutual interaction and/or appropriately timed afferent drive, results in generation of stereotyped population rhythms.

Voltage-dependent and calcium-dependent inactivation of calcium channel current in identified snail neurones

1. The dependence of Ca2+ current inactivation on membrane potential and intracellular Ca2+ concentration ([Ca2+]i) was studied in TEA-loaded, identified Helix neurones which possess a single population of high-voltage-activated Ca2+ channels. During prolonged depolarization, the Ca2+ current declined from its peak with two clearly distinct phases. The time course of its decay was readily fitted by a double-exponential function. 2. In double-pulse experiments, the relationship between the magnitude of the Ca2+ current and the amount of Ca2+ inactivation was not linear, and considerable inactivation was present, even when conditioning pulses were to levels of depolarization so great that Ca2+ currents were near zero. Similar results were obtained when external Ca2+ was replaced by Ba2+. 3. In double-pulse experiments, hyperpolarization during the interpulse interval served to reprime a portion of the inactivated Ca2+ current for subsequent activation. The extent of repriming increased with hyperpolarization, reaching a maximum between -130 and -150 mV. The effectiveness of repriming hyperpolarizations was considerably increased when Ca2+ was replaced by Ba2+. 4. A significant fraction of inactivated Ca2+ channels can be recovered during hyperpolarizing pulses lasting only milliseconds. If hyperpolarizing pulses were applied before substantial inactivation of Ca2+ current, Ca2+ channels remained available for activation despite considerable Ca2+ entry. 5. The relationship between [Ca2+]i and inactivation was investigated by quantitatively injecting Ca2+-buffered solutions into the cells. The time course of Ca2+ current inactivation was unchanged at free [Ca2+] between 1 x 10(-7) and 1 x 10(-5) M. From 1 x 10(-7) to 1 x 10(-9) M, inactivation became progressively slower, mainly due to a decrease of the amplitude ratio (fast/slow) of the two components of inactivation, which fell from about unity to near zero at 1 x 10(-9) M. In double-pulse experiments, recovery from inactivation was enhanced in neurones that had been injected with Ca2+ chelator. 6. We conclude that inactivation of Ca2+ channels in these neurones depends on both [Ca2+]i and membrane potential. The voltage-dependent process may serve as a mechanism to quickly recover inactivated Ca2+ channels during repetitive firing despite considerable Ca2+ influx. 7. The results are discussed in the framework of a model which is based on two states of inactivation, INV and INCA, which represent different conformations of the inactivating substrate, and which are both reached from a lumped state of activation (A). Inactivation leads to high occupancy of INV during depolarization.(ABSTRACT TRUNCATED AT 400 WORDS)

Electrophysiological properties of neurons in the lateral habenula nucleus: an in vitro study

1. The electroresponsive characteristics of neurons in the lateral habenula were studied with intracellular recordings in a brain slice preparation of guinea pig diencephalon maintained in vitro. One hundred and two neurons met the criteria for recording stability, and of these, 18 were analyzed in detail. For these 18 neurons, the mean resting membrane potential was -61.9 mV, the mean input resistance was 124 M omega, and the mean spike amplitude of fast action potentials was 60.3 mV. 2. Lateral habenula neurons were found to have distinct patterns of activity dependent on membrane potential. At membrane potentials more positive than -65 mV, depolarization elicited trains of sodium-dependent fast action potentials. At membrane potentials more negative than -65 mV, slight depolarization elicited a tetrodotoxin-insensitive wave of depolarization, called a low-threshold spike (LTS), from which a burst of fast action potentials were triggered. The principal conductance underlying the LTS is a low-threshold calcium conductance, which is inactivated at membrane potential more positive than -65 mV and deinactivated when the membrane is hyperpolarized to potentials more negative than -65 V. 3. Upon termination of injected hyperpolarizing current, many neurons displayed oscillation in membrane potential at a frequency of 3-10 Hz, thereby generating repetitive bursts of fast spikes. 4. The pattern of neuronal activity in lateral habenula neurons was highly sensitive to slight alterations in membrane potential. The ability of these neurons to fire action potentials in two modes, tonically and in bursts, and the propensity of these neurons to dramatically alter their output in response to transient hyperpolarizing input, indicate that transmission through this relay in the dorsal diencephalic conduction system may be greatly augmented by relatively small hyperpolarizing influences on the individual neurons.

Low-threshold calcium electrogenesis in neocortical neurons

In slices of parietal neocortex, evidence was obtained for the existence of a low-threshold Ca2+ conductance in most neurons. This conductance became apparent when resting membrane potential was held below -60 mV by continuously injected, depolarizing current. Under these conditions, brief hyperpolarizing pulses were followed by generation of tetrodotoxin (TTX)-resistant, Mn2+-sensitive, low-threshold spikes. The results suggest that in neocortex, as in many subcortical structures, low-threshold Ca current may be responsible for burst generation in some neurons.

Expression of rat brain excitatory amino acid receptors in Xenopus oocytes

Xenopus laevis oocytes when injected with rat brain mRNA synthesize neuronal receptors that can be analyzed electrophysiologically. After a post-injection incubation period of 24-72 hours, L-glutamic acid, kainic acid and quisqualic acid caused a dose dependent (10-100 microM) depolarization of the oocyte membrane. The voltage and conductance changes associated with kainate activation were distinguishable from those seen for L-glutamate or quisqualate. There was no response to L-aspartate application and an inconsistent response to N-methyl-D-aspartate. Upon fractionation of the mRNA on sucrose gradients, transcripts greater than 2 Kb in length were obligatory for the synthesis of excitatory amino acid receptors. The electrophysiological response of injected oocytes exposed to L-glutamate was similar to that of native oocytes when exposed to muscarinic agents. This similarity may reflect the activation of the same ionophore and suggests that the active mRNA fraction for glutamate responsiveness either encodes for a binding protein that can be assembled along with native ion channels into the oocyte membrane or encodes for a glutamate binding site with a similar channel.

An N-methyl-D-aspartate (NMDA) receptor antagonist reduces bicuculline-induced depolarization shifts in neocortical explant cultures

We investigated the actions of a specific N-methyl-D-aspartate (NMDA) antagonist, 2-amino-5-phosphonovaleric acid (2-APV), on bicuculline-induced epileptogenesis in organotypic explant cultures from neonatal rat neocortex. Explants were maintained in roller tubes for 3-5 weeks. The late, plateau phase of the intracellularly recorded paroxysmal depolarization shift was sensitive to both intracellularly injected hyperpolarizing currents and 2-APV, suggesting that this component is generated by a voltage-dependent, regenerative process that is mediated by activation of NMDA receptors. The results support the hypothesis that NMDA receptors play an important role in the generation of epileptiform activity by localized circuits of neocortical neurons.

Incidence of neuronal dye-coupling in neocortical slices depends on the plane of section

The fluorescent dye Lucifer Yellow CH was intracellularly injected into neurons in slices of guinea-pig visual neocortex which had been prepared by sectioning either in a plane normal to the pial surface (radial slices) or in a plane parallel to the pial surface (tangential slices). In radial slices 44.3% of the injections resulted in dye-coupling and the number of cells coupled to the impaled neuron per injection followed a Poisson distribution. In contrast dye-coupling was not observed in tangential slices. Incidence of dye-coupling in slices that had been sectioned in both the radial and tangential planes was the same as in intact radial slices, indicating that slicing in the radial plane induced the formation of pathways for dye movement between neurons. The results suggest that formation and/or strengthening of direct intercellular junctions between neocortical neurons may occur as a specific neuronal response to partial dendrotomy.

Cysteamine suppresses kindled seizures in pentylenetetrazol-kindled rats

Rats were kindled by intraperitoneal injection of pentylenetetrazol (PTZ) (30 mg/kg) every 48 h. Once kindled, animals received a single injection of cysteamine (200 mg/kg) and subsequent responses to PTZ were observed. Cysteamine, an agent which depletes brain somatostatin and suppresses kindled seizures in amygdaloid-kindled rats, markedly suppressed the severity of PTZ-induced seizures in PTZ-kindled rats as well. However, it did not alter the convulsive response of non-kindled rats to a submaximal convulsive dose (50 mg/kg) of PTZ. The results support a role for somatostatin in kindling.

Carbon dioxide uncouples dye-coupled neuronal aggregates in neocortical slices

Lucifer Yellow was injected intracellularly into neurons in slices of guinea pig visual cortex. Dye coupling incidence was significantly decreased in slices that were incubated in a high concentration of carbon dioxide. This effect was probably due to intracellular acidification, since exposure to impermeant acid was not effective. The data are consistent with the hypothesis that carbon dioxide interferes with dye coupling in neocortex through its known action as an uncoupler of electronic coupling through gap junctions.

Mechanisms of neocortical epileptogenesis in vitro

1. The cellular mechanisms underlying interictal epileptogenesis have been examined in an in vitro slice preparation of guinea pig neocortex. Penicillin or bicuculline was applied to the tissue, and intracellular recordings were obtained from neurons and glia. 2. Following convulsant application, stimulation could elicit a short-latency excitatory postsynaptic potential (EPSP) and a large, longer latency depolarization shift (DS) in single neurons. DSs in neurons of the slice were very similar to those evoked in neurons of neocortex in vivo in that they displayed an all-or-none character, large shifts in latency during repetitive stimuli, long afterpotentials, and a prolonged refractory period. In contrast to epileptogenesis produced by penicillin in intact cortex, neither spontaneous DSs nor ictal episodes were observed in neocortical slices. 3. In simultaneous recordings from pairs of neurons within the same cortical column, DS generation and latency shifts were invariably synchronous. DS generation in neurons was also coincident with large, paroxysmal increases of extracellular [K+], as indicated by simultaneous recordings from glia. 4. When polarizing currents were applied to neurons injected with the local anesthetic QX-314, the DS amplitude varied monotonically and had an extrapolated reversal potential near 0 mV. In neurons injected with the K+-current blocker Cs+, large displacements of membrane potential were possible, and both the short-latency EPSP and the peak of the DS diminished completely at about 0 mV. At potentials positive to this, the short-latency EPSP was reversed, and the DS was replaced by a paroxysmal hyperpolarization whose rise time and peak latency were prolonged compared to the DS evoked at resting potential. The paroxysmal hyperpolarization probably represents the prolonged activation of the impaled neuron by EPSPs. 5. Voltage-dependent components, including slow spikes, appeared to contribute to generation of the DS at resting potential in Cs+-filled cells, and these components were blocked during large depolarizations. 6. The results suggest that DS generation in single neocortical neurons occurs during synchronous synaptic activation of a large group of cells. DS onset in a given neuron is determined by the timing of a variable-latency excitatory input that differs from the short-latency EPSP. The DS slow envelope appears to be generated by long-duration excitatory synaptic currents and may be modulated by intrinsic voltage-dependent membrane conductances. 7. We present a hypothesis for the initiation of the DS, based on the anatomical and physiological organization of the intrinsic neocortical circuits.

Electrophysiological properties of neocortical neurons in vitro

1. Intracellular recordings were obtained from neurons of the guinea pig sensorimotor cortical slice maintained in vitro. Under control recording conditions input resistances, time constants, and spiking characteristics of slice neurons were well within the ranges reported by other investigators for neocortical neurons in situ. However, resting potentials (mean of -75 mV) and spike amplitudes (mean of 93.5 mV) were 10-25 mV greater than has been observed in intact preparations. 2. Current-voltage relationships obtained under current clamp revealed a spectrum of membrane-rectifying properties at potentials that were subthreshold for spike generation. Ionic and pharmacologic analyses suggest that subthreshold membrane behavior is dominated by voltage-sensitive, very slowly inactivating conductances to K+ and Na+. 3. Action potentials were predominantly Na+ dependent under normal conditions but when outward K+ currents were reduced pharmacologically, it was possible, in most cells, to evoke a non-Na+-dependent, tetrodotoxin-(TTX) insensitive spike, which was followed by a prominent depolarizing after-potential. Both of these events were blocked by the Ca2+ current antagonists, Co2+ and Mn2+. 4. A small population of neurons generated intrinsic, all-or-none burst potentials when depolarized with current pulses or by synaptic activation. These cells were located at a narrow range of depths comprising layer IV and the more superficial parts of layer V. 5. Spontaneous excitatory synaptic potentials appeared in all neurons. Spontaneous inhibitory events were visible in only about 10% of the cells, and in those cases apparently reversed polarity at a level slightly positive to resting potential. Stimulation of the surface of the slice at low intensities evoked robust and usually concurrent excitatory and inhibitory synaptic potentials. Unitary inhibitory postsynaptic potentials (IPSPs) reversed at levels positive to rest. Stronger stimulation produced a labile, long-duration, hyperpolarizing IPSP with a reversal potential 15-20 mV negative to the resting level. 6. Neocortical neurons in vitro retain the basic membrane and synaptic properties ascribed to them in situ. However, the array of passive and active membrane behavior observed in the slice suggests that cortical neurons may be differentiated by specific functional properties as well as by their extensive morphological diversity.

Activity-dependent K+ accumulation in the developing rat optic nerve

Potassium-sensitive microelectrodes were used to study activity-dependent changes of extracellular potassium ion concentration ([K+]o) in rat optic nerves of different postnatal ages (1 day to adulthood). The maximum level to which [K+]o rose with optimal frequencies of stimulation depended on age: mean maximum evoked [K+]o was 17.2 microM in 1- to 3-day-old optic nerves and 9.8 microM in adult nerves. The ceiling [K+]o seen in immature optic nerves, which is uniquely large for a mammalian central nervous system structure, may result from a relatively enhanced rate of evoked K+ release.

Model for simulating cyclic variations in focal epileptogenic excitability

During 0.2 to 1.0 Hz stimulation of an epileptogenic focus, the phenomenon of cyclic spike driving (CSD) occurs, wherein 20- to 40-sec active periods, in which each stimulus triggers an all-or-none interictal event (IE), alternate with quiet periods of similar duration, in which IE are not triggered. CSD was simulated by a model in which individual IE are followed by biphasic response functions which sum linearly to produce long-term oscillations of epileptogenic excitability relative to the IE triggering threshold. The model successfully simulates all of the major characteristics of experimentally induced CSD, including the effects of manipulating stimulus parameters. Results of the simulation provide clues as to the time course of mechanisms which control epileptogenic triggerability.

Dye-coupling between glial cells in the guinea pig neocortical slice

Physiologically identified glial cells in guinea pig neocortical slices were injected with the low molecular weight, fluorescent dye Lucifer yellow CH. The stained aggregates which resulted consisted of one brightly stained, central cell surrounded by numerous lightly stained cells. The central cell had well defined feathery processes and resembled a protoplasmic astrocyte. The surrounding cells appeared also to be glial cells but lacked sufficient detail to be further categorized. This first demonstration of dye-coupling between neocortical glial cells strongly suggests that these cells are connected together via low resistance junctions capable of passing ionic current as well as dye.

Extracellular potassium activity during frequency-dependent conduction block of giant axons in the metathoracic ganglion of the cockroach

In the metathoracic ganglion (T3) of the cockroach, extracellular potassium activity (aK) was measured with ion-sensitive microelectrodes and intracellular recordings were simultaneously made from giant axons (GAs) during high frequency stimulation of the connectives. Blockade of spike conduction through T3 was associated with intraganglionic aK rises of 0.2-0.5 mM, which were only 10% of the periaxonal aK rises suggested from GA depolarizations. When aK in the bath was increased 10-fold, GA conduction block during 1 Hz stimulation did not occur until much higher levels of aK and GA depolarization were achieved. The results suggest that glial sheaths surrounding GAs significantly impede K+ movement, and may thus prevent non-specific axonal interactions, and that stimulus-induced conduction block is not primarily due to K+-induced depolarization and consequent Na+-inactivation.

Dye coupling and possible electrotonic coupling in the guinea pig neocortical slice

Iontophoretic injection of the fluorescent dye Lucifer Yellow CH into single neurons of guinea pig neocortical slices resulted in the staining of more than one cell. Dye-coupled neuronal aggregates were found only in the superficial cortical layers and were often organized in vertical columns. Antidromic stimuli evoked all-or-none, subthreshold depolarizations in some superficial cells. These potentials were not eliminated by manganese and did not collide with spikes originating in the soma, suggesting that they arose from electrotonic interaction between superficial cortical neurons.

Stimulus induced and seizure related changes in extracellular potassium concentration in cat thalamus (VPL)

Extracellular potassium activity (ak) and field potentials (fp) were measured in the nucleus ventro-postero-lateralis (VPL) thalami in order to assess the extent of thalamic participation in cortical seizure activity. Small increases (up to 0.7 mmole/l) or decreases (up to 0.2 mmole/l) in ak were induced by electrical stimulation of the contralateral forepaw. These changes in ak were spatially more limited than the simultaneously recorded fp. Similar observations were made during weak electrical stimulation of the somatosensory cortex and during interictal spikes in a cortical penicillin focus. Large and widespread increases in ak to levels of 11.6 mmoles/l and slow negative fps of 8 mV accompanied seizure generation either in a cortical penicillin focus or during intense repetitive electrical stimulation of the cortical surface. Subsequent to such increases ak fell to subnormal levels. The amplitudes and durations of such undershoots were correlated with the amplitudes of the preceding increases in ak. Sometimes thalamic seizures ceases before cortical epileptic episodes. This resulted in a decrease of cortical EEG amplitudes. After ablation of the sensorimotor cortex seizures in forepaw-VPL could be induced by stimulation of the somatosensory cortex. These results further support the conclusion that specific thalamic nuclei participate in seizure generation and may serve as a subcortical route of seizure spread.

Relation between extracellular potassium concentration and neuronal activities in cat thalamus (VPL) during projection of cortical epileptiform discharge

Neuronal and potassium activities (ak) were measured in the nucleus ventro-posterolateralis thalami (VPL) during propagated epileptiform activity from the somatosensory cortex of cats. Seizures were induced by repetitive electrical stimulation of the cortical surface or by topical application of penicillin. The recruitment of VPL into a seizure resulted in large increases of ak to levels of up to 11.6 mmoles/l, accompanied by increased in neuronal discharge rate to 300/sec. Sometimes the rise in ak preceded active participation of a given thalamo-cortical relay (TCR) neuron in the seizure. After reaching a peak level, ak and neuronal discharge rate slowly declined during an ictal episode. After cessation of seizures all TCR neurons were inhibited, while ak fell to subnormal levels. The duration of these postictal depressions increased with the amplitude of preceding increases and subsequent undershoots in ak and could last up to 120 sec. During decay and undershoot in ak, relay capability of TCR neurons was reduced. Also the probability that action potentials elicited in intracortical endings of TCR cells would antidromically invade their cell bodies was decreased. The duration of these periods varied with the amplitude of undershoot in ak. Seizure threshold was increased during undershoots. These observations are consistent with a long-lasting postictal hyperpolarization of neuronal membranes. The hyperpolarization may be caused by the action of an electrogenic pump, which is probably involved in termination of seizure discharge.

Extracellular free calcium and potassium during paroxsmal activity in the cerebral cortex of the cat

Extracellular calcium and potassium activities (aCa and aK) as well as neuronal activity were simultaneously recorded with ion-sensitive electrodes in the somatosensory cortex of cats. Baseline aCa was 1.2-1.5 mM/l, baseline aK 2.7-3.2 mM/l. Transient decreases in aCa and simultaneous increases in aK were evoked by repetitive stimulation of the contralateral forepaw, the nucleus ventroposterolateralis thalami and the cortical surface. Considerable decreases in aCa (by up to 0.7 mM/l) were found during seizure activity. A fall in aCa preceded the onset of paroxysmal discharges and the rise in aK after injection of pentylene tetrazol. The decrease in aCa led also the rise in aK during cyclical spike driving in a penicillin focus. It is concluded that alterations of Ca++ dependent mechanisms participate in the generation of epileptic activity.