Biomedical subjects

J J Hablitz

Publications and source records attributed to J J Hablitz.

At least 73 records · Page 4Linked to original sources

PubMed1989

Alterations in the microenvironment during spreading depression associated with epileptiform activity in the immature neocortex.

Local changes in extracellular ion concentrations were measured with ion-sensitive microelectrodes in slices of mature (greater than 40 days of age) or immature (16-30 days of age) rat neocortex maintained in vitro. Repetitive stimulation resulted in increases in extracellular potassium ([K+]o) to levels of 8.85 +/- 2.1 mM in slices from adult animals and 12.77 +/- 1.8 mM in slices from immature animals. During exposure to picrotoxin, maximum levels were 11.3 +/- 2.6 and 14.8 +/- 2.5 mM in the mature and immature groups, respectively. Picrotoxin (50 microM) induced spontaneous bursts of repetitive spiking, followed by a slow, negative field potential, associated with spreading depression (SD), in slices from immature animals. [K+]o levels increased to 10.2 +/- 3.9 mM during repetitive spike discharges and reached 30.3 +/- 18.5 mM during SDs. Variations in the size of the extracellular space (ES) were examined during SD. The ES was found to reversibly decrease by 39 +/- 4.5%. Clusters of repetitive spikes were associated with 0.1-0.2 mM decreases in [Ca2+]o, whereas 1.12 +/- 0.06 mM decreases were observed during SDs. Decreases in [Na+]o and [Cl-]o of 56 +/- 10 mM and 41 +/- 9 mM, respectively, were observed during SDs suggesting that a net transmembrane movement of water occurred during SDs. These results indicate that changes in [K+]o associated with epileptiform activity in the immature nervous system are quantitatively different from those observed in the mature brain. These large increases in [K+]o may contribute to the prolonged nature of epileptiform discharges in the developing nervous system.

Aging↗

PubMed1989

Cholinergic modulation of epileptiform activity in the developing rat neocortex.

The effects of carbachol on picrotoxin-induced epileptiform activity and membrane properties of neurons in the developing rat neocortex were examined in an in vitro slice preparation. Intracellular recordings were obtained in layer II-III neurons of slices prepared from rats 9-21 days of age. Epileptiform activity in 9- to 14-day-olds consisted of a sharply rising, sustained (10-30 s) membrane depolarization with superimposed action potentials. Bath application of carbachol (5-50 microM) raised the threshold for evoking epileptiform activity but, when such responses were evoked, their underlying depolarizations were increased in amplitude. Orthodromic stimulation in slices from 15- to 21-day-old animals evoked a prolonged epileptiform burst response that triggered an episode of spreading depression (SD). Carbachol reduced epileptiform responses and suppressed the occurrence of SD. It did not significantly affect the resting membrane potential or the height of the action potential but decreased the rheobase current needed to evoke an action potential and increased the input resistance. All effects of carbachol were antagonized by atropine (1 microM). These results indicate that carbachol has both pre- and postsynaptic effects in the developing neocortex and can significantly modulate neuronal excitability in the immature nervous system.

Action Potentials↗

PubMed1989

Long-term potentiation in frontal cortex: role of NMDA-modulated polysynaptic excitatory pathways.

The present study examined the role of N-methyl-D-aspartic acid (NMDA) receptors in synaptic plasticity in regular-spiking cells of rat frontal cortex. Intracortical stimulation, at levels subthreshold for elicitation of action potentials, evoked a late excitatory postsynaptic potential (EPSP) in layer II-III neurons that was sensitive to the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (APV). This late EPSP showed marked short-term frequency-dependent depression, suggesting that it is polysynaptic in origin. Polysynaptic late EPSPs were selectively enhanced following high-frequency stimulation. This sustained increase in synaptic efficacy, or long-term potentiation, was expressed in regular spiking cells and appeared to result from activation of NMDA receptors on excitatory interneurons. These data demonstrate the existence of an NMDA-modulated polysynaptic circuit in the neocortex which displays several types of use-dependent plasticity.

2-Amino-5-phosphonovalerate↗

PubMed1989

cAMP increases the rate of GABAA receptor desensitization in chick cortical neurons.

During prolonged application of GABA to cultured neurons from the chick embryo cerebrum, whole-cell voltage-clamp recordings show a decline in GABA-gated chloride currents due to desensitization. At a holding potential of -60 mV, desensitization can normally be described as a single exponential process with a time constant (tau) of 7-10 sec at a GABA concentration of 100 microM. After exposure to 50 microM forskolin, the peak amplitude of the GABA-induced currents declined and a fast component of desensitization (tau = 0.92 sec) appeared, whereas the slow component was essentially unchanged. This effect of forskolin was reversible after washing. Similar, although less robust effects were produced by incubation with 8-Br-cAMP, but not by 1,9-dideoxyforskolin. The desensitization process could also be measured by the GABA-dependent uptake of 36Cl- into the cultured neurons. A 20-sec incubation with 10 microM GABA in physiological saline produced a 29% inhibition of subsequent GABA-gated 36Cl- uptake in the presence of 40 mM K+. This inhibition was increased to 64% by the addition of 100 microM forskolin to the preincubation medium. This effect of forskolin was prevented by 100 microM 2',5'-dideoxyadenosine, an inhibitor of adenylate cyclase. A similar acceleration of desensitization was produced by 0.5 mM 8-Br-cAMP or by 0.5 mM isobutylmethylxanthine, but these compounds themselves failed to produce desensitization in the absence of exogenous GABA. In the presence of GABA, cAMP analogs were effective in the order 8-(4-chlorophenylthio)-cAMP greater than 8-Br-cAMP greater than N6,O2'-dibutyryl-cAMP.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗

PubMed1989

EPSPs in rat neocortical neurons in vitro. I. Electrophysiological evidence for two distinct EPSPs.

1. To investigate excitatory postsynaptic potentials (EPSPs), intracellular recordings were performed in layer II/III neurons of the rat medial frontal cortex. The average resting membrane potential of the neurons was more than -75 mV and their average input resistance was greater than 20 M omega. The amplitudes of the action potentials evoked by injection of depolarizing current pulses were greater than 100 mV. The electrophysiological properties of the neurons recorded were similar to those of regular-spiking pyramidal cells. 2. Current-voltage relationships, determined by injecting inward and outward current pulses, displayed considerable inward rectification in both the depolarizing and hyperpolarizing directions. The steady-state input resistance increased with depolarization and decreased with hyperpolarization, concomitant with increases and decreases, respectively, in the membrane time constant. 3. Postsynaptic potentials were evoked by electrical stimulation via a bipolar electrode positioned in layer IV of the neocortex. Stimulus-response relationships, determined by gradually increasing the stimulus intensity, were consistent among the population of neurons examined. A short-latency EPSP [early EPSP (eEPSP)] was the response with the lowest threshold. Amplitudes of the eEPSP ranged from 4 to 8 mV. Following a hyperpolarization of the membrane potential, the amplitude of the eEPSP decreased. Upon depolarization, a slight increase in amplitude and duration was observed, accompanied by a significant increase in time to peak. 4. The membrane current underlying the eEPSP (eEPSC) was measured using the single-electrode voltage-clamp method. The amplitude of the eEPSC was apparently independent of the membrane potential in 8 of 12 neurons tested. In the other 4 neurons, the amplitude of the eEPSC increased with hyperpolarization and decreased with depolarization. 5. Higher stimulus intensities evoked, in addition to the eEPSP, a delayed EPSP [late EPSP (lEPSP)] in greater than 90% of the neurons tested. The amplitude of the lEPSP ranged from 12 to 20 mV, and the latency varied between 20 and 60 ms. The amplitude of the lEPSP varied with membrane potential, decreasing with depolarization and increasing following hyperpolarization. The membrane current underlying the lEPSP (lEPSC) displayed a similar voltage dependence. 6. At stimulus intensities that led to the activation of inhibitory postsynaptic potentials (IPSPs), the lEPSP was no longer observed.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1989

EPSPs in rat neocortical neurons in vitro. II. Involvement of N-methyl-D-aspartate receptors in the generation of EPSPs.

1. Intracellular recordings were obtained from neurons in layer II/III of rat frontal cortex. Single-electrode current- and voltage-clamp techniques were employed to compare the sensitivity of excitatory postsynaptic potentials (EPSPs) and iontophoretically evoked responses to N-methyl-D-aspartate (NMDA) to the selective NMDA antagonist D-2-amino-5-phosphonovaleric acid (D-2-APV). The voltage dependence of the amplitudes of the EPSPs before and after pharmacologic changes in the neuron's current-voltage relationship was also examined. 2. NMDA depolarized the membrane potential, increased the neuron's apparent input resistance (RN), and evoked bursts of action potentials. The NMDA-induced membrane current (INMDA) gradually increased with depolarization from -80 to -40 mV. The relationship between INMDA and membrane potential displayed a region of negative slope conductance in the potential range between -70 and -40 mV which was sufficient to explain the apparent increase in RN and the burst discharges during the NMDA-induced depolarization. 3. Short-latency EPSPs (eEPSPs) were evoked by low-intensity electrical stimulation of cortical layer IV. Changes in the eEPSP waveform following membrane depolarization and hyperpolarization resembled those of NMDA-mediated responses. However, the eEPSP was insensitive to D-2-APV applied at concentrations (up to 20 microM) that blocked NMDA responses. 4. EPSPs with latencies between 10 and 40 ms [late EPSPs (lEPSPs)] were evoked by electrical stimulation using intensities just subthreshold to the activation of IPSPs. The amplitude of the lEPSP increased with hyperpolarization and decreased with depolarization. 5. The lidocaine derivative QX-314, injected intracellularly, suppressed sodium-dependent action potentials and depolarizing inward rectification. Simultaneously, the amplitude of the eEPSP significantly decreased with depolarization. Neither the amplitude of a long-latency EPSP nor the amplitude of inhibitory postsynaptic potentials (IPSPs) was significantly affected by QX-314. 6. Cesium ions (0.5-2.0 mM) added to the bathing solution reduced or blocked hyperpolarizing inward rectification. Under these conditions, the amplitude of the eEPSP increased with hyperpolarization. The amplitude of the lEPSP was unaltered or enhanced. 7. The lEPSP was reversibly blocked by D-2-APV (5-20 microM), although the voltage-dependence of its amplitude did not resemble the action of NMDA on neocortical neurons.(ABSTRACT TRUNCATED AT 400 WORDS)

2-Amino-5-phosphonovalerate↗

PubMed1988

Whole-cell and single-channel recordings of GABA-gated currents in cultured chick cerebral neurons.

1. gamma-Aminobutyric acid (GABA) (10-500 microM) was applied to cultured chick cerebral neurons by pressure ejection, and the resulting currents (IGABA) were recorded using standard whole-cell voltage-clamp techniques. Plots of the peak IGABA as a function of membrane potential were nonlinear with an outwardly rectifying appearance. 2. IGABA decayed during a prolonged application of GABA. This decay was associated with a decline in the conductance of the cell, suggesting that the decline in IGABA was principally due to receptor desensitization. 3. After 5-7 days in culture, whole-cell recordings revealed the presence of spontaneous synaptic currents. These currents were presumed to be GABA-gated inhibitory postsynaptic currents (IPSCs) because they reversed at the Cl- equilibrium potential (ECl-), were blocked by picrotoxin (25 microM), and were prolonged by pentobarbital (50 microM). 4. Synaptic currents were analyzed by fitting exponential functions to their decay. In normal recording saline, 68% of the decays analyzed could be adequately described by a single exponential function. Two exponentials were necessary to describe the decay of the other 32%. The time constant of the decay (for those adequately fitted by a single exponential) increased with depolarization, from an average value of 15 ms at -80 mV to 60 ms at +40 mV. 5. A relationship was noted between IPSC amplitude and decay time constant; IPSCs with larger peak amplitudes had a slower decay. One possible explanation considered for this finding was that transmitter persists in the synaptic cleft and rebinds to the receptors, thus prolonging the decay of the IPSC. 6. Consistent with the above hypothesis was the observation that the decays of miniature IPCSs (examined under conditions of reduced transmitter release) were faster, showed less variability, and were all adequately described by a single exponential function. Furthermore, the decay times were independent of the membrane potential, suggesting that the kinetic parameters of the GABA channel which shape the decay of these miniature IPSCs are independent of voltage. 7. Single-channel activity underlying whole-cell GABA responses could be recorded in isolated outside-out and inside-out patches of membrane. In isotonic choline chloride, single-channel amplitudes were linearly related to voltage and reversed at -1.8 +/- 11.0 mV (n = 12). Under these conditions, the channel had a main conductance state of 20.8 +/- 3.4 pS (n = 12). Transitions were observed from this main conductance state to other conductance states, e.g., two subconductance states of 6 and 12 pS and one supraconductance state of 30 pS.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Extracellular K+ and Ca2+ changes during epileptiform discharges in the immature rat neocortex.

Picrotoxin-induced epileptiform activity was examined in neocortical slices prepared from 8- to 15-day-old rats. This activity consisted of spontaneous bursts of 3-5 discharges that resembled interictal spikes and were interspersed with ictal-like paroxysms lasting 10-30 s. Measurements of extracellular potassium ([K+]o) and calcium ([Ca2+]o) were made during these spontaneous epileptiform events, using ion-sensitive electrodes. Individual interictal spikes were associated with [Ca2+]o decreases of 0.1-0.2 mM, whereas sustained ictal-like discharges were accompanied by decreases of 0.3-0.4 mM. Measurement of [K+]o showed that individual interictal spikes were associated with increases in [K+]o up to 12 mM, whereas increases to more than 20 mM accompanied long-lasting ictal-like discharges. Maximum increases in [K+]o were observed ca. 600 microns below the pial surface. [K+]o increases were followed by undershoots of the resting [K+]o level. The unusually high [K+]o levels associated with epileptiform discharges in the immature neocortex suggest that disturbances in [K+]o regulation may contribute to the generation of the picrotoxin-induced, spontaneous, prolonged ictal-like discharges observed in the 8- to 15-day age group.

Animals↗

PubMed1987

Glutamate-induced ionic currents in cultured neurons from the rat superior colliculus.

The ionic currents induced in cultured rat superior colliculus neurons by rapid application of glutamate (Glut) and the glutamate receptor agonists quisqualate (Quis) and N-methyl-D-aspartate (NMDA) were examined using the whole-cell patch clamp technique. Dissociated cell cultures consisting exclusively of superficial gray layer neurons from rats aged E21-P2 were used. After 7-10 days in vitro, all neurons responded to Glut and the selective agonists, NMDA and Quis. Glut was a mixed agonist, and a variable fraction (10-100%) of Glut-activated currents was due to involvement of NMDA receptors. The NMDA response was strongly regulated by extracellular Ca and Mg levels and modified by exposure to Quis. Quis transiently removed the block of NMDA-activated currents by D-amino-phosphonovaleric acid (APV).

Animals↗

PubMed1987

Conductance changes underlying a late synaptic hyperpolarization in hippocampal CA3 neurons.

1. Single-electrode current- and voltage-clamp techniques were employed to study properties of the conductance underlying an orthodromically evoked late synaptic hyperpolarization or late inhibitory postsynaptic potential (IPSP) in CA3 pyramidal neurons in the rat hippocampal slice preparation. 2. Late IPSPs could occur without preceding excitatory postsynaptic potentials at the resting membrane potential and were graded according to the strength of the orthodromic stimulus. The membrane hyperpolarization associated with the late IPSP peaked within 140-200 ms after orthodromic stimulation of mossy fiber afferents. The late IPSP returned to base line with a half-decay time of approximately 200 ms. 3. As determined from constant-amplitude hyperpolarizing-current pulses, the membrane conductance increase during the late IPSP, and the time course of its decay, were similar whether measurements were made near the resting membrane potential or when the cell was hyperpolarized by approximately 35 mV. 4. When 1 mM cesium was added to the extracellular medium to reduce inward rectification, late IPSPs could be examined over a range of membrane potentials from -60 to -140 mV. For any given neuron, the late IPSP amplitude-membrane potential relationship was linear over the same range of membrane potentials for which the slope input resistance was constant. The late IPSP reversed symmetrically near -95 mV. 5. Intracellular injection of ethyleneglycol-bis-(beta-aminoethylether)-N,N'-tetraacetic acid or extracellular application of forskolin, procedures known to reduce or block certain calcium-dependent potassium conductances in CA3 neurons, had no significant effect on the late IPSP. 6. Single-electrode voltage-clamp techniques were used to analyze the time course and voltage sensitivity of the current underlying the late IPSP. This current [the late inhibitory postsynaptic current (IPSC)] began as early as 25 ms after orthodromic stimulation and reached a peak 120-150 ms following stimulation. 7. The late IPSC decayed with a single exponential time course (tau = 185 ms). 8. A clear reversal of the late IPSC at approximately -99 mV was observed in a physiological concentration of extracellular potassium (3.5 mM).(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

PubMed1987

Spontaneous ictal-like discharges and sustained potential shifts in the developing rat neocortex.

1. Intra- and extracellular recording techniques were used to study epileptogenesis in in vitro slices of immature rat neocortex. Slices of sensorimotor cortex were prepared from animals 5-60 days old. Epileptiform activity was induced by bath application of 50 microM picrotoxin. 2. Convulsant-induced paroxysmal activity was observed only rarely in the youngest age group (5-7 days) and consisted of orthodromically evoked bursts of low-amplitude isolated discharges. This activity was labile and could be evoked only at long interstimulus intervals (greater than 10 s). 3. Extracellular recordings in slices from 8- to 15-day-old rats showed spontaneous epileptiform activity consisting of 10- to 30-s paroxysms of repetitive spike discharges superimposed on a 3- to 5-mV negative steady potential. This steady potential declined slightly during the course of the prolonged discharge and returned quickly to base line following the last spike discharge. 4. Laminar analysis of epileptiform activity in 8- to 15-day-old rats showed that the spike discharges were negative and superimposed on a positive slow wave in superficial cortical layers. At 100 micron below the pial surface, the slow potential reversed polarity and remained negative throughout the remainder of the cortex. Spike discharges reversed polarity 800 micron below the pial surface. 5. In intracellular recordings from slices obtained from 9- to 14-day-old animals, each paroxysm began with a sharply rising membrane depolarization (paroxysmal depolarizing shift, or PDS). A second PDS occurred before the cells repolarized to their resting potential. A series of PDSs then followed, superimposed on a sustained membrane depolarization. This was associated with a 33% decrease in input resistance. Afterhyperpolarizations (AHPs) following termination of the depolarization were low in amplitude or absent. 6. In the 16- to 30-day-old age group, extracellular recordings showed paroxysmal activity consisting of 3-10 initial spikes followed by a sustained, slow, negative-potential shift. This slow potential could be as great as 30 mV in amplitude and could last as long as 180 s. Paroxysmal events recurred spontaneously at intervals of 4-11 min. Spontaneous PDSs and slow, negative-potential shifts were not observed after 30 days of age, although PDSs could still be evoked by orthodromic stimulation. 7. Intracellular recordings in the 16- to 30-day-old group revealed that each paroxysmal event consisted of an initial period of increased synaptic activity and cellular firing, followed by a marked, long-lasting depolarization (LLD), culminating in an AHP.(ABSTRACT TRUNCATED AT 400 WORDS)

Aging↗

PubMed1986

Prenatal exposure to alcohol alters short-term plasticity in hippocampus.

Paired-pulse potentiation was studied in the CA1 region of rats exposed prenatally to alcohol. Pregnant rats were fed a liquid diet containing 35% ethanol-derived calories on days 3 to 21 of gestation. Control animals were fed liquid diets without ethanol. When the pups of both groups were 40 to 60 days old, hippocampal slices were prepared and maintained in vitro. Paired-pulse potentiation was examined at intervals from 5 to 400 ms. Recordings made from these slices showed that the response inhibition typically seen at short interpulse intervals was minimal or absent in the animals with prenatal exposure to alcohol. Potentiation of population spike amplitude was enhanced at intervals to 100 ms.

Animals↗

PubMed1986

Step reductions in extracellular Ca2+ activate a transient inward current in chick dorsal root ganglion cells.

We investigated whether transient step reductions in divalent cations would produce detectable changes in neuronal excitability similar to those reported in the total absence of divalent cations. Using cultured chick dorsal root ganglion cells as a model system, our results indicate that a step reduction in divalent cations induces a transient inward current. This response is mediated by a tetrodotoxin-resistant, Na+-permeable, cation channel that is blocked by cadmium. This, and our observation that the response is abolished by verapamil, suggests that the current passes through calcium channels. This transient inward current was estimated to be activated by decreases in extracellular calcium ([Ca2+]o) as small as 0.5-0.8 mM and thus represents a different response from the one previously observed when steady-state [Ca2+]o levels were reduced to micromolar levels.

Animals↗

PubMed1986

N-methyl-D-aspartate receptor antagonists reduce synaptic excitation in the hippocampus.

The hypothesis that synaptic excitation in the CA1 region of the hippocampus is mediated in part by N-methyl-D-aspartate (NMDA) receptors was tested using intra- and extracellular recording techniques. Synaptic potentials elicited by stratum radiatum stimulation were examined in individual neurons before and after bath application of the NMDA receptor antagonist, DL-2-amino-5-phosphonovalerate (APV). This antagonist reduced both excitatory and inhibitory postsynaptic potentials (EPSPs and IPSPs). When IPSPs were suppressed by the addition of picrotoxin, EPSPs were seen in isolation. APV reduced these EPSPs but did not block synaptic transmission. This antagonist demonstrated anticonvulsant actions when tested against picrotoxin-induced epileptiform activity. These results suggest that, as in the spinal cord and neocortex, synaptic excitation in the CA1 region of the hippocampus is partially mediated by APV-sensitive NMDA receptors. The fact that synaptic activity is not blocked by NMDA antagonists indicates that EPSPs in CA1 neurons are not mediated solely by this receptor.

2-Amino-5-phosphonovalerate↗

PubMed1985

Role of uptake in gamma-aminobutyric acid (GABA)-mediated responses in guinea pig hippocampal neurons.

Intracellular recordings were obtained from hippocampal pyramidal neurons maintained in vitro. Measurements were made of the conductance change induced by iontophoretically applied gamma-aminobutyric acid (GABA) and, using voltage-clamp techniques, of inhibitory postsynaptic currents resulting from activation of inhibitory pathways. Analysis of GABA iontophoretic charge-response curves indicated that there was considerable variation among neurons with respect to the slope of this relation. The placement of the GABA-containing pipette did not appear to be responsible for the observed variation, since vertical repositioning of the pipette did not alter the slope of the charge-response relationship. Steady iontophoresis of GABA from one barrel of a double-barreled pipette markedly affected the charge-response relation obtained when short pulses were applied to the other barrel. The curve was shifted to the left, and the slope was decreased. Concomitantly, the enhanced GABA-induced responses were prolonged. Similar alterations in GABA responsiveness were observed when the uptake blocker, nipecotic acid, was iontophoretically applied. Furthermore, bath application of saline containing a reduced sodium concentration (25% of control) also produced a prolongation of GABA-mediated responses. Under voltage clamp, inhibitory postsynaptic currents were observed to have biphasic decays. The initial, fast decay was prolonged by an average of 18% by nipecotic acid, whereas the later, slow phase was prolonged by 23%. The results of these studies support the hypothesis that a saturable GABA uptake system is responsible for the observed variation in the charge-response curves and, in turn, underlies the apparent sensitizing effect of excess GABA application. The results also suggest that a reduction of transmitter uptake affects the time course of inhibitory postsynaptic currents in the hippocampus.

Animals↗

PubMed1985

Action of excitatory amino acids and their antagonists on hippocampal neurons.

Intracellular recordings were obtained from guinea pig hippocampal neurons maintained in vitro. Current- and voltage-clamp techniques were used to study the effect of microiontophoresis of excitatory amino acid agonists. Modification of agonist responses by bath application of known concentrations of antagonist agents was also examined. All agonists used, glutamate, aspartate, N-methyl-D-aspartic acid (NMDA), and quisqualate, depolarized hippocampal neurons and caused repetitive firing. NMDA was also noted to induce burst-firing in some neurons. Quisqualate and NMDA were more potent than glutamate or aspartate. In slices perfused with a nominally calcium-free saline containing tetrodotoxin and manganese, quisqualate application produced a depolarization associated with a conductance increase. Under those conditions, NMDA-induced depolarizations caused apparent decreases as well as increases in conductance. The apparent decreases in conductance were observed in the voltage range of -40 to -70 mV, whereas increases in conductance were observed at membrane potentials more positive than -35 mV. Under voltage-clamp conditions, quisqualate produced an inward current whose amplitude increased with hyperpolarization and decreased upon depolarization, reversing near 0 mV. The conductance change induced by quisqualate was independent of voltage. NMDA application resulted in an inward current that was maximal around the resting potential and decreased with both hyperpolarization and depolarization. Response reversal was not observed with hyperpolarization to -100 mV but was apparent with depolarization beyond 0 mV. Conductance changes induced by NMDA were voltage dependent, and the application of this agent was associated with the appearance of a region of negative slope conductance in the current-voltage relationship. Apparent decreases in conductance in response to NMDA were reduced when the extracellular magnesium concentration was lowered. Response amplitudes were not affected. The NMDA receptor antagonist DL-2-amino-5-phosphonovalerate (DL-APV) was a potent and selective blocker of NMDA responses, whereas the antagonist DL-2-amino-4-phosphonobutyric acid (DL-APB) was less potent and did not select between NMDA and quisqualate responses. Analysis of iontophoretic dose-response curves indicated that DL-APV was a competitive antagonist. The results of these experiments indicate that hippocampal CA1 pyramidal neurons possess separate receptors for quisqualate and NMDA, with different pharmacological and electrophysiological profiles.

2-Amino-5-phosphonovalerate↗

PubMed1984

Interaction of penicillin and pentobarbital with inhibitory synaptic mechanisms in neocortex.

In this study we characterized the responses of neocortical neurons to iontophoretically applied gamma-aminobutyric acid (GABA) and examined how these GABA responses as well as the inhibitory postsynaptic potentials (IPSPs) were affected by the presence of penicillin or pentobarbital. Intracellular recordings were obtained from slices of rat neocortex maintained in vitro; injection of the dye Lucifer yellow indicated that recordings were primarily from pyramidal neurons. Orthodromically evoked responses were always depolarizing at the cell's resting membrane potential. Such depolarizing responses could easily be reversed in polarity by depolarizing the cell 10-15 mV, suggesting that the response consisted partly of an IPSP. In some cases, depolarization unmasked a small, short-latency excitatory postsynaptic potential (EPSP). Responses to iontophoretically applied GABA were also depolarizing at rest. Biphasic hyperpolarizing-depolarizing responses were occasionally observed upon depolarization of the neuron. Bath application of penicillin (1.7-3.4 mM) decreased the amplitude of the IPSPs and increased their time to peak, an effect associated with the development of epileptiform activity. Penicillin also reduced the maximum response to iontophoretically applied GABA without affecting the dose required to obtain a half-maximal response, suggesting a noncompetitive antagonism. Pentobarbital (100-200 microM) prolonged the time course and increased the amplitude of the IPSPs while producing a leftward shift in the GABA charge-response relation. These results suggest that the convulsant penicillin and the anticonvulsant pentobarbital have opposing actions on GABAergic inhibition in the neocortex.

Animals↗

PubMed1984

Picrotoxin-induced epileptiform activity in hippocampus: role of endogenous versus synaptic factors.

Picrotoxin-(PTX) induced epileptiform activity was studied in guinea pig hippocampal slices maintained in vitro, using intra- and extracellular recording techniques. The observed pattern of spontaneous and evoked epileptiform activity was quite complex. Spontaneous epileptiform events originated in the CA3 region and subsequently spread or propagated to CA1. Activation of CA1 could then reactivate CA3. This reverberation of activity was seen also following stimulation of the mossy fiber afferents from the dentate gyrus to CA3. Stimulation of fibers in the stratum radiatum of the CA1 region could trigger, at short latency, epileptiform activity that either was localized in CA1 or also occurred in CA3, with a late secondary discharge in CA1. This is attributed to a backfiring of the Schaffer collaterals and illustrates the ability of a variety of CA3 inputs to trigger epileptiform activity. Bath-applied PTX, at concentrations of 50-200 microM, had no apparent effect on the resting membrane potential or input resistance of the CA3 cells tested. Depolarizing current pulses elicited characteristic endogenous-burst responses that were not altered by PTX. Synaptic activity evoked by mossy fiber stimulation was altered markedly by PTX. The pattern of observed changes indicated that PTX reduced inhibitory postsynaptic potential (IPSP) amplitudes, resulting in the appearance of repetitive (presumably recurrent) excitatory inputs. Paroxysmal depolarizing shifts ( PDSs ) were generated by the coalescence of these excitatory inputs. Two types of spontaneous bursting were observed after PTX application. The first type was nonepileptiform , all or none in nature, and its frequency was voltage dependent. The second type of spontaneous burst was the PDS. It was epileptiform in character because it was associated with the synchronous discharge of many neurons. It was graded in nature, and its frequency was voltage independent. The graded nature of the PDS was demonstrated by varying the duration and intensity of the orthodromic stimulation. Trains of stimulation could produce PDSs that lasted 500-800 ms. A refractory period was observed following a PDS. By varying the strength of the orthodromic stimulation, it was possible to demonstrate that for the intervals tested this was a relative, not absolute, refractory period. Intracellular recordings in CA3 neurons indicated that each spontaneous PDS was followed by an afterhyperpolarization (AHP).

Animals↗