Mechanisms underlying burst and regular spiking evoked by dendritic depolarization in layer 5 cortical pyramidal neurons

Apical dendrites of layer 5 pyramidal cells in a slice preparation of rat sensorimotor cortex were depolarized focally by long-lasting glutamate iontophoresis while recording intracellularly from their soma. In most cells the firing pattern evoked by the smallest dendritic depolarization that evoked spikes consisted of repetitive bursts of action potentials. During larger dendritic depolarizations initial burst firing was followed by regular spiking. As dendritic depolarization was increased further the duration (but not the firing rate) of the regular spiking increased, and the duration of burst firing decreased. Depolarization of the soma in most of the same cells evoked only regular spiking. When the dendrite was depolarized to a critical level below spike threshold, intrasomatic current pulses or excitatory postsynaptic potentials also triggered bursts instead of single spikes. The bursts were driven by a delayed depolarization (DD) that was triggered in an all-or-none manner along with the first Na+ spike of the burst. Somatic voltage-clamp experiments indicated that the action current underlying the DD was generated in the dendrite and was Ca2+ dependent. Thus the burst firing was caused by a Na+ spike-linked dendritic Ca2+ spike, a mechanism that was available only when the dendrite was adequately depolarized. Larger dendritic depolarization that evoked late, constant-frequency regular spiking also evoked a long-lasting, Ca2+-dependent action potential (a "plateau"). The duration of the plateau but not its amplitude was increased by stronger dendritic depolarization. Burst-generating dendritic Ca2+ spikes could not be elicited during this plateau. Thus plateau initiation was responsible for the termination of burst firing and the generation of the constant-frequency regular spiking. We conclude that somatic and dendritic depolarization can elicit quite different firing patterns in the same pyramidal neuron. The burst and regular spiking observed during dendritic depolarization are caused by two types of Ca2+-dependent dendritic action potentials. We discuss some functional implications of these observations.     

Long-term facilitation of excitatory synaptic transmission in single motor cortical neurones of the cat produced by repetitive pairing of synaptic potentials and action potentials following intracellular stimulation

The effects of postsynaptic firing activity on excitatory postsynaptic potentials (EPSPs) were studied in the motor cortex of anaesthetized cats. Postsynaptic firing was induced by 1-5 nA cathodal current pulses via the recording intracellular microelectrode, while EPSPs were elicited by thalamic, callosal, pyramidal tract and somatosensory stimuli. In 102 cells, EPSP-spike stimulus pairs were applied with 0.2-1/sec frequency and 10-100 msec interstimulus intervals. In 42 neurones, reversible facilitation of paired EPSPs appeared lasting from 4 to 47 min. The synaptic facilitation in most cases was accompanied by membrane depolarization and an increase in input resistance. The effectiveness of current induced action potentials upon test EPSPs provided evidence for the postsynaptic localization of plastic changes occurring in conditioning experiments.     

Presynaptic modulation of GABAergic inhibition by GABA(B) receptors in the rat's inferior colliculus

Whole-cell patch clamp recordings were made from neurons in a brain slice preparation of the inferior colliculus in 11-15-day-old rat pups. Synaptic responses were elicited by applying a current pulse to the lateral lemniscus just below the central nucleus of the inferior colliculus. To examine GABAergic inhibition in the inferior colliculus all excitatory postsynaptic potentials and glycinergic inhibitory postsynaptic potentials were blocked by bath application of their respective antagonists and the contribution of GABA(B) receptors was determined for the remaining inhibitory postsynaptic potentials. For most cells the isolated inhibitory postsynaptic potential was completely blocked by the GABA(A) receptor antagonist, bicuculline, but was unaffected by the GABA(B) receptor antagonist, phaclofen. The GABA(B) receptor agonist, baclofen (10-20 microM), decreased the amplitude of the inhibitory postsynaptic potentials. This effect was completely blocked by phaclofen. Baclofen did not increase the cell membrane conductance or alter the rate of firing produced by depolarization of the cell membrane. In contrast, muscimol, a GABA(A) receptor agonist, greatly increased membrane conductance and lowered the firing rate produced by depolarization. Our results indicate that GABAergic inhibition in the auditory midbrain can be reduced by the activation of GABA(B) receptors and suggest that the effects are presynaptic.     

Voltage-activated sodium channels amplify inhibition in neocortical pyramidal neurons

Inhibitory postsynaptic potentials (IPSPs) in neocortical pyramidal neurons are increased in duration and amplitude at depolarized membrane potentials. This effect was not due to changes in the time course of the underlying synaptic current. The role of postsynaptic voltage-activated channels was investigated by mimicking the voltage change that occurs during an IPSP with current injections. The peak and integral of these 'simulated' IPSPs increased during depolarization of the membrane potential in a tetrodotoxin-sensitive manner. This amplification presumably occurs as the hyperpolarization associated with IPSPs turns off sodium channels that are tonically active at depolarized membrane potentials. IPSP amplification increased the ability of IPSPs to inhibit action potential firing and promoted IPSP-induced action potential synchronization.     

Synaptic and intrinsic control of membrane excitability of neostriatal neurons. I. An in vivo analysis

1. The relationship between membrane properties of neostriatal neurons and spontaneous and evoked synaptic potentials was studied with the use of intracellular recordings from anesthetized rats. Most of these neurons showed regular or irregular spontaneous depolarizing potentials that only in a few cases triggered action potentials at resting level. 2. The stimulation of the ipsilateral substantia nigra or of the sensorimotor cortex produced a relatively fast depolarizing post-synaptic potential (EPSP). In some cells this potential was followed by an inhibitory period that appeared as an hyperpolarization when the cell was depolarized from the resting level (inhibitory postsynaptic potential, IPSP). A late and long-lasting depolarization (LD) followed the EPSP or the EPSP-IPSP sequence. 3. Repetitive discharge with little adaptation was observed during direct depolarization. Most of the neurons tested for current-voltage (I-V) relationship showed nonlinearity of the input resistance in the hyperpolarizing direction. Spontaneous and evoked EPSPs were decreased in their amplitude and duration when the membrane potential was held at levels more hyperpolarized than -85 mV because of the strong rectification at these levels of hyperpolarization. 4. Local microiontophoretic application of bicuculline (BIC) or systemic administration of BIC and pentylenetetrazole (PTZ) produced a reduction of the IPSPs. The reduction of the inhibitory transmission caused a strong increase of the LD. The current-evoked firing pattern was not greatly altered. 5. The intracellular application of cesium increased the amplitude and the duration of the spontaneous depolarizations that triggered bursts of action potentials under this condition. Spikes were broadened and the rectification in the hyperpolarization direction was reduced. 6. Iontophoretically applied cadmium strongly depressed the amplitude of the spontaneous and evoked postsynaptic potentials. During cadmium application, nigral stimulation produced constant latency, all-or-none spikes in the absence of any synaptic potential. 7. Repetitive stimulation of the ipsilateral substantia nigra by electrical shocks (5 Hz, 25 s) produced a progressive and reversible decrease of the spontaneous depolarizing potentials (SDPs) and a decrease of the firing rate. In the same cells, when the train of stimulation was delivered in the ipsilateral cortex, a membrane depolarization coupled with an increase of the firing rate was observed. 8. We conclude that although synaptic circuits mediate a phasic inhibition in neostriatum, the low level of spontaneous firing of most neostriatal neurons is mainly because of the effects that membrane properties exert on the spontaneous and the evoked synaptic depolarizations in the striatum.(ABSTRACT TRUNCATED AT 400 WORDS)     

Facilitation by acetylcholine of tetrodotoxin-resistant spikes in rat hippocampal pyramidal cells

The electrical activity of hippocampal pyramidal cells was studied in slice cultures during blockade of the regenerative Na currents. In the presence of tetrodotoxin, these neurones had a mean resting potential of -68 mV, a membrane input resistance of 87 M omega and displayed marked non-linearities in their current voltage relationship. In response to depolarizing stimuli, pyramidal cells generated action potentials of small amplitude, slow rise and long duration. These tetrodotoxin-resistant spikes were abolished by calcium conductance blockers such as cobalt and cadmium ions. Acetylcholine applied to the bath or by iontophoresis depolarized pyramidal cells, elicited spontaneous tetrodotoxin-resistant spikes and facilitated spiking evoked by depolarizing rectangular current pulses or a current ramp. The effects of acetylcholine were not only slow in onset, but also prolonged; they were completely reversible and sensitive to atropine and calcium-antagonists such as cadmium and cobalt ions which, respectively, reduced and abolished these effects. After hyperpolarizations following injection of depolarizing current pulses were suppressed by acetylcholine and often transformed into depolarizing afterpotentials. Acetylcholine had no effect on voltage-independent conductances as determined by application of hyperpolarizing current pulses. These results could be explained by inhibition of the voltage-dependent K+-current, i.e. the M current (blockade of the calcium current could remove any depolarizing influence resulting from M current inhibition) or by a direct activation of a voltage-dependent calcium current by muscarinic agonists.     

Enhancement of visual responsiveness by spontaneous local network activity in vivo

Spontaneous activity within local circuits affects the integrative properties of neurons and networks. We have previously shown that neocortical network activity exhibits a balance between excitatory and inhibitory synaptic potentials, and such activity has significant effects on synaptic transmission, action potential generation, and spike timing. However, whether such activity facilitates or reduces sensory responses has yet to be clearly determined. We examined this hypothesis in the primary visual cortex in vivo during slow oscillations in ketamine-xylazine anesthetized cats. We measured network activity (Up states) with extracellular recording, while simultaneously recording postsynaptic potentials (PSPs) and action potentials in nearby cells. Stimulating the receptive field revealed that spiking responses of both simple and complex cells were significantly enhanced (>2-fold) during network activity, as were spiking responses to intracellular injection of varying amplitude artificial conductance stimuli. Visually evoked PSPs were not significantly different in amplitude during network activity or quiescence; instead, spontaneous depolarization caused by network activity brought these evoked PSPs closer to firing threshold. Further examination revealed that visual responsiveness was gradually enhanced by progressive membrane potential depolarization. These spontaneous depolarizations enhanced responsiveness to stimuli of varying contrasts, resulting in an upward (multiplicative) scaling of the contrast response function. Our results suggest that small increases in ongoing balanced network activity that result in depolarization may provide a rapid and generalized mechanism to control the responsiveness (gain) of cortical neurons, such as occurs during shifts in spatial attention.     

The role of chloride-dependent inhibition and the activity of fast-spiking neurons during cortical spike-wave electrographic seizures

The conventional view is that the cortical paroxysmal depolarizing shift is a giant excitatory postsynaptic potential enhanced by various intrinsic neuronal currents. Other results point out, however, that synaptic inhibition remains functional in many forms of paroxysmal activities and that intense activation of GABAergic interneurons may accentuate the excitation of target pyramidal cells. To determine the role played by cortical inhibitory neurons in paroxysmal discharges, we used single and dual intracellular recordings from electrophysiologically identified neocortical neurons during spontaneously occurring and electrically induced spike-wave electrographic seizures in vivo. Conventional fast-spiking neurons (presumably local inhibitory interneurons) fired at a very high frequency during paroxysmal depolarizing shifts, which corresponded to the electroencephalogram 'spike' components of spike-wave complexes. The firing of fast-spiking neurons preceded the discharges of neighboring regular-spiking neurons. During electrographic seizures, the reversal potential of the GABA (type A)-mediated potentials in regular-spiking neurons was shifted to positive values by 20-30 mV. Data also show that the prolonged hyperpolarizations during the electroencephalogram 'wave' components of spike-wave electrographic seizures do not contain Cl(-)-dependent inhibitory potentials. Moreover, Cl(-)-dependent mechanisms were reduced or absent during the fast runs that are associated with spike-wave complexes in some paroxysms. We conclude that the strong activity of cortical inhibitory neurons during paroxysmal depolarizing shifts induces Cl(-)-dependent depolarizing postsynaptic potentials in target pyramidal neurons, which facilitate the development of electrographic seizures.     

Sustained potential shifts and paroxysmal discharges in hippocampal formation

Paroxysmal firing was provoked by electric stimulation of afferent pathways in hippocampal formation of intact, urethan-anesthetized rats, of freely moving unanesthetized rats, and in hippocampal tissue slices in vitro. The electric responses of fascia dentata and CA3 zone of the hippocampus of urethan-anesthetized rats were recorded with extracellular microelectrodes. Paroxysmal discharges were provoked by stimulating the ipsilateral angular bundle. During repetitive stimulation, intercurrent paroxysmal discharges (IPaD) took the form of compound action potentials (population spikes) of large amplitude, provoked by but not locked in time to the stimulus pulses. IPaD was often but not always followed by paroxysmal after-discharge (PaAD), usually consisting of bursts of population spikes, sometimes superimposed on a slow wave. Stimulus pulses that were not strong enough to evoke population spikes when applied singly could provoke the paroxysmal firing of large amplitude spikes when applied repetitively. The liminal frequency to provoke paroxysmal firing, with 10-s train duration and with pulses evoking 60 to 80% of maximal amplitude focal postsynaptic potential (PSP) waves, varied between 6 and 15 Hz in urethan-anesthetized rats. The outbreak of IPaD was always accompanied by a marked sustained potential (SP) shift. The polarity of the paroxysmal SP shift was the opposite of the polarity of the PSP waves. We conclude that the extracellular paroxysmal SP shifts in fascia dentata are probably generated mainly by current flowing from the dendritic trees toward the cell somata of granule cells. The amplitude of the population spikes fired during paroxysmal discharges could reach 30-40 mV, indicating the precise coincidence of the impulses fired by many neurons. These spikes often arose without a detectable preceding synaptic potential. We conclude that the synchronization of the action potentials fired by granule and pyramidal cells during paroxysmal discharge is probably due to electric interaction among the neurons. In unanesthetized freely moving rats IPaD and PaAD consisting of bursts of population spikes were provoked. These were similar to those observed in urethan-anesthetized rats. Motor seizures provoked in kindled rats were associated with intense and prolonged spike bursts followed by spikeless positive waves recorded in the granule cell layer of fascia dentata. In hippocampal tissue slices maintained in vitro, paroxysmal firing could be provoked in CA1 zone by repetitive stimulation of Schaffer collaterals. IPaD and PaAD could be provoked in some slices exposed to normal (3.5 mM) [K+] and in all slices exposed to elevated (5.5 or 7.0 mM) [K+].(ABSTRACT TRUNCATED AT 400 WORDS)     

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)     

Potentials evoked by alvear tract in hippocampal CA1 region of rats. I. Topographical projection, component analysis, and correlation with unit activities.

1. The field potentials and unit activities evoked by the alvear tract (AT) in CA1 region of the dorsal hippocampus of rats were studied under sodium pentobarbital anesthesia. 2. The localized activity evoked anterior to an AT stimulus began as a compound action potential, followed by a slower negative wave, and ended in a long-lasting, slow positive wave. Observed with a 64-electrode recording array, topographical projections of the AT in CA1 were seen as parallel strips inclined at an angle of 5-30 degrees medially from the sagittal plane. 3. Three overlapping components in the averaged evoked potentials (AEPs) were distinguished. The first event (component I) was a brief compound antidromic action potential of pyramidal cells. The second field event (component II) reversed from surface negative to deep positive at 200 micrometer from the ventricular surface, increased rapidly with stimulus intensity, potentiated with double shocks, and followed stimulus frequency up to 50/s. The third component was long lasting (up to 200 ms), surface positive and ventral negative (turnover at 150 micron below the pyramidal layer), followed stimulus frequency up to about 10/s, and saturated at a low stimulus intensity (about 3 x threshold). 4. In some preparations, another fast negative peak of about 2 ms duration was found to follow the axon compound action potential on the hippocampal surface and appeared to propagate from the pyramidal layer to the ventricular surface. It was probably of nonsynaptic origin, perhaps due to the centrifugal basal dendritic spikes of the pyramidal cells. 5. Single units were recorded in CA1. Antidromic units were identified by their firing at a fixed latency (1.5 ms) and ability to follow high stimulus frequencies. Units firing at about 2.7 ms latency possessed characteristics of monosynaptic excitation. Under light anesthesia, many of the latter units also showed a late, prolonged suppression of background firing. Tentative interneuronal types fired with peak latencies of 4-5 ms or showed prolonged increase in firing rate. 6. From the correlation with unit post-stimulus time histograms, AEP component II was inferred to be the extracellular, monosynaptic, excitatory postsynaptic potentials, and component III the di- or polysynaptic inhibitory postsynaptic potentials. These postsynaptic potentials were generated by the pyramidal cells and interneurons.     

Role of EPSPs in initiation of spontaneous synchronized burst firing in rat hippocampal neurons bathed in high potassium

1. Spontaneous discharges that resemble interictal spikes arise in area CA3 b/c of rat hippocampal slices bathed in 8.5 mM [K+]o. Excitatory postsynaptic potentials (EPSPs) also appear at irregular intervals in these cells. The role of local synaptic excitation in burst initiation was examined with intracellular and extracellular recordings from CA3 pyramidal neurons. 2. Most (70%) EPSPs were small (less than 2 mV in amplitude), suggesting that they were the product of quantal release or were evoked by a single presynaptic action potential in another cell. It is unlikely that most EPSPs were evoked by a presynaptic burst of action potentials. Indeed, intrinsic burst firing was not prominent in CA3 b/c pyramidal cells perfused in 8.5 mM [K+]o. 3. The likelihood of occurrence and the amplitude of EPSPs were higher in the 50-ms interval just before the onset of each burst than during a similar interval 250 ms before the burst. This likely reflects increased firing probability of CA3 neurons as they emerge from the afterhyperpolarization (AHP) and conductance shunt associated with the previous burst. 4. Perfusion with 2 microM 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX), a potent quisqualate receptor antagonist, decreased the frequency of EPSPs in CA3 b/c neurons from 3.6 +/- 0.9 to 0.9 +/- 0.3 (SE) Hz. Likewise, CNQX reversibly reduced the amplitude of evoked EPSPs in CA3 b/c cells. 5. Spontaneous burst firing in 8.5 mM [K+]o was abolished in 11 of 31 slices perfused with 2 microM CNQX.(ABSTRACT TRUNCATED AT 250 WORDS)     

Kindling-like stimulus patterns induce epileptiform discharges in the guinea pig in vitro hippocampus

In the guinea pig in vitro hippocampal slice preparation, we have demonstrated that the repeated tetanic stimulation of the Schaffer collateral-commissural input to CA1 pyramidal neurones produces a progressive increase in the amplitude and duration of postsynaptic potentials, and stimulus-induced and spontaneous paroxysmal depolarization shifts (PDSs). Both the enhancement of synaptic transmission and the genesis of PDSs were reversibly blocked by the N-methyl-D-aspartate (NMDA) receptor antagonist (+/-)-2-amino-5-phosphonovalerate (APV). These results provide evidence that progressive, stimulation-induced long-term potentiation may serve as the trigger for kindling-induced epileptogenesis, and this process is dependent on the repeated activation of an APV-sensitive receptor.     

N-methylaspartate receptors mediate epileptiform activity evoked in some, but not all, conditions in rat neocortical slices

Using intracellular recordings from pyramidal neurons in isolated slices of rat cerebral cortex epileptiform discharges evoked (1) in the presence of gamma-aminobutyric acid antagonists, and (2) in the absence of Mg2+ were compared. Depolarization shift responses recorded in the presence of bath applied picrotoxin, or electrophoretically applied picrotoxin or bicuculline, were similar in many respects to depolarization shifts reported previously, except that they could be evoked by stimuli subthreshold for evoking discernible postsynaptic potentials in these experiments. Large depolarizations evoked by repetitive activation of an N-methylaspartate receptor mediated synapse in the absence of Mg2+, displayed several properties similar to those of depolarization shifts evoked in the presence of gamma-aminobutyric acid antagonists, i.e. similar shape, latency, inability to follow high repetition rates and a similar voltage relation, suggesting activation of the same cellular mechanism. "Slow spikes" evoked as part of the response to electrophoretically applied N-methylaspartate were augmented, i.e. they were replaced by larger, longer, more complex events, when gamma-aminobutyric acid antagonists were applied. The potentiated response, evoked in the absence of Mg2+, was dependent on the activation of an N-methylaspartate receptor mediated synapse and was blocked by N-methylaspartate antagonists. In contrast, depolarization shifts could be evoked in the presence of large doses of N-methylaspartate antagonists, when gamma-aminobutyric acid antagonists were applied. Spontaneous depolarizations similar to depolarization shifts were recorded when cells were exposed to low, tonic, electrophoretic applications of excitatory amino acids under control conditions. In addition, some potentiation of the N-methylaspartate receptor mediated excitatory postsynaptic potential was achieved in the presence of Mg2+ when cells were depolarized by 10-20 mV. Depolarization shifts evoked when bicuculline was applied electrophoretically to different parts of the dendritic field, some hundreds of microns from the soma, differed in shape, latency and time course and the depolarization shift evoked when bicuculline was applied at one site summed with the depolarization shift evoked when it was applied elsewhere. We conclude that different inputs are required to activate the responses evoked in the presence of gamma-aminobutyric acid antagonists and in the absence of Mg2+. The possibility that both involve activation of dendritic Ca2+ currents and that the magnitude of the response depends on the proportion of the dendritic field activated, is discussed.     

Dendritic calcium spikes in layer 5 pyramidal neurons amplify and limit transmission of ligand-gated dendritic current to soma.

Long-lasting, dendritic, Ca(2+)-dependent action potentials (plateaus) were investigated in layer 5 pyramidal neurons from rat neocortical slices visualized by infrared-differential interference contrast microscopy to understand the role of dendritic Ca(2+) spikes in the integration of synaptic input. Focal glutamate iontophoresis on visualized dendrites caused soma firing rate to increase linearly with iontophoretic current until dendritic Ca(2+) responses caused a jump in firing rate. Increases in iontophoretic current caused no further increase in somatic firing rate. This limitation of firing rate resulted from the inability of increased glutamate to change evoked plateau amplitude. Similar nonlinear patterns of soma firing were evoked by focal iontophoresis on the distal apical, oblique, and basal dendrites, whereas iontophoresis on the soma and proximal apical dendrite only evoked a linear increase in firing rate as a function of iontophoretic current without plateaus. Plateau amplitude recorded in the soma decreased as the site of iontophoresis was moved farther from the soma, consistent with decremental propagation of the plateau to the soma. Currents arriving at the soma summed if plateaus were evoked on separate dendrites or if subthreshold responses were evoked from sites on the same dendrite. If plateaus were evoked at two sites on the same dendrite, only the proximal plateau was seen at the soma. Just-subthreshold depolarizations at two sites on the same dendrite could sum to evoke a plateau at the proximal site. We conclude that the plateaus prevent current from ligand-gated channels distal to the plateau-generating region from reaching the soma and directly influencing firing rate. The implications of plateau properties for synaptic integration are discussed.     

Blockade of GABAA receptors facilitates induction of NMDA receptor-independent long-term potentiation.

An N-methyl-D-aspartate (NMDA)-independent form of long-term potentiation (LTP), which depends on postsynaptic, voltage-dependent calcium channels (VDCCs), has been demonstrated in area CA1 of hippocampus. GABA acting at GABAA receptors limits postsynaptic depolarization during LTP induction. Blockade of GABAA receptors should therefore enhance activation of postsynaptic VDCCs and facilitate the induction of this NMDA receptor-independent, VDCC-dependent LTP. In agreement with this hypothesis, pharmacological blockade of GABAA receptors in the in vitro rat hippocampal slice increased the magnitude of LTP resulting from a normally effective, high-frequency (200 Hz) tetanic stimulation protocol. In addition, GABAA receptor blockade allowed a lower frequency (25 Hz) and normally ineffective tetanic stimulation protocol to induce this form of LTP. Intracellular recordings from CA1 pyramidal cells revealed that blocking GABAA receptors during tetanic stimulation allowed greater postsynaptic depolarization, increased the number of postsynaptic action potentials fired during the tetanization, and also increased the duration of synaptically evoked action potentials. To mimic the increased action potential firing observed when GABAA receptors were blocked, we paired 25-Hz antidromic stimulation with 25-Hz orthodromic stimulation. Paired antidromic + orthodromic 25-Hz stimulation induced NMDA receptor-independent LTP, whereas neither antidromic nor orthodromic stimulation alone induced LTP. Increased action potential firing can therefore at least partially account for the facilitation of NMDA receptor-independent LTP caused by blockade of GABAA receptors. This conclusion is consistent with prior studies demonstrating that action potentials are particularly effective stimuli for the gating of VDCCs in CA1 pyramidal cell dendrites.     

Somatostatin inhibits GABAergic transmission in the sensory thalamus via presynaptic receptors

The action of somatostatin on GABA-mediated transmission was investigated in cat and rat thalamocortical neurons of the dorsal lateral geniculate nucleus and ventrobasal thalamus in vitro. In the cat thalamus, somatostatin (10 microM) had no effect on the passive membrane properties of thalamocortical neurons and on the postsynaptic response elicited in these cells by bath or iontophoretic application of (+/-)baclofen (5-10 microM) or GABA, respectively. However, somatostatin (1-10 microM) decreased by a similar amount (45-55%) the amplitude of electrically evoked GABA(A) and GABA(B) inhibitory postsynaptic potentials in 71 and 50% of neurons in the lateral geniculate and ventrobasal nucleus, respectively. In addition, the neuropeptide abolished spontaneous bursts of GABA(A) inhibitory postsynaptic potentials in 85% of kitten lateral geniculate neurons, and decreased (40%) the amplitude of single spontaneous GABA(A) inhibitory postsynaptic potentials in 87% of neurons in the cat lateral geniculate nucleus. Similar results were obtained in the rat thalamus. Somatostatin (10 microM) had no effect on the passive membrane properties of thalamocortical neurons in this species, or on the outward current elicited by puff-application of (+/-)baclofen (5-10 microM). However, in 57 and 22% of neurons in the rat lateral geniculate and ventrobasal nuclei, respectively, somatostatin (1 microM) reduced the frequency, but not the amplitude, of miniature GABA(A) inhibitory postsynaptic currents by 31 and 37%, respectively. In addition, the neuropeptide (1 microM) decreased the amplitude of evoked GABA(A) inhibitory postsynaptic currents in 20 and 55% of rat ventrobasal neurons recorded in normal conditions and during enhanced excitability, respectively: this effect was stronger on bursts of inhibitory postsynaptic currents(100% decrease) than on single inhibitory postsynaptic currents (41% decrease).These results demonstrate that in the sensory thalamus somatostatin inhibits GABA(A)- and GABA(B)-mediated transmission via a presynaptic mechanism, and its action is more prominent on bursts of GABAergic synaptic currents/potentials.     

Leptin modulates fast synaptic transmission in dog pancreatic ganglia

The aim of this study was to determine whether leptin modulates neuronal activity in intrapancreatic ganglion neurons. Intracellular recordings were made in dog pancreatic neurons. Recombinant mouse leptin (313 nM) was added by superfusion. When leptin was present, fast EPSPs which were subthreshold in normal Krebs solution reached threshold for firing action potentials. However, leptin had no significant (P > 0.05, n = 18) effect on either the resting membrane potential or on membrane input resistance. To determine whether leptin increased the postsynaptic sensitivity to acetylcholine, the response was tested by pressure ejection of acetylcholine. Acetylcholine evoked a 9.4+/-2.2 mV (mean +/- SEM, n = 5) depolarization in normal Krebs solution. In the presence of leptin, the response was not significantly different (9.6+/-2.4 mV, P > 0.05). The results suggest that leptin modulates fast synaptic transmission in pancreatic ganglion neurons by acting on presynaptic nerve terminals.     

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)     

Effects of chloride transport inhibition and chloride substitution on neuron function and on hypoxic spreading-depression-like depolarization in rat hippocampal slices

Chloride fluxes play a crucial role in synaptic inhibition, cell pH regulation, as well as in cell volume control. In many neuropathological processes, cell swelling is a pivotal parameter, since cell volume changes and the dimension of the interstitial space critically modulate synchronized neuronal activity as well as the tissue's susceptibility to seizures or spreading depression. This study therefore focuses on the effects of different Cl(-) transport inhibitors and Cl(-) substitution on neuronal function and hypoxia-induced changes in rat hippocampal tissue slices. Orthodromically evoked focal excitatory postsynaptic potentials were depressed by furosemide (2mM), 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid (1mM) and Cl(-) substitution by methylsulfate, but were enhanced by 4,4'-dinitrostilbene-2,2'-disulfonic acid (1mM). All four treatments induced multiple population spike firing in response to single orthodromic volleys, suggesting reduced synaptic inhibition. Antidromic population spikes increased following Cl(-) withdrawal, were unaffected in the presence of furosemide and 4, 4'-dinitrostilbene-2,2'-disulfonic acid, but were abolished by 4, 4'-diisothiocyanatostilbene-2,2'-disulfonic acid. The amplitude of the hypoxic spreading-depression-like extracellular potential shift was reduced by furosemide, 4,4'-diisothiocyanatostilbene-2, 2'-disulfonic acid and Cl(-) withdrawal, i.e. by the same treatments that depressed orthodromically evoked postsynaptic potentials. Furosemide prolonged the time to onset and the duration of the spreading-depression-like extracellular potential shift, while 4, 4'-dinitrostilbene-2,2'-disulfonic acid shortened the time to onset. Spreading-depression-related cell swelling was recorded as the shrinkage of relative interstitial space, which was measured as tetramethylammonium-chloride space. Neither the Cl(-) transport inhibitors nor Cl(-) withdrawal had any detectable effect on spreading-depression-related cell swelling. CA1 pyramidal neurons usually hyperpolarized during drug application and their input resistance decreased. Cl(-) withdrawal increased their input resistance and caused spontaneous burst firing. Hypoxia caused the expected spreading-depression-like rapid, near complete depolarization of single pyramidal neurons and drastically reduced their input resistance. The three Cl(-) transport inhibitors and Cl(-) withdrawal delayed the onset of the hypoxic depolarization. In low Cl(-) solutions, the apparent threshold potential at which spreading depression was triggered shifted to more positive membrane potentials. The final voltage of the hypoxic depolarization was, however, not affected.It appears from these results that the reduction in the hypoxic spreading-depression-like extracellular potential shifts by Cl(-) transport inhibitors is at least partially attributable to desynchronization of depolarization, not to decreased depolarization in individual cells. Other contributing factors could be changes in recording conditions, depression of swelling-induced amino acid release from glial cells and unspecific side-effects of the applied drugs. Desynchronization could also account for the delayed spreading-depression onset. It is concluded that Cl(-) fluxes play a role in the triggering of spreading depression, but the spreading-depression-like depolarization itself or its self-regenerative character is not mediated by Cl(-).