Repetitive firing of CA1 hippocampal pyramidal cells elicited by dendritic glutamate: slow prepotentials and burst-pause pattern

Cholinergic modulation of neuron spike responses to dendritic and somatic application of excitatory amino acids

The effects of acetylcholine on the spike discharges of neurons induced by iontophoretic application of excitatory amino acids to the bodies and dendrites of cells were studied in 98 neurons in living slices of guinea pig parietal cortex. Acetylcholine applied microiontophoretically to both the bodies and dendrites facilitated improvements in the parameters of responses induced by dendritic activation, with significant decreases in latent periods and increases in the intensity and duration of responses. Thee effects were stably induced at distances of 300 microm from the body and lasted 1 min after exposure to acetylcholine ended. Responses induced by application of excitatory amino acids directly to the cell body did not change significantly in the presence of acetylcholine regardless of the point on the membrane at which they were applied. It is concluded that the predominant effect of acetylcholine is on the efficiency of dendrosomatic conduction.

Decoding of synaptic voltage waveforms by specific classes of recombinant high-threshold Ca(2+) channels

Studies suggest that the preferential role of L-type voltage-sensitive Ca(2+) channels (VSCCs) in coupling strong synaptic stimulation to transcription is due to their selective activation of local chemical events. However, it is possible that selective activation of the L-type channel by specific voltage waveforms also makes a contribution. To address this issue we have examined the response of specific Ca(2+) channel types to simulated complex voltage waveforms resembling those encountered during synaptic plasticity (gamma and theta firing frequency). L-, P/Q- and N-type VSCCs (alpha1C, alpha1A, alpha1B/beta1B/alpha2delta, respectively) were all similarly activated by brief action potential (AP) waveforms or sustained step depolarization. When complex waveforms containing large excitatory postsynaptic potentials (EPSPs), APs and spike accommodation were applied under voltage clamp we found that the integrated L-type VSCC current was approximately three times larger than that produced by the P/Q- or N-type Ca(2+) channels (gamma frequency 1 s stimulation). For P/Q- or N-type channels the complex waveforms led to a smaller current than that expected from the response to a simple 1 s step depolarization to 0 or +20 mV. EPSPs present in the waveforms favoured the inactivation of P/Q- and N-type channels. In contrast, activation of the L-type channel was dependent on both EPSP- and AP-mediated depolarization. Expression of P/Q-type channels with reduced voltage-dependent inactivation (alpha1A/beta2A/alpha2delta) or the use of hyperpolarized intervals between AP stimuli greatly increased their response to complex voltage stimuli. We propose that in response to complex synaptic voltage waveforms P/Q- and N-type channels can undergo selective voltage-dependent inactivation leading to a Ca(2+) current mediated predominantly by L-type channels.

Neurochemical and electrophysiological studies on the functional significance of burst firing in serotonergic neurons

We have previously described a population of 5-hydroxytryptamine neurons which repetitively fires bursts of usually two (but occasionally three or four) action potentials, with a short (<20 ms) interspike interval within a regular low-frequency firing pattern. Here we used a paradigm of electrical stimulation comprising twin pulses (with 7- or 10-ms inter-pulse intervals) to mimic this burst firing pattern, and compared the effects of single- and twin-pulse electrical stimulations in models of pre- and postsynaptic 5-hydroxytryptamine function. Firstly, we measured the effect of direct electrical stimulation (2 Hz for 2 min) of rat brain slices on efflux of preloaded [3H]5-hydroxytryptamine. In this in vitro model, twin-pulse stimulation increased the efflux of tritium by about twice as much as did single-pulse stimulation. This effect was evident in the medial prefrontal cortex (area under the curve: 2. 59+/-0.34 vs 1.28+/-0.22% relative fractional release), as well as in the caudate-putamen (3.93+/-0.65 vs 2.17+/-0.51%) and midbrain raphe nuclei (5.42+/-1.05 vs 2.51+/-0.75%). Secondly, we used in vivo microdialysis to monitor changes in endogenous extracellular 5-hydroxytryptamine in rat medial prefrontal cortex in response to electrical stimulation (3 Hz for 10 min) of the dorsal raphe nucleus. In this model, twin-pulse stimulation of the dorsal raphe nucleus increased 5-hydroxytryptamine by approximately twice as much as did single-pulse stimulation at the same frequency (area under the curve: 50.4+/-9.0 vs 24.2+/-4.4 fmol). Finally, we used in vivo extracellular recording to follow the response of postsynaptic neurons in the rat medial prefrontal cortex to 5-hydroxytryptamine released by dorsal raphe stimulation. Electrical stimulation of the dorsal raphe nucleus (1 Hz) induced a clear-cut poststimulus inhibition in the majority of cortical neurons tested. In these experiments, the duration of poststimulus inhibition following twin-pulse stimulation was markedly longer than that induced by single-pulse stimulation (200+/-21 vs 77+/-18.5 ms). Taken together, the present in vitro and in vivo data suggest that in 5-hydroxytryptamine neurons, short bursts of action potentials will propagate along the axon to the nerve terminal and will enhance both the release of 5-hydroxytryptamine and its postsynaptic effect.

Cholinergic excitation of dendrites in neocortical neurons

Discharge patterns were studied in response to iontophoretic application of acetylcholine to the soma and dendrites of 128 neocortical pyramidal neurons of layer V. Extracellular recordings were obtained from slices of the guinea-pig parietal cortex. All responses found were excitatory and were better expressed in spontaneously firing cells than in silent ones. Sensitivity to acetylcholine was approximately the same at somatic and dendritic sites in all the cells. Activation of muscarinic receptors gave rise to firing patterns with equal latencies and intensities when applied to both soma and dendrites. The latter suggests that membrane excitation elicited in dendrites by binding of acetylcholine to muscarinic cholinoreceptors is likely to propagate towards the soma through intracellular biochemical processes. Modulating effect of acetylcholine on output firing patterns, elicited by dendritic application of excitatory amino acids, included shortening of the somatic response latency and increase of response intensity and duration. We propose that, in contrast to glutamatergic excitation, the spread of cholinergic excitation along dendrites involves intra-cellular chemical signalling and results in changing the electrical properties of dendrites all over their length.

Veratridine-enhanced persistent sodium current induces bursting in CA1 pyramidal neurons

The mechanism of veratridine-induced bursting activity was studied in rat hippocampal CA1 pyramidal neurons. Veratridine (0.1-0.3 microM) induces bursting in previously normal pyramidal neurons. The current-voltage curves of untreated neurons show a slight deviation from the linear Ohmic relation; this deviation is known as the "depolarizing rectification". Veratridine markedly accentuates the depolarizing rectification so that a zero slope or negative slope appears in the current-voltage curve of these neurons. Both the veratridine-induced bursting activity and negative slope resistance are blocked by small concentrations of tetrodotoxin or by raising the calcium concentration of the superfusion medium. Under single-electrode voltage clamping, a subthreshold persistent (slowly inactivating) sodium current, which can be recorded in untreated neurons, is found to be enhanced in the veratridine-treated neurons. This current is thought to be responsible for the slow depolarizing phase of bursting activity and the development of negative slope resistance in the current-voltage relationship. The present results demonstrate that veratridine enhances the slowly inactivating sodium current, leading to the development of negative slope resistance and induction of bursting in rat hippocampal CA1 pyramidal neurons.

Effects of intracellular calcium chelation on voltage-dependent and calcium-dependent currents in cat neocortical neurons

Large neurons from layer V in a slice preparation of cat sensorimotor cortex were impaled with microelectrodes containing KCl plus different concentrations of the Ca2+ chelator 1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid (BAPTA) or two of its derivatives. Impalement with electrodes containing high BAPTA (200 mM) quickly abolished Ca(2+)-dependent afterhyperpolarizations. Spike parameters were normal, but the usual time- and voltage-dependent rectification of subthreshold membrane potential was absent. Normally, this rectification results from activation of two voltage-gated currents, the persistent sodium current (INaP) and the hyperpolarizing inward rectifier current (Ih). Both of these currents were absent during voltage clamp with high BAPTA microelectrodes. Impalement with electrodes containing low BAPTA (2 mM) or derivatives caused a different effect. Injection of a 1-s current pulse evoked phasic firing instead of the tonic firing seen normally. Both the amplitude and the duration of the Ca(2+)-dependent afterhypolarization that followed repetitive firing were much greater than normal. The effectiveness of BAPTA derivatives in altering afterhyperpolarizations and firing properties were similar to their effectiveness in chelating Ca2+. It is assumed that the BAPTA effects result from reduction of intracellular Ca2+ concentration. Results with high BAPTA suggest that (i) both INaP and Ih require a minimal intracellular calcium concentration for normal expression, and that (ii) these voltage-gated currents may be modulated by changes in intracellular calcium concentration. Results with low BAPTA suggest that a small reduction of intracellular calcium concentration preferentially enhances a slow, Ca(2+)-dependent K+ current which then dominates the firing properties of the cell. The transformed firing properties resemble those of hippocampal pyramidal neurons.(ABSTRACT TRUNCATED AT 250 WORDS)

Synaptically triggered action potentials begin as a depolarizing ramp in rat hippocampal neurones in vitro

1. During just-suprathreshold synaptic activation of CA1 pyramidal cells in rat hippocampal slices in vitro the action potential begins as a slow depolarizing ramp, superimposed on the underlying EPSP and forming an integral part of the action potential. We call this ramp a synaptic prepotential (SyPP). 2. In order to examine the SyPP, a procedure for subtraction of the underlying EPSP was necessary. Because action potentials were only elicited by a subset of EPSPs with larger than average amplitude, a subtraction of the mean subthreshold EPSP would not give valid results. Instead, an EPSP to be subtracted was selected from an assemblage of subthreshold EPSPs, so that its amplitude matched the initial part of the spike-generating EPSP. 3. Virtually all action potentials started with a SyPP. Using an amplitude criterion of 1 S.D. of the mean of the matching subthreshold EPSPs, just-suprathreshold EPSPs gave prepotentials in 72-100% of all action potentials from fifteen randomly selected cells. With a criterion of 2 S.D.S, the frequency of occurrence ranged from 36 to 100%. 4. With a constant stimulus strength, there was a certain variability of the spike latencies. Shorter latency spikes had steeper, but smaller SyPPs than later spikes, suggesting that the slope of SyPP influenced the timing of the cell discharge. 5. The SyPP was best fitted by a single, exponentially rising curve, and was both smaller and slower than the large amplitude action potential. Its amplitude was 1-6 mV and the time constant 1-5 ms, which was 10-50 times slower than that of the upstroke of the action potential. 6. A properly timed hyperpolarizing current pulse could block the large amplitude action potential, thereby unmasking the SyPP as an initial depolarizing ramp. 7. The SyPP was more sensitive than the large amplitude action potential to intracellular injection of QX-314, a lidocaine derivative. At the concentrations used (10 or 30 mM) no detectable changes were seen in the large amplitude action potential. 8. Droplet application of a specific N-methyl-D-aspartate receptor antagonist, DL-2-amino-5-phosphonovaleric acid (1 mM), reduced both the EPSP and the firing probability, but did not change the SyPP. 9. The SyPP amplitude and time course depended upon the membrane potential at which the cell was activated. Depolarization enhanced and prolonged the SyPP, while hyperpolarization gave opposite effects. In part, the depolarization-induced amplitude increase could be attributed to membrane accommodation. 10. Antidromically evoked action potentials never started with a prepotential.(ABSTRACT TRUNCATED AT 400 WORDS)

Intrinsic membrane potential oscillations in hippocampal neurons in vitro

Membrane potential oscillations (MPOs) of 2-10 Hz and up to 6 mV were found in almost all stable hippocampal CA1 and CA3 neurons in the in vitro slice preparation. MPOs were prominent for pyramidal cells but less pronounced in putative interneurons. MPOs were activated at threshold depolarizations that evoked a spike and the frequency of the MPOs increased with the level of depolarization. MPOs were distinct from and seemed to regulate spiking, with a spike often riding near the top of a depolarizing MPO wave. Analysis of the periodicity of the oscillations indicate that the period of MPOs did not depend on the afterhyperpolarization (AHP) following a single spike. MPOs persisted in low (0-0.1 mM) Ca2+ medium, with or without Cd2+ (0.2 mM), when synaptic transmission was blocked. Choline-substituted low-Na+ (0-26 mM) medium, 3 microM tetrodotoxin (TTX) or intracellular injection of QX-314 reduced or abolished the fast Na(+)-spike and reduced inward anomalous rectification. About 40% of CA1 neurons had no MPOs after Na+ currents were blocked, suggesting that these MPOs were Na(+)-dependent. In about 60% of the cells, a large depolarization activated Ca(2+)-dependent MPOs and slow spikes. MPOs were not critically affected by extracellular Ba2+ or Cs2+, or by 0.2 mM 4-aminopyridine, with or without 2 mM tetraethylammonium (TEA). However, in 5-10 mM TEA medium, MPOs were mostly replaced by 0.2-3 Hz spontaneous bursts of wide-duration spikes followed by large AHPs. Low Ca2+, Cd2+ medium greatly reduced the spike width but not the spike-bursts. In conclusion, each cycle of an MPO in normal medium probably consists of a depolarization phase mediated by Na+ currents, possibly mixed with Ca2+ currents activated at a higher depolarization. The repolarization/hyperpolarization phase may be mediated by Na+/Ca2+ current inactivation and partly by TEA-sensitive, possibly the delayed rectifier, K+ currents. The presence of prominent intrinsic, low-threshold MPOs in all hippocampal pyramidal neurons suggests that MPOs may play an important role in information processing in the hippocampus.

Effects of depolarization and QX-314 injection on slow prepotentials in rat hippocampal pyramidal neurones in vitro

A slow prepotential preceding the action potential elicited by 40-80 ms depolarizing current pulses injected in CA1 pyramidal cells in rat hippocampal slices was isolated and characterized using a subtraction procedure. The exponentially rising slow prepotential showed enhanced amplitude at depolarized membrane potential levels. In sufficient doses, intracellular injection of the lidocaine derivative QX-314 selectively blocked the slow prepotential, leaving the action potential largely unchanged. These results suggest that the slow prepotential might be mediated by a persistent sodium conductance or threshold channels recently found in various nerve cells, and could trigger action potentials in situations with a long-lasting depolarization.

Dendritic excitation by glutamate in CA1 hippocampal cells

In order to reveal properties and effects of glutamate excitation, CA1 pyramidal cells in rat hippocampal slices were impaled and responses to iontophoresis of glutamate onto sensitive spots in the dendrites were analyzed. The glutamate-elicited response consisted of a steady depolarization; its amplitude was dose-dependent. The cellular response to repeated applications of glutamate showed a striking degree of stability. Both dendritic and somatic depolarization, induced by glutamate and current, respectively, elicited similar discharge patterns. The sensitivity to glutamate was highly localized, corresponding to the dendritic tree of a given cell. Short, repeated glutamate pulses did not interfere with an orthodromic test response, whereas longer glutamate ejections often depressed the EPSP. Combined temporal and spatial pairing of glutamate and orthodromic activation was followed by a lasting increase in synaptic efficiency, similar to LTP.

Glutamate, without GABA antagonists, induces synchronized discharges in intact hippocampus via NMDA receptors

In rats under urethane anesthesia, iontophoresis of high amounts of glutamate (50-150 nA) in hippocampus caused repetitive field potentials. These synchronized discharges were best recorded in the proximal part of stratum radiatum as positive waves of 10-15 ms duration and of 0.5-5 mV amplitude. A tetrodotoxin-sensitive faster component of 2-5 ms duration was frequently superimposed on the peaks of the positive waves and was followed by a negative wave of 1-6 mV and 20-30 ms. Glutamate-evoked discharges were suppressed by iontophoresis of N-methyl-D-aspartate (NMDA) antagonists, MK-801, Mg2+ and ketamine and also by ketamine injection (i.v. 5-10 mg/kg). The population spikes evoked by fimbrial stimulation were not facilitated by glutamate and the synchronized discharges were suppressed for up to 300 ms following the stimulation, suggesting the presence of an efficient inhibition during glutamate-induced synchronized activity. Glutamate also had no effect on paired-pulse inhibition. No synchronized discharges were recorded with a second electrode separated more than 150 microns from the iontophoretic electrode, suggesting that the activity was local. These data demonstrate that high amounts of glutamate evoke synchronized discharges in hippocampus, possibly through activation of NMDA receptors. The model presented may be utilized to study the mechanisms of synchronization without disinhibition.

Action potential repolarization and a fast after-hyperpolarization in rat hippocampal pyramidal cells

1. The repolarization of the action potential, and a fast after-hyperpolarization (a.h.p.) were studied in CA1 pyramidal cells (n = 76) in rat hippocampal slices (28-37 degrees C). Single spikes were elicited by brief (1-3 ms) current pulses, at membrane potentials close to rest (-60 to -70 mV). 2. Each action potential was followed by four after-potentials: (a) the fast a.h.p., lasting 2-5 ms; (b) an after-depolarization; (c) a medium a.h.p., (50-100 ms); and (d) a slow a.h.p. (1-2 s). Both the fast a.h.p. and the slow a.h.p. (but not the medium a.h.p.) were inhibited by Ca2+-free medium or Ca2+-channel blockers (Co2+, Mn2+ or Cd2+); but tetraethylammonium (TEA; 0.5-2 nM) blocked only the fast a.h.p., and noradrenaline (2-5 microM) only the slow a.h.p. This suggests that two Ca2+-activated K+ currents were involved: a fast, TEA-sensitive one (IC) underlying the fast a.h.p., and a slow noradrenaline-sensitive one (IAHP) underlying the slow a.h.p. 3. Like the fast a.h.p., spike repolarization seems to depend on a Ca2+-dependent K+ current of the fast, TEA-sensitive kind (IC). The repolarization was slowed by Ca2+-free medium, Co2+, Mn2+, Cd2+, or TEA, but not by noradrenaline. Charybdotoxin (CTX; 30 nM), a scorpion toxin which blocks the large-conductance Ca2+-activated K+ channel in muscle, had a similar effect to TEA. The effects of TEA and Cd2+ (or Mn2+) showed mutual occlusion. Raising the external K+ concentration reduced the fast a.h.p. and slowed the spike repolarization, whereas Cl- loading of the cell was ineffective. 4. The transient K+ current, IA, seems also to contribute to spike repolarization, because: (a) 4-aminopyridine (4-AP; 0.1 mM), which blocks IA, slowed the spike repolarization; (b) depolarizing pre-pulses, which inactivate IA, had a similar effect; (c) hyperpolarizing pre-pulses speeded up the spike repolarization; (d) the effects of 4-AP and pre-pulses persisted during Ca2+ blockade (like IA); and (e) depolarizing pre-pulses reduced the effect of 4-AP. 5. Pre-pulses or 4-AP broadened the spike less, and in a different manner, than Ca2+-free medium, Cd2+, Co2+, Mn2+, TEA or CTX. The former broadening was uniform, with little effect on the fast a.h.p., whereas the latter affected mostly the last two-thirds of the spike repolarization and abolished the fast a.h.p.(ABSTRACT TRUNCATED AT 400 WORDS)

Thresholds of action potentials evoked by synapses on the dendrites of pyramidal cells in the rat hippocampus in vitro

1. In this study we have measured the thresholds of action potentials and some properties of e.p.s.p.s (excitatory post-synaptic potentials) produced by activation of distal and proximal dendritic synapses in CA1 pyramidal cells in rat hippocampal slices. 2. Simultaneous intracellular and extracellular electrophysiological recordings were made from pyramidal cells. The two recording electrodes were positioned very close together to provide an accurate measurement of the transmembrane potential by subtraction of the extracellular field from the intracellular potential. 3. Two pairs of stimulating electrodes were placed in the stratum radiatum of area CA1. The proximal and distal afferents were stimulated alternately within a 15 s cycle and at an intensity to produce action potentials in about 50% of the trials. 4. At this stimulus intensity there was a slight difference in the mean 10-90% rise times of the e.p.s.p.s produced by stimulation of proximal and distal afferents. The ratio of the distal: proximal rise time was 1.27. 5. In those trials where the action potential failed there was also a significant difference in the time-to-peak of the e.p.s.p.s evoked from proximal or distal sites. The times-to-peak of the e.p.s.p.s from proximal stimulation were between 1.5 and 3.8 ms, whilst the times-to-peak from distal stimulation were longer (2.1-7.2 ms). 6. In 16/18 cells included in this study the thresholds for action potentials evoked by distal stimulation were significantly lower than those following proximal activation. The mean threshold for distal activation was 12 mV compared with 16 mV for proximal activation. 7. The threshold of action potentials activated by depolarizing current pulses appeared to be close to that for action potentials evoked from proximal synapses. 8. Whilst the shape indices of the e.p.s.p.s were slightly different when comparing transmembrane with conventional intracellular recordings, the thresholds of the action potentials were not affected by this procedure. 9. Possible explanations for the low threshold for action potentials evoked from distal synapses are discussed, including an active dendritic membrane and differences in inhibition.