Slow Afterdepolarization Research Articles

Cell-Type–Specific Modulation of Intrinsic Firing Properties and Subthreshold Membrane Oscillations by the M(Kv7)-Current in Neurons of the Entorhinal Cortex

The M-current (current through Kv7 channels) is a low-threshold noninactivating potassium current that is suppressed by muscarinic agonists. Recent studies have shown its role in spike burst generation and intrinsic subthreshold theta resonance, both of which are important for memory function. However, little is known about its role in principal cells of the entorhinal cortex (EC). In this study, using whole cell patch recording techniques in a rat EC slice preparation, we have examined the effects of the M-current blockers linopirdine and XE991 on the membrane dynamics of principal cells in the EC. When the M-current was blocked, layer II nonstellate cells (non-SCs) and layer III cells switched from tonic discharge to intermittent firing mode, during which layer II non-SCs showed high-frequency short-duration spike bursts due to increased fast spike afterdepolarization (ADP). When three spikes were elicited at 50 Hz, these two types of cells reacted with a slow ADP that drove delayed firing. In contrast, layer II stellate cells (SCs) and layer V cells never displayed intermittent firing, bursting behavior, or delayed firing. Under the M-current block, intrinsic excitability increased significantly in layer III and layer V cells but not in layer II SCs and non-SCs. The M-current block also had contrasting effects on the subthreshold excitability, greatly suppressing the subthreshold membrane potential oscillations in layer V cells but not in layer II SCs. Modulation of the M-current thus shifts the firing behavior, intrinsic excitability, and subthreshold membrane potential oscillations of EC principal cells in a cell-type-dependent manner.

Dopamine Induces a Slow Afterdepolarization in Lateral Amygdala Neurons

The amygdala and dopaminergic innervation thereunto are considered to cooperatively regulate emotional states and behaviors. The present experiments examined effects of dopamine on lateral amygdala (LA) neuron excitability by whole cell recordings. Bath application of dopamine induced slow afterdepolarization (sADP). This sADP lasted for >5 s, and its magnitude varied in a concentration-dependent manner. Co-application of the D1 receptor antagonist SKF83566 reduced its amplitude. The D1 receptor agonist SKF38393, applied alone, induced sADP of a smaller amplitude. Induction of the full sADP required 5-HT(2) and noradrenalin alpha(1) receptor activation as well. D2 receptor activation or blockade did not affect sADP induction. The calcium channel blocker cadmium or intracellular calcium chelator bis-(o-aminophenoxy)-N,N,N',N' tetraacetic acid (BAPTA) blocked induction of the sADP, which was suggested to be triggered by calcium influx. Under voltage clamp, membrane conductance decreased at the peak of sADP current (I(sADP)). I(sADP) was suppressed by cesium included in pipettes. The I-V curve of the net I(sADP) was shifted as the external concentration of potassium was raised, and the reversal potential was identical to that of potassium, suggesting that dopamine decreases potassium conductance to induce the sADP. The present sADP may serve as a positive-feedback regulator of excitability in LA neurons.

Physiologic Regulation of a Tetrodotoxin-Sensitive Sodium Influx That Mediates a Slow Afterdepolarization Potential in Gonadotropin-Releasing Hormone Neurons: Possible Implications for the Central Regulation of Fertility

The brain controls fertility through release of gonadotropin-releasing hormone (GnRH), but the mechanisms underlying action potential patterning and GnRH release are not understood. We investigated whether GnRH neurons exhibit afterdepolarizing potentials (ADPs) and whether these are modified by reproductive state. Whole-cell current-clamp recordings of GnRH neurons in brain slices from ovariectomized mice revealed a slow ADP (sADP) after action potentials generated by brief current injection. Generating two or four spikes enhanced sADP amplitude and duration. sADP amplitude was not affected by blocking selected neurotransmitter/neuromodulator receptors, delayed-rectifier potassium channels, calcium-dependent cation channels, or hyperpolarization-activated cation channels but was halved by the calcium channel blocker cadmium and abolished by tetrodotoxin. Cadmium also reduced peak latency. Intrinsic mechanisms underlying the sADP were investigated using voltage-clamp protocols simulating action potential waveforms. A single action potential produced an inward current, which increased after double and quadruple stimulation. Cadmium did not affect current amplitude but reduced peak latency. Pretreatment with blockers of calcium-activated potassium currents (I(KCa)) reproduced this shift and blocked subsequent cadmium-induced changes, suggesting cadmium changes latency indirectly by blocking I(KCa). Tetrodotoxin abolished the inward current, suggesting that it is carried by sodium. In contrast, I(KCa) blockers increased the inward current, indicating that I(KCa) may oppose generation of the sADP. Strong sADPs were suprathreshold, generating repetitive spontaneous firing. I(ADP), sADP, and excitability were enhanced by in vivo estradiol, which triggers a preovulatory surge of GnRH release. Physiological feedback modification of this inward current and resulting sADP may modulate action potential firing and subsequent GnRH release.

Gain modulation by serotonin in pyramidal neurones of the rat prefrontal cortex

Serotonin (5-HT) is widely implicated in brain functions and diseases. The vertebrate brain is extensively innervated by 5-HT fibres originating from the brain stem, and 5-HT axon terminals interact with other neurones in complex ways. The cellular mechanisms underlying 5-HT function in the brain are not well understood. The present study examined the effect of 5-HT on the responsiveness of neurones in the neocortex. Using patch-clamp recording in acute slices, we showed that 5-HT substantially increased the slope (gain) of the firing rate-current curve in layer 5 pyramidal neurones of the rat prefrontal cortex. The effect of 5-HT on gain is confined to the range of firing rate (0-10 Hz) that is known to be behaviourally relevant. 5-HT also changed current threshold for spike train generation, but this effect was inconsistent, and was independent of the effect on gain. The gain modulation by 5-HT was mediated by 5-HT2 receptors, and involved postsynaptic mechanisms. 5-HT2-mediated gain increase could not be attributed to changes in the membrane potential, the input resistance or the properties of action potentials, but was associated with a reduction of the afterhyperpolarization and an induction of the slow afterdepolarization. Blocking Ca2+ entry with Cd2+ increased the gain by itself and blocked 5-HT2- mediated gain increase. Buffering [Ca2+](i) with 25 mM EGTA also substantially reduced 5-HT2- mediated gain increase. Noradrenaline, which blocked the afterhyperpolarization, also induced a moderate increase in gain. Together, our results suggest that 5-HT may regulate the dynamics of cortical circuits through multiplicative scaling.

Further characterization of muscarinic agonist-induced epileptiform bursting activity in immature rat piriform cortex, in vitro

The characteristics of muscarinic acetylcholine receptor agonist-induced epileptiform bursting seen in immature rat piriform cortex slices in vitro were further investigated using intracellular recording, with particular focus on its postnatal age-dependence (P+14–P+30), pharmacology, site(s) of origin and the likely contribution of the muscarinic acetylcholine receptor agonist-induced post-stimulus slow afterdepolarization and gap junction functionality toward its generation. The muscarinic agonist, oxotremorine-M (10μM), induced rhythmic bursting only in immature piriform cortex slices; however, paroxysmal depolarizing shift amplitude, burst duration and burst incidence were inversely related to postnatal age. No significant age-dependent changes in neuronal membrane properties or postsynaptic muscarinic responsiveness accounted for this decline. Burst incidence was higher when recorded in anterior and posterior regions of the immature piriform cortex. In adult and immature neurones, oxotremorine-M effects were abolished by M1-, but not M2-muscarinic acetylcholine receptor-selective antagonists. Rostrocaudal lesions, between piriform cortex layers I and II, or layer III and endopiriform nucleus in adult or immature slices did not influence oxotremorine-M effects; however, the slow afterdepolarization in adult (but not immature) lesioned slices was abolished. Gap junction blockers (carbenoxolone or octanol) disrupted muscarinic bursting and diminished the slow afterdepolarization in immature slices, suggesting that gap junction connectivity was important for bursting. Our data show that neural networks within layers II–III function as primary oscillatory circuits for burst initiation in immature rat piriform cortex during persistent muscarinic receptor activation. Furthermore, we propose that muscarinic slow afterdepolarization induction and gap junction communication could contribute towards the increased epileptiform susceptibility of this brain area.

Voltage‐ and Ca2+‐Dependent Burst Generation in Neuroendocrine Dahlgren Cells in the Teleost Platichthys flesus

The neuroendocrine Type 1 Dahlgren cells of the caudal neurosecretory system of the flounder display characteristic bursting activity, which may increase secretion efficiency. The firing activity pattern in these cells was voltage-dependent; when progressively depolarized, cells moved from silent (approximately -70 mV), through bursting and phasic to tonic firing (< -65 mV). Brief (10 s) evoked bursts of spikes were followed by a slow after-depolarization (ADP; amplitude up to 10 mV, duration 10-200 s), which was also voltage-dependent and could trigger a prolonged burst. The ADP was significantly reduced in the absence of external Ca(2+) ions or the presence of the L-type Ca(2+) channel blocker, nifedipine. BayK 8644 (which increases L-type channel open times) significantly increased ADP duration, whereas the Ca(2+)-activated nonselective cation channel blocker, flufenamic acid, had no effect. Pharmacological blockade of Ca(2+)-activated K(+) channels, using apamin and charybdotoxin, increased the duration of both ADP and evoked bursts. However, action potential waveform was unaffected by either apamin/charybdotoxin, nifedipine, BayK 8644 or removal of external Ca(2+). The short duration (approximately 100 ms), hyperpolarization-activated, postspike depolarizing afterpotentials (DAP), were significantly reduced by nifedipine. We propose that long duration ADPs underlie bursts and that short duration DAPs play a role in modulation of spike frequency.

NK-3 receptor activation depolarizes and induces an after-depolarization in pyramidal neurons in gerbil cingulate cortex

The involvement of tachykinins in cortical function is poorly understood. To study the actions of neurokinin-3 (NK3) receptor activation in frontal cortex, whole cell patch clamp recordings were performed from pyramidal neurons in slices of cingulate cortex from juvenile gerbils. Senktide (500 nM), a selective NK3 receptor agonist, induced a transient increase in spontaneous EPSPs in layer V pyramidal neurons, accompanied by a small depolarization (∼4 mV). EPSPs during senktide had a larger amplitude and faster 10–90% rise time than during control. Senktide induced a transient depolarization in layer II/III pyramidal neurons, which often reached threshold for spikes. The depolarization (∼6 mV) persisted in TTX, and was accompanied by an increase in input resistance. Senktide also transiently induced a slow after-depolarization, which appeared following a depolarizing pulse. The slow after-depolarization persisted in TTX. These data suggest that activation of NK3 receptors on layer II/III pyramidal neurons induce post-synaptic depolarization and an after-depolarization, which could be mediated by blockade of a leak potassium conductance and a non-selective cation conductance, respectively.

Investigation of the role of intracellular Ca(2+) stores in generation of the muscarinic agonist-induced slow afterdepolarization (sADP) in guinea-pig olfactory cortical neurones in vitro.

1. Intracellular recordings were made from guinea-pig olfactory cortical brain slice neurones to assess the possible role of intracellular Ca(2+) stores in the generation of the slow post-stimulus afterdepolarization (sADP) and its underlying tail current (I(ADP)), induced by muscarinic receptor activation. 2. Caffeine or theophylline (0.5 - 3 mM) reduced the amplitude of the I(ADP) (measured under 'hybrid' voltage clamp) induced in the presence of the muscarinic agonist oxotremorine-M (OXO-M, 10 microM) by up to 96%, without affecting membrane properties or muscarinic depolarization of these neurones. 3. The L-type Ca(2+) channel blocker nifedipine (1, 10 microM) also inhibited I(ADP) (by up to 46%), while ryanodine (10 microM) (a blocker of Ca(2+) release from internal stores) produced a small ( approximately 10%) reduction in I(ADP) amplitude; however, neither 10 microM dantrolene (another internal Ca(2+) release blocker) nor the intracellular Ca(2+) store re-uptake inhibitors thapsigargin (3 microM) or cyclopiazonic acid (CPA, 15 microM) affected I(ADP) amplitude. 4. IBMX (100 microM), a phosphodiesterase inhibitor, also had no effect on I(ADP). Furthermore, inhibition of I(ADP) by caffeine was not reversed by co-application of 100 microM adenosine. 5. Caffeine (3 mM) or nifedipine (10 microM) reduced the duration of presumed Ca(2+) spikes revealed by intracellular Cs(+) loading. When applied in combination, nifedipine and caffeine effects were occlusive, rather than additive, suggesting a common site of action on L-type calcium channels. 6. We conclude that Ca(2+)-induced Ca(2+) release (CICR) from internal stores does not contribute significantly to muscarinic I(ADP) generation in olfactory cortical neurones. However caffeine and theophylline, which enhance CICR in other systems, blocked I(ADP) induction. We suggest that this action might involve a combination of L-type voltage-gated Ca(2+) channel blockade, and a direct inhibitory action on the putative I(ADP) K(+) conductance.

Intracellular labeling study of neurons in the superficial part of the magnocellular layer of the medullary dorsal horn of the rat.

Morphology and electrical membrane properties of neurons in the superficial part of the magnocellular layer of the rat medullary dorsal horn (MDH: caudal subnucleus of the spinal trigeminal nucleus) were examined by using horizontal slice preparations. Intracellular recording and biocytin-injection combined with histochemical and immunohistochemical staining were done. Twenty-four neurons were examined successfully and classified into projection neurons (PNs) and intrinsic neurons (INs). The PNs were further divided into type I PNs (I-PNs) and type II PNs (II-PNs). The I-PNs sent axons to the medullary reticular formation; the II-PNs sent axons to the interpolar subnucleus of the spinal trigeminal nucleus but had no axons extending to the medullary reticular formation. The INs that sent no axons to the brain regions outside the MDH were also divided into small INs with spiny dendrites (INSSs) and large INs with aspiny dendrites (INLAs). The dendritic fields of the PNs extended to laminae I and II of the MDH and occasionally further to the spinal tract of the trigeminal nerve, whereas those of the INs were confined within the magnocellular layer of the MDH. The axonal branches of each IN formed a dense axonal mesh around the cell body of the parent neuron. Although the main bodies of the axonal fields of the INs were located in the magnocellular layer, some axonal branches extended to laminae I and II of the MDH. Immunoreactivity for NK1 receptor (substance P receptor) was found in approximately half of the PNs but not in the INs. Although no strong correlation was found between morphology and electrical membrane properties, there were some differences in electrical properties among the morphologically classified neuron groups, e.g., hyperpolarizing sag was observed in some PNs but not in the Ins; inward rectification was observed in some of the INSSs and INLAs but not in the PNs; the slow ramp depolarization and the slow afterdepolarization were observed in all INSSs examined but not in the PNs or INLAs.

Substantia gelatinosa neurons in the medullary dorsal horn: An intracellular labeling study in the rat

Morphologic features and electrical membrane properties of neurons in the substantia gelatinosa (SG) of the caudal spinal trigeminal nucleus (the medullary dorsal horn; MDH) were examined in the rat. Intracellular recording and biocytin-injection combined with histochemical staining were performed in horizontal slices. Twenty-four SG (lamina II) neurons were recorded stably and stained successfully. Both projection neurons (PNs; n = 9) that sent axons to regions outside the MDH and intrinsic neurons (INs; n = 15) that sent axons only to the MDH were observed. The INs were divided into those with dense axonal arborization (INDAs; n = 7) and those with sparse axonal arborization (INSAs; n = 8). In the PNs, the dendrites with spines spread to all MDH layers (laminae I–III). The main axons sent collaterals within the SG and rostrally, caudally, or medially to laminae I and III of the MDH, interpolar spinal trigeminal nucleus, spinal tract of the trigeminal nerve, or upper cervical cord segments. In the INDAs, the dendrites arising from the rostral and caudal pole of the cell bodies mainly extended rostrally and caudally parallel to the rostrocaudal axis of the SG: the dendritic trees were elongated and oval in shape and were confined within the SG. The axonal field of each INDA, a dense mesh of axonal processes, was elongated and oval in shape and almost was confined within the SG. In the INSAs, a small, round cell body was located in the center of each dendritic field, which usually was limited within the SG. Axonal processes ran radially to spread to all layers of the MDH, constituting round or oval axonal fields. The three groups of SG neurons showed more or less different intracellular responses to current injections. In particular, adaptation of spike frequency, hyperpolarizing sag, and rebound excitation were observed in the PNs and INSAs but not in the INDAs. Slow ramp depolarization and slow afterdepolarization were recorded only in INDAs. J. Comp. Neurol. 411:399–412, 1999. © 1999 Wiley-Liss, Inc.

Substantia gelatinosa neurons in the medullary dorsal horn: An intracellular labeling study in the rat

Morphologic features and electrical membrane properties of neurons in the substantia gelatinosa (SG) of the caudal spinal trigeminal nucleus (the medullary dorsal horn; MDH) were examined in the rat. Intracellular recording and biocytin-injection combined with histochemical staining were performed in horizontal slices. Twenty-four SG (lamina II) neurons were recorded stably and stained successfully. Both projection neurons (PNs; n = 9) that sent axons to regions outside the MDH and intrinsic neurons (INs; n = 15) that sent axons only to the MDH were observed. The INs were divided into those with dense axonal arborization (INDAs; n = 7) and those with sparse axonal arborization (INSAs; n = 8). In the PNs, the dendrites with spines spread to all MDH layers (laminae I-III). The main axons sent collaterals within the SG and rostrally, caudally, or medially to laminae I and III of the MDH, interpolar spinal trigeminal nucleus, spinal tract of the trigeminal nerve, or upper cervical cord segments. In the INDAs, the dendrites arising from the rostral and caudal pole of the cell bodies mainly extended rostrally and caudally parallel to the rostrocaudal axis of the SG: the dendritic trees were elongated and oval in shape and were confined within the SG. The axonal field of each INDA, a dense mesh of axonal processes, was elongated and oval in shape and almost was confined within the SG. In the INSAs, a small, round cell body was located in the center of each dendritic field, which usually was limited within the SG. Axonal processes ran radially to spread to all layers of the MDH, constituting round or oval axonal fields. The three groups of SG neurons showed more or less different intracellular responses to current injections. In particular, adaptation of spike frequency, hyperpolarizing sag, and rebound excitation were observed in the PNs and INSAs but not in the INDAs. Slow ramp depolarization and slow afterdepolarization were recorded only in INDAs.

Cellular properties of lateral spinal nucleus neurons in the rat L6-S1 spinal cord.

Conventional intracellular recordings were made from 26 lateral spinal nucleus (LSN) neurons in slices of L6-S1 spinal cord from 10- to 15-day-old rats. At rest, LSN neurons did not fire spontaneous action potentials. With injection of a positive current pulse, action potentials had an amplitude of 72 +/- 7 (SD) mV and duration at half-peak height of 0.75 +/- 0.22 ms. Action potentials were followed by an afterpotential. Most LSN neurons (13/17) exhibited only an afterhyperpolarization (AHP); four neurons exhibited both a fast and a slow AHP separated by an afterdepolarization (ADP). For LSN neurons that exhibited only an AHP, a slow ADP could be identified during bath application of apamin (100 nM). Four of 11 LSN neurons showed a postinhibitory rebound (PIR). Two types of PIR were noted, one with high threshold and low amplitude and the other with low threshold and high amplitude. The PIR with high amplitude was partially blocked in 0 mM Ca2+/high Mg2+ (10 mM) recording solution. Repetitive firing properties were examined in 17 LSN neurons. On the basis of the ratio of the slopes between initial instantaneous firing and steady-state firing frequencies, neurons with low spike frequency adaptation (SFA, 8/17) and high SFA (4/17) were identified. In addition, 2/17 LSN neurons exhibited biphasic repetitive firing patterns, which were composed of a fast SFA, delayed excitation, and low SFA; another two neurons showed only delayed excitation. Plateau potentials also were found in two LSN neurons. Dorsal root stimulation revealed that most LSN neurons (12/13) had polysynaptic postsynaptic potentials (PSP); only one neuron exhibited a monosynaptic PSP. Electrical stimulation of the dorsal root evoked prolonged discharges in low SFA neurons and a short discharge in high SFA neurons. Intrinsic properties were modulated by bath application of substance P (SP). Membrane potentials were depolarized in all eight LSN neurons tested, and membrane resistance was either increased (n = 3) or decreased (n = 2). Both instantaneous firing and steady-state firing were facilitated by SP. In addition, oscillation of membrane potentials were induced in three LSN neurons. These results demonstrate that LSN neurons exhibit a variety of intrinsic properties, which may significantly contribute to sensory processing, including nociceptive processing.

Muscarinic receptors regulate two different calcium-dependent non-selective cation currents in rat prefrontal cortex.

Pyramidal neurons of layer V in rat prefrontal cortex display a prominent fast afterdepolarization (fADP) and a muscarinic-induced slow afterdepolarization (sADP). We have shown previously that both of these ADPs are produced by the activation of calcium-dependent non-selective cation currents. In the present report we examine whether they represent two distinct currents. In most pyramidal neurons recorded with caesium gluconate-based intracellular solution, a calcium spike is followed by a fast decaying inward aftercurrent (IfADP). The decay of IfADP is monoexponential with a time constant (t) of approximately 35 ms. Administration of carbachol (10-30 microm) increases the time constant of this decay by approximately 80% and induces the appearance of a much slower inward aftercurrent (IsADP). IfADP recorded in control conditions and in the presence of carbachol increases linearly with membrane hyperpolarization. In contrast, the carbachol-induced IsADP decreases with membrane hyperpolarization. When the sodium driving force across the cell membrane was reduced, IfADP was found to reverse at around -40 mV whereas IsADP remain inward over the same voltage range tested. Finally, bath administration of flufenamic acid (100 microm-1 mm) selectively blocks the carbachol-induced IsADP without a significant effect on the amplitude of IfADP. These differences in the electrical and pharmacological properties of IfADP and IsADP suggest that they were mediated by two distinct non-selective cation currents.

Ionic mechanism of the slow afterdepolarization induced by muscarinic receptor activation in rat prefrontal cortex.

The mammalian prefrontal cortex receives a dense cholinergic innervation from subcortical regions. We previously have shown that cholinergic stimulation of layer V pyramidal neurons of the rat prefrontal cortex results in a depolarization and the appearance of a slow afterdepolarization (sADP). In the current report we examine the mechanism underlying the sADP with the use of sharp microelectrode and whole cell recording techniques in in vitro brain slices. The ability of acetylcholine (ACh) and carbachol to induce the appearance of an sADP in pyramidal cells of layer V of prefrontal cortex is antagonized in a surmountable manner by atropine and is mimicked by application of muscarine or oxotremorine. These results indicate that ACh acts on muscarinic receptors to induce the sADP. In many cell types afterpotentials are triggered by calcium influx into the cell. Therefore we examined the possibility that calcium influx might be the trigger for the generation of the sADP. Consistent with this possibility, buffering intracellular calcium reduced or abolished the sADP but had little effect on the direct muscarinic receptor-induced depolarization also seen in these cells. These results, coupled to the previous observation that calcium channel blockers inhibit the sADP, indicated that the sADP results from a rise in intracellular calcium secondary to calcium influx into the cell. The ionic basis for the current underlying the sADP (IsADP) was examined with the use of ion substitution experiments. The amplitude of IsADP was found to be reduced in a graded fashion by replacement of extracellular sodium with N-methyl-D-glucamine (NMDG). In contrast no clear evidence for the involvement of potassium or chloride channels in the generation of the sADP or IsADP could be found. This result indicated that IsADP is carried by sodium ions flowing into the cell. However, the dependence of IsADP on extracellular sodium was less pronounced than expected for a pure sodium current. We interpret these results to indicate that the sADP is most likely mediated by nonselective cation channels. Examination of the current underlying the sADP at different voltages indicated that this current was also voltage dependent, turning off with hyperpolarization. We conclude that the sADP elicited by muscarinic receptor activation in rat cortex is mediated predominantly by a calcium- and voltage-sensitive nonselective cation current. This current could represent an important mechanism through which ACh can regulate neuronal excitability in prefrontal cortex.

GABAA mediated afterdepolarization in pyramidal neurons from rat neocortex.

We report a novel slow afterdepolarization (sADP) in layer V pyramidal neurons when brain slices from somatosensory cortex are perfused with gamma-aminobutyric acid (GABA). Whole cell recordings were made from visually identified neurons in slices from 3- to 5-wk-old rats. The firing of action potentials at 100 Hz for 1 s, evoked by a train of brief current pulses, typically is followed by a slow afterhyperpolarization (sAHP). When GABA (1 mM) was applied to the perfusate, the sAHP was replaced by a sADP of approximately 18 mV in amplitude, which on average lasted for 26 s. The sADP was not evoked or terminated as an all-or-none event: it grew in amplitude and duration as the number of evoked action potentials was increased; and when the sADP was interrupted with hyperpolarizing current steps, its amplitude and duration were graded in a time- and voltage-dependent manner. The sADP did not depend on Ca2+ entry into the cell: it could be evoked when bath Ca2+ was replaced by Mn2+ or in neurons dialyzed with 20 mM bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid. We hypothesized that the sADP was generated predominantly in the dendrites because it was associated with the firing of small-amplitude action potentials that continued after the somatic membrane potential was repolarized to -70 mV by steady current injection. We tested this hypothesis by evoking the sADP in neurons with surgically amputated apical dendrites. In those neurons, the average duration of the sADP was 78% shorter than in neurons with an intact apical dendrite and there were no associated small action potentials. The sADP also was evoked by muscimol, but not by baclofen, and was blocked by bicuculline or picrotoxin but not by CGP 35348, indicating that it is mediated through the activation of GABAA receptors. Our results suggest that intense activity in the presence of GABA results in a long-lasting enhancement of excitability in the apical dendrite that in turn could lead to amplification of distal excitatory synaptic potentials.

Layer I neurons of rat neocortex. I. Action potential and repetitive firing properties.

1. Whole cell patch-clamp techniques, combined with direct visualization of neurons, were used to study action potential (AP) and repetitive firing properties of layer I neurons in slices of rat neocortex. 2. Layer I neurons had resting membrane potentials (RMP) of -59.8 +/- 4.7 mV (mean +/- SD) and input resistances (RN) of 592 +/- 284 M Omega. Layer II/III pyramidal neurons had RMPs and RNs of -61.5 +/- 5.6 mV and 320 +/- 113 M omega, respectively. A double exponential function was needed to describe the charging curves of both neuron types. In layer I neurons, tau(0) was 45 +/- 22 ms and tau(1) was 5 +/- 3.3 ms whereas in layer II/III pyramidal neurons, tau(0) was 41 +/- 11 ms and tau(1) was 3 +/- 2.6 ms. Estimates of specific membrane resistance (Rm) for layer I and layer II/III cells were 45 +/- 22 and 41 +/- 11 k omega cm2, respectively (Cm was assumed to be 1 microF/cm2). 3. AP threshold was -41 +/- 2 mV in layer I neurons. Spike amplitudes, measured from threshold to peak, were 90.6 +/- 7.7 mV. AP durations, measured both at the base and half maximal amplitude, were 2.5 +/- 0.4 and 1.1 +/- 0.2 ms, respectively. AP 10-90% rise and repolarization times were 0.6 +/- 0.1 and 1.1 +/- 0.2 ms, respectively. In layer II/III pyramidal neurons, AP threshold was -41 +/- 2.5 mV and spike amplitude was 97 +/- 9.7 mV. AP duration at base and half maximal amplitude was 5.4 +/- 1.1 ms and 1.8 +/- 0.2 ms, respectively. AP 10-90% rise and decay times were 0.6 +/- 0.1 ms and 2.8 +/- 0.6 ms, respectively. 4. Layer I neurons were fast spiking cells that showed little frequency adaptation, a large fast afterhyperpolarization (fAHP), and no slow afterhyperpolarization (sAHP). Some cells had a medium afterhyperpolarization (mAHP) and a slow afterdepolarization (sADP). All pyramidal cells in layer II/III and "atypical" pyramidal neurons in upper layer II showed regular spiking behavior, prominent frequency adaptation, and marked sAHPs. 5. In both layer I neurons and layer II/III pyramidal neurons, changes in membrane potential did not greatly alter AP properties. The duration of APs evoked from -50 to -60 mV was only slightly longer, from -80 to -90 mV. The latency to first spike also was not solely dependent on membrane potential. 6. During repetitive firing, APs broadened in both layer I neurons and layer II/III pyramidal neurons. This was most prominent in pyramidal cells. Broadening was dependent on spike frequency and appeared to result from partial inactivation of both outward potassium and inward sodium currents. 7. In layer I neurons, removing Ca2+ from the bathing solution slightly prolonged spike duration and modestly increased AP firing frequency. These results indicate minimal involvement of Ca2+-dependent K+ currents in AP repolarization. fAHPs were reduced whereas sADPs were abolished. In layer II/III pyramidal neurons, removing Ca2+ reduced or blocked mAHPs and sAHPs and decreased or abolished frequency adaptation. 8. Low concentrations (50 microM) of 4-aminopyridine (4-AP) prolonged APs and induced burst-like firing in layer I neurons. In the presence of 4-AP, the spiking behavior of layer I neurons resembled that of regular spiking layer II/III pyramidal cells. At high concentrations (4 mM), 4-AP could induce a delayed depolarization (DD) after each spike in layer I neurons and in a minority of pyramidal neurons. 9. All layer I neurons had a prominent fAHP that was absent or very small in layer II/III pyramidal neurons. fAHP amplitude was inversely related to AP duration. The reduction of fAHPs by 4-AP or during repetitive firing was accompanied by AP prolongation, suggesting that the current underlying fAHP played an essential role in AP repolarization. The fAHP of layer I neurons could be effectively blocked by 4-AP but only slightly reduced by removing Ca2+ from bathing solution, indicating that the fAHP was mediated primarily by a voltage-dependent transient outward current.(ABSTRACT TRUNCATED)

Cholinergic-Dependent Plateau Potential in Hippocampal CA1 Pyramidal Neurons

Cholinergic stimulation of the hippocampal formation results in excitation and/or seizure. We report here, using whole-cell patch-clamp techniques in the hippocampal slice (34-35 degrees C), a cholinergic-dependent slow afterdepolarization (sADP) and long-lasting plateau potential (PP). In the presence of 20 microM carbachol, action potential firing evoked by weak intracellular current injection elicited an sADP that lasted several seconds. Increased spike firing evoked by stronger depolarizing stimuli resulted in long-duration PPs maintained close to -20 mV. Removal of either Na+ or Ca2+ from the external media, intracellular Ca2+ ([Ca2+]i) chelation with 10 mM bis(2-aminophenoxy)ethane-N,N,N',N'-tetra-acetic acid, or the addition of 100 microM Cd2+ to the perfusate abolished both the sADP and PP. The sADP was depressed and the PP was abolished by either 10 microM nimodipine or 1 microM omega-conotoxin, whereas 1.2 microM tetrodotoxin was ineffective. The involvement of a Na+/Ca2+ exchanger was minimal because both the sADP and PP persisted after equimolar substitution of 50 mM Li+ for Na+ in the external media or reduction of the bath temperature to 25 degrees C. Finally in the absence of carbachol the sADP and PP could not be evoked when K+ channels were suppressed, suggesting that depression of K+ conductances alone was not sufficient to unmask the conductance. Based on these data, we propose that a Ca2+-activated nonselective cation conductance was directly enhanced by muscarinic stimulation. The sADP, therefore, represents activation of this conductance by residual [Ca2+]i, whereas the PP represents a novel regenerative event involving the interplay between high-voltage-activated Ca2+ channels and the Ca2+-activated nonselective cation conductance. This latter mechanism may contribute significantly to ictal depolarizations observed during cholinergic-induced seizures.

Serotonin has different effects on two classes of Betz cells from the cat.

1. Intracellular recording from cat Betz cells in vitro revealed a strong correlation between the dominant effect of serotonin (5-HT) and the Betz cell subtype in which it occurred. In large Betz cells that show posthyperpolarization excitation (termed PHE cells), 5-HT evoked a long-lasting membrane depolarization, whereas 5-HT evoked an initial hyperpolarization of variable duration in smaller Betz cells that show posthyperpolorization inhibition (termed PHI cells). 2. Voltage-clamp studies revealed that 5-HT caused a depolarizing shift of activation of the cation current Ih, which resulted in the depolarization in PHE cells, whereas the hyperpolarization in PHI cells is caused by an increase in a resting potassium conductance. 3. The effect of 5-HT on firing properties during constant current stimulation also differed consistently in the two types of Betz cells. In PHE cells the initial firing rate increased after 5-HT application, but the steady firing was unaffected. The depolarizing shift of Ih activation caused the increase of initial firing rate. 4. In PHI cells 5-HT caused a decrease in spike frequency adaptation. The decrease in adaptation was caused by a combination of two conductance changes. First, 5-HT caused a slow afterdepolarization in PHI cells that could trigger repetitive firing in the absence of further stimulation. The sADP depended on calcium entry through voltage-gated channels and was associated with a decrease in membrane conductance. Second, 5-HT caused reduction of a slow calcium-dependent potassium current that normally contributes to slow adaptation. 5. In conclusion, the effect of 5-HT on excitability differs systematically in Betz cell subtypes in part because they have different dominant ionic mechanisms that are modulated. If we assume that PHE cells and PHI cells represent fast and slow pyramidal tract (PT) neurons respectively, 5-HT will cause early recruitment of fast PT cells and delay recruitment of slow PT cells during low levels of synaptic excitation.

Properties and ionic mechanisms of a metabotropic glutamate receptor-mediated slow afterdepolarization in neocortical neurons

1. Pyramidal neurons from layer V of rat neocortex were recorded intracellularly in a brain slice preparation to study their response to stimulation of metabotropic glutamate receptors (mGluRs) by bath application of the selective mGluR agonist (1S,3R)-1-aminocyclopentane-1,3-dicarboxylic acid (1S,3R-ACPD) and by the nonselective agonists glutamate and quisqualate. 2. The principal postsynaptic effect of mGluR stimulation in the presence of ionotropic glutaminergic and muscarinic cholinergic antagonists was the appearance of a slow afterdepolarization (ADP) after evoked spikes. Only an afterhyperpolarization (AHP) was present in control perfusate. After 20 spikes evoked individually at 100 Hz the ADP peaked at 317 +/- 117 (SD) ms after the spike train, ranged from 1 to 12 mV in peak amplitude, and decayed over 7.4 +/- 4.7 s. This effect was not blocked by L-2-amino-3-phosphono-propionic acid (1 mM). Spikes evoked in the presence of the ionotropic glutamate receptor agonist R,S-alpha-amino-3-hydroxy-5-methylisoxazole-4-proprionic acid (AMPA) did not have an ADP. 3. A detectable ADP appeared at concentrations of 0.1 microM quisqualate or 0.5 microM 1S,3R-ACPD. Maximum ADP amplitude was obtained with 5 microM quisqualate or 100 microM 1S,3R-ACPD. The ADP appeared after a single evoked spike in most cells tested and ADP amplitude increased to a maximum as the number of spikes evoked at 100 Hz was increased to between 5 and 20. 4. The ionic mechanisms underlying the ADP were examined by ion substitution and the application of channel-blocking agents. No difference in ADP amplitude was observed when the recording electrode contained CH3SO4. instead of Cl.. The ADP was present after 3 mM extracellular Cs+ were added to block the hyperpolarization-activated cation current or when 100 microM Ba2+ were included to block voltage-gated K+ currents. The ADP was abolished when Mn2+ was substituted for Ca2+ in the perfusate or when the Ca2+ chelator 5,5'-dimethyl-bis-(o-aminophenoxy)-N,N,N',N'-tetraacetic acid was included in the recording electrode. A large ADP followed Ca2+ spikes evoked in the presence of 1 microM tetrodotoxin with 20 mM tetraethylammonium in the perfusate or with Cs+ substituted for K+ in the recording electrode. The amplitude of the ADP after the Ca2+ spikes was reduced by 49% when extracellular Na+ concentration was reduced from 136 to 26 mM. 5. The voltage dependence of the ADP was examined in relation to K+ equilibrium potential (EK).(ABSTRACT TRUNCATED AT 400 WORDS)

A model of the electrophysiological properties of nucleus reticularis thalami neurons

A model of the electrophysiological properties of rodent nucleus reticularis thalami (NRT) neurons of the dorsal lateral thalamus was developed using Hodgkin-Huxley style equations. The model incorporated voltage-dependent rate constants and kinetics obtained from recent voltage-clamp experiments in vitro. The intrinsic electroresponsivity of the model cell was found to be similar to several empirical observations. Three distinct modes of oscillatory activity were identified: 1) a pattern of slow rhythmic burst firing (0.5-7 Hz) usually associated with membrane potentials negative to approximately -70 mV which resulted from the interplay of ITs and IK(Ca); 2) at membrane potentials from approximately -69 to -62 mV, rhythmic burst firing in the spindle frequency range (7-12 Hz) developed and was immediately followed by a tonic tail of single spike firing after several bursts. The initial bursting rhythm resulted from the interaction of ITs and IK(Ca), with a slow after-depolarization due to ICAN which mediated the later tonic firing; 3) with further depolarization of the membrane potential positive to approximately -61 mV, sustained tonic firing appeared in the 10-200-Hz frequency range depending on the amplitude of the injected current. The frequency of this firing was also dependent on the maximum conductance of the leak current, IK(leak), and an interaction between the fast currents involved in generating action potentials, INa(fast) and IK(DR), and the persistent Na+ current, INa(P). Transitions between different firing modes were identified and studied parametrically.