Calcium-activated cation nonselective current contributes to the fast afterdepolarization in rat prefrontal cortex neurons

Functional specificity of G alpha q and G alpha 11 in the cholinergic and glutamatergic modulation of potassium currents and excitability in hippocampal neurons

In hippocampal and other cortical neurons, action potentials are followed by a slow afterhyperpolarization (sAHP) generated by the activation of small-conductance Ca(2+)-activated K(+) channels and controlling spike frequency adaptation. The corresponding current, the apamin-insensitive sI(AHP), is a well known target of modulation by different neurotransmitters, including acetylcholine (via M(3) receptors) and glutamate (via metabotropic glutamate receptor 5, mGluR(5)), in CA1 pyramidal neurons. The actions of muscarinic and mGluR agonists on sI(AHP) involve the activation of pertussis toxin-insensitive G-proteins. However, the pharmacological tools available so far did not permit the identification of the specific G-protein subtypes transducing the effects of M(3) and mGluR(5) on sI(AHP). In the present study, we used mice deficient in the Galpha(q) and Galpha(11) genes to investigate the specific role of these G-protein alpha subunits in the cholinergic and glutamatergic modulation of sI(AHP) in CA1 neurons. In mice lacking Galpha(q), the effects of muscarinic and glutamatergic agonists on sI(AHP) were nearly abolished, whereas beta-adrenergic agonists acting via Galpha(s) were still fully effective. Modulation of sI(AHP) by any of these agonists was instead unchanged in mice lacking Galpha(11). The additional depolarizing effects of muscarinic and glutamatergic agonists on CA1 neurons were preserved in mice lacking Galpha(q) or Galpha(11). Thus, Galpha(q), but not Galpha(11), mediates specifically the action of cholinergic and glutamatergic agonists on sI(AHP), without affecting the modulation of other currents. These results provide to our knowledge one of the first examples of the functional specificity of Galpha(q) and Galpha(11) in central neurons.

Synchronized population oscillation of excitatory synaptic potentials dependent of calcium-induced calcium release in rat neocortex layer II/III neurons

We examined the roles played by calcium-induced calcium release from ryanodine-sensitive calcium stores in induction of neocortical membrane potential oscillation by using caffeine, an agonist of ryanodine receptors. Intracellular recordings were made from neurons in layer II/III of rat visual cortex slices in a caffeine-containing medium. White matter stimulation initially evoked monophasic synaptic potentials. As low-frequency stimulation continued for over 10 min, an oscillating synaptic potential gradually became evoked, in which a paroxysmal depolarization shift was followed by a 8-10-Hz train of several depolarizing wavelets. This oscillating potential was not induced in a medium containing no caffeine with 2 or 0.5 mM [Mg2+](o). Under blockade of N-methyl-D-aspartate receptors, induction of this oscillating potential failed even with caffeine application. Experiments with the calcium store depletor, thapsigargin, revealed that this oscillating potential is induced in a manner dependent on intracellular calcium release. Dual intracellular recordings revealed that the oscillation was synchronized in pairs of layer II/III neurons. The oscillating potential was detectable by field potential recordings also, suggesting that the present oscillation seems to reflect a network property.

Ionic mechanisms underlying burst firing of layer III sensorimotor cortical neurons of the cat: an in vitro slice study

We examined the ionic mechanisms underlying burst firing in layer III neurons from cat sensorimotor cortex by intracellular recording in a brain slice. Regular spiking was observed in 77.4% of 137 neurons in response to constant intracellular current pulses of 0.5- to 1-s duration. The rest of the neurons showed burst firing. An initial burst followed by regular-spike firing was seen in 71.0% of 31 bursting neurons. The rest of the bursting neurons (n = 9) exhibited repetitive bursting. In the bursting neurons, spikes comprising the burst were triggered from the afterdepolarization (ADP) of the first spike of the burst. We examined the ionic mechanisms underlying the ADP by applying channel-blocking agents. The ADP was enhanced (rather than blocked) by Ca2+ channel blockade. This enhancement of the ADP by Ca2+ channel blockade was apparent even after blockade of the afterhyperpolarization by apamin or intracellular Ca2+ chelation by EGTA. The firing rate of the regular-spiking cells was increased by apamin, intracellular EGTA or Ca2+ channel blockers. In 17.9% of the neurons examined (n = 56), these agents switched the regular-spiking pattern into a bursting one. Burst firing could not be changed to regular spiking by these agents. Four neurons that responded with a single initial burst in control solution responded with repetitive bursting after application of these agents. We conclude that the main function of Ca2+ influx in layer III neurons is to activate Ca2+-dependent K+ conductance, which prevents or limits burst firing. At a time when spike amplitude was unchanged, the ADP was blocked and the burst firing changed to regular spiking by extracellularly applied tetrodotoxin (TTX) or intracellularly applied N-(2,6-dimethylphenylcarbamoylmethyl) triethyl ammonium bromide (QX314). We concluded that a TTX- and QX314-sensitive Na+ current underlies the ADP and therefore contributes to the burst firing of layer III neurons from the cat cortex.

Action potential bursting in subicular pyramidal neurons is driven by a calcium tail current

Subiculum is the primary output area of the hippocampus and serves as a key relay center in the process of memory formation and retrieval. A majority of subicular pyramidal neurons communicate via bursts of action potentials, a mode of signaling that may enhance the fidelity of information transfer and synaptic plasticity or contribute to epilepsy when unchecked. In the present study, we show that a Ca(2+) tail current drives bursting in subicular pyramidal neurons. An action potential activates voltage-activated Ca(2+) channels, which deactivate slowly enough during action potential repolarization to produce an afterdepolarization that triggers subsequent action potentials in the burst. The Ca(2+) channels underlying bursting are located primarily near the soma, and the amplitude of Ca(2+) tail currents correlates with the strength of bursting across cells. Multiple channel subtypes contribute to Ca(2+) tail current, but the need for an action potential to produce the slow depolarization suggests a central role for high-voltage-activated Ca(2+) channels in subicular neuron bursting.

Morphological features and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus. An intracellular recording and labeling study in rat brain slices

The morphology and electrophysiological properties of serotonergic and non-serotonergic projection neurons in the dorsal raphe nucleus (DRN) of the rat were examined in frontal brain slices. Biocytin was injected intracellularly into the intracellularly recorded neurons. Then the morphology of the recorded neurons was observed after histochemical visualization of biocytin. The recorded neurons extending their main axons outside the DRN were considered as projection neurons. Subsequently, serotonergic nature of the neurons was examined by serotonin (5-HT) immunohistochemistry. The general form of the dendritic trees is radiant and poorly branching in both 5-HT- and non-5-HT neurons. However, the dendrites of the 5-HT neurons were spiny, whereas those of the non-5-HT neurons were aspiny. The main axons of both 5-HT- and non-5-HT neurons were observed to send richly branching axon collaterals to the DRN, ventrolateral part of the periaqueductal gray and the midbrain tegmentum. In response to weak, long depolarizing current pulses, the 5-HT neurons displayed a slow and regular firing activity. The non-5-HT neurons fired at higher frequencies even when stronger current was injected. Some other differences in electrophysiological properties were also observed between the 5-HT-immunoreactive spiny projection neurons and the 5-HT-immunonegative aspiny projection neurons.

Initiation of network bursts by Ca2+-dependent intrinsic bursting in the rat pilocarpine model of temporal lobe epilepsy

Chronically epileptic rats, produced by prior injection of pilocarpine, were used to investigate whether changes in intrinsic neuronal excitability may contribute to the epileptogenicity of the hippocampus in experimental temporal lobe epilepsy (TLE). Paired extra-/intracellular electrophysiological recordings were made in the CA1 pyramidal layer in acute hippocampal slices prepared from control and epileptic rats and perfused with artificial cerebrospinal fluid (ACSF). Whereas orthodromic activation of CA1 neurons evoked only a single, stimulus-graded population spike in control slices, it produced an all-or-none burst of population spikes in epileptic slices. The intrinsic firing patterns of CA1 pyramidal cells were determined by intrasomatic positive current injection. In control slices, the vast majority (97%) of the neurons were regular firing cells. In epileptic slices, only 53% the pyramidal cells fired in a regular mode. The remaining 47% of the pyramidal cells were intrinsic bursters. These neurons generated high-frequency bursts of three to six spikes in response to threshold depolarizations. A subgroup of these neurons (10.1% of all cells) also burst fired spontaneously even after suppression of synaptic activity. In epileptic slices, burst firing in most cases (ca 70%) was completely blocked by adding the Ca2+ channel blocker Ni2+ (1 mM) to, or removing Ca2+ from, the ACSF, but not by intracellular application of the Ca2+ chelater 1,2-bis(o-aminophenoxy)ethane-N,N,N ',N '-tetra-acetic acid (BAPTA), suggesting it was driven by a Ca2+ current. Spontaneously recurring population bursts were observed in a subset of epileptic slices. They were abolished by adding 2 M 6-cyano-7-nitro-quinoxaline-2,3-dione (CNQX) to the ACSF, indicating that synaptic excitation is critical for the generation of these events. All sampled pyramidal cells fired repetitively during each population burst. The firing of spontaneously active bursters anteceded the population discharge, whereas most other pyramidal cells began to fire conjointly with the first population spike. Thus, spontaneous bursters are likely to be the initiators of spontaneous population bursts in epileptic slices. The dramatic up-regulation of intrinsic bursting in CA1 pyramidal cells, particularly the de novo appearance of Ca2+-dependent bursting, may contribute to the epileptogenicity of the hippocampus in the pilocarpine model of TLE. These findings have important implications for the pharmacological treatment of medically refractory human TLE.

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.

Li+ and muscarine cooperatively enhance the cationic tail current in rat cortical pyramidal cells

Li+ is known to facilitate the onset of status epilepticus induced by cholinergic stimulation, although the underlying mechanisms are not clear. Under whole-cell current clamp conditions with a CsCl-based internal solution, cortical pyramidal cells display a single plateau-spike followed by a slow depolarizing afterpotential (DAP) in response to injection of a short current pulse. However, the same current pulse generated a burst of plateau-spikes after application of Li+ (2 mM) and muscarine (10 microM). As similar bursts of plateau-spikes were generated through an enhancement of the slow DAP when [K+]o was raised (Kang et al. 1998), we have investigated the effects of Li+ and muscarine on the Ca2+-dependent cationic current underlying the slow DAP, measured as the slow tail current evoked after the offset of depolarizing voltage pulses. Muscarine enhanced the amplitudes of both early and late components of the slow tail current. This effect of muscarine was markedly potentiated by Li+, while Li+ by itself affected the slow tail current only slightly. Intracellular application of heparin (0.5-1 mg/mL) suppressed the effect of muscarine in the presence of Li+. These results suggest that inositol-trisphosphate-induced Ca2+ release is involved in the cooperative enhancement of the slow tail current, and this cooperation may be one of the mechanisms underlying facilitation of the onset of epilepsy induced by these agents.

An oscillatory short-term memory buffer model can account for data on the Sternberg task

A limited number (7 +/- 2) of items can be held in human short-term memory (STM). We have previously suggested that observed dual (theta and gamma) oscillations could underlie a multiplexing mechanism that enables a single network to actively store up to seven memories. Here we have asked whether models of this kind can account for the data on the Sternberg task, the most quantitative measurements of memory search available. We have found several variants of the oscillatory search model that account for the quantitative dependence of the reaction time distribution on the number of items (S) held in STM. The models differ on the issues of (1) whether theta frequency varies with S and (2) whether the phase of ongoing oscillations is reset by the probe. Using these models the frequencies of dual oscillations can be derived from psychophysical data. The derived values (ftheta = 6-10 Hz; fgamma = 45-60 Hz) are in reasonable agreement with experimental values. The exhaustive nature of the serial search that has been inferred from psychophysical measurements can be plausibly explained by these oscillatory models. One argument against exhaustive serial search has been the existence of serial position effects. We find that these effects can be explained by short-term repetition priming in the context of serial scanning models. Our results strengthen the case for serial processing and point to experiments that discriminate between variants of the serial scanning process.

Dopamine decreases the excitability of layer V pyramidal cells in the rat prefrontal cortex

In both primates and rodents, the prefrontal cortex (PFC) is highly innervated by dopaminergic fibers originating from the ventral tegmental area, and activation of this mesocortical dopaminergic system decreases spontaneous and evoked activity in the PFC in vivo. We have examined the effects of dopamine (DA), over a range of concentrations, on the passive and active membrane properties of layer V pyramidal cells from the rat medial PFC (mPFC). Whole-cell and perforated-patch recordings were made from neurons in rat mPFC. As a measure of cell excitability, trains of action potentials were evoked with 1-sec-long depolarizing current steps. Bath application of DA (0.05-30 microM) produced a reversible decrease in the number of action potentials evoked by a given current step. In addition, DA reversibly decreased the input resistance (RN) of these cells. In a subset of experiments, a transient increase in excitability was observed after the washout of DA. Control experiments suggest that these results are not attributable to changes in spontaneous synaptic activity, age-dependent processes, or strain-specific differences in dopaminergic innervation and physiology. Pharmacological analyses, using D1 agonists (SKF 38393 and SKF 81297), a D1 antagonist (SCH 23390), a D2 receptor agonist (quinpirole), and a D2 antagonist (sulpiride) suggest that decreases in spiking and RN are mediated by D2 receptor activation. Together, these results demonstrate that DA, over a range of concentrations, has an inhibitory effect on layer V pyramidal neurons in the rat mPFC, possibly through D2 receptor activation.

Regulation of Ca2+-activated nonselective cationic currents in rat pituitary GH3 cells: involvement in L-type Ca2+ current

Ionic currents were investigated by a patch clamp technique in a clonal strain of pituitary (GH3) cells, using the whole cell configuration with Cs+ internal solution. Depolarizing pulses positive to 0 mV from a holding potential of -50 mV activated the voltage-dependent L-type Ca2+ current (ICa,L) and late outward current. Upon repolarization to the holding potential, a slowly decaying inward tail current was also observed. This inward tail current upon repolarization following a depolarizing pulse was found to be enhanced by Bay K 8644, but blocked by nifedipine or tetrandrine. This current was eliminated by Ba2+ replacement of external Ca2+ as the charge carrier through Ca2+ channels, removal of Ca2+ from the bath solution, or buffering intracellular Ca2+ with EGTA (10 mM). The reversal potential of inward tail current was approximately -25 mV. When intracellular Cl- was changed, the reversal potential of the Ca2+-activated currents was not shifted. Thus, this current is elicited by depolarizing pulses that activate ICa,L and allow Ca2+ influx, and is referred to as Ca2+-activated nonselective cationic current (ICAN). Without including EGTA in the patch pipette, the slowly decaying inward current underlying the long-lasting depolarizing potential after Ca2+ spike was also observed with a hybrid current-voltage protocol. Thus, the present studies clearly indicate that Ca2+-activated nonselective cationic channels are expressed in GH3 cells, and can be elicited by the depolarizing stimuli that lead to the activation of ICa,L.

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.