Mechanisms and roles of muscarinic activation in guinea-pig adrenal medullary cells

Muscarinic receptors are expressed in the adrenal medullary (AM) cells of various mammals, but their physiological roles are controversial. Therefore, the ionic mechanism for muscarinic receptor-mediated depolarization and the role of muscarinic receptors in neuronal transmission were investigated in dissociated guinea-pig AM cells and in the perfused guinea-pig adrenal gland. Bath application of muscarine induced an inward current at -60 mV. This inward current was partially suppressed by quinine with an IC(50) of 6.1 μM. The quinine-insensitive component of muscarine-induced currents changed the polarity at -78 mV and was inhibited by bupivacaine, a TWIK-related acid-sensitive K(+) (TASK) channel inhibitor. Conversely, the current-voltage relationship for the bupivacaine-insensitive component of muscarine currents showed a reversal potential of -5 mV and a negative slope below -40 mV. External application of La(3+) had a double action on muscarine currents of both enhancement and suppression. Immunoblotting and immunocytochemistry revealed expression of TASK1 channels and cononical transient receptor potential channels 1, 4, 5, and 7 in guinea-pig AM cells. Retrograde application of atropine reversibly suppressed transsynaptically evoked catecholamine secretion from the adrenal gland. The results indicate that muscarinic receptor stimulation in guinea-pig AM cells induces depolarization through inhibition of TASK channels and activation of nonselective cation channels and that muscarinic receptors are involved in neuronal transmission from the splanchnic nerve.     

Inhibition of Muscarinic-Coupled Phosphoinositide Hydrolysis by N-Methyl-d-Aspartate Is Dependent on Depolarization via Channel Activation

The intent of this work was to elucidate the mechanism by which N-methyl-D-aspartate (NMDA) receptor agonists inhibit a second messenger system, namely, the stimulation of phosphoinositide (PI) hydrolysis activated by muscarinic cholinergic receptor agonists. NMDA inhibited cholinergic stimulation of PI hydrolysis in a dose- and time-dependent manner. NMDA exerts this effect indirectly through channel activation, because both MK-801 and N-[1-(2-thienyl)cyclohexyl]piperidine (TCP) prevented this action. Prevention of the NMDA effect by removal of sodium, but not calcium, from the incubation buffer suggested that depolarization may be the responsible mechanism. Depolarization alone proved sufficient to inhibit cholinergic activation of PI hydrolysis, because both veratridine and an elevated extracellular potassium level inhibited cholinergic stimulation of PI hydrolysis. The effect of NMDA appeared to require sodium flux through NMDA channels rather than through voltage-dependent sodium channels, because tetrodotoxin failed to inhibit the effect of NMDA. In correlative electrophysiologic experiments, NMDA profoundly inhibited evoked excitatory postsynaptic potentials and population action potentials of CA1 neurons, an effect almost certainly due to depolarization. The dose and time course of the electrophysiologic effects correlated well with the biochemical effects. Taken together, the data support the assertion that NMDA receptor activation inhibits PI hydrolysis by depolarization mediated by sodium flux through NMDA channels.     

Modulatory Effects of Perineuronal Oligodendrocytes on Neuronal Activity in the Rat Hippocampus

Action potentials are fundamental to relaying information from region to region in the nervous system. Changes in action potential firing patterns in neural circuits influence how the brain processes information. In our previous study, we focused on interneuron/perineuronal astrocyte pairs in the hippocampal CA1 region and reported that direct depolarization of perineuronal astrocytes modulated the firing pattern of interneurons. In the current study, we investigated the morphological and electrophysiological properties of perineuronal oligodendrocytes, and examined their modulatory effects on interneuronal firing in the CA1 region. Perineuronal oligodendrocytes only had a few processes, which were crooked, intricately twisted, and twined around the soma and proximal region of the main processes of adjacent interneurons. Whole-cell current patterns of perineuronal oligodendrocytes were homogenous and the current–voltage relationship showed remarkable outward rectification. Although the K⁺ channel blockers, tetraethylammonium and 4-aminopyridine, clearly blocked outward currents, Ba²⁺ did not significantly alter whole-cell currents. Unlike perineuronal astrocytes, the depolarization of perineuronal oligodendrocytes had no effect on interneuronal firing; however, when the interneurons were firing at a higher frequency, the hyperpolarization of perineuronal oligodendrocytes suppressed their action potentials. The suppressive effects of perineuronal oligodendrocytes were inhibited in the presence of a low concentration of tetraethylammonium, which selectively blocked deep and fast afterhyperpolarization. These results suggest that perineuronal oligodendrocytes suppress interneuronal firing through their influence on K⁺ channels, which are responsible for deep and fast afterhyperpolarization.     

Augmentation and suppression of action potentials by estradiol in the myometrium of pregnant rat.

The purpose of this study was to investigate the actions of estradiol on spontaneous and evoked action potentials in the isolated longitudinal smooth muscle cells of the pregnant rat. Single cells were obtained by enzymatic digestion from pregnant rat longitudinal myometrium. Action potentials and currents were recorded by whole-cell current-clamp and voltage-clamp methods, respectively. The acute effects of 17beta-estradiol on action potentials and inward and outward currents were investigated. The following results were obtained. The average resting membrane potential of single myometrial cells was -54 mV (n = 40). In many cells, an electrical stimulation evoked a membrane depolarization, and action potentials were superimposed on the depolarization. In some cells, spontaneous action potentials were observed. Estradiol (30 microM) slightly depolarized the membrane (ca. 5 mV) and attenuated the generation of action potentials by reducing the frequency and amplitude of the spikes. Afterhyperpolarization was also attenuated by estradiol (30 microM). On the other hand, in 5 of 35 cells, estradiol increased the first spike amplitude and action potential duration, while frequency of the spike generation and afterhyperpolarization were inhibited. In voltage-clamped muscle cells, estradiol inhibited both inward and outward currents. Acute inhibition or augmentation of spike generation by estradiol is due to the balance of inhibition of inward and outward currents. Inhibition of both currents also prevented afterhyperpolarization, causing potential-dependent block of Ca spikes.     

Cell-type specific depression of neuronal excitability in rat hippocampus by activation of ATP-sensitive potassium channels

The contribution of ATP-sensitive potassium (K(ATP)) channels to neuronal excitability was studied in different types of pyramidal cells and interneurones in hippocampal slices prepared from 9- to 15-day-old rats. The presence of functional K(ATP) channels in the neurones was detected through the sensitivity of whole-cell currents to diazoxide, a K(ATP) channel opener, and to tolbutamide, a K(ATP) channel inhibitor. The percentages of neurones with K(ATP) channels increase in the sequence: CA1 pyramidal cells (37%)相似文献   

Single guard cell recordings in intact plants: light-induced hyperpolarization of the plasma membrane

Guard cells are electrically isolated from other plant cells and therefore offer the unique possibility to conduct current- and voltage-clamp recordings on single cells in an intact plant. Guard cells in their natural environment were impaled with double-barreled electrodes and found to exhibit three physiological states. A minority of cells were classified as far-depolarized cells. These cells exhibited positive membrane potentials and were dominated by the activity of voltage-dependent anion channels. All other cells displayed both outward and inward rectifying K+-channel activity. These cells were either depolarized or hyperpolarized, with average membrane potentials of -41 mV (SD 16) and -112 mV (SD 19), respectively. Depolarized guard cells extrude K+ through outward rectifying channels, while K+ is taken up via inward rectifying channels in hyperpolarized cells. Upon a light/dark transition, guard cells that were hyperpolarized in the light switched to the depolarized state. The depolarization was accompanied by a 35 pA decrease in pump current and an increase in the conductance of inward rectifying channels. Both an increase in pump current and a decrease in the conductance of the inward rectifier were triggered by blue light, while red light was ineffective. From these studies we conclude that light modulates plasma membrane transport through large membrane potential changes, reversing the K+-efflux via outward rectifying channels to a K+-influx via inward rectifying channels.     

Membrane and firing properties of avian medial vestibular nucleus neuronsin vitro

The intrinsic membrane and firing properties of medial vestibular nucleus (MVN) neurons were investigated in slices of the chick brainstem using intracellular recording and current injection. Avian MVN neurons fired spontaneous action potentials with very regular interspike intervals. The rapid repolarization of all action potentials was followed by an after-hyperpolarization. Intracellular injection of steps of hyperpolarizing current revealed both an inward rectification of the membrane potential during the step and a rebound depolarization following the offset of the step. In some neurons, the rebound depolarization resulted in bursts of action potentials. Steps of depolarizing current applied to spontaneously active neurons evoked increases in firing rate that were higher at the onset of the step than during the steady-state response. The relationship between current and firing rate was linear. The membrane and firing properties of avian MVN neurons were distributed continuously across the population of recorded neurons. These properties appear identical to those of rodent MVN neurons, suggesting that the composition and distribution of ion channels in the MVN neuronal membrane has been highly conserved across vertebrate species.Abbreviations MVN medial vestibular nucleus - VOR vestibulo-ocular reflex - AHP after-hyperpolarization     

Inwardly rectifying potassium channels (Kir) in central nervous system glia: a special role for Kir4.1 in glial functions

Glia in the central nervous system (CNS) express diverse inward rectifying potassium channels (Kir). The major function of Kir is in establishing the high potassium (K+) selectivity of the glial cell membrane and strongly negative resting membrane potential (RMP), which are characteristic physiological properties of glia. The classical property of Kir is that K+ flows inwards when the RMP is negative to the equilibrium potential for K+ (E(K)), but at more positive potentials outward currents are inhibited. This provides the driving force for glial uptake of K+ released during neuronal activity, by the processes of "K+ spatial buffering" and "K+ siphoning", considered a key function of astrocytes, the main glial cell type in the CNS. Glia express multiple Kir channel subtypes, which are likely to have distinct functional roles related to their differences in conductance, and sensitivity to intracellular and extracellular factors, including pH, ATP, G-proteins, neurotransmitters and hormones. A feature of CNS glia is their specific expression of the Kir4.1 subtype, which is a major K+ conductance in glial cell membranes and has a key role in setting the glial RMP. It is proposed that Kir4.1 have a primary function in K+ regulation, both as homomeric channels and as heteromeric channels by co-assembly with Kir5.1 and probably Kir2.0 subtypes. Significantly, Kir4.1 are also expressed by oligodendrocytes, the myelin-forming cells of the CNS, and the genetic ablation of Kir4.1 results in severe hypomyelination. Hence, Kir, and in particular Kir4.1, are key regulators of glial functions, which in turn determine neuronal excitability and axonal conduction.     

Nicardipine inhibits axon conduction but causes dual changes of acetylcholine release in the mouse motor nerve

The effects of nicardipine, a dihydropyridine Ca2(+)-channel antagonist, on neuromuscular transmission and impulse-evoked release of acetylcholine were compared with those of nifedipine. In the isolated mouse phrenic nerve diaphragm, nicardipine (50 microM), but not nifedipine (100 microM), induced neuromuscular block, fade of tetanic contraction, and dropout or all-or-none block of end-plate potentials. Nicardipine had no significant effect on the resting membrane potential and the amplitude of miniature end-plate potentials but increased the frequency and caused the appearance of large size miniature potentials. The quantal contents of evoked end-plate potentials were increased. In the presence of tubocurarine, however, nicardipine depressed the amplitude of end-plate potentials. The compound nerve action potential was also decreased. It is concluded that nicardipine blocks neuromuscular transmission by acting on Na+ channels and inhibits axonal conduction. Nicardipine appeared to affect the evoked release of acetylcholine by dual mechanisms, i.e., an enhancement presumably by an agonist action on Ca2+ channels, like Bay K 8644 and nifedipine, and inhibition by an effect on Na+ channels, like verapamil and diltiazem. In contrast with its inactivity on the amplitude of miniature end-plate potentials, depolarization of the end plate in response to succinylcholine was greatly depressed. The contractile response of baby chick biventer cervicis muscle to exogenous acetylcholine was noncompetitively antagonized by nicardipine (10 microM), but was unaffected by nifedipine (30 microM). These results may implicate that nicardipine blocks the postsynaptic acetylcholine receptor channel by enhancing receptor desensitization or by a use-dependent effect.     

Key role of the nicotinic receptor in neurotransmitter exocytosis in human chromaffin cells

The whole-cell secretory response evoked by acetylcholine (ACh) in human chromaffin cells was examined using a new protocol based on quickly switching from the voltage-clamp to the current-clamp (CC) configuration of the patch-clamp technique. Our experiments revealed that Ca(2+) entry through the nicotinic receptor at hyperpolarized membrane potentials contributed as much to the exocytosis (100.4 +/- 27.3 fF) evoked by 200 ms pulses of ACh, as Ca(2+) flux through voltage-dependent Ca(2+) channels at depolarized membrane potentials. The nicotinic current triggered a depolarization event with a peak at +49.3 mV and a 'plateau' phase that ended at -23.9 mV, which was blocked by 10 mumol/L mecamylamine. When a long ACh stimulus (15 s) was applied, the nicotinic current at the end of the pulse reached a value of 15.45 +/- 3.6 pA, but the membrane potential depolarization still remained at the 'plateau' stage until withdrawal of the agonist. Perfusion with 200 mumol/L Cd(2+) during the 15 s ACh pulse completely abolished the plasma membrane depolarization at the end of the pulse, indicating that Ca(2+) entry through Ca(2+) channels contributed to the membrane potential depolarization provoked by prolonged ACh pulses. These findings also reflect that voltage-dependent Ca(2+) channels were recruited by the small current flowing through the desensitized nicotinic receptor to maintain the depolarization. Finally, muscarinic receptor activation triggered a delayed exocytotic process after prolonged ACh stimulation, dependent on Ca(2+) mobilization from the endoplasmic reticulum. In summary, we show here that nicotinic and muscarinic receptors contribute to the exocytosis of neurotransmitters in human chromaffin cells, and that the nicotinic receptor plays a key role in several stages of the stimulus-secretion coupling process in these cells.     

Antidiabetic sulfonylurea stimulates insulin secretion independently of plasma membrane KATP channels

Understanding mechanisms by which glibenclamide stimulates insulin release is important, particularly given recent promising treatment by glibenclamide of permanent neonatal diabetic subjects. Antidiabetic sulfonylureas are thought to stimulate insulin secretion solely by inhibiting their high-affinity ATP-sensitive potassium (K(ATP)) channel receptors at the plasma membrane of beta-cells. This normally occurs during glucose stimulation, where ATP inhibition of plasmalemmal K(ATP) channels leads to voltage activation of L-type calcium channels for rapidly switching on and off calcium influx, governing the duration of insulin secretion. However, growing evidence indicates that sulfonylureas, including glibenclamide, have additional K(ATP) channel receptors within beta-cells at insulin granules. We tested nonpermeabilized beta-cells in mouse islets for glibenclamide-stimulated insulin secretion mediated by granule-localized K(ATP) channels by using conditions that bypass glibenclamide action on plasmalemmal K(ATP) channels. High-potassium stimulation evoked a sustained rise in beta-cell calcium level but a transient rise in insulin secretion. With continued high-potassium depolarization, addition of glibenclamide dramatically enhanced insulin secretion without affecting calcium. These findings support the hypothesis that glibenclamide, or an increased ATP/ADP ratio, stimulates insulin secretion in part by binding at granule-localized K(ATP) channels that functionally contribute to sustained second-phase insulin secretion.     

Identification of two cardioacceleratory neurons in the isopod crustacean, Ligia exotica and their effects on cardiac ganglion cells

We identified two pairs of cardioacceleratory (CA1, CA2) neurons in the central nervous system of the isopod Ligiaexotica and examined their effects on the cardiac ganglion (CG). CA1 neurons had cell bodies in the 2nd thoracic ganglion and had arborizations in the subesophageal ganglion and the 1st and 2nd thoracic ganglia. CA2 neurons had cell bodies in the 3rd thoracic ganglion and had arborizations in the 2nd, 3rd and 4th thoracic ganglia. They sent axons to the heart through the ipsilateral 3rd roots of the ganglia where their cell bodies were located. Repetitive stimulation of the CA1 axon rapidly increased the burst frequency of the CG, and that of CA2 rather slowly. The increased burst rate caused by the CA1 stimulation was significantly higher than that caused by CA2. Overall depolarization of a quiescent CG cell produced by the CA1 stimulation was significantly larger in amplitude than that produced by CA2. Facilitation was obviously seen in the excitatory post-synaptic potentials evoked by the CA1 stimulation. These results show that the synaptic properties of CA1 and CA2 neurons are different, suggesting that they have different functional roles in heart regulation. Accepted: 19 July 1997     

Evidence that a Ca2+ sparks/STOCs coupling mechanism is responsible for the inhibitory effect of caffeine on electro-mechanical coupling in guinea pig ureteric smooth muscle

Recent studies have highlighted the role of the sarcoplasmic reticulum (SR) in controlling excitability, Ca2+ signalling and contractility in smooth muscle. Caffeine, an agonist of ryanodine receptors (RyRs) on the SR has been previously shown to effect Ca2+ signalling but its effects on excitability and contractility are not so clear. We have studied the effects of low concentration of caffeine (1 mM) on Ca2+ signalling, action potential and contractility of guinea pig ureteric smooth muscle. Caffeine produced reversible inhibition of the action potentials, Ca2+ transients and phasic contractions evoked by electrical stimulation. It had no effect on the inward Ca2+ current or Ca2+ transient but increased the amplitude and the frequency of spontaneous transient outward currents (STOCs) in voltage clamped ureteric myocytes, suggesting Ca2+-activated K+ channels (BK) are affected by it. In isolated cells and cells in situ caffeine produced an increase in the frequency and the amplitude of Ca2+ sparks as well the number of spark discharging sites per cell. Inhibition of Ca2+ sparks by ryanodine (50 microM) or SR Ca2+-ATPase (SERCA) cyclopiazonic acid (CPA, 20 microM) or BKCa channels by iberiotoxin (200 nM) or TEA (1 mM), fully reversed the inhibitory effect of caffeine on Ca2+ transients and force evoked by electrical field stimulation (EFS). These data suggest that the inhibitory effect of caffeine on the action potential, Ca2+ transients and force in ureteric smooth muscle is caused by activation of Ca2+ sparks/STOCs coupling mechanism.     

Modulation of inhibition in the hippocampus in vivo

The nature and mechanisms of septohippocampal transmission have been elucidated by taking advantage of an in situ preparation in experiments with Sprague-Dawley rats under urethane. Both extracellular field potentials and intracellular recordings were made in CA1-3 regions of the hippocampus; and the hippocampal commissure and medial septum stimulated to evoke synaptic activity. Using muscarinic and nicotinic agonists and antagonists it was shown that both acetylcholine and medial septal activity can increase the excitability of pyramidal cells, mainly through muscarinic receptors. The effect of septal stimulation was enhanced by local application of physostigmine and reduced by intraventricular injections of hemicholinium. It was also shown that acetylcholine, when applied in the stratum pyramidale, can reduce the voltage and conductance changes observed during evoked inhibitory postsynaptic potentials (IPSP) without affecting the action of gamma-aminobutyric acid on membrane conductance and voltage. It is therefore proposed that acetylcholine can reduce evoked IPSPs through presynaptic inhibition. Evidence is also presented that medial septal stimulation can reduce the efficacy of evoked IPSPs. These observations provide further support for the existence of a cholinergic septohippocampal pathway.     

EPSP amplification and the precision of spike timing in hippocampal neurons

The temporal precision with which EPSPs initiate action potentials in postsynaptic cells determines how activity spreads in neuronal networks. We found that small EPSPs evoked from just subthreshold potentials initiated firing with short latencies in most CA1 hippocampal inhibitory cells, while action potential timing in pyramidal cells was more variable due to plateau potentials that amplified and prolonged EPSPs. Action potential timing apparently depends on the balance of subthreshold intrinsic currents. In interneurons, outward currents dominate responses to somatically injected EPSP waveforms, while inward currents are larger than outward currents close to threshold in pyramidal cells. Suppressing outward potassium currents increases the variability in latency of synaptically induced firing in interneurons. These differences in precision of EPSP-spike coupling in inhibitory and pyramidal cells will enhance inhibitory control of the spread of excitation in the hippocampus.     

A study of the connection between the interneuron initiating pacemaker activity in a bursting neuron and the bursting neuron of the snailHelix pomatia

The connection between an interneuron initiating pacemaker activity in the bursting RPa1 neuron and the bursting neuron itself (Pin and Gola, 1983) has been analyzed in the snail Helix pomatia. Prolonged depolarization of the interneuronal membrane produced in it a series of action potentials as well as a parallel initiation or enhancement of bursting activity in the RPa1 neuron. If the discharge in the interneuron was evoked by short current pulses of threshold amplitude, no bursting activity was seen in the RPa1 neuron. However, short stimuli delivered on the background of subthreshold depolarization of the interneuronal membrane produced bursting activity in the RPa1 neuron. Under voltage-clamp conditions a slow inward current could be recorded in the RPa1 neuronal membrane after stimulation of the interneuron with a latency of about 2 sec. Short shifts of the holding potential in the hyperpolarizing direction at the maximum of this current produced a transient outward current. Replacement of extracellular Ca2+ by Mg2+ ions, as well as addition of 1 mM CdCl2 to the external solution, prevented the response to the interneuronal stimulation in the RPa1 neuron. Electron microscopic investigation of the interneuron has shown the abundance of Golgi complexes in its cytoplasm with electron-dense granules in their vicinity. It is concluded that the connection between the interneuron and the bursting neuron is of chemical origin, based on secretion by the former of some substances which activate at least two types of ionic channels in the membrane of the RPa1 neuron.     

Relationships among seizures, extracellular amino acid changes, and neurodegeneration induced by 4-aminopyridine in rat hippocampus: a microdialysis and electroencephalographic study

4-Aminopyridine is a powerful convulsant that induces the release of neurotransmitters, including glutamate. We report the effect of intrahippocampal administration of 4-aminopyridine at six different concentrations through microdialysis probes on EEG activity and on concentrations of extracellular amino acids and correlate this effect with histological changes in the hippocampus. 4-Aminopyridine induced in a concentration-dependent manner intense and frequent epileptic discharges in both the hippocampus and the cerebral cortex. The three highest concentrations used induced also a dose-dependent enhancement of extracellular glutamate, aspartate, and GABA levels and profound hippocampal damage. Neurodegenerative changes occurred in CA1, CA3, and CA4 subfields, whereas CA2 was spared. In contrast, microdialysis administration of a depolarizing K+ concentration and of tetraethylammonium resulted in increased amino acid levels but no epileptic activity and no or moderate neuronal damage. These results suggest that seizure activity induced by 4-aminopyridine is due to a combined action of excitatory amino acid release and direct stimulation of neuronal firing, whereas neuronal death is related to the increased glutamate release but is independent of seizure activity. In addition, it is concluded that the glutamate release-inducing effect of 4-aminopyridine results in excitotoxicity because it occurs at the level of nerve endings, thus permitting the interaction of glutamate with its postsynaptic receptors, which is probably not the case after K+ depolarization.     

深度麻醉至脑死亡期间大鼠海马神经元活动的突变

全身麻醉若操作不当可能造成致命的中枢神经系统损伤,因此其安全性受到广泛关注.为了揭示麻醉不断加深的过程中神经元活动的变化规律,本文研究了大鼠在乌拉坦(urethane)深度麻醉至脑死亡期间海马区神经元兴奋性和信号传导功能的变化.利用微电极阵列记录和电刺激技术,在海马CA1区胞体层分别记录Schaffer侧支上正向刺激和海马白质上反向刺激诱发的群峰电位(population spike,PS).以PS的幅值和潜伏期为指标,分析海马神经元活动的变化.结果表明,随着乌拉坦血药浓度的增加,PS幅值逐渐减小,潜伏期逐渐延长,意味着乌拉坦抑制了神经元的兴奋性以及轴突传导和突触传递.特别是这些变化存在明显的转折点(即突变),将整个衰减过程分成慢变和快变2个阶段.快变期的剧烈衰减迅速导致脑死亡.而且,引起突变的决定性因素可能是乌拉坦的血药浓度,而非麻醉时间的长短.但是,当乌拉坦注射速率较慢时,延长的慢变期仍然会使神经元功能的受损加重.这些研究结果为动物实验的麻醉操作和临床麻醉的安全应用提供了重要的信息.     

Synergistic release of Ca2+ from IP3-sensitive stores evoked by synaptic activation of mGluRs paired with backpropagating action potentials

Increases in postsynaptic [Ca2+]i can result from Ca2+ entry through ligand-gated channels or voltage-gated Ca2+ channels, or through release from intracellular stores. Most attention has focused on entry through the N-methyl-D-aspartate (NMDA) receptor in causing [Ca2+]i increases since this pathway requires both presynaptic stimulation and postsynaptic depolarization, making it a central component in models of synaptic plasticity. Here, we report that repetitive synaptic activation of metabotropic glutamate receptors (mGluRs), paired with backpropagating action potentials, causes large, wave-like increases in [Ca2+]i predominantly in restricted regions of the proximal apical dendrites and soma of hippocampal CA1 pyramidal neurons. [Ca2+]i changes of several micromolars can be reached by regenerative release caused by the synergistic effect of mGluR-generated inositol 1,4,5-trisphosphate (IP3) and spike-evoked Ca2+ entry acting on the IP3 receptor.