Effects of quinidine and quinine on the excitability of pyramidal neurons in guinea-pig hippocampal slices

Mitochondrial Calcium Signaling as a Therapeutic Target for Alzheimer's Disease

Mitochondria absorb calcium (Ca2+) at the expense of the electrochemical gradient generated during respiration. The influx of Ca2+ into the mitochondrial matrix helps maintain metabolic function and results in increased cytosolic Ca2+ during intracellular Ca2+ signaling. Mitochondrial Ca2+ homeostasis is tightly regulated by proteins located in the inner and outer mitochondrial membranes and by the cross-talk with endoplasmic reticulum Ca2+ signals. Increasing evidence indicates that mitochondrial Ca2+ overload is a pathological phenotype associated with Alzheimer's Disease (AD). As intracellular Ca2+ dysregulation can be observed before the appearance of typical pathological hallmarks of AD, it is believed that mitochondrial Ca2+ overload may also play an important role in AD etiology. The high mitochondrial Ca2+ uptake can easily compromise neuronal functions and exacerbate AD progression by impairing mitochondrial respiration, increasing reactive oxygen species formation and inducing apoptosis. Additionally, mitochondrial Ca2+ overload can damage mitochondrial recycling via mitophagy. This review will discuss the molecular players involved in mitochondrial Ca2+ dysregulation and the pharmacotherapies that target this dysregulation. As most of the current AD therapeutics are based on amyloidopathy, tauopathy, and the cholinergic hypothesis, they achieve only symptomatic relief. Thus, determining how to reestablish mitochondrial Ca2+ homeostasis may aid in the development of novel AD therapeutic interventions.

Channels to consciousness: a possible role of gap junctions in consciousness

The neurophysiological basis of consciousness is still unknown and one of the most challenging questions in the field of neuroscience and related disciplines. We propose that consciousness is characterized by the maintenance of mental representations of internal and external stimuli for the execution of cognitive operations. Consciousness cannot exist without working memory, and it is likely that consciousness and working memory share the same neural substrates. Here, we present a novel psychological and neurophysiological framework that explains the role of consciousness for cognition, adaptive behavior, and everyday life. A hypothetical architecture of consciousness is presented that is organized as a system of operation and storage units named platforms that are controlled by a consciousness center (central executive/online platform). Platforms maintain mental representations or contents, are entrusted with different executive functions, and operate at different levels of consciousness. The model includes conscious-mode central executive/online and mental time travel platforms and semiconscious steady-state and preconscious standby platforms. Mental representations or contents are represented by neural circuits and their support cells (astrocytes, oligodendrocytes, etc.) and become conscious when neural circuits reverberate, that is, fire sequentially and continuously with relative synchronicity. Reverberatory activity in neural circuits may be initiated and maintained by pacemaker cells/neural circuit pulsars, enhanced electronic coupling via gap junctions, and unapposed hemichannel opening. The central executive/online platform controls which mental representations or contents should become conscious by recruiting pacemaker cells/neural network pulsars, the opening of hemichannels, and promoting enhanced neural circuit coupling via gap junctions.

Gap Junction Blockers: An Overview of their Effects on Induced Seizures in Animal Models

BACKGROUND

Gap junctions are clusters of intercellular channels allowing the bidirectional pass of ions directly into the cytoplasm of adjacent cells. Electrical coupling mediated by gap junctions plays a role in the generation of highly synchronized electrical activity. The hypersynchronous neuronal activity is a distinctive characteristic of convulsive events. Therefore, it has been postulated that enhanced gap junctional communication is an underlying mechanism involved in the generation and maintenance of seizures. There are some chemical compounds characterized as gap junction blockers because of their ability to disrupt the gap junctional intercellular communication.

OBJECTIVE

Hence, the aim of this review is to analyze the available data concerning the effects of gap junction blockers specifically in seizure models.

RESULTS

Carbenoxolone, quinine, mefloquine, quinidine, anandamide, oleamide, heptanol, octanol, meclofenamic acid, niflumic acid, flufenamic acid, glycyrrhetinic acid and retinoic acid have all been evaluated on animal seizure models. In vitro, these compounds share anticonvulsant effects typically characterized by the reduction of both amplitude and frequency of the epileptiform activity induced in brain slices. In vivo, gap junction blockers modify the behavioral parameters related to seizures induced by 4-aminopyridine, pentylenetetrazole, pilocarpine, penicillin and maximal electroshock.

CONCLUSION

Although more studies are still required, these molecules could be a promising avenue in the search for new pharmaceutical alternatives for the treatment of epilepsy.

Quinine suppresses extracellular potassium transients and ictal epileptiform activity without decreasing neuronal excitability in vitro

The effect of quinine on pyramidal cell intrinsic properties, extracellular potassium transients, and epileptiform activity was studied in vitro using the rat hippocampal slice preparation. Quinine enhanced excitatory post-synaptic potentials and decreased fast- and slow-inhibitory post-synaptic potentials. Quinine reduced the peak potassium rise following tetanic stimulation but did not affect the potassium clearance rate. Epileptiform activity induced by either low-Ca(2+) or high-K(+) artificial cerebrospinal fluid (ACSF) was suppressed by quinine. The frequency of spontaneous inter-ictal bursting induced by picrotoxin, high-K(+), or 4-aminopyridine was significantly increased. In normal ACSF, quinine did not affect CA1 pyramidal cell resting membrane potential, input resistance, threshold for action potentials triggered by intracellular or extracellular stimulation, or the orthodromic and antidromic evoked population spike amplitude. The main effects of quinine on intrinsic cell properties were to increase action potential duration and to reduce firing frequency during sustained membrane depolarizations, but not at normal resting membrane potentials. This attenuation was enhanced at increasingly depolarized membrane potentials. These results suggest that quinine suppresses extracellular potassium transients and ictal activity and modulates inter-ictal activity by limiting the firing rate of cells in a voltage-dependent manner. Because quinine does not affect 'normal' neuronal function, it may merit consideration as an anticonvulsant.

Pharmacological isolation of the synaptic and nonsynaptic components of the GABA-mediated biphasic response in rat CA1 hippocampal pyramidal cells

High-frequency stimulation (HFS) applied to stratum radiatum of a rat hippocampal slice in the presence of ionotropic glutamate receptor antagonists evokes a biphasic GABA(A) receptor-dependent response in CA1 pyramidal neurons, with a brief hyperpolarizing IPSP (hIPSP) followed by a long-lasting depolarization. We show now that it is possible to pharmacologically separate the hIPSP and late depolarization from one another. In neurons intracellularly perfused for 1-2 hr with F(-) as the major anion and no ATP, the hIPSP (and the corresponding current, hIPSC) evoked by HFS was blocked, whereas neither the late depolarization nor its underlying current was attenuated. In contrast, internal perfusion with a high concentration (5 mM) of the impermeant lidocaine derivative QX-314 selectively abolished the depolarizing component of the biphasic response and also strongly reduced depolarizations evoked by extracellular microinjection of K(+). Bath application of quinine (0. 2-0.5 mM) or quinidine (0.1 mM) resulted in a pronounced inhibition of the HFS-induced extracellular K(+) concentration ([K(+)](o)) transient but not of the bicarbonate-dependent alkaline shift in extracellular pH. The attenuation of the [K(+)](o) transient was closely paralleled by a suppression of the HFS-evoked depolarization but not of the hIPSP. Quini(di)ne did not affect depolarizations induced by exogenous K(+) either. These data provide direct pharmacological evidence for the view that the HFS-induced biphasic response of the pyramidal neuron is composed of mechanistically distinct components: a direct GABA(A) receptor-mediated phase, which is followed by a slow, nonsynaptic [K(+)](o)-mediated depolarization. The bicarbonate-dependent, activity-induced [K(+)](o) transient can be blocked by quini(di)ne, whereas its depolarizing action in the pyramidal neuron is inhibited by internal QX-314. The presence of fundamentally distinct components in GABA(A) receptor-mediated actions evoked by HFS calls for further investigations of their functional role(s) in standard experimental maneuvers, such as those used in studies of synaptic plasticity and induction of gamma oscillations.

Investigation of the presynaptic effects of quinine and quinidine on the release and uptake of monoamines in rat brain tissue

Quinine and quinidine are reported to potentiate the behavioural effects of serotonergic agents and monoamine uptake inhibitors. We have therefore investigated the presynaptic actions of quinine and quinidine on monoamine uptake and release in rat brain tissue in vitro. Quinidine evoked the release of [3H]5-HT, [3H]noradrenaline and [3H]dopamine from pre-loaded rat brain slices in a concentration dependent manner with EC50 values of 175, 486 and 150 microM, respectively. Quinine induced [3H]monoamine release with similar potencies. Both quinine and quinidine also inhibited the active uptake of [3H]5-HT, [3H]noradrenaline and [3H]dopamine into rat brain synaptosomes with IC50 values in the range 0.13-12.4 microM. The potency of each drug to inhibit [3H]5-HT uptake was significantly higher than that for [3H]noradrenaline or [3H]dopamine. The relative potency of quinidine compared to quinine was more marked in the case of [3H]5-HT (58-fold) than for [3H]noradrenaline (3-fold) or [3H]dopamine (4-fold). The inhibition of [3H]5-HT uptake by quinine and quinidine was competitive in nature and corresponded with the potencies of these drugs to inhibit [3H]paroxetine binding. No correlation was observed between the potencies of quinine and quinidine to induce the release of [3H]monoamines and to inhibit their uptake, suggesting that these effects are mediated by two distinct mechanisms. We conclude that the presynaptic actions of quinine and quinidine on monoamine uptake and release may be implicated in their potentiation of the effects of serotonergic agents and uptake blockers.

The effects of rubidium, caesium and quinine on 5-HT-mediated behaviour in rat and mouse--3. Quinine

It has been shown that caesium, which shares properties with quinine as a K(+)-channel blocker, enhanced 5-HT-mediated behaviour in both rats and mice. It was therefore of interest to investigate the effects of quinine on 5-HT-mediated behaviour in the rat and mouse. Quinine, dose-dependently (ED50 = 5 mg/kg), produced the 5-HT behavioural syndrome in rats pre-treated with tranylcypromine (TCP) (15 mg/kg, i.p.). p-Chlorophenylalanine (i.p., 300 mg/kg x2) or (-)-propranolol (20 mg/kg, i.p.), pindolol (4 mg/kg, i.p.) and ritanserin (0.4 mg/kg, s.c.), all prevented the behavioural syndrome induced by quinine (72 mg/kg, i.p.) plus TCP. The administration of quinine (72 mg/kg, i.p.) enhanced the 5-HT syndrome elicited by p-chloramphetamine (4 mg/kg, i.p.) and the 5-HT agonists, 8-OH-DPAT (0.5 mg/kg, s.c.), 5-MeODMT (2 mg/kg, i.p.), DOI (8 mg/kg, s.c.) and quipazine (25 mg/kg, i.p.) in rats. Pretreatment with quinine also potentiated the 5-HT2-mediated head-twitch in the mouse but had no effect on the hypothermia in the mouse, induced by 8-OH-DPAT (0.5 mg/kg, s.c.). Quinine also enhanced the rate of synthesis of 5-HT in the brain of the rat. On the basis of these findings, together with those in the preceding two papers, it is suggested that the effects of rubidium, caesium and quinine, to enhance differentially various aspects of 5-HT function are mediated by actions on 5-HT-modulated K(+)-channels. This conclusion is also discussed in relation to the actions of lithium and electroconvulsive shock on 5-HT function in brain and the treatment of manic-depressive disease.

Whole cell patch clamping of ciliary epithelial cells during anisosmotic swelling

Anisosmotic cell swelling triggers a regulatory volume decrease (RVD) in cell lines derived from human nonpigmented ciliary epithelium. Measurements of cell volume have indicated that the RVD reflects activation of K+ and/or Cl- channels. We have begun to characterize the putative channels by whole cell patch clamping. The results obtained by altering the external K+ and Cl- concentrations and by adding 20-50 microM quinidine or 1 mM Ba2+ indicate that K+ conductances contribute substantially and Cl- conductances contribute very little to the total membrane conductance (GT) under baseline isotonic conditions. Reducing the external osmolality by 20-50% reversibly and reproducibly increased GT by an order of magnitude. Data obtained from ion substitutions and the channel blockers quinidine and 5-nitro-2-(3-phenylpropylamino)-benzoate indicate that most of the hypotonicity-induced conductance reflects stationary Cl(-)-channel activity. The contribution of new K(+)-channel activity was small at intracellular free Ca2+ concentrations of 10 or 200 nM. We conclude that the RVD triggered by bath hypotonicity primarily reflects increased Cl(-)-channel activity.

Effect of quinine on the release of catecholamines from bovine cultured chromaffin cells

1. The effects of quinine on catecholamine release from cultured bovine chromaffin cells were studied. 2. Quinine (25-400 microM) produced a dose-related inhibition of catecholamine release in response to depolarizing concentrations (12.5-50 mM) of K+. 3. The inhibition of the secretory response to high K+ produced by quinine decreased with the increase in the extracellular concentration of Ca2+. 4. Stimulation of cultured chromaffin cells with 50 mM K+ produced a significant increase in Ca2+ influx. In the presence of 100 microM quinine a 54% inhibition of the K(+)-induced Ca2+ influx was observed. 5. Quinine treatment of chromaffin cell cultures produced a small but significant decrease in membrane resting potential and a less pronounced depolarization in response to 50 mM K+. 6. The results suggest that the inhibition of the K(+)-evoked release of catecholamines produced by quinine is at least partly due to a decrease in Ca2+ influx. Ca2+ influx is lower because quinine reduces the sensitivity of the membrane potential to changes in extracellular K+ but direct effects of quinine on Ca2+ channels cannot be excluded.