Calcium-independent inhibition of GABA(A) current by caffeine in hippocampal slices

Comparison of the effects of caffeine, aminophylline, and saline on the recovery from total intravenous anesthesia in laparoscopic surgeries: A randomized controlled trial

Background and Aims

The return of consciousness (ROC) after general anesthesia (GA) is by stopping the administration of anesthetic agents. At present, no drug is given to reverse the loss of consciousness produced by general anesthetic agents. This study is conducted to find whether caffeine and aminophylline hasten the ROC.

Material and Methods

This study was conducted on 75 American Society of Anesthesiologists (ASA) I and II female patients undergoing laparoscopic hysterectomy, aged between 18 and 60 years. The patients were divided into three equal groups (Group C: caffeine citrate, Group A: aminophylline, and Group S: saline) of 25 each by a computer-generated random number table. GA was induced with propofol, fentanyl, and maintained with propofol infusion. On completion of the surgery, the neuromuscular blocking agent was reversed and then the infusion of propofol was stopped. The study drug was administered intravenously when the BIS 60 was achieved. Time to achieve BIS 90, return of first gag reflex, eye-opening on verbal command, and extubation after study drug administration were noted. Hemodynamic parameters and SpO2 were also monitored.

Results

The time for BIS 60 to 90 was 10 (4.25) min in the caffeine group, 13 (4.25) min in the aminophylline group, and 26 (9.0) min in the saline group. The time to return of gag reflex and time to extubation were shorter in the caffeine and aminophylline group compared to the saline group. The time to eye-opening on verbal command was shorter in the aminophylline group compared to the saline group. Hemodynamic parameters after infusion of the study drug were comparable in all three groups.

Conclusion

Caffeine hastens the recovery from total intravenous anesthesia with propofol and fentanyl in laparoscopic hysterectomy as effectively as aminophylline.

GABA-enriched teas as neuro-nutraceuticals

Teas enriched in GABA are consumed for their beneficial effects on blood pressure, stress and anxiety. These effects may involve actions of GABA on the central and peripheral nervous systems. The anaerobic procedures for the production of GABA-enriched teas increase GABA levels by 10-20 times. They also significantly alter the levels of other constituents that may interact with the actions of GABA. These include epigallocatechin gallate, caffeine and theanine. The possible interactions of these active constituents make the understanding of the effects of GABA-enriched teas complex. More data is needed to establish where and how GABA is acting following consumption of GABA-enriched teas. While there is considerable evidence that such GABA is acting on GABA receptors in the periphery, there is rather less evidence that is acting directly in the brain. Certainly, there is more to the action of GABA-enriched teas than GABA itself.

The Taste of Caffeine

Many people avidly consume foods and drinks containing caffeine, despite its bitter taste. Here, we review what is known about caffeine as a bitter taste stimulus. Topics include caffeine's action on the canonical bitter taste receptor pathway and caffeine's action on noncanonical receptor-dependent and -independent pathways in taste cells. Two conclusions are that (1) caffeine is a poor prototypical bitter taste stimulus because it acts on bitter taste receptor-independent pathways, and (2) caffeinated products most likely stimulate "taste" receptors in nongustatory cells. This review is relevant for taste researchers, manufacturers of caffeinated products, and caffeine consumers.

The effects of intravenous aminophylline on level of consciousness in acute intentional benzodiazepines poisoning in comparison to flumazenil

AIM

Acute intentional benzodiazepine poisoning is marked by a significant loss of consciousness, aspiration pneumonia, and increased rates of mortality and morbidity, especially in older patients with underlying heart or lung disease. These patients may need flumazenil to reverse the respiratory effects of benzodiazepines. The positive effects of aminophylline on respiration and neonatal apnea improvement have been shown previously. However, its possible effects on increasing the level of consciousness have never been evaluated.

METHODS

In a placebo-controlled study, we assessed the effectiveness of aminophylline on increasing the level of consciousness.

RESULTS

Time to full awakening was significantly shorter in those who received aminophylline (72 min vs. 881 min, p = 0.001), compared to those who received a placebo.

CONCLUSION

When "flumazenil" is contraindicated or unavailable, intravenous aminophylline can be used as a second choice.

Caffeine-Induced Suppression of GABAergic Inhibition and Calcium-Independent Metaplasticity

GABAergic inhibition plays a critical role in the regulation of neuron excitability; thus, it is subject to modulations by many factors. Recent evidence suggests the elevation of intracellular calcium ([Ca²⁺]_i) and calcium-dependent signaling molecules underlie the modulations. Caffeine induces a release of calcium from intracellular stores. We tested whether caffeine modulated GABAergic transmission by increasing [Ca²⁺]_i. A brief local puff-application of caffeine to hippocampal CA1 pyramidal cells transiently suppressed GABAergic inhibitory postsynaptic currents (IPSCs) by 73.2 ± 6.98%. Time course of suppression and the subsequent recovery of IPSCs resembled DSI (depolarization-induced suppression of inhibition), mediated by endogenous cannabinoids that require a [Ca²⁺]_i rise. However, unlike DSI, caffeine-induced suppression of IPSCs (CSI) persisted in the absence of a [Ca²⁺]_i rise. Intracellular applications of BAPTA and ryanodine (which blocks caffeine-induced calcium release from intracellular stores) failed to prevent the generation of CSI. Surprisingly, ruthenium red, an inhibitor of multiple calcium permeable/release channels including those of stores, induced metaplasticity by amplifying the magnitude of CSI independently of calcium. This metaplasticity was accompanied with the generation of a large inward current. Although ionic basis of this inward current is undetermined, the present result demonstrates that caffeine has a robust Ca²⁺-independent inhibitory action on GABAergic inhibition and causes metaplasticity by opening plasma membrane channels.

The effects of Aminophylline on clinical recovery and bispectral index in patients anesthetized with total intravenous anaesthesia

OBJECTIVE

Aminophylline, which is clinically used as a bronchodilator, antagonizes the action of adenosine, so it can be used to shorten the recovery time after general anesthesia. Therefore, we wanted to test the hypothesis that the administration of aminophylline leads to an increase in bispectral index (BIS) and clinical recovery in patients anesthetized with total intravenous anesthesia (TIVA). Methods : Ninety two patients who were scheduled for elective inguinal herniorrhaphy were enrolled in this study. All patients were premedicated with midazolam and morphine. Anesthesia was induced with propofol 2.5 mg /kg and remifentanil 2.5 µg/kg without muscle relaxant. For maintenance of anesthesia we used propofol 100µg/kg/min, remifentanil 0.2µg/kg/min and 100% oxygen with stable BIS readings in the range 40-60. After skin closure, aminophylline 4mg/ kg was given to Group A and an equivalent volume of normal saline to Group P. BIS values, heart rate, blood pressure, oxygen saturation and End tidal CO2(ETco2) were determined. Time to eye opening, extubation time and response to command were measured. Results : There were no significant differences in SpO2, ETco2 and anesthesia time. Heart rate and systolic blood pressure were found to be statistically higher (p<0.001) in Group A. Time to eye opening, hand grip and extubation were significantly shorter (p<0.001) in Group A. Bispectral index scores were significantly higher in group A.

CONCLUSIONS

Injection of aminophylline at emergence time led to significant increase in BIS and shortening recovery time from anesthesia.

The buzz on caffeine in invertebrates: effects on behavior and molecular mechanisms

A number of recent studies from as diverse fields as plant-pollinator interactions, analyses of caffeine as an environmental pollutant, and the ability of caffeine to provide protection against neurodegenerative diseases have generated interest in understanding the actions of caffeine in invertebrates. This review summarizes what is currently known about the effects of caffeine on behavior and its molecular mechanisms in invertebrates. Caffeine appears to have similar effects on locomotion and sleep in both invertebrates and mammals. Furthermore, as in mammals, caffeine appears to have complex effects on learning and memory. However, the underlying mechanisms for these effects may differ between invertebrates and vertebrates. While caffeine's ability to cause release of intracellular calcium stores via ryanodine receptors and its actions as a phosphodiesterase inhibitor have been clearly established in invertebrates, its ability to interact with invertebrate adenosine receptors remains an important open question. Initial studies in insects and mollusks suggest an interaction between caffeine and the dopamine signaling pathway; more work needs to be done to understand the mechanisms by which caffeine influences signaling via biogenic amines. As of yet, little is known about whether other actions of caffeine in vertebrates, such as its effects on GABAA and glycine receptors, are conserved. Furthermore, the pharmacokinetics of caffeine remains to be elucidated. Overall behavioral responses to caffeine appear to be conserved amongst organisms; however, we are just beginning to understand the mechanisms underlying its effects across animal phyla.

The Janus face of caffeine

Caffeine is certainly the psychostimulant substance most consumed worldwide. Over the past years, chronic consumption of caffeine has been associated with prevention of cognitive decline associated to aging and mnemonic deficits of brain disorders. While its preventive effects have been reported extensively, the cognitive enhancer properties of caffeine are relatively under debate. Surprisingly, there are scarce detailed ontogenetic studies focusing on neurochemical parameters related to the effects of caffeine during prenatal and earlier postnatal periods. Furthermore, despite the large number of epidemiological studies, it remains unclear how safe is caffeine consumption during pregnancy and brain development. Thus, the purpose of this article is to review what is currently known about the actions of caffeine intake on neurobehavioral and adenosinergic system during brain development. We also reviewed other neurochemical systems affected by caffeine, but not only during brain development. Besides, some recent epidemiological studies were also outlined with the control of "pregnancy signal" as confounding variable. The idea is to tease out how studies on the impact of caffeine consumption during brain development deserve more attention and further investigation.

Novel N-methylated 8-oxoisoguanines from Pacific sponges with diverse neuroactivities

Marine organisms have yielded a variety of metabolites with neuropharmacological applications. Here we describe the isolation and pharmacological characterization of four novel, neurologically active purines 1-4, isolated from Haplosclerida sponges collected in the Republic of Palau. The structures were determined by analyses of spectral and X-ray data. Compound 1 induced convulsions upon intracerebroventricular injection into mice, with a CD50 value of 2.4 nmol/mouse. Purines 2-4 were active in mouse bioassays at higher doses. The seizurogenic activity of 1 was correlated with inhibition of neuronal GABAergic transmission, with only a modest impact on excitatory signaling, in electrophysiological recordings from hippocampal neurons. Despite having a purine template structure, the inhibitory activity of 1 was not prevented by a nonselective adenosine receptor antagonist. Thus, 1 represents a novel substituted purine that elicits convulsions through its actions on inhibitory neurotransmission. These 8-oxoisoguanine analogs comprise a new family of compounds closely related in structure to endogenous neurosignaling molecules and commonly used CNS stimulants.

The potential of caffeine for functional modification from cortical synapses to neuron networks in the brain

Structure and function of the brain are use-dependent variables based on "synapse plasticity". Since synapses are driven by chemical transmitters, synaptic functions are liable to be modified by extrinsic chemicals displaying affinities for synaptic receptors or modulators. Caffeine is a widely used chemical substance that can invade synapses, and has several biochemical and metabolic actions on synaptic activities. This review focuses on the actions of caffeine on changes in structure and function in the region of the hippocampal formation and neocortex, which exhibit high synapse plasticity. At the synapse level, various synaptic receptors and channel activities are modulated by caffeine via mobilization of intracellular calcium, inhibition of phosphodiesterase, antagonism of adenosine receptors and GABA receptors. These actions of caffeine enable neurons to induce plastic changes in the properties of synaptic activities, such as synaptic transmission efficiency and morphology. At the network level, caffeine has the ability to activate cortical neural oscillators that deliver repetitive N-methyl-D-aspartate receptor-dependent signals to surrounding areas, causing strengthening of long-range inter-cortical communications. Caffeine might thus allow reorganization of cortical network functions via synaptic mobilizations.

The effects of aminophylline on bispectral index during inhalational and total intravenous anaesthesia

Aminophylline is usually used during anaesthesia to treat bronchospasm but recent findings suggest that it can also be used to shorten recovery time after general anaesthesia. However, it is unclear whether aminophylline shows similar properties during a steady-state phase of deep surgical anaesthesia. We therefore wanted to test the hypothesis that the administration of aminophylline leads to an increase in bispectral index as a surrogate parameter suggesting a lighter plane of anaesthesia. The study was designed as a double-blind, randomised, controlled trial with two main groups (aminophylline and placebo) and two subgroups (sevoflurane and propofol). We studied 60 patients. The injection of aminophylline 3 mg x kg(-1) was associated with significant increases in bispectral index up to 10 min after its injection, while heart rate and blood pressure did not change. It appears that aminophylline has the ability to partially antagonise the sedative effects of general anaesthetics.

Action-potential-independent GABAergic tone mediated by nicotinic stimulation of immature striatal miniature synaptic transmission

Stimulation of presynaptic nicotinic acetylcholine receptors (nAChRs) increases the frequency of miniature excitatory synaptic activity (mEPSCs) to a point where they can promote cell firing in hippocampal CA3 neurons. We have evaluated whether nicotine regulation of miniature synaptic activity can be extended to inhibitory transmission onto striatal medium spiny projection neurons (MSNs) in acute brain slices. Bath application of micromolar nicotine typically induced 12-fold increases in the frequency of miniature inhibitory synaptic currents (mIPSCs). Little effect was observed on the amplitude of mIPSCs or mEPSCs under these conditions. Nicotine stimulation of mIPSCs was dependent on entry of extracellular calcium because removal of calcium from perfusate was able to block its action. To assess the potential physiological significance of the nicotine-stimulated increase in mIPSC frequency, we also examined the nicotine effect on evoked IPSCs (eIPSCs). eIPSCs were markedly attenuated by nicotine. This effect could be attributed to two potential mechanisms: transmitter depletion due to extremely high mIPSC rates and/or a reduction in presynaptic excitability associated with nicotinic depolarization. Treatment with low concentrations of K(+) was able to in part mimic nicotine's stimulatory effect on mIPSCs and inhibitory effect on eIPSCs. Current-clamp recordings confirmed a direct depolarizing action of nicotine that could dampen eIPSC activity leading to a switch to striatal inhibitory synaptic transmission mediated by tonic mIPSCs.

Ryanodine Receptor Regulates Endogenous Cannabinoid Mobilization in the Hippocampus

Endogenous cannabinoids (eCBs) are produced and mobilized in a cytosolic calcium ([Ca2+]i)–dependent manner, and they regulate excitatory and inhibitory neurotransmitter release by acting as retrograde messengers. An indirect but real-time bioassay for this process on GABAergic transmission is DSI (depolarization-induced suppression of inhibition). The magnitude of DSI correlates linearly with depolarization-induced increase of [Ca2+]ithat is thought to be initiated by Ca2+influx through voltage-gated Ca2+channels. However, the identity of Ca2+sources involved in eCB mobilization in DSI remains undetermined. Here we show that, in CA1 pyramidal cells, DSI-inducing depolarizing voltage steps caused Ca2+-induced Ca2+release (CICR) by activating the ryanodine receptor (RyR) Ca2+-release channel. CICR was reduced, and the remaining increase in [Ca2+]iwas less effective in generating DSI, when the RyR antagonists, ryanodine or ruthenium red, were applied intracellularly, or the Ca2+stores were depleted by the Ca2+-ATPase inhibitors, cyclopiazonic acid or thapsigargin. The CICR-dependent effects were most prominent in cultured or immature acute slices, but were also detectable in slices from adult tissue. Thus we suggest that voltage-gated Ca2+entry raises local [Ca2+]isufficiently to activate nearby RyRs and that the resulting CICR plays a critical role in initiating eCB mobilization. RyR may be a key molecule for the depolarization-induced production of eCBs that inhibit GABA release in the hippocampus.

Modulation of GABAergic transmission by activity via postsynaptic Ca2+-dependent regulation of KCC2 function

Activity-induced modification of GABAergic transmission contributes to the plasticity of neural circuits. In the present work we found that prolonged postsynaptic spiking of hippocampal neurons led to a shift in the reversal potential of GABA-induced Cl- currents (E(Cl)) toward positive levels in a duration- and frequency-dependent manner. This effect was abolished by blocking cytosolic Ca2+ elevation and mimicked by releasing Ca2+ from internal stores. Activity- and Ca2+-induced E(Cl) shifts were larger in mature neurons, which express the K-Cl cotransporter KCC2 at high levels, and inhibition of KCC2 occluded the shifts. Overexpression of KCC2 in young cultured neurons, which express lower levels of KCC2 and have a more positive E(Cl), resulted in hyperpolarized E(Cl) similar to that of mature cells. Importantly, these young KCC2-expressing neurons became responsive to neuronal spiking and Ca2+ elevation by showing positive E(Cl) shifts. Thus, repetitive postsynaptic spiking reduces the inhibitory action of GABA through a Ca2+-dependent downregulation of KCC2 function.

Direct block of human ether-a-go-go-related gene potassium channels by caffeine

The human ether-a-go-go-related gene (hERG) potassium channel is expressed in a variety of cell types, including neurons, tumor cells, and cardiac myocytes. In the heart, it is important for repolarization of the cardiac action potential. Attenuation of hERG current can cause long QT syndrome and cardiac arrhythmias such as torsades de pointes. Caffeine is frequently used as a pharmacological tool to study calcium-dependent transduction pathways in cellular preparations. It raises cytosolic calcium by opening ryanodine receptors and may also inhibit phosphodiesterases to increase cytosolic cAMP. In this study, we show 5 mM caffeine rapidly and reversibly attenuates hERG currents expressed in human embryonic kidney 293 cells to 61.1 +/- 2.2% of control. Caffeine-dependent inhibition of hERG current is not altered by raising cAMP with forskolin, buffering cytosolic calcium with 1,2-bis(2-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid, or inhibition of protein kinase C. Thus, the effects of caffeine are unlikely to be mediated by cAMP or intracellular calcium-dependent mechanisms. Further experiments showed caffeine directly blocks hERG in an open state-dependent manner. Furthermore, caffeine inhibition is greatly reduced by the pore mutants Y562A and F656A hERG, which disrupt block of most previously tested hERG antagonists. Thus, caffeine attenuates hERG currents by binding to a drug receptor located within the inner cavity of the channel. Dietary intake of caffeine is unlikely to cause long QT syndrome because plasma concentrations do not reach sufficiently high levels to significantly inhibit hERG currents. However, the effects of caffeine have implications for its use in examining calcium-dependent pathways in cellular preparations expressing hERG.