The involvement of adenosine neuromodulation in pentobarbital-induced field excitatory postsynaptic potentials depression in rat hippocampal slices

Aminophylline and Ephedrine, but Not Flumazenil, Inhibit the Activity of the Excitatory Amino Acid Transporter 3 Expressed in Xenopus Oocytes and Reverse the Increased Activity by Propofol

We investigated the effects of flumazenil, aminophylline, and ephedrine on the excitatory amino acid transporter type 3 (EAAT3) activity and the interaction with propofol. EAAT3 was expressed in the Xenopus oocytes. L-Glutamate-induced membrane currents were measured using the two-electrode voltage clamp at various drug concentrations. Oocytes were preincubated with protein kinase C- (PKC-) activator, or inhibitor, and phosphatidylinositol 3-kinase (PI3K) inhibitor. To study the interaction with propofol, oocytes were exposed to propofol, propofol + aminophylline, or ephedrine. Aminophylline and ephedrine significantly decreased EAAT3 activity. Aminophylline (95 μM) and ephedrine (1.19 μM) significantly decreased Vmax, but not Km of EAAT3, for glutamate. The phorbol 12-myristate-13-acetate-induced increase in EAAT3 activity was abolished by aminophylline or ephedrine. The decreased EAAT3 activities by PKC inhibitors (staurosporine, chelerythrine) and PI3K inhibitor (wortmannin) were not significantly different from those by aminophylline or ephedrine, as well as those by PKC inhibitors or PI3K inhibitor + aminophylline or ephedrine. The enhanced EAAT3 activities induced by propofol were significantly abolished by aminophylline or ephedrine. Aminophylline and ephedrine inhibit EAAT3 activity via PKC and PI3K pathways and abolish the increased EAAT3 activity by propofol. Our results indicate a novel site of action for aminophylline and ephedrine.

General anesthetics as a factor of effective neuroprotection in ischemic stroke models

Stroke is the second leading cause of death in the world. Unfortunately, only a few drugs have been proved in clinical trials. Drug development of the last decade has been focused substantially on a promising and heterogeneous group of neuroprotective drugs. Hundreds of compounds were suggested as new putative neuroprotectors, which effectiveness was confirmed in preclinical trials only. At the present time discrepancy between results of preclinical studies and clinical trials requires careful analysis. One of the least evaluated and probably the most noticeable reasons is general anesthesia - an obligatory component of an overwhelming majority of existing animal stroke models. The aim of the review is to describe known mechanisms of common general anesthetics influence on ionotropic and metabotropic plasma membrane receptors, and key signal pathways involved in neuronal hypoxic-ischemic injury and survival

Aminophylline (a theophylline-ethylenediamine complex) blocks ethanol behavioral effects in mice

Aminophylline is a complex of theophylline-ethylenediamine, where theophylline is the main component. Theophylline is a methyxanthine and besides inhibiting phosphodiesterase enzymes, it is also a nonselective adenosine antagonist. Several reports suggested the involvement of the brain adenosinergic system in the ethanol-induced motor incoordination. Thus, the objective of this work was to study the effects of the interaction of ethanol with aminophylline as assessed by behavioral tests in mice. Eight groups of male Swiss mice were used. The animals were treated with either distilled water (control) or ethanol (E; 2, 4, and 6 g/kg, orally) for 5 days, or with distilled water for 4 days, and on the fifth day with aminophylline (A; 5 and 10 mg/kg, intraperitoneally). In the association groups (association protocols), the animals were treated with ethanol (E; 6 g/kg, orally) for 4 days, and on the fifth day received aminophylline (A; 10 mg/kg, intraperitoneally), 30 min after the last ethanol administration (first protocol, E/A). In the second association protocol (A/E), ethanol was administered for 4 days, and on the fifth day the animals received aminophylline (A; 10 mg/kg, intraperitoneally), followed again by ethanol (E; 6 g/kg, orally) administration, 30 min later. E (6 g/kg) evoked a central nervous system depressor effect, by decreasing both the locomotor activity and rearing in the open field test, and A (5 and 10 mg/kg) showed opposite effects. However, the E/A or A/E associations blocked the ethanol effect. In the rota rod test, ethanol presented a muscular relaxant effect, which was decreased in both association protocols. In the tail suspension test, while the E/A association decreased immobility, A/E association increased it, as compared with controls. In conclusion, the effects of ethanol were inhibited by its association with aminophylline, suggesting that ethanol acts on the adenosine neurotransmission.

The anticonvulsant action of propofol on epileptiform activity in rat hippocampal slices

We used extracellular electrophysiological recordings from the CA1 region in rat hippocampal slices to investigate the effects of propofol on the field excitatory postsynaptic potential (fEPSP), population spike, and epileptiform activity induced by a Mg(2+)-free condition. Propofol depressed the population spike, fEPSP, and epileptiform activity. Both aminophylline, a nonselective adenosine receptor antagonist, and 8-cyclopentyl-1,3-dipropylxanthine, an A(1) receptor antagonist, significantly reduced the effect of propofol on fEPSP amplitude. However, 3,7-dimethyl-1-propagylxanthine, an A(2) receptor antagonist, did not alter the effect of propofol on fEPSP amplitude. Picrotoxin, a specific chloride channel blocker, partly reduced the effect of propofol on epileptiform activity, but bicuculline, a competitive gamma-aminobutyric acid(A) receptor antagonist, failed to antagonize it. Aminophylline significantly reduced the action of propofol on the epileptiform activity. The anticonvulsant action of propofol was partly reduced by 8-cyclopentyl-1,3-dipropylxanthine, whereas 3,7-dimethyl-1-propagylxanthine failed to affect it. Adenosine depressed the amplitude of fEPSPs in a dose-dependent manner, and propofol enhanced this inhibition. The results demonstrated that, in rat hippocampal slices, propofol inhibits epileptiform activity. In addition, adenosine neuromodulation through the A(1) receptor may contribute to the anticonvulsant action of propofol.

Pentobarbitone modulates calcium transients in axons and synaptic boutons of hippocampal CA1 neurons

Although barbiturates, like other general anaesthetics, depress excitatory synaptic transmission in the central nervous system (CNS), the underlying cellular mechanisms remain unresolved. They may increase the likelihood that an action potential will fail to invade every branch of the axonal arbour, thereby decreasing the synaptic drive to the postsynaptic neurons. Alternatively, they may inhibit calcium entry into the presynaptic terminals, thus reducing transmitter release. To resolve these issues, we have used two-photon microscopy to monitor calcium transients evoked by action potentials in axons, axonal varicosities (synaptic boutons) and fine axon collaterals of hippocampal CA1 neurons. Pentobarbitone (75-300 microM) did not block the invasion of the axonal arbour or the synaptic boutons, but it did reduce the amplitude of the calcium transients recorded from the axons in a concentration-dependent manner. At 150 microM, pentobarbitone reduced the transients to 78+/-4% of the control. Pentobarbitone depressed the calcium transients recorded from the synaptic boutons in a concentration-dependent manner. When 150 microM pentobarbitone was applied, the calcium transients recorded from the boutons were 53+/-3% of the control. This concentration of pentobarbitone also reduced the amplitude and frequency of the spontaneous excitatory postsynaptic potentials to 54+/-4 and 42+/-17% of the control, respectively. The local anaesthetic procaine (500 microM) had no significant effect on action potential invasion of axon collaterals, even though it reduced the action potential amplitude by 25%. This data are consistent with the notion that the pentobarbitone-induced depression of presynaptic calcium transients contributes to its depressant effect on excitatory synaptic transmission in the CNS.

NMDA receptor-mediated mechanism of ketamine-induced facilitation of glutamatergic excitatory synaptic transmission

The effect of ketamine on CA1-field EPSPs (fEPSPs) in rat hippocampal slices was investigated. Ketamine (100 microM) facilitated fEPSPs at 0.05 Hz. The fEPSP facilitation was suppressed completely by AP-5 and partially by propranolol, and also by an increase in stimulation frequency. These results indicate that ketamine facilitates excitatory synaptic transmission by activating NMDA receptors via beta-adrenoceptors under conditions in which NMDA receptor channel block is slight.

Small-dose pentobarbital enhances synaptic transmission in rat hippocampus

We investigated the contribution of bicarbonate ion, gamma-aminobutyric acid-A (GABA(A)) receptors, and N-methyl-D-aspartate (NMDA) receptors to pentobarbital-induced enhancement of excitatory synaptic transmission in the hippocampal slice. Transverse hippocampal slices (400 microm thick) were prepared from 20- to 30-day-old Sprague-Dawley rats and maintained in an interface chamber perfused with warmed (35 degrees C) oxygenated artificial cerebrospinal fluid. Extracellular field potentials, evoked by orthodromic paired-pulse stimulation of the Schaffer collateral CA1 pathway, were analyzed for the population spike (PS) amplitude. Pentobarbital had a concentration-dependent, biphasic effect on PS amplitudes, which were increased approximately twofold (P < 0.001) when the slice was exposed to pentobarbital concentrations of 1 and 5 microM and depressed at drug concentrations larger than 10 microM. Pentobarbital (5 microM) did not increase the PS amplitude when stimulation was stopped during exposure to the drug. The enhancement of PS amplitude was suppressed in the presence of 10 microM acetazolamide, a nonselective carbonic anhydrase inhibitor, and when the slice was bathed in CO(2)/HCO(3)(-)-free artificial cerebrospinal fluid. Pretreatment with 1 microM picrotoxin, a GABA(A) receptor antagonist, or 5 microM 2-amino-5-phosphopentanoic acid, a specific NMDA receptor antagonist, also suppressed enhancement of PS amplitude by 5 microM pentobarbital. The results suggest that small concentrations of pentobarbital (1 and 5 microM) enhance synaptic transmission through mechanisms involving GABA(A) and NMDA receptors and the HCO(3)(-) ion.

IMPLICATIONS

Enhanced hippocampal synaptic transmission after exposure to subanesthetic concentrations of pentobarbital persists during drug washout. This finding may help to explain why some patients experience excitation and enhanced pain during emergence from anesthesia.