[3H]adenosine uptake and release from synaptosomes. Alterations by barbiturates

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.

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

We investigated the contribution of adenosine neuromodulation to mechanisms of pentobarbital-induced depression of excitatory synaptic transmission in vitro. Transverse hippocampal slices were prepared from brains removed from isoflurane-anesthetized male Wistar rats. Field excitatory postsynaptic potentials (fEPSPs), elicited by orthodromic electrical stimulation of Schaffer collateral at 0.05 Hz, were recorded from the CA1 region in oxygenated artificial cerebrospinal fluid. Amplitude of fEPSP was analyzed for assessing drug effects. Pentobarbital (100 microM) transiently depressed fEPSPs (P<0.01); i.e., fEPSP was initially depressed to approximately 60% of control and then recovered to approximately 80% of control. The fEPSP depression was partially suppressed by pretreatment with 50 microM aminophylline, a nonselective adenosine receptor antagonist, and 0.2 microM 3, 7-Dimethyl-1-propagylxanthine, an adenosine A(1) receptor antagonist (P<0.01 each). However, the fEPSP depression was not affected by pretreatment with 10 microM 8-cyclopentyl-1, 3-dipropylxanthine, an A(2) receptor antagonist, or 10 microM bicuculline, a gamma-aminobutyric acid (GABA) A receptor antagonist. The results indicate that adenosine neuromodulation through A(1) receptors and other undefined mechanisms, which are independent from GABAergic mechanisms, are involved in pentobarbital-induced depression of excitatory synaptic transmission.

Hydrogen bonding interaction of diphenylbarbituric acid and 9-ethyladenine. Crystal structure of a 1:1 complex

There are several different types of adenosine receptors found in the brain and molecules capable of interacting with the adenine ring system may have an effect on brain activity. Phenobarbital and diphenylhydantoin, two commonly used antiepileptic drugs have been shown to form hydrogen bonded complexes with adenine derivatives. Although 5,5-diphenylbarbituric acid has anticonvulsant properties equivalent to phenobarbital, neither its structure nor possible interactions with adenine have been investigated. 5,5-Diphenylbarbituric acid and 9-ethyladenine were, therefore, cocrystallized and the three-dimensional 1:1 hydrogen bonded complex was determined by X-ray diffraction. The crystals are monoclinic, C2/c, a = 28.457(9), b = 10.633(3), c = 13.995(4) Å, β = 100.20(2)°, Z = 8, R = 0.067 for 1680 reflections with I > 3σ(I). The molecules propagate along the b crystallographic axis in an alternating fashion in which adjacent adenine and barbiturate molecules interact through a pair of hydrogen bonds. Two-fold symmetry related barbiturate carbonyl groups in neighboring planes of hydrogen bonded ribbons are involved in dipole–dipole attractive interactions. This hydrogen bonding pattern is compared to patterns found in other barbiturate–adenine complexes.Key words: hydrogen bonding, crystal structure, barbiturates, adenine, anticonvulsants.

Effect of acute ethanol on release of endogenous adenosine from rat cerebellar synaptosomes

The effects of pharmacologically relevant concentrations of ethanol on the release of endogenous adenosine from rat cerebellar synaptosomes were investigated. Release was conducted for 5, 10, 30, or 60 s after which time the incubation medium (containing the released adenosine) was rapidly separated from the synaptosomal membranes by vacuum filtration. The adenosine content of the filtrate was measured by HPLC-fluorescence detection. Both basal and KCl-stimulated adenosine release consisted of an initial rapid phase, for the first 10 s, that was followed by a relatively slower phase. Basal endogenous adenosine release was estimated as 199 +/- 14 pmol/mg protein/5 s. Potassium (chloride) increased adenosine release from the basal level to 433 +/- 83 pmol/mg protein/5 s. Ethanol caused a dose-dependent increase of adenosine release. The interaction between dilazep and ethanol indicates that ethanol-stimulated release does not involve the dilazep-sensitive transport system. The results support previous findings that indicate that cerebellar adenosine is involved in the mediation of ethanol-induced motor disturbances in the rat.

Similarities of adenosine uptake systems in astrocytes and neurons in primary cultures

Uptake of extracellular adenosine was studied in primary cultures of astrocytes or neurons. Both cell types showed a high affinity uptake. The Km values were not significantly different (6.5 +/- 3.75 microM in astrocytes and 6.1 +/- 1.86 microM in neurons), but the intensity of the uptake was higher in astrocytes than in neurons (Vmax values of 0.16 +/- 0.030 and 0.105 +/- 0.010 nmol x min-1 x mg-1 protein, respectively). The temperature sensitivity was similar in the two cell types. Adenosine uptake inhibitors and benzodiazepines inhibited the adenosine uptake systems in both astrocytes and neurons with IC50 values in the high nanomolar or the micromolar range and the rank order of potency was similar in the two cell types. In both cell types the (-) isomers of two sets of benzodiazepine stereoisomers were more potent than the (+) isomers. Dixon analysis showed that dipyridamole, papaverine, hexobendine and chlordiazepoxide inhibited the adenosine uptake competitively and clonazepam noncompetitively in both cell types.