Pentobarbital antagonizes the A1 adenosine receptor-mediated inhibition of hippocampal neurotransmitter release

Detection of barbiturate-induced surface changes in cerebrocortical synaptosomes by phase partitioning in an aqueous two-phase system

The barbiturates tested in this work (barbital, phenobarbital, thiopental and pentobarbital) modify the partition of synaptosomes in a Dextran T500-poly(ethylene glycol) 4000 two-phase system. Under adequate experimental conditions, the drugs increase the partition into the upper phase and this effect appears to be due to an action on the biological material and not on the interface potential of the system. This conclusion can be drawn from the fact that synaptosomes preincubated with low concentrations (0.1 mM) of barbital and pentobarbital maintained an increased partition into the upper phase. The extent of the effect observed appeared to be inversely proportional to the hydrophobicity of the drugs since phenobarbital and barbital showed a higher effect than thiopental and pentobarbital. Dithionite-induced anoxia, rotenone and ouabain also induced a similar increase of partition of synaptosomes into the upper phase, suggesting that the surface changes detected by phase partitioning modification of synaptosomes could be somehow related to the bioenergetic maintenance of the membrane ATPase.

Inhibition of N6-[3H]cyclohexyladenosine binding by carbamazepine

The mechanism of action of carbamazepine (CBZ) (Tegretol), despite widespread use in the management of partial and tonic-clonic seizures in adults, is not completely understood. In animals, adenosine and adenosine analogues have anticonvulsant effects that may be due to interactions with central A1 adenosine receptors. CBZ (at therapeutically relevant concentrations) inhibits the binding of agonists and antagonists to brain A1 adenosine receptors, but whether as an agonist/antagonist is not clear. The adenosine agonist, N6-[3H]cyclohexyladenosine ([3H]CHA), binds to membranes from rat cortex and hippocampus at two nanomolar binding sites or states. To clarify the actions of carbamazepine at the A1 adenosine receptor, its inhibitory actions were compared with those of known adenosine agonists and xanthine antagonists using 0.1 nM[3H]CHA, in which almost all binding is to the higher affinity state, or 10 nM [3H]CHA, in which there is a substantial contribution of binding from both states. The ratios of the IC50 values (concentration that inhibits specific binding by 50%) at 10 nM [3H]CHA to the IC50 values at 0.1 nM [3H]CHA were 18-31 for the agonists and 4-10 for the xanthine antagonists. CBZ had a ratio of 3. The inhibitory effects of GTP on [3H]CHA binding were less in the presence of the adenosine agonist, 2-chloroadenosine than were inhibitory effects in the presence of the xanthine antagonist theophylline or CBZ in both cortex and hippocampus. These in vitro studies indicate that CBZ is an antagonist at A1 adenosine receptors in cerebral cortical and hippocampal membranes from rat brain. Agonist activity at A1 adenosine receptors would have been compatible with the sedative anticonvulsant effects of CBZ, but these data do not support a role of the anticonvulsant action of carbamazepine on A1 adenosine receptors in cerebral cortex or hippocampus.

Endogenous adenosine can reduce epileptiform activity in the human epileptogenic cortex maintained in vitro

The effects induced by adenosine and some related compounds upon Mg2+-free epileptogenesis were studied in slices of human epileptogenic neocortex maintained in vitro. Extracellular recordings revealed stimulus-induced and spontaneous epileptiform activity within 1-2 h of perfusion with Mg2+-free medium. A 30-90% decrease of the frequency of occurrence of spontaneous epileptiform discharges was induced by 40-50 microM adenosine while the analog 2-Cl-adenosine exerted a depressant effect (greater than 75% reduction in frequency of occurrence) at 0.3-3 microM. 2-Cl-adenosine also depressed stimulus-induced epileptiform responses and often blocked spontaneous epileptiform activity. Similar effects were seen during bath application of the adenosine uptake inhibitor nitrobenzylthioinosine (10-50 microM) indicating that endogenous adenosine can by itself influence epileptogenicity. Our data demonstrate that in the human epileptogenic neocortex a purinergic mechanism can control Mg2+-free epileptiform activity.

The effects of anesthetics on the concentrations of cholecystokinin octapeptide sulfate-like immunoreactivity in rat brain regions

Cholecystokinin octapeptide sulfate-like immunoreactivity (CCK-8S-LI) was determined by radioimmunoassay in rat brain areas following injections of pentobarbital, halothane and chloral hydrate. Time-dependent changes in the concentrations of CCK-8S-LI were different between pentobarbital and chloral hydrate in all brain regions studied. After pentobarbital injection, CCK-8S-Li peaked at 30-60 min in the frontal cortex, nucleus accumbens, striatum and substantia nigra; after chloral hydrate injection, CCK-8S-LI peaked at 120 min in the hypothalamus, nucleus accumbens and substantia nigra. Both anesthetics induced almost the same sleeping times. Halothane inhalation caused increases in the concentrations of CCK-8S-LI in the amygdala and hippocampus. In addition, following intracardial perfusion of saline for 30 min after pentobarbital anesthesia, the concentrations of CCK-8S-LI increased in the nucleus accumbens, and decreased in the frontal cortex. These results suggest that since different anesthetics cause different changes in CCK levels, anesthetics affect studies of these neurons.

Upregulation of adenosine A1 receptors and forskolin binding sites following chronic treatment with caffeine or carbamazepine: a quantitative autoradiographic study

The effects of feeding a diet enriched in caffeine or carbamazepine (CBZ) were investigated in rats in a quantitative autoradiographic study of adenosine A1 receptors (labeled by [3H]cyclohexyladenosine, [3H]CHA) and adenylate cyclase (labeled by [3H]forskolin). Although regional distribution of [3H]CHA and [3H]forskolin binding sites differed in some areas, chronic CBZ as well as chronic caffeine upregulated both of them. The changes in receptor densities occurred in the same brain microregions, suggesting that caffeine and CBZ act as antagonists at similar subpopulations of adenosine A1 receptors and [3H]forskolin binding sites. Therefore, a selective interaction of these two drugs with distinct adenosine A1 receptors (and adenylate cyclase) probably does not explain the differential effects of caffeine and CBZ on neuronal activity.

Affinities of barbiturates for the GABA-receptor complex and A1 adenosine receptors: a possible explanation of their excitatory effects

The effects of barbiturates on the GABA-receptor complex and the A1 adenosine receptor were studied. At the GABA-receptor complex the barbiturates inhibited the binding of [35S]t-butylbicyclophosphorothionate ([35S]TBPT) and enhanced the binding of [3H]diazepam. Kinetic and saturation experiments showed that both effects were allosteric. Whereas all barbiturates caused complete inhibition of [35S]TBPT binding, they showed varying degrees of maximal enhancement of [3H]diazepam binding; (+/-)methohexital was identified as the most efficacious compound for this enhancement. At the A1 adenosine receptor all barbiturates inhibited the binding of [3H]N6-phenylisopropyladenosine ([3H]PIA) in a competitive manner. The comparison of the effects on [3H]diazepam and [3H]PIA binding showed that excitatory barbiturates interact preferentially with the A1 adenosine receptor, and sedative/anaesthetic barbiturates with the GABA-receptor complex. It is speculated that the interaction with these two receptors might be the basis of the excitatory versus sedative/anaesthetic properties of barbiturates.