Effect of an adenosine antagonist and an adenosine agonist on status entry and severity in a model of limbic status epilepticus

A1 not A2A adenosine receptors play a role in cortical epileptic afterdischarges in immature rats

Endo- as well as exogenous adenosine exhibits anticonvulsant action. Participation of individual types of adenosine receptors was studied in present experiments in immature rats. Cortical epileptic afterdischarges were used as a model in rat pups 12, 18 and 25 days old. CCPA, an agonist of A1 adenosine receptors, decreased markedly duration of afterdischarges whereas DPCPX, an antagonist of A1 receptors, exhibited strong proconvulsant action. Action of either drug was best expressed in 12-day-old rats and it decreased with age. Drugs influencing A2A adenosine receptors (agonist CGS21680 and antagonist ZM241385) did not exhibit systematic effects in our model. Motor phenomena accompanying cortical stimulation or epileptic afterdischarge were never influenced by any of the four drugs studied. A1 adenosine receptors are important in the model of cortical seizures, especially in the youngest group studied.

Early caffeine exposure: transient and long-term consequences on brain excitability

The influence of pre- and postnatal caffeine treatment on brain excitability during development and adulthood is reviewed. Pre- and postnatal exposure to caffeine induces sex- and age-specific long-term neurochemical alterations in the brain and the behavior of rodents. Because adenosine neuromodulation is closely related to the regulation of brain excitability the increased expression in adenosine receptor system due to neonatal caffeine treatment should cause transient and permanent changes in seizure susceptibility. So far, findings have been focused on primarily developmental changes of the brain adenosine modulatory system and have demonstrated that the alterations are not restricted to a single brain region. Neurobehavioral changes and the anticonvulsant effect of early caffeine exposure are dependent on the caffeine dose, developmental stage of exposure and age of testing. Although outcomes of caffeine treatment are still a matter of debate, our review raise questions concerning the impact of early caffeine treatment on regulation of seizure susceptibility during development and adulthood.

Anticonvulsant action of 2-chloroadenosine against pentetrazol-induced seizures in immature rats is due to activation of A1 adenosine receptors

Potentiation of adenosinergic inhibitory modulation is one of possible strategies to develop new antiepileptic drugs. Nonspecific receptor agonist 2-chloroadenosine was tested against pentetrazol-induced convulsions in immature (7, 12, 18 and 25 days old) and adult rats. Doses of 1-15 mg/kg i.p. suppressed tonic phase of generalized tonic-clonic seizures (GTCS) in the two youngest groups, whereas GTCS were abolished in older rats. Minimal clonic seizures in 18-day and older rats were suppressed by high doses of 2-chloroadenosine. The role of A1 and A2A adenosine receptors was studied in 12- and 25-day-old rats. Action of an agonist of A1 receptors CCPA is similar to that of 2-chloroadenosine. An agonist of A2A receptors CGS 21680 exhibits an anticonvulsant action only in the dose-inducing catalepsy; an A2A antagonist ZM 241385 moderately suppressed tonic phase of GTCS only in 12-day-old animals. Anticonvulsant action of adenosine agonists is due to their effects on A1 receptors.

Effects of 2-chloroadenosine on cortical epileptic afterdischarges in immature rats

Adenosine may represent an endogenous anticonvulsant in the brain. This study focused on the possible anticonvulsant action of an adenosine agonist, 2-chloroadenosine, against cortical epileptic afterdischarges (ADs) in immature rats. Three age groups of rat pups with implanted electrodes were studied: 12-, 18- and 25-days-old. The compound, 2-chloroadenosine, was injected after the first successful stimulation at doses of 1, 4 or 10 mg/kg intraperitoneally, and stimulation at the same intensity was repeated three more times. Movements directly elicited by stimulation, as well as clonic seizures accompanying electroencephalography (EEG) ADs, were markedly suppressed in only the 18-day-old animals. The effects in the 12- and especially the 25-day-old rats were moderate. The duration of the ADs decreased in all three age groups with 2-chloroadenosine treatment, and the shortest AD duration was seen in the treated, 12-day-old rats. The AD suppression also lasted longer in this age group than it did in the older animals. After a brief suppression of the second AD, the treated, 25-day-old group exhibited a significant AD rebound during the third and fourth stimulations. Taken together, our data show that 2-chloroadenosine exhibits an anticonvulsant effect that is dose- and age-dependent.

Biphasic effect of chronic postnatal caffeine treatment on cortical epileptic afterdischarges during ontogeny in rats

EEG and motor phenomena elicited by stimulation of sensorimotor cortex were used to study the effects of chronic postnatal administration of caffeine (10 and 20 mg/kg, s.c. from P7 to P11) in rats. Rhythmic electrical stimulation was applied to 12-, 18-, 25- and 67-day-old rats with implanted electrodes. Animals with the higher dose of caffeine exhibited increased thresholds for elicitation of stimulation-bound movements, spike-and-wave afterdischarges (ADs) and clonic seizures accompanying these ADs at the age of 12 days and decreased duration of spike-and-wave ADs at postnatal days (P) 18 and 25. In contrast, chronic administration of the lower dose of caffeine resulted in a proconvulsant effect expressed as a significant prolongation of spike-and-wave ADs in P12, P18 and P25 groups as well as of the second "limbic" type of ADs (significant only in P12 and P25). The biphasic action of chronic postnatal caffeine treatment was transient and was no longer present in 67-day-old rats. Our results demonstrate that early postnatal caffeine exposure results in either pro- or anticonvulsant effect during brain maturation in relation to the dose used. Caffeine is a mixed adenosine receptor antagonist, therefore its effects could be due to a different action on adenosine receptor subtypes; an additional mechanism of action cannot be excluded.

Increase of adenine nucleotide hydrolysis in rat hippocampal slices after seizures induced by quinolinic acid

Quinolinic acid (QUIN), an endogenous convulsant compound, overstimulates the glutamatergic system stimulating N-methyl-D-aspartate receptors, enhancing glutamate release and inhibiting glutamate uptake. Glutamate releases the neuroprotector adenosine, which in turn reduces glutamate release and depresses the neuronal activity. Additionally, adenine nucleotides are an important source of adenosine, by action of ecto-nucleotidases. Here we evaluated the adenine nucleotide hydrolysis in hippocampal slices of adult rats in different times after seizures induced by QUIN. After 45 min, there was an increase of ATP and ADP hydrolysis. After 5 h, there was an increase of ATP, ADP and AMP hydrolysis. After 12 h, there was an increase only of ATP hydrolysis. After 24 h, all hydrolysis returned to control levels. As slice preparations maintain tissue integrity, this study indicates, more than previously observed with synaptosomal preparations, that the extracellular production of the neuroprotector adenosine may be involved in brain responses to seizures.

Adenosine receptor agonists: from basic medicinal chemistry to clinical development

Adenosine is a physiological nucleoside which acts as an autocoid and activates G protein-coupled membrane receptors, designated A(1), A(2A), A(2B) and A(3). Adenosine plays an important role in many (patho)physiological conditions in the CNS as well as in peripheral organs and tissues. Adenosine receptors are present on virtually every cell. However, receptor subtype distribution and densities vary greatly. Adenosine itself is used as a therapeutic agent for the treatment of supraventricular paroxysmal tachycardia and arrhythmias and as a vasodilatatory agent in cardiac imaging. During the past 20 years, a number of selective agonists for A(1), A(2A) and A(3) adenosine receptors have been developed, all of them structurally derived from adenosine. Several such compounds are currently undergoing clinical trials for the treatment of cardiovascular diseases (A(1)and A(2A)), pain (A(1)), wound healing (A(2A)), diabetic foot ulcers (A(2A)), colorectal cancer (A(3)) and rheumatoid arthritis (A(3)). Clinical evaluation of some A(1) and A(2A) adenosine receptor agonists has been discontinued. Major problems include side effects due to the wide distribution of adenosine receptors; low brain penetration, which is important for the targeting of CNS diseases; short half-lifes of compounds; or a lack of effects, in some cases perhaps due to receptor desensitisation or to low receptor density in the targeted tissue. Partial agonists, inhibitors of adenosine metabolism (adenosine kinase and deaminase inhibitors) or allosteric activators of adenosine receptors may be advantageous for certain indications, as they may exhibit fewer side effects.

Proconvulsant effect of aminophylline on cortical epileptic afterdischarges varies during ontogeny

Effect of aminophylline on epileptic afterdischarges (ADs) induced repeatedly by rhythmic electrical stimulation of sensorimotor cortical area was studied in rat pups 12, 18 and 25 days old. The proconvulsant effect of aminophylline (50 and/or 100 mg/kg i.p.) was more expressed in 12- and 18-day-old rats than in the oldest group. In 12-day-old rat pups there was an enormous increase of transition of the spike-and-wave type of ADs into the second, limbic type, a situation observed only exceptionally under control conditions. A prolongation of ADs was related to this transition (limbic ADs are always longer than spike-and-wave ones). Eighteen-day-old rats exhibit this transition less frequently but a marked prolongation of spike-and-wave ADs was recorded in a part of these animals forming a pattern of status lasting some tens of minutes. Aminophylline led only to a transient prolongation of spike-and-wave ADs in the oldest group. The transition into the limbic type of ADs was seen in this age group only exceptionally what is in contrast to age-matched controls in which this transition is common. The effect of aminophylline on cortical ADs which is most marked in the youngest group changes qualitatively during postnatal development.

Markedly increased mesiotemporal lobe metabolism in a case with PLEDs: further evidence that PLEDs are a manifestation of partial status epilepticus

The pathophysiologic and clinical significance of periodic lateralized epileptiform discharges (PLEDs) is unclear; whether PLEDs represent an ictal condition that should be treated remains uncertain. We performed FDG-positron emission computed tomography (FDG-PET) in a patient with PLEDs at 3 days, 18 days, and 10 weeks after onset. During left temporal PLEDs, the initial scan showed intense hypermetabolism of the left mesiotemporal region. The second scan, performed when PLEDs were resolving, displayed reduced hypermetabolism. The follow-up scan, when PLEDs had resolved, showed left temporal hypometabolism. These findings, together with clinical evidence from the literature, are compatible with the interpretation that PLEDs represent partial status epilepticus (SE); whether vigorous therapy is required to prevent neuronal damage from this focal seizure activity remains uncertain.