Adenosine acting via A1 receptors, controls the transition to status epilepticus-like behaviour in an in vitro model of epilepsy

Delayed ischemic electrocortical suppression during rapid repeated cerebral ischemia and kainate-induced seizures in rat

Global cerebral ischemia induces, within seconds, suppression of spontaneous electrocortical activity, partly due to alterations in synaptic transmission. In vitro studies have found that repeated brief hypoxic episodes prolong the persistence of synaptic transmission due to weakened adenosine release. The aim of this study was to investigate in vivo whether the time to ischemic electrocortical suppression (T(ES)) could be altered during energy stress conditions such as rapid repeated global cerebral ischemia and kainate-induced seizures. Experiments were carried out in adult rats under chloral hydrate anaesthesia. Repeated episodes of 1 min of ischemia were induced by transiently clamping the carotid arteries in a 'four-vessel occlusion' model. We devised an automatic method of T(ES) estimation based on the decay of the root mean square of two-channel electrocorticographic recordings. To distinguish the alterations in spontaneous electrocortical activity we compared T(ES) with the ischemic suppression of visual evoked potentials (VEP). During the first ischemic episode, T(ES) was approximately 15 s and remained unchanged when five ischemic episodes were separated by 10-min reperfusion intervals. When ischemia was repeated after 2 min of reperfusion T(ES) progressively increased, reaching a plateau value of approximately 24 s. A similar plateau was reached during kainate-induced seizures. The T(ES) plateau occurred prior to ischemic suppression of VEP. Our data suggest that, under conditions of acute metabolic stress in vivo, the ischemic suppression of spontaneous electrocortical activity may be delayed up to a plateau value. These findings are consistent with the hypothesis of a depletable adenosine pool; however, the restoration of synaptic transmission may be faster in vivo than in vitro.

Adenosine A1 receptors are crucial in keeping an epileptic focus localized

Adenosine is an endogenous neuromodulator with anticonvulsant and neuroprotective properties presumably mediated by activation of adenosine A1 receptors (A1Rs). To study the involvement of A1Rs in neuroprotection during epileptogenesis, we induced status epilepticus by a unilateral intrahippocampal kainic acid (KA) injection (1 nmol) in wild-type C57BL/6 and homozygous adenosine A1R knock out (A1R-KO) mice of the same genetic background. Whereas the KA injection caused non-convulsive status epilepticus in wild-type mice, in A1R-KO mice KA induced status epilepticus with severe convulsions and subsequent death of the animals within 5 days. 24 h after KA injection, brains from wild-type C57BL/6 mice were characterized by slight neuronal cell loss confined to the immediate location of the KA injection. In contrast, KA-injected A1R-KO mice displayed massive neuronal cell loss in the ipsilateral hippocampus, and, importantly, the contralateral hippocampus was also affected with significant cell loss in the hilus and in the CA1 region of the pyramidal cell layer. We conclude that activation of A1 receptors by ambient adenosine is crucial in keeping epileptic foci localized. These results open up a new dimension of the A1 receptor's role in controlling excitotoxic cell death and further demonstrate its importance in preventing the progression of status epilepticus to lethal consequences.

Adenosine A1 receptor knockout mice develop lethal status epilepticus after experimental traumatic brain injury

Adenosine, acting at A1 receptors, exhibits anticonvulsant effects in experimental epilepsy--and inhibits progression to status epilepticus (SE). Seizures after traumatic brain injury (TBI) may contribute to pathophysiology. Thus, we hypothesized that endogenous adenosine, acting via A1 receptors, mediates antiepileptic benefit after experimental TBI. We subjected A1-receptor knockout (ko) mice, heterozygotes, and wild-type (wt) littermates (n=115) to controlled cortical impact (CCI). We used four outcome protocols in male mice: (1) observation for seizures, SE, and mortality in the initial 2 h, (2) assessment of seizure score (electroencephalogram (EEG)) in the initial 2 h, (3) assessment of mortality at 24 h across injury levels, and (4) serial assessment of arterial blood pressure, heart rate, blood gases, and hematocrit. Lastly, to assess the influence of gender on this observation, we observed female mice for seizures, SE, and mortality in the initial 2 h. Seizure activity was noted in 83% of male ko mice in the initial 2 h, but was seen in no heterozygotes and only 33% of wt (P<0.05). Seizures in wt were brief (1 to 2 secs). In contrast, SE involving lethal sustained (>1 h) tonic clonic activity was uniquely seen in ko mice after CCI (50% incidence in males), (P<0.05). Seizure score was twofold higher in ko mice after CCI versus either heterozygote or wt (P<0.05). An injury-intensity dose-response for 24 h mortality was seen in ko mice (P<0.05). Physiologic parameters were similar between genotypes. Seizures were seen in 100% of female ko mice after CCI versus 14% of heterozygotes and 25% wt (P<0.05) and SE was restricted to the ko mice (83% incidence). Our data suggest a critical endogenous anticonvulsant action of adenosine at A1 receptors early after experimental TBI.

Ontogenetic profile of ectonucleotidase activities from brain synaptosomes of pilocarpine-treated rats

Adenosine, a well-known neuromodulator, can act as an endogenous anticonvulsant via the activation of adenosine A1 receptors. This adenine nucleoside can be produced in the synaptic cleft by the ectonucleotidase cascade, which includes the nucleoside triphosphate diphosphohydrolase (NTPDase) family and ecto-5'-nucleotidase. It has been previously reported that ectonucleotidase activities are increased in female adult rats submitted to the pilocarpine model of epilepsy. Several studies have suggested that the immature brain is less vulnerable to morphologic and physiologic alterations after status epilepticus (SE). Here, we evaluate the ectonucleotidase activities of synaptosomes from the hippocampus and cerebral cortex of male and female rats at different ages (7-9, 14-16 and 27-30-day old) submitted to the pilocarpine model of epilepsy. Our results show that ATP and ADP hydrolysis in the hippocampus and cerebral cortex were not altered by the pilocarpine treatment in female and male rats at 7-9, 14-16 and 27-30 days. There were no changes in AMP hydrolysis in female and male rats submitted to the model at different ages, but a significant increase in AMP hydrolysis (71%) was observed in synaptosomes from the cerebral cortex of male rats at 27-30 days. Pilocarpine-treated male rats (60-70-day old) presented an enhancement in ectonucleotidase activities in the synaptosomes of the cerebral cortex (33, 40 and 64% for ATP, ADP and AMP hydrolysis, respectively) and hippocampus (55, 98 and 101% for ATP, ADP and AMP hydrolysis, respectively). These findings highlight differences between the purinergic system of young and adult rats submitted to the pilocarpine model of epilepsy.

Refractory status epilepticus

PURPOSE OF REVIEW

Although conventional anticonvulsant agents can terminate status epilepticus in most cases, a substantial minority of patients develops medically refractory status and requires more aggressive care. This review explores the options available.

RECENT FINDINGS

Increasing numbers of previously unexpected etiologies for refractory status epilepticus continue to be reported. There are also some promising new therapies on the horizon, both for the short and the longer terms.

SUMMARY

Refractory status epilepticus, while a challenge to the intensivist, can be treated with drugs that are commonly used by intensivists. The cooperation of an interested electroencephalographer is vital.