Adenosine A1 Receptor Knockout Mice Develop Lethal Status Epilepticus after Experimental Traumatic Brain Injury

Abstract

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.

Adenosine dysfunction in astrogliosis: cause for seizure generation?

“…All KA-injected Adk-tg and A 1 R knockout mice died within the first three days after KA-injection and displayed excessive SE (Li et al, 2008). Again, Adk-tg and A 1 R knockout mice displayed striking similarity with regard to increased vulnerability to acute brain injury, consistent with earlier reports of lethal status epilepticus in A 1 R knockout mice induced by intrahippocampal kainate or traumatic brain injury (Kochanek et al, 2006).…”

“…To further elucidate the downstream components of the adenosine / epileptogenesis pathways, in the present study we subjected A 1 receptor knockout mice to our paradigm of CA3-selective epileptogenesis and provide the following findings: We previously demonstrated that A 1 R activation is necessary to keep an epileptic focus localized . Likewise, traumatic brain injury in A 1 R knockout mice led to increased seizure spread and lethal status epilepticus (Kochanek et al, 2006). Here we demonstrate for the first time that A 1 R knockout mice develop focal spontaneous seizures in the absence of an epileptogenesis-precipitating injury.…”

“…Consequently, A 1 R agonists are powerful anticonvulsants (Dunwiddie, 1999), effective even in pharmacoresistant epilepsy (Gouder et al, 2003), however associated with prominent cardiovascular and sedative side effects (Dunwiddie and Worth, 1982;Dunwiddie and Masino, 2001;Güttinger et al, 2005). As demonstrated in A 1 R knockout mice, activation of the receptor is necessary to prevent the spread of seizures and seizure-associated cell death; following either kainic acid or traumatic brain injury (Kochanek et al, 2006) induced status epilepticus (SE) all A 1 R knockout animals died within 5 days.…”

The adenosine kinase hypothesis of epileptogenesis

“…Tonic activation of A 1 receptors by an endogenous tone of adenosine is believed to lead to long-distance A 1 receptor mediated inhibition and thus to mediate heterosynaptic depression (Manzoni et al, 1994). In regard to epilepsy, A 1 receptors, which have high expression levels in epilepsy-prone regions such as hippocampus (Fredholm et al, 2001), would thus be uniquely positioned to convey paracrine anticonvulsant functions of adenosine (Güttinger et al, 2005a;Güttinger et al, 2005b) and to prevent the spread of epileptogenic activity Kochanek et al, 2006).…”

“…The generation of mice with moderately reduced levels of ADK in brain rather than complete ADK deficiency seems to be essential, since several lines of evidence suggest that only small changes in ADK levels are permissible: (i) The homozygous disruption of ADK is lethal (Boison et al, 2002b); (ii) ADK is highly conserved in evolution (Spychala et al, 1996), suggesting that mutations are not easily tolerated; (iii) No mutations of human ADK are known in man (OMIM, Online Mendelian Inheritance in Man, Victor A. McKusick et al, Johns Hopkins University) indicating that most mutations are lethal; (iv) Excessive levels of ADK lead to severe deficits in A 1 R activation and to lethal status epilepticus or cell death after otherwise non-lethal triggers Kochanek et al, 2006;Li et al, 2007a;Pignataro et al, 2007b); (v) Inadequate levels of ADK in brain are expected to lead to a rise in adenosine to unacceptably high levels, with likely lethal consequences, e.g. central apnea and perinatal mortality induced by elevated adenosine (Montandon et al, 2006).…”