Astrogliosis in epilepsy leads to overexpression of adenosine kinase, resulting in seizure aggravation

Adenosine kinase (ADK) is considered to be the key regulator of the brain's endogenous anticonvulsant, adenosine. In adult brain, ADK is primarily expressed in a subpopulation of astrocytes and striking upregulation of ADK in these cells has been associated with astrogliosis after kainic acid-induced status epilepticus (KASE) in the kainic acid mouse model of temporal lobe epilepsy. To investigate the causal relationship between KASE-induced astrogliosis, upregulation of ADK and seizure activity, we have developed a novel mouse model [the Adktm1(-/-)-Tg(UbiAdk) mouse] lacking the endogenous astrocytic enzyme due to a targeted disruption of the endogenous gene, but containing an Adk transgene under the control of a human ubiquitin promoter. Mutant Adktm1(-/-)-Tg(UbiAdk) mice were characterized by increased brain ADK activity and constitutive overexpression of transgenic ADK throughout the brain, with particularly high levels in hippocampal pyramidal neurons. This ADK overexpression was associated with increased baseline levels of locomotion. Most importantly, two-thirds of the mutant mice analysed exhibited spontaneous seizure activity in the hippocampus and cortex. This was the direct consequence of transgene expression, since this seizure activity could be prevented by systemic application of the ADK inhibitor 5-iodotubercidin. Intrahippocampal injection of kainate in the mutant mice resulted in astrogliosis to the same extent as that observed in wild-type mice despite the absence of endogenous astrocytic ADK. Therefore, KASE-induced upregulation of endogenous ADK in wild-type mice is a consequence of astrogliosis. However, seizures in kainic acid-injected mutants displayed increased intra-ictal spike frequency compared with wild-type mice, indicating that, once epilepsy is established, increased levels of ADK aggravate seizure severity. We therefore conclude that therapeutic strategies that augment the adenosine system after astrogliosis-induced upregulation of ADK constitute a neurochemical rationale for the prevention of seizures in epilepsy.