Seizure suppression and lack of adenosine A₁ receptor desensitization after focal long-term delivery of adenosine by encapsulated myoblasts

Adenosine is an important inhibitory modulator of brain activity. In a previous ex vivo gene therapy approach, local release of adenosine by encapsulated fibroblasts implanted into the vicinity of an epileptic focus, was sufficient to provide transient protection from seizures (Huber, A., Padrun, V., Deglon, N., Aebischer, P., Mohler, H., Boison, D., 2001. Grafts of adenosine-releasing cells suppress seizures in kindling epilepsy. Proc. Natl. Acad. Sci. U. S. A. 98, 7611-7616). Long-term seizure suppression beyond 2 weeks was precluded by limited life expectancy of the encapsulated fibroblasts. To study the feasibility for long-term seizure suppression by adenosine releasing brain implants, in the present contribution, mouse C₂C₁₂ myoblasts were engineered to release adenosine by genetic inactivation of adenosine kinase. After encapsulation, the myoblasts were grafted into the lateral brain ventricles of epileptic rats kindled in the hippocampus. While seizure activity in animals with wild-type implants remained unaltered, 1 week after grafting all rats with adenosine-releasing implants (n = 25) displayed complete protection from convulsive seizures and a corresponding reduction of afterdischarges in EEG-recordings. The duration of seizure suppression was maintained for a period of 3 weeks in 50% of the animals ranging to a maximum of 8 weeks in one animal. During the course of these experiments, adenosine A ₁ receptors remained responsive to selective agonists and antagonists indicating a lack of desensitization of A₁ receptors after local long-term exposure to adenosine. Furthermore, local release of adenosine did not affect locomotor activity, whereas systemic application of the A₁ agonist 2-chloro-N⁶-cyclopentyladenosine caused strong sedation. Thus, the local release of adenosine by cellular implants provides a feasible option for a potential side-effect free approach for the long-term treatment of focal epilepsies.