The Purinergic P2Y Receptor Family as Novel Drug Target in Epilepsy
Epilepsy is one of the most common chronic neurological diseases and is characterized by recurrent seizures. Despite the existence of numerous anti-epileptic drugs, 30-40% of patients remain drug-refractory. Consequently, we must identify novel target genes with new mechanisms of action. Accumulating evidence suggests neuroinflammation drives seizure pathology and the epileptic phenotype. ATP, usually present at low extracellular concentrations, is released under pathological conditions in the brain where it activates purinergic P2 receptors including the ionotropic P2X receptors and metabotropic P2Y receptors, contributing to changes in glial function and modulation of inflammatory pathways thereby increasing neuronal network excitability.
A role for P2Y receptors has been suggested for several brain diseases and altered expression and function of some subtypes has been reported in animal models of epilepsy, particularly for the P2Y1 receptor subtype. However, to date, no systematic expressional and functional characterization of the P2Y receptor family during seizures has been undertaken.
The aim of the present PhD thesis was to investigate the expressional changes and functional role of P2Y receptors during and following seizures using resected brain tissue from patients suffering from temporal lobe epilepsy, two different mouse models of epilepsy (kainic acid and pilocarpine), P2Y knock-out mice and specific P2Y receptor-targeting agonists and antagonists.
The main findings on expression of the P2Y receptor family include agonist- and P2Y receptor down-stream target specific changes in the levels of the P2Y receptors during status epilepticus. P2Y receptor up-regulation was the main response during chronic epilepsy in experimental epilepsy and in epilepsy patient brain.
Functional studies revealed important effects of P2Y receptors on seizures and epilepsy. Treatment of mice with ADP, a broad-spectrum agonist of P2Y receptors exacerbated seizure severity in a kainic acid model whereas UTP decreased seizure severity during status epilepticus. Antagonism of the P2Y1 receptor prior to kainic acid injection increased seizure severity and seizure-induced brain pathology. In line with these results, pre-treatment with P2Y1 agonists protected the brain from seizure pathology. Surprisingly, and in contrast to results obtained from the pre-treatment regime, P2Y1 antagonist treatment shortly following the induction of status epilepticus strongly reduced seizure severity and neuronal death and P2Y1 agonist aggravated seizure pathology. In support of the P2Y1 driving inflammatory process during status epilepticus, microglial cells were the first to show an increase in P2Y1 levels and anticonvulsive effects were lost in P2Y1 antagonist-treated mice with suppressed brain inflammation.
Finally, P2Y1 antagonist treatment following status epilepticus delayed the emergence of spontaneous seizures and, when given to epileptic mice, reduced the occurrence of spontaneous epileptic seizures by over 50% during treatment.
In conclusion, the targeting of P2Y receptors, particularly P2Y1, may represent a new promising therapeutic strategy to treat status epilepticus and possibly drug refractory epilepsy.