A novel anticonvulsant mechanism via inhibition of complement receptor C5ar1 in murine epilepsy models

- Inhibition of complement C5a receptor 1 (C5ar1) with PMX53 is anticonvulsant.
- Inhibition or absence of C5ar1 reduces neuronal loss following status epilepticus.
- C5ar1 absence during status epilepticus attenuates microglial TNFα increase.
- PMX53 inhibits C5a-induced K+ outward currents on microglial cells.

The role of complement system-mediated inflammation is of key interest in seizure and epilepsy pathophysiology, but its therapeutic potential has not yet been explored. We observed that the pro-inflammatory C5a receptor, C5ar1, is upregulated in two mouse models after status epilepticus; the pilocarpine model and the intrahippocampal kainate model. The C5ar1 antagonist, PMX53, was used to assess potential anticonvulsant actions of blocking this receptor pathway. PMX53 was found to be anticonvulsant in several acute models (6 Hz and corneal kindling) and one chronic seizure model (intrahippocampal kainate model). The effects in the 6 Hz model were not found in C5ar1-deficient mice, or with an inactive PMX53 analogue suggesting that the anticonvulsant effect of PMX53 is C5ar1-specific. In the pilocarpine model, inhibition or absence of C5ar1 during status epilepticus lessened seizure power and protected hippocampal neurons from degeneration as well as halved SE-associated mortality. C5ar1-deficiency during pilocarpine-induced status epilepticus also was accompanied by attenuation of TNFα upregulation by microglia, suggesting that C5ar1 activation results in TNFα release contributing to disease. Patch clamp studies showed that C5a-induced microglial K+ outward currents were also inhibited with PMX53 providing a potential mechanism to explain acute anticonvulsant effects. In conclusion, our data indicate that C5ar1 activation plays a role in seizure initiation and severity, as well as neuronal degeneration following status epilepticus. The widespread anticonvulsant activity of PMX53 suggests that C5ar1 represents a novel target for improved anti-epileptic drug development which may be beneficial for pharmaco-resistant patients.