Saturday, November 9, 2013

Day 1 at #SfN13: Neuroprotection versus Anticonvulsant Action

Day 1:

"The effect of the P2X7 receptor antagonist AZ10606120 in rat brain subjected to seizures by pilocarpine"

Epilepsy is complicated. Although there are some commonalities across the collection of disorders, there is not just one ‘kind’ of epilepsy. There are textbooks on the different types of epilepsy and the seizures that they produce, including the textbook on status epilepticus (seizures lasting more than 5 minutes) that is sitting on my desk at home. Since epilepsy is not a single disorder, but many, it makes sense that when the Society for Neuroscience for the first time ever, curated entire topics for itineraries this year, one of the three topics selected was epilepsy. A great deal of patients diagnosed with epilepsy are controlled by medication (70% by some estimates) but that means there are still 30% that go on to have uncontrolled seizures, which can be life-threatening. Our group is very interested in this 30% cohort and is a major focus of our research. It just so happens that most patients that are resistant to existent medications are the ones with temporal lobe epilepsy (TLE).

How would a non-clinical lab go about modeling such a disorder? Chemoconvulsants (compounds that induce seizures) that act upon the cholinergic system of the brain appear to model TLE. A fringe benefit to modeling TLE with chemoconvulsants is that many of these compounds that act upon the cholinergic nervous system routinely poison people. Treatments identified that can arrest seizures induced chemoconvulsants would therefore have significant potential in a clinical setting. Pilocarpine is one of these chemoconvulsants used to model TLE and in this case, stimulates the muscarinic acetylcholine receptors (mAChRs). Of the 5 subtypes of mAChRs, some studies have suggested that the M1 mAChR is a likely candidate as the initiator of seizure activity. But, difficulty arises in conducting research on mAChRs. mAChRs share an extremely structurally similar binding site for acetylcholine (the orthosteric site). As such, the development of drugs that block activity of a particular mAChRs subtype have been unsuccessful. Genetic tools have provided strong evidence that disrupting M1 mAChR activity can block the induction of cholinergic chemoconvulsants. Deletion of the M1 mAChR, but not the other subtypes completely suppressed seizure induction by pilocarpine in M1 mAChR null-mice, and in 7/8 M1 mAChR heterozygotes (Hamilton et al., 1997; Bymaster et al., 2003). Although exciting, these findings are not directly relevant to a clinical scenario involving anticonvulsant medication.

Candidates in augmenting or suppressing mAChRs signaling may come by targeting other proteins in the cell that interact with mAChRs of interest. The P2X7 receptors may be just that. Kim and Kang, (2011) found that the selective activation of P2X7 receptors negatively regulates M1 mAChR function. But, the understanding of how purinergic receptors modulate synaptic transmission is far less understand than more classical neurotransmitters such as acetylcholine, glutamate, serotonin, and friends.

Today, I spoke with Michelle Ara├║jo (Poster #: 51.17/S7) about her work studying the effect of an antagonist of P2X7 receptors against seizures induced pilocarpine. She found that when she administered a P2X7 antagonist in increasing doses, that this antagonist was increasingly neuroprotective (preventing irreversible cellular loss). I suppose this is not too surprising since the P2X family conducts calcium, and excessive calcium influx during seizure activity can mediate a tremendous amount of damage. However, I would be curious if treatment with a P2X7 agonist post seizure induction might arrest ongoing seizure activity (since this would be negatively regulating M1 mAChR activity). It may very well be that seizure activity would continue and therefore cell death would be increased. The net effect of this action may also cease ongoing seizure activity and thereby prevent a level of calcium influx that continues to result in calcium-mediated cell death. In this case, the amount of cellular loss induced by pilocarpine in the brain region investigated (hippocampus in the case of Michelle's work) may actually be reduced if seizure activity was arrested at an earlier timepoint.

What I always find so fascinating about the topic of neuroprotection in the field of epilepsy is that this does not always go hand in hand with anticonvulsants. In other words, anticonvulsant medications are not always neuroprotective. A hypothese in our field is that seizure induced excitotoxicity is responsible for cellular loss (either directly via calcium-influx, or indirectly through inflammatory mechanisms), but this does not explain why anticonvulsants can arrest seizures without being neuroprotective.

Steven Miller