Unveiling the involvement of connexin43-based hemichannels in in vivo hippocampal physiology and mechanisms of temporal lobe epilepsy

Nowadays, the contribution of Connexin43(Cx43) hemichannels (HCs) to brain functioning and dysfunctioning hasn’t been fully elucidated, mainly because of the paucity of appropriate tools allowing selective targeting of Cx43 HCs. An exception is made for Cx43 mimetic peptides, which inhibit Cx43 HCs but not gap junctions in in vivo settings. In this study, we want to screen Cx43 HCs as novel antiepileptic drug targets through the in vivo application of a newly developed and selective Cx43 HC-inhibitor, TAT-Gap19. We also want to unveil which specific gliotransmitter is exactly released in vivo via these Cx43 HCs, as it is hypothesized that Cx43 HCs are a possible pathway for the release of gliotransmitters. We will elaborate on the physiological relevance of these gliotransmitters by focusing on their involvement in memory mechanisms.
To study the role of Cx43 HCs in spatial memory, we screened TAT-Gap19 in a validated test for spatial memory (Y maze test). We found that an intracerebroventricular (ICV) injection of 1 nmol TAT-Gap19 does not influence the spontaneous alternation in mice in the Y maze. It does however significantly impair the delayed spontaneous alternation in the Y maze. These results show that TAT-Gap19 influences the short-term memory but not the working memory and indicate that Cx43 HCs play a role in spatial memory mechanisms. Other test for spatial (e.g. long-term memory) and declarative memory still need to be performed.
The involvement of Cx43 HCs in in vivo mechanisms of limbic seizures was elucidated by screening TAT-Gap19 in different acute models for seizures. In the acute rat pilocarpine model, we found that an intrahippocampal infusion of 12.5 µM TAT-Gap19 was able to significantly reduce the seizure-related behavioral movements induced by pilocarpine. However, further studies with more animals and with the confirmation of the absence/presence of seizures via EEG monitoring of the animals are needed to confirm our results. Furthermore, we still have to determine the concentration of neurotransmitters (e.g. glutamate and D-serine) in the dialysate samples which were collected every 20 min during the experiment. If TAT-Gap19 attenuates the dialysate levels of glutamate and/or D-serine, then most obviously this results from Cx43 HC-blockade and supports the hypothesis that Cx43 HCs release gliotransmitters in vivo. In the 6 Hertz mouse model, an electrical model for therapy-resistant seizures, we found that an ICV injection of 1 nmol TAT-Gap19 leads to a significant decrease in the seizure duration. These results suggest that Cx43 HCs play a role in seizure generation. Additionally, it is of very high importance to screen TAT-Gap19 in chronic models for epilepsy in order to unravel the role these channels play in epilepsy and epileptogenesis.