Epileptogenesis induces long-term alterations in intracellular calcium release and sequestration mechanisms in the hippocampal neuronal culture modelof epilepsy

Calcium and calcium-dependent processes have been hypothesized to be involv ed in the induction of epilepsy. It has been shown that epileptic neurons h ave altered calcium homeostatic mechanisms following epileptogenesis in the hippocampal neuronal culture (HNC) and pilocarpine models of epilepsy. To investigate the mechanisms causing these alterations in [Ca2+](i) homeostat ic processes following epileptogenesis, we utilized the HNC model of in vit ro 'epilepsy' which produces spontaneous recurrent epileptiform discharges (SREDs). Using [Ca2+](i) imaging, studies were initiated to evaluate the me chanisms mediating these changes in [Ca2+](i) homeostasis. 'Epileptic' neur ons required much longer to restore a glutamate induced [Ca2+](i) load to b aseline levels than control neurons. Inhibition of Ca2+ entry through volta ge and receptor gated Ca2+ channels and stretch activated Ca2+ channels had no effect on the prolonged glutamate induced increase in [Ca2+](i) in epil eptic neurons. Employing thapsigargin, an inhibitor of the sarco/endoplasmi c reticulum calcium ATPase (SERCA), it was shown that thapsigargin inhibite d sequestration of [Ca2+](i) by SERCA was significantly decreased in 'epile ptic' neurons. Using Ca2+ induced Ca2+ release (CICR) cell permeable inhibi tors for the ryanodine receptor (dantrolene) and the IP3 receptor (2-amino- ethoxydiphenylborate, 2APB) mediated CICR, we demonstrated that CICR was si gnificantly augmented in the 'epileptic' neurons, and determined that the I P3 receptor mediated CICR was the major release mechanism altered in epilep togenesis. These data indicate that both inhibition of SERCA and augmentati on of CICR activity contribute to the alterations accounting for the impair ed calcium homeostatic processes observed in 'epileptic' neurons. The resul ts suggest that persistent changes in [Ca2+](i) levels following epileptoge nesis may contribute to the long-term plasticity changes manifested in epil epsy and that understanding the basic mechanisms mediating these changes ma y provide an insight into the development of novel therapeutic approaches t o treat epilepsy and prevent or reverse epileptogenesis. (C) 2001 Harcourt Publishers Ltd.