SEROTONIN BLOCKS DIFFERENT PATTERNS OF LOW MG2-INDUCED EPILEPTIFORM ACTIVITY IN RAT ENTORHINAL CORTEX, BUT NOT HIPPOCAMPUS()

Low Mg2+-induced epileptiform activity in the entorhinal cortex is cha racterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as th e transition between them were blocked by serotonin. In contrast, sero tonin had little effect upon the epileptiform activity in areas CA3 an d CA1 of the hippocampus. Both forms of epileptiform acitvity in the e ntorhinal cortex are sensitive to N-methyl-D-aspartate receptor antago nists and it is shown here that serotonin blocked both types of epilep tiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic pot entials in deep layer entorhinal cortex cells. Serotonin also prolonge d or even prevented the transition between the two types of epileptifo rm activity and we suggest that this may be through activation of the Na+/K+-ATPase. The resistance of epileptiform activity in CA1 and CA3 to serotonin was most likely related to the inability of serotonin to reduce Schaffer collateral-evoked excitatory postsynaptic potentials. Given the strong serotonergic inputs to both the hippocampus and entor hinal cortex, the differential sensitivity of the two regions to serot onin suggests functional differences. In addition since the late recur rent discharges in the entorhinal cortex are resistant to all clinical ly used anticonvulsants, serotonin may open new avenues for the develo pment of novel anticonvulsant compounds. Copyright (C) 1996 IBRO.