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 characterized by an initial expression of seizure-like events followed by late recurrent discharges. Both these forms of activity as well as the transition between them were blocked by serotonin. In contrast, serotonin had little effect upon the epileptiform activity in areas CA3 and CA1 of the hippocampus. Both forms of epileptiform activity in the entorhinal cortex are sensitive to N-methyl-D-aspartate receptor antagonists and it is shown here that serotonin blocked both types of epileptiform activity through an effective concentration-dependent reduction of N-methyl-D-aspartate receptor-mediated excitatory postsynaptic potentials in deep layer entorhinal cortex cells. Serotonin also prolonged or even prevented the transition between the two types of epileptiform 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 entorhinal cortex, the differential sensitivity of the two regions to serotonin suggests functional differences. In addition since the late recurrent discharges in the entorhinal cortex are resistant to all clinically used anticonvulsants, serotonin may open new avenues for the development of novel anticonvulsant compounds.