Seizure activity selectively reduces 5-HT_1A receptor immunoreactivity in CA1 interneurons in the hippocampus of seizure-prone gerbils

Since the correlation between the serotonin (5-hydroxytryptamine, 5-HT) system and seizure activity remains to be clarified, we investigated the 5-HT system in the hippocampus of seizure-resistant (SR) and seizure-sensitive (SS) gerbils. There was no difference of the 5-HT system in the hippocampi of young animals (predisposed and juvenile gerbils) in both SR and SS gerbils. 5-HT immunoreactivity in the dorsal raphe nucleus and the median raphe nucleus was also similarly detected in both animal groups. As compared to SR adult gerbils, only 5-HT_1A receptor immunoreactivity was selectively reduced in CA1 interneurons within SS adult gerbils. (±)-8-Hydroxy-2-(di-n-propylamino)tetralin (a 5-HT_1A receptor agonist, 1 and 2 mg/kg) markedly reduced paired-pulse inhibition in the CA1 region of SS adult gerbils only. These findings suggest that the selective reduction in 5-HT_1A receptor expression on CA1 interneurons of SS adult gerbil may not be developmental defects, but be an acquired compensatory change induced by repeated seizure activity.