Fluorescent tracer in pilocarpine-treated rats shows widespread aberrant hippocampal neuronal connectivity

Neuronal fibres of the hippocampal formation of normal and chronic epilepti c rats were investigated by fluorescent tracing methods using the pilocarpi ne model of limbic epilepsy. Two months after onset of spontaneous limbic s eizures, hippocampal slices were prepared and maintained in vitro for 10 h. Small crystals of fluorescent dye [fluorescein (fluoro-emerald (R)) and te tramethylrhodamine (fluoro-ruby (R))] were applied to different hippocampal regions. The main findings were: (i) in control rats there was no supragra nular labelling when the mossy fibre tract was stained in stratum radiatum of area CA3. However, in epileptic rats a fibre network in the inner molecu lar layer of the dentate gyrus was retrogradely labelled; (ii) a retrograde innervation of area CA3 by CA1 pyramidal cells was disclosed by labelling remote CA1 neurons after dye injection into the stratum radiatum of area CA 3 in chronic epileptic rats; (iii) labelling of CA1 neurons apart from the injection site within area CA1 was observed in epileptic rats but not in co ntrol animals; and (iv), a subicular-hippocampal projection was present in pilocarpine-treated rats when the tracer was injected just below the stratu m pyramidale of area CA1. The findings show that fibre rearrangement in dis tinct regions of the epileptic hippocampal formation can occur as an afterm ath of pilocarpine-induced status epilepticus.