Pilocarpine (PILO)-induced status epilepticus evokes cell death and aberrant sprouting of hippocampal mossy fibers (MFs). In order to learn about alterations in the intrahippocampal circuits, effects of pilocarpine-induced seizures on selected neuronal markers were studied in mice and rats, using immunohistochemistry (IHC). The animals which exhibited intense PILO-induced convulsions for at least 30 min were used in this study. After a 2-month survival period, sprouting of MFs (SMF) was tested in each brain using zinc histochemistry (Timm's silver-sulphide method). In the present study, antibodies were used against neuropeptide-Y (NPY) for a subset of inhibitory neurons, synapsin-I (Syn-I) for synaptic vesicles, calretinin (CR) and calcitonin gene-related peptide (CGRP) for mossy cells (MCs) in mice and rats, respectively. In some untreated mice, the fornix was transected unilaterally to reveal the ratio of the extra- and intrahippocampal CR-immunopositive fibers. Those rodents which displayed zinc-containing ectopic SMF in the supragranular layer (SGL), were considered to be "PILO-reacted" ones, and used for further semi quantitative analyses. Immunoreactivity (IR) of the MCs in the hilum and their processes in the SGL was greatly reduced in rats, but it remained unchanged in the PILO-reacted mice. The latter observation together with the fornix transection experiments indicating the predominance of intrahippocampal CR supposed that the mouse MCs survived the PILO treatment. Increased NPY IR with similar patterns was observed in both species. Most prominently, intense IR appeared in the hilum, the stratum lucidum (SL) and the SGL. The enhancement of the NPY IR might be ascribed to the GABAergic densely-spiny hippocampal cells, which project to the medial septum. Species-differences were found in the Syn-I IR. In the rat, the distribution of Syn-I IR was similar to that of NPY IR. Increased Syn-I IR was found in the SL and in the hilum, but the SGL was moderately elevated in the rat. In contrast, in the mice, Syn-I IR was increased only in the SL, while it was decreased in the hilum, but it did not change in the IML. These differential changes suppose marked loss of interneurons in the mice, but not in rats.

Here, direct evidence of a close relationship between SMF and loss of MCs in the PILO model of rats was provided. Our findings suggest that MCs survive the PILO treatment in the mice. We concluded that in the two animals, similar behaviors to the PILO treatment may result from different changes in neuronal circuits, and moreover, these changes should include extrahippocampal areas, as well.

Grant: 

TÁMOP-4.2.1/B-09/1/KONV-2010-0005