Abstracts

RATIONALE: Following the initial hilar cell loss that is produced by status epilepticus in pilocarpine-treated rats, numerous small neurons gradually accumulate in the hilus. During the first few weeks after status epilepticus, some hilar neurons are labeled for TOAD-64 (Parent et al., 1997), an early marker of post-mitotic neurons. Our goal was to determine the ultrastructural features of both the TOAD-64 labeled cells and small granule-like cells that were observed in the hilus of chronic animals with recurrent seizures. METHODS: Electron microscopic immunocytochemical methods with an antiserum to TOAD-64, kindly provided by S. Hockfield, were used to characterize the labeled hilar cells at three weeks after pilocarpine treatment. Small neuronal cell bodies in the hilus at six months after status epilepticus were also studied with electron microscopy. RESULTS: At three weeks after pilcarpine-induced seizures, numerous TOAD-64 labeled cell bodies and processes were prominent in the hilus at mid rostral-caudal regions of the dentate gyrus. Many of the labeled cell bodies had the ultrastructural characteristics of immature granule cells that included a thin rim of cytoplasm and a relatively undifferentiated nucleus. In chronic pilocarpine-treated animals, small neurons that resembled mature granule cells were present in the hilus. These neurons were often found in clusters and received synaptic contacts on their cell bodies, dendrites and axon initial segments. CONCLUSIONS: The ultrastructural findings confirm the neuronal phenotype of many of the TOAD-64 labeled neurons in the hilus and support suggestions that the labeled cells are immature, newly-generated neurons, rather than mature cells that are re-expressing TOAD-64. It is hypothesized that many of these neurons become the granule-like cells in the hilus of the chronic animals. The presence of synapses on these neurons indicates that they have become integrated into the circuitry of the dentate gyrus and could contribute to the abnormal activity of the dentate gyrus in seizure-prone animals, as suggested previously (Scharfman et al., 1999). Supported by VA Medical Research Funds.