Abstracts

While diffuse cortical malformations (CM) are usually associated with epilepsy, the mechanisms that contribute to this epileptogenic phenotype remain unclear. To gain insight into this issue, investigators have turned to animal models. The reeler mutant mouse is a genetic model of diffuse CM, which exhibits enhanced susceptibility to epileptiform activity in vivo (minimal electroshock) and in vitro (following exposure to bicuculline). Among the brain regions that show enhanced seizure susceptibility in vitro is the dentate gyrus, a region normally resistant to seizures. Since the capacity of a neuronal network to generate synchronized epileptiform activity following bicuculline application depends in part on the presence of recurrent excitatory connections, we hypothesized that a change in the distribution of local excitatory circuits, the mossy fibers, occurs in the dentate gyrus of rl/rl mice. Field potential recordings were used to assess the physiological properties of the dentate gyrus in hippocampal slices from rl/rl mice and controls. The Timm[apos]s stain was used to examine mossy fiber distribution. Adult (2 -7 months) male reeler homozygous mice (rl/rl) and controls (+/?) were used in all experiments. In [ldquo]normal[rdquo] artificial cerebrospinal fluid (ACSF) no differences were noted between groups in orthodromic stimulus response curves, or paired-pulse indices calculated for all interpulse intervals examined. In contrast, in ACSF with bicuculline (30 [micro]m) nearly all slices from rl/rl mice, and none from controls, exhibited spontaneous prolonged negative field potential shifts (duration 600-8000 ms; amplitude 1.5 [ndash] 2.5 mV). Moreover, hilar/CA3 stimulation also evoked prolonged negative field potential shifts in 100% of slices from rl/rl mice in bicuculline, yet 0% of slices from controls. Focal application of the glutamate receptor antagonists DL-2-amino-5-phosphonopentanoic acid (AP5) and 6-cyano-7-nitroquinoxaline-2,3-dione (CNQX) blocked the spontaneous and hilar-evoked prolonged negative field potential shifts reversibly, yet had no affect on the initial antidromic population spike. Histological examination revealed a dramatic difference in the distribution of Timm[apos]s positive stained fibers in rl/rl mice, compared to controls, with a dark band of Timm[apos]s staining found consistently at the border of the molecular and granule cell layers only in reeler mice. These data suggest that aberrant excitatory interconnections (altered mossy fiber distribution) contribute to the increased susceptibility of the rl/rl dentate gyrus to generate epileptiform activity in vitro. Thus, the organization of local excitatory circuits can be disrupted following abnormal neuronal migration and subsequently contribute to the development of a pro-epileptic substrate. (Supported by Epilepsy Foundation.)