NEURONAL MIGRATION DISORDERS - HETEROTOPIC NEOCORTICAL NEURONS IN CA1PROVIDE A BRIDGE BETWEEN THE HIPPOCAMPUS AND THE NEOCORTEX

Neuronal migration disorders have been involved in various pathologies , including epilepsy, but the properties of the neural networks underl ying disorders have not been determined. In the present study, patch c lamp recordings were made from intrahippocampal heterotopic as well as from neocortical and hippocampal neurons from brain slices of rats wi th prenatally methylazoxymethanol-induced cortical malformation. We re port that heterotopic neurons have morphometrical parameters and cellu lar properties of neocortical supragranular neurons and are integrated in both neocortical and hippocampal networks. Thus, stimulation of th e white matter induces both antidromic and orthodromic response in het erotopic and neocortical neurons. Stimulation of hippocampal afferents evokes a monosynaptic response in the majority of heterotopic neurons and a polysynaptic all-or-none epileptiform burst in the presence of bicuculline to block gamma-aminobutyric acid type A inhibition. Furthe rmore, hippocampal paroxysmal activity generated by bath application o f bicuculline can spread directly to the neocortex via the heterotopia in methylazoxymethanol-treated but not in naive rats. We conclude tha t heterotopias form a functional bridge between the limbic system and the neocortex, providing a substrate for pathological conditions.