FOS INDUCTION IN SUBTYPES OF CEREBROCORTICAL NEURONS FOLLOWING SINGLEPICROTOXIN-INDUCED SEIZURES

In adult rats single seizures of varying behavioural severities were c aused by slow, systemic infusion of picrotoxin, an antagonist of the C l- channel at the GABA(A) receptor. We used a double labelling immunoh istochemical method to define the subclasses of neurons that contained Fos protein following seizures. In four cortical regions (piriform, e ntorhinal, motor and sensory) neuronal subclasses were defined with an tibodies against the calcium-binding proteins calbindin D-28K, parvalb umin and calretinin (aspiny neurons), and neurofilament protein (spiny neurons). The remaining spiny neuron population was estimated by subt raction of defined subclasses from total neuronal numbers determined f rom Nissl stain. After seizures, most of the calbindin D-28K immunorea ctive interneurons (> 80%) and many of the unlabelled spiny neurons (6 0-80%) were Fos positive. Co-localisation of Fos was found in about 30 % of parvalbumin, calretinin and neurofilament protein immunoreactive neurons. Paradoxically, mild seizures were associated with induction o f Fos in up to 80% of cortical cells and more severe seizures with 60% , the difference being due to different levels of Fos induction in spi ny neurons. These results also demonstrate that seizures induce Fos pr edominantly in excitatory cortical neurons.