Mapping of neuronal networks underlying generalized seizures induced by increasing doses of pentylenetetrazol in the immature and adult rat: a c-Fos immunohistochemical study.

Previous studies from our group have shown that pentylenetetrazol (PTZ)-induced status epilepticus (SE) leads to age-dependent acute and long-term metabolic and circulatory changes in immature rats. In order to define the neural substrates involved in PTZ seizures according to age, the purpose of the present study was to map the areas of cellular activation during seizures of increasing severity in 10-day-old (P10), 21-day-old (P21) and adult rats. Seizures were induced by repetitive injections of subconvulsive doses of PTZ. The total dose received by the animals ranged from 4 to 125 mg/kg. These doses induced a variety of seizure profiles including absence-like, clonic seizures and SE. The cellular activation was measured as the density of c-Fos immunoreactive cells in animals at 2 h after the onset of the seizures. In P10 rats receiving a behaviourally non-active dose of PTZ, c-Fos immunoreactivity appeared only in the amygdala. The dose of 40 mg/kg that induced absence-like seizures led to a weak c-Fos expression in the medial thalamus, some cortical areas and globus pallidus. Clonic seizures reinforced labelling in the previous areas and induced a spread of c-Fos immunoreactivity to other cortical areas, thalamus, hypothalamus and some brainstem nuclei. At that age, only SE led to a widespread and stronger expression of c-Fos which was, however, totally lacking in the midbrain, and remained incomplete in the brainstem and forebrain limbic system, including the hippocampus. In P21 and adult rats, the inactive dose of PTZ induced c-Fos immunoreactivity in thalamus and hypothalamus. With absence-like seizures, c-Fos labelling spread to the cerebral cortex, amygdala, septum and some brainstem regions. With clonic seizures, immunoreactivity was reinforced in all areas already activated by absence-like seizures, and appeared in the striatum, accumbens, brainstem and hippocampus, except in CA1. After SE, c-Fos was strongly expressed in all brain areas. The intensity of c-Fos labelling was higher in most regions of P21 compared to adult rats. These data are in agreement with the immaturity of cellular and synaptic connectivity in P10 rats, the known greater sensitivity of rats to various kinds of seizures during the third week of life and the nature of the neural substrates involved in PTZ seizures.

Effects of pentylenetetrazol-induced status epilepticus on c-Fos and HSP72 immunoreactivity in the immature rat brain.

Pentylenetetrazol (PTZ)-induced status epilepticus (SE) leads to acute and long-term metabolic decreases in specific brain regions of rats at 10 (P10) or 21 days after birth (P21). These decreases are not related to apparent neuronal damage. Therefore, to better understand the neuronal activation and stress response to PTZ in immature rats, we mapped the expression of c-Fos and of the 72 kDa heat-shock protein (HSP72) in the same model of severe SE induced by the repetitive i.p. injections of subconvulsive doses of PTZ. Rats were sacrificed either at 2 or 24 h after the onset of SE in order to reveal c-Fos immunoreactivity, and at 24 and 72 h for HSP72 expression. Hematoxylin-eosin staining was performed at 24, 72 and 144 h after SE. The expression of c-Fos at 2 h after SE was more marked at P21 than at P10 and was prominent at both ages in the hippocampal dentate gyrus, cerebral cortex and amygdala. Some immunoreactivity was also present in the hypothalamus, thalamus and a few brainstem and cerebellar regions at both ages. There was a good relation between the regions expressing c-Fos and those exhibiting acute metabolic decreases at P21. Conversely, PTZ seizures did not lead to any expression of c-Fos at 24 h after SE or of HSP72 at 24 or 72 h at any age. Cell density was not affected by PTZ-induced SE at any age and at any time. These results suggest that c-Fos is a useful marker of neuronal activation induced by severe and prolonged seizures in the immature brain. The lack of HSP72 and of late c-Fos expression likely reflect the absence of neuronal damage in this model of PTZ-induced SE in the immature rat.