Damage of substantia nigra pars reticulata during pilocarpine-induced status epilepticus in the rat: immunohistochemical study of neurons, astrocytes and serum-protein extravasation.

The substantia nigra has a gating function controlling the spread of epileptic seizure activity. Additionally, in models of prolonged status epilepticus the pars reticulata of substantia nigra (SNR) suffers from a massive lesion which may arise from a massive metabolic derangement and hyperexcitation developing in the activated SNR. In this study, status epilepticus was induced by systemic injection of pilocarpine in rats. The neuropathology of SNR was investigated using immunohistochemical techniques with the major emphasis on the time-course of changes in neurons and astrocytes. Animals surviving 20, 30, 40, 60 min, 2, 3, 6 hours, 1, 2, and 3 days after induction of status epilepticus were perfusion-fixed, and brains processed for immunohistochemical staining of SNR. Nissl-staining and antibodies against the neuron-specific calcium-binding protein, parvalbumin, served to detect neuronal damage in SNR. Antibodies against the astroglia-specific cytoskeletal protein, glial fibrillary acidic protein (GFAP), and against the glial calcium-binding protein, S-100 protein, were used to assess the status of astrocytes. Immunohistochemical staining for serum-albumin and immunoglobulins in brain tissue was taken as indicator of blood-brain barrier disturbances and vasogenic edema formation. Immunohistochemical staining indicated loss of GFAP-staining already at 30 min after induction of seizures in an oval focus situated in the center of SNR while sparing medial and lateral aspects. At 1 h there was additional vacuolation in S-100 protein staining. By 2 hours, parvalbumin-staining changed in the central SNR indicating neuronal damage, and Nissl-staining visualized some neuronal distortion. Staining for serum-proteins occurred in a patchy manner throughout the forebrain during the first hours. By 6 h, vasogenic edema covered the lesioned SNR. By 24 h, glial and neuronal markers indicated a massive lesion in the center of SNR. By 48-72 h, astrocytes surrounding the lesion increased in size, and polymorphic phagocytotic cells invaded the damaged area. In a further group of animals surviving 1 to 5 days, conventional paraffin-sections confirmed the neuronal and glial damage of SNR. Additional pathology of similar quality was found in the globus pallidus. Since astrocytes were always damaged in parallel with neurons in SNR it is proposed that the anatomical and functional interrelationship between neurons and astrocytes is particularly tight in SNR. Both cell elements may suffer in common from metabolic disturbance and neurotransmitter dysfunction as occur during massive status epilepticus.