JUN, FOS, KROX, and CREB transcription factor proteins in the rat cortex: Basal expression and induction by spreading depression and epileptic seizures

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Abstract
The expression of the nuclear c‐JUN, JUN B, JUN D, c‐FOS, FOS B, KROX‐24, and CREB transcription factors was investigated in the cortex of adult rats by immunocytochemistry. The expression patterns were studied in untreated rats and up to 24 hours following topical application of 1 M KCl to the cortical surface (KCl) or i.v. injection of bicuculline (BIC). Topical KCl induced cortical spreading depression and systemic injection of bicuculline evoked generalized tonic‐clonic seizures. In untreated rats, JUN B, c‐FOS, and FOS B were expressed in a small number of neurons in the piriform, perirhinal, entorhinal, and insular cortex and in layers II, III, and VI of all neocortical areas. In contrast, c‐JUN, JUN D, and KROX‐24 were expressed in all cortical layers but with different intensities of immunoreactivity (IR): c‐JUN‐IR was generally weak and predominantly present in layers II, III, and VI. JUN D‐IR was equally strong in all layers. KROX‐24 showed a prominent expression in layers II, IV, and VI. The CREB protein exhibited a high and fairly homogeneous constitutive expression throughout the cortex with a slight preponderance in layer II and piriform cortex. Following KCl or BIC, a strong induction was seen for c‐FOS, JUN B, and KROX‐24, whereas c‐JUN, JUN D, and FOS B showed only a moderate increase compared to basal levels. Changes of CREB‐IR could not be detected. The localization of induced JUN, FOS, and KROX proteins reflected the pattern of labelling in untreated animals but demonstrated a higher intensity of labelling and an increased number of immunoreactive nuclei. The intensity and persistence of IR as well as the number of labelled cells following BIC exceeded those following KCl. Following BIC, increased levels of FOS B and JUN D were still present after 24 hours. Counterstaining with cresyl‐violet and GFAP, a marker for astrocytes, revealed that JUN, FOS, and KROX proteins were expressed in neurons but not in glial cell populations. The present data demonstrate that CREB, JUN, FOS, and KROX transcription factors exhibit a layer‐specific expression in the cerebral cortex with only slight area‐specific differences both in untreated rats and following stimulation with KCl and BIC. The expression of transcription factor proteins indicate complex molecular genetic changes in cortical neurons due to pathophysiological events such as seizure activity and spreading depression.