Research reportThe trophic factors S-100β and basic fibroblast growth factor are increased in the forebrain reactive astrocytes of adult callosotomized rat
Introduction
S-100 is a calcium-binding protein found in the central nervous system (CNS). S-100b (S-100β) and S-100a (S-100α) have been isolated from many CNS regions which have subunit composition ββ and αβ, respectively. In the CNS, the fraction S-100β is predominantly found in glial cells of the CNS. The presence of S-100β immunoreactivity has been detected by immunohistochemistry in subpopulations of oligodendrocytes 39, 62 and in a few specific neuronal cells 29, 35, 54, 73. However, astrocytes are the main S-100β-stained cells in the adult mammalian brain 16, 47. S-100β immunoreactivity is seen predominantly in the cytoplasm and processes of astrocytes 16, 42, 62, 76.
CNS lesions trigger a glial activation which includes morphological and functional alterations 4, 25, 30, 40, 52. It has been postulated that the glial responses to the lesion are related to restoration of homeostasis in the nervous tissue 25, 39. Proliferation and hypertrophy of astrocytes are seen close to the wound 60, 61. Once activated, the astrocyte synthesizes and releases substances which promote wound repair as well as protection of the nervous tissue from further neuronal degeneration 10, 18, 53. Furthermore, increased levels of S-100β and neurotrophic factors have been detected in astrocytes following CNS lesions 26, 33, 67, 72, 75.
The actions of S-100β also include the stimulation of neuronal survival and neurite outgrowth 6, 26, indicating the possible trophic properties of the molecule 5, 26.
Basic fibroblast growth factor (bFGF) is a neurotrophic factor that belongs to the class of heparin-binding growth factors. The actions of bFGF on neurons from several CNS regions have been described. Furthermore, bFGF is able to promote gliogenesis and angiogenesis, actions that indicate its role also in wound repair following a lesion of the CNS 3, 17, 24. Previous reports have extensively described the increased synthesis of bFGF in reactive astrocytes following a brain trauma 18, 22.
In view of these observations, it is possible that astroglial S-100β and bFGF cooperate in the trophic mechanisms of the normal and lesioned CNS.
In the present study, we investigated the spatial and temporal changes in S-100β immunoreactivity by means of immunohistochemistry combined with stereology and also analysed the changes of bFGF immunoreactivity in S-100β immunoreactive cells after a stereotaxic transection of the corpus callosum of adult rats.
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Animal treatment
Thirty-three adult male Wistar rats [body weight (b.w.) 220–250 g] from the Institute of Biomedical Sciences (São Paulo, Brazil) were used in the present study. The rats were kept under controlled temperature and humidity conditions with a standardized light and dark cycle (lights on at 0700 h and off at 1900 h) and with free access to food pellets and tap water.
Neurosurgical procedures
The animals received chloral hydrate anaesthesia (Merck, Germany, 0.6 mg/100 mg b.w.) and were placed in a stereotaxic apparatus
Analysis of S-100β immunoreactivity in the white matter
We found S-100β immunoreactive profiles similar to fibrous astrocytes homogeneously distributed throughout the white matter of the forebrain of the sham-operated rats (Fig. 4A). The S-100β immunoreactive cell profiles had a fusiform shape and a thin cytoplasm which accumulated low to moderate amount of S-100β immunoreactivity. It was possible to see one to three thin S-100β immunoreactive processes projecting from the cytoplasm. The density of the profiles and the intensity of S-100β
Discussion
It is known that an excess of Ca2+ influx is frequently related to activation of N-methyl-d-aspartate (NMDA) receptors, a phenomenon associated with neuronal death [14]. Furthermore, the presence of calcium-binding proteins in the CNS cells can buffer the excess of Ca2+ and thus may help prevent neuronal death [6]. S-100β is one of those substances which may participate in the local trophic phenomena by buffering Ca2+ ions 5, 67.
Many general functions have been attributed to S-100β such as
Acknowledgements
We are grateful to Doctor Andrew Baird (La Jolla, USA) for the generous gift of the bFGF antibody. We wish to thank Professor Kjell Fuxe (Stockholm, Sweden) for valuable advice in the preparation of the manuscript and Doctor Arne Møller (Gostrup, Denmark) for valuable advice concerning the new unbiased stereologic methods. This work was supported by grants from FAPESP (94/3858-3; 95/9060-6; 96/3110-4) and CNPq (523005/94-6), Brazil.
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