A COMPREHENSIVE ANALYSIS OF THE DISTRIBUTION OF FGF-2 AND FGFR1 IN THE RAT-BRAIN

We have examined the cellular distribution of both FGF-2 and FGFR1 imm unoreactivity and their mRNAs throughout the normal adult rat brain in order to reconcile numerous disparate findings in the published liter ature. The results confirm a widespread distribution of FGF-2 and FGFR 1 in the rat brain, and different regions express distinct patterns of FGF-2 and FGFR1 mRNA and protein: neuronal and non-neuronal cells sho w different subcellular distributions that vary according to the area where they are located. The intensity of the staining and hybridizatio n also varies according to the loci examined and the cell type involve d. Astrocytes contain the highest levels of FGF-2 and FGFR1 mRNAs, and characteristically, possess high levels of immunoreactive FGF-2 withi n the nucleus. Amongst non-neuronal cells, oligodendrocytes do not syn thesize or contain significant levels of FGF-2 immunoreactivity howeve r, they do express FGFR1 mRNA. In these cells, immunoreactive FGFR1 is mainly associated with the myelin sheaths of neuronal fibers. In vent ricular systems, ependymal cells synthesize and contain immunoreactive FGFR1. In contrast, only cells lining the lateral wall of the IIIrd v entricle express FGF-2 mRNA. Subependymal cells contain high levels of both FGF-2 and FGFR1 immunoreactivity. Neurons express low levels of FGF-2 mRNA and immunoreactive FGF-2 is localized predominantly to the perikaryon. However, selected populations of neurons, such as CA2 fiel d of the hippocampus, show high levels of FGF-2 mRNA, in which the nuc leus is strongly immunopositive, Similarly, high levels of FGFR1 mRNA are localized to select populations of neurons (e.g. amygdala). FGFR1 immunoreactivity is mainly associated with myelinated fiber tracts (e. g. striatum), and some neurons show immunoreactivity in the perikaryon (e.g. hippocampus), the nucleus (e.g. mesencephalic trigeminal nucleu s), or in axonal projections (e.g. hypothalamus). Remarkably, in many of the areas studied, FGF-2 and FGFR1 mRNA and/or their translated pro tein do not co-localize in neurons (e.g. neo-cortices) or even in the same regions of the brain (e.g. substantia nigra). In other instances, mRNAs for both FGF-2 and FGFR1 colocalize (e.g. supraoptic nucleus). The brain, in contrast to peripheral tissues, contains high levels of FGF-2 and actively expresses its gene under normal physiological condi tions. The highly specific anatomical distribution of immunoreactive F GF-2 in neuronal and non-neuronal brain cells, supports the notion tha t it plays a multifunctional role in the CNS under normal physiology. By correlating the localization and the synthesis of FGF-2 and one of its high affinity receptors, FGFR1, in the CNS, it should be possible to obtain a better understanding of the roles of FGF-2 in normal and p athological conditions.