LOCALIZATION OF THE INSULIN-LIKE-GROWTH-FACTOR-I RECEPTOR IN THE CEREBELLUM AND HYPOTHALAMUS OF ADULT RATS - AN ELECTRON-MICROSCOPIC STUDY

Insulin-Like growth factor I (IGF-I) is an important modulator of cell growth and plasticity in the CNS. Expression of the IGF-I receptor mR NA in brain peaks at times of active cell development perinatally and remains detectable, albeit at lower levels, in the adult. While both a utoradiographic and in situ hybridization studies show a wide and spec ific distribution of IGF-I receptor throughout the adult rat brain, no thing is yet known about its subcellular localization, a critical issu e that will help clarify the biological role of this trophic factor in the adult brain. The present study describes the subcellular localiza tion of IGF-I receptor immunoreactivity in the cerebellar cortex and t he hypothalamic arcuate nucleus by using electron microscopic immunocy tochemistry. In the cerebellum, IGF-I receptor immunoreactivity is pre sent postsynaptically in the dendrites and soma of the Pukinje cell an d presynaptically in axon terminals impinging upon the Purkinje cell s oma, as well as in mossy fibre rosettes in the cerebellar glomeruli. N eurons in the mediobasal hypothalamus also contain IGF-I receptors loc ated pre- and postsynaptically. Endothelial cells, astroglial end-feet surrounding micro vessels throughout all the brain parenchyma, tanycy tes of the third ventricle and oligodendrocytes in the cerebellar whit e matter are also rich in IGF-I receptors. These results strongly supp ort previous observations that IGF-I is a neuromodulator in the adult brain, probably acting as both a pre- and a postsynaptic messenger. Th ey also suggest that glial cells may be involved in the actions of IGF -I in the adult brain.