Identification of Binding Sites for an Insulin-like Growth Factor (IGF-1) in the Median Eminence of the Rat Brain by Quantitative Autoradiography

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Abstract
The microanatomical location of IGF-I binding in the rat brain was determined by in vitro autoradiography with slide-mounted sections of frozen brain. Sections incubated in 0.1 nM [125I]-iodo-IGF-I produced a dense grain concentration in regions of the autoradiographic image corresponding to the external palisade zone of the median eminence; other hypothalamic regions were not so heavily labeled. This reaction was significantly reduced in the presence of 100 nM IGF-l. Measurement of binding by computer digital image analysis of autoradiographic images showed that specific binding for IGF-l in the median eminence was 41.3 .+-. 8 .times. 10-3 fmol/mm2 (mean .+-. SEM); nonspecific binding was 1.19 .+-. 1.8 .times. 10-3 fmol/mm2. In contrast, specific binding to other hypothalamic regions was uniformly lower. In a separate experiment, 1000 nM unlabeled insulin was added. Without insulin, specific binding was 23 .+-. 0.9 .times. 10-3 fmol/mm2; nonspecific binding was 8 .+-. 0.5 .times. 10-3 fmol/mm2. In the presence of 1000 nM unlabeled insulin, specific binding for [125I]-iodo-IGF-l was 23 .+-. 10-3 fmol/mm2. The results suggest that a high concentration of receptors for an IGF-l-like molecule is present in the median eminence.