Quantitative Autoradiographic Evidence for Insulin Receptors in the Choroid Plexus of the Rat Brain

Techniques of in vitro receptor autoradiography were used to visualize binding of 125 I-insulin on slices of frozen rat brain. Slide-mounted sections of frozen rat brain were incubated in 0.05 nM porcine 125 I-monoio-doinsulin, alone or mixed with 1 μM unlabeled porcine insulin, ribonuclease, or glucagon, for 2 h at 22°C. The labeled brain slices were apposed to LKB Ultrofilm to generate autoradiograms. The method permitted equal access of labeled insulin to both sides of the blood-brain barrier and localization of insulin binding sites in small anatomic regions. Quantitative estimates of specific iodoinsulin binding were made by computer digital image densitometry of the autoradiographic film images. High concentrations of specific binding sites for iodoinsulin were present in the choroid plexus of the lateral (26.9 ± 2.0 × 10 3 fmol/mm 2 ), fourth (18.3 ± 3.0 × 10- 3 fmol/mm 2 ), and third (13.2 ± 1.5 × 10 3 fmol/mm 2 ) ventricles (insulin binding is expressed per unit area of autoradiographic image). Binding to the third ventricular choroid plexus was similar to the concentrations observed for liver slices and the external plexiform layer of the olfactory bulb. Specific binding of iodoinsulin in the cingulate cortex and other surrounding regions was less than in choroid plexus. Ribonuclease or glucagon had no measurable effect on binding when mixed with labeled insulin. The results support the hypothesis that the choroid plexus has a high density of receptors for insulin, and suggests that the choroid plexus may be a target of CSF insulin action and/or a site of insulin transport into the CSF.