Immunocytochemical Detection of Insulin in Rat Hypothalamus and Its Possible Uptake from Cerebrospinal Fluid*

Abstract

Insulin-like immunoreactivity (IRI) was detected in the rat hypothalamus, particularly in the paraventricular, periventricular, supraoptic, suprachiasmatic, arcuate and lateral hypothalamic nuclei. The immunostainable IRI was diffusely distributed in comparison to the neuronal concentrations of immunostainable vasopression in the periventricular nucleus, or of IRI in islet B cells, suggesting that immunostainable IRI in the hypothalamus is not concentrated in neuronal perikarya. To determine if insulin in CSF may be a source of some insulin in brain tissue, [125I]iodoinsulin was stereotaxically injected into a lateral cerebral ventricle, and the uptake of radioactivity into periventricular hypothalmus was localized by both quantitative autoradiography of paraffin-embedded brain sections and by measuring the radioactivity present in microdissected brain regions. In brains that received lateral ventricular injections of labeled insulin, the concentrations of radioactivity in the periventricular region of the hypothalamus, as revealed by autoradiographic grains, was significantly greater than that in the periventricular regions of brains that received lateral ventricular injections of labeled insulin mixed with an equimolar excess of an unlabeled insulin mixed with an equimolar excess of an unlabeled peptide (insulin, ribonuclease or both together). The highest levels of radioactivity detected in both autoradiographic and microdissection procedures were in regions nearest to the 3rd ventricle, suggesting that insulin the lateral ventricles has access to the periventricular neuropile in the hypothalamus. The staining pattern of immunostainable insulin in the hypothalamus along with the distribution of radioactivity after CSF injection of labeled insulin are consistent with the hypothesis that insulin is taken up into brain from the CSF.