Morphological alterations of hypothalamic nuclei due to intrahypothalamic hyperinsulinism in newborn rats

In former studies, a temporary, intrahypothalamically localized hyperinsulinism during brain development was shown to result in overweight and metabolic disturbances during later life in rats. Therefore, we tested the hypothesis whether intrahypothalamic insulin treatment during early postnatal life may lead to hypothalamic morphological alterations, i.e., of numerical density of neurons and area of neuronal nuclei or area of neuronal cytoplasm, in this animal model. For this purpose, on the 8th day of age in Wistar rats a long-acting insulin was bilaterally applicated stereotactically into the hypothalamus (12 mIU on each side), while in controls the insulin-free agar-vehicle was given only. By computer-assisted morphometric analysis on the 15th day of life a decrease of the mean area of neuronal nuclei and the mean nucleus-cytoplasm-ratio within the VMN of the insulin-treated animals was observed, as compared to control rats (P < 0.05), while no significant alterations were found in the lateral hypothalamic area (LHA). Analysis of topographically distinct parts of the VMN revealed significant reductions of the mean area of neuronal nuclei (P < 0.001) and nucleus-cytoplasm-ratio (P < 0.05) in the anterior part of the VMN (VMNpa). Furthermore, in the ventrolateral part (VMNpv) a decreased mean neuronal density was observed in the insulin group (P < 0.01). In contrast, the dorsomedial part of the VMN (VMNpd) displayed an increased mean neuronal density in the insulin-treated animals (P < 0.05). In the dorsomedial hypothalamic nucleus (DMN) a significant increase of the mean area of neuronal nuclei (P < 0.01) and the area of neuronal cytoplasm were observed (P < 0.001). These alterations were accompanied by a significantly elevated mean numerical density of astrocytes (positive for glial fibriallary acidic protein; GFAP+) within the periventricular hypothalamic area (PER) of the insulin-treated rats (P < 0.05). These observations speak for a varying vulnerability of LHA, DMN and distinct parts of the VMN to hyperinsulinism during early development, possibly leading to a disturbed organization and, consecutively, permanent dysfunction of these morphologically connected and functionally interacting hypothalamic nuclei.