125I-insulin binding is decreased in olfactory bulbs of aged rats

125I-insulin binding was studied in membrane preparations of olfactory bulb, frontal cortex, hippocampus and hypothalamus from mature (5-month-old) and aged (22-month-old) rats. In the young animals the highest level of specific insulin binding was found in the olfactory bulb, with lower values of specific insulin binding in the frontal cortex, hippocampus and hypothalamus. In the aged rats the specific insulin binding was not changed in the frontal cortex, hippocampus and hypothalamus as compared to the young ones. A significant decrease of total insulin binding was observed only in the olfactory bulbs of aged rats (0.67 +/- 0.04 pmol insulin/mg protein) as compared to the mature animals (1.3 +/- 0.08 pmol insulin/mg protein). Scatchard analysis of insulin binding data revealed that this decrease was due to changes in the number of binding sites rather than to changes in the affinity of insulin receptors. It was suggested that the decrease observed in insulin receptor number in olfactory bulbs of aged rats might be due to the atrophic changes in the structure of olfactory bulbs previously shown by electron microscopy for aged rats.