Functional properties of the subtype of insulin receptor found on neurons

In this report, we have examined the structure, regulation, and function of insulin receptors in cultured neurons from fetal chicken brain. The apparent molecular weight of the alpha-subunit of neuronal insulin receptors, analyzed by photoaffinity labeling and sodium dodecyl sulfate gel electrophoresis under reducing conditions, was 115,000. The number of insulin receptors in the cultures increased from day 2 to day 4 during a period of extensive process formation. After 5 days in culture, there were approximately 40,000 high-affinity insulin receptors per neuron. When neurons were photoaffinity labeled at 16 degrees C and then warmed to 37 degrees C for 30 min, approximately 40% of the cell-surface receptors were recovered in the intracellular, trypsin-insensitive pool. Chronic exposure of neurons to insulin (100 ng/ml) resulted in a time-dependent loss of neuronal insulin receptors with a maximal decrease of 50% after 24 h. Insulin had no effect on glucose transport, glucose oxidation, or glycogen synthase activity in neurons. On the other hand, insulin supported the growth and differentiation of a fraction of neurons isolated from chick forebrain. We conclude that (1) cultured neurons from fetal chicken brain express the same subtype of insulin receptor previously identified in adult rat and human brain, (2) the neuronal subtype of insulin receptor undergoes internalization and down-regulation in response to insulin, and (3) neuronal insulin receptors do not acutely regulate glucose metabolism but mediate growth in neurons.