Brain insulin receptor causes activity-dependent current suppression in the olfactory bulb through multiple phosphorylation of Kv1.3

Insulin and insulin receptor (IR) kinase are found in abundance in discrete brain regions yet insulin signaling in the CNS is not understood. Because it is known that the highest brain insulin-binding affinities, insulin-rece ptor density, and IR kinase activity are localized to the olfactory bulb, w e sought to explore the downstream substrates for IR kinase in this region of the brain to better elucidate the function of insulin signaling in the C NS. First, we demonstrate that IR is postnatally and developmentally expres sed in specific lamina of the highly plastic olfactory bulb (OB). ELISA tes ting confirms that insulin is present in the developing and adult OB. Plasm a insulin levels are elevated above that found in the OB, which perhaps sug gests a differential insulin pool. Olfactory bulb insulin levels appear not to be static, however, but are elevated as much as 15-fold after a 72-h fa sting period. Bath application of insulin to cultured OB neurons acutely in duces outward current suppression as studied by the use of traditional whol e-cell and single-channel patch-clamp recording techniques. Modulation of O B neurons is restricted to current magnitude; IR kinase activation does not modulate current kinetics of inactivation or deactivation. Transient trans fection of human embryonic kidney cells with cloned Kv1.3 ion channel, whic h carries a large proportion of the outward current in these neurons, revea led that current suppression was the result of multiple tyrosine phosphoryl ation of Kv1.3 channel. Y to F single-point mutations in the channel or del etion of the kinase domain in IR blocks insulin-induced modulation and phos phorylation of Kv1.3. Neuromodulation of Kv1.3 current in OB neurons is act ivity dependent and is eliminated after 20 days of odor/sensory deprivation induced by unilateral naris occlusion at postnatal day 1. IR kinase but no t Kv1.3 expression is downregulated in the OB ipsilateral to the occlusion, as demonstrated in cryosections of right (control) and left (sensory-depri ved) OB immunolabeled with antibodies directed against these proteins, resp ectively. Collectively, these data support the hypothesis that the hormone insulin acts as a multiply functioning molecule in the brain: IR signaling in the CNS could act as a traditional growth factor during development, be altered during energy metabolism, and simultaneously function to modulate e lectrical activity via phosphorylation of voltage-gated ion channels.