Insulin in brain: The physiological functions and therapeutic insights for neurodegenerative diseases

This review highlight the function of insulin in the central nervous system in addition to its role in the periphery. The cerebral distribution and mechanisms of insulin and its receptor isoforms are reviewed in detail. We emphasize the essential roles of insulin in the maintenance of cerebral glucose homeostasis, modulation of cognitive performance, regulation of appetite, promotion of cerebrovascular angiogenesis, and exertion of neuroprotective effects. We demonstrate how insulin resistance exacerbates characteristic neuropathological features in Alzheimer's disease (AD) and Parkinson's disease (PD), while insulin-based interventions ameliorate these pathologies through multiple mechanisms including increasing the activity of insulin-degrading enzyme, suppressing Aβ neurotoxicity, and reducing α-synuclein deposition. The review also systematically examines the neuroprotective effects of insulin sensitizers and their potential to reduce the risk of AD, while noting the complexity of their bidirectional regulatory role in PD, which warrants further investigation. Notably, intranasal insulin administration emerges as a promising non-invasive therapeutic approach that bypasses the blood-brain barrier via olfactory and trigeminal pathways, suggesting significant potential for cognitive enhancement and neuropathological mitigation. Nonetheless, it must be noted that the optimal dosage, long-term safety, and sustained efficacy of insulin therapy remain unclear, and the current evidence is derived primarily from preclinical studies or small-scale clinical trials. In summary, this review paper underscores the critical physiological roles of insulin in the brain and outlines novel therapeutic strategies for using insulin in the treatment of AD and PD.