Thyrotropin-releasing hormone protects hippocampal neurons against glutamate toxicity via phosphatidylinositol 3-kinase/AKT pathway and new protein synthesis

Thyrotropin-releasing hormone is best known as a neuropeptide that stimulates the release of thyroid-stimulating hormone and prolactin in hypothalamic-pituitary-thyroid (HPT) axis. Independent from its activity in the HPT axis, TRH also exerts strong neuroprotective activity against neurodegenerative diseases such as Alzheimer’s disease, epilepsy and traumatic brain injury. Although multiple factors have been linked to its neuroprotective action, the cellular mechanism of TRH neuroprotection is still not clear. Here we show that TRH protects hippocampal neurons against glutamate toxicity via phosphatidylinositol 3-kinase (PI3K)/AKT pathway and new protein synthesis. Both adeno-associated virus (AAV) mediated TRH transduction and TRH peptide given exogenously over 24 hours period of time inhibit glutamate-induced lactate dehydrogenase (LDH) release. This effect is not mediated by the decreased intracellular calcium response as TRH treatment (24 hours) has no effect on glutamate-induced increase in intracellular calcium nor the calpain activity. While TRH treatment (10 minutes) significantly inhibits glutamate-induced increase in intracellular calcium, no protective effect is observed when TRH is applied 30 minutes before or after glutamate stimulation. Instead, PI3K inhibitor LY294002 but not mitogen-activated protein kinase (MAPK)/Extracellular signal-regulated kinase (ERK)1/2 inhibitor U0126 completely inhibits the protective effect of TRH. LY294002 also blocks TRH induced AKT activation. In addition, protein synthesis inhibitor cycloheximide inhibits the protective effect of TRH. Taken together, these results suggest PI3K/AKT signaling pathway and new protein synthesis are involved in the protective effect of TRH against glutamate toxicity, thereby providing mechanistic support for its action in neurodegenerative diseases.