Hypothermia and Stem Cells Confer Limited Protection to Mammillary Bodies in a Neonatal Mouse Model of Hypoxia‐Ischemia

Background Hypoxic–ischemic (HI) brain injury due to perinatal asphyxia causes long‐term neurological morbidity in newborns. HI affects brain areas including the cortex, hippocampus, and basal ganglia. More recently the mammillary bodies (MBs), critical for memory, were also shown to be affected in patients, despite treatment with therapeutic hypothermia (TH). Here we characterized the kinetics and extent of MB damage in an established neonatal HI mouse model and evaluated the therapeutic potential of TH or a novel regenerative intervention: intranasal mesenchymal stem cell (MSC) therapy. Methods HI was induced in 9‐day‐old mouse pups; structural and cellular MB damage was assessed at days 1 to 3 and day 28 post HI by immunohistochemistry. For TH, mice were placed at 32 °C for 3 hours directly post HI, and MB damage was assessed at day 10. For MSC therapy, mice received intranasally 0.5×10 6 MSCs at day 10 post HI and MB damage was assessed at day 28 post HI. Results At days 1 to 3 post HI, mice showed cell death, microglia activation, and early gliosis in the ipsilateral MB. Glial scarring was still present at day 28 post HI with concomitant ipsilateral MB atrophy. Additionally, HI induced mammillothalamic tract atrophy and reduced the expression of calbindin + but not parvalbumin + cells. While TH and intranasal MSCs both reduced the HI‐induced cortical/hippocampal lesions, neither treatment reduced MB damage. Conclusions Our novel findings demonstrate early onset and long‐lasting MB damage in a neonatal HI mouse model, mirroring newborn patients. TH and MSCs therapy failed to protect the MBs, highlighting the importance for future research into treatment strategies to protect or repair this vulnerable structure.