Rapamycin Effects on the Brains of C57BL/6 Mouse Models

Background: Rapamycin is a specific inhibitor of mTOR (Mammalian Target of Rapamycin), which regulates multiple biological activities. There have been some researchers looking into rapamycin efficacy against Autism and other neurological diseases, and it has been verified multiple times that rapamycin does have the ability to ameliorate those diseases, but meanwhile, some data also implied that rapamycin could have potential toxicity towards brains.

Objectives: To apply rapamycin on wild type C57BL/6 mouse models and use MR imaging to examine change on whole brain regions, and any reversal change after ceasing the application of drug. Based on the ROIs (regions of interest), more behavioral tests and histology tests will be conducted to verify any correlations between brain volume change and potential adverse effect

Methods: To investigate the potential influence of rapamycin on the brain, we use 6 weeks/10 weeks old wild type C57BL/6 mouse models as subjects or vehicles. Rapamycin was dissolved in 0.25% polyethylene glycol and 0.25% tween prior to usage, and subjects were administered 6 mg/kg every Monday, Wednesday, and Friday by intraperitoneal injections for 4 weeks, while vehicles were administered with the same solution without rapamycin of the same amount and frequency. The drug application ceased from week 5 and all mice are still examined for another 6 weeks for potential reverse symptoms. All mice were examined by 7.0 Tesla MRI scanner at the end of each week, with contrast agent (MnCl2 Solution) applied a day in prior to scanning. All data retrieved by MRI scanner was processed by python and R.

Results: The outcomes of experiment were quite different between subjects of 6 weeks old and 10 weeks old. Upon the initiation of the drug application, subjects (of either 6 weeks old or 10) were experiencing a slower drop of brain volumes, followed by a fast drop from week 3. 10-week subjects' brain volume dropped to the lowest (3.43%) at the end of week 4 and an almost immediate recovery began when stopping using the drug starting from week 5. The volume went all way back to nearly the initial level after 4 weeks and stay steady since. From the t-statistics produced based on the MR images, only a few areas located mainly at cerebral cortex showed high significance of atrophy (0.1 FDR). However, 6-week subjects were experiencing a larger and faster decrease in brain volume and continued to drop after the cessation of drug (as low as 6.44%). T-statistics showed atrophy across the whole brain (0.1 FDR), especially in visual, auditory, motor, and somatosensory cortex regions. The recovery process also took much longer than 10-week subjects, and had only recovered less than 1% at the end of week 8.

Conclusions: The results showed that rapamycin does have the ability to cause brain atrophy and the younger the subjects are, the worse the possible influences seem to be. The adverse effect on the brain is partially recoverable according to the results, and possible influence on the subjects are still examined by detailed behaviour tests and histology tests.