Neuroblast marker Doublecortin expression in the lateral ventricular wall of the brains from Lrig1-T2A-iCreERT2/+; ROSA26-Ai14/+ mice throughout adult life

Lateral ventricular wall whole mount immunofluorescence confocal images from brains of Lrig1T2A-iCreERT2/+; ROSA26Ai14/+ 1c2 mice induced once with a high dose of tamoxifen at 3 months of age. Dcx immunostain throughout adult life.To see why we picked 12 weeks of age as the adult age starting point, see a graph of C57BL/6J mouse weight growth curve at https://www.jax.org/-/media/jaxweb/images/jax-mice-and-services/body-weight-chart-directory/000664-body-weight-01.jpg?h=1004&w=1566&la=en&hash=4C4F0CDD33F2F73DE0087E7D0EFDD4998AB8D351. Also see a graph of C57BL/6J mouse brain weight growth curve at https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3474345/figure/F5/. Mouse body weight increases more rapidly after weaning at ~3 weeks of age and until reaching sexual maturity at ~6 weeks of age. Although the brain weight does not change much between 20 and 70 days, maturation of the brain continues in other ways through and beyond puberty, as is evident from the behavior of the mouse. By 12 weeks of age, the mouse is no longer growing rapidly, and is sexually and behaviorally mature.See image 686_2 at https://doi.org/10.6084/m9.figshare.12555527.v3 for Dcx immunostain on the brains of wildtype C57BL/6J mice to compare to the brains of the Lrig1T2A-iCreERT2/+; ROSA26Ai14/+ mice shown above. The Dcx immunostaining patterns of the wildtype and double heterozygous mice were similar, suggesting that neuroblast generation is unperturbed in the Lrig1T2A-iCreERT2/+; ROSA26Ai14/+ mice that were injected tamoxifen at 12 weeks of age. Consistent with this notion, with our one injection induction protocol of adult mice, no obvious differences in the number of Ki-67+ cells were observed after the tamoxifen injection (compare images 686_2, 773_2, 777_2 at https://doi.org/10.6084/m9.figshare.12555527.v3 to image 689_5 at https://doi.org/10.6084/m9.figshare.12665507.v4).A recent report documented tamoxifen toxicity in the brains of embryonic and juvenile mice (https://doi.org/10.1073/pnas.1918883117). As aforementioned, I didn't observe such toxicity due to tamoxifen (side)effects or high cre activity in the brains of the Lrig1T2A-iCreERT2/+; ROSA26Ai14/+ mice at 13 weeks of age. First, it is noted that the 2 mg per day for 5 days regime for the juvenile mice amounts to approximately 5 times the maximum dose we utilized (i.e., 2 mg into 3 week old mice, at ~10 g --> 200 mg/kg, times 5; our maximum dose was 230 mg/kg once). Second, at the lower dose of 2 mg over 24 h into 3 week old mice (e.g., 2 mg/0.01 kg --> 200 mg/kg), less reduction in the numbers BrdU+ or Ki-67+ cells were observed. Thus, at this time, an attempt to reconcile these observations is that the younger mice (embryonic and juvenile mice) are more susceptible to tamoxifen's dose-dependent (side)effects, and these (side)effects occur at lower tamoxifen doses in younger mice than in adults. As is usual, a minimally toxic dose of tamoxifen might need to be titrated.