Oral and injected tamoxifen alter adult hippocampal neurogenesis in female and male mice

Abstract

Inducible Cre recombinase facilitates temporal control of genetic recombination in numerous transgenic model systems, a feature which has made it a popular tool for adult neurogenesis studies. One of the most common forms of inducible Cre, CreER^T2, requires activation by the selective estrogen receptor modulator tamoxifen (TAM) to initiate recombination of LoxP-flanked sequences. To date, most studies deliver TAM via intraperitoneal injection. But the introduction of TAM-infused commercial chows has recently expanded the possible modes of TAM delivery. Despite the widespread use of TAM-inducible genetic models in adult neurogenesis research, the comparative efficiency and off-target effects of TAM administration protocols is surprisingly infrequently studied. Here we compare a standard, 5 day TAM injection regimen with voluntary consumption of TAM-infused chow. First, we used adult NestinCreER^T2;Rosa-LoxP-STOP-LoxP-EYFP reporter mice to show that 2 weeks of TAM chow and 5 days of injections led to LoxP recombination in a similar phenotypic population of neural stem and progenitor cells in the adult dentate gyrus. However, TAM chow resulted in substantially less overall recombination than injections. TAM administration also altered adult neurogenesis, but in different ways depending on administration route: TAM injection disrupted neural progenitor cell proliferation 3 weeks after TAM, whereas TAM chow increased neuronal differentiation of cells generated during the diet period. These findings provide guidance for selection of TAM administration route and appropriate controls in adult neurogenesis studies using TAM-inducible Cre mice. They also highlight the need for better understanding of off-target effects of TAM in other neurological processes and organ systems.

Significance Statement

Numerous transgenic mouse models use the inducible Cre-Lox system in which a synthetic estrogen tamoxifen is used to initiate recombination of genetic sequences flanked by LoxP sites. Despite widespread use, there is little data available about the effectiveness of different tamoxifen delivery methods. Here, we show that giving adult mice in which inducible Cre is expressed in neural stem and progenitor cells tamoxifen via injection (the most commonly-used route) and chow both induce recombination in those target cells. However, chow delivery is far less efficient than injections and both delivery methods alter adult neurogenesis on their own when compared to vehicle-treated mice. These findings provide guidance for investigators choosing experimental design with inducible Cre systems.