A Daily Injection Paradigm Disrupts the Environmental-Enrichment-Induced Pruning of Miswired Axonal Projections in Developing Mice.

The capacity for enhanced experience, modeled as environmental enrichment (EE) in laboratory animals, to drive positive changes in brain circuitry presents a promising avenue in the development of therapies for neurodevelopmental conditions. Less understood are the underlying mechanisms, or potential interactions of EE with other therapeutic approaches. We have previously shown that early exposure to EE can drive the partial repair of miswired uncrossed retinal projections, and the concomitant rescue of a visual behavior, in the Ten-m3 knockout (KO) mouse. This was associated with a highly spatiotemporally localized upregulation of microglial reactivity in the region where the correction was occurring which peaked around postnatal day (P)25. Aiming to confirm a causal role for microglial-mediated engulfment in this process, we assessed the effect of daily injections of minocycline or vehicle saline from P18 to P24 (inclusive) on measures of microglial reactivity and anatomical corrective pruning in P25 Ten-m3 KO mice. While an effect of EE was confirmed at this timepoint, intriguingly, we found that both the vehicle- and minocycline-treated mice had a similar lack of microglial reactivity and showed a marked absence of corrective pruning of their miswired retinal projections. This suggests that the injection procedure itself disrupted the experience-induced microglial-mediated circuit repair. These results underscore the highly sensitive nature of EE-driven corrective pruning actions of microglia and the critical importance of considering and controlling for all aspects of experience.