Complement receptor <i>C3ar1</i> deficiency does not alter brain structure or functional connectivity across early life development

Genetic deletion of the complement C3a anaphylatoxin chemotactic receptor (<i>C3ar1</i>), a key component of the innate immune response, is reported to induce behavioural phenotypes resembling anxiety and hyperactivity in mice, suggesting a neurodevelopmental role for this gene in health. However, it is not currently clear when and where <i>C3ar1</i> is needed in the brain, which is further complicated by the fact that <i>C3ar1</i> is expressed predominantly by microglia and therefore does not localize to specific brain regions, warranting exploratory and brain-wide assessment through neuroimaging. Resolving when and where <i>C3ar1</i> is needed are questions of significant translational importance because, as a G-protein-coupled receptor, human C3AR1 serves as a potential therapeutic target for disorders associated with complement upregulation, such as schizophrenia. To provide a brain-wide assessment of developmental <i>C3ar1</i> activity, we used longitudinal MRI in male and female adolescent and adult mice (<i>N</i> = 34 <i>C3ar1^{tm1Cge/tm1Cge}</i> and <i>N</i> = 35 <i>C3ar1^+/+</i> ) to estimate regional brain volume using tensor based morphometry, white matter microstructure using fractional anisotropy from diffusion-weighted MRI, and functional connectivity from blood oxygen-level dependent MRI, with behavioural assessment in adulthood. We repeated structural MRI measures in this cohort <i>ex vivo</i> to achieve higher resolution. We further repeated <i>in vivo</i> structural assessment preceded by behavioural testing in adulthood in a second cohort of mice (<i>N</i> <i>=</i> 20 <i>C3ar1^{tm1Cge/tm1Cge}</i> and <i>N</i> <i>=</i> 19 <i>C3ar1^+/+</i> ) to improve confidence in our findings. We achieved low regional brain volume variability, allowing us to resolve previously reported sexually dimorphic effects. We were further able to confirm a well-known developmental increase in fractional anisotropy. Despite being able to detect these established effects, we did not find a robust <i>C3ar1</i>-dependent phenotype in any of the measures we tested, including behaviour, which may be attributed to our study being the first behavioural study in <i>C3ar1</i>-deficient mice to include littermate controls. Therefore, our data do not support neurodevelopmental hypotheses for <i>C3ar1</i>, which is encouraging for therapeutic strategies targeting this receptor since interventions are unlikely to disrupt brain development.