The rat offers a uniquely valuable animal model in neuroscience, but we currently lack an individual-level understanding of the in vivo rat brain network. Here, leveraging longitudinal measures of cortical magnetization transfer ratio (MTR) from in vivo neuroimaging between postnatal days 20 (weanling) and 290 (mid-adulthood), we design and implement a computational pipeline that captures the network of structural similarity (MIND, morphometric inverse divergence) between each of 53 distinct cortical areas. We first characterized the normative development of the network in a cohort of rats undergoing typical development (N=47), and then contrasted these findings with a cohort exposed to early life stress (ELS, N=40). MIND as a metric of cortical similarity and connectivity was validated by cortical cytoarchitectonics and axonal tract-tracing data. The normative rat MIND network had high between-study reliability and complex topological properties including a rich club. Similarity changed during post-natal and adolescent development, including a phase of fronto-hippocampal convergence, or increasing inter-areal similarity. An inverse process of increasing fronto-hippocampal dissimilarity was seen with post-adult aging. Exposure to ELS in the form of maternal separation appeared to accelerate the normative trajectory of brain development - highlighting embedding of stress in the dynamic rat brain network. Our work provides novel tools for systems-level study of the rat brain that can now be used to understand network-based underpinnings of complex lifespan behaviors and experimental manipulations that this model organism allows.
Significance statement: Network models derived from neuroimaging have revolutionized our understanding of human brain development but need translation into animal models to interrogate their underlying mechanisms. Rats provide a valuable model due to their complex behaviors and biological similarities to humans; however, in vivo models of individual-level brain networks remain underdeveloped. In this study, we present a novel computational pipeline to construct such networks from in vivo rat structural neuroimaging data. Our findings highlight the dynamic development and experiential sensitivity of fronto-hippocampal systems in rats, offering a reference for cross-species comparisons and mechanistic insights into brain architecture. To support broader research efforts, we include an open release of code and data for rat MIND similarity network analysis.