The metastatic microenvironment is receiving increasing attention as a target for new breast cancer therapies. The sympathetic nervous system (SNS) is a component of this microenvironment, and our recent studies indicated that SNS activity may support metastasis through beta-adrenergic pathways that recruit macrophages and induce a switch to pro-metastatic gene expression. Development of novel adjunctive therapeutic strategies that target neural regulation of metastasis requires characterization of the relationships between the SNS, the immune system and tumor cells. To address this need, the proposed studies utilize multimodal in vivo imaging techniques to address the following specific aims: 1. characterize SNS regulation of tumor cells in breast cancer metastasis, 2. characterize SNS regulation of tumor-associated macrophages in breast cancer metastasis, 3. characterize SNS regulation of the tumor microenvironment in breast cancer metastasis. These studies will define interactions between SNS nerve fibers, tumor cells and macrophages in the context of the tumor microenvironment and elucidate their collective effects on metastasis. Given recent clinical studies that suggest beta-blockade reduces breast cancer recurrence, the proposed studies are urgently needed to establish a mechanistic foundation for rapid translation of existing compounds (beta-blockers) and development of novel biomarkers of early cancer progression and new anti-metastatic strategies that target SNS regulation of the tumor microenvironment.