Viral Circuit Tracing in Biomedical Pain Research: Methods, Protocols, and Applications.

Viral circuit tracing has transformed pain research by providing detailed insights into the neural circuits underlying pain processing and modulation. This chapter offers a comprehensive overview of the methods, protocols, and applications of viral tracing techniques in pain research, focusing on adeno-associated virus (AAV), rabies virus, and herpes simplex virus (HSV) as key tools. These viral tracers facilitate the mapping of pain-related circuits, such as the periaqueductal gray (PAG)-dorsal raphe (DR) pathway and amygdala-centered circuits, which are pivotal for understanding both the sensory and emotional dimensions of pain. Step-by-step protocols are presented for viral injection, post-injection analysis, and circuit mapping, complemented by highlights of significant studies employing these approaches in acute and chronic pain models. Emerging integrations of viral tracing with optogenetics, chemo-genetics, and single-cell RNA sequencing are explored, underscoring their potential to advance future research. Viral tracing remains an indispensable tool for unraveling the complexity of pain circuits and holds great promise for developing precise, targeted interventions for pain management.