Neuroanatomical Tract Tracers: Not Just for Neural Tracing Anymore.

Neuroanatomical tract tracers (NaTTs) have been used for neural circuit tracing for decades and now find recent applications in disease diagnosis and drug and gene delivery. In this Review, first the different subclasses of NaTTs including nonviral and viral types and their unique properties are discussed. The focus then is shifted to recent developments in improving the design and performance of NaTTs for neural circuit mapping, their role in disease diagnostics, and the emerging applications in drug and gene delivery targeted to the nervous system. In contrast to most molecular and biologic drugs that do not pass through the blood-brain barrier, NaTTs, including certain types of plant lectins, bacterial toxins, and some viruses, are readily taken up by nerve endings in mammalian muscle and efficiently transported within the central nervous system to the brain. Incorporating NaTTs into nanomedicines to bypass biological barriers and to deliver drugs to specific neurons thus presents an exciting direction and offers many possibilities for drug delivery. We hope that this Review will catalyze further discussions and collaborations among neuroscientists, biomedical researchers, and nanotechnologists that lead to innovative therapeutic options for treating neurological diseases.