Mammalian nervous system function involves billions of neurons which are interconnected in a multitude of neural circuits. Here we describe a genetic approach to chart neural circuits. By using an olfactory-specific promoter, we selectively expressed barley lectin in sensory neurons in the olfactory epithelium and vomeronasal organ of transgenic mice. The lectin was transported through the axons of those neurons to the olfactory bulb, transferred to the bulb neurons with which they synapse, and transported through the axons of bulb neurons to the olfactory cortex. The lectin also was retrogradely transported from the bulb to neuromodulatory brain areas. No evidence could be obtained for adverse effects of the lectin on odorant receptor gene expression, sensory axon targeting in the bulb, or the generation or transmission of signals by olfactory sensory neurons. Transneuronal transfer was detected prenatally in the odor-sensing pathway, but only postnatally in the pheromone-sensing pathway, suggesting that odors, but not pheromones, may be sensed in utero. Our studies demonstrate that a plant lectin can serve as a transneuronal tracer when its expression is genetically targeted to a subset of neurons. This technology can potentially be applied to a variety of vertebrate and invertebrate neural systems and may be particularly valuable for mapping connections formed by small subsets of neurons and for studying the development of connectivity as it occurs in utero.
