Demonstration of habenular neurons which receive afferent fibers from the nucleus accumbens and send their axons to the midbrain periaqueductal gray.

After injecting Phaseolus vulgaris leucoagglutinin (PHA-L) and wheat germ agglutinin conjugated with horseradish peroxidase (WGA-HRP), respectively, into the medial part of the nucleus accumbens (NA) and the midbrain periaqueductal gray including the dorsal raphe nucleus (PAG/DR) on one side of single rat brains, we observed that axon terminals labeled anterogradely with PHA-L made possible synaptic contact with neurons in the habenula (Hb) which were retrogradely labeled with WGA-HRP. These Hb neurons were distributed in the medial part of the lateral Hb. The results indicate that the medial part of the lateral Hb relays neuronal information from the medial part of the NA to the PAG/DR.