Projection sites of medial preoptic area and ventral bed nucleus of the stria terminalis neurons that express Fos during maternal behavior in female rats

Medial preoptic area (MPOA) and ventral bed nucleus of the stria terminalis (VEST) neurons are involved in maternal behavior, but the neural sites to which the maternally relevant neurons project have not been determined. Since MPOA and VEST neurons express Fos during maternal behavior, we used a double-labeling immunocytochemical procedure to detect both Fos and a retrograde tracer, wheat germ agglutinin (WGA), in order to determine where these Fos neurons project. On Day 4 postpartum, fully maternal females were separated from their litters. On Day 5, WGA was iontophoretically injected into one of the following regions known to receive MPOA and/or VEST input: Lateral septum, medial hypothalamus at the level of the ventromedial nucleus, lateral habenula, ventral tegmental area, retrorubral field, or periaqueductal gray. On Day 7, females received a 2-h test with either pups or candy, after which they were perfused and their brains were processed for the detection of Fos and WGA. As expected, females tested with pups had more Fos-containing neurons in the MPOA and VEST than did females tested with candy, After WGA injections into several brain sites, the number of double-labeled cells observed in the MPOA and VEST was greater for the maternal females when compared to the non-maternal females, Therefore, these results pinpointed neural circuits that were activated during maternal behavior. For the maternal females, Fos-containing neurons in the MPOA projected most strongly to the medial hypothalamus at the level of the ventromedial nucleus and to the lateral septum, while Fos-containing neurons in the VEST projected most strongly to the retrorubral field, ventral tegmental area, and medial hypothalamus. Although relatively few MPOA and VEST neurons which expressed Fos during maternal behavior projected to the periaqueductal gray, these Fos-expressing neurons made up a relatively large proportion of the MPOA and VEST projection to the periaqueductal gray. This study suggests that MPOA and VEST efferents project to a variety of regions to promote full maternal responsiveness.