Projection Sites of Medial Preoptic Area and Ventral Bed Nucleus of the Stria Terminalis Neurons that Express Fos during Maternal Behavior in Female Rats

Abstract

Medial preoptic area (MPOA) and ventral bed nucleus of the stria terminalis (VBST) neurons are involved in maternal behavior, but the neural sites to which the maternally relevant neurons project have not been determined. Since MPOA and VBST 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 VBST 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 VBST than did females tested with candy. After WGA injections into several brain sites, the number of double-labeled cells observed in the MPOA and VBST 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 VBST projected most strongly to the retrorubral field, ventral tegmental area, and medial hypothalamus. Although relatively few MPOA and VBST 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 VBST projection to the periaqueductal gray. This study suggests that MPOA and VBST efferents project to a variety of regions to promote full maternal responsiveness.

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