Projections of the estrogen receptor-immunoreactive ventrolateral hypothalamus to other estrogen receptor-immunoreactive sites in female guinea pig brain

The ventrolateral hypothalamus in female guinea pigs includes an estrogen receptor dense region adjacent to the ventromedial hypothalamus. This region is reciprocally connected with other estrogen receptor-containing areas suggesting that steroid hormone receptor-containing cells may be directly linked. Phaseolus vulgaris leucoagglutinin, an anterograde tract tracer, was specifically placed in this region with the aim of labeling some projections from estrogen receptor-containing neurons. These projections were colocalized immunocytochemically with the distribution of estrogen receptor-containing cells. Dense ventrolateral hypothalamic innervation was observed in some regions also containing a high concentration of estrogen receptor-containing cells. These regions included the medial preoptic area, the bed nucleus of the stria terminalis, the ventrolateral hypothalamus anterior and posterior to the injection site, and the midbrain central gray. A low density of ventrolateral hypothalamic fibers and terminals was observed in two regions rich in estrogen receptors, the amygdala and the arcuate nucleus. In general, ventrolateral hypothalamic fibers and terminals were present in all regions where estrogen receptors were found except the medial thalamus and habenular region. Labeled terminal boutons and perineuronal baskets were found around estrogen receptor-containing cells in most regions which contained estrogen receptor-containing cells. These close appositions were suggestive of synaptic contacts, suggesting that the ventrolateral hypothalamus may influence steroid-dependent behaviors via the modulation of estrogen receptor-containing cells. Furthermore, ventrolateral hypothalamic projections may include direct connections with estrogen receptor-containing cells, suggesting the presence of a network of interconnected estradiol-sensitive neurons involved in the regulation of estradiol-dependent functions.