While the hypothalamus is now classified into more than ten compartments, uncharacterized areas remain. In this study, we show a new area in the anterior hypothalamus (AH) of mice, a triangular-shaped area between the paraventricular hypothalamic nucleus (PVN) and the fornix, which is enriched in chondroitin sulfate proteoglycans (CSPGs). We designated this region perifornical area of the AH (PeFAH) based on its anatomical location. In Nissl staining, the PeFAH was distinguishable as an area of

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... relatively low density. Immunohistochemical and DNA microarray analyses indicated that PeFAH contains sparsely distributed calretinin-positive neurons and densely clustered enkephalin-positive neurons. Furthermore, the PeFAH was shown to have bidirectional neural connections with the lateral septum (LS). We confirmed enkephalinergic projections from PeFAH neurons to the LS, and inversely, calbindin-positive LS neurons as afferents to the PeFAH. Finally, c-Fos expression analysis revealed that the activity of PeFAH neurons tended to be increased by psychological stressors but not by homeostatic stressors. These findings of neuronal subtypes and projections suggest that the region of the densely clustered encephalin-positive neurons in the PeFAH is comparable with the perifornical nucleus previously identified in rats.