Formation of Projection Pathways from the Arcuate Nucleus of the Hypothalamus to Hypothalamic Regions Implicated in the Neural Control of Feeding Behavior in Mice

The arcuate nucleus of the hypothalamus (ARH) is a critical component of forebrain pathways that regulate a variety of neuroendocrine functions, including an important role in relaying leptin signals to other parts of the hypothalamus. However, neonatal rodents do not lose weight in response to leptin treatment in the same way as do adults, suggesting that certain aspects of leptin signaling pathways in the hypothalamus may not be mature. We tested this possibility by using DiI axonal labeling to examine the development of projections from the ARH to other parts of the hypothalamus in neonatal mice, paying particular attention to the innervation of the paraventricular nucleus (PVH), the dorsomedial nucleus (DMH), and the lateral hypothalamic area (LHA), each of which have been implicated in the regulation of feeding. The results indicate that ARH projections are quite immature at birth and appear to innervate the DMH, PVH, and LHA in succession, within distinct temporal domains. The projections from the ARH to the DMH develop rapidly and are established by the sixth postnatal day (P6), whereas those to the PVH develop significantly later, with the mature pattern of innervation first apparent between postnatal day 8 (P8)-P10. Furthermore, the ability of leptin to activate Fos in the PVH, DMH, and LHA appears to be age-dependent and correlates with the arrival of ARH projections to each nucleus. Taken together, these findings provide new insight into development of hypothalamic circuits and suggest an anatomical basis for the delayed postnatal regulation of food intake and body weight by leptin.