Anterior (AHD) and complete hypothalamic deafferentation (CHD) were performed in female rats to ascertain the origin of LHRH detected in the external layer of ME. Deafferented brains were serially sectioned in a cryostat in the frontal plane. LHRH activity in each section was determined by RIA. While AHD, using a knife with a small radius caused no change in the distribution of LHRH in the brain, AHD, using a knife with a large radius resulted in a significant decrease in LHRH content in the

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... -ME. CHD caused a more marked decrease in the ARC-ME LHRH than AHD. The decrease was more marked in the anterior and posterior portions of the ARC-ME than in the middle one. These effects of CHD on the distribution of LHRH in the ME were ascertained by immunohistochemical studies. In contrast, LHRH content in the POA showed no significant change after AHD and CHD. However, a few LHRH containing fibers were observed in the area lying just proximal to the cut by immunohistochemistry. These results strongly suggest that a part of the LHRH detected in the external layer of ME is derived from that synthesized outside the MBH, possibly in the POA and another part is derived from that synthesized inside the MBH, possibly in the ARC.