Does the hypothalamus of infantile female rats contain a separate follicle-stimulating hormone releasing factor

In an attempt to determine if the high serum follicle-stimulating hormone (FSH) levels of infantile female rats are related to the presence at this age of an FSH-releasing factor different from luteinizing hormone-releasing hormone (LHRH), the FSH and luteinizing hormone (LH) releasing activities as well as the radioimmunoassayable LHRH content of three hypothalamic regions were measured in 12 and 30 day old rats, ages at which circulating FSH levels were elevated and low, respectively. In 30 day old rats, the medial basal hypothalamus-median eminence region (MBH-ME) and the postchiasmatic dorsal area (PCH-DA), which included the dorsal anterior hypothalamic area and parsventricular nuclei, had greater radioimmunoassayable LHRH and LH/FSH releasing activities than in the younger animals. The suprachiasmatic-preoptic region (SCH-POA), however, exhibited similar radioimmunoassayable LHRH and gonadotropin-releasing activities in both age groups. The anterior pituitaries from 12 day old rats, which were employed to assess the biological activity of the hypothalamic extracts, exhibited a greater release of FSH than LH in response to both the extracts and synthetic LHRH. The ratio of FSH to LH released was greater at low doses of the extracts or synthetic LHRH and decreased progressively as the doses were increased. Further dissection of the PCH-DA region of 12 day old rats revealed that the paraventricular nuclei dorsal anterior hypothalamic area (PVN) released more FSH and LH than the ventral anterior hypothalamic area (VAHA). Surprisingly, however, the radioimmunoassayable LHRH content, measured with two different antisera, was distinctly greater in the VAHA than in the PVN region. Administration of an antiserum to LHRH to 11 day old females completely prevented the postcastration rise in both serum FSH and LH. The results indicate that the elevated serum FSH levels present in infantile female rats are not related to the presence of an FSH-RF different from LHRH, but rather can be attributed to an enhanced capability of the pituitary to release FSH. In addition, however, they suggest that the paraventricular nuclei and surrounding areas contain a biologically active form(s) of LHRH which does not exhibit a corresponding radioimmunological activity.