GH-RIH-containing neural elements in the rat hypothalamus

Abstract

With the use of rabbit anti-LH-RH serum, a of delicate nerve fibers and terminals which contain immunoreactive LH-RH can be demonstrated in the median eminence of the rat, corresponding exactly to the tubero-infundibular tract as described earlier with the aid of classical neurohistological methods. The delicate fiber can be traced to the region of the retrochiasmatic area and especially to that of the arcuate nucleus but not farther. Nerve cells do not show any immunopositive reaction for LH-RH in the experimental conditions studied. LH-RH positive nerve fibers terminate on or in the immediate vicinity of the capillary loops penetrating the median eminence. (Endocrinology 96: 135, 1975) THE tubero-infundibular tract was described by Szentagothai (1,2) as the main source of nerve fibers terminating on or in the immediate neighborhood of the capillary loops penetrating the median eminence (ME). On the basis of Szentagothai's neurohistological findings and of Halasz's experimental results concerning the hypophysiotrophic area (3,4,5) they suggested (6) that the tubero-infundibular tract or parvicellular neurosecretory system contains axons of nerve cells which synthesize the so-called among which are luteinizing hormone (LH) and follicle-stimulating hormone (FSH) releasing factors or hormones (LH-RF and FSH-RH). In connection with the localization of the LH-RH-synthesizing neural elements, Halasz et al. (4,5) and Flament-Durand (7) found that the materials present in the hypothalamus would maintain normal gonadotrophic function in hypophysectomized rats only when pituitary grafts were located in the hypophysiotrophic area which includes the arcuate nuclei and the retrochiasmatic area of the hypothalamus. McCann (8) showed that the bulk of LH-RH Received May 2, 1974. 1 This work was supported in part by Population Council Grant No. M72.85 and USPHS Research Grants AM-09094, HD-06555, AM-07467, and VA Administration. activity resided in the ME region of the rat. Smaller but significant amounts could also be found in the part of the hypothalamus designated chiasmatic. The findings by Endroczi and Hilliard (9) again emphasized the high concentration of LH-RH in the ME in the rabbit. (9) In the experiment of Mess et al. (10), LH-RH disappeared from the ME of rats with electrolytic lesions in the preoptic-suprachiasmatic region and in the ventral part of the paraventricular nuclei. Lesions placed in the area of the arcuateventromedial nuclei also brought about some reduction in LH-RH levels in the ME (11). Crighton et al. (12) localized LH-RH activity by sectioning frozen rat hypothalami and assaying the extracts prepared from discrete areas by means of an in vitro assay for LH-RH. Although it confirmed the earlier data of McCann (8), the study of Crighton et al. (12) could not determine the localization of the cell bodies of the neurons that presumably secrete LH-RH. From the results of a subsequent experimental approach, Schneider, et al. (13) concluded that LH-RH secreting neurons might be located in the suprachiasmatic area. This localization was supported by the findings of Quijada et al. (14) who sectioned fresh, frozen hypothalami from normal male rats with a microtome at 400 fim in the frontal, horizontal and sagittal planes. Each section was extracted with 0.1 N HC1 and boiled