Localization of luteinizing hormone-releasing hormone in the preoptic area and hypothalamus of the rat using radioimmunoassay.

To determine the localization of luteinizing hormone-releasing hormone (LHRH), five brains from adult male rats were serially sectioned in a cryostat at - 10 C in either the frontal, horizontal or sagittal planes. Acetic acid-ethanol extracts of each section were assayed for LHRH using radioimmunoassay (RIA) and in some cases using bioassay as well. Approximately 0.2 ng LHRH was concentrated in medial basal preoptic (MB-PO) tissue overlying the rostral portion of the optic chiasm. This LHRH appears to be associated with the organum vasculosum of the lamina terminalis and/or adjacent neural tissue. Uniform, low levels of LHRH were detected in hypothalamic tissue between the preoptic area (POA) and arcuate-median eminence (ARC-ME) region. In the ARC-ME region 2.7 ng of LHRH were concentrated primarily in the median eminence. The lateral distribution of LHRH in the ARC-ME region extended beyond the median eminence into tissue corresponding to the lateral aspect of the ventromedial nucleus. Concomitant bioassay and RIA determinations of LHRH were highly correlated. Of the sections bioassayed, only those sections containing LHRH released FSH. These results confirm the presence of LHRH in the POA and in the rostral hypothalamus of the rat brain. The possible significance of LHRH in the POA for the regulation of LH release is discussed.