Localization of luteinizing hormone-releasing hormone in rat hypothalamus using radioimmunoassay

Radioimmunoassays for LH-RH were performed on frozen rat brain sections cut serially in coronal, parasagittal and horizontal planes. In some of the assays, samples were pooled from corresponding areas in different animals. A clear pattern of distribution of LH-RH rich regions emerged. Two prominent components - a caudal high curve and a rostral smaller hump - were observed, and their variable characteristics discussed. The high curve represents the arcuate-medium eminence (ME) region. Our data suggest that this region is not homogeneous, and three different subdivisions of this arcuate-ME region can be distinguished on the basis of LH-RH content. High values were obtained consistently in the arcuate-ME region, except for females in the late afternoon of dioestrus day 2, at which stage the levels in this region dropped until they were little more than base line. The rostral hump of high LH-RH activity varies both in position and amplitude. These variations are associated with (1) the sex of the animal and (2) the stage of the female cycle. In males this hump appeared in the region of the suprachiasmatic nucleus, while in dioestrous females it appeared in the medial preoptic area, rostral to the male location. Some changes in LH-RH levels are thought to be related to the stage in the female sex cycle. During the afternoon of dioestrus, the caudal high curve representing the arcuate-ME region shrank, whereas the rostral smaller hump (preoptic region) showed much higher levels. Some feed-back take-off may occur from the LH-RH released by the arcuate-ME region. Instead of synthesizing its own LH-RH, the preoptic area may concentrate some of the LH-RH released from the arcuate-ME region, thereafter initiating sexual behaviour as suggested by Moss & McCann (1973).