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The luteinizing hormone-releasing hormone (LHRH) content of the two halves of the mediobasal hypothalamus (MBH) has been studied two weeks after unilateral or bilateral ovariectomy (OVX) in the rat. In control animals, the LHRH content of the right-side MBH was higher than that of the left side. Right-side OVX induced a significant increase in LHRH content on the right-side MBH while left-side OVX was followed by elevated LHRH content on the left-side MBH. In contrast, bilateral OVX induced a significant decrease on the right-side MBH. The observation that unilateral OVX induced increased LHRH content in the ipsilateral half of the MBH to OVX, supports the view that a neural pathway might connect the ovary and the MBH.

Effects of neurotransmitters on in vitro release of lutein-hormone-releasing hormone (LHRH) from fragments of hypothalamic tissue were investigated. Neither acetylcholine (ACh) or histamine (HA) at concentrations of 10(-7) and 10(-5) M, or GABA at 10(-7) to 5 x 10(-5) M affected LHRH release from mediobasal hypothalamic (MBH) fragments or the organum vasculosum of the laminae terminalis (OVLT). In contrast, serotonin (10(-7) and 10(-5) M) significantly inhibited the release of LHRH from the MBH but not from the OVLT. This response to serotonin (5-HT) is no longer observed in presence of methiothepin, a 5-HT receptor antagonist. The present results confirm in vivo data suggesting that the inhibitory effect of serotonin on the release of LHRH from median eminence involves a direct action on neurosecretory nerve-endings.