Subcellular localization of LH releasing activity in the rat hypothalamus

Separation of particulate fractions associated with LH releasing activity has been effected by differential centrifugation followed by sucrose density gradient centrifugation. The fractions were monitored for LH releasing activity by an in vitro incubation with rat pituitaries and for subcellular organelles by electron microscopy. LH released into the incubation medium and contained in the pituitaries was measured by radioimmunoassay (RIA). After differential centrifugation only the "crude mitochondrial fraction" caused an increase of LH release accompanied by a depletion of pituitary LH and a rise in total LH. Further fractionation of this pellet on a discontinuous sucrose gradient resulted in three opaque bands and three clear areas which were removed as sic fractions. Only one fraction, not associated with myelin, synaptosomes, or mitochondria, caused an increase in LH release and total LH while pituitary LH remained unchanged. This fraction appears to be predominantly composed of electron dense vesicles of varying sizes. Also of interest is another fraction which decreased LH release and was again not associated with myelin, synaptosomes, or mitochondria. The decreased release was associated with an increased pituitary LH while total LH remained unchanged. Of the six subcellular fractions obtained from the "crude mitochondrial pellet" only these two caused significant changes in LH release. Two other neural tissues, the cerebellum and cerebral cortex, were similarly processed. None of these subcellular fractions significantly altered LH release and/or synthesis.