Melatonin blocks in vitro generation of prostaglandin by the uterus and hypothalamus.

The effects of melatonin on the motility (isometric tension developed and contractile frequency) of uterine horns isolated from ovariectomized rats as well as on the mechanical responsiveness to added oxytocin or prostaglandin F2 alpha (PGF2 alpha) were explored. The pineal indole (10(-6) M or higher) depressed significantly the spontaneous motility of the uterus and reduced the responses evoked by oxytocin but not those evoked by PGF2 alpha. Melatonin was also tested on the prostaglandin (PG) release into the suspending media from either uterine horns from spayed rats or bovine medial basal hypothalamic (MBH) explants. Melatonin (10(-3) M) diminished the output from the uterus or the MBH of both PGE and PGF-"like material". Similarities between the effects of melatonin and indomethacin as well as the possible physiological relevance of the present findings are discussed.