title: Oxytocin mediates copulation-induced hypoalgesia of male rats.
abstract: Copulatory behavior has been reported to raise the pain threshold in male rats. In this study, we examined the effect of copulatory behavior with or without ejaculation on pain threshold measured by electrical shock via an electrode attached to the tail. It was demonstrated that ejaculation is not necessary to raise the pain threshold in male rats. In addition, we examined whether oxytocin, a hypothalamic neuropeptide, was involved in copulation-induced hypoalgesia. Sexually experienced males were subjected to stereotaxic implantation of a guide cannula targeting the lateral ventricle. After the recovery period, half of the males were intracerebroventricularly treated with an oxytocin antagonist (OTA, 100ng d(CH2)51,Tyr(Me)2,Thr4, Orn8,Tyr-NH29]-vasotocin/1μL saline) and the remaining half were administered saline without anesthesia. Fifteen minutes later, half of each group were given sexual behavior with receptive females. We found no effect of OTA on sexual activity. Immediately after ejaculation, pain threshold was measured. While raised pain threshold was observed after sexual behavior in saline-treated males, no change in pain threshold was found in OTA-treated males even after copulation. The results suggest that central oxytocin mediates copulation-induced hypoalgesia in male rats.
authors.affiliation: Department of Tokyo Judo Therapy, Teikyo University of Science, Japan; Department of Physiology, Nippon Medical School, Japan.
authors.forename: Hiroko
authors.initials: H
authors.lastname: Futagami
authors.affiliation: Department of Physiology, Nippon Medical School, Japan; Health Department of Rehabilitation, University of Tokyo Health Science, Japan.
authors.forename: Yasuo
authors.initials: Y
authors.lastname: Sakuma
authors.affiliation: Department of Physiology, Nippon Medical School, Japan; Department of Animal Sciences, Teikyo University of Science, Japan. Electronic address: ykondo@ntu.ac.jp.
authors.forename: Yasuhiko
authors.initials: Y
authors.lastname: Kondo
mesh_terms: D000818:Animals; D003307:Copulation; D004542:Ejaculation; D005260:Female; D007276:Injections, Intraventricular; D008297:Male; D010121:Oxytocin / Q000502:physiology*; D010146:Pain / Q000503:physiopathology*; D017288:Pain Threshold; D020318:Rats, Long-Evans; D018045:Receptors, Oxytocin / Q000037:antagonists & inhibitors; D014668:Vasotocin / Q000031:analogs & derivatives / Q000494:pharmacology
chemical_list: D018045:Receptors, Oxytocin; C076093:vasotocin, (beta-mercapto-beta,beta-cyclopentamethylenepropionic acid)-O-methyl-Tyr(2)-Thr(4)-Orn(8)-Tyr(9)-NH2; D010121:Oxytocin; D014668:Vasotocin
keywords: Oxytocin; Pain threshold; Rats; Sexual behavior