PII S0196-9781(97)00250-7 Sexual Motivation Promotes Oxytocin Secretion in Male Rats 1 VIVEKA HILLEGAART, 2 PAWEL ALSTER, KERSTIN UVNA ¨ S-MOBERG AND SVEN AHLENIUS Division of Pharmacology, Department of Physiology & Pharmacology, Karolinska Institute, S-171 77 Stockholm, Sweden Received 1 April 1997; Accepted 30 July 1997 HILLEGAART, V., P. ALSTER, K. UVNA ¨ S-MOBERG AND S. AHLENIUS. Sexual motivation promotes oxytocin secretion in male rats. PEPTIDES 19(1) 39 – 45, 1998.—The present study examines plasma oxytocin levels in relation to performance of copulatory behavior in male rats. The animals were divided into three groups: A) home-cage controls, B) sexually naive and C) sexually experienced. Following 15 min of sexual interactions with a sexually proceptive female, brought into estrus by sequential injections of estradiol benzoate (12.5 g animal -1 , -48 h) and progesterone (0.5 mg animal -1 , -6 h), the male rats were decapitated. Trunk blood was collected for preparation of plasma samples, and subsequent radioimmunoassay for oxytocin. Home-cage controls, not exposed to a sexually proceptive female, were decapitated at the same time as experimental animals. It was found that plasma oxytocin levels were significantly elevated in sexually naive rats following exposure to a sexually proceptive female, and that plasma oxytocin levels were highly correlated with intensity of copulatory performance in these animals. In addition, it was also found that plasma prolactin and glucose levels were increased, regardless of sexual experience, in comparison with home-cage controls. It is concluded that the emotional challenge, and the situation-specific demands for action, created by an encounter with a sexually proceptive female, are accompanied by an increased plasma concentration of oxytocin in sexually naive, but not sexually experienced, male rats. © 1998 Elsevier Science Inc. Motivation Sexual behavior Radioimmunoassay Oxytocin Male rat IN a previous series of experiments, specific changes were noted in forebrain monoamine synthesis in male rats, as a result of sexual interactions with a proceptive female. Thus, catecholamine synthesis was increased in circumscribed neocortical areas (prefrontal and cingulate), in the neostri- atum and the amygdala, whereas serotonin synthesis was increased in the cingulate cortex, the neostriatum and pal- lidal areas. Interestingly, this region-selective activation of the monoamine synthesis, produced by sexual interactions, was found in sexually naive, but not sexually experienced, rats and could be differentiated from that produced by physical activity on a treadmill (1,2). Furthermore, mere exposure to a sexually proceptive female, where physical contact was prevented, was sufficient to bring about these changes (46). From these observations it was concluded that the first encounter with a sexually challenging situation recruited neuronal pathways in the limbic forebrain. This conclusion has been confirmed in parallel and subsequent experiments by other investigators (24). The neurohypophysial hormone oxytocin has been shown to be of importance for social recognition and bond- ing in several animal species (9). Such mechanisms could be expected to be vital for kin recognition in intra- or inter- species competitive situations, as well as for cooperative behavior, such as occurs in sexual interactions. Such effects appear separated from mechanisms of copulatory perfor- mance or consummatory aspects, as suggested by observa- tions of gene knock-out mice lacking oxytocin (29). Extra- hypothalamic oxytocinergic projections to various brain structures, including the amygdala, hippocampus and mid- 1 Presented in part at the conference ‘‘The Integrative Neurobiology of Affiliation,’’ organized by the New York Academy of Sciences, Washington DC, March 14 –17, 1996 (16). 2 Requests for reprints should be addressed to Viveka Hillegaart, Ph.D., at the above address; E-mail: viveka.hillegaart@fyfa.ki.se Peptides, Vol. 19, No. 1, pp. 39 – 45, 1998 Copyright © 1998 Elsevier Science Inc. Printed in the USA. All rights reserved 0196-9781/98 $19.00 + .00 39