Paced mating increases the expression of μ opioid receptors in the ventromedial hypothalamus of male rats.

Paced mating induces a positive affective state which is blocked by naloxone, an opioid antagonist. Opioids are released in the medial preoptic area (mPOA) and other brain regions during sexual behavior and μ opioid receptors (MOR) are activated in males that copulate until ejaculation. The aim of the present study was to determine if paced mating increases the expression of MOR in areas involved in the control of sexual behavior in male rats. Sexually experienced rats were assigned to one of the following groups: Paced, males that paced the sexual interaction; Non-Paced, males that did not pace the sexual interaction; Control, males were able to hear, see and smell a sexually receptive female, but no physical contact was possible. Males were sacrificed 4, 8 and 12 h after the behavioral tests. The mPOA, ventromedial hypothalamus (VMH), amygdala, olfactory bulbs (OB) and cortex were dissected and expression of MOR was determined by qPCR. In the VMH, the expression of MOR increased in the Paced group compared to the Non-Paced and Control groups at 4 h. No significant differences were found in the mPOA and the amygdala regardless of the time of sacrifice. In the OB and cortex, expression of MOR was not detectable. Interestingly, we found that the expression of MOR varied at the different times of sacrifice. In conclusion, our results show that the expression of MOR increased in the ventromedial hypothalamus after paced mating and it also varied in different brain areas depending on the time of the day.