Endogenous opioids and sexual motivation and performance during the light phase of the diurnal cycle.

The sexual motivation and performance of sexually experienced male rats were tested during the light phase of the diurnal cycle after treatment with saline or 1 mg.kg-1 naloxone in a bilevel testing box. The sexual motivation during the light phase, as assessed by the increase in anticipatory level changes prior to introduction of a receptive female on subsequent weekly sessions, was comparable to that during the dark phase. Opioid blockade reduced the increase of level changes, suggesting that endogenous opioids are involved in sexual motivation. The sexual performance was impaired during the light phase. Naloxone treatment failed to affect the sexual performance, other than that the post ejaculatory refractory period was increased. This increased latency to re-initiate copulation may be an expression of the reduced sexual motivation. It is concluded that endogenous opioids are not involved in the regulation of the impaired sexual performance during the light phase of the diurnal cycle. In contrast, the sexual motivation, which displays no marked diurnal variation, may be stimulated by endogenous opioids.