Behavioral experiments examined the role of morphiceptin in male rat copulatory behavior. Male copulatory behavior was recorded subsequent to bilateral injections of either 10, 500, or 1000 ng of morphiceptin into the medial preoptic area (MPOA) in sexually vigorous male rats. In the first experiment, all three doses of morphiceptin injected bilaterally into the MPOA produced a dramatic delay in the initiation of copulation. Both mount and intromission latencies significantly increased following injections of all three doses of morphiceptin when compared to saline injected controls. No other parameter was affected. In experiment 2, the inhibitory effects of morphiceptin on male copulatory behavior were abolished by pretreatment with naloxone 20 minutes prior to intracerebral morphiceptin injections. This study represents an attempt at a pharmacological characterization of the inhibitory effects of opioids on male rat sexual behavior. The results of experiment 1 indicate that mu receptors located within the MPOA mediate the initiation of male rat copulatory behavior.



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