Acetyl-RYYRIK-NH2, a purported orphanin FQ antagonist, does not block the orphanin FQ-induced prolactin secretory response in female rats.

Acetyl-RYYRIK-NH2, a drug reported to act as an orphanin FQ/nociceptin (OFQ/N) receptor antagonist, did not block the OFQ/N-induced prolactin increase in female rats. Indeed, Acetyl-RYYRIK-NH2 pretreatment increased the magnitude and duration of the prolactin secretory response. Furthermore, by 15 min after the Acetyl-RYYRIK-NH2 pretreatment, and prior to any OFQ/N administration, circulating levels of prolactin were significantly increased in a dose-related manner, providing further evidence that this drug exhibits agonist activity. In contrast, neither drug affected basal levels of luteinizing hormone nor had any effect on tail flick latency in rats. These results indicate that Acetyl-RYYRIK-NH2 does not antagonize the prolactin secretory activity of OFQ/N.