Metkephamid-induced Flehmen in lambs.

When metkephamid, a systemically active analog of methionine enkephalin, was administered intracisternally to male or female prepubescent lambs as early as the first week of life, we observed a behavioral pattern akin to Flehmen, which is a well-characterized grimace displayed by mature ungulates during mating. This metkephamid-induced Flehmen was preceded by transient bradycardia and apnea followed by somnolence for approximately 1 hour, during which Flehmen was observed intermittently. Pretreatment with the specific opiate antagonist, naloxone, blocked this behavioral response. These observations indicate that endogenous opioid peptides may be involved in Flehmen.