Morphine treatment and reproductive condition alter olfactory preferences for pup and adult male odors in female rats

Administration of morphine sulfate (MS) to pregnancy-terminated and postpartum lactating female rats disrupts both maternal behavior and postpartum aggression. Since the display of these behaviors may be heavily dependent on olfactory cues provided by the stimulus animals (rat pups and adult male rats, respectively), we examined whether MS was affecting the perception of the olfactory stimuli, and whether olfactory perception was modified by reproductive condition. In Experiment 1, lactating rats had their pups removed and were injected with MS (5.0 mg/kg, sc.) or saline. 60 min later they were placed into a two-choice apparatus, one side of which contained bedding soiled by neonates and the other clean bedding. Time spent on each side was recorded for a total of 5 min (300 s; chance = 150 s). Saline-treated mothers spent significantly more time on the pup-odor side, whereas MS-treated females spent significantly less. In Experiment 2, lactating females were treated with MS or saline and exposed to male odors (soiled bedding). MS significantly increased time spent on the side with male odors; when treated with saline, time spent was significantly reduced. Thus, in lactating rats. MS creates an aversion for pup odors while reducing the female's normal aversion toward male odors. In Experiment 3, ovariectomized (ovx) virgin females expressed neither an aversion nor a preference for the odor of pups following saline administration. After MS treatment, however, the virgins showed a distinct preference for pup odors. When exposed to male odors in Experiment 4 ovx virgins showed a marked preference for male odors after MS treatment, and neither a preference nor an aversion after saline. Experiment 5 examined pup odor preferences in intact virgins, early (Day 7), middle (Day 14), late-pregnant (Day 21), and prepartum (Day 22) rats. The pup odor preferences of virgin, Day 7, Day 14, and Day 21 pregnant rats were not different and generally were at chance levels. Day 22 pregnant females exhibited a marked preference for pup odors compared to chance levels, as well as compared to the other four groups. These findings suggest that opiates and endogenous opioids may regulate olfactory preferences and that alterations in this system may underlie normal behavioral changes toward conspecifics prepartum as well as during lactation.