Opioid mediated suppressive effect of milk-derived lactoferrin on distress induced by maternal separation in rat pups.

The present study assessed the effects of bovine milk-derived lactoferrin (bLf) on distress activities induced by maternal separation in 5- to 18-day-old rat pups. The rat pups were injected with BSA (100 mg/kg, i.p.; control) or bLf (100 mg/kg, i.p.) 30 min before the behavioral test. Distress activity was estimated by means of recording body movements or ultrasonic vocalizations (USVs). After 5 min of maternal separation, bLf significantly (P<0.01) suppressed body movements, particularly in the 10-day-old pups. This suppressive effect of bLf was reversed by pretreatment with naloxone, CTOP, and norBNI at doses of 0.1-1 mg/kg. Additionally, USVs were also suppressed by bLf, which was reversed by pretreatment with naloxone. Inhibition of nitric oxide synthase (NOS) with nitro-L-arginine methyl ester (L-NAME) dose dependently (3-10 mg/kg) suppressed separation-induced USV production in 10-day-old pups. Interestingly, the suppressive effect of bLf was completely reversed by pretreatment with a low dose (1 mg/kg) of L-NAME, which did not affect the USVs with single application. These findings demonstrate that milk-derived bLf suppresses distress induced by maternal separation via an opioid-mediated mechanism. Furthermore, bLf possibly activates NOS, and an elevated nitric oxide may cause some modification of the opioid system.