Emotional stress caused by moving to unfamiliar surroundings inhibits oxytocin (OT) release and milk ejection in dairy cows, while blood concentrations of β-endorphin and cortisol are elevated (Bruckmaier et al. 1993). β-Endorphin and cortisol concentrations decrease with repeated relocation to unfamiliar surroundings while milking-related OT release and milk ejection gradually normalize (Bruckmaier et al. 1996). Exogenous cortisol has previously been shown to exert no inhibiting effect on milk ejection in cows (Mayer & Lefcourt, 1987). Therefore, it seems likely that the inhibition of milk ejection in cows – as in other species – is mediated by endogenous opioid peptides. The opioid antagonist naloxone abolishes the inhibiting effect of endogenous opioid peptides on OT release in several species (Haldar et al. 1982; Seckl & Lightman, 1987; Gilbert et al. 1992; Aurich et al. 1996).

Milk ejection is inhibited by α-adrenergic receptor stimulation in the mammary gland without reduction of OT release (Bruckmaier et al. 1997). Opioid-induced inhibition of OT release may be mediated by noradrenergic neurons via β-adrenergic receptors (Crowley & Armstrong, 1992). If adrenergic receptors are involved in the blockade of OT release in cows during milking in unfamiliar surroundings, the inhibition may be abolished by adrenergic blocking agents.

Cortisol synthesis is diminished by administration of metyrapone (Martindale, 1989). Because cortisol has a negative feedback effect on ACTH release, the release of ACTH and concomitantly of β-endorphin is elevated by metyrapone in the rat (Pettibone & Mueller, 1984). Thus, milk ejection should be indirectly inhibited by metyrapone administration via enhanced release of β-endorphin in the familiar barn.

Experiments were designed to investigate the importance of elevated concentrations of β-endorphin in unfamiliar surroundings for disturbed milk ejection in dairy cows.