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
while milking-related OT release and milk ejection gradually normalize
et al. 1996). Exogenous cortisol has previously been shown to
effect on milk ejection in cows (Mayer & Lefcourt, 1987). Therefore,
seems likely that the inhibition of milk ejection in cows – as in
– is mediated by
endogenous opioid peptides. The opioid antagonist naloxone abolishes the
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).
inhibition of OT release may be mediated by noradrenergic neurons via
receptors (Crowley & Armstrong, 1992). If adrenergic receptors are
involved in the
blockade of OT release in cows during milking in unfamiliar surroundings,
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,
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.