Puerperal breast feeding does not stimulate circulating opioids in humans.

ACTH, beta lipotropin (beta LPH), beta endorphin (beta EP), Prolactin (PRL) and Cortisol were measured in the first five days of puerperium at 9:00, before and 30 minutes after suckling, in 7 healthy lactating women. With the exception of PRL plasma levels which decline, although remaining at high concentrations during the observation period, all the other parameters showed a sudden fall from the high levels found at delivery, reaching stable normal levels (beta LPH, beta EP, Cortisol) or concentrations which were 50\% lower than normal (ACTH), from the second day of puerperium. Suckling confirms its capacity to release plasma PRL, while all the other indices remain unmodified. Despite the experimental evidence that serotoninergic neurons are involved both in the PRL response to suckling and the circadian rhythmicity and stress response of proopiocortin-related peptides, the present results suggest that breast feeding is not a stressful situation, as no typical proopiocortin-related peptide response is evident, and that the activation of PRL release is related to stimulation of serotoninergic neurons which differ from those involved in the control of proopiocortin-related peptide secretion.