Ontogeny of the opioidergic regulation of LH and prolactin secretion in lactating sows II: interaction between suckling and morphine administration

Administration of morphine to ten suckled and nine zero-weaned (piglets rem oved immediately after farrowing) sows was used to investigate the apparent absence of opioid regulation of LH and prolactin secretion in early lactat ion. Blood samples were collected at 10 min intervals at 24-30, 48-54, 72-7 8 h post partum, and for a 12 h period. from 08:00 to 20:00 on day 10 after farrowing. Morphine (0.1 mg kg(-1)) was administered as three i.v. bolus i njections at intervals of Ih during the last 3 h of each of the 6 h samplin g periods, and at 6, 7 and 8 h after the beginning of sampling on day 10. T here were significant (P < 0.001) group (zero-weaned versus suckled), time and morphine effects on LH secretion. Plasma LH concentrations increased (P < 0.001) within 48 h of farrowing in zero-weaned sows. Long-term trends of an increase in mean plasma LH in the sampling periods before treatment wer e attenuated in both groups by morphine treatment. Morphine also significan tly inhibited (P < 0.05) prolactin secretion in suckled sows. in zero-weane d sows, plasma prolactin was already low at the start of, sampling and did not change with time or in response to morphine treatment. Therefore, the i nability to demonstrate an opioidergic involvement in the suckling-induced inhibition of LH secretion during the early post-partum period in sows is n ot due to a lack of opioid receptors. Furthermore, in suckled sows, morphin e is stimulatory to systems that have an inhibitory effect on prolactin sec retion.