Opioid control of growth hormone in the suckled sow is primarily mediated through growth hormone releasing factor

Abstract

Endogenous opioid peptides mediate the effect of suckling on LH and PRL in the domestic pig. However, the role of opioids in modulating GH during lactation in swine is not known. Primiparous sows that had been immunized against GRF(1-29) conjugated to human serum albumin (GRF-HSA, n=5) or HSA (n=4) were used to determine changes in GH after naloxone. Treatments were imposed in all sows on day 21 of lactation when antibody titers were 9100 ± 1629. All sows received (LV.) naloxone (0.25 mgfkg) or saline (0.0125 mlfkg) at 15 min intervals for 165 min. Active immunization against GRF-HSAduring lactation decreased (P < 0.05) mean concentration (4.8 ± 0.2 vs 2.6 ± 0.1 ng/ml) and frequency (1.5 ± 0.3 vs 0.4 ± 0.2 peaks/4 hr).Concentrations of LH and PRL were similar in GRF-HSA and HSA immunized sows. Naloxone suppressed (P < 0.05) GH in all sows. In USA sows, naloxone abolished episodic release of GU and decreased average, but not basal, concentrations of GU. In sows immunized against GRF-HSA, naloxone decreased (P <0.05) average and basal GH but failed to decrease frequency of GH release. Naloxone failed to alter frequency of LH release. Concentrations of PRL decreased (P < 0.05) after naloxone in all sows. In conclusion, immunization against GRF-USA blocked most of the effect of lactation on GH. Blocking opioid receptors with naloxone decreased GH and PRL in all sows. In contrast to previous findings naloxone had no effect on LH. Opioids alter concentrations of GH through a GRF dependent and GRF independent pathway.

Effect of feed restriction on serum somatotropin, insulin-like growth factor-I-(IGF-I) and IGF binding proteins in cyclic heifers actively immunized against growth hormone releasing factor

“…This observation is consistent with previous re ports in prepuberal heifers (21), cyclic (32) and lactating (33) cows, steers (34,35), lactat ing sows (36) and cyclic gilts (37).…”

Effect of active immunization against growth hormone releasing factor on concentrations of somatotropin and insulin-like growth factor I in lactating beef cows

“…1 or 2. Other reports have demonstrated that naloxone decreased ST in the suckled rat (47) and pig (7). Naloxone was apparently present in sufficient quantities on day 47 (Exp.…”

“…Exogenous ST enhances retention of nitrogen (4), increases body weight gain in growing ruminants (1) and increases milk yield in dairy cows (3). During lactation in the pig and cow, suckling causes an increase in circulating prolactin and ST and a decrease in luteinizing hormone (LH,(5)(6)(7)(8)(9).…”

“…In the pig, suckling increases ST (7) and decreases LH (5-6) through an increase in endogenous opioid peptides mediated by the hypothalamus. Opioid agonists cause an in crease in ST and a decrease in LH in non-lactating ruminants (13)(14), whereas naloxone, an opioid antagonist, increased serum LH in lactating cows (9).…”

“…The objective of this experiment was to determine whether gestating-lactating beef cows could be actively im munized against GRF and to determine the effects of immunization on concentrations of ST and IGF-I. In addition, we tested the hypothesis that an opioid agonist would increase ST in control, but not in GRF-immunized cows (15), whereas we hypothesized that naloxone would decrease ST in both GRF-immunized and control cows as previously reported for lactating sows (7).…”