The synthetic
met-enkephalin syndyphalin-33 (SD-33) increases feed intake in sheep and transiently increases circulating GH concentrations in sheep, rats, and pigs. Two experiments were performed to evaluate the effects of SD-33 on recently
weaned pigs. In a preliminary experiment, pigs were administered SD-33 (0.5 micromol/kg, given intramuscularly) or saline immediately before a 3-h transport and subsequent placement into group pens. Treatment with SD-33 increased (P = 0.01) daily feed intake; cumulatively, pen intake over 7 d postweaning tended (P = 0.06) to be greater than in control pens. In Exp. 2, pigs were
weaned and fitted with
jugular catheters. The following day, pigs were treated with SD-33 or saline as described above. Transient increases (P < 0.05) in circulating concentrations of GH (at 1 and 1.5 h postinjection) and
cortisol (at 3.5 and 4 h postinjection) were observed in pigs treated with SD-33 relative to controls. No difference in feed intake was observed between treatments over 4 d postinjection. Increased (P < 0.05) numbers of circulating
neutrophils,
lymphocytes, and
monocytes were observed in both
treatment groups over 4 d postinjection, and treatment with SD-33 tended (P = 0.07) to selectively increase
monocyte numbers. Although SD-33 has potential to be used to increase feed intake and decrease the negative effects of stress during
weaning in pigs, further investigation is needed to better understand the timing of effect and to rule out possible
immunosuppressive effects.
