Effect of naltrexone on relaxation induced by flank pressure in pigs

Twenty, 6 to 9 kg Yorkshire piglets were used in 2 trials. Ten piglets received an IM injection of naltrexone at a dose of 1 to 1.3 mg/kg. Ten control pigs received saline. Blind behavioral testing in a "squeeze chute" was conducted 40 minutes after injection. The "squeeze chute" consists of two padded plywood panels hinged on a base to form a V. Each pig was squeezed for 60 seconds. After release, each pig remained in the padded V for 10 minutes. There was sufficient room for the pigs to walk a few steps. Both naltrexone- and saline-treated pigs eventually crouched down in the chute and relaxed against the padded sides of the V. Naltrexone-pretreated pigs had a longer latency to achieve relaxation--311.8 +/- 47.8 seconds vs. 161.8 +/- 30.38 seconds (SE) (p less than 0.02). Each stage of relaxation at induction was rated on a 1-4 scale (1 = squealing and jumping, 4 = relaxed quietly). Naltrexone-treated pigs had significantly lower relaxation ratings than saline-treated pigs (1.90 vs. 3.20) (p less than 0.01). Treatment had no effect on the final degree of relaxation. Naltrexone partially blocked the relaxation response.