MIF-I suppresses deprivation-induced fluid consumption in rats

Rats were injected IP with a 0.1 mg/kg dose of MIF-I, naloxone, dynorphin, [D-Phe 4]-Met-enkephalin, [D-Ala 2, F 5Phe 4]-Met-enkephalin-NH 2, or the diluent vehicle, placed in their home cages for ten minutes, and then given ad lib access to either 20% sucrose, 10% sucrose, water, 0.01% quinine, or 0.02% quinine in a repeated measures design with solutions counter-balanced over five days. Fluid consumption was measured every hour for 4 hours. A mixed analysis of variance yielded significant results for all main effects and the peptides by fluid and hours by fluid interactions. For the 4-hr test period, naloxone and [D-Phe 4]-Met-enkephalin produced reliable increases in consumption while MIF-I produced a reliable decrease. Differences were obtained only with sucrose solutions, and the results clearly suggest that peptides modulate fluid consumption at positive levels of incentive motivation. To reconcile the findings of increased consumption after naloxone with the many studies suggesting a decrease in such paradigms, 0.1, 1.0, and 10.0 mg/kg of naloxone and MIF-I were administered as before but to independent groups of rats and intake was measured every 30 min. These results replicate and extend the above findings by showing that during the first 30-min period, both naloxone and MIF-I suppressed intake in a dose-dependent fashion, with MIF-I being more effective at each dose. The 0.1 mg/kg naloxone group, however, increased consumption over time and achieved a total consumption greater than control animals but comparable to that observed in the first study. It appears that at very low doses naloxone increases consumption over time, but at more commonly tested higher doses it has a suppressant effect. The results support the concept that in many situations MIF-I can produce the same effects as naloxone.