DOPAMINE RECEPTOR ANTAGONISTS BLOCK THE EFFECT OF TYR-MIF-1 (TYR-PRO-LEU-GLY-NH2) ON THE OPIATE FORM OF FOOTSHOCK-INDUCED ANALGESIA

Previous studies have shown that some of CNS actions of an endogenous peptide Tyr-MIF-1, are mediated by dopamine (DA) receptors. To study the effect of DA receptor blockade on the antiopiate properties of Tyr-MIF-1, the opiate form of footshock-induced analgesia was elicited in the rat. The nociceptive responses were determined using the hot-plate test (52.5-degrees-C). Intraperitoneal pre-treatment with haloperidol (500-mu-g/kg), SCH 23390 (150-mu-g/kg), or spiroperidol (1 50-mu-g/kg) potentiated the antinociceptive effect of the footshock and blocked the antagonistic action of Tyr-MIF-1 (200-mu-g/kg and 2.0 mg/kg). A dose of haloperidol too small to potentiate the antinociceptive effect of the footshock (100-mu-g/kg) was still able to block the action of Tyr-MIF-1 (200-mu-g/kg). The results suggest that activation of DA receptors mediates the antagonizing effect of Tyr-MIF-1 on the opiate form of footshock-induced analgesia in the rat.