Rationale: The purpose of the present investigation was to evaluate the effects of the D3 agonist (+/-)-7-hydroxy-dipropylaminotetralin (7-OH-DPAT), various dopamine (DA) agonists and DA antagonists on the antinociceptive effects of mu opioids.
Methods: Antinociception was assessed using a warm-water tail-withdrawal procedure in rats.
Results: The mu opioids morphine (0.3-10 mg/kg) and dezocine (0.03-3.0 mg/kg) produced dose-dependent increases in antinociception with maximal effects obtained at the higher doses tested. Pretreatment with the putative D3 agonist 7-OH-DPAT (1.0-10 mg/kg) produced a dose-dependent attenuation of the antinociceptive effects of morphine and dezocine. At the highest dose of 7-OH-DPAT tested, the morphine dose-effect curve was shifted rightward by approximately 1.5 log units and the dezocine curve by greater than 2.3 log units. The (+)-isomer of 7-OH-DPAT (1.0 and 3.0 mg/kg) also shifted the morphine dose-effect curve to the right in a dose-dependent manner. The DA D3/D2 agonist (-)-quinpirole (0.1-10mg/kg) attenuated the effects of morphine, but these effects were small in magnitude, not dose-dependent and observed only under a limited set of conditions. The DA D2/D3 antagonist spiperone failed to alter the morphine dose-effect curve, but reversed the effects of 7-OH-DPAT on morphine antinociception. Pretreatment with the DA D1 agonist (+/-)-SKF38393 (1.0 and 10 mg/kg) and the D1 antagonist (+)-SCH23390 (0.1 and 1.0 mg/kg) failed to alter the morphine dose-effect curve.
Conclusion: The finding that 7-OH-DPAT markedly attenuated the effects of morphine and that these effects were reversed with spiperone suggests that activity at the D3, and possibly the D2, receptor can modulate mu agonist-induced antinociception.