Effects of four dopamine agonists on l-tetrahydropalmatine-induced analgesia and electroacupuncture analgesia in rabbits.

The effects of icv 4 dopamine (DA) agonists on analgesia caused by iv l-tetrahydropalmatine (THP) 8 mg/kg or by electro-acupuncture (EA) were studied by using the potassium iontophoretic dolorimetry in rabbits. The results showed that both THP-induced analgesia and EA analgesia were markedly attenuated by icv of DA or apomorphine (Apo), 2 mixed D1/D2 agonists. Similar results were obtained when SKF-38393, a selective D1 agonist, was applied. On the contrary, quinpirole hydrochloride (Qui), a selective D2 agonist, was found to enhance the analgesic action of THP or EA. However, DA, Apo, SKF-38393 or Qui per se did not influence the baseline pain threshold. All these observations indicate that functional alterations in DA receptor activities may be involved in THP-induced analgesia and EA analgesia, in which D1 and D2 subtype receptors exert different roles.