Supraspinal D2 receptor involved in antinociception induced by l-tetrahydropalmatine

AIM: To study the role of dopamine (DA) receptors in ;-tetrahydropalmatine (l-THP)-induced antinociception. METHODS: The intraperitoneal tip) and intrathecal (ith) injections were used to give the drugs. The tail-flick test was used to assess the nociceptive threshold of rats. RESULTS: By ip injection, l-THP (10, 20, 40 mg . kg(-1)) as well as D-2 receptor antagonist spiperone (1, 2, 3 mg . kg(-1)) produced dose-dependent antinociceptive effects on the nociception of rats, while D-2 receptor agonist quinpirole, D-1 receptor agonist SKF38393, and D-1 receptor antagonist Sch-23390 showed no antinociception. Moreover, l-THP- or spiperone-induced antinociception was markedly attenuated by quinpirole (2, 3 mg . kg(-1)) but not SKF38393 or naloxone. On the other hand, ith quinpirole (20, 30, 40 mu g . kg(-1)) also induced a dose-dependent antinociception, while ith I-THP, spiperone, SKF38393, and Sch-23390 did not exhibit any antinociception. Furthermore, ith spiperone (20, 30, 40 Ccg.kg(-1)) but not Sch-23390 dose-dependently antagonised the antinociception induced by quinpirole. l-THP (ith, 100, 200, 300 mu g.kg(-1)) also dramatically attenuated the quinpirole-induced antinociception with a dose-dependent relationship. CONCLUSION: Activating the spinal D-2 receptor or blocking the supraspinal D-2 receptor produces antinociception. D-1 receptor might be not directly involved in the antinociception. l-THP (as a D-2 antagonist) as well as spiperone produces antinociception via blocking the supraspinal D-2 receptor.