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