Supraspinal D2 receptor involved in antinociception induced by l-tetrahydropalmatine.
OBJECTIVE
To study the role of dopamine (DA) receptors in l-tetrahydropalmatine (l-THP)-induced antinociception.


METHODS
The intraperitoneal (i.p.) 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 i.p. injection, l-THP (10, 20, 40 mg.kg-1) as well as D2 receptor antagonist spiperone (1, 2, 3 mg.kg-1) produced dose-dependent antinociceptive effects on the nociception of rats, while D2 receptor agonist quinpirole, D1 receptor agonist SKF38393, and D1 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 micrograms.kg-1) also induced a dose-dependent antinociception, while ith l-THP, spiperone, SKF38393, and Sch-23390 did not exhibit any antinociception. Furthermore, ith spiperone (20, 30, 40 micrograms.kg-1) but not Sch-23390 dose-dependently antagonised the antinociception induced by quinpirole. l-THP (ith, 100, 200, 300 micrograms.kg-1) also dramatically attenuated the quinpirole-induced antinociception with a dose-dependent relationship.


CONCLUSIONS
Activating the spinal D2 receptor or blocking the supraspinal D2 receptor produces antinociception. D1 receptor might be not directly involved in the antinociception. l-THP (as a D2 antagonist) as well as spiperone produces antinociception via blocking the supraspinal D2 receptor.
Hu
J Y
JY
Department of Pharmacology, Shanghai Institute of Materia Medica, Chinese Academy of Sciences, China.
Jin
G Z
GZ
D015647:2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine / Q000494:pharmacology; D018712:Analgesics, Non-Narcotic / Q000494:pharmacology*; D000818:Animals; D001552:Benzazepines / Q000494:pharmacology; D001600:Berberine Alkaloids / Q000494:pharmacology*; D018492:Dopamine Antagonists / Q000494:pharmacology*; D065127:Dopamine D2 Receptor Antagonists; D007278:Injections, Spinal; D008297:Male; D017288:Pain Threshold / Q000187:drug effects*; D019257:Quinpirole / Q000494:pharmacology; D051381:Rats; D017207:Rats, Sprague-Dawley; D017447:Receptors, Dopamine D1 / Q000819:agonists / Q000037:antagonists & inhibitors; D017448:Receptors, Dopamine D2 / Q000819:agonists; D013134:Spiperone / Q000494:pharmacology
D018712:Analgesics, Non-Narcotic; D001552:Benzazepines; D001600:Berberine Alkaloids; D018492:Dopamine Antagonists; D065127:Dopamine D2 Receptor Antagonists; D017447:Receptors, Dopamine D1; D017448:Receptors, Dopamine D2; D019257:Quinpirole; C014215:tetrahydropalmatine; D013134:Spiperone; D015647:2,3,4,5-Tetrahydro-7,8-dihydroxy-1-phenyl-1H-3-benzazepine