DL-tetrahydropalmatine-produced hypotension and bradycardia in rats through the inhibition of central nervous dopaminergic mechanisms.

The effects of DL-tetrahydropalmatine (THP; a main active substance of the Chinese herb corydalis), haloperidol (a dopamine D2 receptor antagonist), apomorphine and amphetamine on cardiovascular function and striatal dopamine (DA) release were compared in rats under general anesthesia. Intravenous administration of THP (1-10 mg/kg) or haloperidol (0.5-1.25 mg/kg) produced hypotension, bradycardia and increased DA release in the striatum. On the other hand, amphetamine (0.5-1.25 mg/kg) produced hypertension, tachycardia and increased striatal DA release. However, intravenous injection of apomorphine (0.5-1.25 mg/kg) produced hypotension, bradycardia and decreased striatal DA release. In addition, the THP-induced hypotension was attenuated by pretreatment with spinal transection or amphetamine, while the THP-induced bradycardia was attenuated by pretreatment with bilateral vagotomy or amphetamine. Thus, it appears that THP acts through DA D2 receptor antagonism to induce hypotension and bradycardia in rats.