Effects of d-coclaurine and d-reticuline, benzyltetrahydroisoquinoline alkaloids, on levels of 3,4-dihydroxyphenylacetic acid and homovanillic acid in the mouse striatum.

An intracerebroventricular injection of d-coclaurine (50 micrograms), a benzyltetrahydroisoquinoline alkaloid extracted from Magnolia salicifolia, produced a slight increase in 3,4-dihydroxyphenylacetic acid level and a significant increase in homovanillic acid level in the mouse striatum. Another alkaloid d-reticuline (200 micrograms) increased only homovanillic acid level. An intracerebroventricular pretreatment with d-coclaurine (50 micrograms) did not antagonize suppressive effect of apomorphine on l-dopa formation produced by gamma-butyrolactone (750 mg/kg i.p.) plus aromatic amino acid decarboxylase inhibitor, NSD-1015 (100 mg/kg i.p.). These results suggest that d-coclaurine blocks postsynaptic but not presynaptic dopamine receptors in the mouse striatum.