EFFECT OF TETRAHYDROISOQUINOLINE ALKALOIDS ON ALPHA ADRENOCEPTORS IN RAT BRAIN

About 30 tetrahydroisoquinolines have been investigated for their in vitro affinities to rat brain alpha adrenoceptors. l-Crebanine(1-CBN) and l-tetrahydrocoptisine (l-THC) were found to be the most potent inhibitors of ³H WB 4101 binding to alpha-1 adrenoceptors (K_i=1.9 × 10^-7 and 2.0 × 10^-7 mol/L, respectively), l-Stephanine(l-STP), dehydrostephanine(DHS) and xylopine(XLP) were also shown to be effective on alpha-1 adrenoceptors (K_i=2.8×10^-7, 4.7×10^-7 and 6.5×10^-7 mol/L, respectively). DHS appeared to be the most active in displacing ³H clonidine binding to alpha-2 adrenoceptors (K_i=1.25×10^-6 mol/L). l-tetrahydropalmatine(l-THP) and l-stepholidine (l-SPD) exhibited similar affinities to alpha-1 and alpha-2 adrenoceptors. Berbamine(BBA)interacted moderately with alpha-2 adrenoceptors (K_i=6.16×10^(-6) mol/L). l-STP and XLP have relatively high affinities to alpha-1 adrenoceptors (as above), but they did not show any affinity to alpha-2 adrenoceptors. Their alpha-1 and alpha-2 adrenoceptors binding selectivity ratios K_i (alpha-2)/Kvi(alpha-1) were 357 and 154 respectively. It is suggested that l-STP and XLP are more selective to alpha-1 adrenoceptors.