Alkaloid binding and activation of D-2 dopamine receptors in cell culture

Ergot and pyrrolizidine alkaloids, either extracted from endophyte-infected tall fescue, synthesized, or purchased commercially, were evaluated in cul tured cells to estimate their binding to the D-2 dopamine receptor and subs equent effects on cyclic AMP production in GH(4)ZR(7) cells, transfected wi th a rat D-2 dopamine receptor. Ergopeptide alkaloid (alpha-ergocryptine, b romocryptine, ergotamine tartrate, and ergovaline) inhibition of the bindin g of the D-2-specific radioligand, [H-3]YM-09151-2, exhibited inhibition co nstants (K-I) in the nanomolar range,whereas dopamine was less potent (micr omolar). The lysergic acid amides (ergine and ergonovine) were 1/100th as p otent as the ergopeptide alkaloids. Ergovaline and ergotamine tartrate were equally effective in inhibiting vasoactive intestinal peptide (VIP)-stimul ated cyclic AMP production, with consistent nanomolar effective concentrati on (EC50) values. The remaining ergopeptide alkaloids (alpha-ergocryptine a nd bromocryptine), lysergic acid amides (ergonovine and ergine), and dopami ne were 1/100th as potent. Two representative pyrrolizidines, N-formyllolin e and N-acetylloline, exhibited no binding activity at the D-2 dopamine rec eptor or effects on the cyclic AMP system within the concentration ranges o f nanomolar to millimolar. Our results indicate that the commercially avail able ergot alkaloids ergotamine tartrate and ergonovine may be used interch angeably in the D-2 dopamine receptor system to simulate the effects of ext racted ergovaline and ergine and to examine responses in receptor binding a nd the inhibition of cyclic AMP.