The selectivity of some ergot derivatives for alpha 1 and alpha 2-adrenoceptors of rat cerebral cortex.

The affinities of several ergot derivatives for rat cerebral cortex alpha 1- and alpha 2-adrenoceptors were assessed using radioligand binding techniques. In most cases both [3H]prazosin (labelling alpha 1-adrenoceptors) and [3H]rauwolscine (labelling alpha 2-adrenoceptors) binding were displaced by the ergot derivatives tested, but with markedly different potency. Generally compounds displayed selectivity toward the alpha 2-adrenoceptor, particularly CQ 32-084, which had a Ki value against [3H]rauwolscine binding of 35 nM, but had little effect on [3H]prazosin binding at concentrations in excess of 5 microM. Lisuride was the most potent displacer of [3H]rauwolscine binding with a Ki value of 0.54 nM. Only bromocriptine was relatively alpha 1-selective with a Ki against [3H]prazosin binding of 18 nM and against [3H]rauwolscine binding of 120 nM. The results indicate that the ergot derivatives tested display a marked affinity for alpha-adrenoceptors and that their actions at this receptor class should be considered when interpreting their pharmacological activity in vivo.