Pharmacological properties of [³H]dihydroergokryptine binding sites associated with alpha noradrenergic receptors in rat brain membranes

[³H]Dihydroergokryptine, a mixed agonist-antagonist at alpha noradrenergic receptors, binds in saturable fashion and with high affinity to membranes of rat cerebral cortex. Saturation studies reveal a dissociation constant (K(D)) of 1.6 nM and a receptor density of 21 pmoles/g, similar to the sum of the maximal number of binding sites observed for the [³H]alpha agonist clonidine and the [³H]alpha antagonist WB-4101 alone. The relative affinities of a wide range of drugs indicate that binding to cerebral cortical membranes selectively involves alpha noradrenergic receptors. Dopamine and serotonin inhibit binding only weakly, indicating that labeling of receptors for these neurotransmitters is negligible. The regional distribution of [³H]dihydroergokryptine binding in rat brain coincides with that observed for alpha receptor binding of [³H]clonidine and [³H]WB-4101, except for disproportionately high levels in corpus striatum. This suggests that in striatal membranes [³H]dihydroergokryptine is able to bind to dopamine as well as alpha receptors. The slopes of logit-log plots for the inhibition of [³H]dihydroergokryptine binding by mixed agonist-antagonist are about 1.0, while pure agonists or antagonists display shallower inhibition curves. These data are consistent with a model of the alpha noradrenergic receptor in which agonists and antagonists bind selectively to discrete, noninterconverting sites, while mixed agonist-antagonists can bind to either site.