Selective labeling of alpha-noradrenergic receptors in rat brain with [3H]dihydroergokryptine.

Using concentrations of ³H] dihydroergokryptine between 0.1 and 5 nM, saturable binding can be demonstrated in rat cerebral cortical membranes with a dissociation constant (K_D) of about 0.8 nM. α-Noradrenergic agonists and antagonists compete for the sites labeled by these low concentrations of ³H] dihydroergokryptine with relative potencies characteristics of classical α-noradrenergic receptors. The very low potency of serotonin in competing for these binding sites indicates that, in contrast to findings with higher concentrations of ³H] DHE, low concentrations do not label serotonin receptors. Moreover, the low potency of dopamine in competing for ³H] dihydroergokryptine binding in both striatal and cortical membranes indicates that no detectable portion of binding is associated with postsynaptic dopamine receptors.