[3H]Apomorphine binding to a GTP-sensitive dopaminergic site in bovine anterior pituitary gland

The characteristics of binding of the potent dopaminergic agonist [3H]apomorphine have been studied in bovine anterior pituitary membranes. A high affinity binding site with an apparent dissociation constant (KD) of 0.7 nM and a number of binding sites of 56 fmol/mg protein has been measured when 10(-5) M dopamine was used to assess nonspecific binding. The order of potency of various agonists to compete with [3H]apomorphine binding is consistent with an interaction at a typical dopaminergic receptor: apomorphine greater than dopamine greater than (-)-epinephrine = (-)-norepinephrine much greater than (-)-isoproterenol. Competition for [3H]apomorphine binding by antagonists shows marked stereoselectivity, (+)-butaclamol being 4000 times more potent than (-)-butaclamol. Dihydroergocryptine, a potent dopaminergic agonist on prolactin release, as well as the dopaminergic antagonists spiroperidol, haloperidol, and (+)-butaclamol, compete for [3H]apomorphine binding at nanomolar concentrations. By contrast, adrenergic (phentolamine, propranolol, and clonidine) and serotonergic (serotonin, cyproheptadine, and methysergide) agonists and antagonists do not compete or are weak competitors at micromolar concentrations. Guanosine triphosphate (GTP) decreases [3H]apomorphine binding, the affinity of this ligand for the receptor being decreased 10 times by 300 microM GTP. The close correlation observed between the affinity of a series of agonists and antagonists for the [3H]apomorphine binding sites and their potency as modulators of prolactin release suggests that [3H]apomorphine binding sites are those involved in the control of prolactin secretion.