D-l and D-2 dopaminergic receptors regulate protein phosphorylation in the rat neurohypophysis

Abstract

Dopamine stimulates the phosphorylation of the neuron-specific synaptic vesicle proteins Synapsin I, Protein IIIa and Protein IIIb in the posterior pituitary gland of the rat [ Tsou and Greengard (1982) Proc. natn. Acad. Sci. U.S.A. 79, 6075–6079 . This effect has been characterized in the present investigation. The stimulatory effect of dopamine was mimicked by the selective D-l receptor agonist SKF 38393 and was competitively and potently inhibited by the selective D-l receptor antagonist SKF 83509 as well as by the mixed D-l/D-2 antagonist fluphenazine. Conversely, the effect of dopamine was attenuated by a D-2 receptor agonist (LY 141865) and potentiated by a D-2 receptor antagonist (sulpiride). Norepinephrine also stimulated phosphorylation of the synaptic vesicle proteins, apparently through activation of the D-l receptor. D-l and D-2 dopaminergic receptors may play a role in the regulation of hormone secretion from the neurohypophysis. Evidence exists that in the isolated neurophypophysis activation of D-l receptors facilitates, while activation of D-2 receptors inhibits, release of vasopressin. Further work will be required to determine whether the regulation by D-l and D-2 receptors of the protein phosphorylation in the neurohypophysial peptidergic terminals is related to the regulation by those receptors of the neurohypophysial hormone secretion.