Absence of High Affinity Dopamine Receptors in GH3Cells: A Prolactin-Secreting Clone Resistant to the Inhibitory Action of Dopamine*

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Abstract
Dopamine (DA) and DA agonists bind with high affinity to anterior pituitary receptors which mediate the inhibition of PRL [prolactin] release. Spiperone (SPIP), a DA antagonist, was successfully used to characterize pituitary DA receptors with a Kd of less than 1 nM. The binding of SPIP to GH3D6 cells which secrete only PRL and growth hormone was studied. This clone was derived from a radiation-induced tumor of the rat anterior pituitary. Equilibrium binding of [3H]SPIP to living GH3 cells showed no high affinity receptors, but a low affinity (Kd = 0.83 .mu.M) and saturable (0.06 femtomol/cell) population of sites was observed. In addition, saturable binding with a similar affinity (Kd = 0.57 .mu.M) was noted in broken GH3 cells. The interaction was completely reversible and temperature-dependent. The concentrations of various ligands required to compete for half of the [3H]SPIP binding to whole cells were: chlorpromazine, 0.17 .mu.M; haloperidol, 0.68 .mu.M; pimozide, 0.77 .mu.M; d-butaclamol, 1.16 .mu.M; 1-butaclamol, 1.30 .mu.M; SPIP, 1.49 .mu.M; bromergocryptine, 4.98 .mu.M; apomorphine, 13.9 .mu.M; and DA, 100 .mu.M. The absence of a high affinity site in GH3 cells is consistent with the decreased effectiveness of various agonists and antagonists on PRL secretion. The low affinity interactions observed in GH3 cells apparently are normally present in the anterior pituitary and brain and do not simply represent an alteration of receptor affinity.