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Dopamine D 2 Receptors in the Anterior Pituitary: A Single Population Without Reciprocal Antagonist/Agonist States
Author(s) -
George Susan R.,
Watanabe Masayuki,
Seeman Philip
Publication year - 1985
Publication title -
journal of neurochemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.75
H-Index - 229
eISSN - 1471-4159
pISSN - 0022-3042
DOI - 10.1111/j.1471-4159.1985.tb08740.x
Subject(s) - agonist , antagonist , endogenous agonist , dopamine , dopamine receptor , endocrinology , dopamine antagonist , neuroscience , anterior pituitary , population , receptor , medicine , dopamine receptor d1 , chemistry , biology , hormone , environmental health
Although dopamine agonists can recognize two states of the D 2 dopamine receptor in the anterior pituitary (D high 2 and D low 2 ), we examined whether the dopamine antagonists such as [ 3 H]spiperone could recognize these two sites with different affinities. Using up to 30 concentrations of [ 3 H]spiperone, however, we could only detect a single population of binding sites (porcine anterior pituitary homogenates) with a dissociation constant ( K D ) of 130 p M. When specific [ 3 H]spiperone binding was defined by a low concentration of (+)‐butaclamol (100 n M ), the apparent density was low. When defined by a high concentration of (+)‐butaclamol (10 μ M ), nonspecific sites became detectable, thus revealing two apparent populations of sites for [ 3 H]spiperone, only one of which was specific for dopamine. Sodium chloride reduced the K D of the single population of specific D 2 sites to 64 p M. Guanine nucleotide by itself had no effect on the K D , but enhanced the density by 25%. Since the density‐enhancement could be eliminated by extensive washing of membranes, and could be restored by preincubation with dopamine, the nucleotide‐induced elevation of D 2 density appeared to be a result of the release of tightly bound endogenous dopamine. Thus, monovalent cations and guanine nucleotides appear to have separate regulatory effects on the anterior pituitary D 2 receptor that modulate antagonist‐receptor interactions. Several maneuvers were used to test whether [ 3 H]spiperone could differentiate between the two agonist‐detected subpopulations of sites. Twentyfold different concentrations of [ 3 H]spiperone (47 p M and 1000 p M ) were found to label identical proportions of receptors in the D high 2 and D low 2 states as detected by the agonist 6,7‐dihydroxyaminotetralin (ADTN), suggesting that spiperone labelled equal proportions of D high 2 and D low 2 sites without differential affinity for them. In addition, competition of spiperone for D high 2 sites selectively labelled by the agonist [ 3 H] n ‐propylnora pomorphine (NPA) had a virtually identical K D for spiperone as did the total D 2 receptor population as determined by direct binding studies (75 p M versus 64 p M ). [ 3 H]Spiperone also bound to a uniform population of D low 2 sites induced by preincubation with guanine nucleotide with identical affinity as to the total D 2 population. Thus, these data do not support a “reciprocal model’ for the D 2 receptor (i.e., antagonist having low affinity for D high 2 and high affinity for D low 2 in a manner reciprocal to agonists). It is therefore concluded from these studies that spiperone recognizes the D high 2 and D low 2 states of the receptor with equal affinity and no evidence is provided in support for reciprocal modulation of antagonist/agonist affinities.