Two Classes of [3H]Spiperone Binding Sites in Bovine Neurohypophysis: D-2 Receptors and Putative 5-Ht2Receptors

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Abstract
Binding of [3H]spiperone was studied in membranes obtained from bovine neurohypophyses devoid of intermediate lobe tissue. Non-linear Scatchard plot suggested the presence of more than a single class of binding sites. Competition experiments using ketanserin, a ligand selective for 5-HT2 receptors, were carried out to ascertain whether serotonergic, in addition to dopaminergic receptors, might be responsible for the heterogeneity of [3H]spiperone binding. Computer-assisted modeling suggested the presence of two classes of binding sites for ketanserin (Ka = 1.6 ± 0.2 and 366.7 ± 20.5 nM, respectively). The Ka value for ketanserin binding to the high-affinity sites, as well as the Ka of [3H]spiperone for these sites suggested by the 2 sites model indicate that they represent serotonin 5-HT2 receptors. The [3H]spiperone Ka at the ketanserin low-affinity sites (65 ± 7 pM) and the rank order of inhibitory potencies for several antagonists show that the lowaffinity sites represent dopamine D-2 receptors.