Serotonin 5-HT 2A and/or 5-HT 2C receptors have been implicated in modulating the acquisition of the classically conditioned nictitating membrane response in the rabbit.1 Thus, we have undertaken the characterization of the 5-HT 2A receptor in rabbit brain using the agonist [125I]DOI, and the antagonists [3H]ketanserin and [3H]MDL 100,907. Autoradiographic studies using both [125I]DOI and [3H]ketanserin demonstrate a high density of 5-HT 2A receptors in adult New Zealand rabbit cortex. Therefore, binding studies were performed using membranes prepared from the frontal cortex. (3H]Ketanserin (New England Nuclear) and [3H]MDL 100,907 (Amersham) binding assays were performed at 25° C in 20 mM Tris HCl pH 7.4. Nonspecific binding was defined by 1 µM methysergide and I µM spiperone [3H]ketanserin and [3H]MDL 100,907, respectively. Ketanserin binding (in the presence of 100 nM prazosin) was consistent with 2 classes of binding sites. The high affinity ketanserin site had a Kd of approximately 1 nM, which is consistent with its reported affinity at 5-HT 2A receptors. The GTP analogue, Gpp(NH)p, at concentrations up to 100 mM had no effect on ketanserin binding. In contrast, magnesium dose-dependently inhibited ketanserin binding. The IC50 for DOI displacement of bound [3H]ketanserin was decreased approximately fourfold in the presence of 5 mM MgSO4. More extensive studies were carried out with the highly selective 5-HT 2A receptor antagonist, MDL 100,907. Saturation analysis demonstrated that the binding of MDL 100,907 was consistent with a single class of sites with a Kd of approximately 0.1 nM. As for ketanserin, Gpp(NH)p at concentrations up to 100 mM had no effect, whereas magnesium dose-dependently inhibited MDL 100,907 binding. MgSO4 (5 mM) decreased the affinity of [3H]MDL 100,907 by approximately fourfold. The IC50 for DOI displacement of bound [3H]MDL 100,907 was decreased in the presence of 5 mM MgSO4. As expected, Mg++ shifts the 5HT 2A receptor toward the agonist high affinity state as demonstrated by the increased affinity of the agonist, DOI. Unexpectedly, Mg++ decreases the binding of the putative neutral antagonists ketanserin and MDL 100,907, suggesting that these compounds may actually be inverse agonists at the 5-HT 2A receptor.
1. HARVEY, J. A. 1996 Behav. Brain Res 73: 47-50.
a Supported in part by USPHS MERIT Award
b Corresponding author Tel: 215.842,4750; Fax: 215,843,1515
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