Search

[3H]5-HT revealed the presence of serotonin receptors in cultured rat sensory neurons. [3H]5-CT binding was inhibited by cyanopindolol with an IC50 of 0.87 +/- 0.30 nM, suggesting the expression of the 5-HT1B receptor in these neurons. The presence of 5-HT1B receptors was confirmed by the displacement of [125I]Iodocyanopindolol binding by cyanopindolol with an IC50 of 2.43 +/- 0.81 nM. 5-HT1B receptors are the predominant type of serotonin receptors labeled by [3H]5-HT in cultured DRG neurons, representing approximately 60% of the specific [3H]5-HT binding sites. In addition, 5-HT1D and 5-HT2A receptor binding was also found in these neurons. RT-PCR analysis of RNA isolated from embryonic sensory neurons in culture confirmed the expression of 5-HT1B, 5-HT1D and 5-HT2A receptor mRNA. It also demonstrated the presence of 5-HT1F, 5-HT2C, 5-HT3, 5-HT4, 5-HT5A and 5-HT5B receptor mRNA and the absence of 5-HT1A, 5-HT1E, 5-HT2B, 5-HT6 and 5-HT7 mRNA. The identification of multiple subtypes of serotonin receptors expressed in cultured embryonic sensory neurons suggests that DRG neuronal cultures may be an excellent model to examine the direct effects of serotonin on the activity of these sensory neurons.

One of the most profound increases in vascular responsiveness in hypertension has been observed for serotonin (5-hydroxytryptamine, 5-HT). This study investigates the hypothesis that the increase in vascular responsiveness to 5-HT is the result of altered 5-HT receptor signal transduction. Mesenteric arteries were dissected from deoxycorticosterone- (DOCA) salt hypertensive and sham-normotensive rats for use in isolated tissue experiments. Agonist contractile potencies indicated that a 5-HT2 receptor mediates contraction to 5-HT in both sham and DOCA-salt arteries. In arteries from sham rats, ketanserin (5-HT2A/5-HT2C selective), LY53857 (5-HT2 selective) and spiperone (5-HT2A/5-HT2C selective) shifted contraction to 5-HT (pKB = 8.58, 8.35 and 9.52, respectively) indicating that a 5-HT2A receptor mediates contraction in arteries from normotensive rats. By contrast, ketanserin and spiperone did not shift contraction to 5-HT in DOCA-salt mesenteric arteries (pKB6.52,7.52, respectively). LY53857 did shift the response to 5-HT in DOCA-salt mesenteric arteries (pKB = 7.85). Thus, contraction in arteries from DOCA-salt rats is predominantly mediated by 5-HT2B receptors. Unlike the 5-HT receptor in the sham mesenteric artery and aorta (5-HT2A receptor), the 5-HT receptor in DOCA-salt mesenteric arteries and stomach fundus (5-HT2B receptor) were relatively insensitive to phenoxybenzamine (10-300 nM). These data suggest that the 5-HT2B receptor is insensitive to phenoxybenzamine, is increased in number or, alternatively, has increased G protein coupling. DOCA-salt mesenteric arteries were more sensitive to contraction by the direct G protein stimulator AIF4- (-log EC50 [M]: DOCA-salt = 2.82 +/- 0.04; sham = 2.55 +/- 0.03, P.05). PCR analyses indicated an increase in mRNA for the 5-HT2B receptor in mesenteric arteries of DOCA-salt hypertensive arteries, supporting an increase in receptor number. Taken together these studies demonstrate significant changes in 5-HT receptor signal transduction in DOCA-salt hypertension, both at the level of the receptor and G protein and may provide one reason why ketanserin has proved to be a relatively ineffective antihypertensive agent in some forms of hypertension.

The 5-Hydroxytryptamine2B (5-HT2B) receptor was cloned originally from rat stomach fundus and its pharmacology was determined to be consistent with that of the receptor responsible for contraction of rat fundal tissue in response to 5-HT. Recently, the cloning of the human homolog of the 5-HT2B receptor has been reported and, in this study, we report a detailed pharmacological characterization of this human receptor. The cloned human 5-HT2B receptor has high affinity for [3H]5-HT (Kd = 10.6 +/- 1.5 nM), and the pharmacology of this receptor matches closely the rat 5-HT2B receptor, consistent with the structural relatedness of these two proteins. Most compounds tested show no difference in affinity for the human or rat receptors. There were, however, groups of compounds that discriminated between the human and rat 5-HT2B receptors. Examples include certain ergolines such as methysergide and mesulergine, which have higher affinity for the human than for the rat receptor. Similarly, certain benzoylpiperidines, e.g., ketanserin, pirenperone and pipamperone, and the antipsychotics clozapine and olanzapine have higher affinity for the human 5-HT2B receptor. These pharmacological findings reinforce the desirability of having the human forms of receptors when considering drug actions.