Regulation of the 5-hydroxytryptamine1B receptor in opossum kidney cells after exposure to agonists
Unsworth, C.D.; Molinoff, P.B.
Molecular Pharmacology 42(3): 464-470
1992
ISSN/ISBN: 0026-895X PMID: 1328846 Document Number: 395725
The density of 5-hydroxytryptamine (5-HT)-1B receptors and their coupling to the inhibition of cAMP accumulation were investigated in opossum kidney cells maintained in culture. The density and properties of the receptor were determined using (125I) iodocyanopindolol as the radioligand. The pharmacological specificity of the binding site was consistent with that expected for a 5-HT-1B receptor. Serotonin inhibited forskolin-stimulated cAMP accumulation with an EC-50 of 4-8 nM. Compounds known to show selectivity at the 5-HT-1B receptor, such as trifluoromethylphenylpiperazine and CGS-1 2066B, also inhibited forskolin-stimulated cAMP accumulation, acting as full agonists with efficacies comparable to that of serotonin. Other beta-adrenergic receptor antagonists, including (-)-pindolol and (-)-alprenolol, bound to the receptor with high affinity and acted as partial agonists capable of inhibiting forskolin-stimulated cAMP accumulation. Exposure of cells to 5-HT resulted in a time- and dose-dependent decrease in the density of 5-HT-1B receptors that was not accompanied by a change in the K-d of the binding site for (125I) iodocyanopindolol. A maximum decrease of 60% in the number of 5-HT-1B receptors was evident after a 16-hr treatment with 1 mu-M 5-HT. Concomitant with the observed decrease in the density of receptors was a marked increase in the EC-50 for 5-HT-mediated inhibition of forskolin-stimulated cAMP accumulation. The EC-50 was increased 4-5-fold after a 16-hr exposure to 1 mu-M 5-HT, and the maximal level of inhibition was markedly decreased. Whereas pretreatment with moderate concentrations of 5-HT (100-300 nM) for 16 hr produced significant decreases in the density of 5-HT-1B receptors and increases in the EC-50 for inhibition of forskolin-stimulated cAMP formation, there was little change in the maximal level of inhibition that could be attained. Such a combination of changes could be explained by the presence of "spare" 5-HT-1B receptors on these cells.