Regulation of the 5-hydroxytryptamine1B receptor in opossum kidney cells after exposure to agonists

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-HT1B receptor. Serotonin inhibited forskolin-stimulated cAMP accumulation with an EC50 of 4-8 nM. Compounds known to show selectivity at the 5-HT1B receptor, such as trifluoromethyl-phenylpiperazine and CGS-12066B, 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-HT1B receptors that was not accompanied by a change in the Kd of the binding site for [125I] iodocyanopindolol. A maximum decrease of 60% in the number of 5-HT1B receptors was evident after a 16-hr treatment with 1 microM 5-HT. Concomitant with the observed decrease in the density of receptors was a marked increase in the EC50 for 5-HT-mediated inhibition of forskolin-stimulated cAMP accumulation. The EC50 was increased 4-5-fold after a 16-hr exposure to 1 microM 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-HT1B receptors and increases in the EC50 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-HT1B receptors on these cells.