Different regulation of rat 5-HT2A and rat 5-HT2C receptors in NIH 3T3 cells upon exposure to 5-HT and pipamperone

The 5-HT2A and 5-HT2C receptors belong to the same subtype of the G-protein coupled receptor family and have several agonist and antagonist ligands in common. To gain more insight into the differences in the regulation of the two receptors, we studied the effect of agonist and antagonist pre-treatme nt on radioligand receptor binding and 5-HT-induced inositol phosphate form ation on rat 5-HT2A and rat 5-HT2C receptors stable expressed in NIH 3T3 ce lls. We compared short (15 min) and prolonged (48 h) pre-treatment of the c ells with the natural agonist, 5-HT and with the antagonist pipamperone, wh ich can be readily washed out. The rat 5-HT2C receptor showed an agonist-in duced down-regulation (decrease in B-max of labelled agonist and antagonist binding) and desensitisation (decrease in 5-HT-induced inositol phosphate formation and potency of 5-HT). Antagonist pre-treatment induced an increas e in rat 5-HT2C receptor-mediated inositol phosphate formation as well as i ncreased agonist and antagonist radioligand binding. These findings are con sistent with the classical model of G-protein coupled receptor regulation. In contrast, the rat 5-HT2A receptor expressed in the same host cell behave d differently, unlike the classical model. Pre-treatment with 5-HT for 15 m in and 48 h did not change receptor levels measured by radioligand binding, but the signal transduction response (inositol phosphate formation) was si gnificantly reduced. Pre-treatment with the antagonist pipamperone for 15 m in and 48 h caused an increase in antagonist radioligand binding but a redu ction in agonist radioligand binding and a decrease in inositol phosphate f ormation and potency of 5-HT. Hence, the rat 5-HT2A receptor apparently und ergoes agonist desensitisation without down-regulation of the total recepto r number. Antagonist pre-treatment causes a paradoxical desensitisation, po ssibly by uncoupling of the receptor from G-proteins. The uncoupled recepto r does not bind 5-HT in the nanomolar range but retains its antagonist bind ing properties. Paradoxical antagonist-induced desensitisation of rat 5-HT2 A receptors has also been observed in vivo. (C) 2001 Elsevier Science B.V. All rights reserved.