COMPARATIVE DESENSITIZATION OF THE HUMAN 5-HT2A AND 5-HT2C RECEPTORS EXPRESSED IN THE HUMAN NEUROBLASTOMA CELL-LINE SH-SY5Y

1 We have used previously characterized clones of the human neuroblast oma cell line, SH-SY5Y, constitutively expressing either the human 5-H T2A or 5-HT2C receptor to compare their desensitization profiles after exposure to 5-HT. 2 5-HT stimulated [H-3]-inositol phosphate ([H-3]-I Px) production at both the 5-HT2C (pEC(50)=8.03+/-0.15) and 5-HT2A rec eptors (pEC(50)=7.15+/-0.08), with maximal responses occurring after e xposure to 1 mu M and 10 mu M 5-HT, respectively. 3 Exposure of cells to maximally effective concentrations of 5-HT caused time- and concent ration-dependent desensitization of [H-3]-IPx formation. The 5-HT2A re sponse desensitized slower (t(1/2)=110 min) and with lower sensitivity than that of the 5-HT2C receptor (t(1/2) = 12.5 min). In each case, d esensitization was blocked by co-administration of a specific receptor antagonist. Following exposure to 10 mu M 5-HT for 2 h, both receptor s exhibited extensive desensitization, with subsequent responses to 5- HT reduced by more than 80%. 4 5-HT stimulated Ins(1,4,5)P-3 productio n with a potency similar to that for [H-3]-IPx production at each rece ptor. In both cases Ins(1,4,5)P-3 levels peaked rapidly then returned to basal level within a short time. This peak consistently occurred ea rlier for the 5-HT2C receptor (5 s) than for the 5-HT2A receptor (20 s ). 5 Prior exposure of SH-SY5Y/S-HT2C cells to 5-HT (1 mu M/15 min) ca used a significant decrease in the 5-HT-stimulated peak in Ins(1,4,5)P -3 levels whereas no such change occurred in SH-SYSY/S-HT2A cells foll owing exposure to 10 mu M 5-HT for 15 min. 6 These results indicate th at the human 5-HT2A and 5-HT2C receptors both exhibit desensitization at the level of inositol phosphate formation when expressed in the sam e cellular environment, with the 5-HT2C receptor being more sensitive to 5-HT-mediated desensitization than the 5-HT2A receptor.