Desensitization and down-regulation of 5-HT2 receptors in P11 cells

P11 cells express a high density of 5-hydroxytryptamine2 (5-HT2) receptors coupled to phosphoinositide hydrolysis. The effect of exposure to 5-HT on the expression and function of 5-HT2 receptors in P11 cells was investigated. Exposure of P11 cells to 10 microM 5-HT for as short a time as 20 min led to a decrease in the ability of 5-HT to stimulate phosphoinositide hydrolysis. After 8 hr, phosphoinositide hydrolysis stimulated by 5-HT was greatly reduced or absent. Concentrations of 5-HT as low as 100 nM caused desensitization of phosphoinositide hydrolysis. Stimulation of phosphoinositide hydrolysis by 6-fluoronorepinephrine, an alpha-1 adrenergic agonist, was not affected by prior exposure of cells to 5-HT. Therefore, the decrease in response to 5-HT represented a selective, homologous desensitization of 5-HT2 receptor-mediated phosphoinositide hydrolysis. The density of 5-HT2 receptors in membranes prepared from P11 cells was measured using [125I]lysergic acid diethylamide. Exposure of P11 cells to 5-HT resulted in a decrease in the density of 5-HT2 receptors with no change in the affinity of the receptors for [125I]lysergic acid diethylamide. A decrease in the density of receptors was observed after exposure to 10 microM 5-HT for 1 hr, and after 24 hr the density of 5-HT2 receptors was approximately 30% of the original value. Down-regulation of 5-HT2 receptors in P11 cells may be involved in homologous desensitization of 5-HT2 receptor-mediated phosphoinositide hydrolysis.