The atypical antipsychotics clozapine and olanzapine promote down-regulation and display functional selectivity at human 5-HT(7) receptors

BACKGROUND AND PURPOSE: Classically, ligands of GPCRs have been classified primarily upon their affinity and efficacy to activate a signal transduction pathway. Recent reports indicate that the efficacy of a particular ligand can vary depending on the receptor-mediated response measured (e.g. activating G proteins, other downstream responses, internalization). Previously, we reported that inverse agonists induce both homo- and heterologous desensitization, similar to agonist stimulation, at the G(s)-coupled 5-HT(7) receptor. The primary objective of this study was to determine whether different inverse agonists at the 5-HT(7) receptor also induce internalization and/or degradation of 5-HT(7) receptors. EXPERIMENTAL APPROACH: HEK293 cells expressing 5-HT(7(a, b or d)) receptors were pre-incubated with 5-HT, clozapine, olanzapine, mesulergine or SB269970 and their effects upon receptor density, AC activity, internalization, recruitment of β-arrestins and lysosomal trafficking were measured. KEY RESULTS: The agonist 5-HT and three out of four inverse agonists tested increased internalization independently of β-arrestin recruitment. Among these, only the atypical antipsychotics clozapine and olanzapine promoted lysosomal sorting and reduced 5-HT(7) receptor density (∼60% reduction within 24 h). Inhibition of lysosomal degradation with chloroquine blocked the clozapine- and olanzapine-induced down-regulation of 5-HT(7) receptors. Incubation with SB269970 decreased both 5-HT(7(b)) constitutive internalization and receptor density but increased 5-HT(7(d)) receptor density, indicating differential ligand regulation among the 5-HT(7) splice variants. CONCLUSIONS AND IMPLICATIONS: Taken together, we found that various ligands differentially activate regulatory processes governing receptor internalization and degradation in addition to signal transduction. Thus, these data extend our understanding of functional selectivity at the 5-HT(7) receptor.