Functional Selectivity of D₂Receptor Ligands in a Chinese Hamster Ovary hD_2LCell Line: Evidence for Induction of Ligand-Specific Receptor States

Abstract

There are now several examples of single G protein-coupled receptors to which binding of specific agonists causes differential effects on the associated signaling pathways. The dopamine D 2 receptor is of special importance because the selective activation of functional pathways has been shown both in vitro and in situ. For this reason, the present work characterized a series of rigid D 2 agonists in Chinese hamster ovary cells transfected with the human D 2L receptor using three distinct functional endpoints: inhibition of cAMP synthesis, stimulation of mitogen-activated protein (MAP) kinase phosphorylation, and activation of G protein-coupled inwardly rectifying potassium channels (GIRKs). In this system, S-propylnorapomorphine (SNPA), R-propylnorapomorphine (RNPA), dihydrexidine (DHX), dinapsoline (DNS), and dinoxyline (DNX) all inhibited forskolinstimulated adenylate cyclase activity to the same extent as the prototypical D 2 agonist quinpirole (QP). The rank order of potency was the following: RNPA Ͼ Ͼ QP ϭ DNX Ͼ SNPA Ͼ DHX ϭ DNS. For MAP kinase phosphorylation, DHX, DNS, DNX, and RNPA had efficacy similar to QP, whereas SNPA was a partial agonist. The rank order of potency for MAP kinase phosphorylation was RNPA Ͼ Ͼ QP ϭ DNX Ͼ DHX Ͼ DNS ϭ SNPA. DNX activated GIRK channels to the same extent as QP, whereas DHX and DNS were partial agonists, and RNPA and SNPA caused no appreciable activation. These findings indicate that DHX, DNS, RNPA, and SNPA have atypical functional properties at the hD 2L receptor and display different patterns of functional selectivity. We hypothesize that this functional selectivity may be a result of ligand induction of specific conformations of the D 2L receptor that activate only selected signaling pathways.The proximal event mediating cellular signaling through G protein-coupled receptors (GPCRs) is the binding of ligand, with subsequent conformational changes initiating secondary and distal events. In the past, compounds were believed to cause a single type of functional response for all effectors linked to a given receptor; hence, a compound could be labeled according to its intrinsic efficacy as full agonist, partial agonist, antagonist, or, more recently, inverse agonist. This traditional view of the receptor as operating in a digital fashion, either active or inactive, has been challenged, however, by evidence that some ligands working at a single GPCR cause markedly dissimilar degrees of activation for different effector pathways. This phenomenon that we term "functional selectivity" is vastly different from classic receptor theory. Although it is still not generally appreciated, it is probably universal. It has been demonstrated in many different receptor systems, and termed not only "functional selectivity" [dopamine D 2L receptors (Lawler et al., 1999; This work was supported by National Institutes of Health research grants MH53356 (to R.B.M.), MH40537 (to R.B.M.), NS18788 (to G.S.O.), MH42705 (to D.E.N.), and training grant NS07431.R.B.M. and D.E.N. have a sig...

Aripiprazole has Functionally Selective Actions at Dopamine D2 Receptor-Mediated Signaling Pathways

“…The major point of interest, however, lies in the low potency of aripiprazole for the MAPK effector pathway. With the exception of aripiprazole, prior cAMP inhibition EC 50 data for all compounds correspond fairly well with the MAPK potencies reported in this study (Wilson et al, 2001;Burris et al, 2002;Gay et al, 2004).…”

“…For example, we have shown previously that a number of dopamine D 2 ligands exhibit functionally selective profiles Mottola et al, 2002;Gay et al, 2004). Further evidence for the ability of ligands to activate G-protein-coupled receptor (GPCR)-mediated effectors differentially has been illustrated in serotonin 5-HT 2A and 5-HT 2C (Berg et al, 1998;Kurrasch-Orbaugh et al, 2003), a 2A -adrenergic Kukkonen et al, 2001), b 2 -adrenergic (Ghanouni et al, 2001), cannabinoid CB1 (Glass and Northup, 1999), m-opioid (Allouche et al, 1999), and the oxytocin (Reversi et al, 2005) receptor expression systems, among others, effectively illustrating functional selectivity as a universal mechanism of GPCR effector regulation.…”

Recruitment of β-arrestin2 to the dopamine D2 receptor: Insights into anti-psychotic and anti-parkinsonian drug receptor signaling

“…In addition, RNPA and SNPA have been observed to exhibit functional selective properties with regard to other pathways downstream of the hD2 L receptor (e.g. MAP kinase and GIRK channel activation) (Gay et al, 2004). However, in an other study SNPA was determined to be a partial agonist with regard to cAMP signaling (Kilts et al, 2002).…”

“…2 and Table 1) (Newman-Tancredi et al, 2002;Gay et al, 2004;Urban et al, 2007b). Dopamine and the D2-class selective full agonist quinpirole both acted as full agonists with EC 50 values of 49 nM (95% C.I.…”

“…For the long transcriptional variant of dopamine D2 (D2 L ) receptor, it has been shown that the agonists S(+)-10,11-dihydroxy-N-n-propylnoraporphine (SNPA), R(−)-10,11-dihydroxy-N-n-propylnoraporphine (RNPA) and dihydrexidine are as efficacious as the full agonist quinpirole in inhibition of cAMP production. However, both RNPA and SNPA do not activate G-protein coupled inwardly rectifying K+ (GIRK) channels and SNPA exhibits partial agonism towards D2 L mediated MAP kinase phosphorylation, whereas dihydrexidine shows partial agonism for GIRK channel activation (Gay et al, 2004).…”