Dopamine D2 and D3 receptors. Studies on transduction mechanisms and on the role of a D3 gene polymorphism

Abstract

The neurotransmitter dopamine has been implicated in the pathophysiology of, e.g., Parkinson s disease and schizophrenia, and dopamine receptor-active compounds are commonly employed therapeutically in these and other disorders. An overall aim of this thesis was to illuminate how the signal transduction system involving arachidonic acid (AA) and its metabolite, prostaglandin E2 (PGE2), is influenced by D2 and D3 dopamine receptors, respectively, and to what extent assessment of the effects of ligands with affinity for these receptors on the AA pathway may serve to study phenomena such as partial and inverse agonism. In addition, the possible functional importance of a polymorphism in the gene encoding the D3 receptor was assessed both in vitro, using cAMP and PGE2 as markers of receptor activation, and in vivo, by means of an association study.The results presented in paper I suggest that dopamine potently increases AA formation in D2 receptor-transfected Chinese hamster ovary (CHO) cells. Challenging previous reports, this effect was observed also in the absence of the concomitant administration of a Ca2+-mobilizer. Paper I also outlined the respective roles of G-proteins, Ca2+, and phospholipases for this effect to occur. Whereas ( )-3-PPP, in line with previous studies, displayed the properties of a partial D2 receptor agonist, the alleged dopamine receptor antagonist haloperidol displayed the profile of an inverse agonist. Paper II extends the findings of paper I by showing that the D2-induced induction of AA leads to the formation of PGE2 as the result of the catalytic action of cyclooxygenase 2 (COX-2). In this paper, we also present data supporting the notion that the D2-induced AA mobilization is catalyzed by the cPLA2 phospholipase subtype. Paper III shows that dopamine at low, but not high, concentrations inhibits AA formation in CHO cells transfected with the rat D3 receptor, i.e. an effect opposite to that observed in cells expressing D2 receptors. The dopamine receptor ligand PNU-99194A, which previously has been regarded as a D3 receptor antagonist, displayed agonist-like properties.Paper IV was an attempt to clarify the functional role of a polymorphism in the dopamine D3 receptor gene, the ser9gly polymorphism, using CHO cells transfected with either the ser9 or the gly9 variant of this receptor. The ser9gly polymorphism was found to alter the effect of dopamine on cellular transduction from inhibition of cyclic AMP (ser9) to inhibition of PGE2 (gly9). Haloperidol displayed antagonist properties at ser9-type receptors, and agonist-like properties at gly9 receptors. Finally, paper V aimed at further exploring the notion that the ser9gly polymorphism is functionally important by assessing its possible association with personality traits in healthy humans. The results suggest that the ser9gly polymorphism may be weakly linked to neuroticism in males and females, and to non-conformity traits in women.The main conclusions are (i) that the AA/PGE2 pathway is a useful marker for assessment of D2 and D3 receptor activity in vitro, (ii) that compounds previously assumed to act as neutral antagonists at D2 or D3 receptors may display agonism or inverse agonism vis-à-vis these receptors, hence illustrating the difficulties associated with the characterization of a receptor ligand in terms of intrinsic activity, and (iii) that the ser9gly polymorphism may influence D3 receptor function by means of a hitherto unknown mechanism, i.e. by shifting cellular transduction from one second messenger system (cAMP) to another (PGE2).