LETTER TO JMG The ser9gly SNP in the dopamine D3 receptor causes a shift from cAMP related to PGE2 related signal transduction mechanisms in transfected CHO cells

T he dopamine D3 receptor is a member of the D2 family of dopamine receptors. Several investigators have suggested that D3 receptors are involved in the regulation of locomotion, in the pathophysiology of schizophrenia, and in drug abuse. Dopamine enhances basal and stimulus evoked release of arachidonic acid, as well as the production of one of its metabolites, prostaglandin E2 (PGE2), in CHO (Chinese hamster ovary) cells transfected with the D2 receptor. 4–6 In contrast, dopamine does not influence basal arachidonic acid release but inhibits stimulus evoked release in cells transfected with the rat D3 receptor. 10 On the other hand, with respect to cAMP production, the effect of dopamine is similar in CHO cells transfected with either D2 or D3 receptors, both receptor subtypes mediating a reduction in forskolin induced cAMP formation. 12 A single nucleotide polymorphism (SNP) in the first exon of the dopamine D3 receptor gene, the ser9gly polymorphism, leads to a serine to glycine amino acid substitution in the N-terminal extracellular domain of the receptor protein. An association between this polymorphism and schizophrenia has been suggested, but also questioned, 15 a meta-analysis suggesting the effect to be small but real in white subjects. Other investigators have suggested that the gly-9 allele may be associated with an increased risk of developing antipsychotic induced tardive dyskinesia rather than with schizophrenia per se; this finding also gained support from a report containing both pooled analyses of original data and a meta-analysis, and also from a recent primate study (see also a recent report by Lohmueller and coworkers). In addition, the ser9gly polymorphism has been reported to be associated with therapeutic response to atypical antipsychotic agents, with impulsiveness and attention deficit hyperactive disorder (ADHD), and with obsessive traits. A radioligand binding study has reported that the gly-9 variant of the receptor displays higher affinity for dopamine than the ser-9 variant, but to our knowledge there are no functional investigations in vitro regarding possible differences between the two alleles. Usually, the possible functional consequences of a polymorphism in a gene coding for a receptor are discussed in terms of enhanced or reduced receptor responsiveness. For a receptor coupling to more than one transduction pathway, the possibility that the functional consequence of a polymorphism influencing the structure of the receptor is different for different pathways cannot, however, be excluded. In the present study, we therefore compared CHO cells transfected with either the ser-9 or the gly-9 variant of the human dopamine D3 receptor in relation to the effect of dopamine on both forskolin induced cAMP formation and basal PGE2 production. METHODS Cell culture Both transfected and non-transfected CHO-K1 cells were maintained in Ham’s F-12 medium supplemented with 10% fetal calf serum (FCS), l-glutamine, and penicillin/streptomycin (Biochrome, Berlin, Germany) (= complete medium).