Adenosine A1 Receptor-mediated Modulation of Dopamine D1 Receptors in Stably Cotransfected Fibroblast Cells

Abstract

It has been shown that the binding characteristics of one type of G protein-coupled receptor can be altered by the stimulation of another type of G protein-coupled receptor in crude membrane preparations (1). Such intramembrane interactions have been postulated to represent direct interactions between the receptor molecules and/or to involve G proteins or other mobile molecules associated with the membrane (1). There is increasing evidence suggesting that antagonistic intramembrane interactions between specific subtypes of adenosine and dopamine receptors constitute an important integrative mechanism in the basal ganglia (2, 3). Adenosine A 1 and A 2A receptors antagonistically and specifically modulate the binding characteristics of dopamine D 1 and D 2 receptors, respectively (2, 3). In membrane preparations from rat striatum, the stimulation of A 2A receptors decreases the affinity of D 2 receptors for agonists (4). On the other hand, the stimulation of A 1 receptors was shown to decrease the proportion of D 1 receptors in the high affinity state, without modifying the dissociation constants of high and low affinity D 1 agonist-binding sites (5).Thus, the A 1 receptor agonist had the same effect as that induced by the GTP analogue Gpp(NH)p.1 It was hypothesized that A 1 receptor stimulation might uncouple the striatal D 1 receptor from the G protein (5). There is evidence that the antagonistic A 2A -D 2 and A 1 -D 1 intramembrane interactions are involved in the motor depressant effects of adenosine receptor agonists and the motor stimulant effects of adenosine receptor antagonists, such as caffeine (2-5).The same changes in the binding characteristics of striatal D 2 receptors after A 2A receptor stimulation have been obtained in membrane preparations from a mouse fibroblast cell line (Ltk Ϫ ) stably cotransfected with the dog A 2A receptor and human D 2 (long-form) receptor cDNAs (6). In these transfection studies, it was also found that activation of adenylyl cyclase was not involved in the intramembrane A 2A -D 2 interaction (6). Altogether, these results showed that stably cotransfected cell lines constitute a valuable model to study the mechanistic aspects involved in the intramembrane receptor-receptor interactions. In the present work, this methodology has been applied to study the antagonistic interaction between A 1 and D 1 receptors. The first aim of the study was to demonstrate the existence of an antagonistic A 1 -D 1 intramembrane interaction in mammalian cells stably cotransfected with A 1 receptor and D 1 receptor cDNAs. The second aim was to demonstrate the existence of a functional antagonistic interaction between A 1 and D 1 receptors in the cotransfected cells by means of cAMP accumulation experiments. Finally, the third aim of the study was to find a functional significance of the antagonistic A 1 -D 1 intramembrane interaction. EXPERIMENTAL PROCEDURES Transfection and Maintenance of Fibroblast LtkϪ Cells-Cells from the mouse fibroblast Ltk Ϫ cell line previously transfected with the huma...

Adenosine does not bind to the growth hormone secretagogue receptor type-1a (GHS-R1a)

“…The reduction in adenosine affinity in transfected cells suggests that the latter possibility is more plausible, as a higher K d was detected. In addition, cross-talk between heterodimeric receptor pairs can modify positively or negatively the response to agonists resulting in either enhanced-G-protein activation or cross-inhibition (Ferre et al 1998, Jordan & Devi 1999, even changes in G-protein-coupling specificity (George et al 2000). In this particular case, the presence of GHS-R1a in HEK 293 cells appears to be essential for the qualitative coupling of A2b-R and A3-R to G s -signaling pathway, as cAMP levels were elevated in response to adenosine in HEK-GHS-R1a cells, whereas no increase was determined in HEK 293 cells .…”

Preferential Formation of MT1/MT2 Melatonin Receptor Heterodimers with Distinct Ligand Interaction Properties Compared with MT2 Homodimers

“…Whether both sites are indeed functional and whether ligand binding to both sites is necessary for receptor activation are critical questions to understand the activation mechanism of GPCRs. 125 [␦ and opioid (Jordan and Devi, 1999), and ␦ opioid (George et al, 2000), somatostatin sst1 and sst5 receptors (Patel et al, 2002), adenosine A1 and dopamine D1 (Ferre et al, 1998), somatostatin sst5, and dopamine D2 (Rocheville et al, 2000a)]. …”

Subchronic Caffeine Exposure Induces Sensitization to Caffeine and Cross-Sensitization to Amphetamine Ipsilateral Turning Behavior Independent from Dopamine Release

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