Agonist dependent tolerance property of D3 dopamine receptor: a role for second extracellular loop residues

Our previous studies have shown that the D3 dopamine receptor exhibits, tolerance and slow response termination signaling properties that are not shared with the D2 dopamine receptor. While it has been possible to prevent tolerance in the D3receptor by the introduction of the D2 receptor intracellular loop 2 into a chimeric D3 receptor, it has not been possible to induce tolerance in the D2 receptor by the reciprocal action. Introduction of all intracellular loops from the D3 receptor into a chimeric D2 receptor does not induce tolerance in the chimeric receptor. From these studies it is clear that the second intracellular loop, and in particular absence of a positively charged residue at position 147, of the D3R is necessary but not sufficient for the development of tolerance. The objective of the current study was to evaluate the role of extracellular loops and transmembrane region 4 in D3 receptor tolerance property. To do this, we used chimeric receptors, site‐directed mutagenesis, functional studies and computer modeling. The results from our studies suggest that residues in the extracellular loop 2 (EC2) of D3 dopamine receptor are necessary for the induction of tolerance. Furthermore, the role of D3 receptor EC2 in the development of the tolerance property is agonist‐dependent, suggesting that EC2 of D3 receptor might contribute to functional selectivity.