8 December 2010. Coupling between two different types of dopamine receptor appears enhanced in human depression, and interfering with this interaction has antidepressant-like effects in rats, according to a report in Nature Medicine published online on November 28.
In experimental preparations spanning postmortem brain tissue from humans, non-neuronal cell cultures, and rats, Fang Liu and colleagues at the University of Toronto in Canada examined coupling between dopamine D1 and D2 receptors. Previous work had shown that D1 and D2 could form a complex which activates a G-protein pathway that is not recruited when either receptor is activated alone (see SRF related news story). This suggested that abnormal coupling between D1 and D2 could shift cell signaling into pathological states related to psychiatric disease, and that finding ways to normalize coupling could be a new strategy for treatment.
The new study bolsters these ideas by finding that a direct interaction between D1 and D2 is increased in the brains of people who had major depression. Similarly, using a peptide to interfere with this D1-D2 interaction decreased depression-like behaviors in rats.
The D1-D2 interface
First author Lin Pei and colleagues looked at D1-D2 coupling in postmortem tissue taken from the striatum using a D2-specific antibody. In co-immunoprecipitation experiments, this antibody pulled down about 30 percent more D1 in tissue from 15 people who had had severe depression than from age- and sex-matched controls. Because amounts of D2 bound by the antibody did not differ between the two groups, this suggested that the antibody was snaring more D1 in complex with D2. Consistent with this, over twice as much of the total pool of D1 was in complex with D2 in depression than in controls.
The researchers then identified the regions of the D1 and D2 receptors that were essential for coupling. By engineering fusion peptides containing different parts of each receptor type, they found that the carboxyl tail of D1 directly interacted with the third intracellular loop of D2. Further experiments narrowed in on a 29 amino acid-stretch within this loop: one half of this section bound D1 and the other half did not. The researchers developed a blocking peptide consisting of the D1-binding half to disrupt coupling between endogenous D1 and D2. Treating non-neuronal cells expressing D1 and D2 with this peptide blocked the activation of D1- and D2-induced calcium release in the cells.
To see if interfering with D1-D2 coupling could alleviate depression-like behaviors, the researchers turned to a forced swim test, in which rats are placed in a water bath and the amount of time they spend swimming, climbing, or floating passively—considered a depression-like behavior—is tallied. Rats that had the blocking peptide infused into their prefrontal cortex exhibited less of the passive, immobile behavior than did controls, though overall, their locomotion was normal. The effect size was similar to that obtained by infusing the antidepressant imipramine. The researchers verified that the blocking peptide was working as it should by examining the degree of D1-D2 coupling with co-immunoprecipitation experiments on brain tissue from the different groups of rats subjected to the forced swim test.
The researchers then tested the antidepressant-like activity of this blocking peptide in a learned helplessness paradigm. In the first stage, rats were exposed to the stress of an inescapable foot shock; this elevated levels of coupling between D1 and D2 receptors, which could be mitigated by imipramine. In the second stage three days later, the rats were tested for their reaction to a tone warning of imminent shock in a chamber with two rooms—one that delivered a shock, and one that offered a shock-free retreat. Rats that received the blocking peptide or imipramine after the first stage more frequently escaped the shock than did rats that had received a peptide derived from a section of the D2 receptor that did not disrupt D1-D2 coupling. Rats with the non-disrupting peptide on board seemed stuck in a learned helplessness mode, almost never escaping the shock, whereas rats with the blocking peptide escaped shock in nearly half of the trials, an effect similar to that seen with imipramine.
Figuring out how interfering with D1-D2 coupling alters dopamine signaling will require more experiments. But these results suggest that manipulating the association between these two receptors may be a useful avenue for developing effective treatments for depression that may come with fewer side effects than do current antidepressants, which typically act on D2 receptors.—Michele Solis.
Pei L, Li S, Wang M, Diwan M, Anisman H, Fletcher PJ, Nobrega JN, Liu F. Uncoupling the dopamine D1-D2 receptor complex exerts antidepressant-like effects. Nat Med. 2010 Nov 28. Abstract