Presence of a D2 receptor truncated protein in the brain of the D2 receptor functional knockout mouse, revisiting its molecular and neurochemical features

Null mice for the dopamine D2 receptor (D2R) have been instrumental in understanding the function of this protein in the central nervous system. Several lines of D2R knockout mice have been generated, which share some characteristics but differ in others. The D2R functional knockout mouse, first described in 1997, is functionally null for D2R-mediated signaling but the Drd2 gene was interrupted at the most extreme distal end leaving open the question about whether transcript and protein are produced. We decided to determine if there are D2R transcripts, the characteristics of these transcripts and whether they are translated in the brain of D2R functional knockout mice. Sequence analysis of 3 Rapid Amplification of cDNA Ends showed that D2R functional knockout mice express transcripts that lack only the exon eight. Immunofluorescence showed D2R-like protein in the brain of the knockout mice. As previously reported, D2R functional knockout mice are hypoactive and insensitive to the D2R agonist quinpirole (QNP). However, the heterozygous showed locomotor activity and response to QNP similar to the wild-type mice. Intriguingly, microdialysis experiments showed that heterozygous mice, such as knockouts, have half the normal levels of synaptic dopamine in the striatum. However, heterozygous mice responded similarly to wild-type mice to an acute injection of QNP, showing a 50% decrease in synaptic dopamine. In conclusion, D2R functional knockout mice express transcripts that lead to a truncated D2R protein that lacks from the sixth transmembrane domain to the C-terminal end but retains the third intracellular loop. We discuss the implications of this truncated D2R coexisting with the native D2R that may explain the unexpected outcomes observed in the heterozygous. Finally, we suggest that the D2R functional knockout mouse can be a useful model for studying protein-protein interaction and trafficking of D2R.