The dopamine-deficient (DD) mouse model was created in Richard Palmiter’s lab by Qun-Yong Zhou. In 1995, their first published report documented the profound hypoactivity, adipsia and aphagia of DD mice (1).
The selective removal of dopamine in DD mice was accomplished by a particularly elegant strategy. This strategy was developed to address a significant problem – dopamine is itself a precursor to norepinephrine during catecholamine biosynthesis.
The strategy that was used can be simplified as follows: the ability to make dopamine was removed everywhere, and then returned selectively in only those cells that would use dopamine as a precursor to norepinephrine. In greater detail, the genetic modifications are described below.
First, tyrosine hydroxylase (TH), the enzyme that synthesizes the dopamine precursor L-dopa from tyrosine, was deleted in every cell. With this mutation alone, “TH knockouts” lack all catecholamines. These mice (TH-/-) also have been studied in Dr. Palmiter’s lab. For the DD mice, in contrast, a second mutation was introduced.
In DD mice, a copy of the TH gene was re-inserted under the direction of the dopamine beta-hydroxylase (Dbh) promoter. The enzyme Dbh converts dopamine to norepinephrine. The promoter for Dbh is activated, and gene expression occurs, only in those neurons that naturally make and release norepinephrine rather than dopamine. Dopamine-deficient mice carry a single allele of the modified gene, and one normal Dbh allele (DbhTH/+). In this way, DD mice are able to express both TH and Dbh in noradrenergic neurons while at the same time they lack dopamine in dopaminergic neurons. A diagram of this strategy is presented below. The official notation indicating the genotype of DD mice (Th-/-, DbhTH/+) is given at the bottom, where “+” indicates a wild-type or normal gene copy, “-” indicates the knockout, and a “TH” superscript indicates the insertion of the Th gene.
Another advantage of this strategy has been the ability to replace dopamine to these mice. The product of the deleted enzyme TH is the small molecule L-dopa. Because the AADC gene is unaltered, DD mice are able to convert exogenous L-dopa to dopamine. Therefore, DD mice can be studied in both the absence and the presence of dopamine.