How and why are DD mice treated with L-dopa?
At birth, DD mice resemble control mice, but by 10 – 14 days of age, DD transgenics begin to demonstrate the behavioral signs of profound DA depletion, including hypoactivity, catalepsy and profound hypophagia. In the absence of experimenter intervention, DD mice will starve in the midst of readily available, palatable foods (Zhou and Palmiter 1995). One way to stimulate adequate feeding in DD mice is to replace brain dopamine (DA).
Because DA is a precursor to norepinephrine during catecholamine biosynthesis, the selective removal of DA in DD mice is made possible by two separate genetic manipulations (see below). One advantage of this strategy has been the ability to restore DA to DD mice by treatment with exogenous L-dopa. Because the AADC gene is unaltered, DD mice are able to convert exogenous L-dopa to DA. DD mice can be given exogenous L-dopa by daily intraperitoneal injection.
The transgenic strategy used to create of dopamine-deficient mice is explained in greater detail here. Reference:Zhou and Palmiter, 1995.
Prior to all studies, DD mice have been maintained by daily injection of 3-(3,4 dihydroxyphenyl)-alanine (L-dopa, 50 mg/kg IP), a dopamine precursor. Daily exogenous L-dopa (50 mg/kg) temporarily reverses the Parkinsonian phenotype of DD mice and results in long-lasting (8 -12 h) hyperactivity and voracious feeding. Thus, DD mice are injected with L-dopa to stimulate sufficient voluntary feeding and avoid starvation.
It is important to note that DA replacement is not necessary for the survival of DD mice. We have maintained adult DD mice by intraoral feeding for many weeks in the absence of any DA replacement. DD mice maintained in this way are at least as healthy as those maintained by L-dopa.
What are the behavioral consequences of L-dopa injection in DD mice?
The graph below depicts the locomotor activity, in meters traveled, of a group of DD mice after injection with L-dopa at time zero. Prior to L-dopa injection, these mice demonstrated minimal voluntary activity. Following L-dopa, the mice demonstrate a prolonged period of vigorous hyperactivity. By comparison, the psychomotor activation induced by a large dose of cocaine in a control mouse would peak after 30 minutes, and be virtually absent after a single hour.
Long-lasting locomotor hyperactivity in DD mice following daily L-dopa. The brain DA content of DD mice after L-dopa treatment has measured once, at 2 hours and 18 hours after injection, and was reported (data not shown) in (Zhou and Palmiter, 1995). At 2 hours after L-dopa, when activity of DD mice has peaked, brain DA levels are approximately 10% of what they would be in a normal mouse. Higher doses of L-dopa induced debilitating stereotypies and self-inflicted injury, as was first observed by Ungerstedt in 6-OHDA lesioned rats. Thus, DD mice are extremely sensitive to DA. Within 18-h after L-dopa injection, Zhou and Palmiter reported that the level of DA had fallen to less than 1% that of a normal mouse. There is no hyperactivity following L-dopa in control mice. If anything, control mice are significantly less active in the hour following L-dopa injection.
Immediately following L-dopa injection, both DD and control mice demonstrate the species typical behavioral signs of visceral illness, including elongation of the torso, contraction of the abdominal walls, dragging the belly along the cage bottom, and extension of the hindlimbs and tail. The effects of L-dopa are, in fact, difficult to distinguish from the behavioral effects of a dose of LiCl that is able to induce taste aversion. This is likely to be due to peripheral enteric, sympathetic and hindbrain effects of DA. Within 10 minutes, DD mice begin to move about the cage and are visibly hyperactive within 20 minutes of injection. The movie below depicts typical sequences from within this timecourse.
DD mouse after L-dopa from Claire Cannon on Vimeo.
More information
Questions or comments are welcomed. You may also contact Dr. Cannon at:
