Experimental principle: Injection of reserpine solution can reduce dopamine in healthy mice. ...

(2) According to the meaning of the question (it is now known that "dopa" can be converted into "dopamine" in the human body; However, a drug called reserpine can deplete dopamine in nerve endings. The experimental principle is that dopamine in healthy mice will be exhausted by injecting reserpine solution, which will lead to the appearance of exercise tremor symptoms. If the dopa solution is injected again, it can be converted into dopamine in the body and the symptoms disappear.

(3) According to the principle of univariate and contrast in experimental design, the experimental steps are as follows:

① Material selection and grouping: healthy mice with the same growth and development status were divided into group A and group B with the same number.

② According to the meaning of the question, Group A is the experimental group and Group B is the control group. The experimental group needs to inject a certain concentration of reserpine solution, and the control group needs to inject the same amount of normal saline.

③ Feeding under the same suitable conditions, observing and recording the movement of mice;

④ In order to verify that dopa is the cause of relieving involuntary tremor during exercise, the experimental group A was injected with a certain concentration of dopa solution, while the control group B was injected with the same amount of normal saline.

⑤ Keep feeding under the same suitable conditions, and observe and record the movement of mice.

(4) The experimental results predicted that the mice in group A would have exercise tremor after reserpine injection, and the exercise would return to normal after dopa injection. The mice in group B always exercised normally during the experiment.

(5) The conclusion is that dopamine deficiency is the cause of involuntary tremor during exercise.

So the answer is:

(2) Experimental principle: Injection of reserpine solution will exhaust dopamine in healthy mice, thus causing the symptoms of exercise tremor. If the dopa solution is injected again, it can be converted into dopamine in the body and the symptoms disappear.

(3) Experimental steps:

① healthy mice with the same growth and development status were divided into group a and group B.

② Group A was injected with a certain concentration of reserpine solution, while Group B was injected with the same amount of normal saline.

③ Feeding under the same suitable conditions, observing and recording the movement of mice.

④ Continue the experiment after the expected results appear:

A group was injected with a certain concentration of dopa solution, while B group was injected with the same amount of normal saline.

⑤ Keep feeding under the same suitable conditions, and observe and record the movement of mice.

(4) Experimental results:

Group A: Exercise tremor appeared after reserpine injection, and the exercise returned to normal after dopa injection.

Group B: Exercise was normal all the time during the experiment.

(5) Experimental conclusion: Dopamine deficiency is the cause of exercise tremor.