You are asking difficult questions, my friend. We live in a fallen world, and sickness abounds. Reasons for low dopamine levels can be a result of mercury toxicity. I'll do a C&P, as I'm not really knowledgeable in this area.
------------------------
IV. Toxic and immune reactive effects of mercury
Mercury:
Is neurotoxic (kills or damages brain and nerve cells)
Generates high levels of reactive oxygen species (ROS) and oxidative stress and depletes glutathione and thiols causing increased neurotoxicity from interactions of ROS, glutamate, and dopamine
Kills or inhibits production of brain tubulin cells
Inhibits production of neurotransmitters by inhibiting: calcium-dependent neurotransmitter release, dihydroteridine reductase, nitric oxide synthase, blocking neurotransmitter amino acids, and effecting phenylalanine, tyrosine and tryptophan transport to neurons.
Causes systemic methylation deficiencies, which are documented to commonly be a factor in chronic conditions such as depression and autism.

Numerous studies have found long-term chronic low doses of mercury cause neurological, memory, behaviour, sleep, and mood problems. Neurological problems are among the most common and serious effects of mercury, and include memory loss, moodiness, depression, anger and sudden bursts of anger or rage, self-effacement, suicidal thoughts and lack of strength or force to resolve doubts or resist obsessions or compulsions.

Many studies of patients with major neurological diseases have found evidence amalgam fillings may play a major role in development of conditions such as:

Depression
Schizophrenia
Memory problems
Serious neurological diseases such as MS, ALS, Parkinson’s and Alzheimer’s diseases ... (continued - see below)
-----------
Adding the website address,
<http://www.thenaturalrecoveryplan.com/articles/research-depression-mercury.html>
---Have you talked with your mother's Doctor regarding all these questions? Or is this YOU were are dealing with here? There are supplements that can be taken to raise dopamine levels. Maybe ask Russ? Keep in mind all of our bodies are made differently, and very complicated to the human's eye. Some Doctors are aware of things, but Russ, being the owner of the Herb Forum, is very knowledgeable in this--- So shall we ask him to join in? He may come along. I know he's busy. Best wishes!