Severe reductions in the production of dopamine have been linked with:

Dopamine is a chemical that serves as a neurotransmitter that governs the transmission of signal between the neurons (brain cells) through binding to the receptors. This is also referred to "reward hormone" because this is excreted during activities that causes too much excitement or satisfaction. 

The severe reduction in this hormone is linked primarily to disease that results from the dissociation of the breakdown of this hormone. This causes muscular paralysis, tremor, slow movement. Further this causes difficulty in movement initiation. This disease described above is the Parkinson's Disease.