DDC and Parkinson disease: By analogy with other brain centers containing bienzymatic and monoenzymatic neurons, in the future it is important to assess: (i) what is the regulatory role of the end products of synthesis—L-DOPA and dopamine, within and outside the PeVN; (ii) whether the cooperative synthesis of dopamine by monoenzymatic TH and AADC neurons is activated under systemic failure of the dopaminergic systems of the brain, primarily in Parkinson’s disease; (iii) whether the expression of TH and AADC is associated with the kisspeptin regulation of reproduction.