Parkinson's disease, L-DOPA, and endogenous morphine: a revisit

Clinical observations stemming from widespread employment of restorative L-3,4-dihydroxyphenylalanine (L-DOPA) therapy for management of dyskinesia in Parkinson's Disease (PD) patients implicate a regulatory role for endogenous morphine in central nervous system dopamine neurotransmission. Reciprocally, it appears that restorative L-DOPA administration has provided us with a compelling in vivo pharmacological model for targeting peripheral sites involved in endogenous morphine expression in human subjects. The biological activities underlying endogenous morphine expression and its interaction with its major precursor dopamine strongly suggest that endogenous morphine systems are reciprocally dysregulated in PD. These critical issues are examined from historical and current perspectives within our short review.