Pineal melatonin and sensory symptoms in Parkinson disease.

Sensory symptoms have been reported in 40-60% of patients with Parkinson's disease, and in at least 10% of patients these symptoms precede the onset of the motor disorder. The pathophysiology of these symptoms remains unknown. Diminished brain serotonin concentration has been reported to be associated with sensory symptoms. Serotonin metabolism is regulated by pineal melatonin. The secretory activity of the pineal gland may be diminished in Parkinson's disease. In experimental animals pineal melatonin has been shown to exert analgesic effects by interacting with opiate receptors. In addition, since opioid peptides mediate the analgesic effects of melatonin, decreased opioid peptide functions in Parkinson's disease may be associated with disruption of the "fine-tuning" pain modulatory functions of melatonin and possibly indirectly facilitate the emergence of sensory symptoms.