The significance of eye blink rate in parkinsonism: a hypothesis.

Alterations in blink rate have been reported in several neuropsychiatric disorders presumed to result from abnormal central dopaminergic functions. Increased blink rate in schizophrenia, Tardive dyskinesia, Tourette's syndrome and Meige's disease are associated with enhanced dopaminergic functions. Parkinson's disease is associated with reduced dopaminergic functions and decreased blink rate. Thus, blink rate may reflect striatal and mesolimbic dopaminergic activity. Since acute light exposure suppresses melatonin production and darkness stimulates melatonin secretion, blinking may serve to regulate light-dark exposure to the pineal gland and thus to 'fine tune' melatonin production. As there is evidence to suggest that melatonin inhibits the release of dopamine in the striatum and limbic system, increased blink rate may serve to reduce light exposure, increase melatonin secretion and attenuate dopaminergic functions. Conversely, decreased blinking (as is observed in patients with Parkinson's disease) could reflect a compensatory mechanism to increase light exposure, reduce melatonin production and ultimately increase dopamine functions. This model is novel in that for the first time it suggests a functional link among blink rate, melatonin secretion and striatal dopaminergic functions in movement disorders.