Catecholamine depletion produces irrepressible saccadic eye movements in normal humans

To determine the effect of catecholamine depletion on ocular motor pathways in humans, we studied the eye movements of 3 normal subjects who received the drug metyrosine (alpha-methylparatyrosine). This drug temporarily depleted dopamine and norepinephrine, as measured by a reduction in the metabolite, 3-methoxy-4-hydroxy-phenylethyleneglycol (MHPG). Saccadic, pursuit, and vestibulo-ocular eye movements were recorded using infrared oculography with subjects both on placebo and on metyrosine. The most consistent effect observed with metyrosine was an increase in the amplitude and frequency of saccadic intrusions during fixation and pursuit. Two of the 3 subjects also had shortened time constants for the vestibulo-ocular reflex, attributable in part to the sedative effect of catecholamine depletion. The increase in saccadic intrusions implies that catecholamines modulate the activity of a subpopulation of suppressor motor neurons in the human brainstem.