Apomorphine delays simple reaction time in Parkinsonian patients

Parkinsonian patients have difficulty in the preparation and execution of movements, which translate into delayed performance of simple reaction time (SRT) paradigms.

To examine short-term effects of an acute subcutaneous injection of the dopamine agonist apomorphine on the results of a SRT task.

We studied a SRT paradigm in 26 non-fluctuating idiopathic Parkinsonian patients, before and 30, 60, 90 min after administration of apomorphine.

The reaction time (RT) was significantly delayed after apomorphine injection. We found no significant change in movement time (MT).

Delay of RT and the lack of response of MT to apomorphine administration may result from the sedative effects of apomorphine, overstimulation of postsynaptic dopaminergic receptors with subsequent inhibition of prefrontal cholinergic neurotransmission, and at least partial binding of apomorphine to presynaptic dopaminergic autoreceptors, which cause inhibition of locomotor activity. We suggest that future studies testing the capacity for reaction in Parkinsonian patients should consider the exact timing of the delivery of dopamine substituting drugs prior to the test.