Motor response following repeated apomorphine administration is reduced in Parkinson's disease.

Ten patients with Parkinson's disease (PD) with motor fluctuations under levodopa treatment were given repeated equal subcutaneous injections of apomorphine [minimal effective dose (MED)] in 1 day. The MED was defined as the dose of apomorphine necessary to induce at least 60% reduction of motor disability for a minimum period of 10 min. MED was found for each patient in previous study days. In eight a subcutaneous infusion of apomorphine was performed on a different day. Four patients with simple fluctuations ("wearing off") showed a progressive reduction of the motor response to apomorphine injections, but three of the four had a stable response (continuous "on") to apomorphine infusion. Six patients with complicated fluctuations also exhibited a decreasing response to successive apomorphine injections and often completely failed to respond to some of the boluses. The response to a subcutaneous infusion of apomorphine was unstable in three of four cases. These findings indicate that a reduction of striatal dopaminergic receptor sensitivity is associated with repeated "pulsatile" apomorphine administration in parkinsonian patients with oscillations of motor performance. It is suggested that altered regulation of dopaminergic receptor sensitivity following pulsatile stimulation with levodopa may be a relevant phenomenon in the pathogenesis of motor fluctuations in PD.