The
serotonin (
5-hydroxytryptamine [5HT]) system has recently emerged as an important player in the appearance of
l-3,4-dihydroxyphenylalanine (
levodopa [
l-dopa])-induced
dyskinesia in animal models of
Parkinson's disease. In fact,
dopamine released as a false transmitter from
serotonin neurons appears to contribute to the pulsatile stimulation of
dopamine receptors, leading to the appearance of the abnormal
involuntary movements. Thus, drugs able to dampen the activity of
serotonin neurons hold promise for the treatment of
dyskinesia. The authors investigated the ability of the mixed
5-HT 1A/1B receptor agonist
eltoprazine to counteract
l-dopa-induced
dyskinesia in 6-hydroxydopamine-lesioned rats and in
1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (
MPTP)-treated macaques. The data demonstrated that
eltoprazine is extremely effective in suppressing
dyskinesia in experimental models, although this effect was accompanied by a partial worsening of the
therapeutic effect of
l-dopa. Interestingly,
eltoprazine was found to (synergistically) potentiate the antidyskinetic effect of
amantadine. The current data indicated that
eltoprazine is highly effective in counteracting
dyskinesia in preclinical models. However, the partial worsening of the
l-dopa effect observed after
eltoprazine administration represents a concern; whether this side effect is due to a limitation of the animal models or to an intrinsic property of
eltoprazine needs to be addressed in ongoing clinical trials. The data also suggest that the combination of low doses of
eltoprazine with
amantadine may represent a valid strategy to increase the antidyskinetic effect and reduce the
eltoprazine-induced worsening of
l-dopa therapeutic effects.