Studies on the mechanism of action of the antiparkinsonian drugs memantine and amantadine: no evidence for direct dopaminomimetic or antimuscarinic properties.

The effects of the antiparkinsonian drugs amantadine and memantine and of the noncompetitive NMDA receptor antagonist MK-801 were studied in the model of the electrically evoked release of [3H]dopamine and [3H]acetylcholine in slices of the rabbit caudate nucleus in vitro. In the [3H]dopamine release model, high concentrations (> 10 microM) of the drugs investigated showed a marked increase in spontaneous 3H-efflux, with the following order of potency: memantine > MK-801 > amantadine. The spontaneous 3H-efflux induced by 50 microM memantine was diminished by pargyline (10 microM), but was insensitive to the presence of tetrodotoxin (0.3 microM), or nomifensine (1 microM), or to the omission of Ca2+ in the superfusion medium; in the absence of nomifensine it consisted mainly of 3H-metabolites of dopamine. The latter findings indicate that the memantine-induced spontaneous 3H-efflux is unrelated to the firing of striatal interneurons but point to a direct releasing effect of memantine on dopaminergic storage granules. On the other hand, the electrically evoked release of [3H]dopamine seemed to be unaffected by memantine, was only slightly reduced by amantadine but significantly inhibited by MK-801. In the model of the electrically evoked release of [3H]acetylcholine, only MK-801 (> 50 microM) showed significant inhibitory effects, both on spontaneous and evoked overflow of [3H]acetylcholine; the effects of memantine and amantadine were negligible. Since only high concentrations of amantadine and memantine (exceeding those which, according to the literature, may be reached in vivo) showed effects in the present in vitro dopamine and acetylcholine release models, it is concluded that the clinical efficacy of these antiparkinsonian drugs is most probably unrelated to an enhancement of dopaminergic transmission as suggested from earlier in vitro and in vivo findings. The higher activity of MK-801 in the present model and the recent observation that both memantine and amantadine bind--although with lower affinity--to an MK-801 binding site in the NMDA receptor-linked ion channel, suggest a possible involvement of striatal NMDA receptors in the antiparkinsonian effects of these drugs.