Blockade of glutamate- and cholinergic ion channels by amantadane derivatives

Motor complications in Parkinson's disease

Management of motor complications in advanced Parkinson's disease (PD) can be challenging. The main complications are inadequate dopaminergic tone ("off" time and dose failures) and excess dopaminergic tone (dyskinesia). These motor complications increase as PD progresses. Changing the dose and timing of L-dopa is the main strategy for both scenarios. Reducing "off" time can also be achieved by the addition of adjunctive therapies (dopamine agonists, catechol-O-methyl transferase inhibitors, and monoamine oxidase-B inhibitors). Dyskinesia can improve with amantadine and possibly several other medications. Surgical interventions such as lesioning and deep brain stimulation are considered when pharmacological strategies for motor complications are not satisfactory.

Levetiracetam Potentiates the Antidyskinetic Action of Amantadine in the 1-Methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-Lesioned Primate Model of Parkinson's Disease

Levetiracetam (LEV) (Keppra; UCB Pharma, Brussels, Belgium) has recently been reported to have antidyskinetic activity against levodopa (L-DOPA)-induced dyskinesia in the 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP)-lesioned marmoset and macaque models of Parkinson's disease. Amantadine is frequently used as adjunctive therapy for L-DOPA-induced dyskinesia, but adverse effects limit its clinical utility. The current study was designed to investigate whether LEV can potentiate the antidyskinetic action of amantadine. The antiparkinsonian and antidyskinetic effects of LEV (13 and 60 mg/kg) and amantadine (0.01, 0.03, 0.1, and 0.3 mg/kg), administered alone and in combination, were assessed in the MPTP-lesioned marmoset model of L-DOPA-induced dyskinesia (n = 12). LEV (60 mg/kg) and amantadine (0.3 mg/kg) administered alone significantly reduced l-DOPA-induced dyskinesia without compromising the antiparkinsonian action of l-DOPA. Lower doses were without any significant effects. The combination of LEV (60 mg/kg) and amantadine (0.01, 0.03, 0.1, and 0.3 mg/kg) significantly decreased dyskinesia severity, without compromising the antiparkinsonian action of L-DOPA, more efficaciously than LEV or amantadine monotherapy. These results support the concept that normalization of different pathophysiological mechanisms (i.e., altered synchronization between neurons and enhanced N-methyl-D-aspartate transmission) has a greater efficacy. Combined LEV/amantadine therapy might be useful as an adjunct to L-DOPA to treat dyskinetic side effects and to expand the population of Parkinson's disease patients who benefit from treatment with amantadine alone.

Direct inhibition of the N-methyl-D-aspartate receptor channel by dopamine and (+)-SKF38393

1. Dopamine is known to modulate glutamatergic synaptic transmission in the retina and in several brain regions by activating specific G-protein-coupled receptors. We have examined the possibility of a different type of mechanism for this modulation, one involving direct interaction of dopamine with ionotropic glutamate receptors. 2. Ionic currents induced by fast application of N-methyl-D-aspartate (NMDA) were recorded under whole-cell patch-clamp in cultured striatal, thalamic and hippocampal neurons of the rat and in retinal neurons of the chick. Dopamine at concentrations above 100 microM inhibited the NMDA response in all four neuron types, exhibiting an IC50 of 1.2 mM in hippocampal neurons. The time course of this inhibition was fast, developing in less than 100 ms. 3. The D1 receptor agonist (+)-SKF38393 mimicked the effect of dopamine, with an IC50 of 58.9 microM on the NMDA response, while the enantiomer (-)-SKF38393 was ineffective at 50 microM. However, the D1 antagonist R(+)-SCH23390 did not prevent the inhibitory effect of (+)-SKF38393. 4. The degree of inhibition by dopamine and (+)-SKF38393 depended on transmembrane voltage, increasing 2.7 times with a hyperpolarization of about 80 mV. The voltage-dependent block by dopamine was also observed in the presence of MgCl2 1 mM. 5. Single-channel recordings showed that the open times of NMDA-gated channels were shortened by (+)-SKF38393. 6. These data suggested that the site to which the drugs bound to produce the inhibitory effect was distinct from the classical D1-type dopamine receptor sites, possibly being located inside the NMDA channel pore. It is concluded that dopamine and (+)-SKF38393 are NMDA channel ligands.