Effects of L-DOPA and L-5-HTP on the visual evoked response

Small doses (10 and 20 mg/kg) of L-DOPA inhibited the amplitude of the visual evoked response (VER), while large doses (40 and 80 mg/kg) enhanced it. Though low doses (12.5 and 25 mg/kg) of L-5-HTP caused a slight increase in amplitude of the VER, the simultaneous administration of 12.5 mg/kg of L-5-HTP and 10 mg/kg of L-DOPA produced a marked enhancement. The peak latency was prolonged after the injection of any doses of L-DOPA, L-5-HTP, or both.     

Memantine,amantadine, and L-deprenyl potentiate the action of L-DOPA in monoamine-depleted rats

Summary Some treatments used for Parkinson's disease attenuate locomotor depression in rats treated with reserpine and -methyl-p-tyrosine. In the present study memantine (2.5, 5.0mg/kg), amantadine (10, 20mg/kg) (both uncompetitive NMDA antagonists), and L-deprenyl (1.0, 5.0 mg/kg; MAO-B inhibitor) were tested for possible synergistic interactions with the dopamine agonists: bromocriptine (2.5, 5.0mg/kg) and L-DOPA (50, 100mg/kg, + benserazide, 100 mg/kg). At higher doses, memantine (10 mg/kg), amantadine (40 mg/kg), bromocriptine (5 and 10mg/kg) and L-DOPA (100, 200mg/kg) but not L-deprenyl (up to 10 mg/kg) produced a pronounced increase in locomotor activity when given alone. The combination of memantine, amantadine and L-deprenyl with bromocriptine did not result in synergism of action and, at best, an additive effect was seen. On the other hand the combination of these agents with L-DOPA produced a pronounced synergistic effect. Hence, the clinical observation that coadministration of L-DOPA with either memantine or amantadine results in enhancement of their action is also reflected in an animal model of Parkinson's disease. Such a combination therapy should allow the use of lower doses of both drugs which may reduce the occurrence of side effects and may also be predicted to have additional benefits related to the neuroprotective properties of memantine, amantadine, and L-deprenyl.     

In vivo comparison of the effects of inhibition of MAO-A versus MAO-B on striatal L-DOPA and dopamine metabolism

Summary Utilizing the cerebral microdialysis technique, we have compared in vivo the effects of selective MAO-A, MAO-B, and nonselective MAO inhibitors on striatal extracellular levels of dopamine (DA) and DA metabolites (DOPAC and HVA). The measurements were made in rats both under basal conditions and following L-DOPA administration. Extracellular levels of dopamine were enhanced and DA metabolite levels strongly inhibited both under basal conditions and following L-DOPA administration by pretreatment with the nonselective MAO inhibitor pargyline and the MAO-A selective inhibitors clorgyline and Ro 41-1049. The MAO-B inhibitor deprenyl had no effect on basal DA, HVA, or DOPAC levels. Nervertheless, deprenyl significantly increased DA and decreased DOPAC levels following exogenous L-DOPA administration, a finding compatible with a significant glial metabolism of DA formed from exogenous L-DOPA. We conclude that DA metabolism underbasal conditions is primarily mediated by MAO-A. In contrast, both MAO-A and MAO-B mediate DA formation when L-DOPA is administered exogenously. The efficacy of newer, reversible agents which lack the cheese effect such as Ro 41-1049 are comparable to the irreversible MAO-A inhibitor clorgyline. The possible relevance of these findings for the treatment of Parkinson's disease is discussed.     

Correlación entre la expresión del receptor dopaminérgico D2 y presencia de movimientos involuntarios anormales (MIA) en un modelo de disquinesia en ratas Wistar hemiparkinsonizadas

Parkinson's disease (PD) is characterised by motor alterations, which are commonly treated with L-DOPA. However, long-term L-DOPA use may cause dyskinesia. Although the pathogenic mechanism of L-DOPA-induced dyskinesia is unclear, the condition has been associated with alterations in dopamine receptors, among which D2 receptors (D2R) have received little attention. This study aims to: (i) develop and standardise an experimental model of L-DOPA-induced dyskinesia in rats with hemiparkinsonism; and (ii) evaluate the correlation between D2R expression and presence of abnormal involuntary movements (AIM). We allocated 21 male Wistar rats into 3 groups: intact controls, lesioned rats (with neurotoxin 6-OHDA), and dyskinetic rats (injected with L-DOPA for 19 days). Sensorimotor impairment was assessed with behavioural tests. Dyskinetic rats gradually developed AIMs during the treatment period; front leg AIMs were more severe and locomotor AIMs less severe (P < .05). All AIMs were significantly evident from day 5 and persisted until the last day of injection. D2R density was greater in the striatum and the medial anterior brain of the lesioned and dyskinetic rats than in those of controls. Our results suggest an association between D2R expression and locomotor AIMs. We conclude that RD2 is involved in L-DOPA-induced dyskinesia.     

Impairment of the septal cholinergic neurons in MPTP-treated A30P α-synuclein mice

Dementia in Parkinson's disease (PDD) and dementia with Lewy bodies (DLB) are characterized by loss of acetylcholine (ACh) from cortical areas. Clinical studies report positive effects of acetylcholine esterase (AChE) inhibitors in PDD and dementia with Lewy bodies. We here report that the number of neurons expressing a cholinergic marker in the medial septum-diagonal band of Broca complex decreases in A30P α-synuclein-expressing mice during aging, paralleled by a lower AChE fiber density in the dentate gyrus and in the hippocampal CA1 field. After inducing dopamine depletion by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride (MPTP), no acute but a delayed loss of cholinergic neurons and AChE-positive fibers was observed, which was attenuated by L-3,4-dihydroxyphenylalanine (DOPA) treatment. Expression of nerve growth factor (NGF) and tyrosine receptor kinase A (TrkA) genes was upregulated in 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine hydrochloride-treated wild type mice, but not in A30P α-synuclein expressing animals. In contrast, upregulation of sortilin and p75^NTR genes was found in the A30P α-synuclein-expressing mice. These results suggest that dopamine deficiency may contribute to the impairment of the septohippocampal system in patients with PDD and that L-3,4-dihydroxyphenylalanine may not only result in symptomatic treatment of the akinetic-rigid syndrome but may also alleviate the degeneration of basal forebrain cholinergic system and the cognitive decline.     

L-DOPA infusion mode differentially affects corpus striatal dopamine efflux in the presence of reserpine

Summary In the present experiment we tested the effects of L-DOPA upon dopamine (DA) efflux in vitro from superfused corpus striatal tissue fragments in medium containing reserpine. The purposes of this experiment were first, to evaluate the effects of differing infusion modes of L-DOPA upon DA efflux under conditions in which DA storage capacity has been diminished, and second, to compare this L-DOPA stimulated DA efflux with that of other putative DA secretagogues such as amphetamine and postassium. No differences were obtained in stimulated DA efflux between superfusions performed in the presence or absence of reserpine (10 M) in the medium when L-DOPA (5 M) was infused in a continuous (70 minute) mode during the superfusion. In contrast, a continuous infusion of either amphetamine (10 M) or high potassium (30 mM) resulted in significantly greater stimulated DA efflux in superfusions performed with reserpine in the medium. In addition, when L-DOPA (5 M) was administered for a brief 10-minute infusion period, a significantly greater stimulated DA efflux was obtained with superfusions containing reserpine in the medium. These results suggest that the mode of L-DOPA infusion may be an important factor in regulating DA release under conditions of diminished DA storage capacity.     

PNU-96391A (OSU6162) antagonizes the development of behavioral sensitization induced by dopamine agonists in a rat model for Parkinson's disease

PNU-96391A is a weak dopamine (DA) D(2) receptor antagonist with behavioral stabilizing properties. Previous experiments revealed that PNU-96391A antagonizes the expression of L-DOPA induced behavioral sensitization (dyskinesias) in lesioned primates without inducing akinesia or reducing the anti-Parkinsonian efficacy of L-DOPA. This study evaluated the ability of PNU-96391A to block the development of DA agonist-induced behavioral sensitization in rats with unilateral 6-OH-DA lesions of the median forebrain bundle. Repeated twice daily treatment with L-DOPA and the decarboxylase inhibitor benserazide (15 and 5 mg/kg, IP, respectively), or quinpirole (D(2)/D(3) agonist, 0.1 mg/kg, SC) increased the contralateral rotations measured on day 7 and 14 as compared to day 1. PNU-96391A (10-60 mg/kg, SC, bid.) antagonized the development of behavioral sensitization induced by both agonists. The basal activity of L-DOPA was not affected while a reduction of quinpirole-induced rotations was observed after 30-60 mg/kg, SC of PNU-96391A. Neurochemical analyses confirmed >99 % reductions of striatal DA levels, unilaterally. Concomitant treatment with PNU-96391A and L-DOPA did not affect plasma levels of PNU-96391A indicating that the effects observed are not related to pharmacokinetic interactions. These results suggest that PNU-96391A could be therapeutically useful to prevent the development of behavioral sensitization induced by DA agonists.     

Pergolide protects dopaminergic neurons in primary culture under stress conditions

Summary. Dopamine agonists are an important therapeutic strategy in the treatment of Parkinson's disease. They postpone the necessity for and reduce the required dose of L-3,4-dihydroxyphenylalanine (L-DOPA) medication thus protecting against the development of motor complications and potential oxidative stress due to L-DOPA metabolism. In primary cultures from mouse mesencephalon we show that pergolide, a preferential D₂ agonist enhanced the survival of healthy dopaminergic neurons at low concentrations of 0.001 μM. About 100 fold higher concentrations (0.1 μM) were necessary to partially reverse the toxic effects of 10 μM 1-methyl-4-phenylpyridinium (MPP⁺). Pergolide was equally effective in preventing the reduction of dopamine uptake induced by 200 μM L-DOPA. Furthermore, between 0.001–0.1 μM it also reduced lactate production thus promoting aerobic metabolism. The present findings suggest that pergolide protects dopaminergic neurons under conditions of elevated oxidative stress. Received February 4, 2002; accepted February 21, 2002     

The novel type B MAO inhibitor PF9601N enhances the duration of L-DOPA-induced contralateral turning in 6-hydroxydopamine lesioned rats

Summary. The present study examined the effect of the highly potent and selective MAO B inhibitor PF9601N on L-DOPA-induced rotational behavior in unilateral nigrostriatal 6-hydroxydopamine lesioned rats. Three doses of PF9601N (20, 40 and 60 mg/kg) were administered 30 min before an injection of L-DOPA (25 mg/kg), and both contralateral and ipsilateral rotational behavior was measured. In addition, we also studied the effect produced by another MAO B inhibitor, deprenyl (20 mg/kg), the MAO A inhibitor, clorgyline (20 mg/kg), and the dopamine reuptake inhibitor, GBR2909 (7.5 mg/kg) on L-DOPA-induced rotational behavior. The results showed that PF9601N plus L-DOPA significantly enhanced the duration of contralateral rotational behavior with respect to L-DOPA plus vehicle in a dose-related manner. At the dose of 40 and 60 mg/kg, PF9601N produced significantly more overall contralateral turning than L-DOPA plus vehicle, and at the dose of 60 mg/kg, PF9601N produced significantly more turning behavior than L-DOPA plus deprenyl. These results suggest that PF9601N may be used as a novel tool in the treatment of Parkinson's disease. Received March 18, 1999; accepted September 23, 1999