The neuroregulatory properties of L-DOPA. A review of the evidence and potential role in the treatment of Parkinson's disease

Inhibitory effects of curcumin and cyclocurcumin in 1-methyl-4-phenylpyridinium (MPP+) induced neurotoxicity in differentiated PC12 cells

Development and progression of neurodegenerative diseases like Parkinson’s disease (PD) involve multiple pathways. Thus, effective therapeutic treatments should intervene to address all these pathways simultaneously for greater success. Most of the current pharmacotherapeutic approaches just supplement striatal dopamine. Hence, natural extracts of plants with therapeutic potential have been explored. Curcuminoids belong to one such group of polyphenol which show immense therapeutic effects. Here, we have used intracellular reactive oxygen species (ROS) measurement, and two-photon fluorescence lifetime imaging microscopy (2P-FLIM) of cellular autofluorescent co-enzyme reduced nicotinamide adenine dinucleotide (NADH) to study the inhibitory effects of curcumin and cyclocurcumin in alleviating PD like neurotoxicity of 1-methyl-4-phenylpyridinium (MPP⁺) in neuronal growth factor (NGF) induced differentiated PC12 cells. Our results showed that both cyclocurcumin and curcumin reduced the level of ROS caused by MPP⁺ treatment. Moreover, a significant increase in the free, protein-bound, and average NADH fluorescence lifetimes along with a decrease in the relative contribution of free- vs. protein-bound NADH components in curcuminoids treated cells (pretreated with MPP⁺) were observed compared with those treated with MPP⁺ only. This study, which indicates that cyclocurcumin offers higher neuronal protection than curcumin, may initiate further studies of these compounds in the cure of neurodegenerative diseases.

Distinct effects of intranigral L-DOPA infusion in the MPTP rat model of Parkinson's disease

The potential neuroprotective or neurotoxic effects of 3,4-dihydroxyphenylalanine (L-DOPA) are yet to be understood. We examined the behavioral, immunohistochemical, tyrosine hydroxylase (TH) expression and neurochemical parameters after an intranigral administration of L-DOPA (10 microM) in rats. L-DOPA elicited a 30.5% reduction in dopaminergic neurons, while 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) (100 microg microL(-1)) produced a 53.6% reduction. A combined infusion of MPTP and L-DOPA generated a 42% reduction of nigral neurons. Motor parameters revealed that both the MPTP and L-DOPA groups presented impairments; however, the concomitant administration evoked a partial restorative effect. In addition, MPTP and L-DOPA separately induced reductions of TH protein expression within the substantia nigra. In contrast, the coadministration of MPTP and L-DOPA did not demonstrate such difference. The striatal levels of dopamine were reduced after MPTP or L-DOPA, with an increased turnover only for the MPTP group. In view of such results, it seems reasonable to suggest that L-DOPA could potentially produce dopaminergic neurotoxicity.

Pentobarbital inhibits l-DOPA-induced dopamine increases in the rat striatum: An in vivo microdialysis study

Pentobarbital is reported to inhibit ketamine-induced dopamine (DA) release in the rat nucleus accumbens. The accumbens is a part of the limbic dopaminergic system in the brain, and the dopaminergic neural activity of other components may also be sensitive to pentobarbital. We investigated the effect of pentobarbital administration on DA release in the striatum known as DA-rich basal ganglia, and the interaction between pentobarbital and L-DOPA, using in vivo microdialysis techniques. Male SD rats were implanted microdialysis probe into the right striatum. The probe was perfused with modified Ringer's solution and dialysate was directly injected to an HPLC. Every group of rats was consisted of six to seven animals. In the first experiment, rats were given saline, 25 and 50 mg kg(-1) pentobarbital. The second, each rat was given a local administration of 2 and 5 microg ml(-1) of L-DOPA with perfusate. Finally, other sets of rats were given 5 microg ml(-1) of L-DOPA and 25, 50, or 100 mg kg(-1) pentobarbital. Pentobarbital anaesthesia decreased the extracellular concentration of DA, and local administration of L-DOPA significantly increased DA concentration. Pretreatment with pentobarbital diminished the L-DOPA-induced DA increase. The results of the present investigation demonstrate that administration of pentobarbital might inhibit dopaminergic neural activity not only in the nucleus accumbens but also in the rat striatum. Pentobarbital anaesthesia antagonizes DA increase induced by L-DOPA and suggests the inhibition of metabolism of L-DOPA. The results of some animal experiments on dopaminergic activity under pentobarbital anaesthesia should be reconsidered.

Differential effect of dopamine on mitosis in early postnatal albino and pigmented rat retinae

Insufficient levels of L-DOPA, released from the retinal pigment epithelium (RPE), in albino animals are considered responsible for the abnormal development of the underlying neural retina. L-DOPA normalizes retinal neurogenesis by reducing levels of cell proliferation either by acting on the cells directly or by being converted into dopamine. Here we report the effects of dopamine on mitosis in early postnatal neural retinae from albino and pigmented rats, using 4D (x, y, z and time) confocal microscopy. Exogenous dopamine significantly prolongs mitosis in retinae from albino, but not pigmented, animals. As fewer cells move into and divide in the ventricular zone (VZ) in the presence of dopamine, we conclude that the overall cell cycle is affected. The D1 receptor blocker, SCH 23390, inhibits these effects. Thus, the differential effects of dopamine on neural retinae from pigmented and albino rats in vitro must result from the activation of D1 receptors, which are present in the retina from birth. Immunohistochemical labeling of D1 receptors shows that the pattern of their distribution is similar between pigmentation phenotypes, but levels of expression may be elevated in albinos. Labeling is most intense in the inner plexiform layer but is present throughout the neuroblastic layer. These findings are discussed in light of previous reports of reduced catecholamine levels in the albino retina.

Possibility of autocrine beta-adrenergic signaling in C2C12 myotubes

Levodopa reportedly inhibits insulin action in skeletal muscle. Here we show that C2C12 myotubes produce levodopa and that insulin-stimulated glucose transport is enhanced when endogenous levodopa is depleted. Exogenous levodopa prevented the stimulation of glucose transport by insulin (P < 0.05) and increased cAMP concentrations (P < 0.05). The decrease in insulin-stimulated glucose transport caused by levodopa was attenuated by propranolol (a beta-adrenergic antagonist) and prevented by NSD-1015 (NSD), an inhibitor of DOPA decarboxylase (DDC; converts levodopa to dopamine). Propranolol and NSD both prevented levodopa-related increases in [cAMP]. However, the effects of levodopa were unlikely to be dependent on the conversion of levodopa to catecholamines because we could detect neither DDC in myotubes nor catecholamines in media after incubation of myotubes with levodopa. The data suggest the possibility of novel autocrine beta-adrenergic action in C2C12 myotubes in which levodopa, produced by myotubes, could have hormone-like effects that impinge on glucose metabolism.

A meta-analysis of the response to chronic L-dopa in patients with schizophrenia: therapeutic and heuristic implications

RATIONALE

While it is generally believed that administration of the dopamine precursor levodopa ( L-dopa) exacerbates symptoms of schizophrenia, numerous reports suggest that adjunctive L-dopa may be beneficial. This body of literature has not been critically reviewed.

OBJECTIVES

On the basis of published studies, to determine whether L-dopa administered concomitantly with antipsychotic drugs provides a beneficial response in patients with schizophrenia.

METHODS

This review examined 30 studies involving 716 patients. Due to wide methodological variability and limited statistical information, only five studies encompassing 160 patients could be included in a meta-analysis. The others were evaluated qualitatively.

RESULTS

When L-dopa was added to antipsychotic drugs, the overall improvement was moderate ( d=0.71) and highly significant ( P<0.0001). There were 16 other studies in which L-dopa was added to antipsychotic drugs, but which did not meet criteria for inclusion in the meta-analysis. In these, worsening occurred in less than 20% of patients; the percentage of improved patients varied widely but had a central tendency around 50%.

CONCLUSIONS

. In patients already on antipsychotic drugs, the addition of L-dopa can be beneficial. Dopamine agonists merit further consideration as adjuncts to antipsychotic drugs in the treatment of schizophrenia.