Neurotoxic effect of 1-methyl-4-phenylpyridinium ion on dopaminergic neurons of the retina of goldfish

Intravitreal MPTP drives retinal ganglion cell loss with oral nicotinamide treatment providing robust neuroprotection

Neurodegenerative diseases have common underlying pathological mechanisms including progressive neuronal dysfunction, axonal and dendritic retraction, and mitochondrial dysfunction resulting in neuronal death. The retina is often affected in common neurodegenerative diseases such as Parkinson's and Alzheimer's disease. Studies have demonstrated that the retina in patients with Parkinson's disease undergoes changes that parallel the dysfunction in the brain. These changes classically include decreased levels of dopamine, accumulation of alpha-synuclein in the brain and retina, and death of dopaminergic nigral neurons and retinal amacrine cells leading to gross neuronal loss. Exploring this disease's retinal phenotype and vision-related symptoms is an important window for elucidating its pathophysiology and progression, and identifying novel ways to diagnose and treat Parkinson's disease. 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) is commonly used to model Parkinson's disease in animal models. MPTP is a neurotoxin converted to its toxic form by astrocytes, transported to neurons through the dopamine transporter, where it causes mitochondrial Complex I inhibition and neuron degeneration. Systemic administration of MPTP induces retinal changes in different animal models. In this study, we assessed the effects of MPTP on the retina directly via intravitreal injection in mice (5 mg/mL and 50 mg/mL to 7, 14 and 21 days post-injection). MPTP treatment induced the reduction of retinal ganglion cells-a sensitive neuron in the retina-at all time points investigated. This occurred without a concomitant loss of dopaminergic amacrine cells or neuroinflammation at any of the time points or concentrations tested. The observed neurodegeneration which initially affected retinal ganglion cells indicated that this method of MPTP administration could yield a fast and straightforward model of retinal ganglion cell neurodegeneration. To assess whether this model could be amenable to neuroprotection, mice were treated orally with nicotinamide (a nicotinamide adenine dinucleotide precursor) which has been demonstrated to be neuroprotective in several retinal ganglion cell injury models. Nicotinamide was strongly protective following intravitreal MPTP administration, further supporting intravitreal MPTP use as a model of retinal ganglion cell injury. As such, this model could be utilized for testing neuroprotective treatments in the context of Parkinson's disease and retinal ganglion cell injury.

MPTP-induced apoptosis in the retina of goldfish

Neuronal degeneration observed in the goldfish retina after MPTP administration, displays features of apoptosis, a physiological mechanism of cell death that occurs during development. The ultrastructural features of degenerating retinal neurons, that are seen in the inner nuclear layer two days after intravitreal MPTP administration, are consistent with classic changes observed in the programmed cell death. The DNA strand breaks that characterize apoptotic death are in situ detected by terminal deoxynucleotidyl transferase-mediated dUTP-biotin nick-end labeling.

Quinolinic acid but not MK-801 protects the dopaminergic system from 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine-induced toxicity in goldfish retina

The toxicity of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine, intravitreally injected in goldfish eye, involves interplexiform retinal neurons and depletes tyrosine hydroxylase immunoreactivity and dopamine levels. This induced neurotoxicity was prevented by the concomitant administration in non-toxic doses (10 micrograms) of quinolinic acid, an endogenous structural analogue of N-methyl D-aspartate with excitotoxic properties. Quinolinic acid is ineffective on the retinal degeneration induced by 1-methyl-4-phenylpyridinium ion. This fact suggests that quinolinic acid inhibits the MAO-B oxidation of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine. MK-801, a noncompetitive antagonist of glutamate NMDA-receptors, exerts partial protective effects on MPTP-induced delayed toxicity in mammals. In the goldfish eye, MK-801, injected in low concentration, and in conjunction with 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine or 1-methyl-4-phenylpyridinium ion, did not prevent retinal neurodegeneration. Ten micrograms of MK-801 alone did not affect retinal neurons, while a higher concentration (20 micrograms) causes the chromatolysis of some photoreceptor nuclei.

Spontaneous recovery of MPTP-damaged catecholamine systems in goldfish brain areas

In goldfish, 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) administered for 3 consecutive days (10 mg/kg), produced a marked decrease in dopamine (DA) and noradrenaline (NA) levels in telencephalon, diencephalon and medulla oblongata, without affecting the serotonin (5-HT) content. Furthermore the neurotoxin decreased either [3H]DA high affinity uptake or K(+)-stimulated DA release from synaptosomal (P2) preparations, with concomitant up-regulation of D2 postsynaptic receptors as well. No significant changes of choline acetyltransferase and glutamic acid decarboxylase activity or [3H]glutamate uptake were observed. Moreover the pretreatment with deprenyl (1 mg/kg) or mazindol (10 mg/kg) but not with clorgyline (5 mg/kg) prevented catecholamine depletion. Added in vitro to synaptosomal preparations both MPTP and more potently MPP+, in a concentration-dependent manner, inhibited [3H]DA uptake. Time course study revealed that MPTP-induced alteration of neurochemical parameters in goldfish brain areas were almost completely reversed within 6 weeks, suggesting that catecholamine systems in goldfish brain show a remarkable power of recovery after MPTP lesion.

Effect of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) in goldfish brain

The neurotoxin 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which selectively damages dopaminergic neurons in mammals, caused a marked depletion of tyrosine hydroxylase (TH) immunoreactivity in the goldfish brain. The concomitant ultrastructural observations showed the neurotoxic effect of MPTP on telencephalic, diencephalic and medullar neurons. The affected neurons revealed darkening of the cytoplasm and swelling of the mitochondria and the endoplasmic reticulum. Concomitant significant decreases in dopamine (DA) and noradrenaline (NA) levels were determined in the brain areas where morphological observations were performed. The loss of catecholamine levels was completely prevented by the treatment with the monoamine oxidase (MAO) inhibitor pargyline to prevent MPTP oxidation. The results indicate that in goldfish brain, acute MPTP administration causes selective catecholamine depletion, without altering the serotoninergic system.

Retinal dopamine sensitivity to MPP+ toxicity: electrophysiological and biochemical evaluation

This study concerns the effect of intravitreal injection of 1-methyl-4-phenylpyridinium ion (MPP+) on the electroretinograms (ERG) and on the levels of retinal dopamine (DA) in rabbits. The right eye was injected intravitreously with MPP+ while the other received only the vehicle and served as control. The administration of 7, 40, 70 or 700 micrograms MPP+ resulted in a dose-related decrease of the amplitude of the a and b-waves as well as the oscillatory potentials (OPs) of the ERG, down to extinction. In contrast, the retinal DA content was decreased only with the 700 micrograms MPP+ dose. Fluorescein angiography demonstrated abnormalities in the retinal circulation of all MPP+-treated eyes. These observations indicate that MPP+ causes lesions to the retinal vessels at doses non-toxic to the retinal dopaminergic neurons. These data suggest that intravitreal injection of MPP+ cannot be used to study the physiological role of retinal DA.

Tyrosine hydroxylase immunohistochemistry in the normal and 1-methyl-4-phenyl-tetrahydropyridine (MPP+)-treated retina of goldfish

Dopaminergic interplexiform neurons have been identified in the inner nuclear layer of goldfish retina, with tyrosine hydroxylase (TH) immunocytochemistry in whole-mounted retinae and in cryosections. The neurotoxin 1-methyl-4-phenylpyridinium ion (MPP+), which selectively damages dopaminergic neurons in mammals, caused a marked depletion of TH immunoreactivity in goldfish retina. Three days after intravitreal injection, retinae showed no significant decrease in the number of TH-positive neurons. However most of the TH-immunoreactive cell bodies showed an evident depletion of TH immunoreactivity and their processes, ramified in the inner and outer plexiform layers, disappeared almost completely.

Different sensitivities to MPTP toxicity in primate nigrostriatal and retinal dopaminergic systems: electrophysiological and biochemical evidence

The influences of N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) on dopamine content in the retina and the striatum of monkeys, as well as retinal responses to flash stimulation were studied. Dopamine depletion was observed in the striatum, while retinal dopamine content was similar to controls. The electroretinograms and the oscillatory potentials remained normal, even in monkeys presenting a severe Parkinsonian syndrome, in concordance with the biochemical data. It is proposed that the retinal dopaminergic system is less sensitive to the toxic effects of MPTP than the nigro-striatal pathway.