Nils Gunnar Lindquist

Autoradiography of [14C]paraquat or [14C]diquat in frogs and mice: Accumulation in neuromelanin

The herbicide paraquat has been suggested as a causative agent for Parkinsons disease because of its structural similarity to a metabolite of 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP), which may induce a parkinsonism-like condition. MPTP as well as its metabolite 1-methyl-4-phenylpyridine have melanin affinity, and the parkinsonism-inducing potency of MPTP is much stronger in species with melanin in the nerve cells. Autoradiography of [3H]MPTP in experimental animals has shown accumulation in melanin-containing tissues, including pigmented neurons. In the present whole body autoradiographic study accumulation and retention was seen in neuromelanin in frogs after i.p. injection of [14C]paraquat or [14C]diquat. By means of whole body autoradiography of [14C]diquat in mice (a species with no or very limited amounts of neuromelanin) a low, relatively uniformly distributed level of radioactivity was observed in brain tissue. Accumulation of toxic chemical compounds, such as paraquat, in neuromelanin may ultimately cause lesions in the pigmented nerve cells, leading to Parkinsons disease.

Selective Brain Uptake and Behavioral Effects of the Cyanobacterial Toxin BMAA (β-N-Methylamino-L-alanine) following Neonatal Administration to Rodents

Cyanobacteria are extensively distributed in terrestrial and aquatic environments all over the world. Most cyanobacteria can produce the neurotoxin beta-N-methylamino-L-alanine (BMAA), which has been detected in several water systems and could accumulate in food chains. The aim of the study was to investigate the transfer of BMAA to fetal and neonatal brains and the effects of BMAA on the development of behavioral characteristics during the brain growth spurt (BGS) in rodents. Pregnant and neonatal mice were given an injection of (3)H-BMAA on gestational day 14 and postnatal day (PND) 10, respectively, and processed for tape-section autoradiography. The study revealed transplacental transfer of (3)H-BMAA and a significant uptake in fetal mouse brain. The radioactivity was specifically located in the hippocampus, striatum, brainstem, spinal cord and cerebellum of 10-day-old mice. The effect of repeated BMAA treatment (200 or 600 mg/kg s.c.) during BGS on rat behavior was also studied. BMAA treatment on PND 9-10 induced acute alterations, such as impaired locomotor ability and hyperactivity, in the behavior of neonatal rats. Furthermore, rats given the high dose of BMAA failed to habituate to the test environment when tested at juvenile age. In conclusion, the results demonstrated that BMAA was transferred to the neonatal brain and induced significant changes in the behavior of neonatal rats following administration during BGS. The observed behavioral changes suggest possible cognitive impairment. Increased information on the long-term effects of BMAA on cognitive function following fetal and neonatal exposure is required for assessment of the risk to childrens health.

Neonatal Exposure to the Cyanobacterial Toxin BMAA Induces Changes in Protein Expression and Neurodegeneration in Adult Hippocampus

The cyanobacterial toxin β-N-methylamino-l-alanine (BMAA) has been proposed to contribute to neurodegenerative disease. We have previously reported a selective uptake of BMAA in the mouse neonatal hippocampus and that exposure during the neonatal period causes learning and memory impairments in adult rats. The aim of this study was to characterize effects in the brain of 6-month-old rats treated neonatally (postnatal days 9–10) with the glutamatergic BMAA. Protein changes were examined using the novel technique Matrix-Assisted Laser Desorption Ionization (MALDI) imaging mass spectrometry (IMS) for direct imaging of proteins in brain cryosections, and histological changes were examined using immunohistochemistry and histopathology. The results showed long-term changes including a decreased expression of proteins involved in energy metabolism and intracellular signaling in the adult hippocampus at a dose (150mg/kg) that gave no histopathological lesions in this brain region. Developmental exposure to a higher dose (460mg/kg) also induced changes in the expression of S100β, histones, calcium- and calmodulin-binding proteins, and guanine nucleotide-binding proteins. At this dose, severe lesions in the adult hippocampus including neuronal degeneration, cell loss, calcium deposits, and astrogliosis were evident. The data demonstrate subtle, sometimes dose-dependent, but permanent effects of a lower neonatal dose of BMAA in the adult hippocampus suggesting that BMAA could potentially disturb many processes during the development. The detection of BMAA in seafood stresses the importance of evaluating the magnitude of human exposure to this neurotoxin.

Retention of the cyanobacterial neurotoxin beta-N-methylamino-l-alanine in melanin and neuromelanin-containing cells--a possible link between Parkinson-dementia complex and pigmentary retinopathy.

β‐N‐methylamino‐l‐alanine (BMAA), a neurotoxic amino acid produced by cyanobacteria, has been suggested to be involved in the etiology of a neurodegenerative disease complex which includes Parkinson‐dementia complex (PDC). In PDC, neuromelanin‐containing neurons in substantia nigra are degenerated. Many PDC patients also have an uncommon pigmentary retinopathy. The aim of this study was to investigate the distribution of 3H‐BMAA in mice and frogs, with emphasis on pigment‐containing tissues. Using autoradiography, a distinct retention of 3H‐BMAA was observed in melanin‐containing tissues such as the eye and neuromelanin‐containing neurons in frog brain. Analysis of the binding of 3H‐BMAA to Sepia melanin in vitro demonstrated two apparent binding sites. In vitro‐studies with synthetic melanin revealed a stronger interaction of 3H‐BMAA with melanin during synthesis than the binding to preformed melanin. Long‐term exposure to BMAA may lead to bioaccumulation in melanin‐ and neuromelanin‐containing cells causing high intracellular levels, and potentially changed melanin characteristics via incorporation of BMAA into the melanin polymer. Interaction of BMAA with melanin may be a possible link between PDC and pigmentary retinopathy.

Long-term retention of neurotoxic β-carbolines in brain neuromelanin

Summary.β-Carbolines show structural resemblance to the neurotoxic N-methyl-4-phenyl-1,2,3,6-tetrahydropyridine and are metabolized to mitochondrial toxicants. Humans are continuously exposed to low levels of β-carbolines through cooked food, coffee, alcoholic beverages and tobacco smoke. β-Carbolines have previously been detected in higher levels in the pigmented substantia nigra than in the cortex of humans. The distribution of 3H-labelled harman and norharman in the brain of pigmented and albino mice and in frogs (a species having neuromelanin) was studied by tape-section and light-microscopic autoradiography. Furthermore, the binding of these β-carbolines to dopamine–melanin and melanin granules from Sepia officinalis was examined. The results revealed a high affinity binding to melanin and a long-term retention (up to 30 days) in pigmented tissues, including neuromelanin-containing neurons of frogs after a single injection. The role of long-term exposure to food-related β-carbolines and a retention of these compounds in pigment-containing neurons in the induction of idiopathic Parkinson’s disease should be further considered.

Distribution of an 125I-labelled chloroquine analogue in a pregnant macaca monkey

Whole body autoradiography of a pregnant monkey (Macaca irus) of late gestation was performed 72 h after an intravenous injection of the 125I-labelled chloroquine analogue 4-(3-dimethylaminopropylamino)-7-iodoquinoline (DAPQ). The overall distribution pattern in the monkey was similar to that which was earlier observed in rodents. A few species differences, however, were found in the monkey as compared to the rodents: a high accumulation in the inner part of the adrenal cortex, a high level in the central nervous system, and generally a higher retention in the tissues. The accumulation in the adrenal cortex may be of significance for the cortisone-like effects of the 4-aminoquinolines in rheumatoid arthritis and allied conditions. The fact that no accumulation was found in the adrenal cortex of mice and rats indicates that these species may not be appropriate in studies on the mechanisms involved in the anti-inflammatory action of the 4-aminoquinolines. As was earlier observed in small rodents the melanin containing structures accumulated the drug. In both the mother and the fetus a high concentration was thus seen in the uveal tract of the eye, in the inner ear (in the stria vascularis of the cochlea and the planum semilunatum of the ampullae) and in the hair follicles. This accumulation can be related to reported disturbances--also transplacentally induced--in vision and hearing.

Melanin affinity and its possible role in neurodegeneration

Certain drugs with melanin affinity are known to have caused pigmentary lesions in the eye and skin. This was the basis for the hypothesis that compounds with melanin affinity may cause damage also in other melanin-bearing tissues such as the substantia nigra. The heterogeneity of compounds that binds to melanin is large. Toxins, drugs, and several other compounds have melanin affinity. Compounds showing the highest affinity are mainly organic amines and metal ions. The binding of toxicants to melanin probably protects the cells initially. However, the binding is normally, slowly reversible and melanin may accumulate the toxicant and gradually release it into the cytosol. Several studies indicate that neuromelanin may play a significant role both in the initiation and in the progression of neurodegeneration. MPTP/MPP+ that has been causally linked with Parkinsonism has high affinity for neuromelanin, and the induced dopaminergic denervation correlates with the neuromelanin content in the cells. This shows that the toxicological implications of the accumulation of toxicants in pigmented neurons and its possible role in neurodegeneration should not be neglected. Extracellular neuromelanin has been reported to activate dendritic cells and microglia. An initial neuronal damage induced by a neurotoxicant that leaks neuromelanin from the cells may therefore lead to a vicious cycle of neuroinflammation and further neurodegeneration. Although there are many clues to the particular vulnerability of dopaminergic neurons of substantia nigra in Parkinson’s disease, the critical factors are not known. Further studies to determine the importance of neuromelanin in neurodegeneration and Parkinson’s disease are warranted.

Mechanism of drug-induced chronic otic lesions. Role of drug accumulation on the melanin of the inner ear

Nachweis einer Melanin-Affinität ototoxischer Medikamente im Zusammenhang mit histopathologischen Änderungen in der Stria pigmentierter Tiere, nicht aber bei Albinos, durch Kanamycin.

Testicular lesions of coprine and benzcoprine.

The effect on the testis of the disulfiram-like compounds benzcoprine (N-[1-ethoxycyclopropyl] benzamide) and coprine (N5-[1-hydroxycyclopropyl]-L-glutamine) was studied in rats and dogs. Severe degeneration of the seminiferous epithelium was induced in rats by subacute oral administration of each compound. 60 days after termination of treatment with benzcoprine most seminiferous tubules contained only occasional spermatogonia and the testicular weight was markedly decreased. The blood-testis barrier was unaffected in the benzcoprine-treated rats as judged by a lanthanum tracer technique. In dogs, oral administration of benzcoprine for 1 month caused impaired spermatogenesis, degeneration of germ cells and a decrease in the testicular weight. The results indicate that both compounds act directly on the germ cells. The effect is similar to that of alkylating compounds. Other effects of benzcoprine and coprine (bone marrow depression, lymphocytopenia, positive Ames test in organisms sensitive to base-pair substitution) are well-known properties of alkylating agents. In conclusion benzcoprine and coprine were found to cause severe changes in the testis in rats and dogs, probably due to a direct effect on the germ cells.

Melanin and neuromelanin binding of drugs and chemicals: toxicological implications

Melanin is a polyanionic pigment that colors, e.g., the hair, skin and eyes. The pigment neuromelanin is closely related to melanin and is mainly produced in specific neurons of the substantia nigra. Certain drugs and chemicals bind to melanin/neuromelanin and are retained in pigment cells for long periods. This specific retention is thought to protect the cells but also to serve as a depot that slowly releases accumulated compounds and may cause toxicity in the eye and skin. Moreover, neuromelanin and compounds with high neuromelanin affinity have been suggested to be implicated in the development of adverse drug reactions in the central nervous system (CNS) as well as in the etiology of Parkinson’s disease (PD). Epidemiologic studies implicate the exposure to pesticides, metals, solvents and other chemicals as risk factors for PD. Neuromelanin interacts with several of these toxicants which may play a significant part in both the initiation and the progression of neurodegeneration. MPTP/MPP+ that has been casually linked with parkinsonism has high affinity for neuromelanin, and the induced dopaminergic denervation correlates with the neuromelanin content in the cells. Recent studies have also reported that neuromelanin may interact with α-synuclein as well as activate microglia and dendritic cells. This review aims to provide an overview of melanin binding of drugs and other compounds, and possible toxicological implications, with particular focus on the CNS and its potential involvement in neurodegenerative disorders.