Neuromelanin and its Possible Protective and Destructive Properties

The function of neuromelanin is not known, but some properties of the pigment suggest a protective action. Its unique ability to accumulate and retain several compounds, such as various amines and a number of metals, may protect the pigment-containing neurons from high exposure to harmful substances. This possible mechanism of protection may however in certain instances be of a double-edged nature, as accumulation of neurotoxic agents with a high melanin affinity may cause toxic concentrations in the neuro-melanin-containing cells. MPTP (l-methyl-4-phenyl-l,2,3,6-tetrahydropyridine) seems to be such a compound, as it has been found to preferentially destroy neuromelanin-containing cells. The degree of MPTP neurotoxicity seems to be related to the amount of neuromelanin present in the particular species. It is possible that also manganese, which is known to cause an extrapyramidal disorder resembling Parkinson's disease, causes injury to neuromelanin-bearing neurons due to its melanin affinity. This mechanism may be involved in other forms of chemically induced Parkin-sonism and possibly also in idiopathic Parkinson's disease, although the offending agent remains to be discovered.     

The Cambro - Ordovician boundary

Henningsmoen, G.: The Cambro-Ordovician boundary.
The Cambrian and Ordovician Systems were established in Britain, and it seems appropriate to define the Cambro-Ordovician boundary in Britain or where there is a closely related faunal succession at the junction. When Lapworth erected the Ordovician System in 1879, his intention was that its lower boundary should separate the so-called primordial (or first) and second Palaeozoic faunas. He drew this boundary at the base of the Lower Arenig, but apparently included the Upper Tremadoc in the Lower Arenig. It seems problematic to base the Cambro-Ordovician boundary on grounds of historical priority, and we seem to be free to decide whether to have the boundary at the base of the Tremadocian, at the base of the Arenigian, or within the Tremadocian (e.g. at the base of the Upper Tremadoc). The alternatives are shortly discussed. Arguments are put forward for placing the boundary not at a break, but preferably in a sequence with cosmopolitan fossils, either in a sequence of uniform facies or perhaps rather in a sequence with frequently alternating facies.