New insights on P2X purinoceptors

Significant advances in understanding of P2X purinoceptor pharmacology have been made in the last few years. The limitations of nucleotide agonists as drug tools have now been amply demonstrated. Fortunately, inhibitors of the degrading ecto-ATPase enzymes are becoming available and it has become apparent that the complete removal of all divalent cations can be used experimentally in some systems to prevent nucleotide breakdown. Despite these issues, convincing evidence for P2X receptor heterogeneity, from data with agonists, has recently been reported. A number of new antagonists at P2X purinoceptors have also recently been described which to some degree appear to be more specific and useful than earlier antagonists like suramin. It is now apparent that suramin is a poor antagonist of ATP in many tissues because it potently inhibits ATPase activity at similar concentrations to those at which it blocks the P2X purinoceptor. Advances in the use of radiolabelled nucleotides as radioligands for binding studies has allowed the demonstration of P2X purinoceptors in a variety of tissues throughout the body including the brain. These studies have also provided evidence for receptor heterogeneity. Excitingly, two P2X purinoceptor genes have been cloned but operational studies suggest that more than two types exist. The cloning studies have also demonstrated a unique structure for the P2X purinoceptor which differentiates it from all other ligand-gated ion channel receptors. Further studies on P2X purinoceptor operation and structure are needed to help resolve controversies alluded to regarding the characterization and classification of nucleotide receptors. Hopefully such studies will also lead to a better understanding of the physiological and pathological importance of ATP and its activation of P2X purinoceptors. This will require the identification of better drug tools, in particular antagonists which may also provide the basis for novel therapeutic agents.     

Specific roles of methylcobamide:coenzyme M methyltransferase isozymes in metabolism of methanol and methylamines in Methanosarcina barkeri

An immunochemical approach was employed as a direct test for functional activities of isozymes of methylcobamide:coenzyme M methyltransferase (MT2-M and MT2-A) in the metabolic pathways of methane formation from: methanol, acetate, monomethylamine, dimethylamine, and trimethylamine. Specific removal of the MT2 isozymes from buffer soluble cell extracts of Methanosarcina barkeri was accomplished by use of immobilized, affinity-purified, ovine polyclonal antibodies. Extracts of methanol-grown cells depleted of MT2-M lost entirely the ability to carry out conversion of methanol to 2-(methylthio)ethanesulfonate (methyl-CoM). Methanol:CoM methyl transfer activity was completely restored by addition of purified MT2-M, but no activity was recovered by addition of MT2-A. In contrast, the activity of trimethylamine-grown cell extracts to convert monomethylamine and dimethylamine to methyl-CoM was lost almost entirely by immunosorptive removal of MT2-A. Addition of purified MT2-A, but not MT2-M, to the MT2-A-depleted extract fully reconstituted methyl-CoM formation from both mono- and dimethylamine. Interestingly, in extracts resolved of MT2-A, trimethylamine-dependent methylation of coenzyme M was observed at approximately 20% of the rate of controls not treated with antibody. Furthermore, both isozymes were effective in full restoration of trimethylamine conversion. Tests indicated that neither of the two MT2 isozymes are involved in methane formation from acetate. The results establish that MT2-A plays a specific role in metabolism of methylated amine substrates, whereas, MT2-M functions in methane formation from trimethylamine and methanol.     

The origin of the Australasian marsupial fauna and the phylogenetic affinities of the enigmatic monito del monte and marsupial mole

Alternative hypotheses in higher-level marsupial systematics have different implications for marsupial origins, character evolution, and biogeography. Resolving the position of the South American monito del monte (Order Microbiotheria) is of particular importance in that alternate hypotheses posit sister-group relationships between microbiotheres and taxa with disparate temporal and geographic distributions: pediomyids; didelphids; dasyuromorphians; diprotodontians; all other australidelphians; and all other marsupials. Among Australasian marsupials, the placement of bandicoots is critical; competing views associate bandicoots with particular Australasian taxa (diprotodontians, dasyuromorphians) or outside of a clade that includes all other Australasian forms and microbiotheres. Affinities of the marsupial mole are also unclear. The mole is placed in its own order (Notoryctemorphia) and sister-group relationships have been postulated between it and each of the other Australasian orders. We investigated relationships among marsupial orders by using a data set that included mitochondrial and nuclear genes. Phylogenetic analyses provide support for the association of microbiotheres with Australasian marsupials and an association of the marsupial mole with dasyuromorphs. Statistical tests reject the association of diprotodontians and bandicoots together as well as the monophyly of Australasian marsupials. The origin of the paraphyletic Australasian marsupial fauna may be accounted for by (i) multiple entries of australidelphians into Australia or (ii) bidirectional dispersal of australidelphians between Antarctica and Australia.     

Spatial concepts and perception of physical and diagrammed scenes

The accessibility of objects in mental spatial frameworks depends on their relation to the spatial axes of the world and people's typical interactions with space. The current study investigated perception of space. Subjects viewed either a physical model of a person surrounded by objects (Exp. 1) or diagrams of scenes (Exp. 2). Subjects named objects at directions from their own external perspective. For physical scenes, subjects were faster to name objects at Above/Below locations, followed by Front/Behind locations, followed by Left/Right locations. This finding indicates that subjects used spatial frameworks to locate objects perceptually. For diagrams, response times to name objects did not conform to this pattern, perhaps because the spatial axes of a diagram do not correspond to stable spatial axes of the world.