Thermodynamics and Energetics of the Tonoplast Membrane Operating as a Hysteresis Switch in an Oscillatory Model of Crassulacean Acid Metabolism

The observed endogenous circadian rhythm in plants performing Crassulacean acid metabolism is effected by malate transport at the tonoplast membrane. Experimental and theoretical work asks for a hysteresis switch, regulating this transport via the ordering state of the membrane. We apply a schematic molecular model to calculate the thermally averaged order parameter of the membrane lipid structure in its dependence on external parameters temperature and area per molecule. The model shows a first order structural phase transition in a biologically relevant temperature range. Osmotic consequences of malate accumulation can trigger a transition between the two phases by changing the surface area of the cell vacuole. Estimation of the energy needed to expand the vacuole under turgor pressure because of osmotic changes while acidifying shows that energy needed as latent heat for the calculated change between phases can easily be afforded by the cell. Thus, malate content and the coexisting two phases of lipid order, showing hysteretic behavior, can serve as a feedback system in an oscillatory model of Crassulacean acid metabolism, establishing the circadian clock needed for endogenous rhythmicity. Received: 2 September 1997/Revised: 24 April 1998     

dCAPS, a simple technique for the genetic analysis of single nucleotide polymorphisms: experimental applications in Arabidopsis thaliana genetics

PCR-based detection of single nucleotide polymorphisms is a powerful tool for the plant geneticist. Cleaved amplified polymorphic sequence analysis is the most widely used approach for the detection of single nucleotide polymorphisms. However, this technique is limited to mutations which create or disrupt a restriction enzyme recognition site. This paper presents a modification of this technique where mismatches in a PCR primer are used to create a polymorphism based on the target mutation. This technique is useful for following known mutations in segregating populations and genetic mapping of isolated DNAs used for positional based cloning of new genes. In addition, a computer program has been developed that facilitates the design of these PCR primers.     

Cyclic AMP-Mediated Enhancement of High-Affinity Choline Transport and Acetylcholine Synthesis in Brain

Abstract: Intracerebroventricular administration of N⁶, 2′-O-dibutyryladenosine 3′,5′-cyclic monophosphate (db-cyclic AMP) to mice increased high-affinity choline transport (HAChT) into synaptosomal preparations from the hippocampus, striatum, and frontal cortex in a time-dose-, and brain region-dependent manner. Similar observations were made when the cyclic AMP analogue 8-bromo-cyclic AMP, the adenylyl cyclase activator forskolin, and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine were administered. Inhibition of phosphatase 1 and 2A, with okadaic acid, increased basal choline transport and enhanced the response to db-cyclic AMP. The early increase of HAChT activity induced by db-cyclic AMP was blocked by H-7 and H-89, protein kinase A inhibitors, but not by cycloheximide, a protein synthesis inhibitor. Kinetic analysis of the early changes of HAChT revealed an increase in the apparent V_max without a change of the K_m for choline. Hemicholinium-3 (HC-3) binding was not altered when studied 1 h after db-cyclic AMP administration. In contrast, HC-3 binding and HAChT activity were both elevated when estimated 3 h after the treatment, and pretreatment with cycloheximide partially prevented the db-cyclic AMP-induced HAChT rise. As evidence that enhanced HAChT is associated with a direct action of cyclic AMP-dependent pathways on the cholinergic nerve terminals, addition of 8-bromocyclic AMP to isolated hippocampal synaptosomes induced an increase of HAChT that was prevented by H-89. Choline acetyltransferase activity was not affected at any time during the studies. The synthesis of acetylcholine, however, was enhanced 1 h after db-cyclic AMP addition. Our studies show that cyclic AMP-mimetic compounds appear to modulate the choline carrier by a dual mode: an early increase of the maximal velocity without a change of the number of HC-3 binding sites and a late rise of transport that is accompanied by an increase of HC-3 binding. We postulate that HAChT and consequently acetylcholine synthesis in vivo is modulated, in part, by protein kinase A.     

Epinephrine: A Potential Neurotransmitter in Retina

Abstract: Dopamine (DA), norepinephrine (NE), and epinephrine (EPI) are present in rat retina. DA is the major catecholamine, whereas NE and EPI represent ∼5% of the DA content. DA is contained in a subpopulation of amacrine cells and has been the subject of numerous studies. We investigated the origin and properties of NE and EPI in retina. Following superior cervical ganglionectomy, there was a decrease in NE content, but no decrease in EPI or phenylethanolamine- N -methyltransferase (PNMT) activity. PNMT in retina has many of the substrate-specificity and inhibitor-sensitivity characteristics of other tissues. Enzyme activity is enhanced in newborn rats by treatment with dexamethasone. Exposure to a lighted environment increases retinal EPI in normal and superior cervical ganglionectomized rats. EPI content increased for more than 2 h in a lighted environment. We conclude that most of the NE is contained within the sympathetic neurons that innervate the eye from the superior cervical ganglion, whereas EPI is contained in retinal elements that are responsive to photic stimulation.     

Temporal Variation in Decisions about Parental Care in Bluegill, Lepomis macrochirus

Parental investment theory states that an individual will trade‐off present and future reproductive potential to maximize lifetime reproductive success. Only when parental care is costly in terms of reduced future reproductive potential should individuals be sensitive to changes in the value of current offspring and adjust their care. Here, we examine temporal variation in parental care decision‐making in bluegill (Lepomis macrochirus), in which care is provided by males called ‘parentals’. Previous research has shown that parentals that nest early in the breeding season are in higher energetic condition than those that nest later, and early nesting males appear not to pay an opportunity cost to their care in terms of reduced future reproductive potential. Early nesting males also may have higher paternity in their broods than later nesting males. To examine the parental care decisions made by early and mid‐season nesting parentals, we experimentally reduced males’ perceived paternity by swapping eggs between nests. We found that experimental males that nested early in the breeding season adjusted their brood defence behaviour similarly to control males, which had sham egg swaps performed. Conversely, experimental males that nested mid‐season significantly decreased their brood defence behaviour after the manipulation as compared with control males. Thus, unlike mid‐season nesting males, early nesting males appear relatively insensitive to changes in brood value (paternity), possibly because early nesting males pay little cost in terms of reduced future reproductive potential to providing full care or because these males have a predisposition to high paternity.     

The effects of chronic nitrogen fertilization on alpine tundra soil microbial communities: implications for carbon and nitrogen cycling

Many studies have shown that changes in nitrogen (N) availability affect primary productivity in a variety of terrestrial systems, but less is known about the effects of the changing N cycle on soil organic matter (SOM) decomposition. We used a variety of techniques to examine the effects of chronic N amendments on SOM chemistry and microbial community structure and function in an alpine tundra soil. We collected surface soil (0-5 cm) samples from five control and five long-term N-amended plots established and maintained at the Niwot Ridge Long-term Ecological Research (LTER) site. Samples were bulked by treatment and all analyses were conducted on composite samples. The fungal community shifted in response to N amendments, with a decrease in the relative abundance of basidiomycetes. Bacterial community composition also shifted in the fertilized soil, with increases in the relative abundance of sequences related to the Bacteroidetes and Gemmatimonadetes, and decreases in the relative abundance of the Verrucomicrobia. We did not uncover any bacterial sequences that were closely related to known nitrifiers in either soil, but sequences related to archaeal nitrifiers were found in control soils. The ratio of fungi to bacteria did not change in the N-amended soils, but the ratio of archaea to bacteria dropped from 20% to less than 1% in the N-amended plots. Comparisons of aliphatic and aromatic carbon compounds, two broad categories of soil carbon compounds, revealed no between treatment differences. However, G-lignins were found in higher relative abundance in the fertilized soils, while proteins were detected in lower relative abundance. Finally, the activities of two soil enzymes involved in N cycling changed in response to chronic N amendments. These results suggest that chronic N fertilization induces significant shifts in soil carbon dynamics that correspond to shifts in microbial community structure and function.     

The dopamine receptor adenylate cyclase complex: Evidence for post recognition site involvement for the development of supersensitivity

The dopamine receptor adenylate cyclase complex of a rat striatal membrane preparation became more responsive to dopamine following the injection of 6-hydroxydopamine (6-OHDA) into the median forebrain bundle or following the subcutaneous implantation of morphine pellets. Moreover, the membrane cyclase system was more responsive to activation by GTP, guanyl-5-yl-imidodiphosphate and Mn-ATP. These observations suggest that both 6-OHDA and morphine induce similar biochemical changes in striatum and that the increased responsiveness arises, in part, from modification of the nucleotide regulatory and/or catalytic components of adenylate cyclase.     

Songbird genetic diversity is lower in anthropogenically versus naturally fragmented landscapes

Natural habitats, and the populations they sustain, are becoming increasingly fragmented by human activities. Parallels between ‘true’ islands and ‘habitat’ islands suggest that standing levels of individual genetic diversity in naturally fragmented populations may predict the genetic fate of their anthropogenically fragmented counterparts, but this hypothesis remains largely untested. We compared neutral-locus genetic diversity of individual song sparrows (Melospiza melodia) breeding in a naturally fragmented landscape (small coastal islands) to that of song sparrows in similar-sized ‘urban islands’ separated by roads and housing developments rather than by water. Individuals on coastal islands were more heterozygous and less inbred than those in urban islands. Estimates of population genetic structuring (assessed by pairwise genetic differentiation and Bayesian clustering methods) and contemporary dispersal (based on assignment tests) revealed little structure within either landscape, suggesting that lack of connectivity at the geographic scale we investigated cannot explain the reduced heterozygosity of urban birds. However, within-site genetic similarity was higher in the urban than the coastal landscape. Assuming that historic genetic diversity was similar in these two environments, our findings suggest that anthropogenically fragmented populations may lose genetic diversity faster than their naturally fragmented counterparts.