Litter decomposition in a Cerrado savannah stream is retarded by leaf toughness, low dissolved nutrients and a low density of shredders

1. To assess whether the reported slow breakdown of litter in tropical Cerrado streams is due to local environmental conditions or to the intrinsic leaf characteristics of local plant species, we compared the breakdown of leaves from Protium brasiliense, a riparian species of Cerrado (Brazilian savannah), in a local and a temperate stream. The experiment was carried out at the time of the highest litter fall in the two locations. An additional summer experiment was conducted in the temperate stream to provide for similar temperature conditions. 2. The breakdown rates (k) of P. brasiliense leaves in the tropical Cerrado stream ranged from 0.0001 to 0.0008 day⁻¹ and are among the slowest reported. They were significantly (F = 20.12, P < 0.05) lower than in the temperate stream (0.0046–0.0055). The maximum ergosterol content in decomposing leaves in the tropical Cerrado stream was 106 μg g⁻¹, (1.9% of leaf mass) measured by day 75, which was lower than in the temperate stream where maximum ergosterol content of 522 μg g⁻¹ (9.5% of leaf mass) was achieved by day 30. The ATP content, as an indicator of total microbial biomass, was up to four times higher in the tropical Cerrado than in the temperate stream (194.0 versus 49.4 nmoles g⁻¹). 3. Unlike in the temperate stream, leaves in the tropical Cerrado were not colonised by shredder invertebrates. However, in none of the experiments did leaves exposed (coarse mesh bags) and unexposed (fine mesh bags) to invertebrates differ in breakdown rates (F = 1.15, P > 0.05), indicating that invertebrates were unable to feed on decomposing P. brasiliense leaves. 4. We conclude that the slow breakdown of P. brasiliense leaves in the tropical Cerrado stream was because of the low nutrient content in the water, particularly nitrate (0.05 mgN L⁻¹), which slows down fungal activity and to the low density of invertebrates capable of using these hard leaves as an energy source.     

Cells and viruses with mutations affecting viral entry are selected during persistent infections of L cells with mammalian reoviruses.

Previous studies demonstrated that both cellular and viral mutants are selected during maintenance of persistent infections established in murine L cells with high-passage stocks of mammalian reoviruses. In particular, when one culture was cured of persistent infection, the resulting cells were found to support the growth of viruses isolated from persistently infected cultures (termed PI viruses here) better than that of wild-type (wt) viruses (R. Ahmed, W. M. Canning, R. S. Kauffman, A. H. Sharpe, J. V. Hallum, and B. N. Fields, Cell 25:325-332, 1981). To address the nature of cellular and viral mutations selected during maintenance of persistent reovirus infections, we established independent, persistently infected cultures with L cells and high-passage stocks of wt reovirus. These cultures served as sources of new PI viruses and cured cells for study. We found that although wt viruses grew poorly in cured cells when infection was initiated with intact virions, they grew well in cured cells when infection was initiated with infectious subvirion particles generated from virions by in vitro treatment with chymotrypsin. This finding indicates that the block to growth of wt viruses in cured cells involves an early step that is unique to infection by virions, such as proteolytic processing in an endocytic compartment. We also found that PI viruses grew better than wt viruses in L cells treated with ammonium chloride, a weak base that inhibits the pH decrease in endosomes and lysosomes. Because ammonium chloride blocks an early step in infection by intact virions, probably the proteolytic processing of viral outer capsid proteins by acid-dependent cellular proteases in late endosomes or lysosomes, this finding indicates that PI viruses differ from wt viruses with respect to viral entry into cells. Therefore, these results indicate that both cells and viruses evolve mutations that affect one or more early steps in the viral growth cycle during maintenance of L-cell cultures persistently infected with reoviruses.     

Muscle and Muscle Fiber Type Transformation In Clawed Crustaceans

SYNOPSIS. The first pair of thoracic limbs in many crustaceansis elaborated into claws in which the principal muscle is thecloser. Changes in the fiber composition of the closer muscleduring claw development, regeneration and reversal are reviewedhere and the hypothesis is advanced that such changes are nerve-dependent.In adult lobsters, Homarus amencanus, the paired claws and closermuscles are bilaterally asymmetric, consisting of a minor orcutter claw with predominantly fast fibers and a small ventralband of slow and a major or crusher claw with 100% slow fibers.Yet in the larval and early juvenile stages the paired clawsand closer muscles are symmetric consisting of a central bandof fast fibers sandwiched by slow. Differentiation into a cutteror crusher muscle during subsequent juvenile development isby appropriate fiber type transformation. Experimental manipulationof the claws or the environment in early juvenile stages whenthe claws are equipotent revealed that the determination ofclaw and closer muscle asymmetry is dependent on the convergenceof neural input from the paired claws: the point of convergencemost likely being the CNS. Bilaterally symmetrical input resultsin the development of paired cutter claws while bilaterallyasymmetric input gives rise to dimorphic, cutter and crusherclaws. In the northern crayfish, Orconectes rusticus, wherethe paired claws are bilaterally similar, the closer muscletransforms its central band of fast fibers to slow, both duringprimary development and regeneration. Whether these fiber typetransformations are nerve-dependent is unknown. In adult snappingshrimps, Alpheus sp., the paired claws and closer muscles areasymmetric: the minor or pincer claw has a central band of fastfibers flanked by slow while the major or snapper claw has 100%slow fibers. Claw reversal occurs with removal of the snapperresulting in the transformation of the existing pincer to asnapper and the regeneration of a new pincer at the old snappersite. Transformation of the closer muscle from pincer to snappertype is by degeneration of the fast fiber band and hypertrophyof the slow fibers. Claw transformation can be either preventedif the pincer nerve is sectioned at the time of snapper removalor promoted if the snapper nerve is sectioned: both resultsimplicating a neural basis for muscle transformation.     

CD4+ CD25+ regulatory T cells inhibit the maturation but not the initiation of an autoantibody response

To investigate the mechanism by which T regulatory (Treg) cells may control the early onset of autoimmunity, we have used an adoptive transfer model to track Treg, Th, and anti-chromatin B cell interactions in vivo. We show that anti-chromatin B cells secrete Abs by day 8 in vivo upon provision of undeviated, Th1- or Th2-type CD4+ T cell help, but this secretion is blocked by the coinjection of CD4+ CD25+ Treg cells. Although Treg cells do not interfere with the initial follicular entry or activation of Th or B cells at day 3, ICOS levels on Th cells are decreased. Furthermore, Treg cells must be administered during the initial phases of the Ab response to exert full suppression of autoantibody production. These studies indicate that CD25+ Treg cells act to inhibit the maturation, rather than the initiation, of autoantibody responses.     

Neurotransmitter receptor alterations in Parkinson's disease.

Neurotransmitter receptor binding for GABA, serotonin, cholinergic muscarinic and dopamine receptors and choline acetyltransferase (ChAc) activity were measured in the frontal cortex, caudate nucleus, putamen and globus pallidus from postmortem brains of 10 Parkinsonian patients and 10 controls. No changes in any of these systems were observed in the frontal cortex. In the caudaye nucleus, only the apparent dopamine receptor binding was altered with a significant 30% decrease in the Parkinsonian brain. Both cholinergic muscarinic and serotonin receptor binding were significantly altered in the putamen, the former increasing and the latter decreasing with respect to controls. In addition, ChAc activity was decreased in the putamen. In the globus pallidus, only ChAc activity was significantly changed, decreasing about 60%, with no change in neurotransmitter receptor binding. The results suggest that a progressive loss of dopaminergic receptors in the caudate nucleus may contribute to the decreased response of Parkinsonian patients to L-dopa and dopamine agonist therapy.     

Chronic ethanol feeding produces an increase in muscarinic cholinergic receptors in mouse brain

Chronic ethanol administration (seven days) to mice produces physical dependence while subsequent withdrawal produces seizures and convulsions, especially during the first 24 hours. It is known that ethanol can inhibit brain acetylcholine (ACh) release. Our present study suggests that hippocampal (+25%) and cortical (+14%), but not striatal neurons, can respond to the ethanol-induced reduction in neurotransmitter (ACh) availability by increasing the number of muscarinic cholinergic receptors. It is possible that these supersensitive neurons are involved in the CNS hyperexcitability induced by withdrawal from chronic ethanol.