The NG2 antibody, which recognises an integral membrane chondroitin sulphate, labels a significant population of cells in adult CNS white matter tracts of the rat optic nerve and anterior medullary velum (AMV). Adult NG2+ cells are highly complex with multiple branching processes and we show by EM immunocytochemistry that they extend perinodal processes, which contact nodes of Ranvier. NG2+ cells do not react to conventional immunohistochemical markers for adult glia and so we reservedly term them NG2P cells. In vitro, NG2 labels oligodendrocyte-type-2 astrocyte (O-2A) progenitors that can give rise to oligodendrocytes or type-2 astrocytes, depending on the culture medium. Thus, it is possible that NG2P cells may be derived from the same stem cells as oligodendrocytes. Interestingly, NG2+ cells identified previously in adult CNS displayed phenotypic characteristics of O-2Aadult progenitors and it is possible that, like them, NG2P cells might retain the capacity of generating oligodendrocytes in the adult CNS. This may be an important role of NG2P cells in demyelinating diseases such as multiple sclerosis. It is significant therefore that the perinodal processes of NG2P cells contact the only sites of exposed axolemma in myelinated axons, so that NG2P cells are ideally situated to detect and respond to changes in axonal function during demyelination. A further implication of our finding is that NG2P cells may perform functions at nodes of Ranvier previously attributed to perinodal astrocytes, including the clustering and maintenance of sodium channels in the axon membrane at nodes, during development and following demyelination.

The objective of this study was to determine the extent to which herbivores are able to use conditioned food aversions and preferences to learn about the nutritional and toxic properties of food plants, when food options are simultaneously available. Conditioned food aversions and preferences have been invoked as important mechanisms by which free-ranging herbivores optimize food selection by learning about the negative and positive consequences of consuming particular plant species through a series of encounters. In most previous tests of this hypothesis, access to individual test foods has been separated in time, giving animals the opportunity to associate particular foods with particular post-ingestive effects. We presented animals with a more complex scenario by offering test feeds simultaneously during the learning phase. Such a test is an important step in assessing the importance of conditioned food responses as mechanisms by which herbivores learn to select an optimal diet. We first assessed the ability of goats to learn about test foods and their post-ingestive effects, when different conifer species were offered on separate days during the learning phase and animals were dosed with compounds eliciting positive, negative, or neutral post-ingestive effects. We then investigated the ability of animals to learn to make appropriate choices when all potential test foods were simultaneously available during the learning phase. The results confirmed that goats can learn to associate particular foods with particular post-ingestive effects and adjust their diet selection accordingly. The success with which animals made such associations was greatly reduced when they were presented with test foods simultaneously during the learning phase. When test foods were simultaneously available, animals tended to select a mixed diet, thereby reducing their opportunity to learn about the post-ingestive effects of particular foods. The results suggest that caution is required in extrapolating results of artificial conditioning experiments to free-ranging herbivores. The results also suggest that reducing the risk of toxicity through selection of mixed diets is an important component of a successful foraging strategy.

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Oxalic acid is found in high concentrations in some plants consumed by ruminants and may cause renal toxicity. To determine whether exposure to oxalic acid affects the capacity of the rumen of sheep and goats to degrade the compound, 20 animals (10 sheep and 10 goats) were dosed with free oxalic acid by gelatin capsule twice daily for 3 weeks at one of five levels (0·0, 0·3, 0·6, 0·9 and 1·2 mmol/kg live weight (M) per day). Rumen samples were collected by stomach tube in the week prior to the start of dosing and in each week of the 3-week experiment. Oxalic acid degradation capacity was measured by adding uC-labelled oxalic acid to rumen fluid in vitro and capturing evolved 14CO2. Rates of degradation increased with increasing level of administration (2·30, 4·71, 6·74, 9·83 and 13·90 mmol of oxalic acid degraded per I rumen fluid per day for doses 0·0, 0·3, 0·6, 0·9 and 1·2 mmol/kg M per day, respectively; P < 0·001). Rates of degradation increased during the dosing period (P < 0·001) with the largest increases occurring in the 1st week of dosing. Goats showed a greater response than sheep, with a higher mean oxalic acid degradation capacity (9·04 v. 5·95 mmol of oxalic acid degraded per I rumen fluid, P < 0·05). Oxalic acid administration did not influence plasma calcium concentration or cause renal function impairment as measured by plasma creatinine concentrations. The experiment demonstrated adaptation in the rumen to potential toxins in the host diet and suggests that the rumen micro-organisms of goats may have been more adapted to degrading oxalic acid than sheep.

No abstract