Browsing by Author "Keymer, JE"

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    Acetylcholinesterase and inhibitors: effects upon normal and regenerating nerves of the rat
    (1999) Keymer, JE; Gaete, J; Kameid, G; Alvarez, J
    In peripheral nerves, the function of acetylcholinesterase (AChE) is not related to hydrolysis of acetylcholine. To test for a trophic role, AChE or its inhibitors were administered locally to normal and regenerating nerves of rats. In the normal nerve, neither AChE nor serum albumin affected the cytological pattern of the nerve. BW284c51, a specific inhibitor of AChE, resulted in demyelination, proliferation of Schwann cells and sprouting of axons after 5-7 days. Edrophonium or propidium, other specific inhibitors of AChE, did so to a much lesser extent. Vehicle, and iso-OMPA (inhibitor of pseudocholinesterases) did not affect the cytology of the nerve. Elongation of regenerating axons was evaluated at day 3 post-crush. Native AChE applied distal to the crush reduced the elongation of regenerating axone (-36%), while serum albumin, heated AChE and filtered AChE did not. BW284c51, edrophonium or propidium enhanced the axonal elongation (33%) when they were administered for 2 days before, but not after, the crush. Iso-OMPA or vehicle administered before or after the crush were not effective. Thus, AChE reduces elongation of regenerating axons, while inhibition of AChE enhances elongation and affects the cytology of the normal nerve as well. We propose that AChE has a trophic role in mammalian peripheral nerves.
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    El Nino-southern oscillation-driven rainfall variability and delayed density dependence cause rodent outbreaks in western South America: Linking demography and population dynamics
    (UNIV CHICAGO PRESS, 1999) Lima, M; Keymer, JE; Jaksic, FM
    It is well known that some rodent populations display dramatic density fluctuations in semiarid regions of western South America after the unusual rainfall levels associated with El Nino-southern oscillation (ENSO) disturbances. These correlated phenomena have led some ecologists to believe that rodent outbreaks are determined solely by density-independent factors (e.g., rainfall regime). However, demographic studies have detected strong delayed density-dependent effects in one of the most irruptive rodent species, the leaf-eared mouse Phyllotis darwini. We tested the effects of rainfall and delayed density-dependent factors by constructing a structured model based on demographic data estimated from a capture-mark-recapture study of this species in Chile. A model including both rainfall and delayed density-dependent effects predicts the observed population dynamics rather accurately over a 10-yr period. Interestingly, small changes in model parameters result in large changes in model dynamics, which strongly suggests that local variations in demographic features are important in explaining the asynchronous pattern in outbreak occurrences. These findings suggest that inextricably intertwined endogenous and exogenous forces cause rodent outbreaks in western South America. The former are characterized by delayed nonlinear feedbacks, whereas the latter are characterized by the positive effects of the El Nino phases and the negative effects of the La Nina phases of the ENSO disturbance.