Natural probes for cholinergic sites: L-bebeerine actions on the neuromuscular transmission, the nicotinic receptor/ionic channel complex, and contraction of skeletal muscles

The mechanisms underlying the muscle relaxant of 1-bebeerine (BB), a tertiary alkaloid isolated from the roots of Chondrodendron platyphyllum, were examined in mammalian and amphibian skeletal muscles. Injections of BB (0.05 - 1 g/kg,i.p.) in rats caused a dose-related flaccid paralysis and respiratory arrest at high doses. In isolated rat diaphragmand toad sartorius muscles, BB depressed the indirectly elicited muscles twitches (IC50:228 muM and 5.4 muM, respectively, at 22 degree) and blocked the nerve-elicited muscle action potential. The neuromuscular blockade was not reserved by neostigmine (10 muM). High concentrations of BB (170 and 340 muM) caused muscle contracture unrelated to the junctional blockade, and intensified by increasing the bath temperature. Analysis of the contraction properties showed that BB (40 and 80 muM)increaded the twitch/tetanus ratio (46 percent and 125 percent) and prolonged the relaxation time; the falling phase of the directly elicited action potential in toad sartorius muscle fibers was slower probably by a decreased potasium conductance. BB (0.1 - 340 muM) reduced the binding of [1251]alpha- -bungarotoxin to the junctional AACh receptor of the rat diaphragm (IC50:47.7 muM, at 37 degree. At low concentrations BB (1.5 - 15 muM) induced either opening or blockade of the Ach receptor-ionic channel. The results showed that BB blocked noncompetitively the neuromuscular transmission through a mechanism that affects the Ach recognition site and the ionic channel properties. The alkaloid also produced muscle contracture and changed the contractile properties through its extra-junctional action at the calcium handling by the sarcoplasmic reticulum or the contractile machinery.

Influencia de mecanismos colinergicos nos processos neurodegenerativos relacionados a formacao de amiloide e a fosforilacao da proteina TAU / Cholinergic mechanisms influence in neurodegenerative process related to amyloid formation and phosphorylation of TAU protein

Na doenca de Alzheimer (DA), os principais eventos associados a neurodegeneracao sao a formacao de placas senis e de emaranhados neurofibrilares. Estes fenomenos relacionam-se respectivamente a deposicao de beta-amiloide (Ab) e a alteracoes do estado de fosforilacao da proteina Tau. Esta e componente essencial dos microtubulos, onde se encontra em estado polimerizado. A estabilidade do polimero depende do grau de fosforilacao da Tau, tornando-se mais instavel quanto mais fosforilada a proteina. Consequentemente, a hiperfosforilacao da Tau relaciona-se com menor estabilidade do citoesqueleto, favorecendo a morte neuronal. O Ab e produzido pela clivagem da proteina precursora do amiloide (APP) por acao da enzima beta-secretase, em detrimento da acao mais fisiologica da alfa-secretase, que da origem ao fragmento APPs. As fibras de Ab tem diversos efeitos neurotoxicos, alem de ocorrerem associadamente a uma presumivel perda funcional do metabolito secretado APPs...