Naturally Occurring Acetylcholinesterase Inhibitors and Their Potential Use for Alzheimer's Disease Therapy

Alzheimer's disease (AD) is a main cause of dementia, accounting for up to 75% of all dementia cases. Pathophysiological processes described for AD progression involve neurons and synapses degeneration, mainly characterized by cholinergic impairment. This feature makes acetylcholinesterase inhibitors (AChEi) the main class of drugs currently used for the treatment of AD dementia phase, among which galantamine is the only naturally occurring substance. However, several plant species producing diverse classes of alkaloids, coumarins, terpenes, and polyphenols have been assessed for their anti-AChE activity, becoming potential candidates for new anti-AD drugs. Therefore, this mini-review aimed to recapitulate last decade studies on the anti-AChE activity of plant species, their respective extracts, as well as isolated compounds. The anti-AChE activity of extracts prepared from 54 plant species pertaining 29 families, as well as 36 isolated compounds were classified and discussed according to their anti-AChE pharmacological potency to highlight the most prominent ones. Besides, relevant limitations, such as proper antioxidant assessment, and scarcity of toxicological and clinical studies were also discussed in order to help researchers out with the bioprospection of potentially new AChEi.

Methamidophos: an anticholinesterase without significant effects on postsynaptic receptors or transmitter release

Methamidophos (O,S-dimethyl phosphoroamidothiolate, Tamaron), an organophosphate (OP) anticholinesterase of limited toxicity, is widely used as an insecticide and acaricide. To provide additional insight into the molecular basis of its action, we have used electrophysiological and biochemical techniques to study the effects of methamidophos on the neuromuscular junction of rat and frog and on the central nervous system of rat. Methamidophos has a relatively weak inhibitory action on cholinesterases in rat diaphragm muscle, brain and hippocampal homogenates, with IC50 values on the order of 20-20 microM. An even weaker anticholinesterase activity was found in frog muscle homogenates, with the IC50 being above 300 microM. As further evidence of anticholinesterase activity, methamidophos (1-100 microM) was able to reverse the blockade by d-tubocurarine (0.5-0.7 microM) of neuromuscular transmission in rat phrenic nerve-hemidiaphragm preparations. Inhibition of cholinesterase activity by methamidophos was long lasting, which is consistent with the formation by the agent of a covalent bond with the enzyme's active serine residue. The action was also slowly reversible, which suggests spontaneous reactivation of the enzyme. electrophysiological studies at the rat neuromuscular junction showed that, due to its anticholinesterase activity, methamidophos increased the amplitude and prolonged the decay phase of nerve-evoked and spontaneous miniature end-plate potentials. In contrast to other OP compounds, e.g., paraoxon (Rocha et al., 1996a), methamidophos did not affect neurotransmitter release, nor did it interact directly with the muscle nicotinic acetylcholine receptor. Moreover, it contrast to paraoxon, methamidophos did not affect the whole-cell currents induced by application of acetylcholine, glutamate or gamma-aminobutyric acid recorded to cultured hippocampal neurons. Based on these data, methamidophos appears to have a selective effect on cholinesterase.

Postnephrectomy arteriovenous fistula

The development of the postnephrectomy arteriovenous fistula (PNAVF) between the renal vessels stumps is rare. Here we present a case report of PNAVF, and review the diagnosis, treatment and prevention. The most common clinical features include a loud murmur over the previous nephrectomy scar, and heart failure resistant to common medical treatment. A 58-year-old white woman was admitted to the hospital for a complete evaluation of an unexplained congestive heart failure with no response to common medical treatment. She had had a right nephrectomy for pyonephrosis 13 years before. The diagnosis of PNAVF was suspected because over the right lumbar region a definite trill was palpated, and on auscultation a harsh, machinery-like murmur was heard. The diagnosis was confirmed by aortogram and selective renal arteriography. In May 1989, the right arteriovenous was excised through a right subcostal transperitoneal approach. The renal vessel stumps were individually ligated and sutured separately close to aorta and vena cava. The patient's postoperative course was entirely uneventful in the following seven years. We conclude that during nephrectomy, the renal vessels should be ligated separately, and the transfixation in mass of the stumps avoided to prevent arteriovenous fistula.