Regulation of acetylcholinesterase activity by nitric oxide in rat neuromuscular junction via N‐methyl‐d‐aspartate receptor activation

Abstract

Acetylcholinesterase (AChE) is an enzyme that hydrolyses the neurotransmitter acetylcholine, thereby limiting spillover and duration of action. This study demonstrates the existence of an endogenous mechanism for the regulation of synaptic AChE activity. At the rat extensor digitorum longus neuromuscular junction, activation of N-methyl-d-aspartate (NMDA) receptors by combined application of glutamate and glycine led to enhancement of nitric oxide (NO) production, resulting in partial AChE inhibition. Partial AChE inhibition was measured using increases in miniature endplate current amplitude. AChE inhibition by paraoxon, inactivation of NO synthase by N(x)-nitro-L-arginine methyl ester, and NMDA receptor blockade by DL-2-amino-5-phosphopentanoic acid prevented the increase in miniature endplate current amplitude caused by amino acids. High-frequency (10 Hz) motor nerve stimulation in a glycine-containing bathing solution also resulted in an increase in the amplitude of miniature endplate currents recorded during the interstimulus intervals. Pretreatment with an NO synthase inhibitor and NMDA receptor blockade fully eliminated this effect. This suggests that endogenous glutamate, released into the synaptic cleft as a co-mediator of acetylcholine, is capable of triggering the NMDA receptor/NO synthase-mediated pathway that modulates synaptic AChE activity. Therefore, in addition to well-established modes of synaptic plasticity (e.g. changes in the effectiveness of neurotransmitter release and/or the sensitivity of the postsynaptic membrane), another mechanism exists based on the prompt regulation of AChE activity.

Increase in Maximal Cycling Power With Acute Dietary Nitrate Supplementation

“…Assuming that the observed effects are due to increased NO availability through the exogenous NO 3 − -NO 2 − -NO pathway 1 , animal studies 5,6,7,8 suggest possible mechanisms by which NO may enhance neuromuscular power in people. NO 3 − ingestion appears to increase blood flow and vascular conductance to fast-twitch rat muscle to a greater extent than slow-twitch muscle 21 .…”

“…Increasing blood flow to fast-twitch muscle could increase delivery of circulating NO precursors to the neuromuscular region. There, NO has directly increased the action of acetylcholine in the neuromuscular junction of fast-twitch rat muscle, resulting in greater motor endplate electrical currents 5 . Indirectly, increased NO availability may acutely enhance neuromuscular power by activating soluble guanylate cyclase, and thus increasing cyclic guanosine monophosphate ( C GMP).…”

“…Aside from chronic supplementation, rodent studies 5,6,7,8 highlight potential mechanisms by which increased NO availability could acutely increase neuromuscular power (see DISCUSSION ). Therefore, NO 3 − ingestion could acutely increase muscle power in humans.…”

Motor Axon Synapses on Renshaw Cells Contain Higher Levels of Aspartate than Glutamate

“…High levels of glutamate, EAA transporters and AMPA/NMDA receptors have been detected in motor end-plates [2]–[4] and significant actions of glutamate receptors on NMJ cholinergic neurotransmission have been described. For example, activation of presynaptic metabotropic glutamate receptors modulates acetylcholine neurotransmitter release at the NMJ [5], [6] and postsynaptic NMDA receptor-mediated nitric oxide release regulates acetylcholinesterase activity [7]. However, motor axon postsynaptic actions on normal mammalian muscles are fully blocked by nicotinic acetylcholine receptor antagonists and a contribution from NMDA/AMPA receptors to postsynaptic end-plate currents is not commonly observed.…”

Research on cholinesterases in the Soviet Union and Russia: A historical perspective

“…19) are helping to detect subtle mechanisms of physiological responses to newly synthesized molecules in the cholinergic system. Of particular interest are tissue-specific inhibitors causing a myostatic effect in skeletal muscles with no effect on the heart or respiratory muscles (V. Reznik, K. Petrov and E. Nikolsky) [13].…”