The impact of acetylcholinesterase inhibitors on the extracellular acetylcholine concentrations in the adult rat brain: A meta‐analysis

Abstract

In vivo microdialysis has become a key method in investigating the dynamics of different neurotransmitter systems such as acetylcholine in the extracellular fluid. Depending on the sensitivity of the analytical method applied for measuring acetylcholine levels in brain dialysates, acetylcholinesterase (AChE) inhibitors are often used to increase the basal acetylcholine level up to a detectable magnitude. This artificial manipulation of the system questions the outcome of pharmacological studies and has led to a large number of experiments pursuing the appropriate physiological and pharmacological concentration of the AChE inhibitors in a range between 0.01 and 100 μM. However, the complexity of the action of these substances, particularly through the involvement of muscarinic autoreceptors and the induction of an autoinhibitory effect on acetylcholine release, did not allow this quest to be resolved completely and suggests the application of advanced mathematical methods for the evaluation of acetylcholine baseline levels. Here we performed a meta-analysis on published datasets of in vivo microdialysis measurements to assess the concentration-dependent effects of various AChE inhibitors on acetylcholine levels within the prefrontal cortex, nucleus accumbens, caudate putamen, and hippocampus in adult rats. In total 3255 rats were analyzed and we found that when compared with the minority of studies (14%) that did not use AChE inhibitors (these studies yielded basal levels between 0.55 and 2.71 nM depending on the brain site) an up to 350-fold increase in baseline values after the application of an inhibitor could be detected. Especially, the derivates neostigmine bromide and physostigmine sulfate seem to produce dramatic effects. Furthermore, concentration-dependent effects after the application of AChE inhibitors could not be established. In the case of neostigmine bromide an inverted concentration (0.1-10 μM)-response relationship was even detected. We conclude that although the presynaptic action of AChE inhibitors is well understood the nonphysiological and concentration-independent augmentation of the acetylcholine system requires the use of a standard protocol in order to produce replicable and comparable results. Our meta-analysis suggests the use of 0.1 μM neostigmine which produces an approximately 10-fold boost of brain baseline levels.

Selection of a human butyrylcholinesterase-like antibody single-chain variable fragment resistant to AChE inhibitors from a phage library expressed inE. coli

“…42 No data on the ACh concentrations in the synaptic cleft after AChE inhibitor exposure has been reported in the literature; however, a meta-analysis evaluating results obtained in rat brain studies indicates that this value can increase up to 350-fold over baseline. 43 Moreover, the catalytic efficiency of WZ1-14.2.1 is good, accelerating the spontaneous hydrolysis of about 10 7 times. The enzymatic specificity of WZ1-14.2.1 appears to be more similar to that of BChE than AChE.…”

Attenuation in rats of impairments of memory by scopolamine, a muscarinic receptor antagonist, by mecamylamine, a nicotinic receptor antagonist

“…If scopolamine impairs memory by increasing acetylcholine release, this effect of cholinesterase inhibitors to blunt the actions of scopolamine appears counterintuitive, as it would seem that this would further increase acetylcholine in the synapse. However, microdialysis studies have shown that if the concentration of cholinesterase inhibitors in the dialysate is high, the amount of acetylcholine in the perfusate decreases (Liu and Kato 1994; Noori et al 2012). Therefore, depending on the concentrations used, it is possible that cholinesterase inhibitors in combination with scopolamine can lead to a decreased release of acetylcholine.…”

“…The inclusion of neostigmine protects acetylcholine in the dialysate from being hydrolyzed after collection from the extracellular fluid thereby increasing the measurable amount of acetylcholine in the dialysate (Chang et al 2006; Himmelheber et al 1998; Liu and Kato 1994; Noori et al 2012). …”

Extracellular dopamine, acetylcholine, and activation of dopamine D1 and D2 receptors after selective breeding for cocaine self-administration in rats

“…Approximately 16 hours prior to initial injections, microdialysis probes were inserted through guide cannulas and perfused with artificial cerebrospinal fluid (147 mM NACl, 2.7 mM KCl, 1.0 mM NaH 2 PO 4 , 1.4 mM Na 2 HPO 4 , 2.1 mM MgCl 2 , and 0.1 μM neostigmine, pH 7.4). Neostigmine at this concentration is recommended as part of a standardized protocol to detect acetylcholine in the brain through microdialysis (Noori et al 2012). After overnight flow at 0.1 μL/minute, the rate was increased to 0.75 μL/minute at the onset of the lighted phase (4:00 AM).…”