The effect of cholinesterase inhibitors on the antimuscarinic effect of hemicholinium-3 (HC-3) in the rat

Inhibitors of Acetylcholinesterase Derived from 7-Methoxytacrine and Their Effects on the Choline Transporter CHT1

BACKGROUND

Reversible acetylcholinesterase inhibitors are used in Alzheimer disease therapy. However, tacrine and its derivatives have severe side effects. Derivatives of the tacrine analogue 7-methoxytacrine (MEOTA) are less toxic.

METHODS

We evaluated new derivatives of 7-MEOTA (2 homodimers linked by 2 C4-C5 chains and 5 N-alkylated C4-C8 side chain derivatives) in vitro, using the rat hippocampal choline transporter CHT1.

RESULTS

Some derivatives were effective inhibitors of rat acetylcholinesterase and comparable with 7-MEOTA. All derivatives were able to inhibit CHT1, probably via quaternary ammonium, and this interaction could be involved in the enhancement of their detrimental side effects and/or in the attenuation of their promising effects. Under conditions of disrupted lipid rafts, the unfavorable effects of some derivatives were weakened. Only tacrine was probably able to stereospecifically interact with the naturally occurring amyloid-β isoform and to simultaneously stimulate CHT1. Some derivatives, when coincubated with amyloid β, did not influence CHT1. All derivatives also increased the fluidity of the cortical membranes.

CONCLUSION

The N-alkylated derivative of 7-MEOTA bearing from C4 side chains appears to be the most promising compound and should be evaluated in future in vivo research.

The effect of troxypyrrolidinium, a choline uptake inhibitor, on the excitatory innervation of the rat urinary bladder

Troxypyrrolidinium, a choline uptake inhibitor, reduced but failed to abolish responses of the rat urinary bladder to electrical stimulation at 1-100 Hz although it reduced acetylcholine output during stimulation at 10 Hz to a level similar to that of spontaneous release. Inhibition of the response to stimulation was more complete at faster rates of stimulation and was partially reversed by choline. Troxypyrrolidinium produced a greater inhibition of the 'tonic' component of the response to electrical stimulation than of the 'phasic' component. Hemicholinium-3 or hyoscine produced a similar selective effect on the 'tonic' component. Hemicholinium-3 also reduced acetylcholine output during electrical stimulation to a similar extent as troxypyrrolidinium but hyoscine increased transmitter output. The results support the concept of a second transmitter in the excitatory innervation of the bladder.

Opposing influences on behavior mediated by muscarinic and nicotinic receptors in the rat posterior hypothalamic nucleus

Microinjection of carbachol into the posterior hypothalamic nucleus (PHN) of freely moving rats evoked marked behavioral changes characterized by an escape reaction. This response was quantitated by measuring locomotor activity. In contrast, cholinergic stimulation of the PHN with neostigmine produced sedation and inactivity. Local pretreatment with the nicotinic receptor blocking agent mecamylamine blocked the excitatory effects of carbachol while the muscarinic antagonist atropine abolished the inhibitory effect of neostigmine on motor activity. It is concluded that behavioral changes evoked through cholinergic stimulation of the PHN may be mediated by a muscarinic system which controls sedation and a nicotinic pathway which mediates arousal.

Antagonism of cholinomimetics by troxypyrrolidinium in guinea-pig atria and longitudinal ileal muscle: comparison with hemicholinium-3

The inhibitory effect of troxypyrrolidinium (trox) (10(-6) to 2 x 10(-3) M) on responses to carbachol (CCh) and acetylcholine (ACh) was investigated in the electrically stimulated left atrium and longitudinal ileal muscle of the guinea pig. In both tissues, trox exhibited antimuscarinic activity causing parallel shifts of the concentration--response curves to both agonists with no depression of maximum responses. Responses to CCh were inhibited by trox (5 x 10(-4) M) to a greater extent than responses to ACh and this difference was maintained following inhibition of cholinesterases with dyflos. In the guinea-pig atrium using CCh as agonist dose ratios produced by the higher concentrations of trox (greater than 5 x 10(-5) M) were less than expected resulting in a non-linear Arunlakshana--Schild (A-S) plot and this effect of trox was maintained in the presence of mecamylamine (2 x 10(-5) M). In longitudinal ileal muscle flattening of the A-S plot with high concentrations of trox did not occur. Although hemicholinium-3 (HC-3) produces a non-linear A-S plot for antimuscarinic activity in atria the A-S plot obtained with the longitudinal ileal strip using HC-3 (2 x 10(-5) to 2 x 10(-3) M) did not exhibit flattening at high concentrations. The dose ratios obtained with HC-3 (5 x 10(-4) M) using CCh as agonist were significantly greater than those obtained with ACh. It is suggested that trox, like HC-3, acts as a metaffinoid antagonist at the muscarinic receptor.