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ECPB 2014, 67(3): 7–13
Experimental physiology and biochemistry

Electrophysiological analysis of the neuromuscular complex excitability in experimental menopause

Rodinsky A., Tkachenko S., Mozgunov A.
Abstract

The biological effect of estrogen in mammals is not limited to the effect on target cells in the organs of the reproductive system. Estrogens modulate the activity of all types of nerve cells through many mechanisms. Usage of experimental model allows us to estimate the neurotrophic and neuroprotective effects of estrogen. Such studies are of great interest because estradiol can be an important therapeutic agent for maintaining neural function during aging and in some cases of nervous diseases. The aim of our study was to investigate the electrophysiological activity of the neuromuscular complex under the simulated menopause in rats.

The study was performed on 43 sexually mature female Wistar rats weighing 180– 250 g, that were divided into two groups – control (n=16) and experimental (n=26). In order to reproduce menopause, we provide ovariohisterectomia. In the control group a control surgery – medial laparotomy has been performed. Both groups of animals were kept in standard vivarium conditions (22±2°C, light / dark cycle was 12/12 hrs.) at a standard diet during 120 days, whereupon acute animal experiments were carried out. Sodium thiopental at a dose of 50 mg / kg was used for total anesthesia. Isolated sciatic nerves were placed on bipolar stimulating electrodes, and were submerged in vaseline oil to prevent drying. The registration of the induced action potential of the gastrocnemius muscle and direct stimulation of the muscle was performed by means of two needle electrodes, that were introduced into the muscle with the distance of 10 mm.

We investigated such parameters of neuromuscular complex excitability as the amplitude, latenty period and duration of the induced action potential, threshold and hronaxy of the gastrocnemius muscle. The stimulation threshold has been determined by amperage.

The phenomenon of refractoriness was analyzed by studying the electrical activity of the gastrocnemius muscle in response to indirect stimulation with paired stimuli at intervals of 1 to 20 ms, and a pack of 10 impulses at frequency from 50 to 500 Hz. Irritating stimulus amplitude was equal to two thresholds, duration of 0.3 ms. The study was carried out by using the standard electrophysiological equipment (electronic stimulator with independent power supply, amplifier, analog-to-digital converter with registration and processing of information with personal computer).

It was found that in experimental menopause the excitability of the neuromuscular complex is increase. This was evident by the reduction of the threshold excitation by direct stimulation of the gastrocnemius muscle at 58.82%, and at 44.44% by indirect stimulation, and decrease hronaxy at 26.48% in the case of direct stimulation. Estrogen deficiency leads to prolongation of refractory period and decrease the lability of the nerve. The evidence of that was a significant (p<0,05) decrease at 22.38% of the amplitude of the action potential induced by the second stimulus relative to the first it in conditions of stimulation by paired pulses, and the transformation rate at a frequency of 300 Hz and above during indirect stimulation pulse train. Also, there were a decrease in the degree of synchronization responses of the studied muscle and slowing its response to indirect stimulation, as was shown by increase of latent period at 44.16% and hronaxy at 27.60% when compared to the intact group.

Thus, it was found that in experimental menopause identified changes that lead to an increase in the excitability of the neuro-muscular system and changes in refractory phases, as well as to decrease in the degree of synchronization of responses of the gastrocnemius and in slowing its responses to indirect stimulation in experimental menopause.

Keywords: experimental menopause, electrophysiological studies, action potential

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