The delayed effect of cortagen on the restoration of injured nerve function.

Abstract

It is known that fragments of large peptide molecules affect the growth, development, and regeneration of nerve tissue [1‐3, 5, 9]. The existence of biologically active short peptides allows obtaining their artificial analogues that can stimulate regeneration of injured peripheral nerve fibers. One of these newly synthesized peptides is the tetrapeptide cortagen (Ala-Glu-AspPro), which was obtained using targeted construction based on the amino acid analysis of cortexin, a polypeptide preparation possessing neurotrophic activity. Earlier, we showed that cortagen stimulated the growth of rat sciatic nerve fibers at the initial regeneration stages after their injury [6]. Of special interest are late regeneration stages, when the degree of reinnervation and functional restoration of injured nerve fibers can be estimated. In this work, we performed the study five months after the nerve injury. This time is sufficient for the majority of regenerating fibers to reach the target tissues and complete myelination [7]. Sixteen male Wister rats weighing 200 to 250 g were used. The operations were performed under ketamine narcosis (intramuscular injections of ketamine and rometar (50 and 15 mg/kg, respectively)). The restoration of the sciatic-nerve function was studied after the nerve transsection and suturing its ends with an epiperineural suture [4]. To study the effect of cortagen on regeneration, the animals received intramuscular injections of the peptide (10 μ g/kg) daily for 10 days after the operation. The animals of the control group received intramuscular injections of saline according to the same scheme. The functional state of the nerve was evaluated by recording the compound action potentials in vitro according to Legrand [4]. The functional properties of the regenerating foot skin mechanoreceptors were studied by recording the impulse activity of single fibers with the use of the vaseline bridge technique on a special ebonite platform [1]. The mechanical threshold for each receptor was determined using von Frey monofilaments. The adaptive properties of the receptors were evaluated using mechanical stimulation with the trapezoid stimulus (20‐50 ms; a three-threshold amplitude). The data were statistically processed using Student’s t test. Preparation of the injured sciatic nerve for testing in vitro showed that cortagen caused a decrease in the neuroma located in the suture area. Therefore, simultaneously with measuring the conduction velocity of action potential along the regenerating fragment of the nerve that had already recovered, we also evaluated the conduction velocity through the suture site. We found that the sciatic nerve conduction velocity had, by the moment of the test, been completely restored in the animals of both groups. The method of compound action potential recording did not show the effect of cortagen on conductivity of regenerating nerve fibers. The conductivity of action potential along the distal nerve section was 23.5 ∠ 1.4 and 22.8 ∠ 1.9 m/s in the control and experimental animals, respectively. The conductivity of action potential through the suture was 22.2 ∠ 0.1 and 24.0 ∠ 2.6 m/s in the control and experimental animals, respectively. Note that the compound action potential was recorded in vitro , when standard registration conditions were provided by placing the nerve on a platform with limited interelectrode distance, which allowed determining the conductivity of action potential only along the most rapidly conducting (presumably, motor) nerve fibers classified with group A. It is known that injuries are most harmful for the sensor nerve fibers [10]. The final stage of regeneration of the injured nerve is restoration of the sensitivity of the denervated tissue. With this in mind, we studied the functional properties of foot skin mechanoreceptors (the target tissue of the sciatic nerve) and analyzed the action potentials occurring in single sensory fibers in response to foot mechanical stimulation.