Identification of glutamatergic (VGLUT1; indigo) and cholinergic (VAChT; green) terminals abutting on LG α-motoneuron (α-MN) labeled by means of immunofluorescence (IF).

<p>LG MN perikaryon and proximal parts of dendrites were identified with True Blue (turquoise; A, C, E and G) and all synaptic terminals apposing LG α-MN were identified by synaptophysin IF (red). A-C present single optical section (0.21 μm thick); framed areas on C (merge of A-B) are shown with higher magnification in D to demonstrate contiguity of VAChT or VGLUT1 varicosities with the edge of the α-MN perikaryon. E-H are stacks of 20 optical sections of the same α-MN to show 3D reconstruction of the glutamatergic (indigo) and cholinergic (green) terminals among all synaptic terminals (synaptophysin) abutting on LG α- MN. G–merge of E and F. H—glutamatergic (magenta) and cholinergic (yellow) terminals shown to contact α-MN surface were accepted for quantification while the other (white) terminals, which did not fulfill these criteria, were not analyzed.</p

The number of TB-identified LG α-MNs subjected to the analysis of VGLUT1- and VAChT IF synaptic terminals apposing their cell bodies.

<p>(N)—the number of animals per group. The numbers of analyzed synaptic terminals are shown in square brackets.</p

Electrical Stimulation of Low-Threshold Proprioceptive Fibers in the Adult Rat Increases Density of Glutamatergic and Cholinergic Terminals on Ankle Extensor α-Motoneurons - Fig 4

<p><b>Changes in the number (A) and in the aggregate volume (B) of VGLUT1 IF synaptic terminals contacting LG α-MNs after seven days of stimulation of Ia fibers in the tibial nerve.</b> Data are reported as mean +/- SEM. Both the number and aggregate volume of VGLUT1 terminals were increased on the stimulated comparing to sham-stimulated side (*p = 0.03 and *p< 0.05, respectively, <i>Wilcoxon</i> test).</p

The direct M₁ responses and H₃-reflexes recorded in the rat soleus muscle during stimulation sessions of low-threshold proprioceptive fibers in the tibial nerve.

<p><b>A.</b> Examples of the raw data averaged after 7200 burst repetitions. Arrows indicate stimulus artifacts. <b>B</b>. The mean areas of the H₃-reflexes (blue line) and M₁-responses (green line) during consecutive days of stimulation in six rats. Four 3 min samples daily were taken for the analysis: at the beginning and at the end of the first and fourth stimulation sessions. The data are expressed as a percentage of M_max values for individual animals.</p

Example of typical location of α-motoneurons innervating LG muscle in L5 spinal segment.

<p>The spinal grey matter borderline is marked. Retrogradely labeled motoneurons (framed) are shown enlarged below. Motoneurons were labeled with True Blue fluorescence tracer injected into the LG muscle belly.</p

Time course of amplitude changes of the M- response and H- reflex evoked by electrical stimulation of the tibial nerve in the rat.

<p>Two stimulation patterns were used: continuous bursts of stimuli (solid lines: black – H reflex, grey – M response) or single stimuli (dashed lines: black - H reflex and grey – M response). The averaged responses to the single stimuli consisted of 30 reflexes collected at the beginning of the first and at the end of the last stimulation sessions daily in one animal. The averaged responses to burst of stimuli were collected during 3 min periods at the beginning and the end of every daily session. The strength of stimulation was established near the threshold of activation of the motor fibers, which is higher than that of Ia afferents, therefore stimulation elicited a moderate H-reflex as the majority of Ia fibers are already excited when the direct motor response (M) is at its threshold.</p

Distribution of NT-3 mRNA and BDNF mRNA expression in the tissues involved in circuitry of the H-reflex and their changes caused by continuous bursts of low-threshold, unilateral stimulation of the tibial nerve.

<p><b>A.</b> Relative expression of transcripts in the control group. In the tibial nerve (shaded grey bars), mRNA expression was evaluated by means of high-fidelity reverse transcriptase and random hexamers as primers. The tibial nerve segment enwrapped in the cuff-electrode was analyzed. <b>B.</b> The changes of NT-3 and BDNF level in the spinal cord segments, soleus muscle and tibial nerve after low-threshold stimulation of the tibial nerve. NT-3 mRNA expression tended to increase in the caudal lumbar segments of the spinal cord and the effect was bilateral, similarly as that of BDNF mRNA. In the soleus muscle NT-3 mRNA and BDNF mRNA level decreased comparing to control level. NT-3 mRNA expression in the tibial nerve increased after stimulation but that of BDNF mRNA tended to decrease. Asterisks indicate statistically significant effects (***P<0.001; **P<0.01; *P<0.05, Mann-Whitney U and Wilcoxon tests).</p

Enhancing Proprioceptive Input to Motoneurons Differentially Affects Expression of Neurotrophin 3 and Brain-Derived Neurotrophic Factor in Rat Hoffmann-Reflex Circuitry

<div><p>The importance of neurotrophin 3 (NT-3) for motor control prompted us to ask the question whether direct electrical stimulation of low-threshold muscle afferents, strengthening the proprioceptive signaling, could effectively increase the endogenous pool of this neurotrophin and its receptor TrkC in the Hoffmann-reflex (H-reflex) circuitry. The effects were compared with those of brain-derived neurotrophic factor (BDNF) and its TrkB receptor. Continuous bursts of stimuli were delivered unilaterally for seven days, 80 min daily, by means of a cuff-electrode implanted over the tibial nerve in awake rats. The H-reflex was recorded in the soleus muscle to control the strength of stimulation. Stimulation aimed at activation of Ia fibers produced a strong increase of NT-3 protein, measured with ELISA, in the lumbar L3-6 segments of the spinal cord and in the soleus muscle. This stimulation exerted much weaker effect on BDNF protein level which slightly increased only in L3-6 segments of the spinal cord. Increased protein level of NT-3 and BDNF corresponded to the changes of NT-3 mRNA and BDNF mRNA expression in L3-6 segments but not in the soleus muscle. We disclosed tissue-specificity of TrkC mRNA and TrkB mRNA responses. In the spinal cord TrkC and TrkB transcripts tended to decrease, whereas in the soleus muscle TrkB mRNA decreased and TrkC mRNA expression strongly increased, suggesting that stimulation of Ia fibers leads to sensitization of the soleus muscle to NT-3 signaling. The possibility of increasing NT-3/TrkC signaling in the neuromuscular system, with minor effects on BDNF/TrkB signaling, by means of low-threshold electrical stimulation of peripheral nerves, which in humans might be applied in non-invasive way, offers an attractive therapeutic tool.</p></div

Distribution of TrkC and TrkB mRNA expression in the regions involved in H-reflex circuitry and their changes caused by continuous bursts of low-threshold, unilateral stimulation of the tibial nerve.

<p><b>A.</b> Relative expression of transcripts in the control group. In the tibial nerve (shaded grey bars), the mRNA expression was evaluated by means of high-fidelity reverse transcriptase and random hexamers as primers. <b>B.</b> The changes of TrkC and TrkB level in the spinal cord segments, soleus muscle and tibial nerve after low-threshold stimulation. TrkC mRNA expression was high in the spinal cord of the control group and negligible in the soleus muscle. It tended to decrease after stimulation in the spinal cord and increased in the soleus muscle. Stimulation caused clear decrease of NT-3 mRNA in the tibial nerve. TrkB mRNA expression decreased both in the spinal cord and soleus muscle after stimulation but in the tibial nerve it increased on the sham-side and tended to decrease on the stimulated side. Asterisks indicate statistically significant effects (***P<0.001; **P<0.01; *P<0.05, Mann-Whitney U and Wilcoxon tests).</p

Schema showing the circuitry of the monosynaptic Hoffmann (H) reflex (an analog of the muscle stretch reflex) showing the location of the stimulating and recording electrodes.

<p>Low-threshold continuous burst stimulation of the tibial nerve, addressed to group Ia fibers, was applied to enhance the proprioceptive input to the extensor motoneurons innervating the ankle muscles. The amplitudes of H-reflex and direct M-response were used to control the strength of stimulation.</p