In Vivo Intracellular Recording of Type-Identified Rat Spinal Motoneurons During Trans-Spinal Direct Current Stimulation

Summary

This protocol describes in vivo intracellular recording of rat lumbar motoneurons with simultaneous trans-spinal direct current stimulation. The method enables us to measure membrane properties and to record rhythmic firing of motoneurons before, during and after anodal or cathodal polarization of the spinal cord.

Abstract

Intracellular recording of spinal motoneurons in vivo provides a “gold standard” for determining the cells’ electrophysiological characteristics in the intact spinal network and holds significant advantages relative to classical in vitro or extracellular recording techniques. An advantage of in vivo intracellular recordings is that this method can be performed on adult animals with a fully mature nervous system, and therefore many observed physiological mechanisms can be translated to practical applications. In this methodological paper, we describe this procedure combined with externally applied constant current stimulation, which mimics polarization processes occurring within spinal neuronal networks. Trans-spinal direct current stimulation (tsDCS) is an innovative method increasingly used as a neuromodulatory intervention in rehabilitation after various neurological injuries as well as in sports. The influence of tsDCS on the nervous system remains poorly understood and the physiological mechanisms behind its actions are largely unknown. The application of the tsDCS simultaneously with intracellular recordings enables us to directly observe changes of motoneuron membrane properties and characteristics of rhythmic firing in response to the polarization of the spinal neuronal network, which is crucial for the understanding of tsDCS actions. Moreover, when the presented protocol includes the identification of the motoneuron with respect to an innervated muscle and its function (flexor versus extensor) as well as the physiological type (fast versus slow) it provides an opportunity to selectively investigate the influence of tsDCS on identified components of spinal circuitry, which seem to be differently affected by polarization. The presented procedure focuses on surgical preparation for intracellular recordings and stimulation with an emphasis on the steps which are necessary to achieve preparation stability and reproducibility of results. The details of the methodology of the anodal or cathodal tsDCS application are discussed while paying attention to practical and safety issues.

Introduction

Trans-spinal direct current stimulation (tsDCS) is gaining recognition as a potent method to modify spinal circuit excitability in health and disease^1,2,3. In this technique, a constant current is passed between an active electrode located above selected spinal segments, with a reference electrode located either ventrally or more rostrally⁴. Several studies have already confirmed that tsDCS can be used in managing certain pathological conditions, such as neuropathic pain⁵, spasticity⁶, spinal cord injury⁷ or to facilitate rehabilitation⁸. Researchers suggest that tsDCS evokes alterations in the ion distribution between the intracellular and the extracellular space across the cell membrane, and this can either facilitate or inhibit neuronal activity depending on the current orientation^9,10,11. However, until recently, a direct confirmation of this influence on motoneurons was lacking.

Here, we describe a detailed protocol to conduct in vivo intracellular recording of electrical potentials from lumbar spinal motoneurons in the anesthetized rat with simultaneous application of tsDCS, in order to observe changes in motoneuron membrane and firing properties in response to anodal or cathodal polarization of the spinal neuronal network. Intracellular recordings open several areas of investigation of neuron properties, unavailable for previously used extracellular techniques^9,12. For example, it is possible to precisely measure motoneuron membrane voltage response to direct current flow induced by tsDCS, to indicate voltage threshold for spike generation, or to analyze action potential parameters. Moreover, this technique allows us to determine motoneuron passive membrane properties, such as input resistance, and to observe the relationship between intracellular stimulation current and frequency of rhythmic firing of motoneurons. Antidromic identification of recorded motoneuron, based on the stimulation of functionally identified nerves (i.e., nerves providing efferents to flexors or extensors) allows us to additionally identify types of innervated motor units (fast versus slow), which gives an opportunity to test whether polarization differently influences individual elements of the mature spinal neuronal system. Due to extensive surgery preceding the recording and high requirements on stability and reliability of recordings, this technique is highly challenging but allows direct and long-term assessment of electrophysiological characteristics of one motoneuron: before, during and after application of tsDCS, which is crucial to determine both its acute actions and persistent effects¹³. As a motoneuron directly activates extrafusal muscle fibers¹⁴ and takes part in feedback control of a muscle contraction and developed force^15,16 any observed influence of tsDCS on the motor unit or muscle contractile properties may be linked to modulations of motoneuron excitability or firing characteristics.

Protocol

All procedures connected to this protocol have been accepted by the appropriate authorities (e.g., Local Ethics Committee) and follow the national and international rules on animal welfare and management.

NOTE: Each participant involved in the procedure has to be properly trained in basic surgical procedures and has to have a valid license for performing animal experiments.

1. Anesthesia and premedication

1. Anesthetize a rat with intraperitoneal injections of sodium pentobarbital (an initial dose of 60 mg·kg^-1 for 6-month old male Wistar rats weighing 400‒550g).
   NOTE: This protocol is not limited to the indicated strain, sex or age of rats. Also, alternative anesthesia such as ketamine-xylazine mix, alpha-chloralose or fentanyl+midazolam+medetomidine can be used if more suitable for different research goals or when required by the ethics committee.
2. After approximately 5 min, check the depth of anesthesia by pinching the rat’s hind limb toe with blunt forceps. Proceed with the next steps of the protocol only when no reflex action is observed.
3. Inject 0.05 mL of atropine subcutaneously in order to reduce mucus production after intubation.
4. Inject subcutaneously 5 mL of phosphate buffer containing 4% glucose solution, NaHCO₃ (1 %) and gelatin (14%). This buffer will be absorbed by the cutaneous vessels throughout an experiment and will help maintain fluid balance.
5. Throughout the surgery, periodically check the animal for reflex actions and supplement anesthesia if required (10 mg·kg^-1·h^-1of sodium pentobarbital).

2. Surgery

1. Prepare the animal for surgical treatment by shaving fur over the dorsal part of the left hindlimb, from the ankle to the hip, the backside, from tail to the high thoracic segments, the left side of the chest, and the ventral side of the neck area above the sternum
2. Placement of the intravenous line
   1. Place the rat on its back on a closed-loop heating pad (and secure it with limb fixations).
   2. Using a 21 blade, make a longitudinal cut through the skin from a sternum to a chin.
   3. Hold the skin with forceps and separate it from the underlying tissue.
   4. Using blunt dissection techniques expose the right jugular vein. Carefully dissect the vein from surrounding tissues.
   5. Locate the part of the vein without branching points, slip two 4-0 ligatures beneath it.
   6. Make one loose knot on the proximal end of the previously identified non-branching segment of the vein and one loose knot on the distal end of this segment of the vein. Clamp the vein proximal to the heart, and then ligate the distal part of the vein.
   7. Using iris scissors, make an incision between the clamp and distant ligature. Hold a flap of the vein, and introduce a pre-filled catheter to the point where it is blocked by the clamp.
   8. While holding the vein and the catheter together with forceps, remove the clamp and push the catheter several millimeters into the vein. Secure both ends of the catheter to the vein, and add an additional fixation point to the skin.
3. Introduction of the tracheal tube
   1. Using blunt forceps separate the two mandibular glands covering the sternohyoid muscles. Separate sternohyoid muscles at the midline to expose the trachea.
   2. Slip three 4-0 ligatures beneath the trachea, then make two knots below the tracheal tube insertion point and one knot above.
   3. Locate the cricoid cartilage of the larynx and make an incision below the third tracheal cartilage.
   4. Insert a tracheal tube down the trachea and secure the tube in place with pre-prepared ligatures, then add an additional ligature to the skin.
   5. Place a small piece of cotton wool above the separated muscles, and suture the skin over the operated area.
4. Dissection of hind limb nerves
   1. Using 21 blade, make a longitudinal cut on the posterior side of the left hind limb, from the Achilles tendon to the hip.
   2. Grab the skin with forceps, and using blunt dissection techniques separate the skin from underlying muscles on both sides of the incision.
   3. Locate the popliteal fossa at the back of the knee joint, which is covered by the biceps femoris muscle, and using scissors make a cut between the anterior and posterior part of this muscle.
   4. Moving upwards cut two heads of the biceps femoris all the way to the hip to expose the sciatic nerve. Cauterize as needed to prevent bleeding.
   5. Identify the sural, tibial and common peroneal branches of the sciatic nerve.
   6. Using scissors, separate the lateral from the medial head of the gastrocnemius muscle to expose the tibial nerve and its branches.
   7. Using 55 forceps grab the distal end of the sural nerve, cut it distally and dissect as far as possible.
   8. Repeat the procedure with the common peroneal nerve.
   9. Using a blunt glass rod separate the tibial nerve from surrounding tissues, taking care not to damage the blood vessels, and cut it distally.
   10. Identify the medial gastrocnemius (MG) and the lateral gastrocnemius and soleus (LGS) nerves.
   11. Using 55 forceps, carefully dissect the MG and LGS nerves, disconnecting them from surrounding tissues, but maintaining their connection to the respective muscles.
   12. Place a saline-soaked piece of cotton wool under the exposed nerves.
   13. Close the skin over the operated area.
5. Laminectomy
   1. Using 21 blade make a longitudinal incision from the sacrum up to the thoracic vertebrae.
   2. Separate the skin from underlying muscles.
   3. Cut the longissimus muscle on both sides of the thoracic and lumbar spinous processes.
   4. Using blunt-edged scalpel retract the muscles from the spinal column to expose the transverse processes of each vertebra.
   5. Using blunt tip scissors cut the tendons of muscles connected to the transverse processes along the exposed spinal column. Apply hemostatic agents if necessary.
   6. Identify the Th13 vertebra as the lowest thoracic segment with rib insertion and using fine rongeurs remove spinous processes and laminae from Th13 to L2 vertebrae to expose lumbar segments of the spinal cord. Remember not to damage the L3 spinous process which will be used as a fixation point for spine stabilization.
   7. Remove the Th12 spinous process and smooth the vertebra dorsal surface as much as possible.
   8. Using blunt dissection techniques separate the muscles from Th11 vertebra to create holder insertion points.
   9. Place thin saline-soaked cotton wool over the exposed spinal cord segments.
   10. Move the rat to the custom made metal frame with two parallel bars and two adjustable arms with clamps to support and stabilize the spine.

3. Preparation for the recording and stimulation

1. Vertebral column fixation and nerve arrangement
   1. Place the rat in the custom-made frame on a heating pad, connected to the closed-loop heating system to maintain the animal body temperature at 37 ± 1°C.
   2. Insert ECG electrodes under the skin and connect to an amplifier for heart rate monitoring.
   3. Using the skin flaps, form a deep pool over the exposed spinal cord.
   4. Using metal clamps, fix the vertebral column by putting clamps below Th12 transverse processes and at L3 spinous process.
   5. Make sure that the vertebral column is secured and arranged horizontally, and then apply dorso-ventral pressure on both sides of the column to retract the muscles.
   6. Fill the pool with warm (37 °C) mineral oil and maintain it at this temperature.
   7. Thread a 4-0 ligature through the Achilles tendon, lift and stretch the operated left hind limb so that the ankle is leveled with the hip.
   8. Using the skin flaps make a deep pool over the exposed tibial, MG and LGS nerves.
   9. Fill the pool with warm (37 °C) mineral oil.
   10. Place MG and LGS nerves on bipolar silver-wire stimulating electrodes and connect them to a square pulse stimulator. Use separate stimulation channels for each nerve.
2. Surface electrode placement
   1. Place a silver ball electrode on the left caudal side of the exposed spinal cord, with a reference electrode inserted in the back muscles, and connect both electrodes to the differential DC amplifier. The surface ball electrode will be used to record afferent volleys from nerves.
   2. Using a constant-current stimulator, stimulate the MG and LGS nerves with square pulses of 0.1 ms duration, repeated at a frequency of 3 Hz, and observe afferent volleys.
   3. Determine the threshold (T) for nerve activation, stimulate each nerve at approximately 3·T intensity, and record amplitude of afferent volley for each nerve.
   4. Move the surface electrode rostral and repeat the procedure to identify spinal segments at which amplitudes of the volleys are the highest for each nerve. After determining the maximum volley location, move the surface electrode to a safe distance from the spinal cord.
3. Muscle paralysis and forming a pneumothorax in order to reduce respiratory movements
   1. Paralyze the rat intravenously with a neuromuscular blocker and connect the tracheal tube to an external ventilator in line with a rodent-compatible capnometer (Pancuronium bromide, at an initial dose of 0.4 mg·kg^-1, supplemented every 30 min in doses of 0.2 mg·kg^-1)
   2. Monitor the end-tidal CO₂ concentration and maintain it at about 3‒4% by adjusting ventilation parameters (frequency, air pressure, and flow volumes).
   3. Make a longitudinal incision in the skin between the 5th and 6th rib on a side of the recording.
   4. Using blunt tip scissors cut the overlying muscles to visualize intercostal space between the ribs.
   5. Using small sharp scissors, make a small incision in the intercostal muscles and in the pleura, then insert a tip of a blunt edge forceps into the opening, taking care not to press on the lungs.
   6. Allow forceps to expand or insert a small tube to keep the pneumothorax open throughout the experiment.
   7. After the neuromuscular block, monitor anesthesia depth by checking ECG frequency, and supplement the anesthetic agent if the heart rate exceeds 400 bpm. Muscle paralysis and forming a pneumothorax in order to reduce respiratory movements, which will improve recording stability
4. Opening the dura and pia mater
   1. Using #55 forceps, gently lift the dura mater, and cut it caudally from the L5 segment, rostrally up to the L4 segment.
   2. Using a pair of ultra-thin 5SF forceps make a small patch in the pia covering the dorsal column, between the blood vessels, exactly at the level of the maximum afferent volley from the MG or the LGS nerve.
   3. Use small pieces of saline-soaked and dried gel foam to block bleeding if necessary.
5. tsDCS electrode placement
   1. Make a small incision in the skin on the ventral side of a rat abdomen at the rostro-caudal level corresponding to the location of L4-L5 spinal segments.
   2. Grab the exposed skin flap with a metal clip which will serve as a reference electrode.
   3. Place a saline-soaked sponge on the dorsal side of the Th12 vertebra. Make sure that the sponge size is equal to that of an active tsDCS electrode (circle-shaped stainless steel plate of 5 mm in diameter).
   4. Using a fine manipulator, press the sponge with an active tsDCS electrode to the bone and make sure that the entire surface of the electrode is pressed equally.
   5. Connect both reference and active tsDCS electrodes to a constant-current stimulator unit, capable of delivering a continuous flow of direct current.
6. Preparation of micropipettes
   1. Using a microelectrode puller, prepare a microelectrode.
      NOTE: Both filament and non-filament electrodes can be used, however, remember that the shank of the electrode must be long enough to reach the ventral horn while being thin enough not to compress the spinal cord while descending.
      1. Adjust the puller setting so that the shank entering the spinal cord is approximately 3 mm long, while the tip of the electrode is no more than 1‒2 µm in diameter and microelectrode resistance is between 10 and 20 MΩ.
   2. Fill the microelectrodes with 2M potassium-citrate electrolyte.
   3. Mount the prepared microelectrode on the micromanipulator allowing 1‒2 µm stepping movement and stereotaxic calibration.
   4. Connect the microelectrode to the intracellular amplifier with the reference electrode placed in the back muscles.
7.  After the neuromuscular block, monitor anesthesia depth by checking ECG frequency, and supplement the anesthetic agent so that the rat heart rate does not exceed 400 bpm.

4. Motoneuron tracking and penetration

1. Place the afferent volley recording electrode back on the dorsal surface of the spinal cord, caudally to the location of the recording site, at the level of the L6 segment.
2. Stimulate the MG and LGS nerves with electrical 0.1 ms pulses at a frequency of 3 Hz, and 3T intensity, to activate all the axons of alpha-motoneurons within a selected nerve.
3. Drive the micropipette into a selected patch in the pia with a medio-lateral angle of 15‒20° (with a tip directed laterally).
4. After descending below the surface, calibrate the microelectrode and compensate its capacitance and voltage offset, and continue penetration of the spinal cord when all parameters are stable. An antidromic field potential of the motoneuron pool will be visible at the microelectrode voltage trace while approaching a dedicated motor nucleus during stimulation of the respective nerve.
5. Proceed penetration with the microelectrode at 1‒2 µm steps, and periodically use the buzz function of the intracellular amplifier to clear the electrode tip from any residue.
6. Observe motoneuron penetration which will be characterized by a sudden hyperpolarization of the recorded voltage trace and appearance of an antidromic spike potential.

5. Recording motoneuron membrane and firing properties

1. In a bridge mode of the intracellular amplifier, identify the motoneuron on the basis of the “all-or-nothing” appearance of the antidromic action potential by stimulating respective nerve branches. Record 20 subsequent traces for later averaging.
2.  Implement a strict inclusion criterion to ensure high-quality data: resting membrane potential of at least -50 mV in amplitude; action potential amplitudes greater than 50 mV, with a positive overshoot; membrane potential stable for at least 5 min prior to recording.
3. In a discontinuous current-clamp mode (current switch rate mode 4–8 kHz) of the intracellular amplifier, evoke an orthodromic action potential in a motoneuron using 0.5 ms intracellular depolarizing current pulses. Repeat at least 20 times for offline averaging.
4. Stimulate a motoneuron with 40 short pulses (100 ms) of hyperpolarizing current (1 nA) in order to calculate cell input resistance.
5. Stimulate a motoneuron with 50 ms square-wave pulses at increasing amplitudes to determine the rheobase value as the minimum amplitude of depolarizing current required to elicit a single spike.
6. Inject 500 ms square-wave pulses of depolarizing current, at increasing amplitudes in steps of 0.1–2 nA to evoke rhythmic discharges of motoneurons.

6. Trans-spinal direct current stimulation (tsDCS)

1. While maintaining a stable penetration of the motoneuron, start the polarization procedure by trans-spinal application of direct current. Adjust the current intensity and application time to the experiment design (e.g., 0.1 mA for 15 min).
2. Immediately after switching on the DC, observe the motoneuron membrane potential. Anodal polarization (the active electrode as an anode) should result in depolarization of the membrane potential, while cathodal polarization (the active electrode as a cathode) should evoke an opposite effect. Observe whether a change in the resting membrane potential in response to DC stimulation is constant, which ensures that electrical field intensity is not affected.
3. During continuous current application, repeat steps 5.3‒5.6 in 5 min intervals.
4. Turn off the DC and continue to repeat steps 5.3‒5.6 in 5 min intervals until recordings become unstable or inclusion criteria are compromised.
5. Terminate the experiment and euthanizethe animal using intravenous administration of a lethal dose of pentobarbital sodium (180 mg·kg^-1).

Representative Results

Parameters of action potentials and several membrane properties can be calculated on the basis of intracellular recordings when stable conditions of cell penetration are ensured. Figure 1A presents a typical orthodromic action potential evoked by intracellular stimulation, which meets all criteria for data inclusion (the resting membrane potential of at least -50 mV, and spike amplitude higher than 50 mV, with a positive overshoot). Action potential parameters, such as the spike amplitude, the afterhyperpolarization amplitude or the afterhyperpolarization half-decay time (AHP-HDT) can be measured. A value of the latter parameter in rat motoneurons serves as a reliable criterion for distinguishing between fast and slow motoneurons (AHP-HDT > 20 ms for slow, while AHP-HDT <20 ms for fast motoneurons)¹⁷. Figure 1B shows a cell response to a 100 ms hyperpolarizing current pulse of 1nA, from which both peak and plateau input resistance (IR) of a motoneuron can be determined from the voltage deflection. Figure 1C shows an expanded voltage trace of a rheobasic spike with a clearly marked voltage threshold of the spike, indicating the level of membrane depolarization at which voltage-gated sodium channels are activated to initiate the action potential. All these recordings can be repeated several times during and after tsDCS application, which allows us compare respective parameters as long as the resting membrane potential is stable and other criteria of stimulation and recording protocol are fulfilled.

Several studies have indirectly shown that tsDCS alters motoneuron excitability and firing pattern^9,18. Figure 2 shows examples of intracellular voltage traces from two motoneurons stimulated intracellularly with 500 ms square pulses of depolarizing current before, during and after tsDCS application. Under stable conditions, recordings repeated several minutes one after another can be performed, and motoneuron firing patterns can be reliably compared. Anodal (+) tsDCS was found to act towards increased motoneuron excitability and higher frequencies of rhythmic firing (Figure 2A), while cathodal (-) tsDCS acted towards firing inhibition (Figure 2B). Moreover, the effects of both types of tsDCS outlasted the period of polarization. It is also worth noting that the observed changes in excitability and firing pattern are not merely a result of cell membrane depolarization or hyperpolarization by anodal or cathodal tsDCS, respectively, but display profound alterations not related to the change of a membrane potential, as they persisted despite the fact that this parameter returned to a baseline after the end of polarization.

Finally, it has to be stressed that any deviations from the presented protocol will likely result in a failed experiment, due to deterioration of preparation and/or a profound decline of data reliability. Figure 3 shows examples of recordings when data inclusion criteria were compromised either due to imperfect cell penetration (Figure 3A), neglection to compensate microelectrode resistance and capacitance (Figure 3B) or a spinal cord instability (Figure 3C). It is important that researchers identify such non-optimal recordings, and implement proper corrective actions or disregard such results from the data set.

Figure 1: Parameters of action potentials and membrane properties.
(A) An orthodromic action potential elicited by intracellular stimulation, with indicated basic parameters which can be calculated from this record. AP ampl = action potential amplitude; AHP ampl = afterhyperpolarization amplitude; AHP-HDT = afterhyperpolarization half-decay time. (B) The voltage trace of a membrane response to a short (100 ms) depolarizing current pulse of 1nA intensity, which enables us to calculate input resistance (IR). Notice the peak of a potential deflection (IR Peak) followed by a small decrease and the following plateau phase of the membrane potential (IR plateau). (C) The expanded voltage trace of a rheobasic spike with a dotted horizontal line indicating the spike voltage threshold. Please click here to view a larger version of this figure.

Figure 2: Effects of polarization on motoneuron firing.
(A) Intracellular records from one motoneuron stimulated intracellularly with 7.5 nA for 500 ms, made before (left), during anodal tsDCS (0.1 mA, middle), and 10 min after the end of polarization (right). Note the gradual increase in the motoneuron excitability at the same stimulus intensity. (B) Intracellular records from another motoneuron stimulated intracellularly with 6 nA for 500 ms, made before (left), during cathodal tsDCS (0.1 mA, middle), and 10 min after the end of polarization (right). Note a gradual inhibition of motoneuron firing frequency at the same stimulus intensity. Below recordings, traces of intracellular stimulation current are provided. The calibration bars in the bottom right apply to all presented intracellular recordings. The values of the resting membrane potential are provided to the left of each recording. Frequencies of steady-state firing, calculated from the means of the final three interspike intervals, are given above records. Please click here to view a larger version of this figure.

Figure 3: Examples of suboptimal records as a result of deviations from the experimental protocol.
(A) The antidromic spike recorded from a motoneuron inadequately penetrated. The resting membrane potential is insufficient (-45 mV), and despite an appropriate shape of the spike with all consecutive phases of depolarization, repolarization, and hyperpolarization, its amplitude is too low (41 mV) and without an overshoot. (B) A rheobasic spike generated at an unrealistic voltage threshold (membrane depolarized to +68 mV). This kind of error is usually due to a blocked microelectrode, with uncompensated resistance and capacitance. One can also see that this record is strongly contaminated by 50 Hz electrical noise. (C) A motoneuron rhythmic firing in response to 500 ms depolarizing current, with large fluctuations of a membrane potential, predominantly caused by unstable microelectrode penetration, possibly due to excessive respiratory movements. For all the presented cases the calculated membrane or firing properties would be unreliable. Please click here to view a larger version of this figure.

Discussion

If performed correctly, the surgical part of the described protocol should be completed within approximately three hours. One should take particular care in maintaining stable physiological conditions of an animal during the surgery, in particular body temperature and depth of anesthesia. Apart from obvious ethical considerations, a lack of proper anesthesia can result in excessive limb movements during nerve dissection or laminectomy and lead to damage to the preparation or a premature experiment termination. Upon paralyzing an animal prior to penetrating the spinal cord with a microelectrode, it is crucial to monitor the depth of anesthesia and heart rate and to apply proper ventilation parameters based on animal weight and lung capacity. Any deviations from the desired physiological parameters have to be amended immediately to ensure procedure success. Following the surgery, stable recording conditions should be possible to maintain for at least four hours.

After penetration of a motoneuron, the recording stability is of high importance. It is imperative that a membrane potential remains constant during control recordings, as any fluctuations will significantly influence the rheobase current and a threshold of rhythmic firing. Proper fixation of the vertebral column should provide basic stability, while the goal of a pneumothorax is to decrease the spinal cord movements evoked by respiration. Moreover, one has to be sure that muscle contractions are fully abolished before attempting the penetration and the neuromuscular blocker is administered at regular intervals.

Following a successful penetration antidromic identification of the recorded motoneuron can be performed by stimulation of a respective nerve branch. This is a real advantage of an in vivo preparation, in which motoneuron axons are kept in continuity with the innervated muscles in reference to in vitro intracellular recordings performed on spinal slices, which only recently were possible in adult animals¹⁶, but do not allow identification of recorded motoneuron. However, it is important that the researchers have a clear understanding of the difference between antidromic and orthodromic activation of motoneuron¹⁹ to avoid misinterpretation of the data. It is important to keep the peripheral nerve stimulation as low as possible (less than 0.5 V) to prevent the activation of additional nerves due to the current spread and to pay attention to a constant and short latency of the antidromic spike¹⁹.

Another advantage of the presented technique is that motoneurons can be additionally classified as fast or slow types on the basis of their action potential parameters, namely the AHP-HDT duration¹⁷. Differentiation between motoneurons innervating fast-type and slow-type muscle fibers is crucial in regard to their different contribution to muscle performance during movements. Moreover, fast and slow motoneurons can react differently to the polarization⁹.

To ensure reliable results of polarization one should pay attention to setting proper parameters of tsDCS. Current intensity should, on one hand, provide a desired field density at the selected area to evoke effects on neuronal networks, while on the other hand should be within safety limits for tissue damage²⁰. Size of active and reference electrodes and their placement with regard to a site of recording are also important elements to consider⁴, and the tsDCS duration application time should be sufficient to evoke the desired effects^16,17,22. In this methods paper, the representative results were obtained by application of 100 µA cathodal or anodal polarization for 15 min. Taking into account the electrode shape and diameter, the respective electrical field intensity directly under the electrode was 39.25 µA·mm². However, one should understand that the precise value of the electrical field at the recorded motoneuron site is impossible to pre-determine as motoneuron location with respect to polarization electrode varies, and the e-field density drops significantly with increased depth and decreased electrode size^4,24. Moreover, the orientation of the motoneuron compartments relative to the applied electric field is important for generation of action potentials^22,25,26, and this cannot be predicted for individual cells. In addition, it is highly important to understand that tsDCS effects are not limited to a period of polarization, and that persistent, long-lasting effects are well documented^22,27. Therefore, following even a single, brief polarization session all successive recordings in the same preparation would be performed in post-polarization conditions, which limits the number of possible acute polarization recordings to one per animal.

Additional modifications of the presented procedure can be made to answer specific research questions. This protocol with minimal modifications can be used as a standard for several experimental designs, e.g., when testing various duration and/or amplitudes of applied tsDCS or when comparing short or long-term effects of tsDCS in various pools motoneurons. Use of several genetic disease models (for example SOD1 G93A rat model of amyotrophic lateral sclerosis) or different nerves for antidromic nerve activation (peroneal, tibial, saphenous, etc.) is acceptable. However, one should also be aware of procedure limitations. For example, the use of barbiturates for anesthesia inhibits the activity of persistent inward currents²⁸, while the systemic introduction of specific blockers commonly used in in vitro preparations (e.g., strychnine to block nicotinic receptors) can prove fatal to the animal. It is advisable for researchers to consider these limitations before selecting the proper experimental protocol.

Materials

Name	Company	Catalog Number	Comments
Durgs and solutions	-	-	-
Atropinum sulfuricum	Polfa Warszawa	-	-
Glucose	Merck	346351	-
NaHCO3	Merck	106329	-
Pancuronium Jelfa	PharmaSwiss/Valeant	-	Neuromuscular blocker
Pentobarbital sodium	Biowet Pu?awy Sp. z o.o	-	Main anesthetic agent
Pottasium citrate	Chempur	6100-05-06	-
Tetraspan	Braun	-	HES solution
Surgical equipment	-	-	-
21 Blade	FST	10021-00	Scalpel blade
Cauterizer	FST	18010-00	-
Chest Tubes	Mila	CT1215	-
Dumont #4 Forceps	FST	11241-30	Muscle forceps
Dumont #5 Forceps	FST	11254-20	Dura forceps
Dumont #5F Forceps	FST	11255-20	Nerve forceps
Dumont #5SF Forceps	FST	11252-00	Pia forceps
Forceps	FST	11008-13	Blunt forceps
Forceps	FST	11053-10	Skin forceps
Hemostat	FST	13013-14	-
Rongeur	FST	16021-14	For laminectomy
Scissors	FST	15000-08	Vein scissors
Scissors	FST	15002-08	Dura scissors
Scissors	FST	14184-09	For trachea cut
Scissors	FST	104075-11	Muscle scissors
Scissors	FST	14002-13	Skin scissors
Tracheal tube	-	-	Custom made
Vein catheter	Vygon	1261.201	-
Vessel cannulation forceps	FST	18403-11	-
Vessel clamp	FST	18320-11	For vein clamping
Vessel Dilating Probe	FST	10160-13	For vein dissection
Sugrgical materials	-	-	-
Gel foam	Pfizer	GTIN 00300090315085	Hemostatic agent
Silk suture 4.0	FST	18020-40	-
Silk suture 6.0	FST	18020-60	-
Equipment	-	-	-
Axoclamp 2B	Molecular devices	discontinued	Intracellular amplifier/ new model Axoclamp 900A
CapStar-100 End-tidal CO2 Monitor	CWE	11-10000	Gas analyzer
Grass S-88	A-M Systems	discontinued	Constant current stimulator
Homeothermic Blanket Systems with Flexible Probe	Harvard Apparatus	507222F	Heating system
ISO-DAM8A	WPI	74020	Extracellular amplifier
Microdrive	-	-	Custom made/replacement IVM/Scientifica
P-1000 Microelectrode puller	Sutter Instruments	P-1000	Microelectrode puller
SAR-830/AP Small Animal Ventilator	CWE	12-02100	Respirator
Support frame	-	-	Custom made/replacement lab standard base 51601/Stoelting
Spinal clamps	-	-	Custom made/replacement Rat spinal adaptor 51695/Stoelting
TP-1 DC stimulator	WiNUE	-	tsDCS stimulator
Miscellaneous	-	-	-
1B150-4 glass capillaries	WPI	1B150-4	For microelectrodes production
Cotton wool	-	-	-
flexible tubing	-	-	For respirator and CO2 analyzer connection
MicroFil	WPI	MF28G67-5	For filling micropipettes
Silver wire	-	-	For nerve electrodes