Name: retrograde neuroanatomical tracing of phrenic motor neurons in mice
Uploaded: 2018-02-22T14:00:00
Description: Watch this Scientific Journal Video about Retrograde Neuroanatomical Tracing of Phrenic Motor Neurons in Mice at JoVE.com

We are grateful to Robert Graffin and Pauline Duhant for their technical support.

The experimental protocol was conducted in compliance with the European Communities Council Directives for Animal Experiment (2010/63/EU, 86/609/EEC and 87-848/EEC) and was approved by the Animal Ethics Committee of University of Namur (ethic project n&#176;17-284). Figure 1 depicts the two respective approaches: transdiaphragmatic or transthoracic injections. Use male C57bl/6J mice (n=18), aged from 3 to 4 months in the study.
1. Preparation of CTB Solution
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Q., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Cholera+Toxin+B+Subunit+Shows+Transneuronal+Tracing+after+Injection+in+an+Injured+Sciatic+Nerve.">Cholera Toxin B Subunit Shows Transneuronal Tracing after Injection in an Injured Sciatic Nerve.</a> PLoS One. 10 (12), e0144030 (2015).</li><li>Torgersen, M. L., Skretting, G., van Deurs, B., Sandvig, K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Internalization+of+cholera+toxin+by+different+endocytic+mechanisms.">Internalization of cholera toxin by different endocytic mechanisms.</a> J Cell Sci. 114 (Pt 20), 3737-3747 (2001).</li><li>Chinnapen, D. J., Chinnapen, H., Saslowsky, D., Lencer, W. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rafting+with+cholera+toxin:+endocytosis+and+trafficking+from+plasma+membrane+to+ER.">Rafting with cholera toxin: endocytosis and trafficking from plasma membrane to ER.</a> FEMS Microbiol Lett. 266 (2), 129-137 (2007).</li><li>Fujinaga, Y., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Gangliosides+that+associate+with+lipid+rafts+mediate+transport+of+cholera+and+related+toxins+from+the+plasma+membrane+to+endoplasmic+reticulm.">Gangliosides that associate with lipid rafts mediate transport of cholera and related toxins from the plasma membrane to endoplasmic reticulm.</a> Mol Biol Cell. 14 (12), 4783-4793 (2003).</li><li>Badizadegan, K., Wheeler, H. E., Fujinaga, Y., Lencer, W. I. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Trafficking+of+cholera+toxin-ganglioside+GM1+complex+into+Golgi+and+induction+of+toxicity+depend+on+actin+cytoskeleton.">Trafficking of cholera toxin-ganglioside GM1 complex into Golgi and induction of toxicity depend on actin cytoskeleton.</a> Am J Physiol Cell Physiol. 287 (5), C1453-C1462 (2004).</li><li>Abbott, C. J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Imaging+axonal+transport+in+the+rat+visual+pathway.">Imaging axonal transport in the rat visual pathway.</a> Biomed Opt Express. 4 (2), 364-386 (2013).</li><li>Li, K., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Overexpression+of+the+astrocyte+glutamate+transporter+GLT1+exacerbates+phrenic+motor+neuron+degeneration,+diaphragm+compromise,+and+forelimb+motor+dysfunction+following+cervical+contusion+spinal+cord+injury.">Overexpression of the astrocyte glutamate transporter GLT1 exacerbates phrenic motor neuron degeneration, diaphragm compromise, and forelimb motor dysfunction following cervical contusion spinal cord injury.</a> J Neurosci. 34 (22), 7622-7638 (2014).</li><li>Lepore, A. C. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Intraspinal+cell+transplantation+for+targeting+cervical+ventral+horn+in+amyotrophic+lateral+sclerosis+and+traumatic+spinal+cord+injury.">Intraspinal cell transplantation for targeting cervical ventral horn in amyotrophic lateral sclerosis and traumatic spinal cord injury.</a> J Vis Exp. (55), (2011).</li><li>Llado, J., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Degeneration+of+respiratory+motor+neurons+in+the+SOD1+G93A+transgenic+rat+model+of+ALS.">Degeneration of respiratory motor neurons in the SOD1 G93A transgenic rat model of ALS.</a> Neurobiol Dis. 21 (1), 110-118 (2006).</li><li>Lepore, A. C., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Focal+transplantation-based+astrocyte+replacement+is+neuroprotective+in+a+model+of+motor+neuron+disease.">Focal transplantation-based astrocyte replacement is neuroprotective in a model of motor neuron disease.</a> Nat Neurosci. 11 (11), 1294-1301 (2008).</li><li>Sieck, G. C., Fournier, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Diaphragm+motor+unit+recruitment+during+ventilatory+and+nonventilatory+behaviors.">Diaphragm motor unit recruitment during ventilatory and nonventilatory behaviors.</a> J Appl Physiol. 66 (6), 2539-2545 (1989).</li><li>Janicot, M., Clot, J. 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The protocol described herein can be applied to any strain of adult mice or to any experimental paradigm in which the integrity of the diaphragm-PhMN circuitry should be evaluated. For instance, amyotrophic lateral sclerosis (ALS) and cervical spinal cord injury (cSCI) are conditions associated with PhMN loss, anterograde degeneration of phrenic axons and subsequent respiratory compromise. Animal models of ALS or cSCI mimic histopathological and functional respiratory deficits observed in human diseases. In these models,...

The aim of the study is to present a reliable method to identify phrenic motor neurons (PhMN) on mouse spinal cord sections. Injection of a fluorescent neuroanatomical tracer in the pleural cavity was chosen as the delivery method to reach the phrenic neuromuscular projections onto the diaphragm and use retrograde transport along the phrenic axons to label phrenic cell bodies. Two techniques of intrapleural delivery are described: transdiaphragmatic versus transthoracic.
Phrenic motor neurons are spinal relay cells whose axons converge into phrenic nerves, which ultimately innervate the diaphragm. These are lower motor neurons receiving the inspiratory drive from the bulbar respiratory centers and relaying it to the diaphragm neuro-muscular junctions (NMJ). PhMN are structured into two motor columns, one for each hemicord, running along the mid-cervical spine. In most of mammalian species including humans, the phrenic motor columns spread from levels C3 to C61,2,3. We and others have confirmed that PhMN concentrated in C3-C5 levels in rat and mouse spinal cord4,5,6,7,8. The topographical distribution of phrenic cells is not at random; motor neurons innervating the sternal part of the diaphragm are distributed more densely in the cranial part of the phrenic motor pool (C3), whereas motor neurons innervating the crural part are more caudal (C5)9. Furthermore, PhMN are clustered variously in the ventral horn gray matter. At C3 level, the clusters of phrenic cells lie laterally, then they shift in a ventrolateral direction and are found ventromedially at the most caudal levels10,11.
Given their vital role during inspiration, it is of the utmost importance to accurately identify PhMN in the healthy spinal cord but also follow their fate during pathological conditions, such as degenerative diseases or traumatic injuries of the spinal cord. Since PhMN do not differ morphologically from other cervical motor neurons, identification of PhMN relies on the targeted delivery of neuroanatomical tracers either at the level of primary respiratory centers8, or at the diaphragm NMJ7 or in the phrenic nerve4. The tracer is taken up by the nerve fibers and carried up to the phrenic cell bodies in the cervical spine, where it can be visualized using direct or indirect detection systems. Retrograde or anterograde tracers are commercially available with a broad range of conjugates. Noteworthy, each tracer is endowed with no, low or high abilities for trans synaptic tracing.
In the current study, we chose the beta subunit of the cholera toxin (CTB) functionalized with Alexa Fluor 555 (henceforth referred to as CTB-fluorophore) as a fluorescent label, allowing a direct visualization of PhMN on frozen spinal cord sections. CTB is usually described as a monosynaptic tracer although experimental data tend to show a transneuronal passage12. CTB has the ability to bind the ganglioside GM1 at the plasma membrane of the nerve ending. CTB is internalized via clathrin-dependent or -independent mechanisms and traffics through the trans-Golgi network into the endoplasmic reticulum in a retrograde fashion13,14. The internalization and retrograde transport seem to be dependent on the actin cytoskeleton15,16 as well as on the microtubule network17.
To demonstrate the usefulness of CTB as a retrograde neuroanatomical tracer labeling diaphragm-PhMN circuitry, CTB-fluorophore was delivered intrapleurally. CTB was administered using two techniques: the first one included a laparotomy and multiple transdiaphragmatic injections; the second one, less invasive, used a unique transthoracic injection. Four days later, fluorescently-labeled PhMNs were quantified in the cervical spinal cord from both from healthy and from spinally-injured (C4) animals.

<table>
	<tbody>
		<tr>
			<td>Glass-bead sterilizer Steri 250</td>
			<td>Keller</td>
			<td>31-101</td>
			<td></td>
		</tr>
		<tr>
			<td>Small scissors</td>
			<td>F.S.T.</td>
			<td>14058-00</td>
			<td></td>
		</tr>
		<tr>
			<td>Soft tweezers</td>
			<td>F.S.T.</td>
			<td>11042-08</td>
			<td></td>
		</tr>
		<tr>
			<td>Scalpel blades</td>
			<td>Swann Morton</td>
			<td>No.11 or 15</td>
			<td></td>
		</tr>
		<tr>
			<td>Cholera toxin subunit beta conjugated to Alexa Fluor 555</td>
			<td>Life Technologies</td>
			<td>C22843</td>
			<td>Bring at room temperature before use&#160;</td>
		</tr>
		<tr>
			<td>10ul Hamilton syringue, removable needle</td>
			<td>Sigma-Aldrich</td>
			<td>701RN</td>
			<td></td>
		</tr>
		<tr>
			<td>33-gauge needle for Hamilton syringue, 20mm length, point style 4</td>
			<td>Filter Service</td>
			<td>7803-05</td>
			<td></td>
		</tr>
		<tr>
			<td>500ul insulin syringue MyJector, 27-gauge</td>
			<td>Terumo</td>
			<td>BS05M2713</td>
			<td></td>
		</tr>
		<tr>
			<td>Orientable LED lamp</td>
			<td>V.W.R.</td>
			<td>631-0995</td>
			<td></td>
		</tr>
		<tr>
			<td>Resorbable 4/0 sutures</td>
			<td>S.M.I. AG</td>
			<td>15151519</td>
			<td></td>
		</tr>
		<tr>
			<td>Needle holder</td>
			<td>F.S.T.</td>
			<td>12002-14</td>
			<td></td>
		</tr>
		<tr>
			<td>9mm autoclips</td>
			<td>Bioseb</td>
			<td>205016</td>
			<td></td>
		</tr>
		<tr>
			<td>Autoclip 9mm applier</td>
			<td>Bioseb</td>
			<td>MikRon 9mm</td>
			<td></td>
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	</tbody>
</table>

retrograde neuroanatomical tracing of phrenic motor neurons in mice

Phrenic motor neurons are cervical motor neurons originating from C3 to C6 levels in most mammalian species. Axonal projections converge into phrenic nerves innervating the respiratory diaphragm. In spinal cord slices, phrenic motor neurons cannot be identified from other motor neurons on morphological or biochemical criteria. We provide the description of procedures for visualizing phrenic motor neuron cell bodies in mice, following intrapleural injections of cholera toxin subunit beta (CTB) conjugated to a fluorophore. This fluorescent neuroanatomical tracer has the ability to be caught up at the diaphragm neuromuscular junction, be carried retrogradely along the phrenic axons and reach the phrenic cell bodies. Two methodological approaches of intrapleural CTB delivery are compared: transdiaphragmatic versus transthoracic injections. Both approaches are successful and result in similar number of CTB-labeled phrenic motor neurons. In conclusion, these techniques can be applied to visualize or quantify the phrenic motor neurons in various experimental studies such as those focused on the diaphragm-phrenic circuitry.

Male C57bl/6J mice (n=18), aged from 3 to 4 months were included in the study. At day 0 of the experiment, 8 mice underwent a unilateral C4 contusion, right-sided, according to published protocol7,18. As sham procedure, 10 mice underwent a laminectomy on top of C4 without contusion. At day 3, mice were prepared for the intrapleural injections of CTB-fluorophore according to the two different procedures described above. At day 7, a...

Watch this Scientific Journal Video about Retrograde Neuroanatomical Tracing of Phrenic Motor Neurons in Mice at JoVE.com

Retrograde Neuroanatomical Tracing of Phrenic Motor Neurons in Mice

Here, we describe a protocol for identifying phrenic motor neurons in mice after intrapleural delivery of fluorophore conjugated cholera toxin subunit beta. Two techniques are compared to inject the pleural cavity: transdiaphragmatic versus transthoracic approaches.

Phrenic motor neurons are cervical motor neurons originating from C3 to C6 levels in most mammalian species. Axonal projections converge into phrenic ...

The overall goal of this procedure is to identify phrenic motor neurons in mice using intrapleural delivery of fluorophore-conjugated cholera toxin subunit beta. Two techniques are shown in order to target the pleural cavity, a transdiaphragmatic and a transthoracic approach. This method can help answer key questions in the neuroscience field such as the distribution of the phrenic motor neurons, their morphology as well as the extension of the neural network.The main advantage of the procedure is that the visualization of phrenic motor neurons relies on the accumulation of the fluorescent neuroanatomical tracer that has been retrotransported from diaphragm neuromuscular junctions located in the pleural space avoiding invasive procedure in the central nervous system. Visual demonstration of this method is critical as the steps describing palpation and transthoracic injection are difficult to learn because it is difficult to describe with words the act of feeling with the hand and the dynamic of needle insertion through the thoracic wall. Sterility is essential to the surgery.Use an autoclave or glass bead sterilizer to clean the tools and prepare a clean bench coat. After anesthetizing the animal, check for the absence of reflex and apply ophthalmic ointment. Then shave the ventral skin for the transdiaphragmatic procedure.Alternatively, shave the right-sided thoracic skin for transthoracic procedure. Shave well and broad enough to prevent hair getting into the surgical field. After removing the hair, ensure aseptic conditions by scrubbing the skin with alternating application of 70%ethanol and Betadine.Be careful to scrub from the center of the site toward the periphery. Lastly, ensure that the mouse maintains its body temperature by placing it over a homeothermic pad. For each transdiaphragmatic injection, load 7.5 microliters of CTB solution in a sterile 10 microliter microsyringe with a blunt or short bevel 33 gauge needle.Then with the mouse supine, place a half an inch thick roll of gauze under its neck. Now using a scalpel blade, incise the ventral skin along the midline from the xiphoid process to the umbilical region. To avoid damage to the underlying organs, keep the skin taut and do not apply too much pressure.Next, using small scissors, carefully detach the skin from the abdominal muscles around the incision. This will make it easier to close the incision. Now perform a laparotomy.Open the abdominal cavity using small scissors to make a button-hole incision at the level of the umbilicus. Then incise the abdominal muscles along the white line up to the xiphoid process. To view the abdominal surface of the diaphragm, retract the abdominal muscles using commercial or home-made retractors.Stretch out both sides of the laparotomy and tape down the retractors to the bench coat. Then using an LED lamp, illuminate the abdominal surface of the diaphragm. Next, use soft tweezers to lift up the xiphoid appendix and pull down the lateral lobes of the liver.With small scissors, carefully cut off the suspensory ligament without damaging the gallbladder or the diaphragm. Now inject the three target sites in the right hemi-diaphragm to respectively cover the sternal, the medial and the crural regions. To avoid puncturing the lungs, only penetrate one to two millimeters beyond the diaphragm sheet.Deliver 1/3 of the volume to each of the three sites. To close the laparotomy site, suture the abdominal muscles with interrupted resorbable 4-0 sutures placed every three millimeters. Then staple the skin closed with nine millimeter wound clips placed every five millimeters.Be certain to tighten the staples to prevent the animal from pulling them off, but not so much that they impair healing. For each transthoracic injection, load a sterile 500 microliter insulin syringe with 20 microliters of CTB solution and use a beveled 27 gauge needle. Now position the mouse lateral decubitus and palpate to find the sixth and seventh ribs under the elbow region.While an assistant extends the right fore and hind limbs, insert the syringe under the sixth or the seventh rib tangentially toward the cranium and with the bevel down. Then penetrate three millimeters. Next, elevate the needle gently and the rib should lift upward if the needle is well-positioned in the thoracic cavity.Then stabilize the animal's breathing movements using two fingers to apply light pressure on the chest wall. Now deliver the full bolus of CTB solution in a single injection. Make sure to insert the needle tangentially three to four millimeters in the pleural cavity so that when you raise the needle, the ribs also lift.If you're not sure, remove the needle and do it again. After the injection, immediately position the mouse on the heating pad in a right lateral decubitus orientation. This will help the tracer to spread on the right side of the pleural cavity and it allows for monitoring of the animal's breathing.Next, administer one milliliter of sterile saline solution subcutaneously and provide followup injections if the animal appears dehydrated or listless. Also, subcutaneously administer Buprenorphine twice a day for the first two post-operative days to minimize any potential pain. Three to four-month-old male C57 black 6J mice underwent the described procedure.Three days before, half of these mice underwent a unilateral right-sided C4 spinal contusion or a laminectomy without contusion. One week after the CTB injection, whole spinal cords were harvested for histology. Motor neurons in the ventral horn were identified as a linear column of labeled cells from C3 to C5.These cells were located in the ipsilateral gray matter and looked like motor neurons.In injured animals, a striking loss of labeled neurons was observed at C4 level together with spinal tissue disruption. Next, the CTB-positive phrenic motor neurons were manually counted in every fifth transverse section. The total number of labeled phrenic motor neurons per hemi-cord was slightly lower when using the transdiaphragmatic approach than the transthoracic approach, but this difference was not significant.However, the number of labeled neurons were halved following the C4 injury and this was significant. While attempting this procedure, it is important to remember to consider the animal welfare and to take measures to minimize the pain and discomfort. Following this procedure, other methods like anterograde neuroanatomical tracing or transsynaptic tracing studies can be performed in order to answer additional questions regarding the connectivity of phrenic motor neurons and afferent projection fibers from the rostral ventral respiratory group.Though this method can provide insight into morphological, biochemical, developmental, and functional aspects of phrenic motor neurons, it can also be used to study phrenic circuits in mouse models of amyotrophic lateral sclerosis, spinal muscular atrophy, and spinal cord injury.

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Neuroscience

Assessment of Motor Balance and Coordination in Mice using the Balance Beam

Brain injury, genetic manipulations, and pharmacological treatments can result in alterations of motor skills in mice. Fine motor coordination and balance can be assessed by the beam walking assay. The goal of this test is for the mouse to stay upright and walk across an elevated narrow beam to a safe platform. This test takes place over 3 consecutive days: 2 days of training and 1 day of testing. Performance on the beam is quantified by measuring the time it takes for the mouse to traverse the beam and the number of paw slips that occur in the process. Here we report the protocol used in our laboratory, and representative results from a cohort of C57BL/6 mice. This task is particularly useful for detecting subtle deficits in motor skills and balance that may not be detected by other motor tests, such as the Rotarod.

Deficits in fine motor coordination can be assessed with the balance beam test. Performance on the beam is quantified by the speed at which the beam is traversed and the number of times the mouse slips on the beam.

Brain injury, genetic manipulations, and pharmacological treatments can result in alterations of motor skills in mice.  Fine motor coordination and ...

Multiple-mouse Neuroanatomical Magnetic Resonance Imaging

The field of mouse phenotyping with magnetic resonance imaging (MRI) is rapidly growing, motivated by the need for improved tools for characterizing and evaluating mouse models of human disease. MRI is an excellent modality for investigating genetically altered animals. It is capable of whole brain coverage, can be used in vivo, and provides multiple contrast mechanisms for investigating different aspects of neuranatomy and physiology. The advent of high-field scanners along with the ability to scan multiple mice simultaneously allows for rapid phenotyping of novel mutations.
Effective mouse MRI studies require attention to many aspects of experiment design. In this article, we will describe general methods to acquire quality images for mouse phenotyping using a system that images mice concurrently in shielded transmit/receive radio frequency (RF) coils in a common magnet (Bock et al., 2003). We focus particularly on anatomical phenotyping, an important and accessible application that has shown a high potential for impact in many mouse models at our imaging centre. Before we can provide the detailed steps to acquire such images, there are important practical considerations for both in vivo brain imaging (Dazai et al., 2004) and ex vivo brain imaging (Spring et al., 2007) that should be noted. These are discussed below.

Magnetic resonance imaging (MRI) has become an increasingly popular tool for examining the phenotype of genetically altered mice. This article illustrates the methods necessary to achieve high-throughput phenotyping of genetically altered mice using multiple-mouse MRI.

The field of mouse phenotyping with magnetic resonance imaging (MRI) is rapidly growing, motivated by the need for improved tools for characterizing and ...

Measuring Motor Coordination in Mice

Mice are increasingly being used in behavioral neuroscience, largely replacing rats as the behaviorist's animal of choice. Before aspects of behavior such as emotionality or cognition can be assessed, however, it is vital to determine whether the motor capabilities of e.g. a mutant or lesioned mouse allow such an assessment. Performance on a maze task requiring strength and coordination, such as the Morris water maze, might well be impaired in a mouse by motor, rather than cognitive, impairments, so it is essential to selectively dissect the latter from the former. For example, sensorimotor impairments caused by NMDA antagonists have been shown to impair water maze performance2. 
Motor coordination has traditionally been assessed in mice and rats by the rotarod test, in which the animal is placed on a horizontal rod that rotates about its long axis; the animal must walk forwards to remain upright and not fall off. Both set speed and accelerating versions of the rotarod are available.
The other three tests described in this article (horizontal bar, static rods and parallel bars) all measure coordination on static apparatus. The horizontal bar also requires strength for adequate performance, particularly of the forelimbs as the mouse initially grips the bar just with the front paws. 
Adult rats do not perform well on tests such as the static rods and parallel bars (personal observations); they appear less well coordinated than mice. I have only tested male rats, however, and male mice seem generally less well coordinated than females. Mice appear to have a higher strength:weight ratio than rats; the Latin name, Mus musculus, seems entirely appropriate. The rotarod, the variations of the foot fault test12 or the Catwalk (Noldus)15 apparatus are generally used to assess motor coordination in rats.

Protocols are presented for two established motor coordination tasks, the accelerating rotarod and horizontal bar, also two tests developed in Oxford recently, the static rods and parallel bars. These tests can detect motor impairments potentially of interest in their own right, as well as being possible variables in tests of other areas of behavior.

Mice are increasingly being used in behavioral neuroscience, largely replacing rats as the behaviorist's animal of choice. Before aspects of behavior such ...

Retrograde Fluorescent Labeling Allows for Targeted Extracellular Single-unit Recording from Identified Neurons In vivo

The overall goal of this method is to record single-unit responses from an identified population of neurons. In vivo electrophysiological recordings from individual neurons are critical for understanding how neural circuits function under natural conditions. Traditionally, these recordings have been performed 'blind', meaning the identity of the recorded cell is unknown at the start of the recording. Cellular identity can be subsequently determined via intracellular1, juxtacellular2 or loose-patch3 iontophoresis of dye, but these recordings cannot be pre-targeted to specific neurons in regions with functionally heterogeneous cell types. Fluorescent proteins can be expressed in a cell-type specific manner permitting visually-guided single-cell electrophysiology4-6. However, there are many model systems for which these genetic tools are not available. Even in genetically accessible model systems, the desired promoter may be unknown or genetically homogenous neurons may have varying projection patterns. Similarly, viral vectors have been used to label specific subgroups of projection neurons7, but use of this method is limited by toxicity and lack of trans-synaptic specificity. Thus, additional techniques that offer specific pre-visualization to record from identified single neurons in vivo are needed. Pre-visualization of the target neuron is particularly useful for challenging recording conditions, for which classical single-cell recordings are often prohibitively difficult8-11. The novel technique described in this paper uses retrograde transport of a fluorescent dye applied using tungsten needles to rapidly and selectively label a specific subset of cells within a particular brain region based on their unique axonal projections, thereby providing a visual cue to obtain targeted electrophysiological recordings from identified neurons in an intact circuit within a vertebrate CNS.
The most significant novel advancement of our method is the use of fluorescent labeling to target specific cell types in a non-genetically accessible model system. Weakly electric fish are an excellent model system for studying neural circuits in awake, behaving animals12. We utilized this technique to study sensory processing by "small cells" in the anterior exterolateral nucleus (ELa) of weakly electric mormyrid fish. "Small cells" are hypothesized to be time comparator neurons important for detecting submillisecond differences in the arrival times of presynaptic spikes13. However, anatomical features such as dense myelin, engulfing synapses, and small cell bodies have made it extremely difficult to record from these cells using traditional methods11, 14. Here we demonstrate that our novel method selectively labels these cells in 28% of preparations, allowing for reliable, robust recordings and characterization of responses to electrosensory stimulation.

Retrograde Fluorescent Labeling Allows for Targeted Extracellular Single-unit Recording from Identified Neurons In vivo

Retrograde transport of fluorescent dye labels a sub-population of neurons based on anatomical projection. Labeled axons can be visually targeted in vivo, permitting extracellular recording from identified axons. This technique facilitates recording when neurons cannot be labeled through genetic manipulation or are difficult to isolate using 'blind' in vivo approaches.

The overall goal of this method is to record single-unit responses from an identified population of neurons. In vivo electrophysiological recordings from ...

Extracellularly Identifying Motor Neurons for a Muscle Motor Pool in Aplysia californica

In animals with large identified neurons (e.g. mollusks), analysis of motor pools is done using intracellular techniques1,2,3,4. Recently, we developed a technique to extracellularly stimulate and record individual neurons in Aplysia californica5. We now describe a protocol for using this technique to uniquely identify and characterize motor neurons within a motor pool.
This extracellular technique has advantages. First, extracellular electrodes can stimulate and record neurons through the sheath5, so it does not need to be removed. Thus, neurons will be healthier in extracellular experiments than in intracellular ones. Second, if ganglia are rotated by appropriate pinning of the sheath, extracellular electrodes can access neurons on both sides of the ganglion, which makes it easier and more efficient to identify multiple neurons in the same preparation. Third, extracellular electrodes do not need to penetrate cells, and thus can be easily moved back and forth among neurons, causing less damage to them. This is especially useful when one tries to record multiple neurons during repeating motor patterns that may only persist for minutes. Fourth, extracellular electrodes are more flexible than intracellular ones during muscle movements. Intracellular electrodes may pull out and damage neurons during muscle contractions. In contrast, since extracellular electrodes are gently pressed onto the sheath above neurons, they usually stay above the same neuron during muscle contractions, and thus can be used in more intact preparations.
To uniquely identify motor neurons for a motor pool (in particular, the I1/I3 muscle in Aplysia) using extracellular electrodes, one can use features that do not require intracellular measurements as criteria: soma size and location, axonal projection, and muscle innervation4,6,7. For the particular motor pool used to illustrate the technique, we recorded from buccal nerves 2 and 3 to measure axonal projections, and measured the contraction forces of the I1/I3 muscle to determine the pattern of muscle innervation for the individual motor neurons.
We demonstrate the complete process of first identifying motor neurons using muscle innervation, then characterizing their timing during motor patterns, creating a simplified diagnostic method for rapid identification. The simplified and more rapid diagnostic method is superior for more intact preparations, e.g. in the suspended buccal mass preparation8 or in vivo9. This process can also be applied in other motor pools10,11,12 in Aplysia or in other animal systems2,3,13,14.

Extracellularly Identifying Motor Neurons for a Muscle Motor Pool in Aplysia californica

In animals with large identified neurons (e.g. mollusks), analysis of motor pools is done using intracellular techniques1,2,3,4. Recently, we developed a technique to extracellularly stimulate and record individual neurons in Aplysia californica5. We now describe a protocol for using this technique to uniquely identify and characterize motor neurons within a motor pool.

In animals with large  identified neurons (e.g. mollusks), analysis of motor pools is done using  intracellular techniques1,2,3,4. Recently,  we developed ...

Functional and Morphological Assessment of Diaphragm Innervation by Phrenic Motor Neurons

This protocol specifically focuses on tools for assessing phrenic motor neuron (PhMN) innervation of the diaphragm at both the electrophysiological and morphological levels. Compound muscle action potential (CMAP) recording following phrenic nerve stimulation can be used to quantitatively assess functional diaphragm innervation by PhMNs of the cervical spinal cord in vivo in anesthetized rats and mice. Because CMAPs represent simultaneous recording of all myofibers of the whole hemi-diaphragm, it is useful to also examine the phenotypes of individual motor axons and myofibers at the diaphragm NMJ in order to track disease- and therapy-relevant morphological changes such as partial and complete denervation, regenerative sprouting and reinnervation. This can be accomplished via whole-mount immunohistochemistry (IHC) of the diaphragm, followed by detailed morphological assessment of individual NMJs throughout the muscle. Combining CMAPs and NMJ analysis provides a powerful approach for quantitatively studying diaphragmatic innervation in rodent models of CNS and PNS disease.

Compound muscle action potential recording quantitatively assesses functional diaphragm innervation by phrenic motor neurons. Whole-mount diaphragm immunohistochemistry assesses morphological innervation at individual neuromuscular junctions. The goal of this protocol is to demonstrate how these two powerful methodologies can be used in various rodent models of spinal cord disease.

This protocol specifically focuses on tools for assessing phrenic motor neuron (PhMN) innervation of the diaphragm at both the electrophysiological and ...

Laser-guided Neuronal Tracing In Brain Explants

We present a technique which combines an in vitro tracer injection protocol, which uses a series of electrical and pressure pulses to increase dye uptake through electroporation in brain explants with targeted laser illumination and matching filter goggles during the procedure. The described technique of in vitro electroporation by itself yields relatively good visual control for targetting certain areas of the brain. By combining it with laser excitation of fluorescent genetic markers and their read-out through band-passing filter goggles, which can pick up the emissions of the genetically labeled cells/nuclei and the fluorescent tracing dye, a researcher can substantially increase the accuracy of injections by finding the area of interest and controlling for the dye-spread/uptake in the injection area much more efficiently. In addition, the laser illumination technique allows to study the functionality of a given neurocircuit by providing information about the type of neurons projecting to a certain area in cases where the GFP expression is linked to the type of transmitter expressed by a subpopulation of neurons.

We describe a technique to label neurons and their processes via anterograde or retrograde tracer injections into brain nuclei using an in vitro preparation. We modified an existing method of in vitro tracer electroporation by taking advantage of fluorescently labeled mouse mutants and basic optical equipment in order to increase labeling accuracy.

We present a technique which combines an in vitro tracer injection protocol, which uses a series of electrical and pressure pulses to increase dye uptake ...

Visualization of the Axonal Projection Pattern of Embryonic Motor Neurons in Drosophila

The establishment of functional neuromuscular circuits relies on precise connections between developing motor axons and target muscles. Motor neurons extend growth cones to navigate along specific pathways by responding to a large number of axon guidance cues that emanate from the surrounding extracellular environment. Growth cone target recognition also plays a critical role in neuromuscular specificity. This work presents a standard immunohistochemistry protocol to visualize motor neuron projections of late stage-16 Drosophila melanogaster embryos. This protocol includes a few key steps, including a genotyping procedure, to sort the desired mutant embryos; an immunostaining procedure, to tag embryos with fasciclin II (FasII) antibody; and a dissection procedure, to generate filleted preparations from fixed embryos. Motor axon projections and muscle patterns in the periphery are much better visualized in flat preparations of filleted embryos than in whole-mount embryos. Therefore, the filleted preparation of fixed embryos stained with FasII antibody provides a powerful tool to characterize the genes required for motor axon pathfinding and target recognition, and it can also be applied to both loss-of-function and gain-of-function genetic screens.

Visualization of the Axonal Projection Pattern of Embryonic Motor Neurons in Drosophila

This work details a standard immunohistochemistry method to visualize motor neuron projections of late stage-16 Drosophila melanogaster embryos. The filleted preparation of fixed embryos stained with FasII antibody provides a powerful tool to characterize the genes required for motor axon pathfinding and target recognition during neural development.

The establishment of functional neuromuscular circuits relies on precise connections between developing motor axons and target muscles. Motor neurons ...

In Vivo Two-photon Imaging of Cortical Neurons in Neonatal Mice

Two-photon imaging is a powerful tool for the in vivo analysis of neuronal circuits in the mammalian brain. However, a limited number of in vivo imaging methods exist for examining the brain tissue of live newborn mammals. Herein we summarize a protocol for imaging individual cortical neurons in living neonatal mice. This protocol includes the following two methodologies: (1) the Supernova system for sparse and bright labeling of cortical neurons in the developing brain, and (2) a surgical procedure for the fragile neonatal skull. This protocol allows the observation of temporal changes of individual cortical neurites during neonatal stages with a high signal-to-noise ratio. Labeled cell-specific gene silencing and knockout can also be achieved by combining the Supernova with RNA interference and CRISPR/Cas9 gene editing systems. This protocol can, thus, be used for analyzing the developmental dynamics of cortical neurons, molecular mechanisms that control the neuronal dynamics, and changes in neuronal dynamics in disease models.

In Vivo Two-photon Imaging of Cortical Neurons in Neonatal Mice

We present an in vivo two-photon imaging protocol for imaging the cerebral cortex of neonatal mice. This method is suitable for analyzing the developmental dynamics of cortical neurons, the molecular mechanisms that control the neuronal dynamics, and the changes in neuronal dynamics in disease models.

Two-photon imaging is a powerful tool for the in vivo analysis of neuronal circuits in the mammalian brain. However, a limited number of in vivo imaging ...

Isolation and Culture of Oculomotor, Trochlear, and Spinal Motor Neurons from Prenatal Islmn:GFP Transgenic Mice

Oculomotor neurons (CN3s) and trochlear neurons (CN4s) exhibit remarkable resistance to degenerative motor neuron diseases such as amyotrophic lateral sclerosis (ALS) when compared to spinal motor neurons (SMNs). The ability to isolate and culture primary mouse CN3s, CN4s, and SMNs would provide an approach to study mechanisms underlying this selective vulnerability. To date, most protocols use heterogeneous cell cultures, which can confound the interpretation of experimental outcomes. To minimize the problems associated with mixed-cell populations, pure cultures are indispensable. Here, the first protocol describes in detail how to efficiently purify and cultivate CN3s/CN4s alongside SMNs counterparts from the same embryos using embryonic day 11.5 (E11.5) IslMN:GFP transgenic mouse embryos. The protocol provides details on the tissue dissection and dissociation, FACS-based cell isolation, and in vitro cultivation of cells from CN3/CN4 and SMN nuclei. This protocol adds a novel in vitro CN3/CN4 culture system to existing protocols and simultaneously provides a pure species- and age-matched SMN culture for comparison. Analyses focusing on the morphological, cellular, molecular, and electrophysiological characteristics of motor neurons are feasible in this culture system. This protocol will enable research into the mechanisms that define motor neuron development, selective vulnerability, and disease.

Isolation and Culture of Oculomotor, Trochlear, and Spinal Motor Neurons from Prenatal Islmn:GFP Transgenic Mice

This work presents a protocol to yield homogeneous cell cultures of primary oculomotor, trochlear, and spinal motor neurons. These cultures can be used for comparative analyses of the morphological, cellular, molecular, and electrophysiological characteristics of ocular and spinal motor neurons.

Oculomotor neurons (CN3s) and trochlear neurons (CN4s) exhibit remarkable resistance to degenerative motor neuron diseases such as amyotrophic lateral ...