PDK is supported by a CIHR operating grant (MOP 86523)

1. Surgical Technique
<ol>
 <li>Experiments should be carried out using aseptic technique and following the animal use protocols of your specific institution. Instruments and materials (solutions, test substances, tracers, needles, etc.) coming into contact with living tissue must be sterile to prevent infection and adverse impacts on animal welfare and potential negative impacts on the study.</li>
</ol>
2. Anesthesia
<ol>
 <li>Rats will be anaesthetized using a veterinary isoflurane vaporizer system. Use medical grade oxygen at a rate of 0.8 L/min to vaporize the isoflurane gas. Place the animal in the attached anesthesia box and dial in an isoflurane concentration of 4% until the breathing has slowed and the animal is sedate.</li>
 <li>Next, switch the gas flow to the gas mask attachment for the stereotaxic frame and place the animal in the stereotaxic apparatus. Turn the isoflurane 
	 concentration down to 2% and monitor anesthesia. Larger animals (&gt;300g) may require a higher concentration of isoflurane. Anesthesia should be monitored 
	 during surgery and isoflurane dosage adjusted accordingly. Depth and rate of breathing should be constantly evaluated, and toe pinch evaluation (every 5 min) 
	 for the absence of deep pain should be performed.</li>
 <li>Once the surgery is complete, turn off the isoflurane and allow the animal to breath oxygen for several minutes prior to removal from the stereotaxic device. Body temperature should be maintained by covering the animal with a surgical blanket and/or using a regulated heating blanket during surgery.</li>
</ol>
3. Surgical Approach
<ol>
 <li>Wet the fur on top of the head with 70% ethanol to make the fur easier to cut. Remove the fur from between the eyes using a clipper or sharp scissors. Clean the incision area three times with alternating applications of iodine detergent solution (Proviodine, Betadine, etc) followed by 70% ethanol. Maintain the cornea moist throughout the surgery by applying ophthalmic eye ointment (Tears Naturale P.M.) to the cornea. Spread the ointment over the surface of the cornea by manually opening and closing the eyelids.</li>
 <li>Using a No. 11 blade, make an incision along the midline of the head from approximately 0.5 cm in front of the eyes to 1 cm behind the eyes. Retract the flap 
	 of skin over the eye using forceps and gently tease away the underlying connective tissue with the back of the scalpel. Then, retract the flap of skin laterally 
	 and downward and hold in place with a surgical retractor that can be taped to the base of the stereotaxic instrument.</li>
 <li>Make an incision along the superior rim of the orbital bone while pulling on the overlying fascia with sharp forceps. This will withdraw the fascia overlying the orbit of the eye. The rim of the orbital bone can be clearly demarcated by using forceps to push down on the fascia overlying the orbit. Using a cautery device or scalpel, continue the incision backwards towards the posterior limit of the orbit of the eye. Use the bone of the superior orbit as a guide for the incision. Next continue the incision forwards toward the anterior limit of the orbit. The incision of the superior orbit is best done using a small cautery device in order to prevent bleeding from the underlying vessels that communicate with the venous sinuses.</li>
 <li>If bleeding occurs several steps can be taken. Firstly, apply pressure using sterile surgical swabs or cotton swabs. If bleeding continues, apply cold, sterile phosphate buffer saline (PBS) to the area using a dropper, and maintain pressure. Minor bleeding will stop after several seconds using this procedure. If bleeding persists, apply traction to the tissue with the surgical swab or cotton swab in order to identify the source of bleeding and rapidly cauterize the compromised vessel. After the bleeding is contained, use cold sterile PBS to clean the surgical area of blood. The surgical area should be periodically cleaned in this fashion in order to better visualize structures in the orbit of the eye.</li>
 <li>Once the incision has been cleaned, use forceps and the back of the scalpel to clean the connective tissue at the back of the eye that overlies the orbital contents. The back of the No. 11 scalpel works well as the tip is fine. This will open up deeper portions of the orbital cavity providing a larger surgical window to work in. Use Dumont #7b sharp-curved-serrated forceps when working in the eye as their curvature and fine tips are ideal for manipulating structures in the orbit. Furthermore, the serrations help with gripping structures.</li>
 <li>Next, remove the connective tissue that surrounds the ophthalmic division of the trigeminal nerve which sits near the midline, and remove the nerve using forceps. This step is not necessary, however removing the nerve will provide a larger window of access to the optic nerve later on.</li>
 <li>Following removal of the nerve, use forceps to retract the blood vessel below and completely cauterize the vessel. This step is also not necessary, however cautery of the vessel allows it to be moved anteriorly, thereby creating a larger window once the optic nerve is reached.</li>
 <li>Use a sharp pair of forceps, or forceps that have had their tips bent inward to carefully pick and remove the thin layer of connective tissue over the extraocular muscles and lacrimal gland. Retract the extra ocular muscles from anterior to posterior of the orbit. Grip the proximal part of the muscle with a pair of forceps and use a second pair of curved serrated eye dressing forceps to apply outward traction on the muscle. Make sure that the eye dressing forceps are oriented in the same direction as the muscle when pulling in order to prevent tearing. Remove the most anterior muscle of the orbit (Superior Oblique) in this manner. The muscle will be liberated from deep within the orbit and the remaining length can be retracted in order to rotate the eye outward.</li>
 <li>Repeat Step 3.8 with the next muscle (Medial Rectus) that is located between the lobes of the lacrimal gland and Harderian Gland near the midline of the dorsal surface of the eye. Tape down the retractor to maintain traction on the muscles.</li>
 <li>Gently remove any remaining connective tissue over the surface of the lacrimal gland and lift the gland upward using forceps. Do not compress or squeeze the gland. In order to retract the gland, only a single vessel at the posterior pole needs to be cauterized. Lift the posterior end of the gland upward, and then cauterize the vessel.</li>
 <li>Next, flap the lacrimal gland forward to open the posterior part of the orbit and allow unimpeded access to the muscles that overlie the optic nerve. Keep the area constantly moist using sterile PBS, and drying with surgical swabs or cotton swabs.</li>
 <li>Using sharp forceps once again remove the thin connective tissue that surrounds the muscles of the posterior orbit (Levator Palpebrae Superioris and Superior Rectus) and separate the underlying bundles of muscle. Retract the muscles independently or in unison, using the curved serrated eye dressing forceps, once again, pulling in line with the muscle fibers. Attach the remaining muscle lengths to the retractor along with the muscles that were retracted in Steps 3.8 and 3.9 and tape down the retractor to apply traction. A total of 4 muscles will now be attached to the retractor. This will rotate the eye forward and outward in order to reveal the fat containing sheath that surrounds the optic nerve.</li>
</ol>
4. Accessing the Optic Nerve
<ol>
 <li>Use sharp forceps (Fine tip Dumont) to pull upward on the connective tissue that surrounds the fatty sheath of the optic nerve. Make a longitudinal cut using small Vannas spring scissors. Expand the cut as necessary. The fat contained by the sheath will begin to bulge out once the cut is made. Next remove the flaps of connective tissue by carefully pulling upward from the edge and cutting off the crescent shaped flaps of tissue.</li>
 <li>Remove the fat overlying the optic nerve by using forceps to pull on the fat, while cutting with the Vannas spring scissors. Keep the area clean using sterile PBS and surgical swabs to clean the small amounts of blood that arise from the removal of the tissue.</li>
 <li>The optic nerve is now visible. In order to access the nerve, the meningeal sheath that surrounds the nerve must be removed without damaging the ophthalmic artery which feeds the inner retina. Examine the vascular pattern of the meningeal sheath using forceps to gently rotate the sheath. Look for an area devoid of blood vessels, and permitting a longitudinal cut to be made in the meningeal sheath.</li>
 <li>Using fine tip Dumont forceps, pinch the dura and pull upward. Near the base of the triangular shaped wedge of dura that is created, use the Vannas spring scissors to make a small incision in the sheath. Insert the lower blade of the scissors into the incision and cut the sheath parallel to the direction of the optic nerve, careful not to damage the vasculature with lateral cuts. Use the forceps and scissors to drape the dura to either side of the optic nerve.</li>
 <li>The only remaining covering of the nerve is the arachnoid membrane. It is very thin and transparent. In order to determine if the membrane is still present, use sharp forceps to pinch the surface of the nerve. It the arachnoid is present, pinch the membrane and pull upward to create a triangular wedge of tissue. Make a small incision with the tip of the scissors similar to Step 4.4. Then, insert the lower blade of the scissors and make a longitudinal cut in the arachnoid. Next, use your scissors and forceps to drape the arachnoid to either side of the optic nerve.</li>
 <li>Using a micro-surgical hook, elevate the optic nerve out of the meningeal sheath. Pass the tip of the hook around the outer edge of the nerve and make sure that the hook stays in contact with the nerve so that you do not catch the meningeal membranes with the hook and accidentally transect them. Gently lift the optic nerve out of the meningeal sheath and completely transect the nerve behind the point supported by the hook. The transected optic nerve stump will now have a free end, allowing for the removal of the hook.</li>
 <li>If the RGCs are going to be retrogradely labeled in order to quantify survival, place a small piece of gelfoam soaked in 3% Fluorogold (or another retrograde tracer) over the transected optic nerve stump (see <a href="https://www.jove.com/details.php?id=2261">JoVE protocol 2261</a>).</li>
</ol>
5. Closing and Recovery
<ol>
 <li>Relieve the traction on the extra ocular muscles and return the eye to a neutral position. While doing so, make sure to push the gelfoam down into the orbit of the eye to ensure that as the eye is rotated back into place, the gelfoam remains around the optic nerve stump. Return the lacrimal gland and extra ocular muscles to their natural positions.</li>
 <li>Return the flap of skin to the midline and suture the wound. Apply ophthalmic eye ointment to both eyes. Then, turn off the isoflurane source and allow the animal to breath oxygen for several minutes. Place the animal in a heated cage or a cage underneath a heat lamp to recover. Do not place any bedding in the recovery cage to eliminate the chance of aspirating bedding during recovery.</li>
 <li>Animals should be housed independently after surgery. Post surgical analgesics should be administered according to the guidelines of your animal care authorities, and animals should be carefully monitored after surgery.</li>
</ol>
6. Representative Results:
Transection of the optic nerve results in the loss of 90% of injured RGCs within 14 days postaxotomy 9-11. The main mechanism of RGC death is apoptosis 9, 12. The normal density of RGCs is approximately 2500 cells/mm2. Epifluorescence or confocal imaging can be used to visualize retrogradely labeled RGCs after axotomy. RGC apoptosis is delayed by approximately 4 days after axotomy, leaving a time window for experimental manipulations. At 1 day after axotomy and retrograde labeling with Fluorogold, RGC cell bodies in the ganglion cell layer of the retina and axon fascicles in the nerve fiber layer of the retina are clearly visible when imaging a flatmounted preparation (Fig 1a). By 14 days after axotomy, the majority of RGCs have died, and a few remaining RGCs are interspersed between retinal microglia (Fig 1b). When RGCs undergo apoptosis, microglia phagocytose the dead cells and as a result become transcellularly labeled with the fluorescent tracer that was used to label the RGCs 13, 14. The appearance of the tracer in the phagosomes of microglia is different from that in surviving RGCs. Microglia contain the tracer in highly concentrated and extremely bright phagosomes that are relatively large and scattered throughout their cytoplasm (Fig 1c). RGCs have a more diffuse pattern of staining (Fig 1c) with small punctate vesicles that have been retrogradely transported down their axons filing the cell cytoplasm. These vesicles are much smaller and have less intense fluorescence allowing one to differentiate surviving RGCs from microglia. Furthermore, microglia have much smaller cell bodies and tend to have a stellate or amoeboid morphology as opposed to RGCs that have relatively large and rounded cell bodies. The dendritic trees of RGCs can also help differentiate them from the short bright processes of microglia, when quantifying cell survival. Cell survival can be quantified in different regions of the retina and the density (cells/mm2) can be extrapolated from the area of the corresponding micrographs, since RGCs are found in a monolayer within the ganglion cell layer.
<img src="/files/ftp_upload/2241/2241fig1.jpg" alt="Figure 1" /> Figure 1. Epifluorescence micrographs of Fluorogold labeled RGCs after axotomy and application of the tracer to the optic nerve stump. (A) 1 day after axotomy RGCs and their axon fascicles are labeled with the tracer in a fine punctate manner. (B) By 14 days after axotomy, 90% of RGCs have died and brightly labeled microglia that have phagocytosed dead cells are also labeled with the tracer. (C) Higher magnification illustrating the difference between RGCs and microglia (red arrowheads) at 14 days postaxotomy. Scale bar in A and B is 50 &mu;m. Scale bar in C is 25 &mu;m.

<ol>
	<li>Bahr, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Live+or+let+die+-+retinal+ganglion+cell+death+and+survival+during+development+and+in+the+lesioned+adult+CNS.">Live or let die - retinal ganglion cell death and survival during development and in the lesioned adult CNS.</a> Trends Neurosci. 23, 483-4890 (2000).</li><li>Isenmann, S., Kretz, A., Cellerino, A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Molecular+determinants+of+retinal+ganglion+cell+development,+survival,+and+regeneration.">Molecular determinants of retinal ganglion cell development, survival, and regeneration.</a> Prog Retin Eye Res. 22, 483-543 (2003).</li><li>Koeberle, P. D., Bahr, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Growth+and+guidance+cues+for+regenerating+axons:+where+have+they+gone.">Growth and guidance cues for regenerating axons: where have they gone.</a> J Neurobiol. 59, 162-180 (2004).</li><li>Weishaupt, J. H., Bahr, M. <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+axotomized+retinal+ganglion+cells+as+a+model+for+neuronal+apoptosis+in+the+central+nervous+system+-+molecular+death+and+survival+pathways.">Degeneration of axotomized retinal ganglion cells as a model for neuronal apoptosis in the central nervous system - molecular death and survival pathways.</a> Restor. Neurol. Neurosci. 19, 1-2 (2001).</li><li>Valenzuela, G. a. r. c. i. a., E, S. C. S. h. a. r. m. a. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rescue+of+retinal+ganglion+cells+from+axotomy-induced+apoptosis+through+TRK+oncogene+transfer.">Rescue of retinal ganglion cells from axotomy-induced apoptosis through TRK oncogene transfer.</a> Neuroreport. 9, 3165-3170 (1998).</li><li>Kugler, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Transduction+of+axotomized+retinal+ganglion+cells+by+adenoviral+vector+administration+at+the+optic+nerve+stump:+an+in+vivo+model+system+for+the+inhibition+of+neuronal+apoptotic+cell+death.">Transduction of axotomized retinal ganglion cells by adenoviral vector administration at the optic nerve stump: an in vivo model system for the inhibition of neuronal apoptotic cell death.</a> Gene Ther. 6, 1759-1767 (1999).</li><li>Lingor, P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Down-regulation+of+apoptosis+mediators+by+RNAi+inhibits+axotomy-induced+retinal+ganglion+cell+death+in+vivo.">Down-regulation of apoptosis mediators by RNAi inhibits axotomy-induced retinal ganglion cell death in vivo.</a> Brain. 128, 550-558 (2005).</li><li>Koeberle, P. D., Gauldie, J., Ball, A. K. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Effects+of+adenoviral-mediated+gene+transfer+of+interleukin-10,+interleukin-4,+and+transforming+growth+factor-beta+on+the+survival+of+axotomized+retinal+ganglion+cells.">Effects of adenoviral-mediated gene transfer of interleukin-10, interleukin-4, and transforming growth factor-beta on the survival of axotomized retinal ganglion cells.</a> Neuroscience. 125, 903-920 (2004).</li><li>Berkelaar, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Axotomy+results+in+delayed+death+and+apoptosis+of+retinal+ganglion+cells+in+adult+rats.">Axotomy results in delayed death and apoptosis of retinal ganglion cells in adult rats.</a> J Neurosci. 14, 4368-4374 (1994).</li><li>Villegas-Perez, M. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Influences+of+peripheral+nerve+grafts+on+the+survival+and+regrowth+of+axotomized+retinal+ganglion+cells+in+adult+rats.">Influences of peripheral nerve grafts on the survival and regrowth of axotomized retinal ganglion cells in adult rats.</a> J Neurosci. 8, 265-280 (1988).</li><li>Villegas-Perez, M. P. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Rapid+and+protracted+phases+of+retinal+ganglion+cell+loss+follow+axotomy+in+the+optic+nerve+of+adult+rats.">Rapid and protracted phases of retinal ganglion cell loss follow axotomy in the optic nerve of adult rats.</a> J Neurobiol. 24, 23-36 (1993).</li><li>Quigley, H. A. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Retinal+ganglion+cell+death+in+experimental+glaucoma+and+after+axotomy+occurs+by+apoptosis.">Retinal ganglion cell death in experimental glaucoma and after axotomy occurs by apoptosis.</a> Invest Ophthalmol Vis Sci. 36, 774-786 (1995).</li><li>Thanos, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Specific+transcellular+carbocyanine-labelling+of+rat+retinal+microglia+during+injury-induced+neuronal+degeneration.">Specific transcellular carbocyanine-labelling of rat retinal microglia during injury-induced neuronal degeneration.</a> Neurosci Lett. 127, 108-1012 (1991).</li><li>Thanos, S. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Specific+transcellular+staining+of+microglia+in+the+adult+rat+after+traumatic+degeneration+of+carbocyanine-filled+retinal+ganglion+cells.">Specific transcellular staining of microglia in the adult rat after traumatic degeneration of carbocyanine-filled retinal ganglion cells.</a> Exp Eye Res. 55, 101-117 (1992).</li></ol>

No conflicts of interest declared.

There are many variations of this surgical procedure and several of the steps in this protocol are not necessary. It is only necessary to retract the muscles that overlie the optic nerve in order to gain access to the nerve. However, this results in a very limited working space around the nerve making the critical final stages of transection more difficult. In certain situations it is desirable to transfect the cells from the optic nerve stump and the increased access space afforded by retracting all of the extraocular m...

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		<div>
		<table border="1">
		<thead>
		<tr>
		<th>Material Name</th>
		<th>Type</th>
		<th>Company</th>
		<th>Catalogue Number</th>
		<th>Comment</th>
		</tr>
		</thead>
		<tbody>
		
<tr>
<td>Stereotaxic Frame</td>
<td>&nbsp;</td>
<td>Stoelting, Kopf, WPI</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<tr>
<td>Rat Gas Mask</td>
<td>&nbsp;</td>
<td>Stoelting, Kopf, WPI</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<tr>
<td>Anesthesia System</td>
<td>&nbsp;</td>
<td>VetEquip</td>
<td>901806</td>
<td>&nbsp;</td>
<tr>
<td>Isoflurane (PrAErrane)</td>
<td>&nbsp;</td>
<td>Baxter Corp</td>
<td>DIN 02225875</td>
<td>&nbsp;</td>
<tr>
<td>Surgical Microscope</td>
<td>&nbsp;</td>
<td>WPI, Zeiss, Leica</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<tr>
<td>Fluorogold -(Hydroxystilbamidine bis(methanesulfonate)</td>
<td>&nbsp;</td>
<td>Sigma</td>
<td>39286</td>
<td>&nbsp;</td>
<tr>
<td>Gelfoam</td>
<td>&nbsp;</td>
<td>Pharmacia &amp; Upjohn</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<tr>
<td>Tears Naturale P.M.</td>
<td>&nbsp;</td>
<td>Alcon</td>
<td>&nbsp;</td>
<td>&nbsp;</td>
<tr>
<td>Proviodine</td>
<td>&nbsp;</td>
<td>Medline</td>
<td>MDS093945H</td>
<td>&nbsp;</td>
<tr>
<td>Vannas spring scissors</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>15000-00</td>
<td>&nbsp;</td>
<tr>
<td>Fine tip Dumont forceps</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>11252-00</td>
<td>&nbsp;</td>
<tr>
<td>Micro surgical hook</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>10062-12</td>
<td>&nbsp;</td>
<tr>
<td>Eye dressing serrated forceps</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>11152-10</td>
<td>&nbsp;</td>
<tr>
<td>Dumont #7b sharp curved serrated forceps</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>11270-20</td>
<td>&nbsp;</td>
<tr>
<td>Cauterizer</td>
<td>&nbsp;</td>
<td>Fine Science Tools</td>
<td>18010-00</td>
<td>&nbsp;</td>		</tbody>
		</table>
		</div>

optic nerve transection: a model of adult neuron apoptosis in the central nervous system

Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. 
The optic nerve can be accessed within the orbit of the eye and completely transected (axotomized), cutting the axons of the entire RGC population. Optic nerve 
transection is a reproducible model of apoptotic neuronal cell death in the adult CNS 1-4. This model is particularly attractive because the vitreous
 chamber of the eye acts as a capsule for drug delivery to the retina, permitting experimental manipulations via intraocular injections. The diffusion of chemicals 
through the vitreous fluid ensures that they act upon the entire RGC population. Moreover, RGCs can be selectively transfected by applying short interfering RNAs 
(siRNAs), plasmids, or viral vectors to the cut end of the optic nerve 5-7 or injecting vectors into their target, the superior colliculus 8. 
This allows researchers to study apoptotic mechanisms in the desired neuronal population without confounding effects on other bystander neurons or surrounding glia. 
An additional benefit is the ease and accuracy with which cell survival can be quantified after injury. The retina is a flat, layered tissue and RGCs are localized in 
the innermost layer, the ganglion cell layer. The survival of RGCs can be tracked over time by applying a fluorescent tracer (3% Fluorogold) to the cut end of the 
optic nerve at the time of axotomy, or by injecting the tracer into the superior colliculus (RGC target) one week prior to axotomy. The tracer is retrogradely transported, labeling 
the entire RGC population. Because the ganglion cell layer is a monolayer (one cell thick), RGC densities can be quantified in flat-mounted tissue, without the need 
for stereology. Optic nerve transection leads to the apoptotic death of 90% of injured RGCs within 14 days postaxotomy 9-11. RGC apoptosis has a 
characteristic time-course whereby cell death is delayed 3-4 days postaxotomy, after which the cells rapidly degenerate. This provides a time window for 
experimental manipulations directed against pathways involved in apoptosis.

Watch this Scientific Journal Video about Optic Nerve Transection: A Model of Adult Neuron Apoptosis in the Central Nervous System at JoVE.com

Optic Nerve Transection: A Model of Adult Neuron Apoptosis in the Central Nervous System

Optic Nerve transection is a widely used model of adult CNS injury. Ninety percent of retinal ganglion cells (RGCs) whose axons are completely transected (axotomy) die within 14 days after axotomy. This model is easily amenable to experimental manipulations and highly reproducible.

Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. 
The optic nerve can be ...

optic-nerve-transection-model-adult-neuron-apoptosis-central-nervous

Research

JoVE Journal

Neuroscience

20.5K Views.  University of Toronto. Optic Nerve transection is a widely used model of adult CNS injury. Ninety percent of retinal ganglion cells (RGCs) whose axons are completely transected (axotomy) die within 14 days after axotomy. This model is easily amenable to experimental manipulations and highly reproducible.

Video: Optic Nerve Transection: A Model of Adult Neuron Apoptosis in the Central Nervous System

Methods for Experimental Manipulations after Optic Nerve Transection in the Mammalian CNS

Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. The optic nerve can be accessed within the orbit of the eye and completely transected (axotomized), cutting the axons of the entire RGC population. Optic nerve transection is a reproducible model of apoptotic neuronal cell death in the adult CNS 1-4. This model is particularly attractive because the vitreous chamber of the eye acts as a capsule for drug delivery to the retina, permitting experimental manipulations via intraocular injections. The diffusion of chemicals through the vitreous fluid ensures that they act upon the entire RGC population. Viral vectors, plasmids or short interfering RNAs (siRNAs) can also be delivered to the vitreous chamber in order to infect or transfect retinal cells 5-12. The high tropism of Adeno-Associated Virus (AAV) vectors is beneficial to target RGCs, with an infection rate approaching 90% of cells near the injection site 6, 7, 13-15. Moreover, RGCs can be selectively transfected by applying siRNAs, plasmids, or viral vectors to the cut end of the optic nerve 16-19 or injecting vectors into their target the superior colliculus 10. This allows researchers to study apoptotic mechanisms in the injured neuronal population without confounding effects on other bystander neurons or surrounding glia. RGC apoptosis has a characteristic time-course whereby cell death is delayed 3-4 days postaxotomy, after which the cells rapidly degenerate. This provides a window for experimental manipulations directed against pathways involved in apoptosis. Manipulations that directly target RGCs from the transected optic nerve stump are performed at the time of axotomy, immediately after cutting the nerve. In contrast, when substances are delivered via an intraocular route, they can be injected prior to surgery or within the first 3 days after surgery, preceding the initiation of apoptosis in axotomized RGCs. In the present article, we demonstrate several methods for experimental manipulations after optic nerve transection.

Optic Nerve transection is a widely used model of adult CNS injury. This model is ideal for performing a number of experimental manipulations that target the retina globally or directly target the injured neuronal population of retinal ganglion cells.

Retinal ganglion cells (RGCs) are CNS neurons that output visual information from the retina to the brain, via the optic nerve. The optic nerve can be ...

Multimodal Imaging of Stem Cell Implantation in the Central Nervous System of Mice

During the past decade, stem cell transplantation has gained increasing interest as primary or secondary therapeutic modality for a variety of diseases, both in preclinical and clinical studies. However, to date results regarding functional outcome and/or tissue regeneration following stem cell transplantation are quite diverse. Generally, a clinical benefit is observed without profound understanding of the underlying mechanism(s)1. Therefore, multiple efforts have led to the development of different molecular imaging modalities to monitor stem cell grafting with the ultimate aim to accurately evaluate survival, fate and physiology of grafted stem cells and/or their micro-environment. Changes observed in one or more parameters determined by molecular imaging might be related to the observed clinical effect. In this context, our studies focus on the combined use of bioluminescence imaging (BLI), magnetic resonance imaging (MRI) and histological analysis to evaluate stem cell grafting.
BLI is commonly used to non-invasively perform cell tracking and monitor cell survival in time following transplantation2-7, based on a biochemical reaction where cells expressing the Luciferase-reporter gene are able to emit light following interaction with its substrate (e.g. D-luciferin)8, 9. MRI on the other hand is a non-invasive technique which is clinically applicable10 and can be used to precisely locate cellular grafts with very high resolution11-15, although its sensitivity highly depends on the contrast generated after cell labeling with an MRI contrast agent. Finally, post-mortem histological analysis is the method of choice to validate research results obtained with non-invasive techniques with highest resolution and sensitivity. Moreover end-point histological analysis allows us to perform detailed phenotypic analysis of grafted cells and/or the surrounding tissue, based on the use of fluorescent reporter proteins and/or direct cell labeling with specific antibodies.
In summary, we here visually demonstrate the complementarities of BLI, MRI and histology to unravel different stem cell- and/or environment-associated characteristics following stem cell grafting in the CNS of mice. As an example, bone marrow-derived stromal cells, genetically engineered to express the enhanced Green Fluorescent Protein (eGFP) and firefly Luciferase (fLuc), and labeled with blue fluorescent micron-sized iron oxide particles (MPIOs), will be grafted in the CNS of immune-competent mice and outcome will be monitored by BLI, MRI and histology (Figure 1).

This article describes an optimized sequence of events for multimodal imaging of cellular grafts in rodent brain using: (i) in vivo bioluminescence and magnetic resonance imaging, and (ii) post mortem histological analysis. Combining these imaging modalities on a single animal allows cellular graft evaluation with high resolution, sensitivity and specificity.

During the past decade, stem cell transplantation has gained increasing interest as primary or secondary therapeutic modality for a variety of diseases, ...

Labeling of Single Cells in the Central Nervous System of Drosophila melanogaster

In this article we describe how to individually label neurons in the embryonic CNS of Drosophila melanogaster by juxtacellular injection of the lipophilic fluorescent membrane marker DiI. This method allows the visualization of neuronal cell morphology in great detail. It is possible to label any cell in the CNS: cell bodies of target neurons are visualized under DIC optics or by expression of a fluorescent genetic marker such as GFP. After labeling, the DiI can be transformed into a permanent brown stain by photoconversion to allow visualization of cell morphology with transmitted light and DIC optics. Alternatively, the DiI-labeled cells can be observed directly with confocal microscopy, enabling genetically introduced fluorescent reporter proteins to be colocalised. The technique can be used in any animal, irrespective of genotype, making it possible to analyze mutant phenotypes at single cell resolution.

Labeling of Single Cells in the Central Nervous System of Drosophila melanogaster

We present a technique for labeling single neurons in the central nervous system (CNS) of Drosophila embryos, which allows the analysis of neuronal morphology by either transmitted light or confocal microscopy.

In this  article we describe how to individually label neurons in the embryonic CNS of Drosophila melanogaster by juxtacellular injection of the ...

Direct Intraventricular Delivery of Drugs to the Rodent Central Nervous System

Due to an inability to cross the blood brain barrier, certain drugs need to be directly delivered into the central nervous system (CNS). Our lab focuses specifically on antisense oligonucleotides (ASOs), though the techniques shown in the video here can also be used to deliver a plethora of other drugs to the CNS. Antisense oligonucleotides (ASOs) have the capability to knockdown sequence-specific targets 1 as well as shift isoform ratios of specific genes 2. To achieve widespread gene knockdown or splicing in the CNS of mice, the ASOs can be delivered into the brain using two separate routes of administration, both of which we demonstrate in the video.
The first uses Alzet osmotic pumps, connected to a catheter that is surgically implanted into the lateral ventricle. This allows the ASOs to be continuously infused into the CNS for a designated period of time. The second involves a single bolus injection of a high concentration of ASO into the right lateral ventricle. Both methods use the mouse cerebral ventricular system to deliver the ASO to the entire brain and spinal cord, though depending on the needs of the study, one method may be preferred over the other.

We describe a method to target drugs to the central nervous system by either implanting a catheter or performing a bolus injection into the right lateral ventricle in mice. We focus specifically on the delivery of antisense oligonucleotides. This technique is readily adaptable to other drugs and to rats.

Due to an inability to cross the blood brain barrier, certain drugs need to be directly delivered into the central nervous system (CNS). Our lab focuses ...

In vivo Interrogation of Central Nervous System Translatome by Polyribosome Fractionation

Multiple processes are involved in gene expression including transcription, translation and stability of mRNAs and proteins. Each of these steps are tightly regulated, affecting the final dynamics of protein abundance. Various regulatory mechanisms exist at the translation step, rendering mRNA levels alone an unreliable indicator of gene expression. In addition, local regulation of mRNA translation has been particularly implicated in neuronal functions, shifting &#39;translatomics&#39; to the focus of attention in neurobiology. The presented method can be used to bridge transcriptomics and proteomics.
Here we describe essential modifications to the technique of polyribosome fractionation, which interrogates the translatome based on the association of actively translated mRNAs to multiple ribosomes and their differential sedimentation in sucrose gradients. Traditionally, working with in vivo samples, particularly of the central nervous system (CNS), has proven challenging due to the restricted amounts of material and the presence of fatty tissue components. In order to address this, the described protocol is specifically optimized for use with minimal amount of CNS material, as demonstrated by the use of single mouse spinal cord and brain. Briefly, CNS tissues are extracted and translating ribosomes are immobilized on mRNAs with cycloheximide. Myelin flotation is then performed to remove lipid rich components. Fractionation is performed on a sucrose gradient where mRNAs are separated according to their ribosomal loading. Isolated fractions are suitable for a range of downstream assays, including new genome wide assay technologies.

In vivo Interrogation of Central Nervous System Translatome by Polyribosome Fractionation

This protocol illustrates essential modifications of polyribosome fractionation in order to study the translatome of in vivo CNS samples. It allows global assessment of translation and transcription regulation through the isolation and comparison of total RNA to ribosome bound RNA fractions.

Multiple processes are involved in gene expression including transcription, translation and stability of mRNAs and proteins. Each of these steps are ...

In vivo Optogenetic Stimulation of the Rodent Central Nervous System

The ability to probe defined neural circuits in awake, freely-moving animals with cell-type specificity, spatial precision, and high temporal resolution has been a long sought tool for neuroscientists in the systems-level search for the neural circuitry governing complex behavioral states. Optogenetics is a cutting-edge tool that is revolutionizing the field of neuroscience and represents one of the first systematic approaches to enable causal testing regarding the relation between neural signaling events and behavior. By combining optical and genetic approaches, neural signaling can be bi-directionally controlled through expression of light-sensitive ion channels (opsins) in mammalian cells. The current protocol describes delivery of specific wavelengths of light to opsin-expressing cells in deep brain structures of awake, freely-moving rodents for neural circuit modulation. Theoretical principles of light transmission as an experimental consideration are discussed in the context of performing in vivo optogenetic stimulation. The protocol details the design and construction of both simple and complex laser configurations and describes tethering strategies to permit simultaneous stimulation of multiple animals for high-throughput behavioral testing.

In vivo Optogenetic Stimulation of the Rodent Central Nervous System

Optogenetics has become a powerful tool for use in behavioral neuroscience experiments. This protocol offers a step-by-step guide to the design and set-up of laser systems, and provides a full protocol for carrying out multiple and simultaneous in vivo optogenetic stimulations compatible with most rodent behavioral testing paradigms.

The ability to probe defined neural circuits in awake, freely-moving animals with cell-type specificity, spatial precision, and high temporal resolution ...

Visual Evoked Potential Recording in a Rat Model of Experimental Optic Nerve Demyelination

The visual evoked potential (VEP) recording is widely used in clinical practice to assess the severity of optic neuritis in its acute phase, and to monitor the disease course in the follow-up period. Changes in the VEP parameters closely correlate with pathological damage in the optic nerve. This protocol provides a detailed description about the rodent model of optic nerve microinjection, in which a partial demyelination lesion is produced in the optic nerve. VEP recording techniques are also discussed. Using skull implanted electrodes, we are able to acquire reproducible intra-session and between-session VEP traces. VEPs can be recorded on individual animals over a period of time to assess the functional changes in the optic nerve longitudinally. The optic nerve demyelination model, in conjunction with the VEP recording protocol, provides a tool to investigate the disease processes associated with demyelination and remyelination, and can potentially be employed to evaluate the effects of new remyelinating drugs or neuroprotective therapies.

Focal demyelination is induced in the optic nerve using lysolecithin microinjection. Visual evoked potentials are recorded via skull electrodes implanted over the visual cortex to examine the signal conduction along the visual pathway in vivo. This protocol details the surgical procedures underlying electrode implantation and optic nerve microinjection.

The visual evoked potential (VEP) recording is widely used in clinical practice to assess the severity of optic neuritis in its acute phase, and to ...

Optical Clearing of the Mouse Central Nervous System Using Passive CLARITY

Traditionally, tissue visualization has required that the tissue of interest be serially sectioned and imaged, subjecting each tissue section to unique non-linear deformations, dramatically hampering one's ability to evaluate cellular morphology, distribution and connectivity in the central nervous system (CNS). However, optical clearing techniques are changing the way tissues are visualized. These approaches permit one to probe deeply into intact organ preparations, providing tremendous insight into the structural organization of tissues in health and disease. Techniques such as Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging-compatible Tissue-hYdrogel (CLARITY) achieve this goal by providing a matrix that binds important biomolecules while permitting light-scattering lipids to freely diffuse out. Lipid removal, followed by refractive index matching, renders the tissue transparent and readily imaged in 3 dimensions (3D). Nevertheless, the electrophoretic tissue clearing (ETC) used in the original CLARITY protocol can be challenging to implement successfully and the use of a proprietary refraction index matching solution makes it expensive to use the technique routinely. This report demonstrates the implementation of a simple and inexpensive optical clearing protocol that combines passive CLARITY for improved tissue integrity and 2,2&#8242;-thiodiethanol (TDE), a previously described refractive index matching solution.

Optical clearing techniques are revolutionizing the way tissues are visualized. In this report we describe modifications of the original Clear Lipid-exchanged Acrylamide-hybridized Rigid Imaging-compatible Tissue-hYdrogel (CLARITY) protocol that yields more consistent and less expensive results.

Traditionally, tissue visualization has required that the tissue of interest be serially sectioned and imaged, subjecting each tissue section to unique ...

Partial Optic Nerve Transection in Rats: A Model Established with a New Operative Approach to Assess Secondary Degeneration of Retinal Ganglion Cells

Previous studies have shown that the secondary degeneration of retinal ganglion cells (RGCs) occurs commonly in glaucoma. Partial optic nerve transection is considered a useful and reproducible model. Compared with other optic nerve injury models used commonly for assessing secondary degeneration, e.g. complete optic nerve transection and optic nerve crush models, the partial optic nerve transection model is superior as it distinguishes primary from secondary degeneration in situ. Therefore, it serves as an excellent tool for evaluating secondary degeneration. This study describes a novel operative approach of partial optic nerve transection by directly accessing the area of the retrobulbar optic nerve through the orbital lateral wall of the eyeball. Moreover, we present a newly designed, low cost surgical instrument to assist with transection. As demonstrated by the representative results in distinguishing the boundary of primary and secondary injury areas, the new approach and instrument ensures high efficiency and stability of the model by providing adequate space for surgical operation. This in turn makes it easy to separate the meningeal sheath and ophthalmic vessels from the optic nerve before transection. An additional benefit is that this space-saving operative approach improves the investigators&#39; ability to administer drugs, carriers, or selective RGC tracers to the stump of the partially transected optic nerve, allowing the exploration of mechanisms behind secondary injury in RGCs, in a new way.

Secondary degeneration of retinal ganglion cells (RGCs) occurs commonly in glaucoma. This study describes an innovative operative approach for partial optic nerve transection. The use of this space-saving operative approach extends the model's application range, and allows exploration of secondary injury mechanisms in RGCs in a new way.

Previous studies have shown that the secondary degeneration of retinal ganglion cells (RGCs) occurs commonly in glaucoma. Partial optic nerve transection ...

A Microbiomechanical System for Studying Varicosity Formation and Recovery in Central Neuron Axons

Axonal varicosities are enlarged structures along the shafts of axons with a high degree of heterogeneity. They are present not only in brains with neurodegenerative diseases or injuries, but also in the normal brain. Here, we describe a newly-established micromechanical system to rapidly, reliably, and reversibly induce axonal varicosities, allowing us to understand the mechanisms governing varicosity formation and heterogeneous protein composition. This system represents a novel means to evaluate the effects of compression and shear stress on different subcellular compartments of neurons, different from other in vitro systems that mainly focus on the effect of stretching. Importantly, owing to the unique features of our system, we recently made a novel discovery showing that the application of pressurized fluid can rapidly and reversibly induce axonal varicosities through the activation of a transient receptor potential channel. Our biomechanical system can be utilized conveniently in combination with drug perfusion, live cell imaging, calcium imaging, and patch clamp recording. Therefore, this method can be adopted for studying mechanosensitive ion channels, axonal transport regulation, axonal cytoskeleton dynamics, calcium signaling, and morphological changes related to traumatic brain injury.

This protocol describes a physiologically relevant, pressurized fluid approach for rapid and reversible induction of varicosities in neurons.

Axonal varicosities are enlarged structures along the shafts of axons with a high degree of heterogeneity. They are present not only in brains with ...