The overall goal of this surgical procedure is to enable direct delivery of tracers, substrates and signaling molecules into the cerebrospinal fluid. This method can help to answer key questions in the neuroscience filed regarding CSF and ISF dynamics in the brain. This method allows the delivery of different molecules, tracers, active metabolic substrates into the CSF, cerebrospinal fluid, compartment without damaging the skull or the brain parenchyma.
The implications of this technique extend towards neurological diseases because it allows the evaluation of CSF-ISF fluxes in the brain and also the clearance of metabolites and toxic proteins from the brain parenchyma. So this method provides insight into the dynamics of the CSF-ISF fluxes and additionally we can use it as a fast, brain-wide delivery route for signaling molecules and molecular substrates that could not otherwise pass the blood-brain barrier. To prepare the cannula, break of the beveled metal tip of a 30 gauge dental needle using a needle holder.
Then, insert the beveled metal tip into a 30 centimeter length of PE-10 tubing filled with aCSF. Next, use a one milliliter syringe fitted with a 30 gauge needle to flush the cannula with artificial cerebrospinal fluid. The cannula is then ready for use in the surgery.
After anesthetizing the mouse, position the animal over a heating pad on a stereotaxic apparatus. Check for a lack of pedal reflex with a toe pinch, and wait for the animal's respiration to become slow and steady. Next, shave the head and neck.
Then, wipe away the fur and sterilize the exposed skin. First, scrub the skin with alcohol. Then make two applications of 0.5 per cent chlorohexidine or two per cent iodine solution.
For chronic cannulations, also administer analgesics at the cannulation site. Now, fix the mouse in the stereotaxic frame, making use of either the intraneural arch or the zygomatic arch. Then, tilt the head slightly so that it forms an angle of 120 degrees to the body.
Next, apply ophthalmic ointment and reapply it as needed during the surgery. Find the occipital crest, which is the part of the skull protruding immediately above the neck muscles. Lift the overlying skin using a pair of tweezers and cut an almond-shaped piece of skin of approximately one centimeter along the midline.
Then, using the occipital crest as a reference point, pull apart the superficial connective tissue to expose the neck muscles below. Use cotton swabs or eye spears to control any resultant bleeding. To proceed, separate the muscles at the midline by carefully running the tweezers down the middle of the incision site in the anterior to posterior axis.
Then, with a pair of curved forceps in each hand, join the tips in the middle near the bottom of the skull and pull the muscles aside to expose the cisterna magna. The cisterna magna looks like a tiny inverted triangle, outlined by the cerebellum above and medulla below. Using a surgical eye spear or cotton swab, wipe the dural membrane covering the CM.Next, with curved tweezers in the dominant hand, grasp the cannula near the tube-covered needle.
Then, place the middle finger of the other hand on the ear bar to create a stable rest for the cannula. Now, at the center of the CM, puncture the dura at 45 degrees relative to the mouse head. Only insert the needle to where the bevel is under the dura to avoid penetrating into the cerebellum or medulla.
If necessary, clean off any leaked CSF with a spear or swab. Then, apply a few drops of cyanoacrylate onto the dural membrane surrounding the cannula and add a drop of glue accelerator to cure the glue immediately. Next, combine dental cement and cyanoacrylate glue.
Then, cover the entire incision site and quickly apply a drop of glue accelerator to cure. For injection in mice with acute cisterna magna cannulation, proceed directly to injection of CSF tracers with the aid of an injection pump. For chronic cannulation, cut the tubing down to two or three centimeters and seal it with a surgical weld to retain the inter-cranial pressure levels.
Then, administer carprofen. Now, house the mouse alone and keep the cage over a heating pad until the mouse has regained sternal recumbency. To prepare the cannula, break off the beveled metal tip of a 30 gauge dental needle using a needle holder.
Then, insert the beveled metal tip into a 30 centimeter length of PE-10 tubing filled with aCSF. Using a 30 gauge needle, flush the tubing. Attach the 30 gauge needle to a 100 microliter syringe filled with distilled water and then attach this to a syring pump.
Using the pump, withdraw enough air to create a one centimeter bubble in the cannula. Then, dip the needle into the CSF tracer solution and withdraw 12 microliters into the cannula. Cut the tubing attached to the cannula up to three to four centimeters length and quickly connect the CSF tracer-filled tubing with the implanted cannula with the aid of fine forceps.
To inject CSF tracers into the CM of anesthetized animals, use a syringe pump to deliver tracers at one microliter per minute for five to ten minutes. Then allow the tracers to circulate for 30 minutes without moving the cannula. Gently restrain the animal and cut off approximately one centimeter of the tubing from the implanted cannula.
Then, quickly connect the two cannulas, using the aid of a fine forceps and a pair of curved tweezers if necessary. Now, inject one microliter of tracer per minute over seven to twelve minutes, using a syring pump. After the injection, let the tracer circulate for 30 minutes while taking the necessary measures to ensure that the cannula remains undisturbed and the tubing remains attached.
Then proceed with harvesting the brain. Mouse brains were injected with CSF tracers into the CM as described. A macroscopic dorsal view shows the distribution of the tracers in the subarachnoid cisterns of the cerebellopontine region in the olfactory bulb and in the paravascular space along the middle cerebral arteries.
In the ventral portion of the brain, a macroscopic view shows the CSF tracer distribution along the Circle of Willis. Histological sections of CM-injected brains further reveal the paravascular distribution of tracers within the brain parenchyma. Mice injected under anesthesia or while asleep had much more tracer distribution than mice injected while awake and freely moving in their home cage.
After watching this video, you should gain a good knowledge of how to perform cisterna magna cannulation that will allow you to deliver chemicals or molecules of interest into the cerebrospinal fluid compartment without any operative damage to the skull or to the brain parenchyma. Once mastered, this technique can be performed in between 20 to 30 minutes if performed properly. While doing this procedure, it is important to apply the correct amount of pressure when puncturing the dura mater to avoid major CSF leakage that could interfere with the intercranial pressure.
After its development, this cisterna magna cannulation has paved the way for studies in the CSF-ISF clearance in the brain via the so-called lymphatic system that nowadays is widely used in different animal models of neurological diseases.
