The overall goal of this surgical procedure is to conduct the intracerebroventricular injection of Amyloid beta without stereotactic instruments, and to acutely induce Alzheimer-like behavior abnormalities in normal mice. This method can help answering key questions in the field of neurobiology, such as drug discovery for Alzheimer's Disease. The main benefit of this technique is that we can perform the direct injection of A beta into the intracerebroventricle region of the mice without the help of stereotactic instruments, and it allows us to mimic Alzheimer-like behavior abnormalities in mice without waiting for the aging process.
The implications of this technique extend towards therapy or diagnosis of Alzheimer's Disease, because it can be used to study the downstream pathology of Amyloid beta, and screened for drug candidates. Generally, individual new to this method will struggle, because finding the precise location of bregma on animal skull may be difficult. To begin this procedure, make one millimolar A-beta with DMSO.
Then, dilute it ten fold in PBS. Next, prepare PBS solution containing 10%DMSO as a vehicle for ICV injection. After that, wrap the microsyringe with a 26-gauge stainless steel needle using Parafilm strips in order to adjust the needle length for depth-controlled injection.
Densely wrap the syringe multiple times so that the Parafilm will not be compressed during needle insertion, and the needle length stays constant. Then, cut the layers of Parafilm strips to adjust the needle length to 3.8 millimeters between the needle tip and the Parafilm. Next, wash the syringe twice with 70%ethanol.
Subsequently, rinse the syringe with distilled water. Dry it completely in a fume hood for at least 30 minutes, and leave the clean syringe under a UV light for another 20 minutes before injection. In this step, place a seven week old anesthetized mouse on a warm mat.
Next, apply sterile PBS drops or ointment to both eyes to prevent corneal dryness during the procedure. Afterward, confirm adequate anesthesia by assessing the toe pinch extensor withdrawal reflex. Then, prepare two mirrors with multiple vertical lines drawn on the surfaces to provide references for the syringe's perpendicular injection.
Place one mirror right next to the body of the mouse, and the other in front of the head. Keep M1 parallel to the imaginary midline between the two eyes of the mouse and arrange M2 perpendicular to M1, so that the two planes form a 90 degree angle. Next, spray 70%ethanol on the mouse forehead, and rub it with dry cotton swabs.
To locate the bregma, place the thumb and index fingers directly above the eyes and slightly press the forehead down to the extent that both eyes protrude. At the same time, drag the skin of the forehead backwards in order to minimize the skull movement under it. Locate the injection site with a tailor's tape at 2.4 millimeters depth under the skull.
Fill the syringe with 10 microliters of A-beta solution or vehicle. Place the syringe so that it is perpendicular to the plane of the injection site. Then, align the reflected images of the syringe in both mirrors with the drawn lines on the mirrors.
Begin inserting the needle until the Parafilm wrapping touches the skin. Hold the syringe with one hand, and inject five microliters of the A-beta solution or vehicle with the other hand, slowly over five seconds without pausing, and make sure the syringe remains perpendicular throughout. After completing the injection, wait three to five seconds before removing the syringe.
Place the thumb and index fingers back on the head, and hold it down to prevent the skull from any unneccessary movements. Then remove the syringe without tilting. Subsequently, place the mouse on a warm pad for recovery.
To confirm if A-beta was injected in the target ICV region, check the trace of the needle insertion in the brain tissue to see if it reached one of the ventricles. If the injection was unsuccessful, exclude that subject's data from analysis of the experimental results. In this experiment, the full-length A-beta peptide of 42 amino acids was used to produce a mixture of A beta monomers, oligomers, and fibrils.
Through the HFIP induced monomerization step, relative homogenous monomers were obtained. After the seven day incubation, diverse sizes of A-beta aggregates were developed, in which trimers and tetramers were the dominant species among the oligomeric forms of A-beta. In this study, spacial working memory was assessed in the A-beta injected mice in the Y-maze test, in which the sequence of arm choices and the number of total arm entries were recorded while each mouse was allowed to freely explore the maze.
The A-beta injected mouse group showed significantly lower alternation rates, indicating the development of cognitive deficits. Once mastered, this technique can be done in 20 minutes, including anesthetic procedures, if it is performed properly. While attempting these procedures, it is important to remember to make the needle perpendicular to the plane of injection.
After its development, this technique paved the way for researchers in the field of drug discovery of Alzheimer's Disease to explore in vivo screening of drug candidates in these easily accessible Alzheimer's rodent models. After watching this video, you should have a good understanding of how to conduct intracerebroventricular injection without stereotactic instruments.
