This protocol is significant as it is an adaptable, stereotactic procedure that can be utilized in both mice and rats, and can be applied to many new neuroscientific techniques. The main advantage of this adaptable stereotactic procedure is it's ability to be used to target challenging and difficult-to-reach regions of the brain. Demonstrating the procedure will be Chelsea Faber, a young, talented post-doctoral research fellow in the laboratory.
After confirming that the frame and micromanipulator have been calibrated, place the center height gauge into the socket of the head holder baseplate. Secure the centering scope in the tool holder and sight down the scope. Adjust the position of the micromanipulator until the crosshairs are aligned and focused on the gauge crosshairs, and place the ear bars into the holders such that the indicator lines on both sides are at zero.
Use the medial-lateral and anterior-posterior knobs on the head holder to centerline the ear bars in the X and Y planes above the crosshair of the center height gauge. To align the ear bar position in the Z-axis, remove the ear bars from the holder and remove the center height gauge. Then, replace the ear bars and re-center the bars at zero.
Then, sight down the scope again, and use the vertical shift and coronal tilt knobs respectively to lower and rotate the ear bars until the scope crosshairs remain centered between the ear bars throughout coronal rotation. To align the central axis of rotation for angled coordinates, secure the centering scope in the tool holder and position the micromanipulator to the calculated coordinate. Note that the right/left coordinate for the angled implantation corresponds to the length of side A.Sighting down the scope, mark this coordinate to represent the point at which the cannula will enter the brain, once the head is rotated.
Reposition the micromanipulator over the midline, and use the coronal tilt knob to rotate the head to the calculated angle. If the scope crosshairs do not line up with the reference mark, use the vertical shift knob to adjust the head position in the Z-axis until the crosshairs line up as close as possible to the mark. At this point, the arbitrary point of rotation should be aligned with the center of rotation of the stereotax.
Rotate the head back to the zero degree coronal position before proceeding to subsequent drilling and microinjection steps. After performing the microinjection, return the head to the zero degree level position, and use a hand drill to place two holes, anteriorly, and two holes, posteriorly, far enough from the angled coordinate burr holes to accommodate the ferrule portion of the fiberoptic that sits above the skull. When all of the holes have been drilled, use a small flathead screwdriver to insert the bone screws as gently as possible, such that they sit firmly in the skull without penetrating the brain.
Clamp a fiberoptic cannula into the cannula holder, and place the cannula holder in the tool holder. Rotate the head to the calculated angle, noting that the coordinates of the micromanipulator do not apply to the new tool. Use the center of the angled burr holes as the implantation target.
Lower the fiber optic until it just touches the dura within the center of the burr hole, and zero the micromanipulator in the Z-axis. Slowly lower the fiberoptic to the dorsal ventral angled coordinate, then use cyanoacrylate gel to connect the fiber optic ferrule to the ipsilateral anchor screws. Use a micropipette tip to apply an accelerant.
Once the gel has completely hardened, gently loosen and raise the cannula holder until it is clear of the ferrule. Implant the second fiberoptic to the contralateral angled coordinate, then level the head. For extra security, use the cyanoacrylate gel and accelerant to make an additional connection between the two angled fiberoptic cannulas, and apply a small, relatively thin amount of dental cement to the surface of the skull, taking care to thoroughly cover the anchor screws and the base of the fiberoptic cannulas.
To interrogate the role of hypothalamus ventromedial nucleus neurons in glycemic control, in this representative analysis, a standard, non-angled, stereotactic approach was utilized for the bilateral microinjection of an inhibitory channel rhodopsin virus to the ventromedial nucleus. Given the proximity of the ventromedial nucleus to the midline, space constraints did not permit a non-angled implantation of bilateral fiberoptics, necessitating the development of a surgical strategy for precisely implanting fiberoptics at an angle. The most important things to remember are to simply be patient and to take your time, as being consistent and accurate with your coordinate assignments is critical to success.
This procedure can be adapted and applied to any neuroscientific method requiring microinjection or implantation into the rodent brain, including chemogenetics, optogenetics, and fiber photometry approaches.
