Fig. 2.

Arrangement of headpost and recording chamber on the mouse’s skull and detailed photographs of the headpost and headpost clamp. All drawings are true to scale. (A) Side view of the skull with mountings. The headpost and recording chamber are embedded in acrylic cement, which is anchored to the skull with three screws, two of which are visible in this side view (black arrowheads). A stereotaxic micromanipulator was used to position the caudo-medial corner of the headpost on top of bregma. The vertical dashed line indicates the anterior–posterior coordinates of bregma. The recording chamber was fashioned from a drinking straw and the bottom was shaped with small scissors to match the curvature of the skull bone. (B) Top view of the skull showing the location of all three skull screws in relation to the headpost and recording chamber. This view illustrates that the headpost cannot be mounted more rostral and thus, that the distance between the recording chamber and the post cannot be further increased. The bend in the post increased the space between the recording chamber and the headpost clamp to allow unhindered access with recording equipment (see Fig. 1). The skull bone at the bottom of the recording chamber was removed to provide access to the underlying brain area (cerebellum shown here) through the intact dura. (C) Bottom view and (D) side view of the headpost clamp. The metal plate at the wide end of the fixture was used to attach the clamp to stationary metal post, which was mounted onto the surface of the experimental table. In (D) the headpost is maximally inserted into the opening as it would be during an experiment. The bend serves as a mechanical stop and determines how deep the post can be maximally inserted. This allows the reliable reproduction of z-axis coordinates across experiments. (E) Enlarged top view of the headpost clamp with headpost (post contour marked with dotted line) held in the lower right corner by the set screw. (F) Enlarged view of the headpost, which was cut out of a 3 mm thick aluminum sheet. The bend added three millimeters of distance between the headpost clamp and the recording chamber and also determined how deep the post could be inserted into the clamping fixture. (G) A true-to-scale drawing of the headpost clamp and set screw (dashed outlines) and a mouse’s skull with headpost and chamber. The 5–7 mm distance between the posterior edge of the clamping fixture and the recording chamber were sufficient for accessing the recording chamber with an electrode microdrive.