Biomedical Engineering Reference
In-Depth Information
3. Sheep are anesthetized with midazolam (2 mg/kg) and
ketamine (2 mg/kg) administered through the intravenous
catheter and maintained using isofl urane (0.5-3.0 %) in oxy-
gen delivered through an endotracheal tube.
4. To prevent excessive rumen pressure and to collect refl ux, an
orogastric tube is placed.
5. Continuous monitoring of physical parameters and anesthetic
depth are performed with a rectal temperature probe, electro-
cardiogram, capnograph (end tidal CO
2
), pulse oximeter (O
2
saturation), and oscillometric blood pressure monitor
c
.
6. Sheep are positioned in ventral recumbency and the head fi xed
on a Horsley-Clarke stereotaxic frame.
7. The cranium is clipped and prepared aseptically. A single inci-
sion is made through the skin and periosteum exposing the
skull.
Thalamic Injection
8. Though accurate stereotaxic coordinates can be developed for
certain breeds [
104
], skull landmarks can differ substantially
between rams and ewes, between sheep breeds, and between
animals of the same breed with different horn structure. For
example, sheep of the Jacob breed may have 2-6 horns, which
dramatically alters the position of bregma on the skull.
Therefore, stereotaxic coordinates must be determined by pre-
or intraoperative imaging techniques such as MRI or ultra-
sound. An example of thalamic coordinates (relative to bregma)
for 2-horned Jacob ewes (3-5 months of age) is AP −0.1, ML
±0.7, DV −3.2 cm. However, it is stressed that coordinates
often must be determined for each animal based on breed,
gender, and horn structure/number [
104
].
9. Vector is delivered using a 500
ʼ
L Hamilton syringe with a
non-coring needle (22-25 G).
10. For each thalamic injection site, a single craniotomy is made
directly above each target with a 2 mm burr.
11. The needle is advanced through the meninges and into the
target manually with a micromanipulator at a rate of 1 mm/s.
12. A total of 175
L per thalamus is delivered in ascending steps
separated by 0.15 cm each. Injections are performed at four
steps (3 × 50
ʼ
L/min.
13. The needle remains in the thalamus for 1 min after the fi nal
injection, is then raised halfway, allowed to remain in place for
one more minute, and raised the remainder of the distance
over 30 s
e
.
ʼ
L, 1 × 25
ʼ
L) at a rate of 5
ʼ
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