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Some types of neurophysiology experiments require that a probe be placed in the brain only during the actual experimental sessions, either to stimulate the brain (stimulating electrode), to record electrical activity (recording electrode), or to sample the fluid in the interstitial space (microdialysis probe). Owing to their fragility, or the need to reposition them to sample from a different area of the brain, these probes are removed at the end of each recording session. The use of these types of devices requires the implantation of chronically indwelling hardware called guide cannulae or chambers. When these are implanted, a piece of the skull is removed (craniotomy) and the hardware is placed over the hole and attached to the skull. The hardware is hollow, allowing free access to the brain, and is filled with a sterile solution (typically saline) and capped with a sterile cover to prevent introduction of microorganisms. When the investigator needs to place a probe to begin an experimental session, the cover to the guide cannula or chamber is removed and the probe is introduced into the brain.
Major surgery to implant hardware devices for head restraint, data collection, and stimulation can be accomplished with standard aseptic surgical techniques and typically can be performed in a facility dedicated to aseptic surgery (see Gardiner and Toth, 1999, and Lemon, 1984b, for discussions of surgical issues related to cranial implants). When implanting guide cannulae or chambers, the size of the craniotomy should be large enough to allow access to the structure being studied without unnecessarily exposing neural tissues (Lemon, 1984a). If an implanted device is necessary during the training of the animal, the animal should be conditioned to the training environment prior to any surgery. In this way, animals that will not accept training can be removed from the study before they are subjected to an unnecessary surgery.
Neurophysiology Experiments in Anesthetized Animals
Cases where recordings are made while an animal is anesthetized raise critical questions regarding anesthesia, maintenance of physiologic status, and monitoring of the animal’s condition. The choice of anesthetic must satisfy the need of the experimenter to perturb neuronal status as little as possible while ensuring that the animal remains free of pain and distress. Maintaining an anesthetized (and sometimes immobilized) animal in appropriate physiologic condition is a considerable technical challenge (see “Prolonged Nonsurvival Studies” in Chapter 5). Monitoring both the anesthesia and the animal’s general condition requires careful attention to a number of measures. Although animals in some studies are used in repeated experiments (with intervening recovery periods), in other cases they are maintained under anesthesia for long periods of time for nonsurvival studies (see Chapter 5). If there will be repeated sessions of prolonged anesthesia, special attention should be paid to maintaining the animal’s normal physiologic status between anesthetic sessions.