Before any surgical procedure, it is important to ensure that the animals are in good health by checking their appearance and analyzing their general behavior. A clinical examination may be required to detect certain anomalies and to bring these to the attention of the investigator. In a neurosurgical procedure, it is necessary to eliminate any animal with an abnormality that can cause anatomical or behavioral alterations and thus influence the experimental results (Appendix 1). Signs of reduced appetite; weight loss; deficits in normal exploratory behavior; hyperresponsiveness to handling; vocalization; self-mutilation; prostration; bite marks; scratches; patchy, dull, and/or ruffled fur; and unusual posture or facial expression should immediately exclude the animal from the procedure. Any of these signs (Appendix 3) should be noted on the animal’s follow-up register (Appendix 2B) and reported to the veterinarian to determine the corrective course of action, treatment, prophylactic measures, or eventual euthanasia (Appendix 4).

The investigator responsible for the experiment and the surgical procedure must ensure the availability of the required equipment and its maintenance, preparation, and sterilization. A sufficient amount of time must be given to do all of this in advance, so as not to be caught short at the last moment (Appendix 5). The availability of the required anesthetics and their expiration dates should be verified. The laboratory stock and the administration of any anesthetic should be recorded in a register. This register is maintained by the laboratory personnel in charge of the pharmacy service within the institution of experimentation.

Benchtop instruments, including a cold light source and dental drill handpiece, are placed on the operating table next to the stereotaxic apparatus, along with a receptacle, such as a surgical kidney dish or plastic container, for organic waste (Figs. 5.1 and 5.7). A surgical box containing the sterilized surgical instruments (Figs. 5.2 and 5.7) is placed on a trolley in the clean zone (Fig. 5.8). From our experience, it is necessary to have a minimum of instruments such as those shown in Fig. 5.2 in order to carry out an incision of the head. A surgical box containing the suturing instruments (Figs. 5.3 and 5.7) is kept on a sterile surgical tray pending their use at the end of the surgery.

The rigorous undertaking of a surgical procedure requires the an assistant for the successive opening of different storage boxes containing the surgical drapes, instruments, disposable materials, and suture blister packs so that the gloved surgeon can access them without breaking the chain of asepsis. Sterilized disposable materials, cotton swabs, needles, and sutures (Fig. 5.4) should be placed on the sterile tray next to the box containing the surgical instruments. A sterile drum containing the operating drapes and gauze are placed nearby on a trolley in the “clean” zone (see Fig. 5.8). Surgical caps, masks, gowns, and gloves are made accessible to the operator to facilitate an aseptic procedure. Tips on the selection of suture material are provided in Fig. 5.4 and Table 5.1.

Complications of infection related to a surgical procedure is a permanent risk in animal surgery. Sterilization of the surgical equipment is the first element in preventing postoperative infection. Two sterilization methods are commonly used.

It is important to separate the specialized (and often expensive) equipment from the surgical instruments and materials needed at hand during the operation. Two sets of instruments for the incision and suturing (scalpel, dissecting forceps, drill bits, scissors, and needle holder) allow the cleaning and sterilization of the first set used on animal “X,” while the second set is being used on animal “X + 1.” Special equipment that usually has little or no contact with the tissues of the anesthetized animal can be kept sterile by placing it in a tray covered with a sterile drape between each procedure (Fig. 5.7). The ear and incisor bars can be autoclaved or cleaned simply with a disinfectant towelette.

A coherent organization of a surgical room, operating theater or a lab benchtop space used for surgery, is crucial to carrying out sterile surgery and limiting the risk of breaking the chain of asepsis. The principle of an operational workflow (a forward moving sequence of steps) commonly used in industrial sectors helps in organizing the different levels of cleanliness or sterility required at each stage of the procedure (Fig. 5.8). This organization identifies a “dirty” preparation zone in which shearing of the animal and induction of anesthesia is carried out and an “intermediate” zone where the paws of the animal are washed with antiseptic soap before bringing it into the “clean” zone on a thermostated electric heating blanket or pad equipped with a rectal temperature probe, just prior to being fixed in the stereotaxic apparatus.

After the disinfection of the top of the head (with soap, rinsing, 70 % ethanol, and a disinfectant such as chlorhexidine or polyvinylpyrrolidone iodine¹), a subcutaneous injection of local anesthetic (see Sects. 5.2.3 and 5.2.4) is made on each side of the line of incision. This helps to limit the postoperative pain of the animal. Precision in carrying out the surgical procedure is important for limiting pain, morbidity, and mortality, hence, the importance of proper training in neurosurgical practices.

The first package of sutures opened by the assistant is put it on the surgical tray so the surgeon can take the sterile part of the package and use it at the end of the surgery for the suture.

Each micrometer screw of the stereotaxic apparatus is then disinfected, as well as the handpiece of the dental drill. With the aid of an assistant, the operator then dons a gown and mask from the sterilization drum (see Fig. 5.6), as well as sterile gloves. At this point, the operator proceeds with the set up of the sterile zone with help from the assistant who passes to him the different open boxes of materials. A sterile drape is spread to create the operating field; the different surgical instruments, compresses, and other consumables, as well as any drapes required for the animal, are then successively placed upon the drape. The first package of sutures opened by the assistant is put on the surgical tray so the surgeon can take the sterile part of the package and use it at the end of the surgery for the suture.

During successive procedures, equipment and materials are covered with a sterile drape while the second animal is being prepared and then reused once the operator has changed gloves. Before the skin incision, the operator makes a fresh disinfection of the operating zone ² with chlorhexidine or polyvinylpyrrolidone iodine and then puts the surgical drapes in place.

At the end of the surgical procedure, the animal is placed in the recovery zone, for example, in an incubator or under a heating lamp in the preparation zone. This allows the operator to proceed, if necessary, with an operation on a second animal while monitoring the recovery of the first.

Ideally, all soiled instruments are cleaned or left to soak in the appropriate product in the “intermediate” zone. Once rinsed and dried, they can be autoclaved before reuse. For the second animal, the operator proceeds as before with the shearing and anesthesia in the preparation zone and then the setup of the animal. Normally, the entire procedure of dressing is repeated, in particular a change of surgical gowns, but it is common to limit this to a simple change of gloves.

According to the International Association for the Study of Pain, pain is an “unpleasant sensory or emotional experience caused by actual or potential tissue injury… that provokes protective motor and autonomic reactions leading to a change in the behavior specific to the individual. The inability to communicate does not exclude the possibility that the subject feels pain and requires adequate analgesic treatment.” At the end of the twentieth century, a British survey of 1,000 practicing veterinarians revealed that only half of the animals undergoing an ovarian hysterectomy and two-thirds of those having abdominal surgery received an opioid analgesic or anti-inflammatory drug during their operation. Nearly one surgeon in 10 felt that “the surgical operation in question did not cause enough pain to warrant treatment.” Within the last decade, attitudes have changed significantly – the entire scientific community agrees that pain, if left untreated, increases the risk of morbidity and mortality and constitutes a significant bias for experimental results.

Pain in animals is difficult to assess because the subject is unable to directly communicate what it feels. Only those signs that are secondary to painful phenomenon are expressed and can thus be observed and treated retrospectively. It is nevertheless essential to anticipate and consider it and to deal with it more effectively. This is not only a moral issue but also a scientific and regulatory requirement. As a precautionary principle, it is considered that any procedure perceived as being painful in humans be regarded as such in animals and that a pretreatment thereof with diverse agents (e.g., opioids, steroidal or nonsteroidal anti-inflammatory drugs, local anesthetics, alpha-2 adrenergic receptor agonists) should be implemented. A review of the anatomical mediators of pain transmission is useful here for understanding the concept of more effective multimodal analgesia, which can allow the use of lower doses of drug and thus a reduction in the incidence of their side effects.

Nociception, or pain, is a physiological process that involves five steps: transduction, transmission, modulation, projection, and perception of a painful signal.

Transduction is a process by which nociceptors, specialized free nerve endings that are sensitive to mechanical, chemical, or thermal stimuli, convert these stimuli into signals (Table 5.2) that are relayed by neurons to higher centers. Nociceptors are present in the skin, muscles, joints, bones, and viscera.

Peripheral sensation is transmitted by three types of nerve fibers: Abeta, Adelta, and C. Under normal conditions, the Abeta fibers carry non-painful sensory information such as friction, vibration, pressure, or proprioception. Nociceptors on Adelta and C fibers generate painful sensory information that is carried by the Adelta and C fibers to the gray matter in the dorsal horn of the spinal cord. This process is referred to as transmission (Fig. 5.9).

Within the spinal cord, a synaptic relay allows for a local modulation of the nociceptive signal by the intervention of three types of postsynaptic neurons:

Nociception: a physiological process by which pain signals are routed from pain receptors towards the brain. Nociceptors: pain receptors. Nociceptive stimuli: phenomena capable of activating nociceptors and generating a painful response. NSAIDs : nonsteroidal anti-inflammatory drugs

Regulatory mechanisms are continuously solicited and established by the organism. A nociceptive signal can be amplified by the phenomena of peripheral and central sensitization. While discussed individually here for a better understanding, peripheral and central sensitizations are intricate and inseparable mechanisms (see Fig. 5.9).

“Anesthesia or the use of a sedative, even a powerful one, is not synonymous with analgesia” (Smith et al. 1985). The knowledge and understanding of painful phenomena has led to the development of effective strategies for analgesia. Multimodal analgesia aims to act at different stages of pain signal propagation, based on the combined use of compounds from different drug classes and/or different methods of administration, e.g., from intravenous for rapid and widespread onset and action to epidural or local injection for direct action on nerve fibers. These combinations exploit the complementarity and the eventual synergy between agents, permitting the use of lower doses and thus reducing the risks of adverse side effects (Shavit et al. 2005).

The most commonly used agents for blocking conduction in peripheral nociceptive fibers are the local anesthetics (2 % lidocaine and 0.5 % bupivacaine), which act by inhibiting the operation of sodium channels. They can be injected directly along specific nerve fiber projections (e.g., brachial plexus, cranial and intercostal nerves) or at the level of the nerve root(s) (e.g., epidural injection). Opioids, such as morphine, can also be administered by the epidural route.

The stages described below may vary in duration and intensity, depending on the anesthetic used. In addition, each species has a particular behavior during anesthesia, sometimes with marked individual variations. As anesthesia deepens, reflexes disappear in the following order:

Anesthesia can induce mortality and morbidity by altering the metabolism of the animal. Given their small size, rodents are quite susceptible to hypothermia and dehydration, especially during inhalational anesthesia. The following section aims to define and standardize a rigorous procedure for monitoring the experimental animal before, during, and after surgical interventions. Emphasis will be given to the principal points to consider and will not detail the multiplicity of methods and techniques that can be employed. While the priority is to monitor the proper depth of anesthesia during the operation by checking the various reflexes, attention also needs to be given to maintaining general homeostasis, body temperature, and proper hydration of the animal.