Many decades have seen the conceptual evolution of the neurosciences paralleled by numerous groundbreaking methodological advancements that have paved the way for precise measurement and observation of the animal brain. Today’s knowledge of the central nervous system and its specific functions have benefited from modern imaging techniques such as the MRI, multielectrode electrophysiology, and optogenetics, but unarguably the most reliable and long-established techniques involve an invasive exploration of different brain regions, known as the stereotaxic neurosurgery.
Derived from the Greek stereos, meaning solid and three-dimensional, and tactos, meaning arranged and oriented, the term “stereotaxy” denotes the concept of arrangement in space, as it is used in the context of neurosurgery today.
Stereotaxy was founded on the necessity for developing a method that could allow precise lesioning in specific areas of the brain, a need that exists all the more today, with present technological and scientific advancements in the field of neuroscience. The invention of stereotaxic procedures in animals arose in 1887 when Sir Victor Horsley and R.H. Clarke acknowledged the need for such a method in probing the connections of the cerebellum in the rhesus monkey. Clarke designed a “stereotaxic device” that allowed the stable placement and maintenance of the animal’s head within a three-dimensional system of coordinates modeled in horizontal, sagittal, and frontal planes. Using this device, along with a microtome that could cut through the cranial bone, Clarke and Horseley were able to elaborate models showing the positions of specific brain regions relative to each other, as well as to the surface of the skull. Furthermore, the stereotaxic invention also allowed for an intervening lesioning electrode, minimizing the loss of bone tissue and additional tissue damage to underlying cerebral regions.
Additionally, more than furthering discoveries in the neurosciences, stereotaxic procedures also have significant value in human neurosurgery in the medical setting. Before the development of magnetic resonance imaging, stereotaxic neurosurgery required extensive radiological equipment for the visualization of the brain, as well as the control and positioning of instruments used in invasive procedures. Arteriography and ventriculography were also then used in the reduction of possible risks that could arise from imprecise surgical manipulation. Human neurosurgery methods have since then become increasingly sophisticated with the advent of modern and computerized stereotaxic approaches, yet their foundations are still firmly rooted in pioneer numeric-based imaging. Today, stereotaxic surgery applications in humans include treatment of motor disorders, biopsies of tumoral pathologies and interstitial radiotherapies, and treatment of other conditions such as hydrocephaly and epilepsy. Given these essential contributions, the significance of stereotaxic methods remains resonant with today’s scientific community.
The necessity of animal experimentation for the advancement of research, however, continues to be a sensitive topic of debate among scholars, given the salient role that subjective moral attitudes play. It must be noted that the use of animals in research, particularly in the neurosciences, has been an indispensable source of discovery; much is known about the anatomy, physiology, and behavior of rodents, the applications of which hugely contribute to the understanding of human biology and behavior. Thus, given these justifications, the scientific community is understandably in agreement as to the acceptance of animal experimentation, for as long as no valid alternative is yet to be presented. Stereotaxic surgery among rodents and all animals, in general, is therefore kept in strict adherence to rigorous ethical consideration, through effective implementation of regulatory principles and guidelines of the corresponding committees in different countries.
The refinement of procedures, materials, and technology has henceforth been a necessity in the area of animal research, particularly in the applications of stereotaxic neurosurgery in rodents. Increasing knowledge on the optimization of surgical procedures, as well as heightened public awareness on animal rights and welfare issues and the implementation of strict legislation call for standardization of practices in stereotaxic surgery (Formari et al., 2012). These demands have prompted the development of devices and protocols, to be discussed in the following sections, which researchers can employ for successful experimentation. Rodents are one of the most widely-employed experimental animals in neurobehavioral research due to a number of reasons. To a great degree, the popularity of rats and mice as experimental animals can be attributed to the undeniable convenience and practicality that they offer. Particularly, because of their small size, rodents are manageably easy to house, store, and taken care of. Additionally, the usage of transgenic animals and the availability of pure genetic rodent lines further the usefulness of rodents in the experimental setting (Messier et al., 1998). Numerous rodent models have, therefore, unsurprisingly been instrumental in the research and treatment of various human diseases as well as in the understanding of the different functions of the human brain.
The fundamentals of stereotaxic surgery begin with the preoperative phase, which involves the verification of the rodents’ health status and the preparation and sterilization of surgical equipment. The various methods of pain reduction during and after the surgical procedure along with the importance of adopting proper anesthetic protocols are also taken into consideration (Ferry et al., 2014). Further, the success of stereotaxic surgery is not complete without following standardized protocols for post-surgical care as well as the comprehensive protocols for histology and autopsy procedures.
Because the stereotaxic approach requires anatomical and functional expertise along with exact mathematical positioning of different brain structures of the rodent, preparation in these areas is key to successful experimentation. Importantly, the basics of surgery are also necessary for achieving the right results, in particular, the various methods related to anesthesia, analgesia, and asepsis. A combination of knowledgeability and expertise, as well as the right amount of preparation and the correct usage of stereotaxic equipment, are therefore needed in successfully executing stereotaxic surgical procedures
Parts of the Stereotaxic Apparatus
The main body of the stereotaxic apparatus consists of a U-shaped stereotaxic frame fixed upon a base plate. Three mechanical elements attached to the frame allow for the accurate placement of an electrode or cannula holder along three orthogonal axes, forward and backward, up and down, and side to side. The location of the electrode or cannula holder is manipulated using three adjustable micrometer Vernier screw drives. Two laterally adjustable ear bars and a height-adjustable incisor bar are also fixed onto the main frame to hold the head of the animal in an exact position. The ear bars center the head parallel to the anteroposterior axis of the stereotaxic frame, while the incisor bar determines the plane of inclination of the animal’s head in the “flat-skull position.” The two reference points on the animal’s skull; the bregma and lambda, are kept in the same horizontal plane using the incisor bar.The main body of the stereotaxic apparatus consists of a U-shaped stereotaxic frame fixed upon a base plate. Three mechanical elements attached to the frame allow for the accurate placement of an electrode or cannula holder along three orthogonal axes, forward and backward, up and down, and side to side. The location of the electrode or cannula holder is manipulated using three adjustable micrometer Vernier screw drives. Two laterally adjustable ear bars and a height-adjustable incisor bar are also fixed onto the main frame to hold the head of the animal in an exact position. The ear bars center the head parallel to the anteroposterior axis of the stereotaxic frame, while the incisor bar determines the plane of inclination of the animal’s head in the “flat-skull position.” The two reference points on the animal’s skull; the bregma and lambda, are kept in the same horizontal plane using the incisor bar.Much preparation is necessary before stereotaxic surgery can successfully take place. Firstly, animals must be in good health, as evidenced by their appearance and general behavior on the day of surgery. The maintenance, preparation, and sterilization of surgical equipment is of prime importance, along with the availability and verification of anesthetics that shall be used for the procedure. The usage of anesthesia and extensive awareness of its different effects in varying stages is key to pain management during pre-, intra-, and postoperative treatment. Further, in the usage of anesthesia, much importance is placed on monitoring vital signs, particularly in achieving the proper depth of sedation by checking various reflexes, as well as maintaining general homeostasis, body temperature, and proper hydration while the animal is sedated. Finally, the rodent must undergo shearing in order to eliminate hair and facilitate the disinfection of the skin. Once the stereotaxic setup is ready, the rodent may be placed onto the apparatus, on a thermostatically-controlled heating blanket and fitted with a rectal temperature probe. Simply, the researcher needs to adhere to the typical pre-and-post-operative checklist.
- Setting up the Animal in the Stereotaxic Instrument
The animal is considered ready to be placed on the stereotaxic apparatus once the proper depth of surgical anesthesia is reached, as indicated by regular breathing and a characteristic loss of reflexes.
In securing the animal onto the set-up, one of the two ear bars must first be fixed in the instrument. Then, the head of the animal is gently steered so that the ear canal is guided towards the fixed ear bar. The head is then horizontally maneuvered so that the tip of the bar is positioned behind the aural bone spur. Once the bar is in place, a soft click will be heard. One must be careful to distinguish the sound from a louder snap, characteristic of a possible ruptured eardrum if the bar is inserted too far. Then, the second ear bar is guided into the other ear canal. Once both ear bars are properly placed, the head of the animal will only be able to shift in an Antero- and retroflexed fashion.
Once the ear bars are properly in place, the subject’s head may then be aligned with the center of the stereotaxic frame. The mouth of the animal is opened, and the movable tooth bar is slid behind its upper incisors. The head of the animal is then gently pulled forward so that the tooth bar may slide and click into its correct place.
The final step is locking the snout in place with the usage of the nose bar. The head is immobilized by fixing the nose bar’s setscrew at the proper setting, providing sufficiently firm and stable pressure on the snout. In this manner, no head movement shall be possible during the surgical procedure.
After securing the animal in place, the skin on which the incision will be made is then disinfected by washing with sterile gauze soaked with antiseptic soap, alcohol, and an antiseptic solution. The standard washing procedure involves centrifugal strokes, away from the incision site towards the periphery. The gauze must not touch the edge of the shaved zone. The surgical drapes are used for the maintenance of sterility after the operation zone is cleaned. The drape should cover the entire animal, and can even extend to the adjacent areas, in order to widen the surface of the sterile zone.A scalpel fitted with a new blade is used to incise the scalp. Precise scalpel handling is necessary, involving all layers of skin. The linear incision must start slightly in front of eye level and continue backward or caudally to approximately 0.5 cm behind the ear bars.Once the incision is made, the suture lines of the four bone plates of the skull surface must be visibly identified in order to locate the two reference points correctly; the bregma and lambda. The bregma pertains to the intersection of the sagittal and frontal sutures at the level of the snout, whereas the lambda, named for its resemblance to the Greek letter lambda, is a point located on the midline and at the base of the triangle formed by the lambdoid and sagittal sutures. Once the stereotaxic landmarks have been identified, the operator may carry on with the procedure, beginning with taking the coordinates of the two reference points using the tip of the instrument.Stereotaxic procedures have been around for a long time, up until now have much to contribute to the methodological principles of research in the neurosciences. The importance of knowledge acquired through stereotaxic surgery on rodents and other experimental animals are of considerable magnitude, given the successful extrapolation of findings to research on human biology and behavior. Beyond furthering knowledge, the impact of stereotaxy extends to the symptomatic treatment of motor disorders in humans such as Parkinson’s disease, as well as biopsies of tumoral pathologies and interstitial radiotherapies, and the treatment of other illnesses like epilepsy. Different techniques are used for different research objectives, and these include experimental procedures in vivo for the activation or inactivation of certain brain regions or transmitter systems, permanent selective lesioning, functional neuroanatomy through the use of tracers, and acute or chronic measurements or recordings (Ferry et al., 2014).
One application of stereotaxic procedures on experimental animals lies in the investigation of behavioral functions mediated by different brain regions, by implementing an ablation of function. Procedures such as these involve experimentally inducing lesions of particular brain regions and studying the resulting deficits in function. There are plenty of ways to damage or deplete a brain structure or transmitter system through stereotaxic neurosurgery, such as electrolysis, transection, aspiration, radio frequency, injection of excitotoxins, or by global or focal ischemia through the use of devascularization, photo-irradiation, vessel occlusion, or vasoconstrictive agents. Additionally, stereotaxic neurosurgery can also be used for the temporary inactivation of a brain structure, through microinjection of muscimol, tetrodotoxin, lidocaine, or receptor antagonists at the target system.
Lesioning via electrolysis was first accomplished by Duncan et al., 1975 in which dopamine-containing nerve terminals were unilaterally destroyed through the use of unipolar electrolytic lesions of the medial forebrain bundle. Transection, meanwhile, is another reliable lesioning method that basically involves cutting across target brain structures. Milner and Amaral (1984) successfully created transections across projections of the septal complex to the hippocampus, in the attempt to investigate the existence of a ventral septohippocampal pathway. Moreover, some researchers argue that excitotoxic lesioning remains the most reliable and efficient method that guards against damage to local glial cells or traversing nerve fibers (Kirby et al., 2012). Extensive loss of brain cells surrounding target brain structures can conveniently be avoided by careful practice of excitotoxic focal injection (Jarrard, 2002). Researchers from UCLA and MIT (Kirby et al., 2012) also successfully developed a stereotaxic method allowing for excitotoxic lesions of specific brain areas through an infusion of N-methyl-D-aspartate (NMDA). Specifically, the infusion of NMDA into the brain results in the excitotoxic death of nearby neurons. The established protocol can also be used to infuse other biological compounds, such as viral vectors, and the implantation of more permanent osmotic pumps for prolonged exposure.
Furthermore, stereotaxic surgery also allows for the stimulation of certain brain regions through a number of techniques. These include electrical stimulation through a nonselective activation of a heterogeneous population of neurons, stimulation of NMDA-receptor bearing neurons through a microinjection of NMDA, and laser application succeeding transfection of modified gene sequences (Buchen, 2010). Through a combination of genetics, virology, and optics, optogenetics has provided a tool with which researchers can quickly activate or inactivate specific groups of neurons within particular neural circuits in a highly precise manner that is unachievable through other more traditional methods (Buchen, 2010).
Tracing of Neural Pathways
Retrograde and anterograde tracing, which involve the infusion of such substances as wheat germ agglutinin-conjugated horseradish peroxidase or Fast blue (Milner and Amaral, 1984), enable the tracing of neural pathways and particular brain structures as necessitated by some stereotaxic procedures.
In addition, stereotaxy also allows for sampling techniques in rodents, such as microdialysis. In vivo measurements (measurements in a living organism) can be done through the concentration of extracellular neuromediators by microdialysis, voltammetry (Li et al., 2006), and electropotentials. The process of microdialysis provides quantification of various substances in blood and tissue, including neurotransmitters and neuropeptides, enzyme activity, electrolytes, hormones as well as the monitoring of biochemical and physiological effects of pharmaceutical agents. Furthermore, the method also allows for a more in-depth observation of drug effects on extracellular levels of endogenous substances (Li et al., 2006). Aside from its sampling function, microdialysis also permits the infusion of certain substances into the brain and spinal cord. The stereotaxic procedure would involve the insertion of a microdialysis probe with inlet and outlet tubes for perfusion and sample collection into the fluid or tissue compartment (Zapata et al., 2009).
Rodent Models of Human Diseases
Another extremely useful application of stereotaxic lesioning is the elaboration of rodent models of human diseases. For instance, a model involving platelet aggregation with regard to cerebral ischemia was developed by Watson et al., 1985 as an attempt to investigate the widespread incidence of human stroke cases due to vascular thrombosis. Jumping off of well-established findings on stroke cases, Bergeron (2003) successfully yielded a photothrombotic cortical stroke model by inducing photochemical cortical lesions in the rat brain using stereotaxic surgical procedures. Particularly, a minimally invasive protocol for creating a model of permanent focal ischemia was developed through the use of photochemical cortical lesions.
Finally, stereotaxic procedures are also involved in practices of stem-cell transplantation. The discovery of neural stem cells has opened the door to the possibility of regenerative therapy for many illnesses of the central nervous system, such as Parkinson’s disease. Their properties of self-renewal and multipotency provide the answer to global degeneration or dysfunction, as characterized by loss of discrete neural populations either isolated or dispersed in specific regions of the brain. Neural stem cell transplantation in the rodent brain allows for further investigation into its therapeutic potential for the treatment of certain CNS diseases (Lee et al., 2008).
The importance of stereotaxy is fully emphasized by the numerous possible applications that it ceaselessly presents. The implications of stereotaxic principles during a technologically-advanced time only add to the many possibilities that this branch of science and these sophisticated set of methods can continue to offer.Stereotaxic instruments require appropriate handling and safekeeping for continued use. After every surgical operation, the stereotaxic apparatus along with reusable instruments should be cleaned, decontaminated, and sterilized. The instruments must first be washed with soap and water after use, thoroughly dried, and then autoclaved. More sensitive materials such as cannulae, obturators, and electrodes should be carefully decontaminated or autoclaved, taking into consideration their particular technical tolerances. In cleaning the stereotaxic apparatus, cotton swabs or paper towels soaked with an antiseptic solution of 70% ethyl alcohol are applied to the frame, ear, nose and tooth bars, and verniers, as well as on the heating pad, rectal temperature probe, and workbench. Upon storage, stereotaxic equipment must undergo regular preventive maintenance and service of equipment, depending on their frequency of usage. Measures such as these should effectively maintain the instrument’s precision, as well as prevent the possible stereotaxic error from loose or unsteady components.Bergeron, M. (2003). Inducing photochemical cortical lesions in rat brain. Current protocols in Neuroscience, 9-16.
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