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ILAR Journal V41(1) 2000
The Squirrel Monkey in Biomedical and Behavioral Research
Alan G. Brady
| Alan G. Brady, D.V.M., is Associate Professor in the Department of Comparative Medicine, University of South Alabama College of Medicine, Mobile, Alabama. |
Squirrel monkeys are useful animal models in a variety of different research disciplines in biomedical research. To properly utilize these animals, it is important that research techniques are properly suited to the unique characteristics of this genus. To consider squirrel monkeys as "miniaturized macaques," or a slightly different version of another animal model, is to underestimate both the potential and the pitfalls of using these animals in research.
Characteristics of the Squirrel Monkey Pertinent to Research
Basic Biology and Husbandry
Squirrel monkeys are an arboreal genus that maintain themselves in large groups in the wild. As an arboreal species, they do not readily use the floor of a cage but make maximum use of multiple levels of perches (Williams et al. 1988). Important differences between these animals and the more commonly used macaques include an absolute nutritional requirement of vitamin D3 (cholecalciferol), lack of menses during the female reproductive cycle, and marked seasonality in reproduction--with substantial weight gain ("fatting") in males during the mating season. Normative biology information is shown in Table 1. Hematologic and blood chemistry values for squirrel monkeys have been published (Beland et al. 1979; Richter et al. 1984) and are also available on the Internet (http://www.saimiri.usouthal.edu/prl).
Biosafety Considerations
Concerns regarding transmission of zoonotic diseases are a part of working with any nonhuman primate. Squirrel monkeys are carriers for the Herpesvirus saimiri, which is known to be oncogenic in other primates and can infect and transform human T lymphocytes in vitro (Mansfield and King 1998). They are also carriers for Herpesvirus tamarinus, which is known to cause fatal disease in Aotus and some other species of New World primates (Mansfield and King 1998). Biosafety precautions should always be followed with squirrel monkeys. A sensible approach is to follow the Universal Precautions for Workers Handling Human Blood, Body Fluids and Tissue in the Workplace (CDC 1988) when working with squirrel monkeys, their tissues, and their body fluids.
Squirrel monkeys are not considered to be carriers for Cercopithecine herpes virus 1 (also known as herpes B virus), which can cause deadly central nervous system disease in humans. Therefore, the stringent testing and treatment techniques required for bites, scratches, and splashes inflicted by macaques on humans are not required for cases involving squirrel monkeys (Holmes et al. 1995). Squirrel monkeys are also considered less likely to contract tuberculosis (Osborn and Lowenstine 1998) than macaques.
Importance of Saimiri Species Identification and Pedigree Validation Techniques
Although taxonomists disagree about some of the specific classifications of squirrel monkeys (Lehner 1984), researchers should consider using available techniques for classification of this genus because species identification can have important consequences for research (Moore et al. 1990). Hershkovitz (1984) distinguishes among four species and nine subspecies of Saimiri. Of these, only two species (Saimiri sciureus and Saimiri boliviensis), divided among three subspecies, have been widely used in biomedical research (Abee 1989). Differences among species and subspecies that may have research implications include body weight (Brady et al. 1998; Coe et al. 1985), susceptibility to cholesterol gallstone formation (Portman et al. 1980), endocrine responses (Coe et al. 1985; Martin and McNease 1982), behavior (Mendoza and Lowe 1978), growth and development (Ausman et al. 1985), and susceptibility to malaria (Whiteley et al. 1987).
Techniques available to type animals according to species include physical appearance (Hershkovitz 1984), cytogenetics (Hershkovitz 1984; Moore et al. 1990), and DNA analysis. Only the first two techniques are readily available to investigators. Identification by physical appearance must be regarded as tentative and should be buttressed by the more definitive cytogenetic method. The publication of Hershkovitz (1984) contains drawings and photographs for identification purposes. Photographs for identification may also be found on the "Squirrel Monkey Breeding and Research Resource" Web site (http://www.saimiri.usouthal.edu/prl), which has a wealth of other information about squirrel monkeys that may be useful to investigators.
Cytogenetic analysis for definitive species identification is available. For additional information, contact Dr. Charleen Moore, Department of Cellular and Structural Biology, University of Texas Health Science Center at San Antonio, 7703 Floyd Curl Drive, San Antonio, Texas 78284 (Tel: 210/567-3875).
Biochemical markers have been used for pedigree validation in squirrel monkeys (VandeBerg et al. 1990). Squirrel monkey infants are frequently carried by females in their social group other than by the real dam (allomaternal behavior). As a result of this behavior, pedigree validation may be especially important.
Identification of Animals
Identification of Individual Animals
A variety of methods are available for identification of squirrel monkeys. When properly applied, tattoos (AIMS Inc.1 Budd Lake, New Jersey) provide a durable method of identification. Neck tags that can be read from a distance are advantageous when animals are group-housed. The Squirrel Monkey Breeding and Research Resource at the University of South Alabama uses a modified livestock tag for this purpose (these tags may be purchased in a variety of colors,
further aiding in animal identification from a distance). Collars for neck tags must be carefully fitted. A loose collar may slip over the animal's mandible or catch on parts of the cage. A collar that is too tight may cause skin lesions or, in extreme cases, impede respiration. Collars should be rechecked for proper fit whenever animals are handled because weight loss or gain may change the required collar size.
Implantable microchip transponders have been developed (AVID1 Norco, California; BioMedic Data Systems1 Seaford, Delaware) that broadcast a unique identifying number to an external scanner. When properly implanted, these devices also provide reliable identification.
Determination of Age by Dentition
One major advantage of using purpose-bred squirrel monkeys for research is that the age of an animal can be documented. If feral animals are used, some rough generalizations may be made regarding an animal's age by examining dentition (Brady and Morton 1998; Galliari and Colillas 1985; Long and Cooper 1968; Smith et al. 1994; Tappen and Severson 1971). Information on age of eruption of Saimiri teeth is provided in Table 2.
Hypothermia and Hypoglycemia
Investigators accustomed to using rodents and larger primates in their research should be aware of squirrel monkey characteristics that may require alteration of experimental procedures. Their small size, low body fat, and long extremities probably all contribute to rapid heat loss when they are anesthetized or debilitated. Such animals should be monitored for body temperature on a regular basis. Awake, restrained squirrel monkeys normally have a temperature of 38 to 39.5°C. Healthy, active, struggling animals may rapidly increase their body temperatures as high as 41°C. Temperatures of less than 37.7°C are seen with seriously ill animals and should be investigated immediately. In general, electronic thermometers designed for use on humans and animals may be used with squirrel monkeys. We have also found that tympanic membrane thermometers (Thermoscan® Instant Thermometer,1 Thermoscan Inc., San Diego, California), which register temperatures much faster than most rectal thermometers, can be used with adult squirrel monkeys. This technique is especially useful for rapidly estimating temperatures on multiple animals. In 76 paired measurements with a calibrated mercury rectal thermometer, the correlation was 0.85 with an R2 value of 0.72. The tympanic membrane thermometer tended to register 0.2 to 0.6°C lower than the mercury rectal thermometer. Standard deviation was 0.64°C compared with 0.20°C for the mercury rectal unit (L. E. Williams, University of South Alabama, Mobile, Alabama, personal communication, 1992). Careful positioning of these thermometers over the tympanic membrane is important. In our experience at the University of South Alabama, these thermometers do not work well for infant or smaller juvenile animals.
For hypothermic animals, supplemental heat may be provided in a variety of ways. Awake animals may be housed in incubator-type cages (Thermocare® Inc.,1 Incline Village, Nevada) or on warm water recirculating pads (Gaymar T Pump®,1 Orchard Park, New York). Use of such devices makes care more time consuming. Recirculating pads may also be used for anesthetized animals. Heat packs that use a controlled exothermic chemical reaction can provide a convenient, portable, temporary source of warmth for anesthetized and recovering animals. Many of the packs that use sodium acetate/water reactions (The Heat Solution®,1 Prism Technologies, San Antonio, Texas) are nontoxic if the pack is ruptured. Electrical heating pads should not be used in squirrel monkeys due to the danger of bums and electrical shock.
Squirrel monkeys are also predisposed to hypoglycemia (Abee 1985; Brady et al. 1990b, 1991a). The normal blood glucose concentration for adult Saimiri is 80 -+ 28 mg/dL (Loeb and Quimby 1989). Prolonged research procedures, fasting, or debilitating conditions that result in lack of appetite may place animals at risk for a hypoglycemic crisis. Dry reagent strips for determining blood glucose concentration and the meters that accurately read these strips (Accucheck®, 1 Boehringer Mannheim Corp., Indianapolis, Indiana) are available from most pharmacies. If used in combination with appropriate therapy, these strips are inexpensive and useful tools for protecting the health of squirrel monkeys. Hypoglycemic animals that are still conscious should be given glucose or sucrose solution orally (Gatorade® Corporation, 1 Chicago, Illinois). Unconscious animals should be given sterile 20% dextrose solution by stomach tube (see technique below). Infants are given 1 mL, juveniles 3 to 5 mL, and adults 10 mL. Use of concentrated intravenous glucose solutions to treat hypoglycemia is not recommended.
Restraint Techniques
Restraint techniques used with other primates may be used with squirrel monkeys. Manual restraint, chair restraint, and pole-and-collar methods may all be used for these animals. Restraint equipment is available (Primate Products,1 Redwood City, California).
Techniques of restraint may have important effects on certain types of studies. Both capture and chair restraint can cause adrenal cortical activation and growth hormone release (Brown et al. 1971a,b). Manual restraint can cause elevation in glucagon (Myers et al. 1988). General anesthesia with ether can cause growth hormone release in squirrel monkeys (Brown et al. 1971a). Ketamine anesthesia has been postulated to cause changes in glucose tolerance test results in other nonhuman primates (Streett and Jonas 1982); however, this belief has been disputed by others (Brady and Koritnik 1985; Castro et al. 1981; Kemnitz and Kraemer 1982). The ultrashort-acting barbiturate anesthetic sodium thiopental can cause premature ventricular contractions in squirrel monkeys (Wolf et al. 1969).
Basic Techniques
Gastric Intubation and Vascular Access
Gastric intubation and vascular access techniques are useful for a variety of studies. Gastric intubation may be performed by the orogastric or nasogastric route. Both may be performed using a number 5 French infant feeding tube (Abee 1985). Orogastric intubation requires the use of a speculum in awake animals to prevent the animal from chewing the tube. Nasogastric intubation requires some practice to be performed properly in the squirrel monkey. After marking the distance to the stomach on the tube, a small amount of sterile lubricant jelly (KY® Jelly,1 Johnson & Johnson Co., Arlington, Texas) is applied to the tip, and the tube is gently inserted into the nares. The tube must be directed medially to avoid the blind pouch in the nasal cavity (Abee 1985). When the tube has been inserted into the previously made mark, it should be dipped in water to form a meniscus on the luer adapter. This meniscus should then be observed for respiratory movement because such movement suggests that the tube may have been inserted into the trachea and should be repositioned before use to prevent aspiration pneumonia. Epistaxis may occasionally occur as a result of nasogastric intubation, but it is usually mild and self-limiting when the technique is performed properly.
Blood Sampling and Parenteral Injections
Venous sampling and intravenous injection are best performed with 23- to 27-gauge needles. Large samples (>0.25 mL) may be taken from the femoral vein at the femoral triangle or the lateral tail vein at the tail base. These sites are also suitable for injection. The saphenous veins on the distal legs may be used for intravenous injections and smaller samples. The latter sites are generally suitable only for smaller samples because they collapse easily. Blood sample volumes are limited by the size of squirrel monkeys. As a general rule, no more than 10 mL of blood per kilogram of body weight should be sampled at one time. Animals from which samples are taken repeatedly should have periodic hematocrits to monitor for anemia. At the University of South Alabama, we use the following protocol to treat adult squirrel monkeys that have anemia associated with blood loss or that are included on experimental protocols requiring frequent blood sampling:
- Administer injections of
- iron as iron dextran: 50 mg intramuscularly, once;
- folic acid: 2.5 mg subcutaneously, twice weekly; and
- vitamin B complex includes cyanocobalamin, riboflavin (B2 ), and pyridoxine (B6): dose depends on formulation, subcutaneously, twice weekly.
- Repeat hematocrit weekly. When hematocrit reaches 40%, stop injections.
- Continue to monitor hematocrit weekly. If hematocrit remains >40%, discontinue treatment.
Anesthesia and Analgesia
Anesthesia may be performed in squirrel monkeys with many of the same agents used in other species. The foregoing discussions regarding hypothermia and hypoglycemia in squirrel monkeys are directly applicable to animals that are anesthetized, and especially those animals anesthetized for prolonged periods. Appropriate precautions should be taken to prevent hypothermia and hypoglycemia.
A list of selected anesthetic agents for squirrel monkeys appears in Table 3. Although the list is not comprehensive, the agents that are included are relatively safe and easy to use in a variety of experimental procedures. The use of a long-acting barbiturate such as sodium pentobarbital is recommended in squirrel monkeys only for terminal procedures because long-acting barbiturates cause respiratory depression (Flecknell 1987) and prolonged recovery in squirrel monkeys. If sodium pentobarbital is used at the standard 50 mg/mL concentration, it should be diluted 1:1 with saline before use.
Isoflurane is an excellent agent for inhalation anesthesia of squirrel monkeys. Mask induction may be rapidly performed in animals that are manually restrained, or animals may be induced with an injectable agent, intubated, and maintained under anesthesia with isoflurane. Moms et al. (1997) have described a technique for intubation of squirrel monkeys. Dead space, resistance to flow, and heat and humidity loss in anesthesia circuits are important considerations because of squirrel monkey size (Hartsfield 1996). A non-rebreathing anesthesia circuit such as the Bain Coaxial Circuit (Jorgensen Laboratories,1 Loveland, Colorado) is recommended for inhalation anesthesia.
Mechanical ventilation can be performed in squirrel monkeys under anesthesia. A neurophysiology study of five adult squirrel monkeys used an estimated tidal volume of 6 to 8 mL/kg with a respiratory rate of 70 per minute as initial settings. These volumes were then adjusted to give an end tidal carbon dioxide (collected through a catheter inserted through the endotracheal tube to midtrachea) of 3.5 to 3.8% (Maier et al. 1997; T. H. Moms, SmithKline Beecham Pharmaceuticals, Harlow, UK, personal communication, 1999).
Analgesics should be used in squirrel monkeys after survival from surgical procedures unless specifically contraindicated by experimental protocol or the medical condition of the animal. Buprenorphine at a dose of 0.01 mg/kg intramuscularly or intravenously causes little sedation in squirrel monkeys and is an effective analgesic that may be given every 12 hr (Jenkins 1987). Other analgesic drugs for nonhuman primates have also been described (Flecknell 1987; Jenkins 1987).
Digestive System
Squirrel monkeys are omnivores and so should be provided with supplemental food items (vegetables, fruit, even live food such as mealworms) in addition to a nutritionally balanced staple laboratory diet biscuit (5040 New World Primate Diet,1 PMI Feeds Inc., Richmond, Indiana). When properly selected and presented, such supplementation provides a back-up source of vitamin C in addition to a form of environmental enrichment. Lack of vitamin C in the diet can cause scurvy in squirrel monkeys. These animals can have a unique presentation with this disease, with animals forming cranial hematomas (cephalhematomas) in advanced stages (Kessler 1980).
Squirrel monkeys have been used in studies of Trichomonas infection; an intestinal trichomonad, Tritrichomonas mobiliensis, was first isolated from squirrel monkeys. Procedures for isolation and culture of this organism from rectal swabs have been published (Pindak et al. 1985). Colleagues and I have developed a technique for rendering animals free of intestinal trichomonads by using 25 mg/kg of metronidazole, given by gavage tube twice daily for 5 days, combined with a program of meticulous disinfection (Brady et al. 1988). Although T. mobiliensis has been of considerable scientific interest, the pathogenicity of this organism in squirrel monkeys is controversial.
Cardiovascular System
Squirrel monkeys were one of the first primate models of human atherosclerosis (Malinow et al. 1966, Middleton et al. 1967b). Although other primate species have since supplanted the squirrel monkey in this role, techniques for inducing atherosclerotic plaques in squirrel monkeys by diet manipulation have been well described (Maruffo and Portman 1968; Middleton et al. 1967a). A semipurified diet containing 0.1 g/kg of cholesterol with 45% of calories supplied as fat (Maruffo and Portman 1968) was used to induce the lesions in one study. Squirrel monkeys fed atherogenic diets can have a high incidence of gallstones (Osuga and Portman 1971).
Interest in the squirrel monkey as a model for cardiovascular disease has led to a number of studies of normal cardiovascular function in Saimiri. In one study in which adult squirrel monkeys anesthetized with thiopental had a mean heart rate of 248, each animal's electrocardiogram was characterized. Premature ventricular contractions were commonly observed during the 5 min after induction of anesthesia (Wolf et al. 1969). In another study, mean heart rates in young squirrel monkeys were 271,366, and 333 in 1-day-, 1-mo-, and 1-yr-old squirrel monkeys, respectively. Maturational changes in axis, voltage, and electrocardiographic intervals were similar to those seen in humans (Brady et al. 1991b).
Blood pressure has been studied in chair-restrained squirrel monkeys. Mean arterial blood pressure was 140 + 4 mmHg in 14 chair-restrained squirrel monkeys (Byrd and Gonzalez 1981).
Male Urogenital System
Prostate Gland
Lewis and colleagues (1981) have described the anatomy and pathology of the squirrel monkey prostate gland. Prostatic adenocarcinoma was found in one aged male of the eight males examined.
Semen Collection and Preservation
Semen may be collected by training animals to masturbate and by electroejaculation and vibrostimulation. Of the three techniques, vibrostimulation is recommended because it causes relatively little stress, the sample quality is superior to that obtained with electroejaculation (Yeoman et al. 1997b), and no animal training or anesthesia is required. If electroejaculation is used, care should be taken to avoid excessive tissue temperatures that would result in electrical burns of the rectum. Electroejaculation should be performed with equipment that monitors the temperature of the rectal tissue at the electrode site (Dalzell USA Medical Systems,1 The Plains, Virginia). A technique for cryopreservation of squirrel monkey semen has been described (Yeoman et al. 1997a).
Female Urogenital System
Fertility Assessment and Manipulation
Abee and Aksel (1983) have developed a method for radio-graphically demonstrating the patency of squirrel monkey oviducts. After clamping the cervix, a needle is inserted into the uterus under laparoscopic visualization. Contrast media are injected to demonstrate transit through the oviducts to the abdominal cavity.
Ovulation has been induced in squirrel monkeys. One study utilized a 4-day regime of follicle-stimulating hormone injections followed by injection of human chorionic gonadotropin injection 12 hr after the last follicle-stimulating hormone dose (Dukelow 1979a,b). Oocytes were harvested by laparoscopy 8 to 15 hr after the human chorionic gonadotropin injection. Mizoguchi and Dukelow (1981 ) have described a fixation technique for examination of chromosomes from squirrel monkey oocytes collected by induced ovulation. No evidence of direct effect of gonadotropins on squirrel monkey chromosomes was found.
Fetal Monitoring, Manipulation, and Predicting Pregnancy Outcome
With practice, pregnancy can be determined by manual palpation by 28 to 35 days of the 148- to 160-day gestation period. Measurements of fundal height (from the cranial symphyseal border to the highest point of convexity of the gravid uterus) may also be used to determine gestation (Logdberg 1993). Normal morphometric values for growth and development based on ultrasound imaging have been published for the fetus (Logdberg 1993, Narita et al. 1988). The squirrel monkey fetus has also been imaged using magnetic resonance imaging (Brady et al. 1990a; D'Angelo et al. 1990). Time to delivery during the last half of pregnancy may be approximated without imaging by caliper measurement of fetal biparietal diameter (defined as the distance between the prominences of the parietal bones of the fetal skull) through the abdominal wall of the dam (Brady et al. 1998). Aksel and Abee (1983) have used a technique for measuring the birth canal in squirrel monkeys with radiographs (called "pelvimetry") to predict perinatal mortality. Squirrel monkeys have a high incidence of congenital anomalies, some of which have been characterized in a report by Stills and Bullock (1981).
Cesarean Section
Dystocia is relatively common in squirrel monkeys. Birth weight for squirrel monkey infants is commonly 105-120 g (approximately 15 to 17% of the dam's weight for males and approximately 5 to 8% for females). This characteristic, combined with large head size of term infants, may be responsible for dystocias observed in squirrel monkeys. Criteria for establishing that assistance is necessary in either a vaginal delivery or a cesarean section include
- a weakened or depressed dam or evidence of fetal distress (such as fetal heart rate of fewer than 180 beats per minute);
- failure to progress with labor during a 2-hr period; or
- malpresentation of the fetus (as a general rule, breach presentation of a squirrel monkey fetus requires cesarean section for successful delivery).
Dams undergoing cesarean section frequently reject their infants, requiring foster or nursery rearing. Foster rearing is greatly superior to nursery rearing and should be attempted whenever possible. Ricker and colleagues (1985) and Patino et al. (1995) have described nursery care of squirrel monkey infants. Infants are initially fed 5% dextrose. One hour later, they are fed a 1:1 mixture of 5% dextrose and formula (ZooLogic 20/14 Milk Matrix®,1 Pet Ag Inc., Hampshire, Illinois), followed by undiluted formula (consisting of the Milk Matrix® mixed with powdered milk according to the manufacturer's instructions). Feeding is performed hourly, 15 hr/day.
Endocrine System
The relative resistance of squirrel monkeys to glucocorticoids has been the focus of much study. Squirrel monkeys have cortisol levels that are 12 times those of humans and other nonhuman primates (Cassorla et al. 1982). Fuchs et al. (1997) have described a noninvasive test for salivary cortisol in squirrel monkeys. Reynolds and colleagues (1998) recently developed by transformation with Epstein-Barr virus a B-lymphoblast cell line from squirrel monkeys that expresses this glucocorticoid resistance.
Nervous System
Basic Techniques
Gergen and Maclean (1962) have published a stereotaxic atlas of the squirrel monkey.
Small volumes of cerebrospinal fluid may be collected from Saimiri at the atlanto-occipital articulation. Proper positioning, good anesthesia, and a sharp hypodermic needle are important for safe and successful completion of the procedure. A 1-mL syringe with a 0.5 x 16 mm (25-gauge, 5/8 inch) needle is used (Monoject®,1 Sherwood Pharmaceuticals, St. Louis, Missouri). The animal is anesthetized with a regimen that allows for excellent muscle relaxation, such as ketamine and xylazine (see above). The puncture site is clipped and prepared with antiseptic soap and solution as with preparation of a surgical site. The animal is placed in an upright position with the neck gently flexed and stretched to open the joint space. The joint space is palpated as a depression between the two bony prominences on the midline, and the needle is slowly inserted perpendicular to the skin. Resistance will be felt when the needle contacts the dorsal atlanto-occipital membrane. The needle is then "popped" through the membrane into the cisterna magna without penetrating farther into the spinal cord. A 0.1- to 0.2-mL sample of cerebrospinal fluid may then be slowly aspirated into the syringe. The rare occurrence of seizures resulting from this procedure may be controlled by using midazolam (Versed®, 1 Roche Pharmaceuticals, Nutley, New Jersey) at a dose of 0.1 mg/kg intravenously. Midazolam may be repeated if necessary.
Neurophysiology
Yeshuron and colleagues ( 1981) have designed an interactive computer graphics program for simulating the penetration of a microelectrode into the deep structures of the squirrel monkey brain. Subsequently, Perachio and Correia (1983) developed a slender shaft glass micropipette electrode system, which they have used successfully in the squirrel monkey to record electrical activity in deep brain structures.
Ochikubo and colleagues (1993) have described a methodology for the squirrel monkey electroencephalogram. Kleinlogel (1983) had previously characterized the electroencephalogram during squirrel monkey sleep.
Special Senses
A technique has been developed for assessing olfactory performance in squirrel monkeys. The same methodology has been successfully used in humans (Laska and Hudson 1993).
The squirrel monkey is frequently used in studies of vestibular function. Balance may be tested using the squirrel monkey rail test (Igarashi 1968), and postural ataxia may be characterized using a stabilimetry technique developed by Igarashi and colleagues (1982). Squirrel monkey data have been used to develop a mathematical model of human vestibular responses to some types of rotation (Merfeld 1995).
Behavior
A comprehensive review of experimental techniques for characterizing behavior in Saimiri is beyond the scope of this paper. Nevertheless, it is important to note that squirrel monkey motor activity can be measured in a variety of ways.
Holtzman and Young (1990) described measurement of motor activity using a relatively simple ultrasonic sensor system linked to a personal computer. Andrews (1993) described a video task for squirrel monkeys that requires use of a joystick to place a cursor into a movable target for a food reward. Patel and Migler (1982) described a squirrel monkey conflict test that can be used to test anxiolytic agents.
Necropsy and Perfusion
For protocols that require perfusion before necropsy, the following procedure may be used. A gravity perfusion apparatus is set up at the necropsy table and consists of a parenteral bag or bottle of 500 mL of saline mixed with 1000 units of heparin (for adult animals) (C. Wefers, University of California at Davis, personal communication, 1999) and a l-liter bag or bottle of the fixative of choice for the experimental protocol (such as phosphate-buffered formalin). Tubing from both containers is attached to a three-way stopcock, with outflow from the stopcock leading to a piece of tubing attached to a 16-gauge hypodermic needle. Stopcock positions for flow of saline or fixative must be clearly understood and/or marked before the procedure.
Surgical plane anesthesia with sodium pentobarbital is recommended for the procedure (see Anesthesia and Analgesia above). A thoracotomy is performed, the heart is exposed, and the needle is inserted into the left ventricle. The flow of saline is started, and the right atrium is cut to allow outflow of blood. Saline flow is continued until outflow from the right atrium is clear (do not overflush with saline or improper fixation may result). At this point, the stopcock valve is readjusted to deliver fixative to the animal. Perfusion with as much as 1 liter of fixative may be required to obtain fixation in an adult animal (C. Wefers, personal communication, 1999).
It can be seen that there is a wealth of information available regarding research techniques for use with squirrel monkeys. Careful attention to the unique characteristics of these animals is important to realize this genus' full potential as research animals.
Acknowledgments
The author gratefully acknowledges the assistance of Regina McCreary, Marilyn Holladay, Sheila Morse, and Dr. Susan Gibson in the preparation of this article.
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Table 1 Basic squirrel monkey informationa
| Gestation: | Approximately 150 daysb | ||
| Sexual maturity: | 2.5-3.5 yrc | ||
| Life span: | Approximately 21 yrd | ||
| Median adult weight for Saimiri boliviensis boliviensis: | Male | Breeding season | 1094 g |
| Nonbreeding season: | 892 g | ||
| Female | 700 g |
aL. E. Williams, Department of Comparative Medicine, University of South Alabama, unpublished data.
bData from Abee CR. 1989. The squirrel monkey in biomedical research. ILAR News 31:11-20.
cData from Richter CB, Lehner NDM, Hendrickson RV. 1984. Primates. In: Fox J, Cohen B, Loew F, editors. Laboratory Animal Medicine. San Diego: Academic Press. p 298-379.
dData from Rowe N. 1996. The Pictorial Guide to Living Primates. East Hampton, NY: Pogonias Press.
Table 2 Ages of dental eruption for squirrel monkeysa
| Incisors | Canines | Premolars | Molars | |
| Deciduous (days) | 0-21 | 14-28 | 28-77 | None |
| Permanent (months) | 8-14 | 19-22 | 12-16 | 5-22 |
aAdapted from Long JO, Cooper RW. 1968. Physical growth and dental eruption in captive-bred squirrel monkeys. In: Rosenblum LA, Cooper RW, editors. The Squirrel Monkey. New York: Academic Press. p 193-205.
Table 3 Selected sedation and anesthesia agents for Saimiri
| Class | Agent | Use | Dose | Comment |
| Injectable | Ketamine | Sedation (chemical restraint) | 20-40 mg/kg SC | Best used for short procedures |
| Injectable | Ketamine/xylazine | Anesthesia | 10-20 mg/kg ketamine 4-8 mg/kg xylazine SC | Best used for short procedures. Xylazine reversible with yohimbine at 0.3 mg/kg IV |
| Injectable | Sodium thiopental | Anesthesia | 15-20 mg/kg IV | Reduce induction dose by 50% if ketamine is used as preanesthetica |
| Inhalation | Isoflurane | Anesthesia | 2-4% induction 1-3% maintenance | Indicated for debilitated animals and prolonged procedures |
aData from Flecknell PA. 1987. Laboratory Animal Anesthesia. London: Academic Press.
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