A Practical Approach, Second Edition=Ronald D. Ho.pdf

DEVELOPMENTAL TOXICITY TESTING — METHODOLOGY 223placed in a Plexiglas box that has a narrow V opening in one side. The rodent’s body remains in thebox with the tail extending to the outside through the V opening. Alternatively, the rat can be suspendedin a cone-shaped container (attached to a ring stand) with the tail hanging down in warm water.To ensure that a length of vein will remain patent on subsequent days of dosing, the site of thefirst injection should be as near the tip of the tail as possible. Subsequent injection sites would belocated more proximal to the body. Holding the tip of the tail, the technician inserts the needle(normally 26 gauge attached to a 1-cc syringe) into the vein at a slight angle and slowly injectsthe test article. A localized swelling at the injection site indicates the dose was not delivered i.v.Following withdrawal of the needle, gauze is placed over the injection site, and pressure is applieduntil bleeding stops.Rabbits are dosed via the marginal ear vein, usually at 1 ml/kg of body weight. The rabbit isplaced in a restrainer that allows full access to the ears. The hair from the ear to be dosed is pluckedfrom the tip to the base of the ear at the ear margin to expose the vein. A 25- or 26-gauge needleis used with the appropriate size syringe. The vein is compressed proximal to the injection site.The needle is inserted into the vein (pointing toward the head), the pressure on the vein is released,and the dosing solution is injected slowly. The needle is then removed, and gauze is held firmly overthe injection site until the bleeding stops. The ear used is alternated each day, and the injection sitesshould be selected beginning at the tip of the ear and subsequently moving toward the head. Syringesshould be changed after each dosage group, and needles should be changed after each animal.Subcutaneous — In rodents, subcutaneous (s.c.) injections are normally made in the interscapulararea on the back of the animal. The volume used is generally up to 10 ml/kg of body weight,with needle size dependent on the size of the animal and the viscosity of the dosing formulation.The loose skin behind the neck is grasped between the thumb and forefinger. The needle is insertedthrough the fold of skin but not into the underlying muscle. Loose skin behind the neck or in thehip area of the rabbit is used for s.c. injections. Syringes should be changed between dosage groups,and needles should be changed after each animal is dosed.Intraperitoneal — A rat is restrained by holding its head and thorax. A mouse is grasped bythe scruff of the neck, and the tail is twined around a finger to control the hindquarters. The needleshould be introduced rapidly into a point slightly left or right of the midline and halfway betweenthe pubic symphysis and xiphisternum on the ventral abdominal surface. A 25- or 26-gauge needleis used, and the typical injection volume is 2.5 to 10 ml/kg of body weight.Rabbits are held by the scruff of the neck by a seated technician, with the rabbit’s hips restingin the technician’s lap and the rabbit facing another technician who performs the dosing. The needleis inserted through the skin of the abdomen just lateral to the midline and just posterior to the areaof the umbilicus, pointed toward the spine. The syringe is changed between dose groups, and theneedle is replaced between injections. Intraperitoneal injections in late pregnancy run the risk ofinjection directly into the gravid uterus, since a great deal of the space in the peritoneal cavity istaken up by the uterus.Intramuscular — The site of intramuscular dosing is usually the haunch or upper hind leg(“thigh” region), with the needle inserted into the muscle; repeated dosing should alternate dosingsites (right, left, and repeat).5) Inhalation Exposures — Mated females can be exposed in their home cages (if they arewire-mesh hanging cages), with food and water sources removed (to prevent inadvertent exposurevia ingestion of absorbed or dissolved test material), or the animals can be transferred to exposurecages. Automatic watering systems can be employed within the exposure chambers to providedrinking water ad libitum during the exposure periods. The amount of water exposed to the testatmosphere in the tip of the “nipple” is very small. Therefore, the amount of dissolved test materialin the water available for drinking is also very small. Animals are usually exposed 6 to 8 h/d. Thisinterval is determined from the time the desired concentration is reached at the start of exposure,using a calculated t 99 (time required to attain 99% of the target concentration) until the exposure© 2006 by Taylor & Francis Group, LLC