Physiology& Behavior, Vol. 49, pp. 397-399. ©PergamonPress plc, 1991. Printedin the U.S.A.

0031-9384/91 $3.00 + .00

BRIEF COMMUNICATION

An Inexpensive Gradient for Temperature Selection in Rodents B R I A N S. STUMP, JOHN G. M c C O Y A N D D A V I D D. A V E R Y 1

Department o f Psychology, Colorado State University, Fort Collins, CO 80523 Received 17 April 1990

STUMP, B. S., J. G. McCOY AND D. D. AVERY. An inexpensive gradientfor temperature selection in rodents. PHYSIOL BEHAV 49(2) 397-399, 1991.--The construction of a horizontal temperature gradient is described in detail. The apparatus is built from readily available and inexpensive materials. The chamber is built from PVC tubing, fitted with an aluminum floor, and placed in a sound-attenuated box. A temperature gradient is formed by placing solid CO2 at one end and a hot plate at the opposite end of an aluminum floor. The apparatus described is reliable, dependable, and has proven to be very suitable for use with small rodents. Temperature selection

Thermal gradient

Small rodents

TEMPERATURE gradients are relatively common pieces of equipment in biological, zoological, and psychological laboratories that have proven to be useful in the study of behavioral thermoregulation (1--4, 6, 7). It is also a piece of equipment that experimenters usually have to design and build rather than purchase from an equipment manufacturer. The gradient described here, similar to Stinson and Fisher's (6), was designed to be constructed easily, to be inexpensive to operate, to be easily modifiable, and to be able to accommodate medium to small rodents. The materials needed for construction are listed in Table 1. The first step in construction is to cut 1/2 in. (1.27 cm) plywood into six 1.5 ft by 8 ft (45.72 by 243.84 cm) sheets; two of these are then divided into four 4 ft by 1.5 ft (121.92 by 45.72 cm) sheets. In one 8 ft (243.84 cm) and one 4 ft (121.92 cm) length of the plywood, apertures should be cut in the center of each sheet for the windows: 5 in. by 7.5 ft (12.7 by 228.6 cm) and 5 in. by 3.5 ft (12.7 by 106.68 cm), respectively. Approximately 3/4 in. (1.905 cm), on opposite sides, from the edge of the window openings washers should be loosely attached with 1/2 in. (1.27 cm) wood screws (see Fig. 1). The washers must be loose enough to allow the acrylic windows to be fitted between the wood and the washer, and to allow the windows to be slid open easily. In the bottom section of the chamber, 2 in. (5.08 cm) from the front, eight 1 in. (2.54 cm) diameter holes should be drilled every 1 ft (30.48 cm) to allow for wiring. From these segments of plywood a 12 by 1.5 by 1.5 ft (365.76 by 45.72 by 45.72 cm) chamber is constructed, using 2 by 4 in. (5.08 by 10.16 cm) studs as bracing at both ends and the joint where the 4 ft (121.92 cm) section and 8 ft (243.84 cm) section

Thermoregulation

meet. The sections containing the windows should be attached last with hinges (refer to Fig. 1). Safety hasps are affixed 6 in. (15.3 cm) from each end of the chamber and one is placed 6 in. (15.3 cm) from the joint (refer to Fig. 2). A pneumatic nail gun speeds the construction and reduces splitting of the wood. The sound attenuation board is then cut to fit the inside of the chamber and can be adequately held by wood glue. The two remaining sides of the chamber are constructed with plywood also. In the center of the side panel, nearest the cold end of the gradient, a 6 in. (15.3 cm) diameter opening is cut for a small exhaust fan for removal of excess CO2. This side is attached to the chamber. The other end panel is left unattached for the placement and removal of the gradient. This panel is also removed when an animal is extracted from the gradient. The windows are constructed by dividing the acrylic sheeting into three 4 ft by 6 in. (121.92 by 15.24 cm) pieces. These are then attached to the inside of the front panel by sliding the windows between the washers described previously. The gradient, itself, is constructed by grinding a 7 ft by 1 in. (213.36 by 2.54 cm) slot in the PVC tubing, with a dremel hobby tool, leaving 6 inches (15.24 cm) intact on both ends of the tube. The floor of the gradient is first cleaned with acetone. This will remove the oil from the aluminum so that it is possible to attach thermocouples to the stock with masking tape. Once the thermocouples are attached to the bottom of the flooring, the floor is slid into the PVC and the leads of the thermocouples are fed through the slot in the tubing. The gradient is then placed in the chamber. The gradient is held in place by two cardboard supports. These are constructed by cutting a 8 in. (20.32 cm) diameter cir-

~Requests for reprints should be addressed to David D. Avery.

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398

STUMP, McCOY AND AVERY

TABLE 1

cle in the center of each section of cardboard; then each section of cardboard is folded in half and the opposite sides are coupled with packing tape to form the triangular trusses. The thermocouple leads are placed through the floor of the chamber and then connected to the recorder (Honeywell, Electronics 112). During experimentation, the selected ambient temperature of the subject is determined by visually observing in which section of the gradient the midpoint of the ventral abdomen (6) lies. The temperature of that section is then noted from the recorder. Lighting is supplied by a string of outdoor Christmas lights (20 watts). These were chosen because of the low thermal emission by the bulbs. Therefore, these bulbs have minimal effects on the temperature of the gradient. Only two bulbs, 8 ft (243.84 cm) apart, are left in the string. Each bulb is positioned behind the gradient such that they are not in the subject's visual field. Plugs for the ends of the gradient are made by using the same acrylic as the windows. Eight in. by six in. (20.32 by 15.24 cm) acrylic pieces are cut and placed over each end of the gradient and are held in place by the solid CO z, on one side, and a small brick on the other. Thus the Plexiglas plug located at the end containing a cold source serves as a partition between the air in the gradient and the CO 2 vapor emitted from the solid CO 2, which is in turn removed by the exhaust fan.

CONSTRUCTION MATERIALS FOR GRADIENT Item Description

Quantity

l/>in, plywood: 4 ft by 8 ft 2-in. by 4-in. (pine or fir) studs: 8 ft acrylic sheeting: V4 in. by 2 ft by 4 ft clear poly-vinyl-chloride tubing: 6 in. inner diameter by 8 ft Aluminum bar stock: V4 in. by 10 ft by 4 in. l-in. dowel: 4 ft hot plate 76.5 sq. feet: 2-in. sound attenuation board cardboard: 10 in. by 10 in. by V4 in. exhaust fan: 1800 rpm outdoor (20 watt) Christmas lights: 15 ft eye-screws: 1 in. washers: V4 in. (inner diameter), 1 in. (outer diameter) wood screws: V2 in. safety hasps temperature recorder thermocouples

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FIG, 1. Proportional illustrations of (a) the front view of chamber showing placement of windows, hinges, and safety hasps, (b) the side view of chamber, and (c) the front view of temperature gradient.

TEMPERATURE GRADIENT

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Solid CO 2 and a heating element are placed at either end, as shown in Fig. 2. Solid CO 2, approximately 2 lb (1 kg), is placed directly on the surface of the flooring (see Fig. 2). The hot plate should be placed at least two inches (5.08 cm) from the edge of the PVC, and not be heated to more than 50°C or the tube may deform. Gradient temperatures stabilize within 30 minutes, and follow a nearly linear curve. The cold source reaches 0.0°C when the opposite end of the gradient attains 43°C. The gradient was divided into 16 sections with a wax pencil. These sections designated the temperature which the animal selected. Deer mice have been shown to select according to the temperature of the floor rather than by differences in the vertical gradient of air current above the floor (6). It is presumed that other small rodents select temperatures via peripheral temperature receptors in the same manner. Thus temperature in each section can be continually recorded via the thermocouples in the floor. In our lab, 16 thermocouples were used. The positions of the thermocouples were selected to approximate equal thermal units of 2.5°C rather than equal distances. The individual sections varied in length from 10 to 15 cm, except for the section at either extreme end of the gradient. Since animals rarely selected these extremes in pilot studies, the two extreme sections were extended in length to 23 cm. The removal of a subject and cleaning of the gradient are conducted in, fundamentally, the same manner. The four foot (243.84 cm) dowels are connected together with two eye hooks. When an

animal is to be extracted from the chamber, the side panel of the chamber and the acrylic plug are removed first. Next, a large plastic bag, or wad of cotton sheeting, is placed into the gradient and, with the dowels, is used to push the subject toward the cold end (never the hot end) of the temperature gradient. The window is slid open, the stop on the cold end of the gradient is removed, and the subject is lifted from the apparatus. Cleaning is accomplished by running a damp cotton cloth through the gradient with the dowels also. Feces can be collected by placing a steel pan under the solid CO2. This gradient and these procedures have been used successfully in our laboratory for assessing the effects of intraventricular injections of bombesin (BBS), a potent hypothermic peptide, on temperature selection in the rat (7). Intraventricular injections of BBS resulted in the selection of temperatures that were 9.0-15.5°C colder than those selected by animals following control injections. During control and baseline conditions, the animals selected temperatures between 23.2 and 24.3°C. This is within the range of temperatures (18-28°C) during which thermoregulatory behavior is absent, and neither metabolic heat production nor evaporative water loss is elevated (5). Thus the current authors are in agreement with Poole and Stephenson's (5) assertion that the traditionally accepted thermoneutral zone of the rat (28-32°C) is in fact a temperature range in which heat stress has resulted in decreased motor activity as well as a low rate of metabolism.

REFERENCES 1. Carlisle, H. J.; Dubuc, P. U. Temperature preference of genetically obese mice. Physiol Behav. 33:899-902; 1984. 2. Gordon, C. J. Relationship between autonomic and behavioral thermoregulation in the mouse. Physiol. Behav. 34:887-890; 1985. 3. Laughter, J. S.; Blatteis, C. M. A system for the study of behavioral thermoregulation of small animals. Physiol. Behav. 35:993-997; 1985. 4. Marques, P. R.; Spencer, R. L.; Burks, T. F.; McDougal, J. N. Behavioral thermoregulation, core temperature, and motor activity: simultaneous quantitative assessment in rats after dopamine and

prostaglandin. Behav. Neurosci. 98:858-867; 1984. 5, Poole, S.; Stephenson, J. D. Body temperature regulation and thermoneutrality in rats. J. Exp. Physiol. 62:143-149; 1977. 6. Stinson, R. H.; Fisher, K. C. Temperature selection in deer mice. Can. J. Zool. 31:404--416; 1953. 7. Stump, B. S.; McCoy, J. G.; Avery, D. D. The effects of intraventricular injections of bombesin on temperature selection in the rat. Brain Res. Bull. 25:173-177; 1990.