The Effect of Dividing Walls, a Tunnel, and Restricted Feeding on Cardiovascular Responses to Cage Change and Gavage in Rats (Rattus norvegicus)

Cage change and gavage are routine procedures in animal facilities, yet little is known about whether housing modifications change responses to these procedures. Telemetric activity and cardiovascular parameters were assessed in this experiment. BN and F344 male rats were housed in open or individually ventilated cages, each containing 3 rats, 1 of which had a transponder. A crossover design was used, in which 2 groups were given dividers made of 2 intersecting boards (1 form contained holes loaded with food pellets; the other did not) and 1 group was given a rectangular tunnel. On day 8 of each 2-wk period, the cages were changed; on day 11, rats were gavaged. The parameters were evaluated for the first hour and for the following 17 h. Baseline values for each rat were subtracted from the corresponding response values. The presence of objects did not affect the responses of F344 rats to cage changing or gavage. In BN rats with IVCs, the presence of the plain divider modified the response to both procedures. Responses to procedures appeared to be dependent on both the strain and the cage object, thus complicating the establishment of valid general recommendations.Abbreviations: bpm, beats per minute; HR, heart rate; IVC, individually ventilated cage; MAP, mean arterial pressureThe new European regulations on laboratory rodents^12,15 mandate the provision of sufficient nest material to build a complete, covered nest or, if doing so is not possible, providing a nest box. Because rats are poor nest-builders,¹⁹ they must be provided with objects for this purpose. Moreover, objects that provide cover or divide the cage area may allow the rats to initiate or avoid contact with cagemates.³⁵All these regulations are rather specific, but generally the same for all laboratory rodents, that is, rats, mice, gerbils, hamsters, and guinea pigs. However, all rodent species and even strains and stocks within a species may have different needs. These differences raise the question of whether general guidelines, which may be valid for 1 species, may have a negative effect on welfare in other species and strains.Cage change is a frequent routine procedure in animal facilities that induces temporary, but significant, cardiovascular and behavioral changes in rats.^{9,10,13,27,29,31-33} Similarly the frequency^9,10 and time²⁹ of changing, type of the bedding material,⁹ light intensity, and length of the dark period³ all modify the intensity of the response to cage changing. The effects on physiologic parameters, such as blood pressure and heart rate, after the cage change seem to be a consequence of the transfer procedure itself and of the novel environment.Two features of rats suggest potential advantages of placing objects in the cage. First, rats are known to have a good sense of smell—1493 specific olfactory receptor genes have been identified on the cilia of the olfactory neurons—and smell is their primary sense for monitoring their environment.²⁴ Second, rats have dominance hierarchies in which fighting is essentially territorial, rather than for any specific object.⁴ The term ‘skirmishing’ has been used to describe a pattern of behaviors often assumed to be aggressive in rats; in 1 study,¹⁰ the frequency of skirmishing was increased during the first 15 min after a cage change. Consequently, cage objects may retain a familiar odor cue during cage change; the presence of the old item in the new cage reduces the aggressive behavior of rats that is triggered by regrouping.¹⁰Gavage is a method widely used to administer test compounds into the stomach of laboratory rats. Rats display increased blood pressure and heart rate (HR) immediately after gavage, and these increases may persist for 30 to 60 min after the procedure.^7,40 Furthermore, elevations in plasma corticosterone levels have been measured in rats after gavage.⁸ The selection of the correct administration volume^2,7,8,40 and a suitable probe material⁴⁰ are important to performing this procedure properly, but whether housing can be advantageous is unknown.Housing refinements have not been assessed in regard to their effect on refining the performance of procedures in rats, although housing modifications can alter their physiology and behavior. Rats have lower blood pressure and HR when housed on bedding compared with a grid or plastic floor.²² Rats also prefer a cage with shelter to a barren environment,³⁶ perhaps because they prefer to spend most of the light phase inside the shelter.¹⁴ Rats with a furnished environment are more active than those which lack such objects,³⁸ and the presence of a shelter in the cage decreases fearfulness.³⁶ Finally, the availability of cage objects may allow the rats to exhibit species-specific behaviors.¹¹In many previous studies of the effects of cage changing or gavage,^{7,8,13,27,29,31-33} the presence of objects in the cages was not described. However, 1 study of both Sprague–Dawley and spontaneously hypertensive male rats showed that providing a multifaceted enrichment program over a week did not affect HR or systolic blood pressure responses to placement in a standard rodent restrainer for 60 min.³⁴ However, after removal from the restrainer, the rats showed a secondary increase in HR and systolic blood pressure that was significantly attenuated in enriched compared with nonenriched rats of both strains. Moreover, enriched rats of both strains had lower HR and systolic blood pressure responses to a variety of procedures, including removal of a cagemate, tail-vein injection, and exposure to the odor of urine and feces of stressed male or female rats.³⁴We hypothesized that cage objects would alter the effect of cage change and gavage on telemetrically recorded cardiovascular parameters and locomotor activity. This study was designed to evaluate the effect of an aspen wall divider with or without restricted feeding and of the presence of an aspen tunnel in the cage on these measures after routine cage changing and gavage of laboratory rats in both open-top cages and IVCs.