Effect of housing rats in dim light or long nights on heart rate

Abstract

Housing laboratory animals under lighting conditions that more closely mimic the natural environment may improve their wellbeing. This study examined the effects of dim light or a long-night photocycle on resting heart rate (HR) of rats and their HR responses to acute procedures. Male and female Sprague-Dawley (SD) and spontaneously hypertensive (SHR) rats, instrumented with radiotelemetry transmitters and housed individually under a 12:12-h light:dark photocycle with 10 lx illumination (dim light) or under an 8:16-h light:dark photocycle with 200 lx illumination (long nights), were compared with control rats individually housed under a 12:12-h light:dark photocycle with 200 lx illumination. Dim light and long nights significantly reduced the HR of undisturbed SD and SHR male and SHR female rats during the day and at night; however, the HR of undisturbed SD females was not affected. When rats were subjected acutely to husbandry, experimental, or stressful procedures, dim light or long nights (or both) reduced HR responses to some procedures, did not alter responses to others, and increased responses to yet other procedures. The pattern of effects varied between strains and between male and female rats. Because basal HR was reduced when rats were housed under 10 lx illumination or an 8:16-h light:dark photocycle, we concluded that housing rats under 12:12-h light:dark, 200 lx ambient light conditions was potentially stressful, We also concluded that dim light or long nights did not uniformly reduce the increased HR responses induced by acute procedures.

Figure 1.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on the heart rate (beats/min; bpm) of male Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) during undisturbed periods in the morning (0800 to 0900, panel A), the afternoon (1300 lights off, panel B), and at night (0700 lights off, panel C). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 2.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after routine cage change (panel A) or gentle handling for 1 min (panel B). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 3.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after insertion of a conspecific intruder rat for 1 h (panel A) or removal of an established conspecific cagemate (panel B). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 4.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after hand restraint and SC injection of saline (panel A) or transport in the home cage 100 ft on a cart between the animal housing room and a procedure room plus hand restraint and SC injection of saline (panel B). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 5.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after hand restraint and tail vein injection of saline (panel A) or hand restraint and IP injection of saline (panel B). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 6.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after exposure to a paper towel coated with dried urine and feces from stressed male rats (panel A) or female rats (panel B) that was placed on the cage lid for 15 min. *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.

Figure 7.

Effect of dim light (10 lx) or long nights (8:16-h light:dark) on heart rate responses (area under response curve; AURC) of male and female Sprague–Dawley (SD) and spontaneously hypertensive rats (SHR) after exposure to a paper towel coated with dried rat blood that was placed on the cage lid for 15 min (panel A) or 1 h of confinement in a clear acrylic rat restrainer placed in the home cage (panel B). *, Significantly (P < 0.05) different from value for 12:12-h light:dark, 200 lx; ^#, SHR significantly (P < 0.05) different from SD; ⁺, females significantly (P < 0.05) different from males. Each group contained 6 rats.