Subregional differences in GABAA receptor subunit expression in the rostral ventrolateral medulla of sedentary versus physically active rats.

Neurons in the rostral ventrolateral medulla (RVLM) regulate blood pressure through direct projections to spinal sympathetic preganglionic neurons. Only some RVLM neurons are active under resting conditions due to significant, tonic inhibition by gamma-aminobutyric acid (GABA). Withdrawal of GABAA receptor-mediated inhibition of the RVLM increases sympathetic outflow and blood pressure substantially, providing a mechanism by which the RVLM could contribute chronically to cardiovascular disease (CVD). Here, we tested the hypothesis that sedentary conditions, a major risk factor for CVD, increase GABAA receptors in RVLM, including its rostral extension (RVLMRE ), both of which contain bulbospinal catecholamine (C1) and non-C1 neurons. We examined GABAA receptor subunits GABAAα1 and GABAAα2 in the RVLM/RVLMRE of sedentary or physically active (10-12 weeks of wheel running) rats. Western blot analyses indicated that sedentary rats had lower expression of GABAAα1 and GABAAα2 subunits in RVLM but only GABAAα2 was lower in the RVLMRE of sedentary rats. Sedentary rats had significantly reduced expression of the chloride transporter, KCC2, suggesting less effective GABA-mediated inhibition compared to active rats. Retrograde tracing plus triple-label immunofluorescence identified fewer bulbospinal non-C1 neurons immunoreactive for GABAAα1 but a higher percentage of bulbospinal C1 neurons immunoreactive for GABAAα1 in sedentary animals. Sedentary conditions did not significantly affect the number of bulbospinal C1 or non-C1 neurons immunoreactive for GABAAα2 . These results suggest a complex interplay between GABAA receptor expression by spinally projecting C1 and non-C1 neurons and sedentary versus physically active conditions. They also provide plausible mechanisms for both enhanced sympathoexcitatory and sympathoinhibitory responses following sedentary conditions.

Effects of cage enrichment on heart rate, blood pressure, and activity of female Sprague-Dawley and spontaneously hypertensive rats at rest and after acute challenges.

Here we sought to determine whether a nonsocial cage enrichment program, identical to one we previously used with male rats, was effective in reducing heart rate or systolic blood pressure (SBP) in female Sprague-Dawley rats and spontaneously hypertensive rats (SHR). Young adult rats, each instrumented with a radiotelemetry pressure transmitter, were housed individually under enriched or nonenriched conditions. Heart rate and SBP were monitored at 5- and 1-min intervals, respectively, when the rats were undisturbed or after several different types of experimental manipulations some of which are considered stressful. Cage enrichment did not significantly alter heart rate or SBP of undisturbed rats in either strain at any time during the day or night. However, activity of female SHR was increased in the afternoon and at night under enriched conditions compared with nonenriched conditions. The enrichment program did not significantly reduce heart rate or SBP responses to most acute manipulations in either strain. However, cage enrichment increased the responses to some procedures (Sprague-Dawley: handling, 1-h restraint; SHR: subcutaneous injection, tail-vein injection, handling). We conclude that a nonsocial cage enrichment program did not reduce physiologic indicators of stress in female Sprague-Dawley rats or SHR.

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

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.

Stress-like responses to common procedures in male rats housed alone or with other rats.

The objective of this study was to assess the cardiovascular function and behavior of male Sprague-Dawley rats housed individually or with one or three cagemates during resting conditions and when subjected to common husbandry and experimental procedures and potentially stressful olfactory stimuli. Heart rate (HR), mean arterial blood pressure (MAP), and movement in the cage were collected by using radiotelemetry for 24 h on an experiment-free day and for 2 h before and 3 h after the following acute procedures: cage change, restraint and subcutaneous injection, restraint and tail-vein injection, exposure to the odor of urine and feces from stressed rats, and exposure to the odor of dried rat blood. Home cage behaviors (sleeping, awake, moving, rearing, and grooming) were scored once each minute for 15 min before and 45 min after the acute procedures. Resting HR and MAP values consistently were lower in rats housed four per cage than animals housed alone or with one cage mate. Compared to that of animals housed individually, general activity was higher during the light phase and lower during the dark phase in rats housed four per cage. Rats housed four per cage showed significantly lower HR and MAP in response to acute husbandry and experimental procedures than rats housed alone, and the HR and MAP of rats housed in pairs were not consistently lower than those of rats housed alone. Procedure-induced arousal behaviors were observed in all housing groups after the acute husbandry and experimental procedures, but rats housed four per cage returned to sleeping behavior more quickly than did rats in the other housing groups. In light of these results, we concluded that under resting conditions, rats housed four per cage were less stressed than were rats housed alone, that common procedures induce noteworthy stress-like responses in male rats, and that the magnitude and duration of these responses are reduced by group housing.