Diurnal variation in urodynamics of rat

Abstract

In humans, the storage and voiding functions of the urinary bladder have a characteristic diurnal variation, with increased voiding during the day and urine storage during the night. However, in animal models, the daily functional differences in urodynamics have not been well-studied. The goal of this study was to identify key urodynamic parameters that vary between day and night. Rats were chronically instrumented with an intravesical catheter, and bladder pressure, voided volumes, and micturition frequency were measured by continuous filling cystometry during the light (inactive) or dark (active) phases of the circadian cycle. Cage activity was recorded by video during the experiment. We hypothesized that nocturnal rats entrained to a standard 12:12 light:dark cycle would show greater ambulatory activity and more frequent, smaller volume micturitions in the dark compared to the light. Rats studied during the light phase had a bladder capacity of 1.44+/-0.21 mL and voided every 8.2+/-1.2 min. Ambulatory activity was lower in the light phase, and rats slept during the recording period, awakening only to urinate. In contrast, rats studied during the dark were more active, had a lower bladder capacities (0.65+/-0.18 mL), and urinated more often (every 3.7+/-0.9 min). Average bladder pressures were not significantly different between the light and dark (13.40+/-2.49 and 12.19+/-2.85 mmHg, respectively). These results identify a day-night difference in bladder capacity and micturition frequency in chronically-instrumented nocturnal rodents that is phase-locked to the normal circadian locomotor activity rhythm of the animal. Furthermore, since it has generally been assumed that the daily hormonal regulation of renal function is a major driver of the circadian rhythm in urination, and few studies have addressed the involvement of the lower urinary tract, these results establish the bladder itself as a target for circadian regulation.

Figure 1. Cystometry Setup.

A: Schematic drawing of the surgically implanted intravesical catheter into rat bladder. B: Schematic drawing of the cystometry test chamber, consisting of a wire bottom cage positioned above an analytical balance. A syringe pump is connected to a pressure transducer, which is connected to the intravesical catheter.

Figure 2. Total ambulatory activity during cystometry.

A: CMG recordings were performed during opposite phases of the circadian cycle, 3–7 hours after lights on (ZT3-7, light group) or 3–7 hours after lights off (ZT15-19, dark group). ZT, Zeitgeber time. B: The average ambulatory activity during three consecutive micturition cycles from rats assayed during the light and dark phases. Dark rats showed significantly more activity than light rats (P<0.05).

Figure 3. Normal filling-voiding micturition cycle.

Representative cystometrogram (CMG) from a rat during the light phase in response to continuously-infused saline. Measurements were made of infused and voided volumes and bladder pressure at indicated points of the micturition cycle.

Figure 4. Urodynamic profile of conscious, freely moving rats during continuous infusion of saline into the bladder.

A: Representative CMG recorded during the dark phase when rats are active. Micturition frequency is higher, and bladder capacity is reduced. B: Representative CMG recorded during the light phase when rats are less active or sleeping. Micturition frequency is reduced, and bladder capacity is increased. C: Average micturition interval is increased in light versus dark rats (P<0.05).

Figure 5. Average ambulatory activity during three consecutive micturition cycles from rats assayed during the light and dark phases.

Dark rats showed significantly more activity during micturition events than light rats (P<0.05).

Figure 6. Average infused and voided volumes from micturition events.

Average infused and voided volumes are reduced in dark versus light rats (P<0.05), leading to a decrease in bladder capacity (infused minus voided volume) in the dark rats group.