Rodents are common animal models for studying aqueous humor dynamics and IOP regulation, but accurate measurements of aqueous inflow and outflow are technically challenging in mice and rats. For this reason, previous measurements of outflow facility in rodents have either been conducted in enucleated eyes, cadaveric eyes maintained within the orbit, or in vivo with the animal under anaesthesia. Mohamed and Passaglia1 have made a significant advance by introducing a technology that, for the first time, is capable of measuring outflow facility in conscious rats.
Mohamed and Passaglia1 have made a significant advance by introducing a technology that, for the first time, is capable of measuring outflow facility in conscious rats
The technology relies on a surgically implanted cannula that infuses saline into the anterior chamber with a microfluidic pump via a flow restrictor. Pressure is monitored using a pressure sensor and the flow rate is inferred based on the pressure drop across the flow restrictor. The system was programmed to operate in either a constant flow mode, where the flow rate from the pump is set to a desired level, or constant pressure mode, where the flow rate is automatically adjusted to maintain a desired IOP. Outflow facility was determined by incrementing either the flow or pressure in fixed steps, and then calculating the slope of the flow-pressure relationship using linear regression. Noise-filtering algorithms were applied to minimize the impact of the natural IOP fluctuations associated with eye motion, breathing and the ocular pulse.
The authors validated system performance in vitro, and in anesthetized rats using a cannula to directly record IOP and a flow sensor to record the perfusion flow rate, both of which agreed closely with the system measurements. In the anesthetized animals, the perfusion system exhibited better performance under constant pressure than constant flow, due to the faster response time with at least a four-fold reduction in time required to measure outflow facility (> two hours for constant flow, vs < 30 minutes for constant pressure). In both conscious and anesthetized animals, the authors report measured values of outflow facility that are within the range previously reported in rats using different benchtop systems.2,3
The most important result was the diurnal variation in outflow facility. Measurements of facility several times per day within four individual conscious rats demonstrated a clear diurnal variation in outflow facility, with a higher facility in the day that is consistent with the ~5 mmHg lower daytime IOP reported by the same group in an earlier study4. Similar studies of tonographic outflow facility have reported some evidence for diurnal variations in outflow facility in human eyes along with variations in inflow.5-6 A key question then becomes what factors are responsible for these diurnal fluctuations in outflow facility and the potential role of neural or circulatory signals. Future studies will examine this important question, no doubt advanced by technology developed in the Passaglia lab.
Although there are trade-offs with accuracy compared to benchtop perfusion systems, this unique technology provides exciting new possibilities for glaucoma research, such as longitudinal studies of outflow facility in response to potential therapeutic treatments and imposition of a controllable level of ocular hypertension, which could provide important benefits for studies of pressure-induced optic neuropathy.