Episcleral venous pressure and IOP responses to central electrical stimulation in the rat

PURPOSE: Histological evidence suggests a role for the central nervous system in controlling episcleral venous pressure (EVP). Based on prior studies that identified candidate regions in the brain stem, the present study assessed the effect of electrical stimulation at the location of the superior salivatory nucleus (SSN) on EVP in rats.

METHODS: Male Sprague-Dawley rats (n = 11) were anesthetized using pentobarbital sodium (50 mg/kg intraperitoneally initially, supplemented intravenously [IV] as needed) and paralyzed with gallamine triethiodide (1 mg/kg, IV). The animals were artificially ventilated and the femoral artery and vein were cannulated for blood pressure measurement and drug administration. Carotid blood flow was measured with an ultrasound flow probe and heart rate with a cardiotachometer. IOP was measured through a cannula in the vitreous compartment and EVP was measured through a micropipette in episcleral veins using the servonull technique. After a craniotomy was performed, a unipolar stainless steel electrode was inserted into the brainstem at the coordinates of the SSN using a stereotactic instrument. Stimulations were performed at 20Hz, 9 μA, 1 ms pulse duration, and 200 pulses.

RESULTS: Stimulation at the SSN coordinates increased IOP from 10.6 ± 0.4 to 11.8 ± 0.6 mm Hg (P < 0.01) and EVP from 7.8 ± 1.3 to 10.7 ± 1.1 mm Hg (P < 0.01). Mean arterial pressure, carotid blood flow, and heart rate remained unaltered.

CONCLUSIONS: The present study indicates that the SSN may participate in regulating EVP.