Functional hyperemic response in the rat visual cortex under halothane anesthesia

To establish a model for functional hyperemia in the rat visual cortex, cortical blood flow responses to flash stimulation were measured with the laser Doppler flow (LDF) technique at various levels of halothane anesthesia. The concentration-dependent effect of halothane on arterial pressure and its consequent effect on the hyperemic response were also investigated. Using a stroboscopic light source, 10 flashes at 1 min intervals were delivered to the left eye of 12 Sprague–Dawley rats. LDF responses were measured bilaterally in the monocular primary visual cortex (V1M) at steady state halothane concentrations between 0.4 and 1.4%. In six rats, methoxamine (MX) was infused to prevent halothane-induced hypotension; the remaining rats did not receive MX. In all rats, LDF response to flash commenced within 1 s and peaked at 2.5 s in the contralateral V1M, but not in ipsilateral V1M. The maximum LDF response was 25% at 0.5% halothane and 12% at 1.4% halothane. In rats without MX infusion, mean arterial pressure (MAP) fell from 138 to 90 mmHg when halothane increased from 0.4 to 1.4%. MX infusion prevented the hypotension, but did not influence the LDF response, suggesting that the halothane's effect was direct rather than pressure-mediated. We demonstrate for the first time, a robust functional hyperemic response to discrete flash stimuli in the primary visual cortex of halothane-anesthetized albino rats that can be measured with LDF over a wide range of halothane concentrations and is not fully suppressed at surgical levels of halothane anesthesia.