Sleep-wake and diurnal modulation of nitric oxide in the perifornical-lateral hypothalamic area: Real-time detection in freely behaving rats

•The perifornical-lateral hypothalamic area (PF-LHA) is a wake-promoting region.•Nitric oxide (NO), an endogenous sleep factor, was measured in real-time in the PF-LHA.•In the PF-LHA, NO levels exhibited a waking > REM > nonREM sleep pattern.•NO levels were higher during dark phase and increased during sleep deprivation.•Nitrergic system in the PF-LHA may play a role in sleep-wake regulation.

Nitric oxide (NO) has been implicated in the regulation of sleep. The perifornical-lateral hypothalamic area (PF-LHA) is a key wake-promoting region and contains neurons that are active during behavioral or cortical activation. Recently, we found higher levels of NO metabolites (NOx), an indirect measure of NO levels, in the PF-LHA during prolonged waking (SD). However, NO is highly reactive and diffuses rapidly and the NOx assay is not sensitive enough to detect rapid-changes in NO levels across spontaneous sleep-waking states. We used a novel Nafion®-modified Platinum (NF-PT) electrode for real-time detection of NO levels in the PF-LHA across sleep-wake cycles, dark-light phases, and during SD. Sprague-Dawley male rats were surgically prepared for chronic sleep-wake recording and implantation of NF-PT electrode into the PF-LHA. Electroencephalogram (EEG), electromyogram (EMG), and electrochemical current generated by NF-PT electrode were continuously acquired for 5-7 days including one day with 3 h of SD. In the PF-LHA, NO levels exhibited a waking > rapid eye movement (REM) > non-rapid eye movement (nonREM) sleep pattern (0.56 ± 0.03 μM > 0.47 ± 0.02 μM > 0.42 ± 0.02 μM; p < 0.01). NO levels were also higher during the dark- as compared to the light-phase (0.53 ± 0.03 μM vs. 0.44 ± 0.02 μM; p < 0.01). NO levels increased during 3 h of SD as compared to undisturbed control (0.58 ± 0.04 μM vs. 0.47 ± 0.01 μM; p < 0.05). The findings indicate that in the PF-LHA, NO is produced during behavioral or cortical activation. Since elevated levels of NO inhibits most of the PF-LHA neurons that are active during cortical activation, these findings support a hypothesis that NO produced in conjunction with the activation of PF-LHA neurons during waking/SD, inhibits the same neuronal population to promote sleep.