Can you import sunlight if you live in a cave? Researchers have used light to stimulate neurons on or near the dorsal surface of the brain that express light-activated cation channels. What about neurons that lie beneath the brain surface? Adamantidis et al. report optogenetic control of neurons in the hypothalamus involved in sleep in a recent article in Nature.
The hypothalamus is a ventral brain structure. Hypocretin, which regulates energy expenditure, localizes to the lateral hypothalamus. Loss-of-function mutations in hypocretin are associated with narcolepsy, a disease that makes people fall asleep at inappropriate times.
Channelrhodopsin-2 (ChR2) is a cation channel from algae that depolarizes neurons in the presence of blue light. The authors drove ChR2 expression with the hypocretin promoter in a lentiviral expression vector (Hcrt::ChR2). They cannulated mice and stereotactically injected Hcrt::ChR2 or a control lentivirus into the lateral hypothalamus. Hypocretin neurons infected with Hcrt::ChR2 stably expressed ChR2 for at least 2 months. Blue light induced inward current in hypocretin neurons expressing ChR2, as shown by voltage-clamp recordings in lateral hypothalamic slices. Pulses of blue light or continuous blue illumination induced repetitive action potentials in neurons expressing Hcrt::ChR2.
To stimulate and record from behaving mice, the authors permanently implanted mice with electroencephalography (EEG) and electromyography (EMG) electrodes and inserted an optical fiber into the same cannula used for lentiviral injections. Blue illumination increased the expression of the immediate early gene product c-Fos in lateral hypothalamic neurons expressing Hcrt::ChR2, suggesting that light stimulation from the implanted optical fiber activated lateral hypothalamic neurons, allowing the researchers to study the effects of this activation on sleep.
Mammalian sleep occurs in phases. Slow-wave sleep (SWS) is deep sleep that is characterized in EEG by slow (0.5–4 Hz) delta waves. Rapid eye movement (REM) sleep is a lighter sleep phase than SWS and is characterized by EEG recordings that are similar to the awake state. EMG recordings are nearly silent for mice in REM and SWS sleep but are active in awake mice. After blue light stimulation, Hcrt::ChR2 mice awoke faster from both SWS and REM sleep than did mice treated with control lentiviruses. Durations of sleep and wake periods were similar in control and Hcrt::ChR2 mice. However, photostimulation increased the probability of transitioning to the awake state from both SWS and REM sleep in Hcrt::ChR2 mice.
Does hypocretin secretion regulate wakefulness in the lateral hypothalamus? A hypocretin receptor 1 antagonist blocked the light-induced decrease in sleep-to-wake latency in Hcrt::ChR2 mice, and hypocretin knockout Hcrt::ChR2 mice showed increased latency to wake relative to wild-type Hcrt::ChR2 mice.
Together, these data indicate that the activation of hypocretin neurons induces hypocretin secretion and drives arousal. Therefore, optogenetic tools can be used to selectively activate defined neural populations throughout the brain to determine their roles in complex behaviors.