Hypocretins
Hypocretins, also called Orexins, are hypothalamic neuropeptides that serve important functions in the regulation of an individual’s sleep/wake cycle, homeostatic systems such as thermoregulation, appetite, and rewarding processing. Orexins come in two forms: , which is 33 amino acids long, and , which is 28 amino acids long. Both Orexin-peptides consist of two amphipathic alpha-helixes, which have similar properties to the N-terminal alpha-helixes found on Orexin receptors. Orexin-A can be described as a non-subtype-selective neuropeptide that is capable of binding to either OX1R and OX2R, the two possible Orexin receptors, with roughly equal affinities. However, Orexin-B has been noted to bind to OX2R with an affinity 10 times greater than its affinity for OX1R. has a hydrophilic turn induced by two disulfide bonds, allowing hydrophobic residues to be on one side and hydrophilic residues to be on the other side.
Receptors
Orexin-A and Orexin-B bind with corresponding G-protein-coupled receptors known as and . While both OX1R and OX2R can be found within the brains of mammals, OX1R and OX2R are distributed throughout the brain in varying amounts at different locations. OX1R and OX2R, both, have an alpha-helix in their extracellular N-terminal regions. The N-terminal alpha-helixes serve a critical roll in Orexin-A-mediated neuropeptide activation. Although the structures of the N-terminal alpha helixes of both OX1R and OX2R are similar, the orientation varies between the two receptors. While the N-terminal alpha-helix of is found to interact with the extracellular loop 2 (ECL2), the N-terminal alpha-helix of OX2R is found to be parallel to the helix 8 on the intracellular side of the membrane. Residues at 2.61 and in both OX1R and OX2R play a critical roll in determining the subtype selectivity. Orexin receptors produce neuroexcitation through postsynaptic depolarization by activating non-selective cation channels, inhibiting K+ channels, and activating Na+/Ca2+ exchange. Additionally, OX1R and OX2R stimulate the release of neurotransmitters through presynaptic actions, and OX1R and OX2R modulate synaptic plasticity. Both Orexin receptors have been shown to couple strongly with Ca2+ ion elevations and phospholipase C. Low concentrations of Orexin causes OX1R stimulation, activating a Ca2+ influx. It has been indicated, directly or indirectly, that OX1R and OX2R can couple to Gq, Gi/o and Gs, three of the four heterotrimeric G-protein families. Studies have also revealed that OX2R can couple differently to Gq, Gi/o and Gs proteins depending on which of the various possible types of tissues OX2R can be found in, the OX2R resides within. Since OX1R and OX2R play important rolls in sleep regulation, sleep disorders, such as narcolepsy, can been caused by mutations of these regulators. Narcolepsy, specifically type 1 narcolepsy, can be caused by mutation of the OX2R gene (HCRTR2).
Functions
Sleep-wake Regulation
Low amounts of Orexin and mutations of the OX2R gene (HCRTR2) have been linked to the development of narcolepsy, a sleep disorder characterized by excessive daytime sleepiness, cataplexy, sleep paralysis, and hypnagogic hallucinations. Cataplexy, the loss of muscle tone, hypnagogic hallucinations, hallucinations produced while an individual is falling asleep, and sleep paralysis, the immobilization of an individual’s body while falling or waking from sleep, are thought to be caused when REM sleep is unexpectedly interrupted. Narcolepsy causes a lack of regulation of the sleep/wake cycle, and, thus, narcolepsy frequently causes both interruptions of a person’s wakefulness and a person’s REM sleep. Orexin interacts with several wake-promoting neuronal groups, such as the histaminergic neurons of the tuberomammilary nucleus and the noradrenergic neurons of the locus coeruleus. The locus coeruleus is a structure in the brainstem that produces the neurotransmitter norepinephrine. Therefore, it can be concluded that when Orexin neurons are stimulated the possibility of transitioning to a state of wakefulness increases. However, if Orexin neurons are inhibited the probability of transitioning to sleeping state increases. Narcolepsy is also characterized by fragmented sleep, which is sleep interrupted by many brief periods of arousal. Fragmented sleep is caused by the instability of behavioral state regulation due to the lack of Orexin lowering the thresholds required to transition between sleep and wakefulness. Since narcoleptic individuals tends to fall asleep during the day, but wake up repeatedly at night, it is likely that Orexin functions as a way to stabilize the transition between sleep and wake states, as well as causing the transition between the states.
Thermoregulation
During sleep, the body produces less heat due to reduced muscle activity, lower basal metabolism, peripheral vasodilatation, and sleep-promoting neurons in the preoptic area. Narcoleptic individuals present lower core body temperatures while awake and higher than normal body temperature while asleep. Indicating that narcolepsy does not permit body temperature and basal metabolic rate to decrease or rise properly during the corresponding periods of sleep or wakefulness. Normally, distal skin temperature changes inversely with the core body temperature.
During the day, the core body temperature is high and distal skin temperature is relatively low. The core body temperature decreases at night, while distal skin temperature is relatively high. The onset of sleep can be characterized by a rise in distal skin temperature comparatively to proximal skin temperature. Narcoleptic individuals present higher distal skin temperatures and lower proximal skin temperatures while awake. This could be connected to the feeling of sleepiness felt by narcoleptic individuals since, normally, this combination of temperatures would be presented during periods of sleep. Additionally, narcoleptic individuals are better able to maintain wakefulness when their core temperature and distal skin temperature are altered to simulate the conditions normally experienced during wakefulness. Orexin, also, plays an important role in activating brown adipose tissue to increase the core body temperature. This indicates Orexin has a role in activating both waking and heat production as response to a decrease in ambient temperature. The Orexin system optimizes metabolism when activities, such as food-seeking or reproduction, where enhanced performance is required are preformed, by increasing body temperature during wakefulness. In conclusion, narcoleptic individuals possess an inability to accurately modulate thermoregulatory control in connection to their behavioral state.
Food-seeking behavior and Reward processing
The name for “Orexin” came about largely due to its role in feeding-behavior, since the word orexin means “appetite” in Greek. Hypocretin neurons are inhibited after feeding, reducing arousal, and are activated during fasting, increasing arousal. If Orexin cannot preform its normal function, the increase in wakefulness and foraging behaviors that is expected as a response to a lack of food intake will not occur. The genetic elimination of Orexin results in reduced food anticipatory activity and the expected temporal expression of numerous clock genes. Indicating that Orexin mediates a shifts in circadian rhythms as response to changes in the timing of food availability or nutritional status. Since food-seeking behavior is critical for survival, it carries a potent reward stimulus. Orexin has an active role in reward processing for both food-seeking and drug-seeking behaviors. Even though narcoleptic individuals are often treated with stimulants for years at a time, they rarely abuse stimulants or present drug-seeking behavior. Since frequent use of stimulants can lead to addiction and may cause an individual to abuse said drug in retaliation of growing tolerance of the stimulant’s effect, the lack of drug-seeking behavior and stimulant abuse in narcoleptic individuals heavily indicates Orexin plays a critical roll in addiction. In conclusion Orexin serves an important role in motivation-driven behavior and reward processing.
Cardiovascular
Orexin regulates autonomic and cardiovascular effects associated with sleep and wakefulness. Sleep-associated cardiovascular changes, such as the decrease in blood pressure often referred to as “dipping,” occurs during periods of sleep within a wide range of various species. Orexin plays an important role in regulating blood pressure across behavioral states. A lack of Orexin is linked to lower systemic blood pressure and a failure to properly decrease blood pressure during periods of sleep. Orexin, also, increases sympathetic outflow and has an effect on the hypothalamic-pituitary-adrenal axis that results in an increased release of catecholamine. The activity of Orexin neurons acts as a regulator for the release of corticosterone to the hypothalamic-pituitary-adrenal axis that results in the behavioral responses to stress.
