Ferenc Obál

The Role of Cytokines in Physiological Sleep Regulation

Abstract: Several growth factors (GFs) are implicated in sleep regulation. It is posited that these GFs are produced in response to neural activity and affect input‐output relationships within the neural circuits where they are produced, thereby inducing a local state shift. These GFs also influence synaptic efficacy. All the GFs currently identified as sleep regulatory substances are also implicated in synaptic plasticity. Among these substances, the most extensively studied for their role in sleep regulation are interleukin‐1β (IL‐1) and tumor necrosis factor a (TNF). Injection of IL‐1 or TNF enhances non‐rapid eye movement sleep (NREMS). Inhibition of either IL‐1 or TNF inhibits spontaneous sleep and the sleep rebound that occurs after sleep deprivation. Stimulation of the endogenous production of IL‐1 and TNF enhances NREMS. Brain levels of IL‐1 and TNF correlate with sleep propensity; for example, after sleep deprivation, their levels increase. IL‐1 and TNF are part of a complex biochemical cascade regulating sleep. Downstream events include nitric oxide, growth hormone releasing hormone, nerve growth factor, nuclear factor kappa B, and possibly adenosine and prostaglandins. Endogenous substances moderating the effects of IL‐1 and TNF include anti‐inflammatory cytokines such as IL‐4, IL‐10, and IL‐13. Clinical conditions altering IL‐1 or TNF activity are associated with changes in sleep, for example, infectious disease and sleep apnea. As our knowledge of the biochemical regulation of sleep progresses, our understanding of sleep function and of many clinical conditions will improve.

Brain organization and sleep function

A view of brain organization and sleep function is presented. Sleep is hypothesized to begin at the neuronal group level. Sleep results in the use and thus maintenance, of synapses that are insufficiently stimulated during wakefulness thereby serving to preserve a constancy of a synaptic superstructure. It is further hypothesized that sleep at the neuronal group level is regulated by the production of substances whose rate of production or catabolism is synaptic use-dependent. If sufficient number of neuronal groups are in a sleep state (also called disjunctive state) then the perception of sleepiness occurs. Coordination of neuronal group sleep results from humoral and neuronal projection systems previously linked to sleep regulation. The theory presented is unique in that it: (a) hypothesizes an organizational level at which sleep occurs; (b) hypothesizes that sleep is neuronal--use-dependent, not wakefulness-dependent; (c) hypothesizes that sleep first occurs in evolution when complex ganglia evolved; and (d) hypothesizes the both non-rapid eye movement sleep (NREMS) and REMS serve the same function of synaptic reorganization. The theory is consistent with past theories of sleep function, yet provides a fundamentally new paradigm for sleep research.

Interleukin-6 is pyrogenic but not somnogenic.

Interleukin-6 (IL6) induces acute phase protein production and is hypothesized to mediate systemic and central effects of IL1. To determine whether IL6 possesses somnogenic properties, rabbits were injected intracerebroventricularly with IL6; sleep-wake activity was determined and brain temperatures recorded for 6 hr. IL6 induced fever in a dose-related manner with no effect on sleep-wake activity. IL6, therefore, is the first cytokine reported to elicit fever without promoting sleep. We conclude that the somnogenic action of IL1 is not mediated through IL6.

Cytokines in sleep regulation

The central thesis of this essay is that the cytokine network in brain is a key element in the humoral regulation of sleep responses to infection and in the physiological regulation of sleep. We hypothesize that many cytokines, their cellular receptors, soluble receptors, and endogenous antagonists are involved in physiological sleep regulation. The expressions of some cytokines are greatly amplified by microbial challenge. This excess cytokine production during infection induces sleep responses. The excessive sleep and wakefulness that occur at different times during the course of the infectious process results from dynamic changes in various cytokines that occur during the hosts response to infectious challenge. Removal of any one somnogenic cytokine inhibits normal sleep, alters the cytokine network by changing the cytokine mix, but does not completely disrupt sleep due to the redundant nature of the cytokine network. The cytokine network operates in a paracrine/autocrine fashion and is responsive to neuronal use. Finally, cytokines elicit their somnogenic actions via endocrine and neurotransmitter systems as well as having direct effects neurons and glia. Evidence in support of these postulates is reviewed in this essay.

Inhibition of growth hormone-releasing factor suppresses both sleep and growth hormone secretion in the rat.

To study the possible involvement of hypothalamic growth hormone-releasing factor (GRF) in sleep regulation, a competitive GRF-antagonist, the peptide (N-Ac-Tyr1,D-Arg2)-GRF(1-29)-NH2, was intracerebroventricularly injected into rats (0.003, 0.3, and 14 nmol), and the EEG and brain temperature were recorded for 12 h during the light cycle of the day. Growth hormone (GH) concentrations were determined from plasma samples taken at 20-min intervals for 3 h after 14 nmol GRF-antagonist. The onset of non-rapid eye movement sleep (NREMS) was delayed in response to 0.3 and 14 nmol GRF-antagonist, the duration of NREMS was decreased for one or more hours and after 14 nmol EEG slow wave amplitudes were decreased during NREMS in postinjection hour 1. The high dose of GRF-antagonist also suppressed REMS for 4 h, inhibited GH secretion, and elicited a slight biphasic variation in brain temperature. These findings, together with previous observations indicating a sleep-promoting effect for GRF, support the hypothesis that hypothalamic GRF is involved in sleep regulation and might be responsible for the correlation between NREMS and GH secretion reported in various species.

Brain and core temperatures and peripheral vasomotion during sleep and wakefulness at various ambient temperatures in the rat.

Changes in brain, core and tail skin temperatures (Tbr, Tc and Tt) associated with transitions in the arousal states were recorded in rats throughout the 24-h diurnal cycle at 10 °C, 21 °C and 29 °C. Falling asleep was accompanied by decreases in both Tbr and Tc and vasodilation at 10 δC and 21 °C. At 29 °C, tail vessels were permanently dilated, and further dilation was not found on sleep onset. Tbr and Tc, however, continued to decrease during non-rapid-eye-movement sleep (NREMS); these changes are likely to result from reductions in heat production and increased conductive heat loss. The changes in Tbr, Tc and Tt on awakening mirrored those on falling asleep. It is suggested that the suppression of sleep in the cold and the enhancement of NREMS in the heat promote thermoregulation. Rapid-eye-movement sleep (REMS) was associated with sharp rises in Tbr. The rise in Tbr was the largest in the cold and was attenuated at 29 °C. Tc decreased and Tt increased in the cold, whereas Tc tended to increase and Tt to decrease in the heat. The paradoxical peripheral vasomotion during REMS supports previous suggestions on severe thermoregulatory impairment during REMS in other species.

The effects of immunolesions of nerve growth factor-receptive neurons by 192 IgG-saporin on sleep.

Low-affinity nerve growth factor (NGF) receptors are present on the cholinergic neurons of the basal forebrain. We studied the effects of 192 IgG-saporin, a specific immunotoxin for the NGF receptor-positive, cholinergic basal forebrain neurons, on sleep, the power spectrum of the electroencephalogram (EEG), and body temperature. After 3 d baseline recordings, 12 male rats were injected intracerebroventricularly with 4 micrograms 192 IgG-saporin. EEG, motor activity, and brain temperature were recorded for 23 h on the first, third, fifth, and seventh day after the treatment. 192 IgG-saporin did not affect the total daily amounts but altered the circadian distribution of sleep. On days 1 and 3 after the injection of the immunotoxin, the amount of non-rapid-eye-movement sleep (NREMS) and rapid-eye-movement sleep (REMS) increased during the dark period, whereas during the light both NREMS and REMS decreased. On day 5, these changes were less pronounced and sleep completely returned to the baseline by day 7. The EEG was suppressed in each frequency band and each vigilance state, and, in contrast to sleep, these changes in EEG persisted for 7 days. Brain temperature was decreased from day 3. These results suggest that NGF receptor-positive, cholinergic basal forebrain neurons are not necessary for the maintenance of total sleep time but contribute to the generation of normal EEG and the maintenance of brain temperature.

Diurnal Variations of Tumor Necrosis Factor Alpha mRNA and Alpha-Tubulin mRNA in Rat Brain

The experiments described herein were designed to determine whether tumor necrosis factor alpha (TNF-alpha) displays a diurnal variation in various areas of the normal rat brain. TNF-alpha mRNA transcripts were detected by reverse-transcriptase polymerase chain reaction. To monitor diurnal changes in TNF-alpha and alpha-tubulin expression, rats were sacrificed every 4 h for 24 h starting 1 h after light onset; relative mRNA levels were determined for the cerebellum, cortex, hippocampus, hypothalamus and brainstem. TNF-alpha mRNA was higher during the light than in the dark phase in the hypothalamus and hippocampus. alpha-Tubulin mRNA exhibited a similar diurnal variation in the hypothalamus, hippocampus and cortex. In contrast, beta-actin mRNA was lower during the light phase than the dark phase in the hippocampus and cortex. The observed diurnal variations in TNF-alpha mRNA are consistent with the hypothesis that TNF has a physiological role in the brain.

Prolactin, vasoactive intestinal peptide, and peptide histidine methionine elicit selective increases in REM sleep in rabbits

The purpose of these experiments was to determine whether (1) vasoactive intestinal peptide (VIP) produces effects on rabbit sleep similar to those reported for rats and cats; (2) peptide histidine methionine (PHM), a peptide closely related to VIP, mimics the sleep effects of VIP; and (3) pituitary prolactin (PRL), a pituitary hormone that has a sleep-related secretory pattern and for which VIP and PHM act as releasing factors, has similar effects on sleep. VIP or PHM (0.01, 0.1 and 1.0 nmol/kg) was intracerebroventricularly (i.c.v.) injected; PRL (ovine PRL, 45 and 200 IU/kg) was subcutaneously (s.c.) administered. Sleep-wake activity and brain temperature were recorded for 6 h. For controls, rabbits received artificial cerebrospinal fluid i.c.v. or PRL-vehicle s.c. VIP and PHM promoted rapid eye movement sleep (REMS), although these effects were not dose-dependent. In addition, the high dose of VIP and PHM transiently increased wakefulness. Increases in REMS occurred only during hours 2-6 after i.c.v. injection of VIP and peptide histidine leucine (PHI). After s.c. injection of PRL, REMS started to increase in postinjection hour 3. The effect of the high dose was significantly more pronounced than that of the small dose. Each substance enhanced the frequency of REMS episodes, and the high dose of PRL also increased the duration of REMS bouts. These results are consistent with the hypothesis that VIP is involved in physiological regulation of REMS, and that the VIP- and PHM-induced increases in REMS may be mediated via release of PRL.

Ghrelin-induced sleep responses in ad libitum fed and food-restricted rats

Ghrelin is an endogenous ligand for the growth hormone secretagogue receptor and a well-characterized food intake regulatory peptide. Hypothalamic ghrelin-, neuropeptide Y (NPY)-, and orexin-containing neurons form a feeding regulatory circuit. Orexins and NPY are also implicated in sleep-wake regulation. Sleep responses and motor activity after central administration of 0.2, 1, or 5 microg ghrelin in free-feeding rats as well as in feeding-restricted rats (1 microg dose) were determined. Food and water intake and behavioral responses after the light onset injection of saline or 1 microg ghrelin were also recorded. Light onset injection of ghrelin suppressed non-rapid-eye-movement sleep (NREMS) and rapid-eye-movement sleep (REMS) for 2 h. In the first hour, ghrelin induced increases in behavioral activity including feeding, exploring, and grooming and stimulated food and water intake. Ghrelin administration at dark onset also elicited NREMS and REMS suppression in hours 1 and 2, but the effect was not as marked as that, which occurred in the light period. In hours 3-12, a secondary NREMS increase was observed after some doses of ghrelin. In the feeding-restricted rats, ghrelin suppressed NREMS in hours 1 and 2 and REMS in hours 3-12. Data are consistent with the notion that ghrelin has a role in the integration of feeding, metabolism, and sleep regulation.