Luca Imeri

How (and why) the immune system makes us sleep

Good sleep is necessary for physical and mental health. For example, sleep loss impairs immune function, and sleep is altered during infection. Immune signalling molecules are present in the healthy brain, where they interact with neurochemical systems to contribute to the regulation of normal sleep. Animal studies have shown that interactions between immune signalling molecules (such as the cytokine interleukin 1) and brain neurochemical systems (such as the serotonin system) are amplified during infection, indicating that these interactions might underlie the changes in sleep that occur during infection. Why should the immune system cause us to sleep differently when we are sick? We propose that the alterations in sleep architecture during infection are exquisitely tailored to support the generation of fever, which in turn imparts survival value.

Mutant Prion Protein Expression Causes Motor and Memory Deficits and Abnormal Sleep Patterns in a Transgenic Mouse Model

A familial form of Creutzfeldt-Jakob disease (CJD) is linked to the D178N/V129 prion protein (PrP) mutation. Tg(CJD) mice expressing the mouse homolog of this mutant PrP synthesize a misfolded form of the mutant protein, which is aggregated and protease resistant. These mice develop clinical and pathological features reminiscent of CJD, including motor dysfunction, memory impairment, cerebral PrP deposition, and gliosis. Tg(CJD) mice also display electroencephalographic abnormalities and severe alterations of sleep-wake patterns strikingly similar to those seen in a human patient carrying the D178N/V129 mutation. Neurons in these mice show swelling of the endoplasmic reticulum (ER) with intracellular retention of mutant PrP, suggesting that ER dysfunction could contribute to the pathology. These results establish a transgenic animal model of a genetic prion disease recapitulating cognitive, motor, and neurophysiological abnormalities of the human disorder. Tg(CJD) mice have the potential for giving greater insight into the spectrum of neuronal dysfunction in prion diseases.

Interleukin-1β modulates state-dependent discharge activity of preoptic area and basal forebrain neurons: role in sleep regulation

Interleukin‐1β (IL‐1) is a pro‐inflammatory cytokine that has been implicated in the regulation of nonrapid eye movement (nonREM) sleep. IL‐1, IL‐1 receptors and the IL‐1 receptor antagonist (ra) are present normally in discrete brain regions, including the preoptic area (POA) of the hypothalamus and the adjoining magnocellular basal forebrain (BF). The POA/BF have been implicated in the regulation of sleep–wakefulness. We hypothesized that IL‐1 promotes nonREM sleep, in part by altering the state‐dependent discharge activity of POA/BF neurons. We recorded the sleep–wake discharge profiles of 83 neurons in the lateral POA/BF and assessed the effects of IL‐1, IL‐1ra, and IL‐ra + IL‐1 delivered through a microdialysis probe on state‐dependent neuronal discharge activity. IL‐1 decreased the discharge rate of POA/BF neurons as a group (n = 55) but wake‐related and sleep‐related neurons responded differently. IL‐1 significantly decreased the discharge rate of wake‐related neurons. Of 24 wake‐related neurons studied, 19 (79%) neurons exhibited a greater than 20% change in their discharge in the presence of IL‐1 during waking. IL‐1 suppressed the discharge activity of 18 of 19 responsive neurons. Of 13 sleep‐related neurons studied, IL‐1 increased the discharge activity of five and suppressed the discharge activity of four neurons. IL‐1ra increased the discharge activity of four of nine neurons and significantly attenuated IL‐1‐induced effects on neuronal activity of POA/BF neurons (n = 19). These results suggest that the sleep‐promoting effects of IL‐1 may be mediated, in part, via the suppression of wake‐related neurons and the activation of a subpopulation of sleep‐related neurons in the POA/BF.

Interleukin-1β enhances non-rapid eye movement sleep when microinjected into the dorsal raphe nucleus and inhibits serotonergic neurons in vitro

Interleukin‐1 (IL‐1) and IL‐1 receptors are constitutively expressed in normal brain. IL‐1 increases non‐rapid eye movements (NREM) sleep in several animal species, an effect mediated in part by interactions with the serotonergic system. The site(s) in brain at which interactions between IL‐1 and the serotonergic system increase NREM sleep remain to be identified. The dorsal raphe (DRN) is the origin of the major ascending serotonergic pathways to the forebrain, and it contains IL‐1 receptors. This study examined the hypothesis that IL‐1 increases NREM sleep by acting at the level of the DRN. IL‐1β (0.25 and 0.5 ng) was microinjected into the DRN of freely behaving rats and subsequent effects on sleep–wake activity were determined. IL‐1β 0.5 ng increased NREM sleep during the first 2 h post‐injection from 33.5 ± 3.7% after vehicle microinjection to 42.9 ± 3.0% of recording time. To determine the effects of IL‐1β on electrophysiological properties of DRN serotonergic neurons, intracellular recordings were performed in a guinea‐pig brain stem slice preparation. In 26 of 32 physiologically and pharmacologically identified serotonergic neurons, IL‐1β superfusion (25 ng/mL) decreased spontaneous firing rates by 50%, from 1.6 ± 0.2 Hz (before IL‐1β superfusion) to 0.8 ± 0.2 Hz. This effect was reversible upon washout. These results show that IL‐1β increases NREM sleep when administered directly into the DRN. Serotonin enhances wakefulness and these novel data also suggest that IL‐1β‐induced enhancement of NREM sleep could be due in part to the inhibition of DRN serotonergic neurons.

Changes in the serotonergic system during the sleep-wake cycle: Simultaneous polygraphic and voltammetric recordings in hypothalamus using a telemetry system

Changes in the serotonergic system in the posterior hypothalamus of freely moving rats were related to sleep and wakefulness using in vivo voltammetry (with carbon fiber microelectrodes) and polygraphic recordings. By using an optoelectronic telemetry system for the voltammetric signals, electrical cross-talk between the two settings was avoided and simultaneous neurochemical and electro-physiological recordings could be made so that a detailed time course of events could be obtained. Extracellular levels of the serotonin metabolite, 5-hydroxy-indoleacetic acid, measured every 2 min, increased with wakefulness and decreased with sleep: levels were significantly lower during desynchronized sleep than slow wave sleep. In vivo voltammetry associated with the optoelectronic telemetry system appears to be a useful tool for studying the relationship between neurochemical changes and electrophysiological events.

Interleukin-1 inhibits firing of serotonergic neurons in the dorsal raphe nucleus and enhances GABAergic inhibitory post-synaptic potentials.

In vitro electrophysiological data suggest that interleukin‐1 may promote non‐rapid eye movement sleep by inhibiting spontaneous firing of wake‐active serotonergic neurons in the dorsal raphe nucleus (DRN). Interleukin‐1 enhances GABA inhibitory effects. DRN neurons are under an inhibitory GABAergic control. This study aimed to test the hypothesis that interleukin‐1 inhibits DRN serotonergic neurons by potentiating GABAergic inhibitory effects. In vitro intracellular recordings were performed to assess the responses of physiologically and pharmacologically identified DRN serotonergic neurons to rat recombinant interleukin‐1β. Coronal slices containing DRN were obtained from male Sprague–Dawley rats. The impact of interleukin‐1 on firing rate and on evoked post‐synaptic potentials was determined. Evoked post‐synaptic potentials were induced by stimulation with a bipolar electrode placed on the surface of the slice ventrolateral to DRN. Addition of interleukin‐1 (25 ng/mL) to the bath perfusate significantly decreased firing rates of DRN serotonergic neurons from 1.3 ± 0.2 Hz (before administration) to 0.7 ± 0.2 Hz. Electrical stimulation induced depolarizing evoked post‐synaptic potentials in DRN serotonergic neurons. The application of glutamatergic and GABAergic antagonists unmasked two different post‐synaptic potential components: a GABAergic evoked inhibitory post‐synaptic potentials and a glutamatergic evoked excitatory post‐synaptic potentials, respectively. Interleukin‐1 increased GABAergic evoked inhibitory post‐synaptic potentials amplitudes by 30.3 ± 3.8% (n = 6) without affecting glutamatergic evoked excitatory post‐synaptic potentials. These results support the hypothesis that interleukin‐1 inhibitory effects on DRN serotonergic neurons are mediated by an interleukin‐1‐induced potentiation of evoked GABAergic inhibitory responses.

Selective blockade of different brain stem muscarinic receptor subtypes: effects on the sleep-wake cycle

Changes induced in the sleep-wake cycle by pontine microinjections of muscarinic antagonists were studied in freely moving rats, instrumented for chronic polygraphic recordings. Pirenzepine (PIR), methoctramine (MET) and p-fluoro-hexahydro-siladifenidol (p-F-HHSiD), which are highly selective M1, M2 and M3 antagonists, respectively, were dissolved in 0.1 microliter of sterile isotonic saline (0.2 microliter of distilled water for p-F-HHSiD) and injected into the pontine reticular nucleus, where the administration of 0.5 microgram carbachol (a mixed muscarinic agonist) induced a 52% increase in the amount of desynchronized sleep (DS) over a 6 h recording period. The blockade of M2 receptors was shown to (i) antagonize DS, by increasing its latency and decreasing its percentage, (ii) decrease slow wave sleep, and (iii) enhance wakefulness. These effects were dose-dependent. No changes in the sleep-wake cycle were observed following microinjection of M1 or M3 antagonists. The results support the hypothesis that at the brain stem level only M2 receptors are involved in sleep mechanisms and, particularly, in the generation and maintenance of DS.

Cytokine-Neurotransmitter Interactions in the Brain

The data reviewed in this study show that immune-active molecules, such as infectious agents and their components, and cytokines, may induce profound alterations in several neurotransmitters in the CNS. The activation of the immune system elicits fever, behavioral and neuroendocrine changes and may be involved in neuropathological changes occurring in CNS conditions. These effects may be achieved through and accounted for by the changes induced in central neurotransmitters and in the neuroendocrine system by immune challenges. The present review will summarize the available evidence of the reciprocal interactions between cytokines and neurotransmitters in the CNS.

Blockade of 5-hydroxytryptamine (serotonin)-2 receptors alters interleukin-1-induced changes in rat sleep

Recent data suggest that interleukin-1-induced enhancement of non-rapid eye movement sleep is mediated, in part, by the serotonergic system. To determine if sleep changes induced by interleukin-1 are mediated by a specific serotonergic receptor subtype, we evaluated interleukin-1 effects on sleep in rats pretreated with the 5-hydroxytryptamine (serotonin)-2 receptor antagonist ritanserin. Ritanserin (0.63 mg/kg, intraperitoneally) by itself did not alter sleep-wake behavior, although it did reduce cortical brain temperature. Interleukin-1 (5 ng, intracerebroventricularly) enhanced non-rapid eye movement sleep, suppressed rapid eye movement sleep, and induced a moderate febrile response. Pretreatment with ritanserin completely blocked the febrile response to interleukin-1 and abolished the interleukin-1-induced enhancement in non-rapid eye movement sleep that occurred during postinjection hours 3-4, without altering interleukin-1 effects on rapid eye movement sleep. The present data suggest that serotonin may partially mediate interleukin-1 effects on sleep by interacting with 5-hydroxytryptamine (serotonin)-2 receptors. These results also suggest that interactions between the serotonergic system and interleukin-1 may be important in regulating sleep-wake behavior.

5-Hydroxytryptophan, but not L-tryptophan, alters sleep and brain temperature in rats.

The precise role of serotonin (5-hydroxytryptamine) in the regulation of sleep is not fully understood. To further clarify this role for 5-hydroxytryptamine, the 5-hydroxytryptamine precursors L-tryptophan (40 and 80 mg/kg) and L-5-hydroxytryptophan (25-, 50-, 75-, 100 mg/kg) were injected intraperitoneally into freely behaving rats 15 min prior to dark onset, and subsequent effects on sleep-wake activity and cortical brain temperature were determined. L-5-hydroxytryptophan, but not L-tryptophan, induced dose-dependent changes in sleep-wake activity. During the 12-h dark period, non-rapid eye movement sleep was inhibited in post-injection hours 1-2 by the two lowest L-5-hydroxytryptophan doses tested, while the two highest doses induced a delayed increase in non-rapid eye movement sleep in post-injection hours 3-12. These highest doses inhibited non-rapid eye movement sleep during the subsequent 12-h light period. The finding that L-5-hydroxytryptophan, but not L-tryptophan, induced a dose-dependent and long-lasting decrease in cortical brain temperature regardless of whether or not non-rapid eye movement sleep was suppressed or enhanced contributes to a growing list of conditions showing that sleep-wake activity and thermoregulation, although normally tightly coupled, may be dissociated. The initial non-rapid eye movement sleep inhibition observed following low doses of L-5-hydroxytryptophan may be attributable to increased serotonergic activity since 5-hydroxytryptamine may promote wakefulness per se, whereas the delayed non-rapid eye movement sleep enhancement after higher doses may be due to the induction by 5-hydroxytryptamine of sleep-inducing factor(s), as previously hypothesized. The period of non-rapid eye movement sleep inhibition beginning 12 h after administration of L-5-hydroxytryptophan doses that increase non-rapid eye movement sleep is characteristic of physiological manipulations in which non-rapid eye movement sleep is enhanced. The results of the present study suggest that the complex effects of 5-HT on sleep depend on the degree and time course of activation of the serotonergic system such that 5-HT may directly inhibit sleep, yet induce a cascade of physiological processes that enhance subsequent sleep.