The role of cholinergic basal forebrain neurons in adenosine-mediated homeostatic control of sleep: lessons from 192 IgG-saporin lesions.

A topic of high current interest and controversy is the basis of the homeostatic sleep response, the increase in non-rapid-eye-movement (NREM) sleep and NREM-delta activity following sleep deprivation (SD). Adenosine, which accumulates in the cholinergic basal forebrain (BF) during SD, has been proposed as one of the important homeostatic sleep factors. It is suggested that sleep-inducing effects of adenosine are mediated by inhibiting the wake-active neurons of the BF, including cholinergic neurons. Here we examined the association between SD-induced adenosine release, the homeostatic sleep response and the survival of cholinergic neurons in the BF after injections of the immunotoxin 192 immunoglobulin G (IgG)-saporin (saporin) in rats. We correlated SD-induced adenosine level in the BF and the homeostatic sleep response with the cholinergic cell loss 2 weeks after local saporin injections into the BF, as well as 2 and 3 weeks after i.c.v. saporin injections. Two weeks after local saporin injection there was an 88% cholinergic cell loss, coupled with nearly complete abolition of the SD-induced adenosine increase in the BF, the homeostatic sleep response, and the sleep-inducing effects of BF adenosine infusion. Two weeks after i.c.v. saporin injection there was a 59% cholinergic cell loss, correlated with significant increase in SD-induced adenosine level in the BF and an intact sleep response. Three weeks after i.c.v. saporin injection there was an 87% cholinergic cell loss, nearly complete abolition of the SD-induced adenosine increase in the BF and the homeostatic response, implying that the time course of i.c.v. saporin lesions is a key variable in interpreting experimental results. Taken together, these results strongly suggest that cholinergic neurons in the BF are important for the SD-induced increase in adenosine as well as for its sleep-inducing effects and play a major, although not exclusive, role in sleep homeostasis.

Disrupted sleep-wake regulation in type 1 equilibrative nucleoside transporter knockout mice.

The type 1 equilibrative nucleoside transporter (ENT1) is implicated in regulating levels of extracellular adenosine ([AD]ex). In the basal forebrain (BF) levels of [AD]ex increase during wakefulness and closely correspond to the increases in the electroencephalogram (EEG) delta (0.75-4.5Hz) activity (NRδ) during subsequent non-rapid eye movement sleep (NREMS). Thus in the BF, [AD]ex serves as a biochemical marker of sleep homeostasis. Waking EEG activity in theta range (5-9Hz, Wθ) is also described as a marker of sleep homeostasis. An hour-by-hour temporal relationship between the Wθ and NRδ is unclear. In this study we examined the relationship between these EEG markers of sleep homeostasis during spontaneous sleep-wakefulness and during sleep deprivation (SD) and recovery sleep in the ENT1 gene knockout (ENT1KO) mouse. We observed that baseline NREMS amount was decreased during the light period in ENT1KO mice, accompanied by a weak correlation between Wθ of each hour and NRδ of its subsequent hour when compared to their wild-type (WT) littermates. Perfusion of low dose of adenosine into BF not only strengthened the Wθ-NRδ relationship, but also increased NREMS to match with the WT littermates suggesting decreased [AD]ex in ENT1KO mice. However, the SD-induced [AD]ex increase in the BF and the linear correlation between the EEG markers of sleep homeostasis were unaffected in ENT1KO mice suggesting that during SD, sources other than ENT1 contribute to increase in [AD]ex. Our data provide evidence for a differential regulation of wakefulness-associated [AD]ex during spontaneous vs prolonged waking.

Inducible and neuronal nitric oxide synthases (NOS) have complementary roles in recovery sleep induction.

Sleep homeostasis is the process by which recovery sleep is generated by prolonged wakefulness. The molecular mechanisms underlying this important phenomenon are poorly understood. We have previously shown that nitric oxide (NO) generation increases in the basal forebrain (BF) during sleep deprivation (SD). Moreover, both NO synthase (NOS) inhibition and a NO scavenger prevented recovery sleep induction, while administration of a NO donor during the spontaneous sleep-wake cycle increased sleep, indicating that NO is necessary and sufficient for the induction of recovery sleep. Next we wanted to know which NOS isoform is involved in the production of recovery sleep. Using in vivo microdialysis we infused specific inhibitors of NOS into the BF of rats during SD, and found that an inhibitor of inducible NOS (iNOS), 1400W, prevented non-rapid eye movement (NREM) recovery, while an inhibitor of neuronal NOS (nNOS), L-N-propyl-arginine, decreased REM recovery but did not affect NREM recovery. Using immunoblot analysis we found that iNOS was not expressed during the spontaneous sleep-wake cycle, but was induced by prolonged wakefulness (increased by 278%). A known iNOS inducer, lipopolysaccharide, evoked an increase in sleep that closely resembled recovery sleep, and its effects were abolished by 1400W. These results suggest that the elevation of NO produced by induction of iNOS in the BF during prolonged wakefulness is a specific mechanism for producing NREM recovery sleep and that the two NOS isoforms have a complementary role in NREM and REM recovery induction.

Nitric oxide production in the basal forebrain is required for recovery sleep.

Sleep homeostasis is the process by which recovery sleep is generated by prolonged wakefulness. The molecular mechanisms underlying this important phenomenon are poorly understood. Here, we assessed the role of the intercellular gaseous signaling agent NO in sleep homeostasis. We measured the concentration of nitrite and nitrate, indicative of NO production, in the basal forebrain (BF) of rats during sleep deprivation (SD), and found the level increased by 100 +/- 51%. To test whether an increase in NO production might play a causal role in recovery sleep, we administered compounds into the BF that increase or decrease concentrations of NO. Infusion of either a NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, or a NO synthase inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME), completely abolished non-rapid eye movement (NREM) recovery sleep. Infusion of a NO donor, (Z)-1-[N-(2-aminoethyl)-N-(2-ammonioethyl)amino]diazen-1-ium-1,2diolate (DETA/NO), produced an increase in NREM that closely resembled NREM recovery after prolonged wakefulness. The effects of inhibition of NO synthesis and the pharmacological induction of sleep were effective only in the BF area. Indicators of energy metabolism, adenosine, lactate and pyruvate increased during prolonged wakefulness and DETA/NO infusion, whereas L-NAME infusion during SD prevented the increases. We conclude that an increase in NO production in the BF is a causal event in the induction of recovery sleep.