Nitric oxide production in rat thalamus changes with behavioral state, local depolarization, and brainstem stimulation

Sleep loss mediates the effect of stress on nitrergic signaling in female mice

Nitric oxide (NO) has been implicated as an important neurotransmitter in stress responses and sleep regulatory processes. However, the role of NO in the relationship between stress and sleep remains unclear. The medial septum (MS) and vertical diagonal band (VDB), regions of the basal forebrain involved in sleep regulation, contain nitric oxide synthase (NOS) producing neurons. Additionally, NOS neurons in the dorsal raphe nucleus (DRN) encode information about stress duration. The role of nitrergic neurons in these regions in subserving sex-specific responses to stress and sleep loss has yet to be elucidated. In this study, NADPH-d, an index of NOS activity, was used to examine the effects of acute restraint stress and sleep loss on NOS activity in the MS, VDB, and DRN. We show that NOS activity in response to restraint stress, total sleep deprivation (TSD), and partial sleep restriction (PSR) differs based on sex and region. Initial analysis showed no effect of restraint stress or TSD on NOS activity in the basal forebrain. However, investigation of each sex separately revealed that restraint stress and TSD significantly decrease NOS activity in the MS of females, but not males. Interestingly, the difference in NOS activity between restraint stress and TSD in females was not significant. Furthermore, PSR was not sufficient to affect NOS activity in males or females. These data suggest that restraint stress and sleep loss regulate NOS activation in a sex-dependent manner, and that the NOS stress response in females may be mediated by sleep loss.

Sleep and Cellular Stress

Sleep is a universal phenomenon occurring in all species studied thus far. Sleep loss results in adverse physiological effects at both the organismal and cellular levels suggesting an adaptive role for sleep in the maintenance of overall health. This review examines the bidirectional relationship between sleep and cellular stress. Cellular stress in this review refers to a shift in cellular homeostasis in response to an external stressor. Studies that illustrate the fact that sleep loss induces cellular stress and those that provide evidence that cellular stress in turn promotes sleep will be discussed.

Enhancing oxygen delivery to ovarian follicles by three different methods markedly improves growth in serum-containing culture medium

Invitro ovarian follicle culture systems are routinely used to study folliculogenesis and may provide solutions for infertility. Mouse follicles are typically cultured in standard gas-impermeable culture plates under gas phase oxygen concentrations of 5% or 20% (v/v). There is evidence that these conditions may not provide adequate oxygenation for follicles cultured as non-attached intact units in medium supplemented with serum and high levels of FSH. Three different methods of enhancing follicle oxygenation were investigated in this study: increasing the gas phase oxygen concentration, inverting the culture plates and using gas-permeable culture plates. Follicles cultured under 40% O2 were significantly larger (P P P 2 . These effects were associated with reduced secretion of vascular endothelial growth factor (P P P invivo -matured follicles (~500μm in diameter). Such follicular development is not possible under hypoxic conditions.

Cocoa and Dark Chocolate Polyphenols: From Biology to Clinical Applications

It is well known that cocoa and dark chocolate possess polyphenols as major constituents whose dietary consumption has been associated to beneficial effects. In fact, cocoa and dark chocolate polyphenols exert antioxidant and anti-inflammatory activities switching on some important signaling pathways such as toll-like receptor 4/nuclear factor κB/signal transducer and activator of transcription. In particular, cocoa polyphenols induce release of nitric oxide (NO) through activation of endothelial NO synthase which, in turn, accounts for vasodilation and cardioprotective effects. In the light of the above described properties, a number of clinical trials based on the consumption of cocoa and dark chocolate have been conducted in healthy subjects as well as in different categories of patients, such as those affected by cardiovascular, neurological, intestinal, and metabolic pathologies. Even if data are not always concordant, modifications of biomarkers of disease are frequently associated to improvement of clinical manifestations. Quite interestingly, following cocoa and dark chocolate ingestion, cocoa polyphenols also modulate intestinal microbiota, thus leading to the growth of bacteria that trigger a tolerogenic anti-inflammatory pathway in the host. Finally, many evidences encourage the consumption of cocoa and dark chocolate by aged people for the recovery of the neurovascular unit.

Anandamide and 2-AG are endogenously present within the laterodorsal tegmental nucleus: Functional implications for a role of eCBs in arousal

Previously, we presented electrophysiological evidence for presence in mice brain slices of functional cannabinoid type I receptors (CB1Rs) within the laterodorsal tegmentum (LDT), a brain stem nucleus critical in control of arousal and rapid eye movement (REM) sleep. Further, using pharmacological agents, we provided data suggestive of the endogenous presence of cannabinoids (CBs) acting at LDT CB1Rs. However, in those studies, identification of the type(s) of CB ligands endogenously present in the LDT remained outstanding, and this information has not been provided elsewhere. Accordingly, we used the highly-sensitive liquid chromatography/mass spectrometry (LC-MS) method to determine whether N-arachidonoylethanolamide (Anandamide or AEA) and 2-arachidonyl glycerol (2-AG), which are both endogenous CB ligands acting at CB1Rs, are present in the LDT. Mice brain tissue samples of the LDT were assayed using ion trap LC-MS in selected ion monitoring mode. Chromatographic analysis and product-ion MS scans identified presence of the CBs, AEA and 2-AG, from LDT mouse tissue. Data using the LC-MS method show that AEA and 2-AG are endogenously present within the LDT and when coupled with our electrophysiological findings, lead to the suggestion that AEA and 2-AG act at electropharmacologically-demonstrated CB1Rs in this nucleus. Accordingly, AEA and 2-AG likely play a role in processes governed by the LDT, including control of states of cortical gamma band activity seen in alert, aroused states, as well as cortical and motor activity characteristic of REM sleep.

Prolonged Mild Hypoxia Alters Fetal Sheep Electrocorticogram Activity

OBJECTIVE

To assess the effects of prolonged mild hypoxemia on fetal brain electrocorticogram (ECoG) in late gestation.

STUDY DESIGN

Fetal and maternal catheters were placed under general anesthesia and animals allocated at random to receive intratracheal maternal administration of either nitrogen (n = 8) or compressed air (n = 8). Five days after surgery (125 days' gestational age), nitrogen infusion was adjusted to reduce fetal brachial artery PO2 by 25%. The targeted decrease in fetal oxygenation was maintained for 5 days while fetal ECoG activity and fetal and maternal cardiovascular variables were continuously recorded. Data are presented as mean +/- SEM and were analyzed by two-way analysis of variance (ANOVA) or two-sample t test.

RESULTS

Nitrogen infusion decreased fetal Po2 by 26% (20.5 +/- 1.7 versus 14.3 +/- 0.8) without changing fetal PCO2 or pH. Mild fetal hypoxemia was associated with fetal tachycardia and increased fetal blood pressure (P < .05). Fetal ECoG in hypoxic fetuses showed a significant decrease in the time spent in high voltage (HV) (P < .05) and an increase in the time spent in low voltage (LV) and in the number of low voltage events (P < .05). Also, a significant decrease in the proportion of 1-4 Hz and an increase in the proportion of 13-20 Hz frequencies was observed in LV events without a significant change in the frequency profile of HV events (P < .05).

CONCLUSION

Prolonged mild hypoxemia significantly altered fetal homeostasis as reflected by the sustained tachycardia and increased blood pressure. Fetal ECoG activity was affected significantly in a qualitatively and quantitative manner by mild prolonged hypoxemia.

Age-Related Cognitive Impairment as a Sign of Geriatric Neurocardiovascular Interactions: May Polyphenols Play a Protective Role?

It is known that endothelial dysfunction plays an important role in the development and progression of cardiovascular diseases implicated also in cognitive decline. Experimental studies pointed to the fact that the modification of NO levels via NOS activity may affect the blood pressure level as well as several higher nervous functions—for example, learning and memory. There are emerging evidences from in vitro and animal studies suggesting that polyphenols may potentially have a protective effect on the development of neurodegenerative diseases and may improve cognitive function as well as positively affecting the blood pressure regulatory mechanisms. This review accentuates the need for precisely defined clinically controlled studies as well as for use of adequate experimental procedures discriminating between the human higher brain functions and the only overall activation of the brain cortex. The physiological neurocardiovascular interactions are implicated in the increased healthy life span as well.

The appetite-inducing peptide, ghrelin, induces intracellular store-mediated rises in calcium in addiction and arousal-related laterodorsal tegmental neurons in mouse brain slices

Ghrelin, a gut and brain peptide, has recently been shown to be involved in motivated behavior and regulation of the sleep and wakefulness cycle. The laterodorsal tegmental nucleus (LDT) is involved in appetitive behavior and control of the arousal state of an organism, and accordingly, behavioral actions of ghrelin could be mediated by direct cellular actions within this nucleus. Consistent with this interpretation, postsynaptically mediated depolarizing membrane actions of ghrelin on LDT neurons have been reported. Direct actions were ascribed solely to closure of a potassium conductance however this peptide has been shown in other cell types to lead to rises in calcium via release of calcium from intracellular stores. To determine whether ghrelin induced intracellular calcium rises in mouse LDT neurons, we conducted calcium imaging studies in LDT brain slices loaded with the calcium binding dye, Fura-2AM. Ghrelin elicited TTX-insensitive changes in dF/F indicative of rises in calcium, and a portion of these rises were independent of membrane depolarization, as they persisted in conditions of high extracellular potassium solutions and were found to involve SERCA-pump mediated intracellular calcium stores. Involvement of the ghrelin receptor (GHR-S) in these actions was confirmed. Taken together with other studies, our data suggest that ghrelin has multiple cellular actions on LDT cells. Ghrelin's induction of calcium via intracellular release in the LDT could play a role in behavioral actions of this peptide as the LDT governs processes involved in stimulation of motivated behavior and control of cortical arousal.

Control of sleep and wakefulness

This review summarizes the brain mechanisms controlling sleep and wakefulness. Wakefulness promoting systems cause low-voltage, fast activity in the electroencephalogram (EEG). Multiple interacting neurotransmitter systems in the brain stem, hypothalamus, and basal forebrain converge onto common effector systems in the thalamus and cortex. Sleep results from the inhibition of wake-promoting systems by homeostatic sleep factors such as adenosine and nitric oxide and GABAergic neurons in the preoptic area of the hypothalamus, resulting in large-amplitude, slow EEG oscillations. Local, activity-dependent factors modulate the amplitude and frequency of cortical slow oscillations. Non-rapid-eye-movement (NREM) sleep results in conservation of brain energy and facilitates memory consolidation through the modulation of synaptic weights. Rapid-eye-movement (REM) sleep results from the interaction of brain stem cholinergic, aminergic, and GABAergic neurons which control the activity of glutamatergic reticular formation neurons leading to REM sleep phenomena such as muscle atonia, REMs, dreaming, and cortical activation. Strong activation of limbic regions during REM sleep suggests a role in regulation of emotion. Genetic studies suggest that brain mechanisms controlling waking and NREM sleep are strongly conserved throughout evolution, underscoring their enormous importance for brain function. Sleep disruption interferes with the normal restorative functions of NREM and REM sleep, resulting in disruptions of breathing and cardiovascular function, changes in emotional reactivity, and cognitive impairments in attention, memory, and decision making.

TRANSLATION OF BRAIN ACTIVITY INTO SLEEP

Cytokines including tumor necrosis factor alpha (TNF) play a role in sleep regulation in health and disease. Hypothalamic and cerebral cortical levels of TNF mRNA or TNF protein have diurnal variations with higher levels associated with greater sleep propensity. Sleep loss is associated with enhanced brain TNF. Central or systemic TNF injections enhance sleep. Inhibition of TNF using the soluble TNF receptor, or anti-TNF antibodies, or a TNF siRNA reduces spontaneous sleep. Mice lacking the TNF 55 kD receptor have less spontaneous sleep. Injection of TNF into sleep regulatory circuits, e.g. the hypothalamus, promotes sleep. In normal humans, plasma levels of TNF co-vary with EEG slow wave activity (SWA) and in multiple disease states plasma TNF increases in parallel with sleep propensity. Downstream mechanisms of TNF-enhanced sleep include nitric oxide, adenosine, prostaglandins and activation of nuclear factor kappa B. Neuronal use induces cortical neurons to express TNF and if applied directly to cortical columns TNF induces a functional sleep-like state within the column. TNF mechanistically has several synaptic functions. TNF-sleep data led to the idea that sleep is a fundamental property of neuronal/glial networks such as cortical columns and is dependent upon past activity within such assemblies. This view of brain organization of sleep has profound implications for sleep function that are briefly reviewed herein.

Cerebral microglia mediate sleep/wake and neuroinflammatory effects of methamphetamine

Methamphetamine and modafinil exert their wake-promoting effects by elevating monoaminergic tone. The severity of hypersomnolence that occurs subsequent to induced wakefulness differs between these two agents. Microglia detects and modulates CNS reactions to agents such as D-methamphetamine that induce cellular stress. We therefore hypothesized that changes in the sleep/wake cycle that occur subsequent to administration of D-methamphetamine are modulated by cerebral microglia. In CD11b-herpes thymidine kinase transgenic mice (CD11b-TK(mt-30)), activation of the inducible transgene by intracerebroventricular (icv) ganciclovir results in toxicity to CD11b-positive cells (i.e. microglia), thereby reducing cerebral microglial cell counts. CD11b-TK(mt-30)and wild type mice were subjected to chronic icv ganciclovir or vehicle administration with subcutaneous mini-osmotic pumps. D-methamphetamine (1 and 2 mg/kg), modafinil (30 and 100 mg/kg) and vehicle were administered intraperitoneally to these animals. In CD11b-TK(mt-30) mice, but not wild type, icv infusion of ganciclovir reduced the duration of wake produced by D-methamphetamine at 2 mg/kg by nearly 1h. Nitric oxide synthase (NOS) activity, studied ex vivo, and NOS expression were elevated in CD11b-positive cerebral microglia from wild type mice acutely exposed to d-methamphetamine. Additionally, CD11b-positive microglia, but not other cerebral cell populations, exhibited changes in sleep-regulatory cytokine expression in response to d-METH. Finally, CD11b-positive microglia exposed to d-methamphetamine in vitro exhibited increased NOS activity relative to pharmacologically-naïve cells. CD11b-positive microglia from the brains of neuronal NOS (nNOS)-knockout mice failed to exhibit this effect. We propose that the effects of D-METH on sleep/wake cycles are mediated in part by actions on microglia, including possibly nNOS activity and cytokine synthesis.

Nitric oxide physiology and pathology

Nitric oxide (NO) is just one member of a new class of gaseous signalling molecules with fundamental actions in biology. In higher vertebrates it has key roles in maintaining haemostasis and in smooth muscle (especially vascular smooth muscle), neurons and the gastrointestinal tract. It is intimately involved in regulating all aspects of our lives from waking, digestion, sexual function, perception of pain and pleasure, memory recall and sleeping. Finally, the way it continues to function in our bodies will influence how we degenerate with age. It will likely play a role in our deaths through cardiovascular disease, stroke, diabetes and cancer. Our ability to control NO signalling and to use NO effectively in therapy must therefore have a major bearing on the future quality and duration of human life.

The time course of adenosine, nitric oxide (NO) and inducible NO synthase changes in the brain with sleep loss and their role in the non-rapid eye movement sleep homeostatic cascade

Both adenosine and nitric oxide (NO) are known for their role in sleep homeostasis, with the basal forebrain (BF) wakefulness center as an important site of action. Previously, we reported a cascade of homeostatic events, wherein sleep deprivation (SD) induces the production of inducible nitric oxide synthase (iNOS)-dependent NO in BF, leading to enhanced release of extracellular adenosine. In turn, increased BF adenosine leads to enhanced sleep intensity, as measured by increased non-rapid eye movement sleep EEG delta activity. However, the presence and time course of similar events in cortex has not been studied, although a frontal cortical role for the increase in non-rapid eye movement recovery sleep EEG delta power is known. Accordingly, we performed simultaneous hourly microdialysis sample collection from BF and frontal cortex (FC) during 11 h SD. We observed that both areas showed sequential increases in iNOS and NO, followed by increases in adenosine. BF increases began at 1 h SD, whereas FC increases began at 5 h SD. iNOS and Fos-double labeling indicated that iNOS induction occurred in BF and FC wake-active neurons. These data support the role of BF adenosine and NO in sleep homeostasis and indicate the temporal and spatial sequence of sleep homeostatic cascade for NO and adenosine.

Synapse- and subtype-specific modulation of synaptic transmission by nicotinic acetylcholine receptors in the ventrobasal thalamus

The rodent thalamic ventrobasal complex (VB) which is a subdivision of somatosensory thalamus receives two excitatory inputs through the medial lemniscal synapse, which is a sensory afferent synapse, and the corticothalamic synapse from layer VI of the somatosensory cortex. In addition, the VB also receives cholinergic inputs from the brain stem, and nicotinic acetylcholine receptors (nAChRs) are highly expressed in the VB. Little is known, however, how acetylcholine (ACh) modulates synaptic transmission at the medial lemniscal and corticothalamic synapses in the VB. Furthermore, it remains unclear which subtype of nAChRs contributes to VB synaptic transmission. We report here that the activation of nAChRs presynaptically depressed corticothalamic synaptic transmission, whereas it did not affect medial lemniscal synaptic transmission in juvenile mice. This presynaptic modulation was mediated by the activation of nAChRs that contained α4 and β2 subunit, but not by α7 nAChRs. Moreover, galanthamine, an allosteric modulator of α4β2α5 nAChR, enhanced the ACh-induced depression of corticothalamic excitatory postsynaptic currents (EPSCs), indicating that α4β2α5 nAChRs at corticothalamic axon terminals specifically contribute to the depression of corticothalamic synaptic transmission.

Pedunculopontine and laterodorsal tegmental nuclei contain distinct populations of cholinergic, glutamatergic and GABAergic neurons in the rat

The pedunculopontine tegmental nucleus (PPTg) and laterodorsal tegmental nucleus (LDTg) provide cholinergic afferents to several brain areas. This cholinergic complex has been suggested to play a role in sleep, waking, motor function, learning and reward. To have a better understanding of the neurochemical organization of the PPTg/LDTg we characterized the phenotype of PPTg/LDTg neurons by determining in these cells the expression of transcripts encoding choline acetyltransferase (ChAT), glutamic acid decarboxylase (GAD) or the vesicular glutamate transporters (vGluT1, vGluT2 and vGluT3). Within the PPTg/LDTg complex we found neurons expressing ChAT, vGluT2 or GAD transcripts, these neuronal phenotypes were intermingled, but not homogeneously distributed within the PPTg or LDTg. Previous studies suggested the presence of either glutamate or gamma-aminobutyric acid (GABA) immunolabeling in a large number of PPTg/LDTg cholinergic neurons, leading to the widespread notion that PPTg/LDTg cholinergic neurons co-release acetylcholine together with either glutamate or GABA. To assess the glutamatergic or GABAergic nature of the PPTg/LDTg cholinergic neurons, we combined in situ hybridization (to detect vGluT2 or GAD transcripts) and immunohistochemistry (to detect ChAT), and found that over 95% of all PPTg/LDTg cholinergic neurons lack transcripts encoding either vGluT2 mRNA or GAD mRNA. As the vast majority of PPTg/LDTg cholinergic neurons lack transcripts encoding essential proteins for the vesicular transport of glutamate or for the synthesis of GABA, co-release of acetylcholine with either glutamate or GABA is unlikely to be a major factor in the interactions between acetylcholine, glutamate and GABA at the postsynaptic site.

cGMP-dependent protein kinase type I is implicated in the regulation of the timing and quality of sleep and wakefulness

Many effects of nitric oxide (NO) are mediated by the activation of guanylyl cyclases and subsequent production of the second messenger cyclic guanosine-3′,5′-monophosphate (cGMP). cGMP activates cGMP-dependent protein kinases (PRKGs), which can therefore be considered downstream effectors of NO signaling. Since NO is thought to be involved in the regulation of both sleep and circadian rhythms, we analyzed these two processes in mice deficient for cGMP-dependent protein kinase type I (PRKG1) in the brain. Prkg1 mutant mice showed a strikingly altered distribution of sleep and wakefulness over the 24 hours of a day as well as reductions in rapid-eye-movement sleep (REMS) duration and in non-REM sleep (NREMS) consolidation, and their ability to sustain waking episodes was compromised. Furthermore, they displayed a drastic decrease in electroencephalogram (EEG) power in the delta frequency range (1–4 Hz) under baseline conditions, which could be normalized after sleep deprivation. In line with the re-distribution of sleep and wakefulness, the analysis of wheel-running and drinking activity revealed more rest bouts during the activity phase and a higher percentage of daytime activity in mutant animals. No changes were observed in internal period length and phase-shifting properties of the circadian clock while chi-squared periodogram amplitude was significantly reduced, hinting at a less robust oscillator. These results indicate that PRKG1 might be involved in the stabilization and output strength of the circadian oscillator in mice. Moreover, PRKG1 deficiency results in an aberrant pattern, and consequently a reduced quality, of sleep and wakefulness, possibly due to a decreased wake-promoting output of the circadian system impinging upon sleep.

Excitatory and anti-oscillatory actions of nitric oxide in thalamus

The dorsal lateral geniculate nucleus (dLGN) not only serves as the obligatory pathway for visual information transfer from the retina to neocortex but can also generate intrathalamic rhythmic activities associated with different arousal states and certain pathological conditions. The gating activity of thalamocortical circuits is under neuromodulatory control by various brainstem nuclei as well as intrinsic thalamic neurons (e.g. thalamic reticular nucleus (TRN) neurons and dLGN interneurons). In this study, we examined the effect of the putative neuromodulator nitric oxide (NO) on thalamic neuron excitability. There are multiple potential sources of NO in thalamus: cholinergic terminals originating from brainstem nuclei, GABAergic TRN neurons, and local GABAergic interneurons. Using whole cell recording techniques in in vitro thalamic slices, we found that the NO donor SNAP produced a robust, long-lasting depolarization in TRN neurons, a weaker depolarization in thalamocortical relay neurons, and no effect in local interneurons. SNAP preferentially depolarized stereotypical TRN neurons that could produced strong burst discharge. In contrast, SNAP had little effect on atypical burst and non-burst TRN cells. The NO donor SIN-1 and the endogenous NO precursor, L-arginine, mimicked the SNAP-mediated actions. The NO-mediated depolarizations were blocked by the guanylyl cyclase inhibitor ODQ indicating involvement of the cGMP pathway. In addition, the phosphodiesterase (PDE) inhibitor zaprinast depolarized and occluded the NO-mediated depolarization in TRN neurons. At the circuit level, NO activation significantly attenuated intrathalamic rhythmic activities likely resulting from the shifting of the firing mode of thalamic neurons, perhaps both TRN and thalamocortical neurons, from burst- to tonic-discharge mode. These alterations in thalamic neuron excitability not only change rhythmic circuit activity, but could also influence sensory information processing through thalamocortical circuits.

Neuropeptide interactions and REM sleep: a role for Urotensin II?

Urotensin II (UII) is a peptide with structural similarity to the somatostatin family with potent vasoconstrictor activity. UII receptor is expressed broadly in the periphery, and most notably in the heart and microvessels. In the brain, the UII receptor can be detected in the spinal cord and in cholinergic nuclei in the brainstem known to be involved in REM sleep regulation. Recent data suggest that, in addition to their vasoactive properties, UII receptor ligands may have excitatory activity on a selective group of neurons that modulate REM sleep. This review focuses on the implications of these findings for the neurobiology of REM sleep regulation and discusses the possible impact of UII and other neuropeptides on the balance of the alternation between sleep states.

Phosphodiesterase type 2 and the homeostasis of cyclic GMP in living thalamic neurons

The ubiquitous second messenger cyclic GMP (cGMP) is synthesized by soluble guanylate cyclases in response to nitric oxide (NO) and degraded by phosphodiesterases (PDE). We studied the homeostasis of cGMP in living thalamic neurons by using the genetically encoded fluorescence resonance energy transfer sensor Cygnet, expressed in brain slices through viral gene transfer. Natriuretic peptides had no effect on cGMP. Basal cGMP levels decreased upon inhibition of NO synthases or soluble guanylate cyclases and increased when PDEs were inhibited. Single cell RT-PCR analysis showed that thalamic neurons express PDE1, PDE2, PDE9, and PDE10. Basal cGMP levels were increased by the PDE2 inhibitors erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) and BAY60-7550 but were unaffected by PDE1 or PDE10 inhibitors. We conclude that PDE2 regulates the basal cGMP concentration in thalamic neurons. In addition, in the presence of 3-isobutyl-1-methylxanthine (IBMX), cGMP still decreased after application of a NO donor. Probenecid, a blocker of cGMP transporters, had no effect on this decrease, leaving PDE9 as a possible candidate for decreasing cGMP concentration. Basal cGMP level is poised at an intermediate level from which it can be up or down-regulated according to the cyclase and PDE activities.

Modulation of inhibitory activity by nitric oxide in the thalamus

The dorsal lateral geniculate nucleus (dLGN) is essential for the transfer of visual information from the retina to visual cortex, and inhibitory mechanisms can play a critical in regulating such information transfer. Nitric oxide (NO) is an atypical neuromodulator that is released in gaseous form and can alter neural activity without direct synaptic connections. Nitric oxide synthase (NOS), an essential enzyme for NO production, is localized in thalamic inhibitory neurons and cholinergic brain stem neurons that innervate the thalamus, although NO-mediated effects on thalamic inhibitory activity remain unknown. We investigated NO effects on inhibitory activity in dLGN using an in vitro slice preparation. The NO donor, SNAP, selectively potentiated the frequency, but not amplitude, of spontaneous inhibitory postsynaptic currents (sIPSCs) in thalamocortical relay neurons. This increase also persisted in tetrodotoxin (TTX), consistent with an increase in GABA release from presynaptic terminals. The SNAP-mediated actions were attenuated not only by the NO scavenger carboxy-PTIO but also by the guanylyl cyclase inhibitor ODQ. The endogenous NO precursor L-arginine produced actions similar to those of SNAP on sIPSC activity and these L-arginine-mediated actions were attenuated by the NOS inhibitor L-NMMA acetate. The SNAP-mediated increase in sIPSC activity was observed in both dLGN and ventrobasal thalamic nucleus (VB) neurons. Considering the lack of interneurons in rodent VB, the NO-mediated actions likely involve an increase in the output of axon terminals of thalamic reticular nucleus neurons. Our results indicate that NO upregulates thalamic inhibitory activity and thus these actions likely influence sensory information transfer through thalamocortical circuits.

Nitric oxide-induced adenosine inhibition of hippocampal synaptic transmission depends on adenosine kinase inhibition and is cyclic GMP independent

Adenosine is an important inhibitory neuromodulator that regulates neuronal excitability. Several studies have shown that nitric oxide induces release of adenosine. Here we investigated the mechanism of this release. We studied the effects of nitric oxide on evoked field excitatory postsynaptic potentials (fEPSPs) recorded in the CA1 area of rat hippocampal slices. The nitric oxide donor 1,1-diethyl-2-hydroxy-2-nitroso-hydrazine sodium (DEA/NO; 100 microm) depressed the fEPSP by 77.6 +/- 4.1%. This effect was abolished by the adenosine A1 antagonist 8-cyclopentyl-1,3-dipropylxanthine (DPCPX; 400 nm), indicating that the nitric oxide effect was mediated by adenosine accumulation. The DEA/NO effect was unaltered by the 5'-ectonucleotidase inhibitor alpha,beta-methylene-adenosine 5'-diphosphate (AMP-CP; 100 microm), indicating that extracellular adenosine did not derive from ATP or cAMP release. The guanylyl cyclase inhibitor 1H-[1,2,4]oxadiazole[4,3-a]quinoxaline-1-one (ODQ; 5 microm) did not affect nitric oxide depression of the fEPSPs, indicating that nitric oxide-mediated adenosine release was not mediated through a cGMP signaling cascade. This conclusion was confirmed by the observation that 8-(4-chlorophenylthio)-guanosine-3',5'-cyclic monophosphate (8-pCPT-cGMP; 1 mm) reversibly depressed the fEPSP by 24.9 +/- 4.5%, but this effect was not blocked by adenosine antagonists. Adenosine kinase inhibitor 5-iodotubercidin (ITU; 7 microm) occluded the nitric oxide effects by 74%, suggesting that inhibition of adenosine kinase activity contributes to adenosine release. In conclusion, exogenous nitric oxide evokes adenosine release by a cGMP-independent pathway. Intracellular cGMP elevation partially inhibits the fEPSP but not through adenosine release. Although a direct block of adenosine kinase by nitric oxide can not be excluded, the depression of adenosine kinase activity may be due to inhibition by its own substrate adenosine.

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.

Regional age-related changes in neuronal nitric oxide synthase (nNOS), messenger RNA levels and activity in SAMP8 brain

Background

Nitric oxide (NO) is a multifunctional molecule synthesized by three isozymes of the NO synthase (NOSs) acting as a messenger/modulator and/or a potential neurotoxin. In rodents, the role of NOSs in sleep processes and throughout aging is now well established. For example, sleep parameters are highly deteriorated in senescence accelerated-prone 8 (SAMP8) mice, a useful animal model to study aging or age-associated disorders, while the inducible form of NOS (iNOS) is down-regulated within the cortex and the sleep-structures of the brainstem. Evidence is now increasing for a role of iNOS and resulting oxidative stress but not for the constitutive expressed isozyme (nNOS). To better understand the role of nNOS in the behavioural impairments observed in SAMP8 versus SAMR1 (control) animals, we evaluated age-related variations occurring in the nNOS expression and activity and nitrites/nitrates (NOx^-) levels, in three brain areas (n = 7 animals in each group). Calibrated reverse transcriptase (RT) and real-time polymerase chain reaction (PCR) and biochemical procedures were used.

Results

We found that the levels of nNOS mRNA decreased in the cortex and the hippocampus of 8- vs 2-month-old animals followed by an increase in 12-vs 8-month-old animals in both strains. In the brainstem, levels of nNOS mRNA decreased in an age-dependent manner in SAMP8, but not in SAMR1. Regional age-related changes were also observed in nNOS activity. Moreover, nNOS activity in hippocampus was found lower in 8-month-old SAMP8 than in SAMR1, while in the cortex and the brainstem, nNOS activities increased at 8 months and afterward decreased with age in SAMP8 and SAMR1. NOx^- levels showed profiles similar to nNOS activities in the cortex and the brainstem but were undetectable in the hippocampus of SAMP8 and SAMR1. Finally, NOx^- levels were higher in the cortex of 8 month-old SAMP8 than in age-matched SAMR1.

Conclusion

Concomitant variations occurring in NO levels derived from nNOS and iNOS at an early age constitute a major factor of risk for sleep and/or memory impairments in SAMP8.

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 synthase-containing neurons in the amygdaloid nuclear complex of the rat

The nitric oxide-producing neurons in the rat amygdala (Am) were studied, using reduced nicotinamide adenine dinucleotide phosphate diaphorase (NADPHd) histochemistry. Almost all nuclei of the Am contained NADPHd-positive neurons and fibers, but the somatodendritic morphology and the intensity of staining of different subpopulations varied. The strongly stained neurons displayed labeling of the perikaryon and the dendritic tree with Golgi impregnation-like quality, whilst the dendrites of the lightly stained neurons were less successfully followed. Many strongly positive neurons were located in the external capsule and within the intraamygdaloid fiber bundles. A large number of small, strongly stained cells was present in the amygdalostriatal transition area. In the Am proper, a condensation of deeply stained cells occurred in the lateral amygdaloid nucleus. In the basolateral nucleus, the strongly NADPHd-positive neurons were few, and were located mainly along the lateral border of the nucleus. These cells clearly differed from the large, pyramidal, and efferent cells. The basomedial nucleus contained numerous positive cells but most of them were only lightly labeled. A moderate number of strongly stained neurons appeared in the medial division of the central nucleus, and a larger accumulation of strongly positive cells was present in the lateral and the capsular divisions. The medial amygdaloid nucleus contained numerous moderately stained neurons and displayed the strongest diffuse neuropil staining in Am. In the nucleus of the lateral olfactory tract, the first layer contained only NADPHd-stained axons, in the second layer, there were numerous moderately stained cells, and in the third layer, a few but deeply stained neurons. From the cortical nuclei, the most appreciable number of stained neurons was seen in the anterior cortical nucleus. The anterior amygdaloid area contained numerous NADPHd-positive neurons; in its dorsal part the majority of cells were only moderately stained, whereas in the ventral part the neurons were very strongly stained. The intercalated amygdaloid nucleus lacked NADPHd-positive neurons but an appreciable plexus of fine, tortuous axons was present. In the intra-amygdaloid part of the bed nucleus of the stria terminalis (st) some lightly stained cells were seen but along the entire course of st strongly stained neurons were observed. Some Am nuclei, and especially the central lateral nucleus and the intercalated nucleus, display considerable species differences when compared with the primate Am. The age-related changes of the nitrergic Am neurons, as well as their involvement in neurodegenerative diseases is discussed.

Transmitter modulation of spike-evoked calcium transients in arousal related neurons: muscarinic inhibition of SNX-482-sensitive calcium influx

Nitric oxide synthase (NOS)-containing cholinergic neurons in the laterodorsal tegmentum (LDT) influence behavioral and motivational states through their projections to the thalamus, ventral tegmental area and a brainstem 'rapid eye movement (REM)-induction' site. Action potential-evoked intracellular calcium transients dampen excitability and stimulate NO production in these neurons. In this study, we investigated the action of several arousal-related neurotransmitters and the role of specific calcium channels in these LDT Ca(2+)-transients by simultaneous whole-cell recording and calcium imaging in mouse (P14-P30) brain slices. Carbachol, noradrenaline and adenosine inhibited spike-evoked Ca(2+)-transients, while histamine, t-ACPD, a metabotropic glutamate receptor agonist, and orexin-A did not. Carbachol inhibition was blocked by atropine, was insensitive to blockade of G-protein-coupled inward rectifier (GIRK) channels and was not inhibited by nifedipine, omega-conotoxin GVIA or omega-agatoxin IVA, which block L-, N- and P/Q-type calcium channels, respectively. In contrast, SNX-482 (100 nm), a selective antagonist of R-type calcium channels containing the alpha1E (Cav2.3) subunit, attenuated carbachol inhibition of the somatic spike-evoked calcium transient. To our knowledge, this is the first demonstration of muscarinic inhibition of native SNX-482-sensitive R-channels. Our findings indicate that muscarinic modulation of these channels plays an important role in the feedback control of cholinergic LDT neurons and that inhibition of spike-evoked Ca(2+)-transients is a common action of neurotransmitters that also activate GIRK channels in these neurons. Because spike-evoked calcium influx dampens excitability, our findings suggest that these 'inhibitory' transmitters could boost firing rate and enhance responsiveness to excitatory inputs during states of high firing, such as waking and REM sleep.

Cortical feedback to the thalamus is selectively enhanced by nitric oxide

The brain somehow merges visual information with the behavioral context in which it is being processed, a task that is often attributed to the cerebral cortex. We have identified a new role of the gaseous neurotransmitter, nitric oxide (NO), in the early selective enhancement of corticogeniculate communication that may participate in this process at the level of the thalamus. Visual information is dynamically gated through the thalamus by brainstem neurons that release acetylcholine and NO. Using in vitro electrophysiology, we characterized NO effects on excitatory postsynaptic potentials and currents (EPSCs) elicited from retinal and cortical pathways in the lateral geniculate nucleus of the ferret. NO selectively and reversibly increased cortically-evoked postsynaptic responses, and this effect was mimicked by cyclic guanosine 3',5'-monophosphate (cGMP). Conversely, NO inhibited retinally-evoked responses independently of cGMP. We demonstrated that these differential effects were specific to postsynaptic N-methyl-d-aspartate (NMDA) receptors by studying treatment effects on pharmacologically isolated EPSCs from each pathway. We propose that when brainstem activity is increased during behavioral arousal or rapid eye movement sleep, NO may increase the relative sensitivity of relay neurons to corticogeniculate feedback. The net effect of these changes in synaptic processing may be to selectively suppress peripheral information while unifying data carried by reentrant corticogeniculate loops with the behavioral context in which the visual information is processed.

Moderately different NADPH-diaphorase positivity in the selected peripheral nerves after ischemia/ reperfusion injury of the spinal cord in rabbit

1. To vicariously investigate the nitric oxide synthase (NOS) production after spinal cord injury, NADPH-d histochemistry was performed on the selected peripheral nerves of adult rabbits 7 days after ischemia. The effect of transient spinal cord ischemia (15 min) on possible degenerative changes in the motor and mixed peripheral nerves of Chinchilla rabbits was evaluated. 2. The NADPH-diaphorase histochemistry was used to determine NADPH-diaphorase activity after ischemia/reperfusion injury in radial nerve and mediane nerve isolated from the fore-limb and femoral nerve, saphenous nerve and sciatic nerve separated from the hind-limb of rabbits. The qualitative analysis of the optical density of NADPH-diaphorase in selected peripheral nerves demonstrated different frequency of staining intensity (attained by UTHSCSA Image Tool 2 analysis for each determined nerve). 3. On the seventh postsurgery day, the ischemic spinal cord injury resulted in an extensive increase of NADPH-d positivity in isolated nerves. The transient ischemia caused neurological disorders related to the neurological injury--a partial paraplegia. The sciatic, femoral, and saphenous nerves of paraplegic animals presented the noticeable increase of NADPH-d activity. The mean of NADPH-diaphorase intensity staining per unit area ranged from 134.87 (+/-32.81) pixels to 141.65 (+/-35.06) pixels (using a 256-unit gray scale where 0 denotes black, 256 denotes white) depending on the determined nerve as the consequence of spinal cord ischemia. The obtained data were compared to the mean values of staining intensity in the same nerves in the limbs of control animals (163.69 (+/-25.66) pixels/unit area in the femoral nerve, 173.00 (+/-32.93) pixels/unit area in saphenous nerve, 186.01 (+/-29.65) pixels/unit area in sciatic nerve). Based on the statistical analysis of the data (two-way unpaired Mann-Whitney test), a significant increase (p< or =0.05) of NADPH-d activity in femoral and saphenous nerve, and also in sciatic nerve (p< or =0.001) has been found. On the other hand, there was no significant difference between the histochemically stained nerves of fore-limbs after ischemia/reperfusion injury and the same histochemically stained nerves of fore-limbs in control animals. 4. The neurodegenerative changes of the hind-limbs, characterized by damage of their motor function exhibiting a partial paraplegia after 15 min spinal cord ischemia and subsequent 7 days of reperfusions resulted in the different sensitivity of peripheral nerves to transient ischemia. Finally, we suppose that activation of NOS indirectly demonstrable through the NADPH-d study may contribute to the explanation of neurodegenerative processes and the production of nitric oxide could be involved in the pathophysiology of spinal cord injury by transient ischemia.

Prefrontal cortex-projecting glutamatergic thalamic paraventricular nucleus-excited by hypocretin: a feedforward circuit that may enhance cognitive arousal

The paraventricular thalamic nucleus (PVT) receives one of the most dense innervations by hypothalamic hypocretin/orexin (Hcrt) neurons, which play important roles in sleep-wakefulness, attention, and autonomic function. The PVT projects to several loci, including the medial prefrontal cortex (mPFC), a cortical region involved in associative function and attention. To study the effect of Hcrt on excitatory PVT neurons that project to the mPFC, we used a new line of transgenic mice expressing green fluorescent protein (GFP) under the control of the vesicular glutamate-transporter-2 promoter. These neurons were retrogradely labeled with cholera toxin subunit B that had been microinjected into the mPFC. Membrane characteristics and responses to hypocretin-1 and -2 (Hcrt-1 and -2) were studied using whole cell recording (n > 300). PVT neurons showed distinct membrane properties including inward rectification, H-type potassium currents, low threshold spikes, and spike frequency adaptation. Cortically projecting neurons were depolarized and excited by Hcrt-2. Hcrt-2 actions were stronger than those of Hcrt-1, and the action persisted in TTX and in low calcium/high magnesium artificial cerebrospinal fluid, consistent with direct actions mediated by Hcrt receptor-2. Two mechanisms of Hcrt excitation were found: an increase in input resistance caused by closure of potassium channels and activation of nonselective cation channels. The robust excitation evoked by Hcrt-2 on cortically projecting glutamate PVT neurons could generate substantial excitation in multiple layers of the mPFC, adding to the more selective direct excitatory actions of Hcrt in the mPFC and potentially increasing cortical arousal and attention to limbic or visceral states.

Effects of selective neuronal nitric oxide synthase inhibition on sleep and wakefulness in the rat

The role played by the unconventional messenger Nitric Oxide (NO) upon the sleep-wake cycle remains controversial. Evidence suggests a positive role of NO on Slow Wave Sleep (SWS) and Paradoxical Sleep (PS) regulation, favoring sleep. However, other studies have found a role of NO upon wakefulness and alertness, inhibiting sleep. Divergences have been explained in part because of the use of different inhibitors of nitric oxide synthases (NOS). The aim of this study is to analyse the effects of a highly selective neuronal NOS inhibitor (3-Bromo7-Nitroindazole) on sleep-wake states in rats. Male Wistar rats were stereotaxically prepared for polysomnography. 3-Bromo-7-Nitroindazole (10, 20, 40 mg/kg, i.p.) dissolved in DMSO 10% filled with saline, or vehicle (DMSO 10% in saline) was administered at the beginning of the light period. Three hours of polygraphic recordings were evaluated for stages of vigilance. Results show dose-dependent effects of 3-Bromo7-Nitroindazole upon sleep: 10 mg/kg decreases duration and number of episodes of deep SWS, increasing duration of light SWS. 20 mg/kg decreased duration of light and deep SWS, while active and quiet wake increased. Deep SWS and PS latency increased. Number of episodes of PS decreased, as well as number of cycles of sleep and time spent asleep. 40 mg/kg reduced duration of deep SWS and increased mean episode duration of light SWS. Therefore, sleep states are affected by selective inhibition of nNOS, reducing in all cases deep SWS. These results support the hypothesis that nitric oxide, produced by nNOS, is involved in sleep processes, favoring sleep.

Nitric oxide from the laterodorsal tegmental neurons: its possible retrograde modulation on norepinephrine release from the axon terminal of the locus coeruleus neurons

Nitric oxide released from the cholinergic neurons in the pons may play important roles in sleep-wake regulation. However, there are few reports demonstrating the mechanisms of nitric oxide release in the cholinergic neurons in the pons. The present study investigated the effects of drug delivery of N-methyl-D-aspartic acid on nitric oxide and the neurotransmitters released in the laterodorsal tegmental nucleus (LDT), one of the major cholinergic cell groups in the pons, in rats by in vivo microdialysis with a view to clarifying nitric oxide functions in the cholinergic system. The application of N-methyl-D-aspartic acid (1 mM) into the LDT induced a significant increase in NO(2)and NO(3) for 40 min (P<0.001). Furthermore the same dose of N-methyl-D-aspartic acid induced a significant increase in cyclic GMP for 30 min (P<0.05), as well as in acetylcholine (P<0.001) and norepinephrine for 15 min (P<0.001). 3-(4-Morpholinyl)-sydonone imine hydrochloride (a nitric oxide donor, 5 mM) also induced significant increase in norepinephrine (P<0.05). Pretreatment with 1 mM 2-amino-5-phosphonopentanoic acid (an antagonist of N-methyl-D-aspartic acid receptor) prevented the N-methyl-D-aspartic acid-induced increase in cyclic GMP (P<0.01), acetylcholine and norepinephrine (P<0.01), while that with 1 mM N(G)-nitro-L-arginine (an inhibitor of nitric oxide synthase) prevented the increase in cyclic GMP (P<0.01) and norepinephrine (P<0.01) but not in acetylcholine. These results suggested that nitric oxide release in the LDT induced by activation of the N-methyl-D-aspartic acid receptor on the cholinergic neurons of the LDT, then through the cyclic GMP system, facilitates norepinephrine release from the terminals of noradrenergic neurons in the locus coeruleus. Based on these findings, we propose a possible role of nitric oxide in the LDT is as a retrograde regulator of norepinephrine release from the locus coeruleus.

The effects of intracerebroventricular application of 8-Br-cGMP and LY-83,583, a guanylyl cyclase inhibitor, on sleep-wake activity in rats

Cyclic GMP is the second messenger that mediates most of the neuronal effects of nitric oxide (NO). Several lines of evidence suggest that NO-ergic mechanisms play an integral role in the regulation of vigilance. In the present study, we tested the effects of the activation of cGMP-receptive mechanisms and the inhibitor of guanylyl cyclase (GC), LY-83,583, on sleep in rats. Rats were injected intracerebroventricularly (icv) with 0.16, 4, 100, and 500 microg or 2.5 mg 8-Br-cGMP, a membrane-permeable analogue of cGMP, or 1 and 100 microg LY-83,583. Administration of 4 microg-2.5 mg 8-Br-cGMP increased wakefulness and suppressed rapid-eye-movement sleep (REMS) and non-REMS (NREMS) in rats when given before dark onset but not when given before the light period. The GC inhibitor LY-83,583 strongly promoted NREMS and suppressed REMS during the light period of the day. Furthermore, LY-83,583 induced striking increases in the delta-wave activity of the electroencephalogram (EEG) during NREMS, whereas EEG activity above the 4.5 Hz wave range was suppressed in all vigilance states. Our finding that cGMP has an arousal-promoting activity is in line with the hypothesis that NO/cGMP signaling pathway is involved in the regulation of vigilance.

Day- and nighttime injection of a nitric oxide synthase inhibitor elicits opposite sleep responses in rats

Previous studies suggest that nitric oxide (NO) may play a role in sleep regulation, particularly in the homeostatic process. The present studies were undertaken to compare the sleep effects of injecting a NO synthase (NOS) inhibitor when homeostatic sleep pressure is naturally highest (light onset) or when it is at its nadir (dark onset) in rats. Sleep, electroencephalogram delta-wave activity during nonrapid eye movement sleep (NREMS), also known as slow-wave activity (SWA), and brain temperature responses to three doses of the NOS inhibitor Nω-nitro-l-arginine methyl ester (l-NAME; 5, 50, and 100 mg/kg) injected intraperitoneally at light or dark onset were examined in rats ( n = 6 to 8). The effects of 5 mg/kg l-NAME were determined in both normal and vagotomized (VX) rats. Light onset administration of 50 mg/kg l-NAME decreased NREMS amounts and suppressed SWA and increased rapid eye movement sleep (REMS) amounts. At dark onset, l-NAME injection also dose dependently suppressed SWA; however, unlike light onset injections, both NREMS and REMS amounts were increased after all three doses. Sleep responses to 5 mg/kg l-NAME were not different in control and VX rats, suggesting that the sleep effects of l-NAME are not mediated through the activation of sensory vagal mechanisms. The present findings suggest that timing of the injection is a major determinant of the sleep responses observed after systemic l-NAME injection in rats.

Nitric oxide and sleep

Nitric oxide (NO) is a biological messenger synthesized by three main isoforms of NO synthase (NOS): neuronal (nNOS, constitutive calcium dependent), endothelial (eNOS, constitutive, calcium dependent) and inducible (iNOS, calcium independent). NOS is distributed in the brain either in circumscribed neuronal sets or in sparse interneurons. Within the laterodorsal tegmentum (LDT), pedunculopontine tegmentum and dorsal raphe nucleus, NOS-containing neurons overlap neurons grouped according to their contribution to sleep mechanisms. The main target for NO is the soluble guanylate cyclase that triggers an overproduction of cyclic guanosine monophosphate. NO in neurons of the pontine tegmentum facilitates sleep (particularly rapid-eye-movement sleep), and NO contained within the LDT intervenes in modulating the discharge of the neurons through an auto-inhibitory process involving the co-synthesized neurotransmitters. Moreover, NO synthesized within cholinergic neurons of the basal forebrain, while under control of the LDT, may modulate the spectral components of the EEG instead of the amounts of different sleep states. Finally, impairment of NO production (e.g. neurodegeneration, iNOS induction) has identifiable effects, including ageing, neuropathologies and parasitaemia.

Adenosine and sleep–wake regulation

This review addresses three principal questions about adenosine and sleep-wake regulation: (1) Is adenosine an endogenous sleep factor? (2) Are there specific brain regions/neuroanatomical targets and receptor subtypes through which adenosine mediates sleepiness? (3) What are the molecular mechanisms by which adenosine may mediate the long-term effects of sleep loss? Data suggest that adenosine is indeed an important endogenous, homeostatic sleep factor, likely mediating the sleepiness that follows prolonged wakefulness. The cholinergic basal forebrain is reviewed in detail as an essential area for mediating the sleep-inducing effects of adenosine by inhibition of wake-promoting neurons via the A1 receptor. The A2A receptor in the subarachnoid space below the rostral forebrain may play a role in the prostaglandin D2-mediated somnogenic effects of adenosine. Recent evidence indicates that a cascade of signal transduction induced by basal forebrain adenosine A1 receptor activation in cholinergic neurons leads to increased transcription of the A1 receptor; this may play a role in mediating the longer-term effects of sleep deprivation, often called sleep debt.

Hypocretin/orexin peptide signaling in the ascending arousal system: elevation of intracellular calcium in the mouse dorsal raphe and laterodorsal tegmentum

Dysfunction of the hypocretin/orexin (Hcrt/Orx) peptide system is closely linked to the sleep disorder narcolepsy, suggesting that it is also central to the normal regulation of sleep and wakefulness. Indeed, Hcrt/Orx peptides produce long-lasting excitation of arousal-related neurons, including those in the laterodorsal tegmentum (LDT) and the dorsal raphe (DR), although the mechanisms underlying these actions are not understood. Since Hcrt/Orx mobilizes intracellular calcium ([Ca(2+)](i)) in cells transfected with orexin receptors and since receptor-mediated Ca(2+) transients are ubiquitous signaling mechanisms, we investigated whether Hcrt/Orx regulates [Ca(2+)](i) in the LDT and DR. Changes in [Ca(2+)](i) were monitored by fluorescence changes of fura-2 AM loaded cells in young mouse brain slices. We found Hcrt/Orx (Orexin-A, 30-1,000 nM) evoked long-lasting increases in [Ca(2+)](i) with differing temporal profiles ranging from spiking to smooth plateaus. A fragment of Hcrt/Orx (16-33) failed to evoke changes in [Ca(2+)](i) and changes were not blocked by TTX or ionotropic glutamate receptor antagonists, suggesting they resulted from specific activation of postsynaptic orexin receptors. Unlike orexin receptor-transfected cells, Hcrt/Orx-responses were not attenuated by depletion of Ca(2+) stores with cyclopiazonic acid (CPA; 3-30 microM), thapsigargin (3 microM), or ryanodine (20 microM), although store-depletion by either CPA or ryanodine blocked Ca(2+) mobilization by the metabotropic glutamate receptor agonist (+/-)-1-aminocyclopentane-trans-1,3-dicarboxylic acid (trans-ACPD; 30 microM). In contrast, Hcrt/Orx responses were strongly attenuated by lowering extracellular Ca(2+) ( approximately 20 microM) but were not inhibited by concentrations of KB-R7943 (10 microM) selective for blockade of sodium/calcium exchange. Nifedipine (10 microM), inhibited Hcrt/Orx responses but was more effective at abolishing spiking than plateau responses. Bay K 8644 (5-10 microM), an L-type calcium channel agonist, potentiated responses. Finally, responses were attenuated by inhibitors of protein kinase C (PKC) but not by inhibitors of adenylyl cyclase. Collectively, our findings indicate that Hcrt/Orx signaling in the reticular activating system involves elevation of [Ca(2+)](i) by a PKC-involved influx of Ca(2+) across the plasma membrane, in part, via L-type calcium channels. Thus the physiological release of Hcrt/Orx may help regulate Ca(2+)-dependent processes such as gene expression and NO production in the LDT and DR in relation with behavioral state. Accordingly, the loss of Hcrt/Orx signaling in narcolepsy would be expected to disrupt calcium-dependent processes in these and other target structures.

Spontaneous sleep in mice with targeted disruptions of neuronal or inducible nitric oxide synthase genes

Nitric oxide (NO) affects almost every physiological process, including the regulation of sleep. There is strong evidence that NO plays an important role in rapid eye movement sleep (REMS) regulation. To further investigate the role of NO in sleep, we characterized spontaneous sleep in mice with targeted disruptions (knockout; KO) in the neuronal nitric oxide synthase (nNOS) or inducible (i)NOS genes. REMS in nNOS KO mice was substantially lower than that of their control mice. In contrast, the iNOS KO mice had significantly more REMS than their controls. Inducible NOS KO mice also had less non-REMS (NREMS) during the dark period. Results suggest that nNOS and iNOS play opposite roles in REMS regulation.

Jun, Fos and Krox in the thalamus after C-fiber stimulation: coincident-input-dependent expression, expression across somatotopic boundaries, and nucleolar translocation

Expression of the inducible transcription factors Jun, Fos and Krox is commonly used to map neurons in the brain that are activated by sensory inputs. However, some neurons known to be electrically excited by such inputs do not always express these factors. In particular, stimulation of hindlimb sensory nerve C-fibers induces expression of c-Fos in the medial thalamus (the mediodorsal, intermediodorsal, centrolateral and centromedial), but not in the lateral thalamus (the ventroposterolateral, ventroposteromedial and posterior group). We hypothesized that c-Fos expression might only occur in these lateral areas after more complex stimulation patterns, or that only other transcription factors can be induced in these areas by such stimuli. Thus we examined the effects of single, repeated and coincident C-fiber inputs on expression of six inducible transcription factors in the medial, lateral and reticular thalamus of the rat. A weak C-fiber input caused by noxious mechanical stimulation of the skin of one hindpaw did not induce expression of c-Fos, FosB, Krox-20 or Krox-24; but it did reduce the basal expressions of c-Jun and JunD in both the medial and lateral areas. An intense input produced by electrical stimulation of all the C-fibers in one sciatic nerve also failed to induce expression of c-Fos, FosB, Krox-20 or Krox-24 in the medial or lateral areas. However, in the medial thalamus it increased c-Jun and reduced the basal expression of JunD, whereas in the lateral thalamus it had no effect on c-Jun but again reduced the basal expression of JunD. With repeated stimulation, i.e. when the noxious stimulus was applied to the contralateral hindpaw 6 h after the sciatic stimulation, there was again no induction of c-Fos, FosB or Krox-20 in the medial thalamus; but there was an increase in c-Jun and Krox-24, and a decrease in JunD levels. In the lateral thalamus the repeated stimulation again failed to induce c-Fos, but the expressions of FosB, c-Jun and Krox-24 were increased, and that of JunD was again reduced. With coincident stimulation, i.e. when a stimulus was applied to each hindpaw simultaneously, c-Fos and Krox-24 remained absent; but there was a marked induction of FosB and Krox-20, a strong repression of c-Jun, and no effect or a reduction of the basal levels of JunD. This coincident stimulation also caused FosB to appear in the nucleolus of many thalamic neurons. MK-801, but not L-NAME, blocked all these changes. In summary, noxious stimulation affects the expression of all transcription factors in the medial, lateral and reticular thalamus in a complex manner depending upon the inducible transcription factor considered, the thalamic nucleus, and the stimulation paradigm. The expression of some transcription factors uniquely after simultaneous inputs suggests they act as coincidence detectors at the gene level.

Expression of cGMP-specific phosphodiesterase 9A mRNA in the rat brain

cGMP has been implicated in the regulation of many essential functions in the brain, such as synaptic plasticity, phototransduction, olfaction, and behavioral state. Cyclic nucleotide phosphodiesterase (PDE) hydrolysis of cGMP is the major mechanism underlying the clearance of cGMP and is likely to be important in any process that depends on intracellular cGMP. PDE9A has the highest affinity for cGMP of any PDE, and here we studied the localization of this enzyme in the rat brain using in situ hybridization. PDE9A mRNA is widely distributed throughout the brain with varying regional expression. The pattern of PDE9A mRNA expression closely resembles that of soluble guanylyl cyclase (sGC) in the rat brain, suggesting a possible functional association or coupling of these two enzymes in the regulation of cGMP levels. Most of the brain areas expressing PDE9A mRNA also contain neuronal nitric oxide synthase (NOS), the enzymatic source of NO and the principal activator of sGC. PDE9A is the only cGMP-specific PDE with significant expression in the forebrain, and as such is likely to play an important role in NO-cGMP signaling.

Activity-dependent nitric oxide concentration dynamics in the laterodorsal tegmental nucleus in vitro

The behavioral-state related firing of mesopontine cholinergic neurons of the laterodorsal tegmental nucleus appears pivotal for generating both arousal and rapid-eye-movement sleep. Since these neurons express high levels of nitric oxide synthase, we investigated whether their firing increases local extracellular nitric oxide levels. We measured nitric oxide in the laterodorsal tegmental nucleus with a selective electrochemical microprobe (35 microm diam) in brain slices. Local electrical stimulation at 10 or 100 Hz produced electrochemical responses that were attributable to nitric oxide. Stimulus trains (100 Hz; 1 s) produced biphasic increases in nitric oxide that reached a mean peak concentration of 33 +/- 2 (SE) nM at 4.8 +/- 0.4 s after train onset and decayed to a plateau concentration of 8 +/- 1 nM that lasted an average of 157 +/- 23.4 s (n = 14). These responses were inhibited by N(G)-nitro-L-arginine-methyl-ester (1 mM; 92% reduction of peak; n = 3) and depended on extracellular Ca(2+). Chemically reduced hemoglobin attenuated both the electrically evoked responses and those produced by authentic nitric oxide. Application of the precursor, L-arginine (5 mM) augmented the duration of the electrically evoked response, while tetrodotoxin (1 microM) abolished it. Analysis of the stimulus-evoked field potentials indicated that electrically evoked nitric oxide production resulted from a direct, rather than synaptic, activation of laterodorsal tegmental neurons because neither nitric oxide production nor the field potentials were blocked by ionotropic glutamate receptor inhibitors. Nevertheless, application of N-methyl-D-aspartate also increased local nitric oxide concentration by 39 +/- 14 nM (n = 8). Collectively, these data demonstrate that laterodorsal tegmental neuron activity elevates extracellular nitric oxide concentration probably via somatodendritic nitric oxide production. These data support the hypothesis that nitric oxide can function as a local paracrine signal during the states of arousal and rapid-eye-movement sleep when the firing of mesopontine cholinergic neurons are highest.

Laterodorsal tegmental stimulation elicits dopamine efflux in the rat nucleus accumbens by activation of acetylcholine and glutamate receptors in the ventral tegmental area

Cholinergic and glutamatergic neurons in the laterodorsal tegmentum (LDT) and neighbouring mesopontine nuclei are thought to influence mesolimbic dopaminergic neuronal activity involved in goal-directed behaviours. We measured the changes in dopamine oxidation current (corresponding with dopamine efflux) in the nucleus accumbens (NAc) in response to electrical stimulation of the LDT using in vivo chronoamperometry in urethane-anaesthetized rats. LDT stimulation (35 Hz pulse trains for 60 s, 1 s intertrain interval) evoked a three-component change in dopamine efflux in the NAc: (i) an initial stimulation time-locked increase in the dopamine signal above baseline, followed by (ii) an immediate decrease below baseline, and thereafter by (iii) a prolonged increase in the dopamine signal above baseline. Intra-VTA infusion of the nicotinic receptor antagonist mecamylamine (5 microg/0.5 microL) or the ionotropic glutamate receptor antagonist kynurenate (10 microg/microL) attenuated the first LDT-elicited component. The second suppressive component was abolished by intra-LDT infusions of either the nonselective or the M2-selective muscarinic receptor antagonists scopolamine (100 microg/microL) and methoctramine (50 microg/microL), respectively. In contrast, intra-VTA infusions of scopolamine (200 microg/microL) resulted in a selective attenuation of the third facilitatory component, whereas both second and third components were abolished by systemic injections of scopolamine (5 mg/kg). These results suggest that the initial increase, subsequent decrease, and final prolonged increase in extracellular dopamine levels in the NAc are selectively mediated by LDT-elicited activation of (i) nicotinic and glutamatergic receptors in the VTA, (ii) muscarinic M2 autoreceptors on LDT cell bodies, and (iii) muscarinic receptors in the VTA, respectively.

Neurotransmitters contained in the subcortical extraretinal inputs to the monkey lateral geniculate nucleus

The lateral geniculate nucleus (LGN) is the thalamic relay of retinal information to cortex. An extensive complement of nonretinal inputs to the LGN combine to modulate the responsiveness of relay cells to their retinal inputs, and thus control the transfer of visual information to cortex. These inputs have been studied in the most detail in the cat. The goal of the present study was to determine whether the neurotransmitters used by nonretinal afferents to the monkey LGN are similar to those identified in the cat. By combining the retrograde transport of tracers injected into the monkey LGN with immunocytochemical labeling for choline acetyl transferase, brain nitric oxide synthase, glutamic acid decarboxylase, tyrosine hydroxylase, or the histochemical nicotinamide adenine dinucleotide phosphate (NADPH)-diaphorase reaction, we determined that the organization of neurotransmitter inputs to the monkey LGN is strikingly similar to the patterns occurring in the cat. In particular, we found that the monkey LGN receives a significant cholinergic/nitrergic projection from the pedunculopontine tegmentum, gamma-aminobutyric acid (GABA)ergic projections from the thalamic reticular nucleus and pretectum, and a cholinergic projection from the parabigeminal nucleus. The major difference between the innervation of the LGN in the cat and the monkey is the absence of a noradrenergic projection to the monkey LGN. The segregation of the noradrenergic cells and cholinergic cells in the monkey brainstem also differs from the intermingled arrangement found in the cat brainstem. Our findings suggest that studies of basic mechanisms underlying the control of visual information flow through the LGN of the cat may relate directly to similar issues in primates, and ultimately, humans.

Serotonergic inhibition of action potential evoked calcium transients in NOS-containing mesopontine cholinergic neurons

Nitric oxide synthase (NOS)-containing mesopontine cholinergic (MPCh) neurons of the laterodorsal tegmental nucleus (LDT) are hypothesized to drive the behavioral states of waking and REM sleep through a tonic increase in firing rate which begins before and is maintained throughout these states. In principle, increased firing could elevate intracellular calcium levels and regulate numerous cellular processes including excitability, gene expression, and the activity of neuronal NOS in a state-dependent manner. We investigated whether repetitive firing, evoked by current injection and N-methyl-D-aspartate (NMDA) receptor activation, produces somatic and proximal dendritic [Ca(2+)](i) transients and whether these transients are modulated by serotonin, a transmitter thought to play a critical role in regulating the state-dependent firing of MPCh neurons. [Ca(2+)](i) was monitored optically from neurons filled with Ca(2+) indicators in guinea pig brain slices while measuring membrane potential with sharp microelectrodes or patch pipettes. Neither hyperpolarizing current steps nor subthreshold depolarizing steps altered [Ca(2+)](i). In contrast, suprathreshold currents caused large and rapid increases in [Ca(2+)](i) that were related to firing rate. TTX (1 microM) strongly attenuated this relation. Addition of tetraethylammonium (TEA, 20 mM), which resulted in Ca(2+) spiking on depolarization, restored the change in [Ca(2+)](i) to pre-TTX levels. Suprathreshold doses of NMDA also produced increases in [Ca(2+)](i) that were reduced by up to 60% by TTX. Application of 5-HT, which hyperpolarized LDT neurons without detectable changes in [Ca(2+)](i), suppressed both current- and NMDA-evoked increases in [Ca(2+)](i) by reducing the number of evoked spikes and by inhibiting spike-evoked Ca(2+) transients by approximately 40% in the soma and proximal dendrites. This inhibition was accompanied by a subtle increase in the spike repolarization rate and a decrease in spike width, as expected for inhibition of high-threshold Ca(2+) currents in these neurons. NADPH-diaphorase histochemistry confirmed that recorded cells were NOS-containing. These findings indicate the prime role of action potentials in elevating [Ca(2+)](i) in NOS-containing MPCh neurons. Moreover, they demonstrate that serotonin can inhibit somatic and proximal dendritic [Ca(2+)](i) increases both indirectly by reducing firing rate and directly by decreasing the spike-evoked transients. Functionally, these data suggest that spike-evoked Ca(2+) signals in MPCh neurons should be largest during REM sleep when serotonin inputs are expected to be lowest even if equivalent firing rates are reached during waking. Such Ca(2+) signals may function to trigger Ca(2+)-dependent processes including cfos expression and nitric oxide production in a REM-specific manner.

Nitric oxide-stimulated increase in extracellular adenosine accumulation in rat forebrain neurons in culture is associated with ATP hydrolysis and inhibition of adenosine kinase activity

Adenosine is a putative endogenous sleep-inducing substance, and nitric oxide has been implicated in arousal and sleep mechanisms. We found that various nitric oxide donors, including diethylamine NONOate (DEA/NO), stimulated large increases in extracellular adenosine in nearly pure cultures of forebrain neurons. The effect of DEA/NO could be blocked by 2-phenyl-4,4,5, 5-tetramethyl-imidazoline-1-oxyl-oxide and could not be mimicked by degraded solutions of DEA/NO or by DEA itself; therefore, it was caused by nitric oxide release on hydrolysis of the parent compound. The accumulation of adenosine was not blocked by probenecid or GMP, suggesting that neither extracellular cAMP nor extracellular AMP was the source, and that adenosine was therefore the most likely species transported across the plasma membrane. To pursue this further, we tested the effect of DEA/NO on cellular ATP and found a significant fall in ATP associated with exposure to nitric oxide. In addition, exposure to DEA/NO nearly completely inhibited adenosine kinase activity. It has been found previously that adenosine kinase is inhibited by its substrate, adenosine. We found that exposure to nitric oxide increased intracellular adenosine to 125 +/- 18% of control values (p < 0.01), consistent with the possibility that in our system the inhibition of adenosine kinase is related to an increase in intracellular adenosine, and that the effect of nitric oxide on extracellular adenosine is significantly potentiated by substrate inhibition of adenosine kinase. Furthermore, nitric oxide-stimulated adenosine accumulation may be important in the regulation of behavioral state.

Cuneiform neurons activated during cholinergically induced active sleep in the cat

In the present study, we report that the cuneiform (Cun) nucleus, a brainstem structure that before now has not been implicated in sleep processes, exhibits a large number of neurons that express c-fos during carbachol-induced active sleep (AS-carbachol). Compared with control (awake) cats, during AS-carbachol, there was a 671% increase in the number of neurons that expressed c-fos in this structure. Within the Cun nucleus, three immunocytochemically distinct populations of neurons were observed. One group consisted of GABAergic neurons, which predominantly did not express c-fos during AS-carbachol. Two other different populations expressed c-fos during this state. One of the Fos-positive (Fos(+)) populations consisted of a distinct group of nitric oxide synthase (NOS)-NADPH-diaphorase (NADPH-d)-containing neurons; the neurotransmitter of the other Fos(+) population remains unknown. The Cun nucleus did not contain cholinergic, catecholaminergic, serotonergic, or glycinergic neurons. On the basis of neuronal activation during AS-carbachol, as indicated by c-fos expression, we suggest that the Cun nucleus is involved, in an as yet unknown manner, in the physiological expression of active sleep. The finding of a population of NOS-NADPH-d containing neurons, which were activated during AS-carbachol, suggests that nitrergic modulation of their target cell groups is likely to play a role in active sleep-related physiological processes.