Noor Alam

Sleep–waking discharge patterns of ventrolateral preoptic/anterior hypothalamic neurons in rats

Numerous lesion, stimulation and recording studies in experimental animals demonstrate the importance of neurons within the preoptic/anterior hypothalamic area (POA) in the regulation of sleep induction and sleep maintenance. Recently, a discrete cluster of cells in the ventrolateral POA (vlPOA) of rats was found to exhibit elevated c-fos gene expression during sleep, indicating that these neurons are strongly activated during nonREM and/or REM sleep stages. We examined neuronal discharge during wakefulness and sleep throughout the dorsal to ventral extent of the lateral POA in rats, using chronic microwire technique. We found that neurons with elevated discharge rates during sleep, compared to waking, were localized to the vlPOA. As a group, vlPOA neurons displayed elevated discharge rates during both nonREM and REM sleep. Discharge of vlPOA neurons reflected the depth of sleep, i.e., discharge rates increased significantly from light to deep nonREM sleep. During recovery sleep following 12-14 h of sleep deprivation, vlPOA neurons displayed increased sleep-related discharge, compared to baseline sleep. Neurons in the vlPOA displaying increased neuronal discharge during sleep were located in the same area where neurons exhibit increased c-fos gene expression during sleep. Such neurons are likely components of a rostral hypothalamic mechanism that regulates sleep onset and sleep maintenance.

Sleep-waking discharge patterns of median preoptic nucleus neurons in rats

Several lines of evidence show that the preoptic area (POA) of the hypothalamus is critically implicated in the regulation of sleep. Functionally heterogeneous cell groups with sleep‐related discharge patterns are located both in the medial and lateral POA. Recently a cluster of neurons showing sleep‐related c‐Fos immunoreactivity was found in the median preoptic nucleus (MnPN). To determine the specificity of the state‐related behaviour of MnPN neurons we have undertaken the first study of their discharge patterns across the sleep‐waking cycle. Nearly 76% of recorded cells exhibited elevated discharge rates during sleep. Sleep‐related units showed several distinct types of activity changes across sleep stages. Two populations included cells displaying selective activation during either non‐rapid eye movement (NREM) sleep (10%) or REM sleep (8%). Neurons belonging to the predominant population (58%) exhibited activation during both phases of sleep compared to wakefulness. Most of these cells showed a gradual increase in their firing rates prior to sleep onset, elevated discharge during NREM sleep and a further increase during REM sleep. This specific sleep‐waking discharge profile is opposite to that demonstrated by wake‐promoting monoaminergic cell groups and was previously found in cells localized in the ventrolateral preoptic area (vlPOA). We hypothesize that these vlPOA and MnPN neuronal populations act as parts of a GABAergic/galaninergic sleep‐promoting (‘anti‐waking’) network which exercises inhibitory control over waking‐promoting systems. MnPN neurons that progressively increase activity during sustained waking and decrease activity during sustained sleep states may be involved in homeostatic regulation of sleep.

Effects of lateral preoptic area application of orexin-A on sleep-wakefulness.

Deficiency of orexin, a newly discovered hypothalamic peptide, is thought to lead to abnormal sleepiness and cataplexy in both human narcolepsy and animal models of the disease. As the POA contains extensive orexin terminals and is established as a sleep/arousal regulatory site, we evaluated a hypothesis that this site is a target for the arousal-inducing effects of orexin. Orexin-A was microinjected into lateral preoptic area (IPOA) and the effects on sleep–wakefulness and brain temperature were studied. Compared to saline vehicle control, orexin-A induced an increase in wakefulness for 70 min and suppressed all sleep stages, especially SWS2 and REM for 80 and 90 min, respectively. Brain temperature was not differentially affected by orexin-A compared to saline control. The orexin-induced arousal and REM suppression are consistent with the orexin-deficiency model of narcolepsy. Our results suggest that the IPOA orexin terminal field or adjacent structures may be a locus of arousal regulation by this peptide and a substrate of sleep-wake regulatory deficits in narcolepsy.

Discharge patterns of neurons in cholinergic regions of the basal forebrain during waking and sleep

A subset of neurons recorded in the magnocellular basal forebrain (mBF) of cats and rats exhibit elevated discharge rates during waking and REM sleep, and diminished discharge during sleep with cortical EEG synchrony (nonREM sleep). This pattern is observed in mBF neurons in cats with identified ascending projections, and in neurons located in cholinergic regions of the rat mBF. However, the cholinergic versus noncholinergic nature of recorded cells could not be determined with the extracellular recording method employed. During waking, discharge of mBF neurons is strongly movement-related. Peak discharge rates occur during a variety of head and limb movements. Discharge rates during waking immobility are reduced by >50% compared to rates during waking movement. The absence of movement accounts for more of the variance in discharge across the sleep-wake cycle than does the presence of cortical EEG synchronization. Several factors participate in the regulation of mBF neuronal activity across arousal states. Tonic inhibition mediated by adenosine appears to be present during both waking and sleep. In some mBF neurons, increased GABAergic inhibition contributes to nonREM sleep-related reductions in discharge rate. Fluctuations in mBF cell activity during waking behaviors may reflect changing excitatory input from neurons in the pontine and midbrain tegmentum.

Local preoptic/anterior hypothalamic warming alters spontaneous and evoked neuronal activity in the magno-cellular basal forebrain

Local warming of the medial preoptic/anterior hypothalamus (POAH) promotes sleep, enhances EEG slow-wave activity during sleep, and suppresses arousal-related discharge in neurons of the midbrain reticular formation (MRF) and the posterior lateral hypothalamic area (PLHa). Another important site of sleep and arousal regulation, and a potential site of POAH thermal modulation, is the magnocellular basal forebrain (BF). We examined the ability of local POAH warming during wakefulness to influence the spontaneous and evoked discharge of neurons recorded in the BF of unanesthetized, unrestrained cats. Seventy of 174 BF neurons responded to 60-90 s periods of POAH warming with either increases or decreases in discharge rate. Forty-one of the 70 responsive cells displayed suppression of waking discharge during warming. Discharge rate in these cells declined by an average of 26.04 +/- 2.76%/degrees C of POAH temperature increase. The majority of warming-suppressed BF cells (73%) displayed higher rates of discharge during periods of wakefulness compared to periods of sleep. Twenty-nine of 70 responsive cells responded to POAH warming with an average increase in discharge rate of 43.81 +/- 6.26%/degrees C. A majority of these neurons (62%) exhibited higher spontaneous discharge rates during sleep compared to waking. Orthodromic excitatory responses were evoked in 29 BF cells by electrical stimulation of the MRF or PLHa. Thirteen of 29 cells displayed a waking-related discharge pattern, and responded to POAH warming with a significant suppression of evoked excitation. For a group of 15 behavioral state-indifferent cells (i.e., cells displaying no modulation of spontaneous discharge rate across the sleep-waking cycle), POAH warming had no effect on evoked excitatory responses. These results support the hypothesis that thermosensitive neurons of the POAH exert control of sleep-waking state, in part, via modulation of arousal- and sleep-regulating cell types within the magnocellular BF.

Salubrinal, an inhibitor of protein synthesis, promotes deep slow wave sleep

Previous work showed that sleep is associated with increased brain protein synthesis and that arrest of protein synthesis facilitates sleep. Arrest of protein synthesis is induced during the endoplasmic reticulum (ER) stress response, through phosphorylation of eukaryotic initiation factor 2alpha (p-eIF2alpha). We tested a hypothesis that elevation of p-eIF2alpha would facilitate sleep. We studied the effects of intracerebroventricular infusion of salubrinal (Salub), which increases p-eIF2alpha by inhibiting its dephosphorylation. Salub increased deep slow wave sleep by 255%, while reducing active waking by 49%. Delta power within non-rapid eye movement (NREM) sleep was increased, while power in the sigma, beta, and gamma bands during NREM was reduced. We found that Salub increased expression of p-eIF2alpha in the basal forebrain (BF) area, a sleep-wake regulatory brain region. Therefore, we quantified the p-eIF2alpha-immunolabeled neurons in the BF area; Salub administration increased the number of p-eIF2alpha-expressing noncholinergic neurons in the caudal BF. In addition, Salub also increased the intensity of p-eIF2alpha expression in both cholinergic and noncholinergic neurons, but this was more widespread among the noncholinergic neurons. Our findings support a hypothesis that sleep is facilitated by signals associated with the ER stress response.

Neuronal substrates of sleep homeostasis; lessons from flies, rats and mice

Sleep homeostasis is a fundamental property of vigilance state regulation that is highly conserved across species. Neuronal systems and circuits that underlie sleep homeostasis are not well understood. In Drosophila, a neuronal circuit involving neurons in the ellipsoid body and in the dorsal Fan-shaped body is a candidate for both tracing sleep need during waking and translating it to increased sleep drive and expression. Sleep homeostasis in rats and mice involves multiple neuromodulators acting on multiple wake- and sleep-promoting neuronal systems. A functional central homeostat emerges from A1 receptor mediated actions of adenosine on wake-promoting neurons in the basal forebrain and hypothalamus, and A2A adenosine receptor-mediated actions on sleep-promoting neurons in the preoptic hypothalamus and nucleus accumbens.

Characteristics of sleep-active neurons in the medullary parafacial zone in rats

Growing evidence supports a role for the medullary parafacial zone in non-rapid eye movement (non-REM) sleep regulation. Cell-body specific lesions of the parafacial zone or disruption of its GABAergic/glycinergic transmission causes suppression of non-REM sleep, whereas, targeted activation of parafacial GABAergic/glycinergic neurons reduce sleep latency and increase non-REM sleep amount, bout duration, and cortical electroencephalogram (EEG) slow-wave activity. Parafacial GABAergic/glycinergic neurons also express sleep-associated c-fos immunoreactivity. Currently, it is not clear if parafacial neurons are non-REM sleep-active and/or REM sleep-active or play a role in the initiation or maintenance of non-REM sleep. We recorded extracellular discharge activity of parafacial neurons across the spontaneous sleep-waking cycle using microwire technique in freely behaving rats. Waking-, non-REM sleep-, and REM sleep-active neuronal groups were segregated by the ratios of their discharge rate changes during non-REM and REM sleep versus waking and non-REM sleep versus REM sleep. Parafacial neurons exhibited heterogeneity in sleep-waking discharge patterns, but 34 of 86 (40%) recorded neurons exhibited increased discharge rate during non-REM sleep compared to waking. These neurons also exhibited increased discharge prior to non-REM sleep onset, similar to median preoptic nucleus (MnPO) and ventrolateral preoptic area (VLPO) sleep-active neurons. However, unlike MnPO and VLPO sleep-active neurons, parafacial neurons were weakly-moderately sleep-active and exhibited a stable rather than decreasing discharge across sustained non-REM sleep episode. We show for the first time that the medullary parafacial zone contains non-REM sleep-active neurons. These neurons are likely functionally important brainstem compliments to the preoptic-hypothalamic sleep-promoting neuronal networks that underlie sleep onset and maintenance.