Selective activation of the extended ventrolateral preoptic nucleus during rapid eye movement sleep

Modifications of local cerebral glucose utilization during circadian food-anticipatory activity

Food-anticipatory activity that animals express before a daily timed meal is considered as the behavioral output of a feeding-entrainable oscillator whose functional neuroanatomy is still unknown. In order to identify the possible brain areas involved in that timing mechanism, we investigated local cerebral metabolic rate for glucose during food-anticipatory activity produced either by a 4-h daily access to food starting 4 h after light onset or by a hypocaloric feeding provided at the same time. Local cerebral metabolic rate for glucose measured by the labeled 2-[(14)C]-deoxyglucose technique was quantified in 40 structures. In both groups of food-restricted rats, three brain regions (the nucleus of the solitary tract, the cerebellar cortex and the medial preoptic area) showed a decrease in local cerebral metabolic rate for glucose, compared with control ad libitum animals. In addition, only one structure, the paraventricular thalamic nucleus, was affected by temporal restricted feeding, and not by hypocaloric feeding, compared with ad libitum rats. By contrast, three brain regions, i.e. the intergeniculate leaflets, the paraventricular hypothalamic and the arcuate nuclei, showed specifically metabolic decreases during anticipation of hypocaloric feeding, and not during anticipation of temporal restricted feeding, compared with the ad libitum group. Expression of food-anticipatory activity appears to be regulated by an integrated neural circuit of brainstem and hypothalamic pathways, with hypocaloric feeding involving more extensive forebrain areas than temporal restricted feeding.

Localization of the neurons active during paradoxical (REM) sleep and projecting to the locus coeruleus noradrenergic neurons in the rat

Locus coeruleus (LC) noradrenergic neurons are active during wakefulness, slow their discharge rate during slow wave sleep, and stop firing during paradoxical sleep (PS). A large body of data indicates that their inactivation during PS is due to a tonic GABAergic inhibition. To localize the neurons responsible for such inhibition, we first examined the distribution of retrogradely and Fos double-immunostained neurons following cholera toxin b subunit (CTb) injection in the LC of control rats, rats selectively deprived of PS for 3 days, and rats allowed to recover for 3 hours from such deprivation. We found a significant number of CTb/Fos double-labeled cells only in the recovery group. The largest number of CTb/Fos double-labeled cells was found in the dorsal paragigantocellular reticular nucleus (DPGi). It indeed contained 19% of the CTb/Fos double-labeled neurons, whereas the ventrolateral periaqueductal gray (vlPAG) contained 18.3% of these neurons, the lateral paragigantocellular reticular nucleus (LPGi) 15%, the lateral hypothalamic area 9%, the lateral PAG 6.7%, and the rostral PAG 6%. In addition, CTb/Fos double-labeled cells constituted 43% of all the singly CTb-labeled cells counted in the DPGi compared with 29% for the LPGi, 18% for the rostral PAG, and 10% or less for the other structures. Although all these populations of CTb/Fos double-labeled neurons could be GABAergic and tonically inhibit LC neurons during PS, our results indicate that neurons from the DPGi constitute the best candidate for this role.

The endogenous somnogen adenosine excites a subset of sleep-promoting neurons via A2A receptors in the ventrolateral preoptic nucleus

Recent research has shown that neurons in the ventrolateral preoptic nucleus are crucial for sleep by inhibiting wake-promoting systems, but the process that triggers their activation at sleep onset remains to be established. Since evidence indicates that sleep induced by adenosine, an endogenous sleep-promoting substance, requires activation of brain A(2A) receptors, we examined the hypothesis that adenosine could activate ventrolateral preoptic nucleus sleep neurons via A(2A) adenosine receptors in rat brain slices. Following on from our initial in vitro identification of these neurons as uniformly inhibited by noradrenaline and acetylcholine arousal transmitters, we established that the ventrolateral preoptic nucleus comprises two intermingled subtypes of sleep neurons, differing in their firing responses to serotonin, inducing either an inhibition (Type-1 cells) or an excitation (Type-2 cells). Since both cell types contained galanin and expressed glutamic acid decarboxylase-65/67 mRNAs, they potentially correspond to the sleep promoting neurons inhibiting arousal systems. Our pharmacological investigations using A(1) and A(2A) adenosine receptors agonists and antagonists further revealed that only Type-2 neurons were excited by adenosine via a postsynaptic activation of A(2A) adenosine receptors. Hence, the present study is the first demonstration of a direct activation of the sleep neurons by adenosine. Our results further support the cellular and functional heterogeneity of the sleep neurons, which could enable their differential contribution to the regulation of sleep. Adenosine and serotonin progressively accumulate during arousal. We propose that Type-2 neurons, which respond to these homeostatic signals by increasing their firing are involved in sleep induction. In contrast, Type-1 neurons would likely play a role in the consolidation of sleep.

A quartet neural system model orchestrating sleep and wakefulness mechanisms

Physiological knowledge of the neural mechanisms regulating sleep and wakefulness has been advanced by the recent findings concerning sleep/wakefulness-related preoptic/anterior hypothalamic and perifornical (orexin-containing)/posterior hypothalamic neurons. In this paper, we propose a mathematical model of the mechanisms orchestrating a quartet neural system of sleep and wakefulness composed of the following: 1) sleep-active preoptic/anterior hypothalamic neurons (N-R group); 2) wake-active hypothalamic and brain stem neurons exhibiting the highest rate of discharge during wakefulness and the lowest rate of discharge during paradoxical or rapid eye movement (REM) sleep (WA group); 3) brain stem neurons exhibiting the highest rate of discharge during REM sleep (REM group); and 4) basal forebrain, hypothalamic, and brain stem neurons exhibiting a higher rate of discharge during both wakefulness and REM sleep than during nonrapid eye movement (NREM) sleep (W-R group). The WA neurons have mutual inhibitory couplings with the REM and N-R neurons. The W-R neurons have mutual excitatory couplings with the WA and REM neurons. The REM neurons receive unidirectional inhibition from the N-R neurons. In addition, the N-R neurons are activated by two types of sleep-promoting substances (SPS), which play different roles in the homeostatic regulation of sleep and wakefulness. The model well reproduces the actual sleep and wakefulness patterns of rats in addition to the sleep-related neuronal activities across state transitions. In addition, human sleep-wakefulness rhythms can be simulated by manipulating only a few model parameters: inhibitions from the N-R neurons to the REM and WA neurons are enhanced, and circadian regulation of the N-R and WA neurons is exaggerated. Our model could provide a novel framework for the quantitative understanding of the mechanisms regulating sleep and wakefulness.

Hypothalamic regulation of sleep and circadian rhythms

A series of findings over the past decade has begun to identify the brain circuitry and neurotransmitters that regulate our daily cycles of sleep and wakefulness. The latter depends on a network of cell groups that activate the thalamus and the cerebral cortex. A key switch in the hypothalamus shuts off this arousal system during sleep. Other hypothalamic neurons stabilize the switch, and their absence results in inappropriate switching of behavioural states, such as occurs in narcolepsy. These findings explain how various drugs affect sleep and wakefulness, and provide the basis for a wide range of environmental influences to shape wake-sleep cycles into the optimal pattern for survival.

Wake-promoting actions of medial basal forebrain beta2 receptor stimulation

The locus coeruleus-noradrenergic system exerts an activating influence on forebrain neuronal and behavioral activity states, in part, through the actions of noradrenergic beta receptors located within the medial septal (MS) and medial preoptic (MPOA) areas. The current study examined the extent to which beta2 receptors located within these medial basal forebrain regions modulate behavioral state. In this study, the sleep-wake effects of microinfusion of the beta2 agonist, clenbuterol, into the MS and MPOA were examined. Clenbuterol infusion into both MS and MPOA elicited a dose-dependent increase in time spent awake. These observations indicate that medial basal forebrain beta-sub-2 receptors participate in the noradrenergic-dependent modulation of behavioral state.

Age-related attenuation of the evening circadian arousal signal in humans

The human circadian pacemaker maintains timing and consolidation of sleep-wake behavior by opposing the build-up of homeostatic sleep pressure during the wake episode, particularly in the evening during the 'wake maintenance zone'. We tested whether age-related changes in sleep are a consequence of a weaker circadian arousal signal in the evening. Circadian rhythms and spectral components of the sleep EEG were investigated in 17 young (20-31 year) and 15 older (57-74 year) volunteers under constant posture conditions during a 40-h nap protocol (75/150 min sleep/wake schedule). Quantitative evidence for a weaker circadian arousal signal in aging arose from significantly more sleep occurring during the wake maintenance zone and higher subjective sleepiness ratings in the late afternoon and evening in the older group. In addition, we found a diminished melatonin secretion and a reduced circadian modulation of REM sleep together with less pronounced day-night differences in the lower alpha and spindle range of sleep EEG activity in the older group. Thus, our data indicate that age-related changes in sleep propensity are clearly related to a reduced circadian signal opposing the homeostatic drive for sleep.

Retrograde study of hypocretin-1 (orexin-A) projections to subdivisions of the dorsal raphe nucleus in the rat

A retrograde tracer, WGA-apo-HRP-gold (WG), was injected into each subdivision of the dorsal raphe (DR) nucleus, and subsequent orexin-A immunostaining was performed for the tuberal region of the hypothalamus in order to investigate orexin projections to the DR. Similar to previous studies, the majority of orexin-single-labeled neurons were observed at the dorsal half of the lateral hypothalamus (LH), the circle around the fornix, i.e., perifornical nucleus (PeF), and the area dorsal to the fornix. The present study reports that hypothalamic neurons exhibited differential projections to each subdivision of the DR. Following WG injections into rostral DR, WG-single-labeled cells were observed at the dorsal half of the LH as well as dorsomedial hypothalamic nucleus. The major input to the intermediate DR originates from the ventromedial portion of the LH, PeF, and the area dorsal to the PeF, whereas one to lateral wing DR derived from PeF as well as the ventrolateral portion of the LH. Following WG injections into caudal DR, WG-single-labeled cells were located at ventromedial LH and the ventrolateral portion of the posterior hypothalamus. Following WG injections into each DR subdivision, WG/orexin-double-labeled neurons were observed at LH, PeF, and the area dorsal to the PeF. Only a few double-labeled cells were observed in dorsomedial and posterior hypothalamic nuclei. Our observations suggest that various hypothalamic neurons differentially project to each subdivision of the DR, a portion of which is orexin-immunoreactive. These orexin-immunoreactive DR-projecting hypothalamic neurons might have wake-related influences over a variety of brain functions subject to DR efferent regulation, including affective behavior, autonomic control, nociception, cognition, and sensorimotor integration.

CNS determinants of sleep-related worsening of airway functions: implications for nocturnal asthma

This review summarizes the recent neuroanatomical and physiological studies that form the neural basis for the state-dependent changes in airway resistance. Here, we review only the interactions between the brain regions generating quiet (non-rapid eye movement, NREM) and active (rapid eye movement, REM) sleep stages and CNS pathways controlling cholinergic outflow to the airways. During NREM and REM sleep, bronchoconstrictive responses are heightened and conductivity of the airways is lower as compared to the waking state. The decrease in conductivity of the lower airways parallels the sleep-induced decline in the discharge of brainstem monoaminergic cell groups and GABAergic neurons of the ventrolateral periaqueductal midbrain region, all of which provide inhibitory inputs to airway-related vagal preganglionic neurons (AVPNs). Withdrawal of central inhibitory influences to AVPNs results in a shift from inhibitory to excitatory transmission that leads to an increase in airway responsiveness, cholinergic outflow to the lower airways and consequently, bronchoconstriction. In healthy subjects, these changes are clinically unnoticed. However, in patients with bronchial asthma, sleep-related alterations in lung functions are troublesome, causing intensified bronchopulmonary symptoms (nocturnal asthma), frequent arousals, decreased quality of life, and increased mortality. Unquestionably, the studies revealing neural mechanisms that underlie sleep-related alterations of airway function will provide new directions in the treatment and prevention of sleep-induced worsening of airway diseases.

Retrograde study of projections from the tuberomammillary nucleus to the dorsal raphe and the locus coeruleus in the rat

In the first series of experiments, a retrograde tracer, WGA-apo-HRP-gold (WG), was injected into the dorsal raphe (DR) or the locus coeruleus (LC) and adenosine deaminase immunostaining was subsequently performed for the tuberomammillary nucleus (TMN) in order to investigate projections from the TMN to the two brainstem monoaminergic nuclei. Following rostral DR injections, the majority of retrogradely labeled cells were located in the dorsomedial and ventrolateral subdivisions of the TMN. At middle DR levels, midline injections resulted in labeling mainly in the ventrolateral subdivision, whereas lateral wing injections produced labeling mostly in ventral and caudal TMN subdivisions. When injections were made in the caudal DR, only a few cells were observed along the rostro-caudal extent of the TMN. On the other hand, following rostral LC injections, labeled neurons were observed mainly in ventrolateral and ventral subdivisions of TMN. For principal LC injections, labeled cells were observed mostly in ventrolateral, ventral, and caudal TMN subdivisions, whereas for injections at caudal pole of LC, only a few cells were located along the rostro-caudal extent of the TMN. In the second series of experiments, an iontophoretic injection of fluorogold (FG) into the DR was paired with a pressure injection of WG into the LC to investigate the collateral distribution of TMN axonal fibers to DR and LC. Double-labeled cells were observed in ventrolateral, ventral, and caudal TMN subdivisions. The present study indicated that there exists a robust projection from the TMN to the DR or the LC and that some TMN neurons have axon collaterals projecting to both DR and LC. The TMN neurons with such axon collaterals might provide simultaneous, possibly more efficient, way of controlling the brainstem monoaminergic nuclei, thus influencing various sleep and arousal states of the animal.

Retrograde double-labeling study of common afferent projections to the dorsal raphe and the nuclear core of the locus coeruleus in the rat

Common afferent projections to the dorsal raphe (DR) and locus coeruleus (LC) nuclei were analyzed in the rat by making paired injections of retrograde tracers, gold-conjugated and inactivated wheatgerm agglutinin-horseradish peroxidase (WGA-apo-HRP-gold) and Fluorogold (FG), into the DR and the nuclear core of the LC. Our results demonstrate that the largest number of double-labeled neurons was located at various preoptic regions including medial preoptic area, lateral preoptic nucleus, and ventrolateral preoptic nucleus. The majority of labeled cells were also observed at the lateral hypothalamus, where the number of labeled cells was comparable to that of neurons at the medial preoptic area or lateral preoptic nucleus. A few double-labeled cells were observed at various hypothalamic regions including anterior, medial tuberal, posterior, and arcuate nuclei, as well as mesencephalic areas including substantia nigra compacta and ventrolateral/lateral periaqueductal gray matter. Cells were also observed at prelimbic/infralimbic prefrontal cortices, diagonal band of Broca, bed nucleus of stria terminalis, and pontine/medullary regions including various raphe nuclei, Barrington's nucleus, gigantocellularis, paragigantocellularis, prepositus hypoglossi, subcoeruleus, and dorsomedial tegmental area. Although electrophysiological studies need to be performed, a large number of double-labeled neurons located at preoptic regions as well as lateral hypothalamus might have their major role in simultaneous control over these monoaminergic nuclei as a means of influencing various sleep and arousal states of the animal. Double-labeled cells at the other locations might be positioned to influence a variety of other functions such as analgesia, cognition, and stress responses.

Organization of hypocretin/orexin efferents to locus coeruleus and basal forebrain arousal-related structures

Hypocretin/orexin neurons give rise to an extensive projection system, portions of which innervate multiple regions associated with the regulation of behavioral state. These regions include the locus coeruleus, medial septal area, medial preoptic area, and substantia innominata. Evidence indicates that hypocretin modulates behavioral state via actions within each of these terminal fields. To understand better the circuitry underlying hypocretin-dependent modulation of behavioral state, the present study characterized the degree to which there exists: 1) lateralization of hypocretin efferents to basal forebrain and brainstem arousal-related regions, 2) topographic organization of basal forebrain- and brainstem-projecting hypocretin neurons, and 3) collateralization of individual hypocretin neurons to these arousal-related terminal fields. These studies utilized combined immunohistochemical identification of hypocretin neurons with single or double retrograde tracing from the locus coeruleus, medial preoptic area, medial septal area, and substantia innominata. Results indicate that approximately 80% of hypocretin efferents to basal forebrain regions project ipsilaterally, whereas projections to the locus coeruleus are more bilateral (65%). There was a slight preference for basal forebrain-projecting hypocretin neurons to be distributed within the medial half of the hypocretin cell group. In contrast, hypocretin neurons projecting to the locus coeruleus were located primarily within the dorsal half of the hypocretin cell group. Finally, a large proportion of hypocretin neurons appear to project simultaneously to at least two of the examined terminal fields. These latter observations suggest coordinated actions of hypocretin across multiple arousal-related regions.

Interleukin 1beta enhances non-rapid eye movement sleep and increases c-Fos protein expression in the median preoptic nucleus of the hypothalamus

Interleukin 1beta (IL-1) is a key mediator of the acute phase response in an infected host and acts centrally to coordinate responses to an immune challenge, such as fever and increased non-rapid eye movement (NREM) sleep. The preoptic area (POA) is a primary sleep regulatory center in the brain: the ventrolateral POA (VLPO) and median preoptic nucleus (MnPN) each contain high numbers of c-Fos protein immunoreactive (IR) neurons after sleep but not after waking. We hypothesized that IL-1 mediates increased NREM sleep through activation of these sleep-active sites. Rats injected intracerebroventricularly with IL-1 (10 ng) at dark onset spent significantly more time in NREM sleep 4-5 h after injection. This increase in NREM sleep was associated with increased numbers of Fos-IR neurons in the MnPN, but not in the VLPO. Fos IR in the rostral MnPN was significantly increased 2 h post IL-1 injection, although the percentage of NREM sleep in the preceding 2 h was the same as controls. Fos IR was also increased in the extended VLPO 2 h postinjection. Finally, Fos IR in the MnPN did not differ significantly between IL-1 and vehicle-treated rats that had been sleep deprived for 2 h postinjection, but it was increased in VLPO core. Taken together, these results suggest that Fos IR in the MnPN after IL-1 is not independent of behavioral state and may require some threshold amount of sleep for its expression. Our results support a hypothesis that IL-1 enhances NREM sleep, in part, through activation of neurons in the MnPN of the hypothalamus.

Brain inhibitory mechanisms involved in basic and higher integrated sleep processes

Brain function is supported by central activating processes that are significant during waking, decrease during slow wave sleep following waking and increase again during paradoxical sleep during which brain activation is as high as, or higher than, during waking in nearly all structures. However, inhibitory mechanisms are crucial for sleep onset. They were first identified by behavioral, neuroanatomical and electrophysiological criteria, then by pharmacological and neurochemical ones. During slow wave sleep, they are supported by GABAergic mechanisms located at midbrain, mesopontine and pontine levels but are induced and sustained by forebrain and hindbrain influences. GABAergic processes are also responsible for paradoxical sleep occurrence, particularly by suppression of noradrenaline and serotonin (5-HT) inhibition of paradoxical sleep-generating structures. Hindbrain and forebrain modulate these structures situated at the mesopontine level. For sleep mentation, the noradrenergic and serotonergic silence is thought, today, to be directly, or indirectly, responsible for dopamine predominance and glutamate decrease in the nucleus accumbens, which could be the background of the well-known psychotic-like mental activity of dreaming.

Six-hour selective REM sleep deprivation increases the expression of the galanin gene in the hypothalamus of rats

The effect of short-term selective REM sleep deprivation (RSD) on the gene expression of galanin in the rat hypothalamus was examined using in situ hybridization histochemistry. Monitoring an electroencephalogram (EEG) and electromyogram (EMG) on an on-line computer screen, as the RSD rats entered REM sleep, they were gently stroked on their backs using a brush to wake them during the RSD period. Galanin mRNA levels in the preoptic area (POA) were significantly increased by RSD for a period of 6 h. RSD had no significant effect on the mRNA levels of corticotrophin-releasing factor (CRF), arginine vasopressin (AVP), oxytocin (OXT) or orexins. These results suggest that 6-h selective RSD may not be sufficient to induce the activation of the hypothalamo-pituitary adrenal axis, and that the expression of the galanin gene in the hypothalamus reacts more readily against the loss of REM sleep in comparison to other hypothalamic neuropeptides such as arginine vasopressin, oxytocin and orexins.

Activation of c-fos in GABAergic neurones in the preoptic area during sleep and in response to sleep deprivation

Neurones in the median preoptic nucleus (MnPN) and the ventrolateral preoptic area (vlPOA) express immunoreactivity for c-Fos protein following sustained sleep, and display elevated discharge rates during both non-REM and REM sleep compared to waking. We evaluated the hypothesis that MnPN and vlPOA sleep-active neurones are GABAergic by combining staining for c-Fos protein with staining for glutamic acid decarboxylase (GAD). In a group of six rats exhibiting spontaneous total sleep times averaging 82.2 +/- 5.1% of the 2 h immediately prior to death, >75% of MnPN neurones that were Fos-immunoreactive (IR) were also GAD-IR. Similar results were obtained in the vlPOA. In a group of 11 rats exhibiting spontaneous sleep times ranging from 20 to 92%, the number of Fos + GAD-IR neurones in MnPN and vlPOA was positively correlated with total sleep time. Compared to control animals, Fos + GAD-IR cell counts in the MnPN were significantly elevated in rats that were sleep deprived for 24 h and permitted 2 h of recovery sleep. These findings demonstrate that a majority of MnPN and vlPOA neurones that express Fos-IR during sustained spontaneous sleep are GABAergic. They also demonstrate that sleep deprivation is associated with increased activation of GABAergic neurones in the MnPN and vlPOA.

EP3 and EP4 receptor mRNA expression in peptidergic cell groups of the rat parabrachial nucleus

This study examines the distribution of prostaglandin E2 receptors of subtype EP3 and EP4 among brain stem parabrachial neurons that were characterized with respect to their neuropeptide expression. By using a dual-labeling in situ hybridization method, we show that preprodynorphin mRNA expressing neurons in the dorsal and central lateral subnuclei express EP3 receptor mRNA. Such receptors are also expressed in preproenkephalin, calcitonin gene related peptide and preprotachykinin mRNA positive neurons in the external lateral subnucleus, whereas preprodynorphin mRNA expressing neurons in this subnucleus are EP receptor negative. In addition, EP3 receptor expression is seen among some enkephalinergic neurons in the Kölliker-Fuse nucleus. Neurons in the central part of the cholecystokininergic population in the regions of the superior lateral subnucleus express EP4 receptor mRNA, whereas those located more peripherally express EP3 receptors. Taken together with previous findings showing that discrete peptidergic cell groups mediate nociceptive and/or visceral afferent information to distinct brain stem and forebrain regions, the present results suggest that the processing of this information in the parabrachial nucleus is influenced by prostaglandin E2. Recent work has shown that prostaglandin E2 is released into the brain following peripheral immune challenge; hence, the parabrachial nucleus may be a region where humoral signaling of peripheral inflammatory events may interact with neuronal signaling elicited by the same peripheral processes.

Hypothalamic contribution to sleep–wake cycle development

Infant mammals cycle rapidly between sleep and wakefulness and only gradually does a more consolidated sleep pattern develop. The neural substrates responsible for this consolidation are unknown. To establish a reliable measure of sleep-wake cyclicity in infant rats, nuchal muscle tone was measured in 2-, 5-, and 8-day-old rats, as were motor behaviors associated with sleep (i.e. myoclonic twitching) and wakefulness (e.g. kicking, stretching). Sleep-wake cycles of 2-day-old rats were characterized by short periods of muscle atonia followed by equally short periods of high tone. In 8-day-olds, sleep periods lengthened significantly and disproportionately in relation to awake periods. Next, locus coeruleus (LC) lesions in 8-day-olds resulted in rapid sleep-wake cycling similar to that exhibited by 2-day-olds; in addition, LC lesions had no effect on the duration of awake periods. Finally, transections caudal, but not rostral, to the anterior hypothalamus also reinstated rapid cycling in 8-day-olds, again without affecting the duration of awake periods. This last finding implicates neural structures within the anterior hypothalamus (e.g. ventrolateral preoptic area) in the modulation of sleep-wake cyclicity. The temporal coherence of atonia and myoclonic twitching was not disrupted by any of the manipulations. These results suggest the presence of a bistable mesopontine circuit governing rapid sleep-wake cycling that does not include the LC and that comes increasingly under hypothalamic control during the first postnatal week. This circuit may represent a basic building block with which other sleep components become integrated during ontogeny.

Critical role of dorsomedial hypothalamic nucleus in a wide range of behavioral circadian rhythms

The suprachiasmatic nucleus (SCN) contains the brain's circadian pacemaker, but mechanisms by which it controls circadian rhythms of sleep and related behaviors are poorly understood. Previous anatomic evidence has implicated the dorsomedial hypothalamic nucleus (DMH) in circadian control of sleep, but this hypothesis remains untested. We now show that excitotoxic lesions of the DMH reduce circadian rhythms of wakefulness, feeding, locomotor activity, and serum corticosteroid levels by 78-89% while also reducing their overall daily levels. We also show that the DMH receives both direct and indirect SCN inputs and sends a mainly GABAergic projection to the sleep-promoting ventrolateral preoptic nucleus, and a mainly glutamate-thyrotropin-releasing hormone projection to the wake-promoting lateral hypothalamic area, including orexin (hypocretin) neurons. Through these pathways, the DMH may influence a wide range of behavioral circadian rhythms.

Effects of adenosine on gabaergic synaptic inputs to identified ventrolateral preoptic neurons

The ventrolateral preoptic nucleus (VLPO) is a key regulator of behavioral state that promotes sleep by directly inhibiting brain regions that maintain wakefulness. Subarachnoid administration of adenosine (AD) or AD agonists promotes sleep and induces expression of Fos protein in VLPO neurons. Therefore, activation of VLPO neurons may contribute to the somnogenic actions of AD. To define the mechanism through which AD activates VLPO neurons, we prepared hypothalamic slices from 9 to 12-day-old rat pups and recorded from 43 neurons in the galaninergic VLPO cluster; nine neurons contained galanin mRNA by post hoc in situ hybridization. Bath application of AD (20 microM) to seven of these neurons had no direct effect but caused a significant decrease in the frequency of spontaneous miniature inhibitory postsynaptic currents in the presence of tetrodotoxin, indicating a presynaptic site of action. We conclude that AD-mediated disinhibition increases the excitability of VLPO neurons thus contributing to the somnogenic properties of AD.

A broad role for melanopsin in nonvisual photoreception

The rod and cone photoreceptors that mediate visual phototransduction in mammals are not required for light-induced circadian entrainment, negative masking of locomotor activity, suppression of pineal melatonin, or the pupillary light reflex. The photopigment melanopsin has recently been identified in intrinsically photosensitive retinal ganglion cells (RGCs) that project to the suprachiasmatic nucleus (SCN), intergeniculate leaflet (IGL), and olivary pretectal nucleus, suggesting that melanopsin might influence a variety of irradiance-driven responses. We have found novel projections from RGCs that express melanopsin mRNA to the ventral subparaventricular zone (vSPZ), a region involved in circadian regulation and negative masking, and the sleep-active ventrolateral preoptic nucleus (VLPO) and determined the subsets of melanopsin-expressing RGCs that project to the SCN, the pretectal area (PTA), and the IGL division of the lateral geniculate nucleus (LGN). Melanopsin was expressed in the majority of RGCs that project to the SCN, vSPZ, and VLPO and in a subpopulation of RGCs that innervate the PTA and the IGL but not in RGCs projecting to the dorsal LGN or superior colliculus. Two-thirds of RGCs containing melanopsin transcript projected to each of the SCN and contralateral PTA, and one-fifth projected to the ipsilateral IGL. Double-retrograde tracing from the SCN and PTA demonstrated a subpopulation of RGCs projecting to both sites, most of which contained melanopsin mRNA. Our results suggest that melanopsin expression defines a subset of RGCs that play a broad role in the regulation of nonvisual photoreception, providing collateralized projections that contribute to circadian entrainment, negative masking, the regulation of sleep-wake states, and the pupillary light reflex.

Ventrolateral preoptic nucleus contains sleep-active, galaninergic neurons in multiple mammalian species

The ventrolateral preoptic nucleus (VLPO) is a group of sleep-active neurons that has been identified in the hypothalamus of rats and is thought to inhibit the major ascending monoaminergic arousal systems during sleep; lesions of the VLPO cause insomnia. Identification of the VLPO in other species has been complicated by the lack of a marker for this cell population, other than the expression of Fos during sleep. We now report that a high percentage of the sleep-active (Fos-expressing) VLPO neurons express mRNA for the inhibitory neuropeptide, galanin, in nocturnal rodents (mice and rats), diurnal rodents (degus), and cats. A homologous (i.e. galanin mRNA-containing cell group) is clearly distinguishable in the ventrolateral region of the preoptic area in diurnal and nocturnal monkeys, as well as in humans. Galanin expression may serve to identify sleep-active neurons in the ventrolateral preoptic area of the mammalian brain. The VLPO appears to be a critical component of sleep circuitry across multiple species, and we hypothesize that shrinkage of the VLPO with advancing age may explain sleep deficits in elderly humans.