The median preoptic nucleus reciprocally modulates activity of arousal-related and sleep-related neurons in the perifornical lateral hypothalamus

Metabotropic glutamate receptors in median preoptic neurons modulate neuronal excitability and glutamatergic and GABAergic inputs from the subfornical organ

Cardiovascular and behavioral responses to circulating angiotensin require intact connectivity along the upper lamina terminalis joining the subfornical organ (SFO) with the median preoptic nucleus (MnPO). In the present study on MnPO neurons, we used whole cell patch-clamp recording techniques in brain slice preparations to evaluate the influence of metabotropic glutamate receptor (mGluR) agonists on modulating their intrinsic excitability and SFO-evoked glutamatergic and GABAergic postsynaptic currents. In 22/36 cells, bath application of a mGluR group I agonist (S)-3,5-dihydroxyphenylglycine (DHPG) induced a TTX-resistant inward current coupled with decrease in a membrane K(+) conductance but also a possible increase in a nonselective cationic conductance. By contrast, 27/49 cells responded to a mGluR group II agonist (2S,2'R,3'R)-2-(2',3'-dicarboxycyclopropyl)glycine (DCG IV) with a TTX-resistant outward current and increase in membrane conductance that reversed around -95 mV, suggesting opening of K(+) channels. None of 19 cells responded to the mGluR group III agonist l-(+)-2-amino-4-phosphonobutyric acid (l-AP4). Agonists for all mGluR groups suppressed SFO-evoked excitatory postsynaptic currents and significantly increased paired-pulse ratios, implying a presynaptic mechanism. Only the mGluR group II agonist significantly reduced SFO-evoked inhibitory postsynaptic currents and caused an increase in paired-pulse ratios. These results suggest a complexity of pre- and postsynaptic mGluRs are available to modulate rapid neurotransmission along the upper lamina terminalis from SFO to MnPO.

GABA in pedunculopontine tegmentum increases rapid eye movement sleep in freely moving rats: possible role of GABA-ergic inputs from substantia nigra pars reticulata

Pedunculopontine tegmentum (PPT) has GABA-ergic neurons and receives GABA-ergic projections from substantia nigra pars reticulata (SNrpr). Based on the recent studies from our and other laboratories, it was hypothesized that GABA in PPT promotes rapid eye movement (REM) sleep. In order to further study the role of GABA in PPT in REM sleep regulation, we microinjected GABA-A agonist, muscimol (200 nL, 3.5 mM), into the PPT. Muscimol in PPT significantly enhanced the amount of REM sleep by increasing the mean number of REM sleep bouts. Besides the local interneurons, GABA-ergic afferents from SNrpr are another source of GABA in PPT. In order to understand the contribution of GABA-ergic inputs from SNrpr into PPT for REM sleep regulation, SNrpr was electrically stimulated either alone or simultaneously along with the infusion of GABA-A antagonist, picrotoxin (200 nL, 0.86 mM), into the PPT. The experiment was designed with the premise that stimulation of SNrpr should increase GABA levels in PPT which should increase REM sleep comparable to that after muscimol microinjection in PPT. Further, the effect of stimulation of SNrpr on REM sleep should be antagonized by simultaneous infusion of picrotoxin into PPT. The electrical stimulation of SNrpr did not produce any significant change in sleep-wake states although it was sufficient to counter the effect of picrotoxin injection into the PPT. To overcome the limitations and confounds of electrical stimulation, SNrpr was pharmacologically stimulated by glutamate microinjection (200 nL, 5.34 mM). Infusion of glutamate into SNrpr enhanced REM sleep by increasing the mean number of REM sleep bouts, which was similar and comparable to the effect of muscimol injection into the PPT. The results confirm that GABA in PPT either from local neurons or from SNrpr promotes REM sleep.

c-Fos expression in preoptic nuclei as a marker of sleep rebound in the rat

Thermoregulation is known to interfere with sleep, possibly due to a functional interaction at the level of the preoptic area (POA). Exposure to low ambient temperature (T(a)) induces sleep deprivation, which is followed by sleep rebound after a return to laboratory T(a). As two POA subregions, the ventrolateral preoptic nucleus (VLPO) and the median preoptic nucleus (MnPO), have been proposed to have a role in sleep-related processes, the expression of c-Fos and the phosphorylated form of the cAMP/Ca(2+)-responsive element-binding protein (P-CREB) was investigated in these nuclei during prolonged exposure to a T(a) of -10 degrees C and in the early phase of the recovery period. Moreover, the dynamics of the sleep rebound during recovery were studied in a separate group of animals. The results show that c-Fos expression increased in both the VLPO and the MnPO during cold exposure, but not in a specific subregion within the VLPO cluster counting grid (VLPO T-cluster). During the recovery, concomitantly with a large rapid eye movement sleep (REMS) rebound and an increase in delta power during non-rapid eye movement sleep (NREMS), c-Fos expression was high in both the VLPO and the MnPO and, specifically, in the VLPO T-cluster. In both nuclei, P-CREB expression showed spontaneous variations in basal conditions. During cold exposure, an increase in expression was observed in the MnPO, but not in the VLPO, and a decrease was observed in both nuclei during recovery. Dissociation in the changes observed between c-Fos expression and P-CREB levels, which were apparently subject to state-related non-regulatory modulation, suggests that the sleep-related changes observed in c-Fos expression do not depend on a P-CREB-mediated pathway.

A role for the preoptic sleep-promoting system in absence epilepsy

Absence epilepsy (AE) in humans and the genetic AE model in WAG/Rij rats are both associated with abnormalities in sleep architecture that suggest insufficiency of the sleep-promoting mechanisms. In this study we compared the functionality of sleep-active neuronal groups within two well-established sleep-promoting sites, the ventrolateral and median preoptic nuclei (VLPO and MnPN, respectively), in WAG/Rij and control rats. Neuronal activity was assessed using c-Fos immunoreactivity and chronic single-unit recording techniques. We found that WAG/Rij rats exhibited a lack of sleep-associated c-Fos activation of GABAergic MnPN and VLPO neurons, a lower percentage of MnPN and VLPO cells increasing discharge during sleep and reduced firing rates of MnPN sleep-active neurons, compared to non-epileptic rats. The role of sleep-promoting mechanisms in pathogenesis of absence seizures was assessed in non-epileptic rats using electrical stimulation and chemical manipulations restricted to the MnPN. We found that fractional activation of the sleep-promoting system in waking was sufficient to elicit absence-like seizures. Given that reciprocally interrelated sleep-promoting and arousal neuronal groups control thalamocortical excitability, we hypothesize that malfunctioning of sleep-promoting system results in impaired ascending control over thalamocortical rhythmogenic mechanisms during wake-sleep transitions thus favoring aberrant thalamocortical oscillations. Our findings suggest a pathological basis for AE-associated sleep abnormalities and a mechanism underlying association of absence seizures with wake-sleep transitions.

Selective loss of GABA(B) receptors in orexin-producing neurons results in disrupted sleep/wakefulness architecture

Hypothalamic neurons that contain the neuropeptide orexin (hypocretin) play important roles in the regulation of sleep/wake. Here we analyze the in vivo and in vitro phenotype of mice lacking the GABA(B1) gene specifically in orexin neurons (oxGKO mice) and demonstrate that GABA(B) receptors on orexin neurons are essential in stabilizing and consolidating sleep/wake states. In oxGKO brain slices, we show that the absence of GABA(B) receptors decreases the sensitivity of orexin neurons to both excitatory and inhibitory inputs because of augmented GABA(A)-mediated inhibition that increases the membrane conductance and shunts postsynaptic currents in these neurons. This increase in GABA(A)-mediated inhibitory tone is apparently the result of an orexin receptor type 1-mediated activation of local GABAergic interneurons that project back onto orexin neurons. oxGKO mice exhibit severe fragmentation of sleep/wake states during both the light and dark periods, without showing an abnormality in total sleep time or signs of cataplexy. Thus, GABA(B) receptors on orexin neurons are crucial in the appropriate control of the orexinergic tone through sleep/wake states, thereby stabilizing the state switching mechanisms.

The dual role of the orexin/hypocretin system in modulating wakefulness and respiratory drive

PURPOSE OF REVIEW

Today, numerous studies show that orexin peptides act as regulators of many functions including the control of sleep-wake states, breathing, and central chemosensitivity. However, little is known on neuronal mechanisms by which orexin regulates breathing in a state-dependent manner. This review summarizes recent data on the control of neuronal circuits by orexin, with a special emphasis on breathing, central chemosensitivity, and obstructive sleep apneas.

RECENT FINDINGS

Activity of hypothalamic orexinergic neurons is subjected to maturation and is mandatory to maintain long bouts of wakefulness in adults. At wake onset, this activity progressively builds up as a result of synaptic interactions and reinforces the awake state. Orexin deficiency attenuates the hypercapnic reflex only during wakefulness and is correlated with an increase in sleep apneas. Intrinsic sensitivity to CO2/pH of orexin neurons may impact on brainstem chemosensitive neurons, and this effect likely involves TWIK (tandem of P domains in a weak inwardly rectifying K+ channel)-related acid sensitive K+ (TASK)-like potassium currents.

SUMMARY

Orexin signaling is directly involved in the control of upper airway patency in particular during wakefulness, whereas decreasing activity of orexinergic neurons may contribute to upper airway collapse during sleep causing obstructive sleep apnea. Future research should focus on the role of orexin in upper airway control, which may lead to new clinical strategies for treating breathing disorders associated with sleep.

Inactivation of median preoptic nucleus causes c-Fos expression in hypocretin- and serotonin-containing neurons in anesthetized rat

The median preoptic nucleus (MnPN) of the hypothalamus contains sleep-active neurons including sleep-active GABAergic neurons and is involved in the regulation of nonREM/REM sleep. The hypocretinergic (HCRT) neurons of the perifornical-lateral hypothalamic area (PF-LHA) and serotonergic (5-HT) neurons of the dorsal raphe nucleus (DRN) are mostly active during waking and have been implicated in the regulation of arousal. MnPN GABAergic neurons project to the PF-LHA and DRN. It is hypothesized that MnPN promotes sleep by inhibiting multiple arousal systems including HCRT and other wake-active neurons within the PF-LHA and 5-HT neurons in the DRN. We examined the effects of inactivation of MnPN neurons by locally microinjecting 0.2 microl of 1 mM or 10 mM solutions of a GABA(A) receptor agonist, muscimol, into the MnPN on Fos expression (Fos-IR) in the PF-LHA neurons including HCRT neurons and 5-HT neurons in the DRN in anesthetized rats. Compared to artificial cerebrospinal fluid control, microinjection of muscimol into the MnPN resulted in significantly higher percentages of HCRT and non-HCRT neurons in the PF-LHA and 5-HT neurons in the DRN that exhibited Fos-IR. The percentage of melanin-concentrating hormone (MCH)+/Fos+ neurons in the PF-LHA did not change after muscimol treatments. These results support a hypothesis that the activation of MnPN neurons contributes to the suppression of wake-promoting systems including HCRT and other unidentified neurons in the PF-LHA and 5-HT neurons in the DRN. These results also suggest that MCH neurons may not be under MnPN inhibitory control. These findings are consistent with a hypothesized role of MnPN in sleep regulation.

Nucleus paragigantocellularis afferents in male and female rats: organization, gonadal steroid receptor expression, and activation during sexual behavior

The supraspinal regulation of genital reflexes is poorly understood. The brainstem nucleus paragigantocellularis (nPGi) of rats is a well-established source of tonic inhibition of genital reflexes. However, the organization, gonadal steroid receptor expression, and activity of nPGi afferents during sex have not been fully characterized in male and female rats. To delineate the anatomical and physiological organization of nPGi afferents, the retrograde tracer Fluoro-Gold (FG) was injected into the nPGi of sexually experienced male and female rats. Animals engaged in sexual behavior 1 hour before sacrifice. Cells containing FG, estrogen receptor-alpha (ER(alpha)), androgen receptor (AR), and the immediate-early gene product Fos were identified immunocytochemically. Retrograde labeling from the nPGi was prominent in the bed nucleus of the stria terminalis, paraventricular nucleus (PVN), posterior hypothalamus, precommissural nucleus, deep mesencephalic nucleus, and periaqueductal gray (PAG) of both sexes. Sex differences were observed in the caudal medial preoptic area (MPO), with significantly more FG+ cells observed in males, and in the PAG and inferior colliculus, where significantly more FG+ cells were observed in females. The majority of regions that contained FG+ cells also contained ER(alpha) or AR, indicating sensitivity to gonadal steroids. The proportions of FG+ cells that co-localized with sex-induced Fos was high in the PVN of both sexes and high in the MPO of males but low in the PAG of both sexes despite the large number of PAG-nPGi output neurons and Fos+ cells in both sexes. The characterization of these afferents will lead to a further understanding of the neural regulation of genital reflexes.

Hypothalamic regulation of sleep and arousal

Normal waking is associated with neuronal activity in several chemically defined ascending arousal systems. These include monoaminergic neurons in the brainstem and posterior hypothalamus, cholinergic neurons in the brainstem and basal forebrain, and hypocretin (orexin) neurons in the lateral hypothalamus. Collectively, these systems impart tonic activation to their neuronal targets in the diencephalon and neocortex that is reflected in the low-voltage fast-frequency electroencephalogram patterns of wakefulness. Neuronal discharge in these arousal systems declines rapidly at sleep onset. Transitions from waking to sleep, therefore, involve coordinated inhibition of multiple arousal systems. An important source of sleep-related inhibition of arousal arises from neurons located in the preoptic hypothalamus. These preoptic neurons are strongly activated during sleep, exhibiting sleep/waking state-dependent discharge patterns that are the reciprocal of that observed in the arousal systems. The majority of preoptic sleep regulatory neurons synthesize the inhibitory neurotransmitter GABA. Anatomical and functional evidence supports the hypothesis that GABAergic neurons in the median preoptic nucleus (MnPN) and ventrolateral preoptic area (VLPO) exert inhibitory control over the monoaminergic systems and the hypocretin system during sleep. Recent findings indicate that MnPN and VLPO neurons integrate homeostatic aspects of sleep regulation and are important targets for endogenous sleep factors, such as adenosine and growth hormone releasing hormone.

Why we sleep: the temporal organization of recovery

Why we sleep seems like a simple question, yet it has baffled scientists for generations. Based on recent data, Emmanuel Mignot argues that the function of sleep is essentially a resilient form of cellular recovery, organized anatomically and temporally by natural evolution, that has taken on additional functions over time.

Orexin peptides enhance median preoptic nucleus neuronal excitability via postsynaptic membrane depolarization and enhancement of glutamatergic afferents

Subpopulations of neurons in the median preoptic nucleus (MnPO) located within the lamina terminalis contribute to thermoregulatory, cardiovascular and hydromineral homeostasis, and sleep-promotion. MnPO is innervated by lateral hypothalamic neurons that synthesize and secrete the arousal-promoting and excitatory orexin (hypocretin) neuropeptides. To evaluate the hypothesis that orexins modulate the excitability of MnPO neurons, we used patch-clamp recording techniques applied in rat brain slice preparations to assess the effects of exogenously applied orexin A and orexin B peptides on their intrinsic and synaptic properties. Whole cell recordings under current-clamp mode revealed that 11/15 tested MnPO neurons responded similarly to either orexin A or B (500-1000 nM) with a slowly rising, prolonged (10-15 min) and reversible membrane depolarization. Under voltage-clamp mode, orexin applications induced a tetrodotoxin-resistant inward current of -7.2+/-1.6 pA, indicating a direct (postsynaptic) activation, with a time course similar to the observed membrane depolarization. The orexin-induced responses in 4/7 neurons were associated with a significant decrease in membrane conductance and the net orexin-induced current that reversed at -99+/-5 mV, suggesting closure of potassium channels. Orexins did not attenuate the properties of excitatory (n=4) or inhibitory (n=7) postsynaptic currents evoked by subfornical organ stimulation. By contrast, orexins applications induce a significant increase in both frequency and amplitude of spontaneous glutamatergic postsynaptic currents (5/7 cells) but had no influence on spontaneous GABAergic currents (6/6 cells). Thus, in addition to a direct postsynaptic receptor-mediated excitation, orexins can also increase the excitability of MnPO neurons via increasing their excitatory inputs, presumably through an orexin receptor-mediated excitation of local glutamatergic neurons whose axons project to MnPO neurons.

Imaging unconditioned fear response with manganese-enhanced MRI (MEMRI)

Recent use of manganese-enhanced MRI (MEMRI) to assess the neural circuitry involved in autonomic and somatosensory paradigms has been promising. The current study addresses the feasibility of utilizing this technique to assess more complex cognitive and emotional processes. Since olfactory cues are particularly salient to animals, we utilized odorless air, novel/arousing and novel/fear-inducing scents to assess the neural circuitry sub-serving novelty and unconditioned fear. The present imaging data clearly indicate that animals with no prior exposure to a threat-inducing emotional stimulus selectively activated the unconditional fear neuronal pathway, specifically with heightened amygdala and hypothalamic activation. While animals exposed to the novel/arousing compared to fear-inducing odor demonstrated enhanced uptake in the cingulated and prefrontal cortices. In addition, as expected the hippocampus showed significantly enhanced manganese contrast after novelty exposure. Therefore the current study support the validity of MEMRI in the exploration of highly relevant complex neural circuitries associated with cognition and emotion.