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

Orexin receptors: pharmacology and therapeutic opportunities

Orexin-A and -B (also known as hypocretin-1 and -2) are neuropeptides produced in the lateral hypothalamus that promote many aspects of arousal through the OX1 and OX2 receptors. In fact, they are necessary for normal wakefulness, as loss of the orexin-producing neurons causes narcolepsy in humans and rodents. This has generated considerable interest in developing small-molecule orexin receptor antagonists as a novel therapy for the treatment of insomnia. Orexin antagonists, especially those that block OX2 or both OX1 and OX2 receptors, clearly promote sleep in animals, and clinical results are encouraging: Several compounds are in Phase III trials. As the orexin system mainly promotes arousal, these new compounds will likely improve insomnia without incurring many of the side effects encountered with current medications.

Orexin/hypocretin plays a role in the response to physiological disequilibrium

In the decade since the discovery that pathology of the orexin/hypocretin system is causative for the sleep disorder narcolepsy, considerable progress has been made in understanding the functional role of the neuropeptide. Two, apparently separate functions of orexin have emerged as a consensus from studies to date. The first is the effect on vigilance state boundaries, as exemplified by narcolepsy. Thus the absence of orexin severely limits the ability to maintain prolonged periods of wakefulness or sleep and also allows the unregulated appearance of cataplexy as sudden muscle weakness during wakefulness. The second function is that orexin acts as a signaling molecule in transferring information about physiological disequilibrium to the central nervous system. Orexin activates the central arousal and motor systems during such disequilibrium and so may facilitate the necessary response and adaptation to restore equilibrium. A feasible relationship between these two functions is therefore that the maintenance of prolonged and active wakefulness is an integral part of this adaptive process. Furthermore, the limit placed on the onset of sleep by orexin suggests that these adaptive processes then continue during sleep to become integrated into the development of a coping strategy for the longer term.

GABAergic actions on cholinergic laterodorsal tegmental neurons: implications for control of behavioral state

Cholinergic neurons of the pontine laterodorsal tegmentum (LDT) play a critical role in regulation of behavioral state. Therefore, elucidation of mechanisms that control their activity is vital for understanding of how switching between wakefulness, sleep and anesthetic states is effectuated. In vivo studies suggest that GABAergic mechanisms within the pons play a critical role in behavioral state switching. However, the postsynaptic, electrophysiological actions of GABA on LDT neurons, as well as the identity of GABA receptors present in the LDT mediating these actions is virtually unexplored. Therefore, we studied the actions of GABA agonists and antagonists on cholinergic LDT cells by performing patch clamp recordings in mouse brain slices. Under conditions where detection of Cl(-) -mediated events was optimized, GABA induced gabazine (GZ)-sensitive inward currents in the majority of LDT neurons. Post-synaptic location of GABA(A) receptors was demonstrated by persistence of muscimol-induced inward currents in TTX and low Ca(2+) solutions. THIP, a selective GABA(A) receptor agonist with a preference for δ-subunit containing GABA(A) receptors, induced inward currents, suggesting the existence of extrasynaptic GABA(A) receptors. LDT cells also possess GABA(B) receptors as baclofen-activated a TTX- and low Ca(2+)-resistant outward current that was attenuated by the GABA(B) antagonists CGP 55845 and saclofen. The tertiapin sensitivity of baclofen-induced outward currents suggests that a G(IRK) mediated this effect. Further, outward currents were never additive with those induced by application of carbachol, suggesting that they were mediated by activation of GABA(B) receptors linked to the same G(IRK) activated in these cells by muscarinic receptor stimulation. Activation of GABA(B) receptors inhibited Ca(2+) increases induced by a depolarizing voltage step shown previously to activate VOCCs in cholinergic LDT neurons. Baclofen-mediated reductions in depolarization-induced Ca(2+) were unaltered by prior emptying of intracellular Ca(2+) stores, but were abolished by low extracellular Ca(2+) and pre-application of nifedipine, indicating that activation of GABA(B) receptors inhibits influx of Ca(2+) involving L-type Ca(2+) channels. Presence of GABA(C) receptors is suggested by the induction of inward current by (E)-4- amino-2-butenoic acid (TACA) and its inhibition by 1,2,5,6-tetrahydropyridine-4-ylmethylphosphinic (TPMPA), a relatively selective agonist and antagonist, respectively, of GABA(C) receptors. All of these GABA-mediated actions were found to occur in histochemically-identified cholinergic neurons. Taken together, these data indicate for the first time that cholinergic neurons of the LDT exhibit functional GABA(A, B and C) receptors, including extrasynaptically located GABA(A) receptors, which may be tonically activated by synaptic overflow of GABA. Accordingly, the activity of cholinergic LDT neurons is likely to be significantly affected by GABAergic tone within the nucleus, and so, demonstrated effects of GABA on behavioral state may be mediated, in part, via direct actions on cholinergic neurons in the LDT.

Cytosolic calcium elevation induced by orexin/hypocretin in granule cell domain cells of the rat cochlear nucleus in vitro

Using rat brain slice preparations, we examined the effect of orexin on cytosolic Ca(2+) concentrations ([Ca(2+)](i)) in the granule cell domain (GCD) cells of the cochlear nucleus that carry non-auditory information to the dorsal cochlear nucleus. Application of orexin concentration-dependently increased [Ca(2+)](i), and in two thirds of GCD cells these increases persisted in the presence of tetrodotoxin. There was no significant difference between the dose-response curve for orexin-A and that for orexin-B. Extracellular Ca(2+) removal abolished the [Ca(2+)](i) elevation induced by orexin-B, whereas depletion of intracellular Ca(2+) stores had no effect. The orexin-B-induced elevation of [Ca(2+)](i) was not blocked by inhibitors of reverse-mode Na(+)/Ca(2+) exchanger (NCX) and nonselective cation channel, whereas it was blocked by lowering the extracellular Na(+) or by applying inhibitors of forward-mode NCX and voltage-gated R- and T-type Ca(2+) channels. The ORX-B-induced increase in [Ca(2+)](i) was also blocked by inhibitors of adenylcyclase (AC) and protein kinase A (PKA), but not by inhibitors of phosphatidylcholine-specific and phosphatidylinositol-specific phospholipase C. In electrophysiological experiments using whole-cell patch clamp recordings, half of GCD cells were depolarized by orexin-B, and the depolarization was abolished by a forward-mode NCX inhibitor. These results suggest that orexin increases [Ca(2+)](i) postsynaptically via orexin 2 receptors, and the increase in [Ca(2+)](i) is induced via the AC-PKA-forward-mode NCX-membrane depolarization-mediated activation of voltage-gated R- and T-type Ca(2+) channels. The results further support the hypothesis that the orexin system participates in integrating neural systems that are involved in arousal, sensory processing, energy homeostasis and autonomic function.

Narcoleptic orexin receptor knockout mice express enhanced cholinergic properties in laterodorsal tegmental neurons

Pharmacological studies of narcoleptic canines indicate that exaggerated pontine cholinergic transmission promotes cataplexy. As disruption of orexin (hypocretin) signaling is a primary defect in narcolepsy with cataplexy, we investigated whether markers of cholinergic synaptic transmission might be altered in mice constitutively lacking orexin receptors (double receptor knockout; DKO). mRNA for Choline acetyltransferase (ChAT), vesicular acetylcholine transporter (VAChT) and the high-affinity choline transporter (CHT1) but not acetylcholinesterase (AChE) was significantly higher in samples from DKO than wild-type (WT) mice. This was region-specific; levels were elevated in samples from the laterodorsal tegmental nucleus (LDT) and the fifth motor nucleus (Mo5) but not in whole brainstem samples. Consistent with region-specific changes, we were unable to detect significant differences in Western blots for ChAT and CHT1 in isolates from brainstem, thalamus and cortex or in ChAT enzymatic activity in the pons. However, using ChAT immunocytochemistry, we found that while the number of cholinergic neurons in the LDT and Mo5 were not different, the intensity of somatic ChAT immunostaining was significantly greater in the LDT, but not Mo5, from DKO than from WT mice. We also found that ChAT activity was significantly reduced in cortical samples from DKO compared with WT mice. Collectively, these findings suggest that the orexins can regulate neurotransmitter expression and that the constitutive absence of orexin signaling results in an up-regulation of the machinery necessary for cholinergic neurotransmission in a mesopontine population of neurons that have been associated with both normal rapid eye movement sleep and cataplexy.

Hypocretin/orexin and energy expenditure

The hypocretins or orexins are endogenous neuropeptides synthesized in discrete lateral, perifornical and dorsal hypothalamic neurones. These multi-functional neuropeptides modulate energy homeostasis, arousal, stress, reward, reproduction and cardiovascular function. This review summarizes the role of hypocretins in modulating non-sleep-related energy expenditure with specific focus on the augmentation of whole body energy expenditure as well as hypocretin-induced physical activity and sympathetic outflow. We compare the efficacy of hypocretin-1 and 2 on energy expenditure and evaluate whether the literature implicates hypocretin signalling though the hypocretin-1 and -2 receptor as having shared and or functionally specific physiological effects. Thus far data suggest that hypocretin-1 has a more robust stimulatory effect relative to hypocretin-2. Furthermore, hypocretin-1 receptor predominantly mediates behaviours known to influence energy expenditure. Further studies on the hypocretin-2 receptor are needed.

State-dependent control of lumbar motoneurons by the hypocretinergic system

Neurons in the lateral hypothalamus (LH) that synthesize hypocretins (Hcrt-1 and Hcrt-2) are active during wakefulness and excite lumbar motoneurons. Because hypocretinergic cells also discharge during phasic periods of rapid eye movement (REM) sleep, we sought to examine their action on the activity of motoneurons during this state. Accordingly, cat lumbar motoneurons were intracellularly recorded, under alpha-chloralose anesthesia, prior to (control) and during the carbachol-induced REM sleep-like atonia (REMc). During control conditions, LH stimulation induced excitatory postsynaptic potentials (composite EPSP) in motoneurons. In contrast, during REMc, identical LH stimulation induced inhibitory PSPs in motoneurons. We then tested the effects of LH stimulation on motoneuron responses following the stimulation of the nucleus reticularis gigantocellularis (NRGc) which is part of a brainstem-spinal cord system that controls motoneuron excitability in a state-dependent manner. LH stimulation facilitated NRGc stimulation-induced composite EPSP during control conditions whereas it enhanced NRGc stimulation-induced IPSPs during REMc. These intriguing data indicate that the LH exerts a state-dependent control of motor activity. As a first step to understand these results, we examined whether hypocretinergic synaptic mechanisms in the spinal cord were state dependent. We found that the juxtacellular application of Hcrt-1 induced motoneuron excitation during control conditions whereas motoneuron inhibition was enhanced during REMc. These data indicate that the hypocretinergic system acts on motoneurons in a state-dependent manner via spinal synaptic mechanisms. Thus, deficits in Hcrt-1 may cause the coexistence of incongruous motor signs in cataplectic patients, such as motor suppression during wakefulness and movement disorders during REM sleep.

Orexin A-induced extracellular calcium influx in prefrontal cortex neurons involves L-type calcium channels

Orexins, novel excitatory neuropeptides from the lateral hypothalamus, have been strongly implicated in the regulation of sleep and wakefulness. In this study, we explored the effects and mechanisms of orexin A on intracellular free Ca2+ concentration ([Ca2+]i) of freshly dissociated neurons from layers V and VI in prefrontal cortex (PFC). Changes in [Ca2+]i were measured with fluo-4/AM using confocal laser scanning microscopy. The results revealed that application of orexin A (0.1-1 microM) induced increase of [Ca2+]i in a dose-dependent manner. This elevation of [Ca2+]i was completely blocked by pretreatment with selective orexin receptor 1 antagonist SB 334867. While depletion of intracellular Ca2+ stores by the endoplasmic reticulum inhibitor thapsigargin (2 pM), [Ca2+]i in PFC neurons showed no increase in response to orexin A. Under extracellular Ca2+-free condition, orexin A failed to induce any changes of Ca2+ fluorescence intensity in these acutely dissociated cells. Our data further demonstrated that the orexin A-induced increase of [Ca2+]i was completely abolished by the inhibition of intracellular protein kinase C or phospholipase C activities using specific inhibitors, BIS II (1 microM) and D609 (10 microM), respectively. Selective blockade of L-type Ca2+ channels by nifedipine (5 microM) significantly suppressed the elevation of [Ca2+]i induced by orexin A. Therefore, these findings suggest that exposure to orexin A could induce increase of [Ca2+]i in neurons from deep layers of PFC, which depends on extracellular Ca2+ influx via L-type Ca2+ channels through activation of intracellular PLC-PKC signaling pathway by binding orexin receptor 1.

Hypocretin mechanisms in nicotine addiction: evidence and speculation

BACKGROUND

The hypocretin/orexin system has been implicated in arousal mechanisms, sleep, and sleep disorders, including narcolepsy, and more recently in drug addiction. Theoretically, hypocretin (hcrt) mechanisms appear to be potential substrates for nicotine addiction: arousal and attentional mechanisms influence use and withdrawal symptoms, and hcrt systems overlap anatomically with a number of brain regions associated with nicotine addiction.

OBJECTIVE

This review summarizes the studies that have examined hcrt mechanisms in the effects of nicotine and describes hcrt innervation of, and effects in, several brain regions implicated in nicotine addiction. The review speculates on the possible mechanisms by which hcrt may contribute to nicotine addiction in these regions, with the objective of encouraging research in this area.

RESULTS

In a small literature, both experimenter-administered and self-administered nicotine have been shown to elicit or depend on hcrt signaling. However, although untested in experimental designs, there is compelling evidence that hcrt mechanisms in the ventral tegmental area, the pontine region, thalamocortical circuits, the prefrontal cortex, and the amygdala could have a broad influence on nicotine addiction.

CONCLUSIONS

Evidence reviewed leads to the conclusion that hcrt mechanisms could mediate several dimensions of nicotine addiction, including a multi-faceted regulation of mesocorticolimbic dopaminergic function, but beyond dopaminergic mechanisms, hcrt could influence nicotine use and relapse during abstinence through broadly based arousal/attentional effects. These speculative ideas need to be examined experimentally; the potential gains are a more thorough understanding of the pathophysiology of nicotine addiction, and the discovery of novel targets for the development of pharmacotherapeutics.

Short food deprivation inhibits orexin receptor 1 expression and orexin-A induced intracellular calcium signaling in acutely isolated duodenal enterocytes

Close intra-arterial infusion of the appetite regulating peptide orexin-A stimulates bicarbonate secretion from the duodenal mucosa. The aim of the present study was to elucidate the ability of orexin-A to induce intracellular calcium signaling in acutely isolated duodenal enterocytes. Freshly isolated clusters of enterocytes, obtained from rat duodenal mucosa or human duodenal biopsies, were loaded with fura 2-AM and mounted in a perfusion chamber. Cryptlike enterocytes were selected (caged), and changes in intracellular calcium concentration ([Ca2+]i) were evaluated by fluorescence imaging. Total RNA was extracted from pellets of enterocytes and reverse transcribed to cDNA, and expression of orexin receptors 1 and 2 (OX1R and OX2R) was measured by quantitative real-time PCR. Orexin-A at all concentrations tested (1-100 nM) increased [Ca2+]i in enterocytes isolated from continuously fed rats, and the OX1R-antagonist SB-334867 (10 nM) attenuated the response. The primary [Ca2+]i response was a slow increase to a sustained plateau persisting after orexin-A removal, and a similar response was observed in enterocytes from human biopsies. In contrast to orexin-A, the OX2R agonist (Ala11,D-Leu15)-orexin-B (1-10 nM) did not induce calcium signaling. There were no significant [Ca2+]i responses in enterocytes from animals food deprived overnight, and overnight fasting decreased (P<0.01) enterocyte OX1R as well as OX2R mRNA. Induction of intracellular calcium signaling in isolated duodenal enterocytes is thus mediated primarily by OX1R receptors. Short (overnight) food deprivation markedly depresses receptor expression and inhibits orexin-A induced increases in [Ca2+]i. Studies of enterocyte signaling and intestinal secretion requires particular evaluation regarding feeding status.

Copeptin immunoreactivity and calcium mobilisation in hypothalamic neurones of the rat

Copeptin is cleaved from the C-terminus of vasopressin (VP) prohormone. Immunohistochemical studies have revealed intense copeptin-immunoreactivity (irCOPT) in neurones of the rat hypothalamic nuclei, including paraventricular, supraoptic, suprachiasmatic, periventricular, and accessory secretory. Varicose cell processes emanated from irCOPT neurones, some of which projected caudally and traversed the internal layer of the median eminence, and terminated in the posterior pituitary. Double-labelling hypothalamic sections with copeptin antiserum and VP or oxytocin antiserum revealed an extensive overlapping of irCOPT and irVP neurones. The biological activity of human synthetic nonglycosylated copeptin or VP was evaluated in vivo and in vitro. Copeptin (1, 10, and 20 nmol/kg) injected i.v. caused no significant changes in the mean arterial pressure (MAP) and heart rate of urethane-anaesthetised rats. VP (0.1 nmol/kg) increased MAP, which was accompanied by a small decrease of the heart rate. The ratiometric fluorescence method was employed to assess changes in intracellular Ca2+ concentrations [Ca2+](i) which served as an index of the biological activity of peptides. VP (1 microM) markedly increased [Ca2+](i) of rat hypothalamic neurones or vascular smooth muscle cells, whereas copeptin (100 nm to 1 microM) caused a low amplitude, sustained increase of [Ca2+](i) in a population of hypothalamic neurones, but not in any of the vascular smooth muscle cells tested. The results obtained demonstrate that copeptin is expressed in VP neurones and that the peptide in the concentrations tested, although causing little or no detectable changes of blood pressure and heart rate in anaesthetised rats nor changes in [Ca2+](i) of cultured aortic smooth muscle cells, increases [Ca2+](i) in a small population (< 2%) of hypothalamic neurones tested, indicating that copeptin is biologically active in mammalian neurones.

Dual orexin actions on dorsal raphe and laterodorsal tegmentum neurons: noisy cation current activation and selective enhancement of Ca2+ transients mediated by L-type calcium channels

The hypocretin/orexins (Hcrt/Orxs) are hypothalamic neuropeptides that regulate stress, addiction, feeding, and arousal behaviors. They depolarize many types of central neurons and can increase [Ca2+]i in some, including those of the dorsal raphe (DR) and laterodorsal tegmental (LDT) nuclei-two structures likely to contribute to the behavioral actions of Hcrt/Orx. In this study, we used simultaneous whole cell and Ca2+-imaging methods in mouse brain slices to compare the Hcrt/Orx-activated current in DR and LDT neurons and to determine whether it contributes to the Ca2+ influx evoked by Hcrt/Orx. We found Hcrt/Orx activates a similar noisy cation current that reversed near 0 mV in both cell types. Contrary to our expectation, this current did not contribute to the somatic Ca2+ influx evoked by Hcrt/Orx. In contrast, Hcrt/Orx enhanced the Ca2+ transients produced by voltage steps (-60 to -30 mV) by approximately 30% even in neurons lacking an inward current. This effect was abolished by nifedipine, augmented by Bay-K and abolished by bisindolylmaleimide I. Thus Hcrt/Orx has two independent actions: activation of noisy cation channels that generate depolarization and activation of a protein kinase C (PKC)-dependent enhancement of Ca2+ transients mediated by L-type Ca2+ channels. Immunocytochemistry verified that both these actions occurred in serotonergic and cholinergic neurons, indicating that Hcrt/Orx can function as a neuromodulator in these key neurons of the reticular activating system. Because regulation of Ca2+ transients mediated by L-channels is often linked to the control of transcriptional signaling, our findings imply that Hcrt/Orxs may also function in the regulation of long-term homeostatic or trophic processes.

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.

Hypocretin-1 potentiates NMDA receptor-mediated somatodendritic secretion from locus ceruleus neurons

Our previous observations showed that several stimuli, including high-K(+) solution, glutamate, and voltage pulses, induce somatic noradrenaline (NA) secretion from locus ceruleus (LC) neurons. Hypocretin (orexin), a hypothalamic peptide critical for normal wakefulness, has been shown to evoke NA release from the axon terminals of LC neurons. Here, we used amperometry to test the effect of hypocretin-1 (HCRT) on NMDA receptor-mediated somatodendritic release in LC neurons. Either HCRT or NMDA applied alone dose-dependently induced somatodendritic secretion. Bath application of HCRT notably potentiated NMDA receptor-mediated somatodendritic NA release. This potentiation was blocked by SB 334867, a selective HCRT receptor (Hcrtr 1) antagonist, or bisindolylmaleimide, a specific protein kinase C (PKC) inhibitor, indicating the involvement of Hcrtr 1 and PKC. Consistent with this, phorbol 12-myristate 13-acetate, a PKC activator, mimicked the HCRT-induced potentiation. Furthermore, HCRT enhanced NMDA-induced intracellular Ca(2+) elevation via activation of Hcrtr 1 and PKC, which may contribute to HCRT-potentiated somatodendritic secretion. These results suggest that HCRT modulates LC activity not only by regulating noradrenergic input to its targets, but also by affecting noradrenergic communication in the soma and dendrites.

Changes in glucose do not alter baseline firing rate or chemosensitivity of serotonin neurons cultured from the medullary raphé

A subset of serotonin neurons are putative central respiratory chemoreceptors. To test the hypothesis that serotonin neurons also have intrinsic glucose sensitivity, perforated patch recordings were made from cultured rat medullary raphé neurons after pharmacological blockade of fast glutamatergic and GABAergic synaptic transmission. It has previously been shown that all neurons stimulated by acidosis under these conditions are tryptophan hydroxylase (TPH) immunoreactive. Changes in glucose concentration from 0.1 to 20 mM had no effect on baseline firing rate of 31 neurons that were stimulated by hypercapnic acidosis. The response to hypercapnic acidosis of 25 of these neurons was the same in 0.1, 1, 2, 5 and/or 20 mM glucose as it was in 10 mM glucose. Changes in glucose also did not alter the baseline firing rate of 13 neurons that did not respond to acidosis. Although it is possible that these results were influenced by the use of tissue culture, they do not provide support for intrinsic glucose sensitivity of pH-sensitive serotonin neurons of the medullary raphé.

Association between the G1246A polymorphism of the hypocretin receptor 2 gene and cluster headache: a meta-analysis

The objective of this study was to investigate the association between polymorphisms of the hypocretin receptor 2 gene (HCRTR2) and the risk of cluster headache (CH). The study is a meta-analysis of published case-control studies investigating the association between polymorphisms of the HCRTR2 gene and CH. Pooled odds ratios (OR) were estimated using both random (RE) and fixed effects (FE) models. Three studies, performed in five different European countries, with 593 cases and 599 controls, were included in the study. Allele G of the G1246A HCRTR2 polymorphism was significantly associated with CH (FE OR 1.58, CI 95% 1.27–1.95; RE OR 1.55 (1.14–2.12)). Carriers of the GG genotype showed a higher disease risk compared to the remaining genotypes (FE OR 1.75, CI 95% 1.37–2.25; RE OR 1.69, CI 95% 1.11–2.58). Our data confirm that the G1246A polymorphism of the HCRTR2 gene may modulate the genetic risk for CH.

The Hypothalamic Orexinergic System: Pain and Primary Headaches

The primary headaches are a group of distinct individually characterized attack forms, which although varying in presentation, share some common anatomical basis responsible for the pain component of the attack. The hypothalamus is known to modulate a multitude of functions and has been shown to be involved in the pathophysiology of a variety of primary headaches including cluster headache and chronic migraine. It seems likely that it may be involved in other primary headache disorders due to their episodic nature and may underlie many of their diverse symptoms. We discuss the hypothalamic involvement in the modulation of trigeminovascular processing and examine the involvement of the hypothalamic orexinergic system as a key regulator of this function.

The orexin OX1 receptor regulates Ca2+ entry via diacylglycerol-activated channels in differentiated neuroblastoma cells

We studied the cellular response to orexin type 1 receptor (OX1R) stimulation in differentiated IMR-32 neuroblastoma cells. In vitro differentiation of IMR-32 cells with 5-bromo-2'-deoxyuridine leads to a neuronal phenotype with long neurite extensions and an upregulation of mainly N-type voltage-gated calcium channels. Transduction of differentiated IMR-32 cells with baculovirus harboring an OX1R-green fluorescent protein cDNA fusion construct resulted in appearance of fluorescence that was confined mainly to the plasma membrane in the cell body and to neurites. Application of orexin-A to fluorescent cells led to an increase in intracellular free Ca2+ concentration, [Ca2+]i. At low nanomolar concentrations of orexin-A, the response was reversibly attenuated by removal of extracellular Ca2+, by application of a high concentration (10 mM) of Mg2+, and by the pharmacological channel blocker dextromethorphan. A diacylglycerol, dioctanoylglycerol, but not thapsigargin or depolarization with potassium, mimicked the OX1R response with regard to Mg2+ sensitivity. A reverse transcription-PCR screening identified mRNAs for all transient receptor potential canonical (TRPC) channels, including TRPC3, TRPC6, and TRPC7, which are known to be activated by diacylglycerol. Expression of a dominant-negative TRPC6 channel subunit blunted the responses to both dioctanoylglycerol and OX1R stimulation. The results suggest that the OX1R activates a Ca2+ entry pathway that involves diacylglycerol-activated TRPC channels in neuronal cells.

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.

Hypocretin/orexin selectively increases dopamine efflux within the prefrontal cortex: involvement of the ventral tegmental area

Hypocretins (HCRTs) modulate a variety of behavioral and physiological processes, in part via interactions with multiple ascending modulatory systems. Further, HCRT efferents from the lateral hypothalamus innervate midbrain dopamine (DA) nuclei, and DA cell bodies express HCRT receptors. Combined, these observations suggest that HCRT may influence behavioral state and/or state-dependent processes via modulation of DA neurotransmission. The current studies used in vivo microdialysis in the unanesthetized rat to first characterize the effect of intracerebroventricular infusion of HCRT-1 (0.07, 0.7 nmol) on extracellular levels of DA within the prefrontal cortex (PFC) and nucleus accumbens (Acc). Electroencephalographic/electromyographic measures of sleep-wake state were collected along with select behavioral measures (eg locomotor activity, grooming). HCRT-1 dose-dependently increased PFC dialysate DA levels, and these increases were closely correlated with increases in time spent awake. In contrast, Acc DA levels were unaffected. Additional studies examined whether HCRT-1 acts directly within the ventral tegmental area (VTA) to selectively increase PFC DA efflux and modulate behavioral state. Unilateral infusion of HCRT-1 (0.1, 1.0 nmol) within the VTA increased PFC, but not Acc, DA levels. Importantly, intra-VTA infusion of HCRT-1 increased the time spent awake and grooming. Moreover, HCRT-induced increases in both time spent awake and time spent grooming were significantly correlated with post-infusion PFC DA levels. The current observations predict a prominent modulatory influence of HCRT on PFC-dependent cognitive and affective processes that results, in part, from actions within the VTA. Additionally, these observations suggest that the activation of VTA DA neurons contributes to the behavioral state-modulatory actions of HCRT.

The wake-promoting peptide orexin-B inhibits glutamatergic transmission to dorsal raphe nucleus serotonin neurons through retrograde endocannabinoid signaling

The wake-promoting neuropeptides orexins (hypocretins) play a crucial role in controlling neuronal excitability and synaptic transmission in the CNS. In this study, using whole-cell patch-clamp recordings in an acute dorsal raphe nucleus (DRN) slice preparation, we report that orexin B (Orx-B) depresses the evoked glutamate-mediated synaptic currents in DRN 5-HT neurons. The Orx-B-induced depression is accompanied by an increase in the paired-pulse ratio and the coefficient of variance, suggesting a presynaptic site of action. Orx-B also reduces the frequency but not the amplitude of miniature EPSCs, indicating that depression of glutamatergic transmission is mediated by a decrease in glutamate release. Surprisingly, the Orx-B-induced inhibition of glutamatergic transmission is abolished by postsynaptic inhibition of G-protein signaling with GDPbetaS, suggesting that this effect is signaled by postsynaptic orexin receptors and expressed presynaptically, presumably through a retrograde messenger. Interestingly, the Orx-B-induced depression of glutamate release is mimicked and occluded by the cannabinoid receptor agonist WIN 55,212-2, and is abolished by the CB1 cannabinoid receptor antagonist AM 251. These results imply that the Orx-B-induced depression of glutamatergic transmission to DRN 5-HT neurons is mediated by retrograde endocannabinoid release. Examination of downstream signaling pathways involved in this response indicates that the effect of Orx-B requires the activation of phospholipase C and DAG lipase enzymatic pathways but not a rise in postsynaptic intracellular calcium. Therefore, our findings reveal a previously unsuspected mechanism by which postsynaptic orexin receptors can modulate glutamatergic synaptic transmission to DRN 5-HT neurons.

Mechanisms of orexin A-evoked changes of intracellular calcium in primary cultured cortical neurons

We have investigated the effect of orexin A on the intracellular free calcium concentration ([Ca2+]i) in primary cultured cortical neurons and explored the exact mechanisms of orexin A-evoked changes of [Ca2+]i. In the present study, changes of [Ca2+]i induced by orexin A in primary cultured cortical neurons were first detected by confocal laser scanning microscopy using Ca2+-sensitive dye fluo-4 as a novel calcium fluorescent probe. Our results showed that 1-0.1 microM orexin A induced the increase in [Ca2+]i in cortical neurons. The increase in [Ca2+]i by acute application of orexin A occurred in a dose-dependent manner. Orexin A-induced increase in [Ca2+]i was not observed under the condition of Ca2+-free Dulbecco's modified Eagle's medium. Pretreatment on the cells with 1 microM thapsigargin did not block orexin A-evoked response. These findings first illuminated the fact that orexin A-induced increase in [Ca2+]i may be mainly from extracellular calcium influx in cortical neurons.

OX1 orexin receptors couple to adenylyl cyclase regulation via multiple mechanisms

In this study, the mechanism of OX(1) orexin receptors to regulate adenylyl cyclase activity when recombinantly expressed in Chinese hamster ovary cells was investigated. In intact cells, stimulation with orexin-A led to two responses, a weak (21%), high potency (EC(50) approximately 1 nm) inhibition and a strong (4-fold), low potency (EC(50) = approximately 300 nm) stimulation. The inhibition was reversed by pertussis toxin, suggesting the involvement of G(i/o) proteins. Orexin-B was, surprisingly, almost equally as potent as orexin-A in elevating cAMP (pEC(50) = approximately 500 nm). cAMP elevation was not caused by Ca(2+) elevation or by Gbetagamma. In contrast, it relied in part on a novel protein kinase C (PKC) isoform, PKCdelta, as determined using pharmacological inhibitors. Yet, PKC stimulation alone only very weakly stimulated cAMP production (1.1-fold). In the presence of G(s) activity, orexins still elevated cAMP; however, the potencies were greatly increased (EC(50) of orexin-A = approximately 10 nm and EC(50) of orexin-B = approximately 100 nm), and the response was fully dependent on PKCdelta. In permeabilized cells, only a PKC-independent low potency component was seen. This component was sensitive to anti-Galpha(s) antibodies. We conclude that OX(1) receptors stimulate adenylyl cyclase via a low potency G(s) coupling and a high potency phospholipase C --> PKC coupling. The former or some exogenous G activation is essentially required for the PKC to significantly activate adenylyl cyclase. The results also suggest that orexin-B-activated OX(1) receptors couple to G(s) almost as efficiently as the orexin-A-activated receptors, in contrast to Ca(2+) elevation and phospholipase C activation, for which orexin-A is 10-fold more potent.

Orexin-A-induced Ca2+ entry: evidence for involvement of trpc channels and protein kinase C regulation

The orexins are peptide transmitters/hormones, which exert stimulatory actions in many types of cells via the G-protein-coupled OX(1) and OX(2) receptors. Our previous results have suggested that low (subnanomolar) concentrations of orexin-A activate Ca(2+) entry, whereas higher concentrations activate phospholipase C, Ca(2+) release, and capacitative Ca(2+) entry. As shown here, the Ca(2+) response to subnanomolar orexin-A concentrations was blocked by activation of protein kinase C by using different approaches (12-O-tetradecanoylphorbol acetate, dioctanoylglycerol, and diacylglycerol kinase inhibition) and protein phosphatase inhibition by calyculin A. The Ca(2+) response to subnanomolar orexin-A concentrations was also blocked by Mg(2+), dextromethorphan, and tetraethylammonium. These treatments neither affected the response to high concentrations of orexin-A nor the thapsigargin-stimulated capacitative entry. The capacitative entry was instead strongly suppressed by SKF96365. An inward membrane current activated by subnanomolar concentrations of orexin-A and the currents activated upon transient expression of trpc3 channels were also sensitive to Mg(2+), dextromethorphan, and tetraethylammonium. Responses to subnanomolar concentrations of orexin-A (Ca(2+) elevation, inward current, and membrane depolarization) were voltage-dependent with a loss of the response around -15 mV. By using reverse transcription-PCR, mRNA for the trpc1-4 channel isoforms were detected in the CHO-hOX1-C1 cells. The expression of truncated TRPC channel isoforms, in particular trpc1 and trpc3, reduced the response to subnanomolar concentrations of orexin-A but did not affect the response to higher concentrations of orexin-A. The results suggest that activation of the OX(1) receptor leads to opening of a Ca(2+)-permeable channel, involving trpc1 and -3, which is controlled by protein kinase C.